U.S. patent application number 10/537861 was filed with the patent office on 2006-02-02 for process for producing molten iron.
This patent application is currently assigned to Kabsuhiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Shuzo Ito, Itsuo Miyahara, Koji Tokuda.
Application Number | 20060021469 10/537861 |
Document ID | / |
Family ID | 32463302 |
Filed Date | 2006-02-02 |
United States Patent
Application |
20060021469 |
Kind Code |
A1 |
Ito; Shuzo ; et al. |
February 2, 2006 |
Process for producing molten iron
Abstract
A high purity of molten iron is produced efficiently at a higher
productivity, by feeding a raw material mixture containing a
carbonaceous reducing agent, an iron oxide-containing material and
a CaO-containing material onto a hearth of a moving-hearth reducing
furnace, heat-reducing the raw material mixture in the reducing
furnace, and melting it in a melting furnace melting, wherein a
blending amount of the CaO-containing material in the raw material
mixture is adjusted in such a manner that another feeding of the
CaO-containing material into the melting furnace makes a basicity
of a slag generated in the melting furnace 1.1 or more an feeding
amount of the CaO-containing material becomes 40 kg or less per ton
of the molten iron obtained in the melting furnace.
Inventors: |
Ito; Shuzo; (Hyogo, JP)
; Miyahara; Itsuo; (Hyogo, JP) ; Tokuda; Koji;
(Hyogo, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Kabsuhiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
10-26, Wakinohama-cho
Hyogo
JP
651-8585
|
Family ID: |
32463302 |
Appl. No.: |
10/537861 |
Filed: |
December 4, 2003 |
PCT Filed: |
December 4, 2003 |
PCT NO: |
PCT/JP03/15509 |
371 Date: |
June 6, 2005 |
Current U.S.
Class: |
75/484 |
Current CPC
Class: |
C21B 13/008 20130101;
C21B 13/105 20130101; Y02P 10/136 20151101; Y02P 10/134
20151101 |
Class at
Publication: |
075/484 |
International
Class: |
C21B 13/00 20060101
C21B013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 5, 2002 |
JP |
2002-353525 |
Claims
1. A method of producing a molten iron having a carbon content of
3.0 mass % or more, comprising: feeding a raw material mixture
containing a carbonaceous reducing agent, an iron oxide-containing
material and a CaO-containing material onto a hearth of a
moving-hearth reducing furnace; heating the raw material mixture in
the reducing furnace and thus reducing an iron oxide in the raw
material mixture; generating a solid reduced iron having a
metallization ratio of 80% or more; feeding the solid reduced iron
in the state kept at high temperature into a melting furnace and
further heating the solid reduced iron therein; and reducing an
iron oxide partially remained in the solid reduced iron and melting
the solid reduced iron; wherein a blending amount of the
CaO-containing material in the raw material mixture is adjusted in
such a manner that another feeding of the CaO-containing material
into the melting furnace makes a basicity of a slag generated in
the melting furnace 1.1 or more and that a feeding amount of the
CaO-containing material becomes 40 kg or less per ton of the molten
iron obtained in the melting furnace.
2. The method of producing a molten iron according to claim 1,
further comprising: feeding a powdery carbonaceous material onto
the hearth of the moving-hearth reducing furnace as a hearth
material; and feeding the raw material mixture containing a
carbonaceous reducing agent, an iron oxide-containing material and
a CaO-containing material onto the hearth material.
3. The method of producing a molten iron according to claim 2,
wherein an amount of the carbonaceous material fed as the hearth
material onto the hearth of the moving-hearth reducing furnace is
30 kg or more per ton of the molten iron obtained in the melting
furnace.
4. The method of producing a molten iron according to claim 2,
wherein the feeding amount of the carbonaceous material for the
hearth material is adjusted in such a manner that an amount of the
CaO-containing material blended in the raw material mixture is
sufficient for making the basicity of the slag generated in the
melting furnace 1.1 or more and that an amount of the carbonaceous
material of a non-combustion state fed into the melting furnace
together with the solid reduced iron discharged from the reducing
furnace in a non-combustion state becomes not less than an amount
of the carbonaceous material to be consumed in the melting
furnace.
5. The method of producing a molten iron according to claim 3,
wherein the feeding amount of the carbonaceous material for the
hearth material is adjusted in such a manner that an amount of the
CaO-containing material blended in the raw material mixture is
sufficient for making the basicity of the slag generated in the
melting furnace 1.1 or more and that an amount of the carbonaceous
material of a non-combustion state fed into the melting furnace
together with the solid reduced iron discharged from the reducing
furnace in a non-combustion state becomes not less than an amount
of the carbonaceous material to be consumed in the melting
furnace.
6. The method of producing a molten iron according to claim 1,
wherein a MgO content in the raw material mixture and an amount of
the MgO to be added into the melting furnace are adjusted in such a
manner that the slag generated in the melting furnace has a MgO
content of 6 mass % or more.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method of producing a
molten iron, more specifically, a method of producing a molten iron
by heat-reducing an iron oxide-containing material such as iron ore
together with a carbonaceous reducing agent such as carbonaceous
material in a moving-hearth reducing furnace and then melting a
reduced iron obtained in a melting furnace in particular, which is
improved to produce a high purity of molten iron efficiently at a
higher productivity.
DESCRIPTION OF THE RELATED ART
[0002] As one of the recently developed direct iron making methods,
known is a method of producing a molten iron by beat-reducing in a
rotary type moving-hearth reducing furnace a mixture containing an
iron oxide-containing material (iron source) such as iron ore and a
carbonaceous reducing agent such as carbonaceous material, or a
compact of the mixture containing carbonaceous material in a shape
of pellet, briquette or the like, and then finally reducing and
melting the reduced iron obtained in a melting furnace. This method
is effective to some extent, because heat of high-temperature
exhaust gas generated in the melting furnace can be used
efficiently by introducing the heated gas into the moving-hearth
reducing furnace, and this enhances a thermal efficiency of the
facilities as a whole.
[0003] The direct iron making method using a moving-hearth reducing
furnace connected to a melting furnace as described above has been
studied for a fairly long period, but there are still many problems
to be solved for operation in the industrial scale, for example, in
operational stability, safety, cost, product quality (purity of
molten iron), etc.
[0004] The inventors have been engaged in studies on practical
application of this kind of direct iron making method, and this
time especially studied a method from various aspects aiming at
improvement of smooth operations in a series of processes from
solid-phase reduction of iron sources by using a moving-hearth
reducing furnace such as a rotary, hearth furnace to melting in a
melting furnace, as well as improvement of the quality of molten
iron obtained, further suppression of degradation of processing
facilities, in particular refractory, and extension of lifetime of
the facilities.
[0005] Accordingly, it is an object of the present invention to
provide a method that allows a smoother operation in a series of
processes from reduction in a moving-hearth reducing furnace to
melting in a melting furnace, as well as higher quality of a molten
iron obtained, for further suppression of degradation of processing
facilities, in particular refractory, and extension of lifetime of
the facilities.
SUMMARY OF THE INVENTION
[0006] According to an aspect of the present invention which solved
the above problems, a method of producing a molten iron having a
carbon content of 3.0 mass % or more, comprises: feeding a raw
material mixture containing a carbonaceous reducing agent, an iron
oxide-containing material and a CaO-containing material onto a
hearth of a moving-hearth reducing furnace; heating the raw
material mixture in the reducing furnace and thus reducing an iron
oxide in the raw material mixture; generating a solid reduced iron
having a metallization ratio of 80% or more; feeding the solid
reduced iron in the state kept at high temperature into a melting
furnace and further heating the solid reduced iron therein; and
reducing the partially remaining iron oxide and melting the solid
reduced iron; wherein a blending amount of the CaO-containing
material in the raw material mixture is adjusted in such a manner
that another feeding of the CaO-containing material into the
melting furnace makes a basicity of a slag generated in the melting
furnace 1.1 or more and that an feeding amount of the
CaO-containing material becomes 40 kg or less per ton of the molten
iron obtained in the melting furnace.
[0007] In practicing the above method, it is preferable to further
comprise: feeding a powdery carbonaceous material onto the hearth
of the moving-hearth reducing furnace as a hearth material; and
then feeding the raw material mixture containing a carbonaceous
reducing agent, an iron oxide-containing material and a
CaO-containing material onto the hearth material. At the time, it
is preferable to make an amount of the carbonaceous material fed as
the hearth material, 30 kg or more per ton of the molten iron
obtained in the melting furnace, for effectively increasing the
metallization ratio of the solid reduced iron obtained in the
reducing furnace.
[0008] It is also preferable to blend the CaO-containing material,
whose entire amount is sufficient for making the basicity of the
slag generated in the melting furnace 1.1 or more, with the raw
material mixture and thus eliminate an amount of the CaO-containing
material separately added into the melting furnace in result, for a
smoother melting operation in the melting furnace; and it is
further preferable to adjust the feeding amount of the carbonaceous
material for hearth material in such a manner that an amount of the
carbonaceous material of a non-combustion state fed into the
melting furnace together with the solid reduced iron discharged
from the reducing furnace becomes not less than an amount of the
carbon aqueous material to be consumed in the melting furnace, for
further reliable improvement in the metallization ratio of the
solid reduced iron obtained in the reducing furnace, and for a more
efficient and faster melting operation in the melting furnace.
[0009] It is also preferable to adjust a MgO content in the raw
material mixture or to add MgO additionally into the melting
furnace in such a manner that the slag generated in the melting
furnace has a MgO content of 6 mass % or more, preferably 8 to 13
mass %, for effective suppression of refractory wear in using
magnesia bricks as an inner lining refractory for the melting
furnace. In this case, it is preferable to adjust a blending amount
of CaO-- and MgO-containing materials in the raw material mixture
in such a manner that an amount of the slag-conditioning agents
including CaO and MgO fed additionally into the melting furnace
becomes 40 kg or less per ton of the molten iron obtained in the
melting furnace.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a process flow chart showing one embodiment of the
present invention.
[0011] FIG. 2 is a graph showing a relationship between a
metallization ratio of a solid reduced iron obtained in a
rotary-hearth reducing furnace and a residual carbon amount in raw
materials in practicing the method according to the present
invention and a conventional method.
[0012] (A): rotary-hearth reducing furnace, (B): electric melting
furnace, (C): iron-bath melting furnace, (1): carbonaceous material
(hearth material), (2): raw material mixture (pellet), (3): raw
material-feeding hopper, (4): rotary hearth, (5): burner, (6):
discharge device, (7): exhaust gas duct, (8): feed chute, (9):
electrode, (10): molten slag outlet, (11): molten iron outlet,
(12): exhaust gas duct, (13): top blowing lance, (14):
bottom-blowing nozzle, (15): side-blowing nozzle, Fe: molten iron,
S: molten slag.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] As described above, a molten iron is produced continuously
by solid-phase reduction of an iron oxide-containing material and
melting the reduced material thereof in a series of facilities
comprising a moving-hearth reducing furnace and a melting furnace
installed next thereto in the present invention. Although it is
possible to use a grate-type reducing furnace as a moving-hearth
reducing furnace in the present invention, a rotary-hearth reducing
furnace is most efficient and practical, considering an area for
installation and an operating efficiency of the facility; and thus,
embodiments employing a rotary-hearth reducing furnace will be
mainly described below. However, the present invention is not
limited thereto, and use of any other reducing furnace is possible,
if it is a reducing furnace wherein heat-reduction is carried out
on its moving hearth.
[0014] FIG. 1 is a schematic flow chart illustrating a series of
facilities used in practicing the present invention, wherein (A)
represents a rotary-hearth reducing furnace, (B) an electric
melting furnace and (C) an iron-bath melting furnace; and a
production method according to the present invention is carried out
by using facilities in combination of a rotary-hearth reducing
furnace (A) and an electric melting furnace (B), or in combination
of a rotary-hearth reducing furnace (A) and an iron-bath melting
furnace (C).
[0015] A powdery carbonaceous material (1) to be supplied as a
hearth material and a raw material mixture (2) which contains an
iron oxide-containing material and a carbonaceous reducing agent,
further which is added with CaO, MgO, etc. or as needed with a
small amount of a binder, (or, a compact or semi-compact formed of
these in a shape of pellet and the like as the raw material
mixture) are fed preferably continuously onto a rotary hearth (4)
of the rotary-hearth reducing furnace (A) via a raw
material-feeding hopper (3). More specifically, by installing at
least two said raw material-feeding hoppers, prior to feeding the
raw material mixture (2), the powdery carbonaceous material (1) is
fed onto the rotary hearth (4) from one hopper, and then the raw
material mixture (2) is fed from another hopper adjacent to the
one. In some cases, the powdery carbonaceous material (1) to be fed
as the hearth material may be eliminated.
[0016] The rotary hearth (4) of the rotary-hearth reducing furnace
(A) shown in the figure rotates counterclockwise, normally in a
rotation period of approximately 8 minutes to 16 minutes, although
the frequency varies according to operational conditions. A
plurality of combustion burners (5) are placed in the sidewall
above the rotary hearth (4) and/or in the ceiling in the reducing
furnace (A), and heat is supplied to the hearth area from the
combustion or radiation heat of the burners (5).
[0017] The raw material mixture (2) fed onto the rotary hearth (4)
consisted of refractory is heated by combustion or radiation heat
of the combustion burners. (5) while the mixture moves in a
circumferential direction on the hearth (4) in the reducing furnace
(A), and an iron oxide in the raw material mixture (2) is reduced
to a solid reduced iron while the mixture goes through a heated
zone in the reducing furnace (A).
[0018] When the heat-reduction is carried out by supplying only the
raw material mixture (2) onto the rotary hearth (4), the raw
material mixture (2) is less affected by oxidizing gases present in
the furnace atmosphere in the earlier stage of heat-reducing the
raw material mixture (2), because strongly reducing gases mainly
containing CO gas arc emitted from the carbonaceous reducing agent
blended in the raw material mixture (2) in a great amount to the
area surrounding the raw material mixture (2). However, the amount
of the reducing gases emitted from inside the raw material mixture
(2) decreases in the later stage of reduction, and the raw material
mixture (2) becomes affective to the surrounding oxidizing gases
(generated;by burner combustion, e.g. carbon dioxide, steam, etc.),
occasionally leading to re-oxidation of the reduced iron once
formed by the heat-reduction and consumption of the carbonaceous
reducing agent present inside the raw material mixture (2) by
combustion. As a result, it becomes more difficult to control the
carbonaceous reducing agent blended as a reducing agent in the raw
material mixture (2), sometimes which leads to fluctuation of the
metallization ratio of the solid reduced iron obtained.
[0019] Therefore, it is preferable to lay the powdery carbonaceous
material (1) on the rotary hearth (4) as a hearth material and feed
the raw material mixture (2) thereon, because the oxidizing gases
such as CO.sub.2 generated by burner combustion are reduced again
by the carbonaceous material (1) on the hearth (4), even in the
later stage of heat-reduction, and thus, it is possible to make an
environmental gas surrounding the raw material mixture (2) retain a
high level of reduction potential and to produce a solid reduced
iron having a desirable metallization ratio more consistently and
reliably.
[0020] The solid reduced iron produced in the rotary hearth
reducing furnace (A) as described above is discharged by a
discharge device (6), together with a slag generated in the
heat-reduction process as a by product or the carbonaceous material
(1) used as a hearth: material remaining after combustion. In the
figure, (7) represents an exhaust gas duct.
[0021] In any case, the solid reduced iron generated in the
reducing furnace (A) is discharged in a state kept at high
temperature possibly without cooling by the discharge device (6)
out of the furnace, together with the carbonaceous material (1)
remaining on the hearth (4), and fed into the melting furnace
placed directly connected or close to the rotary-hearth reducing
furnace (A). In the figure, (B) represents an electric melting
furnace and (C) an iron-bath melting furnace, and one of these
melting furnaces is used suitably selected as needed.
[0022] When the electric melting furnace (B) is used, the solid
reduced iron and the residual carbonaceous material at high
temperature discharged from the reducing furnace (A) are fed into
the furnace through a feed chute (8) placed at the top part of the
furnace. An auxiliary material (slag-conditioning agent) containing
CaO Or MgO, if supplied, is fed via a chute identical with or
different from the chute for the solid reduced iron into the
melting furnace (1). In the electric melting furnace (B), heat is
supplied into the furnace from electrodes (9), and unreduced iron
oxide remaining in the solid reduced iron is reduced and the solid
reduced iron is melted by the heat, finally a molten slag (S) and a
molten iron (Fe) are separated and accumulated in the furnace.
Then, these molten iron (Fe) and molten slag (S) are discharged at
a suitable timing respectively via a molten slag outlet (10) and a
molten iron outlet (11) out of the furnace. In the FIG. (12)
represents an exhaust gas duct.
[0023] On the other hand, when the iron-bath melting furnace (C) is
used, the solid reduced iron and the carbonaceous material are fed
into the furnace from the upper part of the melting furnace (C). An
oxygen-containing gas for combustion of the carbonaceous material
in the furnace is generally blown at high speed downward onto the
slag (S) or the molten iron (Fe) by a top-blowing lance (13)
inserted from the top part of the furnace. Alternatively, the
oxygen-containing gas may be blown into the furnace from a
bottom-blowing nozzle (14) placed in the furnace bottom, from a
side-blowing nozzle (15) placed in the furnace wall, or from these
two or more lance and nozzles in a suitable combination.
[0024] In use of this kind of melting furnace, it is necessary to
supply a certain amount of needed heat into the furnace by
combusting a carbonaceous material in the furnace, and thus a
certain amount of fuel is needed; and if no hearth material is fed
or if only using an unburned carbonaceous material (hearth
material) fed from the reducing furnace (A) to the melting furnace
involves scarcity of fuel, the carbonaceous material in short
supply may be added alone or in combination with other auxiliary
materials (CaO, MgO and the like) from the top part of the
furnace.
[0025] In the present invention, in producing a molten iron by
feeding the solid reduced iron obtained in the rotary-hearth
reducing furnace into the melting furnace placed next thereto
continuously or batchwise and then finally reducing and melting the
unreduced iron oxide (FcO) remaining in the solid reduced iron in
the facilities described above, it is quite important to control a
content of a slag former in such a manner that the slag generated
in the melting furnace as a byproduct has a basicity
(CaO/SiO.sub.2) of 1.1 or more, preferably 1.3 or more, for
production of a molten iron having a low-sulfur-content (low
[S]).
[0026] It is for the purpose of lowering an S content in the molten
iron ([S]) by transferring the sulfur content in the molten iron,
[S], toward the slag. If a raw material mixture made of coal, coke
or the like as a carbonaceous reducing agent (generally, a raw
material compact formed of the mixture in a shape of pellet,
aggregate, etc., or a semi-compact, lightly pressed and formed of
the mixture) is used, the solid reduced iron obtained by
heat-reducing the raw material mixture contains a great amount of
sulfur derived from the coal or coke; or if a coal-based powder is
used for a hearth material, the solid reduced iron also contains a
great amount of sulfur; therefore, use of these carbon sources as a
reducing component results in containment of a significant amount
of sulfur in the solid reduced iron.
[0027] When the solid reduced iron is melted in the melting
furnace, the sulfur is distributed between molten iron (Fe) and
molten slag (S) generated in the melting furnace. Results of the
studies by the inventors showed that the partition ratio, {S
content in slag (mass %)/S content in molten iron (mass %):
(S)/[S])}, becomes higher as the slag basicity increases, and thus
it is preferable to feed a CaO source additionally into the melting
furnace in such a manner that the basicity of the slag generated
becomes 1.1 or more, preferably 1.3 or more.
[0028] In addition, the above partition ratio, (S)/[S], is
significantly dependent on an iron oxide (FeO) content in slag and
there is a tendency that the partition ratio decreases drastically
as the iron oxide content increases. Accordingly, even when the
basicity of slag is adjusted, for example, to 1.3 or more, the S
partition ratio toward slag side decreases drastically as the iron
oxide (FeO) content in the slag increases, consequently making it
difficult to lower the content of sulfur in molten iron.
[0029] After studies on the (FeO) content in slag needed for
increasing the above partition ratio, (S)/[S], and lowering the
sulfur content in molten iron, it has been found that the content
may be suppressed about 5 mass % or less and preferably 2 mass % or
less. After further studies on the practical requirement for
reducing the (FeO) content in slag to 5 mass % or less, preferably
2 mass % or less, it has been also confirmed that it is possible to
adjust the (FeO) content in slag to about 2 mass % or less reliably
and consequently raise the S partition ratio toward slag and lower
[S] in the molten iron efficiently by increasing [C] content in the
molten iron to 3.0 mass % or more.
[0030] From the confirmation results above, the basicity of the
slag generated in the melting furnace is adjusted to 1.1 or more,
more preferably 1.3 or more, and the [C] content in the molten iron
obtained to 3.0 mass % or more in the present invention.
[0031] As described above, the amount of CaO component contained as
a gangue component of iron oxide sources (iron ore and others) in
the raw material mixture is not sufficient, and an additional
amount of CaO component to cover shortfall should be added
separately into the melting furnace for adjusting the basicity of
the slag generated in the melting furnace to 1.1 or more. For
promoting the smooth melting of CaO having a high melting point in
the melting furnace, it is necessary to maintain the melting
furnace at a suitable temperature and agitate a byproduct slag
efficiently. However, for example in an electric melting furnace,
it is difficult to maintain a sufficiently high melting rate of CaO
having a high-melting point, which is likely to occur aggregation
and coagulation into a block shape before the slag component fully
melting and consequently to leads to drastic delay of melting as
well as to inhibit a melting of the solid reduced iron.
[0032] MgO-based refractory is commonly used as an inner lining
refractory of the melting furnace, which is exposed to the slag
having a higher basicity; and it is desirable to add an
MgO-containing material separately into the melting furnace such as
dolomite, preferably to a MgO content in the slag generated of 6
mass % or more, preferably 8 to 13 mass %, for increasing the MgO
content relatively in the generated slag during melting of the
solid reduced iron and prevention of the wearing damage of the
MgO-based refractory, but in such a case, supply of MgO, which has
a high-melting point, directly into the melting furnace may cause
similar problems caused by supply of CaO.
[0033] On the other hand, the content or the composition of the
gangues (slag composition) contained in an iron oxide-containing
raw material vary significantly according to grade or properties of
an iron source contained in a raw material mixture (iron ore,
etc.), and the amount of the slag finally generated in the melting
furnace is normally approximately 80 to 300 kg per ton of molten
iron although it varies slightly depending on the desired basicity,
and reaches a level of 400 kg per ton of molten iron when an iron
oxide-containing material lower in iron purity is used as a raw
material. In this manner, the amount of the slag generated in the
melting furnace varies significantly depending on a grade or kind
of iron sources (iron oxide-containing material) used as the raw
material, and thus, the amount of a CaO-containing material (burnt
lime, etc.) to be added into a melting furnace for controlling the
slag basicity, for example, in the range of 20 to 150 kg per ton of
molten iron.
[0034] Thus, for adjusting the basicity of the slag generated in
the melting furnace reliably to 1.1 or more, or for further
preventing the wearing damage of the inner lining refractory, it is
necessary to add solid reduced iron together with a great amount of
slag-conditioning material (CaO, MgO, etc.) into the melting
furnace, making the wearing inhibition described above (delay of
melting of the slag conditioning materials and the solid reduced
iron) a non-negligible issue.
[0035] Focusing on the problems above, the inventors have conducted
studies on the effects of the amount of a slag-conditioning agent
added together with the solid reduced iron into the melting furnace
on the melting inhibition described above and the measures to
overcome the effects, and found the followings: said melting
inhibition becomes more significant, resulting in the significant
adverse effects on the operation of the furnace when the amount of
a slag-conditioning agent added together with the solid reduced
iron into the melting furnace exceeds 40 kg per ton of molten iron;
the amount of a slag-conditioning agent such as CaO (or CaO and
MgO) added into the melting furnace at the time should be reduced
to 40 kg per ton of molten iron or less for prevention thereof; a
slag-conditioning agent component needed in excess should be
blended previously into a raw material mixture to be fed into a
rotary-hearth reducing furnace as raw material; and in this manner,
it is possible to accelerate the melting of the solid reduced iron
and the slag-conditioning agent in the melting furnace.
[0036] Accordingly, when a slag composition contained in a iron
oxide source or others in a raw material mixture have a low
basicity and a CaO-containing material to compensate the shortage
should be added additionally in a melting furnace, the present
invention employs the method of alleviating the abovementioned
melting inhibition drastically: by mixing all or most part of the
additional one to the raw material mixture in advance; then by
sintering or partially melting these slag compositions in the raw
material mixture by using the heat for heat-reduction of the iron
oxide in the rotary-hearth reducing furnace (A) and changing it
into a low melting-point complex oxide; and thus by increasing the
melting rate (slagging rate) of the slag conditioning materials fed
together with the solid reduced iron into the melting furnace and
the slag-generating rate drastically.
[0037] More specifically, when the amount of the slag-conditioning
agent (CaO-- or MgO-containing material) to be added into the
melting furnace is more than 40 kg per ton of molten iron, the
method accelerates the melting of the slag in the melting furnace
and also the melting of the solid reduced iron: by mixing the
excess amount thereof with the raw material mixture to be fed into
the rotary-hearth reducing furnace (A) in advance; then by
sintering or partially melting the slag compositions in the raw
material mixture (containing the added CaO-- or MgO-containing
material) at the same time during heat-reduction of the iron oxide
in the raw material mixture; and thus by changing it into complex
oxides such as CaO--SiO.sub.2, 2CaO--SiO.sub.2, 3CaO-2SiO.sub.2,
2CaO--Al.sub.2O.sub.3--SiO.sub.2, 3CaO-2SiO.sub.2,
CaO--MgO--SiO.sub.2, 3CaO--MgO-2SiO.sub.2, and the like.
[0038] Although the melting point of burnt lime (CaO) per se is
2,572.degree. C., when the CaO-containing material is mixed in
advance into the raw material mixture and heat-reduced at a
temperature in the range of 1,250 to 1,400.degree. C., these form
the complex oxides as described above by sintering or partially
melting together with the other slag components in the raw material
mixture. These complex oxides have melting points significantly
lower than that of CaO itself, and melt rapidly when fed
subsequently into a melting furnace. Among general complex oxides,
the melting point of CaO--SiO.sub.2 is 1,544.degree. C.; that of
CaO--MgO--SiO.sub.2, 1,610.degree. C.; that of
3CaO--MgO-2SiO.sub.2, 1,575.degree. C.; that of 2CaO--SiO.sub.2,
2,130.degree. C.; that of CaO--Al.sub.2O.sub.3-2SiO.sub.2,
1,550.degree. C.; and that of Al.sub.2O.sub.3--SiO.sub.2,
1,810.degree. C., although these melting points may vary according
to the raw material and the blending ratio used, and all of these
melting points are notably lower than that of CaO itself.
[0039] As a result, even when the amount of the slag finally
generated exceeds, for example, 300 kg per ton of molten iron, it
becomes possible to perform a melting operation smoothly and
reliably in a melting furnace without the melting inhibition of the
slag former described above.
[0040] Of course, when the total amount of the slag-conditioning
agent additionally added (CaO-- or MgO-containing material) is less
than 40 kg per ton of molten iron, the operation by previously
blending the total amount of the slag-conditioning agents with the
raw material mixture and adding no slag-conditioning agents such as
CaO or MgO into the melting furnace is also a favorable embodiment
of the present invention, and this method is particularly effective
when an electric furnace, which demands a greater amount of
electric power for heating, is used as a melting furnace. At that
time, it is preferable to feed the solid reduced iron discharged
from the rotary-hearth reducing furnace (A) in a state kept at a
higher temperature as possible, preferably 900.degree. C. or more,
into the melting furnace for suppressing the heat energy needed
additionally in the melting furnace to the minimum.
[0041] In practicing the present invention, the metallization ratio
of the solid reduced iron produced by heat-reduction in the
rotary-hearth reducing furnace is preferably controlled to 80% or
more, preferably 85% or more, and still more preferably 90% or
more. When the metallization ratio is lower than that, the amount
of the unreduced iron oxide (FeO) remaining in the solid reduced
iron generated by heat-reduction becomes larger, and when the solid
reduced iron is fed into the melting furnace, a greater amount of
CO gas is generated due to reduction of the residual iron oxide,
resulting in drastic slag foaming. In addition, a part of iron
oxide is mixed into the generated slag without reduction,
increasing the FeO content of the slag, lowering the S partition
ratio between slag and metal (molten iron), thus disturbing the
decrease of the S content of the molten iron as described above and
drastically accelerating the wearing damage of the inner lining
refractory of the melting furnace. Therefore, the solid reduced
iron generated by heat-reduction in the rotary-hearth reducing
furnace (A) preferably has a metallizalion ratio of 80% or more,
more preferably 85% or more, and still more preferably 90% or
more.
[0042] According to the present invention, in a series of processes
which includes: producing a solid reduced iron by heat-reduction in
a rotary-hearth reducing furnace; feeding and heating the solid
reduced iron obtained subsequently in the melting furnace directly
connected or placed close to the reducing furnace; and thus
reducing the residual unreduced iron oxide and melting the reduced
iron generated into a molten iron, it becomes possible to perform
the series of processes from the production of a solid reduced iron
to the heat melting of the reduced iron efficiently, by developing
solid-phase reduction of the iron source (iron oxide) intensively
as possible in the rotary-hearth reducing furnace (A) and focusing
on melting the solid reduced iron in the melting furnace. Besides,
according to the present invention, it is also possible to reduce
the S content of the molten iron and suppress the wearing damage of
the inner lining refractory of melting furnace to the minimum, by
adjusting to a suitable basicity of the slag.
[0043] In addition in the present invention, during heat-reduction
of a raw material mixture in the rotary-hearth reducing furnace, by
laying a carbonaceous material such as pulverized coal onto the
hearth of the reducing furnace as a hearth material layer prior to
feeding the raw material mixture, and then by feeding the raw
material mixture over the hearth material, it advantageously
becomes possible to produce a reduced iron having a desirable
metallization ratio [percentage of a metal iron weight generated by
reduction of iron oxide to the total amount of iron contained in
the iron source (iron oxide) blended in the raw material mixture]
reliably with a very small fluctuation of the metallization ratio,
specifically of 5 mass % or less, further 2 mass % or less, within
a rotation of the hearth of rotary-hearth reducing furnace.
Therefore, it is preferable.
[0044] The heat needed for heat-reduction in the rotary-hearth
reducing furnace is supplied to the raw material mixture On the
rotary hearth by burner heating. As described above, the reduced
iron once formed may be re-oxidized by oxidizing gases such as
CO.sub.2 gas generated by burner combustion, also, the reduction
ratio will be difficult to increase when the reduction potential
above the hearth [generally represented by
(CO+H.sub.2)/(CO+CO.sub.2+H.sub.2+H.sub.2O).times.100(%) or more
simply represented by CO/(CO+H.sub.2O).times.100(%)] decreases in
the last stage of reduction; but it becomes possible to keep the
reduction potential of the environmental gas surrounding the raw
material mixture high even in the last stage of reduction by laying
a powdery carbonaceous material on the hearth as a hearth material,
because the atmosphere of the furnace, especially the CO.sub.2 gas
diffused into the atmosphere surrounding the raw material mixture
on the hearth as a hearth material is changed once again to CO gas
having reducing power by the carbonaceous material present. As a
result, it becomes possible to ensure a high level of metallization
ratio of 80% or more, 85% or more, and further 90% or more, and to
reduce the fluctuation in metallization ratio to the minimum.
[0045] Because said carbonaceous material fed onto the hearth as a
hearth material is consumed in the reaction with the environmental
gases (in particular, CO.sub.2 gas) as described above, it is
desirable to estimate the consumption amount of the carbonaceous
material in preliminary experiments and to feed the carbonaceous
material in an amount at least ensuring the consumption amount or
more. The consumption amount of the carbonaceous material varies
according to the scale, properties, operational condition and
others of the rotary-hearth reducing furnace used, but the amount
is approximately 20 to 30 kg per ton (dry base) of raw material
mixture fed into the rotary-hearth reducing furnace, or
approximately 30 to 60 kg per ton of molten iron obtained in the
melting furnace by heat-melting, according to the estimation based
on many experiments by the inventors.
[0046] About 30 kg per ton of molten iron of carbonaceous material
fed onto the rotary hearth of the reducing furnace as a hearth
material is mostly consumed for improvement of the atmosphere in
the reducing furnace (A), and the hearth material in an smaller
amount than this has an insufficient atmosphere-improving effect
(reduction potential-retaining effect), prohibiting the desirable
metallization ratio and possibly widening the fluctuation in the
metallization ratio.
[0047] Therefore, in practicing the present invention, for
maintaining the reduction potential of the atmosphere surrounding
the raw material mixture on the hearth at a high level, namely, for
secure prevention of re-oxidation of the reduced iron once formed,
even in the last stage of reduction in the rotary-hearth reducing
furnace, the amount of the powdery carbonaceous material fed as the
hearth material is preferably at least 30 kg or more, more
preferably 50 kg or more, and still more preferably 100 kg or more
per ton of molten iron obtained in the melting furnace.
[0048] In the preferred embodiment of the present invention wherein
a powdery carbonaceous material is laid as a hearth material on the
rotary hearth as described above, it is effective to form a hearth
material layer having a certain thickness for elongating a lifetime
of the hearth refractory, because the hearth material layer becomes
a buffer between the raw material mixture and the hearth refractory
or a protective material of the hearth refractory against a
byproduct slag, etc. In addition, when fed into the melting furnace
in the next process together with the solid reduced iron, the
excessive amount of carbonaceous material discharged without
combustion in the rotary-hearth reducing furnace (A) becomes a
highly active high-temperature carbonaceous material eliminated
volatile components by heating. Thus, the method is also
advantageous in smoother operation of the melting furnace from this
viewpoint. In particular, when the iron-bath melting furnace (C),
that is, a melting furnace having no heating means by electrodes
and using the combustion heat of carbonaceous material as heat
source, is used, it is preferable to supply 100 kg or more per ton
of molten iron of carbonaceous material previously into the
rotary-hearth reducing furnace, because 200 kg or more of
carbonaceous material per ton of molten iron is needed for heating
and melting in the melting furnace.
[0049] However, care should be given to the amount of,the
carbonaceous material, because the excessive amount of carbonaceous
material laid on the hearth of the reducing furnace leads to an
excessively thick hearth material layer (carbonaceous material in
an amount of 200 kg or more results in a layer thickness of about 5
mm or more), and, for example, a layer thickness of more than about
7.5 mm leads to penetration of the raw material mixture into the
hearth material layer on the hearth, causing problems such as
inhibition of the progress of reduction. According to an estimation
based on experiments by the inventors, 100 kg of powdery
carbonaceous material per ton of molten iron is normally equivalent
to a thickness of 2.5 mm, although the thickness depends on the
loading density of the raw material mixture laid thereon.
[0050] The kind of the carbonaceous material to be laid on the
hearth of the rotary-hearth reducing furnace is not particularly
limited; general coal, coke, etc., which is pulverized into
particles, preferably which has a suitable diameter, may be used.
When coal is used, anthracite, which is less fluid and which is
less swelling or viscous on the hearth, is favorable.
[0051] In practicing the present invention, a molten iron having a
carbon content, [C], of 3.0 mass % or more is produced by: feeding
a solid reduced iron with a high metallization ratio in a
rotary-hearth reducing furnace without cooling in a state kept at a
temperature as high as possible, preferably at 900.degree. C. or
more, into the melting furnace; reducing the iron oxide remaining
in the raw material mixture and melting the solid reduced iron
rapidly; and separating a molten iron and a molten slag.
[0052] In addition, in practically applying the method of the
present invention, it is preferable to add a carbonaceous material
into the rotary-hearth reducing furnace in an amount slightly
larger than an amount finally needed, by previously estimating the
amount of the additional carbonaceous material needed in the
melting furnace, and by taking into consideration an amount of the
carbonaceous material consumed at the previous stage for
improvement of the reduction potential of the atmospheric gas in
the rotary-hearth reducing furnace and a carbon amount remaining in
the solid reduced iron after reduction of the carbonaceous reducing
agent blended in the raw material mixture.
[0053] Of course, the amount of carbonaceous material needed in the
melting furnace differ significantly between when an arc-heating
electric furnace (B) is used as the melting furnace and when
heating is performed by using the combustion heat of carbonaceous
material as in the iron-bath melting furnace (C). However in any
case of the present invention, it is possible to produce a solid
reduced iron having a stabilized quality of a fluctuation in
metallization ratio of 5% or less, by previously estimating the
amount of carbonaceous material needed in the rotary-hearth
reducing furnace and the melting furnace; then in the former
process by laying 30 kg or more of carbonaceous material per ton of
final molten iron on the hearth of the rotary-hearth reducing
furnace; and by performing heat-reduction of the raw material
mixture on the carbonaceous material layer. By feeding the solid
reduced iron obtained and the carbonaceous material remaining on
the hearth in a state kept at a temperature as high as possible
into the melting furnace, it becomes possible to conduct a series
of operations from solid-phase reduction to melting smoothly and
efficiently.
[0054] In practicing the inventive method, in the series of
treatments using the rotary-hearth reducing furnace and the melting
furnace, it is preferable to estimate the amount of the
carbonaceous material consumed or needed in all processes and
allocate the carbonaceous material suitably into these processes
according to the amount and the role of the carbonaceous material
needed in each process for more efficient utilization of the
carbonaceous material (carbonaceous reducing agent).
[0055] The amount of the carbonaceous material consumed in all
processes is preferably controlled as grossly classified into the
followings. One is a carbonaceous material (a) which is blended
with a raw material mixture (or its compact, etc.) as a
carbonaceous reducing agent, an amount of the carbonaceous material
(a) is the amount of the carbon consumed if the raw material
mixture is completely reduced; this carbon amount varies according
to the operational condition of rotary-hearth reducing furnace, in
particular its atmospheric gas composition. Another is a
carbonaceous material (b) which is fed onto the moving hearth as a
hearth material; this amount of the carbon, which converts
oxidizing gases, such as, CO.sub.2 generated by burner combustion
in the reducing furnace, back into CO gas by reduction
(CO.sub.2+C=2CO) in the neighborhood of the hearth, prevents
re-oxidation of the solid reduced iron generated over the rotary
hearth, and thus enables production of a solid reduced iron having
a high metallization ratio and a low fluctuation in the
metallization ratio, and is critical for stabilized melting
operation of the solid reduced iron and the slag component in a
melting furnace.
[0056] Still others are a carbonaceous material (c) ensuring the
recarburization (carburization) needed for melting a solid reduced
iron in a melting furnace and producing a molten iron having a
desired carbon content, and a carbonaceous material (d) consumed as
a fuel of the melting furnace when an iron-bath melting furnace
using the combustion heat of carbonaceous material for heating
therein is used. Thus, the carbonaceous material (d) is unnecessary
when an arc-heating electric furnace is used, and such a system is
controlled based on three kinds of carbonaceous materials (a), (b)
and (c).
[0057] The carbonaceous material (a) used as a carbonaceous
reducing agent in a raw material mixture, which is blended in an
amount for efficient promotion of reduction of the iron oxide in
the raw material mixture, is preferably blended in an amount up to
that needed for production of a molten iron having a desired carbon
content in the melting furnace in general. In this way, it is not
necessary to add an additional carbonaceous material into the
melting furnace and it is possible to concentrate only on the
melting of the solid reduced iron. In such a case, the amount
equivalent to the carbonaceous material (b) consumed in the
rotary-hearth reducing furnace is fed into the furnace, but in
either case, the carbonaceous material is allocated suitably so
that the total amount of the carbonaceous materials (a) and (b)
becomes constant.
[0058] Alternatively, when a furnace that combusts carbonaceous
material with oxygen gas separately supplied thereinto and that
uses the combustion heat for melting the solid reduced iron is used
as the melting furnace, the carbonaceous material consumed for
combustion in the melting furnace should be added additionally,
compared with a case when an arc-heating melting furnace is used;
and thus, when coke, for example, is used as the carbonaceous
material, the carbonaceous material in an amount of about 250 kg
per ton of the molten iron finally obtained or more should be added
into the melting furnace, although the amount varies to some extent
according to the operational condition.
[0059] Accordingly in practicing the method of the present
invention, recommended is an operation by blending an amount of
carbonaceous material needed for carburization for ensuring a
carbon content of final molten iron, in principle, previously with
a raw material mixture and laying about 130 to 230 kg/ton (molten
iron) of the carbonaceous material on the hearth of the reducing
furnace, In this case, approximately 50 to 150 kg/ton (molten iron)
of additional carbonaceous material should be added into the
melting furnace.
[0060] When an arc-heating electric melting furnace (B) is used as
the melting furnace, it is necessary to adjust the amount of the
carbonaceous reducing agent in the raw material mixture fed onto
the rotary-hearth reducing furnace as the raw material to a
slightly smaller value, for controlling the carbon content of the
molten iron finally obtained to less than 3.0 mass %, resulting in
widening the fluctuation in carbon consumption amount or the like
in the reducing furnace, and thus it becomes difficult to produce a
solid reduced iron having a metallization ratio of 80% or more,
preferably 85%, and more preferably 90% or more in the reducing
furnace consistently and reliably. Further, the carbon content in
the molten iron finally obtained becomes also unstable, and thus,
the lower limit value of the amount of the carbonaceous material
fed into the reducing furnace as a hearth material is also
restricted significantly.
[0061] On the contrary, when an excessive amount of carbonaceous
material is fed onto a hearth of a moving-hearth reducing furnace
so that the carbon content of final molten iron becomes 3.0 mass %
or more, the carbon content of the molten iron increases gradually
up to the saturated C content, but when it exceeds the saturated C
content, the excessive amount of carbon is discharged out of the
melting furnace together with the slag and the like without being
dissolved into the molten iron in the melting furnace, when the
molten iron and slag are discharged from the melting furnace. Thus,
it is wasteful economically to supply the carbonaceous material
into the reducing furnace in an amount exceeding the saturated C
content of the molten iron.
EXAMPLES
[0062] Hereinafter, the present invention will be described in more
detail with reference to examples and comparative examples, but it
should be understood that the present invention is not restricted
by the following examples and various modifications are possible in
the scope of the present description above and below and included
in the scope of the present invention.
Example 1
[0063] In this experiment, as shown in the following Tables 1 and
2, two kinds of iron ores as iron oxide sources and one kind of
coal as a carbonaceous reducing agent were used, and operational
conditions were compared between when burnt line (CaO) was added
previously to the raw material mixture (here, that is a compact in
a pellet shape) to be fed into a rotary-hearth reducing furnace so
that the slag formed in a melting furnace had a basicity of 1.6
respectively and when burnt lime was not added. TABLE-US-00001
TABLE 1 Major compositions and contents of iron ore (mass %) Iron
ore Total Fe SiO.sub.2 Al.sub.2O.sub.3 CaO MgO S P Ore (A) 68.01
1.08 0.47 0.03 0.06 0.002 0.041 Ore (B) 68.67 3.83 0.19 0.38 0.40
0.002 0.024
[0064] TABLE-US-00002 TABLE 2 Composition of reducing agent (mass
%) Reducing agent FC VM TA Total Coal 71.5 19.9 8.6 100 (Notes) FC:
Fixed Carbon, VM: Volatile Matter, TA: Total Ash
[0065] Raw material compacts (pellets) having the composition shown
in Table 3 were formed by using the two kinds of iron ores and one
kind of carbonaceous material shown in Tables 1 and 2. As described
in FIG. 1, each pellet was first subjected to solid-phase reduction
in a rotary-hearth reducing furnace aiming at producing a solid
reduced iron at a metallization ratio of 92%, and the resulting
solid reduced iron was fed without particular cooling into an
arc-heating electric furnace located close thereto for production
of a carbon-saturated molten iron ([C]: 4.6 mass %) approximately
at 1,525.degree. C. Operational results are summarized in Table 4.
The binder used was wheat flour. TABLE-US-00003 TABLE 3 Raw
material pellet formulations Blending amount (mass %)
Classification Ore Ore Coal Binder Limestone (1) Ore (A) 77.4 21.1
1.5 0.0 (2) Ore (A) 74.4 20.4 1.5 3.7 (3) Ore (B) 78.8 19.7 1.5 0.0
(4) Ore (B) 75.6 18.9 1.5 4.0 (5) Ore (B) 72.4 18.1 1.5 8.0
[0066] TABLE-US-00004 TABLE 4 Summary of experimental results Iron
ore (A) Iron ore (B) Classification Method (1) Method (2) Method
(3) Method (4) Method (5) 1) Main end auxiliary materials Reduced
iron (dry) (kg/t(mi)) 1137 1132 1156 1170 1217 Hearth-fed carbon
material (kg/t(mi)) 54.5 61.9 62.1 Carbon material(added)
(kg/t(mi)) 24.1 -- 21.0 -- -- Burnt lime (kg/t(mi)) 44.9 -- 96.7
54.1 -- Dolomite (kg/t(mi)) -- -- -- -- -- 2) Utility Electric
power (kWh/t(mi)) 507 408 489 437 408 Oxygen (Nm.sup.3/t(mi)) -- --
-- 3) Molten iron [C] (mass %) 4.2 4.5 4.3 4.6 4.6 Molten iron
temperature (.degree. C.) 1475 1525 1505 1510 1525 4) Slag Slag
consumption (kg/t(mi)) 86.6 90.9 184 180.1 179.4
[0067] As apparent from Table 4, in the method (1) using the iron
ore (A), being continuously fed approximately 1,137 kg per ton of
molten iron [represented by kg/t(mi) in the table] of the solid
reduced iron obtained in the rotary-hearth reducing furnace into
the electric arc melting furnace under a high-temperature condition
(about 900.degree. C.), approximately 44.9 kg per ton of molten
iron of burnt lime (CaO) and approximately 24.1 kg per ton of
molten iron of a carbonaceous material (here, that is coke) were
fed at the same time into the melting furnace, for production of a
carbon-saturated molten iron desirably having a [C] content of 4.6
mass %.
[0068] The iron ore used in this experiment was a high-grade ore
containing a small amount of gangue components, but if the targeted
basicity of slag generated in the melting furnace is set to 1.6,
the amount of burnt lime to be added additionally to the melting
furnace became approximately 44.9 kg per ton of molten iron, which
is a slight beyond the range specified in the present invention. As
a result, a phenomenon that a part of the slag aggregates in a
block shape was observed when burnt lime was added to the melting
furnace in the melting process, leading to instability of; the
operation for melting burnt lime and reduced iron and to elongation
of the period needed for complete melting.
[0069] In addition, the carbonaceous material amount needed for
heating in the melting furnace was supplied not onto the rotary
hearth of the rotary-hearth reducing furnace on an upstream side
but directly into the melting furnace, and thus, the reduction
potential of hearth atmosphere declined to some extent at the last
stage of reduction in the rotary-hearth reducing furnace.
Consequently, the solid reduced iron discharged from the reducing
furnace has a relatively low metallization ratio in the range of
78% to 88% and has a larger fluctuation therein (average: 84%), and
thus the molten iron finally obtained has a relatively lower carbon
content of 4.2 mass %. The molten iron temperature also dropped to
1,475.degree. C., lower by approximately 50.degree. C. than the
desired value, and the operation period was also elongated.
[0070] The theoretical value for the amount of carbonaceous
material to be added additionally into the melting furnace is 18.5
kg per ton of molten iron in this case, but 24.1 kg per ton of
molten iron of carbonaceous material, greater by approximately 30%
than the theoretical value, was added to the melting furnace as
fuel, because the operational condition of the melting furnace was
slightly unstable.
[0071] Alternatively in the method (2) using the ore (A) as iron
oxide source, CaO in a total amount needed for adjusting the
basicity of the slag generated in the melting furnace to 1.6 (45 kg
per ton of molten iron) was previously added to the raw material
mixture (pellet); a solid reduced iron having a metallization ratio
of approximately 92% was obtained by feeding the pellet into the
rotary-hearth reducing furnace; and the solid reduced iron was fed
together with approximately 17 kg per ton of molten iron of the
carbonaceous material remaining on the hearth into the next melting
furnace, to give a molten iron having a carbon content of
approximately 4.5 mass %. Approximately 54.5 kg per ton of molten
iron of pulverized coal was fed onto the hearth of the reducing
furnace as a hearth material, and approximately 17 kg per ton of
molten iron thereof was discharged from the reducing furnace in an
unburned state and fed into the melting furnace.
[0072] The combustion ratio of the carbonaceous material in the
rotary-hearth reducing furnace depends on the operational condition
of the reducing furnace, and was approximately 69% in this
experiment. In the method (2), the solid reduced iron obtained in
the reducing furnace had an almost desirable metallization ratio of
92% and an extremely low fluctuation thereof of .+-.1.5%. Further,
it was possible to melt the slag and the solid reduced iron quite
smoothly in the melting furnace even without addition of a
slag-conditioning agent (burnt lime), demonstrating the
advantageous features of the present invention. In addition, the
[C] value of the molten iron finally obtained in this example was
almost desirable 4.5 mass %, and it was also possible to control
the molten iron temperature after treatment almost accurately to a
desired value.
[0073] After sampling the solid reduced iron from the reducing
furnace and separation of the slag from the solid reduced iron,
mineral components in the slag were determined by X-ray analysis.
The results showed presence of mineral components such as
2CaO--SiO.sub.2, 3CaO--MgO-2SiO.sub.2, 3CaO-2SiO.sub.2 and
2CaO--Al.sub.2O.sub.3--SiO.sub.2, demonstrating generation of the
complex oxides superior in solubility intended by the present
invention. The results also showed that the S content of the molten
slag collected from the melting furnace was approximately 1.5 mass
% and the (S) content of the molten iron approximately 0.04 mass %,
indicating that most of sulfur is transferred to the slag.
[0074] The ore (B) contains a great amount of silicon oxide
(SiO.sub.2) in the gangue components as shown in Table 1, and when
this kind of ore is used, it is necessary to add CaO in an amount
greater than that to the ore (A) for generation of a slag having a
desirable basicity in the melting furnace, and the slag consumption
generated in the melting furnace reaches approximately 180 kg per
ton of molten iron. In the method (3), wherein the raw material
compact (pellet) prepared by a carbonaceous reducing agent and a
small amount of a binder with blending the ore (B) was fed alone
without addition of a pulverized coal as a hearth material onto the
rotary hearth and subjected to heat-reduction, it was possible to
achieve a metallization ratio of approximately 88%, but the
fluctuation thereof was slightly larger at .+-.3.5%. In any case,
when melting was carried out by continuously supplying the solid
reduced iron obtained in the rotary-hearth reducing furnace, a
carbonaceous material and a slag-conditioning agent (burnt lime)
into the melting furnace it was necessary in the method (3) to add
a greater amount (96.7 kg per ton of molten iron) of burnt lime
(CaO) for controlling the basicity of the slag generated in the
melting furnace to 1.6, resulting in unsmooth progress of the
melting of the burnt lime fed into the melting furnace, frequent
generations of aggregates (blocks) over the slag during melting,
making it more difficult to continue operation, and consequently,
occurring frequent interruptions of operation.
[0075] While in the method (4), about a half of the burnt lime
needed for adjustment of the slag basicity in the melting furnace
is blended with the raw material pellet previously, and another
half thereof was added to the melting furnace. In this experiment,
all carbonaceous material needed for the melting furnace
(approximately 61.9 kg per ton of molten iron) was fed as a hearth
material onto the hearth of the rotary-hearth reducing furnace on
an upstream side, a part of the carbonaceous hearth material is
consumed in the reducing furnace, and the amount of the residual
carbonaceous material discharged from the reducing furnace was
approximately 17.5 kg per ton of molten iron.
[0076] In this case, the amount of a slag-conditioning agent (burnt
lime) to be added into the melting furnace is approximately 54 kg
per ton of molten iron, which is beyond the range specified in the
present invention. Consequently, a phenomenon that the slag became
partially aggregated in the melting furnace was observed, although
serious troubles leading to interruption of the process for melting
solid reduced iron did not occur. Thus, the amount of the
slag-conditioning agent added additionally to the melting furnace
seems to be approximately 40 kg per ton of molten iron or less.
[0077] On the contrary, in the method (5), the entire CaO source to
be added for controlling the slag basicity in the melting furnace
was blended previously with the raw material pellet, and no CaO
source was added into the melting furnace. In this experiment, all
of the carbonaceous material needed in the entire process was added
in advance into the reducing furnace as a hearth material. A part
of the carbonaceous material fed as a hearth material was consumed
for preservation of the reduction potential in the reducing
furnace, but another part, approximately 18 kg per ton of molten
iron, thereof was fed into the melting furnace together with the
solid reduced iron.
[0078] The solid reduced iron discharged from the rotary-hearth
reducing furnace had a very high metallization ratio of 92% close
to the desired value and a very small fluctuation thereof of
.+-.1.0%; and melting of the byproduct slag and the solid reduced
iron in the melting furnace proceeded rapidly in a more stabilized
manner, allowing stabilized operation even though the slag
consumption was higher at about 180 kg per ton of molten iron.
Example 2
[0079] In this example, used was an iron-bath melting furnace that
uses the combustion heat of carbonaceous material as a heat source
for melting a solid reduced iron. A furnace similar to oxygen
top-blowing converter was used as the melting furnace, and oxygen
for combustion of the carbonaceous material in the melting furnace
was blown onto the molten iron from above by using a top-blowing
lance. Specifically, conducted were two experiments: by a
conventional method of adding into the melting furnace a
CaO-containing material needed for controlling the slag basicity
not being blended with a raw material compact; and by the present
inventive method of blending all of the CaO-containing material
with the raw material compact, where each method uses the same ore
(A) as that used in Example 1 as an iron oxide source. Results are
summarized in Table 5.
[0080] In the conventional method (1), the amount of CaO needed for
adjusting the basicity of the slag generated in the melting furnace
to 1.6 was relatively smaller at about 73 kg per ton of molten
iron, because a high-quality iron ore containing a smaller amount
of SiO.sub.2 in gangue components was used as an iron oxide source;
the amount, which is larger than 40 kg per ton of molten iron i.e.,
the upper limit of the addition amount of the slag-conditioning
agent specified in the present invention, was added to the melting
furnace; and no powdery carbonaceous material was added onto the
hearth of the rotary-hearth reducing furnace for production of
solid reduced iron. Different from the melting method of using an
electric furnace as a melting furnace, the method allowed
relatively smoother melting of the burnt lime added into the
melting furnace, because it is possible to agitate the molten iron
and the slag vigorously by the oxygen blown from the top-blowing
lance. The metallization ratio of the solid reduced iron obtained
after heat-reduction was significantly higher at an average of 90%,
but the fluctuation thereof was extremely larger at .+-.4.5%; thus,
the molten iron temperature fluctuated significantly in the range
of 1,400 to 1,560.degree. C. during melting; and it was difficult
to control the molten iron temperature to the desired value of
1,525.degree. C.
[0081] Then, 275 kg per ton of molten iron of carbonaceous material
was supplied additionally into the molting furnace, based on an
estimation of the amount of the carbonaceous material needed in the
entire process, but the carbon content of the molten iron, [C],
finally obtained was 3.8 mass %, significantly lower than the
desired value of 4.5 mass %.
[0082] Alternatively in the method (2) according to the present
invention, a raw material pellet having a composition identical
with that of the used in Example 1 was prepared, and the pellet and
approximately 210 kg per ton of molten iron of a carbonaceous
material were fed onto the hearth of the rotary-hearth reducing
furnace as a hearth material for heat-reduction. Approximately 30
kg per ton of molten iron of the carbonaceous material supplied
onto the hearth of the reducing furnace is consumed by combustion
for preservation of the reduction potential in the reducing
furnace, and the other, approximately 180 kg per ton of molten
iron, of the carbonaceous material was supplied together with the
solid reduced iron into the melting furnace. Thus, the insufficient
amount, 74 kg per ton of molten iron, of the carbonaceous material
was added additionally to the melting furnace. All burnt lime
needed to adjust the basicity of the slag generated in the melting
furnace to a desired value (1.6) was blended with the raw material
pellet in advance, and no burnt lime was added to the melting
furnace.
[0083] In the method (2) according to the present invention, a
solid reduced iron at approximately 900.degree. C. was fed into the
melting furnace in a similar manner to the conventional method (1);
because the fluctuation of the metallization ratio of the solid
reduced iron was very small at 90 to 93%, the solid reduced iron
melted very smoothly in the melting furnace, giving a molten iron
having a carbon content, [C], of 4.7 mass % (almost identical with
the desired value of 4.6 mass %) at a molten iron temperature of
1,530.degree. C. (desired value: 1,525.degree. C.); the [S] content
thereof is sufficiently low at 0.028 mass %, and the fluctuation in
molten iron temperature was also very small at 1,500 to
1,550.degree. C. TABLE-US-00005 TABLE 5 Summary of experimental
results Iron ore (A) Conventional Inventive Classification method
(1) method (2) 1) Main and auxiliary materials Reduced iron (dry)
(kg/t(mi)) 1090 1155 Hearth-fed carbon material (kg/t(mi)) 210
Carbon material(added) (kg/t(mi)) 275 74 Burnt lime (kg/t(mi)) 73
Dolomite (kg/t(mi)) 2) Utility Electric power (kWh/t(mi)) Oxygen
(Nm.sup.3/t(mi)) 224 209 3) Molten iron [C] (mass %) 3.8 4.7 Molten
iron temperature (.degree. C.) 1500 1530 4) Slag Slag consumption
(kg/t(mi)) 137 133
Example 3
[0084] In this example, shown is a typical method of,producing a
solid reduced iron smaller in fluctuation and higher in
metallization ratio by using a rotary-hearth reducing furnace.
Namely, one case that the heat-reduction carried out by feeding 30
kg or more of a powdery carbonaceous material (per ton of molten
iron obtained in the next melting furnace) onto the rotary hearth
of a rotary-hearth reducing furnace as a hearth material and then
by feeding the raw material pellet over the carbonaceous hearth
material layer; and another case that the heat-reduction carried
out by feeding only the raw material pellet directly onto the
hearth without laying the powdery carbonaceous material onto the
rotary-hearth; were compared. In either case, the residual carbon
amount of the pellet and the metallization ratio were determined by
collecting the raw material pellet under reduction from the
sampling holes placed at different sites of the furnace, while
maintaining the atmospheric temperature in the reducing furnace to
a constant temperature of 1,340.degree. C.
[0085] As shown in FIG. 2, when there was no carbonaceous material
layer on the hearth, the fluctuation in the residual carbon amount
was relatively larger even at the same metallization ratio, and it
was difficult to produce a solid reduced iron having a high
metallization ratio of 90% or more consistently. On the other hand,
as apparent from FIG. 2, when heat-reduction is carried out after
feeding the carbonaceous hearth material over the rotary hearth and
feeding the raw material pellet over there, the fluctuations in
metallization ratio and in residual carbon amount became very small
and a high metallization ratio was obtained; and comparison between
these cases reveals that supply of the carbonaceous material as a
hearth material is extremely effective.
[0086] Accordingly, in practicing the present invention, that is,
in producing a reduced iron having a metallization ratio of 80% or
more by using a rotary-hearth reducing furnace, in particular, by
employing a method of feeding first the carbonaceous hearth
material on the hearth and then feeding the raw material pellet
thereon, it becomes possible to reduce the fluctuation in the
amount of carbon remaining in the solid reduced iron obtained after
heat-reduction (i.e., residual carbon amount) into the range of
.+-.1.0 mass %, and to assure a consistent and reliable operation
in melting the solid reduced iron in the next melting furnace. In
contrast, when compared at the same metallization ratio, the
fluctuation in residual carbon amount when the carbonaceous
material is not fed as a hearth material became .+-.1.0 mass % or
more, indicating that the operation in the melting furnace is
rather instabilized.
[0087] In the present invention, the reason for setting the
metallization ratio of the solid reduced iron to 80% or more is
that when the metallization ratio of solid reduced iron is less
than 80%, the load for reducing the residual FeO in solid reduced
iron in the melting furnace becomes greater and vigorous foaming of
the slag caused by the CO gas bubbles generated during the
reduction of the residual FeO makes the operation more unstable.
Thus, the scope of the present invention is determined.
[0088] In producing a molten iron by heat-reducing a raw material
mixture containing an iron oxide source and a carbonaceous reducing
agent and then heat melting the reduced mixture in facilities in a
combination of a moving-hearth reducing furnace such as
rotary-hearth reducing furnace and a melting furnace, the present
invention described so far in detail allows smoother melting of the
slag and the solid reduced iron in the melting furnace in a shorter
period of time, in particular, by adjusting the addition amount of
the CaO-containing material in such a manner that the basicity of
the slag generated in the melting furnace becomes 1.1 or more and
adjusting the amount of the CaO-containing material fed into the
melting furnace to 40 kg per ton of molten iron or less, adding the
balance of the CaO-containing material to the raw material mixture,
or preferably adding the total amount of the CaO-containing
material to the raw material mixture and eliminating the
CaO-containing material added into the melting furnace.
[0089] In addition, it is possible to keep the reduction potential
consistently at a high level at the last stage of reduction in the
reducing furnace and raise the metallization ratio of the solid
reduced iron by employing the method of feeding a powdery
carbonaceous material onto the moving-hearth reducing furnace as a
hearth material preferably in advance to a raw material mixture;
and in particular, it is possible to perform a series of operations
from solid-phase reduction to heat melting more smoothly by
performing operation by using the carbonaceous material as the
hearth material in an amount of 30 kg or more per ton of molten
iron, more preferably by feeding the total amount of the
carbonaceous material needed in the entire process as the hearth
material.
[0090] In addition according to the present invention it is
possible to raise the carbon content of the molten iron obtained in
the melting furnace to 3.0 or more easily because of the increase
in the metallization ratio of the solid reduced iron obtained in
the reducing furnace and the increase in the amount of the
carbonaceous material used as a hearth material. As a result, the
present invention provides various advantages in industrial
application, such as decrease in the iron oxide content of the
molten slag generated in melting furnace, control of the wearing
damage of the inner lining refractory of melting furnace, increase
in the partition ratio of sulfur toward slag, drastic decrease in
the sulfur content of molten iron, and others.
* * * * *