US20080156144A1 - Method for reducing to metallic chromium the chromium oxide in slag from stainless steel processing - Google Patents

Method for reducing to metallic chromium the chromium oxide in slag from stainless steel processing Download PDF

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Publication number
US20080156144A1
US20080156144A1 US11/964,130 US96413007A US2008156144A1 US 20080156144 A1 US20080156144 A1 US 20080156144A1 US 96413007 A US96413007 A US 96413007A US 2008156144 A1 US2008156144 A1 US 2008156144A1
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slag
furnace
electric arc
chromium
powdered aluminum
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US11/964,130
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English (en)
Inventor
Sang-Yuel JUNG
Yong Hwan Kim
Sun-Min BYUN
Sang-Beom Lee
Hyun-Chul CHUN
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Posco Holdings Inc
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Posco Co Ltd
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Assigned to POSCO reassignment POSCO ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: BYUN, SUN-MIN, CHUN, HYUN-CHUL, JUNG, SANG-YUEL, KIM, YONG-HWAN, LEE, SANG-BEOM
Publication of US20080156144A1 publication Critical patent/US20080156144A1/en
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B7/00—Working up raw materials other than ores, e.g. scrap, to produce non-ferrous metals and compounds thereof; Methods of a general interest or applied to the winning of more than two metals
    • C22B7/04—Working-up slag
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
    • C21C5/52—Manufacture of steel in electric furnaces
    • C21C5/5264—Manufacture of alloyed steels including ferro-alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
    • C21C5/52—Manufacture of steel in electric furnaces
    • C21C5/54—Processes yielding slags of special composition
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/0037—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00 by injecting powdered material
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B34/00—Obtaining refractory metals
    • C22B34/30—Obtaining chromium, molybdenum or tungsten
    • C22B34/32—Obtaining chromium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B5/00—General methods of reducing to metals
    • C22B5/02—Dry methods smelting of sulfides or formation of mattes
    • C22B5/04—Dry methods smelting of sulfides or formation of mattes by aluminium, other metals or silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B21/00—Obtaining aluminium
    • C22B21/0038—Obtaining aluminium by other processes
    • C22B21/0069—Obtaining aluminium by other processes from scrap, skimmings or any secondary source aluminium, e.g. recovery of alloy constituents
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/20—Recycling

Definitions

  • 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.
  • 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.
  • 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.
  • procedures employing high-grade scrap and ferro-alloy having less impurity content are the only ones used.
  • an electric arc furnace is the type of melting apparatus which is mainly used.
  • 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.
  • 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.
  • 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.
  • a reducing agent such as iron-silicon (Fe—Si) or aluminum
  • 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.
  • silicon or carbon which is a component of the molten steel.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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:
  • 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.
  • 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 2 ) is generated.
  • the basicity (CaO/SiO 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.
  • 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.
  • 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.
  • the method herein blows in powdered aluminum dross to the furnace in a thrown-in amount which satisfies the equation:
  • 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.
  • the amount of dross blown in should preferably exceed the chemical equivalent amount needed to reduce all of the chromium in the slag.
  • 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.
  • the basicity of slag in the electric arc furnace is controlled to range from about 1.1 to 1.7, and the alumina (Al 2 O 3 ) content in the slag is maintained at 10% or greater.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the powdered aluminum dross material should have a particle size of no more than about 5 mm.
  • 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.
  • 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.
  • the alumina (Al 2 O 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.
  • 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.
  • an inert, non-flammable carrier gas such as nitrogen or argon
  • a carrier gas pressure above a certain level is required.
  • 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.
  • 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.
  • 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.
  • 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.
  • aluminum dross powder is not blown into the furnace through a hollow electrode such as may be employed in some prior art processes.
  • 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.
  • 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.
  • the reducing agent is typically added to the furnace during the heating up time.
  • 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.
  • 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.
  • the relative quantities of powdered aluminum dross added to, and slag present in, the electric arc furnace should satisfy the following Equation 1:
  • the slag 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.
  • the fluidity of slag becomes optimal when the slag basicity (defined herein as the CaO/SiO 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.
  • 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.
  • Table 1 shows the comparative results of the Cr 2 O 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.
  • 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.
  • 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.
  • 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.
  • 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.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Organic Chemistry (AREA)
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  • Metallurgy (AREA)
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  • Mechanical Engineering (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Environmental & Geological Engineering (AREA)
  • General Life Sciences & Earth Sciences (AREA)
  • Geology (AREA)
  • Refinement Of Pig-Iron, Manufacture Of Cast Iron, And Steel Manufacture Other Than In Revolving Furnaces (AREA)
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  • Carbon Steel Or Casting Steel Manufacturing (AREA)
US11/964,130 2006-12-28 2007-12-26 Method for reducing to metallic chromium the chromium oxide in slag from stainless steel processing Abandoned US20080156144A1 (en)

Applications Claiming Priority (2)

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KR10-2006-136918 2006-12-28
KR20060136918A KR100793591B1 (ko) 2006-12-28 2006-12-28 산화크롬 함유 슬래그로부터의 크롬 금속 환원 방법

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Cited By (6)

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WO2011045755A1 (en) * 2009-10-13 2011-04-21 Petrus Hendrik Ferreira Bouwer Ferrochrome alloy production
EP2554684A4 (en) * 2010-03-29 2017-02-08 Nisshin Steel Co., Ltd. Method for treatment of chromium-containing hot metal and slag
CN108384961A (zh) * 2018-04-11 2018-08-10 武汉科技大学 一种铝铬渣无害化处理的方法
WO2020201869A1 (en) * 2019-04-01 2020-10-08 Sabic Global Technologies B.V. Method for recycling chromium oxide and forming chromium- alloy steel
EP3957756A4 (en) * 2019-04-19 2022-06-15 Nippon Steel Corporation PROCESS FOR PRODUCTION OF MOLTEN IRON CONTAINING CHROME
EP3960880A4 (en) * 2019-04-22 2022-06-22 Nippon Steel Corporation PROCESS FOR THE PRODUCTION OF CHROME CONTAINING IRON SMELTING

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KR101435282B1 (ko) * 2012-01-31 2014-09-01 현대제철 주식회사 슬래그 환원 방법
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JP6451462B2 (ja) * 2015-04-01 2019-01-16 新日鐵住金株式会社 クロム含有スラグからのクロム回収方法
CN104789736B (zh) * 2015-05-18 2017-01-11 安徽富凯特材有限公司 一种降低电弧炉渣中铬元素的方法
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CN106011482B (zh) * 2016-06-30 2019-04-09 华南理工大学 一种铬渣的铬资源回收和脱毒处理方法
CN109628688A (zh) * 2018-12-29 2019-04-16 钢铁研究总院 一种不锈钢渣中铬的在线解毒处理方法
FI20195153A1 (en) * 2019-03-01 2020-09-02 Outokumpu Oy Utilization of metal and metal oxide-containing sidestreams using arc furnace technology
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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2011045755A1 (en) * 2009-10-13 2011-04-21 Petrus Hendrik Ferreira Bouwer Ferrochrome alloy production
EP2554684A4 (en) * 2010-03-29 2017-02-08 Nisshin Steel Co., Ltd. Method for treatment of chromium-containing hot metal and slag
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