Red mud with enhanced dealkalization performance by supercritical water technology for efficient SO2 capture

The total de-alkalization treatment of industrial solid waste red mud (RM) has been a worldwide challenge. Removing the insoluble structural alkali fraction from RM is the key to enhancing the sustainable utilization of RM resources. In this paper, supercritical water (SCW) and leaching agents were used for the first time to de-alkalize the Bayer RM and to remove sulfur dioxide (SO2) from flue gas with the de-alkalized RM slurry. The results showed that the optimum alkali removal and Fe leaching rates of RM-CaO-SW slurry were 97.90 ± 0.88% and 82.70 ± 0.95%, respectively. Results confirmed that the SCW technique accelerated the disruption of (Al–O) and (Si–O) bonds and the structural disintegration of aluminosilicate minerals, facilitating the conversion of insoluble structural alkalis to soluble chemical alkalis. The exchangeable Ca2+ displaced Na+ in the remaining insoluble base, producing soluble sodium salts or alkalis. CaO consumed SiO2, which was tightly bound to Fe2O3 in RM, and released Fe2O3, which promoted Fe leaching. RM-SCW showed the best desulfurization performance, which maintained 88.99 ± 0.0020% at 450 min, followed by RM-CaO-SW (450 min, 60.75 ± 6.00%) and RM (180 min, 88.52% ± 0.00068). The neutralization of alkaline components, the redox of metal oxides, and the liquid-phase catalytic oxidation of Fe contributed to the excellent desulfurization performance of the RM-SCW slurry. A promising approach shown in this study is beneficial to RM waste use, SO2 pollution control, and sustainable growth of the aluminum industry.