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GEOCHEMISTRY OF BAUXITE REFINERY RESIDUES NEUTRALISED BY DIFFERENT METHODS

Johnston M, Clark MW, McConchie DM

Red mud remains the single biggest environmental problem for the alumina industry, because of the fine-grained nature (> 90% is < 10 μm), high pH (>13), high sodium concentration (>50 g/Kg), and high alkalinity (typically ≈ 30 g/kg as equivalent CaCO3). However, neutralisation of red mud provides potential re-use options as a valuable resource and this paper investigates the geochemistry of 4 types of red mud: un-neutralised raw red mud (UNRM), a CO2-neutralised red mud (CNRM), a Basecon™-neutralised red mud (Basecon™), and a CO2-neutralisation followed by a Basecon™- neutralisation (Hybrid) material. Investigations show that although the metal binding capacity for all samples was similar, the neutralisation techniques form two distinct geochemical groups: UNRM and CNRM, and the Basecon™ and Hybrid material. The UNRM and the CNRM have similar total alkalinities and phosphate loading capacities (98 % removal), they are substantially greater than those for either the UNRM or CNRM. Hybrid material at this stage of our investigations appears to provide some environmental advantage to Basecon™ because of an additional ≈17 g/kg CO2 consumption during the alkalinity transformation.