Zinc smelting with coal as the principal heat source and reducing agent
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INTRODUCTION
O V E R the years, there has been continual competition between electrolytic and pyrometallurgical processes in the primary zinc industry.t1,21 Pyrometallurgical reduction of ZnO is more energy efficient. It consumes about 45 G J / t o n of Zn produced compared to about 50 G J / t o n of Zn for electrolytic reductionJ sl The main difficulty with pyrometallurgical production in the 1960s through the 1980s was compliance with environmental regulations on emissions of Cd, Pb, and SO2. At present, more than 80 pct of the primary Zn produced comes from electrolytic plants. Another concern with carbon reduction of ZnO and other metal oxides is the cost and environmental problems associated with the production of coke for blast furnace-type processes. Thus, there is interest in new technology for using coal instead of coke in smelting reactions. For such a process to be attractive, there must be a provision for keeping sulfur in the coal from increasing SO2 emissions and an alternative to fixed bed reactions in which coke provides the needed strength and permeability in the bed. In recent years, there has been an increasing amount of scrap available to the primary Zn industry. This trend has caused a renewed interest in pyrometallurgical processes, because scrap cannot be used in electrolytic processes. A major source of the scrap comes from electric steelmaking plants, where it must be handled as a hazardous waste, t2'41 Efforts are now being made to adapt traditional processes for primary Zn production to reduce Zn compounds in the hazardous waste to Zn vapor, then condense the vapor in a molten Pb splash condenser. Halide and other impurities in the gas phase make the condensation step more difficult. Publications on laboratory-scale reduction of ZnO with carbon and CO are numerous. However, most of these articles were published prior to 1950 and will not be reviewed here. A good example is a paper by Truesdale J.H. SWISHER, Professor, and D. VOHRA, Graduate Student, are with the Department of Mechanical Engineering and Energy Processes, Southern Illinois University at Carbondale, Carbondale, IL 62901. Manuscript submitted May 27, 1992. METALLURGICAL TRANSACTIONS B
and Waring, 151 who measured rates of reduction of ZnO by CO and H 2 as functions of gas flow rate and temperature. Two of the more recent examples are publications of Guger and Manning 161 and Jalan and Rao. t71 The former concluded that rate control of ZnO reduction by CO depends on a diffusion process in series with a surface chemical reaction. Rao and Jalan pelletized ZnO with carbon and concluded that the rate of reduction was controlled by the rate of generation of CO, the active reducing agent. Examination of the potential for applying fluidized bed technology to Zn smelting is just one of a large number of cases where older methods may be or have been replaced by a fluidized bed process. This transition has led to dramatic benefits in capital costs, operating costs, production rates, and emissions levels. Examples of the applicatio
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