MILLS, SILVER. Silver-ores are worked by one of three ways—amalgamation, lixivia tion, and smelting. The selection of the process for treating any particular ore is a question of dollars and cents, and no general rules can be laid down. In Colorado, silver-ores are treated almost exclusively by smelting. There are three great smelting centers in that State and an abundance of silver-lead ore, and, freight rates being comparatively low, siliceous silver-ores even of ordinary grade can be smelted cheaper than they can be milled. On the other hand, the gold-silver ores of the Comstock and the silver ores of Butte. Phillipsburg. and Tuscarora are milled by the amalgamation process, while the lixiviation process is used at Park City, Utah, as well as amalgamation, and at several places in Mexico. In the amalga mation process the silver contents of an ore are recovered by amalgamating them with mer cury. In the lixiviation process they are leached out with certain chemicals and the silver precipitated from the solution by other chemicals. In amalgamation, silver-ores are classed as free-milling, or those in which the silver exists in the form of a mineral which can be amal gamated directly, and non-free-milling, which require a preliminary roasting to convert the silver minerals into such form that they can be amalgamated. Siliceous ores containing native silver, or silver chloride, bromide, or iodide, are typical free-milling ores, while ores in which the silver is carried by sulphides of the base metals constitute the class for which a preliminary roasting is necessary.
Free-milling ores are worked in wet-crushing mills, the customary arrangement of which is as follows: The ore brought in by cars at the top of the mill is dumped over an inclined grizzly or screen and rolls on to the crusher floor. All the small pieces pass through the screen or grizzly into the ore bins underneath. The coarse rock is shoveled into the crusher from the floor, which is on a level with its receiving jaws, and is crushed to the size of wal nuts, falling into the ore-bins, whence it passes into the automatic stamp-feeders through inclined chutes controlled by ore-gates. The automatic feeders, being kept full, supply the
stamps uniformly and as fast as required.
The finely stamped ore suspended in water, and known technically as pulp, flows into large settling-tanks. where excess of water is drawn off. The thick pulp remaining is shov eled in regular charges into a row of amalgamating pans, in which it is worked several hours, first with salt, blue-stone, and other chemicals, then with additions of quicksilver. The con tents of the pans are run into large settlers placed below and in front of the pans, in which the pulp is thinned by additions of water and gentle agitation, and all the quicksilver with precious metals in the form of amalgam settles to the bottom. The pulp is gradually run of from the settlers and flows to waste. The amalgam is strained from the excess of quick silver, retorted to drive off the remaining quicksilver, and the resulting mass of silver and gold melted into bars.
Sometimes values carried in base minerals which will not yield to the above process are caught by concentrators (usually Prue vanners or one of the similar belt machines), which re ceive the waste pulp from the settlers. Generally three pans are supplied with five stamps in this process, so that a 10-stamp mill would require six pans and three settlers. On some ores two pans are sufficient. The quicksilver• is usually elevated and distributed throughout the mill by a special elevator. This process was devised for treating the ores of the Comstock lode, on which account it is frequently known as the Washoe process. and after many years of study and experiment it was perfected there during the ten years between 1870 and 1880. Mr. A. D. Hodges, Jr., in his paper, Amalgamation at the Comstock Lode (Trans. A. I. E., vol. six, p. 195), gives an interesting account of its development. and a description of the methods and apparatus finally adopted. Since that time the process has remained practically unchanged.