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34 Xiao Y. Yan and Derek J. Fray bismuth, antimony or mixtures of these metals, are electrochemically more noble than magnesium indicating that fused salt electrorefining might be a suitable route for separation. In this case, the solvent metal, containing magnesium, is transferred to the electrorefining cell, where the magnesium is transferred to the cathode. It should be emphasised that the reduced activity of magnesium only has a small effect on the cell potential as the potential is related to logarithmically to the activity, as is shown in equation. The combination of carbothermic reduction in a metallic solvent followed by electrorefining could be applied to other metals such as aluminium. The carbothermic reduction is also commercially used to produce silicon metal of 98-99 % purity (metallurgical grade silicon: MG-Si) from SiO2 in an electric arc furnace at very high temperatures. The MG-Si metal produced must be purified to satisfy its high technology applications such as solar cells. Olson and Carleton reported their success in upgrading MG- Si to solar grade silicon (SoG-Si) by molten salt electrorefining [53, 127]. 4.3. Electrolysis Combined with Calciothermic Reduction Calciothermic reduction of TiO2 in molten Ca-CaCl2 has been investigated by Ono and Suzuki (the OS process) for producing titanium powders [61-63]. A schematic diagram of the electrolytic cell for the OS process is shown in figure 11. In the OS process, TiO2 powder feed is charged into a molten bath of Ca-CaCl2 melts, which subsequently reduced by calcium metal in a Ca-CaCl2 melt according to the following reactions: TiO2 + 2Ca = Ti + 2(CaO) (36) where Ca is the calcium metal dissolved in the melt or the calcium droplets present in the melt and is the reductant of Reaction 36, and (CaO) is the CaO dissolved in the melt and is a by-product from Reaction 36. Under the conditions used in the OS process, the dissolved CaO is electrochemically decomposed in-situ in the electrolyte to form CO/CO2 gases at the carbon and metallic calcium at the cathode, with an overall cell reaction being expressed as follows: (CaO) + C = Ca + CO(g) (37) The reduced calcium metal dissolves immediately into the CaCl2 melt and is used as the calcium reductant again for the reduction of TiO2. The OS process demonstrated the effectiveness of calcium metal dissolved in molten CaCl2 in the reduction process. Although the Ca-CaCl2 reaction medium is a powerful reductant, technical improvements have to be established before the process is scaled up for large-scale operations. This is because it is difficult to control the purity and quality of the reduced titanium metal.PDF Image | MOLTEN SALT ELECTROLYSIS
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