U.S. patent application number 13/702409 was filed with the patent office on 2013-03-28 for process for producing granular metal.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). The applicant listed for this patent is Shuzo Ito. Invention is credited to Shuzo Ito.
Application Number | 20130074654 13/702409 |
Document ID | / |
Family ID | 45098024 |
Filed Date | 2013-03-28 |
United States Patent
Application |
20130074654 |
Kind Code |
A1 |
Ito; Shuzo |
March 28, 2013 |
PROCESS FOR PRODUCING GRANULAR METAL
Abstract
A technique that further improves the process for producing
granular metal involves heating agglomerates and reducing and
melting a metal oxide in the agglomerates. The process includes
feeding agglomerates containing a metal oxide and a carbonaceous
reducing agent onto a hearth of a moving hearth reduction melting
furnace, heating the agglomerates to reduce and to melt the metal
oxide, cooling the granular metal obtained by the heating, and
discharging the cooled granular metal out of the furnace to recover
the same. The agglomerates have an average diameter of not smaller
than 17.5 mm are fed onto the hearth when the agglomerates are
heated at a spread density of not lower than 0.5 on the hearth.
Inventors: |
Ito; Shuzo; (Kobe-shi,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ito; Shuzo |
Kobe-shi |
|
JP |
|
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Hyogo
JP
|
Family ID: |
45098024 |
Appl. No.: |
13/702409 |
Filed: |
June 3, 2011 |
PCT Filed: |
June 3, 2011 |
PCT NO: |
PCT/JP2011/062847 |
371 Date: |
December 6, 2012 |
Current U.S.
Class: |
75/363 |
Current CPC
Class: |
B22F 9/20 20130101; C22B
5/10 20130101; C21B 13/105 20130101; C22B 1/245 20130101; C21B
13/0046 20130101 |
Class at
Publication: |
75/363 |
International
Class: |
B22F 9/20 20060101
B22F009/20 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 7, 2010 |
JP |
2010-130124 |
Claims
1. A process for producing granular metal, the process comprising:
feeding an agglomerate comprising a metal oxide and a carbonaceous
reducing agent onto a hearth of a moving hearth reduction melting
furnace; heating the agglomerate, thereby reducing and melting the
metal oxide and obtaining a granular metal; cooling the granular
metal; and discharging the granular metal out of the furnace after
cooling, thereby recovering the granular metal, wherein the
agglomerate has an average diameter of not smaller than 17.5 mm,
and the heating comprises heating the agglomerate at a spread
density of not lower than 0.5 on the hearth.
2. The process according to claim 1, further comprising: spreading
a carbonaceous material on the hearth before the feeding, wherein
the feeding comprises feeding the agglomerates on the carbonaceous
material, thereby obtaining a single layer.
3. The process according to claim 1, wherein the metal oxide is
iron oxide.
4. The process according to claim 1, wherein the moving hearth
reduction melting furnace is a rotary hearth furnace.
5. The process according to claim 1, wherein the metal oxide is
steelmaking dust.
6. The process according to claim 1, wherein the moving hearth
reduction melting furnace comprises: an upstream area having
capable of maintaining a temperature of from 1300.degree. C. to
1450.degree. C. and a downstream area capable of maintaining a
temperature of from 1400.degree. C. to 1550.degree. C.
7. The process according to claim 6, wherein the downstream area is
capable of maintaining a temperature higher than a temperature of
the upstream area.
8. The process of claim 2, wherein the metal oxide is iron
oxide.
9. The process of claim 1, wherein the average diameter of the
agglomerate is not smaller than 18.5 mm.
10. The process of claim 1, wherein the average diameter of the
agglomerate is not smaller than 20 mm.
11. The process of claim 1, wherein the average diameter of the
agglomerate is not smaller than 17.5 mm and not more than 31
mm.
12. The process of claim 1, wherein the average diameter of the
agglomerate is not smaller than 17.5 mm and not more than 28
mm.
13. The process of claim 1, wherein the heating comprises heating
the agglomerate at a spread density of not lower than 0.6.
14. The process of claim 1, wherein the heating comprises heating
the agglomerate at a spread density of from 0.5 to 0.8.
15. The process of claim 1, wherein the heating comprises heating
the agglomerate at a spread density of not lower than 0.5 and not
more than 0.7.
16. The process of claim 2, wherein a thickness of the carbonaceous
material after the spreading is not less than 3 mm.
17. The process of claim 2, wherein a thickness of the carbonaceous
material after the spreading is not more than 30 mm.
18. The process of claim 2, wherein the carbonaceous material has a
particle diameter of not more than 3.0 mm.
Description
TECHNICAL FIELD
[0001] The present invention relates to a process for producing
granular metal by feeding agglomerates configured by a raw material
mixture containing a metal oxide and a carbonaceous reducing agent
onto a hearth, and by heating the same thereon to reduce and to
melt the metal oxide in the raw material mixture.
[0002] Mainly described herein is the process for producing
granular metallic iron, in which the present invention is utilized
most effectively. However, the present invention is not limited to
the above but can be effectively utilized also to a case of heating
and reducing chromium-containing ore or nickel-containing ore, for
example, to produce ferrochromium, ferronickel, or the like.
Moreover, the term "granular" in the present invention does not
necessarily mean a perfectly spherical shape, but also includes
elliptical and ovoidal shapes, as well as any shapes obtained by
slightly flattening these shapes, and the like.
BACKGROUND ART
[0003] There has been developed a direct reduced iron manufacturing
method for obtaining granular metallic iron from agglomerates
configured by a raw material mixture including an iron
oxide-containing material such as iron ore or iron oxide, and a
carbonaceous reducing agent. In this iron producing process, the
agglomerates are charged onto a hearth of a heating furnace and
then heated in the furnace by the gas heat transfer with use of a
heating burner or by radiation heat to reduce the iron contained in
the agglomerates by the carbonaceous reducing agent. Subsequently,
the reduced iron obtained by said heating step is carburized,
melted, and then coalesced in the form of granules while being
separated from sub-generated slag, and the granules are cooled and
solidified to obtain granular metallic iron.
[0004] The above iron producing process does not require a large
scale facility such as a blast furnace and has high flexibility
with regard to resources, for example, because of no need to use
coke, and therefore, in recent years, this process has widely been
studied for practical use. However, this iron producing process
still has many problems to be solved in order to be applied on an
industrial scale, including the stability of operation, safety,
economic efficiency, quality of the granular metallic iron (i.e., a
final product), and productivity. In view of these problems, the
applicant of the present invention previously proposed a method
disclosed in Patent Document 1. In this method, upon heating and
reducing formed products containing a carbonaceous reducing agent
and iron oxide to produce metallic iron, suppressed as much as
possible are the amount of the carbonaceous reducing agent consumed
and the thermal energy necessary for the heating and reducing
process so as to efficiently reduce the iron oxide at lower cost on
a commercial scale. This document discloses an example in which
iron ore, a carbonaceous material, and a binder are blended
together to produce granular pellets having the average diameter of
17 mm, and the pellets are heated and reduced to produce metallic
iron.
PRIOR ART DOCUMENT
Patent Document
[0005] Patent Document 1: Japanese Unexamined Patent Publication
No. H11-241111
SUMMARY OF THE INVENTION
Problem to be Solved by the Invention
[0006] According to above Patent Document 1, the carbonaceous
reducing agent is blended at an amount in consideration of the
stoichiometric amount required to the reduction of iron oxide and
the solution C content into the metallic iron to be generated, and
the heating temperature is appropriately controlled in
consideration of the melting point of the metallic iron upon the
solution of C. Thus, heating and reducing the iron oxide as well as
the separation from slag by melting the iron oxide can be
effectively progressed by using the carbonaceous reducing agent of
the minimum amount required at the heating temperature as low as
possible. As a result, there was established a process for
producing metallic iron more economically and highly practically on
an industrial scale. However, what is required is to further
increase the amount of granular metallic iron produced per unit
area of the effective hearth per unit time, in order to improve the
productivity of the granular metallic iron.
[0007] The present invention was made in consideration of the above
circumstances, and an object thereof is to provide a technique that
further improves the process for producing granular metal by
heating agglomerates containing a metal oxide and a carbonaceous
reducing agent, and reducing and melting the metal oxide included
in the agglomerates.
Solutions to the Problems
[0008] A process for producing granular metal, according to the
present invention is characterized by comprising the steps of:
[0009] feeding agglomerates containing a metal oxide and a
carbonaceous reducing agent onto a hearth of a moving hearth-type
reduction melting furnace;
[0010] heating the agglomerates to reduce and to melt the metal
oxide;
[0011] cooling the granular metal obtained by said heating step;
and
[0012] discharging the cooled granular metal out of the furnace to
recover the same,
[0013] wherein the agglomerates having an average diameter of not
smaller than 17.5 mm are fed onto the hearth when the agglomerates
are heated at a spread density of not lower than 0.5 on the
hearth.
[0014] It is preferable that a carbonaceous material is spread on
the hearth and then the agglomerates are fed on the carbonaceous
material to form a single layer.
[0015] Iron oxide or steelmaking dust is, for example, used as the
metal oxide.
[0016] A rotary hearth furnace is, for example, used as the moving
hearth-type reduction melting furnace.
[0017] It is preferable that the moving hearth-type reduction
melting furnace comprises a upstream area having a temperature
controlled to be from 1300.degree. C. to 1450.degree. C. and a
downstream area having a temperature controlled to be from
1400.degree. C. to 1550.degree. C.
[0018] And it is preferable that the downstream area is set to have
a temperature higher than that of the upstream area in the moving
hearth-type reduction melting furnace.
Effect of the Invention
[0019] In the present invention, the average diameter of the
agglomerates fed onto the hearth and the spread density of the
agglomerates heated on the hearth are appropriately controlled,
which improves the productivity of the granular metal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a plan view schematically showing agglomerates
spread on a hearth.
[0021] FIG. 2 includes pictures in substitution for drawings, which
show states where agglomerates having the average diameter of 18.2
mm are spread.
[0022] FIG. 3 is a graph indicating the relationship between the
distance "r" of adjacent agglomerates and the projected area ratio
or spread density.
[0023] FIG. 4 is a graph indicating the relationship between the
spread density and the amount of agglomerates fed to a furnace.
[0024] FIG. 5 is a graph indicating the relationship between an
average diameter (Dp) of a test material (i.e., agglomerates) and
reaction time.
[0025] FIG. 6 is a graph indicating the relationship between the
average diameter of agglomerates and the productivity index in a
case where granular metallic iron is produced from the agglomerates
spread at a constant density.
[0026] FIG. 7 is a graph indicating the relationship between the
average diameter of agglomerates and the productivity index when
granular metallic iron is produced from the agglomerates (i.e., a
test material) apart from each other at the constant distance "r"
in the hearth.
MODE FOR CARRYING OUT THE INVENTION
[0027] The inventor of the present application conducted diligent
investigations to improve the process for producing granular metal
by feeding onto a hearth of a moving hearth-type reduction melting
furnace and heating thereon agglomerates containing a metal oxide
and a carbonaceous reducing agent to reduce and to melt the metal
oxide included in the agglomerates. The inventor finally found out
that the productivity of the granular metal can be improved by:
(1) preparing the agglomerates so as to have an average diameter of
not smaller than 17.5 mm; and (2) heating the agglomerates that are
spread on the hearth at the spread density of not lower than 0.5,
to achieve the present invention. The details of the achievement of
the present invention are described below.
[0028] In the above Patent Document, when metallic iron is produced
by heating and reducing formed products containing a carbonaceous
reducing agent and iron oxide, pellets (i.e., agglomerates) having
an average diameter of 17 mm are used as the formed products. The
reason why the agglomerates having an average diameter of 17 mm are
used has been thought to be that agglomerates of a larger size will
require longer time to transfer heat to the agglomerates on the
hearth in the furnace, resulting in a longer reaction time and
therefore the deterioration in the productivity of granular
metallic iron.
[0029] However, the inventor of the present application
investigated in more detail on the relationship between the size of
the agglomerates and the productivity to find a new fact that the
productivity of granular metal can be better improved with use of
agglomerates having an average diameter of not smaller than 17.5
mm. This new finding is described with reference to FIG. 7.
[0030] FIG. 7 is a graph referred to in an example to be described
later, indicating the relationship between the average diameter of
agglomerates and the productivity index. In FIG. 7, the
productivity index is a relative value to the productivity that is
set to 1.00 in a case where granular metallic iron is produced with
use of agglomerates having the average diameter of 17.5 mm (i.e.,
1.75 cm). This productivity represents a quantity of granular
metallic iron produced per unit area of the effective hearth per
unit time (to be detailed later).
[0031] As apparent from FIG. 7, the productivity index is larger
and the productivity of granular metallic iron is improved by using
agglomerates having an average diameter of not smaller than 17.5 mm
(more specifically, an average diameter from 17.5 to 32.0 mm) in
comparison to the case of using agglomerates having the average
diameter of 16.0 mm (i.e., 1.60 cm).
[0032] FIG. 7 indicates a result of re-evaluation (i.e.,
simulation), on the basis of the results of various experiments, of
the relationship in the cases where the distance "r" between the
adjacent agglomerates on the hearth is kept constant (in other
words, when the agglomerates are spread on the hearth at different
spread density). The spread density is the density of filled
agglomerates that are spread per unit area of the effective hearth,
and can be calculated from the projected area of the agglomerates
on the hearth (to be detailed later). FIG. 7 indicates the result
of re-evaluation on the basis of the result indicated in FIG. 5. As
seen from the relationship between the average diameter and the
reaction time indicated in FIG. 5, each of the actual measurement
values is slightly varied. Therefore, there was applied the
normalization of the relationship between by the approximation
thereof with a curve that is used in the re-evaluation. This is one
of the approaches of scientific analyses.
[0033] The most important factors in the evaluation of the
productivity of granular metal are the reaction time and the yield
rate (in other words, the product recovery rate). Accordingly,
these properties are particularly normalized in accordance with the
experimental data to conduct the re-evaluation. It is noted that
the apparent density of agglomerates is another important factor
that influences the productivity. However, it is preliminarily
evaluated that agglomerates having a diameter from 16.0 to 32.0 mm,
for example, have small variations in the apparent density as long
as the agglomerates are prepared by using an identical
agglomeration method, and that the apparent density can be
therefore regarded as being substantially constant in the
comprehensive evaluation. According to FIG. 7, as will be referred
to in the example to be described later, the spread density of
agglomerates is increased as the average diameter of the
agglomerates is larger (see Table 6 below). Therefore, it is
understood from FIG. 7 that the productivity of granular metallic
iron can be improved by appropriately controlling the spread
density, as well as by the control of the average diameter of
agglomerates. Consequently, the present invention clarifies that
the productivity of granular metallic iron can be improved by the
control of the spread density as well as the average diameter of
agglomerates.
[0034] Described in detail below is the producing method according
to the present invention.
[0035] Prepared in the present invention are agglomerates having an
average diameter of not smaller than 17.5 mm.
[0036] The agglomerates are prepared by agglomerating a mixture
containing a metal oxide and a carbonaceous reducing agent. The
metal oxide may be an iron oxide-containing material,
chromium-containing ore, nickel-containing ore, or the like. In
particular, what can be used as the iron oxide-containing material
is iron ore, iron sand, steelmaking dust, nonferrous smelting
residue, steelmaking waste, or the like. The carbonaceous reducing
agent may be a carbon-containing material such as coal or coke.
[0037] The mixture may be blended with an additional component such
as a binder, an MgO-containing material, or a CaO-containing
material. The binder may be a polysaccharide (e.g., starch such as
flour). The MgO-containing material may be powdered MgO, those
extracted from natural ore, seawater, or the like, magnesium
carbonate (i.e., MgCO.sub.3), or the like. The CaO-containing
material may be quicklime (i.e., CaO), limestone (i.e., composed
mostly of CaCO.sub.3), or the like.
[0038] The agglomerates are prepared to have an average diameter of
not smaller than 17.5 mm. If the average diameter of the
agglomerates is smaller, the time required to the heat transfer in
the furnace is shortened in general, which also shorten the
reaction time. However, when the average diameter of the
agglomerates is small, it is difficult to spread the agglomerates
evenly on the carbonaceous material laid on the hearth. Moreover,
the particle diameter and unit mass of granular metal are
inevitably decreased, which granular metal is obtained by heating
the agglomerates. Such small granular metal obtained by said
heating step needs to be handled with special care, which results
in the difficulty in feeding the granular metal into a finery such
as an electric furnace or a converter. Furthermore, the small
granular metal is not preferable in view of the melting property.
Therefore, the present invention uses agglomerates having an
average diameter of not smaller than 17.5 mm. The average diameter
of the agglomerates is preferably not smaller than 18.5 mm, more
preferably not smaller than 19.5 mm, and further preferably not
smaller than 20 mm. There is no particular upper limit to the
average diameter of agglomerates. Nevertheless, such agglomerates
having an average diameter of more than 32 mm require too much time
for the heat transfer in the furnace, resulting in longer reaction
time and deterioration in productivity. In addition, the larger
average diameter of agglomerates tends to deteriorate the
granulation efficiency. Therefore, the agglomerates are preferably
prepared to have an average diameter of not more than 31 mm. The
average diameter of the agglomerates is more preferably not more
than 28 mm.
[0039] There is no particular limitation to the shape of the
agglomerates, which may be in the shape of pellets, briquettes, or
the like.
[0040] In order to obtain the diameter of each of the agglomerates,
the longer diameter of the agglomerate and the shorter diameter
thereof in the direction perpendicular to the longer diameter are
measured with use of a vernier caliper, and these longer and
shorter diameters are averaged [diameter=(longer diameter+shorter
diameter)/2]. The average diameter of the agglomerates is obtained
by measuring and averaging the diameters of at least 20 particles
with use of the vernier caliper. In a case where the average
diameter of the agglomerates is equal to .alpha. mm, the diameters
(absolute values) of the agglomerates are preferably distributed in
the range of .alpha..+-.5 mm.
[0041] It is important in the present invention to heat
agglomerates having an average diameter of not smaller than 17.5 mm
which are spread on the hearth at the density of not lower than 0.5
on the hearth. It has been conventionally recognized that
agglomerates having a larger average diameter deteriorate the
productivity. However, the present invention has clarified the
extremely important fact contradictory to the conventional common
knowledge, as to be proved in the examples later. That is, the
productivity is improved in a case where agglomerates having an
average diameter of not smaller than 17.5 mm are heated at the
spread density of not lower than 0.5 on the hearth. However, if the
spread density of agglomerates is lower than 0.5, the density of
the agglomerates spread per unit area of the effective hearth is
too small. In this case, the amount of granular metal generated is
totally decreased even though the particle diameter is increased to
be not smaller than 17.5 mm, which leads to failure in improving
the productivity. Accordingly, agglomerates need to be spread at
the density of not lower than 0.5. The spread density is desirably
set to be as large as possible, and is preferably not lower than
0.6. There is no particular upper limit to the spread density of
agglomerates. However, if agglomerates are fed at a spread density
of more than 0.8, such agglomerates may be laid in two or more
layers. In this case, it is difficult to evenly heat the
agglomerates, which results in difficulty in producing granular
iron of high quality. Therefore, the spread density of agglomerates
is preferably set to have the upper limit of 0.8, and is more
preferably not more than 0.7.
[0042] The spread density of agglomerates is described in detail
below. The spread density of agglomerates is calculated from the
projected area ratio, relative to the hearth, of the agglomerates
spread on the hearth. Described below is the method of calculating
the spread density with reference to FIG. 1.
[0043] FIG. 1 is a plan view schematically showing agglomerates
spread on the hearth. The projected area ratio of the agglomerates
onto the hearth can be calculated by equation (1).
Projected area ratio(%)=[projected area of all agglomerates on
hearth/effective hearth area].times.100 (1)
[0044] The agglomerates are assumed to have a perfectly spherical
shape, and the average diameter of the agglomerates and the
distance of the adjacent agglomerates are expressed by Dp and r,
respectively, the projected area ratio of the agglomerates onto the
hearth can be calculated by the following equation (2):
Projected area
ratio(%)=.pi..times.(Dp).sup.2/4/{(Dp+r).times.(Dp+r).times.3.sup.0.5/2}.-
times.100(%) (2)
[0045] In a case where the distance "r" between the adjacent
agglomerates is set to 0, the projected area ratio has the maximum
value and the maximum projected area ratio has a constant value
(i.e., 90.69%). Assuming that the maximum projected area ratio is
equal to 1, the present invention defines, as the spread density, a
relative value of the projected area ratio that is calculated in
accordance with equation (2) from the average diameter Dp of the
agglomerates and the distance "r" between the adjacent
agglomerates.
[0046] In order to describe the actual cases of the spread density
in more detail, FIG. 2 shows states where agglomerates having the
average diameter of 18.2 mm are spread in containers each in a flat
plate shape of approximately 61 cm square.
[0047] Case (a) in FIG. 2 shows an example of filling in a
container agglomerates weighing 9.3 kg per unit area of 1 m.sup.2,
in which case the spread density was equal to 0.4. The theoretical
amount of agglomerates filled at the spread density of 0.4 weighs
9.33 kg per unit area of 1 m.sup.2. It is therefore found out that
the filled amount and the spread density in Case (a) is
substantially equal to the theoretical values.
[0048] Case (b) in FIG. 2 shows an example of filling in a
container agglomerates weighing 13.9 kg per unit area of 1 m.sup.2,
in which case the spread density was equal to 0.6. The theoretical
amount of agglomerates filled at the spread density of 0.6 weighs
14.0 kg per unit area of 1 m.sup.2. It is therefore found out that
the filled amount and the spread density in Case (b) is
substantially equal to the theoretical values.
[0049] Case (c) in FIG. 2 shows an example of filling in a
container agglomerates weighing 18.5 kg per unit area of 1 m.sup.2,
in which case the spread density was equal to 0.8. The theoretical
amount of agglomerates filled at the spread density of 0.8 weighs
18.66 kg per unit area of 1 m.sup.2. It is therefore found out that
the filled amount and the spread density in Case (c) is
substantially equal to the theoretical values.
[0050] Case (d) in FIG. 2 shows an example of filling in a
container agglomerates weighing 23.2 kg per unit area of 1 m.sup.2,
in which case the spread density was equal to 1.0. The theoretical
amount of agglomerates filled at the spread density of 1.0 weighs
23.33 kg per unit area of 1 m.sup.2. It is therefore found out that
the filled amount and the spread density in Case (d) is
substantially equal to the theoretical values.
[0051] It is quite difficult to spread agglomerates on an actual
hearth at the spread density of 1.0 as shown in Case (d) of FIG. 2.
In an actual case where agglomerates are fed to a furnace in the
amount of the spread density equal to 1.0, there is caused another
problem such as the charged agglomerates being overlaid with each
other. In order to feed agglomerates to the furnace so as not to be
overlaid with each other, it was found out, through the various
demonstration experiments, that the upper limit of the spread
density was preferably set to approximately 0.8, as shown in Case
(c) of FIG. 2.
[0052] On the other hand, as shown in Case (a) of FIG. 2, the
spread density equal to 0.4 causes quite a large number of spaces
on the hearth, which will extremely deteriorate the productivity.
Thus, the ideal lower limit of the spread density will be
approximately 0.5, which is an intermediate value of those of Case
(a) and Case (b) in FIG. 2.
[0053] FIG. 3 indicates the relationship between the distance "r"
of adjacent agglomerates and the projected area ratio or spread
density. In FIG. 3, the marks indicate the results of projected
area ratios, while the marks .quadrature. indicate the results of
spread densities. As apparent from FIG. 3, as the distance "r"
between the adjacent agglomerates is increased, both the projected
area ratio and the spread density of the agglomerates are reduced.
There is recognized a favorable correlation between the projected
area ratio and the spread density relative to the distance "r"
between the adjacent agglomerates.
[0054] FIG. 4 indicates the relationship between the spread density
and the amount of agglomerates fed to the furnace in a case where
the average diameter of the agglomerates is changed in the range
from 14.0 to 32.0 mm. The amount of the fed agglomerates is
indicated by the mass of the fed agglomerates in the effective
hearth area.
[0055] In FIG. 4, a straight line connecting a point (A) and a
point (B) indicates a range of the amount of agglomerates fed to
the furnace in a case where the agglomerates have an average
diameter of not smaller than 17.5 mm and are spread at the density
of 0.5. A straight line connecting a point (C) and a point (D)
indicates a range of the amount of agglomerates fed to the furnace
in a case where the agglomerates have an average diameter of not
smaller than 17.5 mm and are spread at the density of 0.8. As can
be seen from this FIG. 4, the average diameter of the agglomerates
and the amount of agglomerates to be fed to furnace (i.e, the mass
of agglomerates to be fed per effective hearth area) may be
adjusted to control the spread density of the agglomerates on the
hearth to not lower than 0.5.
[0056] The agglomerates are heated in a moving hearth-type
reduction melting furnace to reduce and to melt a metal oxide in
the agglomerates so as to manufacture granular metal. The moving
hearth-type reduction melting furnace and the heating condition in
the furnace are not particularly limited in the present invention,
and there can be adopted a known condition.
[0057] As the above moving hearth-type reduction melting furnace,
there can be used, for example, a rotary hearth furnace. There is
no particular limitation to the width of the hearth of the moving
hearth-type reduction melting furnace. According to the present
invention, it is possible to improve the productivity of granular
metal under an economically advantageous condition even with use of
an actual machine having a hearth width of not smaller than 4
m.
[0058] It is preferable to spread the carbonaceous material
(hereinafter, also referred to as bed material) on the hearth and
then to feed the agglomerates on the carbonaceous material, so that
the agglomerates are fed to form a single layer on the carbonaceous
material layer. The bed material serves as a carbon resource in a
case where the carbon included in the agglomerates is not
sufficient, and also serves as a hearth protective material.
[0059] Although there is no particular limitation to the thickness
of the bed material, the thickness is preferably not less than 3
mm. More specifically, in a case where the moving hearth-type
reduction melting furnace is actually used, the hearth width will
have several meters. Accordingly, it is difficult to spread evenly
the bed material across the width direction and there may be caused
variations in thickness from about 2 to 8 mm. It is preferable to
spread the bed material so as to have a thickness of not less than
3 mm in order to cause no portion on the hearth not covered with
the bed material. The thickness of the bed material is more
preferably not less than 5 mm, and further preferably not less than
10 mm. Because the present invention uses particularly large
agglomerates, such agglomerates are unlikely to be buried even in
the bed material having a large thickness, and the reduction
efficiency will be hardly deteriorated. More specifically, the bed
material having a larger thickness is particularly effective in a
case of using agglomerates that have an average diameter of not
less than 20 mm. There is no particular limitation either to the
upper limit of the thickness of the bed material. However, if the
thickness of the bed material is more than 30 mm, agglomerates may
be buried in the bed material even in the present invention, which
may inhibit the supply of heat to the agglomerates and thus
deteriorate the reduction efficiency. As a result, granular metal
is likely to be deformed or deteriorated in interior quality
thereof. Therefore, the thickness of the bed material is preferably
not more than 30 mm, more preferably not more than 20 mm, and
further preferably not more than 15 mm.
[0060] The carbonaceous material used as the bed material can be
selected from those exemplified as the carbonaceous reducing agent.
The carbonaceous material desirably has a particle diameter of not
more than 3.0 mm, for example. If the particle diameter of the
carbonaceous material is more than 3.0 mm, the molten slag may flow
down through the spaces in the carbonaceous material to reach the
surface of the hearth and erode the hearth. The particle diameter
of the carbonaceous material is more preferably not more than 2.0
mm. However, if the proportion of the particles having a diameter
of smaller than 0.5 mm is too large in the carbonaceous material,
the agglomerates will be buried in the bed material to lead to the
deteriorations in heating efficiency as well as in productivity of
granular metal, which is not preferable.
[0061] The agglomerates are preferably fed onto the hearth so as to
form a single layer over the bed material that is spread on the
hearth. One general idea for the increase in the production
quantity of granular metallic iron will be increasing the amount of
agglomerates to be fed to the furnace. In such a case of increasing
the amount of fed agglomerates, the agglomerates are stacked into
two or more layers on the hearth. In this case, the upper
agglomerates receive sufficient heat from a furnace body to be
reduced and melted, while sufficient heat is not fed to the lower
agglomerates, which are likely to cause residual portions not
having been reduced. If molten iron obtained only from the reduced
and melted upper agglomerates is combined with the lower un-melted
and un-reduced iron and the like, it is impossible to obtain
granular metallic iron of high quality. Therefore, in order to
reliably achieve reduction in the solid state as well as
carburizing and melting inside the furnace as in the present
invention, it is desirable to feed agglomerates onto the hearth so
as to form substantially a single layer.
[0062] Upon feeding agglomerates onto the hearth so as to form a
single layer, a pellet leveler or the like may be used to control
the agglomerates to be spread on the hearth so that the
agglomerates are evenly spread over the effective hearth across the
width direction thereof before the agglomerates fed to the furnace
enter a thermal reaction zone.
[0063] It is possible to apply a common heating condition to the
case where the agglomerates are heated in a moving hearth-type
reduction melting furnace to reduce and to melt the metal oxide
included in the agglomerates. More specifically, the agglomerates
are fed onto the hearth, reduced in the solid state at a
predetermined temperature, and further continuously heated until
being melted, so as to obtain manufactured slag (i.e., oxide)
comprising impurities and granular metallic iron. The agglomerates
on the hearth receive heat from combustion flames of a plurality of
burners installed in an upper portion in the furnace (e.g., on a
ceiling) or on a side wall, or radiation heat from a refractory
material in the furnace, which is heated to a high temperature. The
received heat is transferred from the peripheral portions to the
inner portions of the agglomerates so as to progress the reduction
reaction in the solid state.
[0064] In the upstream area in the furnace, the reduction reaction
progresses while the agglomerates being kept in the solid state. In
the downstream area in the furnace, microscopic particles of
reduced iron in the agglomerates, which have been already reduced
in the solid state, are carburized and then coalesced to each other
in the process of being melted, so as to form granular metallic
iron while being separated from the impurities (i.e., slag
components) in the agglomerates.
[0065] The temperature of the upstream area in the furnace is
preferably controlled to be at approximately 1300.degree. C. to
1450.degree. C. so as to cause the iron oxide in the agglomerates
to be reduced in the solid state. The temperature of the downstream
area in the furnace is preferably controlled to be at approximately
1400.degree. C. to 1550.degree. C. so as to cause the reduced iron
in the agglomerates to be carburized, melted, and coalesced. If the
furnace is heated to be higher than 1550.degree. C., heat is
excessively applied to the agglomerates to exceed the rate of the
heat transferred into the agglomerates. In this case, the
agglomerates are partially melted before being completely reduced
in the solid state. As a result, the reaction progresses rapidly to
cause a molten reduction reaction, which generates abnormal slag
formation.
[0066] The downstream area in the furnace may be set to a
temperature higher than that in the upstream area in the
furnace.
[0067] In the present invention, the productivity of the case where
the agglomerates are heated to reduce and to melt the metal oxide
to produce granular metal is evaluated by the production quantity
(ton) of the granular metal per unit area (m.sup.2) of the
effective hearth per unit time (time), as expressed by equation (3)
below.
Productivity(ton/m.sup.2/time)=production quantity of granular
metal (granular-metal ton/time)/effective hearth area(m.sup.2)
(3)
[0068] In equation (3), the production quantity of granular metal
(granular-metal ton/time) is expressed by equation (4) below.
Production quantity of granular
metal(granular-metal-ton/time)=amount of agglomerates charged
(agglomerates-ton/time).times.mass of granular metal produced from
1 ton of agglomerates
(granular-metal-ton/agglomerates-ton).times.product recovery rate
(4)
[0069] In equation (4), the product recovery rate is calculated as
a proportion of granular metallic iron having a diameter of not
smaller than 3.35 mm to the total mass of the granular metal
obtained [mass of granular metallic iron having a diameter of not
smaller than 3.35 mm/total mass of granular metallic
iron.times.100].
[0070] In Experimental Examples 2 and 3 in the examples to be
described later, in order to quantitatively evaluate the effects of
the present invention, a test material (i.e., agglomerates) having
the average diameter of 17.5 mm is regarded as including standard
agglomerates, and the productivity of each of the agglomerates is
indicated as a relative value (i.e., productivity index) in a case
where the productivity of the standard agglomerates is set to
1.00.
[0071] The present invention will be described in more detail with
reference to the examples. It is noted that the present invention
is never limited to the following examples but can be of course
embodied with appropriate modifications as long as being adaptable
to the purposes of the above statement and the following statement.
Such modifications are also included in the technical scope of the
present invention.
EXAMPLES
Experimental Example 1
[0072] Agglomerates were prepared from a raw material mixture
containing a metal oxide and a carbonaceous reducing agent, and the
agglomerates were fed onto a hearth of a moving hearth-type
reduction melting furnace and were heated thereon to reduce and to
melt the metal oxide in the raw material mixture, so as to produce
granular metallic iron.
[0073] In this case, iron ore having the component compositions
listed in Table 1 below was used as the metal oxide, and coal
having the component compositions listed in Table 2 below was used
as the carbonaceous reducing agent, to produce the agglomerates.
More specifically, the mixture containing the iron ore and the coal
was blended with flour serving as a binder and an auxiliary
material such as limestone or dolomite, to produce agglomerates
(i.e., test materials) in the shapes of pellets having different
average diameters. The blend compositions (i.e., weight
percentages) of the test materials are listed in Table 3 below.
Further, the longer diameters and the shorter diameters of the test
materials were measured with use of a vernier caliper to calculate
the average diameters, which are listed in Table 4 below. Each of
the average diameters of the test materials is obtained by
measuring the sizes of 20 particles of each of the test
materials.
[0074] There are also listed in Table 4 unit mass and an apparent
density of each of the test materials. The unit mass of each of the
test materials is equal to an average value obtained by measuring
the mass of 20 particles. The apparent density of each of the test
materials is obtained by immersing the agglomerates in a liquid
(i.e., mercury) and measuring buoyant forces thereof.
[0075] Each of the test materials thus obtained and having the
different average diameters was heated in a small heating furnace
on a laboratory scale (i.e., the temperature in the furnace being
set to 1450.degree. C.) to reduce and to melt the iron ore included
in the corresponding test material, in order to measure time
required for the reaction (i.e., reaction time). The measurement
results on the reaction time are listed in Table 4 below.
[0076] FIG. 5 indicates the relationship between the average
diameter (Dp) and the reaction time of the test material. In FIG.
5, a dotted curve shows an approximated curve including plotted
points, which is expressed by a quadratic of the average diameter
of the test material. As apparent from FIG. 5, as the average
diameter of the test material increases, the reaction time is
longer.
[0077] According to the results of Experimental Example 1, the
reaction time and the product recovery rate were normalized to
comprehensively evaluate the productivity of a case where the
distance between the adjacent particles of the test material is
changed (see Experimental Example 2 to be described later), or of a
case where the spread density of the test material is changed (see
Experimental Example 3 to be described later).
TABLE-US-00001 TABLE 1 Iron Component composition (mass %) ore
Total Fe FeO SiO.sub.2 CaO Al.sub.2O.sub.3 MgO MnO TiO.sub.2 P S
67.73 29.40 4.54 0.42 0.21 0.47 0.34 0.07 0.018 0.002
TABLE-US-00002 TABLE 2 Component composition (mass %) Fixed carbon
Volatile Ash Total Coal 77.21 16.65 6.14 100
TABLE-US-00003 TABLE 3 Blend composition (mass %) Iron ore Coal
Binder Auxiliary material Test material 71.95 17.01 0.90 11.55
TABLE-US-00004 TABLE 4 Average diameter Unit mass Apparent density
Reaction time No. (mm) (g/Piece) (g/cm.sup.3) (min) 1 17.3 6.06
2.23 8.7 2 18.8 7.58 2.19 8.8 3 19.4 8.46 2.21 9.0 4 21.3 11.16
2.21 10.0 5 23.1 14.60 2.27 10.7 6 25.2 18.77 2.24 12.0 7 27.0
22.98 2.23 13.2
Experimental Example 2
[0078] In Experimental Example 2, test materials, which have
average diameters of 16.0 to 28.0 mm (i.e., 1.60 to 2.80 cm) and
are spread at a constant density on a hearth, were heated in an
actual moving hearth-type reduction melting furnace to produce
granular metallic iron. Comprehensively investigated was how the
average diameter of the test material influences on the
productivity of granular metallic iron thus produced.
[0079] A rotary hearth furnace was used as the moving hearth-type
reduction melting furnace, and each of the test materials was fed
onto the hearth at the spread density of 0.66 and was heated
thereon to reduce and to melt iron ore so as to produce granular
metallic iron. The temperature of the upstream area in the furnace
was set to 1400.degree. C. and the temperature of the downstream
area thereof was set to 1470.degree. C. In the upstream area, the
iron ore in the test material is reduced in the solid state. In the
downstream area, microscopic particles of reduced iron, which are
generated and melted in the test material, are carburized, melted,
and eventually coalesced so as to separate molten iron from
slag.
[0080] The spread density of the test material on the hearth was
controlled by regulating the amount of the test material fed to the
furnace and the moving speed (i.e., rotating speed) of the hearth.
More specifically, the moving speed of the hearth was determined
such that the iron ore was reduced and melted in the heating zone
under an atmospheric condition set in accordance with the result of
the preliminary experiment. The supply amount of the test material
was regulated in consideration of this moving speed, so that the
spread density of the test material on the hearth was controlled to
0.66. Table 5 below shows the distance "r" between the adjacent
particles of the test materials as reference values.
[0081] The productivity of granular metallic iron produced by
reducing and melting each of the test materials was calculated in
accordance with above equation (3), and the productivity of each of
the test materials was indicated as a relative value (i.e.,
productivity index), assuming that the productivity of the test
material No. 12 (i.e., standard agglomerates) has a standard value
(i.e., productivity index equal to 1.00). The productivity indices
of the respective test materials are listed in Table 5 below.
Further, FIG. 6 indicates the relationship between the average
diameter and the productivity index of the test material.
[0082] As apparent from FIG. 6, when the spread density on the
hearth is kept constant, the productivity can be improved by
setting the average diameter of the test material to be not smaller
than 17.5 mm in comparison to the case of setting the average
diameter of the test material to 16.0 mm. In other words, the
productivity is gradually improved as the average diameter of the
test material increases, and the productivity index reaches the
maximum value in the case where the average diameter of the test
material equal to 22.0 mm.
[0083] However, if the average diameter of the test material is set
to be larger than 26.0 mm, the productivity of granular metallic
iron tends to be gradually deteriorated. The productivity will be
deteriorated because the reaction time is longer with the test
material of a larger size. Accordingly, when the spread density is
kept constant, it is found that the productivity can be improved by
setting the average diameter of the test material to the range from
17.5 to 26.0 mm in comparison to the case of using the test
material having the average diameter of 16.0 mm.
TABLE-US-00005 TABLE 5 Average diameter Distance "r" Spread density
Productivity No. (cm) (cm) (--) index 11 1.60 0.37 0.66 0.93 12
1.75 0.37 0.66 1.00 13 1.81 0.42 0.66 1.02 14 1.90 0.44 0.66 1.05
15 2.00 0.46 0.66 1.07 16 2.20 0.50 0.66 1.08 17 2.40 0.55 0.66
1.05 18 2.60 0.60 0.66 1.01 19 2.80 0.64 0.66 0.95
Experimental Example 3
[0084] In Experimental Example 3, assuming test materials each
having an average diameter of 16.0 to 32.0 mm (i.e., 1.60 to 3.20
cm), adjacent particles of each of the test materials being apart
from each other at a constant distance "r" (i.e., 0.42 cm) on the
hearth were heated to produce granular metallic iron in an actual
moving hearth-type reduction melting furnace with the spread
densities of the test materials being changed. In this manner,
investigated was how the spread density of the test material
influenced on the productivity of granular metallic iron.
[0085] In the evaluation in this case, a rotary hearth furnace was
used as the moving hearth-type reduction melting furnace, and each
of the test materials, which have the average diameters listed in
Table 6 below and were fed onto the hearth, was heated to reduce
and to melt iron ore so as to produce granular metallic iron. The
heating condition in the furnace was set identically with that of
Experimental Example 2 described earlier. The spread densities of
the test materials on the hearth are listed in Table 6.
[0086] The productivity of the granular metallic iron produced by
reducing and melting each of the test materials was calculated in
accordance with equation (3) above, and the productivity of each of
the test materials was indicated as a relative value (i.e.,
productivity index), assuming that the productivity of the test
material No. 22 (i.e., standard agglomerates) has a standard value
(i.e., 1.00). The productivity indices of the respective test
materials are listed in Table 6 below. Further, FIG. 7 indicates
the relationship between the average diameter and the productivity
index of the test material.
[0087] As apparent from Table 6 and FIG. 7 below, in the case where
the distance "r" between the adjacent particles of the test
material is kept constant, the spread density of the test material
on the hearth can be increased by setting the average diameter of
the test material to be not smaller than 17.5 mm. Further, the
productivity of the granular metallic iron can be improved by
increasing the average diameter of the test material in comparison
to the case of setting the average diameter of the test material to
16.0 mm. In other words, the productivity is gradually improved as
the average diameter of the test material increases, and the
productivity index reaches the maximum value in the case where the
average diameter of the test material is equal to 24.0 mm.
[0088] However, if the average diameter of the test material is
larger than 24.0 mm, the productivity of the granular metallic iron
tends to be gradually deteriorated. The productivity will be
deteriorated because the reaction time is longer with the test
material of a larger size. Accordingly, it is found that the
productivity can be improved by setting the average diameter of the
test material to the range from 17.5 mm to 32.0 mm in comparison to
the case of using the test material having the average diameter of
16.0 mm.
TABLE-US-00006 TABLE 6 Average diameter Distance "r" Spread density
Productivity No. (cm) (cm) (--) index 21 1.60 0.42 0.63 0.89 22
1.75 0.42 0.65 1.00 23 1.81 0.42 0.66 1.04 24 1.90 0.42 0.67 1.08
25 2.00 0.42 0.69 1.12 26 2.20 0.42 0.71 1.17 27 2.40 0.42 0.73
1.17 28 2.60 0.42 0.74 1.15 29 2.80 0.42 0.76 1.10 30 3.00 0.42
0.77 1.05 31 3.20 0.42 0.78 0.99
[0089] The following conclusion can be obtained by combining the
results of Experimental Examples 2 and 3. As described in
Experimental Example 2, when using agglomerates having a large
average diameter (e.g., agglomerates having an average diameter of
more than 28.0 mm), the productivity of granular metallic iron may
be deteriorated at a constant spread density. However, as described
in Experimental Example 3, if the spread density is increased, the
productivity can be improved even in the case of using the
agglomerates having an average diameter of more than 28.0 mm. In
summary, the productivity can be improved by feeding onto the
hearth at a spread density of not lower than 0.5 the agglomerates
(i.e., test material) having an average diameter of not smaller
than 17.5 mm and heating the agglomerates on the hearth. In other
words, it is possible to productively produce granular metallic
iron by preparing agglomerates having an average diameter of not
smaller than 17.5 mm and feeding the agglomerates onto the hearth
at a spread density of not lower than 0.5 to heat the same in the
furnace.
INDUSTRIAL APPLICABILITY
[0090] The present invention is applicable to improve the
productivity of the granular metal.
* * * * *