tungsten(redirected from Tungsten compounds)
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W, a chemical element in Group VI of the Mendeleev periodic system. Atomic number, 74; atomic weight, 183.85. It is a refractory, heavy metal with a light gray color. Natural tungsten consists of a mixture of five stable isotopes with mass numbers 180, 182, 183, 184, and 186.
Tungsten was discovered and isolated as tungstic oxide (WO3) in 1781 by the Swedish chemist K. Scheele from the mineral tungsten, which was later called scheelite. In 1783 the Spanish chemists, the brothers d’Elhuyar, prepared WO3 from the mineral wolframite and, by reducing WO3 with carbon, were the first to obtain the actual metal, which they called tungsten. The actual mineral wolframite was also known to Agricola (16th century); he called it spuma lupi (wolfs cream) because tungsten, which always accompanies tin ores, interfered with the smelting of tin, converting it into the foam or scum of slags (“it devours tin as the wolf devours the sheep”). In the USA and some other countries the element was also called tungsten (Swedish, “heavy stone”). For a long time tungsten was not used industrially. It was only in the second half of the 19th century that the effects of tungsten on the properties of steel were investigated.
Tungsten is sparsely distributed in nature; its content in the earth’s crust is 1 x 10~4 percent by weight. It does not occur in the free state, but forms its own minerals, mainly tung-states, among which wolframite, (Fe,Mn)WO4, and scheelite, CaWO4, are of industrial importance.
Physical and chemical properties. Tungsten crystallizes in a body-centered cubic lattice system with dimension a = 3.1647 angstroms. Density, 19.3 g/cm3; melting point, 3410° + 20° C; boiling point, 5900° C. Thermal conductivity (cal/cm - sec - ° C), 0.31 at 20° C and 0.26 at 1300° C; specific electrical resistance (ohms · cm x 10~6), 5.5 at 20° C and 90.4 at 2700° C; electron work function, 7.21 x 10~19 joules (J), or 4.55 electron volts (e V); high-temperature radiation coefficient (watts per sq cm), 18.0 at 1000° C, 64.0 at 2200° C, 153.0 at 2700° C, and 255.0 at 3030° C. The mechanical properties of tungsten depend on its previous treatment. The tensile strength (kilograms-force per sq mm [kgf/mm2]) of sintered ingots is 11, and of pressure-treated ingots, 100-430; the elastic modulus is 35,000-38,000 in the case of wire and 39,000-41,000 for single-crystal threads. The Brinell hardness of sintered ingots is 200-230, and of forged ingots 350-400 (1 kgf/mm2 ≈ 10 meganewtons per sq m [MN/m2]). At room temperature the plasticity of tungsten is low.
Under ordinary conditions tungsten is chemically stable. At 400°-500° C the dense metal is appreciably oxidized in air to WO3. Above 600° C steam oxidizes it vigorously to WO2. The halogens, sulfur, carbon, silicon, and boron react with tungsten at high temperatures; fluorine reacts with tungsten powder at room temperature. Hydrogen does not react with tungsten at temperatures up to the melting point. Tungsten forms a nitride with nitrogen above 1500° C. Under ordinary conditions tungsten is resistant to hydrochloric, sulfuric, nitric, and hydrofluoric acids, as well as to aqua regia; at 100° C it reacts slightly with them, and it dissolves rapidly in a mixture of hydrofluoric and nitric acids. Upon heating, tungsten dissolves slightly in solutions of alkalies, and it dissolves rapidly in molten alkalies with the addition of air or oxidizing agents; tungstates are formed during this process. In its com-pounds tungsten has a valence of 2 to 6; the compounds of higher valence are the most stable.
Tungsten forms four oxides, the highest of which is the trioxide WO3 (tungsten trioxide) and the lowest the dioxide WO2, with two intermediate oxides, W10O29 and W4O11. Tungsten trioxide is a lemon-yellow powder, which dissolves in solutions of alkalies with the formation of tungstates. Upon reduction with hydrogen it yields the lower oxides and tungsten successively. Tungstic acid (H2WO4), a yellow powder that is virtually insoluble in water and acids, corresponds to tungsten trioxide. Solutions of tungstates are formed upon its reaction with solutions of alkalies and ammonia. At 188° C, H2WO4 loses water and forms WO3. Tungsten forms a series of chlorides and oxychlorides with chlorine. The most important are WC16 (melting point, 275° C; boiling point, 348° C) and WO2Cl2 (melting point, 266° C; sublimates above 300° C), which are produced by the action of chlorine on tungsten trioxide in the presence of carbon. Tungsten forms two sulfides with sulfur, WS2 and WS3. The tungsten carbides WC (melting point, 2900° C) and W2C (melting point, 2750° C) are solid refractory compounds; they are produced from tungsten and carbon at 1000°-1500°C.
Preparation and use. Wolframite and scheelite concentrates (50-60 percent WO3) are the raw material for the preparation of tungsten. Ferrotungsten (an alloy of iron with 65-80 per-cent tungsten), which is used in steel production, is smelted directly from the concentrates; tungsten trioxide is smelted out to obtain tungsten and its alloys and compounds. Several methods are used in industry to obtain WO3. Scheelite concentrates are decomposed by a soda solution at 180°-200° C in autoclaves (to produce an industrial solution of sodium tungstate) or with hydrochloric acid (to produce industrial tungstic acid):
(1) CaWO4 (solid) + Na2CO3 (liquid) = Na2WO4 (liquid) + CaCO3 (solid)
(2) CaWO4 (solid) +2HC1 (liquid) = H2WO4 (solid) + CaCl2 (liquid)
Wolframite concentrates are decomposed either by sintering with soda at 800°-900° C, followed by leaching of the Na2WO4 with water, or by heating with a caustic soda solution. Decomposition with alkaline reagents (soda or caustic soda) yields a solution of Na2WO4 that contains impurities. After the impurities are removed, H2WO4 is isolated from the solution. (To produce precipitates that are coarser and easier to filter and wash, CaWO4 is first precipitated from the Na2WO4 solution and then decomposed with hydrochloric acid.) The dried H2WO4 contains 0.2-0.3 percent impurities. Calcination of H2WO4 at 700°-800° C yields WO3, from which hard alloys may be produced. To produce metallic tungsten, H2WO4 is further purified by the ammonia method—that is, by dissolving it in ammonia and crystallizing ammonium paratungstate, 5(NH4)2O · 12WO3 · «H2O. Calcination of this salt yields pure WO3.
Tungsten powder is produced by the reduction of WO3 by hydrogen (carbon is also used in making hard metals) in electric tube furnaces at 700°-850° C. Compact metal is made from the powder by the powder metallurgy method—that is, by compressing in steel molds at pressures of 3-5 tons-force per sq cm and heat treatment of the billets. The final stage of the heat treatment, heating to about 3000° C, is performed in special apparatus, with direct passage of an electrical current through the billets in a hydrogen atmosphere. Tungsten that lends itself well to pressure treatment (forging, drawing, and rolling) after heating is produced as a result of this process. Single tungsten crystals are obtained from the billets by the method of crucibleless electron-beam zone melting.
In modern technology, tungsten is widely used both as pure metal and in the form of a number of alloys, the most important of which are alloyed steels, hard alloys based on tungsten carbide, and wear-resistant and temperature-resistant alloys. Tungsten is a component of a number of wear-resistant alloys used for coating the surfaces of machine parts (valves in airplane engines; turbine blades; and so on). Temperature-resistant alloys of tungsten with other refractory metals are used in aviation and rocketry. Its high melting point and low vapor pressure at high temperatures make tungsten indispensable for electric light filaments, as well as for making parts of vacuum-electrical devices in radio electronics and X-ray technology. Several chemical compounds of tungsten are used in different fields of technology—for example, Na2WO4 in the paint and varnish and textile indus-tries, and WS2 as a catalyst in organic synthesis and an efficient solid lubricant for parts exposed to friction.
REFERENCESSmithells, J. Vol’fram. Moscow, 1958. (Translated from English.)
Agte, C., and I. Vacek. Vol’fram i molibden. Moscow, 1964. (Translated from Czech.)
Zelikman, A. N., O. E. Krein, and G. V. Samsonov. Metallurgiia redkikh metallov, 2nd ed. Moscow, 1964.
Khimiia i tekhnologiia redkikh i rasseiannykh elementov, vol. 1. Edited by K. A. Bol’shak. Moscow, 1965.
Spravochnik po redkim metallam. Moscow, 1965. (Translated from English.)
Osnovy metallurgii. Vol. 4: Redkie metally. Moscow, 1967.
O. E. KREIN