The amalgamation.—This is performed in horizontal casks, arranged in rows, each turning upon a shaft which passes through its axis ; and all driven by water wheels. The casks are 2 feet 10 inches long, 2 feet 8 inches wide, inside Ines sure, and are provided with iron ends. The staves are 31 inches thick, and are bound together with iron hoops. They have a double bung-hole, one formed within the other, secured by an iron plug fastened with screws. They are filled by means of a wooden spout terminated by danvass hose ; through which 10 cwts. of the bolted ore-flour are introduced after 3 cwts. of water have been poured in. To this mixture, from 1 to 1. of a cwt. of pieces of iron, 11 inch square, and 1 thick. When these pieces get dis solved, they are replaced by others from time to time. The casks being two thirds full, are set to revolve from 11 or 2 hours, till the ore-powder and water become a uniform pap ; when 5 cwts. of quicksilver are poured into each of them. The casks being again made tight, are put in gear with the driving machinery, and kept constantly revolv ing for 14 or 16 hours, at the rate of 20 or 22 turns in the minute. During this time they are twice stopped and opened, in order to see whether the pap be of the proper consistence ; for if too thick, the globules of quicksilver do not readily combine with the particles of ore ; and if too thin, they fall and rest at the bot tom. In the first case, some water must be added ; in the second, some ore. During the rotation, the temperature rises, so that even in winter it some times stands so high as 104° F.
The chemical changes which occur in the casks are the following ;—The me tallic chlorides present in the roasted ore are decomposed by the iron, whence re sults munate of iron, whilst the den tochloride of copper is reduced partly to protochloride and partly to metallic copper, which throw down metallic sil ver. The mercury dissolves the silver, copper, lead, antimony, into a complex amalgam. If the iron is not present in sufficient quantity, or if it has not been worked with the ore long enough to con vert the copper deutochloride into a protoehloride, previously to the addition of the mercury, more or less of the last metal will be wasted by its conversion into protoehloride (colomel). The water holds in solution sulphate of soda, unde composed sea salt, with chlorides of iron, manganese, &c.
From 300 lbs. of amalgam 7 lbs. of im pure silver mass is obtained, which yields from 10 to 13 parts out of 16 of pure silver—one-fifth being copper. The crude silver is refined in blacklead cru cibles, filled within 2 inches of their brims, and exposed to a high heat. The mass gives oft vapors, and throws a liquid slag, which being skimmed off, the surface is to be strewed over with charcoal powder, and covered with a lid. The heat having been briskly urged for a short time, the charcoal is then re moved along with any fresh slag that may have risen, in order to observe whether the vapors have ceased. If not, fresh charcoal must be again applied, the crucible roust he covered, and the heat increased, till fumes are no longer pro duced, and the surface of the silver be comes tranquil. Finally, the alloy, which contains a little gold and much copper, being now from 11 to 13 parts of fine silver in 16 parts, is cast into iron moulds, in ingots of 60 mares. The loSs of weight
by evaporation and skimming of the slag amounts to 2 per cent. ; the loss in sil ver is quite inconsiderable.
In Mexico a new process has been in troduced which dispenses with amalga mation. The ore is converted into a chloride by common salt, which chloride is then dissolved by a solution of salt. Copper slips are thrown into the liquor, which separate the silver by precipi tation.
Dr. Percy, of England, proposes to separate the silver from the ore by the wet way, using hyposulphite and chloride of lime: the process is recommended as economical.
Till very recently, the only operations employed for separating silver from lead in the English smelting-works, were the following:— 1. Cupellation, in which the lead was converted into a vitreus oxide, which was floated off from the surface of the silver.
2. Reduction of that oxide, commonly called litharge.
3. Smelting the bottoms of the enpels, to extract the lead which had soaked into them, in a glassy state.
Cupellation and its two complementary operations were, in many respects, ob jectionable processes ; from the injunous effects of the lead vapors upon the health of the workmen ; from the very consid erable loss of metallic lead, amounting to 7 per cent. at least ; and, lastly, from the immense consumption of fuel, as well as from the vast amount of manual labor in eurred in such complicated operations. Hence, unless the lead were tolerably rich in silver, it would not bear the expense of cupellation.
The patent process lately introduced by Mr. Pattison, of Newcastle, is not at to the health of workmen; it does not occasion 2 per cent. of loss of lead, and in other respects is so economi cal, that it is now profitably applied in Northumberland to alloys too poor in sil ver to be treated by cupellation. This process is founded on the following phe nomena.
After melting an alloy of lead and silver, if we allow it to cool very slowly, continually stirring it meanwhile with a rake, we shall observe at a certain period a continually increasing number of imperfect little crystals, which may be taken out with a drainer, exactly as we may remove the crystals of sea salt de posited during the concentration of brine, or those of sulphate of soda, ss its agi tated solution cools. On submitting to analysis the metallic crystals thus sepa rated, and also the liquid metal deprived of them, we find the former to be lead almost alone, but the latter to be rich in silver, when compared with the original alloy. The more of the crystalline par ticles are drained from the metallic bath, the richer does the mother liquid become in silver. In practice, the poor lead is raised by this means to the standard of the ordinary lead of the litharge works ; and the better lead is made ten times richer. This very valuable alloy is then submitted to cupellation ; hut as it con tains only a tenth part of the quantity of lead subjected to crystallization, the loss in the cupel will be obviously reduced to one-tenth of what it was by the formei process ; that is, seven-tenths of a per cent. instead of seven.