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TRODUCTION

IN addition to having the highest melting point and the lowest thermal expansion coefficient of all metals, tungsten also comes into prominence with its high density, good electrical and thermal conductivity, and good high temperature mechanical properties. As a result of these properties, tungsten powder cannot be substituted in many technical fields, which include the heavy metal alloy industry, the electric and electronics industry, and chemical applications such as the use of tungsten in cutting tool steels, kinetic energy penetrators, cannon shells, grenades and missiles, light bulbs, X-ray targets, high-energy radiation shields, cathode-ray tubes, vacuum tube filaments, heating elements, rocket engine nozzles, pigments, and catalysts. Currently, the only technically important method of tungsten powder production is the hydrogen reduction of tungsten oxides at temperatures that range from 873 K (600 °C) to 1373 K (1100 °C).[1] The overall reduction reaction can be given as follows: WO3 ðsÞ þ 3H2 ðgÞ ¼ WðsÞ þ 3H2 OðgÞ

½1

It has been estimated that 70 pct of the total world reserves of tungsten is scheelite (CaWO4) and 30 pct is wolframite ((Fe, Mn)WO4). The production of tungsten oxides from these minerals is a complicated and timeconsuming procedure. Furthermore, the Gibbs energy change of reaction (Eq. 1) is not a large negative value, which means there is considerably low driving force for _ METEHAN ERDOG˘AN, Ph.D. Candidate, and ISHAK KARAKAYA, Professor, are with the Department of Metallurgical _ ¨ nu¨ and Materials Engineering, Middle East Technical University, Ino Bulvarı, 06531 Ankara, Tu¨rkiye. Contact e-mail: [email protected]. Manuscript submitted September 28, 2009. Article published online April 14, 2010. 798—VOLUME 41B, AUGUST 2010

the process at the temperatures of reduction. Finally, because reaction (Eq. 1) is endothermic, continuous external heat supply is necessary to attain and preserve the high temperatures required for reduction. Alternative methods to produce tungsten from various raw materials are summarized elsewhere.[1] However, the development of any essentially new method of large-scale tungsten production has not become successful. In studies that aim to produce tungsten by the electrolysis of various molten salts containing tungstenbearing compounds,[2–4] the most important problem is the large particle size of the electrowon tungsten in dendritic deposition,[5] which leads to a relatively porous product when consolidated by the subsequent classic pressing and sintering techniques in addition to short circuiting of electrodes during electrolysis. By the Fray–Farthing–Chen (FFC) Cambridge process, Ti,[6] Cr,[7] Si,[8] Cu,[9] Al, B, Fe, V, Nb, U, Nd, Zr, Hf, Ce, and Ni[10–13] have already been produced in laboratory experiments. In addition, Dring et al.[14] and Bhagat et al.[15] have reported that Ti–W alloys were produced via electrochemical reduction of TiO2–WO3 mixed oxide preforms in molten calcium chloride electrolyte at 1173 K (900 °C). In light of the previously men