Raw materials for direct reduced ironmaking and their manufacturing method

Abstract

Provided is a feedstock for direct-reduction iron manufacturing having excellent reduction resistance and powdering resistance. The feedstock for direct-reduction iron manufacturing comprises: a starting material containing iron oxide; and a coating layer containing particles attached to the surface of the starting material. The particles contain at least one selected from the group consisting of metal oxides, metal hydroxides, and cement. The coverage of the coating layer is 50% or greater, and the standard deviation of the coverage is 15% or less.

Description

[Technical Field]

[0001] The present invention relates to a raw material for direct reduced iron making and a method for producing the same. [Background technology]

[0002] In recent years, against the backdrop of global environmental issues and the depletion of fossil fuels, there has been a strong demand for energy conservation in steelmaking.

[0003] The raw material for iron is mainly iron oxide, such as iron ore. The iron-making process requires a reduction process to reduce the iron oxide. The most common iron-making method in the world is the blast furnace process, which uses a blast furnace. In the tuyere of a blast furnace, coke or pulverized coal reacts with oxygen in hot air (air heated to about 1200°C) to produce CO and H2 gas (reducing gas).The resulting reducing gas then reduces the raw materials (iron ore, etc.) in the blast furnace. Thanks to recent improvements in blast furnace operation technology, the reducing agent rate (the amount of coke and pulverized coal used per ton of molten iron produced) has been reduced to around 500 kg / t. However, the reducing agent rate has already reached its lower limit, and no further significant reductions in the reducing agent rate can be expected.

[0004] On the other hand, in recent years, a direct reduction ironmaking process (also called the "direct reduction process" or "direct ironmaking process") has been developed as an ironmaking process different from the blast furnace process. The direct reduction ironmaking process involves filling a reduction furnace (direct reduction furnace) with raw materials containing iron oxide (such as iron ore), blowing in reducing gas to reduce the raw materials and produce reduced iron, which is then melted in an electric furnace. As the reduction furnace, a shaft furnace is mainly used. Natural gas is the main source of reducing gas. The natural gas is heated and reformed in the thermal reformer together with the furnace top gas discharged from the top of the shaft furnace. This produces reducing gas containing CO and H2. The resulting reducing gas is blown into the shaft furnace and reacts with the raw material (iron oxide) supplied from the top of the shaft furnace, reducing the raw material (iron oxide) and producing reduced iron. The produced reduced iron is cooled in a region (cooling zone) below the position where the reducing gas is injected in the shaft furnace, and then discharged from the bottom of the shaft furnace.

[0005] The raw materials charged into the shaft furnace include lump iron ore (lump ore) and pellets (powdered iron ore solidified into a spherical shape). These raw materials often undergo clustering in the high-temperature reducing atmosphere inside the shaft furnace. Clustering is a phenomenon in which these raw materials fuse together in the high-temperature reducing atmosphere inside the shaft furnace to form large lumps. Clustering can make it difficult to discharge reduced iron from the shaft furnace. It can also cause bridging between raw materials (also known as "hanging") within the shaft furnace, hindering smooth unloading. In either case, the operability of the shaft furnace is significantly reduced.

[0006] For this reason, in normal shaft furnace operation, the maximum reduction temperature is kept low to prevent clustering, but this does not allow the reduction rate to be increased sufficiently, and productivity cannot be improved to a satisfactory level.

[0007] Therefore, in order to suppress the occurrence of clustering, techniques have been developed to coat the surfaces of raw materials containing iron oxide (iron ore, pellets, etc.), and Patent Documents 1 to 3 disclose raw materials for direct reduced ironmaking having a coating layer. Clustering is a sintering phenomenon that occurs when raw materials containing iron oxide are reduced to produce reduced iron (metallic iron). By providing a coating layer on the raw materials, the contact surface between the resulting reduced iron particles is reduced, suppressing sintering and therefore clustering. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 54-93617 [Patent Document 2] Japanese Unexamined Patent Publication No. 62-7806 [Patent Document 3] Japanese Patent Application Publication No. 63-262426 Summary of the Invention [Problem to be solved by the invention]

[0009] When direct reduction ironmaking is carried out, raw materials (iron oxide) such as pellets may become pulverized during the reduction process. This is also called "reduction pulverization." If a large amount of reduction pulverization occurs, the pressure loss inside the shaft furnace increases, which can lead to abnormalities such as raw material hanging.

[0010] In addition, in recent years, from the perspective of carbon neutrality, hydrogen-based reducing gases have been attracting attention as an alternative to conventional natural gas in order to reduce the amount of CO2 emitted from shaft furnaces. When raw material (iron oxide) is reduced using a reducing gas with a high hydrogen ratio, the hydrogen diffuses through the raw material, causing fine cracks in the raw material. These cracks can become the starting point for fracture, promoting reduction disintegration.

[0011] For this reason, there is a demand for the development of raw materials for direct reduction ironmaking that can not only suppress the occurrence of clustering (excellent clustering resistance) but also suppress the occurrence of reduction disintegration (excellent reduction disintegration resistance).

[0012] The present invention has been made in view of the above points, and an object of the present invention is to provide a raw material for direct reduction ironmaking (and a method for producing the same) that has excellent resistance to reduction disintegration. [Means for solving the problem]

[0013] As a result of extensive research, the present inventors have found that the above object can be achieved by employing the following configuration, and have completed the present invention. That is, the present invention provides the following [1] to [8]. [1] A raw material for direct reduction ironmaking, comprising: a raw material containing iron oxide; and a coating layer containing particles adhered to the surface of the raw material, wherein the particles contain at least one selected from the group consisting of metal oxides, metal hydroxides, and cement; a coverage rate of the coating layer is 50% or more; and a standard deviation of the coverage rate is 15% or less. [2] The raw material for direct reduced iron making according to [1] above, wherein the coating layer has a thickness of 5.0 μm or more. [3] The raw material for direct reduced iron making according to [1] or [2] above, wherein the metal oxide and the metal hydroxide contain at least one selected from the group consisting of calcium, aluminum, and magnesium. [4] The raw material for direct reduction iron making according to any one of [1] to [3] above, wherein the amount of the particles adhering to the raw material is 0.10% by mass or more and 3.00% by mass or less. [5] A method for producing raw materials for direct reduced ironmaking according to any one of [1] to [4] above, comprising suspending particles containing at least one selected from the group consisting of metal oxides, metal hydroxides, and cement in water to prepare a slurry-like treatment liquid, coating the surface of a raw material containing iron oxide with the treatment liquid, and heating the raw material coated with the treatment liquid to remove water contained in the treatment liquid. [6] The method for producing raw materials for direct reduced iron making according to [5] above, wherein the metal oxide and the metal hydroxide contain at least one selected from the group consisting of calcium, aluminum, and magnesium. [7] The method for producing raw materials for direct reduced iron making according to [5] or [6] above, wherein the treatment liquid is coated on the surface of the raw materials by an immersion method. [8] The method for producing raw materials for direct reduced ironmaking according to any one of [5] to [7] above, wherein the concentration of the particles in the treatment liquid is changed to change the coverage of the coating layer on the raw materials for direct reduced ironmaking to be produced. [Effects of the Invention]

[0014] According to the present invention, a raw material for direct reduction ironmaking having excellent resistance to reduction disintegration can be provided. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a configuration diagram showing a facility for producing raw materials for direct reduction ironmaking, together with a shaft furnace for reducing the raw materials for direct reduction ironmaking. DETAILED DESCRIPTION OF THE INVENTION

[0016] [Raw materials for direct reduced ironmaking] The raw material for direct reduced iron making of the present invention comprises a raw material containing iron oxide and a coating layer containing particles adhered to the surface of the raw material, the particles containing at least one selected from the group consisting of metal oxides, metal hydroxides, and cement, and the coating layer has a coverage of 50% or more and a standard deviation of the coverage of 15% or less.

[0017] <Raw materials> The raw material for direct reduced iron making of the present invention includes a raw material containing iron oxide (hereinafter also simply referred to as "raw material"). The raw material is, for example, iron ore, and specific examples thereof include lump iron ore (lump ore) and pellets (iron ore powder solidified into a spherical shape). The quality (iron content) of the iron ore used as a raw material is not particularly limited, but from the viewpoint of reduction in a shaft furnace, it is generally preferably 65 mass % or more. However, in recent years, the prices of high-grade ores from South America and other sources are expected to rise, so low-grade ores (iron content: 63% by mass or less), which are inexpensive and abundant resources, such as those from Australia, may also be used as needed.

[0018] <Coating layer> The raw material for direct reduced iron making of the present invention further has a coating layer attached to the surface of the raw material. The coating layer comprises particles as described below.

[0019] <Types of particles> The particles contained in the coating layer contain at least one kind selected from the group consisting of metal oxides, metal hydroxides, and cement.

[0020] (Metal oxides and metal hydroxides) Each of the metal oxide and metal hydroxide preferably contains at least one metal element selected from the group consisting of calcium (Ca), aluminum (Al) and magnesium (Mg). Examples of metal hydroxides include calcium hydroxide, aluminum hydroxide, and magnesium hydroxide. Examples of metal oxides include calcium oxide, aluminum oxide, and magnesium oxide.

[0021] Metal oxides are M x Fe y O z It may also be a composite oxide of metal and iron, represented by the chemical formula (M = Ca, Al, Mg). Ores such as bauxite, dolomite, peridotite, quartzite, and serpentine also contain metal oxides, so these may also be used as the metal oxide.

[0022] Both metal oxides and metal hydroxides may be used. However, when a metal hydroxide is used, it is converted into a metal oxide when heated and a dehydration reaction occurs during the process of producing a raw material for direct reduction ironmaking, as will be described later. Even if dehydration reactions do not occur during the process of producing raw materials for direct reduction steelmaking, the inside of a reduction furnace such as a shaft furnace is at temperatures of 700°C to 1000°C, so dehydration reactions easily occur and most of the metal hydroxides become metal oxides.

[0023] (cement) Examples of cement include self-hardening cement (Portland cement); latent hydraulic cement (such as blast furnace cement); fine powders of blast furnace slag, blast furnace slag, and steel slag; and among these, Portland cement is preferred. Examples of Portland cement include ordinary Portland cement, high-early-strength Portland cement, and high-strength Portland cement. These contain large amounts of 3CaO·SiO2, which exhibits a strong hydration reaction, and have excellent hydraulic properties, resulting in good adhesion to raw materials (such as iron ore). They are also inexpensive and readily available.

[0024] Coverage In the raw material for direct reduced iron making of the present invention, the coverage of the coating layer (the area ratio of the surface of the raw material covered by the coating layer) is at least a certain value because the raw material has excellent resistance to reduction disintegration. The reason for the excellent resistance to reduction disintegration is presumed to be as follows.

[0025] The powdering of the raw material (iron oxide) is thought to occur due to volume destruction caused by collisions between iron oxide particles and wear caused by rubbing of the iron oxide surface. The present inventors have found that in the process of reducing a raw material (iron oxide) having a coating layer, the diffusion of the reducing gas into the interior of the iron oxide is delayed, and as a result, the reduction reaction on the surface of the iron oxide proceeds preferentially, tending to make it easier for metallic iron to be produced on the surface of the iron oxide. The reason why the diffusion of the reducing gas into the interior of the iron oxide is delayed is presumably because the presence of a coating layer on the surface of the iron oxide causes the reducing gas to remain on the surface of the iron oxide, resulting in preferential reduction of the surface of the iron oxide over the interior of the iron oxide. It is believed that the generation of strong metallic iron on the surface of iron oxide can suppress volumetric destruction and wear, which are the causes of powdering. At this time, it is believed that the higher the coverage of the coating layer, the more metallic iron is generated on the surface of the raw material (iron oxide), and therefore the effect of suppressing reduction disintegration is improved.

[0026] In particular, when pellets are used as the raw material, the metallic iron generated on the surface layer of the pellets forms a shell, which improves the strength against external forces in the circumferential direction.

[0027] Also, a higher coverage of the coating layer is preferable because it provides excellent resistance to clustering. As described above, clustering is a solid-phase sintering phenomenon caused by contact between particles of reduced iron (metallic iron) produced by reducing the raw material (iron oxide). The higher the coverage of the coating layer, the more the area of ​​contact between the produced metallic iron particles can be reduced, and therefore it is thought that the effect of suppressing clustering is improved.

[0028] From the above, the coverage of the coating layer is 50% or more, preferably 70% or more, and more preferably 80% or more. The coverage of the coating layer may be 100%.

[0029] The coverage of the coating layer is determined by the method described in the Examples section below.

[0030] <<Standard deviation of coverage>> Furthermore, in the coated raw material of the present invention, the standard deviation of the coverage of the coating layer is a certain value or less because of its excellent resistance to reduction disintegration. The standard deviation is an index representing the degree of dispersion (variation) from the average value, and in the present invention, the standard deviation of the coverage is an index representing the uniformity of the coating layer. If the standard deviation of the coverage is large, there will be a large variation between areas with high coverage and areas with low coverage, and in areas with low coverage, the effect of suppressing powdering due to the generation of metallic iron described above will not be sufficient. Furthermore, if the standard deviation of the coverage is large, areas with high coverage (areas with high strength) and areas with low coverage (areas with low strength) will coexist, and the areas with low strength may become the starting point of destruction, accelerating volumetric destruction. When pellets are used as the raw material, if the standard deviation of the coverage is large, the metallic iron shell will not be formed uniformly, and the effect of improving strength against external forces in the circumferential direction will not be sufficient.

[0031] Also, for the reason that resistance to clustering is excellent, it is preferable that the standard deviation of the coverage is small. When the standard deviation of the coverage is large, sintering of metallic iron is likely to occur in areas with low coverage, whereas when the standard deviation of the coverage is small, sintering of metallic iron can be suppressed uniformly on the surface of the raw material, resulting in a sufficient clustering suppression effect.

[0032] From the above, the standard deviation of the coverage is 15% or less, preferably 13% or less, more preferably 10% or less, even more preferably 8% or less, and particularly preferably 5% or less. The lower limit is not particularly limited, and may be 0%.

[0033] Thickness In the raw material for direct reduced ironmaking of the present invention, the thickness of the coating layer (average thickness in the cross-sectional direction) is preferably 5.0 μm or more, more preferably 10.0 μm or more, and even more preferably 20.0 μm or more, because this provides better resistance to reduction disintegration and clustering. It is believed that the thicker the coating layer, the more delayed the diffusion of the reducing gas into the iron oxide and the more easily metallic iron is produced on the surface of the iron oxide, thereby improving the effect of suppressing reduction disintegration. Furthermore, a thin coating layer is prone to peeling when cracks occur in the raw material, but a thick coating layer is less likely to peel, which is thought to further improve the effect of suppressing reduction disintegration and clustering.

[0034] On the other hand, there is no particular upper limit to the thickness of the coating layer. However, if the coating layer is too thick, carburization (addition of carbon) of the reduced iron obtained from the raw materials for direct reduction ironmaking may be easily inhibited. Therefore, from the viewpoint of suppressing inhibition of carburization, the thickness of the coating layer is preferably 200.0 μm or less, more preferably 100.0 μm or less, and even more preferably 40.0 μm or less.

[0035] The thickness of the coating layer is determined by the method described in the Examples section below.

[0036] <<Amount of particle adhesion>> If the number of particles constituting the coating layer is too small, it may be difficult to obtain the effect of suppressing the occurrence of reduction disintegration and clustering described above. Therefore, the amount of particles that constitute the coating layer is preferably 0.03% by mass or more, more preferably 0.07% by mass or more, and even more preferably 0.10% by mass or more, based on the raw material.

[0037] On the other hand, if the coating layer contains too many particles, the reducibility of the raw material (iron oxide) may decrease, resulting in a decrease in productivity. Therefore, the amount of particles adhering to the coating layer is preferably 8.00% by mass or less, more preferably 5.50% by mass or less, and even more preferably 3.00% by mass or less, based on the raw material.

[0038] The amount of particles attached is calculated from the difference in mass between the raw material before being coated with the treatment liquid described below and the final raw material for direct reduced iron making obtained.

[0039] [Method of manufacturing raw materials for direct reduction ironmaking] Next, the method for producing raw materials for direct reduced iron making according to the present invention will be described with reference to FIG. FIG. 1 is a configuration diagram showing a facility for producing raw materials for direct reduction ironmaking, together with a shaft furnace 7 for reducing the raw materials for direct reduction ironmaking. As shown in FIG. 1, the facility for producing raw materials for direct reduction iron making generally comprises a raw material tank 1, a coating treatment tank 3, a heating furnace 5, and a dehydration furnace 6.

[0040] Raw materials are stored in the raw material tank 1. The raw materials are, for example, iron ore such as pellets or lump ore. Below the raw material tank 1, a charging conveyor 2 is installed. The feeding conveyor 2 carries the raw material supplied from the raw material tank 1 up to a dropping position above the coating treatment tank 3. The coating treatment tank 3 stores a treatment liquid containing particles that will become the coating layer.

[0041] In advance, particles to be coated are suspended in water to prepare a slurry treatment liquid, which is then stored in the coating treatment tank 3 . The particles are at least one selected from the group consisting of metal oxides, metal hydroxides, and cement. The metal oxides and metal hydroxides each contain at least one metal element selected from the group consisting of Ca, Al, and Mg. The metal oxides react with water to become metal hydroxides.

[0042] The concentration of particles in the treatment liquid is not particularly limited, and is adjusted appropriately depending on the desired amount of particles to be attached (and thus the coverage of the coating layer). That is, it is preferable to change the coverage of the coating layer in the final raw material for direct reduced iron making by changing the concentration of particles in the treatment solution. In this case, for example, the coverage is increased by increasing the concentration of particles in the treatment solution.

[0043] The raw material fed from the feeding conveyor 2 into the coating treatment tank 3 is immersed in the treatment liquid. The raw material immersed in the treatment liquid is stirred as appropriate. By this method (immersion method), a raw material whose surface is coated with the treatment liquid (hereinafter referred to as "treated raw material") is obtained.

[0044] The method for coating the surface of the raw material with the treatment liquid (coating method) is not limited to the immersion method. For example, a method of spraying the treatment liquid onto the raw material using a spray (spray method) may be used. However, the immersion method is preferred as the coating method for reasons such as the ease of uniformly providing the coating layer, the ease of controlling the amount of adhesion, and the ease of keeping the standard deviation of the coverage low.

[0045] The coating rate, standard deviation of the coating rate, and thickness of the coating layer in the final raw material for direct reduced ironmaking can be controlled, for example, by adjusting the stirring time after immersing the raw material in the treatment solution. For example, even when the same treatment solution is used, the standard deviation of the coverage of the final direct reduced ironmaking raw material can change if the stirring time or the shape of the stirring blades is changed to change the stirring efficiency in the immersion method.

[0046] The treated raw material is carried out to the heating furnace 5 by the carrying-out conveyor 4a and heated (dried) in the heating furnace 5. In this way, the moisture in the treated raw material (particularly the treatment liquid that coats the raw material) is evaporated and removed. The method for heating the treated raw material is not particularly limited, and examples thereof include methods using steam heating, electric heating, microwaves, and induction current. The heating temperature in the heating furnace 5 is preferably 100°C or higher from the viewpoint of evaporating moisture from the treated raw material. The atmospheric gas inside the heating furnace 5 is preferably a gas with a low CO2 concentration, and more preferably a gas with a CO2 concentration of 1000 ppm by volume or less (for example, an inert gas, air, combustion gas from which CO2 has been removed, heated steam, etc.).

[0047] The treated raw material is heated in the heating furnace 5, and then transported to the dehydration furnace 6 by the discharge conveyor 4b, where it is further heated. Heating in the dehydration furnace 6 causes dehydration reactions represented by the following formulas (1) to (3), forming a dense and porous coating layer. In this way, a raw material for direct reduction ironmaking is obtained, with a coating layer attached to the surface of the raw material. The reactions of the following formulas (1) to (3) are known to proceed at atmospheric pressure in a temperature range of 300 to 500°C or higher, and therefore the heating temperature in the dehydration furnace 6 may be determined appropriately depending on the metal used and at or above this temperature. Ca(OH)2 → CaO + H2O (1) 2Al(OH)3→Al2O3+3H2O···(2) Mg(OH)2 → MgO + H2O (3)

[0048] [Direct reduction steelmaking] The raw materials for direct reduction ironmaking are then transported to a shaft furnace 7, which is a vertical shaft furnace, and reduced by a reducing gas while descending from a furnace top 7a of the shaft furnace 7. Below, we will explain the MIDREX process, a typical direct reduction ironmaking process.

[0049] The shaft furnace 7 is provided with a gas outlet 8 and a gas inlet 9 . The gas inlet 9 is provided at approximately the middle position in the vertical direction of the shaft furnace 7, and supplies reducing gas into the shaft furnace 7. Examples of reducing gases that can be used include natural gas; reformed gas obtained by reforming natural gas and containing CO and H2 as main components; and coal gas (gas generated when coal is gasified). The composition of the reformed gas is, for example, a total concentration of H2 and CO of about 90 mol %, a molar ratio of H2 / (H2+CO) of 0.52 to 0.71, and a CO2 concentration of 0.5 to 3.0 mol %. The temperature of the reducing gas injected from the gas inlet 9 is, for example, 700° C. to 1200° C. The reducing gas reduces the raw materials for direct reduction ironmaking charged from the furnace top 7a, and is then discharged as exhaust gas from the gas outlet 8.

[0050] In the cooling zone 7b at the bottom of the shaft furnace 7, a gas inlet 12 and a gas suction port 11 are provided. Cooling gas and carburizing gas are blown into the shaft furnace 7 through the gas inlet 12. The gas suction port 11 sucks these gases in so as not to intrude into the furnace top 7a. The reduced iron produced by reducing the raw materials for direct reduction ironmaking is cooled by a cooling gas and carburized by a carburizing gas in the cooling zone 7b. N2 is generally used as the cooling gas, and CH4 is mainly used as the carburizing gas, which may contain CO.

[0051] A reduced iron discharge port 13 is provided at the bottom of the shaft furnace 7. The reduced iron 14 after being cooled and carburized is discharged from a reduced iron discharge port 13 .

[0052] As a countermeasure against clustering, a cluster breaker 10 is installed inside the shaft furnace 7 to mechanically break up any clusters that are generated.

[0053] The direct reduction ironmaking method using the shaft furnace 7 has been described above, but the type of reduction furnace (direct reduction furnace) is not limited to this, and methods using a fluidized bed, a rotary kiln, a rotary hearth furnace (RHF), etc. may also be used. [Example]

[0054] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the examples described below.

[0055] <Raw materials> The iron ore used as raw material was pellets from Brazil with particle sizes of 10.0 to 15.0 mm. The pellet composition, in mass%, was T.Fe: 66%, FeO: 0.63%, SiO2: 2.0%, CaO: 2.1%, Al2O3: 0.5%, and MgO: 0.16%.

[0056] <Processing liquid> Equal amounts of commercially available calcium oxide, calcium hydroxide, aluminum oxide, aluminum hydroxide, magnesium oxide, magnesium hydroxide, and cement (Portland cement) were mixed and crushed to a particle size of 1 mm or less. The resulting particles were suspended in water at the concentrations shown in Table 1 below to prepare a slurry treatment solution.

[0057] <Raw material for direct reduction steelmaking> 500 g of raw material (pellets) was weighed and coated with the prepared treatment solution by immersion to obtain a treated raw material. At this time, the stirring time and stirring efficiency after immersion of the raw material in the treatment solution were appropriately adjusted so that the final coating rate, standard deviation of the coating rate, and thickness of the coating layer would be the values ​​shown in Table 1 below. The resulting treated raw material was dried by heating in a heating furnace set at 80°C for 30 minutes, and then further heated in a dehydration furnace set at 500°C for 1 hour. In this way, moisture was removed from the treated raw material to obtain a raw material for direct reduction iron making. In the obtained raw materials for direct reduced iron making, the amount of particles adhering to the raw materials (pellets) was in the range of 0.10% by mass or more and 3.00% by mass or less in all cases.

[0058] <<Coverage rate of the coating layer and standard deviation of the coverage rate>> The surface of the obtained raw material for direct reduction iron making was observed using a microscope at a magnification of 100 times, and microscope images of 10 fields of view were obtained. The area ratio of the coating layer to the surface of the raw material was calculated by image analysis of the obtained microscope image, and the average value of 10 fields of view was taken as the coating ratio (unit: %) of the coating layer. Furthermore, the standard deviation of the coating ratio (unit: %) was calculated from the coverage ratios of the 10 fields of view. The results are shown in Table 1 below.

[0059] <Coating layer thickness> The obtained raw material for direct reduction ironmaking was mechanically polished to expose its cross section. The cross section of the raw material for direct reduction ironmaking was observed using a scanning electron microscope (SEM) at a magnification of 300 times, and SEM images of three fields of view were obtained. For the obtained SEM images, the thickness of the coating layer disposed on the surface of the raw material was measured at 10 points within one field of view, and the average value of three fields of view was taken as the thickness of the coating layer (unit: μm). The results are shown in Table 1 below.

[0060] <Resistance to reduction and disintegration> In order to simulate the direct reduction ironmaking process described based on Figure 1, raw materials for direct reduction ironmaking were reduced using a furnace of a TG (thermogravimetry) device, and then a test was conducted to evaluate the reduction disintegration resistance. Specifically, 500 g of raw materials for direct reduction ironmaking were placed in a carbon crucible with a diameter of 80 mm, and reduction gas (CO: 35 vol%, H2: 55 vol%, N2: 10 vol%, gas flow rate: 15 NL / min) was introduced from the bottom of the furnace at a constant temperature of 900°C, and reduction was carried out until the reduction rate reached 50%. The reduced raw materials for direct reduction ironmaking were rotated 900 times at 30 rpm using a tumbler rotation tester. The ratio of the mass of the sample that fell below the -2.8 mm sieve to the total mass of the sample was then calculated as the reduction degradation rate (unit: mass%). The smaller the reduction degradation rate, the better the resistance to reduction degradation can be evaluated.

[0061] The reduction rate was calculated using the following formula: [Reduction rate (unit: %)] = {([Amount of oxygen in raw materials for direct reduction ironmaking before reduction (unit: mass%)] - [Amount of oxygen in reduced iron (unit: mass%)]) / [Amount of oxygen in raw materials for direct reduction ironmaking before reduction (unit: mass%)]} × 100 The oxygen content in the raw materials for direct-reduced ironmaking was determined as follows. First, chemical analysis was performed to quantify the total iron and FeO content in the raw materials for direct-reduced ironmaking. Next, FeO content in the raw materials for direct-reduced ironmaking was determined by subtracting the Fe in FeO from the total iron content, assuming that the resulting amount was the Fe in FeO3. The total amount of oxygen in FeO and FeO3 was then calculated to determine the oxygen content in the raw materials for direct-reduced ironmaking. The amount of oxygen in the reduced iron was determined as follows. First, chemical analysis was performed to quantify the total iron, Fe, FeO, and metal iron in the reduced iron. Next, Fe3O4 in the reduced iron was quantified by subtracting the Fe in FeO and metal iron from the total iron, assuming that the amount was the Fe in Fe3O4. The sum of the oxygen amounts in FeO and Fe3O4 was then determined to be the oxygen amount in the reduced iron.

[0062] In Table 1 below, the following symbols are used to indicate the reduction disintegration rate. "A", "B", "C", "D" or "E" are preferred because they have excellent resistance to reduction disintegration. A: Less than 1% by mass B: 1% by mass or more and less than 2% by mass C: Reduction disintegration rate is 2% by mass or more and less than 3% by mass D: Reduction disintegration rate is 3% by mass or more and less than 5% by mass E: Reduction disintegration rate is 5% by mass or more and less than 10% by mass F: Reduction disintegration rate is 10% by mass or more and less than 20% by mass G: Reduction disintegration rate is 20% by mass or more

[0063] <Clustering resistance> To evaluate the clustering resistance of raw materials for direct reduction ironmaking, reduction tests were carried out using a load-softening tester. Specifically, 500 g of raw materials for direct reduction ironmaking were placed in a carbon crucible with a diameter of 100 mm, and reducing gas (CO: 35 vol%, H2: 55 vol%, N2: 10 vol%, gas flow rate: 15 NL / min) was introduced from the bottom of the furnace at a constant temperature of 900°C, with a flow rate of 1.5 kg / cm 2 The reduction was carried out while applying a load of 100% until the reduction rate reached 100%. The reduced iron clusters generated after the test were crushed using an I-type rotary tester and rotated 150 times at 30 rpm. A cluster was defined as reduced iron consisting of two or more pellets joined together, and the proportion of clusters to the total sample mass was calculated as the cluster ratio (unit: %). The smaller the cluster ratio, the better the sample's resistance to clustering was evaluated.

[0064] In Table 1 below, the following symbols are used according to the value of the cluster ratio. "A", "B", "C", "D" or "E" are preferred because they have excellent resistance to clustering. A: Cluster ratio is less than 1% B: Cluster ratio is 1% or more but less than 2% C: Cluster ratio is between 2% and 3% D: Cluster ratio is between 3% and 5% E: Cluster ratio is between 5% and 10% F: Cluster ratio is between 10% and 30% G: Cluster ratio is 30% or more

[0065] [Table 1]

[0066] <Summary of evaluation results> As is clear from the results shown in Table 1 above, the raw materials for direct-reduced ironmaking in Examples 1 to 8, in which the coating layer had a coverage rate of 50% or more and the standard deviation of the coverage rate was 15% or less, had better resistance to reduction disintegration than the raw materials for direct-reduced ironmaking in Comparative Examples 1 and 2, which did not satisfy these requirements. Furthermore, the raw materials for direct reduced iron making of Examples 1 to 8 also had good resistance to clustering.

[0067] Furthermore, when comparing Example 4 with Examples 5 and 6, which have similar standard deviations of the coverage rate and thicknesses of the coating layer, Examples 5 and 6, which have a higher coverage rate of the coating layer, have better resistance to reduction disintegration and clustering than Example 4.

[0068] Furthermore, when comparing Example 4 with Examples 7 and 8, which have similar coating layer coverage rates and standard deviations of the coverage rates, Examples 7 and 8, which have thicker coating layers, had better resistance to reduction disintegration and clustering than Example 4. [Explanation of symbols]

[0069] 1: Raw material tank 2: Feeding conveyor 3: Coating treatment tank 4a, 4b: Unloading conveyor 5:Heating furnace 6: Dehydration furnace 7: Shaft furnace 7a: Furnace top 7b: Cooling zone 8: Gas outlet 9: Gas inlet 10: Cluster Breaker 11: Gas suction port 12: Gas inlet 13: Reduced iron outlet 14: Reduced iron

Claims

1. A raw material containing iron oxide; a coating layer containing particles attached to the surface of the raw material; the particles contain at least one selected from the group consisting of a metal oxide, a metal hydroxide, and cement, and the metal oxide and the metal hydroxide contain at least one selected from the group consisting of calcium, aluminum, and magnesium; The coverage of the coating layer is 50% or more, The standard deviation of the coverage is 15% or less, A raw material for direct reduced iron making, wherein the coating layer has a thickness of 40.0 μm or less. However, the coverage rate and the standard deviation are calculated as follows. The surface of the raw material for direct reduced ironmaking is observed using a microscope at a magnification of 100x to obtain microscope images of 10 fields of view. The obtained microscope images are analyzed to calculate the area ratio of the raw material surface covered by the coating layer, and the average value of the 10 fields of view is defined as the coating ratio. Furthermore, the standard deviation is calculated from the coating ratios of the 10 fields of view.

2. 2. The raw material for direct reduced iron making according to claim 1, wherein the coating layer has a thickness of 5.0 μm or more.

3. 2. The raw material for direct reduced iron making according to claim 1, wherein the amount of the particles adhering to the raw material is 0.10% by mass or more and 3.00% by mass or less.

4. 3. The raw material for direct reduced iron making according to claim 2, wherein the amount of the particles adhering to the raw material is 0.10 mass % or more and 3.00 mass % or less.

5. A method for producing a raw material for direct reduced ironmaking according to any one of claims 1 to 4, comprising: a slurry treatment liquid is prepared by suspending particles containing at least one selected from the group consisting of a metal oxide, a metal hydroxide, and cement in water, the metal oxide and the metal hydroxide containing at least one selected from the group consisting of calcium, aluminum, and magnesium; The treatment liquid is applied to a surface of a raw material containing iron oxide, a processing step of heating the raw material coated with the processing solution to remove water contained in the processing solution;

6. 6. The method for producing raw materials for direct reduced iron making according to claim 5, wherein the treatment solution is applied to the surfaces of the raw materials by a dipping method.

7. 6. The method for producing raw materials for direct reduced iron making according to claim 5, wherein the concentration of the particles in the treatment liquid is changed to change the coverage of the coating layer on the raw materials for direct reduced iron making to be produced.

8. 7. The method for producing raw materials for direct reduced iron making according to claim 6, wherein the concentration of the particles in the treatment liquid is changed to change the coverage of the coating layer on the raw materials for direct reduced iron making to be produced.

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