Kinetics of the reaction between hydrogen sulfide and lime particles
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INTRODUCTION
I N recent years, much attention has been directed to the hydrogen reduction of metal sulfides in the presence of lime as an alternative to conventional smelting. ~-7 The overall reaction in this process consists of the following successive reactions: Me~S + H2 = xMe + H2S
[1]
CaO + H2S = CaS + H 2 0
[2]
The role of lime in this process is two-fold: It removes H2S, thus improving the thermodynamics of the first reaction, which has a small equilibrium constant, and it prevents the sulfur-containing gas from escaping with the effluent gas. Therefore, the reaction between hydrogen sulfide and lime is an important part of the overall process. Another example of important industrial process in which this reaction is encountered is the desulfurization of fossil fuels. Calcium oxide is also widely used to remove sulfur dioxide in combustion processes. While a substantial amount of work has been reported on the sulfation of lime, there is little information available on the kinetics of the reaction between hydrogen sulfide and lime. Gavrilova and Al'tshuler s studied the reaction of calcium oxide with a gas mixture of nitrogen containing 0,2 to 0.45 pct hydrogen sulfide over the temperature range 773 to 1273 K, They found that the reaction is diffusion controlled with an apparent activation energy of 13.6 to 18.8 kJ/mol. The reaction rate was retarded by steam addition, and the dissociation of hydrogen sulfide was effectively suppressed by adding hydrogen in the reacting gas. Investigations by Vestal and Johnston 9 and Pell and co-workers ~~revealed that the scavenging reaction of hydrogen sulfide by calcined dolomite is first order with respect to unreacted calcium oxide, but they reported different activation energies of 159.0 and 96.2 kJ/mol, respectively. Pell and co-workers further suggested that the reaction took place between the gas and the S. WON, formerly Graduate Student in the Department of Metallurgy and Metallurgical Engmeenng, Umversity of Utah, xs now Semor Metallurgist with Cyprus Metallurgical Process Corporation, 800 East Plma Mine Road, Tucson, AZ 85725. H.Y. SOHN is Professor xn the Department of Metallurgy and Metallurgical Engineering, Umvers~tyof Utah. Salt Lake Cxty, UT 84112-1183. Manuscript submitted August 17. 1983. METALLURGICALTRANSACTIONS B
calcium oxide part of the dolomite, i.e., magnesium oxide had little reactivity. The kinetics of this reaction was described by an adsorption model. Habashi and Mikhail ~ reported that the reaction between calcium oxide and hydrogen sulfide proceeds in accordance with Jander's parabolic rate equation and has an activation energy of 66.9 kJ/mol between 743 and 1073 K. In a recent study of Westmoreland and co-workers ~2on the reactions between hydrogen sulfide and selected metal oxides, e.g., MnO, ZnO, CaO, and V203, it was suggested that all reactions were first order with respect to gas concentration and that the activation energy for the reaction of hydrogen sulfide with calcium oxide was 21.8 kJ/mol, which was obtained from initial
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