Effects of Magnetic Field on Reduction of CaOContaining Iron Oxides
Yongli JIN1,2,Hai YU2,Jieyu ZHANG1,Zengwu ZHAO3()
1. School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China 2. School of Materials and Metallurgy, Inner Mongolia University of Science and Technology, Baotou 014010, China 3. Inner Mongolia Key Laboratory for Utilization of Bayan Obo Multi-Metallic Resources, Inner Mongolia University of Science and Technology, Baotou 014010, China
Cite this article:
Yongli JIN,Hai YU,Jieyu ZHANG,Zengwu ZHAO. Effects of Magnetic Field on Reduction of CaOContaining Iron Oxides. Acta Metall Sin, 2019, 55(3): 410-416.
Iron ore direct reduction is an attractive alternative route for effective use of low-grade complex symbiotic iron ore resources as well as reducing CO2 emissions from steel making. In this process, solid iron ore pellets are converted to so-called direct reduced iron with a reduction gas such as CO and H2. However, the slow reduction rate of iron oxides at lower temperatures has restricted the productivity of direct reduced iron. We have been studying the use of a magnetic field to enhance the direct reduction process, and investigating the influences of the magnetic field on the reduction of iron oxides and morphology of direct reduced iron. Iron ores are rich in iron oxides and also contain other oxides. The presence of other oxides, for instance CaO, is likely to interact with iron oxides during reduction so as to enhance the reduction rate by varying the lattice structure of solid iron oxides and gas/solid mass transport. In the present work, the effects of magnetic field on the reduction of CaO-containing iron oxides were studied. Isothermal reduction of compact samples of pure Fe2O3 and 2.5%CaO (mass fraction) containing Fe2O3 was carried out at 1073 K under a reaction atmosphere 75%CO+25%CO2 (volume fraction). A constant magnetic field (B=1.02 T) was applied during reduction to compare with the reaction under a normal condition without a magnetic field applied. The results showed that the magnetic field accelerates the reaction rate of Fe2O3 reduction to metallic Fe, and there are no phase compositions change during reduction in a constant magnetic field. The magnetic field promotes the diffusion of Ca in Fe2O3+2.5%CaO, and the reduced sample obtained in a magnetic field appears loose and porous. Thermodynamic calculation indicated that the Gibbs free energy of Fe2O3 reduction and CaFe5O7 phase decomposition is decreased with an interaction of magnetic field, resulting in an increase of the reaction equilibrium constant thus making the reduction of Fe2O3 and decomposition of intermediate phase CaFe5O7 occur more readily.
Fig.1 Schematic of magnetic field reduction furnace (1—gas cylinder, 2—moisture filter, 3—oxygen filter, 4—mass flow controller, 5—gas mixer, 6—Al2O3 work tube with heating elements wrapped around the tube, 7—heat insulation cover, 8—water cooling jacket, 9—permanent magnet, 10—thermocouple, 11—sliding rail for moving heat insulation cover (7) and magnet (9), 12—furnace temperature controller)
Fig.2 Isothermal reduction curves of pure Fe2O3 and Fe2O3+2.5%CaO systems in a constant magnetic field (CMF) 1.02 T, and under the normal condition (NC) without a magnetic field applied
Fig.3 XRD spectra of Fe2O3+2.5%CaO after reduction for various reaction periods in a constant magnetic field 1.02 T (a) and under the normal condition without a magnetic field applied (b)
Point
Fe
O
Ca
1
100.00
-
-
2
83.81
11.59
4.61
3
97.31
1.11
1.58
4
52.11
16.74
31.15
5
80.87
7.74
11.39
6
83.91
13.41
2.68
Table 1 EDS results of points 1~6 in Fig.4 (mass fraction / %)
Fig.4 SEM images of reduced samples for pure Fe2O3和Fe2O3+2.5%CaO after reduction for various periods in a constant magnetic field 1.02 T, and under the normal condition without a magnetic field applied(a) pure Fe2O3, CMF, 20 min (b) Fe2O3, NC, 20 min (c) Fe2O3+2.5%CaO, CMF, 20 min (d) Fe2O3+2.5%CaO, NC, 20 min (e) Fe2O3+2.5%CaO, CMF, 60 min (f) Fe2O3+2.5%CaO, NC, 60 min
Fig.5 SEM image of Fe2O3+2.5%CaO after reduction for 30 min in a constant magnetic field 1.02 T (a) and its element maps of Fe (b), Ca (c) and O (d)
Process
Equation (2)
Equation (3)
Gibbs free energy
J·mol-1
K
Gibbs free energy
J·mol-1
K
NC
= -3.0720×104
31.299
= -3.4127×105
9.4330×103
CMF
ΔGT= -3.1523×104
34.243
ΔGT= -8.4863×105
1.3531×104
Table 2 Gibbs free energies and equilibrium constants for reactions associated with equations (2) and (3)
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