The anomalous behavior of Al 3+ in nickel electrowinning from sulfate electrolytes
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I. INTRODUCTION
THE electrodeposition of metals from aqueous solutions, whether for coatings or primary metal production, requires uniform deposit growth at an optimum current efficiency. To accomplish this, a variety of chemical and physical factors must be balanced and optimized during processing. For example, significant parameters that can affect the electrodeposition of nickel from acid-sulfate electrolytes can include the current density, electrolyte pH, temperature, agitation, and metal ion concentration. The presence of additives and impurities can also be significant, depending on the types and concentrations involved.[1] It has been shown that nickel deposition is sensitive to the presence of metallic impurities, in particular Al, Cr, Co, Cu, Fe, Mg, and Zn, and also sulfur-containing organic compounds. Gogia and Das.[2] studied the effect of Mg21, Mn21, Al31, and Zn21 during the electrodeposition of nickel. They observed that these impurities did not affect current efficiency significantly if the concentrations were sufficiently low, but they had an appreciable effect on the purity, physical appearance, and structural characteristics of the deposits. In a separate study, the effects of Co21, Cu21, Fe21, and Fe31 on nickel electrowinning were determined.[3] Again, the current efficiency did not change significantly in the presence of these impurities over the concentration range studied, but certain changes in the purity and physical appearance of the deposit were observed. Depending on the cation, up to 1000 ppm was necessary to degrade the physical appearance of the deposit. In the case of Al31, only 100 ppm was necessary to significantly degrade the physical appearance of the deposit. Studies by Zhang on the effects of Al31 and Cr31 indicated that significant degradation of both the current efficiency and deposit morphology occurred when these impurities were present in concentrations of only 40 ppm.[4] However, in all cases, the effects of higher Al31 concentrations were not presented. THOMAS J. O’KEEFE, Curators’ Professor Emeritus, is with the Department of Metallurgical Engineering and Graduate Center for Materials Research, University of Missouri-Rolla, Rolla, MO 65409. MICHAEL HOLM, Process Engineer, is with VICOR, Andover, MA 01801. Manuscript submitted December 29, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
In a number of electrolytic processes, it has been observed that impurity levels can be countered by balancing various other deposition parameters. Additives are commonly used for leveling and to counteract detrimental electrolyte impurities, resulting in an improved deposit morphology and current efficiency. A good example of these interactions is observed in the deposition of copper from acid-sulfate electrolytes. For copper, 40 ppm chloride ion and parts per million concentrations of organic additives such as glue, gelatin, and thiourea are used to ensure flat, uniform cathode growth. At low or unbalanced concentrations, these additives can allow the co-deposition of impurity elemen
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