Selective recovery of zinc from spent pickling baths by the combination of membrane-based solvent extraction and electrowinning technologies

Abstract

This work deals with the analysis of an integrated zinc recovery process by means of electrowinning of the stripping solutions coming from the treatment of spent pickling baths (SPB) by a membrane-based solvent extraction (MBSX) process able of increasing the initial Zn/Fe molar ratio. Several stripping solutions containing different concentrations of zinc and iron in acid media obtained previously by the treatment of SPB by MBSX, were subjected to electrowinning to assess the efficiency and selectivity of zinc electrodeposition over iron under different operation conditions. At similar values of the zinc concentration in the stripping solution, the influence of the Zn/Fe molar ratio on the zinc electrodeposition process was negligible. On the other hand, although the variation of the initial concentration of zinc in the stripping solution neither affected the efficiency of zinc electrowinning, it increased the minimum value of zinc concentration in solution beyond which iron co-deposition started. Finally, the increase in the applied current, promoted the increase in zinc fractional conversion and in the zinc space–time yield, while the zinc current efficiency was reduced due to the stronger effect of secondary reactions. Although the change in the stripping characteristics seems not to strongly affect the zinc electrodeposition process, the use of a pretreatment step based on MBSX technology improved the results in terms of zinc percentage recovered and the rest of figures of merit, in comparison with those obtained by the direct electrowinning of SPB.