Magnesia dissolution in steelmaking slags

Basic refractories used in electric arc furnaces (EAF) for steel melting are typically magnesia based. Erosion and wear of the lining results in high costs and production delays to make repairs. Dissolution of magnesia in the slag is one of the main sources of wear and the subject of this thesis. Initial experiments were completed to determine the rate of dissolution in a neutral slag (CaO:SiO2 ratio 1:1) at 1600°C. From these results, it was concluded that magnesia rapidly dissolves and achieves saturation within the first 30 minutes of holding time. Additional experiments were completed to study the effect of slag composition on magnesia dissolution in steelmaking slags over the temperature range of 1600 to 1750°C. Slags with three levels of iron oxide (10, 30, 50 wt.%), two levels of manganese oxide (0 and 20 wt.%) and three levels of CaO:SiO2 ratio (1:1, 2:1, 3:1) were studied. The concentration of magnesia at saturation was shown to increase with temperature and decrease with slag basicity. The saturation limit was also shown to decrease with iron oxide content for acid slags and increase with iron oxide content for basic slags. Statistically, the most important factor for controlling magnesia saturation was observed to be the CaO:SiO2 ratio. The highest magnesia saturation levels occurred with a CaO:SiO2 ratio of 1:1, and decreased dramatically with increased CaO:SiO2 ratio. The second most important factor controlling magnesia saturation was temperature as higher temperatures increased magnesia solubility. This effect was more pronounced for acid slags than for basic slags. The third most important factor for controlling the saturation limit was iron oxide content. Iron oxide (as well as manganese oxide) decreased magnesia solubility for acid slags and increased magnesia solubility for basic slags. The transition from acid to basic was observed at a CaO:SiO2 ratio of approximately 1.5:1. MnO was observed to have little impact on magnesia saturation as a primary factor. However, MnO interacted with both CaO:SiO2 ratio and FetO and therefore has a secondary effect on magnesia saturation --Abstract, page iii