Acid-Resistant Nanofiltration Facilitated Nonsaponified Extraction of Rare-Earth Elements
Wei Lai, Zhenyuan Li, Ju Bai, Luqi Xiao, Peijun Zheng, Lu Bai, Youming Yang, Chunfa Liao, Linglong Shan, Shuangjiang Luo
Abstract
The conventional extraction of rare-earth (RE) elements by alkali saponification consumes plenty of alkalis and produces enormous amounts of salt effluents. Herein, we proposed a novel environmentally friendly nonsaponified RE extraction process facilitated by an acid-resistant nanofiltration (NF) membrane, in which the nonsaponified organic phase directly is extracted with the RE feed repeatedly by a multistage cross-current approach and the exchanged acid in the raffinate is removed by the acid-resistant NF membrane with the deacidified raffinate recycled for nonsaponified extraction. Using YCl 3 as the model sample, the feed RE concentration of the nonsaponified extraction was optimized in the range of 0.1–1.0 mol L –1, and a high RE concentration of 0.21 mol L –1 in the organic phase and an average extraction rate of 89.8% were obtained after seven-stage extraction at a RE feed concentration of 0.1 mol L –1 . In total, 99% of the acid in the raffinate was removed by the commercial Duracid NF1812 acid-resistant NF membrane under an optimized RECl 3 concentration of 4.9 g L –1, along with a recovered pH value of ∼3, resulting in a high total RE yield of 99.6%. The acid-resistant NF-facilitated nonsaponified extraction process featuring a high RE extraction rate and organic RE concentration, the absence of alkali consumption and salinity effluent emission, and eco-recycling of the raffinate renders the separation of RE elements a versatilely sustainable and green process.