The Cyanide Process for the Extraction of Gold ... Second edition, enlarged

Front matter

THE CYANIDE PROCESS FOR THE EXTRACTION OF GOLD BY THE SAME AUTHOR. THE METALLURGY OF GOLD. A Practical Treatise on the Metallurgical Treatment of Gold-bearing Ores. Including the Processes of Concentration, Chlorinatmn, and Extraction by Cyanide, and the Assaying, Melting, and R. fining of Gold. By M. Eissler.M.E. Fourth Edition, Enlarged. With about 250 Illustrations and numerous Folding Plates and Working Drawings. Large crown 8vo, 16s. Cloth. "The work is a storehouse of information and valuable data, and we strongly recommen-i it to all professional men engaged in the gold-mining industry." — Mining Journal. THE METALLURGY OF SILVER. A Practical Treatise on the Amalgamation, Roasting, and Lixiviatton of Silver Ores. Including the Assaying, Melting, and Kenning of Silver Bullion. By M. Eissler, M.E. Third Edition.

Crown 8vo, 10s. 6d. Cloth. "A practical treatise, and a technical work which we are convinced will supply a long-felt want amongst practical men, and at the same time be of value to students and others indirectly connected with the industries." — Mining Journal. THE METALLURGY OF ARGENTIFEROUS LEAD. A Practical Treatise on the Smelting of Silver-Lead Ores and the Refining of Lead Bullion. Including Reports on vaiious Smtlting Establishments and Descriptions of Modern Smelting Furnaces and Plants in Europe and America. By M Eissler, M.E. Crown 8vo, 400 pp., with 183 Illustrations, 12s. 6d. Cloth. "The numerous metallurgical processes, which are fully and exten sively treated of, embrace all the stages experienced in the passage of the lead from the various natural states to its issue from the refinery as an article of commerce.*' — Practical Engineer.

A HANDBOOK ON MODERN EXPLOSIVES. A Practical Treatise on the Manufacture and Use of D}n;imite, Gun- Cotti.n, Nitro-Glycerine and other Explosive Compounds, including Collodion-Cotton. With Chapters on ExpV sives in Practical Application. By M. Eissler, M.E. Second Edition, Enlarged. With 150 Illustrations. Crown 8vo, 12s. 6d. Cloth. " Useful not only to the miner, but also to officers of both services to whom blasting and the use of explosives generally may at any time become a necessary auxiliary." — Nature. LONDON: CROSBY LOCKWOOD AND SON. [Frontispiece* Cyanide Plant (Upper flow os Vats) of the May Consolidated Gold Mining Company, erected by the Rand Central Ore Reduction Compary. THE CYANIDE PROCESS FOR THE EXTRACTION OF GOLD AND ITS PRACTICAL APPLICATION ON THE WITWATERSRAND GOLDFIELDS AND ELSEWHERE BY M. EISSLER MINING ENGINEER ; A.I. M.E. J MEMBER OF THE INSTITUTE OF MINING AND METALLURGY AUTHOR of " the metallurgy of gold," etc., etc. jjjieronrj «Rfttion, (Unlarjjrti LONDON CROSBY LOCKWOOD AND SON 7, STATIONERS' HALL COURT, LUDGATE HILL 1898 [All rights reserved']

Preface

TO THE SECOND EDITION. The continued success of the Cyanide Process for the Extraction of Gold, with the ever-increasing number of mines at which it is employed, has naturally led to a large increase in the number of people interested in the process, and hence the demand for a new and enlarged edition of this work. Mr. Eissler, who is busily engaged in mining operations in Australia, could not himself under take the task of revision, but as he has supplied the bulk of the new matter which has been incorporated in this edition, my task as editor has been comparatively an easy one. In the new matter which has thus been added will be found descriptions of the working of the Cyanide Process in other parts of the world besides the Witwatersrand gold fields — notably at the Gibraltar Cyanide Works, New South Wales, and at Deeble's Cyanide Works, Victoria. An interesting account of the Cyanide Plant re-

PREFACE.

. VI cently constructed at Mercur, Utah, U.S.A., has come to hand in time for insertion in an Appendix (p. 127). I need hardly add that the task of preparing the present volume for the press, although light, has been an agreeable one, and I sincerely hope that this new edition will meet with a reception as favourable as that accorded to its predecessor. E. HENRY DAVIES, M.E., F.G.S. 6, Great Winchester Street, London, E.C. July, 1898. P.S. — For the photograph which has been re produced in the frontispiece to the present edition, illustrating a portion of the Cyanide Plant of the May Consolidated Gold Mining Company, I am indebted to Mr. Ad. Goerz, of London, the Chairman of the Rand Central Ore Reduction Company, who has also kindly contributed a brief article on the Treat ment of Battery Slimes.

A description of the Cyanide Plant mentioned above has been kindly supplied by Mr. Charles Butters. It is right that I should acknowledge here the service Mr. Davies has rendered in carefully carrying out the revision of the book for the present edition in accordance with my wishes. M. EISSLER. Sydney, New South Wales.

Introduction

TO THE FIRST EDITION. It is a great satisfaction to me to be in a position to place before the public, in the present volume, an account of the Extraction of Gold by the Cyanide Process, which I believe will be found by Metal lurgists who desire to avail themselves of the pro cess to be sufficiently full and complete for practical purposes. That I am in a position to do this, is owing to the fact of my having made a lengthened stay in the Witwatersrand gold fields — whence I have just returned — and to my having enjoyed there unique opportunities of studying the process in actual operation. Before entering upon an account of the process, some further introductory remarks will be advisable. Metallurgy, as applied to preparing and winning the gold from its ores, resolves itself, broadly speak ing, into two methods, in me of which mechanical

INTRODUCTION.

. VIII means, and in the other chemical means, are employed. I consider the stamping or crushing of the ore in a battery, and the catching of the gold on amalgamated copper plates, a mechanical process, although the recovery of the gold in chemical combination with mercury might entitle it to be considered otherwise. The whole process is nothing more or less than a simple system of concentration, as the gold, owing to its great specific gravity, separates from the lighter gangue in which it is enclosed ; and when the pul verized ore particles are washed away, the gold grains sink and are arrested on the amalgamated smooth surfaces of the plates over which the stream carries them, or get entangled in the hairs of the blankets, or are stopped in the riffles of the sluice box, or what ever other method is used in the battery to save the free gold.

When gold is in combination with other minerals, more complicated methods have to be employed to collect it by mechanical means, but these it will be unnecessary to describe in the following pages. There are very few chemical means by which gold can be won from its ores, leaving aside the smelting process by which the gold is collected in the smelting furnace by means of, and in combination

INTRODUCTION.

. IX with, other metals. The reason why gold is not easily won by chemical means is because it is a non oxidizable metal, which maintains its purity, and is therefore mostly found in its native or metallic state. Owing to its rarity it has become of such great value, and the medium of interchange since the remotest times. Its solvents are aqua regia, chlorine, and potas sium cyanide. The application of such a corrosive agent as aqua regia for the treatment of ores on a large scale is out of the question, and of the chemical means at our command there remain only the last two. By exposing the gold ores, or the concentrated portion of the same, to the action of chlorine gas, the gold is converted into a soluble chloride of gold by the well-known chlorination methods ; and by the other method the gold is dissolved in solutions of cyanide of potassium.

That gold when in a fine state of division was soluble in cyanide of potassium, was already known in the middle ages, and the gilding of metals was carried out in those remote days by jewellers and alchemists, by the use of gold in cyanide solutions. Of course, gilding by means of fire was usually employed.

INTRODUCTION.

. X Several scientific books, dating back to the begin ning of this century, mention the solubility of gold in potassium cyanide solutions. The application of this solvent for the treatment of auriferous ores was first thought of and patented in the United States in 1867 ; and although the process was tried and experimented with by some eminent metallurgists of that country, no practical or commercial results were obtained. Now, how is it — in spite of repeated failure in America and elsewhere — that within the last few years the cyanide process has come into such pre eminence, and as such a grand success, before the metallurgical world ? In answer to this question, I believe I am safe in stating that the ores of the Witwatersrand gold fields, where the cyanide process was first introduced, carry the gold in a pure and metallic state, in an extremely fine state of division (and even in the pyrites, the gold does not occur in chemical combination, but in a free state) ; and, therefore, all the conditions existed there to make the application of the cyanide process a perfect success.

These facts are not only of interest to the metallurgist, but they should also throw some light on the geological features and conditions under which these peculiar conglomerate beds were formed.

INTRODUCTION.

. XI The mines of this district, it should be said, will for ever have to acknowledge the immense services which the MacArthur-Forrest Company have rendered by developing and introducing the process, as they spent large sums to bring it into practical shape, and to demonstrate its commercial advantages. But for those exertions, enormous values would have remained practically unavailable, preventing many mines from working at a profit, and the production of these fields would not be what it is to-day by 50 per cent. In this connection I can mention that out of a total gold production of 1,478,470 oz. in 1893, there were produced from the tailings 330,510 oz. by the cyanide process ; and in August, 1894, out of the monthly pro duction of 174,977 oz., nearly 58,000 oz. were won by the cyanide process. [See Postscript on p. xiv.]

The great future and potentiality of development which may be anticipated for these gold fields is strikingly illustrated by the opinion of such an emi nent authority as Mr. Hamilton Smith, who (in his report to the house of N. M. Rothschild & Sons), has stated that that portion of the Witwatersrand which lies between the Langlaagte Block B and the Glen cairn Mine— or about nj miles in length, along the strike of the main reef series, — if exploited to a verti-

INTRODUCTION.

. cal depth of 3,000 ft., would yield in gold 215 million pounds sterling; while Bergrath Schmeisser (in his report made to the German Government), taking for estimate a depth of 800 metres, arrives at the yield of 208 million pounds, and for a depth of 1,200 metres, the yield of 349 millions. And when we consider that at least one-third of these prodi gious amounts will be won by the cyanide pro cess, one can hardly overestimate the importance of the work which the MacArthur-Forrest Company has done in bringing that process into its present position of prominence in the Witwatersrand gold fields. Certainly it cannot be that they are entitled to a fair reward for their labours. When I arrived in 1890 on the Witwatersrand gold fields, I undertook numerous experiments for the treat ment of tailings by pan amalgamation, but with un satisfactory results.

The difficulty I had to contend with was the formation of large quantities of iron amalgam in the pan. The quicksilver became dirty and floured, causing very heavy losses, and I had a costly process before me to separate the iron from the gold ; and although others followed me, and put up various devices for treating the tailings, it was only the advent of the cyanide process which solved this

INTRODUCTION.

. XIII most difficult problem, as pan amalgamation would have been too expensive. So far, the process has not achieved similar success in other countries, which to my mind proves that the ores on these fields contain the gold in a free condition ; and to further corroborate this view, I can state that at the Simmer and Jack mine the pyritic concentrates which are daily collected on the blankets yield by pan amalgamation 65 per cent. Of course these blanketings contain a considerable amount of free gold which escapes from the copper plates. Within the last few months, Messrs. Siemens and Halske have successfully introduced on the same fields their patented process, which consists in precipi tating the gold by electricity on sheets of lead ; and, owing to certain economical advantages, it may be anticipated that their process will prove a formidable rival to its predecessor.

In the succeeding chapters will be found an account of the working details of these processes, including some of the observations and researches of those gentlemen who have been most prominent in bringing them to their present state of perfection. I must not conclude these remarks without express ing my deep obligations to the several gentlemen —

INTRODUCTION.

. whose names will be found duly recorded in the following pages — to whom I am indebted for much of the material and information embodied in the volume, and for the opportunities so freely afforded me, during my stay in the Witwatersrand gold fields, of making myself practically acquainted with the working of the cyanide process. It is not myself only but all who are interested — whether as metallurgists or investors — in the gold-mining industry, who have thus been laid under obligation ; and as I have taken no small pains to put to good use the material and opportuni ties which have been so freely placed at my disposal, I trust that the outcome of my efforts in the present volume will prove thoroughly acceptable to that numerous body. M. EISSLER. 37, Belsize Park, South Hampstead, London, N.W. December, 1894.

Postscript. — In 1897 the total output of the mines in the Witwatersrand district was 2,565,164 ozs., of which, on the basis of the returns for December, 1897, the amount recovered by the Cyanide and the Siemens processes w7ould be equal to 32 per cent. of the whole. CONTENTS.

Appendix

. CYANIDE PLANT AT MERCUR, UTAH, U.S.A. . . .127 INDEX LIST OF ILLUSTRATIONS. PAGE FIG. Frontispiece — Cyanide Plant of May Consolidated Gold Mining Company. i. (Plate I.) Section of Princess Works . . facing 4 2, 3. (Plate II.) Mkssrs. Butters and Mein's Automatic Distributor facing 8 4, 5. Do. Do. (Size A) . . .9 6. (Plate III.) Tailing Wheel, Vanner Room, and Cyanide Vats at the Jumpers Mine .... facing 14 7. Staves cut to Circle 15 8. Construction of Filter Vats 16 9. (Plate IV.) Stone Foundation for Filter Vats facing 16 10. Solution Pipes 17 11,12. Butters' Discharge Lid 19 13. Zinc Precipitation Box 23 14. (Plate V.) Deeble's United Cyanide and Pyrites Works facing 26 15 — 24. Deeble's Patent Cyanide Pan .... 30—33 25. (Plate VI.) Zinc Precipitation Boxes at the Princess Works facing 40 26.

(Plate VII.) Electric Precipitation Boxes at the Worcester Works facing 44 27. Rough Sketch of Apparatus for the Treatment of Slimes at the Treasury Mine . . . . .49 28,29. (Plates VIII. & IX.) Worcester Cyanide Plant facing 68 30, 31, 32. (Plate X.) Depositing Box ... ,,70 11. (Plate XI.) General View of the Simmer and Jack Cyanide Plant facing 88 34. (Plate XII.) Simmer and Jack Filter Vats . ,, 90 35,36. (PlateXIII.) Simmer and Jack Extractor House ,, 92 37. (Plate XIV.) Central Works of the Rand Central Ore Reduction Company .... facing 94 38. (Plate XV.) Diagrammatic Sketch of Electric Precipitation Plant at Gibraltar Mine, N.S.W. facing 98 39—43. Cyanide Plant at Mercur, Utah, U.S.A. 129 — THE CYANIDE PROCESS FOR THE EXTRACTION OF GOLD.

Chapter I. Erection Of

A CYANIDE PLANT. Planning the Works. — In the erection of a cyanide plant, before planning the same, some essential points have to be considered. Are the works to be erected to treat an old accu mulated stock of tailings ? Or have they to be laid out to treat the tailings as they come from the battery ? In many cases both these points have to be combined. As tailings reservoirs are generally situated on the lowest point below the battery site, no provision exists below such dams for the erection of works which would permit of further dumping-ground and handling of the stuff by gravitation. At all events, the topographical conditions of Witwatersrand would allow such an arrangement only in rare instances. In localities where the fall of the ground below the reservoirs permits of the erection of the works, I would recommend this to be done, as it permits of the charging of tanks and their discharging by gravitation.

In most cases the opposite course had to be resorted to on these fields • the tailings from the old pits or reservoirs had to be hauled up-hill to the cyanide works, steam power being mostly used. The arrangement is simple enough, as the B dumping-cars are pulled up on an inclined trestle-work above the leaching tanks, and after discharging their contents they run back by gravitation, and are held back by the break of the haul ing drum. In large works five to six trucks, holding 20 cubic feet each, are hauled up at a time. At every mine the mechanical arrangement for the filling of the tanks is different, depending on local conditions. Messrs. Fraser and Chalmers have intro duced a system of mechanical haulage by means of endless wire ropes, which works very well. To work old tailings by the cyanide process offers no difficulty to percolation, as they come to the works in the proper con dition.

They have been cleaned of the slimes by the natural system of concentration, which takes place in the collecting reservoirs. It is very interesting to stand at the discharge end of the launder carrying the tailings to the reservoir, and to see how the tailings arrange themselves according to the natural laws of gravitation, and are prepared here for subsequent treatment. At the head of the tailing pit the coarser tailings accu mulate, and near the dam the finest, and also slimes. The overflow from the first reservoir is collected in a second reser voir, where the slimy, clayey residue accumulates, which, strange to say, is as rich, and even richer, than the tailings in the first reservoir. To lay out plans for an accumulated stock of tailings offers no great difficulty, providing there is near by a sloping ground permitting of the discharging from the leaching tanks and their dumping by gravitation.

If the country is flat, the re-worked tailings will have to be hauled up an incline again, and then dumped. On a flat site it will be necessary to place the leach ing vats on masonry sufficiently high to give room for discharge, and gradient for the flow of the leaching solutions to the preci pitation boxes. When works are erected to treat tailings which are discharged from the battery, important appliances have to be resorted to, to prepare them for the cyanide treatment, and before they are collected in the leaching vats, owing to the physical condition of the powdered ore. SLIMES. 3 The discharge launder which carries the tailings from the battery to the cyanide works should have a grade of at least 3 ft. 6 in. in the ioo ft. to ensure a good flow. In a flat country where no grade exists, the tailings should be elevated by means of bucket wheels to the proper height.

From the experience gained on these fields, tailing pumps have not given satisfaction ; it may be that they were not properly constructed, as I am told that in Australia they are in various places in successful opera tion. There are on several mines here large tailing wheels in use, and I should consider them the best way of elevating tailings, as they require very little attention and repairs when properly constructed and set. Supposing that we desire to erect a cyanide plant directly behind a battery, the following grade would be required for doing the whole work by gravitation. Supposing the plant to be located ioo ft. from the battery — Feet. Inches. The grade for the discharge launders will require . 3 6 Masonry for settling tanks ..... 6 6 Settling tanks ........ 10 o Masonry for leaching tanks 6 6 Settling tanks io o Precipitating boxes and grade for outflow pipes ..60 Total grade 42 6 To this could be added from 6 to 10 ft. of grade for the storage tanks holding the cyanide solutions, wash and alkaline waters, but these are differently placed, and a lack of further grade would present no difficulties, as will be explained later on.

Fig. 1 (Plate I.), showing a section of the cyanide plant of the Princess works, illustrates such a mode of arrangement as is here described. Slimes. —The conglomerates on these fields, after stamping; contain a very large percentage of slimes. By " slimes " are understood the very fine particles of talcose and clayey material mixed with the very fine grains of quartz, iron oxides, and sulphides. If the whole of this fine material be allowed to collect with the coarser grains, the percolation of fluids through the mass becomes impossible, and, therefore, mechanical means have to be adopted, aiming at a separation of the slimes from the coarser material. Two methods have been introduced, aiming at the elimina tion of the slimes — the one by direct filling being the system introduced by Mr. Henning Jennings, the well-known mining engineer ; and the other the intermediate filling adopted by Mr. Charles Butters and Captain Mein, the manager of the Robinson mine.

I will take occasion to remark here that the appliances for the plant were materially changed by Mr. Charles Butters, who has done a great deal for the advancement of the metallurgical treatment of the ores on these fields, as he has introduced a great many practical details, all tending to lessen the cost of the process, and I consider it a very pleasant duty on my part to express my appreciation of his labours and of the good he has done by his work. The elimination of the slimes has an economic bearing on the gold-mining industry of these fields, when it is considered that at least 30 per cent, of the Witwatersrand ores, after crushing, pass away into slime pits : therefore, with a production of 250,000 tons of ore per month (which was the scale of production at the time these pages were first written), more than 75,000 tons would go month by month into the same pits.

Assuming for the Witwatersrand a production of 300,000 ounces of gold (or one million sterling) monthly, if we take the average value of the slimes all around at only 5 dwts. per ton, this would represent nearly 20,000 ounces of gold going into the pits monthly.* Up to the present no cheap method has been devised to deal with the slimes, so as to win the gold from them at a profit. The question of treating the slimes successfully is simply a mechanical one, as there is no chemical difficulty : on the con trary, the solution of the gold can be easily effected, owing to the fine state of division in which the gold exists in the slimes. * See Postscript, p. xiv, ante. Plate I. NOT TO SCALE Pig. 1. — Section os Princess Works. ' To ,„, . EFFECT OF STAMPING ON THE ORE. 5 My impression is that the eventual solution of the problem will be a system of agitation in vats fitted with mechanical stirrers, and after agitation and settling the solution will be decanted and a weaker solution added again, again stirred and decanted, till the last wash waters will only show traces of gold.

This method will become applicable owing to the introduction of the Siemens and Halske process, which effects the precipitation of the gold from the extremely weak solutions which will have to be used. The successful carrying out of this procedure will add largely to the gold production of the Rand. The Effect of Stamping on the Ore. — When ores are stamped in a battery, the resulting product is very uneven, and this is one of the recognised disadvantages of the batteries in use in all gold countries. The majority of the mines on these fields crush fine, using mostly screens of 900 mesh per square inch, and about 50 per cent, of the ore is converted into slimes. To illustrate the fineness to which stamping reduces the ore I may mention that Mr. Williams, the metallurgist of the Crown Reef mine, in formed me that the slimes which flow into their pits do not settle in 24 hours, and that if the overflow is taken and allowed to settle, these second settlings will assay 6 dwts. per ton, being richer, therefore, than the first settlings.

He further stated that after the water leaves the second slime pit it still holds in suspension 2 per cent, of the total gold contained in the ore as it comes from the mine. This is the result from careful experi ments and trials. In regard to the difficulty of settling the fine slimes which remain in suspension, it was ascertained at the Paarl Central works, that the overflow from a large tailing dam carried 8 per cent, of the rock crushed in suspension. As the water on these fields does not run to waste, but is used over and over again in the batteries, there is really no loss incurred, as the suspended slimes eventually settle, the water only retaining a certain constant portion. To give a further illustration of the effects of stamping on the Witwatersrand conglomerates, I will furnish some figures communicated to me by Mr. Bettel, a gentleman whose name has also been closely connected with the cyanide process. 40 lbs. of tailings were caught at a battery in a tub, and at least 30 per cent, passed away as slimes, the ore coming through a 900 mesh screen.

After drying, it was sifted through a screen of 1,600 mesh per square inch, and there remained on the sieve 1-85 per cent. (1) It passed afterwards through three sieves as follows 3,600 mesh and there remained on the sieve 27-93 Per cent. (2) 7,255 „ „ „ „ 2074 „ (3) 14.40° » ,. .. » 7-70 „ (4) The sands passing the last sieve were panned, and A. Remaining in the dish represented . . 1 1 -8o per cent. (5) B. Finest sand panned away . . . 22-34 ,» (6) C. Slimes collected from the panning water . 7-64 ,, (7) Total ioo-oo Each of these grades was assayed, with the following results — (i) . . . . . 3-03 dwts. per ton. (2) 4-00 ,, ,, (3) 4-40 ,, ,, (4) 4-65 » (5) 6-30 „ „ (6) 2-85 „ „ (7) 2*85 „ „ These figures are very instructive, as they show how very fine the ore is crushed in a battery, and that the material, after passing through a 120-mesh sieve per linear inch, can be concentrated, and will yield a concentrate of over double the value of the sands washed away.

No doubt the practical part of these figures will be very soon appreciated on these fields when concentration will receive closer attention than it does at present. After this notice of the important part which the slimes play in the metallurgy of the Rand gold fields, I will revert again to the planning of the works. THE PLANT. 7 The Plant. — The main features of a cyanide plant are the settling vats, the filter vats, the solution storage tanks, and the precipitation-boxes. The filter vats are either made of timber, or are brick vats lined with cement. At the Langlaagte Estate and Gold Mining Company, circular excavations were made in the rocky ground, lined with bricks and cemented, forming tanks 40 ft. in diameter and 10 ft. deep, each holding 400 tons of tailings. Filter vats made of timber will last for years, as contact with cyanide solutions does not seem to destroy it.

I have not been able to obtain the cost of a plant con structed in masonry, but I should consider it more expensive than the timber plant. Where wooden tanks are in use, they are placed in such a position that free access can be had to the bottoms in case of leakage. The number of filtering vats required for a plant depends on the capacity of the battery, and the time it takes to treat a charge of ore. If we want to treat 100 tons of tailings daily, and it takes, say, 3 days to fill, leach, and discharge a vat, it will require 4 leaching vats of 100 tons capacity each (dimensions 22 ft. in diameter, and 5 ft. high), but for safety one extra tank is added. The tendency on these fields is to construct a few large vats for the plant, instead of a large number of small ones. As long as shallow vats were employed, there was no difficulty in shovelling out the tailings over the sides; but with deep vats the bottom discharge was introduced by Mr. Butters.