Leaching of zinc sulfide concentrate from the ganesh-himal deposit of nepal
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ZINC occurs in the earth crust predominantly as sulfides, and sphalerite is its most important ore. Many investigations have been reported for the beneficiation of zinc ore to prepare concentrate from which zinc metal is produced by pyroor hydrometallurgical processes. These processes involve a roasting step, which evolves toxic SO2 gas and requires a sulfuric acid plant to be set up in the smelter. Direct leaching under pressure has several problems associated with maintenance of autoclave.[1,2,3] Among the alternate processes to treat the sphalerite, the hydrometallurgical route without pretreatment, such as direct oxidative[4] leaching, is considered quite attractive. Several reagents have been used for the leaching of sphalerite concentrate. These are ferric ion in chloride and sulfate media, hydrogen peroxide, manganese dioxide, nitric acid, etc. The dissolution of zinc from sphalerite in acidic ferric solution and in acidic bacterial solution containing iron has been widely studied. Kuzminkh and Yakhontova[5] investigated the dissolution of zinc from its concentrates with ferric sulfate in the temperature range 353 to 373 K. The zinc extraction rate increased with an increase in Fe31 concentration up to three times the stoichiometric requirement. The leaching of zinc from sphalerite with ferric ion in different media[6] and also at different hydrochloric acid concentrations[7] was investigated. Crundwell[8] described the kinetics of dissolution of zinc sulfide by electrochemical mechanisms in which the charge transfer from the solid to the oxidant was rate limiting. Fullam and co-workers[9] reported a fused salt electrolysis process for the treatment of sphalerite concentrate on laboratory scale. The process involves electrolytic decomposition of zinc sulfide dissolved in a molten salt to give molten zinc at the cathode and sulfur at the anode. The S.K. SAHU and K.K. SAHU, Scientists, and B.D. PANDEY, Deputy Director, are with the Metal Extraction and Forming Division, National Metallurgical Laboratory, Jamshedpur – 831 007, India. Contact e-mail: [email protected] Manuscript submitted June 15, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
process suffers from the technical problems such as maintenance of low current densities to prevent formation of chlorides. The leaching of zinc in the presence of chloride and pressure conditions[10,11] and kinetic studies of the ZnSFe(III) system in the absence[12,13] and sometimes in the presence of NaCl[14] have been investigated. Parameters on the extraction of zinc from its concentrates were established in hydrochloric acid,[15] hydrogen peroxide,[16] and ferric chloride.[17] Sharma and co-workers[18,19] studied the preferential attack of chloride in the ferric chloride-ZnS system. The oxidation behavior of sphalerite in the presence of pyrolusite in HCl and H2SO4 has been explained[20,21,22] in terms of formation of two corrosion couples, i.e., ZnS-Fe31 and MnO-Fe21, the redox species being generated continuously in a cyclic manner. Leaching of zinc in ni
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