METHOD OF PRODUCING ALUMINUM, ALUMINUM PRODUCTION CELL AND ANODE FOR ALUMINUM ELECTROLYSIS

Abstract: A method of producing aluminum by electrolysis of alumina which is dissolved in a molten cryolite bath is described which uses a dimensionally stable anode compris­ ing a substrate composed of a conductive ceramic, a cermet, a metal, an alloy, an intermetallic compound and/or carbon and a coating thereon comprising a rare earth metal oxide or a rare earth metal oxyfluoride which is preserved by maintaining a concentration of the rare earth metal in the electrolyte. By adding to the bath a contamination inhibiting agent the contamination of the aluminum by substrate components due to corrosion of the substrate by the electrolyte contacting the substrate at imperfections of the coating may be inhibited. The contamination inhibiting agent may be a compound of an alkali or an alkaline earth metal, such as a fluoride, in particular LiF or MgF₂.

Description: The invention relates to a method of producing aluminium by electrolysis of alumina dissolved in a molten cryolite bath using a dimensionally stable anode comprising a substrate which is unstable under the conditions of the electrolysis, said substrate being coated with a layer of cerium oxyfluoride and being preserved by maintaining cerium in the electrolyte. The invention further relates to an aluminium electrowinning cell comprising a dimensionally stable anode having a substrate and a coating thereon, and a molten cryolite bath. The invention finally relates to an anode for the electrolytic production of aluminium by electrolysis of an alumina containing bath of molten cryolite. BACKGROUND ART European Patent Application 0 114 085 discloses a dimensionally stable anode for an aluminium production cell which anode comprises a substrate of a ceramic, a metal or other materials which is coated with a layer of a cerium oxycompound. The anode is stable under conditions found in an aluminium production cell, provided a sufficient content of cerium is maintained in the electrolyte. The anode described in the above European Patent Application performs well with respect to dimensional stability, but contamination of the aluminium by substrate components may occur under certain circumstances. As shown by microphotographs, the cerium-containing coating generally has a non-homogeneous structure leaving small interstices between coated areas, allowing 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 aluminium by substrate components. French Patent Application 2 407 277 discloses a method of electrolysing chlorides of eg. magnesium, sodium, calcium or aluminium in electrolytes at temperatures between 500-800°C using an anode comprising a substrate and a coating of an oxide of a noble metal, while maintaining in the bath a concentration of an oxide or oxychloride of a metal which is more basic than the metal produced. Thus, by increasing the basicity of the bath, the solubility of the anode coating is reduced. This method provides better stability of the anode coating by the addition of melt additives. It relates to the stabilization and protection of the anode coating and not of the substrate as is one of the hereunder defined objects of the present invention. In the above Patent Application, the substrate itself is stable in the chloride bath at the given operating temperature and is effectively protected by the coating. In contrast, in a molten cryolite bath at eg. 960°C an imperfect coating or substrate may not simply be protected against corrosion by modifying the basicity of the bath as described in the French patent but is unstable and corrodes. In a cryolite bath, a mere modification of the basicity would not improve the stability of the substrate. OBJECTS OF THE INVENTION One object to the invention is to provide a remedy for the above described contamination problem. Another object of the invention is to provide a method of producing aluminium using a dimensionally stable anode having a coating with self-healing effect due to bath additions, wherein the contamination of the aluminium by substrate components is inhibited. It is a further object of the invention to provide a technique for inhibiting the contamination of the aluminium by substrate components which is simple to apply, in-expensive and does not require any modifications of the anode itself or of the cell. SUMMARY OF THE INVENTION The above and other objects are met by a method of producing aluminium by electrolysis of alumina dissolved in a molten cryolite bath at a temperature of about 960°C or more using a dimensionally stable anode comprising a substrate which is unstable under the conditions of the electrolysis, said substrate being coated with a substantially continuous and stable layer of cerium oxyfluoride preserved by maintaining cerium in the electrolyte, but having imperfections through which substrate components could diffuse, characterized by the bath further containing at least one compound of Mg or Li as contamination inhibiting agent producing at the imperfections in the cerium oxyfluoride coating a barrier inhibiting contamination of the product aluminium by preventing substrate components diffusing through the imperfections in the coating. The contamination inhibiting agent may in particular be MgF₂ or LiF, the amount of which compared to the total bath composition may be in the range of 1-20w% for MgF₂ and 1-30w% for LiF. The anode substrate may be composed of a conductive ceramic, a cermet, a metal, an alloy, an intermetallic compound and/or carbon, a preferred substrate being eg. SnO₂ or SnO₂-based materials such as described in US patent 3,960,678 comprising sintered SnO₂ and small amounts of other oxides of eg. Fe, Sb, Cu, Mn, Nb, Zn, Cr, Co and W. Other suitable substrates disclosed in US patents 4,187,155 and 4,146,638 comprise a matrix of sintered powders of an oxycompound of at least one metal selected from titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminium, silicon, tin, chromium, molybdenum, tungsten, lead, manganese, beryllium, iron, cobalt, nickel, platinum, palladium, osmium, iridium, rhenium, technetium, rhodium, ruthenium, gold, silver, cadmium, copper, zinc, germanium, arsenic, antimony, bismuth, boron, scandium and metals of the lanthanide and actinide series; and at least one electroconductive agent selected from metallic yttrium, chromium, molybdenum, zirconium, tantalum, tungsten, cobalt, nickel, palladium and silver. Generally, the substrate may also be composed of an electroconductive body covered by a sub-coating of one of the above materials, in particular SnO₂ which in turn is covered by a cerium oxyfluoride coating. The contamination inhibiting barrier may be formed of a substance obtained by adding a contamination inhibiting agent into the bath, the contamination inhibiting agent being a compound of Mg or Li, in particular the fluorides MgF₂ and LiF. The contamination inhibiting barrier may comprise MgAl₂O₄ particularly in the form of a spinel. DETAILED DESCRIPTION OF THE INVENTION The dimensionally stable anodes to which the present invention is related are described in European Patent Application 0,114,085. In use, the anode coatings of cerium oxyfluoride remain stable but there may be a contamination of the aluminium by corrosion of the substrate to which the electrolyte finds limited access by small imperfections of the cerium-containing coating. The principle on which the present invention is based lies in the employment of a contamination inhibiting agent, which per se or in the form of a compound obtained by adding this agent into the electrolyte may infiltrate into the imperfections of the cerium-containing coating to block channels, cracks, open pores and so forth so that contact of the cryolite with the substrate is inhibited. By doing so it is understood that the basic structure of the coating is not changed, only the voids which lead to the exposure of finite portions of the substrate are obstructed. The maintainance of this contamination inhibiting barrier is assured by maintaining in the electrolyte a certain concentration of the agent which forms or produces this barrier, which agents must be non-reduceable by the cathode. It has been found that Mg and Li compounds, in particular fluorides, are effective as such agents. Without limitation to a theory it is believed that in the case of MgF₂ as the contamination inhibiting agent, MgAl₂O₄ comprising a spinel structure precipitates within the voids of the anode coating, inhibiting the electrolyte from contacting the substrate. Another possible explanation of the contamination inhibiting effect of the described agent may be the formation of complexes formed by the said agent and components of the substrate, these complexes forming a barrier along the coating-electrolyte interface comprising a high concentration of such complexes which inhibits access of the electrolyte to the substrate and thereby decreases further corrosion at endangered locations. EXAMPLES Example 1 In a test cell for electrolytic production of aluminium using an SnO₂ anode substrate in the shape of a cylinder with a semi-spherical lower end with dimensions : 12mm diameter and 13mm length, electrolysis was carried out for 30 hours at 960°C. The bath was a basic electrolyte of 88.8w% Na₃AlF₆, 10w% Al₂O₃ and 1.2w% CeF₃ to which were added 20w% LiF. The cathode was a 15mm diameter and 6.2mm high disc of TiB₂. The total current was 1.8 A and the anodic and cathodic current densities were 0.4 A/cm². After the electrolysis, the substrate was coated with a 0.5mm thick layer of a cerium oxyfluoride, weighing 0.89g. The produced aluminium was analyzed for contamination by the substrate, and a Sn concentration smaller than 100 ppm was detected. Under the same electrolysis conditions with a cerium oxyfluoride coating but without the use of any LiF in the cryolite the Sn contamination in aluminium amounted to 1.0 %. Example 2 In a bath comprising the same basic electrolyte to which were added 5w% MgF, electrolysis was carried out at a temperature of 970°C for 118 hours. The dimensions of the SnO₂ anode substrate were : 12.8mm diameter by 21.6mm length. The TiB₂ cathode dimensions were 18mm diameter by 6.2mm height. The total current was 1.8 A with anodic and cathodic current densities of 0.25 A/cm². After the electrolysis, the Sn contamination in the aluminium was found to be 280 ppm. The coating was found to be a fissured layer of fluorine-containing CeO₂ wherein the fissures were at least partially filled with MgAl₂O₄ of spinel structure. Under the same electrolysis conditions, with a cerium oxyfluoride coating but without the use of any MgF₂ in the cryolite, the Sn contamination in aluminum amounted to 1.5 %.

Claims: A method of producing aluminium by electrolysis of alumina dissolved in a molten cryolite bath at a temperature of about 960°C or more using a dimensionally stable anode comprising a substrate which is unstable under the conditions of the electrolysis, said substrate being coated with a substantially continuous and stable layer of cerium oxyfluoride preserved by maintaining cerium in the electrolyte, but having imperfections through which substrate components could diffuse, characterized by the bath further containing at least one compound of Mg or Li as contamination inhibiting agent producing at the imperfections in the cerium oxyfluoride coating a barrier inhibiting contamination of the product aluminium by preventing substrate components diffusing through the imperfections in the coating. The method of claim 1, characterized the substrate being composed of a conductive ceramic, a cermet, a metal, an alloy, an intermetallic compound and/or carbon. The method of claim 1, characterized by the substrate being composed of SnO₂ or a material comprising SnO₂ as a major component. The method of claim 1, characterized by the substrate being coated with SnO₂ or a material comprising SnO₂ as a major component. The method of any preceding claim, characterized by the addition of MgF₂ in an amount of 1-20w% of the total bath composition. The method of any one of claims 1-4, characterized by the addition of LiF in an amount of 1-30w% of the total bath composition. An aluminium electrowinning cell comprising a dimensionally stable anode immersed in a bath of molten cryolite containing dissolved alumina at a temperature of 960°C or more, the anode comprising a substrate which is unstable under the conditions of the electrolysis, said substrate being coated with a substantially continuous and stable layer of cerium oxyfluoride preserved by maintaining cerium in the electrolyte, characterized by the bath further containing at least one compound of Mg or Li as contamination inhibiting agent, there being at the imperfections in the cerium oxyfluoride coating a barrier inhibiting contamination of the product aluminium by preventing substrate components diffusing through the imperfections in the coating, said barrier being produced from said at least one Mg or Li compound. The cell of claim 7, characterized by the substrate being composed of a conductive ceramic, a cermet, a metal, an alloy, an intermetallic compound and/or carbon. The cell of claim 7, characterized by the substrate being composed of or coated with SnO₂ or a material comprising SnO₂ as a major component. The cell of claim 7, 8 or 9, characterized by the bath containing MgF₂ in an amount of 1-20w% of the total bath composition. The cell of claim 7, 8 or 9, characterized by the bath containing LiF in an amount of 1-30w% of the total bath composition. An anode for the electrolytic production of aluminium by electrolysis of an alumina containing electrolyte of molten cryolite, the anode comprising a substrate which is unstable under the conditions of the electrolysis, said substrate being coated with a substantially continuous and stable layer of cerium oxyfluoride which in use may be preserved by maintaining cerium in the electrolyte, characterized by the coating including a contamination inhibiting barrier based on at least one compound of Mg or Li located within imperfections of the cerium oxyfluoride coating for inhibiting contamination of the aluminium produced in use of the anode by preventing substrate components diffusing through the imperfections in the coating. The anode of claim 12, characterized by the substrate being composed of a conductive ceramic, a cermet, a metal, an alloy, an intermetallic compound and/or carbon, and the contamination inhibiting barrier being formed of a substance obtained by adding a compound of Mg or Li as contamination inhibiting agent into an electrolyte in which the electrode is or has been dipped. The anode of claim 13, characterized by the substrate being composed or of coated with SnO₂ or a material comprising SnO₂ as a major component. The anode of claim 13 or 14, characterized by the contamination inhibiting barrier comprising MgAl₂O₄.

Inventors: DURUZ, JEAN-JACQUES
Applicants: MOLTECH INVENT S.A.
Application year: 1986
Main classification: C25C3
IPC codes: C25C3