Thermodynamic behavior of phosphorus in CaO-CaF 2 -SiO 2 and CaO-Na 2 O-SiO 2 systems
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INTRODUCTION
THE hot metal pretreatment process has been developed in order to remove phosphorus and sulfur simultaneously, and many fundamental studies on this process have been conducted. A variety of slags are used for this process, with CaO-based fluxes in especially wide use. The composition of slags after hot metal pretreatment is nearly saturated with CaO and 3CaO. SiO2 in the CaOC a F 2 - S i O 2 temary system. It is important to know the thermodynamic behavior of phosphorus in slags, particularly whether or not the phosphorus is polymerized in slags, because it leads to a different expression of the phosphorus partition between slag and metal, namely, (mass pet P)/[mass pct P[ or (mass pet P)/[mass pct p]2, depending on the species of phosphorus in slags. In the present work, the effect of phosphorus content of CaO-based slags on the partition ratio of phosphorus, together with that of sulfur content, has been investigated using a chemical equilibrium technique.
II.
EXPERIMENTAL
A. Principle of Measurement The molecular formula for dephosphorization of metals is given by Eq. [1], in which n in (P, O25,) denotes the degree of polymerization and K, is the equilibrium constant for Eq. [1]. 5n n[P] + ~ 02 = (PnO2.5n) ap.o2 5.
K, -
.
DSn/4 ap 9xo2
fP.o25." (pct PnOz.5n) f "e" [pct P[ . "Po: 5./4
[1]
[21
ATSUKO TAGAYA, Research Assistant, FUMITAKA TSUKIHASHI, Associate Professor, and NOBUO SANO, Professor, are with Department of Metallurgy, The University of Tokyo, Bunkyo-ku, Tokyo, 113, Japan. HIROYUKI CHIBA, formerly Undergraduate Student, Department of Metallurgy, The University of Tokyo, is with New Materials Research Laboratories, Nissin Steel Co., Ltd., Ichikawa, Chiba. Manuscript submitted September 19, 1990. METALLURGICALTRANSACTIONS B
where a i and f are the activity and activity coefficient of component i relative to 1 mass pct, respectively, pct i is the mass pct of i, and Pe is the partial pressure of i. Equation [2] can be rewritten in a logarithmic form as Eq. [3], where Me is the molecular weight of i. log
n - 1 - -log (mass pct P) [mass pet P] n
(mass pet P)
1 f~ + - log n fPnO2.5n 1 +-log n
[
D5n/4. n" Mp ] K, -o2 Mp.o2 ~,J 9
[3]
When n is equal to 1, Eq. [3] becomes Eq. [4], since the remaining terms of the right-hand side of Eq. [3] are constant at a fixed slag composition and temperature assuming Henrian behavior of the phosphorus both in metal and slag melts. It is reasonable to presume the dilute solution of P, O2.5, even when the phosphorus content of slags is relatively high, because the molar concentration of a species having a large molecular weight is rather small. Similarly, when n is equal to 2, Eq. [3] can be expressed by Eq. [5]: log log
(mass pct P) [mass pct P[
(mass pct P)
= constant
[4]
log (mass pct P) + constant
[5]
[mass pct P[ -
1 2
In accordance with Eqs. [4] and [5], a plot of the partition ratio of phosphorus against the phosphorus content in a slag is to be linear on logarithmic scales.
B. Procedure Two grams of slag and 2 g of Fe-P-Csatd al
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