Oxidation kinetics of zinc vapor in CO:CO 2 mixtures: Part II. Application of plug flow concepts
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I.
INTRODUCTION
THE motives for a new investigation of Zn vapor oxidation kinetics, as outlined in the preceding article,t~] include the following. A lack of consensus among previous investigators as to the oxidation mechanism. This is in spite of numerous common observations relating to ZnO deposition rates and morphologies. The emergence of various new technologies for the recovery of metallic zinc from metallurgical wastes. These processes generally operate with higher zinc partial pressures than those employed in earlier studies of the oxidation kinetics. It was also hoped that by operating with a wider range of gas compositions than those available to Fray and co-workers,t~s] a clearer indication of the potential rate controlling mechanism might emerge. The results obtained substantiate many of the qualitative observations reported by previous investigators. They have not, however, confirmed the rate controlling steps implicit in, or proposed to explain, the empirical rate expressions employed. Since gas mass transport and reaction with molecular oxygen can be eliminated as rate controlling steps, it seems likely that the kinetics of zinc vapor oxidation in CO:CO2 atmospheres is more complex than can be explained by consideration of the reaction Znc~ + C02(g) ~ ZnOc,) + COc~ alone.
L.A. LEWIS, Postdoctoral Research Associate, and A.M. CAMERON, Lecturer, Chemical and Process Metallurgy, are with the Manchester Materials Science Center, University of Manchester Institute of Science and Technology, Manchester, United Kingdom M1 7H5. Manuscript submitted January 7, 1994. METALLURGICALAND MATERIALSTRANSACTIONS B
II.
EXPERIMENTAL
Experimental procedures and the benefits of our modified flow reactor, relative to the original design used by Fray et al., were described in the preceding article, m In this second article, we describe the development of a computer model of the reaction system. The model employs "plug flow" concepts to determine specific rate constants for three competitive reactions which we propose to explain the gas concentrations derived from the experimental mass balances. A personal computer is used to numerically integrate the performance equations of the plug flow reactor. In effect, the computer program systematically varies the specific rate constants of the three reactions proposed to occur and calculates the concentration of the gaseous reactants Zn, CO, and CO2 as a function of distance along the condenser tube. In this manner, the combination of rate constants which most accurately reproduce the results of each individual experiment has been determined. By repeating this procedure for each of the temperatures employed, activation energies for the individual reactions have been estimated. III.
DISCUSSION
A. Plug Flow Theory Our observations have led us to conclude that zinc oxidation in Zn/CO/CO2 mixtures can only be understood by proposing the existence of three competitive consecutive reactions: Zn(g) + C02(g) ~ ZnO(,) + CO(g)
[1]
Zn(g) + CO~) ~ ZnOo) + C(~)
[2]
C(~) + C02(g) ~ 2
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