Effect of ZrO₂ addition to the CaO-SiO₂-MgO-CaF ₂ slags on the sulfur removal from the 16Cr-14Ni stainless steel melts

The equilibrium distribution ratio of sulfur between the 16 mass%Cr-14 mass%Ni stainless steel melts and the CaO-SiO₂-MgO-ZrO₂7 mass%CaF₂ (B(= (mass%CaO)/(mass%SiO₂) = 2.0) slags was measured at 1873 K to clarify the role of ZrO₂ in the desulfurization reaction based on the sulfide capacity concept. The sulfur distribution ratio linearly decreases on increasing the oxygen partial pressure with the slope close to -1/2 in the logarithmic scale at a fixed sulfur potential and temperature. The sulfide capacity of the slags is constant up to about (mass%ZrO₂)/{(mass%ZrO₂) + (mass%MgO)} = 0.3, followed by a linear decrease on increasing the Z/(Z + M) ratio. The effect of ZrO ₂ substitution for MgO on the decrease in the (apparent) sulfide capacity at Z/(Z + M) > 0.3 could be explained by the decrease in the basicity of the liquid slag phase due to the formation of CaZrOj from the computational estimations for the phase equilibria and the thermodynamic activities of slag components. Based on the X-ray diffraction patterns of the slags, it was confirmed that the peak intensity for the Ca₂SiO ₄ is strong through the entire composition range, while the intensity of the peaks corresponding to the CaZrO₃ is relatively strong in the composition of Z/(Z + M) = 0.42 and 0.64. The sulfide capacity can be expressed as a linear function of the mass fraction of CaZrO₃ in the slags investigated in the present study, while that exhibits a wide distribution at a relatively narrow optical basicity region, resulting from the effect of ZrO ₂ on the basicity of the liquid slag phase due to the formation of CaZrO₃.