OA18425A - Electrode cermet material. - Google Patents

Abstract

The invention relates to a cermet material comprising, in mass percentages, at least: - 50% to 90% of a metallic phase (1) containing an alloy of copper (Cu) and nickel (Ni), - 10% to 50% of an oxide phase (2) containing at least iron, nickel and oxygen with the following mass proportion of Ni: 0.2% < Ni < 17%, The invention also relates to an electrode, preferably an anode, comprising said cermet material.

Description

The invention relates to electrode materials, and more particularly to cermet materials used in the composition of anode material for the production of aluminum by electrolysis. It relates more specifically to the cermet materials used for the manufacture of so-called "inert" or "non-consumable" anodes.

The following description refers more specifically to the use of the cermet material, object of the present invention, used in the composition of the inert anode material intended for the manufacture of aluminum by an igneous electrolysis process. However, the mention of this industrial application in the description in no way limits the scope of the invention. The present invention may relate to other industrial applications of the cermet material described below.

Aluminum metal is produced industrially by igneous electrolysis, namely by electrolytic reduction of alumina in solution in a molten cryolite-based bath, called an electrolysis bath, according to the Hall-Hêroult process. Electrolytic reduction is carried out in electrolytic cells comprising an electrolytic cell, fitted with carbon cathode elements, and one or more anodes. The electrolysis bath is contained in the cell and the anodes are partially submerged in the electrolysis bath. The electrolysis current keeps the electrolysis bath at the required temperature by the Joule effect. The electrolysis cell is regularly supplied with alumina in order to compensate for the consumption of alumina produced by the electrolysis reaction.

In standard technology, the anodes are made of carbonaceous material and the electrolysis is carried out at a temperature typically of around 950 ° C. As the anodes made of carbonaceous material are gradually consumed during electrolysis, it is necessary to continuously readjust the height of the part of the anodes which is immersed in the bath and to intervene on the cell to replace the anodes.

In addition, the consumption of anodes:

- produces carbon dioxide (more than two tonnes of carbon dioxide per tonne of aluminum produced), which contributes to the greenhouse effect, and

- releases sulfur dioxide due to the presence of sulfur in the raw materials used for the manufacture of the anode.

The development of an alumina electrolysis technology using "inert" or "non-consumable" anodes seeking to resolve these drawbacks comes up against

inevitably due to the difficulty of producing anodes which meet various competing quality criteria.

Indeed, the anodes must be sufficiently conductive at the operating temperature so as not to inappropriately increase the electrical consumption associated with this technology.

In addition, the anodes must resist corrosion in aggressive cryolite baths in order to have a sufficient life and, in order to avoid contamination of the electrolyte and aluminum with unwanted elements.

Finally, the anodes must be able to be handled. This imposes certain mechanical properties: fragile anodes would be unusable in industrial processes.

In order to satisfy these criteria stated above, namely of conductivity, of corrosion resistance, as well as of solidity, it has been proposed to use monolithic ceramic parts for the manufacture of these inert anodes.

For example, patent applications W002 / 066710, WO02 / 083992 and US2004 / 089558 describe various oxide compositions capable of allowing the production of inert ceramic anodes, for which the amount of metal phase is generally less than 10% in weight. However, the electrical conductivity of this type of anode and their mechanical properties prove to be insufficient.

Furthermore, it has also been proposed to use entirely metallic parts for the manufacture of these inert anodes. For example, patent applications WO99 / 36591 and WO00 / 06803 describe such inert anodes which are entirely metallic. However, the corrosion resistance of these anodes in cryolite baths remains insufficient.

Finally, it has been proposed to use as electrode materials ceramic matrix composite materials containing one or more metallic phases, so as to combine the respective advantages of the metallic phase and the ceramic phase. Such composite materials, which contain at least one ceramic phase and at least one metallic phase, are known under the name “cermet material”.

The metal phases of these cermet materials improve the mechanical properties of the electrodes as well as their conductivity, while the ceramic phases improve the corrosion resistance in cryolite baths.

However, the lack of stability of the various metallic and ceramic phases during a prolonged passage of such anodes made of cermet material in the baths?

cryolites tend to limit their lifespan and cause contamination of electrolyte and aluminum.

Many patent applications relate to this type of anode made of cermet material.

For example, international application WO2004 / 082355 can be cited which describes a process for manufacturing an inert cermet anode of the NiO-NiFe ₂ O4-M type comprising at least one nickel monoxide phase N, a spinel phase of nickel S containing iron and nickel, and a metal phase M containing copper and nickel, said process being characterized in that it comprises:

the preparation of an initial mixture including at least one precursor of said N monoxide and S spinel phases, a precursor of the metal phase M and an organic binder, the proportion of organic binder in the initial mixture being less than 2.0% in weight and the precursor of the metallic phase comprising a metallic powder containing copper and nickel,

- an operation of shaping the mixture, so as to form a green anode of determined shape,

- a sintering operation of the green anode at a temperature above 900 ° C in a controlled atmosphere containing at least one inert gas and oxygen.

This process, which improves the manufacture of the anode by making it possible to reduce the quantity of binder, does not entirely solve the problem of stability of the anode in the cryolite bath. It has been observed in particular that the electrical conductivity of the spinel phase of nickel NiFe ₂ O4 decreases during electrolysis, which conditions the life of the anode.

International application WO2005 / 035813 describes a process for manufacturing an anode containing a cermet material in which one of the constituents of the oxide is a metallic element capable of being reduced in whole or in part during a reduction operation at during the manufacturing process. This process, which improves the manufacture of the anode by reducing metal exudation, also does not completely solve the problem of stability of the anode in the cryolite bath.

International application W001 / 31090 describes an inert cermet anode comprising a ceramic phase represented by the formula Ni _x Fe ₂ yM _z O (3y + x + z) ± B, where M represents at least one metal chosen from Zn, Co, Al, Li, Cu, Ti, V, Cr, Zr, Nb, Ta, W, Mb, Hf and rare earths, preferably Zn and / or Co, x is between 0.1 and 0.99, y is between 0.0001 and 0.9, z is between 0.0001 and 0.5, and δ is between 0 and approximately

0.03. This inert cermet anode also contains a metallic phase, such as

Cu, Ag, Pd, Pt, Au, Rh, Ru, Ir and / or Os. A lifetime compatible with industrial conditions of use has not been demonstrated for this type of anode.

Thus, it is noted that the electrode materials, and in particular the cermet materials, known from the state of the art are not fully satisfactory as materials entering into the composition of the material of inert anodes intended for the production of inert anodes. 'aluminum that function optimally (that is to say with lower power consumption) and with an acceptable lifetime from an industrial point of view.

The present invention sets out to overcome the drawbacks detailed above presented by electrode materials, in particular cermet materials, known from the state of the art.

Indeed, the inventors of the present invention have developed, quite surprisingly, a new cermet material, intended to be used in the composition of the material of an inert anode developed for the production of aluminum and which reconciles the parameters. following:

- toughness and thermal shock resistance superior to formulations of cermet materials already known, and this due to a higher proportion of metal;

- improved electrical conductivity compared to cermet materials known in the state of the art;

- very satisfactory corrosion resistance: no measurable geometric wear up to 506 hours of electrolysis is observed with the cermet material according to the invention;

- the cermet material according to the invention does not induce a loss of metal such as nickel which would be liable to pollute the aluminum produced during electrolysis to a level such that it would not be marketable;

- resistance to high current densities, namely adequate behavior at 1.2 A / cm ² ;

- remarkable resistance to oxidation at 930 ° C in air.

The cermet material according to the invention comprises, in percentages by weight, at least:

- 50% to 90%, preferably 60% to 80%, of a metallic phase which contains at least one alloy of copper (Cu) and nickel (Ni), said alloy comprising in percentages by mass:

• 35% to 75%, preferably 40% to 60%, more preferably 45% to 55%, of nickel;

• 25% to 65%, preferably 40% to 55%, more preferably 45% to 55%, of copper;

• optionally iron (Fe), the percentage by weight of iron in said alloy not exceeding 20%, preferably said percentage by weight of iron is between 2% and 15%, more preferably between 4% and 10%;

- 10% to 50%, preferably 20% to 40%, of an oxide phase containing at least iron, nickel and oxygen with the following mass proportion of Ni: 0.2% "s Nor £ 17%.

In the oxide phase of the cermet material according to the invention, nickel is present in a small amount relative to the other metals forming the oxide phase and in particular relative to iron.

Iron may be the only other metal besides nickel in the oxide phase, in which case the weight ratio of iron in the oxide phase is: 60% £ Fe £ 78%. Iron is thus strongly predominant over nickel.

The oxide phase can also contain at least one metal (M) which is chosen from aluminum (Al), cobalt (Co), chromium (Cr), copper (Cu), manganese (Mn) , titanium (Ti), zirconium (Zr), tin (Sn), vanadium (V), niobium (Nb), tantalum (Ta), yttrium (Y), hafnium (Hf ) or a combination of these metals. These M metals take the place of iron in the oxide structure of the oxide phase. They are more particularly present in small amounts in the oxide phase relative to the iron and advantageously chosen to improve the conductivity of the oxide phase, the sintering of the cermet material and / or the resistance to corrosion by the cryolite bath.

The oxide phase of the cermet material according to the invention can include:

- a monoxide phase of composition Ni _x M _y Fei.x-yO with the following mass proportions:

• 0.3% £ Ni £ 17%, • 60% £ Fe £ 78%, • O £ M £ 1O%, and / or

- a phase of nickel ferrite oxide of composition Ni _x MyFe3-x-yO4 with the following mass proportions:

• 0.2% £ Ni = S13%, • 60% Fe 72%, • 0 £ M £ 8%.

As an indication, the ranges of mass proportions of Ni and Fe mentioned above can still be expressed when:

- the oxide phase comprises a monoxide phase of composition Ni _x Fei. _x O which does not contain any metal M (y = 0) by: £ 0.004 x £ 0.2,

- The oxide phase comprises a phase of nickel ferrite oxide of composition Ni _x Fe ₃ . _x O4 which does not contain any metal M (y = 0) by: 0.01 sxs 0.5.

The nickel ferrite oxide phase of the cermet material has a spinel structure. By spinel structure is meant a crystal structure of AB2O4 type composition where A is a cation in tetrahedral site (surrounded by 4 oxygen) and B are two cations in octahedral site (surrounded by 6 oxygen). This type of compact cubic structure is particularly advantageous under aggressive conditions such as a cryolite bath for the production of aluminum.

In the cermet material according to the invention, the metallic phase and the oxide phase are percolating, ie the networks formed by the oxide phase and the metallic phase are intimately intertwined and pass through the cermet material continuously. This has the advantage that the cermet material according to the invention retains good resistance over time.

Preferably, when the oxide phase of the cermet material comprises a phase of nickel ferrite oxide, said phase of nickel ferrite oxide is of composition Ni _x M _y Fe3- _x -yO4 with the following mass proportions :

- 0.2% £ Ni s 10%, preferably 0.2% s Ni £ 5%;

- 63% s Fe s 72%, preferably 68% £ Fe £ 72%,

- 0 s M £ 4%, preferably 0 £ M £ 2%.

As an indication, these ranges of mass proportions of Ni and Fe mentioned above can also be expressed when the oxide phase comprises a phase of nickel ferrite oxide of composition Ni _x Fe3- _x O4 which contains no metal M (y = 0) by: 0.01 sxs 0.3, and preferably 0.01 sxs 0.1.

Advantageously, when the oxide phase of the cermet material comprises a monoxide phase, said monoxide phase is of composition Ni _x M _y Fei- _x . _y O with the following mass proportions

- 0.3% s Ni 5 13%, preferably 0.3% s Ni s 8%;

- 65% s Fe s 78%, preferably 70% s Fe s 78%,

- 0 s M s 4%, preferably 0 s M s 2%.

By way of indication, these ranges of mass proportions of Ni and Fe mentioned above can also be expressed when the oxide phase comprises a monoxide phase of composition Ni _x Fei. _x O by: 0.004 5x5 0.16, and preferably 0.004 5x5 0.1.

In one embodiment of the invention, the metallic phase of the cermet material further comprises at least one rare earth, preferably a rare earth chosen from yttrium (Y), cerium (Ce), lanthanum (La) and neodymium (Nd).

Rare earth can represent up to 5% of the mass of the metallic phase. More preferably, this percentage by mass of rare earth is less than or equal to 2%. The presence of the rare earth in the metallic phase improves the quality of the grain boundaries and thus prevents the formation of soluble fluorides or oxyfluorides which could contaminate the cryolite bath.

Given the high proportion of the metallic phase in the cermet material according to the invention, the electrical conductivity is close to that of a metal both in its evolution with temperature and in the value reached. The conductivity at room temperature is between 4000 to 5000 S / cm, and at a temperature of 1000 ° C, it is between 2000 to 3000 S / cm.

With the specific composition of the cermet material according to the invention which has been described above, said cermet material has the advantage that it forms on the surface of said cermet material a protective and adherent layer of nickel ferrite with a thickness between about 15 µm and 30 µm, from the start of the electrolysis when the cermet material is used in the composition of the material of an inert anode which is immersed in a cryolite bath or else during a treatment consisting of a pre-oxidation (for example in air between 900 and 1000 ° C., for a period of preferably between approximately 2 and 10 hours) of the inert anode comprising this cermet material and which is produced before its use during an electrolysis.

This protective layer has a composition which tends towards the composition Nio.9M _y Fe2.i-y04. And, during the electrolysis, due to the equilibrium with the cryolite bath containing alumina at the surface of the anode, the protective layer of an oxide phase initially not comprising metal M ( y = 0) tends towards the composition Nio.9Fe2Alo.1O4. This phase of nickel ferrite is less conductive than the oxide phase of the cermet material, but it is more stable, less soluble in the cryolite bath and thus remains on the surface of the anode.

This is why the subject of the invention is also a treated cermet material capable of being obtained after a pre-oxidation treatment of a cermet material as described above. Advantageously, the pre-oxidation treatment is carried out in air between 900 ° C and 1000 ° C for a period of between approximately 2 and 10 hours. The pre-oxidation treatment therefore only has the effect of forming on the surface of the cermet material the protective layer of nickel ferrite as described above.

Another subject of the invention is thus a treated cermet material which is characterized in that it is a cermet material as described above and which is totally or partially coated with a protective layer of composition. Nio.eMyFe ^ i-yO ^ the thickness of this protective layer being advantageously between about 15 and 30 μm.

Thus, in the context of the present invention, by “cermet material according to the invention”, it is a question of both the cermet material as described above and the treated cermet material which has just been described and which is distinguished from cermet material only by the fact that it additionally comprises a protective layer of nickel ferrite.

The cermet material according to the invention comprises a large amount of metallic phase which constitutes a reservoir of metallic species, in particular of nickel. In addition, the oxide phase of the cermet material is rich in iron and low in nickel.

This is why the protective layer of nickel ferrite on the surface of the cermet material according to the invention is constantly regenerated throughout the electrolysis due to the supply of nickel coming from inside the cermet material and of dissolving this same nickel in the oxide phase. The oxidizing environment on the surface of the cermet and nickel-rich material around the very iron-rich nickel ferrite tends to shift the stoichiometry of the nickel ferrite towards an equilibrium where the nickel ferrite contains a greater proportion of nickel, close to Nio , 9M _y Fe2.i-y04.

In other words, the oxide phase of the cermet material which can be a nickel ferrite oxide and / or a monoxide gives rise under oxidizing conditions (electrolysis and / or a pre-oxidation treatment as mentioned above. above) to the formation on the surface of a protective layer of nickel-rich nickel ferrite which is adherent, coherent and which is constantly renewed during electrolysis.

During electrolysis, the oxide phase of the cermet material according to the invention which is low in nickel tends to be attacked by the electrolyte. Therefore, this low nickel oxide phase must be in direct contact with the metallic phase of the cermet material in order to allow a rapid supply of nickel from the metallic phase when this metallic phase is oxidized.

The high proportion of the metallic phase in the cermet material according to the invention gives it very good electrical conductivity when it is used as an inert anode material, whether after a pre-oxidation treatment as described below. above or directly in the cryolite electrolysis bath.

In addition, this high content of the metal phase in the cermet material ensures the maintenance of conductivity when the composition of the cermet material changes during electrolysis, and this due to the equilibrium of the phases that it comprises. .

The upper limit of the content of the metallic phase of the cermet material according to the invention is suitably chosen so that said metallic phase does not interfere with the rapid regeneration from the oxide phase of the protective layer of nickel ferrite described above. -above ; too high a content of this metallic phase would lead to a risk of passivation by formation of nickel fluorides and / or the dissolution of copper oxide.

Furthermore, the lower limit of the content of the metal phase of the cermet material according to the invention is suitably chosen so that the metal phase is present so as to largely occupy the volume of said cermet material. The metallic phase constitutes a percolating phase which is therefore in direct contact with the oxide phase in order to allow a rapid supply of nickel to this oxide phase. The metallic phase also constitutes a large reservoir of nickel to ensure regeneration of the protective layer of nickel ferrite over a long period of time, namely a period which corresponds at least to the periods of use of inert anode in cryolite baths at electrolysis course for aluminum production.

This lower limit of the content of the metal phase in the cermet material is also chosen adequately to ensure good electrical conductivity of the cermet material according to the invention.

The metallic phase of the cermet material according to the invention comprises at least one alloy of nickel and copper. In this alloy, nickel oxidizes before copper and therefore enriches the oxide phase of the cermet material according to the invention; this allows the formation of the protective layer of nickel ferrite on the surface of the anode which is very resistant to the cryolite bath. Nickel also protects copper from oxidation.

The nickel enrichment of the protective layer of nickel ferrite to fill its nickel deficit makes it possible to:

- limit the pollution of the cryolite bath and of the aluminum produced during electrolysis with nickel which would have oxidized and which would have dissolved in said bath in the absence of the formation of this protective layer, and

- avoid passivation of the inert anode by forming dense, non-conductive NiO layers.

The lower limit of the mass content of nickel in the alloy of copper and nickel is chosen appropriately so that the nickel of the metallic phase of the cermet material according to the invention can oxidize over long periods, thus inducing a period of use of the cermet material quite adequate from an industrial point of view, and in particular for its use in a cryolite bath during electrolysis for the production of aluminum.

The copper in the metallic phase which is a more noble element than nickel will remain in the metallic phase of the cermet material for long periods of time; which makes it possible to have reducing conditions which are perfectly favorable for maintaining the composition of the oxide phase, and this at a balance which ensures a good compromise between the resistance to the cryolite bath and the electrical conductivity of the cermet material according to the invention .

The quantity of copper in the metallic phase of the cermet material according to the invention is chosen adequately so that the copper is present throughout the electrolysis, but in a sufficiently low quantity to avoid the melting of the metallic phase during the production of the cermet material according to the invention when it is manufactured by powder metallurgy.

In one embodiment of the invention, iron may be present in the composition of the alloy of copper and nickel of the metal phase of the cermet material before its use as an inert anode material during electrolysis.

The iron content by mass in the metal phase of the cermet material must be low, namely less than 20% and advantageously between 2 and 15%, preferably between 4 and 10%.

This low iron content in the metallic phase can result from the bringing into equilibrium of the phases of the cermet material during its manufacture, and more particularly during the sintering of the cermet material.

The iron content is kept low, and can be zero, in the metallic phase, because the iron in metallic form in the cermet material tends to oxidize rapidly and then dissolve rapidly in the cryolite bath, generating impurities in the cermet material. the bath and the aluminum produced, as well as porosities in said cermet material.

The reference work entitled "Inert anodes for aluminum electrolysis", 1 ^st edition 2007, by loan Galasiu, Rodica Galasiu and Jomar Thonstad indicates on page 447 that for inert anodes based on nickel ferrite, it is appropriate that they contain an excess of NiO due to the solubility of Fe ₂ O ₃ greater than that of NiO. Completely surprisingly and advantageously, the cermet material according to the invention goes against this technical teaching which recommends an excess of NiO to limit the solubility of iron in the cryolite bath for such anodes. In fact, the composition of the cermet material has on the contrary an excess of iron in its oxide phase and it was observed during the electrolysis that the iron in the oxide phase did not pollute the bath so as to make the use of the cermet material for electrolysis inappropriate.

An upper limit of the mass content of iron in the oxide phase has been suitably chosen to create an iron nickel oxide precursor of the nickel ferrite protective layer and to avoid making this phase of the material cermet according to the invention directly soluble in the cryolite bath. In fact, during electrolysis and on the surface of the inert anode, iron is liable to give rise to the formation of Fe ₂ O ₃ or iron aluminates or even iron fluorides which dissolve in the iron. cryolite bath.

The oxide phase of the cermet material will lose some iron during electrolysis. However, because the metallic phase of the cermet material is predominant in this material, the total amount of iron in the oxide phase remains low, so that the loss of iron from the cermet material taken as a whole is not detrimental. the purity of the cryolite bath and the aluminum produced.

Nickel in the oxide phase of the cermet material according to the invention contributes to the electronic balance in said oxide phase. In the oxide phase, the mass content of nickel is preferably chosen so that it is minimal with respect to the mass content of iron in order to limit pollution of nickel in the aluminum produced during electrolysis. , and this due to the inevitable weak dissolution of the oxide phase mentioned above, behind the protective layer of nickel ferrite. When the oxide phase is a monoxide phase, the ratio of the mass content of nickel to that of iron can be in the following range: 0.004s Ni / Fes0.26, and when the oxide phase is a phase of nickel ferrite oxide, the ratio of the mass content of nickel to that of iron may be in the following range: 0.004sNi / Fes0.21.

In addition, during the electrolysis, the nickel originating from the oxidation of the metal phase of the cermet material according to the invention will be dissolved in the oxide phase, due to the substoichiometry in nickel (otherwise said of its low nickel content) of this oxide phase of the cermet material according to the invention. In fact, during electrolysis, the composition of the cermet material according to the invention changes to tend towards a balance of its phases. In other words, the nickel substoichiometry of the oxide phase of the cermet material was adequately chosen to cause displacement of nickel from the metallic phase to the oxide phase under oxidizing conditions.

The raw materials to be used and the associated compositions of the different phases of the cermet material according to the invention can be calculated using thermodynamic software and verified experimentally via measurements with the Castaing microprobe or by EDX (Energy Dispersive X -ray spectrometry) on polished sections examined with the SEM (ie the abbreviation of “scanning electron microscope”).

The cermet material according to the invention can be used either:

1) as a monolithic material of an electrode, preferably an inert anode; said monolithic material may have been subjected prior to its immersion in a cryolite bath to a pre-oxidation treatment (for example between 900 ° C and 1000 ° C in air for a period of between approximately 2 and 10 hours),

2) as a coating material, for example with a thickness of between about 0.5 and 20 mm, on a metallic substrate (in other words a metallic core), with a view to obtaining an electrode, preferably an inert anode,

3) as a material of one or more intermediate layers between a metal substrate and a deposit of a cermet material rich in oxide, that is to say having a percentage by mass of oxide phase greater than the percentage by mass of the oxide phase of the cermet material according to the invention, or of a pure oxide, the assembly constituting an electrode, preferably an inert anode.

When the cermet material according to the invention is used as an intermediate layer material, it creates a chemical gradient and thus makes it possible to accommodate the expansion of the different materials of the electrode.

The subject of the present invention is thus a monolith electrode, preferably an anode, made of the cermet material according to the invention as described above.

Another object of the present invention is an electrode, preferably an anode, comprising a metal core completely or partially covered with at least one layer of a cermet material according to the invention as described above.

In one embodiment of said electrode, the layer of cermet material according to the invention is covered with a deposit of a cermet material rich in oxide or of a pure oxide. The layer of cermet material according to the invention is then an intermediate layer as mentioned above. Thus, the invention relates to an electrode which is characterized in that the layer of cermet material according to the invention is an intermediate layer arranged between the metal core and a layer of cermet material rich in oxide or pure oxide, which further comprises said electrode.

In these embodiments of an electrode according to the invention which comprises a metallic core, said metallic core advantageously comprises at least one alloy of nickel (Ni) and iron (Fe), the mass proportions of Ni and Fe being the following:

- 40% £ Ni 85%, preferably 55% £ Ni £ 80%,

- 15% £ Fe £ 60%, preferably 20% £ Fe £ 45%.

Said metallic core of the electrode material may further comprise copper (Cu) in the following proportion by mass 5% Cu 40%.

Advantageously, the mass proportions of the metal core are as follows:

- 40% £ Ni £ 70%;

- 20% £ Fe £ 45%;

- 7% £ Cu £ 20%.

The metallic core of the electrode material can also comprise at least one metal A chosen from aluminum (Al), cobalt (Co), chromium (Cr), manganese (Mn), molybdenum (Mo), titanium (Ti), zirconium (Zr), tin (Sn), vanadium (V), niobium (Nb), tantalum (Ta) and hafnium (Hf) or is a combination of these metals , the mass proportion of metal A in the metal core being as follows: 0.5% £ A £ 30%.

Advantageously, the mass proportions of the metal core are as follows:

- 40% £ Ni £ 80%;

- 15% £ Fe £ 40%;

- 0 £ Cu £ 20%; <f ^Z

- 0.5% £ A ^ 15%.

The metallic core can further comprise at least one rare earth, preferably a rare earth chosen from among yttrium (Y), cerium (Ce), lanthanum (La) and neodymium (Nd).

Rare earth represents up to 5% of the mass of the metallic core of the electrode.

An electrode according to the invention which comprises a cermet material and a metal core as described above has the following advantages:

- Thanks to the composition of the metal core as described above, the metal core ensures good electrical conductivity and good mechanical stability of the electrode.

- The percentage by mass of iron between 15% and 60% that the metallic core comprises is particularly adequate so that the metallic core can supply the cermet material with iron. In fact, during electrolysis, the loss of iron from the cermet material can be compensated for by the fact that iron will move from the metallic core towards the cermet material (and this due to oxidation phenomena), in particular via cationic defects in the structures of the oxides which the said cermet material comprises.

- In the metal core, nickel confers refractoriness to the Ni-Fe alloy (ie mechanical temperature maintenance) and its resistance to oxidation.

- The composition of the metallic core has been selected so that deterioration of the metallic core by oxidation and / or fluorination is avoided.

- The metal core has the advantage of locally maintaining a reducing potential which counteracts the oxidizing potential of the cermet material on the side of the cryolite bath, said oxidizing potential being linked to the electrolysis reaction in the cryolite bath in which the electrode is immersed. according to the invention.

Preferably, the electrode described above is an anode.

A subject of the invention is also an electrolysis cell comprising at least one electrode as described above.

A subject of the present invention is also a process for manufacturing the cermet material as described above consisting of a powder metallurgy process or a thermal spraying technique, said manufacturing process being characterized in that it uses as materials. at least first:

- iron in metallic or alloy form, and optionally copper and nickel, in metallic or alloy form,

- an oxide chosen from nickel ferrite oxides Ni _x Fe3- _x O4, NiO, Fe ₂ O3, CuO,

Cu ₂ O, CuFeO ₂ , the spinel type Cu _x Fe3- _x O4 with 0 <x £ 1.

In one embodiment of the manufacturing process, a majority proportion of copper is supplied in oxide form to allow the reaction of reduction of copper oxide to metallic copper. The iron is advantageously supplied in metallic form in an amount sufficient to allow the total reduction of the copper oxide to copper metal. This solid-solid reaction of reduction of copper oxide by iron is at the origin of the structure of the cermet material according to the invention consisting of two interlaced networks (or in other words of two percolating phases) and of obtaining an iron oxide and nickel phase rich in iron and poor in nickel.

This oxygen supply for the oxide phase, via a copper oxide is preferred to a direct supply via an iron oxide due to the spatial organization of the oxide and metal phases resulting from the reduction reaction.

Advantageously, 30% to 100% of the copper is provided in the form of copper oxide such as CuO, Cu ₂ O, Cu _x Fe3- _x O4 with 0 <x £ 1, CuFeO2. Preferably, more than 50% of the copper, and even more preferably more than 80% of the copper, is provided in the form of copper oxide.

Advantageously, 30% to 100% of the iron is provided in the form of metallic iron. Preferably, more than 50% of the iron, and even more preferably more than 80% of the iron, is provided in the form of metallic iron.

Thus, the cermet material according to the invention can, for example, be made from a powder metallurgy process.

In the case of powder metallurgy manufacturing, the solid-solid reaction of reduction of copper oxide by iron occurs during sintering of the material after mixing and pressing the raw materials.

Such a manufacturing method is particularly advantageous when the cermet material according to the invention is intended to be used as a monolithic material of an inert anode for electrolysis.

Other methods of manufacturing the cermet material are also conceivable, for example when the cermet material is intended to be applied in the form of a coating on a metal substrate or depending on the shape of the inert anode that is being used. wishes to obtain, as well as the number of layers, their composition and their thickness. These methods can be used separately or in combination with each other. Xf

For example, the metal core of the inert anode can be fabricated by a method selected from casting, casting (preferably, lost wax casting or sand casting), or a hot working technique such as rolling, extrusion or even powder metallurgy.

Then, the different layer or layers of cermet material according to the invention can be applied to the metal core by a method chosen from:

- projection methods, for example:

• plasma projection, • projection by supersonic flame (HVOF, acronym for "High Velocity Oxy-Fuel"), • high pressure projection by supersonic flame (HPVOF, acronym for "High-Pressure High Velocity Oxi-Fuel"), • projection by supersonic air (HVAF, acronym for “High Velocity Air Fuel), • detonation gun, • cold projection,

- methods of applying layers by powder metallurgy such as natural sintering and hot isostatic pressing.

In one embodiment of the invention in which the metallic core is prepared beforehand, the surface of the metallic core is advantageously subjected to a pretreatment so as to remove the casting impurities and to improve the adhesion of the layer of cermet material according to invention.

In one embodiment of the invention, the inert anode can also comprise one or more intermediate layers made of the cermet material according to the invention which are arranged between the metal core and a layer of cermet material rich in oxide, or of a pure oxide.

The intermediate layer formed of the cermet material according to the invention helps to accommodate the mechanical stresses due to the differences in expansion coefficients of the metal core and the layer of cermet material rich in oxide, or pure oxide.

The thickness of the intermediate layer can be between a hundred microns and one to several millimeters.

This inert anode can be manufactured by co-sintering, additive manufacturing or even by using a laser process (for example laser welding or laser sintering).

Upon completion of manufacture, the metal core of the anode is uniformly covered with at least one dense, adherent layer of a cermet material according to the invention.

When the cermet material according to the invention is used as a monolithic anode material or when it is deposited on a metal core in order to obtain an anode, a pre-oxidation treatment can be carried out so as to what forms on the surface of said cermet material, and therefore of the inert anode, the protective layer of dense nickel ferrite, rich in nickel as detailed above and which is poorly soluble in a cryolite bath of electrolysis.

As already explained, without pre-oxidation treatment, the protective layer will form directly during the introduction of the anode material into the cryolite bath and the start of the electrolysis, which create favorable conditions for the oxidation of the surface of the anode material.

Description of figures:

FIG. 1 is a photograph of an observation by SEM in backscattered electrons of a portion at the heart of a monolith anode made of a cermet material according to the invention after a pre-oxidation treatment.

Figure 2 is a photograph of an SEM observation in backscattered electrons of a portion on the surface of the monolith anode made of a cermet material according to the invention shown in Figure 1.

FIG. 3 is a photograph of an observation by SEM in backscattered electrons of a portion on the surface of the monolith anode made of a cermet material according to the invention which is shown in part in the photographs of FIGS. 1 and 2 , and this after 96 hours of electrolysis at a current of 0.6 A / cm ² .

FIG. 4 is a photograph of an observation by SEM in backscattered electrons of a portion on the surface of the monolith anode made of a cermet material according to the invention which is shown in part in the photographs of FIGS. 1 and 2 , and this after 506 hours of electrolysis at a current of 0.8 A / cm ² .

Figure 5 is a graph showing the affected internal thickness of the monolith anode shown in part in Figures 1 to 4 as a function of the electrolysis time.

FIG. 6 is a graph representing the nickel content expressed as a function of the content x of the nickel ferrite oxide phases of composition NixAl _y Fe3- _x -yO4 and monoxide of composition Ni _x Fei. _x O, as well as the mass content of nickel in the metallic phase, as a function of the distance from the surface of the monolith anode shown in part in FIGS. 1 to 4 after 211 hours of electrolysis.

FIG. 7 is a graph representing the nickel content expressed as a function of the content x of the nickel ferrite oxide phases of composition NixAl _y Fe3.x. _y O4 and monoxide of composition NixFei-xO, as well as the mass content of nickel in the metal phase, depending on the distance from the surface of the monolith anode shown in part in Figures 1 to 4 after 506 hours of electrolysis.

Figure 8 is a graph showing the monitoring of electrolysis potentials for 506 hours of the monolith anode shown in part in Figures 1 to 4.

FIG. 9 is a photograph of an observation by SEM in backscattered electrons of a portion of the layer of a cermet material according to the invention which comprises an anode consisting of a metal core covered with said cermet material after sintering.

Figure 10 is a photograph of an SEM observation of backscattered electrons of the same portion of the cermet material layer shown in Figure 9 after 230 hours of electrolysis.

Experimental part :

Cermet materials according to the invention were prepared by mixing the powders and in the following mass proportions:

- 12% of NiFe ₂ O ₄ ;

- 3% Cu;

- 32% deCuO;

- 5% of Ni;

- 48% of a Ni-Fe alloy (the mass contents of nickel and iron in this alloy were 50%).

Then, sintering was carried out under argon at 1250 ° C. so as to obtain monolithic anodes made of a cermet material according to the invention.

Finally, the monolithic anodes thus obtained were subjected to a preoxidation treatment in air at 930 ° C. for 9 hours.

For all the experiments carried out on said anodes, the electrolysis conditions were as follows: a cryolite bath with an initial cryolite ratio equal to 2.2 and containing 5% by weight of CaF ₂ , and 7.5% alumina. The cryolite ratio is the ratio in molar percentages of NaF to AIF3. df

I

The temperature of the bath was maintained at 960 ° C. and with a current of 0.6 A / cm ² to 0.8 A / cm ² . The electrolysis potential was stable with a standard deviation of the order of 0.25 V including the variations of the metal layer, throughout the duration of the experiments.

Observations of the sections of the anodes after electrolysis, coating and cutting showed that the interior of the anodes was very little modified, and this after 96 hours, 211 hours and 506 hours of electrolysis.

These observations testify to the excellent resistance of the cermet material according to the invention when it is used as an inert anode during electrolysis, and this over quite remarkable times (up to 506 hours). These experiments demonstrated the industrial interest of the cermet material according to the invention.

FIG. 1 is a photograph of an observation by SEM in backscattered electrons of a portion at the heart of a monolith anode made of a cermet material according to the invention which was obtained from the mixture of powders and the sintering described. above and after a pre-oxidation treatment.

FIG. 2 is in fact a photograph of an observation by SEM in backscattered electrons of a surface portion of the monolith anode shown in part in FIG. 1.

In the photographs of Figures 1 and 2, we visualize the different phases in the presence of the cermet material:

- metallic phase 1 of an alloy of nickel and copper (the white areas),

- phase 2 of nickel ferrite oxide NixFe3- _x O ₄ (dark gray areas),

- porosities 3 (black spots),

We visualize in addition on the photograph of figure 2:

- a phase 5 monoxide of NixFei-xO (the light gray areas),

- a phase 4 of nickel ferrite of composition Nio.9Fe2.1O4 on the surface of the cermet material (the dark gray areas) which corresponds to the protective layer mentioned above,

- an oxide phase 6 rich in copper.

As explained above, the layer of nickel ferrite which forms on the surface of the cermet material according to the invention is particularly advantageous because it is adherent and coherent; which contributes to the excellent resistance of said cermet material, even under aggressive conditions such as those of a cryolite bath used during electrolysis for the manufacture of aluminum.

In addition, this layer of nickel ferrite will be constantly renewed during the electrolysis, as evidenced by the photographs of Figures 3 and 4 which are photographs of an observation by SEM in backscattered electrons of a portion of l monolith anode which is shown in part in the photographs of Figures 1 and 2, respectively after 96 hours and 506 hours of electrolysis.

In fact, in Figures 3 and 4, we can see the protective layer of nickel ferrite which adheres to the perimeter of the anode. The thickness of this protective layer is approximately 20 to 30 µm. Thus, after 96 hours of electrolysis and even 506 hours, the protective layer of nickel ferrite is still present on the surface of the anode with a substantially identical thickness.

Figure 5 is a graph showing the affected internal thickness of the monolith anode shown in part in Figures 1 to 4 during 506 hours of electrolysis.

By "affected internal thickness" is meant the thickness in which the composition of the material is different from the composition at the anode core, the anode core corresponding to the initial composition of the cermet material before electrolysis.

In view of the graph in Figure 5, it can be seen that the affected internal thickness changes linearly and slightly at a rate of 12 pm / hour during 506 hours of electrolysis. This graph testifies to the excellent stability of the cermet material according to the invention which is resistant to the aggressive conditions of a cryolite bath during electrolysis. The wear of the anode material is very low, less than 0.2mm after 506 hours of electrolysis.

FIG. 6 is a graph showing, after 211 hours of electrolysis, the nickel content expressed as a function of the content x of the phases of nickel ferrite oxide of composition Ni _x AlyFe ₃ -x.yO4 and of monoxide of composition Ni _x Fei.xO, as well as the mass content of nickel in the metallic phase, as a function of the distance from the surface of the monolith anode shown in part in Figures 1 to 4.

FIG. 7 is a graph representing, after 506 hours of electrolysis, the nickel content expressed as a function of the content x of the phases of nickel ferrite oxide of composition NixAl _y Fe3-x-yO4 and of monoxide of composition Ni _x Fei- _x O, as well as the mass content of nickel in the metallic phase, as a function of the distance from the surface of the monolith anode shown in part in Figures 1 to 4.

In view of the graphs of Figures 6 and 7, we note that the profiles of the amounts of nickel in the oxide phases and in the metallic phase are comparable but shifted towards the core of the anode. There is thus a displacement of a certain quantity of nickel from the

metal phase to oxide phases, the deeper from the anode surface as the electrolysis time increases.

The composition of the anode core remains unchanged. The modification front of the cermet material tends to move slowly from the surface to the anode core and a plateau of stable composition forms on the anode surface.

These two graphs of Figures 6 and 7 show that the anode made of a cermet material according to the invention remains stable during electrolysis and is therefore perfectly suitable for its industrial use.

As can be seen in the graph of FIG. 7, the composition of the oxide phase stabilizes at the surface of the anode with a higher nickel content, in particular a phase of nickel ferrite Ni _x Al _y Fe3-x- _y O4 with x close to 0.9, making this layer more resistant to corrosion by the cryolite bath. The nickel in the metallic phase either enters the nickel ferrite or the monoxide phase. Nickel dissolves relatively little in the cryolite bath, which is confirmed by the graph in Figures 6 and 7.

Figure 8 is a graph showing the monitoring of electrolysis potentials for 506 hours of the monolith anode partially shown in Figures 1 to 4. It is noted that the anode has a stable behavior. The advance of the modification front does not influence the electrolysis potential, in particular due to the large part of the metallic phase in the cermet material which maintains a high conductivity.

FIG. 9 is a photograph of an observation by SEM in backscattered electrons of the interface between the metal core of composition N165Fe2sCuio and a cermet material according to the invention of an anode which was obtained after sintering at 1200 ° C. The cermet material comprised 68% nickel ferrite oxide of composition Ni0.04Fe2.9eO4 and 32% metal comprising 50% Ni, 40% Cu and 10% Iron.

Figure 10 is a photograph of a backscattered electron SEM observation of this interface shown in Figure 9 after 230 hours of electrolysis.

In Figures 9 and 10, we can see phase 7 of the metal core.

By comparing the two photographs in Figures 9 and 10, we can see that the metallic core-cermet material interface according to the invention of the anode is very similar and therefore remained stable after 230 hours of electrolysis. The interface is cohesive and there was no infiltration of the bath after 230 hours of electrolysis. This demonstrates the stability during electrolysis of an anode consisting of a metal core covered with a cermet material according to the invention.

In addition, no fluorine was identified at the interface. This means that the anode was not attacked by corrosion after 230 hours of electrolysis.

These experimental results also demonstrate the industrial interest represented by the cermet material according to the invention when it is used as a coating for a metallic core of an electrode, and in particular of an anode. Such an embodiment can be particularly advantageous for increasing the life of the anode due to the regeneration of the cermet material by means of iron migration from the metallic core to the cermet material.

Cabinet Cazenave sari Industrial Property yjpV 500 Yaoundé (Cameroon)

Claims (20)

1. Cermet material comprising, in percentages by mass, at least: - 50% to 90%, preferably 60% to 80%, of a metallic phase (1) which contains at least one alloy of copper (Cu) and nickel (Ni), said alloy comprising in percentages by weight: • 35% to 75%, preferably 40% to 60%, more preferably 45% to 55%, of nickel, • 25% to 65%, preferably 40% to 55%, more preferably 45% to 55%, of copper, - 10% to 50%, preferably 20% to 40%, of an oxide phase (2.5) containing at least iron, nickel and oxygen with the following mass proportion of Ni: 0 , 2% 5 Ni £ 17%. 2. Cermet material according to claim 1, characterized in that the alloy of copper (Cu) and Nickel (Ni) comprises iron (Fe), the mass percentage of iron in said alloy not exceeding 20%, of preferably, said percentage by weight of iron is between 2% and 15%, and more preferably between 4% and 10%. 3. Cermet material according to any one of claims 1 to 2, characterized in that the oxide phase (2.5) further contains at least one metal (M) chosen from aluminum (Al), cobalt. (Co), chromium (Cr), copper (Cu), manganese (Mn), titanium (Ti), zirconium (Zr), tin (Sn), vanadium (V), niobium ( Nb), tantalum (Ta), yttrium (Y), hafnium (Hf). 4. Cermet material according to any one of claims 1 to 2, characterized in that said oxide phase comprises: - a monoxide phase (5) of composition Ni _x MyFei. _x . _y O with the following mass proportions: • 0.3% £ Ni £ 17%, • 60% £ Fe £ 78%, • O £ M £ 1O%, and / or - a phase of nickel ferrite oxide (2) of composition Ni _x M _y Fe3- _x -yO4 with the following proportions by mass: (\ L • 0.2% S Ni S 13%, • 60% £ Fe £ 72%, • £ 0 M £ 8%, M being a metal chosen from aluminum (Al), cobalt (Co), chromium (Cr), copper (Cu), manganese (Mn), titanium (Ti), zirconium (Zr), l tin (Sn), vanadium (V), niobium (Nb), tantalum (Ta), yttrium (Y), hafnium (Hf), or a combination of these metals. 5. Cermet material according to claim 4, characterized in that when said oxide phase comprises a phase of nickel ferrite oxide (2), said phase of nickel ferrite oxide (2) is of composition Ni _x M _y Fe3- _x - _y O4 with the following mass proportions: - 0.2% s Ni s 10%, preferably 0.2% s Ni £ 5%, - 63% £ Fe £ 72%, preferably 68% £ Fe £ 72%, - 0 s M s 4%, preferably 0 £ M £ 2%. 6. Cermet material according to claim 4, characterized in that when the oxide phase of the cermet material comprises a monoxide phase (5), said monoxide phase (5) is of composition Ni _x M _y Fei- _x - _y O with the following mass proportions: - 0.3% s Ni s 13%, preferably 0.3% s Ni £ 8%, - 65% Fe 78%, preferably 70% Fe 78%, - 0 s M s 4%, preferably 0 £ M £ 2%. 7. Cermet material according to any one of claims 1 to 6, characterized in that the metallic phase (1) further comprises at least one rare earth chosen from yttrium (Y), cerium (Ce), lanthanum. (La) and neodymium (Nd). 8. Treated cermet material obtainable after a preoxidation treatment of a cermet material according to any one of claims 1 to 7. 9. Cermet material treated according to claim 8, characterized in that the pre-oxidation treatment is carried out in air between 900 ° C and 1000 ° C for a period of between 2 and 10 hours. 10. Treated cermet material, characterized in that it is a cermet material according to any one of claims 2 to 7 which is completely or partially coated with a protective layer (4) of composition Nio.9M. _y Fe2.i-y04. 11. Cermet material treated according to claim 10, characterized in that the thickness of the protective layer (4) is between 15 and 30 pm. 12. Electrode comprising a metal core fully or partially covered with at least one layer: - a cermet material according to any one of claims 1 to 7, or - of a treated cermet material according to any one of claims 8 to 11. 13. Electrode according to claim 12, characterized in that said metal core comprises at least one alloy of nickel (Ni) and iron (Fe), the mass proportions of Ni and Fe being as follows: - 40% s Ni s 85%, preferably 55% s Ni s 80%, - 15% s Fe s 60%, preferably 20% s Fe s 45%. 14. Electrode according to claim 13, characterized in that said metal core further comprises copper (Cu) in the following mass proportion: 5% Cu 40%. 15. An electrode according to any one of claims 12 to 14, characterized in that the metal core of the electrode material further comprises at least one metal A chosen from aluminum (Al), cobalt (Co), chromium (Cr), manganese (Mn), molybdenum (Mo), titanium (Ti), zirconium (Zr), tin (Sn), vanadium (V), niobium (Nb), tantalum (Ta) and hafnium (Hf) or is a combination of these metals, the mass proportion of metal A in the metallic core being as follows: 0.5% £ A s 30%. 16. Electrode according to any one of claims 12 to 15, characterized in that the metal core further comprises at least one rare earth chosen from yttrium (Y), cerium (Ce), lanthanum (La) and neodymium (Nd). 17. An electrode according to any one of claims 12 to 16, characterized in that said layer of cermet material according to any one of claims 1 to 7 or said layer of cermet material treated according to any one of claims 8 to 11 is an intermediate layer disposed between the metal core and a layer of cermet material rich in oxide or pure oxide, which said electrode further comprises. 18. A method of manufacturing a cermet material according to any one of claims 1 to 7 consisting of a powder metallurgy process or a thermal spraying technique, characterized in that said manufacturing process uses as raw materials at least - iron in metallic or alloy form, and optionally copper and nickel, in metallic or alloy form, an oxide chosen from nickel ferrite oxides Ni _x Fe3- _x O4, NiO, Fe2O3, CuO, CU2O, CuFeO2, spinel of the Cu _x Fe3- _x O4 type with 0 <xs 1. 5 19. The manufacturing method according to claim 18, characterized in that between 30% and 100% of the copper is supplied as an oxide. 20. The manufacturing method according to claim 18 or 19, characterized in that between 30% and 100% of the iron is supplied in the form of metallic iron