Method for reducing iron ore powder

Abstract

A method for reducing an iron ore powder according to one embodiment of the present invention includes: a fluidized bed reduction step in which an iron ore powder, in which the total content of the content of SiO2 and the content of Al2O3 is 4.0 mass% or more and the content of crystal water is 5 mass% or more, is reduced with a reducing gas using a fluidized bed; and a fluidized bed heat treatment step in which the iron ore powder, in which at least a part thereof is metallized by the fluidized bed reduction step, is heat treated with a non-oxidizing gas using a fluidized bed. The temperature in the fluidized bed in the fluidized bed heat treatment step is higher than the temperature in the fluidized bed in the fluidized bed reduction step, and the gas flow rate of the non-oxidizing gas in the fluidized bed in the fluidized bed heat treatment step is 1.5 times or more higher than the gas flow rate of the reducing gas in the fluidized bed in the fluidized bed reduction step, or four times or more the minimum fluidization velocity of the fluidized bed in the fluidized bed reduction step.

Description

Method for reducing iron ore powder

[0001] This application claims priority to Japanese Patent Application No. 2023-172911, filed on October 4, 2023, the contents of which are incorporated herein by reference.

[0002] Direct reduction has been known as one of the ironmaking methods for obtaining iron by reducing raw materials containing iron oxide. Compared to the blast furnace method, the direct reduction method has advantages such as lower plant manufacturing costs and easier operation. Direct reduction methods include methods using a shaft furnace or a fluidized bed. The fluidized bed method does not require prior agglomeration of iron ore powder, which may increase the flexibility of usable iron ore powder. In addition, the direct reduction method can use hydrogen as a reducing agent. By using hydrogen, CO ２ This will enable emissions to be reduced.

[0003] In the reduction of iron ore powder using a fluidized bed (hereinafter, the reduction of iron ore powder using a fluidized bed may be referred to as "fluidized bed reduction"), agglomeration of iron ore powder particles within the fluidized bed, known as sticking or sintering, has traditionally been a major problem, leading to clogging and other problems. Agglomeration during fluidized bed reduction is known to occur due to the interdiffusion of solid metallic iron on the surface of the iron ore powder, the entanglement of whisker-like protrusions, or the formation of low-melting-point slag. Furthermore, because fine powder has a large contact area, the increased contact points between metallic iron particles may increase the likelihood of agglomeration. Agglomeration can lead to uneven fluidization of the iron ore powder, leading to operational shutdowns of the fluidized bed. Therefore, optimizing the operating conditions to avoid agglomeration is important depending on the iron ore powder being used. Because agglomeration is likely to occur when the reaction temperature during fluidized bed reduction is high, it is important to perform the reduction at a low temperature to prevent agglomeration.

[0004] Furthermore, iron ore powder (reduced iron ore powder) reduced in a fluidized bed has a high specific surface area, which can lead to a problem of reoxidation by oxygen in the atmosphere after reduction, resulting in a decrease in the reduction rate. For example, Non-Patent Document 1 examines the reoxidation tendency of reduced iron ore powder. According to Non-Patent Document 1, it is evaluated that the reoxidation tendency differs depending on the reduction temperature and the oxidation temperature, and it is stated that for most iron ore powders, the reoxidation tendency of reduced iron ore powder is highest when the reduction temperature is 600°C. Furthermore, Non-Patent Document 1 states that reoxidation can be suppressed by maintaining the temperature at 700°C, which is higher than 600°C, in a reducing atmosphere.

[0005] Furthermore, Non-Patent Document 2 discloses that when reduced sponge iron powder is heat-treated in a nitrogen atmosphere, it ignites at temperatures below 680° C., but does not ignite at temperatures above 700° C. The reason for this is believed to be that the reoxidation properties change due to the large amount of lattice defects and distortions contained in the powder.

[0006] On the other hand, reduced iron ore powder produced at low temperatures where agglomeration does not occur has a large specific surface area. Therefore, when such reduced iron ore powder is exposed to air, it may rapidly re-oxidize and generate heat. Rapid re-oxidation generates heat, which in turn increases the oxidation rate, resulting in a cyclical decrease in the reduction rate of the reduced iron ore powder.

[0007] Patent Document 1 proposes a method for suppressing the reoxidation of directly reduced iron ore powder by exposing it to a low-concentration oxidizing gas to form a thin oxide layer on the surface and suppressing severe reoxidation. In addition, it states that highly active ore can be suppressed by preliminarily heat-treating it to reduce its activity and then oxidizing its surface.

[0008] Furthermore, Patent Document 2 discloses a method for controlling reoxidation using a treatment gas containing 1.0% or less oxygen, the remainder being substantially nitrogen.

[0009] Patent Document 3 discloses a technique in which, when fine iron ore containing 1.0% by weight or more of water of crystallization is reduced in a fluidized bed, the fine iron ore is heat-treated in advance to reduce the water of crystallization to less than 1.0% by weight, and then supplied to the fluidized bed for reduction.

[0010] Patent Document 4 discloses a method for operating a reduction furnace in which raw materials containing iron oxide are charged into a reduction furnace, a reducing gas that undergoes an endothermic reaction during reduction is introduced into the reduction furnace, and the iron oxide is reduced to obtain reduced iron. In this method, the reduced iron discharged from the reduction furnace is sieved to be classified into reduced iron having a fine particle size range and reduced iron having a coarse particle size range, and the reduced iron having a fine particle size range is recovered. In this technology, a system in which fine ore is reduced in a fluidized bed by a reducing gas is exemplified as a reduction furnace.

[0011] Patent Document 5 discloses a technology in which a granulated powder is produced from raw material fine powder containing iron and having a median diameter of 50 μm or less, and the granulated powder is reduced in a fluidized bed formed by the fluidization of the granulated powder.

[0012] Furthermore, Non-Patent Document 3 describes that aggregation of iron powder does not occur up to 600°C, and above 600°C, the aggregation rate gradually increases as the temperature increases.

[0013] Japanese Patent Publication No. 39-28875 Japanese Patent Publication No. 55-47306 Japanese Patent Publication No. 11-100607 International Publication No. 2022 / 209015 International Publication No. 2023 / 100936

[0014] Yoshiaki Iguchi and Michio Inoue, "Reoxidation Behavior of Sponge Iron Reduced from Iron Oxide and Iron Ore," Iron and Steel, Vol. 56, No. 5, 1970, pp. 507-520; Akimitsu Okura and Yukio Matsushita, "Properties of Reduced Sponge Iron Powder," Iron and Steel, Vol. 51, No. 1, 1965, pp. 11-18; Miyagawa et al., "Mechanism of Sticking in Fluidized Bed Reduction of Iron Ore and Its Prevention," Iron and Steel, Vol. 7, 1992, pp. 1258-1265

[0015] The techniques described in Patent Documents 1 and 2 require the temperature to be raised to a predetermined temperature or higher to suppress reoxidation. Therefore, when attempting to suppress reoxidation by applying these methods to various iron ore powders, depending on the type of iron ore powder, aggregation of the reduced iron ore powder may occur, making it impossible to handle the powder. Lowering the heat treatment temperature is effective in preventing aggregation, but in that case, there is a possibility that the reoxidation suppression effect will not be sufficient. Furthermore, while the technique described in Patent Document 1 forms a thin oxide layer on the surface of the ore, adjusting this layer is difficult.

[0016] In addition, in the conventional method of suppressing reoxidation of reduced iron ore powder by briquetting and densifying the reduced iron ore powder, the volume and weight increase due to briquetting, making it impossible to handle using airflow like powder.

[0017] In addition, when we look at iron ore powder, it is expected that the iron ore powder used in the steel industry will become finer and lower in quality. Therefore, high-quality ore containing almost no gangue will be in high demand and will become difficult to obtain, while low-quality ore containing a relatively large amount of gangue will become relatively easy to obtain. Such easily available iron ore powder contains iron oxides such as hematite (Fe ２ O ３However, goethite may be contained in the form of goethite (FeO(OH)) containing water of crystallization rather than the normal form. Goethite decomposes at approximately 400°C and separates into hematite and water. During this process, a large number of fine pores are formed in the hematite. These pores remain in the reduced iron ore powder after reduction, enhancing the reoxidation property of the reduced iron ore powder. Until now, the iron ore powder that has been primarily targeted for reduction has been high-quality hematite-based iron ore powder. The increase in specific surface area of ​​hematite-based iron ore powder during reduction is smaller than that of goethite-based iron ore powder, and no reoxidation problems have been identified. However, goethite-based iron ore powder, which contains water of crystallization and a high gangue content, has a larger specific surface area as described above, and therefore the reduced iron ore powder obtained from this iron ore powder has a high reoxidation property. For example, reduced iron ore powder obtained by reducing goethite-based iron ore powder at a temperature of about 650°C has a large specific surface area, and when exposed to air, it rapidly heats up to a high temperature of 200°C or higher, resulting in a significant decrease in the reduction rate.

[0018] As described above, reduced iron ore fine produced from iron ore fine with a high moisture content is prone to reoxidation. Therefore, in reducing iron ore fine with a high moisture content using a fluidized bed, it is necessary to ensure operational stability and suppress heat generation from the reduced iron ore fine due to reoxidation after reduction.

[0019] The more gangue components there are, the less likely they are to aggregate. ２ and Al ２ O ３ The techniques described in Patent Documents 4 and 5 do not take into consideration the difficulty of agglomerating iron ore powder due to a high moisture content. ２ and Al ２ O ３ This does not take into account the difficulty of agglomerating iron ore powder due to its high content.

[0020] The present invention has been made in consideration of the above-mentioned problems, and an object of the present invention is to provide a method for reducing iron ore powder, which can obtain reduced iron ore powder that is maintained in a powder state and is inhibited from reoxidation even in air.

[0021] The present inventors have discovered that goethite-based iron ore powder containing a large amount of crystal water can be prevented from reoxidizing by subjecting the reduced iron ore powder to a heat treatment to reduce its specific surface area. Furthermore, they have discovered that when goethite-based iron ore powder is reduced in a fluidized bed, the temperature at which aggregation begins is higher than when conventional hematite-based iron ore powder is reduced in a fluidized bed.

[0022] The gist of the present invention, which has been completed based on the above findings, is as follows: [1] A method for reducing iron ore powder according to one aspect of the present invention is to ２ and Al content ２ O ３[2] In the method for reducing iron ore powder described in [1] above, the temperature in the fluidized bed in the fluidized bed reduction step may be set to 730°C or less, and the temperature in the fluidized bed in the fluidized bed heat treatment step may be set to 720°C or more. [3] In the method for reducing iron ore powder according to the above [1] or [2], the iron ore powder may be reduced in the fluidized bed reduction step until a metallization rate reaches 70% or more. [4] In the method for reducing iron ore powder according to any of the above [1] to [3], the reducing gas may contain hydrogen gas. [5] In the method for reducing iron ore powder according to any of the above [1] to [3], the reducing gas may contain 30% by volume or more of hydrogen gas. [6] In the method for reducing iron ore powder according to any of the above [1] to [5], the fluidized bed in the fluidized bed heat treatment step may be a bubbling fluidized bed, and the gas flow velocity of the non-oxidizing gas in the bubbling fluidized bed may be equal to or higher than a minimum fluidization velocity and equal to or lower than a terminal velocity. [7] In the method for reducing iron ore powder according to any of the above [1] to [6], the non-oxidizing gas may be N ２ [8] In the method for reducing iron ore powder according to any one of [1] to [7] above, the same vessel may be used in the fluidized bed reduction step and the fluidized bed heat treatment step.

[0023] According to the present invention, reduced iron ore powder can be obtained that remains in a powder state and is not easily reoxidized even in the atmosphere.

[0024] 1 is a schematic diagram of an apparatus capable of forming a bubbling fluidized bed; FIG. 2 is a schematic diagram of another example of a container in an apparatus capable of forming a bubbling fluidized bed; FIG. 3 is a schematic diagram of an apparatus capable of forming a circulating fluidized bed; FIG. 4 is a schematic diagram of an apparatus capable of forming a spouting fluidized bed; and FIG. 5 is a diagram of a second embodiment of the present invention showing the sieve-sized mass R of reduced iron ore powder with an opening M after a fluidized bed reduction step. Ｍ The sieve mass H of the iron ore powder after the heat treatment process Ｍ The ratio H Ｍ / R Ｍ 3 is a flowchart showing the flow of calculating the specific surface area of ​​reduced iron ore powder at each holding temperature in Example 1. FIG. 4 is a schematic diagram of the configuration of the apparatus used in Examples 2 to 5. FIG. 5 is a graph showing the specific surface area of ​​reduced iron ore powder at each holding temperature in Example 2. FIG. 6 is a graph showing the specific surface area of ​​reduced iron ore powder at each heat treatment temperature in Example 3. FIG. 7 is a graph showing the maximum temperature reached by reduced iron ore powder after being taken out into the atmosphere for each heat treatment time. FIG. 8 is a graph showing the specific surface area of ​​reduced iron ore powder at each heat treatment temperature in Example 4. FIG. 9 is a graph showing the pore size distribution of each reduced iron ore powder in Example 5.

[0025] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. Note that the proportions and dimensions of each component in the drawings do not represent the proportions and dimensions of each actual component.

[0026] <First embodiment> A method for reducing iron ore powder according to a first embodiment of the present invention is ２ and Al content ２ O ３The method includes a fluidized-bed reduction step in which iron ore powder having a total content of 4.0% by mass or more and a water of crystallization content of 5% by mass or more is reduced with a reducing gas using a fluidized bed, and a fluidized-bed heat treatment step in which the iron ore powder at least partially metallized by the fluidized-bed reduction step is heat-treated with a non-oxidizing gas using a fluidized bed. In this embodiment, the temperature in the fluidized bed in the fluidized-bed heat treatment step is higher than the temperature in the fluidized bed in the fluidized-bed reduction step. Furthermore, the gas flow rate of the non-oxidizing gas in the fluidized bed in the fluidized-bed heat treatment step is 1.5 times or more higher than the gas flow rate of the reducing gas in the fluidized bed in the fluidized-bed reduction step, or is 4 times or more the minimum fluidization velocity of the fluidized bed in the fluidized-bed heat treatment step. Note that, hereinafter, when there is no need to distinguish between the iron ore powder at least partially metallized by the fluidized-bed reduction step and the iron ore powder after the fluidized-bed heat treatment step, both may be referred to as reduced iron ore powder. Furthermore, when describing the iron ore powder that forms the fluidized bed, the iron ore powder before reduction and the iron ore powder after reduction (reduced iron ore powder) will be referred to as iron ore powder without distinction.

[0027] (Fluidized Bed Reduction Step) In the fluidized bed reduction step, SiO ２ and Al content ２ O ３ The iron ore powder having a total content of the above-mentioned elements of 4.0% by mass or more and a content of water of crystallization of 5% by mass or more is reduced with a reducing gas using a fluidized bed.

[0028] [Fluidized Bed] In this process, a fluidized bed is used for reducing the iron ore powder. The fluidized bed may be one or more bubbling fluidized beds (BFB), one or more circulating fluidized beds (CFB), one or more spouted fluidized beds, or a combination thereof. A bubbling fluidized bed is a fluidized bed in which gas bubbles are formed in the fluidized bed of iron ore powder. However, the shape of the bubbles varies depending on the fluidization state, and in some cases, clear bubbles are not formed. A circulating fluidized bed is a fluidized bed in which the gas flow rate is increased to circulate the iron ore powder while entraining it with the gas. A spouted fluidized bed has a protrusion section (gas blow-through section) from which iron ore powder is protruded at a high gas flow rate, and a moving bed in which the iron ore powder accumulates around the protrusion section and moves from top to bottom.The iron ore powder is caught up in the high gas flow rate at the bottom of the moving bed and is repeatedly propelled out of the protrusion section.

[0029] [Iron ore powder] The fluidized bed reduction process is ２ and Al content ２ O ３ The test is carried out on iron ore powder having a total content of 4.0% by mass or more and a content of water of crystallization of 5% by mass or more. ２ and Al ２ O ３ is a gangue component, and if the gangue component is too small, aggregation is likely to occur. ２ and Al content ２ O ３ The total content of the SiO may be 6.0% by mass or more, or 10.0% by mass or more. ２ and Al content ２ O ３ The total content of the SiO ２ and Al content ２ O ３ If the total content of SiO is 11.0 mass% or less, the number of components to be separated is not large, and therefore a decrease in productivity is suppressed. ２ and Al content ２ O ３The total content of the water of crystallization is preferably 10.0% by mass or less. On the other hand, if the water of crystallization is 5% by mass or more, the water of crystallization is removed from the iron ore powder by exposure to high temperatures during reduction, forming a large number of fine pores in the reduced iron powder, and increasing the reoxidation tendency of the reduced iron ore powder after reduction. The water of crystallization may be 8% by mass or more. Alternatively, the water of crystallization may be 9% by mass or less. By undergoing this process, a powdered reduced iron ore powder with reduced reoxidation tendency can be obtained from such iron ore powder.

[0030] SiO ２ The content of Al is determined in accordance with JIS M 8214:1995 "Iron ore powder - silicon determination method". ２ O ３ The content of water of crystallization is determined in accordance with JIS M 8220:1995 "Iron ore powder - Aluminum determination method." The content of water of crystallization is determined in accordance with JIS M 8211:2023 "Iron ore powder - Water of combination determination method - Karl Fischer titration method." Unless otherwise specified, the conditions described in the standards are determined based on standard conditions.

[0031] In this process, a portion of the iron ore powder is metallized. The metallization rate required in this process is determined depending on the requirements of the subsequent process, but it is preferable to reduce the iron ore powder until the metallization rate is 70% or more. By carrying out the fluidized bed heat treatment process described below, reoxidation can be suppressed even if the metallization rate is 70% or more. From the viewpoint of reducing the load of reduction in the subsequent process, the metallization rate is more preferably 90% or more. Furthermore, the metallization rate may be 100% or less, 95% or less, or 90% or less.

[0032] The metallization rate of iron ore powder is measured in accordance with ISO 5416:2006 "Sponge iron - Determination of metallic iron - Bromine-methanol titration method." Unless otherwise specified, the metallization rate of iron ore powder or reduced iron ore powder is measured under the conditions described in the standard.

[0033] The median diameter of the iron ore powder is, for example, 20 μm or more and 5 mm or less. For stable operation, the median diameter of the iron ore powder is preferably 20 μm or more. The median diameter of the iron ore powder is more preferably greater than 50 μm. If the median diameter of the iron ore powder is greater than 50 μm, the iron ore powder exhibits better fluidity and can be reduced more stably by the fluidized bed. Therefore, the median diameter of the iron ore powder is more preferably greater than 50 μm. On the other hand, the median diameter of the iron ore powder is preferably less than 5 mm. The median diameter of the iron ore powder is more preferably 1 mm or less. This is because if the median diameter of the iron ore powder becomes too large due to aggregation, stable fluidization of the iron ore powder may be hindered, and in some cases, this may cause partial blockage of the container forming the fluidized bed. Note that even if the iron ore powder contains coarse particles, this is not a problem as long as it is stably fluidized.

[0034] The median diameter of the iron ore powder can be measured by the following method: the volume-based particle diameter d in the undersize cumulative distribution measured using a laser diffraction particle size measuring device (Malvern Panalytical, Mastersizer 3000), which is a wet measuring device. ５０ is the median diameter of the iron ore powder. The measurement conditions were: dispersion medium: water, dispersion medium refractive index: 1.33, particle refractive index: 2.918 (iron oxide Fe ２ O ３ In the case of iron ore powder containing large particles, the powder is sieved using a dry sieve, and the particle diameter of the powder randomly sampled from the raw material fine powder under the sieve is measured three times using the above laser diffraction particle size measuring device, and the d ５０ The average value can be taken as the median diameter.

[0035] [Reducing Gas] The reducing gas is, for example, CH ４ , CO, and H ２ The reducing gas preferably contains hydrogen gas. In the production of reduced iron using a conventional shaft furnace, natural gas, coal, CO gas, etc. are used in the reduction reaction, which can contribute to global warming.２ On the other hand, if hydrogen gas is used as the reducing gas, CO ２ The reducing gas preferably contains 30% by volume or more of hydrogen gas, and more preferably 50% by volume or more of hydrogen gas. ２ From the viewpoint of reduction, a higher content of hydrogen gas is preferable, and the reducing gas preferably contains 100% by volume of hydrogen gas. Furthermore, a part of the reducing gas may contain an inert gas.

[0036] The flow velocity of the reducing gas is equal to or greater than the minimum fluidization velocity of the fluidized bed. If the flow velocity is less than the minimum fluidization velocity, the iron ore powder will not be fluidized. In order to suppress the scattering of the iron ore powder, the flow velocity of the reducing gas is preferably less than the terminal velocity. If the flow velocity of the reducing gas is less than the terminal velocity, the scattering of the iron ore powder is suppressed, and the frequency of contact between the iron ore powder particles can be maintained high. Furthermore, if the fluidized bed is a bubbling fluidized bed, the loss of the iron ore powder and reduced iron ore powder due to scattering can be suppressed.

[0037] The flow rate of the reducing gas (Bm / s) is the superficial velocity (Bm / s) at which a fluidized bed is realized, and the gas flow rate per unit time (Bm ３ / s) is the cross-sectional area of ​​the fluidized bed (m ２ ) The flow rate of the reducing gas can be measured using a flow meter attached to the gas supply pipe. The "B" attached to the unit of velocity or flow rate indicates the velocity or flow rate under actual conditions (temperature, pressure).

[0038] The minimum fluidization velocity is the minimum gas flow rate at which the pressure drop in the fluidized bed becomes constant with increasing gas flow rate. It can be experimentally measured using the following method. Specifically, the pressure drop in the fluidized bed can be calculated by measuring the pressure difference between the upper and lower parts of the bed where iron ore fines are present (the difference between the pressure in the gas reservoir below the distributor plate and the space above the bed, as described below) and subtracting the pressure difference when no iron ore fines are charged (the pressure drop through the distributor plate alone). The pressure drop in the fluidized bed is plotted against the gas superficial velocity to determine the minimum gas flow rate at which the pressure drop becomes constant. However, to eliminate the dependence on the initial particle packing structure and obtain reproducible data, the gas flow rate is gradually decreased from a value sufficient for fluidization, and the minimum fluidization velocity is determined as the point at which the pressure drop begins to decrease from a constant value. In measuring pressure drop, the pressure measurement positions do not necessarily have to be limited to the gas reservoir below the distributor plate and the space above the bed. The pressure may be measured at, for example, the inside of the fluidized bed or the space above the bed, as long as the pressure loss of the fluidized bed can be measured.

[0039] The terminal velocity ut (m / s) of the iron ore powder can be expressed by the following equation (1) when the iron ore powder is approximated to a spherical shape to the first order.

[0040]

[0041] In the above formula (1), g (m / s 2 ) is the gravitational acceleration, ρ p (kg / m 3 ) is the particle density of iron ore powder, ρ f (kg / m 3 ) is the density of the reducing gas, D p (m) is the median diameter of the iron ore powder, C ｄ (-) is the drag coefficient, which is arranged according to the Reynolds number Re, and is expressed as follows using the Brown and Lawler approximation formula (formula (2)):

[0042] C ｄ =(24(1+0.15Re 0.681 ) / Re)+(0.407 / (1+8710Re -1 ) ...Formula (2)

[0043] The temperature in the fluidized bed can be, for example, 500°C or higher and 900°C or lower. If the temperature in the fluidized bed is 500°C or higher and 900°C or lower, the reduction reaction of the iron ore powder is promoted, and productivity is improved. The temperature in the fluidized bed is preferably 730°C or lower from the viewpoint of suppressing excessive progress of aggregation due to adhesion between iron ore powder particles and performing stable operation. Therefore, the temperature in the fluidized bed is preferably 500°C or higher and 730°C or lower. The temperature in the fluidized bed is more preferably 550°C or higher and 680°C or lower.

[0044] The temperature inside the fluidized bed is measured using a thermocouple whose tip is positioned in the fluidized part of the iron ore powder inside the fluidized bed.

[0045] [Average Residence Time] When the fluidized bed is a bubbling fluidized bed, the average residence time of the iron ore powder in the bubbling fluidized bed depends on the temperature of the reduction reaction, but is preferably 3 minutes or more and 180 minutes or less. An average residence time of 3 minutes or more results in reduced iron ore powder with a high reduction rate. On the other hand, an average residence time of 180 minutes or less maintains high processing efficiency. Furthermore, an average residence time of 180 minutes or less suppresses a decrease in the strength of the reduced iron ore powder due to excessive reduction, or pulverization of the reduced iron ore powder due to collisions between the reduced iron ore powder particles or between the reduced iron ore powder and the equipment, thereby suppressing a decrease in the recovery efficiency of the reduced iron ore powder. Therefore, an average residence time of 180 minutes or less is preferable. The average residence time of the iron ore powder in the bubbling fluidized bed 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.

[0046] When the fluidized bed is a circulating fluidized bed, the average residence time of the iron ore powder in the circulating fluidized bed depends on the temperature of the reduction reaction, but is preferably 3 minutes or more and 120 minutes or less. The average residence time of the iron ore powder in the circulating fluidized bed is more preferably 5 minutes or more and 60 minutes or less. The reason why the above average residence time is preferable is the same as when the fluidized bed is a bubbling fluidized bed.

[0047] When the fluidized bed is a spouted fluidized bed, the average residence time of the iron ore powder in the spouted fluidized bed depends on the temperature of the reduction reaction, but is preferably 3 minutes or more and 180 minutes or less. The average residence time of the iron ore powder in the spouted fluidized bed is more preferably 5 minutes or more and 120 minutes or less. The reason why the above average residence time is preferable is the same as when the fluidized bed is a bubbling fluidized bed.

[0048] The average residence time of the iron ore powder can be calculated by the following method. Specifically, a fixed amount of ore powder with the same median diameter but different gangue components is added as tracer particles, and the time change in the gangue component content of the discharged reduced iron powder is examined. The peak time period obtained, during which the gangue component content characteristic of the added tracer ore powder is highest, is defined as the average residence time of the iron ore powder. Using the above method, it is possible to experimentally measure the average residence time.

[0049] When the vessel used in the fluidized bed reduction step and the vessel used in the fluidized bed heat treatment step are different, the reduced iron ore fine after the fluidized bed reduction step is transferred in a non-oxidizing atmosphere.

[0050] SiO in a fluidized bed ２ and Al content ２ O ３ When iron ore powder having a total content of 4.0% by mass or more and a water of crystallization content of 5% by mass or more is reduced, the temperature at which aggregation begins is higher than when hematite-based iron ore powder is used. While the reason for this is not entirely clear, it is believed that iron ore powder with a low iron content and a high gangue content has a smaller proportion of metallic iron particles in contact with each other during reduction than high-grade hematite ore. This allows the heat treatment temperature to be higher than the reduction temperature in the fluidized-bed heat treatment step that follows the fluidized-bed reduction step.

[0051] (Fluidized Bed Heat Treatment Step) In the fluidized bed heat treatment step, the reduced iron ore powder at least partially metallized in the fluidized bed reduction step is heat treated with a non-oxidizing gas using a fluidized bed.

[0052] [Fluidized Bed] In this process, a fluidized bed is used for the heat treatment of the reduced iron ore powder. The fluidized bed may be one or more bubbling fluidized beds, one or more circulating fluidized beds, one or more spouted fluidized beds, or a combination thereof. The fluidized bed used in this process may be formed in the same vessel used in the fluidized bed reduction process. By using the same vessel used in the fluidized bed reduction process in this process, the reduced iron ore powder does not need to be transferred to a separate vessel, allowing for efficient treatment of the reduced iron ore powder. Furthermore, by using the same vessel, the reduced iron ore powder after the fluidized bed reduction process is not exposed to the atmosphere, thereby suppressing oxidation of the reduced iron ore powder. Furthermore, the type of fluidized bed used in the fluidized bed reduction process and the fluidized bed heat treatment process may be the same.

[0053] [Non-oxidizing gas] The non-oxidizing gas is, for example, a gas that does not oxidize the reduced iron ore powder after the fluidized bed reduction step, and is, for example, N ２ , He, Ne, Ar, Kr, Xe, etc. The reduced iron ore powder is heat-treated in a non-oxidizing gas environment, and the specific surface area thereof is reduced.

[0054] The gas flow velocity of the non-oxidizing gas is at least 1.5 times higher than the gas flow velocity of the reducing gas in the fluidized bed in the fluidized bed reduction step, or at least four times the minimum fluidization velocity of the fluidized bed in the fluidized bed heat treatment step. Basically, the latter is used when the gas flow velocity of the reducing gas in the fluidized bed reduction step is high, and the former is used when it is low. When the fluidized bed in the fluidized bed reduction step is a circulating fluidized bed, the latter is used, i.e., the gas flow velocity of the non-oxidizing gas is at least four times the minimum fluidization velocity of the fluidized bed in the fluidized bed heat treatment step. This makes it possible to maintain a good fluidized state of the fluidized bed.

[0055] The flow rate of the non-oxidizing gas is the superficial velocity at which a fluidized bed is formed, and is calculated by dividing the gas flow rate per unit time by the cross-sectional area of ​​the fluidized bed. The flow rate of the non-oxidizing gas can be measured using a flow meter attached to the gas supply pipe.

[0056] The temperature in the fluidized bed in the fluidized bed heat treatment step is higher than the temperature in the fluidized bed in the fluidized bed reduction step. By setting the temperature in the fluidized bed in the fluidized bed heat treatment step higher than the temperature in the fluidized bed in the fluidized bed reduction step, the specific surface area of ​​the iron ore powder can be reduced. The temperature in the fluidized bed in the fluidized bed heat treatment step can be set, for example, at 700°C or higher and 900°C or lower, under conditions higher than the temperature in the fluidized bed in the fluidized bed reduction step. From the viewpoint of reducing the specific surface area in a short period of time, the temperature in the fluidized bed in the fluidized bed heat treatment step is preferably 720°C or higher, more preferably 750°C or higher. Furthermore, in order to further suppress agglomeration of the reduced iron ore powder, the temperature in the fluidized bed in the fluidized bed heat treatment step is 850°C or lower. Therefore, the temperature in the fluidized bed in the fluidized bed heat treatment step is preferably 720°C or higher and 850°C or lower, more preferably 750°C or higher and 850°C or lower.

[0057] The temperature inside the fluidized bed is measured by a thermocouple whose tip is positioned in the fluidized portion of the fluidized bed where the reduced iron ore powder is flowing.

[0058] In this process, the reduced iron ore powder can be heat-treated at a high temperature, which reduces the specific surface area and suppresses reoxidation of the reduced iron ore powder after heat treatment. Furthermore, no reducing gas is used in the fluidized bed heat treatment process. If a reducing gas containing hydrogen gas were used in the fluidized bed heat treatment process, a large amount of hydrogen gas would be required to maintain the fluidized state. On the other hand, in the fluidized bed heat treatment process, only a small amount of hydrogen is consumed in the reduction of the iron ore powder, and the hydrogen generated in the reduction of the iron ore powder is released outside the vessel. ２ A large amount of hydrogen gas containing O is discharged. In order to reuse the discharged hydrogen gas, the temperature of the hydrogen gas must be lowered for dehydration and then raised again. This results in a very large energy load. Therefore, in the fluidized bed heat treatment process, the energy load can be reduced by performing heat treatment using a non-oxidizing gas that does not contribute to the reaction and can be used inexpensively.

[0059] (Apparatus Configuration Example) Here, examples of apparatus configurations to which the iron ore powder reduction method according to this embodiment can be applied will be described with reference to the drawings. Fig. 1 is a schematic diagram of an apparatus capable of forming a bubbling fluidized bed. Fig. 2 is a schematic diagram of an apparatus capable of forming a circulating fluidized bed. Fig. 3 is a schematic diagram of an apparatus capable of forming a spouted fluidized bed.

[0060] [Bubbling Fluidized Bed Forming Apparatus] As shown in FIG. 1, the bubbling fluidized bed forming apparatus 10 can include a container 11 and a dry dust collector 12.

[0061] As shown in FIG. 1 , the vessel 11 has, for example, a gas supply port 111 disposed at the bottom through which a supply gas is supplied into the interior of the vessel 11, an iron ore powder supply port 112 through which iron ore powder or reduced iron ore powder is supplied, and a dispersion plate 113 disposed above the gas supply port 111.

[0062] The bubbling fluidized bed is formed by fluidizing the iron ore powder with gas that is supplied from the gas supply port 111 and rectified through multiple vent holes in the dispersion plate 113. The gas ventilation method and form for forming the bubbling fluidized bed are not limited to a specific form, and include the use of a flat dispersion plate 113 such as a porous plate or a slit plate, as well as a simple nozzle type, a cap type in which a nozzle tip is provided with a cap with various types of blowing holes, and a pipe type in which a grid tube with multiple holes is disposed on the side of the tube, as long as the supply gas can be supplied into the vessel 11 and the iron ore powder can be blown up to form a fluidized bed.

[0063] The reduced iron ore powder after reduction or the reduced iron ore powder after heat treatment is discharged from an openable and closable reduced iron powder outlet (not shown).

[0064] The bubbling fluidized bed forming apparatus may also have a container for forming multiple bubbling fluidized beds therein. Figure 2 is a schematic diagram of another example of a container in an apparatus capable of forming a bubbling fluidized bed. The container 11A may include, for example, an iron ore powder supply port 112A provided on one longitudinal side, an outlet 114 provided on the other longitudinal side, multiple gas supply ports 111A arranged in parallel in the longitudinal direction, a dispersion plate 113A provided above each gas supply port 111A, and a partition plate 115 provided between adjacent gas supply ports 111A. The space between the inner wall of the container 11A and the partition plate 115, as well as the space between adjacent partition plates 115, are the spaces where the fluidized bed is formed. The height of the partition plate 115 is lower than the height of the bubbling fluidized bed. A container 11A configured in this manner can increase the average residence time of the iron ore powder and increase the achieved reduction rate. It goes without saying that the installation positions and number of iron ore powder supply ports, the installation positions and number of outlets, and the installation positions and number of partition plates are not limited to the embodiment shown in Figure 2 and may be changed as appropriate.

[0065] 3 , the circulating fluidized bed forming apparatus 20 includes a riser section 21 which is a container in which iron ore powder forms a fluidized bed, a cyclone 22 connected to an outlet 214 provided at the top of the riser section 21, and a circulation line 23 extending downward from the bottom of the cyclone 22 and connected to the bottom of the riser section 21. If necessary, the circulating fluidized bed forming apparatus 20 can include a dry dust collector 24 connected to the cyclone 22 and collecting pulverized iron ore powder and reduced iron (dust) contained in the off-gas.

[0066] The riser section 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 section 21 is basically the same as the vessel 11 in the bubbling fluidized bed forming apparatus 10.

[0067] The cyclone 22 collects particles scattered along with the exhaust gas. The collected particles are returned to the riser section 21 through a circulation line 23, and the exhaust gas is discharged outside the circulating fluidized bed forming apparatus 20 via a dry dust collector 24.

[0068] The circulation line 23 is connected to the lower part of the cyclone 22 and includes a downcomer 231, which serves as a flow path for the iron ore powder separated from the gas in the cyclone 22, and a loop seal unit 232, one end of which is connected to the lower end of the downcomer 231 and the other end of which is connected above the dispersion plate 213 of the riser section 21. The loop seal unit 232 provides a sealing effect by temporarily storing iron ore powder. The gas ventilation method and form for forming the circulating fluidized bed are not limited to a specific form, as long as it can supply the supply gas into the riser section 21, blow up the iron ore powder, and form a fluidized bed, and may be a flat dispersion plate 113 such as a porous plate or a slit plate, a simple nozzle type, a cap type in which a nozzle tip is provided with a cap with various types of blowing holes, or a pipe type in which a grid tube with multiple holes is disposed on the side of the tube.

[0069] In the circulating fluidized bed forming apparatus 20, the exhaust gas (off-gas) may contain pulverized iron ore powder (dust). Therefore, the dust contained in the off-gas can be collected using a dry dust collector 24. As the dry dust collector 24, for example, a cyclone, a multiclone, a ceramic filter, or the like can be used. In the circulating fluidized bed forming apparatus 20, for example, a cyclone smaller than the cyclone 22 may be provided in series as the dry dust collector 24 after the cyclone 22.

[0070] The iron ore powder or reduced iron ore powder supplied through the iron ore powder supply port 212 is fluidized by the supply gas supplied through the gas supply port 211 and rectified through the multiple vent holes in the dispersion plate 213. Specifically, the iron ore powder or reduced iron ore powder is transported from bottom to top within the riser section 21, passes through the cyclone 22 and the circulation line 23, and circulates within the circulating fluidized bed forming apparatus 20. Therefore, the interior of the circulating fluidized bed forming apparatus 20 forms a circulating fluidized bed. Note that the iron ore powder remains in the loop seal section 232 for a while. When the reduction or heat treatment using the circulating fluidized bed forming apparatus 20 is a batch process, the treated powder is extracted, for example, from an openable / closable outlet (not shown) provided at the bottom of the cyclone 22 (midway along the downcomer 231). When the reduction or heat treatment is a continuous process, for example, a valve at an outlet that can be opened and closed provided in the riser section 21 is opened at regular time intervals or continuously, and the powder after treatment is extracted, and iron ore powder or reduced iron ore powder is replenished through the iron ore powder supply port 212.

[0071] [Spout-Fluidized Bed Forming Apparatus] The spout-fluidized bed forming apparatus 300 basically has the same configuration as the bubbling fluidized bed forming apparatus 10. However, the spout-fluidized bed forming apparatus 300 differs from the bubbling fluidized bed forming apparatus 10 in that it does not uniformly agitate the entire mixture. For example, as shown in FIG. 4 , a protrusion section 311 (gas blow-through section) is formed above the gas supply port 111, from which iron ore powder is protruded at a high gas flow rate, and a moving bed 312 is formed in which the iron ore powder accumulates around the protrusion section and moves from top to bottom. The iron ore powder at the bottom of the moving bed 312 is entrained in the supply gas flowing through the protrusion section 311 at a high gas flow rate, and the iron ore powder is repeatedly protruded from the protrusion section 311. The fluidized bed formed by the protrusion section 311 and the moving bed 312 is the spout-fluidized bed 310.

[0072] The fluidized bed for reducing the iron ore powder and the fluidized bed for heat-treating the iron ore powder may each be a single circulating fluidized bed, a single bubbling fluidized bed, or a single spouted fluidized bed, or may be multiple circulating fluidized beds, multiple bubbling fluidized beds, or multiple spouted fluidized beds, or a combination of one or more circulating fluidized beds, one or more bubbling fluidized beds, and one or more spouted fluidized beds. In a circulating fluidized bed, the difference between the average flow velocity of the supply gas and the average moving velocity of the iron ore powder (slip velocity) is large, so the gas in contact with the iron ore powder is frequently exchanged. During reduction, the surroundings of the iron ore powder approach an equilibrium state, preventing the reduction reaction from stagnating, and the iron ore powder is efficiently reduced. However, because the average moving velocity of the iron ore powder itself is high, mechanical wear and destruction occur due to collisions between the iron ore powder particles, easily generating dust. Compared to a circulating fluidized bed, a bubbling fluidized bed has a smaller difference (slip velocity) between the average flow velocity of the supplied gas and the average movement velocity of the iron ore powder. Therefore, the reduction efficiency of the iron ore powder using a bubbling fluidized bed is inferior to that using a circulating fluidized bed. On the other hand, dust generation tends to be suppressed more than in a circulating fluidized bed, and energy costs for gas supply can be reduced by reducing the gas flow velocity. A spouting fluidized bed can use iron ore powder with a larger particle size than a circulating fluidized bed. Furthermore, a spouting fluidized bed is advantageous when shortening the residence time of the iron ore powder is desired. It is preferable to determine the configuration of the fluidized bed taking into consideration the characteristics of the circulating fluidized bed, bubbling fluidized bed, and spouting fluidized bed, as well as the median diameter and Fe content of the iron ore powder.

[0073] It goes without saying that the devices shown in the drawings are merely examples and are not limited to the embodiments shown in the drawings.

[0074] Second Embodiment A method for reducing iron ore powder according to a second embodiment of the present invention has the following features (I) to (V). (I) The treatment target is SiO ２ and Al content ２ O ３The iron ore powder has a total content of 4.0% by mass or more and a crystal water content of 5% by mass or more. (II) The iron ore powder reduction method according to this embodiment includes a fluidized bed reduction step in which reduction is performed with a reducing gas using a fluidized bed. (III) The iron ore powder reduction method according to this embodiment includes a heat treatment step in which iron ore powder (reduced iron ore powder) that has been at least partially metallized by the fluidized bed reduction step is heat treated in a non-oxidizing atmosphere. (IV) The heat treatment temperature in the heat treatment step is higher than the temperature in the fluidized bed in the fluidized bed reduction step. (V) The sieve-over mass fraction R of the reduced iron ore powder with openings M of 5 mm, 3 mm, 1 mm, 0.5 mm, and 0.25 mm after the fluidized bed reduction step when sieved using sieves with openings M of 5 mm, 3 mm, 1 mm, 0.5 mm, and 0.25 mm in this order. Ｍ and the sieve mass ratio H of the reduced iron ore powder after the heat treatment process. Ｍ The relationship between H and H is expressed as follows: Ｍ / R Ｍ : 1.0 to 1.2, where the oversized mass of the sieve with an opening size M is the total mass of the reduced iron ore powder remaining on the sieve with an opening size M and larger than that. Ｍ The sieve mass ratio H of the reduced iron ore powder after the heat treatment process is 20% of the initial mass at the same sieve size M. Ｍ Looking for H Ｍ / R Ｍ Since the features (I) and (II) are the same as those in the first embodiment, detailed description thereof will be omitted here.

[0075] (Heat Treatment Step) In the heat treatment step, the iron ore powder (reduced iron ore powder) that has been at least partially metallized in the fluidized bed reduction step is heat treated in a non-oxidizing atmosphere.

[0076] [Non-oxidizing atmosphere] The non-oxidizing atmosphere is, for example, a gas that does not oxidize the reduced iron ore powder after the fluidized bed reduction step, and is, for example, an Ar atmosphere or a N ２ The reduced iron ore powder is heat treated in a non-oxidizing atmosphere, and its specific surface area decreases.

[0077] The temperature of the non-oxidizing atmosphere is higher than the temperature in the fluidized bed in the fluidized bed reduction step in order to reduce the specific surface area of ​​the reduced iron ore powder.

[0078] The temperature of the non-oxidizing atmosphere is measured by a thermocouple placed in the atmosphere, and the temperature is measured by bringing the tip of the thermocouple into contact with the layer of reduced iron ore powder.

[0079] In the heat treatment step, the sieve mass ratio R of the reduced iron ore powder after the fluidized bed reduction step is Ｍ The sieve mass ratio H of the reduced iron ore powder after the heat treatment process having an opening M Ｍ The ratio H Ｍ / R Ｍ The reduced iron ore powder after the fluidized bed reduction step is heat treated under the condition that the H Ｍ / R Ｍ If H is 1.2 or less, it can be said that aggregation of the reduced iron ore powder is suppressed and the heat treatment step is carried out stably. Ｍ / R Ｍ If the value of H exceeds 1.2, the reduced iron ore powder after the heat treatment process is agglomerated, and it can be said that a stable heat treatment process has not been carried out. If there is no change in the degree of particle agglomeration before and after the heat treatment, H Ｍ / R Ｍ is 1.0. Therefore, H Ｍ / R Ｍ may be 1.0 or more. In addition, since the particles may decrease due to the balance, H Ｍ / R Ｍ may be 0 or greater.

[0080] Now, referring to FIG. Ｍ / R Ｍ 5 is a graph showing a method for calculating the oversized mass ratio R of the reduced iron ore powder after the fluidized bed reduction step according to the second embodiment of the present invention. Ｍ The sieve mass ratio H of the reduced iron ore powder after the heat treatment process having an opening M relative to Ｍ The ratio H Ｍ / R Ｍ 10 is a flowchart showing the procedure for calculating

[0081] H Ｍ / R ＭThe sieving for calculating the sieving ratio is basically performed according to JIS M 8706:2015, "Iron ore and reduced iron—Method for measuring particle size distribution by sieving." First, sieves with mesh sizes M = 5 mm, 3 mm, 1 mm, 0.5 mm, and 0.25 mm are prepared. Reduced iron ore powder after the fluidized bed reduction step (reduced powder) and reduced iron ore powder after the heat treatment step (heat-treated powder) are sampled, and these are taken as initial masses (100 parts by mass). The initial mass may be determined arbitrarily, for example. The reduced powder and heat-treated powder to be subjected to sieving are dried at 105±5°C until a constant weight is reached.

[0082] The reduced powder (100 parts by mass) and the heat-treated powder (100 parts by mass) are each sieved using a sieve with an opening M of 5 mm (step S1). The mass ratio R of the reduced powder remaining on the sieve relative to the initial mass is ５ｍｍ (Step S2). The over-sieve mass ratio R of the reduced powder is measured. ５ｍｍ is 20 mass% (20 mass parts) or more (step S2 / YES), the sieved mass ratio H of the heat-treated powder to the initial mass ５ｍｍ Measure H ５ｍｍ / R ５ｍｍ is calculated (step S11).

[0083] The sieved mass ratio R of the reduced powder to the initial mass ５ｍｍ If the mass ratio of the reduced powder to the heat-treated powder is less than 20 mass% (step S2 / NO), the reduced powder and the heat-treated powder (the under-sieve reduced powder and the under-sieve heat-treated powder) that passed through the sieve with an opening M of 5 mm are sieved with a sieve with an opening M of 3 mm (step S3), and the mass ratio R of the reduced powder that passed through the sieve with an opening M of 3 mm to the initial mass is calculated. ３ｍｍ (Step S4) Here, the mass ratio R on the sieve with an opening M of 3 mm is measured. ３ｍｍ is the ratio of the total mass of the iron ore powder remaining on the sieves with openings M = 5 mm and 3 mm to the initial mass. In other words, the mass ratio R on the sieve with openings M = 3 mm ３ｍｍ is the ratio of the sieved cumulative mass to the initial mass. ３ｍｍ is 20 mass% or more (step S4 / YES), the sieve-over mass proportion H ５ｍｍ Measure H ３ｍｍ / R ３ｍｍis calculated (step S11).

[0084] Sieved mass ratio R of reduced powder ３ｍｍ If the mass fraction R of the reduced powder passing through the sieve with an opening M of 3 mm and the heat-treated powder passing through the sieve with an opening M of 1 mm is less than 20 mass% (step S4 / NO), the reduced powder and the heat-treated powder passing through the sieve with an opening M of 1 mm are sieved (step S5), and the mass fraction R of the reduced powder passing through the sieve with an opening M of 1 mm is calculated. １ｍｍ (Step S6) The mass fraction R of the reduced powder on a sieve with an opening M of 1 mm is measured. １ｍｍ is 20 mass% or more (step S6 / YES), the mass proportion H １ｍｍ Measure H １ｍｍ / R １ｍｍ is calculated (step S11).

[0085] Sieved mass ratio R of reduced powder １ｍｍ If the mass fraction R of the reduced powder passing through the sieve with an opening M of 1 mm is less than 20 mass% (step S6 / NO), the reduced powder and the heat-treated powder passing through the sieve with an opening M of 1 mm are sieved using a sieve with an opening M of 0.5 mm (step S7), and the mass fraction R of the reduced powder passing through the sieve with an opening M of 0.5 mm is calculated. ０．５ｍｍ (Step S7) The mass fraction R of the reduced powder on a sieve with an opening M of 0.5 mm is measured. ０．５ｍｍ is 20 mass% or more (step S8 / YES), the mass proportion H ０．５ｍｍ Measure H ０．５ｍｍ / R ０．５ｍｍ is calculated (step S11).

[0086] Sieved mass ratio R of reduced powder ０．５ｍｍ If the mass fraction R of the reduced powder passing through the sieve with an opening M of 0.5 mm and the heat-treated powder passing through the sieve with an opening M of 0.25 mm is less than 20 mass% (step S8 / NO), the reduced powder and the heat-treated powder passing through the sieve with an opening M of 0.5 mm are sieved (step S9), and the mass fraction R of the reduced powder passing through the sieve with an opening M of 0.25 mm is calculated. ０．５ｍｍ (Step S10) The mass fraction R of the reduced powder on a sieve with an opening M of 0.25 mm is measured. ０．２５ｍｍ is 20 mass% or more (step S10 / YES), the mass proportion H ０．２５ｍｍ Measure H ０．２５ｍｍ / R ０．２５ｍｍCalculate.

[0087] Sieved mass ratio R of reduced powder ０．２５ｍｍ If the content is less than 20 mass % (step S10 / NO), it is determined that there is no problem with the fluidity.

[0088] The vibration method and the end point of the sieving described above may comply with JIS M 8706:2015.

[0089] The heat treatment step can be carried out using a furnace capable of controlling the temperature and atmosphere, such as a known rotary kiln. When an internally heated rotary kiln is used for the heat treatment step, the non-oxidizing gas is preferably introduced in the opposite direction to the flow direction of the reduced iron ore powder (countercurrent contact) in order to suppress variations in the atmospheric temperature. When an externally heated rotary kiln is used, the direction of introduction of the non-oxidizing gas is not limited.

[0090] In this embodiment, the heat treatment temperature can be set higher than the reduction temperature in the heat treatment step performed after the fluidized bed reduction step, so that the specific surface area can be reduced and reoxidation of the reduced iron ore powder after the heat treatment can be suppressed. Also, in this embodiment, the energy load can be suppressed for the same reasons as in the first embodiment.

[0091] Next, examples of the present invention will be shown, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to the conditions used in the following examples. The present invention can adopt various conditions as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[0092] Example 1 SiO ２ The content is 6.55% by mass, and Al ２ O ３ Iron ore powder having a SiO content of 3.62 mass%, a water of crystallization content of 8.38 mass%, and a median diameter of 97.8 μm was held in an air atmosphere at 600° C., 700° C., or 800° C. for 2 hours. ２ The content was determined in accordance with JIS M 8214:1995 "Iron ore powder - silicon determination method." ２ O ３The content was determined in accordance with JIS M 8220:1995 "Iron ore powder - Aluminum determination method." The content of water of crystallization was determined in accordance with JIS M 8211:2023 "Iron ore powder - Compound water determination method - Karl Fischer titration method." The median diameter of the iron ore powder was measured using a laser diffraction particle size analyzer, with the dispersion medium being water, the dispersion medium refractive index being 1.33, and the particle refractive index being 2.918.

[0093] The specific surface area of ​​the iron ore powder after being kept under the above conditions was measured. The specific surface area was measured in accordance with JIS Z 8830:2013 "Method for measuring the specific surface area of ​​powder (solid) by gas adsorption." Liquid nitrogen was used as the cooling tank, and N was used as the adsorbate gas. ２ He was used as the calibration gas and carrier gas.

[0094] Fig. 6 is a graph showing the specific surface area of ​​the iron ore powder at each holding temperature in Example 1. As shown in Fig. 6, it was found that the higher the holding temperature, the lower the specific surface area.

[0095] Example 2 The above-mentioned iron ore powder was reduced using the apparatus shown in Figure 7. A dispersion plate made of fired glass beads was placed inside a vessel with an inner diameter of 35 mm, and iron ore powder was packed onto the dispersion plate. The thickness of the iron ore powder layer was 35 mm. The tip of a thermocouple was positioned inside the iron ore powder layer. A heating mechanism was installed on the outer periphery of the vessel, allowing the inside of the vessel to be heated. N ２ Gas is supplied into the vessel, and the temperature is raised while fluidizing the iron ore powder. After the fluidized bed reaches a predetermined temperature, H ２ Gas was supplied. ２The gas flow rate was 0.1 Bm / s. The gas flowing through the vessel was discharged from the top and analyzed. The reduction period was 2 hours from the start of the reaction. Using a pressure probe with its tip positioned inside the iron ore powder layer and another pressure probe with its tip positioned above the iron ore powder layer, the pressure in the iron ore powder layer (fluidized bed) and the pressure in the space above the iron ore powder layer during gas flow were continuously measured to determine whether the fluidized bed was stable. When excessive aggregation occurred or gas bypass (channeling) occurred in the area where the iron ore powder segregated, abnormalities such as pressure loss exceeding the weight of the fluidized bed due to clogging of the entire fluidized bed or pressure loss approaching zero due to gas blowing through the fluidized bed and not contributing to fluidization appeared.

[0096] The temperature in the fluidized bed was set to 600°C, 700°C, or 800°C. ２ After the gas supply was stopped, the reduced iron ore powder was cooled to room temperature and taken out into the atmosphere. The specific surface area of ​​the reduced iron ore powder after being held in the atmosphere was measured by the method described above. Furthermore, the reduced iron ore powder was reduced at a temperature of 600°C or 800°C in the fluidized bed, cooled to room temperature, and then taken out into the atmosphere. The temperature immediately after being taken out into the atmosphere was measured.

[0097] FIG. 8 is a graph showing the specific surface area of ​​the reduced iron ore powder at each holding temperature in Example 2. The reduced iron ore powder corresponding to each plot in FIG. 8 was cooled to room temperature and then discharged into the atmosphere. As shown in FIG. 8 , it was found that the specific surface area decreased as the holding temperature increased. Furthermore, the reduced iron ore powder reduced at 600°C, cooled to room temperature, and then discharged into the atmosphere heated up to over 200°C in the atmosphere. On the other hand, the temperature of the reduced iron ore powder reduced at 800°C, cooled to room temperature, and then discharged into the atmosphere remained unchanged from room temperature. Furthermore, the specific surface area of ​​the reduced iron ore powder in Example 2 was smaller than that in Example 1 at all temperatures.

[0098] Example 3 The iron ore powder was reduced and heat-treated using the same apparatus as in Example 2. The temperature in the fluidized bed during reduction was 600°C, and H ２The gas flow rate was set to 0.08 Bm / s. ２ Gas to N ２ The gas was switched to N and heat treatment was carried out. ２ The gas flow rate was 0.08 Bm / s. ２ The gas flow rate was 5.5 times the minimum fluidization rate. The heat treatment temperature was 800°C, and the holding time was 0, 5, 15, 30, 60, or 90 minutes. ２ After the gas supply was stopped, the reduced iron ore powder was cooled to room temperature and taken out into the atmosphere. Under the condition of 0 minute retention time, cooling started immediately after the temperature in the fluidized bed reached 800°C.

[0099] 9 is a graph showing the specific surface area of ​​the reduced iron ore powder at each heat treatment temperature in Example 3. As shown in FIG. 9, it was found that the specific surface area was reduced by heat treatment at a temperature higher than the reduction temperature. It was also found that the specific surface area was further reduced by extending the heat treatment time.

[0100] 10 is a graph showing the maximum temperature of the reduced iron ore powder after being taken out into the air for various heat treatment times. It was also found that the maximum temperature further decreased as the heat treatment time increased. This is thought to be due to a decrease in the oxidation rate caused by a decrease in the specific surface area.

[0101] Example 4 Using the same apparatus as in Example 2, only reduction or reduction and heat treatment of the iron ore powder was carried out. The temperature in the fluidized bed during reduction was 600°C, and H ２ The gas flow rate was set to 0.08 Bm / s. ２ Gas to N ２ The gas was switched to N and heat treatment was carried out. ２ The gas flow rate was 0.08 Bm / s. ２ The gas flow rate was 5.5 times the minimum fluidization rate. The heat treatment temperature was 700°C or 800°C, and the holding time was 2 hours. ２After the gas supply was stopped, the reduced iron ore powder was cooled to room temperature and taken out into the atmosphere. The maximum temperature and specific surface area of ​​each reduced iron ore powder taken out into the atmosphere were measured. The specific surface area was measured for the iron ore powder after it had reached the maximum temperature and then cooled to room temperature.

[0102] FIG. 11 is a graph showing the specific surface area of ​​the reduced iron ore powder at each heat treatment temperature in Example 4, along with the maximum temperature achieved. The white plots represent samples that were not subjected to heat treatment but were reduced at 600°C. As shown in FIG. 11, the specific surface area decreased with increasing heat treatment temperature, and a decrease in the maximum temperature achieved by the reduced iron ore powder was also confirmed. This result suggests that an increase in heat treatment temperature reduces the specific surface area and, accordingly, the maximum temperature achieved.

[0103] Example 5 Using the same apparatus as in Example 2, only reduction or reduction and heat treatment of the iron ore powder was carried out. The temperature in the fluidized bed during reduction was 600°C, and H ２ The gas flow rate was set to 0.08 Bm / s. ２ Gas to N ２ The gas was switched to N and heat treatment was carried out. ２ The gas flow rate was 0.08 Bm / s. ２ The gas flow rate was 5.5 times the minimum fluidization rate. The heat treatment temperature was 800°C, and the holding time was 0, 15, 30, and 60 minutes. ２ After the gas supply was stopped, the reduced iron ore powder was cooled to room temperature. The pore size distribution of the cooled reduced iron ore powder was measured. Under the condition of 0 minute retention time, cooling was started immediately after the temperature in the fluidized bed reached 800°C.

[0104] The pore size distribution was measured as follows. First, for each reduced iron ore powder that had been cooled to room temperature and then exposed to the atmosphere, an adsorption isotherm was obtained from the amount of adsorbate gas adsorbed, which was measured in accordance with JIS Z 8830:2013 "Method for measuring the specific surface area of ​​powders (solids) by gas adsorption." Liquid nitrogen was used in the cooling tank, and N was used as the adsorbate gas. ２He was used as the calibration gas and carrier gas. Next, the pore size distribution was calculated in accordance with the method for determining mesopore size distribution by Barrett, Joyner, and Halenda (BJH method) described in JIS Z 8831-2:2010 "Pore size distribution and pore characteristics of powders (solids) - Part 2: Measurement of mesopores and macropores by gas adsorption."

[0105] Fig. 12 is a graph showing the pore size distribution of each reduced iron ore powder in Example 5. As shown in Fig. 12, as the heat treatment time increased, the distribution peak shifted toward larger diameters, and the number of pores also tended to decrease. Furthermore, unlike hematite-based iron ore powder, the iron ore powder used tends to remain fluidized even at high temperatures. Therefore, it was found that by using a fluidized bed, the specific surface area can be reduced and reoxidation can be suppressed while maintaining the powder state.

[0106] Example 6 The flow rate of the non-oxidizing gas in the fluidized bed in the fluidized bed heat treatment process and the temperature in the fluidized bed in the fluidized bed heat treatment process were changed to evaluate the fluidity in the fluidized bed heat treatment process and the suppression of reoxidation. In this evaluation, the same equipment as in Example 2 was used in the reduction process and the heat treatment process. ２ and Al content ２ O ３ The total content of the iron ore powder having a crystal water content of 4.0 mass% or more and a crystal water content of 5 mass% or more, and the iron ore powder having a median diameter of 100 μm is subjected to a reducing atmosphere (H ２ After being held at 700°C for 2 hours, the reduced iron ore powder was subjected to heat treatment. ２ The gas was switched to heat treatment.

[0107] The evaluation results of fluidity are shown in Table 1. The pressure of the fluidized bed during gas flow and the pressure in the space above the layer of reduced iron ore powder were continuously measured to determine whether the fluidized bed was stable. If the moving average value of the pressure value over one minute was larger than the range of pressure change due to the ejection of reduced iron ore powder, it was evaluated as poor (B), and if the moving average value was smaller than the range of pressure change due to the ejection of reduced iron ore powder, it was evaluated as good (A). The results are shown in Table 1. The pressure change due to the ejection of reduced iron ore powder occurs when N is blown without reduction. ２ The results when fluidized with gas only were referenced. ２ When fluidized with gas only, the pressure change was about 5%.

[0108] Also, N ２ After the gas supply was stopped, the reduced iron ore powder was cooled to room temperature and taken out into the atmosphere. The maximum temperature reached after taking out each reduced iron ore powder was measured. When the maximum temperature reached was 200°C or higher, the evaluation result was C (poor). When the maximum temperature reached was 100°C or higher but less than 200°C, the evaluation result was B (good). When the maximum temperature reached was less than 100°C, the evaluation result was A (very good). The results are shown in Table 2.

[0109]

[0110] As shown in Table 1, when the gas flow velocity of the non-oxidizing gas in the fluidized bed in the fluidized bed heat treatment process was four times or more the minimum fluidization velocity, the evaluation results were good when the temperature in the fluidized bed in the heat treatment process was 650°C or higher and 800°C or lower.

[0111]

[0112] As shown in Table 2, when the temperature in the fluidized bed in the fluidized bed heat treatment process was 700°C, the evaluation results were good, and when the temperature in the fluidized bed in the fluidized bed heat treatment process was 750°C or higher and 800°C or lower, the evaluation results were extremely good.

[0113] Example 7 SiO ２ and Al content ２ O ３The total content of the iron ore powder is less than 4.0 mass %, the content of water of crystallization is 5 mass % or more, and the median diameter is 100 μm. ２ ), and maintained for 2 hours at the temperature shown in Table 3. The gas flow rate in the fluidized bed reduction step was as shown in Table 3. The fluidity in the fluidized bed reduction step was evaluated in Table 3. The results are shown in Table 3.

[0114]

[0115] As shown in Table 3, SiO ２ and Al content ２ O ３ When the total content of the iron ore powders is less than 4.0% by mass, excessive aggregation due to adhesion between the iron ore powder particles is suppressed even under conditions of a low gas flow rate when the temperature of the fluidized bed is 700°C. However, it was found that there are conditions where the fluidity is poor at temperatures of 750°C or higher.

[0116] Example 8 SiO ２ and Al content ２ O ３ The total content of the iron ore powder is less than 4.0 mass %, the content of water of crystallization is 5 mass % or more, and the median diameter is 100 μm. ２ After being held at 700°C for 2 hours, the reduced iron ore was subjected to heat treatment. ２ The heat treatment was carried out by switching to a non-oxidizing gas. The fluidity in the fluidized bed heat treatment step was evaluated under the conditions shown in Table 4 for the gas flow rate of the non-oxidizing gas in the fluidized bed and the temperature in the fluidized bed in the fluidized bed heat treatment step. In this evaluation, the same equipment as in Example 2 was used for the reduction step and the heat treatment step. The results are shown in Table 4.

[0117]

[0118] As shown in Table 4, SiO ２ and Al content ２ O ３ When the total content of the contents of the above is less than 4.0 mass%, the evaluation result may be poor even if the gas flow velocity of the non-oxidizing gas in the fluidized bed in the fluidized bed heat treatment step is four times or more the minimum fluidization velocity.

[0119] As described above, according to the present invention, reduced iron ore powder can be obtained using a fluidized bed, while maintaining its powder state and suppressing reoxidation even in air. The obtained reduced iron ore powder can be melted and refined in an electric furnace, similar to scrap or ordinary direct reduced iron (DRI) products, and used as an iron source for crude steel production. Furthermore, when a process is designed to achieve a low reduction degree, the reduced iron ore powder can be used as an iron source to be charged into a blast furnace as semi-reduced iron powder, which reduces the reducing material usage rate in the blast furnace.

[0120] DESCRIPTION OF SYMBOLS 10 Bubbling fluidized bed forming device 11, 11A Container 12 Dry dust collector 111, 111A Gas supply port 112, 112A Iron ore powder supply port 113, 113A Dispersion plate 114 Outlet 115 Partition plate 20 Circulating fluidized bed forming device 21 Riser section 22 Cyclone 23 Circulation line 24 Dry dust collector 211 Gas supply port 212 Iron ore powder supply port 213 Dispersion plate 214 Outlet 231 Downcomer 232 Loop seal section 30 Spout fluidized bed forming device

Claims

1. SiO ２ and Al content ２ O ３ 1. A method for reducing iron ore powder, comprising: a fluidized bed reduction step of reducing iron ore powder having a total content of 4.0 mass% or more and a content of water of crystallization of 5 mass% or more with a reducing gas using a fluidized bed; and a fluidized bed heat treatment step of heat treating the iron ore powder at least partially metallized by the fluidized bed reduction step with a non-oxidizing gas using a fluidized bed, wherein the temperature in the fluidized bed in the fluidized bed heat treatment step is higher than the temperature in the fluidized bed in the fluidized bed reduction step, and the gas flow velocity of the non-oxidizing gas in the fluidized bed in the fluidized bed heat treatment step is 1.5 times or more higher than the gas flow velocity of the reducing gas in the fluidized bed in the fluidized bed reduction step, or is 4 times or more the minimum fluidization velocity of the fluidized bed in the fluidized bed heat treatment step.

2. The method for reducing iron ore powder according to claim 1, wherein the temperature in the fluidized bed in the fluidized bed reduction step is 730°C or lower, and the temperature in the fluidized bed in the fluidized bed heat treatment step is 720°C or higher.

3. A method for reducing iron ore powder according to claim 1 or 2, wherein in the fluidized bed reduction step, the iron ore powder is reduced until a metallization rate reaches 70% or more.

4. The method for reducing iron ore fines according to claim 1 or 2, wherein the reducing gas comprises hydrogen gas.

5. The method for reducing iron ore fines according to claim 1 or 2, wherein the reducing gas contains 30 volume % or more of hydrogen gas.

6. A method for reducing iron ore powder as described in claim 1 or 2, wherein the fluidized bed in the fluidized bed heat treatment step is a bubbling fluidized bed, and the gas flow velocity of the non-oxidizing gas in the bubbling fluidized bed is not less than the minimum fluidization velocity and not more than the terminal velocity.

7. The non-oxidizing gas is N ２ 3. The method for reducing iron ore fine according to claim 1 or 2, wherein the gas is gas or Ar gas.

8. The method for reducing iron ore fines according to claim 1 or 2, wherein the same fluidized bed is used in the fluidized bed reduction step and the fluidized bed heat treatment step.

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