Phosphate capacity of CaO-AI 2 O 3 slags containing CaF 2 , BaO, Li 2 O, or Na 2 O

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I.

INTRODUCTION

P H O S P H O R U S is introduced into steel via ore pellets, coke, and limestone at the blast furnace stage and from scrap at the steelmaking stage. The phosphorus specification for high-strength, low--alloy steel is, in some cases, 0.015 pct P or less. Along with raw material selection, phosphorus can be removed in a hot metal pretreatment, in basic oxygen steelmaking, or during secondary steelmaking. The problem with dephosphorization of hot metal is that silicon oxidizes in preference to phosphorus; thus, phosphorus removal is only possible when silicon content in the hot metal is lower than 0.2 pct. The United States and European steel industries are reluctant to adopt this process, because desiliconization before the basic oxygen steelmaking (BOS) process lowers the hot metal energy content. This prevents the addition of significant amounts of scrap. An alternative to dephosphorization in BOS is to carry out dephosphorization in the ladle; however, this process must be done before the deoxidation of the metal. Slag removal should also be carried out to avoid the reversion of phosphorus to steel from slag. In the case of specialty steel, phosphorus removal is more complicated. For example, reduction of phosphorns levels in a stainless steelmaking operation or subsequent refining process is difficult, because chromium will oxidize in preference to phosphorus when conventional slags are used. It is, therefore, necessary to investigate alternative processes for reducing the phosphorus content of stainless steel without losing chromium. Previous work rl~ by the authors has shown that BaOBaF2 slags have high phosphate capacities but are expensive and not practical. Therefore, flux compositions C. NASSARALLA, formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, is Senior Research Engineer, U.S. Steel Technical Center, Monroeville, PA 15146. R.J. FRUEHAN, Professor and Director of Center for Iron and Steelmaking Research, is with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted July 8, 1991. METALLURGICAL TRANSACTIONS

B

which have a high phosphate capacity and can be economically used to dephosphorize steel should be investigated. Lime-based slags are economically used in steelmaking. However, they exhibit very limited phosphorus removal. It is possible that the addition of highly basic oxides to lime fluxes will increase the ability of the flux to absorb phosphorus without extensively increasing the overall cost. This study examined the binary system CaO-AI203, the ternary system CaO-A1203-CaF2, and the effects of adding basic oxides, such as BaO, LizO, and Na20, to the CaO-A1203-CaF2 system. The ability of fluxes to dephosphorize has been determined by measuring the phosphorus distribution ratio between the flux and carbon-saturated iron (Lp) at equilibrium. The phosphate capacities have been calculated from Lp using available thermodynamic dat

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