Manganese electrodeposition — A literature review

Electrolytic manganese metal has been used in a wide range of applications such as production of steel and aluminum alloys. However, manganese electrowinning is technically and commercially challenging due to a low current efficiency and high cell voltage. Rigorous purification and the use of additives are required for commercial manganese electrowinning. Some effective additives, such as selenium compounds, contaminate the metal and have negative environmental impacts. This literature review summarizes the background information on hydrogen evolution on manganese, manganese electrodeposition and its polarization behavior, effects of impurities, and the effects of additives such as selenium compounds, and their mechanisms in order to provide guidelines for development of cleaner and more efficient manganese electrodeposition conditions. Electrolytic manganese is conventionally produced from ammonium sulfate media in a diaphragm cell. Hydrogen evolution on manganese in Mn-free electrolyte is very slow compared to manganese deposition. However, it is catalyzed by manganese deposition. With increasing polarization, the manganese deposit structure changes from γ-Mn to α-Mn. Heavy metal impurities (e.g. Ni and Co) are co-deposited with manganese and catalyze the dissolution of manganese and then hydrogen evolution through the formation of galvanic micro-cells, resulting in a low manganese current efficiency. With increasing deposition time, the manganese deposit gradually becomes rougher and more dendritic, resulting in an increase in the rate of manganese dissolution. Smooth and compact deposition reduces the harmful effect of impurities and increases manganese current efficiency. Various additives (typically sulfite and selenite) have been used to counteract the harmful effects of impurities and improve manganese deposit quality. The addition of selenite (or selenate) and sulfite inhibits hydrogen evolution, increases the overpotential of manganese deposition in the low polarization region and decreases the overpotential of manganese deposition in the high polarization region, facilitates the crystallization of stable α-manganese, improves the leveling effects of the electrolyte, and finally increases the manganese current efficiency. The formation of insoluble selenides and sulfides with impurities eliminates or suppresses the generation of galvanic micro-cells and then contributes to an increase in the current efficiency of manganese deposition. Alternative additives should inhibit hydrogen evolution significantly, but manganese deposition to an appropriate degree, promote the formation of α-Mn, function as a leveling agent, have a minimum contamination of the manganese deposit and reducing capacity to stabilize the electrolyte.