A Critical Review of Limitations of Slag Capacity Concepts in Metallurgical Applications by Taking Sulfide and Phosphate Capacities as Examples

The desulfurization and dephosphorization performances have been considered to be the most challenging operations for refining ultra-clean steel. The distribution ratios of sulfur and phosphorus in the form of L_i = ( % i)/ . -0pt[ % i] as well as slag capacity concepts of sulfide and phosphate capacities in the form of C_i^m - ∝ (% i^m - ) have been extensively studied and widely applied for the description of desulfurization and dephosphorization abilities and potentials, respectively. However, the intrinsic limitations of two slag capacity concepts as sulfide and phosphate capacities regardless of slag oxygen potential p_O_2 have seriously limited their direct applications into guiding the practical refining operations compared with the partitions or distribution ratios L_i . The theoretical limitations of sulfide and phosphate capacities have been extensively reviewed and critically assessed by comparison with the distribution ratios L_i in this contribution by taking two preferred thermodynamic conditions of both basic oxides concentration or slag basicity and slag oxygen potential as criteria. Meanwhile, the merits of two slag capacity concepts as measures for describing slag basicity like optical basicity have also been reviewed to be disruptive because the ratios of activity of free oxygen ion a_O^2 - in slags to activity coefficients f_H,i of sulfide and monomer phosphate PO_4^3 - in slags as implicit parameters in the defined two slag capacity concepts cannot hold constants in most complex slags. Furthermore, the limitations of phosphate capacity in two ways have been restated. Moreover, the oxygen potential p_O_2 shows a great effect on plots of the desulfurization and dephosphorization abilities against two slag capacity concepts of slags at a given temperature. The abnormally higher oxygen potential p_O_2 can result in a suppression of the desulfurization ability of slags with a greater desulfurization potential, while the unreasonably smaller oxygen potential p_O_2 can lead to a diminishing of the dephosphorization ability of slags with a larger dephosphorization potential. It has been verified by five case-studies covering desulfurization and dephosphorization operations that the partitions or distribution ratios L_i related to equilibrium quotients k_i of desulfurization and dephosphorization reactions of Fe-based melts by slags can be in good accordance with two preferred thermodynamic conditions. The reasonable application of two slag capacity concepts is recommended to estimate or determine the distribution ratios between slags and Fe-based melts with the aid of activity a_% ,O of oxygen and activity coefficient f_% ,i of sulfur and phosphorus in Fe-base melts.