E. Seventy Percent Alumina Brick
After World War II, 70% Al2O3 class refractories became a “workhorse” in
industrial furnaces because of their high use or duty ratings and because of
their durability in many processes where slag corrosion or other reactions take
place. This class of refractories can be produced either from bauxitic clays exhibiting
70% Al2O3 or by using appropriate mixtures of bauxite (88% Al2O3) and
fireclay (40% Al2O3). As in the case of 60% Al2O3 brick, the mineral base of
the firebrick makes a profound difference in physical properties and thermal
response of 70% Al2O3 class brick.
The chemical composition and physical properties of the two types of 70%
alumina brick are given in Table 10. The brick based on calcined bauxite exhibits
much higher reheat expansion (PLC) and higher spalling loss than the brick based
on bauxitic clay. On the other hand, the brick based on calcined bauxite may have
superior performance where erosion resistance is required.
The microstructures of the two types of 70% Al2O3 brick are shown in
Figures 7 and 8. In Figure 7, the brick is made from calcined aggregates,
which exist, surrounded by a finely textured matrix. The distribution of
medium-size particles around coarse aggregates is excellent. This microstructure
suggests excellent spalling resistance.
By contrast, the brick made from calcined bauxite has a completely different
appearance. The dark bauxite particles predominate the microstructure, but a
calcined clay aggregate particle is evident in the lower right of the photomicrograph
of Figure 8. A glassy matrix containing mullite surrounds all particles. A
comparison of Figures 7 and 8 provides graphic evidence that brick in the
same alumina class can have completely different compositions, and the microstructures
alone suggest that key reheat (PLC) properties of these brick will be
different.