Phosphate-bonded alumina linings in preheating and calcining

Peak temperatures in preheating and calcining are well below those in the burning zone, and a basic brick is not generally required to hold a coating. What is needed here is enough cold and intermediate strength to resist abrasion and impact from the burden, a porosity low enough to slow alkali-vapor ingress, and a hot face that remains intact through stops, starts and temperature swings.

Phosphate-bonded high-alumina–mullite bricks were developed along exactly those lines. High-purity bauxite and mullite are phosphated and then bonded at intermediate temperature, giving a working lining of high strength, low porosity and good thermal shock. The bond already provides usable mechanical properties in a temperature range where ceramic sintering is not yet complete, which is why the brick suits preheating and calcining, and is also used in rotary kilns that calcine high-moisture burdens or run a similar temperature schedule.

High-RUL phosphate bricks, and phosphate bricks aimed more at wear, are different emphases within the same family: the former keep their shape at a somewhat higher temperature, the latter face more erosive spans. When they appear in the same zone as anti-spalling high-alumina, silica-mullite or lightweight low-conductivity brick, the division of labour is clear — the phosphate bond carries strength and spalling resistance, the lightweight layer carries heat flow, and silica-mullite or high-alumina is matched to the alkali load and the temperature.