Opposite selectivities of tri‐n‐butyl phosphate and Cyanex 923 in solvent extraction of lithium and magnesium

The synergic solvent extraction system of tri‐n‐butyl phosphate (TBP) and FeCl(3) (or ionic liquids, ILs) has been extensively studied for selective extraction of Li from Mg‐containing brines. However, Cyanex 923 (C923), which extracts many metals stronger than TBP, has not yet been examined for Li/Mg separation. Here, we report on the unexpected observation that the C923/FeCl(3) system has opposite Li/Mg selectivity compared to the TBP/FeCl(3) system. Detailed investigations show that the opposite selectivity of the C923/FeCl(3) (or IL) system is due to three factors: (1) the strong extraction of Fe by C923 leads to a low concentration of [FeCl(4)](−) in the system, which is essential for Li extraction; (2) C923 in combination with an IL extracts Mg strongly by an ion‐pair mechanism; (3) most importantly, C923 extracts Mg by solvation, resulting in an insufficient concentration of C923 for Li extraction. The unexpected poor Li/Mg selectivity of C923 highlights the irreplaceable role of TBP in the selective recovery of Li.