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28 Xiao Y. Yan and Derek J. Fray 3.2. Molten Salt Electrolysis of Anode Materials 3.2.1. The Composite Anodes Processes for Aluminium and Magnesium Withers et al. reported fused salt electrolysis cells for the electrolytic deposition of aluminium or magnesium in which the anode consisting of a composite mixture of Al2O3 or MgO and carbon is the sole source of aluminium or magnesium in chloride or mixed chloride and fluoride electrolytes [100-103]. Their experiments with the composite anodes made of Al2O3-carbon have been carried out in electrowinning cells at around 700 °C [102]. The overall reaction in the Al2O3/C case is the same as in the HH process: Al2O3 + 3/2C = 2Al + 3/2CO2 (g) (30) The main advantage of the composite anode is that the Al2O3 and carbon are mixed together in their stoichiometric ratio. A high solubility of Al2O3 in the electrolyte is not necessary and, perhaps, could be regarded as an advantage as this allows lower melting point electrolytes with higher conductivities to be used. Alumina powder is removed from the pot room giving a cleaner operation and the crust breaking procedure to feed alumina to the cells is eliminated. Furthermore, as there is always a supply of alumina, anode effects are no longer observed in the cell. The composite Al2O3/C cell also offers the advantages of the lower electrolysis temperature and less corrosive property of the electrolytes over the HHC for primary Al production. For example, the new process utilising composite anodes also promises Al at 4 kWh/1b and design studies indicate that it can be retrofitted into existing the HH plants. However, these advantages are partially offset by the poor electronic conductivity of the composite anode which can only be overcome by inserting consumable aluminium conductors into the anode. The incentive for the development of this technology to magnesium extraction is perhaps greater as the preparation of MgCl2 feed to the cells consumes about 50 % of the cost and energy consumption for the production of magnesium. Withers and Loutfy utilized MgO-C anodes in mixed chloride-fluoride or all fluoride electrolytes and produced electrolytic magnesium at about 700 °C with only CO2/CO as anode gases. The process has been demonstrated on a laboratory scale with the production of magnesium at less than 4 kWh per 1b. Again, there were problems with the conductivity of the composite anodes which were overcome by inserting rods of magnesium into the anodes [103]. Ratvik studied in more detail composite anodes for the electrolytic production of magnesium and aluminium in molten LiCl containing 10-20 mol% NaF at 700 °C [104]. They found that no anode effect was observed using nominal current densities up to 2 A/cm2 and current efficiencies of as high as 90 % were achieved. The anodic gaseous products were found to be CO2 and CO, with no halogens, HF, or HCl were detected. Balaraju et al. carried out detailed investigations into liquidus behaviour, Al2O3 solubility, electrical conductivity and decomposition potential of Al2O3 dissolved in various compositions of KCl-NaF melts to determine suitable electrolytes for electrolytic production of aluminium using Al2O3/C composite anodes [105]. They concluded that the KCl-NaF system is a more suitable electrolyte than the LiCl-NaF system because of the higher cost of LiCl than KCl and operating problems associated with its hygroscopicity. However, both studied revealed that sludge formation and contamination of the electrolyte by fine carbon particles, causingPDF Image | MOLTEN SALT ELECTROLYSIS
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