Fundamental study of the role of cathode structure on zinc electrowinning

The influence of cathode macromorphology on the current efficiency of zinc electrowinning was determined. The studies have shown that surface roughness affects current efficiency during zinc electrowinning, particularly when the electrolyte contains impurities such as nickel and antimony. Current efficiency does not change with surface roughness as long as the electrolyte is sufficiently pure. Zinc deposited on smooth zinc cathodes tends to retain a more favorable structure and macromorphology, which is less sensitive to the effects caused by antimony and nickel. Glue helps to counteract the effect of the impurities; however, its presence in the electrolyte at certain combinations with other additives produces an increase in surface roughness and a dramatic decrease in current efficiency. A definite relationship was observed between the data obtained by cyclic voltammetry and the long time current efficiency tests. A statistical screening design program was conducted to determine the individual effects and interactions of temperature (35-45°C), antimony (0.02-0.06 mg/l), nickel (1-5 mg/l), glue (5-15 mg/l) and acid concentrations (175-225 g/l) on current efficiency. The results show that smooth zinc cathodes are less susceptible to impurity attack while rough deposits give consistently lower current efficiencies than smooth zinc. Under certain conditions, glue proved to be an important factor in influencing roughness evolution. The influence of cathode current distribution on current efficiency was also investigated. The effect of current density (up to 50 mA/cm2) and sulfuric acid concentration (150-250 g/l) on current efficiency was determined. The results show that low current densities (up to 10 mA/cm2) cause an increase in hydrogen evolution and an eventual dissolution of the zinc deposits. At the same operating conditions of current efficiency tests (12 hour deposition time), cyclic voltammetry tests show that the current rise measurements present a good correlation with current efficiency. Electrolytes containing antimony and glue show that, at low current densities, glue does not completely counteract the effect that antimony and high acid concentrations have on current efficiency. A proper chemical balance of the electrolyte was very critical. The experimental results have shown that surface macromorphology of the electrode was an important factor to be considered in the evaluating the efficiency of zinc electrowinning. In addition, surface macromorphology plays an important role in the interpretation of the impurity effects --Abstract, pages iii-iv