Page:The American Cyclopædia (1879) Volume XV.djvu/58

50 Nevada, particularly in the Comstock lode; miargyrite (AgSbS2), with 36.9 silver, steel-gray to iron-black, metallic lustre, dark cherry-red powder, H. 3, sp. gr. 5.2, occurring in Saxony, Spain, and Mexico; pyrargyrite (Ag3SbS2), dark ruby silver or antimonial silver blende, with 59 silver sometimes a little arsenic, black or by transmitted light deep red, H. 2 to 2.5, sp. gr. 5.759, occurring in Saxony, Baden, Cornwall, Norway, Mexico, South America, and Nevada; and polybasite (Ag9SbS6), with from 64 to more than 72 silver, the antimony being partly and sometimes wholly replaced by arsenic, and the silver partly by copper or to less extent iron and zinc, color iron-black, streak black, H. 2.5, sp. gr. 6.2, occurring in the Hartz, Saxony, Hungary, Mexico, and Nevada. Proustite, or light ruby silver (Ag3AsS3), similar to pyrargyrite, except that the color is lighter and the antimony is replaced with arsenic, occurs in the same localities, but more rarely; it contains 65.4 silver. Copper silver glance or stromeyerite (CuAgS), with 53 silver and 31 copper, iron-black, black shining powder, H. 2.75, sp. gr. 6.2, occurs in Silesia, Chili, and elsewhere. The foregoing are the principal true silver ores. The chief argentiferous ores of other metals are those of lead, copper, and zinc. Iron pyrites and arsenical pyrites, as well as bismuth, cobalt, and nickel ores, may be argentiferous, but it is usually by reason of finely disseminated silver ores throughout their mass. Galena is always more or less argentiferous. In the United States, the galena of the Appalachian range and of the Mississippi valley is usually poor in silver, while that of the Rocky mountains and the interior basin to the Sierra Nevada is highly argentiferous. Oxidized ores are usually poor in silver, but the carbonate, &c., occurring in the limestone of New Mexico, Utah, and the Eureka district, Nevada, are exceptions, being smelted in large quantities for lead and silver. The peculiar ore known as stetefeldtite, which occurs abundantly in Nevada, is an oxidized but massive mineral containing antimony and other base metals, and often very rich in silver. The variable mineral or class of minerals known as tetrahedrite (Fahlerz, argentiferous gray copper, freibergite, tennantite, hermesite) seems to be a combination of metallic sulphides with sulphides of antimony and arsenic, or a sulphide of antimony and copper, in which the antimony may be partly replaced by arsenic, and the copper by iron, zinc, silver, and even, as in freibergite, lead, or, as in hermesite, quicksilver. The percentage of silver varies from a mere trace to 32 per cent. Pure zinc blende is usually poor in silver, but is frequently found in intimate association with true silver ores or native silver, and particularly with argentiferous galena; and in some notable instances the blende is richer than the galena.—The mechanical concentration of silver ores by water is attended with heavy loss, by reason of their usual association with base ores of

nearly the same specific gravity, and their property of cleaving when crushed into fine scales and splinters or dust, which are usually carried away by the current. The yield of silver ores is generally rated in this country in ounces troy to the ton of 2,000 lbs. avoirdupois or 29,167 oz. troy. About 1 per cent. of silver would be equivalent to 292 oz. to a ton. A yield of a little less than 3 oz. is represented by the decimal .0001 or .01 per cent. This small proportion will not pay for the mining and reduction of the ores; but where lead is produced containing .01 per cent. of silver, the latter can still be extracted and saved by refining processes. (See .) The pig lead (variously called work lead, crude bullion, and base bullion) mainly produced from argentiferous galena, carries from 20 to 200 oz. of silver to the ton.—The methods of producing silver from ores and furnace products may be divided into three classes: smelting, amalgamation, and humid extraction. The smelting processes are mostly based upon the capacity of metallic lead, as well as its oxide and sulphate, to separate silver under fusion from its combinations, the liberated silver alloying itself with an excess of lead and accumulating in the metallic bath in the hearth of the furnace. The following chemical equations indicate the typical reactions of the lead smelting processes: Ag2S + Pb + xPb = Ag2,xPb + PbS; Ag2S + PbO = AgPb + SO2; Ag2S + PbSO4 = Ag2Pb + 2SO2. (See .) From the argentiferous lead thus produced the silver is obtained directly by an oxidizing fusion (cupellation), transforming the lead into litharge and leaving metallic silver upon the cupel; or the argentiferous lead is first submitted to treatment in a battery of melting kettles, in which at a low temperature a portion of the liquid mass crystallizes, while another portion, rich in silver, remains liquid; and the crystals being ladled from each kettle to the next, and there submitted to remelting and recrystallization, while the liquid is passed down the series in an opposite direction, the contents of silver are at last chiefly concentrated into a small quantity of so-called rich lead, which is then cupelled (the Pattinson process); or the silver is extracted from the molten lead by means of the superior affinity between silver and zinc, metallic zinc being added to the bath and the zinc-silver alloy rising to the surface and being skimmed off and submitted to further treatment by means of smelting, liquation, or distillation (the Parkes process, with the modifications of Cordurié, Flach, and others). In smelting argentiferous copper ores, the silver is often concentrated in a copper matte or black copper, which may then be smelted with lead, or treated in the humid way. The liquation of argentiferous copper consists in alloying it with a certain quantity of lead, and afterward heating the alloy above the melting point of lead, but below that of copper. The lead “sweats” out, carrying the silver with it, and leaving