PL157722B1 - Method for eletrowinning of metals and anode for elektrowinning of metals - Google Patents

Abstract

1. An anode for use in obtaining metals, specifically aluminum, electrolytically from igneous bath, specifically of a fluoride-based salt, containing a metal, alloy or ceramet base, having a protective anodic active surface which, when being used, is secured by keeping in the molten salt a suitable concentration of a compound of a metal that is less precious than the metal being obtained, wherein on the metal-, alloy- or ceramet-base surface there is an electrolytically conductive oxygen-blocking layer, the layer containing chromium oxide and containing at least one of the elements of the group comprising platinum, palladium or gold, or containing platinum-zirconium alloys or nickel-aluminum alloys having been selected as the said oxygen-blocking layer, furthermore the anode having been coated with a ceramic-oxide layer between its protective coating and the oxygen-barrier layer, the said ceramic-oxide layer holding the protective coating is selected from a group of compounds comprising copper oxide in solid solution with at least one additional oxide, further comprising nickel ferrite, copper oxide and nickel ferrite, non-stoichiometrically doped or partly substituted spinels, and oxides of rare-earth metals, or oxyfluorides.

Description

@ PATENT DESCRIPTION ® EN © 157722 © BI

REPUBLIC

POLAND

Application number: 275618 © IntCl ⁵ :

C25C 3/12

Patent Office of the Republic of Poland

Zgłoszema Date: ⁸⁸ 0 ⁴ .11.19

CZHEUH t nfllH1

Anode for the electrowinning of metals

The holder of the patent:

Moltech lnvent S.A., Luxembourg, LU

Application announced:

May 14, 1990 BUP 10/90

Inventors:

Thinh Nguyen, Geneva, CH Abdelkrim Lazouni, Geneva, CH Kim Son Doan, Geneva, CH

The following was announced about the grant of the patent:

June 30, 1992 WUP 06/92

Proxy:

PHZ '' Polservice, Warsaw, PL

157722 BI

Λ 1. An anode for the electrowinning of metals, especially aluminum, from electrolytes of molten salt, especially fluoride-based salt, containing a metal, alloy or cermet substrate, having a protective active anode surface, which is preserved during use by maintaining the appropriate concentration in the molten salt a compound of a metal less noble than the obtained metal, characterized in that it has an electronically conductive oxygen barrier layer on the surface of the substrate made of metal, alloy or cermet, the oxygen barrier layer being selected as a layer containing chromium oxide and containing at least one of the elements including platinum, palladium or gold or containing platinum-zirconium alloys or nickel-aluminum alloys, the anode also having a pre-deposited ceramic oxide layer between the protective coating and the oxygen barrier layer, and the ceramic oxide layer serving as a support for the protective coating and is selected and from the group of compounds consisting of copper oxide in solid solution with at least one additional oxide, nickel ferrite, copper oxide and nickel ferrite, doped non-stoichiometric or partially substituted spinels, and rare earth oxides or oxofluorides.

C.

ANODE FOR ELECTROLYTIC RECEIVING OF METALS

Claims (5)

Patent claims 1. An anode for electrolytic removal of metals, especially aluminum, from molten electrolytes, soll optite nt fluoride, with a solid substrate of metal, alloy or ceimettl, with the active surface of the molten water, which is fused with the fused melt during use. mettlu mnlej szltchet nego nlż mettle otizymywtny, zntmlenna that posltdt elektionlcznle conductive wtistwę ztpotową dlt oxygen on powleizchnl podłożt formed of mettlu alloy or ceimettlu, Pisa wherein wtistwt ztpoiowt dlt oxygen wybitnt jtko wtistwt ztwleItjąci oxide chiomu l ztwiettjąct which ntjmnlej one spośiód pleiwltstków consisting of platinum, ptlltd or gold or decomposed platinum-cyanine alloys or aluminum-aluminum alloys, where the tnodt posltdt and also the pre-coated oxide oxide film between the cold layer and the solid oxygen molding from a group of compounds including meson oxide in concentrated form with at least one additional oxide, nlglycolate, nlglycolate, nltechlomethic intramedullary or partially substituted mesolite oxygen or istocloxide. 2. Anodt according to ztstlz. 1, zπimίenna this that the injection of the doubtful oxygen it is an integral form of the oxide, or ceimettlu. composed of squłtdnlkt or squłtdnlków a substrate of mettel, alloy 3. Anodt according to ztstiz. 1, zntmlenna this that the substrate is a hard alloy with 10 to 30¼ in weight of chioma, 55 to 90¾ nlkel, kobtltu l / or geltzt l to 15% of aluminum, htfnu, mollbden, nlobu, kizemu which is the reason for the question of the second oxygen , ttnttlu, tyttnu, wolfitm, chiomy oxide ztwleit. wtntdu, ltiu l zicon, pizzas 4. Anodt according to ztstlz. 1, variable this that the oxygen test is a separate material that is stuck to the methylene, alloy or ceimettl material. 5. Anodt according to ztstiz. 1, due to the fact that the molten oxide formed by the carbon monoxide mixed with carbon monoxide or mtngtium oxide. * * * The nucleotide of the extract is tnodt to electrolytic otizymywtnlt mettll, zwtłtszczt aluminum from molten electrolytes, zwtszczt soll optitej nt fluoic. Anodt posltdt a substrate made of mettel, alloy or ceimettel l active tnodic coating, which is a protective surface coating that illuminates the noble mettlum material, i.e. electrolytic mettel. The coating is safe from the oxidation of the liquid and the molten methylene, which is the exact concentration of this molten methylene. The electrolytic method of the electrolytic pathway in the United States of America is well known. Ameiykl and 4,614,569. Usually, the coating is thinned with a hardness of oxygen, softened fluoi, or in combination with a mixture of ceiu l possibly fuzzy pleiwltstków for electrolyte. Elektiollt may be a melted kilollt which solidifies and dissolves alumina, and a melted alumina produced. There have been problems with the tnody substrate so far. Even if it is cumulative, the integrity of this may be mixed. On Friday, when the substrate is methanol, alloy or ceittl, it may oxidize, resulting in a diminished and diminished water background, due to the perfect operation of a protective coating of oxygen fluoro, which the chionl of the substrate is covered with direct electromagnetic detachable material. 157 722 A promising solution to these problems was the use of a cetamic-metal composite material with at least one ceramic phase and at least one metal phase, whitening the mixed oxides of ceiu, aluminum, nickel, iron and / or copper in the form of a backbone of interconnected ceramic oxide grains, which that the skeleton is entwined with a continuous metallic alloy or intermetallic compound with aluminum, nickel, iron and / or copper as described in European Patent Application No. 257,708. and clays because even if they do corrode, they do not lead to corrosion products which contaminate the electrolytically obtained aluminum, but substrate corrosion remains a problem. The materials used as non-usable anodes in molten electrolytes must have good stability in an oxidizing atmosphere, good mechanical properties, good electrical conductivity, and be capable of operating for extended periods of time under polarization conditions. At the same time, the materials used on an industrial scale should be such that their welding and processing do not pose insurmountable problems for a person skilled in the art. Ceramic materials have good chemical corrosive properties, however, their low electrical conductivity and difficulties in making mechanical and electrical contact as well as difficulties in shaping and processing these materials severely limit their use. The use of cermets has been proposed to overcome the difficulties of conductivity and the processing of ceramics. Cermets were obtained by pressing and sintering mixtures of ceramic powders with metal powders. However, cermets with good stability, good electrical conductivity and good mechanical properties are difficult to produce and their production on an industrial scale is problematic. The chemical incompatibilities of ceramics with metals at high temperatures also pose problems. Composite materials have also been proposed, consisting of a metal core inserted into a previously treated ceramic structure or a metal structure covered by a ceramic layer. Cermets have been tried as non-consumable anodes for molten salt electrolysis, but the problems associated with these materials have not been overcome to date. U.S. Patent No. 4,374,050 describes inert electrodes for aluminum production constructed of at least two metals or metal compounds to provide a complex metal compound. For example, an alloy of two or more metals may be surface oxidized to form a complex metal oxide on the surface of an unoxidized alloy substrate. U.S. Patent No. 4,374,761 describes similar blends further containing dispersed metal powder to improve conductivity. U.S. Patent Nos. 4,399,008 and 4,478,693 disclose various metal oxide blends that can be applied as oxide blends to a metal substrate by plating or using plasma. U.S. Patent No. 4,620,905 discloses an oxidized tin-based alloy electrode with nickel, iron, silver, zinc, magnesium, aluminum or yttrium, either as cermet or partially oxidized on the surface. These partially oxidized alloys have various drawbacks in that the formed oxide layers are too porous for oxygen and not sufficiently stable under corrosion conditions. Moreover, it has been observed that at high temperatures the partially oxidized structures still oxidize and this uncontrolled oxidation results in the separation of the metal and / or oxide layer. In addition, processing ceramics and achieving good mechanical and electrical contact with such materials presents problems that are difficult to overcome. Ceramic-metal adhesion is particularly difficult to achieve, and this serious problem has made the use of such simple compounds difficult. The anode according to the invention has an electronically conductive oxygen barrier layer on the surface of a metal, alloy or cermet substrate. The barrier layer for oxygen is ex4 157 722 taken from the group consisting of a layer containing chromium oxide, a layer containing at least one of platinum, palladium or gold, or containing platinum-cyicon alloys or nickel-aluminum alloys. The anode further has a pre-applied ceramic oxide layer between the protective coating and the oxygen barrier layer. This ceiamic oxide layer serves as an attachment for the protective coating and is selected from the group consisting of copper oxide in solid solution with at least one additional oxide, nickel ferrite, copper oxide and nickel ferrite, doped non-stoichiometric or partially substituted spinels, and rare earth oxides or oxofluorides. . The oxygen barrier layer is preferably an integral oxide layer, Composed of a metal, alloy or cermet substrate component or components. The substrate is preferably an alloy containing 10 to 30% by weight of chromium, 55 to 90% nickel, cobalt and / or iron and up to 15% aluminum, hafnium, molybdenum, niobium, silicon, tantalum, titanium, tungsten, vanadium, yttrium and zirconium, with with which the oxygen barrier layer comprises chromium oxide. The oxygen barrier layer is preferably a separate layer applied to the surface of the metal, alloy or cermet substrate. The ceramic oxide layer preferably comprises copper oxide in solid solution with nickel oxide or manganese oxide. An advantage of the invention is to improve the electrowinning of aluminum and other metals from molten salts containing compounds (e.g. oxides) of the recovered metals by improving the protection of the metal, alloy or cermet substrate. The advantage of the invention is to obtain complex anodic structures having good chemical stability at high temperatures in an oxidizing and / or corrosive environment, good electrochemical stability at high temperatures under anodic polarization conditions, low electrical resistance, good chemical compatibility and adhesion between ceramic parts and metal, easy to process, low material and manufacturing costs and easy to adapt to industrial dimensions. The barrier function serves to prevent gaseous or ionic oxygen from penetrating into the substrate and must have good electronic conductivity, while also assisting the attachment of the cerium oxofluoide protective coating or ceramic coating, which in turn maintains the cerium oxofluoide protective coating. The composite anode structure typically has a metal core of an alloy that is resistant to high temperatures, for example chromium with nickel, cobalt or iron, and preferred components, with a ceramic coating that may be an oxidized copper alloy. In addition to 55-90%, typically 55-65%, by weight of the primary nickel, cobalt and / or iron component (e.g. 70-80% nickel with 6-10% iron or 75-85% iron), the core alloy contains 10 up to 30% (preferably 15 to 30%) by weight of chromium but free of copper or other metals which are readily oxidized, ie containing no more than 1% by weight of such components, typically 0.5% or less. Other minor components such as aluminum, hafnium, molybdenum, niobium, silicon, tantalum, titanium, tungsten, vanadium, yttrium and zirconium may be added to the core alloy up to a total of 15% by weight to improve its oxidation resistance at high temperatures. Other elements such as carbon and bot may also be present in trace amounts, usually less than 0.5%. Conveniently for the core they may be used dos ^TEP NE ^hours an ^d lu nadsto ^{py l} last hundred ^dust tr udno ^ pliwe, KtKe I ^k Π ^NC 0NEL ^ ^M, H ^A S ^TAR L0Y ^ ^M, H ^A Ynes ^ ^M, UOIMET ™ , NIM0NIC ™, INCOLOY ™, ^I k ⁿ IEZ ditch their many varieties. In some embodiments, a ceramic coating is used containing an oxidized alloy of 15 to 75% by weight of copper, 25 to 85% by weight of nickel and / or magnesium, up to 5% by weight of lithium, calcium, aluminum, magnesium, or iron, and up to 30% by weight of platinum, gold. and / or palladium, in which the copper is fully oxidized and at least part of the nickel and / or magnesium is oxidized in a solid solution with copper oxide, and the substrate contains 15 - 30% by weight of chromium, 55 - 85% nickel, cobalt and / or iron and to 15% by weight of aluminum, hafnium, molybdenum, niobium, silicon, tantalum, titanium, tungsten, vanadium, yttrium and zirconium, and the interface between the substrate and the ceramic surface coating has an oxygen barrier layer containing chromium oxide. The metal sheath or sheath can be made of a copper-based alloy and is typically 0.1 to 2 mm thick. The copper alloy usually comprises 20 to 60 '. by weight of copper and 40 - 60¾ by weight of another component of which at least 15 - 20¾ is a solid solution with copper oxide. Cu-Ni or Cu-Mn alloys are typical examples of this class of alloys. Cu-Ni alloys available in the Tm Tm of the trade, such as the MONEL 'or CONSTANTAN ¹ grades, can be used. Other ceramic coatings that serve to attach a protective coating held in place, e.g. cerium oxyfluoride, include nickel ferric oxide or copper oxide and nickel ferrite, doped, non-stoichiometric and partially substituted ceramic oxide spinels, containing bonds of divalent nickel, cobalt, magnesium, manganese, and copper. and zinc with divalent or trivalent nickel, cobalt, manganese and / or iron, and preferably dopants selected from Ti **, ^ * \ ^S n **, re **, H ^f * ^{+, M} n " ^, re ' ⁺ , Ni ^3+, Co ³ *, ^M n ^3+, A]: ³ * ^, Cr ^{3+, r} e * ^{-, Mi} ** ^, Co ² *, Mg ² *, ^M n ^C \ ^Z n ^C * ^{and Li +} ( see description ^p Athens ^t mid ^S dances in United States Patent No. 4 552 630) as well as coatings based on rare earth oxides and tlenofluorkach, and in particular the pre-imposed cerium oxyfluoride or in combination with other sklaonikami. The alloy core resists oxidation under oxidative conditions at temperatures up to 1100 ° C due to the formation of an oxygen-impermeable, non-combustible oxide layer on the joining surface. This oxygen impermeable layer is preferably obtained by locally oxidizing the chromium contained in the substrate alloy to form a thin chromium oxide layer or mixed chromium oxide with other alloy components in smaller amounts. On the other hand, the chromium oxide barrier layer could be applied e.g. by spraying plasma onto a substrate made of an alloy containing nickel, cobalt or iron, or other types of electronically conductive oxygen-impermeable barrier layers could be used, such as a platinum-zirconium layer or a nickel-aluminum layer, layers mixed oxides based in particular on chromium oxide, alloys and intermetallic compounds, especially those containing platinum or another noble metal or oxygen-free ceramics such as carbides. Preferably, however, barrier layers containing chromium oxide, alone or with another oxide, will be formed by locally oxidizing a suitable alloy substrate, but for other blends other methods are also possible including torch sputtering, plasma sputtering, sputtering, electron beam evaporation, and electroplating followed by an appropriate oxidation treatment before or after the coating as metal, layers of different metals or as an alloy. The complex metal structure can be of any geometry and shape. The appropriate shapes of the structure can be obtained by cutting, extruding, cladding or welding. In the welding process, the metal used must have the same composition as the alloys of the core or shell. In another method of fabricating complex metal structures, the sheath alloy is deposited as a coating on a cut alloy core. Such coatings can be applied by known deposition techniques such as torch spraying, plasma sputtering, cathode sputtering, electron beam evaporation or electroplating. A shell melt coating may be deposited directly as a desired blend or may be formed by diffusion of various layers of successively deposited components. Following the shaping step, the complex structures are typically subjected to controlled oxidation to convert the shell alloy to a ceramic shell. The oxidation step is carried out at a temperature lower than the melting point of the alloys. The oxidation temperature may be selected such that the rate of oxidation is about 0.005 to 0.010 mm per hour. Oxidation may be performed in air or in a controlled oxygen atmosphere, preferably at about 1000 ° C for 10-24 hours to completely oxidize the copper. 157 722 With certain substrate alloys it has been observed that a substrate component, in particular iron or other constituent metal present in the substrate melt but not present in the coating melt, may diffuse into the ceramic oxide coating during the oxidation phase before oxidation is complete, or diffusion may be induced by heating in neutral. atmosphere before oxidation. Diffusion of the coating component into the substrate may also take place. Preferably following the oxidation step, the blend is heated in air at about 1000'C for about 100 to 200 hours. This annealing or aging step improves the homogeneity of the composition and structure of the formed ceramic phase. The ceramic phase may advantageously be a solid solution of (M _x Cuj _x ) O-, where M is one of the major constituents of the shell alloy. Due to the presence of copper oxide as the main component, which plays the role of oxygen transferring and binding agent during the oxidation step, the shell stupa can be changed completely into a coherent ceramic phase. The stresses that usually arise due to the volume increase during the transformation of the shell alloy are absorbed due to the plasticity of the copper oxide phase, which reduces the risk of cracking of the ceramic layer. When the shell alloy is completely converted to the ceramic phase, the surface of the refractory alloy of the core of the structure reacts with oxygen and forms an oxide layer based on Cr2O, which acts as an oxygen barrier preventing further oxidation of the core. Due to the similar chemical stability of the components of the ceramic phase formed from the copper-based alloy and the chromium oxide core phase, there is no incompatibility between the ceramic shell and the metal core, even at high temperatures. The limited interdiffusion between the chromium oxide based layer at the surface of the metal core and the copper oxide or other ceramic metal shell can provide the copper oxide with good adhesion to the metal core. The presence of CuO gives the ceramic shell layer semiconductor properties. The electrical resistivity of CuO is about 10 to 10 ohm.cm at 1000 ° C and is reduced by a factor of about 100 due to the presence of a second metal oxide such as NiO or MnO®. The electrical resistivity of this ceramic phase can be improved by introducing a soluble noble metal into the copper alloy prior to the oxidation step. The soluble noble metals can be palladium, platinum or gold in an amount up to 20-30 - by weight. In this case, a cermet shell can be obtained with the cross-linking of a noble metal uniformly distributed in the ceramic substrate. Another way to improve the electrical conductivity of the ceramic shell may be to dope a second metal oxide phase, for example the NiO of the ceramic phase prepared from Ni-Cu alloys may be doped with 11 ter. As a result of producing a solid solution with stable oxides such as NiO or MnO®, the copper oxide ceramic shell has good stability under corrosive conditions at high temperatures. Moreover, after the aging step, the composition of the ceramic phase may be more monohydrate, with large grain sizes, whereby the risk of grain boundary corrosion is greatly reduced. The described non-consumable anodes can be used in the electrolysis of molten salts at temperatures in the range 400-1000 ° C as an anode or as an anode substrate for held in place cerium oxofluoride-based anode coatings used in the electrowinning of aluminum. The use of anodes as a substrate for cerium oxyfluoride coatings is particularly advantageous because the cerium oxyfluoride coating can interpenetrate with copper oxide based coatings or other ceramic coatings providing excellent adhesion. In addition, formation of the cerium oxofluoride coating in situ from cerium-containing molten cryolite occurs with no or minimal substrate corrosion. In use of the described structure of the substrate as the anode, it is understood that the resulting electrolytic metal bar b ^Ed branches ^d mo st slag ^h Etna Mz cerium ^{(Ce +} ^) c ^s dissolve ^the so located ^{in L,} and ^so That the required metal is deposited at the cathode of the non-cathodic cerium deposit. Such metals may be selected from the group UHa (aluminum, gallium, indium, thallium), groups IVb (titanium, zirconium, naphthalene), groups Vb (vanadium, niobium, tantalum) and groups VIIb (manganese, flax). Then the ochionna ns coating. ceiu oxyfluoride may be electrolytically deposited on the anode substrate during the initial period of operation in the molten electrolyte in the electrolytic cell, or a protective coating may be applied to the anode substrate prior to inserting the anode into the molten electrolyte in the cell. Preferably, the electrolysis is performed in a fluoride-based molten salt containing dissolved metal oxide and at least one cerium compound, the protective coating being a cerium oxygen compound containing fluorine. For example, the coating may contain fluorine containing cerium oxide with only traces of additives. The advantages of the invention over the prior art are illustrated in the working examples. Example I. Oxidation of a copper-based alloy ^T s the tube has left Monel. ^M ^ 400 (63% Ni - ^2% Fe - ^2.5% Mn - additions of Cu) having a ^COG re ^<y jnic 10 mm length of 50 mm, a wall thickness of 1 mm, is introduced into the furnace is heated to 1000 ° C the air. After 400 hours of oxidation, the luia is completely transformed into a ceramic structure approximately 12 mm in diameter by 52 mm in length with a wall thickness of 1.25 mm. In the optical microscope, the ceramics obtained represent a single-phase structure with large grain sizes of about 200 - 500 micrometers. The mapping of copper and nickel by the Scanning Electron Mictoscopy shows a very homogeneous distribution of the two components; no separation of the blend is observed at the grain boundaries. Measurements of the electrical conductivity of a sample of the obtained ceramic material give the following results: Temperature (* C) 400 700 850 925 1000 Resistivity (ohm cni) 8.30 3.10 0.42 0.12 0.08 Example II. Annealing of an oxidized copper-based alloy Two rounds of onelu ^{4 M} ™ 00 uUenione at 1000'C in an area ^{and t} pestle ^as opiano the spinning land ^ 1 are subjected to further annealing in air at 1000'C. After 65 hours, one batch is removed from the furnace, cooled to room temperature and its cross section is examined with a microscope. The full thickness of the pipe wall is already oxidized and transformed into a single-phase ceramic structure, but the grain bonds are loose and the copper-enriched phase is observed at the grain boundaries. After 250 hours, the second test tube is removed from the furnace and cooled to room temperature. Its cross-section is observed under an optical microscope. Extending the aging step from 65 hours to 250 hours results in an improved, more compact structure of the ceramic phase. There is no visible blend zone at the grain boundaries. Examples I and II thus show that copper-based alloys after oxidation and annealing have interesting properties, however, as demonstrated by the test (Example 5), the alloys themselves are suitable for use as an electrode substrate for aluminum production. Example lila, Illb, IIIc. Preparation of the mixtures according to the invention Example of Ilia. Ruta with a hemispherical end, 10 mm outside diameter and 50 mm long TM is made of Monel 400 bar. The wall thickness of the round is 1mm. A rod made of Inconnl ™ material (type 600: 76% Ni - 15.5% Cr - 8% Fe) with a diameter of 8 mm and a length of 500 mm is mechanically inserted into the Monel tube. The exposed portion of the Inconel bar above the Monel sheath is protected by an aluminum sleeve. The structure is placed in the oven and heated 157 722 cut in the air from room temperature to 1000 ° C for 5 hours. The oven temperature is kept constant at 1000 ° C for 250 ° C, then the oven is cooled to room temperature at about 50 ° C per hour. Optical microscopy examination of the cross-section of the final structure shows a good surface bond between the Inconel core and the ceramic shell formed. Some microcracks are observed in the surface junction zone of the ceramic phase, but no cracks are formed in the outer zones. Inconel core surfaces are partially oxidized to a depth of about 60 to 75 micrometers. The chromium oxide based layer formed on the Inconel surface layer penetrates the mutually oxidized ceramic Monel phase and provides good adhesion between the metal core and the ceramic shell. Example Illb. A cylindrical structure with a hemispherical end, a diameter of 32 mm with l ^{l g th} of ¹⁰⁰ mm es ^{t j yk} in Onan ^p slaughter obrribkę machining of the rod ^and the NCO ⁿ el 6Οθ ^'Μ (typical composition: 76¾ Ni - Cr 15,5¾ - 8¾ Fe + small amounts of components) K maximum: carbon (0.15¾). manganese (1¾). sulfur (0.015¾). silicon (0.5¾). copper (0.5¾). The surface of the Inconel structure is then sandblasted and cleaned successively in a hot alkaline solution and in acetone to remove traces of oxides and grease. After the purification step, the structure is coated successively with a nickel BO layer and a 20 micron copper layer through the electrolytic deposit (s) in a nickel sulphamate and cupric sulphate bath, respectively. The coated structure is heated under an inert atmosphere (argon containing 7 ° water) at 500 ° C for 10 hours, then the temperature is increased successively to 1000 ° C for 24 hours and 1100 ° C for 48 hours. The heating rate is adjustable up to 300'C / hr. After the thermal diffusion step, the structure is allowed to cool to room temperature. The interdiffusion between the nickel and copper layers is complete and the Inconel structure is covered by a shell of Νι-Cu alloy about 100 microns thick. The analysis of the obtained shell coating resulted in the following values of the basic components: t Coating surface Interdiffusion zone coating - substrate Ni (w *.) 71.8 82.8 - 81.2 Cu (w 26.5 11.5 - 0.7 Cl (in ¾) 1.0 3.6 - 12.0 Fe (in Ό 0.7 2.1 - 6.1 After the diffusion step, the coated Inconel structure is oxidized in air at 1000 ° C for 24 hours. The heating and cooling rates in the oxidation step are respectively 300 * C / hour. and 100'C / hour After the oxidation step, the Ni-Cu sheath coating turns into a black, homogeneous ceramic coating with excellent adhesion to the Inconel core. Examination of the cross-section of the final structure shows an outer single-phase nickel / copper oxide coating about 120 micrometers thick and an inner Cr. ^ C ^ j layer 5 to 10 micrometers thick. The interior of the Inconel core remained in its original metallic state without any trace of internal oxidation. Pr zykiad IIIc. The hemispherical end cylindrical structure, 16mm in diameter and 50mm long, is made by machining from a stainless ferrite steel bar (typical composition: 17¾ Cr, 0.05¾ C, 82.5¾ Fe). The structure is successively coated with Ni up to 160 microns and Cu up to 40 microns as described in Example 3b followed by a diffusion step under argon and 7 i hydrogen at 500'C for 10 hours, at 1000'C for 24 hours and 1100'C for 24 hours. Analysis of the resulting shell coating gave the following values for the main components: 157 722 Coating surface Coating diffusion zone - substrate Ni Cw M 61.0 39.4 - 2.1 Cu (in M) 29, B 0.2 - 0 Cr Cw M 1.7 9.2 - 16.0 Fe Cw%) 7.5 51.2 - BI, 9 After the diffusion step, the phenolic invisible steel structure and the final coating are oxidized in the air at 1000 ° C for 24 hours as described in the chart Illb. After the oxidation step, the shell coating is changed to a black, monohydrate, ceiamic coating. The cross-section of the final pattern shows multilayer ceramic coatings composed of: - monohydrate nickel / copper oxide outer coating approximately 150 micrometers, which contains small amounts of nickel / iron oxide, - an intermediate nickel / iron oxide coating of about 50 micrometers, which is defined as the Nif ^ O ^ phase and - a composite metal-oxide layer of 25 to 50 microns followed by a continuous CtoOj layer of 2 to 5 microns. The ferritic stainless steel interior remained initially metallic. Example IV. Testing the mixture according to the invention A composite metal-ceramic structure prepared from the Monel 400-Inconel 600 structure as described in Example Ilia is used as an anode in aluminum electrowinning testing using a corundum crucible as electrolyte bath and a titanium diboride plate as cathode. The electrolyte is composed of a mixture of the cryolite CNajALFg) with 10¾ AltOj and 1¾ CeFj. The operating temperature is maintained at 970 - 980 ° C and fed es ^{t j} s ^{t s} h ow ^AD ano ^d ow ^s og ^E S ^{T S T} of ^{0.4 A /} cm. ^At 6 ° hours of electrolysis, the anode is negative from the test cell. The surface of the submerged anode is homogeneously covered by the blue coating of cerium oxofluoide produced during electrolysis. No visible corrosion of the oxidized Monel ceramic coating is observed, even with the melting line not covered by the coating. The anode section shows, in turn, the Inconel core, the ceramic sheath, and a cerium oxyfluide coating layer approximately 15 mm thick. Due to the interpenetration at the metal / ceramic and ceramic / coating junction surfaces, the adhesion between the layers is excellent. The chemical and electrochemical stability of the anode is improved due to the low levels of nickel and copper impurities in the clay produced at the cathode of 200 and 1000 ppm, respectively. These values are significantly lower than those obtained with a comparable test on a ceramic substrate as shown in Comparative Example 5. Example 5 Comparative study of an oxidized / annealed copper-based alloy TM The ceramic tube made by oxidation / annealing of the Monel 400 'in Example 2 is then used as the anode in the aluminum electrowinning test following the same procedure as in Example 4. After 24 hours of electrolysis, the anode is removed from the test cell. The blue oxofluoride coating is partially formed on the ceramic tube, occupying approximately 1 cm of the direct distance below the melting line. Not overcoating but corrosion of the ceramic substrate in the lower parts of the anode is observed. The impurity of the aluminum produced at the cathode was not measured, however, it is estimated to be about 10-50 times the value measured in Example 4. This result is explained by the low electrical conductivity of the ceramic tube. In the absence of a metal core, only a limited part of the rout below the melting line is polarized 157 722 for coating production. The underside of the anode, not polarized, is exposed to the chemical attack of the cryolite. The material to be fed alone is therefore unsuitable as an anode substrate for an opal coating on an oxyfluide ceiu. It has therefore been found that the composite material according to the invention (i.e. the material of the Ilia pysysystem tested in Example 4) is technically significantly better than the simple oxidized / annealed copper oxide based alloy. Example VI. Investigation of composite material according to the invention Two c ^lived ndiyczne Strul <turn the bicone ^l U °° ™ are ^provided in ^yk onan di squint ^angle Ravana s ^j k described in Example IIIb, and covered by a layer of nickel-miedf about 250 - 300 micrometers by flame spraying an alloy powder 70¾ Ni - 30% Cu (by weight). After the coating step, the structures are attached in parallel to the two ferritic steel conductive bars of the anode support system. The conductive rods are protected by corundum sleeves. The coated Inconel anodes are then oxidized to 1000 "in air. After 24 hours of oxidation, the anodes are transferred immediately to an aluminum electrowinning basin made of a graphite crucible. The crucible has vertical walls shielded by an alumina ring and the bottom is cathodically polarized. The electrolyte is composed of a mixture of cryolite (NajALF?) With 8.3% ALF}, 8.0% A? 0, and 1.4% CeO?. The shell temperature is kept at 970 - 980 ° C. The total immersion depth of the Inconel double-ended nickel / copper oxide electrodes is 45 mm from the hemispherical bottom. The electrodes are then anodically polarized using a total current of 22.5 A during 8 hours. The total current is then gradually increased to 35A and kept constant for 100 hours. During the second period of electrolysis, the cell voltage is in the range of 3.95 to 4.00 volts. After 100 hours of operation at 35 A, both anodes are removed from the test cell. The surface of the submerged anode is uniformly covered by a blue coating of cerium oxofluoride produced during the first electrolysis period. The black nickel / copper oxide ceramic coating of the non-submerged anode parts is covered by a hard coating produced by the condensation of the cryolite pat above the liquid level. Examination of the anode nodes shows in turn: - outer layer of cerium oxofluoride about 1.5 mm thick, - a nickel / copper oxide secondary coating d 300 - 400 micrometers thick and - an inner layer of Cl2O3 0 9 ranging from 5 to 10 micrometers. No oxidation or deterioration of the Inconel core is observed, other than some microscopic pinholes due to the selective diffusion of chromium into the Inconel surface, creating a Ci ^ O oxygen barrier (Kiikendall porosity). Department of Publishing of the UP RP. Circulation of 90 copies