The effect of Na 2 O on dephosphorization by CaO-based steelmaking slags
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I.
INTRODUCTION
PREVIOUS studies till showed that the Na20-based slags can dephosphorize better than the CaO-based slags. Hot metal t r e a t m e n t p r o c e s s e s [5,6,71 have been developed to remove phosphorus and sulfur simultaneously using Na2CO3. However, these NazCO3 treatment processes have high refractory costs and require extra processing and a low Si content in the hot metal; consequently, they are not widely adopted. However, it has been shown that a small addition of Na20 to the traditional CaO-based slags increases the removal of phosphorus from the hot metaltsl and liquid steelY] Na20 addition lowers the melting points of CaO-based slags, improves the slag fluidity, and possibly increases the phosphate capacity of CaO-based slag. In addition, the activity of Na20 in CaO-based slag is known to be high; t1~ consequently, its effect on dephosphorization may be significant even at low concentrations. For carbon steel production, it is often necessary to improve dephosphorization or hinder rephosphorization in the ladle. The addition of Na20"SiO2 has been used to improve dephosphorization in special ladle treatments t91 using oxidizing slags. Furthermore, if any steelmaking slag is carried over to the ladle, most of the phosphorus in the slag is reduced back into the metal after the oxygen potential is decreased by the deoxidation. If Na20 greatly increases the phosphorus distribution ratio of ladle-type slags this reversion could be decreased. The critical thermodynamic aspects of these processes are the equilibrium phosphorus partition ratios, Lp (or (wt pct P)slag/[Wt pct P]met~), between these slags and metal. The purpose of this work is to measure the equi-
J.J. PAK, formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, is Senior Research Engineer with RIST, Pohang Steel, Pohang, Korea. R.J. FRUEHAN, Professor, is with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted March 30, 1989. METALLURGICALTRANSACTIONSB
librium phosphorus distribution between CaO-Na20-SiOz, CaO-Na20-FeOr-SiO2 (CaO or 2CaO'SiO2 saturated), and CaO-Na20-A1203-SiO2 slags and Fe-Csat-P or Fe-P alloys. The Lp between carbon-saturated Fe-P melts and prefused CaO-Na20-SiO2 slags has been measured at 1400 ~ as a function of slag composition at 1 atm CO pressure. In one set of experiments, Na20 was added to a fixed CaO/SiO2 ratio (40/60: molar ratio). In the other set of experiments, the SiO2 content in slag was fixed at 50 wt pct and the ratio of CaO and Na20 was varied to compare the dephosphorization efficiencies of these two basic oxides. Experiments were then carded out to determine (1) the effect of Na20 addition on steelmaking-type slags (CaO-FeOr-SiO2) saturated with CaO or 2CaO.SiO2 for dephosphorization and (2) the effect of Na20 addition to ladle slag (CaO-A1203SiO2) on phosphorous distribution with the aim of retarding rephosphorization. In the case of CaO-saturated
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