Cobalt Powder Production by Electro-Reduction of Co3O4 Granules in Molten Carbonates Using an Inert Anode

Abstract

Electrochemical splitting of Co3O4 into cobalt powder and oxygen was realized by electrolysis in molten Na2CO3-K2CO3 at 1023 K using a solid oxide pellet cathode and a Ni11Fe10Cu alloy inert anode. The electro-reduction mechanism of Co3O4 was investigated in details by cyclic voltammetry measurements and potentiostatic electrolysis in a three-electrode cell combining with the composition and morphology characterization of the electrolytic products. The electro-reduction of Co3O4 in Na2CO3-K2CO3 melt mainly involves two steps by electrochemical formation of intermediate product of CoO and then the CoO is reduced to cobalt metal. NaxCoO2 intermediate was not detected by XRD measurements and EDX analysis, different from the electro-reduction of Fe2O3 in this melt. However, the particle size and morphology of the intermediate CoO is found to be dependent of the applied potential in the range of −0.35 V to −0.75 V vs. Ag/Ag2SO4 reference electrode. Metallic cobalt powder was prepared by potentiostatic electrolysis at potential more negative than −1.25 V (vs. Ag/Ag2SO4) or by constant cell voltage electrolysis under a cell voltage higher than 1.7 V using a Ni11Fe10Cu inert anode. A high current efficiency of 95.3% and a low electrolysis energy consumption of 2.16 kW·h/kg-Co represents a novel high efficiency and green process for Co extraction. The Co product is a mixture of FCC and HCP structure, which is affected by the applied cell voltage and deserves further investigation to recognize the structure controllable condition and mechanism.