Carbonate capacities of CaF2-MgO and CaF2-CaO-MgO slags

The refining abilities of slags for impurities in steel, such as sulfur and phosphorus, can be related to slag basicity and it has been found that carbonate capacity can be employed as a basicity index. In the present study, the carbonate capacities of the binary CaF2-Mgo system and the ternary CaF2-CaO-MgO system were measured in the temperature range of 1360-1480 oC. The carbonate capacity in the CaF2-MgO system increased with increasing MgO concentration. Replacement of CaF2 by CaO at constant MgO and replacement of MgO by CaO at constant CaF2 in the CaF2-CaO-MgO system resulted in an increase in the carbonate capacity. The amount of dissolved CO2 in both the CaF2-MgO and the CaF2-CaO-MgO systems increased with increasing partial pressure of CO2. There was a linear relationship between the partial pressure of CO2 and the mole fractions of both MgO and CaO. In addition, the carbonate capacities of both the binary and the ternary systems decreased with increasing temperature. This demonstrates that the dissolution of CO2 is exothermic. The enthalpy of dissolution of CO2 for the CaF2-MgO binary system was within the range of -116 to -49.61 kJ/mole and for the CaF2-CaO-MgO ternary system it was within the range of -247.30 to -136.23 kJ/mole. Using the thermodynamic data available in the literature, the sulfide capacities were calculated for the CaF2-MgO system. The concept of optical basicity was employed to discuss the results. (author)