Pellet production method

Description

1 / 17 “PELLET PRODUCTION METHOD” Technical Area

[001] The present invention relates to a method of producing pellets that are used as raw material charged into a shaft furnace or similar. Background of the Invention

[002] Coarse ores and pellets are used as feedstocks during reduction in shaft furnaces, which has become a common direct reduction process in iron production. Pellets are produced through several stages of crushing, mixing, granulation, and sintering of ores. In the sintering stage, it is known that as the water present in the iron ore pellets (so-called raw pellets) that have not yet been sintered evaporates, a phenomenon called bursting occurs, in which the raw pellets explode and / or pulverize, thus degrading the pellet yield.

[003] Patent Literature 1 describes a method for producing crude pellets containing an iron ore, auxiliary feedstock and organic binder. According to this production method, the occurrence of bursting during the sintering of the crude pellets can be suppressed by controlling the viscosity of the water in which the organic binder was dissolved in relation to a proportion of the iron ore feedstock composition that contains a large amount of crystalline water. List of Citations Patent Literature Patent Literature 1: JP-A-2020-180371 Summary of the Invention Technical Problem

[004] Currently, as raw material for ore (iron ore) for Petition 870250086688, dated 09 / 25 / 2025, page 10 / 33 2 / 17 pellets, generally high-grade ores containing small amounts of crystalline water are used. However, with the increased demand for high-grade ores in the coming days, it is estimated that a stable supply of high-grade ores will become difficult. Therefore, the use of low-grade ores, with a total Fe content of less than 63% by mass, sourced from Australia, India, and other countries, is under consideration. As the crystalline water content in iron ore pellets (raw pellets) increases, bursting in sintered pellets, as described above, is likely to occur, thus resulting in a demand for improved pellet production methods aimed at suppressing bursting.

[005] In this sense, in the case of the raw pellet production method disclosed in Patent Literature 1, the appropriate particle size of the raw pellet material, the appropriate amount of added water, and the appropriate drying condition(s) thereof are not considered. Thus, the suppression of bursting in sintered pellets is insufficient, which led to the demand for the development of a new pellet production method capable of suppressing bursting more effectively.

[006] The present invention was carried out considering the circumstances mentioned above. One of the objectives of the present invention is to provide a method for producing pellets capable of more effectively suppressing bursting in pellets that are produced by a granulation step of carrying out granulation by adding water to a raw material and a sintering step of sintering the granulated product. Solution to the Problem

[007] The pellet production method of the present invention includes: a granulation step in which granulation is carried out by adding water to a raw material prepared by mixing a powdered substance containing an iron ore with an iron content of 63% by mass or less and Petition 870250086688, dated 09 / 25 / 2025, page 11 / 33 3 / 17 an auxiliary raw material; and a sintering stage which includes a drying and preheating zone and a subsequent sintering zone where a granulated product obtained is sintered, in which before the sintering stage, a specific surface area and the actual density of the raw material and an amount of water added in the granulation stage are measured, followed by the use of the measurement values ​​obtained to estimate appropriate values ​​of a drying temperature and a gas flow rate in the drying and preheating zone of the sintering stage, thus allowing a treatment in the drying and preheating zone of the sintering stage to be carried out using the estimated appropriate values ​​of the drying temperature and gas flow rate.

[008] Here, in the case of the pellet production method of the present invention which is configured in the manner described above, it is considered that more preferable solutions can be brought about when (1) the appropriate values ​​of the drying temperature and gas flow rate in the drying and preheating zone of the sintering step are estimated in such a way that, using the measurement values ​​of the specific surface area and the actual density of the raw material and the measurement value of the amount of water added in the granulation step, the drying temperature and gas flow rate are determined so that a bursting index X calculated by the following formula (1) is not less than 0.100 and not greater than 3.000, X=0.825 x Sm x ρ x 10-4+ 0.061 x M + 0.245 x T x 10-2+ 0.072 x V 1.285 ... (1) where Sm, ρ, M, T and V are defined as follows, Sm: Specific surface area of ​​the raw material (cm2 / g) ρ: Actual density of the raw material (g / cm3) Petition 870250086688, dated 09 / 25 / 2025, page 12 / 33 4 / 17 M: Amount of water added in the granulation step (% by mass) T: Drying temperature (°C) V: Gas flow rate (m / s). Advantageous Effects of the Invention

[009] According to the pellet production method of the present invention, bursting can be suppressed more effectively in pellets that are produced by a granulation step of performing granulation by adding water to a raw material and a sintering step of sintering the granulated product. Brief Description of the Drawings

[010] [FIG.1] is a flowchart that explains the steps of a pellet production embodiment of the present invention. Description of the Modalities

[011] One embodiment of the present invention is described in detail below. Herein, the following embodiment is a set of examples of a device and / or method that embodies the technical concept of the present invention and is not intended to limit the configuration of the present invention to those shown below. That is, various modifications may be made to the technical concept of the present invention within the technical scope described in the claims.

[012] FIG. 1 is a flowchart explaining the steps employed in one embodiment of the pellet production method of the present invention. One embodiment of the pellet production method of the present invention is described below with reference to FIG. 1. The pellet production method of this embodiment includes a granulation step and a sintering step. In the granulation step, granulation is carried out by adding water to a raw material prepared by mixing a powdered substance containing an iron ore with an iron content equal to or less than 63% by mass (so-called low-grade ore) and an auxiliary raw material(s), obtaining Petition 870250086688, dated 09 / 25 / 2025, page 13 / 33 5 / 17 if a granulated product is thus obtained. The sintering step includes a drying and preheating zone and a subsequent sintering zone, in which the granulated product obtained is sintered to produce pellets (sintered). Here, the reason why the present invention is intended only for an iron ore whose iron content is equal to or less than 63% by mass (the so-called low-grade ore) is because the impact of blasting is less significant if an iron ore with an iron content greater than 63% by mass (the so-called high-grade ore) is used.

[013] The characteristics of the pellet production method of this type are as follows. First, before the sintering stage, the specific surface area and actual density of the raw material are measured, along with the amount of water added during the granulation stage. Thus, using these obtained measurement values, appropriate values ​​of a drying temperature and a gas flow rate in the drying and preheating zone of the sintering stage can be estimated. Then, the treatment in the drying and preheating zone of the sintering stage can be carried out using the estimated appropriate values ​​of the drying temperature and gas flow rate. Particularly, in the description below, an iron ore whose iron content is 63% by mass or less is simply referred to as iron ore.

[014] According to the pellet production method of this type, bursting can be suppressed. Here, bursting is a phenomenon in which, when iron ore pellets (raw pellets) are heated, they explode and / or turn to dust as the water in the raw pellets evaporates.

[015] Below are described in detail the specific surface area, actual density, amount of water added, drying temperature, gas flow rate and a bursting index X, in the production method of Petition 870250086688, dated 09 / 25 / 2025, page 14 / 33 6 / 17 pellets of this type. Here, in the following description, to distinguish the pellets before and after sintering, the iron ore pellets (pellets before sintering) can be called raw pellets, while the pellets after sintering can be called sintered pellets. Raw pellets and sintered pellets may contain auxiliary raw materials other than iron ore (e.g., bentonite). Furthermore, there are no particular limitations as to the type and mixture (ore blend) of the iron ore(s) that serve as raw material for the raw pellets. The raw material for the raw pellets may consist of one type of iron ore or be prepared by mixing several types of ores in an arbitrary mixing ratio. Specific surface area and actual density

[016] Bursting occurs when the internal pressure is increased by water evaporated from the raw pellets. For this reason, it is considered that, in the case of raw pellets acting as a compacted powder bed, bursting is likely to occur if the vapor permeation resistivity is high. The specific surface area, due to its measurement method, reflects the resistivity to a fluid permeating the compacted powder bed, making it a crucial element impacting the bursting property.

[017] Furthermore, the actual density is substantially determined by the type(s) of iron ore. Based on this specific surface area and actual density, the average particle size of the packed bed can be estimated using the following formula (2). A small average particle size easily leads to a dense packed structure, which will impact the bursting. Here, the specific surface area can be measured according to JIS R 5021, and the actual density can be measured according to JIS M 8717. [Formula 1] Petition 870250086688, dated 09 / 25 / 2025, page 15 / 33 7 / 17 x 104$mx ρem where d, Sm, and ρ are defined as follows. d: particle size (m) Sm: specific surface area (cm2 / g) ρ: real density (g / cm3)<Quantidade de água adicionada>

[018] The amount of water added to the raw pellets is a crucial element that impacts the bursting property. As mentioned above, bursting occurs when the internal pressure is increased by water evaporated from the raw pellets, which indicates that the amount of water acting as a source of steam generation impacts the bursting property. The amount of water added can be calculated by measuring the variations in the weight of the raw pellets when dried at 105°C. <Temperatura de secagem>

[019] The sintering stage can generally be divided into a drying and preheating zone and a sintering zone. When the raw pellets sent to the sintering stage are heated in the drying and preheating zone, bursting occurs as the water inside the pellets evaporates. Thus, the drying temperature impacts the bursting property. While an excessively high drying temperature makes bursting more likely due to abrupt evaporation, an excessively low drying temperature will prevent sufficient drying of the raw pellets, which can lead to reduced strength and productivity of the sintered pellets. Therefore, the raw pellets need to be dried at an appropriate temperature. Typically, the drying temperature in the drying and preheating zone ranges from 100 to 500°C. <Taxa de fluxo de gás> Petition 870250086688, dated 09 / 25 / 2025, p. 16 / 33 8 / 17

[020] In the drying and preheating zone of the sintering stage, hot air is blown onto the raw pellets to heat them. As the gas flow rate of the hot air becomes faster, the temperature will easily increase due to an increase in the heat transfer coefficient, which will make bursting more likely to occur due to abrupt evaporation. A slow gas flow rate makes it impossible for the raw pellets to be sufficiently dried, which can lead to a decrease in the strength and productivity of the sintered pellets. Therefore, the raw pellets need to be dried at an appropriate gas flow rate. Typically, the gas flow rate in the drying and preheating zone is in the range of 0.1 to 3.0 m / s. <Burning rate index X>

[021] In the present invention, the bursting index X is represented by the following formula (1), which is formulated by modeling, based on real data acquired, the extent of the impacts brought about by the “specific surface area”, “real density”, “amount of water added”, “drying temperature” and “gas flow rate” which are the elements that impact bursting, X=0.825 x Sm x ρ x 10-4+ 0.061 x M + 0.245 x T x 10-2+ 0.072 x V 1.285 ... (1) where Sm, ρ, M, T and V are defined as follows, Sm: Specific surface area of ​​the raw material (cm2 / g) ρ: Actual density of the raw material (g / cm3) M: Amount of water added in the granulation step (% by mass) T: Drying temperature (°C) V: Gas flow rate (m / s).

[022] The bursting index X represented by formula (1) indicates the probability of bursting; it indicates that the higher this value is, Petition 870250086688, dated 09 / 25 / 2025, page 17 / 33 9 / 17 The greater the probability of bursting occurring. When the bursting index X value is 3.000 or less, sintered pellets can be obtained with suppression of bursting. It is preferable that the bursting index X value be 2.000 or less, more preferably 1.000 or less. However, an excessively small bursting index X value will lead to insufficient drying, which will cause the raw pellets to be insufficiently sintered in the sintering step and therefore incur a decrease in strength. When the bursting index X value is 0.100 or greater, post-sintering strength can be guaranteed. Although there are no specific limitations regarding post-sintering strength, a post-sintering strength of 250 kg / pellet or higher, which is generally required in the technical field of the present invention, is preferred.

[023] Next, the granulation and sintering steps of the pellet production method of the present invention are described in detail.

[024] The granulation stage is a stage for obtaining iron ore pellets (raw pellets) by granulating an ore powder. There is no limitation on the granulation method used in the granulation stage. In the granulation stage, for example, a pelletizer can be used to granulate an ore powder. As such, a pan-type granulator (called a pan pelletizer) can, for example, be used. In the sintering stage, the raw pellets, after passing through the drying and preheating zone, are subjected to the sintering zone to be sintered and transformed into sintered pellets. The sintering temperature in the sintering zone is, for example, 1,150 to 1,350°C.

[025] Here, as explained above, bursting is a phenomenon in which, when raw pellets are heated, they explode and / or turn to powder as the water contained in the raw pellets evaporates. A Petition 870250086688, dated 09 / 25 / 2025, page 18 / 33 10 / 17 bursting probability, that is, the bursting property, can, for example, be evaluated based on the proportion of pellets with a size of 5.6 mm or smaller, generated after heat treatment of raw pellets at a given temperature (e.g., 300°C), or after sintering of raw pellets.

[026] Specifically, when bursting occurs in the sintering step or during the heat treatment of raw pellets at a given temperature (referred to as “in the sintering step or similar” hereafter), the raw pellets will explode or pulverize so that small pellet pieces (e.g., those of a size of 5.6 mm or smaller) will be generated. Thus, bursting is likely to occur (the bursting property is high) if pellet pieces are generated in a large quantity in the sintering step or similar, which can then be evaluated as unfavorable in terms of bursting suppression. In contrast, bursting is unlikely to occur (the bursting property is low) if pellet pieces are generated in a small quantity in the sintering step or similar, which can then be evaluated as favorable in terms of bursting suppression.

[027] The amount of pellet fragments generated in the sintering or similar step is 3.0% by mass or less, preferably 1.5% by mass or less, more preferably 1.0% by mass or less, so that degradation in the yield of sintered pellets can be suppressed. Therefore, if the amount of pellet fragments generated in the sintering or similar step is 3.0% by mass or less, the pellets can be evaluated as having good quality, with suppression of bursting. Examples

[028] Raw pellets were produced by the following procedures, and their bursting property was evaluated. Petition 870250086688, dated 09 / 25 / 2025, page 19 / 33 11 / 17

[029] As iron ore (raw material ore) serving as raw material for the crude pellets, ores (Ore A, Ore B, Ore C) with the chemical compositions shown in Table 1 below were used. Here, “LOI” in Table 1 is indicated as a crystalline water content (% by mass), which is a change in weight after holding the ore at 1,000°C for 30 min. Furthermore, in Table 1, “T.Fe” represents the total Fe content in the iron ore in terms of % by mass. Here, “T.Fe” is a quantified value according to a method for determining the total iron content for iron ores, which is specified in JIS M 8212:2022. As shown in Table 1, Ore A, Ore B, and Ore C as raw materials for the crude pellets are each low-grade ores with a T.Fe content of less than 63% by mass. [Table 1] Iron Ore Chemical Composition (% by mass) T.Fe FeO S1O2 CaO Al2O3 MgO LOI Ore A 63.0 0.69 5.31 0.04 1.1 0.07 2.7 Ore B 56.4 0.34 5.34 0.16 3.12 0.20 11.6 Ore C 61.0 0.31 3.93 0.02 2.08 0.08 5.9

[030] Table 2 below presents information on the ores used in the raw pellets (ore properties) and the conditions or results of the evaluation in the granulation and sintering stages. Following the granulation and sintering stages described below, sintered pellets were obtained from the ores with the raw material properties presented in Table 2, from experiments no. 1 year 17.

[031] Initially, the raw material ores shown in Table 2 were crushed with a ball mill (in a batch manner), thus obtaining an ore powder. Then, the specific surface area and the actual density of the ore powder were measured. In addition, Table 2 also Petition 870250086688, dated 09 / 25 / 2025, page 20 / 33 12 / 17 shows the particle sizes calculated from the specific surface areas and actual densities.

[032] Next, bentonite was added to the ore powder in an amount of 1.0% by mass to obtain a mixed powder. Here, although added as a binding element for granulation, bentonite is not essential in this embodiment. The mixed powder was then granulated in a pan-type granulator (pelletizer) to obtain raw pellets (granulation step). During granulation, the mixed powder was rotated while water was added to it, so that the granulation water content (amount of water added) would become various amounts of water addition. Here, the granulation water content can be obtained by measuring the change in weight of the raw pellets before and after keeping them at 105°C for 24 hours. For example, if the rate of change in the weight of the raw pellets after keeping them at 105°C for 24 hours is less than 10% by mass, this means that the water content in the granulation stage is 10% by mass.

[033] Regarding the particle size of the raw pellets, a caliper was used to measure the diameters of the major and minor axes, and an average value (arithmetic mean) was used as the particle size. Specifically, for each group of raw pellets obtained in each experiment number, the particle sizes of 10 pellets were measured, and an average value (arithmetic mean) was used as the particle size of the raw pellets prepared in that specific experiment number.

[034] The bursting property was evaluated based on the amount of pellet fragments generated after the sintering step. Specifically, the bursting property was evaluated as follows. Initially, 500 g of raw pellets were loaded into a vertical furnace (cylindrical in shape and 60 mm in diameter). From below the layer of raw pellets towards its top, air heated to various Petition 870250086688, dated 09 / 25 / 2025, page 21 / 33 Air at temperatures of 13 / 17 was passed through it at various flow rates for 10 minutes. Then, the raw pellets that had been exposed to this air at various temperatures were heated to 1300°C in a different electric furnace and sintered at that temperature for 25 minutes (sintering step). Subsequently, the raw pellets were removed from the furnace and sieved with a sieve with a 5.6 mm opening. The bursting property was evaluated by measuring the passage rate (% by mass) of the pellet pieces that had passed through the sieve. In Table 2, the passage rate of the pellet pieces is shown as “rate of -5.6 mm”. Furthermore, after evaluating the bursting property, the resistance of the pellets in which bursting did not occur was measured. The resistance measurement was performed using an autograph with a displacement speed set to 1.0 mm / min, and an average value obtained from 10 samples was used.In Table 2, the measurement result is shown under "resistance of the sintered pellet". Petition 870250086688, dated 09 / 25 / 2025, page 22 / 33 [Table 2] No. Raw material property Granulation stage Sintering stage Remarks Ore type Specific surface area (cm2 / g) Real density (g / cm3) Particle size (pm) Particle size of raw pellet (mm) Water content (% by mass) Drying temperature (°C) Gas flow rate (m / s) Formula (1) rate of - 5.6 mm (% by mass) Strength of sintered pellet (kg / pellet) 1 A 1211 4.9 10.11 11.0 7.6 200 0.7 0.21 0.17 252 Example of the invention 2 A 1598 4.9 7.66 12.4 8.1 300 1.0 0.66 0.39 297 Example of the invention 3 A 1823 4.9 6.72 13.2 Example of invention 4 B 2400 3.8 6.58 12.6 11.5 300 1.0 0.98 0.44 303 Example of invention 5 B 4230 3.9 3.64 13.1 11.9 300 1.0 1.61 0.85 328 Example of invention 6 C 2505 4.4 5.44 11.3 11.0 300 1.0 1.10 0.68 316 Example of invention 7 C 3306 4.3 4.22 12.8 10.5 300 1.0 1.34 0.76 321 Example of invention 8 C 3306 4.3 4.22 13.2 12.5 300 1.0 1.46 0.71 312 Example of the invention 9 C 3306 4.3 4.22 12.8 10.5 150 1.0 0.97 0.44 301 Example of invention 10 C 3306 4.3 4.22 12.8 10.5 500 1.0 1.83 0.92 328 Example of invention 11 C 3306 4.3 4.22 13.2 12.5 500 1.5 1.98 0.97 341 Example of invention 12 C 3306 4.3 4.22 13.2 12.5 500 2.0 2.02 1.52 344 Example of invention 13 C 6520 4.4 2.09 12.5 12.0 300 1.0 2.62 1.98 351 Example of invention, 14 / 16 Petition 870250086688, dated 09 / 25 / 2025, page 23 / 33 14 C 6520 4.4 2.09 12.5 12.0 400 1.5 2.90 1.96 354 Example of the invention 15 C 1507 4.4 9.05 11.3 8.5 105 0.5 0.07 0.09 188 Comparative Example 16 C 6520 4.4 2.09 12.5 12.0 500 1.5 3.15 3.23 352 Comparative Example 17 A 5980 4.9 2.05 13.2 8.9 500 2.0 3.04 3.17 361 Comparative Example 15 / 16 Petition 870250086688, dated 09 / 25 / 2025, page 24 / 33 16 / 17

[035] The following facts were clarified from the results shown in Table 2. Firstly, regarding the sintered pellets from experiments no. 1 year 14, where the values ​​of the bursting index X expressed by formula (1) were all 3.000 or less, the passage rates (rates of -5.6 mm) of the pellet pieces, which served as indices of the bursting property, were all 3.0% by mass or less. Specifically, there were passage rates of 1.5% by mass or less. That is, in experiments no. 1 year 14, which are associated with this modality, sintered pellets could be obtained while suppressing bursting. In contrast, with regard to experiments no. 16 and no. 17, where the burst index values ​​X expressed by formula (1) were not 3,000 or less, the passage rates (rates of -5.6 mm) of the pellet pieces were greater than 3.0% by mass.That is, in experiments no. 16 and no. 17, the bursting could not be suppressed. Meanwhile, in the case of experiment no. 15, where the value of the bursting index X expressed by formula (1) was less than 0.100, the sintering resistance was 250 kg / pellet or less, therefore not reaching the required resistance.

[036] Based on the results described above, the following can be understood: Regarding the examples of the invention in which the pellets were treated in the drying and preheating zone of the sintering stage with drying temperatures and gas flow rates that were estimated so that the bursting index X expressed by formula (1) was from 0.100 to 3.00, the passage rates (rates of -5.6 mm) of the pellet pieces were all 3.0% by mass or less. Thus, it can be understood that the bursting property was favorable in relation to the sintered pellets. Furthermore, in the examples of the invention, the strengths of the sintered pellets were greater than 250 kg / pellet, indicating that the required strength was met. Industrial Applicability Petition 870250086688, dated 09 / 25 / 2025, page 25 / 33 17 / 17

[037] The pellet production method of the present invention is industrially useful because it is able to suppress bursting more effectively in pellets that are produced by a granulation step of carrying out granulation by adding water to a raw material and a sintering step of sintering the granulated product. Petition 870250086688, dated 09 / 25 / 2025, page 26 / 33

Claims

1 / 2 CLAIMS 1. Method for producing pellets, comprising: a granulation step in which granulation is carried out by adding water to a feedstock prepared by mixing a powdered substance containing an iron ore with an iron content of 63% by mass or less and an auxiliary feedstock;and a sintering stage that includes a drying and preheating zone and a subsequent sintering zone where a granulated product is sintered, CHARACTERIZED by the fact that before the sintering stage, a specific surface area and the actual density of the raw material and an amount of water added in the granulation stage are measured, followed by the use of the measurement values ​​obtained to estimate appropriate values ​​of a drying temperature and a gas flow rate in the drying and preheating zone of the sintering stage, thus allowing a treatment in the drying and preheating zone of the sintering stage to be carried out using the estimated appropriate values ​​of the drying temperature and gas flow rate.

2. Method for producing pellets, according to claim 1, CHARACTERIZED in that the appropriate values ​​of the drying temperature and gas flow rate in the drying and preheating zone of the sintering step are estimated in such a way that, using the measurement values ​​of the specific surface area and the actual density of the raw material and the measurement value of the amount of water added in the granulation step, the drying temperature and gas flow rate are determined so that a bursting index X Petition 870250086688, dated 09 / 25 / 2025, p. 32 / 33 2 / 2 calculated by the following formula (1) not less than 0.100 and not greater than 3.000, X=0.825 x Sm x ρ x 10-4 + 0.061 x M + 0.245 x T x 10-2 + ​​0.072 x V 1.285 ...(1) where Sm, ρ, M, T and V are defined as follows, Sm: Specific surface area of ​​the raw material (cm2 / g) ρ: Actual density of the raw material (g / cm3) M: Amount of water added in the granulation step (% by mass) T: Drying temperature (°C) V: Gas flow rate (m / s). Petition 870250086688, dated 09 / 25 / 2025, page 33 / 33.

Images