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Refining Metals

The art of purifying a metal from dross, or separating it from metallic alloys. More or less impurities remain after the common methods of reduction have been employed, which can only be eliminated by subsequent refining. Copper, for instance, usually contains small quantities of antimony, iron, tin, etc., after reduction in the reverberatory furnace used for this purpose. By re-melting in the refining-furnace and exposing the metal to the oxidizing influence of the air, these foreign metals oxidize and are converted to slag, which is skimmed off as it rises in the crucible. This operation subjects the copper to oxidization also, but the copper oxide is reduced again by adding coal to the surface and stirring the metal with a green-wood pole. The pole emits its gases forcibly, and creates a violent ebullition which exposes every portion to the reducing action of the coal, by which means the oxide of copper is deprived of its oxygen and the copper is made pure. Tin and lead are treated after the same manner ordinarily, but special processes are followed for the separation of silver from the latter metal. Gold is refined by first dissolving the metal in aqua regia (see AQUA REGIA), after which the silver, etc., with which it is usually alloyed may be precipitated by chemicals having no action on the solution of gold. Salt of iron is then employed to precipitate the gold in a fine powder, which is then melted and cast, the product being pure gold. Refined silver is obtained by dissolving the metal in nitric acid, and, after filtering the solution, precipitating it with common salt as a chloride of silver, which, after being mixed with sulphuric acid, is acted upon by bars of zinc, by which means chloride of zinc is formed and the silver again resumes the metallic state. See REDUCING METALS; SEPARATING METALS FROM THEIR ALLOYS. For refining iron, see MALLEABLE IRON; FINERY-FURNACE; etc.