EP1151150B1 - Graphite cathode for electrolysis of aluminium - Google Patents

Abstract

In this cathode, which is a single block, the electrical resistivity is heterogeneous along its longitudinal axis, this resistivity being higher in the end regions of the cathode (3) than in the central region of the latter.

Description

The present invention relates to a graphite cathode for aluminum electrolysis.

In the electrolytic process used in most factories aluminum production, an electrolytic cell includes, in a box metallic sheathed with refractories, a cathode sole composed of several cathode blocks juxtaposed. This set constitutes the crucible which, rendered sealed by pot lining, is the seat of transformation, under the action of electric current, of the aluminum electrolytic bath. This reaction takes place at a temperature generally above 950 ° C.

• 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,
• 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.

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:

• 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 1200 ° C. The graphite cathode does not contain anthracite. The cathode made from these materials is commonly called a carbon cathode,
• 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 over 2400 ° C. This cathode is called a graphite cathode.

It is known to use carbon cathodes, which however have average electrical and thermal characteristics, no longer suitable to the operating conditions of modern tanks, especially high amperage. The need to reduce energy consumption, and the possibility of increasing the intensity of the current, in particular in existing facilities, promoted the use of graphite cathodes.

The graphitization treatment of the graphite cathode, more than 2400 ° C, increases the electrical and thermal conductivities, thus creating sufficient conditions for optimized operation of a electrolysis tank. Energy consumption decreases due to decline the electrical resistance of the cathode. Another way to take advantage of this decrease in electrical resistance consists in increasing the intensity of the current injected into the tank, allowing an increase in production aluminum. The high value of thermal conductivity of the cathode allows the evacuation of the excess heat generated by the increase intensity. In addition, graphite cathode vessels appear less electrically unstable, i.e. with less fluctuation in electrical potentials, than carbon cathode tanks.

However, it turned out that the tanks fitted with cathodes graphite have a shorter lifespan than tanks fitted with carbon cathodes. Graphite cathode vessels become unusable by too high an iron enrichment of aluminum, which results from the attack of the cathode bar by aluminum. Metal hits the bar as a result of erosion of the graphite block. Although erosion of carbon cathodes is also noted, it is much lower and does not affect the duration of life of tanks which become unusable for causes other than erosion of the cathode.

On the contrary, the wear of graphite cathodes is fast enough to become the first cause of death of aluminum electrolysis cells at an age that can be described as early compared to the lifetimes recorded for equipped cells. carbon cathodes. The following wear rates are therefore recorded for the different materials: Cathode wear speed (mm / year) Carbon, semi-graphitic 10-20 Carbon, graphitic 20-40 graphite 40-80

The single figure of the attached schematic drawing shows a block cathode 3, with the cathode bars of current supply 2, the initial profile is designated by the reference 4. The erosion profile 5, represented in dotted, shows that this erosion is accentuated at the ends of the block cathode.

Document FR 2 117 960 describes a cathode for the preparation aluminum by electrolysis. This cathode is made from several semi-graphitic carbon blocks, with different resistivities each other. This complex structure due to the juxtaposition of blocks with the electrical discontinuity which it involves, is justified not by a reduced erosion, since cathodes of this type are not sensitive erosion, but by a decrease in the swelling of the sole in the area Central.

The document FR 2 351 192 describes, in a device for production of aluminum, a cathode assembly comprising a bar cathodic and a carbon block separated by a heterogeneous interface allowing to vary over the length of the cathode assembly, the contact resistance between the cathode bar and the carbon block.

The erosion rate of a graphite cathode block is, by Consequently, its weak point, and its economic attractiveness in terms of gain of production may disappear if the service life cannot be increased.

The calculation of current densities in the cathode shows that these are higher on the outlet side of the cathode bars. These current densities are higher as the electrical resistance of the cathode is weak. Thus the erosion profile of each cathode, and in particular the heavy wear observed at the ends of the cathodes correspond to areas of high current densities in the cathode.

The problem is therefore to reduce erosion of cathodes by graphite, in particular in the end zones thereof.

The object of the invention is to provide a graphite cathode whose service life is increased by limiting the erosion that occurs at ends.

To this end, in the cathode according to the invention, the cathode in graphite is in one piece and its electrical resistivity is heterogeneous along its longitudinal axis, this resistivity being higher in the zones cathode end than in the central area of the cathode, the difference resistivity in the end zones and in the central zone of the cathode being obtained by a different heat treatment in these different zones during the graphitization operation, the end zones being at one temperature lower than that of the central zone.

The average resistivity of the product will remain compatible with a optimized operation of the electrolysis tank. The highest resistivity in the cathode end zones channels the current lines to the center of the tank. Because of this, the high current densities usually recorded to the output of the cathode bars are attenuated, inhibiting thus the erosion mechanism in these areas. The life of the tank is therefore increased. As indicative, the cathode end areas can be considered as located between approximately 0 and 800 mm from each end.

According to one possibility, during the graphitization operation, the cathode end zones are brought to a temperature of the order from 2,200-2,500 ° C, while the central zone is brought to a temperature of the order of 2,700 to 3,000 ° C.

According to a first embodiment, the difference of heat treatment in the end zones and in the central zone of the cathode is obtained by limiting the thermal insulation of the graphitization furnace and / or by having thermal drains in the end zones of the cathodes, to increase heat losses.

According to another embodiment, the difference in treatment thermal in the end zones and in the central zone of the cathode is obtained by creating, during the graphitization operation, modifications local current lines and, therefore, the resulting Joule effect.

It is possible to combine these two phenomena during the same graphitization operation.

In accordance with an embodiment of the cathode according to the invention, in the case where the graphitization operation is carried out simultaneously for several cathodes arranged parallel to each other others inside an oven, for example of the Acheson type, in which the cathodes are separated from each other by a grain lining resistor, for example carbon or coke granules, the difference of heat treatment between the end zones and the central zone is obtained by modulating the resistivity of the resistor grain between two cathodes and / or by having thermal drains in the end zones.

Figure 1 est une vue d'une cathode, avec indication plus spécifique de l'érosion de celle-ci après un certain temps de fonctionnement ;

Figures 2 à 4 sont trois vues, respectivement, de dessus, de face et de côté d'un four de graphitisation de type Acheson ;

Figures 5 à 7 sont trois vues, respectivement, de dessus, de face et de côté d'un four de graphitisation de type longitudinal.

In any case, the invention will be clearly understood with the aid of the description which follows, with reference to the appended schematic drawing showing, by way of nonlimiting examples, several installations for obtaining a cathode according to the invention :

Figure 1 is a view of a cathode, with more specific indication of the erosion thereof after a certain time of operation;

Figures 2 to 4 are three views, respectively, from above, from the front and from the side of an Acheson type graphitization oven;

Figures 5 to 7 are three views, respectively, from above, from the front and from the side of a longitudinal type graphitization oven.

Figures 2 to 4 show an Acheson type oven 6, in which a number of cathodes 3 are arranged parallel to each other others, in several rows, with interposition between the different cathodes of a resistor grain 7. This resistor grain can be constituted, for example by carbon or coke granules. The assembly is arranged inside a heat-insulating grain 8. Electric energy is injected inside the oven, to perform the graphitization operation, the heating resulting from the effect Joule. In an oven of this type, the streamlines are perpendicular to the axis of the cathodes 3. To achieve less heating in the areas end of cathodes 3, the resistivity of the resistor grain is higher in the zones 9 corresponding to the end zones of the cathodes 3, that that of this grain resistor in zone 10 corresponding to the central part cathodes. It is also possible to reduce the thickness of the grain insulation 8 in the end areas of the cathodes, to promote the phenomenon of limitation of the graphitization temperature in these areas end by heat loss.

FIG. 5 represents a longitudinal oven 11 in which several cathodes 3 are arranged end to end, with interposition between two cathodes adjacent to a graphitization joint 12. The graphitization joints are as weak as possible to avoid unwanted heating at the junction between the cathodes. In addition, heat losses materialized by arrows are created in the end zones of the cathodes, by providing a smaller thickness of insulation 8, and / or the presence of thermal drains which can be made of graphite and positioned perpendicular to the cathodes, facing the areas to be cooled.

As is apparent from the above, the invention provides a great improvement to the existing technique by providing a cathode of traditional structure, and obtained by known means, having a higher resistivity in its end zones than in its central zone, thus reducing the current density in the cathode at its ends, and increase resistance to erosion in these areas end.

Claims (5)

1. Graphite cathode for the electrolysis of aluminium with improved resistance to erosion, characterised in that it is made in one piece and in that its electrical resistivity is heterogeneous along its longitudinal axis, this resistivity being higher in the end regions of the cathode (3) than in the central region thereof, the difference in resistivity in the end regions and in the central region of the cathode (3) being obtained by a different heat treatment in these different regions during the graphitisation operation, the end regions being at a temperature below that of the central region.
2. Graphite cathode according to Claim 1, characterised in that, during the graphitisation operation, the end regions of the cathode (3) are brought to a temperature of the order of 2200-2500°C, whereas the central region is brought to a temperature of the order of 2700 to 3000°C.
3. Method of producing a graphite cathode according to either one of Claims 1 and 2, characterised in that it consists in creating a difference in heat treatment in the end regions and in the central region of the cathode (3) by limiting the thermal insulation (8) of the graphitisation furnace (11) and/or by arranging heat sinks opposite the end regions of the cathodes, in order to increase the heat losses.
4. Method of producing a graphite cathode according to either one of Claims 1 and 2, characterised in that it consists in creating the difference in heat treatment in the end regions and in the central region of the cathode (3) by locally modifying the lines of current, and consequently the Joule effect which results therefrom, during the graphitisation operation.
5. Method of producing a graphite cathode according to Claim 4, characterised in that, in the case where the graphitisation operation is carried out simultaneously for a plurality of cathodes (3) arranged parallel to one another inside a furnace (6), for example of the Acheson type, in which the cathodes (3) are separated from one another by a resistor-grain packing (7), for example carbon or coke granules, the difference in heat treatment between the end regions and the central region of the cathode (3) is obtained by varying the electrical resistivity of the resistor grain between two cathodes and/or arranging heat sinks opposite the end regions.

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