Electrode composition

Abstract

The present invention relates to a self-calcining electrode material for electric arc furnaces, containing one or more carbon components and a binder, wherein the binder is hard bitumen and having a needle penetration at 25 C. according to DIN EN 1426 of <50 [per 0.1 mm] and/or a softening point (ring and ball) according to DIN EN 1 427 of at least 65 C. and/or having a density at 25 C. according to DIN EN 52004 of 0.5 to 2 g/cm3, wherein the electrode material has a PAH content of <500 ppm. The hard bitumen is preferably derived by flash distillation from soft and medium-hard bitumen types and has a high sulfur content.

Claims

1. A self-calcining electrode composition for electric arc furnaces, comprising one or more carbon components and a binder, wherein the binder contains exclusively hard bitumen with a needle penetration of <50 per 0.1 mm at 25 C. according to DIN EN 1426, a softening point, ring and ball, of at least 65 C. according to DIN EN 1427, a density of 0.5 to 2 g/cm.sup.3 at 25 C. according to DIN EN 52004, or any combination thereof, and wherein the electrode composition has a polyaromatic hydrocarbon (PAC) content of <500 ppm.

2. The self-calcining electrode composition according to claim 1, wherein the hard bitumen has a high sulfur content of 5-7% and is obtained from crude oil containing a large amount of organically bound sulfur.

3. The self-calcining electrode composition according to claim 1, wherein the electrode composition has a PAC content less than or equal to 10 ppm.

4. The self-calcining electrode composition according to claim 1, wherein the electrode composition has a PAC content less than or equal to 5 ppm.

5. The self-calcining electrode composition according to claim 1, wherein the electrode composition has a PAC content of less than or equal to 1 ppm.

6. The self-calcining electrode composition according to claim 1, wherein the hard bitumen is present in an amount of at most 35% by weight, based on the electrode composition.

7. The self-calcining electrode composition according to claim 1, wherein the one or more carbon components are selected from anthracite, coke, graphite, and mixtures thereof.

8. The self-calcining electrode composition according to claim 4, wherein anthracite (a) is present as gas-calcined anthracite in an amount of at most 60% by weight, based on the electrode composition, or (b) is present as electrically calcined anthracite in an amount of at most 80% by weight, based on the electrode composition.

9. The self-calcining electrode composition according to claim 4, wherein coke is present in an amount of at most 60% by weight, based on the electrode composition.

10. The self-calcining electrode composition according to claim 4, wherein graphite is present in an amount of at most 15% by weight, based on the electrode composition.

11. A baked electrode comprising a sheet metal jacket and a baked electrode composition according to claim 1.

12. A method for producing a self-calcining electrode composition, comprising the steps of mixing one or more carbon components, a binder and optionally additives, shaping the mixture into a predetermined shape, wherein exclusively hard bitumen is used as the binder with a needle penetration of 50 per 0.1 mm at 25 C. according to DIN EN 1426, a softening point, ring and ball, of at least 65 C. according to DIN EN 1427, a density of 0.5 to 2 g/cm.sup.3 at 25 C. according to DIN EN 52004, or any combination thereof.

13. The method according to claim 12, wherein the hard bitumen is obtained by flash distillation of soft and medium-hard bitumen varieties.

14. The method according to claim 12, wherein the hard bitumen has a high sulfur content of 5-7% and is obtained from crude oil containing a large amount of organically bound sulfur.

15. The method according to claim 12, wherein the hard bitumen is present in an amount of at most 35% by weight, based on the electrode composition.

16. The method according to claim 12, wherein the one or more carbon components are selected from anthracite, coke, graphite, and mixtures thereof.

17. The method according to claim 12, wherein coke is present in an amount of at most 60% by weight, based on the electrode composition.

18. The method according to claim 12, wherein graphite is present in an amount of at most 25% by weight, based on the electrode composition.

19. The self-calcining electrode composition according claim 1, wherein the hard bitumen has a high sulfur content of 5-7%.

20. The self-calcining electrode composition according to claim 1, wherein the hard bitumen is present in an amount of 15% to 30% by weight, based on the electrode composition.

21. The self-calcining electrode composition according to claim 1, wherein the one or more carbon components are selected from the group consisting of gas-calcined anthracite, electrically calcined anthracite, coke, graphite and mixtures thereof.

22. The self-calcining electrode composition according to claim 4, wherein anthracite (a) is present as gas-calcined anthracite in an amount of 10% to 40% by weight, based on the electrode composition, or (b) is present as electrically calcined anthracite in an amount of 65% to 80% by weight, based on the electrode composition.

23. The self-calcining electrode composition according to claim 4, wherein coke is present in an amount of 30% to 60% by weight, based on the electrode composition.

24. The self-calcining electrode composition according to claim 4, wherein graphite is present in an amount of 3% to 12% by weight, based on the electrode composition.

25. The method according to claim 12, wherein the hard bitumen has a high sulfur content of 5-7%.

26. The method according to claim 12, wherein the hard bitumen is present in an amount of 15% to 30% by weight, based on the electrode composition.

27. The method according to claim 12, wherein the one or more carbon components are selected from the group consisting of gas-calcined anthracite, electrically calcined anthracite, coke, graphite and mixtures thereof.

28. The method according to claim 12, wherein coke is present in an amount of 30% to 60% by weight, based on the electrode composition.

29. The method according to claim 12, wherein graphite is present in an amount of 3% to 12% by weight, based on the electrode composition.

Description

WAYS FOR CARRYING OUT THE INVENTION

(1) FIG. 1 shows a self-calcining Sderberg electrode for an electric arc furnace in a sectional illustration according to the prior art in a highly simplified manner.

(2) The temperature zones of a self-calcining Sderberg electrode are illustrated in FIG. 1 on the basis of a conventional coal tar pitch binder. The electrode composition 1, pressed into briquettes or cylinders, is supplied to a cylindrical housing 3 and is present in solid form in a zone 2, at a temperature of around 80 C. A power supply 4 is located on the outside of the housing. Electrical energy is supplied to the electrode composition via the contact jaw 5. The thermal energy output by the molten material 9 serves as further energy source. As a result of the energy input, the electrode composition obtains a pasty consistency at approximately 130 C. In the baking zone 6, between 500 C. and 1000 C., the volatile portions escape and the electrode composition transitions into a solid state. In zone 7, between 1000 C. and 1500 C., the carbon is present in amorphous form. In zone 8, above 2000 C., the graphitization occurs. The electrode composition, which is not yet baked, in the temperature range between 80 C. and around 500 C., is electrically non-conductive. The energy input takes place electrically via the electrical resistance of the electrode composition. Starting at around 500 C., the electrical resistance of the electrode composition decreases and it becomes electrically conductive. The graphitized electrode 8 is surrounded by a plasma or electric arc at the electrode tip (not visible in FIG. 1).

(3) By using the bitumen, which is suitable for the invention, the graphitization of the electrode is already possible below the contact jaws.

EXAMPLE 1: ELECTRODE COMPOSITION ANTHRACITE (GAS-CALCINED)

(4) A first example for a self-calcining electrode composition for electric arc furnaces has the following components: 22% of bitumen as binder; 28% of coke in the form of coke dust with a grain size fraction of 0<x0.21 mm; 2% of graphite in the form of graphite dust. Coke dust and graphite dust have a specific surface according to Blaine of 4500-6000 Blaine; 11% of a mixture of gas-calcined anthracite with coke in the mixing ratio of gas-calcined anthracite:coke=3:1 with a fine grain fraction of 0<x0.84 mm; 15% of a mixture of gas-calcined anthracite with coke in the mixing ratio of gas-calcined anthracite:coke=3:1 with an average grain fraction of 0.84-3.36 mm; 17% of a mixture of gas-calcined anthracite with coke in the mixing ratio of gas-calcined anthracite:coke=3:1 with a coarse grain fraction of 3.36-20 mm and 5% of graphite with a grain fraction between 1 and 25 mm.

(5) In response to the production of the self-calcining electrode composition, the above-mentioned components as well as optionally further additives are broken, ground and classified. The obtained dry mixture is subsequently preheated to 120 to 200 C., preferably to 160-180 C., particularly preferably to 175 C. and is mixed at this temperature.

(6) For the addition to the dry mixture, the binder bitumen is heated to 65 C. above the softening point (softening point ring and ball according to DIN EN 12591). The processing of the dry mixture and bitumen binder takes place in batches or continuously in a mixture, for example a temperature-controlled, oscillating mixing and kneading screw, until the desired homogeneity is reached. The obtained mixture is shaped and cooled subsequently, either into briquettes or cylinders, blocks.

EXAMPLE 2: ELECTRODE COMPOSITION COKE

(7) A second example for a self-calcining electrode composition for electric arc furnaces has the following components: 24% of bitumen as binder; 28% of coke in the form of coke dust with a grain size fraction of 0<x0.21 mm; 3% of graphite in the form of graphite dust. Coke dust and graphite dust have a specific surface according to Blaine of 4500-6000 Blaine; 8% of coke with a fine grain fraction of 0<x0.84 mm; 17% of coke with an average grain fraction of 0.84-3.36 mm; 15% of gas-calcined anthracite with a coarse grain fraction of 3.36-20 mm and 5% of graphite with a grain fraction between 1-25 mm.

(8) The above-listed components are mixed at a mixing temperature of 175 C.

(9) In response to the production of the self-calcining electrode composition, the above-mentioned components as well as optionally further additives are broken, ground and classified. The obtained dry mixture is subsequently preheated to 120 to 200 C., preferably to 160-180 C., particularly preferably to 175 C. and is mixed at this temperature.

(10) For the addition to the dry mixture, the binder bitumen is heated to 65 C. above the softening point (softening point ring and ball according to DIN EN 12591). The processing of the dry mixture and bitumen binder takes place in batches or continuously in a mixture, for example a temperature-controlled, oscillating mixing and kneading screw, until the desired homogeneity is reached. The obtained mixture is shaped and cooled subsequently, either into briquettes or cylinders, blocks.

EXAMPLE 3: ELECTRODE COMPOSITION ECA (ELECTRICALLY CALCINED ANTHRACITE)

(11) A third example of a self-calcining electrode composition for electric arc furnaces has the following components: 26% of bitumen as binder; 35% of electrically calcined anthracite in the form of anthracite dust with a grain size fraction of 0<x0.21 mm and a specific surface according to Blaine of 4500-6000 Blaine; 5% of electrically calcined anthracite with a fine grain fraction of 0<x0.84 mm; 5% of electrically calcined anthracite with an average grain fraction of 0.84-3.36 mm, and 29% of electrically calcined anthracite with a coarse grain fraction of 3.36-20 mm.

(12) Comparison of typical characteristic values of an electrode composition with coal tar pitch as binder and a bituminous electrode composition after the calcination at 1000 C. (laboratory test) according to the preceding second example:

(13) TABLE-US-00001 Electrode Electrode composition composition Properties (bitumen) (pitch) Unit Standard Calcined 1.40 1.49 g/cm.sup.3 ISO 12985-1 density Electrical 105 70 m DIN 51911 resistance Bending 3 6 MPa ISO 51902 strength e-module 0.7 1.2 GPa Rheinfelden (static) method e-module 2.9 6.5 GPa Rheinfelden (dynamic) method Compressive 13 25 MPa ISO 18515 strength Thermal 2.2 2.6 W/mK Hot disk conductivity Method Ash content 2.5 2.5 % ISO 8005 Benzo(a)pyrene 0.01 3000 ppm DIN EN ISO 17993 EPA 16 PAC 0.1 25000 ppm DIN EN ISO 17993

(14) Tests from the practice (sample collection from an electric arc furnace for the production of ferrosilicon) will be explained below. In these practical tests, an electrode composition according to the above-described second example was used, for which the laboratory test was conducted as well.

(15) Using the bitumen, which is suitable for the invention, a baked electrode results, which shows the below-listed characteristic material values. The measured samples are cylindrical sample bodies from the baked electrode. They originate from an electric arc furnace for the production of ferrosilicon and were removed 20 cm below the contact jaws.

(16) Analysis values of this bituminous electrode as compared to a coal tar pitch-bonded electrode are illustrated in the below table.

(17) TABLE-US-00002 Electrode Electrode Properties 1 (bitumen) 2 (pitch) Unit Standard Calcined 1.42 1.59 g/cm.sup.3 ISO 12985-1 density Electrical 35 47 m DIN 51911 resistance Bending 3.2 7.5 MPa ISO 51902 strength e-module 2.8 5.8 GPa Rheinfelden (static) method Compressive 10 28 MPa ISO 18515 strength Thermal 31 16 W/mK Hot disk conductivity Method

(18) The differences between the laboratory values and the values, which originate from the production plant, are significant in particular in the case of the electrical resistance and prove the good baking behavior of the bituminous electrode, the improvement of the elastic deformability as well as an easy graphitization of the electrode.