Build-up formation and corrosion of monolithic refractories in cement kiln preheaters

Nowadays, the cost of energy and the environmental standards have encouraged cement manufacturers to replace conventional fuels by alternative fuels (industrial wastes and agricultural and forest residues). Therefore, the increasing uses of these alternative fuels in cement plants have severely increased build-up formation and attack of the monolithic refractories used. All this strongly affects the durability of the refractory castables. In order to investigate this issue a number of samples, taken from five cement kiln preheaters, were studied. The results obtained confirm that the build-up constitution varies broadly and they consist of alternate layers of slightly different compositions as a consequence of changes in operating conditions of the furnaces. Their growth and compactness are due to a vitreous phase formation. On the other hand, the main compounds causing the attack of the monolithic refractories are: K2O, SO3, CaO and Cl−. These give place, in the matrix of the refractory material, to a liquid phase together with minor amounts of different crystalline phases. The depth of the attack depends on the SiC content in the refractory castable, as the proportion of SiC in the matrix increases the depth of attack decreases. The liquid phase brings about the adherence of the ring to the refractory castables. Therefore, when build-up is removed, part of the refractory material adhered to them is also removed. The thickness of the peeled off material depends on the depth of the attack.