Thermodynamics of sulfur in BaO-CaO-CO 2 system
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I.
INTRODUCTION
THE properties of BaO-containing systems should be investigated because of new demands for iron alloys with low and ultralow impurity content, like phosphorus and sulfur. Studies have been published on the phosphate and sulfide capacity of BaO-BaF2 and BaO-BaC121~-4J and on BaCO3 ability to dephosphorize stainless ts~ and high manganese steel. [6] However, the thermodynamic properties of BaO-BaCO3 containing fluxes are not yet fully understood. Moreover, measurements on the phase diagram of the BaOBaCO3 system indicated that there are eutectic points at a lower temperature ~7] and that the CO2 solubility of the BaO-containing system is much greater than that of the CaO-based system, which further enhances interest in the system's properties. This study was undertaken to obtain the thermodynamic data on sulfur and carbon dioxide in the CaOBaO-CO2 system in the temperature range from 1000 ~ to 1350 ~ and with different slag compositions. The results will contribute to the development of a new approach using refining reagents to produce pure steel and its alloys.
lI. C H A R A C T E R I S T I C S O F BA-CONTAINING MIXTURES The phase diagram of BaO-CO2 measured by Baker 171 shows a minimum eutectic point at 1060 ~ for BaCO3 = 64 mole pct and Pco~ = 0.00667 atm. Watanabe e t a l . tSl indicated that BaO-MnO containing a few mass percent of CO2 melted at 1300 ~ Since both BaO and MnO TOSHIHIKO SAKAI, formerly Contract Research Fellow, Institute of Industrial Science, The University of Tokyo, is Chief Engineer, Metallurgical Development Center, Nippon Mining and Metals Company, Ltd., Hitachi, lbaraki 317, Japan. MASAFUMI MAEDA, Associate Professor of Metal Source Technology, is with the Institute of Industrial Science, The University of Tokyo, Minato, Tokyo 106, Japan. Manuscript submitted March 23, 1992. METALLURGICAL TRANSACTIONS B
are basic oxides, this low melting point of the mixtures is very interesting. I n s i t u observation of sintering BaCO3 in a hot thermocouple apparatus t91 was conducted to examine the characteristics of the mixture. This observation revealed that it melted completely at temperatures from 480 ~ to 700 ~ in an Ar-CO2 atmosphere. In this experiment, 1 mg of sample was set on a thermocouple of Pt/Pt 13 pct Rh with 0.5-mm diameter. Because the BaOcontaining mixture easily corroded Pt wire, the maximum temperature of the measurement was about 700 ~ According to the above findings, it is possible to obtain a very well-homogenized sinter of CaO-BaO-CO2 at a temperature below 700 ~ that would be very fine powder at the higher temperature, enhancing the rate of the desulfurization of molten metals.
III.
EXPERIMENTAL PRINCIPLE
In the present study, the ability of the BaO-containing mixture to hold sulfur was determined by equilibrating sulfur in silver and slag under fixed oxygen partial pressure. The following reactions should be considered. 1/2S2 + 02- = 1/202 + S 2-
[1]
1/2S2 = [S]inAg
[2]
Using Eqs. [1] and [2], the sulfide capacity (Cs 2 ) in terms of the property in th
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