Carbothermic reduction of zinc sulfide in the presence of calcium carbonate
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UCTION
THE carbothermic reduction of zinc sulfide in the presence of calcium carbonate is an important reaction in two processes: pyrometallurgical extraction of zinc from zinc sphalerite[1] and zinc recovery from spent zinc oxide catalyst as generated from a steam reforming process.[2] In this reaction, carbon acts as a reductant to reduce zinc sulfide and calcium carbonate as a scavenging agent to fix the sulfur content in zinc sulfide to prevent the emission of sulfur dioxide. Out of seven publications that refer to the carbothermic reduction of zinc sulfide, five employed calcium oxide as the scavenging agent[1,3–6] and three reports used calcium carbonate.[3,7,8] Skopov et al.[3] mentioned that carbothermic reduction occurred after prior decomposition of CaCO3. Employing calcium carbonate as the scavenging agent, Zhang et al.[7] reported that the rate of reduction was higher than that employing calcium oxide and the fixation of sulfur during reduction was excellent. Subsequently, Zhang et al.[8] proposed a reaction mechanism and suggested a rate-controlling step to interpret this reaction. Other information on this reaction is lacking. The objectives of this study were to examine the variations of composition and physical properties of solid sample during reaction and to develop a reaction mechanism and model to interpret the carbothermic reduction of zinc sulfide in the presence of calcium carbonate. To achieve these two objectives, an X-ray diffractometer (XRD), a carbon and sulfur determinator, a scanning electron microscope (SEM) with energy-dispersive X-ray analysis CHE-MING WU, Graduate Student, and CHUN-I LIN, Professor, are with the Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, R.O.C. Taiwan 106. Contact e-mail: [email protected] HSI-KUEI CHEN, Professor, is with the Department of Biochemical Engineering, Hwa-Hsia College of Technology and Commerce, Taipei, R.O.C. Taiwan 235. Manuscript submitted August 31, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
(EDX), and a surface area analyzer were employed in this work. II.
EXPERIMENTAL
A. Materials Argon (Yuang-Ron Gas Co., Taipei, Taiwan) with a minimum purity of 99.9995 pct was employed. Reagent-grade zinc sulfide from Acros Organics (Geel, Belgium) with a minimum purity of 99.99 pct and reagent grade carbon black from Strem Chem. Co. (Newburyport, MA) with a minimum purity of 99.99 pct were used. Calcium carbonate (reagent grade, minimum purity of 99.5 pct) was supplied by Wako Pure Chem. Industries (Osaka, Japan). B. Sample Preparation Zinc sulfide with the original size of average diameter 5.98 3 10"6 m, as determined by a laser diffraction particle size analyzer (LS-230, Beckman Coulter, Miami, FL), was employed; while carbon black and calcium carbonate were screened through a set of Tyler standard screens to obtain the desired sizes. All of these materials were dried at 378 K for 43,200 seconds (12 hours), separately. Powders of zinc sulfide, carbon black, and calcium carbonate of known sizes with
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