Recrystallization of Fe2O3 During the Induration of Iron Ore Oxidation Pellets
Magnetite and hematite concentrates are the two main raw materials for oxidized pellet production. Magnetite concentrates have more predominance due to the heat release by oxidation during roasting and may reduce energy consumption (Jiang et al., 2008; Li et al., 2009). However, with the continuous consumption of iron resources driven by the rapid development of iron and steel industry, magnetite resources are becoming scarce and so the development of pellet production is restricted to some extent. Thus, it is imperative to make better use of hematite resources to meet the raw material supply for pellet production (Xu, 2001).
Isoconversional analysis of thermal dissociation kinetics of hematite in air and inert atmospheres
Dissociation of hematite is an undesirable reaction for iron ore pelletizing process leading to severe deterioration in compressive cold strength and reducibility factors. It was shown that raising temperature in an induration machine would cause hematite’s dissociation, which is either present in the primary ore or formed by oxidation of magnetite in the feed. The oxidation reaction of magnetite is exothermic, which complicates temperature control within the non-isothermal area of preheating. Kinetics of the dissociation reaction is the temperature’s primary function, which controls the extent of the reaction. Pure hematite samples were subjected to several runs of thermal analysis carried out under both air and inert atmosphere, in order to achieve a comprehensive knowledge about the temperature dependencies of dissociation kinetics. Due to the observed uniformity, isoconversional methods were chosen in the present work over isothermal and non-isothermal for calculation of kinetic parameters of the reaction. Respectively, activation energy values of hematite dissociation were found to be 324 and 382 kJ mol−1 in inert and air atmosphere. The high value of activation energy implies strong dependency of the single-step reaction rate on the temperature. It was also observed that the forward reaction had higher activation energy than the backward reaction; hence, an increase in temperature results in an overall acceleration of the dissociation reaction.
Optimizing the operation of straight-grate iron-ore pellet induration systems using process models
Mathematical models of straight-grate pellet induration processes have been developed and carefully validated by a number of workers over the past two decades. However, the subsequent exploitation of these models in process optimization is less clear, but obviously requires a sound understanding of how the key factors control the operation. In this article, we show how a thermokinetic model of pellet induration, validated against operating data from one of the Iron Ore Company of Canada (IOCC) lines in Canada, can be exploited in process optimization from the perspective of fuel efficiency, production rate, and product quality. Most existing processes are restricted in the options available for process optimization. Here, we review the role of each of the drying (D), preheating (PH), firing (F), after-firing (AF), and cooling (C) phases of the induration process. We then use the induration process model to evaluate whether the first drying zone is best to use on the up- or down-draft gas-flow stream, and we optimize the on-gas temperature profile in the hood of the PH, F, and AF zones, to reduce the burner fuel by at least 10 pct over the long term. Finally, we consider how efficient and flexible the process could be if some of the structural constraints were removed (i.e., addressed at the design stage). The analysis suggests it should be possible to reduce the burner fuel lead by 35 pct, easily increase production by 5+ pct, and improve pellet quality.