Penetration and corrosion of magnesia grain by silicate slags

Abstract

Penetration and corrosion resistance of high purity sintered and fused magnesia grain by model EAF (CaO/SiO₂ = 1·38) and BOF ‘late’ slags (CaO/SiO₂ = 3·29) at 1600 and 1700°C were investigated by SEM, EDS, and XRD analysis. Thermodynamic calculations were performed to assist interpretation of the reaction processes involved. At the test temperatures, Fe andMn ions from both model slags diffused into themagnesia grain to form a magnesiowustite, (Mg,Fe,Mn)O. The magnesiowustite directly adjacent to the slag had a much larger crystal size than that of the bulk MgO far from the MgO/slag interface. The large magnesiowustite grains limit the potential for grain boundary penetration into the sintered magnesia. The magnesiowustite layer formed with the EAF slag took up more Fe_xO from the slag than that formed with the BOF slag, which was partially responsible for a lower slag penetration into sintered magnesia grain since the remaining silica rich local liquid was rendered more viscous. The EAF slag was not saturated with respect to MgO, so the magnesiowustite which did form later reacted with Ca and Si ions remaining at the MgO/EAF slag interface to form low melting phases such as merwinite, C₃ MS₂, and then dissolved into the slag, rendering the dissolution process essentially indirect. The BOF late slag was already oversaturated with respect to MgO, so slag penetration only occurred in the sintered magnesia grains.