U.S. patent application number 15/024481 was filed with the patent office on 2016-08-18 for method for manufacturing iron nuggets.
This patent application is currently assigned to KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.). The applicant listed for this patent is KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.). Invention is credited to Taiji HATAKEYAMA, Shuzo ITO, Shorin O, Hiroshi SUGITATSU.
Application Number | 20160237514 15/024481 |
Document ID | / |
Family ID | 52742824 |
Filed Date | 2016-08-18 |
United States Patent
Application |
20160237514 |
Kind Code |
A1 |
ITO; Shuzo ; et al. |
August 18, 2016 |
METHOD FOR MANUFACTURING IRON NUGGETS
Abstract
Provided is a method for manufacturing iron nuggets by which
reduced iron obtained by heating and reducing agglomerates, or iron
nuggets obtained by melting and aggregating the reduced iron can be
prevented from reoxidation inside a movable hearth heating furnace
and quality of the reduced iron can be improved. The method
involves charging and heating agglomerates including iron oxide and
a carbonaceous reducing agent on a hearth of a movable hearth
heating furnace, reducing and melting the iron oxide in the
agglomerates, and then discharging obtained iron nuggets to the
outside of the furnace and recovering the iron nuggets. The
agglomerates have a coating layer, including a fluid carbonaceous
material, on the surface.
Inventors: |
ITO; Shuzo; (Kobe-shi,
JP) ; HATAKEYAMA; Taiji; (Kobe-shi, JP) ; O;
Shorin; (Kobe-shi, JP) ; SUGITATSU; Hiroshi;
(Kobe-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) |
Kobe-shi, Hyogo |
|
JP |
|
|
Assignee: |
KABUSHIKI KAISHA KOBE SEIKO SHO
(KOBE STEEL, LTD.)
Kobe-shi, Hyogo
JP
|
Family ID: |
52742824 |
Appl. No.: |
15/024481 |
Filed: |
August 18, 2014 |
PCT Filed: |
August 18, 2014 |
PCT NO: |
PCT/JP2014/071534 |
371 Date: |
March 24, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21B 13/0046 20130101;
B22F 2998/10 20130101; C21B 11/08 20130101; C21B 2300/02 20130101;
Y02P 10/134 20151101; C21B 13/105 20130101; C22B 1/245 20130101;
Y02P 10/136 20151101; C21B 13/0066 20130101; C22B 1/2406 20130101;
C22B 1/24 20130101; B22F 2998/10 20130101; C22C 33/0235 20130101;
B22F 2201/04 20130101 |
International
Class: |
C21B 13/10 20060101
C21B013/10; C22B 1/24 20060101 C22B001/24; C22B 1/245 20060101
C22B001/245; C21B 11/08 20060101 C21B011/08; C21B 13/00 20060101
C21B013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 25, 2013 |
JP |
2013-198980 |
Claims
1. A method for manufacturing iron nuggets by charging and heating
agglomerates including iron oxide and a carbonaceous reducing agent
on a hearth of a movable hearth heating furnace, reducing and
melting the iron oxide in the agglomerates, and then discharging
obtained iron nuggets to the outside of the furnace and recovering
the iron nuggets, wherein the agglomerates have a coating layer,
including a fluid carbonaceous material, on the surface.
2. The method according to claim 1, wherein the fluid carbonaceous
material is at least one selected from the group consisting of
bituminous coal, subbituminous coal, and lignite.
3. The method according to claim 1, wherein an average thickness of
the coating layer is greater than 0.30 mm.
4. The method according to claim 1, wherein the agglomerates are
obtained by agglomerating a mixture including the iron oxide and
the carbonaceous reducing agent in a first pelletizer to form core
portions, and then forming the coating layer including the fluid
fluid carbonaceous material on the surface of the obtained core
portions in a second pelletizer.
5. The method according to claim 1, wherein a top portion of the
coating layer is not lower than a top portion of the iron nuggets
while the agglomerates are heated.
6. The method according to claim 1, wherein the coating layer
becomes a shell-shaped coke while the agglomerates are heated.
7. The method according to claim 1, wherein the agglomerates are
charged to form a single layer on the furnace hearth.
8. The method according to claim 1, wherein the carbonaceous
reducing agent is placed on the furnace hearth before the
agglomerates are charged on the furnace hearth.
9. The method according to claim 1, wherein a C amount in the iron
nuggets is 2.5 mass % or more.
10. The method according to claim 1, wherein an S amount in the
iron nuggets is 0.120 mass % or less.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for manufacturing
iron nuggets by heating agglomerates including iron oxide, for
example, iron ore, and a carbon-containing reducing agent (also
referred to hereinbelow as "carbonaceous reducing agent") and
reducing and melting the iron oxide contained in the
agglomerates.
BACKGROUND ART
[0002] For example, the technique disclosed in Patent Literature 1
is known as a method for manufacturing iron nuggets by heating
agglomerates including iron oxide and a carbonaceous reducing
agent. This literature indicates that in a method for manufacturing
a solid metal product from carbon including a metal-bearing
compound, the surface of a molded product including carbon and the
metal-bearing compound is coated with a treatment substance and the
covered molded product is supplied on a furnace hearth and heated.
It is also indicated that the coating layer includes a carbonaceous
compound.
CITATION LIST
Patent Literature
[0003] Patent Document 1: U.S. Pat. No. 6,214,087
SUMMARY OF INVENTION
[0004] The agglomerates charged on the hearth of a movable-bed
heating furnace are heated by radiation heat or gas heat transfer
created by a heating burner provided in the furnace, and iron oxide
contained in the agglomerates is reduced by the carbonaceous
reducing agent, thereby generating iron nuggets. However, where a
heating burner is used as a heating means, a flow of atmosphere gas
is created inside the furnace. Since the atmosphere gas includes
oxidizing gases such as carbon dioxide and water vapors, the
reduced iron obtained by heating and reducing the agglomerates and
the iron nuggets obtained by melting and aggregating the reduced
iron can be re-oxidized by the oxidizing gas. Where the reduced
iron or iron nuggets is re-oxidized, the amount of FeO in the slag,
which is formed as a byproduct during the generation of the reduced
iron, increases and the ratio of sulfur amount (S) in the slag to
the sulfur amount [S] in the iron nuggets (hereinafter, referred to
as sulfur distribution ratio and represented by (S)/[S]). Where the
amount of sulfur in the iron nuggets increases, the quality of the
iron nuggets is degraded. Further, FeO in the slag reacts with
carbon [C] contained in the generated semi-molten iron and molten
iron, thereby causing decarburization and reducing the amount of
carbon in the iron nuggets. Following the decarburization reaction,
a large number of fine CO gas bubbles are present in the slag,
thereby causing significant expansion. As a result, intense slag
foaming occurs and the foam covers the iron nuggets in a
semi-molten state and molten state which is being aggregated. The
resultant problem is that heat supplied from the upper section of
the heating furnace is shielded, the reaction time is greatly
increased, and productivity is decreased. Yet another problem is
that where slag foaming is initiated, the iron nuggets assume an
irregular shape, the iron nuggets are insufficiently separated from
part of the slag, and iron nuggets quality is degraded.
[0005] The present invention has been created with the foregoing in
view and it is an objective thereof to provide a method for
manufacturing iron nuggets by which reduced iron obtained by
heating and reducing agglomerates, or iron nuggets obtained by
melting and aggregating the reduced iron can be prevented from
reoxidation inside a movable hearth heating furnace and quality of
the reduced iron can be improved.
[0006] In order to resolve the above-described problems, the
present invention provides a method for manufacturing iron nuggets
by charging and heating agglomerates including iron oxide and a
carbonaceous reducing agent on a hearth of a movable hearth heating
furnace, reducing and melting the iron oxide in the agglomerates,
and then discharging obtained iron nuggets to the outside of the
furnace and recovering the iron nuggets, wherein the agglomerates
have a coating layer, including a fluid carbonaceous material, on
the surface.
[0007] At least one selected from the group consisting of
bituminous coal, subbituminous coal, and lignite can be used as the
coal material. The average thickness of the coating layer is
preferably greater than 0.30 mm.
[0008] The agglomerates are obtained by agglomerating a mixture
including iron oxide and the carbonaceous reducing agent in a first
pelletizer to form core portions, and then forming the coating
layer including a fluid carbonaceous material in a second
pelletizer on the surface of the obtained core portions.
[0009] It is preferred that a top portion of the coating layer be
not lower than a top portion of the iron nuggets while the
agglomerates are heated.
[0010] It is preferred that the coating layer become a shell-shaped
coke while the agglomerates are heated. The agglomerates are
preferably charged to form a single layer on the furnace hearth. It
is preferred that the carbonaceous reducing agent be placed on the
furnace hearth before the agglomerates are charged on the furnace
hearth.
[0011] It is preferred that a C amount in the iron nuggets be 2.5
mass % or more. It is preferred that an S amount in the iron
nuggets be 0.120 mass % or less.
[0012] According to the present invention, when iron nuggets are
manufactured by heating, reducing, and melting the iron oxide
contained in agglomerates, the agglomerates are used which have a
coating layer including a fluid carbonaceous material on the
surface of core portions including iron oxide and a carbonaceous
reducing agent. Therefore, while the agglomerates are heated, the
coating layer is swelled and modified, the so-called coking
proceeds, and a petal-like shell-shaped coke is formed. The
shell-shaped coke acts as a windbreak wall that prevents the
atmosphere gas from oxidizing the core portions and protects the
core portions. As a result, the reoxidation of the reduced iron
obtained by heating and reducing the agglomerates, or the iron
nuggets obtained as a result of melting and aggregating the reduced
iron is suppressed and the increase in the FeO amount in a slag
which is a byproduct generated in the production of the iron
nuggets is suppressed. Therefore, the amount of sulfur contained in
the iron nuggets can be reduced and the quality of the reduced iron
can be increased. Furthermore, since the amount of FeO in the slag
does not increase, the decarburization of carbon [C] contained in
the produced semi-molten iron and molten iron can be suppressed and
the amount of carbon in the iron nuggets can be increased. In
addition intense slag foaming can be also prevented. Therefore, the
generation of iron nuggets of irregular shape can be prevented, the
separation of the iron nuggets and slag is improved, and the iron
nuggets quality can be improved.
BRIEF DESCRIPTION OF DRAWINGS
[0013] FIG. 1 is a schematic diagram illustrating the process of
heating the agglomerates charged on the hearth of the heating
furnace.
[0014] FIG. 2 is a schematic diagram illustrating the stage (4) in
FIG. 1 in greater detail.
[0015] FIGS. 3(1) to 3(3) are photographs of agglomerates taken at
the time of actual heating of the agglomerates in the heating
furnace.
[0016] FIG. 4(1) is a photograph, taken under an optical
microscope, of the cross section of the reduced iron recovered in
the later phase of solid reduction. FIG. 4(2) is a photograph
obtained by image processing of the photograph in FIG. 4(1).
[0017] FIG. 5(1) is a photograph, taken under an optical
microscope, of the cross section of the reduced iron recovered
immediately prior to melting and aggregation. FIG. 5(2) is a
photograph obtained by image processing of the photograph in FIG.
5(1).
[0018] FIG. 6(1) is a photograph illustrating a state in which the
iron nuggets after the completion of melting and aggregation are
covered by intensely foamed slag when the agglomerates of the
related art which have no coating layer are heated. FIG. 6(2) is a
photograph of the recovered iron nuggets. FIG. 6(3) is a photograph
of the recovered slag.
[0019] FIG. 7(1) is a photograph illustrating the state, after the
completion of melting and aggregation, of agglomerates obtained by
forming a coating layer including a fluid carbonaceous material on
the surface of the core portion. FIG. 7(2) is a photograph of the
recovered iron nuggets. FIG. 7(3) is a photograph of the recovered
slag.
[0020] FIGS. 8(1) to 8(4) are schematic diagrams illustrating the
cross section of the petal-like shell-shaped coke formed while
heating the agglomerates when the thickness of the coating layer
was changed.
[0021] FIG. 9(1) is a photograph taken immediately after the
heating and reduction treatment of No. 4 in Table 5. FIG. 9(2) is a
photograph taken immediately after the heating and reduction
treatment of No. 5 in Table 5. FIG. 9(3) is a photograph taken
immediately after the heating and reduction treatment of No. 6 in
Table 5.
DESCRIPTION OF EMBODIMENTS
[0022] The inventors have conducted a comprehensive research aimed
at the prevention of the reoxidation of iron nuggets obtained by
heating, reducing, and melting iron oxide contained in
agglomerates, and at the quality improvement of iron nuggets. In
particular, the research was focused on reducing the amount of
sulfur contained in the iron nuggets, increasing the amount of
carbon contained in the iron nuggets, preventing the occurrence of
iron nuggets of irregular shape, and improving the separation of
the iron nuggets and slag. The results obtained have demonstrated
that the abovementioned problems can be resolved by using
agglomerates having a coating layer including a fluid carbonaceous
material on the surface of core portions including iron oxide and a
carbonaceous reducing agent. This finding led to the creation of
the present invention.
[0023] Initially, the mechanism preventing the reoxidation of iron
nuggets and improving the iron nuggets quality in the method for
manufacturing iron nuggets in accordance with the present invention
will be explained with reference to the drawings. FIG. 1 is a
schematic diagram illustrating the process of heating the
agglomerates charged on the hearth of the heating furnace. FIG. 2
is a schematic diagram illustrating the stage (4) in FIG. 1 in
greater detail. FIG. 3(1) is a photograph illustrating the swelling
and coking state of the coating layer immediately after the
agglomerates have been charged into the heating furnace. FIGS. 3(2)
and 3(3) are photographs of the petal-like shell-shaped coke, iron
nuggets inside thereof, and slag grains, the photographs being
taken after the agglomerates having a coating layer on the surface
of core portions have been heated, melted, and aggregated.
[0024] In the manufacturing method of the present invention, the
agglomerates having a coating layer including a fluid carbonaceous
material on the surface of core portions including iron oxide and a
carbonaceous reducing agent are charged onto the hearth of a
movable hearth heating furnace and heated. The schematic diagram of
the agglomerates charged into the heating furnace is depicted in
FIG. 1(1). In FIG. 1(1), the reference numeral 1 stands for a core
portion, 2--a coating layer, and 3--an agglomerate.
[0025] The core portion 1 includes iron oxide and a carbonaceous
reducing agent and may optionally include a flux or a binder. The
composition of the core portion 1 is the same as in the related art
and will be described hereinbelow in detail.
[0026] The coating layer 2 includes a fluid carbonaceous material
and may optionally include a binder. The composition and thickness
of the coating layer 2 will be described hereinbelow in detail.
[0027] Inside the movable hearth heating furnace, the temperature
is usually raised and held at about 1350.degree. C. to 1550.degree.
C. with a heating burner. Where the agglomerate 3 is charged on the
hearth of the heating furnace, the agglomerate 3 is heated by gas
heat transfer and radiation heat created by the heating burner. At
this time, the coating layer 2 is fluidized, swells as a whole, as
depicted in FIG. 1(2), rapidly solidifies, and forms a shell-shaped
coke. Cracks are initiated at the top portion of the shell-shaped
coke, but the coke is continuous as a whole and forms a
shell-shaped spherical body.
[0028] The state in which the coating layer 2 is swelled by heating
and cracks are initiated in the coating layer 2 in the top portion
of the agglomerate is depicted in FIG. 3(1). As indicted in FIG.
3(1), a large number of cracks have appeared in the coating layer
2, but it is continuous as a whole and forms a shell-shaped
spherical body. Since the shell-shaped spherical body is
constituted by solid coke, it excels in thermal conductivity.
Therefore, where the shell-shaped spherical body is heated by
radiation heat inside the heating furnace, the core portion 1 is
also heated by heat transfer.
[0029] Where the heating is continued, as depicted in FIG. 1(3),
the reduction of iron oxide under the effect of the carbonaceous
reducing agent proceeds in the core portion 1 and solid reduced
iron is formed. At this time, the reduction of iron oxide
constituting the core portion 1 proceeds from the top side of the
core portion 1 and reduced iron 4 is generated.
[0030] Where the heating is further continued, as depicted in FIG.
1(4), iron oxide constituting the core portion 1 is sufficiently
reduced and separated into iron nuggets 6 constituted by reduced
iron and slag 7 produced as a byproduct when the iron nuggets 6 are
generated. The state assumed at this time is depicted in FIG.
3(3).
[0031] Meanwhile, in the coating layer 2 that covers the core
portion 1, as depicted in FIG. 1(3) and FIG. 1(4), the shell-shaped
spherical body is formed around the core portion 1, and this
coating layer 2 is gradually oxidized, consumed, and thinned by the
oxidizing gas included in the atmosphere gas. At this time, the top
portion of the coating layer 2 is oxidized and consumed faster than
the bottom portion and is gradually lost. Therefore, when the
thickness of the coating layer decreases, an opening is formed in
the top portion, as depicted in FIG. 1(3) and FIG. 1(4). The
photograph taken at this time is presented in FIG. 3(2). As
depicted in FIG. 3(2), the shell-shaped coke formed by the coating
layer 2 in which the opening has been formed in the top portion has
assumed a petal-like shape. Where the coating layer becomes
thicker, the formed shell-shaped coke also becomes thicker, the
upper portion of the shell-shaped coke is not opened, and the
reaction is completed in a state in which the core portion is
enclosed in the coke. Therefore, it is clear that a more effective
action is produced to prevent the reoxidation induced by the
atmosphere gas. Both the case in which the upper portion of the
shell-shaped coke is open and the case in which it is not open are
included in the scope of the present invention.
[0032] As depicted in FIG. 1(4), the shell-shaped coke which is
derived from the coating layer 2 and in which an opening is formed
in the top portion is formed around the iron nuggets 6 such as to
enclose the iron nuggets 6. Therefore, the shell-shaped coke acts
to prevent the reduced iron, which has been obtained by heating and
reducing the core portion, and the iron nuggets, which has been
produced by melting and aggregating the reduced iron, from
reoxidation by the atmosphere gas inside the heating furnace. This
action will be explained hereinbelow in detail with reference to
FIG. 2.
[0033] The flow of the atmosphere gas is shown by arrows in FIG. 2.
With the manufacturing method in accordance with the present
invention, while the solid reduction of iron oxide contained in the
core portion 1 is completed and the melting and aggregation
advance, as depicted in FIG. 2, the shell-shaped coke derived from
the coating layer 2 is formed such as to enclose the reduced iron,
which has been obtained by heating and reducing the core portion,
or the iron nuggets, which has been produced by melting and
aggregating the reduced iron 6, and the slag 7. Therefore, the
atmosphere gas in the heating furnace is unlikely to come into
direct contact with the reduced iron, which has been obtained by
heating and reducing the agglomerates in which the coating layer is
present on the surface of the core portions, or the iron nuggets 6
produced by melting and aggregating the reduced iron. Further, the
atmosphere gas includes carbon dioxide gas (CO.sub.2 gas) and
moisture (H.sub.2O), and where the carbon dioxide gas comes into
contact with the shell-shaped coke derived from the coating layer
2, the carbon dioxide gas is reduced by the shell-shaped coke and,
as represented by Equation (1) below, carbon monoxide gas (CO gas)
is generated. Further, where the moisture contained in the
atmosphere gas comes into contact with the shell-shaped coke
derived from the coating layer 2, the moisture is reduced by the
shell-shaped coke and, as represented by Equation (2) below,
hydrogen gas (H.sub.2) and carbon monoxide gas (CO gas) are
generated. As a result, the reduction degree RD of the atmosphere
gas on the periphery of the shell-shaped coke derived from the
coating layer 2 increases, thereby preventing the reoxidation of
the reduced iron, which has been obtained by heating and reducing
the agglomerates in which the coating layer is present on the
surface of the core portions, or the iron nuggets 6 produced by
melting and aggregating the reduced iron. The reduction degree RD
of the atmosphere gas is determined by Equation (3) below.
CO.sub.2+C=2CO (1)
H.sub.2O+C=H.sub.2+CO (2)
RD=[(CO+H.sub.2)/(CO+H.sub.2+CO.sub.2+H.sub.2O)].times.100 (3)
[0034] According to the manufacturing method of the present
invention, while the agglomerates 2 in which the coating layer is
present on the surface of the core portions are heated, the reduced
iron, which has been obtained by heating and reducing the
agglomerates, or the iron nuggets, which have been produced by
melting and aggregating the reduced iron, is sufficiently protected
from an oxidizing gas by the shell-shaped coke derived from the
coating layer 2, and the reoxidation of the reduced iron or iron
nuggets can be prevented. While the agglomerates are heated, the
shell-shaped coke derived from the coating layer 2 assumes a
petal-like shape, and although the height of the shell-shaped coke
derived from the coating layer 2 is not constant and part of the
coke is lost, the effect of the present invention still can be
obtained.
[0035] By contrast, where only the core portions 1 which do not
have the coating layer including a fluid carbonaceous material is
heated in a heating furnace, as in the conventional process,
although the reaction of the core portions 1 as a whole advances in
the solid reduction phase, since the core portions 1 themselves are
directly exposed to the atmosphere gas, the reduced iron, which has
been obtained by heating and reducing the core portions 1, or the
iron nuggets, which has been produced melting and aggregating the
reduced iron, is partially re-oxidized by the oxidizing gas
contained in the atmosphere gas.
[0036] The photos of the reduced iron obtained by heating only the
core portion 1 which does not have the coating layer including a
fluid carbonaceous material are shown in FIGS. 4 and 5. FIG. 4 is a
photograph, taken under an optical microscope, of the cross section
of the reduced iron recovered in the later phase of solid
reduction. FIG. 5 is a photograph, taken under an optical
microscope, of the cross section of the reduced iron recovered
immediately prior to melting and aggregation. Further, in FIGS. 4
and 5, (1) are microscopic photographs of the cross sections, and
(2) are schematic diagrams which show, by shading, the reduced
portions and re-oxidized portions in the cross section depicted in
(1).
[0037] As depicted in FIGS. 4(2) and 5(2), it is clear that in the
upper portion of the reduced iron, part of the generated metallic
iron is re-oxidized into FeO.
[0038] The FeO generated by the reoxidation is rapidly melted in
the slag which is separated and generated in the melting and
aggregation phase, thereby increasing the concentration of FeO in
the slag. Further, when the FeO is melted in the slag, it reacts
with carbon [C]contained in the generated semi-molten iron and
molten iron, thereby causing decarburization. Therefore, a large
number of fine CO gas bubbles are present in the slag and the slag
is greatly expanded. As a result, intense slag foaming is induced
and the foam covers the semi-molten and molten iron nuggets which
is in the process of aggregation. The resultant problem is that
heat supplied from the upper section of the heating furnace is
blocked, the reaction time increases, and the productivity
decreases. Yet another problem arising when slag foaming occurs is
that the iron nuggets assume an irregular shape, the iron nuggets
and part of the slag cannot be sufficiently separated, and quality
of the iron nuggets is degraded. The oxidizing gas is generated by
combustion with the combustion burner which is used for heating in
the heating furnace, combustion of the combustible gas generated by
the reduction reaction, and leakage of the air from the outside
into the heating furnace.
[0039] Further, where agglomerates are used which have the coating
layer including a fluid carbonaceous material, as in the
manufacturing method of the present invention, since the reduced
iron, which has been obtained by heating and reducing the
agglomerates, or the iron nuggets, which has been produced by
melting and aggregating the reduced iron, is prevented from
reoxidation, and the iron nuggets and slag are individually melted
and fused and separated from each other in the melting and
aggregation phase. As a result, slag foaming is not induced.
[0040] As explained hereinabove, the most important feature of the
manufacturing method of the invention is that agglomerates are used
which have a coating layer including a fluid carbonaceous material
on the surface of core portions including iron oxide and a
carbonaceous reducing agent.
[0041] The fluid carbonaceous material, as referred to herein,
means a carbonaceous material demonstrating thermal softening
ability at 350.degree. C. to 400.degree. C. The "carbonaceous
material demonstrating thermal softening ability", as referred to
herein, means a carbonaceous material with a softening and melting
point of 350.degree. C. to 400.degree. C., when the softening and
melting point of carbonaceous material is measured by the method
specified by ISO 10329 (2009).
[0042] It is preferred that at least one coal selected from the
group consisting of fluid bituminous coal, fluid sub-bituminous
coal, and fluid brown coal be used as the fluid carbonaceous
material, and two or more of the same may be used. Among those
carbonaceous materials, bituminous coal is more preferred.
Carbonaceous materials also include anthracite, but anthracite is
not fluid. Therefore, even when anthracite is included in the
coating layer 2, a shell-shaped spherical body is not formed around
the iron nuggets. Therefore, the core portion is exposed to the
atmosphere gas in the heating furnace, and the reduced iron, which
has been obtained by heating and reducing the agglomerates, or the
iron nuggets, which has been produced by melting and aggregating
the reduced iron, is re-oxidized.
[0043] The average thickness of the coating layer 2 is not
particularly limited, but the thickness, for example, more than
0.30 mm is preferred. Where the average thickness of the coating
layer 2 is more than 0.30 mm, the effect of suppressing the
reoxidation of the iron nuggets is further enhanced and a
petal-like outer shell can be formed. Such a thickness also
effectively increases the strength of the coating layer 2 and also
increases the strength of the entire agglomerate. Where the average
thickness of the coating layer 2 is equal to or less than 0.30 mm,
the strength of the coating layer 2 decreases and the thickness of
the shell-shaped spherical body (i.e., petal-like coke) formed by
heating of the coating layer 2 decreases. As a result, the heating
process is accompanied by oxidation and consumption, and the shape
of the iron nuggets is difficult to maintain till the iron nuggets
are melted and aggregated. Therefore, the average thickness of the
coating layer 2 is more preferably 0.50 mm or more, even more
preferably 0.70 mm or more, and still more preferably 1.00 mm or
more. The upper limit of the average thickness of the coating layer
2 is not particularly limited, but where the thickness is too
large, the amount of carbonaceous material used increases, the
amount of iron contained in all of the agglomerates decreases, and
productivity decreases. Such a thickness is also cost inefficient.
Therefore, the average thickness of the coating layer 2 is
preferably 2.00 mm or less, more preferably 1.80 mm or less, and
still more preferably 1.50 mm or less.
[0044] The thickness of the coating layer 2 may be measured by
observing the cross section of the agglomerates under an optical
microscope.
[0045] Described hereinabove are the agglomerates which
characterize the manufacturing method of the present invention.
[0046] A method for manufacturing iron nuggets in accordance with
the present invention will be explained hereinbelow.
[0047] A method for manufacturing iron nuggets in accordance with
the present invention includes, in the order of description:
[0048] a step for agglomerating a mixture including iron oxide and
a carbonaceous reducing agent to form core portions (hereinafter
also referred to as a "core portion forming step");
[0049] a step for forming a coating layer including a fluid
carbonaceous material on the surface of the obtained core portions
(hereinafter also referred to as a "surface coating step");
[0050] a step for charging the obtained agglomerates on the hearth
of a movable hearth heating furnace, heating, and reducing and
melting the iron oxide in the agglomerates (hereinafter also
referred to as a "reducing and melting step"); and
[0051] a step for discharging the obtained iron nuggets to the
outside of the furnace and recovering the iron nuggets (hereinafter
also referred to as a "recovering step").
[0052] [Core Portion Forming Step]
[0053] In the core portion forming step, core portions of the
agglomerates are manufactured by agglomerating a mixture including
iron oxide and a carbonaceous reducing agent.
[0054] Specific examples of iron oxide sources that can be used as
the iron oxide include iron ore, iron sand, steelmaking dust,
non-ferrous smelting residue, and steelmaking wastes.
[0055] A carbon-containing reducing agent, for example, coal or
coke, can be used as the carbonaceous reducing agent. When coal is
used, fluid coal may be used or non-fluid coal may be used.
[0056] A flux may be additionally compounded with the
abovementioned mixture. The flux, as referred to herein, fuses
together with the gangue in the iron oxide source or with the ash
component in the carbonaceous reducing agent and adjusts the
melting point or fluidity of the final slag.
[0057] For example, a CaO-supplying substance, a MgO-supplying
substance, an Al.sub.2O.sub.3-supplying substance, a
SiO.sub.2-supplying substance, and fluorite (CaF.sub.2) can be used
as the flux. For example, at least one selected from the group
consisting of CaO (quicklime), Ca(OH).sub.2 (hydrated lime),
CaCO.sub.3 (limestone), and CaMg(CO.sub.3).sub.2 (dolomite) can be
used as the CaO-supplying substance. At least one selected from the
group consisting of CaMg(CO.sub.3).sub.2 (dolomite), a MgO powder,
Mg-containing substances extracted from natural ores or seawater,
and MgCO.sub.3 may be compounded as the MgO-supplying substance.
For example, an Al.sub.2O.sub.3 powder, bauxite, boehmite,
gibbsite, and diaspore can be compounded as the
Al.sub.2O.sub.3-supplying substance. For example, a SiO.sub.2
powder or quartz sand can be used as the SiO.sub.2-supplying
substance.
[0058] A binder may be further compounded as a component other than
the iron oxide, carbonaceous reducing agent, and flux to the
abovementioned mixture.
[0059] For example, a polysaccharide such as starch, e.g., corn
starch and wheat flour, can be used as the binder.
[0060] The flux is sometimes referred to hereinbelow as an
additive.
[0061] The iron oxide, carbonaceous reducing agent, and optionally
compounded additive and binder may be mixed using a mixer of a
rotary container type or a stationary container type.
[0062] The mixture obtained in the mixer is agglomerated to
manufacture core portions of agglomerates. The average diameter of
the core portions is not particularly limited, but recommended to
be, for example, 18 mm to 22 mm.
[0063] A first pelletizer which is used when agglomerating the
mixture can be, for example, a pan pelletizer, a cylindrical
pelletizer, a twin-roll briquette molding machine, and an
extruder.
[0064] The shape of the core portions is not particularly limited
and may be, for example, a pellet-like or briquette-like shape.
[0065] [Surface Coating Step]
[0066] In the surface coating step, a coating layer including a
fluid carbonaceous material is formed on the surface of the core
portions obtained in the core portion forming step.
[0067] When the coating layer is formed, a binder may be included
in addition to the fluid carbonaceous material. The above-described
binders can be used as the binder.
[0068] The binder included in the coating layer and the binder
included in the core portions may be of the same or different
types.
[0069] For example, a pan pelletizer or cylindrical pelletizer can
be used as a second pelletizer to be used when forming the coating
layer including a fluid carbonaceous material.
[0070] The first pelletizer and the second pelletizer may be of the
same or different types.
[0071] The size of the agglomerates in which the coating layer
including a fluid carbonaceous material is formed on the surface of
the core portions is not particularly limited, but it is preferred
that the maximum particle size be 50 mm or less. Where the particle
size of the agglomerates is too large, the granulation efficiency
is degraded. Further, where the agglomerates become too large, heat
transfer to the lower portions of the agglomerates is degraded and
productivity decreases. The lower limit value of the particle size
of the agglomerates is about 5 mm.
[0072] The agglomerates may be also dried by heating in the heating
furnace in the below-described reducing and melting step, but it is
recommended that the drying be performed before the reduction and
melting step. Further, the coating layer may be formed once the
core portions have been dried after the granulation, but it is
preferred that the drying be performed after the coating layer has
been formed on the surface of the core portions.
[0073] [Reducing and Melting Step]
[0074] In the reducing and melting step, the agglomerates obtained
in the surface coating step are charged onto the hearth of a
movable hearth heating furnace and heated, thereby reducing and
melting the iron oxide in the agglomerates and forming iron nuggets
constituted by reduced iron.
[0075] The movable hearth heating furnace, as referred to herein,
is a heating furnace in which the furnace hearth moves as a belt
conveyor inside the furnace. Examples of such furnaces include a
rotary hearth furnace and a tunnel furnace. In the rotary hearth
furnace, the external shape of the furnace hearth is designed in a
circular or donut shape such that the start point and end point of
the furnace hearth are at the same position, the iron oxide
contained in the agglomerates charged on the furnace hearth is
heated and reduced in one rotation cycle inside the furnace to
generate reduced iron which is then melted and aggregated to
generate iron nuggets and slag. Therefore, in the rotary hearth
furnace, a charging means is provided for charging the agglomerates
into the furnace on the upstreammost side in the rotation direction
and a discharging means is provided on the downstreammost side in
the rotation direction. Because of a rotating structure, the
upstreammost side is actually the directly upstream side of the
charging means. The tunnel furnace is a heating furnace in which
the furnace hearth moves in the linear direction inside the
furnace.
[0076] The agglomerates are preferably heated at a temperature of
1350.degree. C. or higher. Where the heating temperature is below
1350.degree. C., the reduced iron and slag are unlikely to melt and
a high productivity sometimes cannot be obtained. Therefore, it is
preferred that the heating temperature be 1350.degree. C. or
higher, more preferably 1400.degree. C. or higher. However, where
the heating temperature exceeds 15500.degree. C., the exhaust gas
temperature rises, large-scale exhaust gas treatment equipment
needs to be used and the equipment cost rises. Therefore, it is
preferred that the heating temperature be 1550.degree. C. or less,
more preferably 1500.degree. C. or less.
[0077] It is preferred that the agglomerates be charged in a single
layer on the furnace hearth. Where the agglomerates are charged on
the furnace hearth in two or more layers, the agglomerates in the
lower layer are not sufficiently heated, reducing and melting
thereof are insufficient, and iron nuggets are difficult to
produce. The one layer, as referred to herein, means that the
agglomerates are not stacked in the vertical direction with respect
to the furnace hearth, and gaps may be present between the
agglomerates in the transverse direction. Thus, the agglomerates
may be charged sparsely. Further, the agglomerates may partially
overlap each other, but such partial overlapping will not cancel
the effect of the present invention.
[0078] It is preferred that the carbonaceous reducing agent be laid
as a bedding material on the furnace hearth prior to charging the
agglomerates onto the furnace hearth. By laying the bedding
material, it is possible to protect the furnace hearth.
[0079] It is preferred that the particle size of the bedding
material be 3 mm or less so as to prevent the agglomerates or the
melt thereof from submerging. The lower limit of the particle size
of the bedding material is preferably 0.5 mm or more so as to
prevent the bedding material from being scattered by the combustion
gas of the burner.
[0080] [Recovering Step]
[0081] In the recovering step, the iron nuggets obtained in the
reducing and melting step is discharged to the outside of the
furnace, and the iron nuggets are recovered.
[0082] When the iron nuggets are discharged to the outside of the
furnace, since the slag generated as a byproduct and the bedding
material are included in addition to the iron nuggets, the iron
nuggets may be recovered outside the furnace by using, for example,
a sieve or a magnetic separator.
[0083] With the manufacturing method in accordance with the present
invention, it is possible to manufacture iron nuggets with a C
content of 2.5 mass % or more. Further, with the manufacturing
method in accordance with the present invention it is possible to
manufacture iron nuggets with an S content of 0.120 mass % or
less.
[0084] The present application claims priority to Japanese Patent
Application No. 2013-198980, filed on Sep. 25, 2013. The entire
contents of the description of Japanese Patent Application No.
2013-198980 are incorporated by reference in the present
application.
[0085] The present invention is explained in greater detail
hereinbelow on the basis of examples thereof, but the present
invention is not intended to be limited to those example, and it
goes without saying that the present invention can be implemented
by applying modifications within a range compatible with the
aforementioned and below-described spirit, and those modifications
are all included in the technical scope of the present
invention.
Examples
Experimental Example 1
[0086] In the present experimental example, agglomerates having on
the surface a coating layer including a fluid carbonaceous material
and agglomerates having no coating layer were prepared, the
agglomerates were heated in a heating furnace, and it was
investigated whether or not the reoxidation of the obtained iron
nuggets was suppressed.
[0087] Initially the agglomerates including iron oxide and a
carbonaceous reducing agent were manufactured.
[0088] An iron ore of the composition presented in Table 1 below
was used as the iron oxide. In Table 1, T. Fe means "total iron".
The iron ore to be used was ground such that the ore with a
particle size of 44 .mu.m or less constituted 67 mass %.
[0089] A carbonaceous material with the composition presented in
Table 2 was used as the carbonaceous reducing material. In Table 2,
T. C means "total carbon" and F. C means "fixed carbon". The
carbonaceous material to be used was ground such that the coal with
a particle size of 75 .mu.m or less constituted about 55 mass
%.
[0090] Then, a binder, an additive, and an appropriate amount of
water were compounded with the mixture including the iron ore and
carbonaceous material, followed by agglomeration in a first
pelletizer, and granulation into green pellets serving as core
portions. Wheat flour was used as the binder. Limestone, dolomite,
and fluorite were used as the additives. A pan pelletizer was used
as the first pelletizer. The average diameter of the green pellets
was 21 mm. The compounding ratio of the iron ore, carbonaceous
material, binder, and additives is shown in Table 3.
[0091] Some of the obtained green pellets were charged into a drier
and heated for about 1.0 h at 160.degree. C. to 180.degree. C. to
remove the adhered water, thereby producing spherical dry
pellets.
[0092] Other obtained green pellets were used, without drying, to
form a coating layer including a fluid carbonaceous material on the
surface thereof. Fluid bituminous coal was prepared as the fluid
carbonaceous material, the green pellets were charged into a second
pelletizer, a mixture prepared by mixing bituminous coal and a
small amount of binder (wheat flour) was then supplied into the
pelletizer, and a coating layer was formed on the surface of the
core portions. A pan pelletizer was used as the second pelletizer.
The green pellets obtained by forming the coating layer on the
surface of the core portions were cut, the cross section was
observed under an optical microscope, and it was confirmed that the
average thickness of the coating layer was 1.0 mm. The green
pellets in which the coating layer was formed on the surface were
then charged into a drier and heated for about 1.0 h at 160.degree.
C. to 180.degree. C. to remove the adhered water, thereby producing
spherical dry pellets (i.e., agglomerates).
[0093] Then, the spherical dry pellets in which the coating layer
was not formed and the spherical dry pellets in which the coating
layer was formed were charged into a heating furnace (test furnace)
maintained at about 1450.degree. C. and heated, and the iron oxide
in the dry pellets was reduced and melted.
[0094] The heating furnace had a highly oxidizing atmosphere to
simulate the actual furnace. More specifically, the oxidizing gas
was carbon dioxide, and the gas atmosphere in the furnace was a
mixed atmosphere including 40 vol % of carbon dioxide and 60 vol %
of nitrogen. As a result, when the dry pellets were charged into
the heating furnace, the coating layer was swelled, the
carbonaceous material contained in the coating layer was coked
around the core portions, and petal-like outer shells were formed.
The petal-like outer shells acted as windbreak walls preventing the
atmosphere gas from coming into contact with the core portions.
[0095] The iron nuggets obtained after the iron oxide was reduced
and melted in the heating furnace was discharged to the outside of
the furnace and the iron nuggets were recovered. At this time, the
slag which was generated as a byproduct when the iron nuggets were
produced was also recovered. The compositions of the obtained iron
nuggets and slag are presented in Table 4 below.
[0096] The ratio (sulfur distribution ratio) of the S amount (S)
contained in the slag to the S amount [S] contained in the iron
nuggets is also presented in Table 4 below.
[0097] The following conclusions can be made based on the results
shown in Table 4.
[0098] (No Coating Layer)
[0099] When the coating layer was not formed, the amount of FeO in
the slag was as large as 6.53 mass %, as shown in Table 4. As a
result, the sulfur distribution ratio was 1.56, the S amount
contained in the iron nuggets was 0.171 mass %, and the quality of
the iron nuggets could not be improved.
[0100] The reason why the amount of FeO in the slag has increased
is considered as follows. In the solid reduction phase, in the dry
pellet in which the coating layer has not been formed, although the
reduced iron is generated from the top of the pellet, part thereof
is re-oxidized by the oxidizing gas in the atmosphere
(Fe+CO.sub.2=FeO+CO), the generated FeO is melted in the molten
slag, and high-FeO molten slag is produced. As a result, in the
subsequent melting and reduction reaction within the melting and
aggregation phase, a reaction (decarburization reaction) is induced
between the FeO in the slag and [C] in the molten iron nuggets, and
an intense slag foaming effect is demonstrated. The resultant
drawback is that since the decarburization reaction is an
endothermic reaction, the transfer of heat to iron is greatly
delayed and the reaction time is significantly extended. Since the
foamed slag covers the semimolten iron in the course of aggregation
and blocks heat radiation from above, this is one more reason why
the heat transfer to iron is significantly delayed and the reaction
time is greatly extended. Although molten iron nuggets and molten
slag are eventually formed, since the slag is greatly foamed and a
high content of FeO in the slag is still maintained, [S] in the
produced iron nuggets becomes 0.171 mass %. Further, [C] in the
produced iron nuggets becomes 2.49 mass % which is less than the
target value 2.5 mass %. The resultant effect is that the quality
of iron nuggets serving as a product is greatly degraded. Another
adverse result is that the intense slag foaming prevents the
temperature of the iron nuggets from rising, the complete
aggregation does not take place within a predetermined reaction
time, iron nuggets of irregular shape which has taken in part of
the slag at a high ratio is formed, and from the standpoint of
shape, the value of the iron nuggets as a product is greatly
reduced.
[0101] FIG. 6(1) is a photograph of iron nuggets after completion
of melting and aggregation. FIG. 6(2) is a photograph of the
recovered iron nuggets. FIG. 6(3) is a photograph of the recovered
slag.
[0102] In the present experimental example, a sufficient reaction
time (i.e., in-furnace residence time) is ensured. As a result, it
is clear that, as depicted in FIG. 6(2), although the recovered
iron nuggets has irregular shape, the iron nuggets and slag are
separated. However, from the standpoint of productivity of the
actual equipment, it is difficult to ensure a sufficient in-furnace
residence time and the product actually needs to be discharged to
the outside of the furnace before the aggregation is entirely
completed. The unavoidable consequences include further
deterioration of iron nuggets shape and decrease in iron nuggets
product quality and yield which occurs because the product is
discharged to the outside of the furnace in a state in which the
separation of part of the slag and metal is insufficient.
[0103] (With Coating Layer)
[0104] Where the dry pellets on which the coating layer has been
formed are charged into the heating reaction field, the
carbonaceous material contained in the coating layer is rapidly
coked. A very characteristic effect confirmed at this time is that
large cracks appear in the coating layer, but the coating layer
does not peel off or fall down, and as depicted in FIG. 2 a coke
wall is formed and the core portion is enclosed therein. It has
been found that this coke wall advances the reduction reaction of
the iron oxide contained in the core portion, and while the upper
part thereof is gradually oxidized and consumed by the atmosphere
gas (C+CO.sub.2=2CO, C+H.sub.2O=CO+H.sub.2), the coke wall can
assume a petal shape with the removed upper portion. Furthermore,
where the solid reduction is completed, the iron nuggets, which
have been generated inside the core portion, and other oxides are
aggregated while melting in the bottom portion inside the
petal-like coke wall, the molten iron nuggets and molten slag are
separated, and the reaction is completed.
[0105] Thus, the petal-like coke wall plays a very significant role
of protecting the core portion from the oxidizing atmosphere gas,
and a significant difference is confirmed with the reaction
behavior of the conventional dry pellets, in which the coating
layer is not formed, in that the reaction of the core portion is
completed while the reoxidation thereof by the atmosphere gas is
significantly suppressed over substantially the entire phase from
the solid reaction phase to the melting and aggregation phase.
[0106] FIG. 7(1) is a photograph illustrating the state after the
completion of melting and aggregation. FIG. 7(2) is a photograph of
the recovered iron nuggets. FIG. 7(3) is a photograph of the
recovered slag.
[0107] As a result, as shown in FIGS. 7(2) and 7(3), the method of
the present invention makes it possible to obtain iron nuggets of
substantially the same shape and also ensures good separation from
the slag which is also recovered. Further, as indicted in Table 4,
the amount of FeO contained in the slag is 0.29 mass %, and it is
clear that the reoxidation of the iron nuggets is suppressed. In
addition, the sulfur distribution ratio is 14.64, and the amount of
S contained in the iron nuggets has been reduced to 0.059 mass
%.
TABLE-US-00001 TABLE 1 Composition of iron ore (mass %) T. Fe FeO
SiO.sub.2 CaO Al.sub.2O.sub.3 MgO MnO TiO.sub.2 63.28 1.38 5.39
0.08 0.70 0.05 0.67 0.12
TABLE-US-00002 TABLE 2 Composition of carbonaceous material (mass
%) T. C F. C Volatiles Ash 86.18 77.96 15.46 6.58
TABLE-US-00003 TABLE 3 Compounding ratio (mass %) Iron ore
Carbonaceous material Binder Additive 70.23 16.36 1.10 12.31
TABLE-US-00004 TABLE 4 Composition of Composition of slag iron
nuggets (mass %) (mass %) (S)/ C Si Mn S T. Fe FeO S [S] No Coating
2.49 0.03 0.04 0.171 5.95 6.53 0.267 1.56 layer With Coating 3.45
0.07 0.44 0.059 1.35 0.29 0.864 14.64 layer
Experimental Example 2
[0108] In the present experimental example, the agglomerates were
manufactured by changing the thickness of the coating layer formed
on the surface of core portions, the produced agglomerates were
heated in a heating furnace, and it was investigated whether or not
the reoxidation of the obtained iron nuggets was suppressed.
[0109] Initially, green pellets in which the coating layer was
formed on the surface of core portions were manufactured according
to the procedure of Experimental Example 1 by changing the
thickness of the coating portion. The core portions on which the
coating layer was formed were cut and the thickness of the coating
layer was checked by observing the cross section under an optical
microscope. As a result, the average thickness of the coating layer
was 0.30 mm to 2.00 mm.
[0110] The green pellets obtained by forming the coating layer on
the surface of the core portions were charged into a drier and
heated for about 1.0 h at 160.degree. C. to 180.degree. C. to
remove the adhered water, thereby producing spherical dry pellets
(i.e., agglomerates).
[0111] Then, the spherical dry pellets were charged into a heating
furnace (test furnace) maintained at about 1450.degree. C. and
heated, and the iron oxide in the dry pellets was reduced and
melted. The heating furnace had a highly oxidizing atmosphere to
simulate the actual furnace. More specifically, it was a mixed
atmosphere including 40 vol % of carbon dioxide and 60 vol % of
nitrogen. As a result, when the dry pellets were charged into the
heating furnace, the coating layer swelled, the carbonaceous
material contained in the coating layer was coked around the core
portions, and petal-like outer shells were formed. The height of
the petal-like outer shell differed among the samples, but all of
the outer shells still acted as windbreak walls preventing the
atmosphere gas from coming into contact with the core portions.
[0112] The iron nuggets obtained after the iron oxide was reduced
and melted in the heating furnace was discharged to the outside of
the furnace and the iron nuggets was recovered. At this time, the
slag which was generated as a byproduct when the iron nuggets were
produced was also recovered. The compositions of the obtained iron
nuggets and slag are presented in Table 5 below.
[0113] Meanwhile, as a comparison example, Table 5 presents the
results obtained when spherical dry pellets were manufactured by
directly charging green pellets, in which the coating layer was not
formed on the surface, into the drier and drying under the
conditions same as those when the coating layer was formed on the
surface.
[0114] The obtained spherical dry pellets were heated under the
conditions same as those when the coating layer was formed on the
surface, and the iron oxide in the dry pellets was reduced and
melted. The compositions of the obtained iron nuggets and slag are
presented in Table 5 below.
[0115] The ratio (sulfur distribution ratio) of the S amount (S)
contained in the slag to the S amount [S] contained in the iron
nuggets is also presented in Table 5 below.
[0116] The following conclusions can be made based on the results
as shown in Table 5.
[0117] In No. 8, the coating layer was not formed on the surface of
the core portions. Therefore, the reoxidation of the iron nuggets
obtained by reduction could not be prevented, the amount of FeO
contained in the slag increased to 6.53 mass %, and the sulfur
distribution ratio decreased to 1.56. As a result, the amount of S
contained in the iron nuggets increased to 0.171 mass % and the
quality of the iron nuggets could not be improved.
[0118] By contrast, in Nos. 1 to 7, the coating layer was formed on
the surface of the core portions. Therefore, the reduced iron
obtained by reduction of the iron oxide contained in the
agglomerates, or the iron nuggets was prevented from the
reoxidation in the heating furnace, the amount of FeO contained in
the slag reduced to 0.18 mass % to 2.23 mass %, and the sulfur
distribution ratio increased to 41.64 to 2.96. As a result, the
amount of S contained in the iron nuggets decreased to 0.022 mass %
to 0.139 mass %, and the quality of the iron nuggets could be
improved. Further, as clearly indicated in Table 5, the amount of
FeO contained in the slag tends to decrease and the sulfur
distribution ratio tends to increase with the increase in the
thickness of the coating layer. Therefore, it is clear that the
amount of S contained in the iron nuggets can be decreased by
increasing the thickness of the coating layer. In particular, in
Nos. 1 to 6, the amount of S contained in the iron nuggets could be
suppressed to 0.120 mass % or less.
[0119] Meanwhile, in No. 8 in which the coating layer was not
formed on the surface of the core portions, the amount of carbon
contained in the iron nuggets was as low as 2.49 mass %, whereas in
No. 1 to 7 in which the coating layer was formed on the surface of
the core portion, the amount of carbon contained in the iron
nuggets increased to 2.65 mass % to 3.52 mass %, and it is clear
that the quality of the iron nuggets can be improved by forming the
coating layer on the surface of the core portions.
[0120] It was also understood that a large height of the petal-like
outer shell formed after the heating and reduction treatment tends
to be maintained as the average thickness of the coating layer
increases.
[0121] FIGS. 8(1) to 8(4) are schematic diagrams illustrating the
height of the petal-like wall surface which has been formed while
heating the agglomerates and remained after the iron nuggets have
been obtained when the thickness of the coating layer was changed.
FIG. 8(1) illustrates the case in which the average thickness of
the coating layer was, for example, 1.30 mm to 2.00 mm. FIG. 8(2)
illustrates the case in which the average thickness of the coating
layer was, for example, 0.80 mm to 1.20 mm. FIG. 8(3) illustrates
the case in which the average thickness of the coating layer was,
for example, 0.60 mm to 0.80 mm. FIG. 8(4) illustrates the case in
which the average thickness of the coating layer was, for example,
more than 0.30 mm and equal to or less than 0.50 mm. In FIG. 8, the
reference numeral 2 stands for the coating layer, 6--iron nuggets,
7--slag.
[0122] FIG. 9(1) is a photograph taken immediately after the
heating and reduction treatment of No. 4 in Table 5. FIG. 9(2) is a
photograph taken immediately after the heating and reduction
treatment of No. 5 in Table 5. FIG. 9(3) is a photograph taken
immediately after the heating and reduction treatment of No. 6 in
Table 5.
[0123] In No. 7 in which the average thickness of the coating layer
was 0.30 mm, small-scale slag foaming has occurred, but in No. 6 in
which the average thickness of the coating layer was 0.50 mm, no
slag foaming has occurred. Meanwhile, in No. 8 in which the coating
layer was not formed on the surface of the core portions, very
intense slag foaming has occurred.
TABLE-US-00005 TABLE 5 Thick- Composition of Composition of slag
ness iron nuggets (mass %) (mass %) (S)/ No. (mm) C Si Mn S T. Fe
FeO S [S] 1 2.00 3.49 0.08 0.48 0.022 1.38 0.18 0.916 41.64 2 1.50
3.51 0.08 0.48 0.034 1.36 0.19 0.905 26.62 3 1.30 3.52 0.08 0.47
0.042 1.32 0.21 0.890 21.19 4 1.00 3.45 0.07 0.44 0.059 1.35 0.29
0.864 14.64 5 0.70 3.10 0.04 0.33 0.080 1.33 0.49 0.634 7.93 6 0.50
3.01 0.04 0.21 0.117 1.78 1.16 0.513 4.38 7 0.30 2.65 0.03 0.12
0.139 2.50 2.23 0.411 2.96 8 0 2.49 0.03 0.04 0.171 5.95 6.53 0.267
1.56
Experimental Example 3
[0124] In the present experimental example, the agglomerates were
manufactured by using a non-fluid carbonaceous material for
including in the coating layer to be formed on the surface of the
core portions, the agglomerates were heated in the heating furnace,
and it was examined whether or not the reoxidation of the obtained
iron nuggets was suppressed.
[0125] Initially, green pellets in which the coating layer with an
average thickness of 0.50 mm was formed on the surface of the core
portions were manufactured according to the procedure of
Experimental Example 1. In this case, anthracite was used as a
non-fluid carbonaceous material instead of the fluid bituminous
coal. The composition of the anthracite is presented in Table 6
below.
[0126] The green pellets obtained by forming the coating layer on
the surface were charged into a drier and heated for about 1.0 h at
160.degree. C. to 180.degree. C. to remove the adhered water,
thereby producing spherical dry pellets (i.e., agglomerates).
[0127] Then, the spherical dry pellets on which the coating layer
was not formed and the spherical dry pellets on which the coating
layer was formed were charged into a heating furnace (test furnace)
maintained at about 1450.degree. C. and heated, and the iron oxide
in the dry pellets was reduced and melted.
[0128] The heating furnace had a highly oxidizing atmosphere to
simulate the actual furnace. More specifically, it was a mixed
atmosphere including 40 vol % of carbon dioxide and 60 vol % of
nitrogen.
[0129] As a result, when the dry pellets were charged into the
heating furnace, the coating layer swelled, but cracked in a
tortoise shell shape and deposited on the core portions as thin
debris. As a result, the petal-like outer shell constituted by coke
could not be formed. The debris deposited on the core portions was
falling down on the periphery of the core portions with the passage
of time, and the top of the core portions was exposed to the
atmosphere gas.
[0130] The iron nuggets obtained after the iron oxide was reduced
and melted in the heating furnace was discharged to the outside of
the furnace and the iron nuggets were recovered. At this time, the
slag which was generated as a byproduct when the iron nuggets were
produced was also recovered. The compositions of the obtained iron
nuggets and slag are presented in Table 7 below.
[0131] The ratio (sulfur distribution ratio) of the S amount (S)
contained in the slag to the S amount [S] contained in the iron
nuggets is also presented in Table 7 below.
[0132] The following conclusions can be made on the basis of Table
7. Even in the case in which the coating material is formed on the
surface of the core portions, when the carbonaceous material
contained in the coating layer is not fluid, the reoxidation of the
reduced iron obtained by heating and reducing the agglomerates, or
the iron nuggets produced by melting and agglomeration of the
reduced iron cannot be prevented and the amount of FeO contained in
the slag cannot be reduced. As a result, the sulfur distribution
ratio decreases, the amount of sulfur contained in the iron nuggets
increases, and quality cannot be improved.
TABLE-US-00006 TABLE 6 Composition of anthracite (mass %) T. C F. C
Volatiles Ash 79.59 79.69 4.35 15.96
TABLE-US-00007 TABLE 7 Composition of iron Composition of slag
Thickness nuggets (mass %) (mass %) (mm) C Si Mn S T. Fe FeO S
(S)/[S] 0.50 2.64 0.03 0.13 0.147 2.18 2.34 0.404 2.75
LIST OF REFERENCE NUMERALS
[0133] 1 core portion [0134] 2 coating layer [0135] 3 agglomerates
[0136] 4 reduced iron [0137] 6 iron nuggets [0138] 7 slag
* * * * *