Chemistry of the Acid Neutralization Capacity of Bauxite Residue

Bauxite residue (red mud-RM), a byproduct of the Bayer process for refining bauxite to alumina, is highly alkaline (pH similar to 13) and its treatment and storage poses unique waste management challenges. In this work, the acid neutralizing capacity (ANC) of Jamaican RM from a storage facility in Sherwin, Texas, was measured, and the solid and aqueous phase contributions to the ANC were investigated through analyses of the RM composition and chemical equilibrium modeling. The ANC of RM was measured by both rapid and long-term acid titration of RM slurries to pH endpoints of 4.5, 6, 8, and 10. The ANC of Jamaican RM was found to be 1.8 and 0.5 mEq/g RM for long-term titration to pH 4.5 and 10, respectively. Using information on RM composition, a chemical model was formulated in MINEQL+ and used to investigate reactions responsible for the changes in pH for strong acid additions to RM slurry. Through modeling, the ANC was determined to be attributable primarily to dissolution of calcite (CaCO3(s)), tricalcium-aluminate (Ca3Al2O6(s); C3A), sodium-aluminum-silicate (NaAlSiO4(s)), and to the neutralization of Al(OH)(4)(-), NaOH, and NaCO3- in the residue porewater. The presence of the minerals was confirmed through X-ray diffraction analyses of the bauxite residue before and after titration. The chemical model provided insights about the reactions controlling acid-base behavior of RM and the potential for metal leaching from the RM slurry at various pH values. It can be used to estimate the dose of acid required to neutralize a given quantity of RM.