U.S. patent application number 11/964130 was filed with the patent office on 2008-07-03 for method for reducing to metallic chromium the chromium oxide in slag from stainless steel processing.
This patent application is currently assigned to POSCO. Invention is credited to Sun-Min BYUN, Hyun-Chul CHUN, Sang-Yuel JUNG, Yong Hwan KIM, Sang-Beom LEE.
Application Number | 20080156144 11/964130 |
Document ID | / |
Family ID | 39217448 |
Filed Date | 2008-07-03 |
United States Patent
Application |
20080156144 |
Kind Code |
A1 |
JUNG; Sang-Yuel ; et
al. |
July 3, 2008 |
METHOD FOR REDUCING TO METALLIC CHROMIUM THE CHROMIUM OXIDE IN SLAG
FROM STAINLESS STEEL PROCESSING
Abstract
The present invention relates to a method for reducing to
metallic chromium the chromium oxide material found in slag formed
in an electric arc furnace during the process of making stainless
steel in that furnace. The chromium oxide content of the slag can
be effectively reduced to a relatively low concentration by
maintaining the slag in a liquid phase while at the same time
blowing into the furnace via a carrier gas certain amounts of
powdered aluminum dross. In particular, powdered aluminum dross can
be blown into the furnace in amounts ranging from about 10 to 20 kg
of dross per ton of molten steel in the furnace. Alternatively,
powdered aluminum dross can be blown into the furnace in a
thrown-in amount which satisfies the equation:
0.5.ltoreq.[Thrown-In Al dross (ton).times.100]/[Slag in Furnace
(ton).times.Cr.sub.2O.sub.3 (wt %)].ltoreq.1.0. Via such a method
used during the stainless steel making process, the recovery of
valuable chromium and the rate of chromium oxide reduction can be
increased. Further, the cost of the stainless steel making process
can be reduced by using relatively inexpensive powdered aluminum
dross as the chromium oxide reducing agent instead of more
expensive conventional reducing agents.
Inventors: |
JUNG; Sang-Yuel; (Pohang-si,
KR) ; KIM; Yong Hwan; (Pohang-si, KR) ; BYUN;
Sun-Min; (Pohang-si, KR) ; LEE; Sang-Beom;
(Pohang-si, KR) ; CHUN; Hyun-Chul; (Pohang-si,
KR) |
Correspondence
Address: |
ROBERTS MLOTKOWSKI SAFRAN & COLE, P.C.
P. O. BOX 10064
MCLEAN
VA
22102-8064
US
|
Assignee: |
POSCO
Gyeongsangbuk-do
KR
|
Family ID: |
39217448 |
Appl. No.: |
11/964130 |
Filed: |
December 26, 2007 |
Current U.S.
Class: |
75/10.35 |
Current CPC
Class: |
C21C 5/54 20130101; C22B
5/04 20130101; C21C 7/0037 20130101; C21C 5/5264 20130101; C22B
34/32 20130101; C22B 21/0069 20130101; Y02P 10/20 20151101; C22B
7/04 20130101; Y02P 10/216 20151101 |
Class at
Publication: |
75/10.35 |
International
Class: |
C22B 7/04 20060101
C22B007/04 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 2006 |
KR |
10-2006-136918 |
Claims
1. A method for reducing to metallic chromium the chromium oxides
found in slag formed in an electric arc furnace during a stainless
steel making process, which method comprises maintaining said slag
in a liquid phase while at the same time blowing into said furnace,
via a carrier gas, powdered aluminum dross in a thrown in amount
which satisfies the equation: 0.5.ltoreq.[Thrown-In Al dross
(ton).times.100]/[Slag in Furnace (ton).times.Cr.sub.2O.sub.3 (wt
%)]<1.0.
2. A method according to claim 1 wherein the particle size of the
powdered aluminum dross ranges from about 1 mm to 5 mm.
3. A method according to claim 2 wherein the powdered aluminum
dross is blown into said electric arc furnace via a steel tube
along with at least one inert carrier gas selected from the group
consisting of nitrogen (N) and argon (Ar).
4. A method according to claim 3 wherein the amount of powdered
aluminum dross blown into said electric arc furnace exceeds the
chemical equivalent amount needed to reduce all of the chromium in
the slag.
5. A method according to claim 1 wherein the blowing in of powdered
aluminum dross to said electric arc furnace occurs at a point in
time after the addition of any oxygen introduced into said furnace
during the stainless steel making operation has been completed.
6. A method according to claim 1 wherein the blowing in of the
powdered aluminum dross to said electric arc furnace occurs at a
point in time after consumption of power during the operation of
electric arc furnace is between about 300 to 400 kW/ton of metal in
said furnace.
7. A method according to claim 1 wherein the basicity of slag in
the electric arc furnace is maintained between about 1.1 and
1.7.
8. A method according to claim 7 wherein the alumina content of the
slag within the electric arc furnace is maintained at 10 wt % or
greater.
9. A method according to claim 1 wherein the aluminum content of
the blown in powdered aluminum dross is 30 wt % or greater.
10. A method for reducing to metallic chromium the chromium oxides
found in slag formed in an electric arc furnace during a stainless
steel making process, which method comprises maintaining said slag
in a liquid phase while at the same time blowing into said furnace,
via a carrier gas, powdered aluminum dross in a thrown-in amount of
from about 10 to 20 kg of dross per ton of molten steel in said
furnace.
11. A method according to claim 10 wherein the particle size of the
powdered aluminum dross ranges from about 1 mm to 5 mm.
12. A method according to claim 11 wherein the powdered aluminum
dross is blown into said electric arc furnace via a steel tube
along with at least one inert carrier gas selected from the group
consisting of nitrogen (N) and argon (Ar).
13. A method according to claim 12 wherein the amount of powdered
aluminum dross blown into said electric arc furnace exceeds the
chemical equivalent amount needed to reduce all of the chromium in
the slag.
14. A method according to claim 10 wherein the blowing in of
powdered aluminum dross to said electric arc furnace occurs at a
point in time after the addition of any oxygen introduced into said
furnace during the stainless steel making operation has been
completed.
15. A method according to claim 10 wherein the blowing in of the
powdered aluminum dross to said electric arc furnace occurs at a
point in time after consumption of power during the operation of
electric arc furnace is between about 300 to 400 kW/ton of metal in
said furnace.
16. A method according to claim 10 wherein the basicity of slag in
the electric arc furnace is maintained between about 1.1 and
1.7.
17. A method according to claim 16 wherein the alumina content of
the slag within the electric arc furnace is maintained at 10 wt %
or greater.
18. A method according to claim 10 wherein the aluminum content of
the blown-in powdered aluminum dross is 30 wt % or greater.
Description
CROSS-REFERENCE TO PRIORITY APPLICATION
[0001] This application claims Paris Convention priority from
Korean Patent Application No. 10-2006-0136918, filed on Dec. 28,
2006 in the Korean Intellectual Property Office. The disclosure of
this priority application is incorporated herein by reference in
its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a method for recovering
metallic chromium from chromium oxide-containing slag formed in an
electric arc furnace during a stainless steel making process. More
specifically, the method herein involves blowing powdered aluminum
dross and a carrier gas into the slag containing chromium oxide in
order to reduce the chromium oxide therein to metallic chromium and
to thereby increase the rate of recovering valuable metals such as
chromium, etc. from the slag.
[0004] 2. Description of Related Art
[0005] In general, steel making processes, which include processes
for refining stainless steel, involve the use of an electric arc
furnace, a refining furnace with fine control of steel content, and
a continuous casting operation. In order to meet market demand for
the stainless steel with flexibility, production procedures which
first employ an electric arc furnace have generally come into
use.
[0006] The production of molten steel by means of the electric arc
furnace can be largely divided into procedures involving melting
scrap and ferro-alloy and procedures involving melting molten iron
and scrap and by then mixing them. However, for production of
high-grade steel such as the stainless steel, procedures employing
high-grade scrap and ferro-alloy having less impurity content are
the only ones used. In such procedures, an electric arc furnace is
the type of melting apparatus which is mainly used. However,
because in this case costs are higher in comparison with the use of
inexpensive molten iron, an effective method for treatment of
process byproducts to recover valuable metals such as chromium
therefrom is required. Several methods for metal recovery from
byproduct have been proposed. In particular, in the case of the
chromium, chromium removal from slag is necessary since unless it
is removed, chromium can be eluted from slag in a form of
hexavalent chromium. This can create an environmental pollution
problem when the slag is discarded or used.
[0007] Stainless steel generally has a chromium component content
of 10% or more. Since this chromium component has a stronger
affinity for reaction with oxygen than does iron (Fe), the
oxidation of the chromium component inevitably occurs in steel
making process conducted at temperatures of 1500.degree. C. or
higher.
[0008] Also, in the case of the electric furnace, since the oxygen
is inevitably blown into the furnace contents in order to promote
the melting of the scrap iron, a large amount of the chromium
component is oxidized and found in slag formed during the
manufacture of molten stainless steel.
[0009] The chromium oxide content in the slag generated as a
byproduct in the process for manufacturing the stainless steel
using the electrical furnace is relatively high, e.g., on the order
of 5% to 30%. Therefore, in order to reduce manufacturing costs and
more effectively utilize resources, after the ferro-alloy and the
scrap are melted, a reducing agent such as iron-silicon (Fe--Si) or
aluminum is commonly added to the heat-increasing device used to
raise the temperature of such molten steel. Addition of such a
reducing agent reduces the chromium oxide in the slag and increases
the content of metallic chromium found in the molten steel.
[0010] The chromium oxide in the slag present in the
heat-increasing device is partially reduced by means of the added
silicon or carbon, which is a component of the molten steel.
However, in general, a large amount of oxygen is also blown into
the molten steel in the heat-increasing device in order to raise
the temperature of the molten steel while reducing the power
imparted to the electric arc furnace. Accordingly, the chromium
reduction brought about by the addition of the silicon or carbon to
the molten steel is insignificant in comparison with the chromium
oxidation which is brought about by the blown-in oxygen.
[0011] In addition, the iron-silicon or aluminum used as the
reducing agent is expensive so that addition of such reducing agent
materials is limited in amount in order to minimize costs.
Therefore, other attempts intended to suppress the oxidation of the
chromium during the blowing in of oxygen have also been made.
[0012] Korean Laid-open Patent Application No. 2005-0109763
discloses a method which involves maintaining the slag in a liquid
phase at a high temperature advantageous to the reduction reaction
of the valuable metal. This involves raising the temperature of
slag using a burner in recovering the valuable metal in the slag of
the stainless making steel electric arc furnace. Via this method,
chromium oxidation can be suppressed. A reducing agent must still
be used so that the effect of the method is not great. Slag which
is not involved in such a separate reduction process is tapped
together with the molten steel and is slagged off. Chromium in this
slagged off material can be recovered only through a separate
process other than the steel making process.
[0013] Japanese Laid-Open Patent Application No. 2001-316712
discloses a method for reducing chromium oxide in slag by using in
an electric arc furnace at least one electrode which is a hollow
electrode. A reducing agent such as aluminum, aluminum dross,
carbon, etc., together with inert gas, is blown in through the
hollow electrode. This method is limited in application because of
the necessity of using the hollow electrode.
[0014] Further, Korean Laid-Open Patent Application No.
2000-0021329 discloses a method for inducing valuable metal
recovery from slag by blowing carbon powder into an electric arc
furnace. In this case, it is disadvantageous that the reaction of
chromium oxide and carbon occurs at a relatively low temperature,
and accordingly the speed of chromium reduction is slow.
[0015] Also, Korean Laid-Open Patent Application No. 1998-047211
discloses a method for recovering chromium by means of gas stirring
in a ladle after the contents of an electric arc furnace have been
tapped. However, this method has the disadvantage that the chromium
loss in slag skimmed during tapping is large. Post-processing for
recovering the valuable metal from the skimmed slag involves
procedures requiring time and expense, such as crushing, water
separating, magnetic separating, floatation, etc. This kind of
post-processing thus becomes one factor which increases the cost of
a stainless steel making process. Therefore, it would be very
advantageous for economic reasons to recover as much chromium as
possible from molten slag before the slag is skimmed.
[0016] In general, iron-silicon (Fe--Si) alloy on the order of 2 to
3 kg per ton of molten steel is added before tapping the molten
steel fabricated in the electric arc furnace. This enables a
portion of valuable metals such as chromium to be recovered by
means of following reactions:
[0017] Reaction No. 1
(Cr.sub.2O.sub.3)+[Si].fwdarw.(SiO.sub.2)+[Cr]
[0018] Reaction No. 2
(MnO)+[Si].fwdarw.(SiO.sub.2)+[Mn]
[0019] Reaction No. 3
(FeO)+[Si].fwdarw.(SiO.sub.2)+[Fe]
[0020] The iron-silicon alloy introduced into the electric arc
furnace molten steel is melted in the molten steel to raise silicon
content so that chromium in slag is reduced by means of the
interface reaction of the molten steel and the slag. However, in
the case of using the iron-silicon as a reducing agent, most of
silicon is oxidized by the oxygen being blown into the molten
steel. Accordingly, the amount of silicon used for chromium
reduction does not reach 50% of amount of silicon added. Also, when
a large amount of silicon is added in order to increase the amount
of chromium recovered from the slag, a large amount of silicon
oxide (SiO.sub.2) is generated. Accordingly, the basicity
(CaO/SiO.sub.2) of the slag deteriorates so that the fluidity of
slag is diminished. This, in turn, lowers the working efficiency of
the process and represents a disadvantageous condition in the
course of reducing chromium oxide in the slag.
[0021] In order to obtain a desirably high rate of recovery of
valuable metals, including expensive chromium, while also reducing
the manufacturing cost of producing stainless steel, a reducing
agent would need to be identified which is more effective and
efficient than are the known iron-silicon reducing agents.
SUMMARY OF THE INVENTION
[0022] The present invention addresses the above problems
associated with the presence of chromium oxide which forms in slag
during the making of stainless steel. It is an object of the
present invention to provide a method for reducing such chromium
oxides to metallic chromium within the slag so that the resulting
metallic chromium can be recovered from the slag. Accordingly,
chromium oxide concentration should be reduced to especially low
levels within the slag in an electric arc stainless steel-making
furnace.
[0023] In order to address the above object, the method of the
present invention maintains the slag in a liquid phase and, at the
same time, blows in certain specified amounts of powdered aluminum
dross to the furnace via a carrier gas. In one embodiment, the
method herein blows in powdered aluminum dross to the furnace in a
thrown-in amount which satisfies the equation:
0.5.ltoreq.[Thrown-In Al dross (ton).times.100]/[Slag in Furnace
(ton).times.Cr.sub.2O.sub.3 (wt %)].ltoreq.1.0.
In another embodiment, the amount of powdered aluminum dross thrown
in ranges from about 10 to 20 kg of dross per ton of molten steel
within the furnace.
[0024] In preferred invention embodiments, the blown-in powdered
aluminum dross ranges in particle size from about 1 mm to 5 mm and
the powdered aluminum dross is blown into the furnace together with
at least one inert gas comprising nitrogen (N) or argon (Ar)
through a steel tube. Furthermore, the amount of dross blown in
should preferably exceed the chemical equivalent amount needed to
reduce all of the chromium in the slag.
[0025] More preferably, blowing in of the powdered aluminum dross
is carried out at a point in time after any oxygen which is added
to the electric arc furnace during its operation has been
completed. Also such blowing in of the dross is preferably carried
out at a point in time when consumption of power by the furnace
reaches from about 300 to 400 kW/ton of metal in the furnace. In
still further preferred embodiments, the basicity of slag in the
electric arc furnace is controlled to range from about 1.1 to 1.7,
and the alumina (Al.sub.2O.sub.3) content in the slag is maintained
at 10% or greater. These features increase the fluidity of the slag
which in turn facilitates reduction of the chromium oxide therein
as well as recovery of metallic chromium therefrom.
[0026] An advantage of the method herein in terms of the recycling
of waste material and environmental friendliness is that the method
usefully utilizes aluminum dross which is produced in considerable
amounts as refined slag during aluminum refining processes. This
waste from a non-ferrous field can thus be industrially employed in
steel making.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The various embodiments and features of the invention herein
can be more readily appreciated from the description provided
hereinafter, some of which references the accompanying drawings. Of
these drawings,
[0028] FIG. 1 is a graphical depiction showing the steps in a
conventional process for producing stainless steel in an electric
arc furnace;
[0029] FIG. 2 is a graph showing chromium loss rate in slag during
the stainless steel electric arc furnace process;
[0030] FIG. 3 is a diagram showing viscosity variation in slag as a
function of slag composition;
[0031] FIG. 4 is a plan view showing a steel tube suitable for
blowing in powdered aluminum dross to an electric arc furnace using
a carrier gas according to the present invention; and
[0032] FIG. 5 is a graph showing the chromium oxide remaining in
slag when reducing slag chromium content according to the method of
the present invention and in comparison with chromium oxide content
of the slag when reducing slag chromium content according to
methods of the prior art.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0033] Hereinafter, certain exemplary embodiments according to the
present invention will be described with reference to the
accompanying drawings. Also, like reference numerals refer to like
elements throughout. In particular, the method herein for the
reduction and recovery of metallic chromium from slag containing
chromium oxide according to the embodiments of the present
invention are described.
[0034] FIG. 1 shows the steps of a general stainless steel electric
arc furnace (EAF) process. FIG. 2 is a graph showing chromium loss
rate in slag during the stages of the stainless steel electric arc
furnace process. As shown in FIG. 1, a raw material charging step
is generally performed twice or three times during the operation of
an electric arc furnace. This is because steel in a scrap iron
state has a volume which is several tens times larger than the
volume of steel in its molten state. In carrying out a first
charging, scrap iron and ferro-alloy corresponding to about 50% of
eventual amount to be tapped are added to the electric arc furnace.
Electric power is then supplied to the electric arc furnace to melt
the initial charge of metal. Thereafter, the application of
electric power is discontinued, and the roof of the electric arc
furnace is then opened so that the remaining scrap iron and
ferro-alloy can be secondarily added to the furnace. Electric power
is then reapplied to the arc furnace to completely melt all of the
scrap iron and ferro-alloy therein. Once the scrap iron and
ferro-alloy are completely melted, the amount of electric power
applied to the furnace is reduced. Subsequently, oxygen is blown
into the furnace and the furnace is then tapped with a
heat-increasing device so as to increase the temperature of melted
steel in the furnace up to a target value.
[0035] During the operation of the electric arc furnace, since
molten metal always contacts the air therein, chromium in the
molten metal is oxidized, and this causes a loss of this molten
steel component. Therefore, as a chromium reducing agent,
ferrosilicon (FeSi) is conventionally added to the furnace. Loss of
chromium still occurs, however, in the form of slag, skull, and
dust, etc., as shown in FIG. 2. Among other factors, most of this
chromium loss is caused by the flow of oxidized chromium into the
slag. Therefore, the need arises to recover this valuable metal
from the slag by reducing the chromium oxide therein to elemental,
i.e., metallic, chromium.
[0036] In order to maximize the recovery of valuable metal in the
slag, the chemical and physical properties of the reducing agent,
the composition and fluidity characteristics of the slag, and the
temperature of molten steel are all factors which need to be
considered and balanced.
[0037] First, chromium and other oxides found in the slag can be
thermodynamically reduced by the aluminum component of the powdered
aluminum dross which is added. Reduction of such metal oxides by
aluminum takes place according to the following reaction
equations:
[0038] Reaction No. 4
(Cr.sub.2O.sub.3)+2Al(l).fwdarw.2Cr(s)+(Al.sub.2O.sub.3)
[0039] Reaction No. 5
3(MnO)+2Al(l).fwdarw.3Mn(l)+(Al.sub.2O.sub.3)
[0040] Reaction No. 6
3(FeO)+2Al(l).fwdarw.3Fe(l)+(Al.sub.2O.sub.3).
[0041] Thus, when the valuable metal oxides found in the slag of
the stainless steel electric arc furnace contact aluminum at
temperatures (about 1600.degree. C.) which form the molten steel
and slag, the above reactions occur so that the oxides of the
valuable metals can be reduced to the metals in their elemental
form. However, since ideally the melting time for the metal in the
furnace should be relatively short in order to optimize
productivity of the stainless steel electric arc furnace operation,
it may be difficult to practically carry out the above reactions to
the extent needed if the speed of such reduction reactions is
slow.
[0042] The reaction speed for reduction of oxides in the slag is
proportional to a) the speed at which valuable metal oxide material
moves within the slag, and b) the surface area of contact of such
oxides with the reducing agent powder. Therefore, increasing metal
oxide movement speed by agitating the slag or increasing in the
reaction contact area by increasing the surface area of the
powdered aluminum dross are both important adjustments which can be
used to accelerate the speed of reducing the valuable metal oxides
in the slag.
[0043] One technique which can be used to increase the reduction
reaction surface area is to utilize powdered aluminum dross
material having a larger specific surface area. Larger specific
surface area is achieved by using aluminum dross powder of a
relatively smaller particle size.
[0044] Therefore, with respect to the speed of the reduction
reaction, it is advantageous that the powdered aluminum dross
material should have a particle size of no more than about 5 mm.
Also, other components besides aluminum found in the aluminum dross
powder can change the physical properties of slag by reacting with
various materials generally found within the slag. This change in
slag properties can also adversely affect the reduction of chromium
oxide within the slag. Thus when the aluminum content of the
powdered aluminum dross is too low, this factor can also ultimately
inhibit the desired reduction reactions involving the metal oxides
within the slag. Given these considerations, in a preferred
embodiment of the present invention, the powdered aluminum dross
material used in this invention should have an aluminum content of
30 wt % or more and should also have a particle size ranging from
about 1 mm to 5 mm.
[0045] FIG. 3 is a triaxial diagram showing the viscosity changes
in slag according to slag composition. Referring to FIG. 3, if the
valuable metal oxides are reduced by means of aluminum as in the
reduction reaction herein, the alumina (Al.sub.2O.sub.3) component
which is generated by such reactions can play a role in increasing
slag fluidity by lowering viscosity of slag. Therefore, the speed
of oxide material movement within the slag can be increased, making
it possible to desirably accelerate the metal-, e.g., chromium-,
producing reduction reaction speed.
[0046] In another preferred embodiment of the method herein, the
powdered aluminum dross reducing agent can be blown into the slag
layer using an inert, non-flammable carrier gas (such as nitrogen
or argon) in order to bring the powdered aluminum dross particles
into contact with the metal oxides within the slag. In order to
overcome the pressure drop created by the depth of slag layer, a
carrier gas pressure above a certain level is required. In this
preferred embodiment, a steel tube is used to blow in the powdered
aluminum dross using the carrier gas to transport the particles.
Gas and particles can be blown through this tube and into the
furnace from a working opening toward the center of the furnace
mounted with an electrode. As an example, nitrogen gas at a
pressure of from 3 to 4 bar can be used to blow the powdered
aluminum dross into the slag through a steel tube having a 2 inch
nominal inside diameter. Preferably, the steel tube used is made of
lower grade "soft" or "mild" steel, which is steel having a
relatively low carbon content.
[0047] FIG. 4 is a plan view showing an arrangement wherein a steel
tube is used for blowing powdered aluminum dross into an electric
arc furnace in accordance with one embodiment of the present
invention. As shown in FIG. 4, the powdered aluminum dross is blown
into the electric arc furnace 1 via a steel tube 4. This steel tube
4 is distinct from tube 3 which is used to blow oxygen into the
furnace 1 and is also distinct from the electrodes 2 used to
convert electrical energy into heat within the furnace. As shown in
FIG. 4, aluminum dross powder is not blown into the furnace through
a hollow electrode such as may be employed in some prior art
processes. Argon or nitrogen gas is used to carry the powdered
aluminum dross via tube 4 into the furnace 1 to promote agitation
of the slag therein. Such agitation of the slag increases the speed
of the reduction reaction occurring within the slag. The electric
arc furnace 1 apparatus further comprises an electrode 2 which is
one of three electrodes configured in a triple-top electrode
arrangement as shown in FIG. 4. After scrap iron is charged into
the electric arc furnace 1, current is applied to the electrodes of
the furnace. The scrap iron is then melted by means of high heat
such as that generated by the electric arc.
[0048] Generally, in order to melt the scrap metal in the electric
arc furnace (which generally occurs at a constant temperature),
application of electrical power to the furnace in an amount of at
least about 420 kW/ton of metal is required. For example, when the
melting of scrap and ferro-alloy is carried out in an electric arc
furnace holding 90 tons of metal, application of 300 kW/ton of
electrical power is used to melt the scrap and ferro-alloy. Further
application of 120 kW/ton of electrical power can then be used to
increase the temperature of the resulting molten steel and slag in
the furnace above the melting temperature and up to an increased
temperature of 1600.degree. C.
[0049] The period during which the metal in the furnace is melting
is referred to herein as the melting time and period during which
the molten metal is then heated to an increased temperature above
the melting temperature is referred to herein the heating up time.
In order to reduce the oxides of chromium in the slag, the reducing
agent is typically added to the furnace during the heating up time.
However, since oxygen is also typically blown into the furnace
during the heating up time in order to reduce the amount of
electric power applied to the electrodes and also to promote the
agitation of slag and molten steel, it is preferred that the
addition of the reducing agent to the furnace be deferred, for
example until after the blowing in of oxygen has been
completed.
[0050] In the case of some prior art reducing agents, a relatively
longer reaction time is needed to carry out the reduction reaction
of valuable metals which occurs when the reducing agent is added to
the slag. However, the chromium reduction which takes place when
aluminum is the active component of the reducing agent, as in the
present invention, occurs very quickly. Thus sufficient reduction
of chromium can occur in the method of the present invention even
when addition of the powdered aluminum dross reducing agent is
deferred to a point in time after the blowing of oxygen into the
furnace has been completed.
[0051] In order to ensure that the amount of powdered aluminum
dross added to the furnace is above the chemical equivalent amount
needed to effectively reduce the oxides of chromium in the slag, it
is preferred that the powdered aluminum dross reducing agent be
added to the furnace in a thrown-in amount ranging from about 10 to
20 kg of dross per ton of molten steel. Alternatively, the relative
quantities of powdered aluminum dross added to, and slag present
in, the electric arc furnace should satisfy the following Equation
1:
Equation 1
[0052] 0.5.ltoreq.[Thrown-In Al dross (ton).times.100]/[Slag in
Furnace (ton).times.Cr.sub.2O.sub.3 (wt %)].ltoreq.1.0.
[0053] In order to promote a relatively fast time of reaction
between the metal oxides in the slag and the added aluminum dross
reducing agent, the slag should be maintained in a liquid phase,
and the slag should also have sufficient fluidity (e.g.,
sufficiently low viscosity) to permit the desired reduction of the
metal oxides therein. As shown in FIG. 3, the fluidity of slag
becomes optimal when the slag basicity (defined herein as the
CaO/SiO.sub.2 weight ratio) ranges from about 1.1 to 1.7, and the
alumina content of the slag is 10 wt % or more. Maintenance of
appropriate slag fluidity increases the reduction reaction speed
and also promotes the absorption of the reduced valuable metals
from the slag into the molten steel. This prevents the valuable
metals from collecting in the slag.
[0054] Hereinafter, certain embodiments of the present invention
will be described by means of the following examples:
EXAMPLES
[0055] The reduction of chromium oxide in slag by the use of
ferrosilicon conventionally employed in a stainless steel making
electric arc furnace is compared with chromium oxide reduction
achieved by adding powdered aluminum dross in accordance with the
method of the present invention.
[0056] Typically, in a stainless steel making electric arc furnace,
the chromium oxide content of the slag therein after the scrap and
ferro-alloy are melted, and prior to chromium oxide reduction by
added reducing agents, reaches 20 to 25%. Chromium oxide is formed
when the chromium component in the molten steel is oxidized by the
contact of the molten steel with air in the furnace and/or with
oxygen which has been blown into the furnace during its operation
as hereinbefore described. The technical effect provided by the
method of the present invention can be confirmed by comparing
chromium oxide reduction realized using a powdered aluminum dross
reducing agent versus chromium oxide reduction provided by a
conventional ferrosilicon reducing agent.
[0057] The following Table 1 shows the comparative results of the
Cr.sub.2O.sub.3 content reduction in slag using both the powdered
aluminum dross reducing agent of the present invention and the
ferrosilicon reducing agent of the prior art.
TABLE-US-00001 TABLE 1 Operation Test number Cr.sub.2O.sub.3
Content (wt %) in Slag Before Addition of 1 17.45 Reducing Agent 2
31.70 Addition of Powdered 1 1.91 Aluminum Dross 2 3.69 Reducing
Agent 3 3.69 (Present Invention) 4 4.98 5 2.55 Addition of
Ferrosilicon 6 6.83 Reducing Agent 7 7.75 (Comparative Example) 8
8.78 9 8.63 10 11.45
[0058] Referring the Table 1, it can be seen that the remaining
chromium oxide content in the slag after tapping the stainless
molten steel from the electric arc furnace is 2% to 5% when
powdered aluminum dross is blown into the furnace. It can also be
seen that such chromium oxide contents are from of 2% to 8% lower
than chromium oxide contents realized using conventional chromium
reduction methods using ferrosilicon.
[0059] FIG. 5 is a graph showing a comparison of the chromium oxide
content remaining in slag after reducing chromium in accordance
with the present invention and the chromium oxide content after
reducing chromium in accordance with a prior art method.
[0060] As shown in the Comparison Example of FIG. 5, use of
ferrosilicon (at 3 kg per ton of molten steel) in a conventional
valuable metal reduction method, the remaining chromium oxide
content in the slag after tapping reaches 7% to 10% according to
analytical results. In this case, the time of throwing in
ferrosilicon is after the blowing in of oxygen to the furnace has
been completed. This is said to be the addition sequence which is
capable of maximizing the chromium reduction effect. However the
content of chromium oxide remaining is the slag is still relatively
high.
[0061] As also shown in FIG. 5, chromium oxide content of the slag
is also reduced when powdered aluminum dross in added to the
furnace, blown in with a nitrogen carrier gas. In this instance,
the powdered aluminum dross is blown into the furnace in an amount
of 10 kg per ton of molten steel. The time period of blowing in can
be varied according to the nitrogen pressure used. Powdered
aluminum dross addition usually takes about 10 minutes when
nitrogen pressures in the range of 3-4 bar are used. Also in this
instance the powdered aluminum dross reducing agent is blown in
after completion of the blowing of oxygen into the furnace. This
timing of addition serves to maximize the chromium reduction effect
and to prevent the oxidation of aluminum by oxygen.
[0062] The above examples show that the method of the present
invention which involves blowing in of powdered aluminum dross to
the stainless steel-making electric arc furnace via a carrier gas
can improve the reduction of chromium oxide in slag and the
recovery rate of chromium in comparison with similar prior art
methods using ferrosilicon reducing agents. And, the cost of the
stainless steel making process can be reduced by using relatively
inexpensive powdered aluminum dross in comparison with methods
which use more expensive conventional reducing agents.
[0063] Although preferred embodiments of the present invention have
been shown and described, it would be appreciated by those skilled
in the art that changes might be made to such preferred embodiments
without departing from the principles and spirit of the present
invention, the scope of which is defined by the accompanying claims
and their equivalents.
* * * * *