DE2255776A1 - ELECTRODE - Google Patents

Description

Telephone (0811) φ%) 55776 telegrams patemus münctien Posischeck Munich 39418 Bank ReuschelA Co Munich 830820

Dr.-Ing. Roland Liesegang Dipl .-! Ng. Hans-Peter Lieck

SWISS ALUMINUM AG.
Our reference number P 009 41

electrode

The invention relates to an electrode, in particular one for interaction anode determined with an electrolyte of a molten aluminum electrolysis cell, the surface of which is on parts not intended for the passage of current provided with a coating that prevents oxidation of the anode material is.

The cathodically connected tub of an aluminum smelted flow electrolysis cell contains molten aluminum, an electrolyte floating on aluminum that contains aluminum oxide, with the electrolyte on top of the atmosphere facing side forms a solid crust, which in turn is covered with a layer of clay (Aluminum oxide) for periodic enrichment of the electrolyte and thermal insulation of the bath

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ORfGiNAL! NSPECTH)

is covered in the tub. Anodes made of charcoal protrude, the layer of clay and the crust reaching through Conducting the electric current that keeps the electrolysis going in the electrolyte. The crust and the aluminum oxide poured on it usually do not enclose the circumference of the anode sealing, but through rising gases "and other influences, a gap forms around the circumference of the anode. For enrichment of the electrolyte with alumina, the crust is periodically pounded. The released from the electrolysis process and rising mainly through the gaps around the circumference of the anode Gases form one of the above with the surrounding air Air freshening which is detrimental to the longest possible service life of the anodes.

The consumption of an anode during the operation of the cells or also called burn-up, which results from primary and secondary burn-up composed, is based on two oxidation mechanisms that have to be considered separately for this purpose.

During primary burn-up, the oxygen released from the aluminum oxide during the fused-salt electrolysis attacks the Carbon from the anode, forming a rising from the tub Gas mixture of carbon dioxide and monoxide, wherein this reaction, which brings about the majority of the burn-up, exothorates with heating of the electrolyte while reducing the for the Electrolysis necessary energy runs. The anode burn-up caused by this oxidation mechanism is in the case of carbon anides inevitable.

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With the means of the invention, the secondary burn-off, which impairs economic cell management, is affected an anode-causing oxidation mechanism influenced.

The temperature of the bath or the electrolyte in the cathode trough is in the order of magnitude of 950 to 980 ° C, and this heat source gives the carbon anodes a heat content in such a way that there is a gradient between the side of the anode facing the bath and the side facing away from the bath. This one Corresponding to the heat content of the anodes, the surfaces of the anodes have a temperature gradient between approximately 980 C and

C on. The part of the anode protruding out of the bath is surrounded by a gas mixture consisting of hall air and gases rising from the cell, which, favored by the temperature of the anode, has an oxidizing effect on the anode and thus promotes combustion. In contrast to the oxidation mechanism that triggers the primary burn-up, the burn-up reactions that take place make no contribution to the heating of the bath and thus to a reduction in energy, but represent unacceptable losses that can amount to up to 3% of the total consumption of anode material.

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It is known to provide anodes made of synthetic charcoal and intended for use in an aluminum melt-flow electrolyte cell with a coating on the upper and side surfaces thereof in order to reduce the losses caused by secondary burn-up. Due to the requirement that the material of the coating must not contaminate the electrolyte and thus the electrolytically deposited aluminum, the material chosen in the prior art was aluminum, which was applied by spraying or pouring on. Various defects are inherent in coatings of this type. Aluminum spray coatings are applied in a thickness of about 0.3 to 0.5 mm. Cast coatings have a thickness of at least 1 cm, but preferably several cm, which, multiplied by the total surface of all anodes immersed in the cell, results in a total volume that is disproportionately large as a constantly circulating amount of metal in relation to a temporally defined amount of carbon dioxide from the Cell. However, the most serious defect is that the aluminum coating melts at a isotherms on the anode surfaces of about 650 ⁰ C, whereby the anode surface in their lower free several centimeters high range between the bath and the isotherms of about 650 G the oxidizing attack is exposed to ambient air mixture . Even if the aluminum coating that is melting down leaves the natural aluminum oxide skin under it, this skin also tears and does not adhere to the carbon surface or does not adhere well to the surface, so that it cannot provide effective protection against burn-off.

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As mentioned in the introduction, the secondary burn-up amounts to up to 8% of the total consumption of coal for the production of aluminum. If there is an economic benefit to be drawn from the elimination of the secondary burn-up, the costs incurred for this must be kept within the scope of the aforementioned share of coal costs. Through the production of a cast body, the care to be taken in the choice of the coating material, the large amounts of constantly circulating metal with the inevitable additional expenditure of electrical energy for the fused-salt electrolysis as well as the only limited anode protection by melting the coating along below the isotherms of 650 C is the up; ·: and high for the coating known from the prior art. The amount of expenditure in relation to the savings achievable from the secondary burnup does not provide such an incentive for the known coating in terms of costs that it could have prevailed in practice. .

The invention is based on this, and it is based on the object of providing an environment-neutral at the operating temperature non-melting coating on a carbon electrode, especially one for interaction with an electrolyte an aluminum smelting electrolysis cell determined over-

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To create a train on an anode made of coal, the material expenditure necessary for the production of which is kept to a minimum represents the benefit that can be achieved from the elimination of secondary burn-up. The object is thereby achieved according to the invention solved that the coating consists of a flame-sprayed unit based on aluminum oxide, in which the individual grains at least partially from one

are connected to each other at the site of the solidification process. .

To produce such a coating layer, the grains of the ceramic material are suspended in a high-temperature gas jet at least heated to the plastic, preferably to the molten state and through this gas jet is thrown against the surface to be coated. These melt droplets then gradually solidify on the surface to be coated after they have touched each other and more or less fused together.

The structure of the coating according to the invention. surrendered from it as a compact aggregate consisting of grains joined together and solidified in place. That on this type of agglomerated, quenched aluminum oxide is mainly in the gamma modification, from a crystallographic point of view before. The applied coating is overheated

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Avoid as it gradually changes the gamma to the alpha modification transferred »which has the disadvantage in operation, ' that if they are present in excessively high levels in the coating, they are practically in the cryolite electrolyte no longer dissolves and leads to the formation of soil crusts.

Depending on the application conditions and because of the high melting point of the ceramic substance used and the resulting very large temperature drop When solidifying and also because of the roughness of the surface of the synthetic carbon base body, it may happen that individual Insufficiently connect melt droplets when they hit the already applied ones and leave gaps between them, or that as a result of the very rapid solidification and the resulting internal stresses, cracks within or between the individual solidified grains arise, which can lead to a certain porosity of the coating layer. This porosity can result in limited secondary burn-up when the anode is in operation. For elimination this deficiency by means of a statistical closure of the pores is the invention before? partially designed that the coating has a thickness of 0.1-1.0 mm, preferably 0.2-0.5 mm.

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The starting material for the production of the coating can simply be conventional metallurgical clay (aluminum oxide) -be used, which is also used to feed the electrolysis cells.

However, it is possible to add a small amount of flux, e.g. cryolite, to this aluminum oxide with the Purpose to lower the melting point of the oxide, respectively. to obtain a wider melting interval, which in the Ueberzup: s ~ layer a better union of the melt droplets and thus the solidified grains beneath them with He rau setz uric, the Porosity allowed. It is also possible to use a powder whose individual grains already contain the desired additives from a previous manufacturing process.

According to a further advantageous embodiment of the invention the ceramic coating layer described is combined with a metallic layer.

A pure or alloyed metal is used as the material for this into consideration, which is permissible for the electrolytically deposited aluminum in the cell and furthermore preferably has the property of wetting the ceramic coating layer to a certain extent in the liquid state. To this

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Pure aluminum is particularly suitable for this purpose. Such Metal layer can be applied by any suitable method, e.g. by flame spraying, and is without difficulty to get pore-free.

The metal layer can be arranged either below or above the ceramic layer, or the coating can also consisting of several individual ceramic and metallic materials, alternately applied one after the other in a sandwich structure Layers exist. .

Such a combined coating acts when the anode is in operation as follows; · /

On the parts of the anode surface that have a temperature below the melting point of the metal, the dense, pore-free metal layer acts as complete protection against burn-off. In the warmer areas between the isotherm corresponding to the melting point of the metal and the actual bath surface melts the metal, e.g. aluminum and penetrates by capillary action into the pores of the ceramic layer, where it is under the influence of the oxidizing ambient atmosphere partially oxidized. Consequently In this area, the pores of the ceramic layer also melt above the operating temperature Oxidation products, i.e. e.g. with aluminum oxide, which also provides reliable protection against burn-off.

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ensures. I combination with the aluminum oxide layer

So the aluminum layer has an additional effect

that does not occur with conventional aluminum protective layers.

For this purpose, relatively thin metal layers, preferably based on aluminum, with a thickness of 0.05-1.0 mm, preferably 0.1-0.2 mm , are sufficient.

According to a further embodiment of the invention, there is the coating made of a Cerinet-like material, preferably made of aluminum oxide and aluminum sprayed onto one another in a weight ratio of 10: 1 to 2: 1, the individual aluminum oxide grains in the solidified coating at least are fused and bonded directly to one another on part of their circumference and the metallic aluminum fills the gaps between the connection points.

Thanks to its content of relatively plastic, metallic aluminum, such a cermet coating is more pore-free than flame-sprayed coatings on a purely conventional basis. If anodes coated in this way are in operation in a cell, the dense metal component ensures protection against erosion on the relatively cold parts. In the warmer areas near the bath, the aluminum melts and oxidizes to aluminum oxide, which tightly seals the gaps.

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The choice of Kütten quality aluminum oxide for the coating the anode has the advantage of this product anyway is used to feed the electrolyte. Therefore, the material costs can be zero for the purely ceramic coating to be viewed as.

In the case of combined coatings, the same applies "with regard to the Aluminum oxide content; the amount of aluminum required to achieve the desired purpose can also be very small be held, and significantly, less than in the case of known pure aluminum coatings, so that the material costs are kept very low from this side as well can be. It must be taken into account that the aluminum introduced into the bath with the coating is in the vicinity the anode is at least partially reoxidized and therefore has to be re-electrolyzed. The amount of aluminum used is therefore practically to be taken into account at the metal price, but, as explained, can be kept to a minimum.

Finally, the coating of alumina and aluminum easily dissolves in the electrolyte ₃ when the anode is lowered in a known manner, so that no malfunction can be caused in this respect either.

In summary, the coating according to the invention brings clear advantages both in technical and in commercial terms Respect. .

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Preferred embodiments of the invention and an apparatus for producing the coatings according to the invention are shown, for example, in the figures. Ask dar:

1 shows an anode provided with a coating according to the invention in section

Fig. 2, 3, ^, 5 and β special embodiments of Combination coatings'

7 shows an apparatus for forming the coatings according to the invention

For clarity, the thickness of the protective layers is shown in these figures depicted greatly exaggerated.

Fig. 1 shows an inventive, already in an electrolysis cell inserted anode 10. This anode hangs on its anode rod 11 and dips with its lower part in the molten cryolite 12 of an aluminum melting flow electrolysis cell a. A crust floats on top of the cryolite

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13, which in turn with an alumina layer I ^ for periodic Enrichment of the cryolite 12 is covered under control of the heat content of the cell. Usually touched the crust 13 does not cover the surface 19 of the anode 10, so that a gap 16 remains therebetween through which from the. Fused metal electrolysis evolved gases with the formation of an oxidizing effect on the surfaces 19 of the anode 10 Ambient atmosphere rise. To protect the anode surfaces 19 against oxidizing components of the environment the anode IC is provided with a coating 15 made of flame-sprayed aluminum oxide. Before commissioning the The anode covers all surfaces 19 except for the molten cryolite 12 facing. When using the-. Anode in a cell, the molten cryolite 12 dissolves the coating 15 according to the immersion depth of the anode 10 in the cryolite 12 so that the coating up to the surface of the molten cryolite 12 also in the gap 16 is retained. This provides effective protection in the hottest area of the anode and thus at the point of the the most violent secondary burn-up, which occurs with advanced readjustment of the anode 10 in the direction of the anode rod 11 migrates, guaranteed.

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The thickness of the coating 15 is conveniently of idle O ₁ I -. 1.0 π ", preferably 0.2 -. 0.5 mm layers of ceramic materials such as alumina, but are not limited to the above values but can move up The layer thickness is determined according to the resulting statistical distribution of the active ingredient particles for closing any pores, and the stated thickness was used to achieve the purpose pursued according to the invention, the creation of a largely gas-impermeable and one to avoid Crack formations and splintering of the thermal expansion of the anode 10 well-fitting coating 15 found to be sufficient.

According to FIG. 2, the coating applied to the anode 10 is shown 15 a layer 17 of aluminum is deposited from aluminum oxide. This also oxidizes during operation of the anode Aluminum of the layer 17 and its oxidation products close the pores in the coating 15. The thickness of the coating 15 can also be 0.1 to 1.0 mm, preferably 0.2 to 0.5 mm, the thickness of the layer 17 can be between 0.05 and 1.0 µm. With this ceramic-metal-material composite layer one can do the statistical pore closure and the pore closure with the help of oxidation products coordinate in an advantageous manner.

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Pig. 3 shows a further embodiment of the invention at between the surface 19 of the anode 10 and the coating 15 made of aluminum oxide, a cover layer made of aluminum 18, which engages the carbon of the anode 10, is seen from FIG. Components causing oxidation bring about here from the ambient atmosphere after passing through the ·. Cover 15 the material of the cover layer 18 under lock and key the pores of the coating 15 to a conversion. In this embodiment, in which the cover 15 is the above specified thickness and also the cover layer 18 can have the thickness of the layer 17 made of aluminum, are with regard to the Thickness of the cover layer 18 is rather the lower fourth due to the small pore-related gas passage through the coating selectable. - ·

For use in an environment with high proportions of oxidizing, gaseous or other aggregate components is the embodiment according to Pig. t \ determined. A coating 15 made of aluminum oxide is applied to the anode surfaces 19. This carries a layer 17 made of metallic aluminum, which in turn is covered with a cover 20 made of aluminum oxide. The layer 17 made of aluminum has the same mechanism of action as QC in connection with the embodiments explained above.

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forms was described. The thickness of the cover 20 can be 0.1 to 1.0 mm, preferably 0.2 to 0.5 mm, wherein this layer thickness can also be selected in the specified range for the coating 15. For the thickness of the layer 17 are Values between 0.05 and 1.0 mm are advantageous. In these areas, the layer thicknesses are variable, so that the total layer thicknesses can also be formed from the sum of the upper values without actual, merely economically set upper limits, like every total layer thickness as the sum of intermediate values of the marked individual layers.

In the Pig. 5 it is a Training after the Pig. 4. The surfaces 19 of the anode 10 are provided with a base layer 21 on which a Coating 15 is applied with a layer 17 of aluminum arranged thereon, the layer 17 being a cover 2U wears. The coating 15 and the cover 20 are as in the Pig embodiment. 4 made of aluminum oxide, or as already in connection with Pig. 1 mentions a ceramic material which dissolves in the cryolite without contamination. According to the invention, there is layer 17, the cover layer 18, layer 17 in the Pig embodiment. 4 and 5 and the base layer 21 made of aluminum. Aluminum "is for Achieving the goal according to the invention is a preferred means, but the invention is not limited to

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but other metallic materials can also be used, as long as they do not affect cryolite and the melt material produced and have the same effect as described.

According to Fig. 6, the protection of the anode surfaces consists of one Layer 22 of cermet-like material, i.e. of a ceramic-metallic material in mixed form, advantageously the material consists of aluminum oxide and aluminum with a weight ratio of 10: 1 to 2: 1 depending on the lirad of the action of oxidizing components and the total porosity of the applied layer, which in turn depends on the statistical distribution of the layer particles. For economic reasons, the layer thickness expediently ranges from 0.1 to 1.0 mm. In the case of a layer thickness at the lower part of the marked In the range, a weight ratio of aluminum oxide to aluminum will be chosen that is closer to 2: 1 than to 10: 1 comes, with the weight ratio increasing with increasing layer thickness will approach the value 1u: 1. According to the invention, the / connection with the embodiments according to i'ig. 1, 2, 3, 4 and 5 mentioned coatings 15 and covers 20 can be formed from the cermet-like material. Here too are other oermet-like materials suitable for use, if they meet the requirement of the impurity-free solution in the

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Cryolite, not affecting the electrodeposited Aluminum and the lack of impairment of the cell lining while at the same time forming one of the Meet the thermal expansion of the anode, gas-impermeable coating that can be matched.

Coatings made of ceramic and ceramic-metallic materials are applied according to the invention in finely divided form and using thermal energy, to a coherent layer compacted, with the coatings metallic type, for example by flame spraying, can be applied with the aid of an acetylene burner. This kind of Layer formation; delivers economically above the state of the Satisfactory results beyond technology. On the other hand, in order to achieve the optimum from the elimination of the Secondary erosion of the drawable benefit, the device shown in FIG. 7 for the formation of coatings is preferred.

This is a so-called plasma torch 23 which applies the coating material 2k in a finely dispersed form to the anode surface 19 in a molten state while at the same time forming a relatively dense layer.

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An arc burns in a cavity of the plasma torch 23 and a gas is introduced into the cavity, which is ionized by the arc and depending on the setting of the torch 23 is brought to a high energy content in the order of magnitude of up to 10 kcal / kg. Through the passage 25 is the ionized gas jet 26 is fed to the coating material 24 in either powder, rod or molten form therein Finely prepared and applied to the anode surface by means of the high energy inherent in the gas jet 26. The device offers the advantage that the application of the material 24 and the formation of a relatively dense one The simultaneous heating of the inode surface 19 or an area around the application site prevents the applied layer from jumping off and closes the oxidation of the carbon due to the shock heat, or a possibly previously applied metallic layer, s.B an aluminum layer, made of »

Among the high energy inherent in the gas jet 26 is a material that is placed on the material to be applied, for one Order to understand the maximum possible energy content of the gas jet, which can be up to 10 kcal / kg. For the mission a layer of aluminum oxide will adjust the energy content of the ionized gas jet so that the energy optimal for application, however, is not so large that the aluminum oxide evaporates before it reaches the surface to be coated achieved.

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To improve the adhesiveness and stability of those made of aluminum oxide or an oermet-like material Layer, the gas jet should have an oxidizing effect on the layer material. When using a non-oxidizing ionized gas jet it can come to the formation and to the existence of aluminum suboxide and free oxygen, whereby where required, optimal adhesion and stability cannot be achieved. In the presence of nitrogen in the ionized gas jet can form aluminum nitrides, which in equal measure the stability and adhesion of the applied Counteract the layer. Both gas-stabilized and water-stabilized ones can be used to generate an ionized gas jet Find plasma torch use. The burner is created by introducing water through an opening into the cavity water stabilized.

The water-stabilized plasma torch 23 should have a minimum output of 40 KW, preferably of about 150 KW or higher. Achievements in these orders of magnitude are after The current state of technology can be achieved primarily with water-stabilized burners; such burners ensure As already mentioned, the simultaneous rapid heating of a sufficiently large area around the application site so that a An applied layer pops off, as well as an oxidation of the coal and one that may already have been applied to the coal Aluminum layer is avoided.

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Burner 23 of this type are not subject to any restrictions in terms of their design that would allow only one material 24 to be introduced into the ionized gas jet 26. It is thus possible to introduce one or more * materials 24 into the plasma torch 23 or into the ionized gas jet 26, a homogeneous or heterogeneous material being discharged depending on how the device is handled. Consequently, the 'adjustability of their performance is due to an apparatus for ionisation of a gas jet for the formation of environmentally j gebungsneutrai operable anodes according to the embodiments according to FIGS Λ up. 6

The effort for the device for ionizing a gas jet or of the burner 23 in relation to the benefit flowing from them by suppressing the secondary burn-up is relatively small, so that the aim of the invention is not abandoned when, for example, to produce a coating consisting of an aluminum layer with a layer of aluminum oxide deposited thereon, two devices of the aforesaid Kind of use, one being the aluminum schioht and the other that forms from alumina.

Two devices for ionizing a gas jet are for example also for the production of a coating consisting of an aluminum layer with sprayed-on cermet-like

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Layer suitable. A device would be used first 'Aluminum is applied to the anode surface and then under Activation of the second, combining its discharge. The cermet-like layer is built up from aluminum and aluminum oxide, The latter arrangement can also be used to form a coating from a cermet-like layer one burner for aluminum and the other for aluminum oxide. Equally good results on the The process is economical if the aluminum layer is on the anode surface or as an intermediate layer a sandwich coating is applied by flame spraying using an acetylene-oxygen mixture.

The following are practical examples to further clarify of the invention listed:

example 1

The surface of a carbon anode to be protected for molten aluminum electrolysis can be lightly sandblasted with corundum sand in advance. An approximately 0 ^ 1 mm thick layer of aluminum oxide is made using a water-stabilized plasma torch ·; of ISO KV / power consumption and a spray performance of

ih.

about 20 kg / alumina in four consecutive passes applied by painting the surface crosswise. The distance between the anode of the plasma torch and the above ~

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area of the carbon anode is 25 - 30 cm. The rate of deposition of the aluminum oxide is about 16 kg per hour. Not that one deposited aluminum oxide is sucked off, collected and fed back into the process. The grain size of the aluminum oxide is 75 to 150 / Λ.

Example 2

The surface to be protected of a conventional carbon anode for aluminum electrolysis can be lightly sandblasted with corundum sand will. A layer of aluminum about 0.1 mm thick is made with the help of a metallization burner, i.e. an acetylene-oxygen burner, applied. Immediately afterwards, a water-stabilized plasma torch with a power consumption of 150 KW and a spray capacity of 20 kg becomes more technical Alumina an approximately 0.3 mm thick layer of AlpO per hour, by successively brushing the surface three times applied with the plasma flame, with the distance of the anode of the plasma unit from the surface of the carbon anode 25 to 30 cm. The separation efficiency of the AIpO is about 80 #. The AIpO (technical clay) used has a grain size from 75 to

Example 3

The surface to be protected is, as described in Example 2, provided with an approximately 0.1 mm thick layer of aluminum metal. With the help of a water stabilized plasma torch

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inspect »©

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of 150 KW power consumption, which is equipped with a continuously advanced aluminum _{wire anode of 3 t} 5 mm 0 , an amount of approx. 20 kg AlpO and approx. 5 kg Al metal per hour is formed with the formation of a cermet-like layer applied from about 0.4 mm thickness. The other conditions are the same as those given in Example 2.

Example 4

The surface to be protected is, as described in Example 1, optionally sandblasted and then with the help of a water-stabilized plasma torch with 150 KW power consumption, which is provided with a continuously advanced aluminum wire anode of 3.5 mm 0 , with an amount of about 7 kg al and approximately 20 kilograms AIPO, per hour to form a Germet-like layer of 0 _f 5 mm thickness plasmabe- coated. The distance between the surface and the aluminum wire anode is 20 to 25 cm; the layer is applied by wiping the plasma flame four times over the surface. As in the other examples, care must be taken here to direct the plasma jet as perpendicularly as possible onto the surface of the carbon anode.

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Claims (14)

Patent Attorneys Liesegang, Lieck Telephone (0311) 225303 Telegrams patomus many Dr.-Ing. Roland Liesegang Dipl.-Ing. Hans-Peter Lieck SWISS ALUMINUM AG. Our sign P 009 41 claims 1. Electrode, especially to interact with the electrolyte of a Aluminum melt-flow electroJysis cell specific anode, the surface of which provided with a coating that inhibits oxidation of the anode material is, characterized in that the coating (15; 22) consists of a on the basis of aluminum oxide built up f Solidification processes are interconnected, 2. Electrode according to claim!, Characterized in that the coating (15; 22) has a thickness of 0.1 to 1.0 mm. 3. Electrode according to claim 2, characterized in that the coating (15; 22) has a thickness of 0.2 to 0.5 mm. -26- 309821/1046 4. Electrode according to claim 1, 2 or 3, characterized in that that on a coating (15) based on aluminum oxide, a layer (17) Aluminum is applied. 5. Electrode according to claim 4, characterized in that the Layer (17) made of aluminum has a thickness of 0.05 to 1.0 mm. 6. Electrode according to one of claims Ibis5, characterized in that that between the surface (19) of the anode (10) and the coating (15) based on aluminum oxide, a cover layer (18) made of aluminum is provided. 7. Electrode according to claim 6, characterized in that the Cover layer (18) made of aluminum has a thickness of 0.05 to 1.0 mm. 8. Electrode according to one of claims 4 to 7, characterized net that on the layer (17) made of aluminum a cover (20) flame-sprayed aluminum oxide is applied. 9. Electrode according to claim 8, characterized in that the Cover (20) made of flame-sprayed aluminum oxide has a thickness of 0.1 to 1.0 mm. 10. Electrode according to claim 9, characterized in that d'e Cover (20) has a thickness of 0.2 to 0.5 mm. 309821/1046 7255776 11. Electrode according to claim 8, 9 or 10, characterized in that that between the surface (19) of the anode (10) and the coating (15) A base layer (21) made of aluminum is arranged on the basis of aluminum oxide. 12. Electrode according to one of claims 1 to 11, characterized in that that the coating (22) consists of a cermet-like material. 13. Electrode according to claim 12, characterized in that the cermet-like material made of aluminum oxide and aluminum with a There is a weight ratio of 10: 1 to 2: 1. 14. Electrode according to one of claims 1 to 13, characterized in that that the melting point-lowering admixtures are added to the aluminum oxide. 309821/1046