EP1159469A1 - Impregnated graphite cathode for electrolysis of aluminium - Google Patents

Impregnated graphite cathode for electrolysis of aluminium

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Publication number
EP1159469A1
EP1159469A1 EP00901692A EP00901692A EP1159469A1 EP 1159469 A1 EP1159469 A1 EP 1159469A1 EP 00901692 A EP00901692 A EP 00901692A EP 00901692 A EP00901692 A EP 00901692A EP 1159469 A1 EP1159469 A1 EP 1159469A1
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EP
European Patent Office
Prior art keywords
cathode
graphite
product
autoclave
impregnation
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Granted
Application number
EP00901692A
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German (de)
French (fr)
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EP1159469B1 (en
Inventor
Régis Paulus
Jean-Michel Dreyfus
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Carbone Savoie SAS
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Carbone Savoie SAS
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Classifications

    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
    • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
    • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes

Definitions

  • the present invention relates to an impregnated graphite cathode for the electrolysis of aluminum.
  • an electrolytic cell comprises, in a metal box sheathed with refractories, a cathode sole composed of several juxtaposed cathode blocks.
  • This assembly constitutes the crucible which, sealed by pot lining, is the seat of the transformation, under the action of the electric current, of the aluminum electrolytic bath. This reaction takes place at a temperature generally higher than 950 ° C.
  • the cathode block is made from carbonaceous material. These materials range from semi-graphite to graphite. They are shaped by extrusion or by vibro-massage after mixing the raw materials:
  • the graphitization treatment of the graphite cathode allows the increase of the electrical and thermal conductivities, thus creating sufficient conditions for optimized operation of an electrolysis tank.
  • Energy consumption decreases due to the decrease in the electrical resistance of the cathode.
  • Another way to take advantage of this drop in electrical resistance consists in increasing the intensity of the current injected into the tank, thereby allowing an increase in the production of aluminum.
  • the high value of the thermal conductivity of the cathode then allows the evacuation of the excess heat generated by the increase in intensity.
  • graphite cathode tanks appear less electrically unstable, that is to say with less fluctuation of the electrical potentials, than carbon cathode tanks.
  • FIG. 1 of the appended schematic drawing shows a cathode block 3, with the cathode bars of current supply 2, the initial profile of which is designated by the reference 4.
  • the erosion profile 5, represented by dotted, shows that this erosion is accentuated at the ends of the cathode block.
  • the erosion rate of a graphite cathode block is, therefore, its weak point, and its economic appeal in terms of production gain may disappear if the service life cannot be increased.
  • carbon cathodes and graphite cathodes consist, in the finished product, of solid graphite grains, and differ essentially by the heat treatment imposed on the binder.
  • the pitch of the graphitic product is treated during the cooking of the à product at a temperature in the region of 1200 ° C.
  • the binder of the graphite cathode is brought, during graphitization, to a temperature above 2400 ° C. and is therefore transformed into graphite.
  • the porosity of the cathodes, carbon and graphite results from the coking of the binder. However, this porosity is invaded during operation
  • Document JP 02 283 677 relates to electrodes for electroerosion machining.
  • the electrodes are impregnated and annealed before undergoing a graphitization heat treatment at 2600-3000 ° C.
  • the document EP 0 562 591 relates to a method of impregnating at room temperature carbon and graphite blocks, from pitches treated with resins to obtain impregnation yields greater than 40%, after carbonization of the impregnator. This document does not address either the electrolysis of aluminum or the problem of erosion of graphite cathodes.
  • Document JP 54 027 313 relates to an electrode impregnated with resins, for the production of chlorine.
  • the object of the invention is to provide a graphite cathode whose lifetime is increased.
  • this cathode contains, in the porosity of its structure, a carbonaceous product baked at less than 1600 ° C., improving the resistance to erosion by protecting the graphite binder.
  • the carbon product is introduced by impregnation into a graphite cathode obtained in a known manner.
  • the carbonaceous product cooked at less than 1600 ° C. inside the porosity of the cathode provides protection for the graphite binder and improves the erosion resistance of the cathode. This product is deposited on the graphite binder
  • the impregnation product prevents the degradation of the latter by reaction with the components of the bath migrating into the porosity of the cathode. Due to its low temperature heat treatment,
  • the impregnation product is more resistant to attack by the components of the bath.
  • the carbonaceous product protecting the graphite binder is chosen from coal pitches and petroleum pitches.
  • such a cathode consists in injecting the carbonaceous product, protecting the graphite binder, in liquid form into the porosity.
  • the carbonaceous impregnation product is a coal pitch, it is heated to a temperature of the order of 200 ° C. to obtain a satisfactory viscosity.
  • a method of producing the cathode according to the invention consists
  • the autoclave causing the impregnation product to penetrate into the autoclave in liquid form, until the cathode is completely immersed, and to break the vacuum in the autoclave by injecting a gas under pressure to allow depending on the duration of the treatment, partial or total filling of the porosity of the cathode by the impregnation product, to bring the autoclave to atmospheric pressure, to take the cathode out of the autoclave, and finally, after possible cooling, to carry out a heat treatment at a temperature below 1600 ° C., but sufficient to effect hardening and / or coking of the impregnation product, ensuring the formation of a layer of non-graphitized carbon, which protects the graphitized binder from erosion.
  • the heat treatment carried out after impregnation aims to stabilize the impregnation product. It can take place in specialized installations "0 or during the preheating of the electrolysis tank and the operation thereof.
  • the impregnation can be carried out on the whole of the cathode, or only on a part of it. Since only partial impregnation is desired, it is advisable
  • Figure 1 is a schematic view of a cathode
  • Figure 2 is a view of an installation for impregnating a cathode with a carbonaceous product.
  • Figure 1 was previously described to show the erosion profile of a graphite cathode after a certain period of use.
  • FIG. 2 represents an impregnation installation comprising an autoclave 6 intended to receive a graphite cathode 3.
  • This autoclave 6: j o can be placed in communication with a tank 7 for storing the carbonaceous impregnation product, by a conduit 8, as well as with a vacuum source through a pipe 9 and with a source of pressurized gas through a pipe 10.
  • a cathode block 3 is placed in the autoclave 6.
  • the carbonaceous product 12 is stored in the tank 7, and possibly heated to be in a liquid state with a viscosity ensuring easy penetration into the porosity of the cathode.
  • the graphite block 3 and the autoclave are heated to the same temperature.
  • a vacuum is created in the autoclave 6 by opening the duct 9.
  • the carbonaceous product 12 ⁇ is admitted into the autoclave 6 until total immersion of the graphite block 3.
  • the duct 8 then being closed, the vacuum is broken by the injection of a gas under pressure through the conduit 10.
  • the impregnator penetrates into the porosity of the product.
  • the duration of the treatment is calculated to allow total or partial invasion of the porosity of the product.
  • the graphite block 3 is taken out of the autoclave and cooled if necessary.
  • the graphite block can then undergo a heat treatment operation at a temperature below 1600 ° C., this heat treatment being a function of the nature of the carbonaceous product 12.
  • the impregnating pitch is a coal pitch 5 with a Mettler point equal to 95 ° C. and the level of insolubles in toluene is less than 6%.
  • the pitch is preheated to a temperature of 200 ° C for which its viscosity is less than 150 cP.
  • the comparison of the weights of the cathode before and after treatment makes it possible to calculate a weight gain of 19%.
  • a theoretical calculation based on the porosity of the product and the density of the impregnation pitch makes it possible to conclude that with such a recovery the entire porosity of the cathode is filled with impregnator.
  • the product is then cooked in a reducing atmosphere at a temperature close to 1000 ° C. The cooking operation again opens the porosity, leaving part of the impregnating agent in the porosity.
  • the characteristics of the impregnated cathode are compared to that of the non-impregnated cathode: graphite non-impregnated cathode impregnated variation (%) apparent density 1, 593 1, 744 + 9.5 flexural strength (MPa) 10.6 17 , 3 + 63.5
  • the invention brings a great improvement to the existing technique, by providing a graphite cathode of traditional structure, the qualities of electrical and thermal conductivity of which are fully maintained, and the wear of which is greatly limited by compared to a traditional cathode. It goes without saying that the invention is not limited to the sole embodiment of this cathode, nor to the sole embodiment of the method, described above by way of examples, on the contrary, it embraces it. all variants.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Electrolytic Production Of Metals (AREA)
  • Ceramic Products (AREA)
  • Electrodes For Compound Or Non-Metal Manufacture (AREA)
  • Carbon And Carbon Compounds (AREA)

Abstract

This cathode (3) contains, within the pores in its structure, a carbon-containing product fired at a temperature of less than 1600° C., improving the erosion resistance by protecting the graphitized binder.

Description

CATHODE GRAPHITE IMPREGNEE POUR L'ELECTROLYSE DE L'ALUMINIUM IMPREGNATED GRAPHITE CATHODE FOR ALUMINUM ELECTROLYSIS
La présente invention a pour objet une cathode graphite imprégnée pour l'électrolyse de l'aluminium.The present invention relates to an impregnated graphite cathode for the electrolysis of aluminum.
Dans le procédé électrolytique utilisé dans la plupart des usines de production d'aluminium, une cuve d'électrolyse comprend, dans un caisson métallique gainé de réfractaires, une sole cathodique composée de plusieurs blocs cathodiques juxtaposés. Cet ensemble constitue le creuset qui, rendu étanche par de la pâte de brasque, est le siège de la transformation, sous l'action du courant électrique, du bain électrolytique en aluminium. Cette réaction a lieu à une température supérieure en général à 950° C.In the electrolytic process used in most aluminum production plants, an electrolytic cell comprises, in a metal box sheathed with refractories, a cathode sole composed of several juxtaposed cathode blocks. This assembly constitutes the crucible which, sealed by pot lining, is the seat of the transformation, under the action of the electric current, of the aluminum electrolytic bath. This reaction takes place at a temperature generally higher than 950 ° C.
Pour résister aux conditions thermiques et chimiques prévalant lors du fonctionnement de la cuve et satisfaire à la nécessité de conduction du courant d'électrolyse, le bloc cathodique est fabriqué à partir de matériau carboné. Ces matériaux vont du semi-graphitique au graphite. Ils sont mis en forme par extrusion ou par vibrotassage après malaxage des matières premières :To withstand the thermal and chemical conditions prevailing during the operation of the tank and satisfy the need for conduction of the electrolysis current, the cathode block is made from carbonaceous material. These materials range from semi-graphite to graphite. They are shaped by extrusion or by vibro-massage after mixing the raw materials:
• soit un mélange de brai, d'anthracite calciné et/ou de graphite dans le cas des matériaux semi-graphitiques et graphitiques. Ces matériaux sont ensuite cuits à environ 1 200° C. La cathode graphitique ne contient pas d'anthracite. La cathode fabriquée à partir de ces matériaux est communément appelée cathode carbone,• or a mixture of pitch, calcined anthracite and / or graphite in the case of semi-graphitic and graphitic materials. These materials are then baked at around 1,200 ° C. The graphite cathode does not contain anthracite. The cathode made from these materials is commonly called a carbon cathode,
• soit un mélange de brai, de coke avec ou sans graphite dans le cas des graphites. Dans ce cas les matériaux sont cuits à environ 800 ° C, puis graphitisés à plus de 2 400° C. Cette cathode est appelée cathode graphite.• or a mixture of pitch, coke with or without graphite in the case of graphites. In this case, the materials are baked at around 800 ° C, then graphitized at more than 2400 ° C. This cathode is called a graphite cathode.
Il est connu d'utiliser des cathodes carbone, qui cependant ont des caractéristiques électriques et thermiques moyennes, ne convenant plus aux conditions de fonctionnement des cuves modernes, notamment de forte intensité de courant. La nécessité de réduire la consommation d'énergie, et la possibilité d'augmenter l'intensité du courant, notamment dans des installations existantes, a promu l'utilisation des cathodes graphite.It is known to use carbon cathodes, which however have average electrical and thermal characteristics, which are no longer suitable for the operating conditions of modern tanks, in particular of high current intensity. The need to reduce energy consumption, and the possibility of increasing the intensity of the current, especially in existing installations, has promoted the use of graphite cathodes.
Le traitement de graphitisation de la cathode graphite, à plus de 2 400° C, permet l'augmentation des conductivités électrique et thermique, créant ainsi les conditions suffisantes à un fonctionnement optimisé d'une cuve d'électrolyse. La consommation d'énergie diminue en raison de la baisse de la résistance électrique de la cathode. Une autre façon de profiter de cette baisse de résistance électrique consiste à augmenter l'intensité du courant 5 injecté dans la cuve, permettant ainsi une augmentation de la production d'aluminium. La valeur élevée de la conductibilité thermique de la cathode permet alors l'évacuation de l'excès de chaleur généré par l'augmentation d'intensité. De plus, les cuves à cathode graphite apparaissent moins instables électriquement, c'est-à-dire comportant moins de fluctuation des '0 potentiels électriques, que les cuves à cathodes carbone.The graphitization treatment of the graphite cathode, at more than 2,400 ° C., allows the increase of the electrical and thermal conductivities, thus creating sufficient conditions for optimized operation of an electrolysis tank. Energy consumption decreases due to the decrease in the electrical resistance of the cathode. Another way to take advantage of this drop in electrical resistance consists in increasing the intensity of the current injected into the tank, thereby allowing an increase in the production of aluminum. The high value of the thermal conductivity of the cathode then allows the evacuation of the excess heat generated by the increase in intensity. In addition, graphite cathode tanks appear less electrically unstable, that is to say with less fluctuation of the electrical potentials, than carbon cathode tanks.
Toutefois, il s'est révélé que les cuves équipées de cathodes graphite présentent une durée de vie plus faible que les cuves équipées de cathodes carbone. Les cuves à cathodes graphite deviennent inutilisables par un enrichissement trop élevé en fer de l'aluminium, qui résulte de l'attaque deHowever, it has been found that the tanks fitted with graphite cathodes have a shorter service life than the tanks fitted with carbon cathodes. Graphite cathode vessels become unusable by too high an iron enrichment of aluminum, which results from the attack of
15 la barre cathodique par l'aluminium. Le métal atteint la barre par suite de l'érosion du bloc graphite. Bien qu'une érosion des cathodes carbone soit également constatée, elle est beaucoup plus faible et n'altère pas la durée de vie des cuves qui deviennent inutilisables pour d'autres causes que l'érosion de la cathode.15 the cathode bar by aluminum. The metal reaches the bar due to the erosion of the graphite block. Although erosion of the carbon cathodes is also observed, it is much lower and does not affect the life of the tanks which become unusable for causes other than erosion of the cathode.
70 Au contraire, l'usure des cathodes graphite est suffisamment rapide pour devenir la première cause de mortalité des cuves d'électrolyse de l'aluminium à un âge que l'on peut qualifier de précoce par rapport aux durées de vie enregistrées pour les cuves équipées de cathodes carbone.70 On the contrary, the wear of graphite cathodes is fast enough to become the first cause of death in aluminum electrolysis cells at an age that can be described as early compared to the lifetimes recorded for the cells. fitted with carbon cathodes.
Ainsi on enregistre les vitesses d'usure suivantes pour les différentsThus the following wear speeds are recorded for the different
?.5 matériaux :? .5 materials:
Cathode vitesse d'usure (mm/an)Cathode wear rate (mm / year)
Carbone, semi-graphitique 10-20Carbon, semi-graphitic 10-20
Carbone, graphitique 20-40 graphite 40-80Carbon, graphitic 20-40 graphite 40-80
*o* o
La figure 1 du dessin schématique annexé montre un bloc cathodique 3, avec les barres cathodiques d'amenée de courant 2, dont le profil initial est désigné par la référence 4. Le profil d'érosion 5, représenté en pointillés, montre que cette érosion est accentuée aux extrémités du bloc cathodique.FIG. 1 of the appended schematic drawing shows a cathode block 3, with the cathode bars of current supply 2, the initial profile of which is designated by the reference 4. The erosion profile 5, represented by dotted, shows that this erosion is accentuated at the ends of the cathode block.
La vitesse d'érosion d'un bloc cathodique graphite est, par conséquent, son point faible, et son attrait économique en terme de gain de 5 production peut disparaître si la durée de vie ne peut pas être augmentée.The erosion rate of a graphite cathode block is, therefore, its weak point, and its economic appeal in terms of production gain may disappear if the service life cannot be increased.
Bien que partant de matières premières différentes, les cathodes carbone et les cathodes graphite sont constituées, dans le produit fini, de grains solides graphite, et diffèrent essentiellement par le traitement thermique imposé au liant. Le brai du produit graphitique est traité pendant la Ό cuisson du produit à une température voisine de 1 200°C. Le liant de la cathode graphite est porté, durant la graphitisation, à une température supérieure à 2 400 °C et est donc transformé en graphite.Although starting from different raw materials, carbon cathodes and graphite cathodes consist, in the finished product, of solid graphite grains, and differ essentially by the heat treatment imposed on the binder. The pitch of the graphitic product is treated during the cooking of the à product at a temperature in the region of 1200 ° C. The binder of the graphite cathode is brought, during graphitization, to a temperature above 2400 ° C. and is therefore transformed into graphite.
La porosité des cathodes, carbone et graphite, résulte de la cokéfaction du liant. Or cette porosité est envahie pendant le fonctionnementThe porosity of the cathodes, carbon and graphite, results from the coking of the binder. However, this porosity is invaded during operation
35 des cuves par les produits d'électrolyse, principalement des fluorures de sodium et d'aluminium. Ces produits sont donc en contact avec le carbone ou le graphite issu du liant.35 of the tanks by electrolysis products, mainly sodium and aluminum fluorides. These products are therefore in contact with carbon or graphite from the binder.
Le document Chemical Abstract vol. 73 n° 22 enseigne l'imprégnation de cathodes pour boucher la porosité et empêcher laChemical Abstract vol. 73 n ° 22 teaches the impregnation of cathodes to block the porosity and prevent the
20 pénétration de produits réactifs. Cette imprégnation est réalisée par des produits autres que le brai et le goudron qui, selon l'auteur, ne sont pas efficaces car ils ne mouillent pas assez de carbone.20 penetration of reactive products. This impregnation is carried out by products other than pitch and tar which, according to the author, are not effective because they do not wet enough carbon.
Le document JP 02 283 677 concerne des électrodes pour usinage par électroérosion. Les électrodes sont imprégnées et recuites avant 25 de subir un traitement thermique de graphitisation à 2600-3000°C.Document JP 02 283 677 relates to electrodes for electroerosion machining. The electrodes are impregnated and annealed before undergoing a graphitization heat treatment at 2600-3000 ° C.
Le document EP 0 562 591 concerne une méthode d'imprégnation à température ambiante de blocs carbone et graphite, à partir de brais traités par des résines pour obtenir des rendements d'imprégnation supérieurs à 40 %, après carbonisation de l'imprégnant. Ce document ne vise ni ?0 l'électrolyse de l'aluminium, ni le problème de l'érosion des cathodes graphite.The document EP 0 562 591 relates to a method of impregnating at room temperature carbon and graphite blocks, from pitches treated with resins to obtain impregnation yields greater than 40%, after carbonization of the impregnator. This document does not address either the electrolysis of aluminum or the problem of erosion of graphite cathodes.
Le document JP 54 027 313 concerne une électrode imprégnée de résines, pour la production de chlore. Le but de l'invention est de fournir une cathode graphite dont la durée de vie soit augmentée. A cet effet, cette cathode contient, dans la porosité de sa structure, un produit carboné cuit à moins de 1 600° C, améliorant la résistance à l'érosion par protection du liant graphité. 5 Le produit carboné est introduit par imprégnation dans une cathode graphite obtenue de façon connue.Document JP 54 027 313 relates to an electrode impregnated with resins, for the production of chlorine. The object of the invention is to provide a graphite cathode whose lifetime is increased. To this end, this cathode contains, in the porosity of its structure, a carbonaceous product baked at less than 1600 ° C., improving the resistance to erosion by protecting the graphite binder. The carbon product is introduced by impregnation into a graphite cathode obtained in a known manner.
Le produit carboné cuit à moins de 1 600°C assure à l'intérieur de la porosité de la cathode, une protection du liant graphité, et améliore la résistance à l'érosion de la cathode. Ce produit se dépose sur le liant graphitéThe carbonaceous product cooked at less than 1600 ° C. inside the porosity of the cathode provides protection for the graphite binder and improves the erosion resistance of the cathode. This product is deposited on the graphite binder
10 en tapissant la porosité, sans boucher la porosité qui est nécessaire au flux des produits issus du bain d'électrolyse. En s'interposant entre les produits du bain et le liant graphite, le produit d'imprégnation empêche la dégradation de ce dernier par réaction avec les composants du bain migrant dans la porosité de la cathode. De par son traitement thermique à basse température,10 by lining the porosity, without blocking the porosity which is necessary for the flow of products from the electrolysis bath. By interposing between the bath products and the graphite binder, the impregnation product prevents the degradation of the latter by reaction with the components of the bath migrating into the porosity of the cathode. Due to its low temperature heat treatment,
' 5 en comparaison avec un graphite, le produit d'imprégnation est plus résistant aux attaques des composants du bain.In comparison with graphite, the impregnation product is more resistant to attack by the components of the bath.
Le produit carboné protégeant le liant graphité est choisi parmi les brais de houille et les brais de pétrole.The carbonaceous product protecting the graphite binder is chosen from coal pitches and petroleum pitches.
Suivant un mode de mise en oeuvre, le procédé d'obtention d'uneAccording to one embodiment, the method of obtaining a
.20 telle cathode consiste à injecter le produit carboné, protégeant le liant graphité, sous forme liquide dans la porosité. A titre d'exemple, si le produit carboné d'imprégnation est un brai de houille, celui-ci est chauffé à une température de l'ordre de 200°C pour obtenir une viscosité satisfaisante..20 such a cathode consists in injecting the carbonaceous product, protecting the graphite binder, in liquid form into the porosity. For example, if the carbonaceous impregnation product is a coal pitch, it is heated to a temperature of the order of 200 ° C. to obtain a satisfactory viscosity.
Un procédé de réalisation de la cathode selon l'invention consisteA method of producing the cathode according to the invention consists
?5 tout d'abord, de façon connue en soi, à réaliser une cathode à partir de coke, avec ou sans graphite, et de brai subissant un traitement thermique à plus de 2 400° C, à placer cette cathode dans un autoclave après un éventuel préchauffage à une température correspondant à la température à laquelle le produit d'imprégnation possède la viscosité souhaitée, à faire le vide dansFirstly, in a manner known per se, to produce a cathode from coke, with or without graphite, and from pitch undergoing heat treatment at more than 2400 ° C., to place this cathode in an autoclave after possible preheating to a temperature corresponding to the temperature at which the impregnation product has the desired viscosity, to create a vacuum in
30 l'autoclave, à faire pénétrer dans l'autoclave le produit d'imprégnation sous forme liquide, jusqu'à immersion totale de la cathode, et à casser le vide dans l'autoclave par injection d'un gaz sous pression pour permettre, suivant la durée du traitement, le remplissage partiel ou total de la porosité de la cathode par le produit d'imprégnation, à ramener l'autoclave à la pression atmosphérique, à sortir la cathode de l'autoclave, et enfin, après refroidissement éventuel, à réaliser un traitement thermique à une température inférieure à 1 600° C, mais suffisante pour réaliser le 5 durcissement et/ou la cokéfaction du produit d'imprégnation, assurant la formation d'une couche de carbone non graphité, qui protège le liant graphité de l'érosion.30 the autoclave, causing the impregnation product to penetrate into the autoclave in liquid form, until the cathode is completely immersed, and to break the vacuum in the autoclave by injecting a gas under pressure to allow depending on the duration of the treatment, partial or total filling of the porosity of the cathode by the impregnation product, to bring the autoclave to atmospheric pressure, to take the cathode out of the autoclave, and finally, after possible cooling, to carry out a heat treatment at a temperature below 1600 ° C., but sufficient to effect hardening and / or coking of the impregnation product, ensuring the formation of a layer of non-graphitized carbon, which protects the graphitized binder from erosion.
Le traitement thermique réalisé après imprégnation vise à stabiliser le produit d'imprégnation. Il peut avoir lieu dans des installations spécialisées " 0 ou lors du préchauffage de la cuve d'électrolyse et du fonctionnement de celle-ci.The heat treatment carried out after impregnation aims to stabilize the impregnation product. It can take place in specialized installations "0 or during the preheating of the electrolysis tank and the operation thereof.
Il peut être noté que l'imprégnation peut être réalisée sur l'ensemble de la cathode, ou seulement sur une partie de celle-ci. Dans la mesure où l'on ne souhaite qu'une imprégnation partielle, il convientIt may be noted that the impregnation can be carried out on the whole of the cathode, or only on a part of it. Since only partial impregnation is desired, it is advisable
, 5 d'imperméabiliser la surface du bloc à traiter, ou alors de n'immerger que partiellement le bloc dans le liquide d'imprégnation., 5 to waterproof the surface of the block to be treated, or else to only partially immerse the block in the impregnating liquid.
Afin de renforcer l'action du traitement, il est possible de procéder si besoin est, à plusieurs cycles successifs d'imprégnation et de recuisson.In order to enhance the action of the treatment, it is possible to carry out, if necessary, several successive cycles of impregnation and annealing.
De toute façon, l'invention sera bien comprise à l'aide de la 70 description qui suit, en référence au dessin schématique annexé représentant, à titre d'exemple non limitatif, une cathode en graphite, ainsi qu'une installation d'imprégnation d'une cathode :Anyway, the invention will be better understood with the aid of the description which follows, with reference to the appended schematic drawing representing, by way of nonlimiting example, a graphite cathode, as well as an impregnation installation. a cathode:
Figure 1 est une vue schématique d'une cathode ; Figure 2 est une vue d'une installation d'imprégnation d'une 5 cathode par un produit carboné.Figure 1 is a schematic view of a cathode; Figure 2 is a view of an installation for impregnating a cathode with a carbonaceous product.
La figure 1 a été décrite précédemment pour montrer le profil d'érosion d'une cathode graphite après un certain temps d'utilisation.Figure 1 was previously described to show the erosion profile of a graphite cathode after a certain period of use.
La figure 2 représente une installation d'imprégnation comprenant un autoclave 6 destiné à recevoir une cathode graphite 3. Cet autoclave 6 :jo peut être mis en communication avec un réservoir 7 de stockage du produit d'imprégnation carboné, par un conduit 8, ainsi qu'avec une source de dépression par un conduit 9 et avec une source de gaz sous pression par un conduit 10. Après obtention, de façon traditionnelle, d'un bloc graphite destiné à former une cathode, avec opération de graphitisation à plus de 2 400°C, ce bloc cathode 3 est placé dans l'autoclave 6. Le produit 12 carboné est stocké dans le réservoir 7, et éventuellement chauffé pour se trouver à un état 5 liquide avec une viscosité assurant une pénétration aisée dans la porosité de la cathode. Le bloc graphite 3 et l'autoclave sont chauffés à la même température.FIG. 2 represents an impregnation installation comprising an autoclave 6 intended to receive a graphite cathode 3. This autoclave 6: j o can be placed in communication with a tank 7 for storing the carbonaceous impregnation product, by a conduit 8, as well as with a vacuum source through a pipe 9 and with a source of pressurized gas through a pipe 10. After traditionally obtaining a graphite block intended to form a cathode, with a graphitization operation at more than 2400 ° C., this cathode block 3 is placed in the autoclave 6. The carbonaceous product 12 is stored in the tank 7, and possibly heated to be in a liquid state with a viscosity ensuring easy penetration into the porosity of the cathode. The graphite block 3 and the autoclave are heated to the same temperature.
Le vide est réalisé dans l'autoclave 6 par ouverture du conduit 9.A vacuum is created in the autoclave 6 by opening the duct 9.
Tout en conservant l'autoclave sous vide, le produit carboné 12 ι est admis dans l'autoclave 6 jusqu'à immersion totale du bloc graphite 3. Le conduit 8 étant alors fermé, le vide est cassé par l'injection d'un gaz sous pression par le conduit 10. Sous l'action de la pression hydrostatique ainsi créée, l'imprégnant pénètre dans la porosité du produit. La durée du traitement est calculée pour permettre un envahissement total ou partiel de la 5 porosité du produit.While keeping the autoclave under vacuum, the carbonaceous product 12 ι is admitted into the autoclave 6 until total immersion of the graphite block 3. The duct 8 then being closed, the vacuum is broken by the injection of a gas under pressure through the conduit 10. Under the action of the hydrostatic pressure thus created, the impregnator penetrates into the porosity of the product. The duration of the treatment is calculated to allow total or partial invasion of the porosity of the product.
Enfin la pression est ramenée à la pression atmosphérique, le bloc graphite 3 est sorti de l'autoclave et refroidi s'il y a lieu. Le bloc graphite peut alors subir une opération de traitement thermique à une température inférieure à 1 600°C, ce traitement thermique étant fonction de la nature du 0 produit carboné 12.Finally, the pressure is brought back to atmospheric pressure, the graphite block 3 is taken out of the autoclave and cooled if necessary. The graphite block can then undergo a heat treatment operation at a temperature below 1600 ° C., this heat treatment being a function of the nature of the carbonaceous product 12.
Un exemple de traitement de cathode graphite est décrit ci-après.An example of graphite cathode treatment is described below.
ExempleExample
Une cathode graphite entière de dimensions 650*450*3300 est imprégnée par du brai imprégnant. Le brai imprégnant est un brai de houille 5 de point Mettler égal à 95 °C et le taux d'insolubles dans le toluène est inférieur à 6 %. Le brai est préchauffé à une température de 200° C pour laquelle sa viscosité est inférieure à 150 cP. Le produit est chauffé dans un autoclave à la température de 200° C. Une fois la température atteinte, l'autoclave est mis sous vide jusqu'à atteindre un vide résiduel inférieur à 0 10 mm de mercure (760 mm de mercure = 101 300 Pa). Le brai chaud est alors admis dans l'autoclave par aspiration. La cathode étant immergée dans le brai, la vanne d'admission du brai est fermée et de l'azote gazeux est injecté dans l'autoclave à la pression de 10 bars (1 bar = 105 Pa). Après une heure de mise sous pression, l'autoclave est ouvert et le produit est refroidi.An entire graphite cathode of dimensions 650 * 450 * 3300 is impregnated with impregnating pitch. The impregnating pitch is a coal pitch 5 with a Mettler point equal to 95 ° C. and the level of insolubles in toluene is less than 6%. The pitch is preheated to a temperature of 200 ° C for which its viscosity is less than 150 cP. The product is heated in an autoclave at a temperature of 200 ° C. Once the temperature has been reached, the autoclave is placed under vacuum until a residual vacuum of less than 0 10 mm of mercury is reached (760 mm of mercury = 101,300 Pa). The hot pitch is then admitted into the autoclave by suction. With the cathode submerged in the pitch, the pitch inlet valve is closed and nitrogen gas is injected into the autoclave at a pressure of 10 bars (1 bar = 10 5 Pa). After one hour of pressurization, the autoclave is opened and the product is cooled.
La comparaison des poids de la cathode avant et après traitement permet de calculer un gain de poids de 19 %. Un calcul théorique basé sur la porosité du produit et la densité du brai d'imprégnation permet de conclure qu'avec une telle reprise l'ensemble de la porosité de la cathode est remplie d'imprégnant. Le produit est ensuite cuit en atmosphère réductrice à une température voisine de 1 000°C. L'opération de cuisson entraîne à nouveau l'ouverture de la porosité, en laissant une partie de l'imprégnant dans la porosité. Les caractéristiques de la cathode imprégnée sont comparées à celle de la cathode non-imprégnée : cathode graphite non- imprégnée imprégnée variation (%) densité apparente 1 ,593 1 ,744 + 9,5 résistance à la flexion (MPa) 10,6 17,3 + 63,5The comparison of the weights of the cathode before and after treatment makes it possible to calculate a weight gain of 19%. A theoretical calculation based on the porosity of the product and the density of the impregnation pitch makes it possible to conclude that with such a recovery the entire porosity of the cathode is filled with impregnator. The product is then cooked in a reducing atmosphere at a temperature close to 1000 ° C. The cooking operation again opens the porosity, leaving part of the impregnating agent in the porosity. The characteristics of the impregnated cathode are compared to that of the non-impregnated cathode: graphite non-impregnated cathode impregnated variation (%) apparent density 1, 593 1, 744 + 9.5 flexural strength (MPa) 10.6 17 , 3 + 63.5
Après cuisson le gain de poids est de 9,5 % et le gain de résistance à la flexion est très important, ce qui prouve le bouchage des microfissures par le brai imprégnant et ainsi un bon mouillage du brai imprégnant sur le brai graphité. Comme il ressort de ce qui précède, l'invention apporte une grande amélioration à la technique existante, en fournissant une cathode graphite de structure traditionnelle, dont les qualités de conductivité électrique et thermique sont totalement maintenues, et dont l'usure est fortement limitée par rapport à une cathode traditionnelle. Comme il va de soi, l'invention ne se limite pas à la seule forme d'exécution de cette cathode, ni au seul mode de mise en oeuvre du procédé, décrits ci-dessus à titre d'exemples, elle en embrasse au contraire toutes les variantes. C'est ainsi notamment qu'il serait possible de faire subir à un bloc graphite plusieurs traitements successifs, éventuellement à partir de plusieurs produits carbonés différents ou de ne réaliser un traitement que sur une surface du bloc, par exemple correspondant aux extrémités de la cathode, sans que l'on sorte pour autant du cadre de l'invention. La création du vide, la mise sous pression ou l'immersion totale ne sont pas nécessaires si l'on veut réaliser un traitement par trempage ou un traitement localisé d'une zone prédéfinie de la cathode. After cooking, the weight gain is 9.5% and the gain in resistance to bending is very large, which proves the plugging of the microcracks by the impregnating pitch and thus a good wetting of the impregnating pitch on the graphite pitch. As is apparent from the above, the invention brings a great improvement to the existing technique, by providing a graphite cathode of traditional structure, the qualities of electrical and thermal conductivity of which are fully maintained, and the wear of which is greatly limited by compared to a traditional cathode. It goes without saying that the invention is not limited to the sole embodiment of this cathode, nor to the sole embodiment of the method, described above by way of examples, on the contrary, it embraces it. all variants. It is thus in particular that it would be possible to subject a graphite block to several successive treatments, possibly from several different carbon products or to carry out a treatment only on one surface of the block, for example corresponding to the ends of the cathode. , without thereby departing from the scope of the invention. The creation of vacuum, pressurization or total immersion is not necessary if one wants to perform a soaking treatment or a localized treatment of a predefined area of the cathode.

Claims

REVENDICATIONS
1 . Cathode graphite imprégnée pour électrolyse de l'aluminium, caractérisée en ce qu'elle contient, dans la porosité de sa structure, un produit carboné ( 12) cuit à moins de 1 600°C améliorant la résistance à1. Impregnated graphite cathode for aluminum electrolysis, characterized in that it contains, in the porosity of its structure, a carbonaceous product (12) baked at less than 1600 ° C improving the resistance to
5 l'érosion par protection du liant graphité.5 erosion by protecting the graphite binder.
2. Cathode graphite selon la revendication 1 , caractérisée en ce que le produit carboné ( 12) est introduit par imprégnation dans une cathode graphite obtenue de façon connue.2. Graphite cathode according to claim 1, characterized in that the carbonaceous product (12) is introduced by impregnation into a graphite cathode obtained in a known manner.
3. Cathode graphite selon l'une quelconque des revendications 1 Ό et 2, caractérisée en ce que le produit carboné (12) protégeant le liant graphité est choisi parmi les brais de houille et de pétrole.3. Graphite cathode according to any one of claims 1 Ό and 2, characterized in that the carbon product (12) protecting the graphite binder is chosen from coal and petroleum pitches.
4. Procédé de réalisation d'une cathode graphite selon la revendication 3, caractérisé en ce qu'il consiste tout d'abord, de façon connue en soi, à réaliser une cathode (3) à partir de coke, avec ou sans4. Method for producing a graphite cathode according to claim 3, characterized in that it consists first of all, in a manner known per se, of producing a cathode (3) from coke, with or without
15 graphite, et de brai subissant un traitement thermique à plus de 2 400°C, à placer cette cathode dans un autoclave (6) après un éventuel préchauffage à une température correspondant à la température à laquelle le produit d'imprégnation (12) possède la viscosité souhaitée, à faire le vide dans l'autoclave (6), à faire pénétrer dans l'autoclave le produit d'imprégnation 0 (12) sous forme liquide, jusqu'à immersion totale de la cathode (3), et à casser le vide dans l'autoclave par injection d'un gaz sous pression pour permettre, suivant la durée du traitement, le remplissage partiel ou total de la porosité de la cathode par le produit d'imprégnation, à ramener l'autoclave (6) à la pression atmosphérique, à sortir la cathode (3) de l'autoclave, et 5 enfin, après refroidissement éventuel, à réaliser un traitement thermique à une température inférieure à 1 600° C, mais suffisante pour réaliser la cokéfaction du produit d'imprégnation, assurant la formation d'une couche de carbone non graphité, qui protège le liant graphité de l'érosion. 15 graphite, and pitch undergoing heat treatment at more than 2400 ° C., to place this cathode in an autoclave (6) after possible preheating at a temperature corresponding to the temperature at which the impregnation product (12) has the desired viscosity, to create a vacuum in the autoclave (6), to allow the impregnation product 0 (12) to enter the autoclave in liquid form, until the cathode (3) is completely immersed, and to break the vacuum in the autoclave by injecting a pressurized gas to allow, depending on the duration of the treatment, partial or total filling of the porosity of the cathode with the impregnation product, to bring the autoclave (6) at atmospheric pressure, to take the cathode (3) out of the autoclave, and finally, after possible cooling, to carry out a heat treatment at a temperature below 1600 ° C., but sufficient to carry out the coking of the product of impregnation, ensuring the f formation of a layer of non-graphite carbon, which protects the graphite binder from erosion.
EP00901692A 1999-02-02 2000-02-01 Impregnated graphite cathode for electrolysis of aluminium Revoked EP1159469B1 (en)

Applications Claiming Priority (3)

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FR9901322 1999-02-02
FR9901322A FR2789093B1 (en) 1999-02-02 1999-02-02 GRAPHITE CATHODE FOR ALUMINUM ELECTROLYSIS
PCT/FR2000/000233 WO2000046427A1 (en) 1999-02-02 2000-02-01 Impregnated graphite cathode for electrolysis of aluminium

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CN100491600C (en) * 2006-10-18 2009-05-27 中国铝业股份有限公司 Method for preparing carbon block of cathode capable of being humidified
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IS6025A (en) 2001-07-27
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US6723212B1 (en) 2004-04-20
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EP1159469B1 (en) 2004-04-21
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