A clean two-stage Bayer process for achieving near-zero waste discharge from high-iron gibbsitic bauxite

The current Bayer process focused on alumina extraction without considering the comprehensive utilization of other valuable components, such as iron, and the resulting red mud caused severe ecological and environmental safety risks. This work proposed a clean two-stage Bayer digestion technology to obtain iron-rich red mud with low Na2O content from high-iron gibbsitic bauxite processing. The effects of digestion conditions on alumina extraction rate and iron enrichment degree in red mud were studied systematically. The ICP, XRD, SEM, and TEM were used to investigate the chemical compositions, phase structures and microstructures of bauxite and red mud. Results show that approximately 83% Al2O3 was extracted, and about 43% SiO2 was separated simultaneously in gibbsitic bauxite in the first stage, whereas undigested aluminum-bearing minerals mainly originated from substituted aluminum in goethite. Hydrogen was produced by the reaction of glycerol with sodium aluminate solution in the second stage, which promoting the reaction of Al-goethite with titanium-bearing minerals to form Fe(Ⅱ) and Ti(Ⅳ)-substituted hematite. The increase in reductants amount, caustic alkali concentration, reaction temperature, and time favored Al-goethite transformation in digestion. The total Al2O3 recovery and the Fe2O3 in red mud could reach more than 99% and 86%, respectively. Moreover, the content of Al2O3, SiO2, and Na2O in red mud satisfied the iron ore concentrate standard of GB/T 36704-2018, which was attributed to avoiding the generation of desilication products (DSP) in red mud. The 93% silicon could be removed from the liquid phase by two separations of the digested slurry. This work will facilitate the large-scale reduction of red mud discharge through further the value-added use of novel red mud technology in collaboration with steel and aluminum, achieving the green and sustainable development of the alumina industry.