SE2350133A1 - Iron ore pellets and method for producing iron ore pellets - Google Patents

Abstract

An object of the present invention is to provide iron ore pellets which have a characteristic of enabling a further decrease in coke consumption, in a blast furnace operation. Iron ore pellets according to one aspect of the present invention are for use in a blast furnace operation, wherein a porosity of the iron ore pellets which results from large open pores having a pore size of greater than or equal to 4 μm is greater than or equal to 21%, and the iron ore pellets have a crushing strength of greater than or equal to 180 kg/P.

Description

DESCRIPTION IRON ORE PELLETS AND METHOD FOR PRODUCING IRON ORE PELLETS [TECHNICAL FIELD] [000 1 ] The present invention relates to iron ore pellets and a method for producing iron ore pellets. [BACKGROUND ART] [0002] As a blast fumace operation, a method is Well-known in Which pig iron is produced by: altemately stacking, in a blast fumace, a first layer containing an iron ore raW material, and a second layer containing coke; and injecting an auxiliary fuel into the blast furnace from a tuyere and melting the iron ore raW material by using resulting hot blasts. In this method for producing pig iron, the iron ore raw material, being supplied as iron ore pellets, is reduced, Whereby the pig iron is produced. At this time, the coke mainly serves as a spacer to secure gas perrneability. id="p-3" id="p-3" id="p-3" id="p-3" id="p-3" id="p-3" id="p-3"

[0003] As a technique for improving reducibility of this iron ore raW material, iron ore pellets in Which a volume of pores having a diameter of greater than or equal to l0 um is greater than or equal to 0.01 cm3/ g has been proposed (see Japanese Unexamined Patent Application, Publication No. S63-219534). With regard to these iron ore pellets, by controlling the volume of comparatively large pores , a decrease in crushing strength is inhibited and closure of the pores is prevented, Whereby overall porosity is increased. [PRIOR ART DOCUMENTS] [PATENT DOCUMENTS] [0004] Patent Document l: Japanese Unexamined Patent Application, Publication No. S63 -2 1 95 34 [SUMMARY OF THE INVENTION] [PROBLEMS TO BE SOLVED BY THE INVENTION] [0005] In blast fumace operations, due to recent demands to decrease C02, there is a requirement for further decreasing coke consumption. To respond to this, a method in Which the reducibility of the iron ore raW material is improved by increasing pores to enlarge a surface area thereof is conceivable. HoWever, in the case of the above-described conventional iron ore pellets, since the porosity is controlled by the pores having the comparatively large diameter, it is difficult to enlarge the surface area per unit Weight of the iron ore pellets, even if the porosity is increased. In order to improve the reducibility, it is l necessary to greatly increase a volume of the pores having the diameter of greater than or equal to 10 um. In this case, the crushing strength of the iron ore pellets tends to decrease. Since the iron ore pellets are easily pulverized in the blast fumace when the crushing strength decreases, gas perrneation resistance in the blast fiamace increases, thereby increasing the risk of hindering stable operation of the blast fumace. id="p-6" id="p-6" id="p-6" id="p-6" id="p-6" id="p-6" id="p-6"

[0006] In other words, with the conventional iron ore pellets, it is difficult to fiarther improve the reducibility; thus, in order to further decrease the coke consumption, iron ore pellets having novel characteristic(s) are required. id="p-7" id="p-7" id="p-7" id="p-7" id="p-7" id="p-7" id="p-7"

[0007] The present invention was made in view of the foregoing circumstances, and an object of the present invention is to provide iron ore pellets having a characteristic of enabling a further decrease in the coke consumption in a blast fumace operation.

[MEANS FOR SOLVING THE PROBLEMS] [0008] As described above, in order to improve the reducibility of the iron ore pellets, it is necessary to increase the surface area per unit weight of the iron ore pellets, while inhibiting a decrease in the crushing strength. As a result of thorough investigation, the present inventors found that if porosity which results from large open pores having a pore size of greater than or equal to 4 um is controlled, iron ore pellets having improved reducibility can be produced, thereby completing the present invention. id="p-9" id="p-9" id="p-9" id="p-9" id="p-9" id="p-9" id="p-9"

[0009] More specifically, iron ore pellets according to one aspect of the present invention are iron ore pellets for use in a blast furnace operation, wherein a porosity of the iron ore pellets which results from large open pores having a pore size of greater than or equal to 4 um is greater than or equal to 21%, and the iron ore pellets have a crushing strength of greater than or equal to 180 kg/P. [00 1 0] The porosity of the iron ore pellets which results from the large open pores having the pore size of greater than or equal to 4 um is set to greater than or equal to the lower limit. Only the open pores, which connect to an exterior of the pellets, contribute to enlarging the surface area of the iron ore pellets; thus, controlling the porosity which results from these open pores enables directly enlarging the surface area per unit weight of the iron ore pellets, which actually contributes to a reaction. Furthermore, due to the crushing strength being greater than or equal to the lower limit, the iron ore pellets are not easily pulverized in the blast fumace during the blast furnace operation. Thus, the iron ore pellets are superior in reducibility, consequently enabling further decreasing the coke consumption in the blast fumace operation. [001 1] As referred to herein, the "porosity which results from the large open pores having the pore size of greater than or equal to 4 um" means an amount calculated in accordance with 80 >< A+4 /A [%], wherein: so [%] denotes an open porosity, deterrnined by using a mercury intrusion porosimeter (for example, "Autopore III 9400", manufactured by Shimadzu Corporation); A [cm3/g] denotes a total pore capacity per unit weight of the iron ore pellets; and A+4 [cm3/g] denotes a total pore capacity of pores having a pore size of greater than or equal to 4 um per unit weight of the iron ore pellets. It is to be noted that the open porosity means a proportion accounted for by a Volume of total open pores with respect to an apparent volume of the iron ore pellets. [00 12] A content of f1nes having a grain size of less than or equal to 4.7 um is preferably greater than or equal to 8% by mass. When the content of the f1nes having the grain size of less than or equal to 4.7 um is greater than or equal to the lower limit, the crushing strength can be increased while improving the porosity which results from the large open pores having the pore size of greater than or equal to 4 um. [00 1 3] The iron ore pellets preferably have an aggregate structure of f1nes. When the iron ore pellets thus have the aggregate structure of the f1nes, the crushing strength can be increased while improving the porosity which results from the large open pores having the pore size of greater than or equal to 4 um. As referred to herein, the "aggregate structure" means a state in which a plurality of grains of dispersed f1nes gather to form secondary particles, and specifically means a state in which greater than or equal to 5, and preferably greater than or equal to 10 grains of the f1nes are in contact with each other. [00 14] A method for producing iron ore pellets according to another aspect of the present invention includes: a step of balling green pellets by adding, to an iron ore raw material, water for use in the balling; and a step of f1ring the green pellets, wherein a viscosity of the water is greater than or equal to 15 mPa-s. [00 1 5] In the method for producing iron ore pellets, since the viscosity of the water at the time of balling the green pellets is greater than or equal to the lower limit, the iron ore pellets can be easily produced having: the porosity which results from the large open pores having the pore size of greater than or equal to 4 um being greater than or equal to 21%; and the crushing strength which is greater than or equal to 180 kg/P. [00 1 6] As referred to herein, the "viscosity" means a value measured in accordance with JIS-Z8803: 2011 by using a rotary viscometer. [00 1 7] The Water preferably contains an organic binder, and a content of the organic binder in the green pellets is preferably greater than or equal to 0.0l% by mass and less than or equal to 1.0% by mass. When the organic binder is thus contained in the water in a content falling within the above range, the aggregate structure of the f1nes can be forrned in the iron ore pellets to be produced. Accordingly, the crushing strength of the iron ore pellets can be increased while improving the porosity of the iron ore pellets which results from the large open pores having the pore size of greater than or equal to 4 um.

[EFFECTS OF THE INVENTION] [00 1 8] As described above, the iron ore pellets of the present invention have the characteristic of enabling further decreasing the coke consumption in the blast fumace operation. Furtherrnore, by carrying out the blast fumace operation using the iron ore pellets produced by using the method for producing iron ore pellets of the present invention, the coke consumption can be further decreased.

[BRIEF DESCRIPTION OF THE DRAWINGS] [00 1 9] FIG. l is a schematic plan view and a partially enlarged cross sectional view illustrating iron ore pellets according to one embodiment of the present invention.

FIG. 2 is a flow chart illustrating a method for producing iron ore pellets according to an other embodiment of the present invention.

FIG. 3 is a schematic view illustrating a structure of a production apparatus used in the method for producing the iron ore pellets illustrated in FIG. 2.

FIG. 4 is a graph illustrating a relationship between crushing strength, and a porosity which results from large open pores having a pore size of greater than or equal to 4 um in EXAMPLES.

FIG. 5 is a schematic cross sectional view illustrating a structure of a furnace for a large-scale reduction under load test, used for investigating reduction percentages in EXAMPLES.

FIG. 6 is a graph illustrating a temperature profile for heating a sample-packed bed at the time of investigating the reduction percentages in EXAMPLES.

FIG. 7 is a graph illustrating a relationship between a temperature of the sample-packed bed, and a flow rate of gas supplied.

FIG. 8 is a graph illustrating a relationship between the reduction percentage, and the porosity which results from the large open pores having the pore size of greater than or equal to 4 um in EXAMPLES.

[DESCRIPTION OF EMBODIMENTS] [0020] Hereinafter, the iron ore pellets according to the one embodiment of the present invention, and the method for producing iron ore pellets according to the other embodiment of the present invention are described. [002 1 ] Iron Ore Pellets Iron ore pellets 1 shown in FIG. 1 are iron ore pellets for use in a blast fumace operation. Iron ore pellets are a product which is obtained from a pellet feed, iron ore f1nes, and auxiliary material(s) as needed, and is made with characteristics suitable for a blast fumace (for example, size, strength, reducibility, and the like) in order to improve quality. [0022] As shown in FIG. 1, the iron ore pellets 1 are constituted mainly from coarse grains 11, serving as the pellet feed, and f1nes 12, being a pulverized raw material of iron ore, and numerous pores 13 are formed in an interior thereof. As described above, the iron ore pellets 1 may contain the auxiliary material(s). Examples of such auxiliary material(s) include limestone, dolomite, and the like. id="p-23" id="p-23" id="p-23" id="p-23" id="p-23" id="p-23" id="p-23"

[0023] A size of the iron ore pellets 1 is appropriately decided in accordance with, e. g., the blast fumace to be used, and for example, a grain size may be greater than or equal to 10 mm and less than or equal to 25 mm. id="p-24" id="p-24" id="p-24" id="p-24" id="p-24" id="p-24" id="p-24"

[0024] As the coarse grains 11, for example, coarse grains prepared from a blend of one or a plurality of brands of fine-grain pellet feed may be used. The coarse grains 11 are grains having a grain size of greater than or equal to 45 um, and it is preferred that coarse grains having a grain size of less than or equal to 0.5 mm account for greater than or equal to 90% by mass of a total of the coarse grains 11. When the proportion of the coarse grains 11 accounted for by the coarse grains having a grain size of less than or equal to 0.5 mm is less than the lower limit, a surface area may be insuff1cient, whereby the reducibility at the time of the blast furnace operation may decrease. id="p-25" id="p-25" id="p-25" id="p-25" id="p-25" id="p-25" id="p-25"

[0025] The f1nes 12 which may be used are, for example, f1nes prepared by pulverizing, with a pulverizer, the pellet feed for use as the coarse grains 11. The f1nes 12 are grains having a grain size of less than 45 um, and of these, the lower limit of a content of the f1nes 12 having a grain size of less than or equal to 4.7 um is, with respect to the total of the iron ore pellets 1, preferably 8% by mass, more preferably 10% by mass, and still more preferably 20% by mass.

When the content of the f1nes 12 having the grain size of less than or equal to 4.7 um is greater than or equal to the lower limit, the crushing strength can be increased while improving the porosity which results from the large open pores having the pore size of greater than or equal to 4 um. On the other hand, the upper limit of the content of the f1nes 12 having the grain size of less than or equal to 4.7 um is not particularly limited, and may be, for example, 50% by mass. [0026] The iron ore pellets 1 preferably have an aggregate structure 12a of the f1nes 12. As shown in FIG. 1, in the iron ore pellets 1, a plurality of grains of the f1nes 12 gather to come in contact with each other, forrning secondary particles. In other words, in the iron ore pellets 1, there are regions in which a density of the f1nes 12 is higher than elsewhere. When the f1nes 12 thus have the aggregate structure l2a, strength of this aggregated site increases, whereby the crushing strength of the iron ore pellets 1 improves. On the other hand, due to the aggregating, the f1nes 12 become localized and regions in which the f1nes 12 are not present are also localized, whereby a Volume of one pore 13, described later, tends to increase. Consequently, the number of open pores 13a having a large pore size increases. Therefore, when the iron ore pellets 1 thus have the aggregate structure 12a of the f1nes 12, the crushing strength can be increased while improving the porosity which results from the large open pores having the pore size of greater than or equal to 4 um. id="p-27" id="p-27" id="p-27" id="p-27" id="p-27" id="p-27" id="p-27"

[0027] There are two types of the pores 13, being: open pores 13a which connect to an exterior of the iron ore pellets 1; and closed pores 13b which are confined in the interior of the pellets. In other words, as shown in the enlarged cross sectional view of FIG. 1, while a part of the open pores 13a comes in contact with a surface of the iron ore pellets 1, the closed pores 13b are enclosed by the coarse grains 11 and the f1nes 12. Generally, the porosity is decided based on a volume ratio of the total pores 13, being a total of the open pores 13a and the closed pores 13b, but the porosity which results from the open pores 13a is important in order to improve the reducibility of the iron ore raw materials since, of the pores 13 of the iron ore pellets 1, only the open pores 13a come in contact with a reducing gas in the blast fumace. id="p-28" id="p-28" id="p-28" id="p-28" id="p-28" id="p-28" id="p-28"

[0028] Furthermore, in a case in which the porosity is at a certain level, a surface area of the iron ore pellets 1 increases as the pore size of the open pores 13a decreases. However, when the pore size of the open pores 13a is small, dispersion of the reducing gas in the interior of the open pores 13a may be difficult. Thus, it is considered necessary for the pore size of the open pores 13a to be greater than or equal to the certain level. On the other hand, when the porosity increases, the crushing strength of the ire ore pellets 1 decreases, which may lead to a disadvantage in which pulverization tends to occur in the blast fumace. id="p-29" id="p-29" id="p-29" id="p-29" id="p-29" id="p-29" id="p-29"

[0029] As a result of thorough investigation, the present inventors found that if the porosity which results from the large open pores 13a having the pore size of greater than or equal to 4 um is controlled, the reducibility of the iron ore pellets 1 can be improved. In other words, the lower limit of the porosity which results from the large open pores 13a having the pore size of greater than or equal to 4 um is 21%, more preferably 23%, and still more preferably 6 %. When the porosity which results from the open pores l3a is less than the lower limit, improvement of the reducibility of the iron ore pellets 1 may be insuff1cient, whereby sufficiently decreasing the coke consumption in the blast furnace operation may fail. id="p-30" id="p-30" id="p-30" id="p-30" id="p-30" id="p-30" id="p-30"

[0030] On the other hand, since increased porosity results in decreased crushing strength, the upper limit of the porosity which results from the open pores l3a is set to fall within a range not being below a certain value. The lower limit of this crushing strength is 180 kg/P, more preferably 190 kg/P, and still more preferably 200 kg/P. When the crushing strength is less than the lower limit, the iron ore pellets 1 may be easily pulverized in the blast fumace, whereby the blast fumace operation may be difficult. [003 1] The lower limit of a total open pore volume of the large open pores l3a having the pore size of greater than or equal to 4 um is preferably 0.06 cm3/g, and more preferably 0.07 cm3/g. When the total open pore volume is greater than or equal to the lower limit, the reducibility of the iron ore pellets l can be improved. id="p-32" id="p-32" id="p-32" id="p-32" id="p-32" id="p-32" id="p-32"

[0032] Furthermore, an open pore size leading to a maximum change percentage of the open pore volume is preferably greater than or equal to 7 um, and more preferably greater than or equal to 8 um. When the open pore size is greater than or equal to the lower limit, the reducibility of the iron ore pellets l can be improved. id="p-33" id="p-33" id="p-33" id="p-33" id="p-33" id="p-33" id="p-33"

[0033] Advantages The porosity of the iron ore pellets 1 which results from the large open pores l3a having the pore size of greater than or equal to 4 um is set to greater than or equal to 21%. Since only the open pores l3a, which connect to the exterior of the pellets, contribute to enlarging the surface area of the iron ore pellets 1, directly enlarging the surface area per unit weight of the iron ore pellets 1, which actually contributes to the reaction, is enabled by controlling the porosity which results from these open pores l3a. Furthermore, due to the crushing strength being greater than or equal to 180 kg/P, the iron ore pellets 1 are resistant to pulverization in the blast fumace during the blast fumace operation. Thus, the iron ore pellets 1 are superior in reducibility, consequently enabling further decreasing the coke consumption in the blast fumace operation. id="p-34" id="p-34" id="p-34" id="p-34" id="p-34" id="p-34" id="p-34"

[0034] Method for Producing Iron Ore Pellets The method for producing iron ore pellets shown in FIG. 2 includes: a balling step S1; a f1ring step S2; and a cooling step S3, and enables producing the iron ore pellets l of the present invention, shown in FIG. 1. The method for producing iron ore pellets may be carried out by using, for example, a production apparatus with a grate kiln system (hereinafter, may be also merely referred to as "production apparatus 2"), shown in FIG. 3. The 7 production apparatus 2 includes: a pan pelletizer 3; a traveling grate fumace 4; a kiln 5; and an annular cooler 6. [0035] Balling Step In the balling step S1, green pellets P are balled by adding water for use in the balling to an iron ore raw material. Specifically, the water is added to the iron ore, and then this water-containing iron ore is charged into the pan pelletizer 3, serving as the pelletizer, and rolled to produce the green pellets P, having a ball shape. [003 6] The iron ore is constituted from the coarse grains 11 and the f1nes 12 which constitute the iron ore pellets 1. Although surface Characteristics of the iron ore vary greatly depending upon a mining region and a pulverizing/transporting method, the surface characteristics of the iron ore are not particularly limited in the present method for producing iron ore pellets. [003 7] The water constitutes bridges between particles of the iron ore. Strength of the green pellets P balled in the balling step S1 is maintained due to an adhesion force acting between the particles, resulting from this bridging. In other words, a bond between the particles is expressed by means of surface tension of the water between the particles, and the adhesion force between the particles is ensured by a value obtained by multiplying the surface tension by the number of points of contact between the particles. id="p-38" id="p-38" id="p-38" id="p-38" id="p-38" id="p-38" id="p-38"

[0038] In the method for producing iron ore pellets, the lower limit of a viscosity of the water is 15 mPa-s, more preferably 30 mPa-s, and still more preferably 100 mPa-s. When the viscosity of the water is less than the lower limit, the crushing strength of the iron ore pellets 1 to be produced may be insufficient. On the other hand, the upper limit of the viscosity of the water is not particularly limited, and may be, for example, 10,000 mPa-s. [003 9] The water preferably contains an organic binder. As the organic binder, an organic binder having a mo lecular weight of preferably greater than or equal to 104 and less than or equal to 108, and more preferably a substance having a molecular weight of greater than or equal to 104 and less than or equal to 106 is used, and in particular, examples thereof include comstarch, tapioca, potato, guar beans, and the like. id="p-40" id="p-40" id="p-40" id="p-40" id="p-40" id="p-40" id="p-40"

[0040] It is to be noted that with regard to blending of this organic binder, in a case in which the iron ore has suff1ciently retained water, the organic binder alone is preferably added in accordance with an amount of the retained water. Conversely, in a case in which the iron ore has not suff1ciently retained water, the water is preferably added in a state of being brought to a desired viscosity by blending the organic binder with water. In an interrnediate case, the 8 amount of Water retained in the iron ore is considered, and a blending amount of the organic binder is decided such that the Viscosity of the water added has the desired Viscosity. In this case, the blending of the organic binder may be carried out with respect to moisture retained by the iron ore. In other words, the adding of water to the iron ore, and the blending of the organic binder may be carried out concurrently. [004 1 ] The lower limit of a content of the organic binder in the green pellets P is preferably 0.01% by mass, and more preferably 0.1% by mass. On the other hand, the upper limit of the content of the organic binder is preferably 1.0% by mass, and more preferably 0.2% by mass. When the content of the organic binder is less than the lower limit, the aggregate structure 12a of the f1nes 12 may not be suff1ciently formed in the iron ore pellets 1 to be produced, whereby the crushing strength may be insufficient. On the other hand, when the content of the organic binder is greater than the upper limit, the porosity of the iron ore pellets 1 which results from the large open pores having the pore size of greater than or equal to 4 um may increase and tend toward saturation, whereby the effects may be insufficient with respect to a rise in raw material cost. id="p-42" id="p-42" id="p-42" id="p-42" id="p-42" id="p-42" id="p-42"

[0042] The lower limit of a content of moisture in the green pellets P is preferably 7.0% by mass, and more preferably 8.0% by mass. On the other hand, the upper limit of the content of moisture is preferably 110% by mass, and more preferably 10.0% by mass. When the content of moisture is less than the lower limit, the bridge structure, resulting from water, between the particles of the iron ore may be insufficient, whereby the crushing strength may be insuff1cient. ConVersely, when the content of moisture is greater than the upper limit, the porosity of the iron ore pellets 1 which results from the large open pores having the pore size of greater than or equal to 4 um may not suff1ciently increase. id="p-43" id="p-43" id="p-43" id="p-43" id="p-43" id="p-43" id="p-43"

[0043] Firing Step In the firing step S2, the green pellets P are f1red. In the f1ring step S2, the traVeling grate fumace 4 and the kiln 5 are used. id="p-44" id="p-44" id="p-44" id="p-44" id="p-44" id="p-44" id="p-44"

[0044] TraVeling grate fumace As shown in FIG. 3, the traVeling grate fumace 4 has: a traVeling grate 41; a drying chamber 42; a dehydrating chamber 43: and a preheating chamber 44. id="p-45" id="p-45" id="p-45" id="p-45" id="p-45" id="p-45" id="p-45"

[0045] The traVeling grate 41 is configured to be endless, and the green pellets P placed on this traVeling grate 41 can be transferred to the drying chamber 42, the dehydrating chamber 43, and the preheating chamber 44, in this order. id="p-46" id="p-46" id="p-46" id="p-46" id="p-46" id="p-46" id="p-46"

[0046] In the drying chamber 42, the dehydrating chamber 43, and the preheating chamber 9 44, the green pellets P are subjected to: drying by blowing a heating gas Gl downward; dehydrating; and preheating, Whereby preheated pellets H are obtained having strength, imparted to the green pellets P, sufficient to resist the rotation in the kiln 5. id="p-47" id="p-47" id="p-47" id="p-47" id="p-47" id="p-47" id="p-47"

[0047] Specifically, the following procedure is followed. First, in the drying chamber 42, the green pellets P are dried at an atmospheric temperature of about 250 °C. Next, in the dehydrating chamber 43, the green pellets P after the drying are heated to about 450 °C in order to mainly decompose and remove combined Water in the iron ore. Furtherrnore, in the preheating chamber 44, the green pellets P are heated to about l,l00 °C, Whereby carbonate contained in limestone, dolomite, and/or the like is degraded to remove carbon dioxide, and magnetite in the iron ore is oxidized. Accordingly, the preheated pellets H are obtained. [0048] As shown in FIG. 3, the heating gas Gl used in the dehydrating chamber 43 is reused as the heating gas Gl in the drying chamber 42. Similarly, the heating gas Gl in the preheating chamber 44 is reused as the heating gas Gl in the dehydrating chamber 43, and a combustion exhaust gas G2 used in the kiln 5 is reused as the heating gas Gl in the preheating chamber 44. By thus reusing the heating gas Gl, Which is on the doWnstream side and has a high temperature, and the combustion exhaust gas G2, heating cost of the heating gas Gl can be decreased. It is to be noted that bumer(s) may be provided in each chamber to control the temperature of the heating gas Gl. In FIG. 3, bumers 45 are provided in the dehydrating chamber 43 and the preheating chamber 44. Furtherrnore, the heating gas Gl used in the drying chamber 42 is finally discharged from a smokestack C. id="p-49" id="p-49" id="p-49" id="p-49" id="p-49" id="p-49" id="p-49"

[0049] Kiln The kiln 5 is directly connected to the traveling grate fumace 4, and is a rotary fumace having a sloped cylindrical shape. The kiln 5 f1res the preheated pellets H Which are discharged from the preheating chamber 44 of the traveling grate fumace 4. Specifically, the preheated pellets H are f1red at a temperature of about l,200 °C by combustion With a kiln bumer (not shown in the figure) provided on an outlet side of the kiln 5. Accordingly, high-temperature iron ore pellets l are obtained. [005 0] In the kiln 5, as air for combustion, an atmosphere serving as a cooling gas G3 used in the annular cooler 6 is used. Furtherrnore, the high-temperature combustion exhaust gas G2 used for f1ring the preheated pellets H is sent to the preheating chamber 44 as the heating gas Gl. [005 l] Cooling Step In the cooling step S3, the high-temperature iron ore pellets l obtained in the f1ring step S2 are cooled. In the cooling step S3, the annular cooler 6 is used. The iron ore l0 pellets 1 cooled in the cooling step S3 are accumulated and used in the blast fumace operation. [0052] In the annular cooler 6, the iron ore pellets 1 can be cooled by blowing the atmosphere serving as the cooling gas G3 by using a blowing apparatus 61, While transferring the high-temperature iron ore pellets 1 discharged from the kiln 5. id="p-53" id="p-53" id="p-53" id="p-53" id="p-53" id="p-53" id="p-53"

[0053] It is to be noted that the cooling gas G3, Which Was used in the annular cooler 6, resulting in an increase in temperature, is sent to the kiln 5 and used as the air for combustion. [0054] Advantages In the method for producing iron ore pellets, the viscosity of the Water at the time of balling the green pellets P being greater than or equal to 15 mPas enables easily producing the iron ore pellets 1 of the present invention having: the porosity Which results from the large open pores having the pore size of greater than or equal to 4 um being greater than or equal to 21%; and the crushing strength Which is greater than or equal to 180 kg/P. id="p-55" id="p-55" id="p-55" id="p-55" id="p-55" id="p-55" id="p-55"

[0055] Other Embodiments It is to be noted that the present invention is not limited to the above-described embodiments. [0056] In the above-described embodiments, the case in Which the iron ore pellets are constituted from the coarse grains and the f1nes is described, but the iron ore pellets being constituted from only the coarse grains, or only the f1nes also falls Within the intended scope of the present invention.

[EXAMPLES] [005 7] Hereinafter, the present invention is explained in further detail by Way of Examples, but the present invention is not in any Way limited to these Examples. [0058] Example 1, Example 2, Comparative Example 1 In accordance With the method for producing iron ore pellets shown in FIG. 2, iron ore pellets of Example 1, Example 2, and Comparative Example 1 Were produced. [005 9] Balling Step As the Water, a Water containing an organic binder Was employed in Example 1 and Example 2, and a content of the organic binder Was 0.1% by mass in Example 1 and 0.2% by mass in Example 2. As a result, a viscosity of the Water used in the balling Was 17.4 mPa-s in Example 1, and 31.7 mPa-s in Example 2. The organic binder used Was a starch-type 11 organic binder (an organic binder obtained by adding bentonite in a content of 10% by mass, as an excluded amount, to a raw material being a mixture of 60% by mass comstarch, 30% by mass tapioca, and 10% by mass potato). Furthermore, measurement of the viscosity Was performed in accordance With JIS-Z8803: 2011 by using a rotary viscometer. id="p-60" id="p-60" id="p-60" id="p-60" id="p-60" id="p-60" id="p-60"

[0060] On the other hand, the Water of Comparative Example 1 Was Water not containing an organic binder. A viscosity of the Water Was 1 mPa-s. [006 l ] After adding the Water to the iron ore raW material and mixing, ball-shaped green pellets Were produced by: charging a resulting mixture into a pan pelletizer With a diameter of 40 cm, a pan angle of 48 °, a rotation speed of 30 rpm, and a rim height of 95 mm; and rolling. id="p-62" id="p-62" id="p-62" id="p-62" id="p-62" id="p-62" id="p-62"

[0062] Firing Step The green pellets Were charged into a fumace and f1red for 15 min at a temperature of 1,210 °C. It is to be noted that as an atmosphere, a mixture of 1 L of Ng gas and 3 L of air Was employed. Furthermore, each of a heating time period and a cooling time period Was 10 min. id="p-63" id="p-63" id="p-63" id="p-63" id="p-63" id="p-63" id="p-63"

[0063] A porosity Which results from the large open pores having the pore size of greater than or equal to 4 um, and a crushing strength Were measured for the iron ore pellets of each of Example 1, Example 2, and Comparative Example 1. The porosity Which results from the large open pores Was calculated in accordance With so >< A+4/A [%], Wherein: so [%] denotes an open porosity, deterrnined by using a mercury intrusion porosimeter (for example, "Autopore III 9400", manufactured by Shimadzu Corporation); A [cm3/ g] denotes a total pore capacity per unit Weight of the iron ore pellets; and A+4 [cm3/ g] denotes a total pore capacity of pores having a pore size of greater than or equal to 4 um per unit Weight of the iron ore pellets. The crushing strength Was deterrnined by using a Well-known crushing strength tester consisting of a tum table on Which a sample is to be placed, a driving apparatus, and a load cell. The results are shown in FIG. 4. id="p-64" id="p-64" id="p-64" id="p-64" id="p-64" id="p-64" id="p-64"

[0064] From the results in FIG. 4, it is revealed that the present method for producing iron ore pellets, in Which the organic binder Was added and the viscosity of the Water Was greater than or equal to 15 mPa-s, enables easily producing the iron ore pellets in Which the porosity Which results from the large open ore pores having the pore size of greater than or equal to 4 um is greater than or equal to 21%, and the crushing strength is greater than or equal to 180 kg/P. In contrast, it is revealed that With the iron ore pellets of Comparative Example 1, in Which the viscosity of the Water is less than 15 mPa-s, both the porosity Which results from 12 the large open pores having the pore size of greater than or equal to 4 um and the crushing strength are low. id="p-65" id="p-65" id="p-65" id="p-65" id="p-65" id="p-65" id="p-65"

[0065] Reduction Percentage Using the iron ore pellets of Example 1, Example 2, and Comparative Example 1, a large-scale reduction under load test was conducted simulating a peripheral part of a blast fumace to investigate the reduction percentage. id="p-66" id="p-66" id="p-66" id="p-66" id="p-66" id="p-66" id="p-66"

[0066] Fig. 5 illustrates a fumace for a large-scale reduction under load test 7 used in this experiment. A graphite crucible 71 to be packed with a sample was configured to have an inner diameter of 85 mm. A sample-packed bed 72 was constituted of, from the top, an upper coke layer 72a (20 mm in height), an iron ore layer 72b (150 mm in height), and a lower coke layer 72c (40 mm in height). The iron ore layer 72b was a mixture of sintered iron ore (16 to 19 mm in grain size), the iron ore pellets (11.2 to 13.2 mm in grain size), and lump iron ore (16 to 19 mm in grain size). id="p-67" id="p-67" id="p-67" id="p-67" id="p-67" id="p-67" id="p-67"

[0067] While heating the sample-packed bed 72 with a temperature profile shown in Fig. 6 by using an electric fumace 73, gas (reducing gas) of a composition shown in Fig. 7 was supplied thereto. The gas was supplied from a gas inlet pipe 74 provided in a lower portion of the furnace for a large-scale reduction under load test 7, and discharged from a gas outlet pipe 75 provided in an upper portion. A total feed rate of the gas was 51.3 NL/min, and temperature control was carried out by two therrnocouples 76. In addition, a load applied to the sample-packed bed 72 was 1 kgf/cmz. The load was applied by applying a weight of a weight 78 via a loading rod 77. id="p-68" id="p-68" id="p-68" id="p-68" id="p-68" id="p-68" id="p-68"

[0068] Under the aforementioned conditions, the rise in temperature and the supply of gas were stopped when the temperature of the sample-packed bed 72 reached 1,250 °C, and the reduction percentage was calculated from a difference between the pre-reduction weight and the post-reduction weight of the sample-packed bed 72. id="p-69" id="p-69" id="p-69" id="p-69" id="p-69" id="p-69" id="p-69"

[0069] The measurement of the reduction percentage was performed twice. The results are shown in FIG. 8. In the graph in FIG. 8, results of each of two trials are shown with bars, and average values thereof are shown with dots. From the results in FIG. 8, it is revealed that using the iron ore pellets of the present invention results in the reduction percentage being increased, and a fiarther decrease in the coke consumption in the blast furnace operation being enabled.

[INDUSTRIAL APPLICABILITY] [0070] The iron ore pellets of the present invention have the characteristic of enabling 13 further decreasing the coke consumption in the blast fumace operation. Furthermore, conducting the blast fumace operation using the iron ore pellets produced by using the method for producing iron ore pellets of the present inVention enables further decreasing the coke consumption.

[Explanation of the Reference Symbols] [007 l ] l Iron ore pellet ll Coarse grain 12 Fines l2a Aggregate structure l3 Pore l3a Open pore l3b Closed pore 2 Production apparatus 3 Pan pelletizer 4 TraVeling grate fumace 4l Traveling grate 42 Drying chamber 43 Dehydrating chamber 44 Preheating chamber 45 Bumer Kiln 6 Annular cooler 61 BloWing apparatus 7 Fumace for large-scale reduction under load test 7l Graphite crucible 72 Sample-packed bed 72a Upper coke layer 72b Iron ore layer 72c Lower coke layer 73 Electric fiamace 74 Gas inlet pipe 75 Gas outlet pipe 76 Therrnocouple 77 Loading rod 78 Weight P Green pellet H Preheated pellet Gl Heating gas l4 G2 Cornbustion exhaust gas G3 Cooling gas C Srnokestack

Claims (5)

1. Iron ore pellets for use in a blast fumace operation, Wherein a porosity of the iron ore pellets Which results from large open pores having a pore size of greater than or equal to 4 um is greater than or equal to 21%, and the iron ore pellets have a crushing strength of greater than or equal to 180 kg/P.

2. The iron ore pellets according to claim 1, Wherein a content of f1nes having a grain size of less than or equal to 4.7 um is greater than or equal to 8% by mass.

3. The iron ore pellets according to claim 1 or 2, comprising an aggregate structure of fines.

4. A method for producing iron ore pellets, the method comprising: a step of balling green pellets by adding, to an iron ore raW material, Water for use in the balling; and a step of f1ring the green pellets, Wherein a Viscosity of the Water is greater than or equal to 15 mPa-s.

5. The method for producing iron ore pellets according to claim 4, Wherein the Water comprises an organic binder, and a content of the organic binder in the green pellets is greater than or equal to 0.01% by mass and less than or equal to 1.0% by mass.