U.S. patent application number 10/508381 was filed with the patent office on 2005-09-08 for process for producing sponge iron and reduced iron powder sponge iron and charging apparatus.
This patent application is currently assigned to JFE STEEL CORPORATION. Invention is credited to Funatsu, Isao, Itaya, Hiroshi, Kuroki, Takashi, Misumi, Yotsuo, Sakaguchi, Yasuhiko, Sonobe, Akio, Suzuki, Yoshitomo.
Application Number | 20050193862 10/508381 |
Document ID | / |
Family ID | 32830632 |
Filed Date | 2005-09-08 |
United States Patent
Application |
20050193862 |
Kind Code |
A1 |
Suzuki, Yoshitomo ; et
al. |
September 8, 2005 |
Process for producing sponge iron and reduced iron powder sponge
iron and charging apparatus
Abstract
A method for manufacturing sponge iron and an apparatus for
charging in the method are disclosed. Iron oxide powder and
reducing-agent powder are charged such that alternating layers of
the iron oxide powder and the reducing-agent powder are formed and
such that each of the layers is in the form of a helix, and then a
reduction treatment is performed. The method has not only high
reaction efficiency of a gas, high quality, and high productivity,
but also the advantage for a production adjustment because the
amount of charge can be adjusted without the limitation of a
reduction time. The molar ratio of the carbon content in the
reducing agent to the oxygen content in the iron oxide in the
reaction container is preferably 1.1 or more.
Inventors: |
Suzuki, Yoshitomo; (Chiba,
JP) ; Sonobe, Akio; (Chiba, JP) ; Kuroki,
Takashi; (Chiba, JP) ; Sakaguchi, Yasuhiko;
(Chiba, JP) ; Itaya, Hiroshi; (Chiba, JP) ;
Misumi, Yotsuo; (Chiba, JP) ; Funatsu, Isao;
(Chiba, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN & CHICK, PC
220 5TH AVE FL 16
NEW YORK
NY
10001-7708
US
|
Assignee: |
JFE STEEL CORPORATION
2-3, UCHISAIWAI-CHO 2-CHOME, CHIYODA-KU
TOKYO
JP
100-0011
|
Family ID: |
32830632 |
Appl. No.: |
10/508381 |
Filed: |
November 10, 2004 |
PCT Filed: |
January 29, 2004 |
PCT NO: |
PCT/JP04/00866 |
Current U.S.
Class: |
75/469 ;
266/177 |
Current CPC
Class: |
F27D 3/0033 20130101;
F27D 3/08 20130101; C21B 13/00 20130101; F27D 3/003 20130101 |
Class at
Publication: |
075/469 ;
266/177 |
International
Class: |
C21B 013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 31, 2003 |
JP |
2003-024638 |
Jun 26, 2003 |
JP |
2003-182533 |
Aug 4, 2003 |
JP |
2003-286047 |
Claims
1. A method for manufacturing sponge iron, comprising: a charging
step of charging iron oxide powder and reducing-agent powder into a
reaction container; and a reducing step of reducing the iron oxide
powder in the reaction container to produce a mass of sponge iron
by heating from the outside of the reaction container, wherein, in
the charging step, the iron oxide powder and the reducing-agent
powder are charged such that alternating layers of the iron oxide
powder and the reducing-agent powder are formed and such that each
of the layers is in the form of a helix.
2. The method for manufacturing sponge iron according to claim 1,
wherein, in the charging step, the iron oxide powder and the
reducing-agent powder are charged such that layers composed of the
reducing-agent powder are disposed on an inner side-surface of the
reaction container (referred to as "peripheral portion") and
disposed at a central portion along the vertical central axis and
such that the alternating layers that are in the form of helices
are disposed at a portion (referred to as "intermediate portion")
other than the portion of the layers disposed on the inner
side-surface and at the central portion.
3. The method for manufacturing sponge iron according to claim 1,
wherein the iron oxide powder comprises at least one selected from
the group consisting of an iron ore, mill scale, and iron oxide
powder recovered from a waste pickling solution.
4. The method for manufacturing sponge iron according to claim 1,
wherein the reducing-agent powder comprises at least one selected
from the group consisting of coke, char, and coal.
5. The method for manufacturing sponge iron according to claim 1,
wherein a source of a carbon dioxide gas is added to the
reducing-agent powder.
6. The method for manufacturing sponge iron according to claim 1,
wherein the heating temperature is 1000.degree. C. to 1300.degree.
C. in the reducing step.
7. The method for manufacturing sponge iron according to claim 1,
wherein, in the charging step, the thicknesses of the layers of the
iron oxide powder and the reducing-agent powder are variable when
forming the layers that are in the form of helices.
8. The method for manufacturing sponge iron according to claim 1,
wherein, in the charging step, the amounts of iron oxide powder and
reducing-agent powder in the reaction container are controlled such
that the molar ratio of the carbon content in the reducing-agent
powder to the oxygen content in the iron oxide powder is at least
1.1.
9. The method for manufacturing sponge iron according to claim 2,
wherein, in the charging step, the amounts of iron oxide powder and
reducing-agent powder in the reaction container are controlled such
that the molar ratio of the carbon content in the reducing-agent
powder to the oxygen content in the iron oxide powder is at least
1.1.
10. The method for manufacturing sponge iron according to claim 9,
wherein, in the charging step, the amounts of iron oxide powder and
reducing-agent powder in the intermediate portion are controlled
such that the molar ratio of the carbon content in the
reducing-agent powder to the oxygen content in the iron oxide
powder is at least 0.5.
11. A method for manufacturing reduced iron powder, comprising the
steps of: pulverizing sponge iron manufactured by the method
according to claim 1; reducing the resulting pulverized iron; and
repulverizing the resulting reduced iron.
12. Sponge iron having a helical shape.
13. The sponge iron according to claim 12, wherein the sponge iron
has a metallic iron content of at least 97 percent by mass.
14. An apparatus for charging materials used to manufacture sponge
iron into a container, the materials being iron oxide powder and
reducing-agent powder, the apparatus comprising: a charger capable
of rotating and vertically moving in the container when the charger
is disposed in the container; an outlet for the iron oxide powder
and an-outlet for the reducing-agent powder, these outlets being
provided at the bottom of the charger and capable of rotating
together with the charger.
15. The apparatus for charging materials used to manufacture sponge
iron into a container according to claim 14, wherein the opening
areas of the outlet for the iron oxide powder and the outlet for
the reducing-agent powder can be variable.
16. The apparatus for charging materials used to manufacture sponge
iron into a container according to claim 14, wherein the charger
comprises: a cylindrical main body having a diameter of up to 85%
of the inside diameter of the container; and a lower end composed
of part of a cylinder, the horizontal section of the cylinder being
a circle having a diameter of 90% to 95% of the inside diameter of
the container, wherein the horizontal section of the lower end has
the shape of a sector including the center of the circle and part
of the circumference of the circle, or has a shape including the
sector.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for manufacturing
sponge iron used as a material in manufacturing iron powder and a
method for manufacturing reduced iron powder with the sponge iron
manufactured by the method.
[0002] The reduced iron powder is used in the form of powder as-is,
and also used as a material for a sintered product such as a
mechanical component and a magnetic material.
[0003] The present invention also relates to an apparatus for
charging a material of sponge iron manufactured by the method for
manufacturing a sponge iron, and relates to high-purity sponge iron
that can be manufactured by the method.
BACKGROUND ART
[0004] FIGS. 1A and 1B shows a general process for manufacturing
sponge iron. FIG. 1A is a vertical sectional view illustrating a
state of materials charged in a container. FIG. 1B is a horizontal
sectional view illustrating a state of materials charged in a
container.
[0005] Sponge iron is manufactured by the following procedure: Iron
oxide powder 2 and reducing-agent powder 3 are alternately charged
in the form of coaxial cylinders into a cylindrical heat-resistant
reaction container 1 (sagger) that can be equipped with a lid at
the bottom. The charged iron oxide powder 2 and reducing-agent
powder 3 are heated (indirectly heated) at 1050.degree. C. to
1200.degree. C. in the reaction container 1 with a tunnel furnace
or the like. The iron oxide powder 2 in the reaction container 1 is
reduced (roughly reduced) and is sintered by the heat treatment,
thus resulting in metallic iron that is in the form of a sponge,
i.e., sponge iron.
[0006] The iron oxide powder 2 includes iron ore powder and powder
produced by crushing mill scale. The reducing-agent powder 3
includes coke powder and coal powder. Lime powder or the like may
be added to the reducing-agent powder 3, if necessary.
[0007] The above-described techniques are disclosed in "Tekkou
binran", third edition, vol. 5, pp. 457-459 (in particular, page
457, right column, line 10-13) and Japanese Unexamined Patent
Application Publication No. 2002-241822.
[0008] In a known technique for manufacturing sponge iron as shown
in FIGS. 1A and 1B, the iron oxide powder 2 is cylindrically
charged into the reaction container 1 (hereinafter, referred to as
"cylindrical iron-oxide layer"). The reducing-agent powder 3
surrounds this cylindrical iron-oxide layer and is charged into
above, below, and inside of the cylindrical iron-oxide layer.
[0009] When the reaction container 1 is heated after the materials
are charged, in an early stage, a carbon dioxide (CO.sub.2) gas
formed by allowing oxygen that is present in the voids of the
charged layer of the reducing agent to react with carbon in the
reducing agent and formed by the decomposition of limestone added
to the reducing agent reacts with carbon in the reducing agent
according to chemical equation (1) to generate carbon monoxide
(CO), which is a reducing gas, in the charged layer of the
reducing-agent powder 3 (reducing-agent layer).
C+CO.sub.2.fwdarw.2CO equation (1)
[0010] The CO gas thus generated reaches from the reducing-agent
layer to a charged layer of the iron oxide powder 2 (iron-oxide
layer). Then, iron oxide is reduced as the generation of a CO.sub.2
gas according to the following chemical equation (2):
FeOn+nCO.fwdarw.Fe+nCO.sub.2 equation (2)
[0011] The generated CO.sub.2 gas diffuses into the iron-oxide
layer including partially-reduced iron oxide and reaches the
reducing-agent layer again. Then the CO.sub.2 gas reacts with
carbon in the reducing-agent layer to generate a CO gas according
to equation (1). This resulting CO gas diffuses into the iron-oxide
layer again and reacts with unreduced iron oxide according to
equation (2) to produce iron as the generation of a CO.sub.2
gas.
[0012] As a result, all iron oxide powder 2 charged in the reaction
container 1 is reduced to iron powder by repeating the reactions
according to equations (1) and (2) at certain intervals. At the
same time of this reduction reaction, reduced iron particles are
sintered to form cylindrical sponge iron (sintered body). FIG. 2
shows an appearance of a sponge iron produced by a known art (lower
part is omitted).
[0013] An amount of CO gas required for reducing all iron oxide is
theoretically 1 in molar ratio according to equation (2) ((the
number of moles of carbon atoms in the CO gas)/(the number of moles
of oxygen atoms in the iron oxide)). Hence, an amount of reducing
agent required for reducing all iron oxide is 1.0 in molar ratio
((the number of moles of carbon atoms in the reducing agent)/(the
number of moles of oxygen atoms in the iron oxide)). Hereinafter,
(the number of moles of carbon atoms in the reducing agent)/(the
number of moles of oxygen atoms in the iron oxide) is referred to
as "(the carbon content)/(the oxygen content) (molar ratio)".
DISCLOSURE OF INVENTION
[0014] In the above-described process for reducing, diffusion of
the CO and CO.sub.2 gases which are generated in the reaction
container 1 into the iron oxide powder 2 and reducing-agent powder
3 is a main rate-determining factor in the reduction reaction.
However, in a process having the structure charged as shown in FIG.
1, there is a problem in that it takes a long time required for the
reduction because of the long diffusion lengths of the CO and
CO.sub.2 gases.
[0015] For example, in a manufacturing step for an industrial-scale
production with a tunnel furnace for heating, a long time required
for the reduction decreases reaction efficiency (gas use
efficiency); hence, it takes several days from charging materials
to drawing a product, thus leading to low productivity.
Furthermore, heating energy consumption required for the reduction
is significantly large.
[0016] In a process having a charged structure as shown in FIGS. 1A
and 1B, although it is necessary to increase a thickness (radial
direction) of the layer of the iron oxide powder 2 in order to
increase the yield of sponge iron manufactured, in this case, long
reduction time is required. When the thickness of the layer of the
iron oxide powder 2 is reduced in order to shorten the reduction
time, an amount of sponge iron that can be manufactured per
reaction container is decreased. Hence, it does not necessarily
lead to the improvement of the yield per unit time.
[0017] Therefore, a combination of the thickness of the layer of
the iron oxide powder 2 and the reduction time is uniquely
determined so that the largest yield can be achieved. There are
problems with a low degree of flexibility in adjusting the yield as
well as the limitation of the yield.
[0018] In addition, in a process for charging as shown in FIGS. 1A
and 1B, a CO gas generated by the above-described reaction tends to
flow through the layer, which has a lower density, of the
reducing-agent powder 3 and then go out of the reaction container
1. Consequently, the CO gas does not effectively contribute to the
reduction reaction.
[0019] Furthermore, to hold shape of the layer of the iron
oxidepowder 2 in a firing stage, it is necessary to excessively
charge the reducing-agent powder 3 into a portion between the
reaction container 1 and the iron oxide powder 2 and into inside of
the cylindrical iron-oxide layer.
[0020] In the above-described circumstances in a known process,
there is a problem in that a large amount of reducing-agent powder
3 is required, i.e., at least 2.0 in molar ratio, thus resulting in
poor unit requirement of a reducing agent.
[0021] In addition, the lower portion of a cylindrical iron-oxide
layer can swell under its own weight. Hence, there is a problem in
that iron oxide in the swelling portion is insufficiently reduced
within a predetermined reduction time, thus remaining an unreduced
portion.
[0022] It is an object of the present invention to advantageously
solve various problems described above of the known art. That is,
it is an object of the present invention to provide a method for
manufacturing sponge iron, wherein the method has high productivity
and can easily adjust the yield.
[0023] It is another object of the present invention to provide an
apparatus for charging materials into a reaction container, wherein
the apparatus is advantageously used when the above-described
method for manufacturing is performed.
[0024] The inventors have conducted intensive research, and found
that the above-described problems can be advantageously solved by
devising a charged form of iron oxide powder and reducing-agent
powder in a reaction container. Consequently, the present invention
has been completed.
[0025] That is, a first aspect of the present invention, a method
for manufacturing sponge iron includes a charging step of charging
iron oxide powder and reducing-agent powder into a reaction
container; and a reducing step of reducing the iron oxide powder in
the reaction container to produce a mass of sponge iron by heating
from the outside of the reaction container, wherein, in the
charging step, the iron oxide powder and the reducing-agent powder
are charged such that alternating layers of the iron oxide powder
and the reducing-agent powder are formed and such that each of the
layers is in the form of a helix.
[0026] In the above-described first aspect of the present
invention, suitable conditions described below are preferably
applied alone or in any combination.
[0027] (1) In the charging step, the iron oxide powder and the
reducing-agent powder are charged such that layers composed of the
reducing-agent powder are disposed on an inner side-surface of the
reaction container (referred to as "peripheral portion") and
disposed at a central portion along the vertical central axis and
such that the alternating layers that are in the form of helices
are disposed at a portion (referred to as "intermediate portion")
other than the portion of the layers disposed on the inner
side-surface and at the central portion. The peripheral portion and
the central portion along the vertical central axis correspond to a
circumferential portion and a central portion, respectively, in
horizontal sectional view of the container. The intermediate
portion is preferably in the form of a cylinder or a column. When
the reaction container is in the form of cylinder, the vertical
central axis corresponds to the center of the cylinder.
[0028] (2) The iron oxide powder is composed of at least one
selected from the group consisting of an iron ore, mill scale, and
iron oxide powder recovered from waste pickle liquor.
[0029] (3) The reducing-agent powder is composed of at least one
selected from the group consisting of coke, char, and coal.
[0030] (4) A source of a carbon dioxide gas is added to the
reducing-agent powder. The source of a carbon dioxide gas
preferably includes limestone (including calcined limestone). In
this case, the reducing-agent powder to which the powder of the
source of a carbon dioxide gas is added is charged.
[0031] (5) The heating temperature is 1000.degree. C. to
1300.degree. C. in the reducing step.
[0032] (6) In the charging step, the thicknesses of the layers of
the iron oxide powder and the reducing-agent powder are variable
when forming the layers that are in the form of helices. Variably
controlling includes the following meanings: A different thickness
of at least any one of the layers can be set in each reaction
container. A thickness of at least any one of the layers can be
varied with position of the reaction container 1.
[0033] (7) In the charging step, the amounts of iron oxide powder
and reducing-agent powder in the reaction container are controlled
such that the molar ratio of the carbon content in the
reducing-agent powder to the oxygen content in the iron oxide
powder is at least 1.1. The molar ratio is preferably 1.15 or more
and more preferably 1.2 or more.
[0034] (8) In the charging step according to suitable conditions
(1) and (7), the amounts of iron oxide powder and reducing-agent
powder in the charged portion having layered structure are
controlled such that the molar ratio of the carbon content in the
reducing-agent powder to the oxygen content in the iron oxide
powder is at least 0.5. The term "charged portion having layered
structure" represents a cylindrical region formed of helically
deposited layers of iron oxide powder and reducing-agent powder.
The region usually corresponds to a portion other than "layers
composed of the reducing-agent powder" described in (1).
[0035] A second aspect of the present invention is a method for
manufacturing reduced iron powder, the method including the steps
of pulverizing sponge iron manufactured by the method according to
the first aspect; reducing the resulting pulverized iron; and
repulverizing the resulting reduced iron.
[0036] Suitable conditions (1) to (8) in the first aspect of the
present invention can be applied in any combination.
[0037] A third aspect of the present invention is sintered sponge
iron having a helical form. The sponge iron preferably has high
purity, i.e., has a metallic iron content of at least 97 percent by
mass. In the first aspect of the present invention, even a mass of
the high-purity sponge iron having a weight of 100 kg or more can
be manufactured by, for example, particularly applying suitable
condition (7) and subjecting to reduction treatment for sufficient
time.
[0038] A fourth aspect of the present invention is an apparatus for
charging materials used to manufacture sponge iron into a
container, the materials being iron oxide powder and reducing-agent
powder, the apparatus including a charger capable of rotating and
vertically moving in the container when the charger is disposed in
the container; an outlet for the iron oxide powder and an outlet
for the reducing-agent powder, these outlets being provided at the
bottom of the charger and capable of rotating together with the
charger.
[0039] For the method for manufacturing sponge iron according to
the first aspect of the present invention, the fourth aspect of the
present invention is preferably employed to charge the iron oxide
powder and the reducing-agent powder such that alternating layers
of the iron oxide powder and the reducing-agent powder are disposed
and such that each of the layers is in the form of a helix.
[0040] In the fourth aspect of the present invention, the opening
areas of the outlet for the iron oxide powder and the outlet for
the reducing-agent powder is preferably variable. Such a structure
can be preferably used to particularly satisfy suitable condition
(6).
[0041] In the fourth aspect of the present invention, the charger
preferably includes a cylindrical main body having a diameter of up
to 85% of the inside diameter of the container; and a lower end
composed of part of a cylinder, the horizontal section of the
cylinder being a circle having a diameter of 90% to 95% of the
inside diameter of the container, wherein the horizontal section of
the lower end has the shape of a sector including the center of the
circle and part of the circumference of the circle, or has a shape
including the sector. Such a structure can be preferably used to
reduce the thickness of the layer composed of the reducing-agent
powder disposed at the peripheral portion described in suitable
condition (1). Furthermore, even when a projection composed of an
adherent is produced in the reaction container, the above-described
charger can be disposed without interference.
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] FIG. 1A is a cross-sectional view illustrating a known
process for charging iron oxide powder and reducing-agent
powder;
[0043] FIG. 1B is a horizontal sectional view taken along line
IB-IB' in FIG. 1A;
[0044] FIG. 2 is a perspective view showing an appearance of sponge
iron produced by a known process;
[0045] FIG. 3A is a cross-sectional view illustrating an example of
a method for charging iron oxide powder and reducing-agent powder
according to the present invention;
[0046] FIG. 3B is a horizontal sectional view taken along line
IIIB-IIIB' in FIG. 3A;
[0047] FIG. 4A is a schematic diagram showing an example of a
structure of a charger (rotatable charging cylinder) of the present
invention;
[0048] FIG. 4B is a cross-sectional view showing a charging state
when using the rotatable charging cylinder;
[0049] FIG. 5 is a schematic diagram showing another example of a
structure of a charger (rotatable charging cylinder) of the present
invention;
[0050] FIG. 6 is a cross-sectional view illustrating another
example of a method for charging iron oxide powder and
reducing-agent powder according to the present invention;
[0051] FIG. 7 is a perspective view showing an appearance of sponge
iron produced by the present invention;
[0052] FIG. 8 is a cross-sectional view illustrating an
experimental example of a method for charging iron oxide powder and
reducing-agent powder which are in the form of horizontal multiple
layers;
[0053] FIG. 9 is a graph showing the relationship between (the
carbon content)/(the oxygen content) (in molar ratio) (horizontal
axis) in the entire reaction container and time required for
reduction (vertical axis) with reference to various thicknesses of
iron oxide layers in a method of alternating charging;
[0054] FIG. 10 is a cross-sectional view illustrating another
experimental example of a method for charging iron oxide powder and
reducing-agent powder which are in the form of horizontal multiple
layers;
[0055] FIG. 11 is a graph showing the relationship between (the
carbon content)/(the oxygen content) (in molar ratio) (horizontal
axis) in the portion charged in the form of alternating layers and
time required for reduction (vertical axis) with reference to
various thicknesses of iron oxide layers in another method of
alternating charging;
[0056] FIG. 12 is a graph showing the relationship between (the
carbon content)/(the oxygen content) (in molar ratio) (horizontal
axis) in the entire reaction container and time required for
reduction (vertical axis) with reference to various thicknesses of
iron oxide layers in the another method of alternating
deposition;
[0057] FIG. 13 is a graph showing the relationship between the
increment of iron oxide (percent by weight, horizontal axis) and
the purity of metallic iron obtained by the reduction (percent by
mass, vertical axis) with reference to charging in an interwound
helical form (hatching patterned bars) and charging in a
cylindrical form (outline bars);
[0058] FIG. 14A is a cross-sectional view showing yet another
example of a structure of a charger (rotatable charging cylinder);
and
[0059] FIG. 14B is an arrow view showing a cross-section taken
along line XIVB-XIVB' in FIG. 14A (the thickness of the wall is
omitted).
REFERENCE NUMERALS
[0060] 1, 11 reaction container (sagger)
[0061] 2, 12 iron oxide powder
[0062] 3, 13 reducing-agent powder
[0063] 14 apparatus for charging materials
[0064] 14a, 14d partition wall
[0065] 14b rotatable charging cylinder
[0066] 14c cut-out section
[0067] 15 outlet for iron oxide powder
[0068] 16 outlet for reducing-agent powder (used for alternating
charging)
[0069] 16a outlet for delivering reducing-agent powder to the
peripheral portion
[0070] 16b outlet for delivering reducing-agent powder to the
central axial portion
[0071] 17 iron-oxide-powder holding section
[0072] 18 reducing-agent-powder holding section
[0073] 19a, 19b presser plate
[0074] a opening height
BEST MODE FOR CARRYING OUT THE INVENTION
[0075] [Method and Apparatus for Charging Materials]
[0076] The present invention is characterized by a method for
charging materials. The materials are iron oxide powder and
reducing-agent powder. Limestone and the like may be added to the
reducing agent, if necessary.
[0077] As shown in FIG. 1, for example, a process for charging iron
oxidepowder 2 and reducing-agent powder 3, which are in the form of
coaxial cylinders along the axial direction, into an upright
heat-resistant reaction container 1 having a cylindrical shape is
generally applied. Alternatively, the present invention employs a
method for charging iron oxide powder and reducing-agent powder in
helical forms. That is, iron oxide powder and reducing-agent powder
are charged such that a helical layer composed of the iron oxide
powder and a helical layer composed of the reducing-agent powder
are alternately stacked (hereinafter, referred to as "interwound
helical charging").
[0078] By employing a method for interwound helical charging, iron
oxide powder and reducing-agent powder can be charged
simultaneously and continuously. Therefore, a constant thickness of
each layer (the amount of charged powder) can be obtained.
Consequently, the thickness ratio of a reducing agent layer to an
iron oxide layer can also be maintained constant. This thickness
ratio can be set to a desired ratio in each reaction container
depending on a purpose and circumstances.
[0079] In addition, the thickness ratio can also be changed to a
desired value at any time.
[0080] Consequently, the method for interwound helical charging is
useful as a method that is conducive to the improvement of
productivity and yield.
[0081] FIGS. 3A and 3B show an example of the present invention. In
charging materials according to the present invention, it is
preferable to simultaneously charge iron oxide powder 12 and
reducing-agent powder 13 into a cylindrical reaction container 11
(sagger) composed of a heat-resistant material such as silicon
carbide (SiC) with an apparatus for charging materials 14.
[0082] The apparatus for charging materials 14 preferably has a
structure described below.
[0083] The apparatus for charging materials 14 mainly consists of a
rotatable charging cylinder 14b (charger) that is inserted into the
reaction container 11. The cylindrical main body of the rotatable
charging cylinder 14b is separated by a partition wall 14a into two
compartments. The iron oxide powder 12 and the reducing-agent
powder 13 are charged into the two compartments, i.e., an
iron-oxide-powder holding section 17 and a reducing-agent-powder
holding section 18, respectively (each of the material powder is
not shown in the corresponding holding section). Furthermore, an
outlet for iron oxide powder 15 and an outlet for reducing-agent
powder 16 are provided as openings of the holding sections 17 and
18, respectively, at the lower end (bottom or the neighborhood of
the bottom) of the rotatable charging cylinder 14b. The degree of
opening of each outlet (for example, opening height a), that is,
the opening area can be preferably adjusted by a gate such as a
sliding gate (not shown). The position and the direction of each
outlet may be determined according to need. The openings can be
provided at any face selected from among the undersurface, the side
face, and on a cut-out section provided at the undersurface of the
rotatable charging cylinder 14b. Each of the material powder
charged into the corresponding holding section is preferably
delivered by its own weight in principle.
[0084] FIG. 4A is a detail view showing an example of the rotatable
charging cylinder 14b. In this example, a cut-out section 14c that
is in the form of a square cylinder is disposed at a position
extending from the cylinder bottom in a direction perpendicular to
the partition wall 14a. The two outlets (openings) 15 and 16 which
are connected to the holding section 17 and 18 are provided at side
walls that are diagonally opposite each other of the cut-out
section 14c. FIG. 4B is a cross-sectional view showing a state
charged with such a rotatable charging cylinder.
[0085] Modification of this structure includes a structure in which
each of the cut-out sections for iron oxide powder and
reducing-agent powder has the shape of a sector that is about one
quarter of a circle in horizontal section and that is diagonally
opposite each other. In this case, at least part of the outlet 15
and at least part of the outlet 16 are preferably provided at side
faces, which are corresponding to a straight line of the sector, in
the same plane through the axis of the rotatable charging cylinder
14b (a state illustrated in a cross-sectional view of FIG. 3A is
obtained).
[0086] FIG. 5 is a detail view showing another example of the
rotatable charging cylinder 14b.
[0087] To surely charge material powder up to the circumferential
portion of the reaction container 11 under control, the rotatable
charging cylinder 14b preferably has a diameter close to the inner
diameter of the reaction container 11. However, the reaction
container is repeatedly used, and a plurality of cylinders may be
stacked to form a reaction container. Hence, for example, reduced
iron and ash in a reducing agent can adhere to inside of the
reaction container to form a projection. In addition, the container
can slightly incline by strain caused by repeated use. Therefore,
the lower end of the rotatable charging cylinder 14b having a
diameter very close to the inner diameter of the reaction container
11 can come in contact with the reaction container 11, thus causing
damage.
[0088] The purpose of bringing the lower end of the rotatable
charging cylinder 14b closer to the inner diameter of the reaction
container 11 is that openings extending from near the center to
near the circumference of the reaction container are used as the
outlets. Hence, if the positions of the outlets are modified, the
lower end of the rotatable charging cylinder 14b need not have the
shape of the perfect circle in horizontal section. A sector that is
part of this circle (virtual circle) or a shape including at least
the sector is adequate for the lower end.
[0089] FIG. 5 is an example of a lower end having the shape of a
sector. The outlet for iron oxide powder 15 and the outlet for
reducing-agent powder 16 are asymmetrically provided at the side
faces (corresponding to straight lines of the sector) of the
cut-out section 14c that is provided in the same way as shown in
FIG. 4. Although the undersurface of the cut-out section 14c is
open, each powder 12 and 13 is mainly delivered from the side face
because deposited powder functions as the undersurface. Reference
numerals 19a and 19b represent presser plates.
[0090] A desired central angle of a sector may be used. The central
angle is preferably about 180.degree. (i.e., semicircle) or less in
achieving a satisfactorily compact lower end. More preferably, the
maximal diameter of a horizontal section of a cut-out section is
smaller than the diameter of the virtual circle.
[0091] The virtual circle of the lower end desirably has a diameter
closer to the inner diameter of the reaction container in view of
productivity, and preferably has a diameter of about 90% or more of
the inner diameter of the reaction container. On the other hand,
the virtual circle of the lower end desirably has an adequately
small diameter in view of operation, and preferably has a diameter
of about 95% or less of the inner diameter of the reaction
container.
[0092] The rotatable charging cylinder 14b preferably has a
diameter of about 85% or less of the inner diameter of the reaction
container. Leaving a clearance for horizontal displacement in the
container is preferable in order to avoid contact. From the
viewpoint of ensuring the pathway of material powder charged, the
main body of the rotatable charging cylinder has a diameter of
about 30% or more of the inner diameter of the reaction
container.
[0093] Interwound helical charging with such an apparatus for
charging materials 14 is performed as follows: Opening areas (the
degrees of openings) of the outlets 15 and 16 are adjusted. The
rotatable charging cylinder 14b is then inserted into the reaction
container 11 from above. By moving upward the rotatable charging
cylinder 14b at a constant speed while rotating the rotatable
charging cylinder 14b (that is, rotating the outlets 15 and 16),
the materials are charged (alternating charging) via the outlets
such that the ratio of the thickness of the layer of iron oxide
powder and the thickness of the layer of reducing-agent powder is a
constant and such that the layers are wound with each other. In
this way, alternating layers of the iron oxide powder 12 and the
reducing-agent powder 13, which are in the form of helices, is
provided in the reaction container 11.
[0094] Materials are fed into the holding sections 17 and 18 before
charging or during charging into a reaction container, according to
need.
[0095] FIG. 6 shows another example of a method for charging
according to the present invention. The apparatus for charging
materials 14 is shown schematically.
[0096] As shown in FIG. 6, in charging material powder into a
reaction container, a region where interwound helical charging is
performed may be limited to a region other than the peripheral
portion along the axial direction of the reaction container 11. In
addition, a region where interwound helical charging is performed
may be limited to a region other than the central axial portion
along the axial direction of the reaction container 11.
Furthermore, a region where interwound helical charging is
performed may be limited to a region other than both the peripheral
portion and the central axial portion along the axial direction of
the reaction container 1. In all cases, a region where interwound
helical charging is performed is referred to as "cylindrical
intermediate portion". The peripheral portion and the central axial
portion correspond to the circumferential portion and the center of
the container in horizontal section.
[0097] The reducing-agent layer at the peripheral portion can be
necessarily provided from the viewpoint of preventing the
interference between the rotatable charging cylinder 14b of the
apparatus for charging materials 14 and the reaction container 11
and preventing the seizure at the contact regions between the
reaction container and the iron oxide powder. The reducing-agent
layer at the central axial portion can be provided for handling
reasons when removing sponge iron from the container. In such a
case, since a layer composed of a reducing agent alone is provided
at the peripheral portion or the central axial portion, paths of
the reaction gases are formed; hence, the gases diffuse in the
container readily and uniformly. As a result, the effect of
improving the reaction rate can be expected. In addition, the
reducing-agent layer provided at the peripheral portion can also
prevent a product from adhering to the wall of the container.
Therefore, these reducing-agent layers are preferably provided
while optimizing the radial thickness of the layer in view of the
yield of a reducing agent and the molar ratio of (the carbon
content)/(the oxygen content) and the like, if necessary.
[0098] In a cylindrical container, a layer provided at the
peripheral portion preferably has a radial thickness of about 2.5%
to about 5% of the inner diameter of the container. The layer
provided at the central axial portion preferably has a diameter of
about 250 mm or less.
[0099] For example, to provide a reducing-agent layer at the
peripheral portion, an opening is provided at the side of the
rotatable charging cylinder 14a, and then reducing-agent powder may
be delivered to form a layer at the peripheral portion.
Furthermore, to provide a reducing-agent layer at the central axial
portion, a central tube having an opening at its bottom is further
provided at a position where the partition wall 14c is provided,
and then reducing-agent powder may be delivered from the opening to
form a layer at the central axial portion.
[0100] These openings may be connected to the outlet 16 for
providing a helical layer or may be isolated.
[0101] FIG. 14A shows an example of a rotatable charging cylinder
that can charge in a state as shown in FIG. 6. FIG. 14B is a
schematical cross-sectional view taken along line XIVB-XIVB' in
FIG. 14A (the thickness of the wall is omitted for the
simplification). In this example, the outlet for reducing-agent
powder 16 is provided at the undersurface of the rotatable charging
cylinder 14b in order to charge in the form of alternating layers,
for example, to charge in the form of interwound helices.
Furthermore, an opening is provided at the side face of the lower
end of the rotatable charging cylinder 14b, thus constituting an
outlet for delivering reducing-agent powder into the peripheral
portion 16a. In addition, an outlet for delivering reducing-agent
powder into the central axial portion 16b is provided at the center
of the undersurface of the rotatable charging cylinder 14b. A
portion of reducing-agent powder is guided by a partition wall
14d.
[0102] As shown in FIG. 3A, the bottom layer is usually composed of
reducing-agent powder (and limestone and the like) alone. As a
result, the lower end of the iron oxide layer can be surely
reduced, and the seizure between the reaction container and the
iron oxide layer is preferably blocked. The top layer is preferably
composed of reducing-agent powder alone for the same reasons. These
reducing-agent layers can be formed by, for example, closing the
outlet for iron oxide powder 15 of the apparatus for charging
materials 14 or stopping the supply of iron oxide powder to the
rotatable charging cylinder 14b.
[0103] In the present invention, when interwound helical charging
is performed with the above-described apparatus, it is preferable
to variably control the thicknesses of the iron oxide layer and
reducing-agent layer. That is, the thickness of each layer is
preferably maintained constant in each reaction container. However,
it is preferable to be able to adjust the thickness, for example,
to optimize the thickness depending on a material.
[0104] Such a change in thickness of each layer can be achieved by
adjusting at least any two selected from, for example, the rotation
speed and the rising speed of the rotatable charging cylinder 14b
and the degrees of openings of the outlets 15 and 16. In
particular, the adjustment of the degrees of openings of the
outlets 15 and 16 by, for example, opening and closing gates is
preferable because a stable operation can be achieved without the
reductions of diffusibility and the yield and the extension of
reduction time.
[0105] The thickness of each layer can be varied continuously or
discontinuously in theory with the height of the upright reaction
container 11, for example, can be varied at the bottom, the middle,
and the upper portion of the reaction container 11. The present
invention does not exclude such an application. An example of an
application includes that the thickness of the iron oxide layer is
increased at the upper portion where the reduction tends to readily
proceed.
[0106] An iron oxide layer and a reducing-agent layer, which are
provided in the form of helices, preferably have a thickness of at
least about 5 mm. The sum of the thicknesses of the iron oxide
layer and the reducing-agent layer is preferably at least about 10
mm and more preferably at least 40 mm. Excessively small thickness
readily results in an abnormal layer structure because of the
fluctuation of the thickness of each layer. The lower limit of the
thickness of each layer is more preferably at least about 10 mm.
The lower limit of the sum of the thicknesses of the layers is more
preferably at least about 30 mm.
[0107] On the other hand, excessively large thickness increases a
time required for the reduction treatment and reduces the
material-efficiency. Hence, each of the layers preferably has a
thickness of about 100 mm or less. The sum of the thicknesses of
the layers (one iron oxide layer and one reducing-agent layer)
preferably is about 200 mm or less. The upper limit of the
thickness of each layer is more preferably about 80 mm. The upper
limit of the sum of the thicknesses of the layers is more
preferably about 150 mm.
[0108] The ratio between an iron oxide layer and a reducing-agent
layer is usually expressed not by the thickness but by (the carbon
content)/(the oxygen content) (molar ratio). A preferable ratio
will be described below.
[0109] The above-described apparatus for charging materials is an
example. That is to say, in an apparatus for charging iron oxide
powder and reducing-agent powder into a reaction container, the
apparatus preferably includes a charger capable of rotating and
vertically moving; and an outlet for the iron oxide powder and an
outlet for the reducing-agent powder, these outlets being provided
at the charger and capable of rotating together with the charger.
The apparatus can charge the iron oxide powder and the
reducing-agent powder from the outlets in the form of a double
helix by putting the charger into the reaction container and then
moving the charger upward while rotating the charger.
[0110] The charger advantageously has, for example, a cylindrical
shape, but is not limited to this. The charger may have a tubular
shape whose cross-section is in the form of, for example, a sector,
a star, or a multilobal according to the shape of a reaction
container. The holding sections need not be provided by separating
the inside of the charger with a partition wall. Any shape and
position of each holding section may be used. The iron-oxide-powder
holding section and the reducing-agent-powder holding section need
not have the same capacities.
[0111] A fixed or movable guide plate and/or a presser plate are
preferably provided around the outlets 15 and 16 in order to guide
material powder to the direction desired.
[0112] [Material Powder]
[0113] In a method for manufacturing sponge iron according to the
present invention, materials charged into a reaction container
include at least iron oxide powder and reducing-agent powder. The
iron oxide powder preferably includes a powdered iron ore or
powdered mill scale generated in a hot-rolling step of steel. A
pickling step of removing, for example, oxides formed on the steel
products with an acid such as hydrochloric acid results in a waste
acid (pickle liquor). An iron oxide powder obtained by roasting
this pickle liquor is also preferable as the material. Such an iron
oxide powder preferably has an average particle size of about 0.05
to about 10 mm.
[0114] Furthermore, finer iron oxide powder having a particle size
smaller than that of the above-described iron oxide powder, for
example, hematite powder that is industrially controlled so as to
have a specific surface area of at least 2 m.sup.2/g and a particle
size of at least 0.01 .mu.m is added to the mill scale and/or the
iron ore to produce a mixture. The resulting mixture is preferably
used for the material because the mixture improves the quality of
sponge iron.
[0115] Reducing-agent powder includes so-called carbonaceous powder
containing carbon. The carbonaceous powder preferably includes, for
example, coke powder, char (a kind of high-volatile charcoal), coal
powder (noncaking coal is preferable), anthracite powder, and
charcoal. From the viewpoint of the efficient reduction, the
carbonaceous powder preferably has a carbon content of 60% or more.
Reducing-agent powder preferably has an average particle size of
about 0.05 to about 10 mm.
[0116] There is no problem in that reducing-agent powder containing
powder that is a source of a carbon dioxide gas is used as a
material for reducing-agent layers, according to need. The source
of a carbon dioxide gas preferably includes limestone (including
hydrated lime).
[0117] [Reducing Step]
[0118] The iron oxide powder 12 and the reducing-agent powder 13
(including a source of a carbon dioxide gas added and mixed) are
charged into the reaction container 11 with an apparatus for
charging materials 14 shown in, for example, FIGS. 3A and 3B to
provide layers in the form of helices. The reaction container 11
preferably includes, for example, a cylindrical reaction container,
called a sagger, composed of silicon carbide (SiC). The shape of
the reaction container 11 is not limited, but it is believed that a
cylindrical shape is the most advantageous for the reaction
container 11. Furthermore, the dimensions of the reaction container
are not limited. However, in cylindrical shape, the reaction
container preferably has an inner diameter of about 200 to about
800 mm and has a height of about 100 to about 2000 mm. An amount of
the mass of sponge iron manufactured per container is preferably at
least about 10. kg, from the viewpoint of productivity, more
preferably at least about 50 kg, and most preferably at least about
100 kg.
[0119] The reaction container 11 into which the iron oxide powder
12, the reducing-agent powder 13, and, if necessary, limestone and
the like is charged is placed on, for example, a truck and is
disposed at a furnace such as a tunnel furnace. Then, the reduction
is performed by heating the materials charged into the container
for a predetermined time with the container. This reduction is
called a "rough reduction". The purity target (metallic iron
content in sponge iron after the reduction) is determined depending
on an application of the reduced iron powder and is at least about
90 percent by mass, and in an application that requires high
purity, at least about 97 percent by mass. The purity target has no
upper limit. However, the purity achieved within the allowable
costs is about 99.5 percent by mass at the maximum under the
present conditions.
[0120] Unsatisfactory heating temperature for the reduction leads
to the insufficient reduction of iron oxide, thus decreasing the
purity of the resulting sponge iron. The lower limit of the heating
temperature is preferably about 1000.degree. C. On the other hand,
excessively high heating temperature excessively sinters sponge
iron simultaneously with the reduction to harden. As a result,
electric power consumption can be increased when roughly
pulverizing or manufacturing costs can be increased due to wear and
tear on a pulverizing tool. The upper limit of the heating
temperature is preferably 1300.degree. C. Consequently, the heating
temperature is preferably in the range of 1000.degree. C. to
1300.degree. C.
[0121] When a tunnel furnace is used, the reaction container 11
(and iron oxide in the container) that is placed on a truck and
moved in the furnace passes through a preheating zone, where the
temperature is gradually increased, over a period of about 24 hours
(preferably between 20 and 28 hours) and is retained in a firing
zone at about 1000.degree. C. to about 1300.degree. C. for about 60
hours (preferably at least 36 hours and more preferably at least 56
hours; and preferably up to 72 hours and more preferably up to 64
hours). After passing through a cooling zone where the temperature
is gradually reduced (preferably over a period of 20 to 28 hours),
the reduction treatment is completed. The inlet temperature of the
preheating zone and the outlet temperature of the cooling zone are
preferably about 200.degree. C. (about 20.degree. C. to about
400.degree. C.), while the outlet temperature of the preheating
zone and the inlet temperature of the cooling zone are preferably
about 900.degree. C. (about between (the temperature of the firing
zone)-450.degree. C. and (the temperature of the firing
zone)-50.degree. C.), from the viewpoint of, for example, the
protection of the reaction container (refractory).
[0122] Iron oxide is reduced with a reducing agent to produce a
mass of sponge iron by such a thermal reduction reaction. The
resulting sponge iron is necessarily a mass that is in the form of
helix. FIG. 7 shows an example of an appearance (the top end and
the bottom end are omitted) of sponge iron produced by a method of
the present invention.
[0123] A larger height (the axial direction) of the resulting mass
of sponge iron is preferable. However, in view of the limitation of
the size of a reaction container and the reduction of thermal
efficiency resulting from the large size of a reaction container
when heightening a reaction container, a mass of sponge iron
preferably has a height of about 2000 mm or less.
[0124] A method of the present invention can provide high-purity
sponge iron having a purity of 97 percent by mass or more. When the
purity is at least 97 percent by mass, the product characteristics
of sintered components such as mechanical components and magnetic
materials or of reduction iron powder that is used in the form of
powder as-is are advantageously guaranteed. However, a method of
the present invention has the advantage other than purity and thus
is not limited to a method for manufacturing sponge iron having a
purity of at least 97 percent by mass or having high purity. That
is, a method of the present invention can be generally applied to a
usually rough reduction providing sponge iron having a purity of at
least 90 percent by mass. Components other than produced metallic
iron generally contains iron oxide and impurities such as silicon
(Si), manganese (Mn), phosphorous (P), and sulfur (S), the
impurities being in an amount of up to one percent by mass in
total.
[0125] After heating for the rough reduction, produced sponge iron
is separated from a reducing agent and is removed from the reaction
container 11. The resulting sponge iron removed from the reaction
container 11 is roughly pulverized for a finishing reduction into
powder generally having a particle size of about 150 .mu.m or less,
thus resulting in roughly reduced particles. Next, the roughly
reduced particles are disposed in a finish-reducing furnace with a
reducing atmosphere and are subjected to finishing reduction, and
are then further pulverized, thus resulting in reduced iron
powder.
[0126] [Ratio of Iron Oxide to Reducing Agent]
[0127] In charging materials into a reaction container, the ratio
of the amount of iron oxide to the amount of a reducing agent
(solid reducing agent) when the above-described interwound helical
charging is performed, in particular, the ratio of carbon content
in a reducing agent required for oxygen content in iron oxide has
already been described above according to equation (2). That is,
the ratio is determined based on the reduction reaction in which
one carbon atom in a reducing agent reacts with one oxygen atom in
iron oxide ((the carbon content)/(the oxygen content)=1.0 (molar
ratio)). However, a reducing agent needs to generally have a carbon
content larger than the oxygen content in iron oxide. In a known
method, the carbon content in a reducing agent is excessively
charged, that is, is 2.0 to 2.5 times the oxygen content in iron
oxide ((carbon content)/(oxygen content)=2.0 to 2.5 (molar ratio))
because of the above-described reasons. In this case, a reduction
ratio (the purity target of sponge iron) is at least 90 percent by
mass and preferably at least 97 percent by mass in metallic
iron.
[0128] The inventors have investigated the relationship between
(the carbon content)/(the oxygen content) (molar ratio) and the
time required for the reduction in a method for interwound helical
charging by the following experiments.
[0129] As shown in FIG. 8, to simplify the experiments, a method
for charging in the form of not helices but horizontally
alternating charging was employed. That is, the iron oxide powder
12 and the reducing-agent powder 13 are alternately charged to
provide alternating layers that are substantially horizontal. The
horizontally alternating charging produces sponge iron in the form
of a plurality of disks by reduction, thus causing the operation to
be complicated. Therefore, the interwound helical charging has an
advantage over the horizontally alternating charging in actual use.
However, from the viewpoint of the relationship between (the carbon
content)/(the oxygen content) (molar ratio) and the progress of the
reduction reaction, the horizontally alternating charging is
equivalent to the interwound helical charging. Hereinafter, the
horizontally alternating charging and the interwound helical
charging are generically referred to as "alternating charging".
[0130] A reaction container used for the experiments has an inner
diameter of 370 mm, and materials are charged such that the charged
materials have a height of 1400 mm. Iron oxide powder and
reducing-agent powder used were the same materials used in Example
1 described below. Reduction treatment is performed at a maximum
temperature of 1150 .degree. C. A reduction time represents a
retention time at this maximum temperature.
[0131] FIG. 9 is a graph showing the relationship between the ratio
of the carbon content to the oxygen content (in molar ratio) and
the reduction time required for producing metallic iron having a
purity of 97 percent by mass with reference to various thicknesses
of iron oxide layers in a method for horizontally alternating
charging. The molar ratio is the ratio of the carbon content in and
all reducing agent to the oxygen content in all iron oxide.
[0132] As shown in FIG. 9, the filled circle (conventional example
.circle-solid.) represents an example of the result of the same
reduction treatment with a general process for charging in a
cylindrical form (shown in FIG. 1). In this general process, each
of the iron oxide layers had a thickness of 55 mm, (the carbon
content)/(the oxygen content) (molar ratio) was 2.2. The reduction
time required was as much as 53 hours.
[0133] Iron oxide layers having thicknesses of 15 mm (Experimental
Example 4: cross (x)), 20 mm (Experimental Example 3: triangle), 30
mm (Experimental Example 2: square (.box-solid.)), and 50 mm
(Experimental Example 1: rhombus (.diamond-solid.)) provided by
horizontally alternating charging (as shown in FIG. 8) were
reduced. As a result, a smaller thickness of the iron oxide layer
led to the shortening of the reduction time. In the case of a layer
having a thickness of at least 20 mm, when the molar ratio was 1.2
or more, the reduction time was substantially constant. It was
found that the molar ratio did not need to be 2.0 or more.
[0134] When the molar ratio is less than 1.2, it tends to prolong
the reduction time. However, alternating from a process for
charging in a cylindrical form to a method of alternating charging
and the effect resulting from the reduction of the thickness of
layers predominantly counteract the tendency of the prolongation of
the reduction time. That is, more iron oxide can be charged by a
method for helical charging. For example, in this example, a method
for charging iron oxide having a thickness of 30 mm in an
interwound helical form can charge substantially the same amount of
iron oxide charged by a general process for charging in a
cylindrical form. Therefore, in the experimental range where the
molar ratio is 1.1 or more, the effect of the present invention is
sufficiently obtained. In addition, when the molar ratio is 1.15 or
more, the effect of the present invention is more sufficiently
obtained because of a small degree of prolongation of the reduction
time. As a matter of course, when the molar ratio is 1.2 or more,
the reduction time is further shortened.
[0135] When the thickness of each iron oxide layer was 15 mm, the
reduction time was substantially constant at a molar ratio of 1.6
or more. Resulting from repeated experiments under the different
conditions, it was also found that, in an oxygen iron layer having
a thickness of less than 20 mm, the following relationship
holds:
(molar ratio).times.(thickness of iron oxide layer (mm))=2.3 to 2.5
equation (3)
[0136] When the thickness of each iron oxide layer is less than 20
mm, by charging so as to satisfy equation (3), the determination of
the thickness of each iron oxide layer necessarily leads to the
reduction time, thus resulting in a stable operation and a stable
quality of sponge iron produced. However, this relationship can be
due to the difficulty in stably controlling thinner thickness of
each reducing-agent layer rather than an essential relationship
based on the rate of reaction. Hence, it is expected that the
above-described limitation is relaxed as an improvement of a
technique in controlling the thickness of layers.
[0137] From the viewpoint of the yield of a reducing agent, (the
carbon content)/(the oxygen content) (molar ratio) preferably is
not increased. When the molar ratio is less than 2.0, a method of
the present invention has an advantage compared with a general
process for charging in a cylindrical form. The molar ratio is
preferably 1.8 or less.
[0138] As shown in FIG. 6, when a reducing-agent layer is provided
at the peripheral portion in a container or a central axial portion
of the container, the inventors thought that it was necessary to
study whether the regulation of (the carbon content)/(the oxygen
content) molar ratio) in the entire container alone was adequate as
a measure in designing the ratio of the thicknesses of a
reducing-agent layer and a iron oxide layer.
[0139] To determine the amount required of a reducing agent at the
portion of the deposited layers of materials (an intermediated
portion in the form of a cylinder) in a reaction container, the
inventors conducted experiments whether any tendency was observed
in reduction behavior with the ratio of the thicknesses of a
reducing-agent layer and an iron oxide layer.
[0140] The experiment and the result will be described below.
[0141] That is, the molar ratio of the carbon content in a reducing
agent to the oxygen content in iron oxide charged in a reaction
container was fixed at 1.2. An experiment for changing the carbon
content in a reducing agent to the oxygen content in iron oxide in
a portion where the iron oxide and the reducing agent were disposed
in the form of alternating layers excluding the reducing agent
provided at a portion near the wall (peripheral portion) of the
reaction container and at a central portion along the axial
direction was performed.
[0142] This experiment was performed with a method for horizontal
charging as in the above-described experiment. FIG. 10 shows a
schematical cross-sectional view of the state of charged materials.
The reducing-agent layers provided at the top region and the bottom
region of the intermediate portion are also included in the
intermediate portion. Materials and the experimental conditions
were the same as the above-described experiment.
[0143] FIG. 11 is a graph showing the relationship between (the
carbon content)/(the oxygen content) (molar ratio) and the
reduction time with reference to various thicknesses of iron oxide
layers. The filled circles (.circle-solid.) in the graph are the
results from when the process for horizontally alternating charging
as shown in FIG. 8, the reducing-agent layers being not provided at
the peripheral portion and at the central axial portion in the
process.
[0144] As shown in FIG. 11, iron oxide layers that were defined as
four levels, that is, the iron oxide layers having thicknesses of
60 mm (Experimental Example 11: rhombus (.diamond-solid.)), 50 mm
(Experimental Example 12: square (.box-solid.)), 30 mm
(Experimental Example 13: triangle), and 20 mm (Experimental
Example 14: cross (x)) were reduced. As a result, a smaller
thickness of the iron oxide layer led to the shortening of the
reduction time. It was found that when (the carbon content)/(the
oxygen content) (molar ratio) was 0.5 or more, the reduction time
was substantially constant, while when (the carbon content)/(oxygen
content) (molar ratio) was less than 0.5 the reduction time was
prolonged.
[0145] Consequently, to maximally take advantage of the effect
obtained when (the carbon content)/(the oxygen content) (molar
ratio) is 1.2 or more in the entire container, it was found that
(the carbon content)/(the oxygen content) (molar ratio) was
preferably at least 0.5 at the cylindrical intermediate portion,
and the cylindrical intermediate portion being the charged portion
being in the form of helices (interwound helices).
[0146] To verify these results, another experiment was performed as
follows: The molar ratio of the carbon content in a reducing agent
to the oxygen content in iron oxide at the cylindrical intermediate
portion was fixed at 0.8. The amounts of a reducing agent charged
into the peripheral portion and the central axial portion of the
reaction container were varied. FIG. 12 shows the results and is a
graph showing the change in reduction time to (all carbon
content)/(all oxygen content) (molar ratio) in the entire reaction
container. Each of the same symbols used in FIGS. 11 and 12
represents the same thickness.
[0147] As shown in FIG. 12, it was found that when the molar ratio
of (the carbon content)/(the oxygen content) in the entire reaction
container is 1.2 or more, the reduction time is substantially
constant, while when the molar ratio is less than 1.2, the
reduction time is prolonged.
[0148] However, as described above, even if the molar ratio is less
than 1.2, the effect of the present invention can be obtained if
the molar ratio is 1.1 or more and preferably 1.15 or more.
[0149] In summary, in charging iron oxide and a reducing agent into
the reaction container 11 in the form of alternating layers (such
as interwound helical charging) according to the present invention,
the ratio of the reducing agent to the iron oxide charged in the
entire reaction container 11 that includes the peripheral portion,
the cylindrical intermediate portion, and the central axial portion
of the reaction container 11 is determined such that the molar
ratio of the carbon content in the reducing agent to the oxygen
content in the iron oxide is preferably at least 1.1, more
preferably at least 1.15, and most preferably at least 1.2.
[0150] The thickness ratio of a reducing agent layer to an iron
oxide layer at the cylindrical intermediate portion that is charged
in the form of (interwound) helices is preferably determined such
that the molar ratio of the carbon content in the reducing agent to
the oxygen content in the iron oxide is at least 0.5.
EXAMPLES
Example 1
[0151] In this example, experimental levels as shown in Table 1
were defined. Iron oxide and a reducing agent were charged into the
reaction container 11 composed of silicon carbide (SiC) according
to the experimental levels and then roughly reducing treatment was
performed to produce sponge iron. Each of levels A to C and H is an
example of a process for charging in a cylindrical form as shown in
FIG. 1. Each of levels D to F is an example of a method for
interwound helical charging as shown in FIG. 6. Level G is an
example of a method for horizontally alternating charging.
[0152] In Table 1, 20% of the increment of the charge of Levels A
and D represents that the sum of the thicknesses of layers composed
of mill scale in the reaction container 11 was increased by 20%;
40% of the increment of the charge of Levels B and E represents
that the sum of the thicknesses of layers composed of mill scale in
the reaction container 11 was increased by 40%; and 60% of the
increment of the charge of Levels C and F represents that the sum
of the thicknesses of layers composed of mill scale in the reaction
container 11 was increased by 60%. The conditions are described in
detail in Table 2. Under these conditions, each Level was studied
to determine a method for charging, a suitable thickness of a
layer, and purity.
[0153] In this experiment, mill scale generated in a hot rolling
step was dried, pulverized, and screened. The mill scale powder
used included 40 percent by mass of particles that can pass through
60 .mu.m mesh (it was analyzed that the mill scale powder had an
average particle size within a range of 0.05 to 10 mm). A mixture
of limestone powder and carbonaceous powder was used as a reducing
agent that was an auxiliary material. The carbonaceous powder was
produced by mixing coke and anthracite at the coke to the
anthracite ratio of about 7:3. The coke used had an average
particle size of 85 .mu.m and the anthracite used had an average
size of 2.4 mm. The content of the limestone powder having an
average particle size of 80 .mu.m in the entire reducing agent
powder was about 14 percent by mass.
[0154] A reaction container was a cylindrical container having an
inner diameter of 400 mm. For charging in a cylindrical form, iron
oxide was charged so as to form a cylindrical shape having an outer
diameter of 320 mm, having a thickness of each value represented in
Table 2, and having a height of about 1500 mm (the axial
direction). For helical charging, a reducing-agent layer was
provided with a diameter of about 80 mm at the central axial
portion and with a thickness of about 15 mm at the peripheral
portion. Interwound charging was performed at the remaining
cylindrical intermediate portion according to Table 2. The
resulting charged cylindrical intermediate portion had a height of
about 1500 mm. The molar ratio of the carbon content to the oxygen
content in the entire container and at the cylindrical intermediate
portion that is in the form of a cylinder were at least 1.2 and at
least 0.5, respectively.
1TABLE 1 Method for Increment of Charging Step of Level charging
charge time charging A Charging in 20% 45 min Continuous
cylindrical form B Charging in 40% 45 min Continuous cylindrical
form C Charging in 60% 45 min Continuous cylindrical form D
Charging in 20% 35 min Continuous interwound helical form E
Charging in 40% 35 min Continuous interwound helical form F
Charging in 60% 35 min Continuous interwound helical form G
Horizontally 0% 90 min Discontinuous alternating charging H
Charging in 0% 45 min Continuous cylindrical form
[0155]
2TABLE 2 Method for Increment of charging productivity 0% 20% 40%
60% Charging in Thickness of 20 mm 40 mm 60 mm 80 mm interwound
iron oxide helical form layer (vertical direction) Thickness of 30
mm 43 mm 47 mm 45 mm reducing-agent layer (vertical direction)
Charging in Thickness of 57.5 mm 73.5 mm 93.5 mm 122 mm cylindrical
iron oxide form layer (radial direction)
[0156] Horizontally alternating charging was performed in order to
verify the efficiency of charging. That is, the charging was
performed by the following procedure: An apparatus for charging
materials used was the same as for interwound helical charging. The
rotatable charging cylinder was rotated while charging any one of
iron oxide powder or reducing-agent powder and was moved upward.
Next, another powder was charged by the same way. This procedure
was repeated. As shown in Table 1, the horizontally alternating
charging cannot continuously charge and required a longer charging
time than those of the charging in a cylindrical form and the
interwound helical charging. The interwound helical charging had
the shortest charging time.
[0157] Reaction containers 11 each being charged with materials
according to the corresponding Level were placed on one truck and
disposed in a tunnel furnace. The truck passed through a preheating
zone over a period of about one day 200.degree. C. to 900.degree.
C.) and a firing zone 1150.degree. C.) over a period of about three
days and then a cooling zone over 200.degree. C. to 900.degree. C.)
a period of about one day. The truck was removed from the tunnel
furnace, and sponge iron was removed from the container. The purity
of the resulting sponge iron was measured. All resulting sponge
iron weighed 200 kg or more.
[0158] The purity of sponge iron was given by converting the
metallic iron content in a chemical composition determined by a
method for analyzing oxygen. FIG. 13 shows the results.
[0159] As shown in FIG. 13, in the case of the interwound helical
charging (hatching patterned bars), iron oxide was excellently
reduced to produce high-purity sponge iron, which had a purity of
above 97 percent or above 98 percent by mass, when an iron oxide
layer had a thickness of up to 60 mm, i.e., the increment of
productivity is up to 40%. It was found that productivity can be
adjusted by controlling the thickness of the layer up to 40% of the
increment of the charge compared with a known process. In the case
of the charging in a cylindrical form, when the increment of the
charge was 20%, the thickness of the layer was 75 mm and the purity
was 95.65 percent by mass; hence, productivity cannot be improved
compared with the interwound helical charging.
Example 2
[0160] Sponge iron was manufactured according to Inventive Examples
1 to 5 and Conventional Example 1. A method for charging as shown
in FIG. 3A was substantially employed. (The carbon content)/(the
oxygen content) (molar ratio) was 1.2 or more.
Inventive Example 1
[0161] In this Inventive Example, an iron oxide layer having a
thickness of 50 mm and a reducing-agent layer having a thickness of
50 mm were charged in the form of interwound helices. A cylindrical
reaction container was used with a height of 1.8 m and with an
inner diameter of 40 cm. A mixture of coke powder having a particle
size of up to 1 mm and 16 percent by mass of limestone having an
average particle size of about 95 .mu.m was used as the
reducing-agent powder. Pulverized mill scale having a particle size
of up to 0.1 mm (after pulverizing, the mill scale was screened.
The resulting mill scale included 40 percent by mass of particles
that can pass through 60 .mu.m mesh) was used as the iron oxide
powder. Both mill scale powder and coke powder had an average
particle size within a range of 0.05 to 10 mm.
[0162] An apparatus for charging materials as shown in FIG. 4A was
used. The charging was performed as follows: The height of the
opening of the outlet for iron oxide powder 15 was adjusted to 50
mm. The height of the opening of the outlet for reducing-agent
powder 16 was also adjusted to 50 mm. The rotatable charging
cylinder 14b was operated at a rotating speed of 4 rpm and at a
rising speed of 400 mm/min.
[0163] As a result of the charging, charged interwound helices each
having 17 turns were obtained, wherein the iron oxide layer had a
thickness of 50 mm and the layer of solid (powder) reducing agent
had a thickness of 50 mm. The charged iron oxide weighed 339
kg.
Inventive Example 2
[0164] In this Inventive Example, an iron oxide layer having a
thickness of 35 mm and a reducing-agent layer having a thickness of
65 mm were charged in the form of interwound helices. Iron oxide
and a solid reducing agent were charged with the same reaction
container, material powder, and apparatus for charging materials as
Inventive Example 1. The charging was performed as follows: The
height of the opening of the outlet for iron oxide powder 15 was
adjusted to 35 mm. The height of the opening of the outlet for
reducing-agent powder 16 was also adjusted to 65 mm. The rotatable
charging cylinder 14b was operated at a rotating speed of 4 rpm and
at a rising speed of 400 mm/min.
[0165] As a result of the charging, charged interwound helices each
having 17 turns were obtained, wherein the iron oxide layer had a
thickness of 35 mm and the layer of solid reducing agent had a
thickness of 65 mm. The charged iron oxide weighed 237 kg.
Inventive Example 3
[0166] In this Inventive Example, an iron oxide layer having a
thickness of 60 mm and a reducing-agent layer having a thickness of
40 mm were charged in the form of interwound helices. Iron oxide
and a reducing agent were charged with the same reaction container,
material powder, and apparatus for charging materials as Inventive
Example 1. The charging was performed as follows: The height of the
opening of the outlet for iron oxide powder 15 was adjusted to 60
mm. The height of the opening of the outlet for reducing-agent
powder 16 was also adjusted to 40 mm. The rotatable charging
cylinder 14b was operated at a rotating speed of 4 rpm and at a
rising speed of 400 mm/min.
[0167] As a result of the charging, charged interwound helices each
having 17 turns were obtained, wherein the iron oxide layer had a
thickness of 60 mm and the layer of solid reducing agent had a
thickness of 50 mm. The charged iron oxide weighed 406 kg.
Inventive Example 4
[0168] In this Inventive Example, an iron oxide layer having a
thickness of 25 mm and a reducing-agent layer having a thickness of
25 mm were charged in the form of interwound helices. Iron oxide
and a reducing agent were charged with the same reaction container,
material powder, and apparatus for charging materials as Inventive
Example 1. The charging was performed as follows: The height of the
opening of the outlet for iron oxide powder 15 was adjusted to 25
mm. The height of the opening of the outlet for reducing-agent
powder 16 was also adjusted to 25 mm. The rotatable charging
cylinder 14b was operated at a rotating speed of 4 rpm and at a
rising speed of 200 mm/min.
[0169] As a result of the charging, charged interwound helices each
having 34 turns were obtained, wherein the iron oxide layer had a
thickness of 25 mm and the layer of solid reducing agent had a
thickness of 25 mm. The charged iron oxide weighed 339 kg.
Inventive Example 5
[0170] In this Inventive Example, an iron oxide layer having a
thickness of 57.5 mm and a reducing-agent layer having a thickness
of 50 mm were charged. Iron oxide and a reducing agent were charged
with the same reaction container, material powder, and apparatus
for charging materials as Inventive Example 1. The charging was
performed as follows: The height of the opening of the outlet for
iron oxide powder 15 was adjusted to 57.5 mm. The height of the
opening of the outlet for reducing-agent powder 16 was also
adjusted to 50 mm. The rotatable charging cylinder 14b was operated
at a rotating speed of 4 rpm and at a rising speed of 430
mm/min.
[0171] As a result of the charging, charged interwound helices each
having 16 turns were obtained, wherein the iron oxide layer had a
thickness of 57.5 mm and the layer of solid reducing agent had a
thickness of 50 mm. The charged iron oxide weighed 366 kg.
Conventional Example 1
[0172] In this example, charging in a cylindrical form was
performed according to a known process as shown in FIG. 1. The same
reaction container as Example 1 was used. Iron oxide powder was
charged in the form of a cylinder with a thickness of 57.5 mm and
with an outer diameter of 310 mm. A reducing-agent powder was
charged around the iron oxide layer (including the inside of the
cylinder). The same reaction container and material powder as
Inventive Example 1 were used. (The carbon content)/(the oxygen
content) (molar ratio) in the container was about 2.2.
[0173] Reduction treatment was performed with a tunnel furnace. A
time required for the reduction was investigated.
[0174] Table 3 summarizes the results.
[0175] The time required for the reduction represents a retention
time at a firing zone (1150.degree. C.) in order to produce sponge
iron having a purity of 95% or more. Production per hour represents
a value obtained by dividing the weight of charged iron oxide by
the time required for the reduction.
[0176] As shown in Table 3, the method of the present invention
significantly improves productivity compared with the conventional
process.
3 TABLE 3 Inventive Inventive Inventive Inventive Inventive
Conventional Example 1 Example 2 Example 3 Example 4 Example 5
Example 1 Method for Charging in interwound helical form Charging
in charging cylindrical form Thickness of iron 50 35 60 25 57.5
57.5 oxide layer (mm) Thickness of reducing- 50 65 40 25 50
.gtoreq.50 agent layer (mm) Weight of iron 339 237 406 339 366 227
oxide (kg) Reduction time (h) 62 52 78 40 74 75 Productivity 5.46
4.55 5.22 8.47 4.94 3.02 per hour (kg/h)
Example 3
Inventive Example 6
[0177] A layer composed of the reducing-agent powder 13 (coke
powder) was deposited with a thickness of 30 mm at the bottom of
the reaction container 11 with the apparatus for charging materials
as shown in FIG. 4A. Iron oxide powder 12 (mill scale) and
reducing-agent powder 13 were continuously charged onto the bottom
layer such that alternating layers of the iron oxide powder and the
reducing-agent powder were formed and such that each of the layers
was in the form of a helix, the iron oxide layer having a thickness
of 40 mm and the reducing-agent layer having a thickness of 50 mm,
while rotating the rotatable charging cylinder 14b having the
outlet for iron oxide powder 15 and the outlet for reducing-agent
powder 16 and moving upward. Finally, the reducing-agent powder 13
(coke powder) was charged at the top of the reaction container 11.
In this charging, the molar ratio of the carbon content in the
reducing agent to the oxygen content in the iron oxide was 1.6. The
same conditions as EXAMPLE 2 were applied other than those
above-described.
Comparative Example 1
[0178] Charging in the form of horizontal layers as shown in FIG. 8
was performed. In this example, charging was performed according to
the following procedure: In the apparatus for charging materials 14
as shown in FIG. 4A, the reducing-agent powder 3 (coke powder) was
charged to form a layer having a thickness of 50 mm. Next, the iron
oxide powder 12 (mill scale) was charged on the reducing-agent
layer to form a layer having a thickness of 40 mm. This charging
procedure was repeated until the deposited layers reached the top
end of the reaction container 11, providing that the reducing-agent
powder 13 (coke powder) was charged at the top end of the reaction
container 11. The molar ratio of the carbon content in the reducing
agent to the oxygen content in the iron oxide was 1.6.
Conventional Example 2
[0179] charging in a cylindrical form as shown in FIGS. 1A and 1B
was performed as in Conventional Example 1 in EXAMPLE 2, but (the
carbon content)/(the oxygen content) (molar ratio) was 2.5.
[0180] Next, the heat-resistant reaction container 11 containing
materials was placed on a truck and passed through a tunnel furnace
to heat and reduce iron oxide. The tunnel furnace having an entire
length of 100 m was used, and the atmospheric temperature was
adjusted to 1150.degree. C. at the center zone having a length of
40 m. Table 4 summarizes the results of the operations for
manufacturing sponge iron having a purity of 97 percent by mass
under those conditions.
[0181] As is clear from Table 4, in this example of the present
invention, the truck speed was 1.3 m/h compared with 1.1 m/h of the
Conventional Example and was thus 18% faster than the Conventional
Example. The amount of mill scale charged was 256 kg per container
compared with 220 kg per container of the Conventional Example and
was thus 16% greater than the Conventional Example. As a result,
productivity was improved by as much as 38%. A quantity of heat per
unit mass of iron oxide required for heating can be reduced from
11,470 MJ/ton to 8,820 MJ/ton by as much as about 30%.
4 TABLE 4 Inventive Comparative Conventional Example 6 Example 1
Example 2 Method for Charging in Horizontally Charging in charging
interwound alternating cylindrical helical form charging form Truck
speed (m/h) 1.3 1.3 1.1 Amount of mill 256 256 220 scale charged
(kg/container) Retention time at 30.8 30.8 36.4 1150.degree. C. (h)
Heat consumption 8820 8820 11470 rate (MJ/ton)
Example 4
[0182] Sponge iron was manufactured with an apparatus for charging
materials as shown in FIG. 5. The same materials as EXAMPLE 2 were
used. The cut-out section 14c had a semicircular shape (sector
having a central angle of about 180.degree.). A reaction container
having an inner diameter of 400 mm and a height of 2,000 mm was
used. A projection composed of a slag that was formed by a reaction
and adhered (maximum height was about 20 mm) was purposely not
removed and the rotatable charging cylinder was inserted. The main
body of the rotatable charging cylinder had an outer diameter of
310 mm (77.5% of the inner diameter of the container). A virtual
circle at the horizontal cross-section of the cut-out section had a
diameter of 360 mm (90% of the inner diameter of the
container).
[0183] The rotatable charging cylinder can move to an opposite side
when the front end lightly came into contact with the projection or
the reaction container; hence, the rotatable charging cylinder was
able to be inserted to the bottom of the reaction container with no
problems, and there is no problem when charging materials, that is,
260 kg of iron oxide powder was charged with no problems (a layer
composed of iron oxide had a thickness of 50 mm, and a layer
composed of a reducing agent had a thickness of 30 mm).
[0184] After charging, the reduction was performed with no problems
using a tunnel furnace in the same way as EXAMPLE 2. As a result, a
mass of sponge iron having a helical shape was produced with a
purity of 95 percent by mass.
Example 5
[0185] Sponge iron was manufactured according to Inventive Examples
7 to 11, Comparative Example 2, and Conventional Example 3. A
method for charging as shown in FIG. 6 was performed.
[0186] In this EXAMPLE, iron oxide powder composed of mill scale
and/or iron ore was pulverized and screened in order to adjust the
particle size, and was then used as the main material.
Reducing-agent powder composed of at least any one of a simple
substance or a mixture of coke powder, char, coal powder, charcoal
powder, and the like was pulverized and screened in order to adjust
the particle size and was then used as a material. All materials
had an average particle size of about 70 to 90 .mu.m.
[0187] An apparatus was used with a rotatable charging cylinder as
shown in FIG. 14 was used. The operation was performed by the
following procedure: The reducing-agent powder 13 was placed at the
bottom of the reaction container 11, and the iron oxide powder 12
and the reducing-agent powder 13 were charged in the form of
interwound helices while rotating the rotatable charging cylinder
14b of the apparatus for charging materials 14 and simultaneously
with moving upward at a constant speed. The charging was performed
up to the top end of the container 1, provided that the top end of
the reaction container 11 was charged with the reducing-agent
powder 13. To remove a product (sponge iron) from the container, to
prevent sponge iron from adhering to the container, and to enhance
the efficiency of gas diffusion, the central axial portion and the
peripheral portion near the wall were charged with a reducing
agent.
Convention Example 3
[0188] In this example, a general process for charging was employed
as shown in FIG. 1. An iron oxide layer having an outer diameter of
310 mm, an inner diameter of 200 mm, and a length of 1,600 mm was
formed in a heat-resistant reaction container 1 (inner diameter:
400 mm, length: 1,800 mm) (provided that remaining portion was
charged with a reducing agent). (The carbon content)/(the oxygen
content) (molar ratio) was 2.2 in the container. When the purity
target was 97.0 percent by mass, the reduction time 1,150.degree.
C., hereinafter, all reductions were performed at the same
temperature.) was 53 hours.
Inventive Example 7
[0189] In this example, interwound helical charging was performed.
An iron oxide layer had an outer diameter of 390 mm, an inner
diameter of 60 mm, a thickness of 60 mm, and a helical shape. A
reducing-agent layer had a thickness of 45 mm and a helical shape.
The outer diameter and the inner diameter of the reducing-agent
layer was the same as the iron oxide layer. The iron oxide layer
and the reducing-agent layer were simultaneously formed. The molar
ratio of (the carbon content in the reducing agent)/(the oxygen
content in the iron oxide) was 0.8 in the cylindrical intermediate
portion. (The carbon content)/(the oxygen content) (molar ratio)
was 1.2 in the all charged materials. As a result, the amount of
materials charged was increased by 35% compared with Conventional
Example 3. However, the reduction time was as short as 60 hours.
The resulting sponge iron did not adhere to the inner face of the
container and was readily removed from the container.
Inventive Example 8
[0190] In this example, interwound helical charging was performed.
An iron oxide layer had an outer diameter of 365 mm, an inner
diameter of 100 mm, a thickness of 60 mm, and a helical shape. A
reducing-agent layer had a thickness of 28 mm and a helical shape.
The outer diameter and the inner diameter of the reducing-agent
layer was the same as the iron oxide layer. The iron oxide layer
and the reducing-agent layer were simultaneously formed. The molar
ratio of (the carbon content in the reducing agent)/(the oxygen
content in the iron oxide) was 0.5 in the cylindrical intermediate
portion. The molar ratio of the carbon content to the oxygen
content was 1.2 in the all charged materials. As a result, the
amount of materials charged was increased by 35% compared with
Conventional Example 3. However, the reduction time was 59 hours.
The resulting sponge iron did not adhere to the inner face of the
container and was readily removed from the container.
Inventive Example 9
[0191] In this example, interwound helical charging was performed.
An iron oxide layer had an outer diameter of 350 mm, an inner
diameter of 100 mm, a thickness of 60 mm, and a helical shape. A
reduced iron layer had a thickness of 17 mm and a helical shape.
The outer diameter and the inner diameter of the reducing-agent
layer was the same as the iron oxide layer. The iron oxide layer
and the reducing-agent layer were simultaneously formed. The molar
ratio of (the carbon content in the reducing agent)/(the oxygen
content in the iron oxide) was 0.3 in the cylindrical intermediate
portion. The molar ratio of the carbon content to the oxygen
content was 1.2 in the all charged materials. As a result, the
amount of materials charged was increased by 35% compared with
Conventional Example 1. However, the reduction time was 70 hours.
The resulting sponge iron did not adhere to the inner face of the
container and was readily removed from the container. However, the
reduction time was comparable with that in Conventional Example 3
even in view of the increment.
Inventive Example 10
[0192] In this example, interwound helical charging was performed.
An iron oxide layer had an outer diameter of 375 mm, an inner
diameter of 100 mm, a thickness of 60 mm, and a helical shape. A
reducing-agent layer had a thickness of 45 mm and a helical shape.
The outer diameter and the inner diameter of the reducing-agent
layer was the same as the iron oxide layer. The iron oxide layer
and the reducing-agent layer were simultaneously formed. The molar
ratio of (the carbon content in the reducing agent)/(the oxygen
content in the iron oxide) was 0.8 in the cylindrical intermediate
portion. The molar ratio of the carbon content to the oxygen
content was 1.5 in the all charged materials. As a result, the
amount of materials charged was increased by 20% compared with
Conventional Example 3. However, the reduction time was 59 hours.
The resulting sponge iron did not adhere to the inner face of the
container and was readily removed from the container. Inventive
Example 7 with a low molar ratio of (the carbon content)/(the
oxygen content) in the container represented higher production
efficiency per reduction time compared with this example. However,
this example represented excellent results compared with the
Conventional Example.
Inventive Example 11
[0193] In this example, interwound helical charging was performed.
An iron oxide layer had an outer diameter of 395 mm, an inner
diameter of 40 mm, a thickness of 60 mm, and a helical shape. A
reducing-agent layer had a thickness of 45 mm and a helical shape.
The outer diameter and the inner diameter of the reducing-agent
layer was the same as the iron oxide layer. The iron oxide layer
and the reducing-agent layer were simultaneously formed. The molar
ratio of (the carbon content in the reducing agent)/(the oxygen
content in the iron oxide) was 0.8 in the cylindrical intermediate
portion. The molar ratio of the carbon content to the oxygen
content was 1.1 in the all charged materials. As a result, the
amount of materials charged was increased by 40% compared with
Conventional Example 3. However, the reduction time was 78 hours.
The resulting sponge iron did not adhere to the inner face of the
container and was readily removed from the container. In this
example, the reduction time was prolonged. The reduction time was
comparable with that in Conventional Example 3 even in view of the
increment.
[0194] Table 5 summarizes the results.
5 TABLE 5 Conventional Inventive Inventive Inventive Inventive
Inventive Example 3 Example 7 Example 8 Example 9 Example 10
Example 11 Method for charging Charging in Charging in interwound
helical form cylindrical form Outer diameter of 310 390 365 350 375
395 charge (mm) Inner diameter of 200 60 100 100 100 40 charge (mm)
Thickness of iron 55 60 60 60 60 60 oxide layer (mm) Thickness of
reducing- .gtoreq.50 45 28 17 45 45 agent layer (mm) Molar ratio in
2.2 1.2 1.2 1.2 1.5 1.1 container Molar ratio at cylindrical -- 0.8
0.5 0.3 0.8 0.8 intermediate portion Weight of iron oxide 1 1.35
1.35 1.35 1.2 1.4 (relative ratio) Reduction time (h) 53 60 59 70
59 78 Productivity per hour* 0.019 0.023 0.023 0.019 0.020 0.018
*(Weight of iron oxide (relative ratio))/(Reduction time (h))
Industrial Applicability
[0195] As described above, according to the present invention,
sponge iron can be manufactured with high productivity and high
quality (for example, at a purity of 97% or more) by employing the
technique of interwound helical charging. Furthermore, since a
structure formed by charging materials into a reaction container
can be changed to a desired structure easily and readily, quality,
quantity, and a reduction time can be easily adjusted; hence,
production efficiency can be significantly improved. As a result,
high-purity sponge iron can be manufactured at low cost.
* * * * *