The role of hydrogen peroxide in the acid dissolution of indium tin oxide (ITO) in the context of selective indium recovery from secondary sources

Abstract

This paper presents a thermodynamic, kinetic, and mechanistic examination of indium and tin extraction from indium tin oxide (ITO) in acidic sulfate solutions containing hydrogen peroxide (H₂O₂). The experimental approach included a comprehensive characterization of commercial ITO powder and leach residues by different techniques, as well as a systematic investigation of the effects of temperature, sulfuric acid (H₂SO₄) concentration, and hydrogen peroxide concentration on the leaching behavior (yield and rate) of indium and tin. The leaching tests showed that the presence of H₂O₂ in the H₂SO₄ solutions results in a significant increase in the leaching of indium, primarily at moderate temperatures. Furthermore, an increase in the H₂O₂ concentration in the solutions leads to a decrease in the concentration of tin in the leach liquor. The extraction of indium from ITO appears to occur in two steps. The first step involves the spontaneous hydration of In₂O₃ to form indium(III) hydroxide, In(OH)₃, while the second step is the attack of this phase by protons (H⁺) to form soluble In³⁺ species, which is also a spontaneous reaction. The rate-determining step of indium extraction from fine ITO particles (mean particle size = 18 μm) is likely the diffusion of H⁺ through the pores of hydrated In₂O₃ particles and the grains of SnO₂ surrounding these particles. With regard to the extraction of tin, the following sequence of steps was proposed: (i) the spontaneous reduction of the SnO₂ phase in the acid solutions to Sn²⁺ by the active hydroperoxyl (HOO^•) radical, which is formed from the decomposition reaction of H₂O₂; (ii) the spontaneous oxidation of Sn²⁺ ions to Sn⁴⁺ by the oxygen molecules and the active hydroxyl (HO^•) radical, which are also formed from the decomposition reaction of H₂O₂; and (iii) the spontaneous precipitation of SnO₂ from the unstable Sn⁴⁺ ions. A better understanding of the kinetics and mechanism of ITO dissolution in the H₂SO₄-H₂O₂ media may be helpful in the design and improvement of leaching processes targeting high indium recovery by selective leaching.