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Molten Salt Electrolysis for Sustainable Metals Extraction and Materials Processing 3 The mass of product formed is related quantitatively to the quantity of electric charge passed through the circuit by Faraday's laws of electrolysis [2]. 1.2. Molten Salt Electrolysis When an aqueous solution is used as an electrolyte, it is commonly termed aqueous electrolysis. This review will focus on another electrolysis process where the electrolyte used is a pure molten salt or a mixture of molten salts, known as molten salt electrolysis. Molten salt electrolysis is widely used for electrometallurgy, such as electrolytic reduction of metallic compounds to metals (electrowinning) and purification of impure metals to purer metals (molten salt electrorefining) [3]. For instance, molten NaOH can be electrolysed into sodium and oxygen, both of which have important chemical uses. Lithium is produced by electrolysis of a molten LiCl-KCl mixture. It also may cover a process for synthesis of various materials from compounds or a mixture of compounds and elements (fused salt electrosynthesis). 1.2.1. Characteristics of Molten Salt Electrolysis A number of unique properties of molten salts affecting efficiency of electrolysis have been summarised by Fray [3] and Inman [4], The advantages mainly include: • The most common solvent components are the alkali and alkaline earth halides and these all possess very negative Gibbs free energies of formation, i.e., high electrochemical decomposition potential windows. • Overpotentials due to cathodic polarisation are often small in cathodic deposition, which is desirable from an energy efficiency point of view. However, mass transfer- controlled cathodic process generally create dendritic deposits, if the metals are deposited in the solid state, that are difficult to be separated from solidified salts after electrolysis. • The high ionic conductivities (2-9 Ω-1cm-1) and diffusivities with low viscosities of molten alkali and alkaline earth metal salts and their mixtures are other desirable features both from the point of view of power losses and of temperature control by Joule heating. There are some disadvantages associated with using molten salts: • Molten salts are powerful solvents for inorganic materials. However, this can be a disadvantage, for instance in limiting the selection of cell materials for electrodes, linings, and containers. The corrosiveness of molten salts demands the use of expensive refractories for containment. • The use of high temperatures and restricted environments must be considered a disadvantage. There are considerable problems associated with solidification of the electrolyte if the power supply fails or the cell has to be shut down. • Apart from the problems mentioned above, extra energy cost must be incurred to keep the systems in a liquid state. The hydrolysis of many of the hygroscopic salts employed as solutes and solvents can cause difficulties when their use is required onPDF Image | MOLTEN SALT ELECTROLYSIS
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