Generally, decreasing slag volume of blast furnace operation can lead to the lower fuel ratio and higher productivity. For high sinter ratio operation, one of effective ways to obtain a lower slag volume is to reduce the gangue content of sinter. Basically, lowering the amount of serpentine in the sinter mix is a feasible way to produce suitable sinter with the lower gangue content. However, this method may result in lower magnesium oxide content in the final slag that may affect its fluidity. Hence, the objective of this study was to understand the effect of MgO and Al₂O₃ on the fluidity of final slag. The liquidus temperature and viscosity of semi-synthetic slag were measured using optical softening temperature device and viscometer, respectively, and the data were treated to develop the multiple-regression formula of SiO₂–Al₂O₃–CaO–MgO–TiO₂ semi-synthetic slag for liquidus temperature, viscosity equations and iso-fluidity diagrams.
The experimental results indicated that the lower liquidus temperature and the better viscosity stability lay in the area of MgO = 5.4%, Al₂O₃ = 10–15%, TiO₂ = 0.5% and C/S = 1.2 for the range of composition studied. Several observations in the iso-fluidity diagrams of blast furnace final slag have shown that liquidus temperature decreased with decreasing MgO content and the viscosity of slag could be regarded as being independent of MgO content in the range of MgO = 5–9%, Al₂O₃ = 15%, C/S = 1.0–1.2. And, slag fluidity became worse with increasing Al₂O₃ content under the conditions of MgO = 5.4%, C/S = 1.2. This study suggested the MgO contents could be lowered from current 6.5 to 5.4% in the conditions of Al₂O₃ = 15%, C/S = 1.2 under the stable blast furnace operation with high thermal level. Furthermore, this recipe of lower MgO content (5.4%) had been implemented in CSC’s BF operation to reduce the slag volume, and the formula of fluidity developed in this study had been installed in the process computer of all CSC’s BFs to present slag fluidity in real time.