By far the largest use of mullite based products is in refractories. The glass and steel industries are two main markets.
The steel industry is the largest user, where refractoriness, high creep resistance and thermal shock resistance are important. The main use of high-mullite based products is in hot blast stove checker bricks. Many refractories in use in the steel industry have varying amounts of mullite based aggregates in them. Steel ladles, lances, reheat furnaces and slide gates are examples of mullite aggregate based products with various alumina contents (figure 1). The use of monolithic and precast shapes is increasing with the use of bricks declining.
Figure 1. A selection of mullite-based refractory shapes for the steel industry (photo courtesy of Dyson Precision Ceramics, UK)
The glass industry uses mullite based refractories in burner blocks, ports and in checker bricks as well as in the upper structure of the tanks where the glass is melted and in the drawing chambers. Thermal shock resistance, chemical attack resistance, high hot strength and creep resistance are the primary properties valued by the industry.
Mullite based products are also resistant to particulate carryover into the glass melt. This is particularly important in flat glass production, where contamination by low levels of Al2O3 is undesirable.
The next largest user of mullite is the ceramic industry mostly in kiln furniture items such as kiln setter slabs and posts for supporting ceramic ware during firing.
The aluminium and petrochemical industries also use mullite-based aggregates for applications requiring chemical attack resistance, thermal shock resistance and hot-load strength.