Thermodynamics of phosphorus in molten Si-Fe and Si-Mn alloys
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I.
INTRODUCTION
Ferrosilicon and silicomanganese are widely used as deoxidizers in the steel refining process. The phosphorus introduced during the deoxidation process can be hardly removed and is apt to be brought into final products, giving unfavorable effects on steel properties. Accordingly, the dephosphorization of silicon alloys is an important issue for steelmaking. In order to evaluate the dephosphorization process of ferrosilicon and silicomanganese, it is important to know the thermodynamic properties of phosphorus in both alloys. Although thermodynamic properties of phosphorus in Fe-Si alloys and Mn-Si alloys are reported,[1,2] they are restricted only to low silicon content regions. In the present work, the thermodynamic properties of phosphorus in molten Si-Fe and Si-Mn alloys with higher silicon contents up to pure silicon were investigated by equilibrating the alloys in a controlled phosphorus partial pressure. II.
EXPERIMENTAL
activity coefficient of phosphorus in silicon-based alloys relative to an infinitely dilute solution of pure liquid silicon on a mole fraction basis, and PP2 is the partial pressure of phosphorus. Since a graphite crucible was used in the present study, gP in the Si-M-C-P (M: Fe or Mn) system should be expressed as P C ln gP 5 ln g M P 1 ln g P 1 ln g P
where g iP shows the effect of element ‘‘i’’ on the phosphorus activity in molten silicon alloys. Since the solubility of carbon in silicon, which is restricted by the formation of SiC, is 0.0117 mass pct[3] at 1723 K, the effect of carbon content on the phosphorus activity, ln g CP , may be neglected. Miki et al.[4] have reported that the Si-P solution obeys Henry’s law when phosphorus content is lower than 0.1 mass pct. Therefore, the phosphorus content of silicon alloys was controlled not to exceed 0.1 mass pct in the present work, and the self-interaction of phosphorus, ln g PP, is considered to be negligible. Using Eqs. [3] and [4], Eqs. [5] and [6] can be derived by introducing the first- and second-order interacM tion coefficients, ε M P and r P .
A. The Principle of the Measurement Molten Si-Fe and Si-Mn alloys with various compositions were equilibrated in a controlled phosphorus partial pressure. The dissolution of phosphorus into silicon alloys and its Gibbs energy change are expressed as 1 P (g) 5 P(X)in Si 2 2 K5
gP z XP P 1/2 P2
ln K 5 ln gP 1 ln XP 2
[1]
[2] 1 ln PP2 2
M 2 ln K 5 ε M P XM 1 r P X M 1 ln XP 2
ln K 2 ln XP 1
1 ln PP2 2
XM
1 ln PP2 2
M 5 εM P 1 r P XM
[5]
[6]
Measuring XP in silicon alloys and PP2 in the gas phase, the left-hand side of Eq. [6] can be expressed as a function of M XM. Interaction coefficients for each alloy, ε M P and r P , can be determined from the intersection with the ordinate and the slope of a straight line, respectively.
[3]
where K is the equilibrium constant of Eq. [1], gP is the SHIGERU UEDA, Graduate Student, KAZUKI MORITA, Associate Professor, and NOBUO SANO, Professor, are with the Department of Metallurgy, The University of Tokyo, Tokyo 113, Japan. Ma
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