U.S. patent application number 13/575859 was filed with the patent office on 2013-08-01 for method for removing impurities in molten cast iron, and cast iron raw material.
This patent application is currently assigned to KIMURA CHUZOSHO CO., LTD.. The applicant listed for this patent is Tatsuo Atsumi, Takao Fujikawa, Ilgoo Kang, Toshitake Kanno, Kiyoshi Kinoshita, Hirotoshi Murata, Hiromitsu Takeuchi, Nobuya Yamamoto. Invention is credited to Tatsuo Atsumi, Takao Fujikawa, Ilgoo Kang, Toshitake Kanno, Kiyoshi Kinoshita, Hirotoshi Murata, Hiromitsu Takeuchi, Nobuya Yamamoto.
Application Number | 20130195712 13/575859 |
Document ID | / |
Family ID | 44319225 |
Filed Date | 2013-08-01 |
United States Patent
Application |
20130195712 |
Kind Code |
A1 |
Kanno; Toshitake ; et
al. |
August 1, 2013 |
METHOD FOR REMOVING IMPURITIES IN MOLTEN CAST IRON, AND CAST IRON
RAW MATERIAL
Abstract
There is provided a method for obtaining a pure melt in which
the impurities Mn, Al, Ti, Pb, Zn, and B are removed from molten
cast iron and depletion of useful C and Si is suppressed, the
method wherein an excess oxygen flame having a theoretical
combustion ratio of fuel and oxygen (amount of oxygen
(volume).times.5/amount of fuel (volume)) of 1 to 1.5 is directly
exposed to the surface of pre-melted molten cast iron, the
temperature of the molten cast iron is held at 1250.degree. C. or
more and less than 1500.degree. C. while the melt surface is
superheated and an acidic slag is brought into contact with the
melt, and an oxygen-containing gas is injected into the interior of
the molten cast iron.
Inventors: |
Kanno; Toshitake;
(Sunto-gun, JP) ; Kang; Ilgoo; (Sunto-gun, JP)
; Fujikawa; Takao; (Kuwana-shi, JP) ; Takeuchi;
Hiromitsu; (Kishiwada-shi, JP) ; Kinoshita;
Kiyoshi; (Hiroshima-shi, JP) ; Murata; Hirotoshi;
(Yao-shi, JP) ; Yamamoto; Nobuya; (Kochi-shi,
JP) ; Atsumi; Tatsuo; (Toyota-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kanno; Toshitake
Kang; Ilgoo
Fujikawa; Takao
Takeuchi; Hiromitsu
Kinoshita; Kiyoshi
Murata; Hirotoshi
Yamamoto; Nobuya
Atsumi; Tatsuo |
Sunto-gun
Sunto-gun
Kuwana-shi
Kishiwada-shi
Hiroshima-shi
Yao-shi
Kochi-shi
Toyota-shi |
|
JP
JP
JP
JP
JP
JP
JP
JP |
|
|
Assignee: |
KIMURA CHUZOSHO CO., LTD.
SHIZUOKA
JP
MIE PREFECTURE
MIE
JP
SENSHU CORPORATION
OSAKA
JP
NANIWA ROKI CO., LTD.
OSAKA
JP
HITACHI METALS, LTD.
TOKYO
JP
KINOSHITA MANUFACTORY CO., LTD.
HIROSHIMA
JP
|
Family ID: |
44319225 |
Appl. No.: |
13/575859 |
Filed: |
January 24, 2011 |
PCT Filed: |
January 24, 2011 |
PCT NO: |
PCT/JP2011/051195 |
371 Date: |
October 17, 2012 |
Current U.S.
Class: |
420/14 ; 420/9;
75/10.42; 75/10.66; 75/507; 75/556 |
Current CPC
Class: |
C21C 1/04 20130101; C21C
1/08 20130101; C22C 37/10 20130101 |
Class at
Publication: |
420/14 ; 75/507;
75/556; 75/10.42; 75/10.66; 420/9 |
International
Class: |
C21C 1/04 20060101
C21C001/04; C22C 37/10 20060101 C22C037/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 28, 2010 |
JP |
2010-016251 |
Claims
1. A method for removing impurities including manganese (Mn) while
suppressing the depletion of carbon (C) and silicon (Si) included
in pre-melted molten cast iron, the method for removing impurities
in molten cast iron characterized in comprising: holding the
temperature of the molten cast iron at 1250.degree. C. or more and
less than 1500.degree. C.; and directly exposing a surface of the
molten cast iron to an excess oxygen flame having a theoretical
combustion ratio of fuel and oxygen (amount of oxygen
(volume).times.5/amount of fuel (volume)) of 1 to 1.5 while
allowing the melt and an acidic slag layer to come into contact
with each other to superheat the surface.
2. The method for removing impurities in molten cast iron of claim
1, characterized in being a method in which (.DELTA.C/.DELTA.Mn) or
(.DELTA.Si/.DELTA.Mn) is 2.5 or less when the removal efficiency
per unit time of the manganese (Mn) is (.DELTA.Mn/h), the removal
efficiency per unit time of the carbon (C) is (.DELTA.C/h), and the
removal efficiency per unit time of the silicon (Si) is
(.DELTA.Si/h).
3. The method for removing impurities in molten cast iron of claim
1 or 2, characterized in that an oxygen-containing gas is injected
from the interior of the molten cast iron to a surface of the
molten cast iron on which the excess oxygen flame is directly
exposed.
4. The method for removing impurities in molten cast iron of claim
3, characterized in that the oxygen-containing gas is air.
5. The method for removing impurities in molten cast iron of claim
3 or 4, characterized in that the oxygen-containing gas is injected
at 100 to 1600 (L/min) per 1000 kg of melt.
6. The method for removing impurities in molten cast iron of any of
claims 1 to 5, characterized in that elements other than manganese
(Mn) removed as impurities in the molten cast iron are at least one
of the elements selected from lead (Pb), zinc (Zn), titanium (Ti),
aluminum (Al), and boron (B).
7. The method for removing impurities in molten cast iron of any of
claims 1 to 6, characterized in that iron oxide is added to the
molten cast iron.
8. The method for removing impurities in molten cast iron of any of
claims 1 to 7, characterized in that a device for holding the
molten cast iron is a rotary furnace, an electric furnace, a ladle,
a cupola desulfurization ladle, or a turn dish, or a combination
thereof.
9. A cast iron raw material manufactured using the method of claim
1, the cast iron raw material characterized in being 2 to 4 mass %
carbon (C), 0.5 to 4 mass % silicon (Si), 0.1 to 3 mass % manganese
(Mn), 0.0001 to 0.03 mass % lead (Pb), 0.0001 to 1.0 mass % zinc
(Zn), 0.001 to 0.2 mass % titanium (Ti), 0.0001 to 0.5 mass %
aluminum (Al), 0.0001 to 0.04 mass % boron (B), and the remainder
being iron (Fe) and inevitable impurities.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for removing
impurities in molten cast iron, and particularly for removing
manganese (hereinafter referred to as "Mn") while suppressing the
depletion of carbon (hereinafter referred to as "C") and silicon
(hereinafter referred to as "Si"), and also relates to a cast iron
raw material manufactured using this method.
BACKGROUND ART
[0002] Cast iron is an iron alloy in which the main constituent is
iron (hereinafter referred to as "Fe"); in which principal elements
C, Si, Mn, phosphorous (hereinafter referred to as "P"), and sulfur
(hereinafter referred to as "S") are added; in which copper (Cu),
tin (Sn), chromium (Cr), magnesium (hereinafter referred to as
"Mg"), nickel (Ni), cobalt (Co), molybdenum (Mo), vanadium (V),
niobium (Nb), aluminum (hereinafter referred to as "Al"), titanium
(hereinafter referred to as "Ti"), zirconium (hereinafter referred
to as Zr), cerium (Ce), calcium (Ca), barium (Ba), bismuth (Bi),
and other elements are also added in accordance with the intended
purpose; and in which other inevitable impure elements are
contained.
[0003] Cast iron raw material is pig-iron, iron scrap, pig iron
scrap, return pig iron, turnings, or another main iron source; and
a carbon source and other alloy irons for adjusting the C, Si, and
other constituents.
[0004] During the manufacture of cast iron, a large amount of iron
scrap and the like is used as the main iron source for the raw
material in order to reduce cost and energy, and cast iron is an
excellent material in terms of recyclability.
[0005] However, in an iron steel material, which is the origin of
the iron scrap used as the iron source for the manufacture of cast
iron, the added amount of alloy elements other than C and Si tends
to be increased for the purpose of higher efficiency, weight
saving, higher functionality, and the like. In addition, in the
manufacture of high-efficiency iron steel material, comparatively
inexpensive Mn is often added in order to reduce costs due to the
recent rise in prices of rare metals, and the increase in Mn in
iron scrap therefore becomes a problem when iron scrap is used as
cast iron raw material.
[0006] The Mn in cast iron is used as an element for stabilizing
cementite (Fe.sub.3C), which is a compound of Fe and C. In cast
iron, the iron matrix structure is therefore changed into perlite,
which is a layered structure made of Fe.sub.3C and Fe. Hardness and
tensile-strength of the cast iron thereby increases, but
elongation, toughness, and other properties decrease. In spheroidal
graphite cast iron in which graphite in the cast iron is
spheroidized by Mg to improve strength and elongation properties,
the change of the iron matrix structure has a substantial influence
on strength properties. The decrease of elongation and toughness of
the graphite cast iron due to an increase in the Mn content in
particular is dramatic, and is a problem in terms of
manufacturing.
[0007] There is a conventional method for supplying S to molten
cast iron and producing MnS as a method for removing Mn from molten
cast metal (Patent Document 1, Patent Document 2).
[0008] In this method, however, MnS is produced in the molten cast
iron, and problems arise in that the removal treatment is
difficult, and a large amount of S additive is required in order to
improve removal efficiency.
[0009] On the other hand, Mn removal from ordinary molten iron
steel rather than cast iron can be performed employing the
following reaction by refinement by the addition of oxygen using a
converter furnace or the like.
Mn+1/2 (O.sub.2).fwdarw.MnO
[0010] However, this reaction is thermodynamically unstable due to
the oxidation reaction of C and Si in a melt having a high C and a
high Si such as in cast iron, as is apparent from the thermodynamic
Ellingham diagram, and at a temperature of 1400.degree. C. or more.
Accordingly, the useful elements C, Si, and the like are first lost
in the removal of Mn when oxygen is supplied to molten cast iron,
and a problem arises in that the molten cast iron cannot be
formed.
[0011] Because of this, there is proposed a method for
manufacturing spheroidal graphite molten cast iron by beginning
melting from a cold charge using a pure oxygen rotary furnace,
removing Mn using an oxidation reaction while C and Si are
depleted, in the same manner as melting ordinary iron steel, and
then mixing the result together with a separately prepared molten
cast iron having a large amount of C and Si (Patent Document
3).
[0012] In this method, however, the contents of C and Si during
melting are reduced, and the melting point of the melt is therefore
increased, making a further high-temperature operation necessary.
This brings about even further depletion of C and Si. In addition,
this method is not practical in terms of manufacturing costs
because a simultaneous operation in a separate furnace is
necessary.
[0013] In addition, iron oxide or the like is directly added to the
molten cast iron melted in an electric furnace or the like, and
problems arise in that C and Si are lost in the same manner as in a
converter furnace even when the removal of Mn by oxidation is
attempted, a large amount of slag is produced, and the operation
becomes difficult.
[0014] There is disclosed a method in which iron raw material is
heated using the flame of an oxygen burner, and a recarburizer is
added while the raw material is melted to manufacture cast iron
(Patent Document 4).
[0015] However, this method is a method for improving carbon intake
efficiency when carbon is added to molten cast iron, and is a
method for manufacturing cast iron having a high concentration of
carbon.
[0016] As is shown in each of the aforementioned cited documents,
it is difficult to remove Mn in a practical manner using
conventional techniques while preventing the depletion of C and Si
from molten cast iron.
PRIOR ART DOCUMENTS
Patent Documents
[0017] Patent Document 1: Japanese Laid-Open Patent Publication No.
2003-105420
[0018] Patent Document 2: International Application No.
WO2003/083143
[0019] Patent Document 3: Japanese Laid-Open Patent Publication No.
7-268432
[0020] Patent Document 4: Japanese Laid-Open Patent Publication No.
10-8120
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0021] The present invention was devised in order to solve the
aforementioned problems, and therefore an object of the present
invention is to provide a method for removing impurities in cast
iron raw material and obtaining a pure melt in which the impure
elements Mn, Al, Ti, Pb, Zn, and B in molten cast iron are removed
and the depletion of useful C and Si is suppressed.
Means for Solving the Problem
[0022] The present invention provides a method for removing
impurities including manganese (Mn) while suppressing the depletion
of carbon (C) and silicon (Si) included in pre-melted molten cast
iron, the method for removing impurities in molten cast iron
characterized in comprising: holding the temperature of the molten
cast iron at 1250.degree. C. or more and less than 1500.degree. C.;
and directly exposing a surface of the molten cast iron to an
excess oxygen flame having a theoretical combustion ratio of fuel
and oxygen (amount of oxygen (volume).times.5/amount of fuel
(volume)) of 1 to 1.5 while allowing the melt and an acidic slag
layer to come into contact with each other to superheat the
surface.
[0023] The method is also characterized in that
(.DELTA.C/.DELTA.Mn) or (.DELTA.Si/.DELTA.Mn) is 2.5 or less when
the removal efficiency per unit time of the manganese (Mn) is
(.DELTA.Mn/h), the removal efficiency per unit time of the carbon
(C) is (.DELTA.C/h), and the removal efficiency per unit time of
the silicon (Si) is (.DELTA.Si/h).
[0024] The method is also characterized in that an
oxygen-containing gas is injected from the interior of the molten
cast iron to a surface of the molten cast iron on which the excess
oxygen flame is directly exposed, and the oxygen-containing gas is
air in particular. The oxygen-containing gas is injected in a ratio
of 100 to 1600 (L/min) per 1000 kg of melt.
[0025] The method is also characterized in that elements other than
manganese (Mn) removed as impurities in the molten cast iron are at
least one of the elements selected from lead (Pb), zinc (Zn),
titanium (Ti), aluminum (Al), and boron (B).
[0026] The method for removing impurities in molten cast iron of
the present invention is also characterized in that iron oxide is
added to the molten cast iron. The method is also characterized in
that a device for holding the molten cast iron is a rotary furnace,
an electric furnace, a ladle, a cupola desulfurization ladle, or a
turn dish, or a combination of thereof.
[0027] The present invention provides a cast iron raw material
manufactured using the aforementioned method, the cast iron raw
material characterized in being 2 to 4 mass % carbon (C), 0.5 to 4
mass % silicon (Si), 0.1 to 3 mass % manganese (Mn), 0.0001 to 0.03
mass % lead (Pb), 0.0001 to 1.0 mass % zinc (Zn), 0.001 to 0.2 mass
% titanium (Ti), 0.0001 to 0.5 mass % aluminum (Al), 0.0001 to 0.04
mass % boron (B), and the remainder being iron (Fe) and inevitable
impurities.
Effect of the Invention
[0028] The method for removing impurities in molten cast iron
according to the present invention is a method for removing
impurities including Mn while suppressing the depletion of C and Si
contained in pre-melted molten cast iron, wherein the temperature
of the molten cast iron is held at 1250.degree. C. or more and less
than 1500.degree. C., and an excess oxygen flame having a
theoretical combustion ratio of fuel and oxygen of 1 to 1.5 is
directly exposed to the surface of the molten cast iron while an
acidic slag layer is brought into contact with the melt to
superheat the melt surface. It is therefore possible to obtain a
pure molten cast iron in which Mn and other impure elements are
removed from the molten cast iron and the depletion of C and Si,
which are essential elements in cast iron, is suppressed. In
spheroidal graphite cast iron in particular, Mn or other impurities
substantially hinder the elongation and toughness of cast iron raw
material, and the effect of removing Mn or other impure elements is
dramatic.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a schematic view showing a treatment state of
molten cast iron fed into a treatment device;
[0030] FIG. 2 is a view showing a small rotary furnace-type
impurity removal device;
[0031] FIG. 3 is a view showing the C, Si, and Mn contents in the
melt, and the treatment time;
[0032] FIG. 4 is a view showing the Pb, Zn, Ti, Al, and B contents
in the melt, and the treatment time;
[0033] FIG. 5 is a view showing the effect of bubbling or the like
on Mn elimination efficiency in molten cast iron;
[0034] FIG. 6 is a view showing an actual rotary furnace-type
impurity removal device;
[0035] FIG. 7 is a view showing the experiment results of the
actual rotary furnace-type impurity removal device;
[0036] FIG. 8 is a view showing a ladle-type impurity removal
device;
[0037] FIG. 9 is a graph showing experiment results of the ladle
impurity removal device;
[0038] FIG. 10 is a graph in which the Mn removal efficiency is
organized by specific surface area of the melt; and
[0039] FIG. 11 is a graph showing the relationship between the Mn
removal efficiency, and the values of .DELTA.C/.DELTA.Mn and
.DELTA.Si/.DELTA.Mn.
MODE FOR CARRYING OUT THE INVENTION
[0040] In order to remove the impure elements Mn, Al, Ti, Pb, Zn,
and B in molten cast iron and suppress the depletion of useful C
and Si, the present inventors performed extensive research and
perfected a method for removing impurities, which entails directly
exposing flames to the molten cast iron using an oxy-fuel burner,
and superheating the melt surface.
[0041] In the method of the present invention, pig-iron, iron
scrap, pig iron scrap, return pig iron, turnings, and the like are
used as the iron source of the raw material, and iron scrap in
particular is used as the main iron source.
[0042] Iron scrap of recent years has a large amount of
comparatively cheap Mn added for higher efficiency, weight saving,
higher functionality, and other improvements to iron steel
material, and the Mn must be removed. The method of the present
invention can be particularly advantageously applied to iron scrap
that includes a large amount of Mn.
[0043] Elements other than Mn, namely Pb, Zn, Ti, Al, or B, can
also be removed by the method of the present invention. The method
of the present invention is a method for removing these elements,
including Mn, from iron scrap.
[0044] The present invention is a treatment method capable of
removing 0.2 mass % or more of Mn from molten cast iron that has
been pre-melted and holded, the molten cast iron containing, for
example, 3 to 4 mass % C, 1 to 3 mass % Si, and 0.5 to 3 mass %
Mn.
[0045] In the treatment method, (.DELTA.C/.DELTA.Mn) or
(.DELTA.Si/.DELTA.Mn) is 2.5 or less, where .DELTA.Mn is the
removal efficiency per unit time of Mn, .DELTA.C is the removal
efficiency per unit time of C, and .DELTA.Si is the removal
efficiency per unit time of Si. Here, the removal efficiency per
unit time is (element amount prior to treatment-element amount
after treatment) per unit time.
[0046] It is particularly preferred that the removal rate of Mn be
ensured to be at or above 0.6 mass %/h, that .DELTA.C/.DELTA.Mn be
brought to 1, and that .DELTA.Si/.DELTA.Mn be brought to 2 or
less.
[0047] The iron scrap is pre-melted prior to the removal of
impurities, and is fed to a treatment device. The melted scrap can
be made into molten cast iron in the device as long as a treatment
device, for example, an electric furnace or other heating and
melting equipment is provided.
[0048] The temperature of the molten cast iron when the melt is fed
is preferably less than 1500.degree. C., and is more preferably
1250.degree. C. or more and less than 1500.degree. C. When the
temperature is within this range, the treatment for eliminating Mn
while suppressing the depletion of C and Si is facilitated after
the melt has been fed.
[0049] FIG. 1 shows a schematic view expressing the treatment
conditions of the molten cast iron fed to the treatment device.
[0050] An excess oxygen flame 2 is directly exposed by a burner 3
to a surface 1a of molten cast iron 1 fed to the treatment device,
and the melt surface is superheated. In FIG. 1, reference numeral 4
is acidic slag, and 5 is an oxygen-containing gas that is injected
into the molten cast iron 1.
[0051] LPG gas or LNG gas is used as the heat source for the excess
oxygen flame 2, the burner 3 preferably causes combustion while an
amount of oxygen is supplied in excess of the amount of oxygen
necessary for combustion. Examples of a supply source of excess
oxygen include air and pure oxygen gas.
[0052] The theoretical combustion ratio of fuel and oxygen (amount
of oxygen (volume).times.5/amount of fuel (volume)) is 1.5, and is
preferably 1.1 to 1.4. The excess oxygen is insufficient and Mn
elimination does not proceed when the theoretical combustion ratio
is less than 1, and the temperature of the flame does not increase
and Mn elimination does not efficiently proceed when the ratio is
1.5 or more.
[0053] The excess oxygen flame 2 is directly exposed to the surface
1a of the molten cast iron. The temperature of the surface 1a of
the molten cast iron during flame exposure is predicted to be
2000.degree. C. or more, but the temperature inside 1b of the
molten cast iron is held at 1250.degree. C. or more and less than
1500.degree. C.
[0054] Means for holding the temperature inside of the molten cast
iron within this range include means for controlling the amount of
melt and fuel used, the amount of gas 5 injected into the melt in
FIG. 1, the amount of preheat of the treatment device, and
therefore, the temperature of the refractory material in this
case.
[0055] The surface 1a of the molten cast iron is covered by the
acidic slag 4 until the melt temperature reaches 1400.degree. C. in
accordance with the start of the impurities removing treatment, but
the excess oxygen flame 2 is directly exposed to the surface 1a of
the molten cast iron, and the slag 4 on the exposed portion is
thereby eliminated. As a result, the flame 2 makes direct contact
with the surface 1a of the molten cast iron. The surface 1c that is
not exposed to the flame 2 contacts the acidic slag 4 (FIG. 1).
[0056] The melt surface is directly exposed to the flame, the
impurity removal treatment is performed without increasing the
temperature of the entire melt while the remaining melt surface is
made to be in contact with the acidic slag, and it is thought that
the depletion of C and Si can thereby be reduced while oxidation
and removal of Mn progresses.
[0057] The molten cast iron 1 is agitated by the injection of the
oxygen-containing gas 5. Air is preferably used as the
oxygen-containing gas. In addition, the injected amount is
preferably 100 (L/min) or more and 1600 (L/min) or less per melt
weight, and more preferably 200 (L/min) or more and less than 800
(L/min). The force for agitating the melt is insufficient when the
injection amount is low, and the temperature of the melt is
excessively reduced due to the heat removed by the gas when the
injection amount is too high.
[0058] In addition, the oxygen-containing gas 5 is injected from
the interior of the molten cast iron so that bubbles of the gas 5
migrate to the surface 1a of the molten cast iron, which is
directly exposed to the excess oxygen flame 2 and superheated (FIG.
1). In the treatment method of the present invention, the reaction
at the surface 1a of the molten cast iron directly exposed to the
flame 2 is important, and the surface reaction is accelerated by
injecting the oxygen-containing gas 5.
[0059] A treatment device in which the molten cast iron is
accommodated can be used as long as the device is a treatment
device having a shape in which the melt 1 is directly exposed to
the flame 2. Examples of treatment devices that can be used include
a rotary furnace, an electric furnace, a ladle, a cupola
desulfurization ladle, or a turn dish, or a combination
thereof.
[0060] In the cast iron raw material manufactured in accordance
with the present invention, Mn and other impurities can be reduced
while the depletion of useful C and Si is suppressed. It is
possible to readily manufacture a cast iron raw material having 2
to 4 mass % C, 0.5 to 4 mass % Si, 0.1 to 3 mass % Mn (preferably
0.1 to 1 mass % Mn), 0.0001 to 0.03 mass % Pb (preferably 0.0001 to
0.02 mass % Pb), 0.0001 to 1.0 mass % Zn (preferably 0.0001 to 0.02
mass % Zn), 0.001 to 0.2 mass % Ti, 0.0001 to 0.5 mass % Al
(preferably 0.0001 to 0.2 mass % Al), 0.0001 to 0.04 mass % B
(preferably 0.0001 to 0.01 mass % B), and the remainder being Fe
and inevitable impurities.
First Embodiment: Example Using a Small Rotary Furnace-Type
Impurity Removal Device
[0061] Impurities in cast iron raw material were removed by a small
rotary furnace-type impurity removal device shown in FIG. 2. In the
small rotary furnace-type impurity removal device 6, the burner 3
is disposed in the upper part of the device so that the flame 2 of
a burner is directly exposed to the surface of the melt 1
accommodated inside of a furnace 6a to superheat the surface. The
device 6 has the shape of a melt holding furnace disposed in front
of a general cast iron melting furnace, but this shape is not
necessarily required as long as the furnace has a shape in which
the melt 1 can be directly exposed to the flame 2 and superheated.
In FIG. 2, reference numeral 10 indicates an exhaust duct.
[0062] After the device is preheated, the pretreated molten cast
iron shown in FIG. 1 is separately melted in advance, and fed to
the removal device 6. The treatment was carried out with 50 kg of
melt by weight.
TABLE-US-00001 TABLE 1 Element C Si Mn Pb Zn Al Ti B Fe Content
Pre- 3.71 2.25 0.75 0.01 0.007 0.01 0.015 0.005 Remainder (mass %)
treatment Post- 3.5 1.80 0.20 0.0002 0.0003 0.0004 0.003 0.0002
Remainder treatment
[0063] The surface of the pre-treated molten cast iron shown in
TABLE.1 was directly exposed to the burner flame and an impure
element removal experiment was performed. The experiment conditions
are shown below.
[0064] (1) Theoretical combustion ratio (.lamda.) of fuel and
oxygen: three levels of 1, 1.2, and 1.5
[0065] (2) Added amount of iron oxide (% relative to melt amount):
two levels of 0 and 2 mass %
[0066] (3) Treatment time: 80 min (measurement sample taken every
10 min)
[0067] (4) Measured elements: Mn, C, Si, Pb, Zn, Ti, Al, B
[0068] The melt temperature at the start of the experiment was
1300.degree. C., and the heat input was adjusted so that the melt
temperature after about one hour was 1450.degree. C. A test
specimen for chemical composition analysis was collected, and at
the same time the melt temperature, the gas composition at the
exhaust gas outlet, and the amount of dissolved oxygen in the melt
was measured. The melt surface was covered by acidic slag until the
melt temperature reached 1400.degree. C. The composition of the
treated cast iron raw material is shown in Table 1, and the
experiment results are shown in FIGS. 3 to 5.
[0069] FIG. 3 shows the C, Si, and Mn contents in the melt that
change with the progression of the treatment period. The initial
amount of Mn decreased from 0.8 mass % to 0.2 mass %, and the Mn
amount was successfully removed with best efficiency when treatment
was performed at a theoretical combustion ratio (.lamda.) of fuel
and oxygen of 1.2, and 2 mass % iron oxide was added (black
triangular mark in FIG. 3).
[0070] The removal efficiency at this time was 0.6 wt %/h, and
.DELTA.C/.DELTA.Mn was successfully brought to 0.37 and
.DELTA.Si/.DELTA.Mn to 1.7 or less, where the depletion amount of
carbon is LC and the depletion amount of silicon is in relation to
the Mn removal amount .DELTA.Mn. In addition, the removal
efficiency worsened to 0.45 mass %/h when the ratio of fuel and
oxygen (.lamda.) was less than 1. The increase in temperature of
the melt was suppressed due to cooling from the excess oxygen, and
the treatment time was increased along with an increase in the
required fuel amount when the ratio of fuel and oxygen exceeded
1.5.
[0071] FIG. 4 shows the change in Pb, Zn, Ti, Al, and B contents in
the melt that change with the progression of the treatment
period.
[0072] FIG. 5 shows the influence of Si amount, swing, and
air-bubbling in the molten cast iron on the Mn elimination
efficiency. The efficiency increases by reducing the amount of Si,
and it is apparent that the Mn elimination efficiency is further
increased by swing and air-bubbling.
[0073] The flame temperature of the pure oxygen burner used in the
small rotary furnace-type impurity removal device is estimated to
exceed 2000.degree. C. However, the measured temperature of the
molten cast iron was in a range of 1250.degree. C. or more to less
than 1500.degree. C., and did not rise to 2000.degree. C. It is
thought that a high-temperature state is reached only on the
extreme surface of the melt. The melt surface is directly exposed
to the flame and superheated, whereby the impurities are removed
without increasing the temperature of the entire remaining melt,
and it is thought that the depletion of C and Si is suppressed as
oxidation and removal of Mn progresses.
Second Embodiment: Example Using an Actual Rotary Furnace-Type
Impurity Removal Device
[0074] FIG. 6 shows an actual rotary furnace-type impurity removal
device. In an actual rotary furnace-type impurity removal device 7,
three burners 3 are disposed in the upper part of the device so
that the flame 2 of the burners 3 is directly exposed to the
surface of the melt 1 accommodated in a furnace 7a. In addition,
the gas 5 is blown into the lower part of the flame 3 from the
interior of the melt 1 by two lances 9. Impurities in the cast iron
raw material were removed using this device. The treatment was
carried out with 500 kg and 1000 kg of melt by weight fed to the
treatment device 7. The experiment conditions are shown below.
[0075] (1) Theoretical combustion ratio (.lamda.) of fuel and
oxygen: 1.2
[0076] (2) Amount of air injected by the lances: 100 L/min, 200
L/min per lance
[0077] (3) Treatment time: 120 min (measurement sample taken every
10 min)
[0078] (4) Measured element: Mn
[0079] The results of the experiment are shown in FIG. 7.
[0080] The Mn removal efficiency was 0.1 mass %/h when air was not
injected, 0.4 mass %/h when air was injected at 100 L/min, and 1.0
mass %/h when air was injected at 200 L/min, respectively.
Agitation of the melt by the gas improved the Mn removal
efficiency.
[0081] Mn prior to treatment was 0.7 mass % and after treatment was
0.2 mass % when the theoretical ratio .lamda. of fuel and oxygen
was 1.2 and the amount of injected air was 200 L/min, and the time
necessary for treatment was 30 min per 500 kg. In the same way, C
prior to treatment was 3.7 mass % and after treatment was 3.4 mass
%, and Si prior to treatment was 2.7 mass % and after treatment was
2.1 mass %. At this time, .DELTA.C/.DELTA.Mn was 0.6, and
.DELTA.Si/.DELTA.Mn was 1.2.
Third Embodiment: Example Using a Ladle-Type Impurity Removal
Device
[0082] FIG. 8 shows a ladle-type impurity removal device. In a
ladle-type impurity removal device 8, the burner 3 is disposed
above an ordinary ladle for cast iron so that the flame 2 of the
burner 3 is directly exposed to the surface of the melt 1
accommodated in a furnace 8a. In addition, the gas 5 is blown into
the bottom part of the flame 2 from inside of the melt 1 by the
lance 9. Impurities in cast iron raw materials were removed using
this device. The treatment was carried out with 500 kg of melt by
weight fed to the treatment device 8. The experiment conditions are
shown below.
[0083] (1) Theoretical combustion ratio (.lamda.) of fuel and
oxygen: 1.2
[0084] (2) Amount of air injected by the lance: three levels of 100
L/min, 200 L/min, 400 L/min
[0085] (3) Treatment time: 60 min
[0086] (4) Measured element: Mn
[0087] The experiment results are shown in FIG. 9. FIG. 9 shows the
agitation efficiency of the melt based on the injection of air, the
agitation efficiency being converted to agitation energy (W/h) in
the following formula. The Mn elimination efficiency obtained in
the Second Embodiment is shown as well.
. ( W / t ) = 6.18 V g ( N m 3 / min ) T e ( K ) M e ( t ) { ln ( 1
+ h .theta. ( m ) 1.46 .times. 10 - 3 ( Pa ) ) + .eta. ( 1 + T g (
K ) T e ( K ) ) } [ Mathematical Formula 1 ] ##EQU00001##
[0088] where, {dot over (.epsilon.)}: agitation energy of melt
(W/t) [0089] V.sub.g: amount of gas (volume) [0090] T.sub.e:
temperature of melt (K) [0091] M.sub.e: amount of melt (t) [0092]
h.sub.0: length of lance (m) [0093] .theta.: constant [0094]
T.sub.g: temperature of gas (K)
[0095] FIGS. 10 and 11 show a summary of the results obtained in
each of the embodiments. FIG. 10 is a graph showing the results in
which the Mn removal efficiency is sorted by the specific surface
area of the melt per weight of molten cast iron, and FIG. 11 is a
graph comparing the depletion of C and Si to the Mn removal
efficiency.
[0096] The specific surface area of the melt per weight of cast
iron in an actual rotary furnace-type impurity removal device
(Second Embodiment) is 1.7 m.sup.2/t, the specific surface area of
the melt is 3 m.sup.2/t when the treated weight using the small
rotary furnace-type impurity removal device (First Embodiment) is
50 kg, and the specific surface area of the melt using the
ladle-type impurity removal device (Third Embodiment) is 0.2
m.sup.2/t.
[0097] It is apparent from FIG. 10 that the Mn removal efficiency
improves as the specific surface area of the melt increases.
[0098] FIG. 11 shows the values of .DELTA.C/.DELTA.Mn and
.DELTA.Si/.DELTA.Mn when the depletion amount of carbon LC and the
depletion amount of silicon .DELTA.Si are set in relation to the Mn
removal efficiency (.DELTA.Mn/h). When consideration is given to
the practicality of the Mn elimination treatment, the removal
efficiency is preferably 0.55 mass %/h or greater, and at this
time, the area in which .DELTA.C/.DELTA.Mn and .DELTA.Si/.DELTA.Mn
are about 1 or less and about 2 or less, respectively, is the
preferred range that is thought to be the practical region for
operation by a removal device.
INDUSTRIAL APPLICABILITY
[0099] The method for removing impurities of the present invention
is capable of removing Mn or other impure elements from molten cast
iron, and obtaining pure molten cast iron in which the depletion of
C and Si, which are essential elements in cast iron, is suppressed.
This method can therefore be applied to the field of spheroidal
graphite cast iron and the like in which Mn or other impurities
considerably hinder elongation and toughness of cast iron raw
material.
EXPLANATION OF REFERENCE NUMERALS AND SYMBOLS
[0100] 1 Molten cast iron [0101] 2 Excess oxygen flame [0102] 3
Burner [0103] 4 Acidic slag [0104] 5 Gas injected into molten cast
iron [0105] 6 Small rotary furnace-type impurity removal device
[0106] 7 Actual rotary furnace-type impurity removal device [0107]
8 Ladle-type impurity removal device [0108] 9 Lance [0109] 10
Exhaust duct
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