A method for inhibiting abnormal expansion of ultra-high-grade pellet ore by gradient temperature rise reduction in a hydrogen-based shaft furnace

Abstract

The present invention discloses a method for suppressing abnormal swelling of ultra-high-grade pellets by gradient heating reduction in a hydrogen-based shaft furnace, belonging to the technical field of direct reduced iron production. The ultra-high-grade pellets are continuously added from the top of the hydrogen-based shaft furnace and sequentially pass through the first heating reduction zone (heating rate: 10-20 °C / min, temperature reaching 900-930 °C), the second heating reduction zone (heating rate not less than 30 °C / min, temperature reaching 990-1020 °C), the isothermal reduction zone and the cooling zone of the hydrogen-based shaft furnace to obtain direct reduced iron. This method is applicable to hydrogen-based shaft furnaces with various reduction atmospheres. Without reducing the original iron grade of the pellets, the maximum reduction swelling rate of the ultra-high-grade pellets during the reduction process in the hydrogen-based shaft furnace can be less than 15%, and high-purity direct reduced iron with qualified production indexes can be produced, which is beneficial to reducing the energy consumption of subsequent electric furnace smelting and the molten iron refining process.

Description

Technical Field

[0001] The present invention relates to a method for hydrogen-based shaft furnace reduction of ultra-high-grade pellet ore, and in particular to a method for suppressing abnormal expansion of ultra-high-grade pellet ore by gradient heating reduction in a hydrogen-based shaft furnace, belonging to the technical field of direct reduced iron production. Background Art

[0002] With the increasing depletion of high-quality lump ore resources, pellet ore has gradually become the main burden in the hydrogen-based shaft furnace process. Due to the reduction characteristics of the hydrogen-based shaft furnace, the gangue components in the pellet ore cannot be reduced or removed by slagging, resulting in the direct reduced iron retaining the original impurity elements and harmful metal elements in the pellet ore, which then enter the subsequent electric furnace process, increasing smelting energy consumption and process flow. Using ultra-high-grade pellet ore (gangue content less than 0.5%) to produce direct reduced iron can improve the purity of hot metal, reduce the amount of electric furnace slag, and lower smelting energy consumption and costs, thus laying a foundation for realizing low-carbon or carbon-free electric furnace steelmaking and producing high-quality clean steel and special steel. However, there is a technical bottleneck of abnormal expansion in the reduction process of ultra-high-grade pellet ore in the hydrogen-based shaft furnace, which is difficult to meet the production requirements. The literature ("Experimental study on a new process for direct reduction - smelting separation of ultra-high-grade iron concentrate to prepare high-purity iron", Zhao Jiaqi, Northeastern University, master's thesis, 2017) discloses that when using ultra-high-grade pellet ore for hydrogen reduction, the pellet expansion rate is greater than 50%, and the strength of the reduced pellets is lower than 200 N. The literature ("Mechanism study on gas-based reduction swelling behavior of ultra-high grade pellets", Jie Lei, et al., Journal of Materials Research and Technology; 2023; 26: 823-836) discloses that the reduction expansion rate of high-grade pellet ore during hydrogen reduction is greater than 30%. Although the use of calcium and magnesium additives can to a certain extent inhibit the reduction expansion of ultra-high-grade pellet ore, it will inevitably reduce the iron grade of the pellet ore, affect the reduction rate of the pellets, and increase the specific hydrogen consumption during the reduction process. Summary of the Invention

[0003] Aiming at the problem of abnormal expansion during the hydrogen-based shaft furnace reduction process of ultra-high-grade pellets (gangue content less than 0.5%) in the prior art. The purpose of the present invention is to provide a method for suppressing the abnormal expansion of ultra-high-grade pellets by gradient temperature rise reduction in a hydrogen-based shaft furnace. The key of this method lies in regulating the reduction process of ultra-high-grade pellets, carrying out gradient temperature rise reduction in the hydrogen-based shaft furnace, suppressing the abnormal expansion of pellets caused by crystal form transformation and iron whisker growth while ensuring the pellet reduction efficiency. This method is particularly applicable to hydrogen-based shaft furnaces with various reduction atmospheres. Without reducing the original iron grade of the pellets, the maximum reduction expansion rate of ultra-high-grade pellets during the hydrogen-based shaft furnace reduction process can be less than 15%, producing high-purity direct reduced iron with qualified production indexes, which is beneficial to reducing the energy consumption of subsequent electric furnace smelting and the molten iron refining process.

[0004] The ultra-high-grade pellets involved in the present invention are the commonly defined ones in the industry, referring to pellets with a gangue content less than 0.5%.

[0005] To achieve the above technical purpose, the present invention provides a method for suppressing the abnormal expansion of ultra-high-grade pellets by gradient temperature rise reduction in a hydrogen-based shaft furnace. This method is to continuously add ultra-high-grade pellets from the top of the hydrogen-based shaft furnace, and successively pass through the first temperature rise reduction zone, the second temperature rise reduction zone, the constant temperature reduction zone and the cooling zone of the hydrogen-based shaft furnace to obtain direct reduced iron;

[0006] In the first temperature rise reduction zone, the heating rate of the ultra-high-grade pellets is 10 - 20 °C / min. When the ultra-high-grade pellets are heated to 900 - 930 °C and the reduction degree of the ultra-high-grade pellets reaches more than 40%, they enter the second temperature rise reduction zone;

[0007] In the second temperature rise reduction zone, the heating rate of the ultra-high-grade pellets is not less than 30 °C / min. When the ultra-high-grade pellets are heated to 990 - 1020 °C, they enter the constant temperature reduction zone.

[0008] The key improvement of the technical solution of the present invention lies in: adopting a gradient heating reduction method during the reduction process of the hydrogen-based shaft furnace, aiming to make the temperature field of the pellet ore match the reduction process of the pellet ore during the reduction process of the hydrogen-based shaft furnace. On the one hand, it reduces the expansion caused by the internal stress of the crystal form transformation during the initial stage of the reduction of the pellet ore. On the other hand, it regulates the initial morphology of the iron crystal grains on the surface of the particles in the pellet ore, induces the uniform nucleation and aggregation growth of the iron crystal grains, and inhibits the abnormal expansion of the pellet ore caused by the growth of iron whiskers. More specifically, by regulating the appropriate gradient heating rate of the pellet ore at different reduction processes, the present invention can reduce the expansion stress of the pellet ore while ensuring high reduction efficiency. First, by controlling a relatively low heating rate in the first heating reduction zone, the reduction time of the pellet ore is extended and the reduction degree of the reduced pellet ore is controlled to be not less than 40%. On the one hand, it can reduce the internal stress of the crystal form transformation generated by the reduction of Fe2O3 to Fe3O4. On the other hand, it can induce the uniform nucleation of the iron crystal grains on the surface of the particles in the pellet ore and promote the aggregation growth of the iron crystal grains in the pellet ore during the subsequent constant temperature reduction process. On the other hand, it can form an initial metallic iron skeleton structure in the pellet ore before entering the constant temperature reduction, resisting the structural stress and expansion caused by the growth of the iron crystal grains during the high-temperature reduction process. Secondly, based on the increase in the reduction degree of the pellet ore after reduction in the first heating reduction zone, it is beneficial to inhibit the abnormal expansion of the pellet ore, but inevitably increases the reduction time of the pellet ore and affects the subsequent reduction efficiency. Therefore, adopting a faster heating rate in the second heating reduction zone can enable the pellet ore to avoid the temperature range where abnormal expansion is likely to occur. Thirdly, after heating the pellet ore to 990-1020 °C and then entering the constant temperature reduction zone for further reduction, it is not only beneficial to accelerate the reduction and precipitation of wüstite to metallic iron, but also the higher temperature is conducive to the aggregation and growth of the iron crystal grains, thus forming a denser pellet structure and realizing the gradual steady-state reduction of the pellet ore. To sum up, through the gradient heating reduction method, the present invention enables the maximum reduction expansion rate of the pellet ore during the reduction process of the hydrogen-based shaft furnace to be less than 15% without affecting the original iron grade and reduction performance of the pellet ore.

[0009] As a preferred solution, in the first heating reduction zone, when the reduction degree of the ultra-high grade pellet ore reaches 40-70%, it enters the second heating reduction zone. If the reduction degree of the ultra-high grade pellet ore in the first heating reduction zone is too low, it is difficult to induce the uniform nucleation of the iron crystal grains on the surface of the particles in the pellet ore, which is not conducive to the aggregation growth of the iron crystal grains in the pellet ore during the subsequent constant temperature reduction process, and it is impossible to form a preliminary metallic iron skeleton structure, making it difficult to resist the structural stress and expansion caused by the growth of the iron crystal grains during the high-temperature reduction process. However, when the reduction degree of the ultra-high grade pellet ore in the first heating reduction zone is too high, on the one hand, it will significantly increase the reduction time of the pellet and reduce the yield, and on the other hand, it is also not conducive to the aggregation growth of the iron crystal grains in the ultra-high grade pellet ore.

[0010] As a preferred solution, in the second temperature-raising reduction zone, the temperature-raising rate of the ultra-high-grade pellet ore is 30-50 °C / min. If the temperature-raising rate is too low or too high, abnormal expansion of the pellet ore will occur.

[0011] As a preferred solution, in the ultra-high-grade pellet ore, the mass fraction of (SiO2 + CaO + MgO + Al2O3) is less than 0.5%, the mass fraction of (P + S) is less than 0.005%, and the mass fraction of TFe is greater than 69.5%.

[0012] As a preferred solution, the compressive strength of the ultra-high-grade pellet ore is 2500-3000 N, and the reduction index RI 40 is greater than 3. The strength of the ultra-high-grade pellet ore of the present invention should not be too high. When the strength is too high, the pellet structure is too dense, and the pore structure inside and outside the pellet ore is uneven. On the one hand, it will reduce the reduction rate of the pellet ore. On the other hand, it will cause structural stress caused by the structural difference between the inner and outer layers of the pellet ore during the reduction process, resulting in expansion and cracking. By controlling the strength of the ultra-high-grade pellet ore within the range of 2500-3000 N, it has good reduction performance, can ensure its reduction efficiency during the gradient temperature-raising reduction process, and reduce the reduction expansion of the pellet ore. The reduction index RI 40 can be measured according to GB-T 24236-2009.

[0013] As a preferred solution, the ultra-high-grade pellet ore is preheated to 550-650 °C before entering the hydrogen-based shaft furnace. Appropriately increasing the furnace inlet temperature of the pellet ore can reduce the carbon deposition reaction of the hydrogen-rich gas, improve the utilization rate of the reducing gas, and effectively inhibit the pellet expansion phenomenon caused by the carbon deposition reaction on the crystal form transformation.

[0014] As a preferred solution, the composition of the reducing gas introduced into the first temperature-raising reduction zone, the second temperature-raising reduction zone, and the constant temperature reduction zone is: the volume fraction of (H2 + CO) is not less than 90%, and the volume fraction of H2 / (H2 + CO) is not less than 0.6. The present invention can be applied to the hydrogen shaft furnace reduction process with various reducing gas components. Sufficient reducing gas content can accelerate the reduction rate of the ultra-high-grade pellet ore and control the growth morphology of the granular iron crystal grains. In addition to the two main gases H2 and CO in the reducing gas, other gases can be inert gases or nitrogen, etc.

[0015] As a preferred solution, the flow rate of the reducing gas is measured by 1600-2000 m per ton of direct reduced iron 3 and the gas pressure in the hydrogen-based shaft furnace is 0.5-0.9 MPa. Appropriate reducing gas flow rate and furnace gas pressure can ensure sufficient reduction potential and improve gas utilization rate.

[0016] The ultra-high-grade pellet ore reduced in the constant-temperature reduction zone of the present invention has a reduction degree greater than 98%.

[0017] As a preferred solution, the TFe of the direct reduced iron is greater than 99%, and the compressive strength is greater than 400 N. The high-purity direct reduced iron prepared by the present invention has the advantages of low impurity content, compact structure, good strength, etc. Its TFe is greater than 99%, and the compressive strength is greater than 400 N. In particular, it can be used as high-quality furnace charge for low-carbon steelmaking and carbon-free steelmaking, which is beneficial to reducing smelting energy consumption and simplifying the hot metal refining process.

[0018] Compared with the prior art, the beneficial technical effects brought by the technical solution of the present invention are as follows:

[0019] By establishing the matching relationship between the temperature gradient of the hydrogen-based shaft furnace and the reduction process of the ultra-high-grade pellet ore, the present invention regulates the appropriate gradient heating rate and reduction degree of pellets in different reduction processes, and suppresses the abnormal expansion of the ultra-high-grade pellet ore on the premise of ensuring the reduction efficiency. This method does not require adding additives such as calcium and magnesium to the ultra-high-grade pellet ore. Without reducing the original iron grade of the ultra-high-grade pellet ore, the reduction expansion rate of the pellet ore in the hydrogen-based shaft furnace reduction process is less than 15%, solving the problem of abnormal expansion during the hydrogen-rich reduction process of the ultra-high-grade pellet ore. In addition, by setting an appropriate gradient heating interval, it promotes the aggregation and growth of iron grains, forms a more compact pellet structure, and produces high-purity direct reduced iron with qualified quality indicators, which is beneficial to reducing the energy consumption of subsequent electric furnace production and the hot metal refining process. Detailed Embodiments

[0020] To further illustrate the content of the present invention, the following will describe the present invention more comprehensively and meticulously in combination with preferred embodiments, but the protection scope of the present invention is not limited to the following specific embodiments.

[0021] Unless otherwise defined, all professional terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention.

[0022] Table 1 shows the main chemical components of the ultra-high-grade pellet ore used.

[0023] Table 1 Main Chemical Components of Ultra-High-Grade Pellet Ore / wt. %

[0024]

[0025] Example 1

[0026] The method for suppressing abnormal expansion of ultra-high-grade pellet ore by gradient heating reduction in a hydrogen-based shaft furnace provided in this example uses ultra-high-grade pellet ore with a compressive strength of about 2800 N and a reduction index RI40 is 3.2. The components of the reducing gas in the first temperature-rising reduction zone, the second temperature-rising reduction zone, and the constant-temperature reduction zone during the gradient temperature-rising reduction process: the volume ratio of (H2 + CO) is 90%, and the volume ratio of H2 / (H2 + CO) is 1 (100% H2). The flow rate of the reducing gas is 1600 m 3 is introduced per ton of direct reduced iron, and the gas pressure in the furnace is 0.9 MPa.

[0027] First, the ultra-high-grade pellet is preheated outside the furnace to 550 °C, and then enters the first temperature-rising reduction zone and is temperature-risingly reduced to 930 °C at a rate of 20 °C / min. At this time, the reduction degree of the pellet is 41.6%, and then it enters the second temperature-rising reduction zone; in the second temperature-rising reduction zone, it is temperature-risingly reduced to 990 °C at a rate of 30 °C / min, and then enters the constant-temperature reduction zone; the pellet is constantly temperature-reduced at 990 °C until the reduction degree of the pellet reaches 99.2%. The maximum reduction expansion rate of the pellet during the reduction process is 13.2%, the obtained high-purity direct reduced iron TFe is 99.32%, and the compressive strength is 417 N.

[0028] Example 2

[0029] For the method of suppressing abnormal expansion of ultra-high-grade pellets by gradient temperature-rising reduction in a hydrogen-based shaft furnace provided in this example, the ultra-high-grade pellets used have a compressive strength of about 2800 N, and the reduction index RI 40 is 3.5. The components of the reducing gas in the first temperature-rising reduction zone, the second temperature-rising reduction zone, and the constant-temperature reduction zone during the gradient temperature-rising reduction process: the volume ratio of (H2 + CO) is 95%, and the volume ratio of H2 / (H2 + CO) is 0.6. The flow rate of the reducing gas is 2000 m 3 is introduced per ton of direct reduced iron, and the gas pressure in the furnace is 0.5 MPa.

[0030] First, the ultra-high-grade pellet is preheated to 650 °C, and then enters the first temperature-rising reduction zone and is temperature-risingly reduced to 900 °C at a rate of 10 °C / min. At this time, the reduction degree of the pellet is 43.8%, and then it enters the second temperature-rising reduction zone; in the second temperature-rising reduction zone, it is temperature-risingly reduced to 1020 °C at a rate of 50 °C / min, and then enters the constant-temperature reduction zone; the pellet is constantly temperature-reduced at 1020 °C until the reduction degree of the pellet reaches 98.3%. The maximum reduction expansion rate of the pellet during the reduction process is 13.8%, the obtained high-purity direct reduced iron TFe is 99.12%, and the compressive strength is 405 N.

[0031] Example 3

[0032] For the method of suppressing abnormal expansion of ultra-high-grade pellets by gradient temperature-rising reduction in a hydrogen-based shaft furnace provided in this example, the ultra-high-grade pellets used have a compressive strength of about 2800 N, and the reduction index RI 40is 3.2. During the gradient temperature rise reduction process, the reducing gas components in the first temperature rise reduction zone, the second temperature rise reduction zone, and the constant temperature reduction zone: the volume ratio of (H2 + CO) is 95%, and the volume ratio of H2 / (H2 + CO) is 0.72. The reducing gas flow rate is 1800 m per ton of direct reduced iron 3 , and the gas pressure in the furnace is 0.7 MPa.

[0033] First, preheat the ultra-high-grade pellet ore to 600 °C, and then enter the first temperature rise reduction zone to be reduced at a rate of 15 °C / min to 900 °C. At this time, the reduction degree of the pellet ore is 57.2%. Subsequently, enter the second temperature rise reduction zone; in the second temperature rise reduction zone, reduce the temperature at a rate of 40 °C / min to 1020 °C, and then enter the constant temperature reduction zone; the pellet ore is reduced at a constant temperature of 1020 °C until the reduction degree of the pellet ore reaches 99.8%. The maximum reduction expansion rate of the pellet ore during the reduction process is 8.7%. The obtained high-purity direct reduced iron TFe is 99.46%, and the compressive strength is 443 N.

[0034] Example 4

[0035] For the method for suppressing abnormal expansion of ultra-high-grade pellet ore by gradient temperature rise reduction in a hydrogen-based shaft furnace provided in this example, the ultra-high-grade pellet ore used has a compressive strength of approximately 2700 N, and the reduction index RI 40 is 3.2. During the gradient temperature rise reduction process, the reducing gas components in the first temperature rise reduction zone, the second temperature rise reduction zone, and the constant temperature reduction zone: the volume ratio of (H2 + CO) is 95%, and the volume ratio of H2 / (H2 + CO) is 0.86. The reducing gas flow rate is 1900 m per ton of direct reduced iron 3 , and the gas pressure in the furnace is 0.7 MPa.

[0036] First, preheat the ultra-high-grade pellet ore to 550 °C, and then enter the first temperature rise reduction zone to be reduced at a rate of 10 °C / min to 930 °C. At this time, the reduction degree of the pellet ore is 68.2%. Subsequently, enter the second temperature rise reduction zone; in the second temperature rise reduction zone, reduce the temperature at a rate of 40 °C / min to 1020 °C, and then enter the constant temperature reduction zone; the pellet ore is reduced at a constant temperature of 1020 °C until the reduction degree of the pellet ore reaches 98.6%. The maximum reduction expansion rate of the pellet ore during the reduction process is 7.4%. The obtained high-purity direct reduced iron TFe is 99.15%, and the compressive strength is 418 N.

[0037] Comparative Example 1

[0038] Compared with Example 1, the only difference is that the ultra-high-grade pellet enters the first temperature-rising reduction zone and is heated and reduced at a rate of 30 °C / min to 930 °C. At this time, the reduction degree of the pellet is 32.2%, and then it enters the second temperature-rising reduction zone. During the reduction process, the maximum reduction expansion rate of the ultra-high-grade pellet is 25.3%, and the compressive strength of the obtained high-purity direct reduced iron is 298 N.

[0039] Compared with Example 1, due to the use of a faster heating rate in the first temperature-rising reduction zone, the nucleation of iron grains on the surface of the particles is uneven in the initial stage of wustite reduction, which induces the formation of iron whiskers during the isothermal reduction process, increases the internal stress of the pellet, causes abnormal expansion and cracking, and reduces the compressive strength.

[0040] Comparative Example 2

[0041] Compared with Example 1, the only difference is that the pellet is heated to 950 °C in the second temperature-rising reduction zone and then enters the isothermal reduction zone, so that the pellet is isothermally reduced at 950 °C. During the reduction process, the maximum reduction expansion rate of the ultra-high-grade pellet is 21.6%, and the compressive strength of the obtained high-purity direct reduced iron is 366 N.

[0042] Compared with Example 1, the pellet after gradient heating is isothermally reduced at 950 °C. This temperature is conducive to the growth of iron whiskers on the particle surface and inhibits the aggregation and growth of iron grains, resulting in abnormal expansion of the pellet.

[0043] Comparative Example 3

[0044] Compared with Example 1, the only difference is that the ultra-high-grade pellet used has a compressive strength of about 3800 N, and the pellet reduction index is RI 40 is 2.5. During the reduction process, the maximum reduction expansion rate of the ultra-high-grade pellet is 16.7%, and the compressive strength of the obtained high-purity direct reduced iron is 385 N.

[0045] Compared with Example 1, using an ultra-high-grade pellet with a higher compressive strength, due to the more dense pore structure of the pellet, the reduction performance of the pellet is reduced. Due to the difference in the pore structure between the inner and outer layers of the pellet, the internal structural stress of the pellet increases during the reduction process, resulting in expansion and cracking of the pellet.

[0046] Comparative Example 4

[0047] Compared with Example 2, the only difference is that the ultra-high-grade pellet is preheated to 450 °C and then enters the first temperature-rising reduction zone. During the reduction process, the maximum reduction expansion rate of the ultra-high-grade pellet is 22.5%, and the compressive strength of the obtained high-purity direct reduced iron is 326 N.

[0048] Compared with Example 2, when the temperature of the pellet charged into the furnace is 450 °C, a significant carbon precipitation reaction occurs in the pellet ore at the initial stage of reduction, increasing the expansion of the pellet ore caused by the crystal form transformation.

[0049] Comparative Example 5

[0050] Compared with Example 1, the only difference is that during the gradient temperature rise reduction process, the volume ratio of the reducing gas component H2 / (H2 + CO) is 0.5. The maximum reduction expansion rate of the ultra-high-grade pellet ore during the reduction process is 25.8%, and the compressive strength of the obtained high-purity direct reduced iron is 323 N.

[0051] Compared with Example 1, when the proportion of CO in the reducing gas is too high, the reduction rate of the pellet ore decreases, and a significant carbon precipitation reaction occurs in the pellet ore at the initial stage of reduction, increasing the expansion of the pellet ore caused by the crystal form transformation.

[0052] Comparative Example 6

[0053] Compared with Example 1, the only difference is that during the gradient temperature rise reduction process, the flow rate of the reducing gas is 1400 m per ton of direct reduced iron 3 . The maximum reduction expansion rate of the ultra-high-grade pellet ore during the reduction process is 22.3%, and the compressive strength of the obtained high-purity direct reduced iron is 355 N.

[0054] Compared with Example 1, reducing the proportion of the reducing gas decreases the reduction rate of the pellet ore, which is not conducive to the uniform nucleation of iron grains on the surface of the particles during the gradient temperature rise reduction process, triggering the formation of iron whiskers and resulting in abnormal expansion of the pellet ore.

Claims

1. A method for inhibiting abnormal expansion of ultra-high grade pellets by gradient temperature rise reduction in a hydrogen-based shaft furnace, characterized in that: Ultra-high-grade pellet ore is continuously added from the top of the hydrogen-based shaft furnace and sequentially passes through the first temperature-rising reduction zone, the second temperature-rising reduction zone, the isothermal reduction zone, and the cooling zone of the hydrogen-based shaft furnace to obtain direct reduced iron; In the first temperature-rising reduction zone, the heating rate of the ultra-high-grade pellet ore is 10-20 °C / min. When the ultra-high-grade pellet ore is heated to 900-930 °C and the reduction degree of the ultra-high-grade pellet ore reaches more than 40%, it enters the second temperature-rising reduction zone; In the second heating and reduction zone, the heating rate of the ultra-high-grade pellet ore is not less than 30 °C / min. When the ultra-high-grade pellet ore is heated to 990 - 1020 °C, it enters the isothermal reduction zone; the compressive strength of the ultra-high-grade pellet ore is 2500 - 3000 N, and the reduction index RI 40 is greater than 3; The composition of the reducing gas introduced into the first temperature-rising reduction zone, the second temperature-rising reduction zone, and the isothermal reduction zone is as follows: the volume ratio of (H2+CO) is not less than 90%, and the volume ratio of H2 / (H2+CO) is not less than 0.6; The flow rate of the reducing gas is measured as 1600 - 2000 m 3 per ton of direct reduced iron, and the gas pressure in the hydrogen-based shaft furnace is 0.5 - 0.9 MPa; The mass fraction of (SiO2+CaO+MgO+Al2O3) in the ultra-high-grade pellet ore is less than 0.5%, the mass fraction of (P+S) is less than 0.005%, and the mass fraction of TFe is greater than 69.5%; the ultra-high-grade pellet ore is preheated to 550-650 °C before entering the hydrogen-based shaft furnace.

2. A method for inhibiting abnormal expansion of ultra-high grade pellets by gradient temperature rise reduction in a hydrogen-based shaft furnace, characterized in that: The TFe of the direct reduced iron is greater than 99%, and the compressive strength is greater than 400 N.

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