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Molten Salt Electrolysis for Sustainable Metals Extraction and Materials Processing 15 Recently, Rapp et al. investigated the possibility of using molten Na2SO4 as an alternative solvent for Al2O3, with the SOFC-type anode, as they had found that the solubility of α-Al2O3 to be 8 mol% and those for ZrO2 and Y2O3 to be very much lower in a very basic Na2SO4 melt where NaAlO2 was stable at 927 °C [27]. Very little information is available in the literature as to Al-Na2SO4 interactions and cathodic reactions in Na2SO4-based electrolytes with dissolved with α-Al2O3, at around 927 oC, although a better understanding of these behaviours is crucially important to Al electrowinning using these electrolytes. More recently, Yan and Lanyon studied, in detail, the chemical and electrochemical reactions in the Al-Al2O3-Na2SO4 system at 927 oC to evaluate this novel process using Na2SO4 as the alternate solvent for Al2O3, incorporating with a SOFC-type anode, for electrowinning primary aluminium [28]. In their study, cathodic reactions at platinum, gold, and aluminium cathodes in the melts were also investigated using electrolytic cells with a graphite rod or SOFC-type anode. Their results indicated that both aluminium and sodium were co-deposited electrochemically from the Al2O3-Na2SO4 melts in air at 927 and 960 °C, with low current and energy efficiencies. The observed phenomena were explained by the cathodic reduction of O 2- to O2- as well as the back reactions of the aluminium and sodium at (4) Molten Salt Electrorefining of Aluminium The basic design of fused salt electrorefining cells in figure 2 is unsatisfactory as it requires a large floor area and a long and irregular anode-cathode path which results in an excessive voltage drop in spite of the high conductivity of the electrolyte. In order to overcome these difficulties, a three layer refining cell was devised for refining aluminium [29, 30]. In the three-layer cell for aluminium, the density of the impure aluminium anode is increased by a 30 wt% copper addition, and the density of the molten cryolite electrolyte is increased by BaCl2 or/and CaF2 additions so that it falls between that of the high purity electrorefined aluminium and the Al-Cu alloy. However, as the density differences between three layers are small, the interfaces are relatively unstable, the electrolyte layer must be approximately 20 cm thick to prevent physical transfer of anode metal into the cathode layer. As a result, the voltage drop in the electrolyte is very large, around 7 V at 0.4 A cm-2, which can be compared to a thermodynamic potential requirement for transfer of aluminium from Al-30 wt% Cu to pure aluminium of 7 mV. Tiwari and Sharma have used the three-layer approach to electrolytically remove magnesium from Al-Mg alloys [14]. Al-Mg alloys were used as an anode and a cathode was the purified magnesium, with the electrolyte being a molten mixture of CaCl2-MgCl2-KCl-NaCl. The current efficiency exceeded 85 % at the anodic current density of 1.1 A/cm2 and cell voltages of 1.6-4.86 V. 2.4.2. Molten Salt Electrolysis for Magnesium Production (1) Commercial Electrolytic Processes of Magnesium At the present time, electrowinning of magnesium is also a major industrial process. There are two versions of industrial electrowinning processes for magnesium production [31, 32]. In one version, anhydrous MgCl2 is used as the cell feed, this is practised by I.G. Farben Co., and is known as the I.G. Farben process. In the other version, the cell feed is partially dehydrated MgCl2 (MgCl2.1.5-1.7H2O) (a feed contains roughly 2 moles of water per mole of MgCl2) and is used only by Dow Chemical Co. The raw material for magnesium 2 the cathode surface.PDF Image | MOLTEN SALT ELECTROLYSIS
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