Rate control of the flash reduction of zinc calcines
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I.
INTRODUCTION
A process concept for producing zinc pyrometallurgically[1] consists of three stages (Figure 1): (1) dead roasting of zinc concentrates in a conventional fluid bed roaster; (2) reduction and smelting of zinc calcines with carbonaceous material and oxygen in a flash reactor; and (3) condensation of zinc vapor from the process off-gas in an appropriate condenser. The subject of this work was to determine the controlling rate phenomena in the flash reaction shaft of an Outokumpu-type furnace.[2] The material used in this study was Outokumpu zinc calcine containing 57.9 pct ZnO, 22.2 pct ZnFe2O4, 8.0 pct ZnSO4, 2.6 pct ZnSiO2, and 2.5 pct PbSO4. Its specific density was 5.13 g/cm3 and its melting point was 1490 7C. II.
REDUCTION OF SINGLE PELLETS
This work was initiated by a study reported earlier[3] of the rates of reduction of zinc calcine single pellets. Additional experiments were carried out after the previous study in which pellets of pure zinc oxide were reduced at 1100 7C and 1200 7C, under the same conditions as the zinc calcine pellets. These were made by pelletizing wet zinc oxide and then sintering the pellets at 1100 7C to nearly 0 pct porosity. In contrast to the zinc calcine which could be briquetted under pressure, this was the only practical method to produce pure zinc oxide pellets. The rates of reduction of these ZnO pellets were in close agreement with the earlier results by Truesdale and Waring,[4] Guger and Manning,[5] and Bjo¨rling[6] (Figure 2) and confirmed the validity of the data on zinc calcine reduction. The ini-
E.M. WEENINK, Metallurgical Engineer, is with the Minerals Laboratory, BHP Minerals, Reno, NV 89502. N.J. THEMELIS, StanleyThompson Professor of Chemical Metallurgy, is with the Henry Krumb School of Mines, Columbia University, New York, NY 10027. Manuscript submitted July 9, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
tial rate of reduction was determined from the experimental results, correlating the initial linear weight loss vs time as recorded per computer. The carbon monoxide flux (NCO) was thus calculated using the external surface area for each pellet at the start. The NCO for previous authors was calculated from the mass transfer control correlations and initial pellet surface area in pure carbon monoxide atmosphere. As shown in Figure 2, the rates of reduction of zinc calcine pellets at temperatures above 1100 7C decrease appreciably in comparison to the pure zinc oxide. The lower rates of reduction of zinc calcines at temperatures over 1100 7C were attributed to the relatively large amount of iron oxide in the zinc calcine. Cross-sectional cuts of some pellets showed a distinct reaction front: the unreduced core was surrounded by a very thin layer, which in turn was surrounded by the product layer. Scanning electron microscopic analysis of these cross sections[7] showed that both core and shell contained zincite and franklinite (ZnOzFe2O3). However, no iron oxide was found in the core. Initially, the main components in the particle are zincite
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