US20250179595A1

US20250179595A1 - Coating material for use in direct-reduction iron production, and production method therefor

Info

Publication number: US20250179595A1
Application number: US18/843,789
Authority: US
Inventors: Yoshiko Nakahara; Koki TERUI; Sumito Ozawa
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2022-03-11
Filing date: 2023-03-01
Publication date: 2025-06-05
Also published as: WO2023171486A1; JPWO2023171486A1; JP7601226B2; AU2023231853A1; KR20240144318A

Abstract

A coated raw material for use in direct reduction ironmaking is disclosed. The coated raw material includes: a raw material containing an iron oxide; and a coating layer containing particles adhered to a surface of the raw material, and the particles include at least one selected from the group consisting of a calcium compound and cement, and have a particle size of not smaller than 0.010 mm. The particles preferably have a particle size of not smaller than 0.050 mm. An adhesion amount of the particles is preferably not less than 0.10 mass % and not more than 3.00 mass % based on the raw material. The calcium compound preferably contains at least one selected from the group consisting of calcium oxide and calcium hydroxide.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2023/007532 filed Mar. 1, 2023, which claims priority to Japanese Patent Application No. 2022-037767, filed Mar. 11, 2022, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to a coated raw material for use in direct reduction ironmaking, and a method of producing the same.

BACKGROUND OF THE INVENTION

In recent years, with a global environmental problem, a problem of fossil fuel depletion, or the like as background, energy saving is strongly demanded in ironmaking.
The main raw material of iron is iron ore or another iron oxide. An ironmaking method includes a reduction process for reducing an iron oxide as an essential process. The common ironmaking method most widely used throughout the world is the blast furnace method using a blast furnace.
At a tuyere of a blast furnace, coke or pulverized coal reacts with oxygen in hot air (air heated to temperature of about 1,200° C.) to generate CO gas and H₂gas (reducing gas). The raw material (such as iron ore) in the blast furnace is then reduced by the generated reducing gas.
Owing to the recent improvement in the blast furnace operation technology, the reducing material ratio (amount of coke or pulverized coal used to produce 1 t of molten iron) has been reduced to about 500 kg/t. However, the reducing material ratio has reached nearly the lower limit, and it is difficult to expect an additional large reduction in the reducing material ratio.
Meanwhile, the direct reduction ironmaking method (also referred to as “direct reduction method” or “direct ironmaking method”) has been recently developed as a different ironmaking method from the blast furnace method.
The direct reduction ironmaking method involves charging a raw material (such as iron ore) including an iron oxide into a reduction furnace (direct reduction furnace), blowing a reducing gas thereinto, reducing the raw material to produce reduced iron, and thereafter melting the reduced iron by an electric furnace.
As the reduction furnace, a shaft furnace is mostly used.
As a gas source of the reducing gas, natural gas is mainly used. Natural gas, together with a furnace top gas discharged from a furnace top part of the shaft furnace, is heated and reformed in a heating-reforming apparatus. In this manner, a reducing gas containing CO and H₂is generated.
The generated reducing gas is blown into the shaft furnace and reacts with the raw material (iron oxide) supplied from the furnace top part of the shaft furnace. The raw material (iron oxide) is thus reduced, whereby reduced iron is produced.
The produced reduced iron is cooled in a region (cooling section) situated below the blowing position of the reducing gas in the shaft furnace and is thereafter discharged from a lowermost part of the shaft furnace.
As the raw material to be loaded in the shaft furnace, iron ore in a lump form (lump ore), pellet (powdery iron ore being formed into a spherical shape), and the like are used.
These raw materials often cause clustering in a high-temperature reducing atmosphere in the shaft furnace. Clustering is a phenomenon in which these raw materials are mutually fused in a high-temperature reducing atmosphere in the shaft furnace to thereby form a large mass.
When clustering occurs, it is sometimes difficult to discharge the reduced iron from the shaft furnace. In addition, a phenomenon in which raw materials together form a bridge (also called “hanging”) occurs in the shaft furnace, possibly interfering smooth stock's down movement in some cases. The operation performance of the shaft furnace would be significantly lowered in either of the cases.
Hence, in an ordinary operation of the shaft furnace, the highest reducing temperature is limited to a relatively low temperature to thereby prevent occurrence of clustering. As a result, the reduction rate cannot be sufficiently increased, and hence the productivity cannot be improved to a satisfactory level.
Under the circumstances, there have been conventionally developed technologies in which a surface of a raw material containing an iron oxide (such as iron ore) is coated in order to prevent occurrence of clustering (Patent Literatures 1 to 3).
Clustering is a sintering phenomenon that occurs when a raw material such as iron ore is reduced to generate reduced iron (metallic iron). It is assumed that by providing a coating layer on a raw material, surfaces contacting each other in the generated reduced iron decrease to suppress sintering, and hence clustering is prevented.

PATENT LITERATURES

- Patent Literature 1: JP 62-7806 A
- Patent Literature 2: JP 63-262426 A
- Patent Literature 3: WO 2015/16145

SUMMARY OF THE INVENTION

As described above, reduced iron generated through reduction in the reduction furnace is melted in the electric furnace according to the direct reduction ironmaking method. Therefore, carburization of the reduced iron (carbon addition) is essentially required for the purpose of improving the efficiency in operation of the electric furnace.
That is, carburization of the reduced iron makes it possible to obtain the effects including: lowering the melting point of the reduced iron; reducing the remaining iron oxide in the reduced iron; and generating a large amount of energy by blowing oxygen and burning carbon during the melting process in the electric furnace to thereby shorten the melting time.
Carburization of the reduced iron is specifically carried out by, for example, flowing a carburizing gas (e.g., CH₄) in a cooling section situated at a lower part of the shaft furnace.
However, when a coated raw material having a coating layer (Patent Literatures 1 to 3) is used as the raw material for the reduced iron, the coating layer serves as a barrier and inhibits carburization of the resulting reduced iron in some cases.
Aspects of the present invention have been made in view of the above and aim at providing a coated raw material for use in direct reduction ironmaking, with which coated raw material carburization of the resulting reduced iron is not inhibited, and a method of producing the same.
The present inventors found, through an earnest study, that employing the configuration described below enables the achievement of the above-mentioned objects, and aspects of the invention have thus been completed.
Specifically, aspects of the present invention include the following [1] to [8].
[1] A coated raw material for use in direct reduction ironmaking, the coated raw material comprising: a raw material containing an iron oxide; and a coating layer containing particles adhered to a surface of the raw material, wherein the particles include at least one selected from the group consisting of a calcium compound and cement, and have a particle size of not smaller than 0.010 mm.
[2] The coated raw material according to [1], wherein the particles have a particle size of not smaller than 0.050 mm.
[3] The coated raw material according to [1] or [2], wherein an adhesion amount of the particles is not less than 0.10 mass % and not more than 3.00 mass % based on the raw material.
[4] The coated raw material according to any one of [1] to [3], wherein the calcium compound contains at least one selected from the group consisting of calcium oxide and calcium hydroxide.
[5] A method of producing a coated raw material for use in direct reduction ironmaking, the method comprising: preparing a treatment liquid in a slurry form by suspending particles in water, the particles containing at least one selected from the group consisting of a calcium compound and cement and having a particle size of not smaller than 0.010 mm, coating a surface of a raw material containing an iron oxide with the treatment liquid, and removing moisture contained in the treatment liquid by heating the raw material coated with the treatment liquid.
[6] The method of producing a coated raw material according to [5], wherein the particles have a particle size of not smaller than 0.050 mm.
[7] The method of producing a coated raw material according to [5] or [6], wherein the calcium compound contains at least one selected from the group consisting of calcium oxide and calcium hydroxide.
[8] The method of producing a coated raw material according to any one of [5] to [7], wherein an immersion method or a spray method is used to coat a surface of the raw material with the treatment liquid.
According to aspects of the present invention, it is possible to provide a coated raw material for use in direct reduction ironmaking, with which coated raw material carburization of the resulting reduced iron is not inhibited, and a method of producing the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a configuration diagram showing production facility of a coated raw material along with a shaft furnace where the coated raw material is reduced.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[Coated Raw Material]

The coated raw material according to aspects of the invention is a coated raw material for use in direct reduction ironmaking, the coated raw material comprising: a raw material containing an iron oxide; and a coating layer containing particles adhered to a surface of the raw material, and the particles including at least one selected from the group consisting of a calcium compound and cement and having a particle size of not smaller than 0.010 mm.

The coated raw material according to aspects of the invention includes the raw material containing an iron oxide (hereinafter, simply referred to as “raw material”).
An example of the raw material is iron ore, and specific examples thereof include iron ore in a lump form (lump ore), and pellet (powdery iron ore being formed into a spherical shape).
The grade (iron content) of iron ore used as the raw material is not particularly limited and, generally, is preferably not less than 65 mass % from the viewpoint of reduction in a shaft furnace.

The coated raw material according to aspects of the invention further includes the coating layer adhered to a surface of the raw material.
The coating layer contains particles described below.

<<Type of Particles>>

The particles contained in the coating layer include at least one selected from the group consisting of a calcium compound and cement.

(Calcium Compound)

Examples of the calcium compound include calcium oxide, calcium hydroxide, and a calcium iron compound expressed by chemical formula of Ca_xFe_yO_z. Among these, preferred is at least one selected from the group consisting of calcium oxide and calcium hydroxide.
When calcium hydroxide used as the calcium compound is heated in the production process of the coated raw material and undergoes a dehydration reaction, the calcium hydroxide is converted to calcium oxide, as described below.
Even when a dehydration reaction does not take place in the production process of the coated raw material, since the reduction furnace such as a shaft furnace has its interior temperature of 700° C. to 1,000° C., a dehydration reaction easily occurs, and hence calcium hydroxide is mostly converted to calcium oxide.

(Cement)

Examples of the cement include self-hardening cement (portland cement); latent hydraulic cement (e.g., blast furnace cement); and respective fine powders of blast furnace slag, blast furnace water slag, and iron and steel slag. Among these, portland cement is preferred.
Examples of the portland cement include ordinary portland cement, high-early-strength portland cement, and high-strength portland cement. These types of portland cement contain a large amount of 3CaO·SiO₂which exhibits a strong hydration reaction, are excellent in hydraulicity, and thus have good adhesion to the raw material (e.g., iron ore). In addition, these are available at a low price and allow easy access.

(Other Components)

The particles contained in the coating layer can further contain, for example, a magnesium compound such as magnesium oxide or magnesium hydroxide; an aluminum compound such as aluminum oxide or aluminum hydroxide; silicas; and an iron oxide.

<<Particle Size of Particles>>

The particles contained in the coating layer have a particle size of not smaller than 0.010 mm. The reason therefor is described below.
As described above, calcium oxide (CaO) is easily generated also by a dehydration reaction of calcium hydroxide. Besides, calcium oxide is also contained in cement such as portland cement.
Calcium oxide (CaO) causes a reaction expressed by the following formula (1) and hence consumes the carburizing gas (CH₄). Hence, a reaction expressed by the following formula (2) (carburizing reaction) stagnates to thereby inhibit carburization of the reduced iron.
3CH₄+CaO->CaC₂+CO+6H₂ (1)
CH₄->C+2H₂ (2)
The reaction of the above formula (1) occurs on surfaces of calcium oxide (CaO) particles.
When calcium oxide (CaO) particles present on the reduced iron surface have a small particle size, the reactive area per unit mass increases, thus promoting the reaction of the above formula (1), and an amount of the carburizing gas (CH₄) present in the vicinity of the reduced iron decreases. Accordingly, carburizing gas (CH₄) diffusion-limitation occurs, and the reaction of the above formula (2) stagnates.
Therefore, the particle size is increased. As a result, the reactive area per unit mass decreases, and the reaction of the above formula (1) is suppressed, whereby the reaction of the above formula (2) can be prevented from stagnating. In other words, carburization of the reduced iron is not inhibited.
Moreover, with the larger particle size of the particles, gaps between the reduced iron and the particles become larger, and the carburizing gas (CH₄) thus spreads over the reduced iron surface easily, leading to easy occurrence of the carburizing reaction of the above formula (2).
Because of the foregoing reason, the particle size of the particles contained in the coating layer is not smaller than 0.010 mm, preferably not smaller than 0.050 mm, and more preferably not smaller than 0.100 mm.
The upper limit of the particle size of the particles contained in the coating layer is not particularly limited.
Meanwhile, calcium oxide or other constituents of the particles are impurities for the reduced iron. When the particle size of the particles is too large, the proportion of the impurities increases, and the grade (iron purity) of the resulting reduced iron may be thus degraded.
Therefore, from the viewpoint of achieving a good grade of the resulting reduced iron, the particle size of the particles contained in the coating layer is, for example, not larger than 3.000 mm, preferably not larger than 1.000 mm, more preferably not larger than 0.800 mm, and even more preferably not larger than 0.200 mm.

<<Adhesion Amount of Particles>>

When an adhesion amount of the particles contained in the coating layer is too small, the effect of preventing clustering is not sufficiently obtained. Accordingly, because the clustering resistance is excellent, the adhesion amount of the particles is preferably not less than 0.03 mass %, more preferably not less than 0.07 mass %, and even more preferably not less than 0.10 mass % based on the raw material.
On the other hand, when the adhesion amount of the particles contained the coating layer is too large, the reaction of the above formula (1) excessively proceeds, an amount of the carburizing gas (CH₄) thus becomes insufficient, and the reaction of the above formula (2) may be thus inhibited. In addition, the reducibility of the raw material lowers, and the productivity may thus decrease.
Hence, the adhesion amount of the particles is preferably not more than 8.00 mass %, more preferably not more than 5.50 mass %, and even more preferably not more than 3.00 mass % based on the raw material.

[Method of Producing Coated Raw Material]

Next, a method of producing the coated raw material according to aspects of the invention described above is described with reference to FIG. 1 .
FIG. 1 is a configuration diagram showing production facility of the coated raw material along with a shaft furnace 7 where the coated raw material is reduced.
The production facility of the coated raw material is schematically provided with a raw material vessel 1, a coating treatment vessel 3, a heating furnace 5, and a dehydrating furnace 6 as shown in FIG. 1 .
The raw material vessel 1 stores a raw material. The raw material is exemplified by iron ore in the form of, for example, pellet or lump ore. A charging conveyor 2 is installed below the raw material vessel 1.
The charging conveyor 2 carries the raw material supplied from the raw material vessel 1 up to a position above the coating treatment vessel 3 where the raw material is dropped. In the coating treatment vessel 3, a treatment liquid containing the foregoing particles is stored.
The foregoing particles are preliminarily suspended in water to prepare the treatment liquid in the form of slurry, and the thus prepared treatment liquid is stored in the coating treatment vessel 3.
As the calcium compound constituting the particles, use is made of, for example, calcium oxide, calcium hydroxide, a calcium iron compound (Ca_xFe_yO_z) as described above. Calcium oxide reacts with water to be converted to calcium hydroxide.
Types of cement constituting the particles are also as described above.
The particle size of the particles in the treatment liquid does not alter in the processes to be described later (process of coating the raw material surface with the treatment liquid, and process of heating the raw material whose surface is coated with the treatment liquid).
Hence, in order for the particles contained in the coating layer of the resulting coated raw material to have the particle size of not smaller than 0.010 mm, the particle size of the particles in the treatment liquid is also specified to not smaller than 0.010 mm.
The concentration of the particles in the treatment liquid is not particularly limited and is appropriately adjusted depending on the desired adhesion amount.
The raw material charged from the charging conveyor 2 into the coating treatment vessel 3 is immersed in the treatment liquid. As a result, the raw material whose surface is coated with the treatment liquid (hereinafter, referred to as “treated raw material”) is obtained.
The method of coating the raw material surface with the treatment liquid is not limited to this immersion method. For instance, a method involving spraying with a spray (spray method) may be adopted.
Meanwhile, because the coating layer can be evenly provided and the coating amount can be easily controlled, the immersion method is preferably used.
The treated raw material is carried to the heating furnace 5 by a carrying conveyor 4 a and is heated (dried) in the heating furnace 5. In this manner, moisture in the treated raw material (in particular, the treatment liquid coating the raw material) is evaporated to be removed.
The method of heating the treated raw material is not particularly limited, and examples thereof include methods using steam heating, electric heating, microwave, and dielectric current.
The heating temperature in the heating furnace 5 is preferably not lower than 100° C. from the viewpoint of evaporating moisture in the treated raw material. The atmosphere gas in the heating furnace 5 is preferably a low CO₂concentration gas, and more preferably a gas with CO₂concentration of not higher than 1,000 volume ppm (for example, inert gas, air, combustion gas having undergone CO₂removal treatment, and heating steam).
The treated raw material having been heated in the heating furnace 5 is carried to the dehydrating furnace 6 by a carrying conveyor 4 b, and is further heated in the dehydrating furnace 6.
Through the heating process in the dehydrating furnace 6, the dehydration reaction expressed by the following formula (3) occurs, and the coating layer that is dense and porous can be thus formed. Accordingly, the coated raw material in which the coating layer is adhered to the raw material surface can be obtained.
Since the reaction of the following formula (3) proceeds at 580° C. under atmosphere pressure, the heating temperature in the dehydration furnace 6 is preferably not lower than 580° C.
Ca(OH)₂->CaO+H₂O (3)

[Direct Reduction Ironmaking]

The coated raw material is then transported to the shaft furnace 7, i.e., vertical shaft furnace, and is reduced by a reducing gas while descending from a furnace top part 7 a of the shaft furnace 7.
Hereinbelow, a typical direct reduction ironmaking method, MIDREX method, is described.
The shaft furnace 7 is provided with a gas discharge port 8 and a gas blowing port 9.
The gas blowing port 9 is disposed at a substantially middle position in the vertical direction of the shaft furnace 7, and the reducing gas is supplied to the interior of the shaft furnace 7.
As the reducing gas, use is made of, for example, natural gas; reformed gas whose main components are CO and H₂reformed from natural gas; and coal gas (gas generated through gasification of coal).
The reformed gas has a composition having, for example, a total concentration of H₂and CO of about 90 mol %, a molar ratio H₂/(H₂+CO) of 0.52 to 0.71, and CO₂concentration of 0.5 to 3.0 mol %.
The temperature of the reducing gas blown from the gas blowing port 9 is, for example, 700° C. to 1,200° C. The reducing gas reduces the coated raw material loaded from the furnace top part 7 a and is thereafter discharged from the gas discharge port 8 as exhaust gas.
A cooling section 7 b situated at a lower part of the shaft furnace 7 is provided with a gas blowing port 12 and a gas suction port 11.
A cooling gas and a carburizing gas are blown into the shaft furnace 7 through the gas blowing port 12. The gas suction port 11 sucks these gases to prevent the gases from penetrating toward the furnace top part 7 a.
The reduced iron generated as a result of reduction of the coated raw material is cooled in the cooling section 7 b by the cooling gas and is carburized by the carburizing gas.
As the cooling gas, generally, N₂is used. As the carburizing gas, mostly, CH₄is used, and CO may be contained as part of the carburizing gas.
The shaft furnace 7 is provided at its lowermost part with a reduced iron discharge port 13.
Reduced iron 14 having been cooled and carburized is discharged through the reduced iron discharge port 13.
As a countermeasure for clustering, a cluster breaker 10 is disposed inside the shaft furnace 7 and mechanically breaks a generated cluster.
As the direct reduction ironmaking method, the method using the shaft furnace 7 has been described above, but the type of reduction furnace (direct reduction furnace) is not limited thereto, and a method using a fluidized bed, a rotary kiln, a rotary hearth furnace (RHF) or the like may be adopted.

EXAMPLES

Hereinafter, aspects of the present invention will be specifically described with reference to examples. However, the invention is not limited to the examples described below.

As iron ore for the raw material, Brazilian pellets having a diameter of 10.0 to 15.0 mm were used. The pellets had a chemical composition of, by mass, T·Fe: 66%, FeO: 0.63%, SiO₂: 2.0%, CaO: 2.1%, Al₂O₃: 0.5%, and MgO: 0.16%.

Commercial calcium hydroxide and cement (portland cement) were pulverized to fall in each particle size range shown in Table 1 below, and the obtained particles were suspended in water at each concentration shown in Table 1 below, whereby a treatment liquid in the slurry form was prepared.

An amount of 500 g of the raw material (pellets) was weighed and immersed in the treatment liquid for 30 seconds, whereby the treated raw material was obtained. The obtained treated raw material was heated for 30 minutes in a heating furnace being set at temperature of 80° C. and was thus dried.
When the treatment liquid containing calcium hydroxide particles was used, the immersion process and the drying process were further followed by a process of heating for one hour in a dehydration furnace being set at temperature of 600° C.
Moisture in the treated raw material was removed in this manner, whereby the coated raw material was obtained.
The coated raw material thus obtained was subjected to mechanical polishing, and its cross section was exposed. The cross section of the coated raw material was observed using a scanning electron microscope (SEM) at a magnification of 300×, and an SEM image was obtained.
In the obtained SEM image, the particle sizes (equivalent circle diameters) of the particles adhered to the raw material surface were measured, and the average value of 10 particles was regarded as the particle size of the particles contained in the coating layer.
Furthermore, based on the mass difference between the raw material prior to immersion in the treatment liquid and the resulting coated raw material, the adhesion amount of the particles contained in the coating layer (adhesion amount with respect to raw material) was calculated.

For simulation of the direct reduction ironmaking described with reference to FIG. 1 , the coated raw material was reduced using a furnace of a thermogravimetric (TG) apparatus, and thereafter the carburizing test was carried out.
Specifically, 500 g of the coated raw material was placed in a carbon crucible with a diameter of 80 mm, and the reducing gas (CO: 35 vol %, H₂: 55 vol %, N₂: 10 vol %, gas flow rate: 15 NL/min) was flown from a furnace lower part at a constant temperature of 900° C. so that reduction took place until the reduction rate reached 100%, whereby reduced iron was obtained. Subsequently, the carburizing gas (CH₄: 100 vol %, gas flow rate: 5 NL/min) was flown to carburize the reduced iron.
Before and after the carburizing process, the carbon amount in the reduced iron was measured by the infrared absorption method according to JIS G1211-3 “Iron and steel-Determination of carbon,” an increase in the carbon amount (with respect to reduced iron) as a result of carburization was obtained as the carburizing amount (unit: mass %).
With a larger value of the carburizing amount, it can be evaluated that inhibition of carburization was suppressed.
The reduction rate was calculated according to the following equation.
$[Reduction rate (unit : %)] = {([oxygen amount (unit : mass %) in coated raw material before reduction] - [oxygen amount (unit : mass %) in reduced iron]) / [oxygen amount (unit : mass %) in coated raw material before reduction]} \times 100$
The oxygen amount in the coated raw material was determined as described below. First, chemical analysis was performed to determine the quantities of T·Fe and FeO in the coated raw material. Subsequently, the quantity of Fe₂O₃in the coated raw material was determined by calculation in which Fe in FeO was deducted from T·Fe, and the remainder thereof is assumed to correspond to Fe in Fe₂O₃. The total amount of oxygen in FeO and Fe₂O₃was regarded as the oxygen amount in the coated raw material.
The oxygen amount in the reduced iron was determined as described below. First, chemical analysis was performed to determine the quantities of T·Fe, FeO, and M·Fe in the reduced iron. Subsequently, the quantity of Fe₃O₄in the reduced iron was determined by calculation in which Fe in FeO and M·Fe is deducted from T·Fe, and the remainder thereof is assumed to correspond to Fe in Fe₃O₄. The total amount of oxygen in FeO and Fe₃O₄was regarded as the oxygen amount in the reduced iron.
Table 1 below shows “A” and other letters depending on the values of the carburizing amounts. Because inhibition of carburization is suppressed, “A,” “B,” “B⁻,” or “C” is preferred.

- A: carburizing amount was not less than 4 mass %
- B: carburizing amount was not less than 3 mass % and less than 4 mass %
- B⁻: carburizing amount was not less than 2 mass % and less than 3 mass %
- C: carburizing amount was not less than 1 mass % and less than 2 mass %
- D: carburizing amount was less than 1 mass %

For evaluation of clustering resistance of the coated raw material, the reduction test was carried out using load softening test equipment.
Specifically, 500 g of the coated raw material was placed in a carbon crucible with a diameter of 100 mm, a reducing gas (CO: 35 vol %, H₂: 55 vol %, N₂: 10 vol %, gas flow rate: 15 NL/min) was flown from a furnace lower part at a constant temperature of 900° C. while a load of 1.5 kg/cm²was applied, so that reduction took place until the reduction rate reached 100%.
Clusters of reduced iron generated following the test were crushed using an I-type rotation testing machine; 150 rotations were made at 30 rpm. Thereafter, the mass proportion of the sample on a +15 mm sieve with respect to the total mass of the sample was calculated as the cluster proportion (unit: %). With a smaller value of the cluster proportion, it can be evaluated that the clustering resistance is excellent.
Table 1 below shows “A” and other letters depending on the values of the cluster proportions. Because the clustering resistance is excellent, “A,” “B,” or “C” is preferred.

- A: cluster proportion was less than 5%
- B: cluster proportion was not less than 5% and less than 10%
- C: cluster proportion was not less than 10% and less than 30%
- D: cluster proportion was not less than 30%

	TABLE 1

	Treatment liquid (particles)	Coating layer (particles)

		Particle		Particle	Adhesion
		size range	Concentration	size	amount	Carburizing	Clustering
No.	Type	[mm]	[g/L]	[mm]	[mass %]	amount	resistance	Remarks

1	Calcium hydroxide	<0.01	10	0.005	0.05	D	D	CE
2	Cement	<0.01	50	0.008	0.07	D	D	CE
3	Calcium hydroxide	<0.01	100	0.006	0.34	D	C	CE
4	Cement	<0.01	500	0.006	0.12	D	C	CE
5	Calcium hydroxide	<0.01	300	0.005	5.60	D	C	CE
6	Cement	<0.01	1000	0.008	3.10	D	C	CE
7	Calcium hydroxide	0.01 to 0.05	10	0.010	0.09	B⁻	C	EX
8	Cement	0.01 to 0.05	50	0.030	0.05	B⁻	C	EX
9	Calcium hydroxide	0.01 to 0.05	100	0.030	0.46	B⁻	B	EX
10	Cement	0.01 to 0.05	500	0.040	0.27	B⁻	B	EX
11	Calcium hydroxide	0.01 to 0.05	300	0.030	4.80	C	B	EX
12	Cement	0.01 to 0.05	1000	0.040	3.20	C	B	EX
13	Calcium hydroxide	0.05 to 0.1	10	0.050	0.07	B	B	EX
14	Cement	0.05 to 0.1	50	0.080	0.05	B	B	EX
15	Calcium hydroxide	0.05 to 0.1	100	0.070	0.62	B	A	EX
16	Cement	0.05 to 0.1	500	0.080	0.45	B	A	EX
17	Calcium hydroxide	0.05 to 0.1	300	0.070	6.50	B⁻	A	EX
18	Cement	0.05 to 0.1	1000	0.060	3.20	B⁻	A	EX
19	Calcium hydroxide	0.1 to 1	10	0.210	0.08	A	B	EX
20	Cement	0.1 to 1	50	0.380	0.04	A	B	EX
21	Calcium hydroxide	0.1 to 1	100	0.460	1.10	A	A	EX
22	Cement	0.1 to 1	500	0.520	0.52	A	A	EX
23	Calcium hydroxide	0.1 to 1	300	0.660	5.80	B	A	EX
24	Cement	0.1 to 1	1000	0.710	3.60	B	A	EX
25	Calcium hydroxide	0.05 to 1	100	0.120	0.54	A	A	EX
26	Calcium hydroxide	0.05 to 0.5	100	0.230	0.65	A	A	EX
27	Calcium hydroxide	0.5 to 1	100	0.760	2.80	A	A	EX
28	Calcium hydroxide	0.25 to 0.75	100	0.440	2.21	A	A	EX

CE: Comparative Example
EX: Example

As evident from the results shown in Table 1 above, with the coated raw materials of Nos. 7 to 28 where the particles contained in the coating layer had the particle size of not smaller than 0.010 mm, inhibition of carburization of the resulting reduced iron was suppressed, compared to the coated raw materials of Nos. 1 to 6 where this condition was not satisfied.
In addition, the coated raw materials of Nos. 7 to 28 also showed good clustering resistance.
Referring to the coated raw materials of Nos. 7 to 28, the effect of suppressing inhibition of carburization tended to be more excellent, in this order, in Nos. 7 to 12 where the particle size was not smaller than 0.010 mm (smaller than 0.050 mm), Nos. 13 to 18 where the particle size was not smaller than 0.050 mm (smaller than 0.100 mm), and Nos. 19 to 28 where the particle size was not smaller than 0.100 mm.
Referring to the coated raw materials of Nos. 7 to 12, the coated raw materials of Nos. 9 to 12 where the adhesion amount of the particles was not less than 0.10 mass % showed better clustering resistance than that of the coated raw materials of Nos. 7 and 8 where this condition was not satisfied.
In the meantime, the coated raw materials of Nos. 7 to 10 where the adhesion amount of the particles was not more than 3.00 mass % showed more excellent effect of suppressing inhibition of carburization than that of the coated raw materials of Nos. 11 and 12 where this condition was not satisfied.
The same applies to the coated raw materials of Nos. 13 to 18, the coated raw materials of Nos. 19 to 24, and the coated raw materials of Nos. 25 to 28.

REFERENCE SIGNS LIST

- 1: raw material vessel
- 2: charging conveyor
- 3: coating treatment vessel
- 4 a, 4 b: carrying conveyor
- 5: heating furnace
- 6: dehydrating furnace
- 7: shaft furnace
- 7 a: furnace top part
- 7 b: cooling section
- 8: gas discharge port
- 9: gas blowing port
- 10: cluster breaker
- 11: gas suction port
- 12: gas blowing port
- 13: reduced iron discharge port
- 14: reduced iron

Claims

1. A coated raw material for use in direct reduction ironmaking, the coated raw material comprising:

a raw material containing an iron oxide; and

a coating layer containing particles adhered to a surface of the raw material,

wherein the particles include at least one selected from the group consisting of a calcium compound and cement, and have a particle size of not smaller than 0.010 mm.

2. The coated raw material according to claim 1, wherein the particles have a particle size of not smaller than 0.050 mm.

3. The coated raw material according to claim 1, wherein an adhesion amount of the particles is not less than 0.10 mass % and not more than 3.00 mass % based on the raw material.

4. The coated raw material according to claim 1, wherein the calcium compound contains at least one selected from the group consisting of calcium oxide and calcium hydroxide.

5. A method of producing a coated raw material for use in direct reduction ironmaking, the method comprising:

preparing a treatment liquid in a slurry form by suspending particles in water, the particles containing at least one selected from the group consisting of a calcium compound and cement and having a particle size of not smaller than 0.010 mm,

coating a surface of a raw material containing an iron oxide with the treatment liquid, and

removing moisture contained in the treatment liquid by heating the raw material coated with the treatment liquid.

6. The method of producing a coated raw material according to claim 5, wherein the particles have a particle size of not smaller than 0.050 mm.

7. The method of producing a coated raw material according to claim 5, wherein the calcium compound contains at least one selected from the group consisting of calcium oxide and calcium hydroxide.

8. The method of producing a coated raw material according to claim 5, wherein an immersion method or a spray method is used to coat a surface of the raw material with the treatment liquid.

9. The coated raw material according to claim 2, wherein an adhesion amount of the particles is not less than 0.10 mass % and not more than 3.00 mass % based on the raw material.

10. The coated raw material according to claim 2, wherein the calcium compound contains at least one selected from the group consisting of calcium oxide and calcium hydroxide.

11. The coated raw material according to claim 3, wherein the calcium compound contains at least one selected from the group consisting of calcium oxide and calcium hydroxide.

12. The coated raw material according to claim 9, wherein the calcium compound contains at least one selected from the group consisting of calcium oxide and calcium hydroxide.

13. The method of producing a coated raw material according to claim 6, wherein the calcium compound contains at least one selected from the group consisting of calcium oxide and calcium hydroxide.

14. The method of producing a coated raw material according to claim 6, wherein an immersion method or a spray method is used to coat a surface of the raw material with the treatment liquid.

15. The method of producing a coated raw material according to claim 7, wherein an immersion method or a spray method is used to coat a surface of the raw material with the treatment liquid.

16. The method of producing a coated raw material according to claim 13, wherein an immersion method or a spray method is used to coat a surface of the raw material with the treatment liquid.