Effect of carbon excess and milling conditions on the synthesis of nanostructured WC by carbothermic reduction of scheelite (CaWO4)

Mixtures of scheelite and carbon black, with different CaWO4:C weight ratios were prepared using either planetary ball milling (PBM) or ball milling in a custom made milling jar (BM). Three milling conditions were employed: 120 h ball milling (BM-120), 8 h planetary ball milling (PBM-8) and 24 h PBM (PBM-24). SEM and XRD were used to estimate qualitatively the intensity of milling conditions. The most intense milling conditions were attained after 24 h planetary ball milling (PBM-24). Powder mixtures prepared by 120 h ball milling (BM-120) or 8 h planetary ball milling (PBM-8) showed similar results. All mixtures were studied by thermal analysis (TGA–DTA), isothermal annealing at 1200 °C (1 h), X-ray diffraction (XRD) and electron microscopy techniques to determine the combined effect of different amounts of carbon stoichiometric excess and different intensities of milling conditions on the phase purity of tungsten carbide formed by carbothermic reduction of scheelite (CaWO4 + 4C → CaO + WC + 3CO). This reaction occurs through several steps, where the Ca:O atomic ratio goes from 1:4 to 1:1 according to the sequence: CaWO4 → Ca3WO6 → CaO, and concomitant development of CO. Finally, reduced tungsten bearing phases (W2C and/or W) are carburized to give nanostructured WC. The intensity of milling and the presence of a carbon excess do not influence significantly the reduction of scheelite to CaO, which goes to completion at temperatures lower than 1200 °C. However, only more intense milling conditions (PBM-24) allow one to achieve an almost quantitative yield of WC, independent of the presence of a carbon excess in the initial CaWO4:C mixture. Therefore, milling conditions are of crucial importance in order to avoid the use of an excess of carbon in the carbothermal reduction of scheelite.