A handbook of gold milling

Name: A handbook of gold milling
Author: Louis, Henry

Louis, Henry

A handbook of gold milling

Louis, Henry · 1894 · Historical

The art of gold-milling rests upon a twofold scienti basis, the comminution of the ore being a pun mechanical and the extraction of the gold a pun chemical

Overview

A handbook of gold milling is an 1894 historical mining reference by Louis, Henry, preserved in the Mountain Man Mining research library, focused on stamp mill ore. The art of gold-milling rests upon a twofold scienti basis, the comminution of the ore being a pun mechanical and the extraction of the gold a pun chemical…

This 1894 document, A handbook of gold milling, is preserved in the Mountain Man Mining Library for research and reference. Original source: archive.org.

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A Handbook Of Gold Milling

if

A Handbook

Of

Gold Milling

I'

By

Henry Iouis, M.A.

ASSOCIATE OK THK KnVAL SCHOOL OK MINKS

rEl.LOW OK THK INSTITUTK OF CMKMISTRV OK (.KEAT IlKII A(N

Mkmkek Ok Thk Institution Ok .Minin<. Knginfkks

ULMBKR OF THK AMERICAN INSTITUTE OK MINIS*. KN(;iNKKKS, EIC, ETC.

FKOFESSOK OF MININi; IN THK DUKHAM COI.I.Et.K OK SCIENCE

J

MACMILLAN AND CO., Limited

New York : The Macmii.Lan Com T/ Ny

T9O2

Preface To The Fibst Edition

The process of differentiation, so essentially characristic

of all modem development, has of recent years

extended very markedly into the province of the goldttiinc

T. The time is not so long past since any ordinary

Mt in the mill, and it was looked upon as part of

his duty to be equally useful above and below ground.

This state of things did well enough jus long as rich

reefs, or rich [wrtions of reefs, such ixs the superficial

[Krtiuns not uncommonly are, were alone worked ;

enough gold was won to more than pay expenses, and

no one knew or even seemed to care how great the

Ij88 might be in the mill. With the rapid exhaustion

of high grade reefs and the introduction of more

scientific methods of gold extraction, which (together

with improved methods and machinery in mining

proper) have rendered the exploitation of low-grade

ores profitable and therefore possible, a gradual change

viii rh'h/'ACIi

has made itself felt within the last few years. At tli same time a class of men has gradually come int existence, and has been educated — by the hard lessor of practical experience for the most part — to ciirry oi the duties of amalgamator, reduction officer, or mi man, these being the various names by which this poj is designated in various parts of the world — the vei divergence in nomenclature serving to show how vei recently the need for and existence of such a trade ha\ come to be recognised. Under these circumstances is hardly to be wondered at that the technical educatio of the mill man still leaves much to be desired, and th main object of the present volume is to impart so muc of such technical instruction in his duties as ciin 1: included within the covers of a book. Incidentally is hoped that it may also prove of service to managei and managing directors of mines, not only by showin them the most general causes of loss both of mone and material in gold-milling, and by indicating th readiest means of preventing such loss, but also h pointing out to them what are the essential portions c a mill man's duty and to guide them in selecting a dul qualified man for this onerous post, upon the prop( filling of which the prosperity of a mine is so largel dependent. For whilst it is quite true that no one ca make a poor mine into a good one, the converse of th proposition is unfortunately by no means impossibk and it is an undoubted fact that there is in existenc a. laiHTtt number of mines which would give ft

PREFACE ix

better results than they actually do, were the charge of

the mill handed over to a properly qualified mill luan.

It u not too much to say that not one manager in ten

bioii-show much gold he is losing in his mill — what he

is getting is really a secondary question — and this, the

&ult in the first instance of an imperfectly trained mill

man, renders it impossible to detect the shortcomings

of the latter, whose very incapacity thus serves as its

own shield.

It is only quite recently that the science of goldnjilling has come to bo at all fully recognised. In HHist crises this work has been carried on by the light of untr.iine<l instinct, the iffnis fninus th.at has so frf<juently led the way to destruction. It is the object the writer to apply the results of modem science so its to rrplaee Jis far as jH)ssible the uncerttiin methods of crude empiricism, and especially to substitutes scientiHe for unscientific nuMles of rcasoning. Fiiets can usually be trustc'd to taku care of thiansidves, but the sunt? cannot be said of the methods by which these fsicts are arrived at. Scientific reasoning is a process far k*s.s costly and quite as reliable as unscientific groping in thf dark, and the first step towards the foundation of such science is the chissificiition of our knowledge on a syst4*matic bsisis. This is the main object that has been attinpted in the following work, which commences by an account of the physical and chemical properties of gold and also of mercury, the knowledge of the latter being of scarcely leas importance than that of the formex

X Preface

for the scientific amalgamator ; stamp mill constructi is considered in detail, the mechanical principles und< lying the design of each part being throughout elu dated. The theory and practice of concentration, as ; as it refers to gold-milling, is next considered, togeth with the most approved modem method of treating t concentrates and the other products of milling, chapter on the economic considerations involved, a one on the assaying of gold ores and mill products i also appended.

The art of gold-milling rests upon a twofold scienti basis, the comminution of the ore being a pun mechanical and the extraction of the gold a pun chemical process. The mill man should according have a knowledge of the theories of both Mechan (including Physics) and Chemistry. Practically should have served his time as an engineer, or at le; have gone through the shops sufficiently to be a gc fitter and a fair carpenter ; he should also have h some experience in looking after machinery in moti In addition to the above, he ought to have worked a year or so in a chemist's or jissayers laborato where he will have acquired habits of accuracy chemical manipulation, and where he ought to hf learnt some assaying. He is then duly prepared uii(l(irtake the duties of a mill man, and the wri hopi'S that a study of this volume will teach h exactly what are the problems which he will be cal upon to solve, and what are at present the m

Preface Xl

approved methods of solving them. Much of the iofonnation here given may be found scattered through various British, American, and foreign works on Metalluigjr, and more especially in the Transactions of Societies devoted to the cultivation of this and oqgoate sciences. So very many of the fietcts contained in this volume have become the common property, so toqieak, of metallurgists, that, having regard, moreover, to the essentially practical purposes of the present work, it has been considered advisable not to encumber its pages with references to all the sources whence information has been derived ; the author accordingly hopes that this general acknowledgment of obligation to previous writers on this subject will be deemed sufficient.

Lojcdon,

November, 1893.

Preface To The Second And Third

Editions

As the general arrangement of this work seems to bave answered its purpose £urly well, I have left it as fitf as possible untouched, merely bringing it thoroughly up to date. The greatest difficulty has been experienced in keeping down the bulk of the book on account of the great advances that have been made in the last few years in the art of gold extraction ; it is at the same time gratifying to find that many of the mmendations made in the first edition have now P'ed into current gold-milling practice.

'kh CASTLE- U PON -TY N

Jun, 1899. Auguitt, 19U2.

Chapter I

Mode Of Occurrence — Deposits — Para6Eneris — Various

Forms Of Gold

Gold is a metal that has been known to, and prized by, mankind from pro-historic times. Occurring as it does in the native state, its great specific gravity and well-marked colour and lustre must have attracted the attention of savages, even at a very early stage of the development of their powers of observation.

At a period when the extraction of other metals from their ores would have proved a task impossible to their rudimentary resources, the great softness, malleabihty, and ready fusibility of gold (the latter probably discovered after brief experience) would have combined to render it of high value to them for many purposes. And yet, in spite of its having been known and worked for so long a period, the scientific metallurgy of gold is in reality but little more than half a century old. Up till then there seems to have been little or no improvement in the methods employed for its extraction since mediaeval times, gold working and especially gold milling having

2 GOLD MILLING CHr-

remained rude and neglected industries until thejgreat discoveries of gold in California in 1849, and almost immediately afterwards those in Australia, turned the energies of thousands of able, active, intelligent men into this new channel. In the mad race for wealth which then ensued, the primitive methods, all sufficient up to that time, were soon found to be too crude and too slow to satisfy the ambitions of modern gold diggers, whose inventive faculties, stimulated to the highest pitch, were prolific in new methods and improvements upon the old ones. When the rich shallow placers and the rich superficial pockets of gold quartz had been exhausted, and it became necessary to work the poorer alluvials and low-grade reefs, the utmost resources of modem science became even more indispensable, and the present complex system of gold extraction is the ultimate result. Of course this industry is even yet in a comparatively rudimentary and backward state ; the stamp-mill is at best a somewhat cumbersome machine, and the science of the subject is even now but imperfectly understood. Nevertheless the art is very far from stationary, and steady progress is being made in various directions, with the result that sources of gold are now being profitably worked which are actually poorer than the tailings, which were a few years ago allowed to run to waste as worthless.

Occurrence of Gold. — The manner in which gold occurs in nature may be divided for the purpose of the present work into two classes, this division being neither geological nor mineralogical, but simply technical, dependent upon the methods employed for extracting the precious metal from the matrix that carries it :

Clas A. — Gold occurring in a matrix capable of complete disintegration by the action of a stream of water.

Cms B. — Gold occurring in a firm matrix, which qaires the employment of mechanical appliances for its pulverisation.

CUus A. — In this is included the occurrence of gold in aliavial deposits of various ages, in river sands, on seashores, and in gravels of various characters; the gold, which occurs in extremely small proportions as a general role, is readily extracted by the comparatively primitive method of washing. When the mass, consisting of gold in adaiixture with the lighter substances with which it is associated, is agitated in a stream of water, the relatively heavy particles of gold sink to the bottom, whilst the relatively light sand, pebbles and clay are carried off by the current. It is by this method, or some variety thereof, that gold has always been obtained by primitive races of mankind. The washing was originally, no doubt, performed in a dish of some kind, and subsequently in a trough of greater or less length. In time the former developed into the cradle, which consists of a short rectangular box to which a sieve is fitted, and which is supported upon rockers, thus receiving a lateral undulating motion, whilst the auriferous gravel, together with water, is thrown into it. The latter method developed into the ground sluice, which is simply a channel cut into the ground itself, and is often of very great length ; and finally American ingenuity produced the various methods of **hydraulicking," which consists in projecting a powerful stream of water, brought in pipes from a considerable elevation, against the face of a gravel deposit, and then washing down the latter at a very rapid rate, and at a minimum of cost. Mercury is frequently used in most of the modem methods to assist in the separation of gold from the other materials with which

B 2

CrOLD MII.I.INi; HA

r'

it is associated. All the above iiiethods are usually iH' eluded under the general head of "Hydraulic Gol Mining." Their consideration is entirely foreign to the purposes of the present work, which is concerned ex* clusively with the methods employed for the treatment of such auriferous deposits as are included in Class B.

Cldss B, — This class comprises the following modes of occurrence, which differ, as will be seen, more in the manner of their origin than in their intrinsic composition ; in other words, this class is subdivided on geological rather than on mineralogical principles.

(1.) Beefs (known also as lodes, leads or veins). —These may be of any of the well-known types of mineral veins, fissure veins being perhaps the most abundant (Australia, Nova Scotia), though contact veins (Montana) and intercalated veins (Nova Scotia) also frequently occur; a variety of this form of occurrence is that in which a stratified deposit, highly metamorphosed, is traversed by a reticulated mass of veinlets which carry gold. As a notable example of this form, the well-known "Great Mother Lode " of California may be quoted. The country rock is mostly slate or schist ; sometimes the auriferous veinlets are carefully picked out and treated separately, but more often the entire mass of rock and reef is sent to the mill, the method adopted varying with the richness, size, and physical conditions of the deposit. The principal portion of the gangue of which auriferous veins are composed, is always quartz.

(2.) Metamorphosed Ancient Stratified Deposits, — These are at times only an extreme variety of the occurrence last mentioned, where schists or shales, either in the neighbourhood of quartz reefs, or of which such veins form a larger or smaller portion, are impregnated with

' Mode Of Occurrence 5

Again, deposits which have by extreme metamorprism

passed into qoartzites, sometimes carry gold ; a

'K)toUe example of this is the so-called " Sheba reef,"

oear Barberton, South Africa.

(3.) Gravel Deposits of Various Ages, such as are included under Glass il, are sometimes found consolidated by metamorpblc action, or cemented by various binding materials into an auriferous conglomerate, occasionally very liard. In these the gold may be either contemporaneous with the pebbles forming the deposit, and is then like them rounded and waterwom, as is the case in the Cement Mines " of California, or else it has been introduced subsequently to the deposition of the bed, when it presents a crystalline structure, as in the well-known Banket " of Witwatersrand, South Africa.

(4.) Eruptive Bocks. — Some deposits of auriferous eruptive rocks arc known, and worked ; these rocks are generally acidic, like the granites of Treadwcll, Alaska, U.S.A. ; Tiuibarra, New South Wales ; Beresof, Urals ; Sonora, Mexico ; Cripple Creek, Colorado, &c. ; in some at any rate of these instances it would appear that the gold may have formed a primary constituent of the rock mass. Gold has also been said to occur in basic cruptives like diorite and basalt, but the evidence of these occurrences is less convincing.

Whatever their origin, all these divisions of Class B consist essentially of a siliceous matrix carrying gold, mostly in admixture with small proportions of other inineitJs. Quartz is the universal vein-stuff of gold, and as practically all other modes of occurrence of gold appear to be derived from the disintegration of such veins in the first instance, clay being mostly present in some form, the chemical composition of all the above divisioub is \)vac-

Gold Milung

Chap'-

lustily idoutioal, being mostly silica, together with mor 01 Infill Hilioate of alumma, oxide of iron, to. Among gold the practice mostly obtains of speaking of an an- I ifuioua matrix, whatever be its real nature, as "quartz." 'Wis ifsrin is of course inexact, but at the same time it is (|iutru true that the principal portion of such matrix does lislly consist of silica; it is also worth noting that in <ji(5ral the gold miner indiscriminately calls all these ViiiidUM occurrences " reefs," whether they be true veins <'i not.

'lhe following table gives an approximate estimate of Uiu amount of gold produced in the world from deposits iA ( 'lass A and Class B respectively in the year 1897 :

Class A. Alluvial Gutd.

Fine Gold. ! Value in ounces. I Sterling.

Vfih-a .Vi.iiOO

Idol uf North America S30,*00

mtlt America 418,000

Total* 2,9-27,000

£1,m5,500 l,l>96,500 1,401,000 1,775,04)0 3,721.000 1,614,000

Class B. Rvei Gold.

Fine Gold, ounces.

2,170,000 2,7t,00O 1 2,400,000 .MO.OOO 180,000 2'.>5,000 410,000

Value iu Sterling.

7m.&00

Minerals Associated with Gold. — There are a large uuitibor of minerals that accompany gold in the deposits lif (51ai4S J5, some of which are frequently, and some but luioly, associated with gold. It is important that these mi literals should be thoroughly known, and their properties carefully studied, as their presence or absence may wxort a very marked influence upon the milling of the ore.

Mode Of Occurrence 7

These associated minerals may conveniently be divided into four groups :

A. Non-metallic minerals.

B. Metallic minerals, rarely or never auriferous.

C. Metallic minerals, frequently auriferous.

I), Minerals containing gold as an essential element of their composition.

The minerals of Classes B and G occur with gold, and frequently show visible metallic gold enclosed in their substance ; those of Class B rarely contain gold in a state in which it is held invisibly, whereas Class G often holds gold in this condition. The following is a list, as complete as possible, of all minerals that have been known to occur in gold-bearing veins. The rare minerals, which are only known in a few special localities and need not be looked for as a general rule, are in small type, the common ones being distinguished by full-faced type. Many of these minerals are, it will be noted, decomposition products of others that are more essentially the true original constituents of auriferous reefs, and their occurrence is accordingly limited to such portions of the deposits as lie above water level and have been subjected to atmospheric influences. Thus in Class sulphur is a. decomposition product of iron pyrites, and selenite may result from the oxidation of the same mineral, the sulphuric acid so formed in its turn decomposing calcite. A very large number of the minerals comprised in Class B are the results of decomposition or oxidation of some of those in Class C Whenever these secondary minerals are found in the upper portion of a deposit, the minerals from which they are derived may be expected to occur in depth.

ii m

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it

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is 3i,s lilt I?; E s fi|Slf|aa'slfS3.

GOLD MILLING chap.

that as a general rule minerals with which gold is intimately associated (Classes C and have a specific gravity of about 5, These minerals are usually the object of a special series of operations for their collection with a view to the extraction of the gold they contain. When thus collected they form the concentrates" or '*8ulphurets " of the gold miner; either term may be and is used indifferently, but the former is certainly the more correct. These concentrates may obviously contain any of the minerals of Classes C and Z>, and they will also at times contain some of the heavier ones of Class B, notably magnetite ; these latter minerals must, however, be looked upon as diluents or impurities of the concentrates proper. It is interesting to note that marcasite seems to occur only quite exceptionally in auriferous veins, although pyrites is such a very common constituent.

The minerals of Class D were until quite recently looked upon as rarities ; they have now, however, been discovered in considerable quantities in the Cripple Creek region of Colorado, and in various of the gold fields (notably Kalgoorlie) of West Australia, in both of which regions the so-called **telluride ores*' are producing considerable quantities of gold. These telluride ores may contain any or all the minerals of Class C containing tellurium, together with more or less of those of Class Z), and perhaps also some other ill-defined mineral species, the exact composition of which has not yet been determined.

Mode of Occurrence of Beef Gold. — Native gold occurs in reefs in particles of all sizes, from masses of over 100 lbs. in weight (Meroo Creek, New South Wales) down to specks that are only visible under powerful magnifica-

I Mode Of Occurrence 13

tion. It but seldom occurs well crystallised ; it crystallises in the isometric system, the commonest forms being the octahedron, cube, and dodecahedron. The crystals are rarely distinct, mostly aggregated, and frequently distorted and elongated in the direction of one of the axes. It more often occurs in irregular grains, plates, scales, filiform, dendritic, reticulated, or spongy. Sometimes it is pseudomorphous after some other mineral, mostly iron pyrites.

It has no cleavage, and a hackly fracture.

In nature gold never occurs in an absolutely pure state ; it is either alloyed with various metals or else, more rarely, is combined with a few metalloids in the scarce minerals of Class J). The principal metals with which it is alloyed are silver, copper, and iron, the former being universally present.

Native geld thus consists of an alloy of gold with silver or copper, or both, the name of gold being applied to such alloys as long as the gold is present in such quantity that its value considerably exceeds that of the other constituents. No hard and fast line of division between e.g. argentiferous gold and auriferous silver has yet been drawn, the distinction being essentially a technical one, depending upon the methods by which the ore has to be treated (whether by gold-or by silver-extraction processes) and upon the character and after treatment of the bullions produced. I would suggest that a useful practical line of demarcation could be drawn at the alloy containing forty-eight parts of gold to fifty-two of silver, which is of just about the same specific gravity as mercury. Argentiferous gold with more gold than the above would therefore sink in mercury, whilst auriferous silver, with less gold than the above, would

float on it. The alloy containing four parts of gold to one of silver is called electrum. Silver with 10 to 30 per cent, of gold is called kiistelite.

The specific gravity, hardness, colour, and other physical characters of the alloy depend, of course, upon its composition. Some ambiguity may occasionally result from this, strictly speaking, incorrect use of the word gold, to designate both the pure and the native impute rnotal ; but it has been sanctioned by custom, and the context will mostly serve to show whether pure gold or native gold is meant ; wherever there might be any doubt, I shall use the expression pure gold to distinguish it from the native alloy.

The table on the next page gives the compositions and specific gravities of a number of di£ferent specimens of native gold from various sources ; these data have been collected from various publications and are the work of trustworthy observers. They form, of course, only a vory small proportion of the total number of analyses of native gold that have been placed on record. Some of the specimens consisted of alluvial nuggets, but most of them wore reef gold.

In the following table the analyses of gold have been arranged in descending order, according to the amount of pure gold which they contain. It would naturally be expected that their specific gravities should follow the same order ; they certainly do so approximately, but by no means exactly, there being numerous abnormal results, that show specific gravities either markedly higher or markedly lower than the composition of the metal would lead one to expect. The reason for these anomalies is at present quite obscure ; all, however, can scarcely be due to errors of observation. They may be caused by unde-

Mode Of Occurrence

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:ted ca-vities or inclusions in the mass of the metal, lich woald of course reduce the density, or possibly by otiopism of one or other of the constituents,

i6 GOLD MILLING chap.

The purest native gold known is that from Mount Morgan, Queensland, said to contain 99*7 to 99*8 per cent. of gold, a little copper and iron, and a mere trace of silver. An analysis by Leibius gave Au 99*70 per cent., Cu 0*29 per cent. Silver is always present in all native gold and its amount may be judged from the colour of the metal, unless this is masked by other causes. In the latter case a good comparative test consists in fusing a small particle of the metal in a bead of microscopic salt ; a small quantity of silver will make this bead opalescent and yellowish, a larger quantity opaque and distinctly yellow. Less than 0*25 per cent, of silver produces no effect at all.

The average richness of Australian gold is 90 to 92*5 per cent., of Californian gold 88 per cent., of Ural reef gold 92 per cent., of Ural alluvial gold 91 per cent, of pure gold.

Hare Alloys. — In addition to the above-named alloying metals, the following rare native alloys of gold are also known : —

Palladic Gold from Brazil, containing 10 to 25 per cent, of palladium.

Porpezite from Colombia, containing 35 to 60 per cent, of rhodium.

Maldonite from Maldon, Victoria, containing gold 64*5, bismuth 35*5 per cent. (AugBi).

Native Amalgam occurs at Mariposa, California, in small yellow crystals containing: gold 39*02 to 41*63, mercury 60*98 to 58*37 per cent. At Choco in Colombia it also contains silver: gold 38*39, silver 5*00, mercury 57-40, corresponding to the formula ( JAu|Ag)Hg3.

Platinum, osmium, iridium, and antimony also occur alloyed with gold.

I Mode Of Occurrence 17

Kodes of Ocenrrenee of Oold. — Besides the two wellmarked forms of free native gold and chemically combined gold forming a constituent of the gold-bearing minerals of Class jD, gold also occurs in other modes of combination with the minerals of Class the precise nature of which is as yet by no means fully understood. l%e native sulphides of many metals, such as copper, lead, zinc, &c., at times carry gold, whilst iron and arsenical pyrites are very frequently auriferous. The gold is sometimes visible under the microscope in fine threads or plates penetrating the substance of these minerals, but sometimes occurs in some other form indistinguishable even microscopically. It is by no means clear whether it then exists in these sulphides, chemically combined as a sulphide, or whether it is present in an extremely fine state of mechanical subdivision. On the one hand, microscopic examination will not always reveal its presence, and it cannot be completely extracted by substances which have great aflBnity for uncombined gold, unless the sulphides be first destroyed by calcination. On the other hand, such substances (mercury for instance) will extract a certain proportion of gold from auriferous sulphides provided these are very finely ground, and will extract a greater proportion the more finely they are subdivided and the longer continued the grinding. Many theories have been put forward as to the mode of existence of this gold, the principal ones being that it is gold in a very fine state of division, more or less encrusted with a film of some substance such as quartz, or the sulphide of some base metal, &c., or that it is in combination with sulphur, forming a complex polysulphido with the other metal or metals present. Such complex sulphides are, of course, known io exist, and the moro

stable compound always seems to exert a protective influence upon the less stable, and to prevent the decomposition of the latter under conditions in which it could not by itself exist. Neither of these theories fully explain all the phenomena, and it would seem that the true explanation has yet to be found. The facts are, at any rate, well known : if a gold ore containing quartz, auriferous pyrites, and free gold, be ground up with mercury, this latter will extract a portion of the free gold, whilst another portion will not be attacked. If all the free gold be removed and the pyrites ground up with mercury, some gold will be extracted from the pyrites, and the longer the grinding is continued, the more gold will be obtained, but even the most prolonged grinding will fail to extract the whole of the gold. If, however, the pyrites be calcined until the whole of the sulphur has been driven off, the gold can be readily and completely extracted by means of mercury.

In addition to the above modes of occurrence, there is yet another, namely, of free nonamalgamable gold ; this mostly goes by the name of rusty" gold. It is sometimes metallic-looking, of the usual golden colour, but more often brown and lustreless. Many gold ores carry more or less of their valuable contents in this form, in which it resists the action of mercury, and heavy loss is apt to occur in the treatment of such ores by the usual processes. I have found this rustiness particularly marked in ores in which the gold had been deposited upon decomposing crystals of pyrites, or in cavities left by their complete destruction, producing pseudomorphs of gold after pyrites. No satisfactory explanation of the cause of rustiness has as yet been advanced. It is

I Mode Of Occurrence 19

generally said to be dae to the gold being coated by a film of something or other, variously supposed to be silica, oxide of iron, sulphur, some metallic sulphide, some compound of gold, &c., and the fact that rusty gold will partially (but never by any means completely) amalgamate when exposed to prolonged friction with mercury is explained by saying that the coating has been removed by the attrition.

If, however, the ore be calcined at a red heat, the gold is no longer rusty, but is readily amalgamable, and it is difficult to see how this could be the case were the sole cause of rustiness to be due to a coating of silica or oxide of iron.

Lazarus Ercker, who wrote as far back as 1672, points out that it is beneficial to calcine gold ores before treating them by crushing, &c., because the fine subtle gold shrinks and runs together, and assumes a rounded corps4i and such strength that it remains firm in washing an can be caught."

The practice of roasting ores before crushing used at one time to obtain in Cahfornia, when such ores were being treated in the arrastra. It is, of course, obvious that this mode of procedure is only applicable when the ore is rich and fuel plentiful. If the calcining of the ore costs more than the value of the gold saved by it, the process, however correct it may be scientifically, is practically worthless. We are not, however, now considering the question in its economic aspect, but purely from a scientific standpoint.

It may also be noticed that calcination would be effective if the coating were to consist of a thin layer of sulphide of gold, as is suggested by Skey. This chemist

Chem. NewMf xx, 282.

has studied this question in much detail, and has made numerous experiments bearing on it ; he summarises his investigations as follows : — '' (1) Samples of clean-looking gold from some reefs refused to amalgamate though taken direct from the reef and untouched by hand. (2) On such surfaces sulphur is always present. (3) Native gold or pure gold readily absorbs sulphur from moist sulphuretted hydrogen or ammonic sulphide, and absorbs it directly when administered in boiling water. (4) Surfaces so treated refuse to amalgamate, though no apparent change is perceptible in their aspect. (5) Gold so affected is rendered amalgamable by heating in an open fire (unless it contains over 7 per cent, of copper) ; the same effect is produced by the contact of potassic cyanide, chromic and nitric acid, and chloride of lime acidified. (6) This absorption is altogether of a chemical nature."

Of course it is easy enough to imagine circumstances under which reef gold would be subjected to the action of sulphuretted hydrogen or soluble sulphides, seeing how frequently iron pyrites is a constituent of gold reefs, and the above maybe a partial explanation of the phenomenon. Skey himself, however, points out in another place that sulphide of gold is decomposed by mercury, so that if all his observations are correct, a film of sulphide of gold should be no obstacle to amalgamation.

Another possible explanation is that the gold has been subjected to severe pressure, and is therefore in an " unannealed" state (see page 24). Prof. Eggleston has pointed out that unannealed gold is only amalgamable with great difficulty (see page 84) ; of course heating to redness would anneal the gold, and thus account for its amalganiability after calcination.

I myself venture to think that rustiness may very

I MODE OF OCCURRENCE 2i

often, if not in all cases, bo due to the existence of gold in an allotropio state, in which it is not affected by mercury; heating to redness would change the gold from this allotropic to its normal condition, and thus render it amalgamable. I shall enter further into this question in Chapter II., page 47.

Whatever then may be the true causes of the phenomena, it must be remembered that gold occurs in reefs in three distinct forms (exdusiye of its rare occurrence in chemical combination in the minerals of Class 2)) :

I. Ordinary amalgamable gold, usually spoken of as free-milling gold.

II. Gold in some form of intimate physical admixture, or more or less complete chemical combination with sulphides, arsenides, or other compounds of various metals.

III. Rusty gold.

Minerals Mistaken for Oold. — Of the list of minerals given above as liable to occur in gold reefs, a few may at times be mistaken for gold by a careless observer. Thus quartz, and more often mica, stained a bright yellow by oxide of iron, has been mistaken for gold ; the extreme hardness of the former, and the highly perfect cleavage of the latter under the knife are quite sufficient to distinguish them from the soft and sectile, but not fissile metal.

Iron and copper pyrites have both frequently been mistaken for gold when tarnished, as they often are. Here again the knife o£fers a ready means of discrimination, the former mineral being very hard and the latter being scratched with the formation of a gray powder. It is also worth noting that if tarnished pyrites be slowly rotated in the hand, its colour will change with the

varying angles at which light is transmitted through the film of the tarnish, whereas the colour of gold renaains uiialToctod.

liJAnnithito, when wet, much resembles gold; in the proApooting pan it appears to have a bright golden yellow (!()lour, and its great specific weight causes it to settle in tho place where gold would naturally be looked for. When dry, however, its colour changes at once to a pale groonish yellow. Such mistakes as the above are rarely committed by experienced men, even the most superficial glance serving to detect with certainty the presence of gold. In all cases of doubt the knife should, however, bo resorted to, the sectility and malleability .of gold Horving to distinguish it practically from all other minerals likely to be mistaken for it. Even when it is present in the minutest specks, chemical means need nover bo employed if the particles are visible under a Ions.

To determine whether sulphides or tellurides are auriferous it is absolutely necessary to assay them. The appearance of the mineral affords no guide at all as to whether it contains gold or not. It is usually supposed that fine and close-grained iron and arsenical pyrites are richer in gold than are large and well-marked crystals, and this rule often holds good, although it is by no means universal. When an ore is rich in tellurides,'gold may mostly be expected, especially if it comes from an auriferous district, but, as before said, an assay is the onlv safe or reliable test.

Chapter Ii

Physical And Chemical Properties Of Gold

Physical Properties

Gold is certainly dimorphous and may possibly be polymorphous. There are at any rate two well-marked allotropic forms, namely (a) ordinary yellow gold capable of forming crystals, and ()S) brown or black, apparently amorphous, pulverulent gold. Of the properties of the latter very little is known, and in the present state of our knowledge we are forced to assume that such of its physical and chemical properties as are not yet known to differ from those of ordinary gold are probably identical with them. It is however highly probable that the falseness of this assumption may be demonstrated before long. I shall here follow the ordinary custom of restricting the use of the word gold without any qualification to the variety, and shall speak of the variety as amorphous gold.

Specific Gravity. — The specific gravity of gold has been variously determined as between 19*30 and 19*34 ; after melting it is said to be 19*258, which is increased by hammering to 19*367.

Crystallisation. — It crystallises in the cubic system, the

prodominating forms being ocfcahedra, dodecahedra, ano cubes, or combinations of these forms. Native alloys of gold yith silver have also been found crystallised in the same forms as the above.

Malleability, — Of all ntals it is the most malleable and the most ductile ; it can be beaten into sheets thinner than 0000004 of an inch, and has been drawn into wire so fine that 600 feet of it only weigh one grain.

If a piece of gold be hammered or rolled out, it at first extends freely and readily ; it speedily, however, becomes hard and brittle, and if the operation be continued, can easily be broken into pieces. If, however, at this point the mechanical action be interrupted, and the metal heated to a dull red heat, it at once recovers its original inallcability, and can then be further worked until it again becomes brittle, when it can once more be restored to the malleable condition by reheating, and so on, until the limits of extensibility are reached. This operation of restoring malleability by means of heat is known as annealing.'' In the unannealed state the free mobility, relative to each other, of the molecules of gold, which constitutes the condition of malleabihty, is interfered with by these molecules being so driven together as to obstruct each other, each molecule being thus interposed in the path of oscillation of its neighbour. When heated, expansion takes place, the. molecules move a little apart and rearrange themselves, and each molecule is then again free to move without being impeded by the adjoining ones, the state of malleability being thus restored.

Tenacity. — The tenacity of gold is by no means great; it is said that a wire 0*08 inch in. diameter will support a weight of 150 lbs. Later experiments have determined

" Properties Of Gold 25

be tenacity of gold at seven tons per square inch, with

Afl elongation of 30*8 per cent, on a three-inch test piece, is classed among the soft metals, being intermediate between silver and tin in this respect ; on Mohs's scale its hardness is 2-5 to 3. Its elasticity is very low.

Colour. — It is the only yellow metal known, has a brilliant metallic Instre, and is susceptible of a high polish. In thin sheets it appears of a greenish colour by transmitted h'ght, but is only transparent when less than line in thickness. The green light that is transmitted through ordinary gold leaf is turned into a ruby red colour when the thin film of gold is heated to 316 C. ; burnishing the leaf will however restore the original green colour. Some observers explain the variations in colour as being optical phenomena exhibited only by gold because no other metal is sufficiently malleable to be obtained in sheets of the required thinness ; others consider them an intrinsic property of gold, which would thus be markedly dichroic. It emits a greenish light at the moment of solidification from the molten state.

Fusibility. — It is easily fused in any ordinary fire or before the blowpipe, its melting-point being about 1045° C. (1916" F.); various observers give it as ranging from 1037' to 138r C. It is thus somewhat less fusible than copper, but far more so than cast iron. It expands markedly when melting, and contracts on solidifying. It is volatile to some extent at very high temperatures, such as those produced by the electric arc or the oxy-hydrogen blowpipe; even a good mouth blowpipe will volatilise appreciable traces of gold, although an intense white heat is necessary to effect this in perceptible quantity.

Conductivity. — It is a good conductor of heat, ranking in this respect next to silver ; its conductivity is about

' 'Mlin- l, s,,>nu ohstH'veis and 0-9S1 according tc o( Hilwr lH?ing 1. It is also a good conductor* ioliy, having an electrical resistance of tHtiviHl. iiul 1 393 in the hard state, annealed itkou I ; its electrical resistance has been M i>'SUVJ microhms per cubic inch for ♦l uiul 0'Sd47 microhms per cubic inch for hard

(HH)t4r of its volume on being heated from ; itM M|HHMtio heat, referred to water as unity, Viit'iouMly ilotormineil by different experimenters Htu Otvj*)H and OHXW.

Kinoly divided gold (such for instance as tt)iliu\d in buUion '\ssays) is capable of absorbing

Chemical Properties

*toiuij weight of gold has been variously deter- l>iit\v(ion 196-33 and 196B5; it is usually taken

is flutBciontly accurate for ordinary purposes; is Au. It is a triad, and forms two series of aurous and auric, the oxide tj-pes of which are '*®ly AujO and AujOj. It also appears to form an 'liato aeries of aurylio compounds of the AuO, nd compounds corresponding to other stages of also said to exist, but are by no means well

" Properties Of Gold 27

defined. Gold has but few affinities towards other elements, its most stable compounds being with the ; it is affected by very few of the ordinary reagents, 18 not attacked by any of the ordinary acids. Its Qsoal 8olyent is aqua regia (one part of strong nitric and foQT parts of strong hydrochloric acids). It is, however, important to note that gold, although insoluble in pure nitric acid, is somewhat soluble in it when it contains nitrous acid.

It is also attacked by hydrochloric acid in admixture

th any oxidising agent capable of generating chlorine

therefrom. It is attacked by selenic acid with the

formation of selenious acid. It is not oxidisable in the

*ir at any temperature with or without the presence of

njoistnre. It is attacked by fused nitre, but not by fused

potassic chlorate ; fused alkalies attack it in the presence

of air, an aurate being formed in both of the last-named

reactions. It is also attacked by fused alkaline

"Sulphides, a sulphaurate (a fairly stable salt) being

formed. The same effect is said to be slowly produced by

solutions of ammonic and of alkaline sulphides. Finely

divided gold is also soluble in solutions of the alkaline

hyposulphites, with or without the addition of cuprous

readily in the cold ; gold leaf dissolves freely in chlorine

water. It also dissolves in bromine. Iodine in the cold

has no action on it, but attacks it slowly when exposed

to sunlight, or when heated to 50"* C. An aqueous

solution of hydriodic acid has no effect on it, but it is

attacked by an ethereal solution, also by a mixture of

hydriodic and sulphuric acids heated to 300' C. Ferric

chloride and ferric bromide have no effect on it. Finely

divided gold is said to be soluble in ferric sulphate in the

absence of Bamma- <wl1ni bafc AissfeafceiBeHk myiio cmi* firmofeioiL ils tlu LfliHiIb of s wefiJHft cC "— on this sobjeet, I iSmi diftfc pure s&eet oU is fote Jnwlnlih in soinaons of pate ecyniAlfiaed. fisuic dilnriilp in dis* filled wmfe of afiieiigibiui Torfing bom 10 per eent. to sacxnafcionf ftffeo' & iiwinth's esposm to their sfiiion* Gold win, howefec dxaeoffe tnsolBfeEODSol iemedilaEide contoiniDg £cee aod sod eaqpoaed to the air, or in die preeeziee of an oxidisjog eigent.

Fctoseic C7iixde wihrtwm ettirlrw il slowly more raptdl J if finely divided ; it seems Ihsk this setion onlj occois in the presence of str the fesctinw bong profadhfy as follows : —

I hsTe found that pare sheet gold wiU dissolre in solations of commercial potassic cyanide of strengths varying between 10 and 50 per cent, of the salt, at the rate of from 3 to 5 per cent, in a month. Pieces of pore gold were also treated with pare potassie cyanide dissolved in boiled distilled water in sealed tnbes during one month. In this time, one piece in a solution containing 50 per cent, of the salt lost 0*2 per cent.* whilst a similar piece in a 12 per cent, eolation lost 4*7 per cent. No gas was evolved daring the action. The solubility of gold in potassie cyanide sedation was discovered by Bagration in 1843/ and he remarks that corrosion is especially active at the sorftkoe of the Uqaid. Eisner seems to have first enunciated the above equation in 1846. The whole subject has received oarefol investigation more recently from R G. Madaurin; he

' Jfjurii. f. Prakt, Chem, voL xxxi. Pnd, vcJ. xxxvii.

Ii Properties Of Gold 29

comes to the oondnsion that cyanates play no part in the reaction, which depends on the direct absorption of oxygen by the solution ; he also finds that dilate solutions are more active than concentrated ones, and assigns as the reason the fact, which he has proved, that oxygen is more soluble in the former than in the latter. There is accordingly a certain definite degree of dilution which exerts the maximum solvent effect. Madaurin confirms the correctness of the reaction given by Eisner. Gold will also dissolve readily in a mixture of solutions of potassic cyanide and ferricyanide, potassic ferrocyanide being formed. The following is probably the reaction : —

Au + 2K3FeC Vg + 4KCy 2KFeCyg + 2KAuCy,.

Gold also dissolves in a mixture of solutions of potassic cyanide and sulphocyanate, and of potassic cyanide and bromocyanogen (Sulman and Teed's reaction) ; according to the discoverers, the action of this reagent is as follows : —

Auj + 3KCy + CyBr 2KAuCy2 + Br.

Sulphur and sulphuretted hydrogen are usually said to have no effect on gold. Skey says, however, that gold exposed to moist sulphiuretted hydrogen, or immersed in a solution of ammonic sulphide, is superficially converted into sulphide without change of colour, and is in this state not susceptible of amalgamation, and this observation has been independently corroborated by Eggleston.

Phosphorus, arsenic, and antimony combine directly

Chtmirnl XeicH, vol. xxii. p. 282.

' The Jleiaiiunjy o/lSilctr, UM, and Mercury in the United Statts vol. iL p. 59U. 1890.

with gold under the influence of heat. Mercury combines with it readily at all temperatures (see page 84).

Chlorides. — There exist two well-defined chlorides, aurous chloride, AuCl, and auric chloride, AuCl,. There seems also to be good evidence of the existence of a third intermediate one, aurylic chloride, AuCl; higher chlorides are also said to exist, but have scarcely been isolated as yet. These latter compounds have been made the subject of careful study by J. Thomsen.

Aurous Chloride, AuCl. — This is a pale yellow unstable substance, insoluble in water, formed by carefully heating auric chloride to a temperature of 200"* C; heated more strongly, it is resolved into gold and chlorine. It is also produced by the action of water on aurylic chloride, AUjCl. It is slowly acted on by water in the cold, the action being accelerated by sunlight and becoming rapid at the boiUng-point of water, gold and auric chloride being produced.

Caustic potash converts it into aurous hydrate. It combines with alkaline chlorides, forming double salts, the best known of which is potassic aurochloride, KAuCl, ; this substance, of a brownish black colour, is produced by gently heating potassic aurichloride, KAuCl. It is acted on by water, gold being precipitated and potassic aurichloride and potassic chloride being formed.

Auric Chloride, AuClg, is a deep reddish brown, crystalline substance. It is produced by the direct union of its elements, as by introducing gold leaf into chlorine gas. If a current of chlorine gas be passed over a thin strip of gold heated in a glass tube to 300° C, long, deep red needles of auric chloride sublime and condense on the colder parts of the apparatus, the decomposition of the JoanujU/itr Praktiacht Chtmit (-2), vol. xiii. p. 337.

lie chloride with water. It is decomposed slowly action of sunlight, and rapidly on being heated. olable in water, alcohol, and ether, the latter re- ; it entirely from its aqueous solutions, solution is easily decomposed. A stream of )en passed through it throws down metallic gold, )8t metals have the same effect. Platinum, howrill not precipitate gold from the solution of its le, although it will from a neutral solution of auriles. Tin first throws down metallic gold, which awards converted into purple of Gassius. Phos- I, arsenic, antimony, and bismuth immersed in a of auric chloride become coated w ith a covering Gallic gold in the cold, and sulphur and selenium ling. Carbon precipitates gold in the cold when d to light, and so do many native sulphides, such ma, pyrites, zinc blende, stibnitc, cinnabar, Sx. ;

COLO MIIJ.ING HAi

ill order that the precipitation may be quite completi hi the case of oxalic acid the reaction is :—

2AUCI3 + SHgCaO Aug + 6HC1 + GCOg.

Most reducing agents precipitate gold from solution of its chloride. The most usual precipitant is ferrotifi sulphate ; but any protosalt of iron will have the sam

Mercurous nitrate, antimonious chloride, and cuproos chloride dissolved in hydrochloric acid precipitate inetallic gold.

In a cold solution the finely divided gold remains suspended for a very long time, forming a solution which is brown by reflected and purphsh blue by trans III it ted light ; this reaction is perceptible in solutioiis. dilute solutions the precipitate only settles with extreme slowness.

Sulphurous, phosphorous and hypophosphorous acids have the same reducing action, as also has arsenious acid, the latter only slowly in the cold, more rapidly on, heating.

Stannous chloride containing some stannic chloride produces a purple precipitate known as purple of Cassius j it is a very sensitive reaction, a solution containing only -\Jsii\j5i5J5 becoming purple on the addition of the stannous chloride. This is often used as a test reaction for gold, but it must not be forgotten that silver salts also yield a somewhat similar brownish precipitate, which might be mistaken for purple of Cassius.

" Properties Of Gold 33

Potassic iodide produoes in a solution of auric chloride A pi'pitate of yellow aureus iodide and free iodine.

otic nitrate gives a mixed precipitate of argentic and gold oxide, hydrochloric and nitric acids remainiDg in solution.

Strong sulphuric acid precipitates auric chloride from Its concentrated solution, and, on heating, the precipitate decomposes into aureus chloride and chlorine.

Ammonia and ammonic carbonate precipitate the g% explosive ammoniacal oxide of gold known as olminating gold ; the precipitation is not, however, complete.

Caustic potash forms, according to some authorities, a yellowish red precipitate of hydrate, soluble in excess the precipitant ; according to others no precipitate is formed unless there is some organic matter present.

Sulphuretted hydrogen throws down a brownish black precipitate of sulphide, soluble in alkalies and in ammonic sulphide, this being the group-reaction of fturic salts. Sodic phosphate gives no precipitate. Potassic cyanide forms a yellow precipitate soluble in excess of the precipitant.

Potassic ferrocyanide colours the solution bright emerald green.

Auric chloride is further characterised in the dr}' state by imparting a bright green colour to a flame, forming a very distinctive spectrum.

Auric chloride forms with most soluble chlorides a very characteristic and interesting series of salts known as aurichlorides.

Hydric Aurichloride, HAuCl, 3 Aq, crystallises in long square prisms, or in truncated square pyramids ; it ia a

D

-.4 GO/.n M/ll.IXC r'H''''

dark red, deliquescent substance, melts on being and then decomposes, giving off chlorine and hydrochloX

acid.

It is produced by dissolving gold in aqua regia contaitx'' ing but little nitric acid and heating till this is all oorXi'' pletely decomposed, when the double salt crystallise out on cooling.

Auric chloride as ordinarily prepared always contain more or less of this salt.

Amnionic AuricliUn-id, NHAuCl, 3 Aq, forms yelled transparent needles. Aurichlorides of potassium (witb varying amounts of Aq), sodium (with 2 Aq), lithium, magnesium (with 7 Aq), barium, calcium (with 7 Aq), cobalt, nickel, manganese, zinc, cadmium, and silver are known ; they all form reddish to greenish yellow salts, crystallising very readily with various amounts of water of crystallisation.

Aurylic Chloride, AuCl, has been produced by treating amorphous gold with chlorine. It is a hard, dark red, friable substance, hygroscopic, decomposed by water into auric and aureus chlorides.

Aurous Bromide, AuBr, is a gray, unctuous substance, insoluble in water, produced by cautiously heating auric bromide; water decomposes it into gold and auric bromide.

Auric Bromide AuBrg. — Gold dissolves slowly in bromine water ; on evaporating, a blackish gray residue of auric bromide is left, capable of crystallising in scarlet crystals. The solution is bright red, and possesses considerable colouring power. It is also produced by the addition of a solution of hydrobromic acid to a solution of auric chloride, the solution becoming dark red, and hydrochloric acid being produced ; the auric bromide

' " Properties Of Gold 35

r. can be separated by the addition of ether, in which it is :. highly soluble and which removes it completely frdm the aqueous solution.

The affinity of auric oxide for hydrobromic acid is markedly greater than for hydrochloric acid.

The reactions of anric bromide are like those of auric chloride. Auric bromide forms a series of double salts with soluble bromides, many of which have been isolated aod studied.

flyt/nc Auribromide, HAuBr, is a dark red crystalline body.

Auribromides of potassium, sodium, magnesium, harium, zinc, &c., have been produced ; they are all red or reddish brown and highly crystallisable.

Mirylic Bromide, AUgBr, has been produced by the action of bromine on finely divided amorphous gold.

Aiirous Iodide, Aul, is the most stable of the iodides of ?old, and is produced in all reactions of iodine with gold. It is formed by adding potassic iodide to auric chloride : —

AuCL, + 3KI Aul + 3KC1 + 1.. Excess of potassic iodide must be avoided, as this decomposes aureus iodide, and the iodine produced must volatilised at the lowest possible temperature. The salt forms a greenish yellow, crystalline powder, insoluble in water; it splits up into gold and iodine when heated above 60"" C. ]Iost reagents decompose it.

Auric Iodide, Aulj, is a very unstable body, that splits up into aureus iodide and iodine almost as soon as formed. Wlien a solution of auric chloride is added to potassic iodide, a dark green precipitate is produced, soluble in potassic iodide, with the formation of potassic auri-iodide : —

4KI + AuClj KAuI, + 3KC1.

D 2

:;llJn uev '.arm iiiacsisiii. nerailv ocfimotair

ii Jtci S" iaiu na -isiac. toe '3ai> ai5wr -"iia.ii-i lonunic -S- inirvvn " ira- OTiwreieSk

'-it* rTi-i- ji amimukciiiii raiy ' "imy ic or

i-i'-i-si. uiil ji TUts -ixiraisuoii it mhL 'am a lugs

-t i."'Li 2 'fan . ia . A.35 T y". -s k yuilirw . .fraaixifiyr. ccyiialliM

'*i'.'*r.i.''.ii:i iriiiiuaiit iv'aeifccinic actaasu: AiiA-oeannig miBi

uvii:- :i;- isauzki by waa rwa onJy lualinft ' "OL-- -Tz inM i!f:iil nut litiaoiKn.. Its is loldhlp ui

V4V../.r.r-i. Vi-ii .n *:iic lyocijamixiw. also is fmlamif

r- : ".-.:% .ii-,Tihu saLs Tr::cL LiibLe cyanides known M *,.-.:f1r. -.tt sijss izircnArLii betrw[ nfae pocassiiiin ''-''' v. :r/ ,;,r KAnCvj. Tbi apwars be prodneed when r,ffAS Tr'.-G- 5oI::r:oci of ca&ssic cyanide in *.r.*: ;/.-:V:r.ci!j of air. See paee

--t-i;illv yrrAicfA by dissolYing hdminadnggold in a:< . /: rvar. Uk, Ammonia is added lo a solnlion off anrio chlorvi: :infl thft recalling precipitaie washed and treated

K/tai?iic cyanide.

A v]ritir>n <rf potassic aurocyanide dissolTee iodine, prodncln a dark brownish violet, readily crystaDis* **"bfiiancc, potawic auri.iodo-c>-anide, KAuCy.

" Properties Of Gold 37

Aoalogous compounds with chlorine and bromine are abo known.

Potassic Aurocyanide is decomposed by most mineral ftcids, these precipitating aurous cyanide and liberating hydrocyanic acid ; it is very extensively used in electrogiJ<ling, (fcc.

'mc Cnidc, AuCy,, exists only in double salts known as auricyanides.

Potassic Auricijanide, 2KAuCy4, 3Aq, is prepared by mixioghot concentrated neutral solutions of auric chloride aod potassic cyanide. It forms colourless tabular crystals, soluble in warm water and in alcohol ; these give up two molecules of water of crystallisation in dry air, but tile third is only expelled at a temperature of 200° C. At a low red heat the salt is fusible ; slight decomposition sets in about 300' C, but this is not complete even at a kh temperature.

Chlorine and bromine have no efifect upon it. Iodine solves in a hot solution of the salt, displaces two molecules of cyanogen, and produces potassic auri-iodocyanide, KAuCy.J2, Aq, which crystallises in thin, brown, icular crystals ; corresponding chloro-and bromo-salts are known to exist, as also are auricyanides of hydrogen with G Aq), ammonium, sodium, barium, calcium, strontium, and zinc.

Oxides i*f Gnlfl. — Two well-defined oxides are known to exist, aurous and auric oxide, Au.0 an<l .VuOj ; there seems also to be an aurylic oxide, Au.,Oo, and an even higher stage of oxidation, AuOj, is recognised by some authorities ; its existence is, however, doubtful.

Aurous Oxide, AuO. — This is best prepared byaddin* a solution of mercurous nitrate to a neutral solution of auric chloride, avoiding excess 0/ the precipitant : —

r. hJTni> Jaaak junpbab pEBoi umauaAL list Miinr xif nrg'nif

=>'arinuiinru uciE maisuiva: ii. pmSiuniig gold

tnr.r:: >:>5ifi -i;-: iSJi A2i XT.efic: : it is an

i9rxDs a bcownish bUck powder, and

" PROPERTIES OF GOLD jg

state by most reducing agents. It is soluble in concentrated sulphuric and nitric acids; hydrochloric and hydrobromic acids dissolve it, forming the corresponding haloid salts ; hydriodic acid decomposes it, producing metallic gold ; hydrofluoric acid is said by some authorities not to affect it ; according to others it dissolves it.

Ammonia forms with it the highly explosive compound own as fulminating gold.

Alkalies dissolve it, forming salts known as aurates, having the general formula MAuO,.

The potassium salt, KAuO,, 3 Aq, forms pale yellow crystals, very soluble in water; acids decompose its solution, separating auric hydrate. This salt is formed hen nitre is fused with finely divided gold. It is also produced when auric hydrate is boiled with potassic chloride : —

Au,HO + 2KC1 2KAuO, + 2H.p + 2HC1.

With most metallic salts potassic aurate gives a precipitate of an aurate soluble in excess.

A solution of potassic aurate is sometimes used in electrogilding.

Aurylic Oxide, AuOg, has been described byBerzelius and Prat ; its definite existence is still, however, somewhat doubtful.

Oxysalts of Gold

Several of these have been described, but owing to the low chemical aJQ&nities of gold they are all very unstable, and their preparation and investigation are attended with great difficulties.

Auric Hydric Nitrate, AuH(NOs), 3 Aq, is produced when auric hydrate is dissolved in strong nitric acid and

rHA'

iiuiK )UNviiAivi ntnUin at '72' C. forming a basic nitra ii iiV> I . 9UU1 w)pkudy dMomposed at 180" C. iHu c)ivw))]i'fe9M iu alQUaiu fomiiiig auric hydzafeo* ;tai to tA)iM Uir ii aid to have been obtained arx-kUU icnui nvfUl red prisms. The latter I, iu\Mi ,ui!\ nSixxuciv, m concentrated solphnrie acid iiic ntvlui-LU) )3 )ivMti)v\od on dilution, auric hydrate

u .LuilOv' mUi ith sulphites of the aTJcalies,

ikJltiiiu; ivAaksu-' ikuhthii* to s Solution of potassitf' awktftU.au; ii\w or\-siAl$ of a yellow oolonr. It is wUcMuv w \k' \ccviv;aym bv the addition of alcohol III vi\ry Xi'.r\vh.*r luJ; xhu-th iaIcc a veDow tinge on

Willi alisLliiic hvvK'.Ui>ii;:rt

vAivil l*y p!?eivHiiA;iixj; & :v_i\:urf auric chloride and Avviupcri*ti>fl\ '.Jibl;;' sju:. soluMv \\A:or: its sdutioD ivaciioi'S v.i::it:r of ijold !'?r of hyvksulphurous

Silicate of iivld. — When likely divivleigold or a salt of nuchas th chloride mixed with silica and alkalies gc ijyaline earths and fused, as in glass-making, a yeUow gtifialt ol gold or an aurosilicate of the bases present

" Properties Of Gold 41

is prodaoed ; when this glass is heated to redness in the ur, it assumes a beautiful purple shade, the exact cause of which has not yet been determined. Alkaline aurosilicates seem also to be produced by mixing solutions of alkaline aurates and alkaline silicates.

Hrilt of Cassiuit. — This substance was first discovered byCassius of Leyden in 1683. Its constitution has been varioosly explained by different chemists, the two most important and most plausible views being that it is either anaurous stannate, or else a "lake" of stannic hydrate <loiired by finely divided gold. Eiguier, following the formerhypothesiSjgivesitsformulaas Au20(SnOj,)3 + 4Hj0. It is produced by adding a mixture of stannous and stannic chlorides to auric chloride solution, when a fioe purple precipitate is produced ; stannous chloride one gives a brown precipitate. It is also formed by the tioD of metallic tin on a solution of auric chloride, and 'n\*arious similar methods, the object of all of which is obtain the precipitate in a fine state of division.

Heated to 100" C. it retains water ; at a higher temperature it gives off water, but not oxygen, and becomes brick red. It yields no gold up to mercury when triturated with it. It is used in the arts for colouring jlass, incited with which it produces different shades of red var}ing from rose pink to a deep ruby red.

Sulphides 0/ Gold. — Gold has but little alHnity for sulphur, and it is as yet by no means certain what compounds of these elements really exist ; aureus and auric sulphides are usually described as havinj been isolated ; both are blackish brown in colour.

Auroiis Sulphide, Au.S, is said to be produced bypassing sulphuretted hydrogen through a boiling strong solution of auric chloride, and auric sulphide, Aub,

n PROPERTIES OF GOLD 43

seleniuretted and telluretted hydrogien respectively. A nilphotelluride, AuTeS, has been produced by preopititing a solution of auric chloride by potassic mlphotellnride.

6oU and Nitrogen. — No direct compound of these dements has been isolated, but it enters into the compoeitioD of so-called fulminating gold. This is produced when ammonia is added to auric chloride or hydrate ; iu composition is said to be 2(NH,AuN), 3H,0. It can be h&ndled when moist, but when dry the slightest physical cause is capable of determining its explosion with great \'iolence. When it is formed by the addition of ammoDia or ammonic carbonate to solution of the chloride, the resulting precipitate appears to contain chlorine : —

This coinix)und is also fulminating. When auric hydrate

is digested in ammonic sulphate, sulphuric acid is

liberated and fulminating gold produced. It is an olivuweeii

compound, which with great care can be dried at

iOO G. When dr\' it explodes with great violence by

the gentlest friction, by contact with any hard body, by

the action of electricity, or by heating to 140'' C. Its

preparation is dangerous in the extreme, and in fact

neither ammonia nor anv salt of ammonia can be added

to a gold solution without the risk of accident, unless

great care be taken that the resulting compound shall

never be allowed to become diy.

Gold and PhosplioruH . — When finely divided \isjV\ is heated with phosphorus out of contact with air, combination ensues, phosphides of gold being formed. The formulas AuP and AujPj, have been assigned to this

. JO) A :ii oujk- |i:sibile ihM there may be hti. li TIM ii3U~fd l)v hvdrochlorie add, 1

li-.i A-B/i A'fi,**. — A fciarae-piined, easily TAx ars)ci)i iSiii :ii ioud lo be prodnoed when dmi)t>i i?Sk) beuav :n Jirstcjiic npoor (see page

tf*C-.i Aiij S4 , ,'A 'i. — *.rS15 v.'jftiiwiih slioan will 00 wiih n , wii ;Sk 'r.i 4: siliocm ibe ooinpoond i dinv >rllsvf oSiCuT Afi rftjiKj hniile: with 10 pei Wiv biiiUr. T:ie<*i atv avijaschlv ifc£nix* siliddes 0

Allotypic Gold iGoldt

All dlloxropic ir.cviif oAiiw'r.x: ivIvi A;>i>edu$ lobe ]MX) when d vv-ry \iilu:e solu::"" c: c--vi ciiloHde is precip bv uiv'au o: Hk rtxiuo:::*: aitr.:. <uoh ai ferrous sul] uxtilie aoid. \e. The sauio >ub>:ai:ce is perhap Pixk1u*\.h1 \\he!i a:i allw"*) of iiold. \v::h silver or cop[ iiiaiaiicv, is ac:t.xl 0:1 bv r.::rio aoia. :he v*:her metal tlisstlve<l I hereby and :his for:i*. o: remaining When I lie alloys oi gold \v::h svxiivin: or potassioi thrown into water ii:ey an.' iWo:i:jv><OvL the jld left Ik* hind as a tine black i<h Irown iK>wder, a|>iM*ars 10 l>e an exuviMr 101:11 of thi< alloiropic 1 1 forms an amorphous, incyiierenc. brown po having no metallic lustre. Hose has found ths specihc gravity of amorjous gold when precipitatei' fMluiiAD oC the chloride by ferrous sulphate varied

" Properties Of Gold 45

19-55 to 20*72, and when precipitated by oxalie acid was 19*49. I have found the specific gravity of the amorphous gold left behind when the silver is dissolved oat &om a silver-gold alloy to be 19*51, whilst gold similarly left on dissohnng oat the base metal from alloys with lead &nd copper has a specific gravity that may rise to 19*78, or io all cases higher than that of ordinary melted gold. It luis been said by some observers that gold when precipitated from its solutions is crystalline, bat my own observations are opposed to this as a universal statement. Gold when thrown down from moderately strong solutions luis an appearance suggesting crystaUine structure, but hen deposited from dilute solutions it seems under the microscope to form amorphous irregular granules with no indication of cr}*stalline arrangement. When finely 'divided gold, as precipitated on a photograph in the process of toning," is examined under the microscope no crystalline structure can be distinguished in the particles of gold. At my request these observations have been repeated by my friend Mr. F. E. Lott, A.R.S.M., Ac, ho finds that the employment of gold solutions ranging in strength from 10 per cent, to 0*0001 per cent, makes no difference in the form of the precipitate, which under 500 diameters and even under 800 (immersion) has no trace of crystalline apx)earance, nor do the particles show any tendency towards regular arrangement. Beyond this nothing is known of its physical properties. It is immediately converted into a gold by heating to a temperature which I have determined to lie between 400" and XX) C, when the loose pulverulent mass shrinks, becomes

The ipecific gravity of this gold after annealing " was 18 728, ami after niflting 19 186. See Tnins. Am. iMi. Mitt. Ahj/., 18t4, xxiv. p. 705.

mri iiietailic Lnatre of a told. The ame efEecc is whoILT' -iV partially proiinced by violent peicusBioii or fidccfan

liA chemical properties are osoalLy opposed to :<lpntical with those of a Id. Thomsen has howeic found that when fi 'Id is acted on by chlorine or an 17 lie sialts are formed instead of auric ; I h&ve :V>iind a very important difference in one respect* hsu] previously been noted by Knaffi, who states tiomM jold, precipitated by ferrons solphafie, is difffmh of amaJmacion. Pure gold (see page 84) readily eomhinc '.vinh mercury to form amalgams; fi gold obtained faj dpi nation is, however, only with great difficnky Ky mercury, not at all when moist, and very slightly drying on an air bath ; when the mercury is heated. '.o itj boiling' point amalgamation is dightly promoted, Knt ift <4till far from complete ; the addition of a small fragment of oflium to the mercury has no effect unless a few dropA of water are also added, when complete amalgama- r,ir>n at once endues. In this connection I may also refer to t.hf. fU:t\or\ of mercury upon purple of Cassius (see page 41). TVii uhtance ha<i been by many chemists supposed to r;onit of a lake " of stannic hydrate, throughout which fff'.fi metallic gold is suspended, and one of the main ar£imf;nts again (t this view is its indifference to the action of mercury ; if, however, this gold be supposed r.o })fi of ihfi fi allotropic variety, as is probable from its (/)\(A\Tf lis indifference to the action of mercury is quite iritdliiblfj.

Tbomfi,n in his thermochemical investigations' has pf>int#;d out that gold precipitated by sulphurous acid

lHn(§l. Poly J'/nmnl, 16S-282.

' Jmtrnalf, Praki.Chem. xiii. p. 348.

Properties Of Gold

om i flohition of its diloride fmrms a ooheient, ligtooloQred powder, fvfaereas the same reagent precipitates it from itg komide sedation as a very fine dark-o(doared powder, which obstinately retains its polverolent form, ud farther that these two are aUotrcc modificatimis, tt down by the fact that each atom of the latter has a energy of 3210 cal, more than the former. It oeed hardly be said that this forms a strong aigoment m bvoar of the allotropism of gold.

is no reason why /3 gold should not occor in 'toie as well as the a variety, and it is qnite possible ttttt the above-stated indifference of /3 gold to mercory aooount, if not wholly, at any rate partly, for the phenomenon known as the rustiness" of gold, a point which has already been referred to and will be again further on.

PHTSirAl. ANT) <:HEMICAL PHOPEBTTES of JCEBCtTBT

ITse of Mcrcnry. — Every system of gold mfllmg, properly calleii, depends apon the employment of mercui-\- for che extraction of uold from the ore. This "substance being che solvent mainly employed for the reparation of gold, a knowledge of its physical and cheniical properties is indispensable in order that the science of noltl millini? mav be thoroujjhly understood.

Fhysieal Properties of Kereory

Mercnr}' U a metal which differs from all the other common metal. in that it is li<uid at ordinary temperatures, its melting-point being below the usual temperature of the air.

Specific Grai'ify. — Its specific gravity when liquid has been variounly fstimated at between 13 589 and 13-613. one of the lat;st observers giving it at 13*5953 at 0' C. ; the spf;cific gravity of solid raercurjis given as between 14-193 and 14 39L

Its vajKiur density is 6*976.

Thermal Daia.—lt? solidifying point is usually taken it -40' C. ; more accurately it appears to be -39' -44, whilst solid mercur}' commences to melt at - 38**-85 C.

niercunemits vapour howeTer at 15' C, and perhaps at lower temperatures ; so that, if a little mercury be in the bottom of a bottle and a piece of gold suspended in the upper part of the same bottle, the gold will 'Q tiioe be amalgamated and whitened by the mercurial pottrs at ordinary atmospheric temperatures.

Its coefficient of expansion is O'OOOIS for each degree between 0' and lOtT C, increasing slightly with the mperature.

Its specific heat is 0*03247 in the sold state, 0*03312 in the liquid state up to 20* C, and 0*03278 above this iQpenture. Its latent heat of fusion is 2*84 calories, or aboat eighty times as much as is required to raise the perature of solid mercury 1' C. It is the worst conductor of heat of all the metals, its conductivity being only 5 33, that of sUver being taken at 100.

Its electrical conductivity is even lower, being only 1*63, that of silver being taken at 100 ; its electrical resistance per cubic inch is 37*15 microhms.

Optical Properties. — Mercury is pure white in colour, opaque, with a highly brilliant surface ; its vapour is transparent and colourless.

Structural Properties. — Solid mercur\* crystallises in <tahedra : it is soft, sectile and malleable, having a ven* 'o' tenacity. In all these qualities it much resembles lead. liquid mercury does not adhere to any substance except SQch metals as it readily amalgamates with under ordinan*- circumstances ; it wets " such metals, forming a thin adherent film on them, but when placed on other substances to which it does not adhere, its surface tension comes into play, and it forms spherules or takes a convex surface. It is only pure mercury that forms an approximately true

vrLS/X'j

iW

'!'Cti.'*- '-'i* v-eii :: :i:ci"al2< impcriies iu the form i :t;*kls .: :55 sphericity. And the globi ..c.-:;-i:.. i":: :- i s:-u*ikll:i Zd.' .Vliowicg a min .. -ifccK 'I'i'ziic: 1 rvLLli :c*c oc ics purity, as i- 's.-'o. .'i .L ii'r-i.ila' ouiiats :iie globules to assi

: -. I t -Wiid ".Liereury is vigoroi

ii i-vi'. '. s siili-ie "?o'i'j.::ocs s-ioli a solutio

' . : s kTl.: uv ::::-? a numb

- .-". iv:iablt :Lia: the lui

V'-, ::f:ai:v:es tlia: act i

". '- c d v. V '.V : : i 1 la rd , cout

%

t

cral tar less so

Properties Of Mercury 51

vegetable or animal oil. All unctuous substances, even such as cla}'s, also promote this efifect, which appears to be a result of the physical presence of the body determining it, and not a chemical action. Eeducing agents appear to promote to some extent the coalescence of the particles of floured mercury ; this efifect is energetically produced by an electric current, when the negative electrode is placed in the floured mercury, and the positive in a little acidulated water which forms a layer above the mass of floured mercur}'.

The same power is exhibited by a particle of sodium in the presence of water. This effect is in each case attributed to the vigorous reducing action of hydrogen in the nascent state, which is evolved in contact with the luercurv.

Chemical Properties of Mercury

iUneral Chemical Proper tics, — Pure mercury is not

affected by exix)sure to air, eitlier dry or moist, at or-

<iinaiy temperatures ; when impure, its sm*face becomes

covered by a pellicle said to consist of a mixture of

Metallic mercury and mercuric oxide. When heated in

air to 350 C, just below its boiling-point, it is oxidised

uperticially. Water agitated with mercury takes up a

little of it, ac<iuirin a distinct metallic taste ; it appears

not to bu dissolve strictly speaking, but to be taken up

inechanicallv in a state of minute subdivision.

Sulphur combines directly with mercury both in the form of vapour and by rubbing the two substances together. Mercurjvapours at the ordinary temi)erature are absorbed by sulphur.

The haloid elements, chlorine, bromine, and iodine, unite directly with mercury.

K 2

GOLD MiLUHG

N ditti%I\YNl hy hot strong acid with the

T''* authorities, however, chloride of %vnNsi -- th*' presence of air. Hydrobroi xx . !rt,i hy,Wv>lJc add readily, decompose

4i ovNvss by a sohition of potassic per ,v.-v:' -v:v;;>;i oxide in the cold, an

-, -v;:- V ' C:\t fo:-.":vr of which it acts a

\I, .v;;"'.v. X ,'-\v }IcaX. and nicrcuri*

m

.1*. iX'. iN '.VN.S,; jiljin-s with them txx i:.,- j;v:\- ;,--.'v. a:r,algams.

m.MvU iiv 5-.M. m\naX. A.v.,;;':::: fa<M that tl

f

'" Properties Of Mercur Y

Amalgams of all metals have certain (properties in oommoD. They are all white ; when the proportion of mercury is low, they tend to form solid crystalline bodies; as the proportion increases, the amalgams become pasty, and finally liquid. All amalgams appear to be soluble to some extent in pure mercury, and are miscible with mercury in all proportions When such a mixture is subjected to filtration, more or less solid unalgams, having at times approximately constant compositions, are left behind, and the excess of mercury is separated, being, however, saturated with as much of the amalgam as it is capable of holding in solution.

Amalgams are formed in various ways; very many,

sQch as those of potassium, sodium, gold, silver, copper,

nc, A:c., are produced by direct union of the metals at

ordinary temperatures, favoured sometimes by gently

Seating. Some can be formed by electrolysis of a salt

of the metal, mercury-forming the negative electrode —

iron amalgam can be produced in this way — or else by

the action of sodium or ammonium amalgam on solutions

of the metallic salts. In this way amalgams of platinum,

iroo, and aluminium, which cannot be formed by direct

combination, are produced.

As a rule the formation of amalgams causes an absorption of heat with corresponding lowering of temperature ; a few metals, such as potassium, sodium, and cadmium, cause, however, a rise of temperature on amalgamating. Most of the ordinary metals unite with mercury, but platinum, iron, aluminium, chromium, manganese, nickel, and cobalt will not unite directly ; their amalgams can, however, be produced by the two latter methods mentioned above. Mercury and Antimony, — These metals do not unite m

the cold, but on heating a soft amalgain is prodnoed from which antimony gradually separates out as a black powder.

Mercury and Arsenic. — Arsenic heated with mercury forms a gray compound of 5 parts of mercury to 1 of arsenic. When sodium amalgam is rubbed up with moistened arsenious acid, arsenic separates as a blackish IXDwder, but does not combine with the mercury. Several double sulphides of mercury and arsenic are known.

Mercury and Tellurium form a tin-white granular amalgam.

Mercury and Bismuth amalgamate directly at ordinar}* temperatures, forming a white granular crystalline alloy.

Mercury and Zinc form a white crystalline amalgam.

Mercury and Cadmium amalgamate very readily, forming silver-white crystalline alloys; their employment has been proposed in gold amalgamation in the place of sodium amalgam.

Mercury and Tin unite very readily, forming silverwhite alloys, which are largely used for silvering"

mirrors.

Mercury and Lead amalgamate very easily, forming a tin-white granular alloy.

Mercury and Cop2)cr combine freely. The amalgam is silver-white, crystalline, and when well squeezed contains 4-77 parts of mercury to 1 of copper ( Cu2Hg3); 1 ,000 i)arta of mercury will dissolve 004: parts of copper.

Mercury and Silver combine in the cold, but more r(>a(lily when heated. A beautiful crystalline amalgam produced when a globule of mercury is left in a solution of ar'ontic nitrate. Many definite solid crystalline ainalganis of silver arc known, some occurring as natural inincM'als.

'" Properties Of Mercury 55

Mercury and Gold amalgamate readily (see page 84).

Mercury and Iron do not combice directly, bat iron

can te amalgamated by means of sodium amalgam.

Amalgains of iron, containing less than 1*5 parts of iion

to 100 of mercur}' aie fluid, up to 11 parts of iron pasty,

tod above that solid and fusible. When iron amalgam

is exposed to the air, a coating of grayish black oxide

of iron forms on it, but the amalgam is decomposed

completely only after standing some considerable time.

Vhen it is heated in the air, the iron bums, throwing

oat bright scintillations.

Mercury ami Sodium combine energetically, their comjinaiion being attended by the evolution of both heat and light, the maximum effect being produced when the elements are present in such proportions as to form the amalgam HgNa.,. Sodium amalgam is best pre-, paretl by heating pure drj* mercury gently in a flask or 'a>in, and throwing in small, dry, clean chips of sodium freshly cut from a lump of the latter metal, one piece at a ime, until sufficient has been introduced. The combination with each fragment is attended with incandescence. Sxiiiiin amalgam is as white as mercury and as brilliant. When containing 30 parts of mercury to 1 of sodium it rather hard under the file, and has a crystalline, foliated structure. With less mercury than the above it is liichly crystalline and brittle. It forms long pnsmatic crystals, that can be freed from adhering mercury by pressure. According to Berzelius it crystallises in cubes. With 40 parts of mercury' to 1 of sodium it is still solid, but softer. With 60 parts of mercury it forms a mass which is a stiff paste at 21*' C, and seems to be composed of confused interlacing crystals. With 80 parts of mercury it forms a thin paste showing numerous granular

crystals. With 100 parts of mercury it fluid, which seems to be partly solid and f With 130 parts of mercury it is quite fluid.

It is said that when an amalgam contsininf of sodium is allowed to stand under water, ndt the formula separate out.

When gently heated, solid sodium amalga gives off its mercury at a heat-below rednc it decomposes slowly, more . rapidly under solid amalgams high in sodium decompose in rapidly than those low in sodium, ul ducts of the decomposition are mercury and so Sodium amalgam can only be preserved vessels.

Mercury and Potassium. — Potassium amalgs in properties to the last. The evolution of he elements combine, here reaches its maximum proportion is such as to produce an amalg composition HgjKg.

Haloid Compounds Bromides

Mercurous Bromidey HgoBrg, is white, ir water, fusible and volatile below redness ; by the action of mercury on mercuric bror precipitating a soluble mercurous salt by i soluble bromide.

Mercuric Bromide HgBrg, is formed by tli excess of bromine on metallic mercury. It i water, alcohol, and ether ; it is easily decompos and by sulphuric acidi?. It forms double salts

PROPERTtES OF MERCURY 57

kiich are soluble in water. Of these mercuric mide, Hg6r22EBr, may be taken as the type.

Iodides

s Iodide, HgIj, is produced by grinding up mercury and 127 of iodine with a little

all the roerpury has disappeared, forming a }te ; this is thoroughly rubbed up with alcohol, 1 alcohol to remove mercuric iodide, and dried. formed by adding an alkaline iodide to a a mercurous salt ; excess of the iodide must

because alkaline iodides split up mercurous mercuric iodide and free mercury. irk yellowish green powder, apt to decompose n dry into mercury and mercuric iodide ; this ion is accelerated by the presence of numerous It is soluble in ammonia.

Iodide is formed by rubbing up mercury with Lcess, or by precipitating a solution of a merith potassic iodide. The precipitate is soluble [ either reagent. It is markedly dimorphous,

a red and a yellow form, the former being >table. The red form is produced by pre- s above, the yellow by fusion or sublimation, ery soluble in water, but dissolves in many [1 solutions of ammoniacal and other neutral

as sodic and potassic chlorides, &c. ; it is alcohol. It is soluble in solutions of many dies, producing double salts of the general gIgMI and HgljSMI, the latter being always table form. With dyads, compounds of the Hj are produced. I of mercw-y are known to exist.

Chlorides

These are the most important compounds of mercury, and both of them are used in the arts.

Mercurous Chloride, or calomel, HgjClj, is a white amorphous body, insoluble in water, volatile, subliming in square prismatic crystals. It can be made by the direct action of chlorine gas on excess of mercury, by the action of ferric chloride on mercury, and most commonly by the distillation of a mixture of salt and nicrcurous sulphate, when double decomposition ensues. However prepared, it must be carefully washed with Wcater to remove any mercuric chloride that may have been formed, and which is soluble in water. Prolonged boiling with water forms a little mercuric chloride, liberating mercury. The action of hydrochloric acid forms mercuric chloride. Most oxidising agents transform it into mercuric chloride, and most reducing agents into mercury.

Heated w4th sulphur, mercuric chloride and sulphide

aro formed : —

Hg,Cl2+S HgS + HgCl,.

With sulphide of antimony, chloride of antimony and mercuric sulpliide are produced.

Chlorine and aqua regia convert it into mercuric chloride.

Mercuric Chloride or corrosive sublimate, HgCl, is formed by the direct action of chlorine in excess upon mercury, or usually by distilling together mercuric sulphate, salt, and manganic dioxide. It sublimes in white crystalline masses, and is readily soluble in water, alcohol, and ether. Hot hydrochloric acid dissolves it freely, the mass solidifying on cooling. Hot sulphuric acid only attacks it very slowly.

fisan acrid metallic taste, and isiavet violent poison.

ni PROPERTIES OF MERCURY 59

is easily reduced by reducing agents to mercurous chloride, or to metallic mercury. Most metals reduce \i.

Solutions of caustic alkalies in small quantities give a idish brown precipitate of oxychloride, in larger quantities an orange yellow precipitate of mercuric oxide. Carbonates give a precipitate, which is at first white but soon changes to the reddish brown oxychloride. With albumen it forms insoluble compounds. It is very largely used as an antiseptic.

There are only two known : mercurous and mercuric.

Mercurous O.rule, HggO, is a very unstable body ; it is produced as a brownish black powder by the action of alkalies on mercurous chloride. The precipitate must be washed and dried at a low temperature, and not exposed to light ; even diffused light decomposes it into mercuric oxide and metallic mercury.

Mercuric Oxide, HgO. — This oxide is produced when metallic mercury is maintained at a temperature just l)elow its boiling-point for some considerable time in air or oxygen. A higher temperature decomposes it again into metallic mercury and oxygen. It may also be prepared by adding alkalies to a solution of mercuric chloride, when it is precipitated in the anhydrous state as a reddish yellow powder. It appears to be dimorphous, a red and a yellow variety being known to exist. It is very slightly soluble in water, to the extent of 1 part in 200,000 ; even this small amount gives a distinct metallic taste. It is capable of forming compounds with the haloid salts of mercury. At ordinary temperatures it is a very stable cowponnd.

6o GOLD MILLING chA'-

Mercurous Sulphide, Hg,S, is an unstable Uaok sabstance produced by the action of alkaline suljdes o& niercurous salts ; it readily splits up into mercury woA mercuric sulphides, and may be only an intimate nuztoie of these two substances.

Mercuric Sulphide, HgS, is markedly dimoiphooB, there being both a black and a red variety ; the latter is known as cinnabar or vermilion, the former name being applied to the native mineral, which forms the only true ore of mercury, the latter to the artificial oompoond. When formed in the cold, or in the wet way, the black sulphide is produced. This may be sublimed without decomposition when air is excluded, but when condensed the red variety, vermilion, is produced. Sulphur and mercury combine directly when triturated together, forming the black sulphide, these two elements having coTisiderable affinity for each other. It is also produced by the action of alkaline polysulphides on metallio mercury. Sulphuretted hydrogen or alkaline sulphides precipitate it from solutions of mercuric salts. It is not attacked by nitric acid, nor by hydrochloric acid except in the presence of an oxidising agent, or of some metallic chlorides such as ferric or cupric chloride, when mercuric chloride is produced. Boiling concentrated sulphuric acid converts it into mercuric sulphate, sulphurous anhydride being given off : —

HgS + 4H,S0, HgSO, + 4SO2 + 4Hp.

The freshly precipitated black sulphide is partially decomposed by concentrated nitric acid with the separation of sulphur ; it is also slightly soluble in caustic potash. Ordinary mercuric sulphide is, however, practically insoliMe in either caustic alkalies or in ammonic sulphide.

PROPERTIES OF MERCURY ni

Wit'i] heatud iu the air it forms inetallic mercury, and tiiphurous anhydride is evolved. It is decomposed by

Wting with lime, alkaline carbonates, or metals such as

metallic mercury being liberated in each case.

Mercuric sulphide forms insoluble compounds with /Dany other salts of mercury.

SalU of Mercury. — Both oxides of mercury form salts

with acids, the salts so produced being characterised by

a marked tendency to form basic salts containing excess

of oxide of mercury, such, for instance, as HgSO, 2HgO,

excess of mercury with nitric acid diluted with one half its balk of water, until crystals form. As a rule some mercuric nitrate is formed at the same time. It is soluble in a small quantity of warm water, a larger amount splitting it into an acid and a basic salt ; several insoluble basic mercurous nitrates are known.

Mercuric Nitrate, Hg(N03)2, Aq. — When mercury is dissolved in excess of nitric acid, and the solution evaporated, crystals of 2Hg(N03).2, Aq, are formed ; crystals with varying proportions of water of crystallisation have also been produced. Mercuric nitrate is soluble in water, but it splits up readily into acid and basic nitrates ; several insoluble basic mercuric nitrates are known to exist.

Mercurous Sulphate, HggSO, is produced by heating excess of mercury with strong sulphuric acid or by rubbing up mercuric sulphate with metallic mercury ; it is very slightly soluble in water, hut dissolves in dilute nitric and in concentrated sulphuric acids.

Mercuric Sulphate, HgSO, is produced by heating mercury with excess of sulphuric acid and evaporating to dryness, when a white saline mass is left. It forms a white crystalline BnhydrouB powder, soluble in su\p\i\mo

Oj

Cha.

acid, l)ut (lucoiiiposL'd by water into a soluble acid and a. insoluble basic sulphate.

Other Salts, such as chlorates, bromates, iodates, vit rites, phosphates, &c., are known, also carbonates whict' arc very unstable. There are also a large series ot mercurammonic compounds known, some of which ar0 explosive ; fulminating mercury belongs to this class.

The following table shows the characteristic reaction of mercurous and mercuric salts with various reagents : — -

Ucagt'iit.

8iiIi>hurettedHy-\ drovii or solu-j bk> iSulpliideti...;

Putosh or Soda

MercurouB Salts.

Black precipitate, insoluble in Alkaline Sulpliides.

Soluble ludidcs.

fBlack precipitate of Hg]0, insoluble in excess. Alkaline Carbon-1 Dirty yellow precipiUte, be-l

att' / coming black on boiling. /

Ainiiiuniaft Ani-i Blackish grey ammoniacalcomiiionic Carbonate' jiound.

/Green 1 rociiitatr> of HtoLj transfornie<l by 1an;o excess iiit Hgand IlgLj. the latter dissolving.

Sodic riicspliate White precipitate.

Potissic Fcrro-llnruj*- ,'..:4. rymiide \mite precipitate.

I'dlaNKic Ffrri-Vt, i n i i s i

cvanide Reddish brown preciiitatc.

SnhlbIu Gallatcs jHruwiiisb yellow precipitate?.

White precipitate of HaCy.;. PotassicCvanlde ; sjilitting at once into Hg and ( HirCyo.

Mercuric Salts.

Black predpitate (at flrttwUta or yellowj, insoluble in Alkaline Sulphides.

Orange yellow predpitrte of HgO, insoluble in esecH.

Tellowish red precipitate.

White ammoniacal eompomid, soluble in great

'I No precipitate.

Red prt'cipitatc of Hgl2,so]ubIc in exccHii, also in large excess of Mercuric Salts.

/White precipiUte (except with Mercuric Chloride). White precipitate.

White precipitate.

/White precipitate (except with Mercuric Chloride).

Vellowish red preci]imitate. I (Orange precipitate (except with Mercuric Chloride).

Wliite precipiUte of HgCy, soluble in excess (except with 1 Mrrcuric Chloride).

Physiological Effects. — All soluble salts of mercury are violent poisons when taken internally. The symptoms of acute mercurial poisoning are an acrid metallic and astringent taste ; continual expectoration ; severe pains in the stomach and intestines accompanied by vomiting and

in

Properties Of Mercur Y 63

f

diarrhoea ; slow irregular pulse ; severe syncope or somoes convijsions, upon either of which death may follow. The best antidote in cases of mercurial poisoning is the vhite of raw eggs (albumen), which forms an insoluble compound with mercurial salts. Freshly precipitated ferrous sulphide is also a good remedy, forming insoluble sulphide of mercury and ferrous chloride which is harmless; it must, however, be promptly administered. Very fine iron filings may also be given immediately, as they will precipitate metallic mercury.

Slow poisoning is produced by exposure to mercurial vapours, or by protracted handling of mercury ; its s}'mptoms are salivation, skin affections, ulcers of the mucous membrane, and mercurial tremors. There is always liability to salivation when amalgam is being retorted, unless the operator takes especial care not to expose himself to the mercurial fumes. In these cases the inercur}' is slowly eliminated from the system. A strong solution of chlorate of potash is recommended as a mouth wash immediately after exposure to mercurial vapours.

Purijication. — Mercury is generally bought and sold in flasks made of wrought iron closed with a screwed xlug about three-quarters of an inch in diameter. These liasks hold 75 lbs. Spanish, equal to 76 lbs. avoirdupois, of mercury in Europe, and 76i lbs. in California, and themselves weigh about 12 to 14 lbs. Commercial mercury is never quite pure ; it is often contaminated with traces of foreign metals, those most frequently occurring being the more volatile ones, such as lead and zinc. Kedistillation will not effect a complete separation from them, as some portion will distil over with the mercury; in order to avoid this, it has been recommended to distil at a low

temperature in a current of superheated steam. It however, better to employ a chemical method of purifi-' cation. When small quantities of lead or zinc are p- sent, mercury distils much more slowly than when pore; other metals do not seem to exercise this action. Small quantities of mercury may be fairly well cleansed by vigorous shaking in a bottle, not more than a quarter full, with a little crushed loaf sugar, and afterwards filtering through a cone of stout blotting paper which has a small pin-hole at the apex of the cone. This method of filtration may always be used to remove suspended impurities. It may be purified by distilling in a capacious retort, such as is used for retorting amalgam (see page 446), under a layer of cinnabar, the sulphur of which combines with the other metals present, and prevents their passing over. If fairly pure, it may be distilled under a layer of quicklime or iron filings, which will retain any sulphur or arsenic that may be present, and also prevent splashing. The retort should never be more than half full of mercury, and the distillation be conducted ver\- slowly.

If it contains zinc or tin, it maybe purified by digestion with hydrochloric acid or ferric chloride. Lothar Mayer recommends letting it trickle in a very thin stream through a column of ferric chloride about 4 feet in height. Briihl agitates repeatedly with a solution of potassic chlorate and sulphuric acid.

It may also be agitated with moderately concentrated sulphuric acid for some days. When heated with a solution of mercuric nitrate, the contaminating metals are dissolved as nitrates, mercury being precipitated. The most general method and the most satisfactory is by treatment with dilute nitric acid, in which it should be (lia'stcd for twenty-four hours. The best plan is to

Properties Of*Mercury

Scale 9i to 1 ft Fm. 1.

I agitate with dilute nitric acid (about 1 of acid to 3 of water) in a tubulated glass or stoneware receiver (Fig. 1) fanushed with a stopcock at the bottom, and capable of holding about a flask of mercury at a tima After thorough agitation it should be left at rest for a few .vs, and the pure dry mercury then drawn off as required from the bottom of the receiver by means of the stopcock. If crystalline crusts of nitrate of mercury form on the surface, more water, rendered acid by a few drops of nitric acid, must be poured on to it ; the solution will not need renewing for a long time.

Dae care must be taken that the stopcock is a strong one and well secured, as the column of mercury in the receiver exercises a heavy pressure upon it.

By keeping two such receivers in use, the mercury from one of which is being used whilst the other is allowed to purify slowly, being stirred up every few days, a supply of pure mercury may be ensured. It may lie remarked that this method does not remove any of the precious metals, gold and silver, that may be present.

Pure mercury should form small, perfectly spherical globules, uniformly bright, which unite at once when (wrought into contact with each other, should leave no "tail'* at all when allowed to run slowly down an incliiifd ;lass plate, should leave no iilin on rough blotting pajxir, and give no black powder when shaken up in a Ujitle with dry air.

F

Chapter Iv

Alloys And Amalgams Of Gold

Gold is capable of combining with most of the better known metals, forming alloys. An alloy may be defined generally as a compound of two or more metals ; that is to say, it is not merely a mechanical mixture of these metals, capable of being separated into its ingredients by mechanical or physical means alone, but these ingredients are united by so firm a bond as to cause the alloy to partake of the nature of a chemical compound; it differs from a true chemical compound, inasmuch as metals that are capable of forming alloys together, can do so in all proportions irrespective of their chemical combining weights. At the same time there are not wanting indications that more complete, intimate and homogeneous compounds are produced when the metals are combined in proportions according to their equivalents, and therefore capable of being represented by chemical formulas. The evidence that chemical compounds, albeit feebk ones, are formed, is moreover very strong. Thus, wher metals are alloyed, chemicothermal changes are produced the combination being mostly attended with the evOlutior of heat, but sometimes with its absorption. The physica

A/./,OYS OF GO/.n

'>7

i ils uf an alloy are rarely, if ever, the iiuau of Jose of its components. Thus at times two malleable tietals will produce a brittle alloy, or an alloy may be 'usable at a lower or a higher temperature than any of its constituents. The colour of an alloy often differs considerably from those of the metals composing it ; thus tbe red metal copper and the white metal antimony combine to form a violet-coloured alloy. The specific gravity of ao alloy is rarely the arithmetical mean of those of its constituents. The chemical properties of the constituents become greatly changed when they are alloyed. The electric and thermic conductivity of an alloy often varies greatly from those of the metals contained in it. In fact the phenomena which we are accustomed to associate with chemical combination are produced to some extent when an alloy is formed. An alloy of metals in the molten state may accordingly be looked upon as : (1) a chemical compound, or a series of chemical compounds, of the constituent metals ; (2) a solution of one or more of these compounds in excess of any of its constituents ; (3) a solution of one or more metals in another, provided that such metals may then be in an allotropic condition. When the molten alloy is allowed to cool, the soUdified mass may still retain its ingredients in the above form, the dissolved bodies not separating out from their solvent on solidification. At times, however, we see that alloys, which are capable of existing at high temperatures, are decomposed on cooling. The real nature of alloys is at present the subject of investigation by numerous metallurgists, and microscopy has recently been found of especial value for revealing their true structure. ' The opinion most generally held seems to be that an alloy is a solidified solutioij ol a metal or metals

F 2

Gold Milling

their eutectio " compound, an euteotic alloy being in which the constituents exist in such proportions a: produce the lowest possible freezing (or melting) poin Alloys of gold have not yet been very carefully or ' exhaustively studied. A great deal of our knowledge the present day is derived from the labours of G. Hatd who, in 1803, studied the effect of various metals i standard gold. He found, as might be expected, most, if not all, metals tend to reduce the malleability ductility of pure gold, the order in which they do so b as follows, the first on the list having the greatest e in destroying these qualities : —

5. Zinc. G. Cobalt. 7. Manganese.

This subject has received attention from Rob Austen, who has investigated the effect of i quantities of various impurities (about 0*2 per c upon the tenacity of gold. His results are reprod in the table on page 69.

Pure gold was found to have a tensile strength

tons and an elongation of 30'8 per cent. From al

observations he deduces the interesting law that

metals which render gold brittle are those which

liigh positions on Lothar Mayer's curve of the elem

that is to say, which possess a liigh atomic vol

bpecihc gravity/'

VhiL Trana. uf the Iloyal Soc, (if Lmdon, 1888, p. 389.

Alloys Of Gold

TUifle Strength per 8qire Inch.

Blongfttlon per

eent. on S-inch

Feroentageof Imparity Preaent

Kot perceptible.

Leas than 0-2

tt

!

1

ther found that standard gold, consisting of 3 of gold and 83 3 parts of copper per mil. has strength of 18 tons per square inch with an I of 34 per cent, before breaking ; the addition lart of lead reduces this breaking strain to 5*5 quart inch, and part of lead reduces it still 1*84 tons, whilst the elongation has become able.

jcific gravity of alloys of gold with other metals arely, if ever, the calculated mean of their its, there being either contraction or expansion ume of the alloy compared with the volume of its. The following table shows the amount of ge, according to Ilatchett, in alloys made by 1 parts of gold with 1 part of the respective

Contxsrtioa. '

Tin ...

i-?:5

—

f>L-math

1

Zini: ...

—

f.'ohalt

—

'Jilver

—

Xiikel

—

Copper

—

-J

Hatchett further remarks that, generally speakingi those metals which produce brittleness in gold wheo alloyed with it, also cause contraction when these alloys are formed ; and he points out that the result obtained hy him with zinc is not to be depended on, owing to the great volatility of that metal. Although bismuth causes contraction, and lead expansion of the respective alloys in the above proportions, yet when only one-half of a grain of either is present in an ounce of the standard gold-copper alloy, both of these metals cause great expansion, this being in the case of bismuth 4*72, and in that of lead 5*71 per cent.

Matthiessen has done a great deal of valuable work on this subject ; he has particularly investigated the specific gravities of the alloys of gold with tin, lead, bismuth, and silver in varying proportions.

In the case of tin he finds that the specific gravity of the alloy containing tin 100 parts, gold 1 part, calculated from the mean of its constituents, is greater than the obser\'ed specific gravity in the proportion of 1"0007 to 1 ; that for alloys ranging between 30 and 12 parts

rhU Trans, 1860, p. 177.

a/Javs of GOI.n 71

I tin to 1 of gold, the former is less than the latter in the proportion of about 0*99 to 1 ; when the composition is between 8 and 2 parts of tin to 1 of gold, the former is again somewhat greater, and for equal parts of both loetals or for the alloy of 2 parts of gold to 1 of tin again a little less, than the latter. In the alloy of 6 of tin to 1 of gold the two almost agree. In the case of lead the observed specific gravity is always greater tlian that calculated from the mean of the constituents in about the proportion of 1 to 0-99.

In the case of bismuth, the alloy of 90 parts of bismuth tol of gold had a specific gravity very slightly less than tiiat calculated for it, whilst aU the other alloys gave a slightly greater result, the difference increasing as the proportion of gold increased till the composition BiAu was reached.

He remarks that the allo3r8 with lead and tin are all very brittle except where these metals were in great excess.

The following table represents the results of Matthiessen's experiments on the specific gravity of the alloys of silver and gold : —

Composition of AII07.

Silvrr. Gold.

Obseired Specific

Calculated Specific Gravity.

Ratio of 1 Calculated Specific Gravity i

Observed Specific Gravity.

His general deduction from all his experiments is thai the contraction or expansion is a maximum when the nietals are present in about equal parts. He has not however, succeeded in developing any definite law on th( subject.

Alloys of Gold with Various Hetals

Gold and Silver. — The density of the resultant alloy i always very nearly the mean of those of its constituents some observers make it a little higher, others a littl lower. The results obtained by Matthiessen are give: above. Whenever the two metals are combined in atomi proportions, they seem to form a perfectly homogeneous alloy if well incorporated. Thus, alloys having th formulas AugAg, AuAg, AuAg, AuAg, AuAgg are quit homogeneous on cooling.

It is worthy of note that the alloy corresponding t the fornmla AuAgg is of nearly the same specific gravit as is mercury at ordinary temperatures ; all those wit more gold than 48 per cent, will accordingly be heavie and all those with less gold lighter, than an equal bul of mercury.

Generally speaking, the alloys of silver with gold ai harder, paler, and more elastic than pure gold, th hardest consisting of 2 parts of silver to 1 of gold. Tt alloy of 70 parts of gold with 30 of silver is of greenish tinge ; it is used by jewellers, and known c green gold." With equal parts of gold and silver, tb alloy is white with barely a tinge of yellow, and with let old than this, perfectly white. When the alloy contair less than two-thirds of silver, this metal cannot be con pletely extracted from it by nitric or sulphuric aci( and alloys rich in gold are not attacked at all. Tl

Iv Alloys Of Gold 73

phenoraenon known as " spitting," due to the absorption of oxygen by molten silver and the evolution again of the gas on cooling, is shown by alloys of gold and silver provided that the proportion of gold does not exceed onefourth; it even takes place, to some extent, until the Proportion of gold reaches one-third. If molten gold be poured into an equal weight of molten silver, the alloy can no longer retain the oxygen which the latter metal bad absorbed, and it is given off with violent effervescence. Similarly, if the gold be melted and the silver then added in the solid state and allowed to melt without stirring, a Jayer of molten silver will form above the gold, and this layer will absorb oxygen. On stirring so as to incorporate the metals, an alloy is formed, and the oxygen thus liberated similarly escapes with much violence.

This possible source of loss must accordingly be jOiarded against when making gold-silver alloys, which 8 easily done by throwing a few pieces of charcoal upon he surface of the melting metal. The alloy containing 10 per cent, of silver is often spoken of as " electrum," his being the name applied to it by Pliny. It is derived rom the Greek word for amber, and probably refers to he pale colour of the alloy as compared with pure gold. Gold and Copper, — This is an important alloy coiniiercially, forming the gold standard of most of the oinage of the world. British standard gold is twentycarats fine, that is to say, contains oi gold or 16*0 parts per mil. and -r or 83*3 per mil. of copper, ts specific gravity is 17*157; considerable expansion akes place when this alloy is formed, the specific gravity calculated from that of its constituents being 18-47. The pecific gravity of standard gold containing a quarter of a rain of lead to the ounce is 17039, with half a grain

L : 71- -15 '.o-. i.L'i -x-.zz eiija: crniiri" 7 :'L2. Tbi "rettinir-roii:: i:f st*icLinI is 946' C- T'"-; Fr*:r:oh -jrartuiL-'l :or -join i-? pivrts of izold per mil., r - 1 o r y 'av: le r y 7-30 . T!if -a m e co ir. aire st an danl has Iw* ;itl-;rci.-il by r:-..: ozherr rari-rns of :he Latin Union and ry A"!o:i'.':i. T!'--f -Mr- Lin I o: uhe Transvaal was SIG' I : r r . It : h o ci: p ct: r fc e pt rf oc d y pan? . it only lowers tiio Mlloabilirv :: :iie ioM ilizb.:lv. bu: even traces of havo. as previously ir.tlicacei pa;ze 68), a most i:?!porcari: e:*ec: in rh:-? respect. The alloys with copper iuv [larder. more fusible, and soniewhat deeper coloured liian pure iTold. The hardest alloy consists of 7 parts ot old to 1 of copper. When heated in the air, these ;i!lovs rauidlv decor.:e coated with a crust of oxide of ovpvr. Alloys rich in sjold are not affected by exposure to air, under ordinary conditions, but poor ones soon i:inii4i. Alloys ha\'iug the chemical formulas AuCu, V'.iVu. AuCu, Avi.Cu, are quite homogeneous when inoUovl and well -tiLrod. but AuCu and AuCu require to Ik* well stirred, cast into ingots, and these ingots uMii oh od with thoroUiTh stirring several times in order vbiiiin a homoi;oneous alloy. As in the con-responding \Mso of silver alloys, copper can be dissolved out from ilu' alloy when present in large proportion; when the ,ilK>\ I'oiitiiins over \)3'0 per cent, of gold it is not ;viiiirki'\l ly nitric or sulphuric acid.

'ril[lo alloys of gold, copper, and silver are much used I ho nuuuifiieture of jewellery, the silver being em- pUiynl in order to make the colour of low standard paler, and therefore more like gold, than would be if copper alone were used. Some of the very oys also contain a few per cents, of zinc,

vith the same object.

It

Alloys Of Gold

6oU and Tin. — These metals alloy readily ; a considerable percentage of tin makes gold brittle, but less than 2 per cent, does not seem to do it any harm. Alloys hose composition ranges between the formulas SnAu SnAu are vitreous and not crystalline in fracture. SdjAu (45-9 per cent, of gold) shows some tendency to form cnstals, and has a granular fracture. All these oysare of a yellowish gray to a grayish white colour. The alloys ranging between AuoSn and AuSq (40 to 22

Compoiiition of Alloy.

Sjiecir.e Qrmvity.

Gol.l.

Tin.

f

i 1

'I

:J

%

r.o

jXT cent, of gold) crystallise in square prisms ; the alloy Aii.Sn., has this tendency in a very marked degree, fori I ling crystals consisting of secondary and primary prisms and pyramids of the tetragonal system, and having a well-marked basal cleavage. Their colour is tin-white, but on exposure to the air they tarnish to a bronze colour ; these crystals cry " like tin when they arc bent. The alloy containing 11 parts of gold to 1 of tin is pale

yellowish white, brittle, and has an earthy fracture of yellowish gray colour.

Five per cent, of gold alloyed with tin scarcely afGdot either the hardness or the ductility of the latter metals The table on page 75 shows the specific gravities of large number of tin-gold alloys.

The relation of the actual to the calculated speoifio gravities of these alloys has already been indicated.

Standard gold which has one-half of its copper replaced by tin is brittle, pale yellow, and has a close-grained earthy fracture. With eight grains of tin per ounce it is fairly ductile, and rather paler than standard gold. It is, however, brittle at a low red heat.

Gold and Arsenic, — These metals are not easily alloyed by fusion in an open crucible, as most of the arsenic then volatilises; thus 5300 grains of gold melted with 450 grains of arsenic only retained six grains of the latter. This alloy is brittle, but bends slightly under the hammer before breaking. When a plate of gold at a red heat is (xi)osed to the vapour of arsenic, a highly fusible, brittle, alloy forms, which melts and trickles off the surface of the plate. An ounce of gold melted in an atmosphere of arscMiic absorbed 1*5 grains, forming a coarse-grained, gray, brittle alloy. When combination between the metals has taken place, it is very difficult to drive ofif the arsenic by heating the alloy unless the heat be intense and very prolonged. In one experiment, where heating was continued for an hour, six ounces containing 9J dwt. of arsenic lost the whole of the arsenic, and cJso two grains of gold, which were apparently carried ofif in the vapour of the arsenic.

005 per cent, of arsenic will make gold unworkable under the hammer.

'V Alloys Of Gold 77

! and Antimony. — These metals alloy readily, altbongb a little antimony is usually volatilised during

' operation. The alloy containing 8 to 10 per cent, of

witimony is grayish white, very brittle, with a close-oed

, doll gray fracture. An alloy with the formula

AuSb (61-5 per cent, of gold) is white and very brittle ; it

is superficially attacked by nitric acid. In all alloys of

gold with antimony the latter oxidises on heating in the

Standard gold containing eight grains of antimony to

the ounce forms a dull gray, brittle alloy ; with half a

grain it is brittle and close-grained, but shows a little

metallic lustre ; with a quarter of a grain it is still brittle,

but not excessively so. Thus it would seem tiiat 0 05 per

cent, of antimony makes gold unworkable. Gold heated

in the vapour of antimony, either in an open or a closed

crucible, will absorb enough to make it very brittle and to

change its colour to gray.

Gold and Bismuth, — These alloys are all brittle in any ordinary proportions. The alloy of 11 parts of gold and 1 of bismuth is greenish yellow, very brittle, with a finegrained, earthy fracture. Molten gold readily absorbs bismuth vapour and is rendered brittle by it.

Standard gold containing eight grains of bisnmtli per ounce is pale brownish yellow and fine-grained; with four grains its fracture is coarse, witli one grain coarse and spongy, with half a grain very spongy, with a quarter of a grain close granular ; all these alloys are brittle. The table on ijago 78 shows the specific gravities of a number of gold-bisnmth alloys.

In all these alloys, as already stated, there is strojig contraction when the alloy is formed.

Cold Milling

Chap.

It may be noted that the alloy of gold and bismuth cupels readily >vhen heated in the air ; that is to say, that the molten alloy yields a readily fusible oxide of l)ismuth that will sink into and be absorbed by a porous body like a bone-ash cupel, leaving pure gold behind.

Composition of Alloy.

Specific Gravity.

Colli. Bismuth.

Cioid and Lead, — All these alloys are brittle in spite of tlio malleability of the constituent metals; 11 parts of to 1 of lead yield a pale yellow alloy having a pale ]>iown, earthy, fine fracture ; with more lead the alloy becomes of a pale gray colour with a gray, line granular fracture. Alloys with lead remain molten until they have cooled down to a temperature considerably below their melting-point, when they solidify instantaneously, emitting a bright flash, whilst the temperature at once rises to the melting-point of the alloy. Lead vapour combines freely with molten gold in closed vessels. Acetic acid dissolves out nearly all the lead from these alloys.

Tlie following table gives the specific gravity of a series of 'oUl-lead alloys ; -

O V

Alloys Of Gold

CoiuiKJsitiou of Alloy.

Specific Oravity.

Gold.

Lead.

Standard gold containing nineteen grains of lead to the ounce has about the colour of standard gold, with an earthy fracture ; with eight grains of lead the fracture is rather coarse-grained, with one grain coarse grained, with half a grain very spongy, with a quarter of a grain close granular. It is noticeable that both lead and bismuth affect standard gold similarly, half a grain of either metal to the ounce of gold yielding a very spongy alloy. The parallelism of their action is well shown in the following table of the specific gravities of standard gold Nvhcii alloyed with varying amounts of the respective metals : —

Gnins of Impurity IH:r Ounce.

SpeciDc Gravity.

Lead.

Bismuth.

not examined

The specific gravity of the standard gold was 17* 157, of the lead 11-362, and of the bismuth 9-822. (llatchett.)

It is further noteworthy, in this connection, that tlvalloy with lead, like that with bismuth, is capable of bein cupelled, these two being the only ones that possess iM property.

Gold and Ziiic, — If gold and zinc be melted together irt open vessels, much of the latter metftl is volatilised, but in closed vessels there is but little loss. When gold is melted with brass very little of the zinc is driven off. Molten gold readily absorbs zinc vapours. It is stated that an alloy of 7 parts of zinc and 1 of gold can be completely volatilised at high furnace heats. Mr. Pioard states that if a mixture of fbiely divided gold and zinc (as obtained by precipitating cyanidation liquors) be distilled in an ordinary retort, great loss of gold occurs, but that this loss can be reduced to about 0*05 per cent, by making the above mixture up into balls with molasses or strong sugar solution, so as to form a carbonaceous mass in the retort.

An alloy of GO parts of gold with 1 of zinc is very brittle ; 17 parts of gold to 1 of zinc give a pale greenish yellow alloy ; equal parts yield a white, very hard metal, capable of taking a high polish ; 2 parts of zinc to 1 of gold yield a fine-grained alloy, whiter than zinc ; all these alloys are very brittle ; those with more zinc are ductile. I have found that the alloy of 3 parts of zinc to 1 of gold is grayish white, granular and hard, but can be readily rolled into thin strips without annealing.

Standard gold containing nineteen grains of zinc to the ounce is pale yellow, has a coarse fracture, and is very brittle ; with a smaller proportion of zinc it shows a little malleability.

Gold and Cadmium, — These metals alloy freely and

"'i''inniv and brittle alloys, althoiih cadmium is a very malleable metal. J and Iron. — These metals alloy readily ; with 11 [ parts of gold to 1 of iron, the product is pale yellow and ductile, having a specific gravity of 16-885. If cast iron or steel be substituted for pure iron similar results are obtained. So-called " gray gold " consists of 4 to 5 parts of gold to 1 of iron ; it has a grayish yellow colour, and is soQietimes used in jewellery. Equal parts of gold and iron make a gray alloy, 1 of gold to 4 of iron a silverwhite one, hard, magnetic, and capable of being tempered. All these alloys are harder than gold ; 4 per cent, of gold will make steel brittle. Standard gold containing nineteen grains of iron to the ounce is pale grayish yellow and ductile.

Gold and Cobalt, — These metals form alloys ; that containing 11 parts of gold to 1 of cobalt is a dull yellow metl, with a pale earthy fracture, brittle, having a specific gravity of 17*1 12. Standard gold containing more than four grains of cobalt to the ounce is brittle, but with this proportion it commences to show signs of ductility.

Gold and Nickel. — The alloy of 11 parts of gold to 1 of nickel is brass -coloured, brittle, coarse-grained, with an earthy fracture, having a specific gravity of 17068. Other alloys are said to be ductile, yellowish white, hard, as magnetic as nickel, and susceptible of taking a good poUsh.

Standard gold containing nineteen grains of nickel to the ounce is brittle with a fine-grained fracture; with eight grains to the ounce only slightly brittle, and with fr>ur grains ductile, and of about the same colour as standard gold. Gold and Manganese. — These metals are known to forvtv

Q

Sj GOLD MILUNG

an alloy, but nothing is known of its properties. Itil prepared by melting gold at a high temperature in I brasfjued crucible with manganic dioxide and carboB The alloy, which is supjwBed to contain one-eighth ti one-ninth of manganese, ta pale yellowish gray, has I high lustro, and is slightly malleable.

Go]A mid Aluminium. — These alloys have be* examined by Roberts- Austen.' With less than 10 p cent, of aluminium the alloys are pale yellow, but wil this amount brilliantly white; from this point onwari as the proportion of aluminium increases, pink fled appear until the alloy of 22 parts of aluminium to 78 gold is of a splenditi purple colour, in which intense ruby coloured crystals may be recognised. On sf further increasing the proportion of aluminium t alloys lose all red tinge and pass to a gray colour. It also noteworthy that the alloy containing 10 per cent. aluminium melts at about 630°, whilst the purple all has a melting-point of 1070° C, or rather higher than tb of gold itself ; the alloys with more aluminium have agi lower melting-points. A small percentage (say about of aluminium seems to increase the tenacity of gold, fc gold containing 15 to 30 per cent, of aluminium too brittle to tc3t, the tenacity increasing again slight as the percentage of the aluminium is still furtl; increased. All these facts seem to point to the exiatec of a definite purple -col cured chemical compound of gc and aluminium having the formula AuAl. It is ooi ptetely decomposed by hydrochloric acid, alumii chloride being formed, and gold left behind in a ve spongy state,

Gold and Platinum. — The alloy of 2 parts of platini Pnc JIuy. Soe., 1891, vol. xlix. f. 347.

1 of gold is brittle ; with equal parts a ductile alloy,

anng about the colour of gold itself, is produced ; with

2 parts of gold to 3 of platinum the alloy is gray. The

alloy of 11 parts of gold to 1 of platinum is yellowisii

white and ductile. According to E. Matthey. when

a spherical mass consisting of an alloy of gold and platizrnm

is slowly cooled, the platinum is concentrated to

some extent in the centre of the mass.

Gold and Palladium, — These metals alloy in all proportions. The alloy of equal parts is gray, as hard as iron, and less ductile than either of its constituents ; its specific gravity is 11*079. That of 4 parts of gold to 1 of palladium is white, hard, and ductile.

Qold and Rhodium, — The alloy containing a quarter of a part of rhodium is of a golden colour, very ductile, and difScnltly fusible ; with one-sixth of rhodium the alloy is rather more fusible, but less so than pure gold.

Gold and Osmium or Iridium, — These metals do not appear to form sdloys. Iridosmium separates out in black grains, and sinks to the bottom of the crucible in which gold containing it is melted.

Gold and Potassium alloy when heated together ; the alloy is decomposed by water, caustic potash being formed and gold left. The alloy containing 10 per cent, of gold takes fire when thrown into water, leaving the gold behind in the form of a black powder.

Gold and Sodium sdso alloy readily when heated together out of contact of air ; the properties of the alloy are similar to the last-named.

Gold and Tungsten, — The alloy of these metals is yellow, and very difficultly fusible.

Gold and Molybdenum. — The alloy of 2 parts of gold to 1 of molybdenum is a black, brittle substance.

Q 2

Gold Amalgam

As is tho oase with other metals, the alloy of gold wi mercury is known as its "amalgam." Mercury alio with gold with great avidity, this being due not only the marked affinity of the metals but to the fact tli mercury is a metal in the molten state at ordina temperatures. Molten lead, for instance, will alloy -w; and dissolve gold quite as easily as will liquid merea the physical condition of the metal having a marli influence on its chemical relations.

Pure annealed gold placed in contact with clean n cury at ordinary temperatures is at once amalgamat and so is a plate of gold exposed to mercurial vapou mercury and gold are capable of uniting in all proportion but it would seem that the resulting substance is mixture of one or more definite amalgams of gold excess of free mercury, or free gold, as the case d be. Professor Eggleston has recorded the results numerous interesting experiments, in which he sh( that gold that has been heavily pounded, and is there! in the unanncaled condition, that is to say in a stat< tension, will not amalgamate, or only with the greatest difficulty, and he points out that these conditi are liable to be realised in the stamp-mill.

When gold is immersed in an excess of mercur chemical combination of gold and mercury appears t( formed first ; and this combination (or amalgam) disso subsequently in the excess of mercury until the latt( saturated, when the remainder of the gold amalgar deposited on the bottom of the containing vessel.

' The Metallurgy of Silver, Gold, and Mercury in the United S 1 890, vol. ii. 1>. 586.

.iM.lLC.iMS OF GOLD S5

Fotv>,s may be aptly compared to introducing an anliyous salt, such as anhydrous cupric sulphate, into a quantity of water more than sufficient to hydrate it, but ; not sufficient to dissolve all the hydrated sulphate of T copper produced. As in the analogous case just quoted, so : f in the case of the amalgam, if more mercury be added, the - whole of the amalgam formed can be dissolved. If the quantity of mercury be insufficient to dissolve all the amalgam, then the dissolved portion can be separated ] from the imdissolved residue by any mechanical operation, I such as filtration. It must be noted that the mercury wets (that is to say obstinately adheres to) amalgam, so that (just as in the analogous case of cupric sulphate) pressure has to be resorted to in order to free the solid residue as far as possible from the solution. In practice this is usually accomplished by squeezing through chamois leather or some other suitable substance. The solubility of amalgam in mercury is variously estimated by different writers. Lazarus Ercker, one of the first writers on this subject, wrote, in 1672, that a hundredweight of mercury will carry some two or three ounces of gold and silver.*' Henry says that mercury squeezed from gold amalgam carries from a trace to ten grains of gold to the pound. The variations seem to be due to the fact that there are a number of different compounds of gold and mercury, and that the solubilities of these in mercury may vary ; for it must be remembered that gold is not directly soluble in mercury as gold, but that it is gold amalgam, a compound of gold with mercury, that is soluble in mercury. In some experiments on this subject 1 have found that a mixture of 1 part gold with 50 of mercury produced a definite crystalline amalgam containing about 40 per cent, of gold. This was soluble in

Gold Milling

Chap.

mercury to the extent of about 2*29 parts of amalgam per thousand (equal to 0*94 parts of gold per thousand) at a temperature of 15* C. The higher the temperature the greater the solubility of amalgam in mercury. (S page 442.)

Thus with bar gold (containing silver) I obtained the following results : —

Tempeimtare.

Ainalgun diMOlTcd In the per 1000.

src.

These figures suffice to show that the solubility does increase with the temperature, but not to determine the ratio of the increment, as different samples were operated on in each case.

This is corroborated by Kasantzofif, who gives the following table : —

0°C. 100° C.

Parts of gold soluble in lOO of mercury.

This author further states that variations of pressure in squeezing do not affect the solvent powers of the

Bidl. Soc. Chlm., t. xxv. p. 20.

Iv Amalgams Of Cold 87

mercury ; this would indeed be expected from a priori reasoning.

From the above figures it may fairly be assumed that the average solubility of gold in mercury is about ten grains to the pound, and it is interesting to note that this would come to nearly 1*6 ounces to the flask, or just About as much as the value of the mercury itself.

Yarioas more or less definite amalgams of gold have heen isolated, and their characters described.

As already stated, native amalgam has the formula HgAuj, and contains from 3902 to 41*63 per cent, of gold.

By treating 1 part of gold with 50 of boiling mercury, Sleeping heated for some days, allowing to cool and carefully squeezing, I obtained a hard alloy of gold and niercury, crystallising in silver-white, long, delicate, interlacing needles, and consisting of 41-43 of gold and 5857 of mercury, thus corresponding exactly to the composition of native amalgam, but having apparently a different crystalline form.

The following list gives the results obtained by various ex jHjri mentors on this point : —

AuHg is obtained in hard wliite crystalline lainelloi, when precipitated gold is dissolved in mercury at 120" C. and the mixture allowed to cool.

Four-sided crystals of a yellowish white colour, easily fusible without decomposition, are said to liave been produced, containing AuUg.

When a mixture of gold and niercury is heated carefully to a temperature a little above the boilijig-point of mercury, till the weight of the residue remains constant, an amalgam is said to remain, having the composition Au<,Hg.

S8 Gold M/Llmg

Chap.

When gold is dissolved in mercury in aboat the proportion of 1 grain in 1000, and the mercury dissolv in dilute nitric acid at a gentle heat, an amalgam is left in four-sided prisms of a brilliant metallio lustre, which are not affected by boiling nitric acid and do not tarnish in the air. When heated they do not fuse, but give off inercury, and leave pure gold which retains the form and lustre of the original crystals. This amalgam contains about 88 parts of gold and 12 of mercury per cent., corresponding nearly to the formula of AugHg.

Iafll, however, states that on long-continued heating of an amalgam of 20 parts of mercury to 1 of gold, and then acting on the mass with nitric acid, he obtained crystals of gold that obstinately retained small but variable amounts of mercury, which could, however, be leinoved by heating. The amount of mercury retained in the crystals is not definitely stated, and the experiment appears to be somewhat inconclusive.

By amalgamating gold and squeezing, a crystalline amalgam having the formula AuHgg has been produced. It will thus be seen that amalgams of more or less definite composition and form, having formulas respectively corresponding to AujHg, AuHg, AugHg, AuHgj, AuIIg, and Aung,j have been isolated by different observers.

It seems that all these are decomposed into metallic gold and mercury at a heat somewhat above the volatilising point of mercury, but that, whereas those containing the larger proportions of mercury are fusible below this point, those richer in gold are not.

J have found in a series of recent experiments that if

Dingler's Poliftechnischss Jaunial, vol. dxviii. p. 282.

Amalgams Of Gold 89

a soft amalgam of gold be kept heated to the boilingpoiDt of mercury in a hard-glass tube, mercury will volatilise until an amalgam is obtained which is perfectly liquid at this temperature, but which sets hard, forming a silver-white mass, rather brittle, with a granular fracture, when cold ; a good deal of heat is evolved at the moment of solidification, sufficient to volatilise some of the mercury, which forms a mirror roxmd the mass of amalgam, the surface of which is at times left with a slight yellowish tinge. This elevation of temperature and consequent decomposition renders the determination of the exact composition of the hard amalgam a matter of some little difficulty ; I find it to consist of : gold, 37C per cent. ; mercury, 62 4 per cent.

The amalgam generally obtained in gold milling seems to have a composition corresponding to about AuHg, /Making allowance for the silver which it always contains, and this amalgam shows signs of incipient fusion at the temjxjrature at which it commences to decompose. It is impossible to say, in the present state of our knowledge, whether it is a definite compound or a mixture of several of the series of alloys whose compositions are driven above, perhaps also with free gold or free mercury as the case may be.

I have recently found that gold amalgam is attacked ratlier readily by solution of cyanide of potassium. Thus 18 76 grains of hard amalgam containing 376 per Cent, of gold left for a week in a 1 per cent, solution of K(.'y, containing 16 grains of that salt, lost 0179 grain of gold, no mercury at all being dissolved ; in a similar 0 0 per cent, solution, 0*114 grain of gold was dissolved in the same time. This rate of solution is considerably greater than that of gold similarly treated.

Gold Milling

elastic sapling some twelve to fifteen feet long, th end of which is firmly driven into the ground (Fi|

N V

Fit;. 2.

The weight is allowed to strike either on an oh on something equivalent to it which forms an an which is surrounded with a box open at either en< of boards nailed together, or else into a cast-iron about eighteen inches high ; of course, as this is a rude make shift contrivance, built usually i: remote district, the details always vary accor circumstances and the materials available on tl A good dolly will crush about one cwt. of stone fine enough for panning purposes, say to a mesh o in diameter, or about thirty holes to the hnear in

The most rudimentary method of quartz-c practised at the present day is probably that among the Ashantees and other tribes on the Go) ul West Africa. Tliey break the gold-bearing which they manage to extract from the reefs, inl fragments between two stones, and then crush

'0 powder, using a fusiform piece of greenstone or

S5Tmte, which is worked with a rocking motion forwards

and backwards upon a smooth slab of stone, which is

supported inclining gently away from the operator.

The finely-ground stone is caught in flat wooden dishes

one to two feet in diameter, their shape being that of a

segnieDt of a sphere. In these dishes the fine stone is

skilfully washed in a pool of water, the dish receiving a

peculiar circular combined with an undulating motion,

by means of which the light quartz is washed over the

I es of the dish and the gold retained. The tailings

thus washed off are caught in a bigger dish, and rewashed

several times until they yield no more gold. In some

parts of China a very similar plan is employed, only

in this case the crushing stone is very large, being

worked by four men.

In some parts of the Andes a small two-stamp mill driven by water power, and resembling very much in general arrangement the old Saxon stamp mill, is used. It is, however, made without any iron whatever, being constructed entirely of wood, bound together with raw hide, the stamps being shod with granite boulders, and the mortar bottom being also a block of granite. The crushed ore is washed in a similar manner to that mentioned above, the wooden pan used for washing being, however, somewhat flatter. The latter is well known by its Spanish name of batea," and is a useful implement for prospecting purposes.

The Chinese also pound gold quartz under tilt hammers, driven either by foot or by water power, and made entirely of wood except the hammer head, which consists of a boulder of quartzite. The pounded stone is washed in wooden dishes, the section of which is that of

a very obtuse oone, instead of being dtonlary as aza ihoM previously described. I have mentioned the above time crude native methods of crushing gold quarts became they are interesting, inasmuch as each apparatus ii identical with (and perhaps derived from) one which ii used in the preparation of some article of food in eabh of the respective countries. The negroes of West Afm grind maize, which is one of their staple food stufb, between the two stones, whilst the South Americans and the Chinese use mills similar to those described for pulping coffee and husking padi respectively.

In the Middle Ages a mill was used on the continent of Europe which is practically the Saxon stamp mill ae we know it. It was driven by a water-wheel, and had a wooden barrel from which wooden cams projected ; the stamp stems were made of wood, and so was the coffer or battery box. The shoes were of stone or iron, and the ore was stamped dry or wet, the latter method having been introduced as an improvement on the former one about the beginning of the sixteenth century. An interesting wood-cut of one of these mills, taken from the well-known work of Georgius Agricola, Vom Bergwerck, published in 1557, forms the frontispiece of this volume. It gives a very good idea of the construction of a stamp mill three and a half centuries ago. Stamps essentially identical with these are still employed in some of the remoter gold-mining districts of Transylvania. The Saxon stamp mill of modem days and the Cornish tin stamp are practically still the same machine, with their wooden wearing portions largely replaced by iron. The weight of the stamps and the general efficiency of the machine have, of course, been increased, but the principle and many of the details remain unchanged. A series of alterations

Modern Methods 95

each essential portion, still retaining, however, the leading mechanical principle, has evolved the modem Galiforaian gravitation stamp mill, the minute details of which as now oonstmcted will be considered in the following chapters.

Modem Proeeis* — The modem method of treating gold quartz has become a somewhat complex operation. The weight of the ore is usually determined either before or after delivery to the mill ; it is next broken down to proper size for treatment in the stamp mill, where milling operations proper commence, the ore being fed loto the mill and there crashed to a suitable degree of fineness. The first step is the extraction from it of the free gold, and afterwards of the combined and ''rusty" ;old. The free gold is always caught by amalgamating t, and then collecting the amalgam, the great object )eing to secure the vuable metal at as early as possible k stage of the process. The operation of amalgamation nay be carried out in the battery box itself, or outside t on amalgamating tables, in mercury wells, by special imalgamating apparatus (in pans, &c.), or by a combina- ;ion of several of these methods. The heavier portions )f the escaping pulp, which contain in many cases the argest portion of the remaining gold, are then separated Dy some method of concentration, and the concentrates so obtained are treated, to obtain from them the gold :hat they contain. Sometimes when there is comparatively little free gold this is not separated first, but the pulp is subjected to concentration directly it leaves the battery, and the concentrates, which them contain the free gold as well, are put through one or several processes to obtain the gold from them, the free gold being either collected separately, or else got together with

riMi litlJ in tiie sulphurets. If the pulp still contains a notable proportion of gold, it is next run into settling pits, and the sand so collected is subjected to further treatment, now mostly by chemical methods*; in some cases the preceding operation of concentration is omitted, and the whole of the pulp treated chemically as soob as it has left the amalgamating appliances, and in a few extreme cases amalgamation even is dispensed with. The best modem practice consists in amalgamating in the battery box, and then on copper tables, any escaping particles of amalgam being arrested in suitable amalgsm traps. If there is then sufficient gold in the pulp to repay its further treatment, as is mostly the case, it is either treated direct by a chemical method such ii cyanidation, or more often is sized and concentrated, and the concentrates are then treated specially by some metallur<:pcal process that will extract the gold, the tailings being again further treated, if necessary, by some such process as cyanidation. This general scheme may not be universally applicable to all ores. I believe, however, that there are very few indeed that will not give better results by this system than by any other, provided only that scrupulous attciitiou is paid to all its minute details, which must in ever}' case be specially adapted to the ore under treatment. Very often a full knowledge of these minutiaj can only be obtained experimentally, but the proper guide is in every case a thorough study of the chemical and physical characteristics of the ore. It must he noted that there are numerous mills working, and working successfully, on principles at variance with the above, in different parts of the world, and that these lo(?al processes have become firmly established and do not seem likely to be given up very readily. It is quijfce

j

piuLiljle that the above ineLliod, whicli may now bo looked upon as a standard one, namely, battery amal- '-' gamatioD, followed by copper tables, concentration, &c., would give with the ores in question at least as good i and very possibly even better results ; but as these special " processes have been used with success and profit for many years, as the mills have been designed specially for them, as the men are well trained in their practice, &nd as the whole of the operations are thoroughly organised with special reference to them, it is not likely that these systems will now be changed, and it is even Wghly probable that were they changed for the modem oiethod, the latter would not at the outset give better results, even if as good, whatever it might do ultimately. These facts must not, however, bo construed into proof that such local methods are intrinsically better than the i modem standard one. I do not propose in this volume r to enter into details of any methods except the most modem, my object being rather to point out the principles underlying the practice of scientific gold extraction, than to enlarge upon variations in details thereof, which appear to me to possess only local, or at most historical interest.

General Arrangements.— The general arrangements of start|>-inills must vary very greatly according to the circumstances of each individual case. Thus a customs mill— that is to say, a mill that is run for the purpose of crushing ores belonging to outside proprietors, who simply pay for the use of the mill — nmst vary in its structure, as it varies in its objects, from a mill erected for the sole purpose of crushing the ore extracted from a particular mine, and of these, again, the small mill erected to treat the quartz from a small but very rich vein should differ

considerably from a Urge one whose object is io del! with enormouB quantities of low-grade ores. And it v evident that unless the constmction of a mill he strioil; proportioned to the duty required of it, it can neie prove a satisfactory machine.

Weighing Gold Quarti. — In the case of a customs nu! the ore is usually delivered in carts or waggons, moi rarely packed in bags or sacks. In the former case eae vehicle should pass over a jdatform scales, there be oa fully weighed, and the ore dumped into the particular U assigned to it, the tare of each vehicle being also note If the ore is in sacks or bags, their tare, as well as the gross weight, must also be ascertained. When mai different classes of ore have to be treated at a mill, tl ores should always be weighed and not measured, as the specific gravities are liable to vary within very wide limit The condition of dryness or wetness of the ore shou also be recorded at the time of weighing. Disputes as the weight of quartz delivered at the mill are sure to ari unless such precautions are observed ; and to obvia them it is preferable, whenever possible, that the stoi should be weighed in the presence of a representative its owners.

When a mill is run in conjunction with a mine, tl method of determining the weight of the stone by me surement is far preferable to that of weighing it ; on for all, at the commencement of a campaign, a unit measurement should be fixed upon, this being general a car of the size used in conveying the ore from the mil to the mill. In the majority of instances these points a connected by a tramway, and a tramcar properly filli makes the best unit of measurement. Sometimes, account of the physical features of the country, it is four

oiivi'Mient to dump the ore as delivered from tlie mine into a hopper, and to use other cars to tram it thence to the mil] ; in this case the latter car will form the unit, j It should be the duty of the man at the rock-breaker, when this machine is used to sec that the cars are properiy filled, and that a proper tally is kept of the ooniber of cars brought to the mill. When the weight of ore in the unit car is determined, this should be done by weighiDg a large number (say twenty to thirty) cars of ore 00 each of three or four successive days, the percentage of oxHsture in the ore being carefully determined each day on a large average sample. This is easily done by weighing out, say, 200 lbs. of well-mixed ore and drying it on an iron plate over a wood fire, stirring it with a piece of flat iron until no more moisture comes off, but never allowing the iron plate to become hot enough to char a diip of wood laid on it; in the case of pyritous ores special care must be taken not to allow the heat to rise high enough to decompose any of the constituents of the ora The dried ore is reweighed carefully, and the amount of moisture thus determined. This figure will be found most useful in subsequent calculations. The percentage of moisture in gold ore varies within very wide limits according to circumstances ; it may range froin 3 to 15 per cent., or even more occasionally. It must not be forgotten that the specific gravity of ore from different parts of even the same reef is liable to vary considerably ; thus quartz heavily charged with pyrites from a low level will be much heavier than the gozzany quartz from the upper levels, resulting from the decomposition of this same stone. Accordingly a fresh determination of all the data regarding it must be made when ore from new levels or new working faces is first brought to the mill ; such

U2

%A

ores are for many reasons best kept separate throughout the process of milling, as they will probably require different methods of after-treatment.

Of course all that is really required to be known Sssc the purposes of the mining engineer is the amount of goU obtained from a given unit of volume (say a cubic foot) of the stone, as it stands in the reef. This being known, the engineer could frame all his calculations by meaoB of this one datum only. At the same time, however, it is advisable to base all calculations upon the ton of ore, so as to have a standard of comparison to enable the efficiency and economy of the mill in question to be compared with others in the same district, or in other parts of the world. It is, of course, the mine manager's business to know how much a cubic foot of stone as it stands in the reef weighs when extracted and sent to the mill. It is assumed that only milling ore is sent to the mill, all the worthless portions, mullock, casing, &c., being pfcked out before it is loaded into the mill car. In the rare cases, which may occur with very small mills having no rockbreakers, where it is found expedient to do this work either wholly or partly at the mill, a record must be kept of the stone so rejected.

The weight of ores varies within very wide limits, a spongy dry quartz containing little or no metallic sulphurets being obviously far lighter than a wet dense quartz heavily charged with galena, for instance. Broken quartz as sent to the mill will generally range between fifteen and twenty-two cubic feet to the ton. A convenient size of truck for handling and dumping is one carrying about half a ton of stone.

It may be noted that there is a growing tendency to measure gold quartz by the short ton of 2000 lbs. (equal

tuU ;):i loiia ton), this practice obtaining in the United States, in Canada, and in the Transvaal. It is to be hoped that this practice will extend rapidly, and that the long ton of 2240 lbs. will soon become obsolete, together itb such purely local methods of measurement the Colorado cord (equivalent to about seven short tons of ordinary quartz).

Breaking the Quartz. — Ore as it comes from the mine is in lumps too large for feeding direct into the stampmill, which is rarely arranged to take a piece as large as three inches cube. Moreover, even if it could take such large pieces, it is by no means desirable that they should be fed into it, as breaking quartz in the mill is a costly process, and takes up time during which the mill might be nQore profitably employed in fine crushing. Feeding large lumps into the mill is also apt to break the screens and damage the mill in other ways, whilst uniform steady working would bo an impossibility. On all these grounds, but principally on the score of economy, it is advisable to break the ore very small before feeding it into the mill, seeing that it costs far less to break down stone in the rock -breaker than under the stamps. As a general rule it should be broken to pass through a inch ring. It will occasionally, but rarely, happen that ore has to be broken by hand, in which case it cannot well be broken as small as above recommended. This will only occur in the case of a very small mill, when funds have perhaps not proved sufficient to provide a proper rock-breaker. Or, again, a small mill may be working on rich, narrow leaders of quartz in hard, barren country rock, where circumstances render it advisable to put the rich quartz alone, or at any rate as clean as possible, through the mill. It may then become necessary to break the stone

by hand as it comes from the mine, in order to be abl to reject all the barren portions. The conditions node which hand-breaking may deserve the preference ove machine-breaking can be summarised as follows : —

(a) A small mill treating a small output of rich ore.

{d) Very cheap and fairly eflBcient unskilled laboQi whilst skilled labour, such as that of engineers, mechanici &c., commands high prices.

I have notably found these conditions combined i the Kochgar district of the Urals, where the ordinal labourers employed in breaking quartz get only p day, whilst skilled mechanics are scarcely to be got ) all ; all the fuel used has to be imported from a distanc and much of the ore treated is a soft, partially decompo8< granite. Although a mill in this district had a roc breaker erected, it was found cheaper not to run it, b to break the ore by hand. In some parts of West Afri( a somewhat similar state of affairs prevails.

Whenever hand-breaking has to be resorted to, tb should be done in a spacious shed, well lit and floor with earth rammed hard, or heavy plank covered wi) stout sheet iron, so as to admit of the floor being swe; up from time to time, as the fine dust formed in breakii high-grade ore is always found to be very rich in gol Sometimes the breaking is carried on in a portion of tl mill building itself behind the stamp-mill. The breakii sliould be done on a bed of the rock itself, broken smalJ and a light stone-breaker's hammer, double egg-ende about three pounds in weight, should be employed, tl handle being about two feet six inches long and somewhi flexible. A heavy sledge-hammer, say twenty pounds i

weight will be required for the large lumps. Men en- I gaged in this work mostly protect their legs against cuts from sharp splinters of quartz by means of a couple of folds of sacking fastened round their shins, and they sometimes also wear goggles. The latter precaution is unnecessary when the men are used to their work.

Bock-breakers. — There are two main types of these now being largely used — namely, the old type where the stone is crushed between a flat fixed jaw and a reciprocating one, and the newer type in which the fixed jaw is circular, whilst the other one gyrates inside it. In either case, the wearing portion of the jaws consists of dies which are capable of renewal.

To the first type belong the well-known Blake, the Marsden, Dodge, Foster, and numerous other crushers, which may be looked upon as merely variations of the first-named, the Gates and Comet being the best known circular ones. The conditions to be observed in a good rock-breaker are : that the machine must have sufficient weight to work steadily ; that it must be strong enougli to resist the very severe strains to which it is exposed, these strains tending, firstly, to burst open the ends, and, secondly, the sides, of the machine ; that the jaw dies be capable of rapid and easy change, removal, and renewal, and that they be capable of being fixed firmly and adjusted accurately ; that all wearing parts be accessible and capable of renewal; that all parts needing oiling, and above all the driving shaft, be kept as far away from the mouth of the machine as the size of the latter will admit of, in order to prevent the oil, which is used as a lubricant, from finding its way into the quartz ; that it have a heavy fly-wheel or fly-wheels, and a fast and loose pulley with good sliding fork for the belt, in a position convenient !or the man in charge.

I04 GOLD AflLUNG chap.

Ono of the best materials for the dies is good chilled cast-iroD, cast with corrugations about two and a half inches from point to point. Steel plate has been used, but wears very fast ; cast steel, especially manganese or chrome steel, appears to give very satisfactory results. As a rule, the die plates are so constructed as to be capable of being inverted when the lower edge, which has most of th(i work to do, becomes worn out. The average wear of good chilled iron dies may be taken at 0*1 lb. of metal lM>r ton of stone broken. There are several ways of sneuring the dies. The method of running them in at tiie back with lead or some similar readily fusible metal is not a good one, as the dies arc then difficult of removal, and tliero is moreover a risk that particles of this metal may find their way into the quartz and subsequently into the bullion, which they would render brittle. The system of through bolts with heads counter-sunk on the working face of the dies is also not to be recommended. The best one probably is that in which the dies are secured by T-headed bolts entering into slots in the back. These (lies should be cast of chilling metal of such quality as to secure a thoroughly hard face, whilst the back remains suiUciently soft to admit of chipping strips being planed to fit accurately similar planed strips that form a portion of the permanent jaw ; or, if the chiUing quahty of the metal used is too hard for this purpose, it will do almost as well to cast strips of Hat iron into the back of the die, which strips can then be planed as before. The space between the chipping strips should be filled with strips of ih y deal in order to form a solid, slightly elastic cushion for the dies to rest against.

Ar Crusher. — This is too well known to need any

detailed description. It consists of a heavy rectangtdar

/;;/"' ' *he front portion of which forms the fixed

V Rock-Breakers 105

jaw. The swinging jaw is actuated by a toggle-joint at its lower end, the pitman being worked by a powerful eccentric forged on the driving shaft of the machine. Various devices are employed for adjusting the width of the aperture between the bottom ends of the jaw faces. Aver}- good form of this crusher is the so-called Blake - Marsden rock-breaker, in which the driving shaft is set asf&r back as possible from the mouth, the swinging jaw being drawn back at each stroke by the action of the uiachine instead of by a spring.

Sectional crushers are manufactured for special cases here the transport of the heavy castings that compose the ordinary pattern is impossible ; these machines are usually built of thick steel plates, the strain being taken l>y massive steel through bolts. A good pattern is manufactured by the Union Iron Works of San Francisco. !ctional crushers rarely do good work ; in the first place, the total weight of the machine has usually to be J'opt so low that sufficient rigidity cannot be obtained, and [Mirt of the force that should be employed in break - in;: the stone is accordingly spent in racking the machine. Moreover, the severe strains to which it is subject usually end in causing more or less damage to those parts cliiefly exposed to them, however strong these may lie at the outset. Such crushers, and built up crushers generally, should only be employed in case of absolute necessity ; the heavy cast-iron frame is always to be l)ruferred when it can be obtained.

The efficiency of this type of crusher is dependent on liiany circumstances, one of the chief items being of course the quality of the quartz. The following table, the data of which are only approximate, and subject to cfiiibiderable variation according to circumstances, will

give some idea of the capacity of machines of good construction working on typical quarts, the fly-wbeel shaft of the machine making 200 or 300 revolutions pur miotite : —

10' X IS" X 1 20" X 1' WXl 3(rx2

2t ton.

Si .,

Is

Ss

4S

Ihdffe Cruaier, — This machine differs from the Bkke mainly in that the point of suspension of the vibratiog jaw is at the lower instead of at the upper end. The

advantage of this arrangement is that it breaks the ore to a far more uniform size ; at the same time its capacity

kOCK-BREAKBRS

is oorrespondingly diminished. This machine is shown in section in fig. 3, from which it will be seen that the distance between the jaws can be readily adjusted by the screw that shifts the bearings of the swinging jaw inwards or outwards. The following table will give an idea of its average efiSciency : —

DiiufDiiMu of Mouth.

I.H.P. reqoired.

t

tons

ft

NuuiWrof I revuliitioiM ytx Uiinnte.

Jdies Crusher. — This is the best known of ail the g}Tating crushers. As will be seen from Fig. 4, it consists essentially of a cylindrical casting, within which lit A set of dies which together form a jaw of the shajx; of A truncated cone pointing downwards. This is partly closed by the g}'ratiug head that forms the moving jaw, and which carries a die in the form of a cone pointing Qpwards. The quartz is crushed by its descent in the annular space between these two, this interspace being wedge-shaped in cross-section. The crushing head is carried on a shaft slightly inclined from the vertical, suspended at its upper end, the lower end being carried round in a small circle, the head Jbeing so arranged as not to revolve, but only to receive the gyratory' motion of the suspended shaft. This type of crusher is capable of doing a gi'eat deal of work, and does not consume much power, while it runs more steadily and with less jar than the reciprocating crusher. The chief objections to it are its great weight and its slightly more

Grizzlies

complicated construction. There is little to be gained the employment of the smaller sizes of this machine, bat it can be recommended where a large crusher is required, and above all where ore-breaking is done in a separate rock-breaker house, for which it is well suited. The following table exhibits its approximate capacity under normal circumstances : —

of Roiifier.

Dimensions of each of tho

three oiwnings at Mouth.

10' X 12" X 11" X 15" X 20" X 10 24" X 11 30" X 13 42" X 18

6"

—

8"

Weight Machine.

2 A

U

Tons of

Quarts

I>er hour.

I.H.P.

NnmiDor of required, j revolutions

Grizzly.— An important adjunct to the rock-breaker is the grizzly. This consists of an inclined plane of iron bars leading down to the mouth of the rock -breaker, the bars l>eing set at a distance apart equal to the IKice between the bottom ends of the rock -breaker jaws. Thus if the rock -breaker be set to crush to a J-inch cube, the grizzly bars must be inch apart. All the stone that is small enough to go direct to the mill will acconliiijly pass through the grizzly bars, and not ;o into th(i nx;k-breaker at all ; the material passing through the bars drops into a shoot which delivers it into the same hopper into which the rock-breaker itself discharges. The grizzly is thus an arrangement for increasing the

no GOLD MILLING chap

efficiency of the rock-breaker by throwing less worl npon it. It follows that when the rook-breaker's capfl city is considerably superior to that of the mill th grizzly is but little needed, especially when the rod breaker is run by water power. Even then, however it will effect some slight saying in the wear and tear the rock-breaker. This saving will vary in proportion t the size of the fragments which the rock-breaker is s( to deliver. It is evident that if a rock-breaker were 8( to break to 2-inch cube, there might be, say, 20 per can of the entire product of the mine below this size, and grizzly with bars 2 inches apart would thus only leai 80 per cent, of the mine product to be dealt with by tt rock-breaker. Under the same conditions of minin] if the ore had to be delivered to the mill broken 1 j-inch cube, there might probatly be only some 5 p cent, of the mine product below this size, and the savii of this small amount of extra work to the rock-break( might not compensate for the extra expense and trout of erecting and maintaining a grizzly. No general ru can, however, be laid down, and the nature of the o] as it comes from the mine should be carefully studi( before the erection of a grizzly is decided on ; moreover it must not bo forgotten that a grizzly needs a good de of head-room.

Grizzlies are usually from 3 feet to 6 feet wide, ar the length of the bars varies from 10 feet to 15 feel these are usually set at an angle of 45" to 55°, accordir to the nature of the ore. The bars are made of eitht iron or steel, the latter being preferable, about 1 inc wide on the face and about 3 inches deep. It is advi able to have bars specially made for this work having section as shown in Fig. 5, which represents a numb<

Grizzlies

Hi

Scale 3" to I fL Fig. 6.

of the grizzly bars in section, the bars being inch der on their upper than on their lower faces ; this arrangement obviates any risk of their becoming choked. Old steel rails with the heads turned downwards make excellent grizzly bars. They are kept their proper width apart by means of distance pieces at either end, and by having bolts about an inch in diameter with washers on them to fit between the bars running through the entire system. The upper portion of the grizzly should consist of a plate of iron on to which the car is dumped. The grizzly sometimes discharges direct into the mouth of the ore-breaker ; this should only be done when the grizzly is supplied from an ore bin having a door so regulated that just sufficient stone passes out to loep the crusher steadily at work, which at the same time necessitates a certain amount of uniformity in the size of the stone as it comes from the mine. When practicable this is an excellent arrangement for economising labour, as no one is required to attend to the rock-breaker except for such casual supervision as it will receive from the mill man. The more usual arrangement is to discharge the stone on to a rock-breaker floor pet level with the top of the mouth of the machine, and covered with stout iron plates. On this floor stands the man in charge of the breakers. His tools consist of a stout iron hook or scraper to enable him to drag the ore into the mouth of the machine, one or two steel bars to turn over heavy lumps, and a square-mouthed steQl

shovel for small 8tu£f. One man at the breaker and one dumping cars and attending to the grizzly can hands as much stone as the biggest size rock-breaker etn break in a ten hour-shift. As a rule the rock-breakei is run during the day shift only, except in case oi emergency, when the mill is short of ore.

The best possible place for the rock-breaker, especially in the case of a large mill, is in a separate rock-breake house situated either at the mine or between the mim and the mill, wherever sufficient head-room can be ob tained for it. If, for instance, the ore is raised fron a shaft, it is advisable to hoist to a height of, say, 9 feet above the level of the tram-line which supplie the mill, and to deliver the rough ore (after it been picked over if necessary) direct to the shoot suf plying the grizzly or rock-breaker as the case may Ix whilst if the bottom adit is for any reason a simila height above the tram level, this head-room shoul similarly be made available. The best system is t deliver the ore to a large supply bin, whence it passe over the grizzlies and through the rock-breakers, an thence into storage bins below the latter, which bio should be made as large as circumstances admit of, an furnished with shoots and doors so as to automaticall fill the mill cars.

A good rule for the capacities of the storage bins i that they should be able to contain at least a tweuty-fou hours* supply for the mill, so that the latter can ru continuously, even if the mine is stopped for a day.

The system of locating the rock-breaker in a separat breaker house was first employed in California, the one famous Plymouth mine being a good example of it; i this mine the rock-breaker, run by a Pelton wheel, wa

Ore-Bins 113

sd at rach a height in the headgear of the mine-shaft 0 enable cars to be ran from beneath it along a )d tram-line which was laid above the mill bins. method never seems, however, to have become very liar in the United States, but has reached a high e of development in the large mills of the Witwaters- I. Here the so-called " crasher stations " are either ed close to the headgears of the winding shafts, or n a large mill is sapplled from several such shafts maher station is best placed at some intermediate it ; it may then best be constraoted close to the mill, still quite independent of it. Such a crusher station Ains the grizzlies, rock-breakers, storage bins, and, 3me cases, sorting floors, to which reference will be le subsequently. Quite recently the method of using

sets of rock-breakers, one above the other, as ad- .ted for some time by the writer, has come into use

few mines, the very expressive term of tandem " hing being now generally applied to it. In such a case 3re is first tipped on to a coarse grizzly, the coarse ore g direct to the first or coarse crusher, whilst the ore goes through falls on to a fine grizzly ; the fines from atter go direct to the mill, and whatever docs not pass ugh, to the fine crusher. The product from the coarse ler also goes to a fine grizzly, which separates out the

that are ready to be milled, whilst the larger pieces the fine crusher. Sometimes all the ore that has ed the fine crasher is also dropped on to a grizzly.

all pieces that are too coarse are returned to the kers to be crushed finer. In a few cases tandem bing is combined with tandem sorting, the ore under- g a sorting between each crushing, hen head-rooni for a separate rock-breaker house is

not available at the mine, or when for any reasons it is not considered worth while building a separate rook-bresikiiig station, the rock-breaker house has to form part of the general mill-building, a special rook-breaker floor being provided at the right level. This arrangement, which ic still perhaps the most general, is illustrated in several d the views of stamp-mills, Figs. 67 to 71. When possibleii is advisable to have one rock-breaker to every 20 hesd of stamps, although it has been found quite feasible ti supply 40 heads from one machine ; this usually entails however, some little difficulty in duly distributing tb crushed stone to the various battery hoppers. At thi same time it must be distinctly remembered that crushing in the rook-breaker is performed at a far less cost (les than one-fifth as a general rule) than in the stamp-mill It is therefore wise economy to crush as fine as possibl in the rock-breaker, even if it be necessary to double tb' rock-breaker capacity of the mill, and in cases wher very large lumps of ore have to be handled, it ma} as in the case of crusher stations, be advantageous to pass these through two rock-breakers, the first t break, say, to a 3-inch cube, and the second to redo this stuff down to f inch. Double rock-breakers hav been constructed for this purpose, consisting practicall; of two ordinary rock-breakers, one above the other, drivei from one shaft, and with the sizes of their jaws properl; proportioned for continuous work ; these are, however only suitable for very small mills. When two sets o rock -breakers, one above the other, are put in, the uppe ones may with advantage be constructed on the gyrator principle, whilst for the lower ones the Dodge crusher are best.

It is, however, always very advisable to have the rock

V Ore Bins Iis

breakers in a separate boiiding from the mill proper, because the rock-breaker is a machine that must necessarily cause much jar and vibration when working, and this defect is greatly accentuated when one or more of these are supported on a floor 30 feet or so above the floor of the stamp-mill. Again, during their operation much dust is produced, and this fine quartzose dust is &pt to find its way into the bearings of the comparatively delicate pieces of machinery at work in the mill, to— it need hardly be said — their serious injury.

The separate crusher station no doubt adds to the first cost of the plant in most cases, and may also add somewhat to the working cost, especially where the only source of power is a high-priced fuel. On the other hand, it enables fine breaking to bo carried out effectively, and thus increases the working capacity of the mill ; by promoting more regular working of the latter, it also tends to yield better results generally. Moreover, (except where a hillside with a suitable slope is available, it greatly simplifies and cheapens the construction of the mill proper. In flat country, and especially in establishments driven by electric transmission from some distant source of (water) power, the use of the separate rockbreaker station is more especially indicated.

In no case, however, wherever the rock-breakers may Ije placed, should they l)e run by the same motor or driven off the same shaft as the rest of the mill. The work of the rock-breaker is necessarily irregular, much inure power being required when a large piece of rock is between the jaws than when small stuff is going through. This e\il may be to some extent lessened by having very heavy fly-wheels, as already recommended, but, in any case, the irregular action of the rock-breaker causes any

ii6 GOLD MILLING chap.

machinery driven by a common motor or main shaft to run irregularly, and thus seriously disturbs the action oi the mill vrhere uniform driving is an important desidem tum. A separate motor should therefore always b provided for the rock-breakers ; it may be added that a these usually run on the day shift only, the same moto may, by a proper arrangement of shafting, be used fo doing other work at night, if such is necessary.

Sorting. — In all, or nearly all mines, a certain araonr of barren or low grade material, sometimes poor veil stuff and sometimes country rock, is extracted with tl: payable ore. It becomes a question of great econom: importance how this poor material should be treated it is impossible to pick it all out in the mine, and it ma bo taken for granted that the cost of breaking it out an tramming or hoisting it to the surface has necessarily ho incurred, though it is the duty of the mine manager see that its quantity be kept down to the lowest possib limits. Evidently it must either be crushed with tl pay ore or else picked out, the latter operation beii generally spoken of as sorting. The conditions thi favour the former mode of procedure are : —

(h) Relatively cheap skilled and relatively dear u skilled labour.

(c) Gold contents of poor rock to be eliminated at lea* equal to those of the tailings ultimately rejected.

(d) Low cost of total range of processes required treatment of the ore.

{c) Mill capacity at least equal to the total output the mine.

The above conditions exist, for example, in Califomi vvliere, \vith cheap water power and highly paid labourei

V Sorting 117

it is found more profitable to mill the whole of the inaterial stoped out of the mother lode than to attempt to pick out the rich strings of pay quartz from the very poor, partially altered country rock blasted out with it. Od the other hand, the opposite state of affairs prevails &t the Witwatersrand, where there is no water power to be got, the only fuel available being a high-priced coal of only moderate quality. Sorting can be done by native boys at wages, inclusive of food, of about 2. 6<2. per day of 10 hours, whilst skilled mechanics or men in charge of machinery get from 15<. to £1 a day. The waste rock -chiefly quartzite — which is there sorted out but rarely assays much over 1 dwt. per ton, whilst the average nchness of the tailings ultimately rejected is about li dwt. The cost of extracting the gold from the ore is usually from 7. to 8s. per ton inclusive, whilst the cost of sorting is mostly from 2s. to 3s. per ton, and at the same time the producing capacities of many of the mines, when fully developed, are frequently superior to those of their milling plants.

Under these circumstances many of the leading mines, prominent among which are the Ferreira and th( C'rown Reef, have adopted elaborate systems of sorting, and have been able to show in their annual balance sheets handsome profits from the introduction of this practice. The amounts sorted out at the different mines vary from 17 to 50 per cent., about 33 ptT crmt. I>eing an average figure. The sorting is performed either on floors or on moving tables. In the case of the former the ore is dumped over a grizzly, and is, at the same time, washed by a spray of water. The floor is covered with steel plates, and upon this the ore is turned over and the waste picked out and thrown into

Ii8 GOLD MILLING CH

cars, whilst the clean ore is shovelled into a bin. Th are two forms of table in nse. The first consists o round revolving table 3 to 4 feet wide, 20 to 25 feet diameter, driven from a central shaft or by means a pinion and a circular rack bolted to the bottom the framing of the table; the speed is about revolution in li minutes. Tlie other form cons of an endless travelling belt made of steel ph riveted to a couple of chains, the width of the 1 being 2 to 3 feet and its length 30 to 50 feet. B these latter methods admit of very perfect sorting, the wear and tear is rather heavy and the upkeep coi spondintly expensive. Sorting on a floor appears tc the choajxjst method, but requires a good deal of sjm It is diflicult to give exact figures showing the aver eoHt of sorting, as most of the South African coniimi that employ this process include the cost of sort with other items in their accounts, sorting and crushi or sorting, crushing, and tramming to mill being usui put under onc head. Obviously the cost of sorting vary with th(; percentage of waste sorted out, the t( tonnage treat(Ml, and the fineness to which the ore broken hefon? final sorting.

The following tabltj shows tlie recent costs of th items in a few representative mills, being based uj)on unit of on(; (short) ton of ore milled. From the data th iven it would appear that the average cost of crush a ton of ore as raised would amount to about 3t and that of sorting to about \d. The former figure n he looked upon as rather high, compared with la Aniorican mines, where the crushing cost appears to about 2</. to 2'5(/. per ton.

I

Ml

C'Jiaptek Vi

GENERAL ARRANOEHBNTS — THE MOBTAR BOX AND ITS ACCESSORIES — SCREENS — DIES

The Californian Stamp-mill. — The action of the gravitation stamp-mill has already been partially explained. Tlie modern mill, the Californian stamp-mill as it is appropriately called, differs not at all in ultimate principle from its predecessor of four centuries ago, but all its details have undergone extensive modification. The old Saxon mill gradually became the cumbersome Cornish mill of the present day by the replacement of wood by iron in all working parts, thus making it a stronger, more durable, more powerful, and in everyway a better machine. The Californian differs from the Cornish or improved Saxon mill principally in the following points : — The cams, instead of being short iron projections from a huge barrel, are now curved arras threaded on a spindle, the cam shaft. The cams no longer act in the median lino of the stamp stem, but act entirely on one side of it, this lateral action producing a turning moment of the entire stamp upon its axis, the stamp having been made circular in section, instead of rectangular, so as to give due effect to this turning moment. The result of this alteration hsiS been to

H\i V! CrEXE/Af AKNAXGEMI'.X I S

qualisi the wear of all portions of the stamp, and tliiis

to secure uniformity of operation. A great deal has

feen written and repeated in many places as to the

advantage of this rotative motion in producing a grinding

on on the ore in the mortar. This is an entirely

erroneous idea. If the mill is kept in proper running

order and well lubricated, the above-mentioned rotation

place entirely or all but entirely when the stamp

's being lifted, continuing only to a very slight extent,

or not at all (depending on the adjustment of the

des), during the fall of the stamp. This rotation of

stamp is the best marked characteristic of the

Califomian stamp, and that which clearly distinguishes

from other types. Other minor differences are that

the anvil upon which the stamps beat now consists

of separate blocks, one for each stamp, instead of being

one single piece of metal, or simply a bed of stone as

formerly, whilst the mortar has also undergone many

important modifications.

It must not be forgotten that we owe the Californian gravitation gold mill to the ingenuity of American miners, who have, by continual study applied to its improvement in minute details, gradually evolved it from the crude old-fashioned Saxon njill of the niiddlg ages, whilst we have in Cornwall, to our shame be it said, still adhered to the cumbersome original model. And strangely enough we are doing the very same thing over again with regard to the Californian mill. This was elaborated in a country clothed with splendid forests of grand pine trees, which yielded admirable timber for constructive purposes, ready to hand at a time when iron was scarce and dear, and foundries non-existent. No wonder, then, that the American mill employed this timber whenever

possible, and had wooden frames and wooden mortar blocks. It does not, however, follow that because wood was under those circumstances by far the most soitr. able construction material, it should still be so, under conditions entirely different. We have slowly realised this to some extent, and iron-framed mills are coming gradually into use, but at the same time English en(- cers have not yet freed themselves from the trammels of imitativeness, and are still building iron mills which reproduce in iron the pattern of the American wooden ones. The sooner that this conservative policy is abandoned, and that mills are designed with regard only to the mechanical principles involve.d in the problem, and to the best system of solving this by the apication of modern methods of iron and steel manufacture, the sooner may wo hope for essential improvements in mill construction.

The Californian stamp-mill, as already stated, crushes the ore by means of the action of a heavy piece, the stamp, which, lifted by appropriate mechanism, is allowed to fall under the action of gravity upon the ore contained in a mortar. It thus consists of three essential parts : —

li. The stamp.

1. Under this head is included the mortar box proper with its screens and other attachments, the mortar block which forms its foundation, and the dies, which form, so to speak, the replaceable wearing surface of the anvil, upon which the ore is pounded.

2. The stamp consists essentially of a long stem carrying at its lower extremity a head into which is fitted a removable shoe, which constitutes the wearing face of the

li.f'

J;i!iip. With this is usually included also the tappet, i*: T . which is properly speaking a portion of the lifting 3st rA oiecbanism ; as it adds, however, to the effective falling 0, a:; J height of the stamp and is attached to it, it is better rt'ilifJ considered here.

co2lJ 3. The lifting mechanism consists of a horizontal shaft rjl on which are keyed cams acting upon the above-named a:4< I tappets, and also a pulley or spur wheel, which transmits ct J power to the shaft.

1 C2rl The cam shaft bearings may properly be considered loc-:l fere, and so may, for the sake of convenience, the guides, :i:l within which the stamp stems move. : I The proper shape, proportion, and material of each of the above elements constitute essential factors in the success or failure of any mill, and will therefore have to be considered in due detail. There are also various accessories, which, though perhaps not indispensable to the working of a mill, are essential to its successful running, and which will also have to be considered ; such are, for instance, the arrangements for hoisting up tlie stamps, and for supi)orting them when so hoisted, the water bupply, tlie fieding arrangements, &c.

Mortar Block.— The mortar block is usually constructed of los of sound, solid timber, securely bolted together. One of tlie best materials for it is good pitch pine, unless thr weight of this timber, 1-6 times as great as Danzig pine, lie considered an objection. Karri, an Australian wood (a species of Eucalyptus), close-grained and heavy, has also been used with very good results. The length of the logs is from 8 to 15 feet, the usual dimensions of the block being from 20 to 30 inches wide by 48 to 60 inches long. The logs are generally about 16 by 20 inches, and are so arranged as to break joint. They are carefully

ta|- GOLD MiUJlfO

tqmaxA vtA ahoold be mtSi tarred bofbre h ler ; a transveTBe ' strftpfuig pieoe sfacmt

square is usually checked in tor 1 or 2 inche bottom end of the block, bo as to give a wider ba: help in holding the block together. There is

n MORTAR BLOCK 125

Another strapping piece about two-thirdd of the way up the block. Details of such a mortar block are shown in %. 6, in which the upper strapping piece is an iron plate. The mortar block should project from 2 to 4 feet above the flcor level of the mill. i' Mortar blocks have also been made of 2-inch or 3-iTich plank, tarred and spiked together by spikes about 6 inches long. It is claimed that this makes a better block, as it is easier to get sound planks than sound logs of the above dimensions, and moreover it is considered to afford greater 'facilities for cutting out and replacing a defective piece. In either method the whole block is finuly held together by means of strong through bolts.

Too much care and attention cannot possibly be bestowed upon the foundations for the mortar block. It iset in a trench s])ecially excavated for it, which should always be carried well down into solid rock, all decomposed or broken up parts of the rock being cut away and bottom of the trench being then made as level as possible. If the sides of the trench are not wholly in iianl rock, they should be protected by massive retaining Wall*?, built of stone or brick laid in ceniont, or else of concrete. At least 6 inches of good concrete should next i>e well rammed into the bottom of the trench, being levelled ofif as carefully as possible; it may even with advantage receive a surface layer of strong cement in order to make all thoroughly level. Upon this foundation the mortar blocks are then set, great care being bestowed upon their ahgnment and their levelling. The space between them and the sides of the trench should then be rammed with concrete, or else, as is sometimes done in America, some barren quartz may be crushed in the mill and the tailings allowed to run into and fill the trench.

These tailings will pack in quite solid, aided no donl in their settling down tight by the vibration of d mortar blocks in the partly filled trench ; in either oft the packing should be as thorough as possible.

The mortar blocks should have been originally cat inch or so longer than they are intended to be final! When they are in their place, their exact finished heig is determined, and the line of their exact height careful set out across all of them with a delicate spirit level, that the finished blocks may all be of equal height ; th are next all sawn or adzed off according to circoi stances, and finally planed down accurately to this lii As a general rule the erection of the mill framing is nc proceeded with, the top of the mortar blocks being mea while protected by a cover of planks ; when all is rca for the reception of the mortar, a layer of sheet ind rubber }-inch thick or a couple of folds of tarred blan) are laid on top of the block. The hold-down bolts f next dropped into the holes bored for their recepti< the bottom end being secured against a strong fiat ix washer either by means of a cotter or a nut. Short ho down bolts of tliis type are better than long bolts goi far into the block, because, if one of them should brei or its thread strip, there is no difficulty in taking it ( and replacing it by a new one, which could not so read be done in the case of long bolts. The mortar boxes i then lifted into place, this beincj done either with strc tackle or better by means of hydraulic jacks, which i of great service in getting the box into its exact pla The nuts of the hold-down bolts are then put on a screwed down with a long spanner. After the mill 1 been running a couple of days these nuts should again screwed down, putting on, if need be, two men to use

MuRTAR BLOCK

spanner, so as to get the mortar box held clown as liniily as possible. This is necessary so as to obtain a good solid bearing for the mortar, to prevent its shifting or breaking and to minimise the amount of vibration, and is a most important part of the erection of the battery, more especially so when sectional boxes have to be employed. . I Ad improvement that will be found to be very generally if applicable consists in constructing the mortar block not 1 of wood but of a block of concrete. In all cases where . i tbe mortar blocks cannot be obtained on the spot, but r ' have (o be imported with the mill, a large saving both in I first cost and in freight will result from the adoption of - this method. The block must be built up from a foundation of solid rook, well levelled off, either entirely of good concrete, or else of bricks or cut stones laid in cement. Proper bolt-ways must be left for the hold-down bolts, and their heads should bear against stout plate-iron washers, which must be built into the block so as to distribute the strain as evenly as possible. Upon the block thus built is laid a 6-inch thickness of wood, best in two layers of 3-inch plank ; hard wood such as teak is the most suitable for this purpose. Upon the wood again is placed, as before, a sheet of india-rubber or blanketing, and the mortar box is then bolted down as usual. The block so constructed makes a very solid and substantial foundation, and the layer of teak wood furnishes just the small amount of elasticity needed in the structure. A block for a small mill, built in this way according to the writer's design some years ago, has given every satisfaction. This plan has also been adopted in some large mills, e.g. at the Geldenhuis Deep, Witwatersrand, the Banner Mill, Oroville, California, and recently in the new 300-stamp mill of the Alaska Tread-well

Gold Mining Company, where a concrete block with an aDvil block of cast-iron weighing about 9 tons ii employed. Sncb a mortar block is not only cheaper than a wooden one in the first instance, but will far oatlast the latter, and is far less likely to settle irruluij or otherwise work itself nntme. It is manifestly easyto build it up to the exact height required, or perhaps a very little higher, and then to adjust the exact levd A the mortar box by planing down the layer of wood to the correct level.

Mortar Boxes. — Various types of mortar boxes ha?e been used from time to time. The first Califomian mills had '' low" mortars, in which only the sole plate with its rim up to the bottom of the screen frame was made of cast-iron, the rest being built up of plank lined with slicct iron. The only advantages presented by this plan arc the smaller first cost and lighter weight of the mortar, but these are far more than over-balanced by its disadvantages of leakiness and want of durability ; moreover, as regards its effect upon the running of the mill and its crushing capacity, the lightness of the mortar is a serious disadvantage. This form may be said to have gone out of use about the year 1870, and is now never seen except under very exceptional circumstances. None of these crude forms are worth describing in detail, they having all been replaced by boxes of greatly improved construction. A fairly typical mortar box is shown in Fic's 7 to 9. the dimensions being suitable for 800 to 1K)() Ih. stamps. It is made of good cast-iron, and cast ill one piece. Much attention should be paid to the (luality of the iron, which could consist (in Great Britain) of a mixture of good machine scrap, Scotch No. 3 foundry pi,', and No. 3 Bessemer pig-iron. This should give a

Mortar Boxes

JWM for

Plan,

o

Scale of Feet Section at X. Y,

Km. 7.

:i.-ii

Fro/it Elevation, rnjht half in section.

Fig. 8.

K

mixture having a fairly hi percentage of gra; carbon, moderately low silicon, and [diosiAionu nc oeedlng 0*5 per cent, at most. It should be the obj the founder to produce an even and fine-grained which should be perfectly soft in the thinnest part<

yet not show large crystalline grain in the thlckt that a inaximum of strength and toughness may tainecl, in order to withstand the vibration and sti the continuous heavy blows to which it is subj After the boxes have been cast, the founder mi

Sca'Aaxs

I ;i

''ii.iul tu strip ihe heavier portions, sueli as tlie bottom, as soon as possible, in order that the shrinkage strains <iue to cooling may be evenly distributed. The box should then be tested, and if found sound sent to the machine shop. The bottom should first be accurately planed and all other work set out from the surface so obtained. The screen-frame seats must also be planed, and the holes for the hold-down bolts drilled.

The weight of the mortar box is of course proportioned to that of the stamps. The following table shows the \heights usually adopted for modem stamp batteries of various sizes : —

Number of

Weight

Stamps in cftcli Mortar.

of

Mortar.

tons cwt. ;

A five-stamp battery has a mortar box from 45 inches to 60 inches long, 18 inches to 24 inches wide at the base, and 39 inches to 56 inches high. In the exceptional cases where a battery consists of more or less than five stamps, these dimensions will be correspondingly modified.

It is evident that to get the best effect out of the blow of the stamp the weight of the mortar must bear a certain definite proportion to it. The considerations aSec\Ai\

this ratio arc so involved that it would be quite hojKjleS: to attempt to determine it theoretically, and we hav therefore to accept the above empirical data, based oic experience. In order to increase the efficiency of tho blow some makers are setting a cast-iron anvil block for the mortar to rest on when very heavy stamps are used.

The mortar is provided at the back with a feed shoot into which the broken quartz is delivered, either by hand or by automatic mechanical ore-feeders. The upper portion is turned over, as shown in the cross-section, in order to prevent any pieces of stone or pulp from being projected backwards out of the mortar by the action of the stamps. This feed shoot should be about to 3 inches wide in the clear, a httle less at the top, and half an inch more at the feed slot at the bottom ; its position and the slope of the lower part should be such as to direct the stone on to the middle of the dies when these are in their proper position. As the lower part of the shoot is subject to much wear through the dropping on to it of the ore, it should be provided with a hner, best made of good steel plate, which can be replaced when worn out, as shown in Figs. 14 and 15. The feed shoot should be sufliciently high to prevent splashing, and may extend nearly the fuU width of the mortar ; the slot should extend from centre to centre of the two end stamps, this arrangement favouring uniform distribution of the ore.

Screen Frames. — The front of the mortar is closed by the screens. These may be of various materials, but are always fastened to readily removable screen frames. The screen frame is best made of wood, although some few makers employ iron ; this latter material is objectionable on account of its weight and because it is difficult to get as good a joint as when wood is used. The seating against which, the screen frame rests should be accu-

Vi Screens 133

rately planed. The frames are held in place by means of two long steel keys at either end, and one or two shorter sedges at the bottoms. Oak keys are also sometimes used. Other less usual methods of fastening the frames, such as studs or bolts and nuts, cotter bolts, or straps secured by wing nuts have been employed. Of all these latter methods the least objectionable is the use of cotter lots, as the threads of all screws near the face of the screens are soon cut to pieces and destroyed by the scour of the issuing pulp. Yet another method is that of hiogeing the screen frame at its upper edge, so that

instead of being lifted out, it need only be released

nd swung up ; this system presents few advantages

and has some disadvantages, so it is not, on the whole,

be recommended. Keys as shown form by far the

niost satisfactory fastening. They should be made of

forged steel. They must be driven down into thiMi*

places with an ordinary blacksmith's hammer of, say,

-: lbs. weight, and can then readily be reloascfl, provided

'lat they are sufficiently long and have good large heads;.

Tile object is to secure the screen frame so well, that

none of the pulp can find its way round the edges, but

fhat all must pass through the screens. Screen fram(?s

should be made of strips of wood IJ incli thick and

inches broad, well mortised togetlier ; the screens

should be tacked to their inner surface, and then a pioee

of blanket the exact width of the screen frame tackud

nv(fr it. The portions of the outer surface on whi(jh the

stef'l keys bear should be protected by having a strip

of inch sheet iron screwed to them. A complete dupli-

f:ate set of screen frames with screens and blanket strij)s

ready tacked on should always be kept at hand, so

that in case of a screen bursting it may be replaced with

out loss of time, ('hanging screens can be done witln' three minutes in a mill where everything is kept readiness. Sometimes the screen aperture of the mortf is divided into two or more divisions by vertical piecedt which form part of the casting of the mortar, witt the object of stiffening the latter, small square screen frames being then used. This should be unnecessary, and is bad practice, as it diminishes the available dis' charge area ; for the same reason, the screen frame should be made without central stiffening pieces. The length of the screen frame is, of course, determined by that of the mortar. Its height in the clear should be rather more than twice the depth of the pulp above the discharging edge of the screen frame ; this depth is usually 4 to 6 inches, and an average height of 10 or 14 inches in the clear is sufiBcient for the screen frames of most mills. Since screens give way first at the bottom, where most of the wear is, they need only to be turned upside down when the lower part is worn out. For this reason, the top and bottom pieces of the screen frame should be exactly alike. The screen frame should not fill the whole of the front opening ; there should be a space of at least G inches above it, so as to give access to the interior of the mortar whilst the mill is running. This space should be fitted with a piece of thin board held in place by a pair of light keys, or better still by a piece of stout canvas tacked at the top to a strip of wood, and hanging down about an inch inside the frame. This canvas can readily be lifted, and the mill man can then put his hand in to clean the inside of the screen, and take out any chips of wood, bits of rope, grass, twigs, &c., that may be floating on the surface of the pulp in the mortar, and which, if not removed, would choke the screens.

Vi Screens 135

The hand holes with covers held down by bolts, that have been introduced by some makers for this pm:pose, are cumbersomo and unnecessary.

The American practice, which has been most generally followed in England, differs from the Australian, in that the latter is to employ mostly vertical screens, whilst the former prefers them inclining outwards at a small angle, usually to 15*. There are several reasons why an indiDed screen is to be preferred to a vertical one. First of all, with a given height of pulp it provides a larger discharge area ; then, again, it gives more room between the upper part of the screen and the stamp, and thus renders the former less liable to break, without at the same time detracting from its efliciency ; the particles of quartz, too, fall more slowly down an inclined plane than down a vertical one, and, aided by the wash of the pulp, have therefore a better chance of falling through the orifices of the screen. These arguments must not, however, be pushed too far, for if the screen were to incline outwards at too great an angle, particles of crushed quartz would have time to fall back again into the mortar before they reached the screen at all. A compromise has therefore to be arrived at empirically, and experience has shown that an angle of 10" is about the most suitable for the inclination of the screen. Splash Boards. — A splash board should be provided to keep the pulp, which issues from the screen with considerable violence, from being thrown about in all directions. Some English makers favour a sheet-iron one hinged at the top ; but this is cumbersome and unnecessarily heavy, and its use is to be avoided. The mill man cannot see what is going on without lifting up this heavy cover, and he is only too likely to avoid taking this trouble. Far better and cheaper is a stout bit of canvas,

Gold Milung

Chap.

vertically from the top of the screen frame. Another good plan is the use of a splash board as shown in Fig. 10, so placed that the mill man can look over it at his screens without removing it.

Screens.— Screens of various kinds have been used; tliey may, however, all be divided into two main classes, those consisting of plates suitably perforated, and those

woven of wire. Perforated screens of the first class may be either plain

or burred; in the former case, a piece is punched clean out to make the hole, in the latter, the metal is not removed but is simply bent over inwards. The burr must alwa3's be turned towards the inside of the mortar box. The burred form has two main advantages : first, when the hole or slot gets worn larger by the cutting action of the pulp that is continually being driven through it, it can be restored to its original dimensions by laying the screen on a piece of board and beating down the burred edges with a wooden mallet, so that the life of the screen is thereby prolonged; and, secondly, the (;]ifice widening outwards, any particle of quartz that will enter it at all can be driven through it, so that choking is to ;i )('at extent obviated. 1 am not aware that burred screens

.ft.

Scalo

2 ft.

Fid. in.

.Vr/V/:V:"A'.V I S

':a\(; been proved to have any corrosponclin; disadvantage.', . though some suppose them to deliver rather more slowly than plain screens ; it will be seen that their discharging area is as a matter of fact rather less than that of the latter t)'pe. Slot screens may have the slots arranged in various ways. Thus the slots may either break joint or have all their ends in parallel lines, the former being the better arrangement, in that it obviates a tendency of the sheet to split along the lines formed by the ends of the slots. Again, slots may be either horizontal, vertical, or diagonally arranged at various angles ; all these arrangements have their advocates, the two latter being now perhaps the more common. It would seem a priori that the vertical slot is likely to give the readiest discharge, because particles projected towards the screens will tend to fall f towards them in more or less vertical lines ; theoretical reasoning is, however, of but little use on this subject. Punched holes are nearly always arranged quincuncially. In England and Australia punched screens are usually 'le>ii,niate<l by the number of holes per lincar or per S([uaro inch, and slotted screens are taken as corresponding to punched screens in which the diameter of the liole is equal to the width of the slot. This is evidently an utterly barbarous system, conveying no moaning whatever, as it is obvious that the number of holes per square inch is no criterion at all, or only within the widest limits, of the actual size of the hole, which latter is the essentially important factor that determines the suitability of a screen to any given ore. The Americans designate their screens by trade numbers corresponding to the size of needle that can just pass through each ; this is a better plan, but by no means satisfactory in every respect. There is in practice only one really useful way of desiglating

screens of all kinds, and that is by the diameter round hole, or in case of an oval or oblong slot, of tb smaller width of the aperture, expressed in decimals of a inch (or in millimetres). The length of the slot is usual] between inch and inch ; inch is very often adopU by American makers. Some Continental manufacture make screens with a slot inch in length, but these a not much used. Further, the percentage of area of orifi to total screen area should always be known. The tal on the following page gives generally adopted proportic for round punched, and for slot screens.

An American pattern of plain slot screens has all t slots uniformly inch long ; the diagonal slots slope an angle of 30* to the vertical and have 12 to 12*5 si per square inch. Similar slotted screens with vertical horizontal slots parallel ended or breaking joint hi each 12 slots per square inch. Diagonal burred J screens have only 8 slots per square inch, or about thirds the discharge area of plain slot screens of same width of slot.

Of course the proportions and sizes adopted by vari makers are apt to differ considerably from the ab( particularly in the case of round holes.

All these screens are now mostly made of mild s sheets, which form an excellent material for th Hussian sheet iron, which is very malleable and ha Very frood smooth finish, is largely used in Amei T'he quality of the metal should be tested, as the Hf a scrcjon depends principally on the softness and mal of the metal from which it is made. The th U'ss of the screen should bear a definite proportioi the width of the aperture, the relations usually ado] being shown in the following table : —

%

1 i i?.fflS.OMOO

s

Bu

u

i-in

S|-

ml

|SSSg2SSS2=:2

1

!

I40 GOLD MILUNG zm

One of these screens will usually wear under ordinal circumstances from a fortnight to six weeks (or eve more at times), depending largely on the quality of tb rock being crushed, as well as on the shape and size ( the mortar box. As an average for general calculations it may be taken that one screen will outlast the crnshinj of 400 tons of quartz. Some mill men prefer screen nmde of sheet tin, the tin being first burnt off; these bcin| thinner than ordinary sheet-iron screens do not last 8t long, but are supposed to discharge the pulp faster from the same size of orifice. Sheet copper is also some times used ; this is said to wear well, but should not h used where inside amalgamation is practised, as there i: risk of the screens becoming amalgamated and chokinj up with the amalgam collecting on them.

Aluminium bronze has boon used in America, it i said with satisfactory results, its lasting powers beini described as very far superior to those of iron ; in thi j)ractico there appears to be no danger of the screen becoming amalgamated. The bronze employed contain 1)5 per cent, of copper and 5 per cent, of aluminium, an* the old screens can be remelted when the holes hav l)ecome too wide to admit of their further use. In spite however, of the fact that the worn screens have thu some value, they have been found to come too dear, an seem to have gone out of use.

The cost of sheet-steel screens is between 9cZ. and Is. per square foot, or, say, ()5. for a screen ; the wear an tear of such screens may therefore be put down at aboi 01;V/. per ton of quartz, exclusive of carriage, etc.

Woven Wire Screens. — Wire screens differ but slightl in the shape of the mesh used ; most are square, bi some are slightly oblong, there being at times a differeuc

Screens 141

from 10 to 15 per cent, in the length of the longer and

lorter sides respectively. Thus a screen having 1200

aeshes to the square inch is sometimes made, having 33

knd 36 meshes to the linear inch in the directions of the

web and the woof respectively. Wire screens have also

been made having a markedly oblong mesh, the length of

?rhich is three or four times its width ; it is not known

whether there is any definite advantage in using this

form of screen, but it would appear a priori that there

should be, as a stouter wire could be used without

diminishing the discharge area. In the case of wire

screens, as in that of the punched ones, the only logical

S}stem of designation is by the width of the mosh in its

narrower direction. The usual system is by the number

of meshes to the linear inch ; if this represents always the

larj'trof the two numbers, whenever the mesh is not square,

Jnd if the gauge of the wire be known, the size of the

liJesh can be calculated. There is no absolute uniformity

ifl the practice of the different makers, but the table on

page 142 represents one of the most largely followed.

Tlie table refers only to iron or steel wire, which Ijittcr is now vei*y largely used. When the material is fcrass or copper, which are often used for the jBiner sizes, the wire is often of rather heavier gauge than shown in ik table ; for example, the following are dimensions liich arc usual in this country : —

u I V 1 Ganpe of Wire Thickiiesij of w- iii. i

Illftll. IlM'Il.

iO 34 ' GGG92 OGlfiS

Vj 38 Ooooo 0 0005

Gold Milling

Cha

CAST'S t

P

Sooooooopoooooooo cro ooooooooooooooo

H

o

S5

,-i,,-iOOOOOOOOOOOOO

St*

o 4/ t-

a:

-J1 -o ?c -r

i-t r-i i-< CM 00 CO

r

Screens 143

uss aud copper wires have been used whore inside nalgamation is practised, and do not appear to amalgaate. Brass is not, however, a desirable metal to use r any purpose about a stamp-mill, and should be iToided. The chief objection to iron and steel is their indency to rust ; if the screens be kept in rolls, well )ated with grease or composition as they leave the lakers, they can be stored without fear of rusting. Vhen a piece of the required width is cut off from the oU for use, it should be heated for a few minutes over a Ire of chips till the grease is burnt off, a coat of oxide )eiDg thus formed on the wire, which acts as a good )reservative against rusting.

Good wire screens will last about a fortnight if well looked after, and as they are easy to repair, sometiine even longer, corresponding to a crushing capacity of, say, 250 tons. Their cost in England is at the present time about 6/. per square foot. Taking the size of an average screen at about 6 square feet, this would make the cost of wire screens about 0*14 J. per ton of ore crushed as an average figure.

A comparison of sheet and wire screens will show that the chief advantage of the former lies in their greater <lurahility ; as they are dearer than wire screens, the cost ptr ton of ore crushed comes to about the same thing, tlic 'ire screens needing, liowever, more frequent renewal, hence necessitating a little more work on the part of tlie 'nill nmn. The great advantage of wire over sheet scruuns hos in the far greater discharging area of the former, it will be noted is for the same width of orilice rather more than 3 : 1, the dilTerence being even greater than this in the finer sizes. This is so striking a dilTerence that it must not be overlooked, especially in milling

low-grade ores. I have found by experiment on tw< batteries run side by side, one with slotted, the other witi woven screens, that the latter crashed some 15 per cent faster than the former, and this result has recently beei corroborated in America. The employment of perfonitei screens is indicated in the case of rich ores, low ii sulphurets, where inside amalgamation is practised m it is desirable not to discharge too rapidly, whilst woTe screens are specially applicable to the treatment of Ion grade ores, the value of which lies largely in the! sulphurets (which have then to be saved by concentration as a free discharge favours the production of granulB concentrates with a minimum of slimes. Woven screen certainly seem to choke rather more readily than do bun perforated ones, but only to a small extent, and thi difficulty may easily be avoided by a careful mill ma: who looks after his screens well, and always keeps spar frames at hand, ready for changing.

There is no question of more importance to the success ful working of a mill than that of the proper size of th orifice of the screens. Whilst something can be lean by careful study of the ore, yet, after all, the ultimai decision as to what size of mesh should be employed : mostly arrived at empirically. There are two points bo determined in each case : first, what is the size particle which gives the most economical result; am secondly, what size of orifice in the screen will produce maximum number of particles of the required size. Tl first point depends entirely upon the character of the or An ore, the main richness of which lies in its sulphuret should be crushed comparatively coarse, so as to avoi icilucin*' the valuable sulphurets to slimes, which ai (litlicLilt both to save and to treat subsequently. If tl

Vi Screens 14

salphnrets are finely disseminated, the ore must be crushed more finely than when they are in coarser crystals, so as to facilitate their separation from the worthless gangue, the object being to produce a minimum of particles that can consist partly of sulphurets and partly of quartz; obviously the maximum size of the crushed particles must therefore be somewhat less than that of the valuable particles in the original ore.

Very much the same thing applies to the size of the

original particles of free gold. Ore carrying coarse gold

need not be stamped as fine as ore carrying free gold in

state of very fine division. The readiness with which

the gold amalgamates is another factor ; when the gold

amalgamates very freely, it need not be retained in the

battery box for a 3 long as when the opposite is the case,

whilst at the same time over-stamping must be avoided.

Over-stamping, which consists in pounding the particles

gold after they have been rendered fine enough to pass

through the screen, is due to a number of contributing

causes, only one of which is the fineness of the screen.

It is one of the most frequent causes of the loss of gold

in milling. It has already been pointed out that gold

when continually pounded becomes brittle, and moreover

that gold in this state only amalgamates with great

difficulty (page 84). Hence over-stamped gold is apt

to escape past both amalgamating and concentrating

machinery, being in particles so small as to float on water,

and is therefore carried away and lost in the tailings.

The financial aspect of the question too must not he lost sight of ; it may be that coarse crushing will cause the loss of some gold in the tailings that might be saved by stamping finer ; yet if the supply of ore be i)plentiful and the cost of mining low, it may pay to let this gold

escape to waste for the sake of crashing a larger quftntity of stone. Very often the saving of cost per ton ol ore incident on crushing an increased quantity with the same staff and plant may more than compensate for the gold lost, and in such a case coarse crushing is to be recommended. The object of the mill man should be nol so much to produce the cleanest possible tailings as tc conduct the operations of the mill so as to secure fl maximum of profit.

When chemical methods (such as cyanidation) sn applied to the after treatment of the tailings with thi special object of dissolving out the finest particles o gold that may be left, coarser crushing is admissible than when no such chemical process is used, because ai aqueous solution can penetrate and dissolve out gol( through fissures so fine that mercury, on account of it higher surface tension, could not make its way in, so a to come in contact with the gold. This is one reaso: why in the Transvaal the tendency of recent years ha been continuously towards coarser crushing, nor is : by any means certain that the economic limit in th direction has yet been reached.

The second point also depends on a number of coi sideratiohs, the size of the orifices in the screen beii only one of them, the distance of the screen from tl centre of the box, the width of the box, and the dep of discharge being other important factors. It is alwa found that, when quartz is crushed with a screen any given size, only a very small percentage of t crushed pulp is of the maximum size, all the rest bei much finer and some being reduced to slimes, whi latter term is generally applied to particles less than 0*0 inch in diameter.

Screens

'47

The following table, collected from various sources, will give a general idea of some of the results that have been obtained in investigating this subject ; unfortunately the data at our command are scanty in the extreme, and more urgently needed to extend our knowledge on this very obscure poin!i : —

Width or

Ibmieter of

Percentage ofCrnihed Ore.

Left on a Sieve having a Mesh of dinmett-r

:i0

r>2

,

Poued through

dinnictor.

0O-27 0O24 0O22

The percentage of slimes is thus between 25 and 75, whilst the proportion that only just passes through the 'crecn orifices seems not to exceed 10 jxir cent. ; it is pretty clear from this that if a screen is used rather Joarser than the size to which it is desired to reduce he quartz, only about 10 per cent, will be delivered in oocoai*se a state, whilst about 50 per cent, will bo too nc, leaving 40 per cent, in the right condition. Ac- 3rdin'lv all that would bo needed is to recrush this ) per cent, which is too coarse. It will be seen ibserjucntly that such coarse stuff can be separated by )paratns that is very cheap to construct and works itomatically, requiring practically no attention at all ;

that it is probable that in many cases the most Gnomical way of working would be to crush

b 2

rjS '70LD MILLING wvi

nrh ze it <sen?en co 'ieiirer seme 10 per cen'

if 'he omiincr axi sarse. jnd diea temm diis 10 p(

sfnr. M :lie iiacxoy. >7r eise cxtiBiL ic Go die reqmn

<~jrnenulv ipeakine mill niea jre in. die haifaic of cms

ina rhmiuch. uio dne a -screen. A oareful liifEbrenn

itniii; ot .1 parcel or cailuuzs each, sze bein}; assay

hv ir.Lf. Till mc;!! ow widiin whac limits che crnshii

hnniii be comincreii in eacii paracular case. Of coai

'X 'a il-aji) aiiviaahle so use as larse a screen orifice

poihle in ordcur co caise die capftdcv of a batterv to :

maTclrr.Tim Limit: cocsiscenc widi izood woric. Aocordi

L"':*. rc. iicharzirLu oapacirv oc a sivea sere

"xr.r-<< w ;-, -riiere I she iiaziecer of the mesh os<

Fr. '.: :zjzv:i:z T*rr:e3 ot ccmpararive tests made

.\r Af'/.-if-cr. md Brfrmner. die effects of varying t

r.-'r: '-jf rhe; ?cr-;en ere hown. Their results are gii

in :..r/;Ia.r ftrm. rhe las3 colomn giving the t(

r. th; -houM :ri.7i:re:icallv be discharied according

'.'.': a vr? forrr.ViLi: i: :ll be seen that the fijjnres tl

.\Drji*OAirr.a:e fairlv co thiDse act nail v obtain

M'-*i., .,, . . ' on nine tastf

Ir./h.

:J21 0 015 8-90 8-97

mt 0 0210 8-34 8 05

Other Forms of Mortars. — Tlic mortar previoi v.liovvn in T'Ijh. 7 to 1) may be looked upon as

Sca'Eens 149

normal type of modem mortar; there are, however, various modifications of this pattern that have to be considered. Mortar boxes are occasionally made with a double discharge, back and front. In this case the nt screen is arranged as usual, and a back screen about 8 inches high is fitted below the feed hopper, which is a httle higher up than ordinary. This airangcnaent has not, however, found favour with mill men generally, and I have seen several batteries that had originally been built for double discharge, but in which the rear screen had been replaced by a piece of sheet iion, thus making the double into a single discharge box. Theoretically, double discharge should only increase the capacity of a battery whenever it cannot discharge as fast as it stamps. It would seem, at first sight, that doubling the effective discharge area ought to nearly double the crushing capacity of the box, yet the abovequoted experiments of Messrs. Morison and Breniner bave shown that the screen of an ordinary single discharjj'e mortar can pass readily 10 tons per 24 hours, so "that tlie screen duty of about G-55 lbs. of ore per st|uare inch per hour, corresponding to the above conditious, is below the limit at which the screen comincincus to control the crushing capacity of the mill." It has rt'lKjatedly been found in practice that no better results are obtained with a double than witli a single discharge 'iiill, because in the first place the rear screen is continually giving out and requiring to be replaced by a fresh one, which can, of course, not be done without stopping the battery altogether. The rear screen is from its position more liable to damage than the front screen. With an ordinary lift of stamp (say under 10 inches) it is Tran.t. Inst. Min. and Mi U, vol. viii. 1900, 179.

aearceiy posafaie zo Acraage dui oeed ruTppiiE u u ieiivjr ::he iT)ck on .my poTdcuL ot die i&e in fcons ot ;hi iir ::;hird ; oie canaeqaencs amir cunxp&raciTeLv Urp rintnnencd are ::hroxvn backwarris i}v die blew of tbi -itanip. Hi}reQver. when die iieed slac is caised. &s :c luj 50 he m die ywem aow oniier cansideradon. pieces 03 i-ock in tiroppmi; <iown will failaftfamsc die rear aisle cc sbE .novin iieaiL. and are diim dirrjwn Tiulendy aiaicsc chc .*<%ar Kreen. A:: die aome dme die rear 5crn i? ibc mori awkwardly Hcaaced for chantfing : an cct-fedex ciAA CtizAxi zo be diaconnecteil and pushed, one ot die vay- r.fi corned me die men have co crawl under die fc<c 'A.\.r.:cA":.\ jt: i: iie 5<;reen. Mj tha: chanirlii' :h-f rtii <f;r:f:: 4 ".Ak-s 1, fa;' jinLTtc i:2i'r :!iai: ohazLzii: free: i.r:4. foiiz'i :iia: :q acec-izi: or z-i-f.r ir.con- .:r..i;r.-, y.y.r.;''-.rj. r,rie I'iu: icr-itrCj? ar-r apt :o cr ::-fdet::cd r. / .-;... i .Ti-jri -tr.d ill:-.ved ':hok. so ilia: :hc:r full cecett

v:*-/.;":.;.,;:;. of "ih-.j iopai-'rc: aiivantaje. prac::oal-speri- ':r.o': r.,t-. .;ri rjj -.:.- -.i:..'.- conorisiotis a.-? ice atovo tot; r..!./': corr'.fjC.-aV;'i— r.a:.:-Iy. :iiau no beced: is drrivti" fro ;j lu'. iVt of 'rj':":.!.- 'ii-ioj-ar-t boxes. Ic need hard) r,*: -..j.ii :.r..j.', VO-/V f.ic: crushed oro re main in;: It- f...:;; If. r,;.': :;.mortar boxris Causes ir.side amaUamatioi t.o lo--. r.liOi-0 ihly [.-erforrr.od. Accordingly, doubl 'I. v:ri;4rj'': /. oild not b'.- suitablo. wh-n it is desired r:;4U:b mo J. of tii*: ;iol,l inside ttit: box, aud lias littl' t.o r'/:omrriOfiM ir. und-'r anv circumstances. Screens fitted( ic*.'; ti,': *;ii'l-i of box'iS liavo also been tried, but hav \.','.\:i\\j ih r.lj'tir f.ivour, and vc-rv manv disadvautajjes.

Inside Copper Plates. — With many ores (perhaps wit! iiwjii. ii' bo:v, aiirl when this is done, anialjamated cop|)e

n INSIDE PLATES ijl

plates are Urgely used inside the mortar. Usually a front plate only is employed, but sometimes there are both front and rear plates. The front plate is fastened joBt below the screen frame ; it is sometimes attached direct to the mortar, and sometimes (preferably) to a block of wood known as the chock block, which fits just inude the screen frame. The copper plate should be 4 to

SoUb. .K-

Vertical Section of Chock block.

'inches wide, and inclined at an anglu of about 40 <1e- ijrecs to the vertical. When it is fastened dii-oct tu the box ilsulf the casting is arranged to receive it. Its lower end liould then rest against a little step a quarter of an inch ilecp, and it may be secured by buiiig slippod under little projecting lugs cast on either side of the screen seating. .Another arrangement consists in drilling a number, say

Gold .Vilung

F#

Tl

about h&lf a doxen, of holes aboat inch diamei 1( iiudies deep, into the casting ; a plug of hai is driTon tightly into each of these holes, and thi [d&te is eecnred by means of iron-wood screws into the above wooden pings.

The object aimed at is to hold the copper [di firmly, bo that it oanno lodged by any aociden or jar, and yet to a its ready removal for when reqmred.

He other method i in Fig. 11, which givt tical section through tl arrangement. Here j chock block proper, t the copper plate is scr is a piece of IJ-inch pli fully planed, which m the full thickness of th frame D; E ii the c which holds both cho

Croat Section through centre „ . ,

sio. a'-i' C 18 a portion of t

Fio. 1?. casting of the mort

This is, for several

probably the best method of fastening from

plates.

When rear plates arc used, a special recess provided for them underneath the feed shoot a j tion of the bottom of Ohe latter forming a sort to protect the copper plate. This arrangement in section in Fig. 12, which is probably the best

Oi

la

I' /.Vside Plates 153

i'' Hg. 13 U a front elevation of the same mortar box, iliicb is of the well-kuowu "Black Hills" pattern. TK-wfiper plate it> held io poEitioii either by wedgts or ly screws, the arrangement bciii'j identical with that JescriljtJ for the frout jilate.

The uie of a rear plate has certaia advatita<;e3 in some (M. but also uianv atteudaut disadvantages ; it occes-

11

Qi

MiiU iiii;roasert the width of tho inortar Ijox at iiliuui ilii; U-kol of di-icharge of the pulp, and thi is u decided 'liailviuuagf, as it dimiuishes its crushing' oa[>acity. .Moifij\tr, 110 provision is ever made, nur would it lie t-asy (.iltlioUHli quite possible) to iiiuke it, liv which tliu hei;;ht <-'f iIk' reitr amalgatnated plate eould be adjusted to suit tiii; hL'ight of the dies when new and when wuni respectively

. If the jdate is so high as to be in the right plac for new dies, it will be so high as to be partly above tha level of the pulp in the mortar when the dies are worn down, and is then apt to become coated and useless ; % on the other hand, it is in a position to suit the worndown dies, it will be so low as to be cut and scoured when the dies are new. The rear plate is, moreover, difficult of access, and cannot be well got at for cleaning purposes, so that, although at one time very generally adopted, its use is now largely discontinued. Both rear and front plates certainly diminfsh the crushing power of the mortar, but this effect seems to be less marked in the case of the front plate ; at the same time, what the l)atttery loses in efficiency as a crushing, it gains as an amalgamating, machine. Upon the whole, the use of the front plate only is to be recommended in the majority of cases. When it is used, the mortar is some 2 to 3 inches wider at the surface of the discharge than when no plate is used. This is necessary not so much for the sake of making room for the plate itself as to diminish the violence of the scour of the pulp when set in motion by the stamps. In a narrow box this scour is so violent that all amalgam \vould be stripped off the plate as soon as deposited, and the plate itself rapidly cut to pieces. The width of a mortar for 800 to 900 lb. stamps should not be less than 15 inches .at the discharge level, when front inside plates are used ; the use of a rear plate would increase this figure by about another 3 inches. A modern mortar for 1050 lb. stamps, fitted with chock blocks, is shown in Fig. 14, this being the most recent pattern adopted by the Sandycroft Foundry Company, Limited.

In some recent batteries, eg,, those of the Alaskaevation

; right half in section.

Sectional Plan On Ab.

Gold Milling

Treadwell Company, Limited, Alaska, and the N( CrcDSua and the Simmer atid Jack Mines, Transvaal, t back copper plato is replaced by rifles in the back a sides of tlie 1)0X03 and also at times in the front. Tht

i-ilUcH arc gi;iiirally grooves cast into the liner plates of t niortiir (ki-c [wie 159), which arc then usually made i-ii'l Btiifl; these work quite satisfactorily, and do i iifi'ci as much width iia do copper plates, whilst tl ulford similar opportunities to the latter for the acoun

lUli'll Oi

ainalam in places Iroin wliicli thcix is liulc risk of its being dislodged. The construction of such a mortar is shown in Fig. 15, this being the design of the Humboldt Engineering Works, Germany.

Bepth of Discharge. — One of the most important factors in regulating the running of a mill is the depth of discharge, that is to say, the vertical distance from the top of the die to the upper edge of the lower portion of the screen frame over which the pulp is discharged. This varies greatly in different mills, being from 5 to 15 inches. It is obvious that a shallow discharge favours rapid and coarse crushing, whilst the opposite arrangement keeps the pulp longer in the mortar and aids fine crushing. The depth of discharge is therefore determined by the same considerations as to the nature of the ore as govern the coarseness or fineness of the orifices in the screen employed ; it would be decidedly bad practice to use a shallow discharge with a fine screen, or a deep discharge ith a coarse one ; hence these two factors in gold milling first always be regulated so as to correspond to each other and to the character of the ore to be crushed. As the dies wear down, the depth of discharge tends to increase. In order to keep it uniform it is advisable to have chock blocks of different depths, which can be changed j9ari pass7i with the wearing down of the die. The total amount of the latter is usually about 5 inches ; hence the mortar should be so constructed as to admit of chock blocks varying in height by this amount being inserted. If the edge of the casting which forms the lower part of the screen seating be about 1 inch lower than the top of the new die, when in its proper position, a chock block 9 inches deep will give a depth of discharge of 8 inches, which is a very usual amount ; when the die

is worn down completely, a chock block 4 inches de should be used, and the depth of discharge thus k( uniform ; if chock blocks are provided differing in de] by successive amounts of 1 inch, a quite different deg of uniformity will be attained. When in addition chock blocks of varying depths, false bottoms are provi( for insertion below the dies (see page 167), the depth discharge can be completely and accurately control! and this method is now generally adopted in the n: modern> mills.

Inside Linings. — The only portion of the mortar that is liable to wear is that part of the feed shoot u which the ore drops, and those portions of the sides of box against which the pulp is dashed by the action of stamps. These parts should accordingly be protectee a sot of steel plates from half an inch to one inch th cut so as to fit exactly into their places ; their vert edes should bo bevelled at an angle of 45 degrees, so 1 when onco dropped into position, they mutually hold € other ; the sole plates of the dies, too, usually fit aga them a!id help to keep them from shifting. It very ra happens that one of these plates is displaced, and w this does occur, it will be found to be due either to c lossness in putting them in or else to their verl edges being worn. The front plate should come level with the edge of the screen seating; if a cL block is used, this may be protected by having a piec 2 inch sheet steel screwed to its inner face, just be the copper plate. The back plate should reach to bottom of the feed slot, and there should be ano plato bent to the curve of the feed shoot, projec Ronie 12 inches or so up it, and fastened in its place means of bolts ; the side plates should be about 9 in<

Vi Mortar Details 159

h'gher than the level of discharge when the dies are Dew. The use of liner plates allows the weight of the mortar to be considerably reduced without correspondingly shortening its life. Cast steel liner plates up to 1 .1 inches in thickness are sometimes used ; such plates, with grooves cast in them so as to form riffles, have already been referred to. The methods in vogue of inserting and securing liner plates are shown in Figs. 14, 15, and IG.

CoTers. — The top of the mortar box should always be covered. Boxes are mostly cast with a small flange or recess about 1 inch below the top of the mortar, upon which the covers are supported. These covers are in two halves, being divided lengthways, along the centre line of the stamps. Into each half are cut semicircular recesses to allow of the passage of the stamp steins. They are l>cst made of light J-inch plank, their junction beinj; either stepped or bevelled, so as to prevent any splaslies of pulp from spirting up between them. The covers may also be made of light sheet iron ; in this case they should overlap about an inch along the middle of the box. The two covers may be held together by light bolts or by hooks, and should be provided with light handles to finable them to be lifted off readily.

Arrangements for Cleaning Up.— In order to facilitate the removal of the dies from the mortar box when the latter has to be cleaned up, several arrangements have been employed. One consists of a hole about inch in diameter drilled in the bottom of the box, sloping upwards from the back, and coming out in the centre of the middle die ; when the battery is running, this hole is closed by a carefully turned iron pin, which is flrivcn ti'htly into the hole. On cleaning up, this pin is withdrawn, when a bar can be put in and used as a drift for

COI P MU I IXG iiiii

iiijiiai.' .Ul-. Tlic disadviiiUaso of tlii that tho bole soon gets out of true, so

that the pins no longer fit it i eKactly, and leakage takes 1 place ia consequence. A better plan is to have a Betv tion of the front of the box made movable, opposite ths middle die ; this monJile piece is dovetailed and ww fuUy fitted into its place ; it is held down by the sctetfi frame when this latter is in position. When the batter IB to be cleaned up, tluB movable piece is lifted oat, and the middle die can then be prized up by means of a bar or of wedges. Care moat be taken that the apron of the mortar box shall be placed so low down that any accidental leakage that luay take place around this movable block shall find its way over the apron, so as to prevent loss. This arrangement is shown in Fig. 16, which represents a recent pattern of battery box made by Messrs. Bowes

Scott and Western, Limited.

In Morison's new mortar box the entire front is loft

Sectional Plan

v/ MORTAR DETA/I i6i

'pen, the front piece above the screen conmstinc; of a remo\'ab]e plate pressed out of stout sheet steel, so as to give ample stiffness to the entire box, yet capable of being readily swung aside to give access to the interior.

Apron. — The aprcn of the mortar box is usually cast in

one piece with it ; at times it forms a separate casting

which is bolted to the box, but this latter arrangement,

which is indicated in Fig. 15, can only be recommended

coder circumstances where it is necessary to reduce the

weight of the casting to the utmost. However carefully

the joints are faced and jointed, there is always some

risk of leakage, a defect which should be scrupulously

Avoided in a battery. The apron is exposed to a good

deal of wear owing to the scour of the pulp as it issues

from the screen ; when therefore it forms a portion of the

mortar, it should be of sufficient thickness to last as long

any other part of the box, or else it may be protected by

a light replaceable liner plate. When the apron is merely

Mted on, it can of course be readily renewed when worn

out. The apron forms the distributing arrangement from

which the pulp flows into whatever apparatus is to be

next employed for gold extraction. If the pulp is to be

caught in a trough or launder, the apron need have only

one or two apertures through which the pulp is discharged.

In the majority of cases, however, the pulp flows directly

over amalgamating tables or wells, where its uniform disribution

is of great importance. In these cases the front

tf the apron should be pierced by a number of holes about

inch in diameter through which the pulp can flow, thus

iisuring even distribution so long as the apron is level ;

) secure the greatest possible uniformity, the centres of

le holes in question should be set off by the maker

irallel to the planed bottom of the mortar box. This

M

iiviansoiiiecic i:s jiiowii in riys. 7 to it. The apmn shouW always be Bofficiently wide to admit of any necessary work, such as getting out dies aiid shoea, being done on it, without risk of damaging the tables that may be set below it ; it ought to project at least 8 inches beyond tbo front of the screen frame.

Sectional Kortar Bozea.-~Il ia sometimes necessary to construct moriara so that they may be carried into regions difficult of access, wherii the backs of aniintils* or even of men, constitute the sole available means ot transport. When this is the case, the mortar has to mite in sections, so that the weight of no one piece shall exceed perhaps 2 owt. This is under all circumatances an unsatisfactory arrangement ; however well the fitting and jointing be done, there is nearly always some Ic&lc age. Manufacturers have tried many different devices, the best ho far being the arrangBment shown in Figs. l7 to 19. The bottom part only of the niorlar is made of cast-iron, the upper portion being of steel plate rivelei or bolted together. The screen seating is also made of cast-iron. The cast-iron bottom is in vertical sections (one of which is shown in Fig. 19), held together by four bolts as shown, and by a steel dovetailed key that runs iLlong the entire length of the box. The key-way must be planed out and the key made a thorough driving fit. The bolt holes must be carefully bored out and the bolts turned to an accurate driving fit. All the bolts should be furnished with lock nuts, so as to withstand the jar of the stamps. The various sections should be planed and faced with the utmost care, andre fitted together with breaking joint as shown without any jointing material betweeD the sections ; if, on account of bad workmanship, it is absolutely necessary to use the latter, this is best done

SECTIONAL AfORTARS

by painting the planed faces with a solution of indiarubber so as to form a thin, closely adherent film of this substance on the surface. The most difficult joint to keep tight is alwa'S that between the cast-iron bottom and

S/r/e Elevation.

Fn>. 17.

the wrought-iron or steel walls. It would prol)al)ly be

better (although more expensive) to make the l)ottom

sections of cast-stoel ; the sides could then be riveted

finely to them and caulked, which cannot be done in the

ca5*e of cast-iron. Sectional boxes arc, however, always

M 2

ir..iil.lr>*iiio. iiiul >.lioiLia only l)e usc.l in case ol iilisolrif' ncoesdfy. They have, however, been used in some places with success. Thus, for instance, a forty-stamp mill with sectional mortars, built by Fraser and Chalmers, hft been running for eight years at Bosario, Honduras, and has given entire satisfaction.

Diei.— The die forms, as already ssid, the working fwe of the anvil upon which the ore is crushed. Numeroas different forms have been tried at various times. 10 die was formerly made in one pieon oooupying the entire

I

front Vliwi/Batttrn Ban. Right half la

bottoni of the mortar box, but it is now always made in separate pieces, one die to each head. These have bctm nmdo cylindrical, hexagonal, or octagonal, as shown in Fig. 20, a form which on account of its simplicity and suitability for forging is still at times used. Dies have also been made with lugs fitting into recesses in the mortar, but now the form shown in Fig. 21, or one closely approximating to it, is almost universally adopted. The upper portion is cylindrical, coiTesponding in diameter with that of the stamp shoe,

Dies

&ud the foot plate is hexagonal, octagonal, or more frequently square or rectangular. In this latter case the angles should be bevelled off and somewhat overhung, so as to facilitate the lifting out of the die from the mortar box. The foot plates should almost completely fiU the bottom of the mortar ; they should fit against the back and front liner plates, and be so set as to give tke latter no play. The bases of adjoining dies may with advantage be from inch to inch apart. In

Scale.

o

J

fl.

Side View of one of middle sections,

Fio. 19.

recent mills they have betMi set as much as 1 inch

Iirt to facilitate removal ; this is, however, a practice

"y no means to be recommended, it being preferable to

Use one or other of the devices already described (see

]w;,'e 159). In any case J inch should be an ample

Jijjtance.

The weight of the die should be proportioned to that the stamp, being best about 14 per cent, of the weight the latter ; it usually varies between V2 and 15 per cut. of the weight of the stamp. Thus, for a DOU lb.

stamp), 9 incbeB in diameter, the cylindrical part of tb( die should be 9 inches in diameter and 5 inches deep weighing 90 lbs., whilst the foot ptate should be K inches square and inches thick, weighing 40 lbs. making the total weight of the die 130 lbs. For i I 1150 lb. stamp-mill, the cylindrical portion c

t

lliu (Ul- \vu,s iiiaile 'J indites in diameter and (i inch liili, mill llio foot plato UJ iiiclius square and 1 in< tliiek; I lie total \vei(,'ht was 134 lbs., or 11-G per cei of tlie sLiiuip. Ill tliis cast! a false bottom was, liowevc used, wiiieh allows the weight of the die to be Boiiiewh lediiced. If the die bo of good material, it can be wo:

lowu lo the iuuL platf, su tiuu wuli tiie above piupor- I tions, oat of the total weight of 130 lbs., 90 lbs. can he I used up in the mill in the former case, and 106 lbs. out of 134 lbs. in the latter. The question of the material F of which the die should consist will be investigated when treating of stamp shoes, as it is evident that these two must stand in definite relations to each other. Dies must be set with all their upper faces at the same level, and are often laid upon a bed of taihngs well rammed in the bottom of the mortar, about inch in thickness, and when the dies are a good deal worn, this

abed may with advantage be made thicker, say up to 2 inches, according to the requirements of the mill. It is, however, a better plan to insert a cast-iron false bottom " in the mortar to compensate for the wear of the dies; in the best modern mills false bottoms of different thicknesses are provided. These are usually made in two halves with a stepped or sloping joint in the middle, and having a drift-way cored out, so as to allow the upper half to be readily forced up for cleaning up the mortar. Such a false bottom is shown in position in the mortar box, Fig. 14.

Haiti: Ii Vii

THE STAMP— TAPPET — STEW — HE*D — SHOE — lASI SHAFT— CAMS— CAU CCBVK — POWEB UEQCUU

The falling weight of the stamp determines primarii)' the working capacity of a mil), luid batteries are there fore always gauged by thie factor.

The total falling wdgfat ia iade up of —

3. The head ; i. The shoe.

The proportion which thtsc various portions bear 'o the whole weight should be about —

Tappet 14 por cent.

Stem 43 ., ..

In actual practice the wei<;ht of the tappet varies between 11-5 and lG-5 per cent., the stem between 355 and 44, the head between 24 and 33, and the shoe between 15 and 20. A near approximation to the proportions above recommended is, however, advJBable, so

r.iri'i: /.s

wei'ius lor a liOU lb. aiul a liiOU lb. lamp should be respectively —

Tappet about 125 lbs. ... 150 lbs.

Stem „ 390 „ ... 550 „

Head 255 350 „

Shoe „ 130 „ ... 200 „

practice of late years has been to increase the 'height of the stem in proportion to the other parts, ifl some cases to lighten the shoe. Thus in 1B71 iter W. Baymond gave the following as the proportions castomary in Colorado at that time — head : stem : collar : shoe : : 5 : 3 : 1 : 2, whereas the modern practice is given above. It must be mentioned, however, the whole practice of milling has completely changed 10 the last twenty-five years.

Ike Tappet. — Tappets have been made of very many different kinds. The simplest kind, which may for the Ice of economy yet be used for light prospecting stamps ot e.xceeding 300 lbs. in weight, consists of a collaihout 2i inches wude and some 6 inches deep, which is eld in place by a vertical key, the back of the key being QoUowed to fit the stamp stem.

A section and plan of this form of tappet is shown in 22, but, except under the circumstances indicated l>ove, it is now never used.

Another form, which was used in some of the earlier American mills, but abandoned about 1870, althougli still in use in Australia, consists of a collar into which a coarse screw thread has been cut ; a similar screw is cut pon the stamp stem, the upper end of whicli is turned down somewhat so as to allow the tappet to slip over it. By these means the tappet can be adjusted

Statislics of Mines and Miniiuj in the Staler and Territories test of Ricky Mountains, 2871, p. 339,

to ito ex>et pontiaa on &e rtnaa, mnA c&n beld time eith \fj nwAns of s ks; or erf a loekiiig dowD upon tfae te|i(iet. This letter art is shown in Figs. 93 *nd 34, Fig. 33 gtvii a aectko of tbe kapjpet. kod a pUc view of the lod: nat, and Fig. ! view of the stem. It admits of '

1 adjustment of the taiiet, bat i it is ibond that the jam nat, en cxwusts, as shown, of a steel vrhich can be driven down by hammer. neTertheless works looe jar of the stamp. This arrung two great disadvantages : the np the itaiDp stem has, as stated al lumed down, aud is thus sveakei at the same time it is incapabli rcvtrsed when the lower eud of becomes broke or injured. 1 also is not reversible, and has nearing face. An attempt has I to remedy this latter defect by n wetiring face a removable ring which can be bolted to the bo j tappet by means of countcrsi: .Another form of this, known patent tappet, made by Thon , of Castlemaine, Victoria Fi"- SI of a tapered steel body inside

the screw thread is cut ; over the outsid slecvo terminating in a broad flange, which actual tappet face. The sleeve is held in by means of four set screws, the tajJer being, dowuvf&rd. None oi theiie Mrangemeni

Tappets 171

DHjpiiated in Australia, has come into general liToar. ouotLer system consists in making the tappet in

io l.iltVCS, aivi.k-.! VL

I'j-uliLT l)y strong Imlts, which jmii ii uiion thv

TItis tappet shown in Fig. 25. whicli ilIsu shows 11

>lU[iip Mteiu with grooves turnuil u|)0]i it lo prtsveitt thu

ing, it has to be fthiftod the width of a groove, up dowQ as tlie caae may be. This p&tUsrn bas no s] advantage to racommoml il, ts, for maoy reasons, objection

Certainly tlie best pattern tappot is the Califoroiaii gib tap] which is now almost umversi adopted.

It is shown in Fig. 26 and b cylindrical body haTilf [ace at either end. Witliill a recess iu the to]*- pet is a gib made oi' forged steel, and tliis is tightened up to tbe stem by means of two I OF three tapered keys. Two keys can bo used for stamps up to 7M lbs. iu weight, but above that tliree-keycd tappets are preferable. The working face of tbe tappet should be equal in width to that of tbe cam, or even be i(„ 1 ,. a trifle wider, say from

2; to inches. Cbse round the stajnp stem a recess J to i inch wide and J inch deep is turned cut, the object of this being w keep tbe wear ou tbo whole tapiet face uiiitorm, aiid to prevent its asBmniujj any approximation to a ooiiicul

It is obviotifl if the workini; f the tappet were eA to become at conical, a verj- e lateral thrust 1 be set up be- 1 the tappet and cam, tending to the stamp and tppet apart, which they is reduced to liiimm as Ion;; as iTorking faces are tained accurately ontal. The tappet Id be accurately 1 out to fit the 80 as to give a sliding fit, the being less than ich larger than stent. The gib il be of forged plaTied on the and edges, and ront bored to a about J inch )os imeter than the 1 stem ; this arnent ensures a giip of the gib stem. The keys

Plan. Section through c

St.

Kegfo

tappet.

S>.illB.

Km. ao.

should have a gentle taper, which mast be on i side away from the gib ; in a vertical direction keys should be quite inch narrower than the slotf which they work, so as to bear only against the back the gib and the tappet, and nowhere else ; their tendencr jam or to burst the tappet is thus reduced to a minima These keys should be left rough top and bottom, and (k planed and fitted where they bear against the gib a the corresponding back portion of the tappet. When I mill is set up, these keys should be driven well home w a 2]| lb. blacksmith's hammer ; they will then hold 1 tappets firmly in place without any slip, and there will but little danger of bursting the former. Some mak( supply the tappet with a set screw to hold the gib firn in place whilst the keys are being driven home ; the scp is removed before the stamp is set to work. Tappets % usually made of cast-iron ; this should be good, toiif close-grained iron of the best quahty. Chilling t wearing face of the tappets has been tried but cam bo recommended, as the edges are apt to chip. Ma modern mills have been made with cast-steel tappel this, of course, is a better although a more expensi material, and while its adoption may be advised for ve remote localities, freights to which are liigh — especially the case of proved valuable mines, where the first cost the mill becomes a comparatively secondary considerati — it is by no means a necessity, as good cast-iron tapjH will wear for years without renewal.

The Stamp Stem. — The stamp stem forms the main p tion of the entire stamp. The recent tendency has be to increase its diameter so as to make it as stiff as possih A l(ng tliin stem is apt to spring in working, cause excessive friction in the guides and rapidly wearing the

Ii The Stem 175

kway, besides increasing the liability of the stems to

)reak ; a stoat stem is free from these objections, and in

addition gives a more traly vertical and more effective

blow. The diameter should therefore always be as great

as possible within reasonable limits ; this will necessitate

the stems being kept as short as possible, but it is evident

that there is a minimum length below which the stem

cannot be reduced. In practice the stamp stem varies

irom 9 feet to 16 feet long, and is from 2 J to 3 J inches

in diameter. Good dimensions for a 900 lb. stamp stem

are:—

length ... ... ... 11 feet.

The material should be either wrought-iron of first-class quality, preferably good hammered scrap, or else Siemens- Martin or Bessemer steel, very low in carbon, say about 0 2 per cent. The stems may be either cold rolled to gau'.e or else turned all over. The former is slightly the cheaper method, but not by much, because the stems have to be put into the lathe to have their ends tapered down, and once centred, it does not take long to take a finishing cut off the entire length of the stem.

Wlien the screw tappet is used, the stem must, as already explained, be turned down at one end, and have a screw thread cut on it, as shown in Fig. 24 ; the other 'nd of tlie stem is then tapered for driving into the head. When the ordinary pattern of tappet is used, the stem is of the same diameter throughout, both ends being tai)ere(l down, as shown in Fig. 27, to fit into the head. This tapT should be slight, preferably about Oo inch to the

When a large mill is being constructed it is advisable

Gold Milung

Chap.

to tuni up a standard stamp end to the exact sin required : a female gauge to fit this exactly is thao l)ored. and these two standard gauges are kept in the iiiochino shop, all the stems being turned and all the. heads l)orcd to them, as vrell those required for the ori* nal mill as the spare ones required from time to time to replace worn out or damaged parts. By this syston the great advantage will he secured that all stems anil heads will be strictly interchangeable, and that new parts ordered, even after a lapse of several years, will he sure to fit exactly.

Stems are not as a rule worn out. They are apt to l)roak after some years* wear, owing, according to some

Fuj. Ti.

autlioriiies, to crystallisation of the iron, induced by the continual jar ; it has, however, never been proved that iron can thus be crystalHsed by concussion, and it is fliiVKtult to see why, if this explanation be correct, they should so rcjularly break in the same place. In any cjiso a liij'h-class soft steel seems to give even better s(;rice than the best wrought-iron. When stems break, they do so, as a rule, close to the head ; the broken stem can then bo inverted and used again. When both ends are brokeii, fresh pieces can be welded on, or the stems turned down again to fit the heads, so that stems may almost be said to wear an indefinitely long time. The Head. —The usual, it may almost be said the uni*

The Head 177

sal, pattern of head, or boss as it is soinetiincs called, shown in Fig. 28. Its length varies as a rule between and 20 inches, its diameter being the same as that the shoe proper. It consists of a cylinder of iron, one id of which is bored out to receive the tapered end of oe stem, and the other recessed to receive the shank of he shoe ; this latter socket need not be bored out ; it can te cast with quite sufficient accuracy for the purpose, and s, in fact, better left rough, as it thus holds the shoe iiore finely. There are a couple of drift-ways through the head to admit of the insertion of a tapered steel drift, by means of which the end of a stamp stein or the shank of a shoe can respectively be driven out. These drift -ways are sometimes parallel, but more often at right angles to each other, the latter arrangenieni weakening' the head less than the former one. Some niakers shrink a hoop of wrought iron round the top or the bottom end of the boss, or sometimes round both tnds, as shown in Fig. 29. It is quite unnecessary to hoop tlie upper or stem end of the head ; the shank of the shoe is so much-larger than the ei;d of the stem, that there is necessarily much less thickness of metal round the former than round the latter. Hence, if the lower eii<l is sufliciently strong to stand the driving home of the shoe shank even when strengthened by a hoop, the upper end can certainly dispense with such strengLhning, and the upper edge of the casting is preferably neatly rounded. It is by no means necessary to have a oop even round the lower end of the head, but if it is tliou;(ht desirable to use one, care should be taken that lie casting projects J to inch below the hoop. By this "leans, if the top end of the shoe batters against the '-ttoai face of the head, as it will sometimes do, the

oi chit! iacir will bozr op a Uccte and bold ti

in izi place. if the hoop were to

'AAZ bic below zh eaada, this iwiie lnaing i

riilnrf;i: its iliiiinctor, anil it would work loose. It mis' iif>l. forgotUtii tliat a loose hoop _is much worse tl)n noiu' lit ivll, sueiiiK that the casting hasbeeD turned down

The Shoe

I"*

ceive the hoop, and is accordingly weakened by that ant. I am of opinion that hoops are best dispensed altogether, and that the metal of which the heads made should be of such quality as to withstand the ting tendency of the conical pieces which have to be ren into either end. The material of the head is ally cast-iron ; it should be of first-class quality, low phosphorus and silicon, and of close, even texture x)nghout. No English iron should be used in the iture. Of late vears there

m

A been an increasing tendency make stamp heads of casteel instead of cast-iron ; the terence in price is not very t, and there is of course far liahiUty to fracture. Howfer, even a good cast-iron head ill last for many years, as iere is no tendency to wear Jcept in the sockets ; heads dually continue in use until fey are split, an accident hicli is generally caused by ivint in shoe shanks a little too large for their sockets. The Shoe is practically always of the shape shown in g. 30, from which there may be said to be no departure, may be looked u)on as consisting of two portions, tlie lank and the butt. The former is the part tliat enters e head and holds it in its place; the latter is pracally the wearing face of the entire stamp. Soiiictiincs e shank is made hexagonal or octagonal in section ; is practice, formerly more common, has of late years en almost abandoned in favour of the simpler, more

Side View.

Scale, i54=' Fio. nu.

i8o GOLD MILLING chai

convenient, and in every respect better, circular shape The length of the shank is usually between 4 and 6 inches Its taper is a matter of some importance ; if too slight the shoe will not be held firmly in the socket, and if too great there is considerable risk of splitting the head. The taper shown in the figure is very suitable and wdfa well in practice. It may be taken as varying from to inches per foot. As a general rule the diameter ot the base of the shank is half that of the butt, and the diameter of the narrow end of the shank is about twothirds to throe-fourths of that of its base. The shank should not form a sharp angle with the butt, but should be ucatly rounded ofif so as to diminish the liability to fracture at this point. The diameter of the butt is of course the same as that of the head, and the die is usually the same, though some makers seem to prefer the latter a trifle larger ; this plan is not, however, to he recommended. The diameter of the butt of the shoe is an important point, as the eflSciency of the stainj depends upon it ; it should be strictly proportionate t( the total falling weight of the stamp and the charactei of the ore. Thus a 900 lb. stamp for crushing bare quartz has usually shoes 9 inches in diameter, although for softer stone this may be increased to 10 inches ; tli elTectivc weight of the stamp may therefore be taken a between 14 and 11 lbs. per square inch of crushing surface. This figure may be made a trifle less fc lighter stamps, so that the diauietor of the shoe of 750 Ih. stamp should be between 8 J and 9 J inches, an of a GOO lb. stamp between and 8J inches. The abo c.ilculations arc for low drops averaging, say, 6 inches if a liiglier drop bo used, increasing the momentum the stamp, the above-giveu diameters may bo slight

I THE SHOE i8i

xceeded, whilst for a 3-inch drop, tho diameter, even

or a 1,250 lb. stamp, should not exceed 9 inches. The

depth of the butt usually varies between 5 and 7 inches.

The only advantage of a deep butt is that the shoe will

tun longer without requiring to be renewed. If we

ttsame, for the purpose of simplifying the conception,

that the shoe can be worn right down to the shank, it

is evident that, with a deep butt, the proportion of

ineful to useless metal (as far as wear is concerned) will

be greater than with a shallow one, so that there is an

ipparent economy in the use of deep shoes. But, on

the other hand, with a deep butt the difference between

the eflfective weight of a stamp when the shoe is new

and when it is worn out, will be proportionately greater,

and the efficiency of the stamp is thus seriously impaired.

This is a far more important consideration than tho

former one, which becomes, indeed, of very little moment

in the case of big mills which have a foundry attached

to them for the purpose of casting their own shoes.

So much is this the case, that such mills would probably

find it advisable to diminish the depth of the shoo oven

considerably below the minimum figure given above.

On the other hand, mills that have to import their shoes

from a great distance, will find a deep shoo the more

Gnomical. Thus, on the Eand, shoes with butts 9 inches

in diameter and as much as 12 inches deep are in use,

the shanks being inches high, and inches diameter

at the base, tapering to 3 J inches, and weighing altogether

240 lbs., of which quite 200 lbs. can be worn off in work.

The shoe showm in Fig. 30 weighs when new 148 lbs. ;

of this the butt weighs 128 lbs. and the shank 20 lbs.

It is scarcely ever possible to wear the slioe down until

the depth of the butt is less than inch ; indeed this

i8i GOLD MILUNG char

result can only be obtained with exceptionally good material, As a rule, shoes are used until there is IM than an incb of the butt left, when they are thnnn aside and new ones put In. The alteration in the UlinK, weight of the stamp caused by this wear of the shoe iij thus quite sufficient to make a marked difference in tin' efficiency of the mill. In the case of a 900 lb. sttmp using the shoe above illustrated, the loss of weight rf the stamp owing to this cause would amount to about !.-4jrDi*-i- 110 lbs., or 12 per cent. Aa

it is important to keep tbs falling weight approximnley' nnifonn, several devices hva been employed for this purpose. One of the best consists in the employment of a "false shoe" or " cbocfe shoe," which is secui-ed io the head just like an Qxdiaaxf slioe, wliile the partly worn down shoe is in its turn driven into the former. section of such a false shoe. as made by Messrs. Fraser and Clmlmors, Limitt'd, is shown in Fig. 31. False shoes of several different depths, increasing by, say. IJ inches at a time, should ba provided. Another pliiti is to have at hand sets of heads of various depths, iind to put on deeper heads in proportion aa the shoes Wfur. Tliis latter plan is open to the objection tht till' heads have to be so often put on and taken off, lliiit tboy an; apt to wear loot, A suggestion of my own, put forward some years ago, to key discs of iroo.

The Shoe 183

ted 80 as to pass the stem, above the tappets or ve the heads, where they could be held on by set 3W8, has never, as far as I know, been adopted in xstioe, though it presents the advantage that neither ids nor shoes need be removed until entirely worn out ; I tappets would, however, have to be shifted to keep e drop uniform.

The shoe is fastened into the socket of the head by eans of wooden wedges ; these should be made of dry, ftsoned, white pine, sawn into shape, but not planed, hey are usually to inch in thickness, 5 to 6 inches t length, according to the depth of the shank, and ipered to suit the taper of the shank. Thus, for the loe shown above, the wedges would be inch wide at ie base, tapering to 1 inch. It is evident that the width f the wedges, at their top and bottom, must be proortional to the diameters of the top and base of the Wink respectively. These wedges are laid round the lank and tied there. A number of wedges, ready made p into " bracelets " by tying each wedge to its neighbour r means of a bit of string passed round each in succession id knotted, should always be at hand in the mill, whenever a shoe drops out of the socket, one of these ady-made bracelets can be slipped over the shank, as to secure it in its place again without loss of time. ( the sockets are apt to vary a little in diameter, bracelets " made of wedges of various thicknesses ould l)e kept in stock.

hi setting up a stamp, the head is first placed in sition upon a piece of 3-inch plank laid upon the dies, lich latter are supposed to be already in their places, e stems are then dropped in and tightened by a few ws from a heavy sledge hanmier on their upper ends,

a [)l<.ci-ol linjuil llt'illt; iiitortiospil lo kocp toiK>( the

Ht(!tn from being ba>tterd. Unleoa Ibe atem-Boeketa H mot!)) worn, it is host tn tlrivc; L>iu stiriu flireutly into A liofiri. Wl m the! fit is a, Wl tini', n. piece of wxnvM-j of ihin-ehbct iron cut to fit thn socket exactly nroyl wmpped round the oncl of the Btom, hut it ia far to ilispeiise with fvnytliing of the kind. Tliu stm ifl d hoietod up. and the fihoe, with its wsilgeB lie<l on, j on the plank, llio stem and head togethor being t dropped over it ; by d ' the wiiole eoveral tim on tbo plfink, the i homo. As soon as thw

wedges are thon y the Ijftttery water, ihey

expand, and liohl IW- - firmly in its plane.

Tbo plank is then inA a block equal to thff

dusircd lonj!tb of d'-' each die. The tappet i3

then slipiwid over th its gib in plural nwl

allowod to elide (Sum . shea the point of the c*rii.

the latter being in its >Bitioii. The tappet i.e)'

are then driven well home, and the stamp ia allowcil '" rlrop gently a fow times till overy part has been forot into its place.

Haterial of Shoes and Dies. —This is now either hard oiist-iron or forged or cast steel. Tlie production of the two latter kinds is a rather special trade, wl it is rarely that any mill, however laie, would be i" a position to produce its own. Ijarge mills in remote districts, having a foundry attached, will therefore generally find it advisable to use cast-iron. The hutis of the shoe and die should be cast in heavy chills. |1)P thickness of which should not be less than two-thirds of the diameter of the butt, whilst the shank of the shoe and footplate of the die should be moulded in sand, thus keeping the material of the latter portions bb

Shoes And Dies 185

)ssible, whilst the former, if a good chillin<; iron be employed, will be hard and white t. There may be a small amount of mottlin<4 junction of the two grades of metal, but uld not be much, and a little will do no t-steel shoes of open-hearth steel containing 3r cent, of carbon have been largely used, but alts have been obtained of late years by the cial metals, such as manganese-and chromiumbh of the latter seem to be hard, tough materials, adapted for the purposes of the stamprged-steel shoes have been a good deal used, found to wear very irregularly, and were hence ago almost abandoned in favour of cast-steel ; recently Messrs, Fraser and Chalmers, Limited, .he way in introducing improvements in their ire which have again brought them to the front, t the position seems to be that special cast-steel certainly harder than even the best forged steel, lus worn away more slowly than the others, le other hand, are less uniform in structure and to show blow-holes, which may cause thoin to is very likely that there will always he more or fence of opinion as to tlie really best material and dies, but it may be taken as pretty certain e8t material, irrespective of cost, is either lirsted steel or else a high quality of special castthat there is not much to choose between them. ineers assert that the best arrangement is to t steel shoe with a cast-iron die, and that the ich is thus rendered more uniform, and it would t there are substantial reasons for supposing so. Probably a forged-steel die with a chrome-

i:i'in MiijjXG nm

or uiaiigaiiose-steal slip would I'orin a very patisfacloiv combinftUon. The conditions of wear of the sltoe ind die are elightly different, for whereas the faoe of the ilia ia always protected by a layer of quartz, the shoe, oa the other hand, is always pounding npon this auhsUnW- It has already been pointed out, page 11, that quarlv. ia necessarily haifler tijan steel ; henne abrasion of tbe latter must take place, and the consequent wear ol the shoe is greater than that of the die. This used to ba fat more marked in torn of working, when

was customary t laratively deep layer of

quartz over the fith the modem syatfi'"

of shallow stain ?. , may be said that about

twice as much mi i off the shoe as off the

die in the same tim

The life of the b1 depends upon so mftO?

different circumstar scarcely possible to

any accurate data; at fiisi-ii->n vM.ars orif

third to one-half as longaschroLnesteelin most instance' hut this has not been quite a universal experience. Tli* consumption of metal per ton of quartz crushed is best basis of comparison, since tlie length of time that shoe or die will wear of course depends upon its dimeU'' sions as well as on the quality of its material and als' upon the crushing capacity of the mill. Good cast-irof shoes will lose 0-4 to 1'5 lbs. of iron per ton of quart crushed, and good Steel 0*3 to 0-7 lb., but these figure are apt to vary very widely with the quality of atone tiJ be crushed and tbe degree of fineness to which it has to be reduced. Adding the amount lost similarly by the dies, the average loss of cast-iron may be put down as lo lbs. per ton, and of steel 05 lb. to 0'75 lb., as a rough estimate. In some recent comparative tests in South

ni SHOES AND DIES 187

Africa, it was found that good forged-steel shoes and dies lost 0*30 lb. and 0*21 lb. of metal respective} per ton I of ore, the corresponding figures for cast chrome-steel Joeing 0-29 lb. and 0*16 lb. A reduction of an inch in the length of the 8*-inch diameter shoe shown in Fig. 30 corresponds to a loss of weight of about 15 lbs., the total amount available for wear being 110 lbs. Every mill man ought to weigh his worn-out shoes and dies and to note carefully the dates on which he has to put in new ones, and thus determine exactly the consumption of Dietal per ton of quartz crushed. The best metal to employ depends upon local circumstances. A large mill, castinj its own shoes and dies, or a small one, in the ifiiniediate neighbourhood of foundries that will l)uy back ne worn-down shanks and foot-plates, will probably find 't more economical to use cast-iron, but a mill situated 'f a remote district where the high price of transport is 'emain factor in the cost of supplies, should use only "ebest steel that can be procured in spite of its higher fii'tcost. It is worth noting that a good mine smith make excellent quartz-breaking sledges from the ''Qanks of worn-out forged-steel shoes.

lifting Mechanism. — The portion of the mechanism of

lie battery by which the lifting is done consists of the

eaiii shaft, upon which are threaded the cams, together

vith the driving pulley or spur wheel ; and together with

liese may be considered the cam shaft bearing's. It may

noted that the old European system of a cam barrel

of large diameter with short projecting cams has been

utterly abandoned.

The Cam Shaft is usually made sufficiently long to drive the stamps of two batteries, though sometimes a short Cam shaft driving only one battery is employed. The

i88 GOLD AtlLUNG cii

advantage of the latter method is that each battery c be stopped independently of the others, in case repa of any kind are required; its disadvantage is that entails the expense of a double number of drivers, that takes up rather more room, and necessitates more wearii parts. It can only be recommended for a small mill less than twenty heads of stamps. The cam shaft h to be both strong and stiff ; it must be able to transn the full power required to work the battery without ai perceptible deformation due to torsion, and must be ab to support the weight of all the stamps without bendi: or springing in the least. Theory and practice both ooi hi no to point out that a cam shaft for ten heads 1)00 II). stamps should not he less than 5 inches in diamet and is best made inches ; if working only five heac it need not exceed 41 inches in diameter. For heavi stamps (over 1100 Ihs.), these figures are best increas to between G and G.l inches for a ten-head, and 5J 0 inclies for a five-head cam shaft. The best mater for tlic cam shaft is undoubtedly cast steel, preferal open-hearth. Wrought-iron may also be used, and that case it must he the hest hammered scrap ; steel however almost always used for it. It should be ca fully turned to gauge, and if a large mill is being buil standard pair of gauges of the exact diameter of I finished shafts should first be made and kept in I machine shop, as all the cams will have to be bored most accurately to this size.

Drivers. — In the usual American method of mill slrnc'iion, the cam shaft is driven by pulleys and belt off the main lay shaft of the mill. Most Austral; fMiginecrs seem to prefer to place a heavy spur wheel the cam shaft, and to drive by means of pinions on

Pulleys 1S9

lav shaft, which must then he close to the cam shaft, system has its own advantages ; whilst, on the one d, it is at times awkward to get the shafts near <:tough together for the use of gearing, it is also at times difficult to place them far enough apart for the proper of belting. The cost of maintenance of belting is considerably higher than that of gearing, but on the other hand, should any accident happen to stop the stamps suddenly, the belt will slip and injure nothing, whilst m the case of gearing, either the teeth of the latter or else the motor driving the mill may be seriously oamaged ; this last objection may be partly overcome by 4e use of friction gearing or friction clutches.

There is thus no decided advantage in either method ; Ine adoption of one or the other will depend to a groat extent upon the character of the mill framing and the disposition of the mill as necessitated by the i)hysical 'matures of the mill site. Upon the whole, however, niost engineers seem to inchne to the American method pulleys, without perhaps any very sufficient reason.

ulkys. — As there is no outside bearing to a cam haft, the cam pulley overhangs its bearing, and it is uorefore important that it be made as light as possible, insistent with proper strength. The usual si/e for 'hese pulleys is from 4 ft. 6 in. to 7 feet in diameter, and he width of the pulley-face will usually be between 18 inches for a heavy ten-head battery, and 0 inches for a ht five-head one.

Cast-iron pulleys are not suitable for this work ; they very heavy, and the arms, unless exceptionally strong, are apt to be cracked by the vibration of the 'uaciiine. Wrought-iron pulleys with iron or steel arms also unsuitable, because tlie arms are frecjiiently

Gold Milling

Chap.

Scale

Side Elevation

foot

Fic. 3::,

md to break for the same reaBon. If for any cause it coiieidered advisable to use a wrought iron puUej the us should be made of Bpeoially low carbon steel so as be as soft as possible and they should be shrouded

either side up to the nm with 1 incli planking com itti\y filling up the pulley so as to lessen the vibration

. Diuch as passible. Fullejs , ,

lus protected have been run iccessfully. The best class of uUt;y,however,i8 the American uilt-up wooden pulley on a ast-iron boss. This is shown iKletail in Figs. 32 to 34

Tim wood should be well *Miit(l red deal, or prefer ably [litch pine; all joints should be tarred, and the lilaoks of which it is built care 'silly spiked and bolted together

Tliu cust-ii'on boss is in two I'iects. one forming the sleeve bicb keys on to the cam shaft, ilie two discs, one loose and uiL' forming an integral portio

''f the sleeve, being drawn to- ' — ilyX 1"

ii;lbcr by numerous bolts so aa

hold the woo<len plates securely, Thesu pulkys are 'imply first put together and keyed into place, and tlicir i'lis are then finally turned up on the shaft itself to ensure iair running jwrfectly true. Belts grip well on the wooden rim, and the oonii>osite wooden jjuUoy is in every 'ay an excellent and a duriible one, if care is taken to ivc it an occasional coat of paint. The only objection to

Section

Gold Milunc

it is iLs groftt weiyhi illustrated in Pifja. i m cwt. 2 t]ra., out two parts of the apectivelyu cwt. 3q the woodworlc wei 3 qrs.

Gearing. — Whenj it usually takes tl spur wlieel 3 ft. G ii in diameter on tl driven by a pinion to oue half its dit lay Hhaft. It is able that both these be made oE cast-stc ease the pinion sh if the wheel is ma< for the sake of ei anus should be str withstand the jar as well as the direc transniifsion of th rims of both whei shrouded as far circles; the teeth s very etrong, and she be helical V teeth ones, so that thei be quite continuous Insh when thrown gear ia avoided. generally operated

, usual kind, but friction olutoheEl can be

Outage. Too mach care c&noot be be-

Side View. Stale i)4'"i' Front View.

iA on the material and finish of tlie Hpur wiieeln, t is best that they should bo niacin no-ntoulctcd and lily worked up to their ti'uo shape so Q.n to run with

a mintmiDn of lubrication, the necessity for the e

meat of which is one of the ahjections to spur ge

The Cam. — Gams have been made one-two- and

armed, but the two-armed cam is practically tb one ill use now. The patterns generally adopted fi iron aud for steel cams are shown id Figs. 3fi i

m CAAfS 195

tespectively, the departures from these being only slight. Cams may be either right-or left-handed. A right-handed eun 18 one that runs on the right-hand side of the stamp when the observer is looking in the direction in which the vpper arm of the cam is revolving ; hence the boss or bub of the cam is also on the right-hand side, whilst in a leftAianded cam it is on the left-hand side when looking in the direction of revolution. Views of right-and lefthanded cams are shown respectively in Figs. 37 and 38.

Cams are made either of cast-iron or steel. If cast-lion

be used it should be of the very best quality, a tough

ifon.low in phosphorus and of fine uniform grain being

selected; the boss is often strengthened by an iron

koop, as shown in Fig. 36. which represents the usual

shape and dimensions of a cast-iron cam. The proper

material for the cam is, however, cast steel, good

open-hearth steel containing about 0*4 per cent, of car-being

a very satisfactory material for the purpose.

Chrome steel has also been used for cams, but is no

better than ordinary cast steel for this purpose. The

elay occasioned by the breaking of a cam is so great,

that every possible precaution should be taken to prevent

such an accident, which is about the most troublesome

one that can happen in a mill. In a very large mill it

niay be possible to keep a spare cam shaft ready fitted

th cams to replace a cam shaft upon which one of the

cams may have been broken, but this is not customary,

anl, in moderate-sized mills, scarcely possible. When a

cam breaks, all the stamps run by the shaft upon which

IS keyed have to be hung up, and the shaft with its

cam and pulley lifted out of its place by heavy tackle,

which is no easy matter, seeing that a cam shaft completu

o '2

P/o. 37.

Scale I'U,'

io. aa.

CHAP, vn CAMS 107

th cams and pulley for a ten-head mill weighs about two tons. The cams have then to be removed until the broken one can be got at ; meanwhile the necessary keyseats will have been cut in a square cam to replace the broken one. The cams have then to be keyed up in their pbuies again, and the shaft hoisted up into its bearings. This work usually takes the best part of a day, even in veil-fitted mills. At one time an attempt was made to construct cams in two halves held together by strong 'tiiTaps and wedges. It was found, however, that more titmble was entailed by the getting loose and slipping of these sectional cams than was compensated for by the comparative facility of their renewal ; hence this system never came into favour, and, with the great strides made recent years towards the perfection of steel ca.stinjs, "isno\v Ijeen entirely abandoned. Every well-equipped "ill should have cast-steel cams, even thouli iron be "sed for every other part. Steel cams and cast-iron work very smoothly and wear well together.

A cast-iron cam needs also to be very much heavier

han does a steel one. The strengthening rib of the arm

""1st he deei>er and thicker, and the hub also must be

"Miirnjer size. The diameter of a cast-iron hub must ho

'I'lite twice that of the cam shaft, whilst in the case of

J?f>od steel casting IJ times the diameter is sufticicnt,

althougli it is usually made a trifle more. In each case

width of the projecting portion of the boss is made

'>out equal to that of the working face of the cam, the

letter being generally from 2 to 3 inches. The face of

ho cam which comes next to its stamp stem should be

carefully planed so as to be quite true and accurately at

Jght angles to the bore of the cam ; this precaution is

often neglected, in spite of its importance. If the cam is

thus planed, it can safely be set not more than inok away from the stamp stem, and the friction caused by the' one-sidedness of the lift is thus reduoed to a miniiiniiD The cam must be carefully bored out to a " drivuig fit*" on the cam shaft ; as already recommended, a gauge of the exact size should be prepared for this work.

Cams should be fitted with two keys each, the key-way* being 120*" apart ; some makers use only one key, but two keys are always advisable for stamps of over 700 lbs. any rate. The keys should be of ordinary dimensionSi have a considerable taper, and of course be driven towardc the stamp stem and not from it. The whole tendency o the reaction between the stamp tappet and cam is tx force the cam away from the centre of the stamp stenn and this tendency needs, of course, to be guarded against

Within the last few years several methods of attaching the cam to the shaft without the use of key-ways hav*' been introduced. As a key will only hold the cam-bo&J tightly when driven into it longitudinally, it is necessary that each keyway should be at least twice the lengthen the key, and almost the only method of attaining thi - object satisfactorily is by slotting out a continuous ke)' way for the full length of the cam shaft. It will be seec presently that each cam must be placed at a definite angl to all the others, so that, the position of the key-way in the shaft being fixed, each cam requires to have its key-way cut in the exact position to give this definite angle. This difliculty, which renders a cam once slotted only suitable for one particular place, has been very neatly overconu by the Blanton cam, shown in Fig. 39. All the cams an machined by the maker with a wede-shaped recess as shown ; into this fits loosely a piece of the shape showi in the figure, which is practically a wedge curved to th(

Cams

ndins of the cam shaft, and these wedges are suitably distributed along the shaft, and held in their proper relative positions by means of a couple of small set-screws. Each cam can be slid along the cam shaft and slips easily over the wedge; as soon, however, as the shaft is rotated in the proper direction for driving the mill, the weight of the stamps tightens the cams upon their wedges and holds them firmly in place. When a cam breaks and it is

Scale. I inch I f<x>t. Fui. .'I'J

required to take the cams off the shaft, they can bo at once released by a smart blow of a hanimor on the under side of the cams, so as to turn them a little in the opposite direction. Obviously, therefore, the laborious operation 'f renewing a cam is immensely facilitateil, and loss of valuable time is prevented. It would seem at first si'ht as though the shearing strain on the set -screws nuit he enormous ; but in fact the part that these screws really play is merely to keep the wedge from shiftinj until the

cam has gripped it, the arrangement acting from thift moment as a true friction grip, which bites the haidor the greater the strain to be resisted. The Blanton earn ! has proved quite successful in practice, and may be lookai . upon as one of the leading recent improvements of tha ; stamp-mill ; it is made by Messrs. Fraser and Chalmeiftt Limited. Somewhat similar devices are the Davis patent self-tightening cam-fastening, made at Denver, Coloiad(i ; which consists of a short feather, oval in cross-seotioOi which drops into a shallow depression in the shaft, whilst the cam slips over it and tightens against it when the mill is running ; and the cam-fastening introduced by the Humboldt Engineering Company of Cologne, Germsnyi which similarly consists of a short feather, parallel lontudiiially, but wedged-shaped in cross-section, the action of which is quite similar. It is obvious that these two last-named fastenings are applications of the same principle as tlic Blanton grip, but are mechanically inferior; neither of them has yet been employed to any extent. In the most recent modification of the Blanton grip the cam shaft is machined so as to have ten curved taper faces running along its full length ; the cams are correspondingly bored out, so as to slide on to the shaft quite easily ; they are then tightened up with one or two blows of a hannner, and are kept tight by the weight of the stamps as in the older pattern. This new pattern allows any cam to be placed in any required position on the shaft witliout the least fitting, and is a very decided improvement.

When the cams are properly secured in their places so as to be unable to shift laterally — a matter of great importance — the reaction between the tappet and the cam tends to displace the entire cam shaft longitudinally

Vii Cams 20I

in a direction away from the stamp stem towards the tespective cam. Of course this tendency to shifting must be resisted, but it should be clearly borne in mind that there is a tendency to move in this one direction only, so that, if the cam shaft is fitted with collars, only one is needed to prevent the cam shaft shifting laterally away bom the stamps. This tendency can be — and is by lUgood makers — entirely overcome when ten heads of stamps are worked by one cam shaft, by making the cams which drive one set of stamps right-handed and the others kft-handed. When only five stamps are worked by one shaft, this lateral thrust can be overcome by having two cams of one kind and three of the other ; but more usually they are all five either right-or left-handed, the fe\injjj wheel being then keyed on tlie end of the shaft iway from the cam hubs, c?.[jf., if the cams are left-handed the pulley or spur wheel should be on the right-hand end o' the cam shaft, so that its turned boss may work against the turned end of the cam shaft bearing. A cam known Is the " Bally cam was patented with the object of ovorniing this lateral thrust, so arranged that instead of the two arms of each cam working, as is usual, the same stem, they worked adjoining ones. It did certainly prevent wral thrust ; but, as I have already shown, this can h*? (lone in other ways, whilst this "Bally" cam had the disadvantage that it did not continuously rotate the *tainp stem, but simply turned it backwards and forwards through a certain arc, and thus tended to wear the shoes fid dies unequally. It has never really come into use. Another device is that known as the "Harf cam, in which each cam arm consists of two limbs, one of which works on either side of the stem so as to equalise side thrust, whilst, in order to enable it to revolve thostamponc

limb is made slighUy longer than the other. Thissysten seems as unlikely to be adopted as the preceding one.

Cam Cnrre. — The proper corvatore to be given to tbej working face of the cam is a matter of paramount ifrj portance. Fortunately the setting out of this curve is very easy matter. I shall here only enter into the tedh nical portion of the subject, leaving the purely geometrieil part to be dealt with in a separate appendix (page 565) where proofs will' be found of the assertions here advanced. The object of the cam is to convert the unifonD rotary motion of the cam s!iaft into an upward moiiott of the stamp stem, such that the rate of lifting bhsD bo uniform, the action being intermittent, so that tinM is given it to admit of its falling freely with uniformly accelerated velocity under the action of gravity. The curve which will convert uniform rotary motion into a uuifomi lift is one of the involutes to a circle, the radios of this circle being equal to the horizontal distance between the axes of the cam shaft and the stamp stem. It is a property of this involute that the lengths of the arc of ihis circle traversed by the rotating cam in a given time shall he equal to the amount of vertical lift during the same time. Or, if the amount of lift be called h and the radius of the generating circle r, both in inches, the cam moving through a" during the lift /i, then, in the case of the two-armed cam, wliich will alone be considered here —

Trra

h

180 /i Tra

Cam Curve 203

*hese equations provide the means of connecting se three important factors. As a general rule, r is 3d by the conditions of construction of the mill ; it is lal to the sum of the radii of the stamp stem and m shaft plus a small amount of from inch to inch r clearance. The value of r being thus fixed, the igolar motion of the cam corresponding to a lift of

inch will always be r- or . From these data it

perfectly easy to set out the cam curve. Describe a rcle, Fig. 40, with centre C and radius r inches. Draw ny radius CO of the circle, and then set off successive

1 Ua°

idii at angles equal to apart, CI, Ct2, C'3, kc. ;

tthe end of each of these radii set off tangents, making / equal to 1 inch, 2// equal to 2 inches, 3/// equal to

inches, and so on. The curve joining the points 0, /, hUI, &c., will be the curve required. It is clear that le lowest possible point to which tlic tappet can descend I practice is fixed by the radius of the hub of the cam, nee the tappet must never be allowed to strike the ttter. Call this radius then the hnvest possible osition of the tappet will be at a point situated at

height k above the horizontal through 6', whilst its 'Kiest [Kjsition will be k-\-li above it. A length of caniirface corresi>onding to a lift of k-\-h must therefore be tout; the only portion of this curve that ever conies 'to action is that corresponding to h, the portion of the tof the curve corresponding to k being never re(juired ; ' fact, this portion may be given any desired shape proonly that it is so designed that no portion of it can me into contact with the tappet. It is a property of 113 involute that the tangent to it will always be hori-

Gold Milling

zontal, when the line of lift is ▼ertitwl. Hence, by ado[ ing this shape, the whole of the rotative force of the

is converted into vertical upthrust (disregarding friction for the time being). If the cam were to be made a tni6

m CAM CURVE 205

involate throughout the portion of the curve corresponding to ft, the stamp would lifted with uniform velocity, md on leaving the cam it would still be moving upwards with the same velocity, so that the lift actually given would be really somewhat greater than that due to the cam itself. To obviate this, so nmch of the cam curve IB corresponds to the top quarter of an inch of lift is ihaved off, and the curve so modified at the point is the carve actually used for the cam ; the motion of the stamp produced by the curve so modified is practically the same would be produced theoretically by a point moving on t true involute, and may be so regarded.

As an illustration the cam curve required for a 900 lb. stamp of very customary dimensions is here (Fig. 41) wn on a scale of 3 inches to 1 foot ; the lift is taken 7 inches ; the stamp stem is inches in diameter, ind the cam shaft 5 inches ; then, allowing inch for clearance —

Angle corresponding to 1 inch of lift -,.,„r

It is assumed that the cam is to be of steel, having a 9 inches diameter, the value of h being accordingly inches. The method of setting out the curve is as follows : Describe the generating circle with centre C 'd radius r-4"-375. Draw the horizontal diameter and set out from A the perpendicular tangent AD. this set off a height jB A; 4"*5, and a height The horizontal line drawn through B is then evidently the lowest possible position of the tappet,

and that Ihrough D is the highest. From CA anj' suooewiTo nulii CI, (72, C3, . . . CI, ouch at an a 18%' to the one next heforo it, thaC is to Hay the ACl, \C% 2C3, . . . 6C7, muBteM be made eq

D

s

13°6'. Then draw tangents 1/, 2//, 3///, . . .IVl each of the points 1,2, 3. .. 7, equal to — . , . fc + ;i-A( ft), that is to say to 10"-6,9"-S, 8' i"-5 respectively. Join the points D, I, II, . . . V, and from VII continue the curve in any form thi

I CAM CURVE TCfj

yn the desired shape for the cam arm, so as to combine (officlent strength with lightness till it reaches the boss, fake a point a quarter of an inch below D, and from that dnw iu the curve by hand as shown until the curve of the finished cam represented by the black line is obtained, tke dotted line showing the true involute.

The construction of one cam arm only is here shown, the other being quite symmetrical to it. There are various other ways of drawing the involute to a circle, but the one here given is the easiest and most accurate. If preferred, points may be obtained half an inch instead of &D inch apart by making each angle one-half of the above

amount, namely, — , and by decreasing the successive

tangents by half an inch instead of by one inch.

In practice the curve should be thus set out in full size on a sheet of stout drawing paper, pasted on a piece of well-planed board, in which the curve can be accurately cut out when it has been drawn, and metal templates then made from it.

Care must be taken that the point of the cam as seen in plan is cut diagonally to a curve which must be an of the circumference of the tappet, so as to allow the hole tappet to clear the cam and to fall at once, imrne- <lately the cam has passed the centre line of the stamp. fillet thus formed is well seen in Fig. 37.

it is obvious that the above equation, h indieates

the limit of the amount of lift by a two-banned cam

for any given radius. However slowly the cam be made

to revolve, it is impossible for a over to be quite equal

to 180', seeing that the stamp must take some definite

time ill falling, and that the angular speed con espondin

2o8 GOLD MiLUNG ciur.j

to this time most always be something however therefore the maximam possible value of A is less nT. When hier lifts are required, a single-armecl must be used; in this case become 360 and limiting value of h becomes 2irr. Whenever possil single-arined cams are avoided, because they limit speed of the mill and moreover cause greater loss power than do the double-armed cams, seeing that cam shaft has to revolve twice as fast and that a cam-surface is needed for the same speed and lift ; over, the cam shaft is not so well balanced. In the ran cases when it is necessary to produce a lift still gresttf than this, some form of curve other than the normal involute would have to be used, or else r would have to be increased by modifying the arrangement of the mill. Arrangement of Stamps. — It has hitherto been taken for granted that five stamps always compose one batterfi and five is indeed the customary number in a very giea majority of instances, although batteries of from two to six stamps, and occasionally even more, have been built* It cannot be proved mathematically that five is the ab solutely best possible number, but it will be seen from the following considerations that it is the most convenient. Less than three stamps give a very small mortar box, and entail a large number of wearing parts, besides multiplying battery frames, cam shafts, pulleys, &a, whilst more than six stamps would require an inconveniently large and heavy mortar. The end stamps of a battery are always less effective than the central ones, for the reason that the former have a screen surface on either side of them, and the latter on one side only ; for this reason batteries of two or three heads are proportionately less effective than the larger ones. On the other hand, it

Order Of Falling 209

ist as bad to have too many as too few stamps worked the same cam shaft, as any injury to one head may Lse the stoppage of all the stamps on that shaft. It is ind that an odd number of stamps works better than an en number in the mortar, giving a more uniform wash

the pulp and providing a central stamp which will srve to control the action of the battery. Five has ocordingly been fixed upon from the result of long ex perinee as the most convenient number of stamps to work none battery, small portable and prospecting mills having isoally three stamps ; the five-stamp battery is accordingly looked upon as the standard size.

It is not by any means a matter of indifference in what order the stamps of a battery are allowed to fall; the main objects to be attained are uniform working, which shall not admit of the accumulation of crushed qu.irtz in either end of the mortar box, and a steady uniform wash lkwards and forwards of the pulp within the battery ith as little splashing as possible. Two general principles have been enunciated by mill men, compliance with either of which gives satisfactory results, although these prinples are to some extent antagonistic ones. These are : (1) that neighbouring stamps shall never b3 allowed to 'all in succession, and (2) that while any given stamp is Wling, its neighbours shall be rising. Nunihering the stamps successively 1, 2, 3, 4, 5, beginning at the driving -ndof the shaft the first princii)le is complied with by making the succession of falling 1, 4, 2, 5, 3, or I, 3, 0, 4, which two orders are, it will be seen, the same expt that one runs first from the centre towards tlu; *ii;lit hand, and the other from the centre towards the eft hand. Compliance with the second principle demands he orders 1, 5, 2, 4, 3, and 1, 4, 2, 3, 5, which bear to

2Io Gold Millihg

each other the same relation as do the first pair, or other of these four systems is now usoally ado] but the orders 1, 5, 3, 4, 2 and 1, 8, 3, 4, 5, are sometimes used. These latter are not to be reoommeni but any of the foor first named give about equally results and can be used with confidence.

The action of the stamp is twofold — namely, ci the ore in the first place, and afterwards expelling pulp, which consists of crushed ore suspended in wi through the screen apertures by its piston-like As regards crushing power, all the stamps in a batteiy] box are equally effective, but as regards their expulsivs' effect the end stamps do a lesser duty than the central ones, because SJrt of tlpir propulsive power is wasted against the dead ends of the box. There is accordingly a tendency for crushed stone to accumulate under the two end stamps, and if the mortar box and the line of discharge are not accurately horizontal, there will be a tendency for it to accumulate under one or other of the end stamps only. This latter defect can evidently be obviated by taking great care that the horizontality of all parts of the mortar is rigorously maintained, and the former is usually met by giving the end stamps a slightly longer drop (say inch to inch) than the middle ones. Inattention to the due regulation of the water supply is also a frequent cause of similar irregularities in the working of the mill. When a cam shaft works ten stamps, corresponding stamps of the two batteries should fail successively. Thus, if the second-named order be adopted, the order for ten stamps will be 1, 6, 6, 10, 8, 7, 4, 9, 3, 1, and so on for the other orders. The cams must be set so as to produce the desired order of succession, their key-ways being suitably cut by the makers,

Order Of Falling 211

in case of Blanton cams, the holes for the set-screws

lied in their proper places on the shaft. Care

st be taken that their distribution round the cam

it shall be quite uniform, and this is easily done by

king the angle between each two successive (not

ghboaring) cams equal to IBCT divided by the number

cams on the cam shaft. This formula applies only to

o-armed cams, since these give one complete drop for

angular motion of the cam shaft of 180°. Thus, if

ere be ten such cams on one shaft, the angular distance

180° tween successive cams must be ,a- When

are cams are supplied, their key-ways are never cut, as is obviously quite impossible to say which cam is tely to be the first to require replacing. In a bi mill lare cams are key-seated as required by means of a rotting machine, but in smaller establishments they ive to be cut by hand by means of strong flof:;ging lisels driven by a heavy hammer, a reamer being iually driven down afterwards to secure accuracy of fit. : is a good plan to have at hand a block consisting of a lort section of cast-iron or steel of the same depth as le cam-boss, accurately turned to the same gauge as the shaft, and slotted to correspond accurately to the ay-ways of the shaft. This greatly facilitates fitting the sys. The position of one of the key-seats of any one ven cam ought to be accurately known by its distance om the line joining the points of the cam arms, and lowing this, it is easy to calculate the angular distance om it of the key-seats of any of the other cams ; hence ( soon as a breakage occurs, the engineer in charge can : once commence cutting the key-seats in a spare cam , as to get it ready for replacing the broken one. This

p

Gold Milling

Chai

work is, of course, all obviated by using cams on th( Blantou principle.

The rotative power of the cam shaft is exeiiied in various ways besides its normal application to lifting thfl stamp vertically. In making this vertical lift it has not only to overcome the action of gravity but also that <rf

the friction of the stamp in its guides. During the lift the stamp is suspended upon the arm of the cam, and as the point of suspension is to one side of the axiil line of the stamp in which the centre of gravity of the latter lies, there is a tendency on the part of the stamp to assume such an inclined position as will bring its centre of gravity vertically beneath its point of suspension, as shown diagram matically io Fig. 42, where this action is of course greatly exaggerated by exaggerating the distance between the cam and the standi stem. In this diagram G is the cam in cross section, r is the tappet, and Lr the centr< of gravity of the stem, which assumes position in the vertical line XG as shown. This action causes a lateral thrust oi the guides, and at the same time caused a lateral reaction between the tappet an the cam, tending to force the latter awa from the stamp. It can be diminished by bringing th cam as close as possible to the axis of the stamp and ieniovin<; the centre of gravity of the stamp as low down as i)ossiblo, which ellect will be obtained by increasing the weight of the head relatively to that of the other portions of the stamp.

Fi';. 4_'.

Ii Rota Tion Of Stamp 2 1 3

The cam also exerts an action on the tappet that tends to revolve the entire stamp round its axis in the direction in which the cam itself is moving, owing to the friction between the cam and the tappet. The amount of this friction depends upon the velocity of the cam, the weight of the stamp, and principally on the nature and lubrica- [ tion of the two surfaces in contact. Since the revolutions of the tappet and cam take place round axes perpcndicTilar to each other, whilst the contact of the revolving nrCaces takes place in a straight line parallel to the axis of revolution of the cam, it follows that every point on this line of contact on the cam surface is moving at a uniform rate, whilst on the tappet surface every point is moving with a varying velocity, depending on the distance of the point from the axis of revolution. There must therefore be rubbing and not rolling friction tween these surfaces, and lubricants must be employed to diminish this friction as far as possible. The net Jesuit of the friction is, as already statecl, to rotate the entire stamp about its axis, and this circular motion has the great advantage that the wearing surfaces, particularly those of the shoe and die, are affected uniformly, thus contributing greatly to the regularity of working of [ the entire machine. Much nonsense has been written to the eflfect that this whirling movement has a grinding action on the quartz between the stamp and die, but no one who has watched a stamp-mill closely will need to be told that this is not the case. The rotation of the stamp takes place during the lift, and continues very slightly luring the commencement of the descent, being rapidly neutraUsed by the friction of the stamp in the guides, so that it has entirely ceased by the time the shoe strikes the quartz. It is easy enough to prove this by taking a

GOl.n MIUJNC es

dingram by simply holding a piece of chalk stead against a etamp stem whilst working. It will Iheii seen tliat tlm chalk traces a sleep spiral iluring t MOQQt of the stamp, but that during the descent makes an almost vertical line. In practice the stan should never be allowed to rotate through more than 3 at each stroke, so that it should make one comple revolution in never less than twelve drops. If it revol'i faster than this, it is merely wasting power that coul<i I better employed in driving the mill, as the above spec of rotation is ample to secure uniformity of wear.

Lubrication. In order to economise power and I protf-L't the cam anil tqipot tucr's from unilin.' wi-ar, il necessary that they should be thoroughly lubricated, so de&ciency in this respect at once making itself known b the stamps spinning round too rapidly. Many differa lubricants have been used for this purpose. Soma a) anti-friction grease, others merely tallow; a compoi tion of tar, beeswax, and resin, boiled together, is som times used. Some makers of mining machinery sell special composition of this character as a cam lubricai It is best, however, to use some material quite fi from grease, so that if any of it finds its way into t mi>rtar box or on to the plates, the process of ami gamation is not interfered with. Common moUsE may be and often are used, but the best lubricant tor t cam is soft soap thickened with a small quantity finely-ground graphite. A pot of this mixture should kept on the working platform, and whenever a stai shows the least tendency towards too rapid revolutic a little of it should be nibbed on the face of the cam means of a stick 1 ept for the purpose. Many mill m fasten strips of canvas or oilcloth (cut so as to allow t

rii LUBRICATION 215

tamp steins to pass through) balow the lower guides, for the purpose of catching any portions of the lubricant that may become detached and which mighfc fall into the mortar were it not for this shield.

Height of Drop. — The question as to what is the riglit length of drop to be given to the stamps, and the closely oonnected one as to what is the best speed at which to nin, have of recent years undergone a marked modification with the introduction of heavier stamps. There is A pretty general consensus of opinion that heavy stamps, high speeds, and short drops are the conditions that produce the most economical results in crushing, although in some districts, t.g, in Colorado, a different practice prevails owing to reasons that will appear later on (see page 467).

With 900 lb. stamps a six-inch blow is sufficient to crush the firmest piece of quartz completely, and in order to get the maximum effect out of a mill it must, of course, be run at the highest possible speed. This luaximum speed depends upon the height of drop and the distance between the axes of the stamp stem and cam shaft.

It has already been stated that this distance, the height of hft and the angular movement of the cam are

connected by the equation h

Let the cam be so constructed as to give "N drops P'r second ; the cam shaft will then be making 30xV revolutions per minute. Also —

Time of one complete drop + lift in seconds ,.

A

Now let li be the heiiht of the lift ( '-V and

U6 GOLD Ml LUNG CJ

the distance between the centres of the stai

stem and cam shaft, both being expressed in feet, we suppose that the stamp has no interval of rest, th will the time of one drop + time of one lift oocnpy o semi-revolution of the cam shaft ; obviously this will the maximum speed at which the stamp can be drive Now the time occupied by a body in falling in vac

from the height H is / seconds, wlere g is i

accelerating action of gravity (about 32-2 feet per second

It follows from the properties of the involute that i

portion of a semi -revolution of the cam shaft occupied

lifting the tapet is - . The time occupied by the 1 is accordingly ,. ' and

2/i

which formula gives the maximum possible number drops per minute for a given construction of battery ai given depth of drop, supposing the stamp to be worki in vacuo without friction, and to have no interval of k at all. Friction must, however, come into play, and ev the latter condition can never be realised in practice, the stamp must always be allowed to remain at rest 1 a certain time, because in actual work the stamp, aft it strikes the die, rebounds slightly, and then falls agai If the cam were to meet the tappet before the stamp h come to rest for the second time, the shock would

vn HEIGHT OF DROP 1 1 7

violent enough to do serious damage, most probably to break off the arm of the cam. Such a collision, generally ipoken of as " camming/' must accordingly never be risked. The minimum interval of rest which can safely be iQowed in a stamp-mill is one-tenth of second. When this interval of rest is adopted, the above formula would heoome modified thus : —

When r=4-375 inches and 7 inches (i? 0'365 ft., fl= 0*583 ft.), the above formula makes N equal to 1*69, w the maximum possible number of drops per minute ould be 1-69 x 60= 101, still neglecting friction.

1 have quite recently shown what is the actual rate of falling of an ordinary stamp under normal conditions, and that for all practical purposes it can be expressed

substituting in the theoretical equation / , a

<fficient / for g, the mean value of / being 27*5, so that v// 5*24. By substituting this value for Jg in the above equations, a close approximation to practical will be obtained. For example, if the last uation be written thus : —

*nd the same values be again assumed for r and h, will be found to be equal to 1*00, or the maximum DQuiberof drops per nainute would he 9G. In practically

SOimiy- Mill Indicalar Diagrams, Amor. Inst. MIn. Eng., 1898.

COLD MJLUffG

xvsaia"z i -: inpmill, the dimensions of wtioh nspo' i 1 1 r , closely wilh the figures here aasum found '.:iaL L.iiiituiiig juU kvoidud at a a little above 95 drops per minnto.

In practict ihe be)ht of drop is nsiully between 5 IS iochcs, &nd the foUoKiiig Uhlo shows the len time occupied respectively by a body falling in in and by a stamp falling in normal miU practice from raiiotia heights between these extremes, in decimala second: —

Amml

O-inMMnd

e-iM „

tt9M ,.

0-U3 „

t'tti ;:

W ".

Cnuhin; ?oweT. — Disregarding friction fortbemom the power required to lift a stamp of weigbt IP thro the height H is WU foot-pounds.

The crushing effect of a stamp appears to depend mt upon its momentum, this being the product of its i into its velocity.

The momentum of a stamp of weight W, acquire) falling from a height R, is TF /2(f H, when frictic disregarded, or W J'i/H, allowing for friction. Tl fore the power required to lift a stamp varies direct! the height to which it is lifted, whilst the effective I developed by it varies only as the square root cf

Ii Cam Shaft Bearings 219

leight. The maximum effect is accordingly obtained

Erom a given number of foot-pounds employed as a lifting

ler, when W is a maximum and // is a minimum ;

m other words, the most economical way of employing

inver in a stamp-mill is by making the weight of the

sUmp as great and the height of the drop as small as is

OQQsistent with convenience in practice, provided that the

momentum is still sufficient to both shatter the rock and to

propel the shattered particles through the screen. This

is also evident from a comparison of the theoretical and

ictaal velocities of falling, the difference between these,

or in other words the momentum absorbed by friction,

being greater for long drops than for short ones.

furthermore, a short drop allows the number of di'ops

per minute to be increased, and such increase in the rate

falling increases greatly the efficiency of the mill.

Experiments by Messrs. MorisonandBremner have shown

tthe rate of crushing is practically proportional to the

pfeed, the rate of crushing increasing, however, a little

iJore rapidly than the speed when the number of drops

'8 between about 75 and say 110 per minute. Their experi-

"ents also show that the crushing power increases more

'apidly than the weights of the stamps up to a limit of

bout 1,400 lbs. It is worthy of note that these are the

practical conclusions to which modern exi)erience is

feidedly tending ; stamps of 1,200 lbs. and 1,1:50 lbs.

tight are coming into use ; a 1,400 lb. stamp was even

ried in California, but was not found satisfactory.

Cam Shaft Bearings. — The proper shape of the cam shaft bearings defends upon that of the mill frame and iJ the direction and manner in which power is transmitted Motile shaft. A usual form is shown in Figs. 43 to 45, hieh will be fomid thorou{hly satisfactory in practice.

GOLD MtLUNG

Cam Shaft Bearings

cam shaft be driven by a belt, the pull of which inwards, or at any rate not decidedly upwards, as atly the case, the lower bearing will take the whole s work, as it is obvious that the entire weight of the shaft tcether with that of the stamps acts vertically Lwards. Many makers, in fact, dispense with a cap lather, and prefer to let the shaft run in the bearing . its upper half quite unprotected, on the ground that easier to keep the shaft clean and in good running sr and thoroughly lubricated than when it is covered

n

I I

-r

Seal*. I'i' Transverse Section, Y\o. 44.

Plan

Against this view it may be said that there is ususome quartzose dust flying about a stanip-mill, which Qld be apt to work into and cut the uncovered part of shaft journal ; nevertheless open bearings are Uirely d and are found quite satisfactory under ordinary conons. It is at any rate certain that, when the hear- s have caps, these should be, though liht, very 1 fitting and supplied with good automatic needle ricators ; the bolts that hold the cap ncnul only he It ones. Diagonal bearings are sometimes used ; ir employment is only permissible when the belt

;-- .is :o press tlie shaft .-1;: ji tti& hK 10 take any portion

TSf MQ 3BUil Ssr acml cam shaft bearings NC£(n :m!al : hms is :srtnriiai.in used, but is less suit- F*Aai; <teig-acit WqD aaay be meJ where economy

rbkf r:ik>* Tffwfnsxtt cf a earn shaft is, of oooebb,

boc iw vwsfi ihe shaft with its osmSi

- Jiiiiiitiv kiftS cc sbf sampg, is apt to canae ft

vl iikrtC . c frsccctt. sskI fstas aieiiuon should scoord-

- S: :ra.d lae itfecwMkw. All cam shaft beariiigi :-v ?ibs -j btMkTJiic jO a? hkiAi a good deal of oil, XMciksc .rr ib MIS 3tiSiJb cf haK-iroQ and oompietdy -:v;: . ,-Va:' .'c: : ir.y-par.> ai seated times, at

Fvjwr &xjrad — jVAjr icccevi :o ihe cam shaft

ca:: fricnioa of :c vfctr shaft legends oc the weight

m POWER ABSORBED 223

Bonnsts of the weight of the shaft itself, the weigbt of Bie driving pulley or wheel, the weight of the cams, and fhe weight of the total number of stamps continuously being Ufted at the same time. To calculate the number ot stamps being supported at any one time, the formula

— 180P has again to be applied. From this it is

dear that in one complete revolution of the cam shaft, eich stamp is supported for a period corresponding to

sgl a- j. Hence the average number of stamps

worried at any one time is equal to - - where S represents

the total number of stamps driven by the cam Bhaft. This is the number of stamps supported simultaneously, and although these stamps are acting on the cam shaft at different angles and lengths of leverage, yet their total thrust on the bearings will be the sum of their weights. In some cases the pull of the belt also have to be added, especially when this is vcrtic- % downwards. The accepted formula for power so absorbed is : —

Foot-pounds of work absorbed in one revolution Total resultant weight x diameter of shaft in inches. " " 90

From this the horse power is easily ascertained. To take a concrete example, say a battery of ten heads of 900 lb. stamps making 90 7-inch drops per minute, the construction of the cams being that already given

(page 205). In this case the expression - is equal to

51 nearly, and we shall then have for the pressure on the cam shaft bearings : —

Weight of cam shaft 15 feet lonjr, 5 inches in diameter . . 1000

Weight of pulley 2050 „

Weight of 10 cams, each 1411b8. 1410 „

Weightof 5 1 stamps, each 900 lbs 4590 h;

Kesultaucpullof belt, say about 900 „-

Total prossuro 9950

Foot-pounds of work absorbed per revolution

Since the cam shaft makes 45 revolutions per minafte

2. The cam curve being always parallel to the horizontal face of the tappet at the point of contact, there is no lateral thrust developed beyond that due to the friction. The friction may therefore be taken as that duo to dragging a plane surface of length equal to the length of the cam curve underneath a weight equal to that of the stamp, the coefficient of friction being that duo to the nature and luhrication of the surfaces of tb© cam and tappet. This coefficient may fairly be takei at 008 ; it has already been pointed out that this not a case of true rolling friction, seeing that the surfaces in contact are moving at different velocitie- Tlie friction between the cam and the tappet woul evidently be a maxinmm were the stamp so held in tht? guides as to be incapable of revolving ; in that case th friction in the guides due to the motion of revolution would 1)0 nil. If, on the other hand, the tappet were to move (if possible) at the same rate as the cam, the friction at this point would be nil, whilst that in the guides due to the revolution of the stamp stem would be

Power Absorbed 225

maximum. In other words, the effect of the rotation the stamp is to transfer a portion of the friction reloped between the cam and tappet to the surface the stamp stem in contact with the guides. The me reasoning appUes to the shght tendency towards motion of translation in addition to that of rotation, fioerated in the stamp by the movement of the cam. ?bese are both very small amounts ; when a mill is in Spod running order, the grip of the hand is sufficient to keep the stamp from revolving. There will therefore be M perceptible error if the maximum amount of friction possible, namely, that between cam and tappet, is calculated, and considered to include the friction due to the station of the stamp in the guides. The length of the

cwn curve can be shown to be — n where k is, as

At

fore, the height of the starting-point of the tappet, h

height of lift, and r the horizontal distance between

'e axes of the stamp stem and cam shaft, and the

poer absorbed by friction is therefore —

IWi + 72

2r X fiir foot-pounds.

or the case under consideration the expression for the

2 X 4*5 X 7 -h 7"

2x45 ' " inches - 10G7 feet.

"hence, foot-pounds absorbed in each lift of one stamp

10G7x 900x008 708.

'U the rate of 90 drops per minute,

XT x. , 76-8 x 90

3. The friction in the guides due to tlie rotation of

aa6 GOLD MILLING CHi

the stamp h&ving boeu eliminated and included in (9| there remains now to consider the friction caused tj the action of the guides in ni&inlaiiiing the stamp in Ilj vertical jwsition. It was pointed out (page 212) tbd the action of the cam being necessarily on one side o) the axis of the stamp stem, this latter tends to assumel position inclined to the vertical at a small angle. TImM is thus a couple formed, tending to turn the stamp inH this position round its centre of gravity, and it is thi moment of this force that has to be resisted by Ita guides. The angle which this position of repose maJtei nith the vertical is a very small one ; in the case of i typical stamp, the centre ot gravity is about 4 ft. 8 inbelow the tappet, and this angle becomes nearly 3.

The horizontal tlirust will acconlingly be — Hin X W 0'0333 % 900 lbs. 301bs.

This angle is so small and varies so Httle for thi various forms of stamp as now constructed, timt it may be looked upon as constant for them all, and the horizontal thrust always taken at of the weiglit ™ the stamp. Taking the coefficient of friction at 01. the power absorbed in each lift will accordingly be — 'W 7

X i? X 01 30 X j2 " 01 1-75 foot-pounds.

And at 90 drops per minute,

33,000 ' r required to lift the weight of the staD>I ' 12 X 3376

is as before the weight of the stamp in pounds, h th height of Uft in inches, and n the number of drop per minute. This formula gives in the present oas

Power Absorbed 227

]!ollecting the three last-named items, we find that the rse-power required to lift one stamp is —

0-209 + 0 006 + 1-432 1-646 H.P. Aoeordingly that required for tn stamps will be 16*46. Iding to this amount that of the friction of the cam taft in its bearings, we find that it will be necessary communicate a force of 17'22 H.P. to the belt of a sn-head mill of 900 lb. stamps designed to make 90 -inch drops per minute. It will hardly be necessary 0 insist on the great importance of these calculations n practice, so that the engineer can tell exactly what khe indicated power of his motor should be in order to drive his mill under any required conditions. Some niargiii for safety must, of course, always be allowed, but it will be found that the above calculations give an toiple one.

It may be useful to group all the above items into one general formula, calling the number of stamps driven off one cam shaft S, the weight of the cam shaft with cams and pulley IT, and its diameter d ; we have already seen that the diameter of the cam shaft bears a constant relation to that of the cam hub, depending' on the 'natfcrial of which the cam is mad( In the case now Under consideration —

k

now get as the formula for work absorbed (in horsepower), taking the coefficients of friction above given —

Some very important considerations can be ded from the above formulas: —

1. When gearing takes the place of belting, the against the bearings of the cam shaft becomes hori and the resultant thrust is somewhat diminished : but the total amount of power lost from this source is small, owing to the slow rate of revolution of the shaft, the economy so introduced may in practice disregarded. '

2. It will be seen that the loss of power due to frictk| is directly proportional to the length of the cam cnn engaged in lifting the stamp. The formula for the length

of tlie curve was ffiven as — - — or — and this ii

evidently a nnninmin for a given height of lift when isft and increases as increases ; in other words, the lo'cr down that the cam and tappet engage, the less power is wasted, the loss being least when the action commences in the horizontal line through the axis of the cam shaft. la practice this last condition can never be realised, but the best results are obtained when it is approached as nearly as possible. The lowest position of the tappet being deternjined by the diameter of the hub of the cam, it is now clear how a steel tapKt, which can be made with smaller liub than a cast-iron one, causes an economy power in driving the mill. It is curious to note that iri this respect the old Saxon stamp-mill is a better machine mechanicallv than the Californian, as in the former the above condition of 0 is realised; this advantage is however, far more than counterbalanced by attendant (liscidvantagis.

For tlu same values of // and k the expression -

Power Absorbed 229

8 as r increases, so that force would be econo- y keeping the axes of the cam shaft and stamp far apart as possible. This construction would r tend so greatly to increase the weight of the cams

strain on the cam shaft, that economy of power be obtained by diminishing this factor without

the whole machine unwieldy and less efifective r ways.

lie calculations for the friction between the stamp ad the guides only apply when these latter are y adjusted to the work required of them. It would able to so tighten up the guide-blocks as to jam the ud to increase the power required to lift the stamp itely ; but it is evident that any positive pressure 1 by the guides upon the stem will merely result

of power as well as injury to the machine, their 36 being simply to maintain the stamp in a vertical D during its lift, and nothing more.

Chapter Viii

FRAMES — GUIDES — HOISTING GEAB — WATEB SUPPLY— BlXf — GENERAL ARRANGEMENT — QBE-FEEDERS

Mill Framing. — Tlie object of the framing of the mill is primarily to carry the cam shaft, and secondly the guides in whicli tlie stamp stems work ; these simple duties are complicated by the great rigidity of structure required to withstand the constant jar of the stamps and the powerful pull of the belt or thrust of the gearing. The niaia portion of the frame consists of the battery uprigbta whicli carry tho cam shaft bearings. When one cam shaft ojxratos ten stamps it has three bearings, and there are then accordingly three uprights to a battery of ten stamps. When a cam shaft only drives five stamps there are often two uprights to each five-stamp batter}', though here too the arrangement of three uprights to a ten-stamp battery may be adopted, the centre upright carrying bearings which support the inner ends of the two short cam shafts. These uprights are always connected by the n])p('i- and lower guide beams which are fastened to them- The ui)ji4lits are usually supported on horizontal battery sills, and are strengthened laterally against the pull of the belt by struts or framing. Incidentally, too, these

Viii *'A" Frame 231

Lghts usually carry the working platform upon which mill man stands to attend to the cams, tappets, rings, &c.

'he style of framing to be adopted will depend largely the method of transmitting power to the cam shaft, I the position which the lay shaft has to occupy when ting and pulleys are employed. The lay shaft may her be carried on the horizontal battery sills close to 3 ground, or it may be carried high up on a portion of B framing ; it may be either in front of or behind the .ttery.

Wooden Frames are very largely used, especially in merican built mills, which are nearly always driven by alleys. If circumstances admit of the employment of rooden frames, these are perhaps the most generally satisactory. As regards the most suitable timber, almost any rood that can be obtained in beams of the large dimen- Mons required may be used. Pitch pine answers well, but is heavy ; almost any pine-wood may be used, sugar pine and yellow pine being often selected in California, whilst Oregon pine is largely exported to other countries for this purpose. Norway or Dantzig pine also answers perfectly well. Sometimes the superstructure is of light pJne whilst the foundation timbers are of pitch pine or of Karri wood. When there is no construction timber at the spot where the mill has to be erected, and the entire Dll has to be imported, cast-iron frames are perhaps the hest, and where transport is difificult and troublesome, steel frames may be used.

When tlie lay shaft is low down, the so-called .1 frame '8 mostly employed. In this system of framing the upriglit '8 strengthened by diagonal struts and hog chains or tie bolts.

U GOLD Mll.l.lNC

Tile w... mnciiial pattcrna nre shown in Figs. 47, tho chiiif differeDce being that in Fir. -40 tliu stn is turned to the liaok and in Yvf,. 47 to the front of ttd battery : of coui-se tbo lay shaft must be on the same of the upright as the strut, since one of the main fnaotioo

of tlie strut is to brace the upright against the pull of tlie I)elt.

If a uiill be worked with mechanical ore-feeders, the fonuur is probaldy the bettor arrangement, as it leaves tilt' tables in front of the mill entirely open and free to ol)scrviition from every part of the mill building. The lay liiift iiiay be either at A, where it is comparatively out

,1 " A " Frame J33

i the way, or at B, a belt tightenor being required in b latter case, as the belt will be too abort to exercise ffideDt pull vritbout one. Of course the struts and Mte make it difficult to get about on the feed side of the mill, bat as machiDB feeders once adjusted require but Uttle attention, this is not a matter of much importance.

When, on the other hand, niachinc (eedcis itrti not iis(hI, ft'! is still sometimes the case with small mills, it is iininiunt that the back portion of the niill should be as tinencmiibcred as possible, in order to alluw the workiiuui who attends to the tcediiifj; to move about as rapidly ami freely as possible, it will bo noticed tlmt in both

COLD MILUNG fMi

J iG battery sills rest upon three mud silia wbi>

are buried in the ground and may be held in jilnoe 1 hold-down bolts. These mud sills should bo {for i 800 lb. stamp-mill) about IC inches square, or they mi with advantage bo 15 inches wide by 18 inches deep, m ning the full lenjjth of t.liP mill. The battery sills shou be about 15 inchct id checked about 2 inch

deep into the mua two baulks being held t

gether by a couple u bolts nt each joint. Tl

b&ttery uprights posts should bo at le

20 inchofi by 15 i a ten-head stamp-mill, tb

tage be a little wi,... „, lhi1-uL' (m-i, jt take*,

greater strain. The posts should be mortised at least I inches into the sills, care being taken that all the joiuti are very carefully fitted. The mud sills should be coaled with a good layer of pitch put on hot, and the rest ol the frame either welt tarred, or painted with three coati of good paint. The struts should be about 12 inches b' 10 inches, and well mortised into both the uprights an the sills. The hog chains should be made of li|-inch ' Ij-inch round iron, and should be arranged for tighte' ing up either by means of swivels in the middle or long screw threads and large-sized nuts at the upgv ends. Unless the frame is carefully and substantialconstructed of strong sound timber, good results working are impossible. The weight of au A (rarx complete for a ten-head mill of 750 lb. stamps may I taken as between C and 7 tons. These A franies C very well for small and light mills, but ate not to be conuiiondcd for stamps of over 750 lbs. weight. The have been almost entirely replaced of late years by tb so called knee frame, the arrangement of which is show

" A " Frame

n Fig. 48, wliich represents the general arrangement of k mill thus constructed. The mud sills and battery tills should be of at least the same dimensions as given ibove. The battery uprights should be about 14 inches

Side View

.It

Scale '-i to 1 ft. Pt<i. 48.

24 inches ; the central one of a ten-head battery inav vith advantage, together with the battery sill on wliicli stands, be widened to 18 inclus. Tlie knee posts and 'lee beams may be about 14 inches s(juare or else VI "y IG inches. The anle braces should be about 10

COLD mtUNC

ln< ics square. The timbers should be iik and boltod together as eliovrn. A cast-iroD b may with advantage be bolted to tbc battery up to support and stiffen the kuee beam. In this an ment the lay shaft is nearly always carried, as she A, on the knee beams, level with the cam shaft. I times, however, the lay shaft is carried on the bj sill near the knee post, as shown at B and S, oi behind the battery m ;, as at C. Every po

of the lay shaft has ttendant disadvantages

advantages. When canivon the cam shaft leve belt has the advantage of leing horizontal and we of the way of the mil! man. Tlio lay shaft too i of the way whilst its bearings are readily acces At the same time the heavy frame which carries some extent obstructs a clear view of the tables am shuts out much light from them ; moreover, in sp all precautions, the lay shaft bearings are never as as when these bearings are directly on the battery When these bearings are carried as at C, and i fore near the mortar box, the shaft is apt to be c damaged, as it is very difficult to keep it clean fror dust, splashes of pulp, Ac. ; moreover, it is then U! in a very dark place, and receives accordingly less and attention tlian tt would if placed where it i? stantly in view. The vertical pull of the belt if an objection, and the belt is generally so short t tightener must be used, which rapidly wears the so that its life is a comparatively short one, ' objections apply partially but with less force to cases in which the lay shaft is carried on the bi sill, as iti either of tlie positions marked B and h former being the better place. The chief objection t

Knee Frame

system is that the belts are very much in the way of ill man, and prevent ready access to the tables. The frame is well adapted to carrying the lay shaft when itter transmits power to the cam shaft by means of ng. The weight of the woodwork of a knee frame

Side Vieuj

Scale y-a I f t . Fi<i. 4'.i.

plete for a twenty-stamp mill is about IG to 17 tons, ther form of frame which has come neatly into ur is the reversed knee frame, sliown in Fi<'. 49. In system the battery posts are Ijraced aainst tlie fi-ont hts of the ore-bin frames, strengthened at times by

frame. The only positions .TOlabl. for the lay .h.11 m nt A nd B. and it is the sole vahd objection to

frame that it must carry this shaft in a somewhat ii VGiiient place, and as the distance of the battery from the ore-bin uprights is restricted by the array of the ore-bins and ore-feeders, the belt is nec< rather short ; nevertheless, when the shaft is set dose the ore-bin uprights as at the belt is frequently ji long enough not to need any tightener, although one mostly employed, as shown in Fig. 50. This style frame is also very well adapted to driving by the lay shaft being in that case behind the bati The great advantage of this frame is that it affio an uniuterinipted view over the entire table floor of tb! mill, and presents no obstacle to the moving about of tho mill men on that side; it can scarcely, however, be used to advantage unless mechanical ore-feeders are employed. A perspective view of a ten-head battery arranged on this system with reversed knee frame without diagonal hraces, having its lay shaft close to the battery uprights, and using a neat and strong belt tightener, is shown in Fig. 50, the mill in question having been built by the Sandycroft Foundry ('ompany. Limited.

The reversed knee frame is also the most suitable for tlu design of haek-to-hack mills, an aiTangement that is hecomiiig popular for many of the large mills that are rendered necessary by the economic conditions of modem gold mining, though it should never be applied to a smaller number of stamps than 80. The framing of such a hack-to-hack mill, together with that of its ore-bins is shown in Fig. ol, this mill being intended for operation in conjunction with an independent rock-breaker house, a c()U[)l(' oi tracks from which should be carried over the top of the bins. The scantlings of the timbers should he about the same as given for knee frames; it must not

Steel Frame 241

;ten that the ore-bin sills have to carry the whole Df the ore in the bins, and should therefore be a strength, and their mud sills securely bedded, lay be two separate lay shafts as at but it is ; better to drive the entire mill off one main line ling at 3,

Frames. — Wrought-iron frames have occasionally sed for mills, but this material has practically been A by steel, which is in every way more suitable e purpose. Even steel, however, forms by no . a satisfactory frame, as it is much too elastic imits of too much vibration. Its special advantage t of portability. By replacing wooden beams by )und girders to be riveted or bolted together on K)t of erection, the framing of a heavy mill may rried in comparatively small pieces. Tlie mud sills jmetimes made of wood, but more often are re- l by blocks of concrete or of brick or stone laid in it, to which the battery sills and the rest of the

are bolted by strong hold-down bolts. A usual )f steel frame is shown in Fig. 52, which represents nty-head battery with iron frames, shown in perve. It will be noted that in this no attempt has made to dispose the metal to special advantage, ig a simple A frame in which the wooden beams been replaced by compound girders consisting each ) lengths of channel iron held together by bolts suitable distance pieces. A rather better design wn in Fig. 53, where the frame is constructed of ir compound girders, but so arranged as to give ir stiffness. Both these mills were designed and ructed by Messrs. Bowes Scott and Western, Ltd., ndon. Steel frames are, however, whenever pos-

U-D Mil l.tNG

Ue, to be uvoided. The weight of a steel t 3 mill may be taken at aboat five tons

it niUBt not be forguttoii thai tbt: former excU the latter include mud sills, wbioh toriu nearl<

of the total weight of the wooden frames. I belie that the first steel frames made were designed in 18 for a small composite portable mill built by Messj Appleby Bros, for Effiienta, Gold Ck>ast of Africa, which no part exceeded 150 lbs. in weight.

As far as I know, cast-steel frames have never y been employed; in the present advanced condition the art of steel casting, and with the comparatively lo' prices for which steel castings can now be bought, thei construction should offer no difficulty, and it would seen difficult to find any better material, whenever lightoea is an especial consideration.

Cast-Iron Frames have been largely used in Australia and to some extent also on the continent of Europe, h America and in this country they have never been iiiucl used, although there is a very great deal to be said ii favour of this material. It is sufficiently stiff to form i good frame, and elastic enough to stand the jar of th* stamps. Colonial-built mills with cast-iron frames haV' been running in some places for over 30 years, and ai" in as good shape, wliilst they have cost quite as little io repairs, as the best wooden frames. The castings nee* not be of unwieldy size, and can be made in piece capable of being bolted together at their destination without greatly weakening the frame. Moreover, properly designed cast-iron frame is fairly self-containe( and can dispense with the angle struts used in woode and steel frames. The best system is probably some fori of hollow casting, copying more or less the frame of steam liammer. The cast-iron frames for a ten-stam mill of 800 lb. stamps weigh about three tons complel (of course without mud sills or foundation blocks), i tliat this is by no means a heavy system of framing.

very neat pattern of caat-ii-on frame is ehowD in Fig. 5 which gives a view of a five stamp mill built ia Queen Ittnd : it was erected at the Colonial and Indian Exhil tion of IHiiQ in London, and may be taken as representii the hest colonial design. The figure is reproduced fro a photograph ' " by the Queensland Goven

ment Office, iii. the framing are shown i

Fig. 55.

A somewhat iii ern having two oasl-ira

columns ill the making a very good, Bti

frame, is built 1 impson and Co.. of Castlt

maine, Victoria.

A new cast'ii'on frame, mcently designed liv Mcssr: Bowes Scott and Western, Ltd., stiffened, however, by steel strut that ought hardly to be required, is sho"" in Fig. 56.

Lay Shafts. — The proper positions for these bav already been discussed under the various types c frames; with steel and iron frames they are best (*i ried on massive hearings bolted down to heavy blocks c masonry or concrete. As the lay shaft runs usually 8 from twice to thrice the speed of the cam shaft, and ba no strains Co bear except those set up by the transmissic of power, it can be correspondingly lighter. In a long niiH however, the end of the shaft that is nearest the prii"' mover has to transmit the power for driving all the cm' shafts, and must be fitrong in proportion. As power taken off for each successive mill pulley, the diameter of each successive section of the lay shaft can be cof respondingly reduced ; much weight is thus saved altliough the cost of patterns and machining may be ' triple increased, whilst a bigger stock of spare parts suci as brasses, couplings, ic, must also be carried. Tin

Ap. Vi Ii

Guides

y shaft is occasionally coupled direct to the prime M>Ter, hut is usually driven by belting or more rarely y gearing ; for a mill of any size cotton driving ropes kTe undouhtedly the best.

In small or medium-sized mills the motive power is

best placed at one end of the lay shaft, and is often so

mtuated even in large mills, although this position tends

to set up severe lateral strains in a long line of shafting ;

[ m BQch cases the driving power should be applied in the

tenure of the lay shaft. The driving pulleys on the lay

liiaft should be of the ordinary type ; they should be

provided with either ordinary clutches or friction clutches,

Plan.

Scale. J4 '

Fici. &7.

Unless there is a tightener on the battery belt, so as to 'ble anyone battery to be stopped or started iiulepeud- 'tly of the others.

Guides.— The frames of a battery are always connected a pair of horizontal guide-beams, which are bolted to nem, and which serve not only to stiilen the frame but to carry the guides within whicli the stamp stems 'ork up and down. Their function is to withstand tlie Qdency of the stamp stem to deviate from the vertical, already explained. In their simplest form tlu; guides, 'Orm a part of the guide-beam. In the case of wooden

ays GOLD MILLING aU

frames, this arrangement is shov,-n in 57, Hits bd u plan of one complete guide-beam, the h&ttcry postal either end being shown in seution, I

It will be seen that one half of each stem worktl the guide-beam and the other half in a Mngle cap [H8II u'hiuh is bolted to the huatn by u nuiiibur of I4l4 bolts (indicated by dotted linos), its exact position bed adJQsted by means of those bolts and by hard-wood Jin tftuce pieces or wedgeB (not shown iu Fig. 57), "t'I down between the two beams to keep the Etainpelm from being jammed. This arrangement is unsatisfactoqH because, lirtit of all, the motion of the stamp etetn UJun place across the grain of the wood, causing unneeessif friotiuti and wearing the guides out rapidly. Moreowr., it is impostiible to adjust any individual stem that tnlf require it; if, for the sake of economy, it is thought Js-i sirable to adopt this plain guide, there should be li square recess cut out round each stem and a bushin|!*l| hard wood driven down for the stem to work in. Thu bushing can be taken out and renewed when worn, aw. the grain of it will be vertical so as to be parallel to llw motion of the stem. Hancock's guides, made by ibe Sandycroft Foundry Company, Ltd., are quite siiuilitfi except that tiie hard-wood bushing has a projecting square shoulder, to keep it from shifting up or lionf' With iron frames a sonie\\hat similar arrangomfiil i" cast-iron is also at times adopted, only iu this case, shown ill Fig. 5y, each stem has usually its own sepfti*'* cap. Mostly cast-iron guides are simply bored to fit tbe stems, but they are preferably lined with Babbitt metal. Metal guides are not, however, to be recommeuded under any circumstances ; they are apt to work hot and to tiie litems, and their lubrication is less satisfactory tbaf

Guides

ooden sides. More- h wooden guides prac- 1 the wear is confined lide, which is easily and replaceable, instead of g shared by the more tamp stems. rs. Eraser and Chalmeis, nake two patented patof stamp guides, the md the Broughall; either ch is quite satisfactory, specially adapted to named mills only. In g[sides each stem has its ndent pair of wooden ( which are tightened up Ige-shaped iron keys and in the former, and by ng screws and strapping in the latter, which is illustrated in Fig. 59. these guides have the tage that each stamp can Ijusted, and its guideloosened or tightened It interfering with tlie , so that perfect vcrtiof the stamp stems can intained and wear of the taken up without stophe running of the mill ; ishings of any one stamp

KV!-EiH

BSi

ft

s

ft

as*

Gold Milling

can also be taken out entirely, and the stamp drawn repairs or replacement without touching any of the i The objections to them arc tbetr expense and thei plication, each consisting of a number of small The inost satisfactory pattern of guide yet designed one shown in Fig. 60. which sufficiently explains il struction. This is in very general use on the Pacific and can be thoroughly recommended; it can be

-with either wooden or iron guide-beams, and has i advantages above enumerated besides that of I very few loose parts, which is a matter of grei portance in a stamp-mill. All these three last-i systems of guides have the advantage that the gr the guide-bushing Is parallel to the direction of u of the stamp stem. Guides are usually from 12 i to 16 inches deep. Very often the lower guide is

Guides

I deeper than the upper one, but there is no oical instlfication for this practice, as the strains h are practically the same. Especially where any systems of loose guide-blocks here recommended use, it is advisable that the depths of the upper >wer guides should be the same, so that the bushings be interchangeable, this system necessitating the lenance of a smaller stock of spare parts. The

A>r yr bolts I

1

®

r-

@

J

r-

Wf.

' I

' I r

r I

Front Elevation

Section

Fio. 60.

oni of the lower guide should be about 4 inclics to les above the top of the mortar-box ; in the old style rooden mortar-box the lower guides foi*nied the cover he mortar, but this plan has now bec;n discontinued, ire should bo space enough below the lower guidci to the mill man to get at tiie covcm' of the mortar My. It is also well to hang a piece of old rul)l)er tinjj, canvas, or oilcloth, having holes for tlie stamp ms to pass through, below the lower guides, to prevent

GO/jy WLUKG

may (4 tfae lolncaiit used from finding its way idIc mnrur. FfocB tb lop of tb lower gDide-beans to bottom the nppif ihe minimum space must be lengtli ol tli cm from toe to toe plus the height of tOfet and an inch or two for clearaoce. This diSU oui tberefon nere-r be les than aboat i feet and oraally alioDi 6 feet.

LabrieatiOB. — It necessar)- to keep the guides catml Ml as to keep the friction of the stems aguu tbein as low as possilile. Wbeu iron guides are HM Uiey need jinwiically no lubricadon if they are hll bitted with a good suit metal ; if not, antifriction greiH may be used, bot very sparingly. Soft soap is prM able, as being loss injurious to the process of amalgaw lion, when any of the hibricant finds its way into ihi battery box. In the case of wootien guides the bed! lubricant is finely ground graphite made into a stiff palld with a little soft soap. Various lubricatiug composition are sold by makers of milling machinery, but the abon mixture may Ihj thoroughly recommended as the resuMl of extended especience. I

Workiiig Platform.— In order to enable the mill manWJ attend to the tappets, upper guides, i&c., a working phiform is usually arranged about the level of the cam stall or a little below it. This platform may be either at Ihe back or the front of the mill ; it must run along the lull length of it and must be provided with steps or ladders at either end or at any convenient [xiint mar the miiWI" of the mill. American engineers scith t<i pn't.-'i M p"* this platform behind the mill, whilst Australians geneiiilly have it in front ; it is practically unimportant on whiuli side it is placed, except that, if in front and vei? broad, it is liable to cut off too much light from tlw

Jack-Shaft

lalgamating tables below. As a good deal of heavy rk has to be done on the platform, it should be car- d on strong brackets, and itself made of stout plank t less than 2 inches thick ; the planks ought to be ill jointed so as to prevent small tools, nails, kc, from popping through. There should be a s'out railing along e outer edge of the platform.

Stamp Supports. — Occasionally it becomes necessary to

bang up one or more of the stamps — that is to say,

support them so that the lower face of the tappet

tall be about half an inch above the travel of the cam,

-k>

t"

1

h

,C

to

en

l.jUi fur Ji"

Cont. h 4C rttutt

'0

Side View, Scale, ijl"- 1' Front VicLU.

Kuj. r.i.

in order that the cam shaft can contiinu to revolve without touching the stamp or stamps liuiig up. Tlie AaKrican method of doing this is by means of a jackhlmi't and linger bars. The jack-shaft consists of a pieci o( shafting about 3 inches in diameter, of exactly tln '>anii' length as the space between the battery uprights. On each of the inner faces of the batt(My uprights is screwed a bracket such as shown in Fig. ; this is best countersunk into the face of the timber and secured Jjy four H- inch coach screws. Tliese brackets carry the jack-shaft, which need not be tunu'd briOii. A piece of t''>W-iolled shafting answers capitally ; it must be stron*'

to c&ny the ffcigfat rA all the five stamps w loent (lefiinBaliaa. Tbe finders, a usual palt t ia blM>n in Fig. liS, eoaxist of a saddle-s

casting which is lined with leather and rests on th( shaft, tormina ting in a socl(et about 3 inches squai which fits the wooden finger, of requisite length ti ))ort the tappet clear of tbe travel of the cam.

Jack-Shaft

Igers are usually stitipped with ]-inch sheet-iron at I point, aud have an iron handle. When not in use 086 fingers lean back against the edge of the working itfonn, which is often notched to receive them. When becomes necessary to hang up a stamp, the mill man kes in his hand a fid stick, which is best made of a imber of strips of leather or rubber belting 3 inches de, 2 inches deep, and 18 inches long, furnished with vooden handle at one end (or the whole stick may be Mle of wood if desired). He lays this fid stick on the u as it revolves, keeping a firm hold of the handle,

B

Vertical Section

o

Fkj. QX

I then presses his knee firmly against the finger bar. e cam lifts the tappet to a greater height than usual the extra thickness of the fid stick, and is thus liftcMl high that the tappet just clears th(j finger bar, which pushed underneath it by the mill man, who then pulls ck the fid stick. The operation is a simple one, hut quires care and practice ; it is facilitated if the platrm is so placed that the cam is revolving towards the rator. If the stamp has to remain hung up for any ngth of time, it is as well to put a lashing through le handle of the finger bar round both the bar and the

ft

GVLIi 3IILUSG

stamp sliR. so fts to obriate any risk of the former sH ping tiisAn at of the Utter being jan off. In Austral tbe abore uruigemeDt is rcpUoed by two iron bars m lung lengthir&ys along th battery, and a sbort finger fa that rests across these at sacb a height as just to supp( tbe tappet. These bars, a and b, Fig. t>3. are about to inciies deep and to ioches broad, and 4 Brmly bolted to the battery uprights ; tliQ back bul u 2 lo 3 inches higher tbao the front ooe. The bar.c, here made of iroo about inch square, is bent as to oftteh OD the back bar, and when in position j' euppoTts the tappet at the desired height. The fn bar IB notched, 60 that once the finger lias been slip[ into its place it cannot be jerked away again by jar of the stamps. If there are no notches, the fin bar should be lashed to the stamp stem. It is scare necessary to say that this bar is on the side of stem remote from the cam. Tbe finger bars are usui kept out of the way close to the uprights, or else hung up in some convenient spot close at hand u they are required. This makes a very good and pact arrangement for hanging up stamps, is quite effective as the American method, and takes up 1 room.

Stamp Hoiiti.— It occasionally becomes nece&sari hoist a stamp up altogether, as, for instance, whe: tappet slips or a stem breaks. This is almost alw done by a chain-block, which is far the most conveni plan ; a 15 cwt, chain-block does perfectly well. H usually carried by a little traveller or crawl, which r on ft rail above the stamps. A crawl which answ this purpose thoroughly is shown in Fig. 64. It will noticed that this pattern requires two rails. Some

Side Viea;. the front nghthand wheel being removed.

corn iiiujyG

trade requiring only ooe. Tlie rails maybe &a Erom tbo fmtning of tb raill-bailding, or they anpported on the bwwry aprights. as shown ii in 65 and fiG, the latter system being t)

reliable, although it is liable to the objection of i too little head room. In order to attach the tlie chfiiD-block to the stamp, some makers dril into tbe ends of tbe stem and tap it, so thai

Cka Ivls

26t

t can be screwed into it when required. This is not |ood arrangenient, as the hole is apt to get filled with i or the thread to be burred up. A better plan is to p a rinjT such as shown in Fig. 67 over the stem ; as

e*ntre tint of H iilal* 6o/ft' '

I'm 0/ VJ.%t9 bolt; -fp

onal Elevation, centre of Battery,

Scale.

J ft.

Battery uprii'ht posts

"

I"

, Front of Battery

centrt line of stampstem

Fi.i. CJ.

'"tpull of the tackle is somewhat to one sido, this rinj;, *'oich is merely a roughly mado forging, grips tlio stem oite firmly and securely by friction alone ; sonictinies Q additional friction grip is attached to the ring, but it

a6a

Gold Milusg

JH iinnoooHsary. The ring alone answers all

Water Bnpply.— There shotild be a liberal water ply of doan water to the mill whenever it is to olituin thiH desideratum. The principal wateruluuild run the entire length of the mill in any com {Hmition, branches being carried to each separate It IN bv no moans a matter of indifference how the

BidB View.

Plan.

Scale.

- A

, v.jjVa ironi I inch to 1 inch in diameter

-, :,j;y*' oviv">5it< the centres of the second

tho sti-oams should be direct*

,10 o: iho box. Kach pipe must

whf'iv xhc requirements of the mil*

..V -IN The water supply sbouU

ni IVA TEK SUPPL V 263

pmily supplied, and to prevent the crushed quartz from king at one end or the other. Sufficient water must used to carry all the quartz out of the mortar hox soon as it is crushed, and keep it suspended as a y thick pulp inside the mortar. The level of the in the box should stand at from to 5 inches the level of discharge of the screen. If the pulp [Uires to be diluted for the operations outside the batthe necessary water must be added outside. A length of perforated pipe running along just outside the screens is often used for this purpose. In such a cue, this perforated pipe should form a movable arm so ttiat it can be turned out of the way when the screeni iTunie has to be taken out. For each battery there should also be a separate branch pipe fitted with a valve *nd carrj-ing a length of hose with an iron nozzle for ashing down the tables, &c. There should be other branches for washing down the floor of the mill and for scellaneous pui-poses generally. When there is a plentiW water supply, it is always good practice to have branches laid on to all parts of the mill-building. The niount of water consumed by a battery varies within wide limits according to the nature of the ore, be degree of fineness to which it has to be cruslied, 4e mode of treatment adopted for it, <fcc. ; it may be ken as between 80 and 320 cubic feet of water per ton ore crushed, averaging, say, 150 to 200 cubic feet per of ore. Beckoning each stamp to crush 3 to 4 tons per twenty-four hours, the supply of water required for five-stamp battery may bo taken at IJ to 2 cubic feet of water per minute on the average, or say about 20 cubic feet per stamp-head per hour. Battery pulp will thus consist of about 10 per cent, by weight of quartz

tin GOLD MILIJNG !

iiiid 90 per cont. ol water. Wlien the iiaturul w supply is Bcatity, it will be tieccssarj' to nin thti toil from the mill into largu Bt!ttlin{; pits, wlit'it.' tlif l>arii of sand are allowed to settle, and the clean waXei i then ptitnjKid liaek agniu to the tanks that supply d mill, by this means fully half of the total requia monta of the milt may be supplieil. If the water i very uiuddy, it should other be allowed to settle ooinplvtely as possible iu large pita fitted with duowa ous bailie plates, or else filtered through a biiisbwooi filter or some similar contrivance. It is oE the utmo* importance that the mill should be supplied with tl cleanest water possible. If acid, as may happen will certain mine waters, or with water that has heen id contact with decomposing pyritic matter, lime or woodashea may be thrown into the supply tank with raiua advantage. In a few instances sca-water has been aiti in the battery, without injurious effect; brackish wiiWf is similarly used in West Australia.

Flooring. — A stamp-mill should always be floored very substantially, and with some material that will notallov of leakage either of water or of quicksilver. The Iwsl plan is to put in & good cement floor, or, failing this, floor of stout 3-inch plank having the joints carefully caulked like the deck of a ship. The floor ought to have a gentle uniforjn slope downwards from the ore-bio end. At the lower end of the floor there should be gutter delivering into a tank sunk in the floor. Tbe floor should he washed down with a good stream of wafer at least once a day, and all the dirt, &c., thus collected iu the tank put aside for further treatmentt as it is sure to contain gold and probably quicksilver also.

Bins 265

— Every mill ought to be provided with bin y sufficient to hold at least a twenty-four hours' of quartz, and more if possible. These bins are ' triangular in vertical section, terminating at the end by shoots leading one to each mortar box. mechanical ore-feeders are used, the shoot delivers y into the hopper of the ore-feeder. In other it delivers on to a platform at the back of the B, this platform being preferably covered with iron. The aperture of the bin ought to be closed ;ate regulated by a rack and pinion or a screw, and with a proper hand wheel so that the gate can bo any given depth of aperture and the delivery of z thus controlled by the mill man and kept uni- Both the bin and shoot ought to be lined with ' sheet-iron about inch thick for the former and b for the latter, held down to the planking by ended nails or screws. In large mills it is not ible to have one bin to supply more than twenty of stamps. When a separate rock-breaker house d, the tram-line that delivers the crushed ore to lill should come in on top of the mill bins. When Ksk-breakers form a part of the mill structure, the breaker floor is directly above the bins, and the ed ore from the rock-breaker as well as the fine ore passes through the grizzly, drops direct into the ry bin. An arrangement of this kind is shown in in Fig. 68, which illustrates the combination of frangement with reversed knee frames, this being tion of the sixty-stamp mill built for the Montana )any. Limited, by Messrs. Fraser and Chalmers, id, of Chicago. Fig. CD shows the arrangement of breaker floor and ore-bins for an ordinary knee-bore

in luind that this is the mioitnum I ullowod. uml tliftL, uiArcover, il m never poss mn OTK-hin ijuitu lull uji lu the tup. Iii the ' rand dietricC tbe rock-breaker bins are BBCi exceeding 300 tons capacity, but the uitll oi Yury large, haviag a capacity of 100 to 250 ett ftbotit 5 Co 12 lous per ha&d ; the bin of tbo mill of the Glencairn Main Beet Gold Uiniug Limited, has a total capacity of nearly 49 feet, ei)ual to 300 cubic feel or about 14 tons FigH. 70 to 72 show the details of a tws "high" mill (as the oomlii nation of mill anc roclcbreaker above them is called) as congtrae Saiijjcioft Foundry Comiauy. I.iiuitcd. an< taken as tbe type of a good mill of this dcscrif General Arrangement. — A mill of less t stamps is always arranged in one straight line, the most generally suitablo plan. At one a mill-building is usually a snmll fitting-shop fo' repairs that are needed in the mill, and sot machine shop as well. There should also be and ataalgam room and a retorting and melt BO[netiines the retorting and melting) room fon the assay oiBce, which must be far enough fnJ for the balances not to be affected by the jarrj stamps. There should be a largo store-roc ample supplies of mill stores and duplicates parts are kept ready for use. Large mills a built in groups of twenty heads, one grizzly breaker being able easily to distribute ore tc tcrics, when the rock-breakers and mills are in building. With a separate rock-breaker house tion of ore dliitribution is, of course, much i

illAR-J

Ore-Feeders 269

cy large mills of 80 stamps or more are often built on i back-to-back principle illustrated in Fig. 51, the two irs of batteries forming practically independent mills. le manner in which the mill is laid out must depend tirely iix)on local conditions and the nature of the mill ie available, it being always remembered that it is paramount importance to secure a good solid rock andation upon which to erect so heavy a structure. Ore-feeders. — Originally batteries were always fed by jid, and even after the introduction of machine feeding e former method was for some time given the prcfer- 06, as better results were said to be obtained ))y it. mechanical ore-feeders have gradually been improved, has been found that machine feeding is in eviry way l>erior to hand feeding. It increases the Ciipaeity of e mill at least 15 per cent, by keeping up a steady, liform supply of ore, and for tlie saim reason the wear id tear of shoes and dies is less, as the die is always spt covered with a bed of ore, so that there is no ending of metal on metal. There is accordingly also S8 risk of breaking a stamp stem or of splitting a head. must be remembered that in the staminlill the power* ; oidy indirectly appHed to crushing, its direct action eing merely to hft up the stamps ; hence the machine onsunies exactly as much power lion rumiing empty ts when crushing at its fullest capacity, so that an anto- :iiatic feeder actuated by the mill itself is an (economic necessity. In hand feeding too, ore is thrown in ])y t\ic shovelful, so that the mortar is alternately too full too empty; when the former condition prevails "lore is great likelihood of i)ursting a scrc(Mi. The feeder charge of a mill can tell by the sound and feel of stamp when the battery requires more oie ; the

aouad oF metal striking on metal is vory ch&roct nnd the peculiar hanl feel of the blow of die when it is striking on the ilio or on a very ttun luyeti quartz only is also easily recognised with a little pncti It is usually one man's duty to feed two fivo-lil batteries, nnd in small nillla he has also verjotter 1 spall up Ihu stone to the sixo required. Of co band-feodiug at the best is bound to be irregular, whi with a good tuedliaiiical feeder tbe utmost unifonnil is attained if the quartz be delivered to the mac broken down to a small sine. It is the objecl machine-feeding to keep a thin uniform layer of c npon the dies, which ie being constantly kept at 0 definite tbioknesB by the gradual supply of ore to li mortar just in proportion as it is reqiiired. The poiwt ' absorbed by machine lueders is so very little tb&t i' . may be quite neglected. There are very many pattern! , of uiachine (eedors. A very simple device consists U makitij the cud of the ore shoot from the hopper, nwf able about a horizontal hinge. By means of a stronE spring or a counterpoise it is kept up at such an ngl' that the ore cannot run down it : attached to the aid of this hinged shoot is a bumper rod so placed it shall be struck by the tappet of the middle sUmP - whenevfi' tbern is not siiftirifiit ore under thi? sumP to keep it off the die. The shoot is thus jerked dowOt and the ore on it is delivered into the feed shoot of tb mortar. This is a simple and inexpensive arraagemeD and works fairly well with dry ore broken to a uniton"

There are three well-known ore-feeders whose principle is somewhat the same, namely the Victor, the 8tan(ord> and the TuUoch ore-feeders. Among these priority '"

OKE'FEEDElS 271

bt oi time belongs to the Stanford, which appears to ve been the first mechanical feeder invented. Each these machines consists of a hopper carried upon a wooden frame, which is usually on wheels so as to low it to be pushed back out of the way when the mill being cleaned up or repaired ; the hopper is usually bout 2 feet 6 inches square at the mouth and about feet high, so that it has a capacity of about 10 cubic Bet. The hopper is open at the lower end, and delivers Dto a tray or shoot so hung that it is capable of a forward movement when the bumper rod connected with it is struck by the tappet of the stamp. At each forward movement a certain portion of quartz, the quantity of which can be regulated by various devices, is pushed forwards and delivered into the feed shoot of the mortar. Anyone of these ore-feeders answers very well witli dry or clean ore, but if the ore be muddy and sticky, the liopper and tray are apt to become choked, and tlie machine cannot then be relied on for steady fcedinjj. The TuUoch is perhaps the beet machine of the three, 'it should only be used when the stone to be fed is not clayey ; its main advantages are tliat it is a cheap machine, simple and not likely to grt out of order, and easily repaired when worn out ; it is not a heavy machine, weighing G cwt. ready for sliipment. Fig. 73 gives a general view of this feeder.

Another machine that is sometimes used is the roller ore-feeder. In this the bottom of the hopper is closed 'a hollow iron cylinder about 9 inches in diameter; means of a very ingenious friction grip tliis roller is revolved whenever the tappet strikes tlu* humjxr rod, vliieh forms part of the friction grip, antl by the revolution of the roller some of tlie oru is delivered into

aja coin MUjjNG

the mortar bos. Thia inncliinc does not, however, % BHtisfaulorily ; it will not tlo at all good work with c&

oi'es, and wbcn the ores are dry, especially if at all fin it ie apt to deliver so large a quantity at once aa to cbol

Ore'Feeders 273

br. This feeder is accordingly but little used.

ore-feeder at present on the market, and one mbines all the qualities essential to successful 3 Hendy's Challenge ore-feeder. In this machine,

Fig. 74, the bottom of the sheet-iron hopper by an inclined cast-iron plate the revolution of rries the ore into the mortar. By means of bevel his plate is connected with a friction disc, which volved by the action of a kind of friction ratchet ; ;ion gear is actuated by a lever, attached to 7 shaft, and from this shaft a bumping rod is ip to the tappet of the middle stamp. A hole is trough the guide-block of the middle stamp and wiping rod passes through this hole ; its upper end ;ted by a buffer of india-rubber to lessen the hen it is struck by the tappet. By means of a nd wheel turning a screw which controls the play 5ver, the position of the bumper can be adjusted feed regulated with the utmost nicety. When-depth

of ore on the dies falls below the delount, the tappet strikes the bumper ; this blow yed to the rocker shaft, and thence by means of r and grip to the friction disc, which is revolved a greater or lesser arc ; this motion iscommuni- ) the plate, which is in its turn rotated through vc or lesser angle, and carries forward a prokte amount of ore into the mortar. This orevorks well even on the stickiest and finest of d if the ore is broken to at all uniform size can id upon to feed uniformly. Its disadvan- :e higher first cost (about £50) as compared

others and slightly greater weight, namely, cwt. ; it has moreover a large number of work-

T

terence whenever Ua higher coat is not an inauperab objection. A very neat modification of this feeder, know as the suBpeuded Challenge oro-foeder, shown in Fig, 71 is now much used. In this the hopper is dona avra

Osefeedeks

ti, the revolTing iron plate and the friction gear driving leing Buapended directly from the bottom of the feed

'Wt that comes from the bins. The action is of course precisely the same aa with the older pattern, but the 'E*ce token up by the hopper and its frame is economised,

Xt6 GOLD MIIUNG cv

and the back of the batteries rendered more reaffl axrct-ssible. In the newest models, the bumper rod leadil to the tapjiel is done away with, the friction grip lert t<Tiiiiiinting in a fork wliicb embrtuKs the stamp steD and which is worked by a collar secured to the taltei the friction gear, instead of being below the revolvil cant iron plate, is above it, and is thus more accesstbl arid less hable to injury. Various modifications of it original Challenge Feeder are now upon the market.

Chapter Ix

Crushing Machinery — Arba8Tr A — H Untington

Mill

ification of Quartz-Crashing Machines. — It has J been pointed out that the stamp-mill is by far the mportant of all gold quartz-crushing machines ; ly at least 90 per cent, of all the gold quartz d in the world is treated by means of stamps. are, however, a certain number of other machines, possess real advantages, that render their employadvisable under certain circumstances, and these les are worthy of study by the mill man. The numgold-crushiug machines that have been from time B, and are still being invented is something enorscarcely a year passes but that one or two such les spring into existence heralded by the customary liof trumpets, greatly to the benefit, in many cases, interested parties, although they mostly sink into erited oblivion with at least equal rapidity. No purpose can be attained by a description of these 3, and I shall here confine my attention to such les as have proved of real practical use, and which succeeded in maintaining their ground, when local stances are favourable to their adoption. Nothing

ie more certain than that the others will be revived h timn to time, with alight modifications and under n names, and that with most of them history will once iljl repeat itself. Whilst but little harm can result from I trying of new machines upon well-estsblisheii mine; exiBtinj; machinery is safficicnt to earn dividends and ' recoup the losses that may arise from unsuccessful I pcnments in other directions, no well-advised mine-own would thiiilt of relying n(K)n machinery thai has thoroughly stood the test of long practice for developing new mine ; the risks incident to mining proper are ijoit BufScicnt without multiplying them by those attending lb cmployniont of unproved machines.

The various machines (excluding gravitation stamp) used for quartz-crushing may be divided into the followinj classes :—

1. Machines that grind under the action of hoaTy weights ; to this class belong the arrastra, and the Chiiiw n mill with its modifications, such as thu Bryan roller ulH. H &c. Of tiiese the only machine tliat can be eaid to haW ' stood the test of experience is the arrastra. TheCliili*" ' mill ia used on a large scale in several of the gotd-fiel in tile Urals, though its displacement by Btamp-milU probably only a question of time. It has held itsgrwino there up till now, partly because much of the ore treated is very soft, and partly because the only stamp-mills known there up (o the present are defective in almost every deUil- Oi.f of llir host of tlM.-se Chilian niiil plants is to h,i foura' III till' (>iis|ieiiski Mine, lochsar : thtro are two mill' with pans 15 feet in diameter and 30 inches deep, fitted with 3 rollers, 4 feet in diameter by 1 toot wide; these weifih 3 to 4 tons each, and have steel hoops. The bfld arc of steel and the front portion of the pan is fitted wit''

Quartz-Crushers 279

teens of punched sheet-iron with holes about 0*1 inch 1 diameter ; the depth of discharge above the bottom of 16 pan is nearly 12 inches. The speed of working is 12 18 revolutions per minute. Mercury is fed into the pan hen the mill is running on oxidised ores : the pulp runs rer amalgamated copper plates 12 feet long, then over a using table also covered with amalgamated plates, and nally over Embrey vanners. The entire plant treats 0 to 50 tons in 24 hours, and is driven by a 35 H.P. Dgine, which is not, however, worked to its full power. !he ore averages 15 dwt. per ton, and the tailings ibout 3 dwt.

2. Machines that grind by means of balls or rollers inder the influence of centrifugal motion or of gravity, both combined. Of these there are numerous varieties, mch as the Globe mill, the Crawford mill, the Cyclops Bill, the Ball mill, &c., in all of which the actual crusher is spherical; and the Niagara pulveriser, the llowland pulveriser, and the Huntington mill, in which the crusher Ib cylindrical.

These machines have all been grouped together because feliey are characterised by the fact that the crushing action Wees place between one or more rounded crushers and Mnore or less narrow circular track or bed. None of them can be looked upon as machines of proved value for ordinary milling except the Huntington mill.

In some special cases dry crushing may be employed ith advantage, especially in regions where water is scarce, and on ores containing the gold in a state of 'Xtremely fine division. Such ores may he dry crushed then submitted to direct cyanidation. Several machines of this class have given good results when used 0 this way, more especially tlie Jiall mills made by the

aSo GOLD MILLING i

Grusoowerk Company Fried. Krupp). A convei

sizo takes about 11 I.H.P, to drive it, crushes ol J ton of average quartz per hour to a iO mesh, and c about £300. The consideration of tlry crushing is, I ever, foreign to the objects of this book.

3. Disintegrators of various kinds, in which crush is produced by the mutual collision of particles of ore itself at high velocities. These aro all dry crush and are quite unsuitable for the purpose o£ ordini] gold milling.

4. Rolls ; these are inodiScatious of the old Coniii rolls, the difTerencefi consisting chiefly in alteraUoci detail BO as to adapt the machine for fine emsliiiig.* Theoretically, rolls should be one of the moat economical forms of fine crushers, as they substitute steady continuous action for the intermittent action of the stamp. and use up power only in proportion as they do eCfe-ctiw work ; they will probably in time take their place amongst recognised gold-crushing machines, but up to the preeeni they can certainly not be said to have done so, is required in the gold milling practice of to-day is wet crusher that shall at the same time be an efficient amalgamator {not necessarily, however, conducting both operations in the same apparatus), and rolls cannot be said to have attained this position. At present tlier arc hut few isolated instances of gold mines where crush' ing is being done by means of rolls. The so-called KroiO rolls are among the best forms of cruahing rolls; the machine as now constructed is by no means the sanieas originally patented by Krom, and indeed in some details the supposed improvements in which the Krom rolls differed from their prototype, the Coi-nish rolls, have beeP found disadvantageous, and have again been discarded

t KROM ROLLS 281

Is now constructed, the Krom rolls consist of a pair of

M>ller8 about 2 feet in diameter and 15 inches wide, driven

from 80 to 100 revolutions per minute in opposite

iiiections by means of open and closed belts respectively.

TIhe bearings of these rolls are carefully adjusted, and one

of the pair is so supported as to be capable of a certain

amount of play, the rolls being forced together by means of

powerful springs ; they are enclosed in housings, and ore

is supplied uniformly by means of an ingenious ore-feeder.

Pig. 76 shows the arrangement of a Krom roll plant as

vranged by Messrs. Bowes Scott and Western, Limited,

of London. The ore is first broken in two rock-breakers

set one above the other, from the lower one of which it

passes into revolving screens ; all that is fine enough goes

to the rolls, the coarse stone being carried back to the

breakers. A similar revolving screen below the rolls

sUows only the fine to pass through, all that is too coarse

being lifted back into the feeder of the rolls. From the

Krom screens, the fine ore passes into a mixer, where

It 18 mixed with a proper amomit of water, and thence

it passes on to the amalgamating and concentrating

Machinery. This type of plant is said to be in success-hl

operation at gold mines in Australia, Nicaragua, the

Ural Mountains, and Siberia. Nothing, however, is

known of its actual working capabilities, or the quality or

cost of the work performed by it. Among the advantages

claimed for it, as opposed to stamps, are those of economy

in first cost and lightness. A plant such as illustrated,

capable of crushing 40 tons per day of medium quality

<iuartz, through a mesh 0024 inch square, costs about

1500 and weighs about 30 tons. The makers state that

such a plant will require about 32 I.H.P., of wliich the

oUb alone lequiie only 11 I.H.P. Ihcse data would

. Ix Power Stamps 283

iLethe effioienoy of the entire plant about 1*04 cwt. per

LP. per bonr, or little more than two-thirds of that

the Btamp-mill. A very serious objection to the use

ToUs as at present constructed is that they have not

hitherto been adapted to crush any other than dry ore.

be number of mines in the world producing ore so

Iry that, when crushed, it will pass through a fine screen

nihoat clogging is very limited, and there are also very

lew that can afford either to dry or to calcine their ores,

except as a preliminary to direct cyanidation. It would

not seem at all difficult to adapt rolls for wet crushing,

Imt until this has been done and the economy of their

use, when so applied, has been demonstrated, their

sphere of usefulness must of necessity be restricted by

the above limitations. Whatever possibilities rolls may

possess, these are as yet undeveloped, and it remains

for the future to give us an arrangement of crushing

foils thoroughly suitable to the requirements of gold

milling.

5. Power Stamps ; that is to say, stamps in which he momentum of the stamp due to gravity is increased hy mechanical means. To this class belongs the steam stamp, which has been used successfully for the (comparatively coarse) crushing of copper ores, but has apparently proved a failure when applied to gold quartz. For one hing, its proper lubrication presents great difficulties ; moreover, although it seems at first sight that the direct application of steam ought to be far more economical than ts indirect application through cranks, pulleys, cams, &c., yet this is really not the case ; when a gravitation mill is iven by an engine, the most economical form of the latter regards utilisation of steam can be adopted, whereas the steam stamp, owing to the necessity of providing a

raptia

cushion at either end of the stroke, and to the fuclU the cylinder must be of the non-compound type, n much steam is wastecl. A modern pattern, the Trems steam stamp, is stated to have boon used successfully Gunnison, Colorado, as well as at other places. At I first named locality each mortar, 12 inches by 24 iiioli iu area, contains two stamp-heads 0 inches in diamaM and carries screens in three of its sides. Each weighs 300 lbs., but the steam pressure brings its ef blowup to that of a 1,000 lb. stamp; it runsat200 per minute, the length of drops varying from 5 to 8 ini The crushing capacity of the mill averaged 13 tons 24 hours on rather soft ore, with a povjer consurapti'l estimated as equal to 1'2 H.P. The ore was crusbei} through a coarse 20 mesh screen (0 035 inch), but much of it was slimed, 48'5 per cent, of the pulp passing through a 100 mesh screen (about 0005 inch), which portion (srried two-thirds of the total values of the ore. Tbe iniU appears ill-suited to inside amalgamation, fully 95 per cent, of the amalgam obtained coming from the outside plates. The mill is built by the Gates Iron Woflis, Chicago. Of course, no steam stamp could in any case compete with a gravitation stamp driven by water-power. In the Elephant stamp and Dunham's stamp the blow is (ikcri by the propulsiuii tit liigli tijiecil of light bUuo heads, the power being transmitted to the stamp by me&ix of strong springs, so as to protect the machine from inJMj by the jar of the blow. Neither of these machines hftve proved successful in steady work, although elepha"' stamps are sometimes— but very rarely — still used as prospecting plant. These stamps have a very large number of wearing parts that are rapidly destroyed; more especially are the pins that connect tbe spring to tbe

Power Stamps 285

ip-head exposed to an amount of friction that rapidly them to pieces. In pneumatic stamps, such as the (band and the Scholl, the principle is the same as in last-named, except that the spring is replaced by an cushion. A very few are at work in places, but they by DO means satisfactory, and have in more than one e been discarded in favour of the gravitation stamp, isband's pneumatic stamps, manufactured by Messrs. knrey and Co., of Hayle, Cornwall, have been used in ew isolated instances, in Nicaragua (Central America), d in South Africa, but there are no records available of B results obtained by them. The small size of the mortar compared to the output from it, fitted as it is ith screens on three sides, destroys its efficiency as 1 amalgamating machine. According to the makers its ushing capacity is 40 tons (of tin stuff, not of quartz) 'oshed from 4 -inch cube through a mesh 0*048 inch in iameter with an expenditure of 35 1.H.P. This is equal >an output of 0*95 cwt. per I.H.P. per hour, which is y no means great considering how coarse a mesh is mployed. Data are, however, still wanting for an accuite comparison with stamps. The difficulty of proper ibrication seems up to the present to bo a vital objecion to all forms of power stamp, and the comparaively small size of the mortar is also a disadvantage. modern machine, which has, however, not as yet been ested in actual work, is Morison's High Speed Stamp ; this the mortar box, stamp-heads, shoes and dies of the rdinary gravitation pattern are employed, but the cam 8 replaced by a crank moving a cylinder within which 8 contained a piston attached to the uj)per end of the mpstem. Instead of the cylinder being filled entirely ith air, as in the last-named machines, the lower part

s84 GOI.li M/l-f.mG atn

ia filled with wft er, which enters the cylinder frfflB< rosorviiir attached to it through a email post ; in olfal words, Morison's stamp is a. hydro-pneumatic iiisteftd I a peumatio stamp. It works very smoothly at a rate i 130 drops per minute, aud its crushing capacity se* bo decidedly superior to that of the gravitatiou si Iho iuveiitoiclaims that six hends or his high-spM 3taiT)[>s are equal to ten heads of gravitation etampeibq the crushing ofhcioncy of both patterns seems to heal tho same. It remains, however, to be seen how wear and tear of tho new maobino compare with those of tlio oain stamp.

Under this head there may also be mentioned th attempts that have been made to augment the poiver dI the gravitation stamp by malting the head drop under the action of some device that shall supplement the fores of gravity and cause the stamp to descend at a greater tftl* and consequently with more momentum than imder the action of gravity alone. Such are the stamps, also made bf Messrs. Harvey and Co., of Hayle, in which a set of canu above tho tappets drive the latter downwards. Other makers have attempted to use springs for the saw* purpose. It need only be said that all such eKperimenti have proved to he failures even from the point of view oi econorj.y of povrur, .At.d tlie fiuL luusl i.ut ht-lost sigtil of in considering all these machines that the modem stamp is an amalgamatingnoless thanacrushingmachiDB, and that the maximum of efficiency in both function* combined is the object to be striven for.

Tho whole of this part of the subject may be soniaied up in the remark that it is easy to quote instaDces where one or other of the above-named maohinea has bees placed by gravitation stamps, but I know of very (e*

t THE ARRASTRA 387

ns where the gravitatioD etsmp has been discarded in nuT of its would-be oompetitore in gold milliDg. 'The If m&cbineB that need be described in any detail are Pb amstra and the Huntington mill. H Iha Arnutra. — This, in its simpieet form, is a very limitiTe maohine, having been adopted by the pioneers 11 Cklifomian gold-quartz mining from the practice of Ik old Mexican silver-miners. It still holds its own in ttKbun places, because it can be built altogether locally od is very cheap to erect, is simple to work, is adapted

Vertical Sect on

to animal power, and is perhaps the most clficient gold- Uving machine known. It is therefore especially suitye tor the purposes of miners with little or no capital, forking their own claims, particularly where the ore 11 rich and the vein small, and is j'ar ej.-cellence the "uuibine for the prospector and pioneer in remote and "ew mining regions. Its use should be confined to free- "ulling ores, containing a very small proportion of sulpfeorets, such as compose the out-crops of most quartz 'Kfs. It has also the great advantage that it can l)e

28S GOLD MiLUHG

worked by animal power — horses, mules, or oxencan therefore be used in phMs where fael is so that steam-power is out of the question, and where are no streams capable of furnishing water-power, last consideration is a very important one, and must be overlooked, as it wiU sometimes determine the of crushing machinery that will have to be adopted any particular case. In its simplest form the anastaj shown in section in Hg. 77) consists of a shallow bed, which is paved with closely set blocks of hard i upon this pavement two or more heavy drags of are slowly pulled round, thus crushing the ore them. The bed is usually laid upon a foundation of laver of clay beaten down hard and made as firm and soliil as iK)ssible ; this foundation should be two or three feet larger in diameter than the proposed floor of the arrastra proix?r, so as to project all round it and prevent the escape of mercury ; whenever possible this foundation should be made of concrete, and be at least one foot thick. UiK>n this bed there may be a layer of two or three inches of sand rammed hard, and upon this the pavement may be laid, or the blocks, if well dressed, may be laid directly upon the concrete bed. The outer circumference of the arrastra mav be built of blocks of stone, but as the wear upon it is very shght, it may with advantage be made of wooden staves held together by a couple of iron hoops* At times the pavement of the arrastra is well below the surface of the ground, and then the lower part of the walls may simply consist of clay well beaten down- Wooden staves form, however, the most satisfactory wall the height of which generally varies between 1 foot () inches and 4 feet 6 inches. The pavement must consist of blocks of hard stone, carefully dressed and jointedf

nil: ARKASl NA jS,,

laid as evenly and as closely together as possible. be depth of the pavement is usually between 1 and feet, and the stones between 6x6 and 12 x 12 inches area; the larger they are, the better. The joints jomld be filled with good cement, or failing this with fine sand, worked in as thoroughly as possible. best material for the pavement is probably a fineed basalt ; next to this, a granite or quartzite. If ible, stones should be selected that do not assume a th polished surface by wear, but that retain a rough %xtiiie, as these are far more efficient as grinders. At same time the stone should not be markedly softer tttn the ore to be treated. In the centre of the arras tra iilxult in a block of stone or of wood, which carries the cnitral revolving post. This block should carry a step, inferably of cast-iron, in which the pivot of the central post can work. To this post are bolted a number of arms drying between two and eight. It is suitably supported its upper end, and the motive power, whatever this Diay be, is applied to it either by means of one of the above-mentioned arms or by a separate one. When animal power is used a pole is bolted to it, to which the animals are harnessed by suitable means, or else, as in the figure, the harness is fastened to a prolongation of one of the drag arms. Sometimes the upper part of the post canies bevel gearing, and is driven from a main shaft, which may then drive several arrastras, the main shaft being driven by various motors according to circumstances. A very usual arrangement consists of a horizontal hurdy-gurdy wheel having buckets inclined at an angle of 45 degrees, which is attached by arms to the central post, and which surrounds the outer wall of the arrastra. Water is brought to this wheel by means of a flume,

terminating in a vertical penstock, which deliTers i water on to the inclined buckets and thus rotates i central post. The di-ag-stones are attached to the a

proit'ctiiig from the central post ; they are usuilly i the same material as the pavement. Their lowoc be is usually somewhat wider than the up]iur one, and ll are comparatively flat, so as to have no tendency wK over. Into their upper surface a couple of eye-bolts secured, rather in front of the median hne. Theie t bolts may be leaded in or may be screwed into wtujilar plugs driven into holes drilled in the drag-stoui are attached to the arms either by chalas or by tboo' raw hide : there ia often a provision by means of wbitt the length of these attachments may readily be adjusleJ. As a rule, the two are of different lengths, so that lb* front face of the stone when at work forms an angle with the radii of the arrastra. This eniiUreB a thorough taraing over and mixing of the pulp. The stone i: that the front is just oil the bottom, whilst its rear[n presses on it. By this means the drag-stoue rides ova the quartz tying on the iiavement instead of pui>i£>' along before it. The number of drag stones is betwED two and eight, four being the most usual number. The)' should be as large as can conveniently he put in. Sin*" drags, two or three at times to each arm, should ouly be used when large stones are not procurable. That weight varies widely, the limits being two hundredweigtl and one ton respectively. Six or wven hiindredwicli' is a very usual size, four such stones can be readuj drawn by a pair of mules. The diameter of such arrastra is mostly between 8 and 20 feet, and the ividtb of tlic annular bed between 3 and 8 feet. The number of revolutions made is between C and 12 per minute, tbe

The Arrastka 291

speed being that of animal, and the higher that of or steam power.

er the arrastra has been built, it should be run for days on charges of barren quartz sand, until the & and pavement have been worn fairly level, and all ces are thoroughly filled up. The arrastra is then y for use. In the outer wall of the arrastra there ild be a series of holes at various heights closed by 58, and the should deliver into a sluice set on a rather grade, say one inch to the foot, which may be furnished ih amalgamated copper-plates, riffles, or blankets, as eomstances may require. Arrastras of superior design ive been constructed, in which the arrastra proper conats of a wrought-iron pan, about 8 feet in diameter, id a foot or more deep. These pans may be lined with ones, but have more usually a bottom of chilled-iron lates. Many old miners maintain that mercury is least ptto flour in an arrastra where stone works on stone, and lost apt to between two surfaces of metal. We really now so little about the phenomenon of flouring that it ould be unsafe, in the present state of our knowledge, to eny the truth of this statement, although it is difficult to any intelligible reason for it. Tlie arrastra is essenally a fine grinder and amalgamator, and is not suitable T breaking down the ore, which latter should be broken nail before charging, say to about i-inch or |-inch mesh. 1 small establishments, where there are only one or two rrastras driven by animal power, this breaking is usually one by hand. Larger plants driven by power should ways include a rock -breaker and, whenever possible, a lirof Cornish rolls for coarse crusliing. The ore should 3 carefully hand-picked, then cruslied and stored in bins ; possible, the plant should be so arranged as to allow of

S9a . GOLD MfLUNG

the automatic delivery of the crushed ore into the arm tras. The amount charged in will vary with the size oflJ arrastra. the nature of the ore. and the fineness to nhli it is broken in the first instance. It may be calcukteSl form a layer between 1 inch and 2 inches in depth on tbl, bed of the arrasti-a. Ad arrastra 10 feet in diameter k treat about half a ton of ore at a time. This is spi* uniformly all over the bed, damped down with water,ai the arrastra started. At first the front of the drags nuf I BO arranged as to bo about inch above the level of tlK pavement. After an hour or two, the ore will have ba ground fairly fine, when enough water should be addedu form a stiff uniform paste. The requisite amount of mW" Gury is then sprinlsled all over the surface of the pulp,uani being taken to distribute it as regularly as possible. This is perhaps best done by squeezing it slowly through a pie8 of rather fine canvas which will break it up into globnleS' The quantity of mercury used should be about three time! as much as the weight of gold supposed to he present m the ore. In case of doubt on this point it should bi introduced gradually in small quantities, some of the pulp being panned up from time to time in order to examine the character of the amalgam formed ; as long as this Btill shows the shape of the original particles of gold, oris granular.dry.and hard, more mercury isrequired.TintiWbe amalgam assumes about the consistency of putty ; itshoulo never be allowed to get softer than this, and is preferably kept a trifle harder. The mercury should be thoroughly pure and amalgamated with a little sodium, preciaelya " done in the case of mercury charged into the mortar of stamp-mill {see page 305). The most important point to pay attention to, in working thoarrastra, is the consistency of the pulp ; this should be just thin enough to allow the

The Arrastra 293

kgB to work easily and smoothly through it, so as to mix thoronghly, and yet not so thin as to allow the mercury sink tlirougb. the mass, but rather to remain suspended lit, in the form of small globules, until these combine nth the particles of gold. The progress of the amalgamabm can be observed by taking out samples from time to 'fime and panning or 'horning" them; it is complete nhdi no more particles of gold can be found on washing. This usually takes from two to five hours according to circumstances. When this stage has been reached, 4ie pulp should be largely diluted with water [and the ttrastra worked as rapidly as possible. With power irrastras, provision should be made to enable the speed to be increased to about 18 revolutions per minute ; with toimal traction the stones may be so far raised as to bear on the pavement with one edge only ; in this position it needs very much less force to drag them, and their speed may, therefore, be increased. This tends to settle the heavier particles on the bottom of the arrastra, and to suspend the lighter ones throughout the pulp, which should then be run ofif through the plug holes, commencing with the uppermost one, and continuing downwards till all has heen run ofif. The pulp is discharged through the sluice hex; if the gold-is very fine, this box should bo lined on the bottom with a length of 5 to 8 feet of amalgamated copper plates. If the ore contains sulphurets, blanket 8trakes should be employed, and in any case there should he a length of ten to 15 feet of sluice boxes fitted with transverse riffles, about A inch deep and 1 inch wide. Hen the plant consists of several arrastras, they may be arranged as all to discharge into one common sluice. The length of time occupied in working otf one charge is Usually between 6 and 12 hours, according to circumstances

. The quantity of ore treated per day is variable within very wide limits; it may be averagBdi about 30 owt. A twelve-foot arrastra of the best coi tion, driven by power at about 12 revolutions per mil requires about 6 H.P. to drive it, and will, under fai able circumstances, treat 6 tons of ore in 24 hours. As 1 general rule, after a charge has been worked off, arrastra is not cleaned out ; it will, therefore, retain whole practically of the amalgam, a large proportion the sulphurets, and any ore that may not have been groonl fine enough to be suspended in the pulp ; a new ohazgeii then introduced, and the operation repeated. Every week or two the arrastra is cleaned out, all its contents being removed as completely as possible by means of scoops and stifif brushes ; no attempt is, however, made toscrapeout the amalgam that may have found its way down into crevices between the blocks of the pavement. The material collected is washed up in a cradle, or, if small in amount, in pans, and the amalgam saved ; any sulphurets obtained are put aside for treatment. A complete clean-up generally takes places only when the pavement is worn out and has to be replaced. The stones composing it are then taken up, carefully scraped and cleaned so as to leave no amalgam adhering to them, and the sand upon which they are laid taken up and cradled till the hara bed below is reached ; if this consists of clay, the uppe part of it may also be scraped off and washed ; sometimes the pavement is laid upon several beds of sand of different colours, and these layers are taken up and washed one at a time, until it is found that the lowest hmit to which amalgam had penetrated has been reached. A new pavement is then put in as before, and a fresh campaign commenced. The length of time that a pavement vsiH

Huntington Mill 295

is exceedingly variable, depending upon a number Lrcnmstances. The labour required for working the .stra is very little. One man can easily attend to one tstra on a twelve-hour shift, besides spalling up the required for it. A boy is also required for driving mules or other animals that work it. In the case a power arrastra, where the ore is crushed by >per breaking machinery, one man can attend to two three arrastras, or even more if the plant is properly id out.

On account of the cheapness with which it can be mstructed and worked, the arrastra is sometimes used work up the tailings from stamp-mills, which may )ntain enough gold to be worked profitably in the former achine, though they may not pay for retreatraent in Dy other way. Th use of the arrastra is so entirely Bpendent upon local circumstances, that all its details subject to the widest modifications. It has therefore ien necessary to describe its construction and mode of ttion in the most general terms. It will readily be iderstood that the arrastra, as here described, may be apted by alteration of its details to suit the circummces of any given case in which it may be advantageous employ it.

rhe Huntington Mill. — This mill has now been in oration for some years in various parts of the world, i has given satisfactory results when working upon ores table to it. It is not adapted to very hard ores, though (cially suited to clayey ones, and is not, perhaps, as table as are stamps to the requirements of large mines, ere very big quantities of ore have to be handled. If irthing goes wrong with a stamp, it can be hung up hout affecting the others until an opportunity arises

for repairing it; moicoTer owing to tbo great simple of tliH uoobiiw, repairs are usoall}- capable of bcin, cutd repilly and easily, wbilat in the caae of tbe tington mill anj accident usually involves stoppi whole machine. Tlie great disadvantage of the p acting etamp-miil, as compared with grinding IE that the power required lo work it is alwaya the 9 whether there is any ore under tbe stamps or not, wtu a grinding mill runs with far less power when e when it is crushing. It is for this reason, among otbd that the mechanical ore-feeder must be looked upon as an indispensable portion of tbe stamp-mill, while iu importance to a crushing mil) such as the HuntiDgWi is far less. In tact, one of the disadvantages of tte Huntington mill is the fact that any ore-feeder connectol with it is hound to feed at a uniform rate of speed, whtever be tbe rate of crushing of the machine. It oaDnot, as in the stamp-mill, be arranged so as to feed ore only vrhen needed, and can hence scarcely be used when oree of very varying character have to be treated ; in any c&se it requires close attention. Compared with the HuntiDgton mill, the stamp has the disadvantages of greaU' initial cost, greater total weight, and greater time required and cost incurred in its erection. It seems clear, however, that its greater reliability compensates in most cases fo' all these drawbacks.

The Huntington mill, shown in section in Fig. 78, ac in plan in Fig. 79, consists essentially of an iron drunk< in the centre of which rises a pillar, which is made revolve by means of gearing beneath it; the loner portion of the drum is hned with a steel ring, against which the crushing is done. This ring takes most of the wear and tear, and is renewable. Just above tbis ring,

Huntington Mill

GOLD MILLING . j

one-half oi the ctrcumFerQiice of the drum is occupied the screens, of which there are throe to each their depth being about 0 inches. These screens an,, course, curved to the curvature of the machine, and. held in their places by means of iron screen-frames i keys. The remarks already made with referenM battery screens are equally applicable to those of I Huntington mill; owing, however, to the centrifb action of the machine, the pmlp is thrown against screens with considerable violence, and thoy wear out more rapidly than in the case of the stamp-miH any kinds of screens may he and are used. The ring the Huntington corresponds to the dies of the gravitation' mill. Crushing is performed by the action of three tour rollers, which are suspended by means of yokes from arms attached to the revolving spindle, the whole of these arms being in one piece, which forms a di8e-sha])ed cover to the pan. The raints of suspension of the rollers are a little further from the centres of the vertical spioiUe than are the centres of the rollers themselves; consequently the rollers, which swing freely i n a radial direction, are pressed outwards by their own weight against the ring die, thus aiding centrifugal action. The rollers have renewable ateel shells, held in place by wooden wedges, and are capable of revolving freely on the shafts that carry them, there being a provision for the lubrication of tho shaft where the roller works on it, whilst at the same tif the Inbrioant is prevented from finding its way into the pully. The disc that carries the roller yokes also earriesB set of scrapers, in order to turn ovir thi-piUy ami ilriw it towards the ring die. The rollers, like the ring die, are made of steel, so as to minimise their wear, these pieces being, like the shoe and die of the gravitation mill, the

Huntington Mill

nearing [lartsof tbo machiae. The rollers are bo suspeo a.% lo clejir the bottom of the pan by half an inch, men and amalfjam being allowed to accumulate in the so formed. The iron casing is in tliree pieces: a boB pnn with central cone through which the driving spit projects, and the upper part of the drum, which is at in two halves bolted together. The macbiue is usuaUj upon a substantial wooden frame. The crushed ore il livei-ed through the screens to a circular iron lip surroO ing them, whence it is discharged on to the tables orot aiitalgamatiti apparatus that may be used. Mercni] also charged into the machine just as in the correspond case when inside amalgamation is employed in the ma box of the stamp-mill, and the same precautions must observed with regard to this mercury as in the analog case of the stamp-mill. It is in order to prevent t mercury from flouring that the rollers are so suapeni as not to touch the bottom of the pan.

Tiic will is made ia tbi'ca sits as fiillows ; —

Heavier patterns are also made now, especially in 6-foot size, and some of the modern mills have screens extending further round the pan, there being instead of only three. The velocities given above those recommended by the makers, and are suitable comparatively soft material. With hard quartz a soi what higher rate of speed is to be preferred ; as crushing is done by centrifugal action, the speed atwli the milt is driven completely controls the power wb

able to exert. Authentic data are still wanted in r to enable the crushing capacity of the mill to be of by the only reliable test — namely the number crushed to a given size per indicated horse-power per hour. This may be taken at about 1 cwt. per P. per hour through a screen of 0024 inch mesh, Kttle more than two-thirds of the duty that can be lined from a good stamp-mill. The power required drive a 5-foot mill seems to be about 10 I.H.P., a t mill requiring about one-third less, and a 6-foot about one third more power. The actual crushing ity of a 5-foot mill, which seems to be the size used, varies between 10 tons and 30 tons per 24 according to the hardness of the materials and the of the mesh. Quartz of medium softness crushed 30 mesh (say 0*024 inch), which may be taken at about the average, can be put through at the rate of bout 15 to 20 tons per 24 hours.

In working the Huntington mill, the pan is first

charged with a sufficient quantity of clean pure mercury

to amalgamate all the gold that may be liberated in it till

it is cleaned up, which period may vary from a week to a

month. The usual mercury charge is from 30 to 60 lbs.

Ore is then fed in steadily, either by hand or by any of

the machine feeders used for the stamp-mill, which may

be driven by means of cams or by a small shaft taking

its power from the hne shaft of the mill ; the Challenge

ore-feeder is well adapted for this purpose. The ore

should be broken down by a rock-breaker to J-inch cube

&t the outside, before it is fed into the mill. For a large

plant of Huntington mills it may be advisable to use two

rock-breakers, the first for coarse, the second for fine

breaking. Care must be taken that there is no chance

3ea GOLD MILLING

of over-feeding the machiue if aoj-thing, it is pref thut thu ore ahoulcl be fed too slowly rather thai fast. When tlie ore-feeder is once adjusted it i trusted to feed uniformly as long as the quality i ore is unchangeil. As machine feeding is coutrollt tbo speed of the machine and not by its requiiemcuM in the stamp-mill, the ore-feeder will have to be readjui with each variation in the quality of the ore. Fiw the Hmitington mill requires, therefore, constant 0 and watchfulness, and is the most imjiortant factor! successful working. The central portion of the [ut when all is working well, should be practically free froi ore, its accumulation there being a sign of over-feadii The water supply needs careful attention and reguUtiai, the pulp requiring to be a good deal thicker than inth* stamp-mill ; since the pu!p is driven through the screeU hy centrifugal actiou, it doea not require so strong I stream of water to keep the latter clean. Once ouuA the screens on the lip, the pulp may be diluted to anj I desired consistency. As it is difHcult to see whethu all is going well whilst the mill is running, it should b8 stopped once or twice per shift for examination. Car* should be taken that the oil-holes of the central spin and the roller shaft are thoroughly plugged, whilst, neve thelesa, kt-epine Ijotli shatli; well lubricHUa, Tlio wsi of crushing in the Huntington mill naturally varies a good deal in different places, according to circumstances- The largest item of wear and tear is for renewals of tbe rollers and ring die : the amount of this is variously stated at between Id. and lOrf. per ton of ore crushed, bui it is obvious that the hardness or softness of tbe ore the essential factor in determining this. At the BriUiai'' and St. George Mill, Charters Towers, Queensland, there

Huntington Mill 303

K. 5-foot mills making 65 revolutions per minute rushing at the rate of 20 tons per day each. The items of cost are as follows per ton of quartz id:—

i. d.

Wages 3 414

Fuel 2 0-61

Renewals and repairs ... 2 6 05

Stores 1 3-39

Misoellaneons 0 2*32

Total cost per ton . . 9 4*61

the often quoted instance of the Spanish Mine, kda County, California, circumstances are especially irable to the employment of the Huntington mill, ore being practically a decomposed soft clayey slate I quartz stringers ; the total cost of milling by waterer (which has, however, to bo purchased, amounting bout 15 per cent, .of the entire cost) is about 12d, per of ore ; of this amount 2'3d. is the cost of renewing 3r-heads and ring dies, and O'GZ. that of the screens. ;his mine, mercury is charged into the mill at the I of j-oz. per ton of ore crushed in addition to the ntity charged at the commencement of the run ; the I of mercury is very heavy, amounting to 0*25/. per . The mill is cleaned up every month, there being an ingement which admits of the quicksilver and amal- 1 being conveniently discharged from the bottom of pan, when cleaning up the machine.

Chapter X

AMALOAMATIOM — INSIDE PLATES — COPPEB-TABLB8— 1 CUBY WELLS — AMALGAM TRAPSLOSS OF MXBCUH

Gold Extraction. — After the gold quartz hais crushed to the requisite degree of fineness by

of the machinery already described, the process extraction of the gold from it commences. It mirt] be understood that in practice the two operations p] on simultaneously, but, for the sake of clearness, I have considered it advisable to treat the two branchei separately, so as to distinguish between the mechanical process of ore crushing, and the chemical process of goM extraction. It has already been pointed out (page 17) that gold occurs in its ores in various forms, which my be separated into two classes, free or amalgamable gold, and non-amalgamable gold, which has to be extracted by means of special methods. Whenever an ore contains free gold in appreciable quantity, this is always extracted by means of amalgamation. Amalgamation may take place either inside or outside the mortar box (or other crushing machine) ; when outside amalgamation is practised, the apparatus used consists either of amalgamated copper-tables, or of mercury wells, or both. A number of machines have been introduced from time to time under such titles as gold savers, amalgamators, A:c.>

liP. X INSIDE AMALGAMATION 305

t not one of them has come into extended practical e, with the exception perhaps of Laszlo amalgamars and Hungarian mills, both of which are used in ongary ; both consist essentially of shallow iron pans K>at 2 feet in diameter and 4 to 6 inches deep; the >ttoni is covered with a layer of mercury, and inside aem there revolves a muller (of iron in the former nd of wood in the latter machine), by means of which lie stream of pulp is forced into close contact with be mercury. These mills are worked in pairs, a pair eating up to 3 tons of ore per 24 hours; a high Aciency is claimed for them as amalgamating ap £ances. None of the other machines deserve mention. lonalgamation, or the combination of gold with mercury, is a chemical operation, which proceeds by surface contact, and therefore demands a certain time for its completion; all that is needed to insure its completeness 18 that clean gold shall be in contact with a surface of dean mercury for a sufficiently long period for thorough oombination to take place. Whenever these conditions toe realised, complete amalgamation of all free gold will result.

Inside Amalgamation. — As amalgam is less apt to be floured than unamalgamated gold, seeing that it cannot be rendered brittle by percussion, as it moreover has a certain amount of plasticity, and as it is three times as heavy as the gold which it carries, so that the loss of a given weight of it entails a loss in value only one-third W great as the loss of an equal weight of gold, it is advisable to convert gold into amalgam at the earliest possible moment in order to minimise the loss of the precious metaL In its simplest form, inside amalgamation is practised by charging regularly small quantities of

jo6 GOLD MILUNG cM

mercury into the mortar box. As tf role iiiiimiijM charged once every two or three hours. The quiBi used must be about three times as much as that of gold supposed to exist in the ore crushed in the inteik The mercury falling into the mortar is broken into aift globules by the churning action of the stamps and wash of the pulp, and becomes suspended in fJk thoroughly mixed with the latter. There it oodmI contact with the particles of gold and amatgaiDnH them more or less thoroughly. Part of the amalgifl thus produced gradually coheres into small lumps vMH sink down and settle in between the dies; some isl jected towards the screens and passes through theOtwV be caught by the outside appliances, or else, when inside 1 copi>er-plates are used, it may adhere in part to thesa 1 Whether there are or not inside plates, the process of I inside amalgamation is precisely the same. Insidfi I plates, however, facilitate the collection of the amalgam, I and help to retain it in the mortar. They also assist the I process indirectly, because a mortar fitted with an inside plate below the screen is necessarily wider, and must, have a greater depth of discharge than a plain mortar, and the ore, thus retained for a longer time within the mortar, is more completely amalgamated by being kept in contact with mercury during a longer period. The methods of securing and attaching inside plates have already been detailed (page 150). In the Alaska Treadwell and other patterns of mortar, rifHes are used success fully instead of copper-plates for collecting amalgam. The copper plates should not be less than J inch thick, and must be carefully amalgamated. They are best silverplated, and in fact are treated exactly as will presently be described under the head of outside copper-plates.

Inside Amalgamation 307

BS the ore is very rich, these plates are only cleaned ice a week. If this has to be done more frequently advisable to keep duplicate chock blocks with copperts attached. The block in use with its plate is drawn, and a fresh one substituted without loss of , whilst the plate which has been withdrawn can be )ed and cleaned at leisure. In a large mill only one 3 chock block and plate need be kept, if all are, as should be, strictly interchangeable, for in that case >late from the first box can, after cleaning, be set to : in the second box, and this one in turn in the third, so on. The high cost of copper-plates renders this 5 of procedure advisable.

e condition of the escaping amalgam, as caught on rst outside copper-plate, gives valuable indications the progress of the amalgamation, and especially as letter the proper amount of mercury is being charged ihe box. If this amalgam is hard, brittle, granular, clined to crumble under the fingers, there is not jh mercury; if it is thin, soft, and pasty, there is luch, and the supply needs regulating accordingly. ]kmalgam when it settles on the plates outside the ry, should do so in moderately firm, coherent crusts ir when the gold is very fine, softer when it is 3 ; it should not be pasty, but in such a state that ids readily to the pressure of the finger and retains ipression. Machines for automatically feeding any id amount of mercury into the battery have been luced, but are very rarely used, most mill men pre- g to feed it by hand, as the quantity can thus be regulated to the requirements of the battery. A is used for measuring the charge of mercury ; ere shown in half size (Fig. 80) ; the cavity shown

(J inch in diameter and IJ inches long) will hold aba oz. of mercury. The Bpoon ig usually cut out ai piece of soun3, dry, hard wood, the cavity be ont by meatiB of a red hot wire ; the size of ihe ci may readily be varied, as found necessary, wbil frequency with which the charge is put in forms ft ther means of regulation. >fercurj' should be i the mill in closelystoppered bottles of very thick g of such a shape that they can be firmly held in t) Each bottle ought to hold from 3 to S lbs. of inorcniI.| The mercury should be kept in the mercury-room u a layer of mercuric nitrate in a tubulated receiver (|9gtf 66) ; from this the bottle should bo filled, and a little

sodium amalgam added. The sodium amalgam be kept in stock, or else be freshly prepared as wantei ] the latter being the better method. A little mercur)'i' heated gently in a porcelain basin or small flask, ftn then a few chip of clean, dry sodium thrown in, and if necessary pressed below the surface of the mercury with a glass-rod. The amalgam so made is then poured into the mercury bottle, which is filled up with mercury and stoppered. The sodium should he'Used in the proportion of a piece about the size of a pea to each pound of mercury. The usual test for the proper quantity is thai the prepared mercury will just commence to amalgamate iron. A cut nail that baa been filed bright may

Inside Amalgamation 309

Sdmersed in the prepared mercury; if it amalgamates

ceely, there is too much sodium; if the mercury does

not adhere to it at all, too little. The mercury should

mt adhere slightly to the edges of the nail, when the

noportion of mercury may be taken as correct. It must

lot be forgotten that the efficiency of sodium amalgam

lepends entirely upon its being kept from contact with

Bar, as it is decomposed by moisture. The complaint is

often heard that sodium does not save mercury from

wickeniDg in the mill ; but it will generally be found, on

investigation, that such complaints arise from mill men

keeping their sodium amalgam in the open air, or even

M times under water, or solutions of various chemicals !

These men may certainly have taken sodium amalgam

to begin with, but this will very soon have lost its

odium under such treatment, so that they were, in fact,

iJot working with sodium amalgam at all. If flouring

is to be prevented, the prepared mercury must be kept

Jn well-stoppered bottles. The tare of these bottles

should be known, and when filled with mercury, each

bottle should be weighed, and its weight registered in a

special mercury-book kept for the purpose. A check on

the daily consumption of mercury in the mill is thus

obtained.

Cadmium amalgam is stated to have been used with success in California in the place of sodium amalgam, but no data are quoted to substantiate this statement. Inside amalgamation is only unsuitable in the case of ores containing by far the larger proportion of their gold locked up in sulphurets, especially when much antimony or arsenic is present. If, however, care is taken to keep the mercury pure and well charged with ' Eleventh census of the United States, 1889.

3 lo GOLD MILUNC chap.

sodium, but little loss from flouring need be appirehi in any case. When the proportion of free gold is small, it may not be iorth while to use inside gamation. With very poor ores it is sometimes a important consideration to put a very large quantity j through the battery than to save gold very closely ; ia that case there should be no inside plates, and the depth of discharge should be kept low. Nevertheless, merauj : may with advantage be charged into the battery, beeuM: a certain amount of amalgam will always be there istained, and, moreover, this forms the most oonvenieBft mode of keeping the outside plates uniformly suliol with mercury.

Outside Amalgamation. — This is now mostly carried on by means of amalgamated copper-tables. These consist of a sheet of copper, the upper surface of which has been thoroughly amalgamated ; to the copper amalgam so formed, a thin film of mercury will always adhere, and this film immediately amalgamates any gold that may come into contact with it, the gold amalgam so produced adhering to the surface of the copper amalgam unless there is a great excess of mercury present. When in good working order the amalgamated table will thus at once retain any particle of gold in the pulp streaming over it, that may come into contact with it. The theory of table-amalgamation, therefore, demands a clean surface of copper amalgam, carrying a small excess of mercury, with which every particle of the escaping pulp shall be brought into contact.

The lip of the mortar-box discharges on to an upper table sometimes called also an apron, and from this on to a second table, although this practice is distinctly bad. Sometimes a lip-plate is attached to the battery itself.

on SIDE JMA/jJ.lM.n /iKV

kiid isoiiietinies an arrangement is adopted which consists a set of narrow plates, sloping alternately towards id from the screens in a zig-zag manner. There is no cribject to be gained by these arrangements that cannot biB attained equally well by lengthening the apron, unless .uttconomy of space is an object, and the arrangement is ; then adopted merely so as to occupy less room. If r'pOBsible, however, lip-plates should be avoided. The l>e8t arrangement consists of a solid table on independent firm foundations, which are so arranged that its inclination can readily be adjusted. This may be done either by vupporting the lower end of the table upon folding wedges 'Which are carried on solidly-constructed trestles, or else fcy means of bolts which pass through nuts secured to tte trestles, and upon the heads of which bolts the tables then rest ; the latter arrangement is perhaps the better one, but, in either case, provision should be made to prevent the tables, once adjusted, from being displaced the jar of the battery. The upper ends of the table should be hinged to a strongly built trestle or are sometimes hung by iron rods from a portion of the structure independent of the floor and of the frame of the stamps so as not to be affected by the jarring of the machinery. This is important, as irregular jarring prevents the amalgam from adhering properly. The case is, however, far otherwise with a copper-table that receives a regular uniform of vibrating or shaking motion. Such tables have been employed with a longitudinal and a transverse shake. The latter has been used in California in the form of a table 4 feet square vibrating backwards and forwards at right angles to the floor of the pulp at the rate of about 200 1-inch strokes per minute ; such a table needs only a very flat grade, not exceeding half-aniostt

. ani s mii to be a tbcj dBdeoi sx>s wiaoi jSmaei h&km iht mmaal fixed ecyper-l Tllzif uiiei ioiniua ve aAes aboot 6 indiee hi 2if arroBgv-jnittw gjoaM egacdy fit into their plaeet z£ Hszri£ Tx inataff ckaxs or vedtgee. The Tiifaw fOirauAicc he eoabemewed or nailed down to i Uiks. '!l!xisc are two nodss of anaogDmeiii in iiascL re -vJcehKisaivaeaiBfi: in the one, the tabk omrraira&i. eiba £vaa each other aocozately :i£i=:)£ iccai=K ieaaed iqpedwr into one sheet. In rfLbfc.ibfr -uiut 3£hBoSkaipiBUstBtheie being* cc frrcr i TO 6 anehes at each step. When the hWj &rr&ri?fc=teir.i i$ ii is Ibond that nemiiy all tie -.'.-. f- -: -J : : uL'-z 5 r.> f ihe lower cues. When ',.:. : : . 1 -. . : . ;. :- :. .: : y Tirie c : their superior specific V :. . J. ii -. 1 : : . : 1 i : -: : Is : : : : varies direcilv as its mass, '. M : . V :. . : >v. yv : f o: i 1 a rea : i n si nki ng under the

::k ':i.i> be dorie in displacing par- -I. I. :vfro::r.:r. :fce:r cohesion, and the :: A b-xiy :he r.i.re particles of water AZj :r-r:r :: .1 -ylio-. Av.i :ho greater the force required :: Ir rx-:r:-.: cr'-re :hr body can sink. As this force .IvirrL-'. ::: :'::v nif? :: :he boviy. there will be a certain y::n:. a? i:-? surfiCr increases, a: which the work to be d:n-r :r- 'ii?yl.\c:::j panicles of water will be greater thai: :h-r ::rco of i:rav::y which is acting upon it, and in ciis tho c:viy will be unable to sink, although it will i::ivo t ven less tendenov to rise in the water. The relation c: volurr.e to surface <and Therefore of mass to surface for any ;iveii substance; is greatest when the shape of the

m

dy is spherical and least when it is an iniinitely thin t ; and as a sheet must always tend to assume a Horizontal position in a stream of water, the resistance to pbiking is greatest for this form of hody. The very great (inalleability of gold admits of its being beaten into exively thin scales, and therefore there must always in the pulp issuing from a stamp mill, a certain quanty of gold which, in spite of its high specific gravity, no tendency to sink. When the pulp falls over a ip in the plates, these particles are more likely to be iMTOOght into contact with the table and thus amalgamated, than if the stream were allowed to flow on uniformly, without interruption, and for this reason it seems that stepped tables should be preferred to plain ones. The length of the copper-table depends on many consideratioDs, the most important one being the nature of the gold in the ore. If the gold is coarse a short table will suffice, but if fine a proportionately longer one must be ed. The length varies accordingly between 5 and 20 feet. A good average length is 15 feet, or say five plates 3 feet long, with a drop of about 2 inches in depth between each. The grade of the plates must be determined by the character of the ore and the supply of water. Plates must always be so adjusted as to be accurately horizontal at right angles to their length, so that the depth of pulp flowing over them is uniform throughout ; this is an important consideration to which much care and attention must be devoted, as uneven distribution of pulp means a loss of efl&ciency in the action of the table, seeing that one side would be carrying too much and the other too little pulp. It need scarcely be said that no portion of the surface of the tables may ever be allowed to run dry ; great loss of gold would be the immediate result, as dry

particles of gold or amalgam readily float on water! reason of the film of air adhering to them. The gradai the plates and the water supply must be so adjusted each other that each particle of the pcdp may slowly and steadily down the plate, rolling oyer over as it does so. It is better to work with a water supply and a high inclination of table than the opposite conditions, but there should always ample water to make a thinly fluid pulp. Finely-oi ore will require more water and less grade than crushed ; an ore rich in heavy sulphurets — es] galena — will require both more water and a heavier j than a clean quartzose ore, whilst a clayey ore reqmni more water and less grade. The water supply must be such that the sand can nowhere settle upon the plates, hut must always be kept in motion in regular waves across the tahlc. More water should not, however, be used than is necessary just to keep the plates free from any accumulation of sand. Too much care and attention cannot he given to this most important detail ; the due adjustment of the grade of tables and the quantity of water determines to a great extent the efficiency of the mill as an amalgamating machine. The grade of plates varies between inch and 2 inches to the foot, 1 inch being about an average grade. Stepped plates require rather less grade than plain ones, as the velocity acquired by the pulp in its drop tends to carry it forward. The drop should never be so great as to scour the plate upon which the pulp falls ; about 2 inches is a fair depth of drop.

Amalijamaihuj the Plates. — The copper-plates should be J inch thick and of the purest copper procurable. The best brands of Lake Superior or electrotype copper

Copperplates 315

mid alone be used ; any impurities will cause local galaic action, corrosion of the plates, and a foul surface the amalgam. The plates should be carefully rolled, that their surface may be as true as possible, and onld be delivered to the mill in thoroughly good order this respect. They are first annealed, which is usually me by heating them over a fire of shavings and small llets till they are hot enough to char a piece of paper id on them. During this heating the copper-plates Ay be supported on a sheet of iron to keep them from ackling. If at all buckled when cold, they must be fiatmod by gentle blows from a hammer, a piece of wood eing interposed between the plate and the hammer, f they require very much hammering they ought to annealed again, but this is scarcely ever necessary. rhe object of this annealing is to allow the molecules of pper to assume their normal distance apart, and thus Pender the absorption of mercury more easy and re. gnlar. During the heating, the face of the plate which it is intended to amalgamate should be kept uppermost. Pine sand (sea sand if obtainable) is then sprinkled on the upper face of the plate, well moistened and rubbed in with a block of wood until every portion of oxide is imoved and the plate has a uniform red surface, care being at the same time taken not to scratch it. The nd is then washed ofi*, and the plate dried and polished ith fine emery paper folded over a block of wood. A perfectly clean dry surface is thus produced. A mixture then made of about 10 parts of sand to 1 of coarsely pounded sal-ammoniac ; this mixture is damped with ater, and clean pure mercury is sprinkled into it by Ueezing through canvas. This mixture is then rubbed the plate with a piece of canvas or blanket, when

3l6 corn MILLING

amrilgamatioD will at once eommeDce ; inoru muBt be sprinkled on the plate from time to t: the rubbing continued until a uniformly bright ! surface is obtained. As an approximate guide t quantity of mercury required, it may be mentioned li each square foot of copper will retain oz. ai raera this being the actual amount which I found i experimenta on the subject. The effect of the ammoniac is to dissolve off any film of oxide of c that may have formed on the surface of the copper M as to promote metallic contact between the mercury w the copper.

The amalgamated plate is nest well washed water and kept till the following day ; it will t probably be found that the plate is dulled and coveroi vrith a coating of a greenish-grey substance; thin films this substance may show different colours ihw to interference, but when sufficiently thick it is alwiiyf greenish -grey. This substance invariably forms on tbt surface of amalgamated copper-plates, and is due W the oxidation of part of the copper of the amalgam, have analysed it and find it to be a hydrated oside w 1 copper, with sometimes some carbonate ; probably, wb*'' water containing sulphates is used, a basic sulphate mny also form. It is soluble in a number of substaii<< such as dilute acids, ammonia, and potaasic cyani* Usually the plate is rubbed up with a dilute solution of cyanide, a little raoro mercury being at the mdic time rubbed in. It is, however, better to prevent ih formation of this troublesome coating by replacing in* copper in the amalgam adhering to the table by wlVBf' This may be done by rubbing in silver amalgam inate*'' of mercury at this stage. This silver amalgam may

Copper-Plates 317

ared in various ways, but best, perhaps, as follows : sulTicient quantity of silver coin (about oz. per kre foot of surface of the tables) is dissolved in dilute ic acid in a porcelain basin with the aid of a gentle b. The solution is evaporated to dryness very gently, ferably over a water-bath, and then heated till the ne mass commences to fuse, and till all its bluish ;e is turned to greyish-black, this change indicating t all the soluble cupric nitrate is decomposed, inable cupric oxide being left behind. The salt is then solved in a small quantity of distilled water and filtered o a jar or beaker. Pure mercury, to the weight of out three times that of the silver used, is poured in, few drops of nitric acid added, and a few pieces of ight iron floated on the surface of the mercury. The ver will at once commence to precipitate and be abrbed by the mercury forming silver amalgam, the 'ocess taking a few days to complete thoroughly. The Iver amalgam so produced 'should be of a pasty constency. This amalgam is then rubbed hard all over le surface of the amalgamated plate, which is kept lost with a dilute solution of potassic cyanide-; a good ibber for this purpose is made from a strip of pure idiarubber, inch thick and about 6 inches long screwed a strip of wood, which forms its handle, so as to 'object inch. The rubbing must be continued until the hole of the plate is completely coated with silver amalim, which will then keep the plate from tarnishing. A still better method consists in employing elcctro- Iverod copper-plates,which are afterwards amalgamated, nch plates can be obtained from any makers of mining machinery, or, if preferred, they can be silvered at tlie ill. This latter plan should not be adopted unless in

3i8 GOLD MILUNG

m

the case of a very large mill, where plates are coni requiring re-silyering ; otherwise it is cheaper them ready silvered. Electro-silvered plates are amalgamated by rabbiDg with mercury containing sodium amalgam, the plate being kept wet ; of silvered plates require no preliminary polishing, and( to have been annealed before being silvered. Eli vered plates usually carry from 1 to 2 ounces of square foot. Sometimes in preparing new plates, amalgam is used instead of silver, but this is rarely except in mills already in operation where there supply of gold amalgam on hand. Silver is quite effective as gold for keeping plates bright, though lessi for catching gold ; but one or the other should be in order to keep the surface of the amalgamated pi bright, until a film of gold amalgam has been deposit all over them by the operation of milling. It must nc bo forgotten that the first bullion obtained from electro-| silvered plates will be of a lower grade of fineness than! the gold contained in the ore, because it will contain some of the silver removed from the plates.

Various alloys have from time to time been used to replace copper, but none of them have proved successful The only partial exception may be found in the use of Muntz metal, an alloy of zinc and copper, used for the sheathingof wooden ships; this was employed some twenty years ago in the Thames district of New Zealand, because copper-plates were not obtainable. It is best amalgamated in the same way as copper, replacing the salammoniac by dilute sulphuric acid ; Muntz metal is said not to absorb mercury like copper, so that amalgam adheres less firmly to the former than to the latter, whilst the amalgamated alloy is said to be less liable to become

.ted and foul. Tliere is always risk of brittle gold

ring to some of the zinc of the Muntz metal finding its

ky into the final ingots of bullion. No advantage has

m proved to result from its use, and its employment

never spread beyond the district in which it was

Luced by local necessities. Silver plates have been

and answer admirably, the only objection to them

king their high cost.

Working of Copper-Plates, — When the plates have been ly amalgamated and placed in position, it is advisable (t of all to run some barren quartz through the mill and rer the plates. This gives an opportunity of adjusting le water supply to suit the grade of the tables. As soon all is in order, the mill should be stopped and the plates ibbed up with a piece of india-rubber ; if electro-silvered lates are used, a little mercury only need be sprinkled -Over them till the surface just feels soft ; a plate that feels liard has too little mercury, but this latter should never be ded in such quantity as to produce any tendency towards forming distinct drops ; whenever separate globules of mercury form, it is a sign that too much mercury is being xised. When inside amalgamation is employed no mercury need be added outside the mortar. Sufficient will always be carried through with the pulp to keep the plates in order, and, as already mentioned, the condition of the amalgam on the plate nearest the mortar is the best criterion by which the feed of mercury can be regulated. Some mill men prefer to add a part of their mercury inside the boxes, and to sprinkle part over the outside plates. There appears to be not the least objection to this practice, but on the other hand there does not appear to be any advantage in it. Mills run side by side using both methods seem to have given about the same results as regards

3ao GOLD MILUNG

saving of gold With new plates the mill sht stopped every four or six hours, and, when the plaU become " set/' once every shift, for " nibbing np if necessary, for taking off the amalgam, althong is usually done at most once in twenty-four hours, rubbing up the plates, the mill is stopped, the water allowed to run till all the pulp is washed off the taUe necessary, the hose-pipe may be used for this puipoM the plate is discolouredbytheformation of a filmof the| deposit, this must first be removed, the solvent moi employed being a solution of cyanide of potassium ; ioi mill men make a practice of throwing a few peoes cyanide into the mortar from time to time in order t keep their plates bright. This, of course, is only necessaij where non -silvered copper-tables are used, and befon these have become properly set" by the gold amalgaa adherin* to them. The use of cyanide of potassium WM at one time very extended, but of late years it has beea larjj;ely discontinued, and rightly so, seeing that such i solution of cyanide is a ready solvent of gold, which may thus be carried away and lost. Moreover, electro-silverd plates, which are now being very largely used, do not riMiuiro the use of cyanide solution, which should indeed nt'ver ho apphed to an electro-silvered plate. The best solvent for the grey deposit is very dilute acid, (not, hovlivor, nitric acid), whenever this can be obtained. I hav( kept new plates beautifully clean by means of a narrow leaden trough fixed just above the top plate ; this was kept filled with moderately dilute sulphuric acid andfitte( with a syphon formed from a strip of blanket, so as allow a continuous supply of acid to drip on to the plate The acid acted, no doubt, not only by its direct solven action on the deposit, but it also, by attacking the fin

Copper Plates 321

iicles of steel (from the shoes and dies) suspended in I pulp, produced a series of electric couples with the r-plate, which would tend to keep the surface of I amalgam bright. As a rule it is scarcely possible to ep new non-silvered plates bright at first, hence these ist be rubbed up very frequently. After the film has en removed, a little clean mercury charged as usual .ih sodium is sprinkled over the plates by squeezing rough canvas, and this mercury is then rubbed in hard r means of the "rubber" already described, beginning I the top of the plate so as tp distribute the pasty amalun as evenly as possible all over the plates. When the ates have become " set," and it is desired to clean p amalgam from them, this is done before rubbing up. he tools used in cleaning up plates are " amalgam lives (which are simply large-sized painters' palette lives, 6 to 8 inches long, made of good flexible steel, and ept scrupulously clean), scrapers liiade by turning ov(ir leend of a worn-out flat* file for about half an inch and. rinding to a chisel edge, and one or two sbai*]) wood, bisels about 1 inch to inches broad. An enamelled on cup forms the best recipient for the amalgam, bmmeucing at the bottom of each j)latc*, two nien, one n either side, rub all the amalgam upwards, using either :iff brushes, like scrubbing brushes, or the india-rubber rubl)ers ; any amalgam that may adhere too firmly to ethus dislodged, may either be softened by a few drops f mercury, or loosened with the scraper or wood chisel ; 16 whole of the amalgam on each plate is thus swept ito a heap, and then transferred to the cup by means of le amalgam knife. Care must be taken not to scrape 30 closely, so that enough gold amalgam may be left to 3nn a film all over the plate. When the clean up of

JM COUy iilLLlXli

etudb plMe is finiihed. s litUe mereuiy is sprioltled an the pktes, mA ibey nibbed down as before. An d jection to the me of stepped tables is that these take. little looger to clean np tt&n do plain tables : but in opinion this objection does not by any means oattnij the advantages o( the former system. The amalgam locked in a safe and reserved for after treatment, whole process of cleaaiDg and rubbing up a 1 plate sbodld not occupy two men more than ten mi and robbing up alone, five minutes. from the ti stopping the battery to that of restarting it. Usmlt this opportunity ia also utilised for changing scneai goide blocks, Ac, or making any small repairs, may be needed, to the mill.

Gold amalgam gradually accumulates on the pUtH' which camiot be removed by the daily clean up; tlai is usually taken off once a month, when the plates in thoroughly cleaned. For this they are sometimes takenoff the tables and gently heated so as to soften the amalgsiD, and then rapidly scrajwd. A recent American impror* ment on this practice consists in softening the amalgwn by dipping the plate into boiling water, A still betltf plan in mills driven by steam power is to heat up the plate in place by turning a jet of steam on to it, coveiin| it at the same time with a wooden hood to keep tlw heat in , The amalgam may also be softened by meana ol mercury, and the plates thoroughly scraped with wooc chisels without removing them from the tables, but grea care must be taken not to go too deep, so as to avtui scratching or cutting the plates. A film of gold amalgai will always adhere so intimately to the copper as not t he removable in the ordinary way, and this has to b taken off by " burning." The copper plate ia taken o

Cirri:i<ri A I i:s

ke table and heated sharply over a fire of wood-chips, t., till all the mercury is volatilisedi care being taken to inhale the fumes ; the gold can then be stripped in a more or less coherent film. After "burning,'* plates must be re-silvered or amalgamated hke new They always, however, retain some gold, which .rs to soak in by some form of molecular action it becomes truly alloyed with the copper. Hence mill plates have an intrinsic value considerably above of the copper they contain, and in selling them this be taken into account. It is perhaps best to have rttem melted down and to assay the resulting ingots. %i8 circumstance that old mill plates contain valuable quantities of gold is one which is occasionally overlooked, And instances are on record where the purchasers of M old mill have realised very handsome profits on their bargain through such neglect on the part of the Some interesting figures, which must, however, be

) looked upon as exceptional, have been recorded by Mr. B. T. Bayliss at the Montana Mining Company's mill, Montana. A copper-plate, 8 feet by 4 J feet, plated with 1 ounce of silver to the square foot, was in use 3 years and 10 months, during which time 14,942 tons of ore were run over it. Outside amalgamation alone was used, and the plate was only rubbed up and never scraped, until at the end of this period a scale of hard amalgam 0*16 inch thick had accumulated upon it. By daily rubbing this plate had yielded 6,426 ounces of bullion assaying 641*5 of gold and 4439 of silver per mil. It was then burnt," when it yielded 866*1 ounces of bullion that assayed 431*4 of gold and 562*5 of silver

Trant. Amer, Inal, 31 in, Eng., vol. xxvi., 1897, p. 38.

Su Cold Milling

per rail. After " burning." the copper-plale was niell U[i And was found to contaiu 896 ounces of gold, or i other words the plate litul absorbed aboat j ounm gold to the eqiinre foot, which could not be remoTed any meclianical treatment short of removing the niaUi of the plate itself,

The aniount of Rold superficially retained by 00[i plates hae also to be taken loto account in custom ing : it iH uBual to allow the owners of the stone whiob is being milled to scrape the plates for theinselTes, bd they are not aUowed to heat them, and, of course, mol not scrajit: down to the bare copper, but must alvajV leave auch a film of amalgam on the plates as i thi-iij ill working order. Thia is a matter of little or no iinjjortanee when large lots are milled, but in the rniliiag of small test parcels of a few tons, the results are liible to Ijo rendered utterly untrustworthy, because it is impossible to he sure that the amounts of old amalgam it the commencement and end of the run shall be eiactl)' the same. This very obvious source of error is frequently overlooked in test-crush logs, which should never be luaJe ow n, less (]uantity than 25 tons at the least, whilst ll wtiuli; paruol s)iould be put through one luortar box only.

Quantity of Water. —The amount of vrater required for a stamp-mill varies within very wide limits, depending upon the character and composition of the ore, tha detree of fineness to which it has to be crushed, and tlie oiTiciency and arrangement of the battery. Average figures iLiive already been piven on page 263, and it can only be added that the exact amount required in any ciise can only be determined experimentally. As already pointed out, the chemical composition of the battery

Copper'Pla Tes 325

iter is no less important than its freedom from susended clayey matter or slime ; free acid, for example, bond always be neutralised, and greasy water, e.Q.y (mdenser water from an ordinary condensing engine, hold on no account be used ; if its use is absolutely navoidable, it should be treated with lime, and the preipitate formed allowed to settle before it enters the morirs. ore that contains much clay will require more rater than a comparatively clean quartz.

In order to be able to regulate the water supply with preat accuracy, the valves controlling the supply to each lox should be fitted with hand-wheels, projecting over he front of the mortar-box ; the mill man can then vatch the effects on his plates whilst he is in the act of letting his valves. It is an advantage to have tliese Muid-wheels roughly divided by marks, say at every ten legrees, so that they can be at once set to any desired position when restarting the mill after a stoppage.

Ttmyerature of Water. — Tliis is an important point in khe management of copper-plates. A high temperature favours amalgamation, but at the same time tends to K)ften the amalgam and make it so fluid as to be readily sarried off from the plates by the pulp. A low tempera- Qre, on the other hand, not only retards amalgamation, 'Ut also renders the amalgam so brittle that it is apt to ramble to powder, and thus bo carried off and lost. "he most suitable temperature is about 60 to 70 F. As rule, it rarely rises above this, because the pipes bringing water to the mill are mostly (and should always be) )vered up so as to be protected from the direct rays of le sun. In cold climates it is necessary to heat the batry water in winter time, and provision must be made X this when putting in the water mains.

kKid. thm lomr edge of vfaidi j k; th* ladp faM tlias to bme its te wf et with tbe woemary befcm King this eootaet,

to the koMom of the well. Mercary ' iiralia th&Q

tpensi) with even lht I :tMIIy always comliiMd 1 s. Thfre are namtrdU 1

Id tlie first place, thej I ary, in whicVi a ueruin ed up. Where so inimli ] EMfcory is ID I jid to be proportion atelT

greftt, and tbe aRai._j lost is sure to can? of

some gold with it. Moreover, the well is not a vnr etTective aoialgam&tor, as the surface of mercury nbicl it exposes to the pulp is small; it is doubtful whetbei any niercun* well exposes as much surface of oierenrj to the pulp as a table 12 inches long would do.

One of the best forms of mercury well is shown in tbe i annexed figure (Fig. 81). It is made by Messrs. Appleby Bros., Limited, of London. The material is cast-iron, and it is enamelled on the inside. The baffle boafJ slides in a foovc, and can be fixed by wedges at any desired point. Screw plugs are provided in the bottom by means of which the mercury and amalgam can be drawn off when it is desired to clean out the well. Mercury wells are frequently made of bard wood, and I have employed wells made of stout sheet copper

iCOXy WEILS

nated od the iaas

to enable the 1 to moisten the the weU, tbaa ing the OGcnmolasand

, &a., on the and in the comers Tell, which otherften takes pUce. '&c the material of U, the principle is

the same. The hat requires moet

the manipulation

lition of the baffle if this be too deep mercury, the pnlp unable to balance

imn of mercury so and the nell will

ed, with the result

0 pulp at alt will

rough it ; if, on the hand, the baffle

is not immersed

ntly, the pulp will slide over the surthe mercury with-ring

up the latter or coming into

contact with it.

orect position for

o

a:

O

L, however, be very easily determined,

tht; Ijaftle boiird ci even thaoraticall)',

Let Figs. 82 and 83 represent diagrammatically crow sections of a nterctiry welt before and duriiij tlio past of tlie [Hilii r -spBc lively, tde mercury being distiiiguisiirf by Lbo shaded, and the pulp by the dotted jiortioiia the diagi-am. Let the height of the inflow of pulp nlV9 its overflow be called a, the height of mercury above lJi bottom uf the biiflle hoiin! when the pulp is flowing i.

5"

and bcfoie tlie pulp flows A, and the height of the eseapnig pulp i,bo\e the In el of the mercury c ; also let naodw l)e the itiijoctive widths of the inflowing and outflowing sides of tlie "ft til Then in Fig. 83 it is necessary, in order tha,t the pulp iniy flow, that the descending column should be heaviei than the ascending one.

Therefore a + 6+c>c + 13'6& {the specific gravity ot mercury being talten at 13'6) or a>12-66. In Fig. M, if the baffle board he immersed in the mercury to S depth d, it IS clear that the column of mercury whose

Mercury Wells 329

Kiepth is d must be transferred from the inflowing to lie outflowing side ; when thus transferred its depth

will be -fZ, so that the total difference in height be-fc

'veen the two columns of mercury will be ( 1 + j

THierefore b (Z (l+ j. As the pulp will only flow

am

' is

value of d is given by the expression io.fi/ , y Thi

expression accordingly indicates the greatest depth to vrhicli the baffle board may be immersed in the mercury ; it is not strictly accurate because various factors have been neglected, such as the capillarity of the mercury, the weight of sand can-ied by the water, the thickness of the bafllc board, and the depth of the layer of pulp passing beneath it. These errors, however, partly balance each other, and the above formula may be used a fjeneral guide. To take a concrete example : with the well shown in Fig. 82, a 9 inches, n 2 inches

'" 3o inches, and therefore the greatest possible value

'f/will be — -- z — -TT =0-45 in. A convenient niodi-

"cation of the mercury well, much used in Hungary, is hown in section in Fig. 84. In this form the well is ider and deeper than usual, and the bailie board is Replaced by a rectangular block of wood (a), the position of which is regulated by a couple of thumb-screws (n). hich the pulp has to travel, the bottom of the horizon-

:i.W GOLD .V/LL/XG

tal portion of thi; (J consislicig of a surface of i ThflTv U A deptl) of inch of mercury (c) in tl which is coDstnicted of sound, hard wood, and the bli is screwed down to within a inch of it. The width t( the channel on the inflowing side inch, ami c outflowing side j incli. Thus arranged, tliia wel! hM I MpAcity of cxAAe feet of pulp per minute for each fo in )iMi|b of the well, and this can be increased, J DManwy, by lifting the block by means of the ihtui screws. A series fl these wella is asa arranged one belo* the other as shown in ] the section. Them cury and aaialau be drawn off wlw desired through ]npe at one end of tin mercury well, bottom of the well sloping gently in &" directions towardstiis Xy- Wooden merooiy wells are always best cut oul

Wlwn in ojteratiou, amalgam gradually collects ii iv)l : this sinks to U>e bottom unless tlie ore be ver]' in ittlwj. The gold-silver alloy containing W rla of hi\vtt to one of gold has just about the sane fttuvity as mercury, so tliat all alloys coDtaioiiig k*%t than this proporticm will float on the surface d writ iiitM>l of sinking. Such an alloy, however, ii found occurring in gold ores, lien mncb lunUtcd in the well, it is best removed.

: Mercury Wells 331

Shis may be done by means of a small scoop pierced ith holes, but is usually done by hand, the amalgam being roughly squeezed, and the excess of fluid mercury letamed to the well. If necessary, fresh mercury is then Boared in to replace that so removed. The surface of the Mercury in the well is apt to become foul and dirty. When this occurs the mill must be stopped, and all sand, Be, washed as completely as possible off the surface of As mercury, which is then skimmed by means of a ipieoe of indiarubber, or of thick blanketing, until the nrface is once again quite bright. The skimmings thus tibtained are put on one side for subsequent treatment, 'en a complete clean-up is made, all the mercury and amalgam are removed from the well by means of the wrew-plug, or else by means of a small scoop if the well 18 not furnished with a plug, the entire contents being eoUected in mercury pails. These are best made of stout enamelled iron, and should be fitted with a strong iron strap going right under them from side to side, and furnished at the upper end with eyes in which the bale works ; this is better than having the eyes merely riveted to the pail, as much strain is thrown on them. About 1 to U gallons is a suitable size. It should be remembered that if the mercury is allowed to run into a bucket half filled with water, there will be no loss of mercury by splashing. The contents of the mercury well are taken into the mercury room, skimmed and allowed to stand for about a day. At the end of this time the fluid mercury may bo poured or syphoned off from the semiflaid amalgam which will have settled in the bottom of the pail. This fluid mercury must be purified from base metals as previously recommended, charged with a little sodium, and may then be used over again. It will still

be made OBe week to ooee a montfi. i ofcanHUoceft. If b man diScBlt gosH igKw theft oC amalfflm bea vdb owed than in tbt of tables, Iht fnnner gnv no iadieetkins nbeiber ban bevn tam|Kml witb nr ooL

XcRVy Ikt. — It is nsiisl. ami \-ery adrisab place U the foot of the last rupper-plate an array for eatditng any ua)r partJeleg of mercury that haro eaflaped wHh the palp. The osoal (onn mod a boi into which the palp is allowed to Horn, the sUtKtioii r ii -A-r. .: n~: -n down to the N of the box and then rise agaia before it can & The heavier particles, and amoDgsi them mercarj collect in ibe bottom of the box, the lighter porti( ilie palp flowing away. The contents of the bo emptied out from title to time, and pot aside for treatment. Mercury traps may be made either of or of iron ; they should be of fairly large sectional so that the rising colamn of pnip shall not mon rapidly and have time to deposit all the heavier pari .A wooden and a cast-iron mercury trap are show spectively in Figs. 85 and 86 ; their constructia so simple as to need no explanation. Sometime! mercury traps, one below the other, are empl When the pulp leaves the mercury trap, the whole c tree amalganiable gold is supposed to have been extn from it, and the process of extracting the combined then commences. A certain portion of the mercnr} (?5cai.>es with the pulp, and some, but not muci caught in the process of concentration. Some is, ever, so completely floured that the minut put

I Mercury Traps

which it is reduced, defy all attempts at collecting If The value of the mercury thus lost is not very Blii, but its loss always entails some loss of gold, as Ik escaping mercury is probably fully saturated with

Vertical Section.

Plan.

Scale, I '— i' Fkj. 86.

this loss within the narrowest possible limits. Its amount varies very greatly, depending principally upon tbe methods of amalgamation employed. It is probably least when outside plate amalgamation alone is practised, tod greatest when mercury wells are used. Recorded

3JI COLD itILUSG

loM Tmiy between 0-001 lb. txA 1-0 lb. of mercnrjr f, too of ore cnuhd. The losses in the El Calloo iriiicfa mar be looked upon as a tjrpe of a well nu od were bout 003 lb. per ton of ore over as entire and in tb Tnad well -Alaska mill 001 lb. The avsnf loss when inside amalgunation and copper ublee employed may be pat down 003 lb. per Ion, or i a pound of mercury for every thirty tons of ore niS

This loss of mercury represents a cost of about per ton ; in some special cases it has risen to as much ss Qld. per ton, but this &gure is altogether exceptional, and indicates very faulty work. Tn California 1 lb. of mercury-to fifty tons milled seems to be about the average: in Victoria eome of the bast run mills appear to lose 1 lb. to thirty-five tons, whilst in New Zealand the loss seems to be twice as high. In order to keep a check

Loss Of Mercur Y 335

the loss of mercury, a special mercury book should

kept itk which the weight of mercury supplied

I time to time to the mill during the run shall

titered, together with the date, and debited to the

Wben a complete clean-up is made, all the mer-collected

from all sources must again be weighed

entered, on the credit side; the difference will of

ae be the loss during that run. It will thus be seen

ther the loss is either excessive or irregular, and in

er case it should be promptly investigated, so as to

rd against the loss of gold that may accompany it.

aercury book systematically kept affords, moreover,

t of the best checks on any thefts of amalgam that

y be taking place in the mill

Cbaeteb Xi

a him

VtaOv TntBttf ti the Vi.— Wbei

earapfi -r AJ-im trftps i; is suppjsd to

eold. or so little oomht

sitriof;. the palp runs dBodljr

. -'.'.r. which c*rrii it tonsleiil

U a oe<iless to point oat tbil.

:! giiQ site. ftai|de provisiMi sbooU

: -: lir-gs damp leading into aotne raraia.

$H:i:!'. or nTi!F. wluefa H not liable to be choksi Of'

SboaU tb pdipoonuin combiiwil fto\d {oBUi

ihr KkW comhhW ' meMy as k coavbiuent eipnceiM

to Muify pM Uat trtuw to '""'grtrby smli't

coQlKt iiii mereivy, sad not u Bwvwrily inibcftlinj

tht it tt m K slate of oombiiiAtka), tben the

pulp will bn %o nndw farther fetmas in order to

tvctiver this ootnhittod fold. Tins object cu

aicti>nl ID io way : firstly, by tneuis of cuonjUntian'

L.tii ti-'lii'tCs the dunfe-rous portiooa uf tho t4iW

;:,to s comparaciTely small balk for farther treatmeni:

sti.'oiHUy. by means of the more modern method of

Ap. Xi Concemtration

treating the tailings before or after such removal of con- J" centrates by a chemical process, as, for instance, by eyanide extraction.

€oneentration. — A study of Table C, page 10, wbicli includes all those minerals which carry gold with them, will show that all these are comparatively heavy ones. The specific gravity of gold-bearing minerals may be taken at 5*5, or a Httle higher ; whilst that of the nonmetallic and worthless portions of the pulp, which will eoDsist of some or all of the minerals in Table A, mav be taken at about 3 ; owing to their high specific gravity, flome of the minerals enumerated in Table B will usually Aooompany the gold-bearing minerals. The object of concentration is, accordingly, to separate all the minerals of specific gravity of 5 or over from those of 3 or under. The mechanical principles upon which all concentration is based may be very briefly and simply stated. All I bodies that are acted upon by any force would be J propelled by this force at equal velocities, if friction did Dot exist, and if there were no resistance to their move- ? Dient. This theoretical condition is, of course, never I lised, as every body meets with more or less resistftnce from the medium which surrounds it ; its motion '>eing impossible except by displacing the particles of this ijedium, more or less of the force causing motion is absorbed in performing this latter operation, the amount absorbed being greater, other things hein equal, the greater the surface of the body in question. Accordingly, \?hen a body is immersed in a fluid, whether gaseous or liquid, the particles of which otTer resistance to its free motion, then the velocity imparted to it by a given force varies with its mass and inversely with its surface. Assuming the bodies to be spherical, the mass will be a

z

foDotion of d>j, where d is the diameter of the spbn and a the specific gravity of the substaoce compoeiai nhileits surface varies invers(tly as J*; heuce thevelodtt tinpartl to the body will bo a function of ds. In orda that di shall remain a constant, if s be diminiabeil 4 must be proportioD&liy increased, and vice versa: ioj otiier words, a given force will propel a smaller body o(| high specific gravity with the same velocity as a propot-: tionately larger body of lower specific gravity, whai both are suspended in the same fluid medium, and, utKltt, these circumstances, a heavy particle vril! move fBtu,| or wiU move through a greater space in the same periei| of time, than will a light particle of the same siie. Tiieretore, when a number of particles, approximaloij spherical and approximately uniform in size, ara subjected to the action of a force, which may be gravity or may be some mechanical impulse, which shall tend to move them in a given direction, the heavier particlM, Hi'Aiiv further in the same space of time, can tbns In. .I'ariitfd from the lighter particles. The above staiemems are true of particles approximately apheriwl, but it has already been pointed out (page 313) that tha shape of a particle has a most important iuflnence on its behaviour when suspended in a liquid; so that sufficiently thin sheets, for example, have do tendency to sink in a fluid, however much greater their specific Tavity may be than that of the suspending medium, oo accoimt of the very high ratio which their surface beara to their mass ; whilst in spheres, where the ratio of surface to mass is a minimum, the relative effect of varying specific gravity is most readily apparent. Successful concentration, therefore, can only be effected when the particles of crushed matter to be treated approii-

Riffles 339

te to tlie sphere in shape; the object of a crushing chine for preparing pulp for concentration should vrefore be to granulate the material as far as possible. the stamp-mill this object is never very well attained, fe the best results are produced when the crushed ibterial is discharged with the greatest rapidity from B battery box. Inside amalgamation, when inside qpperplates are used, is accordingly prejudicial to good yncent ration, as it precludes working with a low disharge and a narrow mortar box, which are, as we have nen, the conditions which promote rapid discharge of idbepalp.

Concentrators. — The number of appliances designed €or concentrating tailings from stamp-mills is something enormous, and new modifications of them are continually l)ttng brought out. The number of those that have successfully withstood the test of experience, and have held their ground for any length of time, is by no means great ; are reducible to a few simple types, which may be classified as under : —

1. AppHances in which the heavier particles are simply allowed to settle under the action of gravity ; to this class belong strakes, riffles, blanket-tables, &c.

2. Appliances in which the action of gravity is assisted V external means, as in the case of buddies.

3. Appliances in which an impulse is communicated to tlie particles by mechanical action, as in the case of rious forms of shaking tables, rotating concentrators, vanners, &c.

Class I. — Perhaps the simplest, though at the same luie the least efficient, method of collecting concentrates, fsulphurets as they are often called, consists in employ- 'g riffles set in the launder through which the taillings

ran to waflle. Bj' this mBftiis only the Um Imbv7 oiMnl* U coUecled. voA beooo thia sjsIh* neVBT VBsd wboD Um salpharvts uc of ftoy althmgh it Us uses in bdping to ottdi a uua of these ss well as of ofaoW of ma and piticto of amaljun that may haTO eecapoil tbatl The mpknt lonn of nflfeeo&natsaf slateeor wiKxltla 1 iacb vide end 1 toA to inee deep, juunid uiIbbJ finoly ia tlie bottofn of the laoiider at duUH otfhm 6 mehes to 3 fees aiiM. aooocdiiig to the an el tBhwIannw it isdeatedlooUdu Tbeeandaanci tMoiuilT apainet mA of tbsee riflleo, and being in eoastant aptatioo br the totee of the stream o( pa tb bebtin partjicles remaining longer in suspension il> vasbetl orrr ibr tt tif the riffle by the earrent, hiW hearief aad maraer particles falliog more rapiiliji ibe btmon of the uream, aocomatate anst tba HA Fma tttne to thine the hflle-iaoDder b cleaned oA> SSoaUy vhao the miD kstiped foreleamng tip tbe pUltf- Hus osesRy (kNM by plaring a backet at tbe otd tf the hSe-UnodH'. Bftiag ttte rifllee. and wasfaiDg *cwHnhnd oaterial UMO the bodtet by means of abo* pipe auTTiB; a siaall nnaiii of dear water. A lanntf about J wide and 6 inefaai deep b usually ample ibe palp liain tea head of stamps ; it is geoenll; WM a jterper pade thaa tbe eofiier tables, say betvMO 3 and SuwfcesM the fee*, llwwoodea riffles are stfo ttines protecMdfaom wear I9 a sinp of thin bar inn wrevdlothar<9psrsBrfiM. Seteetimes short length v>i bt rails an osed instead of wooden riffie ban, oi rb* uvwittk slats fik TBUCtiaii bfiada set in (tames (V <Mllki Hanganaa rifles), or grid-diad eut-iroo rid" Atv uMii. Boond riffles eoosast of holes aboa If

Blankets 341

shes in diameter bored through short pieces of dh plank, which pieces of plank are then laid in the im of the launder. The holes are usually arranged iancially, the pieces of plank being about 3 or it long. A space of an inch or two is mostly left reen successive planks to act as a transverse riffle. imes longitudinal riffles are used, but these are nob ' effective and cannot be recommended for this vork.

sands so collected are usually piled up and kept for her treatment ; they are often subjected to further centratioD in buddies, just as blanketings are in order >brain cleaner products.

blanket Strokes. — This primitive, but nevertheless cient method of obtaining concentrates, is still gely used. It consists of a shallow launder, the 4tom of which is covered with a series of pieces of ftnket or some similar matelrial. When the stream of dp is sent over these, the heavier particles, falling more ipidly than the lighter ones, settle on the blanket nd are entangled in the nap, being thus held whilst the ghter particles are carried off in the current, whicli of course, strongest and most rapid at the surface of be stream, and slowest at the bottom. As soon as the &p of the blanket has become filled with particles of mineral, it presents a smooth surface, and is naturally nable to catch any more ; before this stage is roac-bed,

is accordingly necessary to wash out the ])articles of lineral from the blanket, and to expose a clean blanket arface to the stream of pulp. In order that tlie process lay continue uninterruptedly, it is necessary to have uplicate blanket strakes, one of which may be filling, Mst the other is being washed. Usually the pulp "om each five-head battery (or sometimes from each ten

jt GOLD MtLUNG m

beads) is ntn orer & set of stokes, ccMisistiDg of 1 Uamlers gi<le by side, or of one wide launder with dJTiskiD dawn mkldle, vhieh thus makes it equivak to two iMcnden. Their lenj is asoally 12 (o 3t> fst and tbotr width 31 Xa 36 inches. 30 mcbes bein on* adopted. Eacii blanket should be 4 to 6 fet ltn| tbure three or foor in each strake. These most I bud BO as to orerlap at least 6 inches, the lower end I the upper ODA, of coataew orerlappiog the npp end d that next below it, like tiles do a kk. At the faeail t each strake there kmU be a bead-board with pins, so to dtnbate the pttlp quite umfoniily across the width the atiake. tUs being a very impottanl niaUer to attend to. Each strake is alo supplied with a gate by which I the polp can be shut off from it. Wheo the puip leaTe the amalgam tnfK, it nsoally nuts into a transverse ili tzihattiiiK laoDder, wliicfa aboidd be suppUed wish valetpipes fined with TahcB. so dat dear water may be nm into it. in order to dilate die polp to any desired extent' Tfai> usually oecessaiy, for a coosisteocy of pulp, whidi worki: we'.I OD ihe oopper-table, is usually too thick for i *Ui>.x'ioJui coDoeniraiion cm blanket strakes. It will generally be found to requite about half as much clear naler, and so:uei:iii as much ajrain as it already contains. Ii is int[<ortjtnt that the tailings should Sow in a (bin, friy ra(ii<i sueani. and loo miKh care cannot be taken lo seciuv itf untfomiitv across the width of the strake. BUnke: sirike-: should be carried like the copper-tables OTi fo'-.i'.!:;; wli:es. or else on screws supported by stoat ::x vveU runk in the ground, and should he adjusted s.'' '.o be accurately horizontal tnnsversely, whilst the ,i;re can be varied at will. A good |dan is to have the upivr end of the strakes suppcwted on some fcam of hiiige>

Blankets 343

st the lower end is capable of adjustment as above, grade is generally flatter than that of the copperas, being mostly between inch and 2 inches to the No definite rales can be laid down for it, as it inds on many circumstances, such as the percentage dphurets present in the ore, their specific gravity, the less of the particles of crushed ore, and the general -acter of the latter. It must be determined by exment in each individual case what grade of strakes what degree of dilution of the pulp gives the cleanest entrates, together with a minimum of loss of sulphu- . The result to be aimed at depends upon the value be concentrates, and the after treatment which these to undergo. Thus if these are very rich and are to he ,ted on the spot, say by chlorination, it will probably K-e economical to aim at saving the largest proportion jible, although by doing so a less clean product may the result ; that is to say, a certain proportion of leless quartzose sand may still remain mixed with concentrated sulphurets. On the otiier hand, if the >hurts are comparatively poor and have to he shipped treatment, it may be advisable to aim at obtaining as ,n a protiuct as possible, at the expense of losing a Ain proportion of the sulphurets in the escaping tail- These results depend not only upon the conditions ady refeiTed to, but also upon the frequency with ch blankets are washed, frequent changes favouring former, and long inten'als the latter form of product, nkets have to be washed at intervals varying from one 'our hours according to ciicunistances, in a trough ride for thepui-pose. There should he one trough to two sets of strakes, and they should be set opposite r lower ends. These troughs are made of plank

COI.n MII.LrNG

naiiBlly IJ inches thick, well bolted togettit

strip ot tarred blanket being interposed between edges, BO as to make them perfectly water-tight. Tliisi a better plan than employing tongued and grooved join e ot leakae tlie former syatom admits of caalk'

: trough is famishi lu inches wide, ranoinj '" to drain into the tronjjb. in Fig. 87.

le blankets are first of 1 then laid in place, in set of strakes, k- )y must be thoroughly

ing, whilst the latter does i with one or two shelve along the sides, and so The general arrangemeui. To work the blanknt s all tboroiighly soakec one (aay the right-ham ranged as already indieaieu.

wet through, or else they are apt to shift when the stream of pulp strikes them. The pulp ia then admitted, great caru being paid to the evenness of its distribution, so that no cliannels nor local accmiiulations of aaud are ever allowed to form. At tlie expiration of the proper interval of liiue — when the top blanket has taken up all the sulphuitts it can boldtbe gate leading to the other (the left-hfimi) strako of eauh set is opened, and the right-hand gate shut dowt). Each blanket ia then folded, beginning with tlie top one ; the lower end of each blanket is lifted and (old over by bringing it to the upi)er end. and the lower end. thus doubled over, is again brought to the top, and so on till tile blanket has been folded four or six times, making' convenient bundle to can-y, and allowing none of theuoiietnti'atcs to escape. The second blanket is siuiiUr')' foldwl, and so on ; often the lower two blankets {wiisn there are four in each strake) only need washing half often iVK the upper two ; iu that case the lower blauVets need not be disturbed. When all the blankets are folded, they aie carried to their respective troughs and laid on

Blankets

lelf ; they are then thrown one by one which must be nearly full of water), unled by being moved rapidly to and fro, las been uppermost in the strake being le trough. When all the mineral has , the blanket is folded again and laid on the shelf until all are washed ; they are the strakes ready for use again as soon blankets require washing ; and so the 3. The blanket trough should be large in a week's accumulation of blanketings

idinal Section

End Elevation

Scale

I 6 O I 2 I.I.I 1 L.

jf..

Fio. 87.

ore than half full. Tlie troughs should week, and the blanket! nTs stacked tnent. Care sliould he exercised in th( 2 blankets ; several firms manufacture of blanketing for this work, which can he my makers of mining machinery. The be not very coarse, strong, and closely ort, stiff nap ; a long nap is apt to hecoiiK dy by the running pulp, and will hold a tity of sulphurets. It is false economy ankets of inferior quality, as tliey wear fT; in cases of emergency, however, any

bL .J ba Baed, socb, for instance, as ihii

Ui U aold for the ose of nalives in

aesai-cii J coon tries. Gnnny sacking is sonic<ttS med instaad of blankets, and is recommeaded b; anthontiee when tbe soipbareta vm very finely di\'idl Huiy older snbeiitiites bsve been employed, uoti pfalsb, wbieli, allhoiigl) very expensive, is said to aaai extremely w [ity of blanket ma;

length of time t tankeis will lal dt

Dpao so many c ivl il is scarcely poail

to give eren ta ; their life, tionei

rarely e&ceeds i wasliing of the blaol

is osoally done boy can easily attend

three sets of stmKes uu.c, .j proportion of snlphi is unnsually high. These two itemsthe cost of labiiW and the wear and tear of the blankets— are the principJ ones in the cost of blanket concentration, and these viD vary so widely in different localities that no price geW ally appltcahte can be fixed. Concentration by blanlii'' used to be far more generally employed in gold mills th*" it is now. hai-ing been to a great extent replaced by more perfect mechanical methods : the great advantage '' presents is that the tirst cost of the installation is extremely low as compared with concentrating raacliinen'i whilst, on the other hand, it is comparatively expensive, though imperfect in its operation. It can only be nvOBmended, therefore, in the case of small mills where is not sufficient capital to equip them with better app' ances, and in new mining dialricts where the value m the concentrates requires to be proved before any lafP sums of money are invested in concentrating machinery. Ciiiiras Tables. — In .\ustriilia blanket slrakes are

Canvas Tables 347

ien followed by canvas tables ; these are long broad likes 80 to 100 feet long and about 4 feet 6 inches de set at a grade of f to inch to the foot. In these B laid light wooden frames to which canvas is nailed ; ily divided pyrites settles readily upon and adheres mparatively firmly to the surface of the canvas, and ben a set of frames is charged with pyritic slimes, it is moved and the pyrites washed off. Three tables are nially arranged side by side, two of which are taking ke pulp whilst the third is being cleaned. Each table the above dimensions will carry about 30 cubic feet I pulp per minute, and one set of tables is considered afficient for a mill crushing 100 tons per day. It is oand that canvas tables will catch pyritic slimes that lacape from ordinary blanket strakes ; they are much n favour with the Chinese in Australia, wlio at times ifrange to be allowed to treat by these means the tailings escaping from stamp-mills ; they are hence, at times, Bpoken of as Chinese tables.

In California very similar canvas plants arc employed, following usually the vanners or other concentrators ; bere, however, a large number (up to 90) of narrow strakes 12 to 22 inches wide are preferred; tlie tal)les arc from 20 to 100 feet in length, and their grade varies from j5 to inch to the foot ; they are laid with li<lit canvas in tlie same way that blankets are arranged in blanket strakes. In Tuolumne County one canvas plant has an area of 2,400 square feet of canvas tables to treat tons of luartz crushed in 24 hours, the stone carrying 3 per cent oJ pyrites. The first cost of these plants is very small, ftnd the chief expense in running them is the labour I'equired, as the canvas lasts a long time.

It would seem as though such canvas tables might with

ifB te wtfhmd fay BtnttoQ ouiTits belts, nU MiairiiaBf : hr u t Imow, bowevor, d MkhiB acnv bi sni to eoDneotlon witb gd iq iffamiy will MlaptMl to this poipM — Ab role, the blAtiketifig , MMUB a TUT Unt proportion- T5 par cent .; at more — of woi ely clean cotieU ' trcAtinent. Thi M Aa M tt: A mmmB of reetftngalAr box 8 niHllillVfeVftel wife, ud 1 foot 6 iRcM' j taft ft anhM Ttus bos art on a steep gc luafclb. Al dM iiffVMd tiiere is a hoad-board ftbont 15 xmtfas tbe fall wiiltfa of the budiHv, I order to regulnm Abave th bead-board is a bo£ into r kang aiao supplied so as to fonti a piilp TIm tail-board of the buddleii p*MWcJ witb ft dodilt row ot bol<s fitted with plugs bd unOiged tb mttr can be drawn off at auy desirfd tevL Aerawt lite tedcUe is laid a plank on wliidt the man wurking it sMbAr ; be proTidt>d with a itif,- ItatutWd bt.'Mxn. Two men an tvquired to each bwldlu: luiv charter's thp blaiiknluii into the head-box. regulate thi-nati-r supply, ami attmds to the phis on the Wnt: the other baniUes the besom. Tbe pulp thus tortiu'il oil the head-board runs smoothly and evenly into tlu' ImUle, where the heavy constituents settle nearesi io ilu-head, the others being depoaited towards the lower

By meuia of the besom the snrfaoe of the accamQmaterul is kept quite smooth and level, and no els are allowed to form in it, the besom being i from below upwards with a lateral movement

I the full width of the buddle. The Kuccesa of iug depends entirely upon keeping' a smooth, -m surface, and supplying the pulp iu a thin, bT stream. As the surface of the sands in the a rises, the lower holes are successively plugged.

COLD MtLUNC kaep the enrfKM? of tfar s&nda &p|inHumM

tJ sDcii a boddle ii |Hr 1 1. 4ucb qtuoUty tA water shod I to M Uk-dt bUnkedngs. The Imddle 4kplh of ban 9 la 13 iiiefae&. \MieD ilhi. Ifce wMbt sapply stopped, uil d t Dts of tfae boddh Biddings, and id irfibeirtkale.>K£airi}dM , Ml aHHlliy qpH

Bubbles 351

trates form the lowest layers. A tossing-tub, in the stirring of the sands as well as the tapping to them, is done by mechanical means, has been fully employed in some gold mills. It should be ibered that blanketings frequently contain a small ddon of floured mercury, which tends to run to- ' into globules under the operations of huddling and gy and due care must be taken that these are not they will, of course, be found in the very lowest of the cleaned concentrates, and can be collected nning these.

M H. — The square huddle above described can 36 used to concentrate the valuable portions of the as they leave the mercury traps. In this case )ad-box is required, the tailing launders delivering i into the head-boards of the buddies. They must orked in pairs, so that one huddle may be filling t its fellow is being cleaned out. On this system nen are required to attend to each pair of buddies. lly a third huddle is set close below each pair for the er concentration of the heads obtained ; it is then anient to make only two divisions of the huddle ints — namely, heads for re-treatment, and tails wliich ot worth the expense of further handling and can he vn away.

r this class of work the round huddle is preferable le square, as it does not require constant manual ir, but only supervision, so that one man can attend veral sets. These also must be built in pairs. The truction of the usual typo of round huddle is sliown g. 89, which represents tlie ordinary convex huddle. lave buddies are also sometimes, but more rarely, , and there are numerous modifications of detail in

Ap. Xi Buddles 353

th types. The one shown here is, however, simple to Dstruct and to operate, and fairly efficient. Its advange is that it can be made by any ordinary carpenter id is inexpensive. It consists of an annular table in the tape of a very obtnse cone, the angle of which to the )rizontal is between and The outer diameter is itween 18 and 24 feet, the inner diameter about 6 feet. he inner portion is occupied by a circular head-board, at a steeper pitch than the huddle itself, and about foot higher at its lower circumference. In the middle this head-board is a conical hopper generally made of on, which is nearly closed by a conical block so set as leave only an annular space of 1 to 2 inches in width the discharge of the pulp. This central conical block uries a vertical spindle, which is caused to revolve by Bvel gearing, and to this spindle are attached two or four rms from which hang thin boards, the lower edges of rhich (set parallel to the face of the circular table) are furished with brushes or strips of stout canvas or blankctig, which, like the besom of the workman in the square uddle, level off the accumulating sands to a smooth even urface. As the huddle gradually fills up with deposited iilings these brushes must be raised, which may be done utomatically, but is usually perfonned by hand by means f some such simple device as is shown in the fi<ure. It easier to adjust these bnishcs if each is made, as shown, 1 two parts. The course of operation is precisely the iinic as that of the square huddle, and similar precauions liave to 1k3 adopted to secure ood results. The rate revolution of the spindle is mostly from 10 to 12 pei* ''nute. A huddle such as is here shown can take 2 to 3 hic feet of pulp per minute, whilst the decree of ution of the latter should be such that each cubic foot

fcminn of As iw— igtiar aoAMe. Thereooait tl BkUiRtflat-Ilia iMwiiiey afaool IS indies wiiie. 13 to 9D laipt (nne. and I Eoot deep. The lover end ema be dtxi hj wrw* frf BBHOW atripe of troop aham 1 inch de J0 llw ont-Bow caa he used to any desired ha Thf (ilnie> ue ran into tbe laander, and a worfcmiD cmtinnaily pasbe tbr sand ttat accomnlates in it. fron the lower mm) towards tbe opperend with alight faoe.tliiK bunliUttnf; llie aurying awsr of Uie lighter siutd vrhilst lh heavier partidts acecmntite od the bottom of ttw ty<-. iha level of the outflow bein grada&llr raised as tb( iilpbaretn necuinoUile in the tye. When the tye is full till! concentnvtca so obtained are worketl over an, nitlicr in a Ntnilar but smaller lye with clean water or in a r(i'-tiint(n1ar huddle. As in the correeponding case ol liuii'llcH, tlii-w! lyes muBt be worked in pairs. They hill liltin iiM.<i), and cannot be recommended except foi Ibi'ir iliitapiK'HR.

Clan III. N<tarly all the modem concentratiif! iiiiMtliiniiry nwd in old mills I)belongs to this class, vbicb

X Shaking Tables 355

3 eludes the various forms of shaking-table, of which kere are endless modifications. Generally speaking, the taking-table consists of a suspended inclined table upon rliich the tailings are delivered in a very thin stream, is table receives a rapid oscillating motion, the effect which is to propel the lighter particles along one path, Ad the heavier ones along another, so that each may low off into receptacles provided for the purpose. These Kiachines are very effective and produce clean concentrates, whilst the loss in the tailings can be kept within rery low limits. The original form of shaking-table is Aiat devised by Bittinger, which has not Ixjcn much improved on since, in which the direction of the shake is right angles to the flow of the pulp. These tallies an* Qsually built in pairs, their general construction being Bhown in Figs. 90 to 92, which represent Rittinger's original design, the first figure giving a section, the next a plan, and the third a front elevation of the machine. The table is strongly made of hard wood, carefully planed and niiide as smooth as possible. It is suspended by four rods which allow its gnule to l)e altered at will. The framing of the table is very sulstantial, and it Carries a stout transverse piece near tlu middle, wliich receives a series of thrusts from a cam which pushes it ajjjiinst a strong wooilen spring. The action of this spring throws it back sharply against a bumj)ing-l)lock, bich gives the shake to the table, the shake thus consisting of a series of sharp shocks. Each tables is Usually H feet long by 4 feet wicUs and has an inclination ef from 3" to 6° to the horizontal. Tlu average number of blows is al)Out 100 jKjr minut:', but in working very *ine sands as many as 150 j)er niinuto are sometimes 'iven, the length of each stroke being about U inches.

A A 2

upper end of the table is fixed u h4 Mil.h lliL sual distri hating -pins. The palp to be Cfnlnitci' B delivered to one-fourth ouly of the 1 boiird, t remainder being supplied with clear i If llie table were at ret, all the particles of the

would tend to rpU down the table in lines parallelt length, the lighter particles achieving the journey riipiiUy than the heavier ones. The impulse due t shaking action acts in a direction at right angles b line of flow, and hence the particles move down in a

Shaking Tables 357

ich is the resultant of these two molioDB. As the locity down the table is continually increasing, whilst Telocity in the direction of the shake remains constant,

Wb resultant takes the form of n paiiiholn. The [*ficlcs moving more slowly, jire expo-jtid for to the action of the traiiaverse impulses thai

3S GOLD AllLUNG c

lighter ones, aud licence are tbrowii foither fn>tn straigbl liriLof Bow. It is tbua possible to obtain a complete sepiiration of tbi' tiul{)iiilocoacenlrat<isaiidb

siinils, the action beiriH quite continuous. This macliiDe unswere well, except in tbe case of vorj' fine aliniea. EmIi double table requii-es about ] indicated H.P. ; onb m*"

I Shaking Tables 359

&Q attend to two double tables without any difficulty. "he capacity of a double table is about to 5 tons per 4 hours ; the consumption of water is considerable, being Itogether about 0*5 to 0*8 cubic foot per minute, three- Durths of which amount constitutes the stream of clear irater. In more modem forms the tables have been oade of planished sheet-iron, of slate, and of plate-glass, whilst metal has been substituted for wood in nearly ifvery part, the supporting frame being light castings, the springs steel, &c.

An end-shaking table has been largely used in Aus- Mia, where it is generally known as the Halley tal)le. Hat shown in Fig. 93 is fairly representative of this class df machine. It was manufactured by Messrs. Appleby Bros., for use on the Gold Coast of Africa, where it is to have done good service. It consists of a table suspended like the Eittinger table, but moved in the direction of its length by a cam (usually a three-throw m, such as is shown separately on a larger scale al)Ove ;he sectional elevation) against a strong spring, which )ashe8 it back with a sharp jerk against a buuiping-pieec, he spring being at the lower end of the table, and the erk, therefore, upwards. When pulp is fed into this able, it is subjected to two opposite forces : that of the tream itself tending to drag the particles downwards, nd that of sucessive jerks tending to throw them awards. The heavier particles are more difficult to lOve by the current of water, whilst they acquire greater lomentum from the impulse of the jerk. Accordingly he barren sands tend to travel down the table and are ischarged at its foot, whilst the sulphuruts travel np the able and are discharged at its head, or, as in the table hown, accumulate in a special division of the taljle,

ylo COf.n M/I.l./.VG.

whence they arw reuioved hy hand. These lablts 4 iii[u{>li! and easy of construction, not expensive, and

Sectional Elbvavoh

f;iii-work except on fine clinics. The inclination of tbi tiihle must 1x1 luljuHted uccurdtiig lo tlio character of tiii

Hendy Concentrator 361

to be treated, and the nature and proportion of the intrates obtained from it. The usual speed is about )lows per minute, and one machine about 4 feet by 8 will treat all the pulp produced by a five-stamp

e Gilpin County bumper, or the Gilt Edge concen- r, as it is often called, is practically identical with sist-described machine, except that it is continuous- g, discharging the concentrates at the upper and the gs at the lower end, and that it is built of iron ad of being largely of wood. These machines arc built ars and are run at 120 to 150 blows per minute; ible machine 7 feet long and 3 feet wide will treat 20 tons of tailings, averaging say 10 per cent, of ntrates, in 24 hours.

e Uendy Concentrator may be looked upon as a rather ly and divergent modification of the shaking-table. )insists of a shallow iron pan 4 feet in diameter, 3rted on a vertical shaft in the centre, and made to ate back and forth by means of cranks on a shaft at dde and joined by connecting-rods to the periphery i pan. The pan has an annular groove at its outer al>out 2 J inches wide and deep, and receives about ihaq) oscillations per minute. The machine is, in a kind of circular shaking-table. When in operation e particles tend to move downwards and to accumu- t the circumference, the heavier sulphurets occupying ower portion and thus collecting in the annular e. This groove is fitted with a gate by means of 1 the discharge from it can be regulated. ch machine is gmiranteecl to treat five tons of tailiii/s lay of twenty-four hours, but a lai'er amount, up [ht tons, can be put through, although in that case

j6a COLD MILUNG

Bome uf the sulphurets may be loet, and the products niU not ba equnlly clean as when a smalkr amount is being treated, A len-stain till will tbure/ore require five oi six Hunily concentrators. Tho weight of each mftcliiu is about nine cwt. and its cost alwul $300 (£60 slerliii|), or say half the price of a Frue vanner. These machina are capable of doing fair work, except on fine slimes, Mi4 were at one time very extenaively used on the Pacifc Coast. They have, however, of late years, been practic replaced by some form or other of Ijelt vannera. A somewhat similar machine is the Duncan concentratoi', which

has a wrought-iron pan instead of a. cast-iron oue, i action being in most respects an imitation of that of tttf Hendy, It has, however, never come into public (avoiT' The WilJU'!/ Table.— This concentrator is one of the most recent of this class of appliances, hut has alreikdy met with marked success. Its general appearance shown in Fig. 94 ; it consists of a flat wooden table. l(i fwl long and 7 feet wide, covered with linoleumi n|K)n which are nailed a series of strips of wood, graduftUy increasing in length from the back to the front of tbe

The Wilfley Table 363

3, and gradually tapering to nothing from a depth it I inch at the motion end. The table slopes s about inch from the motion end, and also forward from the back ; the amomit of this latter ion can be altered at will, according to the nature material that is undergoing treatment. The table 3d by an eccentric combined with a link and so as to have a quick forward and a slow backotion ; a spring keeps the table close against the

the whole time, so that there is no shock or bump yr speaking. The pulp to be concentrated is fed le table from a head-board near the motion end for I of some 3 feet ; the rest of the table receives iter only. When the table is in operation, quartz aer minerals of low specific gravity are canied stream of water down the table in a piactically

line ; heavier bodies sink below tlie ed'e of tlie .re thus unable to escape straight down tlie table, 3 hence gradually moved along it by the series ulses to which they are subjected ; they are pable of being carried by the water current when ve moved clear of the riilles, hence a particle of

on this table moves in a direction that varii's arly parallel with the length of the table to nearly rely across it, according to its size and specific

or, if particles of practically uniform size are alone red, they move according to their respective specific s. Clean tailings run at once to Wiiste, middlings imed by a small raff wheel to the head-hoard, and vier minerals are discharged at dilTerent points of e in accordance with tlu; })rincipl(5 already stated, jat advantage of this tal)le is that it makes a very

dear aiiil dietinrt aoparatiou ui>ou vu-iout) ttpee iniuemlu, it docs not, however, do oquaJIy good work linsizel pulp, benuc it should be preceded hy Spitt! or aaino other forni of automatic classifier. A Uble iuU'udod lo treivt about 30 toDS of ore per 24 hours, has exccplioually been found cupnble of liiltiuf; up 10 ton : in some Autunc&n luills one table is put in tc ten heidi of slumps. U requires idiout 1 H.P. to and should be drivun ut 240 three -quarter-inch strofc( pur minute. The supply of clear water required vanri Itreatly with the character of the Oi'e to be concentrutwl it may be id to rane from S to '1 gallons per aiinDU- The greatest defect in the machine is prolmhly the nS whtid arrangement for returning the niiddhngs. "Rie Weight of the table iu about 22 cwt and its price abooi £90. The nucoesa of this mocbiue baa cnused a very lure number of other-s constructed ou about the saiw principU's lo l)e put oil the market; they all combine iUk features of a transverse shaking-table like the old Rittinger taiile, with transverse grooves or riffles upon its upper surface ; much ingenuity has been shown in making these riffles oblique, sinuous, &c., but the mode of action is in every case the same. Bell Vamxen. — This ckss include some dozen or mon;

each other, and all being modifications of the first one,, the Fruc vimner, which may be looked upon as the typical Ijelt vanning machine. The best known machines of this tyijo arc the Fruc, the Embrey, and the Triumph vanners, and the Liihrig Iwlt concentrator. In the first three the current of pu]|) is iiarallel to the longitudinal direction of motion of tbu Ixilt, in the last it is across this direction ; the first-named has a side shake, the second and third an

I FRUE VANNER 36s

shake. The general principle, is always more or less aesune. fmt Yanntr. — This machine is now so well known Ut any detailed description would be superfluous. It is 3own in perspective in Fig. 95. It consists essentially !an endless rubber belt about 4 feet wide, having flanges ; either side. This runs over a series of rollers set in a une so OS to form a flat table about 12 feet long, which Es at a slight incline at from 3 to 6 inches in the tal length. The belt moves upwards at an average rate

6 feet per minute. About 2 teet 6 inches from the iper end the pulp is discharged on to tin: lM>lt with fji'cat liforniity by means of a distributor, or hciid-boat'd, imi] out foot above this, a number of fine juts of cleiin iter ilHy upon the belt. The \v\mw. currying tlio bt-lt is pt in oscillation at the rutt-of altiiut 200 wli'ok<.;s per inute, the average lengtli of the stroke being 1 inch, le machine is accordingly a form of s]iaking-tal>le, in lich the fixed surface is i-eplucfil by inip moving slowly awards. When the pulp is diBcliufgpd upon tlie tai)le.

jf/, corn Mt! UNG

ctMih particle is snbjwted to two opposite foroea— tliat of tbe stniam of water, which teiiiis to carry it wards to the ff>ot of ihn tiibic ; trnil. secondly, that ol motion of the belt, wliioh teixls to carry it upwards b bead, and the ultimate niotioD of Oiioh particle will It (lecidcul by the extent to which it is actml on by ni other of these forces. The entire pulp Ijciiie kept in state of Agitation by the rapid oscillations of tlie tablo. thai heavier particles tend to sink to the bottom of the Ityir of pulp. lu the bottom layer of pulp, velocity of da I wntur current is least, beio retarded by friction iiatDKltlK' belt, whilst the effect of the upward motion of the Istts iit A iDaximum, it being comnmnioated directly to Ihj ptirticlcB in coTitnet with it ; in the upper layers, on thi other hand, tbe effect of the upward motion of the belt is C(vrcely felt, whilst tbe velocity of the descending current is II inaxirauni. Accordingly, the particles which occupj the upper layers of tbe pulp tend to move down the tabk atiil tlioec occupyinH the lower layers tend to move it 0|i: in .thor words, the li'lit ham'ii particlos .if sami inniv downwards to the foot of tbe belt, whilst the particles of heavy siilphurots move upwards towards the bead. At the sanu! time large particles of barren sand, whose rate of falling through the pulp may be equal to that of smaller IwirliclcR of heavier mineral, project further upwards from the tabic than do the latter, on liccount of their size. The fdTiner are therefore exposed to the action of a more rapid downward curi-ent than the latter, and are hence also caiiifd slowly down tbe lielt. This is tbe reason why this machlLic works well, even on unsized pulp, and why a verj' eonipletc separation of the pidp into barren sands an valuable concentrates is cfTecte;! by it. Of course itgivei even better results if the pulp is first sized, or classified.

Frue Vanner 367

3 different classes, being fed into different vanners. le work of separation is practically done between the ints where the pulp falls on to the belt, and where the is of clear water impinge on it, the action of the latter ing to free the heavy concentrates from the last parti - of sand by such a dilution of pulp as will tend to com- 3te the separation. The application of this theory at ice indicates the main points to which attention must be tected in regulating the machine. In the first place, the machine must be set very subantially, so that only the proper motions may bo com- Qnicated to the pulp, as other irregular impulses-would t)duce cross currents interfering with the proper rnovcent of the particles. The belt must be x>erfectly liorimtal transversely, and the supply of pulp and of clear ater must be perfectly uniform across it, so as to mainin in all parts of the belt an equal and suHicient deptli palp to enable the separation by gravity to take place [ually throughout it. It is found by experience tliat a jpth of about 1 inch of pulp on the belt gives about the conditions in this respect. The velocity of upward ivel of the belt must be such as to counter) )al an ce e downward tendency of the finest jmrticlos of slimed Iphurets ; accordingly, finely-crush<l inaUnnal demands latter belt and a faster speed of u])hill travel than does >re coarsely crushed pulp. Eacli one of those [)oints ist 1x5 carefully regulated by close observation for the iicular class of ore under treatment, so as to enable machine to work at its niaxiniuni cai)acity without 5 of efficiency. Once adjusted, however, tlu niacliine uires but little further attention as long as the condits remain unchanged. Special attention must he paid his latter point. The vanners should he driven by a

l6K GOLD MILLING

special motor, iLiilfpen(Tt;ut from that whicli (lr , mill ibove all, the rock -breakers, and fnrni S*' j

pOBoi iih a Bensitive governor. The 'ounte*''

sh eell looked after, and all the driving belt* '2.

ia ,er. The speed of the machinea will tb''*

kep. rni, Tlie clear writer supply ought to

from a sep fate tank of ample size, and the waterii* leading from it should ' be ot large dianieter. As loW as the stain-mill continues to crash the same cliw material at " ' ing will all the condiriW*

affectiDg thf ilmnged,. and the mac

continue to atisfaotorily.

When the opei-ation, ibe diatributi*

of the pulp G the Ijeat indication to

its working. . is should he so adjusted

that a Bmall tri. sand should show nt caob

of the lower cor . These " sand cotnen"

should not he niist be well marked.

the two should be of equal size. Should they Ije uneqnil tlio fault will Iw found to be either in that the belt is wil accurately level across, that the distributor is not doing its work thoroughly, or that some of the working prl' have not been properly tightened up, so that there are other motions than the nonual ones communicated to tiie Ixjlt ; when once the cause of the want of uniformity lias lieen determined, it ia a very easy matter to remwly it. Too large a corner of sand shows that tlio pulp too thick, whilst absence of any corner indicites tbat it ciiTies too much water, Tlie clear water supply slionld bo only snfBcieiit to keep the laj-er of pulp between tbc distributor and the Wiitfv jets tlioroiighly lind unifonnly wet, and to prevent the formation of any channels throib it, and once the proper rate of supply for this purpose

Frue Vanner 369

las been detennined it should not be altered. If the pttlj as It leaves the mercury traps, is too dense, it must be dimted with clear water, either in the carrying launder or m the distributor ; if it is too dilute, a concentrating r-box, such as is presently to be described, should be ttteiposed. The capacity of the ordinary machine is hoot 6 tons per 24 hours, whilst a wider belt (6 feet in has been known to treat about 12 tons in the me time. The best practice seems to i)e to allow two nners to every five heads of stamps. The amount of yt required is about 0*2 to 0*4 cubic foot per minute the pulp, and about half as much additional for the ir water jets. The entire machine weighs 21 to 22 cwt. I costs $575 (£125). It requires I I.Il.P. to drive it. i man can readily look after sixteen of these machines, sr the belt has passed the head of the machine, it is t down and passes into a tank, where the sulphurets washed off it ; the sulphm*ets accordingly accumulate bese washing tanks, and have to be taken out from time iine ; this is Ixist effected by an ordinaiy long-handled kel. The overflow fi'om these concentrate tanks lys contain b. good deal of slimed sulphurets in sus- $iou, and these are usually the most valuable part of concentrates. Hence the overflow from these tanks lid be made to pass through a series of settling-boxes, re thes€i finely divided sulphurets are deposited. The ing-l)oxes are cleaned out from time to time, usually Q the mill clean-up takes place. For a small battery, ng, say, not more than eight vanners, these are best ;d in a single row. A larger number is best arranged double row, pointing away fiom each other ; that Siiy, with the head ends nearest the centre line; of milding, with a space of about 5 feet between them.

J70 GOLn MILLING ciull

Alon;] the middle of the viuiDer house tbere should be i n&rrow -gauge track upon which run cars, which fiem.- caiTv the imnoentrates to their destination outside lit vanncr house, the attendant shovelling the solphunA from the respective tanks into these cars. I

The pulp, a& it comes out from all the amalgam inn sJiould be run into one main launder, and thence EuppUn in separate launders to each vanner, every launder having a small gate by which the supply of pulp is regnkui These launders are best triangular in section, wilh grade of not less than inch to the toot.

It is obvious, from the dt-soription above given of iht principles of the vonnei:, that a belt with a rough or ooi' rugatcd surf;ice must act more effectively than one with i smooth surface, as it must increase the tendency of he5] particles to work upwards. Ck>rrugated belts have beei inti-oduced, and the new machines thus produced semi to give in some respects Itetter results in that their work iug capacity is grfali.T than that of the old form, od machine being sufficient to treat the pulp from each fivt stamp battery. These "Improved Frue Vannera " U manufactured by Messrs. Fraser, Chalmers, and Co Limited, at u cost of 35 (X170), theur weight being ala slightly greater than that of the old type (about 23 cwt- When pulp is sized before going to the vannera, it seen preferable to treat the coarser portion upon corrugate and the finer upon smooth belts.

As has already been said, the Frue vanner may 1 taken as a type of all vanners, and the above-givi description applies practically to all of them, the difft ences in detail of construction not meriting sepan description ; these mostly refer to the character of t motion, the principle remaining always the same. En

Froe Vanneh 371

a require to be run at a rather higher speed does the Frue vaoDer, 230 blows being about the Lge. Amongst the newer vaDaers the Woodbury and ly may be mentioned, but so far none seems to be rred to the Frue, which is a deservedly popular

le Lilhrig vanner (Fig. 96), which is practically ideowith the Stein- Bilharz machine, is essenli&lly different the above, inasmach as the direction of the belt is at

angles to that of the other machines. The belt is ontal in the direction of its lengttt, but inclined some roasit, receiving impulses in the direction of its travel. usually about 3 feet 6 inches wide und 12 feet long, he dimeneions are often varied ; the rate of travel is i 8 orlO feet per minute, and the number of impulses ; 180, the length of stroke being from i to J inch. pulp is fed on to the belt froni a heitd-box placed lei to the length of the belt clotic to the motion the rest of the belt receiving clear water only. The

B U 2

HtiLcbiae thus resembles a lUttinger side-aJuLkc UiNi: ct Wilflev - a, in wliich the table itself is conlinuill; adva one direction. It has been oiudi ianproveJ

in d ' oanstniction in recent >'<Hirs, and is

gooti I '. machine. It will treat from 3 to 8 lonso'

tailings per -H hours, takes to 1 H,P. to drive it, U mquirea about 5 gallons of cieiir water per uiinatB additiou to that in the pulp. Its ohicl advantage oM the vannera already (loomnh™! that it will make sovenl grades of cono side-shako tabltw, fiiw

working on tnixi opurly is very valuablf ii>

general oro-dre it in gold pulling, viIk

clean tailings trates alone are requiiwiis

of far less ini'

SiEing. — Of viaahility of sizinji; 'If

pulp that comes Ii mill has boon obtruiiinS

itself moi-e aiid x&t. loUce of mill men. W

method has the advantage tbat coinparativulv uOfti crushing wiu l>e used ; it has already been pointetl onl (page 147) that only a small percentage of the total palp is crushed to the uiuximimi size allowed to pass tbrongb the mesh of the l)attery screens. It is an easy matter Hoparale this small proportion, which may be too coarse to admit of proper separation of the gold, and eithw return it to the stamps or treat it iu separate grindinf! niaohinei'y, whilst the rest of the pulp can bo into separate sizes, each of which can be treated on sepirate suitable concentrators. For this purpose one of thi) simplest classifierB ia the y-box (German " Spitzkasteu This has not undergone any substantial improvement Hiiice it was originally designed by Bittinger some thirty years ago. Its principle depends on the fact that it the velocity of a current of water, carrying particles of vMj'-

ig sizes and specific gi-jwities, be deoreased in definite

raccessive ratios, a definite number of these particles

will be deposited, corresponding to the decrease in velocity

o! the current. It has already been pointed out that

a smaller particle of heavier material will fall through

a column of water at an equal rate with a larger but

lighter particle, when the sizes and specific gravities

bear a definite ratio to one another, and the particles

deposited at each diminution of the speed of the current

will accordingly consist of such particles as will fall

through equal spaces in equal times, the entire pulp thus

nndergoing successive classifying. The diminution of the

velocity of the current of pulp is brought about by allow-

Jnj it to flow through a series of boxes of successively

larger sectional area ; in each such box particles are

deposited, whose size is a function of the area of the box,

and nothing more than this is required to effect the sizing

of the pulp. In order, however, that the discharge of

the sized pulp may be effected automatically from each

W, these l)0xe8 are made in the shape of an iiwerted

)ynimid, having a small aperture at the npcx through

which the sized pulp may escape. A series of four such

>yramidal boxes is shown dijigraniniaticiilly in Fig. 07,

I'hich has been taken from Rittingor's book. The first

ox should always be of such a size as to give a width of

(minute. The length of the box is of less ini])ortanee than

:s width, and may for tlu first box taken at aboiit

hree times its width. The widtlns of successive boxes

proportions of 1:2:4:8, ko.. — whilst tiu length may b(

1 arithmetical progression. The angle of th( pyramid

hould l)e at least 50'' to the horizontal, in order that

f

Gold Milumg

partioler may settle on the sides : this pith f>( 50° is given throughout to the transverse wails of the bm, the longitudinal n-alls being best mode vertical dotni le suuh II depth as will admit of a regular square pynwiii ttnniniitin the box. For the larger sie of box it well to iniiko the po'"' -t"'"' lOt of one but o( two ot more pyramids (Fig. & t the angle of 50° mny bt

ninintiuued without ui easing the depth ol H)*

box to such AD extent i it unwieldy. Succesn

boxes may be connected by short carrying launders, which discharge the pulp on to a distribntor, and thence into the box ; it is as well to place a board on edge across the box, a short distance from the side at which the pulp enters, dipping a few inches below the surface of the pul)) in tlie 1k)x, so as to break up any eddies or local ciinvnls across the box. The amount of water discharged wall tlie sized pulp depends primarily upon the size of the aperture at the apex. In order, liowever, not to be

Sizing

bliged to make this too small, a S3rphon discharge is tnployed, which, by bringing the point of actual discharge earer the surface of the pulp in the box, diminishes le velocity of the issuing current, and hence delivers 88 water with the sized pulp. This is accomplished as hown by means of a rising pipe applied outside the box, nd made of such a length that the point of discharge hall not be more than about 2 feet below the surface f the pulp for the fine slimes, and about 3 feet for the

Fuj. 98.

parser material. The diameter of this syphon pipe will ostly vary l>etweon inch and J inch, according to tluj nount of pulp to be discharged. Tliis pij)e must be ted with two valves or plugs, one at the apex of the rramid to l)e used in emptying the box, or when it is jsired to flush out any accumulation of sand from the tttom of the box, and tho other at the mouth of the phon pipe to regulate the flow. In a few exceptional ses, where it is desired to ol)tain a product with very little ater, this may be accomplished by keeping the valve

coin MiUJXC

n>l only opcninp ii inU'miilU'ntlj . Tli.por-

e the pipe to he vsaAAy May advsntafces : (he pUat is very

and nftdOT eoDstnwied In uijordimij eorpenur, qabvs Bule or no ivptin, vA when once well stif''* MBit tMfdlr MI7 ftUcniion. Wbeo the tnttcry, utdcP"' eBBOlK tbt! ficnrnf palp, is stopped. tbelAltortliiHiiil'' Hipiiml iiy n flow of eXuax wsterto prrent nny wUl''ii)f><t of Mud nnd oaiuwqDiit oUttrociwn in the di§cl)U£iti|2 vjkoa. Sa boxs uv now ofteo replied slivel-tron eoBcs.

AnoUier %aA perinpe prttfRnil>lc form of this mudp mMUnc is the " Spitxlattt-." This, ae now coDstnicti'iJ, cyntdata of B aeriM nf pvmnitla] or oookal lioxrs, wliitb tiMj' be UMidii erf citbttr wood or sheet-iron, cairinl n ft sohaUe ftwn*. WiUun Hxtee boxes am sttspnidBl saiiUli-r $>iiniUr ones, the pulp beintz IlvI into tlii.' inl'' spiicis lulwwn thf iwo. The iHivcry valve is iiminp'l like tbAl n( tlip " Spitzkasten." At the npex of the it lljwxis intro<lliced an upward jet of clear water. D[wd iho stifDclh of lhi flow of waler and the area of tte annular spM! Iwtwnrn U>e two eaofs depends the nte of il<-position of tiw sand carried hy the pulp, and fixing ifi cftixkil as in tlio prvvioo' apparatus. A neat form ol S|>)lilaite," constrwclod entirely of iron, is shown in Pi. 99, lilts hdnft the design of the GrasoDwerl Coinpuiy of Xf*ed*-lmrs.

TI'CT'' ansi-voni! mhiT nipsin'! of sijiinj; Inilinc, dt'iK'ndiiig ilie same principle, or vacations of i'l liiH llu' V-l*ox and "Spitzlmie" are prohahly the mtis' ooiivonient of all of them. When the tailings are tlins

sigul, such particles an we too coarse and need foul cmshin mayi as already said, be retreated either the battery or in some form of grintling macbinerl ooartiar portiono of the pulp uhlch do not requini ((riuilinf; may Ixi tvfiated either on bolt vanners, ROme chciii>er fonii of oonoeutrntor, generally n sliu table, oi-even on tine jigs when rerj' conrad. whilst t belt vanners are employed solely on the finer gradts material. It has already been stated that the tl vanner is capable of doing tborougbly goorl w( unsized material, hut at the same time it is alvan in odvantafio, and better resalls will be obtained, when itM only reqiiii'od to treat sized sand, as it is easier to adjoP it with greater accuracy when the size of the stoff it ba to treat in confined within definite limits than wheo tin varies through a very wide range. Moreover, this sj-steo iwlmits of tile employment of a cheaper form of conceo-, trutor for a large hulk of the tailings to be treated, and' bonce more perfect and more economical concentration can l)e obtained when the systeni of sizing is prtictiW. Until the time shall come when the entire hulk nf tbe tailings will be treated by a continuous chemical prows (and it tnay not be very long before tins is accomplished), the Byst*?m of sizing before concentration represents llie most important improvement now available in the actual practice of producing concentrates ; it is, moreoTet. finding a further application in that it will also automatically and cheaply separate out slimes of any required ilegive of fineness, and is thus used for preparing tailings for chrniical treatment; fine slimes oppose the greatest liflR-ully to such treatment, because they pack in amongst thi' coarser sands, making an almost impervious mass

THE El. MORE PROCESS 70

hich practically defies j)ercolation. The separation of es is now accordingly a preliminary step, that is iost always resorted to when battery tailings are to l)e

fitted to cyanidation. BaoreProeefS. — TheElmore processor oil concentration ds upon the selective surface attraction that appears sabsist between certain oils, especially, it would seem, oils ; and the great majority of metals, metallic sularsenides, and similar compounds, whilst oxides and minerals suspended in water are quite indifferent. If, lore, battery pulp containing, for example, quartz, itive gold, and pyrites, be stirred up with oil, the latter rill take up the gold and pyrites, but not the quartz. If oil is present in sufficient quantity, its specific gravity, ven when charged with these minerals, will be considerably below that of water, so that it will form a layer floating upon the water, the quartz, of course, falling to tte bottom of the containing vessel.

In practice the process is carried on by passing the pulp SQocessively through three horizontal mining cylinders boot 10 feet long and 3 feet in diameter, making about bIx revolutions per minute ; there are special ribs inside; the cylinders, which causes the pulp to trend forward steadily, at the same time mixing it with the oil. The Biixed pulp and oil run into a V-box, not unlike a mercury trap, which separates the oil from the pulp, a second similar separator removing the last traces of oil from the latter. The oil charged with mineral goes to a solid basket or drum centrifugal machine in which tlie hulk of the oil is removed, to be used over again. The concentrates are transferred to an open basket centrifugal machine in which most of the remaining oil is recovered.

fkt GOLD mu.iyr,

the oon(ntratei ttojnp now ready for furttier trenlma Siich II unit of )>Unt oan treat n)>oul 25 tons per liay ri-qtiiiTs nboni 8 H.P., logetliei' mith a mipply of nbontl tons of oil.

The process is simple artid cheap, the main i Wxn the cost ot the oil adhering to the finid conccotniM ihcBc carry about 3 per cent, of their weight of nil, a ihat the loi!S of the latter depends npon the percenUji concentrales present in the ore. The oil useil is J coarse residual oil left in the rectification of [lelrolwinN the cost of wich is very low. Any oil will serve is Imi( OS its specific gTHvity is low enough and it is neither limped nor too viscid.

The special advantage of the process consists in fact that it is practically independent of the specib pTvvitj' or of the size of the particles to be seporaleJ.&ni' is hence especially applicable to the ItncHt slimes well l:iLOvvn that many gold-bearing minera.lfi, some which often carry the greater part of tbe gold vnh copper pyrites, t*!lnride3 of gold, &c., are excessively brittle, and therefore forms such extremely fine sUhif* when worked that it is practically impossible to save ibm by liny method of mechanical concentration. It '' prolmble that the Elmore process can lie applied wili advantage in such eases. As the loss of oil is proportional to the percentage of concentrates present, suchco"- contnites as can be collected by mechanical means sbooW l>o removed first, and the Elmore process employfil rt'cover the remainder. The best itrrangenient would In he to send the pulp after leaving the wells on to Wilfl''.v . Oi-some allied form of tables, on which the coarser ew- <ii!ii!i'ates will be titkeo out, the pulp going from these

The Elmore Process

fliinore plant. The process is quite novel, aad has not been submitted to sufficiently exhaustive practical 8, but it seems to promise well, and several gold mines novr erecting Elmore plants. The cost of a plant to bt 100 tons per day is about £2,000, and the cost of Anient should be between Is, and Is. 6(2. per ton.

Chapter Xii

t VtMtCBNTBATES — AMALGAMATION— CStOnV ATIOX — SMELT I NO CYAN I D ATI O N

Tn*tmBt Ooacvntratei.— Afttir the nuriferous sulpluncte ban been obtaineii in the form of oonceutnte, tWy tavp to be (urtlier treated to extract from them tbi- iUl wtiich llwtv contain. At one time (his used to ihtm altuost vtitiMy at the mill by soukmethml of nulwiiMliin. which cxtntcted n varj'ing propoitioD, but tMTvr whole of the gold. Of late ycai-s, howaTer, (he of Bolphurets has been placed on a, more WtteniiAc basis. M>d a now rarely a ptu-t of the mill man's d<MT. MOoeointUS being either sold according to assay outekle wcidu which tteai them, or else, in the case of sull&mMtly lu mills, being h&nded oveito a sepanle dri'culnteul lor the exttsoUou of their precious cotiteots- In either of tb<Me outer cases their treatiuent no lou I within tbo proriitoe of the mill m&n, and it is solE- I p*vnt for tb Utter therefore to understand merely Iha I twinetptw whkh umluorlie tbetr after treatment, in order that he R)*y tumisli product in the best possible c<k>udinou for tbe processes it has to ondero.

Th>rv an-three main mctliods of trv&ting concentntM:

.1, 1h iht' mill— by amalgunalku.

Ji. Uutsitk' ilw mill — I

Xii Amalgamation 383

By wet methods. By smelting.

algamatioii. — It has already been pointed out that by no means certain even now in what, form gold 3 in concentrates. It is certain that some of it is t form of free gold in minute lamellse sealed up so ak in the particles of pyrites, and that some of it is ically combined in tellurides ; it may also exist in forms, e.g, as coated gold, as allotropic gold diffiof amalgamation (except tmder special conditions I tend to convert it into amalgamable gold), and as ically combined gold, or gold so intimately associated anically with other minerals as to present great renee to a comparatively feeble chemical agent, such srcury. Whatever theories be held on these points, 'acts are, at any rate, that when concentrates are id up in intimate contact with mercury, varying )rtions of gold are extracted, the proportion dependiter alia upon the nature of the ore as well as on neness of the grinding and the prolongation of the tion. If, however, the concentrates are first calcined B being amalgamated, practically the whole of the can in most cases be extracted by amalgamation, it is reasonable to suppose that the whole could :tracted were it possible to completely calcine the urets without causing fritting by incipient fusion. . general rule, concentrates are only treated by ods of amalgamation when the mill producing them hated in so remote and inaccessible a district that ost of conveying them to the nearest market is er than the excess of the profit derived from their over their yield when amalgamated, where the cost rriage augments greatly the price of the chemicals*

fi4 GOLD MtU.lNC CM

required for the Kp|)lt(iHtiDu of chemicnJ inothodv, ) wbera Ibe lunount procluued ie so smiJl as not to vnxn Ibo ureotioii of tttiparale works for their tnliiioiil : t dysloiii is boiicu uoufiiicd to small ajid contponiun imperfect instnlUtions. It is vury rare indeed "i aulphurols are calciiic<l before aniivlg&inatiou, iijtiuiq tilllulvaiitHflu of so doing is in most caatis verj' decide III iiiiuiy insCaiiotiti thu oo&tof acalcining furnace isloi upon U8 jirohibitivu, ultlioiifh thurti is ao doubt llis> woold frequuDlly pny for itself in a very abort tin Even pnrliid oftidiiatiou is of advaatagi;, sucli as n QMTwd out liy foriiiiiig beaps upon a thorouglily si and evon floor, say of clay rammed liard, or of briokv* tilths sol in clay puddle. Upon this floor a luyer of wihmJ uImui I fool deep piled, and upon this u of sutphuruts of nliout the same deptii. The pile suiTuuitded on three sides by low waUs wliicli may I |H'niuiuut or movable ; a dntugbt is estubUshed tbroqi iIm centra of Iho pilv from end to end, and the bruahwo tiivd. Uy suitably protecting llie pile from the wind M by (taiMiuti}; il down when the fire becomes too Seraai' H sb-ody uoinbiistion of tJie brushwood may be seuuitr 1 whicli id propKaled to a great extent through the luKt of <wim-iilniu by i)ie combustion of the sulpbiu' coii' tailed tu tbem, and by tliis means a partial calciuatiou obtwiiioL which, althoufih far from perfeci, really facitiittU's ibe libeiaiiou of the gold. When the quaouii t<t' i.H>uot>ntrates is sufficient, and amalgamation has iit-vertliolt<4 to be adopted, it is always worth while to Imikl Minall calcining furnace. A fomace suitable for tlif irtlviiiation of sulphurets is shown in Fig. 100. Od ilii' .'thi'i' hanil. [Mit amalgamation is very rarely appl' tUitii sulp*nin-ts. blanketings being frequently treat<l

Calcination

Pan Amalgamation

J7

:nftlgamatioD is very great indeed. They may nil, how- e-er, be reduced to two types ; the ordinary Galifornian BkD, which originated in America, but is by no means anSned to that continent, and the Berdan pun, used Imost exclusively in tl)e Australian colonies.

Califiimiait Pni.— The simplest lypc of ihis is shown ni ttction in Fifjs. 101 and 102, which diOcr mfioly in some rifling details such as tho methods of s<:cin*in<; thu bIiocr kod dies. The former is uonetructed by Messrs. Bowes

I Pan Amalgamation 389

iott and Western, Limited, and the latter by Messrs. "aser and Chalmers, Limited. Other modifications late almost entirely to alterations in the shape of the uller and pan bottom, and to the airangement of wings I the sides, the former having for object a more eflS- 3nt grinding action, and the latter a more thorough cxjulation of the pulp. The plain pan as shown, gener- y known as the Combination " pan, is, however a very irly efficient machine. These pans are mostly between and 6 feet in diameter, 6 feet being a very usual siae. his pan has a cast-iron bottom, whilst the sides ai'e made wood held together by iron hoops, thus combining length and lightness. In order to protect the bottom om excessive wear and tear, dies are fitted to it by leans of dove-tailed joints, these dies being made usually ' hard cast-iron ; sometimes white iron is employed, but vily chilled iron is better. There are usually eight dies ' a pan. The muller consists of an annular casting, iven by a central spindle, which passes freely up through e conical centre of the pan ; this spindle rests on a step iich carries a case-hardened steel button, the bottom of 5 spindle being rounded off, and also case-hardened to finish friction ; sometimes this bottom end of the spindle ttiade removable, so that it can be renewed when worn, feather fits in a groove in the upper part of the spindle lich drives the muller, at the same time allowing the ter free movement in a vertical direction. The nmller 1 accordingly be raised or lowered by the screw and rid-wheel shown at the upper end of the spindle, a lower nd-wheel carrying a jam nut, which keeps it at any ired height. The lower grinding face of the muller is tected by dies similar to the shoes of the pan. This Q is fairly portable, its total weight, set up, being but

Pan A Ma Lg A Ma Tion 391

sometimes added with the quicksilver, but there seems be DO definite object served by this addition, which probably injurious rather than beneficial. A little cali may be added if there is any fear of the charfe the pan having been contaminated with oil or ease. le amalgamation is continued for some time longer, lually for about six hours altogether. "When the quantity of blanketings or concentrates to J treated is sufficient to keep two pans in operation — lat is to say, when it amounts to more than some 6 tons r 24 hours — it is usual to employ a settler. This is lerely a larger and lighter pan, usually 7 to 8 feet in ameter, having four to eight arms instead of a continuas annular muller; these arms are usually furnished ith wooden shoes, and the bottom is rarely fitted with ies, as the wear is not great. The settler is provided ith a series of discharge pipes with plugs, up one side, that its contents may be drawn off at any desired eight. A special syphon tap, by means of which nierar>' and amalgam may be drawn off from the bottom of le pan, is also provided ; there are usually one or more ixx)ves cast in the lK)ttom of the pan in which the mercury nd amalgam collect, and which comnmnicate with the p-phon tap. The contents of the pan are nin out into le settler and considerably diluted with water, so as to lable the quicksilver to fall freely through the pulp. he spindle carrying the arms is set revolving at about 5 to 20 revolutions per minute, until the mercury and malgam have fairly settled, when the top hole is unlugged and the contents drawn off to that depth, the pindle being kept revolving slowly ; plug after plug is thus pened until all the slimes are run off, when the settler ready to receive a fresh charge. Usually a charge can

39a GOLD MilJ.ING

be worked oB in 3 to 5 hours. Tim slimes sboald Ix; m throufth a lioQ lined with amalgamated copper-plates oalot MJtidea uC lunajgaiu wliicii they may 9

irri' m the anioaiit of conc4'titraU.;s to IkiretiMd

ia D ;a OS to require the jibove plant of two pott

wml u , one pan may be mado to do all the work bj

furnisluriij! it with a series of plug-holes, and by providii tliu counter-shaft that drives it willi u c!one-|mlle>' as to enable the speed of driving to be vri<."d. A sm charge of com i nt the outside, nhouMte

tn'jvteil at a I rather leaa than haU lull

When amalga fed for a sufBcient Itagtt

of time, the half an inah or bo Xvm

the bottom, ,. diluted, and tho niollci

is run at aboi per minute till all

mercury is sett ilp is drawn off jusl as

-when the sepai 1. Tho progross of boiii

amalgamation Id be watched by Ukim

out samples from time to time, and "panning uji" ot "horning" them so as lo sec in what condition ths pnip and quickBilver are. The power i-equired to drive a 6<foot pan is usually between 3 and 4 I.H.P., and & aetller 2 to 3 I.H.P. Care must be taken in examinJD),' covered pans not to bring a light near them as soon they are uncovered. An explosive gas has been known to accumulate inside the upper part of covered pfi& and several accidents have been caused by its ignition. It is not difficult to uiulcrslELnil how hydrogen car generated during the operation of pan amalgamation, the proper ventilation of anialgamatiog pans is a by do means superfluous precaution, which is, however, much too frequently neglected. licrdan Paw.— Usual forms of this pan are shown in

Berdan Pan 393

$8. 103 and 104. It consists essentially of a cast-iron dlow basin having a central cone, which is keyed nly to a spindle set at an angle of about lb"" to the -tical ; usually a row of these pans is driven by means of irel gearing from one counter-shaft. In the annular between the central cone and the nni of the basin 3 either a couple of heavy cast -iron balls running Dse (Fig. 104), or a couple of heavy drags attached by aans of short chains to a portion of the frame through lich the spindle passes (Fig. 103). The former draw- g is from a pan manufactured by Messrs. Appleby res'., and the latter from one by Messrs. Bowes Scott id Western, Limited. The balls or drags, by reason their weight, maintain their approximate position, toving to and fro but slightly in the lower portion of le inclined annular space whilst the pan revolves, the snal speed being about 25 revolutions per minute. The in is preferably lined with a liner piece of hard white chilled iron, which can be renewed when worn out, as lown in Fig. 103. Pulp is fed in at the highest portion f the periphery of the pan, and as the latter continues revolve it is ground between it and the weights (balls r drags) until fine enough to be carried by the stream of ater over the lower edge of the basin, whence it flows flF through suitable sluices. Mercury is fed into the pan om time to time. The process is thus a continuous Qe, and in so far it is preferable to the intermittent ction of the Californian pan. The capacity of the crdan pan dei)ends very largely upon the degree of fineof the pulp fed into it and the degree to which this eds to be ground, the latter being regulated by the Hied of the pan and the amount of the water supply ; an ''oaso in either of these factors increasing the working

Bbrdan Pan 395

mcity of the pan by Bhorteoing the time during which ) pulp renaaine in the pan. On the average its capacity ky be taken as 2 tons pet 24 hours ; usually one pan supposed to take all the blanketings from 5 hea<is of

UopB, The upper portion of the sluices should always alined with amalgainatod copper-plates, and these may advantageously be followed by litHcs. When [)au amalgamation has to be employed at all

tho Berdan pan prosent? several advantages. It u

cheap machine, costing only about £50 ; it ia coDtioU ill its action, and needs pritotieiilly no iittenliou. 1 WISH' and tisitr ia but Blight, and only on cheap reiulily renewal. It does not absorb mnoh power, ab) H I. FT, P. l>ain sufRciKnt to drive ewch pnii. On( other band, it requirew rather morn water than d(i*ll Ciiliroriiiaii uiiial;;ainatin({ pan.

Amalgaviators. — A vast number of machines havelM designed under this name, all of which have fov tbi object the treatment of ttvilings by rubbing them Up,l otiierwise bringing them into intimate contact *il mercury. It need only be said that if there ia no scisnlil justification for the amalgamation of concentrates, there: slill less reason why such a process sliould be applieJ m tailings. No amalgamating appliance can save goid tbi cannot be equally well caught in the Californian stampmill, if the latter is properly handled, and such machintf are only used where milling is imperfectly pei-formed. In Australia, Berdan pans are sometimes used in this waVi iiiid in Hungary the Laszlo amalgamator, a kind of small continuous-acting pan, is in considerable favour.

It is only necessary to conclude the subject of aaiiilpniation of concentrates and tailings by rejJeating tlmt ItT gold ores it is practically a thing of the past. In the Wilwatersrand district, where the treatment of concentnites and tailings is being rapidly elevated to the dignii; of a special science, out of 40,000 ounces of gold exlrsutol from concentrates, under 300 ounces were obtained by pn amalgamation in 1892, since which date this method seeioa to liive been entirely abandoned.

Chlorination.This process was originally invented l? l'eri;>' auJ I'latter independently about the year ISlfl.

Chlortnation 397

. still carried on according to the principles laid down latter, although all its details have undergone much icatioD, and have become the subject of numerous ts ; there are thus some dozen chlori nation processes stance, and several in operation. The process de- I on the fact already stated, that chlorine has a strong ;y for native gold, and readily combines with rmiug the soluble auric chloride (page 30). The Lon containing the gold can be filtered off from the lie, which is now practically free from gold, and the in the solution precipitated. In the Calif ornian fication of Plattner's process, which is on the whole ips still the best mode of carrying it out, the subto be treated, properly moistened, is shovelled into with a double bottom, the upper, false bottom being rated and supporting a suitable filter. Chlorine B conducted into the space below the false bottom, gradually rises till the vat is full of it ; the lid is then I on, and the whole left until the action is complete, 1 the soluble chloride of gold is washed out through liter into other vats, where the gold is precipitated. ous precipitants, such as ferrous sulphate, charcoal, buretted hydrogen, sulphurous acid gas, and copper hide have been used for this pui-pose. It is obvious I this brief description that the presence of any subce attackable by chlorine will cause a waste of that which forms, under suitable conditions, the most ex- >ive item in the treatment by chlorination. Among ly other bodies, iron pyrites and most sulphurcts attacked by chlorine, whilst most peroxides, and ic oxide amongst them, are not. It is therefore issary to calcine the sulphurets produced in concentra- i as completely as possible in order to avoid loss from

jgS GOLD MILLING

this eooroe, wliile calciuHtiou may possiltly also render the gold more readily attackable. of furnaee have been designed for the puii>o3r of lait tioD, and the question of calcining without or with addition of Ball, and the losses of gold thereby incnn have been very fully studied : the simple longcftltiner (Fig. 100) is still perbape the favourite of ftirnoce. The cfalorinating apparatus proper is portion on which most of the patentees have exi their ingenuity. Uears, Thies, and Rothwell in Am and New)>cry and Vaulin in Australia, have replnced Btationary vat by revolving barrels, into which chlorine is either forced, or within which the chlorinei generated by suitable chemical means. Munklll h<t designed a method, used at Puhliiu, in Sweden, by wbid a dilute solution of chemicals capable of generaliai! chlorine shoiild he run over the ore contained in brp vata. Various forms of filters have also been used. a long list might be given of possible precipitants whirft have been proposed, hut the broad principles of the cbhV' ination process, with which alone the mill matt need concern himself, are throughout the same. It is, in the fii place, obvious that thii snlphtirets must, for the wke ol economy, be as clean as possible. It would cost nesrij as much to treat a ton of rough blanketings contniningi say, 90 per cent, of barren ijuartz sand, and 10 I*' cent, of auriferous sulphurets as it would to treot of the clean auriferons sulpbitrets, so that the cost oI extracting a given amount of gold would bo practically ten times as much in the first ease Concentrates intended for chlorination must, t.hereloft, be as clean as the beat type of concentrating uiachiaer)' can make them ; even if it were only for the uviiij

Chlorination 399

freight from the mill to the chlorinatioii works, this im would prove to he an important one. It must he reembered that highly fusihle sulphides, such as those copper and lead, have an injurious action in this process. a account of their low fusibility they tend to soften and it and to cement together, so to speak, the partly-calcined utides of less fusible sulphides, so that properly comuete calcination becomes impossible ; thus not only is :i.lorine wasted, as already pointed out, but gold may he Ksked up in the semi-fused mass and, not l>eing attacked y the chlorine, become entirely lost. Zinc sulphide is, n the other hand, favourable to a thorough calcination, it will stand a high tempcmture in the calcining Umace without softening. If, therefore, a gold ore conains a high percentage of galena, it is not well suited for chlorination, but should preferably h(; treated by a ftnielting process, unless the galena is separated out from the other sulphides by special dressing methods, which its high specific gravity renders by no means dillicult. The Liihrig belt vanner may do good service in this direction. If the gangue of the gold ore he calcareous, even more than onlinaiy attention must be dincted to producing clean sulphurets, and it will be better to accept the alternative of a small loss of the valuable materials rather than to risk their not being quite clean, seeing that Hine has a Kreat affinity for chlorine, and would thus ti'ud to waste a eat d<.'al of this comparatively exK'Hsive rragi'nt.

The table on page 400 shows th<; cost of chlorinating sulphurets in some of the principal establislnuents in North America.

Tlie cost in South Carolina, as given by Mr. Thies, is determined upon roast<;d ore, 1 ton of which is equivalent to IJ tons of raw pyrites ; the barrel process, as used

€SSSg S SS3

in

jS S S 1 £ E . E

S r S S S

® ® : © ® s IS 11 8 sis

Hi

mi

s

J

" - - " - Has

Ilillilljlf 1 f

hP. XII CHLORJNATION 401

sre and in Dakota, is evidently much cheaper than the ler type of vat process, but it muat not be forgotten Bit the former requires a much greater capital outlay Em the latter. At the Utica Mine, California, the cost

chlorinating by the Plattner process is said to be k50 (27.) per ton. The cost of barrel chlorinating in rge iworks at Cripple Creek, including crushing, is said

be about as follows : —

%

labour, including salaries 1.34

Fuel for calcination and power . . . 0.70

Chemicals and supplies 0.72

Miscellaneous 0.77

Total cost per ton of ore . . . 3.53 (145.9(2.)

The total cost of erecting the Sutter Creek works is ated to have been $15,000, say about £3,000. Eecently, owever, such works have been erected more cheaply ; in 389, according to the eleventh U.S. census, the first cost i California of a chlorination works capable of treating

tons in 24 hours was between $6,000 and $7,000, and tiis may be taken as an accurate estimate for the preent day. The same authority gives the total cost of hlorinating a ton of concentrates at about $10, with Ji average recovery of 90 to 92 per cent, of the assay "aloe (of gold) of the ore. The prices for custom chlornating vary somewhat. The usual practice is to buy he concentrates outright at a certain percentage of their essay value — this percentage increasing with their richriess— and deducting a fixed charge for chlorinating. Ihe following is the scale of prices at the Sutter Creek Reduction Works and others in Amador county under the same ownership : —

D D

AXUJ MILLf/TG

ffiag woAs do not ail

( SBife tt> tk ftboTv ; tb Bsnal pnctioe tben Iw Ak gnU ki ;he me of $1 per once, ft ikaiyt dL $19 or bor: per ion. acooidiif ti r w. Iyr te InatiimL Wbes

TwVi am tiftint Ihn if KB akwt $18

f7tDflO|C

Smelting 403

in gold, less f 18 per ton charges of treatment ; . silver is present to a greater value than $10 per ton, tr cent, of its assay value is paid for it. In Australia sual charge for chlorinating is £3 per ton of ore ired at the works, with a guaranteed extraction out 95 per cent. The cost of chlorination in the svaal is also ahout £3 per ton.

inpts have repeatedly been made to replace chlorine r wholly or in part by bromine ; technically, the SB leaves nothing to be desired, but the high pi-ocess mine is a serious obstacle to economic success. letting. — This is the most universally applicable tss, and all concentrates, whatever their composition, le treated by it ; a large percentage of zinc blende is, ver, objectionable, owing to the infusibility of zinc bonds. It must be rembered that smelting can be practised when suitable ores for mixing are availso as to produce a proper furnace charge, and hence lever carried on at the mine, but only by customs dng works, which buy the concentrates from various and mix them to suit their own convenience. The al principles are as follows : When argentiferous 3res, such as galena, are smelted in a blast furnade, lecessary to add a flux, of which oxide of iron is an tial ingredient, the products of fusion being then e bullion," consisting of metallic load, which ins all the silver and gold present in the furnace ;e, and a slag consisting of silicates, usually of iron, alumina, magnesia, <&c., according to the nature of uxes employed. Obviously auriferous concentrates, iting, say, of iron and arsenical pyrites, can he eni- (i in this process by being first calcined as sweet ssible. There will thus he obtained an auriferous

oxidu of iron wliicli could be wddud as a flux to ingredionts of tbe furnooe ahwge. PrarOtically Ibe of blitz gold present will alloy with the lead produce d will be found in the Itaae bullion, from which it Is wurds sepamled by a series of processes. Aocordilig II tbe nature of the other ingredients of the ftimiici! nhuf it may sometiniss he a poailiTe advant'Se to lian iV certain amount of crushed quartz left in the coaociitnMil but as B. general rule the aim of the uiiU nirtn shooU bfl to prfjdiice ti3 clean and as noli ooncentmtes a In the above process copper may be substituted fur ImJI smelting, a bath of the former instead of thf lallt-r moall being used to collect the precious metals. Thta piDCM I is particularly suitable when the concentrates conlAiD notable proportion of copper pyrites, as the copper wosM I thus become a valuable ingredient of the ore, and be fw for accordingly. Tliis copper process is coming ntpiil into favour, coarse copper being produced by a sotm A Buieltiiig processes carried on either in reveal>eratori' or in blast funiacea; the course copper is run into slabs, wliict are then refined electrolj-tically. During the process d electrolysis the silver and gold, in a very impure stole, are deposited as a blackish mud at the bottom of tbe vkls in which the eleotrolylio refining tiikes place. This mud [ is collected and the precious metals obtained from it I ouppcllation. This method ia being practised in sevenl I places in England notably at Swansea, and is applicable to most classes of concentrates ; for concentrates carrying a comparatively large proportion of quartz it is perhaps the most suitable. The first operation usually consiaU in smelting a matte, which will contain all the valu&ble ingredients of the whole charge. If it were feasible to Jo this at the mine at a uioderatd ooat, it would he very

Smelting 405

LTisable to do so, as the matte would be in a better rm for shipment than finely-pulverised concentrates ; Ld as a considerable saving in freight would result, oing that the gold can be concentrated in a matte to very considerable extent without any risk of loss. sere are various smelting works in England, America, id on the continent of Europe, &c., that are prepared purchase auriferous concentrates in any quantity. bis is always done upon assay of a sample, the price ven varying according to the richness of the concentrates id their composition, whilst each smelting works igulates the prices offered according to its needs for the me being of such ingredients for making up the furnace larges. No fixed scale of prices can accordingly be noted. One American smelting works buys at the rate about $18 for the ounce of fine gold, less a vaiyiiig large for treatment. It may be noted that §18 per mce of gold is equal to 87 per cent, of the full value. In the Cripple Creek district smelters buy on th(j ime scale as do the chlorinating and cyaniding mills. I England there are firms that buy concentrates for nelting at London, Swansea, St. Helens, Birmingham, id Newcastle. The following table will serve to give general idea of the prices that concentrates of varying ihness will fetch in England, but, as already said, exact (ures cannot given : —

For OTf assaying per ton of 2,240 lbs. Price i>a:(I ft.r goM icr oz. tny.

Gold Milling

Tho aifi(> thle only i>pli)-s to eulpliumte oonUia bat a siiiaJl proportion — ay under 10 pi-r ccDl.I b Wlion tliitj tneUil is preuent io liu-frt; quantitius, tUc p U lowur ill pn)|HirUi>ii t<i llic aiiinujit ptvtMUil. lu a paring English u uii prioes, it inuiit

membered tliitt "e paid in uet caeh, wli

in the former ootuu ajiuent is inadu in btlb

two month s.

Id West Anati ibarRes of the Dry Oa

fimelting Works ail n: — i

Ov-reu* . , IS fl '

KmiiiflM. I.. 8..r, . J ., IT B

.. 1 ra. 1.J Cot:. . 9:fr. 50 0

ITiiiIri -I 11/ 92-5 50 0

Tlif fiolil is viilued at £4 per oz.

Shipping of ConcentrateE. — Tlii is almost univera lx.'i'FL'tiLi;il in biLs ; as a jjeneral rnle, made of I Hiiniiy siickiiif;, lii)l(linf; about to cwt. each, ('iti]iliive<1. Tli<: iiiiitvriiLl of the luigs should be of clone li.'xliire so us to prt-vent any loss of the coaU which iii'u fii'qnenlly in stute of very line division. is ln;;lily mlviaable to tboronghly dry the concentr lM>foro iMigfjinK them, either hy natural or artificial me Not only will the cxiwnse of paying freight upo certain proportion of water be thus avoided, but, i is even more important, the baga are far more K to arrive at their destination in good condition.

im CYANIDATION 407

txlpbnrets are apt to heat and oxidise, forming sulphates some sulphuric and sulphurous acids, which rapidly K>t and destroy the material of the hags. This is a riatter which must not be lost sight of whenever sul- >l3iirets have to be shipped for long journeys. It is of ionrse a very secondary matter when the chlonnation works are owned by the mine and are consequently close bo the mill. For shipment to a distance, however, it oxight even be advisable to calcine before shipping whenever fuel is cheap at the mine ; calcined ore carries better than raw ore, and moreover, in cases where the ooDcentrates consist chiefly of iron pyrites, a saving in freight would result, pure iron pyrites completely calcined into peroxide of iron losing one-third of its weight, so that 3 tons of raw pyrites only carry as much gold as 2 tons of the same calcined, and naturally cost 50 percent, more for freight. The percentage of nioistun* pn*sent should always be delerniined when the parcel is Vei'lied before bagging. Before a pile of sulphurets is bagged, the pile should be thoroughly turned over, w(;ll 'nixed, and a fair sample taken out for assay, in order to control the results obtained by tlie customs works purcliasing the parcel.

Cyanidation — This comparatively modern process is

Wsed upon the well-known fact of the solubility of gold

in a solution of commercial cyanide of potassium to

to l)e equally suitable to all ores, but works well with

most, and will extract gold from some on which other

processes iiave failed. As tlie solution rm])loyed hns,

unlike chlonne, practically no action upon the native

sulphides usually occinring in concentrates, calcination

is not necessary, except for mechanical reasons in some

4dB GOl.l> Mll.UtfG ad

cases, and tbe process is nouonling]y in so far a cbeif one than chloriuation. It has received its principal i velopment in South Africa, and notably on iho fl watersrand gold fields, where it is now worked on very large scale indeed. Its principal applioition dl ia to tailings from which the sulphurets may or d not have been removed by concentration, bul boiii i pburets and tailings can he and have Iteen treated with grt-at sucoesa. In 1891 the RobiDSon mine ed menced to erect works for tlie treatment of its tAiQ by the cyanide pracess, and in that year already ndl i4,000 ounces of gold were estracted by it. In tl eloye upon 3,800,000 tons of iiiilings wirt-treated, js. ing 1134,545 ounces of old, worth nearly three millu sterling, whilst the treatment of concentrates by cyani tion produced 57,455 ounces of gold. Tlie principle o sists simply in allowing a weak solution of cyan of potassium, containing usually from i to per cent. commercial cyanide, to percolate through tbe cniat ore ; it is found that such a solution dissolves on' large proportion (in favourable cases over 90 per cei of llu; gold contents of tbe ore, whilst scarcely attack any of the Imse metiUs that may be present. Tlic ac tiiiu, which tlieti contains gold in the form of pota* aurocyanide, is filtered and the gold precipiU fmm it: as a general rule the exhausted solution I relains some cyanide and is pumped back to be u over again. Sometiiues it is allowed to run to wa The precipitated gold is melted into bullion. S is briefly the rationnlc of the process of cyanidation full discussion of the details of its practical applical woidd be foreign to the object of the present work,

:i CYANIDATTON 409

will be described only in general terms, so as to supply I8k as much information about it as may be of use to 16 mill man.

'Blani. — This essentially consists of dissolving tanks in 'hich the solution is prepared, storage tanks in which

is diluted to the desired extent, leaching tanks in rhich the liziviation proper is carried out and pre- Lpitating tanks in which the gold is deposited. The haracter and arrangement of the plant will vary with be ph3r8ical features of the site available, and the uantity and nature of the material it is required to reat. The latter consists, in most cases, of tailings which have been allowed to settle in suitable reservoirs Jid are thence trammed to the cyanide plant. The experiment of running tailings direct from the mill into be leaching vats has been tried, but presents considerkble dif!icultics, as these tailings settle very solidly, and )ffer great resistance to the percolation of the solution. present, direct filling is not much in favour. The iist stage, whatever method of filling l)e adopted, is 0 get rid of the slimes, whilst a rough classification ])y means of a few large Spitzkaston or Spitzlutten is often introduced with nmch iwlvantage. In the most modern lants a sy.stem of intermediate filling is used ; in this Mhod the pulp is filled direct, ofUMi by means of disibutors, into a vat, where the sjind is allowed to s(ttle 'J<1 dniin, and wlience it is transferred to the leaching *t. Thest? intermediate vats are now oft(n supj>orted iron or brick columns al)ov( the leaching vats, an

The ri'ador who denircs fuller iiiforination may br leffrri'd with Vantnpfe to Bulletin Xo. of tlie California State Mining Hureau, the cyanide prwoss l>y Dr. A. Schoiih*!.

GOLD MILLING ™,i

mgemetit that reduces the labour of re-hnntllin"W( , though it necessitates the hftmg of the tuS tailings to a uonftiderablti height.

Dissolving Tank. — This is sometimes dispensed wilt the solid cyanide being then placed beneath a sttwo of water flowing into the leaching tank. Usuftlly ii ii n, small tank or vat, into which the orude cyanide llirown, water being run in until it is dissolved, tii strength of the solution is then determined, and it is ran off through a filter of fine steel wire into the aton* tanks, whei-c it is diluted as required. Tlio insoluble impurities pvsent in the crude cyanide settle in this siutJI tank, which has to be cleaned out from time to tinift Crude cyanide is shipped in sealed canisttrs containinf 190 to 195 Ihs. of the cyanide, which usually oonliinf between 75 and 98 per cent, of pure potassic cyaoiiii: the remainder is generally carbonate and oyanale d potash.

Storage Tanks are precisely similar in structure to leaching tanks about to he described, except that tluy contain no liltor.

Leachinij Tanks. — There are several types of tbci cement or brick tanks and wooden and stel vats; tl>e first-named are considered out of date, whilst the ush steel is apparently thouglit very welt of. Ceuieiit tanli* are at times, as at the Langlaagte Estate, sunk belo"' the surface of the ground and tram-lines run over und alongside them for charging and dischai'ging tlieni- The latter process is effected hy means of a steam-crane which lK)islfi a grab capable of filling two trucks at eflcli lift. By these means a tank can l)e emptied in 14 hours at a cost of 2d. per ton. The cyanide solution has a disintegrating action upon cement, and the insides of these

"tiTiks should be painted with a tliick coal of piuaflin

paint ; this precaution is especially necessary with tanks

e'xcavated in the ground, leakage from which is apt to

escape detection. Large vats with bottom discharge arc

Xic?r considered the best, and for this purpose, brick and

cement are unsuitable ; wood or iron vats carried on

brick piers or iron columns with lines of rail running

below the vats have been used in the most modern

plants. Wooden and steel vats are generally preferred ;

they have been made up to 40 feet in diameter, according

to the size of the plant : —

A Tat 14 feet in diameter, 5 feet hi, holds about 50 tons of tailings. M 22 „ „ 6 „ „ 100 „

C M 40 ,, ,, 8 ,, ,, 360 ,, ,,

The above quoted dimensions are all from actual practice, the last-named exceptionally large vats having erected at Roodepoort. Wooden vats should he ade of staves 3 inches to 4 inches thick, of thoroughly ell-seasoned lumber, held in place by hoops of bar-iron, he ends of which are drawn together by powerful bolts. The bottom should be made of good sound 4-incli plank let into the staves. All the work should be most carefully done. The joints should be made with thin whitehead, and when finished the vat should be thickly coated with paraffin paint. The filter consists of a porous false bottom a few inches above the actual bottom of the vat; it is sometimes carried on a wooden grating which rests on a bed of clean gravel, but more often on a frame of stout wooden slats a couple of inches deep, or of tiles pierced with 1-inch round holes. This filter frame is about 1 inch less in diameter than the vat itself; upon it is laid the filter proper, made of very

Btout blanketing, or, better etill, cocoanut-maUtug, lidii in place fav a gaskot of rope driven well dowD aJl ronsl it. are cut in the bottom of the vt tcMlinf

towit nouth of the delivery pipe, which gentnJIj

cons: piece of ven-stout india-rubber bose pipe.

For iron tel vats the fiide plates are best mn*1e f", iiwh to inch tiiiok, vs-itfa vertical joints only, Inp-jninbil. riveted, and caulked ; the bottom plates slioubi he ftbonl inch lliioker thau the sides. Tbe sides sliould bt Btiffened by ; of angle iixin riveU

round them. be paintetl insidi

outside with oe

The eshauat*!- istly discharged ibroogli

aide gates or tb doors of various kinds.

They are best m be, and aro held wakrtight

by meanp it india-rubber aeatings

Wlienevcr posi tailings are slniced

mt of tbe vatR b bead of watr. Wbm

thia is not available they are emptied by hand ; this takes from six to nine hours, and is not a very expensive matltr in the Transvaal, where Kafir "boys" are employed for such duties. It may bo mentioned tbat rwlanguUir wooden tanks, though cheaper to construct tbin round ones, are. not found to work so well in practice.

Vree'tjtiiaiMfi Tanki, or zinc boxes " as tliey are also called, are used for coUectin; tbe gold. The preciintant liithcrto inoe*((y cmiiloyed is zinc sinivings. Tiief*e arc prcpari'd bv tlirciulinfj a number of thin discs of sine on a spindlt and turning the cylinder so formed in a lathe. One man can produce aiK>nt 50 lbs. of zinc shavings in a day's work. These shavings are carried on trays about 20 inches square, the bottoms of wbiob const of iron wire-gauze of J-inch mesh, the trays resting oo cleats

Cyanidation

ftout one-fourth way up in the box. A rule sometimes oUowed is to allow 2 cubic feet of zinc shavings for ker}' ton of solution that passes through the box in t4 hours. The precipitating boxes are usually about 20 30 feet long, 2 to 4 feet wide, and 2 to 3 feet high. Chey are divided into about ten compartments by trans- 9-ierse partitions ; the first one of these does not retich to le top of the box, the next one not quite to the lx)ttom, tad so on, so that the solution is compelled to flow alternately up and down in its passage through the l>ox, as Bhown in the diagrammatic section. Fig, 105. The first

VERTICAL SECTION Scale, 4-inch i foot. Pio. lOS.

compartment is left empty to act as a settling-box for any fine sand that may be carried over by the solution, and the last two are similarly left empty for fine gold to settle in, that might otherwise Ixj carried away by the solution. Each tray holds about 40 lbs. of shavings. There are usually at least two sets of precipitating boxes. From the precipitating boxes the solution runs into wooden vats or cement tanks, as the case may 1x3, to be pumped back as required. All pumps, cocks, valves, (fee, about a cyanidiition plant should be niivde of iron, as brass is slightly attacked by the solution.

Tk( LemAing Process, — The tailings are chargti, I bv hand, into the leochini; tanks until these are fillH u vilfaia fthout G inches froDi the to. An a, nilu, tiulin|(j antvs Tpry recent, contain soluble salts, owing OXtdfttioQ of DKire or less of the sutplmrcts origini pravnl: ihftae sahs (o be washed out. The IDC tMik is ssftptictl vritb a water tap, which k iDDid aoA vnter ran in till the-tank is full. Tlic wiiter it 1 tkrawd to sUnd until these salts are cauijetil]' Miter wbkii has to be ascerlned tu tarn ly fnynriotM* ; ib mUer is then run off. If, u orijy' tbe CM*, the pixaction of these salts has beff mmomfttmi by tltat of fre acid, the latter bas u ba NMUnKBed; this a thitw Ity rtuining in a solution eitha vl nwrt'" soft or, brUer stiU, ol lime-vrater. 1 (he* ran off, aai Awn tha cyanide solution is ran vi. tW iMiMlllh Hub is genenlly between 02 aud OS fir MoL Id 10 b) the kn), hot the most suiUblt slivinh h*s ways to be determined for eiich ore i>x(viuii>rcii. Tliis tjolutkin is allow to reiimtn li)' Ixurs auJ is then ilrawn off. bcino; replsoed bv ItVAh. tki trralnvnt <nritb so-called "strong solutioii' kii$ fnieo Mjebl to twvc Iours. After tbe "attwig *AhltiL<n " Waehtn W OMnpleted, " vreak solution." ccolaitun (Tvxn (KIH O-i per cent, of cyanide, is run Tbv* aolsuiin consists generally of tbe Gnl *i-.u-vi *:ur shr-sjKl has been precijHtated oat o( it. Av-ttwi -i' iv-c I'.T AKvher eight or ten boors, ard 'S iv;- .'rt. TK- "sstvc solution" and tbe "vat " *tT ttn; thrviiiih separate zinc boxes. i-ttaiinvt ftvvit ihe fi.>mter is finer than that ob-ti

C YANIDA TION 415

iQght in tanks, that from the latter is often run to "Bsfce. After the action of the solution is complete, a QaDtity of wash-water is run into the vat about equal 3 that which ultimately remains adherent to the tailings, D that the volume of the solution remains undiminished, kbout half a ton of strong " and half a ton of " weak " dntion are used for each ton of ore in the vat, and le thus kept m circulation. The treatment of each vat akes about three days. Experiments have been tried on system of continuous circulation by which the same caution has been made to pass through several leaching Its, or several times through the same one, but the vantage of this method is by no means established as 'et. It is, however, stated to have been worked with success at the Gold Run Mine, Siskiyou County, California. lere a small four-stamp mill crushes 5 tons of quartz very 24 hours, inside amalgamation being employed, 'hich collects in the mortar 86 to 90 per cent, of the old saved. The pulp passes over the usual amalgamated t>ppertable, and thence direct into a pulp tank '' 4 feet eep and 6 feet in diameter. The overflow from the ulp tank, whilst the latter is being filled, runs into a samp tank," where the slimes settle, to be drawn off as ley accumulate, into a slime tank " for treatment, he water from the sump tank is pumped back into tin? ktter}', Ixjing used continuously. There are four pulp nks, each of which is fitted with a false bottom carrying filter ; when one is full, the stream of pulp is turned to the next one. The pulp is then drained as dry as >88ible, and cyanide solution containing to per cent, cyanide is pumped in. Thenc(; it runs tliroiigh the

Eternith Report of the SUtte Miiuralogist Cnh'foniiit Statr Mining ircaUf ]>uge 430.

,rt 5V/. MILUNC

ifHoe, Had, alter Uw goM hw been pncilated tbok into vwo e >aide aolatiDa teoks. wtiere it U bnmell Bp I iu foil Dwnftt, &Dtl U then nidy br

nOht od. UaiikUj SMrti Uuk vl |iuip i)> Uiwul

four uiin. Il is aikl Uut this pnMess IcavM a mn tnee o( eoU in tho adwuted Uilits. Thv total eal4 milling and ontiniiaaBqnid*tk>o Ua&id to Im $6 p( Inti, whSst th goU aaTcd amount la Iwtweon $50 ai 130 [er too by M u>d (10 to $33 by eysi'

dMion. Cti on is heiD trW a

Aosindasu; ii n to Luvd been

soocessful in So

PrMipifaitOW. 8 ran Ibroogh the sne

boxes alrettdy d >ot Ireating &hoat tbir

tons of solution u Hie xinc shavings id lh>

first trays ani more rapidly thiui in tlie

last oiies ; Um m iosu are therefore naeii U

fill up the front trnys, fresh bright shavings bein Qscii to refill the last oDes. The mlution, then it runs ioU Die xiiic boxes, carries usually from 1 to 3 ounces of guild to the loll, and should not contain more than 12 grain! when it escapes. In a few places zinc fume (fiwlv divided zinc obtained in zinc smelling) ta eniplojed u precipitant.

Collet-ting the Gold. — This is done about once a iofinigitt ; the trays arc lifted out and washed, and the alimes of iTiipure gold ai-e allowed to settle. The clear water eyplioiiud ofT, and the slimes scraped out and rablied through a Giie screen into the auiall tank, the pieces of rutiiaining on this noreen being returned to the trsjs on top of the shaving. The impure gold is again alloweii to Bottlo, and transferred to onaiuetled urou pane, ix

Ii Cyan/Dat/On 41 7

rhich it is dried. It is sometimes simply mixed with orax, sand, and carbonate of soda, and melted in clay racibles, the ordinary melting furnace, to be suhsenently described, being used for this work ; it should, owever, be furnished with dust chambers so as to oUect as much as possible of the zinc fume formed in his operation, which always carries gold. More often he dried slimes are first calcined in reverberatory or Qa£3e furnaces, the bed of which consists of a shallow nm pan. In South Africa the slimes are usually heated nth nitre until most of the base metals present are oxi- Lifled. The bullion produced is very impure, hard, brittle, nd greyish in colour ; it is about 650 per 1000 fine. Asides zinc, it contains, silver, copper, and small quantiies of other metals. It is usually shipped without nrther refining.

Another method, preferred in America, is to treat the sold slimes with dilute sulphuric acid in wooden tubs, which means a large part of the zinc present is dis- K)lved. The remaining slimes are washed by decanta- n, dried in a filter press, and calcined in a mufHe at fc low red heat, and the residue then fused as alx)ve. is said that in this way bullion over 900 per 1000 'ne may be produced.

Whatever method be employed the slags always contain

notable proportion of gold ; some of this can be roovered by crushing, panning, and amalgamating, but >Oic of the gold resists this treatment and can only be Uracted by smelting with lead.

TJieory of the Process. — This is still somewhat imperctly understood. The equation already given (page 28), hich purports to show the actual chemical reaction that kes place when gold is dissolved in cyanide solution, is

E £

awr.

'

T

:bo only reaction which occurs. A solutioo ii'i.U'. evt-n when puiv. uinlergocs numerWH c':.:vfl\ tlirouiih ihi* action of atmosphfric ..T'. -i. f!:!a!os. carbonates, ammonia, ani

L- v.iV'Vv.iii lvin* fornioil, all of which no ov. iv- r. if thi-y ilo not take an act ivo part

: :';.v o.'r.'i:: nipt ion of cyanitlo is accord y txci'f of that re*]uii\Hl by the abow ::-.i.*::(\- i: air.ounts to more than 3 lbs. : ♦. .lO:: . unci' of irolil precipitattnl, or froni ! :;:-. of :.r!'.ini:< ireateil. "."v.'.ve I in tlio precipitation are also .;..:-. :: '- -:i*.': doubtful whether puff

1

si.tlier movi) tlian two-thirds of the total amount of gold

Iracted in the whole world by cyanidation; it is

lleTefore to this country that we have to look chiefly for

Kiable data as to the cost of the operation. This is

between 2. and 4*. per ton treated on the Witwatersrand,

fct may rise to between 75. and 15s. in outlying disti-icts

*iich as De Kaap and Lydenburg. The average cost of

charging and discharging vats is about lOd. per ton, and

of general labour about 6. per ton, whilst power, water,

iiuUQtenance, &c., are very variable. The consumption of

Tanide is usually from 0*3 to 1 lb. per ton, costing say 4d.

te Is. per ton, whilst the consumption of zinc is about

lb. per ton. The following table shows the detailed

<>0Bt6 per short ton for a few typical Witwatersrand mines

ttat are worked on a large scale : —

Tons treated per I annnm

s. d. K. d. s.

clndingfood... 0 444 0 7 28 J

Gcnenil Htores .. i 0 2*96

C'ontractoni ' 0 1 45

d. I s d. s. d.

Total

2 11-01 I 2 11-48 2 1018 2 7*40 3 1 74

I. Jninicr8 Gold Miiihig Comimiiy, LimitcMl. II. Crown Rocf Gold Miniiig Comiiaiiy, Limited. III. LangLiagtc EaUiU mid Gold Mining Company, Limited. IV. City and Suburban Gold Mining Company, Limited, Tailing!. Y. Ditto, Concentratos.

E K 2

T

I GOLD mi.UNC,

of trentiriK cniiwnlnitdn by cii-aniilsWim h II 1 hithcr ihiui that of treatinc Uilin)>. it

which mi eooinc prnot.iciilly prohiliitivc if tlio oonwstriLlts ooi. 1 aiUpliides of copper or of other Iwwc niiUli rwulily fttlnolcMl by cyanide aolution. The cost of tnaiins tailingH st the Langlaagte Estate worka in as shim iibovfi (No, -- - io„ ihe consiiiiiplion of

cyanidti iiml lb. and 0-177 lb, riwjtivcly

; tlio Oi Otratea at the same woib

mounts to 1: mplion of cyauide f>i;in|

In America >eeii used chiefly at 'If

Mercur and le Camp Floyd disiria,

Utah, and at orado ; in the fortnei ll

cost per ton, inciuamg that of crushing the ore, follows (luring 1893 :—

nppUcs, fuel, re|iiiits, Ac. . . . 0..'.7

This figure has since then been considerably reduced. 1" the Cripple Cifck district the cost is said to exceed}' (IGs, 8rf.) per ton, but this figure appears to include tlK co.st of crushing the ore as well its of calcining it, better results being obtiJned with calcined than with raw ore. It is said that this iiiiproTemcnt is due to physical chan merely ; the Cripple Creek ores contain tcUiirides, but il is not known whether these compounds have any defied upon the leaching process, A cyanide plant Itas been is operation tor some time at the property of the Standflri Consolidated Mining Company, Bodie, Mono Count}!

Cyanidation 421

California treating about 80 tons per day, which are liauled a distance of 1,400 feet up an incHne by electrical power, a 10 H.P. motor being used, together with 5 H.P. motor to drive the pumps, &c. The cost of <)perating tlie plant is as follows —

Labour 2 3110

Wood 0 5-230

General supplies . '. . . . 0 1*155

Miscellaneous 0 0*255

Haulage , 1 3315

Totol 5 8*650

A few other cyanide plants are in operation in California *nd in one or two of the other States, but the process 'as made comparatively little headway in America.

In Australasia, cyanidation is being adopted in several of be West Australian mines. In New Zealand it has been ''Uther largely employed, the average cost there being about i. to 5s. per ton. At Hannan's Brownhill Gold Mining mpany. Limited, West Australia, the cost has recently Xxjii 10*. 9*4 J. with a monthly treatment of 1,040 tons. Vt the Lake View Consols, the cost of cyanidation by eaching is given as Is, Sd. per ton, and by filter press as .3*. 5<l ; at the mill of the Ivanhoe CoriK)ration these costs vere as low as 5s. 3d. and Gs. 3(/. respectively ; the cost f cyanidation at the Great Boulder Proprietary is stated o have been lis. {)d. At the Victory Gold Mining ;k>iiipany. Limited, Charters Towers, Queensland, the osi of cyaniding 4,40G tons of tailings amounted to .6s. 9d. per ton, out of which 7s. 2d. per ton was paid in vages. In the Mysore district of India cyanidation has proved

4M aOl.n MILLING

very etioocssful ; the followinn are lli(; oosls t>( ti the leading cotipauteH : —

Mymn (iold Mining Ontnpaiiy, t.bl. :iO,OIli 1 0 Cliiitiit>ion Kuer Gold Milling Cnin-

)wir of IndiH, Limiteil . . . 7S,Siyi 4 V

Nunilydruog Compniij, Limited 35 931 'i

The first cost of a oyanide plutit variae very widd/j. ivcooi'diDg to local conditions mid the charactur ol Ok pliitit itticlf. Broadly speaking, it way cost from £1 eaul) tun to be treated monthly up to three times M amount.

Siemens and Salske Process. — In this process l" inithoil of solution is the siiuio as in the orifjinal oWi hut electrolytic deposition ia subalituted for pretipitiition by zinc. The latter is admittedly the weakest [Njiiit of the Mac Art bur-Forrest process, and it swnn to tail altogether when applied to dilute wluliuns. which latter can be treated successfully by the elooinc current. Therefore in tlio Siemens and Halskiprocess far more dilute cyanide solutions are used, the tiinv Ireattncnt Iteinj; correspondingly extended ; the stroof; Hdlutions contain 0-05 to O'l per cent, of cyanide, and the weak solutions 001 to 0-05 per cent, the time of treat ineiit Imiiig l)etween four and ten days. The prtcii' latiiig lioxes are wooden tanks 20 to 30 feet lonfj. C lu 8 feel wide, and aliout 4 feet hit(h, in wbicli the electi-oik's are suspended, these being so arranged as W cause the solution to travel up and do.vn in thu satuc niaiinci' as in the zinc boxes. The cathodes codbIsI of thin strips or sheets of lead cari'ied on light fnuues, and the anodes of sheets of iron al>out inch in thickness, the latter being enclosed in canvas, so as to retain the Prussian bVue Votmiii inVwift electrolysis of tin-'

'

C Yanida Tion 423

ution. Only a weak current is required, about O'OG ipere per square foot being sufficient; with cathodes

to 2 inches apart a 5 to 7 volt current is ample.

box capable of treating 3,000 tons of tailings per onth, or about 100 tons of cyanide solution per day, quires about 10,000 square feet of cathode surface, ad will need 5 H.P. to generate the requisite current, inder the action of such a current, the gold is deposited 1 a coherent film on the sheet lead ; when the film has eached the desired thickness, the frame carrying the Jad is withdrawn, the gold-bearing lead removed and 3elted up, forming base bullion that may contain up 0 12 per cent, of gold. The base bullion is treated by 'arkes's process, which consists in melting it with zinc, ben an alloy of zinc, gold, and some lead is produced, le bulk of the lead being entirely freed from gold. The inc is then distilled off from the auriferous alloy, and le resulting enriched lead is cupelled, the bullion thus >tained being about 900 per 1,000 line. This process, bilst still to a certain extent on its trial, is already oducing no inconsiderable quantity of gold ; out of the amount of gold stated above (page 408) as having en extracted in 1897 from tailings by cyanidatioii, 2r 150,000 ozs., or about one-sixth of the whole, were 3cipitated by the electrolytic method. Phis process has hitherto been almost confined to Transvaal, hence it is only there that its costs ve Ixien determined with any accuracy. The followfigures for 1896, taken from the Proceedings of the emical and Metallurgical Society of South Africa, 1. I., show (I) the estimated cost of treatment per 1 in a plant working 500 tons of tailings i)er day, d (11) the actual cost of three months' operations at 3 May Cousohdated Gold Mining Company, lAuviUid,

dnriD}! wfaiEb time 37,350 ton§ of tailiags wen; treated, {roductag About 5 dwta. of l>uUJou to the tD :—

SS4 n D D !4

1S9 S 0 0 1 1

a S*5 I 3,088 4 0 i 9'1

Tbo abovt; figures ore esclusive of interest on plwt and i-loyalty. The May Consolidated plant cost £27,500, so tliftt int-'rasl is a heavy item, whilst royalty amouiiliin to alwut I'M. per ton. Upon the whole, it iimy Ik.' atiJ thitt the cost of treating taiBnga by the Siemens-Halske process on the Witwatersrand is a)>out the same as hy llie ordinary cyanidatioti process, althoujh the loss of cyanide is decidedly less; it seems to give a souienbat lower ix.'rccntagu of extraction, and the first cost of the plant is decidedly greater. On the other hand, it produces much finer bullion.

Slimes Treatment. — This forms the most recent developnienl of the cyanidation process. The slimes are either deix>sited in large settUiig pits, and thence transferred

'

Cyantdation 425

the vats, or in the more modern plants are run direct 3 leaching vats or first into intermediate vats. The ching vats are provided with agitators, consisting of tical shafts, provided with arms, or else the pulp in im is agitated by being kept in circulation by centri- ;al pumps or by injecting air, any of these methods gve satisfactory results. The slimes are ated in the vats with three to four times their weight

a very dilute cyanide solution, containing 0*008 per it. of cyanide ; after about two hours' agitation the oies are allowed to settle, and the clear solution is run

into settling tanks, and thence passes to Siemens and ilske electrolysis boxes ; the zinc precipitation method anot be used on these very dilute solutions. The ux- Qsted tailings are now simply discharged, attempts at kshing out the absorbed cyanide solution having so ' proved unprofitable. A charge of about 60 tons

treated in 12 to 18 hours, and an extraction of

to 80 per cent, of the assay value of the slimes is Jised. The costs of slime treatment seem to range between

6rf. and 45. GJ. per ton treated. The following table >ws the cost at the Crown Keef Gold Mining Coinly. Limited, for the year ending March 3lHt, 1898, nng which 40,955 tons of slimes ('22- 12 per cent, of

d

Kuroiicaii wages 0 8 2;i

Native „ 0 :J-40

and materials 1 .T.Vi

Mainttuiance 0 10 "JO

Kleetric pump 0 :ilKS

„ light . . 0 oh;

IJoyalty '. 0 2-27

Total 3 8 12

At the Oeldenhuis Batate a sliuies |i1aiil cnpobl' treating 3,000 toua w tiioiilh bna bceu erecUd &l b of £24,000 : thu cost of tnalmont tliera wiw Gi. 5J. ton for tbo first four moDths of its operation.

First in the Transvmtl and more ruotiutly in V Auelniliii slime treatment has beaoine a sc)iarat(} brHUili I uf thu art of gold extraction in tho latter [iIuk tin I filter press has been suceessfuUy employed for freciiiE''* I treated sliniCE from adhering cyanide solution ; a Imt'n alruady iudinatcd, this adds somewhat to tbc coalrf I troatmint, but renders it possible to treat slinii would otherwise be praolioally intractable.

The introduction of the cyanide process baa very marked eflect upon tlio whole system of s*-'' traction, and it miiy now be looked uiran as the rewuiiisud means of treating tailiii;;s. In spite of tlii: hJilil; ix>isonous character of the solution, fatal accidenta all but unknown ; some men seem especially susceiA' ibic to its effects, and in many cases contact with tbe solution produces painful sores on the arms and hands of men onfjaged in working with it; there is, however, little need to touch the solution, except during the cleanup. This operation, and indeed the whole of the ziw precipitation, would appear to be the weakest part oF entire proeoas. Hitherto, however, nothing has been devised that has succeeded in taking its place. Tbe Uienions and Halske process is stilt on its trial.

Molloy's process was a very promising one, but bss not come into use ; it consists in replacing zinc by sodium, wbicli is employed in the foi'iii of an iinialgam, ibe latter being produced electrolytically and ainiultanoously with its consumption by an ingenious piece of appamtu' Moreover, in this process sodic cyaulde, which is quite

C Yanida Tion 427

Motive a solvent of gold as potassic cyanide, is rated by the precipitation of the gold. Never- i, the process has not been a success practically. 1 inventors have attempted to devise an electronethody using mercury or some amalgam as an >de, but hitherto without success, ordinary battering sands or crushed ores the simple tion method is still considered the best, and it is the whole surprising how very minute a percentthe vast number of patents annually applied to lation processes, can survive even the experimenige. The process does not seem to be equally iible to all ores, although it is diflicult to see what conditions the gold would refuse to dissolve ; isly the physical character of the gold is an tant element in the problem. Cyanidation has found applicable to many ores, and especially to that carried no freely amalgamahle gold. Such an be treated by crushing dry, and cyaniding the td ore. If desired the exhausted tailings could )e sluiced out of the vats and over amalgamated or blankets to catch any coarse gold that might sent. Such methods are in use in several ))laceH ; eat obstacle to their more extended introduction the fact that there does not exist at present any ctory dry crusher that can deal with hard material )st at all comparable with that of wet stamping.

b

CUMTKB Xin

CUUNINO-L*!' — TKKATUtCNT Of AMALOASl — CLEAIilXS,

BttT <!lia-Bp.— Tliia talcBS place weekly, fortni 111 Humility. As II [,vncnil rule, iiisidtplains aru c" ii)! i'vor\' wcoli, but ji ooiopU'to clean-up of the wbi n DiHile iwl> once a luoiitli. To effect this, the ore-i'iii I pHv clis'd. ami iho ore-teodor wheeled Ijack out of lb I WT : lli UnUowed tu ruu until aU thestampsan I {Wuthliit;; U[>on nioUl. only ten heads being thus treiMi I tti Uiiio. If Uw mill is provided with suilablu gea. it I Ath-i)i)iltf ki " bctU down " at a slower speed than the ix<mil cS millinfi, say at 40 to 50 drops per niinatG' Ytwr sUuuiw MV ihcD hun; up, the wuter l>eing aUoweil to I Oiuxinuo ntiiiiiu): until il Domes away quite clivir ; then it aihl scrai-d. as already described. The upper )\Mii.>ii>ot the th)(' rv next oorered over with a stout wwvioH iN'wr. or wiili of plank, which reach ripht .'.'iwvv iIh' in\i iith)(\< if ton hi>ai1s are being cleaned at a Ti,'-*- M-nv for the workman to staud on whilst .l.i.*.;.-.v>; 'ho nwnars. aikl to pmtoct Uie jtes from i'.w .i'.i.t.is. iomI cbock-block are taken oat aod care-

§ T V -.. 77//: iW F.iX Ur

nlly scraiMMl mid cleiiiied, tin; plate hoiii* usually un-

-csrewecl from its seat for this pui-pose. As much sand

kxicl coarse ore as can be got out of tlie mortar by means

short-handled scoops and trowels is then scraped out,

Yid piled either on a clean spot on the mill floor or in a

bin kept for the purpose. The middle die of the mill is

irtext prised out. If the mortar is fitted with any of

ttie arrangements already described to facilitate this, it

becomes an easy matter ; if not, it usually involves a good

deal of work with steel bars and sledge-hammers, cold

ohisels, and wedges, before the die can be lifted. A

tew bars with pointed and chisel-shaped ends should

ways be kept in the mill for this special purpose. Once

the middle die is out, the others are readily got out by

means of a bar. Any sand, &c., adliering to the dies

is scraped and washed off, and added to that already

obtained. The dies are carefully examined, and any

%DQalgam adhering to them, or collected in cavities in

them, carefully removed and put aside. The whole of the

sand contained in the box is then dug out by means of

scoops, trowels, and amalgam knives, and the liner plates

ftre taken out and treated like the dies. Finally, the

inside of the mortar box itself is well cleaned and washed

out, and all amalgam adhering to it scraped off and

put aside. The shoes are next examined and similarly

scraped ; especial attention should be given to the groove

fonned where the head meets the shoe, as this is a

favourite spot for the accumulation of amalgam. Any

worn-out shoes are next knocked out and replaced by new

ones. The liner plates of the mortar are now put back

into their places, or, if worn out, replaced by new ones ;

the dies are put into place upon the false bottoms or

a bed of tailings well rammed down, the interspaces

COt.D MIU-WQ a

n Uiem bpiii); fillnl with smnll oiy>, thi' Rhoek-U nd MRi iu% keyed into thi'ir pUces, tlie oiibsiilr ph ue rubbed up, and tbe buttery is re-atartcd. B; t tiiitu d) next tt>u IteatU will have been beaten dowD,f thoy lire dmund-up iti Uio satuu wuv, and ai on tho cloan-ii|> of the whole ttiill U completed. (M if there a siilTicieiitly large stAff. two or inon head haltericis may be cleaned up (dinultaiicoaaly. customary to so arrange the work, whenever pot that tho double shift (night oa well as day nien) he available for a cle&n-up, and a (cw outside labouK art! also some'lines detailed to aasist. It need baia So eaid that a clean-up offers specially favournble o] tuiiilirs fnr ihfft, so tliiil conHtiUit viyilancc is ikmiUiW on tlie part of the mill manager, who should really never leave the mill from the commencement to the end of the oi>eration. The contents of all the boxes have now been collected, and arc ready for treatment. Here again, if a large staff is available, the treatment of these niaj commence as soon as the first mortar box is emptiei] out, anil may proceed simultaneously with the clcaoinp out of the i"remaining boxes. There are various ways of treating these sands, each of which has its advocalM- In n small mill they arc sometimes simply panned up; the first {Minning is done in large prospecting pans in tub or tank of convenient height filled with water, in which the tailings collect. Tlic final panning of tiK amalgam is performed in pans, the bottom of which ccasists of a sheet of anialgumatcd copper, a little mercuij lieing poured into the pan. This greatly facilitates the clenning-up and collecting of the amalgam, which, Ix'ing softened by the morcurjadded in the pan, aJhei-os to the bottom. Any pieces of iroQ met with in

The Cleanup

g are put on one side for treatment as described on.

etimes these sands are cradled in a common cradle, this is done the cradle should he fitted with good jrse riffles f inch deep, behind which a little merpoured to soften and collect the amalgam. A e cradle for this work is shown in Figs. 106 to rt is of the usual type, only heavier and stronger, ry carefully made. The hopper is a little deeper sual, and its bottom is made of a piece of -inch

Plan, without hopper.

Fio. 106.

ron punched with round holes inch to inch meter. Stout sills should be laid down for the i to work upon, the upper one having a slot into

fits the pin shown projecting from the upper , so as to keep the cradle in its place. A waterinch in diameter, fitted with a valve, and termin- n a couple of feet of hosepipe, should be arranged to supply water to the hopper. The cradle is

worked by hand, but may be driven from any lient portion of the mill shafting. The lower end cradle should deliver into a sluice some 10 feet

3S

GOtXf Mlt.USG

Inng, liw vcffmt portiwi xA which ib linnl nitli nn u gam*tMl Difier-|4ftt(< and thn lnwi i>rtioii wilti HI Thu lain abimU Mirer tatn atuull piL A atiil h nwquuent esaetlf raaemUes the kitow in nil ]utieii! Ihal Um9 emdUt, whieb Es than nhotit 10 UtX I M fixed, )d euutot rock, being fimily hotMfd to thu t tbi> hH4-iron bopper boMom may aim with odTtiA be of mmewhal rtooter iron. In either cmmi the

Longitudinal Vertical Section through ci

is about one-third filled with battery sands, a suf&cienl streati) of water is turned on, and either the cradle in vifjoroiia motion in the former arrangement, or tb* sanils rul>Ied up and down and worked about on the hopiier Imttom liy means of a short stick in the latter oni;. All the hunl lumps, consisting of sand, clay, fiw ore, and amalgam, are thus disintegrated and washed down through the rifHcs. Ac., which retain practically iU the amalgam, any that escapes being caught ia the sluiM

r.

The Cleanup

) contents of the hopper are then examined, any large ips of amalgam (or of more or less completely anialaated gold) picked out and collected, all lumps of iron own together on one side, and the clean stones remainin the hopper thrown out, to he at once returned to I battery. A fresh lot of sand is then thrown into the pper, and so on. Ultimately the hulk of the amalgam 1 be caught in the riffles, and a httle may be cleaned out of the sluice. All the lings from this clean-up will caught in the pit, and may letumed to the mill, or else ited separately in the amalmating barrel, to be presently scribed.

Instead of this method, the ttery sands may be treated once direct in the amalgaming barrel, and this is perhaps e best plan in large mills. In is case all the battery sands, Rether with a sufficiency of iter and a few pounds of merry, are worked in the barrel r a couple of hours until the amalgam is completely parated from the sands. The Californian amalganing pan described in Chapter XII. is also occasionally ed for this purpose, but is less suitable owing to the 5t that big lumps of iron, which would keep the nmller m grinding properly, often find their way into the ids. Whichever of the above processes is adopted, prospecting pan has to be used ultimately to clean - and collect the now pasty or semi-iiuid amalgam, and

Bach View, without hopper.

Scale. Ji'= I fU Fkj. 108.

COLD MILLf.VG

us{>._'ciully Ui sepanUe it from tbc smAller {nnii:JES of ua which ftdbcre obalinrtrty it. 73n mfBOtHi usually DCoom[dUhed by mouia (rf the magneL Hk i in iiufstion i6 ilcnmil in [Hrt trotu the msu-of the ubl ilii?'4, and in [Mil from EnaiMitfi of nriooa peca m&chincry, from bolt endft, nutB. pick poinU, te*.' that fiiKJ their way into the mortw box mlli Um Thu uUiiuaU: result of the buiety to {irtMluou : —

1. Tailings, Taryiog in stBe bom suid ta largo lumpscl om, utid which &rw rotumud to the baUcry :

*2. HucvA of iron to which more or lea flmjwm adliurcs;

:). Putty amalgani rcsorred ita furtbur tnmUnetit.

Willi ruspfct to the iron, it may )m noted that tbil is itpt to Itcuoiue lightty uoialgiunBtod iu apobt by tht Uflu of Bodiuni utnalgain, wliiUt aiualgiun may aldo lodge in &ny creviooB in it. AU the iroa thai b odleded, i) tlirijwii lofiftlioc ill u lioap on luiy smooth, level pioM of tiound (itiir tin; mill, and tlioxti allowed to nist, Jte uxidulioii buing acceli'rutt.'d by nioistcmng from time to tiiut; with ii titron;; solution of sal-ammoniac. When cornpleloly mated through, it is treated with other mill I'uhbisli in tbe barml, and any amalgam it may cootiun

Amalgamatiug Barrel. — In addition la the uses alny desunlicd irj clL-miin-up, the amalgamating barrel must uaoful adjunct to the stamp-mill for gutieral goWsaving purposes. This barrel is shown iu Figs. 109 and 110. It uonsittta of a stout cast-iron barrel, supported on short shafts, which are uorried in bearitiga on a light iro fruiuc. One of these shafts is fitted with fast and loose pulleys by means of which the barrel is caused to rerulve

Itel ri ii

Amalgamating Barrel

e barrel has a man-hole large enough to allow of its ng rapidly discharged, and has one, or sometimes two, nd-holes in the ends ; all these holes are fitted with ter-tigbt covers. A hose is provided which can supply .ter to the interior of barrel through a handle, and there is also a )vable sluice by means

which the contents of e barrel can be run off. In all mills there is 61'ays being produced a rtain quantity of mateil that carries mercury id amalgam, which latter Duld be lost unless this atcrial were specially eated for the recovery

any gold it may coin. Such material is, r instance, found in the ntents of the amalgam !ips. Again, amalgam id mercury are apt to

spilt or to leak from no to time on to the or of the millhouse ; d there is nearly al-lys

a certain amount of splashing of pulp from tin* ttery on to the floor. This Uoor should accordingly he itertight, made either of stout plunks, well caulked, or concrete, and should be so arranged as to slope gently iii all sides towards a small pit at one end. Onc(3 a

F F 2

End Elevation

Fio. 100.

Gold Mtluhg

day lh floor should bi> vruhcd clovn hy means cifibw pipe, ftnd deuted down with a squeegee, evoryitof beJR)! thus ollecU>d iu the pit. where it in loal MtUIc, the water being run off. &ad the oonlenU of pit. from time to time, shovelled out. All chips of taken out of the mortar boxes, and more especially pottHfl of the wedjcs used for wedgiug the stomp shoes inUtllM lieuds, old chock -blocks, screen frames, Ac., should li

collecttHl and drie<l. Every few months a heap of lies* frii'tents should 1m! burnt on a hard, smooth floor, n"" tlie ashes, wliich will be found to contain some gold, eullectud. All this miscellaneous material is worked up, l();;i.'ther witli any other rubbish supposed to conliun ff.i\A, in tbe ariialgamatint; liiirrcl. It is charged int it tliiuufih tile niaii-hole in lots of a few hundredweight a time, llie barrel three-fourths filled with water, and u

:ii BATE A 437

umber of heavy cast-iron balls dropped in. A few ounds of mercury, proportioned to the supposed gold Mtents of the charge, are poured into the barrel, which then closed and allowed to revolve slowly, say 20 to 30 imes per minute. The mercury used should 1x5 purified nd charged with sodium, like that used in the mortar ox; a little potash-lye may also be added with ad- "antage. After a time (usually between 3 and 6 hours), rhen the grinding and amalgamation of its contents are onsidered" complete, one of the hand-hole covers is en off, and the contents of the barrel washed out into he alice-box, the bulk of the mercury being allowed 3 remain in the barrel ; the sluice-l)ox should contain

length of copper-plate and a set of riffles. If it is bought advisable to further treat the ban-el tailings, ey may be allowed to flow into the launders leading the concentrators. When all the fine sands are thus 'ashed out, the hand-hole is closed, the barrel turned ith its man-hole upwards, and a fresh charge thrown in. he cover of the man-hole is then again fastened on, and 'e process repeated. When the mercury in the barrel

considered to be sufficiently charged with gold, the urel is turned with its man-hole pointing downwards, a ercury pail is placed beneath it, and the contents of e barrel collected in the latter. The power required drive a barrel continuously is about 2.V I.H.P. fiatea. — In large mills much of the manual labour of 9 clean-up may be saved by the substitution of the ichanical batea for the usual hand prospicting pan. lis batea, as manufactured by the Risdon Ironworks, n Francisco, is shown in Figs. Ill to 113. It consists an oval or round siiallow pan of cast-iron about 3 fe(t nches or 4 feet in diameter, and about 3 to 4 inches

Gold Milunc

lioep. witJi a roundeil Imtloni Tliis pun is sujiportcHl nt o

I plug in the cet III oil a roller,

Treatment Of Amalgam

ended at the other by a couple of rods of light round , this mode of auspenBion allowing it greftt freedom of on. At the suspended end there ib a small vertical k driving a pin, which forms part of the pan itself, rapid speed, so that the pan receives a gyratory on, not unlike that used in panning-up amalgam in hand-pan, and which serves adniirahly to collect the

iikgilver and amalgam in tlio iiottotn of the shnliow >iron pan, the lighter materials finding their way to sides. Tlie power requiifd to drive the Iwitea is

lit 1 i.n.p.

teatment of Amalgam.— The anmlgam <ihtaiiied off plutcs throughout the inonth's run is usually stored I s|N!ci]il safe until the compli'lii.n of llic cleiin-up. f total mass of arnnlgani eollecU'd may accordingly

feMiia be RtftMl oHL Ae aaHlpm sftte is bed pUeed i tr? :-i-tr. tn-TirrfT to the floor or to nKJ" [HTCi'.T .ii :lw -iiniciiiiremiifss is is a vejy heavjone. The Rvi;' ii:'.';i"'-t hare i sink ifni WiWor supply, and a stroDg work-tLibl?. The Sop if ttii* table should consist ot sint;ie ihib of stone, or of a stout hard-wood piuik 3 itK-hes tiiick. U shvuUt huve a rim a couple of inches iJbM-|> all rvmiul its eil;!. and jut inside this there slioald

Ii Squeezing Amalgam 441

a proove leading to a small depression in one comer ; >inetime8 the whole tahle has a slight inclination towards lis depression. Cups and pails of enamelled ron, malgam knives, a plentiful supply of buckskin and iinvas for squeezing amalgam, should all he at hand, 3gether with the requisite chemicals; a good strong alance capable of carrying 2,000 ounces and turning irith yj ounce, together with the necessary set of weights all preferably made some substance other than brass) omplete the equipment of this room. The amalgam Pom the plates should generally be clean enough to need to further treatment ; it may be sprinkled with a little mercury if very hard, so as to make it slightly pasty, well cneaded, and made up into balls between 20 and 50 ounces in weight. If, on the other hand, it is too soft hat is to say, if it contains too much free mercuiy, it has o be squeezed.

Sqneeimg Amalgam. — This is done by pouring the Necessary amount of soft amalgam into a strong, sound ece of buckskin or chamois leather, which has previously well soaked, or into a bag made of fine canvas. Phe canvas used for this purpose should be strong but not 00 coarse ; the quality which is made for the smaller ails of yachts is best suited to this purpose. It should ie thoroughly soaked before use. The free ends of the iather or canvas are then grasped in the left hand just bove the point occupied by the amalgam and twisted.d )und so as to prevent the latter from escaping; the hole is then immersed in water contained in a mercury ill or pan, and the globular lump so formed twisted rongly with the right hand ; by ibis means considenible ressure is put upon the amalgam contained in the skin r canvas, and the fluid free mercury is squeezed through

dw pom of tbc Uttr, until only a ball of har<) noialpa behind, all the raperflnouB mercury having ban oai of it. Serentl fomtH ot mechiinienl nifTCOTfhkve btKii invrntml, liul none haro pmi>i. mt&arotly meomsfnl lo snpplant hand-aqoef-nut:. (ttt] tho twfit kooini is the hydrauL'o separatur, nbkji Bonaatanf k {wl-ahapad Dat-iron vessel, tho ImtkiiuiJ wfaicli b farmed of pioeo of btidcskin or <:aDVas. wciinlT tied or olbenrise (utened to the TcsseL The amalniaiii wUeli baa to be tapi*KtA is then poured in. and a mvu on, into wlrieh is fastened a sinall water-pipe Cftber with a uuaU fortwt-pump, or with a tuik some eoosiJcnktilc eterotion. By either of Ihue' iiiRiii siRint*. hydraulic pressure is excrciseii on tin iurfno' i>( tlie amalgam vrhich tends to drive tlie In* nicroary throngfa the pores of the leather. Amalgam, bowerer, cMutot be squeezed dry by this appiiratus a) well as by hand, and it is accordingly very little used. A tDCHv modem device consiats of a hydraulic ram, working into a cylindrical cage in which the nnialgam is place*! etmtaiiir in a simnf; canvas hag.

h hAs i' '. t. r-.i'iied ont (pae B5) that wlien iin - 1, hai-d amalgam of a niorr or

k-ss .Ui.u.v ....p.™>,oi. is left bohind, whilst th. es*-;pinf; niereurjis not pure, but consists really of tlnratol !>lution of gold amalgam in mercury.

rtlre.idy mentioned (see paffe 86), I have found as tlio r>siilt of numerous experiments on this subject that tlio iiiiioiinl of amalgam thus dissolved increases wIlli nn inin-;i' in Ceiiiix'raturc of the amalgam treated.

Till-fi'llowiiif; table gives the results of two experiments I'll tliU I'oinl ; ill each case the s<]ueezing was us ci'm(i|iie as pitssihlo hy hand, but it niiist l>e renieinben'd

Squeezing Amalgam

U it is very difficult to ensure that the conditions as gards pressure shall be exactly uniform.

fempentore of Squeezing.

src.

Gold dissolved in the sqaeexed Mercury in pftrtAjier 1,000.

Eiquivalont to Amalgam dissolved in parts per 1,000.

At the same time the residual amalgam was found to 5 proportionally richer in gold, the higher the temperure of squeezing : —

imalgam sf[ueezod at 72" C. retaiued 282*43 parts of gold pr 1,000

„50*C. „ 274-97 „ 18*C. „ 238-76

As it is preferable that the escaping mercury should itain a minimum of gold, it is accordingly advisable to iep the water in which the squeezing is done as cool as )ssible.

Further experiments on the difTercntial squeezing of nail parcels of amalgam gave the cunous result that chess of the escaping mercury is not uniform from iginning to end of the squeezing, but that the first and last )rtions of the mercury are the richest in gold. The results two such difiFerential squeezing are quoted on p. 444. It will be noted that these two experiments were pernned on amalgams of distinctly diflferont character, the st being derived from very finely divided gold, which elded amalgam containing only 22- 1/) jxr cent, of gold, iiilst the second lot of anuilgjiin contained 35-1)55 j)tr nt. The same remark as to the difliculty of securing

fOU? MtLUNG

1.IW r.

fl-TM

a-Bw

I'd

Ito. t.

In* -

1 I2S

i-ass

t-55S

Sit S-M-

lit

ct niiN'KaENttl conditioDS applies to thea V ifc>? ttr-M hiicb of experiments, but I haT L\l :bcrm vptmiiedly &nd have no doubt is t ;nL .i.-ur:u.-y. tfaoG I most admit thkt V vo ?a£i;?£te.-corv espWiAiioD of the obsem 'i inLr>v ifoouib CO onderstajid, and voold exptwne*i, tbac !itf Uss ponioas of mereni .uc iviLii bt> uiu rtefce in Id. bat it -jrh,ii.T-iii,-ii why iQe tires should be richer Cbi i.i{!;tx'i> *t;..vliin;£iG. The pntctical tessootol ,ti -'iv- -f\yvrmeti!K isiifaaB is is not adTtsable it'.iv.'.iitti X' :a)e :"'ir:bes5 poisfriKi? p>int. bat rath iv it-i oi -TTe -nirvn-ry in the amalgi 'iiL-U i)> uoti>(u>.-t.-u :LRiU[Ut;Di Ln the retort.

Ii Cleaning Amalgam 445

Cleaning Amalgam. — The amalgam obtained from bber sources has to be cleaned ; indeed in many mills all :ie amalgam, from whatever source obtained, is thus neated. This operation is performed in the clean-up nii. This machine almost exactly resembles a small amalgamating pan, Fig. 102 ; its diameter is usually 3 feet ul sometimes as much as 4 feet. The muller is furnished vith wooden shoes instead of iron or steel ones, and the ntinuous muller is sometimes replaced by four arms hurrying blocks of wood at their lower extremities, somcfvbat Uke a small settler. The usual speed of the muller a about thirty revolutions per minute, the power rcquu-ed io drive it at this speed being about 2 I.H.P. The pan -s half-filled with water; from 200 to 1,000 ounces of Mnalgam are charged in, and from 20 to 150 lbs. of clean cxiercury. If the presence of grease is suspected, a little strong potash lye may be added to the contents of the pan, but cyanide of potassium, which is sometimes used, ould bo avoided. The lumps of amalgam are soon broken up, and form a uniform pasty mass, any sand or dthcr impmrities present rising to the surface. When the 3peration is complete, the muddy water, with as nmch sand as possible, is run off through a series of plug-holes. rhe muller is then stopped, and the contents of the pan Irawn off into mercury pails ; pieces of iron floating on Lhe surface are removed by the magnet, and sand, (fee., is washed off by means of a small stream of water. Tlie surface of the mercury is usually found to be very foul. This is due to the presence of base metals which have bund their way into the amalgam. The surface of the mercury is skinnned by means of a piece of wut flannel, canvas, or india-rublxir belting, until it remains quite 3right, the skimmings being put aside in a special pail

4f6 COLO MILLING iMJll

for sr|nrte treatnieot. Tho clean pasty am&l( ! thco sqaeezod as iJiviuly detailed, and mnde up M bftlls nmgin from 30 to 60 ounces in weight, md lor retorting.

Ketwtug. — Tho principle of this operation consiatai boating the amaljjani to a temperature above the voUtilil tion point of nieicory, whon this metal distik over. Irann the fi.'cM iu a loose cdlulur stale, in which it is known tf i gold sponge, the mercury being condensed by appropristt eodUng apporattis. Practically there are only two fonis of retort in use in gold mil!*. the pot retort for small quanuties, say up to 1,000 ounces of amalgam, and tht? cylinilnual I retort for larger amonnts. Pol retorts are made in various sizes to hold from 250 to 1.000 ounces ; the former are about 6 inches deep inside, and \ix latter 9 inches, tho dianieli Ijeing about of the dep The usual sliupo is shown id Fig. 114. It vriU be seen Hat

ihc retort consists essentially of two parts, namely the Inxly and the cover, the delivery -pipe being screwed into the latter. These retorts are made of cast-iron and carefully turned inside. It is advisable to have the inside of the cover turned as wdl as the body ; the joints between these two should bo verjaccurately turned and be as true as possihk'. It is advisable to have a V-shaped projector on the face of the flange o( the Ixxly which fits into i eott'L-sponding annular groove in the flange of the cow A piece of good wiuuglu iron piping is screwed into th'

nil RETORTING 447

sover, its other end screwing into a stout Liebig condenser.

This condenser consists of a pipe (of the same diameter

a short piece, 2 to 3 feet long, of a wider pipe so that an

Minular space, closed at both ends, is left all round the

central pipe. An old mercury bottle answers capitally

for the outer pipe. Two smaller pipes communicate with

top and bottom respectively of the annular space,

vater being supplied through the lower and escaping

through the upper one. In its passage up it completely

oools the heated vapours that are passing down through

the small central pipe. The retort is usually supported

on a strong iron tripod, and may have a special furnace

for heating it, although this is not necessary. Often a

fire is built on the ground under and round the retort, the

heat being concentrated upon the retort by laying a few

bricks, or supporting some pieces of sheet-iron round it.

the retort ; sometimes a smith's hearth is used, but this

practice is not to be recommended. There are several

methods by which the cover is secured to the retort body.

Tlie flanges may be clamped together, or they may be

bolted together by three bolts passing through them,

cotter bolts are better than screwed ones. Soiiietiines

a semi-cu'cular bale is used catching under the flange of

the body, whilst a strong set-screw or a wedge presses

3n the top of the cover. Of all these plans the last,

illustrated in the above figure, is the best, as it is the least

ipt to be injured by the action of the fire. Before charging

:he amalgam into the retort, its inside shouKl be coated

kvith some substance to prevent the gold sjxjnge from

;ticking to it. It may Ikj well rubbed in with chalk or

.vhiteniug, but the best coating consists of ecual pai'ts of

4+" r.Ol.n Afif.rjNG ruiF

fijieiy ground fire-oljiy and graphite luiwle up into a lliio paste. The ratort ebould be washed out wilb this, and then 1 a warm place tto as to dry the oontvn. Tk

um re worked up with water to the cousittttiac}

of c be uGcd for o. Iiitiog between the fiui til

the ucxij b. d cover. It is elIso advisable to coat the oulside of the entire retort with a aimilar mixture. whiclt a little £no usbestos has beun added, &b this presems the rtttort from huniLng out rapidly, Tlie balls of auinlfsuii should be broker roe piocca L>acb, and piled

loosuly uiWQ L'tort, which should uevr

be more tliau A dist: of stout uslvsto

millboard, j' illor in diameter than liit

top of the r dropped in, a thin layer

of lute spreiMl . he cover put on. lumal

backwards and i times to ensure a tiiil

fit, and then set The retort is next pliWeii

on its tripod fe id the condenser attiiohed.

The end of the condenser pipe should be a fow inclia atitive the surface of water contained in a mercur)' pail. and should on no account dip below it. A strip canvas should then he tied round the end of the discharge pipe so as to form o. kind of loose tube dipping into the water, hut care must be taken that this tube is not ur tifht. Sometimes a canvas or india-rubber bag is attivohe to the end of the pipe, but the above arrangement is preferable. A fire of small billets of wood, bark, or smill brushwood is then built undor and about the retort, thn fire being preferably so arranged as to burn from above downwards, and the teniperuture very gt-adually ruieed until mercury begins to distil over. The heat of the fiw must then be moderated so as just to keep the mercury distilling over in a gentle stream, but no more. Tbe

to iii\xTal\ire of the retort should nt'V(;r approach redness

as long as any mercury at all distils over. When no more

csonies over, the heat must 1>c raised to redness, and kept

&t this point for a few minutes. The fire can tlien he

removed, and the retort allowed to cool. The entire

operation takes two to four hours as a general rule. It

must not be forgotten that as the mercury distils over

and collects in the pail, tlie level of the water in it will

rise, so that a little must be dipped out from time to

time to prevent its rising above the mouth of the pipe.

When the retort is cool enough to handle, the cover is

taken off, the looting scraped off the flange, and the disc

of asbestos board lifted out. Tlie retort is then inverted

over a piece of stout paper or a prospecting pan, when

the sponge will drop out in one coherent retort piece, if the

operation has been properly carried out. If the retort

has been badly coated, or the heat too great, the sponge

may adhere in places to the retort. It may then mostly

be dislodged by a few taps of a hammer on the l)ottom of

the retort, or, in extreme cases, a light hammer and

chisel may have to l>e used. Tlie object of the disc of

bestos lx>ard is to prevent spirting, and the mechanical

Carrying off of any of the amalgam in the; vapoin-s of

'iiercury. The sponge, when ptjrfectly cold, is weighed and

Cut up with a hammer and chiscil preparatory to melting.

The cylindrical retoi*t is shown in Figs. 115 and 110.

Tliis is usually built into a special furnjico as shown. Ft

Consists of a cylinder of cast-iron 9 to 12 inches in diaineter

and 3 to 4 feet long. Into one end is screwed a

leliver}'-pipe which communicates with a condenser as in

the case of the pot retort. In this case it is, however,

en a lai'ger and more substantial scale, and is usually a

fixture. The other end of the retort is closed by a door

whirh is BcciiroH by bolts or wwlgos like ocm the pol reton. Underneath tlie retort is k fii*-i usually about I foot 2 (et-t when wood is die oseA. iinil siiiatler wlien coal is employed. This is

with ft cast-ii-oii door, as iniiy also lio the ash-pt it, whilst the stuck is furnished with a euitahle dn Siiiiictinios Rix-cial provisions are introduced to p for a nniforni distribution of the heat to all parts retort. There are usually thi-ee or four trays of

Retorting

sheet iron which fit iiito tbii mtoi-t aiicl oarrj' the tiiii ; these should be of such a size iis to Icavi? i iihout the liftfa of its leuh furthest fi'oiu tlic These tmye are coated in the same way as alreiuiy soi-il>e<I for the interior of the pot retort, tliey oliargoil in a similar way, and puhi>d into their pli bcittg lifted nj) by the help of a small orane. which k mostly attached to some portion of the iron franiins "( the fumaco. The cover is then by the same mfsai lifted into its pioce, luted, and secured. Firing Dp then commenced with the same precautions u ooinmonded in the case of the pot retort, namely, nif gradual heating at 'first, until the mercury conimeDM to distil. At this point the temperature is kept ss nearly as possible stationary, until all the mercury bs distilled over, when it is raised to redness. This ustallv takes about six hours. When the retort is cool door is lifted oCT by means of the crane, the trays draiiii out, and the sponge turned out from them. Vnrions minor variations have been made in the form of tb" retort ; sometimes the outlet pipe is at the top, aomi!' times it is centiul. Tiie foi-nier arrangement has tin advantage that it is less liable to get stopped up during the operation ; the latter that when the bottom piut 4. the retort commences to bulge and become defonsfd. owirif; to the greater heat to which it is exposed, ihf retort can be turned upside down, and its lite tliM prolonged. Retorts may be oval or circular in sectioDl some are cast with several projecting ribs round thei" Bo as to strengthen them, and the better to keep Ibwn in shape. Most miils, however, use the plain cylindricil retort. With proper cai-e these returts last tor boiM years, usually two to three.

Ill MELTING

When retorting is carried out with proper precautions, a© loss of mercury is exceedingly small ; at the same aiie, too, the operator runs very little risk of salivation. fc is obvious that these two dangers are intimately onnected, as both mean that mercury vapour is being Hawed to escape into the air. The most frequent cause f this is taking the cover off the retort before it has boroughly cooled down ; it is ver}- rare that the retort racks in use. Eetorted mercury always retains, as beady said, a trace of gold equal to alx)ut 0005 part >er thousand, or, say, one grain to about thirty pounds mercury. This mercury should be purified in the usual way, as it may carry traces of lead, zinc, &c., and ft then fit for use again.

Kelting. — This is always performed in crucibles heated n a pot furnace. Sometimes, in vei-y large mills, a special furnace is built for this work, in which case it is isually built against the side of the retorting furnace, and >pens into the same stack. The assay furnaces described m page 616 do thoroughly well for melting furnaces, j)rorided that it is not considered necessary to melt very arge quantities at a time, and in this case the square vind furnace, shown in Fig. 122, is still the best type of nclting furnace, its dimensions being increased so as ti) ake the requisite crucible. The top of the furnace should lot Ix) too high above the ground, should be kept level, knd covered with a flat cast-iron plate. It is an advanage to have this plate so large that the ingot moulds can ic set upon it. Plumbago crucibles are usually employed or melting gold when no fluxes are required ; when luxes are added, either good clay crucibles, such as the 3attersea pots, or even better, the Salamander crucibles, uade by the same firm (the Morgan Crucible Company,

Liiamnl). Bljould liL- UKed, TJiobo cruiiUts run 1m iilxiut 3 U) 12 inches in hoight by to 6j iuutiuH in dir iiiL-Ur, uutsidt: inuosureruentB. As a (jeDeral piidr, tniiy bo Inkon tliiit u uruoible 10 incht'e in Iidght nil iiolil about 1,500 outiiMiH, whilut otui CJ hiolioe high hold about aOO oLinvtB. Thi; furnaou Ui liisiit iitiy pT* ci-ucililc kIiuuUI hnve & diunivUir about onii iind n luf'l ini:tu'B (Irtipur, By far the best fuel to uav b coke, tii it ia wortii wliile paying u libural price (or it to urn

its Ixiiiig of gocxl quuli ChKrcoal can bu used in ' of need ; iiard-wood o! the l!3t, and it t bt' screened, all that {nsW ' through a jj-inch riddle l>eiW rejected. Wlien chareodl is used a good dranght is in- , dispensable, anil, if reqnirei u blast luay even be used. The trLiuible should always be wanted before U8& Salamander crucibles do not require annealing, but fiK ordinary plumbago cj-ucible this \'a indispensable, and I'vcn for eliLy (Kits it is advisable. In order to anneal b crucible it sliuuld first be thoroughly dried in the ash-pl of the iiiL'Itiiig furnace of a Ixjiler fire, or in sonic similar convenient place. It is then placed inoutb downwards on ji eold fii-e, mid ihc heist gradually raised until the crucible is red hot. The crucible is then taken out, any l)its of fuel or other dirt that may have got in shaken out, and the crucible arranged in the fii'e, with its mouth iijiwiirds ready to receive the sponge. The crucible should 1m; place<l upon a stand made of fire-clay about 3

(lurp iiiid a iittlu lari.T in diainulcr tlian tliu >ottom of the crucible ; a piece of firebrick chipped into makes a very good stand. When a small-sized Wudblc is being used in a coke fire this stand may be lUpensed with, as this fuel burns away but slowly and ixuikes a firm bed for the pot. But in the case of a quickburning fuel like charcoal it is absolutely necessary to tee a stand to support the crucible. Needless to say, stand must be put in its proi)er place, resting firmly on the fire-bars, before the fire is lit. When the crucible has been put into place it is covered with its cover, and fire is made up, fuel being packed all round it. The fire is allowed to bum well through, and the sponge is then charged in by means of the sheet-iron funnel shown in Kg. 117, or, in its absence, a large-sized assayer's scoop r may be used. The sponge will then melt down pretty fast; when completely melted, a fresh lot is charged in, and so on, until the full charge has been transferred to the crucible. Care should be taken in charging never to fill the crucible more than three-four-ths full with the sponge, and the molten gold will then never rise quite as high ; of course the sponge occupies a very much larger bulk than does the molten gold. If thcfii'e at any time burns hollow, it must be well x)oked down and more fuel lulded, the crucible being kept closely covered at such times. When 'charcoal is used, this has generally to be done several times during the course of a melt. Small mills have at times no proper melting furnace ; in this case the crucible may be heated uwn the smith's hearth. The crucible rests on a brick so placed over the tiiydrc that the blast shall rise up on either side of it. A rough wall is then built round it, made of clay, bricks, pieces of old iron plate, Ac. The interior is then tilled up with fuel,

I 456 aot.p hni.i.iNG

anil the Brv ur|ed by riieani* of the btjilows till tl* gold is tliorouRltly iiiftlted. Tliis rouKh-iiHiiready wsia require, howovcr, ooiisidomhlo euro lest tlip cruciUl should crock and th gold niti out : thera ie. bowirm, Utile rc4U' of ultimate loaa, aa the old can alwars panned up ui;aiti from tlie ashca of tliu htsartli.

Whon thii sponge is cluiLii and fru froiii Imsi! mi-Uis, it is host iTiulted by itself without the addition of iny flux. Tl)ia may be judged of by the colour of the Hinnp'; when it iu of a bright yellow colour it needs uu flux, liiiL if it is oyiah or Machish it ueeds refiuiug, othrwisi.' thr resulting ingot may be brittle. lu the first cose Um sponge is charged directly, as already explained, into bliiiikleaii or 9alaioaudr crucibk'. Whan the mull is complete, the surface: should be clettr, brilliant, auil of a greenish yellow colour. There may be a little scion floating on the surfiwe, and this can be scraped off by means of an iron rod the end of which is flattened out Tl 10 crucible is then drawn out from the fire ; for large pots carrying 500 ounces and over it is advisable to uw basket tongs" (Figs. US and 119). An iron link shmild Ir) slipped over ihe liandles to keep ibe longs irom slipping when grasping the crucible. As soon as it has Ik:c[i pulled out, the crucible is released from the baskel tongs, any adhering cindei's arc knocked off, and tlie molten gold is jKiured in a thin steady stream into Die ingot moulds, the crucible being grasped by the aisuyers tongs shown in Fig. 126, jjiige 536, or by those sliown ill Fig. laO.

When the sjwoge is impure and needs refining, it has to be melted down with a refining flux. The most suitable flux consists of a mixture of al>out equal part* of nitre and carbonate of soda with a little bonti-

Kill

Melting

Sufficient of this should he charged with the first lot of sponge to make a layer about half an inch deep on the surface of the molten gold ; or else only alx)ut half this amount may be added at first and a little more with each fresh lot of sponge. Sometimes brittle gold is toughened

Fio. 118.

by throwing small pieces of sal-ammoniac on to the surface of the molten gold. When the gold is ready for iK)uring, the slag should be pushed back from the spout of the crucible, so that the first portions of gold may run free from slag, which will form a layer on top of the ingot. The moment the gold has set and whilst the slag is still liquid, the ingot should be tipped from the mould into a bucket of water, when the slag will come away readily from the ingot ; in case any should adhere it can he

Jl

Scale

J ft.

-i£U£

Fid. no.

loosened by means of a little waiiii dilute nitric acid ; hydrochloric acid must not he used, as gold would bo dissolved by it on account of tlie nitre in t\w, slag.

Ingot Koulds. — Thes(; are best made of cast-iron of the shape shown in Fig. 121. The inside should he

uutvCully |iUiii.-<l uuti IJiiisli(K). unci all the iiti<'l<-s lu'iiiii tDQndud. Tlie ntinia or iiiitiis or luiy othtir distiagiUKliin mark of tlie company owning ttiB bullion mscj ht diBt in the liottoiu of ibc uiuultl ; euro Iiowvvijr luufl tiikcn tliivt tiiose btters ilo nol form aiiy sliarp angle, which would bo likdy lokp Iho in'ot fro III ilropiunn oat TL-Jviily. It is not UbuhI lo unst ingolH larger tliui 1,000 ounces. A sot of moulds for 50. 100. 350, .WO. and 1.000 ounctta is sufficient for tlio requiruuient u( InoKl mills. An ingot mould 8 inches deep, 12 incliM bj 4 incboti at tlie top luid 11 inclios by 3 incbus iit the .bottom, will just about hold un ingot weighing 1,000 oui]<ic& When eiisling ingots of gold mttltud without flujt, ii it bost to cast them under oil. Enough vogettiblc oil (such ftS olive oil) to form a. layer j inch deep is poured into ' till' mould, whitli must bu linitLd up to th>: L.." ' ' [T Jt""'

boil iii-iwi lit of the oil ; j t.- .. Ij

it is Ihcn ready to ri:- Longitudinal Section

coivo the ntoltcn gold.

When tbtrc ui-c fiuxus ' // S..I., ,--.(l

with tliu cold, oil

slioiild not bo used. Cross Section

Thf ingot mould must

llifii be VL-L-y thoroughly liliicklciidcd iind boated so as to

lni perft'clly dry before gold is (wurod into it.

Breakage of Crnciblei. — It occiisiooally happens that a orut;il)ie cracks during tin; pi-ogix'ss of a melt, althoiigb this is a rui'e acoidont if the precautions above enutiieruted ai-e obiH;r\'ed. As soon as a crucible is seen

cm MELTING 459

bo be cracked, it is drawn from the fire and allowed to Qool, or, if possible, the whole or part of its contents are poured, best into water so as to granulate them. The Ere is then allowed to burn itself completely out. The Lnside of the furnace and the fire-bars are scraped down nd the contents of the furnace and ash-pit carefully brushed out. They, together with the cracked crucible, re pounded down and passed through a sieve of 40 holes bo the linear inch, any lumps of gold met with >eing picked out. The pounded material is then panned up, ben the whole of the gold should be recovered. It ust be melted down with nitre and borax, to free it from iron, with which it is pretty sure to have alloyed to ome extent.

Losses in Kelting. — These should be very small indeed Cind are due principally to spirting, or the projection of minute prills of gold out of the crucible, or their adherence to the crucible. They may be reduced to practically nothing by sending all the residues obtained in the melting-house, such as old crucibles, furnace cinders, :flue dust, slags, &c., back either to the stamp-mill or the amalgamating barrel, and this is a precaution which should never be omitted. Similarly, old leathers or pieces of canvas used in squeezing amalgam should Ixi put aside, burnt from time to time, and their ashes also subjected to amalgamation.

Treatment of Skimmings. — The skimmings from the surface of the mercui*y and amalgam (page 445) always ccntain a notable proportion of gold. They sliould be transferred to a small pot retort, wliicli they must not more than one-third fill, and retorttd slowly. The tern ix.*niture nnist be kept down, and sliould not be allowed to reach redness, even at the end of the operation, as the

COLD MtLUffG

iBMiliMi is wMMtUDM very fusible, and mijbt stick to lh ' stitnitly hiatcd. If the amount of skimmingt i is very sd]l, theae may be tiunsferr'd to a crncible, which is then put into the retort, being packed ftll roiBMl uid in its plaoc by means of coke d;ist or nnd. umI the retorting thoe porformod, when there ivill be no dwtger of injnrini; the retort even if the t<TiipenUuTv rises above redness. A black scoriaceoiis mass is usually oblaiued. which may he fused repeatedly with iiilre and bonui. when n button of clean gold -ill oltimately be produced,

Fuira of Pneipitated Gold. — Precipitated gold, v-i oMaim-I from chlorination or cyanidntion, lias ;it liiru'' to be melted. It should be moistened with a strong; solution of borax, forcibly compressed in some appropriate machine whenever practicable, thoroughly dried, iind then chained into the crucible just like sponge. Some workers prefer to omit the borax, and if the gold powder can he compressed by a powerful press this is the lietter plan. The only danger is that some of the finely divideil gold may he carried oEf by the furnace ilruiight, and if the sponge is not thoroughly dry, loss from this source is sure to occur. The proper treatmeiil of gi>l(l slimes, as obtained by the precipitation of cyanidation Hquors hy means of zinc, has already been con sidered tpajje 417).

Chapter Xiv

MODES OF TREATMENT COST OP MILLING GENERAL CON- SIDERATIONS— LABOUR — MILL SITE AND MILL BUILD- ING POWER — ILLUMINATION — RETURNS

Kodes of Treatment. — It is now fairly evident what methods of gold extraction may he looked upon as those that should be recommended in any special case. Obviously, different ores require different modes of treatment, though there is no sound reason that can justify the very wide divergencies of milling practice in various parts of the world. Moreover, it is not sufficiently recognised that the proper method to be employed can always be ascertained beforehand by lalx)ratory tests which can determine the amount of gold that can b( extracted by the various operations already considered. If the bulk of the gold present is not free," the ore should not be treated by milling, direct cyanidation, direct chlorination, or smelting being resorted to, so as to suit the nature and richness of the ore and the circumstances of the locality. It is only when a considerable proportion of the gold can be extracted by amalgamation that milling should be employed at all, and it is only to such ores (constituting the vast majority of gold ore) that reference

will JmuiiuU- Tliiav art' tvrn ln-oiid which oil ntillin ores inj hn diritlvd, bw-cntdi bii-trade ores, the limit betwen them being taken prv!at fts about 7 <lwt. of gold to t3ie ton. Somt-tiiD* it lm[nft iceptjonally thut low-fnulr. ore is tnstnl bj-hih-gnwle methods, or ht|jh-gnK)o ore by low-gwi methods, hut Htich tnstnnceit are raru.

Low-Oraile Op.— Tlie treatment of low-frnwle mv one that may be conaidered as having originikhvi enia in tlw! WMlttm Rtti*s of North .\iiu-rica, whwv ingonnily of gold miners has been aided very powerfuUj by fjreat natural advantages, mich as cheap timba-t uii fael. and, above all, a plentiful supply of water hinli levels. Tlie essential condition of iuccess in tlir tmlment of low-nnule ort-s is that the ore sliall he kepi is continuous movement by purely automatic metliodB, that from thu time the ore leaves the tuiiie cars until i flows out of the mill in the forni of waste lailinflii, aliull never Ixsnlijecteil to Imnillins. whilst alt ill'' operations thai it undergoes shall involvo a rainininin of manual labour. In this connection, too, it is important to rcmemlwr thai tho object of the mill man is ddI meifly to (.-xtract as much gold as possiltle from tbe quartz, nor even always to extract it as cheaply as possible, but to make the largest possible cash profits. TliuB it very often pays to put a larger quantity of ore through the mill, even tboiifjb a little gold be thereby led in the tailings that might Ik-extracted by a slower rale of woi-king, or, although the cost of treatment per ton be slightly increased, provided that the amount of ore to be mined is unlimited for practical purposes, and can be won very cheaply. Thus, for example, a comjpwij having very large reserves of ore would prefer to pit

V Milling Low Grade Ores 463

Qroogh 200 tons of ore per day at a profit of rl?. 9rf. per on rather than 100 tons of ore at a profit of 2., though, as a matter of fact, an incroaso in the (quantity 5f ore crushed nearly always means a reduction in the cost of crushing per ton, as will presently he seen.

One of the hest tj-pes of mill for treating low-grade ore is to be found in the Black Hill region of Dakota. The ore there worked yields ahout 4 dwt. of free gold })or ton; this is milled by means of 850 Ih. stamps, making on an average ahout 80 10-inch dro]>s minute ; the pth of discharge is ahout 10 inches, and the width of the screen apertures 0*024 inch. The crushing capacity of these mills is tons per head per 24 hours. Inside amalgamation is practiced, ahout 60 pea-cent, of the total yield l)eing thus obtained ; a short length of Wanket sluice is placed l)elow the cop|X}r-piatos and the PQlp runs through amalgam traps. No attempt is made further treat the tailings, whose average gold contents Te ahout 16 grains per ton, mostly lockcnl up in sul- >hurts, which latter amount to 3 cent. The district Sbrds a first-rate example of large mills treating low- iTide ore for free gold only and treating it at a profit.

Another system of ore treatment, which is jx'rhaps the lost widely employed, and which may h( looked as le typical modem Californian mntliod, is that of inside nd outside amalgamation, followed by concentration, nd subsequent chlorination of the concentrate's. This is le system adopted in that highly successful mine the laska-Treadwell, in Alaska. The on* from this min( ielded in 1897-1898 at the rate of 2 dwt. 7 grs. of gold er ton, 1 dwt. 12 grs. being fret?, and the remainder eing derived from the conc(*ntrates by chlorination ; the re yielded 1*6 per cent, of concentrates of an average

assay vmlue of 2 iMB. S dwL Then? wen- 240 Kljimps ri9(B

ii tie eft|Mcity of Ibe mill 3-05 toos per

p -a. "nw iMihbonriH niill ol llu.- All

ll III Miniitf! CoRipftDv. wbidi is workd rin'

SMI Ji-s Mid under tliv mutio management, S

doing Klmo Ix-ltWr work. Thin mill oontaias 120flhun[ )30 Iba. ami makJDp 96 8-incli diups }it ' imibing eapwjity is 379 tons prr hnul pf H 'hoiin. "" nearly 2 dwl. ;t jpv

toa : of tti 0 gr& waa fm> Id. titf

nmaindM- Jilorinating the mhikotntes

; tbe Utter was 18 per oot

of the ora, 1 ox. 10 dwt. 12 pff

ton. In spiM of the ore. tl>e pnl In

tiie abort jv i 3fi per ccmil of ibe |m-

dnctimi of g of tlw Alaska Treadmll '

Gold Miiiint; Company consists i>f 300 heiuls of laUII) siaidjK. iiiikking 101 7-ii>ch drops per minute and cmshiiiS at Oh-nile of 4-45 tons per day. Tlie coaivnliates now no longier chlorinated but shipped to the TVom' siii.lttr. The ore yielded tree j-old lo the value of $1-3213 (iilHHii -yi. I./.), and omcentrates worth $0-7362 (aboul nuikins altogether (about 8*. SJ.) pw Ion of on-, Tliis company now owns five mills containiiiK SHO lii-iui* of Riamps.

Most of tile lArfji' milk in CnJiromia work upon ih'* system, whii-h scms lo he the most suitable lutiiefW dtvis-\l for low-jn-ade ore carrying sulpburets. The cWceiitrator now mo>it generally used is the belt vanner in some form, the most popular l>eing probably the Frw. hut talileii of tile \Viltley are also coming lately into

a? MILLING HIGH GRADE ORES 465

. A few mills are, however, beginning to appreciate tlie kdvantages of sizing before concentration, and it is quite ikfily that this method will be more largely adopted in hb near future. In many plants, large canvas tables M)w follow the concentrating appliances in order to save lie very finely divided sulphurets. Occasionally, as for nsiance at the Drumlummon mill, Montana, inside unalgamation is not used. This mill has 60 stamps of fSO lbs. each, making an average of 95 8 -inch drops ler minute. It is said that the use of mercury in the matter is here objectionable, so that the j)ul]) is discharged on to amalgamated tables, followed by Fine 'anners, and these again by launders furnished with iffles.

Uigh-Grade Ore, — The best example of the treatment )f these on modern scientific principles is to be found in he Witwatersrand district of the Transvaal. Here the ttiamps are usually 900 to 1,000 lb. stamps, making, from H) to 100 4-to 7-inch drops ixiv minute, and crushing hrough screens having apertures of an average width 0*024 to 0*035 inch. The average crushing capacity these mills is alxjut 4. J tons per day, sonic, however, oing as much as 5-J tons. During 1897, the average Umber of stamps running was 3,567, wliicb worked 329*69 days and crushed 5,325,355 tons, e(jual to 1 efficiency of 4*53 tons i)er head per 24 hours. Tin* "ocess adopted is inside and outside amalgamation and i times concentration, generally 011 belt vanners, allowed by chlorination or more rarely cyanidation of e concentrates ; the tailings are moru often not eonntrat-ed, but treated together with the suli)hur(*ts. ley are run into settling pits or huge Spitzkasten, lich separate the sands from the slimes, (?ach being

tTbatd by special cyanidation processes. The resntts obtftinable by lliia method in its highest stages of develi uieot are woll shown in the excellent reports isaiai annually l)y some of Uie Witwatersrand niiuee ; thow ol the Crowii Beet Gold mining Company, Limited, lor ia97-9y may be quoted ae typical. The mill consisted d 120 haad of 995 lb. atattps, which ran tor 334-9 dtjB, oruahinf; 1S5,179 tons at the rate of i'608 tons per bad per 24 hours. The roUowing table summariBtiB the resalU obtained : —

I Fsmnlitgf nm gnM Psnynl

a e-;i&

s rie;

2I-J7a

esM

Conranlratoauyanirleil..

Sliius. cyftiiidad

as -176

rtlngs and liyv-iiroduuU.

Slimra ii'it treated

—

D8

S9-913

lO-Ott

The ore contained by assay 14 dwt. 7759 grs. of fine gold to the ton.

A tnelhod which has yielded good results in local use is the so-cidlud " Grass Valley" method, from its having iHjeii introduced in that district of California. Battery an udijai nation was not employed, the pulp being allowed to juii over blanket strakes, and the blanketings 90 collected being amalgamated by special amalgamaUng maebinus, such us the Atwood amalgamator, Eurekt

Xiv Mtlijng High Grade Ores 467

rabber, pans, &c. Amalgamated copper-plates played a very subordinate part in this process, being only used to oatch any free gold that may escape from the blankets. This process required a good deal of hand labour, and was not so perfect a gold-saving method as the more modern ones. Since about ten years this old method has been abandoned in favour of the typical Cahfomian process, namely, milling in mortars fitted with inside copper-plates, followed by outside copper tables and belt concentrators.

An interesting variation on the above process is that of the old St. John del Bey Mine, in Brazil. " This consisted in crushing, collecting the sulphurets upon blanket strakes, and amalgamating the blanketings in barrels, similar to those used for silver amalgamation in Germany. The stamps used weighed G40 lbs., making 75 12-inch drops per minute, the depth of discharge being 18 inches. They crushed two tons per head per 24 hours. A special pattern of reversible blanket strakes was employed. Closely allied to this method is the old practice at Clunes in Victoria. A very similar antiquated process is still in use at Berezovsk in the Urals. The mortars are of the old-fashioned low type ; there are thirty 700 lbs. stamps making about 65 drops per minute ; the screens have about 0*03 inch diagonal slots ; the mill crushes about 55 tons per 24 hours. The pulp inans over blanket strakes, four to each 5-staiiip battery, about 25 feet long and 1 foot wide, three being always in use, whilst the fourth is being cleaned up ; these are followed by three similar strakes containing amalgamated copperplates, in very bad order, which is hardly to be wondered at seeing that only two of these strakes are run at a time, the tbii'd being left dry. The blanketings are washed in a small flat huddle about 7 feet long, 4 feet wide, and

2 feet high. After the sands have beuii cotiaentrntiNi by washing, aljout two tiuistioonrub of mercury art! pound in and worked into the sands with a woodon hoc: tlim till! ooucentrates are waalied off, leavitif; pasly ainalgiuu boliind. Of the total gold got 95 par ciiiit. is from the hlanketiiigs and 5 por cent, off the plates.

Similar remarks may be applied to what is known u the Colorado system of gold milling us to the Gmw Viilley method. In its original form this procuss wi applied to comparatively soft ores, carrying a IwiW percentage (1.0 to 20) of comparatively low grade pj-ritm and very finely divided gold. T}ie stamp is here used u an amalgamating macliine even more than as a i:rusliin£ oni'. The average weight of the stumps is ahout 60(1 lbs., working at the i-ate of 35 to 30 IS-inch dro|is ]Mr minute. The screen ajK-rture is usually about CHfl iriuh, and the depth of discharge 15 inches. The result of this arrangement is that only aiwut one ton is cnisbti] per head in 24 hours, the cost of milling Ijeing proportionately augmented; the loss of mercury is also heav)'. Insirle and outside copper-tables are used, followed raoslly by blanket strakes and end-shake concentrating tallies. The process has developed itself slowly and graduallybut has changed far less within the lust twenty yews than in the case of Californian mill practice. Il cannot he recommended for new countries, as it manifestly presents few advantages over the Witwatersrand system, and has many corresponding disadvantages. At the same time it must be admitted, that probably no olbft method (except perhaps the Hungarian system of stamp milling lollowed hy treatment in Hungarian mills or LiiHzlo amalgamators) could elmet so large a proporUOQ of auialgamahle gold. Whether niiliing is tie proper

Xiv Cost Of Milling 469

process to apply to such ores, or whether they should not rather be treated by the chemical or the smelting methods now in vogue in the Cripple Creek district, for example, is another question.

In Nova Scotia very good work is being done upon the lines of the Calif omian practice, which is closely followed.

In Australia, the most recent gold-producing colony. Western Australia, appears to be following Witwatersrand practice, though several of the mines are tiying direct cyanidation ; the character of the ores in depth has hardly been definitely ascertained, but it seems that the greater portion is not free-milling, and amalgamation is being largely displaced by other methods. In the older colonies there is but little to be learnt, the milling practice being in the great majority of cases far behind the Califomian, though running upon the same general lines. Rock-breakers and self-feeders are conspicuously absent. The weight of the stamps is usually about 900 lbs. and the speed about 80 8-inch drops per minute ; the screens are mostly punched iron and the average crushing capacity rather below 2 tons per head per 24 hours. Inside plates are very rarely used, but mercuiy is at times charged into the mortars. Outside plates are used to a considerable* extent, but much reliance is still placed on mercury wells. Very varied forms of concentrators are used, end-shaking tables being in favour ; these are often followed by blanketstrakes and the latter at times by canvas tables. The rough concentrates so obtained are still largely treated in Berdan pans, although in most larjo milling centres there are customs chloriiiation works, that buy and treat sulphurets at i-easonahlc rates.

New Zealand practice is almost identical with

|4To r.oi.n Mn.irifG .-m

Anstmlinn. Tht; htti grnrli; iif the Aimtrftlian ores iDil Uie c<o&rHL<ne8S x-ni) " froemuts " of much of thv o]il .w llv prababie caust-H of the comparative haikwiiril cotidilaon of sciiMitific gold uiilliiig in Anstndiii, iinutlicr importtul fiutnr tming the conipiiralively small slmi of the rainiftf claims in most of the C-oionies, wliicli but rarely admit tho establishnienl of large aiid n-ell -equipped mills on tk mines themselves.

Com of Milling. — The coat of millin) a ton of ore influenced by so many different conditions that it is ijnite impossible to lay down any broad general rules. It is, of course, a matter of gi-eut importance to be able to estimate beforehand what the a|qHt>ximale cost of treating given on-will be, and this task has often to be atteraphil. In preparing such estimates, the following points wr those that need the most careful consideration.

I. Nature of the Ore, — It is evident that two ores of equal assay value may be of very unequal actual value. lncause the gold contents of the one may cost far more to extract than in the case of the other. Tlius one ore night carry all its gold in the form of free-milling gold. and require no treatment beyond amaJgtuiiatioii, whilst another may need concentration, followed by elabotate after-treatment. One ore might carrjits valuable ingredients in the form of coarse particles, whilst in the other they may be Tt>ry finely divided, so that the secoml ore will need to be crushed tar finer than the first, Uie natural result being thtit the same plant and staff will le able to crush loss of the latter than of the former in same lime, the cost of treatment being corresponding!) increased, whilst tlie loss of tho slimes will be greater A hard tough ore, again, will cost moie to cmsb thai' a friable, easily broken one. Obviously the crusbiiig

XIV COST OF Af/LLING 471

capacity of the mill will also greatly influence the cost of crushing the ore ; thus a mill crushing at the rate of 2 tons per head per 24 hours will work at a tonnage cost very nearly double that of a similar mill crushing 4 tons per head. The cost of labour will be the same in either case, and the cost of fuel but little more, whilst the cost for mercury, spare-parts, &c., should vary almost directly with the tonnage crushed. There is hence a great economic advantage in running a mill at top speed ; for example, a mill running at 60 drops |)er minute was found to crush 50 per cent, faster than the; same mill run at 48 drops per minute, whilst the fuel consumption was very little greater. The fastest crushing compatible with good gold extraction is therfore invariably the best practice.

II. Power Available. — The cheapest form of power obtainable is water power, which costs vciy little more than the interest on the capital expended in constructing the necessary dams and water-courses and erecting the motors, the outlay requisite to keep these in repair being generally a comparatively small item. When water power is not available and steam has to be usud, the quality and price of the fuel obtainal)le is a veiv important element. On an average a ton of quartz requires for its reduction 005 ton of coal or 007 cord of wood, these figures, ranging from 003 to 010 in the case of coal, and 0*03 to 012 in the case of wood, being influenced principally by the quality of the fui;!. The importance of the cost and quality of fuel can therefore readily he appreciated.

III. Labour. — This is an item that may vary within very wide limits. Most of th( lahouirequired about a stamp mill is skilled labour, and this is often dear in

I 4TI GOLD MILUffG ar

pro])Orliii as unekilleil labour is chim]i. For insUccf. in countries where low-prioed native Inhour ia employpii for aII purely mpclianieal purposes, trained wliitr (mln- Qion enerally coiniTiiind liigh waes. T}it; pnkclicn nl wortciiieu, too, in difTiM-eiit parta of tbe world variei gn-Atly ill rctspctot of nuch matters as length of shift. anJ it ie nircly possililcto induce men to work exoepl in Hccordanco with Ihc local customs, whatever these may be.

IV. Hiu of Mill. — This is, of counte. governed liy tb uvuilablti Rupply of oi-e. and ie within certain limits om of the tnoBt ini[x>rtttnt elements of the cost of niiUin);. Thus it takes scarcely nny more men to atleod to e, aixly stiunp than to a ten-stamp mill, so that the cost d labour per ton is but little more for the liUf.T iiiill ihnii one-sixth of that for the smaller mill. On the other band, a 240-stamp mill will require nearly three times as many hands as a sixty-stamp mill ; so that it may be ctHisidered that the limit of economy as refjards mill laiMor is almost I'eached in a mill of sixty to eighty stamp- Of course, in other items of cost, such, for instance, as management, supervision, assay departnient, &c.. the larfjer mill, whatever its size, costs scarcely any more than does the smaller one.

V. Silttation. — As the cost of mill supplies (orma & lai'de element in the cost of milling, the comparatiTe prices of these, which are largely determined by the oosl of transport from the nearest sea-board to the mill, affect greatly the cost of the operation. The wear and tear of a mill amounts, as we have seen, roughly speaking, to some 2 lbs. of metal for each ton of ore crushed, nnd the frciglit on this quantity may readily amount to Very iin)xii'tant item.

In uilililiiin to these pnnc'i]ml (mints, there are usnalb

Cost Of Milling 473

IX of others of smaller importance, depending cal or accidental circumstances, which can

be enumerated here, and which can only be id by investigating the actual costs of milling in iring districts or in others where the conditions are as nearly as possible identical. Costs. — The actual costs of milling in a number al mills, taken from data obtained in practice Bcted from various sources, are here tabulated, I according to the respective countries ; in the M)n of these due regard must be had to the points enumerated*

America. — In the table on page 474 are given the milUng in several mills in the United States, some oaller and worse-conducted estabHshments being

for the sake of comparison. It will be seen that ations are within very wide limits. The slow

js cheaply than the modern mills of California ;

)rmer the rate of crushing is now mostly between

;d 1-25 tons per head per 24 hours. The cost of

tt the Hidden Treasure mill, Gilpin County, was

)s. Id.) per ton, and in the Kansas mill in the same

;l-47 (65. 2d.) per ton in 1896 ; the latter item is

I of : —

$

Labour 0*62

Supplies, fuel, water, &o 0 55

Traiumiug to mill O'SO

th Star mill. Grass Valley, Cal., is an example of )ing excellent and cheap work in spite of the fine, refore relatively slow, crushing which its ore re- Tt must how(n'er he noted that the fijures given hie over leaf include; in this case the cost of con-

I I I I I I 1 1 1

' i 'ill I lilli ''tiiii

r COST OF MILLING 475

ntration, here done on vanners. The additional cost casioned hy this operation was as follows in 1895 : —

%

Labour 0*1006 j>er ton.

Supplira 0 0098

Total 101343 0.

The milling of low-grade ore is a matter that has iceived much attention in the Western States ; it may lily be looked upon as the probable main cause lat has induced American mill men to keep elaborate statistics of their operations, the study of which affords iformation of the greatest use in pointing out where ny excessive outlay occurs, and in effecting correspondig economies. As a most instructive example of the etailed costs of all the items of large and thoroughly yell-run mills, the full particulars of the Alaska Treadrell and Alaska Mexican mills, both situated at Douglas sland, Alaska, are here given for the year 1896. These kills, as already stated (p. 463), are working ores yielding bout two-thirds of their values in the form of free gold nd the rest as sulphurets. The results of the year's

working were : —

Alaska Alaska

Trt'adwell. Mexican.

Fiee gold, per toil 2 00 167

Gold from 8iilihurots 0-97 075

IVrcentigf of sulpliurots . 1 GO 218

OosU of trcahtiCiU per ttm : — $

Mining 0 ni 91 1 14 11

Milling and concentrating' 0 31-7G 0 :59-41

<'lilorination 0 1 1 :J8 0 1599

General cx[KMises ... 0 l.''>"27 0 11(U

OMJ> ttlLUKC

of 340btials.whi IB €0 only during an Bg Uw Twnunder of the yea aB3t,670 tana utd tbo latl

J as follows : —

Uraian.

0 0OT7 O-OSill

garr -

0-M58

T Ti-.i'cr

Iv, ...D,

i7i-f4*s::v

lU;>rc .'.U

t-UiKl

n-,i.wi

oo-m

i"re*t-r -It™*

Ooiw

M'rtitv i

D-Oois

M.'rur Unrs

0 flits

Bi*.ht.U ili . .

0-Ox19

l.-an. shifts 3 1

LHl.ri.iU . . .

o-oois

Oo0S9

Mitviirr l4S flicks!

0-O0S6

0O74S

Kl.vlri.lij:ht.--.Hi.t

O-0103

: o-OOlo

M>s.'elUn<n.ns . .

Ooiss

Totjil supplies

Lul..iir. Jiu>y,-

Xiv Cost Of Mtlltng 477

In both of the above mines the milling costs arc exceptionally low, owing to the large size of the mills, their favourable situation, and the fact that ample water power is available during a large part of the year. The advantage of this item appears from the table on page 474, whence it will be seen that in the year 1893 tlie cost of milling with steam at the Alaska Treadwell mill was 2s. 9d. per ton, whilst it fell to I5. 6d. when running with water, the year's average cost being Is. lOd. The chief economy that seems to have been introduced within the last few years is the replacement of the rockbrcakers of the Blake type by Gates crushers ; with the former the cost for lalK)ur was 0'4460 and of supplies $0*0143 per ton, these items with the latter machines being §00142 and $00070 respectively. The relative economy of a 120 head stamp iTiill as compared with a 60 head, is well illustrated by the figures for the Alaska Mexican mill given in the above table, whilst it will also be noted that the Alaska Treadwell mill of twice the size works very little cheapoi'. During the last seven or eight years, the cost of milling ut the latter mill has only shown trifling, almost accidental, fluctuations, averaging about Is. 6d. per ton. The cost of milling in the new mills is now as low as 1B"59 cents, (say 9|d.) per ton ; the wondeful economy here realised is due not only to the large size of the mills, hut to their excellent equipment and careful management, the strictest attention being paid to every detail. Thus it is worth noting that the cost j)er ton of the steel shoes used is $0*0124, and of the dies, which are of cast iron, $00088.

With the above results may be compared those obtained in another very well-run mill, that of the El Callao Mining Company in Venezuela, which worked high-grade

ore under circumstances of BpooiaJ difficulty. Tie mill was a fflVstanip mill ot good modum const rutrtiffii; during tlio yeiir 1892, it crushed 52,823 tons, which yielded at the rate of 12 dwt. per ton.

Tlio costs per ton, tbo mill crualiing at tho rate of tons per stamp iieatl per 24 hours, weru as follows :

T'ltuUiiiiiiiitsm-t in:

In Mexico the Mcsquital del Oro mine runs ii 50-head mill of eight (650 ll>.) stumps, crushint; at the rale of t'l tons per head per 24 hours ; the fuel used is wood which costs 15s. ix;r cord of 121 cubic feet, the mill using 11 words jxir 24 hours. Tlie costs per ton in 1893 were as follows :

Total ... . . B75

The cost of milling at the Spanish mine, Nevads County, California (already referred to, pajje 303), is worth recording as an example of & mill doing exceptionally good work under highly favourable con<litions. The ore is soft and easily reduced, consistios of a soft talcose slate with stringers of quartz. It is ct'iislied in four Huntitigton mills, whose crushing eintauily is somewhat over 4,000 tons )k.x mouth, or say

Cost Of Milling 479

tons per day. The costs per ton were as follows

It 1894 :

%

Labour 0110

Mercury 0006

Shoes aud dies 0*046

Mill supplies 0*037

Miscellaneous . . 0*016

Water 0*036

Total milling cost $0-249 1. 05.

he ore is exceedingly low grade, yielding only about \d. per ton ; but even at this low figure the operations result in a small profit, the total cost of mining and ing being alK)ut 2s. 2. per ton. The extraction is r, being under 50 per cent, of the value of the ore by

n contrast to this type of work, the cost of ning a good arrastra may be quoted. Such is a iom arrastra running on high grade ores in the son district. The ore is broken to the size of hazel 3 before being charged into the arrastras, of which e are two ; each can work off a charge of about 7 . of ore in 6 to 8 hours, the average capacity of the re plant being 5*4 tons in 24 hours. The power used vater during eight months of the year, and steam ing the remaining four. The charge for treating ores ies with the size of the parcel, the minimum rate ig $15 per ton. The following is the approximate t of treating one ton of ore : —

Ljibour 4 18

Mercury 0*81

Supplies 0*79

n

Total cost £1 13tf. lid.

M-ia aoi.n MILLING rHAf,

Such high coats art of course only permissihie wbai smnti qutmlities of hlfh gruilti ore presenting epraiil diflicultiea have to be trtiateA

An-netrae have, however, the additional advantage thil tliuy can bu operatwi by animal [lower as woU bj stuam or wuLer, itre ood ffoUl siivurs, and an-cliMiJy erected. A pair of flrBt clasti arrastra 12 (eel in iIia meter, driven by a woodon hurdy-gurdy wheol 13 fwi in diameter, cost in California in the year 1895, compk*' in runuing onlur, $700 or about £145. U tiikcs 30 niincr't inches of watr under 100 feet head to drive it ; one uiM, on a IS-hour shift can attend to it, and it treats atioiii 5 tons in 94 hours, each arrastra treating a charge of tons ol ore ill 12 hours.

JVourt Scotia.— few figures are nvaliable showing; tlif cost of milting in Nova Scotia, and are presented i" tabular form on page 482. Fuel and labour are bolii cheap, bonce the costs arc low, having regard to tlie suiall si/.e of the mills. These figures again refer to Ihf short ton of 2,000 lbs.

It may hti noted that at Sherbrooke the charge fw custom milling is $1'15 (4s. 9rf.) por ton. As these figm'es will show, the conditions in Nova Scotia ut fftvoiu-able to extrumoly cheap milling, and if the {."oliiniining industry develops as it should do, so as w permit of the erection of larger mills, this colony will able to rival most other countries in its treatment ot low-grade quartz. It must, however, be remarked thai most of the reefs hitherto worked are comparativelv narrow, hence the cost of mining is proportionately l.iBlic,-.

AiisLnilia. — In Australia a large proixjrtion of all tl'''

Civ Cost Of Milling 481

nills are custom mills, whose interest it is of course that khe exact cost to them of crushing quartz shall not be ascertained. Partly no doubt for this reason, and partly perhaps because gold milling has not become so closely studied a science in the older gold-producing colonies as in other countries which have to treat poorer ores, there are but meagre data available as to the cost of milling. Gold milUng in Western Australia, again, is in a more or less experimental stage, so that the real cost there of milling may be said to be almost unknown up to the present. A few data as to costs are shown in the table on p. 483.

This table shows very extreme variations of milling cost, and would alone suffice to indicate that stampmilling in Australasia is still very far from having advanced the dignity of an exact science. According to an :>official communication, the cost in the different districts New South Wales varies from 2s. 6c?. to 6s. 6rf. per ton. Some of the very best equipped and managed mills in the >lder colonies, and especially in New Zealand, are working it costs not far exceeding 3s. per ton, whilst in the Majority the cost is between twice and thrice that figure, tt is almost as rare to find a mill in these colonies ciiishtig for less than 5s. per ton, as it is to find a Californian iiill in which the cost exceeds this figure. In Queensland t 30 head Customs mill charges 12s. ier ton for cnishing, deluding cartage of the quartz for li miles from mine to >attery. Labour is dear, and fuel (wood) costs about 21s. ier cord. In West Australia, where fuel and water are X)th costly items, milling seems to cost about 10s. per on.

India. — The following arc the costs of milling on two

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i-;3= zl S*S£4S44&'* "&V'

of the moat auccessfal mining properties of tbe Mtvm gold fields in 1897. the ton being here the long lau i' 2,240 lbs. :—

Tolw |n'r hcail piT 24 lion™ I Tun I'ruKlivd per iiniiimi ,

fEaron I %, Nativg f Forf

lOcni'tal Stor™ Total . . .

It will be noticed thiit one half of the milling Mst reprentnts the cost of fuel, due mainly to thfi fact thai coal at the mines is worth some 30s. per ton. The costs at these mines have been very considerably reduced in recent years owing to the increased size and improvements in working of the mills, and largely to the inodernisAtion of the process employed. Thus in 1901 the Mysore mill crushed 127,070 tons at a cost of 4s. %-M. per ton, and the Nundydroog 52,030 tons at ,a cost of 4s. STifpel' ton. These mines used formerly to re-treat the wlioltf of the tailings from the stamp-mills by the antiquiit*J system of pan amalgamation, the cost of which in 1897, when it was still in use for a portion of the mill tailings, i? shown in the following table : —

Cost Of Milling

lantity of tailings treated

'European labour Native lalx)ur Fuel Shoes and Dies . Mereuiy . . . Lubricants . . .General Stores .

Total . . .

he process is not only costly, but also inefficient, the ngs from the pans requiring apparently to be again ted by cyanidation. The cost of this latter process in above mines worked out at 35. 'Id, and 25. Wld. per treated respectively in 1901. It is therefore evident the proper method of working here, as elsewhere I similar ores, is by stamp mill amalgamation followed iyanidation ; and this is the process that has now been $tituted for the old one by these companies. A hbouring company, the Champion Reef, working 140 Is of stamps, obtained the following results in 1898 : —

Tons treated- Cost per ton.

Stamp Mill 89,271 6 3-64

Pan amalgamation . . 28,761 7 0 96

Cyanidation 73,202 4 7-22

et another, the Oongaum mine, crushed 78,125 tons

901, the costs per ton being as follows : —

s. d.

labour 193

Fuel ... 32-5

Shoes and dies 0 3 5

General stores 0 7*7

Total

t-K COl-n Mll.t-im; nur

At thtise iriincis the coHt of cyaiiidatiou iiiiitxmW bi 3s, 0-9rf. per ton, out o/ wLich onc-lmirwas for cyiuiiiie.

In all these miueu nntive klwur is of course cheap TUe yield of gold varies from bout 1 oiuiCti to mt)ier over ounces to the ton of stone crushed.

Grtai Britain. — The cost of milling nt one of the verf few British Gold mines, where systematic working on oonsidorable scale has ever beou attempted, the Hotpai mine in W&les, is a matter of considerublu inturust. It was OS follows at the beginning of 1893

Avisru Damlier of IimkIh ruuiiiug 40

TuLal time of raiuduc . GUI bua&

NninlHT of tons uniKliiit . . . .1,150

Molivu jiower . , . water.

Coat ]>i!r ton — labour ,...,,,. 0 10 „ ,, „ liiaturinl 0 U

Toti 0 111

In the years 1894-1895, the total qimotity milled wu 12,354 tons, at an average cost of 2s. 3-4(W. i>er ton; iIip latter was very irregular, the monthly averages duciiining Ixitween 1*. 4-96d. and 4s. 506rf.

On the Continent of Europe there are but few example* of gold milling, properly speaking. At Vulkoj in Sifbenbtirgeti, Transylvania, a 20-head stamp mill crushts si the rate of 1-75 tons (icr 24 liours ; it is worked by skiii", am) tiie sulphui-ets are concentrated on Frue vaiiiier*. Thi; cost of milling is returned at Gs. per ton. Tb' bu'gest staiiip-inill in Europe is said to be at GurabarZ!, near Unul, Transylvania, containing 190 stamps.

At Bomniel, in Norway, u 50 stamp-mill was in oper*- tion till recently, with a crushing capacity of 134 tons pc'

Xiv Labour 487

head, per 24 hours; in 1896 it cost 21 -3/. for fuel and 5'3cf. for wages per ton crushed.

TrafwtDoaX. — The costs of milling at a few typical mines are given in the table on page 488.

As a rule the best mines on the Witwatersrand mill at a cost of between 2. and 2. 6d. per ton, very few exceedmg Zs, That so low a cost can be attained in spite of dear fuel, and the high costs of stores and wages, is due to the large size of the mills, their very perfect construction, economical management, and also to the fact that coarse crushing is the rule, the fine gold being extracted from the mill tailings by cyanidation. At the Sheba mine the cost is higher, in spite of the existence of available ater power, because a system of much finer crushing is employed ; whether this is an economically sound system is perhaps open to doubt. During 1898 one-sixth of the total ore treated at the Sheba was crushed by the use of electricity, and five-sixths with steam (using coal at 17$. 6Z. per ton as fuel) for the motive power. In the previous year only sixty stamps were run entirely by electricity, and the cost of this power per ton crushed was 25. 3'376/. as against I5. 1*2336?. given above for steam and electricity. The central mill of the Transvaal Gold Mining Estates, Limited, is entirely driven by electricity ; its costs would be lower than they are were it not for the high price of all stores in this somewhat remote region. During the years 1892 to 1895 the cost of crushing in a 20-stamp-niill at the same place, averaged about 45. jK3r ton.

Labour. — It has become a custom almost universally established in mining centres that niill-nien work 12 hour shifts. This indeed they can easily do, as the labour is but light, except when anything goes wrong ; then all hands have to work as hard as they well can in

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c-HAP. XIV LABOUR 489

order that the least time possible may be lost. Whether tbe rock-breakers be in a separate rock-breaker house, or 'whether they occupy their floor in the mill-building, the tnen who attend to them only work a 10-hour shift, rockbreakers never being run during the night shift except in curses of emergency. The capacity of the rock -breaker plant should always be so calculated as to be able to orush easily in 9 hours as much as the mill can do in 24, a.nd if there are ample storage-bins for the crushed ore a.fter it has left the rock-breaker, the rock-breaker plant liiay be even more powerfid with advantage. One man oan readily attend to a rock-breaker, help in dumping the Ore cars as they arrive, keep the grizzlies clear, and do 11 the necessary work, including the keeping of a tally of the number of cars handled, and in case of rotary orushers like the Gates or Comet, which requires practic- 5illy no attention, one man can even attend to several crushers.

A small mill, say of 20 stamps, requires a staff of two amalgamators, one on each shift, and two concentrator men. The former attend to the battery, repair the %>creens, make wedges for the stamp-heads, and do other light work. Larger repairs involve the assistance of a mechanic or a carpenter. The concentrator men see to the proper running of their machines, adjust the water bupply, inclination of the tables, &c., and wheel out the concentrates from time to time. Of course, it is understood that both feeding and concentrating are done i)y machinery. When hand-feeding is employed, one man is required on each shift to every 20 heads, and when blanket-strakes or hand-i)UcUlles are used, a i)oy, or even two, will be required to assist the concentrator man. In addition to the above, assaying, retorting, melting, and

general a

upervision will require pitrt of ttiu tiiiiL' lA

mine f

itendent or manager, wbo should be capsblt

ng all thcBB duties.

larger mills, practically the same flJf

wo

1 iuu rwik-UrMkBr, working 10 lioii™. 1 nmiilgiiiiiitont ... „ 12 ., a conwBtmtOT uiou . ,. ja ,. 1 Ubotircr 10 ,.

At the IIoi Btar Stills in Mm..

of 100 and ] vtsly, thu folluwing is thf staff (A mQQ 01

lakii. OiiMs Ad'

Amalosmaxi - . . %

Eook.ljrmko I . . B

Towhitli iiiuBt lie lidded part oJ the time of foremtn. ciirixjiilei'H, machinists, and watchmen, about oquivalenl to the work of an additional iiian, thus bringing ihi; hImjvu liUMiiiuis up to 10 and 19 resjjec lively. Tlii-se being steam niills, the labour at the luilers has also W l)e included, this being in cacli case 2 engineinen ami 3 lireineii, oi' an addition of men to each staff, nmkini: the toliil nunilMjr of hands cniploycil alx)ut 19 luwl resijoc lively.

In tlie well arranged miilsoftlie Aiastta-Treatlwci! Company (240 stamps) and of the .Muska Mexican Cominy (120 stamps), the' respective labour staffs average aboi us follows: —

JCiv LABOUR 491

Alaska— Alaska—

Treadwell. Mexican.

Foreman and assistant 2 2

Feeders 8 4

Concentrator-men 8 4

Amalgamators 4 2

Orosher-men 3 6

Oilers 2 —

Ditchmen 3 1

Carpenters 2 1

In each case, a little additional time from machinists, Ac., is required, so that the labour force of the two mills together may be taken at about 53 per diem or about 600 hours ; the total quartz that the two mills are capable of crushing is about 1190 tons daily, so that roughly each ton of quartz crushed requires the labour of one man for 0*5 hour.

At a large Transvaal mill, the New Primrose, with 160 stamps, crushing 820 tons of ore per day, the labour stalT is about as follows : —

Battery manager and niill-forcmcn 3

Amalgamators 9

Mechanic and carpenter 2

Engine drivers and stokers 6

European labour .18

Natives 28

Total 46

On account of the high crushing capacity of the sUunps, the labour ratio here is low, amounting to 0l3 hour of European labour and 034 hour of native labour per ton of ore crushed, these figures excluding both rockbreaking and concentrating. It is, however, scarcely possible to compare the lalx)ur efliciency of a native and that of a white man about a stamp-mill.

l4 49> CO/. MtUlNG iHAf.

Taking the aliove (p. 490)staff for ii40-Btampmili, capable of crushing 120 to KiO tons per day, tlie labour etjuiTalenl of one man working itiiout 90 hoiu-s when water-powur, or 140 hoTJTs whfjn steam-power is employed, will ho rt-qiiirud per day ; this will give as the labour equivaltnls for one ton of quartz crushed 0'7 and I'O hour respect tively of one man's work. When the average rale d wages in a district is known, tliese figures give (aJrl)' reliable data for estimating the approKimate cost of labour incurred in milling a ton of quartz in a mill of tbc above size.

Cost of Kill. — Tbo first cost of a stamp mill vuiet considerably according to the quality of the machine ftoJ ' the items included in the; hill of charges. As a rougli eatinmte it may l>e taken that the cost of a mill from rock-breaker to mercury traps, hut excluding motive power or mill buildings, is about £100 [Kr head, and of the hare mill alone about £50 per head, a Httle more lor mills of less than 15 heads, and a httle less for mills of over 40 heads. A first-class English-built mill of 60 nine hunditd stamps mth knee frames eoniplek. including Ilcndy's challenge feetlei-s and good rock-breakurn, cost £(),500, this being exclusive of motive power, A O-stamp mill of seven hundred pound stamps, complete with galvanised iron building, and engine and Iwilers, cost £3,900. In California a 20-8tanip mill ill) nino hundi-ed pound stam[)s, complete with Pelton whet'l ore bins, mill-house, Ac, was erected by contract lor $12,bCK) (say £,600). A first-class 40-Blamp mill con:- [Jlete with Pelton wheel, hut without mill building, cosl $22,000 (say £4,400) in 1892. In 1896 another 40-8tamil mill complete, ready to run. was erected in Cahforoia for $21,000 (say £4,200). A 60-stamp mill of 1020 pound

Xiv Cost Of Mills 493

stamps for Alaska cost $40,600 (say £8,200), freight, erection, buildings, &c., bringing the price in tninning order up to $60,000 (say £12,000). The new 300-8tamp mill seems to have cost (including the mill, site, i&c, about $320,000 (say £65,000). In Australia a lO-starap mill with cast-iron frames, exclusive of everything except the mill and frames, costs about £450, this being about the same price as would be charged in England. The costs of some of the larger mills in the Transvaal are given in the following table, based upon the published accounts of the companies in question : —

No. of

Total cost.

Cost per head.

S,

New Primrose

Geldenhais Estate

Crown Reef

Glencairn Main Reef .

Block B, Langlaagto .

The above figures include mostly the mills erected and in complete running order, with buildings, shops, engines, boilers, water supply, &c., but mostly without rockbreakers and in some cases without'concentrators. It has been estimated that machinery erected on the Witwatcrsrand cost two and a half times as much as its price in England, and this estimate is not far off the mark, though perhaps a little low for the stamp mills quoted above ; three times the home price would seem to bo a fairer figure, though much must natiually deXMid upon the outlay required to get a sufficiently good foundation.

Specifications. —Care should be taken that these are full and inclusive ; attention is here only drawn to a fewspecial points, whilst by way of example a full spticification for a 20 head mill, for which I am indebted to Messrs. Fraser and Chalnuns, Limited, is given in appendix B, page 578.

; i|94 GOLD Mll-UNC nur

Marian. — Their approsiinate weiIits nhoul(l Iip Bpc-oilicd ; they luiist be planed on botUiiii iinil on strn-ti 8Pat9. Each ahouhl ))c nupplied with its sot of sM liner plates, generally five in nniiiber, weighing; toUuir tthout 4 cwt. Cbock-lilocka ami screen fmnies should he aiipplk'd, iind Hcreen keys of forged aleel. Tlie holildown holts aliould lie li inches in diameter.

S(nmp.— The stem should Iw of best hannnered scrsp or forytid Bessemer eteel. The heads may be cast iron or steel. The tappets should be of east iron or steel, bored to gaugo and fitted with steel gibs and kej*B. The shoes should be of chrome or mangfttieae stet:!, and tbe dies of softer cast or forged steel, with a due supply of spiire ones. Drifts for ilriving out the stems and shoes should be supplied.

Cam-Shafts should be of Bessemer steel turned to gauge. Cams should be of good cast steel, planed on the inner face and polished on the working face. The beitrings should lie of cast iron and well habited. The pulleys should be of wood with cast-iron bosses.

Guiiim. — Adjustable guides should lie employed.

Jjwites.— Their nature, the rjuahty of their timber, and dimensions of the principal beams should he cle4vrly stated. Tliey must be painted or tarred. The batten blocks must he of suitable length and quality.

If ore bins are required, ore bin gates should be included ; if Frae vannera, proper launders for distributing the pulp; if rock -breakers, grizzlies must be included.

A proper sized water main and supply tanks should be furnished.

The length and gauge of copper tables should be stated. The I'equisite clean-up gear, say one luurel

'

Xiv Mill Sites 495

one hatea, and one clean-up pan for each forty stamps will be required ; also retort with condenser, and mercury-room fittings. Motive power should 1k taken well in excess of the apparent requironionts of the mill. All the main shafting and counter-shafts should Ix? of ample strength.

Mill Sites. — Economy in working a mill depends to some extent upon its situation. This is generally determined by its requirements in the form of vater. The advantages of using this form of power are so great that it is very often worth while to construct a tram-line of considerable length, whenever by so doing water-power can be utilised ; hence the majority of mills are situated on the banks of streams or rivers. Of course, the watermotor will need to be as low down in the valley as possible, in order to utilise the full available fall of water the more effectually. Care, however, must \yo taken to keep the mill well above flood level. If the waste tailings cannot, for any reason, Ikj carried away by the stream, the mill must l)e at such a height above the bottom of the valley as to leave ample room for tailings' dumps, and this is even more iiiiix)rtant when working an ore of such high grade that the tailings an worth sa\'ing for re-treatment, in which case due provision must be made for the necessary retaining dams or reseiToirs. Whether the mill is worked hv

water or by steam, proper aiTangenients must be made for an ample supply of water to the hatteiy. If possible this should be brought in by gravity, but where this is impossible, pumps of suflicient capjicity, not only to provide the normal quantity used, but to meet any emergency, should be laid down. However the wat(r is obtained, it should be carried into reservoirs or tanks at

such n hiiiKhl Miove tlio rail! floor as to give M Iwist n 30 to 30 feet heA. Tlieat! resurvoirs slioiil.l hold n sn lioure' supply for thi' ontir<( mill. Thuy should in a# of ntwd Ik) preceded by Battling tnnks. or even l)y rough filtcni if tho wiittir is mnddy or foul from any tanw. fiecin how Hi'iioim iTiny bo the InsiwH of oid cnuiwii bv iiaiiiK muddy water in the luittery. IT nt tUI possible. supply o/ (jood olonn water should obtained for Ok mill, lis much of the success of anml(raiiialion depends upon it. In cold climates provitiion must he tnade for beating the water supplied to the mili. This is licM done by mc&ns of steam led into the tanks from Bcparnte boiler, hull t especially for this purpose; htisU' steam from the engines must on no account be used, as it is sure to carry some grease with it. In Nova Scotia, the custom mill at Oldham, which is run by water-power, makes an additional charge of cents of a dollar in winter time for the fuel required to heat the batteiy water.

It occasionally happens that mills have to be erected in places where the water supply is InsuflicJent for tbe uchIs of the battery. When this is the case, tailing reservoirs have to he provided, in which the tailings and slimes are allowed to settle as completely as possible, the water lieing then pumped back to ite used over again. This is done by a well-planned arrangement at the Iligliliind Mill, Dakota ; two reservoirs are formed, one above Ibc other, by throwing two dams at different leveU across the valley of a small creek, the lower reservoir thus formed being four times the size of the upper one. Tlie tailings are run into the upper reseiToir, where the siiniln are allowed to settle, the slimes overflowing from il into the lower one ; the slimes settle theiv, and clear

Mill Sites 497

pumped back from it into the mill supply tank, .ngement is provided by which these reservoirs duiced out when they are filled with accumulated

In case of need, mine water can be used for ig mills, but as this is mostly muddy, provision made to allow the suspended matter in it to Vater from some mines, especially from those where is rich in pyrites, is apt to be acid ; when this case, sufficient lime should be thrown into the tank from time to time to neutralise such acidity precipitate any iron that may be present. n the location of a mill is not determined by its ity to water, there are a few other circumstances luence its selection. Thus it is always advisable, possible, to choose the side of the hill having a apid slope, and consisting of a rock sufficiently form a good foundation for the battery. The height of a mill of the " high type, with a eaker floor above ore bins of normal capacity, taken as 50 to 55 feet from the level of the car that of the concentrator floor, the length over he profile of the mill being rather over 100 feet. a slope of 1 in 2 is the most suitable for a mill Df course provision must be made for the bringing good track from the mine for the ore cars, and easy transport of fuel, where it is required, to the

Some mills have been very successfully supplied by means of wire-rope tramways, which autolly discharge into the proper ore bins. This 3ment is specially advantageous when a comely short distance has to be traversed in very Duntry. 1 the modern methods of transmitting power

K K

4g| COLD .VTLL/UG

eloclriflftliy, the daec proximity of the mill to a of water power U beooming of les import&aco. and Aocount cS the readiness witb which power so conveyed can be cnbdivided ntost of the objections to a pupil>] erastHr boose disppear. It i§ tbercfore becoming tea imporlBnt lo loeate nulls on the sides of steep nJlitysb kod Any witb a Kuffioietit slope to carr' tuliogs is ocmstderod sUisIactor}* provided that il good solid foandatioDs. Under these coudttioDs a siU< elocsc lo the prtocipal mouth ol the mine, or equidistant; from all of them, if there are several, is apt to be pnsferred; tbe greU prinai[de must always Ik: n bered that, for economic woridDg, the ore should oeTsr be lifted from ibe time it ectets the batterj' uclil il escapes after final treatment, in tho fonu of e<ihaustied tailings, but should continuously descend from stai* w stage mo-ed by gravity alone. When tailings have to be submitted to eyanidation after leanag the mill, it is rarely easy to gel a site such thai the tailings can be carried down to the vats by gravity, and it is usual to elevate the tailings from below the mill to the top of the cyaniilt plant. This may be done in various ways ; bucbi elevators or tailing wheels may be used, which >re quite automatic, or else the tailings may be filled into cars, which may be hauled up inclines or hoisted up vtrticaUy.

Kill fioilding. — A mill should always he enclosed in substantial building, admission to which can only obtained at one or two points. In America structures arc mostly all wood, but this material, tbougli poasessitiK sonie advantages, presents great risk ou ire. An iron building is better, hut one mtb

Power 499

ck walls and a galvanized iron roof is better still. e system prevalent in America of building all the in one plane is also not to be recommended, as such oof is especially liable to damage from st-orms. It is iter built in three or four bays, which moreover give 3ellent opportunities for lighting and ventilation. e roof should be carried entirely upon the walls of 5 building, or else upon independent wrought-iron umns ; it should not depend for support on any rtion of the mill framing, which is certain to l)e ected more or less by the vibration of the stamps. is also a good plan in large mills to support the rails on which the crawls used in hoisting the stamps run,

means of the same pillars that cany the roof, instead fastening these to the battery uprights. It is well to ar in mind in designing a mill building that these lars must, if possible, be so placed as not to prevent e mill man from getting an uninterrupted view of thu ales from any part of the mill. It may be taken that B average price of a suitable galvanized roof with lars, &c., for a big mill, will Ixj between Is. G/. and 'la. r square foot, and its weight about 1 cwt. for each 10 uare feet of superficial area covered. Power. — It has already been ix)inted out how the iwer required to drive any given stamp mill may be Iculated. From this, and knowing the aptnoxiniate iwer required to work the various otlua* machines in e mill, the total power required may readily Ixj timated. Thus the power required to work a 40 head .ttery of 900 lb. stamps, designed to make 1)0 sevench drops jHir minute, anil liavifig a capacity of 1 10 tons

quartz iKjr 24 hours, will be as follows : —

K K 2

9D0 Gold Milling

Rock-brralcCT to sniBh 15 loni pet hour . . 30 I.H.F

lUaUiapskl 1-7 l.H.P. each . . . - . ,.

ISFnie vauiera Id ..

1 batss . , 1 ,

113 ,. Add S ]wr cant, (or friction of iliafting . . 6 .

This makes the uffioieiioy of the staiup mil) iilwl ton per I.H.P. pt!T 34 boura including anialgiunilui[(

This is, however, acarcoly a correct way of GSlimatiqi the efficiencj' of & mill, the only really satisfactory

of fti[imu.tLntJ the ufTcutiveness of any given uiocbiue Ijeiiig to culculate the quantity of quartz crushed pa I.H.P, per hour, exclusive of the power required to afMr treatment of the amalgam, 4o,

Thus for the above 40'stamp mill the amount of woii done would be :

1 I. Hp.

Total for craahing 2,800 cwt l,U32 ,, ,,

thus giving — l'45owt. per I.H.P. per hour. To

these figures 5 per cent, may 1x3 added as above for (rictioii of shafting, when we should have 2,800 cwt. worked for 2,039 I.H.P. hours, or 1-38 cwt. (jer I.H.P. per hour.

It has already been pointed out that the effectivs crushing power of the Ivrom roll is alwut 104 cwl, per I.H.P. per hour. For a Huntington mill including rockhreakcr it would be 1-0 cwt. jMr I.H.P. per hour live-foot mill oruahes 20 tons in 24 hours usiofl 12 I.H.P. + rock-breaker VI H-P. during 10 boura).

T

The >tainp mill thus setMiis to he the inosl enicitiit machine, even tiikin the fiires as here fjiven : these fcre. however, exceeded in some cases, seeinjj thiit in Iakota 4i tons, and in the Tmnsvaiil over 5 tons ix?r head per 24 hours are sometimes crushtnl with stamps similar to the above.

Water Power. — It has already been pointed out that the best way of supplying motive power is by water, whenever possible. There are various forms of motors in use, according to the nature of the water power available- Per high falls, the best motor is undoubtedly the Pelton Wheel or some wheel of similar type, such as the Kniiht, these being actuated by a stream of water impinin; from a nozzle against buckets attached to the periphery of a strong wheel. For falls of 100 feet, or more, this is certainly the best type of motor, and it may l)e used with advantage for falls as low as 50 feet. This wheel has the great advantages of cheapness and ease in erection, and a very high degree of efficiency is claimed for it ; it will perhaps be safe to assume its factor of efficiency at 0*75 in practical working. For falls of less than 50 feet the best motor is a Jonval turbine, and where considonihle power is required, a twin Jonval turbine, consisting of two wheels keyed upon one horizontal shaft, with a central supply pipe and each discharging in o])ix)site directions, so that the entire machine is completely balanced, presents many advantages. Not the least of these is that a portion of the fall can he utilised iHjneath the turbine by the employment of a suction tube, which must not, however, exceed al)out 15 feet in length, so that the turbine itself can be placed clear of floods as a general rule. If, however, the water supply is subject to exc(ssive variations of volume, a Girard turbine will 1h foinnl suiKricr

Jipi GOIJ> MILLING nut,

to the Jonval ; the forme' turbine will continue to wi whet) completely submergefl in water, and retiLtn!4 praodcally its full effioienejwhen working with n waler supply pvon very greatly reduced t>e)ow ttie iioniMl (jUMiitity. Turhiaeti oau l>e suoceRsfiilly employoii vbt-ni fftll of not inortt than 5 feet is avaiUhle, provided thtt there is u BiifTioipnt volume o( wntier to produce the poww required. Hrowtly htpcakiug, the efRcieuoy of u weil-constrnoted tui-hine may be put down at 0G5, or soiuewliat less in the onse of very low fulls. Another form of moWf sometimes used for driving small niiils is the Vorleii whel, which is, in fact, only another fonii of tnrfaiDe, rather less efficient than the two fii-st-nanied hut cheaper as to first cost. In order to determine what power can obtained front any given water sujiply, the total effective height and quantity of water available must be known, due allowance being made for the loss of bead caused by friction in the pipes, Ac.

Tjet n represent the number of cubic feet of water available pei' minute ; h the uet available height &her making all deductions :

Tlifii the theoretical horse power developed -.rs-ijin

aa.ooo

of efliciency of the motor selected.

Ill this connection it is well to remember that a miner's inch of water in California is about 1-6 cubic feet per minute, and that a Government sluice head iu AustraliB is CO cuiiic feet per minute. Table I. gives, in a convenient form, the values of the fraction ..o (uyi worke<l oul for A n\unlier of different values of and h.

Xiv Electrical Transmission 503

Electrical TrangmiitioiL — Of recent years great strides hare been made in the application of electricity to the transmission of power to considerable distances, and mining engineers have not been slow in availing themselves of the advantage of this novel method. By its means mills, a good many miles away from water power, can yet be worked by it, the only additional expense incurred in running being interest on the cost of the transmission plant, and the wages of a competent man to Look after it. Even with these additions it costs fai' less than does steam power, and has been applied with gi-eat success, notably in the Western States of America, the Transvaal, and in New Zealand.

The study of the details of any particular case of electric transmission is obviously a matter for the electrician and not for the mill man, so that only a few general bits on the subject will be given here ; for most of these I am indebted to Mr. W. M. Thornton.

The various systems of electrical transmission now considered practicable arc as follows : —

2. Alternating current, either one-, two-or threephase ; using with either of the two latter either synchronous or asynchronous motors.

Continuous current is not well suited to distanc(?s of over 1 ,000 yards, and is best restricted to half that distance ; the plant is simple in character, cheaper than for alt(;r- nating currents, and is well adapted for minute subdivision and for lighting pinpoaes. The motors will start at practically full load and arc* highly (iflficient : hitherto, however, it has b((>n found difVicuit to construct direct current dynamos to work at high pottMitials, so that this system can hardly be employed where any

t got GO/.D MILLING I

lor lUnmint of powor ban tn be trananiittod. Tn e tbui-efore, of ttonie very evident tulv&ntaf;e pofiiieHeil Itv ibi; u(iiilin)ir>nH cnrrBnt system, it is practically only nW wliisre tMiriipumtivttly low powers have to be tmosmilW (or short distiuiws,

This will rarely lie the oaae when stamp mills bavp t<i be electrically driven, so that most modem iitstalUtiaoi I employ altornating currents : electricians are. howerar, by no means agreed which form of alternating current is the most suitable under any given set of conditions. Tb' tbree-pbtiae system requires a smaller consumption d copper tor the conductor than do any of the othera, as shown by the following table of the ratios of the aniounl of copper in a line required to transmit equal currents for the same type of motor : —

fiiiiplo ]>liaNP. Two wires. Ratio of copper requiml 1 T-o ,, Four „ ,, ,, ., 1

Tno ,, Tlireo ., ,, „ „ O'SS

The two-phase motor is moreover more difficult to start than the three-phase against a full load. On the other hand, two-phase transmission seems to be preferred bv some authorities when the current has to be much sul)- divided and to be used for lighting.

In all systems the voltage should be kept high for tlie sake of eoono y n the I ne v e With alternate currents high potet t al ta s ss on s easily obtained even vnth relatively low potent 1 n t1 e dynamo and motor, by the use of stat o a y step pal step-down transformers- The cross s'ct on of t! e con 1 cto to be entployed can be roughly arrvd Ulytl se of Lord Kelvin's law thiU the inte est o tl e fi t co t of the conductor plus allowance fo depreo at o si ould equal the cost of tbe

Xiv Electrical Transmission 505

energy annually dissipated in the conductor." The voltage of a current equal to 1,000 amperes per square inch will drop 86 volts per mile, the loss varynn directly with the number of amperes per square inch and vith the distance. The conductors used in practice are usually between a No. 10 wire for 13 amperes and a 37/15 stranded, for 155 ampdres ; tables showing the conductors to be used for any given current and loss of voltage are issued by all makers of electric cables. Lines are best carried on poles 25 to 30 feet high, of wood or iron according to circumstances, with porcelain insulators ; in wet climates oil insulators may be used. The number of poles may vary from 30 to 60 per mile. In most countries it is found necessary to protect the hne by efficient lightning conductors ; a barbed galvanised wire, stretched a foot or two above the conductors and well grounded at distances not exceeding 200 yards, forms a reliable protection.

Generators capable of giving out not less than 1 K.W. 1*34 H.P.) for each H.P. to be transmitted should be provided ; a low frequency, preferably not exceeding 30 per second, is best for the transniission of power, whilst 60 per second may be looked u])on as the highest nunii)er admissible ; on the other hand, electric lighting is unsatisfactory unless the frequency exceeds 50 per second, so that it is rarely possible in long distance transmission to use the same current lx)th for power and for lighting unless a rotary transformer is employed. The voltage should be 2,000 to 3,000 for small powers, and 3,000 to 10,000 for large, but the modem tendency seems to Ik; to even exceed the latt(;r fijun?. There is also a growing tendency to generate tlu; current at the hi'hest ])ossil)le voltage, and only to use transformers when the (juantity

5o6 GOLD MILLING limi',

nf power to be transmitted nrthe dktanoe of tranfiniiMdon is (;reiit as to necessitate tension rA nvitr 5,000 volte.

-wniinil

"in'imvidi

Jill nf 1.000

o SO 11. P.

a.MO

:i,ooo fi.omj

The question at what tension n line sliniilil workii nrxl wlitithi'.r it iti or is not advantageous to nno tninsformers to attain liigh voltage depends on purely economic considerations ; broadly, it may be suid tliul the longer the distance the gi-eater the economy that can be realised liy the use of transformers and high Toltacs. The economy of oojiper in the line that can be thus obtained is shown by the following table of the number ol ainpdrps required to generate 1 R.H.P. at the motor piilify at different voltages, allowing a motor efficiency of 90 per cent, and a power factor of 0'75 :—

Of t!i{' VMiioiis types of motors there are two systems tliiit are employeil jii the tninaniission of ijower, nimiply, synchronous and asynchronous. The former i-equire a ilireet eiuTcnt exciter and commutator, will only start with ivlatively light loads, have ii s]>eed nearly constant under varying loads, and their power factor at full load is nearly equal to unity.

Asynchronous motors need no exciters, will start under full load with two phase and three-phase currents, but with single phase i-equire to l>e started by means of a sfjeoiiil airaiiyemeut of condenser, whilst their speed

Xiv Electrical Transmission 507

clecreases as the load increases. Their power factor at full load is between 0*65 and 0*85 according to size.

Each type has accordingly its inherent defects iind advantages, but upon the whole the preference seems mostly to have lain hitherto with synchronous motors, the higher power factor being apparently considered by most authorities to compensate for the additional trouble in starting them and getting them into step.

Numerous examples of electrical installations for the purpose of driving stamp mills might be quoted, chiefly from the Western States of America, the Transvaal, and New Zealand, as well as other localities.

The Sheba mill of 60 stamps has been driven by electricity for some three years, and the same power has recently been applied to a second mill of the me size, auxiliary steam power being also provided. The 60 stamp mill was estimated to require 162 H.P. to drive it. The power station is five miles from the mill, power being generated by two Victor turbines capable of producing together nearly 400 H.P. The generators are two-phase alternators with exciters capable of producing 150 E.H.P. at a pressure of 3,300 volts. Tlie armatures are stationary 54 inches in internal diameter by 12 inches wide. The field magnets with 16 poles nin at 400 revolutions per minute, giving a frequency of 53 per second. The conductors consist of three concentric cables laid in a trench nowhere less than 3 feet deep. Each cable contains two conductors each of 19 wires, 0'057 inch in diameter ; the resistance is 88 ohms for each five mile length of double cable. Four 50 kilo-watt transfonnors, nnlucing the tension down to 100 volts, an employcMl. The motors to drive the mill are 50 H.P. motors of the induction type,

LijeS COIM Mll-UNC nrr.

worVing with Iwo-plnwH eurrents at 100 volts iK>tiitial: tbey nin tvt WO rvoliitiori8 pnv minute. craalii-js are driven by two Ifl H-P. itiotom, atifl tlio vnnpr bj' one of tba Mune powor. The efficionoy of the mill moton is iM'-T tx'xA tit full Imui. Tlio cost nf the entire [nt will) iuli]itioii8 tliiit i.'iiitliliKl H tmiismit 3.50 H.F. Bectna to have bev.ti ulmut £35,000, and in 1897 Mixv. mat of electric power and IranHtniafiion per ton onisheti wkh AS follows : —

Totl 2 9-J57

Tills cost is Miid to have lieen reduced since then.

At rilcriiii's Rest the Transviuil Gold Estates have conijileted ii liir;jo electric installation; here Girard turbines drive three-phase dynamos and generate the current, which is transmitted at a potential of 3,000 volts; this is converted by means of step-down transformers into low tension (120 and 220 volt) currents, hy means of which, ainon; other motors, a 65 H.P. three-phase motor driving a 20 stamp mill and a similar 100 H.F. motor driving a 60 stamp mill are supplied with power. The latter motor was foimd caiKihle of workinj* only 50 stamjis, but in all other resi>ccts the installation is ()uiU.' successful.

At Bodie, California, the Standard Mining Company is working l>olh mill and mine by electricity transniitteil for a distance of 12?. miles. The generating plant consists of a Pellon wheel coupled direct to thfe armature shaft ol a 120 K.W. alternate ciirrent single phase eititor rinining jitH6fl revolutions per nnnutc ; the potential is alwut 3,400 volts. The conductuv consists of a double

Xiv Steam Power 509

line of wire 0*29 inch in diameter, carried on glass insulators, the poles being set 100 feet apart. The mill, which contains 20 stamps with pans, vanncrs, &c., is driven by a 120 H.P. synchronous motor, a small 10 H.P. Tesla starting motor being used to start the larger machine by means of a friction pulley. But little difficulty is experienced in starting the motor and getting it into step. The motor also runs at 860 revolutions per minute, the speed being reduced by means of a single countershaft to 80 per minute. The efficiency of the generator is 95*5 per cent., and of the motor 93*9 per cent., whilst the line loss is between 8 and 9 per cent. The total efficiency of the system from water power to motor pulley is therefore about 79 per cent. Tlie entire cost of the plant was $38,000 (say £7,700).

In 1896 it was found in California that the average cost of installations transmitting over 1,000 11. P. to distances of between 13 and 25 miles, ranges between $100 and $140 (£20 and £30) per H.P., the charges for maintenance and working (by water power) varying from $13-50 to $30 (£2 16s. to £6) per H.P. per annum.

Mills are electrically driven at several mines in New Zealand ; it is said that the first battery in the world to be worked by electric transmission was at Skippers* Creek. All the New Zealand installations are, however, on a small scale. In the Transvaal there are electric transmission plants, in addition to those already referred to, at Moodie's, also driven by water power, and a large plant is now being contemplated on the Witwatersrand to transmit 10,000 H.P., the generating station being steam driven in the last named.

Steam Power. — Even where good and cheap fuel is available, this is far the dearest method of diiving a mill,

5ia COLP miUyC a

as will have already appearod from the statistics given, large number of mills are worked with wcxxl as (i and lion ihie can lie obtikiiiod of good quality al jirii ranging from 10s. to &1 per cord, as it can in many mini diBtricts. no lietter fuel can be desired. In tropical con tries, liowever, where a large proportion of the rjipiiil grown wood is soft and worthless for fuel, the price frequently much higher and is often tliu most cxpr.na item in gold milling. Thus at tliu £1 Callao Mt in Venezuela, the cost of tire wood was £2 per cord. the Johannesburg district coal is available ut it n of about 18. to 20s. per ton, but its quality is rallicr poi eioini] of it having a steam raising power of less than or half of that of good Welsh coat. In the case of small mil that Ciiiinot afford a firet-class engineer, it is probal advisable to lay down a good simple high-pressure engii of ample power to do all the work of the mill, and with boiler somewhat larger than is usually considered sul cicnt, MO us to ensure an ample supjjly of steam ; e.g.. foi 10 N.H.P. engine it would be advisable to lay down a N.H.r. boiler, more especially when inferior fuel has to employed. Large and well-equipjjed mills should ho ever hu driven by engines of the most approved consln tion of the compound high-pressure type, with a go economical cut-off; condensing engines may in sot cases be used with advantii.gc. An ample battery Ixiilers should be provided, of such capacity that the may always l>e one standing idle, in order to enable all Ik; kepi well cleaned and in good repair. As a roU( general rule it is usuiil to provide an engine of such si as to give one nominal II. P. for every stamp; c.if., a N.U.P. engine for a 20 stamp mill, Ac. If the engine ar boiler are good ones they will reiwlily give an I.n.P. ihr

Xiv Ill Umtna Tion 51 1

times as great as their N.H.P., and this rule accordingly gives ample power for driving the stamp-mill, and all other machinery that may he required. In large mills it is well to make the engine house an entirely separate huilding from the stamp mill, so as to lessen the chance of grease or oil finding its way from the engine to the mill. For large mills, whatever kind of motor be employed, cotton ropes are to be preferred to belting for chiving the first motion shaft.

Ulumination — It is impossible to expect good work to be done in a mill unless it be thoroughly well lit, so that the mill-men can see what they arc doing. A badly lit mill, moreover, gives oppoi-tunities for the theft of amalgam that would not occur in a well lit one. There should be plenty of windows in the ends as well as in the roof of the mill, and lighting on the night shift should receive careful consideration. There is no hotter method of hghting a mill at night than by moans of electricity, preferably by powerful (50 to 100 c.p.) incandescent lamps. Not only does electricity give a powerful light, but it has also the immense advantage of not requiring any oil or grease about the mill. As rock-breakers are, generally speaking, only run during the day shift, the pow-er thus set free at night-time can be used to advantage in running a dynamo for electric lighting, for which purpose adchtional ix)wer would not therefore need to be provided.

When oil lamps have to be used they should he larg( and powerful ; it should he one man's business to take them down in the morning, clean, trim, and fill thiMn, and put them up in their places again before the night shift comes on, and a special place should he set apart for this work ; in large mills, where an oiler is kepi, ho

GOLD MtLUNG oM.

attend to th Umpe, By this means tbt kiak of oil finding it6 way into the tnonar ix>ses or on loihe jhitm deeidailty lessciunl, and it is always au advaiiU; lo allow only one nuui in the mill to handle all the oil and grease, and not to give this man any other work lo do that will take him near the copper plates or the meruur). iU ShertB. — It is all important for the successful ruuoit of a mill tliat proper complete statistics be ke[A. For this purpose a slale should be kept hanging iu the batter-y- bouse upon which the suptirintendent or head aoialganiator can write his instructions, and the mill mwi in charge ot (he shift can note down all necessary obserrations thai will ratable turn properly lo fill Qp Itia 1 mill-sheet. This he should do every shift, or else in some milU V. is the duty of a clerk to come round and copy out the tigurea on the slate for the past twenty-four hours, aod then to put them into a mill hook. The ("illsheet should be ruled for a month at time, and haTe headings somewhat as follows : —

Culuuiiis may itiso l)e added for the number of concentrulurs at work iitid the quutitity ot uonceutrates produced diiily.

Iv Mill Sheets 513

The following is the form of till sheet recommended y Mr. Nicol Brown : —

Woke Doki Fob The Mokth Op

[e adds that the screen used and the height of discharge lould be noted once on each sheet, and any change in le same should be recorded under " Remarks." A useful check upon the mill men consists of a reilution counter attached to the cam shaft. This enables le superintendent to ascertain at once what amount of ork the mill has been doing during each shift and to leck the speed of the mill and length of stoppages as sported by the mill man. In addition to the millleets, a separate mercury book should always be kept I already recommended, and the results of each cleau- p, the total amalgam produced each month, and the eights of sponge and bullion obtained entered in other book kept specially for that purpose. It is only the careful collection and comparison of accurately orded statistics concerning all the details of the orking that the defecta of any system can be ascerined and improvements introduced.

Chapter Xv

SJkUrUNO AND ASB4TINO OF OKB, TAIWNOS, CONCKKTRITES, AKD BULLION

It is oticn pftrt of a mill man's duty to nssny tlic ffl whicli he if) caliod u[)ou to treat, and the bullion, conccntiutea, and tailings which he produces, so aa to chcdi tiw results obtained in the mill. In laigo mills there is auHicient work to keep an assnyer contintiously engaged. but in smaller ones it is part of the chief amalgamator's duties, and in any case this is vrork that he ouht to Ik !hif to perform it necessary. An assay office is by no DitNins a enslly adjunct to a mill, and it is a most im|<ortAut one, as without it scientiRc work is qoil inipoasible. and in its absence serious losses of goM from pn-vi'i!lwble causes may go on unchecked liecanw uiidisoi'vitrvtl.

AlMJ OAh. — Thb should be near the mill for tlic sakt> of convenience, living only so far off as in)l ii> I*' iitVfclfd 111 hU by the vibration of the stamps. It sh.'uIdo.>iisist of lliix'c rooms: an outer room or shed for siiiupliufi nml rough work ; a furnace room with w.irk IviU'h :mil lunple space for the storage of crucibles. cb.'inionU, Ac, jinil an inner Imlance ix)om. The furnace iwm should U- titled with a sink and water supply, w"l

'"Mm. w rrhw.ici-s

Well-ventilat'jcl draulit clianibur for the ciinying oli' ol

The Sampling Shed should have a well-laid floor of either concrete or stout plank, and in the middle there ould be a stout iron plate about 4 feet square and i inch thick. There should be a large mortar about 2 feet high and 8 inches internal diameter, with a pestle about 4 feet long, and weighing some 50 lbs. ; this is advantageously suspended from a strong spiral or other spring attached to one of the rafters. This large mortar may be supported on trunnions on which it can revolve, being kept in place by a couple of folding wedges. There should also be a couple of smaller iron mortars and a couple of Wedgwood ones, one large and one small. In large mills where big samples of ore have frequently to be examined, it is advisable to have a sample grinder driven off some portion of the mill-shafting in a corner of the mill building, as much time and labour can be saved by it. Various forms of sample grinders are made by makers of mining machinery.

Furnaces. — Two kinds of furnace are needed in gold assaying, the ordinary wind furnace for fusions, and the muffle furnace. The best form of wind furnace is shown in Figs. 122 and 123. It is practically a short shaft about ten inches square, lined with fire brick and covered and held together with iron plates. At a depth of about 16 inches below the top a couple of bearer bars are built into the furnace, upon which the fire-bars are supported. These may either be of cast-iron or else of 1 inch square iron bars QJ inches long, laid about i inch apart. Below the fire-bars is the ash-pit which is closed by a light iron door having a sliding shutter by which the admission of air to the ash-pit may be controlled. The furnace flue

GOLD Mll.UNG

altould be about 3 inches by 4 in area, with a sliding a ii-oii damper. The; flue opens into a short stack ab

Vertical Section

20 feet in height, with which the other fumaoeK also ooinmunicate. The cover coneists of two fire-briolE tilM

Xv Furnaces 517

Htrapped with iiOB. Instead of the above square furnace which has to be built on the spot, the Black furnace may be employed which can be bought ready for aettiiiR to work. The Black furnace is well adapted to the work of the gold-assayer, except that the cover usually furnished with it is quite unsuitable. A piece of boiler-iron about 2 feet square should ;have a circular hole cut in it equal

Section Plan on X.Y.

to tbe size of the fumade top, and by means of three iron brackets this should be riveted to the furnace so as to form a firm level top ; upon this can rest a cover like that used tor the square furnace. The thin sheet-iron stove-pipe usually supplied with the Black furnace is far too light and bums out rapidly ; the first lenRth of 5 or 0 feet next to the furnace at any rate should he made of

5l8 GOLD MILLING cnr.

Stout iron inch tbiok, and the elbow may vjth idvmititge be even thicker. A circular divnipor working on 11 pivot should be insei-t<M( in iLl pipe for controlling tlip drauijiht.

The best fomi of inutile furn&ce is that shown in Fi{!. 124. It is iimnufactured by the Morn Crucible C<miptiny, Liniitet], of Bitttoreea. The Dioet ieuenUly usedil ais is that taking' a. muffle size (8i inches x h iuolie X 3J inches). This furnace should be eel upon u brick pier of such a. height that the inufite itself is it litit< lower than the height of the nssayer's shoulder. U sliould he furnished with a pipe of stout ahoel-iron wl'it'li connects it with the stack, or if preferred it may have ita separate stack in the shape of an iron stove-piptr nliont 10 feet in height. If preferred, a muffle furnace mny be built on the spot. It is ahajjed like a shallow Bfiimre mciliiif,' furnjice wliicli lias been raised some '6 feei from the ground, and which has an opening in front some "J inches aliove the fire-bars, which corresponds to tlie inmilh of the nmfilo.

Balance.— This should he a first-class instrument, siicli iis is constructed expressly for ''''-''ssaying. It need not he a lai-ge one : one capable of carri,-ing 30 grains in fiieh pan iind, when thus loiwled, of turning distinctly with 1)0005 grain is sufficient for all purposes. A portable assa\' haliince, manufactured bv Oertling, may also he usi'<l : it pi-esents the advantajfe of unusual compwtness. of rold assay weights from 20 grains donn lo 0-01 grain, and two ridei-s for sliding along tlie rii.hiiit.-d iH'uni weighing i-e spec lively O'l and O'OS grain AW required. In gold assaying it is advisable to uw oilier standard of weight than the troy grain and tlie ti>.y ouiu-e, .\s all hulliou has to l>e weighed by tlio

r unit, ri 13 best not to attempt to use any other in c assay office. Not only are calculations facilitated by

lis means, but confusion and possible error are avoided.

be advantages claimed for the assay ton system of

J

jao GO/.D MILLING

weighing are in reailty almost imaginary, seeing that in the syBteru here room mended, a simple reference lo tile tahlt-H Riven further on, mid which shoiilil be liun u|) ill every assay office, avoid the uccessily foi" mj calculation.

No description of the assay balance is here given, U the ttsaayer will learn more about its construction in fiw niinutoa' intelligent examination of it than he cooU from many pages of letterpress. Moreover it may bo taken for granted that none will commence assaying without the benefit of a little practical advice from an experienced assayer. The following hiutn as to tbtt miuiageuieot of the balance may however be useful.

TIkbiiiiince i-ase should ahv.iys be kept closoi "lirii not in use. A dish containing a little quicklime should be kept inside the balance case, and renewed from time to time. A camel-hair brush should also be kept io the balance case for dusting the beam and pans when necessary, tlie brush being used for no other purpose than this. The substances to be weighed are always put in the same pan {usually the left). In weighing a button of metal this should be put into the left pan, and a weiglit supposed to be greater than the weight of the button placed in the opposite one ; should it prove to be too small, a larger one must he tried until a weight is found greater than that of the button. This is then replaced by the next smaller weight, and so on until it is found between which two consecutive numbers (being integers of grains) the real weight of the button lies. Tlie sniuUcr number l>eing then left in the pan, tbe tenths of a grain weights are then tried similarly in descending order, and the first decimal of the required weight is thus determined, the remaining two figures

Balance 521

obtained by sliding the O'l grain rider along the kted beam, when its position, as soon as the e is in equilibrium, will give the number of hun- s and thousandths of a grain that go to make up tal weight of the button. When very minute s are being weighed, as often occurs in tailings , it is advisable to use a rider weighing only 0*05 md to divide the result thus indicated by 2 for the eight of the button. In bullion assays, it is advisweigh by substitution, placing a constant weight than that of the body to be weighed in one pan, e body itself in the other, weights being added to ter until the balance is in equilibrium ; the diflferetween the weights in the respective pans is then dght of the body. When the balance is nearly in irium, the oscillations of the pointer on either side zero mark of the scale nmst be read ; when these iial, the balance is in equilibrium. It is never safe mne that the balance is in equilibrium because the when released does not incline to either side ; it be caused to vibrate, and the amplitude of its ons read on the scale. In using the balance care be taken that the beam is released and arrested , without any shock or jar that might damage delicate instrument. The weights nmst never be d with the fingers, but always with the forceps ly provided for this purpose, and which must be )r no other.

weighing out ores, &c., a pair of scales is required, tlie pans of which should he moval)le. The pans not be less than 4J inches in diameter, and the should indicate distinctly grain when loaded 000 grains. It is useful to keep a strong set of

common scales or & eprins-balance neiin. y. to 56 in the aampting room, tiuil us s scnral nde ifae tmllitin Wknuu b kupt in the IhUiukhi luoni of thf a office, tlie assay (umace being Q&eA lor melting dovn ihe goUl sponge.

Saapliiig,— This is one uf the most importnl cfientioos in making assays, and one that ia too freajutill; neglected, .n asaayer may ba%'e to saaiple ; —

(a) The ore as it coiubs the mill.

(b) Ore already estracted, and lying in haps or liiiu, (r) Ore standing in the mine.

(a) When the ore is received at the mill, a shovellnl tiioold be taken fay the rock-breaker man from the can as lliey come from the mine, with a frLHjueney depetidina on the size of the cars and the size of the sample required. Thus a sample may \ie taken from eviiy other car when daily assays of the ore are made, or from every tenth car when only a weekly assay i desii'ed. It lit, perhaps, better to have a shorelful taken ont of the ore feeder hopper at stated intervale, as liy this means a fturer sample nf large and small ore is obtAine<l. In either case all the ore so taken out thrown into a bin or piled in a heap in one comer of ll mill floor.

(6) It is scarcely possible to get a really fair averafit sample from a large heap unless Ihf whole heap i- turned over. This can but rarely be done, so that the Ih'~i plan i-nsi<:ts in cutting a trench about 2 feet nide i'ii:ht ikcross the heap from side to side, and throwing all thf oiv that comes out of this trench into a heap. If an itvft-ige Siunple of several piles of ore has to he taken. e;uh one must be treated in this way. and all the ore so ohijiiiuil thrown together.

Xv Sampling 523

(c) It is sometimes necessary to take a sample from the face of a level or stope in the mine. When this has to be done, the floor should be fii'st of all cleaned up as thoroughly as possible ; ore is then broken down by means of the pick and wedges from the whole surface of the face, taking care that it is taken uniformly from every part. All the sample thus obtained is trannned out and piled in a heap.

The result of any of these three operations will be a pile of ore that should fairly represent the average composition of the entire mass that has been sampled ; it should in no case be less than half a ton, and may amount to several tons. It is first broken down to pass a 2-inch ring if the sample exceeds a ton, or a 1-inch ring if under a ton. It is then thoroughly mixed by means of a long-handled shovel and made into a conical heap ; the top is then flattened down until a circular heap is produced, having the shape of a short truncated cone. Tliis heap is then divided into four equal parts ; this may be done either by marking lines on its surface by means of a pick, or by laying on it a cross-shaped piece of wood, each arm of the cross being of the same length as the radius of the bottom of the heap. Two of the four quarters thus formed are then thrown aside, either of the diagonally opposite pairs being selected. Thus, in Fig. 125, either a and d or b and c may be thrown aside. In the half thus left, all large pieces of ore are again broken down, tli( heap is thoroughly mixed again, and the above operation of halving is repeated until the pile is reduced to 2 or 3 cwt. Tiie rest of the ore is then returned to the ore bins, and the selected heap is sent into the sampling office. An old die does very well for breaking the ore on, a 4 lb.

breaking-hamuier being used. The above opeislim may be performed by a mechamc&l sampler ntid breatoi ftnil subdiviilot; a pnrcel of oru into any i!m nuiwlier of iiqiial pailrS, or tbp breukiri); may be done fcj means of a Rniall rock -breaker, the samplinj; beinfi stilt performed by hand. The sample as sent W tht sampling -houiie io then broken down iu the bkre mortar, and the Buiiie opemtioii of (juartering Afva ooDtinued until a sample weighing about 7 lbs. is produced. The sainple, which should by this time he onished tine enough to pass a sieve of H holes to 'Cat linear inch. still furtlier reduced till u fair sauipW weighing nlKiut half h pound is obtained for fire aswy. remainder of the 7 lb. sample is kiipt for panning and amalgamation assays, if these ore needed. Tbey ore not as J rule required when ore. the character c( which is well known, has to be assayed, but all new samples should be subjei'tci to them, be/ore liie fire assay is pi-ocwde'l witb, UK very much valujible information regarding its chiinittLT is thus obtained.

Sampling TailingLIt is of the utmost importance that constant and careful assays be made of the tailings as they rim to waste. For this purpose a convenient spot slionid lie arranged in the waste tailings launder, where a drop of alxiut a foot can l>e given so as to admit of placing ii bucket, or l)etter still, a small square bos wbicb exactly fits the launder beneath the issuing stroiini of tailings at stated intervals. This vessel should be filled but not allowed to overflow, as in this case more or less concentration of the heaner part of the tailings would ensue. Such a sample should be

Xv Sampling 525

taken at intervals of say three hours, and all the pulp thus obtained poured into a tank or tub. The product of each day's work is allowed to settle completely, and when the supernatant water is perfectly clear, it is syphoned off. The settled pulp and slimes are then scraped or rinsed out through a plug-hole in the bottom of the containing vessel into an iron pan, in which the remaining water is evaporated off so as to get a perfectly dry sample. This is then thoroughly mixed and taken to the sampling room, where it is divided down just as alrepy explained in the case of ore, until about lb. is left, which is then ready for fire assay. If preferred, the tailings obtained throughout the week may be mixed and a fair sample taken of the parcel so obtained. Assays of tailings should always be made at least once a week, and in large mills every day. In some mills a simple automatic tailings sampler is used, which consists of a short length of launder pivoted under the main launder ; in its normal position this short movable launder discharges into another long one, which carries the tailings away to waste. The movable spout is so connected either with a clockwork arrangement, or else with a miniature water wheel driven by the current of the tailings themselves, as to be deflected at given intervals for a short period, during which it discharges the current of pulp into a suitably placed vessel. Another ingenious tailings sampler is on the principle of the Barker's mill with a number of spouts all of different lengths, only one of which can deliver into the vessel in which the sample is being collected. The further treatment of the sample of pulp so obtained is the same as when the sample is taken by hand. In some mills samples are also taken of the pulp as it leaves the

ployed, and tf

W4 Weil as M I to control ttM here be slaWJl

Liallcry scrvens by [tassing a small buuknl nlont: U front of the lip from end to Bud and thus cntchingi portion of the issuing pulp : here again care niut b taken to avoid any concentration of the hea\'ier portions. These samples give but little intnra ation when inside amoigiunnlion is employed, and hence not often token. A better practii the pulp as it runs to the concentrators, tailings after they have left them, so as cRiciency of ttiese machines. It may here ibal assays of tailings should l>e supplemented by thut ocvaaional examination under the lower poWi.'rs ol iniorascope, in order to sec, if possible, in what fottna llie gold is l)eing lost. Sometimes it becomes necessarj til sample heaps of tailings : this may he done in tht Slime way its described above for heaps of ore, namely by cutting a trench through the heap and sampling down the material so obtained. Heaps of tailings oi tailings settled in tiLnks or pits may also he sampled driving iron tulies with a lower cutting edge through tin mass from top to bottom at uniform distances apart alon) lines right across the mass; the cores of tailing>i It'F ill tli<' L;iIh;s form the sample. Files of concentrates an siiiiipled in the same way, hut as it is a difficult mattei to obtain gootl representative average samples of these and as their value is usually high, it is better to takt two quite independent samples from each heap. sample these down and assay them quite independent!} i>r each other imd t-tien take the mean of the two R'sults

Ore Auayi.

Amalgamation Assay —.A convenient quantity for oper Hting on is from two lo three pounds. A good plan is

Ore Assays 527

e a weiglit equal exactly to 224 lbs. (avoirdupois), this exactly one-thousandth part of a ton, unless reits are to be returned by the short ton of 2,000 lbs., rben 2 lbs. will have to be taken. This weight of 2 24 L is obtained by placing 2J lbs. (avoirdupois) in one le pan, and 70 grains (troy) in the other. Shot is ten poured into the second pan until it exactly balances, Vten the weight of the shot wiU be the required weight ; " iifc if preferred, a leaden or brass weight may be thus pre- 't. spired and kept for use. This quantity of ore is then ireighed out It is crushed in one of the smaller iron ; mortars and sieved through a sieve of 60 holes to the "!? linear inch. The best sieve for this purpose is a cylindrical box sieve, about 5 inches in diameter and of the same depth, fitted with a proper cover to prevent loss of ore. If the ore contains coarse gold, this will not pass through the sieve, but will be left on it, as it will flatten under the pestle instead of being crushed by it. There may also be left on the sieve a few pieces of iron derived from the pestle or mortar, which may be removed by means of the magnet. It is advisable to heat these metallic residues with a little dilute nitric acid in a small porcelain basin, so as to remove any adhering crusts of oxide of iron, Ac, which might prevent amalgamation ; the residues are then washed two or three times by decantation and thrown into the parcel of crushed ore. This is then transferred to a Wedgwood ware mortar (or, if preferred, an iron one can be used), of about 12 inches in diameter, and mixed with enough warm wat(?r to make a stiff paste. About an ounce of dry mercury, thoroughly purified and free from gold, is then carefully weighed out ; it is wanted in a small hard-glass flask and a piece of clean sodium about the size of a pea thrown in.

Ab aanw with coplant stirring, tlie i atmy. riMWil oA wilfa mrter. eo Uiit uM I

Ida. entire mam is then frronsl I

faonr. when malganMUon sliould hi 1 . Dw paflle so farmed is ihen Uvor I

Ibar loU to prospectiDR-pan, ul I I np BSiD k rob in nliich the uMngfi Ui Hie mmnoij is eoUcted uul tnmsferred b ilk: 8 it K Bocfa floored and refuses to mto (iiiliili I it inar he slund wiib a small pit ' IB IbU IB the end ot a lass tube, which trill tmmm in to iwi loylhw. IW iDereuy is Uien waalnsd ' mih Gijhj stream irf clear waU'.r unlit it is qoilc clcun. the waair [KiorfJ off iUid jhe mercury driwl by means of I'k'icir.i: pwper. Ii is ihen revveigiied, when its loss of wcL:;b: >htHiiJ not t'seeed 5 per cenl. If it is greater tlian t!:;-i, chf laLlinpj must be panned repeatedly until tlie excels oi is made good. If this cannot be done, the a>iiiy mu>l be rejected and a fresh one commenced. The mervun.- is next tmnsferred to a small Rold annealim; cup wiiioh has been carefully black-leaded inside, ciiverei! with a porcelain or fire-clay cover, and the mercurs' evajKirateil off at a gentle Iieat. This mav be done by placiuy it in the mouth of a muffle, when the fumes will l>e ihawn up the stack, or else in a light sheet iron retort made expressly for this purpose. When all the niecurv is completely volatilised, about 60 grains of assav lead are tbiown into the crucible; this is then pushed a little way into the muftle so as to melt the lead; tlie crucible is then withdrawn, its contents mixed by giving it )i gentle rotary motion, and poured into a small mould such lis is used in fire assays (see page 537). The

esTilting button of lead ia then cupelled, as will he subserviently explained, and the button of bullion so obtained weighed. It may be assumed, without sensible error, 'bat the mercury lost in the operation carried with it same proportion of gold as is contained in the mercury "hich is recovered. Hence the gold contents of the ore "TiU be found by multiplying the weight of bullion <4>tained by the weight of mercury originally taken and hiding the product by the difference between the ight of mercury recovered and the weight of bullion foimd. This figure multiplied by 1,000 gives the richness of the ore in free gold in grains per ton. Thus, suppose, for example, that a lot of 2-24 lbs. of ore was thus treated with 450 grains of mercury ; after amalgamation 442 grains of auriferous mercury were collected which yielded 10'02 grains of gold bullion. Then the amount of amalgamable free gold present in the ore per ton of 2,240 lbs. would be :

oz. dwt. grs.

Panning Assay. — The prospecting-pan is too wellknown an implement to need much description ; it consists of a pan with sloping sides and slightly convex bottom, one or two grooves being run round the upper edge of the sides, which are turned over so as to stiiTeii and strengthen the pan and to enable it to be handled conveniently. The tinned iron pan of the Australian miner is of no use to the mill man. These pans should he made of thin sheet steel or of first-class Russia ahovt iron, having a highly planished surface. They should ho perfectly smooth inside, and kept free from rust. Pans are often used for panning up sands containing amalgam or mercury, and it will be foimd very difficult to freo

M M

com MILUSG

tst tffxa ftil traces tjt mercaiy except by be&t. 1: AairfMa rfiiliiii id kaep a ecud set of puts fm fHMHf aB aataiil ogotainhie mnviiTy. aoai to Timpm thea dkariy bam the (Kbiir& which miKt and iar Am fyc. A Rood pUn ia to ok pui vilk Mm giiMNet far Uw [ovumbt. kmI pans vita on ynwe i— i their nppix edge for painung ttwtefnl (m fc( Pm of two or three diBenat aaci AoM kf It bud. A mool uofo] adjttaci to Uw fMw is faMM (txit to he eootaaoded with tlw meAmini Jmtm, pi0B 431) wfaidi u shallow woods harag the ihafie of ft naall aqpoent o( a splie AiBrikK. It M amit of faazd wood cot acm the ya id it upDcialh' bm<b1 m miAung ores ctmuining lac flD gM tcst fairly dindeii alpfauiM*. Tte tarn h— ry aaainil cimes to the oompantrrdy ivOfih aifatte lh vPOOdL aod is more easUj caught in the hMM ifeiB Ite vbU fan. Tbe hatea Aoold ncm tv Jcff music] carrcin meinirr or amslgaii). Fof ucl t±>e bon: b weneCUDes used : this ooDsUts of a deep eat cat of a large ox-hoin, in eaoly and lajwdly perfonned. frnra enubed ok. Uw \'.,-.:t.>: Ttr-.iT-.j:-*ix>T<J may be used, or else the white :' .iir:':-.tt<}. rukqoe masofactnred for this porpose V:-*i;rv FTkJiK K>i Qialmers. Limited. The mode of ."li liMSe insmimcms is very difficult of descrip- i.'ixT. (CdpiriKrely e*sy of execudoD, and ia- \-,ii;> k i.TA.-i liiii e±n ooiy lie aoqtiired by practice. ii ,i: inipiUTjUK* to the toiil man to be i.v.:-,xji>. V iSiwJi ia their tise, and he should lose no ii'ttv'>:i"~-i' v' wckini: with them all on til he bu ihr For a panning assay the same weight

Xv Panning Assay 531

of ore should be taken as for the amalgamation assay. This is then pounded in the mortar and sieved through a sieve of 40 holes to the linear inch, any gold in the residue left on the sieve being set aside. The pounded ore is then panned, the tailings being allowed to flow into a larger pan and panned again so that nothing may be lost. This operation will show at once whether the ore is rich in sulphurets, and what their nature is. The visible gold should be panned as free as possible from all adhering sulphurets, taking care, however, that none of it is lost. The pan is then dried and its contents, together with any coarse pieces left on the sieve, brushed into a piece of lead foil ; this is rolled up, melted with a Httle assay lead in a scorifier, and then cupelled ; these processes will be presently described iu detail. They result in the production of a button or prill of gold, which can be weighed, and thus the free gold present in the ore determined. The taihngs produced in the panning operation should then be panned over two or three times to collect all the sulphurets present in the ore. These should be dried and weighed, and their percentage in the ore determined ; if weighed in grains troy, this calculation is easily made by remembering that the pound avoirdupois contains 7,000 grains, so that the amount originally taken was 15,680 or 14,000 grains according as the long ton or the short ton is the standard of calculation. These sulphurets may then be assayed by fire assay to determine their value. If preferred, the tailings from the amalgamation assay may be panned up so as to collect the sulphurets present in them, which may then be dried, weighed, and assayed. As, however, some of these are apt to be slimed in the process of amalgamation, the results are less rehable

M M 2

GOLD .WlJJifC

limn vAua BepwMe panning test is init. AutAn I mMliod consisU m not separating the free gold from ti sulptiDivtK, tint in IrvnUa botti to);etlir by &iv aihI tbilMiinning Uir ItiUl goUl pi-eaL-nl in t)ium.

Pin AHsy.

7'ikci>ry o/ fin Jxtoyi. — Let it be supposed thut n 1 to opcTftte on ui ore consisting only of pore iBa 1 and (nw gold, both in a fine state oE division. If Ibis or? be mixed intimalely vitb carbonate of sodfi, litharge, and diannal in proper proportions, and tbe roiitore hiatrd in a eraciblp to tfae fusion point, the following n-aclioQS would lake place : —

1 The charcoal vrould reduce a portion of the litharfie metallic lead — it being taken for granted that not enough charcoal b added to reduce tbe whole of the lithAn:e.

± Pan of tbe silica oa]d combine with the unreduced li:hiiriw lo form silicate of lead.

3 Tlie excess of silica would combine with the soda of the ftirboiiale of soda to form silicate of soda, carbonic acivl tviit; evolved, and any excess of carbonate of so<iii rynM::',i!:i: Hrchanpevl.

t. The U'itit pr\Hfuc*d would alloy with all the melallic sold pr'sent. The contents of tbe crucible would thereicrt' .ilrin: jtcly form two layers : a tower one of auriferous iiA.i, .K'-d :i'a upper one of slag, consisting of silicates of xxit .i::,l .1 jd, ii.ii:ether wiib, perhaps, some carbonate of Ii: I'raotice this latter would attack the crucible, :,'ri--'.:ni; Ai*o silicate of soda. As a gener rule, 100 4;n4::'s o! lithar' re<iuire for their reduction about 3-5 titu)! idii)ary cbarcoal powder. SiUcates of varying

Compositions are produced when different proportions of

Biljca and carbonate of soda are melted together, but it

iHay be taken that to form a readily fusible silicate, 100

grains of silica require about 150 grains of carbonate of

soda.

Gold ores, however, almost always contain other ingredients besides gold and silica, the most important of ihese being oxides of iron and various sulphurets. The oxides of iron are mostly the peroxide FejOs, but sometimes magnetic oxide these both, when fused with silica, form silicates of the protoxide, so that in the case of the former oxide, one equivalent of oxygen (16 parts) is liberated by every 160 parts, and in the latter by every 232 parts of oxide. This oxygen will oxidise a certain proportion of the carbon present in the crucible charge, so ttiat for every 100 grains of peroxide of iron present there will have to be added about 7 grains more of charcoal, and for every 100 grains of magnetic oxide about 5 grains more. When sulphurets are present, both the sulphur and the metal exercise a reducing action, the former being oxidised to sulphurous acid, and the latter to a protoxide, which combines with silica to form a silicate. In the case of iron pyrites, which is the most plentifully occurring sulphide, 100 grains of it would reduce no less than 930 grains of litharge, thus taking the place of about 32*5 grains of charcoal. When there is much pyrites present in the ore it is advisable to substitute red lead for litharge, the former having a high oxidising power ; 100 grains of pyrites reducing about 710 grains of red lead. There arc, however, various other methods by which this difficulty can be overcome.

Preparattcw of the Ore. — The pound of ore which

lutf bETt tiie okimAte revolt of sftmpLuig dowc. ta pefioj dMUibwI. is fxxoA in ui in mortuoDtil Ik lAole of il bas pund Ifantnefa a netv of 100 bolei ki tte Ki— r iadl. Oie*1 cwb most be takeo Um tk winle cf tbe kb is pot tfatani mlhoat say Iok. AoT mrtaffig particles that may be pceaeot w3l, boone, be beaten Sat. and wiQ not pssatbroa tiMMTC. Tfaeae are shaken oat on to a abeet of glased papB, vA eaRAiDy pot od one ode for (be present, fanDiDR Oe ao-eaDed metatBea." "nte sifted ore ia then veigtied. and its'weififat nnoided in the aawy book. Tbe sified OK is then tbonngbly mixed on a ahnet of glued per or. bUlu. of OoaA Amencan dolb by means ol a atali, and liaiuihiiul to a widft-mottlb stoppered boUle.

itnon. — foflomig mlrture is theoweifiadoiit:—

Uaam MO ..

Pn.'l b.wu 00

Tbi; lithaiiie should he first neighed ont accorately and plii-p-l .in a sheet of Anierican cloth, ihen the ore. A-i psacily as the scales will admit of. is placed on lop of the litharge, upon this the charcoal, finally the .MrUinite of soda and half the horas. The last two n-,l ro! hf weighed accurately, but can be measured ou: in spoon? known to hold about the required amount The cbaive is then mixed most iniimalely by meatis of spatula, untii the colour is quite uniform throtighoDt, and no separate particles of the iogredients can be distinguished by the eye. The niislure is then transferred to a crucible about 5 inches in height, and 3 in diameter (sizes F and G of the Morgan Crncible Company.

:v FIRE ASS A Y 535

limited, are the most suitable for tliis work). The remaining 100 grains of borax are then poured on to the heet, rubbed over it with the spatula so as to take up hny particles of the charge that may have been left behind and poured on to the mixture in the crucible so to form a cover for it. The charge should then not Dore than two-thirds fill the crucible. Some assayers irefer to heat the crucible first, in which case the harge is poured into it by means of an assayer's scoop, he 100 grains of borax being placed behind the mixture o as to sweep any particles of the former that may have leen left behind into the crucible.

Meanwhile the fire should have received proper atention and have burnt well through. The crucible hould be placed in the fire with the tongs shown in "ig. 126, which are the best patterns for handling the rucible with. Tongs shaped as in Fig. 127 are useful or packing in fuel, lifting the crucible cover and light k'ork generally. The pots should be put well down in he fire, covered with a proper cover and then packed nth fuel. Coke broken into pieces alx)ut 2-inches cube is he best fuel, but charcoal can be used in case of need ; he latter fuel should be screened through a J-inch creen so as to remove all small stuft' and to ensure a ood draught. The heat is raised slowly, the dampor eing about J open for the first ten minutes, by which ime the charge should commence to frit ; the heat may ben be raised gradually for five minutes and urged for ve minutes more, by which time the crucible should he t a full red heat, and its contents thoroughly liquefied nd in a state of tranquil fusion. The crucible is then fted out and the fused mass poured slowly and steadily ito one or other of the moulds shown in Fig. 128, the

mould baviag been previously well blackleadod i

nntl wftntied. These moulds &re niado oi cast-iron, W

the cavities turned out carefully inl vury smoothly

polished. It is (iB well to liave several of thorn at hiuul-

When thoroughly set. the contents of the mould m

tuniod out on to an iron itlnto and allowed to cooL A

plate of cttst-iron about 1 foot square, and IJ inaliM

thick, with a amouth surface, slioutd be set on tbe

work-ldiiich near the fuinaces

for this purpose. Th

button thus turned out oon-

Btsts of a lower layer of lead

and an upper layer of slag ;

tbe latter should be glassy,

Ijreenialt, brownish, or black, quite uniform in texture

and colour, free fi'oiii spots and stitiiAineso, iinil coiiliuii

no included shots of lead. It is broken off from tbe

lead hy lilows from a light titter's hammer, weighing

to Jh- When the bulk

of till; sJufj is removed, the

i>n the plale and hammered - ,f,

idl round the edge; this causes ' z' ;;:— '

its upjwr and lower faces to

wrinkle up, and loosens the remaining fragments of the slug. Tlie lead button is then well bnished with s stiff hi'iish. such as a small nail-brush, until quite clean and free from overjparticle of slag. Tbe slag should be broken up and examined for shots of lead ; if any an* found llii'v nnist Im' lulded to the main button, Tlii' leml button sli.iuld be marked with its nnnil>er, or in some other distinctive manner, by means of a puncli. sucli njark being entered in the margin of tbe assaycr's

Fire Ass A Y

Qote-book. The lead, which should be perfectly malleable, is then hammered into a roughly cubical shape and weighed. Its weight should be between 350 and 400 grains. If less than the former amount the assay must be repeated, with a proportionately larger quantity of charcoal. If the ore was found on panning to contain a large amount of oxide of iron, which is usually sufficiently indicated by its colour, the above proportion of charcoal in the mixture must be increased in the ratio already pointed out ; in this case, also, 100 or even 200

Vertical Section

Plan

Scale a" to i ft.

Plan

M .

Scale 2 to I ft.

Fn;. IJ*

grains of the carbonate of soda may ha advantajeously replaced by borax, the latter liaving the property of readily fluxing oxide of iron by forming a fusii)]e borate*, whilst some of the oxide of iron, acting as a base and combining with the silica present, takes tin; place of

carbonate of soda.

If there is a large proportion of sulpburets present, or if concentrates are bein4 assayed, the best and simplest plan is to destroy these sulpliid(s by calcination. For this purpose a clay roasting dish, which is a shallow saucer-shaped clay vessel, about 4 inches in diameter, or

r 538

GOf.D MTLLmG

iLs will conveniently go into the muffle, v

nilibtd over insiile willi plunibuo or red ochrp. Tli

I eliiirge of ore, carefully weigliwl out, is transfemiJ i.

I tliia disb. and this is then placed in the muffle, wliicb

, should be barely red-hot. The sulphur in the chiujit;

almost imDiBtl lately takes fire, bnrniug at the surface of

tlie ore with its characteristic bluiab flame when pn?§enl

I in large proportion. The ore must be gently stirred with

an iron rod, about inch in diameter, the end of which

is beaten into a thin flat spatula-shaped point, j incli

I broad and 2 inoLes long. Csje inuat l>e taken to nuw

' tbe heat very gradually, and to keep continiiouslv

Btirring, so us to keep the charge of ore from clotting.

When no more sulphur fumes can be seen, and when the

ore leaves off glowing brightly at each stroke of tbe

stirring rod, the boat can he raised to redness ; stirring

must, however, Ije continued until no more smell ol

sulphurous rwid can bo perceived on taking the dish out

of the muffle, and the ore appeiirs quite dull when it is

stirred. The calcined ore is then allowed to cool, and

when cold is mixed with tbe same charge as the raw ore,

and fused in the same way. It must not, however, be

forgotten that the iron pyrites originally present lus

now become converted into iron peroxide, so that tlie

proportion of reducing agent must he increased, and 900

grains of carbonate of soda and 400 of dried bor

substituted for the quantities in the normal charge.

When concentrates ore being assayed they must be calcined as above. The calcined substance will then consist principally of oxide of iran and contain very little silica. In order to form a fusible slag, silica must therefore be supplied and this ia best done in the form of ordinary green bottle glass ground up and passed IhrDUt;))

Xv Fire Ass A Y 539

a sieve of 80 holes to the inch. 500 grains of concentrates are thoroughly calcined and when cold mixed intimately with : —

Litharge 600 grains.

Charcoal 30 „

Ground Glass 400 ,,

Dried borax 400 „

100 grains of Borax being as usual reserved for a cover.

In case of concentrates containing much galena it is scarcely possible to calcine with any approach to completeness on account of the low fusibility of lead sulphide. In this case it will be best not to calcine, but to prepare the following mixture with the raw concentrates : —

Carbonate of Soda . . 400 ,,

Borax 100 ,,

the borax being used to cover the mixture, ' which is transferred to the crucible in the usual way. A couple of pieces of clean hoop iron 4 inches long inch broad and inch thick are then stuck into the mixture, the pot is put low down and well covered, care being taken that the fire is not too hot. At the end of ten minutes all the mixture should be melted down ; the mass is then well stirred with the pieces of hoop iron and the pot covered again and left for another ten minutes. The pieces of iron are then taken out, the fire is urged for a few minutes more and the contents of the crucible are then poured in the usual way. The pieces of hoop iron should be examined for adherent shots of lead, and if any are found thev must be added to the main button.

When charcoal is used as a fuel it is always advisable

r$fQ GOLD MILLING lUT.

to cliftTgo the mixture into a pot previously hea(d, ud the charge should be such us to )n> very rosdily fusible. For this puqione crude tnrtor can be nsed to rvplnM charcoal. 4 to 5 graiui of the former beinft geoenilly about equal to 1 of the lattr. The proportion of bonis nmy also be iiicrttasod liy another 100 grains.

In hvury cttau the ultlmute result of the fusion is llie protluctioti of a button of lead with which all the goii orinally present in the ore is now alloyed.

CopsUation — Wben u, piece of lead alloyed with gold or ailv(r, or Irath, is heated lu contact with air, it oiette, and the surface of the laolten lead becomes covered wiili II layer of oxide. If this oxide be i-emoved as fast as formed, tbc process of oxidation nill continue until all the Iciul is removed and the precious metals alone are left. Tliisisthe principle of the process of cupel lation, the rtMiu'val of the lead oxide being effected by the device of coiuhmtiiif; the operation in a porous vessel, not ultiuki'd by molten lead oxide, and which can therefore ithscii'b till-oxide as s<x)[i as fonneil, just as Idottin paper aiisDilis wilier. If copi)er is present in the alloy in not t(M> I'n-ai qmnitity it is oxidised, and its oxide is ap[Mr- (Titly (liFsolved in the molten lead oxide and carried into till' poi-ons vessel with it. The vessel in which the o])('ijiti()n is conducted is called a cupel; it is made o( hc.nc iisli and Honerally has the shape shown in Pig. 129; its I'Xiiet sliajK' Ik Tiot. lu)wever, very material, and several dilTiTi'ut patterns are used. Cupels are made fhini boiif iisli, cai'i'fully calcined at a comparatively low tempera' liiiT inilil il Iwciirnrs (juitc white. It is |-.in'ly newssjiri anil nvvi-r iidvisubU for an assayer to prepare bis owii. In lucl I'lipcls f(ir(jold assay are best purchased. French (Dcli'nil) cupels are generally quite satisfactory. If it is

Cupella Tion

Vertical Section

necessary to make them, this is done by means of a cupel

mould, which consists of a cylinder of iron or gun -metal

pierced by a cylindrical hole equal to the diameter of the

cupel it is proposed to make. Into it fits a piston-like

piece, the lower end of which is tmrned into shape so as

to form the cavity of the cupel ; great care must be taken

to have this as smoothly finished as possible. Coarsely

ground bone ash, which will pass through a sieve of

about 20 holes to the linear inch is used. This is stirred

up in water, allowed to settle for a few minutes, and the

milky water carrying any very finely

divided bone ash in suspension is poured

off, and allowed to stand, so as to deposit

this suspended bone ash. Each grade is

dried separately. The coarse bone ash is

then mixed with just sufficient water to

make it cohere when squeezed in the hand.

The mould is filled with this, topped with

a little of the dry, fine, levigated bone ash,

and the piston driven down by a few blows

of a mallet. As the success of the cupels

depends largely upon the due regulation of

the pressure at which they are made, it is better to us( a

pro|)er press insteiul of a mallet for their manufacture.

When made, the cupels are pushed out of the mould from

l>elow, and set aside to dry.

"When the lead buttons produced by the fusion process are ready. for cupellation, the requisite number of cupels are placed in the muffle, the fire having previously been lit. The size of the cupel to be used is determined by the weight of the button of lead ; the cupel should be rather heavier than the lead button it is to carry. The best fuel for the nmffle furnace is coke broken into pieces

Plan

Scale one half. Fio. 12l.

Sl COJ.D MILLING

of 1 U) inches cube, but in c&se o( necessity chutni can be uacd. aJlfaougb the operation is rendered nOliw more li-oublesouie. T>u! ebiuTonl ghoold he in betwuen i aiid li inch cube. &ud free from dust or fine stuff. The muffle ould be brought to a full red heit, the cupels esamiued to sec that Ihey conlaiu no partidet of dirt, £c.. and then the buttons of lead ore nillf charged by means of a pair of loo hght tongs, each into its respective cupel, the position of which in the nmfftr tihiiulil lie noted down ; the muflle door is then d<tsl. lu n few minutes ait the buttons should have melted i>ii'i the door should be very slightly opened : il is itesl to havt? a muffle door made in two pieces, a lower part anil an upper one, the latter being niuch the smaller. If the upper part of the muffle door is now drawn back about of an inch, a sufficient current of air will be admitted into the niufHe. The heat must be kept up to a bright redness, fuel being added if required, but before the fire burns down as far as the top of the muffle, as this would be chilled by contact with the cold fuel. It is best to avoid making up a fire during the course of a cupellation, but this is not always possible when charcoal is the fuel used. The temperature of the muffle is right when tbe molten lead looks bright and luminous and markedly convex, when the spots of oxide are seen to move rapidlv over the face to the edges, and when fumes rise from the cupels without hanging over them and disappear before they reach the top of the muffle. As the operation proceeils the heads rapidly diminish in size and become globulaiin shape ; presently they show a series of iridescent rings, and then suddenly brighten, the rapid motion of the surface appearing to cease abruptly. TTie

Xv Cupellation 543

cupels should be left in a few minutes more and then withdrawn ; no harm is done by leaving them in the muffle for some time in case some of the cupeUations are finished before the others. The cupels are withdrawn by means of light specially-made cupel tongs and allowed to cool. The little buttons or " prills " of gold and silver should be globular in shape, slightly crystalline on the upper surface, frosted-looking rmdemeath, and should not adhere to the cupel. They are lifted up by means of a light pair of long-nosed pliers with half-round points, squeezed and flattened on an anvil. This should be a small stake anvil with a face about 2 inches square, set in a sold block of wood ; it may with advantage be surrounded with a sheet-iron tray about 1 foot square with a rim 1 inch deep ; the middle of this tray has a square hole through which the stake of the anvil just passes, by means of which the tray is kept in position. The hammer used should have slightly-rounded faces and weigh about lb. ; both anvil and hammer faces must be kept thoroughly clean and bright. The lower smrfaces of the prills are well brushed with a small stiff tooth-brush to remove any adhering particles of bone ash, and they are then carefully weighed in the assay balance and their weights noted with the most scrupulous accuracy.

Treatment of Metullics.** — Meantime the "metallies" obtained by sifting and crushing the ore have also to be treated. Any particles of iron are first removed by the magnet, and the residue, which should now consist only of native gold, is wrapped in about 50 brains of lead-foil and cupelled. The prill thus obtained is also carefully weighed.

Fartmg the Frillfl. — The prills still contain silver in

lo |!oM. uul this hiks lo be removed \ty ihe ptK of |rtin£. Wtict) an Alloy of gold and ailw is in ortnc aciiL. Qf ilvi-r can be completeljr diawlnd the lkiy iMDloius least time* u d. Wben workiDg DpoD on noknowD of cilver presenl must be gaessed lk Bulbar of tlu prill, uid eDoagb added to brin tt up In iktiB tone* Uw suppowd wi-ight of gold. When Use coIbiv al Ibe mikn n white a wragltt of silver equal to lfa priO sbnold be KiMcd. It is important tlurt IIm idvvr (?d f(ir ibis pnrpmo be quite pure. Pure (any drcr M prrpand u foUows ; Coin sitter is Batteiud in'm-t, CHI ap by ui<?au$ of a pair of stool ifv*r=. and dis)tved in a mistore of one part of nitric acsi io two jnrts of water. The solution is evaporated lo drvTeii lite dry mass heated to fusion until it blackens, asd tSe jialiDe residue dissolved in warm distilled water azl L.teretl. "Hie filler is well washed, and the filtrate a:'.:iwd lo iwv> pints for each ounce of silver present. soixnsoo is then heated to boiUnp, and hydrochloric virith rvmstanl stirring until the precipitation ao-ivi lo *Kle, ftlien the solution is perfectly clear ,; 7# :.-i3evi a drop of hydrochloric acid to make sure i,-,i: v: ;ht' NtTvr has been precipitated. When tlii >jtfi. rwvci *o.vttipti*hed, the clear solution is carefully *\iVt,t--i .>tfv the vessel is arain filled with boiling water, KLjrrsvL *.jontJ to settle, and the supernatant fluid again H'wii clear, the process being continued - wjtih-water is free from all traces of acid. Tlie ,- .- ,-fW ci siSier is then vrashed into an evaporating .M.> .irjixi ifaorougbty as possible and dried. The

Parting Prills 545

J chloride is mixed with an equal bulk of carbonate soda, and fused in a clay crucible such as is used gold assays, but which the mixture must not more m two-thirds fill. When it is in a state of quiet ion it is poured into any convenient ingot mould. The ;ot of silver is cleaned from adhering slag, washed, lured with a httle fine sand, and is then rolled into hin strip with constant annealing if there is a flatting U at hand ; if not, it must be hammered out as thin as ible, well annealed, and cut into small pieces with the ad shears.

Each of the prills resulting from cupellation, together bh the proper weight of assay silver, is wrapped in grains of lead-foil, which is rolled into a httle ball d cupelled as before. The resulting buttons are carely cleaned and flattened out on the anvil. Each prill then dropped into about oz. of No. 1 parting acid hich has been previously raised to boihng in a small ;t tube) and kept boiling for ten minutes. This No. 1 rting acid consists of 1 part of nitric acid to parts distilled water. It is advisable to add a drop of silver irate solution to every bottle of parting acid, to make re that it shall be free from any trace of chlorine, ber boihng for ten minutes, the acid is poured off and 3 residue boiled for ten minutes more with No. 2 rting acid, consisting of equal parts of nitric acid and ,ter. This acid is then poured off, and the pure gold lich remains in the bottom of the test tube is washed o or three times with distilled water. Tlie test tube then filled with water, covered with a small gold nealing cup, which is a crucible made of porous clay, d the cup and test tube inverted together. The gold

N N

Cold Milling Cum

then falls into the cup. the test tube is remoTed, tbi: water in the cup poured ofT, &nd the gold thoroDglilT dried aod aRDeoied by heating to redness. It is then weighed with all due precautions and its weight reconled. A point that must not be left out of consideratico when reporting the results is that all lead products, Budi as htharge and red lead, contain some eilrer : l]>e amount of this has to be subtracted from the tct weight of the prill obtained. To determine ibis amount, 500 grains of red lead or litharge are heated In a am*!] crucible with about iJO grains of charcoal, tor ten minutes, when the action should be complete. Tbs fused mass is then poured, the button of lead cleftn from any adherent slug, and cupelled in the usual ivst. As a fieneral rule, r500 Lrains of onlinar\- lilhaie or red lead will contain from 001 to 002 grain of

Calculation of the Reanlti. — This is an extremely simple matter by the help of Table No. II. Prom the weight of the prill of bullion the weight of silver present in the oxide of lead is subtracted ; the difference is ibc amount of bullion {gold and silver) actually yielded by 500 griuns of ore. Reference to the table will shiv what this is equivalent to in ounces, penny-weights, and gi-iiins per ton. Similarly the quantity of pure gold per ton is taken out from the table, the difference between the two being the )ield in silver per ton. It is always advisable to make gold assays in duphcate, taking ihe nu'an of the two, which should not differ greatly from eacli otiier, as the true result. The follo\nng esamplf will perhaps render clear the mode of returning ibe assay results:—

Xv Calcula Tion Of Resul Ts 547

Weight of rill of ballion 0046 gr. . 0047 gr.

Weight of silver in 500 grains of litharge . 0*013 „ . 0*013 ,, Weight of billion prodaoed by the ore . . 0*033 ,, . 0 034 ,,

Average bxdlion contents 0*0335 grain

2 oz. 8 dwt 18*6 gms. per ion of 2,240 Ibe.'

Weight of pare gold obtained 0*027 gr. . 0*028 gr.

oz. dwt. gn.

Silver in the ore 0*0060 0 7 20*2

Of course, to the amount of gold and silver thus obtained the amount in the '' metallics " must be added. A simple calculation will show how much of this " metaUic " gold and silver would be yielded by 500 grains, since the amount yielded by the original assay sample (the weight of which has been recorded in grains) is known. The corresponding proportion of ounces, &c., per ton is taken out from the table and added to that previously obtained.

If, for instance, the assay above quoted had been

made on a portion of a parcel weighing 3,760 grains, the

metallics from which had yielded on cupellation a

bullion prill of 0018 grains and after parting a button

of fine gold weighing 0014 grains we should have

And from the tables we find that these figures so obtained correspond to

Bnllion per ton 0 3 3*3

Fine gold per ton 0 2 1 1 6

Silver per ton 0 0 15*7

' This is taken out as follows from Table II. :- -

oz. dwt. gms.

N N 2

bjlB GOLD MILUNG Tho

wbtch unonnte most be added to ibe respective onci previoost}- obtained.

On tbe Continent of Europe it ia nsttal to report 9uch assa;:8 by the number of graininea of gold to the metric (on : TnbleB IV. and V. are conversion tables to enlile returns tlius reported to be converted into ounces, fa., p-r long or short ton and rice versa.

Soorification. — When very rich products, such as tiiatt or high grade concentrates, have to he assayed, the method of scorification is sometimes resorted to. This consists in exiK>sing the substance to be assayed mixed with grana- lKtd lead to a bigfa temperature in the muffle, the mixtuie being contained in a small saucer-shaped vessel of good fire clay, known as a scorifier. The following mixture is

This is well mixed and transferred to the scorifier and covered with 250 grains more of granulated lead, on top of which are placed 50 grains of borax. The muffle muHt be heated to a white heat, and the charg scorifiers are then placed in it by means of specially shaped tongs. The muffle door ia closed until ibe charge is completely melted down, which should take 10 to 15 minutes. Tbe door is then slightly drawn back so as to admit a current of air, and the operation continued as long as tbe molten lead in the scorifier continues to oxidise. In this operation, volatile substances, such as arsenic and sulphur, are almost completely expelled; oxidisahle metals, such as iron, copper, and bismuth, are oxidised and carried into the slag, together with a laie amount of oxide of lead, any siliceous matter present fonning silicates. When the bath of molteq lead is

Xv Scorification 549

completely covered by a layer of slag, about 3 grains of

powdered anthracite wrapped in a little piece of tissue

paper are dropped into each scorifier, with the object of

cleaning the slag. After another five minutes the

scorifier is withdrawn by means of the scori£er tongs

and the contents poured into a button mould. The lead

is cleaned from slag in the usual way ; it should weigh

from 250 to 300 grains and be quite soft and malleable.

If it is not soft or if it is above 300 grains in weight it

must be put back into the muffle in a fresh scorifier (the

same one may be used if not too much corroded) with 20

grains of borax and the process repeated until the

desired result is attained. The button of lead is then

cupelled in the usual way. The weight of the prill

obtained is of course multiplied by 10 in calculating the

quantities per ton. Scorification should be resorted to

whenever the button of lead produced in the crucible

assay is too heavy or is not perfectly soft. It may also

be used with advantage for treating the concentrates

produced in the panning assay if their weight does not

exceed 100 grains.

Tailings Assay. — Tailings are assayed in exactly the same way as ores. In this case, however, as the gold contents are usually very small, it is better to work on a doul)le quantity of ore and fluxes either in a larger crucible or else in two separate lots ; the resulting button or buttons of lead are scorified down until a button weighing about 350 grains is obtained which can then be cupelled as usual. Naturally in calculating the quantities per ton the results must be divided by 2.

Tabulating Results. — It is evident that a complete series of assays of all the products of the mill should show exactly what has become of all the gold contents

Cold Milling

Hw til* ant. In making these calculations it is, boweTor, neoeasknto take care that the errors due to moisture in Ibe ore mi ebiniiiated, otherwise the insults will not ajjrei:. tvt this it tB necessary Uiat the fire assays should be cundoctisd npoa dry on ; Ibe sample prepared for fire assnv K dried in an iron pan over a fire, at a low teiii])enklu:¥. ear* bmni; taken tbat no oxiijalion is set np, and the same UmifE nraet be done nth the concentrates. It htt Inady been pointed out that if the weight of ore going iaio lb* initl is d(<lemiine>d b}- measurement, as reoommvniled. and if the weight of Atj ore coiTKSponding to Cw alandani of lueasureoteot is dulermined, the mill retnnu will then practically be made on the dry ore sent to Ihe

The drv assay gives the total contents of gold per ton of <lry ore. The results of milling give the weight of gold actually obtained per ton of dry ore. The percentage yield of WHventrates per ton of Ar ore is similarly known, and their assay value is also known. The tailings assays, beiitj; made on dry tailings, are similarly reducible to ihe siiie Canditnl. aud these three figures added together should be csqual to the assav value of the ore. From the assay value of the concentrates it is known what loss is iKurrtt in treatiiij: ifaem. \VTien the tailings are further trt'aievl. a:> for iitstauce by the cyanide process, it must fii ty fouinl by envrliueai ou a large scale what weight of (aiUits rvaly for treatment is yielded by a ton of ore, and llic'ii ihi' k'i>:>es in the retreatment of the tailings can be ai.vii[-atflv dettrmined. Of course assays of the exhausted iihti should sv' be made coutinuoualy, so as to check ltu> t'tticacy of the process adopted for their treatment. .artlul tabulation of all these results will not only show at what staples ol tiie process gold is being lost, but forms

Tabulating Results 551

iheck on and consequent preventive agftinst 1 the mill.

ition Assay. — It is sometimes necessary to dehat percentage of the gold can be extracted by from a given ore or tailings. The material

ground to pass through a sieve of 100 holes to

inch ; 1,000 grains are weighed out into a 16 de-mouthed bottle fitted with a well-ground 27 grains of coarsely crushed potassic cyanide n in and then 6 ounces of water carefully Common water is perhaps better than dis- 3r for this purpose. The above proportion — 4*5

commercial cyanide to the ounce of water — )st exactly a 1 per cent, solution of KCy. The well agitated, and allowed to stand in a dark

too cold, for 72 hours ; it should be shaken up

to time and the stopper taken out so as to allow air to the solution. At the end of the above clear solution is decanted off through a coarse an evaporating basin about 7 inches in diameter, rated, best on a water bath. The ore remaining

three times with about 3 ounces of water each the washings gradually transferred to the same

filter pump will be found of great service in this 3 work, as the proper washing of a clayey ore is a 3SS without it.

lution is evaporated to complete dryness, the opened from the dish by means of a steel spatula rised in the dish with a porcelain pestle. It is td in the dish with : —

Litharge 600 grains.

Argol 60 „

ransferred to a suitable crucible and the dish

GOLti aMILLING

rinsed ont hrioo with 100 grains of borax each tbt. rubbtt) down in it with the pestle. This borax is tl ciiarijiHl itilo tho crucihlo as a povcr to the prpfiow niiitunt. It is hcaltid in a xai\wx slow fire, poured mI lh reantlinK leiul button clutnvd mid cupelled as asmL

Anotker nintliod consists in concentrating the sohdiM W abov to two or three ouTicee, and then timnsfeniif (bis to K littlo narrow basin or boat mode by clonng the ends of a strip of Ituul-foil ; this itoat is best pUorf on H itiiuilar lrip of loiul-foil and covered with a itoid piixn* to proveiU spirtin);; tho three pieces of lead-U should toitber woih hulwuen 300 and 400 trains. Hw sohltinn is vu]>omtcd to L-oinjilutu ilrynesK in the Wctn boat, and wlit-n ili'y all the lead is tlirown into a scorifier and melted tip ; the lead button obtained is cleaned and ou]M'lk'd us usual.

In either cuse tho prill produced is weighed and parted as ulwve. It must not bo forgotten that in taking out the residta per ton, tho figures must be divided by two. The ohjeet of the above assay is merely to find out whether thi' ore in question is amenable to cyanide treatment. A eiiinplete cyanidation test is much more elaborate. The degriH; of fineness of the ore, strength of solution and pfno<l of treatment, that give the best results technically and eeonoinicully have to be determined by means of percolation tests on rather larger samples, whilst the acidity and quantity of "cyanide"' substances present must also Iki determined. This is, however, work that bad better le left to the trained chemist ; the simple cyanidation assay here closeril>ed is gonemlly sufficient for the purposes of the assayer.

Bullion Auaya. — It is finally necessary to determine the value of the bullion produced. For this purpose each

Xv Bullion Ass A Y 553

bar of buUion should be assayed. A sample is taken by chipping off small pieces from diagonally opposite corners of each ingot, or else from drillings. In the latter case a fine twist drill of about inch diameter should be used. A hole should be drilled in the middle of the bottom of the ingot, and two holes, one at each end of the upper face, and the drillings thus obtained be mixed together. When pieces are chipped off, they should be flattened in the flatting mills, or beaten out as thin as possible with a hammer, and cut into small pieces by means of shears. Annealing should be avoided, because the composition of the alloy, if rich in copper, is apt to be slightly affected by it. For the same reason it is not advisable to melt together the chips or borings taken into one piece. The only correct way of sampling low grade bulUon is by a sample dipped out when the alloy is molten ; a good plan is to thrust the stem of a clay pipe to the bottom of the crucible, plug the pipe with a little clay, and take the wire of gold thus got as a sample. If the approximate composition of the bullion is known the assay proper may be at once proceeded with, if not, a preliminary assay is made on 5 grains. Tliis amount is carefully weighed out ; 2 J times its weight of assay silver is also accurately weighed out, and the two metals are wrapped in about 50 grains of lead-foil and cupelled. When the cupellation is finished the cupel is slowly withdrawn from tlie muffle, and the button cleaned and weighed. Tlio difference between its weight and that of the bullion and silver originally taken is set down as copper. The button is then rolled or hammered flat, and boiled in two lots of nitric acid, as is done with the prills obtained in the ore assay. The residual fine gold is washed, dried, heated to redness, and

fl|f4 GOLD M!LUNG CH*r

weighed. Its weight is Eot down as pure gold; tbe diffprcnce between the weight orally taken and tlie 8UU1 of the weights of copper and gold is put down silTr. It 19 itceasar>' to have in readiness a qnanlitj al pure gold, which is best prepared as follows:— A qtuntity of the gold cornets from previous assays ii dissolved in iiitro-hydrocbloric acid at a gentle heat, and evvpurated over tbe water bath till the solution conimenwA to cr'sUUise. It then diluted with warm diatiUed water till the solution contains not more lliaii I omee of gold to tlie pint. This is allowed to stand lor MMDc days in a Ufge beaker closely covered up. Tht cle*r fluhl id then nuvfully decanted from any residue that u)ay have been deposited into a large dish or beaker Ii is heated lo boiling, and mixed with a hot strong solution of oxalic acid, an amount of osabc acid beia| useil equal to that of the gold originally taken. After Kvliitg for a few minutes, tbe solution is set aside until al the has settled and tbe supernatant fluid, wbict tuu rvttot strvmgly acid, is colourless. The completeness vM" tho )>nvipiiation may be ascertained by taking out e i:*Ii qUAiility of the solution and testing it with a few drv'ps chloride of tin. When it is complete, the clea: rt;:;d is sxpiwnevi off and the preciiHtated gold washed liiivdkr.aiivvi itiie half a dozen times with hot distiller uAitT. ll is then thrown on to a large Alter, washes sb.<ivHhU with tiot water, and dried. The dried gold i: :;ars:Virv\i to a suitable day crucible, tbe filter burnt, am U-.v ashts adiW, the whole covered with a little drie* cv-tkx aiNt uu'lted. The resxdting button of fine gold i v-U-tx\l ftviu adhenmt slag, well scrubbed, and rolled ou : - :K- dji)t;.u mill into thin foil with constant anneaUng IVv- K.<t$ ol' bttUiou each of 10 grulns are carefuU;

/ Bullion Assa Y 555

eighedont; the amount of pure gold supposed to be resent in each is caleuUted, and so much pure silver added that its weight 'plus that present in the bullion lall be equal to times the weight of the gold present. he metals are then wrapped in a piece of lead-foil, eighing from 50 to 100 grains. A piece of pure gold in weight to the weight of gold supposed to be resent in the 10-grain lot of bullion is then weighed out, >gether with as much assay silver as will equal the >tal weight of that present in the bullion plus that jded, and as much pure electrotype copper as will equal le weight of that supposed to be present in the 10 grains f bullion. These metals are then wrapped up in the Eune weight of lead-foil as was used for the assay. "his latter constitutes a so-called check" assay, and ( made up so as to represent as exactly as possible iie composition of the bullion to be assayed. The two ullion assays and the check assay are then cupelled imultaneously under as exactly as possible the same onditions. When the cupellation is finished, each cupel J covered by inverting an old cupel over it, and the overed cupels are then drawn towards the door of the luffle, which is left open so that they may cool very radually. When cold, the three buttons are detached rom the cupels, carefully cleaned, and accurately weighed, hey are then flattened on a clean bright anvil by means f a hammer about 12 lbs. in weight with rounded faces, which must be kept very smooth and bright. They are hen annealed and rolled in the flatting mill into strips ,bout 2 to 2i inches long, and inch wide. If there kre no flatting mills at hand, the buttons may, with a ittle practice, be drawn out by the hammer alone, but he flatting mills are preferable. The strips should be

MntMtled oooe or twice during tbe operstioD, eare bdu UJnn ikat tb edfKB do not crack or become rough, Wlien t)f%n uuL ihty tare itguin nnneall and rolli! op bptwven the finger uid thumb into spiral coils akiut iocb in diuneler , it is best to keep the dde of the hutton which was nest to the cupel, and which presents a ocoewhftt duller appeftrance than the other, (or tlie (Mrtor nde ol the epinJ.

Tbe diasolTtog oat of the silver is done in assay flasks; thew are small bard glass flasks of a capacity of nboui fi ounces. Into each are poured about 2 ounoes of No. 1 putittf; actil, vrtudi is then beated. As boob as ii has raacbed the faMSng poont, one of the spiral cei\. or (viTPis " as ihy are called, is dropped into each flask. livether with a frmin.eni of burnt fire-clay about the sue i>i a to pivveiK buuipiiig. Afwr lo uiiuuies' builiiig. the acid ptunjd off, ihecoraets are washed, and boiW for 1 j ;;:i!'.u-s lOon? with 2 ounces of No. 2 partiog acid- T!::> the:: poured o9 and the comets again washed v\::h >!:>;il!iAi water. The acid and washings should be |v.:rY>i :ii;o a ressei reserved for the purpose and the j;"vtr riAWered from time to time by precipitating with h_\\:r,vhiono acid and reducing the chloride of silver so i>T\y:-.:v-t\i. A i:>id acneahof: cup is next inverted orer i\*i-h ?i*i. which should have been filled to the top with witty, ai:d when tbe latter is inverted with the cap, the ij.>I>,i iXTtie: fail* ntlv shtuugh the column of water into th' i.".;t\ Tiu' ij then removed, as much water as [v-:b;' iva.rt'vi oJ from the comet, and (he cups with t;v;r i\.x-;t;-.;* dried. are then heated to low kA- i!ss ;n ir.e ciuiSt', and when cold are carefully weighed, l: :,-! w;;r; v" ;h-; rfeck ptiece Las underwne no change, ihi- :r.e weiiihss iM' two pieces repre-

Xv Bullion Ass A Y 557

sents the quantity of pure old present in 10 grains of bullion. If the check piece has gained in weight, its increase must be subtracted from the mean of the weights of the assay pieces ; and if it has lost, the decrease must be added to get the correct quantity of gold present. Similarly the mean of the loss in weight of the two assay pieces after cupellation represents the amount of copper or other impurities present in 10 grains of the bulhon, which must be similarly corrected by the loss of weight — here there cannot be a gain — of the check piece.

The silver is determined by difference. The results are reported millesimally by multiplying by 100. An example will make this plainer : —

Preliminary Assay —

Bullion taken 5*01 grains Fine silver 1260 „

17-61 „ Weight after cupellation 17*44 „

Loss 017 grain=0 339 grains of copper in 10

grains of bullion. Weight afUr parting . . 4450 „ =8*882 „ gold in 10

grains of bullion.

Difference =0*779 grains of silver in 10

grains of bullion.

AcUial Assay, — If the 10 grains of bullion to be taken for the assay proper contain 8*882 grains of gold it will be necessary to alloy this with 4 8*82 22*205 grains of silver. But the bullion already contains 0*779 grains, so there need only be added 22*205-0*179 21*426 grains.

r

COLD Af/Ll.mG

OHck plec*.

Fine gold B'S$3 snins.

Asraysilier 2-205 „

Pure ropptr O'SSB „

Weight fler piiplUtior 3 Lon of weight . - . k-Ttng lois or tata.y

Actual amotmt of copper 0332 ,.

Prt8 of copper per mil SS-a „

No, I. Kr>. II. ChKkpK"-

Wright after |>artiiig . . 8*874 gmiiu . 8*875 grains - S'SSSgniru. Average weight of gold in

nesay jiicces 8*8745 gnine

ofgt

Copper 33-20

When a bar has been assayed it should be carefally weighed. Its weight in ounces and decimals of ao

A'ESr/.TS or ACTUM ASSAY 53.)

ounce troy should be stamped upon it, together with its fineness in parts of gold per 1,000 and the initials of the assayer, a proper set of steel punches being kept in the 'ssay office for this purpose. When its fineness is known, the value per ounce troy can be found from Table No. III., which gives both the full Mint value the Bank value. For instance, the Mint value of the gold per ounce troy of the bullion assayed in the above example is taken out as follows from the table : —

The absolute value of gold is fixed in this country by the Coinage Act, 1870 (33 Vict. Chap. 10), which provides that the standard weight of the sovereign shall be 123*27447 Imperial grains, and its standard fineness 916-6 psr mil. The minting value of gold is thus fixed, whilst the Act further enacts that the Mint shall coin free of charge any bullion brought to it, provided that the buUion does not need refining in order to bring it to standard fineness, the Mint being thus bound to coin but not to refine for the public. The Bank Charter Act, 1844 (7 and 8 Vict. Chap. 32) compels the Bank of England to buy any bullion that may be presented to it for sale at the rate of £3 17s. 9rf. per ounce of standard gold, provided, however, that the bullion must be assayed at the expense of the person tendering it. The minimum Bank rate for gold is thus fixed, the difference between Bank and Mint rate (about per ounce of

Blandaid golti) bting for tbe Bank' for reconpiog loss of iotcrest, ic. Acoonling to the iiBsl! of the market the Bank may pay at tirnea a higher pras' for gold buitioQ than the standard rate. The osuil charges upon bullion are ; —

„ refiniDg . . id, „ „

The value of tbe bar is ealculated on the assay, subject, however, to certain deduotioDs if the bullion is very Itase. as ia generally the case mth cyanide gold precipitated by zinc. These dednctioas are as follows :—

Fioii) asG3V of hose liuUinn over 800 Rn, \eduQt 2 imrts |vr mil,

.; ,, bitu 7(H)aiiil goo Bill-, Jcjiicl SiNirtSTtriuil,

„ ,, „ uiidpr 300 fine, deiluct S parU per mil.

On tbe other hand an estm premium of a irf. per ounce of standard gold is paid by refiners on all gold bullion containing only silver and copper in addition to gold. Cyanide gold being irregular in composition and diffitiilt to assay correctly, duplicate sets of assays arc made on it thus doubling the coit of assaying

The above is the general practice the charges of different bullion brokers and refineis \ar\ing 1 ttk from It hil'it the absolute Mint and Bank \alue* (f gold are thus fixed amounts Ibe actual amount reOiv I depends not onh upon the amount of fine gold prese i hut also upon the fineness of the bullion and the stati, f the nidikct The value so calculated lefers to llie old onh To determine the value of the silver present itquantitj must be determined bv multiplying the weight of the bar bv the parts of silver per mil and dividing by 1000 the vi eight of sjivei is tbtu rated at tbi

Value Of The Assay

current market price of fine silver per ounce. Gold bullion shipped to England is generally consigned to the Bank.

The great value of the bullion assay for the mill man lies in the fact that he is thus enabled to reduce all his calculations to one common standard, namely, ounces, &c., of fine gold yielded by the ton of dried ore; the value of careful tabulation of all the data obtained in milling has already been repeatedly insisted on, and when all these are reduced to this uniform standard, all the elements for exact and scientific calculations are attainable, and thus the foundation laid for accurate and economical work in the various operations of gold milling.

O O

Appendices And Tables

On The Cam Curve

The proper construction of the cam curve is a matter of such g[reat importance to the successful working of the stamp-mill that I have found it advisable to fully investigate its geometrical properties, and propose here to give a short account of these, so as to enable any mill man to work out all problems connected with this portion of the subject, and to find out for himself whether his cams are of the best possible sha|)e, and therefore working under proper economical conditions, or whether he is losing power and straining the structure of the mill owing to defects in their design.

For the proof of several of the following propositions I am indebted to the kind assistance of Mr. A. C. Waters, who has also revised this section for me.

It has already been stated (page 202) that the curve always employed for the cam is an involute to the circle, and the method there given for setting out the cam is that for describing an involute, the general definition of which is that it is the locus of the end of a string that is being unwound from a circle, the string being always maintained tangential to the circle, and having, moreover, in certain cases, the jwwer of uniformly lengthening or shortening as it is unwound. The definition which I shall here employ is a different one, but I propose to prove later on that the curve generated is in either case the same: —

CMJ* jaUJMC

He ciide, ib ed,ii tfce orriedecimbedbf itie atniA fine, and the perpen-

Hta4 ftfal n lo saj, it ttie centnl liu ' liv t III I 4tBfr , ifat tlk cutto] c&m surface reiolresirilli ifar rviLik iCnAiit IiBc linjutiva, lifcioi; hj iu rviuIutiuD tb *ith ttw errii linear rdocitj. Horeorer, the tenh '( 'An £TTE finhe KisKbl line {the nMliuf! of the generating' oTvi-c iirsnaiiMii. twins eiiul to the hiirizontal iliHtsnce biEvcvs ibe axes the am hafl and Ihe stamp KteniK.

I: k ils>i- [VHsiMe i(> find circle, baring the fixed extreniitj i luK-ft.-r a centre. Mich ntdiuH that the arc i>D ih;> .:-.rv iiKluded bv the angular distance traversed byiinuiiua tbrTV"! :n anj peniMl vi lime shall he eijuiil to the linear diK. iai>Le [rtelWd t>T the p<.>int along the line of motion in the same tin>e. Hus circle Ls called the " circle of equal velocities." md il radius is found as follows : If A be the distance alullg iho linr uhii-'U traversed by the jHtint in the unit of lime, "' the allele diK-riUil Jurini: the same time, and r the radius of the cirvle -'f e<)ual vein-ities —

Sow, if d be the radius of the generating circle, d may be either or less than r ; accordingly three

Appendix A

les of inTolute curve exist, corresponding to these three ratios, I known as the curtate, normal, and prolate involutes respec- 3ly. It wiU now be necessary to investigate which of these ee forms of curve is the best suited to the needs of the stamp- 1. Taking the above definition, it appears that the cam curve st be an involute of some kind, in which the axis of the stamp m corresponds to the line of rectilinear motion, whilst the tre of the generating circle is in the axis of the cam shaft. 3 line of motion being vertical in practice, the cam curve has

communicate motion to the horizontal tappet by contact rewith ; it is required to determine under which conditions maximum amount of force can be transferred from the centre otation to the line of vertical motion. Evidently a curve, tangent to which is always horizontal, will bo the curve that fulfils these conditions, 'aking first the general case of prolate and curtate invo-

Cold M/Lu.Vg

lan ukd 131. let be the of molntiuii, ud rrpilkiilu' line of rectilnear motion< C.\ h WitL emLrv C and radiNS r ili*w tlwcild>l KXT i at X dn KS, ttu> \ertit lugtoittoliM whtl*. <m4 piwJId eawBwfWly ut.JP. Noar.lotapoiiitM

through Ihe infinitely snmll apace PQ ; draw the angle PCR which shall be c<|uaI to the angular motion of the cam corredpondin(! ti> the rectilinear ditttAncc P, Pfibeinga )Hirtionof acirrle (IcwrilH-.! witli ountru C ; Ihi-n K will havecoiiicideil with P si the comiiii-'nceDient of the Diuveiiient of the point to (/, and icill have traveraod the arc FB, whilst the point baa moved up U) V, yfi b&n an elemeat of the involut curve, the revolution of

Appendix A 569

which from P to R through the angle PCR, has produced the lift PQ. Let CPf CR cut the circle of equal velocities in X and Y respectively ; join KQ, KP,

Since the distance PQ is supposed to be infinitely small, the triangle PQR may be looked upon as ultimately rectilinear, and QR will ultimately become the direction of the tangent to the curve at point Q.

The angle QPR 180' -(i2PC+i4 PC), and since PiJ is a segment of a circle, RPG will ultimately become a right angle. Hence QPR im''-(W-APC) W-APG.

ButiCCP=z90 -APB.

Therefore QPR=KCP,

Since in the triangles PQR CKPy the two sides PQ, PR are proportional to the sides GK, GP, and the included angles QPR, KGP are equal, therefore the triangles are similar, and the angle PQR PKG=90°:PKS, the upper or lower sign being employed according as the prolate or the curtate involute. Fig. liW or 131, is refen-od to.

When P and Q are infinitely near together, the angle PKS ultimately QKS A QK.

Therefore PQP 90 + AQK,

But KQR PQR±AQK=90'qAQK±AQK.

Which result may be expressed in words by the statement that the tangent to an involute curve at any point on the line of motion is perpendicular to the straight line drawn from that point to a point on the circle of equal velocities, which is on a horizontal radius of that circle, and in the same direction from the centre thereof, as is the line of rectilinear motion. As this result is independent of the ratio of r and d, it is true for all the forms of the involute. If, therefore, either a prolate or a curtate involute be employed for the cam curve, the tangent to this curve would always be inclined to the horizontal face of the tappet, and part of the force that ought to he employed in lifting the weight of the stamp would be wasted by being resolved into a horizontal

Gout Mitum;

Ih* of tbe cam Anii.

porpendicular, in that specwl

hnfB be homonlAl, and Uierenp

. It is accordingly tbu

iaynA Jsr tb* flun. utd it is this curve

Fig. 41, page 206. T"

riik of pndadng a curve at ail

ftUf, but onlf TW7 HlightJj.

rnuJ r d, mid this eiquaticin nwy be Ni'w Itt be slightly greater than r ; then n'

will t>n''>iue slightly less than ii. Heiic to fulfil the alMveconditL,iii ..f very !lii;htly curtate iDvulut, it will be well in [urai-cioe ti> adopt a vue for a rrry di-jhtly Irtf than that given by guation (,2) f.i. in setting i>uC the cam cure illustratd on page 2t>t;. it would be wlvisableto adopt instead of 91'-42' (the vrIuv there found for a smaller value such as 9135', and to take fur (there fuund to be e.|ual to IS"*') the value of 13" S', so m

to iivoid all risk of pri>ducing a pir>late curve. I have obtaineil esi-ellent results in practice by the adoption of this principle.

As iilreivly stateil, il follows from the genera] proponition respecting the tangent to the involute, that the tangent to the nominal involute must always be horizontal, because in the normal involute rf r or CA CK ; hence the line KS coincides with the line -ID, which beoomeB in this case a tangent to

Appendix A

the circle of equal velocities at the point on the horizontal radius of this circle.

This proposition can, however, be proved independently, and this separate proof will be found useful because some of the equations obtained in the course thereof will be afterwards applied in calculating the length of the involute curve.

In Fig. 132, with centre G and radius r describe as before the

Fio. 132.

circle of e({ual velixities AXY, Since r iH now equal to f/, the line of motion now the perpendicular tangent to the circle at ; as before let a point of the tHpi)et be lifted from P through the infinitely small space PQ, let ill be the clement of the involute curve corresponding to this lift, which has been produced by the rotation of the cam through the angle PCiJ, PB. being a segment of a circle described with centre 0. Let GP GR cut the circle AXY in X and Y respectively.

Vp'" Jv Vx Ac

-iRP€+APn. mmd PR. qR i>m

9PR. dCF, tfe tWB aides tiP. PR MR pn-pi

JC Cr Md Ite iBctaded angles (fPR.

k9 B piryi alfiiiifcr to wftiial J D, thct it, is baruooUl-

It .if the

In FL: !:!:! let n'>rnial Lnvoluie be eiupkived to lift

Let the vrcik'iU line Bf' be drnded into a very great number bf-re the oirrespiindii ares BR. ViR,. . . . (?„R„ wliii;fi are the ares described iMund (he common centre C by the respecciTe [hjint i>f the cam curve which come into contact with jR. . . . Z'R„. are successive elements of the cam curve, etich torres pending \.o an element of the lift on the line AD. the sum of y,RT-(Ji, + yjRj+ . . . will the totnl length ..f the cune c-'tresp 'tiding to the total lift ft/*.

BU h. the height .4 ft of the start in'-iH*int above the centre of

the aeni-niting circle it, and the radius CA of this circle r.

Let each element of the lift ft<,, <?,j, &c. x. and let A in;c-

Appendix A

Similarly

0A

Ar+x

it+2a; ,

r

A C A

Pio. 183.

Since BI) h mx, the length of curve I corresponding to the lift BD

Ej74 GOLD MILLmc

But mx eq<ud to h, and when the number of parts is infinitely x, bocoiuLng infinitely small, disappears.

The pnotiual importance of this equation in still designinij hu nlruttily Ihi'ii pointed uut. TIio a.l>ove expression being a miniiniini far n given height of lift I J when jfc 0 and increasing m (

inereMses, it folluwa that a larger surface of the cam cMTve in required fur equal amounts of lift when the tappet is high up ftboTe the centre of the cimi shiifb than when it is nearer ti> i!.

As n concrete example, toking the cum cuitb drawn tin [He Fig. 41, the length of cure involved in raising the tappet through the tirst inch of its lift is

2"x4-376 875

1"14 inches,

whilst the length corresponding to the final inch uf its lift is {lix(6 + 4-5)xl!+l 22 „,, . . In this particular case, therefore, the length of curve involved and the consequent friction generated and power lost during the last inch of lift is 2'2 times as great as during the first inch. Many mill designers ore in the habit of constructing a cam arranged to give a lift several inches greater than the maiimuiu amount ever needeil in practice, and consider that it is a simple matter to shorten the length of drop when desired, by fixing the tappet further up the stem. It is now obvious how very much power is wasted by such an arrangement; it is the mill man'sduty to know what length of divp he will require for milling a given quartx, and to have his cams designed to give exactly that dn and no more, if he wants a machine tliat shall not waste power. Where different kinds uf quartz hnvo to be treated, the cam should bearriinged to give the avemge amount of drop usually required,

Appendix A 575

and not the maTiTnwin drop ever wanted. It is better to crush an oocaaional parcel rather more slowly than to be working continuously daring most of the time under conditions which are mechanically wrong, and involve loss of power. These remarks apply of course with especial force to mills that have not an abundant supply of free water-power.

It may be useful to give an alternative method of drawing the involute curve, and one will now be given which depends on that definition of the curve with which this section commences.

The method given previously (page 203) is a geometrical one for setting out the involute, considering this curve as formed by the tangential unwinding of the circumference of a circle. The old method, well known to miU men, which was based on this same property of the curve, was by actually unwrapping a piece of string which had been wound round a thin disc of wood, the radius of which was equal to the distance between the cam shaft and stamp stem axes ; a pencil was inserted in a loop at the end of this string, which was always kept taut. This rough method is not accurate enough for so important a matter, and the curve should always be set out geometrically.

The alternative method is shown in Fig. 134, which is drawn to a scale of three inches to the foot, and is for the same cam as is represented in Fig. 41 (page 206).

Let 0 be the centre of the cam shaft and AD the centre line of the stamp stems, OA being taken equal to r( On set off, as before, AB equal to the radius of the cam boss /c( 4*5''), and BD equal to the lift 70; divide BD into the spaces Bl, 12, 23 . . . 6£>, each equal to one inch. D is evidently the highest point of the cam curve when the tappet is at the top of its lift. When the tappet was an inch below the top, a corresponding point of the cam curve was at 6 ; we know that during the time that the tappet rose one inch, this point has traversed an

angle equal to =13° '6' as before. Hence the line 08 will

have travelled through that arc by the time the cam is in its highest position, and the point now corresponding to G is foun by joining G6, describing a circle with centre 0 and radius (%, and

COLU itILUHC

nt tbe tit 9Crj qttBl to 13'6'. cnttiiig tlie cM( rf ; Tf ia MewiUy ft point in thu cnrTK ttinillal; le 4tsenbd vith ndii (R, 01 . . . BCR drmvo n|inl nspeettrel j to 9(13

D

S

S?t

J —6

s

c

then joining the points D, VI, V. IV . . . B, thus found, (he cam curve will be obtained, this being a curve such that for ecb

angle of — ( 13"6') of revolution round the ceotre C, it will lift a point one inch on the line A I), the distance GA being equil

It can readily be proved that this ci

& the nomial involaU

Appendix A 577

of the circle whose centre is at 0 and radius (7.4, for in comparing this involute curve as described in Fig. 41 with Fig. 134, let the locus be considered of any similar point in the two curves, such as VII, Fig. 41, and B', Fig. 134. In Fig. 134 describe the circle AEA* with centre G and radius GA ; draw BE tangent to this circle in J&, and join GE, The triangles AEG in both figures are equal by construction. The line GR GB GVI L

nr

but the angle AGVII ACI + 7GAII 107 + ACB

nr

Therefore the angle ACE A CVIIy and the line CE CVII ; therefore the loci of the points E and VII coincide. Similarly it may be proved that any corresponding points of the curves described in Figs. 41 and 134 coincide, and the curves are therefore identical. But the cur\'e in Fig. 41 is the normal involute to the circle drawn with centre C and radius CA , whilst that drawn in Fig. 134 is a curve of uniform lift for uniform angular motion ; therefore this latter curve is the normal involute.

In practice it is advisable to set out the curve in full size by

Ijoth methods, so that any error in the work will be at once

detected by the want of agreement between the curves. It need

hardly be said that the curve obtained in the second method can

be modified precisely in the same way as that obtained by the

previous one, whilst it will also be well to assume for the angle

corresponding to an inch of lift a value very slightly less than that

given by the expression

V

Appendix B

Si'Kuifications For Twenty Stamp Gold Mill

Ukiwiiik, Ac. — OnB-FiWKir luiii Ohitlinora's Iniprovixl Blate I'nlti'ni Stdiif Ui'unkcr. with jnw {jliiteH, beiivy cast fmine, fly H'd-uU, .ailing |il%. uxtciidud oimlt, aud olJ lixtuiui. on.l KttingH. Thu jiiw [ilatiw tu be uf liest steel. The jaw opening Ui Ihi 10 iiichvH by H iiiolnsa.

OiH! Utiwtluy, iiv Oru Screen, 4 fout wide by 10 feet long, witli riHlH, iiuts, H|Mi;iiig u'nshers, &c., complete. The 10 feet ImrH tii Im.' of tii|iei* hucUdii, giving nn opening nt the top of inches.

Fui' Fiivwr tiiiit CliiiliiierH'H Automatic Ure fucdera, of fraciie [ultviii, with Hhout iron hoppura, revolving feed plates, sprin). bmku Hheuls. ningtt, nud nil ncccssorioH. Feed plate linen of inilil Hteul to lie Hup[iliud of removuble jNittem.

Fi'iir Stui'I Fueil ColliirB, borod and fitted to steam with set scruwH, for aetunting the feedons.

Stasii's.— Twenty Stiimp Battery of 1,050 lbs. weight eiicli HtAinp, lUTiuigeil to bo driven by four cam shaftti, by belts and tiglitvnurHfroiii Ktaui{i onuntenshsft. The Battery being caniplute in as follows :-

Fi>iii' Krasvr imd ChiiliiiurHH latust pikttem Homustake Mortars, of h;t[d L-liPSf-grriiufd iait iron, plHnml upon bottom with fouixlatii>n 1 II ill liolt'n di'illud to lemjilatt.', ilitisHud for Hereon fntuies, nd litU'd fur (.'iipiKr lining. MnrUkr to be complete with screen, of ri>iiii<l or slot jinnched liuHHia ii-on or niru cloth an iiiHy be desired, scri.'!.'!! fr.imi.' of hai\l wood. scri;uii frame keys of wrought iron,

Appendix B

chuck blocks of wood with copper lining fitted and fixed, diuck block keys of wrought iron, steel liners, foundation bolts, back copper liner, &c., &c.

Four pieces of best quality Rubber Packing for mortar bottom, for mortar to rest upon, punched for foundation bolts to correspond with mortar bottom.

Twenty Refined Iron Stamp Stems, having both ends tapered 80 as to be reversible. The stems to be turned full length and fitted to heads.

Twenty Best Crucible Oast Steel Tappets, bored for stems, and fitted to suit the cams, and to be reversible, with gibs and keys. Tappets to have hard, broad faces.

Twenty Best Crucible Cast Steel Stamp Heads, with drift holes for stems and shoes, recessed for the head of the shoe and for the tapered ends of the stems.

Twenty Hammered Forged Steel Battery Shoes of Fraser and Chalmerses special mixture of steel.

The weight of the Stamp is made up as follows : —

Total 1,050 lbs.

Twenty Hammered Forged Steel Battery Dies of Fraser and Chalmers's special mixture of steel.

Four Refined Iron or Steel Cam Shafts, with keywaycutfor cam shaft pulleys, each with two collars and set screws ; the cam shafts being turned full length and drilled for cams.

Four Cam Shaft Pulleys, built up of wood segments on a pair of cast iron sleeve fiangcs keyed to the cam shafts, the fianges being bolted through the wwxl before the pulley is turned up to its recjuisite diameter, with crown face, and then {tainted.

Four Outside Cam Shaft Boxes, of close hard cast iron, 12 inches long, fitted with babbitt metal, well liainmered to make it fit perfectly into the casting, and then bored for cam shaft. The

Gold Milling

to be oompleta irith caps and bolts for mme, ami la pW he*ii tnt oun ahkft cullnrH.

Two Dnuble Middle Cam Shaft Boxes, of close hard cast inm,

i SO inchOK long, fitted with babbitt tuetol, well hammered lo nuke

it flt poKoctly into the costing, and then bored for cam shaft.

. Tho boarings to be complete with capH and bolts for same, and

D ba faced for cam shaft collars.

Twtaty Bent Crucible Cast Steel Cams of double arm patten, y bitnid and llttd com shafts, with Blantuo patanl

Puur Jack Shafts nf rolled iron for hanging op the ttimpa.

Eight Jack Shaft ChaitG or bearings uf ctist iron, uidi lag KMwa fur fixing batUiry timbont.

Twunty Hockctn for linger pieces, "f cast iron, leather !inl.

Twoiity Wmiden Finger Pieces for holding up stHiii|is, dresised to lit the Hi>ol>t)ts, and each 6tted with wrought iron ti]>K and malleable iron handles.

Four SotH of Hud Wood Guides, bored tor stems, with bolts, nulH and washers complete. Each set comprising the tvv suctions for top and bottom guides of live stamps.

IhieCouiiiletuSotof Wrought Iron, Lap- Welded, Screwed and Oou[Jl, Water Pipos for 20 stamp battery, with all valves, lees, rMliiccrs. Iieiids and connections for feeding to mortars, and ninin supply piiio of a length to connect to a tank supposed be iiiinmliatiOy nulside the mill building, the main being sufliceiitly fi<r tho full supply of a 30 stamp mill.

Ki'ur Ixutjths of Hose tm washing the nmalgamatd copper l)Utt. with bib cimnectioiis to the water senico ,.t the battery.

t'opju'rs. — Four (hitside Copper Plates 12 feet long by width tkj iui<nar, by J inch thick, tu beof best soft roUi'd snKxxh 9tHr<a>,wp|vrf<>r amalgamating purposes, to lie arranged bcfon? (-ach stam) withiHii noting upon or against the mortars. tk' K' .V. I'rtf as {H<sihle from jar.

t."vat. I W Carriage Crawl aiHl Track Iron for ihc swill', tth H.nnl si-ri'ws for tixiug lo crawl btams. y Khw svt Difcrvntial PuUi-y blocks of 30 cwt. capacity, with

Appendix B 581

Amalgam Safe, &c. — One Amalgam Safe, with sheet iron sides, with strainer and padlock.

Retort, Ac. — Ten inch Gold Retort, dressed out inside, and properly fitted with cover, bale, wedge and condenser pipe.

One Complete Set of Ironwork for a bullion furnace 16 inches diameter, including bars, doors, &c.

Clean-up pan — One Fraser and Chalmers's Clean-up Pan of 30 inches diameter, with iron sides, and with step pulley for driving the same at variable speeds.

Transmission, &c. — One complete Set of Countergear for the whole of the above machinery, in accordance with drawings to be supplied by us, including: —

One Main Line Shaft, with receiving pulley or half coupling to correspond with power connection.

One Clean-up Pan Countershaft.

All the above to be complete with pulleys, key ways and keys, bearings, couplings, collars, set screws, belting and lace leather for driving the whole of the above machinery from steam or water power as specified.

Four Belt Tighteners for battery belts, with shafts, hand wheels, racks, pinions, <&c.

One Belt Tightener, with shaft, hand wheel, rack, pinion, &c., for crusher belt.

Building Bolts. — One Complete Set of Nuts, Bolts, Rods and Washers, Straps, Angles, fcc., fcc., forming all the ironwork for the buildings to enclose this machinery ; the buildings supposed to be of lumber.

Total weight, approximately, 57 tons 3 cwts.

Price £

Frasek and Chalmers, liTD.,

8Pboipicat10Ns For Battkrt Pbahe'Ors

J StainjMPTaiujevnil. d Georgia Pilth Pint. ; comprising Miii- 1, BnUt7 Pils. IImut; , MikIstIU. fJuiie Btms Itvk Strut*. F\-t-.l Fl.-; Poets, JoistsandSupportA, Tightener Guides and Suppurta. &c. all properly cut to aiie, dressed, mortised, tcnuoed and GtMd, B"lt Rules drilled, erected and marked to plac, after which the framework to be properly painted, taken down and protected for ahipmenl.

l>ne Complete St I'f Bolts, Rodn. Tumbucklea. Nuts and Washers, for Battery Fnmework cuaiplete. includ>n|{ Mortal Fi-undation B<>lt&

Phaser *sii Cbalmerm. Ltil, Lundini.

S

so

s

s.

s

a

s

Tarlr II.— SSeWiytt* yiftd of IniUion per bm on eorrafniailt

Ou v-HghU prills oburinfii/rom mt

TItId or BolUon. I

Pn-ToDofMOtl*.

omoss

oi. .Iwt era.

otdwt 0 0 7

0-Ooh

0 B 5-4

0 s la-s

0 G 20

O-OO'l

ono;

0 S 4

n-010

0 Is 1-8

1 e 3-2

0-oao

1 Is 4-8

1 Is 0

3 Is 9-6

o-o;o

otiso

B 1 12-8

Oom

B 17 U-4

6 B 0

S Is 16

O-Mo

Is 12 0

as 6 16

1 osoo

3fi 0 0

o-soo

Gs S 8

U 6 16

aiioo

Iso 19 8

Im 0 0

TvRi.F iir-

-Sho

rijjif

Ji

tfi'ro/'

t

Fineness in parts

Sterling.

per 1000.

American Currency.

Bank

Ratti.

Mint Rate.

£

9,

d.

£

d.

$

cts.

S87

ABLR y. — Convertion of ounces, Ac,, per long and short ton into

grammes per metric ton.

Orunmes

per metric

Orammes

per metric

ton.

Ouneea, Ac,

Ounces, 4kc.,

per ton.

gn.

A.

B.

A.

B.

oza.dwts.

OZ. dwte. gTR.

t

' 0

Note. — If the ounces, &c., are mven on the short ton, the correiponding number of grammes will lye found in cohimn A ; if on the ong ton, in column B.

Index

.bsorptivity of gold, 26 Lccumulatiou of amalgam on

plates, 322 lahesi?e power of mercory, 49 Alaska Treadwell mill, 463, 475

— Mexican mill, 475 Alternating currents, 503 inotropic gold, 20, 23, 44 Uloys, 66

constitution of — , 67 — of gold, 66

specific gravity of

tensile strength of

— , 68 dluvial deposits, 3 gold, production of, 6 duminium and gold, 82 onalffam, cadmium, 80, 309 cleaning of — ,441 differential squeezing of — , 442 gold — , 84

— knives, 321 native — ,16 retorting — , 446

— scales, 441 silver — , 316 sodiimi — , 66, 308 squeezing of — , 441

imalgamating barrel, 434

— pans, 386 Lnialgamation, 383

— assay, 526 inside — , 305 outside — , 310 uan— , 383, 384, 386

iznftlgamators, 396

Amalgamators, Laszlo — , 396 Amalgams, 52

— of gold, 84 Analyses of native gold, 15 Annealing, 24

Antidotes to mercuiial poisoning,

Antimony and gold, 77

— and mercury, 53 Aprons, 161, 310 Arrangement of mill, 268

— of stamps, 208 Arrastra, 278, 287

construction of — , 288 custom — , 480 working of — , 291 Arsenic and gold, 44, 76

— and mercury, 54 Asynchronous motors, 503 Assay, amalgamation, 526

— balance, 518 cyauidation — , 551 fire — , 532

— furnace, 515

— of bullion, 552

— of concentrates, 638

— of ores, 526

— of tailings, 549

— results, 549

— tongs, 535 Assaying, 514

Assays, calculatiuii of, 546 Association uf minerals with gold, 6 Atomic weight of gold, 26 Auric bromide, 34

Auric chLmd*. JO

Brtakus. 103 1

BMking ore, 101 J

BniMiD. and gold. 37, U i

IhuHl.M4* 1

nuKl-,MI 1

-pare— . M7

Bii>ldiiqE.lornill.49$

Awui btoBiali

Bvlliimlr. 552

- Ulanoe. 518

nloe of -. 5SS

-Miiik,u

~Mi4e,n

— ma mcrenry, 54, SO*

CaldnalioD of conixatnif, tt.

— diloni*. 30. M

— ul gold ores, 19, 587

— Mid..

OdeuUtion of 5M

CWifoniiau iii, 387

Calomel. 58

CwaatTe, 20i, 58S

asT— ,SI8

bnUlDIi— , 51B,52S

CmtUL, IM

B.llr — , Ml

Ban mUI. 279

BUdUd — . IBS

Buk Cbtzia Act. 559

Daria — , 208

HMt -, 901 '

— eLloriiation. 39fi

kiy-wajsol— , 198, an

loLrimlion of — . 2M

singlf-irtuwi -, 191, BBS

Baltfry irinU.inuiiuii. V>o

BraritiEs dF camshaft, 219

Bell Tanmrs, 3W

Casting gold, 457

ChallengB orr-rreder, 274

Bills. 265

CliainniB leather, 444

Black furnace. 517

Check assay, 555

Bkck llitis ...clhil, 463

tniitt.ir, 1S3

of merenry, 51

Blake crusbr. 104

Chilian mUl. 278

Blaukel -irakp-. 341

Chiinmiug, 350

— trounhs. a44

Chlorides of men-ury. 58

Klankptiiig!'. a4S

BluikeU. 34S

harrel — , 398

chaigea for 402

Boiling iwint of mi'trurr, i?

coet of — , 399

Bori77

Ftattner praceu of -, S6

Index

blorine and gold, 27, 30 hock (chuck) block, 152 huck shoe, 182

classification of crushing machines,

lean-up, 428 |obalt and gold, 38 oinage Act, 559 olorado process of milling, 468 'flour of gold, 25 otnbination of gold with mercury, pan, 389 ncentrates, 12, 336

amalgamation of — , 382

assay of — , 538

calcination of — , 384, 407

chlorination of — , 396

cyanidation of — , 407

sampling of — , 522

shipment of — , 406

smelting — , 403

treatment of — , 382 'concentration, 337 '-oncentrators, 339

Hendy — , 861 lonductiyity of gold, 25

— of mercury, 49 'ontact veins, 4 V>pper and gold, 73

— and mercury, 54

— plates, 311

absorption of gold by , 324

amalgamation of , 314

cleauing-up , 386

election-siWered , 317

inside , 150, 305

rubbing up , 316, 319

— tables, 310 'omets, 556 )orrosiye sublimate, 58 ]!ost of milling, 470

— of mills, 492 younters, 513 vers for mortar, 159 Cradle, 431 Crawford mill, 279 Jrawl, 258

Crucibles, breakage of, 458

— for melting gold, 453 Crushers, 103

Crusher stations, 113 Crushing machines, classification of, 277 tandem — ,113 Crystallisation of gold, 23

— of mercury, 49 Cupel mould, 541 Cupcllatiou, 540 Cupels, 540 Curtate involute, 567 Customs mill, 97 Cyanidation, 407

— assay, 551 continuous — ,416 cost of — , 418 plant for — , 409 principles of — , 407, 417 tanks for — ,410

Cyanide process — , 407

Cyanide of potassium and gold, 28

crude, 410

Cyanides of gold — , 36 Cyclops mill, 279

Deposits, 4

gravel — , 5

metamorphosed — , 4 Depth of discharge, 157 Dies, 164

material of — ,184

rock-breaker — , 103

wear of — , 186 Dimorphism of gold, 23, 44 Discharge, depth of, 157

double — mortars, 148 Disintegrators, 280 Dodge crusher, 103 Dolly, 91

Drop, height of, 215 Dunham's btauip, 284

El Callao mill, 477 Electric lighting, 511

— precipitation, 422

— transmission, 503 cost of, , 509

Q Q

Bitira neki, aqrifcrav, &

Mas

PilVHttJ.SU

FloariaiE of maaaj, M rani ol gM,

Gnat Britain, milUng il Gnalj. lOU

H&nllKKi, 1 1

— of water, 502 aiaiee —, 502 weight of —,167 HdKbt nf ilrap. 215 HniiJj nMicoutraUr, 361 Ili)-Kide OR*, iM Hoiat Tor aUntp SB8 Horn, 6M Tlowland pnlveriwr. 279

construction of — — , 2S

Wk to back -. 310

povr rainlrcd for

i-ast iron — , m

knei' — , '.'7

workiusof .301

llusUapil's |>ut:uniatic stamp,

Wi-uJeii — . 231

llyilraulic miiiini;, 4

Frmii'iMTk, latlen-, spivifiilions

for. :.<-i

Illumination, 511

lmimritWint-oM.68

Fnio vamier, WS

Inch, miner's, 502

.it.T re-iiiir,'.! for , 3tiS

India, milling ill, 481

F.irl.

Ingot nionlds, 157

Puliiiiii.itiii j.'olii, 43

Fumai-e. ,iiv, 'il,--

Inside niiialKnmation, ;S0li

ma.-k -'. 517

liivolmc, 202, 565

Hina — Mr,

cartate — , 6r

- forfii!.iii;.ti.iii, -".-.l

prolale — , 567

- f,.r H'l "ivltiiii:. Hi

lo.iineBiuisol,i, 27.33

Fiisil.ilitvi.fKul.|, 2.

Iron and gold, 81

Fiisioi, o'r ;iK;iy, ;.34

— and mercury. 55

.Inck-shaft, 255

Ciit.'Sinislirr, 107

.laws of rock-lircaker, 103

ri'il'."t'M'!"' 17'

.limval inrbiup, 501

<iirnr.| liirliirie, 601

Knrri womi, 231

Key for tapjHjls, 17i

fs of cams, 198 ime, 234 eisod, 237 3ll8, 280 :e, 14

imalgamator, 396, 468

g tanks, 410

id gold, 78

nulated — 1648_

lates for mortar, 158

e, 532

'er in — , 646

melting gold, 459

mercury, 333

ide ores, 462

tion of cams, 214

glides, 254

vanner, 364

ite, 16

lility of gold, 24

lese and gold, 81

1 of shoes and dies, 184

al stamp mill, 94

; gold, 453

ics in , 459

cipitated gold, 460 nge, 456 point of gold, 25 of mercury, 48 al |)oisoning, 62 c bromide, 66 oride, 58 npounds, 62 ide, 57 de, 59

eactions of , 62

phide, 60 ms bromide, 56 oride, 58 ni>oiinds, 52 ide, 57

Mercurous oxide, 59

— salts, 61 reactions of , 62

— sulphide, 60 Mercury, 48

chemical properties of — , 51 loss of — , 334

physical properties of — , 48 purification of — , 63

— room, 440 salts of — , 61

— spoon, 307

— trap, 332

— wells, 326 Hungarian , 329

Meshes of screens, sizes of, 143 Metallics, 534, 543 Metamorphosed deposit<i, 4 Mill, arran&ement of, 268

— buildings, 498

— flooring, 264

— framing, 230

— returns, 512

— roof, 498 Mills, cost of, 492

Hungaiian — , 468 staff employed in — , 489

Millsheets, 512

Mill sites, 495

Minerals associated with gold, 6

— mistaken for gold, 21 Miner's inch, 502

Modem process of milling, 95 Moisture in gold quartz, 99 Molybdenum and gold, 83 Moriaon's stamp, 285 Mortar, 128

Black Hills — , 153

— block, 123

— box, 128

— covers, 159

— liner plates, 158 low — , 128 Morison's — , 160 sectional — , 162 weightof — , 131

Motors, 503 Mould, cupel, 541

— for assays, 535

Q Q 2

— gold, la

coninontioii of— — , 1-1

Nrw ZeBLu'l, Tailling in, 481

Nkbel and gold. 81

Nitiates of ffAA, 39

— of mercnrj, 61 Nilrogen aad gold, 43

Nora Scotia, milEing in, 469, 40,

Occurrence of golJ. 2 Oil conraTitntion, 379 Ontnt of f>U of sUlups, 209

Orabii.*, 112,205

suapeinicd — — , 274

SWnfoni — , 270

Tullwli — , 270

Victor 270 Ogmiiiin and fild, S3 OTer-staiiipiiiKi ' Oxidt'sorRold, 37 OxjHtilti ol goM, rnllndic gold, 16 I>a1lu<)iiimsiidRold,83 l>a[i niualgauiution, -183, 36

Bi-rcian — , 393

Califomian — , S87

coniliimtiou — , 3S9

— assaj', 529 Puagrucsis ofgolil, 6 Fartiii;;, f)43, 5&(i

— acid, 545 Pelton wheel, 501 Percussion table, 3S5 Perrorntions of screens, sizes of, 13 rhn<|ili<irus and gold, 43 Physical propprties of gold, 23

PUtTorm, working, of stinip ni

PiienmatiG slampn. Polyiihase cnrrontti, StM Porpudte, 16 Potimillm anil gold. Power required liy %Um

wftter — , 501 — lam™, 383 Precipitating tanks. 113 Preliminvry tnatment of HqM

parting of — , 543

weight of — , 584 Primitive mills, 92 Prolate involute. .'>67 Prospecting nan, 539 Pulleys. 189

irof-

Piirificalinn of nien-iiry, 63 Purple of Casaiiia, 32, 11

Pyrites, 17

assay of — . 638 treatment of — . 382

Qiiiclisityer, ice Monniry

Reactions of gold, i — of mcrcnrv, 112 Kiir plates, ir,2

Index

ng agents, 532 aa, 12

oduction of , 6

9irs, 496

I of assays, 549

lindiical — , 449 t — , 446 ng, 446

tion counter, 513 m and gold, 83

mortar, 156

reakcr liouse, 113

rts of — , 103 '

'tional — , 105

ore-feeder, 271

om, 281

m of stamps, 212, 213

gold, 18, 21, 47

ion, 63

ng, 522

concentrates, 526

ores, 522

tailings, 524

ed, 615

pneumatic stamp, 285

ation, 548

rs, 548

frames, 132

I, 136

nclu'd — , 137

re—, 140

I tanks, 264, 496 g table, 355

istmlian , 359

ttinger's , 355

ilfley , 362

nt of concentrates, 406

iterial of—, 181

Shoes, wear of, 186

Siemens and Halskc process, 422

Silicon and gold, 44

Silver and gold, 72

— and mercury, 64, 316 Sites for mills, 496

Size of crushed particles, 144 Sizing, 372 Skimmings, 445

treatment of — , 459 Slimes, 147

cyanidation of — , 424 Slot screens, 137 Sluice head, 603 Smelting concentrates, 403

charges for , 405

Sodium amalgam, 55, 308

— and gold, 83

— and mercury, 65 Solubility of gold, 27

in mercury, 85

— of mercury, 51 Sorting, 116

cost of — , 119 Spanish mine, 303, 478 Specific gravity of alloys of gold, 68

of associated minerals, 1 1

of gold, 23, 45

of mercury, 48

Sjiecifications for stamp mills, 493,

— for battery framework, 582 Spitting, 73

Spitzlutte, 376

Splash boards, 135

Split tappet. 172

Squeezing amalgam, 441

St. John del Key method of

milling, 467 Staff at the Alaska Tread well

mill, 190

— at the Alaska Mexican mill,

— at the Dakota mills, 190

' Suit at tlii< Nhw PrimroM mill. 491 Tapjwtii, i;il>, 173

HUni{i mill, Culifuruiin, 120 in tlio MfOdlB Aps, 1'4

Stamp, Klophaat, 2i

- iiiiuniiitie, 'isn Hicttm — S9'i

artnngomoiit of — , SOS pnwor roquiroil Iiy — , 25 totting u]-!— , isa (H.'il.if— , 215

Htamv-iuppOTtfl, 2SA

atud&nl golil, 73

Sliinfonl nra-fopilflr, 370

Sulplmtvi of RoM, 40

— of mercury. 111 Sulplinr niul riiUI, 20, 2D, '

— and niprcore , 51 , GO Siilpliiin-tn, 12,.t:l9

SHii

cik'iiiatioii of - cliloriTialion "t fllii[>meiit of — , 4nii HiiiltiD(;of — , 40:) trciitiiipiit — . i'*-2 tynclirotinus luulors, 503

Tnbk'S, ppr,

shaking — , 355 Wiifley — , 362

"Til"ormr.iiry, no

Tailings, assBy nf, 511) oyiiiiiiliktion of 407 rcKurvoirfl fur — , 4i>ti sumpliiigiif — , Wl Irtntiiii'iitof , 33(1

Tmikn fnrlttii'liiiii,', UO

Tonftcityof gold, 24

Teiuile strength ot allow of ggM<

--ofjt

I, 24

Tin ntid iC"l>l, T£> Tonjt*. 468

tiukel — , 456 — for assays, 535 Tuffiing tab, 3St Triuiiways, 437

wireropp — . 4?7 Tmnifiinntrs, 505 Tritnsmissiou of power, rliHrtrli',

'rulluchwfprdtr.'J?!!

Tunsstcn und gold, 83

Turbines, 501

(Jiranl — , r>01 Jonval — , 601

Typ9, 354

nbrey —,364 Fnie — , 364

power required fur ,

water rcqoire.1 for ,

Liihrig — , 364 triumph — , 364

fissure — , 4 Victor i-TB-fccdcr, 270 Volatility of uUl, 25, 90

Index

59

, milling in, 486 , head of, 483, 502 pes, 262 wer, 501, 583

antity of, used in stanij) mill, 263, 324 apply, 262, 495 3f dies and .shoes, 186 ' punched sci'eens, 143 ' woven wire screens, 143 for shoes, 183 ing gold quartz, 98 t of mortar lx)xes, 131 quartz, 100 , Pelton, 501

Wheel, vortex, 502 Wilfley table, 362 Wind furnace, 515 Wire roi>e tramways, 497

— screens, 140 wear of , 143

Witwatersrand, cyanidation the, 408

milling on the — , 465, 487 Wooden frames, 231

— guides, 249

Zinc and gold, 80 — boxes, 412

on

Tjie End.

KlC ilAia> CLAY A.NU SONS, LlMlTtD, LONDON AND IiL NGAY

m

A handbook of gold milling

Overview

Preface To The Fibst Edition

Preface Xl

Preface To The Second And Third

Contents

Chapter I

Chapter Ii

Chapter Iv

Chapter Viii

Chapter Ix

Chapter X

Chapter Xii

Chapter Xiv

Chapter Xv

Appendix B

Appendix B