Thermodynamics of Na 2 O in the molten CaO-CaF 2 -SiO 2 system
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log "}/Na20:
18,600 (+ 1100) --
T
+ 7.23 (---0.08)
It was found that Henry's law holds up to 5 mass pct of Na20 in the flux at 1473 K. The activity of a small amount of NazO in the CaO-CaF2-SiO2 flux is discussed in comparison with that for the Na20-SiO2 system in terms of their refining characteristics.
I.
INTRODUCTION
A small addition of basic oxide such as BaO and Na20 to the CaO bearing fluxes, which are widely in use for hot metal treatment, significantly increases their dephosphorization ability, tl-9J Muraki et al. 12j determined the phosphate capacity of the CaO-CaF2-SiO2 fluxes at hot metal temperatures and found that an addition of as small as 2.5 mass pet Na20 to the CaO-CaF2-SiO2 system enhances the phosphate capacity approximately ten times. The effect of a BaO addition to CaO-CaF2-SiO2 fluxes on the phosphate capacity of the same flux has been also measured by the present authors, t31 A 26 mass pct addition of BaO increases the phosphate capacity of the flux by about six times. Furthermore, it was shown by the authors that the yield of manganese increases and the recovery of niobium, vanadium, and titanium from hot metal to slag is improved by a small addition of Na20 to CaO-CaF2-SiO2 fluxes, m Since the thermodynamic properties such as the activity of Na20 in the CaO-CaF2-SiO2 system are not known, the activity coefficient of Na20 in the CaO-CaF2-SiO 2 flux doubly saturated with CaO and 3CaO" SiO2 has been measured at temperatures of 1423 to 1623 K by using a chemical equilibration technique.
II.
EXPERIMENTAL
In the present study, 20 grams of Pb-Na alloy were equilibrated in a CO2-CO atmosphere at temperatures of 1423 to 1623 K with 13 grams of CaO-NazO-CaF2-SiO2 fluxes doubly saturated with CaO and 3CaO. SiOz. A CaO crucible manufactured by Yoshizawa Lime Industry Co., Ltd. (Ginza, Chuo-ku, Tokyo, Japan) was used. The reaction for the present equilibration experiments and its equilibrium constant are expressed by Eqs. [1] and [21: HISAO K/MURA, Research Assistant, FUMITAKA TSUKIHASHI, Associate Professor, and NOBUO SANO, Professor, are with the Department of Metallurgy, The University of Tokyo, Tokyo 113, Japan. Manuscript submitted May 23, 1994. METALLURGICALAND MATERIALSTRANSACTIONSB
2Na(l, in Pb) + C O 2 (g) = Na20 (1) + CO (g) AG~ = -92,900 + 22.2 T K-
J/mol ~176
[1] [2]
aNa20 " e c o a2a "eco2 ('YNa20 " XNa20) "Pco
[3]
(~/Na in Pb" XNa in Pb)2" eco2
where ai and Ti are the activity and activity coefficient of component i relative to pure substance, respectively. The term Xi is mole fraction of component i, and Pi is partial pressure of i. In the present study the Pco2/Pco ratio was kept constant. Therefore, according to Eq. [3], the activity coefficient of Na20 can be calculated by measuring Na contents of the Pb-Na alloy and fluxes. The activity coefficients of Na in a Pb-Na alloy are reported as 0.315 and 0.582 at 1473 and 1573 K, respectively, by the authors, tnl The activity coefficients at 1423, 1523, and 1623 K were estimated as 0.224, 0.432, and 0.769 at 1423, 1523, and
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