Extraction of Lithium from Magnesium-Rich Solution Using Tri-n-butyl Phosphate and Sodium Hexafluorophosphate

Ionic liquids (ILs) such as 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]) are considered suitable co-extraction reagents of tri-n-butyl phosphate (TBP) for selective extraction of lithium from magnesium-rich salt lake brine. However, ILs are very expensive and the loss of cations can cause the contamination of the aqueous solution. The corresponding inorganic salts of ILs such as sodium hexafluorophosphate (NaPF6) should play the same role as ILs in the extraction of Li+ by TBP, and importantly, they are cheaper and more environmentally friendly. In this study, the effects of NaPF6 concentration, phase ratio, initial pH of the aqueous phase, temperature, and MgCl2 concentration on the extraction of lithium were investigated. It was found that NaPF6 was as good as [C4mim][PF6] in the extraction of Li+ from Mg-rich solution by TBP, and their extraction mechanisms are also the same, which is certified by comparing the Fourier transform infrared spectroscopy (FT-IR) and slope analysis. Hydrochloric acid (HCl) is more effective than sodium carbonate (Na2CO3) in the stripping of Li+, but Na2CO3 can avoid the regeneration of the organic phase and increase the reusability of both TBP/[C4mim][PF6] and TBP/NaPF6 systems. In TBP/NaPF6 system, the schematic of Li extraction/stripping from brine with high Mg/Li ratios using (a) HCl as stripping agent and (b) Na2CO3 as stripping agent