Abstract: Rare-earth elements (REEs) are essential for renewable energy technologies, but their extraction and purification remain difficult due to the presence of competing ions in recycled and natural sources. Conventional separation methods often require large amounts of chemicals and produce significant waste, posing economic and environmental challenges. This study demonstrates a combined nanofiltration (NF) and diafiltration process for efficiently purifying REEs, focusing on La³⁺ separation from mixed-ion solutions.
The combined NF and diafiltration approach achieve a final La³⁺ molar purity of 99%, starting from equimolar mixtures with Na⁺, demonstrating exceptional performance for selective REE concentration. Experimental results demonstrate that highly negative rejections of monovalent ions, such as Na⁺ and H⁺, significantly enhance the purification step (In negative rejection, the concentration of a specific ion is higher in the permeate than in the feed.). In diafiltration, proton concentrations decrease by 3–4 orders of magnitude within 2.4 cell volumes, highlighting the system’s capability to remove acid efficiently while retaining REEs.
We model and optimize this process using rotating membrane cell to achieve a controlled boundary layer, which facilitates accurate estimation of ion permeances across various feed compositions. Using a solution-diffusion-electromigration model, we analyze trends in ion rejection as a function of feed composition, flux, and boundary layer properties.
These findings underscore the potential of membrane-based separations as scalable and environmentally sustainable solutions for REE purification and recovery processes. Future studies will explore broader implementation across REE mixtures, enhanced acid removal at lower pH, and compatibility with ligand-assisted separations to refine selectivity.