Cobalt Powder Production by Electro-Reduction of Co₃O₄Granules in Molten Carbonates Using an Inert Anode

Electrochemical splitting of Co 3 O 4 into cobalt powder and oxygen was realized by electrolysis in molten Na 2 CO 3 -K 2 CO 3 at 1023 K using a solid oxide pellet cathode and a Ni11Fe10Cu alloy inert anode. The electro-reduction mechanism of Co 3 O 4 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 Co 3 O 4 in Na 2 CO 3 -K 2 CO 3 melt mainly involves two steps by electrochemical formation of intermediate product of CoO and then the CoO is reduced to cobalt metal. Na x CoO 2 intermediate was not detected by XRD measurements and EDX analysis, different from the electro-reduction of Fe 2 O 3 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/Ag 2 SO 4 reference electrode. Metallic cobalt powder was prepared by potentiostatic electrolysis at potential more negative than −1.25 V (vs. Ag/Ag 2 SO 4 ) 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.