Sulfide capacity of CaO-CaF 2 -SiO 2 slags
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I.
INTRODUCTION
T H E demand for steels containing very low impurity elements is definitely increasing. One of the processes to produce steel with low sulfur and phosphorus contents is based on the so-called "hot metal pretreatment" which has been adopted by most Japanese integrated steel plants. The purpose of this process is to lower its Si, P, and S contents before charging it to the basic oxygen furnace (BOF). The refining reagent used in the hot metal pretreatment is mainly composed of lime, with a large iron oxide addition when the dephosphorization is the main objective, and some soda addition is made in a special case when higher desulfurization is required. Therefore, a knowledge of the refining capabilities of these slags is of the most importance, not only for selecting the proper refining reagent but also for developing the process and equipment for the treatment. The evaluation of these slags in terms of refining power is simplified if the capacity of slags for impurities is considered. If the problem concerned is desulfurization, it is the sulfide capacity, since this parameter depends only on the composition of the slag and temperature. As far as this concept is concerned, comprehensive reviews on the subject are available in the literature. ]~,2,31 Correlations among capacities of various species in slags have been reported t4] on the basis of experimental measurements. In the present work, the sulfide capacity of CaO-CaF2SiO2 slags saturated with CaO, 3CaO.SiO2, or 2CaO.SiO2 along the liquidus at 1200 ~ 1250 ~ 1300 ~ and 1350 ~ has been determined. The carbonate capacity has also been determined for the same slags, and correlations between these two types of capacities were compared with those between the sulfide capacity and theoretical optical basicity. KATSUJIRO SUSAKI, formerly with the Department of Metallurgy, The University of Tokyo, is with Compania Siderurgica Paulista, COSIPA, Brazil. MASAFUMI MAEDA, Associate Professor, is with the Institute of Industrial Science, The University of Tokyo. NOBUO SANO, Professor, is with the Department of Metallurgy, The University of Tokyo, Tokyo, Japan. Manuscript submitted November 28, 1988. METALLURGICAL TRANSACTIONS B
II.
PRINCIPLE OF THE MEASUREMENT
The sulfide capacity was originally defined by a gasslag reaction (Eq. [1]). It can be expressed in terms of sulfur partition, as Eq. [3], by considering the dissolution of sulfur into a metal (Eq. [2]). 1
1 s2 + o 2- =
o 2 + s 2-
1 2 $2 (g) = [S]
[11
[2]
C 2-s = (pct $2-) 9(poJPs2) '/2 = KI" a o 2 - / f s 2= ((pet S2-)/[pct S]). (Po2) '/2. K 2 / f s
[3]
where [pet M] = (pet M) = fM = Ki =
wt pet of M in metal; wt pet of M in slag; the activity coefficient of M; the equilibrium constant for Reaction [i]; and ao2- = the activity of oxygen ion in slag.
As seen in the second term in Eq. [3], the sulfide capacity depends only on temperature and slag composition. In the present investigation, the sulfide capacity of slags was determined by using metal-slag-gas equilibration. Since Cs 2 does not
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