Preliminary Study of Pure Zinc Recovery from Spent Pickling Liquors by Combining Anion Exchange and Electrodeposition

Zn recovery from spent pickling liquors (SPL) from hot dip galvanization was examined with modelled solutions by combining anion exchange separation and electrodeposition processes. The solution was purified to be suitable for the electrodeposition of Zn with the main goal of eliminating the iron content. The anion-exchange resin bed in a chromatographic column primarily retained Zn, while divalent Fe was removed in the loading and rinsing steps. At the end of the process, the elution of Zn was carried out by a significantly reduced Cl– ion background. In parallel with the purification process, the potentiodynamic study (1 min runs at 40 mV/s polarization speed) of Zn electrodeposition in the series of Fe concentrations was examined. During the separation process, the preliminary reduction of the iron to its divalent state was found to be of utmost importance. The separation of iron could be improved by controlling the preliminary reduction. It was found that the effect of iron concentration on the polarization curves is complex. Initially, hydrogen bubble formation causes inhibition, but at higher iron concentrations a Zn-Fe deposit is soon formed, promoting hydrogen evolution, thereby depressing polarization but also the current efficiency. The loss of the dendritic structure obtained from pure zinc solutions also characterizes the effect of mixed Fe-Zn deposition. In order to recover pure zinc at the cathode, the pre-purification of the Zn electrolyte is essential.