Three-segment control theory of MgO/Al₂O₃ ratio based on viscosity experiments and phase diagram analyses at 1500 °C

In recent years, more and more high Al₂O₃ iron ores were used in East Asia, especially in China, which increased the Al₂O₃ content in blast furnace (BF) slag and resulted in poor metallurgical properties of slag. Adding MgO-bearing flux is one of the methods to improve the metallurgical properties of slag with high Al₂O₃. However, there is lack of theoretical basis for the proper MgO/Al₂O₃ ratio. Therefore, the properties of slag in SiO₂–CaO–MgO–Al₂O₃ system were investigated based on viscosity experiments and phase diagram analyses at 1500 °C, and the proper MgO/Al₂O₃ ratio was explored according to the Al₂O₃ content in slag. The experimental results show that: (1) in the case of Al₂O₃ content less than 14 mass%, there is no limitation of the MgO/Al₂O₃ ratio, and the amount of MgO in slag can be determined according to the hearth temperature and desulfurization; (2) in the case of Al₂O₃ content between 15 and 17 mass%, the proper MgO/Al₂O₃ ratio should be 0.40–0.50; (3) in the case of Al₂O₃ content between 18 and 20 mass%, the proper MgO/Al₂O₃ ratio should be 0.45–0.55. Consequently, the three-segment control theory of MgO/Al₂O₃ for BF slag was built, and the actual BF performances proved the validity and applicability of this theory. The research contents and results can give theoretical guidelines for stable BF operation in a wide range of Al₂O₃ content of 12–20 mass%.