Calculation of sulfide capacities of multicomponent slags
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I.
INTRODUCTION
FOR many years, attempts have been made to correlate empirically the sulfide capacities of multicomponent oxide melts and various basicity scales, such as the "optical basicity" scale. These attempts have met with only very limited success. More recently, Reddy and Blander 11'21 have calculated the sulfide capacities of binary silicate slags a priori from a knowledge of the oxide activities, using a very simple model for the distribution of sulfide ions in the melt. Excellent results were obtained. A simple extension to ternary silicate slags has also been proposed. 131 In the present article, Reddy and Blander's I~'2j model is modified for the case of acid slags. It is shown how A1203 and TiO2 can be included as components. The model is applied to multicomponent slags, so that the sulfide capacity of a melt of any number of components can be calculated if the oxide activities in certain subsystems are known. Finally, the model is combined with our database for oxide activities in multicomponent slags, thereby permitting the rapid calculation of sulfide capacities. Very good agreement with measured sulfide capacities is obtained for SiO2-AlzO3-CaO-MgO-FeOMnO-TiO: slags. The model has been incorporated into the F*A*C*T I41 thermodynamic computing system. With this database, along with the other databases and Gibbs energy minimization programs of the F*A*C*T system, sulfur contents of slags for various conditions of gas/slag/metal/solid equilibria can be directly calculated. The exchange of oxygen and sulfur between an oxide slag and other phases has generally been written in terms of the following reaction: 1
1
A,O (1) + 2 S2 (g) = A,S (1) + 2 02 (g)
[1]
ARTHUR D. PELTON, Co-Director, and GUNNAR ERIKSSON, Associate Researcher, are with the Centre for Research in Computational Therrnochemistry, Ecole Polytechnique, Montreal, PQ, Canada H3C 3A7. ANTONIO ROMERO-SERRANO, formerly Graduate Student with the Centre for Research in Computational Thermochemistry, Ecole Polytechnique, is now Assistant Professor with the Division de Ingenieria Metalurgica, E.S.I.Q.I.E.-I.P.N., C.P. 07300, Mexico, D.F., Mexico. Manuscript submitted October 5, 1992. METALLURGICAL TRANSACTIONS B
where A,,O and A,,S are components of the slag, A being some particular cation. An equilibrium constant may be written for Reaction [ 1]: 1/2
KA = aA~ (Po2]
= exp (-AG~
[2]
aA,,O \Ps2/
where aA,s and aA,o are activities in the slag and dXG~ is the standard Gibbs energy change of Reaction [1]. If the amount of sulfur in solution is small, then Henry's law applies and aA.S varies directly as the amount of dissolved sulfur. Furthermore, for a given oxide slag composition, aa.o is very nearly equal to its value in the sulfur-free slag. Hence, Fincham and Richardson tSl defined the sulfide capacity of a slag as:
Po~ t/2 Cs = (wt pet S) (~--~s~)
[3]
where (wt pct S) is the weight percent of dissolved sulfur. The sulfide capacity so defined will be constant for a given slag as long as (wt pct S) is relatively low. The higher the su
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