Kinetic Model for the Uniform Conversion of Self Reducing Iron Oxide and Carbon Briquettes

Abstract

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; carbon; reduction; oxidation.Boudouard reaction (Reaction 4) displays a highly controlling influence on the overall carbothermic reduction of iron oxide. [1][2][3][4][5] The degree of control the Boudouard reaction displays, as reported by different researchers, however, is seen to vary. There is agreement in literature that the overall self reduction reaction rate is seen to increase with increasing carbon content; increasing carbon surface area; and in the presence of Boudouard reaction catalysing agents (including metallic iron). These effects are consistent with the Boudouard reaction displaying a significant influence on the overall self reduction reaction.The overall self reduction reaction rate, however, is seen to level off at higher carbon contents 12,13) ; higher temperatures 14,15) ; and is seen to be improved by decreasing the iron ore particle size (with the inference of increased surface area).14) These effects suggest that there is a limitation to the control the Boudouard reaction displays, which is not yet fully understood. Such incomplete understanding (regarding the degree of control the Boudouard reaction displays on the overall self reduction rate) introduces difficulty in being able to kinetically describe the reaction system.Adding to the complexity of unclear controlling reactions is the issue of non-isothermal reactions occurring within a self-reducing pellet or briquette. In the self reducing briquette, the reactants are intimately mixed and the reaction proceeds when a sufficient temperature is achieved by all or part of the mixture. The very nature of the having combined, thermally activated reactants raises difficulties when studying the reaction kinetics.Work has been conducted, examining the possibility of briquettes reacting non-homogenously when the temperature is raised. Several researchers have shown the nonisothermal nature of the self-reducing mixture. [16][17][18][19] Work by Seaton et al. 18) showed the temperature profile through the mixture changed with increasing reaction temperature. They indicated that the mode of reaction changed to a shrinking core style with increasing temperature. Accordingly, the complexity of the physical and chemical system makes the selection of an appropriate reduction model difficult. The self reduction process is made up of complimentary oxidation and reduction reactions. However, investigations have only focuss...

Effect of Alumina and Silica on the Reaction Kinetics of Carbon Composite Pellets at 1473 K

“…They concluded that the direct contact of reduced iron with carbon was crucial for low temperature melting and flux addition is the most effective method to remove gangue layer which blocks melting of reduced iron. Our previous work has developed a kinetic model which is suitable for self-reducing iron oxide pellet, 6) and more recently the authors have examined the role of the Boudouard reaction in iron oxide and carbon briquettes.…”

Reduction Behavior of Carbon Composite Pellets Including Alumina and Silica at 1273 K and 1373 K

“…[19][20][21][22] . It is well established idea that the Boudouard reaction is significantly slower than the reduction of iron oxide below 1 473 K (1 200°C), 23,24) so the overall reaction rate is controlled by Eq. (3).…”

“…23,24) When solid carbon reacts with solid iron oxide in carbon composite pellet, the overall reaction is largely influenced by the reaction in Eq. (6).…”

Characterization and Reduction Behavior of Mill Scale

“…Reduction of iron oxides by solid reductants has also been studied, especially when developing the self reducing iron oxide and carbon composite briquettes. [20][21][22][23][24][25][26][27][28] These studies, with different iron oxides and carbon bearing materials, have shown that the self reduction mechanism and kinetics are more complicated because beside the reaction of reduction it includes sensibly endothermic carbon gasification reaction which very often is found to control the overall rate of self reduction. Moreover, other phenomena could influence 28) (like a devolatilization of the reducing agent) or even control 24) (like a heat transfer) the rate of self reduction.…”