Comparative study of the isothermal solid-state reaction systems of kaolinite–Na2CO3 and kaolinite–quartz–Na2CO3 for coal gangue activation

A clear understanding of the solid-state reaction of kaolinite (Kln), quartz (Qtz), and sodium carbonate (Na 2 CO 3 ) is of great significance for the process optimization of coal gangue calcined with Na 2 CO 3 . In this work, a comparative study of the isothermal solid-state reaction systems of Kln–Na 2 CO 3 and Kln–Qtz–Na 2 CO 3 was performed by means of X-ray diffraction (XRD), scanning electron microscope, and energy dispersion spectroscopy (SEM-EDS). The results showed that the calcined products both for these reaction systems mainly contain different kinds of sodium aluminum silicates (e.g., NaAlSiO 4 , Na 1.55 Al 1.55 Si 0.45 O 4 , and Na 1.95 Al 1.95 Si 0.05 O 4 ) and various kinds of sodium silicates (e.g., Na 2 Si 3 O 7 , Na 2 SiO 3 , and Na 6 Si 2 O 7 ). The mass percentage of Na 2 CO 3 played a key role in the phase transformation, determining the Na/Al/Si molar ratio of the formed sodium aluminum silicates. Compared with the reaction system of Kln–Na 2 CO 3 , the existence of Qtz inhibited the formation of sodium aluminum silicates in the reaction system of Kln–Qtz–Na 2 CO 3 . It should be noted that the formed phases both for these reaction systems were slightly different from that of the thermodynamical calculated results of Na 2 O–SiO 2 –Al 2 O 3 using FactSage™ software. According to both the experimental and calculated results, a reasonable batching area for coal gangue activation was proposed that the addition of Na 2 CO 3 should be in the range of 20–50% of the total mass of Kln, Qtz, and Na 2 CO 3 .