Phase Equilibria Studies in the CaO–MgO–Al2O3–SiO2 System with Al2O3/SiO2 Ratio of 0.5 Related to High-Al2O3 BF Slags

With the increasing use of high-alumina iron ores in blast furnace ironmaking, the Al 2 O 3 content in the slags has gradually risen to 15 to 20 wt pct, significantly affecting the phase equilibria and flow properties of the slag system. In this study, the phase equilibria in the CaO–MgO–Al 2 O 3 –SiO 2 quaternary system with a fixed Al 2 O 3 /SiO 2 mass ratio of 0.5 (representative of typical high-Al 2 O 3 blast furnace slags) were systematically investigated using high-temperature equilibration and quenching experiments combined with electron probe microanalysis (EPMA). A pseudo-ternary phase diagram CaO–MgO–(SiO 2 +Al 2 O 3 ) was constructed. At a fixed (SiO 2 +Al 2 O 3 ) content of 55 wt pct, the minimum liquidus temperatures for slags with Al 2 O 3 /SiO 2  = 0.5 occur at MgO/CaO ≈ 0.4 (1360 °C), respectively, and the liquidus temperature of the Al 2 O 3 /SiO 2  = 0.4 slag is consistently lower than that of the Al 2 O 3 /SiO 2  = 0.5 slag over the entire MgO/CaO range studied. At fixed MgO/CaO weight ratio of 0.2 and 0.15, the liquidus temperature first decreases and then increases with increasing quaternary basicity (defined as (CaO+MgO)/(SiO 2 +Al 2 O 3 ) by mass). In the conventional basicity range of blast furnace slag (0.95 to 1.05), the liquidus temperature ranges from 1400 °C to 1500 °C, and the high-alumina slag always exhibits a higher liquidus temperature than the ordinary slag. Significant deviations exceeding 50 °C were observed between the experimental liquidus temperatures and the phase equilibria calculated using the FactSage 8.4 thermodynamic database for high‑alumina blast furnace slags. The compositional characteristics of the melilite solid solution were also analyzed using EPMA‑derived compositional data. Strong linear correlations among Al 2 O 3 , SiO 2 , and MgO in melilite confirm the isomorphous substitution mechanism. When the Al 2 O 3 /SiO 2 ratio in the liquid phase is below 0.5, the Al 2 O 3 /SiO 2 ratio in melilite increases sharply with increasing liquid ratio; when the liquid ratio exceeds 0.5, the Al 2 O 3 /SiO 2 ratio in melilite stabilizes within a narrow range of 1.4 to 1.6, indicating that the incorporation of Al 3 ⁺ in the melilite structure approaches an apparent solubility limit.