SE2351219A1

SE2351219A1 - Method for producing iron ore pellets

Info

Publication number: SE2351219A1
Application number: SE2351219A
Authority: SE
Inventors: Hitoshi TOYOTA; Rikizo Tadai; Tsugunori Kato
Original assignee: Kobe Steel Ltd
Priority date: 2021-04-01
Filing date: 2021-05-13
Publication date: 2023-10-25
Also published as: BR112023018032A2; WO2022208904A1; CL2023002936A1; AU2021439033A1; US20240158886A1; JP2022158000A; EP4303329A1; CA3214315A1; CN116981785A

Abstract

According to an aspect of the present invention, a method for producing iron ore pellets used for operation of a blast furnace and in which a CaO/SiO2 mass ratio is greater than or equal to 0.8 and a MgO/SiO2 mass ratio is greater than or equal to 0.4, includes: balling green pellets by adding, to an iron ore material and dolomite, water for use in the balling; and a step of firing the green pellets, in which the dolomite has a characteristic of being present in a miniaturized state in a structure of the green pellets.

Description

DESCRIPTION METHOD FOR PRODUCING IRON ORE PELLETS [TECHNICAL FIELD] [000 1 ] The present invention relates to a method for producing iron ore pellets. [BACKGROUND ART] id="p-2" id="p-2" id="p-2" id="p-2" id="p-2" id="p-2" id="p-2"

[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 material, and a second layer containing coke; and inj ecting an auxiliary reductant into the blast fumace from a tuyere and melting the iron ore material by using resulting hot blasts. In this method for producing pig iron, the iron ore material, being supplied as iron ore pellets, is reduced, whereby the pig iron is produced. At this time, the coke functions as a reduction agent and 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] The iron ore pellets need to have high reducibility in order to improve production efficiency of pig iron. As iron ore pellets having improved reducibility, for example, iron ore pellets obtained by adding dolomite to make a CaO/SiOg mass ratio greater than or equal to 0.8 and a MgO/SiOg mass ratio greater than or equal to 0.4 are known (see Japanese Unexamined Patent Application Publication No. H1-13693 6). The aforementioned publication further discloses that increasing porosity of the iron ore pellets can improve reducibility.

[PRIOR ART DOCUMENTS] [PATENT DOCUMENTS] [0004] Patent Document l: Japanese Unexamined Patent Application Publication No. Hl- 1 3 693 6 [SUMMARY OF THE INVENTION] [PROBLEMS TO BE SOLVED BY THE INVENTION] [0005] In light of a recent increase in awareness of the environmental problems, a reduction in emission of C02 as the greenhouse gas, specifically an operation with a low reduction agent ratio, is required also in a blast fumace operation. In this case, since pulverization of the iron ore pellets in the blast fumace and the like leads to lowered gas perrneability, a large amount of coke as a spacer for ensuring gas perrneability needs to be charged. An increased charged rate of coke as a reduction agent increases the reduction agent ratio, whereby an operation with a low reduction agent ratio is difficult. Therefore, in order to carry out an operation with a low reduction agent ratio, the iron ore pellets need to have a high crushing strength so as not to be pulverized.

Your Ref 230073SE Our Ref. 21FP-0118/WO/SE id="p-6" id="p-6" id="p-6" id="p-6" id="p-6" id="p-6" id="p-6"

[0006] However, adding dolomite tends to lower the crushing strength. In addition, increasing the porosity of the iron ore pellets necessarily lowers the crushing strength. [0007] The present invention Was made in view of the foregoing circumstances, and an objective thereof is to provide a method for producing iron ore pellets superior in reducibility and high in crushing strength.

[MEANS FOR SOLVING THE PROBLEMS] [0008] The present inventors have thoroughly investigated iron ore pellets obtained by adding dolomite to increase reducibility, and found that adding dolomite treated to be present in a miniaturized state in a pellet structure prior to firing increases crushing strength. Although an exact reason is not clear, the present inventors infer that, by subj ecting dolomite to a predeterrnined treatment, MgO derived from the dolomite is present in a miniaturized state in the iron ore pellets, Whereby an effect of increasing a bonding strength of the pellet structure of the iron ore pellets is produced during firing. In other Words, the bonding strength of the pellet structure is considered to be increased due to the fact that: MgO being miniaturized increases reactivity of MgO and facilitates generation of a magnesioferrite compound, thus contributing to bonding of the pellet structure; and/or MgO having a low bonding strength that may be an origin of fracture of the pellet is miniaturized and less likely to be the origin of fracture. id="p-9" id="p-9" id="p-9" id="p-9" id="p-9" id="p-9" id="p-9"

[0009] In other Words, according to an aspect of the present invention, a method for producing iron ore pellets used for operation of a blast fumace and in Which a CaO/SiOz mass ratio is greater than or equal to 0.8 and a MgO/SiOg mass ratio is greater than or equal to 0.4 includes: balling green pellets by adding, to an iron ore material and dolomite, Water for use in the balling; and f1ring the green pellets, in Which the dolomite has a characteristic of being present in a miniaturized state in a structure of the green pellets. [00 l 0] The method for producing iron ore pellets enables increasing crushing strength of the iron ore pellets to be produced, by adding dolomite that is present in a miniaturized state in a structure of the green pellets prior to f1ring and produces an effect of increasing the bonding strength of the pellet structure of the iron ore pellets. In addition, in the iron ore pellets produced by the method for producing iron ore pellets, a CaO/SiOg mass ratio is greater than or equal to 0.8 and a MgO/SiOg mass ratio is greater than or equal to 0.4, resulting in high reducibility. [00l l] It is preferred that the method for producing iron ore pellets further includes preparing the dolomite, in Which in the preparing, the dolomite is pulverized such that a Blaine specific 2 Your Ref 230073SE Our Ref. 21FP-01 1 8/WO/SE surface area is greater than or equal to 4,000 cm2/ g. Due to the Blaine specific surface area of the dolomite being greater than or equal to the lower limit, the dolomite is miniaturized and integrated into the pellet structure. As a result, reactivity of dolomite can be increased, and MgO can be inhibited from functioning as an origin of fracture in the iron ore pellets to be produced. Therefore, the bonding strength of the pellet structure of the iron ore pellets is increased, whereby the crushing strength of the iron ore pellets can be increased. As used herein, a "Blaine specific surface area" means a value obtained by measurement in accordance with JIS-R-520l:20l5, and, in a case in which a target object is composed of a plurality of powders, indicates a minimum value for an individual powder. [00 12] It is preferred that the method for producing iron ore pellets ﬁirther includes preparing the dolomite, wherein the dolomite is calcined at a temperature greater than or equal to 900 °C in the preparing. As used herein, "calcination" means a heat treatment process of heating a solid such as ore to cause therrnal decomposition and phase transition, and to remove volatile components. Dolomite is a carbonate mineral and represented by CaMg(CO3)2. When dolomite is calcined, the following reaction takes place CaCOs -> CaO + C02, MgCOs -> MgO + C02 and dolomite is therrnally decomposed. At a phase of balling, water is added to MgO generated by the calcination, resulting in a transformation into Mg(OH)2 and miniaturization (dolomite having a large grain size is reduced). As a result, reactivity of dolomite can be increased, and MgO which is generated in the firing and can function as an origin of fracture in the iron ore pellets to be produced can be miniaturized. Therefore, the bonding strength of the pellet structure of the iron ore pellets to be produced is increased, whereby the crushing strength of the iron ore pellets can be increased. [00 1 3] The firing temperature in the f1ring preferably higher than or equal to 1,250 °C.

Due to the firing temperature in the firing being higher than or equal to the aforementioned lower limit, the crushing strength can ﬁarther be increased.

[EFFECTS OF THE INVENTION] [00 14] As explained in the foregoing, by employing the method for producing iron ore pellets according to the present invention, iron ore pellets superior in reducibility and having high crushing strength can be produced.

[BRIEF DESCRIPTION OF THE DRAWINGS] [00 1 5] FIG. l is a ﬂow chart illustrating a method for producing iron ore pellets according to an embodiment of the present invention.

Your Ref 230073SE Our Ref. 21FP-0118/WO/SE FIG. 2 is a schematic view illustrating a structure of a production apparatus used in the method for producing iron ore pellets illustrated in FIG. 1.

FIG. 3 is a graph showing a grain size distribution of dolomite before and after the calcination.

FIG. 4 is a graph showing a relationship between the Blaine specific surface area and the crushing strength in EXAMPLES.

FIG. 5 is a graph showing a relationship between a rate of dolomite particles having a grain size of less than or equal to 20 um and the crushing strength in EXAMPLES. [DESCRIPTION OF EMBODIMENTS] id="p-16" id="p-16" id="p-16" id="p-16" id="p-16" id="p-16" id="p-16"

[0016] Hereinafter, the method for producing pig iron according to each embodiment of the present invention will be described. [0017] First Embodiment The method for producing iron ore pellets illustrated in FIG. 1 includes a preparing step S1, a balling step S2, a firing step S3, and a cooling step S4. For example, as illustrated in FIG. 2, the method for producing iron ore pellets is used for operation of a blast fumace, and can produce iron ore pellets 1 in which a CaO/SiOg mass ratio is greater than or equal to 0.8 and a MgO/SiOg mass ratio is greater than or equal to 0.4, by using a production apparatus with a grate kiln system (hereinafter, may be also merely referred to as "production apparatus 2"). The production apparatus 2 includes: a pan pelletizer 3; a traveling grate fumace 4; a kiln 5; and an annular cooler 6. [001 8] The iron ore pellets 1 are obtained by balling and f1ring finely pulverized ore to form agglomerated ore having a great strength. Regarding production of the iron ore pellets 1, it is known that adding a CaO-containing compound such as limestone to an iron ore material to increase a CaO/SiOg mass ratio in the iron ore pellets 1 improves reducibility of the iron ore pellets 1 (see Patent Document 1). On the basis of this finding, the present method for producing iron ore pellets produces the iron ore pellets 1 having the CaO/SiOg mass ratio of greater than or equal to 0.8. id="p-19" id="p-19" id="p-19" id="p-19" id="p-19" id="p-19" id="p-19"

[0019] In a case in which the raw materials are iron ore (iron oxide) and limestone (CaO- containing compound), a calcium ferrite compound is generated by a solid phase reaction between CaO generated by the therrnal decomposition and iron oxide in the ﬁring, and is simultaneously bound through solid phase diffusion bonding at an interface thereof. Since the bonding is local, fine pores which were present prior to the firing are retained even after the ﬁring, whereby the iron ore pellets 1 are porous bodies in which fine pores are present relatively 4 uniforrnly. [0020] During the blast fumace operation, a reducing gas enters the fine pores diffusively, whereby a reduction reaction proceeds from an outer surface to an inner portion of the iron ore pellets 1. Due to removal of oxygen from the iron oxide by the reduction reaction, the existing fine pores are enlarged and new fine pores are generated, while metallic iron is generated. In a process of shrinkage of an extemal shape of the iron ore pellets 1 due to aggregation of the metallic iron, the fine pores start to decrease. As a result, difﬁasion of the reduction gas into the iron ore pellets 1 is suppressed, whereby the reduction is likely to stagnate. [002 1 ] For suppressing this stagnation of the reduction, addition of a high-melting point component which suppresses loss of the fine pores during an aggregation process of the metallic iron is effective. It is known that particularly adding dolomite as a source of MgO, which is the high-melting point component, to increase a MgO/SiOg mass ratio in the iron ore pellets 1 enables obtaining a powerful effect of suppressing stagnation of the reduction (see Patent Document 1). On the basis of this finding, in the present method for producing iron ore pellets, the iron ore pellets 1 are produced having the MgO/SiOg mass ratio of greater than or equal to 0.4. id="p-22" id="p-22" id="p-22" id="p-22" id="p-22" id="p-22" id="p-22"

[0022] It is preferred that the iron ore pellets to be produced are self-ﬂuxing. Due to the iron ore pellets 1 being self-fluxing, melting down of reduced iron is likely to be accelerated. Note that the self-fluxing property of the iron ore pellets 1 is deterrnined by an auxiliary material and/or the like. id="p-23" id="p-23" id="p-23" id="p-23" id="p-23" id="p-23" id="p-23"

[0023] In the preparing step S1, dolomite is prepared. In the present method for producing iron ore pellets, the dolomite has a characteristic of being present in a miniaturized state in a structure of green pellets P to be balled in the balling step S2 described later. In the preparing step S1, this characteristic is imparted to the dolomite. Specifically, in the preparing step S1, the dolomite is pulverized such that a Blaine specific surface area is greater than or equal to a predeterrnined value. Note that the pulverization can be carried out by using a known pulverizer. id="p-24" id="p-24" id="p-24" id="p-24" id="p-24" id="p-24" id="p-24"

[0024] The predeterrnined value is preferably 4,000 cmz/ g, and more preferably 6,000 cm2/ g. Increasing the specific surface area is considered to be substantially the same as miniaturizing the dolomite. Due to the miniaturization, reactivity of dolomite can be increased, and MgO can be inhibited from ﬁinctioning as an origin of fracture in the iron ore pellets 1 to be produced. Therefore, the bonding strength of the pellet structure of the iron ore pellets 1 to be produced is increased, whereby the crushing strength of the iron ore pellets l can be increased. Note that an upper limit of the Blaine specific surface area of the pulverized dolomite is not particularly limited, but in view of production cost and the like, the Blaine specific surface area of the pulverized dolomite is less than or equal to 10,000 cmZ/g. id="p-25" id="p-25" id="p-25" id="p-25" id="p-25" id="p-25" id="p-25"

[0025] A lower limit of a percentage of particles having a grain size of less than or equal to 20 um in the pulverized dolomite is preferably 35% by volume, more preferably 45% by volume, and further preferably 55% by volume. The percentage of particles having a grain size of less than or equal to 20 um being greater than or equal to the lower limit facilitates an increase in the crushing strength of the iron ore pellets l. Note that the "percentage of particles having a grain size of less than or equal to 20 um" indicates a value obtained from a grain size distribution measured by a grain size distribution measurement apparatus (Microtrac). id="p-26" id="p-26" id="p-26" id="p-26" id="p-26" id="p-26" id="p-26"

[0026] An upper limit of a D50 grain size of the pulverized dolomite is preferably 50 um and more preferably 20 um. The D50 grain size of the dolomite being less than or equal to the upper limit facilitates an increase in the crushing strength of the iron ore pellets l. Note that the "D50 grain size" indicates a value obtained from a grain size distribution measured by a grain size distribution measurement apparatus (Microtrac). id="p-27" id="p-27" id="p-27" id="p-27" id="p-27" id="p-27" id="p-27"

[0027] In the balling step S2, green pellets P are balled by adding water for use in the balling to an iron ore material and the dolomite. As described above, an auxiliary material such as limestone may be added to obtain the CaO/SiOg mass ratio of greater than or equal to 0.8. The MgO/SiOg mass ratio can be adjusted mainly by the dolomite. id="p-28" id="p-28" id="p-28" id="p-28" id="p-28" id="p-28" id="p-28"

[0028] Specifically, in the balling step S2, the water is added to the iron ore material and the dolomite, and then this water-containing mixture (the iron ore material and the dolomite containing the water) is charged into the pan pelletizer 3, serving as the pelletizer, and rolled to produce the green pellets P, having a ball shape. id="p-29" id="p-29" id="p-29" id="p-29" id="p-29" id="p-29" id="p-29"

[0029] The iron ore material is a main material of the iron ore pellets l, and composed of powder of the iron ore (for example, powder of which at least 90% by mass of the total has a grain size of less than or equal to 0.5 mm). 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. id="p-30" id="p-30" id="p-30" id="p-30" id="p-30" id="p-30" id="p-30"

[0030] The water constitutes bridges between particles of the iron ore material. Strength of 6 Your Ref 230073SE Our Ref. 21FP-0118/WO/SE the green pellets P balled in the balling step S2 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. [003 1] In the firing step S3, the green pellets P are fired. In the f1ring step S3, the traveling grate fumace 4 and the kiln 5 are used. [0032] Traveling grate fumace As shown in FIG. 2, 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-33" id="p-33" id="p-33" id="p-33" id="p-33" id="p-33" id="p-33"

[0033] 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-34" id="p-34" id="p-34" id="p-34" id="p-34" id="p-34" id="p-34"

[0034] In the drying chamber 42, the dehydrating chamber 43, and the preheating chamber 44, the green pellets P are subjected to: drying by a heating gas G1; 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-35" id="p-35" id="p-35" id="p-35" id="p-35" id="p-35" id="p-35"

[0035] 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. Furthermore, in the preheating chamber 44, the green pellets P are heated to about 1,100 °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. [0036] As shown in FIG. 2, the heating gas G1 used in the dehydrating chamber 43 is reused as the heating gas G1 in the drying chamber 42. Similarly, the heating gas G1 in the preheating chamber 44 is reused as the heating gas G1 in the dehydrating chamber 43, and a combustion exhaust gas G2 used in the kiln 5 is reused as the heating gas G1 in the preheating chamber 44. By thus reusing the heating gas G1, which is on the downstream side and has a high temperature, and the combustion exhaust gas G2, heating cost of the heating gas G1 can be decreased. It is to be noted that bumer(s) may be provided in each chamber to control the temperature of the 7 Your Ref 230073SE Our Ref. 21FP-0118/WO/SE heating gas G1. In FIG. 2, bumers 45 are provided in the dehydrating Chamber 43 and the preheating chamber 44. Furtherrnore, the heating gas G1 used in the drying chamber 42 is finally discharged from a smokestack C. id="p-37" id="p-37" id="p-37" id="p-37" id="p-37" id="p-37" id="p-37"

[0037] 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 fires the preheated pellets H which are discharged from the preheating chamber 44 of the traVeling grate fumace 4. Specifically, the preheated pellets H are fired 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 1 are obtained. id="p-38" id="p-38" id="p-38" id="p-38" id="p-38" id="p-38" id="p-38"

[0038] A lower limit of the f1ring temperature for f1ring the preheated pellets H is preferably 1,250 °C, and more preferably 1,300 °C. Due to the f1ring temperature being higher than or equal to the aforementioned lower limit, the crushing strength can further be increased. On the other hand, the upper limit of the f1ring temperature is not particularly limited, and may be, for example, 1,500 °C. When the firing temperature is higher than the upper limit, the effect of increasing the crushing temperature tends to be saturated and the effect may be insufﬁcient with respect to the increase in the production cost. In addition, in light of reduction in a cohesion amount of the iron ore pellets 1 according to a rise in temperature, the upper limit is more preferably 1400 °C. [003 9] In the kiln 5, as air for combustion, an atmosphere serving as a cooling gas G3 used in the annular cooler 6 is used. Furthermore, the high-temperature combustion exhaust gas G2 used for firing the preheated pellets H is sent to the preheating chamber 44 as the heating gas G1. id="p-40" id="p-40" id="p-40" id="p-40" id="p-40" id="p-40" id="p-40"

[0040] In the cooling step S4, the high-temperature iron ore pellets 1 obtained in the firing step S3 are cooled. In the cooling step S4, the annular cooler 6 is used. The iron ore pellets 1 cooled in the cooling step S4 are accumulated and used in the blast fumace operation. [004 1 ] 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-42" id="p-42" id="p-42" id="p-42" id="p-42" id="p-42" id="p-42"

[0042] 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. 8 Your Ref 230073SE Our Ref. 21FP-01 1 8/WO/SE id="p-43" id="p-43" id="p-43" id="p-43" id="p-43" id="p-43" id="p-43"

[0043] In the method for producing iron ore pellets, dolomite, being present in a miniaturized state in a structure of the iron ore pellets l and producing an effect of increasing the bonding strength of the pellet structure of the iron ore pellets l, is added. Specifically, due to the Blaine specific surface area of the dolomite being greater than or equal to 4,000 cm2/ g, the dolomite is miniaturized and integrated into the pellet structure. As a result, reactivity of dolomite can be increased, and MgO can be inhibited from functioning as an origin of fracture in the iron ore pellets l to be produced. Therefore, the bonding strength of the pellet structure of the iron ore pellets l is increased, Whereby the crushing strength of the iron ore pellets l can be increased. In addition, in the iron ore pellets l produced by the method for producing iron ore pellets, a CaO/SiOg mass ratio is greater than or equal to 0.8 and a MgO/SiOg mass ratio is greater than or equal to 0.4, resulting in high reducibility. [0044] Second Embodiment According to another embodiment of the present inVention, a method for producing iron ore pellets used for operation of a blast fumace and in Which a CaO/SiOg mass ratio is greater than or equal to 0.8 and a MgO/SiOg mass ratio is greater than or equal to 0.4, includes, as illustrated in FIG. l: a preparing step Sl of preparing dolomite; a balling step S2 of balling green pellets by adding, to an iron ore material and the dolomite, Water for use in the balling; a f1ring step S3 of f1ring the green pellets; and a cooling step S4 of cooling the high-temperature iron ore pellets obtained in the firing step S3. In addition, the dolomite has a characteristic of being present in a miniaturized state in a structure of the green pellets. id="p-45" id="p-45" id="p-45" id="p-45" id="p-45" id="p-45" id="p-45"

[0045] In the method for producing iron ore pellets, the steps except for the preparing step Sl are the same as the corresponding steps in the method for producing iron ore pellets according to the first embodiment. Hereinafter, the preparing step Sl is described and description for the other steps is omitted. id="p-46" id="p-46" id="p-46" id="p-46" id="p-46" id="p-46" id="p-46"

[0046] In the preparing step Sl in the method for producing iron ore pellets, the dolomite is calcined at a temperature greater than or equal to a predeterrnined Value. The present inVentors have found that this treatment imparts to the dolomite a characteristic of being present in a miniaturized state in a structure of the green pellets, Whereby the crushing strength of the iron ore pellets to be produced can be increased. id="p-47" id="p-47" id="p-47" id="p-47" id="p-47" id="p-47" id="p-47"

[0047] The predeterrnined Value is preferably 900 °C, and more preferably l,l00 °C. Note that an upper limit of a calcination temperature is not particularly limited, but in View of 9 Your Ref 230073SE Our Ref. 21FP-01 1 8/W0/SE production cost and the like, the calcination temperature is less than or equal to 1,500 °C. [0048] The effect of enabling an increase in the crushing strength of the iron ore pellets produced by the calcination is discussed. Dolomite is a carbonate mineral and represented by CaMg(C03)2. When dolomite is calcined, the following reaction takes place CaC03 -> Ca0 + C02, MgC03 -> Mg0 + C02 and dolomite is therrnally decomposed. At a phase of the balling step S3, water is added to Mg0 generated by the calcination, resulting in the following hydration reaction Mg0 + H20 -> Mg (0H) to give magnesium hydroxide. id="p-49" id="p-49" id="p-49" id="p-49" id="p-49" id="p-49" id="p-49"

[0049] The present inventors found that miniaturization of the dolomite proceeds in the calcined dolomite due to the hydration reaction. FIG. 3 shows results of measurement of the grain size distribution of the calcined dolomite by a Microtrac before and after the hydration reaction. As shown in FIG. 3, before the hydration reaction, no significant change in grain size is observed between the grain size distribution after the calcination and that of non- calcined dolomite after the hydration reaction; however, it can be observed that the hydration reaction causes a change in grain size, which is considered to be due to a change in crystal structure, and a reduction of large grain-size particles having, for example, a grain size of greater than 20 um, in other words miniaturization, proceeds. Due to the miniaturization, reactivity of dolomite can be increased, and Mg0 which is generated in the f1ring step and can function as an origin of fracture in the iron ore pellets to be produced can be miniaturized. Therefore, the bonding strength of the pellet structure of the iron ore pellets to be produced is increased, whereby the crushing strength of the iron ore pellets can be increased. [005 0] A lower limit of a treatment time of the calcination is preferably 20 minutes, more preferably 50 minutes, and still more preferably l00 minutes. Meanwhile, the upper limit of the treatment time of the calcination is preferably 200 minutes and more preferably l50 minutes. When the treatment time of the calcination is less than the lower limit, therrnal decomposition may not sufficiently proceed and the improvement in the crushing strength of the iron ore pellets may be insufficient. To the contrary, when the treatment time of the calcination is greater than the upper limit, the effect of increasing the crushing temperature tends to be saturated and the effect may be insufficient with respect to the increase in the production cost. [005 l] A lower limit of a percentage of particles having a grain size of less than or equal to 20 um in the dolomite after the hydration reaction (after the balling step S3) is preferably 45% by volume, and more preferably 55% by volume. The percentage of particles having a grain l0 Your Ref 230073SE Our Ref. 21FP-01 1 8/WO/SE size of less than or equal to 20 um being greater than or equal to the lower limit facilitates an increase in the crushing strength of the iron ore pellets. id="p-52" id="p-52" id="p-52" id="p-52" id="p-52" id="p-52" id="p-52"

[0052] In the method for producing iron ore pellets, due to calcining the dolomite at a temperature greater than or equal to the predeterrnined value in the preparing step Sl, the dolomite is present in a miniaturized state in a pellet structure prior to f1ring, and an effect of increasing the bonding strength of the pellet structure of the iron ore pellets is produced. The crushing strength of the iron ore pellets to be produced can thus be increased. In addition, in the iron ore pellets produced by the method for producing iron ore pellets, a CaO/SiOz mass ratio is greater than or equal to 0.8 and a MgO/SiOg mass ratio is greater than or equal to 0.4, resulting in high reducibility. id="p-53" id="p-53" id="p-53" id="p-53" id="p-53" id="p-53" id="p-53"

[0053] [Other Embodiments] It is to be noted that the present invention is not limited to the above-described embodiments. [0054] In the first embodiment, only the method of pulverizing the dolomite in the preparing step such that the Blaine specific surface area is greater than or equal to the predeterrnined value has been described, and in the second embodiment, only the method of calcining the dolomite at a temperature of greater than or equal to the predeterrnined value in the preparing step has been described; however, these methods may be employed in combination. id="p-55" id="p-55" id="p-55" id="p-55" id="p-55" id="p-55" id="p-55"

[0055] In the first embodiment, the method of pulverizing the dolomite in the preparing step has been described; however, dolomite having the Blaine specific surface area greater than or equal to the predeterrnined value may be prepared in advance. Similarly, in the second embodiment, calcined dolomite may be prepared. In this case, the preparing step may be omitted. id="p-56" id="p-56" id="p-56" id="p-56" id="p-56" id="p-56" id="p-56"

[0056] In addition, it is considered that, due to the dolomite being present in a miniaturized state in a structure of the green pellets prior to the f1ring, the crushing strength of the iron ore pellets to be produced can be increased as described above. Therefore, the treatment in the preparing step is not limited to those of the aforementioned embodiments, and the dolomite may be subj ected to another treatment to be present in a miniaturized state in the pellet structure prior to the f1ring. id="p-57" id="p-57" id="p-57" id="p-57" id="p-57" id="p-57" id="p-57"

[0057] In the aforementioned embodiments, the method of producing iron ore pellets by using the production apparatus with the grate kiln system has been described; however, the iron ore l l Your Ref 230073SE Our Ref. 21FP-0118/WO/SE pellets may also be produced by using a production apparatus With a straight grate system. In the production apparatus With the straight grate system, the grate fumace includes a traVeling grate, a drying chamber, a dehydrating chamber, a preheating chamber, and a firing chamber, and the f1ring step is completed only in the grate fumace. Specifically, the green pellets are dried, dehydrated, and preheated by a heating gas in the drying chamber, the dehydrating chamber, and the preheating chamber, and finally f1red in the firing chamber.

[EXAMPLES] id="p-58" id="p-58" id="p-58" id="p-58" id="p-58" id="p-58" id="p-58"

[0058] 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. [0059] [Experiment 1] Iron ore pellets in Which a CaO/SiOg mass ratio Was 1.4 and a MgO/SiOg mass ratio Was 0.8 Were produced by the procedure illustrated in FIG. 1. In the preparing step, the Blaine specific surface area Was changed by pulverization of the dolomite. Note that the ﬁring temperature Was 1,230 °C or 1,250 °C. id="p-60" id="p-60" id="p-60" id="p-60" id="p-60" id="p-60" id="p-60"

[0060] The crushing strength of each of the iron ore pellets thus produced Was measured. The results are shown in FIG. 4. id="p-61" id="p-61" id="p-61" id="p-61" id="p-61" id="p-61" id="p-61"

[0061] The graph in FIG. 4 shows that the Blaine specific surface area of the dolomite being greater than or equal to 4,000 cm2/ g can increase the crushing strength. It is concluded that, particularly in the case of the f1ring temperature being 1,250 °C, the Blaine specific surface area of the dolomite being greater than or equal to 4,000 cm2/ g enables production of the iron ore pellets having a high crushing strength of greater than or equal to 270 kg/P. id="p-62" id="p-62" id="p-62" id="p-62" id="p-62" id="p-62" id="p-62"

[0062] Note that although the CaO/SiOg mass ratio Was 1.4 and the MgO/SiOg mass ratio Was 0.8 in the present experiment in the present experiment, it is inferred that since the CaO/SiOz mass ratio of 0.8 and the MgO/SiOg mass ratio of 0.4, for example, increase the crushing strength, the Blaine specific surface area of the dolomite being greater than or equal to 4,000 cm2/ g gives the crushing strength of greater than or equal to 270 kg/P even in the case in Which the ﬁring temperature is 1,230 °C, by reducing the CaO/SiOg mass ratio and the MgO/SiOz mass ratio. id="p-63" id="p-63" id="p-63" id="p-63" id="p-63" id="p-63" id="p-63"

[0063] [Experiment 2] Iron ore pellets in Which a CaO/SiOg mass ratio Was 1.40 and a MgO/SiOg mass ratio Was 0.83 Were produced by the procedure illustrated in FIG. 1. In the preparing step, the dolomite Was calcined While changing the calcination condition Within ranges of temperature 12 Your Ref 230073SE Our Ref. 21FP-0118/WO/SE from 900 C° to l, l 10 °C and of the treatment time from 80 minutes to 200 minutes. Note that the firing temperature was l,230 °C or 1,250 °C. [0064] Regarding each of the iron ore pellets thus produced, measurements were performed on: a percentage of particles having a grain size of less than or equal to 20 um in the dolomite after the hydration reaction in the balling step; and the crushing strength. The results are shown in FIG. 5. id="p-65" id="p-65" id="p-65" id="p-65" id="p-65" id="p-65" id="p-65"

[0065] The graph in FIG. 5 shows that the calcination at a temperature of greater than or equal to 900 °C can increase the crushing strength. It is concluded that, particularly in the case of the firing temperature being l,250 °C, the percentage of particles having a grain size of less than or equal to 20 um in the dolomite after the hydration reaction being greater than or equal to 45% by volume enables production of the iron ore pellets having a high crushing strength of greater than or equal to 270 kg/P. In addition, it is inferred that also in the case of the firing temperature being 1,230 °C, the percentage of particles having a grain size of less than or equal to 20 um being greater than or equal to 45% by volume gives the crushing strength of greater than or equal to 270 kg/P by reducing the CaO/SiOg mass ratio and the MgO/SiOz mass ratio. [INDUSTRIAL APPLICABILITY] id="p-66" id="p-66" id="p-66" id="p-66" id="p-66" id="p-66" id="p-66"

[0066] By employing the method for producing iron ore pellets according to the present invention, iron ore pellets superior in reducibility and having high crushing strength can be produced. Therefore, the iron ore pellets produced by the present method for producing iron ore pellets can be used in a blast ﬁamace operated with a low reduction agent ratio. [Explanation of the Reference Symbols]

Claims

1.Claims A method for producing iron ore pellets, Which is used for operation of a blast ﬁlmace and in Which a CaO/SiOg mass ratio is greater than or equal to 0.8 and a MgO/SiOz mass ratio is greater than or equal to 0.4, the method comprising: balling green pellets by adding, to an iron ore material and dolomite, Water for use in the balling; and firing the green pellets, Wherein the dolomite has a characteristic of being present in a miniaturized state in a structure of the green pellets. The method for producing iron ore pellets according to claim l, ﬁJrther comprising preparing the dolomite, Wherein in the preparing, the dolomite is pulverized such that a Blaine specific surface area is greater than or equal to 4,000 cm2/ g. The method for producing iron ore pellets according to claim l, ﬁJrther comprising preparing the dolomite, Wherein in the preparing, the dolomite is calcined at a temperature greater than or equal to 900 °C. The method for producing iron ore pellets according claims l, 2, or 3, Wherein a firing temperature in the f1ring is greater than or equal to 1,250 °C.