Kinetics and Mechanism of Indium Leaching from Zinc Oxide Dust in Sulfuric Acid Media and Solvent Extraction

In this study, sulphuric acid was used to leach indium from zinc oxide dust, D2EHPA was applied to extract indium from the leaching solution, and hydrochloric acid was administered to strip indium from the indium-loaded organic phase. The effects of sulfuric acid concentration, temperature, leaching time and liquid-solid ratio on the leaching rate of indium were studied. The optimum leaching conditions for indium were as follows: sulfuric acid concentration of 200 g/L, leaching temperature of 80°C, leaching time of 120 min, and liquid-solid ratio of 8 : 1. Under these conditions, the leaching rates of indium, zinc, iron, and aluminum were 95.67, 97.97, 2.06, and 8.51%, respectively. On the contrary, lead was enriched in the leaching residue. Response surface analysis was carried out to further optimize the experimental conditions. The kinetic effects of temperature and sulphuric acid concentration on the indium leaching process were investigated using a shrinking-core model, and the activation energy of indium leaching was calculated to be 30.9 kJ/mol, with the kinetic model as: 1 – (1 – x)^1/3 = exp(5.11 – 3714/RT)t; 1 – 2x/3 – (1 – x)^2/3 = exp(8.84 + 3.599 ln[H₂SO₄])t. The results showed that the indium leaching process was controlled by a mixture of chemical reaction and diffusion, and the reaction stage of sulphuric acid was 3.599. Meanwhile, the McCabe-Thiel diagram for D2EHPA/HCl extraction/stripping of indium was constructed, and theoretically D2EHPA/HCl extraction/stripping of indium requires 2 stages to complete.