Improved dewatering behaviour of bauxite residue using calcium-magnesium-impregnated acid solution

The generation of large quantities of waste bauxite residue (BR), during the Bayer alumina production process, poses a significant global environmental challenge. Approximately 1 to 2.5 t of waste BR is produced for every ton of alumina manufactured, depending on the chemistry and mineralogy of the raw bauxite sample. This study investigates the effectiveness of hydrochloric acid solution infused with calcium (Ca²⁺) and magnesium (Mg²⁺) ions for the settling and dewatering of BR slurry. Dolomite served as the source of Ca²⁺ and Mg²⁺ ions. Various experimental parameters, including acid concentration, agitation time, temperature, dolomite weight percentage, settling time, and solid weight percentage in the slurry, were optimised to evaluate the dewatering process. The solid percentage in the slurry significantly influences settling efficiency. When the solid percentage is maintained below 15%, settling efficiency ranges between 92 and 98%. However, when the solid percentage exceeds this threshold, settling efficiency slowly declines to 40%. Therefore, tests conducted at higher solid concentrations indicated that either extended settling times or larger volumes of the impregnated acid solution would be necessary to achieve settling efficiencies greater than 90%. Additionally, the effects of dolomite amount and acid concentration were considered during these experiments. The physicochemical characteristics of the BR and the processed product were analysed using techniques such as particle size analysis, ICP-OES, zeta potential, XRD, FTIR, and SEM-EDX studies to support the experimental findings. The incorporation of divalent cations into the low-concentration HCl solution significantly enhances the settling characteristics of BR particles. The possible settling mechanism is discussed based on the experimental evidence, characterisation results, and relevant literature.