2007 Volume 123 Issue 6_7 Pages 351-356
The effect of polyethylene glycol (PEG) on the dissolution behavior of Zn electrodeposited from the electrowinning solutions containing impurities has been investigated by measuring the time-dependence of cathode potential during electrolysis and the partial polarization curves for Zn and impurity deposition. The impurities are classified into four groups ; group I (Ag, Cu, Cd) which changes inherent hydrogen overpotential ηOH by depositing at limiting current density; group II (Ni, Co, Fe) which reduces ηOH by partially depositing at lower current densities than limiting one ; group III (As, Sb, Ge) which catalytically reduces ηOH; and group IV (Al, Pb) which causes no change in ηOH. The PEG affected to the dissolution of deposited Zn in a different way depending on the kind of impurity. In the solution containing Cu of group I as impurity, deposited Zn was more likely to dissolve with PEG than without. Although PEG suppressed Zn deposition, it had no effect on Cu deposition at its limiting current density. Therefore, PEG increased the content of Cu in deposited Zn. Since hydrogen overpotential is lower on Cu than on Zn, codeposited Cu promoted the dissolution of Zn. In the solution containing Ni or Co of group II, PEG suppressed the dissolution of deposited Zn because the evolution of hydrogen was inhibited. Although Sb of group III depolarized the cathodic potential and decreased the current efficiency for Zn deposition, PEG polarized the cathodic potential and increased the cathodic current efficiency. The harmful effects of impurities of group II and III on the dissolution of Zn and the depolarization of cathode potential were decreased with an increase in the molecular weight and the amount of PEG added.
