Recovery of Alumina and Alkali from Red Mud Using NaFeO2 (NF) as an Additive in the Hydrothermal Process

Abstract

Red mud is an alkaline solid residue produced from the extraction of alumina from bauxite. However, the remaining alumina in red mud will cause secondary loss of aluminum resources. In the Bayer process, high alkalinity leads to severe environmental pollution issues and safety hazards. In this study, alkali and alumina were simultaneously recovered by using NaFeO₂ (NF) as an additive in a hydrothermal process. Harmless secondary red mud with low Na₂O content was obtained, avoiding the alkalinity pollution compared with conventional alumina extraction process. The influence of hydrothermal conditions on the extraction rate of alkali and alumina in red mud was also systematically studied. The chemical composition and microstructure of red mud before and after the reaction were studied using various characterization techniques such as XRF, XRD, and SEM. The results show that the A/S (molar ratio of Al₂O₃ to SiO₂) in the secondary red mud is reduced to 0.21, the Na₂O content is 0.43%, and the recovery rate of alumina is 80% with NF as additive. The product is hydroandradite (Ca₃(Fe_xAl_1−x)₂(SiO₄)_y(OH)_9−y) with less aluminum content and hematite. This work will provide a theoretical basis for achieving secondary resource recovery and handling the environmental pollution issues.