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Molten Salt Electrolysis for Sustainable Metals Extraction and Materials Processing 25 and, thereby, allow the inter-electrode gap to be reduced [3, 60]. A centrifugal field acts on a mixture in the same manner as a gravitational field but the former can be varied by changes in rotational speed or the dimensions of the equipment with the result that a much better separation can be attained. In the particular case of molten salt electrolysis, there are three phases – liquid metal, molten salt and gas. According to centrifugal theory, the gas will flow inwards, whereas the denser fluids will be thrown outwards with the net result of a more rapid and efficient removal of the electrolytic products from the inter-electrode gap, improving the current efficiency at small electrode separations. Another possible advantage of the application of centrifugal fields is that the metallic electronically conducting phase will be ejected beyond the edge of the electrodes reducing the chances of the partial short circuiting the electrodes. This may be particularly important for bipolar electrodes. 3. NOVEL FUSED SALT ELECTROLYSIS 3.1. Molten Salt Electrolysis of Cathode Materials The majority of electrolysis processes in molten salts produce liquid metals that deposit on an inert cathode. It is rare that a single salt is used as additions of other salts can raise the electrical conductivity and decrease the melting point of the electrolyte. The additions should, usually, have a higher decomposition potential than the salt to be electrolysed. As an example, magnesium chloride decomposes at a potential about 0.8 V lower than the other salts which may include NaCl, KCl, LiCl, CaCl2 or BaCl2. In most cases, the salt with the lowest decomposition potential electrolyses first even though the cell voltage may be greater than the decomposition potentials of the other salts. This is because the cell voltage (Ecell) is given by Equation 2. It can be appreciated that the polarisation and iR losses can exceed the difference between the decomposition potential of magnesium chloride and the other salts. Provided there is no concentration polarisation, the salt with the lowest potential for decomposition will always decompose first, followed by the next least stable salt. In the case of electrowinning of sodium from a mixture of NaCl, CaCl2, and BaCl2, the decomposition potentials of NaCl and CaCl2 are very similar so that both elements deposit and the calcium is removed by cooling the mixture. Ono and Susuki have electrowon calcium from calcium oxide –calcium chloride mixtures and then used the calcium to reduce other metal oxides [61-64]. In the HHC, the deposition potential for sodium is only slightly less cathodic than the deposition potential for aluminium so that sodium always co-deposits at a low activity with the aluminium [1]. If the carbon cathode is exposed, sodium deposits in preference to aluminium as the sodium is able to intercalate into the graphite. 3.1.1.The FFC-Cambridge Process In 2000, it was reported that it was possible to be able to electrochemically deoxidise a cathode preform made of TiO2 in a molten CaCl2 and this started a new era of intense research and development of producing a low cost titanium metal electrolytically from cost affordable oxides [65, 66].PDF Image | Molten salt electrolysis for sustainable metals extraction
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