A method for preparing high-density sponge iron

Abstract

This invention relates to a method for preparing high-density sponge iron. The method includes: Step 1, preparing green pellets: mixing spherical iron counterweights, iron concentrate powder, and solvent, and pelletizing to obtain green pellets, wherein, based on the mass of the green pellets as 100 wt%, the addition amounts of the counterweights, iron concentrate powder, and solvent are 3%-9%, 36%-39%, and 55%-58%, respectively; Step 2, preparing counterweight pellets: calcining the green pellets to produce counterweight pellets; Step 3, reduction: reducing the counterweight pellets to obtain high-density sponge iron. The sponge iron pellets carrying the counterweights can easily penetrate the foamy slag and enter the slag-steel interface during electric arc furnace steelmaking. At higher speeds, they can even penetrate the liquid surface and enter the molten steel, accelerating the reaction process and increasing the smelting rate.

Description

Technical Field

[0001] This invention relates to a method for preparing high-density sponge iron, belonging to the field of iron and steel metallurgy technology. Background Technology

[0002] The hydrogen-based shaft furnace-electric arc furnace (EAF) short-process smelting technology is one of the core technological pathways for the steel industry to achieve a green and low-carbon transformation. In this process, EAF steelmaking mainly uses sponge iron (direct reduced iron) as raw material. However, a significant problem exists in the current process: although the density of sponge iron is greater than that of foamed slag in the EAF, it is still much lower than that of molten steel. Therefore, when charging the EAF, the sponge iron cannot effectively penetrate the steel-slag interface and sink into the molten steel, but instead floats in the slag-steel interface area for a long time. This state not only slows down the melting and reduction reaction process, but also leads to a significant increase in subsequent EAF smelting energy consumption and may adversely affect the smooth operation of the production process.

[0003] To improve the raw material performance and economy of the hydrogen-based direct reduction process, existing technologies mainly focus on two aspects: first, optimizing the preparation of pellets, and second, optimizing the hydrogen-based reduction process itself.

[0004] In the preparation of pellets, existing technologies aim to improve pellet quality, utilize secondary resources, and reduce production costs. For example, CN120099278A discloses a method for preparing multi-layer composite pellets, which involves layering hematite powder, recycled ore powder, and magnetite powder into pellets, followed by roasting. This method aims to produce high-quality pellets and improve their melting and dripping properties, while utilizing secondary resources to reduce costs. CN120008339A discloses a rotary kiln device for gas-based oxidation or reduction roasting, which aims to increase material reaction rate and throughput by incorporating lifting plates and built-in jet nozzles. CN120060633A discloses a two-stage variable binder and variable speed pelletizing method, which aims to improve green pellet nucleation rate, pelletizing rate, and mechanical strength, while reducing energy consumption and equipment wear, by using organic and inorganic binders in stages and adjusting the pelletizing speed. However, these technical solutions primarily focus on improving pelletizing rate, green pellet strength, or optimizing roasting reaction efficiency. Their core objective is to improve conventional metallurgical indicators or process economics, rather than significantly increasing the bulk density or mass of the pellets themselves. Therefore, they cannot fundamentally solve the problem of sponge iron floating in the electric furnace due to insufficient raw material density. Furthermore, these pelletizing technologies are not specifically targeted at suppressing abnormal expansion, cracking, and pulverization of pellets that easily occur during subsequent hydrogen reduction processes, and their effectiveness is limited.

[0005] In the field of hydrogen-based direct reduction processes, existing technologies focus on optimizing reduction conditions to achieve low-carbon emissions, reduced energy consumption, and controlled expansion. For example, CN120006053A discloses a method for preparing direct reduced iron, which uses hydrogen-rich syngas generated from biomass gasification and matches it with a specific reduction temperature to achieve low-carbon reduction of medium- and low-grade iron ore. CN120099245A discloses a hydrogen-based vertical shaft furnace gradient heating reduction method, which aims to suppress abnormal expansion of ultra-high-grade pellets and reduce their expansion rate to less than 15% by controlling the heating rate and temperature of pellets in different regions of the vertical shaft furnace. CN120041622A discloses a heat compensation system and method for producing direct reduced iron in a pure hydrogen vertical shaft furnace, which aims to achieve low-carbon or even zero-carbon ironmaking by preheating the raw materials to improve the utilization efficiency of thermal energy and hydrogen. Nevertheless, the core objective of these process optimizations is to improve the economics and environmental friendliness of the reduction process (reducing energy consumption and carbon emissions) or control the expansion rate within a specific range. The density of the direct reduced iron (sponge iron) produced did not achieve a substantial and targeted improvement. Therefore, the fundamental problem of sponge iron floating in the electric furnace and low smelting efficiency caused by insufficient density remains unresolved.

[0006] In summary, current improvements to upstream pelletizing technologies (e.g., CN120099278A, CN120008339A, CN120060633A) and core hydrogen-based reduction processes (e.g., CN120006053A, CN120099245A, CN120041622A) do not primarily aim to significantly increase the density of the final sponge iron product. These existing technologies mainly address specific issues in their respective stages, such as cost, pelletizing rate, reaction efficiency, expansion control, or thermal energy utilization. However, they fail to address the core contradiction throughout the short process: sponge iron floats in the electric furnace due to its low density, leading to increased smelting energy consumption.

[0007] Therefore, developing a new method for raw material preparation or processing that can effectively improve the bulk density or quality of sponge iron is of urgent practical need and important technical value for promoting the overall energy efficiency improvement and smooth operation of the hydrogen-based shaft furnace-electric furnace short process. Summary of the Invention

[0008] To address the aforementioned technical problems, the present invention aims to provide a method for preparing high-density sponge iron, which can be achieved by adding counterweights during the manufacturing process of green pellets.

[0009] To achieve the above objectives, the present invention provides a method for preparing high-density sponge iron, comprising: Step 1: Preparation of green pellets: Mix spherical iron counterweights, iron concentrate powder, and solvent, and pelletize to obtain green pellets. The amounts of the counterweights, iron concentrate powder, and solvent added, based on a mass of 100 wt%, are 3%-9%, 36%-39%, and 55%-58%, respectively. Step 2: Preparation of counterweight pellets: The raw pellets are roasted to produce counterweight pellets; Step 3: Reduction: The counterweight pellets are reduced to obtain high-density sponge iron.

[0010] According to a specific embodiment of the present invention, preferably, step one includes: mixing spherical iron counterweights and iron concentrate powder to form a pre-mixed sphere, then adding solvent to fill the gaps to obtain a green sphere.

[0011] According to a specific embodiment of the present invention, preferably, in step one, the spherical iron counterweight is a solid iron sphere with a diameter of 5-8 mm.

[0012] According to a specific embodiment of the present invention, preferably, in step one, the solvent comprises a binder and a fuel. More preferably, based on the sum of the mass of the binder and the fuel being 100%, the addition amounts of the binder and the fuel are 70%-75% and 25%-30%, respectively.

[0013] According to a specific embodiment of the present invention, preferably, the binder is selected from one or more combinations of bentonite, water glass, XPK organic binder and fluorite powder.

[0014] According to a specific embodiment of the present invention, preferably, the fuel is selected from one or more combinations of coke powder, semi-coke powder, and petroleum coke.

[0015] According to a specific embodiment of the present invention, preferably, the water content of the solvent is greater than or equal to 5%.

[0016] According to a specific embodiment of the present invention, preferably, in step one, the particle size of the iron concentrate powder is less than 0.15 mm.

[0017] According to a specific embodiment of the present invention, preferably, in step one, the ball-forming process is carried out using a ball-forming machine, wherein the tilt angle of the ball-forming machine is equal to or less than 60°, more preferably 42°-47°.

[0018] According to a specific embodiment of the present invention, preferably, in step one, the diameter of the green ball is 12-18 mm.

[0019] According to a specific embodiment of the present invention, preferably, in step two, the oxidation degree of the green pellets after calcination is controlled at 72-85%.

[0020] According to a specific embodiment of the present invention, preferably, in step two, the roasting temperature is 1180-1250℃ and the roasting time is 15-25min.

[0021] According to a specific embodiment of the present invention, preferably, in step three, the reduction process includes preheating, first-stage reduction, second-stage reduction, and heat preservation reduction.

[0022] According to a specific embodiment of the present invention, preferably, the preheating temperature is 580-680°C.

[0023] According to a specific embodiment of the present invention, preferably, the temperature of the first stage of reduction is 850-950°C, and the reduction degree of the counterweight pellet is 40-45%.

[0024] According to a specific embodiment of the present invention, preferably, the temperature of the second stage of reduction is 980-1050℃.

[0025] According to a specific embodiment of the present invention, preferably, the heating rate of the first stage of reduction is 15-25℃ / min, and the heating rate of the second stage of reduction is 35-45℃ / min.

[0026] According to a specific embodiment of the present invention, preferably, the reduction degree of the counterweight pellets obtained after the heat preservation and reduction process is above 99%.

[0027] According to a specific embodiment of the present invention, preferably, the first stage reduction, the second stage reduction, and the heat preservation reduction are carried out in a reducing atmosphere, wherein the volume fraction of the reducing gas in the reducing atmosphere is equal to or greater than 85%.

[0028] The increasing proportion of non-metallic materials and non-ferrous metals used in steel production has led to a continuous decline in the quality of scrap steel. As a raw material for electric arc furnace (EAF) steelmaking, scrap steel exhibits significant fluctuations in chemical composition due to its diverse origins, making it difficult to control and manage. This poses considerable challenges to EAF steelmaking operations. This invention discovers that using a certain proportion (e.g., 30-50%) of sponge iron (direct reduced iron) as a diluent in combination with scrap steel can not only increase the uniformity of the steel but also improve and enhance its physical properties, thereby achieving the goal of producing high-quality steel. Therefore, direct reduced iron is not only a substitute for high-quality scrap steel but also an indispensable high-grade raw material for producing high-quality steel. Therefore, this invention has found that when using direct reduced iron (DRI) in electric arc furnace steelmaking, if the height of the charge added to the furnace is too low, its speed decreases rapidly when passing through the foamy slag. By the time it reaches the slag-steel interface, its speed is insufficient to penetrate the interface and enter the molten steel. Even if it does penetrate the interface, its residence time in the molten steel is short, resulting in insufficient reaction between DRI and the molten steel, which is detrimental to the steelmaking process. The preparation method provided by this invention can deliver DRI into the molten steel, thereby producing high-density DRI.

[0029] The weighted sponge iron pellets used in the technical solution of this invention can easily pass through the foam slag and enter the slag-steel interface during electric arc furnace steelmaking. When the speed is high, they can also penetrate the liquid surface and enter the molten steel, thus accelerating the reaction process and increasing the smelting rate. Attached Figure Description

[0030] Figure 1 The image shows the appearance of the pellets prepared in Comparative Example 1.

[0031] Figure 2 The image shows the abnormal expansion and cracking of pellets prepared for Comparative Example 2 in a hydrogen-based vertical shaft furnace.

[0032] Figure 3 The image shows the internal cracking of the pellets prepared for Comparative Example 2.

[0033] Figure 4 The image shows the appearance of the pellets prepared in Example 1. Detailed Implementation

[0034] In order to provide a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention will now be described in detail below, but it should not be construed as limiting the scope of implementation of the present invention.

[0035] As a preferred embodiment, the present invention provides a method for preparing high-density sponge iron, comprising: Step 1: Preparation of green pellets: Mix spherical iron counterweights, iron concentrate powder, and solvent, and pelletize to obtain green pellets. The amounts of the counterweights, iron concentrate powder, and solvent added, based on a mass of 100 wt%, are 3%-9%, 36%-39%, and 55%-58%, respectively. Step 2: Preparation of counterweight pellets: The raw pellets are roasted to produce counterweight pellets; Step 3: Reduction: The counterweight pellets are reduced to obtain high-density sponge iron.

[0036] As a preferred embodiment, step one includes: mixing the spherical iron counterweight and iron concentrate powder to form a pre-mixed sphere, then adding a solvent to fill the gaps to obtain a green sphere.

[0037] As a preferred embodiment, step one specifically includes: The spherical iron counterweight, a portion of iron concentrate powder, and a portion of solvent are initially mixed in a briquetting machine to allow the concentrate powder to adhere to the surface of the spherical iron counterweight. Specifically, the spherical iron counterweight is first placed in the briquetting machine, a portion of iron concentrate powder is added and mixed with the spherical iron counterweight, and finally, solvent is added to fill the gaps between the spherical iron counterweight and the iron concentrate powder, so that the iron concentrate powder and the spherical iron counterweight adhere to each other, resulting in a pre-pressed sphere. Pre-pressed spheres and the remaining iron concentrate powder are added to a pelletizing machine (such as a disc pelletizing machine) to produce green pellets.

[0038] As a preferred embodiment, in step one, the spherical iron counterweight is a solid iron sphere with a diameter of 5-8 mm. In actual smelting, a suitable ferroalloy can be selected as the counterweight according to requirements; for example, ferromanganese alloy can increase the strength of steel, and ferromolybdenum alloy can increase the toughness of steel.

[0039] In a preferred embodiment, in step one, the solvent comprises a binder and a fuel. More preferably, based on the sum of the mass of the binder and the fuel being 100%, the addition amounts of the binder and the fuel are 70%-75% and 25%-30%, respectively. By utilizing solvent combined with processes such as vigorous mixing, grinding, and pelletizing, high-grade green pellets with uniform particle size distribution, high sphericity, and uniform microporous structure and composition can be prepared, effectively avoiding problems such as uneven heating of the pellets.

[0040] As a preferred embodiment, the binder is selected from one or more combinations of bentonite, water glass, XPK organic binder and fluorite powder.

[0041] As a preferred embodiment, the fuel is selected from one or more of coke powder, semi-coke powder, and petroleum coke.

[0042] In a preferred embodiment, the solvent has a water content greater than or equal to 5%.

[0043] As a preferred embodiment, in step one, all raw materials need to be thoroughly mixed, and the specific mixing process can be carried out in a cylindrical mixer.

[0044] As a preferred embodiment, in the pelletizing machine, if all the iron concentrate powder adheres to the surface of the counterweight and the volume meets the size requirements (12-18mm), no solvent needs to be added during the process. If the above requirements are not met, an appropriate amount of solvent needs to be added to promote adhesion.

[0045] As a preferred embodiment, in step one, the iron concentrate powder is high-grade iron concentrate powder, for example, iron concentrate powder with a grade of 68% or higher.

[0046] As a preferred embodiment, in step one, the iron concentrate powder has a particle size of less than 0.15 mm. The ground iron concentrate powder is then screened, and iron concentrate powder with a particle size of less than 0.15 mm is selected for use. Materials with excessively large particles are re-crushed and ground.

[0047] As a preferred embodiment, in step one, the pelletizing process is carried out using a pelletizing machine, and the tilt angle of the pelletizing machine is equal to or less than 60°, more preferably 42°-47°. If the angle of the pelletizing machine is too large, the spherical iron counterweights with concentrate powder adhering to their surfaces will begin to detach from the material, reducing the green pellet forming rate.

[0048] In a preferred embodiment, the diameter of the green pellets in step one is 12-18 mm. During the pelletizing process, if the diameter of the green pellets is too small, water or solvent can be added to increase the adhesion of iron concentrate powder and thus increase the diameter of the green pellets. After pelletizing, the obtained green pellets are screened, and the green pellets with a diameter less than 12 mm and the undersize material are returned to the pelletizing machine for further processing.

[0049] According to a specific embodiment of the present invention, preferably, in step two, the oxidation degree of the green pellets after calcination is controlled at 72-85%. By controlling the oxidation degree within the above range, it is possible to promote uniform consolidation of the inner and outer layers of the pellets, thereby preparing pellets with high strength and uniform microporous structure, which is beneficial to improving the reduction rate of the pellets and inhibiting stress expansion, cracking and pulverization caused by uneven pellet structure.

[0050] According to a specific embodiment of the present invention, preferably, in step two, the roasting temperature is 1180-1250℃ and the roasting time is 15-25min.

[0051] As a preferred specific implementation scheme, in step two, a rotary kiln is preferably used to roast the counterweight pellets. The green pellets roll continuously from the kiln tail to the kiln head inside the rotary kiln. Each green pellet has the same roasting conditions, so the green pellets will not crack due to uneven heating, and the internal counterweight is not easily exposed. The resulting counterweight pellets are uniform and of good quality.

[0052] As a preferred embodiment, in step two, the roasting process includes two stages of drying, with a circulating air system connecting the first and second stages. The first stage uses forced-air drying, while the second stage uses exhaust drying. After drying, the green pellets are preliminarily shaped and enter the preheating, heating, and roasting stages. The roasting process is completed in a rotary kiln. After roasting, the green pellets are cooled to become ore pellets. By employing a two-stage drying followed by roasting, the reduction rate of the pellets can be increased during reduction, stress expansion, cracking, and pulverization caused by uneven pellet structure can be suppressed, thereby improving the yield.

[0053] As a preferred embodiment, in step two, the cooling process adopts a three-stage cooling method, wherein all three stages of cooling are air-cooled. The waste heat generated by air cooling can be recycled to the preheating stage. Using air cooling can control the degree of oxidation during the pellet cooling process, thereby adjusting the mineral composition of ultra-high grade pellets and retaining a portion of iron oxide in the pellet ore to reduce the expansion and pulverization caused by the iron oxide crystal transformation during the reduction process.

[0054] The technical solution of this invention utilizes high-grade iron concentrate powder to prepare counterweight pellets with high strength and uniform microporous structure. These pellets not only have good reducibility, but also effectively suppress expansion, cracking and pulverization during the hydrogen reduction process, ensuring that the counterweight pellets are not exposed during the reduction process.

[0055] As a preferred specific implementation, in step three, high-density sponge iron is prepared by reducing the counterweight pellets.

[0056] As a preferred embodiment, in step three, the reduction process includes preheating, first-stage reduction, second-stage reduction, and heat preservation reduction.

[0057] As a preferred embodiment, in step three, the preheating temperature can be controlled at 580-680℃, preferably 600℃; by heating the counterweight pellets to 580-680℃ in the preheating section before performing the first stage of reduction, the internal expansion behavior of the pellets can be further suppressed.

[0058] As a preferred embodiment, in step three, the temperature of the first stage of reduction can be controlled at 850-950℃. In this stage, the counterweight pellets undergo preliminary reduction with a reduction degree of 40-45%, and the reduction time can be controlled at 30-40 minutes.

[0059] As a preferred specific implementation, in step three, the temperature of the second stage of reduction can be controlled at 980-1050℃; the temperature of the heat preservation reduction is the temperature of the second stage of reduction, that is, after heating to reach the temperature of the second stage of reduction, the heat preservation reduction begins; through the second stage of reduction and the heat preservation reduction, the reduction degree of the counterweight pellets reaches more than 99%, and the total time of the second stage of reduction and the heat preservation reduction can be controlled at 20-30 minutes.

[0060] As a preferred embodiment, in step three, the reduction process is carried out using a hydrogen-based vertical shaft furnace. The counterweight pellets are reduced in the hydrogen-based vertical shaft furnace to produce counterweight direct reduced iron. Specifically, the counterweight pellets are continuously added from the top of the hydrogen-based vertical shaft furnace and sequentially pass through the primary heating reduction zone, the secondary heating reduction zone, the holding reduction zone, and the cooling zone to obtain direct reduced iron.

[0061] As a preferred embodiment, in step three, the heating rate of the first stage of reduction is 15-28℃ / min, and the heating rate of the second stage of reduction is 35-45℃ / min. By employing a gradient high-speed heating reduction method, multi-gradient rapid heating can be achieved, gradually reducing the iron grain shell to form an iron grain shell. This maintains the connection between the counterweight iron core and the iron grain shell, while controlling excessive internal stress expansion to prevent the iron grain shell from breaking and exposing the iron core. It also reduces energy consumption in subsequent electric furnace production. Furthermore, high-speed heating can accelerate the reduction rate in the later stages of pellet reduction, promoting the aggregation and growth of iron grains, ensuring that sufficient metallic iron is subsequently precipitated to fill the iron grain shell, forming a dense direct reduced iron structure.

[0062] As a preferred specific implementation, in step three, to ensure that the expansion of the pellet minerals on the counterweight surface during the preparation of reduced iron does not lead to excessive expansion of the gaps and detachment from the counterweight, the expansion ratio of the pellets during each reduction stage is less than 5%, and the overall expansion ratio is less than 15%. The expansion ratio can be controlled by adjusting the temperature in the reduction zone. When the temperature is below 900℃, the pellet minerals are prone to abnormal expansion, which needs to be avoided. When the reduction temperature is above 1000℃, the degree of flocculation increases, the internal iron grain aggregation increases, and the probability of expansion decreases.

[0063] As a preferred embodiment, in step three, the first reduction stage, the second reduction stage, and the heat-preserving reduction are carried out in a reducing atmosphere, wherein the volume fraction of the reducing gas in the reducing atmosphere is equal to or greater than 85%. By controlling the volume fraction of the reducing gas, the degree of reduction of the pellets during the two reduction stages can be guaranteed to meet the standard. The reducing gas can be hydrogen, etc.

[0064] As a preferred embodiment, in step three, the finished product obtained after reduction treatment can continue to move downward into the cooling section inside the furnace. The reducing gas enters the cooling section from the bottom. The finished product descending from the gas heating section comes into contact with the reducing gas for heat exchange, preheating the reducing gas. After the finished product is cooled, high-density sponge iron is obtained.

[0065] The technical solution of the present invention will be specifically described below through specific embodiments: Example 1 This embodiment provides a method for preparing high-quality direct reduced iron using pellets, including the following steps: Step 1: Place a solid iron ball with a diameter of 6mm into a briquetting machine, add some iron concentrate powder and mix with the ball, and finally add solvent to fill the gap between the ball and the iron concentrate powder, so that the concentrate powder and the ball can bond together to obtain a pre-pressed ball. The pre-pressed spheres and the remaining iron concentrate powder are put into a pelletizing machine (inclined at 45°) to make green pellets. After pelletizing, the pellets are screened to obtain green pellets with a diameter of 14-15mm. Green pellets with a diameter of less than 12mm and the undersize material need to be returned to the pelletizing machine for processing. The mass ratio of solid iron spheres, iron concentrate powder, and solvent is 2:19:29; the proportion of iron concentrate powder added to the briquetting machine is 30%. The prepared green pellets consist of solid iron pellets, iron concentrate powder, and solvent in a ratio of 4%:38%:58%. The iron concentrate powder has a grade of 69% and a particle size of less than 0.15 mm; The solvent contains a binder and a fuel in a mass ratio of 7:3. The binder is bentonite, and the fuel is semi-coke powder. The solvent has a water content of 10%. Step 2: The green pellets are fed into a rotary kiln for roasting, during which the green pellets continuously roll from the tail end of the kiln towards the head end. The roasting process includes two drying stages, with a circulating air system between the first and second drying stages. The first drying stage uses a forced-air drying method, while the second drying stage uses an exhaust drying method. After drying, the green pellets are initially shaped and enter the preheating stage, heating stage, and roasting stage. The roasting process is completed in a rotary kiln. After roasting, the green pellets undergo three stages of cooling to obtain weighted pellets. The oxidation degree of the green pellets after roasting and heating is controlled at 80%. The roasting temperature is 1200-1210℃, and the roasting time is 20 minutes. All three cooling stages use air cooling.

[0066] Step 3: The preheated 600℃ counterweight pellets are continuously added from the top of the hydrogen-based vertical shaft furnace and pass through the first heating reduction zone, the second heating reduction zone, the heat preservation reduction zone and the cooling zone of the hydrogen-based vertical shaft furnace in sequence to obtain direct reduced iron. In each reduction stage, the expansion rate of the pellets is less than 5%, and the overall expansion rate is less than 15%. The temperature for the first stage of reduction is controlled at 890-910℃, the time is controlled at 35 minutes, and the heating rate is about 25℃ / min. At this time, the pellets have undergone preliminary reduction, and the degree of reduction of the pellets is about 42%. The temperature of the second stage of reduction is controlled at 990-1010℃, and the heating rate is about 35℃ / min. When the temperature is higher than 1010℃, it enters the heat preservation reduction zone. The total reduction time is controlled at 25min, at which point the reduction degree of the pellets is increased to 99%. The first-stage heating reduction zone, the second-stage heating reduction zone, and the heat preservation reduction zone all use a reducing atmosphere, in which the volume fraction of the reducing gas (hydrogen) is greater than 85%.

[0067] The finished product, after passing through the reduction zone, continues to move downwards into the cooling section inside the furnace. The reducing gas enters the cooling section from the bottom. The finished product descending from the gas heating section comes into contact with the reducing gas for heat exchange, preheating the reducing gas. After cooling, the finished product enters the warehouse to await use.

[0068] Example 2

[0069] This embodiment provides a method for preparing high-quality direct reduced iron using pellets, including the following steps: Step 1: Place a solid Cr-Si iron alloy sphere with a diameter of 6mm into a briquetting machine, add some iron concentrate powder and mix with the sphere, and finally add solvent to fill the gap between the sphere and the iron concentrate powder, so that the concentrate powder and the sphere are bonded together to obtain a pre-pressed sphere. The pre-pressed spheres and the remaining iron concentrate powder are put into a pelletizing machine (inclined at 45°) to make green pellets. After pelletizing, the pellets are screened to obtain green pellets with a diameter of 14-15mm. Green pellets with a diameter of less than 12mm and the undersize material need to be returned to the pelletizing machine for processing. The mass ratio of solid alloy spheres, iron concentrate powder, and solvent is 3:39:58; the proportion of iron concentrate powder added to the briquetting machine is 32%. The prepared green pellets consist of solid iron pellets, iron concentrate powder, and solvent in a ratio of 3%:39%:58%. The iron concentrate powder has a grade of 70% and a particle size of less than 0.15 mm; The solvent contains a binder and a fuel in a mass ratio of 7:3. The binder is bentonite, and the fuel is coke powder. The solvent has a water content of 10%. Step 2: The green pellets are fed into a rotary kiln for roasting, during which the green pellets continuously roll from the tail end of the kiln towards the head end. The roasting process includes two drying stages, with a circulating air system between the first and second drying stages. The first drying stage uses a forced-air drying method, while the second drying stage uses an exhaust drying method. After drying, the green pellets are initially shaped and enter the preheating stage, heating stage, and roasting stage. The roasting process is completed in a rotary kiln. After roasting, the green pellets undergo three stages of cooling to obtain weighted pellets. The oxidation degree of the green pellets after roasting and heating is controlled at 80%. The roasting temperature is 1200-1210℃, and the roasting time is 20 minutes. All three cooling stages use air cooling.

[0070] Step 3: The preheated 600℃ counterweight pellets are continuously added from the top of the hydrogen-based vertical shaft furnace and pass through the first heating reduction zone, the second heating reduction zone, the heat preservation reduction zone and the cooling zone of the hydrogen-based vertical shaft furnace in sequence to obtain direct reduced iron. In each reduction stage, the expansion rate of the pellets is less than 5%, and the overall expansion rate is less than 15%. The temperature for the first stage of reduction is controlled at 890-910℃, the time is controlled at 35 minutes, and the heating rate is about 26℃ / min. At this time, the pellets have undergone preliminary reduction, and the degree of reduction of the pellets is about 42%. The temperature of the second stage of reduction is controlled at 990-1010℃, and the heating rate is about 38℃ / min. When the temperature is higher than 1010℃, it enters the heat preservation reduction zone. The total reduction time is controlled at 25min, at which point the reduction degree of the pellets increases to 99%. The first-stage heating reduction zone, the second-stage heating reduction zone, and the heat preservation reduction zone all use a reducing atmosphere, in which the volume fraction of the reducing gas (hydrogen) is greater than 85%.

[0071] The finished product, after passing through the reduction zone, continues to move downwards into the cooling section inside the furnace. The reducing gas enters the cooling section from the bottom. The finished product descending from the gas heating section comes into contact with the reducing gas for heat exchange, preheating the reducing gas. After cooling, the finished product enters the warehouse to await use.

[0072] Comparative Example 1

[0073] Comparative Example 1 provides a method for preparing direct reduced iron using pellets, which differs from Example 1 in that: The roasting process includes a drying stage, specifically using a forced-air drying method; Everything else is the same as in Example 1.

[0074] Comparative Example 2

[0075] Comparative Example 2 provides a method for preparing direct reduced iron using pellets, which differs from Example 1 in that: The temperature of the second stage of reduction is controlled at 1050℃, and the heating rate is about 15℃ / min. After the temperature exceeds 1050℃, the pellets enter the heat preservation reduction zone. The total reduction time is controlled at 35min, at which point the reduction degree of the pellets increases to 99%. Everything else is the same as in Example 1.

[0076] Performance testing: The pellets prepared in Example 1 had a density of 6100 kg / m³. 3 The density of the pellets prepared in Example 2 was 5900 kg / m³. 3 The density of the pellets prepared in the comparative proportion was 6000 kg / m³. 3 .

[0077] During use, the sponge iron pellets need to penetrate the slag layer and enter the molten steel with relatively low kinetic energy. For example, when the pellet density is 4 g / cm³... 3 Under certain conditions, the momentum required for sponge iron pellets to penetrate a 400mm thick slag layer and enter the molten steel is 0.942J. As the density and thickness of the foamed slag increase, the momentum required for sponge iron to enter the molten steel also gradually increases.

[0078] The pellets prepared in this embodiment of the invention have a suitable density, allowing them to acquire the momentum required to enter the molten steel at a lower speed, or in other words, reducing the speed required to enter the slag layer. Sponge iron pellets require high kinetic energy to penetrate the foamy slag and enter the molten steel; the higher the density, the lower the speed required to penetrate the molten steel. The initial height required for the sponge iron pellets is also lower, resulting in less initial velocity needed during electric arc furnace smelting and a reduction in overall smelting energy consumption.

[0079] Figure 1 The image shows the appearance of the pellets prepared in Comparative Example 1.

[0080] Figure 2 The image shows the abnormal expansion and cracking of pellets prepared for Comparative Example 2 in a hydrogen-based vertical shaft furnace.

[0081] Figure 3 The image shows the internal cracking of the pellets prepared for Comparative Example 2.

[0082] Depend on Figures 1-3 It can be seen that the pellets have fractured, exposing the internal iron core. Specifically: Figure 1 The situation shown is cracking caused by insufficient oxidation in areas with uneven heating during the roasting process. Compared with Example 1, in order to pursue efficiency, Comparative Example 1 uses a drying stage during the roasting process. At this time, the drying temperature or wind speed is relatively high. After entering the high-temperature environment, the surface moisture evaporates rapidly, forming a dry and hardened layer. However, the faster drying efficiency at this time means that the rate of internal moisture migration to the outside cannot keep up with the rate of surface evaporation, resulting in uneven shrinkage of the pellets inside and outside, which leads to cracking of the outer shell of the pellets.

[0083] Figure 2 , Figure 3 The situation shown illustrates cracking caused by a slow heating rate in a hydrogen-based vertical shaft furnace, resulting in a longer time to reach the optimal temperature in the reduction zone. In Comparative Example 2, during the two-stage reduction, the heating rate was less than 20℃ / min when the temperature reached 1050℃. The excessively long two-stage reduction time led to a rapid increase in the RSI (reduction expansion index), increasing the internal stress of the pellets and causing them to crack.

[0084] Figure 4 This is an image showing the external appearance of the pellets prepared in Example 1. Figure 1 It can be seen that the pellets prepared in Example 1 have no obvious cracks on their surface and there is no cracking.

Claims

1. A method for preparing high-density sponge iron, comprising: Step 1: Preparation of green pellets: Mix spherical iron counterweights, iron concentrate powder, and solvent, and pelletize to obtain green pellets. The amounts of the counterweights, iron concentrate powder, and solvent added, based on a mass of 100 wt%, are 3%-9%, 36%-39%, and 55%-58%, respectively. Step 2: Preparation of counterweight pellets: The raw pellets are roasted to produce counterweight pellets; Step 3: Reduction: The counterweight pellets are reduced to obtain high-density sponge iron.

2. The preparation method according to claim 1, wherein, Step one includes: mixing spherical iron counterweights and iron concentrate powder to form a pre-mixed sphere, then adding solvent to fill the gaps to obtain a raw sphere.

3. The preparation method according to claim 1 or 2, wherein, In step one, the spherical iron counterweight is a solid iron sphere with a diameter of 5-8 mm.

4. The preparation method according to claim 1 or 2, wherein, In step one, the solvent comprises a binder and a fuel; Based on the sum of the masses of the binder and the fuel being 100%, the addition amounts of the binder and the fuel are 70%-75% and 25%-30%, respectively. The binder is selected from one or more of bentonite, water glass, XPK organic binder and fluorite powder; The fuel is selected from one or more of coke powder, semi-coke powder, and petroleum coke. The solvent has a water content greater than or equal to 5%.

5. The preparation method according to claim 1 or 2, wherein, In step one, the particle size of the iron concentrate powder is less than 0.15 mm.

6. The preparation method according to claim 1 or 2, wherein, In step one, the pelletizing is carried out using a pelletizing machine, and the tilt angle of the pelletizing machine is equal to or less than 60°.

7. The preparation method according to claim 1 or 2, wherein, In step one, the diameter of the green pellet is 12-18 mm.

8. The preparation method according to claim 1, wherein, In step two, the oxidation degree of the green pellets after roasting is controlled at 72-85%.

9. The preparation method according to claim 1 or 8, wherein, In step two, the roasting temperature is 1180-1250℃, and the roasting time is 15-25 minutes.

10. The preparation method according to claim 1, wherein, In step three, the reduction process includes preheating, first-stage reduction, second-stage reduction, and heat preservation reduction; The preheating temperature is 580-680℃; The reduction temperature in the first stage is 850-950℃, and the reduction degree of the counterweight pellets is 40-45%. The temperature for the second stage of reduction is 980-1050℃; The reduction degree of the counterweight pellets obtained after the heat preservation and reduction process is over 99%. The first stage reduction, the second stage reduction, and the heat-preserving reduction are carried out in a reducing atmosphere, wherein the volume fraction of the reducing gas in the reducing atmosphere is equal to or greater than 85%. The heating rate of the first reduction stage is 15-25℃ / min, and the heating rate of the second reduction stage is 35-45℃ / min.

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