Separation of indium and tin from ITO powders with short-chain dicarboxylic acid-ChCl deep eutectic solvents: Indium tin leaching and splitting mechanism

Abstract

The substitution of corrosive mineral acids with green deep eutectic solvents (DESs) for extracting rare and precious metals from discarded electronic devices has a sustained appeal for developing clean hydrometallurgical processes. However, the dissolution mechanism of DESs on specific metals still requires an exhaustive understanding. In this work, short-chain dicarboxylic acid-choline chloride DESs were used to dissolve indium tin oxide (ITO) to achieve the separation of indium and tin. Kinetic and thermodynamic studies have shown that the leaching of indium and tin was considered a mixed control of diffusion and chemical reaction, with low activation energies (Ea(In) = 39.93 kJ/mol, Ea(Sn) = 39.52 kJ/mol). The electrospray ionization mass spectrometry (ESI-MS) analysis of the leaching solution and molecular orbital calculations revealed that tin was leached in the main form of [Sn4+(OH−)3(H2O)]+ cation as well as slight [Sn4+MA2−(Cl−)3]− anion, while the dominant ions of indium were [In3+(Cl−)4]− ligand as well as small amounts of [In3+MAH−(Cl−)4]2−, [In3+(MA2−)3]3− and [In3+MA2−(Cl−)2]−. The tin-containing species in the leaching solution can be completely precipitated in the form of cassiterite SnO2 through water dilution and hydrothermal treatment, in which amorphous indium-containing substances were mixed. Most of the indium was retained in the form of [In3+(Cl−)4]− in the liquid. After adjusting the pH of the solution, over 99% of the indium precipitated out as In(OH)3, and after calcination at 600 °C in air for 1 hour, In2O3 was obtained. This study unveiled the leaching mechanism of short-chain dicarboxylic acid-choline chloride DESs on ITO powders. It developed a green, simple, and mild process for potentially recovering ITO-based materials.