BRIQUETTE FOR PRODUCING A FOAMED SLAG EFFECT IN EAF TECHNOLOGY IN STAINLESS STEEL PRODUCTION

Abstract

It is known that a foamed slag can be produced on stainless steel melts in an electric arc furnace by introducing a mixture of metal oxides, limestone, carbon and a binder in the form of briquettes into the furnace so that the briquettes become arranged there in such a way that the metal oxides are reduced by the carbon and the limestone is thermally dissociated at the metal-slag interface beneath the slag and the resulting gases bring about foaming of the slag by bubble formation. This foaming on steels having a high chromium content, due to the significant uptake of chromium oxide by the slag, leads to problems which result from the physicochemical properties of slags having a high chromium oxide content. To provide, according to the invention, briquettes suitable for producing foamed slag having a high chromium oxide content, which is carried out, in particular by the process of WO 2010/003401 A1, all briquette properties to be taken into account and all materials suitable for briquette production are tabulated in the form of a matrix.

Description

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.

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

• • Iron oxide (Fe₂O₃, Fe₃O₄) in any form [such] as scale, converter or EAF/LF dry dust, [or] converter wet dust (sludge),
  • Coke, graphite, carbon (C)
  • Ballast material for all stainless steel types in the form of FeCr, ferritic scrap ballast material for stainless austenitic and duplex steel types in the form of FeCr, ferritic scrap, austenitic scrap, duplex scrap, nickel oxides (NiO_x)
  • Limestone (CaCO₃), lime or fluorspar (CAO or CaF₂)
  • Al oxide (Al₂O₃)
  • Binder in the form of molasses, cement, or another binder.

The goal 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 task posed is accomplished with the characterizing features of claim 1 in that the briquette for use in different melts, such as stainless steel, low alloy steel or high alloy steel, is made up of defined arbitrary mixtures of individual or multiple substances of the following basic components:

O₂ carrier

• • Dust, sludge or slag with ≧10% FeO/Fe₂O₃,
  • Dust, sludge or slag with ≧1% Cr₂O₃,
  • Dust, sludge or slag with ≧1% MnO,
  • Dust, sludge or slag with ≧1% NiO,
  • Scale with ≧10% FeO/Fe₂O₃
    Gas carrier Dust with ≧40% CaCO₃,
    Density adjuster
  • Dust with FeCr,
  • Dust with Fe/low alloy fine scrap,
  • Dust with Cr/ferritic fine scrap,
  • Dust with Ni/austenitic fine scrap/grinding dust,
  • Dust with Mn/ferritic or low-alloy fine scrap,
    Reducing agent Carbon-containing substances ≧90% C,
  • Dust or fine granulate of coke, coal or graphite,
    Binders Molasses, cement, Ca(OH)₂ (slaked lime), in each case ≦5%.

Advantageous embodiments of the invention will become apparent from the subclaims.

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+3CO  (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:

ρ slag<ρ material<ρ 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

• • Iron oxides (Fe₂O₃, Fe₃O₄) in any form [such] as scale, converter or EAF/LF dry dust, [or] converter wet dust (sludge), ore
  • Coke, graphite, carbon (C)
  • Ballast material for all stainless steel types in the form of FeCr, ferritic scrap
  • Ballast material for stainless austenitic and duplex steel types in the form of FeCr, ferritic scrap, austenitic scrap, duplex scrap, nickel oxides (NiO_x)

The additives include:

• • Limestone (CaCO₃)
  • Lime and fluorspar (CaO and CaF₂)
  • Al oxide (Al₂O₃)

As binders:

• • Molasses
  • Cement
  • Or other possible binders.

The composition of the briquette can be specified as follows:

Fe2O3, Fe3O4     10-70 in %
C                2-16 in %
Ballast material 14-78 in %
CaCO3            0-10 in %
CaO, CaF2        0-10 in %
Al2O3            0-10 in %

The assumptions that follow are used to determine the relative density of the foam former, wherein Fe₂O_3,m, m is to be understood as a mixture of Fe₂O₃ with graphite.

• • The relative density of the Fe₂O_3,m, is given by the formula:

$\begin{array}{cc}{\rho }_{{\mathrm{Fe}}_2O_3m}={\rho }_{{\mathrm{Fe}}_2O_3}·\frac{{\%}_{{\mathrm{Fe}}_2O_3}}{100\%}+{\rho }_C·\frac{{\%}_C}{100\%}& \left(8\right)\end{array}$

• •  that of the foam former:

$\begin{array}{cc}\rho ={\rho }_{{\mathrm{Fe}}_2O_{3,}m}·\frac{{\%}_{{\mathrm{Fe}}_2O_{3,}m}}{100\%}+{\rho }_{\mathrm{Balast}}·\frac{{\%}_{\mathrm{Balast}}}{100\%}+{\rho }_{{\mathrm{CaCO}}_3}·\frac{{\%}_{{\mathrm{CaCO}}_3}}{100\%}+{\rho }_{\mathrm{Binders}}\frac{{\%}_{\mathrm{Binder}}}{100\%}& \left(9\right)\end{array}$

• •  wherein ballast means FeCr or scrap and nickel oxide

%_Fe2O3,m+%_Balast+%_CaCO3+%_Binder=100%  (10)

• • The CaCO₃ content can be replaced by CaF₂ contents, wherein in particular it can be assumed that % CaCO₃=0 or CaF₂=0.

The relative density of the foam former can be seen in Table 1 below.

TABLE 1
Relative density of the pure monolithic foam former components used for determining the density of the material
	Component
										Ferritic
	Fe	Cr	Fe2O3	C	CaCO3	CaF2	FeCr (*)	Molasses	Cement	scrap	NiOx
Relative density, [t/m3]	7.86	7.2	5.3	2.25	2.27	3.18	4.09	0.99	2.9	6.51	6.67
(*) 54% Cr—35% Fe—8% C—3% Si

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:

• • The CO/CO₂ gas evolution under standard state (20° C., 1 bar)≧0.15 Nm³/kg briquette,
  • The briquette density is between 2.5 and 7.0 g/cm³,
  • After 24 hr of drying and at ≦4% moisture content, the compressive strength of the briquettes amounts to ≧0.5 N/mm² and the disintegration strength from 2 m onto a 120 mm thick plate is ≧97%.

	TABLE 2
	Stainless	Low alloy	High alloy
	steel	steel	steel	Limestone
	briquette	briquette	briquette	briquette
	Material: O2 carrier
	Individual component or in
	arbitrary mixture with 1-26
1.	Dust FeO/Fe2O3 ≧ 10%	X	X	X
2.	Dust Cr2O3 ≧ 1%	X
3.	Dust MnO ≧ 1%	X
4.	Dust NiO ≧ 1%	X
5.	Scale FeO/Fe2O3 ≧ 10%	X	X	X
6.	Sludge FeO/Fe2O3 ≧ 10%	X	X	X
7.	Sludge Cr2O3 ≧ 1%	X
8.	Sludge Mn ≧ 1%	X
9.	Sludge NiO ≧ 1%	X
10.	Slag FeO/Fe2O3 ≧ 10%	X	X	X
11.	Slag Cr2O3 ≧ 1%	X
12.	Slag MnO ≧ 1%	X
13.	Slag NiO ≧ %	X
	Material: gas carrier,
	individual component or
	in arbitrary mixture
	with 14-19 and 24-26
14.	Dust limestone CaCO3 ≧ 40%				X
	Material: density adjuster
	Individual component or in
	arbitrary mixture with 1-26
15.	Dust FeCr	X
16.	Dust Fe/low alloy fine scrap	X	X	X	X
17.	Dust Cr/ferritic fine scrap	X			X
18.	Dust Ni/austenitic fine	X			X
	scrap/grinding dust
19.	Dust Mn/ferritic or low alloy	X	X	X	X
	fine scrap
	Reducer: Individual component
	or in arbitrary mixture with
	1-26
20.	Carbon-containing dust ≧ 90%	X	X	X	X
21.	Coke (dust, fine granules)	X	X	X	X
22.	Coal (dust, fine granules)	X	X	X	X
23.	Graphite (dust, fine granules)	X	X	X	X
	Binder: Individual component
	or in arbitrary mixture with
	1-26
24.	Molasses ≦ 5%	X	X	X	X
25.	Cement ≦ 5%	X	X	X	X
26.	Calcium hydroxide (slaked	X	X	X	X
	lime) ≦ 5%

Claims

1. 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 introduced into the furnace and arranged there such that below the slag at the metal-slag interface, the metal oxides are reduced by the carbon and the limestone is thermally dissociated and the gases produced in this process cause foaming of the slag by forming bubbles, characterized in that the briquette for use in different melts such as stainless steel, low alloy steel or high alloy steel is made up of defined arbitrary mixtures of individual or multiple substances of the following basic components:

O2 carrier Dust, sludge or slag with ≧10% FeO/Fe2O3, Dust, sludge or slag with ≧1% Cr2O3, Dust, sludge or slag with ≧1% MnO, Dust, sludge or slag with ≧1% NiO, Scale with ≧10% FeO/Fe2O3

Gas carrier Dust with ≧40% CaCO3,

Density adjuster Dust with FeCr, Dust with Fe/low alloy fine scrap, Dust with Cr/ferritic fine scrap, Dust with Ni/austenitic fine scrap, grinding dust, Dust with Mn/ferritic or low alloy fine scrap,

Reducing agent Carbon-containing substances ≧90% C, Dust or fine granulate of coke, coal or graphite,

Binder Molasses, cement, Ca(OH)2 (slaked lime), in each case ≦5%.

2. (canceled)

3. Briquette according to claim 1, characterized in that the briquette for use in foamed slag production for stainless steel is made up as desired from all substances of the six basic components mentioned.

4. Briquette according to claim 1, characterized in that the briquette for use in foamed slag production for low alloy steel or high alloy steel as an O2 carrier contains only mixtures of dust, sludge, slag or scale with in each case ≧10% FeO/Fe2O3 and as a density adjuster only mixtures of dust with Fe/low alloy fine scrap and dust with Mn/ferritic or low alloy fine scrap.

5. Briquette according to claim 1, characterized in that as a briquette additionally usable in all melts, as a pure gas carrier it contains no substances of the basic component O2 carrier.

6. Briquette according to claim 1, characterized in that the CO/CO2 gas evolution in the standard state (20° C., 1 bar) is ≧0.15 Nm3/kg briquette.

7. Briquette according to claim 6, characterized in that the briquette density is between 2.5 and 7.0 g/cm3.

8. Briquette according to claim 6, characterized in that after drying for 24 hr and at ≦4% humidity the compressive strength of the briquette ≧0.5 N/mm2 and the disintegration strength from 2 m on a 120 mm thick plate is ≧97%.

9. Briquette according to claim 6, characterized in that the briquette, the diagonal size of which is between 20 and 100 mm and the height of which is between 15 and 40 mm with a briquette volume of ≦800 cm3, is designed variably as an ellipsoid, as a hexagonal cuboid, as a cuboid, or as a cylinder.