Carbonate and sulfide capacities of calcium fluoride-magnesium oxide and calcium fluoride-calcium oxide-magnesium oxide-containing slags

The refining abilities of slags for impurities in steel, such as sulfur and phosphorus, can be related to slag basicity and carbonate capacity is often employed as a basicity index. In the present study, gas-molten slag equilibration method was selected for the study of the carbonate and the sulfide capacities of the binary CaF₂-MgO system and the ternary CaF ₂-CaO-MgO system in the temperature range of 1380–1450°C. The effect of the chemical composition of the slags on the carbonate and the sulfide capacities was investigated. The effect of temperature on the carbonate capacities of these slags was used to calculate the enthalpy of CO₂ solubility. Furthermore, the effects of the partial pressures of CO₂ , O₂ and S₂ on the solubility of CO₂ and sulfur were studied.; The carbonate and the sulfide capacities in the CaF₂-MgO system increased with increasing MgO concentration. Also, the addition of MgO to the CaF₂-CaO system resulted in a decrease in both the carbonate and the sulfide capacities. Replacement of CaF₂ by CaO at constant MgO, and replacement of MgO by CaO at constant CaF₂ in the CaF ₂-CaO-MgO system resulted in an increase in the carbonate and the sulfide capacities. This is consistent with fact that CaO is more basic than both CaF₂ and MgO.; The amount of dissolved CO₂ in both the CaF₂-MgO and the CaF₂-CaO-MgO systems was measured by the combustion infrared method and found to increase with increasing partial pressure of CO₂ . In addition, the carbonate capacity of both the binary and the ternary systems decreased with increasing temperature. This demonstrates that the dissolution of CO₂ is exothermic.; The sulfur solubility was also measured by combustion infrared method and found to increase with increasing partial pressure of sulfur in both the CaF₂-MgO system and the CaF₂-CaO-MgO system. The X-ray diffraction patterns (XRD) of slags indicated the precipitation of CaS in the CaF₂-CaO-MgO system at values higher than 2000. Thus, the thermodynamic data were used to calculate the activity coefficient of CaS and the activity and the activity coefficient of CaO in the CaF₂-CaO-MgO system.; The data available in the literature on the sulfide and the phosphate capacities of the CaF₂-CaO system and the sulfide capacities of the CaF₂-CaO-MgO system as determined in the present work were employed to calculate the phosphate capacities of the CaF₂-CaO-MgO system. It was demonstrated that the addition of MgO to the CaF₂-CaO system decreased the phosphate capacity. The concept of optical basicity was employed to discuss the results.