Bauxite ‘red mud’ in the ceramic industry. Part 1: thermal behaviour

Samples of red mud, by-products of alumina production from bauxite, are studied in the 120–1400°C interval. An extensive characterization was performed by thermal and X-ray diffraction analyses. The identification of gaseous species released upon heating was carried out by coupling the thermal analizer with a gas-chromatographic/mass spectrometer. Density evolution was also determined as a function of the heat treatment. Results indicate primary H2O release from aluminium hydroxides, followed by carbonate decomposition with CO2 evolution below 900°C. Alkaline oxides, mainly CaO and Na2O, lead to the formation of Ca3Al2O6 and NaAlSiO4 between 900 and 1100°C. At the highest temperatures, reduction of Fe3+ to Fe2+, involving O2 release, promotes the formation of Fe2TiO4, with the disappearance of the rutile-TiO2 phase. The various solid state reactions, ascertained at different stages of the heating process, and possible mass balances are discussed with reference to the state diagrams of principal red mud components.