The paper deals with the possibility of thermodynamic analysis of CaO - SiO2 - Al2O3 - MgO slag system for optimizing of steel desulphurization in a ladle. From the point of view of reducing the sulphur content in steel, the chemical composition of the slag plays a crucial role which affects a critical thermodynamic parameter of the slag, such as the liquidus temperature of the slag, the amount of solid phases in slag at range of refining temperatures, respectively the effective viscosity of slag and CaO saturation in slag. The critical parameters were analyzed in dependence on the content of MgO in slag. The analysis was performed in FactSage 7.2 software. A quaternary graphs of the slag system CaO - SiO2 - Al2O3 - MgO were created with a constant MgO content of 0 wt.%, 5 wt.%, 10 wt.% and 15 wt.%. The results of thermodynamic calculations confirm that the content of MgO among 5% and 10% will have a positive effect on expanding the area of the lowest liquidus temperature and therefore better viscosity of slag. On the other hand, the content of MgO above 8 % led to higher content of solid phases which affected negatively the viscosity.Keywords: FactSage, properties of ladle furnace slag, furnace ladle slag, basicity of ladle furnace slag, temperature of liquid slag
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