2023 Volume 63 Issue 8 Pages 1328-1333
In this study, a new method of microwave-hydrogen synergistic reduction of vanadium titano-magnetite (VTM) was developed to carry out experimental research. Using the theory of the direct low-temperature reduction process, VTM from the area surrounding Chengde, China was used as raw material and H2 was used as a reducing agent. The experimental results and the theoretical analysis proved that VTM can be feasibly reduced via microwave-hydrogen synergistic reduction at low temperature. In addition, H2 reduction of iron titanium oxides was more difficult than that of iron oxides and required a higher reaction temperature. Under microwave heating conditions, increasing the temperature, reduction time, and H2 proportion improved the metallization rate. When reducing for 40 min at 1100°C with 60% H2, the metallization rate reached 92.2%. The reduction product had a porous, sponge-like structure, and it was primarily composed of Fe and Fe9.64Ti0.36 phases. This implies that the Fe9.64Ti0.36 phase may be the enriched phase of Mg, Ca, and Si. During the synergistic reduction process, the metallic iron that precipitated inside the particles migrated to the outer edge of the particles, and the titanium iron oxides that were difficult to reduce inside were coated with metallic iron.
Vanadium titano-magnetite (VTM) is a multi-metal element composite of metal minerals, and it has extensive utilization value because it is the main carrier of iron, vanadium, and titanium.1,2) VTM exhibits a dense structure and complex interaction between valuable elements, and thus valuable metals cannot be directly extracted from it.3,4) Blast furnace and non-blast furnace smelting are usually employed to comprehensively utilize VTM.5) When a blast furnace smelts VTM, the high-temperature reduction of carbon (1300–1400°C) is usually reached,6) but this temperature has the disadvantages of high carbon emissions and high energy consumption. In addition, titanium enters the slag in the form of TiO2, which leads to a waste of titanium resources.7) In contrast, the direct reduction process has the advantages of high efficiency, strong adaptability of raw materials, and stable product composition, and is widely used in VTM research.8,9,10,11,12,13,14) In recent years, the microwave thermal effect has provided a new heating method for metallurgical research, and a number of domestic researchers have used microwaves to execute the direct reduction of polymetallic composite ores. For example, Sun et al.15) applied microwave carbothermal reduction to vanadium titanium magnet concentrate, and the results indicated that microwave heating can reduce the reduction temperature and shorten the reduction time. In addition, Zhao et al.16) directly reduced VTM using anthracite and confirmed that the effect of microwave heating on the reduction process has significant advantages compared to that of conventional heating. Furthermore, Hu et al.17) proposed a new low-temperature method for the rapid and direct reduction of magnetite (called “pre-oxidation-microwave heating deep reduction”), which reached a metallization rate as high as 95.7%. Despite these results, these studies are mostly limited to microwave carbothermal reduction, and there are few studies on the microwave-hydrogen synergistic low-temperature reduction of iron ore.
This study, aimed to preliminarily explore the effect of microwave-hydrogen synergistic low-temperature reduction of iron ore powders and the distribution law of iron and titanium for the purpose of providing a theoretical basis for the separation of iron and titanium. In the experiments, the microwave heating method was applied to produce VTM powders as raw materials, and H2 was used as the reducing agent. In addition, microwave-hydrogen synergy is an important “electricity-hydrogen” combination, and is expected to provide an additional technique for reducing iron ore as well as a potential new method for simultaneously decreasing carbon emissions and comprehensively utilizing ore resources.
The chemical compositions and particle size of ilmenite concentrate in the area surrounding Chengde, China are presented in Tables 1 and 2, respectively. VTM in this area is characterized by poor vanadium and titanium, titanium in magnetite exists in solid solution form. A scanning electron microscopy (SEM) analysis (Fig. 1(a)) showed that the Chengde ilmenite had a dense, smooth surface structure with fine openings. An X-ray diffraction (XRD) analysis (Fig. 1(b)) indicated that the Chengde ilmenite concentrate was primarily composed of Fe3O4, Fe2.75Ti0.25O4, MgO, 2FeO·2Al2O3·5SiO2, and 2FeO·SiO2. Since magnetite predominates in ilmenite, the VTM mentioned below referes to part of the ilmenite in the experimental sample.
| Phase | Fe2O3 | TiO2 | SiO2 | Al2O3 | MgO | CaO | V2O5 | MnO | P2O5 |
|---|---|---|---|---|---|---|---|---|---|
| Weight percentage (wt.%) | 89.00 | 2.92 | 3.05 | 1.23 | 1.84 | 0.87 | 0.50 | 0.11 | 0.10 |
| Particle size (mm) | <0.045 | 0.045–0.074 | 0.074–0.15 | >0.15 |
| Proportion (%) | 82.50 | 10.67 | 4.02 | 2.81 |
(a) SEM image and (b) XRD pattern of the raw materials.
Figure 2 shows a schematic diagram of the microwave heating reduction setup; the microwave equipment parameters are listed in Table 3. In each experimental run, a sample of approximately 15 g was loaded into a corundum crucible. Then the crucible with the sample inside was placed in a heating furnace, and argon was introduced into the system to remove the air out of the furnace, the argon gas flow rate was 500 ml/min. The furnace was then heated from room temperature (around 25°C) up to the desired reduction temperature at a heating rate of 5°C/min in the argon atmosphere. When the reduction temperature was reached and kept warm for 1 hour, Ar–H2 gas mixture was introduced into the furnace to reduce the sample, the total gas flow of the two was 1160 ml/min. The hydrogen gas flow rate was 348–1160 ml/min, and controlled by gas flowmeter. After the experiment, the sample was cooled to room temperature. The thermocouple was used to measure temperature in the experiment. In order to ensure the accuracy of the temperature, the temperature measurement position was about 2 mm above the furnace tube, and a special crucible was used to make the upper edge close to the inner wall of the furnace tube, so that the distance between the sample and the temperature measuring head was as small as possible. The infrared thermometer was used for correction, and the error was about 10°C. Therefore, when the program temperature was setted, it can be reduced about 10°C.
Schematic diagram of an experimental microwave heating setup. (Online version in color.)
| Name | Parameters |
|---|---|
| Manufacturer | Tangshan Renshi Juyuan Microwave Apparatus Co., Ltd |
| Model number | WBAW-4 |
| Frequency | 2.45 GHz |
| Power | 4 KW |
| Working temperature | ≤1500°C |
| Working pressure | ≤0.1 Mpa |
| Heating constant temperature zone | 300 mm |
In this study, the reduction effect of VTM was characterized by the metallization rate, which is defined as (1). Metallic iron (MFe) and total iron (TFe) were detected by chemical titration and inductive coupled plasma emission spectrometer (ICP–MS), respectively.
| (1) |
Where, MFe is the amount of metallic iron in the reduction product and TFe is the total amount of iron in the reduction product.
The reduction of VTM is primarily based on the reduction reactions of iron oxides and iron titanium oxides, and the main oxides are Fe3O4, Fe2TiO4, and FeTiO3. In order to clarify the reduction thermodynamics of the H2-reduced VTM, the calculation results produced by FactSage were plotted, as shown in Fig. 3. The figure indicates that the H2 reduction of the iron titanium oxides was more difficult than that of the iron oxides and required a higher reaction temperature (the reduction of iron oxides have been reported previously18)). The reduction of the iron titanium oxides was also a gradual reduction below 1130°C, and reductions of the iron titanium oxides to Fe and TiO2 also occurred. When the temperature was above 1130°C, the iron titanium oxides were first reduced to FeTi2O5, and then were reduced to Fe and TiO2.
Equilibrium diagram of the H2 reduction of the VTM. (Online version in color.)
Figure 4 shows the effect of temperature on the metallization rate and microscopic morphology under 30% H2 for 40 min. The figure indicates that increasing the temperature increased the metallization rate, and that the microscopic morphology developed into a porous structure. Between the temperatures of 900°C and 1000°C, the metallization rate increased rapidly from 60% to 81.0%. At 900°C, small holes appeared on the surface of the product, but the microstructure was essentially similar to that of the raw material. Between the temperatures of 1000°C and 1100°C, the metallization rate slowly increased from 81% to 82.5%, which indicated that the metallization rate was limited and that the reaction was effectively finished. At 1100°C, the surface of the reducing product was fluffy, large holes appeared, and there were mosaic-like raised substances. These results implied that a high temperature improved the molecular kinetic energy of the H2 and that it accelerated gas diffusion. Thus, the reduction temperature was maintained at approximately 1100°C.
Effect of temperature on the metallization rate and microstructure. (Online version in color.)
Figure 5 shows the effect of the H2 proportion on the metallization rate and microscopic morphology at 1100°C for a duration of 40 min. The figure indicates that the metallization rate increased as the H2 proportion increased, and that the microscopic morphology developed into irregular porous spongy structures. When the H2 proportion increased from 30% to 60%, the metallization rate changed significantly (from 82.5% to 92.2%) due to the diffusion of hydrogen in the lattice and the strengthening of the electron exchange, which promoted the reduction reaction. When the H2 proportion increased from 60% to 100%, the metallization rate increased by only 2.7% and the reduction effect was not significant, but the two reduction products had similar microstructures with surfaces containing small pores. Considering the economy of H2 preparation19) and the fact that the metallization rate reached above 92%, the H2 proportion was maintained at approximately 60%.
Effect of the H2 proportion on the metallization rate and microstructure. (Online version in color.)
The effects of the reduction time on the metallization rate and microscopic morphology at 1100°C and for 60% H2 are shown in Fig. 6. The data points in the figure show that the metallization rate increased as the reduction time increased and that the microstructure changed from a fluffy to a dense form. In the first 30 min of the reduction process, the metallization rate increased sharply from 47% to 79.6%, and the surface was fluffy. In the later stages of reduction, the metallization rate increased slowly as the reduction time was extended, which indicated that there were iron-titanium oxides that would be difficult to reduce. As the reduction time increased, sintering occurred near the reaction interface, making it more difficult for H2 to diffuse into the internal reaction interface. Therefore, the reduction time was limited to approximately 40 min because once this much time elapsed the metallization rate reached as high as 92.2% and there was no sintering.
Effect of the reduction time on the metallization rate and microstructure. (Online version in color.)
In order to gain insight into the reduction mechanism, both raw material and reduced ilmenite samples were analyzed by SEM and energy dispersive spectroscopy (EDS) techniques.
Figure 7(a) presents a SEM image of the raw material, and Fig. 7(b) shows a magnification of the red square shown in Fig. 7(a). The SEM image indicates that the structure was dense and nonporous. Three distinct phases can be observed in Fig. 7(b), and the points in the figure marked “1,” “2,” and “3” correspond to the EDS results shown in Table 4. The combination of these results and the XRD results implied that the white-gray area at point 1 was caused by iron oxides, the black-gray area at point 2 was caused by titanium oxides (which were distributed in a network pattern), and the dark gray area at point 3 was caused by iron-titanium oxides (which were embedded at the edges of the particles).
SEM backscattering micrographs of a cross section of partial raw material: (a) original image; (b) magnified image. (Online version in color.)
| Location | Fe (wt.%) | Ti (wt.%) | V (wt.%) | O (wt.%) |
|---|---|---|---|---|
| Point 1 | 70.95 | 1.70 | 0.23 | 27.12 |
| Point 2 | 1.21 | 58.80 | – | 38.99 |
| Point 3 | 37.04 | 29.09 | 2.45 | 31.22 |
Figure 8 displays a cross-sectional SEM image of the sample reduced for 40 min at 1100°C with 60% H2. The image shows that the product was porous. Three distinct phases were identified, and the points in Fig. 8 marked “4,” “5,” and “6” correspond to the EDS results shown in Table 5 and the XRD results shown in Fig. 9. The SEM-EDS and XRD detections indicated that the white-gray areas in Fig. 8 (points 4 and 5) were caused by the Fe phases, which were derived from the reduction of the iron oxides in the raw material. The dark black-gray area at point 6 was caused by the Fe9.64Ti0.36 phases, which may have been the enriched phases of Mg, Ca, and Si.
SEM backscattering micrographs of a cross section of the reduction products reduced for 40 min at 1100°C with 60% H2. (Online version in color.)
| Location | Fe (wt.%) | Ti (wt.%) | V (wt.%) | O (wt.%) | Mg (wt.%) | Al (wt.%) | Si (wt.%) | Ca (wt.%) |
|---|---|---|---|---|---|---|---|---|
| Point 4 | 96.65 | 2.52 | – | 0.53 | – | – | – | – |
| Point 5 | 95.42 | 3.41 | – | 1.17 | – | – | – | – |
| Point 6 | 4.76 | 51.96 | – | 38.94 | 1.56 | 0.52 | 0.96 | 1.32 |
XRD patterns of the ilmenite powders reduced for 40 min at 1100°C with 60% H2. (Online version in color.)
Figure 10 shows EDS maps of cross sections of the raw ilmenite powders. The Fe was primarily distributed at the edges of the particles, and the Ti was distributed inside the particles, showing a staggered complementary distribution. Hence, the metallic iron phase formed at the surface of the powders and separated from the titanium oxides.
EDS maps of cross sections of the ilmenite powders reduced for 40 min at 1100°C with 60% H2. (Online version in color.)
When the microwave-hydrogen reduction was synergistic, the interiors of the VTM powders were first reduced, and then the metallic iron migrated, aggregated, and grew along the outer layer of the iron oxides to the edges of the particles, locally forming a porous spongy structure. On the one hand, the porous spongy structure may have been caused by the breaking of Fe–O or Ti–O bonds, resulting in oxygen migration and removal.20) On the other hand, because H2 molecules cannot stably adsorb Fe atoms, the H2 could have easily escaped.21)
Under microwave heating conditions, there was no “regular spherical” unreacted core in the ilmenite powders, which was due to the heating effect of the temperature gradient from the inside out.22) In addition, relevant research has demonstrated that microwave field contributes to electric dipole polarization, namely displacement and oriented polarization.23,24,25) The essential difference between the two lies in the change in the magnitude and direction of the distance vector between the positive and negative centers of the dipole.26,27) Therefore, under the action of microwave field, accelerating the diffusion of H2 into the internal surface through the porous spongy metallic iron was the key to improving the metallization rate. These results were used to construct a preliminary schematic model of the microwave-hydrogen synergistic reduction of ilmenite, which is shown in Fig. 11.
Schematic diagram illustrating the process of the microwave-hydrogen synergistic reduction of ilmenite powders. (Online version in color.)
(1) According to the thermodynamic calculations, the initial reduction temperature of the titanium iron oxides was 1130°C, the H2 reduction of the iron titanium oxides was more difficult than that of the iron oxides, and the primary reduction products were TiO2, FeTi2O5, FeTiO3, and FeTiO4.
(2) Performing microwave-hydrogen synergistic reduction for 40 min at 1100°C with 60% H2 was ideal for optimizing the effect of the reduction and for achieving a high metallization rate (which reached as high as 92.2% in the experiment).
(3) The experimental results and the theoretical analysis proved that VTM can be feasibly reduced via microwave-hydrogen synergistic reactions at low temperature, and that the reduction products are primarily composed of the Fe and Fe9.64Ti0.36 phases. In the reduction process, the interiors of the iron ore particles were first reduced, and then the metallic iron migrated and accumulated on the outer edge of the particles to form a porous spongy metallic iron. These results provide a theoretical basis for the separation of iron and titanium.
The authors gratefully acknowledge financial support from the Natural Science Foundation of Hebei Province (E2021209101, E2022209112), Science and Technology Research Project of Higher Education Institutions of Hebei Province (ZD2022125), Tangshan Talent Funding Project (A20220212), and Hebei Graduate Innovation Grant Program (CXZZBS2021100).
