MOLTEN SALT ELECTROWINNING ELECTRODE, METHOD AND CELL

Abstract: A bipolar electrode for electrowinning aluminum or other metals by electrolysis of a molten salt electrolyte containing a dissolved compound of the metal to be won comprises an anodic (5) and a cathodic (6) surface which are both preserved during operation by dissolution of small amounts of a substance in the electrolyte (7) which is capable of being deposited on either surface at a rate compensating the corrosion thereof during electrolysis. The anodic surface (5) is for example cerium oxyfluoride and the cathodic surface cerium hexaboride (6), both surfaces being preserved by addition of cerium compounds, such as oxides, fluorides, hydrides etc. to the melt. The cathodic surface may also include titanium diboride on top of or together with cerium hexaboride.

Description: FIELD OF THE INVENTION The present invention relates to a method of electrowinning a metal by electrolysis of a compound of the metal dissolved in a molten salt electrolyte, and an electrode for this purpose. The invention further relates to a cell for molten salt electrowinning comprising at least one electrode according to this invention. BACKGROUND ART In the electrowinning of aluminium by electrolysis of alumina dissolved in molten cryolite, considerable efforts have been made to provide dimensionally stable materials for cell components which are in contact with the liquid contents of the cell. Such components include the electrodes as well as lining materials and elements which are immersed in the liquid aluminium to restrict bath movements. Among the materials proposed for use under the severe corrosion conditions in a molten salt electrolysis cell are primarily the refractory oxides, the Refractory Hard Metal (RHM) borides, and cermets containing either of them together with an intimately mixed metallic phase for applications where high electrical conductivity is essential. Refractory ceramic and cermet materials are known from numerous publications. These materials are used in a wide variety of applications and their specific composition, structure and other physical and chemical properties may be adapted to the specific intended use. Materials which were proposed for use as anodes in molten salt aluminium electrowinning cells are mainly based on oxides of eg. iron, cobalt, nickel, tin and other metals, which oxides may be provided with enhanced electronic conductivity by doping, non-stoichiometry and so forth. Cathodic materials are mainly based on titanium diboride and similar RHM boride compounds. A completely new concept for a dimensionally stable inert anode for an aluminium-production cell and its manufacture was described in EP-A-0 114 085, wherein a fluorine-containing oxycompound of cerium (referred to as "cerium oxyfluoride") is deposited in-situ on an anode substrate during electrolysis, by maintaining a suitable concentration of a cerium compound dissolved in the melt. In EP-A-0 094 353 it has also been proposed to use materials in a molten salt aluminium electrowinning cell which are composed of a refractory ceramic coated with TiB₂ and wherein the TiB₂ coating is maintained by the addition of titanium and boron to the liquid aluminium. EP-A-0 257 708 which was simultaneously filed with the present application discloses a new substrate material for the above described cerium oxyfluoride anode coating, this new substrate material being a cermet having a ceramic phase basically comprising a mixture of cerium oxide(s) and alumina and a metallic phase comprising an alloy of cerium and aluminium. EP-A-0 115 689 describes an oxide-boride ceramic body which may be part of the cathode of an electrolytic cell, this body being prepared by reaction sintering. As cathodic materials, however, oxide-boride materials are not as stable as simple borides. SUMMARY OF THE INVENTION The invention sets out to provide a metal electrowinning method where an anode surface and a cathodic section can simultaneously be maintained dimensionally stable during operation. The invention also provides an electrode for this method, which electrode may be a bipolar electrode. In the method of producing a metal by molten salt electrolysis according to the invention, as set out in claims 1 to 9, an all-boride cathode which necessarily contains cerium boride, with possible additives of microdispersed aluminium, TiN and CeN, is preserved by cerium ions in the molten electrolyte which also preserve an anodic surface. An electrode for the above method, as set out in claims 10 to 18, has a cerium-boride-containing all-boride cathode part, with possible additives as before. The term "cathode substrate" as used herein includes the special case where both the cathodic substrate and the cathodic surface are made of the same boride(s) of the same metal(s) ie. a bulk material. Thus, the cathodic section of an electrode according to the present invention may, in the case where the entire cathodic section consists of the same material, be made entirely of a bulk material such as cerium boride or, in the case where it comprises a cathodic substrate and a cathodic coating, these two parts may be made of different materials. The cathodic substrate always contains cerium boride alone or together with another rare earth metal boride, alkaline earth metal boride or alkali metal boride. The cathodic substrate must comply with two physical requirements ie. electrical conductivity and thermodynamic stability with the cathodic coating and, in the case of a bipolar arrangement, also with the anodic section. The cathodic substrate necessarily comprises a cerium boride which may be mixed with another boride such as titanium diboride, and the cathodic surface may be a cerium boride, cerium hexaboride being the preferred one, and/or another boride such as titanium diboride or other RHM compounds. The cathodic surface material, ie. the cathodic substrate or the cathodic coating may also comprise additives, namely microdispersed aluminium, TiN or CeN. In a preferred embodiment of the invention, the electrode is a bipolar electrode. In this case, the electrode body has a second, anodically polarized section comprising an anodic substrate and an anodic surface. This anodic surface may be a surface coating or a surface part of a bulk anode section and may be made of or may comprise an oxycompound of cerium, cerium oxyfluoride being preferred. The anodic and cathodic sections of a bipolar electrode according to the invention may be separated by an intermediate stable layer of an alloy or a compound of cerium and another metal such as copper, silver, or a noble metal. In the case where the anodic surface is a coating on an anodic substrate, this anodic substrate may be a cermet having a ceramic phase made of a mixture of cerium oxide(s) and alumina, or mixed oxides, and sulphides nitrides, or phosphides of at least one of cerium and aluminium, and a metallic phase composed of an alloy of cerium and aluminium and optionally silver, and/or at least one noble metal. In bipolar electrodes according to the invention, the anodic surface, be it an anodic coating or a surface part of a bulk anodic section, may be produced in-situ, ie. prior to or during the electrowinning process in the cell by deposition of cerium oxyfluoride onto the anodic section, or ex-situ, by sintering, hot-pressing, spraying or painting and curing of cerium oxyfluoride or a precursor thereof onto the anodic substrate. The cathodic coating is produced ex-situ by sintering, hot-pressing, spraying or painting and curing of cerium hexaboride or, in the case of titanium diboride or another RHM boride compound, by sintering of a powder of TiB₂ or another RHM boride or by reaction sintering a precursor thereof onto the cathodic substrate. An electrode as described above may be used as already mentioned for electrowinning aluminium by electrolysis of alumina dissolved in molten cryolite. However, its use in other metal winning processes using a liquid metal cathode is also contemplated. In the case where the anodic surface consists of cerium oxyfluoride and the cathodic surface consists of cerium hexaboride, cerium or cerium compounds alone may be added to the melt, and a suitable concentration of cerium-containing ions maintained. More generally, cerium together with the other rare earth metal(s), alkaline earth metal(s) or alkali metal(s) included in the cathodic and anodic surfaces, or at least in one of them, are added to the melt. The substance added to the electrolyte in order to maintain a suitable concentration of cerium-containing ions may be selected from oxides, halides, oxyhalides and hydrides of cerium. The concentration of cerium-containing ions in the electrolyte may be chosen well below the solubility limits of the cerium compounds, as the maintenance process of the anodic and cathodic surfaces is not a simple dissolution-deposition mechanism of cerium-containing ions. DETAILED DESCRIPTION OF THE INVENTION The method according to the invention may be performed in a molten salt electrowinning cell in a variety of different cell configurations. Thus, the cathode may be of the drained type, eg. a bulk body of cerium hexaboride maintained dimensionally stable by maintaining cerium ions in the electrolyte. This causes a small concentration of metallic cerium in the electrowon metal such as aluminium in contact with the cathodic surface, which preserves the cathodic cerium hexaboride surface. This cathode is used with an inert anode having an anode substrate coated with a cerium oxyfluoride coating which is simultaneously maintained dimensionally stable by the cerium ions in the electrolyte. The cathode used may also comprise a structure where the cerium (or cerium plus other metal M₁ and/or M₂) is confined to the cathodic substrate, and the cathodic surface is constituted of a coating of eg. titanium diboride or another RHM boride. Another type of electrode according the invention is employed in a bipolar configuration. Each bipolar electrode has an anodic part including a cerium oxyfluoride coating on an appropriate anodic substrate and a cathodic part which may for example be entirely formed of cerium hexaboride or may have a substrate of cerium hexaboride coated with titanium diboride or another RHM boride, or may be cerium boride coated on a composite substrate. The following description concerns one embodiment, namely a bipolar configuration with an anodic surface constituted of cerium oxyfluoride and a cathodic surface or cerium hexaboride. Manufacture of its cathodic and anodic sections are considered separately. The operation and maintenance of this electrode are discussed later. CATHODIC ELECTRODE SECTION In this example, the electrode comprises a bulk cathodic section ie. the entire cathodic section including the cathodic surface consists of the same material throughout. This cathodic section consists of a dense structure of cerium hexaboride produced by sintering cerium hexaboride powder into a sheet of rectangular cross section. This sheet may conveniently be produced by sintering, and the resulting sintered sheet attached to the aforementioned intermediate stable layer prior to or during assembly with the anodic section. This intermediate layer may comprise at least one metal such as copper, silver and the noble metals, this metal being chosen such that its oxide is less stable than cerium oxide. It may further comprise a cerium alloy (eg. cerium-aluminium) or a cerium compound. As the oxides of these metals are less stable than cerium oxide, no reduction of cerium oxide will occur when an anodic cerium oxide layer comes into contact with the intermediate layer. Further, the intermediate layer must be electrically conductive and thermodynamically stable in contact with the anodic section and the cathodic section ie. cerium hexaboride. Alternatively, the bulk cathodic section may be a mixture of cerium hexaboride and a boride of at least one other metal selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mg, Si, Al, La, Y, Mn, Fe, Co and Ni. Further, the cerium hexaboride or the mixture of cerium hexaboride and the boride of these other metals may comprise microdispersed aluminium which improves the electrical conductivity and mechanical properties of the cathodic section. If the anodic substrate is chemically stable in contact with cerium hexaboride, no stable intermediate layer is required. In alternative embodiments, the described cathodic section may comprise, at or adjacent its surface, additions of TiB₂ or a TiB₂/Al cermet, or it may be coated with these materials. Where the electrode according to the invention is a cathode only, it may be produced in a shape which can be fitted in a known aluminium electrowinning cell with drained cathode configuration replacing the classical carbon cathode, eg. in the form of a layer to be arranged on the cell bottom. However, the preferred embodiment of this invention is a bipolar electrode of sheet-like shape with the cathodic section on one side and the anodic section on the other. If the cathodic section is not produced on a stable intermediate layer, it may be combined with such a layer by any suitable process such as cladding, sintering, plasma spraying, bonding or the like. In a subsequent process step or simultaneously, the anodic substrate may be applied to the back surface of the stable intermediate layer by any suitable process including sintering, plasma spraying, bonding or the like. ANODIC ELECTRODE SECTION The anodic substrate may be any electronically conductive material which is sufficiently resistant to corrosion by the electrolyte of an aluminium electrowinning cell to withstand exposure to the electrolyte during its subsequent in-situ coating process without unduly contaminating the bath. Alternatively, if the anodic coating is applied to the anodic substrate ex-situ , eg. by sintering, this requirement is less stringent, as the electrode will only come into contact with the electrolyte after the protective anode coating has been applied. Materials which come into consideration for the anode substrate are doped oxides, such as tin dioxide, zinc oxide, cerium oxides, copper oxides or others, and cermets. Specially preferred is a cermet having at least one of copper, silver and the noble metals optionally associated with a cerium-aluminium alloy as metallic phase and at least one of the following : doped tin dioxide, doped zinc oxide, doped cerium oxides or oxyfluorides, or a mixture of ceriumalumina or a cerium/aluminium mixed oxide optionally associated with other compounds or cerium or aluminium, such as nitrides or phosphides, as ceramic phase. These cermets have suitable physical and chemical properties and should not contain substantial amounts of other substances which would contaminate the liquid contents of an aluminium electrowinning cell by corrosion on start-up or during operation of the electrode. The preferred cermet material may be produced by sintering powders of cerium and aluminium together with their oxides, or by sintering powders of these oxides in a reducing atmosphere or by sintering the metal powders under an oxidizing atmosphere. The preferred method is reaction sintering of aluminium metal with oxides of cerium. An example of the production of cermet by sintering cerium oxide and aluminium is included in Example 2 below. In the case where ceria is present in the anodic substrate material, an intermediate layer must be chosen which is thermodynamically stable therewith, as discussed above. Production of the anodic coating on the anode substrate may be carried ex-situ by sintering, plasma-spraying, hot-pressing, painting and curing, or by any other suitable known method. One preferred process, however, is the in-situ formation of an anodic coating during operation of the electrode in an aluminium electrowinning cell. IN-SITU PRODUCTION OF ANODE COATING AND PRESERVATION OF ANODIC AND CATHODIC COATINGS The electrode prepared according to the above process steps may be introduced into an aluminium electrowinning cell comprising a molten cryolite electrolyte containing up to 10 weight % alumina dissolved therein. Additionally, this electrolyte contains a cerium compound added in a concentration of, for example, about 1-2 weight %. Generally, when cerium is dissolved in a fluoride melt the protective anode coating is predominantly a fluorine-containing oxycompound of cerium, referred to as "cerium oxyfluoride". When dissolved in molten cryolite, cerium remains dissolved in a lower oxidation state but, in the vicinity of an oxygen-evolving anode, oxidizes in a potential range below or at the potential of oxygen evolution and precipitates as a fluorine-containing oxycompound which remains stable on the anode surface. The thickness of the fluorine-containing cerium oxycompound coating can be controlled as a function of the amount of the cerium compound introduced in the electrolyte, so as to provide an impervious and protective coating which is electronically conductive and functions as the operative anode surface, ie. in the present case an oxygen evolving surface. Furthermore, the coating is self-healing or self-regenerating and it is permanently maintained by keeping a suitable concentration of cerium in the electrolyte. The term fluorine-containing oxycompound is intended to include oxyfluoride compounds and mixtures and solid solutions of oxides and fluorides in which fluorine is uniformly dispersed in an oxide matrix. Oxycompounds containing about 5-15 atom % of fluorine have shown adequate characteristics including electronic conductivity; however, these values should not be taken as limiting. For cerium alone, without another metal M₁, the oxycompound can have a composition of the formula CeO x F y where x = 0.05 to 0.15. Additions of tantalum, niobium, yttrium, lanthanum, praseodymium and other rare-earth-element-containing species in small quantities have been reported to increase the density of the cerium oxyfluoride anodic coating, thereby rendering it more impervious, tantalum and niobium or their oxides also improving the electrical conductivity. The metal being electrowon must be more noble than the cerium (Ce³⁺) dissolved in the melt, so that the electrowon metal preferably deposits at the cathode with only a small cathodic deposition of cerium, with a concentration of cerium metal in the molten electrowon metal sufficient to inhibit the dissolution of the cerium hexaboride of the cathodic surface. The metals to be electrowon can be chosen from lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium, barium, aluminium, gallium, indium, thallium, titanium, zirconium, hafnium, vanadium, niobium, tantalum, manganese and rhenium. The concentration of cerium ions dissolved in the lower valency state in the electrolyte will usually be well below the solubility limit of cerium in the melt. For example, when up to 2 % by weight of cerium is included in a molten cryolite-alumina electrolyte, the cathodically won aluminium will contain only 1-3 % by weight of cerium. This can form an alloying element for the aluminium or, if desired, can be removed by a suitable process. The in-situ produced anodic coating provides an effective barrier shielding the anodic substrate from the corrosive action of molten cryolite. Various cerium compounds can be dissolved in the melt in suitable quantities, the most usual ones being halides (preferably fluorides), oxides, oxyhalides and hydrides. However, other compounds can be employed. The compounds can be introduced in any suitable way to the melt before and/or during electrolysis. The cathodic and anodic surfaces will be preserved by maintaining a suitable concentration of cerium ions in the electrolyte. This concentration, of course, depends on the exact bath chemistry and has to be chosen such that an equilibrium is established at both anodic and cathodic surfaces between the rate at which the cerium compounds at the surfaces are corroded by the liquid cell contents and the rate of re-deposition of cerium containing species onto the respective surface. The anodic deposition, be it initial deposition on a blank substrate or continuous deposition once the coating has been formed and is to be preserved, follows the deposition process described above. However, the ex-situ produced cathodic surface of the bipolar electrode only requires to be preserved. BRIEF DESCRIPTION OF THE DRAWINGS The invention is further described with reference to the drawings, in which : Fig. 1 is a schematic illustration of the laminated configuration of a bipolar electrode according to the invention, and Fig. 2 is a schematic view of an aluminium electrowinning cell employing a plurality of bipolar electrodes according to the invention. With reference to Fig. 1, reference number 1 designates the intermediate, stable layer comprised of a Ce/M alloy or intermetallic compound, where M is at least one of copper, silver and the noble metals gold, platinium, iridium, osmium, palladium, rhodium and ruthenium. The layer 1 is coated on one side with a layer 2 constituting the cerium/aluminium-ceria/alumina cermet, constituting the anodic substrate of the electrode. This anodic substrate 3 has a top coating 4, which may be in-situ generated cerium oxyfluoride, in contact with the molten electrolyte 7. Oxygen evolution takes place at the anodic surface 5 and reduction of aluminium ions to aluminium metal occurs at the cathodic surface 6. The anodic surface 5 is preserved and protected against excessive corrosion from the electrolyte by maintaining a concentration of cerium ions in the electrolyte 7. During steady-state operation, these ions deposit on the anodic surface 5 at the same rate as they are dissolved in the electrolyte thereby maintaining the anodic surface dimensionally stable. The cathodic surface 6 is preserved by metallic cerium species present in a surface film 11' of molten aluminium which adheres to the cathodic surface. In practice, the edge portion of the intermediate layer 1 exposed to the electrolyte 7 will be protected by a protective layer which could, eg. be a base layer of cerium oxyfluoride also protecting the edge of the anodic substrate 3. The edge of cathodic layer 2 will be covered and protected by the surface film 11'. Fig. 2 is a schematic representation of an aluminium electrowinning cell having a container 8 for the liquid cell contents 9, and a symmetrically inclined bottom portion 10 which serves to collect the electrowon aluminium 11 in a central through 12. The inner space of the container 8 includes a plurality of bipolar electrodes 13" as illustrated in Fig. 1, as well as an anodic terminal electrode 13 and a cathodic terminal electrode 13'. The anodic terminal electrode 13 comprises an anodic substrate 13a and an anodic coating 13b entirely surrounding the anodic substrate 13a. The cathodic terminal electrode 13" comprises a cathodic body 13d. Each bipolar electrode comprises an anodic substrate 13a, an anodic coating 13b, a stable intermediate layer 13c and a cathodic section 13d. The container 8 is closed at the top by a cover 14. An anodic current feeder 16 extends down from an anodic terminal 18 through the cover 14 and is connected to the anodic terminal electrode 13'. A cathodic current feeder 17 extends down from a cathodic terminal 19 through the cover 14 and is connected to the cathodic terminal electrode 13". Auxiliary equipment of the cell such as electrode supports, alumina feeders and the like is not shown. The cell container 8 has an internal lining 15 which may be made of cerium hexaboride or any other material which is resistant against corrosion by the liquid cell contents 9. Thus, the cell container 8 may be made of an alumina body or packed alumina which is coated on its internal surfaces with borides such as TiB₂, CeB₆ or CeB₄. The bipolar electrodes 13" are all orientated such that their anodic surfaces face the side of the cell at which the cathodic current feeder 16 enters, and their cathodic surfaces face the other side. Electrolysis is carried out by passing current from the anodic terminal electrode 13 across the bipolar electrodes 13" and the interelectrode gaps 20 to the cathodic terminal electrode 13' from where it leaves the cell via the cathodic current feeder 17. EXAMPLES The above described process of producing an electrode is now described by way of examples in which anodic and cathodic parts of the electrode are produced in subsequent steps. Example 1 On a sheet substrate of a Ce/Al/Ag alloy of 100 mm x 100 mm square surface and 5 mm thick, 200 g of cerium hexaboride powder (ALFA 99 % pure, 325 mesh) is consolidated by cold pressing at a pressure of 32 megapascals. Subsequently, the substrate together with the pressed powder are hot pressed at a temperature of 1150°C under a uniformly-maintained pressure of 20 megapascals for one hour. The resulting composite body is a laminate of the original sheet substrate and a dense sintered layer of cerium hexaboride. Example 2 On the uncoated back surface of the laminated sheet as produced in Example 1, 32g of a mixed CeO₂/Al powder containing 82.7 weight % CeO₂ of a grain size between 25 and 35 micrometers (FLUKA AG, of purity higher than 99%) and 17.3 weight % of aluminium (CERAC, of 99.5% purity, 325 mesh) is cold pressed at 32 megapascals to a flat, sheet-like composite body. The density of the pressed CeO₂/Al powder is 57% of the theoretical density. Subsequently, the composite body is hot pressed under 20 megapascals at 1150°C for one hour and at 1250°C for another hour. The cermet part of the consolidated final composite body has a density of 75% of theoretical density. While the substrate has a completely dense structure, the cermet part has a porous central region (the pores have dimensions from 20-50 micrometers) surrounded by a denser region containing only closed macropores. Both of these regions have similar microstructure, ie. a finely dispersed quasi continuous network of cerium aluminate impregnated with a metallic Al₂Ce matrix. The ceramic phase consists of a very finely interconnected grain structure of vermicular or leaf-like grains having a length dimension of 5-10 micrometers and a cross dimension of 1-2 micrometers. Example 3 A laminated sheet as produced in Example 2, comprising an intermediate stable layer of a Ce/Al/Ag alloy with a cerium hexaboride layer on one side and a cerium/aluminium-ceria/alumina cermet on the other side, as well as two terminal electrode sections, one being cathodic and the other anodic, are introduced into a laboratory electrolysis cell comprising a graphite cylinder closed at the bottom by a graphite disc and filled with a powder of cryolite containing 10 weight % alumina and 1.2 weight % of CeF₃. The laminated sheet is arranged in spaced parallel relationship with the terminal electrodes, the flat surfaces facing each other across suitable interelectrode gaps. The cathodic terminal electrode comprises a cerium hexaboride surface facing the anodic substrate of the laminated sheet. The cathodic surface of the laminated sheet faces the anodic terminal electrode comprising an exposed anodic substrate. The anodic terminal electrode is electrically connected with the positive pole and the cathodic terminal with the negative pole of a current source. The assembly is heated to 970°C and upon melting of the cryolite powder the current source is activated to pass current through the electrodes and the interelectrode gaps. During passage of current, cerium oxyfluoride deposits on the anodic substrates of the bipolar electrodes and oil the anodic terminal electrode. After initial deposition of the cerium oxyfluoride on the anodic surfaces, an equilibrium state is reached and a stable cerium oxyfluoride layer is obtained. However, as small amounts of cerium metal are cathodically deposited and withdrawn from the cell together with the electrowon aluminium, cerium compounds should be added from time to time to compensate for these cerium losses. Example 4 200g of cerium hexaboride powder (ALFA 99% pure, 325 mesh) were consolidated by cold pressing at a pressure of 32 megapascals into a sheet measuring approximately 100x100x5mm. The consolidated sheet was then hot pressed at 1600°C for 30 minutes under a pressure of 20 megapascals. A plate of doped cerium oxyfluoride having approximately the same dimensions was produced by cold pressing 200g of a 325 mesh powder mixture, of 93.9% CeO₂, 3.1% CeF₃, 1.0% Nb₂O₅ and 2% Cu at a pressure of 32 megapascals followed by sintering at 1550°C for 1 hour under Argon. The sheets of cerium hexaboride and doped cerium oxyfluoride were then sandwiched together with an interposed 100x100x0.5mm sheet of copper foil, and clad or bonded together to form an assembly by heating at 1100°C under Argon for a suitable time, e.g. about 3 minutes. The resulting assembly is suitable for use as a bipolar electrode in a laboratory-scale aluminium production electrolysis cell as described in Example 3. Example 5 The procedure of Example 4 was followed, except that the copper foil was replaced by a 325 mesh powder mixture of 50 g Cu (metal) and 30 g Ce₂O₃, which formed a layer about 2 mm thick in the sandwich. In this case, it is convenient to extend the hot pressing time eg. to 5 minutes. As before, the resulting assembly can be used as a bipolar electrode, eg. in the laboratory scale cell described in Example 3.

Claims: A method of producing a metal by electrolysis of a compound of said metal dissolved in a molten salt electrolyte, wherein an anodic surface is preserved by maintaining in the electrolyte ions of cerium alone or cerium with another metal M₁ selected from rare earth metals other than cerium, alkaline earth metals or alkali metals, characterized by cathodically polarizing a cathode comprising: a cathodic substrate which consists of one of more borides selected from cerium boride alone or cerium boride together with one or more borides of metals M₁ and/or metals M₂, wherein the metals M₂ are selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mg, Si, Al, La, Y, Mn, Fe, Co and Ni; and a cathodic surface which consists of at least one boride selected from (a) cerium boride alone, (b) cerium boride together with boride of at least one metal M₁ and/or M₂, and (c) boride(s) of metals M₂, with the proviso that the cathodic substrate and/or the cathodic surface may further contain additives from the group consisting of microdispersed aluminium, TiN and CeN; the cerium ions plus other M₁ ions, when present, in the electrolyte serving also to preserve the cathode. The method of claim 1, wherein aluminium is the metal to be electrowon from alumina dissolved in a molten cryolite electrolyte. The method of claim 2, wherein the metal M₁ is selected from lanthanum, calcium and yttrium. The method of any preceding claim, wherein the concentration of cerium ions in the electrolyte is maintained at a suitable level by adding cerium compounds or cerium metal to the electrolyte. The method of claim 4, wherein a compound selected from oxides, halides, oxyhalides and hydrides of cerium is added to the electrolyte. The method of claim 5, wherein the concentration of cerium ions in the electrolyte is well below their solubility limit. The method of any preceding claim, wherein the cathodic substrate comprises cerium hexaboride and the cathodic surface comprises cerium hexaboride and/or titanium diboride. The method of any preceding claim, wherein the anodic and cathodic surfaces are incorporated in bipolar electrodes. The method of claim 8, wherein the anodic surface comprises an oxycompound of cerium and is separated from the cathodic substrate by an intermediate stable layer. An electrode for electrowinning a metal by electrolysis of a compound of the metal dissolved in a molten salt electrolyte according to the method of claim 1, the electrode having a body at least a section of which is cathodically polarized, characterized in that said cathodic section has a cathodic substrate consisting of cerium boride alone, or cerium boride together with one or more borides of metals M₁ and/or metals M₂, wherein metal M₁ is selected from the rare earth metals other than cerium, the alkaline earth and the alkali metals and metal M₂ is selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mg, Si, Al, La, Y, Mn, Fe, Co and Ni; and a cathodic surface which consists of at least one boride selected from (a) cerium boride alone, (b) cerium boride together with boride of at least one metal M₁ and/or M₂, and (c) borides of metals M₂, with the proviso that the cathodic substrate and/or the cathodic surface may further contain additives from the group consisting of microdispersed aluminium, TiN and CeN. The electrode of claim 10, wherein the cathodic substrate comprises cerium hexaboride and the cathodic surface comprises cerium hexaboride and/or titanium diboride. The electrode of claim 10 or 11, wherein the electrode is a bipolar electrode further comprising an anodic section having an anodic surface. The electrode of claim 12, wherein the anodic surface comprises an oxycompound of cerium. The electrode of claim 13, wherein the anodic surface is made of doped cerium oxyfluoride. The electrode of claim 12, 13 or 14, wherein the anodic and cathodic sections are separated by an intermediate stable layer. The electrode of claim 15, wherein the intermediate stable layer comprises at least one metal selected from copper, silver and the noble metals. The electrode of claim 16, wherein the intermediate layer further comprises a cerium alloy or a cerium compound. The electrode of any one of claims 14 to 17, wherein the anodic substrate is made of a cermet comprising at least one metal of copper, silver and the noble metals, optionally together with a cerium-aluminium alloy, as metallic phase, and at least one of doped tin dioxide, doped zinc oxide, doped cerium oxides or oxyfluorides, a mixture of ceria and alumina, and a cerium/aluminium mixed oxide, optionally together with other compounds of cerium or aluminium, including nitrides or phosphides, as ceramic phase. A molten salt electrolysis cell for the electrowinning of aluminium from alumina dissolved in a molten cryolite electrolyte comprising at least one electrode according to any one of claims 10-18, there being a concentration of cerium ions in the electrolyte which preserves the anodic and cathodic surfaces. The cell of claim 19, wherein a plurality of bipolar electrodes according to any one of claims 12 to 18 are arranged in side-by-side relationship.

Inventors: DARRACQ, DOMINIQUE, DURMELAT, CLAUDE, DURUZ, JEAN-JACQUES
Applicants: MOLTECH INVENT S.A.
Application year: 1987
Main classification: C25C7
IPC codes: C25C3, C25C7