The wetting process and crystallisation of Al₂O₃ substrate with molten slags: Addition of CaO‐Fe₂O₃

Abstract

As fluxes, CaO and Fe2O3 are widely applied to effectively improve the melting characteristics of coal ash. However, there are few studies on the effects of CaO and Fe2O3 contents in coal ash on the wetting of refractories used in entrained‐flow slagging gasifiers. This paper investigates the melting properties of ash slag and the wetting process in corundum‐based refractories in relation to the total amount of CaO and Fe2O3 and their proportions in coal ash. The rates of change of the apparent contact angle (ACA) and differential contact angle (DCA) as a function of temperature are employed to evaluate the wetting behaviour of the coal ash during the heating process. The results demonstrate that changes in the ACA result from the expansion or shrinkage of the slag. In particular, two abrupt changes in the ACA are observed at an Fe2O3/CaO ratio of .25. The wettability deteriorates after quenching at ratios ranging from .13 to .67. Differences in the crystal structure and elemental distribution along with the spreading direction are observed. X‐ray diffraction and FactSage calculations show that temperature changes the crystal composition and solid–liquid phase content, supporting the slag wetting process from the perspective of ash fusibility. A fraction of the liquid phase increases twice with increases in temperature, explaining the abrupt change in the ACAs. Overall, the wetting process of the slag can be explained by the surface tension and DCA. The wetting begins at 430 mN m−1 and becomes the most prominent at 435 mN m−1. Finally, an Fe2O3/CaO ratio of .25 is found to be more suitable for the long‐term operation of corundum‐based refractories.