Acid recovery with diffusion dialysis to improve rare earth extraction economics

Acidic Mine Waters (AMWs), rich in dissolved metals and sulphates, contain a significant amount of rare earth elements (REEs), making their recovery attractive. Due to the natural composition of AMWs, ion-exchange (IX) technologies can selectively extract REE, but at the expense of a large excess consumption of sulphuric acid to regenerate the IX resins. As a matter of fact, the higher the sulphuric concentration applied during regeneration, the higher the REE concentration factors achieved in the eluate. This excess of free acid concentration in the eluate makes the REEs recovery both a technical and an economic challenge, due to the large amounts of chemicals needed (i.e., alkali for acidity neutralization and oxalic acid for subsequent REE precipitation). Diffusion dialysis (DD), a membrane-based separation process, has been postulated as an option for the selective recovery of sulphuric acid from the IX eluates, leaving a stream concentrated in REE and other metals rejected by the membrane. This work aimed to obtain the greatest sulphuric acid recovery with the minimum losses of REE by permeation through the DD membrane. An anion exchange membrane (AEM), containing a quaternary ammonium functional group, was used first to characterise ion transport through the AEM in batch configuration, and, second, to assess the effect of flow-rate on acid recovery in dynamic mode. A complex synthetic eluate containing REEs and interfering cations, co-extracted during the IX stage, was used, achieving rejection efficiencies >96 % for major metal ions (Na + , Ca 2+ , Mg 2+ , and Al 3+ ), while maintaining recoveries of sulphuric acid of 78–85 %. This selective transport was governed primarily by Donnan and dielectric exclusion mechanisms, further modulated by ionic charge, hydrated radius, and concentration gradients. The separation resulted in REE-containing solutions with tailored ionic profiles (e.g., sulphuric acid solutions) for subsequent crystallization as oxalates, leading to reduced reagent consumption and higher purity of the solid obtained. • Diffusion dialysis (DD) enables acid recycling, reducing process costs. • Integration of DD in ion-exchange processes for rare earth elements (REEs) recovery • High sulphuric acid recovery (>78 %) via diffusion dialysis • Metal cation rejection exceeds 96 % by Fumasep FAQP-375-PP. • Diffusion dialysis reduces chemical consumption in rare earth processing.