The Washer 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 21/i feet deep and from 4 feet to 5V, 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 (bleed 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 combination 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, known 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 amalgamation process is now nearly obsolete. By it the 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 thin paste. After some hours' rotation mercury and some times a little copper 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 off from the bottom. This amalgam is then distilled. In the pan amalgamation process the crushed ores, after hieing 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 eases 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 beat, and consequently the retort silver, con taining from I per cent. to I% per cent, of mer cury, is 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 products is first converted into a com pound soluble in water or certain aqueous 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 tamed in solution in the form of a chloride are: the Augustin process, using brine as a solvent and metallic copper as a precipitant; the Patera process, where sodium thiosulphate, and the kiss process, where calcium thiosulphate is the solvent. In the Russell process the silver as metal or sulphide is brought into solution by sodium copper thiosulphate, and in the Ziervogel process the silver is converted into a sulphate and dis solved in hot water. The Augustin process is now rarely practiced and need not be mentioned further.
The Patera process is used in Mexico and to some extent in the United States. As ried out in the best mills in the United States the process is briefly as follows: The ore is crushed, dried, and roasted with salt in furnaces. The roasted ore is first treated with water in large vats in order to wash out certain salts of the base metals which are soluble in water. After the water is drawn off the vats are filled with sodium thiosulphate solution, which dissolves the silver chloride. The liquor is then run into other tanks for precipitation. if there is lead in the liquor this is first precipitated by adding sodium carbonate, and the remaining liquor drawn into other vats. Bere the silver is precipitated by adding sodium sulphide. The precipitate is then drawn off and pressed in filter presses to extract the entrained liquor, when it is dried and cupelled with a lead bath to secure the metallic silver.
In the Kiss process a solution of calcium thio sulphate is used for extracting the ores after a chloridizing roasting, the silver being precipi tated from the by calcium sulphide. In the Russell process the silver present in the ore as metal or sulphides is dissolved by a solution of sodium-copper thiosulphate and precipitated by a solution of sodium sulphide. The Zierrope/ process is used in treating copper ores contain ing silver. By careful roasting the silver in such ores is converted into silver sulphate, and this is dissolved out by treating the roasted ore with hot water. From this solution the silver is precipitated by metallic copper.