AU2004222545B2

AU2004222545B2 - Method for the manufacture of an inert anode for the production of aluminium by means of fusion electrolysis

Info

Publication number: AU2004222545B2
Application number: AU2004222545A
Authority: AU
Inventors: Armand Gabriel; Veronique Laurent
Original assignee: Aluminium Pechiney SA
Current assignee: Rio Tinto France SAS
Priority date: 2003-03-12
Filing date: 2004-03-10
Publication date: 2009-03-26
Anticipated expiration: 2024-03-10
Also published as: ATE396290T1; AR043490A1; WO2004082355A3; WO2004082355A2; CN1759207A; RU2005131580A; CN100507090C; EP1601820A2; ES2305745T3; DE602004013935D1; FR2852331B1; IS2627B; AU2004222545A1; NO20054641L; IS8063A; RU2336369C2; EP1601820B1; FR2852331A1; ZA200508244B; SI1601820T1

Description

PROCESS FOR MANUFACTURING AN INERT ANODE FOR PRODUCTION OF ALUMINIUM BY FUSED BATH ELECTROLYSIS Field of the invention The invention relates to aluminium production by fused bath electrolysis. More particularly, it relates to anodes used for this production and manufacturing processes to obtain them.

State of the art Aluminium metal is produced industrially by fused bath electrolysis, namely by electrolysis of alumina in solution in a molten cryolite-based bath called an electrolyte bath, particularly using the well-known Hall-H6roult process. The electrolyte bath is contained in pots called the "electrolytic pots" comprising a steel pot shell lined on the inside with refractory and or insulating materials, and a cathode assembly at the bottom of the pot. Anodes are partially immersed in the electrolyte bath. The expression "electrolytic cell" normally refers to the assembly comprising an electrolytic pot and one or more anodes.

The electrolytic current that circulates in the electrolyte bath and the pad of liquid aluminium through anodes and cathode elements, causes aluminium reduction reactions and also provides a means of maintaining the electrolyte bath at a temperature typically of the order of 950 0 C due to the Joule effect. The electrolytic cell is supplied with alumina regularly so as to compensate for alumina consumption caused by the electrolytic reactions.

In the standard technology, the anodes are made of a carbonaceous material and are consumed by aluminium reduction reactions. The typical lifetime of an anode made of a carbonaceous material is 2 to 3 weeks.

Environmental constraints and costs associated with the fabrication and use of anodes made of carbonaceous material have encouraged aluminium producers to search for anodes made of non-consumable materials, called "inert anodes", for many decades.

Several materials have been proposed, including composite materials containing a so-called "ceramic" phase and a metallic phase. These composite materials are known as "cermets".

Many studies have been carried out on some cermet materials, such as cermet materials for which the ceramic phase contains a mixed iron and nickel oxide.

These studies applied particularly to cermet materials for which the ceramic phase contains a mixed phase of nickel oxide (NiO) and nickel ferrite (NiFe 2 0 4 and for which the metallic phase contains for example iron, nickel or copper. Subsequently, these cermets are called "NiO-NiFe 2

O

4 -M cermets", where M denotes the metallic phase.

For example, as described in American patents US 4 455 211, US 4 454 015 and US 4 582 585, NiO-NiFe20 4

-M

cermets are typically obtained by a process comprising preparation of a mixture of metal powders and powders of one or more iron and nickel oxides, compression of the mixture so as to form a green body with a determined shape and sintering of the green body at a temperature of between 900 and 1500 0 C. The initial iron and nickel oxide powders are typically a precalcinated mixture of nickel oxide (NiO) and iron oxide (typically Fe 2 0 3 or Fe 3 0 4 American patent US 4 871 438, in the name of Battelle Memorial Institute, describes a manufacturing process in which the initial oxide powder is a NiO- NiFe 2 0 4 powder and the initial metal powder is composed of a mixture of 10 to 30% by weight of copper powder and 2 to 4% by weight of nickel. The mass ratio between NiO and NiFe 2 0 4 is between 2 3 0.67) and 3 2 During sintering, copper and nickel form an alloy for which the melting temperature is greater than the sintering temperature, which provides a means of preventing bleed out of the metallic phase, and thus obtaining a final quantity of the metallic phase greater than 17% by weight. The initial mixture does not contain any organic binder. Sintering is done in an argon or nitrogen atmosphere containing 100 to 500 ppm of oxygen.

More recently, American patent US 5 794 112, in the name of Aluminum Company of America, describes a process for manufacturing a cermet in which the initial mixture contains a metal powder composed of copper and or silver and between 2 and 10 parts by weight of an organic binder and in which sintering is done under a controlled argon atmosphere containing between 5 and 3000 ppm of oxygen.

However, there are problems with known manufacturing processes when used for the production of NiO-NiFe 2

O

4 -M cermet parts in which the metallic phase M contains copper and nickel, particularly for the production of large parts (in other words parts for which the smallest dimension (typically the diameter) is greater than or equal to about 20 cm). The applicant found it difficult to satisfactorily control and check the composition and relative proportion of all phases of the cermet. However, the composition and proportion affect the usage properties of the cermet. Furthermore, elimination of evaporation and decomposition products of the binder during sintering depends strongly on the nature of sintering, which makes the process very sensitive to the choice of the binder when its proportion in the initial mixture is high (as in the process described in US patent 5 794 112). Furthermore, the combination of a low thermal conductivity and large dimensions of the green part interrupts evacuation of evaporation and decomposition gases provoked by closing of surface pores. The parts can also crack. These latter difficulties may be partially solved by prolonging the debinding phase, but this solution significantly reduces the productivity of the process.

Therefore, the applicant searched for solutions to solve the disadvantages of known fabrication processes.

Description of the Invention According to a first aspect of the present invention, there is provided a process for manufacturing an inert cermet anode of the NiO-NiFe 2 0 4 -M type comprising at least a nickel monoxide phase N, a nickel spinel phase S containing iron and nickel, and a metallic phase M, containing copper and nickel, the process comprising: preparation of an initial mixture including at least one precursor of the monoxide phase N and the spinel phase S, a precursor of the metallic phase M and an organic binder, the proportion of the organic binder in the initial mixture being less than by weight, the precursor of the metallic phase comprising a metallic powder containing copper and nickel, the precursor of the monoxide and spinel phases comprising a mixture of oxides comprising a nickel oxide and a nickel ferrite, the ratio between the proportion by mass of nickel oxide and the proportion by mass of nickel ferrite being between 0.2/99.8 and 30/70, a mixture shaping operation, so as to form a green anode with a determined shape, a green anode sintering operation, at a temperature higher than 900'C in a controlled atmosphere containing at least an inert gas and oxygen, and wherein the proportion of the monoxide phase in the final cermet is less than 40% by weight.

1949369-IHJG According to a second aspect of the present invention, there is provided use of the process according to the first aspect for the fabrication of inert anodes to be used for the production of aluminium by fused bath electrolysis.

According to a third aspect of the present invention, there is provided use of an inert anode or an assembly of inert anodes, obtained by the process according to the first aspect for the production of aluminium by fused bath electrolysis.

According to a fourth aspect of the present invention, there is provided electrolytic cell intended for the production of aluminium by fused bath electrolysis comprising at least one inert anode produced by the manufacturing process according to the first aspect.

An object of the invention is a process for fabrication of an inert cermet anode, the cermet being denoted by the formula "NiO-NiFe 2

O

4 and comprising a metallic phase M including copper and 1949369-IHJG nickel, and a ceramic phase C, called the mixed phase, comprising at least two distinct phases, namely a "nickel monoxide" phase N and a "nickel spinel" phase

S.

The typical formula of the nickel monoxide phase N is NiO, that may or may not be stoechiometric and may possibly comprise elements other than nickel such as iron. The typical formula of the nickel spinel phase S is NiFe 2 0 4 that may or may not be stoechiometric and may comprise elements other than nickel and iron.

According to the invention, the process for manufacturing an inert cermet anode of the NiO-NiFe 2

O

4 M type comprising at least a nickel monoxide phase N, a nickel spinel phase S containing iron and nickel, and a metallic phase M containing copper and nickel, is characterised in that it comprises: preparation of an initial mixture comprising at least one precursor of the monoxide phase N and the spinel phase S, a precursor of the metallic phase M and an organic binder, the proportion of the organic binder in the initial mixture being low, in other words less than 2.0% by weight, and the precursor of the metallic phase comprising a metallic powder containing copper and nickel, a mixture shaping operation, typically by pressing or isostatic compression, so as to form a green anode with a determined shape, a green anode sintering operation, at a temperature typically more than 900 0 C in a controlled atmosphere containing a small quantity of oxygen, that is typically less than 200 ppm of 02.

6 The applicant had the idea of dissociating the physicochemical functions performed by the binder and the precursor of the metallic phase. In this context, the applicant noted that it was usually sufficient to add a small quantity of organic binder to hold the part at the beginning of sintering (in other words to significantly reduce or to substantially prevent its deformation) and that the role of the chemical reducer of the said binder could be performed by the addition of metallic nickel into the precursor of the metallic phase which is preferably formed from metallic powders.

Dissociating the two functions (mechanical strength of the part and control over the composition of the metallic phase) provides a means of reducing the quantity of binder and consequently reducing emissions of toxic volatile materials under low oxygen contents, reducing the debinding time and reducing risks of cracking and creation of porosity associated with elimination of the binder in the gaseous phase and volatile binder decomposition products in large parts.

The fact of adjusting the composition of the metallic phase of the sintered material by adding nickel not only avoids bleed out of the metallic phase during sintering, but also enables better control over the local chemistry of the ceramic and metallic phases.

Adjustment of the composition of the metallic phase according to the invention also provides a means of making the microstructure of the cermet of large parts more homogeneous.

Use of a small quantity of organic binder makes the process more reliable within the context of an industrial production of anodes (and more generally of parts that will form anodes). In particular, it provides a means of making the process less sensitive to the size of sintered parts.

Sintering causes migration of some of the metallic elements between the different phases. Thus, nickel oxide typically becomes enriched in iron, nickel ferrite becomes non-stoechiometric and the metallic phase becomes enriched in nickel and possibly in iron, usually in smaller proportions. Consequently, the cermet derived from sintering may be described more precisely by the formula Nil-xFexOe±s-NiyFe3-yO4±8-M', where M' is an alloy including the initial metal M, iron and nickel (MFeNi). However, in order to simplify the terminology, the NiO (and more generally Ni 1 _-FexO) and 4 (and more generally NiyFe 3 -y0 4 phases will subsequently simply be denoted by the expressions "monoxide phase" and "spinel phase". Furthermore, the cermet will simply be denoted by the formula "NiO- NiFe20 4 where NiO denotes the monoxide phase 4 denotes the spinel phase and M is the metallic phase.

Inert anodes according to the invention are intended for use in the production of aluminium by fused bath electrolysis. They may possibly be assembled to form anode assemblies comprising several individual anodes such as clusters.

The invention will be better understood after looking at the appended figures and the following detailed description.

Figure 1 represents a preferred embodiment of the manufacturing process according to the invention.

Figure 2A is a micrograph of a typical cermet obtained by the manufacturing process according to the invention.

Figure 2B is a diagrammatic reproduction of the micrograph in Figure 2A.

Figure 3 is a ternary diagram NiO NiFe20 4

M

showing preferred ranges for the initial composition in a preferred embodiment of the invention.

Figure 4 is a truncated ternary diagram Ni Cu oxides showing preferred ranges for the initial composition in a preferred embodiment of the invention.

The metallic powder containing copper and nickel is typically a mixture of a metallic copper powder and a metallic nickel powder. According to the invention, it is also possible to use a metallic powder consisting partly or entirely of a copper and nickel alloy.

Preferably, the size of at least 95% by weight of the grains of the said metallic powder is between 3 and n The proportion of metallic powder in the initial mixture is preferably more than 15% by weight, and more preferably more than 20% by weight. This proportion is preferably less than 35% by weight. It is typically between 15% and 30% by weight and more typically between 20% and 25% by weight. These preferred proportions are shown in the ternary diagram in figure 3 in the case in which the precursor of the said monoxide phase N and spinel phase S is composed of nickel oxide NiO and nickel ferrite NiFe 2 04.

The proportion of nickel in the metallic powder of the precursor for the metallic phase (in other words in the quantity of metallic powder) is preferably more than or equal to 3% by weight, and more preferably between 3 and 30% by weight and typically between 5 and by weight. These preferred proportions are shown in the ternary diagram in figure 4 in the case in which the precursor of the metallic phase is composed of nickel and copper. The preferred ranges for the proportions of Ni and Cu are expressed in terms of the Ni/Cu ratio (for example the ratio 3/97 denotes 3% by weight of Ni in the metallic powder). The expression "Oxides" denotes all constituents of the precursor of the monoxide phase N and the spinel phase S; the proportions of M given in this diagram correspond to the difference to 100% with respect to the total proportion of oxides.

The initial mixture may possibly further comprise at least one element for limiting oxidation of the metallic phase of the cermet, such as silver. This anti-oxidation element is typically added in the form of powder. It may possibly be added to the initial metallic powder. The said anti-oxidation element may optionally be in oxidised form, such as in an oxide (for example Ag 2 that will be reduced during sintering. Anti-oxidation elements may be added in metallic or oxidised form at each step of the preparation of the initial mixture.

The proportion of the organic binder in the initial mixture is preferably between 0.5 and 1.5% by weight. The said binder is preferably capable of keeping the green strength of the shaped green part.

According to the invention, there is no need to use an organic binder with chemical reducer properties since the oxidised phase reduction function is performed essentially by the metallic powder (or the mixture of metallic powders) used in the initial mixture. The said binder is typically APV (polyvinyl alcohol), but .it may be any known organic or organometallic binder such as acrylic polymers, polyglycols (such as glycol polyethylene or PEG), polyvinyl acetates, polyisobutylenes, polycarbonates, polystyrenes, polyacrylates or stearates (such as stearic acid or zinc stearate).

The precursor of the monoxide and spinel phases is typically a mixture of oxides and organometallic compounds capable of forming the said phases during sintering. These oxides or compounds may possibly be added to the initial mixture separately, but it is advantageous to mix them together before adding them to the initial mixture.

The oxides and or compounds of the initial mixture, and particularly the precursor of the monoxide and spinel phases, are preferably in the form of powders. Also preferably, the size of at least 95% by weight of the grains of these powders is between 5 and um.

The precursor of the monoxide and spinel phases typically comprises a mixture of oxides comprising a nickel oxide (typically NiO) and a nickel ferrite (typically NiFe20 4 This mixture of oxides may be obtained in different manners. For example, it may be formed by a mixture of a nickel oxide (NiO) powder and a nickel ferrite (NiFe 2 0 4 powder. It may also be obtained by calcination of a mixture of nickel oxide powder and an iron oxide powder (such as Fe20 3 or Fe30 4 The said mixture of oxides is advantageously obtained by pyrolysis of iron and nickel compounds, which gives an intimate mixture of initial oxides and avoids impurities that are frequently found in industrial iron and nickel oxides. This type of process (known as "spray pyrolysis") typically includes co-precipitation of salts in aqueous solution, hot spraying of salts, grinding and calcination or presintering at a sufficiently high temperature, typically above 900 0

C.

The proportion of nickel ferrite (NiFe20 4 in the initial mixture is typically between 50 and 85% by weight, and preferably between 60 and 85% by weight.

In order to obtain satisfactory densification of the cermet, the proportion of nickel oxide in the initial mixture is typically between 0.1% by weight and 25% by weight. The ratio between the proportion by mass of nickel oxide and the proportion by mass of nickel ferrite (typically NiO/NiFe 2

O

4 is preferably between 0.2/99.8 and 30/70, and more preferably between 0.2/99.8 and 20/80.

The applicant has observed that it is important to precisely adjust the various proportions to obtain a final product with the properties required for use as an anode for the production of aluminium by electrolysis. In particular, the applicant noted the importance of the initial adjustment of the relative proportions of total iron and total nickel (in other words all phases combined) and the relative proportions of nickel oxide and nickel ferrite to obtain a final cermet with the required properties. In particular, the proportions of the precursors of the monoxide, spinel and metallic phases (for example the proportions of nickel oxide, nickel ferrite and metal) in the initial mixture, the sintering temperature and the oxygen content of the sintering atmosphere are advantageously adjusted so as to obtain the required atomic ratio between iron and nickel (Fe/Ni) in the spinel phase of the cermet. This ratio is preferably greater than or equal to 2.4, and more preferably greater than or equal to 2.8.

The initial mixture, in other words the mixture that will be shaped and sintered so as to obtain a cermet part, typically includes water and a dispersing agent in order to facilitate mixing of the constituents and shaping of green parts.

According to a preferred embodiment of the invention, the initial mixture is prepared according to a process comprising: the preparation of a slurry containing water (typically 40% by weight), a dispersing agent to prevent agglomeration of powders (preferably less than 1% by weight) and the initial powder of oxide(s); a slurry desagglomeration operation, typically by stirring, so as to obtain a determined viscosity (typically between 0.1 and 0.2 Pa.sec); addition of the powder of the metallic phase precursor and the organic binder.

The dispersing agent is preferably chemically nonreactive with copper in the metallic phase precursor.

The initial mixture is preferably dried before the shaping operation in order to eliminate water contained in it. This drying is typically done by spray drying.

The shaping operation of the green part is typically done by cold isostatic pressing, in other words by pressing at a temperature capable of preventing excessive evaporation or decomposition of the organic binder. The cold pressing temperature is typically less than 200 0 C. Pressing pressures are typically between 100 and 200 MPa.

The sintering operation of the green part (in other words the green anode or the green anode element) is typically done in a controlled atmosphere containing at least one inert gas and oxygen. The inert gas in the controlled atmosphere used during sintering is typically argon. The said controlled atmosphere preferably comprises between 10 and 200 ppm of oxygen.

A minimum oxygen content is preferable to prevent reduction of oxides in the mixture. A maximum content is useful since it avoids oxidation of the metallic powder(s).

The sintering temperature is preferably between 1150 and 1400 0 C and even better between 1300 and 1400 0 C. It is typically 1350 0 C. The holding time at the sintering temperature is not critical in the process according to the invention. This holding time is typically about two hours in order to maintain uniform sintering. After a step in which the sintering temperature is maintained, the process advantageously includes a slow cooling step at a cooling rate typically of between -100 and -100 0 /hour, between the sintering temperature and an intermediate temperature between about 900 and about 1000 0 C; slow cooling at the beginning of the cooling step increases the electrical conductivity of the anode.

The proportion of the metallic phase of the final cermet is preferably more than 15% by weight and more preferably between 15 and 30% by weight, and typically between 15 and 25% by weight. The proportion of nickel in the metallic phase is preferably greater than or equal to 3% by weight, preferably between 3 and 30% by weight and more preferably between 5 and 25% by weight, so as to increase the resistance of the metallic phase to oxidation when the cermet is used in a molten salt electrolysis process.

The proportion of spinel phase in the final cermet is preferably between 30 and 90% by weight, and typically between 40 and 90% by weight. The spinel phase is preferably non-stoechiometric in order to increase the electrical conductivity. To achieve this, the atomic ratio between iron and nickel (Fe/Ni) in the spinel phase is preferably greater than or equal to 2.4, and even more preferably greater than or equal to 2.8 The spinel phase may possibly comprise at least one substitution element capable of increasing its electrical conductivity, such as tetravalent element (Ti, The proportion of the monoxide phase in the final cermet is preferably less than 40% by weight, to give sufficient resistance of the cermet to electrochemical corrosion.

The applicant observed that, as shown in figures 2A and 2B, the cermet obtained by the process according to the invention comprises a developed spinel phase (S) that surrounds isolated particles of metallic phase (M) and forms a percolation network. The monoxide phase is discontinuous. The applicant has put forward the hypothesis that the high conductivity of the cermet originates largely from the percolation network of the spinel phase in close contact with the metallic phase.

The applicant has also put forward the hypothesis that the percolating nature of the spinel phase can only be obtained for sufficiently high Ni contents in the metallic phase, that is typically more than 5% by weight.

The porosity of the final cermet is typically less than or equal to Its electrical conductivity at a temperature of between 9000C and 10500C is preferably higher than 50 Q-1.cm- 1 and is more preferably higher than 100 Q-1.cm-.

The process according to the invention is advantageously used for the fabrication of inert anodes to be used for the production of aluminium by fused bath electrolysis.

Another object of the invention is the use of inert anodes or inert anode assemblies obtained by the fabrication process according to the invention for the production of aluminium by fused bath electrolysis. In other words, another object of the invention is a process for the production of aluminium by fused bath electrolysis comprising the use of at least one inert anode produced by the manufacturing process according to the invention.

Another object of the invention is an electrolytic cell intended for the production of aluminium by fused bath electrolysis comprising at least one inert anode produced by the manufacturing process according to the invention.

Comparative tests Batch 1 Several cermet anodes have been produced according to the prior art from mixtures of Cu, NiFe20 4 and NiO powders with proportions by weight equal to 17% Cu, 61% 4 22% NiO. The mixture was bonded by 5% by weight of APV in aqueous solution and shaped by cold isostatic pressing. The green anodes were sintered at the maximum temperature of 1350 0 C under a controlled atmosphere (residual oxygen content between 10 and 100 ppm). The density of sintered anodes was 6.10 g/cm 3 giving a residual porosity of 2.84%. The sintered material was composed of 28% by weight of a metallic phase containing 32% by weight of Ni, the proportions of the spinel phase and the monoxide phase being 45.2% and 26.7% by weight respectively. The electrical conductivity of these anodes at 1000 0 C was approximately 77 -1.cm-1.

Batch 2 Several cermet anodes were produced according to the invention from mixtures of Cu, Ni, NiFe 2 0 4 and NiO powders with the proportions by weight equal to 16% Cu, Ni, 57% NiFe 2 0 4 and 22% NiO. The mixture was bonded by 1% by weight of APV in aqueous solution and was shaped by cold isostatic pressing. The green anodes were sintered at a maximum temperature of 1350 0 C under controlled atmosphere (residual oxygen content of between 10 and 100 ppm). The density of sintered anodes was equal to 6.14, giving a residual porosity of The sintered material was composed of 24% by weight of metallic phase containing 28.5% by weight of Ni, the proportions of ferrite in the spinel phase and the monoxide phase being 40% and 36% by weight respectively. The electrical conductivity of these anodes at 10000C was approximately 48 Q- 1 .cm-.

Batch 3 Several cermet anodes were produced according to the invention from mixtures of Cu, Ni, NiFe 2 0 4 and NiO powders with the proportions by weight equal to 19% Cu, 6.4% Ni, 60% NiFe 2 04 and 14.6% NiO. The mixture was bonded by 1% by weight of APV in aqueous solution and was shaped by cold isostatic pressing. The green anodes were sintered at a maximum temperature of 1350 0

C

under a controlled atmosphere (residual oxygen content between 10 and 100 ppm). The density of the sintered anodes was 6.17, giving a residual porosity of 1.95%.

The sintered material was composed of 30.7% by weight of metallic phase containing 32% by weight of Ni, the proportions of the spinel phase and the monoxide phase being 41.6% and 27.7% by weight respectively. The electrical conductivity of these anodes at 10000C was approximately 103 1 .cm 1 Batch 4 Several cermet anodes were produced from mixtures of Cu, Ni, NiFe20 4 and NiO powders with proportions by weight equal to 21% Cu, 4% Ni, 30% NiFe 2 0 4 45% NiO.

The mixture was bonded by 1% by weight of APV in aqueous solution. The green anodes were sintered at a maximum temperature of 1200°C under controlled atmosphere (residual oxygen content between 10 and 100 ppm). The density of the sintered anodes was 6.49, giving a residual porosity of 3.57%. The sintered material was composed of 27.3% by weight of metallic phase containing 24.8% by weight of Ni, the proportions of the spinel phase and the monoxide phase being 21.7% and 51% by weight respectively. The electrical conductivity of these anodes at 1000 0 C was approximately 139 -1.cm 1 The anodes produced in batches 1, 3 and 4 were tested in a test electrolytic cell under the following conditions: electrolysis duration: 10 hours; electrolysis temperature: 960 0

C;

bath composition: cryolite bath with molar ratio NaF/AlF 3 equal to 2.2 (namely with an excess of A1F 3 of 11% by weight) saturated in alumina; electrolytic current density: 1.5 A/cm 2 Table I shows the measured corrosion rates. The "number of tests" column shows the number of anodes tested, one anode being tested in each test. The "Fe/Ni ratio" column corresponds to the Fe/Ni atomic ratio in the spinel phase S measured by X-rays (batches 1, 3 and 4) or by micro-probe (batch 2).

A comparison of the results for batches 1 and 3 shows that anodes can be fabricated according to the invention with low quantities of organic binder while keeping a low corrosion rate and obtaining a high value for the electrical conductivity when hot. A comparison of the results of batches 3 and 4 shows that an excessively high content of the NiO phase in the sintered cermet causes bad resistance to electrochemical corrosion.

Table I Batch Number of Electrical Corrosion Fe/Ni tests conductivity rate ratio at 1000 0 C (cm/an) (O-l.cm-1) 1 4 77 6.4 1 3.039 2 48 2.88 3 2 103 7.2 1.5 3.087 4 2 139 12.8 1.5 2.73

Claims

1. Process for manufacturing an inert cermet anode of the NiO-NiFe 2 O 4 -M type Scomprising at least a nickel monoxide phase N, a nickel spinel phase S containing iron and nickel, and a metallic phase M, containing copper and nickel, the process comprising: preparation of an initial mixture including at least one precursor of the In monoxide phase N and the spinel phase S, a precursor of the metallic phase M and an organic binder, the proportion of the organic binder in the initial mixture being less than by weight, the precursor of the metallic phase comprising a metallic powder containing copper and nickel, the precursor of the monoxide and spinel phases comprising a mixture of oxides comprising a nickel oxide and a nickel ferrite, the ratio between the proportion by mass of nickel oxide and the proportion by mass of nickel ferrite being between 0.2/99.8 and 30/70, a mixture shaping operation, so as to form a green anode with a determined shape, a green anode sintering operation, at a temperature higher than 900 0 C in a controlled atmosphere containing at least an inert gas and oxygen, and wherein the proportion of the monoxide phase in the final cermet is less than 40% by weight.

2. Process according to claim 1, wherein the metallic powder is a mixture of a metallic copper powder and a metallic nickel powder.

3. Process according to claim 1, wherein the metallic powder consists partly or entirely of a copper and nickel alloy.

4. Process according to any one of claims 1 to 3, wherein the size of at least by weight of the grains of the metallic powder is between 3 and 10 irm.

Process according to any one of claims 1 to 4, wherein the proportion of metallic powder in the initial mixture is more than 15% by weight.

6. Process according to any one of claims 1 to 5, wherein the proportion of nickel in the metallic powder of the precursor for the metallic phase is more than or equal to 3% by weight.

7. Process according to any one of claims 1 to 6, wherein the proportion of the organic binder in the initial mixture is between 0.5 and 1.5% by weight.

8. Process according to any one of claims 1 to 7, wherein the precursor of the monoxide and spinel phases is a powder for which the size of at least 95% by weight of the grains is between 5 and 10 pm.

9. Process according to any one of claims 1 to 8, wherein the proportion of nickel ferrite in the initial mixture is between 50 and 85% by weight.

1949369-1HJG Process according to any one of claims 1 to 8, wherein the proportion of nickel ferrite in the initial mixture is between 60 and 85% by weight. 11. Process according to any one of claims 1 to 10, wherein the proportion of nickel oxide in the initial mixture is between 0.1% by weight and 25% by weight. 12. Process according to any one of claims 1 to 11, wherein the ratio between the proportion by mass of nickel oxide and the proportion by mass of nickel ferrite is between 0.2/99.8 and 20/80. 13. Process according to any one of claims 1 to 12, wherein the proportion of the precursors of the monoxide, spinel and metallic phases in the initial mixture and the 1o sintering temperature are adjusted so as to obtain the atomic ratio between iron and nickel (Fe/Ni) in the spinel phase S of the cermet greater than or equal to 2.4. 14. Process according to any one of claims 1 to 13, wherein the shaping operation of the green part is done by cold isostatic pressing. Process according to any one of claims 1 to 14, wherein the controlled is atmosphere comprises between 10 and 200 ppm of oxygen. 16. Process according to any one of claims 1 to 15, wherein the sintering temperature is between 1150 and 1400'C. 17. Process according to any one of claims 1 to 16, wherein it includes a slow cooling step at a rate of between -10' and -100 0 /hour, between the sintering temperature and an intermediate temperature between 900 and 1000 0 C, after a step in which the sintering temperature is maintained. 18. Process according to any one of claims 1 to 17, wherein the initial mixture also comprises at least one element for limiting oxidation of the metallic phase of the cermet. 19. Process according to any one of claims 1 to 18, wherein the spinel phase also comprises at least one substitution element capable of increasing its electrical conductivity. Process according to claim 19, wherein the substitution element is a tetravalent element. 21. Process according to any one of claims 1 to 20, wherein the electrical conductivity of the cermet at a temperature of between 900C and 1050'C is higher than Rl'cm'. 22. Use of the process according to any one of claims 1 to 21, for the fabrication of inert anodes to be used for the production of aluminium by fused bath electrolysis. 1949369-1 HJG F- 22 23. Use of an inert anode or an assembly of inert anodes, obtained by the process according to any one of claims 1 to 21, for the production of aluminium by fused bath electrolysis. 24. Electrolytic cell intended for the production of aluminium by fused bath electrolysis comprising at least one inert anode produced by the manufacturing process according to any one of claims 1 to 21. Dated 2 February, 2009 Aluminium Pechiney Patent Attorneys for the Applicant/Nominated Person SPRUSON FERGUSON 1949369-I HJG