Evaluation of bauxite residues (red muds) of different origins for environmental applications

Bauxite residue (red mud) samples from 11 Australian and overseas Bayer process refineries and variously treated red muds were investigated from the perspective of potential environmental uses for red muds. Red muds had pH (1:5 H2O) values ranging from 8.4 to 12.6 and electrical conductivities of 0.7 to 18.2 mS/cm (1:5 extract), surface area ranged from 15 to 30 m2/g and texture from sandy clay loam to clay. Red muds are mostly composed of crystalline compounds of Fe, Al, Si and Ca with some red muds containing moderate amounts of Ti and Na. Most red muds contain hematite, goethite, quartz, calcite, desilication product (sodalite), gibbsite, boehmite and anatase. Other minerals, including muscovite, halite and gypsum occur in some red muds. The acid buffering behaviour of red muds was investigated by incubation of red muds with various amounts of hydrochloric acid. Buffering curves changed with time and each red mud gave a different buffering curve although curves could be grouped into five different shapes. Most of the buffering occurs between about pH 6 and 8 where the pH is buffered by dissolution of calcite, sodalite and tricalcium aluminate (when present). X-ray diffraction (XRD) analysis of acid treated red muds at different pH values indicate that sodalite and calcite buffer (i.e. dissolve) simultaneously. Buffering at lower pH values (< 4) is due to dissolution of Fe oxides. Each red mud has different pH buffering characteristics so that the use of red mud for liming of acid soil, water, sulphidic mining residues and manufacturing wastes will require detailed characterisation of the buffering reactions for the particular system.