Magnesia-iron spinel and coating in the burning zone

The burning zone is the hottest stretch of a cement rotary kiln. Clinker is sintered here, and the brick face survives under a coating. Magnesia-iron spinel brick has high-purity periclase and iron-alumina spinel as its principal phases; when the spinel morphology is well developed, the brick is flexible in service and takes a coating readily, and can resist the alternating mechanical stresses of a rotary kiln without early failure.

A coating is more than a cover. A stable clinker skin changes the boundary condition at the hot face: chemical attack meets the coating first, and the heat flux into the brick and toward the shell is reduced. The moment the coating is lost, the hot face is exposed to flame and burden, and the thermal stress rises sharply. The central task in the burning zone is therefore to let a coating form, and to let it stay.

Where thermal-shock resistance must be strengthened as well, magnesia-iron spinel is often laid together with a corundum brick: the former continues to carry the coating, the latter adds high-temperature strength and a lower conductivity. Placing the two materials in the same zone is a way of sharing the burning zone’s two principal loads.