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22 Xiao Y. Yan and Derek J. Fray produce titanium sponge continuously and more economically by electrochemical processes. Among major industrial developments, we can cite, the US Bureau of Mines in 1956 [44], Titanium Metal Corporation (TIMET) in the period 1970-1980s [45], Dow-Howmet in the 1980s [46], and RMI and Elettrochimica Marco Ginatta (EMG) in the late 1980s [47]. All these processes were finally aborted either due to engineering issues or for economical reasons. As an example, high purity electrolytic titanium was produced continuously by the Dow- Howmet process developed by a joint venture of The Dow Chemical Company and Howmet Turbine Components Corporation [46]. It was based on a stable diaphragm type cell using a molten LiCl-KCl eutectic with TiCl2 added by first reacting a stoichiometric amount of TiCl4 as a cell feed with an amount of titanium sponge in a feed basket to produce lower valent titanium ionic species. The electrolytic cells operated at 520-600 °C under argon. The cathodes consisted of steel round bar stock, while the graphite rod was used as the anode. TiCl4 was continuously fed into a pre-reduction cathode compartment where reduction to TiCl2 occurred. Since titanium is multivalent, it has been shown that TiCl4 could be reduced to lower valent ionic species of Ti3+, Ti2+, which do show some solubility in some molten salts [48]. However, because of secondary reversibility reactions, which lead to loss in current efficiency and poor quality of metal, no practical process has evolved for electrowinning titanium from a TiCl4 feed. Investigations of separating the anolyte and catholyte to avoid alternating oxidation and reduction with low current efficiency have not proven successful on a commercial scale. 2.4.4. Molten Salt Electrowinning and Electrorefining of Silicon At the present time, carbothermic reduction of SiO2 at around 2000 °C is the commercial process to produce metallurgical grade silicon (MG-Si) of 98-99 % purity. Molten salt electrolysis is an alternative to produce high purity silicon. Progress in molten salt electrowinning and electrorefining of silicon and the possible applications of electrolytic silicon have been reviewed by a number of authors. Cohen discussed some prospective applications of silicon electro-deposition from molten fluorides to solar cell fabrication [49]. Elwell and Feigelson reviewed experimental data on the electrodeposition of silicon from molten salts and considered the potential of this method for photovoltaic applications [50]. Rao and Elwell briefly considered the prospects for economic electrowinning of silicon for more general applications [51]. General reviewers of silicon electrowinning and refining were presented by Elwell and Rao [52], with particular emphasis on molten SiO2-and fluorosilicate-based electrolytes. These reviews concluded that the fused salt electrowinning could become the commercial process of producing silicon of significantly higher purity at a cost comparable with that of aluminium. The relative absence of carbon in the electrodeposited silicon should offer advantages for some high-grade metallurgical applications, and the electrowon silicon should be an attractive starting material for solar applications or as a feed material for the production of semiconductor or detector grade silicon. Olson and Carleton patented a molten salt process to upgrade MG-Si to SoG-Si by electrorefining [53]. In their process, a semipermeable Cu3Si-Si composite was prepared from copper of 99.999 % purity and MG-Si particles. The prepared composite was then used as the anode in their electrorefining cell using a molten mixture of KF-LiF-K2SiF6 as the electrolyte. The electrorefined silicon had greater than 99.999 % purity.PDF Image | Molten salt electrolysis for sustainable metals extraction
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