The use of rotating electrodes in the electrolysis of molten zinc chloride electrolytes
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INTRODUCTION
M O L T E N salt electrowinning as a method of metal production leads to high-purity products and is potentially very energy efficient. However, molten salts are difficult to handle as they are hygroscopic, highly corrosive, have high vapor pressures, and require high operating temperatures, u-4] More importantly, in industrial cells, the energy consumption is relatively high, and the output of metal per unit volume, or space-time yield, is low as a result of the large electrode gap. E2-51 Despite these disadvantages, many metals are produced commercially by molten salt electrolysis. These include refractory metals, alkali metals such as lithium, sodium, and potassium, and more importantly, magnesium and aluminum. Significantly, the demand for these metals is expanding rather than contracting. At the present time, there are three possible types of cell design for molten salt electrowinning. The selection of a particular cell type depends critically upon the relative density of the metal and the electrolyte and also on the corrosive nature of the species within the cell. In the case of aluminum production/3] aluminum is electrowon from a solution of alumina in cryolite into a molten pool of aluminum at the bottom of the cell. At the carbon anode, oxygen reacts with the carbon to produce carbon dioxide. The anodes are therefore consumable, and the cell has to be designed to enable easy replacement of the anodes. Due to the intense magnetic fields, the surface of the molten pool is unstable, and in order to obtain an adequate current efficiency, the anode to cathode spacing is around 5 cm. The overall design, therefore, is a cell essentially consisting of horizontal electrodes. Due to the fact that magnesium and lithium are lighter than the chloride electrolytes, these metals are deposited P.M. COPHAM, formerly Research Student with the Department of Materials Science and Metallurgy, University of Cambridge, is Professor of Mineral Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom. D.J. FRAY, University Lecturer, is with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom. Manuscript submitted July 26, 1989. METALLURGICAL TRANSACTIONS B
on vertical steel cathodes with the chlorine being evolved on vertical carbon anodes, t6 9] and the cells are examples of the second type of cell design. In the I.G. Farbenindustrie cell, the two products are separated by a ceramic diaphragm, and typical performance figures are 7.5 V across the cell, an anode to cathode spacing of 13 cm, and a current efficiency is 90 pct. Overall, the energy consumption is 18.5 k W h / k g , which is roughly 3 times the theoretical minimum energy consumption. The cell design for lithium extraction is similar with a large anode to cathode spacing. In attempts to overcome the poor space-time yields of conventional ceils, bipolar cells, the third cell type, have been investigated, t4,5,1~ Alcoa t1~ developed a bipolar cell for the electrolysis of aluminum from an
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