Title:
METHOD FOR REDUCING TO METALLIC CHROMIUM THE CHROMIUM OXIDE IN SLAG FROM STAINLESS STEEL PROCESSING
Kind Code:
A1


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≦[Thrown-In Al dross (ton)×100]/[Slag in Furnace (ton)×Cr2O3 (wt %)]≦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)
Application Number:
11/964130
Publication Date:
07/03/2008
Filing Date:
12/26/2007
Assignee:
POSCO (Gyeongsangbuk-do, KR)
Primary Class:
International Classes:
C22B7/04
View Patent Images:



Primary Examiner:
YANG, JIE
Attorney, Agent or Firm:
Roberts Calderon Safran & Cole, P.C. (McLean, VA, US)
Claims:
What is claimed is:

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≦[Thrown-In Al dross (ton)×100]/[Slag in Furnace (ton)×Cr2O3 (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

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

1. Field of the Invention

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.

2. Description of Related Art

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.

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.

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° C. or higher.

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.

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.

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.

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.

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.

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.

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.

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.

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:

Reaction No. 1


(Cr2O3)+[Si]→(SiO2)+[Cr]

Reaction No. 2


(MnO)+[Si]→(SiO2)+[Mn]

Reaction No. 3


(FeO)+[Si]→(SiO2)+[Fe]

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 (SiO2) is generated. Accordingly, the basicity (CaO/SiO2) 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.

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

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.

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≦[Thrown-In Al dross (ton)×100]/[Slag in Furnace (ton)×Cr2O3 (wt %)]≦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.

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.

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 (Al2O3) 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.

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

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,

FIG. 1 is a graphical depiction showing the steps in a conventional process for producing stainless steel in an electric arc furnace;

FIG. 2 is a graph showing chromium loss rate in slag during the stainless steel electric arc furnace process;

FIG. 3 is a diagram showing viscosity variation in slag as a function of slag composition;

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

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

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.

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.

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.

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.

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:

Reaction No. 4


(Cr2O3)+2Al(l)→2Cr(s)+(Al2O3)

Reaction No. 5


3(MnO)+2Al(l)→3Mn(l)+(Al2O3)

Reaction No. 6


3(FeO)+2Al(l)→3Fe(l)+(Al2O3).

Thus, when the valuable metal oxides found in the slag of the stainless steel electric arc furnace contact aluminum at temperatures (about 1600° 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.

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.

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.

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.

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 (Al2O3) 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.

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.

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.

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° C.

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.

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.

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


0.5≦[Thrown-In Al dross (ton)×100]/[Slag in Furnace (ton)×Cr2O3 (wt %)]≦1.0.

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/SiO2 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.

Hereinafter, certain embodiments of the present invention will be described by means of the following examples:

EXAMPLES

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.

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.

The following Table 1 shows the comparative results of the Cr2O3 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 1
OperationTest numberCr2O3 Content (wt %) in Slag
Before Addition of117.45
Reducing Agent231.70
Addition of Powdered11.91
Aluminum Dross23.69
Reducing Agent33.69
(Present Invention)44.98
52.55
Addition of Ferrosilicon66.83
Reducing Agent77.75
(Comparative Example)88.78
98.63
1011.45

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.

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.

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.

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.

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.

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.