Recovery of Lithium Ions from Salt Lake Brine with a High Magnesium/Lithium Ratio Using Heteropolyacid Ionic Liquid

A fluoride-free ionic liquid 1-butyl-3-methylimidazolium phosphotungstate ([Bmim]3PW12O40) was synthesized and first used as co-extraction reagent for recovery of lithium ions from salt lake brine with a high Mg/Li ratio. Fourier transform infrared spectra (FT-IR), nuclear magnetic resonance (NMR) and thermogravimetric analysis (TGA) were used for characterizations of the prepared ionic liquid. Tributyl phosphate (TBP), [Bmim]3PW12O40, and dimethyl phthalate (DMP) were used as extractant, co-extraction reagent, and diluent, respectively. Effects of molar ratio of [Bmim]3PW12O40 to Li+, O/A phase ratio, volume fraction of TBP,and carbon chain length of ionic liquid on extraction efficiency and separation factor of Li+ were investigated. The overall extraction efficiency of Li+ was 99.23% after five stages cross-current extraction experiments under optimal conditions. The washing and stripping experiments of the organic phase were conducted, and the parameters were optimized. The reusability of the organic phase was investigated. A total of 10 regeneration cycles of extraction processes showed that the organic phase was highly stable for selective extraction of Li+. Finally, the extraction mechanism using the proposed extraction system was also explored. The results showed that the highly selective extraction of lithium ion was realized by cation exchange and coordination between TBP and Li+.