| Summary: | With the raw materials for ironmaking becoming increasingly complex, more accurate phase equilibrium information on the slag is needed to refine the blast furnace operation to reduce the energy cost and CO<sub>2</sub> emissions. CaO-SiO<sub>2</sub>-Al<sub>2</sub>O<sub>3</sub>-MgO is a basic system of ironmaking slag in which CaO and MgO mainly come from the flux, SiO<sub>2</sub> and Al<sub>2</sub>O<sub>3</sub> are mainly from raw materials. The effect of flux additions on the phase equilibrium of the slag can be described by a pseudo-ternary system CaO-MgO-(Al<sub>2</sub>O<sub>3</sub>+SiO<sub>2</sub>) at a fixed Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub> ratio of 0.4. Liquidus temperatures and solid solutions in the CaO-MgO-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> system with Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub> weight ratio of 0.4 have been experimentally determined using high temperature equilibration and quenching techniques followed by electron probe microanalysis. Dicalcium silicate (Ca<sub>2</sub>SiO<sub>4</sub>), cordierite (2MgO·2Al<sub>2</sub>O<sub>3</sub>·5SiO<sub>2</sub>), spinel (MgO·Al<sub>2</sub>O<sub>3</sub>), merwinite (3CaO·MgO·2SiO<sub>2</sub>), anorthite (CaO·Al<sub>2</sub>O<sub>3</sub>·2SiO<sub>2</sub>), mullite (Al<sub>2</sub>O<sub>3</sub>·SiO<sub>2</sub>), periclase (MgO), melilite (2CaO·MgO·2SiO<sub>2</sub>-2CaO·Al<sub>2</sub>O<sub>3</sub>·SiO<sub>2</sub>) and forsterite (Mg<sub>2</sub>SiO<sub>4</sub>) are the major primary phases in the composition range investigated. A series of pseudo-binary phase diagrams have been constructed to demonstrate the application of the phase diagrams on blast furnace operation. Composition of the solid solutions corresponding to the liquidus have been accurately measured and will be used for the development of the thermodynamic database.
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