Elsevier

Thermochimica Acta

Volume 447, Issue 1, 1 August 2006, Pages 89-100
Thermochimica Acta

Kinetics of reduction of iron oxides by H2: Part I: Low temperature reduction of hematite

https://doi.org/10.1016/j.tca.2005.10.004Get rights and content

Abstract

This study deals with the reduction of Fe2O3 by H2 in the temperature range of 220–680 °C. It aims to examine the rate controlling processes of Fe2O3 reduction by H2 in the widest and lowest possible temperature range. This is to be related with efforts to decrease the emission of CO2 in the atmosphere thus decreasing its green house effect.

Reduction of hematite to magnetite with H2 is characterized by an apparent activation energyEa’ of 76 kJ/mol. Ea of the reduction of magnetite to iron is 88 and 39 kJ/mol for temperatures lower and higher than 420 °C, respectively. Mathematical modeling of experimental data suggests that the reaction rate is controlled by two- and three-dimensional growth of nuclei and by phase boundary reaction at temperatures lower and higher than 420 °C, respectively.

Morphological study confirms the formation of compact iron layer generated during the reduction of Fe2O3 by H2 at temperatures higher than 420 °C. It also shows the absence of such layer in case of using CO. It seems that the annealing of magnetite's defects around 420 °C is responsible for the decrease of Ea.

The rate of reduction of iron oxide with hydrogen is systematically higher than that obtained by CO.

Introduction

Reduction of iron oxide is probably one of the most studied topics. This is due to the importance of iron and steel in the current and future technologies. More than 2 tonnes of carbon dioxide are generated for the production of 1 tonnes of iron metal. It is well known that CO2 increases the green house effect. For these reasons, this study explores the advantages and disadvantages of using pure hydrogen for the reduction of iron oxides at low temperature.

From the industrial point of view, direct reduction of iron ores with H2 or natural gases could have several technical advantages, such as:

  • 1.

    replacement of the expensive metallurgical coke as reducing agent;

  • 2.

    low carbon content of the produced iron;

  • 3.

    generated gases are essentially composed of H2O and H2, thus avoiding the release of CO and CO2 and by the same token avoid the projected Ecotax.

However, reduction of iron ores with hydrogen leads to compact iron layers that could slow their reduction rate. Moreover, in some cases the produced iron is pyrophoric. Finally, and in spite of serious effort [1], [2], [3], the cost of hydrogen production is currently high.

This paper is focused on the reduction of hematite with hydrogen in the temperature range of 220–680 °C.

Section snippets

Literature review

Reduction of hematite by hydrogen proceeds in two or three steps, under and above 570 °C, respectively, via magnetite (Fe3O4) and wüstite (Fe(1−x)O) according to the Bell's Diagram (Fig. 1) and the following equations:3Fe2O3 + H2  2Fe3O4 + H2OFe3O4 + 4H2  3Fe + 4H2O(1−x)Fe3O4 + (1−4x)H2  3Fe(1−x)O + (1−4x)H2OFe(1−x)O + H2  (1−x)Fe + H2O

Generally, it is admitted that wüstite is unstable below 570 °C under thermo-dynamic equilibrium. However, as shown by Fig. 1, wüstite could be an intermediate product during the

Material and experimental procedure

Hematite used in this study had a Fe2O3 content higher than 99.8% supplied by Merck. Impurities are essentially traces of Ca2+, Mg2+, Cl and SO42−. The specific surface area of Fe2O3 was 0.51 m2/g. The porosity volume is 3.3 cm3/g. The SEM indicates that the grain size of the hematite is about 1–2 μm.

Thermogravimetric (TG) tests were performed using 100 mg of sample and a CAHN microbalance (Fig. 2). It has a sensibility of 20 μg. The sample is reduced in gold boat having a surface area of about 2.2 

Results

Fig. 3 groups the isotherms of hematite reduction versus the reaction time at different temperatures. As can be observed, most of the isotherms have a plateau at about 11% of reduction extent percentage ‘R%’ that corresponds to the reduction of hematite to magnetite. One may underline that the time for full reduction of hematite is about 1500, 40 and 5 min at temperatures 237, 349 and 472 °C, respectively. This suggests that the apparent activation energy ‘Ea’ is relatively important. Fig. 4 is

Discussion

As indicated in Table 1, Ea depend on the raw material and its purity and the physical state of this raw material. Moreover, for the same solid, different experimental conditions leads to different values of Ea. Several authors confirm these conclusions as indicated in Table 4. Results obtained in this work agreed with those of Refs. [9], [10], [20], [23], [24]. The suggested mechanisms of reduction of hematite match with those of Refs. [6], [9], [10]. On the other hand, the evolution of Ea

Conclusions

Results of the reduction of iron oxides with hydrogen in the temperature range of 200–680 °C leads to the following conclusions:

  • 1.

    The reduction of hematite in magnetite by H2 is characterized by an apparent activation energy of about 76 kJ/mol.

  • 2.

    The reduction path of magnetite to iron is function of the reaction temperature. At temperatures lower than 420 °C, Fe3O4 is reduced directly to iron. At 450 < T < 570 °C, magnetite and wüstite are present with iron. At T > 570 °C, magnetite is fully reduced to

Acknowledgments

Part of this work was performed in ‘Laboratoire de Chimie du Solide’ of the University of Nancy, France. The authors thank Dr. Ch. Gleitzer for his help and discussions. They also indebted to Mrs. Ch. Richard for her kind help in technical and administration work.

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    Laboratoire Environnement et Minéralurgie, rue du Doyen M. Roubault, BP 40, 54501 Vandœuvre Cedex, France.

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    Centre National de la Recherche Scientifique, 3 rue Michel-Ange, 75794 Paris Cedex, France.

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    École Nationale Supérieure de Géologie, rue du Doyen M. Roubault, BP 40, 54501 Vandœuvre Cedex, France.

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    Institut National Polytechnique de Lorraine, 2 rue de la Forêt de Haye, 54501 Vandœuvre Cedex, France.

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