Metallurgy

Art concerned with the extraction of metals from their ores, and refining them. The metallurgist makes use of the principles of chemistry, combined with the physical forces, particularly heat and electricity, which facilitate chemical reactions. Metals occur in three distinct forms, native metals, oxygen ores, non-oxygen ores, and the treatment differs in each case. Native metals occur in ores in the metallic state; they need only a mechanical treatment to separate the metal from the gangue. This treatment may consist in: 1. Crushing the ore and separating the metal by virtue of its high specific gravity, as by washing or jigging, as with copper. 2. Crushing, and heating on an inclined plate, by which the metal is melted and runs down the plate and away from the gangue, as with bismuth. 3. Melting the ore with the addition of a flux; which unites with the gangue, forming slag, which floats upon the melted metal, and can be poured, skimmed, or tapped off, as with antimony. 4. Crushing, and bringing the ore in contact with a liquid metal which can alloy with the metal in the ore, as with gold and silver by the use of mercury. 5. Melting and fluxing the ore in contact with a metal which can alloy with the native metal; silver and gold are thus readily alloyed with lead. 6. Crushing, and di olving out the metal by an acid or salt solution, the metal being subsequently separated from the solution by precipitation or by electrolysis, as with the sodium-cyanide proce for gold. Oxygen ores are mostly oxides and carbonates, and may be divided into those reducible by carbon at ordinary furnace heats and those reducible by carbon only in the electric furnace; in both cla es there are the volatile and the nonvolatile metals. In the first cla the ores of the volatile metals are mixed with carbon and heated in closed retorts. fitted with a condenser in which the metallic vapor is condensed, as with zinc and sodium. With the non-volatile metals the ores are mixed with carbon and a flux, and the mixture smelted either in a crucible, as with antimony; on an open hearth, as in the early extraction of iron; on an inclosed hearth, as with lead and copper; or in a shaft furnace, as with iron, lead, copper and tin: this cla of ores is also reducible by mixing with the sulphide of the same metal, the sulphur and oxygen combining and leaving the metal free, as with lead ores. The ores, reducible only in the electric furnace, are mixed with carbon and a metal or the oxide of a metal which is not volatile, a powerful electric current raises the whole ma to a temperature of 2,000° to 3,000° C., and reduction ensues; the result is an alloy of the metal with the other non-volatile metal, as with alloys of aluminium with copper or iron. Such oxides, mixed with carbon and smelted in the electric furnace, yield sometimes the pure metal, but. more frequently a compound of the metal and carbon, a carbide, which is in many cases entirely different in its properties. from the pure metal, as calcium carbide. Most of the metallic carbides, however, differ from the metals only in being harder, more brittle, and more easily fusible. The pure metal can in many cases be obtained from them by grinding them, mixing with more of the metallic oxide, and again heating in the furnace without admixture of carbon. The carbon in the carbide combines with the oxygen of the oxide, and is thus expelled, leaving the pure metal. Several other methods of attacking these refractory oxides in the electric furnace are: (1) direct reduction by another metal, as silica, heated with powdered magnesium, is reduced to silicon; (2) solution in a molten bath, with reduction by another metal, as manganese in molten cryolite reduced by aluminium; or by electrolysis, as alumina in a bath of melted cryolite; (3) conversion into a non oxygen compound. and reduction of this by another metal; red hot magnesia with carbon is converted into magnesium chloride by chlorine gas and this is reduced by sodium. Most of the fusible chlorides, fluorides, sulphides, etc., can be directly decomposed by electrolysis, when melted; many of them can be decomposed in aqueous solution by the use of a current of great density, as with chromium salts. Non-oxygen ores are compounds of the metals with sulphur, arsenic, antimony, selenium or tellurium. Silver is found combined with chlorine, from which it is easily reduced by amalgamation with mercury. The ores are frequently concentrated by crushing and washing or jigging, or by liquation on a hot inclined surface, or by smelting down with flux to molten slag and metallic sulphide (matte) or to an arsenide (spei ). The reduction to metal may proceed as follows: (1) direct reduction by another metal, as lead sulphide melted with scrap-iron; (2) roasting in air, as with sulphide of mercury, thus forming sulphur dioxide and mercury vapor, the latter being condensed; the sulphides of almost all other metals roast to metallic oxide or sulphate; (3) roasting to oxide, with subse quent reduction, as with nickel sulphide; (4) roasting partly to oxide, with subsequent reaction of the oxide on the sulphides, as with lead sulphide; (5) roasting to sulphate, with subsequent solution in water and reduction by precipitation by another metal, as copper sulphate precipitated by iron, or deposition by electrolysis, as with zinc sulphate, or precipitation as oxide, IMG:content-1330.jpg:[blocks in formation] with subsequent reduction, as zinc sulphide, roasted to sulphate, precipitated by ammonia; (6) direct electrolysis reduces anti-animals during develop- mony sulphide, when molten; (7) indirect electrolysis, as when ores or the concentrated sulphides are made the anodes in an IMG:content-1331.jpg:[graphic] CH..NH,.SOH(1:3). Met- amido benzene sulphuric acid; monobasic acid, crystallizing in fine needles or prisms; used in the preparation of the azodye, METANIL YELLOW (q.v), isomeric with SULPHANILIC ACID (q.v.). Chandler's Encyclopedia