EP1711973A1

EP1711973A1 - Method for the production of graphite electrodes

Info

Publication number: EP1711973A1
Application number: EP04803711A
Authority: EP
Inventors: Ludwig Kolb
Original assignee: SGL Carbon SE
Current assignee: SGL Carbon SE
Priority date: 2003-12-18
Filing date: 2004-12-10
Publication date: 2006-10-18
Also published as: WO2005060027A1; FR2864417A1; MY146459A

Abstract

The invention relates to a method for producing a graphite electrode comprising an electrode body and at least one connecting element that is disposed on one of the two faces of the electrode body. According to said method, the electrode body and the at least one connecting element are produced separately by forming a carbon-containing material, baking the same, impregnating the same once or several times, and graphitizing the same before the electrode body and the connecting element are joined together. In order to increase the physical stability, especially the bending strength, of the graphite electrode, the electrode body and/or the connecting element is/are impregnated and refired once again following graphitizing.

Description

Process for the production of graphite electrodes

The present invention relates to a method for producing a graphite electrode consisting of an electrode body and at least one connecting element arranged on one of the two end faces of the electrode body, in which the electrode body and the at least one connecting element are separated from one another by a method comprising the steps of forming a carbon-containing mass , Firing, impregnation and graphitization are produced before the electrode body and the at least one connecting element are connected to one another, and the graphite electrodes, electrode body and connecting elements obtainable with such a method.

Graphite electrodes are used, for example, in electric arc furnaces for steel production. When a voltage is applied to the graphite electrode, an arc is formed which leads from the lower end of the electrode to the melting material and which melts the melting material, for example steel scrap or sponge iron. Caused by the arc and the high temperatures inside the furnace, the lower end of the electrode burns slowly and the electrode is consumed. For this reason, electrode strands are usually used, which are pushed into the arc furnace to the extent that the lower end of the electrode burns off during furnace operation.

Such an electrode string usually consists of individual, interconnected cylindrical electrodes or electrode bodies, which over

Fasteners are mechanically and electrically connected. The individual electrode bodies have for receiving the connecting elements truncated cone-like depressions with an internal thread on their two end faces, while the connecting elements have the shape of a double cone, that is to say two truncated cones joined together with their base surface, on the outer surfaces of which an external thread is arranged. To compensate for the electrode erosion during furnace operation, the electrode strands are pushed in the direction of the melt by appropriate movement of a support arm which is connected to the upper end of the electrode strand. As soon as the electrode string falls below a defined minimum length, a new electrode body is screwed onto the upper end of the electrode string via a connecting element and the support arm is shifted upwards accordingly.

When operating the arc furnaces, the electrode strands are subjected to high mechanical loads. Particularly in the case of strong inhomogeneities in the steel melt, for example due to scrap parts floating on it, strong bending forces occur at the upper end of the electrode strand in the vicinity of the support arm, which lead to a breakage of the connecting elements on their equator or the electrode body on their recesses arranged on the end face can. The broken electrode strand then falls into the melt, which results in considerable production downtimes. A break in the upper part of the electrode strand in particular has particularly serious consequences, since in this case the entire strand falls into the melt and has to be recovered in a considerable amount of time.

In order to address these problems, a number of measures have already been proposed to increase the mechanical strength of graphite electrodes.

From US 6,500,022, for example, specially shaped connecting elements for connecting electrodes are known in which the thread forms end side walls of the double cone with respect to the central axis have an angle between 75 and 90 °. This special arrangement is intended to reduce the mechanical stresses in the area of the connection to the electrode bodies. However, the production of these connecting elements is very expensive.

US Pat. No. 6,058,133 discloses a cylindrical graphite electrode body which has a slot running helically along the electrode body. This slot is intended to prevent cracks from spreading by breaking through the slot as a result of the high temperature as it spreads, thereby preventing further spreading. However, the crack phenomena on electrodes are very diverse, so that in general it cannot be predicted in which direction and at what depth cracks will form. In many cases, the slots on the electrode that were originally introduced to contain the cracks can themselves be the starting points for cracks or prevent the spreading of cracks at all or only insignificantly.

It is also known that the mechanical strength of electrode bodies and connecting elements can be increased by multiple impregnation cycles before the graphitization. An impregnation cycle generally comprises the impregnation of the shaped bodies with pitch under vacuum, followed by afterburning at about 800 ° C. (cf. Ullmann's Encyclopedia for Industrial Chemistry 2002, Vol. 6, page 313). However, the mechanical strength can only be increased slightly as the number of impregnation cycles increases. However, this slight improvement is offset by the relatively high costs of a further impregnation step. The cost-benefit ratio also deteriorates with every further impregnation cycle. In addition, with increasing number of impregnation steps, the penetration of the shaped body through the Impregnant severely restricted. There are undesirable inhomogeneities across the cross section of the resulting strand elements.

The invention is therefore based on the object of cost-effectively and effectively improving the strength of graphite electrodes.

According to the invention, this object is essentially achieved in a method of the type mentioned at the outset in that the electrode body and / or the at least one connecting element are at least one more time impregnated and re-fired after the graphitization.

Surprisingly, graphite electrodes with excellent mechanical stability can be obtained by the additional impregnation after the graphitization, so that they reliably withstand the mechanical loads, in particular bending forces, that occur during the operation of an electric arc furnace. In view of the only slight increase in strength due to additional impregnation cycles before graphitization, it was unexpected for the person skilled in the art that an additional impregnation step after graphitization leads to an incomparably greater increase in the mechanical strength, in particular the flexural strength, of the treated electrode body and / or connecting element , For example, with the method according to the invention, after two impregnations before the graphitization, an additional impregnation step after the graphitization can be used to obtain connecting elements with a bending strength that is 15 to 20% higher, whereas an additional impregnation step before the graphitization with otherwise the same treatment only for connecting elements with a leads to 5 to 10% higher bending strength.

It was also surprising that electrode strands with a mechanical strength sufficient for use in electric arc furnaces were not can only be obtained by additional impregnation of the electrode body and connecting elements, but also if only either the electrode body or the connecting element of the graphite electrode is subjected to an impregnation step after the graphitization.

After molding, the electrode bodies or connecting elements are burned in a manner known per se, preferably at temperatures between 700 and 1,300 ° C., particularly preferably at about 800 ° C., for about 500 hours in an oven under an inert gas or reducing atmosphere before the Electrodes are subjected to an impregnation step one or more times and then graphitized at temperatures of more than 2,500 ° C., particularly preferably more than 2,750 ° C. and very particularly preferably at about 3,000 ° C.

According to the invention, the impregnation of the electrode body and / or the connecting element to be carried out after the graphitization can be carried out in any manner known to the person skilled in the art, in particular with the customary impregnation agents. The same impregnating agent, particularly preferably impregnating pitch, is preferably used both in the at least one impregnation step to be carried out before the graphitization and in the at least one impregnation step after the graphitization. Alternatively, it is of course also possible, although less preferred, to use different impregnation agents or conditions for the individual impregnation steps.

In a development of the inventive concept, it is proposed to carry out all or at least some of the impregnation steps under vacuum.

The objects to be treated are incubated with the impregnating agent for a time known to the person skilled in the art for this purpose. Preferably, the electrode body and / or the at least one connecting element incubated with the impregnating agent for 2 to 10 hours, particularly preferably 4 to 6 hours.

According to a particular embodiment of the present invention, after each impregnation step, regardless of whether it is an impregnation before or after the graphitization, afterburning takes place between 600 and 1000 ° C., preferably between 700 and 900 ° C. and very particularly preferably between 750 and 850 ° C. The afterburning is preferably carried out for 60 to 180 hours, particularly preferably 75 to 100 hours.

The method according to the invention is not limited with regard to the number of impregnation steps to be carried out before or after the graphitization. Particularly good results are obtained, however, if an electrode body is impregnated once before the graphitization and after the graphitization. For connection elements, however, a double impregnation before the graphitization followed by an impregnation after the graphitization is preferred. It goes without saying that the mechanical strengths, in particular bending strengths, both of the electrode body and of the connecting elements can be increased further if they are each subjected to two impregnation cycles after the graphitization.

Electrode strands with the best possible mechanical properties are obtained if both the individual electrode bodies and the connecting elements are subjected to an impregnation after the graphitization. However, electrode strands with sufficient mechanical strength, in particular sufficient bending strength at the upper end, can also be obtained if only the individual electrode bodies or the individual connecting elements are subjected to an impregnation after the graphitization. Depending on the intended use, it may be sensible to impregnate only the electrode body or the connecting elements according to the invention after the graphitization.

Surprisingly, the graphite electrodes, electrode bodies and connecting elements obtainable with the method according to the invention are distinguished by improved mechanical properties, in particular higher mechanical strengths against bending forces, compared to those produced by a method according to the prior art. Depending on how the process is carried out, the mechanical strength can be increased by up to 20%.

Electrode bodies with a diameter of 500, 600, 700 or 800 mm or connecting elements, for example with a diameter of 325 mm, obtainable with the method according to the invention by a single impregnation step after the graphitization typically have the following properties:

Electrode body density [g / cm ³ ] 1, 7 to 1, 8 modulus of elasticity * [GPa] 9 to 13 bending strength * [MPa] 12 to 17 Specific electrical resistance [μΩ m] 5.0 to 5.7

Connecting element density [g / cm ³ ] 1, 8 to 2 modulus of elasticity * [GPa] 23 to 30 flexural strength * [MPa] 25 to 35 Specific electrical resistance [μΩ m] 3.0 to 3.6 ^* parallel to the direction of extrusion, ie longitudinal Another object of the present invention are graphite electrodes consisting of an electrode body and at least one connecting element, which can be obtained with the method according to the invention. These graphite electrodes are distinguished from the electrodes known from the prior art by their higher mechanical strength, in particular by their higher flexural strength.

Furthermore, the present invention relates to an electrode body, which for the purposes of the present invention are also graphite electrodes, obtainable by a method comprising the steps of forming a carbon-containing mass, firing, single or multiple impregnation and graphitization, in which the electrode body has at least one after the graphitization is impregnated and re-fired again. In addition, the present invention relates to a connecting element obtainable by a method comprising the steps of forming a carbon-containing mass, firing, single or multiple impregnation and graphitization, in which the connecting element is impregnated and after-fired at least one more time after the graphitization.

Both the electrode bodies according to the invention and the connecting elements according to the invention are distinguished by a higher mechanical strength compared to those known from the prior art, in particular with respect to bending forces, and are therefore ideally suited for producing electrode strands to be used in arc furnaces.

Examples

In the following, the invention is explained on the basis of examples demonstrating the inventive idea, but not restricting these. All materials known to the person skilled in the art for graphite electrodes as suitable, in particular pyrolysis products of pitch and coal, such as, for example, petroleum coke, pitch coke, metallurgical coke or anthracite, together with the known binders can be used as starting materials for both the electrode body and the connecting element. The two components, namely the pyrolysis product and the binder, are mixed with one another by a suitable method before molding. Needle coke is preferably used as the starting material for the electrode bodies and the connecting elements as starting materials as a pyrolysis product in a mixture with coal tar pitch, the maximum particle size of the pyrolysis products for electrode bodies preferably being less than 35 mm, particularly preferably less than 30 mm and very particularly preferably a maximum of 25 mm and for connecting elements is preferably less than 10 mm, particularly preferably less than 5 mm and very particularly preferably a maximum of 3 mm. The carbon-containing mass is preferably shaped by extrusion, although other methods known to the person skilled in the art for this purpose, such as isostatic molding or vibration molding, can also be used.

Example 1 (electrode body)

A molding compound was produced from 100 parts of needle coke with a maximum particle size of 25 mm and 25 parts of coal tar pitch. The mass was mixed for 0.5 hours in a kneading mixer at 160 ° C, then extruded at 120 ° C into a cylindrical strand and then fired in a conventional furnace at 800 ° C for 500 hours. The fired cylindrical carbon bodies were then impregnated once with an impregnation pitch for 5 hours and post-fired at 800 ° C. for 80 hours. Subsequently, the conventionally impregnated and afterburned carbon bodies were graphitized at about 3,000 ° C. in a manner known to the person skilled in the art. The graphite blanks thus obtained were again impregnated with impregnating pitch for 5 hours and after-baked at 800 ° C. for 80 hours.

Finally, the blanks obtained in this way were brought into their final shape (600 mm in diameter and 1,800 mm in length) using CNC milling machines and measured. The properties of the electrode bodies thus obtained are shown in Table 1.

Practical tests showed that an electrode string, which was composed of electrodes produced according to Example 1, showed no cracks due to its increased mechanical strength.

Comparative Example 1

An electrode body was produced and measured by a method according to the prior art, namely analogously to Example 1 but without impregnation and afterburning after the graphitization. The inherent shadows of these electrode bodies are summarized in Table 1.

Example 2 (connecting element)

A molding compound was produced from 100 parts of needle coke with a maximum particle size of 3 mm and 25 parts of coal tar pitch. The mass was mixed in a kneading mixer at 160 ° C for 0.5 hours, then extruded into a cylindrical strand at 120 ° C and then fired in a conventional furnace at 800 ° C for 500 hours. The fired cylindrical carbon bodies were then impregnated twice with an impregnation pitch for 5 hours and post-fired at 800 ° C. for 80 hours. it the conventionally impregnated and afterburned carbon bodies were then graphitized at about 3,000 ° C. in a manner known to the person skilled in the art.

The graphite blanks thus obtained were again impregnated with impregnating pitch for 5 hours and after-baked at 800 ° C. for 80 hours.

The resulting cylindrical blanks for connecting elements with a diameter of 325 mm and a length of 2,350 mm were measured. The properties of these blanks are summarized in Table 1.

Comparative Example 2

A connecting element blank was produced and measured using a method according to the prior art, namely analogously to example 2, but without impregnation and afterburning after the graphitization and with a total of three impregnation steps carried out before the graphitization. The properties of these electrode bodies are summarized in Table 1.

A comparison of the results shows that, with the same number of impregnation steps and thus constant production costs, the connecting elements according to the invention have significantly higher mechanical strengths than the connecting elements produced according to the prior art. Table 1 :

parallel to the direction of extrusion, ie longitudinal

Claims

claims:

1. A method for producing a graphite electrode consisting of an electrode body and at least one connecting element arranged on one of the two end faces of the electrode body for connecting two electrode bodies, in which the electrode body and the at least one connecting element are separated from one another by a method comprising the steps of forming a carbon-containing one Bulk, firing, single or multiple impregnation and graphitization are produced before the electrode body and the at least one connecting means are connected to one another, characterized in that the electrode body and / or the at least one connecting element is impregnated and after-fired at least one more time after the graphitization ,

2. The method according to claim 1, characterized in that the at least one impregnation step before the graphitization and the at least one impregnation step after the graphitization are carried out under the same conditions.

3. The method according to any one of the preceding claims, characterized in that pitch, particularly preferably impregnating pitch, is used as the impregnating agent.

4. The method according to any one of the preceding claims, characterized in that the impregnation of the electrode body and / or the at least one connecting element is carried out under vacuum.

5. The method according to any one of the preceding claims, characterized in that the electrode body and / or the at least one connecting element is incubated with the impregnating agent for 2 to 10 hours, preferably for 4 to 6 hours.

6. The method according to any one of the preceding claims, characterized in that the electrode body and / or the at least one connecting element after the impregnation at 600 to 1,000 ° C, preferably at 700 to 900 ° C and in particular between 750 and 850 ° C, is afterburned ,

7. The method according to any one of the preceding claims, characterized in that the electrode body and / or the at least one connecting element is afterburned for 2 to 10 hours, preferably for 4 to 6 hours.

8. graphite electrode consisting of an electrode body and at least one connecting element obtainable by a method according to any one of the preceding claims.

9. Electrode body obtainable by a method comprising the steps of forming a carbon-containing mass, firing, one or more impregnation and graphitization, in which the electrode body is impregnated and re-fired at least one more time after the graphitization.

10. Connection element obtainable by a method comprising the steps of forming a carbon-containing mass, firing, one or more impregnation and graphitization, in which the connection element is impregnated and after-fired at least one more time after the graphitization.