Thermal behavior and chemical reactivity of coal gangue during pyrolysis and combustion

Abstract

The efficient extraction of Al from coal gangue plays a significant role in achieving the high-value utilization of coal gangue and alleviating the shortage of bauxite resource. Coal gangue is usually subjected to thermal activation to stimulate its chemical reactivity before extracting Al through acid method. The thermal-activation atmosphere is a key ingredient influencing the activation effect and is thus worthy of in-depth study. Herein, a series of comparative studies were performed to gain insights into the thermal behavior and chemical reactivity of coal gangue in nitrogen atmosphere (pyrolysis) and air atmosphere (combustion). First, the pyrolysis and combustion characteristics of coal gangue were determined by thermogravimetric analysis at various heating rates of 5, 10, 15, 20, and 25 °C/min. Second, the kinetics of the two thermal activation processes was analyzed using the iso-conversional method and model-fitting method. Results showed that the pyrolysis and combustion reactions of coal gangue conformed to the nucleation-growth model, and the mechanism functions were determined as A1/2 and A2/3, respectively. We found that the intense combustion of combustible substances did not change the reaction mechanism, but reduced the apparent activation energy required for thermal activation from 177.47–185.17 kJ/mol to 140.24–144.39 kJ/mol. Phase-evolution analysis also revealed that combustion lowered the temperature required for inert kaolinite activation from 550–650 °C to 450–550 °C, indicating that combustion effectively promoted the activation of coal gangue. Finally, HNO3 was used to establish the correlation between the thermal activation and chemical reactivity of coal gangue in view of its significant advantage of easy regeneration. Leaching results further demonstrated that combustion presented obvious benefits for the efficient extraction of Al and the effective inhibition of impurity Fe.