Reoxidation kinetics of flash reduced iron particles relevant to a novel flash ironmaking process

A novel flash ironmaking process based on hydrogen containing reduction gases is under development at the University of Utah. This process will directly reduce fine iron oxide concentrate particles in suspension in a flash reactor. The goal of this work was to study the possibility of reoxidation of iron particles in various gas mixtures from the kinetic point of view. The last stage of hydrogen reduction of iron oxide, i.e., the reduction of wustite, is limited by equilibrium. As the gas particle mixture cools down in the lower part of the flash reactor, the reoxidation of iron could take place because of the decreasing equilibrium constant and the high reactivity of the freshly reduced fine iron particles. The effects of temperature (823 - 973 K) and H2O partial pressure (40 - 100 pct.) on the reoxidation rate were examined. The nucleation and growth model was shown to best describe the reoxidation kinetics. Pressure dependence is first order with respect to water vapor, and the activation energy is 146 kJ/mol. A complete rate equation that adequately represents the experimental data was developed. For oxidation in O2-N2 gas mixtures, effects of temperature (673 - 873 K) and O2 partial pressure (5 - 21 pct.) were studied and the nucleation and growth model was used to describe the initial period of oxidation. Pressure dependence is first order with respect to oxygen and the activation energy is 14.4 kJ/mol. The oxidation in pure CO2 gas was investigated in the temperature range of 873 - 1073 K and the results imply that the oxidation will be negligible in the flash reduction process where CO2 from partial combustion of natural gas with oxygen accounts for less than 10% in the gas mixture.