Magnesia-alumina spinel in the upper transition zone: thermal shock and alkali attack

The upper transition zone sits on the feed side of the burning zone. The coating comes and goes, the brick face is heated and cooled again and again, and alkali and sulphur dioxide in the gas are more active than in the burning zone. The first demand on the material is thermal-shock stability; the second is that it should be attacked as slowly as possible in that atmosphere.

Magnesia-alumina spinel brick has periclase and magnesia-alumina spinel as its main phases. The two phases expand differently, and a network of microcracks forms in the microstructure, absorbing the strain of thermal shock — which is the structural reason for its thermal-shock stability. Spinel itself also resists alkali and sulphur dioxide reasonably well, which is why the brick belongs in the upper transition zone.

Distributing spinel as a coating on magnesia aggregates spreads the spinel phase more evenly and more continuously, without raising the overall alumina content, and can improve resistance to cement-clinker attack and mechanical strength while thermal-shock resistance is maintained. In the upper transition zone, the phase distribution at the aggregate surface matters as much as the bulk chemistry, if thermal shock and corrosion resistance are to be held together.

When the alkali and chlorine load in the transition zone rises further, or when heat loss through the shell becomes the sharper problem, a corundum brick or a composite lining with a lightweight layer appears as another path in the same zone. The starting point is still the thermal shock and the gas chemistry of this band.