The Cyanide Process for the Extraction of Gold and Its Practical Application on the ...

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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 undertake 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-

Vi PREFACE.

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.G.

RS. — For the photograph which has been reproduced 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 Treatment 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 Metallurgists who desire to avail themselves of the process 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 speaking, into two methods, in )ne of which mechanical viii INTRODUCTION.

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 gangiie in which it is enclosed; and when the pulverized 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 whatever 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 nonoxidizable 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 potassium 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 gfilding 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.

X INTRODUCTION.

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 preeminence, 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, ir 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 production of 174,077 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 eminent 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 Glencairn Mine — or about iij miles in length, along the strike of the main reef series, — if exploited to a vertixii 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 prodigious amounts will be won by the cyanide process, 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 ^insaid 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 treatment of tailings by pan amalgamation, but with unsatisfactory 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 precipitating 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 ol 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 expressing my deep obligations to the several gentlemen —

Xiv 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 opportunities 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. V

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 retxims for December, 1897, the amount recovered by the Cyanide and the Siemens processes would be equal to 32 per cent, of the whole.

(xv)

Appendix

CYANIDE PLANT AT MERCUR, UTAH, U.S.A. . . .127

INDEX 139

(xvi)

LIST OF ILLUSTRATIONS.

FIG. PAGE

Frontispiece — CYANiDp Plant of May Consolidated Gold

Mining Company.

1. (Plate I.) Section of Princess Works . . facing 4

2, 3. (Plate II.) Messrs. Butters and Mein's Automatic

Distributor facing 8

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

33. (Plate XI.) General View of the Simmer and Jack

Cyanide Plant facing 88

34. (Plate XII.) Simmer and Jack Filter Vats . ,,90 35» 36. (Plate XIII.) 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 Pre-

cipitation Plant at Gibraltar Mine, N.S.W. facing 98 39_43. Cyanide Plant at Mercur, Utah, U.S.A. 129—136

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 accumulated 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

2 ERECTION OF A CYAOTDE PLANT.

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 hauling 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 introduced 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 condition. They have been cleaned of the slimes by the natm-al 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 accumulate, and near the dam the finest, and also slimes. The overflow from the first reservoir is collected in a second reservoir, 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 ofi'ers no great difficulty, providing there is near by a sloping ground permittmg of the discharging firom 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 leaching vats on masonry sufficiently high to give room for discharge, and gradient for the flow of the leaching solutions to the precipitation 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 loo 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 operation. 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 durectly behind a battery, the following grade would be required for doing the whole work by gravitation. Supposing the plant to be located loo 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 10 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. I (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 froqf the coarser material. *

Two methods have been introduced, aiming at the elimination 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,ocx) 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 goldfi-om them at a profit. The question of treating the slimes successfully is simply a mechanical one, as there is no chemical difficulty ; on the contrary, 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

\2-(7

•— 1 r i

y

A

'. 7" ./<*<■<■ /. |.

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 efifects 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, informed 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 experiments 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 sliipes eventually settle, the water only retaining a certain constant portion.

To give a further illustration of the eff'ects 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, (i)

It passed afterwards through three sieves as follows :

3,600 mesh and there Femained on the sieve 27*93 P®r cent. (2)

14,400 „ „ „ „ 770 „ (4)

The sands passing the last sieve were panned, and

A. Remaining in the dish represented . . i i*8o per cent. (5)

B. Finest sand panned away . . . 22*34 »» (6)

C. Slimes collected from the panning water . 7*64 „ (7)

Total . loo-oo

Each of these grades was assayed, with the following results —

per ton.

4-00 „

4-40 »

630 » 2-85 „

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 constructed 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 firee 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 1 00 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.

The time allowed for the zinc process is generally between 60 and 84 hours, vats of 300 tons or 400 tons capacity being usually discharged and filled in one working day of 10 to 12 hours, giving four days as the total time required per vat with the zinc process. In the Siemens and Halske electrical process the minimum time occupied is 5 days. Where 2 days are occupied in charging and discharging, six vats should be erected.

Before the tailings go to the leaching tanks they have to be

S ERECTION OF A CYANIDE PLANT.

freed of their slimes, and I shall now describe the two methods adopted for this purpose.

Intermediate Filling. — Messrs. Butters and Mein's Distributors. — The first attempts at intermediate filling were made by running the battery tailings to the centre of a circular vat, and allowing the overflow to take place at one point. This did not prove successful, because the sand piled up in a central conical heap, and the slimes settled in the deeper water around the sides of the tank. The next plan was to run the pulp into the vat through a series of stationary launders, delivering at several fixed points. This method improved the distribution, but the result was still unsatisfactory. Then, in order to give an uniform overflow at every point of the periphery of the vat, a circular trough was fixed round the top to collect the overflow and deliver it to a launder.

Each of these alterations was a step in the right direction, but the system of settling could not be considered successful until after the introduction of an automatic revolving distributor. This appliance consists of a central casting, with a vertical spindle a revolving in a footstep bearing b, which casting carried a conical hopper e and a number of radial pipes c with bent ends, as shown in section in Figs. 2, 3 (Plate II.), and 4 and 5.

The distributor is fixed on an iron column in the centre of the vat. The bends at the end of the pipes cause the apparatus to revolve by the reaction of the pulp as it leaves the pipes. Each pipe has a diflferent length, in order to distribute over a number of concentric circles. This also had its faults, as it was found that the slimes collected in narrow rings between the outlets of each pipe, giving rings of clean sand alternately with rings of slime. The difficulty was overcome by attaching flattened nozzles to the ends of the pipes, causing the pulp to spread over a wider area, and also by increasing the number of pipes.

As is noticed by this plan, the arrangement is a hemispherical bowl fi:om which radiate 8, 12, to 16 pieces of pipes