WET PROCESSES. Of the various combined wet and dry processes for extracting silver, the amalgamation process is the first which demands consideration. In the amalgamation process, the silver in ores or metallurgical products is con verted into a mercury alloy, or amalgam, which is subsequently distilled, the silver being left behind and the mercury condensed and used over again. The various amalgamation processes may be grouped into three classes: (1) Amalgama tion with mercury alone; (2) amalgamation with mercury and certain reagents without roast ing: and (3) amalgamation with mercury and reagents after a chlorodizing roasting.
(1) Amalgamation with mercury alone, usu ally called direct amalgamation, is practiced only with ores consisting chiefly of native silver. It was formerly extensively used in Peru, Chile, and Mexico, and is yet used to some extent in those countries where suitable ore is available. The process consists in rubbing the crushed ores with mercury, the crushing either going on at the same time or having been done previously, and is of comparatively limited application.
(2) Amalgamation with reagents and without roasting is employed when the silver exists in sulphur. arsenic, and antimony compounds, and includes what are known as the Gam, Kranke, Patio, and Washoe processes. Of these the Patio and Washoe processes are the most important and they only will be described further. The Polio process is extensively used in Mexico, and to a less extent in South American countries. In carrying it out the first operation is to crush and grind the ore. The coarse crushing is usu ally performed in edge-runner mills, stamps, rolls, or rock-crushers (see GRINDING AND CRUSH ING MACHINERY), while the fine grinding is done in special mills called arrastras. Described brief ly, the arrastra is a circular pit, the sides and bottom of which are paved with hard stone such as quartz or porphyry. In the centre of the pit floor is a pyramidal stone with a hole in its top into which pivots a vertical post supported at its upper end by a horizontal beam. This post car ries two or fen• horizontal arms, to each of which are attached by chains or thongs one or more rectangular blocks of porphyry weighing from 6 to 12 cwt. These blocks are attached in such a way that their front edges are about two inches above the floor while their rear edges drag on the floor. By revolving the vertical shaft these stone
blocks are dragged round and round the pit, grind ing the crushed ore which is deposited on the floor. Revolution of the shaft is effected by horse power, water power. or steam power. Crude as this 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 the form of a thin mud which is termed lama. The lama is first placed on the amalgamating floor or patio in small heaps to drain and these heaps are then shoveled together into a fewer number of large heaps or tortes. The patio is simply a spacious area paved with cement or some other material as impervious as possible to mercury. When first formed the tortes 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 tortes and mixed by a similar method of shoveling and treading. Mercury is then added in a finely subdivided state by plac ing it iu 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 tortes lasts from three to six weeks, and is considered complete when 75 per cent. of the silver contents of the tortes 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.