The role of nanofiltration modelling tools in the design of sustainable valorisation of metal-influenced acidic mine waters: The Aznalcóllar open-pit case

Abstract

Acid Mine Waters (AMWs) are considered the most challenging environmental problem in the Iberian Pyrite Belt (South of Spain) so far, being characterised by an acidic pH (H2SO4-based), high content of heavy metals (e.g. Co, Pb, rare earth elements (REEs)) and non-metals (e.g. As). Nowadays, AMWs are being contemplated as a source of valuable metals (e.g. Zn, Cu, REEs), and different recovery routes have been studied (e.g. ion-exchange, membrane technologies). Among them, nanofiltration (NF) provides the possibility of water (or H2SO4) recovery and concentration of metals, which can be recovered in a subsequent stage. This work evaluates the performance of the Desal DK NF membrane to treat AMW from Aznalcóllar open-pit, which presents a low pH (2.6) and high contents of Zn (920 mg/L), Mg (2540 mg/L) and REEs (17 mg/L). The effect of solution pH on membrane performance was evaluated. Results showed that the equilibrium SO42-/HSO4- controlled the transport of species. Rejections of sulphate were around 84 % at pH 2.1 (prevalence of HSO4-) and increased to 92 % at pH 3.1 (prevalence of SO42-). Metal rejections remained higher than 74 % (except for monovalent species). Additionally, membrane permeances were determined using the Solution-Electro-Diffusion model coupled with reactive transport, and later used to assess the design of the process treatment of the AMW. Results demonstrated the possibility of concentrating metals and recovering H2SO4 as permeate, but also showed that gypsum and anhydrite precipitation can occur. Additionally, CAPEX and OPEX were estimated for a NF plant treating 2 Hm3/year.