Influence of CaO and SiO_2 on the Reducibility of Wiistite Using H_2 and CO Gas

摘要

CaO- (0-20 mass%) and SiO_2-containing (0-30 mass%) wustite ('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 dif-fusivity (D_e) 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.