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Molten Salt Electrolysis for Sustainable Metals Extraction and Materials Processing 23 Most of the above studies were conducted in the 1980s. Recently, Pistorius and Fray reported their success in electrolytic production of silicon in molten CaCl2 at 900 °C by electro-deoxidation of sold SiO2 [54]. The work opened up a new opportunity to produce high purity silicon at lower costs, with less energy consumptions, and in a more environmentally friendly manner. 2.5. Novel Designs of Cell with Fused Salt Electrolytes 2.5.1. Diaphragm Cells The anode-cathode distance can be reduced if a porous diaphragm saturated with molten salt were used to separate the liquid metal pools. A development of the diaphragm cell was patented by Alcoa for aluminium refining in which the diaphragm was a porous carbon membrane which was permeable to the molten alkali chloride-AlCl3 electrolyte, but impervious to the molten metal [55]. As the carbon membrane was electronically conducting, it was also used as the anode feeder electrode. No chlorine evolution was detected as the potential for this reaction is much larger than the anodic dissolution of aluminium. Schwarz and Wendt used the divided electrorefining cell in which the electrolyte contents of the diaphragm were continuously exchanged by gravity driven flow to improve purity of refined aluminium [56]. 2.5.2. Bipolar Cells As well as materials issues with existing cells, the other major problem with electrolysis cells is the low space-time yield due to the two dimensional nature of the electrode arrangements. The space-time yield can be increased if a bipolar cell, such as that shown in figure 5, is used [3]. With this arrangement, electronically conducting plates are inserted in the electrolyte between an anode and cathode so that during electrolysis, the intermediate plates become bipolar, with alternate anodic and cathodic surfaces down the stack. One of the advantages of this arrangement is that there is only one set of electrode contacts for many plates, thereby, reducing resistance losses. However, of greater significance, is the greater surface area per unit volume and hence output which increases the space-time yield. Overall, a lower energy consumption is possible with a bipolar cell because of the reduced overall resistance per pair of electrodes. The most advanced industrial scale project involving a bipolar cell is Alcoa’s aluminium chloride cell {section 2.41 (3)}, which, again, uses the evolved chlorine to improve circulation in the cell by sweeping the aluminium off the cathode surface and drawing in fresh electrolyte [18]. Coupled with the fact that graphite electrodes are effectively inert in the aluminium chloride based melt, the electrode gap could be reduced to 10 mm, as compared with approximately 50 mm in the case of the conventional HHC. This resulted in an energy consumption of less than 9.5 kWh per kg Al which is about 66 % of the value achieved in modern HHC [12, 29]. Unfortunately, although the bipolar cells operated successfully, problems associated with the preparation of the AlCl3 feed to the cell have led to the curtailment of the project. The Alcan process uses MgCl2 from titanium production operations as cell feed using bipolar cells. Alcan has also developed a bipolar cell to extract magnesium from MgCl2 which uses the evolved chlorine to improve circulation of the electrolyte [57]. In this case, thePDF Image | Molten salt electrolysis for sustainable metals extraction
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