Nanostructured tungsten carbide synthesis by carbothermic reduction of scheelite: A comprehensive study

Mixtures of scheelite and graphite or carbon black, with different CaWO4:C weight ratios, have been mechanically milled for 24 h. One mixture was mechanically milled for 8 h to assess the role of a shorter milling time. The resulting powders have been studied by thermal analysis, isothermal annealing at different temperatures (1100–1400 °C), X-ray diffraction (XRD) and scanning electron microscopy (SEM) to determine the effect of temperature and content of carbon on microstructure and phase purity of tungsten carbide formed by carbothermic reduction of scheelite (CaWO4 + 4 C → CaO + WC + 3 CO). The experimentally observed reduction reaction sequence showed the formation of the intermediate phases Ca3WO6, W and W2C. A thermodynamic analysis proved that tricalcium tungstate (Ca3WO6) is a stable phase that forms independently of the partial pressure of CO, and that – under inert gas flow – the reaction can take place at temperatures as low as 700 °C. However, the onset of the reaction was affected by the carbon content and heating rate, and therefore kinetics effects determine the temperature at which the reaction occurs. All powders subjected to 1 h isothermal annealing exhibited micrometer-sized CaO particles and submicrometer-sized WC aggregates consisting of crystallites with an average size ranging between 40 and 90 nm, depending on carbon content and annealing temperature. A shorter milling time did cause a small increase of residual W2C in the powder after heat treatment. A slight stoichiometric excess of carbon black was sufficient to ensure complete reduction of scheelite and formation of high phase purity WC, with very small amounts of W2C. After leaching with diluted HCl, the obtained powder is suitable for manufacturing ultra fine grained cemented carbides tools and wear parts.