Selective recovery of Zn and Fe from spent pickling solutions by the combination of anion exchange and membrane electrowinning techniques

Although the environmental industry has developed many modern techniques to treat spent pickling liquors, including also the membrane separation, still neither of them can be applied alone for the efficient treatment of HCl containing spent pickling solutions originating from hot dip zinc galvanizing plants. In this area, the most frequently applied treatment is still the common precipitation–filtration process. However, this technique generates large amounts of hazardous waste to be deposited at high costs and does not recover the acid. Our novel approach to treat such spent liquors consists of an ion-exchange separation and a membrane electrolysis step to recover HCl, Fe and ZnCl2 as saleable products. The experimental part of this paper concentrates on the anion-exchange separation of the components in the raw liquor and the cathodic deposition of iron at varied pH, temperature and cathode materials by using potentiodynamic testing technique. As a result, a direct anion-exchange with a strongly basic resin can separate the main components (Fe and Zn) of the raw solution loaded at a moderate flow rate into the column. The recommended conditions for the electrolytic deposition of iron are pH>1 and nickel cathode.