Recovery of sulphuric acid and added value metals (Zn, Cu and rare earths) from acidic mine waters using nanofiltration membranes

Abstract

Acidic mine waters (AMWs) contain, in addition to moderate concentrations of sulphuric acid, relatively high concentrations of transition elements (e.g. Fe, Al, Cu, Zn) and, in some cases, minor amounts of valuable rare earth elements (REEs). The established management routes for AMWs, based on the neutralisation and metal stabilisation as hydroxides, are limited by the associated costs, such as reagent consumption. Nanofiltration (NF)-based treatment processes could improve these management routes by recovering the sulphuric acid and simultaneously increasing the concentration of valuable elements (e.g. Zn, Cu and REEs) for further valorisation after the proper removal of Fe. Three different types of NF membranes with (a) a poly(piperazinamide) active layer (NF270), (b) a double active layer (poly(piperazinamide)/proprietary polyamide) (Desal DL) and (c) a sulphonated poly(ethersulphone) active layer (HydraCoRe 70pHT) were evaluated. The influence of Fe(III) concentration on the sulphuric acid recovery from solutions at pH 1 was characterised. NF270 showed the higher permeate fluxes and the higher heavy and REE metal rejections (i.e. the higher concentration factors). As the Fe(III) concentration increased, higher acid permeation was obtained, which helped to decrease the neutralisation costs during its post-treatment. The membrane chemistry of the active layer (nature and acid-base membrane properties) and structure (single/double layer) were found to be strong parameters in the membrane separation performance. Ion transport data were modelled according to a Solution-Electro-Diffusion model coupled with Ion Reactive Transport and the membrane permeances to ions were determined.