Flow properties and crystallization behaviors of coal ash slag at different atmospheres

The flow properties of coal ash slag, which depend closely on the ash chemical composition [1] and the crystallization behaviour [2], are important for the tapping process of entrained flow gasifiers. Iron in the slag exists as different valence states [3] and complicates the slag viscosity [4]. In this study, the viscosities of coal ash slag with different iron states were investigated. The slag crystallization behaviors were analyzed by quenching method together with XRD, thermodynamic calculation and BSE. Results showed that the Fe2+ and Fe3+ coexisted in the slag under the inert atmosphere and Fe2+ was the main valence state under mildly reducing atmosphere. However, part of iron was reduced to metallic iron for the slag under the strongly reducing atmosphere. At high temperatures, the effect of iron states on slag viscosity was hardly to be detected, even for the slag with metallic iron. However, the slag viscosity acted tremendously different as the temperature dropped, which was attributed to the different crystallization behaviors. The slag containing metallic iron showed a sharp increase of viscosity as the temperature approached to the crystallization temperature of anorthite. The slag viscosity under mildly reducing atmosphere was lower than that under inert atmosphere. This was associated with the crystallization of minerals. The coexistence of Fe2+ and Fe3+ in the slag favored the precipitation of spinel and anorthite while only anorthite was found for the slag with Fe2+. The BSE results indicated that, the metallic iron acted as the nucleation centers, leading to the quick increase of anorthite. But, the crystallization behavior in the slag with only Fe2+ or with both Fe2+ and Fe3+ required the degree of supercooling. Besides, the crystallization kinetics should be taken into consideration during the prediction of slag viscosity.