A remarkable improved Li+/Mg2+ selectivity and Li+ recovery simultaneously by adding crown ether to tributyl phosphate-ionic liquid extraction system as co-extractant

The high similarity in chemical properties between Li+ and Mg2+ poses a substantial challenge for their selective separation from salt lake brine with high Mg-Li (m/m) ratios. Herein, a highly efficient solvent extraction system was reported for Li+/Mg2+ selective separation, with tributyl phosphate (TBP) and di(aminobenzo)-14-crown-4 (CE) as extractants, [C4mim][NTf2] (IL) as co-extractant, and methylene chloride as a diluent. Simulated brines included 0.01 M LiCl and 0.01 and 0.2 M MgCl2 with Mg/Li mass ratios of 3.5 and 70, respectively. Molecular dynamics (MD), density functional theory (DFT), and 1H NMR analysis were employed to reveal the hydrogen bond interaction and compatibility between CE, TBP, and IL. Under the optimal extraction conditions containing a concentration of 20 mg/mL CE and 0.61 mmol/mL IL, the maximum single-stage Li+ extraction rate of the TBP-CE-IL extraction system was 86 %, and the separation factor of Li to Mg was up to 515. Compared with the TBP-IL extraction system, the addition of CE effectively improved the extraction rate of Li+ and the selectivity of Mg-Li, which was due to the hydrogen bond interaction between CE and TBP/IL and the cavity size effect between CE and Li+, improving CE coordination and endowing the complex generated in the organic phase with greater stability. The possible structure of the complex in the organic phase was determined to be 2TBP·Li·NTf2-CE·Li·NTf2 through Li+ extraction dynamic analysis. In summary, this work provides an efficient separation technology for Li+ extraction from salt lake brine.