Recovery of High-Purity α-Alumina from Thermal Power Plant Ash via Sodium Carbonate Roasting and Seeded Precipitation

The valorization of thermal power plant ash as a secondary source of aluminum can reduce industrial waste generation and provide an alternative raw material for high-value alumina production. This study investigates the physicochemical basis of a combined process for recovering high-purity aluminum oxide from ash residue through sodium carbonate roasting, aqueous leaching, seeded precipitation, and calcination. Aluminum-bearing phases in the ash were activated by high-temperature roasting with Na₂CO₃ to form water-soluble sodium aluminate. The resulting solution was purified, and aluminum hydroxide was precipitated by a seeded crystallization method before being converted to alumina by calcination. The effects of the sodium carbonate dosage, roasting temperature and duration, precipitation temperature, seed addition, and mixing time were evaluated. The elemental and phase compositions of the intermediate and final products were characterized using X-ray fluorescence and X-ray diffraction analyses. The most effective process conditions were achieved at an ash-to-Na₂CO₃ mass ratio of 1:0.8, a roasting temperature of 1250 °C, and a residence time of 1 h. Aluminum hydroxide precipitation from the sodium aluminate solution was subsequently performed at 35–50 °C, followed by calcination at 1000 °C. Under these conditions, the final product contained 99.7 wt.% Al₂O₃, while the SiO₂ content was reduced to 0.346 wt.%. X-ray diffraction confirmed that α-Al₂O₃ was the predominant crystalline phase. The results demonstrate that the proposed process provides high selectivity for aluminum recovery and enables the production of high-purity alumina from technogenic ash waste. The developed approach offers potential for industrial waste valorization, resource conservation, and the production of import-substituting alumina-based materials.