Experimental and geochemical modelling investigations on the weathering behaviour of bauxite residue : effect of pH

This study attempted to predict the weathering behaviour of bauxite residue, which is of primary concern to understanding the environmental risks of leachates from the ever-growing quantity of reserves. Leaching experiments for bauxite residue were carried out with deionised water at various solution pHs between 5–13. A model with no adjustable parameters was developed using PHREEQC with the XRD mineralogical composition being used as direct inputs to the model. Acid neutralisation curves obtained experimentally compared favourably with PHREEQC modelling results over the entire pH range. Sodalite (7.5–5.7) and calcite (8.0–5.5) were found to be critical buffering minerals during weathering of bauxite residue. Leaching experiments showed that Na, Al, V and, to some extent Ca attained high concentration in the leachate. The PHREEQC model was validated by the experimental trends for Na, Ca and Al. The pH was found to strongly influence the distribution of the species of different elements in the leachate. Above a pH of 6.5, Al(OH)4− was the dominant Al species; HCO3− dominated up to a pH of 9.0 before CO32− became the dominant C species. H4SiO4 was found to be the dominant Si species before H3SiO4− became prominent at pH > 10 and H2SiO42− at pH > 12.5. The kinetics of the weathering process at solution pH 8.35, 9.23, 10.12 and 11.06 showed first order chemical reaction resistance to be rate-controlling.