The thermodynamic behavior of sulfur in BaO-BaF2 slags
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I.
INTRODUCTION
and defined by Eq. [2]
BARIUM-bearing fluxes have recently attracted much attention of metallurgists for their high basicity and low melting temperatures in connection with their application in hot metal pretreatment. In the last several years, much research was carried out on these fluxes in Japan, probably because of the increaseA use of barium-bearing fluxes in steel refining. R was shown by Tabuchi and Sano m and Hara et al. t2] that the BaO-BaF2 system has one of the highest phosphate capacities at 1473 and 1573 K. Iwase et al.]3] measured the phosphate capacity and the activity of P205 in BaO-BaCI2-P205 and discussed the possibilities of their use to control phosphorus in chromium-containing hot metals. Also, some investigations have centered on the carbonate capacity of barium f l u x e s . [4'5'6! To the knowledge of the authors, no information is available on the sulfide capacity of these fluxes. The main objective of this investigation is to determine the sulfide and carbonate capacities of BaO-BaF2 fluxes and to examine the influence of composition, temperature, and partial pressure of oxygen on these parameters. It is also of great interest to check whether the concept of basicity as proposed by Wagner I71 is applicable as a quantitative measure of the basicity of these slags. II.
THEORETICAL ASPECTS
The concept of sulfide capacity was first introduced by Fincham and Richardson t8] considering the gas-slag Reaction [ 1] 1
1
$2 (g) + 0 2. (slag) = 2 O2 (g) + S z- (slag)
[11
1~2-~ , D I / 2 (wt pct ~, J -o:
K1 "ao~-
DI/2 "S2
fs 2
CS2- =
where (wt pct S 2-) ao2fs2Pi
= = = = Ki =
[2]
wt pct S in slag; activity of oxygen ion in slag; activity coefficient of sulfur in slag; the partial pressure of i; and the equilibrium constant of Reaction [i].
It provides a directly measurable value, which is interesting from both practical and theoretical points of view. If we consider the reaction of sulfur dissolution into a liquid metal 1
2 $2 (g) = [S] (metal) the sulfide capacity can be expressed by Eq. [4]: 01/2 "K3 (wt pct S 2-) "-o5
Cs:- =
[wt pct SI "ftsl
METALLURGICAL TRANSACTIONS B
[4]
It is seen from Eq. [4] that the experimentally determined sulfide capacities can be used in practice to calculate the sulfur partition at a certain temperature, provided that thermodynamics of sulfur dissolution into the metal and the partial pressure of oxygen are known. If the sulfide capacity of a certain slag is known, it is possible to calculate the activity of the basic oxide (BaO, CaO) using the data of the solubility of the sulfide, t91 By analogy with sulfide capacity, the carbonate capacity is defined on the basis of Reaction [5]: CO2 (g) + 0 2- (slag) = CO~- (slag)
IVAN P. RACHEV, Graduate Student, is with the Department of Metallurgy, The University of Tokyo, and on leave from the Department of Metallurgy, Higher Institute of Chemical Technology, Sofia, Bulgaria. FUMITAKA TSUKIHASHI, Associate Professor, and NOBUO SANO, Professor, are with the Department of Metallurgy, The Univ
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