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ployed with advantage, however, to clean the quicksilver, or to collect it when floured or granulated. Gum catechu also is a cleansing agent.
The mill process at Owyhee is essentially the Washoe process. In the Owyhee mill the ore is broken in a Blake crusher, and delivered to the stamp-batteries, where it is pulverized with water, and discharged though screens, having 144 holes per square inch, into tanks. In these the pulp settles, and the water passes through other settling-tanks and out of the mill to the slime reservoirs, of which there are five. In these the light slimes are precipitated, to be reconveyed by means of a tramway, bull-wheel, rope, and car, to the mill for further treatment.
The pulp is taken by means of a car from the battery-tanks to the pans. Here it is mixed, ground, and amalgamated for six hours, steam and chemicals being employed to assist the process. From the pans the charge passes into settlers, thence into agitators, thence to Hungerford concentrators and Evans's riffles; and finally, the tailings pass over a string of blanket-sluices. The average yield of this mill, without reckoning the results of the reworking of the slimes, is 92 per cent. of the fire assay. This must certainly be regarded as the most successful application of the Washoe process in the country. The character of the ores no doubt facilitates their economical reduction; but the extraordinary efficiency of the mill is certainly due in large part to well-constructed machinery and to most skillful and faithful superintendence, coupled with constant study of the mechanical and metallurgical problems involved.
Those who find in the supposed imperfections of this or that process an excuse for heavy losses of the precious metals in reducing ores would do well to note such instances as this and profit by the example.
The Washoe process, as practiced by the Meadow Valley Mining Company in Ely District, Nevada, is described in the article upon Lincoln County in this report.
The extraction of gold by chlorination was introduced by the cele. brated metallurgist, Plattner, a professor in the School of Mines, at Freiberg, Saxony. It is practiced in a few localities in Silesia, Hungary, Transylvania, etc., and is acknowledged to be the most complete method of gold extraction on a large scale. It was first introduced in this country, thirteen years ago, by G. W. Deetken, of Grass Valley, California, a skillful metallurgist, who has added some mechanical improvements to the process, and successfully overcome many difficulties arising in its local application. In the present chapter a general outline of the method will be given. For more detailed information recourse may be had to the work of Mr. Guido Küstel on Concentration and Chlorination, pub. lished in 1868 at San Francisco; and for later improvements and particulars not contained in that work, to Mr. Deetken himself, who still resides in Grass Valley, and may be considered the best authority ou the subject.
The principle involved is the transformation of metallic gold,, by means of chlorine gas, into soluble chloride of gold, (the aurum potabile of the alchemists,) which can be dissolved in cold water, and precipitated in the metallic state by sulphate of iron, or as sulphide of gold by sulplureted hydrogen gas. This precipitate may then be filtered, dried, and melted with suitable fluxes, to obtain a regulus of malleable gold.
From this brief statement, it follows that the following conditions are necessary to the success of the process :
1. The gold in the material subjected to the chlorine must be in a metallic state, and accessible to the gas.
2. There must be no other substances in the charge which will unite with free chlorine, since this would occasion a great waste of gas, and a failure in the desired separation of gold from other metals.
3. There must be nothing in the chlorine employed which will attack and render soluble other metals or bases; since this would render the subsequent solution and precipitate impure.
4. There must be no reaction in the mass treated with chlorine which will prematurely precipitate the gold before the final solution is obtained and drawn off.
5. In a word, it is required that all the gold, and, if possible, nothing else, shall be obtained in the final solution. Precipitation and melting then present no special difficulties.
The process naturally divides itself, therefore, into the preparation of the ore for the action of chlorine; the preparation of the chlorine; the chlorination proper; and the lixiviation, precipitation, and melting.
PRELIMINARY TREATMENT OF THE ORE.
Ores, consisting of quartz and free gold, without admixture of other earths or sulphurets, require no further preliminary treatment than reduction to powder. As the material subjected to chlorination has almost invariably been concentrated mechanically beforehand, no appa
I. Ex. 10—27
ratus for crushing or concentration is usually connected with chlorination works. In this account it will be assumed that the raw ore has in all cases been finely pulverized.
Ores containing sulphurets or arseniurets are prepared by roasting. That this class must comprise most of the ores treated by chlorination is evident when it is considered that the presence of these compounds is the chief obstacle to successful amalgamation, and that the concentration of the tailings from amalgamating mills is practically a collection of the sulphurets and arseniurets which they contain.
The object of roasting is to convert the base metals into oxides that will not unite with the chlorine, and to leave the gold in a condition suitable for its chlorination. The latter object may be defeated by one of three causes. The gold in the ore may be in coarse particles, which chloridize too slowly for practical economy. For this reason, ore containing coarse gold is not treated by the chlorination process. Again, the gold may be alloyed with silver, the chloride of which is insoluble, and may form a coating upon the particles, preventing the complete chlorination of the gold. Kiistel thinks that gold of very low fineness, containing from 40 to 50 per cent. of silver, will probably resist the chlorination, unless it is in the finest state of pulverization. In such a case, as indeed wherever there is silver to be extracted, some inodifica tion of the Plattner process, such as the lisiviation with chlorinated brine, must be employed. This will be alluded to hereafter. Finally, it is possible, though perhaps not demonstrated as yet, that other substauces, such as oxide of iron, for example, may coat the gold and hinder the action of the chlorine. This evil (if it exists) is remedied by an addition of salt in roasting. I speak with some doubt on this point, though the efficacy of the use of salt in roasting some ores for chlorination is undoubted. But the nature of this benefit may be otherwise explained than by supposing it to cousist in the removal of a coating from the gold. If the latter were the case, then, it seems to me, salt would always be necessary in the preliminary roasting; but this is not the case. Experiments in Colorado (see page 346 of my last report) have indicated that a coating is left upon gold in the roasting of auriferous sulphurets, that it is probably oxide of iron, and that it may be removed by the addition of salt toward the end of the roasting ;* but while this coating may be sufficient to prevent close contact of gold and quicksilver, and so hinder amalgamation, it does not necessarily follow that it will prevent the action of chlorine gas.
Turning to consider the first object of roasting, the oxidation of the base metals, we find that it must be conducted with great skill and care to insure the success of the subsequent chlorination. The following
* Of the beneficial effect of salt in the roasting of auriferous ores, preliminary to amalgamation, I entertain little doubt. The experiments in Colorado, here referred to, were thoso of Mr. Brückner, with his roasting-cylinders. They are, perhaps, corrobo rated by the unexpected results of the first Stetefeldt furnace, erected at the Twin River Mill, Nyo County, Nevada, and tested upon silver ores. It was found that the pan amalgamation of the roasted ore yielded a small percentage of gold in the silver bullion, which had not been tho case when ordinary reverberatories were used. In this case, however, salt was employed in the reverberatories as well as in the Stetefeldt furnace; and the only. explanation I can at present suggest for the difference in results is, that the finely pulverized condition of the ore in tho latter, and the complete access afforded the chloridization agents to cach particle, effect a complete chloridizing, and allow an action upon fine particles of gold not secured in the ruder reverberatory process. The reverberatories used in chlorination works, however, differ from those in ordinary silver-mills, and the roasting is more careful and thorough. In this case, salt added in the late stages of the process may be more likely to act upon the supposed coating of the gold. But its true function under euch circumstances is, I think, in its action upon magnesia, lead, and perhaps liine.
conditions are involved, together with others, less important, or less peculiarly characteristic of this process:
1. In all roasting operations a high initial temperature is likely to cause a sintering of the sulphurets, and a formation of matte, which cannot be oxidized without a new pulverization. This evil is particularly to be dreaded when galena is present. A low heat and diligent stirring are therefore required at the beginning of the process.
2. Under these conditions, the sulphur of the sulphurets is set free, combining with the oxygen of the air to sulphurous acid, which escapes in gaseous form, and to sulphuric acid, which unites with the oxidized metals (especially iron) to form sulphates.
3. The sulphate of iron (protoxide) is, as has been already remarked, a precipitant of gold from its chloride solution. Hence its presence in the roasted ore will defeat the object of chlorination and lixiviation; and it is therefore necessary to destroy the sulphates. This is accomplished by gradually increasing the temperature until a point is reached at which these combinations are dissociated. The formation and sub. sequent decomposition in the charge of arseniates is governed by similar laws.
4. Lime and magnesia, as well as lead, exert an unfavorable influence on the chlorination, after roasting. For some time after the process had been successfully employed in Grass Valley, the concentrated sulphurets from the Eureka mine, in that district, presented a mysterious obstacle to its application. Chlorine was absorbed by them after roasting; but it seemed to be wasted upon some other substance than the gold. Yet the oxidation in the furnaces appeared to be reasonably complete. Mr. Deetken, who was called into consultation, succeeded in overcoming the difficulty, and became the manager of the chlorination works of the company. According to his experiments, it seems probable that lime, magnesia, (and lead oxide ?) are attacked by the chlorine, forming hypochlorites, or oxychlorides and chilorides, after the manner of alkalies. Mag. nesia, at least, undoubtedly shares with the alkaline bases this property of combining with chlorine. The indifference of the ordinary metallic oxides may, perhaps, be less complete than has been usually supposed. In the case of the Eureka sulphurets the troublesome constituent appears to have been magnesia from the gangue or country rock, The cure was the addition of salt toward the close of the roasting, and at high temperature, by which means the magnesia (sulphate ?) appears to have been chloridized.
5. The presence of any soluble metallic salts is injurious, since they at once react upon the oxide of iron, and the latter precipitates the gold from its solution before lixiviation.
6. Chemists anderstand that metallic oxides, which do not readily, or do not at all, react with chlorine to form chlorides, may decompose readily with hydrochloric acid, since the latter contains hydrogen, which satisfies the oxygen of the base. To explain the matter rudely, (and according to the old-school formulas which are still current among metallurgists,) the equation R 0+CI=RCI+0 represents a reaction which practically does not take place, the affinity of the metal for chlorine not being sufficient to expel free oxygen. But in the reaction expressed by RO+H CI=R CI+H10, the affinity of the metal for chlorine is reënforced by that of hydrogen for oxygen, and the formation of a metallic chloridle and of water simultaneously occurs. But this formation of any other soluble chloride than that of gold is to be avoided, because it leads, as I have indicated, to a premature precipitation of gold. Hence,
the chlorine gas employed in this process must be carefully freed from muriatic acid.
7. Muriatic or hydrochloric acid acts injuriously in another way, namely, when by reason of incomplete roasting the charge contains metallic sulphurets. Decomposing these, the acid produces chlorides, setting free sulphureted hydrogen gas, which is a precipitant of gold from its chloride solution.
8. The free chlorine is both annoying and destructive of health. Care should therefore be taken to ventilate the works thoroughly and to protect the workmen as much as possible against the inhalation of
The roasting is performed, as I have said, in a reverberatory furnace. This kind of furnace derives its name from the fact that the ore treater! in it lies upon a hearth, over which is a low arched roof; and the flame from the fuel in the tire-place at one end, passing over a dividing all called the fire-bridge, "reverberates” along the roof, and is reflected upon the charge. In a muffie-furnace the flame is not allowed to come in contact with the ore, but surrounds and heats the muffle or small oren containing the latter, while the actual oxidation is effected by fresh air introduced from outside. Naturally there is a loss of heat in this form. and the reverberatory, which gives in the case now under discussiou equally satisfactory results, is preferred on account of the saring of fuel. The Gerstenhöfer or the Stetefeldt furnace may perhaps hereafter be applied to this use with still greater economy.
Küstel gives several examples, with descriptions and diagrams, illus•trating different forms of reverberatories employed. They may be classified as single and double furnaces. The latter has two hearths, one over the other; and the roasting is began upon the upper and finished upon the lower hearth. The furnace erected by Mr. Deetken at the Enreka works, which appears to be one of the best forms, has the lower hearth placed, not immediately under the upper, but in continuation of it, on a level 7 feet 10 inches lower. The tiro are connected by a step-flue. The upper hearth is 6 feet wide by 39 long, and has six working-doors on each side. Through these the charge is stirred and pushed along, as desulphurization advances. The lower heartb, imme diately next the fire-place, is much shorter. Here the final roasting takes place, with the addition of salt. A draught through the whole is maintained by means of a chimney 25 feet high and 28 inches square in the clear. Mr. Küstel says of this furnace that it requires more room than the ordinary double furnace, but the work of stirring is less tiresome, since the roaster is not obliged to step constantly up and down. Another advantage is the extent of the upper hearth, which receives nine tons of ore without difficulty, whereas the charging of a furnace two (or even three or four) stories high is troublesome if not favored by sloping ground. It takes about twenty hours to finish the roasting of a charge of 2,000 pounds of sulphurets; but by employing a large and long furnace, such as is here described, over ten tons can be treated continuously, the latest charge receiving its preliminary while the earliest receives its final roasting. The capacity of such a furnace appears from the following brief description of the process, nearly as given by Mr. Küstel.
The heat in the lower hearth is always kept bright. One ton is roasted below, and about nine tons are spread
on the upper long hearth
. Two roasters are constantly at work, mainly at the separate hearths but together, when required, at either. The ore on the upper hearth is pushed along as the process proceeds, until it arrives at the flue leadiug