Size tailoring of WC particles in the carbothermic reduction of scheelite (CaWO4)

Pure scheelite (CaWO4) and carbon black mixtures, containing either 0–6 wt% cobalt or 0–2 wt% nickel were prepared by 24 h planetary ball milling (PBM). CaWO4:C mixtures were then subjected to 1 h annealing at 1200 or 1400 °C in tubular furnace under flowing Ar (heating ramp: 3 °C/min). Following the carbothermic reduction, the obtained powders were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). CaWO4:C mixtures with no added metal gave an almost quantitative WC yield (≥ 96%) after 1 h annealing at both temperatures. All mixtures containing Co or Ni gave rise to a WC yield exceeding 99% after annealing at 1400 °C. Mixtures annealed at 1200 °C exhibited a slightly lower yield, being the estimated percentage of WC ≥ 98% with cobalt dopant, and 95% with 2 wt% Ni. For the first time we here show that Co or Ni addition has a noteworthy effect on the size of WC particles produced by carbothermic reduction of CaWO4 and subsequent leaching with diluted hydrochloric acid. In particular, the size of WC particles could be tailored from 0.2–0.3 μm to 2–3 μm when Co was increased from 0 to 5–6 wt% and the reaction temperature raised to 1400 °C. These findings are particularly appealing from both industrial and sustainability point of view, since they further validate the carbothermic reduction of scheelite as a simpler, cheaper and less energy-intensive process for the synthesis of nano- and micro-structured WC powders to be employed for manufacturing different hardmetal grades.