Hints on amalgamation and the general care of gold mills

Front matter

THE LIBRARY

OF

LOS ANGELES

Table Of Contents

Introduction 7

Chapter;i— General Process 11

Arrangement of Mortars ...... 17

Introduction

Gold has been known from the earliest ages, from its occurrence in a metallic state in sedimentary deposits as well as in veins. On account of its great specific gravity, it can be collected from placers and river beds, by separation with water alone, and in veins after the matrix is crushed, a great amount is saved by the same simple method. When only rich deposits were exploited, anybody could manage as it apparently required only labor and opportunity, while as long as money was made, the losses that occurred did not trouble the owners. From this very simplicity of working, the majority of owners and investors imagined that no special training was requisite to insure success, but that any trustworthy man, skilled in mercantile pursuits, was the proper person for the place of general manager. To-day the same opinion is held by very many, in spite of the warnings from hosts of disastrous failures. One cause for this belief has been the lack of literature for this special branch of metallurgy; in fact, very near 1890, there was practically no works written at all which spoke of gold except as a subsidiary product of the metallurgy of other metals. Volumes, both practical and scientific, have been published which deal with the special branches of the metallurgy of lead, copper, and silver, but the metallurgy of gold seemed so simple,

it only required an occasional article in a paper or

magazine.

In spite of this dearth of written knowledge, practical men in all parts of the world were rapidly improving the methods of treatment, both mechanically and with the aid of chemistry, in their several localities; but, through lack of exchange of thought, each section had a different process, on which faith was pinned to the exclusion of all other processes.

As the rich deposits gave out, attention was turned to those of lower grade, which are now found to be very profitable with improved machinery and increased knowledge, but, even to-day, we are in the infancy of this science. The treatises on gold, now before the public, can still be counted on the fingers of one hand, and are either devoted to the scientific discussions of processes now in vogue, the history past and present of gold milling, or the mechanics pure and simple of the various machinery used. They are all good for the education of the world, but are lacking in definite directions of the best way to employ the methods they describe, and are generally understoqd by those only who have enjoyed a previous technical education. This absence of intelligent application of these known principles in the great majority of the mills in this State (the home of gold milling), has been a constant source of wonder to me, and yet I have a fellow feeling from my first experiences in gold-mill practice. Graduating from a celebrated school of mining engineering, I thought myself eminently fit to at once assume the absolute management of the biggest mining concern on earth, but, thanks to the kind influence of an elder brother, already well known in the mining world, I was started at the bottom of the ladder in a gold mill, to begin my real education. I very soon found that there was no bigger tenderfoot in California, as my technical education had only taught the chemistry and scientific mechanics in general, with no practical rules of how to apply this knowledge. These practical rules have been learned through bitter experience, extending through many years, and to try and help others who are just beginning, I have set down in the following pages the results of the observations I have made, all of which have been practically demonstrated as correct by application on several very difficult ores, with marked success in every case. It has been a work of pleasure to contribute this mite of knowledge, and if I am instrumental in increasing the production of only one gold mill, I will feel amply repaid for my labor.

I ask the indulgence of the reader, to overlook the

and take the will for the deed. I have done the best

I am able, and can only leave the verdict to the public.

W. J. ADAMS, E. M.

San Francisco.

Chapter I. General Process

It is not the purpose, in this book, to enter into any detailed description of "gold milling," as the ground has already been amply covered by E. B. Preston, in his report on "Gold Milling," published by the "State Mineralogist," of California.

Some other points also may seem too well known and appreciated to be mentioned, but these require constant repetition, and even then very little practical application seems to be made by the average millman.

In order clearly to understand the entire method, we must start with the ore as it comes from the mine. This ore must first be carefully examined, as its character and structure as well as its value, determine the most advantageous method to pursue. First, the ore may be hard and tough, requiring force to disintegrate it, though entirely free-milling; seco'nd, the ore may be soft, giving a great deal of slimes; third, the ore may be a mixture of the above two divisions.

First: Here, the r.re is first dumped over a grizzly, with the bars set 1% inch apart, the coarse lumps going through a rock-breaker of some form, to mix with the fines in the ore bin. From this bin it must be carried to stamps, preferably of heavy weight, notwithstanding all assertions to the contrary by patentees and makers of other pulverizing devices which are "just as good as stamps."

Second: When the ore is soft, it also requires to be passed through a breaker, to enable the particles to be of uniform size; but here, very frequently any kind of stamp, and always those of heavy weight, will slush through the bed of pulp, wearing out iron uselessly, and increase the proportion of slimes, always to be avoided as much as possible. Here we find a great advantage in the different types of the Chilian mill, such as the Huntington and Bryan, from both of which the pulp escapes as soon as it is reduced to the requisite size, as determined by the screen, and with the minimum of slimes to hinder future operations.

Third: In an ore containing both soft, clayey or talcose material, and hard bunches of quartz, stamps are generally the best, as the grit of the hard quartz prevents pounding of iron on iron, and causes sufficient splash to keep the screens from choking, while all operations can be kept under close observation, which cannot be done with any pan or roller mill — one of the greatest objections to the general use of these mills. In some few cases, however, where the output from the mine is sufficiently large to warrant the outlay of capital, it will be found of great benefit to separate the ore partially and automatically into two classes. Dumping the ore over a long grizzly, the soft and fine will go into one bin, while the coarse, after being put through the breaker, falls into a second bin. That from the first bin is fed to one or more roller-pan mills, while the hard broken quartz goes to the stamp mill, very largely increasing in this way the output through the two methods combined.

The pulverized ore, technically known as "battery pulp," is now carried by water down silver-plated copper plates, by which means as much of the gold as possible is abstracted, varying very considerably under different millmen's methods: a low percentage saved, being excused by such terms as "floured gold," that floats in water, rusty gold, "plumbago in the ore," or a similar state of affairs.

However, there is generally sufficient gold left in this pulp, which could never be saved with quicksilver, to require further treatment, while the value is too low to treat all the pulp en masse. This gold is either, mechanically or chemically, locked up with other metalliferous substances, generally sulphides or minerals of a higher specific gravity than the accompanying gangue.

This specific gravity is the keystone to all the concentrating devices, and the special method required is determined by only two facts, the value of the concentrates when pure, and the percentage of them carried by the ore. Should these concentrates be of low value and moderately coarse, a simple bumping table, like the Gilpin County bumping table, or its improvement, the Wilfley, could be used, as they require little care, are reasonable in price, and save concentrates free from foreign matter, while the final tailings, though still carrying a large percentage of mineral, will be economically valueless owing to the intrinsically low value of the sulphurets. When, however, the concentrates are of value, for instance above $50 per ton, a mechanism of much greater delicacy must be employed, and of all the devices, none has proved so satisfactory as an endless moving belt, shaking without a jar. As a rule, the higher the grade of sulphurets the more friable they are, and the finest slimes, most easily affected by agitation, carry the richest particles, reaching a culmination in the tellurides. Should the "battery pulp" carry a high percentage of sulphurets, that is, above 12 per cent., it very often becomes necessary to use a double concentration, and in this case two machines of the same type are used, the

14 HINTS OX AMALGAMATION.

tailings from the first passing directly to the second, after removing the surplus water if there is too much added on the first machine.

In spite of all these precautions, it is very frequently found that the concentrator tailings still carry an appreciable value, and most thorough tests must be made to determine whether or not this can be economically saved. The first and commonest method is to allow the tailings to run over a large surface, covered with canvas, which is cleaned and swept several times a day, but, generally, this only saves a small percentage of the loss, and its only advantage is its cheapness. With proper care and experience the concentrators should save all that the canvas plant could do, and any further reduction must treat economically with the entire mass.

So far the only way to do this is by employing some of the modifications of the "cyanide process," and this requires a large tank capacity, so that the tailings can flow directly to the filtering tanks.

This is concisely the method of gold milling, but there are several points to observe, to insure complete success on the majority of ores.

First, foremost and at all times is the question of grease. Guard against its excessive use day and night, month aud year. Keep all the bearings and machinery where it is used as scrupulously clean as a New England housewife does her home; do not grudge the use of clean waste, but see that the employees observe the maximum of cleanliness. Bits of candles do no harm, or very little, if made of stearic acid, but beware of tallow dips. Start in with the rock breaker, and place dripping pans underneath the journals, making frequent examinations to see that all the drops are caught. See that none of the oil used on the running gear of the self-feeders drops on the feeder floor, being very careful about working the leakage of ore always found underneath the feeders, if there is the slightest possibility of its being fouled. Clean the stems of the stamps, the cams and tappets very frequently, and do not use liquid oil on them, but make a swab of blanket and apply a minimum of axle grease, to the cams several times a day, to the stems only as needed. When applied to the stems, only touch them above the guides. It is generally better to tack some canvas between the camshaft floor and the top of the lower guides, giving to it a decided belly, to prevent any grease thrown by the cams from falling into the feeder or throat of mortar, or on the plates. In one mill, where very poor work had been done, though the builder and superintendent claimed most thorough knowledge, we found boards placed below the cams to prevent the grease from falling on the plates, and set at such an angle that there was a constant rolling of pellets of grease directly into the throat of the mortar.

We now come to the concentrators, and it can be safely stated, with hardly an exception, that the cleanliness of the machines is a sure indication of the quality of work done, and the closeness of saving accomplished.

Where rubber belts are used, the oil rots them, the sand and grit get into the bearings, causing bumping, inequality of the plane of the table, and a little grease getting into the pulp will prevent the saving of the finest and richest of the sulphurets. The very act of cleaning the machines brings all parts under close scrutiny, and many a serious break-down is averted, observed in time during this cleaning at regular intervals. Therefore, be sure and watch for grease, daily caution the employes about it, spare no pains nor expense to have every part

HINTS ON AMALGAMATION.

Modem Machinery.

b'lQ. 1. 1 Copperplate. 2— Iron strip. 3— Wood backing.

of the mill at all times as if on parade, and the close saving and freedom from break-downs will amply repay all the trouble and outlay.

Within certain limits the shape of the mortar does not affect the economic results nearly as much as has been supposed— that depending mostly on the man attending to them. A mortar must be narrow, as the object is to get the ore out of the battery as fast as crushed. The back should be curved from the throat, so as to throw the ore directly on the center of the "die." This back must be solid metal, and on no account have copper plates in the back. Their advantage is very dubious, and in nine cases out of ten their use is a very great detriment to successful work. Without them the copper plates on the "chuck-blocks" catch as much amalgam as both combined; there is less surface to scour, particularly with low-grade ore, and with them the additional width of the mortar prevents the splash being exerted exclusively against the screen, and reduces the crushing capacity of the battery. (See Fig. 1.)

The discharge, that is, the distance between the top of die and bottom of screen, should be within a fraction of four inches, new dies requiring wooden strips under the screens to keep this height. The "drop" of the stamps should be 4% to 5 inches, and the speed always above 95 drops per minute, preferably above 100 drops. A slow drop, besides decreasing the duty of the stamps, allows the quicksilver to settle and diminishes its comminution so that too much is liable to be used at one time, a sure loss if the gold is floured. A quick, short drop more nearly approaches the action of the pan in pan amalgamation, keeping all the pulp in constant agitation, and mixing the quicksilver uniformly, so that all parts of the ore come in contact with it before leaving the battery.

18 HINTS OX AMALGAMATION.

The "chuck-block," covered with the copper plate, should be entirely separated from the strip of wood on which the screen rests. The plate should touch the bottom of the screen itself, at as slight an angle as possible, and shoiild be very nearly vertical and five inches deep. Its backing, generally of wood, must be as slight as possible, so as to place it far from the dies. It is held

FIG. 3.

Uodern Machinery.

FIG. 4.

in place by a strip of iron, which extends into the shoulders on each side of the mortar.

Two chuck-blocks should be made with the blocks to conform to the height of the screen as the dies wear down, as with dies half worn the wooden strip underneath the screen is removed, bringing the discharge down to normal height, and the copper must not project above the bottom of the screen-discharge.

This sketch, made without a scale, as the width of mortars varies, will better explain it. Figure 2 shows both cross-section and longitudinal section.

The screens (see Fig. 3) are preferably made with openings eight inches high and the entire length of the mortar. Abstain from putting any cross-pieces, which obstruct the freedom of discharge. In placing the screens the chuck-block is first put in place, then the strip of wood on which the screen rests, having a lining of blanket tacked to both edges, and this strip of wood is keyed by the shoulders cast on the lip of the mortar. Then the screen is put in place and keyed by the wedges driven down the shoulders on each side of mortar. There still remains a largo opening between the top of screen and the bottom of the upper casting of the mortar itself. This should be wide enough, even with new dies, to allow of the easy insertion of the arm to the shoulder. In most mills this is very foolishly closed solidly by a wooden false screen and keyed as tightly as the screen itself. The proper way is to take a strip of wood the full outside width of the mortar in length, and 2x1 inches, and on this tack very strongly either extra-heavy canvas or a piece of rubber belting, which just fills the opening in width and extends a little below the upper edge of screen-frame, in depth. The advantages of this should be obvious, but a few of them may be named: First, it permits the inspection of the interior of the battery at all times, without stopping the stamps; second, the cleaning, twice daily, of all accumulated chips and other clogging of the inside of screen; third, the examination by touch of the amalgam accumulating on chuck-block; and, fourth, in many cases, the driving on and replacing of shoes without removing the screen.

The greatest advantage of all is that the mill is stopped as little as possible, and hence the most perfect amalgamation maintained, while no hard amalgam is broken away, as always occurs when the screen Is removed. In fact, as my first great point was "avoid grease," my second is, "Never remove the screen of a battery till the run is ended, if it can possibly be avoided, and the finer the gold the more important this is."

It takes a very appreciable time, after a battery is started, no matter how short the stop may be, before all the pulp is in constant agitation, and the quicksilver mixed with it.

This rule also brings us to the proper material of which to have the screens made, and in this as well as everything else, the best is always the cheapest. Theoretically, wire screens expose the most discharging surface, but they clog the quickest, and have to be removed to be cleaned by a wire brush, a very serious fault, as stated above. Round-punched holes present too much dead surface, being exactly the reverse of the wire screens. This only leaves us the "burr-slot" and the •'punched-slot," and of these the burr-slot, though clogging the least of any, wears coarser more quickly than the punched-slot. Therefore the advice we give is to use in stamp batteries "punched-slot screens," of the best material to be had. Of material we have tinplate, Russia iron and steel. Tinplate is the cheapest, but the screens are worn out within a few days, and are really the most expensive in the results that can possibly be employed. Russia iron will not break easily, but the orifices wear so coarse within 48 hours, particularly on hard quartz, that the ore will not be pulverized as fine as required. This leaves us only the steel, which breaks before it wears coarse, on account of its hardness. Of course there is steel and steel, but if a point is made about it, steel screens can be procured which will not break for over a month under rough usage, and even in that time the size of the apertures has not appreciably increased, i. e., sufficiently to cause any loss in free gold.

Here we have the best condition, the battery in steady operation for a month, the pulp in steady and constant agitation, no disturbance of the hard amalgam, by removing a screen; and the greater the quantity of amalgam in the battery, the more of the fine and floured gold is caught there. Therefore use a "steel, punched-slot screen," of -the size found by experiment to be the best. Several factors determine this size.

With coarse gold this is not of so much moment, but should the gold be fine, it is necessary to crack the ore that much more to free it. This degree of pulverizing depends on the grade of the ore, as in many cases it costs more to save the finest gold than the total amount saved, and it then becomes necessary in practical reduction to use a larger sized screen, put through more rock, and pay little attention to additional loss in the tailings. However, it is best to use as fine a screen as is conformable with a profit in working the mine.

The outside of the screen must be scraped several times a day, and as good a device as any is a copper spade with the edge turned over at an angle of 45 degrees; wooden handle, copper riveted, as shown in Pig. 4.

There should be a long expanse of copper-plate surface below each battery, though some of it is only of use in case of faulty amalgamation.

These plates are technically divided into:

INSIDE COPPER CHUCK-BLOCK, a description of which has already been given, and there is only to add that it is an unnecessary expense to have it silver-plated.

LIP-PLATE, which must be heavily silvered, at least three ounces of silver to the square foot being put on it.

This plate is just the length of the mortal' and is \vide enough to extend from the inside-edge against which the bottom of screen is braced, to a half-inch beyond the iron lip of mortar. Two rectangular holes are cut in it, to allow the shoulders of lower keys to project, and the wooden strip on which the screen rests is placed on top of the plate and holds it solidly in position.

SPLASH PLATE.— This is rarely found, even in the ftftftf/xtr'Ad fo ffdf frii\4

Modern Machinery.

best mills, but is of the utmost importance; the more so. as the gold is floured. In fact, it is absolutely necessary if very fine gold is to be saved. It should be 12 inches wide, by a length just sufficient to fill the space between the shoulders of the mortar. There are several ways of adjusting, but In every one the point to be observed is, that it shall stand in front of the screens at an angle never less than forty-five degrees and better if over fifty degrees from horizontal; that the bottom shall be at least an inch below the bottom of the screen, and the space between the screen-frame and it shall not exceed three-quarters of an inch. Take this plate and fasten it to an inch board, leaving a margin of the board below the copper plate, to act as a brace to the stalactites slowly formed, and then by back supports, stand it on the edge of the next plates, so that it is held firmly against the mortar and yet can be instantly removed, as shown in Fig. 5.

Another way of great simplicity, which, however, has the objection of causing too much jar, is to rivet a piece of track iron on each shoulder, at the angle the plate shall stand, and slide the plate down, keeping it away from the screen by a false strip tacked to the front of the board, along each side of the plate itself.

By studying the effect of this its advantages can easily be seen. Twenty years ago it was universal to have a spray of clear water constantly dropping on the apron-plate to form a slight stoppage of the pulp and cause the amalgamated gold to settle and attach itself to the plate, but, as future working of the pulp was very much hindered by this excess of water, it is now rarely used. Again, to a certain extent, the output of the mill is increased the greater the amount of water allowed to flow into the battery, and as a result, all the water that could be used and not hinder future operations, is allowed to run into the mortars.

No matter how fine the gold may be, if it is free, under proper amalgamation, it will adhere to an amalgamated surface if brought into actual contact with it, and this the splash-plate does in two ways. First, half of the pulp is thrown against it in a coarse spray, aiding adhesion by the force; second, it then runs down the plate, dropping off the bottom exactly similar to the spray from the clear water troughs used in earlier times, forming a dam and eddies, which cause the other half of the pulp to precipitate its fine amalgam at this point.

From here, the object to be attained is to have no rapid and strong currents. To turn the pulp over and over, so that all parts are frequently brought in contact with the amalgamated surface, and to spread this pulp as thin as consistent with keeping it in constant motion and allowing no settling of even the heavy sulphurets on the plates. It is also found that an occasional drop of one inch to one and one-half inches is a benefit.

Now we come to one of the greatest mistakes made by 90 per cent, of all amalgamators, an error held to like the grip of death by men of broad technical education, causing the failure of many properties which should be successful.

This is the grade given to the large plates. These should never be placed with less than 2^ inches to each foot, and in most cases 3 inches would be better, and even more, while better work can be done at an angle of forty-five degrees than with a grade less than 2y2 inches.

Take a case where the plates are set below 2^ inches, we find an excess of water required to keep all the pulp moving and, therefore, too great a depth, so that the fine gold is held in^ftfmufif ad*?! never touches the amalgamated surface, when we truly have float gold, while an equally bad feature, almost universal, is that it allows too much quicksilver to be used, and fine gold will never be saved with even a slight excess of quicksilver. This point will be more fully explained under the discussion of amalgamation.

With the plates at the proper grade and the requisite amount of water, the pulp travels down the plates in a succession of waves, the lower edge of each wave rolling over and over and bringing every particle of water even, in contact several times with the amalgamated surface. The more sulphides the ore carries, the steeper must be the grade of the plates, to clear them and still not interfere with the concentration of the tailings.

After this explanation we can continue the description of the plates.

From the lip-plate the pulp has a drop of nearly two inches and falls on the first plate separate from the mortar.

The apron-plate should be divided into two sections, the first easily removed at all times. This should be the full length of the mortar, and two feet wide, and that end nearest the battery resting on the wooden shelf fastened to wooden mortar-block, and held in place by the lip of the mortar or a strong hook on each side, while the lower end rests on the second section. This second section must have the same width, and be from 4 to 6 feet long. It is supported by extension legs firmly secured to the floor. By using this form of support the grade of the table can be changed to suit the ore, after practical experiment. Start with a grade of 'ufa to 3 inches, and, if too much water is required to keep all the pulp in steady motion, lower the second section (the first assuming the same grade, as it rests on it), until the pulp is seen to move freely and in successive waves with a minimum of water.

On most ores this amount of plate surface is all that is necessary, but if there is still sufficient fall to allow the tailings to be readily carried away automatically, the pulp is now passed over "sluice plates," as a safeguard against accidents, after passing through a quicksilver trap.

These sluice plates are 18 inches wide and from 8 to 10 feet long, and are set at a minimum angle of 1% inch per foot. All the plates should have a coating of silver of three ounces to every square foot, although on the sluice plates two ounces will do.

There are several devices for traps, but most of them are only receptacles where a slight obstruction to the flow of the current is given and this is not enough. We want to accomplish two objects: first, retain all the amalgam and quicksilver that may have escaped the copper plates; second, secure this with a minimum of sand and sulphurets.

A large open box is always filled with sand which is gradually replaced by coarser sand and sulphurets according to the length of time before its complete removal, while only a modicum of loss is arrested here. The next result of experimenting caused the pulp to descend on one side and rise on the other of a partition, the higher specific gravity of the valuable portions aiding their retention, and It is on this principle, that the most successful traps are now constructed; the best of all being those used in the mills of the "Homestake" in the Black Hills, and designated as the "Black Hills Trap."