NO146543B - ELECTRODE FOR USE BY ELECTROLYSE, SPECIAL FOR ELECTROLYSE OF A MELTED SALT - Google Patents

Description

The present invention relates to an electrode for use in electrolysis, in particular for the electrolysis of a molten salt.

Dimensionally stable electrodes for anodic and cathodic reactions in electrolytic cells have gradually come into use in the electrochemical industry and have replaced those previously known for the use of electrodes made of carbon, graphite and lead alloys. These new electrodes are particularly applicable in liquid mercury cathode cells and in diaphragm cells for . production of chlorine and caustic soda, in metal electro-extraction cells where pure metal is recovered from aqueous chloride or sulphate solutions as well as for cathodic protection of ship hulls and other metal structures.

Such dimensionally stable electrodes usually consist of

a base of a metal such as Ti, Ta, Zr, Hf, Nb and W, where under anodic polarization a corrosion-resistant but non-electrically conductive oxide layer or "barrier layer" is developed, and which is partially covered with an electrically conductive and electrically catalytic layers of metal oxides or metals from the platinum group (see US Patent Nos. 3,711,385, 3,632,498 and 3,846,273). Electro-conductive and electrocatalytic coatings produced from or containing platinum group metals or oxides of the same metals are, however, very expensive and will be consumed or deactivated in certain electrodes, and consequently a reactivation or a new coating is necessary to reactivate the used electrodes.

Furthermore, electrodes of this type cannot be used in a number of electrolytic processes. For example, in molten salt electrolytes a dissolution of the base metal will easily occur, after which the thin protective oxide layer either does not form at all or is very quickly destroyed by the molten electrolyte, after which a subsequent dissolution of the base metal and thereby loss of the catalytic precious metal coating. Furthermore, in many aqueous electrolytes, such as fluoride solutions or seawater, the breakdown voltage on the protective oxide layer over the exposed base metal will be too low, and the base metal will therefore often corrode under anodic polarization.

In recent times, a number of new types of electrodes have been proposed to replace the rapidly degradable carbon anodes and carbon cathodes of the type that have hitherto been used under strongly corrosive conditions, e.g. an electrolysis of molten metal salts, e.g. of the type found in molten fluoride baths where aluminum is produced from molten cryolite. In this particular electrolytic process, which is of great economic importance, the carbon anodes are consumed in an amount of 450 to 500

kg carbon per tonnes of aluminum produced, and a very expensive adjusting device is necessary to maintain a small and even gap between the corroding anode surface and the liquid aluminum cathode. It is estimated that the aluminum factory uses something over 6 million tonnes of carbon-ano there per year. year. The carbon anodes are burned away after the following reaction:

but the real consumption is much higher due to the disintegration and disruption of carbon particles and due to a strong spark formation that occurs across anodic gas films that often form over the anode surface itself, after carbon is poorly wetted by the molten salt electrolytes, or on due to short circuits caused by "bridges" of conductive particles that either come from the corroded carbon anodes or from dispersed particles from the deposited metal.

British Patent No. 1,295,117 describes anodes for molten cryolite baths consisting of a sintered ceramic oxide mater-

iale consisting essentially of Sn02 and smaller amounts of other metal oxides, namely oxides of Fe, Sb, Cr, Mb, Zn, W,

Zr, Ta in a concentration of up to 20%. Although electrically conductive sintered SnO2 with minor additions of other metal oxides such as oxides of Sb, Bi, Cu, U, Zn, Ta, As etc., has long been used as a durable electrode material in glass melting furnaces where alternating current is used (see US patent no. 2,490,825, 2,490,826, 3,287,284 and 3,502,597) such a material has significant wear and corrosion when used as an anode material in a molten salt electrolysis. Wear amounts of up to 0.5 g per hour per cm 2 from samples of the composition described in the above-mentioned patents, when such electrodes are used in a molten cryolyte electrolyte at 3000 a/m . The high wear found on sintered SnO„ electrodes is assumed to be due to several factors: a) chemical attack by halogens after which Sn IV produces complexes of high coordination number with halogen ions, b) reduction of Sn02 by aluminum which is dispersed in the electrolyte; and c) - mechanical erosion of anodic gas development and salt precipitation inside the pores of the material.

Japanese Patent Application No. 112589 (Publication No. 62,114 from 1975) describes electrodes with a conductive substrate of titanium, nickel or copper or an alloy thereof, as well as carbon-graphite or other conductive materials coated with a layer consisting essentially of spinel and /or metal oxides of the perovskite type as well as alternative electrodes produced by sintering mixtures of said oxides. Spinel oxides and perovskite oxides belong to a group of metal oxides which show good electrical conductivity and which have previously been proposed as suitable electroconductive and electrocatalytic anode coating material for dimensionally stable metal anodes (see US patent no. 3,711,382 and 3,711,297 and Belgian patent no. 780.303).

In British patent no. 996,970, an electrode for a thermionic device is described and rare earth metals in the lanthanum group are specified, meaning lanthanum and the lanthanides such as cerium, neodymium and praseodymium. Yttrium is not mentioned or suggested and neither electrolysis of molten salt.

French Patent No. 2,246,657 describes an electrode for producing aluminum from a salt melt. However, electrodes with a sintered matrix consisting of a main component of yttrium oxide and a smaller amount of an electrically conductive material are not described and the electrode surface does not contain any electrocatalyst. The patent instead concerns an electrode where the outer layer is a metal oxide with a spinel or perovskite structure and a cobalt-yttrium oxide spinel is not the same as the yttrium used in the electrodes according to the present invention.

It has been found that coatings of particulate spinels and/or perovskites are mechanically weak because the bond between the ceramic coating and the metal or carbon substrate itself is weak, which is due to the crystal structure of the spinels or that the perovskites are not isomorphic with the oxides in the metal substrate. Different binders such as oxides, carbides, nitrides and borides have been tried without this leading to any particular improvement. In molten salt electrolytes, the substrate material will quickly be attacked due to unavoidable pores in the spinel oxide coating, after which the coating will quickly peel off from the corroded substrate. Furthermore, spinels and perovskites are not chemically or electrochemically stable in molten halide salt electrolytes and show significant wear due to an attack by halide ions and a reducing effect of the dispersed metal.

In the electrolytic production of metals from molten halide salts, it has been found that the previously known anodes have several disadvantages. A significant dissolution of the ceramic oxide material occurs and this brings metal cations into the solution which are deposited on the cathode together with the metal that is produced, and it happens that the impurity content in the produced metal is so high that the metal can no longer be used for the purposes where electrolytically pure metal is required. In such cases, one will completely or partially lose the economic advantages one has in an electrolytic process where one obtains a much higher degree of purer metal than in other types of refining processes.

An electrode material that is to be used with good results under strongly corrosive conditions of the type found in an electrolysis of molten halide salts, especially molten fluoride salts, should primarily be chemically and electrochemically stable under the operating conditions used. Furthermore, the material must be catalytic with regard to the anodic evolution of oxygen and/or halides, so that the anode overpotential is as low as possible so that you get as high an effect as possible from the electrolytic process. The electrode should also be thermally stable at the operating temperatures used, i.e. from approx. 200 to approx. 1100°C, have good electrical conductivity and should be sufficiently resistant to accidental contact with the molten metal cathode. If one ignores coated metal electrodes, since no metal substrate could withstand the highly corrosive conditions found in an electrolysis of molten fluoride salts, a large number of sintered ceramic electrodes with different compositions have been systematically tested.

It has now been found that very efficient insoluble electrodes can be obtained by sintering yttrium oxide and at least one electroconductive agent into a self-supporting body and on the surface of this providing at least one electrocatalyst.

The sintered yttrium oxide electrodes according to the present invention are particularly usable during electroextraction processes where it is desired to produce various metals such as aluminium, magnesium, sodium, potassium, calcium, lithium or other metals from molten salts. Yttrium oxide and at least one electroconductive agent have been shown by direct current electrolysis of molten salt electrolytes to be exceptionally inert and dimensionally stable and can be used as anodes with sufficient electrical conductivity, and when one or more oxide electrocatalysts such as CO 3 O 4 , Ni 3 O 4 , are provided on the surface of said anode Mn62, Rh203, Ir02, Ru02, Ag20 etc, have been shown to have high electrocatalytic activity, especially for chlorine evolution.

The term "sintered yttrium oxide" means a self-supporting,

substantially rigid body consisting substantially of yttrium oxide and at least one electroconductive agent produced by a known method used in the ceramic industry for the production of sintered bodies, e.g. by applying temperature and pressure to a powdered mixture of yttrium oxide and other metals, so that the mixture is shaped to the desired size and shape, or by casting the material into molds, by extrusion or by using binders etc., after which the shaped is sintered the body at high temperature into a self-supporting electrode.

According to the present invention, an electrode is provided for use in electrolysis, in particular for the electrolysis of a molten metal, and this electrode is characterized in that it comprises a self-supporting body of sintered powders consisting of more than 50% by weight of yttrium oxide and at least one electrically conductive metal or metal oxide which is provided over part of the electrode's surface together with at least one electrocatalyst in the form of a metal, metal oxide or mixtures thereof. It is preferred that the electrically conductive material constitutes less than 50% by weight of the sintered electrode and is an oxide of at least one metal selected from the group consisting of zirconium and tin. Furthermore, it is preferred that said electrically conductive material constitutes less than 50% by weight of the sintered electrode and is at least one metal selected from the group consisting of yttrium, chromium, molybdenum, zirconium, tantalum, tungsten, cobalt, nickel, palladium and silver.

The electrical conductivity in the sintered ceramic elec-

trodes can be improved by adding to the composition from 0.1 to 20% by weight of at least one electrically conductive agent selected from the group consisting of (A) doping oxides, typically of metals with a valence lower or higher than the valence of the metals of the oxides that make up the matrix, such as alkaline earth metal oxides of calcium, magnesium," Sr and Ba,

and metals such as Zn, Cd, In,,, Tl,,, As2,Sb2, Bi,, and Sn;

(B) oxides showing electrical conductivity due to intrinsic redox systems such as spinel oxides, perovskites

oxides etc; (C) oxides showing electrical conductivity due to metal to metal bonds such as Cr02, Mn02, Ti20, TiO etc, borides, silicides, carbides and sulfides of metals such as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W or the metals Y, Ti,

Zr, Hf, V, Nb, Ta, Cr, Mo, W, Pd and Ag or alloys thereof

these or mixtures of (A) and/or (B) and/or (C).

By mixing the powder of the matrix material with a smaller amount, typically from 0.5 to 30% with powders of a suitable electrocatalytic material and then sintering the mixture into a self-supporting body, an electrode is obtained that shows satisfactory electrical conductivity and electrocatalytic properties and retains its chemical stability even when the added catalyst would not be resistant in itself to the electrolytic conditions.

The catalyst can be a metal or an inorganic oxy compound. The preferred mixed catalyst powders are powdered metals, e.g. Ru, Rh, Pd, Ir, Pt, Fe, Co,

Ni, Cu and Ag, then especially platinum group metals, powdered oxy compounds of Mn, Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt, Ag, As, Sb and Bi and especially oxy compounds of the platinum group metals.

Particularly preferred are MnO 2 , Co 3 O 4 , Rh 2 O 3 , IrCl 2 , RuO 2 ,

Ag20, Ag202, Ag203, As2C>3, Sb2C>3, Bi203, CoMn204, NiMn204, CoRh204 and NiCo204 as well as mixtures of the aforementioned powdered

metals and oxygen compounds.

It has been found particularly advantageous to add to the yttrium oxide a material such as stannous oxide and zirconium oxide and also to add a smaller amount of at least one metal

end to the group consisting of yttrium, chromium, molybdenum, zirconium, tantalum, tungsten, cobalt, nickel, palladium and silver, because both the mechanical properties and the electrical conductivity of the sintered yttrium oxide electrodes are improved without lowering their chemical and electrochemical corrosion resistance.

These additives are added in powder form and mixed with the powdered yttrium oxide in quantities such as varies from 40 to 1% calculated on the basis of the weight of the metal content. Furthermore, other organic and/or inorganic compounds can be added to the powdered mixture to improve the bonding of the particles during casting and sintering.

Anodes containing a larger amount of ^2°3 have a new melting point that is well above the temperature of the molten salt electrolytes in which they are used, and no phase change is obtained under the working conditions used during electrolysis. Furthermore, the thermal expansion coefficient is not very different from that found in halide salts, and this makes it easier to maintain a suitable electrode distance between the anode and the cathode, and avoid expansions and contractions that could break the salt crust on top of the molten salt electrolyte of that type one finds during a normal aluminum electrowinning process.

The conductivity in sintered yttrium oxide electrodes according to the present invention can be compared with that found in graphite. The matrix has an acceptable processability during forming and sintering and in use will form a thin layer of oxy-halides on the surface under anodic conditions.

The free energy of formation for yttrium oxide is more negative than for the oxide of the corresponding molten halide salt found in the electrolyte phase, so that these sintered yttrium oxide anodes have a high degree of chemical stability.

The sintered yttrium oxide electrodes according to the present invention can also be used as bipolar electrodes. In this embodiment, the sintered yttrium oxide electrodes will be suitable. be produced in the form of a plate where one of the two main surfaces is equipped with a layer containing the anodic electrocatalyst, e.g. the oxides Co^O^, Ni^O^j, MnC^, Rt^O^/IrO2, Ru02 , Ag20 etc, while the other main surface is equipped with a layer containing a suitable cathodic material such as carbides, borides, nitrides, sulphides, carbonitrides etc,

of metals, and particularly preferred are yttrium, titanium and zirconium.

The self-supporting sintered body consisting of a larger amount of yttrium oxide can be produced by grinding the materials together, or separately, preferably to a grain size of between 50 and 500 microns, whereby a powder mixture containing a range of grain sizes is obtained so that a better degree of compaction is obtained . It is preferred that the powder mixture is mixed with water or with an organic binder so that a plastic mass is obtained which has suitable flow properties for the particular forming process used. The material can be shaped in a known manner either by tamping or by pressing the mixture into a mold or into a mold of plaster or the material can be expelled through an ejector nozzle of different shape.

The formed electrodes are then dried and heated to a temperature where the desired bonding can take place, usually between 800 and 1800°C for between 1 and 30 hours, after which there is a slow cooling to room temperature. The heat treatment is preferably carried out in an inert atmosphere or one that is weakly reducing, e.g. i + N2(80%), when the powder

ized mixture essentially consists of yttrium oxide and

a smaller amount of other metal oxides or metals.

When the pulverized mixture also contains metallic powders, it is preferred to carry out the heat treatment in an oxidizing atmosphere, at least in part of the heat treatment, in order to promote the oxidation of metallic particles on the outside of the electrodes. The metal particles inside the sintered material will improve the electrode's conductivity.

The forming process can be followed by the sintering process at a

high temperature as mentioned above, or the shaping and sintering can be simultaneous, i.e. that pressure and temperature are applied to the powder mixture at the same time, for example, in the case of electrically heated molds. Conductor rails for the supply of current can be cast into the ceramic electrodes during casting and/or sintering or attached to the electrodes after sintering or shaping. You can of course also use other forms of shaping, compacting and sintering the yttrium oxide powder mixture.

The electrocatalyst which is usually applied to the electrode surface due to the high cost should have high stability, low anodic overpotential for the desired anodic reaction, as well as high anodic overpotential for undesired reactions. In connection with chlorine development, it will be possible to use oxides of cobalt, nickel, iridium, rhodium, ruthenium or mixed oxides, e.g. RuO^- Ti°2'and ^ f°r~bonding with fluoride-containing electrolytes where oxygen evolution is the desired anodic reaction, it is preferred to use oxides of silver and manganese. Other oxides that can be used as electrocatalysts are oxides of platinum, palladium and lead.

Poisons to restrain unwanted anodic reactions can often be used, e.g. to restrain an oxygen evolution from chloride electrolytes. One should use poisons that give a high oxygen overpotential, and suitable materials are oxides of arsenic, antimony and bismuth. These oxides which fcruke in small quantities,

can be applied together with the mentioned electrocatalytic oxides in amounts of from 1 to 10% of the electrocatalyst calculated on the basis of the respective metal weights.

The application of the electrocatalyst and any poisons can be carried out in a number of different ways. Preferably, the electrocatalyst and optionally mentioned poisons are applied to the sintered yttrium oxide electrodes as a solution of decomposable salts of metals. The sintered yttria-

body is impregnated with the solution containing suitable metal salts and then dried. The electrode is then heated in air or another oxygen-containing atmosphere to convert the salts into the desired oxides.

Generally, the porosity of the sintered yttrium oxide body as well as a method used to impregnate its surface should be such that the metal salts penetrate to a depth of at least 1 to 5 mm, preferably 3 mm, so that after the heat treatment an electrocatalyst is obtained in the pores of the sintered yttrium oxide body to a certain depth.

Alternatively, different powder mixing methods can be used, where the electrocatalyst oxides and possibly also the poisoned oxides have been formed in advance, and these can be ground into powder form and added to the powder mixture during casting on

such a way that the outer layers of the cast electrodes are enriched with powders of electrocatalyst oxides and possibly the poisoned oxides, during the formation itself whereby electrodes with the specified electrocatalysts on the surface are already obtained during sintering.

The sintered yttrium oxide electrodes according to the present invention can be used as bipolar electrodes. In such cases, the yttrium oxide electrodes on one surface are coated with the anodic electrocatalyst and possibly with a poisoning agent for unexpected anodic reactions as indicated above, while the other surface can be equipped with a coating of a suitable cathodic material. The surface of a bipolar electrode that will function as a cathode during an electrolysis can, e.g. be equipped with a layer of metal carbides, borides, nitrides, sulphides and/or carbonitrides of yttrium, tantalum, titanium, zirconium etc.

A preferred method of applying a layer is to use

a plasma jet technique whereby the powder of the materials used is sprayed and fixed on the surface of the sintered yttrium oxide body by means of a flame under controlled atmosphere. Alternatively, the powdered material used can be added during shaping to the yttrium oxide powder mixture itself and then sintered together with this so that a cathodic surface of the bipolar electrode is provided which is equipped with a layer of a suitable cathodic material.

The sintered yttrium oxide activated with a suitable electrocatalyst can be used as a non-consumable electrode during the electrolysis of molten salts for other processes where an electric current is passed through an electrolyte for the purpose of decomposing the electrolyte, to carry out an oxidation and reduction of organic and inorganic compounds or to apply a cathodic potential to a metal structure to be protected against corrosion, in addition to primary and secondary batteries containing molten salts such as aluminum halides - alkali metal halides. Electrodes according to the present invention can be polarized as anodes or as cathodes or can be used as bipolar electrodes, whereby one side or one end of the electrode acts as an anode while the opposite side or the other end acts as a cathode with respect to the electrolyte contacting each side of the electrode respectively, something that is well known from ordinary electrolysis.

An electrolysis cell for use in connection with the present electrode consists of a cell equipped with at least one set of an anode and cathode with a certain distance as well as devices for passing an electrolysis current through said cell, and where said anode is a dimensionally stable three-component electrode of the type as mentioned above. The cell can preferably be used for the electrolysis of aluminum chloride.

The following examples describe several preferred embodiments

of the invention. The percentages of the electrode components are calculated as weight-% free metal based on the total metal content of the composition.

Example 1

About. 250 g of a mixture of a matrix material and an additive material as indicated in Table I was ground in a mixer approx. 20 minutes, after which the powder mixture was poured into cylindrical plastic molds and manually pre-compressed with a steel cylinder press. Each mold was placed in an iso-static pressure chamber, and the pressure was raised to approx. 1500 kg/cm<2>

for 5 minutes and then reduced to zero within a couple of seconds. The samples were taken out of the plastic molds and polished. The pressed samples were then placed in an electrically heated oven and heated from room temperature to 1200°C under a nitrogen atmosphere over 24 hours, held at the maximum temperature from 2 to 10 hours and then cooled to 300°C over 24 hours. The sintered samples were then removed from the furnace and after cooling to room temperature they were weighed and their apparent density and electrical conductivity at 25 and at 1000°C were then measured. The results are shown in Table I.

The results from Table I show that the electrical conductivity

in the sintered ceramic electrodes at high temperatures of 1000°C is from 5 to 10 times higher than that found at 25°iC. An addition of oxides with conductivity equivalent to that found in metals to the essentially non-conductive

ceramic oxides in the matrix, the conductivity of the electrode increases by an order of magnitude of 10 2 , which is evident from electrodes A and No. 1. Addition of a metal which is stable to molten salts, such as yttrium or molybdenum etc, to the ceramic electrodes according to the present invention , the electrical conductivity of the electrode increases from 2 to 5 times.

Example 2

Operating conditions in an electrolytic cell for the production of aluminum metal from a molten cryolite bath were simulated in a laboratory test cell. In a heated graphite crucible, a layer of liquid aluminum was provided on the bottom and a melt consisting of cryolite (80 to 85%), alumina (5 to 10%) and AlF, (from 1 to 5%) was poured over the liquid aluminium. The electrode samples with a working surface of approx. 3 cm 2 prepared as indicated in example 1, and to which a Pt wire was attached, made it easy to conduct electric current to the electrode, and the electrodes were then dipped into the salt melt and held at a distance of approx. 1 cm from the liquid aluminum layer. The crucible was maintained at a temperature from 950 to 1050°C, and the current density was 0.5 A/cm 2 and the cell was operated for 2000 hours. The data obtained are shown in table II. The sample number indicates that the tested electrodes correspond to the samples described in table I. with the same number.

The test electrodes were very good as anodes in the cryolite melt, and the observed amounts of wear are acceptable for the electrolytic production of aluminum from molten cryolite. Both electrodes tested showed low wear during 2000 hours of operation. Generally, the wear on electrodes containing heat stabilizers such as oxy compounds of metals from group III in the periodic table will be approx. 10 times less than what is found in electrodes without heat stabilizers.

Example 3

Electrode No. 1 described in Table I was used as an anode for the electrolysis of a molten aluminum chloride electrolyte in a sample cell described in Example 2. The electrolysis conditions were as follows:

The tested electrodes achieved good results and the weight loss was completely negligible after 2000 hours of operation.

Example 4

Sintered anodes consisting of 70 wt% Y^O^ and 30 wt% ZnO 2 and measuring 10 x 10 x 10 mm were impregnated with an aqueous solution of the chloride salt of the metals listed in Table III and then heated in air from 300 to 650°C. This was repeated until the amount of metal catalyst was

10 g/m 2 calculated as metal. The electrodes were then used as anodes during an electrolysis of a 5:12 mixture of AlCl--NaCl with a current density of 1000 A/m at 750°C. The initial anode potential and the potential as well as the wear after 100 hours were measured. The results are shown in Table III.

The electrodes showed excellent characteristics in the electrolysis of molten aluminum chloride.

Example 5

Sintered anodes which measured 10 x 10 x 10 mm and which consisted of

70% by weight ^ 2°3 and 30% by weight ZrC>2 were impregnated with the metals

as set forth in Table IV, using the procedure of Example 4. The anodes were used for electrolysis of molten metal carbonate fluoride salt, and wear and potential were measured as set forth in Example 4. The results are set forth in Table

IV.

The anodes showed good results in the electrolysis of molten carbonate-fluoride salt where oxygen was developed at the anode.

Example 6

Electrode sample no. 4 from example 1 was used alternatively as anode and as cathode in an electrolysis of synthetic seawater in a test cell where the electrolyte was pumped through the electrode distance of 300 mm at a speed of 3 cm/sec. The current density was kept at 1500 A/m 2 and the electrolyte used contained from 0.8 to 2.4 g of sodium hypochlorate with a faraday effect of more than 88%. The weight loss of the electrode after 200 hours of operation was negligible.

Example 7

Electrode sample No. 3 from Example 1 was used as an anode during electrolysis of an aqueous acidic copper sulfate solution in a cell with a titanium cathode. The electrolyte contained from 150 to 200 g/l of sulfuric acid and 40 g/l of copper sulfate as metallic copper, and the anode current density was 300 A/cm<2>. The electrolyte temperature was from 60 to 80°C, and on average 6 mm of copper was deposited on the flat cathode with a faraday effect that varied from 92 to 98%. The quality of the metal was good and free of dendrites, and the anode overvoltage was very low, varying from 1.81 to 1.95 volts.

Example 8

A block of sintered Y203:Sn02:Y metal in a ratio of

7:2:1 per weight of free metals, a block of sintered Y203:Zr02:Zr metal in a ratio of 6:3:. per weight as free metals, and a block of sintered Y203:Pd metal in a ratio of 9:1 per weight free metals were activated by impregnating the sintered samples with an aqueous solution of CoCl3 followed by drying and heating in air from 300 to 650°C to convert the chloride to CO,0.. This was repeated so that a final coating was obtained on the electrodes of 15 g/m 2 of CO^ O^

on the anode surface. The activated anodes were used for electrolysis of molten A1C13+ NaCl electrolyte, and anode potential and wear are shown in Table V.

Example 9

A block sintered Y^C^-ZrC^ and Pd metal in a weight ratio of 7:2.5:0.5 based on the amount of the free metals was impregnated on one of its larger surfaces with 15 g/m surface of Co ^ O^ by the method described in Example 8. The opposite larger surface was coated with a 1 mm thick layer of zirconium boride applied by flame spraying under a nitrogen atmosphere. The block was placed between two opposite electrodes of graphite which were connected electrically. The space between the electrodes was filled with molten A1C13+ NaCl, and the graphite electrode that stood against the zirconium boride surface' of the sintered bipolar electrode was connected to the positive pole of a direct current source, while the other graphite electrode was connected to the negative pole of the same source. The sintered electrode was thus a bipolar electrode, and molten aluminum metal flowed down into the zirconium-coated surface and was recovered from the bottom of this electrode, while chlorine was developed on the Co 3 O 4 -activated surface of the electrode. The electrolysis was run satisfactorily for 28 hours, when the sample cell itself, which was mainly made of graphite, began to break down. After this period of operation, the bipolar electrode showed no signs of destruction and no wear could be detected.

Other electrocatalysts that can be used in electrolysis

of molten halide salts for halide ion discharge are RuO^/as well as oxides such as As2C>3, Sn2O3 and Bi2°3 which can

is added in a percentage amount of up to 10% by weight based on the amount of metal, so that the oxygen overpotential is raised without affecting the halide ion discharge potential.

For anodes to be used in molten fluoride electrolytes where oxygen is evolved, the catalyst may be those indicated in Example 5 or Rh 2 O 3 , PbO 2 and IrO 2 -TiO 2 .

The components listed in the anode are listed in % by weight of free metal based on the total amount of metal in the anode.

The electrolyte may contain other salts than those used

in the examples, e.g. alkali metal chloride or fluoride as well as salts of the metal subjected to electrolysis. Metal halides, which reduce the melting point of the salt being electrolysed, will thus make it possible to lower the temperature while maintaining the salt bath in a molten state.

The above examples include molten metal salt electrolysis, mainly electrolysis of molten aluminum chloride or fluoride salts. Similarly, molten chlorides of other metals such as alkali metal or alkaline earth metals can be electrolyzed by forming anodes according to common practice. In addition to this, other molten salts, e.g. molten nitrates, are electrolysed in the same way.

A molten alumina cryolite electrolyte or a similar alkali metal aluminum fluoride can be electrolyzed to produce molten aluminium.

These electrodes can be used instead of the graphite anodes used in standard aluminum electroextraction cells, where either the aluminum ore is fed into a cryolite bath or where aluminum chloride is fed into a bath which essentially consists of said aluminum chloride.

The use of these sintered yttrium oxide anodes for the recovery of undesirable metals from molten salts of the metals will result in a reduced power consumption per weight unit of metal produced as well as higher purity of the recovered metals. The electrodes are dimensionally stable during operation and there is therefore no need for frequent interruptions to adjust the optimum distance from the cathode surface, which is necessary when using the usual known consumable anodes.

The sintered yttrium oxide anodes according to the present invention can also be used in aqueous or non-aqueous solutions of electrolytes for the recovery of one or more components in the electrolytes.

Claims (6)

1. Electrode for use in electrolysis, in particular for electrolysis of a molten salt, characterized in that it comprises a self-supporting body of sintered powders consisting of more than 50% by weight of yttrium oxide and at least one electrically conductive metal or metal oxide which is provided over part of the surface of the electrode together with at least one electrocatalyst in the form of a metal, metal oxide or mixtures thereof. 2. Electrode according to claim 1, characterized in that the electrically conductive material makes up less than 50% by weight of the sintered electrode and is an oxide of at least one metal selected from the group consisting of zirconium and tin. 3. Electrode according to claim 1, characterized in that said electrically conductive material constitutes less than 50% by weight of the sintered electrode and is at least one metal selected from the group consisting of yttrium, chromium, molybdenum, zirconium, tantalum, tungsten, cobalt, nickel, palladium and silver. 4. Electrode according to claim 1, characterized in that the electrocatalyst is selected from the group consisting of oxides of cobalt, nickel, manganese, rhodium, iridium, ruthenium, silver and mixtures thereof. 5. Electrode according to claim 4, characterized in that the electrocatalyst is formed in situ on said sintered electrode from a solution of salts of said metals and converted to oxides on the sintered electrode body. 6. Electrode according to claim 4, characterized in that the electrocatalyst consists of powdered oxides of said metal sintered into the outer layer of the electrode.