Electrode mass

Abstract

Self-calcining electrode compound for electric arc furnaces, containing one or more carbon components and a binder, characterized in that the binder is exclusively hard bitumen with a needle penetration at 25°C according to DIN EN 1426 of 0 to 20 per 0.1 mm and / or a softening point (ring and ball) according to DIN EN 1427 of 80°C to 110°C, and with a density at 25°C according to DIN EN 52004 of 0.5 to 2 g / cm³ 3 is, and that the electrode mass has a polycyclic aromatic hydrocarbon or PAH content of <500 ppm, and that the hard bitumen is obtained by flash distillation of soft and medium-hard bitumen grades.

Description

TECHNICAL AREA

[0001] The invention relates to a bitumen-bound self-calcining electrode mass for Söderberg electrodes of an electric arc furnace and a baked electrode produced from the self-calcining electrode mass. STATE OF THE ART

[0002] The technology of Söderberg electrodes dates back to the beginning of the 20th century. This electrode technology is primarily used in the field of aluminum molten salt electrolysis and as a Söderberg electrode in electric arc furnaces, for example in melt-reduction furnaces.

[0003] The term Söderberg electrode refers to self-baking or self-calcining electrodes based on the following technical principle: An electrode mass (solid at room temperature) comprising carbon supports such as anthracite, petroleum coke, graphite, and a coal tar pitch binder melts under process heat at 120–200°C, forming a liquid to pasty mass that fills the metal casing and all cavities of the conductive plates. At approximately 500°C, the electrode mass transitions into a solid state, and its electrical resistance decreases. The coke formed from the binder is present in amorphous form. Graphitization of the entire electrode begins at temperatures above 1800°C.

[0004] The Söderberg electrode for melting reduction furnaces comprises a sheet metal casing with ribs (so-called guide plates) arranged on its inner surface. The casing is continuously filled with electrode material, for example, in the form of briquettes, blocks, or cylinders. To extend the electrode during operation, additional sheet metal casings are welded on. The Söderberg electrode for melting reduction furnaces used in the production of silicon metals is a special type of Söderberg electrode without guide plates, in which a graphite electrode, known as ELSA or composite electrodes, is embedded within the electrode material. The energy input that transforms the electrode material into the baked, electrically conductive electrode results from both the process heat of the furnace and the current flowing through the contact plates into the electrode.

[0005] In almost all commercially available electrode compounds for Söderberg electrodes used in electric arc furnaces, coal tar pitch is used as a binder because it has a high coke residue content with excellent binder properties. Despite its high content of potentially harmful polycyclic aromatic hydrocarbons, coal tar pitch has become the standard. All attempts to use non-toxic binders have failed to date.

[0006] With the aim of replacing coal tar pitch as a binder, the use of bitumen in self-calcining anodes for aluminum molten salt electrolysis was investigated in more detail in the mid-1980s. Unlike coal tar pitch, bitumen contains polycyclic aromatic hydrocarbons only in exceptionally small concentrations that are not harmful to health. EP0155230A1 proposes an electrode compound for self-calcining electrodes which, in addition to the usual components anthracite, graphite, petroleum coke, and pitch coke, necessarily includes a mixture of 70–90 wt.% hard bitumen and 10–30 wt.% soft bitumen as a binder. EP0155230A1 explicitly states that neither soft bitumen nor hard bitumen alone is suitable as a binder for the carbon compound. The use of hard bitumen or soft bitumen alone was deemed impractical.

[0007] Hard bitumen is defined as bitumen with a ring and ball softening point of approximately 80-110°C, while soft bitumen has a softening point of approximately 40-65°C, with the density of both types of bitumen not exceeding 1.1 g / cm³. 3(at 25°C). As mentioned in EP01 55230A1, bitumen, due to its low coke residue of approximately 38 wt.%, is generally considered an unsuitable binder for self-calcining electrodes, in contrast to coal tar pitch, which has a coke residue of approximately 50-60 wt.%. Besides the low coke residue, bitumen has a high proportion of volatile components compared to coal tar pitch. In aluminum molten salt electrolysis, aluminum oxide is dissolved in molten cryolite and converted to metallic aluminum at temperatures of around 960°C in an electrolysis cell. The electrolysis is carried out in carbon-lined steel tanks, the bottom of which simultaneously serves as the cathode. The electrolyte and the molten aluminum are contained in the tank. When using self-calcining or self-baking anodes, current is supplied via vertical or horizontal current bars. The anode is immersed in the electrolyte.At the aforementioned process temperature, the volatile components, particularly those of the binder used, escape. The baked anode is therefore characterized by a porous carbon body that is immersed in the electrolysis bath. In practice, it was observed that the anode becomes saturated with liquid electrolyte and is thereby sealed gas-tight. The inevitable escape of the volatile components through the molten mass further increases the porosity. The anodes became mechanically unstable and fell off the current rods. Attempts to replace coal tar pitch as a binder in Söderberg anodes were not pursued further. PRESENTATION OF THE INVENTION

[0008] The object of the present invention is to develop a self-calcining electrode compound for the electric arc furnace which does not have the disadvantages of known self-calcining electrode compounds.

[0009] According to the invention, the problem is solved by a self-calcining electrode mass according to claim 1.

[0010] Furthermore, claim 8 claims a baked electrode made from the self-calcining electrode mass according to the invention.

[0011] When the term electrode material for Söderberg electrodes is used below, this includes Söderberg electrodes with and without a graphite core.

[0012] Electrode mass refers to the mass of binder and carbon components and optional additives, which at the end of the mixing and kneading process is formed, cast or cut into a shape of different sizes suitable for the intended application, e.g. briquettes and cylinders.

[0013] A baked electrode is understood to be an electrode in its solid state, which forms from the so-called baking zone and in zones below this baking zone. This solid state occurs when the electrode mass, originally solid in a metered starting material (briquettes and cylinders), is first transformed into a paste-like mass and then into a solid, i.e., baked, electrode.

[0014] The term "bitumen" used in this document refers to a low-volatility mixture of various organic substances obtained during the processing of crude oil, whose viscoelastic behavior changes with temperature (see also, for example, Römpp Chemie Lexikon, 9th corrected edition, Georg Thieme Verlag, Stuttgart, New York and especially DIN 55946).

[0015] The bitumen suitable for the application according to the invention is exclusively hard bitumen, which is obtained by additional flash distillation of soft and medium-hard bitumen products and is characterized by a hard to spring-hard consistency. The soft and medium-hard bitumens are themselves obtained as residues from a first distillation carried out under atmospheric pressure. The two successive distillation steps are also called two-stage distillation.

[0016] The term “flash distillation” (also called high-vacuum distillation) used for the present application refers to the additional processing step of soft and medium-hard bitumen which is carried out under vacuum (e.g. 2 to 120 mm Hg) and elevated temperature (e.g. 310 to 370°C).

[0017] Particularly preferred for the present application is hard bitumen, which is characterized by a high sulfur content, preferably 5-7%, and is obtained from a crude oil with a high content of organically bound sulfur.

[0018] Furthermore, this hard bitumen suitable for the invention has a coke residue of 25-45%.

[0019] The applicants have surprisingly found that the specific properties of the hard bitumen obtained according to the above selection criteria acted synergistically for self-calcining electrode masses and baked electrodes made from these self-calcining electrode masses, and are an excellent binder for electric arc furnaces.

[0020] Due to the higher proportion of volatile components in the hard bitumen suitable for the invention, compared to conventional coal tar pitch-bound materials, the resulting baked electrode exhibits significantly higher porosity than a baked electrode containing coal tar pitch as a binder. The mechanical properties, specifically flexural strength, compressive strength, and static and dynamic modulus of elasticity, are lower than those of conventional coal tar pitch-bound electrodes. The high porosity of the baked electrode facilitates the venting of volatile gases into the furnace chamber of the electric arc furnace. Cracking processes on the hot electrode carbon (1000–1500°C) convert some of the volatile components into binder-active carbon. The high sulfur content of the hard bitumen promotes the crosslinking of this binder carbon and improves the electrode's elasticity.The higher elasticity compensates for the reduction in mechanical strength.

[0021] The graphitization of the electrode that follows baking occurs more rapidly and already in a zone just below the contact jaws.

[0022] The hard bitumen suitable for the present self-calcining electrode mass is characterized by properties acquired through additional flash distillation (favorable for the present application), such as softening temperature, needle penetration and / or density.

[0023] Hard bitumen with the following properties is therefore particularly suitable: (i) a needle penetration test at 25°C according to DIN EN 1426 from 0 to 20 [per 0.1 mm], preferably 0 to 6 [per 0.1 mm] and / or (ii) a softening point (ring and ball) according to DIN EN 1427 of 80 to 110°C, preferably 85 to 100°C; and (iii) a density at 25°C according to DIN EN 52004 of 0.5 to 2 g / cm³ 3preferably 1.0 to 1.2 g / cm³ 3 .

[0024] According to one embodiment, the hard bitumen has penetration grades of 30 / 45 or 20 / 30 (according to DIN EN 12591) or 10 / 15 (according to DIN EN 13305).

[0025] Hard bitumen is particularly preferred, characterized by a combination of needle penetration at 25°C according to DIN EN 1426 between 0 and 6 [per 0.1 mm] and / or a softening point (ring and ball) according to DIN EN 1427 of 85 to 100°C and a density at 25°C according to DIN EN 52004 of 1.0 to 1.2 g / cm³. 3 distinguishes.

[0026] The amount of hard bitumen used for the production of the electrode mass according to the invention is at most 35 wt.%, preferably 15 to 30 wt.%, more preferably 20 to 25 wt.% based on the electrode mass.

[0027] Hard bitumen is obtained by flash distillation of soft and medium-hard bitumen types.

[0028] Particularly preferred for the present application is hard bitumen, characterized by a high sulfur content, preferably 5-7%, and obtained from crude oil with a high content of organically bound sulfur. In addition to the hard bitumen, the self-calcining electrode mass according to the invention is produced with a dry mixture consisting of one or more carbon components, preferably coke and / or anthracite and / or graphite, and optionally additives such as sulfur, CaF₂, CaO, and metal oxides such as Fe₂O₃ and Al₂O₃.

[0029] The term coke refers to any conventional, commercially available coke product, such as petroleum, needle, and pitch coke, as well as low-ash metallurgical cokes. A coke with a particle size distribution of 0 < x ≤ 50 is preferred, preferably 0.2–25 mm. In a particular embodiment, the coke is present in an amount of at most 60 wt.%, based on the electrode mass. More preferably, the coke is present in an amount of 30 to 60 wt.%, preferably 35 to 55 wt.%, based on the electrode mass.

[0030] The term anthracite preferably refers to calcined anthracite, in particular electrically calcined or gas-calcined anthracite. In a preferred embodiment, gas-calcined anthracite is present in an amount of at most 60% by weight, based on the electrode mass. More preferably, gas-calcined anthracite is present in an amount of 10 to 40% by weight, preferably 15 to 35% by weight, based on the electrode mass. In another preferred embodiment, electrically calcined anthracite is present in an amount of at most 80% by weight, based on the electrode mass. More preferably, electrically calcined anthracite is present in an amount of 65 to 80% by weight, preferably 70 to 75% by weight, based on the electrode mass.

[0031] The term graphite refers specifically to graphite powder or graphite that has been crushed, for example, by breaking and grinding. Furthermore, the term graphite encompasses both synthetic and natural graphites. Synthetic graphites can be either primary or derived from recycled graphite. Recycled graphite includes, for example, electrode remnants that are processed to the desired particle size for the production of electrode material.

[0032] Preferably graphite with a particle size in the range of 0.01 µm [micrometers] to 1 mm, preferably in the range of 1 to 300 µm [micrometers], most preferably in the range of 2 to 20 µm [micrometers].

[0033] Furthermore, the graphite is also present in granular form. A preferred particle size range for the grains is between 0 < x ≤ 50, preferably between 0.2 and 25 mm.

[0034] In a preferred embodiment, graphite is present in an amount of at most 25 wt.%, based on the electrode mass. More preferably, graphite is present in an amount of 3 to 12 wt.%, preferably 5 to 10 wt.%, based on the electrode mass.

[0035] For the production of the electrode mass according to the invention, the aforementioned components can have different grain sizes. In a preferred embodiment, the components coke, anthracite (gas-calcined or electrically calcined), and graphite have a grain size between 0 < x ≤ 50, preferably 0.2–25 mm.

[0036] Furthermore, this hard bitumen suitable for the invention has a coke residue of 25-45%.

[0037] In certain embodiments based on gas-calcined anthracite, the self-calcining electrode masses according to the invention therefore preferably contain (i) 15-30 wt.%, preferably 20-25 wt.%, hard bitumen and (ii) 10-40 wt.%, preferably 15-35 wt.%, gas-calcined anthracite,

[0038] These mixtures may additionally contain as a third component a maximum of 60 wt.%, in particular 30-60 wt.%, preferably 35-55 wt.%, coke.

[0039] As an alternative third or fourth component, these electrode masses can further contain a maximum of 15 wt.%, in particular 3-12 wt.%, preferably 5-10 wt.% graphite.

[0040] In other embodiments based on electrically calcined anthracite, the self-calcining electrode masses according to the invention preferably contain (i) 20-35 wt.%, preferably 25-30 wt.%, hard bitumen and (ii) 65-80 wt.%, preferably 70-75 wt.% electrically calcined anthracite.

[0041] Laboratory tests have shown that bitumen-bonded electrode materials, after calcination at approximately 1000 °C, exhibit lower mechanical properties with regard to flexural strength, compressive strength, and static and dynamic modulus of elasticity compared to coal tar pitch-bonded electrode materials. It has also been shown that the bitumen-bonded electrode materials have a higher electrical resistance and lower thermal conductivity than the coal tar pitch-bonded materials. Under normal operating conditions in the electric arc furnace, corresponding mechanical stresses occur in the electrode body during the phase transition from pasty to baked electrode material. In the temperature range between 500 °C and 1000 °C, the electrode body shrinks, while above 1000 °C, expansion is observed.The bitumen-bonded electrode material results in a baked electrode body with a higher porosity than, for example, a coal tar pitch-bonded material. This higher porosity of the electrode body can be advantageously used, for example, to dissipate gases that arise during the phase transformation of the electrode material and to minimize the resulting mechanical stresses.

[0042] An equally important advantage is that, due to the bitumen used, the electrode masses according to the invention have extremely low polycyclic aromatic hydrocarbons or PAH contents of < 500ppm and therefore no toxic fumes and dusts are produced when used in the electric arc furnace. WAYS TO IMPLEMENT THE INVENTION Fig. Figure 1 shows a highly simplified, sectional view of a self-calcining Söderberg electrode for an electric arc furnace, according to the state of the art.

[0043] In Fig.Figure 1 shows the temperature zones of a self-calcining Söderberg electrode based on a conventional coal tar pitch binder. The electrode mass 1, pressed into briquettes or cylinders, is fed into a cylindrical housing 3 and exists in solid form in zone 2 at a temperature of approximately 80 °C. A power supply 4 is located on the outside of the housing. Electrical energy is supplied to the electrode mass via the contact jaws 5. The thermal energy emitted by the melt 9 serves as an additional energy source. Due to the energy input, the electrode mass reaches a paste-like consistency at approximately 130 °C. In the baking zone 6, between 500 °C and 1000 °C, the volatile components escape and the electrode mass transitions into a solid state. In zone 7, between 1000 °C and 1500 °C, the carbon exists in amorphous form. Graphitization occurs in zone 8, above 2000 °C.The unbaked electrode mass, in the temperature range between 80°C and approximately 500°C, is electrically non-conductive. Energy is transferred electrically via the electrical resistance of the electrode mass. Above approximately 500°C, the electrical resistance of the electrode mass decreases, and it becomes electrically conductive. At the electrode tip, the graphitized electrode 8 is surrounded by a plasma or arc (in . Fig. 1 not visible).

[0044] Using the bitumen suitable for the invention, graphitization of the electrode is achieved even below the contact jaws. Example 1: “Electrode compound anthracite (gas calcined)”

[0045] A first example of a self-calcining electrode compound for electric arc furnaces has the following components: 22% bitumen as a binder; 28% coke in the form of coke dust with a particle size fraction of 0 < x ≤ 0.21 mm; 2% graphite in the form of graphite dust. Coke dust and graphite dust have a specific surface area according to Blaine of 4500–6000 Blaine; 11% of a mixture of gas-calcined anthracite with coke in a gas-calcined anthracite:coke ratio of 3:1 with a fine particle fraction of 0 < x ≤ 0.84 mm; 15% of a mixture of gas-calcined anthracite with coke in a gas-calcined anthracite:coke ratio of 3:1 with a medium particle fraction of 0.84–3.36 mm. 17% of a mixture of gas-calcined anthracite with coke in a mixing ratio of gas-calcined anthracite:coke = 3:1 with a coarse grain fraction of 3.36 - 20 mm and 5% graphite with a grain fraction between 1 and 25 mm.

[0046] In the production of the self-calcining electrode compound, the aforementioned components, as well as optionally further additives, are crushed, ground, and classified. The resulting dry mixture is then preheated to 120 to 200°C, preferably to 160 to 180°C, and particularly preferably to 175°C, and mixed at this temperature.

[0047] The bitumen binder is heated to 65°C above its softening point (ring and ball softening point according to DIN EN 12591) before being added to the dry mix. The dry mix and bitumen binder are processed batchwise or continuously in a mixer, such as a temperature-controlled, oscillating mixing and kneading screw, until the desired homogeneity is achieved. The resulting mixture is then shaped and cooled, either into briquettes, cylinders, or blocks. Example 2: “Electrode mass coke”

[0048] A second example of a self-calcining electrode compound for electric arc furnaces has the following components: 24% bitumen as a binder; 28% coke in the form of coke dust with a particle size fraction of 0 < x ≤ 0.21 mm; 3% graphite in the form of graphite dust. Coke dust and graphite dust have a specific surface area according to Blaine of 4500–6000 Blaine; 8% coke with a fine particle size fraction of 0 < x ≤ 0.84 mm; 17% coke with a medium particle size fraction of 0.84–3.36 mm; 15% gas-calcined anthracite with a coarse particle size fraction of 3.36–20 mm; 5% graphite with a particle size fraction of 1–25 mm.

[0049] The aforementioned components are mixed at a mixing temperature of 175°C.

[0050] In the production of the self-calcining electrode compound, the aforementioned components, as well as optionally further additives, are crushed, ground, and classified. The resulting dry mixture is then preheated to 120 to 200°C, preferably to 160 to 180°C, and particularly preferably to 175°C, and mixed at this temperature.

[0051] The bitumen binder is heated to 65°C above its softening point (ring and ball softening point according to DIN EN 12591) before being added to the dry mix. The dry mix and bitumen binder are processed batchwise or continuously in a mixer, such as a temperature-controlled, oscillating mixing and kneading screw, until the desired homogeneity is achieved. The resulting mixture is then shaped and cooled, either into briquettes, cylinders, or blocks. Example 3: Electrode material ECA (electrically calcined anthracite)

[0052] A third example of a self-calcining electrode compound for electric arc furnaces has the following components: 26% bitumen as a binder, 35% electrically calcined anthracite in the form of anthracite dust with a particle size fraction of 0 < x ≤ 0.21 mm and a specific surface area according to Blaine of 4500-6000 Blaine; 5% electrically calcined anthracite with a fine particle size fraction of 0 < x ≤ 0.84 mm; 5% electrically calcined anthracite with a medium particle size fraction of 0.84 - 3.36 mm and 29% electrically calcined anthracite with a coarse particle size fraction of 3.36 - 20 mm.

[0053] Comparison of typical characteristic values ​​of an electrode mass with coal pitch as a binder and a bitumen-bound electrode mass after calcination at 1000 °C (laboratory test) according to the preceding second example: Characteristics Electrode compound (bitumen) Electrode mass (pitch) Unit standard Calcined density 1,40 1,49 g / cm 3 ISO 12985-1 Electrical resistance 105 70 µΩm DIN 51911 Flexural strength 3 6 MPa ISO 51902 E-modulus (static) 0,7 1,2 GPa Rheinfelden Method E-modulus (dynamic) 2,9 6,5 GPa Rheinfelden Method compressive strength 13 25 MPa ISO 18515 Thermal conductivity 2,2 2,6 W / mK Hot Disk Method Ash content 2,5 2,5 % ISO 8005 Benzo[a]pyren 0,01 3000 ppm DIN EN ISO17993 EPA 16 PAK 0,1 25000 ppm DIN EN ISO17993

[0054] The following describes practical experiments (sampling from an electric arc furnace for the production of ferrosilicon). In these practical experiments, an electrode compound corresponding to the second example described previously was used, which was also used in the laboratory experiment.

[0055] Using the bitumen suitable for the invention, a baked electrode is produced which exhibits the material properties listed below. The measured samples are cylindrical specimens from the baked electrode. These originate from an electric arc furnace for the production of ferrosilicon and were taken 20 cm below the contact jaws. The following table presents analytical values ​​of this bitumen-bonded electrode in comparison to a coal tar pitch-bonded electrode. Characteristics Electrode 1 (bitumen) Electrode 2 (pitch) Unit standard Calcined density 1,42 1,59 g / cm 3 ISO 12985-1 Electrical resistance 35 47 µΩm DIN 51911 Flexural strength 3,2 7,5 MPa ISO 51902 E-module 2,8 5,8 GPa Rheinfelden Method compressive strength 10 28 MPa ISO 18515 Thermal conductivity 31 16 W / mK Hot Disk Method

[0056] The differences between the laboratory values ​​and the values ​​from the production plant are particularly significant in terms of electrical resistance and demonstrate the good baking behavior of the bitumen-bonded electrode, the improvement in elastic deformability and a slight graphitization of the electrode. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] EP 0155230A1 [0006, 0007]

Claims

[1] Self-calcining electrode compound for electric arc furnaces, containing one or more carbon components and a binder, characterized by , that the binder consists exclusively of hard bitumen with a needle penetration at 25°C according to DIN EN 1426 of 0 to 20 per 0.1 mm and / or a softening point (ring and ball) according to DIN EN 1427 of 80°C to 110°C, and with a density at 25°C according to DIN EN 52004 of 0.5 to 2 g / cm³ 3 is, and that the electrode mass has a polycyclic aromatic hydrocarbon or PAH content of <500 ppm, and that the hard bitumen is obtained by flash distillation of soft and medium-hard bitumen grades. [2] Self-calcining electrode compound according to claim 1, characterized by that the hard bitumen has a high sulfur content, preferably 5-7%, and is obtained from crude oil with a high content of organically bound sulfur. [3] Self-calcining electrode compound according to any one of the preceding claims, characterized by that the hard bitumen is present in an amount of at most 35 wt.%, preferably 15 to 30 wt.% based on the electrode mass. [4] Self-calcining electrode compound according to any one of the preceding claims, characterized by that the one or more carbon components are selected from anthracite, preferably gas-calcined or electrically calcined anthracite, coke and graphite. [5] Self-calcining electrode compound according to claim 4, characterized by , that anthracite (a) is contained as gas-calcined anthracite in an amount of at most 60 wt.%, preferably 10 to 40 wt.% based on the electrode mass or (b) is contained as electrically calcined anthracite in an amount of at most 80 wt.%, preferably 65 to 80 wt.% based on the electrode mass. [6] Self-calcining electrode compound according to claim 4, characterized bythat coke is present in an amount of at most 60 wt.%, preferably 30 to 60 wt.% based on the electrode mass. [7] Self-calcining electrode compound according to claim 4, characterized by that graphite is present in an amount of at most 15 wt.%, preferably 3 to 12 wt.% based on the electrode mass. [8] Self-calcining electrode compound according to any one of the preceding claims, characterized by , that the hard bitumen has a needle penetration at 25 °C according to DIN EN 1426 of 0 to 6 per 0.1 mm and / or a softening point ring and ball according to DIN EN 1427 of 85°C to 100°C, and a density at 25 °C according to DIN EN 52004 of 1.0 to 1.2 g / cm3. [9] Baked electrode obtained by baking an electrode mass according to one of the preceding claims.

Images