Fundamental studies on chlorine behavior as related to zinc electrowinning from aqueous chloride electrolytes
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I.
INTRODUCTION
THE electrowinning
of zinc is currently practiced using acidic zinc sulfate solutions. This process is characterized by a large anodic overvoltage, which is required to discharge oxygen at a lead anode. If electrowinning were applied to an acidic zinc chloride solution instead, anodes originally developed for the chlor-alkali industry could be used. These anodes discharge chlorine with very low overvoltages and at very high current densities, permitring a combination of energy savings and/or possible plant productivity increases, depending on the choice of current densities. O f the many investigations into electrowinning of zinc from chloride solutions, almost all were based on the use of a diaphragm around the anode to trap dissolved chlorine and to avoid a cathodic current loss due to reduction back to chloride ions. This use of a diaphragm was the basis of the study by MacKinnon and Brannen. m According to Thomas and Fray, tEl a solid chlorine hydrate can also form as an anodic product below 282 K and can be separated as a slurry from anolyte, but most of the chlorine is discharged as a gas. When a diaphragm is used, it adds to the anode-cathode distance and to the electrolyte resistance, leading to an increase in energy consumption that negates the advantage of a lower anodic overvoltage. Thomas and Fray tEt showed that at current densities of 2200 to 2500 A / m E with air sparging and periodic current reversal (PCR), current efficiencies of about 90 pct could be obtained without a diaphragm. For these reasons, a fundamental study was undertaken to establish (a) the solubility of chlorine in various
HIROSHI MAJIMA, Professor, YASUHIRO AWAKURA, Associate Professor, and KEISUKE TSUGUI, Graduate Student, are with the Department of Metallurgy, Kyoto University, Sakyo-ku, Kyoto 606, Japan. ERNEST PETERS, Professor, is with the Department of Metals and Materials Engineering, The University of British Columbia, Vancouver, BC V6T 1W5, Canada. Manuscript submitted July 20, 1989. METALLURGICALTRANSACTIONS B
aqueous chloride solutions suitable for zinc electrowinning, (b) the cathodic reduction rate of dissolved chlorine in these solutions, and (c) some information on the rote of removal of dissolved chlorine by sparging with air. These data were used to evaluate previous work and to predict the performance of cells for zinc chloride electrolysis in which diaphragms are left out.
II.
EXPERIMENTAL PROCEDURES
Figure 1 is a schematic diagram of the apparatus used for the determination of chlorine gas solubility in aqueous solutions. Chlorine gas from gas cylinder A was dissolved by an aqueous solution contained in the saturator, F, via a gas trap and a presaturator, E. Excess chlorine gas was removed in two steps by absorption towers I and J, containing 1 mol dm -3 K I and 1 mol dm -3 NaESO3. The volume of the saturator F was 140 c m 3, and the chlorine flow rate was set at 20 c m a / m i n throughout each experiment. Both saturators, E and F, were in a thermostatted bath, and the test saturator, F,
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