Method for producing direct reduced iron from iron-containing raw material with near zero carbon

Abstract

The application discloses a method for preparing direct reduced iron from iron-containing raw materials with near-zero carbon, and belongs to the technical field of steel metallurgy. The method comprises the following steps: uniformly mixing iron-containing raw materials, an organic-inorganic composite binder, biomass carbon and water, and then sequentially performing high-pressure forming and drying and solidification to obtain cold briquetting; and the cold briquetting is reduced by pure hydrogen to obtain direct reduced iron. The method can prepare cold briquetting suitable for the performance requirements of a shaft furnace for direct reduction at a relatively low temperature (<300 DEG C), and further prepares direct reduced iron by pure hydrogen reduction. The method has a short process flow, near-zero carbon emission, and provides a new technical route for the green and low-carbon transformation of the steel industry.

Description

Technical Field

[0001] This invention relates to a method for preparing reduced iron from iron-containing raw materials, and particularly to a method for preparing direct reduced iron from iron-containing raw materials through near-zero carbon agglomeration and all-hydrogen reduction, belonging to the field of iron and steel metallurgical technology. Background Technology

[0002] The steel industry is a crucial basic industry, but also a major energy consumer and a key emitter of carbon dioxide. Current steel production has long relied on a long "blast furnace-converter" process, heavily depending on fossil fuels such as coke, coal, and heavy oil. This process generates significant greenhouse gas emissions during ore roasting, coke preparation, and blast furnace ironmaking. Therefore, traditional long-process ironmaking faces immense pressure to reduce carbon emissions and achieve green transformation. Developing low-carbon or even near-zero-carbon pre-ironmaking technologies has become a crucial direction for technological advancement in the steel industry. Direct reduction ironmaking is a key technological path to replace some blast furnace ironmaking and promote the development of short-process electric arc furnaces. Currently, most engineering applications use natural gas reforming-based reduction or coal-based reduction systems, which can reduce unit carbon emissions to some extent, but still heavily rely on fossil fuels. Furthermore, existing direct reduction processes generally rely on pelletized ore obtained through high-temperature roasting, resulting in high energy consumption and carbon emissions during the initial agglomeration process, contradicting the near-zero carbon goal.

[0003] For existing lean iron ore resources, beneficiation often produces fine-grained iron concentrate, which must be agglomerated before it can be used in blast furnaces or direct reduction furnaces. While traditional sintering and roasting pelletizing can efficiently utilize fine-grained iron ore, it requires the addition of fuel at high temperatures, resulting in high energy consumption and high carbon emissions. In contrast, cold-pressing pelletizing using room-temperature or low-temperature conditions has the potential to reduce or replace some high-temperature sintering and pelletizing processes, significantly reducing carbon emissions from ironmaking processes. However, iron concentrate generally has a fine particle size and poor self-binding properties, resulting in insufficient compaction when using conventional cold-pressing processes. The cold-pressed pellets have low strength and often require a large amount of binder to barely meet the requirements for furnace feeding. At the same time, magnetite concentrate has poor reducibility and requires internal carbon addition to improve its reducibility. If fossil carbon such as coal and coke is still used as the main carbon-containing additive, it is difficult to achieve near-zero carbon targets. With the development of hydrogen metallurgy technology, direct hydrogen reduction using hydrogen as the main reducing agent is considered a key path to achieve near-zero carbon emissions from ironmaking processes. Hydrogen reduction, with water vapor as the main product, can significantly reduce CO2 emissions. However, current hydrogen reduction methods are primarily hydrogen-rich, and the reducing gas still contains a certain proportion of CO, resulting in some carbon emissions. Furthermore, the hydrogen reduction reaction is vigorous, placing more stringent demands on the strength of the furnace charge, reduction pulverization, and reduction expansion behavior, requirements that existing cold-pressed pellets often fail to meet. A Chinese patent application (publication number: CN120796696A) discloses a method for preparing reduced iron from iron-containing raw materials through low-carbon agglomeration and hydrogen reduction. Specifically, the method involves mixing iron-containing raw materials (including hematite and magnetite) with a composite binder and water to obtain a mixture; sequentially homogenizing the mixture, high-pressure molding, and drying to obtain cold-pressed pellets; and then subjecting the cold-pressed pellets to hydrogen reduction to obtain metallized pellets. The obvious drawbacks of this method are: firstly, the reducing gas used in its reduction process is hydrogen-rich gas, which still requires the addition of an appropriate proportion of carbon-containing gases such as carbon monoxide. This not only causes a certain degree of carbon emissions, but also the reducing power of hydrogen-rich gas is weaker than that of pure hydrogen gas. Therefore, the reduction process still requires a higher temperature and a longer reduction time, resulting in higher energy consumption. Secondly, the particle size distribution of the iron-containing raw materials is still not scientific enough. Simply optimizing the particle size distribution by matching coarse and fine particle sizes has limited effect on improving the compressive strength of the cold-pressed blocks. Chinese patent (authorization publication number: CN120700272B) discloses a method of sequentially mixing iron-containing raw materials (magnetite concentrate of different particle sizes) with an organic-inorganic composite binder, a multifunctional additive (the multifunctional additive includes fillers, highly reactive biochar, and low-reactive biochar), and water, followed by high-pressure roller molding and solidification to obtain the finished cold-pressed blocks.The main drawbacks of this method are: firstly, the cold-pressed briquettes prepared are suitable for blast furnace reduction, but the carbon emissions during the reduction process are still relatively high; secondly, the applicable iron-containing raw materials are limited to magnetite concentrate; thirdly, the particle size distribution of the iron-containing raw materials is not scientific enough, and the proportion of filler used as core particles is low, resulting in limited improvement on the compressive strength of the cold-pressed briquettes; fourthly, the quality of the biochar introduced is poor, with a high ash content, requiring a high amount of biochar to ensure the overall reducibility of the cold-pressed briquettes, thus introducing more impurities; and fifthly, to meet the basicity requirements of the blast furnace burden, a certain proportion of filler needs to be added, which will reduce the iron grade of the cold-pressed briquettes. Summary of the Invention

[0004] To address the technical problems of high carbon emissions in existing direct reduction processes for producing reduced iron, this invention aims to provide a method for producing direct reduced iron from iron-containing raw materials with near-zero carbon content. This method involves preparing cold-pressed briquettes that meet the performance requirements for direct reduction in vertical shaft furnaces at low temperatures (<300℃). These cold-pressed briquettes can then be reduced to reduced iron through full hydrogen reduction. Compared to traditional direct reduction processes using oxide pellets, this invention eliminates the need for high-temperature oxide pellet preparation and utilizes a hydrogen-coupled biomass char reduction process, resulting in better reducibility. It also boasts significant advantages such as a shorter process flow and near-zero carbon emissions. Compared to existing cold-bonded pellet (cold-pressed pellet) blast furnace reduction processes, the cold-pressed pellets of this invention have a higher overall iron content, and the use of pure hydrogen gas as a reducing agent greatly reduces carbon emissions during the reduction process.

[0005] To achieve the above-mentioned technical objectives, the present invention provides a method for preparing direct reduced iron from iron-containing raw materials with near-zero carbon content. The method involves mixing the iron-containing raw materials with an organic-inorganic composite binder, biochar, and water, and then sequentially subjecting the mixture to high-pressure molding and drying to obtain cold-pressed pellets. The cold-pressed pellets are then reduced with pure hydrogen to obtain direct reduced iron.

[0006] The iron-containing raw materials include hematite powder, hematite concentrate, and magnetite concentrate, and the mass percentage composition of hematite powder, hematite concentrate, and magnetite concentrate is: 5%~10%:10%~20%:70%~85%;

[0007] The mass proportion of the hematite powder with particles smaller than 3 mm is not less than 75%, the mass proportion of particles larger than 0.1 mm is not less than 85%, and the maximum particle size is less than 4.5 mm.

[0008] The mass proportion of particles >0.074mm in the hematite concentrate is not less than 30%, the mass proportion of particles <0.045mm is not more than 40%, and the maximum particle size is less than 0.25mm.

[0009] The magnetite concentrate contains more than 65% by mass of particles smaller than 0.074 mm.

[0010] The biochar comprises highly reactive biochar and low reactive biochar, and the mass percentage composition of highly reactive biochar and low reactive biochar is 20%~35%:65%~80%;

[0011] The high-reactivity biochar contains no more than 10% of particles >0.074mm in mass, no less than 60% of particles <0.045mm in mass, and has a fixed carbon content of no less than 87%, an ash content of no more than 4%, and a volatile matter content of no more than 9%.

[0012] The low-reactive biochar shall contain at least 30% particles larger than 0.074 mm and at least 40% particles smaller than 0.045 mm, with a fixed carbon content of at least 90% and ash and volatile matter content of at least 5%.

[0013] The core idea of ​​this invention is to obtain cold-pressed pellets that meet the performance requirements of direct reduction in a vertical shaft furnace by low-temperature forming of iron-containing raw materials. These cold-pressed pellets are then further reduced to direct reduced iron through internal biomass char-based solid-based reduction coupled with hydrogen-based gas reduction. Compared with existing technologies, this invention has significant advantages: the entire process is extremely short, and carbon emissions are extremely low.

[0014] The key to the near-zero carbon emissions of the entire process for preparing direct reduced iron from iron-containing raw materials in this invention lies in two aspects: First, the temperature during the entire process of preparing cold-pressed pellets from iron-containing raw materials does not exceed 300℃, resulting in extremely low carbon emissions. Second, the reduction process of the cold-pressed pellets involves solid-based reduction of biochar coupled with pure hydrogen gas-based reduction within the cold-pressed pellets, also resulting in near-zero carbon emissions throughout the entire reduction process (Note: Biochar originates from biomass, and the carbon dioxide released during the process of biochar acting as a reducing agent is equal to the carbon dioxide fixed during biomass formation; therefore, it merely achieves carbon dioxide recycling, and this process is considered "near-zero carbon"). Compared with the conventional "hydrogen-rich hydrogen reduction" process, the "gas-solid coupling reduction" process of this invention has significant advantages. Conventional hydrogen-rich hydrogen reduction processes (MIDREX, HYL-III) use hydrogen gas with a hydrogen content of 40%~75%, still containing a certain proportion of carbon-containing gases, which would increase carbon emissions.

[0015] This invention utilizes iron-containing raw materials to prepare direct reduced iron. Through the high coupling between biochar-based solid-base reduction and hydrogen-based gas-based reduction within cold-pressed pellets, the utilization rate of H2 during the reduction process and the reducibility of the cold-pressed pellets can be significantly improved. Firstly, biochar can directly react with iron oxides inside the cold-pressed pellets at high temperatures, while the hydrogen medium accelerates the reduction rate; both synergistically improve the reducibility of the cold-pressed pellets. Simultaneously, biochar has an in-situ pore-forming effect; the pores formed in situ after the reaction at high temperatures facilitate the diffusion of hydrogen into the internal reaction and also facilitate the discharge of gases such as water vapor, further improving hydrogen reduction efficiency and the reducibility of the cold-pressed pellets. Secondly, biochar enables hydrogen recycling, improving hydrogen utilization. Water vapor generated during the hydrogen reduction process inside the cold-pressed pellets can react with biochar in a water-gas reaction (H2O + C = CO + H2), allowing H2 to participate in the reduction reaction, thereby increasing the utilization rate of H2 in the reduction process and reducing H2 consumption per ton of ore produced. Thirdly, using pure hydrogen as the reducing gas requires a slightly lower reduction temperature compared to conventionally using hydrogen-rich hydrogen. This lower reduction temperature reduces the reactivity of the biochar, allowing it to continuously participate in the reduction reaction and preventing rapid, concentrated reactions at high temperatures. Fourthly, the reduction of iron oxide by hydrogen is endothermic, while the reduction of iron oxide by biochar is exothermic. The participation of an appropriate amount of biochar in the reduction reaction can, to some extent, compensate for the technical problem of maintaining the reduction temperature due to the endothermic reaction during the hydrogen reduction of iron oxide.

[0016] This invention utilizes iron-containing raw materials to prepare direct reduced iron. By optimizing the composition and particle size of the iron-containing raw materials and the activity of biochar, combined with a "gas-solid coupled reduction" process, the reduction rate of cold-pressed pellets can be accelerated, the reduction time shortened, and the reduction expansion performance improved. More specifically, on the one hand, by controlling the ratio between low-reactivity biochar and high-reactivity biochar, the biochar can continuously undergo the reduction reaction throughout the reduction heating process, avoiding the expansion of cold-pressed pellets caused by concentrated reduction. On the other hand, by appropriately combining the finest magnetite concentrate, relatively fine hematite concentrate, and coarsest hematite powder in a suitable proportion to meet the particle size distribution curve, the strength of cold-pressed pellets can be significantly improved, the amount of binder used can be reduced, thereby further improving the iron grade of the cold-pressed pellets and reducing the influence of inorganic components in the binder on the reducibility. Meanwhile, by controlling the ratio of magnetite to hematite to regulate the ratio of Fe2O3 and Fe3O4 in iron-bearing raw materials, the reduction expansion performance, reduction pulverization performance, and reduction performance of cold-pressed pellets can be synergistically improved. This is mainly based on the fact that when Fe2O3 is reduced to Fe3O4, the crystal transformation causes volume expansion and cracking, resulting in pulverization and expansion. Fe3O4 exhibits only slight volume expansion during reduction. Adding magnetite can improve the reduction expansion and reduction pulverization performance of cold-pressed pellets. However, magnetite has poor reduction performance, while hematite has good reduction performance. Adding an appropriate amount of hematite can improve the reduction performance of cold-pressed pellets. Therefore, optimizing the blending of iron-bearing raw materials can not only synergistically improve the strength and metallurgical performance of cold-pressed pellets, but also has a wide range of applicable raw materials. In addition to conventional fine-grained iron concentrates, coarse-grained iron concentrates with poor pelletizing performance and even larger-grained powders can all be used in the cold-pressing pelleting process, thus broadening the raw material sources for cold-pressed pellets.

[0017] The iron-containing raw materials of this invention, through optimized blending of iron concentrate composition and particle size, can meet the compositional requirements of cold-pressed pellets and improve metallurgical performance. Magnetite concentrate has relatively poor reducibility but exhibits slight expansion during reduction, while hematite concentrate has better reducibility but suffers from severe expansion during reduction. Blending these two types of iron concentrate in appropriate proportions can improve the malignant expansion and severe pulverization during hydrogen reduction while ensuring the reducibility of the cold-pressed pellets. Conversely, if the proportion of magnetite concentrate is too high, it will worsen the reducibility of the cold-pressed pellets, while if the proportion is too low, it will affect the reduction pulverization performance and reduction expansion performance of the cold-pressed pellets. Meanwhile, the iron-bearing raw material is composed of an appropriate ratio of the finest magnetite concentrate, the finest hematite concentrate, and the coarsest hematite powder. This is primarily to create a fine-to-medium-to-coarse particle size distribution, closely approximating the gradation curve. This significantly improves the density of the cold-pressed pellets, thereby enhancing their strength. Furthermore, the coarser particle size of the highly reducible hematite and the finer particle size of the less reducible magnetite balances the overall reducibility. If the particle size of the hematite powder and concentrate is too coarse or too fine, it will not optimize the particle size composition and improve the strength of the cold-pressed pellets. Optimizing the particle size of the iron-bearing raw material significantly improves the strength of the cold-pressed pellets, reduces the amount of binder required, and further increases the iron grade of the cold-pressed pellets while reducing the impact of inorganic components in the binder on reducibility.

[0018] The biochar of this invention is prepared from agricultural biomass (such as corn stalks or forestry biomass, specifically wood) through a carbonization process (such as high-pressure hydrothermal carbonization). The reactivity of the biochar of this invention is mainly achieved by controlling the ratio of low-reactivity biochar to high-reactivity biochar, ensuring continuous reduction reaction during the reduction heating process and avoiding expansion of cold-pressed pellets caused by concentrated reduction. The proportion of low-reactivity biochar should not be too high or too low. If it is too high, the cold-pressed pellets will be concentratedly reduced at high temperatures, which leads to rapid reduction and easy expansion and pulverization of the pellets. If it is too low, the reaction will be concentrated at lower temperatures, which reduces the reduction rate and lowers the utilization efficiency of the reducing agent. The ash content of the biochar should not be too high, as this introduces more impurities and affects the quality of direct reduced iron. The volatile matter content should not be too high, as this results in a lower effective reducing component (fixed carbon) and higher activity, reducing the overall utilization rate.

[0019] As a preferred embodiment, the TFe content in the iron-containing raw material is not less than 67%.

[0020] As a preferred embodiment, the organic-inorganic composite binder is composed of an organic binder and an inorganic binder in a mass percentage ratio of 15%~25%:75%~85%. As a more preferred embodiment, the organic binder includes at least one of sodium humate, sodium carboxymethyl cellulose, hydroxypropyl methyl cellulose, sodium carboxymethyl starch, gelatinized starch, starch, phenolic resin, epoxy resin, polyester resin, polyacrylamide, and polyurethane. As a more preferred embodiment, the inorganic binder includes at least one of bentonite, silica sol, Na₂SiO₃, K₂SiO₃, CaSiO₃, clay, sodium silicate, and potassium silicate. As a more preferred embodiment, the amount of the organic-inorganic composite binder is 2.0%~5.5% of the mass of the iron-containing raw material. The dosage and composition of the organic-inorganic composite binder of the present invention are optimized based on comprehensive considerations such as cost, bonding effect, and impurity content. Organic binders offer advantages such as significantly improved green pellet performance and low impurity content, but are relatively expensive and have poor high-temperature resistance, easily decomposing at high temperatures. Inorganic binders, on the other hand, are inexpensive and do not decompose at high temperatures, providing high-temperature strength to cold-pressed blocks, but have a higher impurity content. Therefore, throughout the entire process of cold-pressing blocks from room temperature molding to high-temperature reduction, the ratio of organic and inorganic binders can be adjusted to achieve functional complementarity through the synergistic effect of chemical bonds. Furthermore, regarding the amount of composite binder added, too little results in insufficient strength for the cold-pressed blocks, while too much leads to excessively high costs.

[0021] As a preferred embodiment, the amount of biochar used is 0.2% to 5.0% of the mass of the iron-containing raw material.

[0022] Biochar is a renewable resource with near-zero carbon emissions during the reduction process. It has a high fixed carbon content and low ash content, meaning it doesn't introduce many impurities when added to cold-pressed pellets. At high temperatures, biochar can directly react with iron oxides internally, accelerating the reduction rate. It also has a pore-forming effect; the pores formed in situ after the high-temperature reaction facilitate gas diffusion, further improving reducibility. However, the amount of biochar added should not be too high, as excessive amounts will reduce the strength of the cold-pressed pellets.

[0023] As a preferred embodiment, the amount of water used accounts for 3.5% to 7.5% of the total mass of the iron-containing raw materials, organic-inorganic composite binder, and biochar. Under high-pressure molding conditions, a suitable moisture range for molding is required. If the amount of water is too high, the compaction effect will be affected due to the incompressible nature of water; if the amount of water is too low, the binder will be difficult to disperse evenly.

[0024] As a preferred embodiment, the high-pressure forming employs a high-pressure roller briquetting method. During the briquetting process, the linear pressure is 0.8 t / mm to 1.4 t / mm, resulting in flattened round or ellipsoidal pellets with dimensions of Φ15mm×10mm to Φ35mm×20mm. Both excessively high and low linear pressures are detrimental. Excessive linear pressure can cause the iron-containing raw material to be over-compressed, leading to cracking, and due to elastic aftereffects, it will significantly affect the cold-pressed pellet forming effect and strength. Insufficient linear pressure results in insufficient forming pressure, making it difficult to meet strength requirements. If the cold-pressed pellet size is too large, it increases the drying and consolidation time and affects reducibility; if the cold-pressed pellet size is too small, it will affect the output of the cold-pressed pellet forming process.

[0025] As a preferred embodiment, the drying and consolidation process employs hot air heating. During hot air heating, the temperature is 100~250℃, the air velocity is 1.0~2.5m / s, and the drying time is 25~45min. Under these preferred drying and consolidation conditions, the compressive strength of the consolidated pellets is not less than 2500N / P, the drum index (+6.3mm) is not less than 90%, and the abrasion resistance index is not higher than 5%. (The compressive strength testing standards are GB / T 14201-2018, ISO 4700:2015; the drum index and abrasion resistance index testing standards are GB / T24531-2009, ISO 3271:2007.)

[0026] As a preferred embodiment, the conditions for pure hydrogen reduction are: introducing pure hydrogen gas and reducing at 800~950℃ for 60~150 min. Under these preferred conditions, the metallized pellets obtained have a compressive strength of not less than 450 N / P, a TFe content of not less than 90%, a metallization rate of not less than 90%, a dynamic reduction pulverization index (LTD+6.3) of not less than 80%, and a reduction expansion index of not more than 15%. (The TFe content testing standard is GB / T 6730.5-2007, ISO 9507:1990; the metallization rate testing standard is GB / T 24235-2009, ISO 11257:2007; the dynamic reduction pulverization index testing standard is a modified ISO11257:2022, changing the reducing atmosphere in the standard to 100% H2). The reduction product of pure hydrogen is H2O, with no carbon emissions. The hydrogen reduction medium can accelerate the reduction rate and improve the reducibility of cold-pressed pellets. In particular, the H2O generated during the reduction process inside the cold-pressed pellets can react with biochar in a water-gas reaction (H2O + C = CO + H2), allowing H2 to circulate and participate in the reduction reaction, thereby increasing the utilization rate of H2 in the reduction process and reducing the H2 consumption per ton of ore produced. Pure hydrogen has significant advantages over conventional hydrogen-rich gas reduction processes: firstly, it can further reduce carbon emissions during the reduction process. Conventional hydrogen-rich reduction processes (MIDREX, HYL-III) use hydrogen with a hydrogen content of 40%–75%, still containing a certain proportion of CO (10%–36%). Pure hydrogen reduction can further reduce carbon emissions. Secondly, compared with pure hydrogen reduction, it can further accelerate the reduction rate, shorten the reduction time, and improve reduction expansion performance. Therefore, the temperature required for the reduction process can be appropriately reduced, and lowering the temperature can further improve reduction expansion and adhesion phenomena.

[0027] As a preferred embodiment, the pure hydrogen reduction is achieved via a vertical shaft furnace.

[0028] As a preferred embodiment, the mixing is performed using a high-intensity mixing method, with a mixing time of 1-5 minutes. Because the mixture contains a high proportion of fine-particle materials, a high-intensity mixing device is required. If the mixing time is too short, it will be difficult to achieve uniform mixing; if the mixing time is too long, it will increase energy consumption.

[0029] The method for preparing direct reduced iron from iron-containing raw materials with near-zero carbon content provided by this invention includes the following specific steps:

[0030] (1) Add 2.0%~5.5% organic-inorganic composite binder, 0.2%~5% biochar and water to the iron-containing raw material, accounting for 2.0%~5.5% of the total weight of the iron-containing raw material. The iron-containing raw material is composed of hematite powder, hematite concentrate and magnetite concentrate in a mass percentage ratio of 5%~10%:10%~20%:70%~85%, and the TFe content in the iron-containing raw material is not less than 67%. Among them, the mass proportion of <3mm particles in the hematite powder is not less than 75%, and the mass proportion of >0.1mm particles is not less than 75%. The mass proportion of particles larger than 0.074 mm in hematite concentrate is not less than 30%, the mass proportion of particles smaller than 0.045 mm is not more than 40%, and the maximum particle size is less than 0.25 mm; the mass proportion of particles smaller than 0.074 mm in magnetite concentrate is greater than 65%; the organic-inorganic composite binder is composed of organic binder and inorganic binder in a mass percentage ratio of 15%~25%:75%~85%. The organic binder includes at least one of sodium humate, sodium carboxymethyl cellulose, hydroxypropyl methyl cellulose, sodium carboxymethyl starch, gelatinized starch, starch, phenolic resin, epoxy resin, polyester resin, polyacrylamide, and polyurethane; the inorganic binder includes at least one of bentonite, silica sol, Na₂SiO₃, K₂SiO₃, CaSiO₃, clay, sodium silicate, and potassium silicate; the biochar is composed of highly reactive biochar and low-reactive biochar in a mass percentage ratio of 20%~35%:65%~80%; the mass proportion of particles >0.074mm in the highly reactive biochar is no more than 10%, and the mass proportion of particles <0.045mm is no less than 60%; the mass proportion of particles >0.074mm in the low-reactive biochar is no less than 30%, and the mass proportion of particles <0.045mm is no more than 40%; the fixed carbon content of the highly reactive biochar is... The content of carbon is not less than 87%, the ash content is not more than 4%, the volatile content is not more than 9%, the fixed carbon content of low reactive biochar is not less than 90%, and the ash content and volatile content are not more than 5%; after further mixing for 1~5 minutes by strong mixing, a mixture with a moisture content of 3.5%~7.5% is obtained; (2) The mixture is subjected to high pressure molding and drying and solidification in sequence; the high pressure molding adopts the high pressure roller pressing method, the linear pressure is 0.8t / mm~1.4t / mm, the shaped pellets are flat round or ellipsoidal, and the size is Φ15mm×10mm~Φ35mm×20mm; the drying and solidification adopts the hot air heating method, the hot air temperature is 100~250℃, the wind speed is 1.0~2.5m / s, and the drying time is 25~45min; after drying and solidification, cold-pressed pellets are obtained, the compressive strength is not less than 2500N / P, the drum index (+6.3mm) is not less than 90%, and the wear resistance index is not more than 5%;

[0031] (3) The cold-pressed pellets are reduced by hydrogen. The reduction process is carried out in a vertical furnace. The volume percentage of H2 in the reducing gas entering the furnace is 100%. The reduction is carried out at 800~950℃ for 60~150min to obtain direct reduced iron. The metallized pellets have a compressive strength of not less than 450N / P, a TFe content of not less than 90%, a metallization rate of not less than 90%, a dynamic reduction pulverization index (LTD+6.3) of not less than 80%, and a reduction expansion index of not more than 15%.

[0032] Compared with the prior art, the technical solution of the present invention brings the following beneficial effects:

[0033] (1) The method for preparing direct reduced iron from iron-containing raw materials with near-zero carbon is proposed in this invention. The process is short and the production process has near-zero carbon emissions. The temperature of the cold-pressed pellet preparation process is no higher than 300°C, and the carbon emissions are extremely low. The reduction process of the cold-pressed pellet is coupled with hydrogen gas-based reduction by adding high biomass carbon to the cold-pressed pellet. The entire reduction process has near-zero carbon emissions.

[0034] (2) The method for preparing direct reduced iron from iron-containing raw materials with near-zero carbon proposed in this invention highly couples the solid-base reduction of biomass charcoal and the gas-based reduction of hydrogen during the reduction process of cold-pressed pellets, which can improve the utilization rate of H2 and improve the reduction performance of cold-pressed pellets. On the one hand, biomass charcoal can directly react with iron oxides inside at high temperature, and the all-hydrogen reducing medium can also accelerate the reduction rate. The two work together to improve the reducibility of cold-pressed pellets. Biomass charcoal also has a pore-forming effect. The pores generated in situ after the reaction at high temperature are conducive to the diffusion of gas medium, further improving the reducibility. On the other hand, H2O generated during the reduction process inside the cold-pressed pellets can react with biomass charcoal in a water-gas reaction (H2O+C=CO+H2), so that H2 can participate in the reduction reaction in a cycle, thereby improving the utilization rate of H2 in the reduction process and reducing the H2 consumption per ton of ore.

[0035] (3) The method for preparing direct reduced iron from iron-containing raw materials with near-zero carbon proposed in this invention, employing a "gas-solid coupling reduction process," has significant advantages over the conventional "hydrogen-rich gas reduction process." On the one hand, the "gas-solid coupling reduction process" can further reduce carbon emissions during the reduction process, while the conventional "hydrogen-rich gas reduction process" (MIDREX, HYL-III) has a hydrogen ratio of 40%~75% and still contains a certain proportion of CO (10%~36%). Full hydrogen reduction can further reduce carbon emissions. On the other hand, the "gas-solid coupling reduction process" can further accelerate the reduction rate, shorten the reduction time, and improve the reduction expansion performance. Therefore, using the "gas-solid coupling reduction process" can appropriately reduce the temperature required for the reduction process, and lowering the temperature can further improve the reduction expansion and adhesion phenomena.

[0036] (4) The biochar used in the near-zero carbon preparation of iron-containing raw materials proposed in this invention has optimized reactivity. By adjusting the ratio between low-reactivity and high-reactivity biochar, the reduction reaction continues during the reduction heating process, avoiding the expansion of cold-pressed pellets caused by concentrated reduction. In addition, the biochar has a high fixed carbon content and low ash content, so it will not introduce more impurities when added to the cold-pressed pellets.

[0037] (5) The method for preparing direct reduced iron from iron-containing raw materials with near-zero carbon proposed in this invention uses pure hydrogen reduction, which requires a slightly lower temperature than conventional hydrogen-rich reduction. Lowering the temperature can also adjust the overall reactivity of biochar, allowing biochar to continuously participate in the reduction reaction and avoiding rapid concentrated reaction of biochar at high temperatures. At the same time, the process of biochar reducing iron oxides is an exothermic process, which can compensate for the problem of endothermic reduction of iron oxides by pure hydrogen to a certain extent.

[0038] (6) The method for preparing direct reduced iron from iron-containing raw materials with near-zero carbon content proposed in this invention optimizes the blending of iron-containing raw materials, which can not only synergistically improve the strength and metallurgical properties of cold-pressed pellets, but also broaden the sources of iron-containing raw materials. On the one hand, by optimizing the particle size distribution of iron-containing raw materials, the finest magnetite concentrate, the finer hematite concentrate, and the coarsest hematite powder are blended together to meet the particle size distribution curve, which can significantly improve the strength of cold-pressed pellets and reduce the amount of binder, thereby further improving the iron grade of cold-pressed pellets and reducing the influence of inorganic components in the binder on reducibility. On the other hand, by controlling the ratio of Fe2O3 and Fe3O4 in iron-containing raw materials, the reduction expansion performance, reduction pulverization performance, and reduction performance of cold-pressed pellets are synergistically improved. When Fe2O3 is reduced to Fe3O4, the crystal transformation causes volume expansion and cracking, resulting in pulverization and expansion. Fe3O4 exhibits only slight volume expansion during reduction. Adding magnetite can improve the reduction expansion and pulverization performance of cold-pressed pellets. Furthermore, magnetite has poor reduction performance, while hematite has good reduction performance; adding hematite can improve the reduction performance of cold-pressed pellets. In addition, compared to pelletizing processes, cold-pressing pelletizing has a wider range of applicable raw materials. Besides conventional fine-grained iron concentrates, coarse-grained iron concentrates with poor pelletizing properties and even larger fine ore particles can be used in cold-pressing pelletizing. Optimizing the ore blend can broaden the raw material sources for cold-pressed pellets. Detailed Implementation

[0039] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0040] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The patent terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of the invention.

[0041] Unless otherwise specified, the various reagents and raw materials used in this invention are commercially available products or products that can be prepared by known methods.

[0042] Example 1

[0043] An organic-inorganic composite binder comprising 4.5% of the total weight of the iron-containing raw material, biochar comprising 2.5% of the total weight of the iron-containing raw material, and an appropriate amount of water are added. The iron-containing raw material is composed of hematite powder, hematite concentrate, and magnetite concentrate in a mass percentage ratio of 7%:16%:77%, and the TFe content in the iron-containing raw material is 67.5%. Among them, the mass percentage of particles <3mm in the hematite powder is 92%, the mass percentage of particles >0.1mm is 87%, and the maximum particle size is 3.3mm; the mass percentage of particles >0.074mm in the hematite concentrate is 46%, the mass percentage of particles <0.045mm is 25%, and the maximum particle size is 0.23mm; the mass percentage of particles <0.074mm in the magnetite concentrate is 74%; the organic-inorganic composite binder is composed of organic binder and inorganic binder in a mass percentage ratio of 22%:78%. The organic binder is composed of sodium humate, gelatinized starch, and epoxy resin in a mass percentage ratio of 15%:50%:35%; the inorganic binder is composed of bentonite, silica sol, CaSiO3, and sodium silicate in a mass percentage ratio of 5%:25%:10%:60%; the biochar is composed of highly reactive biochar and low-reactive biochar in a mass percentage ratio of 20%:80%; in the highly reactive biochar, the mass percentage of particles >0.074mm is 8%, and the mass percentage of particles <0.045mm is 70%; in the low-reactive biochar, the mass percentage of particles >0.074mm is 40%, and the mass percentage of particles <0.045mm is 30%; the fixed carbon content of the highly reactive biochar is 88%, the ash content is 4%, and the volatile matter content is... The content of the low-reactivity biochar is 8%; the fixed carbon content of the low-reactivity biochar is 90%, and the ash and volatile matter are both 5%; after further mixing by strong mixing for 2.5 min, a mixture with a moisture content of 4.5% is obtained; the mixture is then subjected to high-pressure molding and drying and solidification in sequence; high-pressure molding adopts high-pressure roller pressing method, with a linear pressure of 1.0 t / mm, and the formed pellets are flat round with a size of Φ25mm×14mm; drying and solidification adopts hot air heating method, with a hot air temperature of 150℃, a wind speed of 1.9m / s, and a drying time of 40 min; after drying and solidification, cold-pressed pellets are obtained; the cold-pressed pellets are then reduced by hydrogen in a vertical furnace, with the volume percentage of H2 in the reducing gas entering the furnace being 100%, and the reduction is carried out at 850℃ for 140 min to obtain direct reduced iron.

[0044] The cold-pressed pellets have a compressive strength of 3002 N / P, a drum index (+6.3 mm) of 92%, and a wear resistance index of 3.3%.

[0045] The metallized pellets have a compressive strength of 674 N / P, a TFe content of 92%, and a metallization rate of 90%; the dynamic reduction pulverization index (LTD+6.3) is 84%, and the reduction expansion index is 11%.

[0046] Example 2

[0047] An organic-inorganic composite binder comprising 2.4% of the total weight of the iron-containing raw material, biochar comprising 3.5% of the total weight of the iron-containing raw material, and an appropriate amount of water are added. The iron-containing raw material is composed of hematite powder, hematite concentrate, and magnetite concentrate in a mass percentage ratio of 10%:10%:80%, and the TFe content in the iron-containing raw material is 68%. Among them, the mass percentage of particles <3mm in the hematite powder is 95%, the mass percentage of particles >0.1mm is 86%, and the maximum particle size is 3.1mm; the mass percentage of particles >0.074mm in the hematite concentrate is 50%, the mass percentage of particles <0.045mm is 20%, and the maximum particle size is 0.21mm; the mass percentage of particles <0.074mm in the magnetite concentrate is 70%; the organic-inorganic composite binder is composed of organic binder and inorganic binder in a mass percentage ratio of 15%:85%. The organic binder is composed of sodium carboxymethyl cellulose, hydroxypropyl methyl cellulose, and phenolic resin in a mass percentage ratio of 30%:40%:30%; the inorganic binder is composed of Na₂SiO₃, CaSiO₃, and clay in a mass percentage ratio of 80%:10%:10%; the biochar is composed of highly reactive biochar and low-reactive biochar in a mass percentage ratio of 25%:75%; in the highly reactive biochar, the mass percentage of particles >0.074mm is 5%, and the mass percentage of particles <0.045mm is 75%; in the low-reactive biochar, the mass percentage of particles >0.074mm is 44%, and the mass percentage of particles <0.045mm is 35%; the fixed carbon content of the highly reactive biochar is 90%, the ash content is 3%, and the volatile matter content is 7%; the fixed carbon content of the low-reactive biochar is 9%. The mixture contains 3% ash and 4% volatile matter; after further mixing for 3 minutes using a strong mixing method, a mixture with a moisture content of 4.8% is obtained; the mixture is then subjected to high-pressure molding and drying and solidification sequentially; high-pressure molding uses a high-pressure roller pressing method with a linear pressure of 0.9 t / mm, and the formed pellets are flat round or ellipsoidal in shape with a size of Φ30mm×20mm; drying and solidification uses a hot air heating method with a hot air temperature of 190℃, a wind speed of 2.0m / s, and a drying time of 35min; after drying and solidification, cold-pressed pellets are obtained; the cold-pressed pellets are then subjected to hydrogen reduction in a vertical furnace, with H2 volume percentage in the reducing gas entering the furnace being 100%, and the reduction is carried out at 900℃ for 100min to obtain direct reduced iron.

[0048] The cold-pressed pellets have a compressive strength of 2871 N / P, a drum index (+6.3 mm) of 90.5%, and a wear resistance index of 4.7%.

[0049] The metallized pellets have a compressive strength of 542 N / P, a TFe content of 92.7%, and a metallization rate of 93%; the dynamic reduction pulverization index (LTD+6.3) is 83%, and the reduction expansion index is 14%.

[0050] Example 3

[0051] An organic-inorganic composite binder comprising 3.2% of the total weight of the iron-containing raw material, biochar comprising 2% of the total weight of the iron-containing raw material, and an appropriate amount of water are added. The iron-containing raw material is composed of hematite powder, hematite concentrate, and magnetite concentrate in a mass percentage ratio of 10%:20%:70%, and the TFe content in the iron-containing raw material is 68.4%. Among them, the mass percentage of particles <3mm in the hematite powder is 93%, the mass percentage of particles >0.1mm is 91%, and the maximum particle size is 3.4mm; the mass percentage of particles >0.074mm in the hematite concentrate is 40%, the mass percentage of particles <0.045mm is 18%, and the maximum particle size is 0.24mm; the mass percentage of particles <0.074mm in the magnetite concentrate is 80%; the organic-inorganic composite binder is composed of organic binder and inorganic binder in a mass percentage ratio of 20%:80%. The organic binder is composed of sodium carboxymethyl starch, epoxy resin, and polyacrylamide in a mass percentage ratio of 20%:40%:40%; the inorganic binder is composed of silica sol, Na₂SiO₃, and sodium silicate in a mass percentage ratio of 20%:45%:35%; the biochar is composed of highly reactive biochar and low-reactive biochar in a mass percentage ratio of 20%:80%; in the highly reactive biochar, the mass percentage of particles >0.074mm is 6%, and the mass percentage of particles <0.045mm is 80%; in the low-reactive biochar, the mass percentage of particles >0.074mm is 35%, and the mass percentage of particles <0.045mm is 37%; the fixed carbon content of the highly reactive biochar is 87%, the ash content is 4%, and the volatile matter is 9%. The low-reactivity biochar has a fixed carbon content of 91.5%, an ash content of 3.5%, and a volatile content of 5%. After further mixing for 3.5 minutes using a strong mixing method, a mixture with a moisture content of 4.3% is obtained. This mixture is then subjected to high-pressure molding and drying / consolidation. High-pressure molding uses a high-pressure roller pressing method with a linear pressure of 0.95 t / mm, producing flat, round pellets with dimensions of Φ18mm × 13mm. Drying / consolidation uses hot air heating at a temperature of 140℃ and a wind speed of 2.5 m / s for 45 minutes. Cold-pressed pellets are obtained after drying and consolidation. These cold-pressed pellets are then subjected to hydrogen reduction in a vertical furnace. The reduction process is carried out at 930℃ for 90 minutes, with H2 comprising 100% of the volume in the reducing gas. Direct reduced iron is obtained.

[0052] The cold-pressed pellets have a compressive strength of 3248 N / P, a drum index (+6.3 mm) of 92.3%, and a wear resistance index of 3.8%.

[0053] The metallized pellets have a compressive strength of 538 N / P, a TFe content of 93%, and a metallization rate of 94%; the dynamic reduction pulverization index (LTD+6.3) is 85%, and the reduction expansion index is 14.5%.

[0054] Comparative Example 1

[0055] Compared with Example 1, the only difference is that: no full hydrogen gas was used during reduction, and the volume of H2 in the reducing gas was 70%, with the remaining components being 20% ​​CO, 3% CO2, 4% H2O and 3% CH4. Specifically, no full hydrogen reducing gas was used to improve the reduction performance, reduction pulverization performance and reduction expansion performance, and carbon emissions were not reduced to the maximum extent.

[0056] The cold-pressed pellets have a compressive strength of 3002 N / P, a drum index (+6.3 mm) of 92%, and a wear resistance index of 3.3%.

[0057] The metallized pellets have a compressive strength of 414 N / P, a TFe content of 86%, and a metallization rate of 87%; the dynamic reduction pulverization index (LTD+6.3) is 72%, and the reduction expansion index is 25%.

[0058] Comparative Example 2

[0059] The only difference compared to Example 2 is that no biochar was added; specifically, no biochar was added to improve the reduction performance, and no coupling with full hydrogen reduction was used to improve hydrogen utilization.

[0060] The cold-pressed pellets have a compressive strength of 2954 N / P, a drum index (+6.3 mm) of 90.9%, and a wear resistance index of 4.2%.

[0061] The metallized pellets have a compressive strength of 475 N / P, a TFe content of 86%, and a metallization rate of 85.4%; the dynamic reduction pulverization index (LTD+6.3) is 78%, and the reduction expansion index is 18%.

[0062] Comparative Example 3

[0063] Compared with Example 2, the only difference is that: no low-activity biochar was added, and all the biochar was high-activity biochar. Specifically, the reduction activity of the biochar was not controlled to allow the reduction reaction to continue, resulting in the biochar reacting at a lower temperature. At low temperatures, the reduction rate is lower, which reduces the utilization efficiency of the reducing agent and fails to improve the reducing properties.

[0064] The cold-pressed pellets have a compressive strength of 2904 N / P, a drum index (+6.3 mm) of 90.4%, and a wear resistance index of 4.5%.

[0065] The metallized pellets have a compressive strength of 531 N / P, a TFe content of 87.8%, and a metallization rate of 86.7%; the dynamic reduction pulverization index (LTD+6.3) is 79%, and the reduction expansion index is 13.8%.

[0066] Comparative Example 4

[0067] The only difference compared to Example 3 is that hematite (hematite powder and hematite concentrate) was not added, that is, the iron-containing raw material was 100% magnetite concentrate. Specifically, the iron-containing raw material optimization blending was not implemented to improve the strength and reducibility of the cold-pressed blocks.

[0068] The cold-pressed pellets have a compressive strength of 2411 N / P, a drum index (+6.3 mm) of 87.2%, and a wear resistance index of 7.4%.

[0069] The metallized pellets have a compressive strength of 628 N / P, a TFe content of 83%, and a metallization rate of 82%; the dynamic reduction pulverization index (LTD+6.3) is 82%, and the reduction expansion index is 13.5%.

[0070] Comparative Example 5

[0071] The only difference compared to Example 3 is that hematite powder was not added. The iron-containing raw material was composed of hematite concentrate and magnetite concentrate in a mass percentage ratio of 22.22% : 77.78%. Specifically, the iron-containing raw material optimization blending was not implemented (coarser hematite powder was not added to act as core particles) to improve the strength and reducibility of the cold-pressed briquettes.

[0072] The cold-pressed pellets have a compressive strength of 2478 N / P, a drum index (+6.3 mm) of 88.1%, and a wear resistance index of 6.3%.

[0073] The metallized pellets have a compressive strength of 545 N / P, a TFe content of 86%, and a metallization rate of 84.6%; the dynamic reduction powdering index (LTD+6.3) is 81%, and the reduction expansion index is 14.2%.

Claims

1. A method of producing direct reduced iron from an iron-containing feedstock with near zero carbon, characterized by: The iron-containing raw material is mixed with an organic-inorganic composite binder, biomass charcoal and water, and then sequentially subjected to high-pressure forming and drying to obtain cold-pressed pellets, and the cold-pressed pellets are reduced by pure hydrogen to obtain direct reduced iron. The iron-containing raw material comprises hematite powder ore, hematite concentrate and magnetite concentrate, and the mass percentage composition of the hematite powder ore, hematite concentrate and magnetite concentrate is 5%-10%:10%-20%:70%-85%. The mass percentage of <3mm particle size in the hematite powder ore is not less than 75%, the mass percentage of >0.1mm particle size is not less than 85%, and the maximum particle size is less than 4.5mm. The mass percentage of >0.074mm particle size in the hematite concentrate is not less than 30%, the mass percentage of <0.045mm particle size is not more than 40%, and the maximum particle size is less than 0.25mm. The mass percentage of <0.074mm particle size in the magnetite concentrate is more than 65%. The biomass charcoal comprises high-reactivity biomass charcoal and low-reactivity biomass charcoal, and the mass percentage composition of the high-reactivity biomass charcoal and the low-reactivity biomass charcoal is 20%-35%:65%-80%. The mass percentage of >0.074mm particle size in the high-reactivity biomass charcoal is not more than 10%, the mass percentage of <0.045mm particle size is not less than 60%, the fixed carbon mass content is not less than 87%, the ash content is not more than 4%, and the volatile matter content is 7-9%. The mass percentage of >0.074mm particle size in the low-reactivity biomass charcoal is not less than 30%, the mass percentage of <0.045mm particle size is not more than 40%, the fixed carbon mass content is not less than 90%, and the ash content and the volatile matter content are both not more than 5%.

2. A process for the production of direct reduced iron from an iron-containing feed material with near zero carbon according to claim 1, characterized in that: The organic-inorganic composite binder is composed of an organic binder and an inorganic binder in a mass percentage of 15%-25%:75%-85%.

3. The method according to claim 2, characterized in that: The organic binder comprises at least one of sodium humate, sodium carboxymethyl cellulose, hydroxypropyl methyl cellulose, sodium carboxymethyl starch, starch, phenolic resin, epoxy resin, polyester resin, polyacrylamide and polyurethane. The inorganic binder comprises at least one of bentonite, silica sol, Na2SiO3, K2SiO3, CaSiO3, clay, sodium water glass and potassium water glass.

4. A process for the production of direct reduced iron from a ferrous feed material with near zero carbon according to any one of claims 1 to 3, characterized in that: The amount of the organic-inorganic composite binder is 2.0%-5.5% of the mass of the iron-containing raw material.

5. A process for the near zero carbon production of direct reduced iron from an iron-containing feed material according to claim 1, 2 or 3 characterised in that: The amount of the biomass charcoal is 0.2%-5.0% of the mass of the iron-containing raw material.

6. A process for the near zero carbon production of direct reduced iron from an iron-containing feed material according to claim 1, 2 or 3 characterised in that: The amount of water is 3.5%-7.5% of the total mass of the iron-containing raw material, the organic-inorganic composite binder and the biomass charcoal.

7. A process for the near zero carbon production of direct reduced iron from an iron-containing feed material according to claim 1, 2 or 3 characterised in that: The high-pressure forming adopts a high-pressure roller pressing method, the linear pressure during the pressing process is 0.8t / mm-1.4t / mm, the formed pellets are flat round or ellipsoidal, and the size is Φ15mm×10mm-Φ35mm×20mm.

8. A process for the near zero carbon production of direct reduced iron from an iron-containing feed material according to claim 1, 2 or 3 characterised in that: The dry consolidation adopts a hot air heating mode, the hot air heating process has a hot air temperature of 100-250 DEG C, a wind speed of 1.0-2.5 m / s, and a drying time of 25-45 min.

9. A process for the near zero carbon production of direct reduced iron from an iron-containing feed material according to claim 1, 2 or 3 characterised in that: The pure hydrogen reduction is performed by passing pure hydrogen at a temperature of 800-950 DEG C for 60-150 min.

10. A process for the production of direct reduced iron from an iron-containing feed material with near zero carbon according to claim 9, characterized in that: The pure hydrogen reduction is achieved by a shaft furnace.