Abstract

Bauxite processing residues (mud and sand) originating from a major source of bauxite being mined and processed for alumina production in Australia were collected and characterized. Mixtures consisting of 0:100, 25:75, 50:50, 75:25 and 100:0% v/v residue mud: residue sand were incubated for four weeks and a portion was then leached bi-weekly for another 14 weeks. The macroporosity of sand was much greater than that of mud. Adding increasing proportions of sand to mud decreased total porosity and increased macroporosity but the increase was only appreciable with the presence of 75% sand. The main elements present in residues were Fe, Al and Si and the main crystalline minerals present were hematite, sodalite and goethite. The amorphous component made up 32 to 42% of the mineral content. Due to seawater neutralization of the residues, they had a high EC and soluble Na and Ca content. Leaching caused a decrease in the EC of residues by at least one order of magnitude and ammonium acetate-extractable Na levels were reduced by more than half. The pH in leachates from residues increased progressively until they were about two units higher than initial values and the pH of residues also increased. This was attributed to dissolution of solid phase alkalinity present in the residue.