Extraction of Scandium from Critical Elements-Bearing Mining Waste: Silica Gel Avoiding in Leaching Reaction of Bauxite Residue

Scandium is one of the most valuable rare-earth elements, and its extraction is centralized in China. For this reason, the search for new scandium sources is required to supply the demand. Bauxite residue is considered a secondary scandium resource. It has estimated around 4 billion tons of bauxite residue worldwide with 20–200 mg/kg of scandium. The hydrometallurgical route is considered advantageous due to its low content. However, the acid leaching generates silica gel reducing the extraction rate and making the solid/liquid separation difficult. H2O2 can be used to avoid its synthesis, and the parameters during the leaching must be explored. Despite the studies in the literature about scandium extraction from bauxite residue, there is a lack of investigation about suppressing the synthesis of silica gel in the acid reaction. The present study aimed at the scandium extraction from bauxite residue by H2SO4 and H3PO4 leaching. The use of H2O2 was evaluated to avoid silica gel formation. The effect of time, temperature, and acid concentration was evaluated. Results showed that scandium extraction is directly related to iron extraction. The H2O2 contributed to silicon oxide formation during the acid reaction but declined scandium extraction by half. The leaching rate increased from 25 to 90 °C achieving 99% for iron and aluminum and 91% for scandium. The increase of H2SO4 concentration increased the titanium leaching. The leaching of metals using H3PO4 was similar to H2SO4 leaching. Moreover, the extractive process focused on near-zero-waste generation is essential to avoid waste stored in dams.