Carbothermal Reactions of Quartz and Kaolinite with Coal Char

Abstract

Reactions of quartz and kaolinite separately with coal char were examined at temperatures up to 1600 °C in argon and nitrogen atmospheres, by using thermogravimetric analysis combined with off-gas analysis. X-ray diffraction analysis was utilized to identify the reaction products. The carbothermal reactions of quartz and kaolinite began, respectively, at temperatures as low as 1200 and 1150 °C, with the evolution of CO. The formation of reaction products was quite dependent on heating conditions. Under Ar, the main product(s) were β-SiC for quartz, and β-SiC and α-Al2O3 for kaolinite. Above 1550 °C, Al2O3 gradually formed Al4C3. Under N2, the carbothermic nitridation occurred for both quartz and kaolinite in company with the formation of SiC. For quartz, the products were SiC and hexagonal β-Si3N4 at 1600 °C; however, no crystalline nitrides were distinctly observed at 1400 °C. For the case of kaolinite, β-Si3N4 was appreciably formed at 1400 °C and it disappeared at 1600 °C; above 1500 °C, AlN was formed from Al2O3. Similar changes of crystalline mineral phases were examined on heating three coals in the same manner. Silicon carbide (SiC) and aluminum nitride (AlN) were the major products in the coals treated under nitrogen at 1600 °C for 10 min. The formation of these compounds could be attributed primarily to the carbothermal reactions of kaolinite and quartz inherent in coal.