Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
  • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes
  • C25C3/12—Anodes

Definitions

• the invention relates to the production of aluminum by igneous electrolysis. It relates more particularly to the anodes used for this production and the manufacturing processes which make it possible to obtain them.
• Aluminum metal is produced industrially by igneous electrolysis, namely by electrolysis of alumina in solution in a bath based on molten cryolite, called electrolyte bath, in particular according to the well-known Hall-Héroult process.
• the electrolyte bath is contained in cells, called “electrolysis cells”, comprising a steel box, which is coated internally with refractory and / or insulating materials, and a cathode assembly located at the bottom of the cell. Anodes are partially immersed in the electrolyte bath.
• electrolysis cell normally designates the assembly comprising an electrolysis cell and one or more anodes.
• the electrolysis current which circulates in the electrolyte bath and the liquid aluminum sheet via the anodes and cathode elements, operates the aluminum reduction reactions and also makes it possible to maintain the bath.
• electrolyte at a temperature typically of the order of 950 ° C. by the Joule effect.
• the electrolysis cell is regularly supplied with alumina so as to compensate for the consumption of alumina produced by the electrolysis reactions.
• anodes are made of carbon material and are consumed by the reduction reactions of aluminum.
• the typical lifespan of an anode made of carbonaceous material is 2 to 3 weeks.
• the environmental constraints and costs associated with the manufacture and use of anodes made of carbonaceous material have, for many decades, led aluminum producers to seek anodes made of non-consumable materials, called “inert anodes”.
• Several materials have been proposed, including in particular composite materials containing a so-called “ceramic” phase and a metallic phase. These composite materials are known by the name "cermet".
• cermet materials have been the subject of numerous studies, such as cermet materials whose ceramic phase contains a mixed oxide of iron and nickel. These studies particularly focused on cermet materials whose ceramic phase contains a mixed phase of nickel oxide (NiO) and nickel ferrite (NiFe 2 O 4 ) and whose 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.
• NiO-NiFe 2 O 4 -M cermets are typically obtained by a process comprising the 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 of determined shape and sintering of the green body at a temperature between 900 and 1500 ° C.
• the initial iron oxide and nickel powders are typically a precalcined mixture of nickel oxide (NiO) and iron oxide (typically Fe 2 O 3 or Fe 3 O 4 ).
• the initial oxide powder is a NiO-NiFe 2 O 4 powder and the initial metal powder consists of a mixture from 10 to 30% by weight of copper powder and from 2 to 4% by weight of nickel.
• Copper and nickel form, during sintering, an alloy whose melting temperature is 2004/082355
• the initial mixture does not include an organic binder.
• the sintering is carried out in an argon or nitrogen atmosphere containing from 100 to 500 ppm of oxygen.
• the subject of the invention is a process for manufacturing an inert anode in a cermet, said cermet being designated by the formula “NiO-NiFe 2 O -M” and comprising a metallic phase M including copper and nickel, and a phase ceramic C, known as mixed, comprising at least two distinct phases, namely a phase N known as “nickel monoxide” and a phase S known as “nickel spinel”.
• the nickel monoxide N phase typically corresponds to the formula NiO, which may be non-stoichiometric and which may optionally include elements other than nickel, such as iron.
• the nickel S spinel phase typically corresponds to the formula NiFe 2 O 4 which may be non-stoichiometric and which may optionally include elements other than nickel and iron.
• the method of manufacturing an inert anode in a cermet of the NiO-NiFe 2 O -M type comprising at least one nickel monoxide phase N, a spinel phase of nickel S, containing iron and nickel, and a metallic phase M, containing copper and nickel is characterized in that it comprises: - the preparation of an initial mixture comprising at least one precursor of the monoxide N and spinel S phases, a precursor of the metallic phase M and an organic binder, the proportion of organic binder in the initial mixture being low, namely less than 2.0% by weight, and the precursor of the metallic phase comprising a metallic powder containing copper and nickel, - a setting operation in the form of the mixture, typically by pressing or isostatic compression, so as to form a green anode of determined shape, - a sintering operation of the green anode, at a temperature typically above 900 ° C. and in a cont Role containing a small amount of oxygen, typically less than 200 ppm of O
• the Applicant has had the idea of dissociating the physicochemical functions fulfilled by the binder and the precursor of the metallic phase.
• the nickel oxide is enriched in iron
• the nickel ferrite becomes non-stoichiometric
• the metallic phase is enriched in nickel and possibly in iron, generally in smaller proportions. Consequently, the cermet resulting from sintering can be more precisely described by the formula Ni 1-x Fe x O ⁇ ⁇ ⁇ - NiyFe 3 - y O + ⁇ -rVr, where M 'is an alloy including the initial metal M, iron and nickel (MFeNi).
• NiO and more generally Ni ⁇ -x Fe x O and NiFe 2 O (and more generally Ni y Fe 3- yO 4 ) phases will be, by the continuation, simply called respectively by the expressions “monoxide phase” and “spinel phase”.
• the cermet will simply be designated by the formula "NiO-NiFe 2 O 4 -M", where NiO denotes the monoxide phase (N), NiFe 2 O denotes the spinel phase (S), and M the metallic phase.
• the inert anodes according to the invention are intended for the production of aluminum by igneous electrolysis. They can optionally be assembled to form anode assemblies comprising several individual anodes, such as clusters.
• FIG. 1 represents a preferred embodiment of the manufacturing method of the invention.
• Figure 2A is a micrograph of a typical cermet obtained by the manufacturing process of the invention.
• Figure 2B is a schematic reproduction of the micrograph of Figure 2A.
• FIG. 3 is a ternary NiO / NiFe 2 O 4 / M diagram showing the domains of preference of the initial composition in a preferred embodiment of the invention.
• Figure 4 is a truncated Ni / Cu / Oxide ternary diagram showing the domains of preference of 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. It is also possible according to the invention to use a metal powder comprising, in all or part, an alloy of copper and nickel. Preferably, at least 95% by weight of the grains of said metallic powder have a size of between 3 and 10 ⁇ m.
• the proportion of metallic powder in the initial mixture is preferably greater than 15% by weight, and more preferably still greater 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 represented in the ternary diagram of FIG. 3 in the case where the precursor of said monoxide N and spinel S phases consists of nickel oxide NiO and nickel ferrite NiFe 2 O.
• the proportion of nickel in the metallic powder of the precursor of the metallic phase is preferably greater than or equal to 3% by weight, more preferably between 3 and 30%. by weight, and typically between 5 and 25% by weight.
• These preferred proportions are represented in the ternary diagram of FIG. 4 in the case where the precursor of the metallic phase consists of nickel and copper.
• the preferred ranges of the proportions of Ni and Cu are expressed in terms of Ni / Cu ratio (for example, the ratio 3/97 corresponds to 3% by weight Ni in the metal powder).
• the expression “oxides” designates all of the constituents of the precursor of the monoxide N and spinel S phases; the proportions of M given in this diagram correspond to the difference of 100% with respect to the total proportion of the oxides.
• the initial mixture may optionally further comprise at least one element capable of limiting the oxidation of the metallic phase of the cermet, such as silver.
• This anti-oxidation element is typically added in the form of a powder. It can optionally be added to the initial metal powder.
• Said antioxidant element can optionally be in an oxidized form, such as in an oxide (for example Ag 2 O), which will be reduced during sintering.
• the anti-oxidation elements, in metallic or oxidized form can be added at any stage of the preparation of the initial mixture.
• the proportion of organic binder in the initial mixture is preferably between 0.5 and 1.5% by weight. Said binder is preferably capable of ensuring that the shaped part is held bare.
• an organic binder having chemical reducing properties because the reduction function of the oxidized phases is essentially ensured by the metallic powder (or the mixture of metallic powders) used in the initial mix.
• Said binder is typically PVA (polyvinyl alcohol), but can be any known organic or organometallic binder, such as acrylic polymers, polyglycols (such as polyethylene glycol 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 or organometallic compounds capable of forming said phases during sintering. These oxides or compounds can optionally be added separately to the initial mixture, but it is advantageous to mix them together before adding them to the initial mixture.
• the oxides and / or compounds of the initial mixture, in particular the precursor of the monoxide and spinel phases, are preferably in the form of powders. More preferably, at least 95% by weight of the grains of these powders have a size of between 5 and 10 ⁇ m.
• 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 NiFe 2 O).
• This mixture of oxides can be obtained in different ways. For example, it can be formed by mixing a nickel oxide powder (NiO) and a nickel ferrite powder (NiFe 2 O 4 ). It can also be obtained by calcination of a mixture of a nickel oxide powder and an iron oxide powder (such as Fe 2 O 3 or Fe 3 O 4 ). Said mixture of oxides is advantageously obtained by pyrolysis of iron and nickel compounds, which makes it possible to obtain an intimate mixture of the initial oxides and to avoid the impurities which are frequently found in industrial iron and nickel oxides.
• Such a process typically comprises the co-precipitation of salts in aqueous solution, a high temperature spraying of the salts, grinding and calcination or chamotte at a sufficient temperature, typically above 900 ° C.
• the proportion of nickel ferrite (NiFe 2 O 4 ) in the initial mixture is typically between 50 and 85% by weight, and more 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 mass proportion of nickel oxide and the mass proportion of nickel ferrite (typically NiO / NiFe 2 O 4 ) is preferably between 0.2 / 99.8 and 30/70, and more preferably 0 , 2 / 99.8 and 20/80.
• the Applicant has found that it is important to precisely adjust the different proportions in order to obtain a final product having the desired properties for use as an anode for the production of aluminum by electrolysis.
• she noted the importance of the initial adjustment of the relative proportions of total iron and total nickel (i.e. all phases combined) and the relative proportions of nickel oxide and ferrite. nickel to obtain a final cermet having the desired properties.
• the proportions of the precursors of the monoxide, spinel and metallic phases for example, the proportions of nickel oxide, nickel ferrite and metal
• the sintering temperature and the oxygen content of the The sintering atmosphere is advantageously adjusted so as to obtain the desired 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 still greater than or equal to 2.8.
• the initial mixture typically comprises water and a dispersant to facilitate the mixing of the constituents and the shaping of the raw parts.
• the initial mixture is prepared according to a process comprising:
• the dispersant is preferably capable of not reacting chemically with the copper of the precursor of the metallic phase.
• the initial mixture is preferably dried before the shaping operation in order to remove the water which it contains. This drying is typically carried out by spray drying.
• the operation of shaping the green part is typically carried out by cold isostatic pressing, that is to say by pressing at a temperature capable of avoiding excessive evaporation or decomposition of the organic binder.
• the cold pressing temperature is typically less than 200 ° C.
• the pressing pressures are typically between 100 and 200 MPa.
• the sintering operation of the green part is typically carried out in a controlled atmosphere containing at least one inert gas and oxygen .
• the inert gas of the controlled atmosphere used during sintering is typically argon.
• Said controlled atmosphere preferably comprises between 10 and 200 ppm of oxygen.
• a minimum oxygen content is preferable in order to avoid reduction of the oxides in the mixture.
• a maximum content is advantageous because it avoids the oxidation of the metal powder (s).
• the sintering temperature is preferably between 1150 and 1400 ° C and more preferably between 1300 and 1400 ° C. It is typically 1350 ° C.
• the holding time at the sintering temperature is not critical in the process of the invention. This holding time is typically about two hours in order to ensure the homogeneity of the sintering.
• the method advantageously comprises a step of slow cooling, at a cooling rate typically between -10 ° and -100 ° / hour, between the sintering temperature and an intermediate temperature between about 900 and about 1000 ° C; slow cooling, at the start of the cooling step, increases the electrical conductivity of the anode.
• the proportion of the metallic phase of the final cermet is preferably greater than 15% by weight, 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 to metal phase oxidation when using cermet 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-stoichiometric in order to increase the electrical conductivity.
• the atomic ratio between iron and nickel (Fe / Ni) in the spinel phase is preferably greater than or equal to 2.4, and more preferably still greater than or equal to 2.8.
• the spinel phase can optionally include at least one substitution element capable of increasing its electrical conductivity, such as a tetravalent element (TL Zr, ).
• TL Zr tetravalent element
• the proportion of monoxide phase in the final cermet is preferably less than 40% by weight, in order to obtain sufficient resistance of the cermet to electrochemical corrosion.
• the cermet obtained by the method of the invention comprises a developed spinel phase (S) which surrounds the islets of metallic phase (M) and forms a percolation network.
• the monoxide phase (N) is discontinuous.
• the Applicant hypothesizes that the high conductivity of the cermet comes largely from the percolation network of the spinel phase in close contact with the metallic phase. It also hypothesizes that the percolating nature of the spinel phase can only be obtained for sufficient Ni contents in the metallic phase, typically greater than 5% by weight.
• the porosity of the final cermet is typically less than or equal to 5%. Its electrical conductivity at a temperature between 900 ° C and 1050 ° C is preferably greater than 50 ⁇ ⁇ .cm "1 , and more preferably still greater than 100 ⁇ ⁇ .cm " 1 .
• the method according to the invention is advantageously used for the manufacture of inert anodes intended for the production of aluminum by igneous 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 igneous electrolysis.
• the subject of the invention is also a process for the production of aluminum by igneous electrolysis comprising the use of at least one inert anode produced by the manufacturing process according to the invention.
• the invention also relates to an electrolysis cell intended for the production of aluminum by igneous electrolysis comprising at least one inert anode produced by the manufacturing process according to the invention.
• cermet anodes have been developed according to the prior art from mixtures of Cu powders, NiFe 2 O and NiO having the following proportions (by weight): 17% Cu, 61% NiFe 2 O 4 , 22% NiO.
• the mixture was bound by 5% by weight of PVA in aqueous solution and shaped by cold isostatic pressing.
• the raw anodes were sintered at the maximum temperature of 1350 ° C. under a controlled atmosphere (residual oxygen content between 10 and 100 ppm).
• the density of the sintered anodes was 6.10 g / cm 3 , ie a residual porosity of 2.84%.
• the sintered material consisted of 28% by weight of metallic phase containing 32% by weight of Ni, the proportions of spinel phase and of monoxide phase being respectively 45.2% and 26.7% by weight.
• the electrical conductivity of these anodes at 1000 ° C was approximately 77 ⁇ ⁇ 1 .cm ⁇ 1 .
• cermet anodes were developed according to the invention from mixtures of Cu, Ni, NiFe 2 O 4 and NiO powders having the following proportions (by weight): 16% Cu, 5% Ni, 57% NiFe 2 O 4 and 22% NiO.
• the mixture was bound by 1% by weight of PVA in aqueous solution and shaped by cold isostatic pressing.
• the raw anodes were sintered at the maximum temperature of 1350 ° C. under a controlled atmosphere (residual oxygen content of between 10 and 100 ppm).
• the density of the sintered anodes was 6.14, ie a residual porosity of 2%.
• the sintered material consisted of 24% by weight of metallic phase containing 28.5% by weight of Ni, the proportions of ferrite of spinel phase and of monoxide phase being respectively 40% and 36% by weight.
• the electrical conductivity of these anodes at 1000 ° C was approximately 48 ⁇ ' cm "1 . Lot 3
• cermet anodes were developed according to the invention from mixtures of Cu, Ni, NiFe 2 O and NiO powders having the following proportions (by weight): 19% Cu, 6.4% Ni, 60 % NiFe 2 O and 14.6% NiO.
• the mixture was bound by 1% by weight of PVA in aqueous solution and shaped by cold isostatic pressing.
• the raw anodes were sintered at the maximum temperature of 1350 ° C. under a controlled atmosphere (residual oxygen content of between 10 and 100 ppm).
• the density of the sintered anodes was 6.17, ie a residual porosity of 1.95%.
• the sintered material consisted of 30.7% by weight of metallic phase containing 32% by weight of Ni, the proportions of spinel phase and of monoxide phase being respectively 41.6% and 27.7% by weight.
• the electrical conductivity of these anodes at 1000 ° C was approximately 103 ⁇ "1 .cm " 1 .
• cermet anodes have been produced from mixtures of Cu, Ni, NiFe 2 O 4 and NiO powders with the following proportions (by weight): 21% Cu, 4% Ni, 30% NiFe 2 O 4 , 45% NiO.
• the mixture was bound by 1% by weight of PVA in aqueous solution.
• the raw anodes were sintered at the maximum temperature of 1200 ° C. under a controlled atmosphere (residual oxygen content of between 10 and 100 ppm).
• the density of the sintered anodes was 6.49, ie a residual porosity of 3.57%.
• the sintered material consisted of 27.3% by weight of metallic phase containing 24.8% by weight of Ni, the proportions of spinel phase and of monoxide phase being respectively 21.7% and 51% by weight.
• the electrical conductivity of these anodes at 1000 ° C was approximately 139 ⁇ "1 .cm " 1 .
• the anodes produced in lots 1, 3 and 4 were tested in a test electrolysis cell under the following conditions: - duration of the electrolysis: 10 hours; - electrolysis temperature: 960 ° C; - composition of the bath: cryolite bath with NaF / AlF 3 molar ratio equal to 2.2 (ie with an AlF 3 excess of 11% by weight) saturated with alumina;
• the measured corrosion rates are given in Table I.
• the column “number of tests” corresponds to the numbers of anodes tested, one anode being tested at each test.
• the column “Fe / Ni ratio” corresponds to the atomic Fe / Ni ratio in the spinel phase S measured by X-rays (lots 1, 3 and 4) or by microprobe (lot 2).
• the comparison of the results on batches 1 and 3 shows that the anodes can be manufactured according to the invention with small quantities of organic binder while maintaining a low corrosion rate and while obtaining a high value for the hot electrical conductivity.
• the comparison of the results of lots 3 and 4 shows that an excessive content of NiO phase in the sintered cermet leads to poor resistance to electrochemical corrosion.

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