Recovery of indium by solvent extraction with crown ether in the presence of KCl and stripping with HCl: A mechanistic study

Hydration of In³⁺ 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 InCl₄⁻ of low charge density, which is expected to possess much weaker hydration compared with In³⁺, 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 InCl₄⁻ 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 ¹H 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 InCl₄⁻ to create the complex of CE-KInCl₄, realizing the efficient indium extraction. Moreover, B18C6 showed excellent selectivity toward In³⁺ over the competing ions such as Fe³⁺, Al³⁺, Zn²⁺, Sn²⁺ and Ca²⁺ 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%.