Application of a high temperature calcined andalusite in fired bricks and low cement castables: Fid-Bau Portal

Application of a high temperature calcined andalusite in fired bricks and low cement castables

Hubert, P.

Andalusite is a well known aluminosilicate mineral largely employed in the refractory industry in order to provide high mechanical properties at high temperature and thermal shock resistance. Several authors have shown that these properties are related to the unique microstructure that occurs during andalusite mullitisation. Mullitisation is a topochemical and topotactic process that leads to a composite mullite-glassy siliceous phase. This process begins after 24 h at 1200 deg C for very fine grains. (D50=5 micron) and is achieved in one hour if the temperature reaches 1600 deg C (grains coarser than 1 mm). Consequently, the coarse andalusite aggregates in unfired refractories are fully transformed only in the first centimeters of a refractory lining even if the furnace or metal temperature reaches 1600 deg C. In order to provide to the refractory industry a fully transformed aggregate, a high quality andalusite was calcined in an industrial rotary kiln up to 1550 deg C. After firing, the material was crushed and screened in three fractions 0-1 mm, 1-3 mm, 3-5 mm. A mineralogical study by SEM and XRD was carried out to check the microstructure of the final product and the mullite content. In addition, several properties such as friability and porosity were determined. The behaviour of this new aggregate so called mullitised andalusite was tested in fired brick and low cement castables with the help of reference formulas. Due to its low friability and large reactive surface, the mechanical resistance after firing of the mullitised andalusite based refractories is high in comparison to raw adnalusite. In fired bricks, mullitised andalusite shows a similar behaviour to sintered mullite. Low expansion during firing and high mechanical strength in a large temperature range should provide good thermal shock resistance to LCC. The mullitised andalusite gives better alkalis resistance than andalusite, high alumina chamotte and bauxite in LCC. Finally, this new aggregate with its specific microsturcture and interesting properties should lead to the development of new innovative refractories.