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Molten Salt Electrolysis for Sustainable Metals Extraction and Materials Processing 19 Yan et al. studied effects of the niobium doping level and the temperature, time, and atmosphere of sintering on microstructure and electrical conductivity of the synthesized polycrystalline niobium-doped TiO2 (rutile) and also investigated the possibility of using the polycrystalline niobium-doped TiO2 as the inert anodes for magnesium electrorefining [35, 36]. They found that the properties of niobium-doped TiO2 anodes were greatly influenced by the factors studied and a niobium-doped TiO2 anode with 3 mol% Nb2O3 doping level, sintered at 1300 °C under argon showed the most promising performance during electrolysis at the current densities of 0.75-1.25 A/cm2, with the cell voltages of about 5-6 V. The results demonstrated that the polycrystalline niobium-doped TiO2 was suitable for the use as an inert anode material in the industrial Dow cell for magnesium production. Furthermore, Yan et al. measured liquidus temperatures of the molten MgCl2-NaCl and BaCl2-NaCl systems using the in-bath liquidus sensor with the aim to develop an on-line sensor for determining liquidus temperatures and superheats of magnesium electrowinning baths [37]. Such a sensor could be useful in better controlling cell operations and prolonging the cell life in fused salt magnesium electrowinning. (3) Alternative Molten Salt Electrowinning of Magnesium Sharma at General Motors has explored the electrolysis of MgO dissolved in a melt of NdCl3, a process that exploits the fact that MgO will dissolve in neodymium chloride to form neodymium oxychloride and MgCl2 [38, 39]. MgO + NdCl3 = NdOCl + MgCl2 (14) MgCl2 +C+NdOCl=NdCl3 +Mg+CO (15) 2MgCl2 + C + 2NdOCl = 2NdCl3 + 2Mg + CO2 (16) The NdCl3 regenerated in Reaction 15 or 16 can react with further MgO due to Reaction 14, giving an overall process reaction below in the case of producing CO2 only: 2MgO + C = 2Mg + CO2 (17) The reversible cell voltage for Reaction 17 to occur is 1.52 V at 750 °C, which can be compared with 2.50 V for MgCl2 and 2.75 V for NdCl3. Recently, Lu et al. reported a similar technique of producing magnesium by direct electrolysis of MgO dissolved in molten LaCl3-MgCl2 electrolytes in a 5KA magnesium electrowinning cell with the steel cathode and the carbon anode at 700 °C [40]. In their process, the cathodic and anodic current densities were 0.81 and 0.539 A/cm2, with the cell voltage of 4.6 V and, under these conditions, the cathode and anode products were electrolytic magnesium of 99 % purity and CO gas, respectively. The current efficiency achieved was 85 % higher than that in the Dow process (80 %), with the power consumption of 12 kWh per kg Mg. They indicated that this process has the advantages of less energy consumption, high energy efficiency and is environmentally sound. One of the disadvantages of both methods is the cost of the rare earth chlorides. The above alternative magnesium electrowinning process obviates the need for carbochlorination of MgO in order to convert it into the chloride which is important asPDF Image | MOLTEN SALT ELECTROLYSIS
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