The kinetics of dissolution of sphalerite in ferric chloride solution
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I.
INTRODUCTION
L E A C H I N G processes, which are reactions between solids and liquids, assume importance in the solution mining and hydrometallurgical processing of a number of basic metals in use today. Two reasons for increased interest in hydrometallurgical processing are (1) to replace or supplement smelting due to air quality restrictions, (2) to recover metal values from low grade deposits either by dump leaching or solution mining. During the last twenty-five years, attention has been given to the leaching of zinc sulfide ores, including dissolution by aqueous sulfuric acid,~-8 by nitric acid o by chlorine and hydrochloric acid solutions)~0-~4 by ammonia, ~5 and by aqueous solutions of ferric ion. 7 tr-_q Some of these studies and others involve the use of pressures above atmospheric, t,3,8 20 22-24 Current methods of recovery of zinc from zinc sulfide concentrates involve roasting the concentrate to zinc oxide or sulfate, leaching the resultant calcine with dilute sulfuric acid, and electrodepositing of the zinc from the purified leach solution. Although this method has been established for many years, the direct leaching of the zinc sulfide concentrates, therefore eliminating the roasting step, has the following advantages: (1) elemental sulfur is produced rather than sulfur dioxide; (2) high extraction of zinc may be achieved regardless of the iron content of the zinc concentrates. Because zinc commonly occurs as a zinciron sulfide containing up to 20 pct iron, zinc ferrite (ZnO'Fe203) forms during the roasting process. It is insoluble causing low zinc recoveries in the leach-electrolysis process. i Ferric chloride is well established as a leaching agent. It has been used to treat chalcopyrite, 2~'26 stibnite, 27 tetrahedrite, 2* and galena. 19 It has also been used to treat sphalerite.~629 30 These investigations indicate that sphalerite can be dissolved readily under certain conditions. However, few kinetic studies of sphalerite dissolution have been reported. Hence, the objectives of this investigation were to study the important factors involved in the leaching of sphalerite by G. E BOBECK is Associme Professor of Metallurgy. Department of Metallurgical and Mining Engineering. College of Mines and Earth Resources, University of Idaho, Moscow, ID 83843. H SU is with Minerals Management Service, Department of the Interior, Denver. CO 80225 Manuscript submitted November 7. 1983 METALLURGICAL TRANSACTIONS B
ferric chloride such as temperature, ferric ion concentration, particle size, impurities, and stirring rate; and to determine whether the rate-determining step is the diffusion of ferric ion or products through the porous sulfur layer, the chemical reaction at the liquid-solid interface, or a combination of these.
II.
BASIC CONSIDERATIONS
A. T h e r m o d y n a m i c s
When sphalerite is leached in acidic ferric chloride solution, a variety of reaction products can result such as zinc chloride, Zn(II) chloride complexes, elemental sulfur, ferrous chloride, Fe(lI) and Fe(III) complexes, and s
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