Analysis of the oxidation reactions of CaS
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THE thermochemical behavior of the Ca-SrO system at high temperatures has been of interest to the steel and cement industry for many years. As a result, studies examining the molecular compounds formed in the Ca-S-O system and the physical and thermodynamic properties of such compounds have been conducted since the early 1900s. Zawadski et al, 1 and Curtis 2 reported in early investigations the existence of the compounds 2CaO 9 SO3 and 3CaO 9 2CaSO 4. These compounds were not, however, observed by others, who fou_nd at temperatures between 1173 and 1873 K only CaO(s), CaS(s), flCaSO4(s), otCaSO4(s), and CaSO4(/). Moreover, Schwitzgebel and Lowell,3 utilizing thermodynamic data, demonstrated that CaSO 3 readily decomposes to form CaO and SO2 at atmospheric pressure and temperatures greater than 1273 K, while at temperatures below 1273 K the sulphite disproportionates, forming C a S O 4 and CaS. Newman, 4 using differential thermal analysis, found the/3 to a transition of C a S O 4 t o occur at approximately 1487 K. His results were confirmed by Dewing and Richardson, 5 and Gutt and Smith. 6 Gutt and Smith also investigated the melting point of pure CaSO4 using differential thermal analysis. By heating a CaSO4 sample in an evacuated and sealed platinum capsule, they found melting to occur at 1768 K. Grievson and Turkdogan, 7 however, found the melting point to be 1738 • 10 K. Grievson and Turkdogan arrived at the lower value by melting samples of C a S O 4 in a platinum crucible contained in an evacuated and sealed silica capsule, and it is possible that CaO as an impurity may have lowered the melting temperature thus observed. Newman found that
D. C. LYNCH is Assistant Professor of Metallurgy, Department of Mining, Metallurgical and Ceramic Engineering, University of Washington, Seattle, WA 98195. J. F. ELLIOTT is Professor of Metallurgy, Massachusetts Institute of Technology, Room 4-138, Cambridge, MA 01239. Manuscript submitted December 10, 1979.
and CaO form a low-temperature eutectic at 1638 K. The most recent studies of the CaO-CaSO4, CaSCaSO4, and CaS-CaO equilibria are summarized in Table I, where the chemical reactions involved, the experimental techniques employed, and the experimental results are listed. The work of Turkdogan et al 8 for the temperature range 1273 to 1573 K is the most comprehensive investigation to date of the CaOC a S O 4 and CaS-CaO equilibria. The results of this study are in excellent agreement with the other investigations listed in Table I except for the CaO-CaSO 4 equilibria at 1573 K. The results obtained by Turkdogan et al indicate that at this temperature the experimentally determined equilibrium constant for the reaction CaSO 4
CaO(s) + SO2(g) + CO2(g) = aCaSO4(s) + CO(g)
[1] was larger by a factor of 1.6 than that estimated by extrapolation of the data obtained at lower temperatures. The investigators have postulated that the above observation suggests that the CaO is soluble in aCaSO4, so much so that the activity of aCaSO4 is thereby lowered to 0.63. That hypothesis coul
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