A highly efficient ternary extraction system with tributyl phosphate/crown ether/NaNTf2 for the recovery of Li+ from low-grade salt lakes

A great challenge is to develop suitable techniques to extract Li+ resources from low-grade salt-lake brines with high Mg/Li ratios. In this study, an efficient ternary extraction system has been constructed using tributyl phosphate (TBP) and crown ether (CE) as the extractant and sodium bistrifluoromethylsulfonimide (NaNTf2) as the auxiliary extractant to extract Li+. Results showed that the single-stage ELi% and SFLi-Mg were up to 74.1 % and 757, respectively, when the O/A was 1, 0.02 mol/l Li+ and 0.232 mol/l Mg2+. It is noteworthy that the elution of Li+ was also as high as 100 % at an O/A volume ratio of 5, when 0.5 mol/l HCl was used as the counter-extractant to recovery Li+. The NMR and FTIR analysis and molecular dynamics (MD) and density functional theory (DFT) revealed that the exceptional performance of the three-component extraction system was attributed to the optimal matching between the diameter of Li+ and the cavity size of the CE, as well as the strong affinity exhibited by CE, TBP, and NTf2- towards Li+. Specifically, the 2TBP-LiNTf2-CE-LiNTf2 complex structure with the highest binding energy displayed the most structure. In summary, this work paves the way for the recovery of Li+ from low-grade salt-lake brine.