The Evaluation of Reduction Kinetics of Iron Ore Fines in a Fluidized Bed Reactor System using Hydrogen as a Reducing Agent

The iron and steel industry is an energy-intensive industry, responsible for approximately 7 – 9% of global anthropogenic CO2 emissions. The necessity to reduce these emissions demands new iron and steel-making technologies. Once such technology that offers significant reduction in these emissions is the fluidized bed reduction of iron oxide. This production pathway allows iron ore fines to be reduced directly by gaseous reductants without any prior agglomeration steps. This study deals with the investigation of reduction behavior of hematite fines using solely hydrogen as a reducing agent. The influence of water vapour addition to the reducing gas mixture was also thoroughly investigated.
The experiments were conducted between 600 – 750 °C for two series of test conditions – 1. At 12% gas oxidation degree (GOD); 2. At 20% gas oxidation degree. The thermodynamic driving force was kept constant within each test series. The results indicated that the reduction behavior was influenced profoundly by water vapour addition. Increasing the partial pressure of water vapour in the reducing gas mixture resulted in a change in the concentration gradient of both gas components, which in turn led to a blockage of free reaction sites and thus hindered the diffusion of hydrogen to the reaction interface. Furthermore, water vapour also affected the chemical reaction rate. At 12% GOD, the metallization was insignificant due to low driving force for reduction, except for 750 °C where a good reduction behavior was observed. No metallization was observed at higher gas oxidation degrees.

The apparent activation energy values were determined through the isoconversional method. At 12% GOD, the apparent activation energy for the reduction steps Fe2O3→Fe3O4, Fe3O4→FeO and FeO→Fe were 50.8 – 70.3 kJ/mol, 70.3 – 97.6 kJ/mol and 38.6 – 79.5 kJ/mol, respectively. On the contrary, the apparent activation energy at 20% GOD for the reduction steps Fe2O3→Fe3O4 and Fe3O4→FeO were 25.8 – 143.2 kJ/mol and 30.9 – 143.2 kJ/mol, respectively. The values lie within the range reported in the literature. The disparity in the values appears mainly due to difference in operating conditions, experimental equipment and raw materials used. In the last part of the study, three model-fitting procedures were applied to determine the rate-limiting mechanisms. The results of fitting procedures indicate that the reduction was one-dimensional diffusion-controlled at lower gas oxidation levels and changed to three-dimensional diffusion control as the gas oxidation level increased.