Recovery of rare earth elements from deep-sea mud using acid leaching followed by selective solvent extraction with N1923 and TBP

Deep-sea mud has received significant attention from the international community as a novel resource of rare earth elements (REEs). In this study, a novel approach is proposed for the recovery of REEs from deep-sea mud. The mud samples were characterized, and the occurrence states of REEs were clarified. The results revealed that REEs in mud occur almost entirely in hydroxyapatite minerals, which can be easily reached via acid leaching. The optimized leaching percentage of REEs reaches 89.50 %, 88.94 %, 83.81 %, and 86.52 %, respectively, by using 2.0 mol/L HCl, 2.0 mol/L HNO3, 1.0 mol/L H2SO4, and 2.0 mol/L H3PO4 as leaching agents with a liquid–solid ratio of 4:1 and a stirring speed of 300 r/min at 60 ℃ for 30 min. After that, the REEs were leached into the leaching solution along with the impurities of Al, Fe, Mn, Ca, and Mg. The sulfate leaching solutions were directly used for solvent extraction of REEs using N1923 and TBP. A total of 97.50 % REEs and 100 % Fe were co-extracted through five-stage countercurrent extraction using an organic system consisting of 10 %(v/v) N1923 and 10 %(v/v) TBP in sulfonated kerosene at an O/A ratio of 1:1.4. In contrast, the percentage extraction of other metals was negligible. Furthermore, 99.33 % REEs in the loaded organic system were selectively stripped through a five-stage countercurrent stripping process using the (0.1 mol/L HCl + 4.9 mol/L NaCl) solutions with an O/A ratio of 1:1, leaving Fe in the organic system. The remaining Fe-loaded organic material could be stripped using a 0.5 mol/L HNO3 solution followed by Na2CO3 neutralization for regeneration. Finally, 98.81 % of the REEs in the stripped solution were precipitated through Na2C2O4 precipitation followed by calcination at 900℃for 120 min. Throughout the entire separation and purification process, 80.20 % of REEs in the deep-sea mud resource were recovered as pure enrichment of REEs oxide with a content of 81.82 %, while Al, Fe, Mg, Ca, Mn, and P were less than 0.15 %.