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21

2024

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03

Effect of Humic Acid Binder on the Preparation of Oxidized Pellets from Vanadium-Bearing Titanomagnetite Concentrate (1)

Keywords:

vanadium-bearing titanomagnetite ,oxidized pellets preparation ,binder ,bentonite

Source: internal company


3. Results and Discussion
3.1. Effect of HA Binder on Balling Characteristic
Figure 3 displays the properties of the green balls as bentonite, HA binder, and limestone were blended. Generally, green balls with drop strength > 3 times/0.5 m, compressive strength > 10 N/pellet, and decrepitation temperature > 400 °C are considered qualified for the subsequent sintering process. As can be seen, to produce qualified green balls, the minimum requirement for bentonite dosage is 1.5%. Its drop strength and compressive strength reach 3.6 times/0.5 m and 12.7 N/pellet, respectively. These two indexes further increase to 4.5 and 13.5 with bentonite dosage rising to 2.0%. However, bentonite dosage does not show any apparent impact on the decrepitation temperature of green balls. It always remains higher than 600 °C when bentonite is used as binder. The dosage of binder can be markedly reduced when the HA binder is blended in. It shows that qualified green balls with drop strength of 3.2 times/0.5 m and compressive strength of 12.7 N/pellet can be obtained with 0.75% of HA binder (only half the bentonite dosage). The decrepitation temperature drops to 520 °C as compared with bentonite green balls, which can probably be attributed to the decomposition of the organic components in the HA binder, but it still can meet the standard of green balls. 0~12% of limestone blending does not show any negative influence on the drop and compressive strength of green balls based on the 0.75% of HA binder dosage. When the limestone dosage reaches 8.0%, the compressive strength of the green ball reaches the level of 18 N/pellet. The decrepitation temperature of green balls declines notably when more than 5% of limestone is blended in. Limestone with D50 of 6.75 μm is much finer than the VTM concentrate. Certain amounts of fine particles in the raw material can improve the density and strength of the green balls, but too much may lead to overly dense structure of balls and, therefore, the decrease in decrepitation temperature.

 

3.2. Effect of HA Binder on Sintering Characteristic
It provides the compressive strength of bentonite, HA, and limestone bound pellets obtained by different preheating/roasting conditions. As the most important index of oxidized pellets, the compressive strength requirement for medium and large blast furnaces is 2000~2500 N/P or higher.


For the 1.5% bentonite-blended pellet, its compressive strength shows a steady upward trend with the preheating temperature (Figure 4a). The compressive strength of pellets reaches 2512 N/P when preheated at 950 °C, which meets the requirement of the blast furnace. The strength then slightly increases to 2654 N/P as the preheating temperature rises to 1000 °C. Prolongation of the preheating time seems to show a greater impact on the compressive strength, as demonstrated in Figure 4b. To be specific, the compressive strength increases from 2512 N/P to 2722 N/P with the time prolonged from 15 min to 18 min. The increase is more apparent than that caused by temperature rise. Similar change rules are also observed in the preheating of the 0.75% HA-binder-blended pellet.


Differences also exist between the bentonite- and HA-bonded pellets. For the first, the compressive strength of the HA-bonded pellets obtained by the same sintering condition is always lower than that of bentonite-bonded pellets, but it still can reach the level of >2000 N/P. In addition, the HA-bonded pellets require a longer preheating time. This is possibly related to the decomposition of the HA binder during the preheating process. Then, qualified oxidized pellets can be obtained with a roasting temperature ≥ 1250 °C and time ≥ 8 min both for bentonite- and HA-bonded pellets. Based on the 0.75% of HA binder blending, the compressive strength of sintered pellets peaking at 5% of limestone dosage reaches 2573 N/P under the optimized sintering condition (preheating at 950 °C for 15 min and then roasting at 1250 °C for 10 min). The basicity of pellets reaches 0.6 at this point. Limestone addition induces the formation of calcium ferrite (CaO·Fe2O3, a low melting point phase) during the high-temperature sintering process, which helps to improve the strength of the HA pellets. However, excessive low-melting-point substance formation could hinder the transfer of oxygen in pellets, which can obstruct the oxidation and crystallization of the iron oxide phase, and, consequently, lowers the compressive strength of pellets.


3.3. Effect of HA Binder on the Phase Structure of Sintered Pellets
From the pelletizing results discussed above, qualified pellets can be prepared by VTM with 1.5% bentonite, 0.75% HA, or 0.75% HA + 5% limestone, respectively.  There is no big difference in the phase characteristics among the three pellet samples. Similar to all other oxidized pellets, hematite constitutes their major mineral phase. Beyond that, an extra minor phase (pseudobrookite) is found in bentonite- and limestone-doped pellets. Pseudobrookite, as an iron-containing brookite, is commonly seen in the VTM oxidation process, and trace magnesioferrite spinel phase is detected only in the limestone-doped fluxed pellets. Magnesioferrite spinel is a common phase during fluxed pellet production. It is always formed by the solid-phase reaction of MgO with hematite or magnetite above 600 °C. The lattice of magnetite could be stabilized with MgO penetrated in, which is unfavorable for the oxidation and induration of pellets.


The morphology of the hematite phase in the three sintered pellet samples were observed by an optical microscopy, as shown in Figure 6. In the bentonite-doped pellet (Figure 6a,b), single hematite (bright white, fine grain shape) develops to a relatively complete plate-like state, leaving tiny pores (black, irregular shape) between the interconnected hematite grains. As the strength of the oxidized pellets mainly depends on the crystallization of hematite grains, the physical and chemical conditions for hematite upgrowth during the sintering process is especially crucial. However, the crystalline state is visibly poor, with more and bigger pores between the hematite grains in the HA-doped pellet. This explains why the strength of the HA pellets is slightly lower. HA pellets always need more sintering time for the crystallization of the hematite grains, but 5% of limestone blending promotes the growth of the hematite grains of HA pellets. The melting substance formed in limestone-doped pellets could, to some extent, improve the diffusion of hematite grains. We observe that fine grains develop to plump ones (size: 10~20 μm) with full interlocking. Therefore, the induration of HA pellets can be strengthened by proper limestone addition.


The morphology of the pseudobrookite and magnesioferrite spinel phase in the sintered pellet sample were further identified by an SEM equipped with EDS. From the SEM images (cross-section), a symbiotic relationship between the pseudobrookite and hematite can be found. The pseudobrookite phase is needle-like, but in fact flake-like in the unbroken pellet. It wraps around the iron oxides, which leads to the oxidation of VTM that is more difficult than that of ordinary magnetite.


The magnesioferrite spinel in a piece shape (dark grey in Figure 8), wraps around other minerals, forming large pores around it. The liquid phase (containing magnesium, aluminum, and silicon components) induces the generation of magnesioferrite spinel phase at high temperatures. Then, it shrinks during the pellet-cooling process, leaving large pores around it. The formation of this structure may cause strength deterioration in the final pellets.


Hematite is the major phase of the final pellets. The well-crystallized hematite grains ensure the necessary strength of the pellets. However, the interconnection state of hematite is cut off when pseudobrookite emerges. This undesirably inhibits the upgrowth of hematite grains. The formation of a connecting neck or inter-solution can be found between the magnesioferrite spinel and pseudobrookite phase. However, almost no interconnection forms between the magnesioferrite spinel and hematite. Thus, strengthening the VTM oxidation and regulating the magnesioferrite spinel formation during the sintering process are crucial to improve the induration of HA-bonded pellets.


4. Conclusions
(1) Qualified oxidized pellets with compressive strength higher than 2000 N/P can be prepared by VTM concentrate with 0.75% of HA binder. The dosage of the binder can be reduced by 50% when HA is used instead of bentonite. The compressive strength of HA pellets can be further improved to higher than 2500 N/P when 5% of limestone is blended in;


(2) For the sintered pellets with HA binder, fine hematite grains crystallize, with more pores embed in. However, 5% of limestone blending drives the fine hematite grains to evolve to a plump interlocking state. The formation of pseudobrookite and magnesioferrite spinel phase during sintering treatment could hinder the crystallization of hematite grains, and cause strength decline in the final pellets.

 

Li, G., Zhang, Y., Zhang, X., Meng, F., Cao, P., & Yi, L. Effect of Humic Acid Binder on the Preparation of Oxidized Pellets from Vanadium-Bearing Titanomagnetite Concentrate. Sustainability, 15(8), 6454. https://doi.org/10.3390/su15086454

© 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

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