The effect of sulfur on the gaseous reduction of solid calciowustites
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I.
INTRODUCTION
D U R I N G a series of investigations on the reduction of calciowustites (Fe, Ca)O in CO/CO2 mixtures it was found that significant differences in product microstructure were obtained when experiments were carried out using different sets of apparatus of the same design. For example, the same initial material reduced in nominally the same gas composition, in the one apparatus produced samples having a porous iron morphology (type A).t Using another apparatus the final product consisted of an unusually open iron structure in which the iron appeared as whiskers or needles] The effect of the whisker morphology was to produce catastrophic swelling of the samples as reduction proceeded. On inspection of the reduction apparatus in which catastrophic swelling was obtained, it was discovered that the gas stream in the furnace was contaminated with sulfur. The sulfur emanated from vacuum grease derived from animal fats and from rubber stoppers which had inadvertently come in contact with sulfuric acid used in the gas train. On removing the sources of sulfur contamination from the system, catastrophic swelling of the samples on reduction was completely eliminated and porous iron product morphologies were obtained. These results indicate that sulfur has a significant effect on the morphology of the iron produced on reduction. Previous workers 3 have also reported that sulfur can promote whisker growth on iron oxide reduction. Since sulfur is present in all commercial reduction operations which use coke, coal, oil, or natural gas as reductants, this phenomenon is of obvious importance and worthy of further investigation.
II.
EXPERIMENTAL TECHNIQUES
The materials, their preparation, and the experimental techniques used in this investigation have been adequately described in previous publications.*'4 From the equilibrium constant for the reaction H2 (g) + 89$2 (g) ~ H2S(g)
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D.H. St. JOHN, formerly Postdoctoral Fellow, University of Queensland, is now Lecturer, Department of Metallurgy and Mining, RMIT, Melbourne, Austraha. F. NAKIBOGLU, formerly Graduate Student, Department of Mining and Metallurgical Engineering, University of Queensland, Brisbane, Australia, is Postdoctoral Fellow, Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB, Canada. P.C. HAYES is Senior Lecturer in Metallurgy, University of Queensland, Brisbane, Austraha. Manuscript submitted November 13, 1984. METALLURGICALTRANSACTIONSB
i.e., Kt = PH,_s/PH2" 1/P~, it can be seen that the sulfur activity at any temperature is determined by the H2S/H2 ratio in the gas. By using a cylinder of p r e m i x e d 0.486 pct H:S/H2 gas (nominally 0.5 pct H2S/H2), close control over the H2S/H2 ratio in the reaction gas was possible even at low H2S/H2 ratios. The limits of solubility of sulfur in iron were calculated from data given by Rosenqvist and Dunicz. s At 1273 K this gas mixture is just insufficient to result in the formation of iron sulfide. III.
RESULTS
A. Supply of Sulfur to the Sample In a series of exper
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