Aqueous hybrid liquid membrane process for metal separation - Part II. Selectivity of metals separation from wet-process phosphoric acid

Selective separations of heavy metal ions from industrial wet-process phosphoric acid (WPA) were studied using an aqueous hybrid liquid membrane (AHLM) system. Polyvinylsulfonic acid (PVSA) aqueous solution was used as a liquid membrane (LM) phase, separated from the feed and strip streams by cation-exchange membranes. Heavy metals, such as cadmium and copper, were successfully recovered from the WPA: average fluxes (in mol/m(2) s) to the strip phase were 3.7 x 10(-7) and 6.7 x 10(-7), respectively, at a Cu/Cd selectivity of 2.7. The transfer of iron and zinc species to the strip phase was below detection levels during the 144 h of the AHLM experiment. Selectivity is not a constant parameter in the AHLM separation of metal ions. The idea of dynamic selectivity and techniques for its determination were introduced. Selectivity can be controlled by adjusting the concentration, volume and flow rate of the circulating bulk LM phase. Such a control of the selectivity is one of the advantages of the AHLM system in comparison with other LMs and Donnan dialysis technologies. A theoretical model has been developed for the simulation of the AHLM transport selectivity. Satisfactory correlation between experimental and simulated data was achieved. Selectivity parameters, needed for the AHLM module design, may be obtained by performing series of experiments with feed, carrier and strip flow rate variations, and of membrane-based extraction. Preliminary optimization of the AHLM module may be achieved by simulation of the feed- and strip-side membrane areas, concentrations and volumes of the bulk LM and strip phases, and flow rates. (C) 2000 Elsevier Science B.V. All rights reserved.