U.S. patent application number 10/482403 was filed with the patent office on 2004-09-09 for method of manufacturing metal iron.
Invention is credited to Ito, Shuzo, Kikuchi, Shoichi, Kobayashi, Isao, Tanigaki, Yasuhiro, Tokuda, Koji, Tsuge, Osamu.
Application Number | 20040173054 10/482403 |
Document ID | / |
Family ID | 19047826 |
Filed Date | 2004-09-09 |
United States Patent
Application |
20040173054 |
Kind Code |
A1 |
Tsuge, Osamu ; et
al. |
September 9, 2004 |
Method of manufacturing metal iron
Abstract
The present invention is intended to provide a method for
producing metallic iron, which comprises the steps of supplying a
mixture containing a carbonous reducing agent and iron oxides onto
a hearth of a reduction melting furnace of the moving hearth type,
heating the mixture for reduction melting of the iron oxides,
cooling thus-obtained metallic iron, and discharging the metallic
iron to the outside of the furnace for recovery. The method can
easily remove or repair the surface of a hearth even when metallic
iron powder is buried in the hearth surface or even when the hearth
surface suffers from slag infiltration and erosion, can increase an
availability factor and maintainability of the hearth, and is
suitably practiced for long-term continuous operation. The present
invention resides in a method for producing metallic iron, wherein
a hearth material is laid in the form of a layer on the hearth
prior to supply of the mixture, thereby forming a renewable hearth
capable of being renewed, and the metallic iron is produced while
renewing a part or the whole of the renewable hearth, which has
deteriorated during operation, with the hearth material.
Inventors: |
Tsuge, Osamu; (Kobe-shi,
JP) ; Tanigaki, Yasuhiro; (Kobe-shi, JP) ;
Kobayashi, Isao; (Kobe-shi, JP) ; Tokuda, Koji;
(Kobe-shi, JP) ; Kikuchi, Shoichi; (Kobe-shi,
JP) ; Ito, Shuzo; (Kobe-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Family ID: |
19047826 |
Appl. No.: |
10/482403 |
Filed: |
January 9, 2004 |
PCT Filed: |
June 17, 2002 |
PCT NO: |
PCT/JP02/05995 |
Current U.S.
Class: |
75/485 |
Current CPC
Class: |
C21B 13/10 20130101;
C21B 13/105 20130101; Y02P 10/134 20151101; C21B 13/0046 20130101;
C22B 5/10 20130101; C22B 1/245 20130101; C21B 13/0006 20130101;
C21B 13/0066 20130101; C21B 13/008 20130101 |
Class at
Publication: |
075/485 |
International
Class: |
C21B 011/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 12, 2001 |
JP |
2001-212714 |
Claims
1. A method for producing metallic iron, the method comprising the
steps of supplying a mixture containing a carbonous reducing agent
and iron oxides onto a hearth of a reduction melting furnace of the
moving hearth type, heating the mixture for reduction melting of
the iron oxides, cooling thus-obtained metallic iron, and
discharging the metallic iron to the outside of said furnace for
recovery, wherein a hearth material is laid in the form of a layer
on said hearth prior to supply of the mixture, thereby forming a
renewable hearth capable of being renewed, and the metallic iron is
produced while renewing a part or the whole of said renewable
hearth, which has deteriorated during operation, with the hearth
material.
2. A method for producing metallic iron, the method comprising the
steps of supplying a mixture containing a carbonous reducing agent
and iron oxides onto a hearth of a reduction melting furnace of the
moving hearth type, heating the mixture for reduction melting of
the iron oxides, cooling thus-obtained metallic iron, and
discharging the metallic iron to the outside of said furnace for
recovery, wherein a hearth material is laid in the form of a layer
on said hearth prior to supply of the mixture, thereby forming a
renewable hearth capable of being renewed, and the metallic iron is
produced while a hearth surface is renewed by charging the hearth
material to lie in the form of a layer on said renewable hearth
which has deteriorated during operation, or by supplying the hearth
material onto the surface of the deteriorated renewable hearth.
3. The producing method according to claim 2, wherein the hearth
material is filled in dents formed in the surface of the hearth
layer during operation of said reduction melting furnace.
4. The producing method according to any one of claims 1 to 3,
wherein the whole or a part of said renewable hearth, which has
deteriorated during operation, is removed periodically or
continuously.
5. The producing method according to any one of claims 1 to 4,
wherein a thickness of said renewable hearth is adjusted.
6. The producing method according to claim 4 or 5, wherein after
removing said renewable hearth is renewed by supplying the hearth
material after removing said renewable hearth.
7. The producing method according to any one of claims 1 to 6,
wherein the hearth material contains a substance having a high
melting point and being corrosion-resistant against produced
slag.
8. The producing method according to claim 7, wherein the hearth
material further contains a carbonous substance.
9. The producing method according to any one of claims 1 to 8,
wherein the substance having a high melting point contains oxides
including alumina and/or magnesia, or silicon carbide.
10. The producing method according to any one of claims 1 to 9,
wherein the hearth material further contains a sintering
accelerator.
11. A method for producing metallic iron, the method comprising the
steps of supplying a mixture containing a carbonous reducing agent
and iron oxides onto a renewable hearth capable of being renewed,
which is formed by charging a hearth material to lie in the form of
a layer, heating the mixture for reduction melting of the iron
oxides, cooling thus-obtained metallic iron, and discharging the
metallic iron to the outside of said furnace for recovery, wherein
after supplying a coolant to said renewable hearth, which has
deteriorated during operation, to solidify molten iron residing on
the surface of said renewable hearth, said renewable hearth is
removed together with the residing iron, and the metallic iron is
produced while renewing a part or the whole of said renewable
hearth with the hearth material.
12. The producing method according to any one of claims 1 to 11,
wherein said renewable hearth is softened before renewing said
renewable hearth.
13. The producing method according to any one of claims 1 to 12,
wherein after charging the hearth material to lie in the form of a
layer on said hearth to form said renewable hearth, an atmosphere
modifier containing a powdery carbonous substance is laid in the
form of a layer and the mixture is then supplied.
14. The producing method according to claim 13, wherein the hearth
material is mixed in said atmosphere modifier.
15. The producing method according to claim 13 or 14, wherein said
atmosphere modifier is laid in the form of two or more layers.
16. The producing method according to any one of claims 1 to 15,
wherein a carbonous material layer is provided between said hearth
and said renewable hearth or between said renewable hearth and a
renewable hearth supplied onto said renewable hearth.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for producing
metallic iron, and more particularly to a method for producing
metallic iron, which is employed to produce the metallic iron by
supplying a mixture containing a carbonous reducing agent, such as
coal, and iron oxides, such as iron ore, onto a moving hearth of a
reduction melting furnace of the moving hearth type, heating the
mixture for reduction melting of the iron oxides, and then cooling
thus-obtained metallic iron, and which is improved to be able to
perform continuous operation of the above production process with
stability while minimizing damages of the hearth or repairing
damaged surface areas of the hearth during the operation.
BACKGROUND ART
[0002] As a method for producing reduced iron, there is known a
process of charging a mixture of iron oxides, such as iron core,
and a carbonous reducing agent, such as coal, onto a hearth of a
rotary furnace or a reducing furnace of the moving hearth type,
e.g., of the straight grate type, heating the mixture with
radiation heat in the furnace while the mixture is moving in the
furnace, and then discharging reduced iron, which is obtained by
reduction of the iron oxides with the carbonous reducing agent, to
the outside of the furnace from the hearth using any suitable
discharging means, such as a screw mechanism.
[0003] In the case of providing the mixture as agglomerates in the
form of, e.g., pellets, however, when the agglomerates are charged
onto the hearth, powder generated from the agglomerates with drop
impacts, etc. is accumulated on the hearth. The accumulated powder
is heated and reduced along with the agglomerates, whereby the
accumulated powder becomes powdery reduced iron and the
agglomerates become granular reduced iron. The granular reduced
iron is discharged out of the furnace by a discharging screw, while
the powdery reduced iron is pressed into the hearth surface by the
discharging screw. In continuous operation, therefore, a problem
has occurred in that, as the amount of the reduced iron powder
pressed into the hearth surface increases, the reduced iron powder
coheres together under pressing forces repeatedly applied from the
discharging screw and forms an iron sheet on the hearth surface. In
the reducing furnace of the moving hearth type, since heating and
reducing zones are at high temperatures, but a raw-material
charging zone and a discharge zone are at relatively low
temperatures, the iron sheet formed on the hearth surface tends to
crack or warp because of such a temperature difference between the
zones. Then, if the discharging screw is caught by the deformed
iron sheet, a trouble, such as operation shutdown, has
resulted.
[0004] A technique for solving the above problem has been
previously proposed by the inventors (Japanese Patent No. 3075721).
According to the proposed technique, powder entering a furnace in
company with agglomerates and generated from the agglomerates is
accumulated on the surface of a hearth to form an iron oxide layer
on the hearth, and a discharging device is intermittently or
continuously moved toward the furnace ceiling during the operation,
while adjusting a gap between the discharging device and the iron
oxide layer formed on the moving hearth surface. It is hence
possible to suppress powdery reduced iron from being pressed into
the hearth surface by the discharging device, and to prevent
formation of an iron sheet on the hearth. Further, the accumulated
layer of the reduced iron powder is periodically scraped off so
that the continuous operation is enabled. The proposed technique is
intended to enable the operation to be continued by scraping off an
iron sheet formed on the hearth surface and periodically renewing
and repairing the hearth surface, but it is not intended to scrape
off the hearth itself.
[0005] Also, as a method for producing metallic iron, there is
known a process of charging a mixture of iron oxides and a reducing
material into a reduction melting furnace of the moving hearth
type, such as a rotary hearth furnace; heating the mixture with
radiation heat in the furnace while the mixture is moving in the
furnace; reducing the iron oxides with the reducing material;
separating carburized, molten and aggregated slag; cooling reduced
iron for solidification to form granular solid metallic iron; and
then taking the granular solid metallic iron out of the furnace. In
this connection, the inventors have previously proposed in, e.g.,
Japanese Unexamined Patent Application Publication No. 2000-144224,
a technique for forming a vitreous berth layer made up of iron
oxides, carbon and a silica compound on the hearth surface of a
rotary hearth furnace, thereby preventing damages of the hearth
caused by molten iron. However, since the vitreous layer
deteriorates because of slag infiltration (permeation) and erosion
when the operation is continued, there still remains a room for
improvement to realize stable and continuous operation.
[0006] In view of the above-described state of the art, an object
of the present invention is to provide a method for producing
metallic iron, which can easily remove or repair the surface of a
hearth even when metallic iron powder is buried in the hearth
surface or even when the hearth surface suffers from slag
infiltration and erosion, which can increase an availability factor
and maintainability of the hearth, and which is suitably practiced
for long-term continuous operation.
DISCLOSURE OF THE INVENTION
[0007] The present invention having succeeded in overcoming the
problems described above resides in a method for producing metallic
iron, the method comprising the steps of supplying a mixture
containing a carbonous reducing agent and iron oxides onto a hearth
of a reduction melting furnace of the moving hearth type, heating
the mixture for reduction melting of the iron oxides, cooling
thus-obtained metallic iron, and discharging the metallic iron to
the outside of the furnace for recovery, wherein a hearth material
is laid in the form of a layer on the hearth prior to supply of the
mixture, thereby forming a renewable hearth capable of being
renewed, and the metallic iron is produced while renewing a part or
the whole of the renewable hearth, which has deteriorated during
operation, with the hearth material.
[0008] Also, the present invention resides in a method for
producing metallic iron, wherein a hearth material is laid in the
form of a layer on the hearth prior to supply of the mixture,
thereby forming a renewable hearth capable of being renewed, and
the metallic iron is produced while a hearth surface is renewed by
charging the hearth material to lie in the form of a layer on the
renewable hearth which has deteriorated during operation, or by
supplying the hearth material onto the surface of the deteriorated
renewable hearth.
[0009] When practicing the method of the present invention, the
metallic iron may be produced while filling the hearth material in
dents to repair the dents, which are formed in the surface of the
hearth layer during operation of the reduction melting furnace.
[0010] In the present invention, it is recommended that the whole
or a part of the renewable hearth, which has deteriorated during
operation, be removed periodically or continuously. Preferably, a
thickness of the renewable hearth is adjusted. In a preferred
embodiment of the present invention, the renewable hearth is
renewed by supplying the hearth material after removing the
renewable hearth. The hearth material preferably contains a
substance having a high melting point and being corrosion-resistant
against produced slag. Additionally, the hearth material preferably
further contains a carbonous substance. It is recommended that the
substance having a high melting point contain oxides including
alumina and/or magnesia, or silicon carbide. In a preferred
embodiment of the present invention, a sintering accelerator is
mixed in the hearth material.
[0011] In the present invention, preferably, after charging the
hearth material to lie in the form of a layer on the hearth to form
the renewable hearth, an atmosphere modifier containing a powdery
carbonous substance is laid in the form of a layer and the mixture
is then supplied. Also, in a preferred embodiment of the present
invention, the hearth material is mixed in the atmosphere modifier.
In the present invention, preferably, after supplying a coolant to
the renewable hearth, which has deteriorated during operation, to
solidify molten iron residing on the surface of the renewable
hearth, the renewable hearth is removed together with the residing
iron, and the metallic iron is produced while renewing a part or
the whole of the renewable hearth with the hearth material.
[0012] In the present invention, the renewable hearth may be
softened before renewing the renewable hearth. Also, the atmosphere
modifier may be laid in the form of two or more layers. When
practicing the present invention, a carbonous material layer may be
provided between the hearth and the renewable hearth or between the
renewable hearth and a renewable hearth supplied onto the former
renewable hearth.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic explanatory view showing one example
of a reduction melting furnace of the circular moving hearth type
to which the present invention is applied.
[0014] FIG. 2 is a sectional view taken along the line A-A in FIG.
1.
[0015] FIG. 3 is an explanatory view showing a section of the
reduction melting furnace in the developed form as viewed in the
rotating direction of a moving hearth in FIG. 1.
[0016] FIG. 4 is a schematic explanatory view showing a state in
which a renewable hearth is initially formed.
[0017] FIG. 5 is a schematic explanatory view showing an ordinary
operation.
[0018] FIG. 6 is a schematic explanatory view showing deterioration
of the renewable hearth.
[0019] FIG. 7 is a schematic explanatory view showing a state in
which the renewable hearth is renewed.
[0020] FIG. 8 is a schematic explanatory view showing a state in
which the renewable hearth is renewed.
[0021] FIG. 9 is a schematic explanatory view showing a state in
which the renewable hearth is renewed.
[0022] FIG. 10 is a schematic explanatory view showing a state in
which the renewable hearth is renewed.
[0023] FIG. 11 is a schematic explanatory view showing an operation
employing an atmosphere modifier.
[0024] FIG. 12 is a schematic explanatory view showing an operation
employing two layers of an atmosphere modifier.
[0025] FIG. 13 is a schematic explanatory view showing an
improvement of easiness in removing the renewable hearth with the
aid of carbonous material layers.
BEST MODE FOR CARRYING OUT THE INVENTION
[0026] Embodiments of the present invention will be described below
in detail with reference to the drawings. It is, however, to be
noted that the following embodiments are illustrated merely as
typical examples and the present invention is not limited to the
illustrated examples.
[0027] FIGS. 1 to 3 are schematic explanatory views showing one
example of a reduction melting furnace of the moving hearth type
(rotary furnace). The furnace is of a dome-shaped structure having
a rotary moving hearth in the doughnut form. Specifically, FIG. 1
is a schematic plan view, FIG. 2 is an elevational sectional view
taken along the line A-A in FIG. 1, and FIG. 3 is a schematic
explanatory view showing a section of the reduction melting furnace
in the developed form as viewed in the rotating direction of the
rotary moving hearth in FIG. 1 for easier understanding. In the
drawings, numeral 1 denotes a rotary hearth, and 2 denotes a
furnace body covering the rotary hearth. The rotary hearth 1 is
constructed such that it can be driven by a driving device (not
shown) to rotate at a proper speed.
[0028] As shown in FIG. 2, by way of example, a plurality of
combustion burners 3 are disposed at appropriate places in a wall
surface of the furnace body 2. Combustion heat and radiation heat
generated by the combustion burners 3 are transmitted to a mixture
containing a carbonous reducing agent and iron oxides (hereinafter
referred to also as a "raw-material mixture"), which is placed on
the rotary hearth 1, for performing heating reduction of the
raw-material mixture. In the following, the present invention is
described in connection with the case of employing, as the
raw-material mixture, agglomerates containing a carbonous reducing
agent and iron oxides (hereinafter referred to as "raw-material
agglomerates"). However, the present invention is not limited to
the use of agglomerates, but may use a powdery raw-material mixture
as well. Also, the agglomerates can be prepared in various shapes,
such as pellets and briquettes.
[0029] FIG. 3 shows a preferred example of the furnace body 2. The
interior of the furnace body 2 is divided by partition walls
K.sub.1 to K.sub.3 into a plurality of zones ranging from a
reducing zone Z.sub.1 to a cooling zone Z.sub.4. A raw-material
agglomerate charging means 4, an atmosphere modifier charging means
7, and a hearth material charging means 5 are disposed in an
opposed relation to the rotary hearth 1 at the upstream side in the
rotating direction of the furnace body 2. A discharging device 6 is
provided at the most downstream side in the rotating direction (in
other words, at the side immediately upstream of the charging means
5 because of the rotary structure).
[0030] In operation of such a reduction melting furnace, the rotary
hearth 1 is rotated at a predetermined speed, and the raw-material
agglomerates are supplied from the charging means 4 onto the rotary
hearth 1 such that a layer of the raw-material agglomerates has a
proper thickness. The raw-material agglomerates charged on the
rotary hearth 1 are subjected to combustion heat and radiation heat
generated by the combustion burners 3 while moving in the reducing
and melting zones Z.sub.1 to Z.sub.3. Iron oxides in the
raw-material agglomerates are reduced under heating with the aid of
carbon monoxide generated upon the reaction between the iron oxides
and the carbonous reducing agent in the raw-material agglomerates.
Then, reduced iron produced with almost complete reduction of the
iron oxides is further heated under a carbon-rich atmosphere,
whereby granular molten metallic iron is obtained through a process
in which the reduced iron is carburized, melted and aggregated
while separating from slag produced as a by-product. Thereafter,
the granular molten metallic iron is cooled by any suitable cooling
means C in the cooling zone Z.sub.4 for solidification, and is
successively scraped out by the discharging device 6 provided
downstream of the cooling zone Z.sub.4. Simultaneously, the slag
produced as a by-product is also discharged. After passing a hopper
H, the granular metallic iron and the slag are separated from each
other with any suitable separating means (such as a sieve or a
magnetic screening device). Finally, granular metallic iron having
iron purity of not less than about 95%, more preferably of not less
than about 98%, and containing a very small amount of slag
components can be obtained.
[0031] In the present invention, when producing metallic iron of
high purity using the reduction melting furnace of the moving
hearth type as described above, the prime aim is particularly
focused on protection of the hearth constituted as the rotary
hearth 1. The following description is, therefore, made of
primarily methods for repairing and renewing the hearth. As a
matter of course, however, the construction of the reduction
melting furnace of the moving hearth type, to which the present
invention is applied, is not limited to the shape and structure
shown in FIGS. 1 to 3. So long as the reduction melting furnace
includes a moving hearth as a constituent element, various
reduction melting furnaces of the moving hearth type having any
other structures, e.g., the straight grate type, can also be
effectively employed in the present invention.
[0032] The present invention is implemented in a plant for
producing metallic iron, in which a mixture containing iron oxides,
e.g., iron ore, as an iron source and a carbonous reducing agent,
e.g., coal, serving as a reducing agent for the iron oxides are
supplied onto the hearth of the reduction melting furnace of the
moving hearth type and heated for reduction melting of the iron
oxides, and thus-obtained metallic iron is cooled and discharged to
the outside of the furnace. Then, the present invention is intended
to protect the hearth that serves as a support layer when the
metallic iron is successively produced through the steps of
heating, reducing, carburizing and melting, and to enable stable
operation to be continued by renewing the surface of the hearth
formed with charging of the raw-material mixture.
[0033] The basic concept of the present invention resides in a
method for producing metallic iron, the method comprising the steps
of supplying a mixture containing a carbonous reducing agent and
iron oxides onto a hearth of a reduction melting furnace of the
moving hearth type, heating the mixture for reduction melting of
the iron oxides, cooling thus-obtained metallic iron, and
discharging the metallic iron to the outside of the furnace for
recovery, wherein a hearth material is laid in the form of a layer
on the hearth prior to supply of the mixture, thereby forming a
renewable hearth, and the metallic iron is produced while renewing
a part or the whole of the renewable hearth, which has deteriorated
during operation, with the hearth material.
[0034] FIGS. 4 and 5 are schematic sectional explanatory views
showing one preferred embodiment of the present invention. At the
start of the operation, the hearth material is laid in the form of
a layer on a hearth refractory 8 of the reduction melting furnace
of the moving hearth type prior to supply of the raw-material
agglomerates, thereby forming a renewable hearth 9 that is capable
of being renewed as required. The method of charging the hearth
material is not limited to a particular one, and it is recommended
that the hearth material be charged to lie on the hearth refractory
at a uniform thickness by using the hearth material supply
apparatus 5 while the hearth is rotated. It is also recommended to
level and compact the hearth material by using the discharging
device 6, while the hearth is rotate, after charging the hearth
material on the hearth refractory. This is because the renewable
hearth having an appropriate strength and smoothness can be formed
at any desired thickness. Alternatively, a separate leveling device
(not shown) may be used instead of the discharging device 6.
[0035] The thickness of the renewable hearth is not limited to a
particular one, but it is recommended that the hearth thickness be
preferably not less than 5 mm, more preferably not less than 10 mm,
from the viewpoints of suppressing the molten slag from
infiltrating to the hearth refractory and giving the renewable
hearth a sufficient strength endurable to the operations of
charging the raw-material agglomerates or discharging the metallic
iron as a product and the slag.
[0036] After forming the renewable hearth, raw-material
agglomerates G are supplied onto the renewable hearth by using the
raw-material supply apparatus 4 while the hearth is moved. As
described above with reference to FIGS. 1 to 3, the raw-material
agglomerates are subjected to combustion heat and radiation heat
generated by the burners while moving in the zones Z.sub.1 to
Z.sub.3 of the reduction heating furnace, whereby iron oxides in
the raw-material agglomerates are reduced in a solid state and
become reduced iron. The reduced iron is further heated so that it
is carburized to have a lower melting point and then melted. The
molten iron coheres together to aggregate and grow into relatively
large granular metallic iron Fe while separating from slag that is
produced as a by-product and aggregated into by-product slag Sg.
Then, the granular metallic iron Fe and the by-product slag Sg are
cooled at the position immediately upstream of the discharging
device as described above, and are moved to the position where the
discharging device is installed. Thereafter, the granular metallic
iron Fe and the slag Sg both solidified under the cooling are
scraped out of the hearth surface by the discharging device.
[0037] The production of metallic iron is continued in that way.
With the operation continued for a long period, however, the
renewable hearth deteriorates gradually and the stable production
of metallic iron cannot be continued any more. FIG. 6 shows
examples of deterioration of the renewable hearth. For example, a
part of the molten slag produced as a by-product in the
above-described reduction melting process contacts with the
renewable hearth and penetrates into the renewable hearth. Also,
with the continued operation, the amount of the slag penetrated
into the renewable hearth increases, whereupon the renewable hearth
is corroded or softened because of a reduction in melting point
thereof, thus resulting in metamorphic expansion. As a result, the
renewable hearth loses the strength and smoothness required for the
hearth, and the stable production of metallic iron can no longer be
continued. If the infiltration of slag further proceeds, the spread
of the slag infiltration and erosion reaches the hearth refractory.
This eventually leads to such an event that the furnace must be
stopped and the hearth refractory must be repaired.
[0038] Further, with the operation of discharging the granular
metallic iron Fe and the slag Sg both having been solidified, the
metallic iron Fe and the slag Sg are often buried into the
renewable hearth under pressing by the discharging device. In
particular, the granular metallic iron Fe and the slag Sg tend to
be easily buried into the renewable hearth when it is softened as
described above. The slag Sg buried in the renewable hearth is
moved back into the furnace with the rotation of the hearth and
melted again because of being subjected to high temperature.
Therefore, the buried slag Sg penetrates into the renewable hearth
similarly to the above-mentioned molten slag. The metallic iron Fe
buried in the renewable hearth is also moved back into the furnace
with the rotation of the hearth and melted again because of being
subjected to high temperature. Therefore, the buried metallic iron
coheres with each other or with metallic iron Fe produced from the
newly supplied raw-material agglomerates G, and grow into enlarged
metallic iron. With a further increase in size of the metallic
iron, the thus-enlarged metallic iron cannot be sufficiently cooled
and solidified by the cooling ability available in the cooling
zone, and hence reaches a discharge section while it is in a state
of molten iron. It is difficult to discharge the metallic iron in
such a state out of the furnace by the discharging device. Though
depending on the type of discharging means used, the metallic iron
Fe and the slag Sg that tend to be easily buried into the renewable
hearth are often in the form of fine metallic iron Fe and slag Sg
that are not sufficiently aggregated and grown in the
above-described melting process.
[0039] Further, in the discharging operation, the renewable hearth
having expanded metamorphically is sometimes caught by the
discharging device and is partly peeled off. In other cases, the
enlarged metallic iron residing in the renewable hearth is removed
and a dent is formed there. The metallic iron Fe and the slag Sg
are apt to reside in the thus-formed dent, thereby accelerating the
slag infiltration into the renewable hearth and rendering the
metallic iron to increase its size and to remain molten.
[0040] Renewing of the renewable hearth performed in the present
invention is intended to restore the function of the renewable
hearth and to continue the stable operation of producing the
metallic iron. Examples of the renewing method are shown in FIGS. 7
to 10.
[0041] In FIG. 7, numeral 9a denotes a deteriorated area of the
renewable hearth 9. After discharging the metallic iron Fe and the
slag Sg to the outside of the furnace by the discharging device 6,
the hearth material is charged to the surface of the renewable
hearth deteriorated area 9a prior to supply of the raw-material
agglomerates G so that the stable operation of producing the
metallic iron can be continued. At that time, the lower end of a
blade of the discharging device is positioned on the surface of the
renewable hearth deteriorated area 9a, and with the rotation of the
hearth, it removes a part of the metallic iron Fe and the slag Sg
residing on a surface layer of the renewable hearth deteriorated
area and presses the hearth material, which is newly supplied onto
the surface of the renewable hearth deteriorated area, into the
surface layer of the renewable hearth deteriorated area, thereby
restoring the function of the renewable hearth deteriorated area.
The charging of the hearth material is not necessarily continued at
all times, but may be stopped when the function of the renewable
hearth deteriorated area is restored. Then, the similar renewing
step may be repeated in a stage in which deterioration of the
renewable hearth has progressed again with further continuation of
the operation.
[0042] Stopping the production of the metallic iron to carry out
the step of renewing the renewable hearth results in a reduction of
the availability factor. However, when the renewable hearth is
badly deteriorated, e.g., when a large dent is formed in the
renewable hearth, this remarkably accelerates the slag infiltration
into the renewable hearth and renders the metallic iron to increase
its size and to remain molten as described above. Accordingly, the
production of the metallic iron may be stopped once.
[0043] With the above-mentioned renewing method, since the lower
end of the blade of the discharging device is positioned on the
surface of the renewable hearth deteriorated area 9a, most of the
hearth material other than a part thereof, which is pressed into
the renewable hearth deteriorated area, is discharged out of the
furnace by the discharging device. The amount of the hearth
material consumed is therefore increased.
[0044] FIG. 8 shows another example of the renewing method. After
discharging the metallic iron Fe and the slag Sg out of the furnace
by the discharging device 6, the hearth material is charged so as
to lie in the form of a layer on the surface of the renewable
hearth deteriorated area 9a prior to supply of the raw-material
agglomerates G so that the stable operation of producing the
metallic iron can be continued. At that time, the lower end of the
blade of the discharging device is positioned at a level slightly
away above the surface of the renewable hearth deteriorated area
9a. Therefore, a part of the metallic iron Fe and the slag Sg
residing on the surface layer of the renewable hearth deteriorated
area cannot be removed, but a new layer of renewable hearth is
formed on the renewable hearth deteriorated area with the newly
charged hearth material, thereby restoring the function of the
renewable hearth. The charging of the hearth material is not
necessarily continued at all times, but may be stopped when the new
layer of renewable hearth is formed.
[0045] The thickness of the new layer of renewable hearth laid on
the renewable hearth deteriorated area is not limited to a
particular one, but it is preferably not less than 2 mm to prevent
the new layer of renewable hearth from being adversely affected by
the renewable hearth deteriorated area. Then, the similar renewing
step may be repeated in a stage in which deterioration of the
renewable hearth has progressed again with further continuation of
the operation.
[0046] With the above-mentioned renewing method, the amount of the
hearth material consumed can be reduced by stopping the charging of
the hearth material at the time when the new layer of renewable
hearth is formed. Additionally, as with the case of FIG. 7, the
production of the metallic iron may be stopped once to carry out
the step of renewing the renewable hearth.
[0047] As another renewing method, the hearth material may be
filled in a dent formed in the surface of the hearth layer during
the operation of the reduction melting furnace.
[0048] FIG. 9 shows still another example of the renewing method.
After discharging the metallic iron Fe and the slag Sg out of the
furnace by the discharging device 6, a part or the whole of the
renewable hearth deteriorated area 9a is removed prior to supply of
the raw-material agglomerates G for causing a new surface of the
renewable hearth with less deterioration or free from deterioration
to appear, so that the function of the renewable hearth can be
restored and the stable operation of producing the metallic iron
can be continued. The method of removing the renewable hearth
deteriorated area is not limited to a particular one, and any
suitable removing means (not shown) can be used. When using the
discharging device 6 to remove the renewable hearth deteriorated
area, it is possible to discharge the metallic iron Fe and the slag
Sg out of the furnace, and simultaneously to remove the renewable
hearth deteriorated area. Then, the similar renewing step may be
repeated in a stage in which deterioration of the renewable hearth
has progressed again with further continuation of the operation.
Further, in a stage in which the thickness of the renewable hearth
has reached a minimum limit, the hearth material may be newly
charged for restoring the renewable hearth to have the
predetermined thickness. Though not shown, whenever the renewable
hearth deteriorated area is removed, the hearth material may be
newly charged for restoring the renewable hearth to have the
predetermined thickness.
[0049] With the above-mentioned renewing method, the amount of the
hearth material consumed is reduced as with the case of FIG. 8. In
some cases, however, a difficulty arises in removing the renewable
hearth deteriorated area into a flat and uniform state and an
addition of the hearth material is required depending on the nature
or the degree of deterioration of the renewable hearth, such as
experienced when the renewable hearth is unevenly deteriorated or
when deterioration penetrates a deep portion of the renewable
hearth in some places. Additionally, as with the case of FIG. 7,
the production of the metallic iron may be stopped once to carry
out the step of renewing the renewable hearth.
[0050] FIG. 10 shows still another example of the renewing method.
After discharging the metallic iron Fe and the slag Sg out of the
furnace by the discharging device 6, a part or the whole of the
renewable hearth deteriorated area 9a is removed prior to supply of
the raw-material agglomerates G, and the hearth material is charged
to lie in the form of a layer on an exposed surface of the
renewable hearth after the removal, so that the function of the
renewable hearth can be restored and the stable operation of
producing the metallic iron can be continued. As with the case of
FIG. 9, the method of removing the renewable hearth deteriorated
area is not limited to a particular one. Also, as with the case of
FIG. 8, the thickness of a new layer of renewable hearth laid on
the surface exposed after the removal is not limited to a
particular one, but it is preferably not less than 2 mm to prevent
the new layer of renewable hearth from being adversely affected by
the remaining renewable hearth deteriorated area. Then, the similar
renewing step may be repeated in a stage in which deterioration of
the renewable hearth has progressed again with further continuation
of the operation.
[0051] With the above-mentioned renewing method, a difficulty
arises sometimes in removing the renewable hearth deteriorated area
into a flat and uniform state as with the case of FIG. 9, but there
occurs no problem because a renewable hearth is newly formed on the
exposed surface of the underlying renewable hearth.
[0052] In addition, as with the case of FIG. 7, the production of
the metallic iron may be stopped once to carry out the step of
renewing the renewable hearth. Further, in the cases of FIGS. 9 and
10, when removing a part or the whole of the renewable hearth
deteriorated area, a portion of the renewable hearth which is not
deteriorated may also be removed together.
[0053] The operation of removing the metallic iron and the slag
residing on the surface layer of the renewable hearth or pressing a
new hearth material into the surface layer of the renewable hearth
deteriorated area, or the means for removing the renewable hearth
deteriorated area can be performed by using not only a discharging
device such as of the scraper or screw type, but also any other
suitable removing means such as a milling machine.
[0054] Further, the means for adjusting the thickness of the
renewable hearth is not limited to a particular one, but may be a
discharging device used for removing the metallic iron and the slag
residing on the surface layer of the renewable hearth, or a
removing device for removing the deteriorated renewable hearth, or
a leveling device. Anyway, the thickness of the renewable hearth
can be adjusted by regulating the spacing between the lower end
(e.g., the blade end position) of such a device, which is installed
in the furnace, and the renewable hearth.
[0055] Moreover, the method of ascending and descending the
discharging device or the removing means is not limited to a
particular one, but may be implemented using a jack, a hydraulic or
pneumatic cylinder, etc.
[0056] While several examples of the method of renewing the
renewable hearth have been described above, any suitable renewing
method other than the above-illustrated ones may also be employed,
or those methods may be combined with each other.
[0057] Because the renewable hearth is exposed to high temperatures
in the furnace and is subjected to infiltration and erosion of the
molten slag as described above, the hearth material is preferably a
substance having a high melting point and being corrosion-resistant
against the molten slag. Such a hearth material contains, e.g.,
oxides including alumina and/or magnesia, or silicon carbide. Any
other suitable substance may also be used so long as it has the
above-mentioned property. In the present invention, the hearth
material may be one or two or more kinds of materials in proper
combination; namely, there is no particular limitation on the
number of kinds of hearth materials used. Also, by using the
above-mentioned hearth material to form the renewable hearth,
deterioration of the renewable hearth due to erosion by the molten
slag can be delayed. As a result, it is possible to increase the
availability factor of the plant and to reduce the amount of the
hearth material used.
[0058] Further, when the hearth material contains a carbonous
substance (when the hearth material is a mixture of a
corrosion-resistant material having a high melting point and a
carbonous substance), the renewable hearth has a porous structure
as a result of burning of the carbonous substance in the furnace,
whereby metamorphic expansion attributable to infiltration of the
molten slag can be suppressed and the surface of the renewable
hearth can be maintained in a flat and uniform state for a longer
time. The porous structure of the renewable hearth is also
preferable in points of facilitating removal of the renewable
hearth deteriorated area when the renewable hearth is renewed, and
reducing a wear of the blade end of the means for removing the
renewable hearth deteriorated area, e.g., the discharging
device.
[0059] A mixing ratio of the material having a high melting point
to the carbonous substance is not limited to a particular value,
but it is recommended to fall in the range of preferably from 20:80
to 80:20 and more preferably from 70:30 to 30:70. If the amount of
the carbonous substance is too small, the number of pores in the
renewable hearth would be reduced, thus resulting in a reduction of
the effect of suppressing metamorphic expansion attributable to
infiltration of the molten slag and a difficulty in removing the
renewable hearth deteriorated area. Conversely, if the amount of
the carbonous substance is too large, the renewable hearth could
not have a predetermined level of strength and continuous supply of
the hearth material would be required because the carbonous
substance is burnt and worn in the furnace, thus resulting in an
undesired result of increased cost. Using coal as the carbonous
substance is more preferable in that ash in the coal additionally
develops the effect as a binder for binding the high-melting-point
material together and hence renders the renewable hearth to have an
appropriate strength endurable to the operation of charging the
raw-material agglomerates or the operation of discharging the
metallic iron as a product and the slag Sg. When using coal with
main intent to develop the binding effect of ash contained in the
coal, the mixing ratio of the high-melting-point material to the
carbonous substance may be selected so as to develop the desired
binding effect without being limited to the above-mentioned mixing
rate of the carbonous substance.
[0060] In the present invention, the hearth material may contain a
sintering accelerator. Mixing a sintering accelerator in the hearth
material is preferable in that the sintering accelerator develops
the effect as a binder for binding the high-melting-point material
together and hence renders the renewable hearth to have an
appropriate strength endurable to the operation of charging the
raw-material agglomerates or the operation of discharging the
metallic iron as a product and the slag. The sintering accelerator
is, e.g., a silica compound such as kaolin. However, any other
suitable substance may also be used so long as it develops the
effect as a binder.
[0061] The mixing rate of the sintering accelerator is not limited
to a particular value so long as the binding effect can be
developed, and it is usually in the range of about 3 to 15%.
Because a silica compound or the like as an example of the
sintering accelerator has low corrosion resistance against the
molten slag, it is not preferred to mixed a large amount of the
sintering accelerator in the hearth material.
[0062] The grain size of the high-melting-point material, the
carbonous substance and the sintering accelerator, which are
contained in the hearth material, is not limited to a particular
value, but it is recommended to be preferably not larger than 4 mm
in average, more preferably not larger than 2 mm in average, from
the viewpoints of suppressing infiltration of the molten slag and
taking a proper balance between the strength endurable to the
operation of charging the raw-material agglomerates or the
operation of discharging the metallic iron as a product and the
slag and easiness in removing the renewable hearth deteriorated
area.
[0063] As shown in FIG. 11, an atmosphere modifier containing a
powdery carbonous substance may be laid in the form of a layer on
the renewable hearth 9 prior to supply of the raw-material
agglomerates, and the raw-material agglomerates G may be then
supplied onto that layer. Forming a layer of an atmosphere modifier
10 is effective to suppress an oxidizing burner combustion gas,
which contains CO.sub.2 and H.sub.2O, from impeding a reducing
atmosphere in the vicinity of the raw-material agglomerates G and
to efficiently promote reduction, carburizing and melting of the
raw-material agglomerates G. Another effect is in that the amount
of FeO remaining in the molten slag is reduced, and hence slag
infiltration and erosion into the renewable hearth can be
suppressed. In addition, since the atmosphere modifier enhances the
reducing atmosphere in the vicinity of the raw-material
agglomerates G and thereafter serves as fuel when burnt in the
furnace, it is possible to reduce the amount of burner fuel
consumed, such as natural gas. Furthermore, the atmosphere modifier
serves to suppress the molten slag from infiltrating into the
renewable hearth, to facilitate removal of the metallic iron Fe and
the slag Sg from the renewable hearth, and to achieve smoother
discharge out of the furnace.
[0064] Examples of the atmosphere modifier include coal powder,
petrocoke powder, and coke breeze. The thickness of the atmosphere
modifier is not limited to a particular value, and a very thin
layer of the atmosphere modifier is enough to effectively develop
the effects of enhancing the reducing atmosphere in the vicinity of
the raw-material agglomerates and smoothing discharge of the
metallic iron and the slag. Usually, the intended purposes can be
obtained even with a thickness of about 1 to 10 mm. Additionally,
it is desired that the atmosphere modifier be continuously supplied
because it is burnt and worn in the furnace.
[0065] The grain size of the atmosphere modifier is not limited to
a particular value, but the grain size is recommended to be
preferably not larger than 5 mm in average, more preferably not
larger than 2 mm in average,
[0066] The method of charging the hearth material is not limited to
a particular one, but it is recommended that the hearth material be
charged to lie on the hearth refractory at a uniform thickness
using the powder supply apparatus 5 while the heart is rotated.
[0067] Also, mixing an appropriate amount of the hearth material in
the atmosphere modifier is recommended as a simple manner to
develop the effect of restoring the function of the renewable
hearth deteriorated area. The hearth material mixed in the
atmosphere modifier is moved up to the discharging device 6 with
the rotation of the hearth, and is pressed into the surface layer
of the renewable hearth deteriorated area under the action of the
discharging device, whereby the function of the renewable hearth is
restored. A mixing ratio of the hearth material to the atmosphere
modifier is not limited to a particular value, but the mixing ratio
is usually preferably in the range of 30 to 70%. If the mixing rate
of the hearth material is too small, the effect of restoring the
renewable hearth deteriorated area would be reduced. Conversely, if
the mixing rate of the hearth material is too large, the effect of
adjusting the atmosphere would be reduced. Mixing the hearth
material in the atmosphere modifier is not always required, and the
mixing may be performed only when restoring the function of the
renewable hearth deteriorated area. Further, the method of mixing
the hearth material in the atmosphere modifier is preferable in
that the plant cost and the installation space can be reduced
because one supply apparatus can be shared for supplying the hearth
material and the atmosphere modifier.
[0068] FIG. 12 shows another process of the operation using an
atmosphere modifier. An atmosphere modifier containing a powdery
carbonous substance is laid in the form of two layers on the
renewable hearth 9 prior to supply of the raw-material
agglomerates, and the raw-material agglomerates G are then supplied
onto those layers. In the process of the operation using no
atmosphere modifier as shown in FIG. 7 or in the process of the
operation in which the atmosphere modifier is laid in the form of
one layer as shown in FIG. 11, the lower end of the blade of the
discharging device 6 is always contacted with the surface of the
renewable hearth, which is formed of the hearth material containing
the high-melting-point material with high abrasiveness, such as
alumina and magnesia, and therefore the blade end is remarkably
worn. In the case of forming two layers of the atmosphere modifier,
however, the blade end of the discharging device is positioned on
an upper surface of the atmosphere modifier layer on the lower side
and is kept from coming into direct contact with the renewable
hearth having high abrasiveness. As a result, the life of the blade
can be prolonged and the availability factor of the plant can be
increased. Herein, the expression "forming two layers" means steps
of forming a first layer of the atmosphere modifier, and leveling
the surface of the first layer and then forming another (second)
layer of the atmosphere modifier for convenience of the operation.
By forming those two layers of the atmosphere modifier, the blade
end of the discharging device can be held in contacted with the
first layer of the atmosphere modifier layer and can be prevented
from coming into direct contact with the renewable hearth.
Accordingly, even when the atmosphere modifier is laid in the form
of one layer, for example, the same purpose can be achieved by
forming the atmosphere modifier as a thick layer to such an extent
that the blade end can be held at a position not contacting with
the renewable hearth, without forming the second layer. Also, in
the case of forming two layers of the atmosphere modifier, it is a
matter of choice in practice whether the composition of the
atmosphere modifier is the same or not between the first and second
layers.
[0069] Though not shown, the method of renewing the renewable
hearth based on the process of the operation using an atmosphere
modifier can be practiced similarly to the above-described method
of renewing the renewable hearth based on the process of the
operation using no atmosphere modifier.
[0070] As described above, when the renewable hearth is badly
deteriorated, it sometimes happens that enlarged metallic iron
cannot be sufficiently cooled and solidified in the cooling zone
and reaches the discharge section while being in a molten iron
state, whereby the operation can no longer be continued because of
a difficulty in discharging such enlarged metallic iron out of the
furnace by the discharging device. In that event, by supplying a
coolant to the surface of the renewable hearth to solidify the
molten iron, it is possible to discharge the molten iron and to
continue the operation. In the present invention, the coolant is
not limited to a liquid or gas, but may be a high-melting-point
material, such as alumina and magnesia. The molten iron may be
cooled and solidified, for example, by supplying the hearth
material containing the high-melting-point material, such as
alumina and magnesia, to an area of the molten iron. Alternatively,
the molten iron may be cooled and solidified by providing a water
spray device and supplying water to an area of the molten iron.
[0071] In the above-described step of removing the renewable hearth
deteriorated area, the removal is not easy to perform depending on
the nature of the renewable hearth, but the renewable hearth can be
smoothly removed by softening it in such a case. The method of
softening the renewable hearth is not limited to a particular one,
and the renewable hearth can be softened by increasing the amount
of burner combustion to raise the temperature in the furnace and
then the temperature of the renewable hearth, or by providing a
burner dedicated for directly heating the renewable hearth to raise
the temperature of the renewable hearth. The temperature of the
renewable hearth in that case is not limited to a particular value,
but may be set as required depending on the nature of the renewable
hearth. In the renewable hearth deteriorated area where
infiltration of the molten slag has progressed, however, the
temperature of the renewable hearth is preferably in the range of
1300 to 1550.degree. C., more preferably in the range of about 1450
to 1550.degree. C.
[0072] As another method, the renewable hearth may be softened by
supplying to it, e.g., an additive that has the effect of lowering
the melting point of the renewable hearth. Examples of such an
additive include calcium oxide, sodium carbonate, and calcium
fluoride.
[0073] From the viewpoint of facilitating removal of the renewable
hearth deteriorated area, as shown in FIG. 13, a carbonous material
layer 10a may be formed by laying a carbonous material, e.g., a
powdery carbon substance, in the form of a layer between the hearth
refractory 8 and the renewable hearth 9 or between the renewable
hearth 9 and a renewable hearth laid on the renewable hearth 9.
Anyway, the renewable hearth deteriorated area is removed by
descending the blade end of the discharging device to any desired
position in the carbonous material layer. Since the carbonous
material layer usually provides a powdery brittle layer, the
renewable hearth can be easily removed upon separation at the
carbonous material layer.
[0074] While the raw-material agglomerates in the form of pellets
are used as the raw-material mixture in the above description, the
advantages of the present invention can also provided when a
powdery material is used as the raw-material mixture.
[0075] The present invention will be described below in detail in
connection with Example. It is, however, to be noted that the
following Example is not purported to limit the present invention,
and various modifications made without departing from the purports
of the present invention mentioned above and below are all involved
in the technical scope of the present invention.
EXAMPLES
Example 1
[0076] Agglomerates (diameter: about 16 mm) containing iron ore and
coal were charged into the reduction melting furnace of the moving
hearth type shown in FIG. 1, and then subjected to heating
reduction in a solid state until a metallization rate of not less
than about 90% was obtained, while the atmosphere temperature in
the furnace was controlled to about 1350.degree. C. The
raw-material agglomerates were then melted in the melting zone
(atmosphere temperature: 1450.degree. C.). Metallic iron thus
produced and slag as a by-product were cooled down to about
1000.degree. C. for solidification, and then discharged out of the
furnace by the discharging device (time from the charging of the
raw materials to the discharge was about 12 minutes). Thus-obtained
granular metallic iron (diameter: about 10 mm) had a high iron
grade (iron: about 97% and carbon: about 3%).
[0077] Prior to the charging of the agglomerates, a hearth material
was laid on the hearth in the form of a layer at a thickness of 15
mm through an auxiliary raw-material charging means (not shown),
thereby forming a renewable hearth. Also, an atmosphere modifier
(first layer, material: coal) was laid (thickness: 2 mm) on the
renewable hearth and leveled by the discharging device. Further, an
atmosphere modifier was laid (thickness: 3 mm) on the first layer
of atmosphere modifier. Thereafter, the operation was started by
supplying the agglomerates onto the two layers of atmosphere
modifier. After the cooling and solidifying steps, the metallic
iron, etc. were recovered by the discharging device provided at the
most downstream side. On that occasion, the blade end (lower end)
of the discharging device was positioned on the surface of the
first layer of the atmosphere modifier layer, and the atmosphere
modifier remaining in the second layer was discharged together with
the metallic iron. The atmosphere modifier forming the second layer
was always supplied prior to the supply of the raw materials.
Further, the blade end of the discharging device was descended to
the surface of the renewable hearth once per day for discharging
the first layer of the atmosphere modifier and removing the
metallic iron and the slag residing on the surface of the renewable
hearth. Then, the hearth material was added to restore the function
of the renewable hearth deteriorated area, and two layers of the
atmosphere modifier (which were the same as the above-mentioned
first and second layers of the atmosphere modifier at the start of
the operation) were formed again. The operation was continued in a
similar way by repeating the above steps. After two weeks from the
start of the operation, the blade end of the discharging device was
descended 5 mm from the surface of the renewable hearth to remove
the deteriorated area in the surface layer of the renewable hearth.
Then, after forming the renewable hearth, the first layer of the
atmosphere modifier, and the second layer of the atmosphere
modifier again (which were the same as those formed at the start of
the operation), the operation was continued in a similar way by
repeating the above steps. In this Example, the operation was
continued for three weeks. As a result, the stable continuous
operation was realized and a high availability factor (91%) was
obtained.
Comparative Example
[0078] Metallic iron was produced in the same manner as in above
Example except that the renewable hearth and the atmosphere
modifier layers both formed at the start of the operation were
neither removed nor renewed. After two days from the start of the
operation, the surface of a part of the hearth layer was softened,
and a pool of residing molten iron was formed. Thus, the hearth had
to be repaired under shutdown of the operation, and the stable
continuous operation was not obtained.
INDUSTRIAL APPLICABILITY
[0079] According to the present invention constructed as described
above, the availability factor of a hearth can be drastically
increased and the long-term stable operation of producing metallic
iron can be achieved by charging a hearth material to lie in the
form of a layer on a hearth refractory prior to supply of a
raw-material mixture, thereby forming a renewable hearth capable of
being renewed, and by removing the whole or a part of the renewable
hearth, which has deteriorated with continued operation upon
infiltration of molten slag, burying of metallic iron and slag into
the renewable hearth, or formation of a dent due to peeling and
hollowing, or by charging a new hearth material to restore the
function of the renewable hearth.
* * * * *