Phase Equilibrium Studies Of CaO-MgO-Al₂O₃-SiO₂-TiO₂ System And Their Applications In The Optimization Of Ironmaking Slags

With the rapid development of the steel industry and proposal of the energy-saving and emission-reducing policies,competition in the steel industry has become increasingly fierce.Reduction in energy consumption and production costs has become a major task for enterprises.Optimization of the burden structure and the slag composition is one of the important measures to achieve low-carbon and low-cost ironmaking.Therefore,steel enterprises are facing new challenges:on the one hand,using low-grade iron ore with high impurities such as Al₂O₃,extending the life of the furnace,and improving the utilization of China’s abundant vanadium-titanium magnetite resources to reduce production costs.On the other hand,the ironmaking industry is transitioning towards largescale and intelligent operations,with increased demands for systematic and precise fundamental theories.Currently,the fundamental thermodynamic dada of ironmaking slag systems cannot meet the requirements of modern ironmaking operations and the development and optimization of thermodynamic databases.Therefore,studying the phase equilibria of ironmaking slag systems not only has important theoretical guidance for the efficient utilization of iron resources but also can provide accurate experimental data for the development of thermodynamic models.This study is focused on the CaO-MgO-Al₂O₃-SiO₂ and CaO-MgO-Al₂O₃-SiO₂-TiO₂ systems that are related to the conventional blast furnace slag,high-TiO₂ blast furnace slag,and high-TiO₂ slag produced by HIsmelt process.By using high-temperature equilibration,quenching and electron probe microanalysis（EPMA）technique,the phase equilibria of the quaternary and quinary slag systems are systematically studied,including the liquidus temperatures and corresponding solid solution compositions.Pseudo-ternary phase diagrams of（CaO+MgO）-SiO₂-Al₂O₃ and CaO-MgO-（SiO₂+Al₂O₃）were constructed based on the average composition of the conventional blast furnace slag.A series of pseudo-binary phase diagrams were used to investigate the effects of raw material and flux compositions on the liquidus temperature of the slag,providing theoretical guidance for adapting to different raw materials and identifying the optimal fluxing conditions in ironmaking process.Two pseudo-ternary phase diagrams of（CaO+MgO）-（SiO₂+Al₂O₃）-TiO₂ were constructed with a fixed Al₂O₃/SiO₂ mass ratio of 0.4 and MgO/CaO mass ratios of 0.2and 0.4,respectively,to investigate the effects of TiO₂,quaternary basicity,and MgO/CaO mass ratio on the liquidus temperatures of TiO₂-containing blast furnace slag formed by adding TiO₂-bearing materials for furnace protection or by smelting vanadium-titanium magnetite in a blast furnace.It was found that small proportion of TiO₂ in blast furnace slag can decrease the liquidus temperature while acting as a furnace protector.When smelting vanadium-titanium magnetite in a blast furnace,calcium titanate and ilmenite were the major primary phases.By adjusting the MgO content,the ratio of vanadium-titanium magnetite to ordinary iron ore,and quaternary basicity,the liquidus temperature of the slag can be lowered,providing a theoretical basis for the smelting of vanadium-titanium magnetite in a blast furnace and the recovery of TiO₂ from high-titanium slag.A pseudo-ternary phase diagram of CaO-TiO₂-（SiO₂+Al₂O₃+MgO）with a fixed MgO/SiO₂ mass ratio of 0.58 and MgO/（SiO₂+Al₂O₃）mass ratio of 0.33 was constructed for the smelting of vanadium-titanium magnetite and ilmenite using the HIsmelt technology.Specific slag compositions and temperature ranges were proposed for the mixed vanadium-titanium magnetite and ilmenite in the HIsmelt process.The use of vanadium-titanium magnetite and ilmenite in the HIsmelt process can produce hot metal and high-titanium slag simultaneously,providing a solid theoretical basis for the industrial application of the HIsmelt technology for smelting vanadium-titanium magnetite.By comparing the experimental data obtained in this study for the CaO-MgO-Al₂O₃-SiO₂ and CaO-MgO-Al₂O₃-SiO₂-TiO₂ systems with the thermodynamic predictions by Fact Sage software,the limitations of the current thermodynamic databases for these multicomponent oxide systems were identified,and suggestions were made for optimizing the thermodynamic databases.