EP4600388A1

EP4600388A1 - Method for determining high temperature properties of iron ore pellets, method for producing iron ore pellets, and iron ore pellets

Info

Publication number: EP4600388A1
Application number: EP22963549.5A
Authority: EP
Inventors: Koichi Morioka; Yuki KAJIYAMA; Tsugunori Kato; Yuta TAKIGUCHI
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2022-10-27
Filing date: 2022-11-25
Publication date: 2025-08-13
Also published as: WO2024089903A1; CN119948178A; CL2025001247A1; JP7853885B2; EP4600388A4; JP2024064028A

Abstract

An aspect of the present invention is a method for producing iron ore pellets for use in blast furnace operation, wherein the iron ore pellets being self-fluxing, the method including: raw material blending, wherein an auxiliary material containing CaO and MgO is blended into an iron ore material such that 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; pelletizing, wherein green pellets are made from a raw material mixture obtained in the raw material blending; and agglomerating, wherein strength is imparted to the green pellets, wherein a temperature T1 represented by the following formula 1 is greater than or equal to 1,100 °C, or a temperature T2 represented by the following formula 2 is greater than or equal to 1,350 °C:

T 1 = 1, 155 - 0.095 \times {Po}^{2} + 15 \times {FeO}^{0.5}

T 2 = 220 \times C / S + 13.1 \times M / S - 23.13 \times TFe + 2, 600

wherein Po denotes a porosity [%] of the iron ore pellets, FeO denotes a proportion [% by mass] of FeO, C/S denotes the CaO/SiO₂ mass ratio, M/S denotes the MgO/SiO₂ mass ratio, and TFe denotes a proportion [% by mass] of a total iron content.

Description

[TECHNICAL FIELD]

The present invention relates to a method for determining high-temperature characteristics of iron ore pellets, a method for producing iron ore pellets, and iron ore pellets.

[BACKGROUND ART]

As blast furnace operation, a method is known in which iron ore and/or fired iron ore containing iron oxide as well as coke as a carbon source are charged from an upper portion of the blast furnace, the air and/or oxygen are blown from a tuyere in the lower portion to promote generation of carbon monoxide and a reduction reaction for removing oxygen from the iron oxide in the furnace, and then pig iron is tapped from the lower portion of the furnace.
To smoothly conduct the continuous operation, smooth blast is important. For this purpose, it is desired that the blast pressure is low and stable, i.e., that the gas permeability is favorable. The blast pressure depends on characteristics of burdens. Of the burdens, iron ore, sintered iron ore, and iron ore pellets are each turned to be a mixture of metallic iron and an oxide when exposed to high temperatures and a reducing atmosphere and subjected to a reduction reaction. At the same time, they are softened and transformed by a load in the blast furnace. The softening and transformation cause gaps between the burden particles to be filled, thereby impairing the gas permeability in the furnace. An event caused principally by this phenomenon is referred to as a furnace lower-portion pressure loss, and a reduction thereof is intended.
As iron ore pellets which enable a reduction in the furnace lower-portion pressure loss, self-fluxing pellets having a CaO/SiO₂ mass ratio of greater than or equal to 0.8, a MgO/SiO₂ mass ratio of greater than or equal to 0.4, and a predetermined grain diameter distribution are known (see Japanese Unexamined Patent Application, Publication No. 2008-280556 ).
With regard to the iron ore pellets, the reducibility at high temperatures is improved by setting the CaO/SiO₂ mass ratio to greater than or equal to 0.8 and setting the MgO/SiO₂ mass ratio to greater than or equal to 0.4, and the gas permeability is ensured by controlling the grain diameter distribution.

[PRIOR ART DOCUMENT]

[PATENT DOCUMENT]

Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2008-280556

[SUMMARY OF THE INVENTION]

[PROBLEMS TO BE SOLVED BY THE INVENTION]

Blast furnace operation is performed at high temperatures and needs much energy, and thus, it is required to lower the energy. To enable blast furnace operation at lower energy, it is important to reduce heat which is conducted from a furnace wall to the outside and is not involved with a furnace reaction (furnace wall heat loss). The heat loss in the blast furnace increases as the temperature of the furnace wall becomes higher. The temperature of the furnace wall is high when a furnace gas in the vicinity of the wall has a high temperature and a high flow rate; therefore, it is important to reduce the flow rate of the gas in the vicinity of the furnace wall.
The gas permeability in the blast furnace significantly changes beyond a cohesive zone formed by ore through softening at high temperatures. Burdens such as solid ore and the like are present on a low-temperature side with respect to the cohesive zone, i.e., in an upper region of the furnace, and it is known that when burdens are controlled such that burdens having low gas permeability are located in the vicinity of the furnace wall, the flow rate of the gas in the vicinity of the furnace wall can be reduced.
On the other hand, only liquid iron and slag as well as solid coke are present on a high-temperature side with respect to the cohesive zone, i.e., in a lower region of the furnace, and thus, it is difficult to control a channel of the gas. Accordingly, to enable blast furnace operation at lower energy, it is effective to locate the cohesive zone on the lower side to expand a region in which the flow rate of the gas in the vicinity of the furnace wall can be reduced by the burdens.
In a case of iron ore pellets containing CaO and MgO, a CaO-FeO compound is softened and transformed by melting, whereby formation of the cohesive zone is started. A temperature at which formation of the cohesive zone is started can be represented by a fusion start temperature at which the shrinkage percentage reaches 10% in a load reduction test. When the temperature is further increased, a MgO-FeO compound, which has been present in a solid state, is also melted and rapidly shrinks, and formation of the cohesive zone is ended. The temperature at which the rapid shrinkage occurs is referred to as a rapid shrinkage temperature.
When formation of the cohesive zone is ended, there remains no means for reducing the flow rate of the gas in the vicinity of the furnace wall and the temperature of the furnace wall increases; therefore, it is considered that to enable blast furnace operation at lower energy, the fusion start temperature or the rapid shrinkage temperature is preferably high. However, it cannot be said that a technology for increasing the fusion start temperature or the rapid shrinkage temperature of iron ore pellets has been established.
The present invention has been made in view of the foregoing circumstances, and an object of the present invention is to provide: a method for determining high-temperature characteristics of iron ore pellets which enables determining that the fusion start temperature or the rapid shrinkage temperature is high; a method for producing iron ore pellets by using the method for determining high-temperature characteristics; and iron ore pellets.

[MEANS FOR SOLVING THE PROBLEMS]

An aspect of the present invention is a method for determining high-temperature characteristics of iron ore pellets for use in blast furnace operation, the iron ore pellets being self-fluxing and having a CaO/SiO₂ mass ratio of greater than or equal to 0.8 and a MgO/SiO₂ mass ratio of greater than or equal to 0.4, wherein the following formula 1 representing a fusion start temperature T1 or the following formula 2 representing a rapid shrinkage temperature T2 is used: $T 1 = 1, 155 - 0.095 \times {Po}^{2} + 15 \times {FeO}^{0.5}$ $T 2 = 220 \times C / S + 13.1 \times M / S - 23.13 \times TFe + 2, 600$

wherein in the above formula 1, Po denotes a porosity [%] of the iron ore pellets, and FeO denotes a proportion [% by mass] of FeO with respect to the iron ore pellets, and
in the above formula 2, C/S denotes the CaO/SiO₂ mass ratio of the iron ore pellets, M/S denotes the MgO/SiO₂ mass ratio of the iron ore pellets, and TFe denotes a proportion [% by mass] of a total iron content with respect to the iron ore pellets.

Another aspect of the present invention is a method for producing iron ore pellets for use in blast furnace operation, wherein the iron ore pellets are self-fluxing, the method including: raw material blending, wherein an auxiliary material containing CaO and MgO is blended into an iron ore material such that 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; pelletizing, wherein green pellets are made from a raw material mixture obtained in the raw material blending; and agglomerating, wherein strength is imparted to the green pellets, wherein a temperature T1 represented by the following formula 1 is greater than or equal to 1,100 °C, or a temperature T2 represented by the following formula 2 is greater than or equal to 1,350 °C: $T 1 = 1, 155 - 0.095 \times {Po}^{2} + 15 \times {FeO}^{0.5}$ $T 2 = 220 \times C / S + 13.1 \times M / S - 23.13 \times TFe + 2, 600$

Still another aspect of the present invention is iron ore pellets for use in blast furnace operation, the iron ore pellets being self-fluxing, wherein 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, and a temperature T1 represented by the following formula 1 is greater than or equal to 1,100 °C, or a temperature T2 represented by the following formula 2 is greater than or equal to 1,350 °C: $T 1 = 1, 155 - 0.095 \times {Po}^{2} + 15 \times {FeO}^{0.5}$ $T 2 = 220 \times C / S + 13.1 \times M / S - 23.13 \times TFe + 2, 600$

[EFFECTS OF THE INVENTION]

The method for determining high-temperature characteristics of iron ore pellets of the present invention enables determining that the fusion start temperature or the rapid shrinkage temperature is high. The method for producing iron ore pellets of the present invention, the method using the method for determining high-temperature characteristics of iron ore pellets, enables producing iron ore pellets having a high fusion start temperature or a high rapid shrinkage temperature. Furthermore, the iron ore pellets of the present invention have a high fusion start temperature or a high rapid shrinkage temperature.

[BRIEF DESCRIPTION OF THE DRAWINGS]

FIG. 1 is a flowchart illustrating a method for producing iron ore pellets according to one embodiment of the present invention.
FIG. 2 is a schematic view illustrating a configuration of a production apparatus used in the method for producing iron ore pellets in FIG. 1.
FIG. 3 is a graph showing a correlation between the fusion start temperature and an estimated fusion start temperature T1.
FIG. 4 is a graph showing a correlation between the rapid shrinkage temperature and an estimated rapid shrinkage temperature T2.
FIG. 5 is a flowchart illustrating a method for producing iron ore pellets according to an embodiment different from that in FIG. 1.

[DESCRIPTION OF EMBODIMENTS]

Description of Embodiments of Invention

As a result of intensive studies on the fusion start temperature T1, the present inventors have found that the fusion start temperature T1 can be approximated using the porosity and the proportion of FeO. That is to say, the above formula 1 can be used to accurately estimate the fusion start temperature T1. Furthermore, as a result of intensive studies on the rapid shrinkage temperature T2, they have found that the rapid shrinkage temperature T2 can be approximated using the C/S, the M/S, and the TFe. That is to say, the above formula 2 can be used to accurately estimate the rapid shrinkage temperature T2. Accordingly, by using the above formula 1 or the above formula 2, a formation temperature of the cohesive zone in the blast furnace operation can be easily determined.
Both the above formula 1 and the above formula 2 are preferably used. By thus using both the above formula 1 and the above formula 2, the formation temperature of the cohesive zone can be more accurately determined.
Another aspect of the present invention is a method for producing iron ore pellets for use in blast furnace operation, wherein the iron ore pellets are self-fluxing, the method including: raw material blending, wherein an auxiliary material containing CaO and MgO is blended into an iron ore material such that 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; pelletizing, wherein green pellets are made from a raw material mixture obtained in the raw material blending; and agglomerating, wherein strength is imparted to the green pellets, wherein a temperature T1 represented by the following formula 1 is greater than or equal to 1,100 °C, or a temperature T2 represented by the following formula 2 is greater than or equal to 1,350 °C: $T 1 = 1, 155 - 0.095 \times {Po}^{2} + 15 \times {FeO}^{0.5}$ $T 2 = 220 \times C / S + 13.1 \times M / S - 23.13 \times TFe + 2, 600$

In the method for producing iron ore pellets, the temperature T1 represented by the above formula 1 or the temperature T2 represented by the above formula 2 is greater than or equal to the lower limit. The T1 determined according to the above formula 1 accurately approximates the fusion start temperature; therefore, by setting the T1 to be greater than or equal to the lower limit, the fusion start temperature of the iron ore pellets to be produced can be easily increased. Furthermore, the T2 determined according to the above formula 2 accurately approximates the rapid shrinkage temperature; therefore, by setting the T2 to be greater than or equal to the lower limit, the rapid shrinkage temperature of the iron ore pellets to be produced can be easily increased. Therefore, by using the method for producing iron ore pellets, in which the T1 or the T2 is greater than or equal to the predetermined temperature, iron ore pellets which enable blast furnace operation at low energy can be produced.
It is preferred that the temperature T1 is greater than or equal to 1,100 °C and that the temperature T2 is greater than or equal to 1,350 °C. By thus setting the temperature T1 to greater than or equal to 1,100 °C and setting the temperature T2 to greater than or equal to 1,350 °C, iron ore pellets which enable blast furnace operation at lower energy can be produced.
A CaO amount, a MgO amount, a SiO₂ amount, and an iron amount are preferably adjusted in the raw material blending. By thus adjusting the CaO amount, the MgO amount, the SiO₂ amount, and the iron amount in the raw material blending, the value of the T2 can be controlled.
The strength imparted in the agglomerating is preferably due to firing of the green pellets, and an FeO amount is preferably adjusted by a firing temperature. Since the FeO amount increases as the firing temperature becomes higher, the value of the T1 can be controlled. Furthermore, a proportion of oxygen in the iron ore pellets is reduced, and the iron content (TFe) is increased; thus, the value of the T2 can also be controlled.
The firing temperature is preferably greater than or equal to 1,200 °C and less than or equal to 1,300 °C. By thus setting the firing temperature within the above range, a sintering effect in which the surface tension of the iron ore pellets is enhanced by the high-temperature firing can be obtained to reduce the porosity. Thus, the value of the T1 can be increased.
The auxiliary material preferably contains a calcium ferrite mineral, a magnesium ferrite mineral, and a binder, and an FeO amount is preferably adjusted in the raw material blending. In the case in which the auxiliary material thus contains the calcium ferrite mineral, the magnesium ferrite mineral, and the binder, the FeO amount can be directly adjusted by the amounts of the calcium ferrite mineral and the magnesium ferrite mineral in the raw material blending, and thus, the controllability of the T1 is high.
Still another aspect of the present invention is iron ore pellets for use in blast furnace operation, the iron ore pellets being self-fluxing, wherein 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, and a temperature T1 represented by the following formula 1 is greater than or equal to 1,100 °C, or a temperature T2 represented by the following formula 2 is greater than or equal to 1,350 °C: $T 1 = 1, 155 - 0.095 \times {Po}^{2} + 15 \times {FeO}^{0.5}$ $T 2 = 220 \times C / S + 13.1 \times M / S - 23.13 \times TFe + 2, 600$

Since being self-fluxing and having a CaO/SiO₂ mass ratio of greater than or equal to 0.8 and a MgO/SiO₂ mass ratio of greater than or equal to 0.4, the iron ore pellets have high reducibility. Since the T1 determined according to the above formula 1 accurately approximates the fusion start temperature, the fact that the T1 is greater than or equal to the lower limit means that the fusion start temperature of the iron ore pellets is high. Furthermore, since the T2 determined according to the above formula 2 accurately approximates the rapid shrinkage temperature, the fact that the T2 is greater than or equal to the lower limit means that the rapid shrinkage temperature of the iron ore pellets is high. Therefore, by using the iron ore pellets in which the T1 or the T2 is greater than or equal to the predetermined temperature, blast furnace operation at low energy is enabled.
It is preferred that the temperature T1 is greater than or equal to 1,100 °C and that the temperature T2 is greater than or equal to 1,350 °C. By thus using the iron ore pellets in which the temperature T1 is greater than or equal to 1,100 °C and the temperature T2 is greater than or equal to 1,350 °C, blast furnace operation at lower energy is enabled.
It is to be noted that the shape of each of the iron ore pellets in the present invention is not limited to a spherical shape, and any three-dimensional shape may be employed.

Details of Embodiments of Invention

Hereinafter, the method for determining high-temperature characteristics of iron ore pellets, the method for producing iron ore pellets, and the iron ore pellets according to the embodiments of the present invention will be described with reference to the drawings as appropriate.

Method for Producing Iron Ore Pellets

First Embodiment

A method for producing iron ore pellets illustrated in FIG. 1 includes a raw material blending step S1, a pelletizing step S2, an agglomerating step S3, and a cooling step S4. In the method for producing iron ore pellets, strength imparted in the agglomerating step S3 is due to firing of green pellets, and pellets to be produced are so-called fired pellets.
In the method for producing iron ore pellets, self-fluxing iron ore pellets 1 for use in blast furnace operation can be produced using a production apparatus with a grate kiln system (hereinafter, may be simply referred to as "production apparatus 2"). The production apparatus 2 includes a pan pelletizer 3, a grate furnace 4, a kiln 5, and an annular cooler 6.

Raw Material Blending Step

In the raw material blending step S1, an auxiliary material containing CaO and MgO is blended into an iron ore material such that 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.
In the case in which the strength imparted in the agglomerating step S3 is due to firing of the green pellets, limestone serving as a CaO source and dolomite serving as a MgO source are blended as the auxiliary material.
As needed, the iron ore material and the auxiliary material are preferably pulverized in a ball mill or the like in advance or after the blending to adjust the grain size of the raw material mixture in which the iron ore material and the auxiliary material are mixed.
At this time, when a raw material grain size index is appropriately controlled, a porosity of green pellets P is controlled. As referred to herein, the "raw material grain size index" can be determined by the following method. First, a grain size distribution of the raw material mixture is measured. In the measurement, one of JIS-A-1204:2010, JIS-A-8815:1994, and JIS-Z-8825:2022 can be employed. Next, a sum Σ3/Pi·mi in a range from 3 µm to 1,000 µm is calculated using a proportion mi by mass or volume in each grain size range Pi (representative value) and defined as the raw material grain size index.
The relation between the raw material grain size index and the porosity of the green pellets P holds in the case of a raw material mixture in which iron ore and an auxiliary material under the same brand are blended in the same proportion; however, for example, if the iron ore is under a different brand, the coefficient of proportionality may change owing to influences of the surface shape, wettability, and the like. Hence, a suitable value of the raw material grain size index can be determined by the following method. First, as raw material mixtures having a specific mixing ratio, raw material mixtures having at least two types of raw material grain size indices are prepared, green pellets P are produced, and the porosities thereof are measured. From the results, the relation between the raw material grain size index and the porosity can be calculated. Consequently, a raw material grain size index at which the porosity required for the iron ore pellets 1 is obtained can be determined; therefore, the grain size of the raw material is adjusted such that this raw material grain size index can be obtained. It is to be noted that the adjustment of the grain size also includes purchasing a raw material having such a grain size.
Alternatively, a specific surface area represented by a Blaine index may be used as an indicator for the raw material grain size. The lower limit of the specific surface area is preferably 1,000 cm²/g and more preferably 2,000 cm²/g. On the other hand, the upper limit of the specific surface area is preferably 5,000 cm²/g and more preferably 4,000 cm²/g. When the specific surface area is less than the lower limit, it may be difficult to set the T2, which is an indicator for the rapid shrinkage temperature described later, to greater than or equal to 1,350 °C. Conversely, when the specific surface area is greater than the upper limit, a bursting phenomenon may occur in the agglomerating step S3. As referred to herein, the "specific surface area" means a value measured according to JIS-R5201(2015).
A binder such as bentonite or the like may be appropriately blended into the raw material mixture to obtain the strength of the green pellets P required for transportation in the production process.

Pelletizing Step

In the pelletizing step S2, the green pellets P are made from the raw material mixture obtained in the raw material blending step S1. The green pellets P can be made using a rotary pelletizer. As the rotary pelletizer, the pan pelletizer 3 illustrated in FIG. 2, a drum pelletizer, a disc pelletizer, or the like may be used.
Specifically, in the pelletizing step S2, moisture (water) is added to the raw material mixture, and then this water-containing mixture (the raw material mixture containing the water) is charged into the pan pelletizer 3 and rolled to produce the green pellets P having a ball shape.
The lower limit of the porosity of the green pellets P is preferably 15% and more preferably 17%. On the other hand, the upper limit of the porosity is preferably 25% and more preferably 20%. When the porosity is less than the lower limit, a bursting phenomenon may occur in the agglomerating step S3. Conversely, when the porosity is greater than the upper limit, it may be difficult to set the T1, which is an indicator for the fusion start temperature described later, to greater than or equal to 1,100 °C.
The porosity is preferably controlled by the raw material grain size in the raw material blending step S1 and by the balling time in the pelletizing step S2. By thus controlling the porosity, the porosity can be easily controlled to be a desired value, and thus, the T1 can be more surely set to greater than or equal to 1,100 °C. It is to be noted that when a volume percentage of a pore size of less than or equal to 20 µm in a pore size distribution is set to preferably greater than or equal to 80% and more preferably greater than or equal to 85%, the T1 can be still more surely set to greater than or equal to 1,100 °C. As referred to herein, the "volume percentage of a pore size of less than or equal to 20 µm in a pore size distribution" can be measured according to JIS-R-1655:2003.
Furthermore, a grain size range of the green pellets P is preferably adjusted in the pelletizing step S2 such that a grain diameter after the agglomerating step S3 is greater than or equal to 4 mm and less than or equal to 20 mm, more preferably greater than or equal to 6 mm and less than or equal to 15 mm. By thus setting the grain diameter after the agglomerating step S3 within the above range, a decrease in the upper gas permeability resistance in the blast furnace can be inhibited while maintaining the reducibility at high temperatures.
For the adjustment of the grain size range of the green pellets P, classification by a sieve group including an oversize screen (upper limit sieve) and a seed screen (lower limit sieve) which are each adjusted to have a predetermined sieve mesh size. By thus adjusting the grain size range of the green pellets P by classification, the grain diameter after the agglomerating step S3 can be easily and surely adjusted. It is to be noted that non-standard products removed by the classification are preferably disintegrated and reused as the raw material mixture.

Agglomerating Step

In the agglomerating step S3, strength is imparted to the green pellets P. According to the method for producing iron ore pellets, the green pellets P are fired in the agglomerating step S3. In the production apparatus 2 illustrated in FIG. 2, the grate furnace 4 and the kiln 5 are used in the agglomerating step S3.

Grate Furnace

As illustrated in FIG. 2, the grate furnace 4 includes a traveling grate 41, a drying chamber 42, a dehydrating chamber 43, and a preheating chamber 44.
The traveling grate 41 is configured to be endless and can transfer the green pellets P placed on the traveling grate 41 to the drying chamber 42, the dehydrating chamber 43, and the preheating chamber 44 in this order.
In the drying chamber 42, the dehydrating chamber 43, and the preheating chamber 44, the green pellets P are subjected to drying, dehydrating, and preheating by a heating gas G1, whereby strength sufficient to resist the rotation in the kiln 5 is imparted to the green pellets P to obtain preheated pellets H.
Specifically, the following procedure is followed. First, in the drying chamber 42, the green pellets P are dried at an ambient temperature of approximately 250 °C. Next, in the dehydrating chamber 43, the green pellets P after the drying are heated to approximately 450 °C to mainly decompose and remove water of crystallization in the iron ore. Furthermore, in the preheating chamber 44, the green pellets P are heated to approximately 1,100 °C, whereby carbonate contained in limestone, dolomite, and/or the like is decomposed to remove carbon dioxide, and magnetite in the iron ore is oxidized. Accordingly, the preheated pellets H are obtained.
As illustrated 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 downstream heating gas G1 or combustion exhaust gas G2 which has a high temperature, heating cost of the heating gas G1 can be reduced. It is to be noted that a burner 45 may be provided in each chamber to control the temperature of the heating gas G1. In FIG. 2, the burners 45 are provided in the dehydrating chamber 43 and the preheating chamber 44. Furthermore, the heating gas G1 used in the drying chamber 42 is eventually discharged from a smokestack C.

Kiln

The kiln 5 is directly connected to the grate furnace 4 and is a rotary furnace having a sloped cylindrical shape. The kiln 5 fires the preheated pellets H discharged from the preheating chamber 44 of the grate furnace 4. Specifically, the preheated pellets H are fired by combustion with a kiln burner (not illustrated) provided on the outlet side. Accordingly, the iron ore pellets 1 having a high temperature are obtained.
The lower limit of a firing temperature at which the preheated pellets H are fired is preferably 1,200 °C and more preferably 1,220 °C. On the other hand, the upper limit of the firing temperature is preferably 1,300 °C and more preferably 1,280 °C. When the firing temperature is less than the lower limit, the pellets fail to be sintered, and furthermore, when the firing temperature is greater than the upper limit, coarse crystal grains are likely to be generated, resulting in a possibility that pores in the iron ore pellets 1 may become larger. Conversely, by setting the firing temperature within the above range, a sintering effect in which the surface tension of the iron ore pellets 1 is enhanced by the high-temperature firing can be obtained to reduce the porosity. Thus, the value of the T1 can be increased.
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.

Cooling Step

In the cooling step S4, the high-temperature iron ore pellets 1 obtained in the agglomerating 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 furnace operation.
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.
It is to be noted that the cooling gas G3 used in the annular cooler 6 and having an increased temperature is sent to the kiln 5 and used as the air for combustion.

Rapid Shrinkage Temperature and Fusion Start Temperature

In the method for producing iron ore pellets, the temperature T1 represented by the following formula 1 is greater than or equal to 1,100 °C, and the temperature T2 represented by the following formula 2 is greater than or equal to 1,350 °C: $T 1 = 1, 155 - 0.095 \times {Po}^{2} + 15 \times {FeO}^{0.5}$ $T 2 = 220 \times C / S + 13.1 \times M / S - 23.13 \times TFe + 2, 600$

wherein in the above formula 1, Po denotes the porosity [%] of the iron ore pellets, and FeO denotes the proportion [% by mass] of FeO with respect to the iron ore pellets, and
in the above formula 2, C/S denotes the CaO/SiO₂ mass ratio of the iron ore pellets, M/S denotes the MgO/SiO₂ mass ratio of the iron ore pellets, and TFe denotes the proportion [% by mass] of the total iron content with respect to the iron ore pellets.

The above formula 1 is an estimate formula for estimating the fusion start temperature, and the above formula 2 is an estimate formula for estimating the rapid shrinkage temperature. By using these estimate formulae, the high-temperature characteristics of the iron ore pellets can be determined, and the formulae themselves are embodiments of the present invention. Hereinafter, the method for determining high-temperature characteristics of iron ore pellets by using these estimate formulae will be described.
An aspect of the present invention is a method for determining high-temperature characteristics of iron ore pellets for use in blast furnace operation, the iron ore pellets being self-fluxing and having a CaO/SiO₂ mass ratio of greater than or equal to 0.8 and a MgO/SiO₂ mass ratio of greater than or equal to 0.4, wherein the following formula 1 representing a fusion start temperature T1 and the following formula 2 representing a rapid shrinkage temperature T2 are used: $T 1 = 1, 155 - 0.095 \times {Po}^{2} + 15 \times {FeO}^{0.5}$ $T 2 = 220 \times C / S + 13.1 \times M / S - 23.13 \times TFe + 2, 600$

Fusion Start Temperature

As a result of intensive studies on the fusion start temperature T1, the present inventors have found that the fusion start temperature T1 can be approximated using the porosity and the FeO. FIG. 3 shows a correlation between the fusion start temperature and the fusion start temperature T1 estimated according to the above formula 1. As shown in FIG. 3, these temperatures agree well with each other. That is to say, the above formula 1 can be used to accurately estimate the fusion start temperature T1, and thus, the formation temperature of the cohesive zone in the blast furnace operation can be easily determined.
The present inventors consider that the fact that the fusion start temperature T1 can be approximated according to the above formula 1 can be explained in terms of denseness of the structure of the iron ore pellets 1 at around 1,100 °C. In other words, in a case in which a dense metallic iron shell is maintained at around 1,100 °C, at which a CaO-FeO compound is melted, the strength of the iron ore pellets 1 is maintained, and the fusion start temperature T1 increases. To maintain the dense metallic iron shell, the generation rate of metallic iron in a reduction reaction is preferably low. In the case of a low porosity, diffusion of a reducing gas can be stagnated to lower the generation rate of metallic iron. Furthermore, a pellet base structure containing FeO is holohyaline and does not include a pore. Accordingly, the T1 tends to decrease when the porosity Po is high and to increase when the FeO is high.

Rapid Shrinkage Temperature

As a result of intensive studies on the rapid shrinkage temperature T2, the present inventors have found that the rapid shrinkage temperature T2 can be approximated using the C/S, the M/S, and the TFe. FIG. 4 shows a correlation between the rapid shrinkage temperature and the rapid shrinkage temperature T2 estimated according to the above formula 2 . As shown in FIG. 4, these temperatures agree well with each other. That is to say, the above formula 2 can be used to accurately estimate the rapid shrinkage temperature T2, and thus, a formation end temperature of the cohesive zone in the blast furnace operation can be easily determined.
The present inventors consider that the fact that the rapid shrinkage temperature T2 can be approximated according to the above formula 2 can be explained in terms of a thermodynamic state. In other words, the amounts of CaO and MgO affect depending on their ratios to SiO₂ and each approximate the melting point of an oxide. As the CaO/SiO₂ or the MgO/SiO₂ increases, the melting point becomes higher. Furthermore, the TFe indicates that the CaO and the MgO less affect as the iron content increases and approximates an influence of FeO during reduction on the melting points of the oxides CaO and MgO. Details are as follows: deficiency in CaO leads to a decrease in the reducibility with respect to metallic iron, and much FeO, which is an unreduced oxide, remains. MgO forms, together with the remaining FeO, a MgO-FeO compound having a high melting point and increases the rapid shrinkage temperature T2; however, if there is a deficiency in MgO, free FeO, which does not form a MgO-FeO compound, remains. Moreover, if the iron ore pellets contain much FeO, the total iron content TFe increases. In this regard, the effects of the amounts of CaO and MgO themselves are reflected as coefficients of the C/S and the M/S; therefore, the effect of the FeO is represented by adding the term TFe to the rapid shrinkage temperature T2, and as the FeO increases, i.e., as the TFe increases, the melting point becomes lower.
It is considered that the estimate formulae T1 and T2 are effective until 1,597 °C, which is the melting point of magnetite and Wüstite. The accuracy is high in a range in which the TFe in the iron ore pellets is greater than or equal to 55% by mass, and the accuracy is particularly high in the case of iron ore pellets in which the volume percentage of a pore size of less than or equal to 20 µm in a pore size distribution is greater than or equal to 80%.
In the method for producing iron ore pellets, the temperature T1 represented by the above formula 1 is greater than or equal to 1,100 °C. For example, the melting points of a CaO-FeO compound and an Al₂O₃-CaO-FeO compound are about 1,100 °C. Furthermore, in the method for producing iron ore pellets, the temperature T2 represented by the above formula 2 is greater than or equal to 1,350 °C. For example, some Al₂O₃-CaO-SiO₂ compounds have melting points of about 1,350 °C. It can also be considered that controlling the above formulae 1 and 2 to be greater than or equal to the predetermined temperatures means inhibiting the generation of compounds having such melting points. In this regard, the amount of alumina (Al₂O₃) contained in the iron ore pellets 1 is preferably less than or equal to a certain level, and a content thereof is preferably less than or equal to 3.0% by mass.

Control of T1 and T2

The values of the T1 and the T2 can be adjusted by a variety of methods.
With regard to the porosity Po included in the T1, the porosity Po can be reduced when fine or coarse powder raw materials are used in the raw material blending step S1. Conversely, when a carbonate, a hydrate, and/or the like are/is added, such a material volatilizes during the firing, whereby the porosity Po tends to increase. Furthermore, as described above, the porosity Po is reduced by a sintering effect in which the surface tension of the iron ore pellets 1 is enhanced by the high-temperature firing. When the Po decreases, the T1 becomes higher; when the Po increases, the T1 becomes lower.
The FeO included in the T1 varies depending on an increase or decrease of magnetite ore, which is a FeO-containing raw material, and/or an iron oxide scale and also increases due to FeO remaining owing to reduction of the iron ore pellets 1 in the air by the high-temperature firing and to rapid cooling thereof. When the FeO increases, the T1 becomes higher; when the FeO decreases, the T1 becomes lower.
When the CaO amount, the MgO amount, the SiO₂ amount, and the iron amount are adjusted in the raw material blending step S1, the value of the T2 can be controlled in accordance with an increase or decrease of each amount.
The amounts of CaO, MgO, and SiO₂ can be adjusted by selecting raw materials containing them. For example, dolomite contains carbonates of CaO and MgO, and magnesite contains MgO and SiO₂. Limestone contains a carbonate of CaO, and silica contains SiO₂. By adjusting a compounding ratio of these materials, the values of the C/S and the M/S can be adjusted. When the C/S or the M/S increases, the T2 becomes higher; when the C/S or the M/S decreases, the T2 becomes lower.
The amount of TFe can be adjusted by the choice of iron ore and the compounding ratio. For example, hematite has a high iron content, whereas most gangue components have low iron contents. Furthermore, the amount of TFe also varies by increasing or decreasing the total amount of CaO, MgO, and SiO₂. When the total amount increases, the amount of TFe decreases. Alternatively, when the FeO amount is increased by raising the firing temperature, the proportion of oxygen in the iron ore pellets 1 decreases and the TFe increases. When the TFe increases, the T2 becomes lower; when the TFe decreases, the T2 becomes higher.

Advantages

In the method for producing iron ore pellets, the temperature T1 represented by the above formula 1 is greater than or equal to 1,100 °C, and the temperature T2 represented by the above formula 2 is greater than or equal to 1,350 °C. The T1 determined according to the above formula 1 accurately approximates the fusion start temperature; therefore, by setting the T1 to be greater than or equal to the lower limit, the fusion start temperature of the iron ore pellets to be produced can be easily increased. Furthermore, the T2 determined according to the above formula 2 accurately approximates the rapid shrinkage temperature; therefore, by setting the T2 to be greater than or equal to the lower limit, the rapid shrinkage temperature of the iron ore pellets to be produced can be easily increased. Therefore, by using the method for producing iron ore pellets, in which the T1 and the T2 are greater than or equal to the predetermined temperatures, iron ore pellets which enable blast furnace operation at low energy can be produced.
Furthermore, by setting the temperature T1 to greater than or equal to 1,100 °C and setting the temperature T2 to greater than or equal to 1,350 °C, iron ore pellets which enable blast furnace operation at lower energy can be produced.

Second Embodiment

A method for producing iron ore pellets illustrated in FIG. 5 includes a raw material blending step S11, a pelletizing step S12, and an agglomerating step S13. In the method for producing iron ore pellets, the strength imparted in the agglomerating step S13 is due to a binder, and pellets to be produced are so-called non-fired pellets.
According to the method for producing iron ore pellets, self-fluxing iron ore pellets for used in blast furnace operation can be produced.

Raw Material Blending Step

In the raw material blending step S11, an auxiliary material containing CaO and MgO is blended into an iron ore material such that 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.
In the raw material blending step S11, the auxiliary material contains, in addition to CaO and MgO, a calcium ferrite mineral (CaO·Fe_xO), a magnesium ferrite mineral (MgO·Fe_xO), and a binder (wherein 0.667 ≤ x ≤ 1.0). The auxiliary material is compounded in accordance with the iron grade of iron ore (pellet feed) which is the iron ore material. It is to be noted that in the case in which the auxiliary material contains a ferrite mineral, the CaO which determines the CaO/SiO₂ mass ratio incorporates CaO contained in the CaO·Fe_xO as well as simple CaO; the MgO which determines the MgO/SiO₂ mass ratio incorporates MgO contained in the MgO·Fe_xO as well as simple MgO.
As the calcium ferrite mineral and the magnesium ferrite mineral, materials which have already been synthesized may be used. The calcium ferrite mineral and the magnesium ferrite mineral can be synthesized by a method in which iron oxide, limestone, dolomite, and magnesite are fired or melted at a high temperature in an electric furnace, a sintering furnace, or the like to react with each other, and the resulting material is then cooled and crushed. The valence of the iron oxide varies depending on a temperature history at this time, and the value of x varies in a range of greater than or equal to 0.667 and less than or equal to 1.0. This value of x represents the FeO concentration. In the case of x = 0.667, entire iron is accounted for by Fe³⁺ ions, and in the case of x = 1, entire iron is accounted for by Fe²⁺ ions. In the case in which x is a value intermediate therebetween, both ions are mixed, wherein the proportion of Fe³⁺ ions increases as x becomes closer to 0.667, whereas the proportion of Fe²⁺ ions increases as x becomes closer to 1.
Furthermore, examples of the binder include cement, sodium silicate, starch, a synthetic polymer agent, and the like. It is to be noted that examples of the synthetic polymer agent include an acrylic resin-based one, a urethane resin-based one, an ether cellulose (carboxymethylcellulose (CMC)), and the like.
It is to be noted that in a manner similar to that in the raw material blending step S1 of the first embodiment, as necessary, the grain size of the raw material mixture may be adjusted by pulverization.

Pelletizing Step

In the pelletizing step S12, green pellets are made from the raw material mixture obtained in the raw material blending step S11.
As a method for making the green pellets, a rotary pelletization method using a pan pelletizer, a drum pelletizer, a disc pelletizer, or the like may be used as in the pelletizing step S2 of the first embodiment, or a pressure welding method in which the raw material mixture is put in a mold or the like and compacted, a molding method in which the raw material mixture is put in an extruder, extruded from an extrusion die, and appropriately cut to form a molded product, or the like may be used. It is to be noted that in the case of non-fired pellets, in the rotary pelletization method, the porosity of the green pellets is controlled by appropriately controlling the raw material grain size index and the balling time as in the case of the first embodiment. In the pressure welding method and the molding method, the porosity of the green pellets is controlled by pressure conditions at the time of compacting or molding.
In any method, the porosity and grain size range of the green pellets preferably fall within ranges similar to those in the first embodiment.

Agglomerating Step

In the agglomerating step S13, strength is imparted to the green pellets P.
In the method for producing iron ore pellets, an air curing method, a steam curing method, or the like is selected in the agglomerating step S13 in accordance with the type of the binder. The air curing method is a method in which the green pellets are left to stand still until predetermined strength is obtained and can be used, for example, in the case in which the binder is cement. The steam curing method is a method in which the green pellets are left to stand still in high-temperature steam until predetermined strength is obtained and can be used, for example, in the case in which the binder is sodium silicate or cement.
The agglomerating step S13 may be performed simultaneously with the pelletizing step S12. For example, depending on the type of the binder, sufficient strength is imparted through pelletizing. In this case, it is not necessary to perform the agglomerating step S13 separately from the pelletizing step S12, and the agglomerating step S13 can be completed in the pelletizing step S12.

Rapid Shrinkage Temperature and Fusion Start Temperature

In the method for producing iron ore pellets, by setting the temperature T1 represented by the above formula 1 to greater than or equal to 1,100 °C and setting the temperature T2 represented by the above formula 2 to greater than or equal to 1,350 °C, effects similar to those described in the first embodiment can be obtained. Therefore, detailed description is omitted.

Control of T1 and T2

The values of the T1 and the T2 can be adjusted by a variety of methods.
With regard to the porosity Po included in the T1, the porosity Po can be reduced when fine or coarse powder raw materials are used in the raw material blending step. When the Po decreases, the T1 becomes higher; when the Po increases, the T1 becomes lower.
The FeO included in the T1 varies depending on the calcium ferrite mineral and the magnesium ferrite mineral, which are FeO-containing raw materials. When the FeO increases, the T1 becomes higher; when the FeO decreases, the T1 becomes lower. Since the FeO amount can be directly adjusted by controlling the amounts of the calcium ferrite mineral and the magnesium ferrite mineral, the controllability of the T1 is high.
Furthermore, when the CaO amount, the MgO amount, the SiO₂ amount, and the iron amount are adjusted in the raw material blending step S11, the value of the T2 can be controlled in accordance with an increase or decrease of each amount.
The amount of TFe can be adjusted by the choice of iron ore and the compounding ratio. For example, hematite has a high iron content, whereas most gangue components have low iron contents. Furthermore, the amount of TFe also varies by increasing or decreasing the total amount of CaO, MgO, and SiO₂. When the total amount increases, the amount of TFe decreases. Alternatively, when the FeO amount is increased by increasing the compounding amounts of the calcium ferrite mineral and the magnesium ferrite mineral, the proportion of oxygen in the iron ore pellets decreases and the TFe increases. When the TFe increases, the T2 becomes lower; when the TFe decreases, the T2 becomes higher.

Advantages

As in the first embodiment, by using the method for producing iron ore pellets, in which the T1 and the T2 are greater than or equal to the predetermined temperatures, iron ore pellets which enable blast furnace operation at low energy can be produced.

Iron Ore Pellets

The iron ore pellets according to still another aspect of the present invention are self-fluxing iron ore pellets for use in blast furnace operation. The iron ore pellets 1 are high- strength agglomerated ore obtained by pelletizing and firing finely pulverized ore or by adding a binder, and can be produced, for example, by the above-described method for producing iron ore pellets.
Regarding the production of the iron ore pellets 1, it is known that adding a CaO-containing compound such as limestone or the like to an iron ore material to increase the CaO/SiO₂ mass ratio in the iron ore pellets 1 improves the reducibility of the iron ore pellets 1. On the basis of this finding, the CaO/SiO₂ mass ratio in the iron ore pellets 1 is set to greater than or equal to 0.8.
In a case in which the raw materials are iron ore (iron oxide) and limestone (CaO-containing compound), calcium ferrite compounds are generated in the firing step by a solid phase reaction between the iron oxide and CaO generated by thermal decomposition, and are simultaneously bound at the interfaces thereof through solid phase diffusion bonding. Since this 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 uniformly.
During the blast furnace operation, a reducing gas diffusively enters the fine pores, 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 in which aggregation of the metallic iron causes shrinkage of an external shape of the iron ore pellets 1, the fine pores start to decrease. As a result, diffusion of the reducing gas into the iron ore pellets 1 is inhibited, whereby the reduction is likely to stagnate.
To inhibit this stagnation of the reduction, addition of a high-melting point component which inhibits a loss of the fine pores during the aggregation process of the metallic iron is effective. Particularly, it is known that adding dolomite as a source of MgO, which is a high-melting point component, to increase the MgO/SiO₂ mass ratio in the iron ore pellets 1 enables obtaining a powerful effect of inhibiting the stagnation of the reduction. On the basis of this finding, the MgO/SiO₂ mass ratio in the iron ore pellets 1 is set to greater than or equal to 0.4.
The iron ore pellets 1 are self-fluxing. Due to the iron ore pellets 1 being self-fluxing, melting down of reduced iron is easily accelerated. It is to be noted that the self-fluxing property of the iron ore pellets 1 is determined by an auxiliary material and/or the like.
In the iron ore pellets 1, the temperature T2 represented by the following formula 1 is greater than or equal to 1,350 °C: $T 2 = 220 \times C / S + 13.1 \times M / S - 23.13 \times TFe + 2, 600$ wherein in the above formula 1, C/S denotes the CaO/SiO₂ mass ratio of the iron ore pellets, M/S denotes the MgO/SiO₂ mass ratio of the iron ore pellets, and TFe denotes the proportion [% by mass] of the total iron content with respect to the iron ore pellets.
Furthermore, in the iron ore pellets 1, the temperature T1 represented by the following formula 2 is greater than or equal to 1,100 °C: $T 1 = 1, 155 - 0.095 \times {Po}^{2} + 15 \times {FeO}^{0.5}$ wherein in the above formula 2, Po denotes the porosity [%] of the iron ore pellets, and FeO denotes the proportion [% by mass] of FeO with respect to the iron ore pellets.

Advantages

Since being self-fluxing and having a CaO/SiO₂ mass ratio of greater than or equal to 0.8 and a MgO/SiO₂ mass ratio of greater than or equal to 0.4, the iron ore pellets 1 have high reducibility. Since the T1 determined according to the above formula 1 accurately approximates the fusion start temperature, the fact that the T1 is greater than or equal to the lower limit means that the fusion start temperature of the iron ore pellets 1 is high. Furthermore, since the T2 determined according to the above formula 2 accurately approximates the rapid shrinkage temperature, the fact that the T2 is greater than or equal to the lower limit means that the rapid shrinkage temperature of the iron ore pellets is high. Therefore, by using the iron ore pellets 1, in which the T1 or the T2 is greater than or equal to the predetermined temperature, blast furnace operation at low energy is enabled.
Moreover, by using the iron ore pellets in which the temperature T1 is greater than or equal to 1,100 °C and the temperature T2 is greater than or equal to 1,350 °C, blast furnace operation at lower energy is enabled.

Other Embodiments

It is to be noted that the present invention is not limited to the above embodiments.
In the first embodiment of the method for producing iron ore pellets, the method for producing iron ore pellets by using the production apparatus with the grate kiln system has been described; however, a production apparatus with a straight grate system may also be used in the production. In the production apparatus with the straight grate system, the grate furnace includes a traveling grate, a drying chamber, a dehydrating chamber, a preheating chamber, and a firing chamber, and the agglomerating step is completed only in the grate furnace. 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 fired in the firing chamber.
In the above embodiments, the case in which in the method for producing iron ore pellets, the T1 is greater than or equal to 1,100 °C and the T2 is greater than or equal to 1,350 °C has been described; however, setting both the T1 and the T2 to be greater than or equal to the predetermined temperatures is not an essential constituent feature. Blast furnace operation at lower energy can be enabled only by setting the T1 to greater than or equal to 1,100 °C or only by setting the T2 to greater than or equal to 1,350 °C.
Similarly, with regard to the iron ore pellets described in the above embodiments, setting the T1 and the T2 to greater than or equal to 1,100 °C and greater than or equal to 1,350 °C, respectively, is not an essential constituent feature. Even when only the T1 is greater than or equal to 1,100 °C or even when only the T2 is greater than or equal to 1,350 °C, blast furnace operation at lower energy can be enabled.

EXAMPLES

Hereinafter, the present invention will be described more in detail by way of Examples; however, the present invention is not limited to these Examples.

No. 1

Iron ore was prepared as an iron ore material, and limestone, dolomite, and bentonite were prepared as an auxiliary material. A raw material mixture was obtained by blending the auxiliary material into the iron ore material such that the CaO/SiO₂ mass ratio (C/S) and the MgO/SiO₂ mass ratio (M/S) were the respective values shown in Table 1.
Green pellets were produced in the following manner: the raw material mixture was pulverized by a ball mill pulverizer, and then, the pulverized raw material was charged into a disc pelletizer machine and rolled while adding moisture, whereby the raw material was pelletized to have a grain diameter of greater than or equal to 10 mm and less than or equal to 12 mm.
The green pellets were put in a grate furnace and then dried and prefired by heating using high-temperature air as a heating gas. The preheated pellets which had been prefired were put in a kiln furnace and then heated to obtain iron ore pellets No. 1.
Measured values of the TFe, the FeO, the porosity, the fusion start temperature, and the rapid shrinkage temperature of the iron ore pellets No. 1 and values of the T1 and the T2 based on the above formulae 1 and 2 were as shown in Table 1. It is to be noted that the measured values were obtained by a load reduction test. As the fusion start temperature, a temperature at which the shrinkage percentage reached 10% was calculated. Furthermore, the rapid shrinkage temperature was defined as a temperature which fell within a temperature range from a temperature indicating a maximum pressure loss to the end of meltdown (shrinkage percentage: 100%) and at which the shrinkage percentage first reached a value greater than or equal to 1%/min.

No. 2

Iron ore pellets No. 2 were obtained in the same manner as No. 1, except that a raw material mixture was prepared such that the C/S and the M/S were values shown in Table 1. Each parameter of the obtained iron ore pellets is as shown in Table 1.

No. 3

In the iron ore pellets No. 1 and No. 2, both the measured values and the estimated values (T1 and T2) of the fusion start temperature are less than 1,100 °C, and those of the rapid shrinkage temperature are less than 1,350 °C. Then, iron ore pellets No. 3 were obtained by adjusting the TFe, the FeO, the C/S, and the M/S to values shown in Table 1 such that the T1 was greater than or equal to 1,100 °C and the T2 was greater than or equal to 1,350 °C. Each parameter of the obtained iron ore pellets is as shown in Table 1.

No. 4

Iron ore pellets No. 4 were intended to have a fusion start temperature of greater than or equal to 1,100 °C and a rapid shrinkage temperature of greater than or equal to 1,350 °C while their porosity was decreased by 5% relative to the iron ore pellets No. 1. To decrease the porosity by approximately 5%, the ore and the auxiliary material were pulverized to prepare a raw material having a grain size, i.e., a specific surface area represented by the Blaine index being 1.8 times to 2.2 times. The iron ore pellets No. 4 were obtained by using the pulverized raw material and adjusting the TFe, the FeO, the C/S, and the M/S to values shown in Table 1 such that the T1 was greater than or equal to 1,100 °C and the T2 was greater than or equal to 1,350 °C. Each parameter of the obtained iron ore pellets is as shown in Table 1.

Table 1

Pellet name	Physical property values					Measured characteristic values		Estimated values
	TFe	FeO	C/S	M/S	porosity	fusion start temperature (°C)	rapid shrinkage temperature (°C)	fusion start temperature T1 (°C)	rapid shrinkage temperature T2 (°C)
	(% by mass)	(% by mass)	-	-	(%)	fusion start temperature (°C)	rapid shrinkage temperature (°C)	fusion start temperature T1 (°C)	rapid shrinkage temperature T2 (°C)
1	64.9	0.26	1.00	0.51	29.5	1,078	1,324	1,080	1,326
2	64.9	0.16	0.97	0.63	30.2	1,028	1,336	1,074	1,320
3	63.6	0.91	0.98	0.62	18.7	1,139	1,359	1,136	1,352
4	63.6	0.20	1.21	0.52	21.9	1,144	1,374	1,116	1,403

As described above, in each of the iron ore pellets No. 1 and No. 2, in which the estimate formulae T1 and T2 were not used, the fusion start temperature was less than 1,100 °C, and the rapid shrinkage temperature was also less than 1,350 °C. In contrast, in each of the iron ore pellets No. 3 and No. 4, in which the estimate formulae T1 and T2 were used and each parameter was adjusted such that the fusion start temperature was greater than or equal to 1,100 °C and the rapid shrinkage temperature was greater than or equal to 1,350 °C, the fusion start temperature was greater than or equal to 1,100 °C, and the rapid shrinkage temperature was also greater than or equal to 1,350 °C. It has been found that by thus using the method for producing iron ore pellets of the present invention, iron ore pellets having a high fusion start temperature and a high rapid shrinkage temperature, which have not been achieved, can be first obtained.

[INDUSTRIAL APPLICABILITY]

[EXPLANATION OF THE REFERENCE SYMBOLS]

1: Iron ore pellets
2: Production apparatus
3: Pan pelletizer
4: Grate furnace
41: Traveling grate
42: Drying chamber
43: Dehydrating chamber
44: Preheating chamber
45: Burner
5: Kiln
6: Annular cooler
61: Blowing apparatus
P: Green pellets
H: Preheated pellets
G1: Heating gas
G2: Combustion exhaust gas
G3: Cooling gas
C: Smokestack

Claims

A method for determining high-temperature characteristics of iron ore pellets for use in blast furnace operation, the iron ore pellets being self-fluxing and having a CaO/SiO₂ mass ratio of greater than or equal to 0.8 and a MgO/SiO₂ mass ratio of greater than or equal to 0.4,
wherein formula 1 representing a fusion start temperature T1 or formula 2 representing a rapid shrinkage temperature T2 is used: $T 1 = 1, 155 - 0.095 \times {Po}^{2} + 15 \times {FeO}^{0.5}$ $T 2 = 220 \times C / S + 13.1 \times M / S - 23.13 \times TFe + 2, 600$

wherein in the formula 1, Po denotes a porosity [%] of the iron ore pellets, and FeO denotes a proportion [% by mass] of FeO with respect to the iron ore pellets, and

in the formula 2, C/S denotes the CaO/SiO₂ mass ratio of the iron ore pellets, M/S denotes the MgO/SiO₂ mass ratio of the iron ore pellets, and TFe denotes a proportion [% by mass] of a total iron content with respect to the iron ore pellets.
The method for determining high-temperature characteristics of iron ore pellets according to claim 1, wherein both the formula 1 and the formula 2 are used.
A method for producing iron ore pellets for use in blast furnace operation, wherein the iron ore pellets are self-fluxing, the method comprising:
raw material blending, wherein an auxiliary material comprising CaO and MgO is blended into an iron ore material such that 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;

pelletizing, wherein green pellets are made from a raw material mixture obtained in the raw material blending; and

agglomerating, wherein strength is imparted to the green pellets,

wherein a temperature T1 represented by formula 1 is greater than or equal to 1,100 °C, or a temperature T2 represented by formula 2 is greater than or equal to 1,350 °C: $T 1 = 1, 155 - 0.095 \times {Po}^{2} + 15 \times {FeO}^{0.5}$ $T 2 = 220 \times C / S + 13.1 \times M / S - 23.13 \times TFe + 2, 600$

wherein in the formula 1, Po denotes a porosity [%] of the iron ore pellets, and FeO denotes a proportion [% by mass] of FeO with respect to the iron ore pellets, and

in the formula 2, C/S denotes the CaO/SiO₂ mass ratio of the iron ore pellets, M/S denotes the MgO/SiO₂ mass ratio of the iron ore pellets, and TFe denotes a proportion [% by mass] of a total iron content with respect to the iron ore pellets.
The method for producing iron ore pellets according to claim 3, wherein the temperature T1 is greater than or equal to 1,100 °C, and the temperature T2 is greater than or equal to 1,350 °C.
The method for producing iron ore pellets according to claim 3 or 4, wherein a CaO amount, a MgO amount, a SiO₂ amount, and an iron amount are adjusted in the raw material blending.
The method for producing iron ore pellets according to claim 3 or 4, wherein
the strength imparted in the agglomerating is due to firing of the green pellets, and

an FeO amount is adjusted by a firing temperature.
The method for producing iron ore pellets according to claim 6, wherein the firing temperature is greater than or equal to 1,200 °C and less than or equal to 1,300 °C.
The method for producing iron ore pellets according to claim 3,
wherein
the auxiliary material comprises a calcium ferrite mineral, a magnesium ferrite mineral, and a binder, and

an FeO amount is adjusted in the raw material blending.
Iron ore pellets for use in blast furnace operation, the iron ore pellets being self-fluxing,
wherein
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, and

a temperature T1 represented by formula 1 is greater than or equal to 1,100 °C, or a temperature T2 represented by formula 2 is greater than or equal to 1,350 °C: $T 1 = 1, 155 - 0.095 \times {Po}^{2} + 15 \times {FeO}^{0.5}$ $T 2 = 220 \times C / S + 13.1 \times M / S - 23.13 \times TFe + 2, 600$

wherein in the formula 1, Po denotes a porosity [%] of the iron ore pellets, and FeO denotes a proportion [% by mass] of FeO with respect to the iron ore pellets, and

in the formula 2, C/S denotes the CaO/SiO₂ mass ratio of the iron ore pellets, M/S denotes the MgO/SiO₂ mass ratio of the iron ore pellets, and TFe denotes a proportion [% by mass] of a total iron content with respect to the iron ore pellets.
The iron ore pellets according to claim 9, wherein the temperature T1 is greater than or equal to 1,100 °C, and the temperature T2 is greater than or equal to 1,350 °C.