Application of centrifugal fields in fused salt electrowinning with a view to reducing electrolytic energy consumption
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I.
INTRODUCTION
FUSED salt electrowinning techniques are a rapid way of producing high-purity light metals,[1,2,3] such as lithium, sodium, and magnesium, with a minimum waste of starting materials but are often criticized for their large energy consumption and low space-time yields relative to pyrometallurgical reduction techniques.[4] Resolving these two problems will result in a major advancement in the light metals industry, with the demand for expensive metals, such as lithium, predicted to increase. This article is concerned with reducing the energy consumption in fused salt electrowinning processes using a novel centrifugal separation technique. Factors influencing the cell energy consumption are the presence of parasitic side reactions, nonuniform current distribution, poor heat insulation, and large reduction potentials. However, one of the main problems that has proved difficult to resolve is the current use of a large electrode spacing. Reducing this spacing from about 100 mm (typical of commercial cells) to less than 10 mm significantly reduces the resistance between the electrodes. However, this enhances recombination of electrolysis products, resulting in poor current efficiencies. The current efficiency f may be calculated from the expression f5
zFw Mit
[1]
where z is the number of electrons transferred, F is Faraday’s constant, w is the mass of recovered metal, M is the relative atomic mass of the metal, i is the constant electrolysis current applied, and t is the time of the electrolysis. Apart from direct fluid mechanical factors discussed so far, other factors may also affect the current efficiency. The presence of electrochemical side reactions at the electrode surfaces may reduce the current that would otherwise drive the primary electrode reactions. The electrolytic decompo-
ANTONY COX, Research Fellow, and DEREK J. FRAY, Professor, are with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, UK. Manuscript submitted February 1, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS B
sition potentials for potassium chloride and zinc chloride at 500 7C are 3.75 and 1.60 V, respectively.[5] Consequently, potassium deposition is not feasible in the zinc chloride / potassium chloride molten salt system. Nonuniform current densities may cause uneven accumulation of zinc on the cathode which could lead to recombination that may not occur under conditions of uniform deposition of zinc. However, the parallel equisized plate geometry adopted in the apparatus significantly reduces the risk of current distortions developing on the electrodes. The principal factor affecting the current efficiency in the novel cell used in this work is thought to be the fluid dynamics established within the volume bound by the electrode spacing. Segregation of products by diaphragms[6] has been attempted but imposes a resistive overpotential which counters the original intention of using a small electrode spacing. The factor to be addressed in this work is the reduction of the la
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