U.S. patent application number 11/508189 was filed with the patent office on 2006-12-14 for method of producing stainless steel by re-using waste material of stainless steel producing process.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho ( Kobe Steel, Ltd). Invention is credited to Itsuo Miyahara, Hiroshi Sugitatsu.
Application Number | 20060278042 11/508189 |
Document ID | / |
Family ID | 29717463 |
Filed Date | 2006-12-14 |
United States Patent
Application |
20060278042 |
Kind Code |
A1 |
Sugitatsu; Hiroshi ; et
al. |
December 14, 2006 |
Method of producing stainless steel by re-using waste material of
stainless steel producing process
Abstract
A method of producing stainless steel includes the steps of
melting a raw material in an electric furnace to form molten steel,
and then refining the molten steel by a refining furnace to produce
stainless steel in a stainless steel producing process. In the
method, a carbonaceous reducing agent is added to a zinc-containing
waste material produced in the stainless steel producing process,
the resultant mixture is agglomerated by a briquette press to form
agglomerates incorporated with a carbonaceous material, the
agglomerates incorporated with the carbonaceous material are heated
in a rotary hearth furnace to reduce and evaporate zinc to form
dezincified agglomerates, and then the dezincified agglomerates are
charged as a coolant in an oxidation period of the refining
furnace.
Inventors: |
Sugitatsu; Hiroshi;
(Kobe-shi, JP) ; Miyahara; Itsuo; (Kobe-shi,
JP) |
Correspondence
Address: |
C. IRVIN MCCLELLAND;OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho (
Kobe Steel, Ltd)
Kobe-shi
JP
|
Family ID: |
29717463 |
Appl. No.: |
11/508189 |
Filed: |
August 23, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10438013 |
May 15, 2003 |
|
|
|
11508189 |
Aug 23, 2006 |
|
|
|
Current U.S.
Class: |
75/503 ; 75/531;
75/567 |
Current CPC
Class: |
C21C 5/54 20130101; Y02P
10/216 20151101; C21B 13/0046 20130101; C22B 1/245 20130101; Y02P
10/232 20151101; C22B 7/02 20130101; Y02P 10/20 20151101; C21C
5/5264 20130101; C21C 5/005 20130101; C21C 5/36 20130101; C22B
1/005 20130101; C21C 5/56 20130101; C22B 7/001 20130101; C22B 1/248
20130101; Y02P 10/23 20151101; C21B 13/006 20130101; C22B 19/30
20130101; C22B 5/16 20130101; Y02P 10/214 20151101; C22B 1/24
20130101 |
Class at
Publication: |
075/503 ;
075/531; 075/567 |
International
Class: |
C21C 7/04 20060101
C21C007/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 18, 2002 |
JP |
2002-177005 |
Claims
1. A method of producing stainless steel comprising: a stainless
steel producing step of melting a raw material to form molten steel
in a melting furnace, and then refining the molten steel to produce
stainless steel in a refining furnace; a reducing agent adding step
of adding a carbonaceous reducing agent to a zinc-containing waste
material produced in the stainless steel producing step to form a
mixture; a heat treatment step of heating the mixture to evaporate
and remove zinc and form a dezincified mixture; and a charging step
of charging, the dezincified mixture as a coolant into the refining
furnace, and optionally the melting furnace, in the stainless steel
producing step, wherein the dezincified mixture is charged as the
coolant into the refining furnace in a final stage of an oxidation
period or/and an initial stage of a reduction period in the
stainless steel producing step.
Description
[0001] This is a continuation application of U.S. Application Ser.
No. 10/438,013, filed May 15, 2003.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a method of producing
stainless steel by re-using waste materials such as dust and scales
produced in a stainless steel producing process.
[0004] 2. Description of the Related Art
[0005] Stainless steel is generally produced by melting scraps, and
raw materials such as Fe--Cr, Fe--Ni, and Ni metal in an electric
furnace, and then refining molten steel by a refining furnace
(stainless steel producing process). Conventionally, the electric
furnace corresponds to a melting period in which raw materials are
melted, but an oxidation period (referred to as a
"pre-decarbonization period") in which molten steel is decarbonized
by oxygen blowing may be further provided. The refining furnace
corresponds to the oxidation period in which molten steel is
decarbonized by oxygen blowing, a reduction period in which Cr
oxidized in the oxidation period and transferred into slag is
re-reduced to a metal which is recovered to the molten steel, and a
finish refining period in which the molten steel is deoxidized, and
the steel components and temperature are controlled. The exhaust
gas discharged from the electric furnace and refining furnace (VOD,
RH, AOD, MRP, etc.) contains dust. Since the dust contains
components such as Fe, Ni, Cr, and the like, the dust is preferably
re-used as a raw material. Moreover, the dust contains Cr.sup.6+,
and thus a great cost is required for disposing the dust.
Therefore, from an economical viewpoint, it is desirable to recycle
the dust. However, the dust contains Zn mainly derived from scraps,
and thus when the dust is returned to the electric furnace and
refining furnace without any treatment, Zn is reduced and
evaporated, and scattered in the exhaust gas, thereby concentrating
Zn in the dust. The dust containing concentrated Zn adheres to the
inner surfaces of a throat and exhaust gas piping to cause the
problem of coating the inner surfaces. In addition, oxidation and
reduction of Zn are repeated at each time of recycle of dust,
thereby causing the problem of deteriorating energy efficiency.
Reduction of Zn contained in the dust is an endothermic reaction,
and is effected in a furnace, thereby consuming heat energy. On the
other hand, oxidation of Zn is an exothermic reaction and is
effected in an exhaust gas system, thereby uselessly discharging
most of the heat energy of the exhaust gas.
[0006] Therefore, a method has been proposed in which dust is
re-used after it is reduced in a process apart from the stainless
steel producing process, and then returned to the stainless steel
producing process.
Prior Art 1
[0007] Japanese Unexamined Patent Application Publication No.
56-93834 discloses a method in which a carbonaceous reducing agent
is added to mill scales, dust and sludge, the resultant mixture is
pelletized, and heated and reduced in a rotary hearth furnace to
produce metal-containing pellets, and then the metal-containing
pellets are melted by an electric arc furnace for producing pig
iron to separate and recover valuable metals such as Fe, Ni, Cr,
Mo, etc. The recovered valuable metals are contained in a molten
metal, and the molten metal is poured into a mold of a continuous
casting machine from the electric arc furnace to form metal lumps.
This publication discloses an example (example III) in which metal
lumps containing 2.95% by mass of carbon are added to an electric
arc furnace for producing stainless steel.
Prior Art 2
[0008] Japanese Unexamined Patent Application Publication No.
9-209047 discloses a method of re-using a waste material of a
stainless steel producing process, the method comprising a
pelletization step of pelletizing a mixture of coke and a
chromium-containing blend obtained by adding an appropriate amount
of chromium ore to a chromium-containing waste material produced in
the stainless steel producing process to produce pellets, a
reduction step of heating, by a combustion gas, the pellets allowed
to stand on a hearth of a rotary hearth furnace to produce
chromium-containing iron pellets with minimizing breakdown and fine
generation, a waste heat recovering step of recovering, as steam,
sensible heat possessed by an exhaust gas of the reduction step,
and a zinc-containing dust recovering step of separating and
collecting zinc-containing dust produced in the reduction step and
contained in the exhaust gas discharged from the waste heat
recovering step to recover the zinc-containing dust. This
publication also discloses an example in which chromium-containing
iron pellets are melted in an electric furnace, and used as a part
of raw materials for producing chromium-containing pig iron.
[0009] In the above-described prior arts 1 and 2, in heating the
pellets in the rotary hearth furnace, Zn contained in the pellets
is reduced with the carbonaceous reducing agent, and evaporated and
removed from the pellets. Therefore, even if the pellets after
reduction are supplied to the electric furnace, the dust is not
enriched with Zn, thereby preventing the problem of coating in an
exhaust gas system.
[0010] However, in the above-described prior arts 1 and 2, the
pellets after reduction are charged into the electric melting
furnace, and used for producing chromium-containing pig iron having
a high carbon content. Therefore, the content of carbon remaining
in the pellets (metal lumps or chromium-containing iron pellets)
after reduction is relatively high. Namely, as described above, in
the example of the prior art 1, the carbon content of the metal
lumps is 2.95% by mass. In example 2 of the prior art 2, the carbon
content of the chromium-containing iron pellets is not specified,
but 4.7% by mass of carbon is present in 125 parts by mass of
chromium-containing pig iron, and the pig iron is produced from 211
parts by mass of chromium-containing iron pellets. Therefore, the
carbon content of the chromium-containing iron pellets, which is
estimated in consideration of the carbon content consumed by
chromium reduction in the electric furnace, is 2.8% by mass or
more. The chromium-containing pig iron is decarbonized to a target
carbon level in a next oxidation period, and then reduced and
finish-refined to produce stainless steel. Since decarbonization is
performed by blowing oxygen into molten steel, Cr is oxidized with
the progress of decarbonization, and is transferred into slag.
After decarbonization is completed, Fe--Si is added as a reducing
agent in the reduction period to reduce a Cr oxide to return the
oxide to metal Cr, thereby recovering Cr in the molten steel.
[0011] However, Cr contained in the chromium-containing iron
pellets is not sufficiently reduced by heating in the rotary hearth
furnace (generally, a Cr metallization degree is about 40% or
less), and most of Cr remains in an oxide form. The unreduced Cr
oxide is metallized by reduction with carbon remaining in the
pellets and carbon contained in the molten steel in the electric
furnace in the melting period, and recovered in the molten steel.
However, a part of the Cr oxide remains in the slag and is
discarded together with the slag (electric furnace slag). Cr
recovered in the molten steel is partially oxidized in a subsequent
oxidation period (or pre-decarbonization period), and transferred
into the slag. The Cr in the slag is again reduced in a subsequent
reduction period and recovered in the molten steel, but a part of
Cr remains in the slag, and is discarded together with the slag
(refining furnace slag). In this way, the unreduced Cr oxide
contained in the chromium-containing iron pellets is reduced in the
melting period, and then oxidized in the oxidation period (or
pre-decarbonization period), and further reduced in the reduction
period. Therefore, an endothermic reduction reaction requires
excess reduction energy, thereby causing an energy loss. Also, Cr
remains in both the electric furnace slag and the refining furnace
slag, thereby causing the problem of a low yield of Cr recovered to
the molten steel.
SUMMARY OF THE INVENTION
[0012] Accordingly, it is an object of the present invention to
provide a method of producing stainless steel capable of decreasing
the energy required for reducing Cr, and increasing a Cr yield of
molten steel when a waste material such as dust produced in a
stainless steel producing process is re-used.
[0013] According to the present invention, a method of producing
stainless steel comprises a stainless steel producing step of
melting a raw material to form molten steel, and then refining the
molten steel to produce stainless steel, a reducing agent adding
step of adding a carbonaceous reducing agent to a zinc-containing
waste material produced in the stainless steel producing step to
form a mixture, a heat treatment step of heating the mixture to
evaporate and remove zinc and form a dezincified mixture, and a
charging step of charging the dezincified mixture as a coolant into
a furnace in the stainless steel producing step.
[0014] The method of producing stainless steel further comprises a
step of agglomerating the mixture to form agglomerates incorporated
with a carbonaceous material.
[0015] In the method of producing stainless steel, the agglomerates
incorporated with the carbonaceous material are heated in the heat
treatment step to evaporate and remove zinc, to form dezincified
agglomerates, and the charging step comprises charging the
dezincified agglomerates as a coolant into the furnace in the
stainless steel producing step.
[0016] In the method of producing stainless steel, the amount of
surplus carbon in the mixture is controlled by controlling the
amount of the carbonaceous reducing agent added so that the amount
of residual carbon in the dezincified mixture is 2% by mass or
less.
[0017] In the method of producing stainless steel, the amount of
surplus carbon in the agglomerates incorporated with the
carbonaceous material is controlled by controlling the amount of
the carbonaceous reducing agent added so that the amount of
residual carbon in the dezincified agglomerates is 2% by mass or
less.
[0018] The method of producing stainless steel further comprises a
dezincification agglomeration step of agglomerating the dezincified
mixture to form dezincified agglomerates.
[0019] In the method of producing stainless steel, the charging
step comprises charging the dezincified agglomerates as a coolant
into the furnace in the stainless steel producing step.
[0020] In the method of producing stainless steel, the amount of
surplus carbon in the mixture is controlled by controlling the
amount of the carbonaceous reducing agent added so that the amount
of residual carbon in the dezincified agglomerates is 2% by mass or
less.
[0021] In the method of producing stainless steel, the molten steel
is agitated after the dezincified mixture is charged.
[0022] In the method of producing stainless steel, the molten steel
is agitated after the dezincified agglomerates are charged.
[0023] In the method of producing stainless steel, the dezincified
mixture is charged as the coolant into the furnace in an oxidation
period or/and reduction period in the stainless steel producing
step.
[0024] In the method of producing stainless steel, the dezincified
agglomerates are charged as the coolant into the furnace in the
oxidation period or/and reduction period in the stainless steel
producing step.
[0025] In the method of producing stainless steel, the dezincified
mixture is charged-as the coolant into the furnace in a last stage
of the oxidation period or/and an initial stage of the reduction
period in the stainless steel producing step.
[0026] In the method of producing stainless steel, the dezincified
agglomerates are charged as the coolant into the furnace in the
last stage of the oxidation period or/and the initial stage of the
reduction period in the stainless steel producing step.
[0027] In the present invention, a carbonaceous reducing agent is
added to a zinc-containing waste material produced in the stainless
steel producing step to form agglomerates incorporated with a
carbonaceous material, and the agglomerates incorporated with the
carbonaceous material are heated to evaporate and remove zinc,
producing dezincified agglomerates having a residual carbon content
of 2% by mass or less.
[0028] In the present invention, a carbonaceous reducing agent is
added to a zinc-containing waste material produced in the stainless
steel producing step to form a mixture, the mixture is heated to
evaporate and remove zinc, producing a dezincified mixture, and
then the dezincified mixture is agglomerated to produce dezincified
agglomerates having a residual carbon content of 2% by mass or
less.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is an equipment flowchart illustrating an example of
a stainless steel producing process according to an embodiment of
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] The present invention will be described in detail below.
[0031] An embodiment of the present invention will be described in
further detail below with reference to the drawing.
[0032] In the embodiment of the present invention, an example of a
stainless steel producing process 1 is described, in which an
electric furnace 11 for melting raw materials (main raw material E
and fluxing agent F) to form molten steel G, and an AOD (Argon
Oxygen Decarbonization) furnace 12 serving as a refining furnace
for refining the molten steel G are provided, as shown in FIG.
1.
[0033] A zinc-containing waste material A such as an electric
furnace dust generated from the stainless steel producing process 1
is mixed with a carbonaceous reducing agent B such as coal
(reducing agent adding step), and the resultant mixture is
agglomerated by an agglomeration machine 2 such as a briquette
press or the like (carbonaceous material adding and agglomeration
step). Besides the electric furnace dust, mill scales, mill sludge,
AOD dust or other refining furnace dust, or an appropriate mixture
thereof may be used as the zinc-containing waste material A.
Besides coal, coke fines, charcoal, waste toner, or other carbides,
or an appropriate mixture thereof may be used as the carbonaceous
reducing agent B. Also, a secondary raw material and binder may be
added according to demand. Besides a compression molding machine
such as the briquette press, a tumbling granulator, an extrusion
molding machine, or the like may be used as the agglomeration
machine 2.
[0034] The resultant agglomerates C incorporated with the
carbonaceous material are charged into a rotary hearth furnace 3
serving as a reducing furnace. As the reducing furnace 3, a
multiple hearth furnace, a rotary kiln, or the like, as well as the
rotary hearth furnace, may be used. When the agglomerates C
incorporated with the carbonaceous material have a high moisture
content, the agglomerates C may be dried with a dryer not shown in
the drawing before they are charged into the reducing furnace
3.
[0035] The agglomerates C incorporated with the carbonaceous
material are heated to 1100 to 1400.degree. C. in the reducing
furnace 3 to evaporate and remove heavy metals such as Zn, Pb, and
the like by reduction. At the same time, metal compounds of Fe, Ni,
Cr, Mo, and the like are reduced in a solid state, and metallized
to obtain dezincified agglomerates D (heat treatment step).
However, the metallization degree of Cr is not so high, and even
when the amount of the carbonaceous reducing agent B added to the
agglomerates C incorporated with the carbonaceous material, and the
heating temperature in the reducing furnace 3 are appropriately
controlled, the metallization degree is about 40%. On the other
hand, the metallization degrees of Fe and Ni can be increased to
90% or more by controlling the amount of the carbonaceous reducing
agent B added to the agglomerates C incorporated with the
carbonaceous material, and the heating temperature in the reducing
furnace 3.
[0036] In the stainless steel producing process 1, the main raw
material E comprising scraps, Fe-Cr, Fe-Ni, and Ni metal, and the
fluxing agent F such as calcined lime are charged into the electric
furnace 11, and the resultant mixture is melted by arc heating to
produce the molten steel G. When the AOD furnace 12 is used in the
subsequent step, the electric furnace 11 is charged only with the
melting period of simply melting the raw materials. Next, the
molten steel G is transferred to the AOD furnace 12 to refine the
molten steel G. The AOD furnace 12 is charged with the oxidation
period in which Ar and oxygen are blown into the molten steel G to
decarbonize the molten steel G, the reduction period in which the
molten steel G is agitated by blowing only Ar to metallize Cr,
which is oxidized in the oxidation period and transferred into
slag, by reduction with C contained in the molten steel G, and to
recover Cr into the molten steel G, and the finish refining period
in which a deoxidizing agent (reducing agent) such as Fe--Si and
alloy elements are added to the molten steel G, and the molten
steel G is agitated by blowing only Ar to deoxidize the molten
steel G, and to control the components and temperature of the
molten steel G. The molten steel G is refined by the AOD furnace 12
to produce stainless steel H which is then transferred to a next
casting step 4.
[0037] The dezincified agglomerates D obtained by the rotary hearth
furnace 3 are charged in the oxidation period and/or reduction
period of the AOD furnace 12 (charging step). Since zinc is
sufficiently removed from the dezincified agglomerates D before the
dezincified agglomerates D are charged into the AOD furnace 12, Zn
is not concentrated in dust, thereby causing no problem of coating
in the exhaust gas system, unlike in the prior arts 1 and 2.
[0038] Furthermore, since the dezincified agglomerates D are
charged directly in the oxidation period and/or reduction period of
the AOD furnace 12, not in the melting period of the electric
furnace 11, Cr derived from the dezincified agglomerates D does not
remain in the electric furnace slag, thereby causing the effect of
increasing the Cr yield as compared with the prior arts 1 and 2. In
addition, a useless path in which a Cr oxide contained in the
dezincified agglomerates D is reduced and then again oxidized is
omitted, to eliminate the need for excess reduction energy, thereby
causing the effect of improving energy efficiency.
[0039] The dezincified agglomerates D are more preferably charged
in a last stage of the oxidation period (near the time when oxygen
blowing for decarbonization is finished) and/or an initial stage of
the reduction period (near the time when the deoxidizing agent is
added) because re-oxidation of Cr can be further prevented.
[0040] Furthermore, the amount of the carbonaceous reducing agent B
(i.e., the amount of surplus carbon) in the agglomerates C
incorporated with the carbonaceous material is preferably decreased
to a level which does not cause an excessive decrease in the
dezincification degree of the agglomerates (dezincified
agglomerates) D after heating in the rotary hearth furnace 3. This
is because with the large amount of the carbonaceous reducing agent
B contained in the agglomerates C incorporated with the
carbonaceous material, the amount of residual carbon in the
dezincified agglomerates D is increased. Therefore, when the
dezincified agglomerates D are used as the coolant in the oxidation
period of the AOD furnace 12, the amount of oxygen used for
decarbonization is increased, and the oxidation period is extended
to deteriorate productivity. The possible reason why the oxidation
period is extended is that with the molten steel having a low
carbon content (for example, 0.4% by mass or less), a
rate-determining process of decarbonization reaction is thought to
be a process of diffusing C in the molten steel, and carbon in the
dezincified agglomerates D slowly diffuses into the molten steel as
compared with C contained in the molten steel. Also, when carbon
diffuses into the molten steel, the carbon content of the
dezincified agglomerates D is higher than that in the surrounding
molten steel, thereby decreasing the diffusion rate.
[0041] In this way, when the amount of the carbonaceous reducing
agent B in the agglomerates C incorporated with the carbonaceous
material is limited, the metallization degree of iron in the
dezincified agglomerates D is decreased, but the effect of the
coolant is increased due to an increase in the amount of iron oxide
in the dezincified agglomerates D. In this case, the cooling effect
is increased to about two times as high as the cooling effect of
ordinary scraps. Furthermore, as the amount of residual carbon
decreases, strength of the dezincified agglomerates D increases to
decrease the fine generation at the time of transport and storage,
or charging into the refining furnace, thereby improving the
yield.
[0042] Although the dezincified agglomerates D charged as the
coolant in the oxidation period of the AOD furnace 12 are dissolved
in the molten steel, unreduced Cr contained in the dezincified
agglomerates D can be efficiently recovered by charging a reducing
agent such as Fe--Si and strongly agitating with Ar gas in the
reduction period in the subsequent step.
[0043] Besides the AOD furnace, a VOD (Vacuum Oxygen
Decarbonization) furnace, a MRP (Metal Refining Process) furnace,
or the like can be used as the refining furnace 12. When a VOD
furnace is used as the refining furnace 12, the oxidation period
(pre-decarbonization period) and the reduction period are provided
after the melting period in the electric furnace 11 so that the VOD
furnace is charged only with the finish refining period. Therefore,
the dezincified agglomerates D are charged as the coolant in the
oxidation period (pre-decarbonization period) and/or reduction
period in the electric furnace 11. In this case, unreduced Cr in
the dezincified agglomerates is reduced and recovered into the
molten steel in the reduction period of the next step in the
electric furnace 11, and then the molten steel is transferred to
the VOD furnace. In the finish refining period of the VOD furnace,
the amount of the coolant used is small in order to suppress Cr
oxidation, and the Cr yield is lower than that in the AOD furnace.
Therefore, in the use of the VOD furnace, the effect of the present
invention is smaller than that in the use of the AOD furnace.
[0044] The method described in this embodiment comprises
agglomerating a mixture of the zinc-containing waste material A and
the carbonaceous reducing agent B, and then heat-treating the
agglomerates in the reducing furnace 3 to obtain the dezincified
agglomerates D. However, the mixture of the zinc-containing waste
material A and the carbonaceous reducing agent B may be charged
into the reducing furnace 3 without agglomeration, and then
heat-treated in the reducing furnace 3. Also, the dezincified
mixture obtained by heat treatment may be charged as the coolant.
Furthermore, the dezincified mixture after heat treatment may be
agglomerated to form the dezincified agglomerates D
(dezincification agglomeration step). The dezincified agglomerates
may be charged as the coolant.
[0045] The dezincified agglomerates D and/or dezincified mixture
can be used as the coolant in the oxidation period (or the
pre-decarbonization period) and/or the reduction period, and also
used as the main raw material and/or additional raw material of the
electric furnace 11 and the refining furnace 12.
EXAMPLES
Example 1
[0046] Coal was added to a mixture of electric furnace dust
generated from the stainless steel producing process and mill
scales, and the resultant mixture was agglomerated into
pillow-shaped agglomerates of 21 mm.times.37 mm.times.9 mm by a
briquette press, to produce agglomerates incorporated with a
carbonaceous material having the composition shown in Table 1.
TABLE-US-00001 TABLE 1 (% by mass) Agglomerates incorporated with
carbonaceous material Amount of T. Fe M. Fe FeO C surplus carbon
37.28 4.78 20.21 8.48 T. Ni M. Ni T. Cr M. Cr Zn -1.26 4.048 0.96
1.948 0.35 2.128
[0047] The amount of surplus carbon is defined as follows:
[0048] Amount of surplus carbon (% by mass)=[amount (% by mass) of
carbon in the agglomerates incorporated with the carbonaceous
material]-[amount (% by mass) of oxygen combined with Fe, Ni and Zn
contained in the agglomerates incorporated with the carbonaceous
material].times.12/16
[0049] When the mixture is used without agglomeration, the amount
of surplus carbon is defined as follows:
[0050] Amount of surplus carbon (% by mass)=[amount (% by mass) of
carbon in the mixture]-[amount (% by mass) of oxygen combined with
Fe, Ni and Zn contained in the mixture].times.12/16
[0051] The agglomerates incorporated with the carbonaceous material
were heated in a small heating furnace at various temperatures in
the range of 1150.degree.C. to 1350.degree.C., and the compositions
of the agglomerates (dezincified agglomerates) incorporated with
the carbonaceous material after heating were measured by chemical
analysis to determine the metallization degree of each metal and
the dezincification degree. The heating atmosphere was a nitrogen
atmosphere, and the heating time was 5 to 8 minutes. Table 2 shows
the compositions of the agglomerates (dezincified agglomerates)
incorporated with the carbonaceous material after heating, and
Table 3 shows the metallization degree and the dezincification
degree. TABLE-US-00002 TABLE 2 Heating Test Temperature Composition
(% by mass) No. (.degree. C.) T. Fe M. Fe C T. Ni M. Ni T. Cr M. Cr
Zn SM-1 1150 50.21 28.77 2.24 6.524 6.48 2.173 0.26 1.099 SM-2 1200
52.65 29.51 1.90 6.467 6.42 2.133 0.20 0.563 SM-3 1250 49.94 31.89
1.64 7.416 7.21 2.264 0.17 0.427 SM-4 1300 52.95 31.71 1.55 6.628
6.60 2.198 0.18 0.470 SM-5 1350 52.34 31.26 0.58 6.530 6.53 3.010
0.07 0.212
[0052] TABLE-US-00003 TABLE 3 Test Metallization degree (%)
Dezincification Crush strength No. Fe Ni Cr degree (%) (kg/B) SM-1
57.30 99.33 11.97 61.65 67 SM-2 56.05 99.27 9.38 81.27 71 SM-3
63.86 97.22 7.51 85.02 100 SM-4 59.89 99.58 8.19 84.45 121 SM-5
59.72 100.00 2.33 92.90 160
[0053] Tables 2 and 3 indicate that the amount of residual carbon
and the dezincification degree of the dezincified agglomerates vary
with the heating temperature. At a heating temperature of
1200.degree.C. or more, the dezincification degree is desirably 80%
or more. It is also found that the amount of residual carbon
decreases as the heating temperature increases. The amount of
residual carbon is preferably smaller than an amount necessary for
reducing iron oxide, nickel oxide and chromium oxide remaining in
the dezincified agglomerates. With a carbon residue content
exceeding the necessary amount, excess oxygen is required for
removing (decarbonizing) carbon from the molten steel in the
stainless steel producing process.
[0054] In this example, stainless steel was produced in the
producing process comprising an electric furnace and AOD furnace by
using the dezincified agglomerates of SM-4, and the behavior of Cr
contained in the dezincified agglomerates was measured with
reference to operation data such as a Cr yield of an actual
machine. As a result of the measurement, assuming that the Cr yield
(the ratio of Cr remaining in stainless steel to Cr contained in
the dezincified agglomerates) of the dezincified agglomerates used
as part of a raw material in an electric furnace is 100, the Cr
yield of the dezincified agglomerates used as a coolant in a final
stage of the oxidation period of the AOD furnace is 105, and thus
the Cr yield is improved. Also, assuming that the energy necessary
for reducing Cr, which is contained in the dezincified agglomerates
used as part of the raw material in the electric furnace and which
remains in stainless steel, is 100, the reduction energy of the
agglomerates used as the coolant in the oxidation period of the AOD
furnace is 95, and the energy consumption is decreased. This is
because when the dezincified agglomerates are charged as a part of
the raw material into the electric furnace, a part of chromium
reduced in the electric furnace is again oxidized in the oxidation
period of the AOD furnace, and thus re-reduction is required after
the oxidation period. On the other hand, when the agglomerates are
used as the coolant in the oxidation period (and/or the reduction
period) of the AOD furnace, reduction in the electric furnace is
not required.
Example 2
[0055] Next, a relationship between the amount of surplus carbon in
agglomerates incorporated with a carbonaceous material and the
amount of residual carbon in the agglomerates (dezincified
agglomerates) incorporated with the carbonaceous material after
heat treatment was examined. The amount of coal added to the
agglomerates incorporated with the carbonaceous material was
changed to produce three types of samples having different amounts
of surplus carbon, as shown in Table 4. TABLE-US-00004 TABLE 4 (%
by mass) Amount of Test surplus No. T. Fe M. Fe FeO C T. Cr M. Cr
Zn carbon SD-1 27.63 1.20 3.18 11.75 0.436 0.08 16.53 0.47 SD-2
27.13 1.83 3.57 11.58 0.490 0.07 21.81 -0.27 SD-3 27.46 1.63 5.96
10.09 0.445 0.06 17.09 -0.85
[0056] Each of the samples was heat-treated at 1300.degree.C.
(constant) in the same small heating furnace as that used in
Example 1 in the same atmosphere for the same heating time as in
Example 1. Table 5 shows the composition of each sample
(dezincified agglomerates) after heating, and Table 6 shows the
metallization degree of each metal element, and the dezincification
degree. TABLE-US-00005 TABLE 5 (% by mass) Test No. T. Fe M. Fe C
T. Cr M. Cr Zn SD-1 46.60 42.22 2.35 0.810 0.31 0.179 SD-2 48.76
43.93 1.78 0.899 0.09 0.463 SD-3 47.33 40.86 0.98 0.806 0.09
0.261
[0057] TABLE-US-00006 TABLE 6 Metallization degree (%)
Dezincification Test No. Fe Cr degree % SD-1 90.60 38.27 99.36 SD-2
90.09 10.01 98.82 SD-3 86.33 11.17 99.11
[0058] As described above, the amount of residual carbon in the
agglomerates incorporated with the carbonaceous material is
preferably smaller than a necessary amount for reducing an
unreduced metal oxide. However, the analysis results shown in Table
5 indicate that an appropriate amount of residual carbon can be
selected. Table 4 and 5 indicate that the amount of residual carbon
in the agglomerates (dezincified agglomerates) incorporated with
the carbonaceous material after heating can be controlled by
controlling the amount of surplus carbon in the agglomerates
incorporated with the carbonaceous material. However, the reduction
ability of a metal oxide and the form of present carbon vary with
the generation source of a zinc-containing waste material used as a
raw material of the agglomerates incorporated with the carbonaceous
material and the type of the carbonaceous reducing agent added.
Therefore, an appropriate numerical range of the amount of surplus
carbon varies with the types of the zinc-containing waste material
and the carbonaceous reducing agent, and a combination thereof.
Also, as be seen from Example 1, the amount of residual carbon in
the dezincified agglomerates varies with the heating temperature.
Therefore, in consideration of these points, for example, the same
test as in Examples 1 and 2 must be previously performned by using
each of combinations and types of the zinc-containing waste
material and carbonaceous reducing agent to determine an
appropriate numerical range of the amount of surplus carbon.
[0059] The present invention having the above-described
construction can provide the method of producing stainless steel
capable of decreasing Cr reduction energy and increasing a Cr yield
of molten steel when a waste material such as dust produced in the
stainless steel producing process is re-used.
* * * * *