Unveiling the mechanism of liquid-liquid extraction separation of Li+/Mg2+ using tributyl phosphate/ionic liquid mixed solvents

Ionic liquids (ILs) based liquid-liquid extraction (LLE) is one of the most efficient and green techniques for mining Li from salt lake brines, however, the extraction mechanism still needs to be clarified. Here, tributyl phosphate (TBP) and four kinds of ILs were used as extractant/co-extractant for recovering Li from Mg2+/Li+ aqueous solution. The extraction conditions including the IL selection, IL/TBP ratio, Li+ concentration and Li+/Mg2+ ratio were analyzed in detail. A Li+ recovery of 65.3 % and Li+/Mg2+ separation factor of 68.4 at the Li+/Mg2+ ratio of 1/50 wt% were achieved. This high Li extraction efficiency and Li+/Mg2+ selectivity were analyzed by in-situ FTIR and 31P NMR, the results turned out that this is attributed to the strong interactions between Li+ and TBP/[NTf2]−, and much larger than that of Mg2+. This strong interaction facilitated the [C4mim]+ cation in IL exchange with Li+ between organic and aqueous phase that finish the extraction process, which was further proved by EDX and XPS results. Additionally, the binding energy of the extraction intermediates were calculated by density functional theory (DFT), and the most stable one is 3TBP•Li•H2O•NTf2, suggesting one H2O combined with Li+ enter into the organic phase during the extraction. As a result, the Gibbs free energy of the extraction between Li+ and ILs/TBP mixture can be obtained to be −132.2 kJ mol−1. In summary, our work showed an in-depth understanding of the extraction mechanism and provides guidelines for the ionic liquids-based Li extraction process in the future.