Magnesium reduction of WO 3 in a self-propagating high-temperature synthesis (SHS) process
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H.Y. Sohn Department of Metallurgical Engineering, University of Utah, Salt Lake City, Utah 841112-1183 (Received 4 November 1994; accepted 19 December 1994)
High-purity tungsten was prepared by the self-propagating high-temperature synthesis (SHS) process from a mixture of WO 3 and Mg. The MgO in the product was leached with an HC1 solution. The complete reduction of WO 3 required a 33% excess of magnesium over the stoichiometric molar ratio Mg/W0.3 of 3. The product tungsten had a purity of 99.980% which was higher than that of the reactant WO 3 . This is because the impurities were either volatilized at the high temperatures generated during the rapid exothermic reaction or dissolved into the HC1 solution during leaching.
Tungsten has the useful characteristics of high melting point (3410 °C) as well as high-temperature strength and electrical conductivity. Thus, it finds a wide use in illumination, electronics, electrical contact, and heatresistant structure. It is also the main raw material for the ultrahard alloys used in cutting tools, antifriction tools, and high-speed steels.1 Currently, tungsten is produced by the reduction of WO 3 at 1000 to 1100 °C with carbon, H 2 or calcium, by the hydrogen reduction of WF 6 at about 600 °C, or by the decomposition of WC16 at high temperatures.2 Lately, many pure substances have been prepared by the self-propagating high-temperature synthesis (SHS) process.3"5 This process can be used to prepare a fine powder of high-temperature materials at 2000 to 4000 °C using their high exothermic heats of reaction. The process is relatively simple and rapid without requiring much external heat supply. Many different materials including carbides, borides, nitrides, and silicides have been synthesized by this process.6-7 In this work, tungsten powder was prepared by the SHS reaction between WO 3 and magnesium, producing tungsten and MgO. The effects of various experimental conditions were examined. The tungsten trioxide used in this experiment was of 99.9% purity and 10 ± 5 fim size. Magnesium particles of 99% purity and 10 to 100 mesh size were obtained by crushing and grinding a magnesium ingot. Predetermined amounts of the reactants were mixed in an alumina ball mill and pressed into pellets of 33 mm diameter and 15 to 20 mm height under various compaction pressures. The SHS reaction was carried out under argon. The experimental variables were the mixing ratio of the two reactants, compaction pressure, and mixing time. The product mixture was very porous, and J. Mater. Res., Vol. 10, No. 4, Apr 1995
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much magnesium oxide dust was formed. The product mixture, which was easily breakable in a mortar, was leached with 10 to 30% HC1 solution to remove MgO under various temperatures and for various lengths of time. The product tungsten powder was analyzed to XRD to examine its crystal structure and by SEM and TEM to examine its microstructure. Its chemical composition was determined by ICP. The reaction involved is: WO3 + 3Mg
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