Zinc electrowinning from acidic sulphate solutions Part IV: effects of perfluorocarboxylic acids

The effects of perfluorobutyric acid, perfluoroheptanoic acid and perfluorooctanoic acid, in the presence and absence of antimony(III), on the cathodic current efficiency, power consumption and polarisation behaviour of the cathode during the electrowinning of zinc from acidic sulphate solutions were investigated. The surface morphology and crystallographic orientations of the zinc deposits were also examined. Addition of any of these perfluorocarboxylic acids increased the current efficiency, decreased power consumption and produced better surface morphologies. Voltammetric studies indicated that polarisation of the electrode in the presence of these additives was in the order PFOA > PFHA > PFBA. The nature of the electrode reactions was investigated through measurements of exchange current densities, Tafel slopes and transfer coefficients. Perfluorocarboxylic acids were found to be better additives for zinc electrodeposition when Sb(III) was absent from the zinc electrolyte.