Influence of CAO and Sio ₂ on the reducibility of wüstite using H ₂ and co gas

CaO- (0-20 mass%) and SiO ₂-containing (0-30 mass%) wüstite ('FeO') compacts were isothermally reduced at 1 273 K under CO and H ₂ gas. Prior to reduction, the phase of dicalcium ferrite (Ca ₂Fe ₂O ₅) and fayalite (Fe ₂SiO ₄) was equilibrated with 'FeO' at 1 273 K under 50%CO/50%CO ₂ and identified using X-ray diffraction and scanning electron microscopy. The rate of reduction for CaO-containing 'FeO' compacts under both H ₂ and CO increased up to the vicinity of 2.5 mass% CaO, and then decreased with higher CaO dependent on the formation of an intermediate phase of dicalcium ferrite. For SiO ₂-containing 'FeO', the rate decreased with SiO ₂ additions. When the dense fayalite is present reduction using CO was limited, while considerable reduction was observed using H ₂. The reduction was affected by three distinct reduction mechanisms of interfacial chemical reaction, gaseous mass transport, solid state diffusion of oxygen or a combination of these individual mechanisms termed the mixed control. The contribution of each mechanism with the content of CaO or SiO ₂ affecting the reduction behavior was determined. The compact porosity increased when CaO was added to approximately 2.5 mass% and subsequently decreased with higher CaO, but continuously decreased with SiO ₂ additions. The ratio of the effective diffusivity (De) to molecular interdiffusivity (D) was highest at the vicinity of 2.5 mass% CaO and thus the maximum reduction rate was obtained when the porosity was highest.