Integration of nanofiltration membranes in recovery options of rare earth elements from acidic mine waters

Acid mine waters (AMWs) are characterized by a low pH, high concentrations of transition metals (iron, copper, zinc), and the presence of highly valuable metals such as rare earth elements (REE) at lower concentrations. Sustainable management of AMW implies the reduction of acidity and Fe/AI concentrations and the recovery of valuable metals by ion-exchange processes. Nanofiltration (NF) offers a good opportunity compared to conventional methods to treat AMW, such as good passage of mono-charged ions (e.g. hydrogen sulphate, proton) for further recovery of sulphuric acid and high rejection of multi-charged ions, such as transition metals and REE. Two membranes with a different active layer, a double membrane (poly-piperazinamide/proprietary polyamide) (Desal DL) and a sulphonated polyethersulphone (HydraCoRe 70pHT), were evaluated with a solution mimicking acidic streams generated after the removal of iron by precipitation and its subsequent treatment with ion exchange resins. The recovery of sulphuric acid and metal rejections were evaluated by changing the sulphuric concentration/pH (1.0-1.5) and by varying the concentration of aluminium in the solution (0.6-2.2 g/L). The transport of ions through NF membranes was described according to the solution-electro-diffusion model coupled with reactive transport of ions, and the membrane permeances to ions were determined as a function of aluminium concentration. Desal DL showed better results than HydraCoRe 70pHT in terms of acid recovery and metal rejections for both tested conditions. Membrane chemistry of the active layer (nature and acid-base membrane properties) and chemical speciation were found to be strong parameters in the separation performance of the membranes. Finally, a process scheme for recovering the REE is proposed. (C) 2018 Elsevier Ltd. All rights reserved.