Understanding the electro-reduction of metal oxides in molten salts

Recent work has demonstrated that solid metal oxides can be directly electro-reduced to the respective metals or alloys in molten salts. Typically, for the electrolysis of Cr2O3, less than 0.2 wt % oxygen could be achieved in the metal powder product with the current efficiency and energy consumption being 75% and 5 kWh/kg, respectively. However, the electrolysis of TiO2 was found to be less efficient. Based on experimental observations and hypothetical models, this paper illustrates various possible steps of the reduction mechanism, including (1) separated electron and oxygen transfer at the three phase interline (boundary), (2) metal atom aggregation and oxide surface renewal, (3) consecutive metallisation of oxide particles, (4) three-phase interline propagation and oxide cathode surface metallisation, (5) intercalation of calcium into the metal oxide cathode, (6) oxygen transport in the solid and liquid phases, and (7) interactions between molten salt and cathode before and after reduction.