Effect of CaO on Dephosphorising Ability of Deoxidation Slag for Effective Utilisation of Phosphorus in Steel

Recent proposal of utilisation of impurities in steel requires preservation of phosphorus in molten steel during its refining process. To know the phosphorus absorbing ability of deoxidation by-product slag having CaO, the phosphorus partition has been investigated between the MnO–SiO2–FetO–CaO–MgO–P2O5 slag and molten iron in this study. The phosphate capacities (CPO43–) for the present slag having CaO content from 4.73 to 19.6 mass% have been determined to be from 2.22×1015 to 1.25×1017 at temperatures from 1843 to 1923 K. Focused on the slag basicity, a thermodynamic discussion gives the capacity as functions of optical basicity (Λ) and temperature (T) as follows; logCPO43– = (11300Λ + 54560)/T + 18.30Λ –27.43 Estimated values of phosphorus distribution ratio ranges from 0.0003 to 0.008 between the present slags and molten iron in practical operations according to empirical phosphate capacities. CaO is useful to improve phosphate capacity of this system to some extent.