Effect of pre-oxidation on the kinetics of reduction of ironsand

The kinetics of reduction raw and pre-oxidized ironsand with hydrogen was studied using a combination of thermogravimetric analysis technique and microstructural characterization. Temperature range of 1073–1273 K (800–1000 °C) and H2 partial pressure of 0.1–1.0 atm were covered. It was found that the reaction is first order with respect to pH2 at 1173 K (900 °C). The reduction reaction was considerably faster for the pre-oxidized ironsand, owing to the presence of micro-cracks in the particles. The extent of reduction was quantified against time at different temperatures and the results were analyzed using both conventional kinetic models, as well as an iso-conversional method. The results indicated that for the raw ironsand, the reduction rate is first limited by nucleation and growth of products and later by first order chemical reaction. At last, it is controlled by three-dimensional diffusion of reactants. The apparent activation energy of reduction increased from ∼50 to 105 kJ mol−1 when the conversion degree increased from 0.2 to 0.6. On the other hand, the activation energy for the pre-oxidized ironsand remained stable at 67.2 kJ mol−1 over the entire process, following a first order chemical reaction control step. The diffusion-limited step does not appear to be limiting the rate for the pre-oxidized ironsand reduction process due to the micro-cracks formed in this material, facilitating the gas access to solids.