Dephosphorization and desulfurization pretreatment of molten iron with CaO-SiO₂-CaF₂-FeO-Na₂O slags

The present work estimated the ability for the removal of phosphorus and sulfur from hot metal with CaO-SiO₂-CaF₂ fluxes which contain 20 mass% CaF₂ and [mass%CaO/mass%SiO₂]=4. Silicon losses were also determined along the treatment. Some additions of FeO (4 to 8 mass%) and Na₂O (1 to 6 mass%) were used to change the oxygen potential of the molten iron. The experiments were carried out in an open 10kg induction furnace using Al₂O₃ refractory in the temperature range 1 350-1 400°C. The oxygen potentials of the melt, continuously measured with a high temperature galvanic cell, were between 10^-13.4 atm (before the treatment) and 10^-12.0 atm (after the treatment). FeO and Na₂O contents in the slag directly affect the oxidation of the alloy elements of the liquid iron. With high silicon contents in the hot metal (0.6%) the maximum removal of silicon was about 80% whereas the maximum phosphorus and sulfur removal were about 14 and 90%, respectively. By low silicon contents (0.16%) the maximum removal of silicon, phosphorus and sulfur were 80, 40 and 95%, respectively. A thermodynamic analysis was carried out to predict the final composition of the hot metal and the species that were formed in the slag. Both results, calculated and experimental, show that sulfur and silicon were diminished in the melt in a higher extent than phosphorus and manganese. The species in the slag predicted by the model are in agreement with the X-ray analysis of the slag.