Effect of pre-concentration on membrane solvent extraction process for the recovery of rare earth elements from dilute acidic leachate

Pre-concentration and separation processes of Rare Earth Elements (REEs) were investigated in terms of several factors. Nitric acid leachate of e-waste was first pre-treated by increasing pH and filtering through microfiltration. For pre-concentration, pre-treated leachate was concentrated by nanofiltration. While a 70% permeate recovery ratio was kept constant, the rare earth elements concentrations were triplicated under optimum conditions. A flat sheet supported liquid membrane process with a polyvinylidene fluoride (PVDF) support membrane was successfully used to extract REEs from the pre-treated leachate. Of the two extractants evaluated, bis-2-ethylhexyl phosphoric acid (D2EHPA) displayed a higher REE separation efficiency than did di-2,4,4,-trimethylpentyl phosphinic acid (Cyanex 272). However, Cyanex 272 separated Sc more selectively. For direct membrane solvent extraction (MSX) and MSX with pre-concentration, pre-treatment pH and D2EHPA concentrations were optimized at values of 1.5% and 15%, respectively. When comparing the results of MSX for direct and pre-concentrated configurations, it was seen that REEs and HREEs recoveries were increased 10% and 30% in MSX with pre-concentration at the end of single-stage MSX. Pre-concentration not only increased the MSX process efficiency but also enabled acid recovery from nanofiltration permeate. A more environmentally friendly and economical process scheme was proposed, including acid recovery from both NF filtrate and post-MSX leaching residue by two different membrane distillation configurations.