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

Abstract

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.

APPENDIX: External Gas Mass Transfer Determination

The mass transfer coefficient of CO–CO₂ (m_CO-CO2) that was used to compute the reduction rate associated with external gas transfer was determined indirectly by measuring the evaporation rate of Mg under Argon-5 pct H₂ and H₂ at 625 °C. The equipment, flow and geometric conditions of the evaporation experiments were identical to those conditions of the iron oxide reduction. The results are shown in Figure A1.

Fig. A1

Weight loss of Mg powder as a function of time at 620 °C under a flow of 5 lpm of Ar-5 pct H₂ and H₂

The evaporation rate of magnesium in Argon (R_Mg-Ar), which is controlled by external gas phase mass transfer, is expressed in Eq. [A1], where m_Mg-Ar is the mass transfer coefficient of Mg in Argon, and p ^S_Mg and p ^B_Mg represent the partial pressure of Mg in the surface and bulk stream, respectively.

$$ {\frac{{dn_{\rm Mg} }}{dt}} = - {\frac{{m_{{\rm Mg} - i} \times A_{\delta } }}{\text{RT}}}(p_{\rm Mg}^{S} - p_{\rm Mg}^{B} ) $$

(A1)

Using Eq. A1, the mass transfer coefficient (m _Mg-Ar) is calculated with the experimental evaporation rate and by taking A_δ as the crucible area; $p_{\rm Mg}^{S}$ is calculated as the equilibrium vapor pressure of Mg, whereas $p_{\rm Mg}^{B}$ is assumed to be negligibly small. The mass transfer coefficient of Mg–Ar at 625 °C then is extrapolated to the temperature range of interest (600 °C–1000 °C) and converted to mass transfer of CO–CO₂ which accounts for the differences in gas diffusivities. This methodology was validated using the Mg–H₂ evaporation results also shown in Figure A1.