Kinetics of the Oxidation of CaS
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2CaO(s) + 2S0 2 (g )
was examined by continuous thermogravimetric analysis. The experiments covered a temperature range of 1673 to 1853 K, Ar-02-S02 mixtures varying from 1 to 40 pet O2 and 0 to 20 pet S02, and gas velocities ranging from 20 to 57 cm/s at the reaction temperature. Analysis of the data shows that the initial rate of the reaction is controlled by the mass transfer of O2 through the gaseous boundary layer of the pellet and is subsequently controlled by the diffusion of O 2 through the porous layer of the reaction products. To verify these results, the permeation rates of S02 through the reaction products were measured at room temperature and compared to values calculated from the thermogravimetric data .
IN steelmaking operations, gases containing sulfur may be evolved when hot cinder from desulfurization operations is exposed to the atmosphere. In many cases, much of the sulfur bearing material in the cinder is CaS. Although a growing concern for the environment has resulted in an increasing interest in determining possible methods for minimizing the contamination of the atmosphere by sulfur bearing gases, little has been done to examine the oxidation kinetics of CaS. Previous investigations have dealt indirectly with the desulfurization of the sulfide, and as a consequence, only sketchy information is avallable.t'" As a result, the work reported in this study was undertaken to examine the factors influencing the oxidation of caS in a carefully defined system, namely, the oxidation of high purity caS in Ar-02 and Ar-02S02 atmospheres. The use of a carefully defined system has permitted a simple analysis of a complex reaction sequence. Dense pellets of the sulfide were reacted in Ar-02S02 mixtures varying from 1 to 100 pet 02 and 0 to 20 pet S02, at temperatures in the range of 1223 to 1853 K, and with gas velocities at the reaction temperature ranging from 20 to 57 cm/s. The oxidation of the sulfide was found to be accompanied by a variety of processes, weight gain, weight loss, and a combination of weight gain-weight loss with oscillatory behavior depending on the range of variables involved. Consequently the results of the investigation will be reported in three papers. In this paper, the first of the three, the kinetics of the weight loss reaction: 2CaS(s) + 30 2 (g ) -
2CaO(s) + 2S0 2 (g )
[1]
is treated. The experimental variables employed in the investigation of the kinetics of this reaction are given in Table I. The reaction of the caS pellets was topochemical; producing a uniform product layer on the outside of the pellet as shown in Fig. 1. The process involved five steps in sequence: D. C. LYNCH, formerly Research Assistant with MIT, is now Assistant Professor of Metallurgy, University of Washington, Department of Mining, Metallurgical and Ceramic Engineering, Seattle, WA 98195. J. F. ELLIOTT is Professor of Metallurgy and Fellow of TMS and ASM, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted February 23, 1978.
1) Transfer of O2 from the flowing gas st
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