Direct reduction carburisation of scheelite with carbon

Thermogravimetry, XRD and SEM have been used to study the direct reduction-carburisation of CaWO4 with C at 1500K. CaO, W, WC are the products of the process. The reduction process was studied with solid carbon under an argon atmosphere and under vacuum. Reduction under an atmosphere of CO was then studied and it was found that, compared to argon and vacuum, reduction under CO was very slow and the products were not fully carburised. This led to the conclusion that the first step in the reduction process is a solid-solid reduction reaction with carbon producing tricalcium tungstate (Ca3WO6) as an intermediate product. This tricalcium tungstate is then further reduced to W before carburisation to WC. The overall rate of the process was found to be controlled by the W-WC stage.