Fluidized-bed electrodeposition of zinc
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I. INTRODUCTION AND PREVIOUS INVESTIGATIONS
THE fluidized bed electrode has been the subject of many investigations concerned with the electrowinning of metals from aqueous solution. The electrode might offer advantages ~n terms of capital cost (as a result of the high. spacetime yield achievable in such a cell), in automation, and possibly in terms of reduced electrical energy consumption. 22 A further possible advantage of the fluidized bed electrode in the case of zinc electrowinning is that the top of the cell can be sealed permitting the recovery of hydrogen codeposited with the zinc. This would be advantageous in terms of process economics (hydrogen being sold or used as fuel) and would alleviate the "acid mist" problem occurring in conventional cells. In this report, the use of the electrode in zinc electrowinning from acidic sulfate solutions was investigated. The equilibrium potential (with respect to the standard hydrogen electrode) for zinc in contact with a solution containing zinc ions at unit activity is -0.763V. In other words, from a solution containing both hydrogen and zinc ions at unit activity, thermodynamics predicts that hydrogen, rather than zinc, would be electrodeposited. The deposition of zinc from commercial acidic electrolyte (often containing 100 to 200 g/liter of sulfuric acid) would appear even more unlikely. The fact that zinc is electrowon on an industrial scale from such electrolytes is a consequence of the more favorable kinetics for deposition of zinc on zinc compared to the deposition of hydrogen, or, expressed in electrochemical terms, there is a large overpotential for hydrogen deposition on zinc. This overpotential is sensitive to the detailed chemistry of the solution (presence of impurities) and to the nature of the zinc surface; those phenomena that lower the overpotential will increase the hydrogen evolution rate and lower the current efficiency. The negative effects of impurities in conventional electrowinning of zinc from acid sulfate electrolytes have been studied by many investigators in recent years.~-~ It is well known that zinc deposition is very sensitive to small quantities of certain impurities. Impurity behavior is not well understood, and mechanisms by which certain concentrations of impurities become active or detrimental to current V. JtRICNY, Engineer, is with Institute of Chemical Process Fundamentals, Czechoslovak Academy of Science, 165 02 Praha 6-Suchdol, Czechoslovakia. J.W. EVANS, Professor of Metallurgy, is with the Department of Materials Science and Mineral Engineering, University of California, Berkeley, Berkeley, CA 94720. Manuscript submitted February 10, 1984. METALLURGICALTRANSACTIONS B
efficiency have not been clearly identified. In addition, synergism among impurities may occur and causes unpredictable difficulties. The maximum levels of impurities 6 must be considered in relation to the process parameters and factors such as zinc and acid concentrations, current density, temperature, time of electrolysis, etc. Robinson and O'Keefe ~showed
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