Method for producing reduced iron

Abstract

This method for producing reduced iron uses a fluidized bed and produces reduced iron by bringing a reducing gas into contact with a raw material powder containing iron oxide. The method includes a fluidized bed reduction step in which the raw material powder, which has a crystallization water content of 3.5 mass% or less, is reduced at a temperature of 400°C to 590°C.

Description

Reduced iron manufacturing method

[0001] The present invention relates to a method for producing reduced iron. This application claims priority based on Japanese Patent Application No. 2024-036243 filed on March 8, 2024, and Japanese Patent Application No. 2024-053725 filed on March 28, 2024, 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. In the method using a shaft furnace, raw materials are agglomerated into pellets or the like. In the method using a fluidized bed, solid particles are blown up with a fluid such as gas to create a suspended state, and the large contact area is utilized to efficiently promote chemical reactions, heat exchange, and the like. When a fluidized bed is used in the ironmaking process for obtaining reduced iron from powdered iron ore, the costs and CO2 associated with agglomeration are reduced compared to the method using a shaft furnace. ２ Furthermore, these direct reduction methods can use hydrogen as a reducing agent. By using hydrogen, CO ２ This will enable emissions to be reduced.

[0003] When reducing 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"), it is important to avoid a phenomenon of agglomeration of the iron ore powder within the fluidized bed, known as "sticking," and stagnation of the reduction reaction.

[0004] If sticking occurs, the iron ore fines will not flow uniformly, causing blockages inside the treatment vessel, which is an operational problem. Therefore, it is important to operate the treatment vessel under efficient conditions that allow the iron ore fines to be reduced to the desired reduction degree while avoiding sticking.

[0005] Typically, when processing powder in a fluidized bed, a gas flow rate is set according to the specific gravity and particle size of the powder, and a fluidized bed is created in the processing vessel at a flow rate equal to or greater than the minimum gas flow rate (minimum fluidization velocity) required for fluidizing the powder. In fluidized bed reduction techniques for obtaining reduced iron, processing is performed at as high a temperature as possible to maximize the reduction rate within a range considered to achieve stable fluidization without sticking. However, it is known that sticking is more likely to occur at higher temperatures and with a higher iron concentration in the raw iron ore powder. Therefore, in conventional processes using hydrogen gas, in order to avoid sticking and increase productivity, operation is often performed at a temperature range of approximately 600 to 750°C. For example, the technique described in Non-Patent Document 1 operates at a temperature of 630 to 650°C. This is because, in the case of iron ore for direct reduced iron production, which contains approximately 64% by mass or more of iron, sticking often occurs at temperatures above approximately 750°C.

[0006] Regarding the stagnation of the reduction reaction of iron ore powder (hereinafter, the stagnation of the reduction reaction may be referred to as reduction stagnation), it is known that when metallic iron is generated on the surface layer of the iron ore powder during the reduction process, the progress of the reduction reaction may stagnate depending on the form of the metallic iron that covers the iron ore powder, and the reduction rate may become more difficult to improve as the reduction progresses.

[0007] For example, Non-Patent Document 2 describes that when a magnetite reagent is used, a significant reduction stagnation phenomenon occurs when dense iron is produced from a dense magnetite phase. Non-Patent Document 2 also describes that, as a method for avoiding the production of dense iron, it is effective to perform an oxidation treatment in advance to convert the magnetite phase into a hematite phase, and to refine the particle structure as the phase changes from the hematite phase. However, unlike reagents, the raw iron ore used in actual operations varies in chemical composition, particle size distribution, properties, etc. depending on the production area and mine. Therefore, in actual operation process design, it is necessary to consider different measures depending on the type of ore.

[0008] Furthermore, Non-Patent Document 3 discloses a study on the reduction rate using actual iron ore. Non-Patent Document 3 considers that the reduced iron produced on the periphery of the iron ore sinters at a temperature of about 700°C, which reduces the permeability of water vapor generated during reduction, making the unreduced material inside the iron ore difficult to reduce.

[0009] The reduction described above is one of the major factors that reduces productivity even in the process using a fluidized bed.

[0010] For the purpose of suppressing stagnation of reduction in a fluidized bed, for example, Patent Document 1 discloses a technology in which reduction conditions in a preheated fluidized bed, which is the first stage of a multi-stage fluidized bed, are adjusted to suppress the generation of dense, hard-to-reduc magnetite from hematite in the preheated fluidized bed, thereby increasing the metallization rate of the final reduced product.

[0011] Furthermore, Patent Document 2 describes a technology in which, in order to suppress the generation of hardly reducible magnetite in the first fluidized bed in a multi-stage fluidized bed reduction process, the residence time at about 400 to 580°C is set as short as possible, thereby suppressing the generation of a high-density magnetite phase.

[0012] Japan Special Table No. 2005-502790 Japanese Special Table No. 2001-515144

[0013] Steven A. Elmquist, Peter Weber, and Heinz Eichberger, “Operational results of the Circored fine ore direct reduction plant in Trinidad”, Stahl unt eisen 122, (2002), 2, p.59-64. THOMAS WOLFINGER, DANIEL SPREITZER, HENG ZHENG, and JOHANNES SCHENK, “Influence of a Prior Oxidation on the Reduction Behavior of Magnetite Iron Ore Ultra-Fines Using Hydrogen”, Metall Mater. B, 53B(2022) p.14-28, JB Hendarson, “A Study of the Rate and Mechanism of the Hydrogen Reduction of Fine Hematite Ores”, AIME Physical Chemistry of Process Metallurgy (1959) p.671-688

[0014] As described above, conventional reduction of iron ore powder using a fluidized bed is carried out in the range of 600 to 750°C. To achieve this temperature, not only is the reaction vessel maintained at the above temperature, but also preheating of the raw iron ore to about 850 to 900°C and high-temperature treatment of the reducing gas to about 700 to 900°C are carried out. This necessitates the selection of furnace materials that can withstand these temperatures, the installation of a cooling mechanism for the mechanical drive units, and the prevention and replacement of refractory deterioration due to the temperature difference between rising and falling temperatures, all of which contribute to increased equipment costs.

[0015] From the viewpoint of preventing sticking and reducing equipment costs, a lower reduction temperature is preferable. However, since the reduction rate of iron increases with increasing temperature, a temperature range of about 600 to 750°C has been selected in the prior art, at which sticking does not occur.

[0016] Furthermore, Non-Patent Document 3 does not point out that the tendency of reduction stagnation itself varies depending on the ore type. According to the knowledge of the present inventors, the occurrence and degree of reduction stagnation in actual iron ore fines varies depending on the ore type. Therefore, for various ore types and their differences in specific components and properties, the tendency of reduction stagnation cannot be determined until reduction is actually performed, making process design difficult. Furthermore, in actual plant operation, it is conceivable that a plurality of ore types may be mixed and used. In this case, when ore types with different characteristics are mixed, it is difficult to design a process in terms of the reduction behavior of the ore fines after mixing.

[0017] The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for producing reduced iron that can efficiently obtain reduced iron with a high reduction rate while avoiding sticking and stagnation of the reduction reaction in a process for reducing iron ore powder using a fluidized bed, and that can reduce equipment costs.

[0018] As described above, the reduction rate increases with increasing temperature. Therefore, it is common to perform reduction at as high a temperature as possible within the range where sticking, which is essential in fluidized bed reduction processes, does not occur. Furthermore, it is believed that the reduction reaction significantly slows down at temperatures below 590°C, and reduction treatment has generally been performed at temperatures between 600 and 800°C. However, the present inventors have discovered that for iron ore powder with a low content of combined water (CW), reduction stagnation significantly occurs in the range of 600 to 800°C. The present inventors have also found that this reduction stagnation in the range of 600 to 800°C when the content of combined water is low also occurs in iron ore powder containing a mixture of multiple ore species. Furthermore, through extensive research, the present inventors have found that reducing iron ore powder with a low content of combined water of crystallization at a lower temperature than conventionally used, i.e., 590°C or below, avoids the tendency for reduction stagnation and achieves a higher reduction rate in a shorter time than conventional reduction at 600 to 800°C.

[0019] The gist of the present invention, which was completed based on the above findings, is as follows. [1] A method for producing reduced iron according to one aspect of the present invention is a method for producing reduced iron by bringing a reducing gas into contact with a raw material powder containing iron oxide using a fluidized bed, and includes a fluidized bed reduction step of reducing the raw material powder, which has a crystallization water content of 3.5 mass% or less, at a temperature of 400°C or higher and 590°C or lower. [2] In the method for producing reduced iron according to [1] above, the raw material powder may not have been subjected to a crystallization water removal step by heat treatment at 105°C or higher. [3] In the method for producing reduced iron according to [1] or [2] above, the raw material powder may have a crystallization water content of 3.0 mass% or lower. [4] In the method for producing reduced iron according to [1] or [2] above, the raw material powder may have a crystallization water content of 1.0 mass% or lower. [5] In the method for producing reduced iron according to any one of [1] to [4] above, the raw material powder may be a mixed raw material powder comprising a mixture of a plurality of different iron ore powders containing iron oxide, and at least one of the plurality of different iron ore powders may have a crystallization water content of 5.0 mass% or more. [6] In the method for producing reduced iron according to any one of [1] to [5] above, the temperature may be 400°C or higher and 560°C or lower. [7] In the method for producing reduced iron according to any one of [1] to [5] above, the temperature may be 500°C or higher and 530°C or lower. [8] In the method for producing reduced iron according to any one of [1] to [7] above, the reducing gas may be hydrogen gas. [9] In the method for producing reduced iron according to any one of [1] to [8] above, the raw material powder may be reduced in the fluidized bed reduction step until a metallization rate reaches 70% or higher.

[10] The method for producing reduced iron according to any one of [1] to [9] above may include an oxidation suppression step of suppressing oxidation of the reduced iron after the fluidized bed reduction step.

[11] In the method for producing reduced iron according to the above item

[10] , the oxidation suppression step may include a step of reducing a surface area of ​​the reduced iron after the fluidized-bed reduction step before the raw material powder after the fluidized-bed reduction step is taken out into an oxidizing atmosphere.

[12] In the method for producing reduced iron according to the above item

[11] , the step of reducing the surface area of ​​the reduced iron may include a heat treatment of the reduced iron at a temperature of 720°C or higher in an inert atmosphere.

[13] In the method for producing reduced iron according to

[10] , the oxidation suppression step may be a heat treatment step of heat treating the reduced iron after the fluidized-bed reduction step with a non-oxidizing gas.

[14] In the method for producing reduced iron according to

[13] , the heat treatment step may be a fluidized-bed heat treatment step of heat treating the reduced iron after the fluidized-bed reduction step with a non-oxidizing gas using a fluidized bed that is the same as or different from the fluidized bed used in the fluidized-bed reduction step, 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 may be 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 4 times or more the minimum fluidization velocity of the fluidized bed in the fluidized-bed reduction step.

[15] In the method for producing reduced iron according to

[14] , the temperature in the fluidized bed in the fluidized-bed heat treatment step may be 720°C or higher.

[16] In the method for producing reduced iron according to the above

[14] or

[15] , the fluidized bed in the fluidized bed heat treatment step may be a bubbling fluidized bed.

[17] In the method for producing reduced iron according to any one of the above

[13] to

[16] , the non-oxidizing gas may be N. ２

[18] In the method for producing reduced iron according to any one of the above

[14] to

[16] , the same fluidized bed may be used in the fluidized bed reduction step and the fluidized bed heat treatment step.

[0020] According to the present invention, in a process for reducing iron ore powder using a fluidized bed, it is possible to efficiently obtain reduced iron with a high reduction rate while avoiding sticking and stagnation of the reduction reaction, and to reduce equipment costs.

[0021] FIG. 1 is a schematic diagram of a bubbling fluidized bed forming device. FIG. 2 is a schematic diagram of another example of a bubbling fluidized bed forming device. FIG. 3 is a schematic diagram of a circulating fluidized bed forming device. FIG. 4 is a schematic diagram of a spouting fluidized bed forming device. FIG. 5 is a schematic configuration diagram showing an example of reduced iron production equipment equipped with one circulating fluidized bed forming device and three bubbling fluidized bed forming devices as reduction devices. FIG. 6 is a graph showing the transition of the reduction rate during hydrogen reduction of each raw material powder in Example 1. FIG. 7 is a graph (A) showing the transition of the reduction rate and graph (B) showing the transition of the reduction rate during hydrogen reduction of raw material powder B in Example 1. FIG. 8 is a graph showing the transition of the reduction rate during hydrogen reduction of each raw material powder in Example 3. FIG. 9 is a graph showing the transition of the reduction rate during hydrogen reduction of each mixed powder in Example 3.

[0022] 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.

[0023] <First Embodiment> A method for producing reduced iron according to an embodiment of the present invention is a method for producing reduced iron by bringing a reducing gas into contact with a raw material powder containing iron oxide using a fluidized bed, and includes a fluidized bed reduction step of reducing raw material powder having a crystal water content of 3.5 mass % or less at a temperature of 400°C or higher and 590°C or lower.

[0024] (Fluidized Bed Reduction Step) In the fluidized bed reduction step, the raw material powder having a crystal water content of 3.5 mass % or less is reduced at a temperature of 400°C or higher and 590°C or lower.

[0025] [Fluidized Bed] In this process, a fluidized bed is used for the reduction of the raw material 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 raw material powder. However, the shape of the bubbles varies depending on the fluidization state, and in some fluidization states, clear bubbles may not be formed. A circulating fluidized bed is a fluidized bed in which the gas flow rate is increased to circulate the raw material powder while entraining it with the gas. A spouted fluidized bed has a protrusion section (gas blow-through section) from which the raw material powder is protruded at a high gas flow rate, and a moving bed in which the raw material powder accumulates around the protrusion section and moves from the top to the bottom, repeating the behavior of being caught up in the high gas flow rate at the bottom of the moving bed and being protruded from the protrusion section. Each fluidized bed is described in detail below.

[0026] [Raw Material Powder] The raw material powder is a powder containing iron. The raw material powder may be a powder obtained by mixing multiple different iron ore powders containing iron oxide (hereinafter, this may be referred to as a mixed raw material powder or a mixed powder). When reducing a raw material powder containing a mixture of iron ore powders with different compositions and properties, it is possible to estimate, from the average crystallization water content of the mixed raw material powder, whether reduction at a low temperature is advantageous depending on the crystallization water content under various mixing conditions. The raw material powder may be, for example, iron ore powder mined from an iron ore mine, iron ore powder obtained by crushing and sieving such iron ore powder, or powder discharged and recovered from various iron-making processes. Specific examples of raw material powder include sinter feed, which has traditionally been used as a raw material for sintered ore, pellet feed, which is used as a raw material for pellets, magnetite concentrate, and iron-making dust. The term "dust" as used herein includes converter dust generated in converters, as well as fine particles of partially unreduced ore and fine particles of reduced iron that may be generated in direct reduced iron manufacturing equipment, such as fluidized bed reducers and shaft furnace reducers. The mixed powder is obtained by mixing iron ore powders obtained from different supply sources. In particular, in ironmaking processes, such as the blast furnace process, it is important to adjust the components of the raw materials as much as possible to obtain the desired steel products. Therefore, it is preferable to use a mixed powder that combines iron ore powders from different mountain regions and brands. Raw material powders used for iron production generally contain approximately 50 to 70% by mass of Fe. From the perspective of reducing the energy required for the process of separating impurities and suppressing the ore consumption rate, it is preferable to use raw material powders containing 55 to 68% by mass of Fe. Therefore, it is preferable to use a mixed powder that contains 55 to 68% by mass of Fe.

[0027] The crystal water content of the raw material powder is 3.5% by mass or less. Raw material powder with a low crystal water content has a dense mineral structure of particles constituting the raw material powder. When reduced at a certain temperature (approximately 600°C) or higher, metallic iron formed on the surface of the mixed powder in a dense state covers the mixed powder in a two-dimensional film shape. This makes reduction stagnation more likely to occur. When the crystal water content of the raw material powder is 3.5% by mass or less, reduction stagnation in a low temperature range of 590°C or less can be suppressed, making it possible to efficiently obtain reduced iron with a high reduction rate. The crystal water content is preferably 3.0% by mass or less, and more preferably 1.0% by mass or less. The lower limit of the crystal water content is not particularly limited, and the crystal water content may be below the detection limit. The raw material powder used in this embodiment preferably has a crystal water content of 3.5% by mass or less without being subjected to a heat treatment such as calcination at temperatures above 105°C, except for a drying treatment performed to remove adhered moisture. Examples of processes in which the heating temperature exceeds 105°C include an ore preheating process, a granulation process, and a pre-oxidation process. The reason for this is that in the case of ores containing a large amount of crystallization water, the crystallization water is released during the temperature increase process before the reduction process, causing cracks and pores to form in the particles that make up the raw material powder, eliminating the tendency for dense metallic iron to form on the surface layers of the particles and making it unnecessary to take special measures to avoid reduction stagnation. The drying process is a process that is carried out in a dry air environment or an inert gas atmosphere at a temperature of 200°C or less, for example, in the range of 100 to 105°C, and mainly removes attached moisture.

[0028] When the raw material powder is a mixed powder of multiple different iron ore powders, it is preferable that the crystal water content of at least one of the iron ore powders is 5.0 mass% or more. Iron ore powder with a high crystal water content is not suitable for low-temperature reduction by itself, but the inventors have found that by using it in combination with iron ore powder with a low crystal water content, it is possible to effectively apply a reduction process in a low temperature range. Even if the crystal water content of at least one iron ore powder is 5.0 mass% or more, as long as the crystal water content of the mixed powder is 3.5 mass% or less, reduction stagnation can be suppressed in a low temperature range of 590°C or less, and reduced iron with a high reduction rate can be efficiently obtained.

[0029] The content of water of crystallization is determined in accordance with JIS M 8211:2023 "Iron ore powder - Method for determining water of synthesis - Karl Fischer titration method." That is, it can be determined as the weight ratio of water generated while heating iron ore powder from 105°C to 950°C to the weight of iron ore powder.

[0030] If the median diameter of the raw material powder is less than 50 μm, it will exhibit poor fluidity in treatment in a fluidized bed, so the median diameter of the raw material powder is 50 μm or more. On the other hand, if the median diameter of the raw material powder is too large, the gas flow rate required for fluidization will become too high, exceeding the capacity of the equipment that can be realistically installed, so the median diameter of the raw material powder is desirably 8 mm or less. Therefore, the median diameter of the raw material powder is preferably 50 μm or more and 8 mm or less. A particle size of 8 mm or less is within the particle size range of iron ore powder used as a typical sinter feed. In the fluidized bed, reduction using gas is performed. Therefore, in order to increase reactivity with the reducing gas and achieve an efficient reduction treatment, the median diameter of the raw material powder is preferably a small particle size within the above range that results in a relatively large surface area. The median diameter of the mixed powder is more preferably 50 μm or more and 5 mm or less, and even more preferably 50 μm or more and 2 mm or less.

[0031] The median diameter of the raw material powder can be measured by the following method. That is, the volume-based particle diameter d in the under-sieve cumulative distribution measured using a dry sieve and a laser diffraction particle size measuring device (Malvern Panalytical, Mastersizer 3000), which is a wet measuring device. ５０ The setting conditions for measurement in the laser diffraction particle size measuring device were: dispersion medium: water, dispersion medium refractive index: 1.33, particle refractive index: 2.918 (iron oxide Fe ２ O ３(Refractive index of refraction). Specifically, the raw material powder is first sieved using dry sieves with nominal mesh sizes of 8.0 mm and 1.0 mm, and the mixed powder on the 8.0 mm sieve is removed. The mixed powder on the 1.0 mm sieve is sieved using a plurality of sieves with different mesh sizes, for example, 6.7 mm, 5.6 mm, 4.75 mm, 4.0 mm, 3.35 mm, 2.8 mm, 2.36 mm, 2.0 mm, 1.7 mm, 1.4 mm, and 1.18 mm, and the particle size distribution is measured. For the raw material powder that falls below the 1.0 mm sieve, raw material powder is sampled from any five locations among the sample, and the particle size distribution is measured for each location using the laser diffraction particle size measuring device described above. The particle size distribution is then calculated by averaging the measurement results from the five locations. Next, the average particle size distribution of particles over 1.0 mm measured by sieving and the particle size distribution of particles 1.0 mm or less measured by a laser diffraction particle size analyzer are combined to determine the overall particle size distribution of the raw material powder that sieves under the 8.0 mm sieve. Finally, using this overall particle size distribution, the particle size distribution is integrated from the smaller particle size distribution to determine the particle diameter d at which the integrated value of the relative particle amount on a volume basis becomes 50%. ５０ is the median diameter.

[0032] The method for mixing the plurality of raw material powders is not particularly limited. For example, a plurality of feeders may be provided in front of a mixed powder supply port that supplies the mixed powder to a container forming a fluidized bed therein, and different raw material powders may be supplied to each feeder, and these raw material powders may be mixed inside the container. Alternatively, a mixing container may be provided between each feeder and the container, and the raw material powders may be mixed in the mixing container. Alternatively, a mixed powder obtained by mixing the raw material powders in advance may be supplied to the container from the feeder through the mixed powder supply port.

[0033] The crystal water content and median diameter of the raw material powder may be quantified for each lot of the raw material powder. The crystal water content and median diameter of the mixed powder may be quantified for the mixed powder, or may be quantified for each raw material powder before mixing, and calculated based on each quantified value and the mixing ratio. Each raw material powder is divided into 100 g portions using a dividing machine capable of dividing the charged powder into equal portions, and then 2 g portions are removed using the cone-quartering method. The quantification is performed using powder samples taken at random three times.

[0034] [Reducing Gas] The reducing gas may be any reducing gas. Examples of the reducing gas include hydrogen gas, a mixed gas of hydrogen and nitrogen, a mixed gas of hydrogen and Ar, a mixed gas of hydrogen and water vapor, a mixed gas of hydrogen, water vapor, and nitrogen, CO gas, and synthesis gas (a mixed gas of carbon monoxide and hydrogen). The reducing gas may be CH ４ The reducing gas may contain an inert gas. The raw material powder is reduced by the reducing gas. The reducing gas is preferably a gas containing hydrogen gas, more preferably hydrogen gas. Hydrogen gas has a higher reduction rate than CO gas and does not generate carbon dioxide, which is generated when CO gas or synthetic gas is used, so it has a small environmental impact. The reducing gas preferably contains 30% by volume or more of hydrogen gas, more preferably 50% by volume or more of hydrogen gas. CO ２ From the viewpoint of reduction, a higher content of hydrogen gas is preferable, and the reducing gas preferably contains 70% by volume or more of hydrogen gas.

[0035] The flow velocity of the reducing gas is equal to or greater than the minimum fluidization velocity of the fluidized bed. The minimum fluidization velocity is the minimum gas flow velocity at which the pressure loss in the fluidized bed becomes constant with increasing gas flow velocity, and iron ore powder will not fluidize below the minimum fluidization velocity. When a circulating fluidized bed is used as the fluidized bed, there is no particular upper limit to the gas flow velocity, but from the viewpoints of reducing power energy and maintaining a high gas utilization rate, it is preferable to practically suppress the maximum gas flow velocity. The flow velocity of the reducing gas is, for example, 0.02 m / s or more and 20 m / s or less. The flow velocity of the reducing gas is preferably 0.03 m / s or more and 10 m / s or less. From the viewpoint of stably fluidizing raw material powder with a large particle size, the flow velocity of the reducing gas is preferably about 1.2 times or more the minimum fluidization velocity of the raw material powder.

[0036] When the fluidized bed is a bubbling fluidized bed, the flow velocity of the reducing gas is preferably 0.2 m / s or more and less than 1.0 m / s. The flow velocity of the reducing gas for forming the bubbling fluidized bed is lower than the flow velocity of the reducing gas for forming the circulating fluidized bed, and very little raw material powder escapes from the bubbling fluidized bed. The flow velocity of the reducing gas for forming the bubbling fluidized bed is more preferably 0.3 m / s or more and 0.8 m / s or less.

[0037] When the fluidized bed is a circulating fluidized bed, the flow velocity of the reducing gas is, for example, 1.0 m / s or more and 20 m / s or less. From the viewpoint of improving the reaction efficiency with the gas, which is one of the advantages of a circulating fluidized bed, the flow velocity of the reducing gas is preferably set so that the difference between the average gas velocity and the average velocity of the particles (slip velocity) becomes large. As an example of the raw material powder, in view of the specific gravity and particle size range of general iron ore, the flow velocity of the reducing gas is preferably 3.0 m / s or more and 10 m / s or less.

[0038] The flow rate of the reducing gas is the superficial velocity at which a fluidized bed is formed, and is the value obtained by dividing the gas flow rate per unit time by the cross-sectional area of ​​the fluidized bed. The flow rate of the reducing gas can be measured with a flow meter attached to the gas supply pipe.

[0039] The minimum fluidization velocity can be experimentally measured using the following method. For example, the pressure drop in a fluidized bed can be determined by measuring the pressure difference between above and below the bed containing the raw material powder (the difference in pressure between the gas reservoir below the distributor plate and the void space above the bed, as described below), and subtracting the pressure difference when no raw material powder is 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 dependency 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 the pressure drop, the pressure measurement positions are not limited to the gas reservoir below the distributor plate and the void space above the bed. The pressure measurement positions may be, for example, the inside of the fluidized bed and the void space above the bed, as long as they allow measurement of the pressure drop in the fluidized bed.

[0040] The terminal velocity ut (m / s) of the raw material powder can be expressed by the following formula (1) when the raw material powder is approximated to a spherical shape to the first order.

[0041]

[0042] In the above formula (1), g (m / s 2 ) is the gravitational acceleration, ρ p (kg / m 3 ) is the particle density of the raw material powder, ρ f(kg / m 3 ) is the density of the reducing gas, D p (m) is the median diameter of the raw material powder, C ｄ (-) is the drag coefficient, which is arranged according to the Reynolds number Re, and is expressed as follows using the approximate formula of Brown and Lawler (formula (2)):

[0043] C ｄ =(24(1+0.15Re 0.681 ) / Re)+(0.407 / (1+8710Re -1 ) ...Formula (2)

[0044] The temperature during reduction in the fluidized bed (reduction temperature) is 400°C or higher and 590°C or lower. When the temperature in the fluidized bed is 400°C or higher and 590°C or lower, the reduction reaction is promoted while avoiding reduction stagnation for the raw material powder with a low content of crystal water, and reduced iron can be efficiently obtained with a high reduction rate. The reason why reduction stagnation is avoided at 590°C or lower is not entirely clear. However, based on the Fe—O phase diagram, the inventors speculate that the wüstite phase, which is thermodynamically stable at 570°C or higher, may contribute to the densification of iron. The inventors speculate that reduction stagnation is avoided at 590°C or lower because the wüstite phase is less likely to be generated during the reduction process. Since the reduction reaction occurs in the fluidization zone within the fluidized bed, the temperature of the fluidization zone is set to 400°C or higher and 590°C or lower. The reduction temperature is preferably 500°C or higher. The reduction temperature is preferably 560°C or lower, more preferably 530°C or lower.

[0045] The method for controlling the temperature in the fluidized bed is not particularly limited, but a feature of the fluidized bed is that if the raw material powder is sufficiently fluidized, the gas and the substance are mixed very well, and the temperature in the fluidized bed is uniform, making temperature control in the fluidized bed easier than other reaction control methods. The reaction temperature in the fluidized bed can be adjusted by surrounding the reaction vessel in which the fluidized bed is placed with a heat insulating material and supplying and mixing particles and gas that have been preheated to a predetermined temperature. It is also possible to heat the reaction vessel from the outside and perform heat exchange with the vessel wall to adjust the temperature in the fluidized bed to a predetermined temperature.

[0046] The temperature inside the fluidized bed is measured by a thermocouple whose tip is positioned in the fluidized part of the raw material powder inside the fluidized bed.

[0047] [Average Residence Time] The average residence time of the raw material powder in the reaction vessel may be determined in consideration of the type of fluidized bed, the time required to achieve a desired reduction rate depending on the reducing gas components and temperature, and the like.

[0048] The average residence time of the raw material powder retained in the bubbling fluidized bed is preferably 3 minutes or more and 180 minutes or less. When the average residence time is 3 minutes or more, reduced iron 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 of the reduction apparatus can be maintained at a high level. Furthermore, when the average residence time is 180 minutes or less, a decrease in the strength of the raw material powder due to excessive reduction or pulverization of the raw material powder due to collisions between the raw material powder particles or between the raw material powder and the apparatus during circulation can be suppressed, resulting in a decrease in the recovery efficiency of reduced iron. Therefore, the average residence time is preferably 180 minutes or less. The average residence time of the raw material powder retained in the bubbling fluidized bed is more preferably 5 minutes or more and 60 minutes or less. The average residence time can be controlled by changing the reducing gas velocity, the amount of particles retained in the reaction vessel, the particle withdrawal rate, etc.

[0049] 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 180 minutes or less. The average residence time of the raw material 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.

[0050] When the fluidized bed is a spouting fluidized bed, the average residence time of the raw material powder in the spouting 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 raw material powder in the spouting 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.

[0051] The average residence time of the raw material powder can be calculated by the following method. That is, for example, 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 content of gangue components in the discharged reduced iron powder is examined. The peak time period obtained, during which the content of gangue components that characterize the added tracer ore powder is highest, is taken as the average residence time of the raw material powder. Using the above method, it is possible to experimentally measure the average residence time.

[0052] In this process, at least a portion of the raw material powder is reduced. The reduction rate is preferably 70% or more and 100% or less. As the reduction rate increases, the proportion of metallic iron on the surface of the raw material powder increases. When the proportion of metallic iron on the surface of the raw material powder increases, the metallic iron covers the surface of the raw material powder in a two-dimensional film, causing reduction stagnation at temperatures between 600 and 800°C. On the other hand, at temperatures below 590°C, the surface of the raw material powder is prevented from being covered by metallic iron in a two-dimensional film, allowing a high surface area to be maintained. When the reduction rate is low, the amount of metallic iron produced is small, and the difference due to the reduction temperature does not occur, thereby preventing the advantages of reduction at low temperatures from being realized. Therefore, the reduction rate is preferably 70% or more. The reduction rate is more preferably 80% or more. On the other hand, when reduction treatment is performed with the aim of achieving a high reduction rate of over 95%, although this depends on the characteristics of the raw material powder, the improvement in the reduction rate may stagnate even when reduced at low temperatures, thereby reducing the significant difference due to the reduction temperature. Therefore, a reduction rate of 95% or less is more preferable.

[0053] The reduction degree is set according to the purpose. When the method for producing reduced iron according to this embodiment is used as a general method for producing reduced iron for refining in an electric furnace, the achieved reduction degree is preferably 90% or more. If the reduction degree is 90% or more, the reduced iron can be provided as a final product to a user who performs refining in an electric furnace, for example. Furthermore, if the reduced iron is charged as a raw material into a blast furnace for the purpose of reducing the usage rate of a reducing agent such as coke in the blast furnace or for use in an electric furnace for producing molten iron, the reduction degree does not need to exceed 90% and may be, for example, about 70 to 80%.

[0054] The reduction rate can be calculated, for example, by the following method. That is, about 0.1 g of raw material powder is weighed into a quartz cell in a glove box under a nitrogen atmosphere, and the raw material powder is immersed in benzene to prevent the raw material powder from coming into contact with air. The quartz cell is placed in a thermobalance (TGD7000, manufactured by Shinku Riko Co., Ltd.), and the system is evacuated. Then, nitrogen is introduced at a rate of 2.00 × 10 －４ m ３ / min, and the temperature is raised to 200°C at a heating rate of 20°C / min to evaporate the benzene. Thereafter, the temperature is raised to 700°C at a heating rate of 20°C / min, and after the temperature and the balance have stabilized, oxygen is introduced into the system and maintained until there is no weight increase. Next, the system is cooled to 100°C or below, evacuated, substituted with nitrogen, and again heated to 700°C at a heating rate of 20°C / min. Then, 2.00 x 10 hydrogen gas is introduced. －４ m ３ The flow rate is maintained at 1 / min until no weight change is observed. Based on the weight change, the reduction rate is calculated using the following formula (3).

[0055] X = {(m Ｆｅ２Ｏ３ -m ｓａｍｐｌｅ ) - 0.329 × (m Ｆｅ２Ｏ３ -m Ｆｅ )} / {0.671×((m Ｆｅ２Ｏ３ -m Ｆｅ )} ...Equation (3) In the equation, X is the reduction rate (%), m Ｆｅ２Ｏ３ is the weight of the raw material powder after oxidation (the weight of the raw material powder when the weight increase stops after oxygen is introduced), and m ｓａｍｐｌｅ is the mass of the raw material powder, m Ｆｅ is the weight of the raw material powder after reduction (the weight of the raw material powder when there is no weight increase after hydrogen gas is introduced). The chemical forms of the raw material powder after oxidation and reduction using a thermobalance were determined by X-ray diffraction to be Fe, ２ O ３ and Fe.

[0056] (Oxidation suppression step) Reduced iron after the fluidized bed reduction step has a high specific surface area and may be oxidized by oxygen in the atmosphere, resulting in a decrease in the reduction rate. To suppress oxidation, it is preferable to perform an oxidation suppression step after the fluidized bed reduction step. The oxidation suppression step is not particularly limited as long as it is a treatment that suppresses oxidation of the reduced iron after the fluidized bed reduction step. As examples of the oxidation suppression step, a surface area reduction step and a heat treatment step will be described. Hereinafter, reduced iron after the fluidized bed reduction step may be simply referred to as reduced iron.

[0057] <Surface Area Reduction Step> In the surface area reduction step, the reduced iron is hot-briquetted in an inert atmosphere. By hot-briquetting the reduced iron into hot briquette iron (HBI) or compact iron, the surface area of ​​the reduced iron can be reduced, thereby reducing its oxidizability. A known hot briquetting method can be used to obtain HBI, such as the method described in JP 2009-79292 A. The briquetting method described in JP 2009-79292 A is a method for producing HBI by sandwiching a powdered or granular raw material containing a large amount of reduced iron between a pair of rollers with a concave mold at a relatively high temperature of 1000°C or less, for example, 500 to 800°C. The HBI is cooled to room temperature using a water-cooling device. In this hot-briquetting method, the reduced iron is pressed and molded. Therefore, to sufficiently reduce the oxidizability of the reduced iron obtained by fluidized bed reduction, it is desirable to obtain a high-density molded product that minimizes voids between the reduced iron particles. The atmosphere in the surface area reduction step is preferably an inert atmosphere. The molding temperature is preferably 720°C or higher.

[0058] <Heat Treatment Step> The heat treatment step is a step of heat treating the reduced iron after the fluidized-bed reduction step with a non-oxidizing gas. As an example of the heat treatment step, a fluidized-bed heat treatment step using a fluidized bed will be described. The fluidized-bed heat treatment step is a step of heat treating the reduced iron after the fluidized-bed reduction step with a non-oxidizing gas using a fluidized bed that is the same as or different from the fluidized bed used in the fluidized-bed reduction step.

[0059] [Fluidized Bed] In this step, a fluidized bed is used for the heat treatment of the reduced iron. 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 step may be formed in the same vessel as that used in the fluidized bed reduction step. By using the same vessel as that used in the fluidized bed reduction step in this step, the reduced iron does not need to be transferred to another vessel, allowing for efficient treatment of the reduced iron. Furthermore, by using the same vessel, the reduced iron after the fluidized bed reduction step is not exposed to the atmosphere, further suppressing oxidation of the reduced iron. When the vessel used in the fluidized bed reduction step and the vessel used in the fluidized bed heat treatment step are different, it is preferable to transfer the reduced iron after the fluidized bed reduction step in a non-oxidizing atmosphere.

[0060] [Non-oxidizing gas] The non-oxidizing gas is, for example, a gas that does not oxidize the reduced iron after the fluidized bed reduction step, and is, for example, N ２ , He, Ne, Ar, Kr, and Xe. Reduced iron is heat-treated in a non-oxidizing gas environment, and its specific surface area is reduced. The non-oxidizing gas is preferably N ２ gas or Ar gas.

[0061] The flow velocity of the non-oxidizing gas is preferably at least 1.5 times higher than the 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 flow velocity of the reducing gas in the fluidized-bed reduction step is high, and the former is used when the flow velocity is low. When the fluidized bed in the fluidized-bed reduction step is a circulating fluidized bed, the latter is used, i.e., the 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. The flow velocity of the non-oxidizing gas for forming the bubbling fluidized bed is lower than the flow velocity of the non-oxidizing gas for forming the circulating fluidized bed, and very little raw material powder escapes from the bubbling fluidized bed. Therefore, when the fluidized bed is a bubbling fluidized bed, the flow velocity of the non-oxidizing gas may be at least the minimum fluidization velocity and at most the terminal velocity.

[0062] 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.

[0063] The temperature in the fluidized bed in the fluidized bed heat treatment step is preferably 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 reduced iron 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 suppress aggregation of the reduced iron, 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.

[0064] The temperature inside the fluidized bed is measured using a thermocouple whose tip is positioned in the fluidized part of the reduced iron in the fluidized bed.

[0065] In this process, the reduced iron can be heat-treated at a high temperature, which reduces the specific surface area and suppresses oxidation of the reduced iron 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 raw material powder, and the hydrogen generated in the reduction of the raw material powder is released outside the vessel. ２A large amount of hydrogen gas containing O is discharged. 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. 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 is inexpensive to use. Furthermore, if a reducing gas is used, further reduction will proceed during heat treatment at a temperature higher than that in the reduction treatment process, and new active metallic iron will be generated on the surface, which may cause sticking.

[0066] The heat treatment of reduced iron is not limited to the above-mentioned fluidized bed heat treatment process, and can be carried out using a furnace capable of controlling the temperature and atmosphere, for example, a known rotary kiln. When an internally heated rotary kiln is used for the heat treatment process, the direction of introduction of the non-oxidizing gas is preferably opposite to the direction of movement of the raw material 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.

[0067] For example, in the heat treatment step, when the pre-heat treatment step raw material powder, which is the raw material powder after the fluidized bed reduction step and before the heat treatment step, and the post-heat treatment step raw material powder, which is the raw material powder after the heat treatment step, are each sieved using a sieve with a mesh size of 5 mm, a sieve with a mesh size of 3 mm, a sieve with a mesh size of 1 mm, a sieve with a mesh size of 0.5 mm, and a sieve with a mesh size of 0.25 mm in this order, the heat treatment is carried out under conditions such that the ratio of the total sieved mass of the post-heat treatment step raw material powder on the sieves with the specified mesh size when the total sieved mass of the pre-heat treatment step raw material powder reaches 20 mass% or more is 1.0 to 1.2 times. If the ratio is 1.0 to 1.2 times, it can be said that aggregation of the raw material powder is suppressed, and the heat treatment step is carried out stably.

[0068] (Apparatus Configuration Example) Here, with reference to the drawings, an example of an apparatus configuration to which the reduced iron manufacturing method according to this embodiment can be applied will be described. The reduction apparatus may be, for example, one or more circulating fluidized bed forming apparatuses that form a circulating fluidized bed, one or more bubbling fluidized bed forming apparatuses that form a bubbling fluidized bed, one or more spouting fluidized bed forming apparatuses that form a spouting fluidized bed, or a combination thereof. FIG. 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.

[0069] [Bubbling Fluidized Bed Forming Apparatus] As shown in FIG. 1, the bubbling fluidized bed forming apparatus 10 includes a container 11 and a dry dust collector 12 .

[0070] As shown in FIG. 1 , the vessel 11 has, for example, a gas supply port 111 disposed at the bottom and through which a reducing gas is supplied into the vessel 11, a raw material powder supply port 112 through which raw material powder is supplied, and a dispersion plate 113 disposed above the gas supply port 111.

[0071] The bubbling fluidized bed is formed by fluidizing the raw material powder with gas that is supplied from the gas supply port 111 and rectified through multiple vent holes in the dispersion plate 113. Note that the specific gas ventilation method and form for forming the bubbling fluidized bed are not limited to those using a flat dispersion plate 113 such as a porous plate or a slit plate, as long as it is possible to supply a reducing gas into the vessel 11, blow up the raw material powder, and form a fluidized bed, such as 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.

[0072] The reduced iron after reduction or the reduced iron after heat treatment is discharged from an openable and closable reduced iron powder outlet (not shown).

[0073] The bubbling fluidized bed forming apparatus may also have a container for forming multiple bubbling fluidized beds therein. FIG. 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, a raw material 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 spaces in which a 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 raw material powder and increase the achieved reduction rate. It goes without saying that the locations and number of raw material powder supply ports, the locations and number of outlets, and the locations and number of partition plates are not limited to the embodiment shown in FIG. 2 and may be changed as appropriate.

[0074] 3 , the circulating fluidized bed forming apparatus 20 includes a riser section 21 which is a container in which the raw material 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 which extends downward from the bottom of the cyclone 22 and is connected to the lower part 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 for collecting pulverized iron ore powder and reduced iron (dust) contained in the off-gas.

[0075] The riser section 21 has a gas supply port 211, a raw material powder supply port 212, and a dispersion plate 213 disposed above the gas supply port 211. The riser section 21 is basically similar to the vessel 11 in the bubbling fluidized bed forming apparatus 10.

[0076] 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.

[0077] 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 raw material 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 distribution plate 213 of the riser section 21. The loop seal unit 232 provides a sealing effect by using the raw material powder temporarily stored therein. Note that the specific form and method of gas ventilation for forming a circulating fluidized bed are not limited to those described above, and may include, in addition to the use of a flat distribution 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, as long as the reducing gas can be supplied into the riser section 21 and the raw material powder can be blown up to form a fluidized bed.

[0078] In the circulating fluidized bed forming apparatus 20, the exhaust gas (off-gas) may contain 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.

[0079] The raw material powder supplied through the raw material powder supply port 212 is fluidized by the reducing gas, which is supplied through the gas supply port 211 and rectified through the multiple vent holes in the dispersion plate 213. Specifically, the raw material 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. The raw material powder remains in the loop seal section 232 for a while. When the reduction or heat treatment in 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 through the downcomer 231). When the reduction or heat treatment is a continuous process, for example, a valve at the openable / closable outlet provided in the riser section 21 is opened at regular intervals or continuously, and the treated powder is extracted, while raw material powder is replenished through the raw material powder supply port 212.

[0080] [Spout-fluidized-bed forming apparatus] The spout-fluidized-bed forming apparatus 30 basically has the same configuration as the bubbling fluidized-bed forming apparatus 10. However, the spout-fluidized-bed forming apparatus 30 differs from the bubbling fluidized-bed forming apparatus 10 in that it does not uniformly stir the entire mixture. For example, as shown in FIG. 4 , a protruding section 311 (gas blow-through section) from which the raw material powder is protruding at a high gas flow rate above the gas supply port 111 and a moving layer 312 in which the raw material powder accumulates around the protruding section and moves from top to bottom are formed. The raw material powder at the bottom of the moving layer 312 is entrained in the reducing gas flowing at a high gas flow rate through the protruding section 311, and the raw material powder is repeatedly protruding from the protruding section 311. The fluidized bed composed of the protruding section 311 and the moving layer 312 is the spout-fluidized bed 310.

[0081] The fluidized bed for reducing the raw material powder and the fluidized bed for heat-treating the reduced iron may each be a single circulating fluidized bed, a single bubbling fluidized bed, or a single spouting fluidized bed. Alternatively, they may be multiple circulating fluidized beds, multiple bubbling fluidized beds, or multiple spouting fluidized beds. A circulating fluidized bed has a large difference (slip velocity) between the average flow velocity of the reducing gas and the average movement velocity of the raw material powder, resulting in frequent exchange of the gas in contact with the raw material powder. During reduction, the surroundings of the raw material powder approach an equilibrium state, preventing the reduction reaction from stagnating. This results in efficient reduction of the raw material powder. On the other hand, the average movement velocity of the raw material powder itself is also large, resulting in mechanical wear and destruction due to collisions between the raw material powder particles, which can easily generate dust. Compared to a circulating fluidized bed, a bubbling fluidized bed has a smaller difference (slip velocity) between the average flow velocity of the reducing gas and the average movement velocity of the raw material powder. Therefore, the reduction efficiency of the raw material powder using a bubbling fluidized bed is inferior to that of 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 rate. A spouting fluidized bed can use raw material powder with a larger particle size than a circulating fluidized bed. A spouting fluidized bed is also advantageous when shortening the residence time of the raw material 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 raw material powder.

[0082] <Configuration Example of Reduced Iron Production Facility> Here, a configuration example of a reduced iron production facility having a plurality of fluidized beds will be described with reference to Fig. 5. Fig. 5 is a schematic configuration diagram showing an example of a reduced iron production facility equipped with one circulating fluidized bed forming device 20 and three bubbling fluidized bed forming devices 10 as reduction devices. In Fig. 5, solid arrows indicate the flow of powder, and dashed arrows indicate the flow of gas.

[0083] The reduced iron production equipment 1 shown in FIG. 5 includes one circulating fluidized bed forming device 20 and three bubbling fluidized bed forming devices 10 as the reduction device 2. For example, in the reduced iron production equipment 1 shown in FIG. 5, reduced iron 41 is produced as follows. Raw material powder 40 is supplied to the riser section 21 of the circulating fluidized bed forming device 20. The raw material powder 40 supplied to the riser section 21 is reduced by a reducing gas 42 in the circulating fluidized bed. The partially reduced raw material powder 40 is sent to the first-stage vessel 11 of the bubbling fluidized bed forming device 10, where reduction of the raw material powder 40 proceeds in the bubbling fluidized bed formed by the raw material powder 40 and the reducing gas 42. The raw material powder 40 in the vessel 11 is sequentially supplied to the second-stage vessel 11 and the third-stage vessel 11 and reduced there. The raw material powder 40 is finally reduced in the bubbling fluidized bed in the third-stage vessel 11 to produce reduced iron 41.

[0084] Dry dust collectors 12 connected to each of the vessels 11 recover raw material powder 40 that may be contained in the off-gas, and the recovered raw material powder 40 is fed back into each vessel 11 .

[0085] The off-gas containing dust separated by the cyclone 22 of the circulating fluidized bed forming apparatus 20 and the dry dust collector 12 of the bubbling fluidized bed forming apparatus 10 is sent to the dry dust collector 24. The dust is separated from the off-gas in the dry dust collector 24 and collected.

[0086] As described above, reduced iron can be produced by a reduced iron production facility equipped with, for example, one circulating fluidized bed former 20 and three bubbling fluidized bed formers 10 as reduction devices. The reduction device 1, in which the bubbling fluidized bed former 10 is provided downstream of the circulating fluidized bed former 20, can shorten the reduction time in the early stage of reduction, where the reduction reaction tends to proceed rapidly due to the supply rate of the reducing gas reaching the surface of the raw material powder. This can avoid excessive use of reducing gas in the later stage of reduction, where the reduction rate tends to stagnate due to the rate of material diffusion within the ore. Furthermore, by providing multiple bubbling fluidized bed formers 10, the average residence time of the raw material powder can be ensured and the variation in residence time can be suppressed, thereby enabling reduced iron with a desired reduction rate to be obtained with little variation in quality.

[0087] 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.

[0088] For example, from the viewpoint of achieving stable operation, the pressure loss in the fluidized bed may be monitored at any time. When excessive agglomeration occurs or when gas bypass (channeling) occurs in the region where the raw material powder segregates, 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 may occur. In such cases, large-scale maintenance of the manufacturing equipment may be required. However, if pressure loss is monitored at any time, it becomes possible to shut down the equipment at an appropriate time before clogging or segregation progresses and the maintenance burden becomes too great. Such a configuration is realized by a pressure measuring device.

[0089] The reduced iron production method according to the present embodiment can efficiently produce reduced iron from raw material powder containing iron, even when a mixture of iron ore powders of different ore types has a low crystal water content, which would otherwise exhibit significant reduction stagnation in conventional reduction processes. Furthermore, the reduced iron production method according to the present embodiment deliberately selects a low temperature range of 590°C or less, which is lower than the conventional process temperature. This reduces equipment costs by addressing equipment issues associated with using reducing gas at high temperatures, particularly the deterioration of furnace materials due to high-temperature hydrogen, eliminating heat-resistant measures for mechanical drives, and downgrading furnace materials. Furthermore, lower reduction temperatures also lead to reduced operating costs. Furthermore, even when a raw material powder containing an ore with a high crystal water content that does not exhibit reduction stagnation is mixed with a raw material powder containing an ore with a low crystal water content, the average crystal water content of the mixed raw material powder can provide guidelines for setting conditions for efficient reduction. This indicates that it will be possible to select low-temperature operating conditions using ore with a high crystallization water content while avoiding reduction stagnation, which will lead to the realization of an economical steelmaking process using a variety of iron ore raw materials in the future, including ores with low iron content.

[0090] The technical scope of the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. Furthermore, the components in the present embodiment can be replaced with well-known components as appropriate without departing from the spirit of the present invention.

[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 In order to estimate the transition of the hydrogen reduction rate in a fluidized bed reduction vessel for the raw material powders A to H shown in Table 1, the transition of the reduction rate during hydrogen reduction in a stationary state was measured by thermogravimetry (TG).

[0093] Table 1 shows the iron content (T.Fe), crystal water (CW), and gangue components of each raw material powder, SiO ２ , Al ２ O ３ The contents of CaO and MgO are shown. Components other than those shown in Table 1 are impurities such as MnO, P, S, etc., and the contents of all of them were in the range of 0.001 to 0.3 mass%. Impurities are components that have little effect on reduction using a fluidized bed. Raw material powders A to C contain hematite (Fe ２ O ３ Raw material powders A to C are fine ores that have been subjected to ore dressing treatment to increase the iron content, and have a relatively sharp particle size distribution. Raw material powders D to H have been dry-classified to 0.25 mm or less.

[0094]

[0095] Thermogravimetric analysis was performed using a high-temperature differential thermobalance TG-DTA / H (Rigaku Corporation, Thermo plus EVO2). 15 mg of raw material powder was placed in an alumina sample container, and the sample was heated to the following set temperature at a heating rate of 10°C / min under a nitrogen gas flow (214 cc / min). The nitrogen gas flow rate was set within a range that ensured sufficient rapid gas replacement within the apparatus without the sample being carried away by the gas flow. The reduction temperature was set at six levels between 500°C and 1000°C in 100°C intervals. After holding at each temperature for 60 min to stabilize the weight, hydrogen gas was introduced (214 cc / min or 54 cc / min) in addition to nitrogen gas while maintaining each temperature, and the weight loss associated with reduction at a hydrogen concentration of 50% or 25% by volume was measured. Since goethite changes to hematite as water of crystallization is released, the reduction rate was calculated based on the weight of the sample before hydrogen was introduced, and the weight when oxygen in the hematite was completely removed was calculated as 100% reduction rate from the weight loss data. The measurement results are shown in Figure 6.

[0096] Figure 6 shows graphs of the transition of the reduction degree for each raw material powder shown in Table 1, arranged in order of the crystal water content. For raw material powders A to E with a low crystal water content, particularly raw material powders A to E with a crystal water content of 3.5% by mass or less, it was found that the time to reach a reduction degree of over 90% was shorter when reduced at 500°C than when reduced in the 600 to 700°C range, as indicated by the dashed line in Figure 6 . That is, for raw material powders A to E with a crystal water content of 3.5% by mass or less, reduction stagnation in the 600 to 700°C range was significant. The inventors speculate that the reason for this is as follows: A high crystal water content causes a transformation from the goethite phase to the hematite phase due to dehydration during the heating process before reduction, and cracks and pores are formed in the raw material powder during this process. It is speculated that these pores inhibit the formation of a dense, two-dimensional metallic iron film, which causes reduction stagnation during metallic iron production. Furthermore, it was found that if the content of water of crystallization is 1.0 mass % or less, the reduction rate reaches 90% more quickly when reduced at 500°C than when reduced at 800°C.

[0097] Figure 7 shows the transition of reduction rate during hydrogen reduction at five temperatures between 500°C and 600°C in 25°C intervals (Figure 7(A)), and a graph of the reduction rate versus reduction rate (Figure 7(B)). Raw material powder B is a hematite concentrate in which significant reduction stagnation was observed. As can be seen from Figures 7(A) and (B), at 500°C and 525°C, the reduction rate reached 100% and no reduction stagnation was observed. Reduction stagnation was clearly observed above 550°C, and as can be seen from Figure 7(A), it was found that the higher the reduction temperature, the lower the reduction rate at which the reduction reaction began to stagnate. Furthermore, with regard to the reduction rate, a peak was observed around 10% reduction rate at 500 to 550°C. This is thought to be due to the rate starting to decrease after the magnetite phase was formed. Under the conditions of 575°C and 600°C, the reduction rate peaked near a reduction degree of 20 to 30%, and then dropped significantly at a reduction degree of 50 to 70%. This significant drop in the reduction rate is thought to be due to the formation of two-dimensional metallic iron films on the surface of the raw material powder, with a dense wüstite phase as the parent phase. Considering these factors and the fact that the temperature range in which the wüstite phase is unstable on the reduction equilibrium diagram overlaps with temperatures below 590°C, it is thought that the reason reduction stagnation is avoided at temperatures below 590°C when raw material powder with a low content of crystallized water is because the formation of the wüstite phase is suppressed.

[0098] Example 2 Fluidized bed reduction was carried out on raw material powders B, D, and E from Example 1. Specifically, a dispersion plate made of baked glass beads was placed inside a vessel with an inner diameter of 35 mm, and raw material powder was packed onto the dispersion plate. The thickness of the raw material powder layer was 35 mm. The tip of a thermocouple was positioned inside the raw material powder layer. A heating mechanism was installed on the outer periphery of the vessel, allowing the interior of the vessel to be heated. N was blown from below the vessel. ２ Gas was supplied into the vessel, and the temperature was raised while fluidizing the iron ore powder. After the fluidized bed reached the temperature shown in Table 2, H ２ Gas 90% by volume and N ２A mixed gas containing 10% by volume of gas was supplied. The gas flow rate of this mixed gas was as shown in Table 2. The minimum fluidization velocity of raw material powders B, D, and E was 0.02 m / s, and the terminal velocity was 0.3 m / s. The reduction time was 2 hours from the start of the reaction. Using a pressure probe with its tip positioned inside the iron ore powder layer and a pressure probe with its tip positioned above the iron ore powder layer, the pressure in the iron ore powder layer (fluidized bed) during gas flow and the pressure in the space above the iron ore powder layer were continuously measured to determine whether the fluidized bed was stable. If excessive agglomeration occurs or if gas bypass (channeling) occurs in the area where the iron ore powder segregates, 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 will occur.

[0099] In the reduction treatment, the time required for the reduction rate to reach 90% was measured and compared with the time required for the reduction rate to reach 90% when the reduction treatment was performed at 600°C. If the time required for the reduction rate to reach 90% was shorter than when the reduction treatment was performed at 600°C, the reduction efficiency was evaluated as A, if it was about the same, it was evaluated as B, and if the reduction rate did not reach 90%, it was evaluated as C. An evaluation of A was considered to be pass, and an evaluation of B or C was considered to be fail.

[0100] The time required for the reduction rate to reach 90% was measured as follows: The residual hydrogen concentration of the exhaust gas passing through the fluidized bed was measured by gas chromatography, and the amount of hydrogen gas consumed by the reduction reaction during passage through the fluidized bed was calculated from the difference between this and the supplied hydrogen gas concentration. The percentage of iron oxide reduced out of the weight of iron oxide converted to hematite, which was estimated in advance based on the input amount and chemically analyzed component values, was calculated, and the time elapsed from the start of hydrogen gas flow until the reduction rate reached 90% was measured.

[0101] Furthermore, cases where sticking did not occur during the reduction treatment were rated as A (pass), and cases where sticking occurred were rated as B (fail). The occurrence of sticking was judged as follows. That is, when the pressure loss in the fluidized bed portion measured during the reduction treatment was found to have decreased significantly by more than the weight loss due to reduction, the powder removed after the reduction treatment was sieved through a sieve with meshes corresponding to the maximum particle size of the raw material powder originally charged, and when residue remained on the meshes, it was judged that sticking had occurred.

[0102] In Example No. 8 in Table 2, a hot-molded reduced iron compact was produced using the reduced iron without contact with an oxidizing atmosphere, and then the compact was taken out into the atmosphere. In Examples Nos. 9 to 16, 18, 19, and 21 to 25, the reduced iron was heat-treated using a fluidized bed without contact with an oxidizing atmosphere, and then taken out into the atmosphere. The atmosphere ("gas" in Table 2) used in these treatments was the gas flow rate and treatment temperature shown in Table 2. In Examples Nos. 1 to 7, 17, and 20, the temperature of the reduced iron after reduction treatment was lowered to less than 40°C and then taken out into the atmosphere. If the maximum surface temperature of the reduced iron measured with a radiation thermometer within 1 minute after taking out into the atmosphere was 50°C or less, the oxidizing property was evaluated as A. If the maximum surface temperature of the reduced iron was 200°C or more, the oxidizing property was evaluated as B. A rating of A was considered a pass, and a rating of B was considered a fail.

[0103]

[0104] In Example No. 1, the reduction temperature was too low at 300° C., so the reduction rate did not reach 90%. Note that, since Example No. 1 did not reach 90% reduction rate, it was excluded from the evaluation of oxidizing properties.

[0105] In Example No. 2, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 400°C, and the evaluation result of the reduction rate was acceptable. In Example No. 2, the cooled reduced iron was taken out directly into the air after reduction, so oxidation occurred, generating heat to a high temperature, and the evaluation of oxidizability was unacceptable.

[0106] In Example No. 3, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 500°C, and the evaluation result of the reduction rate was pass. In Example No. 3, the cooled reduced iron was taken out directly into the air after reduction, so oxidation occurred and heat was generated to a high temperature, and the evaluation of oxidizability was fail.

[0107] In Example No. 4, the temperature during reduction was 600° C., and the evaluation result of the reduction rate was unacceptable. In Example No. 4, the cooled reduced iron was taken out directly into the air after reduction, which caused oxidation and generated heat at a high temperature, and the evaluation of the oxidizability was unacceptable.

[0108] In Example No. 5, the temperature during reduction was 700° C., and the evaluation result of the reduction rate was unacceptable. In Example No. 5, the cooled reduced iron was taken out directly into the air after reduction, which caused oxidation and generated heat at a high temperature, and the evaluation of the oxidizability was unacceptable.

[0109] In Example No. 6, the reduction temperature was 800° C., and the evaluation result of the reduction rate was acceptable. However, excessive aggregation occurred due to reduction at a high temperature.

[0110] In Example No. 7, the reduction temperature was 900° C., and the evaluation result of the reduction rate was acceptable. However, excessive aggregation occurred due to reduction at a high temperature.

[0111] In Example No. 8, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 400°C, and the evaluation result of the reduction rate was acceptable. In Example No. 8, HBI was produced to reduce its surface area and then taken out into the air, so the evaluation of oxidizability was acceptable.

[0112] In Example No. 9, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 500°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 750° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0113] In Example No. 10, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 525°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 750° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0114] In Example No. 11, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 750° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0115] In Example No. 12, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 575°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 750° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0116] In Example No. 13, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was acceptable. In Example No. 13, BFB was used and heat treatment was performed at 750°C in an Ar gas atmosphere, thereby reducing the surface area of ​​the reduced iron, and therefore the evaluation of the oxidizability was acceptable.

[0117] In Example No. 14, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 750° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0118] In Example No. 15, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was acceptable. ２Although the heat treatment was carried out in a gas atmosphere, the heat treatment temperature was low at 550° C., so the surface area of ​​the reduced iron was not reduced sufficiently, and the evaluation of the oxidizability was unacceptable.

[0119] In Example No. 16, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 800° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0120] In Example No. 17, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was pass. In Example No. 17, the cooled reduced iron was taken out into the air as it was, and oxidation occurred, generating heat at a high temperature, and the evaluation of oxidizability was fail.

[0121] In Example No. 18, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was acceptable. ２ Although the heat treatment was carried out in a gas atmosphere, the heat treatment temperature was low at 650° C., so the surface area of ​​the reduced iron was not reduced sufficiently, and the evaluation of oxidation resistance was unacceptable.

[0122] In Example No. 19, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 800° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0123] In Example No. 20, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was pass. In Example No. 20, the cooled reduced iron was taken out into the air as it was, and oxidation occurred, generating heat to a high temperature, and the evaluation of oxidizability was fail.

[0124] In Example No. 21, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was acceptable. ２ Although the heat treatment was carried out in a gas atmosphere, the heat treatment temperature was low at 650° C., so the surface area of ​​the reduced iron was not reduced sufficiently, and the evaluation of oxidation resistance was unacceptable.

[0125] In Example No. 22, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 800° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0126] In the example of No. 23, a raw material powder having a crystal water content of 3.5 mass% or less was used, and a two-stage fluidized bed using CFB and BFB was used, and the temperature during reduction was set to 550°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 750° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0127] In the example of No. 24, a raw material powder having a crystal water content of 3.5 mass% or less was used, and a two-stage BFB was used at a reduction temperature of 550°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 750° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0128] In Example No. 25, a raw material powder having a crystal water content of 3.5 mass% or less was used, and a two-stage CFB was used at a reduction temperature of 550°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 750° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0129] Example 3 For iron ore powders A to C shown in Table 3, in order to estimate the transition of the hydrogen reduction rate in a fluidized bed reduction vessel, the transition of the reduction rate during hydrogen reduction in a stationary state was measured by thermogravimetry (TG).

[0130] Table 3 shows the iron content (T.Fe), crystal water (CW), and gangue components of each iron ore powder. ２ , Al ２ O ３ The contents (mass%) of CaO and MgO are shown. Components other than those shown in Table 3 are impurities such as MnO, P, and S, and the contents of each were in the range of 0.001 to 0.3 mass%. The impurities are components that have little effect on reduction using a fluidized bed. Iron ore powder A contained hematite (Fe ２ O ３ Iron ore powder A is a hematite-based concentrate mainly composed of iron ore powder B, hematite-based ore powder B, and goethite (FeO(OH))-based ore powder C. Iron ore powder A is a fine ore that has been subjected to a dressing process to increase the iron content, and has a relatively sharp particle size distribution. Iron ore powders B and C have been dry-classified to 0.25 mm or less.

[0131]

[0132] The iron ore powders A to C were mixed in the proportions shown in Table 4, and the crystal water content, gangue content, and iron content of the resulting mixed powder are shown in Table 4.

[0133]

[0134] Thermogravimetric analysis was performed using a high-temperature differential thermobalance TG-DTA / H (Rigaku Corporation, Thermo plus EVO2). 15 mg of raw material powder was placed in an alumina sample container, and the sample was heated to the following set temperature at a heating rate of 10°C / min under a nitrogen gas flow (214 cc / min). The nitrogen gas flow rate was set within a range that ensured sufficient rapid gas replacement within the apparatus without the sample being carried away by the gas flow. The reduction temperature was set at six levels between 500°C and 1000°C in 100°C intervals. After holding at each temperature for 60 min to stabilize the weight, hydrogen gas was introduced (214 cc / min or 54 cc / min) in addition to nitrogen gas while maintaining each temperature, and the weight loss associated with reduction at a hydrogen concentration of 50% or 25% by volume was measured. Since goethite changes to hematite as water of crystallization is released, the reduction rate was calculated based on the weight of the sample before hydrogen was introduced, and the weight when oxygen in the hematite was completely removed was calculated as 100% reduction rate from the weight loss data. The measurement results are shown in Figure 6.

[0135] Fig. 8 is a graph showing the transition of the reduction rate for each iron ore powder shown in Table 3, arranged in order of the crystal water content. The reduction rate was measured for each iron ore powder by changing the reduction temperature. Reduction stagnation was noticeable for iron ore powder A, which had a high iron content, while reduction stagnation was not observed around 700°C for iron ore powder C, which had a high crystal water content. For iron ore powder B, whose crystal water content was between that of iron ore powder A and that of iron ore powder C, the reduction behavior showed an intermediate tendency between that of raw material powder A and that of raw material powder B.

[0136] Fig. 9 shows graphs of the transition of the reduction degree for raw material powders (cases) b to f and h to l shown in Table 4, arranged in order of the water of crystallization content. Note that raw material powders (cases) a, g, and m in Table 4 correspond to iron ore powders A, C, and B, respectively. Therefore, the graphs for raw material powders (cases) a, g, and m are shown in Fig. 8. Furthermore, the graphs with the legend "500" in each graph are graphs showing the transition of the reduction degree when the reduction temperature was set to 400 to 590°C, and the graphs with the legend "700" in each graph are graphs showing the transition of the reduction degree when the reduction temperature was set to 600 to 700°C.

[0137] As shown in Figure 9, reduction stagnation depends on the crystallization water content of the raw material powder, with ores with lower crystallization water content showing a stronger tendency for reduction stagnation. Reduction stagnation was more pronounced for raw material powders with low crystallization water content, particularly those with a crystallization water content of 3.5% by mass or less. Reduction at 400 to 590°C achieved a reduction rate of over 90% faster than reduction at 600 to 700°C. This is presumably because a high crystallization water content causes a transformation from goethite to hematite due to dehydration during the heating process before reduction. This process forms pores in the ore, preventing the formation of a dense wüstite phase, which causes reduction stagnation in metallic iron production. Furthermore, when the crystallization water content was 2.0% by mass or less, reduction at 400 to 590°C achieved a reduction rate of 90% more quickly than reduction at 600 to 700°C, demonstrating its superiority.

[0138] Example 4 Fluidized bed reduction was carried out on raw material powders (cases b and c) in Example 3. Specifically, a dispersion plate made of baked glass beads was placed inside a vessel with an inner diameter of 35 mm, and raw material powder was packed onto the dispersion plate. The thickness of the raw material powder layer was 35 mm. The tip of a thermocouple was positioned inside the raw material powder layer. A heating mechanism was installed on the outer periphery of the vessel, allowing the interior of the vessel to be heated. N was blown from below the vessel. ２ Gas was supplied into the vessel, and the temperature was raised while fluidizing the raw material powder. After the fluidized bed reached the temperature shown in Table 5, H ２ Gas was supplied. The gas flow rate was set under the conditions shown in Table 5. The minimum fluidization velocity of raw material powders (cases b and c) was 0.03 m / s, and the terminal velocity was 1.4 m / s. The reduction time was 2 hours from the start of the reaction. In addition, using a pressure probe with its tip positioned inside the raw material powder layer and a pressure probe with its tip positioned above the raw material powder layer, the pressure in the raw material powder layer (fluidized bed) during gas flow and the pressure in the space above the raw material powder layer were continuously measured to determine whether the fluidized bed was stable. If aggregation progressed excessively or if gas bypass (channeling) occurred in the region where the raw material 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 would occur.

[0139] In the reduction treatment, the time required for the reduction rate to reach 90% was measured and compared with the time required for the reduction rate to reach 90% when the reduction treatment was performed at 600°C. If the time required for the reduction rate to reach 90% was shorter than when the reduction treatment was performed at 600°C, the reduction efficiency was evaluated as A, if it was about the same, it was evaluated as B, and if the reduction rate did not reach 90%, it was evaluated as C. An evaluation of A was considered to be pass, and an evaluation of B or C was considered to be fail.

[0140] The time required for the reduction rate to reach 90% was measured as follows: The residual hydrogen concentration of the exhaust gas passing through the fluidized bed was measured by gas chromatography, and the amount of hydrogen gas consumed by the reduction reaction during passage through the fluidized bed was calculated from the difference between this and the supplied hydrogen gas concentration. The percentage of iron oxide reduced out of the weight of iron oxide converted to hematite, which was estimated in advance based on the input amount and chemically analyzed component values, was calculated, and the time elapsed from the start of hydrogen gas flow until the reduction rate reached 90% was measured.

[0141] Furthermore, cases where sticking did not occur during the reduction treatment were rated as A (pass), and cases where sticking occurred were rated as B (fail). The occurrence of sticking was judged as follows. That is, when the pressure loss in the fluidized bed portion measured during the reduction treatment was found to have decreased significantly by more than the weight loss due to reduction, the powder removed after the reduction treatment was sieved through a sieve with meshes corresponding to the maximum particle size of the raw material powder originally charged, and when residue remained on the meshes, it was judged that sticking had occurred.

[0142] In Example No. 33 in Table 5, a hot-molded reduced iron compact was produced using the reduced iron without contact with an oxidizing atmosphere, and then the compact was taken out into the atmosphere. In Examples Nos. 34 to 41 and 43 to 47, the reduced iron was heat-treated using a fluidized bed without contact with an oxidizing atmosphere, and then the compact was taken out into the atmosphere. The atmosphere used in these treatments ("Gas Type" in Table 5) was the gas flow rate and treatment temperature shown in Table 5. In Examples Nos. 26 to 32 and 42, the temperature of the reduced iron was lowered to 40°C after reduction and then the reduced iron was taken out into the atmosphere. If the maximum surface temperature of the reduced iron measured with a radiation thermometer within 1 minute after taking out into the atmosphere was 50°C or less, the oxidizing property was evaluated as A. If the maximum surface temperature of the reduced iron was 200°C or more, the oxidizing property was evaluated as B. A rating of A was considered a pass, and a rating of B was considered a fail.

[0143]

[0144] In Example No. 26, the reduction temperature was too low at 300° C., so the reduction rate did not reach 90%. Note that Example No. 26 was excluded from the evaluation of oxidizing properties because the reduction rate did not reach 90%.

[0145] In Example No. 27, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 400°C, and the evaluation result of the reduction rate (reduction efficiency) was acceptable. In Example No. 27, the cooled reduced iron was taken out directly into the air after reduction, which caused oxidation and generated heat at a high temperature, and the evaluation of oxidizability was unacceptable.

[0146] In Example No. 28, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 500°C, and the evaluation result of the reduction rate was pass. In Example No. 28, the cooled reduced iron was taken out directly into the air after reduction, so oxidation occurred and heat was generated at a high temperature, and the evaluation of oxidizability was fail.

[0147] In Example No. 29, the temperature during reduction was 600° C., and the evaluation result of the reduction rate was unacceptable. In addition, in Example No. 29, the cooled reduced iron was taken out directly into the air after reduction, so oxidation occurred and heat was generated at a high temperature, and the evaluation of oxidizability was unacceptable.

[0148] In Example No. 30, the temperature during reduction was 700° C., and the evaluation result of the reduction rate was unacceptable. In Example No. 30, the cooled reduced iron was taken out directly into the air after reduction, so oxidation occurred, generating heat to a high temperature, and the evaluation of oxidizability was unacceptable.

[0149] In Example No. 31, the reduction temperature was 800° C., and the evaluation result of the reduction rate was acceptable. However, excessive aggregation occurred due to reduction at a high temperature.

[0150] In Example No. 32, the reduction temperature was 900° C., and the evaluation result of the reduction rate was acceptable. However, excessive aggregation occurred due to reduction at a high temperature.

[0151] In Example No. 33, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 400°C, and the evaluation result of the reduction rate was acceptable. In Example No. 33, HBI was produced to reduce its surface area and then taken out into the air, so the evaluation of oxidizability was acceptable.

[0152] In Example No. 34, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 500°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 750° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0153] In Example No. 35, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 525°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 750° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0154] In Example No. 36, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 750° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0155] In Example No. 37, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 575°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 750° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0156] In Example No. 38, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was acceptable. In Example No. 38, the surface area of ​​the reduced iron was reduced by heat treatment at 750°C in an Ar gas atmosphere using BFB, and therefore the evaluation of the oxidizability was acceptable.

[0157] In Example No. 39, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 750° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0158] In the example of No. 40, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was acceptable. ２ Although the heat treatment was carried out in a gas atmosphere, the heat treatment temperature was low at 550° C., so the surface area of ​​the reduced iron was not reduced sufficiently, and the evaluation of the oxidizability was unacceptable.

[0159] In Example No. 41, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was acceptable. ２The surface area of ​​the reduced iron was reduced by heat treatment at 800° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0160] In Example No. 42, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was acceptable. In Example No. 42, the cooled reduced iron was taken out into the air as it was, and oxidation occurred, generating heat to a high temperature, and the evaluation of oxidizability was unacceptable.

[0161] In the example of No. 43, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was acceptable. ２ Although the heat treatment was carried out in a gas atmosphere, the heat treatment temperature was low at 650° C., so the surface area of ​​the reduced iron was not reduced sufficiently, and the evaluation of oxidation resistance was unacceptable.

[0162] In the example of No. 44, the content of water of crystallization was 3.5 mass% or less, the temperature during reduction was 550°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 800° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0163] In the example of No. 45, a raw material powder having a crystal water content of 3.5 mass% or less was used, and a two-stage fluidized bed using CFB and BFB was used, and the temperature during reduction was set to 550°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 750° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0164] In the example of No. 46, a raw material powder having a crystal water content of 3.5 mass% or less was used, and a two-stage BFB was used at a reduction temperature of 550°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 750° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0165] In Example No. 47, a raw material powder having a crystal water content of 3.5 mass% or less was used, and a two-stage CFB was used at a reduction temperature of 550°C, and the evaluation result of the reduction rate was acceptable. ２ The surface area of ​​the reduced iron was reduced by heat treatment at 750° C. in a gas atmosphere, and the evaluation of the oxidizability was found to be acceptable.

[0166] Example 5 For the examples of raw material powders A to H in Example 1 and mixed powders (cases) c, k, and l in Example 3, the time required to reach a reduction rate of 90% when the reduction temperature was 400 to 590°C or 600 to 700°C is shown in Table 6. In Table 6, the reduction temperature "500" indicates that the reduction temperature was 400 to 590°C, and "700" indicates that the reduction temperature was 600 to 700°C.

[0167]

[0168] It was found that for raw material powders with low crystal water contents, particularly raw material powders A to E and mixed powder c with 3.5 mass % or less, the time to reach a reduction rate of over 90% was shorter when reduced at 500°C than when reduced at 700°C. On the other hand, for raw material powders F to H and mixed powders k and l with crystal water contents of over 3.5 mass %, the time to reach a reduction rate of over 90% was shorter when reduced at 700°C than when reduced at 500°C.

[0169] 10 Bubbling fluidized bed forming device 20 Circulating fluidized bed forming device 30 Spout fluidized bed forming device

Claims

1. A method for producing reduced iron by bringing a reducing gas into contact with a raw material powder containing iron oxide using a fluidized bed, the method comprising a fluidized bed reduction step in which the raw material powder, which has a crystal water content of 3.5 mass% or less, is reduced at a temperature of 400°C or higher and 590°C or lower.

2. The method for producing reduced iron according to claim 1, wherein the raw material powder is raw material powder that has not been subjected to a process for removing water of crystallization by heat treatment at 105°C or higher.

3. A method for producing reduced iron according to claim 1 or 2, wherein the crystal water content of the raw material powder is 3.0 mass % or less.

4. The method for producing reduced iron according to claim 1 or 2, wherein the content of water of crystallization in the raw material powder is 1.0 mass % or less.

5. A method for producing reduced iron according to any one of claims 1 to 4, wherein the raw material powder is a mixed raw material powder comprising a mixture of a plurality of different iron ore powders containing iron oxide, and at least one type of raw material powder among the plurality of different iron ore powders has a crystal water content of 5.0 mass% or more.

6. A method for producing reduced iron according to any one of claims 1 to 5, wherein the temperature is 400°C or higher and 560°C or lower.

7. A method for producing reduced iron according to any one of claims 1 to 5, wherein the temperature is 500°C or higher and 530°C or lower.

8. The method for producing reduced iron according to any one of claims 1 to 7, wherein the reducing gas is hydrogen gas.

9. A method for producing reduced iron according to any one of claims 1 to 8, wherein in the fluidized bed reduction step, the raw material powder is reduced until a metallization rate reaches 70% or more.

10. The method for producing reduced iron according to any one of claims 1 to 9, further comprising an oxidation suppression step of suppressing oxidation of the reduced iron after the fluidized bed reduction step.

11. The method for producing reduced iron according to claim 10, wherein the oxidation suppression step comprises a process for reducing the surface area of ​​the reduced iron after the fluidized bed reduction step before the raw material powder after the fluidized bed reduction step is taken out into an oxidizing atmosphere.

12. The method for producing reduced iron according to claim 11, wherein the treatment for reducing the surface area of ​​the reduced iron comprises heat treating the reduced iron in an inert atmosphere at a temperature of 720°C or higher.

13. The method for producing reduced iron according to claim 10, wherein the oxidation suppression step is a heat treatment step in which the reduced iron after the fluidized bed reduction step is heat treated with a non-oxidizing gas.

14. A method for producing reduced iron as set forth in claim 13, wherein the heat treatment step is a fluidized bed heat treatment step in which the reduced iron after the fluidized bed reduction step is heat treated with a non-oxidizing gas using a fluidized bed that is the same as or different from the fluidized bed used in the fluidized bed reduction step, 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 reduction step.

15. The method for producing reduced iron according to claim 14, wherein the temperature in the fluidized bed in the fluidized bed heat treatment step is set to 720°C or higher.

16. The method for producing reduced iron according to claim 14 or 15, wherein the fluidized bed in the fluidized bed heat treatment step is a bubbling fluidized bed.

17. The non-oxidizing gas is N ２ The method for producing reduced iron according to any one of claims 13 to 16, wherein the oxygen gas is a nitrogen gas or an Ar gas.

18. The method for producing reduced iron according to any one of claims 14 to 16, wherein the same fluidized bed is used in the fluidized bed reduction step and the fluidized bed heat treatment step.

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