Formation of micronised WC from scheelite (CaWO4)

A stoichiometric mixture of graphite and scheelite have been mechanically milled together for 40 h. The resultant powder, and an unmilled powder, have been examined by thermal analysis, isothermal annealing and X-ray diffraction to determine the effect of milling on the reduction of scheelite to tungsten carbide. The milled powder underwent a rapid reduction reaction at 800°C, 300°C lower than the unmilled powder. The reduction sequence was shown to be multistage with Ca3WO6, W2C, and W all formed as intermediate phases prior to WC. The milled powder showed complete reduction to WC in 1 h at 1000°C, whereas the unmilled powder was incompletely reduced, even after 1 h at 1200°C. Unoptimised acid leaching of the reduced powder showed that separation of the final phases was readily achievable, leaving highly porous WC particles which were composed of grains around 200 nm in size.