Reduction of Iron Oxide Fines to Wustite with CO/CO₂ Gas of Low Reducing Potential

The reduction of iron oxide fines to wustite between 590 °C and 1000 °C with a CO–CO₂ gas mixture of low reducing potential was studied. The reduction kinetics and the dominating reaction mechanism varied with the temperature, extent of reduction, and type of iron oxide. Reduction from hematite to wustite proceeded in two consecutive reaction steps with magnetite as an intermediate oxide. The first reduction step (hematite to magnetite) was fast and controlled by external gas mass transfer independently of the oxide type and the temperature employed. The second reduction step (magnetite to wustite) was the overall reaction-controlling step, and the reduction mechanism varied with the temperature and the oxide type. Moderately porous oxide fines followed the uniform internal reaction for the temperature range studied. For highly porous oxides, the second reduction step was controlled by external gas mass transfer above 700 °C. Below that temperature, a mixed regime that involves external gas mass transfer and limited mixed control, which comprises pore diffusion and chemical reaction, takes place. The rate equations for this mixed control reaction mechanism were developed, and the limited mixed control rate constant (k_lm) was computed. For denser oxides under uniform internal reaction, the product of the rate constant and pore surface area (k·S) was calculated.