Acid-Resistant Nanofiltration Facilitated Nonsaponified Extraction of Rare-Earth Elements

The conventional extraction of rare-earth (RE) elementsby alkalisaponification consumes plenty of alkalis and produces enormous amountsof salt effluents. Herein, we proposed a novel environmentally friendlynonsaponified RE extraction process facilitated by an acid-resistantnanofiltration (NF) membrane, in which the nonsaponified organic phasedirectly is extracted with the RE feed repeatedly by a multistagecross-current approach and the exchanged acid in the raffinate isremoved by the acid-resistant NF membrane with the deacidified raffinaterecycled for nonsaponified extraction. Using YCl3 as themodel sample, the feed RE concentration of the nonsaponified extractionwas optimized in the range of 0.1-1.0 mol L-1, and a high RE concentration of 0.21 mol L-1 inthe organic phase and an average extraction rate of 89.8% were obtainedafter seven-stage extraction at a RE feed concentration of 0.1 molL(-1). In total, 99% of the acid in the raffinatewas removed by the commercial Duracid NF1812 acid-resistant NF membraneunder an optimized RECl3 concentration of 4.9 g L-1, along with a recovered pH value of & SIM;3, resulting in a hightotal RE yield of 99.6%. The acid-resistant NF-facilitated nonsaponifiedextraction process featuring a high RE extraction rate and organicRE concentration, the absence of alkali consumption and salinity effluentemission, and eco-recycling of the raffinate renders the separationof RE elements a versatilely sustainable and green process.