Chlorination kinetics of ZnO with Ar-Cl 2 -O 2 gas and the effect of oxychloride formation
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INTRODUCTION
IT is well known that huge amounts of refining dust are generated every year in Japan: approximately 5 Mt from blast furnace and converter and 0.5 Mt from electric arc furnace (EAF).[1] Blast furnace dust and converter dust have been used and recycled as sintering material, refining material, cement material, etc. However, 5 to 30 mass pct of zinc and 3 to 5 mass pct of lead are contained on average in EAF dust,[2] which mainly originates from scraps, especially from galvanized steel sheets for zinc. The amount of heavy metals in EAF dust is expected to increase due to the increase in waste scrap from used cars and household appliances. If these kinds of heavy metals are charged in refining processes, these will circulate and accumulate in the system, which causes increased impurities in products, unstable operations, and also production cost. Therefore, direct recycling of dust containing heavy metals is strictly regulated. These dusts containing heavy metals, especially containing considerable amounts of zinc, have been treated by the electrothermal distillation method, Waelz kiln, or MF process[3] by the reduction/evaporation method to recover zinc and to reduce waste disposal cost because EAF dust is designated as special management industrial waste and the disposal of dusts is expensive. However, recovered zinc is low-quality crude zinc oxide containing chlorine, lead, or cadmium and could not be used as a resource for electrowinning of zinc directly. Recovered zinc oxide is treated as zinc resource in the ISP process, in which maximal amounts of impurities are 1.0 mass pct of Cl, 0.1 mass pct of Cd, and 0.2 mass pct of F.[3] Moreover, some zinc HIROYUKI MATSUURA, Graduate Student, and FUMITAKA TSUKIHASHI, Professor, are with the Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan. Contact e-mail: [email protected] Manuscript submitted August 2, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
contained in EAF dust is difficult to recover because of low-reducibility zinc compound, mainly ZnFe2O4. As a result, about 30 pct of EAF dusts have been landfilled annually in Japan,[3] and the amount of dumped zinc is expected to be 30,000 t,[2] which is large compared with the annual zinc production of about 600,000 t in Japan. Thus, development of an efficient zinc recovery process is an urgent subject. Recently, a new recovery process for heavy metals has been proposed in which heavy metals in the dust react with waste polyvinyl chloride to form heavy metal chlorides.[4–7] Steelmaking dusts mainly contain iron, lead, and zinc oxide. The stable phase of iron is oxide and those of zinc and lead are chloride at high partial pressures of oxygen and chlorine. These heavy metal chlorides show high vapor pressures at low temperature; for example, the boiling points of zinc and lead chlorides are 1004 K and 1223 K, respectively.[8] Therefore, heavy metals may be expected to be selectively chlorinated and re
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