| Due to the excellent properties of inorganic ceramic materials compared to organic polymer membrane and the development of inorganic material science, ceramic membranes are applied in increasing fields, such as environmental protection, energy, food, chemical engineering, pharmaceuticals and so on. However, some problems still exist during its applications like high fabrication cost, limited materials, etc. Aimed at solving the problems of high fabrication cost and unity of porosity and mechanical strength, anorthite-cordierite based cetamic membrane was synthesized by using coal fly ash and dolomite as starting materials, and whisker-shaped mullite based membrane by using coal fly ash and Al(OH)3 as raw materials and Mo O3 as an additive, which can be briefly described as the following two aspects:1. Low-cost fabrication of anorthite-cordierite based porous ceramic membraneAnorthite(Ca O·Al2O3·2Si O2) and cordierite(2Mg O·2Al2O3·5Si O2) are widely used in ceramic, refractory and electronics because of their low expansion coefficient, thermal conductivity and dielectric constant. This work demonstrated a new method converting coal fly ash(CFA) industrial waste to anorthite-cordierite based porous ceramic membranes(ACCM), through high-temperature sintering with dolomite mineral. Characterizations of as-obtained ACCM were concerned, including phase composition, pore structure, N2 and water permeability, microstructure and mechanical property. From dilatometric measurement, the densification temperature of sample with 28.43 wt.% dolomite addition was extended to 1100 oC compared to 1060 oC for that of sample without any dolomite. For samples sintered at 1150 oC, their open porosity increased from 25.23 ± 0.47% to 46.54 ± 0.21% when dolomite addition was arose from 0 to 28.43 wt.%. It suggested that this level of dolomite addition significantly inhibited the sintering behavior of CFA. When further increasing the content of dolomite, the densification was promoted. It was ascribed to the glassy phase formed by excess Mg O and Si O2. The biaxial flexural strength decreased exponential with the increasing open porosity. The sample with 28.43 wt.% dolomite sintered at 1175 oC exhibited good properties with open porosity 38.9 ± 0.8%, average pore size 1.1 μm, biaxial flexural strength 41 ± 2 MPa, N2 flux 3457.63 m3·m-2·h-1 and water flux 27.91 m3·m-2·h-1 at 3.0 bar, and its crystalline phases were mainly composed of 84 wt.% anorthite and 16 wt.% cordierite.2. Fabrication of whisker-shaped mullite based ceramic membrane Mullite is a kind of classic ceramic material, but generally, its secondary mullitization temperature is above 1200 oC. In this work, whisker shaped mullite with inter-locking structure in porous ceramic membrane was synthesized by in situ reaction sintering with coal fly ash and Al(OH)3 as starting materials and Mo O3 as sintering additive. The secondary densification temperature was improved to 1445 oC compared to 926 oC by the addition of Mo O3. From the XRD patterns and SEM images, it can be conclued that the decrease of secondary mullitization temperature was induced by the increasing content of liquid phase formed at low temperature, which was resulted from the formation of Ca Mo O4 with low melting point. Also the volatilization of Mo O3 and Ca Mo O4 provided space for the growth of mullite whiskers. The open porosity increased from 41.65 ± 0.13% to 58.14 ± 0.15% with increasing amount of Mo O3, while shrinkage percent and pore size decreased. For membranes with 20 wt.% Mo O3, the biaxial flexural strengths were higher than that of membranes without Mo O3 between 1100 and 1300 oC, and the open porosity was higher when biaxial flexural strengths were at the same level. |
PDF Full Text Request