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Molten Salt Electrolysis for Sustainable Metals Extraction and Materials Processing 5 • Pyrometallurgical processes are generally more energy efficient as reduction, using carbon, occurs directly rather than having to burn the carbon to generate heat which eventually is converted into electricity via the Carnot cycle, • The higher operating temperatures, corrosive nature of the melts, and the tendency to fume, due to the high vapour pressures of molten salts, causing difficulties associated with containment and process control, • For metals whose melting point exceeds the boiling point of the electrolyte, the metals are deposited in a solid, dendritic form for which, unless special precautions are taken, there may be difficulties in extracting the product, and • The output of metal per unit volume and the metal production rate (space-time yield) are relatively low. For many metals, the last point is, perhaps, the most significant and this is a result of the cell design which, in many cases, can be regarded as a two-dimensional reactor with large inter-electrode spacing. This can be contrasted with pyrometallurgical reactors where the reactions occur three-dimensionally, throughout the whole reactor, with very short diffusion distances and high surface areas. As well as a poor production rate per unit volume or floor surface area, the net result is a high energy consumption which is usually at least two or three times the theoretical minimum. Despite of these disadvantages, a few metals are produced commercially by molten salt electrolysis and these include the refractory metals, alkali metals such as lithium, sodium, and potassium and, more significantly magnesium and aluminium. It is apparent that to make molten salt electrolysis processes more attractive, a “three- dimensional” reactor is required with substantially reduced inter-electrode spacing which would increase both the space-time yield and the energy efficiency. Some advances have already been made in this area [6-10]. 1.2.3.Comparison with Aqueous Electrometallurgical Processes The advantages over aqueous extraction are as follows: • The much higher conductivities and diffusivities result in much lower iR losses and higher current densities can be achieved at modest voltages, • The elevated temperatures of operation result in larger exchange currents which give rapid kinetics and lower activation polarisation for the electrode reactions, • The absence of water as a solvent means that it is not necessary to consider the evolution of hydrogen as a competing cathodic reaction. Furthermore, alkali halide salts, often used as solvents in fused salt electrolytes, have high decomposition potentials, and • Molten salts generally are mutually soluble and, therefore, concentration polarisation effects are minimised. The disadvantages include: • difficulties when operating at high temperatures, • high volatility of electrolytes and corrosion of refractories, and • when the product is a solid, it is difficult to separate the solid from the solidified electrolyte.PDF Image | MOLTEN SALT ELECTROLYSIS
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