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Molten Salt Electrolysis for Sustainable Metals Extraction and Materials Processing 7 During electrolysis, the voltage required to drive the process is substantially higher than the reversible potential as calculated by the Nernst equation, which for metal halide (MXn) in a multi-component molten salt solution and in equilibrium with pure metal (M) and pure gas (X2) at atmospheric pressure can be expressed as the following: ∆Gο EMXn = − MXn − RT ln aMXn (1) nF nF where ∆Gο is the standard Gibbs free energy of formation of MXn, F is the Faraday MXn constant (9.64853×104 C), R is the gas constant (8.314 J mol-1 K-1), T is the temperature in Kevins and aMXn is the activity of MXn in the electrolyte solution. EMXn is the reversible potential or equilibrium decomposition potential of MXn as a function of electrolyte composition and temperature. The difference between the equilibrium potential and the applied voltage is due to kinetic factors which increases the energy consumption of the electrowinning cell. The imposed cell voltage for the electrolytic decomposition of MXn is given by the relationship below: Ecell = EMXn + (ηa + ηc )cathode + (ηa + ηc )anode + ηelectrodes + iR (2) where Ecell is the applied cell voltage, ηa and ηc are the activation and concentration overpotentials at the electrode, respectively, and are a function of electrolyte composition, current density and temperature; ηelectrodes is the voltage drops associated with the electrodes themselves, with the bus bars and the electrical contacts between them, i is the current, R is the cell resistance, and iR is the ohmic overpotential (or iR drop) and is due to the electrical resistance of the electrolyte and is proportional to current density and interelectrode distance and inversely proportional to the electrical conductivity of the electrolyte and to the electrode area. Therefore, cell voltages depend upon individual melt chemistry, electrode materials, and cell design. Existing industrial molten salt electrolysis cells, such as the HHC for aluminium and Dow cell for magnesium, operate at voltages three or five times the value of the Nernst potential. The productivity of an electrolytic cell is expressed by several figures. Current efficiency is frequently defined as the ratio of the number of equivalents of metal product to the number of moles of electrical charge delivered to the cell by the power supply. As such, current efficiency is essentially a measure of compliance with Faraday’s laws of electrolysis. In molten salt electrowinning, current densities of about 1 A/cm2 are common and cathodic current efficiencies of 60-80 % are usual. These figures are well below those achieved for the HHC for electrowinning of aluminium, which typically attains current efficiencies exceeding 95 % [12]. Voltage efficiency is defined as the ratio of the reversible decomposition potential to the imposed cell voltage. It is an extent of the deviation from the Nernst equation and is a measure of inefficiency due to kinetic factors. For the electrowinning in molten fluoride electrolytes, voltage efficiencies are typically below 50 %, when using molten chloride electrolytes values of as low as 25 % or less have been reported.PDF Image | Molten salt electrolysis for sustainable metals extraction
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