Metalized pellets with high metallization rate and low expansion index, and preparation and application thereof

Abstract

This invention belongs to the field of metallurgical technology, specifically relating to a method for preparing metallized pellets that achieves both high metallization rate and low expansion index. The method involves oxidizing iron ore pellets by high-temperature roasting and then subjecting these pellets to a three-stage gas-cooling process to obtain metallized pellets. The atmosphere A in the first stage of gas cooling and the atmosphere C in the third stage are each at least one of N2 and an inert gas, while the atmosphere B in the second stage of gas cooling is an atmosphere containing H2 and / or CO. High metallization rate and low reduction expansion are achieved synergistically by controlling the cutoff temperature, cooling rate, and gas flow rate during the second stage of gas cooling. The method proposed in this invention features low energy consumption, low pollution emissions, and convenient operation; the resulting metallized pellets are of excellent quality and have promising application prospects.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, and more particularly to the preparation of iron ore metallized pellets. Background Technology

[0002] Steel is an important metallic material with wide applications in modern industrial development and urban construction. my country has ranked first in the world in crude steel production for many consecutive years and is one of the major crude steel exporters. my country's steel industry structure is mainly based on a long process of blast furnace ironmaking-converter steelmaking and a short process of direct reduction-converter steelmaking, with the long process accounting for as much as 90% of crude steel production. However, the long process has disadvantages such as high energy consumption, long process length, and high carbon emissions. The carbon emissions from the ironmaking process account for more than 70% of the entire process, mainly in the coking, sintering, and blast furnace ironmaking stages. Because blast furnaces mainly use carbonaceous reducing agents such as coke, carbon emissions during steelmaking remain high. Some studies have proposed injecting hydrogen into the blast furnace to replace coke for reducing iron ore; however, due to the endothermic nature of hydrogen reduction, insufficient heat is prone to occur in the lower part of the blast furnace, leading to increased iron ore pulverization and reduced blast furnace permeability, seriously affecting blast furnace output and molten iron quality. Furthermore, sinter accounts for as much as 75% of blast furnace charge, and sintering is one of the reasons for high carbon emissions. In contrast, short-process production has the advantages of not using blast furnaces and sinter. The raw materials for direct reduction are usually natural lump ore and pellets. With the depletion of iron ore resources, the reserves of high-grade natural lump ore are decreasing, and pellets have become the main raw material for direct reduction. Compared to sintering, the carbon emissions of the pelleting process are only 30% of those of sintering. Therefore, using pellets instead of sinter in production can significantly reduce carbon emissions from steel production.

[0003] Direct reduction is a crucial step in short-process steelmaking. High-quality direct reduced iron (DRI) is essential for reducing electric arc furnace energy consumption and producing high-quality steel. Key metrics for DRI include reduction degree, metallization rate, total iron content, compressive strength, and reduction expansion index. However, reduction efficiency and compressive strength are typically negatively correlated with the reduction expansion index. During reduction, pellet expansion is often difficult to control. Generally, expansion below 20% is considered normal, 20-40% is considered abnormal, and above 40% is considered catastrophic. Excessive reduction expansion leads to decreased compressive strength and increased porosity in the metallized pellets, causing pulverization during smelting, deteriorating the furnace environment, and affecting steel quality. Therefore, controlling pellet reduction expansion during reduction is necessary to provide high-quality raw materials for subsequent steelmaking.

[0004] The reduction expansion of pellets is typically influenced by multiple factors, such as gangue content, alkali metal content, reducing atmosphere, and thermal regime. Patents CN103305687A, CN113073196A, CN113817918B, and CN104232886A disclose methods for suppressing the reduction expansion of pellets. However, their basic approach is to increase the amount of bentonite added, which primarily increases the content of gangue minerals such as SiO2 in the pellets, thereby inhibiting the growth of iron whiskers during reduction and reducing the reduction expansion. However, excessively high gangue content will lower the iron grade of the pellets, making them unsuitable for steelmaking. Lowering the reduction temperature, shortening the reduction time, and reducing the reducing gas flow rate can also achieve the preparation of low-reduction-expansion pellets, but this severely affects the degree of reduction and metallization rate of the pellets. Therefore, in previous studies, it has been difficult to achieve the preparation of metallized pellets that combine high reduction degree, high metallization rate, and low reduction expansion without increasing the amount of additives. Summary of the Invention

[0005] To address the challenges of high and difficult-to-control reduction expansion in existing direct reduction technologies for ore pellets, and the difficulty in achieving both high metallization rates and low reduction expansion, this invention aims to provide a simple, low-cost, and environmentally friendly method for preparing metallized pellets that achieves both high metallization rates and low expansion indices. The method aims to prepare metallized pellets with excellent metallization rates and low expansion rates through a special multi-stage atmosphere co-regulation without modifying the oxidation pellet preparation process or requiring auxiliary additives.

[0006] The second objective of this invention is to provide metallized pellets prepared by the aforementioned method and their application in steel smelting.

[0007] A method for preparing metallized pellets that balances high metallization rate and low expansion index involves subjecting iron ore pellets to three-stage gas cooling treatment while hot to obtain oxidized pellets.

[0008] Among them, the first stage of air-cooled atmosphere A and the third stage of air-cooled atmosphere C are each at least one of N2 and an inert gas; the second stage of air-cooled atmosphere B is an atmosphere containing H2 and / or CO.

[0009] The cutoff temperature T1 for the first stage of air cooling is 1100~1300℃;

[0010] The cutoff temperature T2 in the second stage of air cooling is 300-800℃, and the flow rate of atmosphere B is 0.3-1.5L / min, with the cooling rate controlled at 1.0-5.0℃ / min.

[0011] The cutoff temperature T3 of the third-stage air cooling is below 100℃.

[0012] This invention innovatively applies the three-stage controlled gas cooling treatment to high-temperature oxidized pellets, and further coordinates the gas cooling atmosphere and conditions of each stage, especially the joint control of the cutoff temperature, gas cooling rate and gas flow parameters of the second stage. This achieves synergy and optimizes the reduction behavior of the pellets. As a result, while improving the reduction metallization rate, it can also significantly suppress pellet expansion, thereby achieving a balance between high metallization rate and low expansion index of the metallized pellets.

[0013] In this invention, the oxidized pellets can be obtained by conventional means, namely through processes such as pelletizing, drying, preheating and calcination, and there are no specific requirements for the amount of additives or binders used.

[0014] In order to make full use of energy, the oxidized pellets after high-temperature roasting can be directly processed by this invention.

[0015] In this invention, the starting temperature of the three-stage gas cooling treatment for oxidized pellets is between 1100 and 1400°C.

[0016] In this invention, the particle size of the oxidized pellets is between 10 and 18 mm.

[0017] In this invention, the cooling rate of the first stage of gas cooling treatment (also known as the first stage of gas control treatment process) is not subject to any special requirements, for example, it is below 1.0℃ / min.

[0018] In this invention, the flow rate of the first stage air-cooled atmosphere A is not particularly required. For example, it can be 0.1 to 1.5 L / min, and considering the processing efficiency, it can be further 0.8 to 1.2 L / min.

[0019] In this invention, the processing time of the first stage of air cooling is not particularly required. Considering the processing efficiency, it can be within 30 minutes, or even 10 to 30 minutes.

[0020] In this invention, after the first stage of gas cooling treatment, the atmosphere is changed to atmosphere B for the second stage of gas cooling treatment. Thus, the pellets are cooled based on atmosphere B, and direct reduction is performed using the decreasing cooling process. On this basis, by further coordinating the control of the cutoff temperature, cooling rate and flow rate of atmosphere B, the cooling and reduction behavior of the pellets can be optimized, thereby improving the metallization rate of the pellets and controlling their expansion.

[0021] In this invention, the atmosphere B may contain, in addition to functional gases including H2 and / or CO, at least one dilution gas selected from nitrogen and inert gases.

[0022] In this invention, the content of functional gas in atmosphere B is above 50% v%.

[0023] In this invention, the temperature of T2 is preferably 400-600°C, and more preferably 430-470°C.

[0024] In this invention, during the second air cooling stage, the flow rate of atmosphere B is preferably 0.35 to 0.8 L / min, for example, it can be a value and parameter range of 0.35 L / min, 0.40 L / min, 0.45 L / min, 0.5 L / min, 0.55 L / min, or 0.6 L / min, and can be further set to 0.4 to 0.6 L / min.

[0025] In this invention, the cooling rate in the second air-cooling stage is preferably 1.5 to 4 °C / min, for example, it can be 1.5 °C / min, 2 °C / min, 2.5 °C / min, 3.5 °C / min, 3.72 °C / min, 4 °C / min and their parameter ranges, and more preferably 2 to 2.5 °C / min.

[0026] In this invention, after the second stage of gas cooling, a third stage of gas cooling treatment (also known as the third stage of gas control treatment) is performed to obtain the metallized pellets.

[0027] In this invention, during the third stage of air cooling, the cooling rate is 1-4℃ / min, and considering the processing efficiency, it can be further increased to 2-3℃ / min. The gas flow rate is 0.8-1.5L / min, and can be further increased to 1-1.2L / min.

[0028] The present invention also includes metallized pellets prepared by the aforementioned preparation method.

[0029] In this invention, thanks to the preparation method described above, the prepared pellets can be endowed with special physicochemical characteristics, enabling them to achieve both excellent metallization rate and low expansion index.

[0030] The present invention also provides the application of the metallized pellets obtained by the method, using them as raw materials for electric arc furnace steelmaking.

[0031] The metallized pellets prepared by this invention have both excellent metallization rate and low reduction expansion index. When used in the steel smelting process, they can achieve better smelting results.

[0032] Beneficial effects:

[0033] This invention provides a method for the direct reduction of oxidized pellets through a three-stage controlled gas cooling process. Based on the joint control of the parameters of the three-stage controlled gas cooling process, synergy can be achieved to obtain high-quality metallized pellet products with high metallization rate and low reduction expansion.

[0034] The present invention found that the metallization rate of the prepared metallized pellets was 89.17% and the reduction expansion index was 3.68%, achieving a balance between high metallization rate and low reduction expansion.

[0035] This invention has the advantages of low energy consumption, low pollution emissions, convenient operation, and high product quality. Attached Figure Description

[0036] Figure 1 This is a flowchart of the three-stage gas control process of the present invention. Detailed Implementation

[0037] The present invention will be further described in detail below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0038] To avoid repetition, the raw materials involved in this specific embodiment are described uniformly as follows, and will not be repeated in the specific embodiments:

[0039] The oxidized pellets used in the examples can be any conventional oxidized pellets in the industry, for example, obtained through pelletizing (time: 10-20 min; moisture: 6-12 wt.%), drying (temperature: 100-110℃; time: 6-12 h), preheating (temperature: 800-950℃; time: 5-20 min) and calcination (temperature: 1100-1300℃; time: 5-20 min); its main chemical composition is TFe≥64.00 wt.%, SiO2≥8.51 wt.%, FeO≤1.48 wt.%, Al2O3≤0.42 wt.%, CaO≤0.18 wt.%, Na2O≤0.18 wt.%, MgO≤0.15 wt.%, K2O≤0.10 wt.%; its particle size is between 14 and 16 mm.

[0040] It should be noted that the gases used in the air cooling process of this invention are all at room temperature. Furthermore, the gas flow rates are all set at the flow meter's parameters (design flow rates). Of course, an error of ±10% of the design flow rate is permissible for the instantaneous gas flow rate.

[0041] Furthermore, the cooling rate in the air cooling process refers to the average cooling rate [(air cooling start temperature - air cooling stop temperature) / air cooling time]. In this invention, the entire air cooling process is maintained at a relatively uniform cooling rate, but reasonable control errors are allowed. For example, the instantaneous cooling rate in the air cooling process can be at the level of the average cooling rate ±0.2℃ / min.

[0042] Example 1

[0043] First stage of gas control: The obtained high-temperature oxidized pellets are passed into an N2 atmosphere (atmosphere A) with a flow rate of 1.0 L / min and kept for 20 to 25 min, and the cutoff temperature T1 of the first stage of gas control is controlled at 1150 to 1200℃.

[0044] Second stage of gas control: Subsequently, cooling is carried out at a rate of 2.00℃ / min in a pure H2 atmosphere (atmosphere B) with a flow rate of 0.4L / min, and the cutoff temperature T2 is controlled at 450±20℃.

[0045] Third stage gas control: The atmosphere is then changed to N2 (atmosphere C) for the third stage of gas cooling, wherein the flow rate of atmosphere C is 1.0 L / min, the cooling rate is 2.20℃ / min, and the cutoff temperature T3 is controlled below 50℃ to obtain metallized pellets.

[0046] The metallized pellets obtained in this embodiment were measured to have a metallization rate of 89.17%, a reduction expansion index of 3.68%, a reduction degree of 93.36%, and a compressive strength of 840 N / p.

[0047] Example 2

[0048] Compared to Example 1, the only difference lies in the conditions for controlling the second stage of gas control. The experimental groups are as follows:

[0049] Group A: Cutoff temperature T2 is 550±20℃, cooling rate is 2.00℃ / min, and gas flow rate is 0.4L / min;

[0050] Group B: Cut-off temperature T2 is 650±20℃, cooling rate is 2.00℃ / min, and gas flow rate is 0.4L / min;

[0051] Group C: Gas flow rate is 0.8 L / min, cutoff temperature T2 is 450±20℃, and cooling rate is 3.72℃ / min;

[0052] The test was conducted according to the method in Example 1, and the results are as follows:

[0053] Group A: Metallization rate 87.59%, reduction expansion index 3.30%, reduction degree 93.28%, compressive strength 1320 N / p;

[0054] Group B: Metallization rate is 83.12%, reduction expansion index is 2.98%, reduction degree is 90.73%, and compressive strength is 1470 N / p;

[0055] Group C: Metallization rate is 87.44%, reduction expansion index is 4.66%, reduction degree is 91.45%, and compressive strength is 1014 N / p.

[0056] Comparative Example 1

[0057] Compared with Example 1, the only difference is that the first stage of gas control treatment was not performed. Instead, the pellets at the same T1 temperature as in Example 1 were subjected to the subsequent second and third stages of gas control, and the conditions were the same as in Example 1.

[0058] The metallized pellets obtained in this comparative example were measured to have a metallization rate of 87.62%, a reduction expansion index of 4.97%, and a reduction degree of 91.83%.

[0059] Comparative Example 2

[0060] Compared to Example 1, the only difference is that after the second stage of gas control, there is no third stage of gas control; instead, the gas is cooled along with the furnace. The processes and parameters of the first and second stages of gas control, as well as other operations and parameters, are the same as in Example 1.

[0061] The metallized pellets obtained in this comparative example were measured to have a metallization rate of 78.57%, a reduction expansion index of 3.91%, and a reduction degree of 82.64%.

[0062] Comparative Example 3

[0063] Compared with Example 1, the only difference is that the three-stage gas control process is not performed. Instead, hydrogen is used in both the first and third stages of gas control. All other operations and parameters are the same as in Example 1.

[0064] The metallized pellets obtained in this comparative example were measured to have a metallization rate of 89.42%, a reduction expansion index of 5.66%, a reduction degree of 93.69%, and a compressive strength of 876 N / p.

[0065] Comparative Example 4

[0066] Compared with Example 1, the only difference is that the cutoff temperature, cooling rate and gas flow rate in the second stage of gas control process are not controlled within the required range. The difference is that the cooling rate is 5.20℃ / min, the cutoff temperature T2 is 450±20℃, and the gas flow rate is 0.4L / min.

[0067] The test was conducted according to the method in Example 1, and the results were as follows: metallization rate was 31.14%, reduction degree was 52.00%, and compressive strength was 710 N / p.

[0068] Comparative Example 5

[0069] The difference from the example is that the conventional direct reduction process in the industry is adopted. That is, the reduction of the oxide pellets is carried out under isothermal conditions. Specifically, the oxide pellets are first heated to 1000°C in an N2 atmosphere with a gas flow rate of 1.0 L / min at a rate of 10°C / min, and then reduced at an isothermal temperature for 40 min in a pure H2 atmosphere with a gas flow rate of 0.4 L / min. After the reduction is completed, the pellets are cooled to room temperature in an N2 atmosphere with a gas flow rate of 1.0 L / min to obtain metallized pellets.

[0070] The metallized pellets obtained in this comparative example were measured to have a metallization rate of 90.44%, a reduction expansion index of 23.74%, a reduction degree of 93.65%, and a compressive strength of 518 N / p.

[0071] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be regarded as exemplary and non-limiting in all respects. The scope of the invention is defined by the appended claims rather than the foregoing description, and all changes falling within the meaning and scope of the equivalents of the claims are intended to be included within the present invention.

[0072] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style of the specification is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other implementation schemes that can be understood by those skilled in the art.

Claims

1. A method for preparing metallized pellets that balances high metallization rate and low expansion index, characterized in that, Metallized pellets are obtained by oxidizing and roasting iron ore pellets and then subjecting them to a three-stage gas cooling process while hot. Among them, the first stage of air-cooled atmosphere A and the third stage of air-cooled atmosphere C are each at least one of N2 and an inert gas; the second stage of air-cooled atmosphere B is an atmosphere containing H2 and / or CO. The cutoff temperature T1 for the first stage of air cooling is 1100~1300 ℃; The cutoff temperature T2 in the second stage of air cooling is 300~800 ℃, and the flow rate of atmosphere B is 0.3~1.5 L / min, with the cooling rate controlled at 1.0~5.0 ℃ / min; The cutoff temperature T3 of the third-stage air cooling is below 100 ℃.

2. The preparation method according to claim 1, characterized in that, The starting temperature for the three-stage gas cooling treatment of the oxidized pellets is 1100~1400 ℃; The particle size of the oxidized pellets is between 10 and 18 mm.

3. The preparation method according to claim 1, characterized in that, The first stage of air cooling takes less than 30 minutes.

4. The preparation method according to claim 1, characterized in that, In the atmosphere B, in addition to functional gases containing H2 and / or CO, dilution gases containing at least one of N2 and inert gases are also permitted.

5. The preparation method according to claim 4, characterized in that, In the atmosphere B, the content of functional gas is above 50% v%.

6. The preparation method according to claim 1, characterized in that, The temperature of T2 is 400~600 ℃.

7. The preparation method according to claim 1, characterized in that, During the second stage of air cooling, the flow rate of atmosphere B is 0.35~0.8 L / min.

8. The preparation method according to claim 1, characterized in that, In the second stage of air cooling, the cooling rate is 1.5~4 ℃ / min.

9. The preparation method according to claim 1, characterized in that, During the third stage of air cooling, the cooling rate is 1~4 ℃ / min, and the air flow rate is 0.8~1.5 L / min.

10. A metallized pellet prepared by the preparation method according to any one of claims 1 to 9.

11. An application of the metallized pellets prepared by the method according to any one of claims 1 to 9, characterized in that, It is used as a raw material for electric arc furnace steelmaking.

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