Recovery of lithium from high Mg²⁺/Li⁺ ratio brine using a novel solvent extraction system TOP-FeCl₃-MIBK

The dissolution loss of extractant into aqueous solution during lithium solvent extraction from salt lakes poses a significant issue. We found that branched chains of neutral organophosphorus extractants provide suitable steric hindrance, enhancing lithium extraction efficiency and Li/Mg selectivity. Additionally, increasing the number of alkyl chains in organophosphorus extractants increases their hydrophobicity, thereby reducing extractant dissolution loss during the extraction process. Based on these findings, a novel extraction system comprising tri(2-ethylhexyl) phosphate (TOP), FeCl₃, and 4-methyl-2-pentanone (MIBK) was explored. The extraction conditions, including the Fe/Li molar ratio, diluents, phase ratio, and stripping conditions, were thoroughly investigated and optimized. The extraction mechanism was elucidated at the molecular level using FT-IR, Raman, NMR, and density functional theory (DFT) calculations. The results demonstrated that the TOP-FeCl₃-MIBK system achieved an 85 % extraction efficiency for Li⁺ in a single stage, with a Li⁺/Mg²⁺ separation factor reaching 5842, surpassing previously reported data in the literature. ³¹P NMR and ⁷Li NMR analyses revealed that the TOP and tributyl phosphate (TBP) complexes existed in different forms within the organic phase. Slope analysis indicated that the stoichiometric ratio of the extraction complex involving Li⁺, TOP, and MIBK was LiFeCl₄·TOP·MIBK. Furthermore, DFT calculations indicated that the TOP complex was more stable than the TBP complex after extraction. We anticipate that our findings will pave the way for the development of a series of novel extractants with higher Li/Mg selectivity by enhancing steric hindrance and incorporating longer alkyl chains to minimize dissolution loss. We expect this research to inspire the development of similar neutral organophosphorus extractants.