Thermodynamics of Ca-CaF 2 and Ca-CaCI 2 systems for the dephosphorization of steel
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I.
INTRODUCTION
THEproperties of alloy and stainless steels change markedly at low phosphorus content, tl'Ej For instance, ultra-low phosphorus stainless steel with phosphorus contents of less than 0.005 pct has significantly improved corrosion resistance.[3] Dephosphorization of chromium steel, however, is very difficult to achieve by the conventional oxidizing refining processes because chromium is a less noble element than phosphorus and consequently may be oxidized before phosphorus. Recently, a number of laboratory and plant studies have been conducted to remove phosphorus as a phosphate ion with alkaline earth metal oxides or alkali metal carbonates such as BaO, t4] LiECO3,15] and Na2CO3 .[6] There also have been many trials of the dephosphorization under reducing conditions of stainless steel in which phosphorus is removed as a phosphide ion using Ca-CaF2, tT'sJ Caf2-faF2,19] CaC2,11~ and Ca-CaC12 .till In spite of numerous practical experiments, few papers have reported the thermodynamic properties of slags used for dephosphorization under reducing conditions. Tabuchi et al. t~Ej measured the activity coefficient of Cal.sPO 4 and Cal.5P in CaO-CaF2 melts in the presence of carbon. Krhler e t a / . Hal also measured the equilibrium phosphorus distribution ratio between Ca-Ca halide slags saturated with CaO and liquid iron in a high pressure induction furnace. There has not, however, been any systematic research on the thermochemical properties of phosphorus in Ca-Ca halide slags containing relatively large amounts of calcium. The objective of this study is to measure the activity coefficients of calcium and calcium phosphide in Ca-Ca halide slags. In order to control the calcium and phosphorus potentials, the slag was equilibrated with both a Ag-Ca alloy N. MASUMITSU, formerly a Graduate Student at Carnegie Mellon University, is with Nippon Steel Corporation, Japan. K. ITO, Research Associate, and R.J. FRUEHAN, Professor, are with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellc,a University, Pittsburgh, PA 15213. Manuscript submitted December 4, 1987.
METALLURGICALTRANSACTIONSB
and pure solid iron. The equilibrium phosphorus distribution ratios between these slags and molten iron-chromium alloys were calculated using the activity coefficient of calcium phosphide and available thermodynamic data.
II.
EXPERIMENTAL
A. Experimental Principle
Since calcium has a high vapor pressure at elevated temperatures (0.42 atm at 1400 ~ and 1.41 atm at 1600 ~ for pure Ca), it is difficult to keep a constant calcium concentration in the slag during an experiment. Therefore, a Ag-Ca alloy was used as a calcium reservoir to maintain a constant calcium activity. The reaction in this experiment can be expressed as reaction [1], and the equilibrium constant by Eq. [2]. 1p Ca(l) + ~- 2(g) = Cal.sP(s)
gl =
aca~p
3/2~ 1/2 aca/"P2
[1] [2]
where ac,l 5P and ac, are the activities of calcium phosphide and calcium in the slag phase or flux and PP2 is the partial pressure of phosphorus. Anot
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