EP0452599A2 - Arc furnace with consumable electrode - Google Patents

Abstract

This arc furnace (10), for the treatment at very high temperature of a conductive pulverulent material, comprises a vertical sleeve (14); an upper electrode (20) coaxial with the sleeve; a lower electrode producing with the first an arc column (11); means (17,22) for tangentially injecting a gas into the sleeve, creating a vortex therein; means (24) for introducing the material into the upper part (16) of the sleeve in order to form a coating of particles falling into the sleeve, projected against its internal wall (18) by centrifugal force and completely covering this wall to protect it , while being processed by the arc column; a crucible located under the sleeve, to collect the molten particles which flow from the lower end of the sleeve, and positioning means (26; 28,30,32) for vertically adjusting the upper electrode, which can slide through from the upper end of the sleeve and is made of a consumable material, such as graphite. Thanks to this arrangement, this electrode does not have to be cooled with water.

Description

The present invention relates to an improved arc furnace, comprising a consumable electrode and intended to be used in the processing of ores or other metallic or non-metallic compounds, at very high temperatures, with the aim of transforming them physically or chemically.

Arc furnaces are very well-known devices, which have been the subject of much research and development in recent decades. By definition, such furnaces use a heat-generating arc column, created between a set of electrodes, to heat the ores or compounds to be treated at very high temperatures and thus make it possible to obtain reactions which could not be obtained. other. The arc column usually consists of a mixture of excited and / or dissociated molecules, positively charged ions and free electrons obtained from a gas (called "plasma gas") subjected to partial ionization by means of 'an electric arc (usually direct current) formed between the two electrodes.

The arc furnace, to which the present invention applies, is of the type comprising an upper electrode situated in the upper part of a sleeve, and a lower electrode in electrical contact with the conductive molten ore collected in a crucible under the electrode superior. The arc column which forms between the upper and lower electrodes ensures the melting of the pulverulent material introduced into the sleeve and causes the desired physical and chemical transformation, the molten material subsequently falling into the crucible. Such an oven is described for example in American patent application No. 399,997, filed on August 29, 1989 and assigned to the Applicant.

The arc furnaces of the type described above, to which the invention applies, usually employ so-called "non-consumable" electrodes. In practice, the duration of such "non-consumable" electrodes is quite short, since it varies between three and a thousand hours depending on the operating conditions. Replacing these electrodes is expensive and it often happens that the process has to be interrupted.

In general, non-consumable electrodes must also be cooled with water to avoid excessive erosion. This makes them prone to water leaks which can be the source of explosions due to the reaction of the water and the material under treatment at high temperature.

The present invention relates to an arc furnace of the type described above, which uses a consumable electrode which, on the one hand, it is not necessary to cool with water, and which, on the other hand, can be advanced as it wears out, thus increasing the operating period of the oven.

The consumable electrode used in the oven according to the invention is preferably made of graphite.

The oven comprises a vertical sleeve, electrically insulated, and provided, at its upper end, with a first electrode which, as previously indicated, can be made of graphite and be of conventional structure, which serves to maintain an arc column generated between its lower end and a bath of molten material contained in the crucible of the furnace. This first electrode can be lowered so as to be brought closer to the surface of the bath to allow easy striking of the arc. Once this has started, the electrode is reassembled in the chamber defined by the sleeve.

The material to be treated is introduced, in powder form, inside the sleeve at the top thereof, next to the electrode. The material is projected by centrifugation against the internal wall of the sleeve by a tangential jet of gas injected inside the sleeve so as to provide a coating, essentially cylindrical and uniform, of particles falling into the sleeve. These particles completely cover the internal wall of the sleeve and protect it while being simultaneously treated by the heat given off by the arc column. The oven further includes a crucible located under the sleeve to collect the treated molten particles which trickle from the lower end of the sleeve. A second electrode is provided at the bottom of the crucible to complete the electrical circuit formed by the graphite electrode, the arc, the conductive molten bath and the external electrical circuit connected to a source of electrical energy.

It is known that consumable electrodes, preferably graphite, are highly reliable "tools" when used in arc furnaces for different applications at power levels up to 50 megawatts. The use of such electrodes has hitherto never been suggested for arc furnaces having the configuration described above.

• un manchon vertical, isolé électriquement, ayant une extrémité supérieure, une extrémité inférieure, et une paroi interne cylindrique ;
• une électrode supérieure, montée coaxialement au manchon à l'extrémité supérieure de celui-ci ;
• une électrode inférieure, coopérant avec l'électrode supérieure par connexion appropriée de ces deux électrodes à une source d'énergie électrique, ces deux électrodes étant aptes à produire, entre elles, une colonne d'arc ;
• des moyens permettant l'injection tangentielle d'un gaz à l'intérieur du manchon, de façon à y créer un tourbillon ;
• des moyens permettant l'introduction du matériau pulvérulent à traiter à l'intérieur du manchon près de l'extrémité supérieure de celui-ci, à côté de l'électrode supérieure, de façon à former un revêtement, essentiellement cylindrique et uniforme, de particules tombant dans le manchon, les particules étant projetées contre la paroi interne du manchon par la force centrifuge exercée par le tourbillon et recouvrant entièrement la paroi interne pour la protéger, tout en étant simultanément traitées par la colonne d'arc ;
• un creuset situé sous le manchon, pour recueillir les particules traitées en fusion qui ruissellent de l'extrémité inférieure du manchon, le matériau en fusion étant en contact électrique avec l'électrode inférieure, et
• des moyens de positionnement pour ajuster verticalement l'électrode supérieure, cette dernière pouvant coulisser au travers de ladite extrémité supérieure du manchon.

More specifically, the invention relates to an arc furnace for the treatment of a conductive pulverulent material at very high temperatures, this furnace comprising:

• a vertical sleeve, electrically insulated, having an upper end, a lower end, and a cylindrical inner wall;
• an upper electrode, mounted coaxially with the sleeve at the upper end thereof;
• a lower electrode, cooperating with the upper electrode by appropriate connection of these two electrodes to a source of electrical energy, these two electrodes being capable of producing, between them, an arc column;
• means allowing the tangential injection of a gas inside the sleeve, so as to create a vortex there;
• means allowing the introduction of the pulverulent material to be treated into the interior of the sleeve near the upper end thereof, next to the upper electrode, so as to form a coating, essentially cylindrical and uniform, of particles falling into the sleeve, the particles being projected against the internal wall of the sleeve by the centrifugal force exerted by the vortex and completely covering the internal wall to protect it, while being simultaneously treated by the arc column;
• a crucible located under the sleeve, to collect the treated molten particles which trickle out from the lower end of the sleeve, the molten material being in electrical contact with the lower electrode, and
• positioning means for vertically adjusting the upper electrode, the latter being able to slide through said upper end of the sleeve.

According to the invention, this furnace is characterized in that the upper electrode is made of a consumable material.

The electrode according to the invention may include a bore allowing the temporary or continuous supply of a stream of plasma gas, such as argon, to facilitate the initiation of the arc or even allow a more stable operation of the arc.

The positioning means preferably comprise means for advancing the upper electrode and means for monitoring this advancement, so as to ensure that the upper electrode is advanced as it wears out. that the length of the arc remains constant.

According to a first possible embodiment, these means for monitoring the progress of the upper electrode may include an assembly formed by a detector radiation, an optical tube connecting the detector and said inner wall, and a gas supply connected to the optical tube to prevent obstruction by particles falling into the sleeve, and be arranged so that the detector can detect the presence or absence of radiation emitted by the arc or by the upper electrode in the direction of the optical tube and thus allow the control of said advancement means. In particular, the progress control means may comprise two such assemblies aiming at two points arranged vertically in different positions so that one of these assemblies can detect the radiation emitted by the arc, and the other assembly, the radiation emitted by the upper electrode.

According to a second possible, preferred embodiment of the invention, these same means for controlling the progress of the upper electrode may include means for weighing the upper electrode and means for measuring the length of the upper electrode outside the oven so that the vertical position of the upper electrode inside the sleeve can be calculated, the upper electrode being made of a material of known density.

• 1a Figure 1 est une vue schématique en section verticale du manchon d'un four à arc pourvu d'une électrode supérieure consommable selon l'invention ;
• la Figure 2 est une section horizontale selon aa de la Figure 1 ; et
• la Figure 3 est une vue schématique de l'électrode supérieure dans trois positions différentes relativement à un des moyens de contrôle de sa position selon l'invention.

The invention and its advantages will be better understood on reading the following description of a given preferred embodiment with reference to the drawings in which:

• 1a Figure 1 is a schematic view in vertical section of the sleeve of an arc furnace provided with an upper consumable electrode according to the invention;
• Figure 2 is a horizontal section along aa of Figure 1; and
• Figure 3 is a schematic view of the upper electrode in three different positions relative to one of the means for controlling its position according to the invention.

Figure 1 illustrates the upper electrode and the supply system of an arc furnace 10 according to the invention. This upper electrode 20 can be moved using a roller drive system 26, so that its position can be adjusted in the vertical direction.

According to the invention, the upper electrode 20 is a solid electrode of graphite or any other similar consumable material.

The material 12 which is to be treated in the oven enters via supply lines 24 provided around the upper periphery of the sleeve 14. Holes below called "optical tubes" 32 are oriented towards the lower end 21 of the electrode 20. Any possible light emitted by the end 21 passes through the tubes 32 to reach the detectors 28. A continuous supply of gas 30 is connected to the tubes 32 to ensure that the ends of these tubes 32 which open onto the wall cylindrical 18 are not obstructed by the material 12. An alarm system (not shown) is connected to the gas supply 30, and is triggered when the gas flow falls below a determined control value. The detectors 28 can be fitted with optical filters to filter or reduce the light intensity of the arc 11.

In Figure 2, it is shown that the plate 16, forming the upper end of the sleeve 14, is provided with an annular channel 17, into which is injected, in several places, a propellant gas from another supply in gas 22. This upper end plate 16 is made of steel and resistant to wear, and the channel 17 is shaped inside the sleeve, that is to say under the plate. The gas from the supply 22 is tangentially injected into the annular channel at four points equally spaced from one another. The injected propellant gas drives the material to be treated 12 in a rotational movement and projects the material thus accelerated by centrifugal force against the cylindrical wall 18 of the sleeve. The material 12 is introduced into the upper part of the sleeve, dropping it at four equidistant points 24 (two of these points being illustrated in Figure 1). The material 12 thus introduced is entrained and forms a film or coating on the cylindrical wall 18 of the sleeve, as illustrated in FIG. 1, and this coating is heated by the radiation from the arc 11.

Referring again to Figure 1, the arc 11 is formed between the upper electrode 20 and the molten material 12 collected in a crucible (not shown), under the sleeve 14. In practice, a bottom electrode is connected electrically to the molten material 12 and to a source of electrical energy (not shown), forming a circuit between the electrode 20 and the molten material 12 in the crucible. The material 12 in the crucible is kept hot by the passage of the current going to the electrode at the bottom thereof.

At the start of the treatment, the drive system 26 is used to lower the electrode 20, so that its lower end 21 approaches either the bottom electrode or the molten material which has been heated to beforehand in the crucible. Once the electrode 20 has been lowered, the arc 11 can easily be struck. The electrode 20 can then be raised to its normal position which is that shown in FIG. 1.

The position of the end 21 of the electrode 20, which slides through the upper end 16 of the sleeve 14, can be adjusted to ensure that there is adequate energy transfer to the coating of material 12. According to a first embodiment of the invention, this adjustment can be controlled by the light detectors 28, one of them picking up the radiation from the arc 11, and the other picking up the radiation emitted by the red-hot end 21 of the electrode 20. The optical tubes 32, passing through the external wall of the sleeve 14 and through the cylindrical wall internal 18 thereof, create a path to radiation from the arc 11 and the end 21 of the electrode to the detectors. The light that strikes the lower of the two detectors 28 can be attenuated by a high density filter 29. Each light detector 28 emits a voltage signal V1 and V2 which is proportional to the light intensity striking its surface.

Each voltage signal is compared to a reference voltage by comparators (not shown), the output signals of which act on control means making it possible to raise or lower the electrode 20 by means of the drive system 26 .

Figure 3 shows the various situations that can be encountered in the form of phases A, B and C. At A, the two optical tubes 32 point towards the light arc 11. The two voltage signals V1 and V2 are greater than their respective reference voltages and, consequently, a signal is sent to the control means to lower the electrode 20. At B, the two optical tubes 32 point towards the light electrode 21. In this case, the two signals voltage V1 and V2 are lower than their respective reference voltages. It follows that a signal is sent to the control means to lift the electrode. In C, the upper optical tube 32 points towards the end of the electrode 21, while the lower optical tube 32 points towards the light arc 11. In this case, no signal is sent, and the electrode 20 remains stationary.

According to another possible embodiment, the positioning means may include means for weighing the electrode 20 and means for measuring the height thereof outside the furnace 10. By knowing the density of the electrode 20 , of uniform construction, the position of the end 21 can be calculated and adjusted by means of the actuation system 26, as required.

Although the illustrated means for introducing the material 12 are the supply lines 24 to the through which the material 12 falls, it is also possible to inject this material 12 by using the gas supply 22 or separately from it but in a similar tangential direction.

The electrode 20 may be of a solid construction, but it may also be provided with a central bore, of small diameter, into which a gas serving to stabilize the arc can be injected, this gas preferably being, argon. The gas thus injected can also be chosen so as to participate in the reaction occurring inside the furnace.

Claims (10)

Arc furnace (10) for the treatment of a powdery material (12), conductive, at very high temperature, of the type comprising: - a vertical sleeve (14), electrically insulated, having an upper end (16), a lower end, and a cylindrical internal wall (18); - an upper electrode (20), mounted coaxially with the sleeve (14), at the upper end (16) thereof; - a lower electrode, cooperating with the upper electrode (20) by appropriate connection of these two electrodes to a source of electrical energy, these two electrodes being capable of producing, between them, an arc column (11); - Means (17,22) allowing the tangential injection of a gas inside the sleeve (14), so as to create a vortex there; - Means (24) allowing the introduction of the pulverulent material to be treated (12) inside the sleeve (14) near said upper end (16), next to the upper electrode (20), so as to forming a coating, essentially cylindrical and uniform, of particles falling into the sleeve (14), these particles being projected against the internal wall (18) of the sleeve (14) by the centrifugal force exerted by the vortex and completely covering the internal wall ( 18) to protect it, while being simultaneously treated by the arc column (11); - a crucible located under the sleeve (14), to collect the treated molten particles which trickle out from the lower end of the sleeve (14), the molten material (12) being in electrical contact with the lower electrode, and - positioning means (26; 28, 30, 32) for vertically adjusting the upper electrode (20), the latter being able to slide through said upper end (16) of the sleeve (14), said oven (10) being characterized in that said upper electrode (20) is made of a consumable material. Oven according to claim 1, characterized in that the positioning means comprise means (26) for advancing the upper electrode (20) and means (28, 30, 32) for controlling this advancement, for s' ensure that the upper electrode (20) is advanced as it wears out. Oven according to claim 2, characterized in that the means for monitoring the progress of the upper electrode (20) comprise an assembly (28, 30, 32) formed by a radiation detector (28), an optical tube (32) connecting the detector (28) and the internal wall (18) of the sleeve (14), and a gas supply (30) connected to the optical tube (32) to prevent obstruction by them particles falling into the sleeve (14), and are arranged so that the detector (28) can detect the presence or absence of radiation emitted by the arc (11) or by the upper electrode (20) in the direction of the optical tube (32) and thus allow the control of the advancement means (26). Oven according to claim 3, characterized in that the progress control means comprise two assemblies (28, 30, 32) aiming at two points arranged vertically in different positions so that one of these assemblies can detect radiation emitted by the arc (11), and the other together, the radiation emitted by the upper electrode (20). Oven according to claim 2, characterized in that the means for monitoring the progress of the upper electrode (20) comprise means for weighing the upper electrode (20) and means for measure the length of the upper electrode (20) outside the oven (10), so that the vertical position of the upper electrode (20) inside the sleeve (14) can be calculated, the upper electrode (20) being made of a material of known density. Oven according to one of claims 1 to 5, characterized by the fact that the upper electrode (20) is tubular, and provided with a bore of small diameter, and by the fact that means are provided for introducing a gas into the hollow electrode for the purpose of injecting gas in the direction of the arc (11), thereby stabilizing the latter. Oven according to one of claims 1 to 5, characterized in that the upper electrode (20) is tubular, and provided with a narrow bore, and in that means are provided for introducing a gas into the hollow electrode for the purpose of injecting gas in the direction of the arc (11), thereby stabilizing the latter, the gas thus injected being chosen so as to participate in the reaction occurring inside the furnace. Oven according to one of claims 1 to 5, characterized by the fact that the upper electrode (20) is tubular, provided with a bore of small diameter, and by the fact that means are provided for introducing a gas into the 'hollow electrode for the purpose of injecting gas in the direction of the arc (11), to thereby stabilize the latter, the gas thus injected being argon. Oven according to one of claims 1 to 5, characterized in that the means (24) allowing the introduction of the pulverulent material comprise a set of openings in the upper end (16) of the sleeve (14) around the 'upper electrode (20), through which the powder material (12) is brought into the sleeve (14), near the inner wall (18) thereof. Oven according to one of claims 1 to 5, characterized in that the consumable material of the upper electrode (20) is graphite.

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