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Molten Salt Electrolysis for Sustainable Metals Extraction and Materials Processing 21 consumption of 9-12 kWh per kg Mg, more efficient than the conventional industrial MgCl2 electrolyzers. The magnesium would leave the cell as a vapour and be condensed as a high purity product. 2.4.3. Molten Salt Electrowinning of Titanium The properties of titanium have long been recognised as a light, strong, and corrosion resistant metal, which has led to many different approaches over the past several decades to extract titanium from its ore. Most of the titanium produced worldwide is obtained by the Kroll process invented in the late 1930s and commercialised in the mid 1940s [43]. The Kroll process is basically a batch process that reduces gaseous titanium tetrachloride (TiCl4) with molten magnesium inside a steel retort followed by vacuum evaporation or inert gas sweep to remove by-products at 1000 °C. Despite the many other methods investigated to produce titanium, the only method currently utilised commercially are the Kroll and Hunter processes. These processes utilise TiCl4 which itself is produced from the carbo-chlorination of TiO2- rich feedstocks such as weathered ilmenite, natural and synthetic rutiles, and titanium slags or a refined TiO2 according to the reaction: TiO2(s) + 2Cl2(g) + 2C(s) = TiCl4(g) + 2CO(g) (18) In the Kroll process, TiCl4 is reduced at around 800 °C under argon to produce titanium sponge: 2Mg(l) + TiCl4(g) = Ti(s) + 2MgCl2(l) (19) The titanium sponge produced is then crushed, hand sorted, compacted into briquettes that are electron-beam welded into electrodes, the electrodes are then vacuum-are-melted into a round ingots, which is finally machined into to remove the outer skin. The MgCl2 is then separated and recycled electrolytically to produce magnesium as the reductant to further reduce the TiCl4 and chlorine for the carbochlorination reaction. In the Hunter process, sodium is used as a reductant according to the reaction: 4Na(l) + TiCl4(g) = Ti(s) + 4NaCl(l) (20) The titanium produced by either the Kroll or Hunter process must not only be separated from the reductant halide by vacuum distillation and/or leaching in acidified solution to free the titanium sponge for further processing to useful titanium forms, but also require the regeneration of the reductant by electrolysis of the molten magnesium chloride. This makes the entire process for preparing titanium highly labour and energy intensive. Because of these multiple steps, the resultant titanium is quite expensive which limits its use to cost insensitive applications. Alternative processes to the Kroll and Hunter processes have been investigated, including plasma techniques, molten chloride salt electrolytic processes, molten fluoride methods, the Goldschmidt process, and alkali metal-calcium techniques. Other processes investigated have included aluminium, magnesium, carbothermic and carbo-nitrothermic reduction of TiO2 and plasma reduction of TiCl4, without measurable success. Kroll predicted that titanium will be made competitively by fused salt electrolysis but, to date, this has not been realised. In the past 50 years, numerous attempts have been made toPDF Image | MOLTEN SALT ELECTROLYSIS
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