A thermodynamic study of dephosphorization using BaO-BaF 2 , CaO-CaF 2 , and BaO-CaO-CaF 2 systems
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I.
INTRODUCTION
H I G H - C A R B O N alloyed metal can be produced from stainless scrap or from carbon steel and high-carbon ferrochromium in an electric arc furnace. The phosphorus in stainless steel tends to increase through recycling scrap and by the addition of raw materials, such as chromium ore and coke. It is known that the presence of phosphorus is detrimental to the mechanical properties t~j and corrosion resistance t21 of stainless steel. However, it is difficult to reduce the phosphorus levels in a stainless steelmaking operation or subsequent refining processes, because chromium will oxidize in preference to phosphorus when using conventional slags. It is, therefore, necessary to investigate alternative processes for removing the phosphorus in stainless steel without chromium loss. It is theoretically possible to remove phosphorus from carbon-saturated stainless steel if the phosphate capacity is sufficiently high. Generally, highly basic slags have high phosphate capacities. Numerous practical experiments have been done; however, few of them have dealt with the basic thermodynamic properties of slags used for dephosphorization under oxidizing conditions. Tabuchi and Sano t3'41 studied the thermodynamics of phosphate and phosphide in CaO-CaF2 and BaO-BaF2 melts by equilibrating these melts with silver in a graphite boat. They reported that BaO-BaF2 fluxes have a larger phosphate capacity than CaO-CaF2 fluxes. However, this study was only done for a small composition range for BaO C. NASSARALLA, Graduate Student, and R.J. FRUEHAN, Professor, are with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. D.J. MIN, formerly Research Associate, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, is with Research Institute of Science and Technology (RIST), Pohang, Korea. Manuscript submitted July 13, 1990. METALLURGICALTRANSACTIONSB
in BaO-BaF2 melts. Hara and Sano t51 extended this work for higher BaO contents in the melts for 1200 ~ and 1300 ~ by equilibration with Fe-Csat alloys. Fluxes such as BaO-BaF2 are very expensive and, in most' cases, not feasible for use on an industrial scale. Therefore, there is a need for establishing an economical dephosphorization technique for chromium-containing iron. Lime-based slags are normally used in steelmaking but are ineffective for stainless steel. It may be possible that the addition of highly basic oxides to lime fluxes will increase the abilities of the flux to absorb phosphorus without being too expensive. This work examines the ability of the removal of phosphorus for CaO-CaF2 and BaO-BaF2 fluxes at 1400 ~ and the effect of the addition of BaO to lime systems by measuring their phosphorus distribution ratio between the slag and metal and determining the phosphate capacity and activity coefficient of POzs. Also, the activity of BaO in a lime system was measured and used to explain the behavior of these fluxes in the phosphorus removal.
II.
THERMODYNAMIC
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