Patent Application
20140352496
1. Field of the Invention
The invention relates to a briquette for producing a foamed slag on stainless steel melts in an electric arc furnace, containing a mixture of metal oxides, limestone, carbon, and a binder which is placed in the furnace and arranged there such that the metal oxides are reduced and the limestone is thermally dissociated beneath the slag at the metal-slag interface and the gases formed in this process make the slag foam by forming bubbles.
2. Description of the Prior Art
In the operation of electric arc furnaces, packed-in solid materials, primarily scrap and alloys, are melted with the electric arcs of the electrodes, which project from above into the furnace chamber. In this process the slag, aside from its primary function of removing undesirable constituents from the melt, performs a protective function, since it partially fills the space between the electrode end and the metal surface and protects the refractory lining of the furnace from the radiant energy of the electric arc furnace. This protective function of the slag can be improved if the slag is made to foam by suitable methods. The foaming of the slag is one of the decisive process sequences in stainless steel production in an electric arc furnace that affect the increase in efficiency of steel production and reduce the production costs. Therefore this is a generally employed method in the production of standard carbon steels.
However, the situation is different when this is to be applied to foams on steels with high chromium content, since the significant uptake of chromium oxide by the slag causes problems. This is a consequence of the physicochemical properties of slags with high chromium oxide content.
To be sure, methods for foaming slag with high chromium content are known, but all of them are unsatisfactory. For example in EP 0 829 545 B1 a method is suggested for producing a foamed slag on molten stainless steel in an electric arc furnace by introducing into the slag, using an injection medium such as nitrogen, a powder composed of a metal oxide--either zinc oxide or lead oxide--and carbon. The oxide contained in the powder is reduced when it reacts with the carbon. In this process bubbles are formed in the slag; these which essentially consist of carbon monoxide and make the slag foam. Because of the relatively large surface area associated with the powder form, a brief, vigorous reaction with the slag occurs, and it takes place in a limited area near the point of injection or blowing into the melt bath.
To avoid the drawbacks of introducing powdered substances, it is suggested in WO 2004/104232 that the materials used for foaming the slag, a mixture of metal oxide and carbon, be loaded into the electric arc furnace in the form of pressed molded pieces. The density of these molded pieces is adjusted such that they float in the slag, preferably close to the foam/slag interface.
To minimize unwanted loss of valuable material in the case of high chromium oxide content in the slag, it is suggested in WO 2008/095575 A1 that the pellets or briquettes, composed of a defined mixture of an iron source as the ballast material, carbon or carbon and silicon as the reducing agent, and a binder, be charged into the electric arc furnace in such a way that they undergo a chemical reduction reaction with the metal oxides of the slag, especially with the chromium oxide present, under the slag layer, wherein the reaction gases produced, primarily carbon monoxide, support slag foaming.
In addition, WO 2010/003401 A1 describes a method for producing foamed slags with which the foaming of a slag with high chromium oxide content can be achieved. The core of this process is that the material contains an iron oxide source, carbon source, high carbon-containing ferrochrome and/or scrap or nickel oxide (other than ferritic steel) as admixtures as well as limestone and possibly additional fluorspar, binders such as molasses and/or cement, and additional gas sources for the foaming process. This material is to be present in briquette form or as pellets with different sizes. The mixture to be introduced in the form of molded pieces, such as briquettes or pellets, contains as its basic components
The object of the invention is to supply suitable briquettes for the production of foamed slags with high chromium oxide content, performed particularly according to the method of WO 2010/003401 A1, wherein all briquette properties to be considered as well as all materials suitable for briquette manufacturing are summarized in the form of a matrix.
The object of the invention is achieved with a briquette for use in different melts, such as stainless steel, low alloy steel or high alloy steel, is made up of mixtures of individual or multiple substances of the following basic components:
O2 carrier Dust, sludge or slag with ≧10% FeO/Fe2O3,
Gas carrier Dust with ≧40% CaCO3,
Density adjuster 10%-16% by weight
Reducing agent Carbon-containing substances ≧90% C,
Binders Molasses, cement, Ca(OH)2 (slaked lime), in each case ≦5%.
The use of foaming of the slag in EAF metallurgy provides a number of advantages, such as improvement of the thermal efficiency of the furnace because of the low thermal conductivity of the foam, low consumption of refractory material and electrodes, and stabilization of the electric arc furnace and the noise level.
To achieve effective foaming, a high gas production must be achieved at the metal/slag interface. The dominant factors here are the gas CO which serves as the foam generator and CO2. These gases form during the reduction of iron oxide and chromium oxide and the thermal dissociation of the limestone as follows:
Fe2O3+3C=2Fe+3CO (1)
FeO+C=Fe+CO (2)
Cr2O3+3C=3CO+2Cr (3)
CrO+C=Cr+CO (4)
CaCO3=CaO+CO2 (5)
In these reactions, the degree of reduction of the iron oxide by the carbon is very high, while the reduction of chromium oxide by the carbon is less effective.
It should be noted that the slags in the manufacturing of stainless steel contain very little iron oxide but a large amount of chromium oxide, so that the low efficiency of CO generation in such slags is understandable. More effective gas generation may be achievable by the systematic addition of synthetic materials such as scale and limestone.
The relative density of the additives plays an important role for the foam production, specifically compared with that [relative density] of the slag or the metal. It contributes to bringing the gas production reaction to the slag/metal interface so that the foaming becomes more effective and better controllable.
The density can be influenced by the suitable selection of very dense materials (metals), so-called ballast materials such as ferritic scrap and/or ferrochrome, and less dense materials (oxides).
The principal component in the foam formers is iron oxide (Fe2O3), with added carbon as a reducing agent. Thus the following reaction takes place:
Fe2O3+3C=2Fe+3 CO (6)
wherein the foam mixture of Fe2O3 and graphite contains 18.37% graphite and as the remainder, 81.63% Fe2O3.
The composition is completed by ferrochrome (FeCr) with high chromium content, ferritic scrap and limestone CaCO3.
In the case of slags of austenitic steels, nickel oxide may also be added.
Ferrochrome and ferritic steel make the foam-forming additives heavier because of their high relative density. Thus the relative density is between the specific densities of slag and metal according to:
p slag <p material <p metal (7)
In consequence the material is systematically positioned at the slag-metal interface by buoyancy.
In this process it dissolves in the metal bath, increasing the bath weight.
Thermal dissociation of limestone results in production of CO2, which supports foaming, while calcium oxide dissolves in the slag and increases the viscosity and basicity of the slag. In addition the slag viscosity can also be adjusted by the addition of fluorspar (CaF2).
The foam former according to the invention consists of basic components such as
The additives include:
As binders:
The composition of the briquette can be specified as follows:
The assumptions that follow are used to determine the relative density of the foam former, wherein Fe2O3,m, is to be understood as a mixture of Fe2O3 with graphite.
that of the foam former:
wherein ballast means FeCr or scrap and nickel oxide
%Fe
The relative density of the foam former can be seen in Table 1 below.
The relative density data shown relate to monolithic material. On the other hand, if the material for foam formation is used in the form of briquettes, its relative density is naturally lower.
The briquettes are produced by pressing the material; different densities are obtained depending on the percentage composition.
The relative density of slags produced in steel manufacturing is in the range of 2.5 to 3 g/cm3.
A pressed composition that contains Fe2O3 and carbon in the mixture mentioned in practice has a density of 3.2 g/cm3, while computationally for the individual components a density of 4.7 g/cm3 results. A density of 2.9 g/cm3 was found experimentally for the slag being considered.
From viewpoint of the desired foam forming effect, the relative density of the foam former should fall in the range of 2.8-6.0 t/m3.
At low geometric dimensions for the additives (pellets or briquettes) the gas is released rapidly, since the total reaction surface is larger in the case of smaller dimensions.
It was already mentioned that the foam-forming mixture should be added in the form of briquettes or pellets. In this case the briquettes are produced in a specially designed press.
Dimensions of diagonal 20-100 mm and height 15-40 mm have proven advantageous for the briquettes.
The pellets or briquettes can be produced with addition of molasses or cement in a drum before pressing, but other binding techniques, ensuring that the desired properties in terms of hardness, fracture strength and compressive strength are achieved, are also possible.
The briquettes produced by pressing have a briquette volume of ≦800 cm3, a diagonal length between 20 and 100 mm, and a height between 15 and 40 mm and are variably designed as ellipsoid, hexagonal cuboids, cuboids or cylinders.
The possible compositions of the briquettes that can be produced from the preceding statements as well as the materials that can be used are summarized in Table 2 in the form of a matrix.
According to this, the briquettes for use in foamed-slag production for stainless steel can be made up as desired from all of the listed substances 1-26 of the basic components summarized in Table 2.
For use in foamed slag production for low alloy steel or high alloy steel, the briquettes contain as the O2 carrier only mixtures of dust, sludge, slag or scale with in each case ≧10% FeO/Fe2O3 and as the density adjuster only mixtures of dust with Fe/low alloy fine scrap and dust with Mn/ferritic or low alloy fine scrap.
A limestone briquette additionally usable in all melts contains as the pure gas carrier no substances of the basic component O2 carrier.
The physical properties of the possible briquettes produced according to this Table 2 then fall in the following ranges:
| Number | Date | Country | Kind |
|---|---|---|---|
| 102010011712.9 | Mar 2010 | DE | national |
| 102010022692.0 | Jun 2010 | DE | national |
This application is a continuation-in-part of application Ser. No. 13/634,710 filed Oct. 12, 2012 and which is a National Stage Application based on a PCT application PCT/EP2011/050880 filed on Jan. 24, 2011 and claiming priority of German applications No. 10 2010 011 712.9 filed Mar. 17, 2010 and No. 10 2010 022 692.0 filed Jun. 4, 2010, all of the applications mentioned herein being incorporated by reference thereto.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13634710 | Oct 2012 | US |
| Child | 14459527 | US |
