Method for reducing iron ore powder

Abstract

A method for reducing iron ore powder according to one embodiment of the present invention comprises: a fluidized bed reduction step for reducing iron ore powder having a total SiO2 content and Al2O3 content of 4.0 mass% or more and a crystal water content of 5 mass% or more by using a reducing gas using a fluidized bed; and a fluidized bed heat treatment step for heat-treating the iron ore powder at least partially metallized by the fluidized bed reduction step with a non-oxidizing gas using the fluidized bed, the temperature in the fluidized bed in the fluidized bed heat treatment step being higher than the temperature in the fluidized bed in the fluidized bed reduction step, and the temperature in the fluidized bed heat treatment step being lower than the temperature in the fluidized bed in the fluidized bed reduction step. The gas flow rate of the non-oxidizing gas in the flow layer in the flow layer heat treatment step is 1.5 times or more greater than the gas flow rate of the reducing gas in the flow layer in the flow layer reduction step, or is 4 times or more than the minimum flow rate of the flow layer in the flow layer reduction step.

Description

Technical Field

[0001] This invention relates to a method for reducing iron ore powder. This application claims priority based on Japanese Patent Application No. 2023-172911, filed on October 4, 2023, the contents of which are incorporated herein by reference. Background Technology

[0002] Previously, direct reduction (DR) was known as one of the ironmaking methods for reducing iron oxide-containing raw materials to obtain iron. Compared with the blast furnace method, DDR has advantages such as lower equipment manufacturing costs and easier operation. DDR can utilize either a shaft furnace or a flow bed. In the flow bed method, the pre-agglomeration of iron ore powder is not required, increasing the flexibility in the availability of usable iron ore powder. Furthermore, hydrogen can be used as a reducing agent in DDR. By utilizing hydrogen, CO2 emissions can be reduced.

[0003] In the reduction of iron ore powder using a flow layer (hereinafter, the reduction of iron ore powder using a flow layer is sometimes referred to as "flow layer reduction"), the agglomeration of iron ore powder within the flow layer, known as adhesion or sintering, leading to blockages, has long been a major problem. Agglomeration in flow layer reduction is known to occur on the surface of the iron ore powder due to the interdiffusion of solid metallic iron, the entanglement of fibrous protrusions, or the formation of low-melting-point slag. Furthermore, it is believed that the large contact area of ​​the fine powder increases the number of contact points between metallic iron particles, making agglomeration more likely. If agglomeration occurs, the iron ore powder cannot flow uniformly, causing the flow layer to stop operating. Therefore, optimizing the operating conditions to avoid agglomeration is important, depending on the iron ore powder being used. Agglomeration is more likely to occur at high reaction temperatures during flow layer reduction; therefore, it is important to perform reduction at low temperatures to prevent agglomeration.

[0004] Furthermore, iron ore powder reduced in the flow layer (reduced iron ore powder) sometimes suffers from re-oxidation due to atmospheric oxygen after reduction due to its high specific surface area, resulting in a decrease in the reduction rate. For example, Non-Patent Literature 1 studied the re-oxidation property of reduced iron ore powder. According to Non-Patent Literature 1, the re-oxidation property varied for each reduction temperature and oxidation temperature, and among most iron ore powders, the re-oxidation property was highest at a reduction temperature of 600°C. Additionally, according to Non-Patent Literature 1, as a method to suppress re-oxidation, maintaining the temperature at 700°C, which is higher than 600°C, under the reducing atmosphere can suppress re-oxidation.

[0005] Furthermore, Non-Patent Document 2 discloses that when reduced sponge iron powder is heat-treated in a nitrogen atmosphere, it ignites below 680°C, but does not ignite when heat-treated above 700°C. The reason given is that the re-oxidizability caused by the presence of numerous lattice defects and deformation changes due to heat treatment.

[0006] On the other hand, reduced iron ore powder produced at low temperatures without agglomeration has a large specific surface area. Therefore, if such reduced iron ore powder is exposed to air, it may rapidly re-oxidize and generate heat. If rapid re-oxidation occurs, the accompanying heat generation and the resulting increase in the oxidation rate occur cyclically, significantly reducing the reduction rate of the reduced iron ore powder.

[0007] In Patent Document 1, a method for suppressing reoxidation of directly reduced iron ore powder is proposed, which involves contacting the powder with a low concentration of oxidizing gas to create a thin oxide layer on the surface, thereby suppressing severe reoxidation. Furthermore, it describes a method for suppressing the oxidation of highly reactive ores by performing a heat treatment beforehand, oxidizing the surface while reducing its reactivity, thereby inhibiting reoxidation.

[0008] In addition, Patent Document 2 discloses a method for controlling re-oxidation by using a treatment gas containing less than 1.0% oxygen and the remainder being substantially composed of nitrogen.

[0009] Patent document 3 discloses the following technology: when reducing powdered iron ore containing more than 1.0% by weight of water of crystallization in a flow layer, the powdered iron ore is preheated to reduce the water of crystallization to less than 1.0% by weight before being supplied to the flow layer for reduction.

[0010] Patent Document 4 discloses an operating method for a reduction furnace. A raw material containing iron oxide is charged into the furnace, and a reducing gas, which accompanies the endothermic reaction during reduction, is introduced into the furnace to reduce the iron oxide to obtain reduced iron. This operating method involves screening the reduced iron discharged from the furnace, classifying it into reduced iron with a fine particle size range and reduced iron with a coarse particle size range, and recovering the reduced iron with the fine particle size range. In this technology, an example of a reduction furnace is the reduction of fine ore using a reducing gas in a flowing layer.

[0011] Patent document 5 discloses the following technology: granulating granulated powder with a median particle size of less than 50 μm containing iron, and reducing the granulated powder by utilizing the flow layer formed by the flow of the granulated powder.

[0012] Furthermore, Non-Patent Document 3 describes that no iron powder agglomeration occurs up to 600°C, but the agglomeration rate gradually increases with temperature above 600°C.

[0013] Existing technical documents

[0014] Patent documents

[0015] Patent Document 1: Japanese Patent Publication No. 39-28875

[0016] Patent Document 2: Japanese Patent Application Publication No. 55-47306

[0017] Patent Document 3: Japanese Patent Application Publication No. 11-100607

[0018] Patent Document 4: International Publication No. 2022 / 209015

[0019] Patent Document 5: International Publication No. 2023 / 100936

[0020] Non-patent literature

[0021] Non-Patent Literature 1: Yoshiaki Iguchi, Michio Inoue, "Reoxidation Behavior of Sponge Iron After Reduction with Iron Oxide and Iron Ore", Iron and Steel, 1970, Vol. 56, No. 5, pp. 507-520

[0022] Non-Patent Literature 2: Ozō Myōkō and Matsushita Yūō, “On the Properties of Reduced Sponge Iron Powder”, Iron and Steel, 1965, Vol. 51, No. 1, pp. 11-18

[0023] Non-Patent Literature 3: Miyagawa et al., “Mechanism of Adhesion Generation and Prevention Method in the Reduction of Flow Layer of Iron Ore”, Iron and Steel, 1992, No. 7, pp. 1258-1265 Summary of the Invention

[0024] The technical problem that the invention aims to solve

[0025] In the technologies described in Patent Documents 1 and 2, the temperature needs to be raised to a specified level or higher to suppress re-oxidation. Therefore, if this method is applied to suppress re-oxidation for various iron ore powders, depending on the type of iron ore powder, the reduced iron ore powder may sometimes agglomerate, making it unsuitable for powder processing. Lowering the heat treatment temperature is effective in preventing agglomeration, but in this case, it may not achieve a sufficient re-oxidation suppression effect. Furthermore, the technology described in Patent Document 1 forms a thin oxide layer on the surface of the ore, but its adjustment is difficult.

[0026] In addition, in the previous method of suppressing the re-oxidation of reduced iron ore powder by agglomerating and densifying it, the volume and weight increase due to agglomeration, so it cannot be processed by airflow like powder.

[0027] Furthermore, focusing on iron ore powder, it is predicted that the iron ore powder used in the steel industry will be micronized and reduced in grade. Therefore, it is believed that high-quality ore with almost no gangue is in high demand and difficult to obtain, while low-quality ore containing more gangue is relatively easy to obtain. In such easily obtainable iron ore powder, iron oxide is sometimes not hematite (Fe₂O₃), but rather exists as goethite (FeO(OH)) containing water of crystallization. Goethite decomposes at around 400℃, separating into hematite and water, but at this time, a large number of fine pores are formed in the hematite. These pores also remain in the reduced iron ore powder after reduction, increasing the reoxidizability of the reduced iron ore powder. To date, the main iron ore powder targeted for reduction is high-grade hematite-based iron ore powder. Compared with goethite-based iron ore powder, hematite-based iron ore powder shows a smaller increase in specific surface area during reduction and does not exhibit problems related to reoxidation. However, as mentioned above, goethite-based iron ore powder containing water of crystallization and a high content of gangue components has a larger specific surface area, resulting in high re-oxidation properties of the reduced iron ore powder obtained from it. For example, reduced iron ore powder obtained by reducing goethite-based iron ore powder at a temperature of around 650°C has a high specific surface area, and if it comes into contact with the atmosphere, it rapidly heats up to a high temperature of over 200°C, significantly reducing the reduction rate.

[0028] As mentioned above, reduced iron ore powder made from iron ore powder with high moisture content is prone to re-oxidation. Therefore, in the reduction of iron ore powder with high moisture content using a flow layer, it is necessary to ensure operational stability and suppress the heating of the reduced iron ore powder caused by re-oxidation after reduction.

[0029] The higher the gangue content, the more difficult it is to agglomerate. Patent Document 3's technology does not consider the difficulty of agglomerating iron ore powder due to high SiO2 and Al2O3 content. The technologies described in Patent Documents 4 and 5 do not consider the re-oxidation of reduced iron ore powder due to high moisture content, nor the difficulty of agglomerating iron ore powder due to high SiO2 and Al2O3 content.

[0030] The present invention was made in view of the above-mentioned problems. The object of the present invention is to provide a method for reducing iron ore powder, which can obtain reduced iron ore powder that inhibits re-oxidation even in the atmosphere while maintaining the powder state.

[0031] Technical solutions for solving technical problems

[0032] The inventors have obtained the following insights: for goethite-based iron ore powder containing a large amount of water of crystallization, re-oxidation can be suppressed by subjecting the reduced iron ore powder after reduction treatment to a heat treatment that reduces its specific surface area. Furthermore, they have obtained the following insights: when goethite-based iron ore powder is reduced in a fluidized bed, the temperature at which agglomeration begins is higher compared to the conventional case of reducing hematite-based iron ore powder in a fluidized bed.

[0033] The main points of the present invention, based on the above insights, are as follows.

[0034] [1] A method for reducing iron ore powder according to one aspect of the present invention comprises: a flow layer reduction step, wherein an iron ore powder having a total content of SiO2 and Al2O3 of 4.0% or more and a content of water of crystallization of 5% or more is reduced by a reducing gas using a flow layer; and a flow layer heat treatment step, wherein the iron ore powder having been metallized in at least a portion by the flow layer reduction step is heat treated by a non-oxidizing gas using a flow layer, wherein the temperature in the flow layer heat treatment step is higher than the temperature in the flow layer reduction step, and the gas flow rate of the non-oxidizing gas in the flow layer heat treatment step is 1.5 times or more greater than the gas flow rate of the reducing gas in the flow layer reduction step, or is 4 times or more the minimum flow rate of the flow layer in the flow layer heat treatment step.

[0035] [2] In the reduction method of iron ore powder described in [1] above, the temperature in the flow layer in the flow layer reduction process can be set to below 730°C, and the temperature in the flow layer in the flow layer heat treatment process can be set to above 720°C.

[0036] [3] In the reduction method of iron ore powder described in [1] or [2] above, the iron ore powder may be reduced to a metallization rate of 70% or more in the flow layer reduction process.

[0037] [4] In any of the reduction methods of iron ore powder described in any of [1] to [3] above, the reducing gas may also include hydrogen.

[0038] [5] In any of the above-mentioned methods for reducing iron ore powder, such as [1] to [3], the reducing gas may contain more than 30% by volume of hydrogen.

[0039] [6] In any of the above-mentioned methods for reducing iron ore powder [1] to [5], the flow layer in the flow layer heat treatment process may be a bubble flow layer, and the gas flow rate of the non-oxidizing gas in the bubble flow layer may be above the minimum flow velocity and below the terminal velocity.

[0040] [7] In any of the reduction methods of iron ore powder described in any of [1] to [6] above, the non-oxidizing gas may be N2 gas or Ar gas.

[0041] [8] In any of the above-mentioned methods for reducing iron ore powder [1] to [7], the same flow layer may be used in the flow layer reduction process and the flow layer heat treatment process.

[0042] Invention Effects

[0043] According to the present invention, it is possible to obtain reduced iron ore powder that is not easily re-oxidized even in the atmosphere while maintaining the powder state. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of a device capable of forming a bubble flow layer.

[0045] Figure 2 This is a schematic diagram of another example of a container in a device capable of forming a bubble flow layer.

[0046] Figure 3 This is a schematic diagram of a device capable of forming a circulating flow layer.

[0047] Figure 4 This is a schematic diagram of a device capable of forming a jet flow layer.

[0048] Figure 5 This refers to the sieve mass H of the iron ore powder with mesh M after the heat treatment process, as described in the second embodiment of the present invention. M The sieve mass R of reduced iron ore powder with mesh size M after the fluidized bed reduction process M The ratio of H M / R M The flowchart of the process.

[0049] Figure 6 This is a graph representing the specific surface area of ​​each iron ore powder held at a specific temperature in Example 1.

[0050] Figure 7 These are schematic diagrams of the device structures used in Examples 2-5.

[0051] Figure 8 This is a graph representing the specific surface area of ​​each reduced iron ore powder maintained at a specific temperature in Example 2.

[0052] Figure 9 This is a graph representing the specific surface area of ​​the reduced iron ore powder at each heat treatment temperature in Example 3.

[0053] Figure 10 It is a graph representing the highest temperature reached by the reduced iron ore powder after each heat treatment time when it is removed from the atmosphere.

[0054] Figure 11 This is a graph representing the specific surface area of ​​the reduced iron ore powder at each heat treatment temperature in Example 4.

[0055] Figure 12 This is a graph showing the pore size distribution of each reduced iron ore powder in Example 5. Detailed Implementation

[0056] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Furthermore, the ratios and dimensions of the constituent elements in the drawings do not represent the actual ratios and dimensions of the constituent elements.

[0057] <First Implementation Method>

[0058] The iron ore powder reduction method of the first embodiment of the present invention includes: a flow layer reduction step in which iron ore powder having a combined SiO2 content and Al2O3 content of 4.0% or more and a water of crystallization content of 5% or more is reduced using a reducing gas in a flow layer; and a flow layer heat treatment step in which iron ore powder that has been at least partially metallized by the flow layer reduction step is heat-treated using a non-oxidizing gas in a flow layer. In this embodiment, the temperature in the flow layer heat treatment step is higher than the temperature in the flow layer reduction step. Furthermore, the gas flow rate of the non-oxidizing gas in the flow layer heat treatment step is at least 1.5 times greater than the gas flow rate of the reducing gas in the flow layer reduction step, or at least 4 times the minimum fluidization velocity of the flow layer in the flow layer heat treatment step. It should be noted that, hereinafter, when it is not necessary to distinguish between iron ore powder that has been at least partially metallized by the flow layer reduction step and iron ore powder after the flow layer heat treatment step, both are sometimes referred to as reduced iron ore powder. In addition, when describing the iron ore powder that forms the flow layer, the iron ore powder before reduction and the iron ore powder after reduction (reduced iron ore powder) are not distinguished and are referred to as iron ore powder.

[0059] (Flow layer reduction process)

[0060] In the flow layer reduction process, a flow layer is used to reduce iron ore powder with a combined SiO2 and Al2O3 content of 4.0% by mass or more and a water of crystallization content of 5% by mass or more using reducing gas.

[0061] [Flow layer]

[0062] In this process, a flow layer is used in the reduction of iron ore powder. The flow layer may employ one or more bubble-fluidized beds (BFB), one or more circulating fluidized beds (CFB), one or more jet-fluidized beds, or a combination thereof. A bubble-fluidized bed is a flow layer in which gas-based bubbles are formed within the iron ore powder-based flow layer. However, the bubbles vary in shape depending on the flow state, and there are cases where no clearly defined bubbles are formed, depending on the flow state. A circulating fluidized bed is a flow layer in which the iron ore powder circulates with the gas by increasing the gas flow rate. A jet-fluidized bed has an exit section (gas blowout section) that ejects iron ore powder at a high gas flow rate, and a moving layer around the exit section where iron ore powder accumulates and moves from top to bottom. This process repeatedly involves the iron ore powder being drawn into the lower part of the moving layer by a high gas flow rate and ejected from the exit section.

[0063] [Iron ore powder]

[0064] The flow layer reduction process is performed on iron ore powder with a combined SiO2 and Al2O3 content of 4.0% by mass or more and a water of crystallization content of 5% by mass or more. SiO2 and Al2O3 are gangue components; if the gangue content is too low, agglomeration is likely to occur. The combined SiO2 and Al2O3 content can be 6.0% by mass or more, or 10.0% by mass or more. Alternatively, the combined SiO2 and Al2O3 content can be 11.0% by mass or less. If the combined SiO2 and Al2O3 content is 11.0% by mass or less, the components to be separated are insufficient, thus suppressing the decrease in productivity. The combined SiO2 and Al2O3 content is preferably 10.0% by mass or less. On the other hand, if the water of crystallization content is 5% by mass or more, the iron ore powder is exposed to high temperatures during reduction, thereby removing the water of crystallization from the iron ore powder and forming numerous fine pores in the reduced iron powder. This improves the reoxidizability of the reduced iron ore powder. The water of crystallization content can be 8% by mass or more. Alternatively, the water of crystallization content can be 9% by mass or less. Through this process, a powdered reduced iron ore powder with improved and suppressed reoxidizability can be obtained from such iron ore powder.

[0065] The SiO2 content was quantified according to JIS M 8214:1995 "Method for Quantitative Analysis of Silicon in Iron Ore Powder". The Al2O3 content was quantified according to JIS M 8220:1995 "Method for Quantitative Analysis of Aluminum in Iron Ore Powder". The water of crystallization content was quantified according to JIS M8211:2023 "Method for Quantitative Analysis of Water in Iron Ore Powder - Karl Fischer Titration". Unless otherwise specified, quantification was performed under standard conditions.

[0066] In this process, a portion of the iron ore powder is metallized. The metallization rate determined in this process is determined according to the requirements of subsequent processes, but it is preferable to reduce the iron ore powder to a metallization rate of 70% or higher. By implementing the flow layer heat treatment process described later, even with a metallization rate of 70% or higher, re-oxidation can be suppressed. From the viewpoint of reducing the reduction load in subsequent processes, a metallization rate of 90% or higher is more preferable. Alternatively, the metallization rate can be 100% or less, 95% or less, or 90% or less.

[0067] The metallization of iron ore powder was determined according to ISO 5416:2006 "Sponge iron - Determination of metallic iron - Bromine-methanol titration method". Unless otherwise specified in this standard, the metallization of iron ore powder or reduced iron ore powder was determined under standard conditions.

[0068] The median particle size of the iron ore powder is, for example, 20 μm or more and 5 mm or less. For stable operation, a median particle size of 20 μm or more is preferred. A median particle size of more than 50 μm is more preferable. If the median particle size of the iron ore powder exceeds 50 μm, the iron ore powder exhibits superior flowability, and the iron ore powder can be reduced more stably through the flow layer. Therefore, a median particle size of more than 50 μm is more preferable. On the other hand, a median particle size of less than 5 mm is preferred. A median particle size of less than 1 mm is more preferable. This is because if the median particle size of the iron ore powder becomes too large due to agglomeration, it may hinder the stable flowability of the iron ore powder, and depending on the situation, may cause localized blockage in the container forming the flow layer. Furthermore, even if the iron ore powder contains coarse particles, there is no problem as long as stable flowability is achieved.

[0069] The median particle size of iron ore powder can be determined by the following method: The particle size d, based on the volumetric distribution in the undersize cumulative distribution, is measured using a laser diffraction particle size analyzer (Malvern Panalytical, Mastersizer 3000) as a wet-scale measuring apparatus. 50The median particle size of the iron ore powder was used as the measurement condition. The settings were: dispersion medium: water; refractive index of the dispersion medium: 1.33; particle refractive index: 2.918 (refractive index of iron oxide, Fe2O3). In cases where the iron ore powder contained large particles, it was sieved using a dry sieve. The particle size of three random samples of powder taken from the sieve was measured using the aforementioned laser diffraction particle size analyzer. 50 The average value can be used as the median particle size.

[0070] [Reducing gases]

[0071] The reducing gas includes, for example, at least one of CH4, CO, and H2. At least a portion of the iron ore powder is reduced using the reducing gas. The reducing gas preferably includes hydrogen. In conventional reduced iron production using shaft furnaces, the use of natural gas, coal, CO gas, etc., in the reduction reaction generates CO2, which can contribute to global warming, resulting in a high environmental load. On the other hand, if hydrogen is used as the reducing gas, CO2 is not generated by the reduction reaction, thus suppressing the environmental load. The reducing gas preferably contains 30% by volume or more hydrogen, more preferably 50% by volume or more hydrogen. From the viewpoint of reducing CO2, a higher hydrogen content is preferred; the reducing gas preferably contains 100% by volume hydrogen. Additionally, a portion of the reducing gas may contain inactive gases.

[0072] The flow rate of the reducing gas is above the minimum flow velocity of the flow layer. If the flow rate is below the minimum flow velocity, the iron ore powder will not flow. To suppress the dispersion of iron ore powder, the flow rate of the reducing gas is preferably lower than the terminal velocity. If the flow rate of the reducing gas is lower than the terminal velocity, the dispersion of iron ore powder can be suppressed, and a high frequency of contact between iron ore powder particles can be maintained. Furthermore, when the flow layer is a bubble flow layer, the loss of iron ore powder and reduced iron ore powder caused by dispersion can be suppressed.

[0073] The reducing gas velocity (Bm / s) is the empty tower velocity (Bm / s) that achieves the flow layer, and the gas flow rate (Bm) supplied per unit time. 3 / s) divided by the cross-sectional area of ​​the flow layer (m²) 2 The value obtained is obtained by measuring the flow rate of reducing gases using a flow meter installed on the gas supply piping. The "B" marking the unit of velocity or flow rate indicates the velocity or flow rate under actual conditions (temperature, pressure).

[0074] The minimum flow velocity is the minimum gas velocity at which the pressure loss in the flow layer becomes constant with increasing gas velocity, and can be experimentally determined by the following method. For example, the pressure loss of the flow layer is obtained by measuring the pressure difference between the lower and upper parts of the layer containing iron ore powder (the pressure difference between the gas pool portion below the dispersion plate and the space portion above the layer, described later), and subtracting the pressure difference when no iron ore powder is loaded (pressure loss of the dispersion plate only). The pressure loss in the flow layer is plotted relative to the gas tower velocity to determine the minimum gas velocity at which the pressure loss becomes constant. However, to eliminate the dependence on the initial particle packing structure and obtain reproducible data, the minimum flow velocity is determined by gradually decreasing the gas velocity from a sufficient level for flowability, using the point where the pressure loss begins to decrease from a certain region. In the measurement of pressure loss, the measurement location need not be limited to the gas pool portion below the dispersion plate or the space portion above the layer. The measurement location can be, for example, inside the flow layer or the space portion above the layer, as long as the pressure loss of the flow layer can be measured at any location.

[0075] When iron ore powder is approximately spherical, the terminal velocity ut (m / s) of the iron ore powder can be expressed by the following equation (1).

[0076] [Formula 1]

[0077]

[0078] In the above formula (1), g (m / s 2 ρ is the acceleration due to gravity. p (kg / m 3 ρ is the particle density of iron ore powder. f (kg / m 3 ( ) represents the density of the reducing gas, D p (m) represents the median particle size of iron ore powder, C d (-) is the drag coefficient, which is rearranged using the Reynolds number Re and expressed using the approximation of Brown and Lawler (Equation (2)) as follows.

[0079] C d = (24(1+0.15 Re) 0.681 ) / Re)+(0.407 / (1+8710 Re -1 ...Equation (2)

[0080] The temperature within the flow layer can be set, for example, to 500°C or higher and 900°C or lower. If the temperature within the flow layer is 500°C or higher and 900°C or lower, the reduction reaction of the iron ore powder is promoted, and productivity is increased. From the viewpoint of suppressing excessive agglomeration caused by the adhesion of iron ore powder to each other and achieving stable operation, the temperature within the flow layer is preferably 730°C or lower. Therefore, the temperature within the flow layer is preferably 500°C or higher and 730°C or lower. More preferably, the temperature within the flow layer is 550°C or higher and 680°C or lower.

[0081] Temperature measurement within the flow layer is performed by a thermocouple positioned with its front end located within the flow section of the iron ore powder.

[0082] [Average stay time]

[0083] When the flow layer is a bubble flow layer, the average residence time of the iron ore powder retained in the bubble flow layer depends on the temperature of the reduction reaction, but is preferably 3 minutes or more and 180 minutes or less. If the average residence time is 3 minutes or more, reduced iron ore powder with a high reduction rate can be obtained. On the other hand, when the average residence time is 180 minutes or less, the processing efficiency is maintained at a high level. In addition, if the average residence time is 180 minutes or less, the reduction in strength of the reduced iron ore powder caused by excessive reduction or the micronization of the reduced iron ore powder caused by collisions between reduced iron ore powders or between reduced iron ore powders and the apparatus can be suppressed, and the reduction in the recovery efficiency of the reduced iron ore powder can be suppressed. Therefore, the average residence time is preferably 180 minutes or less. The average residence time of the iron ore powder retained in the bubble flow layer is more preferably 5 minutes or more and 150 minutes or less. The average residence time is adjusted by adjusting the amount of iron ore powder extracted per unit time.

[0084] When the flow layer is a circulating flow layer, the average residence time of iron ore powder retained in the circulating flow layer depends on the temperature of the reduction reaction, but is preferably 3 minutes or more and 120 minutes or less. More preferably, the average residence time of iron ore powder retained in the circulating flow layer is 5 minutes or more and 60 minutes or less. The reasons for these preferred average residence times are the same as those for the case where the flow layer is a bubble flow layer.

[0085] When the flow layer is a jet flow layer, the average residence time of iron ore powder retained in the jet flow layer depends on the temperature of the reduction reaction, but is preferably 3 minutes or more and 180 minutes or less. More preferably, the average residence time of iron ore powder retained in the jet flow layer is 5 minutes or more and 120 minutes or less. The reasons for these preferred average residence times are the same as those for the case where the flow layer is a bubble flow layer.

[0086] The average residence time of iron ore powder can be calculated using the following method: As tracer particles, a certain amount of ore powder with equal median particle size but different gangue compositions is introduced, and the temporal variation in the gangue content of the discharged reduced iron powder is investigated. The time period of the peak with the highest gangue content characteristic of the introduced tracer ore powder is taken as the average residence time of the iron ore powder. According to the above method, the average residence time can be experimentally determined.

[0087] When the container used in the flow layer reduction process is different from the container used in the flow layer heat treatment process, the reduced iron ore powder after the flow layer reduction process is transferred in a non-oxidizing atmosphere.

[0088] When reducing iron ore powder with a combined SiO2 and Al2O3 content of 4.0% by mass or more and a water of crystallization content of 5% by mass or more in the flow layer, the agglomeration initiation temperature is higher compared to when hematite-based iron ore powder is used. The reason for this is not entirely clear, but it is believed that compared to high-grade hematite ore, iron ore powder with a lower iron grade and higher gangue content has a smaller proportion of metallic iron in contact with each other during reduction. Therefore, in the flow layer heat treatment process following the flow layer reduction step, the heat treatment temperature can be higher than the reduction temperature.

[0089] (Heat treatment process for the flow layer)

[0090] In the flow layer heat treatment process, a flow layer is used to heat-treat reduced iron ore powder that has been partially metallized by the flow layer reduction process using a non-oxidizing gas.

[0091] [Flow layer]

[0092] In this process, a flow layer is used in the heat treatment of reduced iron ore powder. The flow layer may be one or more bubble flow layers, one or more circulating flow layers, one or more jet flow layers, or a combination thereof. The flow layer used in this process can be formed from the container used in the flow layer reduction process. By using the same container in this process as in the flow layer reduction process, the reduced iron ore powder does not need to be transferred to other containers, thus enabling efficient processing of the reduced iron ore powder. Furthermore, by using the same container, the reduced iron ore powder after the flow layer reduction process is not exposed to the atmosphere, thus suppressing oxidation of the reduced iron ore powder. Additionally, the type of flow layer used in the flow layer reduction process and the flow layer heat treatment process can be the same.

[0093] [Non-oxidizing gases]

[0094] Non-oxidizing gases are those that do not oxidize the reduced iron ore powder after the flow layer reduction process, such as N2, He, Ne, Ar, Kr, or Xe. Heat treatment of reduced iron ore powder in a non-oxidizing gas environment reduces its specific surface area.

[0095] The flow rate of the non-oxidizing gas is at least 1.5 times greater than the flow rate of the reducing gas in the flow layer during the reduction process, or at least 4 times the minimum flow velocity of the flow layer during the heat treatment process. Basically, the latter applies when the flow rate of the reducing gas in the reduction process is higher, and the former applies when it is lower. When the flow layer in the reduction process is a circulating flow layer, the latter, i.e., the flow rate of the non-oxidizing gas, is set to at least 4 times the minimum flow velocity of the flow layer during the heat treatment process. This maintains a good flow state in the flow layer.

[0096] The flow rate of non-oxidizing gases is the empty tower velocity that achieves the flow layer, which is obtained by dividing the gas flow rate per unit time by the cross-sectional area of ​​the flow layer. The flow rate of non-oxidizing gases can be measured using a flow meter installed on the gas supply piping.

[0097] The temperature within the flow layer in the flow layer heat treatment process is higher than the temperature within the flow layer in the flow layer reduction process. By making the temperature within the flow layer in the flow layer heat treatment process higher than the temperature within the flow layer in the flow layer reduction process, the specific surface area of ​​the iron ore powder can be reduced. The temperature within the flow layer in the flow layer heat treatment process is set to be higher than the temperature within the flow layer in the flow layer reduction process, for example, 700°C or higher and 900°C or lower. From the viewpoint of reducing the specific surface area in a short time, the temperature within the flow layer in the flow layer heat treatment process is preferably 720°C or higher, more preferably 750°C or higher. Furthermore, to further suppress the agglomeration of the reduced iron ore powder, the temperature within the flow layer in the flow layer heat treatment process is 850°C or lower. Therefore, the temperature within the flow layer in the flow layer heat treatment process is preferably 720°C or higher and 850°C or lower, more preferably 750°C or higher and 850°C or lower.

[0098] Temperature measurement within the flow layer is performed by a thermocouple positioned with its front end located within the flow section of the reduced iron ore powder.

[0099] In this process, the reduced iron ore powder can be heat-treated at high temperatures, thus reducing the specific surface area and suppressing the re-oxidation of the heat-treated reduced iron ore powder. Furthermore, no reducing gas is used in the flow-layer heat treatment process. If a reducing gas containing hydrogen were used in the flow-layer heat treatment process, a large amount of hydrogen would be required to maintain the flow. On the other hand, in the flow-layer heat treatment process, the amount of hydrogen consumed due to the reduction of the iron ore powder is small, while a large amount of hydrogen containing H2O generated during the reduction of the iron ore powder is discharged outside the container. To reuse the discharged hydrogen, it is necessary to cool the hydrogen to dehydration and then reheat it. Therefore, the energy load becomes very large. Therefore, in the flow-layer heat treatment process, by using a non-oxidizing gas that does not contribute to the reaction and is inexpensive, the energy load can be suppressed.

[0100] (Example of device structure)

[0101] Hereinafter, an example of the apparatus structure for the reduction method of iron ore powder according to this embodiment will be described with reference to the accompanying drawings. Figure 1 This is a schematic diagram of a device capable of forming a bubble flow layer. Figure 2 This is a schematic diagram of a device capable of forming a circulating flow layer. Figure 3 This is a schematic diagram of a device capable of forming a jet flow layer.

[0102] [Bubble Flow Layer Forming Apparatus]

[0103] like Figure 1 As shown, the bubble flow layer forming apparatus 10 may include a container 11 and a dry dust collector 12.

[0104] For example, such as Figure 1 As shown, the container 11 has: a gas supply port 111 disposed at the bottom for supplying gas into the interior of the container 11; an iron ore powder supply port 112 for supplying iron ore powder or reduced iron ore powder; and a dispersion plate 113 disposed above the gas supply port 111.

[0105] The iron ore powder is circulated by gas supplied from the gas supply port 111 and rectified through multiple vents of the dispersion plate 113, thereby forming a bubble flow layer. It should be noted that the ventilation method and form of the gas used to form the bubble flow layer are not limited to using a flat dispersion plate 113 such as a porous plate or a slit plate. Any structure that can supply gas into the interior of the container 11 and blow up the iron ore powder to form a flow layer can be used, such as a simple nozzle type, a cap type with a cover of various shapes with a blow-out hole at the nozzle tip, or a pipe type with a grid tube with multiple holes on the side of the pipe. The specific form is not limited.

[0106] Reduced iron ore powder or heat-treated reduced iron ore powder is discharged from the openable and closable reduced iron powder outlet (not shown).

[0107] Alternatively, the bubble flow layer forming apparatus may also have a container in which multiple bubble flow layers are formed internally. Figure 2 This is a schematic diagram of another example of a container in an apparatus capable of forming a bubble flow layer. The container 11A may include, for example: an iron ore powder supply port 112A disposed on one side along its length; an outlet 114 disposed on the other side along its length; a plurality of gas supply ports 111A arranged side-by-side along its length; a dispersion plate 113A disposed above each gas supply port 111A; and a partition plate 115 disposed between adjacent gas supply ports 111A. The space between the inner wall of the container 11A and the partition plate 115, and the space between adjacent partition plates 115, forms the flow layer. The height of the partition plate 115 is lower than the height of the bubble flow layer. This structure of the container 11A can prolong the average residence time of the iron ore powder and improve the reduction rate. It should be noted that the location and number of iron ore powder supply ports, the location and number of outlets, and the location and number of partition plates are not limited to... Figure 2 The method shown can, of course, be modified as appropriate.

[0108] [Circulating Flow Layer Forming Apparatus]

[0109] For example, such as Figure 3 As shown, the circulating flow layer forming apparatus 20 includes: a rising section 21 serving as a container for forming a flow layer of iron ore powder; a cyclone separator 22 connected to an outlet 214 located at the upper part of the rising section 21; and a circulation pipeline 23 extending downward from the bottom of the cyclone separator 22 and connected to the lower part of the rising section 21. The circulating flow layer forming apparatus 20 may be equipped with a dry dust collector 24, which is connected to the cyclone separator 22, to recover micronized iron ore powder and reduced iron (dust) contained in the exhaust gas.

[0110] The riser 21 has a gas supply port 211, an iron ore powder supply port 212, and a dispersion plate 213 disposed above the gas supply port 211. The riser 21 is basically the same as the container 11 in the bubble flow layer forming apparatus 10.

[0111] Cyclone separator 22 captures particles that are dispersed along with the exhaust gas. The captured particles are returned to riser 21 through circulation pipeline 23, and the exhaust gas is discharged to the outside of circulation flow layer forming device 20 via dry dust collector 24.

[0112] The circulation pipeline 23 includes: a downcomer 231, which is connected to the lower part of the cyclone separator 22 and serves as the flow path for the iron ore powder separated from the gas by the cyclone separator 22; and an annular seal 232, one end of which is connected to the lower end of the downcomer 231 and the other end of which is connected to the upper part of the dispersion plate 213 of the riser 21. The annular seal 232 achieves a sealing effect by temporarily accumulating iron ore powder. It should be noted that the ventilation method and form of the gas used to form the circulating flow layer are not limited to using a flat dispersion plate 113 such as a porous plate or a slit plate. Any structure that can supply gas to the interior of the riser 21 and blow up the iron ore powder to form a flow layer is acceptable, except for simple nozzle type, cap type with various shapes of caps with blow-out holes at the nozzle tip, or pipe type with grid pipes with multiple holes on the side of the pipe.

[0113] In the circulating flow layer forming apparatus 20, the exhaust gas sometimes contains finely pulverized iron ore powder (dust). Therefore, a dry dust collector 24 can be used to capture the dust contained in the exhaust gas. As the dry dust collector 24, for example, a cyclone separator, a multi-stage dust collector, a ceramic filter, etc., can be used. In the circulating flow layer forming apparatus 20, for example, a cyclone separator smaller than the cyclone separator 22 can also be connected in series after the cyclone separator 22 as the dry dust collector 24.

[0114] Iron ore powder or reduced iron ore powder supplied from iron ore powder supply port 212 is fluidized by the rectified supply gas supplied from gas supply port 211 and passing through multiple vents of dispersion plate 213. Specifically, the iron ore powder or reduced iron ore powder is conveyed from bottom to top inside the riser 21, circulating inside the circulating flow layer forming device 20 via cyclone separator 22 and circulation line 23. Therefore, the interior of the circulating flow layer forming device 20 becomes a circulating flow layer. It should be noted that the iron ore powder is temporarily retained in the annular seal 232. In the case where the reduction or heat treatment based on the circulating flow layer forming device 20 is an intermittent process, the processed powder is extracted, for example, from an openable and closable outlet (not shown) located at the bottom of cyclone separator 22 (midway in downcomer 231). In the case where the reduction or heat treatment is a continuous process, for example, the valve of the openable and closable outlet provided in the riser 21 is opened at certain time intervals or continuously to take out the processed powder, and iron ore powder or reduced iron ore powder is replenished from the iron ore powder supply port 212.

[0115] [Jet Flow Layer Forming Device]

[0116] The jet flow layer forming apparatus 300 has a structure substantially the same as that of the bubble flow layer forming apparatus 10. However, the jet flow layer forming apparatus 300 differs from the bubble flow layer forming apparatus 10 in that it does not uniformly agitate the entire structure. For example, as Figure 4 As shown, above the gas supply port 111, a flyout section 311 (gas blowout section) is formed, through which iron ore powder is ejected at a high gas flow rate. A moving layer 312 is formed around the flyout section, around which iron ore powder accumulates and moves from top to bottom. The iron ore powder at the bottom of the moving layer 312 is drawn into the high-velocity supply gas flowing through the protrusion 311, repeating the action of the iron ore powder being ejected from the protrusion 311. The flow layer formed by the protrusion 311 and the moving layer 312 is a jet flow layer 310.

[0117] The flow layer of reduced iron ore powder and the flow layer of heat-treated iron ore powder can be a circulating flow layer, a bubble flow layer or a jet flow layer, or multiple circulating flow layers, multiple bubble flow layers or multiple jet flow layers, or a combination of more than one circulating flow layer, more than one bubble flow layer or more than one jet flow layer.

[0118] In the circulating flow layer, the difference between the average velocity of the supplied gas and the average moving velocity of the iron ore powder (slip velocity) is large. Therefore, the gas in contact with the iron ore powder is replaced frequently. During reduction, this prevents the area around the iron ore powder from approaching equilibrium and causing the reduction reaction to stagnate, resulting in efficient reduction of the iron ore powder. On the other hand, because the average moving velocity of the iron ore powder itself is also high, mechanical wear and damage occur due to collisions between iron ore powder particles, easily generating dust.

[0119] Compared to the circulating flow layer, the bubble flow layer has a smaller difference (slip velocity) between the average velocity of the supplied gas and the average moving velocity of the iron ore powder. Therefore, the reduction efficiency of the bubble flow layer for iron ore powder is lower than that of the circulating flow layer. On the other hand, regarding dust generation, in addition to the tendency to be suppressed compared to the circulating flow layer, the energy cost for gas supply can also be suppressed by inhibiting the gas flow rate.

[0120] In the jet flow layer, iron ore powder with a larger particle size than that in the circulating flow layer can be used. Furthermore, the jet flow layer is advantageous when it is desirable to shorten the residence time of the iron ore powder.

[0121] The composition of the flow layer is preferably determined by considering the characteristics of the circulating flow layer, the bubble flow layer and the jet flow layer, as well as the median particle size and Fe content of the iron ore powder.

[0122] Furthermore, the apparatus shown in each of the accompanying drawings is merely one example, and is certainly not limited to the manner shown in the drawings.

[0123] <Second Implementation Method>

[0124] The iron ore powder reduction method of the second embodiment of the present invention has the following features (I) to (V).

[0125] (I) The object of processing is iron ore powder with a total content of SiO2 and Al2O3 of 4.0% by mass or more and a content of water of crystallization of 5% by mass or more.

[0126] (II) The reduction method of iron ore powder in this embodiment includes a flow layer reduction process using a flow layer and reducing gas for reduction.

[0127] (III) The reduction method of iron ore powder in this embodiment includes a heat treatment process in which a small portion of the iron ore powder (reduced iron ore powder) that has been metallized by the flow layer reduction process is heat-treated in a non-oxidizing environment.

[0128] (IV) The heat treatment temperature in the heat treatment process is higher than the temperature inside the flow layer in the flow layer reduction process.

[0129] (V) The proportion of reduced iron ore powder with mesh size M after the fluidized bed reduction process, after being sieved sequentially using sieves with mesh sizes M = 5mm, 3mm, 1mm, 0.5mm and 0.25mm. M The mass ratio H of the reduced iron ore powder after heat treatment on the sieve with mesh M is... M The relationship is shown below.

[0130] ·H M / R M : 1.0~1.2

[0131] The "screen mass of mesh M" refers to the total mass of M and the reduced iron ore powder remaining on a screen with a larger mesh size. The ratio R is the screen mass of the reduced iron ore powder after the fluidized bed reduction process. M Given identical mesh sizes M, representing 20% ​​of the initial mass, calculate the percentage H of the reduced iron ore powder remaining on the sieve after heat treatment, based on the mesh size M. M Calculate H M / R M Features (I) and (II) are the same as in the first embodiment, therefore detailed descriptions are omitted here.

[0132] (Heat treatment process)

[0133] In the heat treatment process, iron ore powder (reduced iron ore powder) that has been at least partially metallized through the flow layer reduction process is heat treated in a non-oxidizing atmosphere.

[0134] [Non-oxidizing ambient gas]

[0135] Non-oxidizing ambient gases are gases that do not oxidize the reduced iron ore powder after the flow layer reduction process, such as Ar or N2 ambient gases. When reduced iron ore powder is heat-treated in a non-oxidizing ambient gas, its specific surface area decreases.

[0136] In order to reduce the specific surface area of ​​the reduced iron ore powder, the temperature of the non-oxidizing ambient gas is higher than the temperature inside the flow layer in the flow layer reduction process.

[0137] Temperature measurement of non-oxidizing ambient gas is performed using a thermocouple placed in the ambient gas, with the tip of the thermocouple in contact with a layer of reduced iron ore powder.

[0138] In the heat treatment process, the mass ratio H of the reduced iron ore powder on the sieve after the heat treatment process is... M The proportion of reduced iron ore powder on the sieve relative to the reduced iron ore powder after the fluidized bed reduction process, R M The ratio of H M / R M Under conditions where H is below 1.2, the reduced iron ore powder after the flow layer reduction process is subjected to heat treatment. If H M / R M If the concentration is below 1.2, it can be said that the agglomeration of reduced iron ore powder is suppressed, and the heat treatment process is carried out stably. In H... M / R M When the value exceeds 1.2, the reduced iron ore powder agglomerates after the heat treatment process, indicating that a stable heat treatment process was not implemented. If the degree of particle agglomeration remains unchanged before and after heat treatment, H... M / R M The value is 1.0. Therefore, H M / R M It can be above 1.0. Additionally, since the number of particles decreases at equilibrium, H... M / R M It can be 0 or higher.

[0139] Here, refer to Figure 5 Explain how to calculate H M / R M The method. Figure 5 This refers to the proportion of the reduced iron ore powder with a mesh size M after the heat treatment process, as shown in the second embodiment of the present invention, in terms of the sieve mass ratio H. M The proportion of reduced iron ore powder on the sieve with a mesh size M relative to the reduced iron ore powder after the fluidized bed reduction process, R M The ratio of H M / R M The flowchart of the process.

[0140] For H M / R MThe calculated sieve size was determined by sieving according to the method based on JIS M 8706:2015 Iron ore and reduced iron - determination of particle size distribution by sieving. First, sieves with mesh sizes M = 5 mm, 3 mm, 1 mm, 0.5 mm, and 0.25 mm were prepared.

[0141] Reduced iron ore powder (reduced powder) after the fluidized bed reduction process and reduced iron ore powder (heat-treated powder) after the heat treatment process are collected and used as initial masses (100 parts by weight). The initial masses can be determined arbitrarily. The reduced powder and heat-treated powder supplied for sieving are powders dried to a constant weight at 105±5℃.

[0142] The reducing powder (100 parts by weight) and the heat-treated powder (100 parts by weight) were sieved separately using a sieve with a mesh size of M=5mm (step S1). The mass ratio R of the reducing powder on the sieve relative to the initial mass was determined. 5mm (Step S2).

[0143] The mass ratio R of the reducing powder on the sieve 5mm When the content is 20% by mass (20 parts by mass) or more (step S2 / yes), determine the sieve mass ratio H of the heat-treated powder relative to the initial mass. 5mm Calculate H 5mm / R 5mm (Step S11).

[0144] The mass ratio R of the reducing powder on the sieve 5mm If the initial mass percentage is less than 20% (step S2 / No), the reduced powder and heat-treated powder (undersize reduced powder and undersize heat-treated powder) that have passed through a sieve with a mesh size of M=5mm are sieved separately using a sieve with a mesh size of M=3mm (step S3), and the mass percentage R of the reduced powder on the sieve with a mesh size of M=3mm is determined. 3mm Relative to the initial mass (step S4). Here, the mass ratio R on the sieve with a mesh size M = 3 mm is... 3mm It is the ratio of the total mass of iron ore powder remaining on sieves with mesh sizes M=5mm and 3mm to the initial mass. In other words, the mass proportion R on the sieve with mesh size M=3mm. 3mm It is the ratio of the cumulative mass on the sieve to the initial mass.

[0145] The mass ratio R of the reducing powder on the sieve 3mm When the content is 20% or more by mass (step S4 / yes), determine the mass percentage H of the heat-treated powder on the sieve. 5mm Calculate H 3mm / R 3mm (Step S11).

[0146] The mass ratio R of the reducing powder on the sieve 3mmIf the content is less than 20% by mass (step S4 / No), the reduced powder and heat-treated powder that have passed through a sieve with a mesh size of M=3mm are sieved separately using a sieve with a mesh size of M=1mm (step S5), and the mass ratio R of the reduced powder and the powder that passes through the sieve with a mesh size of M=1mm is determined. 1mm (Step S6).

[0147] The mass ratio R of the reducing powder on a sieve with a mesh size M = 1 mm 1mm When the mass percentage is 20% or more (step S6 / Yes), determine the mass percentage H of the heat-treated powder on a sieve with a mesh size M = 1 mm. 1mm Calculate H 1mm / R 1mm (Step S11).

[0148] The mass ratio R of the reducing powder on the sieve 1mm If the mass percentage is less than 20% (step S6 / No), the reduced powder and heat-treated powder that have passed through a sieve with a mesh size of M=1mm are sieved separately using a sieve with a mesh size of M=0.5mm (step S7), and the mass ratio R of the reduced powder and the powder that has passed through the sieve with a mesh size of M=0.5mm is calculated. 0.5mm Perform the measurement (step S7).

[0149] The mass ratio R of the reducing powder is measured on a sieve with a mesh size of M = 0.5 mm. 0.5mm When the mass percentage is 20% or more (step S8 / Yes), the mass percentage H of the heat-treated powder on a sieve with a mesh size M = 0.5 mm is determined. 0.5mm Calculate H 0.5mm / R 0.5mm (Step S11).

[0150] The mass ratio R of the reducing powder on the sieve 0.5mm If the content is less than 20% by mass (step S8 / No), the reduced powder and heat-treated powder that have passed through a sieve with a mesh size of M=0.5mm are sieved separately using a sieve with a mesh size of M=0.25mm (step S9), and the mass ratio R of the reduced powder and the powder that passes through the sieve with a mesh size of M=0.25mm is determined. 0.5mm (Step S10).

[0151] The mass ratio R of the reducing powder on a sieve with a mesh size M = 0.25 mm 0.25mm When the content is 20% or more by mass (step S10 / yes), the mass percentage H of the heat-treated powder on a sieve with a mesh size M = 0.25 mm is determined. 0.25mm Calculate H 0.25mm / R 0.25mm .

[0152] The mass ratio R of the reducing powder on the sieve 0.25mmIf the percentage is less than 20% by mass (step S10 / No), it is determined that there is no problem with fluidity.

[0153] The vibration method and the endpoint of the screening process described above shall be in accordance with JIS M 8706:2015.

[0154] The heat treatment process can be carried out using a furnace capable of controlling temperature and ambient gas, such as a known rotary kiln. When using an internally heated rotary kiln in the heat treatment process, to suppress deviations in ambient gas temperature, the direction of introduction of the non-oxidizing gas is preferably opposite to the direction of travel of the reduced iron ore powder (convective contact). When using an externally heated rotary kiln, the direction of introduction of the non-oxidizing gas is not limited.

[0155] In this embodiment, the heat treatment process following the flow layer reduction step can also be performed at a temperature higher than the reduction temperature, thereby reducing the specific surface area and suppressing the re-oxidation of the reduced iron ore powder after heat treatment. Furthermore, in this embodiment, for the same reasons as in the first embodiment, the energy load can also be suppressed.

[0156] Example

[0157] The following describes embodiments of the invention. However, the conditions in these embodiments are merely examples used to confirm the feasibility and effectiveness of implementing the invention, and the invention is not limited to the conditions used in the following embodiments. The invention can employ various conditions to achieve its objectives without departing from its spirit.

[0158] <Example 1>

[0159] Iron ore powder with a SiO2 content of 6.55% by mass, an Al2O3 content of 3.62% by mass, a water of crystallization content of 8.38% by mass, and a median particle size of 97.8 μm was kept at 600℃, 700℃, or 800℃ for 2 hours under atmospheric conditions. The SiO2 content was quantified according to JIS M 8214:1995 "Iron Ore Powder - Quantitative Method for Silicon". The Al2O3 content was quantified according to JISM 8220:1995 "Iron Ore Powder - Quantitative Method for Aluminum". The water of crystallization content was quantified according to JIS M 8211:2023 "Iron Ore Powder - Quantitative Method for Compound Water - Karl Fischer Titration". The median particle size of the iron ore powder was determined using a laser diffraction particle size analyzer with water as the dispersion medium, a refractive index of 1.33 for the dispersion medium, and a particle refractive index of 2.918.

[0160] The specific surface area of ​​the iron ore powder after being maintained under the above conditions was determined. The specific surface area was determined by the following method: JIS Z 8830:2013 "Method for determination of specific surface area of ​​powders (solids) based on gas adsorption". Liquid nitrogen was used in the cooling tank, N2 was used as the adsorbate gas, and He was used as the calibration gas and carrier gas.

[0161] Figure 6 This is a graph representing the specific surface area of ​​each iron ore powder held at a specific temperature in Example 1. For example... Figure 6 As shown, the higher the temperature, the lower the specific surface area.

[0162] <Example 2>

[0163] use Figure 7 The apparatus shown is used for the reduction of iron ore powder as described above. A dispersion plate containing sintered glass beads is placed inside a container with an inner diameter of 35 mm, and iron ore powder is filled onto the dispersion plate. The thickness of the iron ore powder layer is 35 mm. The tip of a thermocouple is positioned inside the iron ore powder layer. A heating mechanism is installed around the outer periphery of the container to heat the interior. N2 gas is supplied into the container from below, and the iron ore powder is heated while flowing through it. Once the flow layer reaches a predetermined temperature, H2 gas is supplied. The H2 gas flow rate is 0.1 m / s. The gas flowing through the container is discharged from the top outwards, and the discharged gas is analyzed. The reduction time is defined as 2 hours from the start of the reaction. Furthermore, pressure probes positioned inside and above the iron ore powder layer are used to continuously measure the pressure of the iron ore powder layer (flow layer) and the pressure in the space above the iron ore powder layer during gas flow to determine the stability of the flow layer. In cases of excessive agglomeration or gas bypass (channeling) occurring in areas of iron ore powder segregation, abnormalities may occur, such as excessive pressure loss exceeding the weight of the flow layer due to overall blockage of the flow layer, or pressure loss approaching zero due to gas passing through the flow layer without contributing to fluidization.

[0164] The temperature within the flow layer was set at 600°C, 700°C, or 800°C. After the supply of H2 gas was stopped, the reduced iron ore powder was cooled to room temperature and then exposed to the atmosphere. The specific surface area of ​​the reduced iron ore powder after being exposed to the atmosphere was measured using the method described above. Additionally, for reduced iron ore powder that underwent reduction at a flow layer temperature of 600°C or 800°C, was cooled to room temperature, and then exposed to the atmosphere, its temperature immediately after exposure to the atmosphere was measured.

[0165] Figure 8 This is a graph representing the specific surface area of ​​the reduced iron ore powder at each maintained temperature in Example 2. (Compared to...) Figure 8The reduced iron ore powder corresponding to each of the drawings was removed and released into the atmosphere after being cooled to room temperature. For example... Figure 8 As shown, the higher the holding temperature, the lower the specific surface area. Furthermore, the reduced iron ore powder reduced at 600°C and cooled to room temperature before being exposed to the atmosphere heated to over 200°C in the atmosphere. On the other hand, the reduced iron ore powder reduced at 800°C and cooled to room temperature before being exposed to the atmosphere did not change temperature compared to room temperature. Additionally, compared to Example 1, the reduced iron ore powder in Example 2 had a smaller specific surface area at any temperature.

[0166] <Example 3>

[0167] The reduction and heat treatment of the iron ore powder described above were performed using the same apparatus as in Example 2. The temperature within the flow layer during reduction was set to 600°C, and the flow rate of H2 gas was set to 0.08 Bm / s. Then, the gas supplied to the container was switched from H2 gas to N2 gas for heat treatment. The flow rate of N2 gas was 0.08 Bm / s. Therefore, the flow rate of N2 gas was 5.5 times the minimum flow velocity. The heat treatment temperature was 800°C, and the holding time was 0 minutes, 5 minutes, 15 minutes, 30 minutes, 60 minutes, or 90 minutes. After stopping the supply of N2 gas, the reduced iron ore powder was cooled to room temperature and exposed to the atmosphere. Cooling was initiated immediately after the temperature within the flow layer reached 800°C, with a holding time of 0 minutes.

[0168] Figure 9 This is a graph showing the specific surface area of ​​the reduced iron ore powder at each heat treatment temperature in Example 3. (Example:) Figure 9 As shown, the specific surface area decreases by performing heat treatment at a temperature higher than the reduction temperature. Furthermore, it can be seen that extending the heat treatment time further reduces the specific surface area.

[0169] Figure 10 This is a graph showing the maximum temperature reached by the reduced iron ore powder after being removed from the atmosphere for each heat treatment time. Furthermore, it can be seen that extending the heat treatment time further reduces the maximum temperature reached. This is attributed to the decrease in oxidation rate due to the reduced specific surface area.

[0170] <Example 4>

[0171] Using the same apparatus as in Example 2, the reduction of the iron ore powder described above was performed, or a combination of reduction and heat treatment was performed. The temperature within the flow layer during reduction was set to 600°C, and the flow rate of H2 gas was set to 0.08 Bm / s. Then, the gas supplied to the container was switched from H2 gas to N2 gas for heat treatment. The flow rate of N2 gas was 0.08 Bm / s. Therefore, the flow rate of N2 gas was 5.5 times the minimum flow velocity. The heat treatment temperature was 700°C or 800°C, and the holding time was 2 hours. After stopping the supply of N2 gas, the reduced iron ore powder was cooled to room temperature and exposed to the atmosphere. For each piece of reduced iron ore powder exposed to the atmosphere, the highest reaching temperature and specific surface area after exposure were measured. For iron ore powder that had reached its highest reaching temperature and then cooled to room temperature, the specific surface area was measured.

[0172] Figure 11 This is a graph showing the specific surface area of ​​the reduced iron ore powder at each heat treatment temperature in Example 4, and the highest temperature reached is also recorded. The white curve is for the sample that underwent reduction at 600°C without heat treatment. Figure 11 As shown, the specific surface area decreases with increasing heat treatment temperature. Furthermore, a decrease in the maximum reach temperature of the reduced iron ore powder was also confirmed. These results indicate that as the heat treatment temperature increases, the specific surface area decreases, accompanied by a decrease in the reach temperature.

[0173] <Example 5>

[0174] Using the same apparatus as in Example 2, the reduction of the iron ore powder described above was performed, or a combination of reduction and heat treatment was performed. The temperature within the flow layer during reduction was set to 600°C, and the flow rate of H2 gas was set to 0.08 Bm / s. Then, the gas supplied to the container was switched from H2 gas to N2 gas for heat treatment. The flow rate of N2 gas was 0.08 Bm / s. Therefore, the flow rate of N2 gas was 5.5 times the minimum flow velocity. The heat treatment temperature was set to 800°C, and the holding times were set to 0 minutes, 15 minutes, 30 minutes, and 60 minutes. After stopping the supply of N2 gas, the reduced iron ore powder was cooled to room temperature. The pore size distribution of the cooled reduced iron ore powder was measured. Cooling was initiated immediately after the temperature within the flow layer reached 800°C, with a holding time of 0 minutes.

[0175] The fine pore size distribution was determined by the following method. First, for each reduced iron ore powder exposed to the atmosphere after cooling to room temperature, an adsorption isotherm was obtained by measuring the adsorption amount of the adsorbate gas according to JIS Z 8830:2013 "Method for determination of specific surface area of ​​powders (solids) based on gas adsorption". Liquid nitrogen was used in the cooling bath, N2 was used as the adsorbate gas, and He was used as the calibration gas and carrier gas. Next, the fine pore size distribution was derived according to the method for determining the mesopore size distribution (BJH method) of Barrett, Joyner, and Halenda described in JIS Z 8831-2:2010 "Fine pore size distribution and micropore characteristics of powders (solids) - Part II: Method for determination of mesopores and macropores based on gas adsorption".

[0176] Figure 12 This is a graph showing the pore size distribution of each reduced iron ore powder in Example 5. For example... Figure 12 As shown, the longer the heat treatment time, the more the peak value of the distribution changes towards a larger diameter, and the number of peak values ​​also tends to decrease. Furthermore, it is known that the iron ore powder used differs from hematite-based iron ore powder, exhibiting a tendency to remain fluid even at high temperatures. Therefore, by using a flow layer, the powder state can be maintained, and the specific surface area can be reduced to suppress re-oxidation.

[0177] <Example 6>

[0178] By varying the flow rate of the non-oxidizing gas and the temperature within the flow layer during the heat treatment process, the flowability and re-oxidation inhibition during the heat treatment process were evaluated. In this evaluation, the reduction and heat treatment processes used the same apparatus as in Example 2. Reduced iron ore powder with a combined SiO2 and Al2O3 content of 4.0% by mass or more, a water of crystallization content of 5% by mass or more, and a median particle size of 100 μm was heat-treated after being held at 700°C for 2 hours in a reducing atmosphere (H2). Then, the gas supplied to the container was switched from atmospheric gas to N2 gas for further heat treatment.

[0179] The results of the fluidity evaluation are shown in Table 1. The stability of the fluid layer was determined by continuously measuring the pressure of the flow layer in the gas flow and the pressure in the space above the reduced iron ore powder layer. A condition where the 1-minute moving average of the pressure value was greater than the range of pressure changes caused by the ejection of reduced iron ore powder was evaluated as poor (B), and a condition where the moving average was smaller than the range of pressure changes caused by the ejection of reduced iron ore powder was evaluated as good (A). The results are shown in Table 1. The pressure change caused by the ejection of reduced iron ore powder was compared with the results when fluidized with N2 gas alone without reduction. Furthermore, the pressure change when fluidized with N2 gas alone without reduction was approximately 5%.

[0180] In addition, after stopping the supply of N2 gas, the reduced iron ore powder was cooled to room temperature and then exposed to the atmosphere. For each piece of reduced iron ore powder exposed to the atmosphere, the highest temperature reached after exposure was measured. A maximum temperature above 200°C was rated C (poor); a maximum temperature above 100°C but below 200°C was rated B (good); and a maximum temperature below 100°C was rated A (extremely good). The results are shown in Table 2.

[0181] [Table 1]

[0182]

[0183] As shown in Table 1, when the gas flow rate of the non-oxidizing gas in the flow layer during the heat treatment process is more than 4 times the minimum flow velocity, the temperature in the flow layer during the heat treatment process is above 650℃ and below 800℃, and the evaluation results are good.

[0184] [Table 2]

[0185]

[0186] As shown in Table 2, when the temperature inside the flow layer in the flow layer heat treatment process is 700℃, the evaluation result is good; when the temperature inside the flow layer in the flow layer heat treatment process is above 750℃ and below 800℃, the evaluation result is extremely good.

[0187] <Example 7>

[0188] Iron ore powder with a combined SiO2 and Al2O3 content of less than 4.0% by mass, a water of crystallization content of more than 5% by mass, and a median particle size of 100 μm was kept in a reducing atmosphere (H2) at the temperatures shown in Table 3 for 2 hours. The gas flow rates in the flow layer reduction process are shown in Table 3. The flowability of the flow layer in the reduction process is evaluated in Table 3. The results are shown in Table 3.

[0189] [Table 3]

[0190]

[0191] As shown in Table 3, when the combined content of SiO2 and Al2O3 is less than 4.0% by mass, and the temperature of the flow layer is 700°C, excessive agglomeration caused by the adhesion of iron ore powder is suppressed, even under low gas flow rate conditions. However, it is evident that poor flowability exists above 750°C.

[0192] <Example 8>

[0193] Iron ore powder with a combined SiO2 and Al2O3 content of less than 4.0% by mass, a water of crystallization content of more than 5% by mass, and a median particle size of 100 μm was subjected to heat treatment at 700°C for 2 hours under a reducing atmosphere (H2). Then, the gas supplied to the container was switched from atmospheric gas to N2 gas for further heat treatment. The flowability of the flow layer in the heat treatment process was evaluated under the conditions shown in Table 4, including the gas flow rate of the non-oxidizing gas in the flow layer and the temperature in the flow layer. In this evaluation, the reduction and heat treatment processes used the same apparatus as in Example 2. The results are shown in Table 4.

[0194] [Table 4]

[0195]

[0196] As shown in Table 4, when the combined content of SiO2 and Al2O3 is less than 4.0% by mass, even if the gas flow rate of the non-oxidizing gas in the flow layer during the flow layer heat treatment process is more than 4 times the minimum flow rate, the evaluation results are sometimes unsatisfactory.

[0197] Industrial availability

[0198] As described above, according to the present invention, it is possible to obtain reduced iron ore powder that, by utilizing a flow layer, suppresses re-oxidation even in the atmosphere while maintaining the powder state. The obtained reduced iron ore powder, like waste and conventional direct reduced iron (DRI) powder, can be melted and refined in an electric furnace and used as an iron source for crude steel manufacturing. Furthermore, in processes designed to achieve low reduction rates, it can also be used as a semi-reduced iron powder, reducing the utilization rate of reducing materials in the blast furnace, as an iron source fed into the blast furnace.

[0199] Explanation of reference numerals in the attached figures

[0200] 10: Bubble Flow Layer Forming Device

[0201] 11, 11A: Container

[0202] 12: Dry dust collector

[0203] 111, 111A: Gas supply port

[0204] 112, 112A: Iron ore powder supply port

[0205] 113, 113A: Dispersion plate

[0206] 114: Export

[0207] 115: Divider

[0208] 20: Circulating Flow Layer Forming Device

[0209] 21: Ascending section

[0210] 22: Cyclone Separator

[0211] 23: Circulation pipeline

[0212] 24: Dry dust collector

[0213] 211: Gas supply port

[0214] 212: Iron ore powder supply port

[0215] 213: Dispersion plate

[0216] 214: Exports

[0217] 231: Downcomer

[0218] 232: Ring seal

[0219] 30: Jet Flow Layer Forming Device

Claims

1. A method for reducing iron ore powder, characterized in that, have: The flow layer reduction process uses a flow layer and reducing gas to reduce iron ore powder with a total SiO2 content and Al2O3 content of 4.0% or more and a water of crystallization content of 5% or more. The flow layer heat treatment process involves using a flow layer and a non-oxidizing gas to heat-treat the iron ore powder that has been at least partially metallized in the flow layer reduction process. The temperature inside the flow layer in the heat treatment process is higher than the temperature inside the flow layer in the reduction process. The gas flow rate of the non-oxidizing gas in the flow layer during the heat treatment process is more than 1.5 times greater than the gas flow rate of the reducing gas in the flow layer during the reduction process, or more than 4 times the minimum flow velocity of the flow layer during the heat treatment process.

2. The method for reducing iron ore powder according to claim 1, characterized in that, The temperature within the flow layer during the flow layer reduction process is set to below 730°C. The temperature within the flow layer during the heat treatment process is set to 720°C or higher.

3. The method for reducing iron ore powder according to claim 1 or 2, characterized in that, In the flow layer reduction process, the iron ore powder is reduced to a metallization rate of 70% or higher.

4. The method for reducing iron ore powder according to claim 1 or 2, characterized in that, The reducing gas contains hydrogen.

5. The method for reducing iron ore powder according to claim 1 or 2, characterized in that, The reducing gas contains more than 30% by volume hydrogen.

6. The method for reducing iron ore powder according to claim 1 or 2, characterized in that, The flow layer in the heat treatment process is a bubble flow layer. The gas flow rate of the non-oxidizing gas within the bubble flow layer is above the minimum flow velocity and below the terminal velocity.

7. The method for reducing iron ore powder according to claim 1 or 2, characterized in that, The non-oxidizing gas is N2 gas or Ar gas.

8. The method for reducing iron ore powder according to claim 1 or 2, characterized in that, The same flow layer is used in both the flow layer reduction process and the flow layer heat treatment process.

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