The Stamp Milling of Gold Ores

Preface

. This book offers a careful description of the milling practice of gold-mining districts on both sides of the equator. It records the results of investigations made by the author in the course of professional work during the past ten years, and is published with the hope that the data which it contains may prove of service to comrades in the same field of industrial endeavor. The author's inquiry into the principles of this important branch of metallurgy sprang from the first observation of the apparently contradictory practice of Colorado and California, in both of which states (during the years between 1885 and 188S) he had charge of the operation of typical milling plants. The search for the scientific principles explaining methods so diverse led him to the general study of the reduction of gold ores, and induced him to spread the inquiry over a field nearly commensurate with existing mining regions.

The matter thus collected is rather a painstaking description of practice than a discussion of the chemistry and mechanics of the stamp-milling process. It will, therefore, it is believed, be serviceable to those engaged in the actual work of the mill, no less than to the student in the laboratory or lecture room. The ordinary operations of the stamp mill do not involve a knowledge of nice chemical formulae nor intricate mechanics. A good millman has the clarified common sense which lies at the basis of all true science, and has won a knowledge of the bed-rock principles of this ancient process of gold extraction such as no book learning can equal. To him the author has talked, and with him he has worked. To him he now humbly commends this book with the wish that the millman may VI

PREFACE.

. never do a tiling without a reason for it, and always remember that where so great improvement has been made much more must yet be possible. To tlie student starting out in his career as mining engineer or metallurgist, the author trusts the information gathered by a comrade, not much his senior, will be serviceable in em phasizing the many-sidedness of the stamp mill, and in awakening an intelligent interest in a process whose history covers the evolution of a great industry. To those about to choose a specialty in metallurgy, a science now covering a field so extensive as to be in its completeness beyond the mastery of any individual, he suggests gold milling as a profitable and fascinating study. To the juniors in the profession the author would like to say that the gathering of correct information is a work which brings its own reward.

The publication of data thus collected may not be of much moment to this busy world, but it clarifies tlie contributor's ideas and gives a compact crystalline form to a quantity of amorphous knowledge, rendering the routine work of the profession vastly more productive and pleasant. There is no better way of winning knowledge than to give away the little that you have ; all that we all of us know is but a small matter. The worst of all waste is the waste of experience. Such have been the feelings that prompted the making of this book, and with such the writer offers it now to his fellow workers. T. A. Rickard. Denver, Col., January, 1897. PAGE, 1-10 TABLE OF CONTENTS. Chapter I. — The Philosophy of the Stamp-Milling Process. The stamp and mortar resemble hammer and anvil — The Colorado and California practice viewed from this standpoint — The ore like a nut to be broken open — The adjustment of the mill to this end — The applicability of the pro cess to certain classes of ore — Milling a matter of business.

Chapter II. — Gilpin County, Colorado. The history of this old mining district — The evolution of a peculiar local practice — A description of the work done by a representative mill — A comparison with other neighboring plants — The character of the ores treated — Criticism of the methods in vogue — The millmen of Black Hawk — Suggestions for the future 11-34 Chapter III. — The Typical Stamp Mills of California. Grass Valley in Nevada County — The history of this region — Description of the North Star mill and its operation — Working costs — The Empire and Idaho mills — Comparisons — Choice of screens — The amalgamating plates and their arrangement — Recent changes — The tendency of these changes toward a greater degree of battery amalgamation 35-56 Chapter IV. — Californian Practice in Amador County.

The eariy history of Amador — The growth of its mining industry — Description of the Gover mill and its operation — Working costs — Other mills in this district — Comparisons — The weight of the stamps — Changes in the depth of discharge — The arrangement of the amalgamating tables — This defect of the Californian mills — The general results obtained by the Amador mills — The excellence of the work done ........ 57-74 table of contents. Chapter V.— The Profitable Working of Large Bodies of Low-Grade Ore. The Black Hills— Their early development— The lode structure of the region— The various stamp mills—Com parisons — The Homestake Company's plant— The rapidity of the crushing— Rock-breakers— The regulating of the dis charge by the use of chock-blocks— Screens— The consumption of mercury 75-86 Chapter VI.— Milling in the Black Hills, South Dakota.

The arrangement of the Homestake plant — The Highland and Deadwood-Terra mills — The order of the drop — The concentration of the pyrites — The process of amalgamation — Incompleteness of the treatment — Shoes and dies — The Homestake mortar and its excellence — Working costs — Criticism of the arrangement of the amalgamating tables — The process as a whole 87-101 Chapter VII. — Early Australian Methods. The first Australian stamp mill — The history of Clunes and the Port Phillip and Colonial Gold Mining Co. — The arrangement of the old mill — The blanket tables — The wells — Barrel treatment of blanket washings — Loss of mer cury — Other mills in the district — The South Clunes United — The mortar and its arrangement — Double discharge — The records of extraction — Character of the ores — Its simplicity — Suitability of the processes employed — General excellence of the milling methods 102-118 Chapter VIII. — More Modern Australian Methods.

The first gold discoveries at Ballarat — Leading features of representative mills — The Star of the East plant — Its arrangement — The old and new mill compared — Incom pleteness of the equipment — The Brittania United mill — The use of hot water in the battery — The addition of lime to the ore — The New Normanby mill — Comparisons — Absence of rock-breakers and feeders — General remarks . . 119-132 Chapter IX. — Gold Milling at Bendigo, Victoria. The story of early discovery— Importance of the district — A list of representative mills — The Fortuna crushing works — The Pearl mill and its arrangement — The New Chum Consolidated— Comparisons — Iron-framed batteries — The character of the ore — Criticism of the treatment — Absence of labor-saving machinery— Hand-feeding — Gen eral remarks 133-153 table of contents.

IX Chapter X.— Double-Discharge Mortars in Victoria. The Ovens district in Victoria— The Harrietville mill- Its arrangement — The peculiar design us the mortar box — Double discharge — Comparison between plated and plain copper — Treatment of the concentrates— The clean-up — Iron-framed battery — The Oriental mill — A simple ore feeder — Pan treatment of tailings — The cost — Other mills in the district — Derivation of practice from Clunes — General remarks 154-166 Chapter XI. — The Use of the Stamp Mill for Ores Unsuited for such Treatment. The Thames or Hauraki gold field in New Zealand — Its brilliant record — Representative mills — The Saxon and Moanataeri mills — Their arrangement — Comparisons — The wear of the screens — Complexity of the ores — Their refractory character — Inadequacy of the methods of extraction — The tailings plants — Suggestions — The use of Muntz metal plates — A mill test — Confirmation of author's criticisms. . 167-183 Chapter XII. — The Stamp Mills of Otago, New Zealand.

The Province of Otago — Its output — List of mills — Imita tion of Australian practice — The Phœnix mill — Its arrangement — The wells and blankets — Barrel treatment — The screens — Character of the ores — The process versus plate amalgamation — Tests — The concentration of the pyrites — Incomplete equipment — The process viewed as a whole . 184^-195 Chapter XIII. — A Review of Australian Practice. Wet processes versus dry — The simple nature of most Australian gold ores — The refractory ores of Queensland and New South Wales — A comparative table — Clunes, Bendigo, and Ballarat — Similarity of the mills — The con trast between Otago and the Thames, New Zealand — Treat ing ores unsuited to the stamp mill — Charters Towers in Queensland — Chlorination of concentrates — General sur vey 196-204 Chapter XIV.— The Wear and Tear of a Mill.

Shoes and dies — Their time of service — A comparative table — Cost — Varieties of material used in different regions — Colorado, California, Arizona, Australia, and New Zea land — The use of rock-breakers diminishes wear and tear — Results of general experience 205-215 X TABLE OF CONTENTS. Chapter XV. — The Flouring of Mercury. The terms " sickening" and "flouring" — Carelessness in their usage — Definitions — Flouring not necessarily injurious — How caused — The sickening of mercury ami the cause of it — The use of condenser water — Subdivision of mercury in patent pulverizers — Remedies 216-219 Chapter XVI. — Comparisons. The actual cost of milling in various districts — Figures — Labor and machinery — Comprehensive table of treatment — Weight of stamps — Iron-framed batteries — Depth of dis charge — Rate of crushing — Wire screens — The character of the ores — LTse of the stamps in concentrating mills — Need of sizing apparatus — Supplementary leaching processes — The wide field covered by stamp milling 220-235 Chapter XVII. — Mills and Millmen.

Need of technical knowledge in milling — Gradual de velopment of goldwinning processes — Arrangement of the amalgamating table — Lack of accurate data of work done in mills — Waste of experience — Rarity of careful investigation of methods and appliances — Disinclination of millmen to experiment 236-243 Chapter XVIII. — The Future of the Stamp Mill. Failures in the design of stamp mills and their cause — An example in Idaho — Comparison between stamp-milling and other processes — Battery amalgamation — Failure of steam stamps in amalgamation — Relation between pulveri zation and amalgamation in the stamp mill — New con trivances proposed to replace the stamp mill, and their results — Conclusion 244-249 Glossary of Stamp-Milling Terms 250-153 LIST OF ILLUSTRATIONS. PAGE. Central City and Black Hawk, Colo 13 Interior of the Hidden Treasure Mill, Black Hawk, Colo. ... 19 Gilpin County bumping table SO Type of a California ten-stamp mill 38 Mortar used at the North Star mill, Grass Valley, Cal 41 Battery screen frame 42 Amalgamating tables at the North Star mill, Grass Valley, Cal. 43 North Star mill, Grass Valley, Cal 47 North Star mill, Grass Valley, Cal 48 The Union mill, San Andreas, Cal 51 Impaired wire screen 53 Interior of the Gover mill, Amador County, Cal 59 Sketches showing arrangement of amalgamating tables .... 63 Mortar for an Amador stamp mill 68 Section of an Amador stamp mill 73 Deadwood-Terra Company's open cut, South Dakota 77 Interior view of Golden Star mill, Homestake Mg.

Co., South Dakota 85 The Homestake Company's stamp mills, South Dakota 88 Longitudinal and cross-sections of the Highland mill, South Dakota 88 Plans of the Homestake mortar, Homestake Mining Co., South Dakota 94 The stamp mill of the South Clunes United Co., Clunes, Victoria 107 Amalgamating barrel 110 Crushing mill at Ballarat, Victoria 120 An iron-framed battery as erected at Bendigo, Victoria .... 149 Details of battery at Bendigo, Victoria 150 Double-discharge mortar used at the Harrietville mill, Victoria, 156 Harrietville mill, the Ovens District, Victoria 161 Mills in the Thames district, New Zealand 180 An Arizona stamp mill 210 The Elmore mill, Rocky Bar, Idaho 232 Plan of forty-stamp gold mill, Aztec Gold M. and M. Co. . . . 233 txij

Chapter I

The Philosophy of the Stamp-Milling Process. Milling is a process of ore reduction whereby the extraction of the valuable metals is effected at a minimum of expense. Gold stamp milling is that particular process in which a heavy cylindrical body of iron is made to fall upon the ore in such a manner as to crush it, and thereby facilitate a separation between the gold and the valueless minerals by which the gold is encased. The latter weigh less than the former, and are removed by the aid of water. The gold is then collected through the agency of mercury with which it readily forms an alloy or amalgam. From this combination it is finally ex tracted by the distillation or retorting of the mercury. The mechanism of the stamp acts on principles similar ta those underlying the crudest devices used by man.

It may be likened to a hammer, of which the shoe is the hammer head, the stamp stem is the handle, and the die the anvil. The ore itself has been compared to a nut struck by a hammer whose blow has separated the valueless shell (the quartz) from the valuable kernel(the gold). Similes such as these may be carried further. The hammer falls, the anvil is fixed ; the same relation exists between the stamp and the die inside the mortar. The anvil is made of softer metal than the hammer, so also the die is often, and should be always, of iron or steel less hard and more tough than that composing the shoe. The movement of the hammer and the drop of the stamp are both intermittent. There is in both a loss of power due to inelastic impact ; in both, also, there is an apparent loss of time due to the interval separating the blows.

In the case of the hammer that interval of time is utilized in the shifting of the material which the hammer blows are shaping ; in the case of the stamp the time is turned to advantage in the opportunity which it gives for the water to assist in the separation of the gold from the quartz. The hammer falls upon the dry and wide surface of an anvil, the stamp, however, drops upon a face of iron (the die) confined within a narrow box (the mortar). Water covers the die and the ore lying upon it. The blow of tlie falling stamp not only crushes the ore, but also causes a violent pulsation of the water. That pulsation becomes converted into an irregular splash against the sides of tlie mortar. The latter has an opening in front, through which the water is discharged, carrying with it the crushed ore.

This, called tin pulp, spreads itself over tables placed on an incline, which are lined with a metal, usually copper, having an amalgamated surface such as will arrest the particles of gold and at the same time permit the grains of quartz and other valueless material to pass over it and out of the mill. The work done by the hammer is dependent upon its weight, the rapidity of its blow, and the distance through which it strikes. In stamp milling this simple analogy has many departures from it. These can best be explained by examining two extreme types, such, for instance, as are afforded by the divergent methods of Colorado and California. If you go to Black Hawk, in Gilpin County, Colorado, and walk up the long straggling street, you will hear the stamps falling so slowly that the drop of individual ones can be dis tinguished.

At one time the rate used to be about 24 per minute, now the speed is regulated at an average of 30 drops per minute. If, on the other hand, you should visit Angel's Camp, in Calaveras, or Sutter Creek, in Amador County, Cal ifornia, you would hear the muffled roar of stamps falling so fast as to make it quite impossible to recognize the drop of any single one. There the rate would average 90, with an occasional maximum of 105 drops per minute. Upon further inquiry we shall find that in Colorado the stamp weighs 500 to 600 pounds, the most common figure at Black Hawk being 550. In California the extremes are greater, from 750 to 1000 pounds, with a frequent repetition of 850. In Colorado the drop ranges from 16 to 20 inches, in Cali fornia from 4 to 6 inches. If we now multiply these three factors of weight, drop and speed, we find that the theoretical work done is nearly equal and is approximately 1 horse power.

But the results aimed at in stamp milling cannot be measured in foot-pounds. The efficiency is gauged rather by the amount of ore crushed and the completeness wherewith the gold is extracted. The stamp mill is both a crushing machine and an amalgamating contrivance. Therefore, not withstanding the approximate equality in the sum total of theoretical work, the Colorado mill crushes only one ton per 24 hours ; while in California, with an ore of similar hardness, the amount is from two and one-half to three times as much. Why is this difference ? Should it exist ? Is it the result of ignorance or the outcome of experience, ? In answering these questions we shall haveto discuss the bed-rock principles of stamp milling. To the first question the answer comes promptly. The difference is due to the dissimilarity of the ores treated.

That which goes to the mills at Black Hawk contains an average of 15 per cent, of pyrite. The gold which it carries is in a very fine state of subdivision, and is intimately associated with the pyrite. On the other hand, the mill stuff treated at Angel's Camp, or Sutter Creek, carries only from 1 per cent. to 2 per cent, of pyrite, and the accompanying gold is not nearly so intimately mingled with it, but rather occurs in clean quartz and in a comparatively coarse condition. The immediate result is that coarse crushing, which is, of course, more rapid and therefore more economical than fine crushing, will do for the California simple quartz ores what it can not do for Colorado's complex pyritic material. Let us return for a moment to the simile of the hammer and the nut. The hammer which cracks open the nut may liberate the kernel without crushing it.

I have seen gold bearing quartz which could be likened to such a nut. The gold occurred in cavities in quartz of a honeycombed kind. In dealing with such an ore there is just a certain blow which will break the brittle quartz and liberate the ductile gold. Such conditions are ideal. Usually the hammer will not only break the shell but also crush the kernel, so that the particles of both become confused together. This more nearly resembles the ordinary action of the stamp, which has, moreover, to deal with a material in which the gold and its envelopment of gangue, the valuable and the valueless constituents, are so irregular in size and so intermixed that the one is often crushed too much and the other too little. Particles of <y\d will remain attached to or enveloped by pieces of quartz, rock, or pyrite, unless crushed to a fineness such as compels a separation.

Without such a complete separation amalgamation will not take place. It is obvious, therefore, that when the gold in an ore is coarse the process of crushing need not be carried so far as when the gold is in a state of minute subdivision.* Again, it is evident that the longer a particle of ore remains inside tlie mortar the more frequently, other things being equal, will it undergo the blow of the stamp, and that the more often it is struck by the stamp the finer the condition into which it will be pulverized. How are these factors regulated? In two ways, by the depth of discharge and by the mesh of the screen. ■ The ore upon the die is under water. After it has been sufficiently crushed it is ejected by the splash of the water through the screen or grating which covers the opening in the front of the mortar.

The surface of the water is approxi mately level with the bottom of the aperture occupied by the screen. In Colorado the depth of discharge (also termed the issue), as measured by the distance from the bottom of the screen to the top of the die, is 14 inches. In California it is 4 inches only. Here we have found one of the reasons for the diffeience in the crushing capacity of the mills. Though the same amount of power be expended in lifting the stamps, and though the screen used lie of a similar mesh, yet in Colorado less ore is crushed in a given time because the stamp falls through 10 inches more of water and has to eject the crushed ore at a level 10 inches higher than in the California mill. The greater thickness of the cushion of water deadens the blow of the stamp and weakens the force of the splash.

What is the result upon the ore ? It is found that even with screens of equal mesh the crushed ore remains inside the deep ♦Only microscopic examination can give one an adequate idea of the minuteness ot the particles of gold in certain jres, or of the extent of its diffusion through the mass. Colorado mortar longer than in the shallow one used in Cali fornia, and that, therefore, the pulp discharged from the former has a fineness much greater than that delivered from the latter. The screen is a controlling, but by no means final, factor in regulating the rapidity of the discharge and the size of the particles of the pulp. In California 30-mesh wire cloth is ordinarily employed ; in Colorado, slot-punched screens equivalent to 50 mesh. The one represents 900 and the other 2,500 holes per square inch.

In neither case, however, is the ore crushed just to that particular size permitting of exit through the screen, but much of it is crushed to an unnecessary and far greater degree of fineness. It is found with the use of screens of similar mesh, say 40, that in a California batterj' about 50 per cent., and in a Colorado mill fully 70 per cent., of the pulp will pass through a 100-mesh sieve. This is due to the pause which occurs between the successive drops. Particles of ore which have been pulverized to a size which would allow of their discharge through the screen are enabled to fall back and are recrushed by the succeeding blows of the stamp. In the fast (California) mill the interval between the drops is two-thirds of a second; in the slow (Colorado) mill it is two seconds. The irregular splash of the water inside the mortar causes an unequal sizing of the pulp.

The particles of pulverized ore strike the screen in a haphazard -way. It is a question of hit or miss whether any particular particle be thrown against an opening or a blank. If it fail to pass through, it is drawn back by the recession of the water and undergoes further pulverization. This action must obviously take place to a greater extent in a deep mortar with a punched screen which offers less opportunity for free discharge than in a shallow mortar with a wire screen which offers increased facility for the issue of the pulp. Hence the tendency to a greater degree of excessive pulverization in the Colorado mill as compared to the Californian. This excessive pulverization is desired by the Colorado millman because it enables him to break the close intimacy between the finely divided gold and its associated pyrite.

He therefore has combined the features of a long and slow drop with a deep discharge. The comparatively light weight of orks. the stamp is necessitated by the excessive height of the drop He has also designed a mortar which is wide and roomy. This causes the splash of the water inside the battery to be weak, and compels the pulp to remain inside until pulverized to a fineness much exceeding that required merely for its passage through the screen openings. Then, having obtained conditions enabling him to separate the gold from the pyrite in the ore, he catches it upon amalgamated copper plates which are arranged along the front and back of the mortar. To sum up, he makes the crushing feature of his mill sub servient to amalgamation. Now turn to the Californian. His ore does not require such fine pulverization, because it does not present such difficulties to the separation of the gold which it contains.

He finds he can readily save the gold in the pulp, after it has been ejected from the battery, by catch ing it upon tables covered with amalgamated sheets of copper. He, therefore, has made his battery primarily a fast-crushing machine, and only incidentally an amalgamator. If gold is caught within the mortar, as some usually is, so much the better ; but, I insist, amalgamation inside the battery is sec ondary to the obtaining of conditions facilitating rapid crushing. The Californian has consequently found that for his purpose the best arrangement is that of a stamp mill having heavy, fast dropping stamps which will induce rapid pulveri zation ; he has designed a narrow mortar, thereby diminish ing the opportunities for the resettling of particles of pulver ized ore ; he has utilized a shallow discharge, thereby increas ing the force of the splash of the water and accelerating the exit of the pulp through the screen.

In this case the intro duction of plates for amalgamation is rarely admissible, because in mortars having so shallow a discharge as 4 inches, the violent agitation of the pulp abrades or scours the surface of such plates. We have given the reason for the marked difference existing between the milling methods of two districts in the same country. Next comes the query, Should such a difference exist? Yes, if stamp milling is to be the process employed in the reduction of the ore. The first axiom of successful milling is that the process which is the best adapted to the ore and the cheapest under any given conditions is the one which if should be chosen. It has been claimed by many that the stamp mill is not the best machine, nor amalgamation the best process, for the treatment of the ores of Gilpin County, the home of the Colorado method which has been described.

It has been stated that the practice which is in vogue there is incorrect from a metallurgical standpoint, and out of date. Concentration has been suggested as the proper process. The examination of this question is an excellent example of the relation existing between the commercial and technical sides of metallurgical practice. Take, for instance, the case of 10 tons of ore, each carrying $8 worth of gold. The yield by amalgamation would be, say 65 per cent., equivalent to $52, to be supplemented by a further yield of 15 per cent., or $12, in the form of one ton of pyrites concentrates. There would thus be obtained $64 out of the original contents ($80) of the ore, equal to an extraction of 80 per cent. The ton of concentrates would be shipped to Denver, where the smelter would pay 95 per cent, of the assay value, less the charge for treatment, $5, and freight, $1.50.

The net return would be $&S0>--lhis; last figure added to the gold obtained by amalgamation, which is usually sold to a local bank, gives $56.90 as the final return from the 10 tons of ore. Now, suppose concentration to be substituted for the pres ent method of milling. The 10 tons of millstuff would be concentrated into one. Supposing the extraction to be 80 per cent., then the one ton of concentrates would have an assay value of $64. When forwarded to the smelter this material would be required to pay a higher treatment charge because of its higher value and its greater silica contents. The charge would be from $8 to $10 per ton. Let us take the lower figure, $8, and add to it the freight, which would be $2, or 50 cents more than that levied on the lower grade material. Add $1 for sampling,* and the total deductions, the smelter paying 95 per cent, of the assay value, foot up to $14.20, leaving $49.80 as the final return.

There is, therefore, a balance of $7.10 in favor of the present method. In the above example the costs of milling and concentration *A voided in the case of the very low grade, finely pulverized and homogeneous stamp-mill concentrates which are hand-sampled by means of a tube resembling a cheesetaster. have been supposed equal, and an extraction of 80 per cent. allowed in both cases. There are some ores in Gilpin County which, being more heavily charged with pyrites, galena, etc., are more suitable for concentration than stamp milling; similarly there are others so poor in pyrite and so free milling as to put concentration out of court. On the whole, however, the above-cited example will be sufficient to indicate how largely local commercial considerations enter into the choice of the method of ore reduction.