Kinetics of the zinc slag-Fuming process: Part III. model predictions and analysis of process kinetics
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I.
INTRODUCTION
THE mathematical model presented in Part III of this paper, based on the kinetic description developed in Part I, 2 was found to give a quantitatively coherent picture of slag fuming. The analysis of industrial data from five different fuming operations yielded a consistent set of kinetic parameters describing coal entrainment in the slag, coal combustion, and oxygen utilization. These results verify that the model is a useful description of the process. However, although the model itself presents a detailed mechanism of process operation, it does not immediately allow identification of the rate-controlling steps. The purpose of this paper is to elucidate these steps and thereby outline strategies for process improvement for both batch and continuous fuming. Before pursuing this, however, it is important to demonstrate the relative insensitivity of the model to various parameters such as slag circulation velocity which can only be estimated with some uncertainty. Only w,hen this condition holds are the model predictions of value. II.
'average' of the different fuming operations analyzed in Part II.1 These process parameters, many of which are taken from the discussion in Part II, ~ are given in Table I. The fuming cycle for the standard case, expressed as zinc, ferric iron, and temperature levels of the slag plotted against time, is shown as solid curves in Figure 1. The zinc elimination curve is seen to be virtually linear although the rate of zinc oxide reduction declines below about 3.5 pct Zn. The ferric iron and temperature curves indicate that within 10 to 15 minutes quasi steady-state conditions are established. The temperature rises initially, during the period in which ferric-iron reduction dominates, but once the ferric iron has reached its quasi steady-state level, zinc-oxide reduction, which is more endothermic, assumes greater significance. The increased thermal load on the furnace then brings about a decline in temperature. Eventually, as low zinc concentrations are approached, the zinc reduction rate begins to fall and consequently the slag temperature starts to rise. Melting of frozen wall slag and continued ferrous iron oxidation then gradually raise the ferric iron level. The overall fuming time from 14 to 2 pct Zn is 93.6 minutes.
STANDARD FUMING CYCLE
In order to discuss model predictions and to assess the sensitivity of these predictions to different parameters, a standard fuming case was selected which represented an G.G. RICHARDS. formerly Graduate Student, now Assistant Professor, and J. K BRIMACOMBE, Stelco Professor of Process Metallurgy. are with The Centre for Metallurgical Process Engineering, Department of Metallurgical Engineering, The Untversity of Brmsh Columbia, Vancouver, BC V6T 1W5, Canada. Manuscript submitted July 11, 1984.
METALLURGICAL TRANSACTIONS B
III.
SENSITIVITY ANALYSIS
The model contains a number of parameters which have been assigned values with varying degrees of uncertainty. These include: [i] slag circulation velocity, v~; [ii] coal particl
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