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BY-NC-ND 3.0 license Open Access Published by De Gruyter January 26, 2016

Research on the reduction of Guizhou oolitic hematite by hydrogen

  • Huang Run

    Huang Run has been an Associate Professor at Guizhou University since 2013. He obtained his PhD in metallurgic engineering from Chongqing University, China. His primary research interests include chemical engineering, ironmaking and steelmaking, and the comprehensive use of resources.

    , Shanshan Bi

    Shanshan Bi has started his MSc at Guizhou University, China, where he is currently carrying out a research on the comprehensive use of resources, metallurgy, and chemical engineering under the supervision of Professor Jinzhu Zhang.

    , Pengsheng Liu

    Pengsheng Liu has started his MSc at Guizhou University, China, where he is currently carrying out a research on the comprehensive use of resources, metallurgy, and chemical engineering under the supervision of Professor Jinzhu Zhang.

    , Benjun Xu

    Benjun Xu is an Associate Professor at Guizhou University. He obtained his PhD in mineral engineering from Central South University, China. His primary research interests include mineral process engineering and hydrometallurgy.

    and Jinzhu Zhang

    Jinzhu Zhang is a PhD supervisor at Guizhou University and is mainly engaged in chemical engineering and physical chemistry of process metallurgy. He has received some awards, such as special government allowances of the state council and tube experts of Guizhou Province.

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Abstract

Oolitic hematite abounds in the Guizhou Province of China but is hard to use with the traditional process. The reduction of Guizhou oolitic hematite by H2 was studied. Ore microscopy, phase change, and the metallization rate of oolitic hematite under different reduction conditions were characterized by optical microscopy, X-ray diffraction, and chemical analysis, respectively. The metallization ratio of the reduced sample increased with the increase of reduction time. The metallization ratio reached 80.75% when the temperature was 1000°C and the reduction time was 120 min. The new phases of FeAl2O4 and Fe2SiO4 can be generated during the reduction process, which hindered the reduction of the oolitic hematite.

1 Introduction

In China, high-quality iron ore resources are deficient and thus are typically imported from Australia, India, and Brazil, and the dependence on its importation is as high as 63% [1]. As a result, this poses a serious influence on the healthy development of ironmaking and steelmaking industries. Strengthening the research and utilization of low-grade ores has an important strategic meaning to the tight supply situation. Oolitic hematite is approximately 3.23 billion tons, accounting for 10% of the iron ore reserves, and the iron grade is approximately 30%–40% with a high content of P that is usually between 0.4% and 1.1% [2]. Oolitic hematite has a complex mineral structure and extremely fine particle size. On the contrary, it is often associated or locked with siderite and oolite chlorite and contained phosphate rocks, which are hard for monomer dissociation [3]. According to the above characteristics, researchers have taken many studies [4–9], such as selective aggregation-reverse flotation, high-gradient magnetic separation, magnetic roasting and separation, direct reduction methods, acid leaching, chlorination roasting-acid leaching, and microwave-assisted separation. However, employing these methods has been proven difficult in getting the requirements of the ironmaking process.

Meanwhile, direct reduction-magnetic separation was studied by Xu et al., who obtained an iron concentrate with a grade of 90.23%, and the P contents decreased from 0.82% to 0.06% [10, 11]. The direct reduction methods of ironmaking can solve the shortage of coking coal resources, reduce the energy consumption of iron and steel production, improve the quality of steel products, and conduce the separation of iron-phosphors [12], so the direct reduction process is more apt for the use of oolitic hematite. Direct reduction is divided into coal-based direct reduction and gas-based reduction. The existing research was mainly about the former. Therefore, the reduction of oolitic hematite by H2 was investigated in this paper.

2 Materials and methods

2.1 Materials

The ore used in the experiment came from Guizhou Province, China, and the chemical composition of which is shown in Table 1. The reductant was H2 (99.999%) and the high-purity Ar was chosen as the shielding gas.

Table 1

Chemical analysis of raw materials (wt%).

TFeFeOSiO2Al2O3CaOMgOSP
51.209.449.788.711.560.710.0270.68

The X-ray diffraction (XRD) pattern results of the oolitic hematite are shown in Figure 5. The iron element mainly existed in the form of hematite, and the main gangue mineral was kaolinite.

2.2 Method

The oolitic hematite ores were crushed and ground into powder with a size of <0.074 mm, dried at 120°C for 4 h, mixed with water, put into a cylindrical mould with an inner diameter of 20 mm, and pressed like a 20 mm high cylindrical sample. The samples were dried at 120°C for 4 h again. The cylindrical sample was put into the KSS-1600°C tubular resistance furnace and heated in the argon atmosphere (50 ml/min). When the temperature has increased to the design temperature (5°C/min) and held for 20 min, the Ar was switched with H2 (300 ml/min). The time was recorded at the same time. When the time was reached as the setting time, the furnace was turned off and H2 was used instead of Ar. The set-up of the experimental apparatus is shown in Figure 1. When the sample was cooled to room temperature, the mass of the sample was weighed by a high-precision electronic balance. After reduction, the pellet in each experiment was cut into two. One was used for chemical analysis of total iron (TFe) and metallic iron (MFe), whereas the other was used for optical microscopy and XRD. The mass loss ratio and metallization ratio were defined as follows:

Figure 1: Set-up of the experimental apparatus.
Figure 1:

Set-up of the experimental apparatus.

(1)Masslossratio=(mo-mt)mo×100% (1)

where mo and mt are the mass of the unreduced material and the mass of the sample after reduction at any time, respectively.

(2)Metallizationratio=mMFe/mTfe×100% (2)

where mMFe and mTFe are the mass fraction of the metallic iron and total iron in the reduced samples, respectively.

3 Results and discussion

3.1 Optical microscopy

The oolitic hematite reduction experiments were carried out at the reduction temperature of 1000°C (30, 60, 90, and 120 min) and the reduction time of 120 min (900°C, 1000°C, and 1100°C) with H2 (300 ml/min). After reduction, the cooled sample was cut into two from the middle, and one of them was observed using optical microscopy. The results are shown in Figure 2. The oolitic structure was obviously clear, the diameter of which was approximately 100 μm. The white regions were iron phase, the gray regions were silicate, and the dark regions were pores. The metallic iron particles began to accumulate and grow up in external oolitic with the increase of reduction time. Meanwhile, the reduction of oolitic hematite by H2 was from the outside to the inside of the oolitic structure. When the temperature was 1000°C and the reduction time was 60 min, the inside of oolitic has obviously not been reduced; however, the whole oolitic was reduced after 120 min. Under the same conditions of temperature, the oolitic structure was not destroyed with the increase of the reduction time. However, when the temperature was 1100°C and the reduction time was 120 min, the oolitic structure was destroyed. On the contrary, the metallic iron particles migrated outwards and accumulated to grow up. In the end, it was easier to separate the metal iron from the slag.

Figure 2: Optical microscopy images of the reduced samples.
Figure 2:

Optical microscopy images of the reduced samples.

3.2 Mass loss ratio

During the reduction of oolitic hematite with H2, there was mass loss of the samples although losing oxygen from Fe2O3 or FeO. The effect of reduction time or temperature on the mass loss ratio of the reduced samples is shown in Figure 3. The change of mass loss ratio presents three stages. When the temperature was 1000°C, the mass loss ratio of the sample was increased fast with the increase of reduction time from 10 to 30 min, the speed of the growth decreased from 30 to 90 min, and the speed of growth became smooth after 90 min. The mass loss ratio was more than 10% before 10 min, which was mainly produced by the samples volatile during the process of increasing temperature. With that and combined with Figure 4, the metallization ratio of the sample was nearly zero in the initial stage. When the reduction time was 30 min, the mass loss ratio of the sample was not changed below 900°C, but the mass loss ratio of the sample increased with the increase of temperature when the temperature was higher than 900°C. When the reduction time was 120 min, the mass loss ratio of the sample increased with increasing temperature. When the temperature was 1100°C, the mass loss ratio of the sample was approximately 24.2%.

Figure 3: Effect of reduction time or temperature on the mass loss ratio of the reduced samples.
Figure 3:

Effect of reduction time or temperature on the mass loss ratio of the reduced samples.

Figure 4: Effect of reduction time or temperature on the metallization ratio of the reduced samples.
Figure 4:

Effect of reduction time or temperature on the metallization ratio of the reduced samples.

3.3 Metallization ratio

The metallization ratio of the reduced samples with different reduction conditions is shown in Figure 4. The metallization ratio gradually increased with the increase of reduction time. The tendency of the growth of the metallization ratio resembled the mass loss ratio of the reduced samples. When the reduction temperature was higher than 1000°C, the metallization ratio of the reduced sample was not changed with the reduction time of 30 min. When the reduction time was 120 min, the metallization ratio of the reduced sample was 50% with the temperature of 800°C and 83.26% with the temperature of 1100°C.

3.4 Phase change

The XRD patterns of the reduced samples with different holding times are shown in Figure 5. When the holding time was 10 min, ferric iron was reduced to ferrous iron. Some ferrous iron existed in the form of FeO, and the others reacted with Al2O3 or SiO2 to form FeAl2O4 or Fe2SiO4, which were difficult to reduce. When the holding time was 30 min, the phase of FeO disappeared, and metal Fe was found in the product. Meanwhile, the phases of FeAl2O4 and Fe2SiO4 still existed. When the holding time was 90 min, the new phase SiO2 appeared and Fe2SiO4 cannot be detected. Fe2SiO4 could be seen as 2(FeO) and 1(SiO2) when FeO was reduced gradually, so there was not enough FeO to form Fe2SiO4. Fe2SiO4 was decomposed, so the phase of SiO2 was found. When the holding time was 120 min, the main phases were metal iron and FeAl2O4. Adding alkaline oxides could improve the reduction of oolitic hematite.

Figure 5: XRD pattern of the samples with different reduction times at 1000°C.
Figure 5:

XRD pattern of the samples with different reduction times at 1000°C.

4 Conclusion

The reduction of Guizhou oolitic hematite by H2 is clarified in the present study. The conclusions can be summarized as follows.

The reduction of oolitic hematite is a diffusion process, which takes place from the outside to the inside of the oolitic particles and samples. High temperature and long reaction time would help in the diffusion of H2, and the cyclic structure of oolitic particles would be destroyed when the reduction temperature was 1100°C and the reduction time was 120 min.

The mass loss ratio and metallization ratio of the reduced sample had the same change tendency with the variation of time, which increased with the increase of reduction time and were divided into three stages. The phases of fayalite and spinel could be formed during the reduction that would cause a lower reaction speed.


Corresponding author: Jinzhu Zhang, School of Materials and Metallurgy, Guizhou University, Guiyang 550025, China; and Guizhou Province Key Laboratory of Metallurgical Engineering and Energy Process Energy, Guiyang 550025, China, e-mail:

About the authors

Huang Run

Huang Run has been an Associate Professor at Guizhou University since 2013. He obtained his PhD in metallurgic engineering from Chongqing University, China. His primary research interests include chemical engineering, ironmaking and steelmaking, and the comprehensive use of resources.

Shanshan Bi

Shanshan Bi has started his MSc at Guizhou University, China, where he is currently carrying out a research on the comprehensive use of resources, metallurgy, and chemical engineering under the supervision of Professor Jinzhu Zhang.

Pengsheng Liu

Pengsheng Liu has started his MSc at Guizhou University, China, where he is currently carrying out a research on the comprehensive use of resources, metallurgy, and chemical engineering under the supervision of Professor Jinzhu Zhang.

Benjun Xu

Benjun Xu is an Associate Professor at Guizhou University. He obtained his PhD in mineral engineering from Central South University, China. His primary research interests include mineral process engineering and hydrometallurgy.

Jinzhu Zhang

Jinzhu Zhang is a PhD supervisor at Guizhou University and is mainly engaged in chemical engineering and physical chemistry of process metallurgy. He has received some awards, such as special government allowances of the state council and tube experts of Guizhou Province.

Acknowledgments

The authors are especially grateful to the Natural and Science Foundation of China (grant nos. 51274074 and 51404080).

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Received: 2015-8-18
Accepted: 2015-11-12
Published Online: 2016-1-26
Published in Print: 2016-1-1

©2016 by De Gruyter

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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