Selective Recovery of Zinc over Iron from Spent Pickling Wastes by Different Membrane-based Solvent Extraction Process Configurations

This paper reports the selective separation of zinc over iron from spent pickling wastes or effluents using two different membrane-based solvent extraction process configurations, Nondispersive solvent extraction (NDSX) and emulsion pertraction technology (EPT). The process aims to obtain a highly concentrated zinc solution with a negligible content of iron to allow for Zn electrowinning. The effect of the following process variables on the kinetics and selectivity of zinc separation has been evaluated: (i) process configuration NDSX and EPT, (ii) extractant concentration, tributylphosphate (TBP) in the range of 20−100% v/v and (iii) stripping phase/feed phase volume ratio in the range Vs/Va 0.2−2. The transport of iron, chloride and free acid has been also monitored to gain insight into the separation fundamentals. EPT configuration overcame NDSX in terms of zinc and iron separation kinetics, although separation selectivity (at 30 min) was higher for NDSX configuration, αZn/Fe = 22, compared to EPT process αZn/Fe = 15. The optimum TBP content in the extractant phase was found to be 50% v/v. A further increase did not improve the Zn recovery kinetics and reduced the Zn/Fe selectivity. The increase of the Vs/Va ratio improved the process efficacy in terms of kinetics and zinc recovery.