The selective oxidation of mixed metal sulfides with lime in the presence of steam without emitting sulfur-containing po
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I. I N T R O D U C T I O N
IN recent years, it has become necessary to treat complex sulfide minerals from scattered and small deposits of low grade ores. Conventional pyrometallurgical methods are not suitable for the treatment of these ores, mainly because of the complex processes required. Air pollution due to the sulfur bearing gases produced is also a serious problem in processing sulfide ores. Recently, the lime-steam oxidation process has been invented and studied by Sohn and Kim t~,2J for single metal sulfides. In this process, metal sulfides are reacted with lime with an initial charge of steam. The possibility of emitting hydrogen sulfide to the atmosphere can be eliminated by the formation of solid calcium sulfide, and the metal sulfides are transformed to metal oxides. The emphasis of this present study is on mixed sulfides. Only certain metal sulfides are thermodynamically amenable to this treatment; thus, selective oxidation for mixed sulfides may be achieved. The oxidation reactions of mixed metal sulfides can be represented by the following general scheme: MeiS + H20 = MeiO + H2S
[1]
H2S q- CaO -- CaS + H 2 0
[2]
MeiS + CaO = MeiO + CaS
[3]
Two combinations of mixed metal sulfides with lime were studied: ZnS-PbS and ZnS-FeS. This present study also extends the previous work by Sohn and Kim t2~ concerning the analysis of the overall rate equation to mixed sulfide systems. Using the individual kinetics data, the conversion in the mixed sulfide system can be predicted: S. SOEPRIYANTO, Graduate Student, F.A. OLSON, Professor, and H.Y. SOHN, Professor, are with the Department of Metallurgy and Metallurgical Engineering, University of Utah, Salt Lake City, LIT 84112-1183. This paper is based on a presentation made in the T.B. King Memorial Symposium on "Physical Chemistry in Metals Processing" presented at the Annual Meeting of The Metallurgical Society, Denver, CO, February, 1987, under the auspices of the Physical Chemistry Committee and the PTD/ISS. METALLURGICAL TRANSACTIONS B
II. T H E R M O D Y N A M I C A N D KINETIC CONSIDERATIONS
A. Thermodynamics The standard free energy and enthalpy changes for reactions of the various metal sulfides studied are given in Table I. As shown in this table, the reaction between metal sulfide and steam is generally unfavorable due to the positive free energy change of the reaction. However, the free energy change of the reaction between lime and hydrogen sulfide gas is sufficiently negative to make that of the overall reaction (Eq. [3]) negative for ZnS and FeS so that the reactions may proceed, except for the one involving lead sulfide. This would provide selectivity. The overall reactions, shown in Table I, in general have small endothermic enthalpy changes.
B. Kinetics The formulation of the following kinetics expressions is based on the previous work of Sohn and Szekely [SJ and extends the previous work by Sohn and Kim. [2] In obtaining the intrinsic kinetics data, it is important that the reaction system being studied is controlled only by chemical ki
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