The synergic solvent extraction system of tri-n-butyl phosphate (TBP) and FeCl₃ (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/FeCl₃ system has opposite Li/Mg selectivity compared to the TBP/FeCl₃ system. Detailed investigations show that the opposite selectivity of the C923/FeCl₃ (or IL) system is due to three factors: (1) the strong extraction of Fe by C923 leads to a low concentration of [FeCl₄]⁻ 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.

中文翻译：

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磷酸三正丁酯和 Cyanex 923 在溶剂萃取锂和镁中的相反选择性


磷酸三正丁酯(TBP) 和 FeCl ₃ （或离子液体，IL）的协同溶剂萃取系统已被广泛研究用于从含镁盐水中选择性萃取锂。然而，Cyanex 923 (C923) 提取的许多金属强度都比 TBP 强，但尚未针对 Li/Mg 分离进行测试。在这里，我们报告了意外的观察结果，即与 TBP/FeCl ₃系统相比，C923/FeCl ₃系统具有相反的 Li/Mg 选择性。详细研究表明，C923/FeCl ₃ (或IL)体系的相反选择性是由于三个因素造成的：(1) C923对Fe的强烈萃取导致体系中[FeCl ₄ ] ^-浓度较低，这导致对于锂的提取至关重要； (2) C923与IL结合通过离子对机制强烈提取Mg； (3)最重要的是，C923通过溶剂化提取Mg，导致C923提取Li的浓度不足。 C923 出乎意料的差的 Li/Mg 选择性凸显了 TBP 在 Li 选择性回收中不可替代的作用。