Room temperature reduction of scheelite (CaWO 4 )
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Room temperature reduction of scheelite (CaWO4 ) N. J. Welham Petrochemistry and Experimental Petrology, Research School of Earth Sciences and Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, Australian National University, Canberra. ACT 0200, Australia (Received 3 December 1997; accepted 23 July 1998)
A mixture of scheelite and magnesium has been mechanically milled together for 100 h, either with graphite or in a nitrogen atmosphere, with the intention of forming tungsten carbide or nitride. The resultant powders were examined by thermal analysis, isothermal annealing, and x-ray diffraction to determine the effect of milling on the reduction of scheelite. With graphite, nanocrystallite W2 C was the exclusive tungsten product; WC was not detected even after annealing at 1000 ±C. No nitride formed in the system milled with nitrogen; however, 10 nm crystallites of elemental tungsten were formed. The unwanted phases, MgO and CaO, were readily removed by leaching in acid, leaving a fine powder composed of impact welded aggregates of either carbide or 99% pure tungsten metal.
I. INTRODUCTION
Tungsten carbide is probably the most common hard material in use today for cutting edges and other high strength applications. It is usually made by a somewhat circuitous route from either the ore wolframite (FeWO4 ) or from scheelite (CaWO4 ). Wolframite is dissolved in acid and the tungsten separated from impurities by lime precipitation as CaWO4 . The precipitated CaWO4 , or scheelite, is dissolved in acid and precipitated using ammonia to form ammonium tungstate which is subsequently calcined to form WO3 . The trioxide is thermally reduced with hydrogen to form metallic tungsten.1 For carbide formation the tungsten is blended with carbon and heated at high temperatures (,2000 ±C) in a hydrogen atmosphere, or as shown recently,2–6 by reaction during extended ball milling. For use in cutting tool manufacture the hard material powder should be extremely fine grained, as it has been shown that the smaller the particles of tungsten carbide the harder the final composite.7 A route that reduces the number of steps is potentially of commercial importance. Previous attempts to produce WC directly from ores, or their high purity laboratory equivalents, have shown that direct reduction is possible using excess carbon at temperatures .1200 ±C.8–10 However, complete conversion of scheelite to CaO and WC was not achieved, even after 20 h at 1200 ±C.8 Ball milling of a stoichiometric mixture of scheelite and graphite has been shown to result in complete reduction to WC and CaO within an hour at 1100 ±C, although W2 C and W were both present as intermediate phases.11 The separation of the final powders was readily achieved by simple acid leaching, leaving particles composed of partially sintered 200 nm grains. Tungsten forms several characterized nitrides,12 the most common of which are W2 N and WN. The nitrides J. Mater. Res., Vol. 14, No. 2,
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