Influence of CaO and SiO2 on the Reducibility of W^|^uuml;stite Using H2 and CO Gas

Abstract

CaO- (0–20 mass%) and SiO2-containing (0–30 mass%) wüstite (‘FeO’) compacts were isothermally reduced at 1273 K under CO and H2 gas. Prior to reduction, the phase of dicalcium ferrite (Ca2Fe2O5) and fayalite (Fe2SiO4) was equilibrated with ‘FeO’ at 1273 K under 50%CO/50%CO2 and identified using X-ray diffraction and scanning electron microscopy. The rate of reduction for CaO-containing ‘FeO’ compacts under both H2 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 SiO2-containing ‘FeO’, the rate decreased with SiO2 additions. When the dense fayalite is present reduction using CO was limited, while considerable reduction was observed using H2. 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 SiO2 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 SiO2 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.