Method for manufacturing iron ore pellets

Abstract

To provide a method for producing iron ore pellets that are excellent in reducibility at high temperature and can reduce upper ventilation resistance of a blast furnace.SOLUTION: A method for producing iron ore pellets according to one aspect of the present invention is a method for producing self-fusing iron ore pellets used in blast furnace operation, and comprises: a raw material blending step of blending an auxiliary raw material containing CaO and MgO with an iron ore raw material such that a CaO / SiO2 mass ratio becomes 0.8 or more, and a MgO / SiO2 mass ratio becomes 0.4 or more; a granulation step of granulating raw pellets with porosity of 15% or more and 22% or less from a mixed raw material obtained in the raw material blending step; and a firing step of firing the raw pellets at a temperature of 1200°C or higher and 1300°C or lower.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for producing iron ore pellets and iron ore pellets.

Background Art

[0002] As a blast furnace operation, iron ore containing iron oxides, sintered ore, and coke as a carbon source are charged from the upper part of the blast furnace, and air or oxygen is blown from the tuyere at the lower part to cause the generation of carbon monoxide and the reduction reaction of removing oxygen from iron oxides in the furnace, and pig iron is taken out from the lower part of the furnace. This method is well-known.

[0003] In order to smoothly progress the continuous operation, it is important to smoothly perform the blowing. For this purpose, it is desirable that the blowing pressure is low and stable, that is, the air permeability is good. This blowing pressure depends on the properties of the charged materials. Among the charged materials, iron ore, sintered ore, and iron ore pellets are exposed to a high temperature and a reducing atmosphere, undergo a reduction reaction, and become a mixture of metallic iron and oxides. At the same time, they are softened and deformed under the load in the blast furnace. This softening and deformation fills the gaps between the charged material particles and hinders the air permeability in the furnace. The phenomenon that mainly causes this is called the lower furnace pressure loss, and it is aimed to reduce this.

[0004] As an iron ore pellet capable of reducing this lower furnace pressure loss, there is a known self-fusing pellet having a CaO / SiO2 mass ratio of 0.8 or more and a MgO / SiO2 mass ratio of 0.4 or more and having a predetermined particle size distribution (see Japanese Patent Application Laid-Open No. 2008-280556).

[0005] In the above iron ore pellets, by setting the CaO / SiO2 mass ratio to 0.8 or more and the MgO / SiO2 mass ratio to 0.4 or more, the reducibility at high temperature is enhanced, and by controlling the particle size distribution, the air permeability is ensured.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

[0007] While conventional iron ore pellets allow for some degree of permeability, their effect is relatively limited, and they tend to worsen airflow resistance (upper pressure drop) in the relatively low-temperature region at the top of the shaft furnace. Furthermore, recently, there has been an increase in operations where pulverized coal is blown into the blast furnace through the tuyeres to reduce the amount of expensive coke used. As a result, with the increase in the amount of pulverized coal blown in, the amount of coke that has supported the permeability within the blast furnace decreases, and the overall airflow resistance tends to increase. Therefore, there is a particular need to reduce the airflow resistance at the top of the blast furnace.

[0008] The present invention has been made based on the circumstances described above, and aims to provide a method for producing iron ore pellets that exhibits excellent reducibility at high temperatures and reduces the upper ventilation resistance of a blast furnace, as well as the iron ore pellets themselves. [Means for solving the problem]

[0009] A method for producing iron ore pellets according to one aspect of the present invention is a method for producing self-fluxing iron ore pellets used in blast furnace operation, comprising: a raw material blending step of blending auxiliary raw materials containing CaO and MgO with ore raw materials such that the CaO / SiO2 mass ratio is 0.8 or more and the MgO / SiO2 mass ratio is 0.4 or more; a granulation step of granulating raw pellets having a porosity of 15% or more and 22% or less from the mixed raw materials obtained in the raw material blending step; and a firing step of firing the raw pellets at a temperature of 1200°C or more and 1300°C or less.

[0010] Another embodiment of the present invention is a self-fluxing iron ore pellet used in blast furnace operation, having a CaO / SiO2 mass ratio of 0.8 or more, a MgO / SiO2 mass ratio of 0.4 or more, and an average crushing strength of 270 kg / p or more. [Effects of the Invention]

[0011] The present invention provides a method for producing iron ore pellets that exhibit excellent reducibility at high temperatures and reduce the upper airflow resistance of the blast furnace. Furthermore, the iron ore pellets produced by the present invention exhibit excellent reducibility at high temperatures and reduce the upper airflow resistance of the blast furnace. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1 is a flowchart showing a method for manufacturing iron ore pellets according to one embodiment of the present invention. [Figure 2] Figure 2 is a schematic diagram showing the configuration of the manufacturing apparatus used in the iron ore pellet manufacturing method shown in Figure 1. [Figure 3] Figure 3 is a graph showing the relationship between the percentage of particles smaller than 5 mm generated during the transportation of iron ore pellets and the average crushing strength. [Modes for carrying out the invention]

[0013] [Description of Embodiments of the Invention] The inventors of this invention diligently investigated the upper airflow resistance of a blast furnace and found that even if the proportion of small-diameter iron ore pellets, which worsen pressure loss during pellet production, is suppressed, it is unavoidable that the pellets will break and generate powder during subsequent transportation and blast furnace insertion processes. They then discovered that reducing the powder generated from the iron ore pellets due to transportation and impact within the blast furnace reduces the upper airflow resistance of the blast furnace. Furthermore, the inventors found that increasing the average crushing strength to 270 kg / p is effective in reducing powder.

[0014] However, iron ore pellets with a CaO / SiO2 mass ratio of 0.8 or higher and an MgO / SiO2 mass ratio of 0.4 or higher, which are required to increase the reducibility of iron ore pellets in blast furnace operation, have not previously been found to have an average crush strength of 270 kg / p or higher. This is thought to be because coarse crystal grains of the CaO, MgO, SiO2, and Fe2O3 compounds are formed during firing, reducing the strength. In other words, when the crystal grains are large, the direction of the slip planes, where dislocations move easily, tends to align, making the pellets more susceptible to fracture due to dislocation movement at low stresses.

[0015] The inventors conducted further investigations and found that the formation of the above-mentioned coarse crystal grains occurs at high firing temperatures. Crystals in mineral structures are formed and expand by diffusion. The diffusion coefficients of Fe, Ca, Si, Mg, Al, etc., in the solid and liquid phases increase with increasing temperature. In other words, the higher the firing temperature, the more the crystals diffuse and the larger they become, resulting in the formation of coarse crystal grains. Therefore, the inventors hypothesized that the formation of coarse crystal grains could be suppressed by lowering the firing temperature.

[0016] On the other hand, if the firing temperature is too low, the pellets will not sinter properly. In other words, the distance between ore particles will increase, and the contact points between particles will decrease, weakening the forces that make up the pellet strength and tending to reduce the crushing strength. Therefore, even if the firing temperature is lowered to suppress the formation of coarse crystal grains, the crushing strength will not increase sufficiently.

[0017] Here, the inventors focused on the fact that if the firing temperature is low and the pellets do not sinter properly, the porosity does not decrease. The decrease in porosity occurs because the iron ore particles move closer together and coalesce through diffusion to reduce the surface area, thereby lowering the surface energy of the iron ore particles. For this reason, it is thought that in high-porosity pellets, the distance between ore particles remains wide, and the contact points between particles decrease, thus weakening the forces that constitute the pellet strength. Therefore, the inventors considered that if the porosity could be controlled independently of the firing temperature, the crushing strength could be sufficiently increased. They then discovered that the crushing strength could be sufficiently increased by controlling the porosity at the raw pellet stage, which had not been done conventionally, and completed the present invention.

[0018] That is, the method for manufacturing an iron ore pellet according to one aspect of the present invention is a method for manufacturing a self - fluxing iron ore pellet used in blast furnace operation, comprising a raw material blending step of blending a secondary raw material containing CaO and MgO into an ore raw material so that the mass ratio of CaO / SiO2 is 0.8 or more and the mass ratio of MgO / SiO2 is 0.4 or more, a granulation step of granulating green pellets with a porosity of 15% or more and 22% or less from the mixed raw material obtained in the raw material blending step, and a firing step of firing the green pellets at a temperature of 1200°C or more and 1300°C or less.

[0019] The iron ore pellet manufactured by the method for manufacturing an iron ore pellet is self - fluxing, has a CaO / SiO2 mass ratio of not less than the above lower limit, and a MgO / SiO2 mass ratio of not less than the above lower limit, so it has high reducibility. Further, with the porosity in the green pellet stage within the above range and the green pellets fired at a temperature within the above range in the firing step, the crushing strength of the manufactured iron ore pellet can be sufficiently increased. Therefore, by using the method for manufacturing an iron ore pellet, it is possible to manufacture an iron ore pellet that is excellent in reducibility at high temperatures and can reduce the upper ventilation resistance of a blast furnace.

[0020] In the above granulation step, a rolling granulator may be used, and the porosity may be controlled by the raw material particle size in the raw material blending step and the rolling time in the granulation step. By controlling the porosity in this way, it is easy to control the porosity to a desired value, and the crushing strength can be more reliably increased.

[0021] The particle size range of the green pellets may be adjusted in the granulation step so that the particle size after the firing step is 4 mm or more and 20 mm or less. By setting the particle size after the firing step within the above range in this way, it is possible to suppress a decrease in the upper ventilation resistance of the blast furnace while maintaining the reducibility at high temperatures.

[0022] For adjusting the particle size range of the green pellets, classification using a screen group having an oversize screen and a seed screen adjusted to a predetermined mesh size may be used. By adjusting the particle size range of the green pellets in this way by classification, the particle size after the firing step can be adjusted easily and reliably.

[0023] The iron ore pellet according to another aspect of the present invention is a self-fluxing iron ore pellet used in blast furnace operation, having a CaO / SiO2 mass ratio of 0.8 or more, a MgO / SiO2 mass ratio of 0.4 or more, and an average crushing strength of 270 kg / p or more.

[0024] Since the iron ore pellet is self-fluxing with a CaO / SiO2 mass ratio of not less than the above lower limit and a MgO / SiO2 mass ratio of not less than the above lower limit, it has high reducibility. Also, since the iron ore pellet has an average crushing strength of not less than the above lower limit, it is possible to reduce the powder generated from the iron ore pellet due to transportation or impact in the blast furnace and reduce the upper part ventilation resistance of the blast furnace.

[0025] It is preferable that the mass ratio of the crushing strength of 100 kg / p or less is 10% or less. By setting the mass ratio of the crushing strength of 100 kg / p or less to not more than the above upper limit in this way, it is possible to further reduce the powder generated from the iron ore pellet and further reduce the upper part ventilation resistance of the blast furnace.

[0026] Here, the "crushing strength" is the strength defined in JIS-M8718:2017, and the "average crushing strength" refers to the average value of the crushing strengths of any at least 10 iron ore pellets.

[0027] [Details of Embodiments of the Present Invention] Hereinafter, a method for manufacturing an iron ore pellet and the iron ore pellet according to an embodiment of the present invention will be described with appropriate reference to the drawings.

[0028] [Method for Manufacturing Iron Ore Pellets] The method for manufacturing iron ore pellets shown in Figure 1 comprises a raw material blending step S1, a granulation step S2, a calcination step S3, and a cooling step S4. As shown in Figure 2, this method for manufacturing iron ore pellets can produce self-fluxing iron ore pellets 1 used in blast furnace operation using a great kiln type manufacturing apparatus (hereinafter also simply referred to as "manufacturing apparatus 2"). Manufacturing apparatus 2 comprises a pan pelletizer 3, a great furnace 4, a kiln 5, and an annular cooler 6.

[0029] <Raw material blending process> In the raw material blending process S1, auxiliary raw materials containing CaO and MgO are blended into the ore raw material so that the CaO / SiO2 mass ratio is 0.8 or higher and the MgO / SiO2 mass ratio is 0.4 or higher.

[0030] Specifically, in the raw material blending process S1, limestone, which is a source of CaO, and dolomite, which is a source of MgO, are blended as auxiliary raw materials according to the iron grade of the iron ore (pellet feed) which is the ore raw material mentioned above.

[0031] The above-mentioned ore raw materials and auxiliary raw materials may be crushed in a ball mill or the like beforehand or after blending, as needed, to adjust the particle size of the mixed raw material containing the above-mentioned ore raw materials and auxiliary raw materials. The inventors of the present invention know that the porosity of raw pellet P is proportional to the particle size index of the raw materials. In other words, by appropriately controlling the particle size index of the raw materials, the porosity of raw pellet P can be controlled, and the strength of the iron ore pellet 1 can be controlled by this porosity of raw pellet P.

[0032] Here, the "raw material particle size index" can be determined by the following method. First, measure the particle size distribution of the mixed raw materials. For this measurement, one of the following standards can be used: JIS-A-1204:2010, JIS-A-8815:1994, or JIS-Z-8825:2022. Next, using the mass ratio or volume ratio mi in each particle size range Pi (representative value), calculate the sum Σ3 / Pi·mi from 3 μm to 1000 μm, and use this as the raw material particle size index.

[0033] The relationship between the raw material particle size index and the porosity of raw pellet P is valid for mixed raw materials made by blending the same brand of iron ore and auxiliary materials in the same ratio. However, if the brand of iron ore is different, for example, the proportionality coefficient may change due to the influence of surface shape, wettability, etc. Therefore, a suitable value for the raw material particle size index can be determined by the following method. First, prepare raw materials with at least two different particle size indices in a mixed raw material with a specific mixing ratio, produce raw pellet P, and measure the porosity. From this result, the relationship between the raw material particle size index and porosity can be calculated. As will be described later, there is also a proportional relationship between porosity and the strength of iron ore pellet 1, so the required porosity can be calculated from the required strength of iron ore pellet 1. Then, the raw material particle size index that results in the required porosity can be determined, and the particle size of the raw material can be adjusted to achieve this raw material particle size index. Note that adjusting the particle size also includes purchasing raw materials that have such a particle size.

[0034] Furthermore, the above mixed raw materials may contain binders such as bentonite as appropriate to obtain the necessary strength of the raw pellets P for transport during the manufacturing process.

[0035] <Granulation process> In the granulation process S2, raw pellets P with a porosity of 15% to 22% are granulated from the mixed raw materials obtained in the raw material blending process S1. A rolling granulator can be used to granulate the raw pellets P. As the rolling granulator, a pan pelletizer 3 or a drum pelletizer as shown in Figure 2 can be used.

[0036] Specifically, in the granulation process S2, water (granulation water) is added to the above mixed raw materials, and then this granulation water-containing mixture (the above mixed raw materials containing granulation water) is put into the pump pelletizer 3 and tumbled to produce mud-ball-shaped raw pellets P.

[0037] In the method for manufacturing iron ore pellets, the porosity of the raw pellets P is controlled as described above. The lower limit of the porosity is 15%, with 17% being more preferable. On the other hand, the upper limit of the porosity is 22%, with 20% being more preferable. If the porosity is below the lower limit, there is a risk of a steam explosion (bursting phenomenon) occurring in the firing process S3. Conversely, if the porosity exceeds the upper limit, there is a risk of a decrease in the crushing strength of the iron ore pellets 1.

[0038] The porosity described above can be controlled by the particle size of the raw materials in the raw material blending process S1 and the rolling time in the granulation process S2. By controlling the porosity in this way, it becomes easier to control the porosity to the desired value, and the crush strength can be increased more reliably.

[0039] Furthermore, it is preferable to adjust the particle size range of the raw pellets P in the granulation process S2 so that the particle size after the firing process S3 is 4 mm to 20 mm, more preferably 6 mm to 15 mm. By keeping the particle size after the firing process S3 within the above range, it is possible to maintain reducibility at high temperatures while suppressing a decrease in the upper ventilation resistance of the blast furnace.

[0040] To adjust the particle size range of the raw pellets P, it is preferable to use a classification system with a sieve group having oversized screens (upper limit sieve) and seed screens (lower limit sieve) adjusted to predetermined sieve sizes. By adjusting the particle size range of the raw pellets P in this way through classification, the particle size after the calcination process S3 can be easily and reliably adjusted. It is preferable that any off-spec products that do not meet the specifications during the classification process are crushed and reused as raw materials for mixing.

[0041] <Firing Process> In the firing process S3, the raw pellets P are fired at a temperature of 1200°C to 1300°C. In the manufacturing apparatus 2 shown in Figure 2, a grate furnace 4 and a kiln 5 are used in the firing process S3.

[0042] (Great Furnace) As shown in Figure 2, the grate furnace 4 comprises a traveling grate 41, a drying chamber 42, a water-freezing chamber 43, and a preheating chamber 44.

[0043] The traveling grate 41 is configured in an endless manner, and the raw pellets P placed on this traveling grate 41 can be moved in the order of drying chamber 42, water-free chamber 43, and preheating chamber 44.

[0044] In the drying chamber 42, the water-free chamber 43, and the preheating chamber 44, the raw pellets P are dried, water-free, and preheated with heating gas G1 to obtain preheated pellets H that have been given sufficient strength to withstand rolling in the kiln 5.

[0045] Specifically, the following procedure is followed. First, the raw pellets P are dried in the drying chamber 42 at an ambient temperature of approximately 250°C. Next, in the syneresis chamber 43, the dried raw pellets P are heated to approximately 450°C to decompose and remove the water of crystallization mainly in the iron ore. Furthermore, in the preheating chamber 44, the raw pellets P are heated to approximately 1100°C to decompose the carbonates contained in limestone, dolomite, etc., remove carbon dioxide, and oxidize the magnetite in the iron ore. This yields preheated pellets H.

[0046] As shown in Figure 2, the heating gas G1 used in the water-freezing chamber 43 is reused as the heating gas G1 for the drying chamber 42. Similarly, the heating gas G1 for the water-freezing chamber 43 is reused from the heating gas G1 for the preheating chamber 44, and the combustion exhaust gas G2 used in the kiln 5 is reused as the heating gas G1 for the preheating chamber 44. By reusing the high-temperature heating gas G1 or combustion exhaust gas G2 from the downstream side in this way, the heating cost of the heating gas G1 can be reduced. Burners 45 may also be provided in each chamber to control the temperature of the heating gas G1. In Figure 2, burners 45 are provided in the water-freezing chamber 43 and the preheating chamber 44. The heating gas G1 used in the drying chamber 42 is ultimately discharged from the chimney C.

[0047] (Kiln) Kiln 5 is directly connected to the grate furnace 4 and is a cylindrical rotary furnace with a gradient. Kiln 5 calcines the preheated pellets H discharged from the preheating chamber 44 of the grate furnace 4. Specifically, the preheated pellets H are calcined by combustion using a kiln burner (not shown) located on the outlet side. This yields high-temperature iron ore pellets 1.

[0048] The lower limit of the firing temperature for firing the preheated pellets H is 1200°C, with 1220°C being more preferable. On the other hand, the upper limit of the firing temperature is 1300°C, with 1280°C being more preferable. The inventors have found that when the firing temperature is within the above range, there is a proportional relationship between the porosity of the raw pellets P and the strength of the iron ore pellets 1. In other words, if the firing temperature is below the lower limit, the pellets will not sinter properly, and if the firing temperature exceeds the upper limit, coarse crystal grains are more likely to form, which may reduce the crushing strength of the iron ore pellets 1. Conversely, once the desired strength is determined, the porosity of the raw pellets P can be determined from that proportional relationship.

[0049] In kiln 5, the air used as combustion air is the cooling gas G3 that was used in the annular cooler 6. In addition, the high-temperature combustion exhaust gas G2 used for firing the preheated pellets H is sent to the preheating chamber 44 as heating gas G1.

[0050] <Cooling process> In the cooling process S4, the high-temperature iron ore pellets 1 obtained in the calcination process S3 are cooled. An annular cooler 6 is used in the cooling process S4. The iron ore pellets 1 cooled in the cooling process S4 are collected and used in blast furnace operation.

[0051] In the Annula Cooler 6, the high-temperature iron ore pellets 1 discharged from the kiln 5 are moved while air, which is the cooling gas G3, is passed through by the ventilation device 61 to cool the iron ore pellets 1.

[0052] Furthermore, the cooling gas G3, which has been used in Annular Cooler 6 and whose temperature has risen, is sent to Kiln 5 and used as combustion air.

[0053] <Advantages> The iron ore pellets 1 produced by this iron ore pellet manufacturing method are self-soluble, have a CaO / SiO2 mass ratio of 0.8 or higher, and a MgO / SiO2 mass ratio of 0.4 or higher, thus exhibiting high reducibility. Furthermore, by setting the porosity of the raw pellets P to 15% to 22% and then firing the raw pellets P at a temperature of 1200°C to 1300°C in the firing process S3, the crushing strength of the produced iron ore pellets 1 can be sufficiently increased. Therefore, by using this iron ore pellet manufacturing method, it is possible to produce iron ore pellets 1 that exhibit excellent reducibility at high temperatures and reduce the upper ventilation resistance of the blast furnace.

[0054] [Iron ore pellets] Another aspect of the present invention relates to an iron ore pellet 1, which is a self-fluxing iron ore pellet used in blast furnace operation. This iron ore pellet 1 is produced by granulating fine ore and firing it to form a high-strength agglomerate ore, and can be manufactured, for example, by the iron ore pellet manufacturing method described above.

[0055] In the production of iron ore pellets 1, it is known that adding CaO-containing compounds such as limestone to the ore raw material increases the CaO / SiO2 mass ratio of the iron ore pellets 1, thereby improving the reducibility of the iron ore pellets 1. Based on this knowledge, the CaO / SiO2 mass ratio of the iron ore pellets 1 is 0.8 or higher.

[0056] When the raw materials are iron ore (iron oxide) and limestone (CaO-containing compound), during the calcination process, a solid-phase reaction occurs between the CaO produced by thermal decomposition and the iron oxide, generating a calcium ferrite compound. Simultaneously, solid-phase diffusion bonding occurs at the point of contact between these compounds. This bonding is localized, and the micropores that existed before calcination are maintained after calcination, resulting in the iron ore pellet 1 becoming a porous body with relatively uniform micropores.

[0057] During blast furnace operation, reducing gas diffuses into these micropores, causing a reduction reaction to progress from the outer surface to the interior of the iron ore pellet 1. The reduction reaction removes oxygen from iron oxide, leading to the expansion of existing micropores and the generation of new ones, while simultaneously producing metallic iron. As the outer shape of the iron ore pellet 1 shrinks due to the aggregation of this metallic iron, the number of micropores decreases. As a result, the diffusion of reducing gas into the interior of the iron ore pellet 1 is suppressed, and the reduction process tends to stagnate.

[0058] To suppress this reduction stagnation, the addition of high-melting-point components that inhibit the disappearance of micropores during the aggregation process of metallic iron is effective. In particular, it is known that adding dolomite as a source of MgO, which is a high-melting-point component, and increasing the MgO / SiO2 mass ratio of iron ore pellet 1 yields a high reduction stagnation suppression effect. Based on this finding, the MgO / SiO2 mass ratio of iron ore pellet 1 is 0.4 or higher.

[0059] The iron ore pellet 1 is self-fluxing. Making the iron ore pellet 1 self-fluxing facilitates the dissolution of reduced iron. The self-fluxing properties of the iron ore pellet 1 are determined by the auxiliary materials used.

[0060] The lower limit of the average crushing strength of the iron ore pellet 1 is 270 kg / p, with 300 kg / p being more preferable. As mentioned above, based on the knowledge that reducing the amount of powder generated from the iron ore pellet 1 due to transportation and impact in the blast furnace reduces the upper ventilation resistance of the blast furnace, the inventors diligently investigated and concluded that the amount of powder can be controlled by the average crushing strength. Figure 3 shows the relationship between the mass fraction of powder with a particle size of 5 mm or less (5 mm or less powder ratio) generated by the transportation of the iron ore pellet 1 and the average crushing strength. This result is based on the results of empirical verification along the transportation route. Figure 3 includes those with a CaO / SiO2 mass ratio of less than 0.8 and a MgO / SiO2 mass ratio of less than 0.4, but regardless of these properties, it can be seen that the 5 mm or less powder ratio is consistently low in iron ore pellet 1 with an average crushing strength above the lower limit. In this invention, the upper limit of the average crushing strength of the iron ore pellet 1 is not particularly limited, but in reality, for example, the upper limit is 500 kg / p.

[0061] In the iron ore pellets 1, it is preferable that the mass ratio of pellets with a crushing strength of 100 kg / p or less is 10% or less, more preferably 5% or less, and even more preferably 1% or less. Even if the average crushing strength of the iron ore pellets 1 is high, if there is a large variation in strength between individual pellets, it is conceivable that the absolute amount of iron ore pellets 1 that turn into powder will be large. In this regard, by keeping the mass ratio of pellets with a crushing strength of 100 kg / p or less below the above upper limit, the amount of powder generated from the iron ore pellets 1 can be further reduced, and the upper ventilation resistance of the blast furnace can be further reduced.

[0062] <Advantages> The iron ore pellets 1 are self-soluble, have a CaO / SiO2 mass ratio of 0.8 or higher, and a MgO / SiO2 mass ratio of 0.4 or higher, thus possessing high reducibility. Furthermore, the iron ore pellets 1 have an average crushing strength of 270 kg / p or higher, which reduces the amount of powder generated from the iron ore pellets 1 during transportation and impact within the blast furnace, thereby reducing the upper ventilation resistance of the blast furnace.

[0063] [Other embodiments] However, the present invention is not limited to the embodiments described above.

[0064] In the above embodiment, a method for producing iron ore pellets using a great kiln type manufacturing apparatus was described, but it is also possible to produce them using a straight-grate type manufacturing apparatus. In a straight-grate type manufacturing apparatus, the great furnace comprises a traveling great, a drying chamber, a water-freezing chamber, a preheating chamber, and a firing chamber, and the firing process is completed using only the great furnace. Specifically, in the drying chamber, water-freezing chamber, and preheating chamber, the raw pellets are dried, water-freezed, and preheated with heating gas, and the final firing takes place in the firing chamber. [Examples]

[0065] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0066] [No.1] Iron ore was prepared as the raw material for the ore, and limestone, dolomite, and bentonite were prepared as auxiliary materials. The auxiliary materials were blended with the ore raw material so that the CaO / SiO2 mass ratio was 1.2 and the MgO / SiO2 mass ratio was 0.4 to obtain a mixed raw material.

[0067] Using the above mixed raw materials, raw pellets were granulated using a disc pelletizer and calcined at 1260°C using a calcination apparatus consisting of a fixed grate furnace and a kiln furnace to obtain iron ore pellet No. 1. Table 1 shows the raw material particle size index, the porosity of the raw pellets, and the average crushing strength of the iron ore pellets for iron ore pellet No. 1.

[0068] [No.2] Iron ore pellets No. 2 were obtained in the same manner as No. 1, except that the above mixed raw materials were crushed in a ball mill for 30 minutes. Table 1 shows the raw material particle size index, the porosity of the raw pellets, and the average crushing strength of the iron ore pellets No. 2.

[0069] [No.3~No.5] Iron ore pellets No. 3 to No. 5 were obtained in the same manner as No. 1, except that the raw material particle size index for No. 1 and No. 2 was set to 1.8 times, 2.0 times, and 2.2 times, respectively, based on the raw material particle size index for No. 1, so that the average crushing strength would be 270 kg / p or more. Table 1 shows the raw material particle size index, porosity of the green pellets, and average crushing strength of the iron ore pellets for No. 3 to No. 5.

[0070] [Table 1]

[0071] The results in Table 1 show that even with iron ore pellets having a CaO / SiO2 mass ratio of 0.8 or higher and a MgO / SiO2 mass ratio of 0.4 or higher, by adjusting the porosity of the raw pellets to 15% or more and firing them at a temperature of 1200°C or more and 1300°C or less, iron ore pellets with an average crushing strength of 270 kg / p or more can be obtained. [Industrial applicability]

[0072] The present invention provides a method for producing iron ore pellets that exhibit excellent reducibility at high temperatures and reduce the upper airflow resistance of the blast furnace. Furthermore, the iron ore pellets produced by the present invention exhibit excellent reducibility at high temperatures and reduce the upper airflow resistance of the blast furnace. [Explanation of Symbols]

[0073] 1. Iron ore pellets 2 Manufacturing equipment 3. Pumpelette 4 Great Furnaces 41 Traveling Great 42 Drying room 43 Sewer room 44 Preheating chamber 45 burners 5 kilns 6 Anura Cooler 61 Ventilation device P Raw Pellets H Preheating pellets G1 Heating Gas G2 Combustion exhaust gas G3 Cooling Gas C Chimney

Claims

1. A method for producing self-fluxing iron ore pellets used in blast furnace operation, wherein the average crushing strength is 270 kg / p or more, and the mass ratio of pellets with a crushing strength of 100 kg / p or less is 10% or less, CaO / SiO ２ A mass ratio of 0.8 or higher, and MgO / SiO ２ A raw material blending step in which auxiliary raw materials containing CaO and MgO are blended with ore raw materials so that the mass ratio is 0.4 or more, A granulation process is performed to granulate raw pellets with a porosity of 15% to 22% from the mixed raw materials obtained in the above raw material blending process, A firing process in which the above raw pellets are fired at a temperature of 1200°C to 1300°C. Equipped with, The above firing process, The procedure for drying the above raw pellets in the drying chamber of the Great Furnace, A procedure to decompose and remove the crystalline water in the ore raw material by raising the temperature of the dried raw pellets to a higher temperature than the drying procedure in the water-free chamber of the great furnace, A procedure to remove carbon dioxide from the raw pellets from which the above-mentioned crystal water has been decomposed and removed, by heating them to a higher temperature than the above-mentioned decomposition and removal procedure in the preheating chamber of the great furnace, The procedure for firing the raw pellets from which carbon dioxide has been removed in a kiln and A method for producing iron ore pellets having [a certain characteristic].

2. In the above granulation process, a tumbling granulator is used. A method for producing iron ore pellets according to claim 1, wherein the porosity is controlled by the particle size of the raw materials in the raw material blending step and the rolling time in the granulation step.

3. A method for producing iron ore pellets according to claim 1 or claim 2, wherein the particle size range of the raw pellets is adjusted in the granulation step so that the particle size after the above-mentioned firing step is 4 mm or more and 20 mm or less.

4. A method for producing iron ore pellets according to claim 3, wherein classification is performed using a sieve group having oversized screens and seed screens adjusted to predetermined sieve sizes to adjust the particle size range of the raw pellets.

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