Alkali Vapor Attack Behavior of Al₂O₃-SiO₂ Refractory Raw Materials

Sewage sludge and household garbage are used as an alternative fuel in recent production of cement. However, the alternative fuel causes serious alkali attack and spalling of bauxite-SiC refractories for cement kiln lining, thus having an impact on the operation of the cement kiln. In this paper, the alkali vapor exposure was used to evaluate alkali attack on some raw materials like bauxite, mullite (M60) and molochite, which were used to prepare mullite-SiC composite refractory as a lining of cement kiln. The results show that the alkali attack resistance of these raw materials is closely related to their chemical composition, phase composition and microstructures. Bauxite with corundum and mullite as main phases is attacked by alkali to form a kaliophilite phase and induce cracks and loose structure, further enhancing gaseous alkali penetration and serious damage. Mullite (M60) with mullite as a main phase is attacked by alkali to form leucite and transient liquid phase. The resultant liquid phase prevents from the penetration of alkali, and the attack only occurs on the surface layer, so mullite (M60) exhibits a superior alkali attack resistance. Molochite with mullite and glass (the content of 46%) reacts with alkali vapor to form more liquid phase and leucite. As alkali vapor is dissolved in the glass phase of molochilte, the integrity of molochite is destroyed by alkali attack.