Continuous Fuming of Zinc-Bearing Residues: Part I. Model Development
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INTRODUCTION
THE disposal of zinc containing wastes is an important sustainability issue facing the metallurgical and wider community because these materials may pose potential environmental problems. Major sources of these materials at present are zinc leach residues and electric arc furnace dusts, the latter originating during steel recycling. The development of technologies that can provide technical and economic solutions to the processing of these materials remains an ongoing problem. One of the technologies currently used to process Zncontaining wastes is zinc slag fuming. Conventional zinc-fuming processes are batch operations involving the injection of fine, pulverized coal, air, and the introduction of Zn-containing wastes into molten slags. The Zncontaining wastes dissolve in the molten slag bath, and the oxides of zinc, lead, and other elements are removed from the slag bath by the reduction and generation of volatile metal species.[1,2] Supplementary (tertiary) air is introduced above the slag bath, oxidizing the gaseous zinc to fine particulate solid ZnO. Supplementary (tertiary) air is essential for the better utilization of fuel. The coal acts as both a heat source, through combustion KAREL VERSCHEURE, Research assistant, BART BLANPAIN, and PATRICK WOLLANTS, Full Professors, are with the Department Metaalkunde en Toegepaste Materiaalkunde (MTM), Katholieke Universiteit Leuven, B-3001 Heverlee, Belgium. Contact e-mail: [email protected]. MAURITS VAN CAMP, Senior Metallurgist, is with Umicore Research, B-2250 Olen, Belgium. PETER HAYES, Professor, and E. JAK, Associate Professor, are with the Pyrometallurgy Research Centre, PYROSEARCH School of Engineering, The University of Queensland, Brisbane, Qld 4072, Australia. Manuscript submitted July 25, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS B
with the primary and secondary air, and as a reductant through the production of CO gas. This CO is then available to reduce metal oxides dissolved in the slag phase. This process has been used to remove zinc from lead blast furnace slags. The chemistries of these slags are described by the simplified system with CaO/SiO2 CaO-FeO-Fe2O3-PbO-SiO2-ZnO between 0.7 and 1.2.[3] The initial zinc oxide concentrations in these slags are in the range 15 to 20 wt pct ZnO, and the liquidus temperatures are in the range 1150 C to 1250 C. The rates of reduction of these slags progressively decrease with decreasing zinc oxide concentration in the slag. Zinc-fuming processes may potentially provide technical and economical solutions to Zn-containing waste treatment if these processes can be operated in a continuous mode. Continuous operation of zinc-fuming processes requires high fuming rates down to low zinc concentrations in the slag. It is now clear as a result of extensive industrial experience and a number of studies[3,4,5] that high process temperatures (about 1350 C), low oxygen partial pressures, and good mixing of the molten bath are essential to obtain high fuming rates down to low Zn concentrations i
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