Silver being a precious metal a small per cent of it in lead or copper ores makes it the metal of chief value. Even when it is the metal of lower value a few ounces in a ton of ore means the difference between profit and loss in working a mine. Of the lead-silver ores by far the most important is galena, lead sulphide (PbS). Practically all galena contains silver, the extremes being represented by galena from Carinthia, Austria, containing .05 ounce per ton, and from Idaho, up to 2,040 ounces per ton. In galena the silver is present either as isomorphous silver sulphide or some finely dis seminated silver mineral. In oxydized lead ores, cerussite, etc., the silver is mostly present as chloride. Of the strictly copper silver ores, the most important is tetrahednte, or gray cop per ore, which is essentially a sulphide of cop per and antimony (4CuS.Sb.Ss), though actually of very varying composition, and the related mineral tennantite (4Cu,S.As,S,), also called gray copper. These minerals are of common occurrence in Gunnison, Clear Creek, Summit and Gilpin counties, Colo. Silver is also found associated with chalcopyrite, chalcocite, bornite and other copper minerals, also with iron pyrite, zinc blende, hematite and various minerals, all of which may be classed as silver ores when the percentage of silver is enough to make its extraction profitable. To-day the largest part of the world's silver supply comes from mines not worked for silver alone but rather for gold, copper or lead, the silver being a by-product. Thus it happens that the opera tion of many silver-producing mines and the resulting output of silver depend not on the market price of silver but on the price of lead or copper. This is not true, however, of the Cobalt district, Ontario, where silver is the leading metal and cobalt and nickel are by products.
Metallurgy of Silver Silver ores as previously noted may be divided into silver ores, proper, silver-lead and silver-copper ores. They may also be divided into (1) free-milling; (2) refractory and (3) smelting ores. The first includes native silver, also the chloride and bromide of silver, the metal being extracted by its forming an amalgam with mercury. In so called refractory ores, usually silver sulphide with or without the sulphides of arsenic and antimony, the ores are roasted with salt or otherwise treated prior to amalgamation. The smelting ores include particularly silver-lead and silver-copper ores. The oldest of the methods of extracting silver by amalgamation, one still used in Mexico, is the patio process. In this the ore is coarsely crushed by stamps or by a Chilean mill, then transferred to an ar rastra, a circular space paved with stone, where the ore mixed with water is ground fine under heavy stones drawn around by mules. By the addition of mercury all gold and free silver are extracted. The wet ore is then spread on the patio or amalgamating floor, thoroughly mixed with 3 to 5 per cent of salt, by mules treading it, allowed to stand a few days and then copper sulphate and mercury are added and the ore well trodden for several days, mercury being added from time to time to collect the silver as an amalgam. The chemical reactions which
take place are complicated. Finally the mass is washed to remove non-metallic material, the heavy silver amalgam is collected and the mer cury recovered by distillation, leaving the silver. Of the modern amalgamation processes the Washoe was developed for treating the complex silver-gold ores of the Comstock lode. The ore was broken in a crusher, reduced to pulp by stamps and then transferred to iron pans four to six feet in diameter, known as amalga mating pans. The pans had a false bottom or die, on which revolved an iron plate, or muller, having cast-iron shoes. The ore pulp, one to three tons, with the necessary amount of salt, copper sulphate and mercury, was heated with a steam coil or by blowing in steam and ground three to five hours. Then the light material was washed away and the amalgam collected. Vari ous modifications of the Washoe process have been devised and there are still many mills using some form of pan amalgamation, but the extrac tion is usually not very high. Silver chloride is soluble in a saturated soluble solution of salt and more soluble in sodium hyposulphite. Hence processes have been devised which aim to con vert the silver in an ore to a chloride by roast ing with salt, and then remove it by leaching. In the Augustin process the roasted ore is leached with a solution of salt, and in the Rus sell process with sodium hyposulphite and cuprous sodium hyposulphite. Neither process is now in use in the United States.
Smelting, where practicable, has the advan tages of simplicity, speed and often high ex traction as compared with all other methods of treating ores not free-milling. It is the regular method of treating lead-silver and copper-silver ores. Lead ores are smelted in the usual way (see LEAD) ; the silver goes into the lead and is recovered by the zinc, or Parkes, process. In the copper ores (see COPPER) the silver goes with the copper and may be separated by elec trolytic refining. Gold-silver alloys obtained from milling or smelting ores of many varieties are also treated electrolytically (see Gout). The silver obtained from some silver-lead ores is often 'purified by cupellation. A small re verberatory furnace with a hearth of bone ash is used. The lead bullion is melted, the lead oxydized to litharge by ajet of air until the silver bath is clear, then the silver is cast in molds forming silver pigs or bars. Many ores carrying silver go to concentrating mills equip ped with jigs, vanners, etc., for separating the ore from the gangue. The concentrates thus obtained are usually smelted.