Iron production from Fe3O4 and graphite by applying 915 MHz microwaves
- a Kyoto University, Gokasho, Uji-shi, Kyoto 611-0011, Japan
- b National Institute for Fusion Science, 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan
- c Department of Chemistry and Materials Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan
- d Microwave Processing and Engineering Center, Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA
- e Department of Conservation Science, Tokyo University of the Arts, 12-8 Ueno Kouen, Taito-ku, Tokyo 110-8714, Japan
- Received 20 February 2012
- Revised 9 July 2012
- Accepted 11 July 2012
- Available online 20 July 2012
The chemical behavior of iron ore reduction was experimentally studied under electric and magnetic fields of microwaves. The microwave magnetic field rapidly heated the powdery ores so that the reduction of the ore to metal ions was completed. The mechanism was investigated theoretically. It was found that the combined effect of thermal energy and a portion of the microwave magnetic field enhanced deoxidation and that the microwaves act as a dynamic catalyst at high temperatures. High-frequency magnetic fields were found to enhance the antibonding character of oxygen in materials with unpaired spins above the Fermi level of the 3d shell.
- Microwave heating;
- Metal oxides;
- 915 MHz;
- Microwave chemistry;
The imminent shortage of high-quality iron ore and fossil fuels such as coal and oil has recently been compelling researchers to develop new steel-making methods based on energy transfer processes that do not involve thermal energy generated by high-temperature gases ,  and . Microwave processing has attracted interest for steel making due its ability to realize volumetric heating and its high energy efficiency  and . 2450 MHz microwave irradiation has recently been used to reduce, melt, and cast iron ores (magnetite or hematite) in the presence of carbon  and . Ishizaki et al. constructed a high-power (120 kW) continuous microwave furnace and Huang et al. built a 915 MHz, 225 kW microwave furnace to scale up steel production  and . Huang et al. obtained sponge iron, but since they did not reach the melting stage using 915 MHz microwave heating, they employed DC arcing to complete iron ore reduction. If the physical mechanism of the various reactions that occur during microwave heating of a mixture of iron and carbon was determined, a better method could be developed for completing the reduction and melting of iron. In this study, we experimentally investigate the chemical behavior of ore reduction using microwaves.
2. Experimental procedures
The experimental system consists of waveguides with a 915 MHz magnetron oscillator, a three-stub tuner, a plunger, an isolator, and a TE102 rectangular waveguide cavity coupled through an iris (Fig. 1). The cavity was tuned to 915 MHz using the plunger at the end of the waveguide. This system provides separation of the electric and magnetic fields inside the cavity . A quartz cylinder was installed in the cavity. It was evacuated by a rotary pump and purged by high-purity (99.9%) N2 gas. A precursor mixture of iron ore and carbon was placed in a crucible (φ12 mm×12 mm) and it was insulated by a layer of alumina–silica fiber board. The surface temperature of the reactants was monitored using an infrared radiation pyrometer. In this study, a 89 wt% Fe3O4–11 wt% C (10 g; Fe3O4:1 μm pass) mixture was placed at an electric field node (where the magnetic field is zero), a magnetic field node (where the electric field is zero), and a location where E and H fields coexist (referred to as the E–H mixed mode), as shown in Fig. 1.
3. Results and discussion
Fig. 2 shows the temporal evolutions of the sample surface temperature for the E-field, the E–H mixed mode, and the H-field, respectively. The E–H mixed mode and the H-field generated temperatures of over 1300 and 1150 °C at 3 min, respectively. In these experiments, the holding time was fixed at 15 min (for which Emax and Hmax generated temperatures of 1100 and 1260 °C, respectively). For the E–H mixed mode, the sample temperature exceeded 1300 and 1360 °C after 3 min and 6 min, respectively. The E–H mixed mode provided unsteady heating and the temperature gradually increased to 1370 °C in 17 min. This unsteady heating is due to microwave absorption changing as different chemical reactions are promoted. Fig. 3 shows XRD 2θ spectra obtained after processing in the E-field, the E–H mixed mode, and the H-field. The H field gave lower FeO peaks and higher α-Fe peaks than the E–H mixed mode. The E–H mixed mode gave higher temperature than the E-field, which indicates that pure H-field heating enhances ore reduction (FeO=Fe+1/2O2).