US20130055853A1

US20130055853A1 - Method for producing metallic iron

Info

Publication number: US20130055853A1
Application number: US13/696,412
Authority: US
Inventors: Takeshi Sugiyama; Takao Harada
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2010-05-06
Filing date: 2011-05-02
Publication date: 2013-03-07
Also published as: WO2011138954A1; JP2011252226A

Abstract

Disclosed is a technique for preventing the adhesive of metallic iron and/or wustite (which is a material produced by the heat reduction of iron oxide contained in a powder derived from an agglomerate that comprises, as a raw material, a mixture containing a iron-oxide-containing substance and a carbonaceous reducing material) on a heath of a movable furnace heath type heating furnace without largely changing the design of a facility for the production, in the production of metallic iron by placing the agglomerate on the heath and heating the agglomerate in the heating furnace to reduce iron oxide contained in the agglomerate. A heath-forming material for preventing the cohesive of metallic iron and/or wustite (which is a material produced by the heat reduction of iron oxide contained in the powder derived from the agglomerate) on the heath is charged into the furnace together with the agglomerate.

Description

TECHNICAL FIELD

The present invention relates to a method for producing massive metallic iron by placing an agglomerate, which is produced from a raw material mixture containing an iron oxide source, such as iron ore or iron oxide, and a carbon-containing reducing material, on a hearth of a movable hearth furnace and heating the agglomerate to reduce iron oxide in the agglomerate.

BACKGROUND ART

A direct reduction ironmaking process has been developed in which metallic iron is produced from a raw material mixture containing an iron oxide source (hereinafter also referred to as an iron-oxide-containing substance), such as iron ore or iron oxide, and a carbon-containing reducing material (hereinafter also referred to as a carbonaceous reducing material). In accordance with this ironmaking process, an agglomerate formed of the raw material mixture is placed on a hearth of a movable hearth furnace and is heated in the furnace utilizing gas heat transfer or radiant heat of a heating burner to reduce iron oxide in the agglomerate with the carbonaceous reducing material, yielding massive metallic iron. In the ironmaking process, however, part of the agglomerate is pulverized to a powder by rolling, collision, or drop impact. When the agglomerate is placed on the hearth, the powder derived from the agglomerate accompanies the agglomerate and accumulates on the hearth to form an accumulation layer. Like the agglomerate, the accumulation layer is heat reduced in the furnace to form metallic iron or wustite (FeO). Metallic iron or wustite left in the furnace accumulates on the hearth and raises the hearth level, which makes operation difficult. To avoid this, the accumulation layer is usually removed with a discharger. However, because of its small thickness, the accumulation layer on the hearth sometimes remains on the hearth even after the removal of massive metallic iron, which was formed by the heat reduction of iron oxide in the agglomerate, from the furnace. Thus, the accumulation layer is compressed with the discharger and finally forms a large solid that cannot be discharged from the furnace. Furthermore, the discharge of a lump formed by the aggregation of metallic iron or wustite from the furnace sometimes forms unevenness on the hearth, which makes operation difficult. Patent Literatures 1 to 3 propose a technique for solving these problems.
Patent Literature 1 proposes a method for preventing the formation of a steel sheet on a hearth. The method involves the use of a discharger for discharging reduced iron, which is produced by the reduction of a carbon composite iron oxide agglomerate, from a movable hearth reduction furnace and the operation of maintaining the gap between the surface of the moving bed and the discharger. According to this technique, the gap can prevent a powder derived from an agglomerate and accompanying the agglomerate in the furnace from being pressed on the hearth and prevent the formation of a rigid steel sheet.
Patent Literature 2 proposes a method for removing a substance adhered on a hearth of a rotary hearth reduction furnace from the hearth surface, which involves quenching the hearth surface to cause cracks in the adhesive material on the hearth before removing the adhesive material from the hearth.
Patent Literature 3 proposes a method for maintaining the cleanliness of the hearth surface of a rotary hearth furnace by removing a metallic iron powder left on the hearth or adhesives on hearth bricks or by preventing the metallic iron powder from remaining on the hearth. According to this maintenance method, reduced iron powder left on the hearth is removed from the hearth by blowing off the reduced iron powder with a gas jet between the outlet for the reduced iron and the inlet for the raw materials.

CITATION LIST

Patent Literature

PTL 1: Japanese Patent No. 3075721
PTL 2: Japanese Unexamined Patent Application Publication No. 2002-12906
PTL 3: Japanese Unexamined Patent Application Publication No. 11-50120

SUMMARY OF INVENTION

Technical Problem

The techniques disclosed in Patent Literatures 1 to 3 require a design change of the discharger for discharging reduced iron from the movable hearth reduction furnace, the construction of an apparatus for quenching the hearth surface, or the construction of an apparatus for blowing off reduced iron powder, which increases the capital investment.
In view of the situations described above, it is an object of the present invention to provide a method for producing metallic iron by placing an agglomerate, which is produced from a raw material mixture containing an iron-oxide-containing substance and a carbonaceous reducing material, on a hearth of a movable hearth furnace and heating the agglomerate to reduce iron oxide in the agglomerate. This technique prevents metallic iron or wustite, which is produced by the heat reduction of iron oxide contained in a powder derived from the agglomerate, from sticking to the hearth without significantly changing the design for facilities.

Solution to Problem

A method for producing metallic iron according to the present invention that can solve the problems described above has a main point in that, in the production of metallic iron by placing an agglomerate (having a particle size, for example, in the range of 5 to 50 mm), which is produced from a raw material mixture containing an iron-oxide-containing substance and a carbonaceous reducing material, on a hearth of a movable hearth furnace and heating (for example, 1200° C. to 1400° C.) the agglomerate to reduce iron oxide in the agglomerate, a hearth-forming material for preventing metallic iron and/or wustite, which is produced by heat reduction of iron oxide contained in a powder derived from the agglomerate, from sticking to the hearth is charged into the furnace together with the agglomerate.
(a) When the carbon content of the agglomerate is 122% (which herein means % by mass) or more of the carbon content required to reduce iron oxide in the agglomerate, the composition of the hearth-forming material is preferably controlled such that the CaO, SiO₂, and Al₂O₃contents of the total composition of the powder derived from the agglomerate and the hearth-forming material satisfy the following formulae (1) and (2):
[CaO]/[SiO₂]=0.25 to 1.20 (1)
[Al₂O₃]/[SiO₂]=0.2 to 0.7 (2)
wherein each [ ] in the formulae (1) and (2) denotes the content (% by mass) for the component specified in the square brackets.
In the case of (a), the composition of the hearth-forming material is preferably controlled such that the total CaO, SiO₂, and Al₂O₃content is in the range of 3.0% to 7.0% of the total composition of the powder derived from the agglomerate and the hearth-forming material.
(b-1) When the carbon content of the agglomerate is less than 122% of the carbon content required to reduce iron oxide in the agglomerate, the composition of the hearth-forming material is preferably controlled such that the total carbon content of the total composition of the powder derived from the agglomerate and the hearth-forming material is 122% or more of the carbon content required to reduce iron oxide in the agglomerate and that the CaO, SiO₂, and Al₂O₃contents of the total composition of the powder derived from the agglomerate and the hearth-forming material satisfy the following formulae (3) and (4):
[CaO]/[SiO₂]=0.25 to 1.20 (3)
[Al₂O₃]/[SiO₂]=0.2 to 0.7 (4)
wherein each [ ] in the formulae (3) and (4) denotes the content (% by mass) for the component specified in the square brackets.
(b-2) When the carbon content of the agglomerate is less than 122% of the carbon content required to reduce iron oxide in the agglomerate, the composition of the hearth-forming material is preferably controlled such that the total carbon content of the total composition of the powder derived from the agglomerate and the hearth-forming material remains less than 122% of the carbon content required to reduce iron oxide in the agglomerate and that the CaO, SiO₂, Al₂O₃, and MgO contents of the total composition of the powder derived from the agglomerate and the hearth-forming material satisfy at least one of the following formulae (5) to (9):
[CaO]/[SiO₂]<0.25 (5)
[CaO]/[SiO₂]>1.20 (6)
[Al₂O₃]/[SiO₂]<0.2 (7)
[Al₂O₃]/[SiO₂]>0.7 (8)
[MgO]/[SiO₂]>0.4 (9)
wherein each [ ] in the formulae (5) to (9) denotes the content (% by mass) for the component specified in the square brackets.
In the case of (b-2), the composition of the hearth-forming material is preferably controlled such that the total CaO, SiO₂, and Al₂O₃content is more than 7.0% of the total composition of the powder derived from the agglomerate and the hearth-forming material.
A hearth-forming material having a particle diameter in the range of 0.5 to 2 mm preferably constitutes 50% by mass or more of the total amount of hearth-forming material charged in the furnace.

Advantageous Effects of Invention

According to the present invention, a hearth-forming material charged into a hearth of a movable hearth furnace together with an agglomerate can prevent metallic iron or wustite, which is produced by heat reduction of iron oxide contained in a powder derived from the agglomerate, from sticking to the hearth. This can prevent a large adhesive material, such as a steel sheet, that cannot be discharged from the furnace from being formed on the hearth and prevent the rising of the hearth level. Thus, metallic iron can be efficiently produced without significantly changing the design for facilities.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the relationship between temperature and deformation ratio when a pellet is reduced at different [MgO]/[SiO₂] ratios.

FIG. 2 is a photograph substituted for drawing of a cross section of a reduced pellet.

FIG. 3 is a graph of 40% contraction temperature as a function of [MgO]/[SiO₂].

FIG. 4 is a graph of 40% contraction temperature as a function of [CaO]/[SiO₂].

FIG. 5 is a SiO₂—MgO—FeO ternary equilibrium diagram.

FIG. 6 is a CaO—SiO₂—MgO ternary equilibrium diagram.

FIG. 7 is a CaO—SiO₂—Al₂O₃ternary equilibrium diagram.

DESCRIPTION OF EMBODIMENTS

The techniques proposed in Patent Literatures 1 to 3 require significant design changes of facilities and significant capital investments. Thus, the present inventors have made extensive studies in order to provide a method for efficiently producing metallic iron with a minimum capital investment, in which metallic iron or wustite produced by heat reduction of iron oxide contained in a powder derived from an agglomerate in a furnace is prevented from sticking to a hearth, which prevents the formation of a large adhesive material, such as a steel sheet, that cannot be discharged from the furnace on the hearth and prevents the rising of the hearth level. As a result, it was found that a hearth-forming material can be charged into the furnace together with the agglomerate. More specifically, the present invention was completed by finding that, with consideration given to the carbon content of the agglomerate charged into the furnace and the carbon content required to reduce iron oxide contained in the agglomerate, the hearth-forming material can be charged into the furnace while the composition of the hearth-forming material is appropriately controlled such that the total composition of a powder derived from the agglomerate and the hearth-forming material satisfies predetermined conditions.
A method for producing metallic iron according to the present invention is characterized in that a hearth-forming material for preventing metallic iron and/or wustite, which is produced by heat reduction of iron oxide contained in a powder derived from an agglomerate, from sticking to a hearth is charged into a furnace together with the agglomerate. The powder derived from the agglomerate adhered on the hearth is based on a powder accompanying the agglomerate charged into the furnace and a powder produced by disintegration of the agglomerate caused by rapid heating in the furnace. Thus, when the agglomerate is charged into the furnace together with a hearth-forming material, the hearth-forming material can be mixed with the powder derived from the agglomerate on the hearth. Appropriate control of the composition of the hearth-forming material in consideration of the composition of the powder derived from the agglomerate can prevent metallic iron or wustite produced by heat reduction of iron oxide contained in the powder derived from the agglomerate from sticking to the hearth. This can prevent the formation of an adhesive material, such as a steel sheet, or the rising of the hearth level, thus increasing the metallic iron production efficiency.
The hearth-forming material is added before the agglomerate is charged into the furnace and preferably when the hearth-forming material is blended with the agglomerate.
When the hearth-forming material is added before the agglomerate is charged, for example, the hearth-forming material may be added to the agglomerate on a conveyor for charging the agglomerate into a hopper, and a mixture of the agglomerate and the hearth-forming material may be placed on the hearth. Among the charged mixture, a powder derived from the agglomerate and granules of the hearth-forming material accumulate on a lower portion of the agglomerate and move as a mixture when the agglomerate is leveled off with a leveler.
A material for preventing metallic iron or wustite produced by heat reduction of iron oxide contained in a powder derived from an agglomerate from sticking to a hearth may be charged as the hearth-forming material. More specifically, paying attention to the carbon content of the agglomerate, the hearth-forming material may be charged into the furnace while the composition of the hearth-forming material is controlled in a manner that depends on whether the carbon content is (a) 122% or more or (b) less than 122% of the carbon content required to reduce iron oxide in the agglomerate. When the carbon content of the agglomerate is 100% of the carbon content required to reduce iron oxide in the agglomerate, this means that the iron oxide in the agglomerate is entirely (100%) reduced. When the carbon content is 122% of the carbon content required to reduce iron oxide in the agglomerate, this means that the carbon content is in excess of 22%, and the carbon content of 22% corresponds to approximately 5% of the residual carbon in the agglomerate after reduction.
The carbon content of the agglomerate and the carbon content required to reduce iron oxide in the agglomerate can be calculated from the composition of the raw material mixture composing the agglomerate. The carbon content of the agglomerate after heat reduction of iron oxide in the agglomerate can be determined, for example, by charging the agglomerate into an electric furnace, heating the agglomerate in an inert atmosphere (for example, N₂atmosphere) at 1300° C. (representative temperature), and measuring the amount of residual carbon in the agglomerate after reduction reaction by infrared analysis. The carbon content of the agglomerate before heating can be calculated backwards from the total of this analytical value and the carbon content required to reduce iron oxide in the agglomerate.

(a) 122% or More

When the carbon content of the agglomerate is 122% or more of the carbon content required to reduce iron oxide in the agglomerate, the composition of the hearth-forming material may be controlled such that the CaO, SiO₂, and Al₂O₃contents of the total composition of the powder derived from the agglomerate and the hearth-forming material satisfy the following formulae (1) and (2):
[CaO]/[SiO₂]=0.25 to 1.20 (1)
[Al₂O₃]/[SiO₂]=0.2 to 0.7 (2)
wherein each [ ] in the formulae (1) to (2) denotes the content (% by mass) for the component specified in the square brackets.
Thus, when the carbon content of the agglomerate is higher than the required carbon content, and carbon remains after heat reduction, iron oxide contained in the agglomerate is completely reduced, and the resulting metallic iron forms separate fine granules. Furthermore, excessive carbon in the agglomerate promotes the carburization of metallic iron through heat reduction, thus separating one metallic iron granule from another with a hard and brittle slag phase. Thus, even if an adhesive material, such as a steel sheet, is formed on the hearth, the adhesive material can be easily crushed and removed from the furnace.
Thus, when the carbon content of the agglomerate is 122% or more of the required carbon content, it is effective to further promote the granulation of metallic iron so as to facilitate the discharge of metallic iron from the furnace. In order to promote the granulation of metallic iron, the present invention focuses on slag associated with the production of metallic iron, and the melting point of the slag is decreased to promote the aggregation and granulation of metallic iron. More specifically, the composition of the hearth-forming material is controlled such that the CaO, SiO₂, and Al₂O₃contents of the total composition of the powder derived from the agglomerate and the hearth-forming material satisfy the formulae (1) and (2).

Regarding Formula (1)

When [CaO]/[SiO₂] is preferably in the range of 0.25 to 1.20, the melting point of the slag can be decreased to promote the granulation of metallic iron. [CaO]/[SiO₂] is more preferably 0.3 or more and 1.1 or less.

Regarding Formula (2)

When [Al₂O₃]/[SiO₂] is preferably in the range of 0.2 to 0.7, the melting point of the slag can be decreased to promote the granulation of metallic iron. [Al₂O₃]/[SiO₂] is more preferably 0.6 or less, still more preferably 0.4 or less.
When the carbon content of the agglomerate is 122% or more of the required carbon content, the composition of the hearth-forming material is preferably controlled such that the total CaO, SiO₂, and Al₂O₃content is in the range of 3.0% to 7.0% of the total composition of the powder derived from the agglomerate and the hearth-forming material. A higher amount of molten slag results in promotion of the carburization of metallic iron after heat reduction. Thus, when the total amount of the components described above is preferably 3.0% or more, the granulation of metallic iron can be promoted. The total amount is more preferably 4.5% or more, still more preferably 5.0% or more. However, the total amount of more than 7.0% results in excessively increased molten slag, which may flow downward and erode the hearth. Thus, the total amount is preferably 7.0% or less, more preferably 6.5% or less.

(b) Less Than 122%

When the carbon content of the agglomerate is less than 122% of the required carbon content,
(b-1) the composition of the hearth-forming material is controlled such that the total carbon content of the total composition of the powder derived from the agglomerate and the hearth-forming material is 122% or more of the required carbon content, or
(b-2) the composition of the hearth-forming material is controlled such that the total carbon content of the total composition of the powder derived from the agglomerate and the hearth-forming material remains less than 122% of the required carbon content.
In the case of (b-1), it is important that the composition of the hearth-forming material is controlled such that the total carbon content of the total composition of the powder derived from the agglomerate and the hearth-forming material is 122% or more of the required carbon content, and the composition of the hearth-forming material is controlled such that the CaO, SiO₂, and Al₂O₃contents of the total composition of the powder derived from the agglomerate and the hearth-forming material satisfy the following formulae (3) and (4):
[CaO]/[SiO₂]=0.25 to 1.20 (3)
[Al₂O₃]/[SiO₂]=0.2 to 0.7 (4)
wherein each [ ] in the formulae (3) and (4) denotes the content (% by mass) for the component specified in the square brackets.
More specifically, when the carbon content of the agglomerate is less than 122% of the required carbon content, this results in slightly insufficient carbon, and part of iron oxide contained in the powder derived from the agglomerate may remain unreduced, for example, as wustite. This also results in less carbon involved in the carburization of metallic iron and promotes the formation of a metallic iron sheet instead of the granulation of metallic iron. Thus, in order to completely reduce iron oxide in the powder derived from the agglomerate and sufficiently carburize the iron oxide for granulation, a carbonaceous reducing material is blended as the hearth-forming material, the deficiency in the carbon content of the agglomerate is compensated for, and the composition of the hearth-forming material is controlled such that the total carbon content of the total composition of the powder derived from the agglomerate and the hearth-forming material is 122% or more of the required carbon content.
In this case, the CaO, SiO₂, and Al₂O₃contents of the total composition of the powder derived from the agglomerate and the hearth-forming material must satisfy the formulae (3) and (4). The formulae (3) and (4) are the same as the formulae (1) and (2) and are defined on the basis of the same finding. More specifically, the melting point of the slag can be decreased to further promote the granulation of the metallic iron, thus facilitating the removal of the metallic iron from the furnace.
In the case of (b-2), it is important that the composition of the hearth-forming material is controlled such that the total carbon content of the total composition of the powder derived from the agglomerate and the hearth-forming material remains less than 122% of the required carbon content, and that the CaO, SiO₂, Al₂O₃, and MgO contents of the total composition of the powder derived from the agglomerate and the hearth-forming material satisfy at least one of the following formulae (5) to (9):
[CaO]/[SiO₂]<0.25 (5)
[CaO]/[SiO₂]>1.20 (6)
[Al₂O₃]/[SiO₂]<0.2 (7)
[Al₂O₃]/[SiO₂]>0.7 (8)
[MgO]/[SiO₂]>0.4 (9)
wherein each [ ] in the formulae (5) to (9) denotes the content (% by mass) for the component specified in the square brackets.
When no carbonaceous reducing material is blended as the hearth-forming material, and the total carbon content of the total composition of the powder derived from the agglomerate and the hearth-forming material remains less than 122% of the required carbon content, it is effective to appropriately control the composition of gangue components. More specifically, the melting point of the gangue components can be increased to form solid gangue between metallic iron or wustite particles and thereby increase the distance between the metallic iron or wustite particles, which can prevent the aggregation of the metallic iron or wustite particles. This can prevent the metallic iron or wustite particles from sticking to the hearth or sticking to the hearth and forming a lump to raise the hearth level.
Metallic iron produced by the reduction of iron oxide contained in the powder derived from the agglomerate is minute and has very low cohesion. Depending on the composition of gangue components, such as CaO, SiO₂, and Al₂O₃, the resulting slag may have a low melting point, and the formation of molten slag during heat reduction facilitates the movement of Fe atoms on the metallic iron surface in the vicinity of the molten slag, which promotes the coalescence of metallic iron to form a reticulated metallic iron coalescence layer. The compression of the metallic iron coalescence layer results in the formation of a dense metal steel sheet (adhesive material), making it difficult to remove metallic iron from the furnace.
Insufficient reduction of iron oxide also results in the formation of wustite (FeO). Even in this case, the presence of the molten slag facilitates the movement of Fe atoms on the wustite surface and promotes the coalescence of wustite to form coarse wustite particles. The coarse wustite particles forms large blocks with the molten slag and become difficult to remove from the furnace.
Thus, it is believed that the prevention of coalescence between metallic iron or wustite particles or between metallic iron particles and wustite particles facilitates the removal of metallic iron or wustite from the hearth. On the basis of such findings, when no carbonaceous reducing material is blended as the hearth-forming material, and the total carbon content of the total composition of the powder derived from the agglomerate and the hearth-forming material remains less than 122% of the required carbon content, it is important to increase the melting point of the resulting slag to reduce the formation of molten slag.

Formulae (5) and (6)

When [CaO]/[SiO₂] is preferably less than 0.25 or more than 1.20, the resulting slag can have a high melting point, which can prevent the coarsening of metallic iron or wustite particles. [CaO]/[SiO₂] is more preferably 0.20 or less or 1.25 or more.

Formulae (7) and (8)

When [Al₂O₃]/[SiO₂] is preferably less than 0.2 or more than 0.7, the resulting slag can have a high melting point, which can prevent the coarsening of metallic iron or wustite particles. [Al₂O₃]/[SiO₂] is more preferably 0.18 or less, still more preferably 0.16 or less, or more preferably 0.8 or more.

Formula (9)

MgO can reduce the formation of molten slag and prevent the coarsening of metallic iron or wustite particles. Among gangue components, a component having a lower melting point melts earlier during temperature rise. Dissolution of a component that can increase the melting point in the gangue components solidifies the molten component. Repetition of these yields molten gangue. Thus, even when the average composition of gangue has a high melting point, a bonded substance may be partly formed. Since MgO can easily diffuse into solid FeO, a higher MgO content results in a higher melting point of slag. Thus, MgO can reduce the formation of molten slag.
As is clear from FIG. 5 described below, the melting point of molten slag changes greatly with [MgO]/[SiO₂]. Thus, the MgO content may be controlled while the balance between the MgO content and the SiO₂content is taken into consideration. More specifically, when [MgO]/[SiO₂] is preferably more than 0.4, the formation of molten slag can be reduced to increase solid slag. [MgO]/[SiO₂] is more preferably 0.45 or more, still more preferably 0.5 or more. [MgO]/[SiO₂] is 0.9 or less, for example.
At least one of the formulae (5) to (9) may be satisfied to increase the melting point of the resulting slag.
When the carbon content of the agglomerate is less than 122% of the required carbon content, and the total carbon content of the total composition of the powder derived from the agglomerate and the hearth-forming material remains less than 122% of the required carbon content, the composition of the hearth-forming material is preferably controlled such that the total CaO, SiO₂, and Al₂O₃content is more than 7.0% of the total composition of the powder derived from the agglomerate and the hearth-forming material. When the total amount is more than 7.0%, the amount of gangue can be increased, and the solid slag can be increased. This can prevent metallic iron or wustite from becoming coarse through aggregation and adhering to the hearth to raise the hearth level. The total amount is more preferably 7.5% or more, still more preferably 8% or more. The total amount is 10% or less, for example.
A material serving as a CaO source, a SiO₂source, an Al₂O₃source, or a MgO source may be blended as the hearth-forming material. Examples of the CaO source include calcium oxide (CaO) and limestone (main component: CaCO₃). Examples of the SiO₂source include silica sand and mixtures with other components, such as serpentinite. Examples of the Al₂O₃source include bauxite and mixtures with other components, such as alumina-containing iron ore. Examples of the MgO source include MgO-containing slag, Mg-containing substances extracted from seawater, magnesium carbonate (MgCO₃), and dolomite.
In order to control the composition of the hearth-forming material such that the total composition of the powder derived from the agglomerate and the hearth-forming material satisfies the requirements described above, the mass of the powder derived from the agglomerate must be measured.
Examples of the powder derived from the agglomerate include powders of two types: a powder that is produced by disintegration of part of an agglomerate produced from a raw material mixture containing an iron-oxide-containing substance and a carbonaceous reducing material or disintegration due to impact or abrasion of the agglomerate (hereinafter also referred to as a powder I) or a powder that is produced by disintegration of the agglomerate during heat reduction in a furnace (hereinafter also referred to as a powder II).
The mass of the powder I is measured, for example, by measuring the total mass of agglomerate charged into a furnace and, after classification into the agglomerate and the powder derived from the agglomerate, directly measuring the mass of the powder derived from the agglomerate. In the present invention, a powder is defined to have a particle diameter of 3 mm or less.
The method for directly measuring the mass of the powder derived from the agglomerate cannot be applied to cases where the characteristics of the agglomerate vary while the agglomerate is continuously charged into a furnace. As described in an example described below, a rotation strength test that simulates a transfer process up to charging a formed agglomerate into a furnace may be performed to measure the mass of a powder having a particle diameter of 3 mm or less and estimate the mass of the powder derived from the agglomerate.
The mass of the powder II may be measured by heating the agglomerate in an electric furnace and measuring the mass of a powder having a particle diameter of 3 mm or less produced by rapid heating (for example, a heating rate of 10° C./min or more) to estimate the mass of the powder derived from the agglomerate.
On the basis of such estimation of the mass of the powder derived from the agglomerate, the total composition of the powder derived from the agglomerate and the hearth-forming material can be represented by the following formulae (21) to (24):
CaO(kg/h): H_CaO=(L_CaO×W_L+C_CaO×CW_L+S_CaO×SW_L+A_CaO×AW_L+M_CaO×MW_L)/100 (21)
SiO₂(kg/h): H_SiO2=(L_SiO2×W_L+C_SiO2×CW_L+S_SiO2×SW_L+A_SiO2×AW_L+M_SiO2×MW_L)/100 (22)
Al₂O₃(kg/h): H_Al2O3=(L_Al2O3×W_L+C_Al2O3×CW_L+S_Al2O3×SW_L+A_Al2O3×AW_L+M_Al2O3×MW_L)/100 (23)
MgO(kg/h): H_MgO=(L_MgO×W_L+C_MgO×CW_L+S_MgO×SW_L+A_MgO×AW_L+M_MgO×MW_L)/100 (24)
In the formulae (21) to (24), L_CaO, L_SiO2, L_Al2O3, and L_MgOdenote the percentage (% by mass) of CaO, SiO₂, Al₂O₃, and MgO, respectively, in the agglomerate, and W_Ldenotes the mass (kg) of the powder derived from the agglomerate charged into a furnace per unit time (h).
C_CaO, C_SiO2, C_Al2O3, and C_MgOdenote the percentage (% by mass) of CaO, SiO₂, Al₂O₃, and MgO, respectively, in the CaO source of the hearth-forming material, and CW_Ldenotes the mass (kg) of the CaO source in the hearth-forming material charged into a furnace per unit time (h).
S_CaO, S_SiO2, S_Al2O3, and S_MgOdenote the percentage (% by mass) of CaO, SiO₂, Al₂O₃, and MgO, respectively, in the SiO₂source of the hearth-forming material, and SW_Ldenotes the mass (kg) of the SiO₂source in the hearth-forming material charged into a furnace per unit time (h).
A_CaO, A_SiO2, A_Al2O3, and A_MgOdenote the percentage (% by mass) of SiO₂, CaO, Al₂O₃, and MgO, respectively, in the Al₂O₃source of the hearth-forming material, and AW_Ldenotes the mass (kg) of the Al₂O₃source in the hearth-forming material charged into a furnace per unit time (h).
M_CaO, M_SiO2, M_Al2O3, and M_MgOdenote the percentage (% by mass) of CaO, SiO₂, Al₂O₃, and MgO, respectively, in the MgO source of the hearth-forming material, and MW_Ldenotes the mass (kg) of the MgO source in the hearth-forming material charged into a furnace per unit time (h).
When the target compositions are represented by the following formulae (25) to (28), the total composition of the powder derived from the agglomerate and the hearth-forming material is represented by the following formulae (29) to (32) on the basis of the formulae (21) to (24):
[CaO]/[SiO₂]=1.3 (25)
[Al₂O₃]/[SiO₂]=0.3 (26)
[MgO]/[SiO₂]=0.5 (27)
CaO+Al₂O₃+SiO₂=7 (28)
H_CaO/H_SiO2=1.3 (29)
H_Al2O3/H_SiO2=0.3 (30)
H_MgO/H_SiO2=0.5 (31)
(H_CaO+H_Al2O3+H_SiO2)/(W_L+CW_L+SW_L+AW_L+MW_L)×100=7 (32)
Since no SiO₂source is generally added as the hearth-forming material, SW_Lmay be 0. When the SiO₂source is added, the SiO₂source is set at a temporary value, and another additive amount that gives the target component ratio is determined. When the result does not reach the target value, the amount of SiO₂source to be added is changed until the solution is obtained.
With respect to the hearth-forming material, a hearth-forming material having a particle diameter in the range of 0.5 to 2 mm preferably constitutes 50% by mass or more of the total amount of hearth-forming material charged into a furnace. Although the hearth-forming material having a smaller particle diameter is easier to mix with the powder derived from the agglomerate, the hearth-forming material having an excessively small particle diameter is blown off by wind while the hearth-forming material is charged into a furnace or heated in the furnace and cannot achieve the intended effects. Thus, the hearth-forming material having a particle diameter of 0.5 mm or more preferably constitutes 50% by mass or more. However, the hearth-forming material having an excessively large particle diameter is difficult to mix with the powder derived from the agglomerate and cannot achieve the intended effects. In order to rapidly melt CaO or MgO in molten gangue when the gangue begins to melt and thereby promote the solidification of slag, it is recommended that the hearth-forming material have an increased surface area. Thus, the hearth-forming material having a particle diameter of 2 mm or less preferably constitutes 50% by mass or more.
The agglomerate is produced by shaping a raw material mixture containing an iron-oxide-containing substance and a carbonaceous reducing material. The iron-oxide-containing substance may be iron ore, iron sand, or a nonferrous smelting residue. The carbonaceous reducing material may be a carbon-containing substance, for example, coal or coke.
The raw material mixture may contain another component, such as a binder, a MgO source, or a CaO source. The binder may be a polysaccharide (for example, starch, such as wheat flour). The MgO source or the CaO source may be one exemplified as the MgO source or the CaO source to be blended into the hearth-forming material.
The agglomerate may have any shape, for example, a pellet or briquette form. The agglomerate may have any size and may have a particle size (maximum diameter) of 50 mm or less. The particle size is approximately 5 mm or more. The particle size of the agglomerate in a briquette form may be a sphere-equivalent diameter.
The agglomerate may be heated in the furnace to an agglomerate temperature in the range of 1200° C. to 1400° C. to reduce iron oxide in the raw material mixture.
The furnace may be a movable hearth furnace, for example, a rotary hearth furnace.
The temperature of the agglomerate is particularly preferably 1250° C. or more. At an agglomerate temperature of 1250° C. or more, the melting time of metallic iron and slag can be decreased. However, an excessively high agglomerate temperature may result in erosion of a hearth with molten metallic iron, raising the hearth level. Thus, the temperature of the agglomerate is preferably 1350° C. or less.
The agglomerate may be heated with a burner, and the temperature of the agglomerate may be controlled with the combustion conditions of the burner.
Although the present invention is more specifically described in the following examples, the present invention is not limited to these examples. Various modifications may be made to these examples without departing from the gist described above and below. These modifications are also within the technical scope of the present invention.

EXAMPLES

In Experimental Example 1, an agglomerate produced from a raw material mixture containing an iron-oxide-containing substance and a carbonaceous reducing material was heated in a furnace to reduce iron oxide in the raw material mixture, producing metallic iron. The composition and strength of the metallic iron were determined to examine the relationship between fixation on a hearth and the composition. In Experimental Example 2, the effects of CaO, SiO₂, and MgO on the deformation ratio of agglomerate were examined to determine the relationship between the composition and the formation behavior of molten slag. In Experimental Example 3, a ternary phase diagram was used to examine the relationship between the melting temperature of an Al₂O₃slag component and the composition.

Experimental Example 1

An agglomerate having the composition listed in the following Table 1 was produced as an agglomerate produced from a raw material mixture containing an iron-oxide-containing substance and a carbonaceous reducing material. The shape of the agglomerate was a pillow-shaped briquette having a sphere-equivalent diameter (maximum diameter) in the range of approximately 22 to 26 mm for Nos. 1, 6, and 7 and a spherical pellet having a particle size (maximum diameter) in the range of approximately 12 to 20 mm for Nos. 2 to 5 in Table 1. In Table 1, TFe denotes the total iron content, TC denotes the carbon content (in Table 1, the total carbon content of the agglomerate), and FC denotes the percentage of carbon that is not converted into gas at 970° C. Table 1 lists [CaO]/[SiO₂], [Al₂O₃]/[SiO₂], [MgO]/[SiO₂], and [CaO]+[Al₂O₃]+[SiO₂] based on the composition of the agglomerate.
The agglomerate was heated in a furnace to 1300° C. to reduce iron oxide contained in the agglomerate, thus yielding metallic iron. The heating time in the furnace was listed in the following Table 2.
Table 2 listed the measurements of the composition of the agglomerate after heating. In Table 2, MFe denotes the metallic iron content, TC denotes the carbon content (in Table 2, the total carbon content after heating), TC/TFe×100 denotes the ratio of the total carbon content to the total iron content, and Metal Fe denotes the metallization ratio [=metallic iron content (%)/total iron content (%)×100].
In Table 2, RCs denotes the carbon content of the residue (agglomerate) after heating based on the carbon content of the agglomerate before heating. Subtracting RCs from TC of the agglomerate before heating yields the carbon content used for reduction (RedC). Table 2 lists the ratio of the residual carbon after heating to carbon required for reduction (RCs/RedC×100). As is clear from Table 2, the carbon content at which the residual carbon after heating is approximately 5% of the carbon content required for reduction is approximately 22%.
The strength of massive metallic iron (agglomerate) after heating was measured in a rotation strength test.

Rotation Strength Test

The residue was sieved in a rotating container at a total number of revolutions of 500 in accordance with three particle diameters: 1 mm or less, more than 1 mm and 2 mm or less, and more than 2 mm. The rotating container is a cylinder having a diameter of 113 mm and a length of 205 mm and has two barrels, which rotate at a rotation speed of 30 rpm.
Table 2 lists the ratio of a powder having a particle diameter of 1 mm or less to the mass of the sieved powders. An increase in the percentage of the powder having a particle diameter of 1 mm or less indicates that the residue can be easily pulverized, is not adhered on a hearth, and has good removability. In the present invention, removability is considered to be excellent when the percentage of the powder having a particle diameter of 1 mm or less is 29% or more (working examples) and poor when the percentage is less than 29% (comparative examples).
The following are discussions based on Table 2. With respect to Nos. 1, 2, 3, and 5, the carbon content of the residue is 5% or more (RCs/RedC×100 is 22% or more), and the carbon content of the agglomerate is 122% or more of the carbon content required to reduce iron oxide contained in the agglomerate. With respect to Nos. 1, 2, and 3 of these, among the compositions of the agglomerate, [CaO]/[SiO₂] is in the range of 0.25 to 1.20, and [Al₂O₃]/[SiO₂] is in the range of 0.2 to 0.7, which satisfy the formulae (1) and (2). Thus, Nos. 1, 2, and 3 are weakly adhered on the hearth. In contrast, with respect to No. 5, among the compositions of the agglomerate, [CaO]/[SiO₂] is 0.23, which does not satisfy the formula (1). No. 5 also has a total CaO, Al₂O₃, and SiO₂content of less than 3.0%, and metallic iron is easily sintered. Thus, residue removability is not improved.
With respect to Nos. 4, 6, and 7, the carbon content of the residue is less than 5% (RCs/RedC×100 is less than 22%), and the carbon content of the agglomerate is less than 122% of the carbon content required to reduce iron oxide contained in the agglomerate. With respect to No. 6 of these, among the compositions of the agglomerate, [CaO]/[SiO₂] is 0.14, which satisfies the formula (5). Thus, slag has an increased melting point, and the residue has low cohesion, is easily separable, and has good removability. With respect to Nos. 4 and 7, among the compositions of the agglomerate, [CaO]/[SiO₂] is in the range of 0.25 to 1.20, [Al₂O₃]/[SiO₂] is in the range of 0.2 to 0.7, and [MgO]/[SiO₂] is 0.4 or less, which do not satisfy the formulae (5) to (9). Thus, Nos. 4 and 7 are strongly sticked on the hearth.

	TABLE 1

	Composition of agglomerate (% by mass)		[CaO] + [Al₂O₃] +

No.

TFe

FeO

TC

FC

CaO

SiO₂

Al₂O₃

MgO

[CaO]/[SiO₂]

[Al₂O₃]/[SiO₂]

[MgO]/[SiO₂]

[SiO₂]

1	46.24	0.32	21.55	17.35	2.11	2.44	1.44	0.31	0.86	0.59	0.13	5.99
2	47.95	0.34	20.65	17.99	1.98	2.50	1.49	0.25	0.79	0.60	0.10	5.97
3	52.19	3.42	16.61	14.47	1.14	1.96	1.33	0.08	0.58	0.68	0.04	4.43
4	53.68	25.02	17.10	14.96	1.32	2.22	0.92	0.40	0.59	0.41	0.18	4.46
5	53.57	23.09	17.53	15.34	0.38	1.62	0.61	0.29	0.23	0.38	0.18	2.61
6	46.96	1.21	16.21	10.95	0.55	3.82	1.62	0.12	0.14	0.42	0.03	5.99
7	46.96	0.72	16.18	10.93	1.86	5.54	1.56	0.12	0.34	0.28	0.02	8.96

TABLE 2

Time	Composition after heating (% by mass)	Percentage of	Carbon content (% by mass)

No.	(min)	TFe	FeO	MFe	TC	TC/TFe × 100	MetalFe	1 mm or less (%)	RCs	Red C	RCs/Red C × 100

1	16.00	80.1	0.51	79.47	11.0	13.73	99.2	68.30	6.37	15.18	42
2	16.00	81.1	0.28	80.16	10.5	12.95	98.8	86.20	6.21	14.44	43
3	16.00	87.2	0.53	86.58	5.0	5.73	99.3	29.31	2.98	13.63	22
4	16.00	88.0	0.50	87.20	3.6	4.09	99.1	7.12	2.19	14.91	15
5	16.00	86.6	0.40	86.26	5.1	5.89	99.6	0.40	3.16	14.37	22
6	9.95	84.1	0.90	83.11	3.1	3.69	98.8	67.24	1.73	14.48	12
7	8.83	80.9	0.92	80.13	1.3	1.61	99.0	5.82	0.75	15.43	5

MetalFe = MFe/TFe × 100

Experimental Example 2

It is difficult to accurately observe the formation behavior of molten slag in the reduction of iron oxide in the presence of CaO, SiO₂, and MgO. Because of the coexistence of solid and liquid states and nonuniform presence of each oxide, it is not clear what state of molten slag contributes to metallic iron sintering and the promotion of coarse coalescence of wustite.
A MgO source magnesite and a CaO source limestone were blended with iron ore containing SiO₂as a gangue component to form a pellet (agglomerate). The pellet was fired in the air in an electric furnace at 1300° C. for 10 minutes. While the pellet was reduced with a gas, the deformation ratio of the pellet was measured. The effects of CaO, SiO₂, and MgO on the deformation of the pellet were examined. The results were described in “High Temperature Reduction and Softening Properties of Pellets with Magnesite” (Transactions of the Iron and Steel Institute of Japan, issued by The Iron and Steel Institute of Japan, vol. 23 (1983), No. 2, p. 153).
The reduction of the fired pellet with a gas was performed under a load of 0.5 kg/pellet with a reducing gas (CO gas:N₂gas=30% by volume:70% by volume) while the fired pellet was heated to 1500° C. at 10° C./min. The SiO₂content of the pellet was 0.3%, and [MgO]/[SiO₂] was changed in the range of 0.01 to 1.32. The deformation ratio of the pellet was measured before and after reduction. The results were described in the literature described above. FIG. 1 shows the results.
The deformation of the pellet is based on contraction resulting from the reduction of iron oxide to metallic iron and deformation resulting from the formation of molten slag. Deformation at 1100° C. or more predominantly results from the latter deformation. This is demonstrated by observing structure photographs of a cross section of the pellet described in the literature. FIG. 2 shows structure photographs described in the literature. FIG. 2 shows photographs substituted for drawing of a cross section of a pellet reduced by heating a fired pellet in which SiO₂is 4.5% and [MgO]/[SiO₂]=0.59 to 1300° C. FIG. 2(1) shows the outer area of the pellet, and FIG. 2(2) shows the inner area of the pellet. (1) includes much metallic iron shown in white, and (2) includes wustite shown in gray and molten slag shown in black. The wustite particles are coarse and have a molten round surface. Thus, it is clear that deformation at 1100° C. or more results from the formation of molten slag.
In the literature, the effects of the pellet composition on the deformation of the pellet are examined by measuring the temperature for 40% contraction (hereinafter also referred to as 40% contraction temperature) of the pellet. FIGS. 3 and 4 show the results.
FIG. 3 shows the results for different [MgO]/[SiO₂] with a SiO₂content of 4.4% (circles) or 8.3% (crosses) without CaO. FIG. 4 shows the results for different [CaO]/[SiO₂] with [MgO]/[SiO₂]=0.72.
As is clear from FIG. 3, in the absence of CaO, the 40% contraction temperature monotonously increases with [MgO]/[SiO₂]. In the presence of CaO, the 40% contraction temperature is lowest at [CaO]/[SiO₂]=0.45. The composition at which the 40% contraction temperature is 1350° C. is [MgO]/[SiO₂] of more than 0.4 in the absence of CaO as illustrated in FIG. 3 or [CaO]/[SiO₂] of less than 0.18 or more than 1.05 in the presence of CaO as illustrated in FIG. 4. Although this result is different from the range described above in the presence of CaO and MgO, regarding the results of Example 1 as important, [CaO]/[SiO₂] is defined to be less than 0.25 or more than 1.20.
It is also qualitatively clear from a ternary equilibrium diagram that such a way of thinking is reasonable.
FIG. 5 is a SiO₂—MgO—FeO ternary equilibrium diagram. In FIG. 5, [MgO]/[SiO₂]=0.4 gives a straight line, and the melting point under this condition is 1450° C. even for different FeO contents, indicating that the melting point monotonously decreases with [MgO]/[SiO₂].
Even having a melting point of 1450° C., for example, all the gangue components are not necessarily solid at 1350° C. Since part of the gangue components melt at approximately 1200° C. or more, a higher melting point is only indicative of a smaller amount of melt.
FIG. 6 is a CaO—SiO₂—MgO ternary equilibrium diagram. In FIG. 6, [MgO]/[SiO₂]=0.4, [MgO]/[SiO₂]=0.72, [CaO]/[SiO₂]=0.25, [CaO]/[SiO₂]=0.45, and [CaO]/[SiO₂]=1.20 give straight lines. Even when [MgO]/[SiO₂] is constant, with a change in [CaO]/[SiO₂], [CaO]/[SiO₂] of 0.45 almost results in a compound CaO.MgO.2SiO₂having a melting point of approximately 1400° C. Thus, getting closer to the composition of a low-melting-point compound, the molten gangue increases. As shown by the dotted lines in FIG. 6, in order for the melting point of the slag to be approximately 1450° C. or more, [CaO]/[SiO₂] may be less than 0.25 or more than 1.20.

Experimental Example 3

The effects of Al₂O₃on the deformation of the pellet was examined in Experimental Example 2.
FIG. 7 is a CaO—SiO₂—Al₂O₃ternary equilibrium diagram. In FIG. 7, [Al₂O₃]/[SiO₂]=0.2, [Al₂O₃]/[SiO₂]=0.7, [CaO]/[SiO₂]=0.25, and [CaO]/[SiO₂]=1.20 give straight lines. In a region surrounded by these straight lines, a low-melting-point slag having a melting point of approximately 1250° C. is partly formed. Outside this region is a high-melting-point region. Thus, the amount of molten slag can be decreased in the outside of this region.

INDUSTRIAL APPLICABILITY

According to the present invention, a large adhesive material, such as a steel sheet, that cannot be discharged from a furnace can be prevented from being formed on a hearth, and the hearth level can be prevented from being raised. Thus, metallic iron can be efficiently produced without significantly changing the design for facilities.

Claims

1. A method for producing metallic iron, the method comprising:

placing on a hearth of a movable hearth furnace an agglomerate comprising a raw material mixture comprising a carbonaceous reducing material and a substance comprising iron oxide, and

heating the agglomerate to reduce iron oxide in the agglomerate,

wherein a hearth-forming material that prevents metallic iron, wustite, or both, from sticking to the hearth is charged into the furnace together with the agglomerate, and

wherein the metallic iron, wustite, or both, are produced by heat reduction of iron oxide in a powder derived from the agglomerate.

2. The method of claim 1, wherein, when a carbon content of the agglomerate is 122 mass % or more of a carbon content required to reduce the iron oxide in the agglomerate, the hearth-forming material has a composition such that CaO, SiO₂, and Al₂O₃contents of a total composition of the powder derived from the agglomerate and the hearth-forming material satisfy formulae (1) and (2):

[CaO]/[SiO₂]=0.25 to 1.20 (1)

[Al₂O₃]/[SiO₂]=0.2 to 0.7 (2)

wherein each [ ] in the formulae (1) and (2) denotes the content (% by mass) in mass % for the component specified in the square brackets.

3. The method of claim 1, wherein

when a carbon content of the agglomerate is less than 122 mass % of a carbon content required to reduce the iron oxide in the agglomerate,

the hearth-forming material has a composition such that

a total carbon content of a total composition of the powder derived from the agglomerate and the hearth-forming material is 122% or more of the carbon content required to reduce the iron oxide in the agglomerate, and

CaO, SiO₂, and Al₂O₃contents of the total composition of the powder derived from the agglomerate and the hearth-forming material satisfy formulae (3) and (4):

[CaO]/[SiO₂]=0.25 to 1.20 (3)

[Al₂O₃]/[SiO₂]=0.2 to 0.7 (4)

wherein each [ ] in the formulae (3) and (4) denotes the content (% by mass) in mass % for the component specified in the square brackets.

4. The method of claim 1, wherein

the hearth-forming material has a composition such that

a total carbon content of a total composition of the powder derived from the agglomerate and the hearth-forming material remains less than 122% of the carbon content required to reduce the iron oxide in the agglomerate, and

CaO, SiO₂, Al₂O₃, and MgO contents of the total composition of the powder derived from the agglomerate and the hearth-forming material satisfy at least one formula selected from the group consisting of formulae (5) to (9):

[CaO]/[SiO₂]<0.25 (5)

[CaO]/[SiO₂]>1.20 (6)

[Al₂O₃]/[SiO₂]<0.2 (7)

[Al₂O₃]/[SiO₂]>0.7 (8)

[MgO]/[SiO₂]>0.4 (9)

wherein each [ ] in the formulae (5) to (9) denotes the content (% by mass) in mass % for the component specified in the square brackets.

5. The production method according to of claim 2, wherein the hearth-forming material has a composition such that a total CaO, SiO₂, and Al₂O₃content is 3.0% to 7.0% by mass of the total composition of the powder derived from the agglomerate and the hearth-forming material.

6. The method of claim 4, wherein the hearth-forming material has a composition such that a total CaO, SiO₂, and Al₂O₃content is more than 7.0% by mass of the total composition of the powder derived from the agglomerate and the hearth-forming material.

7. The production method according to claim 1, wherein at least 50% by mass of the hearth-forming material has a particle diameter of 0.5 to 2 mm.

8. The method of claim 1, wherein the agglomerate has a particle size of 5 to 50 mm.

9. The method of claim 1, wherein the agglomerate is heated in the movable hearth furnace at a temperature of 1200° C. to 1400° C.

10. The method of claim 2, wherein [CaO]/[SiO₂] is 0.3 to 1.1.

11. The method of claim 2, wherein [Al₂O₃]/[SiO₂] is 0.2 to 0.6.

12. The method of claim 2, wherein [Al₂O₃]/[SiO₂] is 0.2 to 0.4.

13. The method of claim 3, wherein [CaO]/[SiO₂] is 0.3 to 1.1.

14. The method of claim 3, wherein [Al₂O₃]/[SiO₂] is 0.2 to 0.6.

15. The method of claim 3, wherein [Al₂O₃]/[SiO₂] is 0.2 to 0.4.

16. The method of claim 5, wherein the total CaO, SiO₂, and Al₂O₃content is 4.5% to 6.5% by mass of the total composition of the powder derived from the agglomerate and the hearth-forming material.

17. The method of claim 5, wherein the total CaO, SiO₂, and Al₂O₃content is 5.0% to 6.5% by mass of the total composition of the powder derived from the agglomerate and the hearth-forming material.

18. The method of claim 6, wherein the total CaO, SiO₂, and Al₂O₃content is more than 7.5% by mass of the total composition of the powder derived from the agglomerate and the hearth-forming material.

19. The method of claim 6, wherein the total CaO, SiO₂, and Al₂O₃content is more than 8% by mass of the total composition of the powder derived from the agglomerate and the hearth-forming material.

20. The method of claim 6, wherein the total CaO, SiO₂, and Al₂O₃content is more than 7.0% and less than 10% by mass of the total composition of the powder derived from the agglomerate and the hearth-forming material.