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36 Xiao Y. Yan and Derek J. Fray be possible to apply the concept to more complicated electrode arrangements, thereby, approaching the output of pyrometallurgical reactors. 5.1.2. Discussion of Electrorefining Although fused salt electrorefining is considerably less energy intensively than electrowinning, the only commercial application is in purifying aluminium from the HH cells and this is very energy inefficient. The limited application of fused salt electrorefining is due almost entirely to the present design of the cells which do not take advantage of the high electrical conductivity of fused salts and the very low thermodynamic potential required for electrorefining. Designs of cell have been suggested and evaluated on a laboratory scale which indicate that voltages as low as 0.2 V can be achieved at acceptable current densities. It appears that with an energy efficient cell, the possibilities for fused salt electrorefining are quite extensive, ranging from treatment of scrap, combination with carbothermic reduction and, lastly, in combination with electrowinning. It is perhaps in combination with carbothermic reduction that the greatest possibilities exist as carbothermic reduction, although energy efficient and rapid, generally gives a product which is impure and needs further treatment. Efficient fused salt electrorefining should give a very pure product at relatively low energy consumption. In addition, unlike purification using air or chlorine, all the metals remain in the metallic state and, therefore, retain their commercial value rather than appearing as a residue. The electrolytic extraction processes of aluminium and magnesium must be improved to yield abundant quantities of metal at process competitive with ferrous alloys. There is also a growing need for refractory metals both as the primary constituents of structural components and as surface coatings on less expensive base-metal components. The potential of molten salt electrolysis to process the refractory metals remains largely untapped. Clearly, a broad new research initiative in fused salt electrometallurgy is in order. Overall, environmental concerns will prompt a shift from thermochemical approaches to electrochemical approaches. We see evidence of this with the announced new magnesium smelters where ore bodies that historically were processed by metallothermic reduction are now being electrolysed. If we couple this trend to carbon-free generation of electricity, the results will be “greener” metallurgy. The second point is that the transitional technologies will play an uncertain role. This is largely due to the fact that the metals industry is very conservative due to the very high capital costs associated with technology. Whether we consider incremental changes in current technology or radical innovation, the future looks bright for molten salts. Research specific to fused salt electrometallury: The HHC used to produce aluminium and fused salt cells for magnesium, titanium, etc., have problems not shared by other electrometallurgical operations. These problems provide worthwhile research opportunities, e.g.: • Re-oxidation of metal product is a phenomenon that reduces productivity and wastes energy; the phenomenon is presently poorly understood. • Transport phenomenon in aluminium cells, require further study. • New materials are required to withstand the hostile high temperature, corrosive environment, e.g., materials for cathodes, anodes, containment, sensors and diaphragms.PDF Image | Molten salt electrolysis for sustainable metals extraction
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