Improved dewatering behaviour of Bauxite Residue using Calcium-Magnesium-impregnated acid solution

The generation of large quantities of red mud, or bauxite residue (BR), during the Bayer process of alumina production from bauxite poses a significant global environmental challenge. Approximately 1 to 2.5 tons of waste BR is produced for every ton of alumina manufactured, depending on the chemistry and mineralogy of the feed bauxite sample. Effectively managing, safely storing, and finding productive uses for this waste are three major concerns for global aluminum producers. This study investigates the effectiveness of a hydrochloric acid solution impregnated with calcium (Ca²⁺) and magnesium (Mg²⁺) ions for the settling and dewatering of BR slurry. Dolomite was used 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 optimized to evaluate the dewatering process. Initial comparison experiments using 1.5-12N HCl and 1.5-12N HCl with Ca²⁺ and Mg²⁺ revealed the effectiveness of the acid solution enhanced by these divalent cations. The solid percentage in the slurry significantly influenced the settling efficiency; as the solid weight percentage increased from 2.5% to 15%, the settling efficiency gradually declined from 98% to 92%. However, when the solid weight percentage increased beyond this range, the settling value sharply dropped to 40%. The experiments maintained 5 minutes of slurry agitation followed by 2 minutes of settling using 1.5N HCl with 5g of dolomite. Further studies at higher solid concentrations indicated that either extended settling times or greater 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 also considered during these experiments. The physicochemical characteristics of the BR and the processed product were analyzed using techniques such as Particle Size Analysis, ICP-OES, Zeta-Potential, XRD, FTIR, and FESEM-EDX studies to support the experimental findings. The incorporation of divalent cations like Ca²⁺ and Mg²⁺ into the low-concentration HCl solution significantly enhances the settling characteristics of BR particles. The possible settling mechanism is discussed based on experimental evidence, characterization results, and relevant literature.