Direct preparation of macroporous mullite supports for membranes by in situ reaction sintering

The natural mineral kaolin combined with additive Al(OH)₃ and AlF₃ as alumina sources was used to directly prepare macroporous mullite ceramic membrane supports through in situ reaction sintering. The effects of composition and the sintering temperature on the micro-morphology, phase development and pore structure of porous mullite support were investigated extensively. It was found that the excess SiO₂ in kaolin was consumed rapidly by adding the alumina precursors with the formation of secondary mullite in the temperature range of 1300-1500 °C. During the sintering process, pore structure and stiff skeleton needle-like structure mullite formed in the support, which resulted in good pore structure and high mechanical strength. Thus, the support is suitable for the preparation of asymmetric ceramic membranes. The pore structure and the micro-morphology of the support could be controlled by adjusting raw material Al₂O₃ composition from 50 to 72 wt.% of Al₂O₃ and the sintering temperature from 1300 to 1550 °C. The porous mullite supports with the porosity of 27-55.6%, average pore size of 0.73-1.50 μm and mechanical strength of 15.5-66.7 MPa were prepared after optimization.