In modern iron blast furnace operations,utilising low grade iron ores and poor quality fuels is essential for low cost ironmaking.The accurate information of phase equilibria for complex slag system is necessary for better understanding the changes of slag chemistry in blast furnace.The compositions and liquidus temperatures of primary slag in cohesive zone was determined with the reduction of industrial sinters.The phase equilibria in the “FeO”-CaO-SiO2-Al2O3-MgO system with metallic iron saturation have been comprehensively investigated by quenching technique and EPMA analysis.The pseudo-ternary section(CaO+SiO2)-Al2O3-MgO at 50 wt%“FeO” was constructed and a series of pseudo-binaries were derived to be easily used in industrial applications.The major primary phase was found to be oxide[(Fe,Mg)O] and the liquidus temperatures increase significantly with increasing MgO in this primary phase field.Al2O3 has little effect on liquidus temperature in this primary phase field.Liquidus temperatures increase in melilite primary phase field and decrease in Ca2SiO4 primary phase field with increasing Al2O3 concentration.Liquidus temperatures are not sensitive to MgO concentration in both melilite and Ca2SiO4 primary phase fields.