Effect of firing temperature and atmosphere on sintering of ceramics made from Bayer process bauxite residue

Bauxite residue, the principal waste from the Bayer process, was dried, pressed and studied for its thermal and sintering behaviour under different atmospheres, up to 1100 °C. For sintering in air and N₂, shrinkage begins at 800 °C and ranges from 2.6% to 13.9%, after firing at 1000–1100 °C. Bulk density varies from 1.7 to 2.3 g/cm³ whereas water absorption from 31.5% to 17.7%. The main crystalline phases identified on firing in air were hematite (Fe₂O₃), gehlenite (Ca₂Al₂SiO₇) and perovskite (CaTiO₃) whereas magnetite (Fe₃O₄) was also found on firing in N₂. Microstructures are characterised by irregularly shaped, <20 μm Feret diameter, pores in a ceramic matrix with interconnected porosity. The average pore size is greater in samples fired in N₂. On sintering in 4%H₂/Ar, shrinkage begins at 710 °C. After firing at 1100 °C, shrinkage is 20.1% and water absorption 1%. The main crystalline phases are magnetite, wustite (FeO), gehlenite and perovskite. Microstructures are characterised by a compact heterogeneous matrix, with isolated <15 μm Feret diameter, closed pores. The grains have reacted with the adjacent phase and their shape is rounded with no sharp facets. Increased sintering temperature results in improved physical properties for all atmospheres tested and in higher average pore size when sintering takes place in air and N₂. The use of magnetite-reducing sintering conditions can potentially assist in the production of a variety of ceramic compositions containing bauxite residue.