Kinetics of iron carbide formation from reduced iron ore in CO-H2-H2S mixtures

The rate of iron carbide formation by the reaction of completely reduced limonite with CO-H-2-H2S mixtures of a(s) = 0.5 was gravimetrically measured. The weight gain by the reaction was evidenced to directly correspond to the carburization, i.e. taking in carbon from the gas phase, to form iron carbide by comparing the weight gain with the amount of carbon in iron carbides analyzed by Mossbauer spectroscopy. The rate was measured in the varied total flow rates and varied compositions of the reaction gas, and the varied weight and varied size of iron ore particles. It was concluded that the carburization rate on pore surface of completely reduced iron ore particles controls the iron carbide formation rate. By applying the integrated rate equation for the first order reaction to f(theta) versus t curves, the carburization rate was evaluated. The rate of the reduced limonite ore was much faster than the rate of a reduced hematite ore by the factor about 6. From the rate being nearly proportional to p(CO) to 80% CO, it was concluded that the main carburization reaction is CO+H-2 =[C] + H2O and the rate controlling step is the dissociative adsorption of CO molecules. The carburization rate decreased with increasing the reduction temperature but it hardly changed with the carburization temperature because the rate was strongly dependent on the specific surface are of reduced limonite. The dependencies of the rate on the reduction temperature for the carbide formation at 973K and on the carbide formation temperature from the reduced limonite at 1173K were quantitatively discussed.