Home Oxidizing Roasting Performances of Coke Fines Bearing Brazilian Specularite Pellets
Article Open Access

Oxidizing Roasting Performances of Coke Fines Bearing Brazilian Specularite Pellets

  • Tiejun Chun EMAIL logo and Deqing Zhu
Published/Copyright: August 5, 2015
Download Article (PDF)
Become an author with De Gruyter Brill
Submit Manuscript Author Information
High Temperature Materials and Processes
From the journal High Temperature Materials and Processes Volume 35 Issue 6

Abstract

Oxidized pellets, consisting of Brazilian specularite fines and coke fines, were prepared by disc pelletizer using bentonite as binder. The roasting process of pellets includes preheating stage and firing stage. The compressive strength of preheated pellets and fired pellets reached the peak value at 1.5% coke fines dosage. During the initial stage of preheating, some original Fe2O3 was reduced to Fe3O4 because of partial reduction atmosphere in pellet. During the later stage of preheating and firing stage, coke fines were burnt out, and the secondary Fe2O3 (new generation Fe2O3) was generated due to the re-oxidization of Fe3O4, which improved the recrystallization of Fe2O3. Compared with the fired pellets without adding coke fines, fired pellets with 1.5% coke fines exhibited the comparable RSI (reduction swelling index) and RDI+3.15 mm (reduction degradation index), and slightly lower RI (reducibility index).

Keywords: Brazilian specularite pellets; coke fines; compressive strength; metallurgical performances

Introduction

With the rapid development of iron and steel industry in China, there has been a soaring demand for iron ores in the past decade. The imports of iron ores have increased from 148 million tonnes in 2003 to 933 million tonnes in 2014. Brazilian specularite iron ores, with characteristics such as high iron grade, low impurities and low price, have been used for pelletizing plants in recent years. However, the specularite pellets requires higher firing temperature to achieve the requirement of compressive strength. In many pelletizing plants, mixtures of magnetite fines and specularite fines are used to produce oxidized pellets. In order to adequate compressive strength for blast furnace requirement, ratio of magnetite needs to exceed 20% [1–2].

In order to improve the compressive strength and metallurgical performances of oxidized pellets, efforts have been made by modifying the basicity (the ratio of CaO/SiO2) and MgO content of pellets [3–4]. Meanwhile, the technology of adding a certain proportion of solid fuel to pellets is another way to improve the compressive strength of pellets. However, most of research has focused on carbothermic reduction to product direct reduction iron (DRI) for non-blast furnace [5–6]. With the increasing requirement for blast furnace fuel economy, adding solid fuel to pellets as an energy saving measure could be greatly developed in the oxidized pellets. The firing temperature was decreased because of the combustion of solid fuel during the roasting process [7]. In view of the commercial prospects of specularite pellets containing solid fuel, the oxidized pellets consisting of Brazilian specularite fines and coke fines were conducted in this investigation.

Experimental

Materials

The materials included Brazilian specularite fines, bentonite and coke fines. Coke was ground in laboratory ball mill to obtain the required fineness for pelletizing. The chemical composition and particle size distribution of all the materials used for pelletizing are given in Tables 1 and 2, respectively. The particle morphologies of Brazilian specularite fines under scanning electron microscope (SEM) are depicted in Figure 1. It can be seen that the surfaces of specularite fines are smooth, which is different from that of common hematite and magnetite.

Table 1:

Chemical composition of raw materials/mass%.

ElementSpecularite finesBentoniteCoke ash
Fetotal66.375.8914.48
FeO0.11−−
Na2O0.031.42−
K2O0.040.07−
SiO21.2251.8247.83
Al2O31.4221.7722.27
CaO0.010.308.03
MgO0.012.601.18
S0.010.02−
P0.040.01−
LOI1.4313.20−
Table 2:

Particle size distribution of materials used for pelletizing/mass%.

Size range (μm)SpeculariteBentoniteCoke fines
+1507.100.003.30
−150 +7412.100.005.40
−74 +4513.4014.506.30
−45 +387.8023.1010.70
−3859.6062.4074.30
Figure 1: Particles morphologies of specularite fines under SEM.
Figure 1:

Particles morphologies of specularite fines under SEM.

Bentonite is usually employed as binder in the pelletizing plants. The bentonite contains 90.50% montmorillonite, higher swelling volume of 57.5% and the water adsorption of 299%. Coke fines were used as the solid fuel, and its proximate analysis tested according to GB/T 212–2008 is presented in Table 3.

Table 3:

Industrial analysis of coke fines/mass%.

MoistureAshVolatile matterFixed Carbon
0.8020.593.3775.24

Methods

In each batch, 4 kg mixture consisting of Brazilian specularite fines and a given proportion of coke fines was blended with 1.2% bentonite. Green pellets were prepared from the mixture in a disc pelletizer of 0.8 m in diameter and 0.2 m rim depth, rotational speed at 38 r/min and inclined at 47° horizontally. The green pellets were obtained at sizes of 10 mm to12.5 mm in diameter by screening and the moisture of green pellets was about 8–9%.

1 kg green pellets were dried in the oven at 105°C (105°C = 378 K) for 4 h until the weight unchanged. Dried pellets were preheated and fired in a horizontal tube furnace of 50 mm in diameter and 600 mm in length. Both ends of tube were open and air could flow into the tube furnace. First, the dried pellets were put into a corundum crucible of 100 mm in length, 40 mm in width and 20 mm in height, and then the crucible was pushed into the preheating zone of the furnace step by step and preheated at a given temperature for a given period. Then, the preheated pellets were taken out of the furnace and cooled in the air or directly pushed forwards into a higher temperature zone for firing. Finally, the fired pellets were taken out and naturally cooled in air.

The chemical compositions of iron ore and bentonite were analyzed using XRF (X Ray Fluorescence). The compressive strength and metallurgical performances of the cooled pellets were measured to evaluate the qualities as the burden for blast furnace. The preheated pellets and fired pellets were tested for their compressive strength according to ISO 4700. Cold fired pellets were tested for RI (reducibility index) according to ISO 7215. RSI (Reduction swelling index) and RDI (reduction degradation index) were also tested according to ISO 4698 and ISO 4696–2, respectively. Apparent density and true density of fired pellets were tested by pycnometer method. The porosity (ε = 1−ρ1/ρ2) of fired pellets was obtained by the results of apparent density (ρ1) and true density (ρ2). After pellets samples have been mounted and polished, microstructure of pellets was measured by using Leica optical microscopy and SEM with EDS.

Results and discussion

Compressive strength

Roasting process includes preheating stage and firing stage. Figure 2 shows the effect of coke fines dosage on the compressive strength of preheated pellets at different preheating temperatures. With increasing coke fines dosage from 0 to 1.5%, the compressive strength of preheated pellets was improved significantly and then decreased sharply when coke fines dosage was beyond 1.5%. Meanwhile, with increasing temperature also enhanced the compressive strength of preheated pellets.

Figure 2: Effect of coke fines dosage on compressive strength at different preheating temperature (Preheating for 15 min).
Figure 2:

Effect of coke fines dosage on compressive strength at different preheating temperature (Preheating for 15 min).

The effect of coke fines dosage on the compressive strength of fired pellets at different firing temperatures is indicated in Figure 3. When the coke fines dosage was 1.5%, the compressive strength of fired pellets reached the peak value. At the firing temperature of 1,250°C, the compressive strength of fired pellets increased from 1,820 to 2,774 N/pellet when coke fines dosage increased from 0 to 1.5%, and then decreased when coke dosage was higher than 1.5%. Therefore, the suitable dosage of coke fines was recommended at 1.5%.

In the preheating stage, according to reaction, (1) partial reducing atmosphere was formed in pellets because of combustion of coke fines. First, original Fe2O3 was reduced to Fe3O4 according to the following reaction (2). According to eq. (3) and Figure 4, the reaction (2) is easy to occur at low temperature and low CO concentration. When preheating time was extended, coke fines have already burnt out, and secondary Fe2O3 grains (new generation Fe2O3) formed due to the re-oxidization of Fe3O4. The compressive strength of fired pellets was improved because of the recrystallization of Fe2O3 [9].

(1)2C(s)+O2=2CO
(2)3Fe2O3(s)+CO=2Fe3O4(s)+CO2ΔGTθ=-52131−41.0TJ/mol
(3)lnKp=lnpco2pco=6270.3T+4.93
Figure 3: Effect of coke fines dosage on compressive strength at different firing temperature (Preheating at 1,050°C for 15 min and firing for 15 min).
Figure 3:

Effect of coke fines dosage on compressive strength at different firing temperature (Preheating at 1,050°C for 15 min and firing for 15 min).

Figure 4: Equilibrium diagram of reduction of iron oxide by carbon [8].
Figure 4:

Equilibrium diagram of reduction of iron oxide by carbon [8].

In order to confirm that some original Fe2O3 was reduced to Fe3O4 firstly, and then Fe3O4 was re-oxidized to secondary Fe2O3, the effect of roasting time on Fe3O4 content of pellets with 1.5% coke fines was investigated, as shown in Figure 5. In initial stage of preheating, the Fe3O4 content reached the high value of 15.75% when preheating for 5 min. It can be explained that the reduction rate of original Fe2O3 to Fe3O4 is faster than the re-oxidation rate of newborn Fe3O4 to secondary Fe2O3 in the initial stage. However, the reduction rate decreased when preheating time was prolonger than 5 min. The results indicated that the original Fe2O3 was firstly translated to Fe3O4, and then the newborn Fe3O4 was re-oxidized to secondary Fe2O3. In the firing stage, Fe3O4 content continued to decrease from 2.58% to 0.48%, which presented that Fe3O4 was continued to re-oxidize in firing stage.

Figure 5: Effect time on Fe3O4 content of pellets containing 1.5% coke fines dosage at different stages (Preheating at 1,050°C and firing at 1,250°C).
Figure 5:

Effect time on Fe3O4 content of pellets containing 1.5% coke fines dosage at different stages (Preheating at 1,050°C and firing at 1,250°C).

However, when coke fines dosage level was added at 2.0%, reaction (4) would occur to form FeO, as shown in Figure 6. The FeO (Point 2) surrounded by Fe2O3 (Point 1) was detected in the fired pellets, and some Fe2SiO4 (Point 2) was also found.

(4)Fe3O4(s)+CO=3FeO(s)+CO2;ΔGTθ=35380-40.16TJ/mol(T>570°C)
(5)lnKp=lnpco2pco=−4255.5T+4.83
Figure 6: SEM-EDS results of fired pellets containing 2.0% coke (Preheating at 1,050°C for 15 min and firing at 1,250°C for 15 min).
Figure 6:

SEM-EDS results of fired pellets containing 2.0% coke (Preheating at 1,050°C for 15 min and firing at 1,250°C for 15 min).

As shown in Figure 7, compared with the fired pellets without adding coke fines, the fired pellets with 1.5% coke fines presented the lower porosity, and the inner structure was much more compact, as shown in Figure 8(b) and Figure 9(b), which resulted in the higher compressive strength. When coke fines dosage level was higher, more coke fines dosage also led to more cracks and holes formed during the fired pellets. As shown in Figure 8(c) and Figure 9(c), more cracks and holes were appeared in the fired pellets with 2.0% coke fines, indicating the lower compressive strength of pellets.

Figure 7: Effect of coke fines dosage on porosity of fired pellets (Preheating at 1,050°C for 15 min and firing at 1,250°C for 15 min).
Figure 7:

Effect of coke fines dosage on porosity of fired pellets (Preheating at 1,050°C for 15 min and firing at 1,250°C for 15 min).

Figure 8: Inner-structures of fired pellets at different coke fines dosage (Preheating at 1,050°C for 15 min and firing at 1,250°C for 15 min).
Figure 8:

Inner-structures of fired pellets at different coke fines dosage (Preheating at 1,050°C for 15 min and firing at 1,250°C for 15 min).

Metallurgical performances

The fired pellets prepared for metallurgical performances tests were conducted under the conditions of preheating at 1,050°C for 15 min and firing at 1,250°C for 15 min. Table 4 illustrates the effect of coke fines dosage on metallurgical performances. RI was decreased from 70.25% to 65.45% when coke fines dosage was increased from 0 to 2.0%. As seen in Figure 7, compared with the fired pellets with 1.5% coke fines, the fired pellets without adding coke fines exhibited the higher porosity. During the reduction, reduction gas could easily penetrate through these pores into the pellets core, and improved the reducibility. However, even though the pellets with 2.0% coke fines also presented high porosity, some amount of iron silicate formed in the pellets, such as 2FeO.SiO2 (as shown in Figure 6, point 2), which was difficult to reduce into metallic iron, and presented low reducibility [10–11].

Table 4:

Effect of coke fines dosage on metallurgical performances of fired pellets/mass%.

Coke fines dosageRIRSIRDI +3.15 mm
070.2522.3286.46
1.067.3622.9087.58
1.566.8521.0692.36
2.065.4523.2285.92

For the fired pellets with 1.5% coke fines, the RSI was slightly improved, and the RDI+3.15 mm was much higher than that of other fired pellets. It can be explained that the fired pellets containing 1.5% coke fines presented the larger size of Fe2O3 grains, and strong bonds, as shown in Figure 9(b), which resisted the reduction swelling and reduction degradation.

Figure 9: Optical microstructures of fired pellets at different coke fines dosage (Preheating at 1,050°C for 15 min and firing at 1,250°C for 15 min).
Figure 9:

Optical microstructures of fired pellets at different coke fines dosage (Preheating at 1,050°C for 15 min and firing at 1,250°C for 15 min).

Conclusions

  1. The technology of adding a certain proportion coke fines into Brazilian specularite pellets is an effective way to improve the compressive strength of fired pellets. The suitable dosage of coke fines was recommended at 1.5%.

  2. In preheating stage, some original Fe2O3 was reduced to Fe3O4, and then new born Fe3O4 was re-oxidized to secondary Fe2O3. In firing stage, Fe3O4 content continued to decrease, and secondary Fe2O3 was continued to form. The compressive strength of fired pellets was improved due to the recrystallization of Fe2O3. However, when coke fines dosage level was higher than 1.5%, more cracks and holes were formed, leading to the lower compressive strength.

  3. With increasing the coke fines dosage, RI decreased. The fired pellets with 1.5% coke fines presented the lower RSI and higher RDI+3.15 mm because the strong bonds formed, which resisted the reduction swelling and reduction degradation.

References

[1] G. Li, X. Li, Y. Zhang, G. He and T. Jiang, Ironmaking & Steelmaking., 36 (2009) 393–396.10.1179/174328109X410325Search in Google Scholar

[2] T. Chun, D. Zhu and J. Pan, J. Cent. South. Univ. T., 18 (2011) 1875–1880.10.1007/s11771-011-0924-ySearch in Google Scholar

[3] S. Dawrapudi, T.K. Ghosh, A. Shankar, V. Tathavadkar, D. Bhattacharjee and R. Venugopal, Int. J. Miner. Process., 99 (2011) 43–53.10.1016/j.minpro.2011.03.004Search in Google Scholar

[4] S. Dawrapudi, T.K. Ghosh, A. Shankar, V. Tathavadkar, D. Bhattacharjee and R. Venugopal, Int. J. Miner. Process., 112 (2012) 56–62.10.1016/j.minpro.2012.06.006Search in Google Scholar

[5] B.K. Pandey and T. Sharma, Int. J. Miner. Process., 59 (2000) 295–304.10.1016/S0301-7516(99)00080-0Search in Google Scholar

[6] S. Halder and R.J. Fruehan, Metall. Mater. Trans. B., 39B (2008) 809–817.10.1007/s11663-008-9201-3Search in Google Scholar

[7] R.J. Fruehan, Metall. Mater. Trans. B., 8B (1977) 279–286.10.1007/BF02657657Search in Google Scholar

[8] X. Chen, Pyrometallurgical Process Physical Chemistry (Chinese), Metallurgy Industry Press, Beijing (1984).Search in Google Scholar

[9] Z. Tang, G. Li, X. Li, H. Zhang, Y. Zhang and T. Jiang, Ironmaking & Steelmaking., 40 (2013) 69–73.10.1179/1743281212Y.0000000024Search in Google Scholar

[10] S.C. Panigraphy, B.C. Jena and M. Rigaud, Metall. Miner. Trans. B., 21B (1990) 463–474.10.1007/BF02667858Search in Google Scholar

[11] D. Zhu, T. Chun, J. Pan and J. Zhang, Int. J. Miner. Process., 125 (2013) 51–60.10.1016/j.minpro.2013.09.008Search in Google Scholar

Received: 2014-12-15
Accepted: 2015-6-20
Published Online: 2015-8-5
Published in Print: 2016-6-1

©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.

Articles in the same Issue

  1. Frontmatter
  2. Research Articles
  3. Effect of Activating Agent on the Preparation of Bamboo-Based High Surface Area Activated Carbon by Microwave Heating
  4. Research on the Microstructure and Mechanical Property of Ti-7Cu Alloy after Semi-Solid Forging
  5. Synthesis and Characterization of BiFeO3 Ceramic by Simple and Novel Methods
  6. Synthesis and Characterization of Lead Sulfide Nanostructures with Different Morphologies via Simple Hydrothermal Method
  7. Comparison of Brazed Residual Stress and Thermal Deformation between X-Type and Pyramidal Lattice Truss Sandwich Structure: Neutron Diffraction Measurement and Simulation Study
  8. Influence of V–N Microalloying on the High-Temperature Mechanical Behavior and Net Crack Defect of High Strength Weathering Steel
  9. Numerical Simulation on Optimization of Center Segregation for 50CrMo Structural Alloy Steel
  10. Direct Extraction of Ti and Ti Alloy from Ti-Bearing Dust Slag in Molten CaCl2
  11. Constitutive Equation for 3104 Alloy at High Temperatures in Consideration of Strain
  12. Microstructural Properties and Four-Point Bend Fatigue Behavior of Ti-6.5Al-2Zr-1Mo-1V Welded Joints by Electron Beam Welding
  13. Oxidizing Roasting Performances of Coke Fines Bearing Brazilian Specularite Pellets
  14. Purification of Niobium by Electron Beam Melting
  15. Void Formation during Diffusion – Two-Dimensional Approach
https://doi.org/10.1515/htmp-2014-0233
Keywords for this article
Brazilian specularite pellets; coke fines; compressive strength; metallurgical performances
Creative Commons
BY-NC-ND 3.0

Articles in the same Issue

  1. Frontmatter
  2. Research Articles
  3. Effect of Activating Agent on the Preparation of Bamboo-Based High Surface Area Activated Carbon by Microwave Heating
  4. Research on the Microstructure and Mechanical Property of Ti-7Cu Alloy after Semi-Solid Forging
  5. Synthesis and Characterization of BiFeO3 Ceramic by Simple and Novel Methods
  6. Synthesis and Characterization of Lead Sulfide Nanostructures with Different Morphologies via Simple Hydrothermal Method
  7. Comparison of Brazed Residual Stress and Thermal Deformation between X-Type and Pyramidal Lattice Truss Sandwich Structure: Neutron Diffraction Measurement and Simulation Study
  8. Influence of V–N Microalloying on the High-Temperature Mechanical Behavior and Net Crack Defect of High Strength Weathering Steel
  9. Numerical Simulation on Optimization of Center Segregation for 50CrMo Structural Alloy Steel
  10. Direct Extraction of Ti and Ti Alloy from Ti-Bearing Dust Slag in Molten CaCl2
  11. Constitutive Equation for 3104 Alloy at High Temperatures in Consideration of Strain
  12. Microstructural Properties and Four-Point Bend Fatigue Behavior of Ti-6.5Al-2Zr-1Mo-1V Welded Joints by Electron Beam Welding
  13. Oxidizing Roasting Performances of Coke Fines Bearing Brazilian Specularite Pellets
  14. Purification of Niobium by Electron Beam Melting
  15. Void Formation during Diffusion – Two-Dimensional Approach
Sign up now to receive a 20% welcome discount
Institutional Access
How does access work?
Have an idea on how to improve our website?
Please write us.
© 2025 De Gruyter Brill
Downloaded on 11.9.2025 from https://www.degruyterbrill.com/document/doi/10.1515/htmp-2014-0233/html
Scroll to top button