Hydration of In is the main factor limiting its extraction efficiency from an aqueous solution during a liquid-liquid extraction process. In this study, KCl was introduced into the aqueous solution to facilitate the formation of InCl of low charge density, which is expected to possess much weaker hydration compared with In, promoting the solvent extraction of indium. The crown ethers (CEs) with varied cavity sizes, benzo-18-crown-6 (B18C6), benzo-15-crown-5 (B15C5), and benzo-12-crown-4 (B12C4), were synthesized. The extraction performance of the CEs toward indium in the presence of sufficient KCl in the aqueous solution was investigated. The liquid-liquid extraction process was analyzed theoretically based on density functional theory (DFT) from the aspects of thermodynamics, geometric structure optimization, electrostatic potential (ESP), and independent gradient model (IGM). The theoretical evaluations agreed well with the experimental results that the hydration of indium could be significantly weakened through the formation of InCl and the complexation ability of the CEs toward indium is in the order of B18C6 > B15C5 > B12C4. The complexation mechanism between the CEs and indium during the extraction process was further explored with the assistance of H NMR spectrum and SEM-EDS. The results indicate that crown ether coordinates with K to form [CE-K] at the two-phase interface, which further associates with InCl to create the complex of CE-KInCl, realizing the efficient indium extraction. Moreover, B18C6 showed excellent selectivity toward In over the competing ions such as Fe, Al, Zn, Sn and Ca in a complex system. Indium could be efficiently recovered from the loaded organic phase by using 1 M HCl as the stripping agent with a stripping efficiency of 98.1%.

中文翻译：

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在 KCl 存在下用冠醚溶剂萃取并用 HCl 反萃回收铟：机理研究


在液-液萃取过程中，In的水合是限制其从水溶液中萃取效率的主要因素。在这项研究中，KCl被引入到水溶液中，以促进低电荷密度InCl的形成，与In相比，InCl的水合作用预计要弱得多，从而促进了铟的溶剂萃取。合成了具有不同空腔尺寸的冠醚（CE）：苯并-18-冠-6（B18C6）、苯并-15-冠-5（B15C5）和苯并-12-冠-4（B12C4）。研究了在水溶液中存在足够的 KCl 的情况下 CE 对铟的萃取性能。基于密度泛函理论（DFT），从热力学、几何结构优化、静电势（ESP）和独立梯度模型（IGM）等方面对液液萃取过程进行了理论分析。理论评估与实验结果非常吻合，即通过形成InCl可以显着削弱铟的水合作用，CEs对铟的络合能力为B18C6> B15C5> B12C4。借助核磁共振氢谱和扫描电镜-能谱分析，进一步探讨了萃取过程中CE与铟之间的络合机理。结果表明，冠醚与K在两相界面配位形成[CE-K]，并进一步与InCl缔合形成CE-KInCl配合物，实现了铟的高效萃取。此外，在复杂的体系中，B18C6 对 In 的选择性优于 Fe、Al、Zn、Sn 和 Ca 等竞争离子。采用1 M HCl作为反萃剂，可以有效地从负载的有机相中回收铟，反萃效率为98.1%。