Investigation On Factors Affecting Properties Of Micropored Lightweight Mullite Aggregate

Reducing heat lose of industrial furnaces is an important aspect of energy saving. Using light weight refractories is an effective way for insulating purpose. Development and application of high performance light weight refractory castables is meaningful to energy saving, for which developing high duty insulating refractory aggregate materials is very necessary.This work prepared light weight micropored mullite aggregate with pore sizes in microns, by using industrial alumina and natural silica as starting materials and anthracite coal as burning-out material, wet co-grinding, then naturally sedimentating and drying to become green body and finally firing at suitable high temperature. Factors affecting properties of the synthesized mullite aggregate were investigated, including heating schedule, additives and the phase composition of industrial alumina. Thermal behavior of the green body consisting of mixed industrial alumina, silica and anthracite coal powders was investigated by means of DSC-TG to find that the coal is burned out at 600°C and the mullitization occurs at around 1200°C. Two types of heating schedule were compared on the influence on the mullite formation ratio, one being heating up to reach a certain temperature and then then soaking at it for a certain time, and the other being two-step soaking at a high and a lower temperature respectively. Phase composition of the synthesized mullite was analysed by XRD. It is found that temperature is the most affective factor to mullitization. Mullite formation is much more dependent on temperature than soaking time. It is favorable to both mullite formation and energy consumption by adopting soaking first at a high temperature for short time and then at a lower temperature for necessary time. Adding 1%, 3%, 5% and 7% repectively pre-synthesized sintered mullite as a mullite seeding additive leads to some increase in bulk density (BD) and cold compressive strength(CCS), while the gain in mullite ratio is not significant, all around 60%. Introducing respectively 1%, 3%, 5% and 7% of yttria(Y2O3) brings about increased BD and CCS. When 1%, 3%, 5% and 7% aluminum fluoride(AlF3) is added, however, BD and CCS decline. Phase composition of industrial alumina as a starting material has significant influnence on properties, that withγ-Al₂O₃ as the main phase has higther reactivity, facilitating mullite convertion, and hence leading to lower BD and higher porosity of the aggregate. Determination on pore size distribution by means of mercury porosimetry analysis revealed that the pore sizes center around a range of 3⁵μm and are uniformly distributed. Microstructure of the aggregate was observed and analysed by SEM.The micropored insulating mullite aggregate developed by this work is featured by Al₂O₃ content around 70%, mullite ratio above 50%, porosity more than 50% and BD under 1.2g/cm3. This work indicates that an appropriate firing temperature for such synthesized light weight mullite aggregate material is 1350^℃1450^℃. Since in such highly porous aggregate the contacts of the powders are quite limited, and the mass trasferis through solid phase is rather difficult, hence the mullite conversion ratio and the sintering stateas a function of temperature and time changes little. Properties of the prepared insulating mullite aggregate maintain stable up to 1500^℃and significant sintering shrinkage does not occur. On the other hand, the mullite ratio is controlled at 50%～65%, there remain some unreacted corundum and silica, and mullitization can thus further occur when subjecting to further heating to compensate high temperature sintering shrinkage.