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

TITLE OF INVENTION: METHOD FOR PRODUCING REDUCED IRON TECHNICAL FIELD

[0001] The present invention relates to a method for producing reduced iron. Priority is claimed on Japanese Patent Application No. 2024-036243, filed March 8, 2024, and Japanese Patent Application No. 2024-053725, filed March 28, 2024, the contents of which are incorporated herein by reference. BACKGROUND ART

[0002] In the related art, a direct reduction method has been known as one of ironmaking methods for obtaining iron by reducing raw materials containing iron oxide. The direct reduction method has advantages over a blast furnace method, such as lower plant manufacturing costs and easier operation. The direct reduction method includes a method using a shaft furnace or a fluidized bed. In the method using the shaft furnace, a raw material agglomerated into pellets or the like is used. The method using the fluidized bed is a processing method in which solid grains are blown up by a fluid such as a gas to be in a floating and suspended state, and a chemical reaction, heat exchange, and the like are efficiently performed by using a size of a contact area. When the fluidized bed is used in an ironmaking process for obtaining reduced iron from iron ore fines, costs and CO2 emissions associated with agglomeration are not required as compared with the method using the shaft furnace. In addition, hydrogen can be used as a reducing material in these direct reduction methods. By using hydrogen, it is possible to reduce CO2 emissions. In the reduction of iron ore fines using a fluidized bed (hereinafter, the reduction of iron ore fines using a fluidized bed may be referred to as “fluidized bed reduction”), it is important to avoid an agglomeration phenomenon of iron ore fines, which is called sticking, and a stagnation of the reduction reaction within the fluidized bed.

[0004] The occurrence of sticking results in an operational problem in which the iron ore fines do not uniformly fluidize, leading to clogging of the inside of a treatment vessel. Therefore, it is important to operate under efficient conditions in which the iron ore fines can be reduced to a desired reduction degree while avoiding sticking.

[0005] In general, when processing fines in a fluidized bed, a gas flow velocity corresponding to a specific gravity and a grain size of the target fine is set, and a fluidized bed is realized in a treatment vessel at a flow velocity equal to or higher than a minimum gas flow velocity (minimum fluidization velocity) required for the fines to enter a fluidized state. In the fluidized bed reduction technique for obtaining reduced iron, the process is performed at as high a temperature as possible to maximize the reduction rate within a range in which stable fluidization in which sticking does not occur can be realized. However, it is known that sticking is more likely to occur at a higher temperature and is more likely to occur as an iron concentration contained in raw material iron ore fines is higher. Therefore, in the process in the related art, from the viewpoint of increasing productivity while avoiding sticking, the operation is often conducted in a range of about 600°C to 750°C when hydrogen gas is used. For example, in the technique disclosed in Non-Patent Document 1, the operation is conducted at 630°C to 650°C. This is because sticking occurs at about 750°C or higher in many cases in iron ore fines for producing direct reduced iron containing about 64 mass% or more of iron, which has been applied in the conventional operation using actual equipment.

[0006] Regarding the stagnation of the reduction reaction of the iron ore fines (hereinafter, the stagnation of the reduction reaction may be referred to as reduction stagnation), it is known that, when metallic iron is formed on a surface layer of the iron ore fines in a reduction process, the progress of the reduction reaction may be stagnated depending on a formation morphology of the metallic iron covering the iron ore fines, making it difficult for the reduction degree to improve as the reduction proceeds.

[0007] For example, Non-Patent Document 2 discloses that, when a magnetite reagent is used, a remarkable reduction stagnation phenomenon occurs when dense iron is generated from a dense magnetite phase. Non-Patent Document 2 shows that, as a method for avoiding the generation of the dense iron, it is effective to perform an oxidation treatment in advance to convert the magnetite phase into a hematite phase and then refine a structure of grains in association with a phase change from the hematite phase. However, unlike the reagent, the raw material iron ore fines used in an actual operation vary in their chemical components, grain size distribution, properties, and the like depending on their place of origin and mine. Therefore, in the design of an actual operation process, it is required to consider different measures depending on the ore type.

[0008] In addition, Non-Patent Document 3 discloses a study related to the reduction rate using actual iron ore fines. In Non-Patent Document 3, it is considered that reduced iron formed in an outer circumferential portion of the iron ore fines is sintered at a temperature of about 700°C, and the permeability of water vapor generated during reduction deteriorates, so that unreduced substances inside the iron ore fines exhibit poor reducibility.

[0009] The above-described reduction is one of major factors that reduce productivity even in a method using a fluidized bed.

[0010] In order to suppress the reduction stagnation in the fluidized bed, for example, Patent Document 1 discloses a technique for adjusting reduction conditions in a first-stage preheating fluidized bed of a multi-stage fluidized bed to suppress the formation of dense and poor-reducibility magnetite from hematite in the preheating fluidized bed, thereby increasing a metallization rate of a final reduced product.

[0011] In addition, Patent Document 2 discloses a technique for suppressing the formation of a high-density magnetite phase by setting a residence time in a range of about 400°C to 580°C as short as possible, in order to suppress the formation of poor-reducibility magnetite in a first-stage fluidized bed in a multi-stage fluidized bed reduction process. Citation List Patent Documents

[0012] Patent Document 1: Published Japanese Translation No. 2005-502790 of the PCT International Publication Patent Document 2: Published Japanese Translation No. 2001-515144 of the PCT International Publication Non-Patent Documents

[0013] Non-Patent Document 1: 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 to 64 Non-Patent Document 2: THOMAS WOLFINGER, DANIEL SPREITZER, HENG ZHENG, and JOHA NNES SCHENK, “Influence of a Prior Oxidation on the Reduction Behavi or of Magnetite Iron Ore Ultra-Fines Using Hydrogen”, Metall Mater. B, 53B(2022) p.l4to 28, Non-Patent Document 3: J.B.Hendarson, “A Study of the Rate and Mechanism of the Hydrogen Reduction of Fine Hematite Ores”, A.I.M.E Physical Chemis try of Process Metallurgy (1959) p.671 to 688 SUMMARY OF INVENTION Technical Problem

[0014] As described above, the reduction of iron ore fines using a fluidized bed in the related art is performed in a range of 600°C to 750°C. In order to achieve this temperature, not only is a reaction vessel maintained at the aforementioned temperature, but high-temperature treatments are also performed, such as preheating the raw material iron ore fines to about 850°C to 900°C and setting the temperature of the reduction gas to about 700°C to 900°C. Therefore, the selection of furnace materials that can withstand these temperatures, the installation of a cooling mechanism for a mechanical drive unit, the prevention of refractory degradation due to temperature differences in heating and cooling, and the replacement of refractories occur, which are factors that increase equipment costs. From the viewpoint of suppressing the above-described sticking and reducing equipment costs, a low reduction temperature is preferable. However, since the reduction rate of iron increases as the temperature increases, in the related art, a temperature range of about 600°C to 750°C, which is a range in which sticking does not occur, has been selected.

[0016] In addition, Non-Patent Document 3 does not point out that the tendency of reduction stagnation itself differs depending on the ore type. According to the findings of the present inventors, in actual iron ore fines, the presence or absence of the occurrence of reduction stagnation and the degree thereof vary depending on the ore type. Therefore, for various ore types and differences in their specific components and properties, it is not possible to determine the tendency of reduction stagnation until the reduction is actually performed, making the process design difficult. Furthermore, in actual plant operations, it is also considered to mix a plurality of ore types for use; in such a case, when ore types with different characteristics are mixed, the process design is also difficult in terms of predicting what kind of reduction behavior the mixed iron ore fines exhibit.

[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, which can efficiently obtain reduced iron with a high reduction degree while avoiding sticking and stagnation of a reduction reaction in a reduction process of iron ore fines using a fluidized bed, and can reduce equipment costs. Solution to Problem As described above, since the reduction rate increases as the temperature increases, the reduction is typically performed at as high a temperature as possible within a range in which sticking to be avoided in the fluidized bed reduction process does not occur. In addition, it is considered that the reduction reaction significantly decreases in a low temperature range of 590°C or lower; therefore, in most cases, the reduction has been performed at 600°C to 800°C. However, the present inventors have found that reduction stagnation significantly occurs in a range of 600°C to 800°C for iron ore fines having a low combined water (CW) content rate. The present inventors have found that the reduction stagnation in the range of 600°C to 800°C when the combined water content rate is low occurs even in a case of iron ore fines in which a plurality of ore types are mixed. Furthermore, through intensive studies by the present inventors, it has been found that, by reducing iron ore fines having a low combined water content rate at a lower temperature than in the related art, that is, at 590°C or lower, the tendency of reduction stagnation is avoided, and a higher reduction degree than that in the reduction performed in the related art at 600°C to 800°C is reached in a short amount of time.

[0019] The gist of the present invention completed based on the above findings is as follows. [1] A method for producing reduced iron according to one aspect of the present invention uses a fluidized bed and produces reduced iron by bringing a reduction gas into contact with raw material fines containing iron oxide, the method including: a fluidized bed reduction step of reducing the raw material fines having a combined water content of 3.5% by 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], the raw material fines may be raw material fines that have not undergone a step of removing combined water by a heat treatment at 105°C or higher. [3] In the method for producing reduced iron according to [1] or [2], the raw material fines may have a combined water content of 3.0% by mass or less. [4] In the method for producing reduced iron according to [1] or [2], the raw material fines may have a combined water content of 1.0% by mass or less. [5] In the method for producing reduced iron according to any one of [1] to [4], the raw material fines may be mixed raw material fines obtained by mixing a plurality of different iron ore fines containing iron oxide, and at least one raw material fine of the plurality of different iron ore fines may have a combined water content of 5.0% by mass or more. [6] In the method for producing reduced iron according to any one of [1] to [5], 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], 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], the reduction gas may be hydrogen gas. [9] In the method for producing reduced iron according to any one of [1] to [8], in the fluidized bed reduction step, the raw material fines may be reduced until the metallization rate reaches 70% or more.

[10] The method for producing reduced iron according to any one of [1] to [9] may further 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

[10] , in the oxidation suppression step, before the raw material fines after the fluidized bed reduction step are taken out into an oxidizing atmosphere, a treatment of reducing a surface area of the reduced iron after the fluidized bed reduction step may be performed.

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

[11] , in the treatment of reducing the surface area of the reduced iron, the reduced iron may be heat-treated 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 a fluidized bed used in the fluidized bed reduction step, the temperature inside the fluidized bed in the fluidized bed heat treatment step may be higher than the temperature inside the fluidized bed in the fluidized bed reduction step, and a gas flow velocity of the non-oxidizing gas in the fluidized bed in the fluidized bed heat treatment step may be at least 1.5 times as large as the gas flow velocity of the reduction gas in the fluidized bed in the fluidized bed reduction step, or may be at least 4 times 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 inside the fluidized bed in the fluidized bed heat treatment step may be set to 720°C or higher.

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

[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

[13] to

[16] , the non-oxidizing gas may be N2 gas or Ar gas.

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

[14] to

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

[0020] According to the present invention, it is possible to efficiently obtain reduced iron with a high reduction degree while avoiding sticking and stagnation of a reduction reaction in a reduction process of iron ore fines using a fluidized bed, and to reduce equipment costs. BRIEF DESCRIPTION OF DRAWINGS

[0021] [FIG. 1] A schematic diagram of a bubbling fluidized bed forming apparatus. [FIG. 2] A schematic diagram of another example of the bubbling fluidized bed forming apparatus. [FIG. 3] A schematic diagram of a circulating fluidized bed forming apparatus. [FIG. 4] A schematic diagram of a spouted fluidized bed forming apparatus. [FIG. 5] A schematic configuration diagram showing an example of a reduced iron production facility provided with one circulating fluidized bed forming apparatus and three bubbling fluidized bed forming apparatuses as reduction apparatuses. [FIG. 6] A graph showing a transition of a reduction degree during hydrogen reduction of each raw material fine in Example 1. [FIG. 7] A graph (A) showing a transition of a reduction degree during hydrogen reduction of a raw material fine B in Example 1 and a graph (B) showing a transition of the reduction rate. [FIG. 8] A graph showing a transition of a reduction degree during hydrogen reduction of each raw material fine in Example 3. [FIG. 9] A graph showing a transition of a reduction degree during hydrogen reduction of each mixed fine in Example 3. DESCRIPTION OF EMBODIMENTS

[0022] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. Ratios and dimensions of constituent elements in the drawings do not represent actual ratios and dimensions of the constituent elements.

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

[0025] [Fluidized Bed] In this step, a fluidized bed is used for the reduction of the raw material fines. As the fluidized bed, one or more bubbling fluidized beds (BFBs), one or more circulating fluidized beds (CFBs), one or more spouted fluidized beds, or a combination thereof is used. The bubbling fluidized bed is a fluidized bed in a state where bubbles are formed by gas in a fluidized bed formed of the raw material fines. It should be noted that the bubbles have various morphologies depending on a fluidized state, and there are cases where clear bubbles are not formed depending on the fluidized state. The circulating fluidized bed is a fluidized bed in which the raw material fines are circulated together with gas by increasing a gas flow velocity. The spouted fluidized bed includes a spouting section (gas blow-through section) where the raw material fines are blown upward at a high gas flow velocity, and a moving bed where the raw material fines accumulate around the spouting section and move from an upper portion to a lower portion, in which the raw material fines repeat a behavior of being entrained into the high gas velocity flow at a lower portion of the moving bed and being blown upward from the spouting section. Details of each fluidized bed will be described below.

[0026] [Raw Material Fines] The raw material fines are fines containing iron. The raw material fines may be fines obtained by mixing a plurality of different iron ore fines containing iron oxide (hereinafter, may be referred to as mixed raw material fines or mixed fines). Even when reducing the raw material fines obtained by mixing iron ore fines having different components and properties, it is possible to estimate a case where the reduction in a low temperature range is superior due to a combined water content under various mixing conditions from an average combined water content of the mixed raw material fines. The raw material fines are, for example, iron ore fines mined from iron ore mines, iron ore fines obtained by crushing and sieving such iron ore fines, or fines discharged and recovered from various ironmaking processes. Specific examples of the raw material fines include sinter feed, which has been conventionally used as a raw material for sintered ore, pellet feed, which has been used as a raw material for pellets, magnetite concentrate, and ironmaking dust. In addition, the dust referred to herein includes so-called converter dust generated in a converter, fine grains of partially unreduced ore that can be generated in a direct reduced iron production apparatus such as a fluidized bed type reduction apparatus or a shaft furnace type reduction apparatus, fine grains of reduced iron, and the like. The mixed fines are obtained by mixing iron ore fines obtained from different supply sources. In particular, in an ironmaking process represented by a blast furnace method, it is important to adjust components as much as possible on the raw material side to obtain a desired steel product, and it is preferable to use mixed fines obtained by mixing iron ore fines of different origins or brands. The raw material fines used for iron production generally contain about 50% to 70% by mass of Fe. From the viewpoint of reducing energy in a step of separating impurity components, reducing specific ore consumption, and the like, it is preferable to use raw material fines containing 55% to 68% by mass of Fe. Therefore, it is preferable to use mixed fines containing 55% to 68% by mass of Fe.

[0027] The raw material fines have a combined water content of 3.5% by mass or less. The raw material fines having a low combined water content have high density as a mineral structure of grains constituting the raw material fines, and the metal iron formed on the surface layer of the mixed fines by reduction at a certain temperature (about 600°C) or higher covers the mixed fines in a two-dimensional film shape in a dense state. As a result, reduction stagnation is likely to occur. When the combined water content of the raw material fines is 3.5% by mass or less, it is possible to suppress reduction stagnation in a low temperature range of 590°C or lower, and it is possible to efficiently obtain reduced iron having a high reduction degree. The combined water content is preferably 3.0% by mass or less, and more preferably 1.0% by mass or less. The lower limit of the combined water content is not particularly limited, and the combined water content includes a case where the combined water content is less than the detection lower limit. The combined water content of the raw material fines to be used in the present embodiment is preferably 3.5% by mass or less in a state where no heat treatment exceeding 105°C, such as calcination, has been performed, except for a drying treatment conducted for the purpose of removing adhered moisture. Examples of the treatment in which the heating temperature exceeds 105 °C include an ore preheating step, a granulation step, and a pre-oxidation step. This is because, in a case of ore containing a large amount of combined water, cracks or pores are formed in the grains constituting the raw material fines as the combined water is dehydrated during the temperature-rising process before the reduction step; therefore, a tendency for dense metal iron to be formed on the surface layer of the grains is eliminated, and special measures to avoid reduction stagnation are not required. The drying treatment is a treatment mainly for removing adhered moisture, which is performed in a dry air environment or an inert gas atmosphere at a temperature of 200°C or lower, for example, in a range of 100°C to 105°C.

[0028] When the raw material fines are mixed fines formed of a plurality of different iron ore fines, the combined water content of at least one of the iron ore fines is preferably 5.0% by mass or more. The present inventors have found that, although iron ore fines having a high combined water content are not suitable for low-temperature reduction on their own, the iron ore fines can be effectively used in a reduction process in a low temperature range by being mixed with iron ore fines having a low combined water content. Even when the combined water content of at least one iron ore fine is 5.0% by mass or more, when the combined water content of the mixed fines is 3.5% by mass or less, it is possible to suppress reduction stagnation in a low temperature range of 590°C or lower, and it is possible to efficiently obtain reduced iron having a high reduction degree.

[0029] The amount of the combined water is determined in accordance with JIS M 8211:2023 “Iron ores-Determination of combined water-Karl Fischer titrimetric method”. That is, the combined water content can be obtained as a weight proportion of moisture generated while heating the iron ore fines from 105 °C to 950°C with respect to the weight of the iron ore fines.

[0030] When the median diameter of the raw material fines is less than 50 pm, poor fluidization property is exhibited in the treatment in the fluidized bed, so that the median diameter of the raw material fines is 50 pm or more. On the other hand, when the median diameter of the raw material fines is too large, the gas flow velocity required for fluidization becomes too large to exceed the capacity of the practically installable equipment. Therefore, it is desirable that the median diameter of the raw material fines is 8 mm or less. Therefore, the median diameter of the raw material fines is preferably 50 pm or more and 8 mm or less. The grain size of 8 mm or less is within a grain size range of the iron ore fines used as typical sinter feed. In the fluidized bed, reduction is performed using a gas. Therefore, from the viewpoint of increasing the reactivity with the reduction gas and realizing an efficient reduction treatment, the median diameter of the raw material fines is preferably a small grain size in which the surface area is relatively large within the above-described range. The median diameter of the mixed fines is more preferably 50 pm or more and 5 mm or less, and still more preferably 50 pm or more and 2 mm or less.

[0031] The median diameter of the raw material fines can be measured by the following method. That is, a volume-based grain size dso in a cumulative distribution under the sieve, measured using a dry sieve and a laser diffraction-type grain size analyzer (Mastersizer 3000 manufactured by Malvern Panalytical Ltd.,), which is a wet measurement device, is defined as the median diameter of the mixed fines. Setting conditions during the measurement in the laser diffraction-type grain size analyzer are a dispersion medium: water, a refractive index of the dispersion medium: 1.33, and a grain refractive index: 2.918 (refractive index of iron oxide Fe20s). Specifically, the raw material fines are first sieved using dry sieves with nominal openings of 8.0 mm and 1.0 mm, and the mixed fines on the 8.0 mm sieve are excluded. For the mixed fines on the 1.0 mm sieve, a plurality of sieves having different apertures, for example, sieves having openings of 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 are used for sieving, and the grain size distribution is measured. For the raw material fines under the 1.0 mm sieve, the raw material fines are sampled from five arbitrary locations therein, the grain size distribution is measured at each of the five locations using the above-described laser diffraction-type grain size analyzer, and then the grain size distribution obtained by averaging the measurement results at the five locations is calculated. Next, the average grain size distribution for grains more than 1.0 mm measured by sieving and the grain size distribution for grains of 1.0 mm or less measured by the laser diffraction-type grain size analyzer are combined to obtain the overall grain size distribution of the raw material fines under the 8.0 mm sieve. Finally, using this overall grain size distribution, the grain size dso. at which the cumulative value of the relative grain amount on a volume basis becomes 50% when integrated from the smaller grain size side, is defined as the median diameter.

[0032] The method of mixing the plurality of raw material fines is not particularly limited. For example, a plurality of feeders may be provided upstream of a mixed fine supply port for supplying the mixed fines to a container in which a fluidized bed is formed, and different raw material fines may be supplied to the respective feeders to mix the raw material fines inside the container. In addition, a mixing container may be further provided between each feeder and the container, and the raw material fines may be mixed in the mixing container. In addition, the mixed fines obtained by mixing the raw material fines in advance may be supplied to the container from the feeder through the mixed fine supply port.

[0033] The combined water content and the median diameter of the raw material fines need only be quantified for each lot of the raw material fines. In addition, the combined water content and the median diameter of the mixed fines may be quantified for the mixed fines, or may be quantified for each of the raw material fines before mixing and calculated based on the respective quantified values and the mixing ratio. Each of the raw material powders is divided down to 100 g by a sample divider capable of equally dividing the charged fines, and then taken down to 2 g by a coning and quartering method; the quantification is performed using the fine grains arbitrarily sampled three times therefrom. [Reduction Gas] The reduction gas need only be a reduction gas. The reduction gas is, for example, hydrogen gas, a gas mixture of hydrogen and nitrogen, a gas mixture of hydrogen and Ar, a gas mixture of hydrogen and water vapor, a gas mixture of hydrogen, water vapor, and nitrogen, CO gas, or synthesis gas (a gas mixture of carbon monoxide and hydrogen). The reduction gas may contain CH4. In addition, the reduction gas may contain an inert gas. The raw material fines are reduced by the reducing gas. The reduction gas is preferably a gas containing hydrogen gas, and more preferably hydrogen gas. The hydrogen gas has a higher reduction rate than the CO gas, and does not generate carbon dioxide generated when the CO gas or the synthesis gas is used, and thus has a small environmental load. The reduction gas preferably contains 30% by volume or more of hydrogen gas and more preferably contains 50% by volume or more of hydrogen gas. From the viewpoint of CO2 reduction, it is preferable that the amount of the hydrogen gas be high, and it is preferable that the reduction gas contain 70% by volume or more of the hydrogen gas.

[0035] The flow velocity of the reduction gas is equal to or higher 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 is constant with respect to an increase in the gas flow velocity, and the iron ore fines are not fluidized at a gas flow velocity lower than the minimum fluidization velocity. When a circulating fluidized bed is adopted as the fluidized bed, there is no upper limit to the gas flow velocity, but it is preferable to practically suppress the maximum gas flow velocity from the viewpoint of reducing power energy and maintaining a high gas utilization rate. The flow velocity of the reduction gas is, for example, 0.02 m / s or more and 20 m / s or less. The flow velocity of the reduction gas is preferably 0.03 m / s or more and 10 m / s or less. From the viewpoint of stably fluidizing the raw material fines having a large grain size, the flow velocity of the reduction gas is preferably equal to or more than about 1.2 times the minimum fluidization velocity of the raw material fines.

[0036] When the fluidized bed is a bubbling fluidized bed, the flow velocity of the reduction gas is preferably 0.2 m / s or more and less than 1.0 m / s. The flow velocity of the reduction gas for forming the bubbling fluidized bed is lower than the flow velocity of the reduction gas for forming the circulating fluidized bed, and the amount of the raw material fines blown from the bubbling fluidized bed is extremely small. The flow velocity of the reduction 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 reduction 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 an advantage of the circulating fluidized bed, it is preferable that the flow velocity of the reduction gas be set such that a difference (slip velocity) between the average gas velocity and the average velocity of the grains is large. As an example of the raw material fines, the flow velocity of the reduction gas is preferably 3.0 m / s or more and 10 m / s or less, based on the specific gravity of general iron ore fines and the grain size range.

[0038] The flow velocity of the reducing gas is a superficial gas velocity at which the fluidized bed is achieved, and is a value obtained by dividing a gas flow rate per unit time to be supplied by a cross-section area of the fluidized bed part. The flow velocity of the reducing gas can be measured by a flowmeter attached to a gas supply pipe.

[0039] The minimum fluidization velocity can be experimentally measured by the following method. For example, the pressure loss of the fluidized bed is obtained by measuring a pressure difference between the lower and upper portions of the bed where the raw material fines are present (a difference in pressure between a gas plenum section below a distributor and a space section above the bed, which will be described below) and subtracting a differential pressure (pressure loss of the distributor alone) when the raw material fines are not charged. The pressure loss in the fluidized bed is plotted against a superficial gas velocity, and the minimum gas flow velocity at which the pressure loss becomes constant is obtained. It should be noted that in order to eliminate dependence on an initial grain packing structure and acquire reproducible data, the minimum fluidization velocity is determined as a point at which the pressure loss begins to decrease from a region where the pressure loss is constant by gradually decreasing the gas flow velocity from a value sufficient for fluidization. In the measurement of the pressure loss, the measurement position of the pressure is not necessarily limited to the gas plenum section below the distributor and the space section above the bed. The measurement position of the pressure may be set to, for example, the inside of the fluidized bed and the space section above the bed, and need only be any measurement position of the pressure at which the pressure loss of the fluidized bed can be measured.

[0040] The terminal velocity ut (m / s) of the raw material fines can be represented by Equation (1), assuming a first-order approximation of the raw material fines as spheres.

[0041] - * * Equation (T)

[0042] In the above Equation (1), g (m / s2) represents the gravitational acceleration, pp (kg / m3) represents the grain density of the raw material fines, pf (kg / m3) represents the density of the reducing gas, Dp (m) represents the median diameter of the raw material fines, and Cd (-) represents the drag coefficient, which is organized by the Reynolds number Re and is represented below using the approximate expression (Equation (2)) of Brown and Lawler.

[0043] Cd = (24(1 + 0.15Re0681) / Re) + (0.407 / (1 + 8710Re4) ... Equation (2)

[0044] The temperature during the 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 with respect to the raw material fines having a low combined water content described above, and thus reduced iron having a high reduction degree can be efficiently obtained. The reason why the reduction stagnation is avoided at 590°C or lower is not necessarily clear, but the present inventors speculate that, from the Fe-O phase diagram, there is a possibility that the wustite phase that is thermodynamically stable at 570°C or higher contributes to the densification of iron. The present inventors speculate that, when the temperature is 590°C or lower, the wustite phase is less likely to be generated in the reduction process, thus avoiding the reduction stagnation. Since the reduction reaction is performed in the fluidized section in the fluidized bed, the temperature of the fluidized section is set to 400°C or higher and 590°C or lower. The reduction temperature is preferably 500°C or higher. In addition, the reduction temperature is preferably 560°C or lower, and more preferably 530°C or lower.

[0045] The method of controlling the temperature inside the fluidized bed is not particularly limited, but, as a characteristic of the fluidized bed, in a case where the raw material fines are sufficiently fluidized, the mixing of the gas and matter is extremely good and the temperature inside the fluidized bed becomes uniform. Therefore, the temperature control of the fluidized bed is easier compared to other reaction control methods. As the heating method, the reaction temperature inside the fluidized bed can be adjusted by surrounding a reactor equipped with the fluidized bed with a heat insulating material, and supplying and mixing grains and gas that have been heated in advance to a predetermined temperature. In addition, it is also possible to adjust the temperature inside the fluidized bed to a predetermined temperature by heating the reactor from the outside and exchanging heat with a reactor wall surface.

[0046] The temperature inside the fluidized bed is measured using a thermocouple installed such that its tip is positioned in the fluidized section of the raw material fines in the fluidized bed.

[0047] [Average Residence Time] The average residence time of the raw material fines in the reactor need only be determined in consideration of the type of the fluidized bed, the time required to achieve a desired reduction degree depending on the reduction gas composition and the temperature, and the like.

[0048] The average residence time of the raw material fines retained in the bubbling fluidized bed is preferably 3 minutes or longer and 180 minutes or shorter. When the average residence time is 3 minutes or longer, reduced iron having a high reduction degree can be obtained. On the other hand, when the average residence time is 180 minutes or shorter, the processing efficiency of the reduction apparatus is maintained high. In addition, when the average residence time is 180 minutes or shorter, it is suppressed that the recovery efficiency as reduced iron decreases due to the pulverization of raw material fines, which is caused by a decrease in the strength of the raw material fines from excessive reduction, or by collisions between the raw material fines themselves or between the raw material fines and the apparatus during circulation. Therefore, the average residence time is preferably 180 minutes or shorter. The average residence time of the raw material fines retained in the bubbling fluidized bed is more preferably 5 minutes or longer and 60 minutes or shorter. The average residence time can be controlled by changing the reduction gas velocity, the amount of grains retained in the reactor, the grain discharge rate, and the like.

[0049] When the fluidized bed is a circulating fluidized bed, an average redisence time of the iron ore fines retained in the circulating fluidized bed depends on the temperature of the reduction reaction, but is preferably 3 minutes or longer and 180 minutes or shorter. The average redisence time of the raw material fines in the circulating fluidized bed is more preferably 5 minutes or longer and 60 minutes or shorter. The reason why the above average redisence time is preferable is the same as when the fluidized bed is a bubbling fluidized bed.

[0050] When the fluidized bed is a spouted fluidized bed, the average redisence time of the raw material fines retained in the spouted fluidized bed depends on the temperature of the reduction reaction, but is preferably 3 minutes or longer and 180 minutes or shorter. The average redisence time of the raw material fines in the spouted fluidized bed is more preferably 5 minutes or longer and 60 minutes or shorter. The reason why the above average redisence time is preferable is the same as when the fluidized bed is a bubbling fluidized bed.

[0051] The average redisence time of the raw material fines can be calculated by the following method. That is, as tracer grains, for example, ore fines having the same median diameter but different gangue components are charged in a fixed amount, and a time-dependent change in the amount of the gangue components of the discharged reduced iron fines is examined. The time zone in which the amount of the gangue components characterizing the charged tracer ore fines reaches its peak is determined as the average redisence time of the raw material fines. From the above method, the average redisence time can be measured experimentally.

[0052] In this step, at least a part of the raw material fines is reduced. The reduction degree is preferably 70% or more and 100% or less. As the reduction degree increases, the proportion of metallic iron on the surface of the raw material fines increases. As the proportion of metallic iron on the surface of the raw material fines increases, at 600°C to 800°C, the reduction stagnation occurs because metallic iron covers the surface of the raw material fines with a two-dimensional film. On the other hand, at 590°C or lower, covering of the surface of the raw material fines with a two-dimensional film of metallic iron is avoided, and a high specific surface area can be maintained. As the reduction degree decreases, the amount of metallic iron generated decreases, the difference due to the reduction temperature does not occur, and the advantages of the reduction at a low temperature are not realized. Therefore, the reduction degree is preferably 70% or more. The reduction degree is more preferably 80% or more. On the other hand, when the reduction treatment is performed to obtain reduced iron having a high reduction degree of more than 95%, the improvement in the reduction degree may stagnate even in the reduction at a low temperature although this depends on the characteristics of the raw material fines. As a result, the significant difference due to the reduction temperature decreases. Therefore, the reduction degree is more preferably 95% or less.

[0053] The reduction degree is set according to the purpose. In a case where the method for producing reduced iron according to the present embodiment is used as a general method for producing reduced iron for refining in an electric furnace, the final reduction degree is preferably 90% or more. When the reduction degree is 90% or more, the reduced iron can be provided as a final product of reduced iron to, for example, a user who performs refining in the electric furnace. In addition, in a case where the purpose is to charge the reduced iron into a blast furnace as a raw material to lower the consumption rate of reducing agents in the blast furnace, such as coke, or in a case where it is intended for a molten iron-producing electric furnace, the reduction degree does not necessarily have to exceed 90%; for example, the reduction degree may be about 70% to 80%. The reduction degree can be calculated, for example, by the following method. That is, about 0.1 g of the raw material fines is weighed into a quartz cell inside a glove box in a nitrogen atmosphere, and the raw material fines are immersed in benzene to avoid contact with air. The quartz cell is placed in a thermobalance (TGD7000 manufactured by ULVAC-RIKO Inc.,), and the interior of the system is evacuated. Thereafter, nitrogen is allowed to flow at 2.00 x IO4 m3 / min, the temperature is raised to 200°C at a temperature increase rate of 20 °C / min, and benzene is evaporated. Then, the temperature is raised to 700°C at a temperature increase rate of 20 °C / min, and, after the temperature and the balance are stabilized, oxygen is introduced into the system and is held until there is no further increase in weight. Next, the interior of the system is cooled to 100°C or lower, evacuated, and purged with nitrogen, and the temperature is raised again to 700°C at a temperature increase rate of 20 °C / min. Then, the hydrogen gas is flowed at 2.00 x IO4 m3 / min and is held until a change in weight is not recognized. The reduction degree is obtained from Equation (3) based on the above-described change in weight.

[0055] X = {(mpe203 - msample) - O.329x(mpe203 - mpe)} / {0.671 X((mFe203 - mFe)} ... Equation (3) Here, in the equation, X represents the reduction degree (%), mpe203 represents the weight of the raw material fines after oxidation (the weight of the raw material fines when the weight increase disappears after oxygen introduction), msampie represents the mass of the raw material fines, and mpe represents the weight of the raw material fines after reduction (the weight of the raw material fines when the weight increase disappears 27 after hydrogen gas introduction). It can be confirmed by X-ray diffraction that the chemical forms of the raw material fines after oxidation and after reduction by the thermobalance are Fe2Os and Fe, respectively.

[0056] (Oxidation Suppression Step) The 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 degree. In order 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 for suppressing the oxidation of the reduced iron after the fluidized bed reduction step. As an example of the oxidation suppression step, a surface area reduction step and a heat treatment step will be described. Hereinafter, the 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-formed under an inert atmosphere. By hot-forming the reduced iron into hot briquetted iron (HBI), which is a hot-formed product of reduced iron, or compact iron, the surface area of the reduced iron can be reduced, thereby making it possible to suppress its oxidizability. A known hot briquetting method can be applied to the hot briquetting method for obtaining HBI, and for example, a method disclosed in Japanese Unexamined Patent Application, First Publication No. 2009-79292 can be applied. The forming method disclosed in Japanese Unexamined Patent Application, First Publication No. 2009-79292 is a method of manufacturing HBI by nipping a powdery or granular raw material containing a large amount of reduced iron between a pair of rollers having concave molds at a relatively high temperature of 1000°C or lower, for example, at a temperature of 500°C to 800°C. The HBI is cooled to room temperature by a water cooling apparatus. In this hot-forming method, the reduced iron is pressed and formed. Therefore, in order to sufficiently reduce the oxidizability of the reduced iron obtained by the fluidized bed reduction, it is desirable to obtain a high-density formed product such that cavities between the reduced iron grains are minimized as much as possible. The atmosphere in the surface area reduction step is preferably an inert atmosphere. In addition, the temperature during the forming 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. As the fluidized bed, one or more bubbling fluidized beds, one or more circulating fluidized beds, one or more spouted fluidized beds, or a combination thereof is used. The fluidized bed used in this step may be a fluidized bed formed in the container used in the fluidized bed reduction step. By using the same container in this step as in the fluidized bed reduction step, the reduced iron does not need to be transferred to another container, thus enabling efficient processing of the reduced iron. In addition, since the same container is used, the reduced iron after the fluidized bed reduction step is not exposed to the atmosphere, and thus the oxidation of the reduced iron is further suppressed. When the container used in the fluidized bed reduction step and the container used in the fluidized bed heat treatment step are different from each other, it is preferable to transfer the reduced iron after the fluidized bed reduction step under a nonoxidizing 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, N2, He, Ne, Ar, Kr, or Xe. The reduced iron is heat-treated under a non-oxidizing gas environment, and the specific surface area thereof is reduced. The non-oxidizing gas is preferably N2 gas or Ar gas.

[0061] It is preferable that the gas flow velocity of the non-oxidizing gas be at least 1.5 times as large as the gas flow velocity of the reduction gas in the fluidized bed in the fluidized bed reduction step, or be at least 4 times the minimum fluidization velocity of the fluidized bed in the fluidized bed heat treatment step. Basically, the latter applies when the gas flow velocity of the reduction gas in the fluidized bed reduction step is high, and the former applies when the gas flow velocity is low. When the fluidized bed in the fluidized bed reduction step is a circulating fluidized bed, the latter applies, that is, the gas flow velocity of the non-oxidizing gas is set to at least 4 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 the amount of the raw material fines blown from the bubbling fluidized bed is extremely small. Therefore, when the fluidized bed is a bubbling fluidized bed, the gas flow velocity of the non-oxidizing gas may be equal to or higher than the minimum fluidization velocity and equal to or lower than the terminal velocity.

[0062] The flow velocity of the non-oxidizing gas is a superficial gas velocity at which the fluidized bed is achieved, and is a value obtained by dividing a gas flow rate per unit time to be supplied by a cross-section area of the fluidized bed part. The flow velocity of the non-oxidizing gas can be measured by a flowmeter attached to a gas supply pipe.

[0063] The temperature inside the fluidized bed in the fluidized bed heat treatment step is preferably higher than the temperature inside the fluidized bed in the fluidized bed reduction step. By setting the temperature inside the fluidized bed in the fluidized bed heat treatment step to be higher than the temperature inside the fluidized bed in the fluidized bed reduction step, the specific surface area of the reduced iron can be reduced. Under the condition in which the temperature inside the fluidized bed in the fluidized bed heat treatment step is higher than the temperature inside the fluidized bed in the fluidized bed reduction step, the temperature can be set to, for example, 700°C or higher and 900°C or lower. From the viewpoint of decreasing the specific surface area in a short amount of time, the temperature inside the fluidized bed in the fluidized bed heat treatment step is preferably 720°C or higher and more preferably 750°C or higher. In addition, in order to suppress the aggregation of the reduced iron, the temperature inside the fluidized bed in the fluidized bed heat treatment step is 850°C or lower. Therefore, the temperature inside the fluidized bed in the fluidized bed heat treatment step is preferably 720°C or higher and 850°C or lower, and more preferably 750°C or higher and 850°C or lower.

[0064] The temperature inside the fluidized bed is measured by a thermocouple installed such that its tip is positioned in the fluidized section of the reduced iron in the fluidized bed.

[0065] In this step, since the reduced iron can be heat-treated at a high temperature, the specific surface area can be reduced, thereby suppressing the oxidation of the reduced iron after the heat treatment. In addition, the reducing gas is not used in the fluidized bed heat treatment step. In a case where a reducing gas containing hydrogen gas is used in the fluidized bed heat treatment step, a large amount of hydrogen gas is required in order to maintain the fluidized state. On the other hand, in the fluidized bed heat treatment step, the amount of hydrogen consumed by the reduction of the raw material fines is small, and a large amount of hydrogen gas containing H2O generated by the reduction of the raw material fines is discharged to the outside of the container. In order to reuse the discharged hydrogen gas, it is necessary to lower the temperature of the hydrogen gas for dehydration and then raise the temperature again. Therefore, energy load is extremely large. In the fluidized bed heat treatment step, the heat treatment is performed using a non-oxidizing gas that does not contribute to the reaction and can be used at a low cost, thereby suppressing the energy load. In addition, when a reducing gas is used, further reduction proceeds during the heat treatment performed at a temperature higher than that in the reduction treatment step, and sticking may occur due to the generation of fresh active metallic iron on the surface. The heat treatment of the reduced iron is not limited to the above-described fluidized bed heat treatment step, and can be performed using a furnace capable of controlling the temperature and the atmosphere, and can be performed by, for example, a known rotary kiln. When an internal heating rotary kiln is used in the heat treatment step, in order to suppress variations in the atmosphere temperature, it is preferable that an introduction direction of the non-oxidizing gas be opposite to a traveling direction of the raw material fines (countercurrent contact). When an external heating type rotary kiln is used, the introduction direction of the non-oxidizing gas is not limited.

[0067] For example, in the heat treatment step, the heat treatment is performed under the condition that, when raw material fines before the heat treatment step, which are the raw material fines after the fluidized bed reduction step and before the heat treatment step, and raw material fines after the heat treatment step, which are the raw material fines after the heat treatment step, are sieved in the following order using a sieve having an opening of 5 mm, a sieve having an opening of 3 mm, a sieve having an opening of 1 mm, a sieve having an opening of 0.5 mm, and a sieve having an opening of 0.25 mm, the ratio of the total mass on a sieve having a predetermined opening in the sieving of the post-heat treatment raw material fines to the total mass on a sieve having a predetermined opening in the sieving of the pre-heat treatment raw material fines is 1.0 times or more and 1.2 times or less, where the predetermined opening is the one at which the total mass on the sieve reaches 20% by mass or more in the sieving of the raw material fines before the heat treatment step. When the ratio is 1.0 times or more and 1.2 times or less, it can be said that the aggregation of the raw material fines is suppressed and the heat treatment step is stably performed. (Apparatus Configuration Example) Here, an example of an apparatus configuration to which the method for producing reduced iron according to the present embodiment can be applied will be described with reference to the drawings. As the reduction apparatus, 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 spouted fluidized bed forming apparatuses that form a spouted fluidized bed, or a combination thereof is used. 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 the apparatus capable of forming the 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 spouted fluidized bed.

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

[0070] For example, as shown in FIG. 1, the container 11 includes a gas supply port 111 that is disposed at a bottom part and through which a reduction gas is supplied to the inside of the container 11, a raw material fine supply port 112 through which raw material fines are supplied, and a distributor 113 disposed above the gas supply port 111.

[0071] The bubbling fluidized bed is formed by the raw material fines flowing by a gas that is supplied from the gas supply port 111 and that is rectified through a plurality of vent holes of the distributor 113. Examples of a ventilation method and form of the gas for forming the bubbling fluidized bed include, in addition to a flat sheet-type distributor 113 such as a porous plate or a slit plate, a simple nozzle type, a cap type provided with a cap with various forms of blowout holes at a nozzle tip, and a pipe type with a grid tube having a plurality of holes in a side surface of a tube. Specific forms thereof are not limited as long as the reduction gas is supplied into the container 11 to allow the raw material fines to be blown up and form a fluidized bed.

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

[0073] In addition, the bubbling fluidized bed forming apparatus may have a container therein for forming a plurality of bubbling fluidized beds. FIG. 2 is a schematic diagram of another example of a container in the apparatus capable of forming the bubbling fluidized bed. A container 11A may include, for example, a raw material fine supply port 112A provided on one side surface in a longitudinal direction, an outlet 114 provided on the other side surface in the longitudinal direction, a plurality of gas supply ports 111 A arranged in parallel in the longitudinal direction, a distributor 113A provided above each of the gas supply ports 111 A, and a partition plate 115 provided between the gas supply ports 111 A adjacent to each other. A space between an inner wall of the container 11A and the partition plate 115 and a space between the partition plates 115 adjacent to each other are spaces in which the fluidized bed is formed. The height of the partition plate 115 is lower than the height of the bubbling fluidized bed. In the container 11A having such a configuration, an average redisence time of the raw material fines can be increased, and a final reduction degree can be increased. It goes without saying that the installation positions and the number of the raw material fine supply ports, the installation positions and the number of the outlets, the installation positions and the number of the partition plates, and the like are not limited to the aspect shown in FIG. 2 and may be changed as appropriate.

[0074] [Circulating Fluidized Bed Forming Apparatus] As shown in FIG. 3, a circulating fluidized bed forming apparatus 20 includes, for example, a riser section 21 that is a container in which the raw material fines form a fluidized bed, a cyclone 22 connected to an outlet 214 provided at an upper portion of the riser section 21, and a circulation line 23 that extends downward from a bottom part of the cyclone 22 and that is connected to a lower portion of the riser section 21. The circulating fluidized bed forming apparatus 20 can include, as necessary, a dry dust collector 24 that is connected to the cyclone 22 and that recovers pulverized iron ore fines or reduced iron (dust) contained in off-gas.

[0075] The riser section 21 includes a gas supply port 211, a raw material fine supply port 212, and a distributor 213 disposed above the gas supply port 211. The riser section 21 is basically the same as the container 11 in the bubbling fluidized bed forming apparatus 10.

[0076] The cyclone 22 collects grains scattered together with exhaust gas. The collected grains are returned to the riser section 21 through the circulation line 23, and the exhaust gas is discharged to the outside of the circulating fluidized bed forming apparatus 20 via the dry dust collector 24.

[0077] The circulation line 23 includes a downcomer 231 that is connected to a lower portion of the cyclone 22 and that serves as a flow path of the raw material fines separated from the gas in the cyclone 22, and a loop seal section 232 having one end connected to a lower end of the downcomer 231 and the other end connected to a side above the distributor 213 of the riser section 21. The loop seal section 232 provides a sealing effect through the temporarily accumulated raw material fines. Examples of a ventilation method and form of the gas for forming the circulating fluidized bed include, in addition to a flat sheet-type distributor 113 such as a porous plate or a slitted plate, a simple nozzle type, a cap type provided with a cap with various forms of blowout holes at a nozzle tip, and a pipe type with a grid tube having a plurality of holes in a side surface of a tube. Specific forms thereof are not limited as long as the reduction gas is supplied into the riser section 21 to allow the raw material fines to be blown up and 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 by using the dry dust collector 24. As the dry dust collector 24, for example, a cyclone, a multiclone, or a ceramic filter can be used. In the circulating fluidized bed forming apparatus 20, for example, as the dry dust collector 24, a cyclone smaller than the cyclone 22 may be provided in series behind the cyclone 22.

[0079] The raw material fines supplied through the raw material fine supply port 212 are fluidized by the reduction gas supplied from the gas supply port 211 and rectified through the plurality of vent holes of the distributor 213. Specifically, the raw material fines are transported upward from below inside the riser section 21, and pass through the cyclone 22 and the circulation line 23, thereby circulating inside the circulating fluidized bed forming apparatus 20. Therefore, the inside of the circulating fluidized bed forming apparatus 20 forms a circulating fluidized bed. In the loop seal section 232, the raw material fines are temporarily held. When the reduction or the heat treatment by the circulating fluidized bed forming apparatus 20 is a batch-type process, the treated fines are discharged, for example, through an openable and closable outlet (not shown) provided at the lower portion of the cyclone 22 (in the middle of the downcomer 231). When the reduction or the heat treatment is a continuous-type process, for example, a valve of an openable and closable outlet provided in the riser section 21 is opened at regular time intervals or continuously, whereby the processed fines are discharged, and the raw material fines are replenished through the raw material fine supply port 212.

[0080] [Spouted Fluidized Bed Forming Apparatus] A spouted fluidized bed forming apparatus 30 basically has the same configuration as the bubbling fluidized bed forming apparatus 10. It should be noted that the spouted fluidized bed forming apparatus 30 is different from the bubbling fluidized bed forming apparatus 10 in that it does not agitate the entire contents uniformly. For example, as shown in FIG. 4, a spouting section 311 (gas blow-through section) where the raw material fines are spouted at a high gas flow velocity above the gas supply port 111 and a moving bed 312 where the raw material fines accumulate around the spouting section and move from the upper portion to the lower portion are formed. The raw material fines repeat a behavior in which the raw material fines in the lower portion of the moving bed 312 are entrained by the reduction gas flowing through the spouting section 311 at a high gas flow velocity, and are blown upward from the spouting section 311. The fluidized bed formed of the spouting section 311 and the moving bed 312 is a spouted fluidized bed 310.

[0081] Each of the fluidized bed for reducing the raw material fines and the fluidized bed for heat-treating the reduced iron may be one circulating fluidized bed, one bubbling fluidized bed, or one spouted fluidized bed, may be a plurality of circulating fluidized beds, a plurality of bubbling fluidized beds, or a plurality of spouted fluidized beds, or may be a combination of one or more circulating fluidized beds, one or more bubbling fluidized beds, and one or more spouted fluidized beds. Since the circulating fluidized bed has a large difference (slip velocity) between an average flow velocity of the reduction gas and an average movement velocity of the raw material fines, the gas that comes into contact with the raw material fines is exchanged frequently. Accordingly, during the reduction, stagnation of the reduction reaction due to the surroundings of the raw material fines approaching an equilibrium state is avoided, and the raw material fines are efficiently reduced. On the other hand, since the average movement velocity of the raw material fines itself is also high, mechanical wear or fracture occurs due to collision between the raw material fines or the like, and dust is likely to be generated. Since the bubbling fluidized bed has a smaller difference (slip velocity) between the average flow velocity of the reduction gas and the average movement velocity of the raw material fines than the circulating fluidized bed, the reduction efficiency of the raw material fines by the bubbling fluidized bed is inferior to the reduction efficiency of the raw material fines by the circulating fluidized bed. On the other hand, the generation of dust tends to be suppressed more than in the circulating fluidized bed, and, by suppressing the gas flow velocity, it is possible to reduce the energy cost for supplying the gas. In the spouted fluidized bed, the raw material fines having a larger grain size can be used as compared with the circulating fluidized bed. In addition, the spouted fluidized bed is advantageous even in a case where it is desired to shorten the redisence time of the raw material fines. It is preferable to determine the configuration of the fluidized bed in consideration of the characteristics of the circulating fluidized bed, the bubbling fluidized bed, and the spouted fluidized bed, and the median diameter, the Fe content, and the like of the raw material fines.

[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 provided with one circulating fluidized bed forming apparatus 20 and three bubbling fluidized bed forming apparatuses 10 as reduction apparatuses. In FIG. 5, a solid line arrow indicates a flow of the fines, and a broken line arrow indicates a flow of the gas.

[0083] A reduced iron production facility 1 shown in FIG. 5 includes one circulating fluidized bed forming apparatus 20 and three bubbling fluidized bed forming apparatuses 10 as reduction apparatuses 2. For example, in the reduced iron production facility 1 shown in FIG. 5, reduced iron 41 is produced as follows. Raw material fines 40 are supplied to a riser section 21 of the circulating fluidized bed forming apparatus 20. The raw material fines 40 supplied to the riser section 21 are reduced by a reduction gas 42 in the circulating fluidized bed. The partially reduced raw material fines 40 are fed into a first stage container 11 in the bubbling fluidized bed forming apparatus 10, and the reduction of the raw material fines 40 proceeds in a bubbling fluidized bed formed by the raw material fines 40 and the reduction gas 42 in the container 11. The raw material fines 40 in the container 11 are sequentially supplied to a second stage container 11 and a third stage container 11 and are reduced. The raw material fines 40 are finally reduced to the reduced iron 41 in the bubbling fluidized bed in the third stage container 11.

[0084] The dry dust collector 12 connected to each of the containers 11 recovers the raw material fines 40 that can be contained in the off-gas, and the recovered raw material fines 40 are fed into each of the containers 11 again.

[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 by the dry dust collector 24 and is recovered.

[0086] As described above, for example, the reduced iron can be produced by the reduced iron production facility provided with one circulating fluidized bed forming apparatus 20 and three bubbling fluidized bed forming apparatuses 10 as the reduction apparatuses. With the reduction apparatus 1 in which the bubbling fluidized bed forming apparatus 10 is provided at a rear stage of the circulating fluidized bed forming apparatus 20, a reduction time can be shortened in an initial stage of reduction in which the reduction reaction tends to rapidly progress due to limited supply of the reduction gas reaching the surface of the raw material fines, and excessive use of the reduction gas can be avoided in a later stage of reduction in which the reduction rate tends to stagnate as it becomes controlled by the mass diffusion rate within the ore. In addition, by providing multiple stages of the bubbling fluidized bed forming apparatuses 10, the average redisence time of the raw material fines can be secured while suppressing variations in the redisence time, so that reduced iron with a desired final reduction degree can be obtained with less variation in quality.

[0087] It goes without saying that the apparatus shown in each drawing is merely an example and is not limited to the aspect shown in the drawing.

[0088] For example, from the viewpoint of realizing stable operation, the pressure loss in the fluidized bed may be configured to be monitored at any time. When agglomeration proceeds excessively, or when a gas bypass phenomenon (channeling) occurs in a region where the raw material fines are segregated, abnormalities occur in that the pressure loss becomes excessively large by the weight of the fluidized bed or more due to clogging of the entire fluidized bed, or the pressure loss approaches zero due to a state in which the gas blows through the fluidized bed without contributing to fluidization. In this case, large-scale maintenance may be required for the production facility. However, in a case where the pressure loss is monitored at any time, it becomes possible to stop the facility at an appropriate time before the maintenance load increases due to the progress of clogging or segregation. Such a configuration is realized by a pressure measurement device.

[0089] In the method for producing reduced iron according to the present embodiment, even in a case where iron ore fines of a plurality of different ore types are mixed, the reduced iron can be produced with high productivity from the raw material fines containing iron, which have a low combined water content and exhibited a significant reduction stagnation phenomenon in the conventional reduction process. In addition, in the method for producing reduced iron according to the present embodiment, by intentionally selecting a low temperature range of 590°C or lower, which is lower than the process temperature adopted in the conventional method, it is possible to reduce the facility cost, such as solving facility issues when using a reducing gas at a high temperature, particularly reducing the refractory deterioration due to high-temperature hydrogen, omitting heat resistance measures for a mechanical drive unit, and downgrading the refractory material. Furthermore, a decrease in the reduction temperature leads to a decrease in the operating cost. Furthermore, even when raw material fines containing ores having a high combined water content that does not exhibit reduction stagnation and raw material fines containing ores having a low combined water content are mixed and used, operating guidelines for efficient reduction can be derived from the average combined water content of the raw material fines after mixing. This indicates that it is possible to select low-temperature operating conditions using ores having a high combined water content while avoiding reduction stagnation, which leads to the realization of an economical ironmaking process using diverse iron ore raw materials, such as ores with low iron grade, in the future.

[0090] The technical scope of the present invention is not limited to the abovedescribed embodiment, and various modifications can be made without departing from the gist of the present invention. In addition, it is possible to appropriately replace the constituent elements in the present embodiment with well-known constituent elements without departing from the gist of the present invention. Examples

[0091] Next, examples of the present invention will be shown, but conditions in the examples are one example of conditions adopted to confirm the feasibility and effect of the present invention, and the present invention is not limited to the conditions used in the following examples. The present invention may adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

[0092] <Example 1> For raw material fines A to H shown in Table 1, the reduction degree transition in hydrogen reduction under a stationary state was measured by thermogravimetry (TG) in order to estimate the hydrogen reduction degree transition in a fluidized bed reduction furnace.

[0093] Table 1 shows the contents of iron (T. Fe), combined water (CW), and gangue components, SiCh, AI2O3, CaO, and MgO, in each raw material fine. Components other than the components shown in Table 1 are impurities such as MnO, P, and S, and the contents thereof were all in a range of 0.001% to 0.3% by mass. The impurities are components having a small influence on the reduction using a fluidized bed. The raw material fines A to C are hematite-based concentrates mainly containing hematite (Fe2O3), the raw material fines D and E are hematite-based iron ore fines, and the raw material fines F to H are goethite (FeO(OH))-based iron ore fines. Here, the raw material fines A to C are fine iron ores that have undergone a beneficiation treatment to increase the iron content, and have a relatively sharp grain size distribution. The raw material fines D to H were dry-classified to 0.25 mm or less.

[0094] [Table 1] Raw material fines T. Fe (%by mass) CW (%by mass) SiO2 (%by mass) AI2O3 (%by mass) CaO (%by mass) MgO (%by mass) A 65.9 0.16 4.90 0.37 0.08 0.05 B 67.8 0.34 1.90 0.27 0.05 0.05 C 67.1 0.48 2.23 0.67 0.01 0.01 D 62.5 0.73 7.07 0.52 0.05 0.06 E 64.5 2.87 1.87 1.38 0.02 0.07 F 61.0 5.05 4.21 2.34 0.19 0.07 G 53.6 8.38 6.55 3.62 0.59 0.21 H 58.2 10.03 4.18 1.48 0.07 0.09 5

[0095] For the thermogravimetric analysis, a high-temperature differential thermobalance TG-DTA / H (Thermo plus EVO2 manufactured by Rigaku Corporation) was used. 15 mg of the raw material fines was put into an alumina sample container, and the sample was heated to the following set temperature at a temperature increase rate 10 of 10 °C / min under a nitrogen gas flow (214 cc / min). The flow rate of the nitrogen gas was set within a range that allowed the internal space of the apparatus to be replaced with a gas sufficiently quickly, while ensuring that the sample was not discharged by being entrained in the gas stream. The reduction temperature was set to six levels at intervals of 100°C between 500°C and 1000°C. After holding the sample at each temperature for 15   60 minutes for weight stabilization, 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 the reduction at a hydrogen concentration of 50% by volume or 25% by volume was measured. Since goethite changes to hematite with the removal of combined water, the reduction degree was calculated based on the sample weight before the introduction of hydrogen, and the weight loss data was converted into a reduction degree, where 100% reduction corresponds to the weight when oxygen in the hematite was completely removed. FIG. 6 shows the measurement results.

[0096] FIG. 6 is a diagram in which graphs of the reduction degree transition for the respective raw material fines shown in Table 1 are arranged in order of the combined water content. It was found that, for raw material fines having a low combined water content, particularly the raw material fines A to E having a combined water content of 3.5% by mass or less, the time required for the reduction degree to reach more than 90% was shorter in the reduction at 500°C than in the reduction in a range of 600°C to 700°C, as indicated by broken lines in FIG. 6. That is, for the raw material fines A to E having a combined water content of 3.5% by mass or less, reduction stagnation was remarkable in a range of 600°C to 700°C. The present inventors speculate the reason for this as follows. Due to the high combined water content, a change from the goethite phase to the hematite phase occurs along with dehydration during the temperature-rising process before the reduction, and cracks or pores are formed in the raw material fines during this process. It is speculated that these pores inhibit the formation of dense, twodimensional film-like metallic iron, which would otherwise cause reduction stagnation during the generation of metallic iron. Furthermore, it was found that, when the combined water content was 1.0% by mass or less, the reduction degree reached 90% earlier in a case of the reduction at 500°C than in a case of the reduction at 800°C.

[0097] FIG. 7 shows, for the raw material fines B, the reduction degree transition (FIG. 7(A)) during hydrogen reduction at five temperature levels set at intervals of 25°C from 500°C to 600°C, and a graph (FIG. 7(B)) of the reduction rate with respect to the reduction degree. The raw material fines B are a hematite concentrate for which the reduction stagnation was remarkably confirmed. From FIGS. 7(A) and 7(B), at 500°C and 525°C, the reduction degree reached 100%, and no reduction stagnation was observed. The reduction stagnation was clearly exhibited at 550°C or higher, and, as can be seen from FIG. 7(A), it was found that the higher the reduction temperature, the earlier the reduction reaction starts to stagnate at a low reduction degree. In addition, regarding the reduction rate, under the conditions of 500°C to 550°C, the peak of the reduction rate was observed near a reduction degree of 10%. It is considered that this is due to the fact that the rate starts to decrease after the formation of the magnetite phase. Under the conditions of 575°C and 600°C, the peak of the reduction rate was observed near a reduction degree of 20% to 30%, and then the reduction rate was significantly decreased near a reduction degree of 50% to 70%. It is considered that the significant decrease in the reduction rate is due to the generation of two-dimensional film-like metallic iron having a dense wustite phase as a primary phase on the surface of the raw material fines. Due to these factors and the fact that a temperature range in which the wustite phase is not stable on the reduction equilibrium diagram overlaps with a temperature of 590°C or lower, it is considered that the avoidance of reduction stagnation at 590°C or lower when using the raw material fines having a low combined water content is due to the suppression of the formation of the wustite phase.

[0098] <Example 2> The raw material fines B, D, and E in Example 1 were subjected to reduction by a fluidized bed. Specifically, a distributor made of sintered glass beads was installed inside a container having an inner diameter of 35 mm, and the raw material fines were loaded onto the distributor. The thickness of the bed of the raw material fines was set to 35 mm. The tip of the thermocouple was positioned inside the bed of the raw material fines. A heating mechanism was installed on the outer periphery of the container, making it possible to heat the inside of the container. N2 gas was supplied into the container from below the container, the temperature was raised while fluidizing the iron ore fines, and, after the fluidized bed reached the temperature shown in Table 2, a gas mixture of 90% by volume of H2 gas and 10% by volume of N2 gas was supplied. The gas flow velocity of the gas mixture was set to the conditions shown in Table 2. The minimum fluidization velocity of the raw material fines B, D, and E was 0.02 m / s, and the terminal velocity was 0.3 m / s. The reduction time was set to 2 hours from the start of the reaction. In addition, by using a pressure probe whose tip was positioned inside the bed of the iron ore fines and a pressure probe whose tip was positioned above the bed of the iron ore fines, a pressure of the bed of the iron ore fines (fluidized bed) during gas flow and a pressure of a space above the bed of the iron ore fines were continuously measured, and it was determined whether the fluidized bed was stable. When agglomeration proceeds excessively, or when a gas bypass phenomenon (channeling) occurs in a region where the iron ore fines are segregated, abnormalities occur in that the pressure loss becomes excessively large by the weight of the fluidized bed or more due to clogging of the entire fluidized bed, or the pressure loss approaches zero due to a state in which the gas blows through the fluidized bed without contributing to fluidization.

[0099] In the reduction treatment, the time until the reduction degree reached 90% was measured and compared with the time required for the reduction degree to reach 90% when the reduction treatment was performed at 600°C. The reduction efficiency was evaluated as A when the time was shorter than the time required for the reduction degree to reach 90% when the reduction treatment was performed at 600°C, the reduction efficiency was evaluated as B when it was equivalent, and the reduction efficiency was evaluated as C when the reduction degree failed to reach 90%. A case where the evaluation was A was classified as a pass, while a case where the evaluation was B or C was classified as a fail.

[0100] The time until the reduction degree reached 90% was measured by the following method. That is, the residual hydrogen concentration of the exhaust gas passing through the fluidized bed was measured by gas chromatography, the amount of hydrogen gas consumed by the reduction reaction during the passage through the fluidized bed was calculated from a difference from the supply hydrogen gas concentration, and the percentage of iron oxide reduced in the weight of iron oxide in terms of hematite assumed based on the charge amount and the chemical analysis component values in advance was calculated, thereby measuring the elapsed time from the start of the hydrogen gas flow to the reduction degree of 90%.

[0101] In addition, a case where sticking did not occur in the reduction treatment was classified as A (pass), and a case where sticking occurred was classified as B (fail). The presence or absence of sticking occurrence was determined by the following method. That is, in a case where the pressure loss was significantly reduced beyond the weight reduction amount due to the reduction from the pressure loss of the fluidized bed portion being measured during the reduction treatment, the fines taken out after the reduction treatment were sieved using a sieve with an aperture corresponding to the maximum grain size of the raw material fines initially charged, and a case where residue remained on the sieve was determined as an occurrence of sticking.

[0102] In addition, in the example of No. 8 in Table 2, hot briquetted iron was produced using the reduced iron after the reduction without being exposed to an oxidizing atmosphere, and then taken out into the atmosphere. In the examples of Nos. 9 to 16, 18, 19, and 21 to 25, the reduced iron after the reduction was subjected to a heat 5 treatment using a fluidized bed without being exposed to an oxidizing atmosphere, and then taken out into the atmosphere. The atmosphere (“gas” in Table 2), the gas flow velocity, and the treatment temperature used in these treatments are as shown in Table 2. In the examples of Nos. 1 to 7, 17, and 20, after the reduction treatment, the temperature of the reduced iron was lowered to less than 40°C, and then taken out into the 10 atmosphere. After being taken out into the atmosphere, when the maximum value of the surface temperature of the reduced iron measured by a radiation-type thermometer within 1 minute was 50°C or lower, the oxidizing property was evaluated as A, and, when the maximum value of the surface temperature of the reduced iron was 200°C or higher, the oxidizing property was evaluated as B. A case where the evaluation was A was 15 classified as a pass, while a case where the evaluation was B was classified as a fail. [Table 2] No. Reduction treatment Oxidation suppression treatment Note Raw material fines Combined water content (% by mass) Fluidized bed Gas flow velocity (m / s) Reduction temperature (°C) Reduction efficiency Sticking Treatment method Gas type Gas flow velocity (m / s) Temperature (°C) Oxidizing property 1 B 0.34 BFB 0.6 300 C A - - - - - Comparative Example 2 B 0.34 BFB 0.6 400 A A - - - - B Inventive Example 3 B 0.34 BFB 0.6 500 A A - - - - B Inventive Example 4 B 0.34 BFB 0.6 600 B A - - - - B Comparative Example 5 B 0.34 BFB 0.6 700 B A - - - - B Comparative Example 6 B 0.34 BFB 0.6 800 A B - - - - A Comparative Example 7 B 0.34 BFB 0.6 900 A B - - - - A Comparative Example 8 B 0.34 BFB 0.6 400 A A HBI n2 - 650 A Inventive Example 9 B 0.34 BFB 0.6 500 A A BFB n2 1 750 A Inventive Example 10 B 0.34 BFB 0.6 525 A A BFB n2 1 750 A Inventive Example 11 B 0.34 BFB 0.6 550 A A BFB n2 1 750 A Inventive Example 12 B 0.34 BFB 0.6 575 A A BFB n2 1 750 A Inventive Example 13 B 0.34 BFB 0.6 550 A A BFB Ar 1 750 A Inventive Example 14 B 0.34 BFB 0.6 550 A A CFB n2 8 750 A Inventive Example 15 B 0.34 BFB 0.6 550 A A BFB n2 1 550 B Inventive Example 16 B 0.34 BFB 0.6 550 A A BFB n2 1 800 A Inventive Example 17 D 0.73 BFB 0.6 550 A A - - - - B Inventive Example 18 D 0.73 BFB 0.6 550 A A BFB n2 1 650 B Inventive Example 19 D 0.73 BFB 0.6 550 A A BFB n2 1 800 A Inventive Example 20 E 2.87 BFB 0.6 550 A A - - - - B Inventive Example 21 E 2.87 BFB 0.6 550 A A BFB n2 1 650 B Inventive Example 22 23 E B 2.87 0.34 BFB CFB + BFB 0.6 0.6 (BFB) 550 550 A A A A BFB BFB n2 n2 1 1 800 750 A A Inventive _____Example_____ Inventive Example 24 B 0.34 Two-stage BFB 0.6 (first), 0.3 (second) 550 A A BFB n2 1 750 A Inventive Example 25 B 0.34 Two-stage CFB 8 (first), 5 (second) 550 A A BFB n2 1 750 A Inventive Example In the example of No. 1, since the temperature during the reduction was too low at 300°C, the reduction degree did not reach 90%. In the example of No. 1, since the reduction degree did not reach 90%, the oxidizing property was not evaluated.

[0105] In the example of No. 2, the combined water content was 3.5% by mass or less, the temperature during the reduction was 400°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 2, since the cooled reduced iron after the reduction was taken out into the atmosphere as it was, oxidation occurred, heat was generated at a high temperature, and the oxidizing property was evaluated as a fail.

[0106] In the example of No. 3, the combined water content was 3.5% by mass or less, the temperature during the reduction was 500°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 3, since the cooled reduced iron after the reduction was taken out into the atmosphere as it was, oxidation occurred, heat was generated at a high temperature, and the oxidizing property was evaluated as a fail.

[0107] In the example of No. 4, the temperature during the reduction was 600°C, and the evaluation result of the reduction rate was a fail. In addition, in the example of No. 4, since the cooled reduced iron after the reduction was taken out into the atmosphere as it was, oxidation occurred, heat was generated at a high temperature, and the oxidizing property was evaluated as a fail. In the example of No. 5, the temperature during the reduction was 700°C, and the evaluation result of the reduction rate was a fail. In addition, in the example of No. 5, since the cooled reduced iron after the reduction was taken out into the atmosphere as it was, oxidation occurred, heat was 5 generated at a high temperature, and the oxidizing property was evaluated as a fail.

[0109] In the example of No. 6, the temperature during the reduction was 800°C, and the evaluation result of the reduction rate was a pass. However, excessive aggregation occurred due to the reduction at a high temperature. 10

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

[0111] 15           In the example of No. 8, the combined water content was 3.5% by mass or less, the temperature during the reduction was 400°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 8, since HBI was produced to reduce the surface area and taken out into the atmosphere, the oxidizing property was evaluated as a 20 pass.

[0112] In the example of No. 9, the combined water content was 3.5% by mass or less, the temperature during the reduction was 500°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 9, since the heat treatment was performed at 750°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass.

[0113] In the example of No. 10, the combined water content was 3.5% by mass or less, the temperature during the reduction was 525°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 10, since the heat treatment was performed at 750°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass.

[0114] In the example of No. 11, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 11, since the heat treatment was performed at 750°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass.

[0115] In the example of No. 12, the combined water content was 3.5% by mass or less, the temperature during the reduction was 575°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 12, since the heat treatment was performed at 750°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass. In the example of No. 13, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 13, since the heat treatment was performed at 750°C in the Ar gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass.

[0117] In the example of No. 14, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 14, since the heat treatment was performed at 750°C in the N2 gas atmosphere using the CFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass.

[0118] In the example of No. 15, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 15, the heat treatment was performed in the N2 gas atmosphere using the BFB, but, since the heat treatment temperature was as low as 550°C, the reduction of the surface area of the reduced iron was not sufficient, and the oxidizing property was evaluated as a fail.

[0119] In the example of No. 16, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 16, since the heat treatment was performed at 800°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass.

[0120] In the example of No. 17, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 17, since the cooled reduced iron after the reduction was taken out into the atmosphere as it was, oxidation occurred, heat was generated at a high temperature, and the oxidizing property was evaluated as a fail.

[0121] In the example of No. 18, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 18, the heat treatment was performed in the N2 gas atmosphere using the BFB, but, since the heat treatment temperature was as low as 650°C, the reduction of the surface area of the reduced iron was not sufficient, and the oxidizing property was evaluated as a fail.

[0122] In the example of No. 19, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 19, since the heat treatment was performed at 800°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass. In the example of No. 20, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 20, since the cooled reduced iron after the reduction was taken out into the atmosphere as it was, oxidation occurred, heat was generated at a high temperature, and the oxidizing property was evaluated as a fail.

[0124] In the example of No. 21, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 21, the heat treatment was performed in the N2 gas atmosphere using the BFB, but, since the heat treatment temperature was as low as 650°C, the reduction of the surface area of the reduced iron was not sufficient, and the oxidizing property was evaluated as a fail.

[0125] In the example of No. 22, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 22, since the heat treatment was performed at 800°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass.

[0126] The example of No. 23 is an example in which a two-stage fluidized bed using the CFB and the BFB was used for the raw material fines having a combined water content of 3.5% by mass or less at the reduction temperature of 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 23, since the heat treatment was performed at 750°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass.

[0127] The example of No. 24 is an example in which a two-stage BFB was used for the raw material fines having a combined water content of 3.5% by mass or less at the reduction temperature of 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 24, since the heat treatment was performed at 750°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass.

[0128] The example of No. 25 is an example in which a two-stage CFB was used for the raw material fines having a combined water content of 3.5% by mass or less at the reduction temperature of 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 25, since the heat treatment was performed at 750°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass.

[0129] <Example 3> For iron ore fines A to C shown in Table 3, the reduction degree transition in hydrogen reduction under a stationary state was measured by thermogravimetry (TG) in order to estimate the hydrogen reduction degree transition in a fluidized bed reduction furnace.

[0130] Table 3 shows the contents (% by mass) of iron (T. Fe), combined water (CW), and gangue components, SiO2, AI2O3, CaO, and MgO, in each iron ore fines sample. Components other than the components shown in Table 3 are impurities such as MnO, P, and S, and the contents thereof were all in a range of 0.001% to 0.3% by mass. The impurities are components having a small influence on the reduction using a fluidized bed. The iron ore fines A are hematite-based concentrates mainly containing hematite (Fe2O3), the iron ore fines B are hematite-based iron ore fines, and the iron ore fines C are goethite (FeO(OH))-based iron ore fines. Here, the iron ore fines A are fine iron ores that have undergone a beneficiation treatment to increase the iron content, and have a relatively sharp grain size distribution. The iron ore fines B and C were dry-classified to 0.25 mm or less.

[0131] [Table 3] T. Fe (%by mass) CW (%by mass) SiO2 (%by mass) AI2O3 (%by mass) CaO (%by mass) MgO (%by mass) Iron ore fines A 67.8 0.34 1.90 0.27 0.05 0.05 Iron ore fines B 64.5 2.87 1.87 1.38 0.02 0.07 Iron ore fines C 53.6 8.38 6.55 3.62 0.59 0.21

[0132] The iron ore fines A to C were mixed at the proportions shown in Table 4, and the combined water content, the gangue content rate, and the iron content of the obtained mixed fines are shown in Table 4.

[0133] [Table 4] case Mixing condition (% by mass) Component (% by mass) Iron ore fines A Iron ore fines B Iron ore fines C CW Gangue T. Fe a 100 0 0 0.34 2.27 67.8 b 83.3 0 16.7 1.68 3.72 65.5 c 66.7 0 33.3 3.02 5.17 63.2 d 50.0 0 50.0 4.36 6.62 60.8 e 33.3 0 66.7 5.70 8.07 58.5 f 16.7 0 83.3 7.04 9.52 56.2 g 0 0 100 8.38 10.97 53.8 h 0 16.7 83.3 7.46 9.70 55.7 i 0 33.3 66.7 6.54 8.43 57.5 j 0 50.0 50.0 5.63 7.16 59.4 k 0 66.7 33.3 4.71 5.88 61.2 1 0 83.3 16.7 3.79 4.61 63.0 m 0 100 0 2.87 3.34 64.9

[0134] For the thermogravimetric analysis, a high-temperature differential thermobalance TG-DTA / H (Thermo plus EVO2 manufactured by Rigaku Corporation) 5 was used. 15 mg of the raw material fines was put into an alumina sample container, and the sample was heated to the following set temperature at a temperature increase rate of 10 °C / min under a nitrogen gas flow (214 cc / min). The flow rate of the nitrogen gas was set within a range that allowed the internal space of the apparatus to be replaced with a gas sufficiently quickly, while ensuring that the sample was not discharged by being 10 entrained in the gas stream. The reduction temperature was set to six levels at intervals of 100°C between 500°C and 1000°C. After holding the sample at each temperature for 60 minutes for weight stabilization, 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 the reduction at a hydrogen concentration of 50% by volume or 25% 15 by volume was measured. Since goethite changes to hematite with the removal of combined water, the reduction degree was calculated based on the sample weight before the introduction of hydrogen, and the weight loss data was converted into a reduction degree, where 100% reduction corresponds to the weight when oxygen in the hematite was completely removed. FIG. 6 shows the measurement results.

[0135] FIG. 8 is a diagram in which graphs of the reduction degree transition for the respective iron ore fines shown in Table 3 are arranged in order of the combined water content. For each iron ore fines sample, the reduction degree was measured by changing the reduction temperature. The reduction stagnation was remarkable in the iron ore fines A having a high iron content, while the reduction stagnation was not observed around 700°C in the iron ore fines C having a high combined water content. In the iron ore fines B whose combined water content is between the combined water content of the iron ore fines A and the combined water content of the iron ore fines C, the reduction behavior showed an intermediate tendency between the reduction behavior of the raw material fines A and the reduction behavior of the raw material fines B.

[0136] FIG. 9 is a diagram in which graphs of the reduction degree transition for the raw material fines (cases) b to f and h to 1 shown in Table 4 are arranged in order of the combined water content. The raw material fines (cases) a, g, and m in Table 4 correspond to the iron ore fines A, C, and B, respectively. Therefore, the graphs of the raw material fines (cases) a, g, and m are shown in FIG. 8. In addition, in each graph, a graph with the legend of “500” is a graph showing the reduction degree transition when the reduction temperature was set to 400°C to 590°C, and a graph with the legend of “700” is a graph showing the reduction degree transition when the reduction temperature was set to 600°C to 700°C.

[0137] As shown in FIG. 9, it was found that the reduction stagnation depends on the combined water content of the raw material fines, and the ore having a smaller combined water content tends to have a stronger reduction stagnation. It was found that, for raw material fines having a low combined water content, particularly the raw material fines having a combined water content of 3.5% by mass or less, the reduction stagnation was remarkable, and the time required for the reduction degree to reach more than 90% was shorter in the reduction at 400°C to 590°C than in the reduction in a range of 600°C to 700°C. It is estimated that this is because the high combined water content leads to a change from the goethite phase to the hematite phase along with dehydration during the temperature-rising process before the reduction, and the formation of pores in the ore during this process inhibits the formation of a dense wustite phase, which otherwise causes reduction stagnation during the production of metallic iron from the wustite phase. When the combined water content was 2.0% by mass or less, the reduction at 400°C to 590°C was even faster and superior to the reduction at 600°C to 700°C in terms of the time required for the reduction degree to reach 90%.

[0138] <Example 4> The raw material fines (cases) b and c in Example 3 were subjected to reduction by a fluidized bed. Specifically, a distributor made of sintered glass beads was installed inside a container having an inner diameter of 35 mm, and the raw material fines were loaded onto the distributor. The thickness of the bed of the raw material fines was set to 35 mm. The tip of the thermocouple was positioned inside the bed of the raw material fines. A heating mechanism was installed on the outer periphery of the container, making it possible to heat the inside of the container. N2 gas was supplied into the container from below the container, the temperature was raised while fluidizing the raw material fines, and, after the fluidized bed reached the temperature shown in Table 5, H2 gas was supplied. The gas flow velocity was set to the conditions shown in Table 5. The minimum fluidization velocity of the raw material fines (cases) b and c was 0.03 m / s, and the terminal velocity was 1.4 m / s. The reduction time was set to 2 hours from the start of the reaction. In addition, by using a pressure probe whose tip was positioned inside the bed of the raw material fines and a pressure probe whose tip was positioned above the bed of the raw material fines, a pressure of the bed (fluidized bed) of the raw material fines during gas flow and a pressure of a space above the bed of the raw material fines were continuously measured, and it was determined whether the fluidized bed was stable. When agglomeration proceeds excessively, or when a gas bypass phenomenon (channeling) occurs in a region where the raw material fines are segregated, abnormalities occur in that the pressure loss becomes excessively large by the weight of the fluidized bed or more due to clogging of the entire fluidized bed, or the pressure loss approaches zero due to a state in which the gas blows through the fluidized bed without contributing to fluidization.

[0139] In the reduction treatment, the time until the reduction degree reached 90% was measured and compared with the time required for the reduction degree to reach 90% when the reduction treatment was performed at 600°C. The reduction efficiency was evaluated as A when the time was shorter than the time required for the reduction degree to reach 90% when the reduction treatment was performed at 600°C, the reduction efficiency was evaluated as B when it was equivalent, and the reduction efficiency was evaluated as C when the reduction degree failed to reach 90%. A case where the evaluation was A was classified as a pass, while a case where the evaluation was B or C was classified as a fail. The time until the reduction degree reached 90% was measured by the following method. That is, the residual hydrogen concentration of the exhaust gas passing through the fluidized bed was measured by gas chromatography, the amount of hydrogen gas consumed by the reduction reaction during the passage through the fluidized bed was calculated from a difference from the supply hydrogen gas concentration, and the percentage of iron oxide reduced in the weight of iron oxide in terms of hematite assumed based on the charge amount and the chemical analysis component values in advance was calculated, thereby measuring the elapsed time from the start of the hydrogen gas flow to the reduction degree of 90%.

[0141] In addition, a case where sticking did not occur in the reduction treatment was classified as A (pass), and a case where sticking occurred was classified as B (fail). The presence or absence of sticking occurrence was determined by the following method. That is, in a case where the pressure loss was significantly reduced beyond the weight reduction amount due to the reduction from the pressure loss of the fluidized bed portion being measured during the reduction treatment, the fines taken out after the reduction treatment were sieved using a sieve with an aperture corresponding to the maximum grain size of the raw material fines initially charged, and a case where residue remained on the sieve was determined as an occurrence of sticking.

[0142] In addition, in the example of No. 33 in Table 5, hot briquetted iron was produced using the reduced iron after the reduction without being exposed to an oxidizing atmosphere, and then taken out into the atmosphere. In the examples of Nos. 34 to 41 and 43 to 47, the reduced iron after the reduction was subjected to a heat treatment using a fluidized bed without being exposed to an oxidizing atmosphere, and then taken out into the atmosphere. The atmosphere (“gas type” in Table 5), the gas flow velocity, and the treatment temperature used in these treatments are as shown in Table 5. In the examples of Nos. 26 to 32 and 42, after the reduction treatment, the 5 temperature of the reduced iron was lowered to 40°C, and then taken out into the atmosphere. After being taken out into the atmosphere, when the maximum value of the surface temperature of the reduced iron measured by a radiation-type thermometer within 1 minute was 50°C or lower, the oxidizing property was evaluated as A, and, when the maximum value of the surface temperature of the reduced iron was 200°C or higher, 10 the oxidizing property was evaluated as B. A case where the evaluation was A was classified as a pass, while a case where the evaluation was B was classified as a fail. [Table 5] No. Reduction treatment Oxidation suppression treatment Note Mixed fines Combined water content (% by mass) Fluidized bed Reduction gas Gas flow velocity (m / s) Reduction temperature (°C) Reduction efficiency Sticking Treatment method Gas type Gas flow velocity (m / s) Temperature (°C) Oxidizing property 26 b 1.68 BFB h2 0.6 300 C A - - - - - Comparative Example 27 b 1.68 BFB h2 0.6 400 A A - - - - B Inventive Example 28 b 1.68 BFB h2 0.6 500 A A - - - - B Inventive Example 29 b 1.68 BFB h2 0.6 600 B A - - - - B Comparative Example 30 b 1.68 BFB h2 0.6 700 B A - - - - B Comparative Example 31 b 1.68 BFB h2 0.6 800 A B - - - - A Comparative Example 32 b 1.68 BFB h2 0.6 900 A B - - - - A Comparative Example 33 b 1.68 BFB h2 0.6 400 A A HBI n2 - 650 A Inventive Example 34 b 1.68 BFB h2 0.6 500 A A BFB n2 1.0 750 A Inventive Example 35 b 1.68 BFB h2 0.6 525 A A BFB n2 1.0 750 A Inventive Example 36 b 1.68 BFB h2 0.6 550 A A BFB n2 1.0 750 A Inventive Example 37 b 1.68 BFB h2 0.6 575 A A BFB n2 1.0 750 A Inventive Example 38 b 1.68 BFB h2 0.6 550 A A BFB Ar 1.0 750 A Inventive Example 39 b 1.68 BFB h2 0.6 550 A A CFB n2 8.0 750 A Inventive Example 40 b 1.68 BFB h2 0.6 550 A A BFB n2 1.0 550 B Inventive Example 41 b 1.68 BFB h2 0.6 550 A A BFB n2 1.0 800 A Inventive Example 42 c 3.02 BFB h2 0.6 550 A A - - - - B Inventive Example 43 c 3.02 BFB h2 0.6 550 A A BFB n2 1.0 650 B Inventive Example 44 c 3.02 BFB h2 0.6 550 A A BFB n2 1.0 800 A Inventive Example 45 b 1.68 CFB + BFB h2 8 (CFB), 0.6 (BFB) 550 A A BFB n2 1.0 750 A Inventive Example 46 b 1.68 Two-stage BFB h2 0.6 (first), 0.3 (second) 550 A A BFB n2 1.0 750 A Inventive Example 47 b 1.68 Two-stage CFB h2 8 (first), 5 (second) 550 A A BFB n2 1.0 750 A Inventive Example In the example of No. 26, since the temperature during the reduction was too low at 300°C, the reduction degree did not reach 90%. In the example of No. 26, since the reduction degree did not reach 90%, the oxidizing property was not evaluated.

[0145] In the example of No. 27, the combined water content was 3.5% by mass or less, the temperature during the reduction was 400°C, and the evaluation result of the reduction rate (reduction efficiency) was a pass. In addition, in the example of No. 27, since the cooled reduced iron after the reduction was taken out into the atmosphere as it was, oxidation occurred, heat was generated at a high temperature, and the oxidizing property was evaluated as a fail.

[0146] In the example of No. 28, the combined water content was 3.5% by mass or less, the temperature during the reduction was 500°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 28, since the cooled reduced iron after the reduction was taken out into the atmosphere as it was, oxidation occurred, heat was generated at a high temperature, and the oxidizing property was evaluated as a fail.

[0147] In the example of No. 29, the temperature during the reduction was 600°C, and the evaluation result of the reduction rate was a fail. In addition, in the example of No. 29, since the cooled reduced iron after the reduction was taken out into the atmosphere as it was, oxidation occurred, heat was generated at a high temperature, and the oxidizing property was evaluated as a fail.

[0148] In the example of No. 30, the temperature during the reduction was 700°C, and the evaluation result of the reduction rate was a fail. In addition, in the example of No. 30, since the cooled reduced iron after the reduction was taken out into the atmosphere as it was, oxidation occurred, heat was generated at a high temperature, and the oxidizing property was evaluated as a fail.

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

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

[0151] In the example of No. 33, the combined water content was 3.5% by mass or less, the temperature during the reduction was 400°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 33, since HBI was produced to reduce the surface area and taken out into the atmosphere, the oxidizing property was evaluated as a pass.

[0152] In the example of No. 34, the combined water content was 3.5% by mass or less, the temperature during the reduction was 500°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 34, since the heat treatment was performed at 750°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass.

[0153] In the example of No. 35, the combined water content was 3.5% by mass or less, the temperature during the reduction was 525°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 35, since the heat treatment was performed at 750°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass.

[0154] In the example of No. 36, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 36, since the heat treatment was performed at 750°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass.

[0155] In the example of No. 37, the combined water content was 3.5% by mass or less, the temperature during the reduction was 575°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 37, since the heat treatment was performed at 750°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass. In the example of No. 38, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 38, since the heat treatment was performed at 750°C in the Ar gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass.

[0157] In the example of No. 39, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 39, since the heat treatment was performed at 750°C in the N2 gas atmosphere using the CFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass.

[0158] In the example of No. 40, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 40, the heat treatment was performed in the N2 gas atmosphere using the BFB, but, since the heat treatment temperature was as low as 550°C, the reduction of the surface area of the reduced iron was not sufficient, and the oxidizing property was evaluated as a fail.

[0159] In the example of No. 41, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 41, since the heat treatment was performed at 800°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass.

[0160] In the example of No. 42, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 42, since the cooled reduced iron after the reduction was taken out into the atmosphere as it was, oxidation occurred, heat was generated at a high temperature, and the oxidizing property was evaluated as a fail.

[0161] In the example of No. 43, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 43, the heat treatment was performed in the N2 gas atmosphere using the BFB, but, since the heat treatment temperature was as low as 650°C, the reduction of the surface area of the reduced iron was not sufficient, and the oxidizing property was evaluated as a fail.

[0162] In the example of No. 44, the combined water content was 3.5% by mass or less, the temperature during the reduction was 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 44, since the heat treatment was performed at 800°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass. The example of No. 45 is an example in which a two-stage fluidized bed using the CFB and the BFB was used for the raw material fines having a combined water content of 3.5% by mass or less at the reduction temperature of 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 45, since the heat treatment was performed at 750°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass.

[0164] The example of No. 46 is an example in which a two-stage BFB was used for the raw material fines having a combined water content of 3.5% by mass or less at the reduction temperature of 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 46, since the heat treatment was performed at 750°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass.

[0165] The example of No. 47 is an example in which a two-stage CFB was used for the raw material fines having a combined water content of 3.5% by mass or less at the reduction temperature of 550°C, and the evaluation result of the reduction rate was a pass. In addition, in the example of No. 47, since the heat treatment was performed at 750°C in the N2 gas atmosphere using the BFB to reduce the surface area of the reduced iron, the oxidizing property was evaluated as a pass. <Example 5> For the examples of the raw material fines A to H in Example 1 and the mixed fines (cases) c, k, and 1 in Example 3, Table 6 shows the time to reach the reduction degree of 90% when the reduction temperature was set to 400°C to 590°C or 600°C to 700°C. In Table 6, the reduction temperature “500” indicates that the reduction temperature was 400°C to 590°C, and “700” indicates that the reduction temperature was 600°C to 700°C.

[0167] [Table 6] Reduction temperature [°C] Time required for reduction degree to reach 90% [min] A B C D E F G H c k 1 500 22.5 16.2 15.1 17 19.8 21.6 >60 33.1 53.1 57.7 18.2 700 >60 >60 >60 >60 37.1 18.8 38.4 8.4 >60 22.4 7.4

[0168] It was found that, for raw material fines having a low combined water content, particularly the raw material fines A to E and the mixed fines c having a combined water content of 3.5% by mass or less, the time required for the reduction degree to reach more than 90% was shorter in the reduction at 500°C than in the reduction at 700°C. On the other hand, for the raw material fines F to H and the mixed fines k and 1 having a combined water content of more than 3.5% by mass, the time required for the reduction degree to reach more than 90% was shorter in the reduction at 700°C than in the reduction at 500°C. REFERENCE SIGNS LIST

[0169] 10 Bubbling fluidized bed forming apparatus 20 Circulating fluidized bed forming apparatus 30 Spouted fluidized bed forming apparatus

Claims

1. A method for producing reduced iron, the method using a fluidized bed and producing reduced iron by bringing a reduction gas into contact with raw material fines containing iron oxide, the method comprising:a fluidized bed reduction step of reducing the raw material fines having a combined water content of 3.5% by mass or less 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 fines are raw material fines that have not undergone a step of removing combined water by a heat treatment at 105°C or higher.

3. The method for producing reduced iron according to Claim 1 or 2,wherein the raw material fines have a combined water content of 3.0% by mass or less.

4. The method for producing reduced iron according to Claim 1 or 2,wherein the raw material fines have a combined water content of 1.0% by mass or less.

5. The method for producing reduced iron according to any one of Claims 1 to4,wherein the raw material fines are mixed raw material fines obtained by mixing a plurality of different iron ore fines containing iron oxide, and at least one raw material fine of the plurality of different iron ore fines has a combined water content of 5.0% by mass or more.

6. The 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. The 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 reduction gas is hydrogen gas.

9. The method for producing reduced iron according to any one of Claims 1 to 8,wherein, in the fluidized bed reduction step, the raw material fines are 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, in the oxidation suppression step, before the raw material fines after the fluidized bed reduction step are taken out into an oxidizing atmosphere, a treatment of reducing a surface area of the reduced iron after the fluidized bed reduction step is performed.

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

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

14. The method for producing reduced iron according to Claim 13, wherein the heat treatment step is a fluidized bed heat treatment step of heattreating 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 a fluidized bed used in the fluidized bed reduction step,a temperature inside the fluidized bed in the fluidized bed heat treatment step is higher than a temperature inside the fluidized bed in the fluidized bed reduction step, anda gas flow velocity of the non-oxidizing gas in the fluidized bed in the fluidized bed heat treatment step is at least 1.5 times as large as the gas flow velocity of the reduction gas in the fluidized bed in the fluidized bed reduction step, or is at least 4 times 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 inside 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 method for producing reduced iron according to any one of Claims 13 to 16,wherein the non-oxidizing gas is N2 gas or 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 andthe fluidized bed heat treatment step.

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