NO134706B - - Google Patents

Abstract

Carbon anode for use in the production of aluminum by melt electrolysis.

Description

The present invention relates to a carbon anode for use in the production of aluminum and whose surface is provided with a coating that inhibits oxidation of the anode material.

The cathodically connected vessel in a melting electrolysis cell for the production of aluminum contains molten aluminum as well as an electrolyte that floats on top of the aluminum and contains aluminum oxide. On the side of the electrolyte facing the external atmosphere, a solid crust is formed, which in turn is covered with a layer of alumina (aluminum oxide) for periodic enrichment of the electrolyte and thermal insulation of the bath in the electrolysis vessel.

Anodes of a synthetically manufactured carbon material are passed through the aforementioned alumina layer and the crust and extend down into the electrolyte to supply the electric current that maintains electrolysis processes. The crust and the alumina placed on top of it end

usually does not close tightly around the anode circumference, but forms a gap around the anode circumference due to gas emissions and other influences. To enrich the electrolyte with aluminum oxide, the crust is broken

in at regular intervals. The gases that develop during the electrolysis process, and which mainly rise up through the cracks around the anodes' circumference, form a mixture with the surrounding air above the electrolysis bath, and this mixture has a detracting effect on the maximum achievable lifetime of the anodes. The consumption of anode material during the operation of the electrolysis cell is known under the term "burn-off" and consists of a primary and a secondary burn-off which are each based on their own oxidation process, which for the present purpose will be considered separately.

During the primary burning, the oxygen released from the aluminum oxide under the melting electrode attacks the carbon material in the anode, so that a gas mixture of carbon dioxide and carbon monoxide is formed, which rises from the electrolytic bath. This reaction, which causes the largest part of the burn-off, takes place exothermicly with heating of the electrolyte and reduction of the necessary supplied energy for the electrolysis work. This primary burning cannot be avoided with carbon anodes.

However, the invention relates to agents for influencing the oxidation process which. produces the secondary burning of the anode, which adversely affects the economic operation of the cell.

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The temperature of the bath, which means the temperature of the electrolyte in the cathode vessel, is around 950 to 980°C, and this heat source transfers part of its heat content to the carbon alloys, so that a temperature gradient occurs between that side. of the anode that faces the bath and the side that faces away from the bath. Equivalent;;

the heat supplied to the anodes, the anode surfaces will show a

temperature gradient between' a temperature maximum of 980°C and a lowest value of 400°C. The part of the anode that extends well above the bath is surrounded by a gas mixture containing air from the electrolysis hall and gases rising from the cell, these gases at the current anode temperature affecting the anode on

'oxydérenae way, and thus causes a faster burning of this.

The termination reactions, which take place below,/.produce,

in contrast to the oxidation process which produces the primary burning, no contribution to the heating of the bath and thus to the aforementioned energy reduction, but instead causes an unproductive loss of carbon, which can amount to 8% of the total consumption of anode material.

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To reduce the losses that.; occurs during secondary combustion, it is known to provide the anodes with a coating on their upper side surfaces, when the anodes consist of carbon and are intended for immersion in a melting electrolysis cell for the production of aluminium. Based on the requirement that the coating material must not contaminate the electrolyte - and thus the electrolytically separated aluminum - according to known technology, aluminum is used as coating material, which can be applied by spraying or casting. There are, however, various disadvantages with coatings of this type. Sprayed-on aluminum coatings are produced in a thickness between 0.3 and 0<;>.5 mm, while cast coatings have a thickness of at least 1 cm, but preferably several cm, which multiplied by the total surface for all anodes in the cell gives a total volume, considered as a continuous..circulating amount of metal, is disproportionately large in relation to the produced amount of raw aluminum in the cell. The most prominent disadvantage, however, lies in the fact that the aluminum coating melts along an isotherm of around 650°C on the anode surfaces, so that each anode surface will have a lower exposed zone with a height of several cm between the bath and this isotherm of around 650°C, whereby said zone will be freely exposed to oxidizing attack from the surrounding gas mixture. Although the aluminum coating, when it melts away, will leave behind an internal natural aluminum oxide layer, this layer will also corrode away, as it adheres poorly or not at all to the carbon surface. Effective protection against burning cannot therefore be ensured in this way.

It is also known to provide carbon electrodes with a ceramic coating based on finely divided aluminum oxide which is rolled or hammered into the anode surface before the anode is burned in. During firing, however, nowhere near the melting point of aluminum oxide is reached. The oxide grains cannot therefore fuse together, and the coating will consequently not be tight.

Finally, it is known to apply a coating of aluminum with an oxide content of 1 - 10% to an electrode by introducing liquid aluminum into a directed air stream, so that an oxide layer is formed on the surface of the aluminum droplets. In this method, it is of essential importance that an aluminum oxide content of 10% is not exceeded, as this will cause combustion of the carbon anode. However, with a lower content of aluminum oxide, the aforementioned disadvantage of a pure aluminum coating will easily occur.

As stated above, the secondary burn can amount to up to 8%

of the total carbon consumption for a given amount of aluminum produced. If an economic benefit is to be achieved by eliminating the secondary combustion, the costs used for this purpose must be kept within the scope of the above-mentioned part of the carbon costs.

The invention is based on this basis, and its purpose is to provide an electrode coating which is neutral to the environment and does not melt at the current operating temperatures, for coating a carbon anode intended for melt electrolysis in the production of :•..•>•'/>:• •*.- • ■ v*. • ....... •aluminium. Here, it is a significant feature that the necessary "material consumption for the production of said coating entails minimal disadvantages in relation to the advantages achieved by eliminating the secuhdær burn-off.

This purpose is achieved according to the invention by the fact that the coating consists of flame-sprayed material particles based on aluminum oxide, the individual particles being at least partially joined together by solidification in place.

For the production of such a coating, the material particles which are to make up the coating are preferably heated in a gas jet at high.

temperature' at least to the plastic state and preferably to

molten state, at the same time that the particles are flung towards the surface to be coated by means of this gas jet. The molten liquid droplets then solidify in turn on the surface to be coated, after they have come into contact with each other and have fused to a greater or lesser extent. However, the individual aluminum oxide particles which have become more or less compressed due to the impact against the surface can still usually be separated from each other by viewing the coating under strong magnification.

The rapidly cooled aluminum oxide agglomerate which appears in this way is crystallographically mainly in the gamma modification. Any overheating of the applied coating should be avoided, because the gamma modification will then eventually turn into the alpha modification, which in operation exhibits the disadvantage that, if it is present in too large a quantity in the coating, - will not be able to dissolve in cryolite - the electrolyte bg

will thus lead to the formation of bottom crust.

Due to the material's high melting point and the strong temperature drop during solidification, as well as due to the surface roughness of the underlying carbon body, it may happen that the individual falling droplets do not combine to a sufficient extent with the already applied droplets, so that the spaces are thereby created, or that as a result of the very rapid solidification and the resulting internal stresses, cracks will form inside or between the individual solidified grains, which in turn can lead to a certain porosity of the coating. This porosity can, when the anode is in operation, lead to a limited secondary burning. In order to eliminate this effect on the basis of statistical closing of the pores, the coating should preferably have a thickness of 0.1 to 1.0 mm, preferably between 0.2 and 0.5 mm.

As starting material in the production of the coating, ordinary industrial alumina (aluminium oxide), of the same type as the material supplied to the electrolysis cell, can simply be used.

However, it is possible to add a small amount of flux, e.g. cryolite, to this aluminum oxide, with the intention of lowering the melting point of the oxide or to provide a wider melting area, which will enable a better joining together of the molten droplets in the coating, and thus also of the thereby solidified particles, which will result in a reduction of the porosity. It is also possible to add: a powder whose individual grains already contain the desired state, which has been added in a previous manufacturing process.

According to a further advantageous embodiment of the invention, said coating is combined with a metal layer. For the sake of simplicity, a coating with at least a major proportion of aluminum oxide,

be termed a ceramic coating.

Materials that can come into consideration for use as a metallic coating are pure or alloyed metal that can be allowed in connection with the aluminum that is electrolytically separated in the cell, and furthermore preferably has the property that the material moistens the ceramic coating to a certain extent when it is present in liquid state. Pure aluminum is particularly well suited for this purpose. Such a metal layer can be applied using any suitable method, e.g. by flame spraying, and should thereby be able to be kept free of pores without difficulty.

The metal layer can either be arranged below or on top of the ceramic layer, or the coating can also consist of several separate ceramic layers and metallic layers applied alternately on top of each other.

In operation, an anode works with such a combined coating applied

in the following way:

On those parts of the upper anode surface that have temperatures below the melting point of the metal, the impermeable non-porous coating will act as a complete protection against burn-off. In the warmer zones between the isotherm that corresponds to the metal's melting point and the bath's surface, the metal, e.g. the aluminium, melt and by capillary action penetrate into the pores of the ceramic layer, where it is partially oxidized under the influence of the surrounding oxidizing atmosphere. In this zone, the pores in the ceramic layer are thus closed by oxidation products, i.e. e.g. with aluminum oxide, which likewise has a melting point above the operating temperature and thus ensures reliable protection against burning. In combination with the aluminum oxide layer, the aluminum layer thus performs a secondary function, which

will not take place with conventional aluminum protective layers.

For this purpose, it will be sufficient with relatively thin metal layers, preferably on an aluminum basis and in a thickness of 0.05 to 1.0 mm, preferably between 0.1 and 0.2 mm.

In a further embodiment of the invention, the coating is formed

of a material of the cermet type and consisting of aluminum oxide and aluminum sprayed on simultaneously in a mutual weight ratio of 10:1 to. 2:1, whereby in the solidified coating the individual aluminum oxide grains, at least over part of their outer surface, fuse directly, while the metallic aluminum will fill up the gaps between

the connection points. Thanks to its content of relatively plastic, metallic aluminum, such a cermet coating over its total thickness will be more pore-free than flame-sprayed coatings on a pure ceramic basis. When anodes coated in this way with a cermet material are in operation in a cell, over the relatively cold areas of the anode surface, protection against burning will be ensured by the impermeable metallic components. Above the warmer areas of the surface, in the vicinity of the bath, the metallic aluminum will melt and oxidize to aluminum oxide, which impenetrably closes the existing pore spaces.

Choosing aluminum oxide of industrial quality for coating anode surfaces has the advantage that this same product can also be used for enrichment of the electrolyte without further ado. The material costs for such a pure* ceramic coating can therefore be estimated to be almost zero.

In the case of combined coatings, the same will be the case with regard to the aluminum oxide component, while the amount of aluminum required to fulfill the specified task can also be kept at the same time

a low level, and in particular significantly less than with the known pure aluminum coatings. Under these conditions, material costs can thus also be kept very low. It must in this connection as well

account must be taken of the fact that the aluminum introduced into the bath together with the coating near the anode will be reoxidised at least partially, and thus must be electrolysed again. The amount of aluminum used? must therefore in practice be taken into account with the current metal price, but can, as just explained, be limited to a minimum.

The coating of alumina and aluminum is easily dissolved in the electrolyte, when the anode is immersed in a known manner, so that there will be no operating disturbances in this respect.

The carbon anode according to the invention thus differs from previously known designs of a similar nature in that effective protection against secondary combustion is achieved in a simple and commercially applicable way. The applied protective coating thus does not become liquid and thereby ineffective when a temperature of approx. 650°C near the bath, and the coating loses

nor any substantial part of its action due to pores

or rice. The aluminum oxide particles in the coating solidify on the spot and filling of any spaces between the different particles can be ensured by the following measures: Statistically closing all pores by giving the coating a certain layer thickness.

Application of an aluminum layer on top of the ceramic coating.

Arrangement of an aluminum layer between the anode surface and the ceramic coating.

Application of a cermet layer.

Preferred embodiments of the invention and a device for producing coatings according to the invention will now be described with reference to the attached drawings, in which:

Fig. 1 shows an anode provided with a single layer,

Fig. 2-6 shows special combined coatings, and

Fig. 7 shows a device for applying the coatings.

For the sake of clarity, the thickness of the different protective

layers shown greatly exaggerated in the aforementioned figures.

Fig. 1 shows an anode 10 which is already mounted in an electrolysis cell. This anode hangs in its anode stem 11 and its lower part is immersed in the molten cryolite 12 in a melt electrolysis cell. On top of the cryolite floats a crust 13 which in turn is covered with an alumina layer 14 for periodic enrichment of the cryolite 12 and regulation of the cell's heat content. Usually, the crust 13 is not in contact with the surface 19 of the anode 10, so that there will be a gap 16 between the crust and the anode, and through this gap gases will rise which are developed during the melting electrolysis, whereby an ambient atmosphere is formed which on oxidizing way affects the surface 19 of the anode 10. For protection of the anode surface 19

against the oxidizing components of the environment, the anode 10 is equipped with a coating 15 of flame-sprayed aluminum oxide. Before the anode is put into operation, the coating is applied to all surface parts 19 except for the parts that will be in contact with the molten cryolite 12.

Upon immersion of the anode in a cell, the molten cryolite 12 dissolves the coating 15 to an extent corresponding to the immersion depth of the anode

10 in the cryolite 12, so that the coating is retained down to the surface of the molten cryolite 12, i.e. also in the gap 16. By these means, effective protection is ensured in the anode's hottest surface zone, which constitutes the area for the most serious secondary burning.

i By successive downward displacement of the anode 10 in the usual manner to compensate for the primary burning of carbon, the coating is melted

gradually, but will all the time still extend down to the surface of the cryolite. The thickness of the coating 15 is suitably between 0.1 and 1.0 mm, preferably between 0.2 and 0.5 mm. Layer of ceramic materials, e.g. aluminum oxide, however, is not limited to the given values, but can go beyond these both upwards and downwards.

It is actually required that the layer thickness be made sufficient to

ensure, on the basis of the static distribution of the particle, that there are no continuous pores that penetrate through the entire thickness of the coating. The specified values for the thickness have been found to

be satisfactory for achieving the stated purpose of the invention, namely the production of a coating 15 which is impermeable to gas and is well adapted to the thermal expansion of the anode 10, in order to avoid cracking and flaking.

According to fig. 2, a further layer 17 is applied on the outside

the coating 15 of aluminum oxide on the anode 10. During operation of the anode, the aluminum in the layer 17 is also oxidized, and the produced oxidation products close the pores in the coating 15. The thickness of the coating 15

can likewise in this case amount to 0.1 to 1.0 mm,

preferably between 0.2 and 0.5 mm, while the thickness of the layer 17 can amount to between 0.5 and 1.0 mm. With this combined coating of ceramic and metallic material, both statistical closing of the pores and closing of these by means of the generated oxidation products can be counted on in an appropriate manner, as these effects add to each other.

Fig. 3 shows a further arrangement whereby, between the surface 19 of the anode 10 and the coating 15 of aluminum oxide, a base layer 18 of aluminum is arranged which is in direct contact with the carbon material in the anode 10. In this case, such components of the surrounding atmosphere which produces oxidation after having penetrated the coating 15, cause a change of the material in the base layer 18 in such a way that the pores in the coating 15 are closed. In such a construction, in which the coating 15 has the thickness indicated above, 18 have a thickness of the same order of magnitude as the aluminum layer 17 in fig. 2, or a thickness of the base layer 18 below the lowest specified value can be chosen, because the gas flow through the pores in the coating 15 is only quite weak.

The embodiment shown in fig. 4, is suitable for use in environments with large quantities of constituents which are oxidising in their effect, as well as being in gaseous form or possibly other aggregate states. A coating 15 of aluminum oxide is applied to the anode surface 19. This is provided with a layer 17 of metallic aluminum which in turn is covered by a coating 20 of aluminum oxide. The layer 17 of aluminum is suitable for the same operating conditions that have previously been described in connection with fig. 2. The thickness of the coating 20 can amount to 0.1 to 1.0 mm, preferably between 0.2 and 0.5 mm, and this layer thickness can also be used for the coating 15 under the present conditions. Values between 0.05 and 1.0 mm in one or more application layers are advantageous for the thickness of the layer

17. In practice, the thickness of the individual layers or of the deposits during the individual application processes cannot be kept uniform, but it is possible to eventually achieve an overall coating of uniform thickness. Fig. 5 shows a further development of fig. 4. The surface 19 of the anode 10 is equipped with a base layer 21, on which is applied a coating 15 with an outer layer of aluminum 17, which in turn is equipped with a cover layer 20. The coating 15 and the cover layer 20 are made of aluminum oxide, as as in the embodiment shown in fig. 4, or as already described in connection with fig. 1.

According to the invention, the cover layer 17 in fig. 2, the base layer 18 in fig. 3, the intermediate layer 17 in fig. 4 and 5 as well as the base layer 21 in fig. 5 all of aluminium.,

Aluminum constitutes a preferred material for achieving the purpose of the invention. in this connection, but the invention is not limited to this material, and other metallic materials can also be used, possibly in addition to aluminium, as long as the materials used do not affect the cryolite and the melt produced by it, as well as

has the same effect as previously described.

In fig. 6, the protection of the anode above the song consists of a layer 22 of

cermet material, which means a mixture of ceramic and metallic materials. Preferably, this material consists of aluminum oxide and aluminum in a mutual weight ratio of 10:1 to 2:1. The choice of weight ratio within this range depends on the available amount of environmental components with an oxidizing effect as well as the total porosity of the applied coating, which in turn depends on the statistical distribution of the particles that

make up the coating. The layer thickness is also chosen for economic reasons within the range of 0.1 to 1.0 mm. For a layer thickness at the lower limit of the area, a weight ratio between aluminum oxide and aluminum should be chosen that is closer to 2:1 than 10:1, while. the weight ratio with increasing layer thickness will approach the value 10:1. The coatings 15 and the outer layers 20 which are described in connection with the designs shown in fig. 1-5 can also be formed from the cermet material. Other types of cermet materials can also be suitable for the present application, if they satisfy the requirement for a contamination-free solution in cryolite, no influence on the electrolytically separated aluminum, and also no influence on the cell lining, at the same time that the material in question must have the ability to form a gas impermeable coating and. is adapted to the anode's thermal expansion.

Coatings of ceramics and of ceramic-metallic materials are applied according to the invention preferably in finely divided form and compressed into a coherent layer by the application of thermal energy, while the coatings of metallic type e.g. can be applied by flame spraying, by. using an acetylene torch. This type of layer formation produces commercially satisfactory results, compared to known designs. In order to achieve the optimal benefits of eliminating secondary combustion, this is used

connection preferably the device shown in fig. 7 for forming the coatings.

This device is a so-called plasma burner 23, which applies the coating material 24 in a molten state to the anode surface 19, the material being finely distributed and able to form a relatively

impermeable coating.

An arc burns in a cavity in: the plasma torch 23, and a gas is introduced into the cavity to be ionized by the arc, whereby, in accordance with the setting of the torch 23, the gas is brought to a high energy content of the order of up to 10 Kcal/kg. The coating material 24 is introduced into the ionized gas jet 26 through a passage 25, either as powder, wire or in molten form. The material is converted in the gas jet into finely divided form and is applied to the anode surface with the help of the high thermal and kinetic energy present in the gas jet 26. This device has the advantage that the application of the material 24 and the formation of a relatively impermeable layer can ^ibéd-- at the same time. The simultaneous heating of the anode surface 19, i.e. a certain zone around the present application site, prevents any throwback of the applied layer, and excludes, on the basis of shock heating, oxidation of the carbon material or any previously applied metal layer, e.g.

a pre-applied aluminum layer. By "high energy content" for the gas jet 26 is to be understood an energy content which is set in advance to the maximum possible level for the present material and is sufficiently high for filtering this material. The energy content can be as high as 10 5 Kcal/kg. For applying a coating of aluminum oxide, the energy content of the ionized gas beam should be set so that the energy is optimal for the coating process, but not so great that the aluminum oxide evaporates before it reaches the surface to be coated.

To increase the adhesion and stability of a coating being made up

of aluminum oxide or an eermet-type material, the gas jet should affect the layer material in an oxidizing manner. When using a non-oxidizing ionized gas beam, aluminum suboxide and free oxygen can be formed, so that optimal adhesion and stability are not achieved exactly where it is desired. In the presence of nitrogen in the ionized gas jet, nitrides can be formed, which also counteract the desired stability and adhesion of the applied coating. Both gas-stabilized and water-stabilized plasma torches can be used to produce an ionized gas beam. A burner is water stabilized by :• introducing water through an opening in the cavity.

The plasma burner 23 is water stabilized, and should have a minimum input power of 40 kw, and preferably around 150 kw or higher. Input effects of this order of magnitude can, at the present technological level, only be achieved with water-stabilized burners, and as already mentioned, such burners simultaneously enable rapid heating of a sufficiently large area around the application site so that any throwback of an applied layer or oxidation of the carbon material or any applied aluminum layer in the carbon material, can be avoided.-

Burners 23 of this type are not limited by their construction

so that only one material 24 can be introduced into the ionized gas stream 26. It is thus possible to introduce one or more materials 24 into the plasma torch 23, that is to say into the ionized gas jet 26, since depending on the handling of the device, a homogeneous or heterogeneous material is obtained in the coating. A device for producing an ionized gas beam is therefore, on the basis of its adjustable output beam, well suited for the production of anodes according to the constructions shown in fig. 1 to 6, and which in operation are neutral with regard to their surroundings.

The price for such devices for producing an ionized gas beam, e.g. the burner 23, is relatively low in relation to the advantages that are obtained by means of such devices by suppressing the secondary combustion, so that the purpose of the invention is still achieved if, e.g. two devices of the type described are used for the production of a coating consisting of an aluminum layer with an overlying layer of aluminum oxide, one of said devices producing the aluminum layer and the other device the aluminum oxide layertw: Two devices for ionizing a gas stream can also be suitably used for production of a coating consisting of an aluminum layer with an overlying cermet-type coating. When producing such a double coating, aluminum is first applied to the anode surface from one device, after which the cermet-type layer is built up by also connecting the other device, so that the two devices combine their released materials,

which respectively: consists of aluminum and aluminum oxide. The latter

arrangement is of course also well suited for applying a cermet coating alone by using one burner for applying aluminum and the other burner at the same time for spraying aluminum oxide. Alternatively, equally good results can be achieved with regard to the economy of the method, if the aluminum layer is applied to the anode surface or as an intermediate layer in a multi-layer coating by means of flame spraying using an acetylene-oxygen mixture. '"

In the following, some practical manufacturing examples for anodes according to the invention will be indicated.

Example 1

On a carbon anode for aluminum electrolysis, the surface is preferably first sandblasted using corundum sand. An aluminum oxide layer with a thickness of approximately 0.4 mm is applied using < a water stabilized plasma torch with 150 kw input power and a material spraying of approximately 20 kg/hour, in four successive coats across the surface. The distance from the anode of the plasma torch to the surface of the carbon anode amounts to 25 to 30 cm. The application rate for aluminum oxide amounts to around 16 kg/hour.

The aluminum oxide that is not applied is absorbed, collected and returned to the process. The grain size of the aluminum oxide amounts to between 75 and 150 μm.

Example 2

The surface which. must be protected on a regular carbon anode for aluminum electrolysis, sandblasted preferably with corundum sand to begin with. An aluminum layer of 0.1 mm thickness is applied by means of a metallization torch, which means an oxygen-acetylene torch. Directly on top of this layer, a coating of A^O^ with a thickness of approximately 0.3 mm is applied, using a spring-stabilized plasma torch with an input power of 150 kw and a sprayed material amount of 20 kg/hour, by successive strokes of the plasma flame three times above the surface, the distance between the anode of the plasma torch and the surface for the carbon anode being 25 to 30 cm. The application efficiency for K\ £>^ amounts to" about...

80%. The applied ^2°3 (industrial alumina) has a grain size between 75 and 150 µm.

Example 3

The surface to be protected is equipped, as described in example 2, with a layer of approximately 0.1 mm thickness of metallic aluminium. A quantity of around 20 kg Al2°3 and around 5 kg

metallic aluminum per hour is then applied using a water-stabilized plasma torch with 150 kw input power, the torch being continuously supplied with an aluminum wire with a diameter of 3.5 mm as anode, to form a coating of the cermet type with a thickness of about 0.4 mm. The remaining conditions completely correspond to those stated in example 2.

Example 4

The surface to be protected is sandblasted as described in example 1, and then plasma coated with an amount of about 7 kg aluminum and about 20 kg Al^ p^ Per* hour» ve<^ using a water-stabilized plasma torch, with eh 150 kw input power, as the burner is continuously supplied as anode with aluminum wire with a diameter of 3.5 mm, to form a cermet layer of 0.5 mm thickness. The distance between the surface and the burner anode of aluminum wire amounts to 20 to 25 em, and the layer is applied by guiding the plasma flame four times over the surface. As with the other examples, it must be ensured that the plasma jet is directed as perpendicularly as possible to the surface of the carbon anode.

Claims (13)

1. Carbon anode for use in the production of aluminum by melt electrolysis, and whose surface is provided with a coating that inhibits oxidation of the anode material, characterized in that the coating consists of flame-sprayed material particles based on aluminum oxide, the individual particles being at least partially joined together by solidification in place. 2. Anode as stated in claim 1, characterized in that the coating (15) based on aluminum oxide has a thickness of 0.1 to 1.0 mm, preferably between 0.2 and 0.5 mm. 3. Anode as stated in claim 1 or 2, characterized in that a layer (17) of aluminum is applied to the outside of the coating (15) based on aluminum oxide. 4. Anode as specified in claim 3, characterized in that the aluminum layer has a thickness between 0.05 and 1.0 mm. 5. Anode as stated in claim 1 or 2, characterized in that a covering layer (18) of aluminum is arranged between the surface (19) of the anode (10) and the coating (15) based on aluminum oxide. 6. Anode as specified in claim 5, characterized in that the aluminum cover layer (18) has a thickness between 0.05 and 1.0 mm. c 7. Anode as specified in claims 1 - 4, characterized in that a cover layer (20) of flame-sprayed aluminum oxide is applied to the outside of the layer (17) of aluminium. 8. Anode as specified in claim 7, characterized in that the cover layer (20) of flame-sprayed aluminum oxide has a thickness between 0.1 and 1.0 mm, preferably between 0.2 and 0.5 mm. 9. Anode as specified in claims 7 and 8, characterized in that between the surface (19) of the anode (10) and the coating (15) based on aluminum oxide, a base layer (21) of aluminum is arranged. 10. Anode as specified in claims 1-9, characterized in that the coating consists of a cermet material consisting of aluminum oxide and aluminium. 11. Anode as specified in claim 10, characterized in that the weight ratio between aluminum oxide and aluminum in said cermet material is between 10:1 and 2:1. 12. Anode as specified in claim 10, characterized in that the coating has a thickness of between 0.1 and 1.0 mm, preferably between 0.2 and 0.5 mm. 13. Anode as specified in claims 1-12, characterized in that the aluminum oxide is l in added melting point lowering additives. in