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24 Xiao Y. Yan and Derek J. Fray magnesium is lighter than the electrolyte and the electrodes are arranged vertically with a weir system at the top to draw off the metal. Rapid circulation of the electrolyte is achieved by using small inter-electrode spacing and a high current density, which results in a lift rate generated by the large amount of chlorine produced. The cell has been run with gaps as small as 8 mm, compared to 50 mm or more in existing commercial cells, and a current efficiency of 70 %. However, no overall energy consumption is quoted and it may not be significantly more efficient than existing cell designs, as a result of the large amount of gas between the electrodes, leading to a relatively large resistance. 2.5.3. Cells Using Recessed Electrodes It has been realized that anodic depletion, which causes co-dissolution of impurities in an impure anode and contaminating refined metal, is a major problem in fused salt electrorefining. The problem was overcome by using a flow-through electrochemical cell in which the anolyte was continuously removed, passed through a purification reactor and returned to the cathode side of the cell. In order to address this problem, Fray patented cells using recessed electrodes which allowed molten metal to flow down the electrodes, separated by a ceramic fibre or blanket [58]. The great advantage of this cell compared to the other designs is that although the current density on the diaphragm can be very high, the current density on each individual droplet of metal, as it meanders down the cell, can be very low due to the large surface area of droplets in the cathode. This will obviously decrease the likelihood of surface depletion but, perhaps, more significantly the motion of the droplets in the bed ensures that the metal is continuously agitated, thereby, increasing mass transfer within the droplet. Furthermore, unlike other designs, the load of metal on the diaphragm will be virtually insignificant as droplets are on the order of a few mms in diameter. Unlike electrowinning where there are many examples, it is perhaps instructive to consider areas in which fused salt electrorefining could find applications. Recently, Cox and Fray successfully removed 99 % of magnesium and 46 % manganese from beverage cans, an aluminium alloy based on AA5182 and AA3004, using the RCC in a molten MgCl2-NaCl electrolyte at 680-740 °C [59]. They reported that the magnesium formed at the cathode contained on average about 0.5 wt% Al, while the aluminium contained 0.0135-0.033 wt% Mg and 0.6485-1.079 wt% Mn. The electrical power consumption of the cell was only 2.7 kWh per kg Mg transferred, compared to 15 kWh per kg using a three-layer cell. They also indicated that such a cell has many economic and environmental advantages over conventional methods to remove magnesium from scrap alloy, least of which are a minimum of 80 % power savings based on chlorination and the three-layer cell, pollution-free operation, a very useful by-product, and the possibility of producing aluminium master alloys in situ. 2.5.4. Use of Rotating Electrodes It is apparent that a further reduction in cell voltage can only be achieved if the anode-to- cathode distance can be decreased but, with existing technology, this would be at the expense of increased back reaction of the products and a lower current efficiency. Elimination of the back reaction can only be accomplished if the products of electrolysis can be removed more efficiently. One possible way of meeting this objective is to rotate the electrodes in order to apply a modest centrifugal field to encourage the separation of the products of electrolysisPDF Image | Molten salt electrolysis for sustainable metals extraction
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