Research on the reduction of Guizhou oolitic hematite by hydrogen

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.

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.

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

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

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.

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 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.