Kinetic Model for the Uniform Conversion of Self Reducing Iron Oxide and Carbon Briquettes
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ISIJ International, Vol. 43 (2003), No. 8, pp. 1136–1142 Kinetic Model for the Uniform Conversion of Self Reducing Iron Oxide and Carbon Briquettes Jeremy MOON and Veena SAHAJWALLA School of Materials Science and Engineering, University of New South Wales, Sydney 2052 NSW, Australia. (Received on September 2, 2002; accepted in final form on February 5, 2003 ) A kinetic model has been developed to describe the uniform conversion of a self reducing mixture of iron oxide and carbon. The model takes into account the reaction kinetics of both the iron oxide reduction and carbon oxidation. The model is validated with experimental data. Rate constants are compared with those in the literature. The combination of existing reaction analysis techniques coupled with the model developed has shown that for the experimental conditions used here, the Boudouard reaction controls the self reduction kinetics. KEY WORDS: mathematical modelling; kinetics; uniform conversion model; self-reduction; iron oxide; car- bon; reduction; oxidation. that for practical situations, self reducing mixtures undergo 1. Introduction indirect reduction (solid iron oxide–gaseous intermediary– Extensive research regarding the self-reducing mixture of solid carbon). These reactions are outlined in Reaction 1 to iron oxide and carbonaceous materials has been report- Reaction 4. ed.1–5) Iron oxide fines are generated at every stage of iron Reduction oxide processing, leading to either a loss of the iron re- source or the burden of higher utilisation costs. Being able Reaction 1: 3Fe O ϩCO→Fe O ϩCO to utilise, economically, such iron oxide fines has the poten- 2 3 3 4 2 tial to provide significant benefits to the iron industry. The Reaction 2: Fe O ϩCO→3FeOϩCO self reducing mixture represents a possible solution to fine 3 4 2 iron oxide utilisation. In order to realise and optimise this Reaction 3: FeOϩCO→FeϩCO potential of the self reducing mixture, it is necessary to un- 2 derstand the fundamental reaction mechanisms. In this Oxidation paper, a shortcoming in an existing fundamental kinetic model is addressed in order to better understand the self re- Reaction 4: C ϩCO →2CO duction reactions. (s) 2(g) (g) Common methods used to analyse the carbothermic re- For the iron oxide–gas reaction (Reaction 1–Reaction 3), duction of iron oxide have either centred around the first Turkdogan9) examined the carbon monoxide reduction of order irreversible unimolecular model (i.e.: lnԽ1ϪXԽϭϪkt), iron oxide, and determined an activation energy of variants of the half-life method (e.g.: dX/dt ) or using a 47 kJ/mol. Of reactions which make up the iron oxide re- tϭto rate equation to describe the gasification of carbon (the duction process, the wüstite to iron step (Reaction 3) has 1) Boudouard reaction) by CO2. Interestingly, all three meth- been shown to be the slowest step. ods require constant gas concentrations (whether describing The oxidation of carbon by CO2 is an extremely impor- iron oxide reduction or carbon gasification). Self reducing tant reaction in general, and consequently has been widely iron oxide–carbon reactions do not display constant gas studied. However, activation energies for the CO2 oxidation concentrations during reactions.6,7) of graphite vary from 130 kJ/mol (Tiwari et al.10)) to Additionally, while there is little doubt that the 370 kJ/mol (Turkdogan and Vinters11)). While there is a Boudouard reaction displays a strong influence on the car- wide variance in reported activation energies for the CO2 bothermic reduction of iron oxide, the degree of control it oxidation of graphite, it can be seen that the activation ener- displays is unclear. This may well be due to the use of inap- gy for CO2 oxidation of graphite is still considerably higher propriate kinetic models to describe the carbothermic re- than that for iron oxide reduction. Therefore the CO2 oxida- duction process. tion of graphite is a more temperature sensitive reaction Haque and Ray8) reviewed the topic of solid–solid reac- than iron oxide reduction. tion for the carbothermic reduction of iron oxide, showing There is also a consensus among researchers that the © 2003 ISIJ 1136 ISIJ International, Vol. 43 (2003), No. 8 Boudouard reaction (Reaction 4) displays a highly control- cal system makes the selection of an appropriate reduction ling influence on the overall carbothermic reduction of iron model difficult. The self reduction process is made up of oxide.1–5) The degree of control the Boudouard reaction dis- complimentary oxidation and reduction reactions. However, plays, as reported by different researchers, however, is seen investigations have only focussed on the kinetics of either to vary. There is agreement in literature that the overall self the oxidation or reduction reactions. The aim of this study reduction reaction rate is seen to increase with increasing is to provide a method to describe the carbothermic reduc- carbon content; increasing carbon surface area; and in the tion of iron oxide, taking into account both the iron oxide presence of Boudouard reaction catalysing agents (includ- reduction reactions and the Boudouard reaction. The effect ing metallic iron). These effects are consistent with the of the temperature profile on such an approach is also ex- Boudouard reaction displaying a significant influence on amined. the overall self reduction reaction. The overall self reduction reaction rate, however, is seen 2. Theoretical Background to level off at higher carbon contents12,13); higher tempera- tures14,15); and is seen to be improved by decreasing the iron 2.1. Determination of Controlling Reaction ore particle size (with the inference of increased surface Analysing the reaction off-gas provides an effective and area).14) These effects suggest that there is a limitation to dynamic method to assist in the determination of the reac- the control the Boudouard reaction displays, which is not tion controlling the self reduction process. The oxygen po- yet fully understood. Such incomplete understanding (re- tential of the reaction off gas can be calculated in terms of garding the degree of control the Boudouard reaction dis- PCO/PCO2 and can then be compared against equilibrium 1) plays on the overall self reduction rate) introduces difficulty PCO/PCO2 of the individual component reactions. Figure 1 in being able to kinetically describe the reaction system. shows an example of reaction equilibrium PCO2/PCO values Adding to the complexity of unclear controlling reac- from a standard phase diagram, for the temperature of tions is the issue of non-isothermal reactions occurring 1 000°C. within a self-reducing pellet or briquette. In the self reduc- The component reaction which is controlling could then ing briquette, the reactants are intimately mixed and the re- be determined by a process of elimination. However, as dis- action proceeds when a sufficient temperature is achieved cussed earlier, it is assumed that if the iron oxide reduction by all or part of the mixture. The very nature of the having is rate limiting, then the slowest step is the reduction of combined, thermally activated reactants raises difficulties wüstite. when studying the reaction kinetics. To determine which of the reactions are controlling, it is Work has been conducted, examining the possibility of first necessary to identify the reaction at equilibrium. A briquettes reacting non-homogenously when the tempera- given reaction is at equilibrium when the off gas PCO/PCO2 ture is raised. Several researchers have shown the non- concentration falls on the equilibrium PCO/PCO2 value for isothermal nature of the self-reducing mixture.16–19) Work that reaction. Accordingly, the complimentary (reduction of by Seaton et al.18) showed the temperature profile through wüstite/oxidation of carbon) reaction is taken to be the con- the mixture changed with increasing reaction temperature. trolling reaction. For example, if the off-gas PCO/PCO2 con- They indicated that the mode of reaction changed to a centrations indicate that the wüstite–iron reaction is at equi- shrinking core style with increasing temperature. librium, then it can be inferred that the complimentary Accordingly, the complexity of the physical and chemi- Boudouard reaction is the controlling reaction. Fig. 1. Illustration of the method used to determine equilibrium (P /P ) for a given temperature from the iron–oxygen CO2 CO phase diagram. 1137 © 2003 ISIJ ISIJ International, Vol. 43 (2003), No. 8 2.2. Controlling Kinetic Model CO(g) molecule (Reaction 5). The terms i1 and j1 are the rate The complexity of the reactions involved in the carbo- constants for the forward and reverse reactions respectively. thermic reduction of iron oxide has led to difficulty in the The second stage is the desorption of the C(O) from the selection of an appropriate model. There are several meth- carbon solid to form a gaseous CO molecule (Reaction 6), ods generally used to describe the kinetics of the carbother- the rate constant of which is the term j3. mic reduction of iron oxide, such as: iP1 CO • The first order unimolecular irreversible uniform conver- Rateϭ 2 Ϫ j j sion of a self reducing briquette (i.e. ln(1 X) vs. time, ϩϩ1 1 1,2) 1 PCOP CO and its variants), that is ; 2 j3 j3 ln(lϪX)ϭϪk·t ..............................(1) iP1 CO ϭ 2 .................(5) ()1ϩϩkP kP • The comparison of dX/dt at a given time (e.g. tϭ0). A 23CO CO2 variant on this is the examination of the time required to j attain a given degree of reduction (e.g. t/t )4,5); and Cϩϩ CO←�1→ C(O) CO X Reaction 5: f 2(g)j 1 (g) • Given the high degree of control that the Boudouard reac- tion displays on the overall carbothermic reaction, there →j 3 have been attempts to describe iron oxide reduction rates Reaction 6: C(O) CO(g) in terms of carbon gasification rates (Langmuir– Hinshelwood style equation, see Eq.