Comparative Study of Four Acidic Extractants as Modifiers in the TBP/FeCl₃ Solvent Extraction System for Lithium and Magnesium Separation

The TBP/FeCl3 solvent extraction (SX) system exhibits a high selectivity of lithium over magnesium from magnesium-rich brines, but it suffers from the difficulty of lithium stripping. The addition of 2-ethylhexyl phosphonic acid mono-2-ethylhexy ester (P507) as a modifier to the TBP/FeCl3 system significantly enhances the stripping efficiency of the extraction system by controlling the Fe3+ coordination. However, the performance of other acidic extractants similar to P507 has not yet been investigated. In this work, we systematically compared the performance of the four most commonly used commercial acidic extractants, namely, P507, di(2-ethylhexyl) phosphoric acid (P204), bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272), and neodecanoic acid (Versatic Acid 10) as modifiers in the TBP/FeCl3 system. It was found that the addition of P204, P507, or Cyanex 272 slightly suppressed the extraction of lithium but Versatic Acid 10 did not; the coextraction of magnesium followed the order P507 > P204 approximate to Cyanex 272 > Versatic Acid 10. Importantly, the ability of the modifiers to keep Fe3+ in the organic phase is correlated with their acidity, i.e., P204 > P507 > Cyanex 272 >> Versatic Acid 10. Overall, P204 and P507 exhibited comparable performance, and they both surpassed Cyanex 272 and Versatic Acid 10.