Di-(2-ethylhexyl) phosphinic acid as extractant for recovering lithium from the desorption solutions of high Mg/Li ratio brine

With the amid surging demand for lithium, solvent extraction has gradually developed into a key method for lithium recovery in industry. Due to the problems of large dissolution loss and high extraction equilibrium pH of β -diketone system, di-(2-ethylhexyl) phosphinic acid (P227) was selected to recover Li + from the desorption solution of salt lakes (SL-DS). Compared with di-2-ethylhexyl phosphoric acid (P204), 2-ethylhexyl phosphoric acid mono 2-ethylhexyl ester (P507), and bis (2,4,4-trimethylpentyl) phosphinic acid (Cyanex272), P227 showed better separation ability for Li + and Na + . The extraction conditions, including saponification degree, initial pH, extractant concentration and phase ratio were studied. It could be found that the extraction efficiency of Li + was maintained at a high level in the initial pH range of 2.25–12.61. A cascade process consisting of eight-stage extraction and five-stage scrubbing was performed to verify the feasibility of this technology. 99.9 % of Li + from the SL-DS was extracted at the equilibrium pH of 7.60, and P227 exhibited superior selectivity for Li + over Na + . Moreover, under the optimal condition of 3.8 mol/L HCl and O/A of 20:1, the stripping ratio of Li + reached 99.76 % via a four-stage stripping experiment. The Li + concentration of stripping liquor attained 23.11 g/L and a high-purity (99.5 %) Li 2 CO 3 product was obtained by precipitation. After five cycles, P227 demonstrated excellent regeneration stability. Eventually, the composition of extraction complex was revealed as LiR•NaR by slope analysis, and the cation exchange mechanism was further confirmed using FT-IR and 31 P NMR.