Reduction of alkalinity in bauxite residue during Bayer digestion in high-ferrite diasporic bauxite

• Alkali decreases greatly as lime addition, digestion temperature and time increase.
• XRD and SEM confirm existence of two kinds of hydrogarnets.
• Mineralogical compositions of bauxite residues were accurately calculated.
• Andradite–grossular hydrogarnet determines Al2O3 extraction efficiency.
• New technology with zero waste was proposed to process high-ferrite bauxite.

Bauxite residue is a hazardous solid waste generated during the Bayer process of extracting alumina from bauxite ore, the high alkalinity of which limits its large-scale industrial applications. In order to reduce the alkalinity of bauxite residue, rich lime additions from 0 to 30 wt.% and high digestion temperatures from 250 °C to 300 °C were performed on a high-ferrite diasporic bauxite during the Bayer digestion. The alumina extraction efficiency from bauxite and the Na2O to SiO2 weight ratio of the bauxite residue were systemically investigated. The alkalinity in the bauxite residue decreases considerably with the increasing lime addition, digestion temperature and duration. The bauxite residues after digestion are mainly comprised of hematite, zeolite, grossular hydrogarnet, andradite–grossular hydrogarnet and perovskite, but their proportions vary greatly. A new method to accurately calculate the mineralogical compositions is proposed according to both the chemical compositions and XRD analyses of the bauxite residues. The mechanisms of alkalinity reduction in the bauxite residue and performance change in bauxite digestion are discussed.