Study of Structural Transformation and Chemical Reactivity of Kaolinite-Based High Ash Slime during Calcination

Abstract

The transformation of kaolinite into metakaolinite by thermal activation to obtain highly active aluminosilicate is commonly known. In addition to kaolin, the high content of kaolinite in coal mining waste is another potential source for obtaining an aluminosilicate precursor, thereby protecting the environment and adding value to industrial wastes. In this paper, the kaolinite-based high ash slime (KAS) was calcined at temperatures ranging from 400 °C to 1000 °C under air, N2, and CO2 atmospheres, respectively. The thermal behaviors and structural evolution of each component in KAS were analyzed by thermal analysis (TG-DSC), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Moreover, the chemical reactivity of Al2O3 and SiO2 in calcined KAS was evaluated by HCl and NaOH leaching methods. The results show that the applied KAS in this study primarily consisted of kaolinite and carbon, while the minor mineral phases included quartz, calcite, and pyrite. Additionally, the structural transformation of kaolinite during calcination included dehydroxylation, sintering, and the formation of mullite. Crystalline kaolinite completely decomposed into semicrystalline metakaolinite at 600–800 °C, accounting for the increase in chemical reactivity. The interlayer sintering of metakaolinite and the recrystallization of amorphous components led to the decrease in chemical reactivity after 800 °C. Furthermore, the thermal behaviors of carbon in KAS are greatly affected by the calcination atmosphere. The presence of carbon reduced the chemical reactivity of calcined KAS.