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Molten Salt Electrolysis for Sustainable Metals Extraction and Materials Processing 39 All processes that involve the electro-reduction of oxides in molten salts could benefit from inert oxides as all existing processes emit carbon dioxide. An inert anode would allow the overall reaction to be: MO = M + 1/2O2 (38) totally eliminating the production of carbon dioxide, provided the electricity comes from a renewable source. The aluminium industry has led the research for an inert anode and has explored many oxides, including tin oxide, and has concluded that cermets, consisting of mixtures of metals and ferrites may be suitable. In the FFC-Cambridge Process, there is an additional complication in using carbon anodes and that is carbon transfer from the anode to the cathode. Alternative anodes have been sought for this process. Tin oxide was shown to be unsuitable as an insulating layer of calcium stannate formed on the anode. By far the most promising was calcium titanate which is stable in the melt and, in order to increase its electronic conductivity, it was sintered with a small amount of calcium ruthenate to from a semi-conducting solid solution [128]. What are the challenges and opportunities for the future? One important area is new electrode materials enabling the use of inert anodes and drained cathodes. At the moment slightly less than 1⁄2 kg carbon per kg of Al produced is consumed. Eliminating carbon from the HH cells is by far the change that would have the greatest environmental impact. Installing cathodes made of a material that is wetted by molten aluminium and positioning such cathodes above the floor of the cell would eliminate the need to maintain a deep pool of liquid metal on the bottom of the cell. The materials showing the most promise is titanium diboride (TiB2) which has graduated from laboratory testing to installation in industrial cells for long-term performance assessment. Novel refractories as a replacement for Si3N4-bonded SiC need to be developed for use as the HHC sidewall materials on which a frozen layer termed “side ledge” may be no longer required. With such a ledge-free sidewall, the heat within the HHC is kept rather than removed. This leads to a potentially 30 % saving in energy that would otherwise be lost through the sidewalls when operating the HHC with a side ledge. Furthermore, the cell capacity and productivity may be most increased with a ledge-free sidewall cell configuration. New electrolyte chemistry is also interest. In particular, there has been growing interest in so-called low-ratio baths, i.e., baths containing much greater amounts of AlF3 (about 40 wt% in excess of cryolite stoichiometry) than is currently the practice (about 10 wt% in excess of cryolite stoichiometry). At such high levels of AlF3 the liquidus temperature drops below 700 °C, almost 300 °C below the operating temperature of contemporary cells. The hope is that operation at lower temperatures will reduce the wear on cell components and perhaps even allow the sustained performance of inert anodes that have been found to be unworkable at 960 °C. Indeed, it may well be that if low-ratio bath is to be used at all, the cell will need to be free of carbon anodes. However, recent work by one of the authors suggests that at 800 °C electrolysis of low-ratio bath (B.R. = 0.56) on carbon anodes generates perfluorocarbons at normal operating voltages. There is certainly a need for new electrode materials in the Dow cell where carbon consumption rates are very high; hence, an inert anode would constitute an improvement. There is also a need for a new process for preparing anhydrous MgCl2 in a manner that is energy efficient and gives a product of high purity. As mentioned above, the development of O2-evolving inert anodes for the FFC-Cambridge Process appears to be very significant forPDF Image | MOLTEN SALT ELECTROLYSIS
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