RU2336369C2 - Method of fabricating inert anode for production of aluminium by means of electrolisis in melt - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention refers to method of fabricating inert anode out of cermet NiO-NiFe₂O₄-M. The method of fabricating inert anode out of cermet NiO-NiFe₂O₄-M, containing single oxide nickel phase N, spinel nickel phase S, containing iron and nickel, and metallic phase M, containing copper and nickel, includes preparation of a source mixture containing at least a precursor of single oxide phase N and spinel phase S, a precursor of metallic phase M and organic binding with its contents in the source mixture less, than 2.0% wt; at that the precursor of the metallic phase contains a metallic powder including copper and nickel; the precursor of the single oxide and spinel phase contains mixture of oxides out of nickel oxides and nickel ferrite; ratio of weight contents of nickel oxide to weight contents of nickel ferrite lies within the range of 0.2/99.8 to 30/70; then the method includes forming this mixture for producing of a raw electrode of a certain shape, then sintering of the raw anode at the temperature above 900°C in controlled ambiance, containing inert gas and oxygen; at that contents of single oxide phase in cermet is less, than 40% in weight.

EFFECT: facilitating reduction of toxic volatile substances discharge at low supply of oxygen and continuous exhaust of binding, and curtailing chances of cracks and porosity in electrodes.

21 cl, 4 dwg, 1 tbl, 4 ex

Description

Technical field

The present invention relates to the production of aluminum by melt electrolysis. In particular, it relates to the anodes used in this production, and to the methods used for their manufacture.

State of the art

In industry, metallic aluminum is produced by melt electrolysis, namely, by electrolysis of alumina (aluminum oxide) dissolved in a bath based on molten cryolite, called an electrolyte bath, in particular using the well-known Hall-Heroult method. The electrolyte bath is contained in containers called “electrolysis baths,” including a steel casing that is lined internally with refractory and / or insulating materials, and a cathode device located at the bottom of the electrolysis bath. The anodes are partially immersed in the electrolyte bath. The term "cell" is usually called a complex containing an electrolysis bath and one or many anodes.

The electrolysis current that flows in the electrolyte bath and the liquid aluminum layer through the anodes and cathode elements causes aluminum reduction reactions and also allows the electrolyte bath to be maintained at a temperature of usually about 950 ° C due to the Joule effect. Alumina is regularly charged into the electrolysis bath in order to compensate for the consumption of alumina during electrolysis reactions.

In classical technology, the anodes are made of carbon material and are consumed due to aluminum reduction reactions. Typically, the service life of a carbon material anode is 2 to 3 weeks.

Environmental protection requirements and costs associated with the manufacture and disposal of anodes made of carbon material force manufacturers to search for ways to make anodes from non-consumable materials called “inert anodes”. Many materials have been proposed, among which, in particular, are composite materials containing a phase called “ceramic” and a metal phase. These composite materials are known as cermet (cermet).

Some cermets have been the subject of numerous studies, such as cermets, whose ceramic phase contains a mixed oxide of iron and nickel. These studies were carried out, in particular, on cermet materials, the ceramic phase of which contains a mixed phase of nickel oxide (NiO) and nickel ferrite (NiFe ₂ O ₄ ), and the metal phase of which contains, for example, iron, nickel or copper. In the future, these cermets began to be called "NiO-NiFe ₂ O ₄ -M cermets," where M denotes the metal phase.

As described, for example, in US patents US 4455211, US 4454015 and US 4582585, NiO-NiFe ₂ O ₄ -M cermets are usually obtained using a method comprising preparing a mixture of metal powders and powders of one or more iron and nickel oxides, pressing this mixture for molding raw (non-sintered) bodies of a certain shape and sintering of these raw bodies at a temperature in the range from 900 to 1500 ° C. The starting powders of iron oxide and nickel are typically a pre-baked mixture of nickel oxide (NiO) and iron oxide (typically Fe ₂ O ₃ or Fe ₃ O ₄ ).

US Pat. No. 4,871,438, issued to the Batelle Memorial Institute, discloses a manufacturing method in which said starting oxide powder is a NiO-NiFe ₂ O ₄ powder and the starting metal powder is a mixture of 10-30% by weight of copper powder and 2-4% by weight of nickel. The mass ratio of NiO to NiFe ₂ O ₄ is between 2: 3 (≈0.67) and 3: 2 (= 1.5). During sintering, copper and nickel form an alloy whose melting point is higher than the sintering temperature, which avoids bleeding out of the metal phase (in English terminology) and thus obtain a final amount of the metal phase in excess of 17% by weight. The initial mixture does not contain an organic binder. Sintering is carried out in an atmosphere of argon or nitrogen containing from 100 to 500 ppm of oxygen.

In a later US patent US 5794112, issued in the name of "Aluminum Company of America", disclosed is a method for producing cermet, in which the initial mixture contains a metal powder containing copper and / or silver and from 2 to 10 parts by weight of an organic binder, and which sintering is carried out in a controlled atmosphere of argon containing from 5 to 3000 ppm of oxygen.

However, the known manufacturing methods give rise to problems in the production of parts from cermet NiO-NiFe ₂ O ₄ -M, the metal phase M of which contains copper and nickel, in particular, in the manufacture of large parts (that is, parts whose minimum size is usually a diameter of greater than or equal to about 20 cm). The applicant encountered difficulties in regulating and satisfactorily managing the composition and relative content of all phases of the cermet. However, the composition and content mentioned affect the performance of the cermet. In addition, the removal of the products of evaporation and decomposition of the binder during sintering mainly depends on its nature, which makes the method very sensitive to the choice of binder, when its content in the initial mixture is significant (as in the method disclosed in US patent 5794112). In addition, the combination of low thermal conductivity and the large size of the wet part leads to a stop of the removal of evaporation gases and decomposition due to clogging of pores on the surface. Parts may also crack. These problems can be partially solved by lengthening the binder removal step, but such a solution significantly reduces the productivity of the method.

Therefore, the applicant has set himself the task of finding such solutions that can eliminate the disadvantages of known manufacturing methods.

Disclosure of invention

The object of the present invention is a method of manufacturing an inert anode from cermet, wherein said cermet is designated by the formula "NiO-NiFe ₂ O ₄ -M" and contains a metal phase M containing copper and nickel, and a ceramic phase C, called mixed and containing at least two different phases, namely: phase N, called the "monoxide nickel phase", and phase S, called the "spinel nickel phase"("nickelspinel").

The monoxide nickel phase N usually corresponds to the formula NiO, which may be non-stoichiometric and which, if necessary, may contain other elements than nickel, such as iron. The spinel nickel phase S usually corresponds to the formula NiFe ₂ O ₄ , which may be non-stoichiometric and which, if necessary, may contain other elements than nickel and iron.

According to the invention, the method of manufacturing an inert anode from a cermet of the type NiO-NiFe ₂ O ₄ -M containing at least a monoxide nickel phase N, a spinel nickel phase S containing iron and nickel, and a metal phase M containing copper and nickel are different in that it includes:

- preparation of the initial mixture containing at least a precursor of the monoxide phase N and spinel phase S, the precursor of the metal phase M and an organic binder, while the content of the organic binder in the initial mixture is low, namely less than 2.0% by weight, and the metal phase precursor comprises a metal powder containing copper and nickel;

- the operation of molding this mixture, as a rule, by pressing or isostatic pressing, to obtain a crude anode of a certain shape;

- the operation of sintering the crude anode at a temperature, as a rule, of more than 900 ° C and in a controlled atmosphere containing a small amount of oxygen, namely, as a rule, less than 200 ppm O ₂ .

The applicant has come up with the idea to separate the physicochemical functions performed by the binder and the precursor of the metal phase. In this context, he found that it is usually sufficient to use a small amount of an organic binder to maintain the shape (strength) of the part at the beginning of sintering (that is, to significantly limit and even prevent its deformation) and that the role of the chemical reducing agent of the binder can be ensured by adding metallic nickel to the metal phase precursor, which is preferably formed by metal powders.

This separation of the two functions — providing mechanical strength and controlling the composition of the metal phase — reduces the amount of binder and, therefore, reduces the release of toxic volatile substances with a small supply of oxygen, reduces the duration of removal of the binder and limits the risks of cracking and porosity associated with the removal of the binder in the gas phase and volatile decomposition products of the binder in large parts. The adjustment of the composition of the metal phase of the sintered material due to the addition of nickel allows not only to avoid sweating of the metal phase during sintering, but also to better control the local chemistry (chemical composition) of the ceramic and metal phases. Correction of the composition of the metal phase according to the invention also allows for better uniformity of the cermet microstructure in large parts.

The use of a small amount of organic binder can improve the reliability of the method in the industrial production of anodes (and, in a more general case, parts intended for forming anodes). In particular, it allows to reduce the sensitivity of the method to the size of the sintered parts.

Sintering leads to the migration of part of the metal elements between different phases. So, as a rule, nickel oxide is enriched in iron, nickel ferrite becomes non-stoichiometric, and the metal phase is enriched in nickel and, optionally, iron, as a rule, in smaller proportions. Therefore, the cermet obtained as a result of sintering can be more accurately expressed by the formula Ni _1-x Fe _x O _{1 ± δ-} Ni _y Fe _3-y O _{4 ± δ} -M ', where M' is an alloy including the starting metal M, iron and nickel (MFeNi). However, to simplify the terminology in the future, the phases NiO (and, in the more general case, Ni _1-x Fe _x O) and NiFe ₂ O ₄ (and, in the more general case, Ni _y Fe _3-y O ₄ ) will be denoted by the terms “monoxide phase” and “spinel phase”, respectively. In addition, the cermet will simply be denoted by the formula “NiO-NiFe ₂ O ₄ -M”, where NiO denotes the monoxide phase (N), NiFe ₂ O ₄ denotes the spinel phase (S), and M the metal phase.

Inert anodes according to the invention are intended for the production of aluminum by melt electrolysis. If necessary, they can be connected to form anode assemblies consisting of many separate anodes, such as anode blocks.

The present invention will become more apparent from the following accompanying figures and detailed description.

Figure 1 shows a preferred embodiment of a manufacturing method according to the invention.

Figa is a micrograph of a typical cermet obtained by the manufacturing method according to the invention.

Fig. 2B is a schematic reproduction of the micrograph shown in Fig. 2A.

Figure 3 is a triple diagram of NiO / NiFe ₂ O ₄ / M showing preferred regions of the starting composition in a preferred embodiment of the invention.

FIG. 4 is a truncated triple diagram of Ni / Cu / Oxides showing preferred regions of the starting composition in a preferred embodiment of the invention.

A metal powder containing copper and nickel is typically a mixture of a metal copper powder and a metal nickel powder. According to the invention, it is also possible to use a metal powder containing, in whole or in part, an alloy of copper and nickel. Preferably, at least 95% by weight of the particles (grains) of said metal powder have a size of from 3 to 10 microns.

The content of the metal powder in the starting mixture is preferably greater than 15% by mass, and even more preferably greater than 20% by mass. This content is preferably less than 35% by weight. As a rule, it is in the range from 15% to 30% by weight, and even more often from 20% to 25% by weight. These preferred contents are shown in the ternary diagram of FIG. 3 in the case where the precursor of said monoxide phase N and spinel phase S is nickel oxide NiO and nickel ferrite NiFe ₂ O ₄ .

The nickel content in the metal powder of the metal phase precursor (i.e., in the taken amount of the metal powder) is preferably greater than or equal to 3% by weight, and even more preferably is in the range of 3 to 30% by weight, and typically in the range of 5 up to 25% by weight. These preferred contents are shown in a ternary diagram in FIG. 4 in the case where the precursor of the metal phase is nickel and copper. Preferred ranges of Ni and Cu are expressed as the Ni / Cu ratio (for example, the ratio 3/97 corresponds to 3 wt.% Ni in the metal powder). The term “Oxides” means the combination of the components of the precursor of the monoxide phase N and the spinel phase S; the contents of M shown in this diagram correspond to the difference between 100% and the total oxide content.

The starting mixture may, if necessary, also contain at least one element capable of limiting the oxidation of the metal phase of the cermet, such as silver. This antioxidant element is usually added in powder form. It can, if necessary, be added to the starting metal powder. Said antioxidant element may optionally be in an oxidized form, such as an oxide (e.g., Ag ₂ O), which will be reduced during sintering. Antioxidant elements, in metallic or oxidized form, can be added at any stage in the preparation of the initial mixture.

The content of the organic binder in the feed mixture is preferably in the range of 0.5 to 1.5% by weight. Said binder is preferably capable of providing strength to the formed part in its raw form. According to the invention, it is not necessary to use an organic binder having the properties of a chemical reducing agent, since the function of reducing the oxidized phases is mainly performed by the metal powder (or a mixture of metal powders) used in the initial mixture. Said binder is typically PVA (polyvinyl alcohol), but can also be any known organic or organometallic binder, such as acrylic polymers, polyglycols (such as polyethylene glycol or PEG), polyvinyl acetates, polyisobutylene, polycarbonates, polystyrenes, polyacrylates or stearates (such as stearic acid or zinc stearate).

The precursor of the monoxide and spinel phases is usually a mixture of oxides or organometallic compounds capable of forming said phases during sintering. These oxides or compounds may, if necessary, be separately introduced into the feed mixture, but are preferably mixed with each other before being added to the feed mixture.

The oxides and / or compounds in the starting mixture, in particular the precursor of the monoxide and spinel phases, are preferably present in the form of powders. Even more preferably, at least 95% by weight of the particles of these powders have a size in the range of 5 to 10 microns.

Typically, the monoxide and spinel phase precursor contains a mixture of oxides containing nickel oxide (typically NiO) and nickel ferrite (typically NiFe ₂ O ₄ ). This mixture of oxides can be obtained in various ways. For example, it can be formed by mixing a powder of nickel oxide (NiO) and a powder of nickel ferrite (NiFe ₂ O ₄ ). It can also be obtained by calcining a mixture of nickel oxide powder and iron oxide powder (such as Fe ₂ O ₃ or Fe ₃ O ₄ ).

The above-mentioned mixture of oxides is mainly obtained by pyrolysis of iron and nickel compounds, which allows to obtain a homogeneous (thin) mixture of the starting oxides and to avoid the ingress of impurities, which are often present in industrial iron and nickel oxides. This method (known as “spraying and pyrolysis”, from the English “spray pyrolysis”) usually involves the joint precipitation of salts in an aqueous solution, spraying (dispersing) these salts at high temperature, grinding and firing, or “chamotte” (from French “Chamottage”) at a sufficiently high temperature, usually in excess of 900 ° C.

The content of Nickel ferrite (NiFe ₂ O ₄ ) in the initial mixture is usually from 50 to 85% by weight, and preferably from 60 to 85% by weight. To achieve sufficient cermet compaction, the nickel oxide content in the feed mixture is typically in the range of 0.1% by weight to 25% by weight. The ratio of the mass content of Nickel oxide to the mass content of Nickel ferrite (usually NiO / NiFe ₂ O ₄ ) is preferably in the range from 0.2 / 99.8 to 30/70, and even more preferably from 0.2 / 99.8 to 20 / 80.

The applicant has determined that it is important to accurately adjust the various contents to obtain a final product having the necessary properties for use as an anode in the production of aluminum by electrolysis. In particular, he noted the importance of initially adjusting the relative contents of the total amount of iron and the total amount of nickel (that is, in any phases combined) and the relative contents of nickel oxide and nickel ferrite to obtain the final cermet with the desired properties. In particular, the contents of the monoxide, spinel and metal phase precursors (for example, the content of nickel oxide, nickel ferrite and metal) in the initial mixture, the sintering temperature and the oxygen content in the sintering atmosphere are mainly adjusted so as to obtain the necessary atomic ratio of iron to nickel (Fe / Ni) in the spinel phase of the cermet. Preferably, this ratio is greater than or equal to 2.4, and even more preferably greater than or equal to 2.8.

The initial mixture, that is, a mixture intended for molding and sintering to form a part from cermet, typically contains water and a dispersant to facilitate mixing of the components and molding of the raw parts.

According to a preferred embodiment of the invention, the initial mixture is prepared using a method including:

- the preparation of a slip containing water (typically 40% by weight), a dispersant capable of preventing the formation of powder agglomerates (preferably less than 1% by weight), and the initial oxide powder (s);

- a slip deagglomeration operation, usually by stirring, so as to obtain a certain viscosity (usually from 0.1 to 0.2 Pa · sec);

- adding a metal phase precursor powder and an organic binder.

The dispersant is preferably not capable of reacting chemically with copper from the metal phase precursor.

Before the molding operation, the feed mixture is preferably dried to remove the water contained therein. This drying is usually carried out by spraying (spray drying, from the English "spray drying").

The operation of forming a wet part is usually carried out by cold isostatic pressing, that is, pressing at a temperature that avoids excessive evaporation or decomposition of the organic binder. Typically, the cold pressing temperature is less than 200 ° C. Pressing pressures typically range from 100 to 200 MPa.

The sintering operation of a crude component (i.e., a crude anode or a raw anode element) is usually carried out in a controlled atmosphere containing at least one inert gas and oxygen. Typically, the inert gas in the controlled atmosphere used in sintering is argon. Said controlled atmosphere preferably contains from 10 to 200 ppm oxygen. A minimum oxygen content is preferred to prevent the reduction of oxides in the mixture. The maximum content is predominantly that avoids the oxidation of metal powder or metal powders.

The sintering temperature is preferably in the range from 1150 to 1400 ° C., and even more preferably from 1300 to 1400 ° C. As a rule, it is 1350 ° C. The exposure time at sintering temperature is not critical in the method according to the invention. Typically, this holding time is approximately two hours in order to ensure uniformity of sintering. After the holding step at the sintering temperature, the method preferably includes a slow cooling step with a cooling rate typically from -10 ° to -100 ° / h from the sintering temperature to an intermediate temperature ranging from about 900 to about 1000 ° C; slow cooling at the beginning of the cooling stage allows to increase the conductivity of the anode.

The content of the metal phase in the final cermet is preferably more than 15% by mass, even more preferably from 15 to 30% by mass, and typically ranges from 15 to 25% by mass. The nickel content in the metal phase is preferably greater than or equal to 3% by mass, preferably in the range of 3 to 30% by mass, and even more preferably in the range of 5 to 25% by mass in order to increase the resistance of the metal phase to oxidation during use in the process electrolysis in molten salt.

The content of the spinel phase in the final cermet is preferably from 30 to 90% by weight and, as a rule, is in the range from 40 to 90% by weight. The spinel phase is preferably non-stoichiometric in order to increase the electrical conductivity. To this end, the atomic ratio of iron to 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, if necessary, contain at least one substitution element capable of increasing its conductivity, such as a tetravalent element (Ti, Zr, etc.).

The content of the monoxide phase in the final cermet is preferably less than 40% by mass to ensure sufficient cermet resistance to electrochemical corrosion.

The applicant has found that, as can be seen from figa and 2B, the cermet obtained using the method according to the invention contains a developed spinel phase (S), which surrounds the islands of the metal phase (M) and forms a percolation lattice (grid). The monoxide phase (N) is an intermittent (discrete) phase. The applicant hypothesized that the increased conductivity of the cermet is mainly due to the percolation lattice of the spinel phase, which is in close contact with the metal phase. He also hypothesized that the percolation nature of the spinel phase can only be achieved with sufficient Ni contents in the metal phase, typically exceeding 5% by weight.

The porosity of the final cermet is typically less than or equal to 5%. Its conductivity at a temperature of from 900 ° C to 1050 ° C is preferably more than 50 Ohm ^-1 · cm ^-1 , and even more preferably more than 100 Ohm ^-1 · cm ^-1 .

The method according to the invention is mainly used for the manufacture of inert anodes intended for the production of aluminum by melt electrolysis.

The invention also relates to the use of inert anodes or assemblies of inert anodes obtained by the manufacturing method according to the invention for the production of aluminum by melt electrolysis. In other words, an object of the present invention is also a method for producing aluminum by melt electrolysis, comprising the use of at least one inert anode obtained by the manufacturing method according to the invention.

The object of the invention is also an electrolyzer intended for the production of aluminum by melt electrolysis, containing at least one inert anode obtained by the manufacturing method according to the invention.

Comparative tests

Party 1

Several cermet anodes were made in accordance with the prior art, based on mixtures of powders of Cu, NiFe ₂ O ₄ and NiO, taken in the following proportions (by weight): 17% Cu, 61% NiFe ₂ O ₄ , 22% NiO. To this mixture was added 5% by weight of PVA in an aqueous solution and molding was carried out by cold isostatic pressing. Raw anodes were sintered at a maximum temperature of 1350 ° C in a controlled atmosphere (residual oxygen content from 10 to 100 ppm). The density of the sintered anodes was 6.10 g / cm ³ , i.e., the residual porosity was 2.84%. The sintered material consisted of 28% by weight of the metal phase containing 32% by weight of Ni, while the contents of the spinel phase and the monoxide phase were 45.2% and 26.7% by weight, respectively. The electrical conductivity of these anodes at 1000 ° C was approximately 77 Ohm ^-1 · cm ^-1 .

Party 2

Several cermet anodes were made in accordance with the present invention, based on mixtures of powders of Cu, Ni, NiFe ₂ O ₄ and NiO, taken in the following proportions (by weight): 16% Cu, 5% Ni, 57% NiFe ₂ O ₄ and 22% NiO. To this mixture was added 1% by weight of PVA in an aqueous solution and molding was carried out by cold isostatic pressing. Raw anodes were sintered at a maximum temperature of 1350 ° C in a controlled atmosphere (residual oxygen content from 10 to 100 ppm). The density of the sintered anodes was 6.14 g / cm ³ , i.e., the residual porosity was 2%. The sintered material consisted of 24% by weight of the metal phase containing 28.5% by weight of Ni, while the contents of the spinel phase and the monoxide phase were 40% and 36% by weight, respectively. The conductivity of these anodes at 1000 ° C was approximately 48 Ohm ^-1 · cm ^-1 .

Party 3

Several cermet anodes were made in accordance with the present invention based on mixtures of powders of Cu, Ni, NiFe ₂ O ₄ and NiO, taken in the following proportions (by weight): 19% Cu, 6.4% Ni, 60% NiFe ₂ O ₄ and 14.6% NiO. To this mixture was added 1% by weight of PVA in an aqueous solution and molding was carried out by cold isostatic pressing. Raw anodes were sintered at a maximum temperature of 1350 ° C in a controlled atmosphere (residual oxygen content from 10 to 100 ppm). The density of the sintered anodes was 6.17 g / cm ³ , i.e., the residual porosity was 1.95%. The sintered material consisted of 30.7% by weight of the metal phase containing 32% by weight of Ni, while the contents of the spinel phase and the monoxide phase were 41.6% and 27.7% by weight, respectively. The conductivity of these anodes at 1000 ° C was approximately 103 Ohm ^-1 · cm ^-1 .

Party 4

Several cermet anodes were made based on mixtures of powders of Cu, Ni, NiFe ₂ O ₄ and NiO, taken in the following proportions (by weight): 21% Cu, 4% Ni, 30% NiFe ₂ O ₄ , 45% NiO. To this mixture was added 1% by weight of PVA in an aqueous solution. Raw anodes were sintered at a maximum temperature of 1200 ° C in a controlled atmosphere (residual oxygen content from 10 to 100 ppm). The density of the sintered anodes was 6.49 g / cm ³ , that is, the residual porosity was 3.57%. The sintered material consisted of 27.3% by weight of the metal phase containing 24.8% by weight of Ni, while the contents of the spinel phase and the monoxide phase were 21.7% and 51% by weight, respectively. The electrical conductivity of these anodes at 1000 ° C was approximately 139 Ohm ^-1 · cm ^-1 .

Anodes manufactured in batches 1, 3 and 4 were tested in an experimental electrolyzer under the following conditions:

- duration of electrolysis: 10 hours;

- electrolysis temperature: 960 ° C;

- the composition of the electrolyte bath: cryolite bath with a molar ratio of NaF / AlF ₃ equal to 2.2 (that is, with an excess of AlF ₃ in 11% by weight), saturated with alumina;

- electrolysis current density: 1.5 A / cm ² .

The measured values of the corrosion rate are given in the table. The column "number of tests" corresponds to the number of anodes tested, with each test being carried out on one anode. The column "Fe / Ni ratio" corresponds to the atomic ratio Fe / Ni in the spinel phase S, measured by the X-ray method (parties 1, 3 and 4) or using a microanalyzer (party 2).

A comparison of the results for parties 1 and 3 shows that the anodes can be made according to the present invention using small amounts of an organic binder while maintaining a low corrosion rate and achieving a high hot conductivity. A comparison of the results for batches 3 and 4 shows that too high a NiO phase content in the sintered cermet leads to poor resistance to electrochemical corrosion.

Table The consignment Number of tests Electrical conductivity at 1000 ° С (Ohm ^-1 · cm ^-1 ) Corrosion Rate (cm / year) Fe / Ni Ratio one four 77 6.4 ± 1 3,039 2 - 48 - 2.88 3 2 103 7.2 ± 1.5 3,087 four 2 139 12.8 ± 1.5 2.73

Claims (21)

1. A method of manufacturing an inert anode from cermet type NiO-NiFe ₂ O ₄ -M containing at least monoxide nickel phase N, spinel nickel phase S containing iron and nickel, and a metal phase M containing copper and nickel, including preparing the original a mixture containing at least a precursor of said monoxide phase N and a spinel phase S, a precursor of the metal phase M and an organic binder, wherein the content of the organic binder in the initial mixture is less than 2.0% by weight, the metal precursor phase contains a metal powder containing copper and nickel, the monoxide and spinel phase precursor contains a mixture of oxides containing nickel oxide and nickel ferrite, the ratio of the mass content of nickel oxide to the mass content of nickel ferrite is in the range from 0.2 / 99.8 to 30 / 70, molding this mixture to obtain a raw anode of a certain shape, sintering the raw anode at a temperature of more than 900 ° C in a controlled atmosphere containing at least one inert gas and oxygen, while the content of the monoxide phase cermet is less than 40% by weight. 2. The method according to claim 1, characterized in that said metal powder is a mixture of metallic copper powder and metallic nickel powder. 3. The method according to claim 1, characterized in that said metal powder contains, in whole or in part, an alloy of copper and nickel. 4. The method according to any one of claims 1 to 3, characterized in that at least 95% by weight of the particles of said metal powder have a size of from 3 to 10 microns. 5. The method according to any one of claims 1 to 3, characterized in that the content of the metal powder in the initial mixture is more than 15% by weight, more preferably more than 20% by weight. 6. The method according to any one of claims 1 to 3, characterized in that the nickel content in the metal powder of the metal phase precursor is greater than or equal to 3% by weight, more preferably in the range of 3 to 30% by weight. 7. The method according to any one of claims 1 to 3, characterized in that the content of the organic binder in the initial mixture is in the range from 0.5 to 1.5% by weight. 8. The method according to any one of claims 1 to 3, characterized in that the precursor of the monoxide and spinel phases is a powder of at least 95% by weight of particles of which have a size of from 5 to 10 microns. 9. The method according to any one of claims 1 to 3, characterized in that the nickel ferrite content in the initial mixture is in the range from 50 to 85% by weight. 10. The method according to any one of claims 1 to 3, characterized in that the content of Nickel ferrite in the initial mixture is in the range from 60 to 85% by weight. 11. The method according to any one of claims 1 to 3, characterized in that the content of Nickel oxide in the initial mixture is in the range from 0.1 to 25% by weight. 12. The method according to any one of claims 1 to 3, characterized in that the ratio of the mass content of nickel oxide to the mass content of nickel ferrite is in the range from 0.2 / 99.8 to 30/70, more preferably from 0.2 / 99, 8 to 20/80. 13. The method according to any one of claims 1 to 3, characterized in that the content of the monoxide, spinel and metal phase precursors in the initial mixture and the sintering temperature are controlled to obtain an atomic ratio of iron to nickel (Fe / Ni) in the spinel phase S of the cermet exceeding or equal to 2.4, preferably greater than or equal to 2.8. 14. The method according to any one of claims 1 to 3, characterized in that the raw anode is molded by cold isostatic pressing. 15. The method according to any one of claims 1 to 3, characterized in that said controlled atmosphere contains from 10 to 200 ppm of oxygen. 16. The method according to any one of claims 1 to 3, characterized in that the sintering temperature is in the range from 1150 to 1400 ° C, more preferably from 1300 to 1400 ° C. 17. The method according to any one of claims 1 to 3, characterized in that after exposure to the sintering temperature, slow cooling is carried out at a rate in the range from -10 to -100 ° / h, from the sintering temperature to an intermediate temperature in ranges from 900 to 1000 ° C. 18. The method according to any one of claims 1 to 3, characterized in that the initial mixture further comprises at least one element capable of limiting the oxidation of the metal phase of the cermet, such as silver. 19. The method according to any one of claims 1 to 3, characterized in that the spinel phase further comprises at least one substitution element capable of increasing its electrical conductivity. 20. The method according to claim 19, characterized in that said substitution element is a tetravalent element. 21. The method according to any one of claims 1 to 3, characterized in that the specific conductivity of the cermet at a temperature in the range from 900 to 1050 ° C is more than 50 Ohm ^-1 · cm ^-1 , more preferably more than 100 Ohm ^-1 · cm ^-1 .

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