NO147490B - ELECTRODE FOR USE BY ELECTROLYSE, SPECIAL FOR ELECTROLYSE OF MELTED SALT - Google Patents

Description

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

Dimensionally stable electrodes for anodic and cathodic reactions in electrolysis cells have gradually come into use in the electrochemical industry and replaced the previously known for the use of carbon, graphite and lead alloy electrodes. These new electrodes are particularly applicable in liquid mercury cathode cells and in diaphragm cells for the production of chlorine and caustic soda, in metal electroextraction cells where pure metal is extracted 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, in which, 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 layer of metal oxides or metals from the platinum group (see US Patent Nos. 3,711,385, 3,632,498 and 3,846,273). Electrically 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 ones the electrodes.

Furthermore, electrodes of this type cannot be used in

a variety 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 fluorideppp 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 special electrolytic process, which is of great economic importance, the carbon anodes are consumed in an amount of approx. 450 to 500 kg of 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 anodes 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 which 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 which consist of a sintered ceramic oxide material consisting essentially of SnO^ and minor amounts of other metal oxides, namely oxides of Fe, Sb, Cr, Nb, Zn, W, Zr, Take in a concentration of up to 20$>. Although electrically conductive sintered SnOg 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 o 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 cryolite electrolyte at 3000 A// m 2. The high wear found on sintered Sn0„ electrodes is assumed to be due

IV

several factors: a) chemical attack by halogens after which Sn gives complexes of high coordination number with halogen ions, b) reduction of SnO,, by aluminum dispersed in the electrolyte; and c) mechanical erosion of anodic gas evolution and salt precipitation inside pores in 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»7H»382 and 3,711,297 and Belgian patent no. 78O. 3O3).

However, 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 isomorphous 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 be quickly attacked due to unavoidable pores in the spinel oxide coating, after which the coating is quickly peeled 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 for which requires electrolytically pure metal. 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 highly 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 development of oxygen and/or halides, so that the anode overpotential is as low as possible and that the electrolytic process has as high an effect as possible. The electrode should also be thermally stable at the operating temperatures used, i.e. from 200 to 1100°C, have good electrical

tric 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.

According to the present invention, it is provided

an electrode for use in electrolysis, in particular for electrolysis of molten salt, and this electrode is characterized in that it comprises a self-supporting matrix of sintered powders avetoxide of at least one metal selected from the group consisting of titanium, tantalum, zirconium, vanadium, niobium, aluminum , silicon, tin, manganese, iron, cobalt, nickel, silver, arsenic, bismuth, lanthanum, ytterbium, thorium and yttrium, and at least one electrically conductive metal or metal oxide, and where said electrodes at least over part of their surface are equipped with at least one electrocatalyst in the form of a metal, metal oxide or mixtures thereof.

It is preferred that said electrically conductive material constitutes less than 50% by weight of the sintered electrodes.

The "sintered" electrode is thus a self-supporting, essentially rigid body consisting essentially of an oxymetal compound and at least one electroconductive agent produced by a known method of the type used in the ceramic industry, e.g. using temperature and pressure

a powdered mixture of the materials, whereby the mixture is formed into the desired size and shape, or by casting the material in a mold, by extrusion or by using binders etc., and then sintering the shaped body at high temperatures into a self-supporting electrode . The oxyhalide compounds are preferably oxychlorides and oxyfluorides.

The electrical conductivity of sintered ceramic electrodes is improved by adding to said electrodes 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 lower or higher valence metals than the valence of the metals included in the oxides that make up the matrix, such as alkaline earth metals (oxides of ) Ca, Mg, Sr and Ba and metals such as Zn, Cd, In2, Tl^, Asg, Sb^, Bi,, and Snj (b) oxides that have good electrical conductivity due to an intrinsic redox system,

such as spinel oxides, perovskite oxides etc.j (c) oxides showing electrical conductivity due to metal to metal bonds such as CrO₂, MnO₂, TiO, Ti₂O₂ etc.; borides, silicides, carbides and sulfides of metals such as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and ¥ or the metals Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Pd and Ag or alloys thereof or mixtures of (A) and/or (b) and/or (c).

The preferred electrocatalysts are selected from the group of metals consisting of ruthenium, rhodium, palladium, iridium, platinum, iron, cobalt, nickel, copper and silver and mixtures thereof, as well as oxides of metals from the group consisting of manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, iridium, platinum, silver, arsenic, antimony, lead and bismuth, as well as mixtures of these.

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 by sintering the mixture into a self-supporting body, when this body is used as an electrode, a satisfactory electroconductive and electrocatalytic property while the body retains its chemical stability even when the added catalyst itself would not be resistant under the conditions that occur during electrolysis.

The electrocatalyst can be a metal or an inorganic oxygen compound. The preferred added catalyst powders are powdered metals such as Ru, Rh, Pd, Ir, Pt, Fe, Co, Ni, Cu and Ag, then especially the platinum group metals, powdered oxy compounds of Mn, Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt, Ag, As, Sb and Bi and especially the oxy compounds of the platinum group metals.

Particularly preferred are |3MnO2, Co^O^, Rh^O^, IrO^, RhO2, Ag2O, Ag2O2, Ag2O3, ASgO^, SbgO , Bi.,0^ CoMn^, NiCo.,0^ as well as mixtures of the aforementioned powdered metals and oxygen compounds.

It has been particularly found advantageous to add to the oxymetal compound another material such as tin oxide, zirconium oxide and the like, in addition to adding a smaller amount of at least one metal selected from 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 electrodes are improved without lowering or weakening their chemical and electrochemical corrosion resistance.

These additives can be added in powder form and mixed with the powdered metal oxide in percentages varying from kO to 1$ based on the weight of the metal content. Furthermore, other organic and/or inorganic compounds can be added to the powder mixture to improve the bonding of the particles both during forming or casting and sintering.

The anodes have a high melting point that is well above the temperature of the molten salt electrolytes where they are used,

and no phase change occurs in the electrodes under the working conditions used during electrolysis. Furthermore, the thermal expansion coefficient is not much different from that found in the halide salts used in the molten salt baths, and this means that you better maintain suitable electrode gaps between the anode and cathode and avoid expansions and

contractions that could destroy the salt crust on top of the molten salt electrolyte e.g. as found in a normal aluminum electrowinning process.

The conductivity in the sintered electrodes according to the present invention can be compared to that found in graphite. The matrix has acceptable workability during forming and sintering and during use will form thin layers of oxyhalides on the surface under anodic conditions. The free energy of formation for the metaloxy compounds is more negative than the corresponding energy for the oxide of the molten salt electrolyte which is in the halide phase, so that these sintered anodes have high chemical stability.

The sintered electrodes according to the present invention can also be used as bipolar electrodes. In this embodiment, the sintered electrodes will preferably be produced in the form of a plate or the like where one of the two larger surfaces of the electrode is equipped with a layer containing an anodic electrocatalyst such as the oxides CO^O^, Ni^O^, MnOg, Rh^O^, IrO^, RuO^, Ag^O etc, while the other larger surface is equipped with a

layer containing a suitable cathodic material such as carbides, borides, nitrides, sulphides, carbonitrides etc. of metals, preferably of the metals yttrium, titanium and zirconium.

The self-supporting sintered body consisting of a larger part of an oxymetal compound can be produced by grinding the materials together, or separately, preferably to a grain size between 50 and 500 µm, whereby a powder mixture is obtained

ing that contains varying grain sizes so that a better degree of compaction is achieved. According to a preferred method, the powder mixture can be mixed with water or with an organic binder so that a plastic mass is obtained with suitable flow properties for the forming process that one wishes to use. The material can be shaped in a known manner either by tamping or by pressing the mixture into a mold or by pouring the mixture into a regular mold, or the material can be extruded through a die into varying shapes.

The formed electrodes are then subjected to drying and heated to a temperature where the desired bond is obtained, usually between 800 and 1800°C for between 1 and 30 hours, then followed by a slow cooling to room temperature „ The heat treatment is preferably carried out in a inert atmosphere or one that is weakly reducing, e.g. in H"2 + N2 (80%), when the powdered mixture essentially consists of an oxymetal compound 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 for part of the heating time,

thereby promoting the oxidation of the metallic particles on the outside of the electrodes. The metallic particles that remain inside the sintered material itself will thus improve the electrode's electrical conductivity.

The shaping itself can be followed by sintering at high temperature as mentioned above, or shaping and sintering can be carried out simultaneously, i.e. that pressure and temperature are applied simultaneously, e.g. using electrically heated molds. Supply wires or rails can be melted into the ceramic electrodes during casting or sintering or can be attached to the electrodes after sintering or "shaping". Other methods can also be used to shape, compress and sinter the powdered mixtures if this is desired.

The electrocatalyst, which is usually only applied to the electrode surface due to cost, should have high stability, low anodic overpotential for the desired anodic reaction, and high anodic overpotential for undesired side reactions. In connection with chlorine evolution, oxides of cobalt, nickel, iridium, rhodium, ruthenium or mixed oxides such as RuOg - Ti02 etc. can be used, while in connection with fluoride-containing electrolytes where oxygen evolution is desired, oxides of silver and manganese. Other oxides that are used as electrocatalysts can be oxides of platinum, palladium and lead.

Poisons can be used to hold back unwanted anodic reactions, e.g. to avoid oxygen evolution from chloride electrolytes. One should use poisons that have a high oxygen overpotential, and suitable compounds are oxides of arsenic, antimony and bismuth. These oxides, which are used in small amounts, can be applied together with the electrocatalyst 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 as divided as mentioned above can be carried out in a manner known to each. Preferably, the electrocatalyst and possibly said poison are applied to the sintered electrodes as a solution of decomposable metal salts. The sintered body is impregnated with the solution containing a suitable metal salt and is then dried. The electrode is heated in air or in an oxygen-containing atmosphere to thereby convert the salts into the desired oxides.

The sintered body should have such good porosity and the method used to impregnate the surface layers of the sintered body with the metal salts should be such that the penetration of the solution should decrease to at least 1 to 5 mm, preferably 3 mm measured from the surface of the electrode, such that the electrocatalyst is present in the pores of the sintered body down to a certain depth from the surface itself after sintering.

Alternatively, with suitable powder mixing techniques, forming electrocatalyst oxides and possibly formed poisoning oxides can be taken and ground into powder form and added to the powder mixture during the casting of the electrodes in such a way that the outer layers of the cast electrodes are enriched with the powders of the electrocatalyst oxides and possibly the poisonous oxides so that after sintering the electrocatalyst is provided on or near the surface of the electrodes.

The sintered electrodes according to the present invention can also be used as bipolar electrodes. According to this execu-

see, the electrodes on a surface can be equipped with an anodic electrocatalyst, possibly with a poisoning agent to avoid unwanted anodic reactions, e.g. by means of one of the methods mentioned above, while the other surface can be provided with a coating of a suitable cathodic material.

For example, the surface of a bipolar electrode which will function as a cathode during electrolysis can be equipped with a layer of metal carbides, borides, nitrides, sulphides and/or carbonitrides of yttrium, tantalum, titanium, zirconium, etc.

A preferred method for applying a layer is by means of a plasma flow technique whereby the powder of the selected materials is sprayed on and adheres to the surface of the sintered body by means of a flame under a regulated atmosphere. Alternatively, the powdered mixture selected therein can be added during the forming process and thus sintered on the cathodic surface of the bipolar electrode.

The electrodes can be used effectively for the electrolysis of many electrolytes. They can be particularly advantageously used as anodes in electrolytic cells of the type used for the electrolysis of molten salt electrolytes, e.g. molten cryolite baths, molten halides of aluminium, magnesium, sodium, potassium, calcium, lithium and other metals. Thus, aluminum halides can be electrolyzed using the Hall process or using the methods described in US Patent Nos. 3,464,900, 3,518,712 or 3,755,099 (these patents are incorporated by reference in this patent application) using electrodes as described here as anodes. The temperature during the electrolysis is sufficiently high to melt and keep the salts of the metal to be recovered in a molten state, and the metal is deposited in a molten state and collected as a molten cathode where the molten metal is taken out from the bottom of the layer.

The electrodes can also be used effectively as anodes and/or cathodes during direct current electrolysis of other molten salt electrolytes that typically contain halides, oxides, carbonates or hydrates for the production of aluminum, beryllium, calcium, cerium, lithium, sodium, magnesium, potassium, barium, strontium, cesium and other metals.

When electrodes according to the present invention are used as bipolar electrodes for electrolysis of molten salts, the composition of the cathodic part of the electrodes must be such that it is not reduced by cathodic reaction or attacked by the metal deposited on the cathodes, especially when the electrode is partly composed of an oxy compound. For this reason, it is desirable to have the composition on the cathode side of the bipolar electrode so that it is inert to the cathodic reaction and for a reducing effect of the molten metal.

The electrodes can also be used as anode and/or as cathode in electrochemical processes such as the following: Electrolysis of aqueous chloride solutions for the production of chlorine, NaOH, hydrogen, hypochlorite, chlorates and perchlorates, electroextraction of metals from aqueous sulphate or chloride solutions for the production of copper, zinc, nickel, cobalt and other metals, electrolysis of molten metal salt electrolytes typically containing halides, oxides, carbonates or hydrates for the production of aluminium, beryllium, calcium, cerium, lithium, sodium,' magnesium, potassium, barium, strontium , cesium and other metals, as well as electrolysis of bromides, sulphides, sulfuric acid, hydrochloric acid and hydrofluoric acid. Generally, the electrodes can be used in all electrolytic processes.

In the following, several preferred embodiments are described to illustrate the invention. Percentages of the components in the electrode are calculated as a percentage per weight and calculated on the basis of free metals based on the total metal content in the composition.

Among the preferred anodes are those where the main part of the self-supporting body is tin dioxide either alone or together with up to 20% by weight of cobalt oxide equipped with a coating of cobalt oxide which gives the electrodes improved mechanical properties and electrocatalytic properties for chlorine evolution. Other preferred additives are Y 2 O 3 , TiO 2 and TaO 5 .

The electrocatalytic coating can be protected against wear by a simultaneous or subsequent application of a protective agent such as a metal oxide, e.g. TiO,,, Ta^ O^ or SiO„ as well as mixed oxides such as AgRe„0 , TiCoo0, and AgW0_.

Example 1

Three sintered electrode samples 80$ Sn02 + 20$ cobalt (type A), 80$ Sn02 + 10$ Co + 10$ Mo (sample B) and 100$ Sn02 (sample C) were prepared and then carefully washed with water and dried in vacuum. The resulting electrodes were then immersed under vacuum in a solution as indicated in Table I and then dried by heating to 370°C for 20 minutes in a circulation oven with air passage. The samples were then brushed with the same solution and again heated for 15 minutes at 350°C in the same oven, and this procedure was repeated several times until the electrodes had a weight increase of 5 g/m 2 .

The electrodes were then used as anodes in a test cell for the electrolysis of aluminum chloride at 750°C and an anodic current density of 1000 A// m 2. The cell voltage was 5 volts, and the electrolyte was a 5-1-1 weight mixture of aluminum chloride, sodium chloride and potassium chloride. The anodic potential was determined at the beginning, after 500 hours of operation, while the weight loss of the electrodes was determined at the same time. For comparative purposes, a standard graphite electrode was used under the same conditions, and the results obtained are given in Table I.

The results from Table I show that the coated electrodes have a lower overpotential for chlorine evolution without a significant increase in weight loss. Sample C, which had too high an overpotential without post-treatment, was not suitable for the electrolysis reaction, while the treated sample C was this. The average faraday effect during the test was 96$. A common graphite electrode was used in the same way and compared to the reference graphite electrode, showing a voltage of approx. 0.8 volts.

Example 2

Samples consisting of 90% tin dioxide and 10% cobalt were sintered, and the electrodes were then provided with a coating of cobalt oxide as described in example 1, and a layer of

10 g/m 2 of said cobalt oxide. The electrodes were then used to electrolyze the electrolytes listed in Table II under the operating conditions indicated. The anode potential was measured after 300 hours of operation and the wear at the same time, and the results are shown in Table II.

Table II clearly shows that electrodes according to the present invention have lower wear and lower anode potential even after 300 hours of operation.

Example 3

Disc-shaped electrodes with a diameter of 10 mm and a thickness of 5 mm were produced from powders with a mesh number from 100 to 250. The powders were press-formed at a pressure of

y , 2

, 1000 kg/cm and then sintered in an induction furnace under the conditions indicated in Table III, which also shows the composition of the powders.

The sintering was carried out in a furnace through which the specified gas was circulated or maintained at atmospheric pressure. The outer surfaces and possibly some of the inner pores were thus exposed to an oxidizing atmosphere at the specified temperature, and the exposed metal on the surface was oxidized so that the electrocatalyst was formed.

The conductivities of samples 1 to 7 were measured at 500°C, and were between 0.01 and 1.0 and the density of the sintered electrodes varied between 5 and 8.5 g/cm . The electrodes were used as anodes in a test cell for the electrolysis of aluminum chloride at 750°C and an anodic current density of 1000 A/m'* was used. The cell voltage was 5 volts, and the electrolyte was a 5-1-1 mixture of aluminum chloride, sodium chloride, and potassium chloride. The anodic potential was determined at the beginning and after 500 hours of operation, and the weight loss was determined after said 500 hours. For comparative purposes, a reference graphite electrode was also used under the same conditions, and the results are given in Table IV.

The results in Table IV show that electrodes 1 to 7, which contain a larger amount of an oxide and a smaller amount of a metal, have a lower overpotential for chlorine evolution and have very low wear. Electrode 8, which did not contain any added electroconductive metal, had a significantly higher overpotential for chlorine evolution, and the reference graphite electrode had an overpotential above that found for electrodes 1 to 7 and much higher wear. The reference graphite anode needed various adjustments during the electrolysis, and had to be replaced early on with a new one. The average effect during the trial was 97$. All samples 1 to 7 were less brittle than sample No. 8.

Example h

About. 250 g of a mixture of the matrix material and the added materials listed in Table I was ground in a mixer for 20 minutes, and the powder mixture was then poured into cylindrical plastic molds and manually pressed with a cylindrical steel press. Each mold was placed in an isostatic pressure chamber, and the pressure was raised to approx. 1500 kg/cm for 5 minutes, and then reduced to zero within a few seconds. The samples were then taken out of the plastic molds and polished. The pressed samples were placed in an electrically heated oven and heated from room temperature to 1200 oC under a nitrogen atmosphere during 24 hours, held at the maximum temperature from 2 to 5 hours and then cooled to 300 o C over 2k for hours. The sintered samples were then taken out of the furnace, and after cooling to room temperature they were weighed, and their density and electrical conductivity were measured at 25 and 1000°C. The results are shown in Table V.

The results from Table V show that the electrical conductivity in the sintered ceramic electrodes at high temperatures such as 1000°C is from 5 to 10 times higher than the conductivity at 25°C. Addition of oxides with conductivity equivalent to that found in metals to the essentially non-conductive ceramic oxides in the matrix increases the conductivity of the electrodes by an order of magnitude of 10 2. Addition of a metal that is stable to the molten salts, such as yttrium or molybdenum etc., to the ceramic electrodes according to the present invention, increases the electrical conductivity of the electrodes by from 2 to 5 times.

Example 5

The conditions in an electrolysis 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 added to 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 top of the aluminum. Test electrodes with a working surface of 3 cm 2 , prepared as described in example kt and to which a platinum wire had been soldered so as to obtain a light device for the supply of electric current, were dipped into the salt melt and held at a distance of about. 1 cm from the liquid aluminum layer. The crucible was kept at a temperature from 950 to 1050°C, and the current density was 0.5 A/cm<2> and the cell was run for 2000 hours. The experimental data are as shown in Table VI.' The sample number indicates the electrode that was tested, and this corresponds to the sample described in table V with the same number.

The electrodes could be used with good results as anodes in the cryolite melt, and the wear observed is within the acceptable limits that can be achieved for the electrolytic production of aluminum from molten cryolite. All the tested electrodes had low wear during the aforementioned 2000 hours of operation. Usually, the wear on electrodes containing heat stabilizers such as oxy compounds of metals from group III in the periodic system will be approx. 10 times less than for electrodes without such heat stabilizers.

Example 6

The electrodes k and 5 described in Table V were used as anodes for the electrolysis of a molten aluminum chloride electrolyte in the sample cell, which is described in Example The electrolysis conditions were as follows:

The tested electrodes were used with good results,

and the weight loss after 2000 hours of operation was negligible.

Example 7

Electrodes Nos. 10 and 11 from Example 1 were used as anodes for the electrolysis of an aqueous bromide solution to produce bromine, and where a diaphragm-type test cell with an asbestos diaphragm was used to separate the cathode part with a steel cathode from the anode part with the test electrode as the anode. The electrolysis was carried out with an aqueous solution containing from 200-220 g/l of sodium bromide, and the electrolyte temperature was 80 to 85°C with a current density of 2000 A/m<2>. The power output was 95$ and after 1000 hours of operation the weight loss on the test electrode was negligible.

Example 8

The electrode samples no, 10, 11 and 13 from example 1 were used alternatively as anode and as cathode during an electrolysis of synthetic seawater in a test cell where the electrolyte was pumped through the electrode distance of 3 mm at a speed of 3 cm/sec. The current density was maintained at 1500 A/m , and the electrolyte used contained from 0.8 to 2.4 sodium hypochlorate with a faraday effect of more than 88$. The weight loss of the electrode after 200 hours of operation was negligible.

Example 9

The electrode samples no. 12 and 1k from example 1 were used as anodes for the electrolysis of an aqueous acidic copper sulphate solution in a cell with a titanium cathode. The electrolyte contained from 150 to 200 g/l of sulfuric acid and k0 g/l of copper sulfate as metallic copper, and the anode current density was 300 A/cm . 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 (NHE).

Other electrocatalysts which can be used in the electrolysis of molten halide salts for halide ion discharge are RuOg and oxides such as ASgO^, Sn^O^ and Bi^O^ which can be added in percentages of up to 10 wt-$ free metal based on the total metal content, thereby to raise the oxygen overpotential without affecting the halide ion discharge potential.

For anodes to be used in molten fluoride electrolytes where oxygen is evolved, one can use catalysts of the type indicated in example 5 or RhgO^, V\ >0^ and IrO^.TiO^.

The components in the anodes are stated in the examples as weight-$ free metal based on the total metal content in the anode

it.

The electrolyte may contain other salts than those used in the examples, such as alkali metal chloride or fluoride as

as well as salts of the metals subjected to electrolysis.

The metal halides can very effectively reduce the melting point of the salt subjected to electrolysis, whereby a lower temperature can be used while keeping the salt bath in a molten state.

The above examples include molten metal salt electrolyses, in particular electrolysis of molten aluminum chloride or fluoride salts. In a similar way, molten chlorides of other metals such as alkali metals or alkaline earth metals can be electrolysed with the indicated anodes according to common practice. In addition, other molten salts such as molten nitrates can be electrolysed in the same way. Furthermore, a molten bath of aluminum oxide and cryolite or the like can e.g. alkali metal aluminum fluoride is electrolysed so that molten aluminum is produced.

These electrodes can be used instead of graphite anodes

in standard aluminum electro-extraction cells either where the aluminum ore is fed to a cryolytic bath or where aluminum chloride is fed to a bath essentially containing aluminum chloride.

The use of these sintered metal oxide anodes for the recovery of desired metals from molten salts of said metals can result in a reduced power consumption per unit weight of metal produced, as well as higher purity of the recovered metals. The electrodes are dimensionally stable during operation and there is thus no need for frequent adjustments to maintain an optimal distance from the cathode surface, which is necessary with previously known consumable anodes.

Claims (6)

1. Electrode for use in electrolysis, in particular for electrolysis of molten salt, characterized in that it comprises a self-supporting matrix of sintered powders of an oxide of at least one metal selected from the group consisting of titanium, tantalum, zirconium, vanadium, niobium, aluminium, silicon, tin, manganese, iron, cobalt, nickel, silver, arsenic, bismuth, lanthanum, ytterbium, thorium and yttrium, and at least one electrically conductive metal or metal oxide, and where said electrodes are equipped over at least part of their surface 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 said electrically conductive material constitutes less than 50% by weight of the sintered electrodes 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 the electrically conductive material makes up less than 50% by weight of the sintered electrodes and consists of 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 and silver. 5. Electrode according to claim 1, 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 said sintered electrode. 6. Electrode according to claim 4, characterized in that the electrocatalyst consists of powdered oxides of said metals sintered on the outer layers of said electrode.