(19)
(11)EP 0 203 884 B1

(12)EUROPEAN PATENT SPECIFICATION

(45)Mention of the grant of the patent:
06.12.1989 Bulletin 1989/49

(21)Application number: 86810183.3

(22)Date of filing:  22.04.1986
(51)International Patent Classification (IPC)4C25C 7/02, C25C 3/12, C25D 9/06, G01N 27/56

(54)

Dimensionally stable anode for molten salt electrowinning and method of electrolysis

Formstabile Anode für die Schmelzflusselektrolyse und Elektrolyseverfahren

Anode de dimensions stables pour électrolyse en sel fondu et procédé d'électrolyse


(84)Designated Contracting States:
CH DE FR GB IT LI NL SE

(30)Priority: 17.05.1985 EP 85810235

(43)Date of publication of application:
03.12.1986 Bulletin 1986/49

(73)Proprietor: MOLTECH Invent S.A.
2320 Luxembourg (LU)

(72)Inventor:
  • Durus, Jean-Jacques
    CH-1204 Geneva (CH)

(74)Representative: Cronin, Brian Harold John 
c/o MOLTECH S.A. 9, Route de Troinex
1227 Carouge/GE
1227 Carouge/GE (CH)


(56)References cited: : 
EP-A- 0 030 834
EP-A- 0 114 085
  
  • CHEMICAL ABSTRACTS, vol. 98, 1983, page 178, abstract no. 57142a, Columbus, Ohio, US; M. TAKASHIMA et al.: "Preparation and physical properties of rare earth fluoride oxides. 1. Preparation of neodymium fluoride oxides and application to electrocatalysts or solid electrolytes", & NIPPON KAGAKU KAISHI 1982, (12), 1896-902
  
Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention).


Description


[0001] The invention relates to conductive substrates carrying a coating comprising an oxyfluoride of cerium providing enhanced resistivity against reducing as well as oxidizing environments up to temperatures of 1000°C and higher.

[0002] The invention further relates to a method of manufacturing said coating.

[0003] Coated substrates according to the invention may be used to form non-consumable anodes for electrowinning metals from molten salts, but there are also other possible applications, e.g. sensors for the chemical composition of fluids, such as oxygen sensors for gases or liquid metals. Further the coatings may privide corrosion protection of the substrates at high temperature, and are generally useful in applications where electronic and/or ionic conductivity combined with chemical stability at high temperatures are desirable. Enhanced chemical stability at high temperatures is desired e.g. for protective coatings of heat exchangers exposed to corrosive environments.

Background of the Invention



[0004] The European Patent Application 0 114 085 published on July 25, 1984 discloses a dimensionally stable anode for an aluminum production cell comprising a conductive substrate of a ceramic, a metal or other materials which is coated with a layer of a cerium oxycompound. The anode is essentially stable under conditions found in an aluminum production cell, provided that a sufficient content of cerium is maintained in the electrolyte.

[0005] The anode as described in the above European patent application performs well in respect of dimensional stability, however, contamination of the produced aluminum by substrate components may occur under certain circumstances. As shown by microphotographs, the cerium containing coating can be comprised of a non-homogeneous and non-continuous structure leaving small interstices between coated areas, which provide access of the electrolyte to the substrate. In such cases, the electrolyte may corrode the substrate leading to a limited but undesired contamination of the aluminum by substrate components.

[0006] It had also been speculated that the above described coating may consist of other rare earth metals such as praseodymium, samarium, europium, terbium, thulium or ytterbium in a suitable concentration. However these elements are not easy to be coated under the conditions provided in the above publication which does not contain any instructions as to how these elements may be coated onto the substrate, nor in which ranges of concentration. Further, it does not contain any suggestion as to a possible beneficial effect of these elements.

[0007] French patent application 2 407 277 discloses a method of electrolyzing chlorides of e.g. magnesium, sodium, calcium or aluminum in electrolytes at temperatures between 500 - 800°C using an anode comprising a substrate and a coating of an oxide of a noble metal, whereby a certain concentration of an oxide or oxychloride of a metal which is more basic than the metal, produced is maintained in the bath. Thus, by increasing the basicity of the bath the solubility of the anode coating is reduced.

[0008] This method provides better stability of the anode coating by the addition of melt additives, however, these additions relate to the stabilization of the coating rather than to the improvement of the coating morphology and does therefore not contribute to the improvement of the substrate protection, which is not always completely satisfactory in the case of a pure cerium oxycompound coating. The substrate itself which is essentially protected by the coating and only subject to corrosion at finite deficient locations thereof may not simply be protected against corrosion by modifying the basicity of the bath as described in the French patent, since the anode substrate according to the present invention is inherently unstable in a fluoride bath at e.g. 960°C and needs therefore to be completely shielded therefrom. A mere modification of the basicity would not improve the stability of the substrate as it does with a coating of an oxide of a noble metal which is essentially stable in the bath per se.

Objects of the Invention



[0009] It is one object of the invention to provide a remedy for the above described contamination problem.

[0010] It is another object of the invention to provide a dimensionally stable anode for electrowinning of a metal from a molten salt electrolyte containing an oxide of said metal, the anode having a coating which essentially completely inhibits the access of the electrolyte to the substrate.

[0011] It is a further object of the invention to provide a method of producing aluminum or other metals using a dimensionally stable anode comprising a coating wherein the formation of crevices and other deficiencies which allow access of the electrolyte to the substrate is substantially eliminated.

[0012] It is a further object of the invention to provide a simple technique for inhibiting the contamination of the aluminum by substrate components by a method which is simple to apply, which is inexpensive and which does not require any modifications of the anode itself or of the cell.

[0013] Finally, it is an object of the invention to provide a coated substrate with improved properties for general applications where at least one or a combination of the following properties - electronic and ionic conductivity and chemical stability against oxidizing as well as reducing environments at high and low temperatures - are desirable.

Summary of the Invention



[0014] The above and other objects are met by a conductive substrate carrying a coating comprising an oxyfluoride of cerium, characterized in that the coating is a substantially impervious layer further comprising at least one doping element selected from the group consisting of yttrium, lanthanum, praseodymium and other rare earth metals, the concentration of the doping element(s) in the coating being less than 10 w.-% in respect of cerium.

[0015] The coating may comprise the doping element(s) in a concentration between 0, 1 - 5 w.-% of the cerium content.

[0016] The coating may be deposited onto a substrate being a metal, an alloy, a ceramic material, a cermet and/or carbon. A particularly preferred substrate is Sn02, or Sn02 based materials.

[0017] The substrate may be coated by deposition of the coating constituents onto the substrate immersed in an electrolyte containing said constituents in dissolved state.

[0018] The coated substrate according to the invention may serve as an anode for electrowinning of metals by molten salt electrolysis, in particular for the production of aluminum from alumina dissolved in molten cryolite.

[0019] However, other uses are intended and covered by the scope of the invention. Such other possible uses and applications were already mentioned in the preamble of this specification and comprise chemical sensors, corrosion protection, and chemically stable coatings for high and low temperatures.

[0020] In accordance with the invention a method of coating a substrate as described above is characterized by adding sufficient amounts of compounds of cerium and at least one doping element selected from the group consisting of yttrium, lanthanum, praseodymium and other rare earth metals to a molten salt electrolyte and passing electrical current therethrough with the substrate and coating kept under anodic polarization.

[0021] Good results for the morphology of the coating have been achieved with concentrations of the doping element(s) in respect to cerium ranging from approximately 1 : 1 in example 2 to approximately 4.7 : 1 in Example 3. The cerium concentration in the electrolyte was 1.2 w.-% in both cases. The concentration of the doping elements in the deposit does not significantly change with variations of their concentration in the electrolyte above a certain level, since a maximum concentration of the doping element in the coating is expected which corresponds to the thermodynamic solubility of the doping elements in the cerium oxyfluoride crystal lattice. On the other hand, however, the given values for the concentration of the doping elements may not be substantially decreased without affecting the coating composition and morphology. According to the different doping elements and parameters of the coating process the concentration of the doping elements in respect to cerium may vary from 0.1 : 1 to 100 : 1.

[0022] It is convenient for the bath chemistry if the compounds of the doping elements are oxides and/or fluorides.

[0023] Further features of the invention are the employment of the above described method of manufacture for the production of non-consumable anodes for metal electrowinning from the metal oxide dissolved in a molten salt electrolyte such as the production of aluminum by electrolysis of alumina dissolved in molten cryolite, which method comprises adding to the electrolyte prior to or during a preliminary period under special electrolysis operating conditions or during normal electrolysis a sufficient amount of compounds of cerium and at least one doping element selected from the group comprising yttrium, lanthanum, praseodymium and other rare earth metals. Continuing operation of the anode, during metal electrowinning, may be assured by maintaining sufficient concentrations of cerium and, if necessary, the doping element throughout normal electrolysis.

[0024] The initial production of the coating on the substrate may be carried out outside a molten salt electrowinning cell prior to the use of the anode in said cell, or during preliminary or normal electrolysis operating conditions within said electrowinning cell.

[0025] The choice and concentration of the doping elements from the mentioned group comprising yttrium, lanthanum, praseodymium and other rare earth metals may be carried out according to the intended use of the coating, and will generally be governed by considerations of how the particular element influences the morphological, chemical and electrical properties of the coating. Some of the mentioned doping elements such as yttrium create enhanced ionic conductivity, which may be of interest for the sensor application, however, for its use as coating for dimensionally stable aluminum electrowinning anodes the electronic conductivity should prevail. Since the raise of the ionic conductivity with the addition of most doping elements of the above group is dependent on the concentration thereof, this concentration should not be too high in cases where the electronic conductivity is the desired form of conductivity, provided the morphology of the coating is sufficiently improved.

Detailed Description of the Invention



[0026] The invention is now described in view of its application for dimensionally stable anodes for electrowinning of metals by molten salt electrolysis.

[0027] The dimensionally stable anodes over which the anodes of the present invention are an improvement are described in European Patent Application 0 114 085.

[0028] Known anode coatings comprised of cerium oxyfluoride lead to a contamination of the aluminum by corrosion of the substrate to which the electrolyte finds limited access by small imperfections of the cerium-containing coating.

[0029] The present invention is based on the finding that the addition of small amounts of doping elements which coprecipitate with the cerium on the anode substrate modifies the coating morphology in such beneficial manner that the coating is developed with a continuous coherent structure, thereby providing a substantially impervious layer on the substrate, which completely sheathes the latter and avoids thereby any access of electrolyte.

[0030] The cerium based coating including these doping elements selected from yttrium, lanthanum, praseodymium and other rare earth metals may be applied outside the electrolysis cell or within the cell during preliminary operating conditions, or it may be established during normal operation by immersing an uncoated substrate into the electrolyte to which controlled amounts of compounds such as oxides and/or fluorides of cerium and the doping elements are added to the electrolyte and maintained at a certain concentration.

[0031] The mentioned doping elements and their oxyfluorides do not precipitate on anode substrates such as Sn02 other than together with the cerium compounds and even in the presence of cerium the doping elements precipitate onto the anode substrate in a rate which is substantially lower than could be expected according to their concentration in respect of the cerium content in the electrolyte. The doping elements or their oxyfluorides are completely dissolved in the solid cerium oxyfluoride phase of the coating. It may therefore be possible that the content of the doping elements at least in an inner region it hereof be kept at its initial level, thus maintaining the imperviousness in this region even without further doping elements being added to the electrolyte, whereby only the concentration of cerium needs to be maintained. Alternatively, in order to maintain the concentration of cerium and the doping elements in the molten salt electrolyte, Misch metal oxides may be added thereto which contain a major amount of cerium oxide and minor amounts of other rare earth metal - as well as yttrium - oxides. A suitable composition among a variety of different natural ores containing Misch metal oxides may be chosen according to the final use of the coating.

[0032] The doped oxyfluoride coating is extremely resistant to strong reducing as well as oxidizing environments such as found in a Hall-Heroult cell. The material is resistant to oxygen which is released in substantial amounts from the melt in the case of non-carbon anodes, and against fluorine or fluorides from the cryolite. The coating is resistant against these gases since it is an oxyfluoride compound which is inert against further attack by fluorine and oxygen. However, the cryolite in such cells contains a certain concentration of dissolved metallic aluminum which is highly reducing in particular at the temperatures involved. The above coating, however, is neither reduced by liquid aluminum in bulk nor dissolved in cryolite, since the oxides of cerium and the other doping elements are more stable than aluminum oxide.

[0033] These very slowly dissolving anode coatings may be operated under constant conditions, whereby an equilibrium between the dissolution rate of the coating in the electrolyte and the deposition rate of the dissolved constituents is maintained, or the operation conditions may be controlled intermittently, whereby the anode is operated until a minimum coating thickness representing a safety limit is achieved, beyond which contamination of the bath and the product metal by corrosion of the substrate may not be avoided. Alternative methods may then be provided which comprise regrowing the coating by adding to the electrolyte the necessary compounds as mentioned above or withdrawing the spent anodes to put in new ones, and recoating the used anodes outside the cells for further use.

[0034] The choice of a particular doping element depends - as already mentioned - on the intended application. In the case of use for aluminum electrowinning anodes it should be considered that oxyfluorides of the metals in question have electronic and also ionic conductivity. Electronic conductivity is the preferred form, while ionic conductivity leads under particular conditions to the formation of a sub-layer between the substrate and the coating, this sub-layer being depleted of oxygen and comprising substantially pure fluorides of cerium and the doping elements. For this application, the doping elements should therefore not substantially enhance the ionic conductivity over that of cerium oxyfluoride. Praseodymium, yttrium, lanthanum and some others are in this respect acceptable candidates. While lanthanum would be acceptable in this respect, in an aluminum electrowinning cell its electrowinning potential is such that it coprecipitates with the aluminum produced, so that the contamination of the product metal is unacceptable. However, the employment of doping elements which are not suitable for aluminum electrowinning anodes may be envisaged for other applications.

[0035] The invention is further described by three examples and microphotos demonstrating the improvement of the coating morphology by addition of the above described doping elements.

[0036] Fig. 1 shows a coating achieved by immersion of a Sn02 substrate into a bath as described in the Examples but without any doping element, only with 1.2 % cerium. It is apparent that the coating 1 covers the substrate 2 in a non-satisfactory manner. Large crevices 3 and voids 4 are visible in the coating which allow access of the electrolyte to the substrate which is not resistant to the latter. In addition to these large imperfections, very fine microcracks 5 are visible which, however, are due to the thermal shock to which all samples were subjected when they were removed from the hot test cell. These microcracks which are also visible in the other Figures do not occur under normal operation.

[0037] Figs. 2 to 4 show coatings which were made according to the Examples including the doping additives. As compared to Fig. 1, the coatings 1 in Figs. 2, 3 and 4 are substantially improved in respect of their sealing effect for the substrate, i.e. their imperviousness. All large imperfections have disappeared, only the above mentioned microcracks which are due to the sample preparation are still visible. It is perceivable that such improved anode coatings are highly beneficial in order to reduce corrosion of the anode substrate by the electrolyte and the contamination of the metal produced.

Examples


Example 1:



[0038] To 340g electrolyte comprising 90 w.-% cryolite and 10 w.-% A1203 were added 4 g CeF3 and 17 g Y203. Electrolysis was carried out for 30 hours at 960°C with an anodic current density of approximately 0.2 A/cm2. After the electrolysis, the anode was found to be coated with a 0.44 mm thick layer comprising approximately 98 w.-% cerium oxyfluoride and approximately 2 w.-% yttrium oxyfluoride. The microphoto (Fig. 2) shows a continuous coherent coating which is free from the aforementioned crevices and holes, whereby no portions of the substrate are exposed to the electrolyte. The microcracks 5 do not have any influence on the coating performance, since they are due to the sample preparation and would not occur in normal operation.

Example 2:



[0039] To 340 g electrolyte comprising 90 w.-% cryolite and 10 w.-% A1203 were added 4 g CeF3 and 3.5 g PrsO". Electrolysis was carried out for 30 hours at 960°C with an anodic current density of approximately 0.2A/cm2. After the electrolysis, the anode was found to be coated with a 0.37 mm thick layer comprising approximately 97 w.-% cerium oxyfluoride and approximately 3 w.-% praseodymium oxyfluoride. The microphoto (Fig. 3) shows a continuous coherent coating which is free from the aforementioned crevices and holes, whereby no portions of the substrate are exposed to the electrolyte.

Example 3:



[0040] To 340 g electrolyte comprising 90 w.-% cryolite and 10 w.-% A1203 were added 4 g CeF3 and 17 g LaF3. Electrolysis was carried out for 30 hours at 960°C with an anodic current density of approximately 0.2A/cm2. After the electrolysis, the anode was found to be coated with a 0.44 mm thick layer comprising approximately 99 w.-% cerium oxyfluoride and approximately 1 w.-% lanthanum oxyfluoride. The microphoto (Fig. 4) shows a continuous coherent coating which is free from the aforementioned crevices and holes, whereby no portions of the substrates are exposed to the electrolyte.


Claims

1. A conductive substrate carrying a coating comprising an oxyfluoride of cerium, characterized in that the coating is a substantially impervious layer further comprising at least one doping compound of an element selected from the group consisting of yttrium, lanthanum, praseodymium and other rare earth metals, the concentration of the doping element(s) in the coating being less than 10 w.-% of the cerium.
 
2. The coated substrate of claim 1, characterized by comprising a structure of oxyfluorides of cerium and the doping element(s), the concentration of the doping element(s) being between 0.1 - 5 w.-% of the cerium concentration.
 
3. The coated substrate of claim 1 or 2, characterized by the substrate being a metal, an alloy, a ceramic material, a cermet and/or carbon.
 
4. The coated substrate of claim 3, characterized by the substrate comprising Sn02.
 
5. A method of producing a coated substrate according to any preceding claim, characterized by anodically polarizing the substrate in a molten salt electrolyte containing cerium and said at least one doping element selected from the group consisting of yttrium, lanthanum, praesodymium and other rare earth metals.
 
6. The method of claim 5, characterized by the electrolyte being cryolite.
 
7. The method of claim 5 or 6, characterized by the concentration of the doping element(s) in the electrolyte being in the range of 0.1 to 100 times the concentration of cerium.
 
8. The method of claim 5, 6 or 7, characterized by the compounds of the doping elements being oxides and/or fluorides.
 
9. A dimensionally stable anode for electrowinning of a metal from an oxide thereof dissolved in a molten salt electrolyte, the anode comprising a coated substrate according to any one of the claims 1 - 4 or as produced by the method of any of claims 5 - 8.
 
10. A method of producing a metal by electrolysis of a compound of said metal dissolved in a molten salt electrolyte using an anode according to claim 9, characterized by adding to the electrolyte a compound or compounds containing at least one doping element selected from the group comprising yttrium, lanthanum, praseodymium and other rare earth metals, and maintaining sufficient concentrations of cerium and optionally of the doping element throughout normal electrolysis.
 
11. The method of claim 10, characterized by the electrowon metal being aluminum.
 
12. The method of claim 10 or 11, characterized by producing the coating on the substrate outside a molten salt electrowinning cell prior to the use of the anode in said cell, or during preliminary or normal electrolysis operating conditions within said electrowinning cell.
 
13. Use of the coated substrate of any one of claims 1 - 5 as chemical sensor for oxygen and/or fluorine containing gas.
 


Revendications

1. Un substrat conducteur portant un revêtement comprenant un oxyfluorure de cérium, caractérisé en ce que le revêtement est une couche substantiellement imperméable comprenant au moins un élément dopant sélectionné dans le groupe consistant de l'yttrium, du lanthane, du praséodyme et d'autres métaux terreux rares, la concentration de l'élément(s) dopant(s) dans le revêtement étant inférieur à 10 en poids par rapport au cérium.
 
2. Le substrat revêtu de la revendication 1, caractérisé par la présence d'une structure d'oxy- fluorures de cérium et d'élément(s) dopant(s), la concentration de l'élément(s) dopant(s), la concentration d'élément(s) dopant(s) étant entre 0,1 - 5 % en poids de-concentration de cérium.
 
3. Le substrat revêtu de la revendication 1 ou 2, caractérisée par le substrat étant un métal, un alliage, un matériau céramique, un cermet et/ou du carbone.
 
4. Le substrat revêtu de la revendication 3, caractérisée par le substrat comprenant Sn02.
 
5. Une méthode de production d'un substrat revêtu selon n'importe quelle revendication précédente, caractérisée par la polarisation anodique du substrat dans un électrolyte de sel fondu contenant du cérium et ledit au moins un élément dopant sélectionné du groupe consistant de l'yttrium, du lanthane, du praséodyme et des autres métaux terreux rares.
 
6. La méthode de la revendication 5, caractérisée en ce que l'électrolyte est la cryolithe.
 
7. La méthode de la revendication 5 ou 6, caractérisée en ce que la concentration d'élément(s) dopant(s) dans l'électrolyte est dans l'intervalle de 0,1 à 100 fois la concentration de cérium.
 
8. La méthode de la revendication 5, 6 ou 7, caractérisée en ce que les composés des éléments dopants sont des oxydes et/ou des fluorures.
 
9. Une anode dimensionnellement stable pour la récupération électrolytique d'un métal à partir d'un oxyde de celui-ci dans un électrolyte de sel fondu, l'anode comprenant un substrat revêtu se-Ion n'importe quelle des revendications 1 - 4 ou produite par la méthode de n'importe quelle des revendications 5 - 8.
 
10. Une méthode pour la production d'un métal par l'électrolyse d'un composé dudit métal dissout dans un électrolyte de sel fondu utilisant une anode selon la revendication 9, caractérisée par l'addition à l'électrolyte d'un composé ou de composés contenant au moins un élément dopant sélectionné du groupe comprenant l'yttrium, le lanthane, le praséodyme et les autres métaux terreux rares, et en maintenant des concentrations suffisantes de cérium et optionnellement de l'élément dopant pendant la totalité de l'électrolyse normale.
 
11. La méthode de la revendication 10, caractérisée en ce que le métal récupéré électriquement est l'aluminium.
 
12. La méthode de la revendication 10 ou 11, caractérisée par la production du revêtement sur le substrat à l'extérieur de la cellule de récupération électrolytique à sel fondu avant l'utilisation de l'anode dans ladite cellule, ou pendant des conditions d'opération d'électrolyse préliminaire ou normale à l'intérieur de ladite cellule.
 
13. L'utilisation du substrat revêtu de n'importe quelle des revendications 1 à 5 comme senseur chimique pour un gaz contenenant de l'oxygène et/ou du fluor.
 


Ansprüche

1. Leitfähiges Substrat mit einer Ceroxyfluorid enthaltenden Beschichtung, dadurch gekennzeichnet, daß die Beschichtung eine im wesentlichen undurchlässige Schicht ist, welche ferner mindestens ein Element ausgewählt aus der Gruppe bestehend aus Yttrium, Lanthan, Praseodym und anderen Seltenen Erdmetallen als Doping-Verbindung enthält, wobei die Konzentration der (des) Dopingelemente(s) in der Beschichtung weniger als 10 Gew.-% des Cer beträgt.
 
2. Beschichtetes Substrat gemäß Anspruch 1, dadurch gekennzeichnet, daß es eine Struktur aus Ceroxyfluoriden und der (demi) Dopingelement(en) enthält, wobei die Konzentration der (des) Dopingelemente(s) zwischen 0,1 und 5 Gew.-% der Cerkonzentration liegt.
 
3. Beschichtetes Substrat gemäß Anspruch 1 oder 2, dadurch gekennzeichnet, daß das Substrat Metall, eine Legierung, ein keramisches Material, ein metallisch-keramischer Composit-Werkstoff und/oder Kohlenstoff ist.
 
4. Beschichtetes Substrat gemäß Anspruch 3, dadurch gekennzeichnet, daß das Substrat Sn02 enthält.
 
5. Verfahren zur Herstellung eines beschichteten Substrats gemäß einem der vorangegangenen Ansprüche, dadurch gekennzeichnet, daß man das Substrat in einem Elektrolyten in Form einer Salzschmelze, welche Cer und mindestens ein Dopingelement ausgewählt aus der Gruppe bestehend aus Yttrium, Lanthan, Praseodym und anderen Seltenen Erdmetallen enthält, anodisch polarisiert.
 
6. Verfahren gemäß Anspruch 5, dadurch gekennzeichnet, daß der Elektrolyt Kyrolit ist.
 
7. Verfahren gemäß den Ansprüchen 5 oder 6, dadurch gekennzeichnet, daß die Konzentration der (des) Dopingelemente(s) in dem Elektrolyt im Bereich des 0,1 - bis 100fachen der Cerkonzentration liegt.
 
8. Verfahren gemäß den Ansprüchen 5, 6 oder 7, dadurch gekennzeichnet, daß die Verbindungen der Dopingelemente Oxide und/oder Fluoride sind.
 
9. Dimensionsstabile Anode zur elektrolytischen Gewinnung eines Metalls, welches in Form seines Oxides in einem geschmolzenen Salz als Elektrolyt vorliegt, wobei die Anode ein beschichtetes Substrat gemäß einem der Ansprüche 1 bis 4 oder ein mit Hilfe des Verfahrens gemäß einem der Ansprüche 5 bis 8 hergestelltes Substrat enthält.
 
10. Verfahren zur Herstellung eines Metalls durch Elektrolyse einer Verbindung des Metalls, welche in einer Salzschmelze als Elektrolyt gelöst vorliegt, unter Verwendung einer Anode gemäß Anspruch 9, dadurch gekennzeichnet, daß man zu dem Elektrolyt eine Verbindung oder Verbindungen zufügt, welche mindestens ein Dopingelement ausgewählt aus der Gruppe enthaltend Yttrium, Lanthan, Praseodym oder andere Seltene Erdmetalle enthält, und daß man während der gesamten normalen Elektrolyse ausreichende Konzentrationen an Cer und gegebenenfalls an den Dopingelementen aufrecht erhält.
 
11. Verfahren gemäß Anspruch 10, dadurch gekennzeichnet, daß das elektrolytisch gewonnene Metall Aluminium ist.
 
12. Verfahren gemäß Anspruch 10 oder 11, dadurch gekennzeichnet, daß man die Beschichtung auf dem Substrat außerhalb einer Zelle zur elektrolytischen Metallgewinnung aus Salzschmelzen vor Verwendung der Anode in der Zelle oder zu Beginn oder während der normalen Elektrolyse unter Betriebsbedingungen innerhalb der Zelle zur elektrolytischen Metallgewinnung aufbringt.
 
13. Verwendung des beschichteten Substrats irgendeines der Ansprüche 1 bis 5 als chemischer Sensor für Sauerstoff und/oder Fluor enthaltendes Gas.
 




Drawing