High-Temperature Interactions between Coal Char and Mixtures of Calcium Oxide, Quartz, and Kaolinite

The interaction between coal char and mixtures of calcium oxide, quartz, and meta-kaolinite were investigated by heating to 1600 °C under Ar. Calcium oxide was gradually reduced by coal char in the temperature range 950−1450 °C, and then rapidly carbided above 1450 °C, releasing CO. In the case of CaSiO3, the carbothermal reduction of the SiO2 component started at 1200 °C in preference of the CaO component, forming SiC and Ca2SiO4; at higher temperature, both of the SiO2 and CaO components in Ca2SiO4 were carbided significantly. When CaO was mixed with meta-kaolinite, the two phases reacted with each other below 1200 °C. In the chemically bound CaO−SiO2−Al2O3 system, the reactivity sequence of carbothermal reduction was SiO2 > CaO > Al2O3; the reduction of the SiO2 component also began at 1200 °C, forming SiC; upon heating to 1600 °C, the presence of a calcium alumimum oxide carbide (approximately CaAl1.9O4C0.4) was notable. CaO became difficult to reduce as it was chemically combined with Al2O3. On the other hand, it was found that two demineralized coal chars were appreciably graphitized in the presence of calcium species. Free CaO exerted a strong catalytic effect on the graphitization of coal char, mainly forming the graphitic carbon at 1600 °C. As CaO incorporated with meta-kaolinite or quartz, the action of CaO toward the graphitization of coal char weakened, primarily resulting in the turbostratic carbon at 1600 °C.