Efficient selective leaching of scandium from red mud through ammonium persulfate–activated roasting

Scandium (Sc) is crucial for many high-tech industries because of its excellent alloying, electrochemical, and optical properties. Red mud (RM), produced during the Bayer process of alumina production, is rich in rare-earth elements and is a vital Sc source. However, selectively extracting Sc from RM presents significant challenges. Herein, we employed ammonium persulfate–activated roasting followed by water leaching, releasing Sc bound to hematite through crystalline-phase transformation of RM minerals. Sc was leached efficiently and selectively, while the impurity elements—iron (Fe), aluminum (Al), silicon (Si), and titanium (Ti)—were retained in the leaching residue. Results showed that under optimal conditions, the Al leaching efficiency was 4.83 %; Fe, Si and Ti were minimally leached (<1%); and the Sc leaching efficiency reached 98 %. Two mineral phase transformation pathways occur during ammonium persulfate activation and roasting: 1) a direct reaction with the raw ore at low temperatures, and 2) a heterogeneous reaction with oxygen and sulfur dioxide, which are decomposed by ammonium persulfate at high temperatures through mineral adsorption. The behavior and mechanisms of sulfidation at mineral interfaces were investigated by density functional theory calculations. The findings indicate that the nonhomogeneous-phase formation process of scandium and iron sulfates is thermodynamically preferred in an O2 environment and the thermal stability of the formed Sc2(SO4)3 (−7.40 eV) is higher than that of Fe2(SO4)3 (−5.66 eV). Overall, the proposed method provides a new theoretical basis and technical support for efficient selective leaching of rare-earth metals from industrial solid wastes.