The American Cyclopædia (1879)/Amalgamation

AMALGAMATION, the process of extracting gold and silver from the gangues in which they occur in nature by combining them with mercury. The ores are crushed and then washed through different machines in which mercury is placed. This seizes upon the little particles of the metals that come in contact with it, and brings them together into one mass, from which the earthy matters are all washed away. Any greasy substance present almost wholly prevents this effect, the grease adhering in a film upon the surface of the mercury, and thus rendering impracticable the close contact necessary for their union. The amalgam is from time to time taken out of the washing machines, squeezed through cloth or dressed deerskin, the liquid portion replaced, and the solid distilled in an apparatus suitable for saving the mercury, which is then ready for use upon another lot of ore. The silver residue from distillation needs refining to render it perfectly pure. There are two processes for separating silver from its ores by amalgamation—the European process in barrels, and the American in heaps. The ore is treated in the European process by pulverizing the ore and roasting it with an admixture of common salt; by this means all the silver, which was originally a sulphide, is converted into the chloride. The roasted ore is then placed in barrels which can be revolved upon a vertical axis, and is thoroughly loosened and stirred up with water. Iron in the form of nails or scrap is then added, which takes the chlorine from the silver and yields the latter in the metallic state. Mercury is then added in larger quantity than is sufficient to amalgamate all the silver. After the barrel has revolved for an hour or more the mercury will have taken up the silver, and is then drawn off. This mercury is filtered and distilled as above described. The American process of amalgamation in heaps has the advantage of simplicity, and although not so perfect in its extraction of silver, it does not require fuel and expensive apparatus. The ore is first broken up to the size of a pea by means of rude stamps, and then ground to fine powder in round cylindrical tubs with bottoms of stone. Each tub has a horizontal arm revolving with its centre upon the vertical axis of the tub, and having at each end a chain attached to a stone weighing from 50 to 100 pounds. When the arm is revolved, these stones are dragged round and pulverize the ore. The ore is then placed on an amalgamating floor built of stone, and is mixed with a little salt and mercury. After some days the mercury is collected, filtered, and distilled as above described, to save the silver.