Roles of tannic acid and gelatin in Zn electrowinning and their inhibition mechanisms investigated via electrochemical methods

In this study, the roles of tannic acid and gelatin in Zn electrowinning were investigated. The results indicated that the addition of 10 mg/L of gelatin promoted Zn electrowinning and increased its current efficiency (CE) from 89.55% to 91.8%. However, the CE was only 77.47% when the electrolyte contained 50 mg/L of gelatin. As the concentration of tannic acid in the electrolyte increased from 10 mg/L to 400 mg/L, the CE decreased from 85.73% to 72.09%, which represented declines of 4.27% and 19.5%, respectively, compared with that of normal Zn electrowinning conditions in the absence of tannic acid. With increase in the concentrations of tannic acid and gelatin, the cell voltage increased and CE decreased sharply, which eventually resulted in a significant increase in the unit consumption of direct current (DC). The mechanisms by which tannic acid and gelatin inhibited the kinetics of Zn plating were additionally researched using electrochemical methods. The results showed that tannic acid and/or gelatin in high concentrations in the electrolyte significantly inhibited the deposition of Zn on the cathode by increasing the overpotential, reducing the deposition rate, and covering the electrode surface, which led to the appearance of agglomerates and needle-like structures on the surfaces of the Zn sheets.