Method for estimating the fragmentation rate of iron ore pellets

Abstract

To estimate a rate of powdering of iron ore pellet caused by impact acted on the iron ore pellet during the transportation of the iron ore pellet.SOLUTION: A powdering rate is a ratio of a mass of a powdered substance to a mass of an iron ore pellet before powdering. First, a gap portion included in the iron ore pellet is extracted, on the basis of a three-dimensional image obtained by X-ray CT scanning of the iron ore pellet before the powdering. Subsequently, in the extracted gap portion, a gap portion having a spheroidicity of a threshold or less is specified as a crack, and a crack volume rate that is a ratio of the total volume of the crack to an apparent volume of the iron ore pellet is determined. Subsequently, a powdering rate is determined from the determined crack volume rate, on the basis of a correlation (primary function) between the crack volume rate and the powdering rate.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for estimating the degree of pulverization of iron ore pellets when the pellets are pulverized by impact during transportation. [Background technology]

[0002] When iron ore pellets are transported, they may be subjected to physical impacts, which may cause some of the iron ore pellets to pulverize. If the iron ore pellets containing the pulverized pellets (hereinafter referred to as "pellet chips") are charged into a blast furnace or a direct reduction shaft furnace, the pellet chips will deteriorate the gas permeability inside the furnace. For this reason, the iron ore pellets are usually sieved to remove the pellet chips before being charged into a blast furnace or the like. The removed pellet chips are usually transported to a sintering plant, where they are agglomerated and then charged into a blast furnace.

[0003] On the other hand, with regard to the pulverization of iron oxide raw materials (pellets and sintered ore) charged into a blast furnace or the like, Patent Document 1 discloses a technique for evaluating the reduction pulverization of pellets, and Patent Document 2 discloses a technique for determining the fracture toughness value of sintered ore.

[0004] In Patent Document 1, the porosity and average pore diameter of ironmaking pellets are measured, the reaction zone width is estimated from these measurements, the total crack area is estimated from this reaction zone width, and the reduction disintegration index of the ironmaking pellets is estimated from this total crack area. Here, the reaction zone width is the reciprocal of the slope representing the change in magnetite concentration in the radial direction of the pellet due to reduction. Furthermore, the total crack area is the total crack area per unit volume in the pellet caused by reduction.

[0005] In Patent Document 2, the length of a crack formed when a Vickers indenter is pressed into a sample (sintered ore) is calculated, and the fracture toughness value of the sample is calculated based on the length of the crack, the length of the indentation formed when the Vickers indenter is pressed into the sample, and the Vickers hardness of the sample. Here, the crack length is calculated from the AE energy calculated from the AE wave when the Vickers indenter is pressed into the sample, the thickness of the sample, and the porosity of the sample. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-60920 [Patent Document 2] Japanese Patent Application Publication No. 2018-44210 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to estimate the pulverization of iron ore pellets due to impacts that occur during transportation. Patent Document 1 focuses on pulverization when steelmaking pellets are reduced (reduction pulverization), but does not focus on pulverization of iron ore pellets due to impacts during transportation, as in the present invention. Furthermore, Patent Document 2 estimates the fracture toughness value of sintered ore, but does not estimate pulverization of iron ore pellets due to impacts during transportation, as in the present invention. [Means for solving the problem]

[0008] The present invention provides a method for estimating the pulverization rate, which is the ratio of the mass of pulverized material to the mass of the iron ore pellets before pulverization, for iron ore pellets that are pulverized by impact during transportation. First, voids contained in the iron ore pellets are extracted based on three-dimensional images obtained by X-ray CT scanning of the iron ore pellets before pulverization. Next, of the extracted voids, voids with a sphericity below a threshold are identified as cracks, and a crack volume rate, which is the ratio of the total volume of cracks to the apparent volume of the iron ore pellets, is calculated. Next, the pulverization rate is calculated from the calculated crack volume rate based on the correlation between the crack volume rate and the pulverization rate. Here, the correlation between the crack volume rate and the pulverization rate is expressed as a linear function and is calculated in advance.

[0009] The threshold value can be set to 0.5 [-]. The pulverized material can be particles with a particle size of 6.3 mm or less. The correlation between the crack volume fraction and the pulverization rate can be calculated for each iron ore pellet transport path. The sphericity can be calculated from the volume and surface area of ​​the voids in the three-dimensional image. [Effects of the Invention]

[0010] According to the present invention, by focusing on the correlation between the crack volume fraction and the pulverization rate, the pulverization rate can be estimated from the crack volume fraction of the iron ore pellets before pulverization. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a flowchart illustrating a method for estimating the fineness rate of iron ore pellets. [Figure 2] FIG. 1 is a diagram showing an example of an X-ray image of an iron ore pellet in one cross section. [Figure 3] FIG. 1 is a diagram showing an example of an image in which voids are extracted from an X-ray image of an iron ore pellet in one cross section. [Figure 4] FIG. 1 is a diagram showing an example of an image in which voids (cracks) are extracted from an X-ray image of an iron ore pellet in one cross section. [Figure 5]FIG. 1 is a diagram showing an example of an image in which voids (pores) are extracted from an X-ray image of an iron ore pellet in one cross section. DETAILED DESCRIPTION OF THE INVENTION

[0012] This embodiment estimates the pulverization rate when iron ore pellets are pulverized by impacts occurring during transportation. The pulverization referred to here does not include pulverization of iron ore pellets due to reduction in a furnace. The transport route of iron ore pellets is the route taken from immediately after the iron ore pellets are produced until they are charged into a blast furnace or the like. However, the iron ore pellets are most susceptible to impacts during loading and unloading (e.g., loading and unloading from a ship), making them more likely to pulverize. The estimation of the pulverization rate is based on the premise that the transport route of the iron ore pellets is the same, and it is assumed that the impacts acting on iron ore pellets along the same transport route are the same.

[0013] Iron ore pellets are fired products made by agglomerating fine iron ore of a specified particle size (for example, 100 μm or less) into a size of about 10 mm, and are used as a raw material in the blast furnace process and direct reduction process. The manufacturing process of iron ore pellets mainly includes a granulation process and a firing process. In the granulation process, fine iron ore whose particle size and moisture have been adjusted is granulated using a granulator to produce green pellets. In the firing process, the green pellets are heated and fired to produce iron ore pellets.

[0014] The pulverization of iron ore pellets due to impact can be broadly divided into pulverization due to wear of the surface of the iron ore pellets (hereinafter referred to as "abrasion pulverization") and pulverization due to cracks inside the iron ore pellets (hereinafter referred to as "crack pulverization"). Cracks in iron ore pellets occur when the internal pressure of the raw pellets increases due to the rapid evaporation of water during the firing process and the thermal decomposition of the water of crystallization in the iron ore.

[0015] It is believed that abrasion and powdering occur significantly when the matrix strength of the iron ore pellets is reduced due to the firing temperature being lower than the target temperature or the firing time being shorter than the target time (so-called insufficient firing) during the firing process. However, in a normal firing process, the firing temperature and firing time are properly controlled, so abrasion and powdering are unlikely to occur. Therefore, cracking and powdering are thought to be the dominant form of iron ore pellet powdering due to impact.

[0016] (Method for estimating the powdering rate of iron ore pellets) A method for estimating the iron ore pellet fragmentation rate Rp will be described using the flowchart shown in Figure 1. As will be described later, the iron ore pellet fragmentation rate Rp is estimated based on the crack volume rate Rvc of the iron ore pellet. First, the fragmentation rate Rp and the crack volume rate Rvc will be described below.

[0017] The pulverization rate Rp is the ratio (wt%) of the total mass M2 of the pulverized material generated by the pulverization of the iron ore pellets due to the impact to the mass M1 of the iron ore pellets before pulverization, and is expressed by the following formula (1).

[0018]

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[0019] The iron ore pellets before pulverization may be iron ore pellets immediately after being produced by the above-described production process and before being transported, which causes pulverization. As described above, considering that the iron ore pellets are most susceptible to impact during loading and unloading (e.g., loading and unloading from a ship), the iron ore pellets before pulverization may be iron ore pellets before loading and unloading (e.g., before being loaded onto a ship). Meanwhile, pulverized material may be defined as particles having a predetermined particle size or less, and the predetermined particle size may be, for example, 6.3 mm.

[0020] The crack volume ratio Rvc is the ratio of the total volume V2 of all cracks present in the iron ore pellets (before pulverization) to the apparent volume V1 of the iron ore pellets (before pulverization), and is expressed by the following formula (2).

[0021]

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[0022] The apparent volume V1 is determined from the surface shape of a three-dimensional image obtained by performing an X-ray CT scan on the iron ore pellets (before pulverization). The iron ore pellets contain voids, and the volume of these voids is also included in the apparent volume V1. The total volume V2 of cracks is determined by identifying cracks in the three-dimensional image obtained by performing an X-ray CT scan on the iron ore pellets (before pulverization). In other words, all cracks contained in the iron ore pellets (before pulverization) are identified, and the total volume V2 is the sum of the volumes of these cracks. The method for identifying cracks will be described later.

[0023] In step S101, an X-ray CT scan is performed on the iron ore pellets (before pulverization). By the X-ray CT scan, multiple X-ray images of the iron ore pellets can be obtained for cross sections perpendicular to the scanning direction. FIG. 2 shows an X-ray image (one example) of the iron ore pellets in one cross section. Note that a three-dimensional image of the iron ore pellets can be obtained by superimposing the multiple X-ray images described above.

[0024] In step S102, void portions contained in the iron ore pellets are extracted based on the three-dimensional images obtained by the X-ray CT scan in step S101. Specifically, by performing binarization processing on the X-ray images (cross-sectional images) of the iron ore pellets, void portions are distinguished from non-void portions (hereinafter referred to as "non-void portions") in each of all X-ray images. The non-void portions correspond to the matrix of the iron ore pellets. Here, the matrix refers to the combined portion of the solid and minute pores (pores smaller than the spatial resolution) that cannot be detected by the X-ray CT scan.

[0025] In the binarization process, a threshold value is set for brightness values ​​between 0 and 255, and portions showing brightness values ​​equal to or greater than the threshold are distinguished from portions showing brightness values ​​less than the threshold. Here, the brightness value of void portions is smaller than the brightness value of non-void portions, so portions showing brightness values ​​less than the threshold are void portions, and portions showing brightness values ​​equal to or greater than the threshold are non-void portions. This makes it possible to extract the void portions (three-dimensional shapes) contained in the iron ore pellets. In addition to the binarization process, image processing (e.g., top-hat transformation processing) required to extract the void portions can be performed.

[0026] In extracting voids, one continuous space surrounded by non-voids constitutes one void, and iron ore pellets usually have multiple voids. Figure 3 shows an example of an image in which voids have been extracted from an X-ray image of an iron ore pellet in a cross section (corresponding to Figure 2).

[0027] In step S103, the sphericity Ψm of each void portion is measured. The sphericity Ψm is calculated using the following formula (3). The sphericity Ψm is a value of 1.0 [-] or less, and a sphericity Ψm of 1.0 indicates a perfect sphere.

[0028]

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[0029] In the above formula (3), Ψm is the sphericity [-], and Vv is the volume of one void [mm 3 ], and Av is the surface area of ​​one void [mm 2 The volume Vv and surface area Av can be determined from a three-dimensional image obtained by X-ray CT scanning. That is, the volume Vv and surface area Av can be determined by identifying each void portion contained in the iron ore pellet.

[0030] In step S104, based on the sphericity Ψm measured in step S103, cracks are identified from the void portion extracted in step S102. Normally, pores and cracks are included in the void portion contained in the iron ore pellet, but in order to calculate the above-mentioned crack volume fraction Rvc, only the cracks included in the void portion are identified. Considering the respective formation processes of pores and cracks, pores have a shape that is close to a sphere, whereas cracks have a shape that cannot be said to be spherical. Therefore, pores and cracks can be distinguished by their shapes (i.e., sphericity Ψm).

[0031] Specifically, a threshold value Ψth of the sphericity Ψm for distinguishing between pores and cracks is set, and voids where the sphericity Ψm is equal to or less than the threshold value Ψth are identified as cracks. Voids where the sphericity Ψm is greater than the threshold value Ψth are considered to be pores. Figure 4 shows an example of an X-ray image (corresponding to Figure 2) of an iron ore pellet in a cross section in which voids (cracks) have been extracted. Figure 5 shows an example of an X-ray image (corresponding to Figure 2) of an iron ore pellet in a cross section in which voids (pores) have been extracted.

[0032] The threshold value Ψth is a boundary value of the sphericity Ψm for determining whether a void portion corresponds to a pore or a crack, and can be a value smaller than 1.0 [-]. Here, the threshold value Ψth is preferably 0.5 [-]. If the threshold value Ψth is smaller than 0.5, the evaluation for identifying cracks is likely to be underestimated, and void portions that are actually recognized as cracks may not be identified as cracks. On the other hand, if the threshold value Ψth is larger than 0.5, the evaluation for identifying pores is likely to be underestimated, and void portions that are actually recognized as pores may be identified as cracks.

[0033] In step S105, the crack volume fraction Rvc of the iron ore pellet is calculated. The crack volume fraction Rvc is calculated from the above formula (2), and the total volume V2 shown in the above formula (2) is the sum of the volume Vv of voids identified as cracks among the volume Vv shown in the above formula (3). Note that the apparent volume V1 shown in the above formula (2) can be calculated from the surface shape of the three-dimensional image obtained by the X-ray CT scan in step S101, as described above.

[0034] In step S106, the powdering rate Rp is calculated from the crack volume rate Rvc calculated in step S105. It has been found that there is a correlation between the powdering rate Rp and the crack volume rate Rvc, which is expressed as a linear function of the following formula (4). If this correlation is calculated in advance, the powdering rate Rp can be calculated from the crack volume rate Rvc.

[0035]

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[0036] In the above formula (4), a and b are coefficients that can be determined in advance. The coefficient a is a positive value, and the coefficient b is a positive or negative value. The coefficients a and b depend on the transport route and type of iron ore pellets, so they can be determined in advance for each of these dependent factors.

[0037] The crushing rate Rp and the crack volume fraction Rvc can be measured in advance by using the same type of iron ore pellets as the iron ore pellets for which the crushing rate Rp is to be estimated and transporting them along the same transport route as the iron ore pellets for which the crushing rate Rp is to be estimated. Based on these measurements, the correlation between the crushing rate Rp and the crack volume fraction Rvc (the above formula (4)) can be obtained in advance.

[0038] As described above, the estimation of the powdering rate Rp is based on the assumption that the iron ore pellets are transported along the same transport route and that the impacts acting on the iron ore pellets during transport are equivalent. Therefore, when the iron ore pellets are transported along different routes, the correlation between the powdering rate Rp and the crack volume fraction Rvc is calculated for each transport route, and the powdering rate Rp is estimated from the crack volume fraction Rvc using this correlation. However, since the degree of powdering due to impacts may differ depending on the type of iron ore pellet, the correlation between the powdering rate Rp and the crack volume fraction Rvc is calculated for each type of iron ore pellet, and the powdering rate Rp is estimated from the crack volume fraction Rvc using this correlation.

[0039] According to this embodiment, if the crack volume fraction Rvc of iron ore pellets before pulverization is measured from a three-dimensional image obtained by X-ray CT scanning, the pulverization fraction Rp can be estimated based on the correlation (linear function) between the pulverization fraction Rp and the crack volume fraction Rvc. This makes it possible to grasp the mass ratio (pulverization fraction Rp) at which the iron ore pellets pulverize during transportation.

[0040] In addition, the mass of the pulverized iron ore pellets (excluding the pulverized material) can be determined, and the pellet ratio when charging the iron ore pellets into the blast furnace can be estimated in advance. Here, if the charging amount of the iron ore pellets does not reach the target amount, the iron ore pellets to be charged into the blast furnace can be increased in advance according to the shortage. [Example]

[0041] First, the correlation between the powdering rate Rp and the crack volume fraction Rvc was determined using eight iron ore pellets (No. 1 to No. 8) that differed only in lot. Specifically, the eight iron ore pellets were conveyed along a predetermined conveying path, and the powdering rate Rp and the crack volume fraction Rvc were determined for each iron ore pellet. Based on these results, the correlation (linear function) between the powdering rate Rp and the crack volume fraction Rvc was determined. This will be explained in detail below.

[0042] After loading, 50 kg of iron ore pellets (corresponding to the mass M1 shown in the above formula (1)) were collected and dried at 105°C for 24 hours to facilitate separation of pellet chips (powdered material) adhering to the iron ore pellets. After this, the pellets were sieved using a sieve with a mesh size of 6.3 mm, and the mass of pellet chips with a particle size of 6.3 mm or less (corresponding to the mass M2 shown in the above formula (1)) was measured.

[0043] As a result, the powdering rate Rp was calculated for each of the eight iron ore pellets based on the above formula (1). Although powdering has already occurred in the iron ore pellets after loading, it is assumed that the pellet chips remain attached to the iron ore pellets, and the mass of the iron ore pellets after loading is set to the mass M1 shown in the above formula (1) (the mass of the iron ore pellets before powdering).

[0044] For each of the eight iron ore pellets described above, 20 iron ore pellets were sampled before loading and unloading, and an X-ray CT scan was performed on each individual iron ore pellet using a three-dimensional measurement X-ray CT device (TDM3000H-FP, Yamato Scientific Co., Ltd.). The imaging conditions for the X-ray CT scan are shown in Table 1 below. The pixel size was set to a spatial resolution (16.6 μm / pixel) that would not interfere with the detection of voids (including pores and cracks) contained in the iron ore pellets.

[0045] [Table 1]

[0046] A binarization process was performed on a three-dimensional image of the iron ore pellet obtained by X-ray CT scanning to extract voids (including cracks and pores). Here, the threshold value for the binarization process was set to 110, and areas with a brightness value of less than 110 were identified as voids. Next, the threshold value Ψth was set to 0.3 (Example 1), 0.5 (Example 2), 0.7 (Example 3), and 1.0 (Comparative Example), and cracks were identified from among the voids. The crack volume ratio Rvc was then calculated as described above.

[0047] In this example, 20 iron ore pellets were collected from each of the eight iron ore pellets, and the crack volume fraction Rvc was calculated for each iron ore pellet, and then the average value Rvc_ave of the crack volume fractions Rvc of the 20 pellets was calculated.

[0048] The measurement results of the above-mentioned powdering rate Rp and crack volume rate Rvc_ave are shown in the following Table 2. Table 2 below also shows the porosity, which is the ratio of voids (including cracks and holes) to the apparent volume of the iron ore pellets.

[0049] [Table 2]

[0050] In each of Examples 1 to 3 and Comparative Example, the correlation (approximation) between the crushing rate Rp and the crack volume rate Rvc was determined based on the crushing rate Rp and the crack volume rate Rvc_ave in the iron ore pellets (No. 1 to No. 8), and it was possible to express it as a linear function as shown in Table 3 below. Table 3 below shows the coefficient of determination R 2 Also shown.

[0051] [Table 3]

[0052] As can be seen from Table 3 above, by making the threshold value Ψth smaller than 1.0, the coefficient of determination R 2 In addition, when the threshold value Ψth was set to 0.5, the coefficient of determination R 2 Therefore, it can be seen that the correlation between the powdering rate Rp and the crack volume rate Rvc is best expressed by setting the threshold value Ψth to 0.5.

[0053] Next, the crack volume fraction Rvc was measured for the iron ore pellets for which the breakage rate Rp was to be estimated. These iron ore pellets were the same type as the iron ore pellets used when the correlation (linear function) between the breakage rate Rp and the crack volume fraction Rvc was calculated, but the lot was different. The crack volume fraction Rvc was measured using the same method as that used to measure the crack volume fraction Rvc_ave for the iron ore pellets (No. 1 to No. 8) described above.

[0054] When the crack volume fraction Rvc was measured with the threshold Ψth set to 0.5, the crack volume fraction Rvc was found to be 2.3 [vol%]. Furthermore, when the correlation when the threshold Ψth was 0.5 (Example 2 shown in Table 3 above; Rp = 2.72 × Rvc + 3.27) was used to calculate the powder fraction Rp from the measured crack volume fraction Rvc, the powder fraction (estimated value) Rp was found to be 9.53 [wt%].

[0055] On the other hand, for the iron ore pellets (50 kg) for which the powdering rate Rp was to be estimated, the mass of the pellet chips after loading was measured to determine the powdering rate Rp, and the powdering rate (measured value) Rp was 9.50 [wt%]. The estimated results and measured results of the above-mentioned powdering rate Rp are shown in Table 4 below. As can be seen from Table 4 below, the powdering rate (estimated value) Rp was almost consistent with the powdering rate (measured value) Rp. Therefore, by estimating the powdering rate Rp as in this example, the estimation accuracy of the powdering rate Rp can be ensured.

[0056] [Table 4]

Claims

1. A method for estimating a powdering rate, which is a ratio of the mass of powdered iron ore pellets to the mass of iron ore pellets before powdering, for iron ore pellets that are powdered due to impact during transportation, comprising: Extracting voids contained in the iron ore pellets based on a three-dimensional image obtained by X-ray CT scanning of the iron ore pellets before pulverization; Among the extracted void portions, void portions having a sphericity equal to or less than a threshold value are identified as cracks, and a crack volume ratio, which is the ratio of the total volume of the cracks to the apparent volume of the iron ore pellet, is calculated; calculating the powdering rate from the calculated crack volume rate based on a correlation between the crack volume rate and the powdering rate, which is expressed by a linear function and which has been calculated in advance; A method for estimating the powdering rate of iron ore pellets, comprising:

2. The method for estimating the iron ore pellet powdering rate according to claim 1, characterized in that the threshold value is 0.5 [-].

3. 3. The method for estimating the powdering rate of iron ore pellets according to claim 1, wherein the powdered material has a particle size of 6.3 mm or less.

4. 3. The method for estimating the fineness rate of iron ore pellets according to claim 1, wherein the correlation is determined for each conveying route of the iron ore pellets.

5. 3. The method for estimating the powdering rate of iron ore pellets according to claim 1, wherein the sphericity is determined from the volume and surface area of ​​the void portion in the three-dimensional image.

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