PREPARATION, CHARACTERIZATION AND USES OF MULLITE GRAIN

The properties were compared of grain formed at low temperature from a homogeneous mullite precursor gel, and grain formed from the manufactured material produced at high temperature by fusion or by sintering. A homogeneous precursor gel with the oxide stoichiometry of mullite was prepared by treating technical ethyl silicate with aluminium chlorohydrate, with or without dibutyltin oxide as hydrolysis/gelation catalyst. The gel was dried and heated for 5 h at 700 ° C to remove organic residues then ground to 8 μm nominal size to obtain a mullite precursor grain. Crystallites form even at this moderate temperature. Commercially produced fine mullite grain manufactured by fusion or by high temperature sintering was also ground to a give a grain of 8 μm nominal size to provide a comparative standard with well known material. Each of the three grain materials, homogeneous precursor gel, fused mullite and sintered mullite, each ground to 8 μm nominal size, representative of the 'fines' fraction of a grain mix to be used in producing refractory shapes, was made into compacts which were sintered, and, when cold, tested for compressive and bend strength. Materials of this size were chosen for comparison because of their significance in applications where properties at temperature are important. The precursor gel compacts were 95% crystalline mullite and reached 86% of theoretical density. At 1500 ° C, heated in a 90 deg min-1 schedule, they had strength comparable to sintered mullite; both materials were much stronger than fused mullite. The results show that sintering procedure has a profound effect on strength, and indicated that in the absence of binder, fused mullite is less reactive than sintered mullite. Some properties of refractory shapes and bricks made from the sintered or fused mullite grain are discussed and some uses in refractory shapes are considered. © 1990 Chapman and Hall Ltd.