Sidebilder
PDF
ePub

poured upon its surface. If the borax is added sooner, it acts too much on the lower part of the pot; and, if thrown in cold, is apt to chill the gold. The clay-pipe which is to convey the chlorine to the bottom of the melted gold is now introduced. (It is necessary to carefully heat the lower portion of this pipe for some ten minutes before introducing it into the molten gold, or it is apt to split.) At the moment of its entering the melted gold, the screw compression-clamp is slightly loosened, so as to allow a small quantity of gas to pass through it, and thus prevent any metal rising and setting in the pipe, which is then gradually lowered to the bottom of the molten gold, where it is kept by means of a few small weights attached to the top. The compression-tap is now quite relaxed, and the gas is heard bubbling up through the melted metal, which it does quietly, and without projection of globules from the pot.

Sufficient hydrochloric acid must be added to the generators, from time to time, to keep up a rapid evolution of chlorine. A rough general rule is to allow one imperial quart of acid of 1.15 specific gravity to every 10 ounces of silver in the alloy operated on. The column of liquid in the safety-tube, acting, as it does, like a barometer, affords a ready means of knowing the pressure in the generator, and of judging of the rate of production of the gas, as well as at once showing by its fall, if anything irregular has occurred-such as a leak or a crack of the chlorine pipe or pot. From 16 to 18 inches in the safety-tube correspond to and balance 1 inch of gold in the refining crucible. When the chlorine is first introduced into the melted gold, a quantity of fumes are seen to pass up from the holes in the crucible-lid; these are not chloride of silver, but the volatile chlorides of some of the baser metals, and they are especially dense when much lead is present in the alloy under treatment, forming a white deposit on any cold substance presented to them. After a time, longer or shorter, according to the impurities in the gold, these fumes cease. So long as any decided quantity of silver is presented in the molten gold, the whole, or nearly the whole, of the chlorine is absorbed, little, if any, appearing to escape, and to be thus wasted; and it is found that the better the supply of chlorine the quicker is the operation.

It is a curious circumstance that, though, in toughening with corrosive sublimate, this substance is only thrown on the surface of the melted gold, yet the whole mass is toughened by its action. It seems essential, in using chlorine, that the gas should pass to the very bottom to effect a complete refinage.

As soon as the operation is nearly over fumes of a darker color than those observed at the commencement make their appearance; and the end of the refinage is indicated by a peculiar flame or luminous vapor of a brownish yellow color, (occasioned by the free and now waste chlorine escaping,) which can be seen on removing a small plug which fits into a hole in the lid of the pot. This, however, of itself, is not a sufficient indication; the process is not complete until this flame imparts to a piece of white tobacco-pipe, or similar substance, when held in it for a moment, a peculiar reddish or brownish yellow stain; so long as it gives any other color, the refinage is unfinished. When these appearances are observed (usually for gold containing about 10 per cent. of silver in about an hour and a half from the introduction of chlorine) the gas is shut off, and the pots removed from the fire, the white crucible lifted out of the black one, and, together with its contents, allowed to stand seven minutes, until the gold becomes cool enough to set or solidify. The chloride of silver, which remains liquid much longer, is then poured off into iron molds. The crucible is then inverted on an iron table, when the still red-hot gold falls out in the shape of a cone; this is slightly scraped, and then thrown, hissing, into a concentrated solution of common salt to free it from any adherent chloride of silver.

An alloy containing originally 89 per cent. of gold, 10 per cent. of silver, and 1 per eent. of base metals, will yield, on an average, a cake of chloride weighing, with a little adherent borax, 16 ounces for every 100 ounces operated on.

It is necessary very carefully to dry and heat the molds into which the chloride of silver is poured, as the slighest moisture causes the latter to be violently dispersed while red-hot, to the great risk of the bystanders. With ordinary care, this will never happen; but attention is called to the point, as a very deliquescent chloride of iron is apt to form on the molds.

The gold is now fine, and simply requires remelting into ingots.

As before stated, it is found that all these operations can readily be performed, and about 2,000 ounces refined per day in three common melting-furnaces, between 9 a. m. and 2 p. m.; 98 per cent of the gold originally contained in the alloy operated on is then ready for delivery. The other 2 per cent. remains with the chloride of silver, partially in the metallic state, and partly in a state of combination with chlorine, and probably silver.

To free the chloride of silver from this combined gold (that mechanically mixed being eliminated at the same time) it is melted in a boraxed white pot, with the addition of from 8 to 10 per cent. of metallic silver, rolled to about inch thickness. The chloride of gold is, by this means reduced at the expense of the metallic silver, chloride of silver being formed; while the liberated gold sinks, and melts into a button at the bottom of the pot. As soon as the whole is thoroughly melted, the pot is removed

from the furnace, and allowed to stand about ten minutes, and the still liquid chloride of silver is then poured into large iron molds, so as to form slabs of a convenient thickness for the next operation; that is, its reduction to the metallic state.

After the fusion of the chlorides, a small quantity of a curious spongiform substance adheres to the sides of the crucible used, probably consisting of sub-chloride of silver; but since it always contains a little gold, care has to be taken in pouring off the fluid chlorides to prevent this auriferous sponge from falling out and mixing with them.

The fusion of the chlorides with metallic silver does not remove every trace of gold; but, with proper care, the amount remaining in the silver produced need not exceed three parts in 10,000, or about two grains of gold in every pound (troy) of silver-a quantity too small to pay for further extraction in this colony.

The slabs of chloride of silver are reduced without difficulty by plates of wrought iron or zinc, in the usual way; but my friend and colleague, Dr. Leibius, has contrived a very excellent apparatus for this purpose.

The manager of the Bank of New South Wales has kindly allowed me the use of 500 ounces of Queensland gold to illustrate this paper. This quantity was divided into two nearly equal parts; one portion weighing 248 ounces was left in its original unrefined condition, as seen in the ingot on the table; the other portion weighing 252 ounces was refined in the manner described above, and the resulting bar of fine gold, assaying 995, is placed by the unrefined ingot for comparison, and the silver extracted weighing 38.8 ounces, and assaying 991.1 lies beside it.

Besides the separation and recovery of the silver as above described, another useful end is gained by this process.

A very large proportion of the gold of Australia (more especially that obtained by amalgamation from our quartz-veins) is more or less brittle-an effect generally due to the presence of small quantities of lead or antimony, rendering the bullion quite unfit for coinage or manufacture until it has undergone some process to render it tough.

The methods usually employed for this purpose are either fusion with niter and borax, melting with oxide of copper, or the addition of corrosive sublimate (bichloride of mercury) to the melted gold. The two former of these plans are troublesome, from the corrosive action they exert on the crucibles, and the last (namely, the employment of corrosive sublimate, which is that usually adopted) is most objectionable, from the dense and highly injurious fumes evolved.

In Victoria this is regarded as so serious a matter in a public and sanitary point of view, as to have induced the municipal council of Melbourne to institute an action at law against the Union Bank to compel them to abate the nuisance thus created by their gold-melting establishment. The passage of chlorine-gas through the melted gold is found to effect the complete toughening of the metal by the elimination, as volatile chlorides, of the materials which render it brittle, while the evolution of the deleterious mercurial fumes is avoided.

In the metallurgic treatment of the precious metals some loss is always sustained; but that incurred in the process here described is not found to be excessive.

The average loss of gold in operating hitherto has been found to amount to nineteen parts in every 100,000 of alloy treated, which is considerably less than would be met with in toughening an equal amount of gold with corrosive sublimate in the ordinary

manner.

The loss of silver has amounted to 240 parts in every 100,000 of alloy operated on (containing, originally, say 10 per cent. of silver.) There is no doubt that a considerable portion of both these losses would be recovered on further treating the pots and ashes remaining after the operation; and it is found that, as manipulatory skill is acquired, the proportional loss of silver appears to be decreasing. In refining on the large scale, gold containing 10 per cent. of silver, the cost of the operation in Sydney, including labor and the above losses of gold and silver, but exclusive of rent of premises and superintendence, is about five farthings per ounce, but varying with the quantity of silver present in the alloy operated on.

In England, where hydrochloric acid is a waste product of the alkali works, and all apparatus is cheaper, the cost of refining by this method would be proportionally less. The fineness of the gold produced by this process varies from 991 to 997 in 1,000 parts, the average, as found on a refinage of many thousand ounces, being 993.5, or 23 carats, 34 grains. The remaining 64 thousanths are silver; and this compares favorably with any of the previously known practical processes, none of which leave less silver than this in the resulting fine gold.

If the refined gold be subjected to a re-refinage by chlorine, the amount of silver left in it can be reduced to 0.2 per cent., just as in the refinage by the ordinary sulphuric acid process, the same result can be obtained by subjecting the refined gold to a further refinage with bisulphate of potash. For practical working, however, this would probably never be attempted.

The silver resulting from this method of refining is tough, but its quality varies somewhat according to the gold originally operated on; if the alloy treated contains

much copper, the greater part of this remains with the resulting silver, but the other metals are nearly all eliminated.

The fineness of the silver hitherto obtained has varied from 918.2 to 992.0 in 1,000 parts, the average being 965.6. Analysis of the silver resulting from the refinage of gold known originally to have contained, amongst the base metals in the alloy, copper, lead, antimony, arsenic, and iron, gave the following result:

[blocks in formation]

A very extended series of experiments have been made at the Sydney Branch of the the Royal Mint to test the value of this process; and the result has been (as mentioned by the honorable the Colonial Treasurer, in his speech on the Budget, October 14th, 1869) that "active steps are now being taken to bring the system into operation" into that establishment.

I have already, in the paper read before the Chemical Society, acknowledged the obligation I feel under to my brother officers, Mr. Robert Hunt and Dr. Leibius, for their kind help and encouragement in perfecting this process of refining; but my especial thanks are also due to Professor Smith, of the Sydney University, who, in the kindest manner, placed his laboratory at my disposal, to assist me in this matter, and also to Dr. Thompson and Mr. Edward Hill for their valuable and friendly help.

In a paper subsequently read before the same society, Dr. A. Leibius, Assayer to the Sydney Branch of the Royal Mint, described as follows a new apparatus for reducing the chloride of silver, which is employed in connection with the foregoing method.

In the refinage of gold bullion by Miller's new chlorine process, the silver contained in the alloy thus treated is eliminated from the latter in the state of argentic chloride, which, by a subsequent process, is reduced to metallic silver.

This reduction has always been effected in the usual manner, viz, by placing the slabs of fused argentic chloride between plates of wrought iron or zinc, with the addition of acidulated water. Although a perfect reduction to metallic silver has always been achieved, yet it required a considerable amount of time and manipulation, since the thick slabs of fused argentic chloride were, after two or three days, only partially converted into metallic silver, and had to be rearranged in order to expedite their complete reduction. Such manipulations, however, were not only found to be very objectionable on account of the time they required, but more so on account of the very disagreeable work which they caused to the operator. The reduced spongy silver was broken up, by hand, into small pieces, in order to ascertain its complete reduction, and was then boiled in acidulated water to free it from iron or zinc.

It remained, therefore, a desideratum to effect the reduction of the fused masses of argentic chloride in a manner which would at the same time be quicker in its execution, and also obviate the just-alluded-to manipulations.

In 1868, Messrs. De la Rue and Hugo Miller, in London, constructed a galvanic battery, one pole of which consisted of fused argentic chloride the thickness of a goose-quill, the other pole of cylinders of zinc. Adopting this principle, I have endeavored to construct an apparatus which should fulfill the requirements before

referred to.

After operating successfully with a small model which allows the reduction of about 250 ounces of argentic chloride in one operation, I have, with slight modifications, constructed an apparatus which will reduce from 1,400 to 1,500 ounces of argentic chloride in twenty-four hours. The apparatus and its dimensions are as follows:

Two thick boards, 15 inches long, are joined together on both ends by three strong battens, so as to form an open box without a bottom, 13 inches long by 14 inches wide, and 15 inches high, (inside measurement.) The two boards forming the length of the box or frame contain seven vertical grooves, inch wide, and inch deep, at intervals of 14 inches from each other. These grooves are cut down to a length of 12 inches, leaving 3 inches of each board forming the legs of the frame.

At the termination of these grooves passes horizontally a narrow slit, inch deep, and along the whole length of each board, into which a strip of metallic silver, inch wide, and the thickness of about a threepenny-piece, is tightly fixed, projecting on one side of the frame about 18 inches beyond each board.

The seven grooves already alluded to are for holding zinc plates, inch thick, 14 inches long, and 12 inches high, which rest on both sides on the strips of silver, which, as just described, are jammed horizontally into the sides of the two boards. A connection is thus established between the seven zinc plates and these strips of silver.

The second part of the apparatus consists of a wooden frame, cut out of a solid board 1 inch thick, and supplied with two large iron handles. This frame is the same length as the box holding the zinc plates, but 3 inches narrower. It contains on each side, parallel to the direction of the zinc plates, twelve slits inch long, which hold silver bands inch broad, and the thickness of a threepenny-piece. These silver bands are passed through the slits in the board, so as to form on cach side of it six loops, 114 inches in length, and 4 inch wide. The six loops on one side are exactly opposite to those on the other side of the board, at a distance of about 9 inches. They are intended to hold the slabs of argentic chloride, which are 12 inches long, 10 inches high, and about inch thick, and are put through these loops lengthwise, projecting on each end about 1 inch beyond the silver bands.

The whole frame holds, as before stated, six of these slabs of argentic chloride, which are placed between the six spaces formed by the seven zinc plates, from which latter they are about inch apart on each side.

The projecting horizontal strips of silver jammed into the sides of the lower frame are then connected with the ends of the silver forming the loops in which the argentic chloride is suspended; and the whole apparatus thus charged is placed in a tub filled with water. After a short time, galvanic action is discernible; the liquid gets gradually warmer, and a strong galvanic current is observed. After about twentyfour hours, the action has nearly ceased, and the whole argentic chloride is found to be completely reduced to metallic silver, which retains in the silver loops the same shape, and outwardly also, nearly the same appearance as when first introduced as argentic chloride. The latter contains always more or less chloride of copper, (eliminated, together with the silver during the operation of refinage by chlorine,) which is reduced together with the chloride of silver; in fact, this soluble chloride of copper helps to act as an exciting liquor for the battery. In the first experiments, a weak solution of salt (chloride of sodium) was used as exciting liquor; but it was found that this could be dispensed with, and only common water used, (the action, however, is, in this case, a little retarded and does not become powerful until about two hours after the battery is set.) By using a part of the resulting liquor from a previous reduction of argentic chloride, and which contains chloride of zinc, it has been found that the galvanic action sets in very rapidly, and accelerates thereby the completion of the reduction.

No acid is used; and, therefore, the amount of zinc used in each reduction has invariably been found to be almost the theoretical quantity required to combine the chlorine of the argentic chloride treated with the metallic zinc, in order to form chloride of zinc.

The quantity of metallic zinc thus used was always from 24 to 25 per cent. of the weight of the argentic chloride reduced.

The reduced silver is boiled out in acidulated water, in order to remove the basic and oxy-chlorides, and finally in pure water, while still suspended in the silver loops. As soon as it is taken off the last boiling, it is immediately ready for the melting pot, since the heat from the boiling water dries the porous mass of silver sufficiently to allow of its immediate melting. The seven zinc plates, when first used, weigh about 140 pounds avoirdupois; the six slabs of argentic chloride, of the dimensions already given, weigh about 1,400 ounces troy.

The zinc plates are used over again, until too thin for that purpose, when they are remelted, and cast into new plates. It has been found that the quantity of zinc used is little, if at all, increased by prolonging the time of connection with the silver plates after the reduction is completed; the whole apparatus, when once set in operation, can therefore be left to itself until it is found convenient to melt the reduced silver.

While this apparatus reduces the argentic chloride much quicker than if the latter is simply placed in contact with zinc or iron plates, it obviates any handling of the argentic chloride from the time the latter has been placed in the silver loops until the reduced silver is ready for the melting-pot-advantages which have been fully appreciated by those who formerly had to resort to tedious and disagreeable manipulations.

CHAPTER XVI.

SMELTING SILVER ORES.

The base metals used for the extraction of silver from its ores by smelting are lead and copper, and the different methods employed may be accordingly classified under the general headings of "extraction by means of lead," and "extraction by means of copper." The latter is as yet not in operation in the United States, and I therefore pass over this subject for the present. The extraction of silver from its ores by means of lead has, on the contrary, assumed such proportions in the West during the last year, that a discussion of this business at the present time seems important. Although it is here impracticable to go into the details of all the different processes in use in various parts of the world, I may still hope to do some good by dwelling especially on those evidently suited best for the conditions under which the extraction of silver by smelting may be most economically carried on in our western mining districts.

Silver extraction by means of lead is classified according to the shape of the furnaces used for the purpose. Thus we have:

I. Smelting in the open hearth;

II. Smelting in reverberatory furnaces; and

III. Smelting in shaft furnaces.

All these processes have one common purpose, the reduction of the lead to the metallic state and the concentration of the metallic silver in it; but the chemical reactions by which this is accomplished often differ greatly, and the efficiency of each method varies with local circumstances. To know therefore the reactions, and to weigh the circumstances in their economical bearings, is the first duty of those who wish to select a process for a particular locality.

I. SMELTING IN THE OPEN HEARTH.

This method is the oldest and simplest; and up to the present day very few improvements have been made in its original features. It has been and is still employed principally in the United States, and in Scotland and the north of England. The process as practiced in the American hearth is distinguished from the method followed in England and Scotland, chiefly by the employment of hot blast in smelting very pure raw ores. The ores smelted in the Scotch hearth must likewise be free from silica, but not necessarily from other gangue. They are prepared for smelting by roasting in reverberatories, and the blast employed in smelting is cold. In both processes, inferior kinds of fuel, such as wood, peat, &c., can be used. The first condition of the economical use of the hearth in smelting lead ores or a mixture of these with silver ores is therefore purity of ore, especially absence of silica and of foreign sulphureted metals. The ore ought to be in the form of pieces, not crushed. If brought to the smelting-works in the latter condition, it ought to be agglomerated in reverberatories before it is smelted in the hearth, but if this has to be done it would be really more economical to finish the smelting process also in the reverberatory.

The above conditions being primarily requisite for successful smelting in the hearth, and a large loss of both lead and silver by volatilization being certain, unless a very extensive and costly system of condensing

« ForrigeFortsett »