Metal Recovery from Wastewater Using Membranes

Abstract

This paper outlines the requirements for metal recovery from wastewater, with particular reference to electroplating. The technical features of three alternative membrane processes are described. Nanofiltration is shown to separate ionic species on the basis of coulombic interactions or hydrated ion size, which leads to either a ‘charge' pattern or a ‘hydration' pattern of rejection. These rejection patterns provide ion selectivity. Ultrafiltration (UF), coupled with ion-complexing polymers or ion exchange resin, also provides efficient removal of metal ions, at high flux. The effectiveness of the UF-resin process is considerably increased as the resin size is decreased. Solvent extraction in a Liquid Membrane Contactor (LMC), which is based on microporous hollow fibre modules with aqueous and organic phases circulating through the shell or fibre lumens, achieves high fluxes of metal ions. A limitation of the LMC is the need to avoid phase leakage. The factors governing the critical displacement pressure and the effective transmembrane pressure are discussed. An LMC with a high packing density of fibres in the shell is preferred. Finally, the paper discusses criteria for selection of ‘user friendly' technologies for the electroplating industry. The membrane technologies, particularly in combined form, score highly except in terms of simplicity. This aspect needs further development.