Electrolysis of solid MoS 2 in molten CaCl 2 for Mo extraction without CO 2 emission

Abstract

Sintered (300 °C) porous pellets of MoS 2 were electrolysed to elemental S and Mo in molten CaCl 2 (800–900 °C) under argon at 1.0–3.0 V for 1–20 h. On a graphite anode, the product was primarily S (but traces of CS 2 could not yet be excluded by this work) and evaporated from the molten salt, allowing the electrolysis to continue. It then condensed to solid at the lower temperature regions of the system. The anode remained intact after repeated uses. The MoS 2 pellet was highly conducting at high temperatures and could be fast electro-reduced to fine Mo powders (0.1–1.0 μm) in which the S content could be below 1000 ppm. No reduction occurred at voltages below 0.5 V. Partial reduction was seen at 0.5–0.7 V, and converted MoS 2 to a mixture of MoS 2 and Mo 3S 4, or Mo 3S 4 and Mo with the Mo content increasing with the voltage. Cyclic voltammetry of the MoS 2 powder in a Mo-cavity electrode, together with the electrolysis results, revealed the reduction mechanism to include two steps: MoS 2 to Mo 3S 4 at −0.28 V (potential vs. Ag/AgCl), and then to Mo at −0.43 V.