Combined nanofiltration and diafiltration for isolation of rare-earth ions

Abstract

Rare earth elements (REEs) are critical materials due to their utilization in high-flux magnets for wind turbines and electric motors. Purification and recycling of REEs are vital for maintaining their supply. This work explores and models nanofiltration (NF) followed by diafiltration for concentrating La3+ and polishing monovalent ions from the concentrated solution. Highly negative monovalent-ion rejections greatly enhance this polishing step, and proton concentrations decrease 3–4 orders of magnitude in 2.4 cell volumes of diafiltration. Studies with rotating membranes give a controlled, uniform boundary layer above the membrane to enable estimation of salt and ion permeances at various feed compositions. Insertion of these permeances in the solution-diffusion-electromigration model yields trends in ion rejections as a function of composition, solution flux, and boundary layer thickness. Simulated rejections of monovalent ions become much more negative with lower solution fluxes (up to a point) and higher ratios of La3+ to monovalent ions. La3+ rejections are much higher in the rotating membrane than in a stirred cell where the diffusion boundary layer is thicker and nonuniform. The combination of NF and diafiltration provides concentrated La3+ with a molar purity of 99 % when starting with equimolar mixtures of La3+ and Na+. Acid removal from La3+ solutions via diafiltration could prove useful for subsequent separations among REEs or other transition metals.