GB2097638A

GB2097638A - Arc furnace electrode

Info

Publication number: GB2097638A
Application number: GB8208739A
Authority: GB
Original assignee: Arc Technologies Systems Ltd
Current assignee: Arc Technologies Systems Ltd
Priority date: 1981-04-23
Filing date: 1982-03-25
Publication date: 1982-11-03
Also published as: DK181782A; PT74764A; ES8400644A1; AU8219982A; NO820908L; HU186006B; EP0063711B1; TR21231A; PL236124A1; FI821029L; PT74764B; DE3265539D1; JPS5894794A; BR8202309A; DE3116221A1; GR75550B; EP0063711A1; ZA822054B; DD202362A5; ATE15120T1

Abstract

An arc furnace electrode comprises a top portion (5) of metal and a consumable bottom portion (6) of carbon material having a substantially cylindrical form, the portions being connected to each other by a screw nipple (1) or the like, or indirectly. The top portion (5) has a liquid cooling device comprising a feed duct (2) and a return duct (3) and the bottom region of the top portion can advantageously be protected by a high-temperature resistant covering (4). The bottom portion (6) is formed of fine grained, high-tensile, high-graphite carbon material with a bulk density of at least 1.70 g/cm<3>. The electrode is used with advantage for the production of electrosteel. <IMAGE>

Description

SPECIFICATION Arc furnace electrode The invention relates to an arc furnace electrode comprising a top portion of metal and a consumable bottom portion of carbon material, having a substantially cylindrical shape and being connected to each other by means of screw nipple or the like or indirectly, and the top portion is provided with a liquid cooling device with a feed duct and a return duct and the bottom region of the top portion can advantageously be protected by a high temperature resistant covering. The invention also relates to a method of making steel using such an electrode. Arc furnaces for producing electrode steel, copper, corundum, cobalt, silicon etc. were hitherto operated with graphite electrodes as the current carrying elements. An electrode strand usually comprises a plurality of graphite units which are joined to each other by screw connections orthe like.Frequently, three electrode strands are used as current carrying elements for each furnace in such electrothermal high-temperature melting processes. The prior art also discloses combination electrodes for arc furnace operation, comprising a metal shank to which a carbon material tip is joined by a screw connection such as nipple and the like. For example, the German Offenlegungsschrift 1 565 751 discloses electric arc furnace electrodes comprising a top metallic headpiece, a bottom metallic headpiece, electrical conductors for joining the two, a ceramic compound enclosing the said conductors and the bottom headpiece and an electrode tip which is interchangeably attached to the bottom headpiece. A liquid cooled electrode is also disclosed by the German Offenlegungsschrift 28 45 367 which is provided with a cylindrical clamping part attached to the electrode support arm, a metallic cooling system, attached to the former and adapted to carry the electrode current and supporting at the free end a screwthread portion for screwmounting the electrode tip and the tubular heat shield which contains the cooling system in the region exposed to the furnace atmosphere at a distance and in a fixed spatial co-ordination thereto. The European Offenlegungsschrift 12 573 discloses a combination electrode in which the metallic contact of the metal shank disclosed laterally on the outside is supported in insulated manner with respect to the internally disposed metallic cooling system. A ceramic covering, secured by means of hooks, is provided in the bottom part of the metallic cooling shank and extends approximately to the height of the screw nipple connection by means of which a carbon part is attached. The principle of such combination electrodes has been known for a long time, for example from the German Patent Specification 268 660 issued in 1012. Substantial losses of carbon material, inter alia by side oxidation, occur in conventional electrode strands. Tests have therefore been performed on solid strand electrodes of carbon material, by impre nation or by the application of protective coverings, such as ceramic and/or metallic coatings, to counteract this undesirable effect. These steps on the one hand have only a limited effect and on the other hand render the electrodes more expensive. Depending on the construction and length of the metal shank which is introduced into the furnace, side oxidation of the previously-mentioned combination electrodes, comprising a metal shank with a screwmounted carbon part, can be reduced. But these electrodes also require further improvement with regard to a reduction of the side and tip oxidation losses. The metal shank and carbon material connected thereto also requires constant improvement in order to obtain optimum operating conditions and operation with the least amount of malfunctioning while minimizing breakage losses of the electrodes. It is therefore the object of the invention to disclose an arc electrode of the kind described hereinbefore, in which the metal shank and the carbon material are adapted to each other so that operation of the electrode is possible with nipple malfunctioning. More particularly, it should be possible to reduce the consumption of carbon material due to side oxidation and high breakage rates, in particular when the electrodes are subjected to extreme current loadings. At the same time it is intended to achieve a reduction of the electrode down times and a simplification of the manufacturing process of the carbon electrodes which form the bottom part of the electrode. This problem is solved by the provision of an electrode of the kind described hereinbefore, which is characterised in that the bottom portion is formed of fine-grained, high-tensile high-graphite carbon material with a bulk density of at least 1.70 g/cm . The bottom portion is usually connected to the top portion of metal by means of a nipple which consists of metal such as cast iron or copper but preferably of graphite. A different kind of fastening to the top portion of the metal can be selected instead of the nipple. The latter is provided with a liquid cooling device which is usually formed of at least one feed duct and one return duct. The feed duct of the cooling device advantageously also reaches the upper outer region of the nipple which is to be preferred. Alternatively, it is possible for the cooling system itself to extend through the nipple. Within the scope of the invention it is advantageous if the top portion extends over 40 to 80%, more particularly 60 to 80% of the overall length of the electrode. Although the advantages of the invention are already achieved if the bottom portion is formed of fine-grained, high-tensile, high-graphite carbon material with a bulk density of at least 1.70 g/cm , particularly advantageous results are achieve with bulk densities in the region of 1.75 to 1.92 g/cm . The use of the last-mentioned carbon materials is therefore particularly advantageous. The carbon material which forms the bottom portion of the electrode according to the invention can have a specific electrode resistance of less than 6 ohms mm /m in a preferred embodiment of the invention. It is also advantageous if the carbon material has a thermal conductivity of more than 200 W/mK. Finally, the fine-grained, high-tensile, high-graphite carbon material, which forms the bottom portion, can advantageously be selected so that the bending strength exceeds 15 N/mm . An electrode of the above-mentioned kind, in which the bottom portion of find grained, highgraphite carbon material has a bulk density of 1.75 to 1.92 g/cm , a specific electric resistance of ^ 6 ohms mm /m, a thermal conductivity of ? 200 W/mK and a bending strength in excess of 15 N/mm2 is therefore particularly advantageous. In manufacturing the bottom portion it is particularly advantageous to proceed from a carbon material with a maximum particle size in the region of 1 to 3 mm. The carbon material which is used in the bottom portion of the electrode according to the invention can be produced with particular advantage from high-grade premium coke with the use of bonding and impregnating media. When using the above-mentioned or, where appropriate, other starting materials, particularly good bottom portions are obtained with graphiting temperatures in excess of 2900.C. According to one preferred embodiment of the electrode according to the invention, the diameter of the bottom portion is smaller than that of the top portion of metal and also smaller than that of solid graphite electrodes for the given loading. The diameter of the bottom portion is advantageously in the region of 150 to 500 mm. According to one embodiment of the invention, the bottom portion is provided at one end face with female screwthreading and with a screwed stud on the other end face. It is thus possible to connect the bottom portion directly, without an interposed nipple, to the top portion of metal and furthermore to screw-mount the residual member of the previously used bottom portion to the underside of the new bottom portion. In the embodiments of the electrode according to the invention which dispense with the interposition of a nipple, advantages are also obtained since the transition zone from the top to the bottom portion is particularly trouble prone in view of the temperature difference which prevails at that place and because of the different coefficients of thermal expansion of the appropriate materials in known combination electrodes. It is also possible to provide the bottom portion with a central bore of 20 to 50 mm diameter which is open at both ends, similarto tubular electrodes, but better still is not open at both ends. Furthermore, the external surface of the bottom portion can advantageously remain unmachined. The provision of the electrode according to the invention achieves a number of advantages. For a given loading the electrodes can have smaller dimensions than conventional electrodes. Furthermore, it has been found that the electrodes have a substantial shock resistance and greater stability against side erosion. By constructing the carbon part to smaller dimensions it is possible for the carbon electrode parts to be pressformed, fired, impregnated and graphited in a manner which is simpler than that for electrodes of larger dimensions. Advantageously, the electrode according to the invention can be used for the production of nonferrous metals such as copper and cobalt, but also for the production of corundum, silicon and the like. The electrode is however employed preferentially in the production of electrosteel. The electrode according to the invention is particularly suited for the production of steel in the so-called "high-power" or "ultra high power" region with currents of 40 to 80 kA where the diameter of the bottom portion can be in the region of 400 to 600 mm. A specially preferred current loading for the electrodes according to the invention is in the region of 50 to 75 kA given the previously-mentioned carbon part diameters. The accompanying drawings shows a longitudinal section through one embodiment of the electrodes according to the invention but the invention is not confined thereto. In the illustrated electrode, the cooling medium, usually water, is supplied through the feed duct 2 and returned through the return duct 3. The cooling medium also enters into a chamber within the screw nipple 1, which is formed, for example, of cast iron. The top portion 5 of metal in this case comprises an upper region of larger diameter and a lower region of smaller diameter extending into the screw nipple 1 which represents the connection to the bottom portion 6 of cabon material, comprising fine-grained, high-tensile, high-graphite carbon material with a bulk density of at least 1.70 g/cm . The high temperature resistant covering 4 comprises a plurality of individual mouldings which can be supported by a bearing 7. The high-temperature resistant insulation 4 is adjoined by an electrically conductive intermediate stratum 11 which is defined towards the inside by the extending, internally disposed metal shank or its portion of smaller diameter 12. Bores, through which inserted pins ensure proper seating of the high temperature resistant mouldings via a spring, can be provided in addition to the cooling bores 15. The subject of the invention is however not confined to the construction shown in the illustration. Constructions with modifications of the electrode kind shown in the illustration are particularly advantageous within the scope of the invention. In such electrodes, which are preferred within the scope of the invention, the metal shank has a substantially constant diameter. Rings of high temperature resistant material - more particularly those of graphite - can be screwmounted on said shank. The cooling system can advantageously be constructed so that cooling medium flows around the top outer region of the nipple but does not enter the nipple itself. An electrically conductive intermediate stratum is not always provided in such constructions. These and other embodiments of the electrode according to the invention are included within the scope of the invention provided the carbon material of the consumable bottom portion is formed of fine-grained, high-tensile, high-graphite carbon material with a bulk density of at least 1.7 g/cm3. The use of the electrode according to the invention is explained by reference to the example hereinbelow: Example: An electrode was used in which the top portion consisted of copper which was cooled with water via a system comprising a feed and return duct. The copper shank, disposed within the furnace atmosphere, was protected by a high temperature resistant covering. The bottom portion was screwmounted on the metal shank by means of a graphite nipple. The diameter of the bottom portion was smaller than that of the top portion of approximately 350 mm. The specific electric resistance was 5.1 ohms mm /m. The electrode had a central bore of 30 mm diameter. Three electrodes were introduced into a furnace with a capacity of 50 tonnes and containing bulky scrap as charge material. The furnace was operated with three phases having a maximum phase current of 50 kA with a voltage of 490 V. The electrode according to the invention could be used in continuous operation resulting in a graphite consumption of 3.1 kg per tonne of liquid steel.

Claims

1. An arc furnace electrode comprising atop portion of metal, a consumable bottom portion of carbon material and having a substantially cylindrical shape, means for connecting said top portion to said bottom portion, and a liquid cooling device provided for said top portion and defining a feed duct and a return duct, said bottom portion being formed of fine-grained, high tensile, high-graphite carbon material with a bulk density of at least 1.70 g/cm3.

2. An electrode as set forth in claim 1, wherein said connection means comprises a screw nipple and said top portion has a bottom region which is protected by a high temperature resistant covering.

3. An electrode as set forth in claim 1, wherein the bulk density of said material is in the region of

1.75 to 1.92 g/cm .

4. An electrode as set forth in claim 1, wherein the specific electric resistance of said material is less than 6 ohms mm /m.

5. An electrode as set forth in claim 1, wherein the thermal conductivity of said material is greater than 200 W/mK.

6. An electrode as set forth in claim 1, wherein the bending strength of said material is greater than 15 N/mm .

7. An electrode as set forth in claim 1, wherein the bulk density of said material amounts to between

1.75 and 1.92 g/cm , the specific electric resistance of said material is 6 ohms mm /m, the thermal conductivity of said material amounts to 3= 200 W/mK and the bending strength of said material is greater than 15 N/mm .

8. An electrode as set forth in claim 1, wherein said bottom portion of carbon material has a maximum particle size of the order of 1 to 3 mm.

9. An electrode as set forth in claim 1, wherein said carbon material is produced from high-grade premium coke with the use of bonding and impregnating media.

10. An electrode as set forth in claim 9, wherein graphiting is performed at a temperature above

2900.C.

11. An electrode as set forth in claim 1, wherein the diameter of said bottom portion is less than that of said top portion of metal and is also less than that of a solid graphite electrode for a given loading.

12. An electrode as set forth in claim 1, wherein the diameter of said bottom portion is of the order of 150 to 500 mm.

13. An electrode as setforth in claim 1, wherein said bottom portion has one end face with female screwthreading and another end face provided with a screwthreaded stud.

14. An electrode as setforth in claim 1, wherein said bottom portion defines an open central bore of 20 to 50 mm diameter.

15. An electrode as set forth in claim 1, wherein the external surface of said bottom portion is unmachined .

16. A method of producing electrosteel comprising passing an electric current through an arc furnace electrode comprising a top portion of metal, a consumable bottom portion of carbon material and having a substantially cylindrical shape means for connecting said top portion to said bottom portion, and a liquid cooling device provided for said top portion and defining a feed duct and a return duct, said bottom portion being formed of fine-grained, high tensile, high-graphite carbon material with a bulk density of at least 1.70 g/cm .

17. A method as set forth in claim 16, wherein said electric current is between 40 and 80 kA and the diameter of said bottom portion is between 400 and 800 mm.

18. An arc furnace electrode substantially as described with reference to the accompanying drawings.