Phases Transition and Consolidation Mechanism of High Chromium Vanadium-Titanium Magnetite Pellet by Oxidation Process

Effect of roasting temperature on compressive strength of HCVTM oxidized pellet

The compressive strength (CS) of HCVTM oxidized pellet at different roasting temperatures was tested as Chinese GB/T 14201–1993, illustrated in Figure 5. Thus, CS increased with the temperature and the changes were described as three sections according to the TG-DSC curve of HCVTM (Figure 3). When the roasting temperature was less than 1,173 K, CS had a slight increase from 50 N to 516 N corresponding to the increase in temperature from 573 K to 1,173 K. With the temperature increased to 1,373 K, the rate of CS also increased. While the temperature was above 1,373 K, CS improved significantly.

Figure 5:

Effect of roasting temperature on the CS of HCVTM oxidized pellet.

The reasons that the CS was advanced by the increasing temperature could be summarized as follows: as the temperature rising, all the physical and chemical reactions inside pellet were accelerated and the recrystallization was promoted, which were both conducive to the sufficient oxidation. And then the structure of pellet was compact and the CS improved [18, 19]. When the roasting temperature exceeded 1,523 K, CS was 2,847 N which could meet the requirement of gas-based shaft furnace production.

Effect of roasting temperature on phase compositions of HCVTM oxidized pellet

HCVTM oxidized pellets roasted at different temperatures were analyzed by XRD and shown in Figure 6 and Table 2. At room temperature, the main phases inside HCVTM pellet were magnetite (Fe₃O₄), titanomagnetite (Fe_2.75Ti_0.25O₄), chromite (FeCr₂O₄) and coulsonite (Fe₂VO₄). When the roasting temperature was below 773 K, the main phase compositions nearly kept the same. It was just because that the oxidation was slow leading to the low reaction rate as well as the lazy diffusion among particles, which made the phase has a slight change and difficult to be detected out. This phenomenon provided a proper evidence for the not so obvious improvement of CS at the low temperature (Figure 5).

Figure 6:

XRD patterns of HCVTM pellet roasted at different temperatures.

With the increase of the roasting temperature to 973 K, both the oxidations of magnetite (Fe₃O₄) and titanomagnetite (Fe_2.75Ti_0.25O₄) were promoted. Pure magnetite was oxidized to hematite directly above 773 K [20]. Several new peaks appeared in the XRD pattern which could be attributed to the newly formed phase of titanohematite (Fe₉TiO₁₅). Besides, FeCr₂O₄ was “corroded” by magnetite and generated the solid solution of Cr_1.3Fe_0.7O₃, but the vanadium in Fe₂VO₄ happened to transition and generated the solid solution of (Cr_0.15V_0.85)₂O₃.

At the roasting temperature of 1,173 K, Fe₃O₄ was not detected out and had been already totally oxidized to Fe₂O₃, so the peaks of Fe₂O₃ became stronger. As the temperature increased continuously, the oxidation was promoted and all the oxidation reactions were enhanced. At 1,373 K, except for the stronger diffraction peaks of each phase, the main phases in HCVTM oxidized pellet had no changes. However, because of the overlapping peaks, the decomposition from Fe₉TiO₁₅ to Fe₂O₃ and FeTiO₃ was not verified at 973–1,373 K. When the temperature was up to 1,573 K, the titanium was present in the form of Fe₉TiO₁₅ and pseudorutile (Fe₂Ti₃O₉) generated from the further oxidation of FeTiO₃. But ulvospinel (Fe₂TiO₄), FeTiO₃ and rutile (TiO₂) were not detected out.

Effect of roasting temperature on the microstructure of HCVTM oxidized pellet

Figure 7 showed the SEM images with EDS analyses of HCVTM oxidized pellet at different temperatures. From the result, when the temperature was low, the particles were independent and angular, and the pores were coexisted as three shapes of big pores, small pores and intervals among particles; in general, the structure of pellet was loose. As the temperature increasing to 973 K, the number of pore decreased obviously. Magnetite was oxidized to hematite, and the corresponding magnetite of V, Ti and Cr was also oxidized to their own hematite or generated some special and complicated solid solutions (Points E and F in Figure 7(c)). The structure of HCVTM pellet became compact. Further increasing the temperature, both the recrystallization and crystal growth were enhanced, and the crystal bridges among particles were developed but still un-widespread, so the whole pellet structure was not contiguous. At 1,573 K, the crystal bridges further transited and grew, and there were plenty of liquid phases uniformly distributed in the pellet. Then the particles were interconnected and the inside structure was compact, which were both contribute to obtain a better CS. Combining the elemental proportions of point H obtained from the EDS spectrum and the results of XRD analysis, the phase composition of H was initially identified as a mixture of Fe₉TiO₁₅ and Fe₂Ti₃O₉.

Figure 7:

SEM images with EDS analyses (mass %) of HCVTM oxidized pellet at different temperatures.

The atomic iron to titanium ratio (Fe/Ti) of the white phase along analyzed lines in the HCVTM oxidized pellet at different temperatures was compared in Figure 8. The analyzed white phase contained Fe, V, Ti, Cr and O. Formation of a new titanohematite solid solution (Fe₉TiO₁₅) was seen from the XRD pattern (Figure 6), the SEM image (Figure 7) and the Fe/Ti ratio (Figure 8), and its percent was increased with the roasting temperature. At 973 K, the exsolved titanohematite had a higher content of Ti than that in the titanomagnetite matrix, which could be assigned to the higher mobility of Fe³⁺ in comparison with Ti⁴⁺. In the process of titanomagnetite oxidation, the concentration of Fe³⁺ increased, causing a compressive stress in the titanomagnetite lattice and triggering the structural transition of cubic spinel to rhombohedral titanohematite. During this transition, the Fe³⁺ migrated out of the fresh was formed rhombohedral lattice what accelerated the further oxidation of titanomagnetite. This process was characterized by the fluctuation of Fe/Ti ratio at a low roasting temperature (Figure 8(a)). With the increase of roasting temperature, the oxidation reactions were stable gradually resulting in the relatively uniform Fe/Ti ((Figure 8(b) and (c)). Generally, the Fe/Ti decreased and to be stable as the temperature increased.

Figure 8:

The Fe/Ti ratio of the white phase in HCVTM oxidized pellet at 973 K (a), 1,173 K (b), and 1,373 K (c).

Phase transitions of valuable elements in HCVTM oxidized pellet

Combining the above analyses, reactions of the phases containing Ti in HCVTM pellet during oxidation process were summarized as follows:

The oxidation of titanomagnetite with the formation of a titanohematite solid solution (Fe₉TiO₁₅) and ilmenite (FeTiO₃) at 773–973 K:

The further oxidation of ilmenite with the formation of pseudorutile (Fe₂Ti₃O₉) at 1,373–1,573 K:

From the ternary phase diagram of FeO-Fe₂O₃-TiO₂, the oxidation paths of Ti-bearing phases in HCVTM were also illustrated in Figure 9. As shown, ulvospinel and magnetite formed a solid solution of titanomagnetite (spinel, line ①); ilmenite and hematite formed a solid solution of titanohematite (rhombohedral, line②). There was a miscibility gap between titanomagnetite and titanohematite [21, 22]. The oxidation path followed the line marked [1], [2] and [3] corresponding to the reactions of (1), (2) and (3) respectively. The reaction (1) included the oxidation of Fe²⁺→ Fe³⁺, migration of iron ions (Fe²⁺ and Fe³⁺) and structural transition of spinel to rhombohedral. However, the reaction (2) included the structural transition of spinel to trigonal (ilmenite). At 1,373–1,573 K, the reaction (3) started; however, the degree of the transition from ilmenite to pseudorutile was quite low.

Figure 9:

The ternary system of FeO-Fe₂O₃-TiO₂.

The phase containing V transformed from coulsonite spinel (Fe₂VO₄) to the solid solution of V and Cr after 773 K with the reactions:

Accordingly, the Cr₂O₃, transformed from the chromite spinel (FeCr₂O₄), generated the solid solution with V₂O₃ and Fe₂O_3, respectively, together with the reaction (5) and the following reactions:

Consequently, the phase transition behaviors in HCVTM during the oxidation process were as follows: (1) Fe₃O₄ → Fe₂O₃; (2) Fe_2.75Ti_0.25O₄ → Fe₉TiO₁₅ + FeTiO₃ → Fe₉TiO₁₅ + Fe₂Ti₃O₉; (3) Fe₂VO₄ → V₂O₃ → (Cr_0.15V_0.85)₂O₃; (4) FeCr₂O₄ → Cr₂O₃ → Cr_1.3Fe_0.7O₃ + (Cr_0.15V_0.85)₂O₃.

Oxidation consolidation process of HCVTM oxidized pellet

As well as the ordinary magnetite pellet, the solid phase consolidation was the main method of oxidation consolidation for HCVTM pellet. Through the analyses on the change rules of CS, morphology evolutions and phase transitions, the detail oxidation consolidation process of HCVTM pellet was divided into three stages and presented as follows:

The first stage was oxidation. When the roasting temperature was less than 1,173 K, the main reactions were the oxidations of pure magnetite and titanomagnetite. In this stage, the phase composition was mainly composed of iron oxide. The mineral particles were angular and the number of pore in the pellet was more (Figure 7(a), (b) and (c)). The CS of pellet was relatively low, less than 600 N (Figure 5, section (1)).

The second stage was recrystallization consolidation. The temperature range was 1,173–1,373 K. Hematite was recrystallized and the crystal bridges among mineral particles appeared during this stage. Besides, the phases containing V, Ti and Cr continued to consolidate. The crystal bridges among particles developed but still un-widespread, so the whole pellet structure was not contiguous (Figure 7(d) and (e)). Therefore, the CS reached to 774 N (Figure 5, section (2)).

The third stage was the particle refining and recrystallization-consolidation by the attending of liquid phase. The temperature range was 1,373 ～ 1,573 K. In this stage, the silicate liquid phase generated gradually, by which the hematite and solid solution were separated to many small particles. And these small particles recrystallized and grew quickly. In addition, the small particles were conjoined by the crystal bridges and a small amount of slag. Then the pellet formed the compact whole and the oxidation consolidation was almost complete (Figure 7(f)). So, the HCVTM pellet had a relative high CS during this stage (Figure 5, section (3)).

It was noticed that the oxidation consolidation process almost corresponded to CS changes (Figure 5), microstructure transformations (Figure 7) and the TG-DSC curve in air (Figure 4). There were two similar key temperature points that just divided the oxidation process into three stages described as the above. Therefore, the analyses for oxidation consolidation process of HCVTM pellet were rational and credible.

Effect of roasting time on compressive strength of HCVTM oxidized pellet

Effect of roasting time on the CS of HCVTM oxidized pellet (1,573 K) was presented in Figure 10. By prolonging the roasting time, the CS showed an increasing tendency but not obvious. When the roasting time is short, the HCVTM pellet formed the double-layer structure due to the incomplete oxidation. The outer sphere was the recrystallization of hematite and titanohematite, while the core was magnetite recrystallization and slag consolidation. The concentric cracks resulted from the asynchronous shrink of the outer and core might weaken the CS. With the increasing time, the layer of hematite and titanohematite became thicker gradually, and then the complete oxidation of pellet had been achieved which was benefit to enhance the CS of pellet. When the roasting time was 10 min, the CS was 2,608 N which could meet the requirement for gas-based production.

Figure 10:

Effect of roasting time on the CS of HCVTM oxidized pellet (1,573 K).

Effect of roasting time on the phase compositions of HCVTM oxidized pellet

The XRD patterns of HCVTM pellet roasted for different times (1,573 K) were shown in Figure 11. In any experimental case, the phases were almost the same just different on the intensity of diffraction peak for each phase. When the roasting time was short, the pores among particles were big which could provide the wide and enough contact areas for gas–solid reactions, and the interior diffusion resistance was small due to the thin oxidation product layer. Then the oxidation reaction was quick and violent, and the diffraction peaks of all oxides included Fe₂O₃, Fe₉TiO₁₅, Fe₂Ti₃O₉, Cr_1.3Fe_0.7O₃ and (Cr_0.15V_0.85)₂O₃ were strengthened evidently in the initial stage. Prolonging the roasting time, the link by the crystal bridge was close-knit leading to the decreasing reaction contact area. Moreover, the recrystallization of hematite and titanohematite was complete gradually and the oxidation reaction became slow. Therefore, the intensity of diffraction peaks increased inconspicuously.

Figure 11:

XRD patterns of HCVTM pellet roasted at 1,573 K at different roasting times.

Effect of roasting time on the microstructure of HCVTM oxidized pellet

SEM images with EDS analyses of HCVTM pellet roasted for different times (1,573 K) are shown in Figure 12. Due to incomplete recrystallization of hematite and titanohematite at the beginning of roasting for 10 min, the mineral particles were single granular and the structure of the pellet was loose. By prolonging the time to 20 min, the recrystallization was accomplished gradually. Furthermore, the less pores in the pellet and the connection of single particles made the pellet become a compact whole, then the pellet had a good CS. If the roasting time was to prolong further, the excessively compact and thick oxidation product layer might cause some negative influences on the diffusion. Therefore, the microstructure of the pellet had no obvious changes after 20 min.

Figure 12:

SEM images with EDS analyses (mass %) of HCVTM pellet roasted at 1,573 K at different times.

The Fe/Ti ratio along the analyzed line in the phase containing Fe, V, Ti, Cr and the Fe/Ti ratio at different roasting time was obtained in Figure 13. As shown, the Fe/Ti ratio decreased and became stable around 5 with the prolonging roasting time.

Figure 13:

The Fe/Ti ratios of the white phase in HCVTM pellet roasted at 1,573 K for 5 min, 10 min, and 15 min.