DE9115816U1 - Mass for securing a screw connection between carbon or graphite electrodes - Google Patents

Description

SIGRI GmbH 8901 Meitingen

Description

Mass for securing a screw connection between carbon or graphite electrodes

The invention relates to a mass that can be at least partially melted and coked under the influence of temperature for securing a screw connection between electrodes made of carbon or graphite by means of a threaded nipple made of carbon or graphite that engages in the front threaded sockets of the electrodes to be connected or by means of a threaded extension located on a front side of a first electrode that engages in the thread of a front recess of a second electrode that is complementary to the extension and its thread.

Electrodes made of carbon and/or graphite are used on a large scale in electrothermal and electrochemical processes due to their electrical conductivity and good temperature and media resistance. The metallurgical processes carried out in the arc furnace are of the greatest importance here, along with calcium carbide or silicon production, for example, and melt electrolytic processes.

In all of these processes, the electrodes are used up as a result of the chemical reactions and the thermal and mechanical conditions in the reaction vessel. They are therefore constantly replaced as they are used up.

consumption. In important processes, this is done, such as in the arc furnace in steel production, by coupling another electrode to an electrode piece in the system by placing one of the faces of a second electrode on the face of the electrode facing away from the reaction zone and by mechanically connecting both electrodes using anchoring elements on their faces. In practice, electrode strings made up of three or more electrodes are constructed and operated in this way.

Threaded connections have long proven themselves as anchoring elements, with two main types being distinguished. The currently most commonly used type has threaded, central holes in both electrode faces, so-called nipple boxes. When coupling two electrodes together or when building or completing an electrode string, the two ends of the electrodes to be coupled are connected in a form-fitting and firm manner by screwing a threaded nipple into the threaded holes in the faces of each electrode, which are complementary to the nipple in terms of shape and thread design, and then by tightening the screw connection with a defined force. In the second method, each electrode has a threaded, central hole in one face and an extension on the other face that rises above this face and complements the hole and the thread on the other face. To connect two electrodes, one electrode is inserted with its extension into the hole of the other electrode and the positive and firm connection of the two

Electrodes are made by tightening the threads that create the coupling.

In practice, it has now been shown that such screw connections can loosen and even come off, especially under the influence of unavoidable vibrations and electric fields. Since such occurrences lead to serious operational disruptions, means have been sought to lock such connections in a way that cannot be released under operating conditions. The German patent DE 867 268 and the German publications DE-AS 10 34 788 and 10 30 482 disclose the introduction of pitch into the threads that hold the electrode connection together. The pitch is introduced into holes in the threaded sockets of the electrodes or in the nipples. It then softens when the electrodes are heated during operation and flows through connecting channels into the threads, where it cokes when heated further during operation, thereby creating a firm, material-locking connection and improving the current transfer. The pitch can be introduced into the holes or supply spaces in ground solid form, in liquid form or in the form of pitch sticks. It can also contain additives such as fillers.

In the German publication No. 15 65 076 a putty is described for securing a nipple connection between carbon electrodes, which is effective even at temperatures below 400 ⁰ C, ie at temperatures that are below the coking temperature of pitch. This putty contains synthetic resin, pitch, fillers and dextrin. With the right composition it can be made into shapes that are suitable for later processing and it is said to have an unlimited shelf life. When handling

However, in the continuous use of this putty in operation, it has been shown that due to its synthetic resin content, it ages during storage, or that slow hardening processes take place in it, so that during operational use, the putty cannot penetrate sufficiently into the threads of the electrode connections due to its high viscosity.

A significant disadvantage of the known pitch-containing masses for locking electrode connections is their content of substances that can be carcinogenic. These substances, which consist of highly condensed aromatic ring systems, form from low molecular weight aromatic units under the influence of higher temperatures during the production process in the pitches and are therefore present in all pitches. The reference substance for such carcinogenic polycyclic aromatic hydrocarbons (PAH) is the compound benzo(a)pyrene (hazardous substances ordinance as announced on September 25, 1991). The limit content of PAH in substances or compositions above which a hazard is assumed is currently 50 ppm (hazardous substances ordinance as announced on September 25, 1991). The content of benzo(a)pyrene in pitches is approximately 100 times this value. As a result, there is a general need to limit or prevent the handling and possible release of such substances as far as possible.

The invention was therefore based on the object of creating a composition or mass for securing screw connections between electrodes made of carbon or graphite, which have a benzo(a)pyrene content of less than 50 ppm or whose binder has a benzo(a)pyrene content of less than

50 ppm, which is at least partially meltable and cokable, has good expansion properties, which develops a good locking effect at temperatures from 300 °C, i.e. in a temperature range in which pitches are not yet coked, and whose locking effect is also as good or better than that of locking masses containing pitch.

The object is achieved according to the invention in that the mass contains a substance which can be melted and coked under the influence of temperature and which consists essentially of carbon and hydrogen (hereinafter referred to as binder), which is characterized as follows: softening point (DIN 52025): 50 to 120 ⁰ C, content of substances insoluble in toluene: maximum 2.5%, content of benzo(a)pyrene: less than 50 ppm, viscosity at 160 "C: 300 to 8000 m Pa s.

Embodiments and further developments of the inventive solution emerge from the subclaims.

The binder consists of a complex mixture of heat-polymerized low-molecular hydrocarbons and monocyclic aromatic hydrocarbons, which can contain alkyl side chains and hydroxyl groups. Such a substance is commercially available, e.g. under the name Rauxolit (supplier: Verkaufsgesellschaft für Teererzeugnisse (VFT mbH)). The content of benzo(a)pyrene in this binder is less than 50 ppm, but preferably less than 20 ppm and thus considerably below the concentration that is considered to be declarable and questionable. The binder can be used alone or in a mixture with fillers and other additives as a locking agent. In the latter application, the content of benzo(a)pyrene in the mixtures must be reduced again.

reduced depending on the proportion of additives. When the binder is heated to a temperature of 550 ⁰ C according to DIN 51905, a coke residue of approx. 50 wt. % remains. When compositions according to the invention are coked in the threads of an electrode connection, the coke which forms anchors itself to the thread surfaces of the parts to be connected and thus, if necessary with the assistance of additional coke bridges formed between a filler and the binder, causes a material-locking, firm locking of the connection. To increase the coke yield, condensing agents such as sulphur, phthalonitrile, aldols, chloronaphthalenes or Lewis acids such as anhydrous AlCl ₃ or FeCl ₃ can also be added to the compositions according to the invention. The addition of sulphur to the composition is preferred here, since with a corresponding increase in temperature sulphur also has a swelling effect in the mixture in addition to a condensing effect. Additions of 4 to 12 % sulphur are advantageous. In addition to agents that increase the coke yield, fillers from the group carbon or graphite can also be added to the binder. All types of petroleum, coal tar pitch or mine coke as well as graphite or soot can be used here. The addition of graphite powder has proven to be particularly advantageous, as this simultaneously promotes the transfer of the electrical current in the electrode connection. The grain size of the filler must be selected so that the functionality and processability of the mass is guaranteed. A powder with a grain size of 80% smaller than 200 μηη has proven to be advantageous. However, it is easy to deviate from this specification, e.g. specifically towards finer grain sizes. The expert can determine by simple tests which grain fineness is best for the respective application.

The content of solid fillers in the mass should be between 0 and 62% by weight, which conversely corresponds to binder contents between 38 and 100% by weight. For the most common applications, compositions with good effectiveness and processability are obtained within the following formulation limits:

Binder 38 to 100 wt.%, filler 0 to 50 wt.%, sulfur 4 to 12 wt.%.

In addition to a good expansion effect, sufficient flowability at high temperatures is a prerequisite for the mass to function well. The binder therefore has a viscosity of between 300 and 8000 m PA s measured at 160 °C using standard laboratory equipment.

To produce the locking compound, the recipe components in powder form are mixed without the influence of temperature according to a first process. The powder mixture produced in this way can be introduced into the receiving spaces provided for the locking compound in the connecting elements of the electrodes and in the nipples. The receiving spaces, e.g. holes, are closed in a known manner after the powder has been introduced, e.g. with textile or cardboard plugs or pads, in order to prevent the locking agent from falling out prematurely. The powder can also be pressed into the openings after it has been filled. A closure with a plug is then unnecessary. According to another process, the recipe components are thoroughly mixed in a heatable mixer, e.g. a Z-arm kneader at temperatures of 100 to 160 °C. The mass can then be mixed in liquid form according to a process variant, if necessary with an intermediate

a heatable storage vessel with a stirring device in which the mass is kept in a molten state, into the receiving spaces in the elements connecting the electrodes, where it immediately cools down and can no longer flow out. According to another variant, the liquid or plastic mixture obtained after hot mixing is processed into cylindrical bodies using known methods, e.g. using an extrusion press. For the purposes of intermediate storage, these bodies have a larger diameter (approx. 10 cm), for the purposes of immediate further processing they are thin rods that have the diameter dimensions of the holes in the connecting elements that will later receive them in the form of rod sections, for example 20 mm. Of course, the thin rods can also be stored, although they are more fragile. From the large-format bodies intended for intermediate storage, thin rods can then later be pressed out as needed in a manner known to every expert using an extrusion press. It is also possible to press directly from such mass cartridges or cartridges into corresponding holes in nipples using an extrusion device integrated into a nipple processing line. Which of the described methods is used depends on the respective operational conditions and application.

The storage of the inventive compositions or of the binder alone is possible without any problems at normal ambient temperatures, ie up to 40 ^° C, over long periods of time without any change in the flow behavior at high temperatures, the expansion behavior and the locking effect.

In summary, the invention has the following advantages:

- the content of benzo(a)pyrene as a lead substance for carcinogenic substances in locking masses consisting solely of binders or in locking masses which, in addition to the binder, also contain fillers and possibly other additives, is less than 50 ppm and thus at least a factor of 100 lower than the benzo(a)pyrene content of the pitches previously used for this purpose (see Table 1).

- the expansion behavior of the binder alone and of the locking agents produced using the binder is at least as good as that of the pitches used previously or the locking masses produced with them. In most cases, the new binder results in a better expansion effect and thus a better penetration of the locking agent into the threads of the electrode connection than with pitch (see Table 1).

Table 1

Ar
Example
1 retiermitte
Example
2 »I , Erf i nc
Example
3 lation
Example
4 Example
5 Locking
Example
6 i ttel, Ver
Example
7 even
Example
8th Example
9 Mass with
Rauxolite
EP72OC Mass with
Rauxolite
EP 114 ⁰ C Rauxolite
EP 72 ⁰ C Rauxolite
EP114OC Mass with
Bad luck
EP 6O ⁰ C Mass with
Bad luck
EP 7O ⁰ C Mass with
Bad luck
EP 8O ⁰ C Mass with
Bad luck
EP 85 ⁰ C Mass with
Bad luck
EP 95 ⁰ C Bloating test &bgr;,
storage
1 day > 10 >10 >10 >10 4-5 6-7 4-5 5-6 3-4 Bloating test /2>,
storage
8 weeks >10 >10 >10 > 10 4-5 6-7 4-5 4-5 3-4 Salary of
Benzo (a ) -
py ren
[ppm] < 20 <20 12 11 > 50 > 50 >50 8000 9000

- the locking effect of the masses according to the invention is significantly improved compared to coked pitch or pitch-bound locking masses (see also Table 2). In contrast to pitches, it starts at 300 °C. This makes it possible to dispense with sulphur as a recipe ingredient, which is advantageous for reasons of environmental and occupational safety.

- Compositions according to the invention have an unlimited shelf life under normal ambient conditions. They can be processed by all known methods.

The described teaching of the invention is supplemented by the following embodiments.

Example 1:

54 parts by weight of rauxolite, softening point (DIN 52025) 72 °C and 37.5 parts by weight of electrographite powder with a grain size of 0.04 to 0.2 mm were mixed in a Z-arm kneader at 110 to 12 0 "C for 15 minutes. 8.5 parts by weight of sulfur powder were then added to the mixture and mixed for a further 10 minutes while maintaining the temperature of 110 to 120 ⁰ C. The hot mixture was transferred to a mold with an inner diameter of 200 mm, compacted therein to form a pre-pressed part and, after cooling to approx. 80 °C, transferred as a pre-pressed part to an extrusion press and pressed by this press while maintaining the temperature of approx. 80 °C into rods with a format of 20 mm diameter and 1400 mm length. The rods cooled on a cooling line were then stored on wooden pallets.

Example 2:

Using Rauxolit, softening point (DIN 52 025) 114 °C, rods with dimensions of 20 mm diameter x 1400 mm length were produced using essentially the same procedure as described in Example 1. In contrast to Example 1, the temperature during mixing in the Z-arm kneader was 150 to 155 ^° C and the temperature during pressing of the rods was 120 °C.

Examples 3 and 4:

On an extrusion press, rods of the same dimensions as in the preceding examples were produced from Rauxolit, softening point (DIN 52 025) 72 °C, alone, without additives (Example 3) and from Rauxolit, softening point (DIN 52 025) 114 °C ₁ also without additives (Example 4) in a generally known manner.

Examples 5, 6, I ₁ 8 and 9 (comparative examples):

In essentially the same way as in Examples 1 and 2, rods were made from five commercially purchased binding pitches, as are usually used to make locking compounds. These were pitches with the following softening points (DIN 52 025):

EP 60 ⁰ C (Example 5),

EP 70 °C (Example 6),

EP 80 °C (Example 7),

EP 85 ⁰ C (Example 8),

EP 95 ⁰ C (Example 9).

In contrast to Examples 1 and 2, the temperature during mixing in the Z-arm kneader corresponded to the temperature of the softening point of the pitch used plus 40 ^° C and the temperature during pressing of the rods corresponded to the temperature of the softening point of the pitch used plus approx. 20 °C.

The following tests were carried out using the locking masses according to Examples 1 to 9:

a) Expansion test after 1 day storage at 55 ± 2 ⁰ C

b) Bloating test after 8 calendar weeks storage at 55 ± 2 ⁰ C.

c) Benzo(a)pyrene content.

The value for the expansion test is a dimensionless quantity ß. It was determined as follows: rod sections with the dimensions 2 0 mm diameter x 40 mm length were placed in the middle of a glass tube (diameter 22 mm, length 500 mm) open on both sides at room temperature. The samples prepared in this way were placed in an oven heated to 280 "C in a precisely horizontal position and left there for 60 minutes. After removing the sample from the oven and allowing it to cool, the increase in length caused by the expansion was determined (see Figure 1) and this value was set in relation to the length of the rod section before insertion into the oven according to the relationship ß = ^T.

The determination of the benzo(a)pyrene content was carried out by extracting the sample to be examined with toluene and subsequent gas chromatographic analysis of the eluate with mass spectrometric detection using D 10 - anthracene and D 12 - perylene as internal standards. In the case of the rauxolites,

This method allows the toluene extract to be further concentrated.

The measured values are shown in Table 1. They clearly show that the masses according to the invention are far superior to pitch-containing masses from the point of view of health and environmental protection and that they also have significantly better expansion properties with the same storage life.

In order to determine the locking effect of the inventive masses in comparison to known masses, sections of rods corresponding to the length of the bores, which had been manufactured according to the conditions of the embodiments 2, 4 and 7, were pressed into the bores of externally threaded, double-conical graphite nipples for graphite electrodes, 500 mm diameter, which were usually provided for receiving locking masses. Each of the nipples contained two bores open on both sides, 20.5 mm in diameter, which were guided through the longitudinal axis at right angles to the longitudinal axis of the nipple at 1/5 and 4/5 of the height of the nipple and were completely filled with locking mass. Two electrodes provided with corresponding threaded nipple boxes were connected by screwing through each of these nipples in a known manner. The tightening torque was 1765 Nm in all cases. After tightening, the unscrewing torque of the electrode connections that had not yet been locked was first determined. The electrode connections provided with locking compound according to Example 4 were then heated to 350 ¹ C, the electrode connections provided with locking compound according to Examples 2 and 7 were heated to 590 ⁰ C and each was kept at this temperature for about 4 hours. After cooling to room temperature, a

The tightening torque is determined by the electrode connections. The measured values are listed in Table 2.

Table 2

Tightening torques for locked electrode connections. Tightening torque 1765 Nm. All values given in Nm

Connection
Example 4,
Rauxolite, EP 114 ⁰ C ng with locking mass
Example 2,
Rauxolite, EP 114 ⁰ C
+ Filler + Sulphur according to
Example 7 , Comparison,
Pitch, EP 80 C + filler
+ Sulphur Ond torque
which is not thermal
nimble I ten
Links 2634
2493
2962
^Wed \ ^teL " 2696
value: 2579
2550
2955
Medium-
value: 2952
2314
3276
Middle- ₂₈₄₇
value: -===--- Torque
which at 350 ⁰ C
nimble 11 en
links 37226
45763
42454
Medium- ₄
value: Torque
which at 490 ⁰ C
treated
Links 30098
47338
44298 17800
16025
21007
Middle- ₁₈₂₇₇
value:

The measured values convincingly show that the new locking masses have a significantly better locking effect than the known compositions produced using binding pitch and that the new binding substance already develops a strong locking effect well below the coking temperature of pitch.

Claims (8)

1. Mass that can be melted and coked at least partially under the influence of temperature for securing a screw connection between electrodes made of carbon or graphite by means of a threaded nipple made of carbon or graphite which engages in the front threaded sockets of the electrodes to be connected or by means of a threaded extension located on a front side of a first electrode which engages in the thread of a front-side recess of a second electrode which is complementary to the extension and its thread,
characterized in that the mass contains a substance which can be melted and coked under the influence of temperature and which consists essentially of carbon and hydrogen (referred to as binder in the following claims), which is characterized as follows: Softening point (DIN 52025): 50 to 120 °C, content of substances insoluble in toluene: maximum 2.5%, Content of benzo(a)pyrene: less than 50 ppm, viscosity at 160 °C: 300 to 8000 m Pa s. 2. Mass according to claim 1, characterized in that the content of benzo(a)pyrene in the binder is less than 20 ppm. 3. Mass according to claims 1 or 2, characterized in that their binder content is between 0 and 100%. 4. Mass according to claims 1 or 2, characterized in that it contains fillers from the group carbon or graphite. 5. Mass according to claim 4, characterized in that the fillers have a grain size of 80% smaller than 200 M . 6. A composition according to claim 4 or 5, characterized in that the filler is graphite. 7. A composition according to claims 1, 2, 4, 5 or 6, characterized in that it contains coke yield-increasing agents from the group consisting of sulphur, phthalonitrile, aldols, chloronaphthalenes and Lewis acids. 8. Mass according to claims 1, 2, 4, 5 or 6, characterized in that it consists of 0 to 50 percent by weight of fillers, 38 to 100 percent by weight of binder and 4 to 12 percent by weight of sulfur.