Investigation of a novel alkali baking–leaching process for dissolution of Fe, Al, Ti, Sc, and Ga values and generation of alumina and titania from red mud

The current study investigates alkali baking using sodium hydroxide followed by water and sulfuric acid leaching to extract Al, Fe, Ti, Sc, and Ga values. Water leaching of baked mass selectively dissolves Al. Subsequent, sulfuric acid leaching of water-leach residue dissolves Fe, Ti, Sc, Ga, and remaining Al and Si values. The alkali baking forms sodium metal oxide (NaFeO2, Na2TiO3, NaAlO2, NaAlSiO4) phases, which improves the dissolution in sulfuric acid compared to the oxide phases of metals. The alkali dose was a prominent factor in forming sodium metal oxides. Baking at 600 °C, 1 h, 75 wt% NaOH yielded 60% Al dissolution during water leaching and 82% Ti, 83% Fe, 97% Sc, and 64% Ga dissolution during acid leaching. Scandium dissolution strongly correlates with titanium during leaching, and TEM analysis confirms the apparent enrichment of scandium in the anatase and hematite matrix. Carbonation of water leach solution precipitates alumina and dawsonite (NaAlCO3(OH)2) phases with a product having 85–87% Al2O3 purity and 12 wt% yield. Thermal hydrolysis of acid leach solution generates Ti(OH)4 precipitate which is calcined to obtain anatase with a 95–97% TiO2 and 13 wt% yield. The leach residue contains unreacted hematite and anatase phases having <10% Al2O3 and SiO2 with 6.5 wt% yield. The hydrolyzed solution contains Sc(III) (6 mg/L), Ga(III) (5 mg/L), and Fe(II) (28.4 g/L) ions. The proposed process consumes 0.75 kg NaOH and 1.40 kg H2SO4 per kg red mud and generates 0.14 kg alumina and 0.13 kg titania product with 0.07 kg residue enriched with 48% anatase.