Oxidation mechanism of C in MgO-C refractory bricks

Kinetics of oxidation of MgO-C refractory bricks containing 14.3% graphite, 80.9% magnesia, and 4.8% phenolic resin is investigated comprehensively. Effect of temperature and grain size distribution on the rate of oxidation of graphite is evaluated by application of a mathematical model developed on the basis of shrinking core/progressive conversion regime to the gas-solid processes involved. Incorporation of the experimental data into the mathematical model indicates that a mixed controlling mechanism governs the oxidation rate. Three activation energies were recognized: (a) 68 kJ/mol for chemical adsorption of gas on the surface of the graphite flakes, (b) 22 kJ/mol for pore diffusion of the gases within the decarburized layer, and (c) 140 kJ/mol for internal diffusion of the reactants toward the active reaction sites. The first process was influential at all temperatures, the second was confined to the temperatures above 800 degrees C and the third was attributed to the temperatures lower than 800 degrees C. Empirical data indicated a much lower weight loss rate at temperatures lower than 800 degrees C.