Method for reducing hydrogen-rich reduction expansion of pellets based on hydrogen-based shaft furnace step-by-step reduction

Abstract

The invention discloses a method for reducing hydrogen-rich reduction expansion of pellets based on hydrogen-based shaft furnace step-by-step reduction, and belongs to the technical field of direct reduction iron production. The method comprises the following steps: sequentially reducing pellets in a first-stage hydrogen-based shaft furnace (the temperature is 780-880 DEG C, the volume fraction of H2 + CO is greater than 80%, and the volume ratio of H2 / (H2 + CO) is 0.6-0.8) and a second-stage hydrogen-based shaft furnace (the temperature is 980-1040 DEG C, the volume fraction of H2 + CO is greater than 90%, and the volume ratio of H2 / (H2 + CO) is greater than or equal to 0.8), and cooling to obtain the direct reduction iron. The maximum reduction expansion rate of the whole pellet ore in the reduction process is reduced to 10% or below, the reduction efficiency of the hydrogen-based shaft furnace is guaranteed, meanwhile, direct reduction iron with the quality index meeting the requirements and the higher iron grade is produced, and clean, efficient and short-process utilization of iron ore resources is achieved.

Description

Technical Field

[0001] The present invention relates to a method for reducing the reduction expansion of pelletized ore in a hydrogen-based shaft furnace, and particularly to a method for reducing the hydrogen-rich reduction expansion of pelletized ore based on stepwise reduction in a hydrogen-based shaft furnace, belonging to the technical field of direct reduced iron preparation. Background Art

[0002] The iron and steel industry, as an energy-consuming intensive industry, is one of the manufacturing industries with the highest carbon emissions. It is very important to promote the green and low-carbon transformation of the iron and steel industry. The carbon emissions of the traditional blast furnace-converter long process account for more than 73% of the total emissions of the iron and steel industry. Although with the continuous development of low-carbon blast furnace technologies such as large-scale pellet smelting technology, hydrogen-rich smelting technology, and top gas recycling-oxy-fuel blast furnace, the carbon emissions of the long process smelting process have decreased, the maximum carbon emission reduction is only 24%, and it is difficult to achieve the carbon neutrality goal. Compared with the long process, the carbon emission reduction effect of the hydrogen-based shaft furnace-electric furnace short process can reach more than 50%. It is an effective measure to optimize the iron and steel process flow, energy structure and product structure, and also the future development direction of the iron and steel industry.

[0003] However, the pellet for hydrogen-based shaft furnace has a high iron grade and few gangues. Under the condition of hydrogen-rich reduction, the crystal form transformation of hematite and the difference in reduction rates between the inner and outer layers of the pellet will cause significant accumulation of internal stress in the pellet, and it is easier to induce the formation of iron whiskers, promoting the reduction swelling of the pellet. The greater the reduction swelling of the pellet during the reduction process in the hydrogen-based shaft furnace, the lower the strength of the reduced pellet. Moreover, cracks and disintegration may occur to the pellet during the reduction process, resulting in a decrease in the permeability index in the furnace, severely restricting the productivity release of the hydrogen-based shaft furnace and the stable production of high-quality direct reduced iron. The existing technologies mainly reduce the reduction swelling of the pellet by optimizing the thermal regime in the pellet preparation process and adding calcium-based, magnesium-based fluxes or boron-containing components. For example, the literature ("Study on gas-based reduction mechanism of ultra-high-grade pellets with high basicity by experiment tests and first-principles calculation", Jie Lei, et al., Powder Technology; 2024; 436: 119462) discloses a method of reducing the reduction swelling rate of high-grade pellets by adding calcium oxide. In addition, although improving the pellet strength by optimizing the preheating and roasting regime can resist the internal stress generated during pellet swelling to a certain extent, the effect is limited, and it will increase the energy consumption in the pellet preparation process. When the pellet strength is too high, it will also affect the reduction rate, and cracks are more likely to occur during the reduction process. Using pellet additives faces the problem of applicability to different iron-containing raw materials, and inevitably reduces the iron grade of the pellet ore, resulting in an increase in the slag volume, an increase in smelting energy consumption, and a decrease in productivity during the subsequent smelting process. Summary of the Invention

[0004] Based on the problem that the pellet for hydrogen-based shaft furnace is prone to reduction swelling due to its high iron grade and few gangues, the existing technologies mainly reduce the reduction swelling of the pellet by increasing the pellet strength, adding calcium-based, magnesium-based fluxes or boron-containing components. However, the effect of suppressing reduction swelling by increasing the pellet strength is limited, and it will increase the energy consumption in the pellet preparation process. When using pellet additives, on the one hand, there will be problems with the applicability to different iron-containing raw materials, and on the other hand, it will inevitably reduce the iron grade of the pellet ore and increase the smelting energy consumption.

[0005] In view of the above technical problems existing in the prior art, the object of the present invention is to provide a method for reducing the hydrogen-rich reduction swelling of pellet ore based on step-by-step reduction in a hydrogen-based shaft furnace. The key of this method lies in adopting the way of step-by-step reduction in a hydrogen-based shaft furnace. After the pellet ore is reduced to a certain extent at medium and low temperatures first, then high-temperature rapid reduction is carried out in a hydrogen-rich atmosphere, so that the maximum reduction swelling rate of the pellet ore can be controlled below 10%. This method has simple process, easy operation and strong raw material applicability. Without using pellet additives such as calcium and magnesium, it can produce direct reduced iron with quality indexes meeting the requirements and higher iron grade, which is conducive to realizing the clean, efficient and short-process utilization of iron ore resources.

[0006] To achieve the above technical object, the present invention provides a method for reducing the hydrogen-rich reduction swelling of pellet ore based on step-by-step reduction in a hydrogen-based shaft furnace. In this method, the pellet ore is successively reduced in the first-stage hydrogen-based shaft furnace and the second-stage hydrogen-based shaft furnace, and then cooled to obtain direct reduced iron;

[0007] In the first-stage hydrogen-based shaft furnace, the reduction temperature is 780-880 °C, and the reducing gas satisfies that the volume fraction of H 2 +CO is greater than 80%, and the volume ratio of H 2 / (H 2 +CO) is 0.6-0.8, and the reduction degree of the pellet ore is controlled at 45-60%;

[0008] In the second-stage hydrogen-based shaft furnace, the reduction temperature is 980-1040 °C, and the reducing gas satisfies that the volume fraction of H 2 +CO is greater than 90%, and the volume ratio of H 2 / (H 2 +CO) is ≥0.8, and the reduction degree of the pellet ore is controlled above 95%.

[0009] The key to the technical solution of the present invention lies in adopting a step-by-step reduction method during the hydrogen-based shaft furnace reduction of pellet ore, which can control the maximum reduction expansion rate during the entire pellet ore reduction process below 10%, and this is an unexpected effect. More specifically, first, the pellet ore is preliminarily reduced in the first-stage hydrogen-based shaft furnace at a temperature of 780-880 °C, so that the reduction degree of the pellet is 45-60%. The low temperature in the first-stage hydrogen-based shaft furnace can not only effectively reduce the internal stress of the pellet caused by crystal form transformation and thermal expansion effect while ensuring the reduction efficiency of the pellet, but also effectively reduce the carbon deposition problem in the initial stage of hydrogen-based shaft furnace reduction, improve the utilization rate of reducing gas, and reduce the pellet expansion or fragmentation caused by carbon deposition reaction and carbon deposition. At the same time, the temperature in the first-stage hydrogen-based shaft furnace is controlled within an appropriate range, which can reduce the thermal stress expansion generated due to the sudden temperature change when the pellet enters the second-stage hydrogen-based shaft furnace. The pellet ore preliminarily reduced in the first-stage hydrogen-based shaft furnace is subjected to high-temperature hydrogen-rich rapid reduction in the second-stage hydrogen-based shaft furnace. Under the action of high temperature and hydrogen-rich reducing gas, the reduction rate in the later stage of pellet ore reduction can be accelerated, promoting the aggregation, growth and consolidation of iron grains, so that the maximum reduction expansion rate of the pellet ore during the hydrogen-rich reduction process is less than 10%, and direct reduced iron with a dense structure, good strength and high iron grade is obtained. To sum up, by adopting a step-by-step reduction method and controlling conditions such as temperature and atmosphere during each stage of reduction, the present invention can control the maximum reduction expansion rate during the entire pellet ore reduction process below 10%, and at the same time produce direct reduced iron with quality indicators meeting the requirements and higher iron grade.

[0010] As a preferred solution, the average particle size of the pellet ore is 12-14 mm, and the compressive strength is 2500-3500 N. The strength of the pellet ore is preferably controlled within a suitable range. When the strength is too high, the structure of the pellet ore is too dense, and the pore structures inside and outside the pellet ore are uneven. On the one hand, it will reduce the reduction rate of the pellet ore. On the other hand, it will cause structural stress due to the difference in pore structures inside and outside the pellet ore during the reduction process of the pellet ore, resulting in pellet ore expansion and cracking. However, if the strength of the pellet ore is too low, it is difficult to meet the requirements of hydrogen-based shaft furnace reduction.

[0011] As a preferred solution, the pressure of the reducing gas in the first-stage hydrogen-based shaft furnace is 0.5-0.7 MPa, and the flow rate of the reducing gas is 1400-1600 m relative to each ton of direct reduced iron 3 . When the temperature in the first-stage hydrogen-based shaft furnace is determined, by regulating the pressure and flow rate of the reducing gas, the reduction rate of the pellet ore in the first-stage hydrogen-based shaft furnace can be ensured, and the utilization rate of the reducing gas can be improved. In the present invention, the temperature of the first-stage hydrogen-based shaft furnace is controlled at medium and low temperatures, and at relatively low reducing gas pressure and flow rate, to control the relatively low reduction rate of the pellet ore, which can effectively reduce the internal stress caused by crystal form transformation and thermal expansion effect in the initial stage of pellet reduction, and induce uniform nucleation and layer-by-layer growth of iron grains on the particle surface.

[0012] As a preferred solution, the pressure of the reducing gas in the second-stage hydrogen-based shaft furnace is 0.8 - 1.0 MPa, and the flow rate of the reducing gas is 1600 - 1800 m per ton of direct reduced iron 3 . In the present invention, the temperature of the second-stage hydrogen-based shaft furnace is controlled at a high temperature, and at the same time, under relatively high pressure and flow rate of the reducing gas, the rapid reduction of pellet ore is controlled, which can inhibit the formation of iron whiskers during the reduction of pellet ore in the second-stage hydrogen-based shaft furnace and improve the utilization rate of the reducing gas.

[0013] As a preferred solution, the top gas generated in the second-stage hydrogen-based shaft furnace is purified and then used to prepare the reducing gas in the first-stage hydrogen-based shaft furnace. This operation can effectively realize the recycling of the reducing gas and the efficient utilization of waste heat.

[0014] As a preferred solution, the apparent density of the direct reduced iron is 2.0 - 2.4 t / m 3 , and the compressive strength is greater than 500 N.

[0015] The pellet ore involved in the present invention is a conventional pellet ore in the art. It is obtained by mixing and pelletizing iron concentrate with a binder and then subjecting it to oxidation roasting. Among them, the iron concentrate includes at least one of magnetite concentrate, hematite concentrate, limonite concentrate, and vanadium-titanium iron concentrate; the binder contains at least one of bentonite, sodium carboxymethylcellulose, and polyacrylamide. The preparation process of pellet ore is also well-known in the prior art.

[0016] After being reduced in the first-stage hydrogen-based shaft furnace, the pellet ore of the present invention has a compressive strength greater than 1000 N, which is beneficial to resisting the thermal stress expansion of the pellets in the second-stage hydrogen-based shaft furnace.

[0017] Compared with the prior art, the beneficial technical effects of the technical solution of the present invention are as follows:

[0018] The method provided by the present invention adopts the method of gradually reducing in a hydrogen-based shaft furnace. First, the pellet ore is reduced to a certain extent at medium and low temperatures, and then high-temperature rapid reduction is carried out. It can effectively reduce the internal stress caused by the crystal form transformation and thermal expansion effect in the initial stage of pellet reduction, induce uniform nucleation and layer-by-layer growth of iron grains on the particle surface, promote the aggregation and growth of iron grains in the pellet during the subsequent reduction process, and make the maximum reduction expansion rate of the pellet ore less than 10% during the hydrogen-rich reduction process.

[0019] The method provided by the present invention can effectively reduce the problem of carbon deposition in the initial stage of reduction in the hydrogen-based shaft furnace of the prior art while ensuring the reduction efficiency of the pellet. On the one hand, it is beneficial to improve the utilization rate of the reducing gas. On the other hand, it can reduce the pellet expansion or fragmentation caused by the carbon deposition reaction and carbon deposition.

[0020] The reducing atmosphere adopted by the method provided by the present invention is conducive to the uniform nucleation and aggregation growth of iron grains. At the same time, the top gas in the second-stage hydrogen-based shaft furnace is purified and used as the reducing gas in the first-stage hydrogen-based shaft furnace, realizing the recycling of the reducing gas and the efficient utilization of waste heat.

[0021] The method provided by the present invention has a simple process, is easy to operate, and has strong raw material applicability. It can produce direct reduced iron with a dense structure, good strength, and higher iron grade without using pellet additives such as calcium and magnesium, which is conducive to the clean, efficient, and short-process utilization of iron ore resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is the process flow chart of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0023] 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.

[0024] 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.

[0025] Example 1

[0026] Magnetite concentrate and hematite are mixed into iron concentrate in a weight ratio of 70:30, and then the iron concentrate, bentonite, and sodium carboxymethylcellulose are prepared into pellet ore through mixing and pelletizing and oxidation roasting in a weight ratio of 99.3:0.6:0.1. The average particle size of the pellet ore is 12 - 14 mm, and the compressive strength is 2850 N.

[0027] During the step-by-step reduction process in the hydrogen-based shaft furnace, the reduction temperature in the first-stage hydrogen-based shaft furnace is 880 °C, and the gas pressure is 0.5 MPa. The volume fraction of the reducing gas (H 2 +CO) in the first-stage hydrogen-based shaft furnace is 84%, the volume ratio of H 2 / (H 2 +CO) is 0.6, and the gas flow rate is 1600 m 3 / t. After reduction in the first-stage hydrogen-based shaft furnace, the reduction degree of the pellet ore is 60%, and the compressive strength is 1154 N. The reduction temperature in the second-stage hydrogen-based shaft furnace is 1040 °C, and the gas pressure is 0.8 MPa. The volume fraction of the reducing gas (H 2 +CO) in the second-stage hydrogen-based shaft furnace is 95%, the volume ratio of H 2 / (H 2+(CO) is 0.8, and the gas flow rate is 1800 m 3 / t. The maximum reduction swelling rate of the pellets during the step-by-step reduction process is 7.3%, and the apparent density of the obtained direct reduced iron is 2.4 t / m 3 , and the compressive strength is 583 N.

[0028] Example 2

[0029] Mix magnetite concentrate and vanadium-titanium iron concentrate in a weight ratio of 80:20 to form iron concentrate, and then prepare pellet ore by mixing, pelletizing, and oxidative roasting the iron concentrate, bentonite, and polyacrylamide in a weight ratio of 99:0.8:0.2. The average particle size of the pellet ore is 12 - 14 mm, and the compressive strength is 3057 N.

[0030] During the step-by-step reduction process in the hydrogen-based shaft furnace, the reduction temperature in the first-stage hydrogen-based shaft furnace is 780 °C, and the gas pressure is 0.7 MPa. In the first-stage hydrogen-based shaft furnace, the volume fraction of the reducing gas (H 2 +CO) is 90%, and the volume ratio of H 2 / (H 2 +CO) is 0.8, and the gas flow rate is 1400 m 3 / t. After reduction in the first-stage hydrogen-based shaft furnace, the reduction degree of the pellet ore is 45%, and the compressive strength is 1082 N. The reduction temperature in the second-stage hydrogen-based shaft furnace is 980 °C, and the gas pressure is 1.0 MPa. In the second-stage hydrogen-based shaft furnace, the volume fraction of the reducing gas (H 2 +CO) is 95%, and the volume ratio of H 2 / (H 2 +CO) is 1.0 (100% H 2 ), and the gas flow rate is 1600 m 3 / t. The maximum reduction swelling rate of the pellets during the step-by-step reduction process is 9.2%, and the apparent density of the obtained direct reduced iron is 2.0 t / m 3 , and the compressive strength is 534 N.

[0031] Example 3

[0032] Mix magnetite concentrate and limonite concentrate in a weight ratio of 85:15 to form iron concentrate, and then prepare pellet ore by mixing, pelletizing, and oxidative roasting the iron concentrate, bentonite, and sodium carboxymethylcellulose in a weight ratio of 99.5:0.4:0.1. The average particle size of the pellet ore is 12 - 14 mm, and the compressive strength is 2963 N.

[0033] During the step-by-step reduction process in the hydrogen-based shaft furnace, the reduction temperature in the first-stage hydrogen-based shaft furnace is 820 °C, and the gas pressure is 0.6 MPa. In the first-stage hydrogen-based shaft furnace, the volume fraction of the reducing gas (H 2 +CO) is 86%, and the volume ratio of H 2 / (H2 The (H₂ + CO) is 0.72 and the gas flow rate is 1500 m³ / t. After the first-stage hydrogen-based shaft furnace reduction, the reduction degree of the pellet ore is 50% and the compressive strength is 1034 N. The temperature of the second-stage hydrogen-based shaft furnace reduction is 1020 °C and the gas pressure is 0.9 MPa. In the second-stage hydrogen-based shaft furnace, the volume fraction of the reducing gas (H₂ + CO) is 92%, and the volume ratio of H₂ / (H₂ + CO) is 0.9. The gas flow rate is 1700 m³ / t. During the step-by-step reduction process, the maximum reduction expansion rate of the pellet is 5.1%, and the apparent density of the obtained direct reduced iron is 2.1 t / m³, and the compressive strength is 615 N. 3 / t. After the first-stage hydrogen-based shaft furnace reduction, the reduction degree of the pellet ore is 50% and the compressive strength is 1034 N. The temperature of the second-stage hydrogen-based shaft furnace reduction is 1020 °C and the gas pressure is 0.9 MPa. In the second-stage hydrogen-based shaft furnace, the volume fraction of the reducing gas (H₂ + CO) is 92%, and the volume ratio of H₂ / (H₂ + CO) is 0.9. The gas flow rate is 1700 m³ / t. During the step-by-step reduction process, the maximum reduction expansion rate of the pellet is 5.1%, and the apparent density of the obtained direct reduced iron is 2.1 t / m³, and the compressive strength is 615 N. 2 +CO) volume fraction is 92%, volume ratio of H 2 / (H 2 +CO) is 0.9, gas flow rate is 1700 m 3 / t. The maximum reduction expansion rate of the pellet during the step-by-step reduction process is 5.1%, and the apparent density of the obtained direct reduced iron is 2.1 t / m 3 , and the compressive strength is 615 N.

[0034] Example 4

[0035] Magnetite concentrate, bentonite and sodium carboxymethylcellulose are made into pellet ore after being mixed, pelletized and oxidized and roasted according to the weight ratio of 99.7:0.2:0.1. The average particle size of the pellet ore is 12 - 14 mm, and the compressive strength is 3287 N.

[0036] During the step-by-step reduction process in the hydrogen-based shaft furnace, the temperature of the first-stage hydrogen-based shaft furnace reduction is 840 °C and the gas pressure is 0.6 MPa. In the first-stage hydrogen-based shaft furnace, the volume fraction of the reducing gas (H₂ + CO) is 86%, and the volume ratio of H₂ / (H₂ + CO) is 0.72. The gas flow rate is 1500 m³ / t. After the first-stage hydrogen-based shaft furnace reduction, the reduction degree of the pellet ore is 60% and the compressive strength is 1123 N. The temperature of the second-stage hydrogen-based shaft furnace reduction is 1020 °C and the gas pressure is 0.9 MPa. In the second-stage hydrogen-based shaft furnace, the volume fraction of the reducing gas (H₂ + CO) is 92%, and the volume ratio of H₂ / (H₂ + CO) is 0.9. The gas flow rate is 1700 m³ / t. During the step-by-step reduction process, the maximum reduction expansion rate of the pellet is 3.7%, and the apparent density of the obtained direct reduced iron is 2.3 t / m³, and the compressive strength is 656 N. 2 +CO) volume fraction is 86%, volume ratio of H 2 / (H 2 +CO) is 0.72, gas flow rate is 1500 m 3 / t. After the first-stage hydrogen-based shaft furnace reduction, the reduction degree of the pellet ore is 60% and the compressive strength is 1123 N. The temperature of the second-stage hydrogen-based shaft furnace reduction is 1020 °C and the gas pressure is 0.9 MPa. In the second-stage hydrogen-based shaft furnace, the volume fraction of the reducing gas (H₂ + CO) is 92%, and the volume ratio of H₂ / (H₂ + CO) is 0.9. The gas flow rate is 1700 m³ / t. During the step-by-step reduction process, the maximum reduction expansion rate of the pellet is 3.7%, and the apparent density of the obtained direct reduced iron is 2.3 t / m³, and the compressive strength is 656 N. 2 +CO) volume fraction is 92%, volume ratio of H 2 / (H 2 +CO) is 0.9, gas flow rate is 1700 m 3 / t. During the step-by-step reduction process, the maximum reduction expansion rate of the pellet is 3.7%, and the apparent density of the obtained direct reduced iron is 2.3 t / m 3 , and the compressive strength is 656 N.

[0037] Example 5

[0038] Magnetite concentrate and hematite are mixed into iron concentrate in a weight ratio of 30:70. Then, the iron concentrate, bentonite, and polyacrylamide are mixed and pelletized, and oxidized and roasted according to a weight ratio of 98.8:1:0.2 to prepare pellet ore. The average particle size of the pellet ore is 12 - 14 mm, and the compressive strength is 3166 N.

[0039] During the step-by-step reduction process in the hydrogen-based shaft furnace, the reduction temperature in the first-stage hydrogen-based shaft furnace is 860 °C, and the gas pressure is 0.6 MPa. The volume fraction of the reducing gas (H 2 +CO) in the first-stage hydrogen-based shaft furnace is 92%, and the volume ratio of H 2 / (H 2 +CO) is 0.8, and the gas flow rate is 1500 m 3 / t. After reduction in the first-stage hydrogen-based shaft furnace, the reduction degree of the pellet ore is 55%, and the compressive strength is 1047 N. The reduction temperature in the second-stage hydrogen-based shaft furnace is 1020 °C, and the gas pressure is 0.9 MPa. The volume fraction of the reducing gas (H 2 +CO) in the second-stage hydrogen-based shaft furnace is 92%, and the volume ratio of H 2 / (H 2 +CO) is 0.92, and the gas flow rate is 1700 m 3 / t. During the step-by-step reduction process, the maximum reduction expansion rate of the pellet is 8.5%, and the apparent density of the obtained direct reduced iron is 2.1 t / m 3 , and the compressive strength is 514 N.

[0040] Comparative Example 1

[0041] Compared with Example 1, the only difference is that the reduction temperature in the first-stage hydrogen-based shaft furnace is 720 °C. After reduction in the first-stage hydrogen-based shaft furnace, the compressive strength of the pellet is 942 N. During the step-by-step reduction process, the maximum reduction expansion rate of the pellet ore is 18.3%, and the apparent density of the obtained direct reduced iron is 1.93 t / m 3 , and the compressive strength is 385 N.

[0042] Compared with Example 1, due to the lower reduction temperature of the pellet in the preliminary stage, it is not conducive to the uniform nucleation and layered growth of iron grains on the particle surface, and the pellet strength is lower. Moreover, due to the large temperature difference between the reduction in the first-stage hydrogen-based shaft furnace and the reduction in the second-stage hydrogen-based shaft furnace, the pellet is more likely to generate thermal stress expansion and cracking when entering the second-stage hydrogen-based shaft furnace.

[0043] Comparative Example 2

[0044] Compared with Example 1, the only difference is that the reduction temperature in the first-stage hydrogen-based shaft furnace is 930 °C. After reduction in the first-stage hydrogen-based shaft furnace, the compressive strength of the pellet is 793 N. During the step-by-step reduction process, the maximum reduction expansion rate of the pellet ore is 23.8%, and the apparent density of the obtained direct reduced iron is 1.87 t / m3 The compressive strength is 341 N.

[0045] Compared with Example 1, due to the too high temperature of the initial reduction of the pellets, the internal stress generated by the crystal form transformation during the reduction process of the pellets is greater, and iron whiskers are more likely to be generated on the surface of the particles in the pellets at high temperature, resulting in an increase in the reduction expansion rate of the pellets and a lower strength of the pellets.

[0046] Comparative Example 3

[0047] Compared with Example 1, the only difference is that: the reduction degree of the pelletized ore after the first-stage hydrogen-based shaft furnace reduction is 30%. The compressive strength of the pellets after the first-stage hydrogen-based shaft furnace reduction is 1067 N. The maximum reduction expansion rate of the pelletized ore during the step-by-step reduction process is 25.6%, and the apparent density of the obtained direct reduced iron is 1.85 t / m 3 The compressive strength is 327 N.

[0048] Compared with Example 1, the pellets are reduced to 30% in the first-stage hydrogen-based shaft furnace and then enter the second-stage hydrogen-based shaft furnace for continuous reduction. On the one hand, there is still a small amount of hematite in the pellets that has not been completely reduced, causing expansion stress during the high-temperature reduction process; on the other hand, since the iron grains on the surface of the pellet particles have not completed sufficient nucleation, the growth of iron whiskers during the high-temperature reduction process is caused, and the reduction expansion rate of the pellets increases.

[0049] Comparative Example 4

[0050] Compared with Example 1, the only difference is that: after the pellets are reduced by the first-stage hydrogen-based shaft furnace, they enter the second-stage hydrogen-based shaft furnace at 950 °C for continuous reduction. The maximum reduction expansion rate of the pelletized ore during the reduction process is 16.5%, and the apparent density of the obtained direct reduced iron is 1.96 t / m 3 The compressive strength is 422 N.

[0051] Compared with Example 1, due to the lower temperature of the second-stage hydrogen-based shaft furnace, it is not conducive to the aggregation and growth of iron grains in the pellets in the later stage of reduction, the expansion rate of the pellets increases, and the strength of the pellets decreases.

[0052] Comparative Example 5

[0053] Compared with Example 1, the only difference is that: the average particle size of the pelletized ore used is 14 - 16 mm, and the compressive strength is 4055 N. The maximum reduction expansion rate of the pelletized ore during the reduction process is 17.6%, and the apparent density of the obtained direct reduced iron is 2.04 t / m 3 The compressive strength is 557 N.

[0054] Compared with Example 1, the pelletized ore used has a larger particle size and a higher compressive strength of the pellets, resulting in a greater difference in the crystallization consolidation and pore structure between the inner and outer layers of the pellets, leading to an increase in the internal stress in the pellets during the reduction process and an increase in the reduction expansion rate of the pellets.

[0055] Comparative Example 6

[0056] Compared with Example 1, the only difference is that during the reduction process in the first-stage hydrogen-based shaft furnace, the volume fraction of the reducing gas (H 2 +CO) is 70%, and the volume ratio of H 2 / (H 2 +CO) is 0.5. During the reduction process in the second-stage hydrogen-based shaft furnace, the volume fraction of the reducing gas (H 2 +CO) is 80%, and the volume ratio of H 2 / (H 2 +CO) is 0.7. The maximum reduction swelling rate of the pellets during the reduction process is 19.4%, and the apparent density of the obtained direct reduced iron is 1.88 t / m 3 , and the compressive strength is 388 N.

[0057] Compared with Example 1, the proportion of the reducing gas decreases, and the volume fraction of H 2 in the reducing gas decreases. On the one hand, it affects the reduction rate of the pellets, and on the other hand, it promotes the formation of iron whiskers in the pellets. Therefore, the reduction swelling rate of the pellets increases.

Claims

1. A method for reducing the expansion of pellets by hydrogen-rich reduction based on step-by-step reduction in a hydrogen-based shaft furnace, characterized in that: The pellets are reduced in a first-stage hydrogen-based shaft furnace and a second-stage hydrogen-based shaft furnace in sequence, and then cooled to obtain direct reduced iron; The reduction temperature in the first-stage hydrogen-based vertical furnace is 780-880°C, the reducing gas satisfies that the volume fraction of H2+CO is greater than 80%, and the volume ratio of H2 / (H2+CO) is 0.6-0.8, and the reduction degree of the pellets is controlled at 45-60%; The reduction temperature in the second-stage hydrogen-based vertical furnace is 980-1040°C, the reducing gas satisfies that the volume fraction of H2+CO is greater than 90%, and the volume ratio H2 / (H2+CO) is ≥0.8, and the reduction degree of the pelletized ore is controlled at above 95%.

2. The method of reducing the expansion of pellets by hydrogen-rich reduction based on hydrogen-based vertical furnace step-by-step reduction according to claim 1, characterized in that: The average particle size of the pellets is 12-14 mm, and the compressive strength is 2500-3500 N.

3. The method of reducing the expansion of pellets by hydrogen-rich reduction based on step-by-step reduction in a hydrogen-based shaft furnace according to claim 1, characterized in that: The pressure of the reducing gas in the first stage hydrogen-based vertical furnace is 0.5-0.7 MPa, and the flow rate of the reducing gas is 1400-1600 m / t of direct reduced iron. 3 .

4. The method of reducing the expansion of pellets by hydrogen-rich reduction based on step-by-step reduction in a hydrogen-based shaft furnace according to claim 1, characterized in that: The pressure of the reducing gas in the second-stage hydrogen-based vertical furnace is 0.8-1.0 MPa, and the flow rate of the reducing gas is 1600-1800 m / t of direct reduced iron. 3 .

5. The method for reducing the expansion of pellets by hydrogen-rich reduction based on step-by-step reduction in a hydrogen-based shaft furnace according to claim 1, characterized in that: The top tail gas generated in the second-stage hydrogen-based vertical furnace is purified and used to prepare the reducing gas in the first-stage hydrogen-based vertical furnace.

6. A method for reducing the expansion of pellets by hydrogen-rich reduction based on step-by-step reduction in a hydrogen-based shaft furnace according to any one of claims 1 to 5, characterized in that: The apparent density of the direct reduced iron is 2.0-2.4 t / m 3 , compressive strength is greater than 500 N.

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