A Manual of Fire Assaying

Charles Herman Fulton · 1911 · Historical

The subjects of assay furnaces, assay fluxes, cupellation and special methods of assay have been added to. Recent progress in Assaying has made it desirable

This 1911 document, A Manual of Fire Assaying, is preserved in the Mountain Man Mining Library for research and reference. Original source: archive.org.

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Fire Assaying

Published by the

McGraw-Hill BookCompmvy

Successors to the Book Departments of the

McGraw Publishing Company Hill Publishing" Company

Publishers of Books for Electrical World The Engineering and Mining; Journal

Engineering Record American Machinist

Electric Railway Journal Coal Age

Metallurgical and Chemical Engineering f\>wer

J S'mtf YORK i-AiJLICUBRltY

A*TOR, LENt* AM*

A Manual

Of

Fire Assaying

By

CHARLES HERMAN EULTON, E.M.,D.Sc

PROFESSOR Or METALLURGY, CASE SCHOOL OF APPLIED SCIENCE.

SECOND EDITION Entirely Rewritten and Enlarged

McGRAW-HILL BOOK COMPANY

239 West 39Th Street, New York 6 P Street, London, E. C.

Copyright, 1907, by the Hill Publishing Co.

COPYBIGHT, 1911, BY McGRAW-HiLL BOOK COMPANY.

All rights reserved

Printed and Electrotyptd ' The Maple Pres York. Pa.

Go tie Aotbet

This Book Is Lovingly Dedicated Bt The Author

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Preface To The Second Edition

Recent progress in Assaying has made it desirable to add to the book and revise it.

The subjects of assay furnaces, assay fluxes, cupellation and special methods of assay have been added to.

The author desires to express his thanks to Mr. W. J. Sharwood for additional material furnished for the book and for a search of the first edition for errors.

He also acknowledges the kindness of Messrs. Jay A. Carpenter, K. Van L. Smith, O. A. Anderson and others in furnishing new material.

He will appreciate greatly the courtesy of assayers, or metallurgists who will call his attention to errors or pertinent omissions in this edition.

Charles H. Fulton

Cleveland, Ohio, December, 1911.

I'KKKvrK niK K1R**T EDITION

.;ii'i. i i '"'n- t.'i-iv n -h'm-ix 'rii.r.cii 'v *in? subject of fire

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Contents

Pages Preface vii, ix

Chapter I

Assay Furnaces and Tools 1-26

Assay Furnaces. Fuels. Coal-burning Muffle-Furnaces. Construction. Dimensions. Wood-burning Furnace. Coke Furnace. Gasolene Furnace. Gas Furnace. Capacities of Furnaces and Cost per Assay for Fuel with Different Furnaces. Muffles. Furnace Tools, Tongs, etc. Multiple Scorifier Tongs. Cupel Charging Device. Molds. Crucibles. Scorifiers, Roasting Dishes, etc. Crushing and Pulverizing Apparatus.

Chapter Ii

Definitions; Reagents; the Assay of Reagents 27-35

Definitions of Assaying. General Method for the Determination of Gold and Silver. Reagents Used in Assaying. Assay of Reagents.

Chapter Iii

Sampling 36-41

Methods of Sampling. Classification of Sampling Methods. Principles of Sampling. Sampling in Mills and Smelters. Coning and Quartering. Sampling by Alternate Shovel. Sampling Apparatus in the Assay Laboratory. Preparation of the Assay Sample. Control Assays. Umpire Assays. Mode of Settlement. Sampling Lead and Copper Bullion.

Chapter Iv

Weighing; Balances and Weights 42-52

The Assay Balance. Method of Setting Up. Construction. Discussion of the Principles of the Balance. Sensibility. Weighing. Determination of Length of Balance-arms. Pulp Balance. Practical Notes on the Assay Balance. Weights. The Assay-ton System.

Chapter V

"eduction and Oxidation Reactions 53-63

Reduction. Oxidation. Reduction of Lead from Lithargo 1, by Carbo" Kv Flour, by Sulphides. Influence of

Xll CONTENTS

Pag Soda. Reduction of Lead from Lead Silicates. Oxidation of Impurities by Niter. Niter Reactions with Reducing Agents. Reaction between Metallic Lead and Niter. Niter and Carbon. Niter and Pyrite. Influence of Silica on the Reactions. Charges to Determine Reducing and Oxidizing Powers. Oxiding Power in Ores.

Chapter Vi

The Crucible Assay; Assay Slags 64-75

Nature of the Crucible Assay. Influence of Fineness of Crushing. Mode of Occurrence of Gold and Silver, Physical Properties of the Slag, Chemical Properties of the Slag. Formation Temperatures of Assay Slags. Nature of Assay Slags and their Mode of Formation. t Influence on the Assay of the Formation Temperature. Constituents of Assay Slags. Classification by Silicate Degree. Influence of Silicate Degree and Different Bases on Formation Temperature. Eutectic Compositions of Slags. Table of Assay Slags. Table for the Calculation of Slags. Example of the Calculation of an Assay Slag. Composition of Ores. Assay Slags Commonly Made. Color of Slags.

Chapter Vii

Cupellation 76-106

Object of Cupellation. Bone Ash Cupels. Making Cupels. Magnesia Cupels. Cement Cupels. Process of Cupellation. Sprouting of Beads. Freezing-point Curves of Lead-Silver and Lead-Copper. Temperature of Cupellation. The "Uncovering" and "Freezing" of Lead Buttons. "Feathers" in Cupellation. Influence of Impurities on <he Process of Cupellation. Influence of Copper. Influence of Tellurium. Cupellation with Cupels of Different Materia.

Chapter Viii

Pasting 107-110

Ratio of Gold to Silver Necessary to Part with Nitric Acid. Inquartation. Strength of Acid. Temperature of Acid. Parting Devices. Annealing.

Chapter Ix

IflBAT of Okes Containing Impurities 111-130

IMnition of Impurities. Common Impurities. Effect of

, on Assay. Production of a Matte. Effect of Silica.

4 Soda. Kind of Impurities. Standard Methods of

48ting. Niter Method. Miller's Oxide-Slag

"tins' Excess-litharge Method. Niter-Iron

Contents

Pages Method. Nature of the Iron-Nail Fusion. The Cyanide

Method. Comparison of the Different Crucible Methods of Assay. Scorification. Scorifiers. Amount of Lead used. Amount of Ore. Process of Scorification. Temperature of Scorification. Rescorification. Order of Oxidation of Metals. Applicability of Scorification. Combination Method. For Blister Copper. For Mattes. For Cyanide Precipitates. Precautions to be Observed in the Method.

Chapter X

Special Methods of Assay 131-159

Telluride Ores. Assay by Cripple Creek Flux. By Excesslitharge Flux. Behavior of Tellurium in the Fusion. Amount of Tellurium Present in Ores. Losses Caused by Tellurium. Assay of Complex Tellurides with Different Fluxes. Results Obtained. Assay of Copper-bearing Material. By Scorification. Results Obtained. Excess-litharge Method for Blister Copper. By Crucible Assay. Assay of Material Containing Zinc. Effect of Zinc. Scorification for Zinc Ores. Crucible Fusion for Zinc Ores. For Cyanide Precipitates. Assay of Material Containing Graphite. Assay of Antimonial Gold-silver Ores. Arsenical Ores. Difficulties Experienced with these Ores. Assay of Heavy Sulphides. Assay of Material Containing Metallic Scales. Assay of Ores Containing Free Gold. The Assay of Slags and Cupels. Amalgamation Test to Determine Free Gold Present. Assay of Cyanide Solutions. Assay of Material Containing Metallic Iron.

Chapter Xi

Errors in the Assay tor Gold and Silver 160-173

Losses in the Cupellation of Pure Silver. Of Pure Gold. How the Losses Occur. Effect of Temperature*. Influence of Different Types of Cupels. Curves Showing Cupellation Losses. Losses in the Cupellation of Gold-Silver Alloys. Relative Amount of Loss by Absorption and Volatilization. Slag Loss and Cupel Absorption in Telluride Ores. In Zinciferous Material. In Highgrade Silver Ores. In Cupriferous Material. General Discussion of Losses. Other Errors. Retention of Lead or Copper in Beads. Retention of Silver by Gold after Parting. Loss of Gold by Solution in Acid. Occluded Gases. Error in Weighing. Resumed

Chapter Xii

The Assay of Bullion 174-185

Classification of Bullions. Assay of Lead Bullion. The Assay of Silver Bullion. Cupellation Method, Preliminary Assay,

xiv 'contents

Pages Check Assay, Regular Assay. Gay-Lussac Method for Silver Bullion. Standardization of Solution, Apparatus Required, the Assay, Calculations. The Assay of Gold Bullion for Silver by a Wet Method. The Assay of Gold Bullion. Preliminary Assay, Check Assay, Proof Alloys. Regular Assay. Preparation of Proof Gold and Silver.

Chapter Xiii

The Assay of Ores and Alloys Containing Platinum, Iridium,

Gold, Silver, etc 180-192

Difficulty of Assay. Composition of Platinum Nuggets. Cupellation of Lead Containing Platinum, etc. Appearance of Cupeled Bead. Action of Acids on Metals Contained in the Platinum-Silver bead. Nitric Acid. Sulphuric Acid, Nitrohydrochloric Ac'd. Methods of Assay to Obtain Lead Button. For Ores Containing Metallic Grains. For Alloys. Method of Assay by Parting Silver-Platinum Alloys in Sulphuric Acid, etc. Method of Assay by Dissolving IxmuI Button in Nitric Acid. Results Obtainable.

Chapter Xiv

V:-ik As* ay ok Tin, Mercury, Lead, Bismuth and Antimony. 193-201

iieueral Remarks on the Fire Assay for Base Metals. The

Assay vf Tin Ores. Causes of Loss in the Assay. Preparaiou

the Ore for Assay. The Cyanide Method. The Gcr-

-:uui Method. Results Obtainable. The Mercury Assay. Ap-

atatu* Required. The Conduct of the Assay. Results

"tamable. .Way of Lead Ores. Inaccuracies of it. Lead

'ixMfttiuxL Soda- Argol Method. Cyanide Method. Reac-

i.u the Assay. Results Obtainable. Tho Assay of

and Bismuth Ores.

Table for Weights. Table of Assay Valuations. of Bullion and Alloys of Precious Metals. Weight of Fine Gold and Silver.

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List Of Illustrations

Fio. Page

1. Two-Muffle Furnace. Perspective View 2

2. Two-Muffle Furnace. Cross Section 3

3. Two-Muffle Furnace. Longitudinal Section 4

4. Three-Muffle Furnace. Cross-Section 5

5. Three-Muffle Furnace. Longitudinal Section 6

6. Oil-burning Furnace. Cross-Section 7

7. Wood-Burning Muffle-Furnace. Cross-Section 8

8. Wood- Burning Muffle-Furnace. Longitudinal Section 9

9. Wood-Burning Muffle-Furnace 10

10. Muffle-Furnace for Burning Coke 11

11. Combination Muffle and Pot Furnace 12

12. Combination Muffle and Pot Furnace 12

13. Gasolene Furnace Apparatus 13

14. The Cary Gasolene Burner 13

15. Gasolene Tank and Pump Apparatus 14

16. Gasolene-Burning Crucible Furnace 15

17. Gasolene-Burning Muffle-Furnace 16

20. Muffle Furnace with Special Supports 19

21. Gas-Burning Muffle-Furnace 20

22. Crucible Tongs. Undesirable Model 21

32. Fire-Clay Annealing-Cup Tray 23

39. Diagram of Assay Balance 43

42. Non-Column Type of Assay Balance 50

A Manual Of Fire Assaying

Chapter I Assay Furnaces And Tools

FURNACES. — The furnaces used in assaying are many in design, varying mainly with the kind of fuel used. The furnaces are classified as follows: (1) Pot furnaces, in which the assay is in direct contact with the fuel; (2) Muffle-furnaces, in which a muffle or receptacle containing the assay is externally heated.

As the muffle-furnace is practically essential 1 for carrying on the operations of scorification and cupellation, and crucible fusions can be made satisfactorily in the muffle if it be large enough, muffle-furnaces have largely replaced pot furnaces for general assaying. In general, they are cleaner, more easily operated, better controlled as to temperature, and if large enough are of great capacity, which makes them especially desirable for smelter, mill and mine assay offices, where frequently a great number of assays are performed daily. The choice of fuel for heating the furnaces is usually dependent on locality. Bituminous and lignite coal, coke, anthracite, crude oil, gasolene or kerosene, wood, fuel and illuminating gas are all used. Of these, coke and anthracite are the fuels least desirable for mufflefurnaces, for burning without flame they must surround the muffle. This makes the firing difficult, requiring considerable' attention. The best fuel, usually also the most easily obtainable, is bituminous or good lignite coal, yielding a long or reasonably long flame. One-, two-and three-muffle furnaces, constructed of fire-clay tiling, fire-brick, and common hard brick, tightly bound with stays and rods, are in common use, and for general utility, wheTe much work must be performed, are very desirable.

"Koenig's Furnace," in Trans. A. I. M. E., XXVIII, 271. This furnace is practically a pot furnace fired by gasolene, and with an air blast can be used to scorify and cupel without a muffle.

A Manual Of Fire Assaying

Coal, Coke and Oil Furnaces. — Fig. 1 shows such a twomuffle furnace in perspective, and Figs. 2 and 3, in cross-section, The essential parts of the furnace, as the tiling, A, B, L, K, etc., can be readily purchased, although the interior of the furnace may also be built of fire-brick. The tiling furnace, however, is more easily set up and is more durable. In the design of the soft-coal furnace, the essential dimensions are: area of fire-grate; distance from the grate to the bottom of the lower muffle; the

" fire space," i.e., the distance between muffles and the side and end walls of the furnace, and between the top of the upper muffle and the roof of the furnace, giving the proper space for combustion of the gases. These dimensions depend upon the nature of the coal. In Figs. 2 and 3 the grate dimensions are 17.25 X21.0 in.; distance from grate to lower muffle, 18 in.; fire space, 2.5 in.; external dimension of muffle, 19 in. long, 12.25 in. wide, 7.75 in. high. The flue area should be from one-sixth to one-eighth of the grate area. The flue is best placed forward of a line through the center of the muffles to get the full sweep of the flame around them, although this arrange-

Fio. 1.— Two-muffle Fuhnace. ment w J t 1 poor d ra ft i 8 apt to Cause Perspective view.

smoky muffles. The walls of the furnace are thick (13 in.) to prevent radiation. The front of the furnace above the muffle is arched. The arch tiling has in it a duct, leading to the flue, to carry off lead fumes. The muffles are supported by two sets of tiles, placed into the side walls and sometimes by an additional set in the rear end wall. These tiles frequently prove weak, and in falling away leave the muffle without support, causing it to be short lived. The supports are best made in such shape, of two pieces, that they will join under the center line of the muffle and arch over, supporting each other. The writer has used supports of this type, which were perfectly satisfactory and increased the life of the muffles greatly. A furnace of the kind described has a capacity of 25 to 30 fusions (20-gram crucible) per hour, including the necessary cupellations. If the fusions are made in 30-gram

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. scorifiers, the capacity is from 20 to 24. s furnace burns from 37 to 47 lb. of coa! >7.00 per ton, makes the cost per assay for 0.80 cents to 1.00 cent, when assaying conwhat more when the furnace is not charged opacity. With a good grade of coal (6500

calories), a maximum temperature of 1150° to 1200° e obtained in this furnace after 4 hours firing. Figs. 4 off a three-muffle furnace of similar type. irnaces may also be readily modified to burn crude oil. bo done by placing tiling in the fire-box, and making isary pipe and burner connections. 1

ivinun. "I'ruilo Oil for Fire Assaying," Proc. Colo. Sci Sop.. VII, 311.

4 A Manual Of Fire Assaying

Fig. 6 shows such a furnace. The burner is a J in. pipe connected by a T to the oil line, also a J in. pipe. A £ in. steam pipe passe.-* into the burner pipe at the rear through a packing nut which permits of the adjustment of the distance between the nozzle of the burner pipe and the nozzle of the steam pipe. By varying this distance the flow of oil may be affected

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independent of the steam and oil inlet valves. The nozzle of the burner pipe is an J in. hole and that of the steam pipe an i in. hole.

The grate bars in the furnace are covered with fire brick as shown in the illustration. The placing of the fire brick is of importance as the successful working of t ho furnace is dependent upon their position.

In starting the fire a piece of oiled waste, box just back of the burner. When k oil and steam are turned on sinuiltAH valves are then set to give the pi should be used to furnish a plato V

Assay Furnaces And Tools

to ignite the oil, otherwise explosions are apt to occur. The steam used should be dry, and to insure this the Bteam pipe leading to the burner may be passed around the flue as shown in the figure. The valve DV at the end of the steam line is kept slightly open during working to permit the escape of water of condensation. A small steam coil may also be placed in the oil tank to keep the oil more fluid. The furnace may be heated to a red heat 15 to 20 minutes after starting. The furnace has a capacity of 25 to 30 assays, including cupellations in 1$ to 2

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hours and 50 to 60 assays in 3 hours. The amount of oil used varies from 4.2 to 5.3 gals, per hour. With oil at 8J cents per gal. the cost per assay for fuel is 2.2 cents to 2.8 cents. Figs. 7 and 8 show a wood-burning muffle-furnace. In some districts wood ia the only available cheap fuel. If the fire-box and fire paces are properly designed (i.e., of larger size than in the Mil furnace) and a deep bed of fuel is provided for (i.e., the distance from the grate surface to the bottom of the fire-door is from 8 to 10 in.), sufficient temperature for ordinary assaying oanbe attained in this type of furnace. Almost any wood may used.

A Manual Of Fire Assaying.

In the furnace shown in Figs. 7 and 8, the grate is 18 in. wide and 26 in, long; the distance from the grate bars to the bottom of the muffle is 26 in. and the fire space is 2.5 in. wide at the sides and 3.5 in. at the top. Fig. 9 shows a wood burning furnace of somewhat different construction. 1 With pifion pine or fir wood at $4.50 per cord, the cost of fuel is 65 cents for a daily run of .10 to 40 assays. With a poor grade of wood at

per con), in another instance, 30 assays cost 93 cents, or ;t. I cnus per assay, including cupellations.

Coke and anthracite inuHlo-furnaces when used are usually smaller, although lure furnace* may be specially designed and (mill of the pliers! type of the coal furnaces described.

Fig. 10 shows !i small coke or anthracite furnace. The fuel is led in at the lop and kepi well heaped around the muffle. A furnace of the kind shown in Fig. 10 will consume front 33 to 38 lb. of coke per hour, according to draft. With MflKfiLXM in., 10 assays per hour, including cup

Assay Furnaces And Tools

ftVitl*-/ muffin for f.imww are made, in varying iiaes and 4hpM. Th* hart for (rental iwe is one of nearly rectnnffuUi* #*tifrt i , *ifh but a alljefady arched top. The lar- #t:*t. rtfttHfi ordinarily mwi are 19 in. long, in. wide and IV, in hi*h 'ottf,idft dimension.-). Jluffies 19 in. long. 12 in. ftfid 7.7.', in. hijrh a: very common in coal furnaces. The * haw: two holfia in t.hft rear end to induce an air draft

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Tlif tin' I In id uf diijumi'l tin' muffle in the furnace has great lumiliin "it I lu-life of tlio muffle. Muffles inadequately suplunli'd muni I'nii'k nmt full In pieces. It is perhaps better to muffle* liv one substantial rather broad support near the middle and Hero** the whole bottom, and by resting the limit end on the furnace wall Mini tiif rear end on two replaeable i'l:i\ "iioi'olls. than to have more nuiiierous supports extending ithtltumw onl\ beyond the walls on the bottom. Muffles vliould be stored in a dry warm plate to prevent their absorbing moisture and when new muffle* are placed in a furnace. ;- should be tiled bsliilv w ith wood chips for an hour to anneal the muffles.

Assay Furnaces And Tools

before heavy firing is begun. The spilling of slag and lead in the Xnuffle rapidly leads to corrosion and softening of the bottom and consequent destruction. To avoid this deterioration in part, muffle bottoms should be covered with a layer about £ in. thick of bone ash, silica sand, or Portland cement, to act as an absorbent. Muffles are also subject to destruction from the fluxing action of the ashes of the fuel burnt on the grate. Ashes high in iron oxide are the worst in this respect.

In setting muffles, it is essential for the attainment of the best heating conditions to thoroughly lute up the space around

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the edge of the muffle and the arch opening in the front of the furnace into which it fits. If this space be of considerable size, it is best filled in roughly with chips of broken crucibles, etc., before applying the luting material. Luting material may be of fire clay and crushed fire brick or crucibles, one-fourth of the former to three-fourths of the latter, mixed with sufficient water to make a plastic mass. The fire brick may be crushed to pass an eight mesh screen. Raw fire clay has too great a shrinkage to be used alone. One-quarter fire clay, one-quarter shredded asbestos, and one-half crushed fire brick makes a good luting material.

Figs. 1 1 and 12 represent a combination of crucible pot furnace and muffle furnace, such as is used in England. Tbe furnace may be built of ordinary fire brick and is dimensioned in such a manner as to avoid cutting brick as much as possible. It is fired by coke. The two crucible furnaces S connect with the main flue, 9 X9 in. in size, by the flues -V. The crucible furnace nearest the muffle also connects with the muffle furnace by the extra flue 0. By these means the hot gases of combustion may

lie diverted to the muffle furnace, instead of directly to the stack. The pot furnaces are 14X14 in. in section, and on account of the sloping top, are 17 in. deep at the front and 23 in. at the liaek. The top of the furnace is best made of a sheet steel plate cut required. The doors are made of two tiles, each 20X10 X4 in., held together by two pieces of 1.5 in. channel iron clamped by two 0.5 in. rods. To the ends of these rods, four 3 in. iron wheels are fastened on which the doors run. The tiles are secured in the frame in such a manner that there is a clearance

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Assay Fdknacbb And Tools 1!

of 0.25 in. above the furnace top to freely move the doors. Each furnace has 8 grate bars of 1 in. square wrought iron resting at the cuds on two similar bars, placed on the brickwork. The bars are 14 in. long except the two center ones which are 18 in. loog, and may be withdrawn through the opening G for dumping the fire.

There are four muffles H, in the furnace, 15X9X6 in. outside measurements. The lower muffle and perhaps the next upper one may be used for scorification and cupellation. The other two muffles will not heat to a high enough temperature for anything except annealing and roasting. The muffles rest at the

back and sides on the ends of bricks cut to a level as shown and projecting from the furnace body. At the front they lie flush on 1.5 in. angle irons A.

The furnace is built with the front entirely open; the grate bars are the same as described for the crucible furnace. When the muffles have been placed in position the space around the front of the muffles is filled with a mixture of fire clay and silicate of soda to a depth of 3 in. A strong solution of silicate of soda or "water glass" is mixed with 3 times its weight of water until homogeneous. This solution is then mixed with fire clay to a stiff paste; usually 1 part of solution is required for 7 parts of fire clay. The mixture usually contracts on heating and shrinks away at the edges. These cracks then have to be filled again.

1'2 A MA.Nl'AL OF FIltK ASSAYING

The top of the muffle furnace is covered with tile laid in I. .5 in angle irons A'. The flue V from the muffle furnace into the stack is 12 x3 in. in size. The draft of the furnaces is controlled by placing sheet iron plates in front of the ash pit doors.

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Gasolene -fired Furnaces. — Furnaces of this type are in comii] use, and for small offices, where the pressure of work is t ri '.-it, they afford a convenient and cheap method of operation.

on account of case of transportation and great calorific Power, is alto employed in out-of-the-way districts for iwtensiv. ''aily work. Where coal is reasonably cheap, nut. above $6.0 Per ton. gasolene a! 30 cents per gal, r mi compete with ir i

large offices or school where the assay furnaces are operated cOntbmously for the greater part of the day.

Kg. 13 shows a gasolene furnace apparatus. The furnace, divided into crucible and muffle compartments, is made of fireclay tiling, bound with sheet iron. Il is heated by a brass ami copper burner, provided with a generating device. The burner*

ace made i" varying size. to suii different furnaces. The gasolene 1.- stored in a steel tank, of or 10 gal. capacity, provided wrjtfa an aii-pump In furnish pressure, A pressure gauge is at- n 1 "1 Hie Ian!, (lencrally, to 0.375-in. piping joins d the burner. The burner and piping air connected

Assay Furnaces And Tools

Fig. 15 shows the tank and pump apparatus. It is best t place this at a considerable distance from the furnace, in order to avoid accidental explosions. Fig. 16 shows a crucible furnace, and Fig. 17 a large gasolene muffle-furnace. The writer ha* attained a temperature of 1350° C. in small gasolene furnaces, .-uili as Fie. 1 represents, and 1250° C. in large furnaces, as represented by Fig. 17. By a special construction of furnace, with graphite muffle and heavy insulation against radiation, with good draft, the writer has attained (for metallurgical experi- Mentation) temperatures of 1500° to 1530° C, after three hours,

with a2-in. gasolene burner as shown in Fig. 14, with gasolene ate pnnare of 55 lb. and a consumption of 1.53 gal. per hour. A 2-in. Cary burner, under 10 lb. pressure, will consume from 0.65 to 0.75 gal. per hour. A No. 31 Cary combination furnaee, holding at a charge in the crucible compartment six 20-gram enwibiee and having a muffle 7X10.5X4.5 in. in size, has a capacity of 10 fusions, per hour, including cupellation. Willi gasolene at 30 cents per gallon, the cost of fuel per assay is 2.25 rents.

Kg. In shows the Ciis,e burner for gasolene or similar distillate. When in the proper position it is inverted, i.e., the preheating i at the top instead of at the bottom as in the Carj

Manual Of Fiiie Assaying

burner. The generator or boss in which the gasolene is vaporized is east in one piece with the mixing climber which is in the form of a truncated cone and very much shorter than in other gasolene burners. The burner is smaller and more compact than the ordinary burner of the same capacity. The fact that it is inverted permits the gas formed from the gasolene

in the generator to pass freely upward to the valves. The valves are of special design. Ordinarily the needle valve i- iwd in burners of this type, that is, a pointed hard steel in-.-dle works in the circular valve orifice, making an annular (("Hint! for the escape of the gas. This annular opening varied lirii'-n.-ioiis according to the position of (lie needle, and may v <-!<.-<"d completely by screwing the needle up ns far as it will

W jili use, the tendency) of the needle is to enlarge tlic valve

Bee and cause increased consumption ol gasolene. fha valve

borne is closed by the valve seal meeting a shoulder

the valve stem, both planed surfaces. The opening for the

h m gu is annular as before, bu1 the end of the Muni i ah o

I'iti rjoee not oloee the valve The burner is made of phosphor

!B, and operates besl under a pressure of from 40 i" BO lb,

Fig. IS BBowa a gas"burner for assay furnaces. The air supply ; - controlled by the butterfly valve .1. The gas issues from the i viiLli opening /' and mixes with the air from the annular

ipeoing C, for combustion, The gas flow is regulafcd by the ock G. The burner is provided with the pilot tube to ignite be gas in starting the burner. Fig. 20 shows ;i Case gasolene, oil, or gas fired muffle furnace new design. Ii is provided with ;i heating chamber th* eaturet of which are first— a of fire-clay blocks so contracted aa to form channels or flues under the muffle to direct - Bame and bo1 products ol combustion, insuring a uniform ic:.it distribution and acting aa a firm support for the muffle; ami second — sets of vertical ribbed channels or flues in the side -ill-to guide the hoi gasi - and accomplish an en distribution ! the ln.it The channels between the ribs arc wider near the

onl thi t the rear of the furnace ii ler to lessen friction

. the gases in (his purl . thus ciiiisine I lie lieu ling of the fr

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, iu.wal or fire usimra

The fire clay blocks or "muffle heaters" may be readily replaced by new ones when necessary.

Gas Furnaces. — Where municipal illuminating gas or other gaseous fuel is available, gas-fired furnaces are convenient anil cheap of operation. The Reichhelm furnace (American Gas Furnace Company) is frequently used. The furnaces require air at low pressure, which is mixed with gas in proper proportion before it enters the furnace through the several burners IlH

proportion of gas to air is controlled by valves. Fig. 21 shows

tlir Itiniiu'i'. lia.s furnaces permit of close control of heat and are

inperature work.

V URHACH TOOLS. — Convenient tools are necessary for the hand-

Korifiers and cupels. The features o w on t lg

ii y be light, grasp the crucible, etc.,

of lipping, and take up little room in the

.n of a tool deficient in these quolitii

Fig. 22 is given. This shows a pair

to grasp the body of the crucible. It

Bo full of crucibles, owing to the space

Jo shows a pair of crucible tongs icible, and operating in little space.

Assav Pubnacbs And Tools- 19

Fig. 24 shows two types of cupel tongs. Fig. 25 shows a good form of scorifier tongs, and Fig. 26 another form.

For large offices where much work must be quickly accomplished, special forms of tools may be used. Figs. 27 and 28 -luiu .i multiple tongs 1 for scorifiers. This apparatus will handle scarifiers, practically a muffleful at one time. It is composed of quintuple tongs, corresponding to the five longitudinal rows of scorifiers in the muffle. The lower part of each pair of the tODge constate of a fork on which the scorifiers rest, and one of

whose prongs is rectilinearly extended through two betting in :i frame and held in position by collars. Thin extension is free to [Vrolve on the hearings, and it is the axis of rotation of the tongs. To each of them is attached, at a right angle, a lever extending Upward at Vi°, and all the levers are connected by slotted joints to a cross-rod. Therefore if, by means of a crank fastened l" the end of one of the extended prongs, one of the forks is turned and the scorifiers tilted to the desired angle, the others rotate to the same extent. The center of gravity of the scorifiers lies to

one nde Df Urn rotation point, and thej would, therefore, on being lifted, till mi thai direction; this, however, is preventoi omas-bar resting against a po t at

which the inclination tends. The Bcorificrs arc clutched by the upper pronga of the tongs, which is fastened to a spring on a post of ths fork below, and which is free to move in a vertical plane, the pivotal point lying over the spring and post. By bringing

presauni on the extended ends of these clutch bars behind tbr pivot, their other end "ill rise above the scorificrs, and thai.

rnloaae them, dt permit the placing of the to the tongs, lln

pressure uxcrted on the rear ends the clutches is accomplished

bj ,i, of i is-bar fastened to a spring bar, which is itaeS

fastened to the handle of the instru nt. An ordinary mold

wltli 20 lioli . arranged to receive Lhp rontenta of the scorifiesm, imiIi the longs.

Assay Fu Knacks And Tools

Fig. 29 shows a device to charge 30 cupels at one time. It comprises a top sliding plate with openings corresponding exactly to the position of the cupels. The openings in the lower plate correspond with those of the upper one; the plate, however, rests on two adjacent sides extended downward at right angles to the plate and to each other, thus forming two closed sides of the instrument; one at the front and the other at the

Fin. 22. — Oritciblk Toncw. Undesirable model.

Fi<;. 21. — CYi'KL Toxos.

Fir,. 26. -Scorn fikr Toxiss.

right-hand side. The height of these sides is such that when resting on the bottom of the muffle the bottom plate will be some distance above the cupels, and by a slight pull forward and a push to the left with the handle of the instrument the set of cupels will be perfectly alined in both directions and the apertures in the lower plate will exactly cover the tops of the cupels. The lead buttons are placed in the apertures of the upper plate

Assay Fu11Xaces And Tools

ami rest on the lower plate before introducing the instrument into the furnace, and when it is placed over the cupels, which have been properly alined in the muffle, the Upper plate is pushed

forward to a stop-point, bringing the apertures of the two plates into register, thus causing the lead buttons to drop down into the cupels. The handle of the upper plate runs through guides

c ' -r" o

fixed to the handle of the lower plate; both handles are connected with a spring, which acts as a brake when the upper plate is pushed forward to drop the buttons, and also serves to bring it

back into its original position, in which the buttons cannot drop through the apertures in the lower plate.

Molds — Fig. 30 shows machined cast-iron molds (o receive

_'l A MANUAL OF FIHE ASSAYING

I hi' molten fusions. The sharp cone-shaped mold is preferable the shallow hemispherical type, as the lead buttons are the <uui well defined and separate easily from the slag. The mold is be*! made with a screw-handle, so as to be easily repaired

in case of breakage. The inner surface of the molds should be machined smooth, to permit the ready separation of slag and lead button from the mold. For scorification fusions, smaller

I hi i ln< i mini it uf iiipHs to the parting mom. iron cupel travs,

,l,, in I'in III. I"''' i"'d. The handle is removable, and

,,„ In, „)!,: irne" I'M' n number of trays. For the annealing of ,,1,1 i,,.,,,!,, ,.i coMielN, lire-clay trays as shown in Fig. 32 arc m|<lnM'd i'bi\, however, is very easily broken, and more ,h tll ii.i s inn i me made of sheet iron and heavy asbestos

nuillltUi* ANU SCORIFIERS.— Fire-clay crucibles are largely i i n, i In I n 1 1 I'd Slate, and fire-clay ware for assay purposes is ,.ie I Imi'.e in Hiinic nf the western Slates. Following

tuition

iii 1 1 pa Mai

i.viifi percent.

Sil.m

Ferric Oxide

Una

Albiln-

Tlir crucibles arc rated by gram capacity, thai is, by the Dumber of grama of ore with the proper amount of flaxes necessary for fusion which the crucible will hold. The chief bubs are 5, 10, i-'. 15, 20, 30 and 10 grama; of these the 20-and 30-gram niea are raostij used, the 20-gram crucible for the 0,5 assay ton, and the 30-gram for the I assay ton Fusions. Big. 33 shows the

parlous shapes employed. Imported Eiessian triangular crucibles nd sand crucibles are also used, but in Mind! quantities,

Imported Bnltersco el ay crunbles give g I satisfaction ami

are used by soi [ i lie large asaaj offices in preference to dome*

tir Bre clay goods, for the reason (hat their quality is generally Uniform and thai tln-y lasl for n larger number of Fusions than the

A Manual Of Fire Assaying

poorer grade of domestic goods which are sometimes sold. highest grade of domestic material is, however, in most eases be preferred as being fully as long lived and cheaper.

The special mixture of clays and their treatment for crucibles manufacture is generally a trade secret, jealously guarded and little information concerning the subject is available.

Scorificrs are made of the same clays as the crucibles and ar& designated in size by their outside diameters; I.5-, 2-, 2.5-and 3.5-in. sizes arc made. These will hold a volume of 15 e.c w

'J.'i iM'„ ;!7 im'. and UH> re . respectively. The 2">-in. scorifierie the imc commonly used Fiji. M show* tin-ordinary type of scoritiers. (toasting ilUltcs aiv shallow tire-clay dishes similar loscoriliers. but not j-o thick l'hc\ are rated by their diameters; ihv common >i.o- being 3. I. o and in. lii;. ;!,1 shows the ordinary buck Ivard and mullet, aud So buck board broahes. For the description of ot Hoi minos tool-and apparatus, as screens, pliers, and crushing and gnudiug ma. !;;;;et> . necessary to the assav laboratory. the reader sofesivd she ol'.;v.;:nous aod well-illustrated catalogues of tlu< vi-N hose Balaam.

cussed in their respect :*c clip'.e!-

Chapter Ii

Definitions; Reagents; The Assay Of

Reagents

Assaying includes all those operations of analytical chemistry liich have for their object the determination of the constituents ores and metallurgic products. Three methods are used: C 1) Fire assaying (dry methods) ; (2) gravimetric analysis (wet Xiethods) ; (3) volumetric and colorimetric analysis (wet methods) . "Chis work treats of fire assaying only, with a few exceptions. "Xhe quantitative determination of the following metals is discussed: gold, silver, platinum, etc., lead, antimony, bismuth, tin nnd mercury; chiefly, however, gold and silver.

Fire assaying comprises the separation of the metal sought from the other components of the ore, by heat and suitable fluxes, and then the weighing of .it in a state of greater or lesser purity.

Gold and Silver. — Gold and silver are determined in their ores, or metallurgic products, by collecting them with lead, forming an alloy, which may be accomplished either by the crucible or the scorification fusion, the lead being then driven off by cupellation, and the resultant bead of the gold and silver alloy weighed. The separation of gold from silver is accomplished by parting in most instances with nitric acid, rarely by sulphuric acid.

In order to successfully collect the precious metals by means of lead, it is essential that the ore be mixed with suitable fluxes, so that in fusion the ore is thoroughly decomposed chemically, and a liquid slag of the proper constitution produced, enabling the lead with its alloyed gold and silver to settle from the slag by gravity, thus affording a ready separation.

M i- i

i,, li 1 1 1 1 1 1

ii. ... i,

i . i . i*n i. ilir:i ;t r i I borax glai

ii . i ii

iii.. ii.it- i id i .ri| li\ hrat iti the ci .i in ii i nii!i mil iiniimIIv ivuchotl

1 W i" " ."' " r 'i 1 ' :

,

V

The Assay Of Reagents 20

111 c Xa 2 0, with silica, forms sodium silicates, as Xa 2 Si0 3 , etc., which are very fusible. It also possesses the property of readily forming sulphides and sulphates and, in the presence of metallic of freeing lead in the charge from sulphur.

Xa 2 C0 3 melts at 814° C.

Assayers frequently use sodium bicarbonate in place of calcined sodium carbonate, particularly in the United States, on account of its lower cost. Thus while refined sodium carbonate Cc >sts 8 cents per pound, sodium bicarbonate costs but 3 cents Pr pound, at commercial centers. When the salts are calculated to the basis of the base (Xa 2 0) contained, the difference in c °st is not so wide, still the bicarbonate is cheaper. Xevcrthe- e s$j it is preferable to use the carbonate, since the great amount °f gas evolved in the decomposition of the bicarbonate is apt to e **Vise mechanical losses in the assav. Crude sodium carbonate ° r soda-ash may be used costing about 2 cents per pound. In crucible under the influence of heat the bicarbonate decomposes as follows:

4. Potassium carbonate acts in a similar manner to sodium e *Xrbonate. It melts at 88.")°

.5. Silica is a powerful acid flux and combines with the metallic °Xides or bases present in the charge to form the slag, which is composed of silicates. It is present in most ores in considerable quantity, ranging from small amounts in basic ores to the main bulk of the ore in quartz ores. It melts at 1775° (V (Quartz).— (Roberts- Austen, 1890.)

6, 7. Anhydrous boric acid (B 2 3 ), Horax (Xa 2 ().2B 2 () 3 , 10H 2 O), and Borax glass (Xa 2 0.2B 2 3 ) or anhydrous sodium bi-boratc. Boric acid 2 readilv forms borates on fusion at comparatively high temperature with lithium, potassium, sodium, and silver oxides, generally forming orthoborates (3Xa 2 ().B 2 3 ). Boric acid does not readily dissolve silica, but sodium or potassium meta-borate (Xa 2 ().B,0 3 , K 2 ().B 2 3 ), formed probably during the fusion of borax glass, or sodium bi-borate (Xa 2 0.2B 2 3 ), with bases, will readily dissolve silica, as well as alumina and chromic oxide. The alkaline meta-borates are markedly volatile when molten and deliquesce in tin* air.

1 Day and Shopard nivo (In* niHung-point of Si() ; at approximately 10"J.VC; Jnur. \m. ('hem. Soc., XXVIII, 1090.

- W. Uuertler, Sprechsual, XLV. 61.': Jour. Soc. Chcni. Intl., 1VI. l."i, luos. i."is

Borates are classified as follows :Ortho-borates, e.g. 3CaO.B,0, pyro-borates, e.p.CaO.BjO,; sesqui-borates, e.g., 3CaO.2B,0,: meta-borates 9 e.. y CaO.B 3 O s ;andbi-borates 9 CaO. 2 B,O r Tfc following borates are of interest to the assayer: Magnesium ortho-borate, 3MgO.B,0,; magnesium pyro-borate, 2MgO.B 2 0,: the corresponding borates of nickel and cobalt; the ortho-,pyrometaand bi-borates of calcium, strontium and barium. Le&i oxide forms glasses with boric acid and borax, of which PbO.B,0, is hard like flint glass, and 3PbO.B,0„ may be softened in boiling oil. Other substances which may not be compounds are: 3Zn0.2B,0,; 3ZnO.B,O s ; MnO.B,0,; 3MnO.B,0,; SMnOBjO, CuO.B,0,; 3Cu,0.2B a O,; and 3B,O f .2Fe0.2Fe 2 O s . Bismuth antimony and arsenic also form borates.

Borax and borax glass are fluxes used frequently by assayer*. They are considered acid fluxes, but it will be noted from the above that they have the power of dissolving silica and alumina and will hence corrode crucibles. They can be used to flu* silica to a certain extent, a use, however, to which they are not put. Sodium bi-borate has the property of passing gradually from the liquid to the solid state (amorphous) and vice verso, under ordinary conditions with no definite freezing-or meltingpoint. It can be made to crystallize or freeze at a definite temperature only under the influence of vibration from rapidly repeated shocks. Crystallized sodium bi-borate melts at 742° C. 1

The use of borax glass as a flux to form easily fusible borates with metallic bases is dependent upon the liberation of boric acid from the bi-borate, in the presence of the free bases. What particular borates form is largely a question of temperature attained. The use of much borax gives rise to hard stony slags, very tough, from which the lead button separates with difficulty. Often a film of lead will adhere to the slag, causing mechanical loss. Slags containing much borax will often fly to pieces suddenly. especially when touched with a sharp instrument, while cooling. 1 This is due to devitrification of amorphous glassy borates and the formation of definite crystallized borates.

In fluxing ores containing zinc it is to be noted that boric oxide, either alone or mixed with one-half its weight of borax, will flux zinc oxide into a very fluid slag, which is, however, very corrosive to clay crucibles.

1 Day and Allen, Am. Jour. 8c. , XIX, 102.

E. Clennell, Eng. and Min. Jour., LXXXVII, 696.

The Assay Of Reagents 31

8. Fluorspar is occasionally used in assaying. It melts at a comparatively high temperature, 1330° C, but when fused is very thinly fluid. The greater part of it remains unchanged throughout the fusion, and hence its lime cannot be considered as available for fluxing silica. It gives the slags containing it a stony appearance. Owing to its great fluidity, it has the property, shared by soda and litharge to some extent, of holding in suspension unfused particles, thus still making a fluid slag. Where the decomposition of the ore to be assayed is essential, as it is in most cases, its use is not to be advocated.

9. Lime is used either as the carbonate or as the oxide or hydrate. In the crucible it is converted into oxide, the carbonate beginning to lose its C0 2 at 800° C. In itself it is extremely infusible (1900° C; Hempel, 1903), but with silica, when joined with other bases and in moderate quantities, it makes very desirable slags. It is found in many ores. Magnesia acts in a similar way. Its melting-point is 2250° C. (Hempel, 1903.)

10. Hematite, or natural ferric oxide, and limonite, are of frequent occurrence in ores, and are sometimes added as a flux. Ferric oxide has a high melting point, about 1560° C. In the crucible it is converted by reducing agents, such as argol, charcoal, etc., to ferrous oxide (FeO), and then unites with silica to form silicates. The fact that it is reduced to ferrous oxide, conversely gives it an oxidizing power. Manganese oxides acting in a similar way are also frequently found in ores. Alumina, Al 2 O s , is often found in ores, and unites with silica to form silicates. It has no oxidizing power. Al 2 O s melts at 1880° C. (Hempel, 1903.)

11. Test lead and sheet lead are used chiefly in the scorification assay and in cupellation. In both of these operations the lead is oxidized by the oxygen of the air (2Pb 4- 2 2PbO) to litharge. In the scorification assay part of this PbO volatilizes; the greater part becomes fluid . and holds in suspension and solution other metallic oxides derived from ores, thus forming what is termed an oxide slag. In cupellation, part of the lead is volatilized as PbO, and part is absorbed by the cupel as PbO. Lead melts at 326° C.

12. Argol is a crude bitartrate of potassium, separating out in wine casks, from the wine on standing. On heating, it breaks up as follows:

2KHC 4 H 4 6 + heat K 2 + 5H 2 + 6CO + 2C

ili A MANUAL OF FIRE ASSAYING

The carbon and carbon monoxide set free gives it its reducing power. The K 2 left acts as a basic flux.

13, 14, 15. Charcoal, coke, coal dust, sugar and flour are reducing agents by virtue of the carbon or hydrogen, or both, that they contain.

16. Lead flux is a ready-prepared flux used mainly in the assay of lead ores for lead. It has the following composition:

Sodium bicarbonate 16 parte

Potassium carbonate 16 parts

Borax glass 8 parts

Flour 4 parts

It is also made up in other proportions.

17. Black flux is made of 1 part KX0 3 and 3 parts argoK deflagrated. It is sometimes used in the tin and lead assay.

18. Black flux substitute consists of 3 parts of flour and 10 parts of XaHC0 3 . It is used in the tin assay.

19. The alkaline cyanides are powerful poisons and when powdering them for use as a flux great care must be taken not to inhale the dust. The mortar in which the pulverizing is done should be covered by a cloth during the operation, which is best conducted at an open window. Two kinds of commercial cyanide may be readily purchased on the market. 1. What is known as " potassium cyanide," but which consists of the mixed cyanides of sodium and potassium, containing varying amounts of impurities such as alkaline carbonates, sulphates, etc. The quality is expressed by the cyanogen content, in terms of KCX. Thus "98 per cent. KCX" is in common use. Without going into detail, it is to be noted that salts of this type may contain considerable impurity, although rated as "98 per cent. KCX," and unless known to be good should not bo used in the tin assay. Pure potassium cyanide, c.p. can be obtained only at a comparatively high price. 2. Sodium cyanide. This is a commercial salt that may be obtained nearly pure. When its cyanogen contents are rated at 12.5 to 130 per cent. KCX, it may be used with safety as a flux for the tin assay

A sample of commercial "98 per cent. KCX," impurities not known, had a freezing-point of 326° C, as determined in the author's laboratory.

When heated somewhat above its inciting point in the presence of air, alkaline cyanide forms cyanate and then decomposes with the liberation of cyanogen. Crucibles in which it is used

The Assay Of Reagents 33

s **ould be covered. The alkaline cyanides are used mainly in assay of base metals as bismuth, lead, tin and antimony.

It is a powerful reducing and desulphurizing agent, acting as follows:

PbO -f KCiY KCNO + Pb

PbS + KCN KCNS + Pb

20. Potassium nitrate or niter is used as an oxidizing agent. "VVith metallic lead it acts as follows:

7Pb 4- 6KN0 3 7PbO + 3K 3 + 3N 3 + 40 2 (approximately). It is frequently used in assaying to oxidize impurities in the charge, such as sulphur, arsenic, etc. It acts as a basic flux. Potassium nitrate fuses at 339° C.

Sodium nitrate or Chile saltpeter is sometimes used in place of xiiter, but as it deliquesces much more than the latter it is not so convenient.

Other oxidizing agents such as potassium permanganate, potassium ferri cyanide, etc., may be used in the assay of impure ores, but are more expensive and not any better. It is desirable to dry niter at 100° C. before use and then keep it in a closely stoppered bottle, otherwise it will be weakened per unit weight on account of the absorbed moisture.

21. Salt (NaCl) is used as a cover. It is very thinly fluid and is not decomposed during the fusion. It freezes at 801° C l

THE ASSAY OF REAGENTS. - It is essential for the assayer to be assured of the fact that his reagents are pure, or at least to know to what extent they are impure and what the impurity consists of. For this reason it is necessary to examine lots of reagents from time to time, as they come into the laboratory, by approved chemical methods, to determine their purity. Sometimes reagents or fluxes, as a result of being left exposed in the laboratory, become accidentally or purposely "salted" or contaminated with gold, silver or base-metal values. A blank assay for metals on the reagents will readily determine this. In general, it may be stated that the labeling of a chemical " c. p." does not necessarily make it so. Borax has been found to contain platinum. 2

It is necessary to determine the silver in litharge and test lead, as these two reagents frequently contain some silver, 4ue to their being usually made from lead bullion refined by the Parkes' or zinc-desilverizution process, which leaves some silver

W. P. White, Am. Jour. 6'c*., XXVIII, 470.

2 J G. Ilosc, Jour. . Md. ami Min. Soc. S. A , IX, 168.

in them. As litharge is almost invariably used in the crucible assay, and test lead in the scorification assay, any silver or. possibly, gold introduced into the results by their use must be subtracted, so as not to be ascribed to the ores. Most assay supply houses now furnish practically silver-free litharge and lead containing only traces of silver and no gold.

The method of determining silver and gold in litharge ami test lead is as follows:

The following charge is weighed out in duplicate:

Litharge 3 assay tons

Sodium carbonate 20 grams

Silica 7 grams

Argol 2 grams

The various ingredients are put from the scale pan on a sheet of glazed paper and thoroughly incorporated by mixing. It is essential to weigh the litharge and argol as accurately as possible with the pulp balances in use.

The incorporated charge is then transferred to a 20-gram crucible, a shallow cover of borax glass being put on top of the charge, and then fused in the muffle-furnace for from 25 to 35 minutes at a yellow heat (1000° C). The fusion is considered complete when the charge is in quiet fusion, that is, when there is no more bubbling and boiling in the charge and when the onlv motion observable is that due to convection currents. The charge is then poured into an iron mold and allowed to solidify, which takes approximately 10 minutes. The lead button is then separated from the slag by the hammer and formed into a cube. It is weighed and its weight recorded in grams and tenths of a gram in the assay note-book, a definite assay number being assigned to this assay and its duplicate. The lead button is then cupeled, the cupel being first placed in the muffle for 10 to 12 minutes before the lead button is dropped into it. If the button weighs from 15 to 20 grams, as it should, it will take 25 or 30 minutes to finish the cupellation, that is, to drive off the lead. The end of this operation, in this particular instance, is denoted by the darkening of the small silver bead. The bead is then removed from the cupel after tin's has become cold, flattened on a small anvil with a blowpipe hammer, cleaned of adhering bone-ash from the cupel by a button brush, and weighed carefully on the assay balances, the weight being recorded in milli

The Assay Of Reagents 35

grams and hundredths of a milligram. The weight of the bead, divided by the number of assay tons (3) taken in the assay, gives the number of ounces contained in a ton (2000 lb.) of litharge, or the number of milligrams per assay ton of litharge. 1 If the presence of gold is suspected in the litharge, the silver bead from the cupellation, after weighing, is dropped into a parting-cup filled with hot nitric acid (9 parts water to 1 part concentrated nitric acid, sp. gr. 1.42), which will dissolve the silver and leave the gold as a black residue. This residue is washed three times by decantation with cold distilled water, carefully dried and annealed at a red heat in the muffle; after cooling it is weighed as already described for silver. The weight of the gold is recorded and then subtracted from the weight of the original gold and silver bead. The difference in weight gives the amount of silver.

To determine the silver and gold in test lead, weigh out 3 assay tons, place in a 2.5-in. scorifier, add a pinch of borax glass, and scorify in the muffle at a yellow heat (1000° C). As the lead oxidizes to litharge, this melts and forms a slag which, owing to the convexity of the meniscus of molten lead, falls to the side of the surface and forms the slag ring, leaving a disk of fresh lead exposed. The scorification is finished when the slag finally covers all the lead. The charge is then poured into an iron mold, the further method of procedure followed being identical with the one described for the litharge assay.

It is possible to obtain test and sheet lead with only traces of silver, and litharge practically free from silver. It is often desirable that the litharge should contain a uniform amount of silver, for whenever low-grade gold ores, deficient in silver, are assayed, silver will have to be added at some stage of the assay in order to insure parting, or the complete separation of the gold from the silver. In assaying very low-grade gold ores, in which practically only gold is present, the final bead might be so small as to sink into minute cracks in the cupel and thus be lost. The addition of silver in this case, either by adding it in the metallic state or by its presence in the litharge, obviates this difficulty

Litharge will frequently contain from 0.20 to 0.32 mg. of silver per assay ton. It is, however, not safe to assume the above figures. The test lead ordinarily bought from the supply houses contains only traces of silver.

1 For a discussion of weights used in assaying, cupellation and weighing, reference should be made to these subjects.

"i

i

' t

reiarioi:

:r:it.-Ie< are

:"r:;e on

W V.S.Mr

Sampling 37

Gilpin County, Colorado, ores, carrying from 1 to 4 oz. gold, illustrates this:

Diam. of largest piece, in inches 0.04 0.08 0.16 0.32 0.64 1.25 2.50

Minimum weight of sample, in lbs 0.0625 0.50 4 32 256 2048 16348

The proper weight of sample for any desired size of ore particle is obtained by multiplying the known weight for the given size by the cube of the ratio of the desired size to that of the given size.

As an example of mill practice by machine sampling on Cripple Creek ores of from 2 to 6 oz. gold per ton, the following is given:

The ore is crushed to pass a 1.5-in. ring, and from the total bulk a Vezin sampler cuts out one-fourth. This is passed to crushing rolls, which reduce it to 0.25-in. size. It is then elevated to another Vezin sampler, which takes out one-tenth of the bulk, the final sample being one-fortieth of the ore, or 2.5 per cent. This is then cut down and crushed finer and sampled in the usual way (alternate shovels, etc.), described further on. In smelting works, where it is desirable to have the product going to the furnaces as coarse as possible, the above method is modified by not crushing so fine and by taking larger samples; or hand sampling is employed. The size of the sample depends not only on the size of the ore particles, but also on the nature of the ore. If the values are uniformly distributed, smaller samples will do than are necessary where they are " spotted " or irregularly distributed. 1 While machine sampling, with properly constructed apparatus, is largely in use, and is most desirable when applicable, hand sampling may be accurately performed; it is still widely used by smelting plants, as it avoids crushing a large part of the ore. 2

Coning and Quartering. — The method of " coning and quartering " has been in use for jnany years, and is still employed, but it is being displaced largely by the "alternate-shovel" method. Coning and quartering, unless carefully performed, which is difficult to do, is apt to be inaccurate. In this method, the thorough mixing of the ore is essential, and the mixing is supposed to be effected by coning. The cone is built up by men moving around the circumference of a circle and shoveling the ore upon

1 L. T. Wright, Element of Chance in the Sampling of Ores, Min. Mag., Ill, 353 (1910).

2 For a good discussion of Machine Sampling, consult A. W. Warwick, "Notes on Sampling," published by the Industrial Pub. Co., Denver, Colo.

Chapter Iii

Sampling

Proper sampling is of the utmost importance, for unless the sample to be assayed accurately represents the lot of ore or metallurgies product from which it is taken, in other words, unless it. is a true sample, the greatest care in the assay itself means nothing. Large amounts of money are involved in settlements made on the assay of final samples representing many tons of rich ore, matte, bullions, etc. Mills and smelters purchase ores the carload on the assay of the final sample, and even slight errors mean loss either to the shipper or the purchaser. Where so-called "specimen" assays are made, the sampling of the small amount of pulp is usually a simple matter, although accuracy is also required. Tn mosi cases the samples, representing large lots, are handed to the assaver, so that he is usually not directly concerned as to how the samples were obtained; but in general he should be familiar as to how sampling is conducted. Sampling may be classified under two heads:

b. Alternate shovels.

KitHim*. Machine samplim*.:

d Pail of the Mivam for the whole time.

b The whole i he ore Mream pari of the tllttC.

WhaieNcv the M.eih.,1 o! dimpling distinct relation

must br'. :he ue.ht of the -:iinplo and the size oH .1"

ore njini.'.. IV.:-. :. : he o-.e policies a iv large (10 to 1-

. n i . i-t the particles

. v IM ,K 1 rule in force

Gilpin County, (.'oiaiad*. UluatratBB thw:

Tha proper weiem m

IS ulitainri.i by till' cut* ftixe.

As an -

ore*- of ir is given:

The ore balk a VeziB samplecrushing r :

bulk, the final auupb This is il' usual wj

will do thttii bj

P* Lt-

put,

of

Here

then

Lishing

ii inin no moiH-

bbe per-nils

are

ieatcd at

the sampling

divided into

unsaved by the

i liner; a third

I., ore und those made hj

i iiiu limit, depending on

lurchs er' a aj

if t.lioy do noi

and tiler to rcatiaay their

ipti I reaasay. If they

. i.,, -in who makes an

"iluunuf* ui Sourov nl Krrut in Ajmu

3S A Manual Of Fikk Assaying

the point of a cone formed by the angle of repose of the material falling vertically upon one point. The samplers — from 4 to 8 men — move so as to be always diametrically opposite each other.

In order to fix the point of the cone, a rod is driven into the ground as a guide. It is evident that the shoveling must be very conscientiously done in order to have the ore distribute itself uniformly (fine and coarse) over the surface of the cone; but this uniformity is essential to the obtaining of a true sample. When the cone has been built up, it is then pulled down by the men walking around the pile and scraping the ore from the apex to the base, until a flat plaque of ore is made about 12 or 18 in. thick. Then, in the form of a cross, plates of iron are carefully centered on the pile and driven in, dividing the plaque into quarters. Two opposite quarters are removed to the bins, and the other two, representing the sample, are reshovelcd into a cone and the operation repeated. The ore is then recrushed and coned and quartered again, until finally a sample of from 25 to 30 lb. is obtained. The number of recrushings depends upon the size of the first sample and the nature of the ore. The sample is then ground fine and prepared for the assay office by cutting down with a split sampler or other approved device. The whole process is slow and laborious. Three men can handle from 20 to 25 funs of sample per shift at a cost of from 45 to 50 cents per ton.

Thv Altcrnate-shovd Method. — The fundamental law of sampling may be stated thus: In order to properly take a sample of ore, it is necessary to take the sample frequently, or in as many places as possible, and to take the same quantity each time at regular intervals. These conditions are fulfilled by the " alternate-shovel" method, which is conducted as follows:

The ore from the cars is dumped on a platform and men with the proper sized and shaped shovels put it into the bins, taking out for the sample a certain number, dependent on the nature and size of the ore pieces; e.g., nine shovels are thrown into %h& bins and every tenth shovel is taken as a sample. If the ore is* ditlicult to sample, sample shovels may be taken more frequently* - or if tin* ore is uniform, less frequently. It is usual to cut from one-fifth to one-twentieth of the ore. The alternate-shove*! method posse*>es the following advantages:

1. It is more reliable and accurate than coning and quarterin

*J. It ij* cheaper in operation.

It is quicker.

The " quartering " and the "split-shovel" methods are not reliable and need not be described.

At the plant of the Standard Smelting Company, at Rapid City, S. Dak., the shovel sample is passed to a Blake crusher with a 9 X 15 in. mouth opening, having an A discharge, so as to halve the crushed sample. One of the halves is fed directly to a pair of 24x12 in. rolls, the discharge from which is again automatically halved. If a 100-ton lot is taken as a unit, the sample at this point is 2.5 tonB (taking every tenth shovel), with no particle larger than 0.375 in. in diameter. The rolls discharge directly upon a plate-iron floor, where the ore is reshoveled, every fifth or tenth shovel being taken as a sample, which now amounts to 1000 or 500 lb. This is put through a pair of 12X 12 in. sampling rolls and crushed fine, and then sampled by a large Jones split or rime sampler, which takes halves, until finally a sample of between 15 and 20 lb. is arrived at. This is put through a small cone grinding mill, and after a determination of moisture on the sample floor is sent to the assay office. Here it is cut down to about 2 lb. by a small Jones sampler, and then crushed on a buck board to pass a ]2fcmesh screen, furnishing the assay sample. This sample is supposed to contain no moisture, as this was eliminated on the sample floor, where the percentage of moisture is determined; but as all settlements are made on dry samples, the final assay sample is again heated at 100° C. fpr some time in order to expel any moisture which the sample may have absorbed in its passage from the sampling works to the assay office. 1 The assay sample is divided into 4 parts and put in paper sacks. One part is assayed by the seller of the ore or product; one part by the purchaser; a third part is kept for emergency; and a fourth part is laid aside for an umpire assay, if such becomes necessary.

The assays made by the seller of the ore and those made by the purchaser of the ore are called control assays. If the seller and purchaser agree within a certain limit, depending on the value of the ore, settlement is made on the purchaser's assay, or sometimes on the average of the two assays. If they do not agree, it is the practice for the buyer and seller to reassay their own samples or to exchange pulp samples and reassay. If they do not then agree, an umpire assayer is chosen who makes an

' G. A. , Etta, and Min. Jour.. XC. 1047. "Moisture at a Source of Error in Asa Report* "

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Balances And Weights

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4. . rhe central knife-edt-'.

D. . Adjuatment far center of gravity of the baliui

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V . pointer-arm.

r. . distance of deflection of center of gravity, or

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Balances and weights 45

sion of <the pointer. The lower the center of gravity of the balance systenr, the more rapid the oscillation of the balance. The higher or the nearer the point of suspension, the slower the oscillations' and the greater the sensibility.

Weighing. — Before weighing, the balance is always thoroughly cleaned in every part from dust by a soft camel' s-hair brush, made perfectly level by adjusting the leveling screws, and the pointer standardized to o by the little thumb-screws C, C. To do this, the balance is set in motion until the pointer swings to from 5 to 8 divisions on the scale each side of the zero mark. If the balance-arms are equal in moment, the pointer will swing practically an equal number of divisions on each side, losing, however, a trifle on each swing, thus: +8,-7.75, + 7.5, — 7.25, +7,-6.75, etc., the loss being due to friction and to a gradual settling back into equilibrium. If the swings are not as outlined, the adjustment is made until they become so. The balance is then tested for sensibility as described, and the adjustment made for it, if necessary, by moving the center of gravity. If the balance-arms are suspected of being unequal in length (though this is rare in good balances), weighing by "substitution," or double-weighing is adopted. In this method, the object to be weighed is placed first in one pan and weighed, and then in the other, the true weight being the square root of the product of the two weights found. When the sensibility of the balance is accurately known, no adjustment for equal moment of arms need be made, but weighing may be done by deflection, after the true zero or equilibrium point is found. This is found as follows: Start the balance swinging and count swings to the left as minus and to the right as plus. Suppose the swings are as follows: — 8, +3, — 7.5. The zero-point then is

or the true zero, or true " point of rest" is 2.375 divisions to the left of the zero mark on the scale.

Then place the particle to be weighed on the right-hand pan and weigh again to determine the point of rest under these conditions. The swings are as follows: — 10. +2,-9.5. The sensi-

Balances And Weights 47

o 4 L L A + B

K A+B±m±n R 2 A

If ni — n, or the reversal of the masses shifts the zero- Point exactly as much to one side as it was before on the other

9 the actual o, the balance has equal arms; i.e., 1. should

R R

n °t exceed 1± 0.000003.

Some assayers weigh by "no deflection." They adjust the

balance to the true zero, place the bead to be weighed in the

Sht-hand pan, and then by the addition of weights and the

Moving of the rider by repeated trials, balance the bead, so that

when the balance is lowered gently on its knife-edge, no

flection of the pointer takes place. This method, however, is

recommended, as it disregards friction and inertia, and for

8 **lall weights gives inaccurate results.

Practical notes on the assay balance. - in those laboratories where the balance cannot be supported on stone piers trouble may be experienced from jarring of the balance. This can largely be eliminated by supporting the levelling screws on truncated pyramids cut out of rubber packing, making the lower base of the support 2 in. square and the upper one 1 in. square, with a thickness of about 1 in. A small square of ground glass may be cemented to the top of each support to take the thrust of the levelling screw. Another method of avoiding the jar is to bore four holes A in. deep into the balance table top, and insert No. 5 rubber stoppers on top of which a small piece of heavy sheet lead is placed, about f in. thick. The level screws should be sunk into the lead about T in. deep, for the best effect 1 .

One source of trouble with delicate assay balances is their tendency to become magnetized or charged with static electricity which will cause them to act in a very erratic manner during weighing. Balance beams constructed of material subject to magnetization should be avoided. When a balance of this kind is in vise it may become necessary to change its position to avoid in part the magnetizing forces. For instance the balance beam should not be parallel to a north and south line. Balances constructed of non-magnetic material may be subject to similar

D. M. Liddeil, Eng. and Min.Jour., LXXXIX, 30.3.

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B m.vm I.- AND WEIGHTS

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pwa the "non-column" type of button balance. The wry \ir.n column of these balance- l.y -Imh jiiTijf the length of the

i gers, tends to concentr the vable mass near its

inrul .I-..I- thus giving -j: r ( j i r uf |.iui.-r. while preserving s; the pointer extends upward, and the Boale is above Eta beam. In some forms the pointer is horizontal and the scale rsi tical, placed to the aide of the beam.

WEIGHTS. -Tin; weights used in weighing beads are milligram weights, usually from I nig. up to mg., the units being as EoUowb: i. .'. 5, i". -'it, 50, 100, 200 500 and 1000 mg. They best made of platinum, as the material musl be nol readil; ©ded, so thai the weight will remain constant, Riders are l in determine weights up to 1 nig, the balance-beams being divided into 100 equal spaces, each space being equivalent to K01 mg, with a I -nig rider Kilters are made of fine platinum il for assaj balances usually 'nun-as0.,V and 1-inj! riders. One-milligram riders itre coinriKinlv used. Where the balance i readily be made sensitive iii Dim.") in-., li.5-mg, riders can 'li |UTitii ; ntlierwise l-mg. rider.- arc preferable, as

MAXt.VL OF FIRL

they are uot so readily injured by handling. Riders are frequently sold which are not of true weight, and it is essential u check them before using. The same is true of weights. It u desirable for every assay office to have a set of standardized

weights for comparison. These standardised weights can purchased from the balance firms; or a set may be corrected by the Government Bureau of Standards. 1

ran Of Standard*. IVjil l

Balances And Weights 51

The Assay-ton System. — Gram and assay-ton weights arc Used to weigh pulp and rluxes. The assay-ton system was devised by Professor Charles F. Chandler, of Columbia University, Xew York, and reconciles the difficulties arising from the fact that all ores, etc., are weighed by the avoirdupois system, while precious metals are weighed by the troy system. The basis of

the assay ton is the number of troy ounces in 1 ton (2000 lb.) avoirdupois.

Item =2000 lbs.;

I lb. (avoirdupois) 7000 troy grains; , 1 ton =14,000,000 troy grains,

, , 14,000,000 therefore, ' =29,166 oz. (troy),

Then, taking 1 mg. as the unit, 1 assay ton 29,166 mg., or 29.166 grams, and 1 mg. bears the same relation to 1 assay ton as 1 oz. troy bears to 1 ton of 2000 lb. avoirdupois.

From this it follows that if 1 assay ton of ore is taken, and the silver and gold from this is weighed in milligrams, this weight will represent ounces troy per ton of ore. Fig. 43 shows a set of platinum assay weights; Figs. 44 and 45 show a set of assay-ton and gram weights, respectively.

b

Chapter V Reduction And Oxidation Reactions

REDUCTION. — A reduction reaction, as particularly defined for assaying, is one in which a metal is reduced from its compounds by some reducing agent. The chemical definition is also applicable in that, in assaying, we frequently reduce a compound from a state of higher oxidation to a lower state of oxidation by means of a reducing agent.

An oxidation reaction is one in which a metal or a com pound is changed to a compound of a higher state of oxidation; for example, Pb to PbO, S to S0 2 , or PbO to Pb0 3 . Reduction and oxidation reactions frequently occur in assaying, and it is essential that the assayer be thoroughly familiar with the theory and facts. In speaking of reducing agents and reduction with special reference to assaying, we have chiefly in mind such reagents as reduce metallic lead from litharge in the crucible. The chief of these are: (1) argol, (2) charcoal or coke or coal dust, (3) flour or sugar. These are added to the charge in sufficient quantity to produce the proper size of lead button in the crucible assay. It often happens that an ore will contain reducing agents, chiefly sulphides, so that it becomes unnecessary to add an extraneous agent. In fact, it may contain an excess of reducing agent, requiring an oxidizing agent to destroy the excess.

The reduction of lead by argol is expressed by the following equation:

10 PbO + 2KHC 4 H 4 6 lOPb + 5H 3 + K 2 + 8C0 2

One gram of argol will reduce 5.50 grams of lead from 5.93 or more grams of PbO. The above formula for argol is that of pure bitartrate of potassium. Argol contains as impurity a certain amount of carbonaceous matter, so that its reducing power will be increased. It will be found that the actual reducing power of 1 gram of argol varies between 7 and 9.5 grams of lead, dependent on the argol used.

The reduction of lead by charcoal is expressed by the following reactions:

2PbO + C 2Pb + CO,

Reduction And Oxidation Reactions 55

FeS 3 + 7PbO =FeO + 2S0 8 + 7Pb 2FeS 3 + 15PbO =Fe 2 3 + 4S0, + 15Pb

The first equation will give 12 grams of Pb per gram of pyrite, and the second will give 12.9 grams. The accompanying table gives the reducing powers of the various substances as determined by the litharge-soda charge given for pyrite.

Table Ii.— Reducing Powers Of Agents

Quantity of lead in grams Name of reducing agent reduced by 1 gram of

reducing agent

Argol 9.61

Flour 10.53

Sugar 11.78

Charcoal ' 26.0

Pyrite ' 12.24

Pyrrhotite 8.71

Stibnite ' 7.17

Chalcocite 4.38

Sphalerite 8. 16

When no soda is present to induce the formation of alkaline sulphates, the following reaction takes place, sulphur dioxide (SO a ) being formed:

FeS 2 + 5PbO FeO + 2S0 2 + 5Pb ;

or 1 gram of pyrite reduces 8.6 grams of lead.

In the assay, as ordinarily performed, the foregoing conditions are modified by the presence of other substances, in the main by silica. Lead oxide readily forms silicates with silica, and the mono-, bi-, and tri-silicates are easily fusible, while those of a higher degree are fusible with difficulty. When a reducing agent (argol, sulphides, etc.) is fused with a silicate of lead, or with a charge containing litharge and silica, only a little lead is reduced when the silica is present in amounts to form a trisilicate or above, and only somewhat more when the silica is present in amounts to form a mono-or bisilicate. The reason for this is that the silicates of lead are not reduced by sulphides or carbonaceous reducing agents at temperatures below about 1000° C. 1 Above

1 Consult Mdallurgie, IV, 647.

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Reduction And Oxidation Reactions 0/

When carbonaceous reducing agents are used to obtain the required lead button, the nature of the charge, as regards acidity (clue to Si0 2 or borax), has little influence on the size of button, provided sufficient bases, outside of PbO, are present to decompose lead silicates formed, and the silicate degree does not exceed a monosilicate. The amount of litharge present has some influence. The quantity of carbonaceous reducing agent remaining constant, the size of button will increase somewhat with increasing amounts of PbO in the charge. When the reducing agent is a sulphide (often a natural constituent of the ore), the acidity of the charge influences, to a certain extent, the size of button obtainable. It is, however, the amount of alkaline base present (K 2 0,Na 2 0) that exerts the most powerful influence, its presence inducing the formation of S0 3 and, consequently, sulphates, thus reducing larger amounts of lead than when no alkaline bases are present, the sulphur going off as S0 2 .

OXIDATION. — Oxidation of impurities in ores is frequently necessary in order to obtain good results in the assay. When ores contain an excess of sulphides, arsenides, etc. (by an excess is meant a quantity above that which will give the required size of lead button), an oxidizing agent is required to oxidize this excess, enabling it to be volatilized or slagged. Oxidation of impurities is accomplished in one of two ways.

1. By the addition of potassium nitrate (KX0 8 ) to the charge (or other oxidizing agents).

2. By roasting the ore, thus using the oxygen of the air for the oxidation of impurities.

When niter is added to an assav, it reacts with the most easily oxidizable compounding the charge, which is usually the reducing agent, i.e., the sulphide present. Extraneous reducing agents, such as argol, flour, or charcoal, are present simultaneously with niter only when it is desired to determine the oxidizing power of niter against these reagents. For the sake of convenience, the oxidizing power of niter is expressed in terms of lead. If finely divided lead is fused with niter, the fusion reaching a temperature of 1000° C. after one-half hour, the following reaction takes place, approximately:

7Pb+6KNO s 7PbO + 3K 3 + 3N 2 + 40 2 ; " or 1 gram of niter oxidizes 2.39 grams of lead. The actual number of grams of lead oxidized, determined by a considerable number of experiments, has been found to be 2.37. The analysis of

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Reduction And Oxidation Reactions 59

fore, 1 gram of niter in this instance would oxidize 12.22X0.39, or 4.76, grams of lead. The accompanying table 1 shows actual results obtained for the oxidizing power of niter against different reducing agents.

Table Iv.— Oxidizing Power Of Niter

, Oxidising power of niter

Reducing agent in tenn8 of left

Pyrite 4.73 grams

Charcoal 5 . 15 grams

Flour 5.09 grams

Argol 4 . 76 grams

It follows, therefore, that the oxidizing power of niter varies with the reducing agent used.

When the assay charge contains silica and borax glass, the above figures no longer hold, for in their presence oxygen is evolved by the niter, which escapes from the charge, as in the case of the oxidation of metallic lead by niter. The amount of oxygen lost (thus reducing the oxidizing power of niter) is probably a function of the rate of rise of temperature, but evidence also points to the fact that silica reacts with the niter, setting free oxygen, at a temperature very close to that at which niter reacts with charcoal, or at which oxygen will react with carbon. Niter fuses at 339° C, but does not give off oxygen when fused alone until 530° C. is reached. Charcoal ignites at temperatures 2 ranging from 340° C. to 700° C, depending upon the temperature at which it was burnt, while silica begins to react with niter at very nearly 450° C, probably according to the following reaction:

2KNO, + Si0 2 K,SiO, + 50 + N,

Thus, during the period in which the temperature in the crucible gradually rises to a yellow heat (that of the muffle), oxygen escapes during the range from 400° C. to 500° C, etc., this last being taken as an average temperature at which charcoal will begin actively to oxidize. 8

'From a number of experiments by the author, willow charcoal was found to begin reaction with niter at very close to 440° C. This is offered tentatively, as an explanation of what occurs.

Niter will begin to react with argol and pyrite at practically its melting-point.

The oxidizing power of niter against charcoal in charges containing silica will frequently vary between 3.7 and 4.2 grams of lead, averaging about 4 grams. This is 1.1 grams lower than in the litharge-soda charge. The oxidizing power of niter against sulphides is but little lowered by the presence of silica or borax glass. When the oxidizing power of niter against pyrite (sulphides) is considered, and expressed in terms of lead, the varying reducing power of suphides in different charges has to be taken into account. Taking as an example a charge containing considerable silica, so that a large part of the soda (alkaline base) is absorbed as a silicate, leaving but little to form sulphate from the oxidation of the pyrite, it is found that the reducing power of pyrite is 9 grams of lead, as already noted. In this charge, niter will react with pyrite as follows:

4FeS 2 + 10KNO 8 4FeO + 5K 2 S0 4 + 3S0 3 + 5N 2 ;

or 1 gram of niter oxidizes 0.475 gram pyrite. The oxidizing power of niter expressed in lead is then 9 X 0.475, or 4.275 grams. Actually, it will be very little lower than this, as but little oxygen escapes without action. The actual figure obtained by experiment is very close to 4.20.

It is evident from this that the oxidizing power of niter varies with the type of charge used. It ranges, for pyrite, from about 4 grams in acid charges to 4.76 in basic charges (containing no silica). It varies still more with other sulphides. It has been the practice of assayers in making the niter fusion to run a preliminary assay in a comparatively basic charge (approximately the litharge-soda type), and use the figure obtained for the reducing power of the ore in this charge in calculating the amount of niter for the final fusion, usually made quite acid. In this way discordant results are obtained, for both the reducing power of the ore and the oxidizing power of niter vary in the different charges.

Supposing that the preliminary assay showed the reducing power of a nearly pure pyrite to be 12 grams of lead per gram of ore. Using a 0.5 assay ton in the final fusion, on this basis the amount of lead reduced would be 12x15, or 180 grams. Subtracting the weight of the lead button, 20, from this leaves the equivalent of 160 grams of lead to be oxidized. Taking

Reduction And Oxidation Reactions 01

as the oxidizing power of niter in the final charge, 40 grams of

niter would be added. But in the final charge, owing to its

acidity, the reducing power of the pyrite is but 10 grams of lead

per 1 gram of ore, and the total reducing power of 0.5 assay ton

is 150 grams. It therefore follows that the final result will show

no button. The oxidizing pow r er for niter which should have been

used is VX4, or 5.3, and 31 grams of niter added. This, then,

would give approximately the proper sized button. As the

range of reducing power for pyrite is from about 9 to 12.2 grams

of lead, according to whether the charge is acid and contains

httle soda, or is of the litharge-soda type, the most satisfactory

ay to determine the amount of niter to add is to have the nature

°f the preliminary charge the same as that of the final charge,

a Ud then use the figure 4 to 4.2 as the oxidizing power of niter. 1

The following charges arc recommended to determine oxidizing

a nd reducing powers:

Preliminary Assay, Xo. 1 Preliminary Assay, No. 2

5 grams of pyritous ore 5 grams of pyritous ore

8 grams of SiO., 8 grams of SiO a

100 grams of PbO 100 grams of PbO

12 grams of Na 2 C0 3 12 grams of Na 2 C0 3

Borax glass cover 3 grams of KN0 3

Borax glass cover

The difference in weight of the lead buttons of preliminary Assays Nos. 1 and 2, divided by 3, will give the oxidizing power Of niter in the type of charge used. The weight of the button of preliminary assay No. 1, divided by 5, gives the reducing power of the ore.

Preliminary Assay, Xo. 3 5 grams of pyritous ore 12 grams of Na./X),

It will be noted that the reducing power of the ore is greater than that obtained in preliminary assay No. 1. In order to determine the reducing power of argol and charcoal, make up the following charges in duplicate:

Preliminary Assay, No. 4 Preliminary Ash ay, Xo. 5

10 grams Na a C0 3 10 grams Xa 2 0O 3

2 grams argol 1 gram charcoal or coke or coal

Borax glass cover dust

Borax glass cover 1 This has reference to pure dry KN( )j.

In order to determine the oxidizing power of niter as compared to charcoal, make up the following charge in duplicate:

Preliminary Assay, No. 6

5 grams Si0 2 1 gram charcoal, etc.

10 grams Xa 3 C0 3 Borax glass cover

Calculate results as directed for niter in pyritous ores.

Certain basic ores will have an appreciable oxidizing power, so that when the usual amount of reducing agent is added to the charge to obtain a 20-gram lead button, it is found that, due to the oxidizing power of the ore, the button is deficient in size. The oxidizing ingredients of an ore are generally hematite (Fe a O s ) y magnetite (Fe 3 4 ), &nd manganese oxides; e.g., MnO a . The reaction which takes place is as follows:

2Fe 2 3 + C 4FeO + C0 2

One gram of Fe 2 3 requires 0.037 gram of carbon to reduce it to FeO.

In order to determine the oxidizing power of an ore, make up

the following charge, if the ore consists mostly of base. When

considerable silica is present in the ore, decrease the silica in the

charge:

1 assay toil of ore 15 grams Si0 2

20 grams Na 2 C0 3 1 . 5 grams coal

90 grams PbO Borax glass cover

Chapter Vi

The Crucible Assay; Assay Slags

In almost every instance, when a crucible assay is to be made, the ore and the fluxes added are thoroughly incorporated by mixing, so that, theoretically at least, every particle of the ore is in contact with a particle or particles of fluxes and reducing agent, the most favorable condition to produce a thorough reaction among them. The separation of the precious metals is dependent upon their affinity for metallic lead, forming an alloy of lead, gold and silver, in which lead greatly preponderates, and which readily settles by gravity from the balance of the ore and fluxes which have united to form a slag. The ore to be assayed must in all instances be in a finely crushed condition, varying in American practice, from 80-mesh up to 200-mesh material. What takes place within the crucible depends upon some or all of the following factors:

1. The fineness of crushing. Are all the particles of gold and silver or their alloy present, entirely set free from the inclosing gangue? In some ores this takes place with much coarser crushing than in others. In other ores the metals are so finely disseminated that all are not set free within the limits of crushing as carried out.

2. The mode of occurrence of the gold and silver. Is it in the free state, as is most generally the case with gold, or are the precious metals in the form of a more or less complex mineral compound (tellurides, argentite, etc.), which must be decomposed before the gold and silver will alloy with the lead?

3. The physical properties of the slag produced; e.g., its formation point, its fluidity at temperatures somewhat above its formation point, and its fluidity after superheating.

4. The chemical nature of the slag, its acidity or basicity, the nature of the bases present, more particularly copper, zinc, antimony, manganese, iron, etc.

If a crucible be broken open and its contents examined shortly after fusion has commenced, these will be found to consist of a

heterogeneous mass through which are scattered innumerable particles of lead, both microscopic and macroscopic. The larger particles have been formed by the coalescence of the smaller particles gradually settling through the charge toward the bottom of the crucible to form the final lead button as the temperature rises and the charge becomes more fluid and less resistant. It is evident that the completeness of the collection of the precious metals depends upon the main factors already outlined. The temperature at which carbon begins to react with PbO to form Pb 1 is 530 to 5oo° C, well below 884° C, the melting-point of PbO. The formation point of a borate silicate, PbO, Xa 2 0, 4Si0 2 , 2B 2 3 . (Seger Cone Xo. 0.022) the constituents of which are contained in nearly all assay charges, is .")90° C

In the fusion of a mixture containing silica, various bases and borax glass, that silicate-borate having the lowest formation point will form, and then as the temperature rises absorb either silica or base or both, as these are in excess of the ratio required to form the lowest formation-point compound. If the temperature does not rise high enough to cause this absorption, the excess of silica or base or both will remain in suspension in the formed silicate-borate, practically in an unaltered condition. If the formed silicate, etc., constitutes the greater part of the mass, there will be an imperfect non-homogeneous slag; if the excess of silica or base forms the greater part of the material, there will be a slightlv fritted mass.

Taking the simplest case, and also the most uncommon, that of an ore containing free gold completely liberated by crushing, the particle of lead,- formed at a comparatively low temperature, can unite at once, as soon as formed, with the gold particle not inclosed in gangue and commence settling to the bottom to form the lead button. It is evident that in this instance the homogeneous fusion and chemical decomposition of the ore are immaterial. Taking, however, the far more common case, in which the metals are not completely liberated by crushing, it is evident that the particle of gold still inclosed within the gangue cannot be reached by the lead already reduced, and it becomes practically essential to hold the lead in place until the ore particle containing the gold

1 Doeh/. iiml irauiiiaim . M'tnllnnjii , 1 V, 11.M). According to Ror - — - - immm

7Vi/i'.w on Chrmistru, II. Kti'i 1007). reacts with PbO ivart* with PhO to form IM :it :;iO" Motowitseh, M*"

-'Then' will many particles of lead for no will escape for link of lc;il.

The Crucible Assay 65

is thoroughly broken up chemically and liquefied, so that the lead can absorb the gold. If the lead settles through the charge before this decomposition takes place, gold will remain in the slag. The only way to control this condition is:

(a) By fine crushing, liberating the metals as completely as possible.

(6) By the choice of a slag having the proper physical properties, i.e., a low formation point and a viscous nature near the formation point.

(c) By a comparatively slow fusion during the early stages of the assay, to prevent as much as possible the rapid settling away of the lead particles through the still existing interstices of the charge.

Where compounds of the precious metals are in the ore, such as argentite (Ag 2 S), tellurides, calaverite and sylvanite, (AuAgTeJ, etc., these are readily decomposed by the litharge as follows:

Ag 2 S + 2PbO 2Pb Ag + S0 2

The tellurides will be especially considered in Chapter X, on Special Methods of Assay."

ASSAY SLAGS. — An assay slag from the crucible assay consists in most instances of silicates and borates of metallic bases. "While usually of a homogeneous nature, a slag is rarely a chemical compound. It is to be considered in most cases as a complex "solid solution," this term as applied here including both the crystalline isomorphous mixtures, or " mixed crystals, " and the amorphous glasses. As an example: Litharge with silica forms certain silicates which are chemical compounds, but which have not been definitely determined, though very likely Pb 2 Si0 4 is one of them, judging by cooling curves which have been taken. 1 This silicate is capable of dissolving either PbO or Si0 2 and forming homogeneous "solid solution" within certain limits, the solid solutions in cases when the silica contents arc above 11.94 per cent. — corresponding to Pb 2 Si0 4 — being glasses.

In a similar way all the common bases, Na 2 0, K a O, FeO,

OaO, MgO, Al 2 O s , ZnO and MnO form silicates which are sol-in

each other when molten, and when frozen will form

complex isomorphous mixtures or amorphous glassy

' rtions." An assay slag is therefore usually a complex

taoh, Metallurgy, IV, 651. S. Hilpert, Metallurgir, V, 535.

A Manual Of Fire Assaying

"solid solution." Boric acid and alkaline borates act similarly to silica, and if borax is used in the fusion the final slag will be a complex " solid solution" of silicates and borates of PbO, Xa,0, FeO, CaO, etc., dependent upon the bases in the ore and the fluxes used.

Silicates are defined in degree by the ratio of oxygen in the base to that in the acid. The chemical classification is as follows:

Name

Table V— Silicate Degrees

Oxygen Ratio. Base to Acid Example

Orthosilicate Metasilicate . Sesquisilicate Bisilicate

1 to 1 1 to 2 1 to 3 1 to 4

MgO.FeO.SiO, MgO.CaO.2SiO, K,O.Al,0,.6SiO, Ca0.2SiO,

The metallurgical classification is made on the same basis, i'.\. oxygen in the base to that in the acid, but is somewhat different. It is the one adopted in these notes.

Table Vi.— Silicate Degrees

Formula, RO vba$*°

Name

4RO s;o,

2Ro S.O.

Subsilicate

Monosilioato

2R,0 3SiO t

Sosojuiilicato

4R/V 98iO,

Bisilioate

R.O,. 3SiO t

Triplicate

Borate :v.ay Iv in a somewhat similar manner.

lr. sror.oval. it :ua\ Iv stated that the higher the silicate degree, the :v.ow ir.thtsihle is the a::.: that a polybasic mixture, or.e o: ;v..v:.\ is r.totv easily tv.siKe than one of few. These jce-oral statements are net wit! 1 nt exceptions, for certain bi- >:lh-atos ane. trisilieato> ha\ e a :usl:;c point than the corresivno.ins; % et; It also oli'ivr. greatly upon tie the :\:>-'v v o.' -:v <:1 .-ateswill "v. PbO. Xa,0, and

The Crucible Assay

K a O give easily fusible silicates; FeO and MnO give comparatively readily fusible silicates; Al 3 O s , CaO, and MgO give difficultly fusible silicates. When, however, silicates of all these various bases are mixed and go into solution as a homogeneous mass, the effect of this mixture on the melting-point of the mass is often to lower it. In fact, the silicate mixtures are to be looked upon from the same point of view as metallic alloys; there may be eutectic mixtures, i.e., mixtures of two or more constituents which have a lower melting-point than either of the constitutents, as is illustrated in the accompanying diagram. 1

uoo*c

Hypersthene a ;

Rhodonite

Fia. 46. — Freezing-point Curve; Riiodonitk-hypersthene.

The eutectic mixture, or the composition of lowest meltingpoint in the series occurs at 20 per cent, hypersthene (the bisilicate of iron) and 80 per cent, rhodonite (the bisilicate of manganese). The melting-point of this mixture is 985° C, which is considerably lower than that of either constituent alone. In the series CaSi0 3 — Na 2 SiO s a minimum occurs in the freezing-point curve at a composition of 80 per cent. Na 2 Si0 3 and 20 per cent. CaSiO s , the freezing temperature being 920° C. while the freezing point of Na 2 Si0 3 is about 1010° C. and that of CaSiO, is 1505° C. 2

Typical Assay Slags. — A slag of low formation temperature and considerable viscosity at that temperature corresponds to Seger Cone No. 0.022- Xa 3 O.Pb().4Si0 3 .2B 2 3 . r >W° This may be written: Ph0.4SiO,.Na,B l O.-

1 J. H. L. Vogl, Die Silikatitcftmtizloisunycn, II, ChriMiuria.

2 11. C. Wallace, Zrit. Anory. . LXIII, 2

A Manual Of Fire Assaying

By calculation from the atomic weights the following charge will yield this slag:

PbO 33.3 grams

SiO, 36.2 grams

Na, B 4 0- 30.4 grams

The slag, corresponding to Seger Cone 0.017 and melting at 740° C, may be desirable for aluminous ores:

(XajO.PbO.AljOj.eSiOBjO,), which may be written (Na,B 4 7 . PbO. Al,O s . 6SiO,).

The following charge will yield this slag:

Na t B 4 T 22.9 grams Al s O a 11 5 grams

PbO 24.9 grams SiO, 40.7 grams

Table Vii— Assay Slags 1

Formula

Approximate

Silicate degree

temperature

at which fluid

Remarks

Monotfilical*. .

Vitreous, colorless, trans parent.

Ristticste

Stony, white, crystalline.

Mono$tUr*:o

Vitreous, fight yellow,

Vitreous, fight yellow,

MvvtivvsilicA'.o

Very fluid, stony black.

Vitreous, black.

Resinous, black.

Nmo

Vitreous, yellow-green.

Bv*hcAt*

Uw

Vitreous, yeDow-creep.

Moo.xXfttv*:*'

uw

Vitreous, black.

MocrxYtlix*Atv

Vitreous, black, cootaim sq. crystals-

Iuv

Mlreous, black.

M.NUvVfthCA'M*

uw

S*suy. sight yellow.

Vtseovs. Mow. STST-

; vv

v;vco;r.; w," ,;vv*w ,s,X\ lowov :ho :Vrr.:**;oa points. Rwv v;;,:. :i> !VO v .v \UO Mr.O RaO. arid Al a O, are

The Crucible Assay

outlined must necessarily be made. The easily fusible bases PbO and Na a O serve to lower the formation point of the slag. If it is accepted that the composition of the slag in the assay is practically the constant factor, it is evident that when the approximate composition of the ore is known, we will add either basic or acid fluxes, in such proportions as to produce the proper slag decided upon. The most desirable constitution for an assay slag in general, is that of a monosilicate or a sesquisilicate, sometimes, but more rarely, a bisilicate. If the ore is basic a bisilicate may be approached, if acid a monosilicate, or even a sub-silicate, in order to insure complete decomposition of the ore. %

The accompanying table will simplify slag calculations:

TABLE VIII.— THE CALCULATION OF SLAGS 1 Unit Molecular Base Ratio; e.g., PbO: Na,0: FeO, etc.=-1: 1: 1

One part

of base by

weight

Parts of other bases necessary

Na 2 PbO CaO Al a O, FeO ZnO

Parts of SiO a

necessary for

monosilicate

Na,0

PbO.

FeO.

CaO.

A1,0,

CuO..

ZnO.

One part by weight of i Na 8 PbO CaO Al,O a FeO , ZnO CuO 8iO t requires to form 2.07 7.36 1.86 1.14 2.40 2.70 2.63 the monosilicate parts parts parts parts parts ; parts parts

When a bisilicate is to be calculated, the silica required for a monosilicate is determined and then multiplied by two. Vice versa, when the bases for the monosilicate have been calculated and a bisilicate is to be formed, the bases must be divided by two. The same reasoning applies to other silicate degrees.

1 Baaed on Balling's table.

Example of the Calculation of an Assay Slag. — The problem is to calculate a charge to produce the following monosilicate: Na 2 O.PbO.FeO.Ca0.2SiO a . Taking as the unit 10 grams of Na 2 0, it follows from the preceding table that the weights of the substances required are:

Na a O 10X1 =10.0 grams

PbO 10X3.59 -35.9 grams

FeO 10X1.16 -11.6 grams

CaO ...10X0.903= 9.03 grams

The silica required will be:

for the Na,0 10 X0. 486 4. 86 grams

PbO 35. 90X0. 136=4. 86 grams

FeO 11. 60X0. 419=4. 86 grams

CaO 9.03X0.539=4.86 grams

Total 19.44 grams SiO,

The silica may be determined by calculating it for one base and multiplying that figure by the number of oxygen molecules in the bases present, after having reduced the slag formula to its lowest possible terms. Before making up the charge, it is essential to remember that the Na 2 in this instance is furnished in the form of NaHC0 8 , which contains approximately 40 per cent, of Na 2 0, and that the FeO is furnished by an iron ore of the following approximate composition:

Fe 2 8 , 80 per cent.; Si0 2 , 17 per cent.

The lime is furnished by limestone, CaCO,, practically pure- It is also necessary to provide a lead button; so extra litharge

must be furnished. To reduce the lead, coal dust is added.

Some of the coal will be used up to reduce the Fe 2 O s to FeO.

Hence the following calculations are to be made: 10 grams Na 2

are required; therefore — X 100 25 grams of NaHCO, must

be added. PbO contains 92 per cent, of Pb; therefore, in order

to obtain a 20-gram lead button, - - =22 grams of PbO

must be added, in addition to the 35.9 grams for the silicate— a total of 57.9 grams of PbO. Eleven and six-tenths grams of FeO are required. Fe 2 0, consists of 90 per cent, of FeO and 10 per

cent, of 2 ; and as the ore is 80 per cent, of Fe 2 O a , — — — - - 2 F 2 3 ' 90X80

The Crucible Assay 71

16.1 grams of ore will be required. The limestone contains 54

per cent. CaO; therefore, — — 16.7 grams of limestone

will be required.

The coal in use has a reducing power of 20 grams of lead per gram of coal.

The following reaction takes place between carbon and the Fe 2 8 .

2Fe a 8 + C 4FeO + CO a .

One gram of Fe 2 O s requires -— =0.037 gram of charcoal.

' 20 X 100 But as the coal used is only wr~- =58 per cent, as strong as

charcoal, the following quantity will have to be added to the 16.1 grams of Fe 3 3 to reduce it:

To this must be added 1 gram for the reduction of the 20-gram lead button, giving 1.82 grams of coal to be added.

Since the iron ore contains silica, this is to be deducted from the silica calculated. The amount of SiO a in the ore is 16.1 X 17 per cent. 2.74 grams.

The correct charge then is:

25 grams NaHCO, 16.7 grams limestone

57.9 grama PbO 16.7 grams silica (19.44-2.74)

Salt cover

Following is the calculation of the same slag, but for a quartz ore containing 95 per cent. SiO a . The formula for the slag is: Na 2 O.PbO.FeO.Ca0.2SiO a . Taking as the unit 1 assay ton of ore, or, in round numbers, 30 grams, this will contain 28.50 grams of SiO a . These 28.5 grams are to be divided into 4 equal parts to satisfy the 4 bases present. Therefore, 7.1 grams of SiO a will go to such an amount of each base as will form a monosilicate.

7 . 1 grams Si0 2 require 7 . 1 X 2 . 07 14 . 7 grams Na a O 7 . 1 grams SiO a require 7 . 1 X 7 . 36 52 . 25 grams PbO 7.1 grams SiO, require 7.1X2.40 17.04 grams FeO 7.1 grams SiO, require 7.1X1.86*13.20 grams CaO

The bicarbonate of soda required is — =37 grams.

The PbO required is 52.25 + 22 74.25 grams including . the lead button.

The FeC0 3 (siderite) required is ' -- =27 grams.

Tk r a 13.20X100 OAA

Ihe limestone required is =24.4 grams.

The complete charge is:

1 assay ton ore 27 grams FeCO,

37 grams NaHCO a 24.5 grams CaCO,

Salt cover

In one case the ore is of a basic nature — hematite and limestone (17 grams of each), and in the other case it is of an acid nature — quartz; yet the slag produced is the same in both cases. This brings out the fact that the slag is the constant and that fluxes are added of such nature and in such quantity, determined by the ore, as to produce a slag of fairly constant composition. It is to be noted that the slag made in the two assays contains four bases, PbO, Na 2 0, FeO, CaO, and that these are present in unit molecular base ratio. As a matter of fact, the assayer rarely adds CaO or FeO as fluxes, but when these are present in the slag, they are derived from the ore. The bases added as fluxes are practically limited to three, PbO, Na,0 and, at times, K 2 0, so that when an ore consisting chiefly of SiO a is to be assayed, the slag made will approximate a monosilicate and borate of lead oxide and soda.

The tabic of assay slags given mentions only those in which the bases are present in the unit molecular ratio. It is evident that where an ore is considered in which numerous bases are present, these are not contained in the unit molecular ratio, so that the formula of the slag made will rather have this general form :

in which, for a monosilicate, considering the letters as oxygen coefficients, x -f y z -j - 1 — 2v. In order to get a slag of low formation point, the coefficients of the more infusible bases, such as CaO, MgO, Al 2 O s , will have to be materially smaller than those of the more fusible bases, PbO, Na 2 0, and FeO.

The Crucible Assay

In assay practice, it is neither possible nor desirable to make analyses of ore before assaying for gold and silver. The assayer, however, is supposed to have a good working knowledge of lithology and mineralogy, which will enable him to form a correct judgment of the contents of his ore within fair limits. It will be comparatively easy for him to tell at once whether he has limestone or dolomite, or an ore containing much limonite or hematite or the iron sulphides; or whether magnesia, barium or other bases are present, and in what general proportions. Following are analyses of silicious and lead-antimonial ores:

Table Ix.— Silicious Ores

No. 1

No. 2

No. 3

No. 4

No. 5

No. 6

Gold 0.63 oz.

Silver 2.00 oz.

per cent.

Silica 65.38

Iron 13.40

Sulphur 11.40

Arsenic 0.90

Antimony trace

Tellurium 0.003

Zinc

Copper 0.02

Manganese trace

Alumina 5.43

lime 2.10

Magnesia 0.20

! 1.75 oz. per cent

I 0.62 oi. per cent.

1 1.00 oi. per cent.

per cent.

1 per cent.

trace

trace

trace

trace

trace

trace

trace

I trace

trace

trace

Table X.— Lead-Antimonial Ores

No. 1

No. 2

No. 3

Silica 60 . 1 per cent.

Ferrous oxide 5.2 per cent.

Alumina 9.5 per cent.

Magnesia 2.68 per cent.

Lime trace

Lead 10.6 per cent.

Antimony 4.4 per cent.

Sulphur 0.5 per cent.

Water 0.3 per cent.

trace

trace

.Able Xii.— M

i i trill.*. ; i'

"nit ;t

l;t*:i:t-.:i " ii:.f

".o.-iL -t-r :r.

. ...a 'inn . \.II .liliU'" :""i!!i .ir'-I *V7ji-- ". :ir.

. "u iii- -":.s. ::. i.>iiy.r. ".v:iirii

. . .li' i-iiij'i'i'.y :' .:— -Ivin r li"I::nz i"

..liii.iu .iii.- - ; "V";ill:-: xi'is. T::-'"

-

Vt!lO.

v l l .

v .i.ua VitUV

" .vs-siv Ton

Co

as Ci

of tin

N"iL> 14 crams

P" V' cranio

I £ra:v.

Coal . 1 cram

H -ax il:i* cover

The Crucible Assay 75

All of the above charges will yield satisfactory slags in an ore assay if the ore is of the nature described. No. 3 is the cheapest in point of cost; No. 2 is the one most frequently made.

Color of Slags. — Most slags from ore assays will be from light to very dark green in color or almost black, this color being due to various proportions of ferrous silicate. When iron is absent, the color of lead silicates (yellow) may predominate, or white and gray or colorless slags, due to silicates of CaO,MgO,ZnO, etc., be produced. Copper produces red slags, due to cuprous silicate. Cobalt gives blue slags. When much lime is present in an ore, this is best calculated to a bisilicate or even higher, while the other bases can be calculated to the monosilicate.

Chapter Vii Cupellation

Cupellation has for its object the oxidation of the lead in the gold, silver, etc., alloy to PbO, which in part (98.5 per cent.) is absorbed by the cupel, and in part (1.5 per cent.) volatilized. The silver and gold of the alloy are left as a metallic bead. The process is carried out in cupels. Cupels are shallow porous dishes, made generally of bone-ash, or magnesia, produced by calcining magncsite. Portland cement may be used as a cupel material.

Leached wood-ashes (particularly from beech-wood) and lime and magnesia have also been used for cupels. A mixture of bone-ash and leached wood-ashes, in the proportion of 1 to 2 and 2 to 1 respectively, has been used, and is said to give a much smaller absorption of the precious metals than bone-ash cupels. 1

Bone Ash Cupels. — The bone which yields the bone-ash on calcining has the following composition. 1

Sheep bones Cattle bones

iVulW tVJ.70 per cent. 58.30 per cent.

rnlH),... 7.00 per cent. 7.00 per cent.

% l 59 per cent. 2.09 per cent.

Organic mutter "JtVM per cent. 30.58 per cent.

Tlioao honor will produco Kuusash of the following composition:

No. l Xo. 2

$1 S9 per cent. S3. 07 per cent.

9 V.* ivr cent. 10.00 per cent.

4 k per cent. 3.SS per cent.

4 x I A per oer.t -9$ per cent.

Cupellation

The bone-ash used for cupels must be specially treated by washing with an aqueous solution of ammonium chloride (this all to the extent of 2 per cent, of the weight of the bone-ash to be treated). 1 This reacts with CaCO a and any CaO present, converting them into CaClj, which is removed by washing with water. The presence of CaCO, is very undesirable in bone-ash for cupels, art it begins to give off CO a at 800° C, about the temperature of the beginning of cupellation, causing a serious spitting of the lead button, which entails a loss of the preciouB metals. Cupels should not be kept where the nitrous fumes from parting can be absorbed by them, u these will form a ), with any CaO that may be present, which also is decomposed about the temperature of eupettation. Bone-ash melts at about 1450° C. (Hempel).

The physical nature of the cupel, especially as regards porosity, very important. For this reason there should be a careful adjustment of the relative amounts of different sized particles present. Practically, only the fraction of 1 per cent, of the bone-ash should remain on a 30-mesh screen. If there is an insufficiency of line particles in the bone-ash, the cupel will be too porous and cause a relatively heavy absorption of gold and silver. If the bone-ash is too fine, the cupels made from it will be too dense, prolonging the cupellation and causing losses, mainly by increased volatilization.

The following is a screen analysis of the bone-ash commonly purchased, but which is rather coarse:

Through a 20-mesh

On a 30-raesh

?cm?n, 2 90 per cent.

On a 40-mesli

screen, 0.40 per cent.

Oil u 60-tneah

screen, 10 . 04 per cent.

On a 80-mesh

screen, 2.00 per not.

On a 100-

screen, 11.20 percent.

Through a 100-ntesh

screen, 68.88 percent.

wuld be as unifor

iTi-iry,

r this reason are best made by machine, in which a constant

sure may be obtained, rather than by hand molds. Fi. 47

8 a good type of cupel machine. Considerable pressure may

used, and tfie cupels made quite firm. It is not possible to

>ectfy the proper condition in definite terms, but a batch of

;, after being made up and carefully dried for at least three

i month, should be tested by cupeling a weighed quan-

1, Pnc. CAtw. ami MM. Sot. of .S. A., 1 1. SM

7X A Manual Of Fibs Assaying

tity (200 mgs.) of c. p. silver with 20 grams of lead at the prope temperature, 850° C, and the loss noted. It should oi from 1.5 to 1.8 per cent.

I lif bons-ash to be made into cupels is mixed with t" i 6 I

I 'J per cent, of water, in which is dissolved a little K,CO„ or to which has been added a Utile molasses or stale beer. -After making, the cupels should be carefully and Blowly dried, tl

- in' several months old before using. In the Royal British Mint no cupels less i ira old ar*

If cupels are too rapidly dried, or have :..

and check when placed in ihe furnace and make i no I in Asm unreliable. The importance of cupels cannot Ik? o veresti matedmqoentlv

cupel. Tin-shape of 1

hie fly in i

t he loss

u-s and h;

Cupellation 79

exposed to oxidation is increased, but as the absorption of precious metals is probably a function of the area exposed, it will be large in shallow cupels. 1

Magnesia Cupels. — Of recent years the so-called " patent " cupels have come into wide use especially in England and South Africa and to a lesser extent in the United States. These cupels are made almost invariably of a magnesia base. This magnesia is produced by calcining crude Austrian, Californian or Turkish magnesite, and is used largely in the steel industry for basic refractory brick.

The compostion is about 90 per cent MgO, and 10 per cent, of impurities, chiefly CaO, Fe,O s , A1 2 0, and Si0 2 . The cupels are invariably very hard and firm, of a brown color and are formed under high pressure. The exact composition of the cupels is generally a trade secret. Magnesia cupels cannot very readily be made in the laboratory like bone-ash cupels, and in almost all instances their cost is higher. A number of brands are on the market, as the Morganite cupel, made by the Morgan Crucible Co., Battersea Works, London, those made by Deleuil, Paris, and the Mabor, Scalite, Velterite, Star, etc., brands. Morganite cupels, 31.5 m.m. top diameter (about 1.25 in.) the common size, cost $3.35 per 100 in St. Louis.

The properties of various types of cupels are discussed in a following section.

Portland Cement Cupels. — Satisfactory cupels may be made of ordinary Portland cement provided the amount of mixing water is carefully adjusted. 2 The amount of water should be 8 per cent, of the weight of the cement. If less than 5 per cent, water is used the cupels are too fragile, if 20 per cent, is used they will not readily pass the cupel machine. Upon heating, cupels with less than 5 per cent, and with more than 15 per cent, water cracked about the edges. Cupels made of one-half cement and one-half bone-ash give good results.

Cement cupels are very cheap as compared to bone-ash. Cement will cost from 35 cents to $1.00 per 100 lb., while boneash costs from $5.00 to $8.00 per 100 lb. Cement cupels should be thoroughly dried before use, otherwise they will develop cracks during heating.

1 H. K. Edmonds, Bng. and Min. Jour., LXXX, 245.

2 T. P. Holt and N. C. Chris tensen, "Experiments with Portland Cement Cupels," Eng. and Min. Jour., XC, 560. J. W. Merritt, "Cement vs. Bone- Ash Cupels," Min. and Sci. Press, C. 649.

&#x27;V

-.yel loiul buttons, the cupels

water has been used in

:." :he remaining moisture is

violently through the molten

-.::g." i.e., the projection of

. : aiiJ silver from the cupel.

;-. nuaining CaCO s , will coni-

.-. ..as proceeded for some time

.ibove 800° C. This can ho

\::ct spitting, is to be consid-the

cupels absorb gases at

t'f.iv rat ure rises, are again ex-

, % ray-black scum. If the lead

kivuM be, this black scum dis-

.:n.vvering M of the lead button.

. ; -*t. begins to "drive," and va.tticcuvs. Lead buttons should

.. iic muffle. If other and more

niicr and the temperature re-

.liiMM Those foreign metals should,

the .a iK % surface of the molten lead

used *rf the button, and are absorbed

If . " ' The process of cupellation is

I hey i urfaee of the cupel to that

assays " w Slhargo which is formed by

Tlie tfifcwncc between the surface

Very in aiuI while litharge can "wet*'

cupel. I r jafcwrbod. molten lead cannot

precious i J&H u ' hence is not absorbed,

shallow, m ir*P*l gold. left on the cupel by

Cupellation 81

the oxidation of the lead will not be absorbed by the cupel. As will be noted further on there is always a loss of precious metal during cupellation, the greater part of which is caused by absorption by the cupel. Whether this absorption is due to some small part of the lead alloy passing into the cupel, or to an oxidation of some silver with consequent absorption has never been definitely determined. It is true that the different cupel materials and the physical condition of the cupel as regards porosity influence absorption, the greatest factor, however, is temperature of cupellation, a comparatively slight increase of temperature causing a marked increase in absorption. Whether this increased absorption is due to an increased oxidation of the precious metal, or a decrease in the surface tension of the lead alloy is open to question. This subject is again referred to on page 133.

The temperature of cupellation is the most important single factor in the operation. Three distinct temperatures must be considered, (1) the temperature of the cupelling lead; (2) the temperature of the muffle, by which is meant the temperature of the interior of a blank cupel, directly adjoining the one containing the lead, and (3) the temperature of the air in the muffle, near the cupel. The vital temperature is that of the cupelling lead, but as this is difficult to measure except by special apparatus, the "muffle temperature," which always bears a distinct relation to the temperature of the cupelling lead is used hereafter in designating the "temperature of cupellation."

The temperature of cupellation for pure lead buttons should be 850° C. to "uncover" the button, this may be lowered to about 770° C. during the major part of the cupellation, but must be raised again to about 830° C. near the end to finish the operation. This applies to bone-ash cupels. The temperature of the lead itself during cupellation is higher than that indicated by the blank cupel near it, owing to the rapid oxidation of the lead. This is shown by the brighter color of the lead.

Any foreign metals, as Cu, Sb, Fe, Zn, etc., which are present are oxidized (some by the PbO formed), and absorbed by the cupel, if not present in too large amounts.

Zn+PbO=Pb + ZnO.

Such elements as Sb, As, and Zn, when present in the button, are in part volatilized as oxides, and in part absorbed. When

cupellation for silver is carried on, the temperature should not be above 820° C, in which case crystals of litharge (feathers) form on the side of the cupel toward the muffle mouth. If the temperature is too low for the cupel to successfully absorb practically all of the PbO, these feathers form low down in the cupel. When the temperature is about right, they form near the upper rim of the cupel. It is, however, to be noted that the draft through the muffle influences the formation of feather litharge; i.e., if the draft is strong, feathers will form, although the temperature is somewhat above 820° C. During cupellation, the door of the muffle should never be left wide open, but should be set slightly ajar, so that the cold air will not strike directly upon the cupels. When silver and gold are cupelled for, owing to the higher melting-point of the silver-gold alloy, the finishing temperature will have to be 860° C. at least.

As the cupellation proceeds, the percentage of lead in the alloy decreases and that of Ag and Au increases. The litharge thrown off from the center of the button is in larger specks, and brilliant, and the button assumes a more rounded form. When this phenomenon appears, the cupel should be pushed back into the hotter part of the furnace or the temperature of the furnace raised somewhat. When the last of the Pb goes off, large buttons are covered with a brilliant film of colors (interference colors) and the button appears to revolve axially. The colors then disappear, the bead becomes dull, and then again takes on a silvery tinge.

If now the temperature of the muffle is below that of the melting-point of silver (962° C), or below that of the gold-silver alloy constituting the bead, or if the cupel be withdrawn from the furnace, the "blick" or "brightening" or "flash" of the bead takes place; i.e., the bead suddenly becomes very bright, at the moment of solidification, owing to the release of the latent heat of fusion, which raises the temperature of the bead very much for a short time. The bead has been in a state of surf usion, i.e., in a state of fusion below its true freezing-point, toward the last of the cupelling operation; and if it be lightly jarred or the temperature allowed to drop still lower (by taking it out of the muffle), it suddenly congeals and assumes a state normal (solid) to the temperature existing. The release of the latent heat, raising the temperature of the bead, causes the brightening. The "brightening" of very small beads is rarely noticeable.

Cupellation 83

Silver and gold beads still containing small amounts of Pb or Cu do not brighten so noticeably. If even minute quantities of rhodium, iridium, ruthenium, osmium, or osmium-iridium, are present, buttons will not flash. Platinum and palladium are excepted.

Silver beads after cupellation, and at the moment of solidification, also "sprout." According to Gay-Lussac molten silver dissolves 22 times its volume of oxygen, at the freezing-point. Later researches 1 prove this practically correct. At 1020° C. molten silver will hold 19.5 volumes of oxygen (at 760 mm. and 0° C) and at the melting-point somewhat more. For any given temperature the oxygen dissolved is proportional to the square root of the oxygen pressure. In air at 760 mm. pressure the oxygen has a partial pressure of 150 mm. and the volume of oxygen dissolved by molten silver under assay conditions is 9.65 volumes at the freezing-point of silver. The oxygen is dissolved either as monatomic oxygen or as silver oxide (Ag 2 0), in dilute solution. It is probable that this silver oxide, not being soluble in solid silver is dissociated with explosive violence, with the liberation of oxygen, when the silver solidifies.

This oxygen, suddenly expelled when the bead solidifies, causes a cauliflower-like growth on the bead. Small particles of silver may even be projected from it and cause a serious loss. When gold is present in the silver bead to the extent of 33 per cent, or more, sprouting does not take place. Silver beads containing small quantities of Pb, Cu, Zn, Bi, etc., will not sprout, so that if a button does sprout it is a sign of purity.

Buttons below 5 mgs. in weight do not sprout readily; large buttons, however, do. Sprouting can be prevented by slow cooling in the muffle, or by having ready a hot cupel which can be set, inverted, over the one holding the bead, and withdrawing both from the muffle, thus cooling the bead slowly. Sprouted beads are to be rejected as an assay.

When cupelling for silver alone, or for silver and gold, it is necessary to watch the end of the cupellation carefully, and to promptly remove the cupel about 30 seconds to 1 minute after the bead has become dull. A heavy loss of silver commences if the silver buttons are kept beyond that time in the furnace. If silver is not to be determined, but gold only, the buttons may

i Donnan and Shaw, Jour. Soc. Chem. Ind. t XXIX, 987. Sieverta und Hagenacker, Zeit Phya. Chem., LXVIII, 115.

/

be left in for 5 to 10 minutes without loss of gold. Gold beads will retain minute amounts of lead which cannot be removed by permitting the bead to stay in the muffle.

It is to be noted, however, that silver lead alloys containing between 80 and 90 per cent, of silver also show the phenomenon of sprouting or developing a cauliflower-like growth on solidification. 1

The bead, when cold, is taken from the cupel with a pair of pliers, and cleaned of bone-ash by flattening somewhat with a hammer. It should be examined with a glass to make sure that no bone-ash adheres to it.

The bead should be either white or yellow, depending on the amount of gold present, round and not flat (the latter indicating the presence of foreign metals), and should possess a crystalline surface where it adhered to the bone-ash. It should be firmly attached to the bone-ash of the cupel. If it is not, this fact indicates that lead is still present. It should also have no rootlets extending into the cupel. The cupel, after cupellation, should be smooth and firm, not fissured and cracked, and of a light yellow color when cold. Other colors indicate the presence of foreign metals.

The freezing-point curve of lead-silver (Fig. 48) will give some idea of the proper temperature of cupellation. A lead button is to be considered as an alloy of lead and silver (or gold) which in the process of cupellation undergoes the change from practically pure lead to that of pure silver (or gold).

A 20-gram button containing 200 mgs. of silver contains 1 per cent, of Ag. An alloy of lead and silver containing 4 per cent, of Ag is of "eutectic composition" and melts at 303° C, the melting-point of pure lead being 327° C. Most assay buttons will contain very much less than 1 per cent, of silver and will melt practically at the melting-point of lead. Leaving out of consideration for the moment that lead "uncovers" at 850° C. in an oxidizing atmosphere, and the proper temperature required to cause a ready absorption of PbO by the cupel, it is evident that for a lead button weighing 20 grams and containing 20 mgs. of silver (0.1 per cent.), the temperature required to keep the button molten ranges from 327° C. to 303° C, until the button has decreased f $ in weight by the loss of Pb, practically the entire time of cupellation.

When the button has reached ¥ V °f original weight, the

K. Friedrich, Metallurffie, III, 398.

Cupellation

temperature required to kep it molten will rapidly increase, according to the curve, as more lead is oxidized, until, in order to prevent freezing and get pure silver, a temperature of 910° C

Fig. 48. — Freezing-point Curve, Lead-silver

CuO PblOO

T

Pio. 40. — Freezing-point Curve, Lead-copper

and slightly above must finally be reached. 1 In order, however, to cause a rapid formation of PbO and its ready absorption by the cupel, and not have heavy losses of Au and Ag, it is found

1 While the melting point of silver is 962° C, this temperature is not necessary as surfusion takes place.

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Cupellation

the muffle as determined by that of a blank cupel, adjoining the one containing the lead; and (3) the temperature of the air immediately surrounding the cupel. This is invariably lower than the first two temperatures, which accounts for the low

j c.

So

Noto

Minute*

Fio. 50. — Temperature-Curve Showing Difference between Air in Muffle

and Cupelling Lead.

temperature figures that have been assigned to the cupellation process. It is to be noted that there is an air draft through the muffle during cupellation, cold air constantly entering at the mouth of the muffle, so that the air in the muffle does not attain the temperature of the muffle walls. The actual air temperature

is also probably somewhat lower than the thermo couple junction shows, since this absorbs heat radiated from the muffle walls more rapidly than the air. Fig. 50 shows the two temperature curves, one, the actual temperature of the cupelling lead and the other that of the air in the muffle, close by and at a level with the top of the cupel. The general form of the curve is due to fluctuations of temperature in the muffle, caused by firing and attempts to regulate the temperature, by draft and otherwise. It will be noted that the temperature of the cupel rises rapidly after oxidation has commenced, attaining a maximum of 940° C. and then falling as the muffle cooled. The interesting data is the difference between the temperatures of the air in the muffle and the cupel, which is greatest during the period of active oxidation. The maximum difference is 145° C. The lead "froze" or was covered over with a coating of PbO, preventing further cupellation at 802° C, the air in the muffle being then at 675° C. The actual minimum temperature of cupellation in this case was therefore 802° C, 127° higher than the air temperature. 1

Experiment. — To determine the temperature of the "opening or uncovering ,, of the button; i.e., the beginning of cupellation, and the "freezing " of the button; i.e. , where cupellation is stopped by the formation of PbO which is not absorbed.

In this experiment, 134 grams of lead were used. The presence of gold or silver has no influence on these critical temperatures, as the melting-point of the alloys is usually far below the " uncovering " temperatures and the precious metals form no oxides which would complicate matters. The influence of such metals as copper will be referred to further on. The set was run with a blank, at the same temperature as the cupel before the lead was added. Fig. 51 gives the curves plotted as before. The results show that the button begins to uncover at 800° C. and 804° C. and begins to "freeze" at 804° and 788° C. These are the actual cupel temperatures. A repetition of the experiment in the same cupel shows "uncovering" at 832° C, 829° C, and 834° C. and a freezing at 850° C. Other results show the beginning of "uncovering" at 797° C. and completely open at 805° C. Another shows an opening to occur at 811° C. Another shows

1 In order to definitely prove the difference in temperature to be due to .the oxidation of the lead, a aet was run in which the lead in the oupel was covered by a clay dish luted on, practically preventing oxidation. In this instance the muffle and cupel were at nearly the same temperature for the space of an hour, first one being a little higher and then the other

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is also probably somewhat lower than the the shows, since this absorbs heat radiated fr more rapidly than the air. Fig. 50 shows curves, one, the actual temperature of the - other that of the air in the muffle, close 1 the top of the cupel. The general form fluctuations of temperature in the muffle attempts to regulate the temperature, I It will be noted that the temperature n' after oxidation has commenced, attaint) and then falling as the muffle cooled. 'I difference between the temperatures oi the cupel, which is greatest during the The maximum difference is 145" C. covered over with a coating of PbO. tion at 802° C, the air in the muffle actual minimum temperature of therefore 802°C, 127° higher than i

Experiment. — To determine the or uncovering" of the button; i.e. and the "freezing" of the button; /. by Uiv formation of PbO which is m

In (his experiment, 134 grams - ciu'c of gold or silver has no mfi i 1-iirrn, an the melting-point of "uncovering" temperatures aa-, oxides which would cumplica melnls as copper will be rc-fem wilh a blank, at the hhiiio te lead wan added, Fig. ol give* result* show thai the butt SIH" and logins to -fre* the actual cupel lemperatl in the same cupel " SIM" V. and a freezing at iting of " uneovcring" at Another shows an open

ol llip W.I. 4i it nu run in

i ue cupel

the lead,

jii 1 to 150° C.

. ii'shed that the

- ss* apparent that

- idte itself. In Fig.

*jjy above the muffle

a j0t m soon as the

ol onler to get a

uur," thus pre-

&quot;In 91

"elation to volume

i in the uncover-ii

silica or borax,

in marks the "free-

i all the temperature

freezing" are well bc-is

here self-evident that

i temperature is 900° C.

. Iii'h "opening" is observed

1-Nty.s practically always have

- of silicious slag, and the bone-

|iiantities of Si0 2 . When the lead

.sis slag and fine, loose bone-ash go to

tilm. According to recent research

silicates— 6PbO.Si0 2 , 5PbO.Si0 2 and

iil at 794° C, 796° C. and 726° C. respec-

:iije composition of these silicates is as

Litharge

Silica

l>.

- i- -nt that when the very small amount of litharge is the film is considered, that minute quantities of sil- :tr( necessary to materially lower the "opening" temof the button. From these facts it follows that the or "uncovering" temperature is not a fixed tempering will depend upon the following factors: The presence of silica in a condition to combine with (very probably also of borax). Where this silica comes in has already been mentioned.

J. The vaporization of solid litharge. As the rate of vapor- /.ation depends upon the temperature, and the relation of rea exposed to volume present, a button with a thick covering vill not open at as low a temperature as one with a thin coverng. This can be demonstrated by placing a button in the cupel it a temperature below 700° C. and permitting it to form a heavy lira of PbO, then raising the temperature to the usual "uncov-

Wl. Moatowitach, Afc/n/Zurr/iV, IV, 017.

ering" point, and placing another button into a second heated cupel. The last button will uncover first, as its thinner cover of litharge will vaporize in less time.

3. The presence of foreign metals in the lead, such as copper, iron, etc., will raise the "uncovering" temperature. This is a frequently observed fact, and the reasons for it are practically obvious. If the temperature of the cupel at the moment of uncovering could remain fixed, the increase in the oxidation of the lead would very soon balance the vaporization at that temperature and the button would again freeze, but it has already been noted that a very sharp rise in temperature at once occurs automatically; i.e., independent of the muffle, due to the rapid oxidation of the lead; this effects a marked increase in vaporization, keeping the button open and soon in most instances the temperature of the button itself passes to and beyond the melting point of litharge (884°) and cupellation proceeds rapidly. Cupellation, however, can seemingly be carried on below the melting-point of litharge, as Figs. 51 and 52 will show. The particles of litharge formed on the surface of the button, though solid, are pasty and capable of being absorbed by the cupel, or the surface of the cupelling lead being the area of the most active oxidation is at or above the temperature of melting litharge, which the thermo-j unction at the bottom of the lead does not indicate. 1

In one experiment, containing considerable silver, it was noted that very near the end of the cupellation when the amount of silver was large and that of lead small, the button was cupelling at an indicated cupel temperature of 750° C, the button then solidified and proved to be a lead-silver alloy. The temperature of 750° evidently did not represent the surface temperature of the button as was indicated by the brightness of the PbO specks formed; i.e., the amount of heat liberated by the small amount of lead oxidized was insufficient to make any material impression on the thermo-j unction.

The "Freezing" of the Button. — When the temperature of the muffle falls so that the heat of oxidation of lead is no longer

1 In ma experiment to shed light on this point, 70 grams of pure PbO were placed in a cupel and heated to 815° C. for the time of 20 minutes. The litharge showed Yaporiiatioo, but none was absorbed by the cupel. In a duplicate experiment the temperature was raised to 883° C. just below the melting-point, and while the litharge did not melt, all of it was rapidly absorbed by the cupel. In the first ease the mass of litharge was sintered. Absorption thus probably occurs in the "pasty" stage mentioned.

Cdpbllation 93

enough to keep its temperature such that the rate of vaporization ia in excess of the rate of oxidation, the molten button will become covered by a film of litharge and cupellation ceases.

"Feathers" are crystals of solid litharge sublimed from the

v *por and deposited .>n the cupel walls. That the cupel walls

ar e invariably cooler than the cupelling lead is self-evident.

Feathers will therefore form when the temperature of the cupel

H 'all is below or near that of "uncovering." They will not form

above about 820° C. The cooler wall and that on which feathers

m 08t usually form is that toward the muffle mouth, due to the

'direct impingement of cooler air currents. These feathers form

( best guide to the temperature of cupellation ordinarily

av a.ilable. During their formation the actual temperature of the

11 l-> lead is usually from 840° to 900° C, although it may be

a higher if the oxidation be rapid. The rapidity of

l oxidation depends largely on the air supply, and this heavy

air current striking the cupel may cool the walls sufficiently to

a heavy sublimation of feathers, although the true temper-of

cupellation; i.e., that of the button may be unduly high.

-ttiaumi. — From the foregoing, it appears that the "uncov-

*~Viig" of the button occurs at from 800° to 840" G. dependent on

"Vcral factors, and that the actual minimum temperature of

x pcllation, may be placed at about 850° C, but usually rises

-Viovc this, i.e., independent of the muffle, frequently to 930°

Vd 940°, unless the muffle temperature is lowered after uncover-

g. The necessary finishing temperature is, however, higher

tian 850° C.

Experiment. — To determine the phenomena incident to the ' finishing" of a cupellation containing silver, i.e., that of surfusion, " " sprouting, " freedom of the silver bead from lead, temperature necessary to finish, etc.

It has frequently been noted that a cupellation containing silver and gold, or both, could seemingly be "finished," i.e., all the lead eliminated therefrom when the temperature of the muffle was well below that of the melting-point of silver; i.e., 962° C, or that of the gold-silver alloy. From the foregoing, it is evident that the temperature of the muffle is not by any means the same as that of the cupellation. Roberts- Austen 1 quotes Dr. Van Riemsdijk, stating that "he observed that a globule of

3 the Study of Metallurgy, I

gold or silver in a fused 'state will pass below its solidifying point without actually solidifying, but the slightest touch with a metallic point will cause the metal to solidify and the consequent release of its latent heat of fusion is sufficient to raise the globule to the melting-point again, as is indicated by the brilliant glow which the button emits. " Rose 1 also quotes the same author, and it is evident that the gold and silver globules mentioned are derived from cupellation.

Six sets of experiments were carried on in this connection, some of which are plotted in Figs. 52, 53, and 54. It is evident from these Figs, that surfusion unquestionably occurs, and in a most marked manner, the greatest degree of surfusion noted, being 77° C. All of the buttons "sprouted," i.e., showed cauliflower-like growths of silver on final solidification. This sprouting has always been considered a sign of purity of the silver, 2 particularly pointing to the absence of lead.

In order to test this point, some of the silver buttons from the experiments were very carefully examined for lead in quantities of a gram, and showed but traces of it, quantities not determinable. Some showed minute quantities of copper. In effect they were all "fine silver." The surfusion is therefore very real. In the authorities cited on surfusion, the statement is made that on solidification from surfusion, the "flash" of the button occurs, showing the raising of the temperature to the melting-point of the silver. H. M. Howe s states: "Once freezing sets in (in the surfused metal or alloy) the heat which it evolves raises the temperature toward, and more often quite to, the true freezingpoint, where it remains during the remainder of the freezing.''

In experiments carried on with the following quantities of silver, 10, 14, 18, 30.4 and 30 grams, the "flash" was not observable, neither by the eye nor by any actual rising deflection of the galvanometer pointer, although a repeated and careful search was made for this. In order to determine whether the size of the button had any influence on the "flashing," various amounts of silver, beginning with 350 mgs. and varying by 50 mgs. up to 850 mgs., were cupelled so as to finish with surfusion. It was found that the beads up to and including 650 mgs. flashed markedly, that of 700 mgs. faintly only, and those above showed no "flash."

1 Metallurgy of Gold. 4th Ed., p. 598.

7 Rose. Metallurgy of Gold, p. 477. Collins. Metallurgy of Silver. 1900. p. 2. SennaM. MetaU-Huettenkunde, 1901. p. 605, 2nd Ed. 3 Iron, Steel and Other Alloy*. 1903, p. 20.

Cupellation

If the differences in temperature between the cupelling alloy and the muffle blank at any time interval be plotted as ordinates from a basal line, it is readily shown by the different curves, that the greatest difference occurs at the close of the cupellation; in some instances, just as the last of the lead oxidizes (play of colors). The differences noted show the marked evolution of heat at the "finishing" of the cupellation, and are due to the release of the latent heat of fusion. In the case of the large

OS 4 e 8 10 12 14 10183022 2120883033 3430 3840 43 44 40 48 80 5204 50 68 00 0204 00 08 70

Mluutea

Fig. 52. — Curve Showing Temperature During Cupellation of Pb Ag.

buttons, however, this does not seem to be sufficient to cause an actual rise of temperature in the cupel, when the muffle temperature is actively sinking, as was the case in experiments shown by Figs. 52, 53 and 54. As already stated, however, no "flash" was observable to the eye in the larger silver buttons, nor did the galvanometer indicate it, as surely might be expected. The. "lag" or time interval between the occurrence of a temperature and its recording by the galvanometer, is not great when an iron protective tube is employed. This is shown very plainly by

A Manual Of Fire Assaying

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Cupellation 97

the marked "jogs" in the cupel curves when the lead alloy is added to the cupels (Figs. 53 and 54). The lowest "finishing'' temperature found showed suffusion extending to about 885° C. or 77° C. below the melting-point of silver. The temperature indicated by the "muffle blank," which at the start was 10° below the cupel, was 845° C. The last of the lead went off from the cupel at 910°. This represents about the minimum "finishing temperature," judging by the general appearance of the cupellation. It is to be noted, however, that this "finishing temperature" is reached automatically in the cases where the muffle temperature is such as to afford "uncovering" of the button and prevention of freezing; i.e., approximately 830° to 840° C. on the average, in the case of pure lead buttons. One experiment was carried out in which the cupelling lead alloy showed a temperature of 750 . 1 not far from the end of cupellation, but at this temperature the button solidified into a lead-silver alloy. The approximate composition of the silver-lead alloy freezing at this temperature is 70 per cent. Ag, 30 per cent Pb. RSsumt. — It appears from the foregoing that:

1. In the case of the lead buttons not containing any appreciable amount of copper or iron, etc., a muffle temperature of at least 800° C. and, better, one of 850° is required to "uncover" or start cupellation.

2. That this temperature may be lowered to about 770° C. during the oxidation of the greater part of the lead.

3. That toward the end of the cupellation or the "finishing," in case of silver, it must again be raised to about 830° C. in order to get a pure silver button.

4. That the actual temperature of the cupelling lead is always appreciably higher than the muffle temperature.

5. That the actual finishing temperature of the cupellation cannot safely be carried below about 910° C.

6. That the greatest observed surfusion of silver was 77° C. and that this is probably very near the maximum.

7. That silver beads finishing with surfusion are free from lead.

8. That "feathers" or crystals of sublimed litharge on the cupel are an indication of the proper cupellation temperature, provided the air draft is not excessive.

9. That it is just as essential to regulate the air draft of the muffle as its temperature.

1 For an explanation of this seemingly low cupellation temperature see p. 92.

Where rri-y .u: curare -iineihirion xork La required, such as in bullion -essay:iii: ami v:iere -lie iQiount of xork justifies it, a iman 1 -lose "en:pear:ire ami air control is practically essential. In *-;e T v .f "ae :-v:enr improvement in elect rica/Iy heated furnaces, in vnirh -em pera ran*s 'ran be rapitUy and accurately nr:-'.ile. imi "iie niujfle heated uniformly, practically eiiminari:i:r ":ie -iierm:ii iradienr. a furnace of this type wouJ'i -seem *-esr adapted for "he v.rk.

WFLCTEJCE OF BASE METAL IMPURITIES.— When the lead buttons are -v-ii-aniinared -v*>:i :i.-- merais. such as copper, the temperar:: ./upeilarh n :i.i;sr !-e higher in onier to prevent freezing. The n "his is reailily apparent when rhe rreezir. in: -e V:.z. 4'.* .f -he lead-copper series of allov- i-i insrr The fi*-:.:: in* of an allov contain 1 "

While rhe ... r.a! *:per i er !, trn*a a in rhe lead button -vii* -r/.ail. .'' ot:-r :-es not: xidize as reaiiilv as the 1 and vr.u, r., -i "rare ::: -h* V-;rri..n. rapidly raising t h e

r-'iine.-! :..- -!.- v.. !:. -n.all aii:-;;:L:-. If it is less x

..,1/ p:-<-'-:, fc ir. uc ". ."or. 'he 'T';.:i;.le as.-ay, the base meta-I 1

I rfi|'iM' ,-j'J i/.i*.-o:i.-- ,'j:'*' ih/ri-.j by the behavior of s/'tit'u )tiiu. u,(t<'i t >ir,'j r-opper. err-., rend to make it hardin fh': k,.'l i,-i f *or, n.-iK-i.. jr bring*-. pi,() often found in Wli'iv the t"iM nri'l ilv#-r eon tents, of the loal button approach ;;0 ]uv rent, of it-- .'.eiirtit , it. i-. britt:.

However, impnrih'- in the \t- At will not ahvavs be inolicjited by liriltleiie orluinlii/ ; without characteristics, impurities rruiy . fc, till he pie,-.< :n t. in -uffieient ninount to cause loss. All impurif.ie, 1 . tin not mu-.e lil:#- ,'iuiounls of loss in cupellation. Tlii! losr-. due lo I he jnesenre of inipuriiif-s is chiefly in by Mie cupel, nii'l cornpnnit ivel y ,.m;i|| by volatilisation.

t

Cupellation

The accompanying table 1 shows the influence of impurities.

Twenty-five-gram lead buttons were cupelled, containing 1 gram

of the impurity specified, 4 mgs. of Ag, and 1 mg. of Au. The

temperature of cupellation was 1000° C, in order to prevent

freezing as a result of impurity.

The high losses are due in part to the high temperature employed. The table really gives the relative influence of the impurities. Bismuth has been used in place of lead for cupellation. While in the table bismuth is stated to be the cause of a very heavy absorption, this is not substantiated by other researches. 3 When it is present in the lead button it tends to concentrate during the cupellation, and is removed by oxidation toward the last of the operation. Some of it is very apt to be retained by the precious metal bead. Cupellation may be carried on with bismuth, but the absorption is much higher. 3 The presence of Bi in the cold cupel may be recognized by the fact that the place which the silver button occupies is brown and surrounded by concentric rings of a yellow and blackish-green color. Copper colors the cupel from a dirty green to a black, dependent on the amount of copper.

Table Xiii.— Influence Of Impurities

Impurity

Loss of Gold

Loss of Silver

Remarks

None 1.2 per cent. 11.8 per cent.

Tin 2.0 per cent. 13.9 per cent.

Arsenic 3.9 per cent. 16.3 per cent.

Antimony 5.3 per cent. 13.3 per cent.

Zinc 9.3 per cent. 17.6 per cent- Cadmium 3.5 per cent. 13.1 per cent.

Iron 4.0 per cent. 16.6 per cent.

Manganese 13.6 per cent. 24 . 3 per cent.

Molybdenum 11.0 per cent. 26.2 per cent.

Vanadium 7.7 per cent. 21.7 per cent.

Copper 10 . per cent. 32 . 6 per cent.

Bismuth 4 21.8 per cent. 27 . 9 per cent.

Thallium 23 . 1 per cent. 34 . 4 per cent.

Tellurium 55 . 8 per cent. 67 . 9 per cent.

Selenium 54 . 1 per cent. 64 . 5 per cent.

Most of this loss, even with Te and Se, is cupel absorption.

1 T. K. Rose, in Jour. Chem. Met. and Min. Nor. of S. A. % Jan., 1905. 2 K Sander, Berg-und Hurttenmacnnischc Zeitung, 1903, p. 81. 8— XII, 244.

3 Smith, in Jour. . Soc, 1891, 863.

4 Doubtful.

r -n.

Jl 3C.kXtT.kI. OP FTR£ 13&JLYTSG

Tin T arsenic, zinc. cadmium, iron, and manganese cause scons to form on the cupeL due zo the formation of oxides which are not readily absorbed. Iron caTises a iark coloration of the cupel. Antimony in considerable quantity causes the cupel to check and crack. The same may be said of copper.

Copper. — This metal Is oiiiized with more difficulty than lead. the Cu 2 forming fcj aid of the action of PbO; however, Cu,0. again coming into contact wi:h metallic lead, is reduced to Cu. and in this way is persistent toward the end of the eupeOation. although a large excess of Pb over Cu is present, and finally some remains with the Au and Ag. The loss of silver during the cupellation is due mainly to absorption, in large part as oxide. This oxidation of the silver in the presence of much lead is not to be ascribed to the action of atmospheric oxygen, but rather to "oxygen carriers," such as PbO. Cu 2 0. etc. It is very probabl e that CujO acts peculiarly in this manner, and the high absorption** noticed when Cu is present is due to this fact. It is to be note** that losses in silver occur toward the end of the cupellation, an*i occur in great part just before finishing; the small black-gree rings, surrounding the place where the silver bead rests, located most of the silver. It is the concentration of the copper, silver and gold that causes the high absorption. Lodge 1 shows ther influence of small amounts of copper on the cupellation of silver and gold.

Tablk Xiv. Copi&#x27;Kk Is Cupellatio.V Of Silver And Gold

Hilver milligram*

gram*

Copper CT.'irri.-.

Percentage

of copper

in lead

Temperature degrees centigrade 2

Percentage of loss

Ratio Pb toCu

2o:t

o o:io:i

20J

Cu retained

f'NotnH dii Annny'inu." V ' rl '"'/ 1 T#mpriluro of nir in mtil!!i\

Cupellation 101

Gold T , Percentage Temperature ..

.„. Lead f ' j n Ratio Pb

miluof copper degrees cen- Percentage of loss .

grams . £f r. , , to Cu

grams ° in Pb tigrade 1

no.

io ;

10

775 0.20 copper on ', 500 to 1

Gold is more retentive of copper than silver. It is to be noted that even with a ratio of 200 Pb to 1 Cu, it is not possible to Remove all copper, and beads obtained from mattes and heavy copper ores should be examined for copper; otherwise silver Results may be high. Retained copper in these silver beads will compensate for loss of silver, but the amount retained is so variable that this error cannot be considered to compensate the loss. Tellurium. — Tellurium has a great affinity for gold and silver, ajad if present in an ore in any appreciable amount, some of it go into the lead button with the gold and silver, and thus have its influence on the cupellation. It tends to concentrate during the cupellation and is with difficulty removed by oxidation. When there is present in the lead button more than 15 per cent. of the gold and silver weight in tellurium, the beads resulting from cupellation have a dull and frosted appearance. Larger amounts than this cause the beads to divide and split up in the cupel. F. C. Smith 3 shows the influence of tellurium on the 'cupellation as follows, these results being confirmed by J. C. Bailar 4 and others.

1 Temperate of air in muffle.

1 Actual losses; copper retained, 0. 16 per cent Gold about the same weight as before cupellation.

"The Occurrence and Behavior of Tellurium in Gold Ores," etc., in Trans. A. I. M. E, XXVI, 495.

4 "West Chem. and Met.," I, 119.

A Manual Of Fire Assaying

Table Xv.— Tellurium In Cupellation Of Gold And

Silver

Containing Loss by absorption aation

bullion added

Au Ag Au Ag Au Ag

per cent, per cent. , per cent, per cent.

Cupelled with 12 grams of lead

Note the similar effect of selenium.

Antimony. — The presence of antimony causes increased losses by absorption, although its effect is not as pronounced as that of copper or tellurium. During the cupellation litharge and antimony combine to form antimoniate of lead, which, if present in considerable amount, may cause the formation of scoria on the cupel. Small amounts of antimony tend to remain with the gold and silver, as with copper and tellurium.

As a guide in cupellation, the following scale of color temperatures is given. 2

Degrees Centigrade

Lowest red visible in the dark 470

Dark blood-red or black-red 532

Dark red, blood-red, low red 566

Dark cherry-red 635

Cherry-red, full red 746

Light cherry, light red 843

Orange 900

Light orange 941

Yellow 1000

Light yellow 1080

White" 1205

CUPELLATION IN CUPELS OF DIFFERENT MATERIAL. — The

cupel inn tori al has a decided influence on the progress of a cupellation. What has preceded refers more particularly to bone-ash

Solonmm ImkUmmI uf tellurium.

Whit* mul Taylor, m Am. ?kv. Mch. En* . XXI, 2S. H. M. Howe, in

Cupellation 103

cupels. In cupek with a magnesia base the process as regards temperature differs somewhat, due to the different thermal properties of the two types of material. The following difference in thermal properties may be noted. 1 Bone-ash cupel, mean specific heat between 15° and 100° C. is 0.185. Magnesia cupel, mean specific heat for same temperatures is 0.215. A bone-ash and magnesia cupel of identical volumes weigh respectively 22 and 29 grams. The heat conductivity of magnesia cupels is very much greater than that of bone-ash cupels. When the two types of cupels are heated to 90° C. in a steam bath, at the end of 14 minutes the magnesia cupels are at 90° C. and the bone-ash cupels at only 60° C. During cupellation of lead at the end of 6 minutes from the addition of the button the magnesia cupel showed practically the same temperature in the cupelling lead as in the bottom of the cupel, viz. 920° C, while the bone-ash cupel in the same muffle showed a temperature of 990° C. for the cupelling lead, and only, 932° C. in the bottom. The total heat capacity of a magnesia cupel is more than 50 per cent, greater than that of a bone-ash cupel of the same volume, so that on cooling the two types of cupel the magnesia cupel retains a higher temperature somewhat longer than the bone-ash cupel in spite of its greater diffusivity of heat. From this data the reason of the behavior of magnesia and bone-ash cupels during cupellation is apparent. It will be noted: (1) That in magnesia cupels the lead is less bright and hence at a lower temperature than in bone-ash cupels, although the muffle temperature is the same. This is due to the fact that the extra heat generated by the combustion of the lead is diffused as rapidly as generated by the superior diffusivity of the magnesia cupel and hence cannot serve to raise the temperature of the lead, as is the case in the bone-ash cupel. Hence for the same "muffle temperature" the actual cupellation temperature of the lead in the magnesia cupels is 50° to 60° C. lower than in the bone-ash cupels. To this fact is due the lower losses of precious metal in magnesia than in bone-ash cupels. From the discussion under " cupellation temperature" it will have been noted that with bone-ash cupels, if once the muffle has attained a temperature sufficiently high to cause the uncovering of the button, the rise in temperature of the lead due to its oxidation, is sufficient to carry the cupellation to a finish provided the muffle temperature is not lowered at the end of the

Bannister and Stanley, "Thermal Properties of Cupels," Bui. 56, I. M. M. (1909).

A Manual Of Fire Assaying

operation. This is not the case with magnesia cupels for now obvious reasons, and it will be necessary to raise the muffle temperature toward the end of the operation or what amounts to the Haine thing, piwh the cupel to the hotter part of the muffle. Assayors who are used to bone-ash cupels, therefore, have some difficulty at first due to "freezing" of buttons when using magnesia cupels.

2. Magnesia cupels retain a higher temperature longer than hone-ash cupels when withdrawn from the furnace or moved to the cool part of the muffle, and hence silver buttons show a loHNor tendency to sprout, due to the slow cooling they undergo.

The load in magnesia cupels seems to open somewhat more readily and cupels slightly faster than in bone-ash cupels.

The accompanying tables give data of results obtained by boneash ami magnesia cupels on pure silver and on a copper matte. 1

TABLE XVI.— COMPARISON OF BONE-ASH AND MAGNESIA OITKI.S OX C. P. SILVER CUPELLED WITH 10 GRAMS

Sheet Lead

Amount of silver Bone-ash cupels. , M .. x

4.S5

Is 92

9

u.

t

2ft $4

If A

$£Lywt.. OspdcKaMt m.

rr*.

Z to- HTQeii

: IL.T t fr*lT"_r , f"5

i —

Tl i5 i v: Itf

ihe :s&- tt:

I r£

lis =Ukj r-kx snail

acsrc

amounts of other metals notably, lead, 1 to which may be due in somfe cases the higher results obtained.

Table Xvii.— Comparison Of Bone-Ash And Magnesia

Cupels On A Copper Matte

No.

1 Weight , lead

Matte taken

Bone-ash cupels

Magnesia

button

a. t.

1 grams

1 Au+Ag .

Au

i Au+Ag

Ag ' Au

' 21.3

The assays given in the table were made by the excess litharge method. The average result stated in ounce per ton is as follows : for bone-ash cupels, gold 12.86 oz., silver 202.67 oz.; for magnesia cupels, gold 13.16 oz., silver 222.76 oz. These results are uncorrected assays, viz., do not include the slag or cupel absorption. In practice it was found necessary to make these corrections to obtain concordant results. It will be noted that the magnesia cupels give higher results on gold and very much higher results on silver. This last is without question due in large part to the retention of copper by the beads, and calls for caution in the use of magnesia cupels on this type of material.

Portland Cement Cupels. — During cupellation Portland cement cupels act very similarly to bone-ash cupels. The loss is somewhat higher than in bone-ash cupels. The accompanying table gives losses in Portland cement cupels and bone-ash cupels and those made of one-half of each material. The temperatures are average temperatures during cupellation, from the opening of the button to the " blick. " One hundred mgs. of silver were

i D. M. Iiddell, Bug. and Min. Jour., LXXXIX, 254.

10G A Manual Of Fire Assaying

cupelled with about 20 grams of lead. 1 The temperatures were measured by inserting a thermocouple into a hole bored beneath the bowl of the cupel. They hence represent a temperature which is a mean between that of the cupelling lead and a muffle "blank" cupel.

Table Xviii.— Cupellation Losses With Different

Types Of Cupels.

Average U. S. Portland R. D. Portland One-half cement, Bone temp. cement, cement, one-half bone-ash, I ash, loss

deg. 0. loss per cent. loss per cent. loss per cent. .per cent.

Another test to determine the relative absorption of bone-ash and cement cupels 2 gave the following results: On 10 mgs. silver with 15 grams lead, at an orange heat (very high) cement cupels showed 6.64 per cent, absorption and bone-ash cupels, 6.38 per cent. At a light cherry heat, cement cupels showed 4.91 per cent, and bone-ash 4.62 per cent, absorption. It is to be noted that the percentage absorption other factors being equal is dependent on the amount of precious metal cupelled (see p. 163). In using cement cupels, the beads must be carefully cleaned otherwise when parting in nitric acid insoluble silica is apt to remain which will be weighed as gold. The bead on cement cupels is likely to be more flat than on bone-ash cupels.

1 Holt and Christensen, Eng. and Min. Jour., XC, 560. "Experiment* with Portland Cement Cupels."

2 J. W. Merritt, "Cement vs. Bono-ash Cupels," Min. and Sci. Press., C, 649.

Chapter Viii Parting

Parting is the separation of gold from silver by means of acid. In assaying, nitric acid is almost exclusively used, although sulphuric acid may be employed. In order to separate silver from gold by means of acid, it is essential that there be present at least twice as much silver as gold. When less silver is present, it is impossible to separate all of the silver from gold by means of acid (see assay of gold bullion, in Chapter XII). When the above-stated amount is present, it requires acid of not less than 1.26 specific gravity, boiling for at least 20 or 30 minutes, to separate the silver from gold. The ratio of 2 and 2.5 to 1 is used practically only in the bullion assay.

In parting beads from ore assays, it is considered necessary to have at least five times as much silver as gold present. The addition of silver to gold or to the gold-silver alloy in order to prepare for parting is termed "inquartation," from the fact that at least 3 parts of silver to 1 part of gold were formerly considered necessary. The nitric acid used for parting must be free from hydrochloric acid and chlorine in order not to have a solvent action on the gold. 1 Nitric acid should be examined for chlorides before being used for parting. In order to part silver from gold successfully, the following points must receive careful consideration: (1) The strength of the acid used; (2) the temperature of the acid; (3) the ratio of gold to silver in the bead to be parted.

1. The proper strength of acid is of great importance. Formerly, most authorities recommended that acids of 1.16 and 1.26 sp. gr. respectively — 2 parts water to 1 of acid (1.42 sp. gr.) and 1 of water to 1 of acid — be used, first the weak acid and then the stronger acid. T. K. Rose recommends 4 parts acid to 3 parts water, which strength, if the acid be heated, will not break up the gold in the bead into fine particles, even if 50 parts of silver are present to 1 part of gold. Gold is less apt to break up when it is less than 0.10 nig. in weight. Keller 3 recommends

1 Consult the caption "Solution of Gold by HN0 3 ," in Chapter XI. a Keller, Trans. A. I. M. E., XXXVI, 3.

acid of the following strength: 1 part acid (sp. gr. 1.42) to 9 parts distilled water. In this strength of acid the gold almost invariably remains in a coherent mass, even when the silver is 500 times as much as the gold. This is the strength of acid recommended for ordinary assay purposes. The beads should be boiled in the acid for at least 10 to 15 minutes in order to insure parting.

2. It iH essential to have the acid at the boiling-point before dropping in it the bead to be parted. Putting the bead into cold acid and heating up gradually is almost certain to leave the gold, especially where the ratio of silver to gold is high, in a powdered, fine condition, very apt to cause losses in washing and subsequent handling of the gold. Cold acid should not be used.

3. While the best ratio of silver to gold, for parting ordinary beads, iH 5 to 1, this ratio is not always under control, since the assayer must be content in many cases with the ratio that the ore furnishes him, when this is more than 5 to 1. If less than 5 to 1, silver should be added in order to bring it up to this ratio. The silver mav be added directlv to the crucible or scorification fusion, or to the lead button during cupellation if it is not essential to determine the silver in the ore.

If it is essential to determine the silver, and inquartation is necessary, the bead from the cupellation is first weighed, the requisite amount of silver is added to the bead, both wrapped up in about - grams of sheet lead, and then it is recupelled and parted.

Heads which need inquartatiou may also be fused with silver, on a piece of charcoal, by means of the blowpipe; but this method is not to be recommended, as it frequently occasions loss.

Many assay ers. it they suspect an ore to be deficient in silver for parting, avid silver to the crucible, not determining the in this asav. but runtime, a separate scorification assav for this mtrivso

Vuothcv w .v\ v to avlvi to :uv charge a vlosired number of cubic vvutiuvto-.s VitNv\ solution of >u;h strength that 1 c.c. contain I u.c Ve I'ito u\\;v.-;t:o:: ;sr. then be made from

in order to remove all trace of silver nitrate. The black stain occurring in parting cups after heating for the annealing of the gold is due to metallic silver reduced from silver nitrate by the

heat, showing insufficient washing. Parting may be carried on in small porcelain crucibles called "parting cups," or in testtubes or in flasks similar to copper-assay flasks. In order to

part in flasks or test-tubes, it is essential to have the gold stay as a coherent mass, so as to prevent loss in transference. When parting cups are used, after washing, the gold is carefully dried and the gold annealed at a dull-red heat, either in the muffle or by means of the blowpipe. After acid treatment, the gold is left as a soft black mass, probably an allotropic condition of the gold; but upon heating this is changed to the normal yellow metallic state in which it is weighed. Fig. 55 shows a convenient parting bath with test-tubes; Fig. 50 shows parting flasks commonly in use.

Chapter Ix

The Assay Of Ores Containing Impurities

Impurities, from the assayer's point of view, are such substances, contained in ores, furnace products, or other material, necessitate some particular method of assay or treatment, or the observing of special precautions not included in the ordinary crucible assay as already outlined.

Common impurities are sulphur, arsenic, tellurium, antimony, zinc, copper, etc. Of these sulphur is by far the most common. In performing an assay it is usually the aim of the assayer, Aarhenever this is possible, to produce by direct fusion, either by t:he crucible or scorification method, a pure lead button weighing approximately 20 grams. If the button is smaller than this, here is danger of not collecting the values; if larger, cupellation is too prolonged and losses are increased. In the assay of lowgrade gold ores it may be desirable to produce lead buttons of 25 to 30 grams in order to obtain the best results. The impurities mentioned affect either the size of the button, or the purity of the button, or both. To show the effect of sulphur the following definite example is taken.

Given an ore containing pyrite, which, in a charge yielding the ordinary type of monosilicate slag, gives a reducing power of 5 grams of lead per gram of ore. If the following charge,

15 grams of ore 70 grams of PbO

30 grams of Na 2 C0 8 8 grams of SiO a

Borax glass cover

be made up and fused, a 60-gram button (approximately) will be produced, on top of which will be a small quantity of " matte, " i.e., an artificial sulphide of the metals, in this case iron and lead. This matte is brittle and may contain some silver and a little gold. On hammering the button, it is lost. In general, it is an undesirable product to make. A small amount of matte is produced in this case, since the ore has the power to reduce 75 grams of lead from PbO, while only 70 grams of PbO are present, so that the

excess sulphide of the ore not acted upon by the PbO remains in the charge, uniting with some of the lead to form a sulphide of iron and lead. The button is also much too large to cupel. If in the charge the PbO is materially increased, the ore will react to the extent of its full reducing power, a lead button of 75 grams will be produced, no matte will be found, and the slag will be improved, owing to the addition to it of the fusible base PbO- If the PbO in the charge be materially reduced, the lead button will be much smaller (owing to the dearth of PbO available fo reduction), considerable matte will be formed, and the slag will be poor.

If the silica be increased, so that sufficient be present to forn* the higher silicates with all the bases present, practically no lead will be reduced, for the sulphide has not the power to reduce much Pb from lead soda silicates unless a free base be present, e.g.,

In this way the sulphur remains in the charge in the form of sulphide sulphur.

Soda will cause the formation of S0 3 , if PbO is present to furnish the oxygen, and if it can act as a free base, i.e., if it is not combined with silica (see Chapter V, on Reduction and Oxidation Reactions). An increase of soda without an increase of PbO or Si0 2 will lessen the amount of matte, as sulphur will tend to combine to some extent with the Na 2 to form, with the FeS, a double sulphide of iron and soda, etc., which will be dissolved in the slag. The above outlines the effect of such impurities as sulphur and arsenic, and shows the necessity of special methods of assay directed toward the getting rid of impurities.

The impurities mentioned may be divided into two classes:

(a) Those which can be volatilized by oxidation or otherwise, e.ij. y sulphur, arsenic, and antimony.

(b) Those which cannot be volatilized, e.g., copper, zinc, etc. Some of these may be partly volatilized, as antimony and

zinc. For the removal of all of them, however, whether by volatilization or by slagging, oxidation is essential.

In one method employed on light sulphide or arsenic ores, the iron-nail method, sulphur and arsenic are carried into the slag as a double sulphide or arsenide r Wi and iron, etc.

Crucible Fusions.

Assay Op Ores Containing Impurities 113

The following methods are standard methods for the assay of impure ores, and are discussed in detail:

2. The niter method, (a) The common niter method. (6) Miller's oxide slag method, (c) Perkins' excess-litharge method.

6. The combination wet-and-dry method

(removal of impurities by solution) .

THE ROASTING METHOD. — It is usual to carefully weigh out 0.5 or 1 assay ton of the ore to be assayed, and place it in a roasting dish of sufficient size to permit of stirring without loss by Spilling. The dish is placed in the muffle, the temperature of Vrhich is not above a "black red" and the firing of which is under good control, so that the temperature will not rise too rapidly. In the case of an ordinary sulphide ore, such as a pyrite, or, for example, a chalcopyrite and quartz, the following reactions take place, if the roasting is carried on slowly at a low heat: 1

3CuFeS 2 + 180 + heat Cu 3 S + 3FeS0 4 + CuS0 4 + SO,

At 590° C. the ferrous sulphate decomposes spontaneously, sulphatizing the balance of the copper:

Cu 3 S + 2FeS0 4 + 60 2CuS0 4 + Fe 2 O s + SO s

At 655° C. the copper sulphate decomposes into basic sulphate and S0 8 , and at 700° C. into CuO and S0 3 , as follows:

2CuSO< CuO.CuS0 4 + S0 3 ,

CuO.CuS0 4 2Cu0 4- S0 3 ;

so that the final products of the roast, when carried to above 700° C, are ferric and cupric oxide, with a complete removal of the sulphur. If the temperature is not carried above 700° C, sulphur remains in the charge as sulphate, which may again be reduced in the crucible to sulphides:

2CuS0 4 + 3C Cu 2 S 4- S0 2 + 3C0 2

If, for any reason, it is not desirable to carry the temperature as high as 700° C, the ore, after roasting until no further smell

*R. H. Bradford, Trans. A. I. M. K., XXXIII. 68.

of S0 a is discernible, is cooled and mixed with 5 to 10 grams of powdered (NH 4 ) 2 CO s , and reroasted at a low heat, the sulphuric anhydride (SO s ) being eliminated as volatile ammonium sulphate, (NH 4 ) a S0 4 :

Any silver in the ore that has been roasted will be in the form of Ag 3 S0 4 , or if arsenic and antimony are present, partly in the form of arseniates and antimoniates. If the roasting temperature is carried to 870° C. and above, the silver sulphate will be decomposed, leaving the silver in the form of metallic silver. In order to avoid loss of silver it is best not to carry the temperature above 700° C.

In roasting simple pyrite ores, the reactions are similar, but simpler, and the temperature need not be carried above 600° C During roasting, the ore should be stirred frequently in order to expose fresh surfaces to oxidation.

When ores contain arsenic and antimony, the roasting operation is more difficult and complex, and considerable care and skill arc required to eliminate the greater part of these two volatile elements. The reason for this is that the arsenic and antimony pass by roasting first to the state of the lower oxides As,O s , Sb a O s , which are volatile, and then to the state of the higher oxides As,0 5 , Sb a O a , forming arseniates and antimoniates of certain metals present in the ore, some of which are stable even at high temperatures, thus fixing the arsenic and antimony in the roasted ore, and not eliminating it. The arseniates (or antimoniates) which ordinarily form are those of copper, iron and silver. The best conditions for the elimination of arsenic and antimony are alternate oxidation and reduction at a low heat. The presence of sulphur tends to aid the elimination of arsenic and antimony by the formation of the volatile sulphides of these elements. The reducing action necessary for the elimination of arsenic and antimony is best obtained by mixing with the ore equal volumes oi coal dust or charcoal, ami roasting at a dark red heat until the coal is burnt off. then cooling, adding more coal dust, and reroasting. In this way the greater part of the arsenic and antimony can be readily volatilized, except in very rich silver ores. When galena ores are to bo roasted, the ore is best mixed with an equal volume of silica and roasted at a very low heat. In this roast Pb80 4 is formed to a considerable extent.

A88Ay Op Ores Containing Impurities 115

which at a higher heat is decomposed by the SiO a present, as follows:

PbS0 4 + SiO, PbSiO s + S0 3

Care must be taken with this roast as, at the formation point of lead silicate, silver losses are apt to occur. A successful roast will be indicated by a yellow color (lead silicate), and an unsuccessful one by a black or gray color (fused, undecomposed sulphides). In general, heavy sulphide ores that contain their chief value in gold may be roasted, when this is carefully done, without loss of gold; but silver ores, especially when of high grade, are apt to give low results.

In making up the charge for the roasted ore, it is to be noted that from a sulphide ore (pyrite, etc.) the product is frequently of an oxidizing nature and basic, which must be taken into account in adding the fluxes. In galena ores, when silica has been added, this must be accounted for.

The roasting method is frequently used for heavy sulphide ores, especially when they have a low value in gold and silver, as it permits of a large amount of ore being taken (1 assay ton and more), which after roasting presents no difficulty in making the proper fusion.

THE NITER METHOD. — The first step in the niter method is the making of a preliminary assay according to the directions already given. The precautions concerning the reducing power of the sulphides in different types of charges must be carefully noted; it is best to have the preliminary charge of the same composition as the final assay charge. Or else the reducing power may be determined by the soda-litharge charge and this cut down by 25 per cent., 20 grams deducted for the lead button, and the remainder divided by 4 to get the amount of niter to add, in grams, if the monosilicate slag is to be made in the assay.

The amount of ore taken for the niter assay varies according to the grade of the ore in gold and silver and according to the amount of impurity present. It is rarely desirable to add more than 20 grams of niter to the charge, as larger amounts cause difficulty through the evolution of too much gas. One-half assay ton is the amount of ore most frequently taken. Sometimes, with ores containing much impurity, 0.10 to 0.25 assay ton is used. Twenty-gram crucibles (170 c.c. capacity) are used for amounts of 0.5 assay ton of ore and less, and 30-gram crucibles (240 c.c. capacity) for 1 assay ton of ore.

MILLER'S OXIDE-SLAG METHOD.— This method is a modified niter method applicable to such ores as contain practically no silica; i.e., heavy nulphide ores, such as pyrites, arsenopyrite, mattes, etc. It is based on the fact that PbO has the power to hold in solution and in suspension oxides of such metals as copper, iron, etc. (see p. 122, where "scorification" is discussed), in certain amounts. Niter is added to oxidize the sulphides, etc., and NiijCO, to aid in the complete oxidation of the sulphur by the formation of sulphates, in the manner already discussed. The first stop, as in the ordinary niter method, is the preliminary assay, according to the following charge:

Oru 3 grams

PbO 50 grams

The final charge is as follows:

Oro 0.5 assay ton

PbO 70.0 grams

Nii.CO, 12.0 grams

KNO, icalcuated for a 20-gram button)

Quick tiros, 1100° W minutes, are found to be best. The slags art* usually dull black and pour readily, and the button separates easily from the slag. (In slags high in silica or containing much bora\ % the lead buttons are apt to adhere closely to the slag With the oxide-slag method, trouble is sometimes experienced through the lead refusing to collect and remaining shotted through the slag. The difficulty is usually due to too much soda especially if considerable niter is used) although too low a temperature of fusion is also a factor.

The method gives reliable results on gold and silver, comparing well with the other standard methods. 1

PERKINS* EXCESS-UTH ARG E METHOD. This method is based on the fact that PbO will oxides of other metals and, if present in great excess, will to a large extent, the reductiv v u of o'hov !v.o:als. >uch as Cu a:v: Six The presence of so

v.\uch rbO also iv.>v.:vs a ox:.i::;:v£ rendencv in the

Assay Op Ores Containing Impurities 117

some litharge, but leaving much litharge uncombined in the charge.

The following table shows the proportion of PbO required to form fusible compounds with the principal metallic oxides: 1

TABLE XIX.— PbO REQUIRED WITH METALLIC OXIDES

One part of CutO CuO ZnO Fe304 FejOi MnO SnOa SbjOi Aa*Os

Requires parte of PbO.. 1.5 1.8 8 4 10 10 13 5 1

In order to carry out the excess-litharge method intelligently, it is necessary to know the approximate composition of the ore, so as to provide the proper amount of PbO and SiO a . The best fusion exhibits, in a section of the cone of the slag after breaking, silicates of lead, iron, etc., on the outer surface, gradually passing to crystalline litharge toward the center. The temperature of fusion should not exceed 1050° to 1100° C. It must be above 884° C. (melting-point of PbO). The first step is the making of a preliminary assay in order to determine the amount of niter to be added. 3

The final charge most frequently used is:

Ore 0.25 to 0.5 assay ton Na 2 C0 3 12 grams

X*bO 8 to 10 assay tons SiO, 10 grams

Niter to obtain 20-gram button

The button is generally clean, and separates easily from the slag.

The excess litharge method will give somewhat low results on silver, especially on high grade ores but will give good results on gold. In ores of. the following analysis, SiO a 40 to 60%; Fe, 5%; CaO, 2%; Pb, 15 to 40%; Zn, 2%; Ag, 20 to 80 oz.; S, 1%; and a trace of copper, the results in the accompanying table were obtained by the use of charges A, B and C. s

Charge A Charge B Charge C

PbO 25 grams 50 grams 75 grams

Borax glass. .. . 4 grams 4 grams 4 grams

Flour 2.25 grams 2.25 grams 2.25 grams

NaHCO, 25 . grams 25 . grams 25 . grams

K t CO, 25.0 grams 25.0 grams 25.0 grams

Ore 0.5 a. t. 0.5 a. t. 0.5 a. t.

1 Hofman, "Metallurgy of Lead," p. 7.

' In place of niter, it may be necessary, in this method or in Miller's method, to add argol, if ore is not reducing.

Kenneth Williams, Jour. Ind. and Eng. Chan., II, 406.

TABLE XX.— AVERAGE RESULTS SHOWING EFFECT OF AN INCREASE OF PbO ON SILVER RESULTS

Ore No.

Charge A, Charge B, Charge C,

ounces Ag per ton ounces Ag per ton ! ounces Ag per ton

An ore from Cobalt, Canada, 1 containing 5.06 per cent. Ni and 9.12 Co, chiefly as niccolite and smaltite, and some free silver was assayed by the following charge:

Ore 0.05 a. t.

NaHCO, 10 grams

Borax glass 10 grams

Argol 1.5 grams

Litharge as given in table.

TABLE XXL— AVERAGE RESULTS SHOWING THE EFFECT OF AN INCREASED AMOUNT OF PbO ON SIL VER RESU LTS

Silver, Silver in slag, Silver in cupel,

Litharge, Lead button, ounces ounces per ounces per

grams grams ton ton ton

so

So

19S4.S

So

THE IRON-NAIL METHOD. — This method does not attempt

to oxidize impurities, but aims to carry sulphur, etc., into the slag. The ore is decomposed by the iron nails added to the charge and by the PbO present. As iron reduces PbO to Pb, the amount of litharge added to the charge is limited to 25 to 30 grams. The amount of soda needed is large, as this flux is depended upon to carry the sulphur into the slag. The slag should be below a monosilicatc in degree, and high in soda, as basic alkaline dags have a high solver.* power for sulphides.

R. W. ]xviiw\ The .v Hijrh Z:.thnrec i- :he Crucible S A 1 M F . XXXV111. £XS.

Assay Of Ores Containing Impurities

A typical charge on an ore that has a reducing power of about 4 grams of Pb per gram of ore is: 1

Ore 0.5 assay ton SiO, 2 grains

NaHCO, 30 grams borax 8 grams

PbO 30 grams nails 17 grams

Salt cover

The soda should usually be twice the amount of ore in the charge. The reactions that take place are approximately as follows: 7PbO + FeS 3 + 4NaHC0 3 7Pb + 2Na 3 S0 4 + FeO + 4CO, + 2H,0

Part of the ore is decomposed by the PbO, and part of the S may go off as S0 3 , as discussed in previous pages. The iron nails decompose the balance of the sulphides: FeS 2 +Fe 2FeS PbS+Fe=Pb-f FeS (if galena is present or lead sulphide forms).

The iron sulphide (FeS) is dissolved by the alkaline slag, forming probably double sulphides of soda and iron.

To show the nature of the iron-nail fusion, the following results of two fusions on a pyrite ore containing 39.5 per cent. S — a reducing power equal to about 8 — are given: 3

Charge 1 Charge 2

1 assay ton ore

4 grams SiO,

4 nails

. 5 assay ton 30 grams 30 grams

4 grams

The following results were obtained:

No. 1

Slag 60 grams

Matte 23 . 5 grams

Lead 24 . 5 grams

Crucible and charge before" fusion 685 grams

Crucible and charge after fusion 665 grams

Loss in weight 20 grams

Nails before fusion 64 grams

Nails after fusion 43 grams

Loss of iron 21 grams

Per cent, of S in slag 6.73

8 in slag 4 . 03 grams

S in ore 1 1 . 85 grams

8 passed off as S0 2 . 95 grams

in matte 6 . 87 grams

""dge, "Notes on Assaying," p. 99. ie, ibid., p. 101.

No. 2 65 grams none

5 . 92 grams . 96 grams

none

A Manual Of Fire Assaying

It will be noted that the charges are identical as far as the fluxes are concerned, but that the amount of ore differs. It is desirable in heavy sulphide ores to keep the ore down to 0.5 assay ton and lower if necessary.

Care must be taken not to have the slag above a monosilicate in degree, for if higher in SiO a there will be particular danger in this charge of not having the sulphides oxidized by the PbO, more sulphide being retained in the charge than it can dissolve, and forming a matte, even with small amount of ore.

THE NITER-IRON METHOD.— This method is in principle the same as the iron-nail method. An amount of niter is added at random, sufficient to oxidize but a portion of the sulphides; the balance being decomposed by the nails.

THE CYANIDE METHOD. — Sometimes, when no other fluxes are at hand, or w r hen a rapid assay is to be made in which accuracy is not essential, a fusion of ore with cyanide may be made, and the resultant button cupelled for silver and gold. The method is a rapid one and gives good malleable buttons, but is apt to be low in gold and silver, especially in silver. The cyanide used should be pure, free from carbonates or other impurities, and the fusion should be made at a low temperature. The following charge is used:

Ore. . 5 to 1 assay ton

PbO 25 grams

KCN 3 assay tons

When the ore contains copper and other base-metal impurities, these are reduced and enter the lead button. Sulphur is taken up by the slag as potassium-sulpho-cyanate (KCNS). In general, it is a method not to be recommended. The following results show the loss in silver which takes place in this method. 1

Table Xxii.— Loss Of Silver In Cyanide Method

Niter method Cyanide method

Silver, by uncorrected assay

Silver in slag

Silver from cupel

563.73 mgs. I . 10 mgs. 7.81 mgs.

i K. H. Miller, "Corrected Assays," iu So*. Mines Quart., XIX, November, 1897.

Assay Of Ores Containing Impurities 121

The results are averages of duplicate assays. The loss of gold in the slag by cyanide fusion is not nearly so marked as that of silver.

A COMPARISON OF THE DIFFERENT CRUCIBLE METHODS OF ASSAY FOR IMPURE ORES. — In very impure ores, containing large amounts of sulphur, arsenic, etc., the roasting method is applicable when gold only is to be determined, or when silver results need not be very accurate. The roasting method gives uniformly lower silver results than most of the other methods, although to a large extent this is due to roasting at too high a temperature. The roasting method has the advantage that when ores are low grade large quantities of ore can be taken, which is not always possible with the other methods. Roasting, however, must be skillfully conducted in order to be successful.

The niter method is a desirable and clean method of assav giving accurate results. Where large quantities of niter are employed, the oxidizing action in the crucible is greatly increased, and it is probable that thereby losses in silver are apt to occur by the slagging of the silver.

There is no accumulated evidence on this subject, but many assayers holds this opinion. 1 The niter method is desirable for such ores as do not contain amounts of sulphur requiring extraordinary amounts of niter. Usually, the limit of niter in a charge is placed at about 20 grams; if the ore should require more than this, it is generally considered advisable to reduce the quantity of ore taken for the assay. This has the disadvantage of multiplying the error of the assay, when finding the value per ton.

The modified niter methods discussed offer advantages in the slagging of base-metal impurities. This is particularly true of copper and zinc. It is very much easier to cause copper to enter the slag when an oxide slag is made than when a silicate is made. This is partly due to the oxidizing nature of the high litharge charges. The best method for the slagging of base-metal impurities is the excess-litharge method.

The iron-nail method is a standard method, which can be successfully applied to most sulphide ores and, with care, to arsenical ores. It is not applicable to ores containing base- Ities, such as copper, for, being essentially reducing Tactically all of the base-metal impurities will lead button. When used with arsenical ores,

and Set. Press, CII, 301, gives data which rather tends to

the temperature employed should be low, not above 1050° C; otherwise speiss (an artificial arsenide of iron) is apt to form, which may carry values. It also has the objection, in the case of very impure ores, that small quantities must be taken for assay, involving serious risk of multiplying an error of assay.

SCORIFICATION. — This is the oxidizing fusion of ore with metallic lead in the muffle-furnace, producing, in the main, a litharge slag, i.e., an oxide slag. It is a method of assay which requires no previous preparation of the ore or preliminary assay, and as practically only one flux is employed, it is both a cheap and a rapid method. It is also a thoroughly reliable method, when proper precautions are taken and when it is employed on material suitable for the purpose. The operation is performed in shallow fire-clay dishes, called scorifiers.

The sizes commonly used are:

1 . 5-in. scorifiers; cubic contents 15 c.c.

2.0-in. scorifiers; cubic contents 25 c.c.

2 .5-in. scorifiers; cubic contents 37 c.c.

3 .5-in. scorifiers; cubic contents 100 c.c.

The dimensions referred to are outside dimensions. The size most commonly employed is the 2.5-in. one. Before these dishes are used it is usual to line the inside with ferric oxide. This is done by preparing crushed iron ore or ochre, mixing with water, and painting the inside of the dishes. This gives them a basic lining, and to some extent prevents the oxide slag from attacking the silica in the clay.

Some scorifier slags, especially if they contain copper, are very corrosive. The amount of ore taken for scorification varies from 0.10 assay ton to 0.25 assay ton; but 0.10 assay ton is the amount most frequently taken. The la rger amounts are rarely used, unless the ore contains practically no bases. Sometimes, for very impure material, as little as 0.05 assay ton is taken. The amount of test lead varies according to the nature of the ore. The more impure the ore the larger will be the ratio of lead to ore. With 0.10 assay ton the test lead will vary from 40 to 100 grams. A common charge is 40 to 50 grams of test lead for ordinary ores. As already pointed out, certain quantities of litharge are required in order to make

ible compounds with the metallic oxides. If the ore consmall amounts of the metallic oxide, the test lead will be

Assay Of Okes Containing Impurities 123

small in amount; if it contains appreciable quantities of ferric oxide (Fe 2 8 ) or Cu, etc., large amounts of test lead will be required. It is best to add a small amount of borax glass to the charge, from 1 to 1.5 grams, scattering it over the surface of the lead. This aids in the solution of the bases present. When the ore contains the basic oxides mentioned, borax glass up to 3 and 4 grams will materially aid in forming good slags, without infusible scoria. This infusible scoria often appears in ores containing large amounts of bases, and is very apt to give low results by entangling unfused portions of ore within itself. It is best to mix the weighed-out portion of ore with one-half of the test lead to be used, and then cover over with the balance.

The scorification may be divided into the following distinct steps:

1. Melting. In this stage the lead melts, and the ore, being of a lesser gravity, rises to the surface of the molten lead and floats there.

2. Roasting. The ore on the surface of the lead is attacked by the oxygen of the air and roasts in the same way as described under "Roasting of Ores."

3. Scorification Proper. The lead commences to oxidize, forming litharge. A small percentage (3) volatilizes and the balance forms a fusible slag. This now absorbs the oxides formed by the roasting, dissolving them and forming an igneous solution. The silver and gold, liberated, are absorbed by the remaining metallic lead. The slag, as it forms, drops to the side, forming a slag ring, with the center of the lead bath open to the atmosphere. The reason for this is that the meniscus of molten lead is convex, thus causing the collecting of the slag on the rim of the scorifier. The scorification continues until the whole of the lead is covered over with slag. It is then considered finished and the assay is poured. Should the assay be left in the muffle, the lead will still continue to oxidize, although none is exposed to the air, the interchange of oxygen taking place by means of the litharge and other oxides present. The size of the lead button desired from this assay ranges from 15 to 20 grams. If the scorification is continued to produce smaller buttons, losses are apt to occur by oxidation of the silver, especially if this is present in considerable amounts, thus forming rich slags.

A Manual Of Fire Assaying

.eiuperature of scorification ranges from 1000° C. to 11 .tioougti with pure ores higer temperatures may be employ" &&- impure ores containing much base metal are scorifi' . utcons from the scorification are very apt to be contar** 1 " vitu metal, especially copper, and will then "vacorified with more test lead, in order to get a e uiioa -'or cupellation.

u: uetals are to some extent oxidized simultaneously Uw mixture of metals may be roughly separated by success! e w;auou. each metal in turn partially protecting the met: u order, while the latter may act as an oxygen carrier 1 -oi-timor. 1 The order of oxidation is as follows:

bV io * , Cu to Cu a O

£n to ZaO Pt to —

Pt> to PbO Ag to Ag,0

Ni to Xi,O t Au to AuO

r*io o:*ior oi oxidation of the following elements is not so

.vc&u.

Vx t O, Te to TeO,

£twn in the table shows the difficulty encountered

% v vocal copper by scorification, as lead stands ahead

, s . v oxier removal, and it is very difficult and requires

x- . n. ; Vx ax n xery readily removed by scorification (oxida-

vV: \\ctv%ettt like Te and Se, are difficult to remove

k and may tend to concentrate with the Au

4 v. v 'V. cuwlUtion.

Ao verification assay should be homogeneous

; ..v W au earthy appearance, it is an indication

s.:- .vi.4-,ure having been used, and the button is

. v. : to contained PbO. White patches of

: , ,- . V sla& after pouring also indicate rather

. x ., .. o *u" scorification, as this sulphate forms at

v ., . v ,-'.*de: xlow oxidation.

. ., o.- 'uci'tod is a reliable one on most materials,

x , , enumerated below. As the usual quantity

.. v- u Mi..iu*u. civ . Oxygen Gas," in Trans. I. M. M. t April,

Assay Of Ores Containing Impurities 125

taken for assay is 0. 1 to 0.2 assay ton, it is evidently not a suitable method for low-grade ores, especially low-grade gold ores, where at least 0.5 to 1.0 assay ton must be taken in order to get accurate results, and avoid the multiplication of the error of weighing. It is practically impossible to get reliable results on $5 to $10 gold ores by ordinary scorification. If, however, 10 assays of 0.1 assay ton are made, the buttons from these combined and re-scorified into one button, which is then cupelled, the results are reliable, but not so good as from the crucible assay on the same total amount, on account of the multiplicity of weighing and other operations, which occasion errors and losses. The method in this instance would also be more costly of time and materials.

For ordinary and rich silver ores,. and very rich gold ores or furnace products, such as bullions, mattes, etc., the method is a desirable one. It requires no preliminary operations and thus saves valuable time. The slag loss is frequently somewhat higher than in the crucible assay. It is, as ordinarily performed (in duplicate), a cheap method as regards fluxes, etc. It does not give good results on very basic ores, i.e., those containing hematite, manganese oxides, etc., as in this case, unless a great deal of lead is used, scoria are apt to form in the slag, which may entangle lead and undecomposed ore. Neither does it give good results on telluride ores, cyanide precipitates, or ores that contain chloride of silver.

When basic material is to be scorified, small additions of SiO . up to 1 gram, may prove advantageous. In general, however, the addition of fluxes, except test lead, is not to be recommended. Scorification may be modified by the addition of considerable amounts of borax glass, litharge, silica, when it approaches the crucible assay in character with none of its advantages.

THE COMBINATION METHOD.— The trouble arising from the presence of considerable amounts of base metals, such as copper and zinc, has been fully discussed in previous pages, as well as the difficulty of their removal by fusion methods. For this reason the combination wet-and dry-method has been developed, to remove the objectionable impurities by solution. The method is used chiefly on copper-bearing material, such as heavy copper ores, copper mattes, blister copper, and to a lesser extent on zinc ores, and on cyanide precipitates produced by zinc, and has been advocated for telluride ores.

Van Liew's Method for Blister Copper. — This is a standard

method for copper material. Weigh out duplicate samples of 1 assay ton each of copper borings, add 350 c.c. cold water and 100 c.c. HNO s ( S P- g r - 1-42), and set in a cool place for 20 hours, stirring from time to time. Then, if the copper is not dissolved, add from 5 to 30 c.c. Jnore of concentrated acid. At the end of 26 to 28 hours the solution of the copper is complete. Do not apply heat in order to minimize as much as possible the solution of small quantities of gold, by whatever action this may take place. The oxides of nitrogen in the solution are removed by blowing air into it for 20 to 30 minutes.

Salt solution (containing 5.4207 grams of NaCl per 1000 c.c.) is added in sufficient quantity to precipitate the Ag present as chloride. 1 c.c. of this solution will precipitate 1 mg. of Ag, and an excess of 4 to 8 c.c. above that required for the Ag should be added. If the amount of Ag in the copper is small, add 10 c.c. of a saturated solution of lead acetate and 2 c.c. of concentrated H 2 S0 4 in order to form PbS0 4 , to aid in settling the silver chloride. Let this stand for about 12 hours and filter the precipitate into the proper sized filter, and wash it well into the point of the filter paper. Dry the filter carefully in the air bath, and when dry, add 8 grams of test lead on top of the precipitate, and carefully transfer to a scorifier containing 2 grams of lead. This is placed in the muffle, heated just to incipient redness, and the filter papers burnt off, but only until the flame disappears, and not into ash. This takes only a minute or so, the precaution being taken to prevent loss of silver by volatilization as AgCl, the lead and carbon present reducing the AgCl to Ag. Then add 3 to 4 grams of PbO, and the same amount of borax glass, raise the heat until well molten, and pour. No scorification is necessary, as no impurities are present. The lead button will weigh 5 to 8 grams and is cupelled with feather litharge. The results should check within 0.2 to 0.3 oz. for Ag and very closely for gold. 1 Sulphuric Acid Method for Blister Copper. 2 — To 80 c.c. of cone. H 2 S0 4 add 25 c.c. of a solution of CuS0 4 (160 grams per 1000 c.c.) using a low wide No. 5 beaker. Heat to such a temperature that on the addition of the copper borings action commences immediately; add 1 a. t. borings, spreading them over the bottom of the beaker. Heat until all dissolving action has ceased,

1 It. W. Van Liew, in Eng. and Min. Jour., LXIX, 498 ct aeq.

2 F. F. Hunt, "Determination of Gold in Copper Bullion," Eng. and Min. Jour., LXXXVII, 465.

Assay Of Ores Containing Impurities

usually from 1 to 1J hours; then cool and add 400 c.c. of distilled water, stirring to prevent caking of the crystals. Bring to juat a boil, filter, and wash the beaker thoroughly, using a rubbertipped glass rod as a stirrer. Place the filter-paper with the residue in a 2.5-in. Bcorifier, dry and bum off the paper; add 35 grams test lead and 1 gram silica, scorify to a button of about 9 grams, cupel, and part as usual.

Silver may be determined by adding salt solution, as in Van [dew's method, and 10 c.c, of a 10 per cent, solution of lead acetate, stirring well and letting stand over night. Then filter with the usual precaution, and add the paper and precipitate to the same scorifier containing the gold, and proceed as in the case of gold only.

In place of cupric sulphate, mercuric nitrate or mercuric sulphate 1 may be used, the equivalent of about 100 mgs. of mercury for an assay ton of borings. The mercury salt is best added to the copper borings, stirring a little and then adding the 80 c.c. sulphuric acid and boiling on a hot plate for three-quarters of an hour. Then proceed as already described. When mercury salt is used in the above quantity on low-grade bullions containing front 10 to 50 ok. Ag per ton all the silver is thrown down with the gold. If more silver is present salt solution should be added in sufficient quantity to precipitate the silver and any mercury that has passed into solution.

The object of the addition of cupric sulphate or the mercuric salt is lo prevent the formation of copper sulphides, which will remain in the residue and make necessary more than one scorification to remove the copper before cupellatidn.

The sulphuric-acid method is stated to give results equal to the "all fire" method (p. 139) on gold.

Combination Assay for Malic. — Van Liew's method of treating in the cohl is rarely suitable for mattes, as heat is usuallyessential in order to insure a decomposition of the matte in a reasonable length of time. Take 3 duplicates of i assay ton each and treat

i large beakers, provided with watch-glass covers, with 100 c.c. of distilled water and 50 c.c. UNO, (sp. gr. 1.42). After the violent chemical action subsides, add 50 c c more of concentrated acid, aud warm the beakers on a hot plate until everything soluble is dissolved: usually the residue is white or grayish.

. i , i ,

ii i i , 1 1. ... i . ..

""' r a i. The

frequently

'" " : : Iy excepted)

-h.-in the standard

-'-:.-: Hy ascribed to the

"'""-' -"lotion, which, in

' llill-I-r.-,i,.|" ,|i sput0 (hu

Assay Op Ores Containing Impurities 129

The solution may be due to the formation of H a S0 4 during solution, as the mixture of this acid and HN0 8 has a solvent action, or to the presence of impurities like chlorides or HC1, etc., or possibly to the presence of nitrates, particularly those of iron or copper.

It has been demonstrated that gold is soluble in hydrochloric acid solutions of iron alum, and of cupric chloride, but not in pure HCL 1

The fact that the combination method on copper-bearing material gives low results on gold is, however, well established.

Owing to the number of manipulations in the combination assay, it is often apt to give low results in the hands of inexperienced chemists, mainly due to the mechanical losses in handling. The directions given should be carefully followed, especially those regarding amount of solution, strength of acid, temperature, time, etc. Neatness is indispensable. The HN0 8 must be pure. The directions regarding the burning off of the filter-paper must be closely followed. The amount and strength of the salt solution must be carefully adhered to and it must be added at the proper time. Some assayers, instead of adding salt solution at the same time as H a S0 4 and a H 8 02) 2 , filter off the residue containing the gold and make a separate precipitation for the silver, believing that the addition of a salt solution may cause a slight redissolving of the gold. At this point of the assay that is, however, hardly probable. A large amount of NaCl is to be avoided, as AgCl is very appreciably soluble in brine. C. Whitehead recommends NaBr or KBr instead of NaCl for this reason.

Combination Method for Precipitates from the Cyanide Process. 2 — Where the troublesome base-metal impurity is zinc instead of copper, as in this case, sulphuric acid can be substituted with advantage for HN0 8 . The method is as follows:

Of the precipitates 0.10 assay ton is taken, placed in a beaker, and 20 c.c. of sulphuric acid (concentrated) and 60 c.c. of water are added. This is heated on a hot plate for about one hour, or until zinc and zinc oxide are in complete solution. Add salt solution of the strength already mentioned in the paragraph on Van Liew's method for blister copper, in slight excess, to precipitate the silver present, remembering that 1 c.c. will precipitate

i W. J. McCaughey, Jour. Am. Chem. Soc, XXXI, 1261.

2 Fulton and Crawford, "Notes on Assay of Zinc Precipitates Obtained in the Cyanide Process," in School of Mine* Quart., XXII, 163.

1 mg. of silver. Stir briskly with glass rod to agglomerate the silver-chloride formed.

The residues are then filtered through the proper sized filter, carefully washed with hot water into the point of the filter-paper, and dried in the air bath at a low heat. After drying, transfer to a 20-gram crucible containing 1 assay ton of litharge, and burn the filter-paper off in the manner already described. Then add 15 grams of soda and 2 grams of argol, mix thoroughly, and cover with a heavy cover of borax glass. Fuse and cupel the resultant lead button. Weigh the gold and silver bead, and from a preliminary assay determine the proper amount of silver necessary in order to inquart the bead. The amount of silver should be jiiHt about 2.5 times the amount of gold. Roll out the bead, after flattening with a hammer, until, after repeated rollings, the fillet will have about the thickness of a visiting card. It is best to anneal the bead at a red heat between the various rollings, in order to prevent cracking on the edges. Then part in a parting flask in hot nitric acid having a specific gravity of 1.20. Boil twice for at least 20 minutes each time, in order to insure the complete removal of the silver. This method of parting leaves the gold in one coherent mass, termed a "cornet," and is identical with the method practised in the gold bullion assav.

Chapter X Special Methods Of Assay

TELLURIDE ORES. — Gold ores containing the precious metals in the form of tellurides of gold and silver, mainly calaverite and sylvanite, are more difficult of assay than ordinary gold ores, and special methods are essential in order to get good results. The scorification assay is not reliable for telluride ores, giving almost uniformly low results. It is not used by assayers and chemists of the great telluride ore district in Colorado — Cripple Creek. It seems that in scorification the main cause of loss is volatilization, for while the slag loss is higher than for ordinary ores, slag and cupel corrections still leave the results from this assay far below those of the crucible assay when properly performed.

Of recent years selenium gold ores have been found 1 and in general the precautions necessary for the assay of telluride ores apply also to selenium gold and silver ores.

Tellurium has a great affinity for gold and silver that for silver being greater than that for gold; and if a high-grade telluride ore be assayed, even by special method, the beads from the cupellation will frequently still contain tellurium. 2 In the crucible assay the losses, which are somewhat greater than in ordinary ores, occur in the slag, and from the presence of the Te in the lead button, causing absorption of precious metals by the cupel. The aim in the crucible assay is to remove the tellurium from the gold and silver and slag it. This is best accomplished by the presence of considerable litharge as an oxidizing agent, and otherwise properly balancing the flux. The flux recommended quite generally by Cripple Creek assayers is made up as follows:

Potassium carbonate 7 parts Flour 1.0 part

Sodium carbonate 6 parts Litharge 30.0 parts

Borax glass 5.5 parts

i "Selenium Gold Ore," Eng. and Attn. Jour., XC, 418; Min. and Sci. Press, C, 224. 2 E. C. Woodward, "Cupel Losses in Telluride Ores," in West. Chem. and Mel., 1, 120.

This is for the ordinary silicious Cripple Creek ores. About 75 grams of this flux is used with 0.5 assay ton of ore. This gives the following charge:

Ore 0.5 assay ton Borax glass 8.5 grams

PbO 45.5 grams NTa,CO, 9.0 grams

Flour 1.5 grams K 2 CO, 10.5 grams

The heat recommended is such that a temperature of 1063° C, the melting-point of gold, is reached at the mouth of the muffle. Some assayers recommend a somewhat greater temperature to insure the decomposition of the tellurides. The time of fusion should be about 45 to 50 minutes.

In most telluride ores the silver contents are not great enough to permit of the parting of the bead obtained from cupellation. It is therefore necessary to add silver at some stage before parting and in this instance it is best done during the crucible assay, since by doing this there is apt to be less absorption of gold during cupellation on account of the presence of silver in the lead button.

It is essential to recognize that the flux recommended above for tellurides does not make what can be strictly termed an " excess-litharge charge.' 7

Hillebrand and Allen 1 recommend the following charge for Cripple Creek ores:

Ore 1 assay ton Borax glass 10 grams

NaHC0 3 1 assay ton Reducing agent (if necessary)

PbO 6 assay tons Salt cover

This approaches more nearly the excess-litharge charge.

The salt as a cover may with advantage be replaced by litharge. The fusion should be conducted slowly and at a temperature not exceeding 950° to 1000° C.

It is essential in telluride ores to have the sample crushed to 120-or, better, to 150-mesh. The reason for this is that, owing to the irregular distribution of values in these ores, fine crushing is required to get a true .sample, and also because the low meltingpoint of the charge usually employed makes this essential.

The precise behavior of tellurium in the cruciWmaM.v. ad during scorification or cupellation, has not a'

1 "A Comparison of a Wet and Crucible- Fire Met* Ores," Bull. No. 253, U. S. G. Survey.

Special Methods Of Assay

with scientific thoroughness though some preliminary work has been done. 1 The following facts are reasonably well established: 1. The great affinity of tellurium for gold and silver, resulting in tellurium passing to the lead button with the precious metals, unless a charge be used that is essentially oxidizing in its character, and effecting the slagging of the tellurium. 2. During the oxidation of the lead button by cupellation or scorification the tellurium tends to concentrate in the remaining lead-goldsilver alloy although in different degree in the two operations, the concentration being more pronounced in scorification. 3. The effect of tellurium on the lead-gold-silver alloy is to very greatly decrease its surface tension, so much in fact that if proportionately sufficient tellurium be present the surface tension is changed sufficiently to cause the alloy to " wet " the cupel and be absorbed as alloy, thus causing heavy losses of precious metals. If the proportion of lead to tellurium to gold should attain the concentration of 10:1:1, complete absorption may take place leaving no gold bead. Such a result will, however, occur only in exceptional cases, as in the direct cupellation of telluride mineral, etc. The accompanying data due to S. W. Smith, shows the relative elimination of tellurium during cupellation and scorification. Lead buttons of 20 grams containing 0.05 gram each of gold and tellurium were submitted to cupellation and scorification, and the process interrupted at intervals for the determination of tellurium.

TABLE XXIII.— SHOWING ELIMINATION OF TELLURIUM DUR- ING CUPELLATION AND SCORIFICATION

Cupellation

Scorification

Cupelled

Per cent, of

Per cent, of

Scorified

; Per cent, of

Per cent.

down to w t.

original lead

tellurium in ' i

down to wt.

original lead

tellurium in

grams

remaining

the button ' ,

grams

1 remaining

the button

L&amp;2 M

t&l.gsg

Trana. I. M. M., Bids. 44, 45 and 47 (1908). Holloway and Pearse, Trans. 89. 40, and 45 (1907 and 1908).

It will be noted that during cupellation the tellurium at first concentrates in the lead and then begins to be eliminated, the percentage decreasing. In the case of scorification there is a very decided concentration of tellurium in the lead toward the end of the operation. The difference is probably due in large part to the fact that in cupellation the tellurium is in part absorbed by the cupel as a lead tellurium alloy, which action cannot take place in scorification. The removal of tellurium by oxidation from lead, thus passing into cupel or slag, is evidently a difficult process. In an alloy of lead, gold and silver and tellurium, the tellurium can form compounds with both the precious metals and lead. It will be distributed between the two according to the relative masses present and the relative chemical affinities. If then in the alloy relatively much lead be present (100 to 200 parts Pb per part Au and Te) by far the larger part of the tellurium will be eliminated by absorption as lead telluride, and only a little will stay with the precious metals. This small amount will not be sufficient to materially lessen the surface tension of the bead at the end of the cupellation and hence absorption will be small. If, however, the amount of lead be small so that the relative amount of tellurium and gold be increased the absorption of the latter may be very heavy. 1 It follows therefore that in lead buttons obtained in the crucible assay, which may contain tellurium, it is better to cupel directly in order to avoid heavy absorption in the cupel. Scorification might be resorted to in this case for two reasons: 1. To reduce a large 'lead button; 2. in the mistaken idea of eliminating tellurium.

From what has gone before it is evident that a large button is not disadvantageous as it really tends to decrease the absorption of precious metal when tellurium is present.

The preliminary scorification of lead buttons from crucible assays of telluride ores has been shown to give low results. 1 The cause for the almost universally low results on telluride ores by the scorification method is also to be attributed in part to the above reasons.

4. Silver seems to exert a protective action on the gold and lessen the absorption of the latter, due probably to the greater atlinitv of silver for tellurium, thus forming silver telluride to the exclusion of the formation of much gold telluride. consequently

Special Methods Of Assay 135

lessening the absorption of gold. It is therefore desirable to perform the assay in the presence of considerable silver, which will have to be added anyway at a later stage to give a bead that will part.

In the assay of telluride ores the general object therefore will be to remove as much tellurium from the gold and silver before cupellation as is possible. This is best done by the performance of a crucible assay with an oxidizing charge. The oxidizing properties of the charge are obtained by the use of an excess of litharge. If we consider the ordinary telluride ore as composed of a silicious or shaly gangue containing the precious metals as tellurides and containing also certain amounts of sulphides, then when this is subjected to fusion with litharge (a large excess) the telluride minerals and sulphides are oxidized, the tellurium probably forming tellurate of lead or, in the presence of soda, tellurate of soda. (See behavior of sulphur, Chapter V.) If, however, an insufficient amount of PbO is present so that it forms lead silicates only with the silica of the ore, the oxidizing effect will be much diminished, since lead silicates form at a low temperature and do not readily give up oxygen. It is therefore desirable to form a slag which has the characteristics of an excess litharge charge, viz., is not glassy, but of an earthy dull appearance. Considerable soda should be present to aid the oxidation of the impurities. The borax glass should not exceed 5 to 10 grams, and the lead button made should be large, 25 to 30 grams. The fusion should be made slowly, particularly at first, and the temperature not exceed about 1000° C, since there is a possibility of dissociating the tellurium compounds in the slag and sending the tellurium into the lead button.

It is probably impossible to remove all the tellurium from gold and silver by such an oxidizing fusion for the reason that the reduction of lead from some of the litharge at a certain stage of the assay for the collection of the gold and silver, also again reduces some of the tellurium which has been oxidized. It is desirable to obtain the full oxidizing effect of the litharge before the reduction of lead takes place and for this reason charcoal is to be recommended as the reducing agent, when this needs to be employed, instead of argol or flour, since the CO evolved by the two latter begins to reduce Pb from PbO at about 300° C. less than solid carbon, which acts at about 550° C. (page 64).

13G A Manual Of Fihe Assaying

For ordinary silieious telluride ores of only slight reducing power the following charge is recommended:

Ore,

a. t.

PbO,

gr.

Na,CO a ,

gr.

Borax glass,

gr-

Charcoal,

gr.

Silver foil,

PbO cover,

g.

The fusion should be conducted slowly at first, the final temperature not much exceeding 1000° C.

If the button from the fusion is thought to contain tellurium, as is probably the case in the assay of a high-grade ore, it will be desirable to place it in a 20-gram crucible, cover with 30 grams PbO, mixed with 2 grams borax glass and bring to fusion, then pour and proceed as usual. This treatment will eliminate considerable tellurium from the lead. (S. W. Smith.)

It is stated 1 that in the oxidizing roasting of Cripple Creek telluride ores, in their preparation for chlorination or cyanid a " tion, the greater part of the tellurium in the raw ore is found in the roasted ore as a tellurite of iron. Some assayers add an ir° n nail to the assay, not so much to desulphurize as to provide n excess of iron for the purpose of combining the tellurium with- l as in the case of sulphur.

For the quantity of tellurium present, its influence on assay is certainly profound. The following table gives an icJ-ea of the quantity present:

Table Xxiv.— Quantity Of Tellurium In Ores

vi m- w Lrt i to Black HiUa Black Hi Ua

hlcment (. ripple (. reek Ore C ripple Creek Ore , , . , .

Cambrian Cambnax*

Tellurium 0.0712 per cent. . 0.092 percent.. . 0033 per cent . 0.010 per

Cold 0.0506 per cent.. 0.060 percent.. 0.0026 per cent.. 0.003 per

Silver 0.0075 per cent. . 0.0103 per cent. .

As already stated, tellurium is with difficulty separated from gold and silver, and in spite of an oxidizing charge is frequent/' carried down in the lead button. The loss then tkflB place m

the cupel, tellurium causing a heavy ab' " """

however, takes place by volatilization.

Special Methods Of Assay 137

higher slag loss in the telluride assay than in the assay of ordinary ores. 1 Hillebrand and Allen, already quoted, assayed telluride ores by the combination wet-and-dry assay, getting the gold and silver free from tellurium, but found that the crucible assay as ordinarily performed for telluride ores gave just as satisfactory, if not better, results.

A STUDY OF THE ASSAY OF BLACK HlLLS CAMBRIAN ORES. — These ores are probably complex tellurides. The ores were oxidized and of the following average composition:

Si0 2 71.5 per cent.; Fe 2 3 16.3 percent.; Al 2 O s 4.8per cent.; CaO 1.5 per cent.; Gold 0.79 oz.; Ag 0.10 oz.

Samples of this type of ore, representing controls on car-load lots, were assayed by assayers A and B in the same laboratory, with the same kind of cupels, and great regard to temperature of cupellation. Assayer A made fusions on one assay ton lots, in triplicate, with the following stock flux:

Na 2 C0 3 3 . 25 parts Borax glass ... 5 . 00 parts

K 2 CO, 2 .25 parts Argol 1 .00 parts

PbO 18.00 parts

The amount of flux used was 4 assay tons per assay ton of ore, with quite a heavy borax glass cover. Fusions made at 1100° C, approximately.

The stock flux is equivalent to the following charge:

Ore 1 assay ton PbO 73.2 grams

Na,C0 3 13.2 grams Borax glass 20.3 grams

K 2 CO a 9.1 grams Argol 2 4.0 grams

On account of the negligible quantity of Ag present, every assay was salted with Ag. The beads were parted in acid 1 to 9, and were in each case required to check against each other in weight. The beads were then weighed together and the resultant weight divided by 3 to obtain the amount of gold.

Assayer B made assays on the same pulp samples with the following stock flux:

Na a C0 3 3.25 parts Borax glass 2.00 parts

K 2 C0 3 2.25 parts Argol 0.75 to 1 .00 part

PbO 22.00 parte

iC. H. Fulton, School of Mines Quart., XIX. F. C. Smith, Trans. I. M. M..IX, 344 Min. Rep., LI, 163. Hillebrand and Allen, Bull. No. 253, U. S. G. Survey, 12, 14.

2 This amount of argol required because ores are oxidizing. The button produced usually 22 to 25 grams.

Three assay tons of flux were used to each 0.5 assay ton ore, with a soda cover one-quarter inch thick. Assays were mad in quadruple, all fusions being salted with Ag, parted in 1 to acid, and the beads required to check against each other in weigh and then weighed together, and the sum divided by 2 to get th< value per ton.

The stock flux is equivalent to the following charge:

Ore 0.5 assay ton PbO 67

Xa,CO, 9 grams Borax glass 6

K t C0 3 6 grams Argol 2.5

The results of these series of assavs were as follows:

Lot Xo. . Assater A Assater B

Oi. Au per ton Os. Au per ton

8S823 0.82 0.98

Sss1S 0.S5 0.91

Ssm0 79 0.84

Sss53 0.S5 0.91

Sss90 .0 Si 0.83

SS$2tf 0.77 0.82

3S4o 77 0.81

Ss7S0 So 0.88

9S509 69 0.79

Assay ers A and B then exchanged fluxes, and as they checked each other's previous results closely, it Invame evident that the flux of assay or A was ill-balanced and wouKl not give good results. Slag an J cupel co mictions were ir.ade by Assayer A on assays made with his :t;;x. In:: oven these corrections added failed to bruns his results u'o to those of assaver B.

The OjUcstior. arises as to what is the specific trouble with rlux A. Or. oxatr.ir.atior.. it w : % d :v found to contain an excessive armour.t ot borax class. when the cover is considered. It is ver\ probable that the ae:dit\ o: the charge although a good rbud slac is obtained is so crea; % 1qU> account both the silica of the otv sr.d the Ivrax t ore k not com-

Special Methods Of Assay 139

pletely decomposed by the basic ingredients of the charge; i.e., the soda and litharge become saturated with borax and then do not completely decompose the silicious ore. The fact that reassays of the slag do not bring the results up to the figures obtained by assayer B does not necessarily imply that the slag does not contain these values, as the charge used to flux the slags and cupels again contains much borax glass, so that practically the same conditions obtained as before.

THE ASSAY OF COPPER-BEARING MATERIAL,— Copper-bearing material includes ores containing copper and furnace products, chiefly mattes, blister copper, etc. Copper, which in the assay has a strong tendency to go into the lead button, causes, when present in sufficient quantity, serious losses by cupel absorption. Therefore all methods of assays for this class of material endeavor to eliminate copper from the lead button to be cupelled. A standard method for the assay of material high in copper, especially for Ag, is the combination assay for blister copper and mattes, described in Chapter IX.

Another standard method, especially for gold, and one that is carried out frequently as a check to the above, is the scorification or "all fire" method. This is performed as follows:

Ten samples, of 0.10 assay ton each, are taken and placed in 3-inch scorifiers with 50 grams of test lead (the silver content of which is accurately known) ; 25 grams of the lead are mixed with the matte, or borings, etc., and the other 25 grams used as a cover. On top of the charge is placed 1 gram each of silica and borax glass. The scorification is carried on at a moderate temperature until the assays are just about to slag over, which takes usually about 25 minutes, and then they are poured. The resultant button will weigh about 15 to 16 grams and be quite hard with copper. The buttons, cleaned from slag, are scorified, test lead being added to make the total lead up to 40 grams. The second scorification will take about 30 minutes and the resultant buttons will weigh from 10 to 12 grams. These are cupelled in 10 separate cupels, placed so as to be subject to uniform temperature, i.e., in one horizontal row across the muffle. Cupellation should be conducted at as low a temperature as is feasible. The beads are weighed separately and then together. They are then grouped in two lots of 5 each, which are parted in acid, strength 1 to 9, the beads being kept in this acid at nearly boiling temperature for 20 minutes and finished for 5 minutes with

] .42 sp. gr. acid (full strength). The ten cupels are taken in lots of two each (only the litharge-stained part is taken) , crushed to pass 100-mesh and assayed by the following charge:

100 grams PbO 45 grams borax glass

20 grams Na 2 C0 3 3 grams argol

Soda cover

The lead buttons are cupelled, and the silver and gold obtained added to the first weights. The scorification slags may also be reassayed and this correction added, but in practice the cupel correction is the only one usually allowed. Sometimes no correction is allowed. It is to be noted that, even with a rescorification of the first button of the assay, the final silver beads, from 55 per cent. Cu matte containing 180 oz. Ag per ton and 2.31 oz. gold, will contain from 2.5 to 4 per cent, copper, which must be deducted in order to get correct silver results. (For a further discussion of scorification slag losses and cupel absorption in assaying copper-bearing material, see Chapter XI.)

The scorification method is generally employed for the determination of gold in mattes, and the combination method for the determination of silver. Of recent years, special crucible methods for copper mattes and copper-bearing material have been developed with considerable success. 1

A satisfactory method on copper mattes, up to 20 per cent, copper and high in gold and silver, was practised by the Standard Smelting Company, at Rapid City, S. Dak. The matte sample is put through a 120-mesh screen, and for controls 4 assays of 0.25 assay ton each are made, with the following stock flux:

Silica 11 parts Sodium carbonate 25 parts

Litharge 70 parts Niter 5 parts

An 0.25 assay ton matte is run with a 3.5 assay ton flux and a thin borax glass cover. The flux figured to the charge is as follows:

. 25 assay ton matte 24 . grams Na,CO,

10.5 grams Si0 2 5.0 grams KNO,

The heat used is high and the fusion short, giving a clean fluid slag and a bright button of approximately 20 grams. These buttons are cupelled directly for gold and silver. One cupel and

14, An All-fire Method for the Assay of Gold and Silver in Blister Copper," in Trans. A. I. M. E. t XXXIII 670. Perkins, "The Lithargo Process for the Assay of Copperbearing Ores," ibid.. XXXI, 913.

Special Methods Op Assay 141

one slag are then re-run in the same crucible that the original fusion was made in, and the result of the four corrections added to the sum of the original buttons. No scorification is made before cupellation. The average correction, on the usual grade of matte (5 oz. Au, 40 oz. Ag), is 2.5 per cent, gold and 5.5 per cent, silver. Below is a comparison of this method with the standard scorification assay, including cupel and slag correction. The copper content of this matte was 19.98 per cent.

Table Xxv.— Comparison Of Methods In Assay

Crucible method

Scorification method

Original assay

Correction

The returns on this pulp by the refiner were: gold, 4.19 oz.; silver, 36.71 oz. Matte No. 1545; copper content, 17.6 per cent.

Table Xxvi.— Comparison Of Methods In Assay

Crucible method Scorification method

Original assay 3.42 , 31 .94 3.40 31 .86

Correction 0.10 1.85 0.11 1.93

The following table shows results by this method with correction and refiners' results (by same method without correction)

A Manual Of Fire Assaying

Table Xxvii. Corrected And Uncorrected Assays

On Copper Matte

Lot No.

Crucible method :

Standard Smelting

Company

Crucible method: Refiner

Gold

Silver

Gold

Silver

Copper

oz. per ton

oz. per ton

oz. per ton

!

oz. per ton

%

A typical sample of matte on which these assays were made analyzes as follows:

Gold 4 . 10 oz. per ton

Silver 31 . 55 oz. per ton

Copper 17.4 per cent.

Iron 45.9 per cent.

Zinc 2.5 per cent.

Silica 3.3 per cent.

Lime 0.5 per cent.

Sulphur 29. 1 per cent.

Lead trace

The crucible charge employed can readily be modified to apply to mattes higher in copper or greater in reducing power.

Perkins' excess-litharge method has already been described. He states that for low-grade copper-bearing material (2 to 4 per cent.), 5 assay tons of PbO to 0.5 assay ton of ore will remove most of the copper, if the balance of the fluxes is properly proportioned, i.e., if there is ample free PbO to dissolve copper oxides. For high-grade mattes, etc. — 48 to 60 per cent, copper

Special Methods Of Assay 143

— 8 assay tons of PhO to 0. 1 assay ton of matte will remove most of the copper. Perkins also developed a crucible method for metallic copper, as follows:

Weigh out 0.25 assay ton of copper borings, divide it into 3 approximately equal parts, anil place in 20-gram crucibles. In this way weigh out 4 sets, getting 12 assays. Into each crucible put 800 ings. of powdered sulphur, mix thoroughly with the copper, and then on top of this put the following flux, being careful no1 Co mix the flux with the copper:

Na,CO, 0.25 assay ton PbO S.Oassayton

K,CO, 0.25aBsayton SiO,.. . 5 assay ton

Salt cover

Place the crucibles into a dark-red muffle and gradually raise the temperature for 45 minutes to a yellow heat. The temperature regulation is important, and it is necessary to produce a neutral or reducing atmosphere in the muffle by the presence of coal or coke. The buttons, weighing about 18 grams each, are put together in lots of three, representing 0.25 assay ton, and scorified at a low heat. The resultant buttons should weigh .5 to 6 grams. Each of these buttons is now rescorified with 25 grams of lead at a low heat, until 6-gram buttons are obtained. These are cupelled with feathers. This method is stated to give results on gold equal to the all -scorifi cation method, and on silver equal to the combination method.

THE ASSAY OF ZINCIFEROUS ORES AND METALLURGIC PRODUCTS COHTAIHIBG ZIHC.— Zinc most frequently occurs In. ores as the sulphide, sphalerite, and, in certain metallurgical products, &h the metal (zinc cyanide precipitates). Zinc boils at &40°C, and rapidly volatilizes. Zinc oxide volatilises slowly at 1180°, and rapidly at 1400°. Zinc silicates alone are difficultly fusible, hut are readily so when mixed with borax or boricacid or ferrous silicate. 1 The presence of zinc in material to be assayed calls for certain precautious, and in general the assay is difficult. Metallic zinc has a great affinity for gold and silver, greater than lead, as is shown by the Parkes process for the desilverization of lead bullion. Under oxidizing influences 1 the formation of zinc oxide and its volatilization causes losses of gold ami silver. That this hiss is mechanical does not make it leas serious. The boiling

-point of zinc occurs at a temperature somewhat below the normal for ordinary scorification, and it is this fact, coupled with the fact that the zinc oxide formed is with difficulty soluble in litharge, that make accurate assay-results hard to obtain, especially in scorification. Zinc containing gold and silver may be distilled off and volatilized with very little loss of gold and silver, if the conditions are reducing. 1

Scorification is frequently employed for zinciferous ores, although it is not generally satisfactory. When used, it is best carried out in a way similar to that adopted for copper-bearing material, using from 0.05 to 0.10 assay ton of ore with from 50 to 80 grams of test lead, 2 grams of borax glass, and 1 gram of silica, the last being essential to flux the zinc oxide formed. Otherwise insoluble scoria and crusts form on the scorifier. Slag and cupel corrections are generally necessary and from 5 to/10 assays are made, the results being averaged. As zinc is readily oxidized, lead buttons contaminated with zinc are not to be feared and rescorification is rarely necessary. Among the most important zinciferous material presented for assay are the zinc-gold precipitates from the cyanide process. Scorification is not desirable for these. 2 They are best assayed by the crucible method or by one of the combination methods already described.

Crucible Method. — The crucible method best suited for unoxidized zinc ores is the niteV method, with sufficient silica present to form at least the monosilicate with zinc. Borax glass and much litharge is also desirable. On a practically pure sphalerite the following charge will give good fusions at temperatures of about 1100° C:

Ore 0.5 assay ton Si0 2 8 grams

Na 2 CO s 15 grams KN0 3 22 grams

PbO 150 grams Heavy borax glass cover.'

This charge can be modified, as regards niter and silica, to suit any sphalerite ore.

A good crucible charge for cyanide precipitates, containing up to 50 per cent, zinc, is:

Precipitates 0.1 assay ton Si0 2 5 grams

Na,C0 3 5 grams Na 2 B 4 0- 2 pams

PbO 70 grams Flour 1 /ram

Light borax glass cover

1 Rose, ibid., and references.

3 "Notes on the Assay of Zinc Precipitates, etc.," in School of Mines Quart., XX, 1£* 3 A similar charge is recommended by Lay, for complex zinc-lead con centra; Min. Ind., XIII, 287.

Special Methods Of Assay 145

The following method 1 is used on cyanide precipitates containing 12,000 to 22,000 oz. Ag, and 300 oz. Au per ton at the mill of the N. Y. and Honduras Rosaria Min. Co. in Honduras, C. A.

In a 20 gram crucible mix 27 grams test lead, 2.5 grams borax glass, 0.5 gram silica with 0.1 a. t. of the precipitates, tapping the crucible to make certain that no material adheres to the sides. In another crucible mix 33 grams of PbO, 25 grams of Na 3 CO s , 4.5 grams borax glass, 1.5 grams of silica, and 0.15 grams charcoal. After mixing transfer this second charge on top of the contents of the first crucible, and make the fusion as usual. The button, separates very cleanly from the slag. The slag and cupel are reassayed, and one weighing made on the three beads recovered, from the cupellation. The slag corrections are small, amounting to about 25 oz. Ag per ton of precipitates containing 14,000 oz. The gold loss in the slag is very small.

The assays are run in triplicate. In the assay of such highgrade material weighing of precipitates must be done on analytical balances.

Assay of Plumbago Crucibles for Gold and Silver. — Graphite or plumbago crucibles are extensively used in the smelting of cyanide-zinc precipitates, and the old discarded ones are usually sold in lots to some smelter; they often contain considerable gold and silver. These pots present difficulty in assaying, chiefly on account of the graphite and zinc contained. From a given weight ef sample, tlje metallics and scales are separated by passing the material through a 150-mesh screen, and a regular scale assay is made as outlined at the end of this chapter. The pulp is assayed as follows: 2

From 0.05 to 0.10 assay ton is taken and mixed with a little more than one-half its weight of niter and 30 grams of litharge, placed in a 2.5 in. scorifier, covered with 30 grams of litharge and afterward with a thin cover of borax glass, placed in a muffle, and fused finally at a yellow heat. The buttons are cupelled, weighed, and parted as usual.. Crucible assays may also be made on this material by the niter excess-litharge fusion, with a charge as fcvow*?

0.J ass,*r conn phite 5 grams Na 3 CO s

k " rams SiO, carbon contents of pulp)

Borax glass cover

Privat3 communication, E. Van L. Smith Name of the originator of the method not know, the author.

3 A modification of T. L. Carter's method; see Eng. and Min. Jour., LXVIII, 155.

In both methods it is essential that the amount of pulp, usually, should not exceed 0.1 assay ton, the carbon giving difficulties with greater amounts than this.

The Assay of Residues from Zinc Distillation (containing considerable carbon) for Silver and Gold. 1 — From 0.10 to 0.5 assay ton of the powdered residue is mixed with 35 grams of niter and 10 grams of Na 2 2 (sodium peroxide), and dropped, in lots of 5 grams each, into a red-hot crucible which can be readily covered, and the oxidation reactions permitted to complete themselves. The flux then added consists of 70 grams of litharge, 10 grams of borax glass, 10 grams silica, 2 grams argol and alight borax glass cover. The fusion is carried out at a yellow heat and the buttons cupelled as usual.

THE ASSAY OF AftTIMONIAL AND ARSENICAL ORES FOR GOLD AND SILVER. — Gold-and silver-bearing antimonial ores, such as stibnite, jamesonite, etc., are usually assayed by the niter method, 2 in the presence of considerable soda and niter, to induce the formation of the antimoniate of soda. A preliminary assay to determine the amount of niter is essential. The following charge is recommended for nearly pure stibnite: 8

Ore 0.5 assay ton KNO, 18 grams

PbO 120 grams Borax glass 6 grams

Na 2 C0 3 10 grams SiO a 10 grams

Salt cover

The fusion should be conducted slowly and at a low temperature. The button will usually contain very little antimony, the cupel not showing scoria or cracks. If it does contain enough to cause losses in cupellation, the buttons should be scorified. Smith 4 gives the following charge for ore containing approximately 75 per cent, stibnite. The niter, etc., can be varied for the ore as the gangue increases:

Ore 1 assay ton Borax glass 8 grams

PbO 75 grams KNO a 20 to 25 grams

Na 2 C0 3 25 grams Salt cover.

Another method, practically as good as the niter method, is the roasting with charcoal or coke-dust. 5 The sample of ore,

i K. Sander, in Ena. and Min. Jour., LXXIII, 380.

2 William Kitto, "The Assay of Antimonial Gold Ores," in Trans. I. M. M., 1906, Nov. 8 and Doc. 13.

3 Smith, "The Assay of Complex Gold Ores," in Trans. I. M. M., IX, 332.

Smith, ibid.

5 Sulman, Trans. I. M. M.. IX, 340.

Special Methods Of Assay 147

usually i assay ton, is mixed with approximately its own volume of coke-dust or coal-dust, placed in a 5-in. roasting dish, covered with another dish, and roasted in a muffle with closed door, at a temperature not exceeding a dark cherry-red (635° C), for about 35 to 40 minutes. This will cause the volatilization of 95 to 96 per cent, of the antimony as sulphide without appreciable loss of gold. The roast should have a yellow appearance when finished, and can be fused with the following charge:

Roasted ore SiO, 7 grams

PbO 70 grams Argol 2 grams

Na 2 CO, 20 grams Borax glass cover

This method gives good results on jamesonite ores.

Arsenical ores are assayed by the same methods as the antimonial ores; also by the iron-nail method, although this last is not generally to be recommended. The subject of the best method of assay of antimonial and arsenical ores still lacks thorough investigation. The chief points may be outlined as follows:

1. In the roasting, unless great care is taken as regards temperature, mechanical loss of gold and silver takes place, owing to the rapid disengagement of the arsenic and antimony oxides, or sulphides of these metals. Unless the roast is conducted at a low heat and in the presence of considerable carbon, arseniates and antimoniates of base metals or silver may form, holding values which later on are not completely decomposed in the crucible, owing to their stability at a high temperature, the result being appreciable slag losses.

2. In the niter method, the presence of much niter, with its powerful oxidizing effect, may also induce the formation of arseniates and antimoniates, containing silver and possibly gold, which will remain in the slag.

3. In the iron-nail method, unless the fluxes are carefully adjusted and the temperature kept below 1100° C, speiss carrying values is very apt to form above the lead button, and thus necessitate a re-assay, or a treatment of this speiss.

The Assay of Arsenical Nickel-cobalt Silver Ore. 1 — Two types of ores may be considered. 1. Those high in Ag and also high in Ni and Co contents, and 2. those low in Ag, but high in Xi

D. K. Bullens, Eng. and Mxn. Jour., XC, 809. Lodge, Trans. A. I. M. E., XXXVIII ,

and Co contents. It is essential to flux the Ni and Co in the slag since these elements seriously interfere with cupellation causing low results. Ni present in the lead button to the amount of 0.5 per cent, causes a scum of NiO to be left on the cupel. More than this causes the "freezing" of the button. The effect of cobalt is not so pronounced as that of nickel.

For the ores high in silver the scorification assay is to be recommended with the following charge:

Ore 0.05 to 0.10 a. t.

Lead 65 to 75 grams

Borax glass 3 to 5 grams

Silica 1 to 3 grams

Slag and cupel corrections should be made. It is desirable at times to check results by wet analysis for silver.

For ores low in silver the crucible assay with high litharge gives better results than the scorification assay. Small amounts of ore, 0.10-0.2 a. t., should be used, for the nickel, cobalt and arsenic in the ores are apt to form a speiss in the assay. For ores containing metallic silver in any amount the "scale assay" should first be made.

THE ASSAY OF SULPHIDES, MAINLY PYRITB, BUT CON- TAINING SMALL AMOUNTS OF COPPER, ZINC SULPHIDES, ETC. — Where gold only has to be determined ores of this character, the roasting method is satisfactory. This, however, proves unreliable for silver, and in many cases (as at Leadville) the silver contents of these sulphides are the most important. The best method, after many trials, was found to be the niter fusion on comparatively small lots of ore. The ore has the following analysis :

Iron 33 to 44 per cent. Zinc 4 to 8 per cent.

Sulphur 38 to 45 per cent. Copper 0.5 to 3.5 per cent.

Insoluble... 4 to 20 per cent. Lead to . 4 per cent.

Four assays are made on 0.25 assay ton each, with 3 to 4 assay tons of the following flux, the amount depending on the reducing power; i.e., on the amount of sulphides present:

PbO S parts Si0 2 1.5 parts

KN0 3 1.5 parts Borax glass 1.5 parts

Na 2 C0 3 3.0 parts

Special Methods Of Assay 149

Either a salt or a soda cover is used. The temperature of fusion is brought up gradually to a yellow heat. With 4 assay tons this gives the following charge: 1

Ore . 25 assay ton Na a CO, 24 grams

PbO 62 grams Borax glass 11 grams

KXO, 12 grams Si0 2 11 grams

The buttons are usually clean, and separate well from the slag.

Another method which may be used on this type of ore is the niter-iron method. This has the advantage that no preliminary assay is necessary to determine the amount of niter for the proper size button, but that only sufficient niter is added to partially oxidize the sulphides, the iron nails being relied upon to decompose the balance of the ore. On ores of the class shown by the analysis, the following charge is successful :

Ore 0.5 assay ton SiO a 8 grams

Na 3 CO, 25 grams Borax glass 8 grams

PbO 30 grams Iron nails 2 to 3 tenpenny

KNOj 15 grams Thin borax glass cover

If the ore has a lesser reducing power than shown by the analysis given, niter and silica should be decreased in the charge.

Rapid Methods for Sulphide Ores. 2 — The approximate percentage of sulphides in ores may be quickly determined by vanning with sufficient accuracy for thje addition of the proper amount of niter. An ordinary color or spotplate, used in volumetric chemical analysis is best used for the vanning. Small quantities of ore are placed in the four outside depressions and carefully vanned in a basin of water until only the sulphides are left. The quantity of these is estimated in per cent, of the total amount of ore taken for the vanning test. In gaining experience with this method it may be desirable for the assayer to make comparisons with ores of known sulphide contents. If pyrite be taken as the sulphide of unit reducing power, then chalcopyrite, blende, pyrrhotite, and arsenopyrite will have a reducing power of §, stibnite of £, and galena and chalcocite of J that of pyrite. In the complex sulphide ores the relative amounts of the different sulphides are estimated and the amounts converted into terms of pyrite. In the ordinary excess litharge charge with a fair amount of borax and soda and with 0.5 a.t. of ore, 15 per cent.

1 See also W. G. Vail, "Niter Assay for Sulphide Ores," in West. Chem. and Met., II, 14. a F. O. Harley, Eng. and Min. Jour., LXXXIX, 1221, XC, 647; also Min. and Sci. Press, CI, 147, and E. T Hall, Min. and Sci. Press, CI, 345.

of pyrite will reduce a 22 gram button. For every 5 per cent. of pyrite present above 15 per cent., 2.1 grams of niter are necessary to destroy the excess reducing power. Two stock fluxes are used in the assay of ores. 1. The reducing flux, designed to give a 22 gram button with a neutral ore on a charge of 0.5 a.t. ore and a measure or scoop of flux (84 grams). This flux is made as follows: PbO, 15 parts; Na 2 CO s , 4 parts; borax, 2 parts; flour 0.44 parts. When 84 grams of flux are used this gives the following charge:

Ore 0.5 assay tons

PbO 60 grams

Na 2 CO, 16 grams

Borax 8 grams

Flour 1 .75 grams

2. Non-reducing flux to be used in connection with niter for sulphide ores which will give a button larger than 22 grams. This flux is made as follows: PbO 15 parts; Na 3 CO„ 3.5 parts; borax 2.5 parts; silica 0.5 parts. When 84 grams of this flux are used it gives the following charge:

Ore 0.5 assay ton

PbO 60.0 grams

Na,CO, 14.0 grams

Borax 10.0 grams

Silica. 2.0 grams

Niter As necessary

When sulphide ores are assayed which do not contain sufficient sulphides for a 22 gram button, the reducing and non-reducing fluxes are mixed in such proportion as to obtain the correct result. Thus — suppose an ore contains 10 per cent, pyrite, its reducing power would be }-gx22 14.6 grams lead on the basis of 0,5 a.t. The deficiency in lead is therefore 22—14.6 7.4 grams. In order to obtain 7.4 grains lead the following amount of reducing flux is required: Si \7. 4 28.27 grams. The balance of the charge of 84 grains will be made up of non-reducing flux, and the whole charge will be:

Ore 0.5 assay ton

tttnluviag thix 2$ 27 grams

N vui -reducing flux 55.73 grams

IW and niter are measured by volume in properly IumahkI or measures.

Special Methods Of Assay 151

For high sulphide ores when very accurate results are required a preliminary assay is made as follows:

Ore 3 . 64 grams

Non-reducing flux 50 . grams

This is run in a 10 gram crucible. This charge will give a lead button weighing as much as the niter necessary to oxidize all the sulphides in 0.5 a.t. of the ore. Place the lead button obtained in one scale pan of the pulp ifccale and from the hook above the other pan suspend by fine wire a weight so that with the wire it amounts to 6 grams. Then add niter to the pan having the 6 gram weight until the scale is in balance. This amount of niter is the proper amount necessary to produce a 22 gram button with the ore and the non-reducing flux if 0.5 a.t. of ore is taken for assay. (Consult Chapter V.)

For important assays it is desirable to make 4 assays, combine the buttons from 2, and scorify into one button each. Make the two cupellations, weigh the beads separately for Ag, combine them for parting and make one weighing on gold.

If the ores assayed contain more than 12 per cent, copper, it is desirable to take the lead buttons from the assay and place them into crucibles with 50 grams of litharge and 2 grams Si0 2 , place in the muffle and leave there four or five minutes after the PbO has melted. Then withdraw the crucible and with the tongs give the coiltents a rapid swirling motion for a few minutes and then pour. This treatment eliminates most of the copper remaining in the button. Then cupel and part as usual. It is to be noted that the methods described may have to be modified to suit particular conditions.

THE ASSAY OF MATERIAL CONTAINING METALLIC SCALES. — Ores of this kind are difficult to assay and obtain correct results from, as the metallic particles (usually gold or silver) are so unevenly distributed as to make it practically impossible to obtain an accurate sample. Two methods of assay are available:

(a) Approximately 500 grams of ore (or less, if deemed advisable) are weighed out, crushed, and put through a 150-or 200-mesh screen, care being taken to separate out the scales as closely as possible. Screening and crushing should frequently succeed each other. When all the scales have been separated out, they are transferred to a parting cup and dissolved in 3 to 5 c.c. of nitro-hydrochloric acid, if gold, or in nitric acid if silver or copper. The pulp is then heaped up into a cone in a large porcelain dish,

of pyritc will reduce u. 2\£ t of pyrite present above Lit h eary to destroy the exuben u used in the assay of orea. ,1 a 22 gram button with u a measure or scoop ol ii, follows: PbO, 15 parte, 0.44 parts. When SI ( following charge:

Ore

PbO . Na.CO,

Flour. .

2. Non-reduce I 1 sulphide ores wh This flux is made borax 2.5 parts; nitti are used it give?

Ore

PbO.. . Na.CO,. . Borax..

Niter. . . .

When sulphidi I sulphides for a 22 i fluxes are mixed n Thus — suppose i i power would 0.5 u.t. The* In order to obt-i.. duciug Hux is i of the charge of anil the whole c

s a: r lie cone, and the

r -rz. t-l.— -iipiuled water,

siefH." Tiah it out. The

- ir-jorb all of the

-a ii-h. The pulp is

--Tf'UTi-~ =Jxed on glazed

I: is then assayed

- vwah*d out, crushed j iwervwd above. The Xl_ -ae loss in dusting rui'c: the lead button ur-i.-i Then 15 grams . with the proper 9MC cupelled, and the t 'Stained, the total tm is calculated, con- i dver, respectively, r the original weight i in ounces per ton. i CHIEF VALUE e ores are difficult I free gold particles them uniformly r proper way to assay to 1500 grams of reen, place in a six to a rather thick B.tf. of mercury from should be free of precious metals s some gold. The : two hours, best in . -wparate the mercury ; all the pulp, in vf the fine slimes of be necessary to add t amalgam during i wreurv. The pulp carefully poured When dry it

Special Methods Of Assay 153

is mixed on a cloth, and 1 a.t. samples taken and assayed by a proper method. The mercury is carefully transferred from the pan to a porcelain dish, washed with water to free from sands, dried with filter-or blotting-paper and then transferred to a 20 gram crucible in which 20 grams of lead have been placed. To the crucible is then added a charge consisting of 30 grams PbO, 10 grams Na 2 C0 8 , 5 grams borax glass and 0.5 gram argol and silver foil enough to part the gold.

The fusion is made by raising the heat very gradually; it is best to use a muffle that has not yet become red, and has a good draft through it, to prevent the escape of mercury fumes into the room. The button from the fusion is cupelled in the usual manner. The gold is weighed in mgs. and the weight divided by the grams of ore taken and multiplied by 29.166, gives the oz. gold per ton present as "free" gold. This figure added to the assay results from the pulp gives the total contents of the ore in oz. per ton.

Another method 1 is carried out as follows:

Take 6 a.t. of the sample crushed to pass 80 mesh, add sufficient water and a small amount of sulphuric acid to make a thin paste in an 8 in. porcelain mortar, add 8 grams of redistilled mercury and grind thoroughly for 30 minutes. Then separate the tailings from the mercury by washing them off with a stream of water obtained, say by attaching a hose to a hydrant. During ' the washing the mortar should be given a rotary motion. Collect the overflow from the mortar in a large gold pan and treat the pannings as described in the method above. When the mercury in the mortar is quite clean from sands give it a final wash with water, then dry it with filter-paper and transfer to a crucible containing enough litharge with reducing agent to give a 20 grams button. Add enough silver to part the gold and start the fusion at a very low heat in the furnace. Cupel the lead button and weigh.

Divide the weight of the gold by the number of assay tons of ore taken and add this to the figure obtained from the assay of the pulp in order to get the total value of the ore in ounces per ton. When the ore to be assayed contains arsenopyrite and graphite some of these will adhere to the mercury. In order to overcome this difficulty add to the washed mercury in the porcelain mortar 5 c.c. of cone. HNO s and enough silica to make a thin paste.

A. T. Roob, Mining World, XXXII, 319.

Grind for a few minutes and then wash the silica and acid off with water and proceed as before.

AMALGAMATION TEST TO DETERMINE THE AMOUNT OF "FREE" GOLD PRESENT. 1 — One hundred grams of crushed ore are weighed out into a citrate of magnesia bottle, 150 c.c. of water added and then 2 c.c. of pure mercury from a burette. The stopper is clamped, the bottle rolled in a piece of cloth and placed in a moving shaker for two hours. It is then removed, opened, covered with the thumb, shaken, and inverted over a 3 in. porcelain dish, and as much clean mercury as possible allowed to run out. A little more water is added and more mercury allowed to run out into another dish and so on as long as any comes out. If the mercury is not floured nearly all is removed in two operations. All the clean mercury is then put into a 250 c.c. beaker. The bottle is then shaken well and again inverted to let a little sand run out into a dish. This sand is then panned into an enameled dish, usually only a few globules of mercury being obtained. If much is found from this third inversion the whole charge must be panned and if the mercury is floured a small globule of liquid sodium amalgam should be added to the pan. In ordinary routine work the tailings from the panning are discarded. For special purposes as when the tailings are to be tested by cyaniding, concentration, etc., they are allowed to settle completely, decanted and if necessary dried for further tests or for assay. To the mercury after its collection in the 250 c.c. beaker 0.5 gram of pure silver is added (if this has been carefully prepared and cleaned by treating in cyanide solution or weak nitric acid or by slightly amalgamating the surface it may be used to pick up the small globules of mercury collected in panning) about 150 c.c. of HN0 3 , sp. gr. 1.14, previously warmed to about 70° C. is now poured into the beaker which is set into an enameled pan on a hot plate and left there till all the mercury has dissolved. If not too hot it is unnecessary to cover the beaker. As soon as the mercury disappears the liquid is filtered on a 12.5 cm. paper previously wetted. The residue is rinsed on to the paper, washed once or twice with very dil. HNO, (not over 5 per cent) and once with water. Test lead is then sprinkled on the paper, it is folded, placed on test lead in a scorifier and enough lead added to make a 20 gram button.

1 Method used at the Homestake Mine, S. D. Communicated by Wm. J. Sharwood.

Special Methods Of Assay

Silver is added to insure parting and also a few grams of borax glass. The scorifier is then charged into the muffle, the paper burned, the charge scorified for a few minutes, poured, the button cupelled and the bead parted and the gold weighed. From a 100 gram ore sample each mg. of gold represents 0.29166 oz. or $6.03 free gold per ton.

Notes on the Carrying Out of the Amalgamation Test. — The amalgamation test is carried out for the purpose of determining the amount of precious metals that can be recovered from the ore in milling operations by means of amalgamation. The size of the crushed ore will influence the results; therefore, in different tests the degree of fineness must be nearly constant. The exact fineness used depends upon conditions. Temperature has its influ-

Fig. 57. — Homestake Agitator for Amalgamation and Cyanide Tests.

ence; if the tests are carried out at temperatures higher than the normal daily temperature results will be higher. If the temperature be low results will be lower. The addition of silver to the mercury reduces the time required for its solution by about onehalf. Extreme care must be taken to get mercury and silver practically free from gold. It is desirable to run a blank assay on say 20 c.c. of mercury and 5 grams of silver (representing ten times the quantity of each used in the assay) . The mercury and silver are dissolved in acid as stated in the amalgamation test and the residues treated as described. The best mercury obtained after testing a number of new flasks and when used with commercial proof silver required a correction of 0.02 mg. of gold for 2 c.c. mercury and 0.5 gram silver. The silver contained nearly half of the gold thus found. If the amount of silver in the ore recoverable by amalgamation is to be determined the parting of the mercury by nitric acid must be replaced by the crucible

fusion of the mercury as described in method 1 for the assay of ores containing free gold.

Equipment Required for Routine Amalgamation Teste.— 1 . Shaking box with 24 compartments each 3 in. square and 6 in. deep as shown in Fig. 57. The following are the details of construction. Sides and bottom made of 1.75 in. lumber; partitions of 0.5 in. lumber; sills 4 X4 in. lumber; connecting rod lJXf in. oak lumber, 4 ft. long; supports of light steel 1.5 X-jt in. and 2 ft. long; shaft 1 in. 250 r. p. m.; throw of eccentric, 2 in. A frue vanner eccentric rod and supports may be used in the construction of the agitator. A pad should be placed at the bottom of each pocket. Pieces of canton flannel, 12 to 15 in. square, are used to wrap each magnesia bottle.

2. Twenty-four citrate of magnesia bottles with spring clamps and rubber washers. These have a capacity of 350-370 c.c.

3. Twenty-four beakers, capacity about 250 c.c.

4. Two enameled iron pans to hold 12 beakers each.

5. Six porcelain dishes, 3 in. in diameter and 2 enameled iron pans, 8 in. in diameter and 2 in. deep.

6. Filtering rack for 12-2 J in. funnels. Twelve extra beakers.

7. Copper weighing scoop and copper funnel with steep sides for charging bottles.

8. Cylinder graduated to 100, 150, and 200 c.c.

9. Glass stopper burette standing in enameled iron pan. 10. Supplies as mentioned in the assay.

THE ASSAY OF CYANIDE SOLUTIONS. Method I. 1 — Measure out any convenient volume into a beaker (preferably 10 or 20 a.t using beakers of 500 to 700 c.c. capacity). Add 10 to 20 c.c. of lead acetate solution containing 10 to 20 per cent, of the salt, then introduce 3 to 4 grams of zinc dust in the form of an emulsion or suspension in water and stand on a hot plate. When moderately heated but before boiling, acidify with about 20 c.c. strong hydrochloric acid, either c.p. or of the best commercial grade. Boil until action nearly ceases, and the reduced lead has collected into a spongy mass. Filter on a " quick" paper, and wash precipitate twice with hydrant water. Remove the filter-paper and precipitate and squeeze out as much water as possible. H&e* in a 2 in. scorifier with 10 to 15 grams of test lead an4 %JfiAiw grams of borax glass. Place at once in the muffle, bi scorify for only a few minutes, pour, cupel lead ]

1 Duo to Mr. Allan J. Clark, Hornet) take Mining Co.

Special Methods Of Assay 157

weigh. Unless silver is to be determined, silver foil should be added to the scorifier for inquartation, or a measured volume of dilute AgN0 8 solution may be added to the beakers from a burette.

Notes on the Method. — About 100 grams zinc dust are usually mixed with 300 c.c. of water in a bottle with an J-in. glass tube passing through the cork. This mixture is shaken into a capsule of the proper size used as a measure. The other reagents must be roughly measured. Their proportions should be varied slightly until conditions are found which yield a " sponge " of lead quickly with the particular solutions regularly assayed. Impure hydrochloric acid does not give good results, nor do other acids. It is essential that nearly all the zinc be dissolved before filtering. Comparatively cheap filter-papers answer well. If a 300 c.c. flask be regraduated to deliver 301.45 c.c. every mg. of gold obtained from this volume represents $2.00 gold value per ton. If copper is present in solutions a somewhat longer scorification than above stated may be desirable.

The method was suggested by Chiddey's method 1 in which zinc shavings and lead salt are used to produce a lead sponge. In this original method the lead sponge is recovered by hand and not filtered and then cupelled direct without a preliminary scorification.

Clark's method gives somewhat better results on low grade solutions than evaporation methods with litharge or litharge bearing flux. It has the particular advantage of being an exceedingly rapid method as compared to the tedious evaporation methods.

Method 2. Evaporation Method. — Measure out 5 to 10 a. t. or more of solution by means of a properly graduated flask and transfer to either porcelain or agate ware evaporating dishes, of 300 to 500 c.c. capacity. To the solution add 50 to 60 grams of litharge and place the dishes in a sand bath on a hot plate and carefully evaporate to almost complete dryness. If agate ware dishes are used it is essential that the agate lining be unbroken, otherwise precious metals will precipitate on the iron surface and adhere to the same, giving low results in the assay. Tin dishes MaihuilkLjiot be used. When practically dry transfer contents by

>nd Min. Jour., LXXV, 473. Consult abo W. H. Barton, M'ent. *nd A. Whitby, Jour. Chem. Met. and Min. Soc. S. A., X,

means of a spatula to a glazed paper, and remove any adhering litharge from the dish by means of a moist piece of filter-paper, thoroughly wiping out the dish. If the evaporation has not been carried too far this can readily be done. Then mix in a 20 grams crucible, 25 grams litharge, 15 grams Na a CO s , 2 grams argol, 2 grams Si0 2 and 5 grams borax glass, and transfer the litharge from the evaporation and filter-paper to the crucible and again mix with a spatula. Fuse the charge and proceed as usual. Unless silver is to be determined add silver foil to the crucible before fusion.

Evaporation methods conducted in dishes made of lead foil have the disadvantage of permitting the use of comparatively small quantities of solution only and very frequently give low results.

THE ASSAY OF SLAGS AND CUPELS FOR THE CORRECTION ASSAY. — (a) Slags: The charge for these depends upon whether they are acid or basic. Particular care must be taken to get a charge that will completely decompose the original slag. If this is acid, the charge should aim to make a new slag more basic, and vice versa. The lead button should be from 25 to 30 grams in weight. Many assayers frequently add simply litharge and reducing agent to the slag in making the fusion. This is not always desirable, for if the slag already has much litharge in it, soda, etc., may with profit be added as the extra base in place of litharge.

(6) Cupels: The bone-ash of the cupel will not unite with fluxes to form slags, but remains suspended in the fusion. For this reason the cupel should be put through a 150-to 200-mesh screen before assaying, the litharge-stained portion only being taken. For one large cupel, or two small ones, the charge is as follows:

Cupel Borax glass 45 grams

PbO 60 grams Argol 2.5 grams

Na 2 CO, 25 grains Soda cover

Fluorspar is not desirable in the assay of cupels, as it merely adds another ingredient in suspension.

Magnesia cupels may be fluxed with the following charge:

Cupel Borax glass 20 grams

PbO 40 grams Silica 10 grams

Na,CO, 20 grams Argol 2.5 grams

Borax cover

Special Methods Op Assay 159

Cement cupels are more easily fluxed and an ordinary crucible charge for a somewhat basic ore will answer very well.

The Assay Of Material Containing Metallic Iron. 1 —

Material of this kind will be obtained in the clean up of mortar boxes of stamp mills, the iron being present as pellets, and much larger pieces mixed with sand, pebbles, etc. It cannot be crushed and is assayed in the state received. Its correct sampling is practically impossible. Crucible fusions are made in the presence of bisulphate of soda and niter. The charge is as follows:

Material to be assayed 1 assay ton

Bisulphate of soda 8 to 24 grams

Na,C0 3 25 grams

SiO, 10 grams

Borax glass 25 grams

Litharge 35 grams

Niter 1 to 4 grams

The fusion should be conducted at a high heat for about 45 minutes. Then add to the crucible 15 grams of PbO mixed with 2 grams argol and continue fusion for 20 min. more until quiet.

The action of the bisulphate is probably as follows. It breaks up on heating.

2NaHS0 4 Na 3 S0 4 + H 2 + S0 3

The metallic iron is converted into FeS0 4 by the SO s in the early stage of the fusion, and is then converted into ferrous silicate as the temperature rises. The litharge and niter aid in the oxidation of the iron. Practically all of the PbO is reduced by the metallic iron. Interaction also takes place between the NaHS0 4 and the Na a CO, dependent on the quantities present. Na 2 CO s may with advantage be replaced by lime for this reason. The charge may have to be modified considerably in quantities of the reagents present to suit the material to be assayed.

"Modification of Method of H. R. Jolly," Jour. Chem. Met. and Min. Soc. S. A., VIII.

Chapter Xi Errors Ix The Assay For Gold And Silver

Losses In The Cupellation Of Pure Gold Akd Silver.—

These losses may be divided into (1) losses by absorption, (2) losses by volatilization. The losses of gold and silver in the cupellation are functions of (a) the temperature of cupellation; (6) the amount of lead with which the gold and silver is cupelled; (c) the physical nature of the cupel; (d) the nature and amount of impurities present; (e) the influence which silver has on the gold loss, and rice versa.

There is considerable literature extant upon losses in cupellation of the two precious metals, but in the older researches the temperature influence is but vaguely defined, owing to the lack of means for ready and satisfactory temperature measurements, a deficiency which is now supplied by the LeChatelier platinumrhodium pyrometer. Losses are also expressed as percentages of the total amount of metal cupelled, and then the average percentage losses are indicated. That this is very deceptive is made evident by reference to the curve of losses accompanying this chapter.

It is for this reason that the statement of results given by Mason and Bowman, 1 that the average loss in cupellation of pure silver under normal conditions is 1.99 per cent, and for gold 0.296 per cent., does not convey any very definite idea, unless the amount of metal cupelled is accurately specified, as well as the temperature. This fact has been noted by other observers, 2 but no effort has been made to express results coordinately.

The following data show the losses which occur:

1 Jour. Am. . Soc, XVI, 305.

Kaufman, in Eng. and Min. Jour., LXXIII, 829. Miller and Fulton, in "School of Mine* Quart.;' XVII, 169

Errobs In The Assay For Gold And Silver

Table Xxviii.— Cupellation Of Pure Silver

Amt. of silver

Amt. of lead

Temperature

Total losses

milligrams

grains

deg. Cent.*

per cent.

Table Xxix.— Cupellation Of Pure Silver

Amt. of lead grams

Approximate

Temp. deg. Cent.

of air in muffle

Amt. of silver Mgs.

Total loss in per cent.

1 Lodge, "Notes on Assaying," p. 59. Of air in muffle, directly above cupel. 8 Trans. A. /. M . E., XXVI, 473.

ti

.

:

ell:. I'

silva amor

HO I'fl'i.

Tin-

1 Jo.;-; .1 K:ui!':i.-

Errors In The Assay For Oold And Silver

uioi 10 lass-led

A

F "ia£ ixsAYING

Anit. of silver Mgs.

25

Parentheses made up iv.-q.n-. The results, vii . equal merit, different typrminO. with ill-

The agreeni' when the l'aci oases no pivrithat the anim::

it? jn5triL i ti*il from figures in Mr.

praip*s ire taken, and while

3? -jnier :han the best work call?

-. .error rwognition of the precise

:m "ae best and most complete

'flowing the relation between

-e Tercentage loss. I refrain

5-.;. i: in equation covering the

-iu utron is clearly discernible - "V ernr'erature variations will

t .'Osi'-ierably less than that for i i*: bullion assav. He - no ize. under normal temperarr "JJOO. of which 82 per cent. T - ti . -jiaciliiation (probably), and Ui aiculated to percentage on t?r :ent. for the highest los?. i-jx ioi-ition loss.) -.'aaun interesting data resorption and volatilization, to

ALLOYS.— The loss of gold different when both gold Tien either metal alone

Mx Of Gold

Authority

Kaufman

l.iiiaoV. ?

us

it

ar.i :h* *.;';a*ion!i

Errors In The As8Ay For Gold And Silver

Table Xxxii.— €Upellation Of Gold

Amt. of Amt. of ( Approximate i T ,. I . Total loss

gold lead Temp. deg. Cent, i . , volatilized

° per cent, absorption .

mgs. i grams I of air in muffle x per cent.

increased

increased

increased

increased

back of muffle

muffle increased increased increased back of muffle

Table Xxxiii.— €Upellation Of Gold

Temp, of cup-

Amt. of gold

Amt. of silver

Amt. of Pb

ellation deg.

Total loss gold

mgs.

mgs.

grams

Cent.

per cent.

air in muffle

A

Bull. No. 263, U. S. G. Survey.

1 Eng. and Min. Jour., LXXIX, 708.

A Manual Of Fire Assaying

Table Xxxiv.— Cupellation Of Gold-Silver Alloys

All Cupellations made with 25 Grams of Lead

Total lorn

Absorbed by Cupel

Amount gold mgs.

Amount silver mgs.

Temp. °C.

increased

increased

increased

back of muffle. . . .

front, 750°

increased

increased

increased

increased

back of muffle. . . .

front, 750°

increased

increased

increased

increased

back of muffle. . . .

Gold

per

cent

Silver

per

cent

Gold per

Silver per

cent I cent.

Gold : SUter cent, cent

Rose shows (Table XXXIII) the protective action that silver exercises over gold, the total loss of gold decreasing as the amount of silver present increases. Hillebrand and Allen show how the total loss is distributed between absorption by the cupel and volatilization. It is evident that while the total loss of gold is decreased by the presence of silver, the volatilization loss of gold is increased by the presence of silver (compare Tables XXXII and XXXIV). When gold and silver are present in the, ratio of 1 to 2, the averages are as follows:

Of the total gold loss, 68 per cent, is absorbed, 32 per cent, is volatilized.

Of the total silver loss, 71 per cent, is absorbed, 29 per cent, is volatilized.

However, as the total loss is determined by the difference in weight between the proof gold and silver and the weights of the cupelled bead and parted gold, and the volatilization item by the difference between the total loss and the amount recovered by the re-assay of the cupel, it is evident that certain errors obtain which apparently make the volatilization loss appear greater

Errors In The Assay For Gold And Silver

than it really is. The error, however, cannot be very great. The data are inconclusive regarding the influence of the temperature on the relative losses by absorption and volatilization, but it seems indicated that the volatilization loss is proportionately greater with an increase of the temperature of cupellation.

LOSSES IN THE ASSAY OF ORES.— Table XXXV, etc., show losses of gold and silver in the assay of ores, during fusion and cupellation, as influenced by the presence of certain impurities.

Table Xxxv.— Telluride Ores

Woodward/in "West. Ckem. and Met.," I, 12.

5 Fulton, in "School of Mines Quart.," XIX, 410.

' Lodge, in "Tech. Quart." 1809, XII, 171 (averages).

4 Butt. No. 253, U. S. Q. Survey (averages; Hillebrand and Allen).

8 Average of 34 fusions, tellurium in all beads.

6 Average of 10 fusions.

7 Cripple Creek flux.

Excess-litharge charge.

Weight of lead button

Method of

Slag loss

Cupel absorption

fusion

Milligrams

Grams

Per cent.

Per cent.

1 Crucible 7

Crucible i Crucible Crucible Crucible Crucible Crucible Crucible Crucible Crucible Crucible Crucible Crucible Crucible Crucible Crucible Crucible Crucible 8 Crucible Crucible Crucible

A Manual Of Fire As8Aying

Table Xxxvi.— Zinciferous Material, Etc.

Amount of Au and Ag in Weight weight of ore j of lead taken for a*- button say.

Slag loss

Cupel absorption

Method of assay

Remarks

Au mgs.

Ag

mgs.

Grams

1 Scorification after i acid treatment

' Scorification after I acid treatment

Direct crucible fusion

Crucible fusion 1 after acid treatment

1 Direct crucible fusion

i

I Crucible fusion niter method

1 Crucible fusion niter method

!

, Crucible fusion niter method

Au per

Ag i Au per per

Ag per

cent. cent. I cent.! cent.;

0.06 0.40| 0.11, 1.30 Zn. ppt containing ] I 42.3 per cent Zn

0.04 0.34 0.08! 1.10, Figures represent ! I averages.

t

0.06 0.51 0.18| 1.181 Zn. ppt. containing I I ! 14.3 per cent. Zn,

1 9.1 per cent. Cu. 0.15 2.73 0.16| 1.29 Figures represent

! averages

taining some copper Silicious ore containing some copper

1 Fulton and Crawford, in School of Mines Quart., XXII, 153.

2 Lodge, in Trans. A. I. M. E., XXXIV, 432. Miller, in School of Mines Quart., XIX, 43.

Errors In The Assay For Gold And Silver

-High-Grade Carbonate And Sulphide Silver Ores&#x27;

Amt. of Au '

and Ag in i Weight 1

weight of ore of lead

taken for button ;

assay ,

Method

of

assay

Slaglo*. !

; absorption

Au mg8.

Ag mgs.

Grams

Au Ag Au Ag per per , per per cent cent. cent. cent.

Second correction from fusion of slags 1 and cupels of first I correction per cent.

Crucible fusion Crucible fusion Crucible fusion Double amt. of

fluxes Crucible fusion

(Double amt. of fluxes Scorification Crucible fusion Scorification

Table Xxxviii.— Cupriferous Material

Amt of Au and Agin

weight of ore

taken for

assay

Weight! of lead button I

Au mgs.

Ag i Grams

Total loss recovered including

Method

slag and cupel

of

! Remarks

assay

—

Au

Ag

per cent.

per cent

Crucible fusion

!

! Mattes containing ' about 20 per cent. Cu.

Crucible fusion

Crucible fusion

Crucible fusion

Scorification

Scorification

The foregoing tables represent for the most part averages, and in every case the losses for the normal assay; i.e., in the case of the fusion, the charge known to yield the best results, and the proper temperature for cupellation. The losses are therefore

1 First fire results on lead carbonate ore, last two on silver sulphides. All results reprcmd

Fulton, ibid, XVII, 160.

to be ascribed to the nature of the material assayed, chiefly to the influence of certain elements present. In considering the percentage of loss, it must be recalled that this varies inversely with the amount of precious metal in the charge, i.e., with the size of the gold-silver bead. The sum of the cupel absorption and the slag loss (which can, in part, be recovered) is not the total loss, as it does not include that by volatilization, which is small in most cases, but in some cases, again, may be quite appreciable, as in the case of telluride ores. What the loss is in slag, when no element like tellurium, copper, zinc, etc., is present, may be seen by reference to Table XXXVI, to those assays fused after acid treatment, and to Table XXXVII, showing crucible fusions on lead carbonate ore. The slag loss in gold and silver for these ores is very small. In cases where the impurity present and causing loss is nearly all eliminated in the fusion, e.g., zinc, antimony, etc., the cupel absorption is practically that for pure silver and gold under the same circumstances. Where the impurity is tellurium, or selenium, or copper, the cupel absorption is decidedly increased. One fact is to be noted, the fact that the slag losses present no regularity, even for the same material. This is probably due partly to differences of slag composition among different experimenters, and partly to difference of temperature of fusion; and also to the method of refusion of slag.

The high loss in scorification slags shown in Table XXXVII for lead carbonate ores containing silver is due to the general unsuitability of the ore for scorification, although scorification slags show higher losses than crucible slags. That, in spite of this, scorification assays, on silver-bearing material show equally good and better results in many cases than the crucible assay, is due to the fact that the silver beads retain small quantities of lead and copper (see further on) , and to the fact that in the multiplication of the weight of the silver bead by 5 or 10, or whatever the assay-ton factor may be, this error is multiplied, giving an apparently better result.

The amount of slag has comparatively little influence on the amount of precious metals retained, provided the amount of collecting lead is ample. Buttons of less than 18 to 20 grams should not be made, and if the amount of slag is great or the quantity of silver and gold in the charge is more than 500 mgs., 25-and 30-gram buttons are essential. In k k *rf large

buttons which contain no impurity, it rect>

Errors In The Assay For Gold And Silver 171

if possible, rather than rescorify to smaller size, as* the rescorification causes greater loss than the direct cupellations.

During scorification there is also an appreciable loss of the precious metals by volatilization, which is absent in the crucible assay. This, in the case of telluride or zinciferous ores, may become so great as to put scorification out of the question.

OTHER ERRORS. — Retention of Lead in Cupelled Beads. — Small quantities of lead are almost invariably retained in the gold and silver beads with ordinary temperatures of cupellation. Hillebrand and Allen, 1 in two careful experiments on sets of three beads, approximately together 90 mgs. gold, found that 0.30 per cent, and 0.37 per cent., respectively, of lead were retained. This retention of lead cannot be corrected by leaving the bead in the muffle for some length of time after the blick, as this is, of course, prohibitive in the case of silver, and in the case of gold seems to actually cause an increase of weight. It has already been stated that copper and tellurium are very apt to be present in the final bead, when in the ore in any appreciable quantity. The retention of base metal by the bead causes a plus error in silver, but will not effect the result on gold unless the parting is by H,S0 4 ; and where the weight of the bead is multiplied by a factor to get results per ton, the final error in silver may be very appreciable. The presence of copper in the final bead practically insures the complete removal of the lead.

In order to show what is usually termed "fine silver " the following analysis of Government fine silver is appended.

Ag, 99.929%; Cu, 0.056%; Pb, 0.003%; Au, 0.007%; As, 0.001%; Sb, 0.002%; Fe, 0.001%; Zn, trace. 2

Retention of Silver by the Parted Gold. — Ordinary parted gold, after the proper treatment with weak and strong acid, retains from 0.05 to 0.10 per cent, of silver. In the assay of gold bullion after the first acid treatment of the quart ation alloy, the gold on the average retains 0.25 per cent, silver. After the second acid treatment, the final silver retention is from 0.06 to 0.09 per cent., depending on the time of boiling. If the amount of silver to gold in the quartation alloy is less than 2.5 to 1, somewhat more than the above amount of silver will be retained. 3

Silver can, practically, be completely extracted by more than

Bull. No. 253, U. S. G. Survey.

*Min.Ind., XV, 545.

Rom, "Metallurgy of Gold," p. 453.

two treatments with acids, according to Hillebrand and Allen. 1 In the ordinary assay for ores as usually darned out, it is safe to assume that some silver is invariably retained by the gold, and frequently much more than is supposed; however, with lowgrade ores, this retention is negligible.

Solution of Gold by Acid. — It is essential that the nitric acid used for parting be free from impurities, especially from hydrochloric acid and chlorine; otherwise solution of gold is sure to follow. Gold is quite soluble in mixtures of hot sulphuric and nitric acid, 3 and is again precipitated by dilution.

According to Hillebrand and Allen, 8 nitrous acid (HNO a ) and mixtures of HN0 8 and HN0 3 do not dissolve gold, though there is much earlier literature to the contrary. Nitrous acid has frequently been considered in this connection, as it is formed to some extent by the action of HNO s on silver.

According to Rose, 4 some gold is dissolved by nitric acid on continued boiling to constant gravity of acid. This solution is placed in the bullion assay at 0.05 per cent, or 0.5 parts per 1000. Hillebrand and Allen state that the loss of gold by solution is very small and irregular. It may be disregarded in the ore assay. The solubility of gold in HNO s is readily demonstrated when large quantities of gold are used. F. P. Dewey 5 in careful experiments showed the solution of gold to the extent of 660 mgs. per liter of of cone, acid, on boiling about 25 grams gold for two hours. He states that the temperature (120° C.) required to boil cone, acid has as decided an influence as the strength of the acid.

Occluded Gases. — Parted gold beads and " cornets" retain about twice their volume in occluded gases after annealing. The principal gas is stated to be carbon monoxide. Two volumes amount to 0.02 per cent, by weight, which is already allowed for in the silver retention.

Errors in Weighing. — The best scales are accurate to 0.01 mg., and scales can be obtained weighing to 0.005 mg. This last is used in assay offices, where great accuracy is required, on such material as bullions, rich mattes, etc. It is usually an unnecessary refinement in the ordinary ore assay, for the reason that the probable error in the assay is greater than this.

i Bull. 2.53, U. S. G. Survey. '

2 Lenher, in "Jour. Am. Chenu Soc," XXVI, 552

3 Ibid.

4 Ibid., p. 507.

5 Jour. Am. Chem. Soc, XXXII, 318.

Errors In The Assay For Gold And Silver 173

The errors in the assay for gold and silver may be summarized as follows:

1. Losses by absorption in the slag of the fusion.

2. Losses by volatilization during fusion.

3. Losses by absorption during cupellation.

4. Losses by volatilization during cupellation.

5. Errors by gain in weight of bead, due to retention of foreign elements. This affects results on silver chiefly.

6. Errors in weight of gold after parting by the retention of silver and occluded gases.

7. Losses of gold by solution in nitric acid.

The chief losses are Nos. 1 and 3, which can be recovered by "corrected assay," i.e., by re-assay of slag and cupel, to the extent of about 80 to 85 per cent. Wherever considerable accuracy is required, corrected assays should always be made. The losses by volatilization are usually slight, although from the foregoing data these are sometimes seen to be considerable. The retention of foreign metals by the bead is a plus error in favor of silver, and the retention of silver in the parted gold is a plus error in favor of gold. Silver losses are considerably greater in magnitude than gold losses. The total amount of precious metal recovered by the assay varies with the nature of the material. Designating the total amount of gold and silver in an ore or product as 100, the corrected assay will show from 99 to 99.8 per cent, of the gold, and from 98 to 100+ per cent, of the silver, the high silver result in some cases being due to retention of foreign metal.

In the bullion assay for gold, the algebraic sum of the errors outlined, the losses being designated minus and the gains plus, is called the "surcharge." In the gold bullion assay this will vary from +0.025 per cent, in very pure gold bullion, to —0.25 per cent, in base bullion, passing to zero for a bullion about 800 fine.

Chapter Xii The Assay Of Bullion

GENERAL. — Bullion is classified as follows:

1. Lead bullion, usually the product of the lead blast-furnace; 95 per cent, and more lead, containing some copper, antimony, etc., silver and gold.

2. Base bullion, containing from 100 to 925 parts of silver per 1000, gold in varying amounts, and a large percentage of base metals, chiefly copper, zinc, lead, etc. Produced most frequently by cyanide mills.

3. Dore* bullion, containing 925 to 990 parts of silver per 1000, some gold, and base metals, mostly copper, but also lead, antimony, zinc, etc.

4. Fine silver bullion, free from gold, containing 990 and more parts silver per 1000, but some base metals, usually copper.

5. Silver bullion, containing little base metal and less than half its weight in gold.

6. Gold bullion, containing little base metal and more than half its weight in gold.

7. Fine gold bullion, free from silver, containing from 990 to 1000 parts gold per 1000.

Silver and gold in all bullions but lead bullion are estimated in parts per thousand, and bullion is said to be so many parts fine. Thus, if 1 gram (1000 mgs.) of bullion is taken for assay and it contains 925 mgs. gold, it is said to be 925 fine.

In the assay of gold bullion the "miHime" system of assay weights is used, a millieme being 0. 5 mg., and the assay is reported in parts of 10,000, or the fineness with one decimal added. Thus the above bullion would be reported as 925.0 fine. In this system the 500-mg. weight is stamped 1000, the 250-mg. weight 500, etc. The scales used must therefore be sensitive to 0.05 mg., or 0.1 millieme. This presents no difficultyvj ordinary assay balances are sensitive to 0.01 mg. with a 0.5 gram.

Lead bullion is recorded in oz. per ton, in tt for ores.

THE ASSAY OF LEAD BULLION.— The sample of bulHon may be melted under charcoal and granulated in cold water, or it may be rolled out into a strip in the rolls, and the pieces cut at intervals from this for the sample. If lead bullion is free from copper, antimony, zinc, sulphur and arsenic, etc., it may be cupelled directly for gold and silver. In this case, 4 portions of 0.5 assay ton each are wrapped in about 7 grams of sheet lead, placed in the hot cupels, and cupelled with feathers. The cupels are fused with the following charge:

Stained part of cupel SO grama PbO 15 grams Na,CO a

45 grams Iiurax glass 2 grams argol Thin litharge cover

The buttons from this fusion are cupelled and the weight of the gold and silver added to that obtained from the first cupellation.

If the bullion contains base metals which will influence the results of the eupellation, 4 portions of either 0.5 or 1.0 assay ton are weighed out and mixed with 30 to 50 grams of test lead; 1.5 grams of borax glass and 0.5 gram of silica are put on top of the lead and the charge scorified. The resultant buttons, which should Weigh about 15 grams, are then cupelled. The scorifier slag and cupel are re-assayed by the above charge and the correetion added.

THE ASSAY OF SILVER BULLIOH 1 (also applicable to Base Builion, Dort Bullion, etc.). COPELLATION METHOD.— This method i.- wed :i an approximation for bullions in which silver is to be determined accurately, serving as a preliminary assay for the salt titration, mint, or Gay-Lussac method.

(a) PrelimiiiaTij Assay.— Exactly 500 mgs. of bullion are wmghed out on an assay balance in order to save calculation, wrapped in 10 grams of sheet lead, and cupelled at 850°. C, or with ample feathers of litharge. The silver bead is cleaned. weighed and parted in 1 to 9 HN0 3 for at least 20 minutes; then, if any gold shows, heated for 5 minutes more in concentrated washed, and the gold dried, annealed and weighed. The mnt of found, subtracted from the weight of the bead, the approximate silver, and the weight of the bead, subfrom the amount of bullion taken [500 mgs.). give-the

iuc nf mIvw l.nlJi „,,. "! iSulil Bullion." Inlrr iii this I I.

base metal. This base metal is usually copper, and its presence? may be detected by the coloring of the cupel.

(b) Making the Check Assay. — As the loss of silver and gold is a question of temperature, amount of precious metal present, amount of lead of cupellation, and amount and kind of base metal present, it is desirable to have the regular cupellation, accompanied by a check assay, made up as nearly as possible to the composition of the bullion to be assayed, and cupelled under the same conditions. The check assay is therefore made up from data obtained in the preliminary assay. As the silver determined in this preliminary assay is low, due to absorption and volatilization, a correction of 1.2 per cent, is added as an approximation or, rather, the amount of Ag found is considered as 98.8 per cent, of that present, and this amount of proof silver weighed out. To this is added, in proof gold, the amount of gold found in the preliminary assay. The difference between the sum of the corrected silver and the gold, and 500, is the amount of base metal to be weighed out for the check. As already stated, the base metal is usually copper, and in making up the check c.p. sheet copper is used. The check thus weighs 500 mgs. and approximates very closely the composition of the bullion. Duplicates of 500 mgs. of bullion are now weighed out, and these and the check each wrapped in the proper amount of sheet lead, as determined from the table below:

Table Xxxix.— Lead Ratio In Cupellation

Fineness in

Amount of

Amount of lead

silver

copper present

for

cupellation

Ratio of lead to base metal

Milliemes

Milliemes

Grams

72tol

60tol

nrp l

il.'U'Pll 111 51 1

onrncc iht

muffle, so as to be exposed as nearly as possible to the same

The Assay Of Bullion 177

temperature, and three more cupels are placed near them to act as covers for the cupellation when finished, in order to prevent sprouting. When the cupels have had all volatile matter expelled the assays are dropped into them, the check in the center one, and the cupellations carried on in the usual way, with feathers. After the blick, the cupels are drawn to the front of the muffle and covered with extra cupels. Sprouted buttons must be rejected. The beads are now cleaned, weighed, and rolled out, parted in flasks, with the acids as described for the preliminary assay, and the gold weighed.

The difference between the silver actually used in the check and that found by assay is the correction to be added to the mean silver result of the two bullion assays made, which should not differ by more than a millieme (0.5 point fineness). This correction may be plus or minus, according to the amount of copper in the bullion; for with much copper, some of this may be retained by the silver and give rise to a minus correction. The gold is corrected in the same way as the silver. The subtraction from 500 of the sum of the corrected silver and gold gives the amount of base metal. The individual results obtained, express the assay results in fineness.

When metals of the platinum group are present, the method must be modified as outlined, in Chapter XIII, for the assay of platinum, etc.

WET METHODS: GAY-LUSSAC OR MINT METHOD.— This method is a most accurate one and is based on the complete precipitation of Ag as AgCl in a nitric acid solution by means of sodium chloride. The reaction is as follows:

AgNO, + NaCl AgCl + NaNO, 1 part Ag 0.54207 NaCl

The standard solution of NaCl usually employed is of such strength that 100 c.c. precipitate 1 gram of Ag, so that 5.4207 grams of c.p. NaCl are dissolved per liter of distilled water to give the standard solution. This solution can also be made up by using a saturated salt solution at 60° F., and then adding 2.07 parts of this to 97.93 parts of distilled water. The last method of obtaining the solution is not as good as the first, owing to the difficulty of obtaining the precise temperature of 60° F. and keeping it there. Aside from the standard solution mentioned, there is required another of one-tenth its strength (obtained

by taking 1 part of the standard NaCl solution and adding to it 9 parts of distilled water), and an acidulated solution of AgXO s , obtained by dissolving 1 gram of proof silver in 15 c.c. of HNO tT 1.26 sp. gr., and diluting with distilled water to 1000 c.c. It follows from the above that 1 c.c. of the one-tenth solution will just precipitate the Ag in 1 c.c. of the acidulated silver nitrate solution.

The standard NaCl solution is termed the "normal salt" solution in the assay, although not properly so; the weak solution is termed the "decimal salt solution," and the silver nitrate solution the "decimal silver" solution.

Standardizing Solutions. — The apparatus required is:

1. A large bottle or carboy, containing the normal salt solution placed on an elevated shelf so that the solution may be siphoned by means of glass tubing and rubber hose to the main 100-c.c. pipette.

2. Liter bottles containing respectively the decimal salt and the decimal silver solutions.

3. An accurate 100-c.c. pipette, clamped to a suitable stand, and provided at the top with a glass overflow-cup containing a moistened sponge to catch the overflow of the normal salt solution.

4. Two small graduated 10-c.c. pipettes, one for the decimal salt and one for the decimal silver solution. Burettes may be used in place of these.

5. A number of strong 8-to 12-oz. bottles, similar to reagent bottles, provided with rubber corks.

The standardizing of solutions is carried out as follows: Two portions of exactly 1002 mgs. proof silver are dissolved in 15 c.c. of 1.26 sp. gr. HNOg, the nitrous fumes are removed by boiling, the solution is transferred to the titration bottles and water added to bring up the amount of solution to 125 c.c. The 100-c.e. pipette is then filled with normal salt solution to the mark, after washing out with salt solution to prevent dilution. The filling is done by fastening the siphon hose to the bottom of the pipette, opening the clamp on the hose, and letting the pipette fill, with a little overflow. The solution is then shut off by clamping the hose, a finger placed on the top opening of the pipette to prevent the solution running out, and the hose removed. The pipette is then permitted to drain to the 100-c.c. mark, and the solution held there by closing the top of the pipette with the finger. The

The Assay Of Bullion 179

lie containing the dissolved proof silver is then placed under pipette and the normal salt solution permitted to completely n into it. The bottle is then violently shaken for three or minutes, either by hand or a mechanical agitator, and the allowed to settle, leaving the supernatant liquid clear. If normal solution is made up correctly, it will have precipitated i, 1000 mgs. of silver, leaving 2 mgs. unprecipitated. One c.c. decimal salt solution is now added to the bottle by means of "? of the 10-c.c. pipettes or a burette, which, if the solution ill contains Ag unprecipitated, gives rise to a white cloud of rCl. The bottle is again shaken, the precipitate allowed to 'ttle, and another c.c. of decimal salt solution added. If this tils to give a precipitate, then 100.1 c.c. of normal salt solution re equivalent to 1002 mgs. of silver (1 c.c. of decimal salt solution -0.1 c.c. normal salt solution). If the second addition of t.'cimal salt solution gives a precipitate, the shaking and settling ..re repeated, and a third and fourth, etc., addition made, until no . urther cloud appears. The assayer soon learns to judge by the lensity of the cloud whether only part of the c.c. has been used up. In this way he should be able to judge to the fourth of a c.c. or the half of a millieme. If the first addition of decimal salt solution fails to give a precipitate, the normal solution contains an excess of salt, and 2 c.c. of decimal silver solution are now added, one of which neutralizes or precipitates the 1 c.c. of decimal salt solution added, the other acting on the excess of salt in the solution. The decimal silver solution is added until no further cloud appears, in the same way as described for the decimal salt solution. In this way the exact strength of the normal salt solution is determined in duplicate. If it is incorrect to the extent of more than 2 points fineness either way (i.e., either strong or weak), it is corrected by the addition of either water or salt, and restandardized, and, when correct, a new decimal salt solution made up from it. Its strength is finally recorded on the bottle as follows: 100 c.c. 1000 mgs. Ag, or whatever it may actually be.

The Assay. — It is evident from the preceding that the amount of bullion to be taken for assay must contain as nearly as possible 1000 mgs. Ag in order to make the titration with solution as short as possible, and avoid undue additions of the decimal solutions. For this reason the bullion on which the silver determination is to be made is first assayed by the cupellation method,

or at least a preliminary assay, described under this method, is made, and from these data the amount of bullion containing 1000 mgs. of silver calculated. For instance, suppose the cupellation method shows the bullion to be 900 fine in silver, then

fineness : amt. of bullion :: silver : amt. of bullion.

or 1111.11 mgs. bullion contains 1000 mgs. Ag. This amount of bullion is then weighed out in duplicate and dissolved in acid, placed in titration bottles, as described above, under " Standardization of Solutions," and titrated.

The calculation for fineness is as follows: Suppose the strength of the normal solution is 100 c.c. 1001 mgs. Ag, and that 99.8 c.c. of normal solution were used in the titration (100 c.c. normal salt, and 2 c.c. decimal silver) ; then

the x, or amount of silver in bullion, equaling 998.99 mgs.; and the fineness is

the y, or fineness, equaling 899.1.

The only metal interfering with the salt titration is mercury, which will be precipitated by the NaCl as Hg a Cl 3 ; the addition of 20 c.c. sodium acetate and a little free acetic acid to the assay will prevent the precipitation of the mercury. Mercury can be detected in the titration if the AgCl has not turned dark as the result of exposure to sunlight. Mercury will be found sometimes in mill bullions which have been retorted at too low a temperature. The assay and standardization of the solution should be carried out where there is no sun, and where light is not too strong.

THE ASSAY OF GOLD BULLION FOR SILVER BY A WET METHOD. — The accurate estimation of silver in bullions containing a large proportion of gold is not all that can be desired by the ordinary fire method. The Gay-Lussac method is generally not applicable on account of the large amount of bullion that must be taken for a sample in order to get 1 gram of silver. The following wet method 1 will yield good results. Take 0.5 gram of the bullion, fuse with 1.5 gm. of pure cadmium under a cover of potassium cyanide in a porcelain crucible in the flame of a blast lamp. Enough cyanide must be used to

1 E. H. Taylor, Australian Mining Standard, August 26, 1908, 235. Consult also J. E. Clennel, Eng. and Min. Jour., LXXXIII, 1099.

The Assay Of Bullion 181

cover the cadmium. Five minutes is sufficient to insure fusion. Allow to cool, place in stream of running water which will rapidly dissolve the cyanide and leave the alloy. Transfer this to a flask with 20 c.c. of water, add 40 c.c. of HNO, in installments of 10 c.c. each while boiling for one hour. Dilute to 150 c.c. and add 10 c.c. of ferric alum indicator and titrate with the standard solution of NH 4 CNS. - This solution is made as follows: 1.6 grams of pure NH 4 CNS are dissolved in 1000 c.c. of distilled water. This is standardized against pure silver foil dissolved in HNO s and diluted to 150 c.c. 1 c.c. of the solution equals approximately 4.483 parts of Ag per 1000 under the conditions described above. The indicator is a saturated solution of ferric alum. The appearance of the red color marks the end point. Copper in amounts of 100 parts per 1000 in the bullion does not interfere with the delicacy of the end point. In case the bullion is very high in gold the cadmium must be increased. The parted gold is recovered from the residues in the flasks.

THE ASSAY OF GOLD BULLION. 1. Sampling.— Bullion bars and retort sponge, as shipped to the United States assay offices and mints, is remelted into bars to make the deposit uniform. These are sampled by taking chips from diagonally opposite corners, each of which is rolled into a fillet and assayed by different assayers, who are required to check with each other within narrow limits; if they do not, the bar is remelted, stirred thoroughly, and recast; then sampled again and assayed. If base bullion, or one which liquates seriously on cooling, is to be assayed, dip-samples are taken from the molten bullion by means of a small graphite ladle, and the sample granulated in warm water. Silver bullion is sampled in the same manner.

2. Preliminary Assay. — This is made in the way described for silver bullion, except that in the assay of gold bullion no determination of silver is made by cupellation; but if this is to be determined, the mint wet method is used. Experienced assayers can judge the approximate fineness of gold bullion by the color, and add the proper amount of silver necessary to insure parting. In the San Francisco mint, 2 parts of Ag to 1 of Au are used. 1 The British royal mint formerly used 2.75 parts of Ag. to 1 of Au, a but now uses 2 to 1. More than 3 parts Ag

John W. Pack, "Assaying o! Gold and Silver in U. S. Mint", in Min. and Set. Press LXXXVII, 317. Rose, in Bng. and Min. Jour., LXXX, 402.

The Assay Of Bullion 183

is retained by the gold bead to toughen it, aud it can be easily rolled without Tracking, if, between reductions by the rolls, the iilict is annealed at n dull-red heat. The presence of copper in the button aids in the total removal of lead during the cupellation. 1

The fillet is then again annealed and rolled into a spiral, railed a "cornet," and parted in a parting flask. This is rilled with 30 c.c. of HNO, sp. gr. 1.20, free from CI, H.SO„ HjSO,, fir any sulphide, and heated to boiling (or at least 90° C.) for 20 minutes. The acid is then decanted off, and the cornet washed carefully several times with hot distilled water by deeautation. Then 30 c.e. of boiling nitric acid, sp. gr. 1.30, are added to the flask, and the cornet boiled again for 20 minutes, after which (he acid is decanted, and the washing with hot water repeated. During the boiling, a parched pea added to the flask prevents bumping. The flask is now filled to the very top with cold distilled water, a suitably sized porcelain parting-cup placed over the mouth, fitting reasonably tight, and the flask inverted. The cornet will settle into the parting-cup, and the flask is then gently tipped to permit the water to escape, the water is decanted from the parting-cup, and the cornet gently dried. When dry, the cornet is transferred to a clay annealing cup, the cover is put on, and -the cup is placed in the muffle, and the comet annealed at a full-red heat. It is then weighed. The weight of the gold plus that of the added silver, subtracted from the weight of the cupelled bead, gives the approximate amount of silver in the assay. This added to the weight of the gold and subtracted from 500 mgs. (the weight of bullion taken) gives the approximate amount of base metal. If the amount of silver added to part the gold has raised the ratio of Ag to Au over 3 to 1, the gold will probably have broken up, or at least parts will have broken from the edges of the cornet; care must, in this case, be taken to collect all of it in the washing. If the results show that the ratio of Ag to Au has been less than 2 to 1, the cornet must be recupelled with 2.5 parts Ag and parted as described.

The Assay.— The final assay is made up from data obtained in the preliminary assay. Duplicates on 1000 milliemes are run, with a check assay made, up in compositions near to that of the bullion as possible, as described for the cupellation assay of silver. In making up the cheek, proof gold and proof silver are

' Row, Befinit* CJold Bullion," in I. M. M., April 13. 1005.

used, and c.p. copper foil. The United States mints use various proof alloys in the making up of check assays. For the assay of fine gold bars (990 fineness and above), a proof alloy of 1000 gold, 2000 silver, and 30 parts copper is used. For coin metal (900 parts fine), a proof alloy of gold 900 parts, silver 1800 parts, copper 100 parts is used. For the determination of base metal (the difference between the gold and silver, and the 500 mgs. taken for assay), a proof alloy of gold 900 parts, silver 90 parts, copper 10 parts is used. 1 In this last the gold need not be proof gold, but may be remelted cornets. It is to be noted that these proof alloys are made up on the assumption that 2 parts of Ag to 1 of gold are used in parting. The British mint uses a proof alloy, or trial plate, 916.6 fine in gold.

For the assay of crude gold bullion, i.e.,. mill bullion, the proof alloy for fine gold bars is generally used.

The amount of lead used in the cupellation is as follows: 2

Table Xl.— Lead Ratio In Cupellation

Amount of gold per . , - . ,

i A Amount of lead

1 Ratio of lead to copper i (base metal present) Milligrams Grams '

96tol

68tol

48to 1

38tol

27tol

To the duplicates of the 1000 milliemes of bullion, the proper amount of Ag is added, to bring the ratio of Ag to Au to 2 to 1, and then they are wrapped in the proper amount of c.p. sheet lead. The check is made up as indicated by the preliminary assay, and the three assays cupelled as described for the assay of silver bullion. The three beads are then treated and parted, as described for the preliminary assay. The two bullion assays should not differ by more than 0.25 part of a millieme. The

1 John Pack, ibid.

2 Rose, "Metallurgy of Gold," 1902, p. 494.

The Assay Of Bullion 185

correction as indicated by the check should then be applied, whether this be plus or minus. The difference between the fine gold in the check and that obtained by the assay of the check is the surcharge, which is more definitely defined in Chapter XI, on "Errors in the Assay for Gold and Silver." This surcharge will usually amount to about for a bullion of about 700 to 800 fine; above that there will be a "plus surcharge/' and below that a " minus surcharge." The plus surcharge will be subtracted and the minus surcharge added.

THE PREPARATION OF PROOF GOLD.— This is prepared by dissolving practically pure gold (cornets) in nitro-hydrochloric acid, permitting the solution, after some dilution, to stand for four days to allow AgCl to settle out. It is then decanted very carefully by siphoning. The gold chloride solution is then evaporated almost to dryness, taken up with plenty of distilled water, a few c.c. of NaBr or KBr solution added, allowed to stand for some days, and again decanted by siphoning, after which operation it is slowly dropped from a burette into a beaker containing c.p. aluminium foil. When precipitation is complete, HC1 is added to dissolve the excess of Al, and the residual gold is washed thoroughly with water by decantation, and then dried and melted into a bead in a fresh cupel (but not cupelled with Pb) . The gold is then rolled into a thin strip for use. 1

Proof silver is prepared by dissolving c.p. silver foil in HN0 8 , and then precipitating with HC1 after filtering. The AgCl is thoroughly washed with diluted HC1 and converted into metallic silver by Al in the presence of HC1, all Al being dissolved out. The washed silver is then fused in a porcelain crucible, and rolled into strips. 3

Consult also Rose, "Metallurgy of Gold," p. 10, and Pack, ibid. 2 John Pack, ibid.

Chapter Xiii

THE ASSAY OF ORES AND ALLOYS CONTAINING PLAT- INUM, IRIDIUM, GOLD, SILVER, ETC.

Materials containing some of the above elements are presented to the assayer for determination in the shape of sands containing chiefly platinum, alloys and jewelers' sweeps, and, more rarely ores containing platinum in the form of the mineral sperrylite, etc.

The assay for platinum and associated metals is a difficult one, due to the fact that in the parting of the precious metal beads, by acids, complex reactions take place, by which platinum, palladium, silver, etc., both go into solution and are retained in the residue, unless certain well established ratios of metals present are observed and the parting operation repeated several times. The alloys of platinum and silver have been most thoroughly investigated in this connection. 1 When the alloy is more complex, i.e., contains also gold, palladium, iridium, rhodium, etc., the difficulties of the assay are increased; the data at present available are meager.

Platinum nuggets from the Urals contain: 2 Pt, 60 to 86.5 per cent.; Fe, up to 19.5 per cent.; Ir, up to 5 per cent.; Rh, up to 4 per cent.; Pd, up to 2 per cent.; also Os, Ru, Cu, Au, and iridosmium.

When material containing Au, Ag, Pt, Pd, Ir, Rh, Ru, Os, and IrOs is fused by the crucible assay or melted with lead, the Au, Ag, Pt, Pd, Ir, Rh, IrOs are collected by the lead and the Ru, and Os only partially so. If the resultant lead button is cupelled, the final bead will contain the Au, Ag, 8 Pt, Pd, Ir, Rh, IrOs, and a comparatively small portion of the Os and Ru, the most of these two metals being lost by oxidation. The presence of any considerable amounts of Os and Ru in the lead button,

1 Thompson and Miller, in Jour. Am. Chem. Soc, XXVIII, 1115. See this paper for other references.

2 Kemp, in Eng. and Min. Jour., IJCXIII, 513 (Notes on Platinum and Associate Metals).

8 Exclusive of losses by absorption and volatilization.

The Assay Of Ores And Alloys 187

owing to the fact that they will not alloy readily, causes them to appear as a black scum or as spots on the bead, near the end of the cupellation. The presence of the platinum group of metals, raising the melting-point of the gold-silver alloy, renders necessary a high temperature of cupellation in order to remove lead. Even then, when the ratio of Ag to Pt, etc., is less than 5 to 1, lead will be retained in varying proportions at the cupellation temperature of gold bullion. To get rid of the lead, the proportion should be 10 to l. 1 The following points on the first cupellation of the lead buttons, resulting from the assay of material containing Pt, etc., will give the assayer an idea of what is present. When Pt alone, or with very little silver is present, the bead from the cupellation (at a comparatively high temperature) is rough, dull gray, flat, and contains lead.

If more silver is present, but less than 2 parts of Ag to 1 of Pt, the beads are rough, flat, and have a crystalline surface.

If more than 2 parts of Ag are present and not more than \o, the bead approaches more nearly the appearance of a normal silver bead, but has a more steely appearance and is flatter in proportion to the Pt, etc., contained.

Beads containing more platinum than 1 in 16 will not blick or flash. 2

The effect on the appearance of the bead of Pd, Rh, Ir is similar to that of Pt, but not identical.

Owing to the difficulty in alloying iridium, this, when present, is apt to be found at the bottom of the bead, in the shape of fine black crystalline particles. 3

THE ACTION OF ACID ON THE ALLOY BEADS.— A great deal of literature exists on this point; but most of it is very conflicting; some facts, however, have been definitely established.

Nitric Acid. — In an alloy of Pt and Ag treated by HX0 3 , platinum goes into solution in various proportions, depending on the ratio of Ag to Pt, and probably to some extent on the strength of acid. It has been stated that when the ratio of Ag to Pt is 12 or 15 to 1, this solution of Pt is complete in one treatment, but this has been disproved by later investigation. 4 In order to accomplish the solution of Pt, the acid treatment

1 Sharwoc* f! ™llation on Platinum Alloys, containing Ag and An," in Jour. Soc. Chem. S.

VIII, 397. Id," p. 514. Jour. Am. Chem. Soc, XXVHU115.

luiiUil LUillj. 1 .-

The Assay Of Ores And Alloys 189

aqua regia, 1 to 5, leaving Ir, IrOs, and Rh, and some Ru and Os, if present. This last residue, treated with strong aqua regia, removes Ir, leaving iridosmium and rhodium as a final residue.

METHODS OP ASSAY. 1. Ores. — Rich ores, carrying Pt, etc., in grains, present difficulty in sampling, inherent to any ore containing " metallics. " It is best to take from 30 to 50 grams of the sample and fuse it with 6 times its weight of lead in a crucible, fluxing the gangue. The lead is poured, and after cooling the slag is detached carefully, the lead platinum alloy being brittle, weighed and remelted under charcoal in order to insure a uniform alloy, and then granulated as fine as possible by pouring into a large volume of cold water from a considerable height. The resultant sample iB then dried and is ready for assay. An amount containing approximately 200 mgs. Pt is weighed out and scorified with 50 grams Pb into a 20-gram button.

If, in the low-grade ores, the Pt, etc., is present as grains, a weighed quantity is concentrated by panning and the concentrates scorified with 20 to 25 times their weight of test lead, and the button treated according to method No. 1 or 2, as below. If the ore contains the rare metal in other form, crucible fusions are made on 1 assay ton, as with gold and silver ores, and if very low grade, the buttons from 4 to 5 fusions are scorified into one button, final duplicates being made as usual. The lead buttons are treated as below.

2. Alloys. — An amount of drillings or filings (representing a true sample of the alloy), containing, if possible, not to exceed 200 mgs. of Pt, etc., is weighed out and scorified with 80 grams of test lead, to a button of about 18 to 20 grams. The lead buttons are treated as outlined below.

First Method. — The lead button obtained by any of the foregoing methods is cupelled at a temperature of at least 900° C, or, better, 950° C, and the resultant bead examined. If, from the foregoing description of the appearances of a bead, it is thought that the ratio Ag to Pt, Au, etc., is less than 10 to 1, the button is removed, the necessary silver added to bring it up to the above ratio, recupelled with 5 to 8 grams of lead at a temperature of 900° C, and weighed. The bead is then flattened and rolled out into a cornet, if large and not too brittle, and parted with 15 c.c. H 2 S0 4 concentrated, boiling for 15 to 20 minutes. The acid is then decanted into a beaker and saved,

the residue re-treated with 5 c.c. more of acid for 10 minutes, and the residue and acid washed into the beaker containing the first acid. The acid is then diluted and the residue separated by filtration through a small ashless filter, and thoroughly washed with hot water to insure removal of Ag 2 S0 4 . The filter-paper is dried and carefully transferred to a porcelain parting-cup or an annealing cup, and the carbon burnt off in the muffle. The annealed residue is brushed out on the scale pan of the bead balance and weighed. It consists of gold, platinum, iridium, iridosmium, rhodium, and possibly osmium and Ru (if any escaped oxidation during the cupellation), and perhaps some palladium. Its color will be gray or black, if the rare metals are present to any extent. If not, the characteristic gold color will show. The palladium is largely in the filtrate. (It is questionable how complete this solution is. 1 ) If it has been unnecessary to add Ag to the cupellation to get the 10 to 1 ratio, the difference in weight between the original bead and the weight of the residue represents the Ag. If silver had to be added and the bead recupelled, the weight of the added silver plus that of the residue, subtracted from the weight of the recupelled bead, gives the silver. Allowance must, however, be made for considerable loss of silver as a result of high cupellation temperature. If accurate silver results are required, a duplicate assay on the material must be run, and the silver requisite to bring the ratio up to 10 to 1 is added at once to the lead button, one cupellation only being made. At the same time this is run, a check assay is run beside it, made up of the same weight of lead, and the proper weight of silver, i.e., the amount added tq the first cupellation plus the amount approximately known to be in the assay. The loss in this will give the correction to be added to the assay for Ag. It may be desirable to determine Ag in the wet way. (See " The Assay of Silver Bullion. ")

The residue is now wrapped in 8 to 10 grams of lead foil with at least 20 times its weight in silver and cupelled again at a high temperature. The bead, if large, is rolled out and heated to boiling in a mattrass or flask for 20 minutes with HN0 8 , sp. gr. 1.20, after which the acid is decanted into a beaker, and the treatment repeated with HN0 8 of 1.26 sp. gr. The residue, if finely divided, should now be filtered through an ashless filter and washed as already described. If not, the filtrate can be

1 Ricketta and Miller, in "Notes on Assaying," state that the Pd dissolves with the Ag.

The Assay Of Ores And Alloys 191

decanted and the residue. washed. The residue consists of Au, Ir and iridosmium, and some Rh and Ru. If there is a suspicion that any platinum, etc., remains, the residue must be re-treated with acid until of constant weight. The platinum is in the filtrate, which will be colored brown or black.

The difference between the weights of the first and second residues is platinum, the result possibly being somewhat high if palladium is present in the material assayed. The second residue is now warmed in a mattrass with dilute aqua regia 1 (1 to 5) for 15 minutes. This dissolves the gold, some of the Ru and very little Rh, leaving the Ir, iridosmium and Rh, with some Ru. The residue is either filtered or decanted, as necessary, dried, annealed, and weighed. The difference in weight between the second and third residues represents gold, somewhat high, if the Ru has partly escaped oxidation and volatilization during cupellation. The gold can be recovered by precipitation with oxalic acid, as described in the second method.

If the third residue is treated with strong aqua regia, and boiled, it dissolves out the iridium, leaving as a residue the iridosmium and most of the Rh. This is dried, annealed, and weighed, the difference in weight between the third and fourth residues representing iridium, and the weight of the fourth residue representing iridosmium and Rh. The method determines Ag, Pt, Au, Ir, and iridosmium plus Rh. The probable errors in the determination have been pointed out. Palladium can be satisfactorily determined only by wet analysis.

Second Method. 2 — Take the lead button from the ore or alloy assay, and scorify at a high heat, with additional test lead, if necessary, to a weight of 8 to 10 grams. It should contain less than 5 per cent. Pt. etc., in order to be malleable. Roll out the button into a long thin fillet and place in a large beaker with 200 c.c. of HNO s , sp. gr. 1.08, s and heat until all action ceases. Filter through a small ashless filter and wash the residue with hot water. Dry the residue and paper, transfer to a large-size parting-cup and ignite in the muffle, to burn off the carbon, and oxidize any Pb not dissolved. Then heat to boiling in the cup with HN0 8 , 1.08 sp. gr., decant, wash thoroughly with hot water, dry, anneal, and weigh the residue. This consists of Au, Pt, Ir, iridosmium, and most of the Rh, as well as the Ru and Os which

1 Concentrated aqua regia is 1 part HNOa, sp. gr. 1.42, and 3 parts HC1, sp. gr. 1.20.

3 E. H. Miller, in School of Mine* Quart., XVII, 26.

81 parts distilled HsO to 19 parts HNOi cono. (sp. gr. 1.42).

i

the residue re-treated and the residue ami the first acid. The separated by filtratim oughly washed with The filter-paper is : the muffle. Tinpan of the bead b:.' num, iridium, irid Ru (if any escape some palladium metals are pre.- color will show questionable liunnecessary tu the different i- of the residue : the bead rem:- the residue. gives the .-il\ siderable loss If accurate material inu-' up to 10 to I onlv beinn is run beside proper wcig! lation plus r i The loss in : : for Ag. It (See "The A The resid' at least l'm high tcinpei to boiling i? sp. gr. 1.20. I he irealmei if tinelv divi' and washed

Hi. mv< fiii'l V

*?r- - CHAPTER XIV

OF TIN, MERCURY, LEAD, BISMUTH AND Y*Urite— ANTIMONY

of ores for base metal by fusion is still carried out in J- L.;_ .. pp 0C a iiy f or i ca d anc j tin. The fire assay gives, not

n li -r-pt metal content, but the yield obtainable in smelting, - -'h in metallurgic operations the yield may be greater or '' Tho smelter, therefore, purchases lead, tin, and copper

iiJ ' (he basis of the "dry" or fire assay. The fire assay of

- 1 ' is practically no longer in use, except in part of the Lake district, on metallic copper concentrates, and in purcopper ores the assay is made by the standard electromethod, or a volumetric method, and a percentage of 1 to 1.5 deducted to indicate dry assay. The usual dcducis 1.3 per cent. Thus the dry assay of copper on an ore is !\-:ilcnt to the percentage obtained by the electrolytic method 1 .3 per cent. hile wet methods, with a deduction, will in all probability employed eventually for all lead ores, as it is now for impure id ores, pure lead ores are still assayed by the fire method. m ores are almost invariably assayed by the fire method, as the ..ot analysis of tin is long and tedious.

THE ASSAY OF TIN ORES.— The fire assay of tin ores is applicable only to those ores in which tin exists as cassiterite, the oxide (SnOj). The chief reasons for inaccuracies in the fire assay o/ tin are:

1. Some of the tin, reduced in the assay from the oxide, is apt to be volatilized at the temperatures necessarily employed.

2. Metallic tin may be slagged by alkaline carbonates used in some of the methods of assay, forming stannates.

3. Foreign metals present in the ore arc apt to be reduced and enter the button.

4. Sulphides present carry tin into the slag. If sulphates are present, they are reduced to sulphides.

5. Silica and silicates, always present in the ore, even after very careful concentration, carry tin into the slag, as silicate.

while the Sn0 2 passes through the lower stage of oxidation in being reduced to metallic tin.

6. The cassiterite, before reduction, is apt to combine with basic fluxes present in the assay, and be carried into the slag as stannates.

From this, therefore, it is evident that the fire assay for tin is only an approximation, although in many cases a very close one. If the result on a tin ore by the fire method checks that of the standard wet method (the modified Rose method 1 ), it is to be ascribed to a balancing of errors, due to the presence of other metals in the ore, which have been reduced into the tin button.

Preparation of the Ore for Assay. — It is essential to remove all the gangue of the ore and have for the assay nothing but the cassiterite, as far as this is possible. The ore is roughly crushed on a buck board and put through a 40-mesh screen, crushings and screenings succeeding each other at frequent intervals in order to avoid the " sliming " of the cassiterite. If the ore is low-grade, i.e., below 2 per cent. Sn, 1000 grams of the crushed ore is weighed out and carefully panned in a gold pan, the first pannings being saved for repanning. The ore is concentrated just as much, as possible without incurring loss of cassiterite. The concentrates from the repanning of the tailings of the first treatment are added to the main lot of concentrates. Some or all of these will, unless the ore is very pure, contain probably garnets, feldspar, tourmaline, magnetite, zircons, wolframite, columbite, sulphides, quartz, etc. The concentrates are carefully transferred to a porcelain dish, dried, and roasted at a bright-red heat in order to decompose sulphides and sulphates. While the concentrates are still red-hot, they are transferred into a beaker containing water in order to make garnet and other silicates soluble (all except uvarovite), and after decanting water, treated with nitro-hydrochloric acid to remove most of the contaminating minerals, except quartz, wolframite, and some garnet. The concentrates are then filtered off and dried. If quartz is present, this can be removed by transferring the filtered concentrates to a platinum dish and treating with HF. This, however, will rarely be necessary. The concentrates are then crushed in an agate mortar to pass a 100-mesh screen and treated as described below.

The Assay. — The two best methods for assay are the cyanide

i Hofman, "The Dry Assay of Tin Ores," in Trans. A. I. M. E., XVIII, 1.

with black flux substitute. s generally to be preferred, as rite have less influence on the ilization is reduced to a miniti'iiiperaturc employed, is essential to use only the purest. he cyanide process. Such impurities as hides in cyanide cause serious losses in line cyanide to use is sodium cyanide, procured at the present time. Some of I liyanide known as "potassium cyanide" Bperaturc that the concentrates sink to s before reduction, and when reduction p little globules of tin are found to be very i order that the fusion may be successful, I directions closely. It is best to use 10 . amount near that; usually llie % obtained from the concentration of the if the proper amount of ore is chosen for wo grams of powdered cyanide are firmly 20-gram crucible, the concentrates are mixed ! of cyanide, placed in the crucible, and with ;") grams more. The crucibles are placed in (lie r n n full-red heat (750° C), and are kept at this temper-about

15 to 20 minutes. The charge will become very :iiul will be a brown-red. The temperature should not - . high as to cause the cyanide to boil and evolve heavy It may, however, be kept too low, in which case the i ni<:nl reactions will not complete themselves and the tin .1 fail to collect into a button. If the concentrates still coin some foreign minerals, the fusion takes longer than 20 .iiinutes. The crucibles are then withdrawn, cooled, anil the Imttori recovered by breaking the crucible. There will be two distinct slags, the lower one. surrounding the button, usually light green, amorphous and suhtrunslucciit. and the upper one or fused cyanide, opaque, milk-white and coarsely granular, soluble in water. The tin button should be while and soft; if not, h contains foreign metals.

The German JMft/.--Tho Unman method is based mi ihu

1 Holm*", ibid

fusion of the cassiterite concentrates with charcoal and black flux substitute, which has the composition, 2 parts K 2 CO s , 1 part flour. Five grams of the concentrates are intimately mixed with 1 gram of pure wood charcoal and put into a No. D lead crucible or an ordinary 20-gram crucible. On top of this are placed 15 grams of black flux substitute, with which 1.25 grams borax glass have been mixed. Finally a pure salt cover is added, and a piece of charcoal, the crucible covered with a clay cover, placed in the muffle, and heated at a moderate heat until boiling of the charge has ceased, and then for one-half to three-quarters of an hour more at a white heat. The crucible is then removed from the muffle, allowed to cool, and broken for the tin button. This should be white and soft, as in the cyanide fusion.

During the fusion, as the temperature rises, the charcoal reduces the stannic oxide to metallic tin, while any ferric oxide is reduced to ferrous oxide, if the heating is gradual, and is taken up by the slag. As the temperature rises, the flour in the black flux substitute partially decomposes, liberating carbon throughout the charge, which, as fusion takes place, prevents any stannic oxide not as yet reduced from uniting with the alkali of the flux. The slag, after cooling, should be crushed and panned for any prills of tin which have not entered the button. These are weighed and added to the weight of the button.

Results Obtainable. — Black Hills cassiterite concentrates, roasted, quenched, and treated with nitro-hydrochloric acid. 1

Wet method of Rose-Chauvenet

with K 2 CO a =67.84 per cent. Sn

German method 67 . 58 per cent. Sn

Cyanide method 67 . 49 per cent. Sn

Stream tin from Durango, Mexico, 2

German method 63 . 92 per cent. Sn

Cyanide method =65. 19 per cent. Sn

It is to be noted that while the dry methods approach very closely to the wet analysis, which gives the actual tin in the ore, the dry assay results are due more or less to a balancing of errors. Frequently dry assays will give higher results than the analysis; this is due usually to reduced iron.

1 Hofman, ibid.

3 E. H. Miller, "The Assay of Tin Ores," in "School oj Mines Quart.," XIII, No. 4.

The Assay Of Tin, Mercury, Lead, Etc. 197

Of the influence of foreign minerals left in the cassiterite concentrates, quartz has the worst, causing heavy losses. Feldspar and tourmaline have similar effect, but not to so marked a degree. Mica and garnet give high results, due to the reduction of iron, although tin is lost in the slag. Columbite acts in a similar manner. With the German method the result is much more seriously affected by these impurities than with the cyanide fusion. 1

THE ASSAY OF MERCURY. — Mercury occurs in ores chiefly as cinnabar (HgS), and may with accuracy be determined by Chism's method. 2 For low-grade ores, the method is especially satisfactory, and has the advantage of being rapid and short. It is based on the fact that mercury is distilled from HgS, etc., in the presence of iron filings, and can be caught on silver-foil. The difference in weight between the mercury-impregnated silverfoil and the foil before the assay gives the mercury. The apparatus required is as follows:

3. A piece of carefully annealed silver-foil 1.5 in. square, which is fitted and bent down to make a reasonably tight cover for the annealing cup.

4. A flat silver or copper dish, holding 20 to 25 c.c. of water. A silver crucible may be used in place of this.

5. A piece of asbestos board, 4 in. square and about 0.20 in. thick, in the center of which a circular hole has been carefully cut, into which the annealing cup will fit so as to project about 0.5 in. below the bottom of the board.

6. A small alcohol lamp, of about 60 c.c. capacity.

7. A wash-bottle with cold water, and a glass tube for a siphon. The silver-foil is carefully fitted over the top of the annealing cup, the edges being bent down so as to make a closefitting cover and prevent the escape of mercurial vapor. The silver dish should be polished on the bottom, and be in close contact with the foil, so that the cooling effect of the water will be fully transmitted.

The Assay. — For low-grade ores from 0.5 to 1 gram is taken and mixed with from 30 to 50 parts of iron filings. These filings

1 Hofman, ibid.

2 R. E. Chiam, in Trans. A. I. M. E., XXVIII, 444.

Consult also, G. A. James, Eng. and Min. Jour., XC, 800 and W. W. Whitton, Calij Tech. Jour., Sept., 1904; M in. lnd. t XVII, 751.

should all pass a 40-mesh screen. A select lot of filings are best digested with alcohol for some time to remove oil and grease, then heated in a muffle to a dull-red heat for 10 minutes, cooled, and stored in a tight bottle. It is essential to have the filings free from oil and grease, else this will be deposited on the silverfoil with the mercury. The amount of mercury in the ore should not be so great as to cause too heavy a coat on the silver-foil. For high-grade ores, not more than 0.1 to 0.2 gram should be used. Very small amounts of mercury can be detected by this method.

The ore, mixed with filings, is placed in the annealing cup, ►which is set into the asbestos board on the ring-stand, the silverfoil weighed accurately, after igniting, to within 0.1 mg., and fitted to the cup, and the silver dish, filled with cold water, placed on the foil. The alcohol flame is then allowed to play just on the bottom of the cup, but not to spread around the sides. The flame should be about 1.25 in. high and is best shielded by a screen to steady it. The bottom of the crucible should not become more than a dull red, otherwise mercury will escape condensation. The time of heating should be from 10 to 15 minutes. It is best to heat for about 10 minutes, then cool, and reheat for 3 to 5 minutes. Longer heating than this causes loss of mercury. The degree and time of heat are very important.

During the heating the water in the dish should be replaced once or twice. It can easily be removed by a bent tube that has been filled with water, acting as a siphon. While the warm water is being removed, cold water is added from a wash-bottle. After the proper heating, the alcohol lamp is removed, the assay allowed to cool somewhat, the silver dish removed, and the silver-foil with the mercury transferred by forceps to a desiccator and then weighed. The difference in the weight of the foil after and before the assay is the weight of the mercury, from which the percentage is calculated. The foil can be used again after driving off the Hg at a red heat in the muffle, or with a Bunsen burner. A piece of foil can be used about six times. It should be weighed before each assay. The method also serves as a very sensitive and easily applied qualitative test on ores.

The following figures will serve to show the accuracy of the method: 1

CJ X. lWhi'Mer. in "School of \Iin,s Qu.trt." XXIII.

The Assay Of Tix, Mercury, Lead, Etc.

By Electrolysis prom Cyanide

Solution Ore No. 1 12.37 per cent.

Ore No. 2 67.26 per cent.

Bi Ch ism's Method 12.44 per cent.

The accompanying illustration (Fig. 60) shows the apparatus employed.

THE ASSAY OF LEAD ORES.— The fire assay of lead ores will probably pass out of use in time, just as the fire assay of copper has done. At the present time it is still largely used, although for complex ores containing much copper or bismuth, or antimony with the lead, it is not in vogue. It is, however, still the criterion

Mekcuhi A

in the purchase of pure sulphide and oxidized lead ores, and also such complex ores as furnished by the Leadville, Colorado, district. Unoxidized ores of this type contain pyrite, blende, galena, some little chalcopyrite and gangue. Oxidized ores contain cerrusite, anglesite, calamine, limonite, etc., and gangue. The object of the assay is to bring the lead of these ores down into a button, free from other base metals, such as Cu, Zn, Bi, Sb, Fe, and free also from S and As. The loss of lead by volatilization and slagging and the reduction of base metals should be kept to a minimum. As already stated, this is a difficult thing

to do; so that pure ores will invariably give low results, and impure ones high.

There are three methods of assay, differing in the flux used; (1) the lead flux method; (2) the soda-argol method; (3) the cyanide fusion. Of these, the lead flux method is chiefly used throughout the West. The soda-argol method is a good one on ores not basic. The cyanide method is only applicable to pure ores. With impure ores it tends to reduce other base metals, due to its powerful reducing action. Various mixtures of lead flux are used, of which three are made up as follows:

No. 1 No. 2 No. 3

4 parts NaHCO, 2 parts NaHCO a 6 . 5 parts NaHCO,

4 parts K,CO, 2 parts K a CO, 5 parts K 2 CO,

2 parts flour 1 part flour 2 . 5 parts flour

1 part borax glass 1 part borax glass 2 . 5 parts borax glass

Flux No. 3 is probably the best for most purposes, as determined on a series of ores, the results with it being slightly higher. 1 For assay, 10 grams of ore (100-mesh fine) are mixed with 30 grams of flux, placed in a No. 6 or D crucible, or in a 20-gram crucible, covered with 8 grams more of flux, and put into the muffle at a low heat, which is then raised to a light yellow (1080° C). The fusion should take about 30 to 35 minutes. Nails are added to the charge, two tenpenny nails for heavy sulphides, one for light sulphides or oxidized ores. When the charge is taken from the muffle, the nails are removed from the crucible by a pair of short hand tongs, care being taken to wash off all adhering lead globules. The crucible is then shaken and tapped thoroughly, and poured. The lead buttons are cleaned by hammering and then weighed. The percentage is obtained by multiplying by 10.

The reactions in the crucible are as follows: 7PbS + 4K 2 CO s 4Pb + 3 (K 2 S, PbS) + K 2 S0 4 + 4C0 2 K 2 S, PbS + Fe Pb + K 2 S + FeS 2PbO + C 2Pb + C0 2

The carbon liberated in finely divided particles from the flour on heating reduces any lead oxides or carbonates in the ore, while the iron reduces lead from its sulphides and sulphates. The assay should check (in triplicate) within 0.5 per cent.

1 McElvenny and Izett, in "The Chemic.il and Fire Methods of Determining Lead Ores/ Min /2ep.,XLVIII, 26.

The Assay Of Tin, Mercury, Lead, Etc.

The soda-argol method uses the following flux:

NaHCO, 6 parts

Argol 1 part

For 10 grams of ore, 35 grams of flux are taken, with a light flux cover. The fusion is performed as described for the lead flux method. The method is good on ores containing some silica, but not on basic ores or pure galenas, as all acid is lacking in the flux. A borax glass cover is best where the method is employed on basic ores.

In the cyanide method, pure cyanide should be used, and the temperature should be kept much lower than for the other two methods. For the regulation of temperature, reference is made to the assay of tin by the cyanide fusion.

For the fusion, 10 grams of ore are mixed with 35 grams cyanide, and a light cyanide cover used. Concerning the accuracy of the method the following figures are appended: 1

Ore

Fire

Per cent.

Per cent.

4. Pyrite, Sphalerite, Galena

THE ASSAY OF ANTIMONY AND BISMUTH ORES.— For accurate and satisfactory determinations on these ores, wet methods must be resorted to. Antimony occurs chiefly as the sulphide, stibnite, although the oxides and some native metal are found as ore. Bismuth as an ore occurs chiefly as the native metal, but is found also in combination with oxygen, sulphur, etc. For the assay, the following charge is best:

Ore 10 grams

Cyanide 40 to 50 grams

Cover of cyanide.

Fuse at a full red heat, as given for tin, for 30 minutes. The resultant buttons are brittle and cannot be hammered.

1 Determination of Lead in Ores, I. T. Bull, School of Mines Quart., XXII, 348.

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A Manual Of Fire Assaying

Table Xliii.— Fineness Of Bullion And Alloys Of

Precious Metals

Denomination

Equivalent in

Milliemee or Parts

Per Thousand

One carat.

One grain per marc. .

24 carats — 1 pound troy (England) 24 carats — 1 mark (Germany, etc.) 4,608 grains — 1 marc ol 8 ounces (France, [ Spain, etc.) 8 ounces — 1 marc (France, Spain, etc.) 16 loth — 1 mark (Germany, etc.)

Table Xliv — Volume And Weight Of Fine Gold And

Silver

One Cubic Centimeter

One Cubic Inch

One Cubic Foot

Fine Silver:

Weight: grams

Weight: troy ounces.

Fine Gold:

Weight: grams. Weight: troy ounces.

Value: U. S. dollars I12.82 57

Value: pounds sterling.

The foregoing tables are due to Mr. W. J. Sharwood and were first published in Mines and Minerals, XXIX, 250.

Baes of Computation. — The gram is taken as 15.4320 grains. The value of a troy ounce of fine gold is assumed as being exactly $20.67, instead of $20.6718346+, resulting in an error of less than one in 10,000. Values in English coin are based on the assumption that an ounce of fine gold is worth 4.25 pounds sterling, or 85 shillings, 1,020 pence; this is too high by about one part in 2,000, the true value being 1,019.45 pence. It is useless to attempt a closer approximation in practical work, for the simple reason that gold bullion assays are rarely reported closer than the nearest half millieme, or to within one part in 2,000. At the values adopted one dollar is equivalent to 4.1 1224 shillings, and one pound sterling to $4.86353.

Foreign and Obsolete Values. — The adarme (27.7 grains or of the Spanish ounce), sometimes used by Mexicans colloquially

Appendix 205

and especially with reference to placer work, is about 1.8 grams, which in fine gold would be worth $1.20. For practical purposes, however, an adarme of ordinary gold may be taken as equivalent to $1, and this exactly true for gold 830 fine.

Russian reports state values in zolotniks per 100 poods, but for low-grade placer deposits doli per 100 poods are used. As a dola is tj zolotnik, we may take T of the values given in the table for the zolotnik, without serious error.

Marcos per cajon were formerly used in some South American countries, one marco per cajon apparently ranging between 100 and 70 parts per million. One oitavo per quintal corresponds practically to 2 ounces per ton.

The loth per centner, used in the older German works, corresponds to one part in 3,200, which is nearly 9 ounces per short ton or 10 ounces per long ton. In some cases there seems to have been a considerable variation from this ratio, the value being sometimes taken as one part in 3,520, or of that used in these tables — the centner being then assumed as 110 instead of 100 pounds. The quentchen was i, and the denar of the loth.

Index To Authors

Name Paob

Allen, E. T., see Hillebrand and Allen.

Ames and Bliss 48

Anderson, O. A 104

Austin and Hunter 48

Bachelder, G. N 198

Bailar, J. C 101

Balling, C 69

Bannister and Stanley 103

Barton, W. H 157

Bettel, W 77

Bowman, F. C 3

Bowman and Mason 160

Brunton, W. D 36

Bull, I. T 201

Bullens, D. K . 147

Carter, T. L 145

Chiddey, A 157

Chism, R. E 197

Clark, A. J 156

Clennell, J. E 30, 180

Collins 94

Crawford, C. H 129, 168

Day and Allen 30

Day and Shepard 29

Dewey, F. P 172

Doeltz, 28, 90

Doeltz and Graumann 64, 90

Donnan and Shaw 83

Eager and Welch 162, 164

Edmands, H. K 79

Flinn, F. B 127

Friedrich, K 84

Fulton, C. H 86, 104, 129, 134, 137, 160, 167, 168, 169

Godshall, L. D 161, 162

Gottschalk, V. H 42

Guertler, W 29

Hall, E. T 149

Hawley, F. G 108, 149

Hempel, A 31, 77

Hemtz, F 76

Hillebrand, W. F., and Allen .128, 132, 137, 164, 165, 166, 167, 171, 172

Hofman, H. O 117, 194, 195, 196, 197

Holloway and Pearse 133

Name Paci

Holt and Christensen 79, 106

Howe, H. M 94, 102

Hunt, F. F 126

Huntoon, L. D 40

Izett and McElvennv 200

James, G. A 39, 197

Janin, Jr., Louis 6

Jolly, H. R 159

Kaufman, W. H 160, 162

Keller, Edward 19, 41, 107

Kemp, J. F 186

Kerl, B 76

Kitto, Wm 146

Koenig, G. A 1

Lay, F 144

Lenher, Vict 172

Liddell. D. M 41, 47, 105, 162

Lodge, R. W 118, 119, 147, 161, 164, 167, 168, 1SS

Mason and Bowman 160

McCaughey, W. J 129

Merritt, J. W 79, 106

Miller, E. H. . . . 54, 59, 116, 120, 160, 168, 169, 186, 187, 188, 191, 196

Mostowitsch, Wl 28, 64, 65, 90, 91

Nutter, E. H 6

Orton, E 24

Ostwald, W 48

Pack, J. W 181, 184, 185

Perkins, W. G 116, 140, 142

Roberts, G. M 41

Roberts-Austen 29, 93

Roos, A. T 153

Rose, J. G 33

Rose, T. K 94, 99, 107, 124, 134, 143, 144, 164, 165, 171, 172, 181,

Sander, K 99, 146

Schnabel, C 94

Schiffner, M 187

Sharwood, W. J. 154,187,188,202

Shepard and Day 29

Schorlemmer and Hoscoo 64

Sieverts and Hagenacker §3

Smith, E 99

Smith, E. A 146

Smith, F.C 101,137

Smith, S. W 133, 134, 136

Sulman, H. L 146

Taylor, E. II . . 180

Thompson, T 186, 187

Index To Authors 209

Name Page

Vail, W. G 149

Van Liew, R. W 126

Van Nuys, C. C 164

Vogt, I. H. L 67

Wallace, R. C 67

Warwick, A. W 37

West, E. E 68

Whitby, A 157

White, W. P 33, 54

White and Taylor 102

Whitton, W. W 197

WilliamB, J. D 143

WilliamB, K 117

Woodward, E. C 121, 131, 167

Wraith, W 41

Wright, L. T 37

Index

A

Page

Accuracy of the gold-silver assay 173

Alternate shovel method of sampling 38

Alumina 31

Amalgamation Test 154

Analysis of bone ash 76

of copper matte 142

of hematite 74

of fine silver 171

of fire clay for crucibles 25

of lead-antimonial ores 73

of limestone 74

of sheep and cattle bones 76

of silidous ores 73

Antimony, behavior of, in roasting 114

ores, assay of 201

Antimonial ores, method of assay of 146

Argol 31

reducing power of 53

Arsenic, behavior of, in roasting 114

Arsenical ores, method of assay of 146

nickel-cobalt silver-ores 147

method of assay of r 147

Assay balance 48

reagents 28

valuations, table of 203

Assaying, definition of 27

Assay-ton system of weights 51

B

Balances 42

construction of 42

for weighing pulp 49

non-column type of 49

practical notes on 47

theory of 44

Basic ores 62

Bismuth ores, assay of 201

Black flux 32

substitute 32

Paoe

Blister Copper, method of assay of 125, 126

sampling of 41

Bone-ash, analysis of 76

screen-analysis of 77

Borates 30

Borax 29

glass 29

influence of on assay 30, 138

melting point of 30

Boric acid 29

Bullion, assay of 174

classification of 174

fineness of 174,204

Burners, Case type 15

consumption of oil by 15

for gasolene 13

size of 14

for oil 4

for gas . . . 17

pressure of gasolene in 14, 15, 17

Cary burner 14

Case gasolene burner 15

Charcoal 32

reducing power of 54

temperature at which reaction begins with 59

use of, in roasting 114

Check assav 176, 184

Chism'8 method for mercury 197

Coning and quartering, sampling by 37

Control assays 39

Combination method of assay 125, 127

for cyanide precipitates 129, 144

precautions to be observed in 128

Copper mattes, method of assay of 127

results on 141

method of assay of blister 125, 126

sampling of blister 41

Copier-bearing material, assay of 125, 127.. 139, 140, 142, 151

losses in assav 169

Cost of fuel for assaying 3, 5, 6, 8, 15

Crucibles 24

Battcrsea 25

capacity of . . 25

domestic 25

fire-clav for 25

Index 213

Page

Crucible assay, methods 113

theory of 63

charges for quartz ores 74

for basic ores 70

Cupels, assay of 140, 158, 175

comparison of bone ash and magnesia 104, 105

cost of, 79

influence of shape of 78

of bone ash 76,103

of magnesia 79,103,104

of Portland Cement 79, 105, 106

properties of 77,78,103

Cupel-charging device 21

Cupel machines 77

Cupellation, appearance of beads from 84

when platinum is present 187

"flash" of beads after 94

formation of "feathers" during 82,93

"freezing" of lead-button in 88,92

influence of copper on 100, 101

of antimony on 102

of foreign metals on 81, 92, 98

of tellurium on 101,102,133

losses in 160

nature of 76

process of 80

retention of lead in beads from 171

silver and gold absorption during 78, 104, 105, 106

sprouting of silver after 83

surfusion of silver during 82, 93, 94, 97

temperature of 81, 86,93

uncovering of lead-button in 88

Cyanide 32

of "Potassium ' 32

of sodium 32

Cyanide method for antimony ores 201

for bismuth ores 201

for gold and silver ores 120

results by 120

lead ores 201

tin ores 195

Cyanide Solutions, assay of 156, 157

D

£ Errors in the assay for gold and silver 173

Page

Excess litharge method 116,142

results obtainable from 117, 118

F

Ferric oxide 31

Flour 32

reducing power of 54

Fluor spar r . . 31

in the reassay of cupels 158

Furnaces, capacity of 2, 5. 6, 15

combination pot and muffle 10

for assaying 1

for burning coal 2

coke 6

gas 17,18

oil 4,17

wood 5,6

fuel consumption in 3, 5

gasolene-fired 12

temperature attainable in 15

Furnace tongs 18, 19

tools 18

annealing-cup tray 23

cupel-charging device 21

cupel-tray 23

molds , 23

multiple scorifier tongs 19

G

Gasolene as fuel in assaying 12, 15

German method of assay for tin 195

Gold, losses of during cupellation 164

occluded gases 172

preparation of proof 185

silver retained by after parting 171

solution of by acid 172

weight and volume of fine 204

Gold bullion 174

assay of 181

for silver in 180

surcharge in assay of 173, 185

ores containing "free " gold, assay of 152,154

-silver alloys, losses during cupellation of 164

Graphitic material, method of assay for 145, 146

H Hematite, effect of in ores 62

Index 215

Page

Impurity, definition of Ill

Impurities in ores 111,112

Inquartation 108

Iridium, determination of 191, 192

effect of adds on 188

in platinum nuggets 186

Iron, assay of metallic 159

nail method 118

reactions in 119

J Jones riffle sampler 39, 40

Lead 31

bullion, assay of 175

sampling of 41

flux 32,200

ores, assay of 199

fluxes used in assay of 200

reactions during assay of 200

results obtained in assay of 201

silicates 65

formation of in roasting 115

melting-point of 91

reduction of 55

Lime 31

Litharge 28

melting-point of 28, 90

reduction of 53, 55, 64, 135

required to fuse metallic oxides 117

silver and gold in 34, 35

Luting material 9, 11

M

Matte produced in crucible assay 111,112

Mercury, assay of 197

results obtained in 199

Method of assay for antimony 201

for bismuth 201

for copper bearing material 125, 127, 139, 151

for cyanide solutions 156, 157

for free gold ores 152, 154

for lead 199

for mercury 197

Page

Excess litharge method 116, 142

results obtainable from 117,118

F

Ferric oxide 31

Flour 32

reducing power of 54

Fluor spar r . . 31

in the reassay of cupels 158

Furnaces, capacity of 2, 5. 6, 15

combination pot and muffle 10

for assaying 1

for burning coal 2

coke 6

gas 17, 18

oil 4. 17

wood 5, 6

fuel consumption in 3, 5

gasolene-fired 12

temperature attainable in 15

Furnace tongs 18, 19

tools 18

annealing-cup tray 23

cupel-charging device 21

cupel-tray 23

molds 23

multiple scorifier tongs 19

G

Gasolene as fuel in assaying 12,15

German method of assay for tin 195

Gold, losses of during cupellation 164

occluded gases 172

preparation of proof 185

silver retained by after parting 171

solution of by acid 172

weight and volume of fine 204

Gold bullion 174

assay of 181

for silver in 180

surcharge in assay of 173, 185

ores containing "free" gold, assay of 152,154

-silver alloys, losses during cupellation of 164

Graphitic material, method of assay for 145, 146

Hematite, effect of in ores 62

Page

for platinum ores 189

for telluride ores 131

for tin 193

Methods of assay 113

combination method 125, 127

comparison of 121

cyanide method 120

iron nail 118

Miller's oxide slag method 116

niter method 115

niter-iron method 120

Perkins 1 excess litharge method 116

roasting method 113

soorification method 122

Miller's oxide slag method 116

Molds 23

Muffles, method of support of S

size of 8

N

Niter . 33

iron method of assay 120

manner of action of 59

method of assay 115

oxidizing power of 57, 58, 60

against various reducing agents 59

variation of oxidizing power of 60

Nitric acid, action of on platinum alloys 187

used in parting 107

, Nitro-hydrochloric acid, action of on platinum alloys 188

O

Oxidation 57

metals, sequence of 124

Oxidizing power of an ore 62

Palladium, behavior of during assay 190

effect of acids on 187

Parting 107

precautions during 108

ratio of gold to silver necessary 107, 108, 181

strength of acids used in 107

Inoix

t*latinum alloys, behavior of with ,

and allied metals, behavior of during

ores, assay of . .

methods of assay of Potassium carbonate

Preliminary assay

Pyrite, reducing power of

It

Reagents for assaying

Reducing agents

amount of load reduced by

power of an ore .

Reduction

Roasting ...

dishes

Salt

Sampling . .

by alternate aim vol methods

by coning and quartering

by machine . .

of copper and lead bullion

of gold and silver bullion

salting during

Stale ores, mcllmilK of unsay of Scorifi cation

application of method

errors in method , .

for copper bearing material . .

for line ores

process of

Scorifiers

eapacitj' of , .

Selenium gold ores

Silica

Silver, behavior of in roasting . . - -

bullion

cupellation method for

Gay-Lussac method for . . . .

gold alloys, losses during cupellatior

losses of during cupellation . . . .

method of adding to assay

Page

method for in gold bullion X80

preparation of proof 185

retained by parted gold 171

weight and volume of 204

Slags, assay 65

calculation of 59

color of 75

formation temperature of 67, 68

nature of 67

assay of 158

silicate degree of 66

Soda-argol method for lead ores 201

Sodium bicarbonate 29

carbonate 28

influence of on reduction of litharge 54

melting-point of 29

chloride 33

cyanide 32

sulphate, formation of in assay 54

Sprouting of silver beads 83

Sulphides, assay of 148

rapid method for 149

reducing power of 54, 55, 149

Sulphuric acid, action of on platinum alloys 157

Surcharge 173, 188

Surfusion of silver during cupellation 82, 93, 94, 98

Telluride ores, losses in assay of 167

method of assay of 131

complex 137

Tellurium, effect of, on assay 133

elimination of during cupellation and scorification 133

quantity of, in ores 136

Temperature color scale 102

Test lead " 31

Tin ores, assay of 193

inaccuracies in 193

cyanide assay for 194

German method of assay for 195

results obtained from 196

Tongs 18,19

multiple for scorifiers 19

Umpire assay 40

Index 219

W

Page

Weighing 45

errors in 172

Weights, assay ton system of 51

conversion table for 202

for assaying 49

millieme system of 174

platinum 49

rider 49

standardized 50

Z

Zinc, behavior of, in assay 143

Zinciferous ores, assay of 143

crucible method for 144

losses in assay of 168

r

M

J