U.S. patent application number 11/296320 was filed with the patent office on 2006-04-06 for granular metallic iron.
This patent application is currently assigned to MIDREX INTERNATIONAL B. V. Zurich Branch. Invention is credited to Keisuke Honda, Shuzo Ito, Shoichi Kikuchi, Isao Kobayashi, Yasuhiro Tanigaki, Koji Tokuda, Osamu Tsuge.
Application Number | 20060070495 11/296320 |
Document ID | / |
Family ID | 18991356 |
Filed Date | 2006-04-06 |
United States Patent
Application |
20060070495 |
Kind Code |
A1 |
Ito; Shuzo ; et al. |
April 6, 2006 |
Granular metallic iron
Abstract
Metallic iron nuggets made by reducing-melt of a material
containing a carbonaceous reductant and a metal-oxide-containing
material, the metallic iron nuggets comprising at least 94% by
mass, hereinafter denoted as "%", of Fe and 1.0 to 4.5% of C, and
having a diameter of 1 to 30 mm are disclosed.
Inventors: |
Ito; Shuzo; (Hyogo, JP)
; Tanigaki; Yasuhiro; (Hyogo, JP) ; Kobayashi;
Isao; (Hyogo, JP) ; Tsuge; Osamu; (Hyogo,
JP) ; Honda; Keisuke; (Hyogo, JP) ; Tokuda;
Koji; (Hyogo, JP) ; Kikuchi; Shoichi; (Hyogo,
JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
MIDREX INTERNATIONAL B. V. Zurich
Branch
Zurich
CH
|
Family ID: |
18991356 |
Appl. No.: |
11/296320 |
Filed: |
December 8, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10332951 |
Jan 14, 2003 |
|
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PCT/JP02/04677 |
May 15, 2002 |
|
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11296320 |
Dec 8, 2005 |
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Current U.S.
Class: |
75/503 |
Current CPC
Class: |
Y02P 10/134 20151101;
C21B 13/008 20130101; C21B 2100/42 20170501; C21B 13/0006 20130101;
C22C 37/10 20130101; C21B 13/0046 20130101; F27B 9/39 20130101;
B22F 2998/10 20130101; C21B 13/105 20130101; F27B 9/16 20130101;
C22C 37/00 20130101; B22F 9/06 20130101; B22F 2999/00 20130101;
C22C 38/02 20130101; Y02P 10/136 20151101; B22F 2998/10 20130101;
B22F 9/20 20130101; B22F 9/06 20130101; B22F 2999/00 20130101; B22F
9/06 20130101; B22F 2201/30 20130101 |
Class at
Publication: |
075/503 |
International
Class: |
C21B 15/00 20060101
C21B015/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 15, 2001 |
JP |
2001-145566 |
Claims
1-7. (canceled)
8. A method of making metallic iron nuggets, the method comprising
mixing together a carbonaceous reductant and a metal oxide to form
a mixture; and heating the mixture to produce the metallic iron
nuggets, wherein almost all of the metallic iron nuggets produced
by the heating of the mixture have a diameter in a range of from 1
to 30 mm and comprise at least 94 mass % of metallic Fe and 1.0 to
4.5 mass % of C.
9. The method according to claim 8, wherein 98% or more of the
metallic iron nuggets produced by the heating of the mixture have a
diameter in a range of from 1 to 30 mm and comprise at least 94
mass % of metallic Fe and 1.0 to 4.5 mass % of C.
10. The method according to claim 8, wherein all of the metallic
iron nuggets produced by the heating of the mixture have a diameter
in a range of from 1 to 30 mm and comprise at least 94 mass % of
metallic Fe and 1.0 to 4.5 mass % of C.
11. The method according to claim 8, wherein the metallic iron
nuggets further comprise 0.20 mass % or less of sulfur.
12. The method according to claim 8, wherein the metallic iron
nuggets further comprise 0.02 to 0.5 mass % Si and less than 0.3
mass % Mn.
13. The method according to claim 8, wherein the heating reduces
the metal oxide in a solid state to form a reduced iron; and the
method further comprises heating the reduced iron in a reducing
atmosphere to carburize and melt the reduced iron.
14. The method according to claim 8, further comprising adding a
CaO source to the mixture to adjust the basicity (CaO/SiO.sub.2) of
a slag component in the mixture to 0.6 to 1.8.
15. The method according to claim 8, wherein the metallic iron
nuggets further comprise less than 0.08 mass % of sulfur.
16. The method according to claim 8, further comprising adjusting
an amount of the carbonaceous reductant in the mixture so that the
residual carbon content at a metallization ratio of 100% after
reduction by heating is 1.5 to 5.0%.
17. The method according to claim 10, wherein the metallic iron
nuggets have a uniform diameter.
18. The method according to claim 8, wherein 98% or more of the
metallic iron nuggets produced by the heating of the mixture have a
diameter in a range of from 5 to 30 mm and comprise at least 94
mass % of metallic Fe and 1.0 to 4.5 mass % of C.
Description
TECHNICAL FIELD
[0001] The present invention relates to metallic iron nuggets made
by reducing-melt of a material containing iron oxide, such as iron
ore, and a carbonaceous reductant, such as coke, the metallic iron
nuggets having a high Fe purity, specified C, S, Si, and Mn
contents, and a specified diameter.
BACKGROUND ART
[0002] A direct iron-making process for making reduced iron by
direct reduction of an iron oxide source such as iron ore using a
carbonaceous substance or a reducing gas has long been known.
Extensive research has been conducted as to the specifics of the
reducing process and continuous reduction equipment.
[0003] For example, Japanese Unexamined Patent Application
Publication No. 11-337264 discloses a rotary hearth that allows
efficient continuous production of reduced iron, in which, during
reduction by heating of green pellets prepared by solidifying a
mixture of an iron oxide source such as steelmaking dust or fine
ore and a carbonaceous substance using a binder, explosions which
occur when undried green pellets are rapidly heated are prevented
due to installation of a preheating zone.
[0004] In the technology, including the above-described technology,
for making metallic iron by heating and reducing compacts
containing an iron oxide source and a reductant, a considerable
amount of a slag component becomes mixed in the resulting metallic
iron due to the use of the iron ore or the like. In particular, in
a method for making sponge metallic iron, the Fe purity is
drastically low because the separation of the slag component that
became mixed in the metallic iron is difficult. Thus, a preliminary
treatment for removing this considerable amount of slag component
is required when these materials are used as an iron source.
Moreover, nearly all of the metallic iron obtained by a known
direct iron-making process is sponge-shaped, and thus the handling
thereof as an iron source is difficult since such metallic iron is
fragile. In order to actually use such metallic iron as a material
for making iron, steel, or alloy steel, a process such as a
secondary process to make briquettes therefrom is required, and the
expenses for additional equipment therefor are considerable.
[0005] Japanese Unexamined Patent Application Publication No.
9-256017 discloses a method for making metallic iron nuggets having
a high metallization ratio, the method including heating and
reducing compacts containing iron oxide and a carbonaceous
reductant until a metallic iron sheath is formed and substantially
no iron oxide is present in the inner portion while forming nuggets
of the produced slag in the inner portion, continuing heating so as
to allow the slag inside to flow outside of the metallic iron
sheath so as to separate the slag, and further performing heating
so as to melt the metallic iron sheath.
[0006] In the known processes, including these conventional
techniques, for making metallic iron nuggets, no technology capable
of efficiently making metallic iron having a diameter within a
predetermined range while fully considering the quality and
handling convenience of materials for making iron, steel, or iron
alloy has been established. As for the purity of the metallic iron
nuggets, although high-purity metallic iron nuggets with a low
contaminant content are naturally preferred, no specific idea for
specifying the optimum carbon content in the metallic iron nuggets
used as the material for iron making and steelmaking has been
formulated. Moreover, no specific manufacturing technology for
controlling the carbon content within a predetermined range has
been established.
[0007] Furthermore, when metallic iron is made by reducing iron
oxide such as ore, coke or a coal powder is generally used as the
reductant. However, these reductants normally have a high sulfur
(S) content. Since the reductant becomes mixed in the metallic iron
produced, the resulting metallic iron nuggets normally have a high
S content. Accordingly, the metallic iron nuggets must be subjected
to desulfurization before they are actually used as the material
for making iron or steel. This is also one of the main reasons for
the degradation in quality of the metallic iron nuggets.
[0008] Accordingly, in order to make metallic iron nuggets of high
value by a reducing-melt process, it is not sufficient to merely
hope to increase the purity. A technology that can reliably make
metallic iron, in which the contaminant content, such as a sulfur
content, is specified and the size of which is optimized in view of
production possibility and handling quality, the technology also
being capable of satisfying the demands in the market such as a
greater flexibility in the choice of material for making iron,
steel, or various alloy steels, and reduction of the cost required
for making iron or steel using, for example, an electric furnace,
is required to be established.
[0009] The present invention is developed based on the
above-described background. An object of the present invention is
to provide metallic iron nuggets of stable quality that have an
optimum size in view of the overall production possibility and
handling quality as an iron source, and in which the contaminant
content of the metallic iron nuggets, such as carbon and sulfur
contents, is specified. The metallic iron nuggets of the present
invention can thus satisfy the demands in the market such as a
greater flexibility in the choice of material for making metallic
iron and a reduction of the cost required for making iron or steel
using, for example, an electric furnace.
DISCLOSURE OF INVENTION
[0010] Metallic iron nuggets of the present invention that overcome
the above-described problems are metallic iron nuggets having an Fe
content of 94% (percent by mass, contents of components are in
terms of percent by mass) or more, a C content of 1.0 to 4.5%, a S
content of 0.20% or less, and a diameter of 1 to 30 mm, the
metallic iron nuggets being made by reducing-melt of a material
containing a carbonaceous reductant and an iron-oxide-containing
material.
[0011] The metallic iron nuggets of the present invention need not
be spherical. Granular substances having an elliptical shape, an
oval shape, and slightly deformed shapes thereof are also included
in the metallic iron nuggets of the present invention. The diameter
of the nuggets ranging from 1 to 30 mm is determined by dividing
the total of the lengths of the major axis and the minor axis and
the maximum and minimum thicknesses of a nugget by 4.
[0012] Preferably, the metallic iron nuggets further include 0.02
to 0.50% of Si and less than 0.3% of Mn.
[0013] The metallic iron nuggets are prepared by heating the
material so as to react a metal oxide contained in the material
with the carbonaceous reductant and a reducing gas produced by such
a reaction and to reduce the metal oxide in the solid state, and
further heating the resulting reduced iron in a reducing atmosphere
so as to carburize and melt the resulting reduced iron and allow
the reduced iron to cohere while excluding any by-product slag.
During this process, a CaO source is added to the material to
adjust the basicity of the slag components in the material, i.e.,
CaO/SiO.sub.2, within the range of 0.6 to 1.8. In this manner,
sulfur contained in the material can be efficiently captured by the
slag produced during reducing-melt, and metallic iron nuggets
having a S content of 0.08% or less can be obtained.
[0014] The amount of the carbonaceous reductant is adjusted so that
the remaining carbon content during the step of reducing-melt of
the material is in the range of 1.5 to 5.0% when the metallization
ratio of the metallic iron nuggets after the solid reduction is
100%. In this manner, the resulting carbon content can be
controlled within the above-described range.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is an explanatory schematic view showing an example
of reducing-melt equipment for making metallic iron nuggets of the
present invention.
[0016] FIG. 2 is a cross-sectional view taken along line A-A in
FIG. 1.
[0017] FIG. 3 is an explanatory cross-sectional view in which FIG.
1 is developed in the longitudinal direction.
[0018] FIG. 4 is a graph showing the transitions of the atmosphere
temperature, the temperature of material compacts, the reduction
ratio, and the amount of CO and CO.sub.2 gasses throughout a
solid-reduction period and a melting period when a two-stage
heating process is employed in the present invention.
[0019] FIG. 5 is a graph showing the transitions of the residual Fe
content and the metallization ratio of the metal oxide in the
material compacts throughout the solid-reduction period and the
melting period.
[0020] FIG. 6 is a graph showing the relationship between the
residual carbon content in the reduced iron when the metallization
ratio is 100% and the residual carbon content of the end product
metallic iron nuggets.
[0021] FIG. 7 is a graph showing the relationship between the
metallization ratio and the reducing degree.
[0022] FIG. 8 is a graph showing a change in the reducing degree of
an atmosphere gas and in the temperature of the interior of the
material compacts when a coal powder is used as an atmosphere
adjustor and when the coal powder is not used as an atmosphere
adjustor.
[0023] FIG. 9 is a photograph showing the state of metallic iron
and slag immediately after carburization and melting obtained by a
manufacturing experiment.
[0024] FIG. 10 is an experimental graph demonstrating that the
sulfur content of the metallic iron nuggets can be decreased by
adjusting the basicity of the slag by intentionally adding a CaO
source to material compacts.
[0025] FIG. 11 is a graph showing the relationship between the
sulfur content of the metallic iron nuggets and the basicity of the
product slag.
[0026] FIG. 12 is an explanatory diagram showing the composition of
the material, and the ratio and the composition of the products
such as metallic iron nuggets produced by a manufacturing process
employed in Example.
[0027] FIG. 13 is a photograph of metallic iron nuggets prepared in
Example 1.
[0028] FIG. 14 is an explanatory diagram showing the composition of
the material, and the ratio and the composition of the products
such as metallic iron nuggets produced by a manufacturing process
employed in another Example.
[0029] FIG. 15 is a photograph of metallic iron nuggets prepared in
Example 2.
[0030] FIG. 16 is a graph showing the relationship between the
diameter of the material compacts (dry pellets) and an average
diameter and an average mass of the produced metallic iron
nuggets.
BEST MODE FOR CARRYING OUT THE INVENTION
[0031] Metallic iron nuggets of the invention are granular metallic
iron made by reducing-melt of a material containing a carbonaceous
reductant and an iron-oxide containing material. The metallic iron
nuggets contain 94% or more (more preferably 96% or more) of Fe and
1.0 to 4.5% (more preferably 2.0 to 4.0%) of C. Preferably, the S
content of the metallic iron nuggets is 0.20% or less, more
preferably, 0.08% or less, and the diameter is in the range of 1 to
30 mm (more preferably 3 to 20 mm). The reasons for setting these
ranges are as follows.
[0032] The Fe content of the metallic iron nuggets is the primary
factor that controls the quality of the metallic iron nuggets.
Naturally, the higher the Fe purity, i.e., the lower the
contaminant content, the better. In the present invention, the
required Fe purity is 94% or more, and more preferably, 96% or
more. The reason for this is as follows. When metallic iron nuggets
having a contaminant content exceeding 5% are used as a material
for iron and steelmaking, the contaminants contained in the
material float on the surface of a bath and form slag, which is
difficult to remove. Moreover, because elements, such as S, Mn, Si,
and P, dissolved in a molten steel adversely affect the physical
properties of the end products made using the resulting metallic
iron, processes such as desulfurization, dephophorization, and
desiliconization are necessary during an refinning step. These
preliminary treatments require substantial time and effort.
Accordingly, the Fe content of the metallic iron nuggets of the
present invention must be at least 94%, and more preferably, at
least 96%.
[0033] The C content of the metallic iron nuggets is essential in
securing the required amount of C to suit the steel grade when the
metallic iron is used as a material for steelmaking, and is
important in view of increasing versatility as material iron.
Accordingly, the C content of the metallic iron nuggets is
preferably at least 1.0%, and more preferably, at least 2.0%. When
the metallic iron contains excessive amounts of carbon, the
tenacity and the shock resistance of steel or alloy steel made from
such metallic iron are adversely affected, and thus the steel or
alloy steel becomes fragile. Thus, a decarburization process such
as blowing becomes necessary during the process of refinning. In
order to use the metallic iron nuggets as a material for iron and
steelmaking without being burdened by these additional processes
and without hindrance, the C content must be 4.5% or less, and more
preferably, 4.0% or less.
[0034] Sulfur adversely affects the physical properties of steel
and is thus usually considered undesirable, although sulfur can be
used to increase machinability of some types of steel grade. The
metallic iron nuggets of the invention used as a material
preferably contain 0.20% or less, and more preferably, 0.08% or
less of sulfur. In order to increase the applicable range of the
metallic iron nuggets as an iron source so that the metallic iron
nuggets can be used in various steelmaking processes, the Si
content should be in the range of 0.02 to 0.5%, and the Mn content
should be less than 0.3%.
[0035] The metallic iron nuggets of the invention having the
above-described C, S, Si, and Mn contents are particularly
advantageous when compared to most commonly used pig iron made
using blast furnaces. The pig iron made using blast furnaces
generally contains 4.3 to 4.8% C, 0.2 to 0.6% Si, and 0.3 to 0.6%
Mn, although the contents of C, S, Mn, Si, and the like in the pig
iron made using a blast furnace vary according to the type of metal
oxide and coke used therein, operation conditions, and the like.
Especially in blast furnace iron making, the produced molten
metallic iron is carburized at the bottom part of the blast furnace
in a high reducing atmosphere in the presence of a large amount of
coke; hence, the C content is nearly saturated. Since SiO.sub.2,
which is included as a gangue component, is readily reduced in a
high-temperature atmosphere in the presence of a large amount of
coke, approximately 0.2 to 0.6% of Si is contained in the molten
metallic iron, and it is difficult to obtain molten metallic iron
having a Si content of less than 0.20%. Moreover, since MnO is
easier to reduce than SiO.sub.2, MnO is readily reduced in a highly
reducing atmosphere when a large amount of MnO is included in the
material iron ore. As a result, the Mn content in the molten
metallic iron becomes inevitably high.
[0036] In contrast, the metallic iron nuggets of the present
invention made by a process described below contain 1.0 to 4.5% C,
0.02 to 0.5%, and more preferably less than 0.2%, Si, and less than
0.3% Mn. The metallic iron nuggets of the present invention differ
from common metallic iron described above in the composition.
Furthermore, as described below, the S content of the metallic iron
nuggets of the present invention is reduced by using a CaO source
during the step of making a material compact so as to increase the
basicity of the slag components. The metallic iron nuggets of the
present invention is distinguishable from metallic iron made
according to a common process in that the S content is 0.08% or
less.
[0037] It is essential that the metallic iron nuggets of the
present invention have a diameter in the range of 1 to 30 mm.
Minute particles having a diameter less than 1 mm cause quality and
handling problems because fine slag components easily become mixed
with such minute particles and such minute particles of metallic
iron fly off easily.
[0038] The upper limit of the diameter is set in view of reliably
obtaining a predetermined level of the Fe purity within required
manufacturing restrictions. In order to obtain large nuggets having
a diameter exceeding 30 mm, large compacts must be used as a
material. With such large material compacts, the time taken to
conduct heat toward the inside of the material compacts during a
process of solid reduction, carburization, and melting,
particularly during solid reduction, for making metallic iron
nuggets, is long, decreasing the efficiency of solid reduction.
Moreover, the incorporation of the molten iron after carburization
and melting due to cohesion does not proceed uniformly. As a
result, the produced metallic iron nuggets have complex and
irregular shapes, and metallic iron nuggets having a uniform
diameter and quality cannot be obtained.
[0039] The size and shape of the iron nuggets are affected by
various factors including the size of the material compacts as
described above, the composition of the material (the type of metal
oxide source and the composition of the slag), the carburization
amount after solid reduction, the furnace atmosphere temperature
(particularly the atmosphere temperature in the region where
carburization, melting, and cohesion are performed), and the supply
density at which the material compacts are supplied to the
reducing-melt furnace. The supply density and the size of the
material compacts have the same influence. The higher the supply
density, the likelier it is for the molten metallic iron produced
by carburization and melting to form large nuggets on a hearth due
to cohesion and incorporation. By gradually increasing the supply
density of the material compacts and eventually stacking the
material compacts on a hearth, the chance that molten metallic iron
incorporates to form large nuggets can be increased. However, when
the supply density is excessively high, the heat conduction ratio
in the furnace decreases, and thus the solid reduction ratio cannot
be increased. Moreover, uniform cohesion and incorporation become
difficult, and the resulting metallic iron nuggets will have
complex and irregular shapes. Metallic iron nuggets having a
uniform diameter and a uniform shape cannot be obtained.
[0040] These problems derived from the size of the material
compacts and the like are particularly acute when metallic iron
nuggets having a diameter of 30 mm or more as products are made. No
such problems occur in making nuggets having a diameter of 30 mm or
less, and nuggets having a relatively uniform diameter of 30 mm or
less and a relatively uniform shape can be obtained. In view of the
above, the diameter is limited to 30 mm or less in the present
invention. It should be noted that nuggets having a highly uniform
diameter, shape, and quality can be obtained at a diameter of 3 to
15 mm.
[0041] The size of the produced metallic iron nuggets is also
affected by the type and the characteristics of the iron ore
contained in the material compacts. Generally, the cohesion
property is satisfactory when magnetite iron ore is used as an iron
oxide source. However, not all of the iron content in one material
compact necessarily coheres into one metallic iron nugget. The iron
content in one material compact frequently forms two or three
nuggets. The cause of such a phenomenon is not precisely known, but
a complex combination of difference in oxygen content, in crystal
structure of iron ore, in slag composition derived from the gangue
composition are considered as possible causes. In any case,
metallic iron nuggets having a relatively uniform diameter and
shape can be obtained at a diameter of the product nuggets of 30 mm
or less.
[0042] The metallic iron nuggets of the present invention satisfy
all of the requirements described above and can be effectively used
as an iron source for making iron, steel, or alloy steel using
various facilities for iron, steel, or alloy-steelmaking, such as
an electric furnace.
[0043] An embodiment of a method for making metallic iron nuggets
satisfying the above requirements will now be described in detail
with reference to the drawings.
[0044] FIGS. 1 to 3 are schematic illustrations showing an example
of a reducing-melt furnace of a rotary hearth type developed by the
inventors used for making metallic iron nuggets of the present
invention. The reducing-melt furnace has a ring-shaped movable
hearth and a dome-shaped structure. FIG. 1 is a schematic
illustration thereof, FIG. 2 is a cross-sectional view taken along
line A-A in FIG. 1, and FIG. 3 is a cross-sectional view of the
movable hearth, developed in a moving direction to promote
understanding of the structure. In the drawings, reference numeral
1 denotes a rotary hearth, and reference numeral 2 denotes a
furnace casing that covers the rotary hearth. The rotary hearth 1
is configured to rotate at an adequate speed by a driver not shown
in the drawing.
[0045] A plurality of combustion burners 3 is provided at suitable
positions of the wall of the furnace casing 2. The combustion heat
and the radiant heat thereof from the combustion burners 3 are
applied to material compacts on the rotary hearth 1 so as to
perform heat reduction of the compacts. The furnace casing 2 shown
in the drawing is a preferable example and is divided by three
partitions K.sub.1, K.sub.2, and K.sub.3 into a first zone Z.sub.1,
a second zone Z.sub.2, a third zone Z.sub.3, and a fourth zone
Z.sub.4. At the uppermost stream in the rotation direction of the
furnace casing 2, a feeder 4 for feeding material and an auxiliary
material, the feeder 4 facing the rotary hearth 1, is provided. At
the lowermost stream in the rotation direction, i.e., the position
upstream of the feeder 4 because of the rotatable structure, a
discharger 6 is provided.
[0046] In operating this reducing-melt furnace, while allowing the
rotary hearth 1 to rotate at a predetermined speed, material
compacts containing iron ore or the like and a carbonaceous
substance are supplied from the feeder 4 until an adequate
thickness is reached. The material compacts placed on the rotary
hearth 1 receive the combustion heat and the radiant heat thereof
from the combustion burners 3 during the course of traveling
through the first zone Z.sub.1. The metal oxide contained in the
compacts is reduced while sustaining its solid state due to the
carbonaceous substance in the compacts and carbon monoxide produced
by burning the carbonaceous substance. Subsequently, the material
compacts are further reduced by heating in the second zone Z.sub.2.
The resulting iron, which is substantially completely reduced, is
then further heated in a reducing atmosphere in the third zone
Z.sub.3 so as to carburize and melt the reduced iron while allowing
the reduced iron to separate from by-product slag and form nuggets,
i.e., metallic iron nuggets. Subsequently, the resulting metallic
iron nuggets are cooled and solidified in the fourth zone Z.sub.4
by a suitable cooling means C, and are sequentially discharged by
the discharger 6 at the downstream of the cooling means C. At this
time, the by-product slag derived from the gangue component, etc.,
in the iron ore is also discharged. The by-product slag is
separated from the metallic iron by suitable separating means, such
as a screen and a magnetic separation apparatus, after the slag and
the metallic iron is fed to a hopper H. The resulting metallic iron
nuggets have an iron purity of approximately 94% or more, and more
preferably, 96% or more, and contain a significantly low amount of
the slag component.
[0047] It should be noted that although the fourth zone Z.sub.4 in
the drawing is of an open-air type, the fourth zone Z.sub.4 is
preferably provided with a cover so as to prevent heat dissipation
as much as possible and to suitably adjust the atmosphere inside
the furnace in actual operation. Moreover, although, in this
embodiment, the rotary furnace is divided into the first zone
Z.sub.1, the second zone Z.sub.2, the third zone Z.sub.3, and the
fourth zone Z.sub.4 using three partitions K.sub.1 to K.sub.3, the
zone configuration of the furnace is not limited to this structure.
Naturally, the zone configuration may be modified according to the
size of the furnace, the required manufacturing capacity, the
operation mode, or the like. However, in order to efficiently
manufacture the metallic iron nuggets of the present invention, a
structure in which a partition is provided at least between the
solid-reduction area of the first half period of the heating
reduction, and the carburization, melting, and cohesion area of the
second half period of the heating reduction so that the furnace
temperature and the atmosphere gas can be separately controlled is
preferable.
[0048] During the above reducing-melt process, when the atmosphere
temperature during the reduction (solid reduction period) is
excessively high, i.e., when the atmosphere temperature exceeds the
melting point of the slag components including the gangue
component, unreduced iron oxide, and the like during a certain
period in the reduction process, iron oxide (FeO) in the material
melts before it is reduced. As a result, smelting-reduction rapidly
occurs due to the reaction of the molten iron oxide with carbon
contained in the carbonaceous substance. Note that
smelting-reduction is a phenomenon in which a material is reduced
in a molten state, and is different from solid reduction. Metallic
iron can still be produced by smelting-reduction; however, when
reduction occurs in the molten state, the separation of reduced
iron from by-product slag is difficult. Moreover, the reduced iron
is obtained in the form of a sponge, which is difficult to make
nuggets therefrom, and the slag content in the reduced iron becomes
high. Accordingly, it becomes difficult to achieve an Fe content
within the range specified by the present invention. Furthermore,
the molten metallic iron formed by incorporation due to cohesion
may flow on the hearth and may become planular instead of
granular.
[0049] FIG. 4 shows the state of the reaction when material
compacts (pellets having a diameter of 16 to 19 mm) containing iron
ore as an iron oxide source and coal as a carbonaceous reductant
are fed to a furnace having an atmosphere temperature of
approximately 1,300.degree. C. (the straight line 1 in the graph)
so as to solid-reduce the material compacts until a reduction ratio
of 100% (the elimination ratio of oxygen in the iron oxide in the
material compacts) is reached, and then the resulting reduced iron
is fed to a melting zone controlled at approximately 1425.degree.
C. (straight line 2) beginning at the time indicated by straight
line 3 in the drawing so as to melt the resulting reduced iron. In
the graph, the temperature inside the compacts, the atmosphere
temperature of the furnace, and changes of carbon dioxide and
carbon monoxide over time produced during the reduction process are
also shown. The temperature inside the compacts is continuously
measured using a thermocouple inserted into the material compacts
in advance.
[0050] As is apparent from this graph, in order to maintain the
solid state of the material compacts fed into the furnace and to
reduce the material compacts to a reduction ratio (oxygen
elimination ratio) of 80% (point A in FIG. 4) or more, and more
preferably, 94% (point B in FIG. 4) or more, the furnace
temperature is preferably maintained in the range of 1,200 to
1,500.degree. C., and more preferably, 1,200 to 1,400.degree. C.,
to perform solid reduction, and subsequently increased to 1,350 to
1,500.degree. C. to reduce the remaining iron oxide while allowing
the produced metallic iron to form nuggets by carburization and
melting. According to this two-stage heating process, metallic iron
nuggets having a high Fe purity can be reliably and efficiently
manufactured.
[0051] The time indicated by the horizontal axis in FIG. 4 may vary
depending on the composition of the iron ore or the carbonaceous
substance constituting the material compacts. Normally, solid
reduction of the iron oxide, melting, cohesion, and incorporation
can be completed and metallic iron nuggets can be made within 10 to
13 minutes.
[0052] If solid reduction of the material compacts is stopped at a
reduction ratio of less than 80% and melting is started therefrom,
sponge-shaped metallic iron is produced, and formation of nuggets
from such metallic iron is difficult. Moreover, it is difficult to
achieve an Fe content of 94% or more in the resulting metallic
iron. In contrast, when the solid reduction is performed until a
reduction ratio of 80% or more, and more preferably 94% or more is
reached and then the subsequent step of carburization, melting, and
cohesion is performed, the remaining FeO in the material compacts
can be effectively reduced regardless of the type and the
composition of the iron ore in the material compacts. Moreover, in
the subsequent step of carburization and melting, nuggets can grow
while excluding the by-product slag. Thus, metallic iron nuggets
having a high Fe content and a relatively uniform diameter can be
obtained.
[0053] In the solid-reduction region shown in the first part of
FIG. 4, the preferable furnace temperature that can securely
achieve a high reduction ratio is 1,200 to 1,500.degree. C., and
more preferably 1,200 to 1,400.degree. C. At a furnace temperature
of less than 1,200.degree. C., the solid reduction reaction
progresses slowly, and thus the dwell time in the furnace must be
made longer, resulting in poor productivity. At a furnace
temperature of 1,200.degree. C. or more, and particularly
1,500.degree. C. or more, the metallic iron nuggets incorporate
with one another to form large nuggets of irregular shapes. Such
metallic iron nuggets are not preferable as a product.
[0054] The metallic iron nuggets may not incorporate with one
another to form large nuggets in a temperature range of 1,400 to
1,500.degree. C. depending on the composition and the amount of the
iron ore in the material. However, this possibility and frequency
are low. Thus, the temperature during the solid reduction period is
preferably 1,200 to 1,500.degree. C., and more preferably 1,200 to
1,400.degree. C. In actual operation, it is possible to adjust the
furnace temperature to 1,200.degree. C. during the early stage of
the solid reduction period and then increase the furnace
temperature to 1,200 to 1,500.degree. C. during the latter stage of
the solid reduction.
[0055] The compacts subjected to the required reduction in the
solid-reduction zone are transferred to a melting zone having a
high furnace temperature of 1,425.degree. C. The temperature inside
the compacts increases as shown in FIG. 4, drops after reaching a
point C, and then increases again until a predetermined temperature
of 1,425.degree. C. is reached. The temperature drop at point C is
caused by latent heat accompanying melting of the reduced iron,
i.e., the point C can be considered as the starting point of the
melting. This starting point is substantially determined by the
residual carbon content in the reduced iron particles. Since the
melting point of the reduced iron drops as a result of the
carburization by the residual carbon and a CO gas, melting of the
reduced iron is accelerated.
[0056] In order to rapidly melt the reduced iron, a sufficient
amount of carbon for carburization must remain in the reduced iron
after the solid reduction. The content of the residual carbon is
determined by the amount of the iron ore and the carbonaceous
substance used in making the material compacts. The inventors have
confirmed through experiments that when the amount of the
carbonaceous substance is initially adjusted so that the residual
carbon content, i.e., the excess carbon content, in the
solid-reduced substance is 1.5% at the time the final reduction
ratio during the solid-reduction period reaches 100%, i.e., at the
time the metallization ratio reaches 100%, the reduced iron can be
rapidly carburized, thereby causing a drop in the melting point.
Accordingly, the reduced iron can rapidly form nuggets having a
suitable diameter by cohesion and incorporation in a temperature
range of 1,300 to 1,500.degree. C. Note that when the residual
carbon content of the solid-reduced carbon is less than 1.5%, the
melting point of the reduced iron does not drop sufficiently due to
the shortage of carbon for carburization, and the heating
temperature must thus be increased to 1,500.degree. C. or more.
[0057] When the carburization amount is zero, i.e., when pure iron
is involved, the melting temperature is 1,530.degree. C., and the
reduced iron can be melted by heating at a temperature exceeding
this temperature. However, in actual furnaces, the operating
temperature is preferably low to reduce heat load imposed on
furnace refractories. The operating temperature is preferably
approximately 1,500.degree. C. or less. In particular, the
operating conditions are preferably adjusted to allow a temperature
increase of approximately 50 to 200.degree. C. after the staring
point C of melting, which is the beginning of the melting and
cohesion period. In order to smoothly and effectively perform the
above-described solid reduction, carburization, and melting, the
temperature during the carburization and melting is preferably 50
to 200.degree. C., and more preferably, 50 to 150.degree. C.,
higher than the temperature during the solid reduction.
[0058] In this invention, the final carbon content in the end
product metallic iron nuggets must be in the range of 1.0 to 4.5%,
and more preferably, 2.0 to 4.0%. The final carbon content is
substantially determined by the amount of the carbonaceous
substance used in making material compacts and atmospheric
adjustments during the solid-reduction period. Especially, the
lower limit of the carbon content is determined by the residual
carbon content in the reduced iron during the final stage of the
solid reduction and the retention time (carburization amount)
during the period following the period of solid reduction. If a
reduction ratio of 100% is nearly achieved during the final stage
of the solid reduction as described above while securing 1.5% of
the residual carbon content, the end product of the metallic iron
nuggets can have a carbon content of 1.0% or more. Moreover, the
inventors have also confirmed that when the residual carbon content
in the reduced iron upon completion of the solid reduction is 5.0%
and carburization, melting, and cohesion of this reduced iron are
performed during the subsequent period of melting and cohesion, the
carbon content in the resulting metallic iron nuggets can be
increased to 4.5%. However, in order to reliably obtain metallic
iron nuggets having a final carbon content of 2.0 to 4.0%, the
residual carbon content in the reduced iron after completion of the
solid reduction is preferably controlled in the range of 1.5 to
4.5%.
[0059] As for the atmosphere gas in the process, during the period
in which solid reduction is rapidly progressed, a large amount of
CO is generated by the reaction of the metal oxide with the
carbonaceous substance in the material compacts, and the region
adjacent to the compacts is maintained at a high reducing
atmosphere due to the self-shielding effect. However, during the
latter stage of the solid reduction and during the subsequent
carburization and melting period, the amount of the CO gas produced
drastically decreases. Thus, prevention of reoxidation due to the
self-shielding effect cannot be expected.
[0060] FIG. 5 shows results of examination on the relationship
among the metallization ratio, the residual FeO, and the residual
carbon in the resulting material of the solid reduction. As shown
in the graph, FeO decreases as solid reduction progresses, that is,
as the metallization ratio increases. Up to straight line 1 in the
graph, solid reduction of the material compacts progresses inside
the furnace controlled at a temperature of 1,200 to 1,500.degree.
C. Subsequently, carburization, melting, and cohesion of the
reduced iron progress during the melting period in which the
temperature is controlled in the range of 1,350 to 1,500.degree. C.
in a highly reducing atmosphere. During this period, the
relationship among the metallization ratio, the residual FeO and
the residual carbon changes as shown by the portions of the curves
included in the right section of the graph from the straight line
1.
[0061] Curves (1) and (2) in FIG. 5 show the relationship between
the metallization ratio and the residual carbon content. The curve
(1) is when the residual carbon content is 1.5% when the
metallization ratio is 100%. The curve (2) is when the residual
carbon content is 3.0% when the metallization ratio is 100%. In
order to obtain the metallic iron nuggets of the present invention,
the amount of the carbonaceous substance is preferably controlled
during the process of making material compacts so that the residual
carbon content is above the curve (1).
[0062] Note that even when a predetermined amount of the
carbonaceous substance is used in making material compacts, the
residual carbon content at the metallization ratio of 100% slightly
varies depending on the reducing degree of the atmosphere gas
inside the furnace. Accordingly, the amount of the carbonaceous
substance should be suitably adjusted according to the reducing
degree of the atmosphere gas used in the operation. In any case,
the initial amount of the carbonaceous substance is preferably
adjusted so that the final residual carbon content is 1.5% or more
at a metallization ratio of 100%.
[0063] FIG. 6 shows the results of the examination on the
relationship between the residual carbon content at a metallization
ratio of 100% and the C content of the resulting metallic iron
nuggets. When the residual carbon content is 1.5 to 5.0%, the
resulting metallic iron nuggets can securely have a C content of
1.0 to 4.5%. When the residual carbon content is 2.5 to 4.5%, the
resulting metallic iron nuggets can securely have a C content of
2.0 to 4.0%.
[0064] In the description above, two indices, i.e., the
metallization ratio and the reduction ratio, are used to indicate
the state of FeO reduction. The definitions of the metallization
ratio and the reduction ratio are described below. The relationship
between the two is, for example, shown in FIG. 7. The relationship
between the two changes depending on the type of the iron ore used
as an iron oxide source. FIG. 7 shows the relationship between two
when magnetite (Fe.sub.3O.sub.4) is used as an iron oxide source.
Metallization ratio=[metallic iron nuggets produced/(metallic iron
nuggets produced+iron in iron ore)].times.100(%) Reduction
ratio=[amount of oxygen removed during the reduction process/amount
of oxygen in the iron oxide contained in the material
compacts].times.100(%)
[0065] The reducing-melt furnace used in making the metallic iron
nuggets of the present invention employs burners to heat the
material compacts, as described above. During the solid-reduction
period, as described above with reference to FIG. 4, the iron oxide
source and the carbonaceous substance in the material compacts fed
into the furnace react with each other to produce a large amount of
CO gas and a small amount of CO.sub.2 gas. Accordingly, the region
adjacent to the material compacts is maintained at a sufficient
reducing atmosphere as a result of the shielding effect of the CO
gas emitted from the material compacts themselves.
[0066] However, during the latter stage and the final stage of the
solid reduction period, the amount of the CO gas decreases rapidly,
resulting in a decrease in the self-shielding effect. Accordingly,
the reduced iron becomes vulnerable to the exhaust gas, i.e., an
oxidizing gas such as CO.sub.2 and H.sub.2O, produced by burner
heating, and reoxidation of the reduced metallic iron may occur.
Moreover, after completion of the solid reduction, melting and
cohesion of the minute particles of reduced iron progress due to
the carburization of the reduced iron using the residual carbon in
the compacts and a decrease in the melting temperature resulting
from the carburization. During this stage also, since the
self-shielding effect is poor, the reoxidation of the reduced iron
may readily occur.
[0067] In order to efficiently perform carburization, melting, and
cohesion after the solid reduction to secure an Fe purity of 94% or
more and to thereby obtain metallic iron nuggets of a suitable
diameter while preventing a decrease in the Fe purity resulting
from such reoxidation as much as is feasibly possible, the
composition of the atmosphere gas in the carburization and melting
regions is preferably optimized.
[0068] In view of the above, the examination of atmosphere
conditions for efficiently performing carburization and melting
while preventing the reoxidation of the reduced iron during the
carburization and melting period after completion of the solid
reduction was conducted. The results of the examination will now be
described with reference to FIG. 8. In the experiments, a box
furnace was used, and coal powder was used as an atmosphere
adjustor during the carburization and melting stage. On a hearth, a
coal powder was bed to an adequate thickness so as to keep a highly
reducing atmosphere during the carburization and melting.
[0069] In particular, coal powders having different particle
diameters were used as atmosphere adjustors. The coal powder was
bedded to a thickness of approximately 3 mm on an alumina tray, and
50 to 60 material compacts having a diameter of approximately 19 mm
were placed on the bed of the coal powder. A thermocouple was
provided to one of the material compacts. The material compacts
were fed into the box furnace. The temperature of the composite
during heating was measured, and the composition of the gas
produced was measured to determine the possibility of the
reoxidation of the produced metallic iron. Note that the
temperature inside the electric furnace was adjusted so that the
maximum furnace temperature is approximately 1,450.degree. C. The
initial composition of the atmosphere gas inside the furnace was
CO.sub.2: 20% and N.sub.2: 80%.
[0070] FIG. 8 shows the results of the experiments in which the
temperature of the material compacts detected by the thermocouple
described above and the composition of the atmosphere gas when the
temperature inside the electric furnace is gradually elevated were
measured over time. The horizontal axis shows changes in
temperature, and the vertical axis shows a simplified reducing
degree (CO)/(CO+CO.sub.2) of the atmosphere gas. In the graph, four
experimental results are plotted. Curve (3) shows the result of the
experiment where no atmosphere adjustor was used. Curve (4) shows
the result of the experiment where a coarse coal powder having an
average diameter exceeding 3.0 mm was used as an atmosphere
adjustor. Curves (1) and (2) show the results of the experiments
where fine coal powders A and B having a diameter of 2.0 mm or less
were used. In the graph, an FeO--Fe equilibrium curve and an
Fe.sub.3O.sub.4--FeO equilibrium curve are also included. The
circled regions indicate periods during which the solid reduction
nearly completes and the carburization, melting, and cohesion of
the reduced iron begin in these experiments. The control of the
atmosphere gas during these periods is particularly important for
preventing reoxidation of the iron oxide and for obtaining metallic
iron nuggets of a high Fe purity.
[0071] As is apparent from this graph, the curve (3) of the
experiment where no atmosphere adjustor was used, the region C
indicating the beginning of the carburization, melting, and
cohesion of the reduced iron, was far below the FeO--Fe equilibrium
curve. This demonstrates that the entire reduced iron melted while
a portion thereof underwent the reducing-melt. The metallic iron
was still obtained in this experiment, but, as described above,
when reducing-melt occurs, the resulting iron is likely to be
sponge-shaped and is thus easy to make nuggets therefrom. Moreover,
the Fe purity of the metallic iron was insufficient.
[0072] In contrast, the curves (1) and (2) show the results of the
experiments in which fine coal powder was used. As is apparent from
the graph, the reducing degree of the atmosphere gas was
significantly improved. Moreover, the region A in which the
carburization, melting, and cohesion of the reduced iron occurred
was above the FeO--Fe equilibrium curve, meaning that the
generation of FeO was prevented in these experiments. The curve (3)
shows the results of the experiment using a coarse coal powder. In
this experiment, the region B in which the carburization, melting,
and cohesion of the reduced iron occurred was slightly below the
FeO--Fe equilibrium curve. This means some degree of reoxidation
might have occurred. However, the composition of the produced
metallic iron was examined, and the results confirmed that
substantially no reoxidation occurred in this experiment.
[0073] It was also confirmed that the metallic iron nuggets having
an Fe content of 94% or more and a carbon content of 1.0 to 4.5%
can be highly effectively manufactured by controlling the reducing
degree of the atmosphere gas to at least 0.5, more preferably, at
least 0.6, yet more preferably, at least 0.7, and most preferably
above the FeO--Fe equilibrium curve, at least during the beginning
stage of the carburization, melting, and cohesion period. In this
manner, carburization, melting, and cohesion can be smoothly
performed without allowing the reoxidation of the reduced iron
produced by solid reduction.
[0074] Direct analysis of the experimental data shown in FIG. 8
suggests that a substantial degree of reoxidation may occur at a
simplified reducing degree of 0.5 to 0.7. However, this experiment
examines the reoxidation degree of the atmosphere gas only; the
inner portions of the actual material compacts or the atmosphere
near the actual material compacts are maintained at a highly
reducing atmosphere because of the presence of the residual carbon
inside the material compacts and the atmosphere adjustor. Moreover,
an oxidizing gas such as CO.sub.2 and H.sub.2O in the atmosphere of
the upper portion of the hearth is readily reduced by the
carbonaceous atmosphere adjustor when the oxidizing gas enters the
section near the material compacts. Thus, it is assumed that no
reoxidation occurs even when the measured reducing degree of the
atmosphere is 0.5 to 0.7. Note that at a reducing degree of less
than 0.5, the produced metallic iron is readily reoxidized,
cohesion of the metallic iron and formation of metallic iron
nuggets become difficult due to insufficient carburization, and
metallic iron nuggets having a diameter in the range of the present
invention are difficult to obtain.
[0075] After carburization, melting, and cohesion of the reduced
iron are completed, the reducing degree of the atmosphere gas
decreases rapidly. However, in actual operation, the metallic iron,
which has been melted and cohered, is nearly completely separated
from the by-product slag by this time. Thus, the metallic iron is
hardly affected by the atmosphere gas, and metallic iron nuggets
having a high Fe content and a low inclusion slag content can be
effectively made by cooling and solidifying this metallic iron.
[0076] As is apparent from above, a coal powder used as an
atmosphere adjustor is preferably pulverized to a diameter of 3 mm
or less, and more preferably, 2 mm or less to further reliably
prevent the reoxidation during carburization, melting, and
cohesion. In view of the yield and operation of the furnace in
actual operation, the diameter of the coal powder is most
preferably in the range of 0.3 to 1.5 mm. No limit is imposed as to
the thickness at which the coal powder is bedded, but the thickness
of the coal powder bed is preferably approximately 2 mm or more,
and more preferably 3 mm or more since the amount of the coal
powder as the atmosphere adjustor is insufficient at an excessive
small thickness. No limit is imposed as to the upper limit of the
thickness. However, since the atmosphere adjusting effect saturates
at an excessively large thickness, it is practical and
cost-effective to restrict the thickness to preferably
approximately 7 mm or less, and more preferably, approximately 6 mm
or less. Any material can be used as an atmosphere adjustor as long
as it releases CO. Examples of such materials include coal, coke,
and charcoal. These materials may be used alone or in
combination.
[0077] The atmosphere adjustor may be bedded on a hearth before the
material compacts are fed on a hearth. In such a case, the
atmosphere adjustor also functions to protect the hearth refractory
from the slag bleeding during the reducing-melt process. However,
since the atmosphere adjustor exerts its effect during the
carburization, melting, and cohesion period after the solid
reduction, it is also effective to sprinkle the atmosphere adjustor
from above the hearth immediately before the carburization and
melting of the material compacts begin.
[0078] According to the above method, the reoxidation of the
reduced iron can be prevented and carburization, melting, and
formation of nuggets can be effectively performed since the
reducing degree of the atmosphere gas during the carburization and
melting period is enhanced. Thus, metallic iron nuggets having a
high Fe content and a suitable size can be efficiently
manufactured. During the process, in order to effectively perform a
series of steps from solid reduction to the carburization, melting,
and cohesion, the temperature and the atmosphere gas are preferably
separately controlled according to the step. In particular, the
temperature during the solid reduction period is preferably 1,200
to 1,400.degree. C. to prevent reducing-melt reaction, as described
above. The temperature during the carburization, melting, and
cohesion period is preferably 1,300 to 1,500.degree. C. More
preferably, the temperature during the solid reduction period is 50
to 200.degree. C. lower than the temperature during the
carburization, melting, and cohesion period.
[0079] As for the atmosphere gas conditions, since a large amount
of CO gas that is produced by the burning of the carbonaceous
substance inside the material compacts maintains a highly reducing
atmosphere during the solid reduction period, the atmosphere gas
inside the furnace does not require extensive control. In contrast,
during the carburization, melting, and cohesion period, emission of
the CO gas from the material compacts drastically decreases. As a
result, reoxidation caused by the oxidizing gas produced by the
combustion of the burners may readily occur. Thus, in order to
obtain metallic iron nuggets having an adequate carbon content, it
is essential to suitably adjust the atmosphere gas inside the
furnace from this period on. The atmosphere gas can be adjusted by
using an atmosphere adjustor, for example.
[0080] In order to suitably adjust the temperature and the
atmosphere gas composition inside the furnace according to the
progress of the reducing-melt, the reducing-melt furnace is
preferably divided into at least two zones in the traveling
direction of the hearth by using a partition, as shown in FIGS.
1-3. Preferably, the upstream zone is configured as a solid
reduction zone, and the downstream zone is configured as a
carburization, melting, and cohesion zone so as to separately
control the temperature and the atmosphere gas composition of each
zone. Note that FIG. 3 shows as example in which the furnace is
divided into four zones using three partitions to allow more
stringent control of the temperature and the atmosphere gas
composition. The number of zones can be adjusted to suit the scale
and the structure of the reducing-melt facility.
[0081] The metallic iron nuggets of the present invention made by
the above-described process contain substantially no slag component
and have an Fe purity of 94% or more, and more preferably 96% or
more, and a carbon content of 1.0 to 4.5%. The diameter thereof is
in the range of 1 to 30 mm. These metallic iron nuggets are used as
an iron source in known facilities for steelmaking, such as a
electric furnace and a converter. When using the metallic iron
nuggets as a material for steelmaking, the sulfur content therein
is preferably as low as is feasibly possible. The investigation has
been conducted to remove sulfur contained in the iron ore and the
carbonaceous substance as much as possible during the process of
making the metallic iron nuggets and to obtain metallic iron
nuggets having a low sulfur content.
[0082] As a result, it has been found that the sulfur content in
the end-product metallic iron nuggets can be reduced to 0.08% or
less by intentionally adding a CaO source, e.g., burnt lime, slaked
lime, or calcium carbonate, during making the material compacts
using the iron ore and the carbonaceous substance so as to adjust
the basicity (i.e., the ratio of CaO/SiO.sub.2) of the overall slag
components contained in the material compacts to 0.6 to 1.8, and
more preferably 0.9 to 1.5, the overall slag components including
the gangue component in the iron ore, etc.
[0083] Note that coke or coal, which is the most commonly used
carbonaceous reductant, normally contains approximately 0.2 to 1.0%
of sulfur. The majority of sulfur contained therein is captured in
the metallic iron. If basicity adjustment intentionally using a CaO
source is not performed, the basicity calculated based on the slag
composition in the material compacts is usually 0.3 or less,
although the basicity significantly varies according to the type of
iron ore. In slag having such a low basicity, sulfur cannot be
prevented from becoming mixed into the metallic iron during the
solid reduction process or the subsequent process of carburization,
melting, and cohesion. Approximately 85% of total sulfur in the
material compacts will be included in the metallic iron. As a
result, the sulfur content of the metallic iron nuggets is
increased, and the quality of the end-product metallic iron is
degraded.
[0084] It was confirmed that by intentionally adding a CaO source
during the step of making material compacts so as to adjust the
composition of the slag component to exhibit a basicity of 0.6 to
1.8, sulfur can be fixed in the by-product slag which is produced
during solid reduction and carburization, melting, and cohesion. As
a result, the sulfur content in the metallic iron nuggets can be
dramatically reduced.
[0085] The sulfur content reduction is considered to occur when
sulfur contained in the material compacts is allow to react with
CaO and is thus fixed as CaS (CaO+S=CaS). Conventionally, when the
above-described reducing-melt mechanism was not clearly known, it
was considered that desulfurization effect comparable to that of a
hot metal desulfurization cannot be achieved by the addition of
CaO. However, the inventors have confirmed that CaO in the slag
captures sulfur when the reduced iron melts, forms nuggets, and
becomes separated from the slag due to the carburization caused by
the residual carbon inside the reduced metal, and thus the sulfur
content in the resulting metallic iron nuggets can be dramatically
decreased.
[0086] Such a sulfur reduction mechanism is different from a normal
hot metal desulfurization using CaO-containing slag and is
considered as a reaction unique to the above-described process. Of
course, if carburized and melted reduced iron is sufficiently put
into contact with the by-product molten slag under appropriate
heating conditions, a liquid-liquid (molten iron-molten slag)
reaction may determine the ratio of the S content in the slag (S %)
to the S content in the metallic iron nuggets [S %], i.e., the
distribution ratio of sulfur (S %)/[S %]. However, as can be
confirmed by the photograph shown in FIG. 9, the slag-metal contact
area of the produced molten iron and the molten slag is small.
Thus, a large sulfur reduction cannot be expected from the
slag-metal equilibrium reaction after the reduced iron is
carburized, melted, and cohered. Accordingly, it can be assumed
that the desulfurization mechanism of intentionally adding CaO into
the material compacts employed in the above process includes a
sulfur trapping reaction peculiar to CaO during carburization,
melting, and cohesion of reduced iron, the sulfur trapping reaction
preventing the sulfurization of the metallic iron nuggets.
[0087] The amount of the CaO added to adjust the basicity should be
determined based on the amount and the composition of the gangue
component contained in iron ore or the like and on the type and the
amount of the carbonaceous substance added to the material. A
standard amount of CaO required to adjust the basicity of the
overall slag component in the above-described range of 0.6 to 1.8
is, in terms of pure CaO, 2.0 to 7.0%, and more preferably 3.0 to
5.0%, of CaO in the entirety of the composites. When slaked lime
[Ca(OH).sub.2] or calcium carbonate (CaCO.sub.3) is used, the
amount thereof should be converted to CaO. It was confirmed that
when 4% CaCO.sub.3 was contained in the material compacts to adjust
the basicity of the slag component to approximately 0.9 to 1.1, an
apparent desulfurization ratio of 45 to 50% was obtained. The
apparent desulfurization ratio was determined by the equation
below. When 6% CaCO.sub.3 was contained in the material compacts to
adjust the basicity of the slag component to approximately 1.2 to
1.5, an apparent desulfurization ratio of 70 to 80% was
obtained.
[0088] Apparent desulfurization ratio (%)=[S content (%) in the
metallic iron nuggets made from CaO-added material compacts/S
content (%) in the metallic iron nuggets made from material
compacts not using an additive CaO].times.100.
[0089] The effect of adding a CaO source to the material on
reduction of sulfur will now be described based on experimental
data taken using a box furnace. FIG. 10 shows changes in sulfur
content when reducing-melt is performed as described above using
iron ore, a carbonaceous substance, a small amount of binder
(bentonite, or the like), and an adequate amount of CaO.
[0090] In FIG. 10, "dry compact" shows that, of 100% sulfur
contained in the material before reducing-melt, approximately 89%
was contained in the carbonaceous substance (coal) and
approximately 11% was contained in the iron ore. When the compacts
were subjected to reducing-melt, approximately 85% of sulfur
remained in the reduced iron upon completion of the solid reduction
explained above with reference to FIG. 4. Approximately 12% of
sulfur evaporated and was discharged from the furnace. When
compacts containing no additive CaO source (the calculated basicity
of the slag component in the composite being 0.165) were used,
74.8% of sulfur was trapped in the end-product metallic iron
nuggets, and 10.2% of sulfur was trapped in the slag.
[0091] When material compacts having their basicity of the slag
component adjusted to 1.15 by adding 3% of a CaO source were used,
the amount of sulfur captured in the metallic iron nuggets
decreased to 43.2%, and the amount of sulfur trapped in the slag
was increased to 48.8%. The amount of sulfur evaporated and
discharged outside the furnace during the manufacturing process
reduced to approximately 8%. When material compacts having their
basicity of the slag component adjusted to 1.35 were used by adding
5% of a CaO source, the amount of sulfur captured in the metallic
iron nuggets decreased to 18.7%, and the amount of sulfur trapped
in the slag was increased to 78.8%. The amount of sulfur evaporated
and discharged outside the furnace during the manufacturing process
was reduced to 1.5%.
[0092] The above basic experiments using a box furnace demonstrated
that the basicity adjustment by adding a CaO source was
particularly effective in reducing the amount of sulfur contained
in the metallic iron. The same experiment was conducted using a
demonstration reactor. In the experiment, the effect of the
basicity on the sulfur reduction of the metallic iron nuggets was
quantitatively examined by varying the amount of the CaO source to
yield different slag basicities. The results are shown in FIG.
11.
[0093] This graph illustrates the relationship between the final
basicity of the slag and the sulfur content in the metallic iron
nuggets. In the experiment, the slag was produced while varying the
amount of the CaO source, and each of the points in the graph shows
an actual result. The shaded region in the graph shows the results
of the above-described basic experiments using a box furnace. Since
the basic experiments employed an electrical heating method and
used an inert gas as an atmosphere gas, the oxidation potential of
the atmosphere was low, which advantageously affects the apparent
desulfurization ratio. In contrast, the demonstration furnace
employed burner combustion, and thus the reducing degree of the
atmosphere gas was low due to the generation of combustion gas
compared to that of the basic experiments. The sulfur content in
the metallic iron nuggets was higher than the results of the basic
experiments. However, the basic tendency was substantially the same
as that shown by the results of the basic experiments. It could be
confirmed that when no CaO source was added, the sulfur content in
the metallic iron nuggets in the region A was approximately 0.12%.
When the basicity was adjusted to approximately 1.0, the S content
was reduced to 0.05 to 0.08%, as shown in region B, and the
apparent desulfurization ratio was approximately 33 to 58%. When
the basicity was increased to 1.5, the sulfur content in the
metallic iron was reduced to approximately 0.05%, as shown in
region C.
[0094] When a CaO source is added to increase the basicity of the
slag to 1.8 or more, the melting point of the produced slag
increases, and the operating temperature must thus be increased to
an excessively high level. As a result, the damage on the furnace
is accelerated, and the heat economy is degraded. Moreover, the
cohesion property of the reduced iron is degraded, and the
resulting metallic iron is obtained as minute particles smaller
than 1 mm having a low product value.
[0095] As is apparent from these experiments, when an adequate
amount of a CaO source is intentionally added to the material
compacts to increase the basicity of the slag component to
approximately 0.6 or more, the produced slag captures a
significantly larger amount of sulfur, and the amount of the sulfur
captured in the metallic iron nuggets can thus be significantly
reduced. As a result, metallic iron nuggets that satisfy the level
of the sulfur content required in the present invention, i.e.,
metallic iron nuggets having a sulfur content of 0.08% or less, can
be easily manufactured. Furthermore, as described above with
reference to FIG. 10, the amount of sulfur discharged outside the
furnace as SO.sub.X or the like during a series of metallic iron
nuggets manufacturing steps can be drastically reduced. Thus, air
pollution due to effluent gas can be minimized. Moreover, load of
desulfurizing the effluent gas can be significantly reduced if
desulfurization treatment of the effluent gas is performed.
[0096] When the CaO source is added to reduce the S content, as
described above, bleeding of low-melting point slag which leads to
dissolution of the hearth refractories may occur during the
reducing-melt period due to a decrease in the melting point of the
by-product slag depending on the amount of the CaO source added. In
implementing the above-described process, a two-stage heating
method including a solid reduction period and a carburization,
melting, and cohesion period is preferably performed. During the
solid-reduction period, the temperature is preferably adjusted to
1,200 to 1,400.degree. C., and during the carburization, melting,
and cohesion period, the temperature is preferably adjusted to
1,350 to 1,500.degree. C. In this manner, the solid reduction can
be sufficiently performed below the melting point of the by-product
slag, and, subsequently, the reduction of the remaining FeO, and
carburization, melting, and cohesion of the reduced iron can be
performed to minimize undesirable bleeding of the by-product
slag.
[0097] In making metallic iron by first solid-reducing material
compacts containing iron ore and a carbonaceous substance and then
carburizing, melting, and cohering the resultant material, the
amount of the carbonaceous reductant in the material compacts, the
temperature conditions during solid reduction, and the composition
of the atmosphere gas and the temperature conditions during
carburization and melting, and the like should be suitably
adjusted. In this manner, reduction, carburization, melting,
cohesion, and incorporation can be efficiently performed, and
metallic iron nuggets having a high Fe purity, a suitable carbon
content, and a suitable diameter can be obtained. Under these
conditions, the resulting metallic iron nuggets have a Si content
of 0.02 to 0.5%, and a Mn content of less than 0.3%. The sulfur
content of the metallic iron nuggets can be reduced by
intentionally adding CaO in the material compacts so as to adjust
the basicity of the slag component.
[0098] The resulting metallic iron nuggets of the present invention
have a high Fe purity, a suitable carbon content, a uniform shape,
and a size of 1 to 30 mm. Thus the metallic iron nuggets of the
present invention exhibit high handling quality and can thus
effectively used as an iron source for making iron, steel, or
various alloy steels.
EXAMPLES
[0099] The present invention will now be described in detail using
examples. These examples do not limit the scope of the present
invention. Various modifications are possible without departing
from the scope of the invention described herein. These
modifications are included in the technical scope of the present
invention.
Example 1
[0100] Material compacts having a diameter of approximately 19 mm
were made by uniformly mixing hematite ore, i.e., an iron source,
coal, and a small amount of a binder (bentonite). Metallic iron was
made using these material compacts. The material compacts were fed
inside a reducing-melt furnace of a rotary hearth type shown in
FIGS. 1 to 3, and solid reduction was performed at an atmosphere
temperature of approximately 1,350.degree. C. until a metallization
ratio of approximately 90% was reached. Subsequently, the resulting
material compacts were transferred to a carburization, melting, and
cohesion zone at an atmosphere temperature of 1,440.degree. C. so
as to perform carburization, melting, and cohesion, and to separate
by-product slag to make slag-free metallic iron nuggets.
[0101] In this process, coal powder, i.e., an atmosphere adjustor,
having a diameter of 2 mm or less was bedded on a hearth to a
thickness of approximately 5 mm before the material compacts were
fed to the furnace so as to control the reducing degree of the
atmosphere gas during the carburization, melting, and cohesion
period in the range of 0.60 to 0.75. The material composition, the
composition of the reduced iron after completion of solid
reduction, the composition of the end-product metallic iron, the
composition of the produced slag, etc., are shown in FIG. 12.
[0102] The metallic iron that had been melted, cohered, and
substantially completely separated from the slag was then
transferred to a cooling zone to be cooled to a temperature of
1,000.degree. C. and solidified, and was discharged outside the
furnace with a discharger. The production ratios and the
compositions of the recovered metallic iron nuggets, the by-product
slag, and the excess carbonaceous substance were analyzed. The
reduced iron immediately before the carburization and melting was
sampled from the reducing-melt furnace to analyze the composition
of the reduced iron immediately before the carburization and
melting. The results demonstrated that the metallization ratio was
approximately 90%, and the residual carbon content was 4.58%. The
time taken from feeding of the material compacts to discharging of
the metallic iron was remarkably short, i.e., approximately 9
minutes. The resulting metallic iron had a carbon content of 2.88%,
a Si content of 0.25%, and a S content of 0.165%. The resulting
metallic iron could be easily separated from the by-product slag. A
photograph of the produced metallic iron nuggets is shown in FIG.
13. The metallic iron nuggets had a diameter of about 10 mm and a
substantially uniform size.
Example 2
[0103] Material compacts having a diameter of approximately 19 mm
were made by uniformly mixing magnetite ore, i.e., an iron source,
coal, a small amount of a binder (bentonite), and 5% of CaCO.sub.3
as a slag basicity adjustor and forming the resulting mixture into
compacts.
[0104] The material compacts were fed on a bed of coal powder
(average diameter: approximately 3 mm) having a thickness of
approximately 3 mm, the bed of coal powder being formed on a
hearth. The coal powder was used as an atmosphere adjustor. The
solid reduction was performed as in Example 1 while maintaining the
atmosphere temperature at approximately 1,350.degree. C. until the
metallization ratio reached nearly 100%. Subsequently, the
resulting material compacts were transferred to a melting zone
maintained at 1,425.degree. C. so as to perform carburization,
melting, cohesion, and separation of by-product slag so as to make
slag-free metallic iron. The material composition, the composition
of the reduced iron after completion of solid reduction, the
composition of the end-product metallic iron, the composition of
the produced slag, etc., are shown in FIG. 14.
[0105] The metallic iron that had been melted, cohered, and
substantially completely separated from the slag was then
transferred to a cooling zone to be cooled to a temperature of
1,000.degree. C. and solidified, and was discharged outside the
furnace with a discharger. The production ratios and the
compositions of the recovered metallic iron nuggets, the by-product
slag, and the excess carbonaceous substance were analyzed. The
reduced iron immediately before the carburization and melting was
sampled from the reducing-melt furnace to analyze the composition
of the reduced iron immediate before the carburization and melting.
The results demonstrated that the metallization ratio was
approximately 92.3%, and the residual carbon content was 3.97%.
[0106] The time taken from feeding of the material compacts to
discharging of the metallic iron was remarkably short, i.e.,
approximately 8 minutes. The resulting metallic iron had a carbon
content of 2.10%, a Si content of 0.09%, and a S content of 0.07%.
Since a CaO source was added to decrease the S content in this
example, the S content was lower than that in Example 1. A
photograph of the produced metallic iron nuggets is shown in FIG.
15, and 98% or more of the iron nuggets had a diameter in the range
of 5 to 30 mm.
[0107] In this example, because the melting point of the by-product
slag was decreased due to the addition of the CaO source, bleeding
of the molten slag was feared during the latter period of the solid
reduction. However, the example employed a two-stage heating
process in which the temperature during the solid-reduction period
was adjusted to 1,200 to 1,400.degree. C. to produce reduced iron
having a high metallization ratio by solid reduction, and then the
resulting reduced iron was heated at 1,350 to 1,500.degree. C.
Moreover, because the coal power, i.e., the atmosphere adjustor,
was bedded on a hearth, a problem of dissolution of hearth
refractories due to bleeding of molten slag never occurred.
[0108] The microscopic structure of the reduced iron at the end
stage of the solid reduction was examined in detail. In Example 1
not using a CaO source, Fe--(Mn)--S was present on the surface of
the reduced iron at a high concentration. It was confirmed that
during the carburization and melting, Fe--(Mn)--S was captured
inside the molten iron. In contrast, in Example 2 using a CaO
source, most sulfur was allowed to react with the CaO source and
was fixed during the end stage of the solid reduction. It was
confirmed that sulfur was prevented from entering the molten iron
during the step of carburization and melting.
[0109] Another experiment was conducted as in the above-described
experiment but by replacing the coal powder used as the atmosphere
adjustor to fine-particle coal powder, having a particle size of
2.0 mm or less. It was confirmed that the S content in the
resulting metallic iron was decreased to 0.032%.
Example 3
[0110] An experiment was conducted under the same conditions as
those in Example 1 and an actual furnace. In this experiment, the
diameter of the material compacts (pellets) was varied within the
range of 3 to 35 mm to examine the effect of the size of the
material compacts on the average diameter and the average mass of
the resulting metallic iron nuggets. The results are shown in FIG.
16.
[0111] As is apparent from this graph, metallic iron nuggets having
a diameter in the range of 5 to 20 mm, i.e., the type of metallic
iron nuggets exhibiting superior handling quality as the
end-product metallic iron, could be effectively manufactured from
material compacts (dry pellets) having a diameter of approximately
10 to 35 mm.
INDUSTRIAL APPLICABILITY
[0112] The present invention having the above-described
configuration provides metallic iron nuggets having a high Fe
purity, an adequate C content, and a suitable size for handling
ease. The metallic iron nuggets further has low S, Si, and Mn
contents, are easy to handle as an iron source, and has a reliable
quality. As described above, these metallic iron nuggets can be
efficiently and reliably manufactured with a high reproducibility
by suitably controlling the manufacturing conditions.
* * * * *