Page:The American Cyclopædia (1879) Volume VIII.djvu/93

Rh establishments, the current conveying the sediments is led through a succession of apparatus, each machine, sluice, or other contrivance being intended to catch a portion of the gold carried past the preceding one. The refuse finally escaping is called tailings, and usually contains: native gold, so finely divided that it has been swept by the current through all the apparatus employed; minute particles of amalgam and “floured” quicksilver, carried off in the same way; coarser particles of gold adhering to fragments of rock; and, finally, gold associated mechanically or chemically with iron or copper pyrites, blende, galena, and mispickel. The tailings are usually run into reservoirs, allowed to settle, and then stored in heaps. Sometimes these heaps are again amalgamated, with or without a preceding concentration by washing. It has been found in many instances that some kinds of pyrites slowly decompose by exposure and thus set free fine gold. The metallic sulphurets are, however, in many cases, separated from other tailings by washing immediately after the first amalgamation of the ore, and various devices have been employed for the treatment of such concentrated pyrites, which is often the richest in gold of all the constituents of the vein stuff. Following the analogy of natural decomposition, it has been repeatedly attempted, by roasting the pyrites in reverberatory furnaces, to drive off the sulphur and oxidize the metallic bases, so as to obtain a product containing fine particles of free gold. The objection brought against this treatment, that the vapors of roasting carry off mechanically fine particles of gold, seems to be ill-founded. More serious objections are the cost of the roasting process, and the circumstance that the roasted product does not contain the gold in a condition suitable for amalgamation. It is supposed that the particles when thus artificially and rapidly reduced become coated with a film of oxide of iron, preventing the intimate contact with quicksilver upon which amalgamation depends. Very careful roasting in cylinders, with the addition of salt, is said to have obviated this difficulty; but the question of expense remains.—The present methods of treatment for pyritous gold ores are: 1, the extremely fine pulverization of the ore, liberating, as far as this is mechanically possible, the particles of gold; 2, the amalgamation of the pyritous residues in pans, with the addition of chemicals intended to facilitate decomposition; 3, chlorination; 4, smelting. The chlorination process was introduced by Prof. Plattner of Freiberg, Saxony, for the treatment of auriferous residues in Silesia. As improved by Deetken, it has been employed in this country for about 15 years. The principle involved is the transformation of metallic gold by means of chlorine gas into soluble chloride of gold (the aurum potabile of the alchemists), which can be dissolved in cold water and precipitated in the metallic state by

sulphate of iron. This precipitate may then be filtered, dried, and melted with suitable fluxes, to obtain a regulus of malleable gold. It is necessary that all the gold, and if possible nothing else, shall be obtained in the final solution. If this is secured, the precipitation and melting are easy. To render the gold in the ore accessible in a metallic state to the chlorine gas, and at the same time to convert the base metals into oxides which will not unite with the chlorine, the raw ore is finely pulverized and (if sulphurets or arseniurets are present) roasted. The cost of this treatment, amounting in the Pacific states and territories to from $12 to $25 a ton, excludes its use for low grade ores; and hence it cannot supersede the stamp mill and amalgamation process, though it is acknowledged to be metallurgically the most complete method of gold extraction on a large scale. Ores containing iron, copper, gold, and silver may be roasted and deprived of their copper and iron by leaching with dilute sulphuric acid, of their silver by boiling with concentrated sulphuric acid, and of their gold by treating the auriferous residuum with aqua regia. If lead is present, the whole residuum after the removal of copper must be melted with lead and cupelled. This process is not now used in the United States, though it is recommended by high authority. Telluric ores are treated in Transylvania in a somewhat similar way. The smelting processes for the extraction of gold are the same as those for silver. Since the two minerals always occur in nature together, the final result of smelting is argentiferous gold or auriferous silver. The separation of the two metals is effected: 1, by dissolving the silver in nitric acid or boiling sulphuric acid, which leaves behind a brown powder of gold; 2, by treating the alloy with aqua regia, in which gold is dissolved as chloride, while the chloride of silver is but slightly soluble; or 3, by passing a current of chlorine gas through the alloy while in a melted state. For separation with nitric acid, the alloy should contain 2½ parts of silver to 1 part of gold. For the separation with sulphuric acid, the best results are obtained with alloys containing not much less than 3 or more than 4 parts of gold in 16 parts, the remainder being silver and copper. It is usually necessary in treating native gold to melt it with at least 2½ times its own weight of silver, and then to separate by the action of acids the silver thus added, and also that originally contained in the gold. It is said that the chlorine process effects a complete separation of the silver in one operation, at the time the gold is melted, and thus saves much time, material, machinery, and interest on capital. Nitric acid and sulphuric acid processes are used in the mints of the United States, and the chlorine process is employed in some of the British colonial mints.—Among the most recent authorities on this subject are: Phillips, “The Mining and Metallurgy of Gold and Silver”