Analysis of the transformation behaviors of a Chinese coal ash using in-/ex-situ XRD and SEM-EXD

Crystal and phase transitions of a Chinese Datong coal ash were evaluated using room temperature X-ray diffraction (RT-XRD), in-situ high temperature (HT)-XRD, and scanning electron microscope combining energy dispersive X-ray analysis (SEM-EDX) as temperature varied from 300 ° C to 1600 ° C. The transforming behaviors of ash were related to its melting properties. Results revealed that the transforming behavior of minerals from in-situ HT-XRD was different with that from routine RT-XRD. Low-temperature Datong ash contained quartz and clays (such as kaolinite); the changes in minerals at low-temperature region (300-1000 ° C) were mainly due to the minerals decomposition and partial phase transition. Significant transition of quartz to cristobalite occurred at ca. 1100 ° C; the continuous transition and reaction of silica and alumina at ca. 1200 ° C accelerated the softening of the ash (ST ≈ 1290 ° C). An X-ray amorphous liquid phase was formed mainly by the eutectic solution of silica and mullite at ca. 1400 ° C, which obviously affected the hemisphere temperature (HT ≈ 1490 ° C) of ash; meanwhile, ultra fine alumina solid particles, which were probably derived from kaolinite, were found in the melted ash. Virtually, the fusing (FT > 1500 ° C) of Datong ash was considered to be determined by the formation of solid particles and eutectic solution. Eventually, the semi-quantitative evaluation of transformations of ash was successfully obtained through the combined methods.