Degradation of polymer additives during long-term Zn electrowinning

The time dependence of molecular weight of gelatin, the cathode potential, and the morphology of deposited Zn were investigated during long-term electrolysis of Zn to evaluate the degradation of gelatin and polyethylene glycol (PEG) added to the electrolyte for Zn electrowinning. Gelatin and PEG acted as a polariser for Zn deposition, shifting the cathode potential to a less noble direction. The polarisation caused by the additives gradually decreased with an increase in electrolysis duration. The effective duration of additives depended significantly on molecular weight, and was longer in PEG-containing solution than in a gelatin-containing solution. Gel permeation chromatog-raphy revealed that the molecular weight of gelatin in an electrolyte decreased in with an increase in electrolysis duration. In particular, the molecular weight of gelatin decreased more rapidly at the anode than at the cathode. The decrease in molecular weight of gelatin by electrolysis brought about a shift of the cathode potential to a more noble direction, showing that the molecular weight of degraded gelatin was related to the deposition potential of Zn. Even in the case without electrolysis, the molecular weight and polarisation effect of gelatin decreased with a lapse of time. On the other hand, the morphology and the crystal orientation of Zn obtained by long-term electrolysis corresponded to the change of the cathode potential with time regardless of the kinds of additives. With an increase in the cathodic polarisation, the deposits showed the smaller grains with preferred orientation of {1010}. The morphology and the crystal orientation of deposited Zn became similar to that in the additive-free solution when the polarisation was decreased as a result of the degradation of additives.