Electrode mass transfer under conditions of natural and forced convection
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I. I N T R O D U C T I O N O N E of the most challenging shortcomings and demanding problems of aqueous electrowinning cells is the poor space-time yield. This can be improved only by increasing the current passed through the cell, but in order to operate at high current densities forced convection must be applied to raise the limiting current so that smooth, compact, and uniform deposits can be produced. This can be achieved for both anodic and cathodic reactions by fluidized bed techniques, I1'2"31 gas sparging and mechanical stirring, I5"61jetting of the electrolyte, I7,8] electrode oscillation, ]9'~~ or ultrasonicsJ 61 Various methods have been applied for determining mass transfer coefficients at specific locations on electrode surfaces, including electrolyte freezing, t~ll interf e r o m e t r y y I and electrochemical techniques. I13'14'151 The latter provide the most direct measure of cation mass transfer coefficients but have generally relied upon the local limiting current density for the species in question. This limits the applicability of the data since the mass transfer coefficient at the limiting current need not be the same as at the lower currents used in commercial practice, because the mass transfer coefficient can change with current. In one method, c~31 a number of electrically insulated potential probes are mounted within and coplanar with the cathode surface to determine the local limiting current as the whole electrode is polarized. Only relatively dilute solutions can be used to avoid massive copper deposition that would short the probes to the cathode as well as generate surface roughness. For more concentrated electrolytes, a technique has been u s e d [~41 where a small probe electrode facing the anode is held close to the cathode in a specific region and polarized to the limiting current. Although this can be used in the true operating environment of a working electrowinning cell, the limiting current density at the probe is A.V. COOKE, formerly Graduate Student, Department of Materials Science and Metallurgy, University of Cambridge, is Manager, Technology Commercialization, Martin Marietta Laboratories, Baltimore, MD 21227-3898. J.P. CHILTON and D.J. FRAY are University Lecturers, Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, United Kingdom Manuscript submitted February 25, 1987. METALLURGICAL TRANSACTIONS B
not necessarily representative of that of the electrode itself, because the metal deposited on the probe at the limiting current has a higher surface area, and the agitation regime at the probe is not identical to that at the electrode. To address these shortcomings, Ettel e t a / . [7'81 have investigated a method for the determination of local mass transfer coefficients to the cathode during copper electrowinning on commercial size electrodes under conditions of natural and forced convection. In brief, the method entailed electrowinning copper from a sulfate bath under standard operating conditions while codepositin
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