Agglomerated ore for blast furnace

Abstract

Provided is an agglomerated ore for a blast furnace, the agglomerated ore having higher reducibility. The agglomerated ore for a blast furnace has a cross-sectional structure in which the area ratio of hematite is 40% or more and the area ratio of magnetite is 30% or less, wherein the average Feret diameter of hematite particles in the cross-sectional structure is 30 μm or more as calculated from the distribution of crystal orientations of the hematite.

Description

Blast furnace agglomerate ore

[0001] This invention relates to agglomerate ore for blast furnaces.

[0002] Agglomerate ore is an artificial ore produced primarily from iron ore and used in the operation of blast furnaces. The quality of agglomerate ore is evaluated based on its reducibility, etc. If agglomerate ore is highly reducible, the amount of reducing agent such as powdered coke used in blast furnace operation can be reduced, thus reducing CO2 emissions. ２ It is expected to have an effect on reducing emissions.

[0003] Agglomerate ores have a texture containing multiple mineral phases, and the texture of agglomerate ores is one of the factors that influences their reducibility. Therefore, the relationship between the phase fraction of each phase in the texture of agglomerate ores and the reducibility of agglomerate ores is being investigated.

[0004] For example, Non-Patent Document 1 shows that sintered ores mainly composed of hematite and calcium ferrite are more reducible than sintered ores mainly composed of magnetite and slag.

[0005] Patent Document 1 proposes a sintered ore in which the relative proportion of hematite is 16% by mass or more and 24% by mass or less, and the relative proportion of magnetite is 46% by mass or more and 54% by mass or less. According to Patent Document 1, when the sintered ore is reduced while being heated from room temperature, the reduction rate at which 1200°C is reached can be made 71% or more.

[0006] Japanese Patent Publication No. 2020-12141

[0007] N. Maeda, Y. Ono: Tetsu-to-Hagane, 72(1986), 775.

[0008] However, even when the phase fractions of each phase in the microstructure of agglomerated ore were controlled according to the policy outlined in Non-Patent Document 1, there was still room for improvement in reducibility. Furthermore, the technology proposed in Patent Document 1 also had room for improvement in reducibility. Moreover, while these prior arts evaluated reducibility under conditions where a blast furnace was operated using a general reducing gas, there was a need for agglomerated ore that exhibited high reducibility when used under operating conditions where a gaseous reducing agent containing hydrocarbons was injected into the blast furnace.

[0009] This invention has been made in view of the above circumstances, and aims to provide agglomerate ore for blast furnaces that has even higher reducibility.

[0010] As a result of diligent research, the inventors have found that the above objective can be achieved by adopting the following configuration.

[0011] 1. Agglomerated ore for blast furnaces, wherein the cross-sectional structure of the agglomerated ore has a hematite area ratio of 40% or more and a magnetite area ratio of 30% or less, and the average Ferret diameter of the hematite particles in the cross-sectional structure, calculated from the distribution of the crystal orientation of the hematite, is 30 μm or more.

[0012] 2. The blast furnace agglomerate according to claim 1, wherein the cross-sectional structure comprises at least one selected from calcium ferrite, slag, and pores.

[0013] According to the present invention, it is possible to provide agglomerate ore for blast furnaces that has even higher reducibility.

[0014] The present invention will be described below. The following description illustrates preferred embodiments of the present invention, and the present invention is not limited in any way by the following description.

[0015] First, we will describe the experiments conducted in completing this invention. Agglomerate was reduced, and the cross-sectional structure of the reduced agglomerate was observed. The reduction conditions were: reducing gas composition: CO3 1% - H ２ 19%-N ２ The reduction was performed at 50%, with a reduction temperature of 800°C and a reduction time of 3 hours. Observations revealed that hematite contained in agglomerate ore could be classified into two types based on the formation of reduced iron. Specifically, hematite had two types of structures: one that was difficult to reduce, forming a metallic iron shell around it during reduction, and another that was easily reduced, forming metallic iron with a striped structure without forming a metallic iron shell. Furthermore, the observations also revealed that magnetite forms a metallic iron shell during reduction. Based on these experiments, the present invention was completed through further improvements.

[0016] The agglomerate according to the present invention will be described in detail below. In the cross-sectional structure of the agglomerate according to the present invention, the area ratio of hematite is 40% or more, and the area ratio of magnetite is 30% or less. Furthermore, the average Ferret diameter of the hematite particles in the cross-sectional structure of the agglomerate, calculated from the distribution of crystal orientations of the hematite, is 30 μm or more.

[0017] Hematite area percentage: 40% or more. Hematite is a reducible structure when it satisfies the conditions for hematite particles described later. If the hematite area percentage is less than 40%, the reducibility decreases. Therefore, in the cross-sectional structure of the agglomerate ore according to the present invention, the hematite area percentage is set to 40% or more. The upper limit of the hematite area percentage is not particularly limited, but it may be, for example, 100% or less, or 75% or less.

[0018] Magnetite area percentage: 30% or less. Magnetite is a structure that is difficult to reduce, forming a metallic iron shell during reduction. When the amount of magnetite exceeds 30% by area, the reducibility decreases. Therefore, in the cross-sectional structure of the agglomerate according to the present invention, the area percentage of magnetite is set to 30% or less. The lower limit of the area percentage of magnetite is not particularly limited and may be 0%, or it may not be present, or it may be present, and the area percentage of magnetite may be, for example, greater than 0%, or 5% or more.

[0019] The cross-sectional structure of the agglomerate may further include at least one selected from calcium ferrite, slag, and pores. The cross-sectional structure of the agglomerate may consist of hematite, magnetite, calcium ferrite, slag, and pores.

[0020] (Calcium ferrite) There is no particular lower limit to the area percentage of calcium ferrite. However, calcium ferrite is a structure that is easily reduced without forming a metallic iron shell during reduction. Therefore, in order to further increase its reducibility, it is preferable that the calcium ferrite is 15 area percent or more. The calcium ferrite may be 60 area percent or less, or 50 area percent or less.

[0021] (Slag) The area percentage of slag is not particularly limited. However, reducing the area percentage of slag can improve the permeability of the agglomerate. Therefore, in order to further increase the reducibility, it is preferable that the slag is 10 area percent or less. The lower limit of the area percentage of slag is not particularly limited and may be 0%, or it may not be present at all.

[0022] (Stomata) The area ratio of stomata is not particularly limited. However, stomata serve as pathways for reducing gases to enter. Therefore, in order to further increase the reducibility, it is preferable that the stomata be 10 area percent or more. The stomata may be, for example, 60 area percent or less, or 45 area percent or less.

[0023] The area ratio of each tissue is determined by observing the cross-sectional structure of the agglomerate under an optical microscope. Specifically, it can be determined by the method described in the examples.

[0024] The average Ferre diameter of hematite particles calculated from the distribution of crystal orientations of hematite is 30 μm or more. Hematite contained in agglomerated ore is composed of multiple hematite particles. As mentioned above, hematite can be classified into two types of structures with different reducibility, but hematite particles contained in structures that are easily reducible have a larger Ferre diameter than hematite particles contained in structures that are not easily reducible. If the average Ferre diameter of hematite particles in the cross-sectional structure of agglomerated ore is less than 30 μm, the agglomerated ore contains many structures that are not easily reducible, and its reducibility is low. Therefore, according to the present invention, the average Ferre diameter of hematite particles in the cross-sectional structure of agglomerated ore is set to 30 μm or more. The upper limit of the average Ferre diameter of hematite particles is not particularly limited, but for example it may be 50 μm or less.

[0025] The average Ferret diameter of hematite particles is calculated from the distribution of hematite crystal orientations. Since hematite is reduced at the particle level where the crystal orientation is the same, particle size based on crystal orientation is important. Specifically, the cross-sectional structure of agglomerate is observed with a polarizing microscope, and information on the distribution of hematite crystal orientations is obtained based on the brightness of the hematite region in the polarizing microscope image. The grain boundaries of the hematite particles are then defined from the obtained distribution information. More specifically, it can be determined by the method described in the examples. Bright-field images from an optical microscope do not provide information on crystal orientation, so there is a risk of overestimating the particle size in areas where small primary hematite particles are aggregated. For this reason, it is necessary to observe the structure using a method that allows the crystal orientation of hematite to be determined, such as the polarizing microscope described above.

[0026] Agglomerate ore may be sintered ore or calcined pellets. Sintered ore is produced by sintering raw materials containing iron ore (especially powdered ore). Calcined pellets, on the other hand, are produced by granulating and calcining raw materials containing iron ore (especially powdered ore).

[0027] The agglomerated ore has a reducing gas composition of: CO3 1% - H ２ 19%-N ２ The reduction rate (RI) when reduced under the conditions of 50%, reduction temperature: 800°C, and reduction time: 3 hours may be between 80% and 100%. Specifically, RI can be determined by the method described in the examples.

[0028] The agglomerate according to the present invention is used in a blast furnace. By charging the above agglomerate into the blast furnace and operating it, reduced iron can be produced, and CO2 is produced during this process. ２ Emissions can be reduced. The following describes the operation method of a blast furnace using the above-mentioned agglomerated ore.

[0029] The above lump-shaped ore can be used under the operating conditions of a general blast furnace. For example, it can be used under the operating conditions of blowing hot air (generally air heated to 1000 to 1300 °C) from the tuyere. Further, the above lump-shaped ore can also be suitably used under the operating conditions of blowing a gaseous reducing agent containing at least one of hydrocarbons, hydrogen, and ammonia into the blast furnace. Examples of the hydrocarbon that can be contained in the gaseous reducing agent include methane. Note that the gaseous reducing agent containing at least one of hydrocarbons, hydrogen, and ammonia may be a mixture of multiple types of gases. For example, nitrogen or the like may be contained as an impurity. When blowing a gaseous reducing agent containing at least one of hydrocarbons, hydrogen, and ammonia into the blast furnace, oxygen may be further blown into the blast furnace. The above lump-shaped ore has high reducibility when used under any of the above-described operating conditions.

[0030] The blowing amount of the gaseous reducing agent containing at least one of hydrocarbons, hydrogen, and ammonia into the blast furnace is preferably 100 Nm ３ / t or more in terms of converted value. Here, the blowing amount is expressed as the amount per ton of hot metal produced. Also, the converted value of the blowing amount is obtained by converting the amount of the gaseous reducing agent blown into the blast furnace to the amount of the gaseous reducing agent after conversion when the gaseous reducing agent blown into the blast furnace is converted to another gaseous reducing agent in the blast furnace. For example, when 1 Nm ３ of methane is blown into the blast furnace, methane reacts with oxygen near the tuyere to be converted into 1 Nm ３ of carbon monoxide and 2 Nm ３ of hydrogen, and these function as gaseous reducing agents. Therefore, the blowing amount of the gaseous reducing agent when 1 Nm ３ of methane is blown into the blast furnace is calculated as 3 Nm ３ in terms of converted value. Also, for example, when 1 Nm ３ of ammonia is blown into the blast furnace, ammonia is thermally decomposed into 1.5 Nm ３ of hydrogen and 0.5 Nm ３ of nitrogen. Since nitrogen does not function as a gaseous reducing agent, the blowing amount of the gaseous reducing agent when 1 Nm ３ of ammonia is blown into the blast furnace is 1.5 Nm ３ in terms of converted value.It is calculated. Similarly, when other gases such as nitrogen are included in the gas blown into the blast furnace, the converted value of the blowing amount is calculated. For example, when a gas containing nitrogen is blown into the blast furnace, since nitrogen does not function as a gas reducing material, the blowing amount of nitrogen is not counted in the above converted value. Also, when a gas containing hydrogen or carbon monoxide is blown into the blast furnace, since these gases function as gas reducing materials as they are, the blowing amounts of hydrogen or carbon monoxide are directly counted in the above converted value.

[0031] Also, in the gas reducing material containing at least one of hydrocarbons, hydrogen, and ammonia, the concentration of the gas reducing material that actually contributes to reduction is preferably 90% by volume or more, and more preferably 95% by volume or more. Here, the gas reducing material that actually contributes to reduction refers to hydrogen, carbon monoxide, and gases (such as methane, ammonia, etc.) that are converted into one or both of carbon monoxide and hydrogen in the furnace. Also, the remainder other than the gas reducing material that actually contributes to reduction may be composed of impurities.

[0032] Next, an example of the method for producing the agglomerated ore will be described. Note that the present invention is not limited in any way by the following description.

[0033] The raw materials for the agglomerated ore are not particularly limited, and for example, it may be a mixture of components such as iron ore fines, secondary raw material fines, miscellaneous raw materials, and solid fuel. As the iron ore fines, iron ore fines of multiple grades may be used. Examples of the secondary raw material fines include limestone, silica, and serpentine. Examples of the miscellaneous raw materials include dust, scale, and returned ore. The solid fuel is a raw material for burning in a sintering apparatus to promote the sintering reaction, and examples thereof include pulverized coke. The ratio of the above components can be changed as appropriate.

[0034] When producing sintered ore, after granulating the raw materials, they may be sintered by a sintering device, and the particle size of the obtained sintered body may be adjusted. When producing fired pellets, the raw materials may be granulated into pellets, and the obtained pellets may be fired by a sintering device. In the sintering of sintered ore, the granulated raw materials are filled into a sintering device, ignition is carried out at the upper part of the packed bed, and the gas introduced into the sintering device is sucked from the upper part to the lower part of the packed bed to perform a sintering reaction. At that time, the sintering reaction stops when the combustion zone reaches the lower part of the packed bed. Examples of the sintering device include a Dwight-Lloyd type sintering machine and a sintering pot. Examples of the sintering pot include a Grinawalt type sintering machine. In the firing of pellets, the granulated pellets may be introduced into a sintering device and a sintering reaction may be carried out by high-temperature firing gas. Examples of the sintering device include a shaft furnace method, a grate method, and a grate kiln method.

[0035] In the cross-sectional structure of the above lump ore, in order to make the area ratio of hematite, the area ratio of magnetite, and the average Feret diameter of hematite particles within the above-mentioned ranges, the content of solid fuel in the raw materials and the oxygen concentration in the gas introduced into the sintering device may be controlled. By increasing the solid fuel, the particles of hematite can be coarsened. On the other hand, if the solid fuel is excessively contained, there is a risk of a decrease in the area ratio of hematite or an increase in the area ratio of magnetite due to excessive heat. Also, by increasing the oxygen concentration in the gas introduced into the sintering device, the amount of melt generated during sintering can be increased to coarsen the particles of hematite. On the other hand, if the oxygen concentration becomes excessively high, there is a risk of a decrease in the area ratio of hematite or an increase in the area ratio of magnetite.

[0036] Hereinafter, the present invention will be described based on examples.

[0037] First, various lump ores were produced by controlling the content of solid fuel in the raw materials and the oxygen concentration in the gas introduced into the sintering device. A laboratory-scale sintering pot was used as the sintering device. For the obtained lump ore, the cross-sectional structure was observed and the reducibility was evaluated by the following method.

[0038] (Observation of cross-sectional structure) The obtained agglomerated ore was crushed to a particle size of 1-2 mm using a crusher, then embedded in resin, and the surface to be evaluated (observation surface) was mirror-polished. Polishing was performed using SiC paper from #120 to #400, followed by polishing with diamonds of 9 μm, 3 μm, 1 μm, and 0.25 μm.

[0039] Bright-field and polarized light microscope images were acquired of the observation surface of the polished agglomerate using an optical microscope. These images were acquired using an optical microscope equipped with imaging capabilities at a magnification of 100x, capturing the entire observation surface of the evaluation sample while continuously changing the field of view. Here, bright-field and polarized light microscope images were acquired using bright-field mode and polarized light mode, respectively, for the same field of view. When acquiring polarized light microscope images, the polarizer and analyzer were shifted by several degrees from crossed nicols to ensure clear contrast.

[0040] The obtained bright-field microscope images were segmented based on color tone using TWS (Trainable Weka Segmentation), a plugin for ImageJ, and classified into hematite, magnetite, calcium ferrite, slag, and pores. The area percentage of each tissue after classification was calculated using the Measure function of ImageJ. The obtained area percentages of hematite, magnetite, calcium ferrite, slag, and pores are shown in Table 1.

[0041] Next, using the segmentation results performed on the bright-field microscope image, the hematite region was identified in the polarized light microscope image, and the regions other than hematite were masked to obtain a polarized light microscope image of hematite only. The brightness values ​​output in 256 levels in the above polarized light microscope image of hematite only were classified into 10 levels, and the distribution information of crystal orientation was obtained by assigning regions with the same brightness level to the same crystal orientation.

[0042] Next, in the distribution of crystal orientations, the outer edges of regions with continuous luminance of the same gradation were defined as the apparent grain boundaries of hematite particles. Then, the hematite particles defined by the apparent grain boundaries were classified into aggregate hematite particles, which exist as aggregates of three or more hematite particles in contact with each other, and discrete hematite particles. Next, using the distance between the centers of the discrete hematite particles and their crystal orientations, the determination that two or more discrete hematite particles constitute a single hematite particle was made for all discrete hematite particles, starting with those with the largest perimeters. As a criterion for determination, when a specific discrete hematite particle is targeted, discrete hematite particles located within a distance of the target hematite particle's perimeter and having the same crystal orientation are considered to be the same hematite particle as the target hematite particle. It should be noted that this may result in cases where three or more discrete hematite particles are determined to be a single hematite particle in a chain reaction. For example, regarding discrete hematite particles A to C, it is possible that A and B are determined to be the same hematite particle, and that B and C are also determined to be the same hematite particle. In that case, regardless of whether A and C were determined to be the same hematite particle or not, A to C were all determined to be the same hematite particle. Next, for two or more discrete hematite particles determined to be a single hematite particle, the rectangle circumscribing those two or more discrete hematite particles was defined as the grain boundary of that single hematite particle. For the remaining hematite particles, the apparent grain boundaries were used to define the grain boundaries. From the defined grain boundaries of the hematite particles, the Ferret diameter of each hematite particle was calculated, and the average Ferret diameter was calculated for all hematite particles. The obtained average Ferret diameters are shown in Table 1.

[0043] (Evaluation of Reducibility) Next, the reducibility of the agglomerate was evaluated. The above agglomerate was crushed to a particle size of 19-21 mm using a crusher, and after classification, 500 g of the obtained agglomerate was charged into a vertical reduction test furnace. Composition of reducing gas: CO3 1%-H ２ 19%-N ２A reduction test was conducted under the conditions of 50%, reduction temperature: 800°C, and reduction time: 3 hours. Note that in order to determine the JIS-RI, which is the reducibility index specified in JIS M8713, approximately 30% CO - N ２ The evaluation will be conducted using a reducing gas with a composition of approximately 70%. In contrast, this evaluation condition simulates the operating conditions in which a gaseous reducing agent containing hydrocarbons is injected into a blast furnace. ２ A reducing gas containing [a specific substance] was used. In particular, the amount of gaseous reducing agent containing hydrocarbons injected into the blast furnace was 100 Nm³ (converted value). ３ The operating conditions were simulated under conditions of 1 / t or more, and where the concentration of the gaseous reducing agent that actually contributes to reduction was 90% by volume or more. The reducibility index (RI) was calculated from the mass and chemical analysis values ​​of the agglomerate ore before and after the reduction test, in the same manner as the calculation method for the achievable JIS reduction rate specified in JIS M8713. The results are shown in Table 1. The agglomerate ore that met the conditions of the present invention had high reducibility.

[0044]

Claims

1. Agglomerated ore for blast furnaces, wherein the cross-sectional structure of the agglomerated ore has a hematite area ratio of 40% or more and a magnetite area ratio of 30% or less, and the average Ferret diameter of the hematite particles in the cross-sectional structure, calculated from the distribution of the crystal orientation of the hematite, is 30 μm or more.

2. The blast furnace agglomerate according to claim 1, wherein the cross-sectional structure comprises at least one selected from calcium ferrite, slag, and pores.

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