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4 Xiao Y. Yan and Derek J. Fray • • • • a large scale. Purification is often a prolonged process which necessities considerable peripheral handling facilities. Other disadvantages arise from the high volatility of many of the high oxidation state certain metal halides (particularly chlorides), causing to fuming and loss of electrolyte. Specific disadvantages arise from anodic reactions where energy considerations are often in conflict with efforts to maintain cathode product purity. The corrosiveness of molten salts demands that use of expensive refractories for containment, and in many cases, inert atmospheres are required. Furthermore, with present cell designs the thermodynamic potential is usually insignificant compared to the resistive losses in the electrolyte which can be as high as several volts. The most commonly used salts are chlorides, fluorides, and chloride-fluoride mixtures. Chlorides offer the advantages of a lower operating temperature and of a greater choice of electrode and container materials. As to disadvantages, chlorides react with moisture; some are hygroscopic, even deliquescent, while others decompose by hydrolysis. Hydrolysis prevents dehydration – simple heating will not result in the removal of the water of hydration from certain metal chlorides such as LiCl or NbCl5. Fluorides, on the other hand, have the advantage of being less reactive with moisture and, additionally, can dissolve oxides directly, avoiding fluorination, which requires reaction with a fluorine containing compound. In principle, the use of an oxide-based cell feed simplifies the process flow sheet and reduces capital and operating costs. Unfortunately, the higher melting points and greater corrosion properties of the fluorides severely limit the choice of cell materials. Another disadvantage is the relatively low solubility of refractory metal oxides in molten fluorides. Choice of suitable electrodes for molten salt electrolysis depends mainly on chemical reactivity of the electrodes towards an electrolyte and cost of manufacture of the electrode. There are advantages and disadvantages in producing solid metal or liquid metal, or metal dissolved in a metallic solvent as a liquid alloy [5]. When the solid metal is a cell product, it can be simply recovered by removing it from its cathode substrate. The disadvantage is that the solid metal deposited from molten salts is invariably dendritic and this results in salt entrainment with the need for some form of subsequent treatment for salt removal. The production of the liquid metal solves the morphological problem and facilitates the easy removal of the product from the cell by siphoning. The majority of metals can be deposited from fused salt but as commercial production is restricted to the more reactive elements, it is worthwhile considering the advantages and disadvantages of fused salt electrolysis as compared to smelting and aqueous electrolysis 1.2.2. Comparison with Pyrometallurgical Processes Compared to smelting, the advantages are generally perceived to be a higher purity product and, perhaps, intrinsically cleaner processes. However, as few metals are produced or refined using fused salt electrolysis, these advantages are normally outweighed by the disadvantages which include [3]:PDF Image | MOLTEN SALT ELECTROLYSIS
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