Page:The New International Encyclopædia 1st ed. v. 18.djvu/222

* SILVER. 178 SILVER. ing the crushed ore which is deposited on the Hour. Revolution of the shaft is ell'ected by horse power, water power, or steam power. Crude as tliis mill appears, it has been found that no other form of grinding apparatus serves the purpose so well. The ore is ground with enough water so that when it is removed from the arrastras it is in tlie form of a thin ruud which is termed lama. The lama is first placed on the amalgamating Moor or patio in small heaps to drain and these he.ips are then shoveled together into a fewer nunilier of large heaps or iortas. The patio is simply a spacious area paved with cement or some other material as impervious as possible to mercury. When first formed the tortus are of about the consistency of thick mud. They are then covered with a sprinkled layer of salt and turned with a shovel, after which they are trod by mules or horses driven round and round for several hours. Another turning with the shovel follows, and is succeeded by an- other period of treading. After a sufficient num- ber of repetitions of these alternate processes, sulphate of copper in one form or another is sprinkled over the tortas and mixed by a similar method of shoveling and treading. !Mercury is then added in a finely subdivided state by plac- ing it in bags of sail cloth, which are carried by men walking over the heaps, the metal falling from the bags in the form of a fine rain of globules. This mercury is in turn mixed by turning and treading. Altogether this treatment of the tortas lasts from three to six weeks, and is considered complete wlien 75 per cent, of the silver contents of the tortas have been extracted. The next step is to separate the amalgam from the other materials, and this is accomplished by agitating the torta in vats with water. The heavy amalgam settles to the bottoms of the vats and the water and lighter matter are drawn off. The amalgam is collected and pressed into bags, molds, or bottles, and is then ready for distillation in the manner described farther on. The WasJioe process of amalgamation is the one most extensively used in the United States. The ore is first crushed in stone-breakers and then stamped fine in stamp mills with water. From the stamps the wet powder passes to the amalgamating pans. These are cylindrical ves- sels of cast iron, or having cast-iron bottoms and wooden sides. They are from 2 feet to 2% feet deep and from 4 feet to 51/2 feet in diameter. A vertical shaft in the centre of the pan carries a number of arms extending downward and having at their ends shoes which bear against the bot- tom of the pan. This agitating and grinding apparatus is called a muller. The ore is intro- duced into the pan with mercury, sulphate of copper, and salt, and the contents are heated by steam. The stirring and heating process con- tinues from two to three hours, when the amalga- mation is, completed. The contents of the pan are then transferred to another similar vessel where they are agitated with water, this agita- tion serving to keep the lighter material sus- pended while the heavier amalgam settles. At suitable intervals the water is decanted off a portion at a time until only the amalgam re- mains. This is placed in canvas bags and the excess mercury filtered off, when it is ready for distillation. There are several modifications of the Washoe process in use. the two chief ones being the comitnation process, in which the ores are submitted to a preliminary concentration before amalgamation, and the Boss process, in which the amalgamation is not conducted in a single pan, but in a series of pans through which the pulp flows continuously. (3) Amalgamation with reagents and with roasting is carried out by three processes, Icuown as barrel amalgamation, pan amalgamation, and Tina amalgamation. As a preliminary to all of these processes the ores are dried and crushed and then roasted in furnaces generally with salt. The Barrel aiiialf/uination process is now nearly obsolete. Bj' it tlie crushed ore is first roasted with salt to reduce the silver to chloride and is then charged into rotating barrels with scrap- iron and enough water to make a tliin paste. After some hours' rotation mercury and some- times a little cojipcr sulphate are added and the rotation continued for a longer period. The barrels are then filled with water and the mer- cury holding silver in solution is run oil' from the bottom. This amalgam is then distilled. In the pan umalyaiiuition process the crushed ores, after being roasted with salt, are fed into pans and agitated with water for one or two hours. Mercury is then added, and the agitation con- tinued until amalgamation is complete. Except that the pans are of wood, their construction and operation are the same as in the Washoe process. In the Tina process the pans have copper bot- toms, the mullers are of copper, and the salt is added to the roasted ore in the pan. In the barrel and pan processes the brine formed by the salt and water dissolves the silver chloride, and the iron, in the form of scrap in the barrel proc- ess and in the muller blades in the pan process, reduces this to metallic silver. In the Tina proc- ess the copper of the pan and muller serves the same purpose as the iron in the other two proc- esses. Distillation is the final operation by which the silver-mercury alloy or amalgam resulting from all the amalgamation processes is separated into silver and mercury. The vessel or retort in which the distillation is performed varies in shape, but the most common forms are the vertical cast-iron cylinder retort used in Mexico and the horizontal cast-iron cylindrical retort used in the United States. In all cases the vessel is closed except for a tube to carry off the mercury gas and con- vey it to suitable condensers, and the process consists simply in charging it with amalgam and heating it in a furnace until the mercury is vaporized and only the silver remains. Silver absolutely free from mercury cannot be secured in retorts without danger to these vessels from the heat, and consequently the retort silver, con- taining from 1 per cent, to 1% per cent, of mer- cury, ^s refined in small reverberatory furnaces or in crucibles. The second class of wet processes to be con- sidered is that in which the silver is received by precipitation from aqueous solutions. In this process the silver contained in ores or metal- lurgical pi'oduets is first converted into a com- pound soluble in water or certain aqvieous solu- tions, and then precipitated as an insoluble compound by suitable reagents and the precipi- tate worked up for the metal. The soluble silver compound is either the chloride, which is soluble in salt or sodium thiosulphate solution, or else the sulphate, which is soluble in hot water. The principal processes in which silver is ob-