Thermodynamic Studies of the Fe-Pt System and “FeO”-Containing Slags for Application Towards Ladle Refining

Abstract: In the present work, the thermodynamic activites of ironoxide, denoted as "FeO" in the slag systems Al2O3-"FeO", CaO-"FeO", "FeO"-SiO2, Al2O3-"FeO"-SiO2, CaO-"FeO"-SiO2and "FeO"-MgO-SiO2were investigated by employing the gasequilibration technique at steelmaking temperatures. Thestrategy was to expose the molten slag mixtures kept inplatinum crucibles for an oxygen potential, determined by aCO/CO2-ratio. A part of the iron reduced from the "FeO"in the slag phase was dissolved into the Pt crucible.In order to obtain the activites of "FeO", chemical analysisof the quenched slag samples together with thermodynamicinformation of the binary metallic system Fe-Pt is required.Careful experimental work was carried out by employing asolid-state galvanic cell technique as well as calorimetricmeasurements in the temperature ranges of 1073-1273 K and300-1988 K respectively. The outcome of these experiments wasincorporated along with previous studies into a CALPHAD-type ofthermodynamic assessment performed with the Thermo-Calc?software. The proposed equilibrium diagram enabledextrapolation to higher temperatures.The experimentally obtained activites of "FeO" in thepresent work, along with earlier investigations were assessedwith the KTH slag model, THERMOSLAG©. New binaryparameters were evolved and incorporated in THERMOSLAG©.The present model calculations are compared with othercommercially available software such as F'A'C'T?andThermo-Calc?. The validity of the modified model wasinvestigated by measurements carried out in case of Al2O3-"FeO"-SiO2, CaO-"FeO"-SiO2and "FeO"-MgO-SiO2ternary slags. The potential of the model tocompute the activities in the case of multicomponent slags wasdemonstrated.A correlation between the activity of a metallic oxide in aternary slag system and the sulphide capacity of the slag wasinvestigated by using the solubility of sulphur in the binarysystems CaO-SiO2and Al2O3-CaO along with the sulphide capacity of the Al2O3-CaO-SiO2system. The estimated values of the activitieswere found to be in good agreement with the measured values.This correlation also gives the possibility to elucidate theapplicability of Henry's law to the activity of a metallicsulphide and to determine the order in the affinity of a cationto sulphur between two metallic oxides in a slag.Model calculations were performed with THERMOSLAG©, by using plant data from the ladle refiningprocess at OVAKO Steel, Hofors, Sweden. It was found thatoxygen estimations in the metal from the "FeO" analyses ofslags, obtained by conventional sampling and analysis methodwere less reliable. Reliable estimation of the oxygen levelsutilising the sulphur partition between the slag and the metalwere carried out using THERMOSLAG© software.Keywords:Thermodynamics, Activity, Galvanic cell,Calorimetry, Gas equilibration technique, Iron-platinum alloys,FeO, Slags, Modelling, Ladle