The Role of Oxalic Acid in the Leaching System for Recovering Indium from Waste Liquid Crystal Display Panels

In this study, oxalic acid showed better performance than inorganic acid in the indium leaching system from waste liquid crystal displays. The In3+ leaching efficiency with the powder dosage of 50 g/L came up to 100% within 45 min, using 0.5 M oxalic acid and stewing at 70 degrees C. Ultrasonic cavitation can improve the powder dosage. Combined with the substance analysis using ICP, XPS, SEM, and XRD, In2O3 dissolution depended on the distributive H+ concentration. Metal hydrolysis was a general negative phenomenon for ions loss in an acid leaching system. According to the Arrhenius model, the apparent activation energy for the indium leaching reaction was 43.622 kJ/mol, indicating that the indium leaching rate was mainly affected by the ion diffusion process through the solid-liquid interface and the chemical reaction process. Oxalic acid can maintain an appropriate H+ concentration to reduce the acid consumption from nontarget substances to relieve the powder surface corrosion for reuse and to inhibit metal hydrolysis, and its C2O42- ions also could precipitate some metal impurities such as Ca2+ and Sr2+ in the leachate.