| dc.description.abstract | Over 4 billion tonnes of bauxite residue, the by-product of alumina refinery, has been generated globally. High alkalinity and high-water content of the residues are two of the major challenges in dealing with its safe storage and management. Limited studies have evaluated the treatments of neutralization of the alkalinity of long-term storage bauxite residues and the enhanced settling performance after alkalinity neutralization simultaneously. The objectives of this study were to: 1) investigate the mineral compositions and transformation of a wet bauxite residue in a more-than-50-year old storage pond, 2) examine the efficiencies of four compounds: H2SO4, CaCl2, FeCl3, and NaH2PO4 in neutralizing alkalinity; and 3) evaluate the effectiveness of the above treatments with and without additional surfactants and polymers in enhancing the settling of bauxite residues.
Mineral quantification indicated that Fe-oxides dominated the land area residue and lower portion of the lake sediments whereas aluminum hydroxides/oxyhydroxide dominated the upper portion of the sediment. The uncommon dominance of Al-hydroxide could be attributed to precipitation of the bayerite and nordstrandite [Al(OH)3] at lower pH when atmospheric CO2 dissolved in lake water reducing the pH of the residue during storage. The precipitation of aluminum hydroxide created an Al concentration gradient, drove aluminate ions from lower sediments to diffuse upward in the bauxite deposit column. This also explained the formation of a hard surface crust, consisting of bayerite, nordstrandite, and calcite, at the disposal pond land area.
The four chemicals (i.e., H2SO4, CaCl2, FeCl3, and NaH2PO4) successfully reduced the pH of the bauxite residue from 10 to 8, but none of them improved the settling of the residue compared to the NaCl control treatment. Anionic surfactant (SDS) improved the settling of bauxite residue before but not after alkalinity neutralization. Neutral and anionic polyacrylamide (PAM) promoted the flocculation and settling of bauxite residue both before and after alkalinity neutralization, while cationic PAM showed little effect. It appeared that binding phosphate on the surfaces of oxides altered the surface properties of bauxite residue and reduced the PAM settling efficiency.
Among the surfactant and polymers tested, anionic and nonionic polyacrylamide enhanced the settling of residue by forming larger flocs more quickly. Forming stable large flocs also promoted the formation of large pores, which hindered the consolidation of the particles to smaller volumes. | |
