The calcium-phosphorus and the simultaneous calcium-oxygen and calcium-sulfur equilibria in liquid iron
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I.
INTRODUCTION
DEPHOSPHORIZATION is one of the more important steps in the pretreatment of hot metal and the production of ferroalloys and stainless steels, and, in the latter case, dephosphorization by conventional oxidation refining is impossible because of the preferred oxidation of the morereactive alloying elements present. Investigations, conducted over the last decade, have shown that effective dephosphorization can be achieved by adding metallic calcium or calcium compounds to the melt 11-61 to produce Ca3P 2 as the product of dephosphorization.[6'7'81 The equilibrium between Ca3P 2 and liquid iron containing Ca and P has not received much attention until recently. [8'~' 10] Study of equilibrium between CaO and liquid iron containing Ca and O and the equilibrium between CaS and liquid iron containing Ca and S is made difficult by the high vapor pressure of Ca and the low solubility of Ca in liquid iron. Equilibrium constants for the reactions CaO[s) = [Ca] + [O]
[11
CaS(s) = [Ca] + IS]
[21
and
taken from the literature, 18'u-19]and listed in Tables I and II, respectively, show a variation of several orders of magnitude. In most of these studies the systems were either open or under argon atmospheres during sampling and the consequent release of calcium could have disturbed the equilibrium. Also, in the treatment of some of the data, the sulfur content of the iron was not taken into account in the consideration of the Ca-O equilibrium or the oxygen content of the iron was not taken into account in the consideration of the Ca-S equilibrium. Furthermore, some of the thermodynamic data for Ca, such as its standard free energy change for solution in liquid iron, may not be accurate.
QIYONG HAN, Professor, X I A O D O N G Z H A N G , Lecturer, and DONG CHEN and PENGFEI WANG, formerly Graduate Students, are with the Beijing University of Iron and Steel Technology, Beijing, China. Manuscript submitted July 10, 1987. METALLURGICAL TRANSACTIONS B
In the present study the equilibrium constant for the reaction Ca3P2(s) = 3[Ca] + 2[P]
[3]
and the interaction coefficient e c" have been determined using a vapor-liquid equilibration technique and the equilibrium constants for the reactions given by Eqs. [1] and [2] and the interaction coefficients e Ca o and e Ca s have been determined by equilibrating liquid iron contained in CaO-CaS crucibles with varying vapor pressures of Ca.
II.
EXPERIMENTAL
A. Study of the Equilibrium Ca~P2(s~ = 3[Ca] + 2[P] Iron containing 0.002 wt pct C, 0.005 wt pct Si, 0.008 wt pct S, 0.005 wt pct P, 0.001 wt pct Mn, 0.02 wt pct Ni, and 0.18 wt pct O was melted and deoxidized to less than 30 ppm O using magnesium vapor in a sealed molybdenum container, and calcium phosphides were prepared from metallic Ca (99.9 pct pure) and P (99.9 pct pure) in the twotemperature zone furnace shown in Figure 1. X-ray diffraction analysis of the reaction product showed the presence of CaP and Ca3P2 (Figure 2). According to the Ca-P phase diagram CaP exists below 1200 ~ TiN crucibles were fabricated
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