SE437679B - BIPOLER ELECTROD FOR MELT SALT ELECTROLYSIS - Google Patents

Description

lb 30 800507648 Furthermore, electrodes of this type are not active in a number of electrolytic processes. For example, in the case of molten salt electrolytes, the support means for the anodic oxide film-bearing metal dissolves rapidly, since the thin protective oxide layer is either not formed at all or is rapidly destroyed by the molten electrolyte with subsequent dissolution of the anodic oxide film-forming metal base and loss of the catalytic noble metal coating. Furthermore, in the case of several water electrolytes, eg fluoride solutions, or in seawater, the breakdown voltage of the protective oxide layer on the exposed metal base is too small and the metal base often corrodes during anodic polarization.

Recently, other types of electrodes have been proposed to replace the rapidly consumed carbon anodes and carbon cathodes, which have hitherto been used in highly corrosive applications, such as electrolysis of molten metal salts, typically for the electrolysis of molten fluoride baths, e.g. was used to make aluminum from molten cryolite. In this particular electrolytic process, which is of great economic importance, carbon anodes consume at a rate of about 450-500 kg of carbon per ton of aluminum produced and expensive constant adjustment equipment is required to maintain a small and uniform gap between the corrosive anode surfaces and the the liquid aluminum cathode. It can be estimated that over 6 million tonnes of carbon anodes are consumed in one year by aluminum manufacturers. The carbon anodes burn out due to the reaction in A1203 + 3/2 C - to 2 Ål + 3/2 CO2 but the actual consumption rate is much higher due to the brittleness and breakdown of carbon particles and intermittent spark formation, which takes place through the anode gas films , which are often formed over the anode surface, because carbon is little wetted by the molten salt electrolytes, or short circuit caused by "bridges" of conductive particles coming from the corrosive carbon anodes and from dispersed particles from the deposition metal. The British patent 1,295,117 describes anodes for molten cryolite baths consisting of a sintered ceramic oxide material consisting essentially of SnO 2 with minor amounts of other metal oxides, namely oxides of Fe, Sb, Cr, Nb, Zn, W, Aw, Ta in concentration 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 been used for a long time as a durable electrode material in AC glass furnaces (see U.S. Patents 2,490,825, 2,490). .826, 3,287,284 and 3,502,597), they usually show considerable wear and corrosion when used as anode materials in the electrolysis of molten salts. It has been found that wear or abrasion rates of up to 0.5 g / h per cm 2 are achieved from samples of the compositions described in said patent, when used in narrow cryolite electrolyte at 3000 A / m 2. The high wear rate of SnO 2 sintered electrodes appears to be due to several factors: a) a chemical attack of halogens, and in fact Snlv gives complex compounds of high coordination numbers with halogen ions, b) reduction of SnO 2 by aluminum dispersed in the electrolyte and c ) mechanical erosion through anode gas evolution and salt deposition within the pores of the material.

Japanese Patent 112,589 (LL 62,114) discloses electrodes having a conductive support member of titanium, nickel or copper or an alloy thereof, carbon graphite or other conductive material coated with a layer consisting essentially of spinel and / or perovskite type metal oxides and alternatively electrodes obtained by sintering mixtures of said oxides. Spinel oxides and perovskite oxides belong to a group of metal oxides, which typically exhibit good electrical conductivity, and have previously been proposed as suitable electroconductive and electrocatalytic anode coating materials for dimensionally stable anodes of anodic film-forming metals (see U.S. Patents 3,711.292 2,711.292 , Belgian patent 780,303). 8005Û76 = 8w 15 20 25 30 However, coatings of particulate spinels and / or perovskites have been found to be mechanically weak, since the bond between the particulate ceramic coating and the metal or carbon substrate is inherently weak, due to the crystal structure of spi the crystals and perovskites, since the crystal structures of the spinels and perovskites are not isomorphic with the oxides of the metal carrier and various binders such as oxides, carbides, nitrides and borides have been tested with little or no improvement. In the case of molten salt electrolytes, the substrate material is rapidly attacked due to the unavoidable pores through the spinel oxide coating and the coating is quickly peeled from the corrosive substrate. Furthermore, spinels and perovskites are not chemically or electrochemically stable in molten halogen salt electrolytes and exhibit a considerable wear rate due to halogen ion attack and the reducing action of dispersed metal.

In the electrolytic production of metals from molten halide salts, said anodes according to the prior art have been found to have another disadvantage. The considerable dissolution of the ceramic oxide material brings metal cations into the solution, which deposit on the cathode together with the metal generated and the contaminant content of the recovered metal is so high that the metal can no longer be used for applications requiring electrolytic purity. . In such cases, one will completely or partially lose the economic benefits of electrolyte processes, which are largely due to the high purity achieved compared to smelting processes.

An electrode material which is to be used successfully in various corrosive conditions, such as in the electrolysis of molten halide salts and in particular of molten fluoride salts, must be primarily chemically and electrochemically stable under the operating conditions. It must also be catalytic with regard to the anode evolution of oxygen and / or halides, so that the anode overpotential is lowest for high overall efficiency in the electrolysis process. The electrode must also have thermal stability at operating temperatures of, for example, approximately 200 - 110006, good electrical conductivity and be sufficiently resistant to temporary contact with the molten metal cathode. With the exclusion of coated metal electrodes, since hardly any metal substrate can withstand the extremely corrosive conditions found in molten fluoride salt electrolysis, the applicant has systematically examined the properties of a very large number of sintered substantially ceramic electrodes of various compositions.

An object of the present invention is to provide new bipolar electrodes for melting salt electrolysis and the characteristic is seen in that the self-supporting body is a matrix of sintered powders comprising a main component of an oxy compound and a smaller part of an electroconductive material, the oxy compound comprising at least one element selected from the group consisting of titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminum, silicon, tin, chromium, molybdenum, tungsten, lead, manganese, beryllium, iron, cobalt, nickel, platinum, palladium , osmium, iridium, rhenium, technetium, rhodium, ruthenium, gold, silver, cadmium, copper, zinc, germanium, arsenic, antimony, bismuth, boron, scandium and metals from the lanthanoid and actinoid series and that the cathode surface is provided with a layer of cathode material selected from the group consisting of metal carbides, borides, nitrides, sulphides and carbonitrides and mixtures thereof.

Preferred metals in the lantanoid and actinoid series are lanthanum, terbium, erbium, thorium and ytterbium.

The term "sintered electrode" is intended to mean a self-supporting, substantially rigid body consisting essentially of an oximetallic compound and at least one electroconductive material made by any of the known methods used in the ceramic industry. industry, for example by applying temperature and pressure to a powder mixture of the materials to form the mixture to the desired size and shape or by casting the material in molds, extrusion or by using adhesives, etc. and then sintering the shaped body at high temperature to a self-supporting electrode. The oxyhalide compounds are preferably oxychlorides or oxyfluorides.

The electrical conductivity of the sintered ceramic electrodes is improved by adding to the composition 0.1 to 20% by weight of at least one electroconductive material selected from the group consisting of (A) doping oxides, typically metals with a valence, which is lower or higher than the valence of the metals in the oxides that make up the matrix, such as the alkaline earth metals Ca, Mg, Sr and Ba, and such metals as An, Cd, Inz, TI2, Asz, Sbz, Biz and Sn, (B) oxides exhibiting electroconductivity due to internal redox systems, eg spinel oxides, perovskite oxides, etc., (C) oxides exhibiting electroconductivity due to metal-to-metal bonds such as CrO 2, MnO 2, TiO, Ti 2 O 3, etc., borides, silicides , carbides and sulphides of the anodic oxide film-forming 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 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 and oxides of metals within the group consisting of manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, iridium, platinum, silver, arsenic, antimony, lead and bismuth and mixtures thereof. qeitpi fl - 15 20 25 30 8Û05Û76 ~ 8 By adding to the powder in the matrix material a small amount, typically 0.5 - about 30%, of powder of a suitable electrocatalytic material and by sintering the mixture in the self-supporting body it was found that when used as an electrode it has satisfactory electroconductive and electrocatalytic properties which maintain its chemical stability even though the added catalyst would not in itself be resistant to the electrolysis conditions.

The electrocatalyst can be a metal or an inorganic oxy compound. The preferred added catalyst powders are powders of the metals Ru, Rh, Pd, Ir, Pt, Fe, Co, Ni, Cu and Ag, 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, Rh 2 O 3, IrO 2, RhO 2, Åg 2 O, Ag 2 O 2, Ag 2 O 3, As 2 O 3, Sb 2 O 3, Bi 2 O 3, CoMn 2 O 4, NiMn 2 O 4, CoRh 2 O 4 and NiCo 2 O 4 and mixtures of said powder metals and oxy compounds.

It has been found to be particularly advantageous to add to oxymetal compounds a second material, for example stannous oxide, zirconia or the like, and also to add a small amount of at least one metal belonging to the group comprising yttrium, chromium, molybdenum, zirconium, tantalum , tungsten, cobalt, nickel, palladium and silver, whereby both the mechanical properties and the electrical conductivity of the sintered electrodes are improved without appreciable reduction in their chemical and electrochemical corrosion resistance.

These additives are added in powder form and mixed with the powder metal oxide in percentages which can range from 40 to 1% based on the weight of the metal content. Optionally, additional other organic or inorganic compounds may be added to the powder mixture to improve the bonding of the particles during both casting and sintering. 120012 QUALITY asnvs-o s, IU 15 2U 25 The conductivity of the sintered electrodes of the invention is comparable to that of graphite. The matrix has acceptable function in shaping and sintering and in use forms a thin layer of oxyhalides on its surface under anodic conditions. The release energy of the metal oxy compounds is more negative than the oxide release energy of the corresponding salt electrolyte of the halide phase, so that these sintered anodes have a high degree of chemical stability.

The sintered metal oxy compound electrodes of the invention are bipolar electrodes. In this case, the sintered electrodes can suitably be produced in the form of a disk or a plate, one of the two main surfaces of the electrode being provided with a layer containing the anodic electrocatalyst, such as the oxides CO 3 O 4, Ni 3 O 4, MnO 2, Rh 2 O 3, RuO 2, Al 9 O etc. and the second main surface is provided with a layer containing suitable cathode materials, such as carbides, borides, nitrides, sulfides, carbonitrides, etc. of metals, especially of the anodic oxide film-forming metals and most preferably of yttrium, titanium and zirconium.

The self-supporting sintered body, which consists of a main part of oxy-metal compound, can be prepared by grinding the materials or separately grinding preferably to a grain size between 50 and 500 microns to provide a powder mixture containing a range of grain sizes to obtain a better degree of compactness.

According to one of the preferred methods, the powder mixture is mixed with water or with an organic binder to obtain a plastic mass which has suitable flow properties for the particular molding process used. The material can be cast in a known manner either by stamping or pressing the mixture into a mold or by slurry casting in a fired gypsum mold or the material can be extruded through a hollow die into various shapes. The cast electrodes are then subjected to a drying process and heated to a temperature at which the desired bonding can take place, usually between 800 DEG-180 DEG C. for a period of 1 to 30 hours, normally followed by slow cooling to room temperature. . The heat treatment is preferably carried out in an inert atmosphere or one which is somewhat reducing, for example in H 2 + N? (80%), as the powder mixture is composed essentially of oximetallic compounds with a minor amount of other metal oxides or metals.

As the powder mixture also contains metal powder, it is preferable to carry out the heat treatment in an oxidizing atmosphere, at least during part of the heat treatment cycle, in order to promote the oxidation of the metal particles in the outer layers of the electrodes.

The metal particles, which remain inside the body of the sintered material, improve the electrical conductivity properties of the electrode.

The molding process can be accompanied by the sintering process at high temperature as mentioned above or the molding process and the sintering process can be simultaneous, i.e. pressure and temperature can be applied to the powder mixture simultaneously, for example by means of electrically heated molds. Input conductor connectors can be fused to the ceramic electrodes during the casting and sintering processes or attached to the electrodes after sintering or casting. Other methods of shaping, compressing and sintering the powder mixture can, of course, be used.

Due to the cost, the electrical catalyst which is usually applied to the electrolyte must have a high stability, a low anodic overpotential for the desired anode reaction and a high anodic overpotential for undesired reactions. In the case of chlorine evolution oxides of cobalt, nickel, iridium, rhodium, ruthenium or mixed oxides thereof, such as RuO 2 - TiO 2, etc. can be used and in the case of fluoride containing electrolytes, where the oxygen evolution is the desired anode reaction, oxides of silver and manganese are preferred. Other oxides for use as electrocatalysts may be oxides of platinum, palladium and lead.

Toxins to suppress unwanted anodic reactions can be used, for example to suppress oxygen evolution from chloride electrolytes. Toxins, which show a high oxygen overpotential, should be used and suitable materials are oxides of arsenic, antimony and bismuth. These oxides, which are used in small percentages, can be added together with the electrocatalyst oxides in percentages from 1-10% of the electrocatalyst calculated on the weight of the respective metals.

The application of the electrocatalyst and the optional toxin can be accomplished by any known coating method. Preferably, the electrocatalyst and optionally the poison are applied to the sintered electrodes as a solution of decomposable salts of the metals. The sintered body is impregnated with the solution containing the appropriate metal salts and dried. Thereafter, the electrode is heated in air or in another oxygen-containing atmosphere to convert the salts to the desired oxides.

Generally, the porosity of the sintered body and the method used to impregnate the surface layer of the sintered body with the metal salts will allow penetration of the solution down to a depth of at least 1-5 millimeters, preferably 3 nm, inwardly from the electrode surface, so that after the heat treatment the electrocatalysts are present in the pores of the sintered body down to a certain depth inwards from the surface of the electrodes.

Alternatively, by suitable powder mixing techniques, preformed electrocatalytic oxides and optionally preformed poison oxides can be ground to 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 electrolyte catalyst. the toxic oxides during the forming process, whereby after sintering the surface of the electrodes will already be provided with the electrocatalyst.

The sintered bipolar electrodes of the invention are provided on a surface with the anodic electrocatalyst and optionally with the poisoning agent for the undesired anode reaction by one of the methods described above, while the other surface is provided with a coating of a suitable cathode material. The surface of the bipolar electrode, which is to act as a cathode during the electrolysis process, may, for example, be provided 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 by plasma beam technique, whereby powders of selected materials are sprayed and adhered to the surface of the sintered body with a flame under a controlled atmosphere. Alternatively, the selected powder materials can be added to the powder mixture during the forming process and then sintered together, whereby the cathode surface of the bipolar electrode is provided with a layer of the selected cathode material.

The electrodes can also be effectively used as anodes and cathodes in DC electrolysis of molten salt electrolytes typically containing halides, oxides, carbonates or hydrates for the production of aluminum, beryllium, calcium, cerium, lithium, sodium, magnesium, potassium, barium, strontium, cesium and others. metals.

When the bipolar electrodes according to the invention are used as electrodes for melting salt electrolysis, the composition of the electrodes must be such that it is not reduced by the catalysis in PGoRiQEÅLEQÉQ 8005076-8 20 Pooa debates-- _. The reaction or is attacked by the metal deposited on the cathodes, especially when the electrode composition is an oxy compound.

For this reason, it is desirable to have the composition for the cathode side of the bipolar electrode inert to the cathode reaction and reducing effect of the molten metal.

The electrodes can also be used as anodes and cathodes in electrochemical processes such as electrolysis of chloride aqueous solutions for the production of chlorine, caustics, hydrogen, hypochlorite, chlorates and perchlorates, electro-extraction of metals from sulphate or chloride aqueous 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 aluminum, beryllium, calcium, cerium, lithium, sodium, magnesium, other metals and electrolysis of bromides, sulphides, sulfuric acid, hydrochloric acid and hydrofluoric acid. In general, the electrodes are usable for all electrolytic processes.

The following examples describe various preferred embodiments to illustrate the invention. It will be appreciated, however, that the invention is not intended to be limited to the particular embodiments. The percentages of the compounds in the electrodes are calculated as a percentage by weight and as free metal based on the total metal content of the composition.

Among the clay-coated anodes are those in which the main component of the self-supporting body is tin dioxide alone or with up to 20% by weight of cobalt oxide, provided with a cobalt oxide coating which provides electrodes with improved mechanical properties and electrocatalytic properties for chlorine production.

Other preferred additives are Y 2 O 3, TiO 2 and Ta 2 O 5.

IO 80050 76-8 13 The electrocatalyst coating can be protected against wear by simultaneous or subsequent application of a protective agent such as oxide of an anodic oxide film-forming metal, eg TiO 2 and Ta 2 O 5 or SiO 2, mixed oxides such as AgRe 2 O 3, TiCo 3, 04 and AgXwo 3.

Example 1 Three sintered electrode samples, 80% SnO 2 + 20% cobalt (sample A), 80% SnO 2 + 10% Co + 10% Mo (sample B) and 100% SnO 2 (sample C) were prepared and then washed thoroughly with water. and dried under vacuum. The resulting electrodes were then immersed under vacuum in the solution listed in Table I and then dried followed by heating at 370 ° C for 20 minutes in a forced air oven. The samples were then brushed with the same solution and heated for 15 minutes at 35006 in the same oven and this procedure was repeated several times, until the electrode had a weight increase of 5 g / m2.

The electrode samples were then used as anodes in a sample cell for electrolysis of aluminum chloride at 75,000 and below an anode current density of 1,000 A / m 2. The cell voltage was 5 volts and the electrolyte was a 5-1-1 mixture by weight of aluminum chloride, sodium chloride and potassium chloride. The anode potential was determined from the beginning and after 500 operating hours and weight losses of the electrode were determined after 500 hours.

For comparison purposes, a standard graphite electrode was also used under the same conditions and the results are shown in Table I. t a ~ Fifty 'Å 8ÛÛ5Û76 ~ 8 14 Table I Sample Solution_ Anode potential Weight loss after soo mm. g / mz solvent salt Initial V (RCGE) +) after 500 hours. 5 A Water and CoC12 0.45 0.47 0.8 formaldehyde B Hydrogen chloride- IrCl3 0.45 0.45 0.5 acid C Hydrogen chloride- IrCl3 0.5 0.6 0.2 acid A untreated 0.6 0.6 0.5 B "0.55 0.55 0.5 C" 1.0 1.3 none +) RCGE: Chlorine graphite reference electrode. The results in Table I show that the coated electrodes have an even lower overpotential for chlorine evolution without any 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 is. The average faraday efficiency during the test was 96%. A standard graphite electrode used in the same manner and compared to the reference graphite electrode showed a voltage of about 0.8 volts.

Example 2 Samples of 90% by weight of tin dioxide and 10% by weight of cobalt were sintered and the electrodes were then coated with a coating of carbon dioxide as in Example 1 to obtain a layer of 10 g / m 2 of cobalt oxide. The electrodes were then used to electrolyze the electrolytes of Table II under operating conditions set forth therein.

The anode potential after 300 operating hours and the degree of wear after 300 hours were determined and are given in Table II.

Table II Electrolyte- Electrolyte- Current Density- Avg. Anodpo- Weight- smmnansättn. temp. OC hot A / m2 faraday- tential for- and weightfbr- effect- after lust holding white 300 hours. g / m2 V (RCGE) AlCl 3 + KC] 750 1000 92% 0.5 0.5 (511) CaCl 2 + KCl 450 1000 94% 0.6 0.5 (5: 1) PhCl? + KCl 450 1000 90% 0.6 0.5 (5: 1) Table II shows that the electrodes according to the invention have a low degree of wear and a low anode potential even after 300 operating hours.

Example 3 Disc-shaped electrodes with a diameter of 10 mm and a thickness of 5 mm were prepared from powder with sieve No. 100 - 250 (mesh size 0.149 - 0.62 mm). The powders were compression molded at a pressure of 1000 kg / cm 2 and then sintered in an induction furnace under conditions shown in Table III, which also shows compositions of the powders.

The sintering was carried out in a furnace through which the specified gas was circulated or maintained at atmospheric pressure. Thus, at least the outer surfaces and perhaps a portion of the inner pores were exposed to an oxidizing atmosphere at the indicated temperature and the exposed metal in the surfaces was oxidized to form the electrocatalyst.

Table III Síntrlng Sample Components and Heating No 5% by weight Temp. OC Atmospheric time 1 Sn02 80% 1250 Forced air- 2 h Circulation 2 Sn0¿ 80% C0 10% 1500 "2 h 10 Mo 10% 3 Sn02 80% Mo 10% 1250" 2 h Ni 10% 4> Sn02 75% Ambient - Ib Laå03 10% 1500 luft 2 h CO 5 Sn02 60% C02Ní04 30% 1500 "2 h Co 10% 20 6 Sn02 60% Si03 10% La20ë 1000" 2 h Co2N104 10 A Cu 10% 10 20 25 8005076-8 17 Table III cont 7 Sn02 95% Forced Co 2.5% 1500 air clr- 2 h Mo 2.5% culture 8 Sn02 100% 1500 Air 10 h The electroconductivities of samples 1 - 7, measured at 500 ° C, were between 0.01 and 1.00 ohm'1 and ohm "1 cm1 and the density of the sintered electrodes varied between 5 and 8.5 9 / cm3. The electrode samples were used as anodes in a sample cell for electrolysis of aluminum chloride at 75000 and an anode current density of 1000 A / m2.

The cell voltage was 5 volts and the electrolyte was a 5-1-1 mixture of aluminum chloride, sodium chloride and potassium chloride. The anode potential was determined initially and after 500 hours of operation and the weight loss of the electrode was determined after 500 hours. For comparison purposes, a graphite reference electrode was also used under the same conditions and the results are shown in Table IV.

Table IV Test Anode potential Weight loss after no. V v. Ma) *) son um g / mz Initial After 500 hrs. 1 0.6 0.6 0.5 2 0.55 0.55 0.5 3 0.60 0.6 0.8 4 0.55 0.6 0.5 5 0.6 0.6 None 6 0 .65 0.65 0.5 7 0.55 0.6 None 8 1.0 1.3 None Graphite 0.85 0.85 105 +) RGE: graphite reference electrode.

POOR QUÄLTTY 800-50 76 ~ 8 10 l5 20 18 The results in Table IV show that the electrodes 1 - 7 containing a major part of an oxide and a minor part of a metal have a low overpotential for chlorine development and a very low degree of wear. The electrode 8, which did not contain any addition of electroconductive metal, had a significantly higher overpotential for chlorine evolution and the graphite reference electrode had an overpotential over the values of the electrodes 1 - 7 and a high degree of wear. The reference anode required significant adjustments during electrolysis and early replacement. The average efficiency during the test was 97%. All samples 1 - 7 were less fragile than sample 8.

Example 4 Approximately 250 g of a mixture of the matrix material and the additive listed in Table I was ground in a mixer for 20 minutes and the powder mixtures were poured into cylindrical plastic molds and pre-compressed manually with a steel cylinder press. Each mold was placed in an isostatic pressure chamber and the pressure was increased to about 1500 kg / cm 2 and pre-compressed manually with a steel cylinder press. Each mold was placed in an isostatic pressure chamber and the pressure was increased to about 1500 kg / cm 2 in 5 minutes and then reduced to zero in a few seconds. The samples were then taken out of the plastic molds and polished. The compressed samples were placed in an electrically heated oven and heated from room temperature to 120006 under a nitrogen atmosphere for a period of 24 hours, kept at the maximum temperature for 2 - 5 hours and then cooled to 30096 for the following 24 hours. The sintered samples were taken out of the oven and after cooling to room temperature they were weighed and their apparent density and electrically active at 25 ° C and at 1000 ° C were measured. The results are shown in Table V.

C 20 25 Table V Sample Sample composition.

Sintering time _aooso76-e Apparent Electrical conductivity density white no and weight percentage at max. Temp. __ g / cm3 at 1000 ° at 25% E.-1cm'1 ßvó-1cm "1 1 zroz ao% í Y203 10% 5.1 0.1 - YOF IrO2 2 Ta205 50% La203 5% SiO2 5% 5, 3 0.4 - V02 20% Co304 20% 3 ZrO2 50% Ti203 20% 5.1 0.3 - Zr0C12 20% Rh203 10% 4 NDZO5 30% 1102 20% YOF 20% 5.6 0.5 - A920 20 % Sb203 10% 5 Y303 20% La303 20 A ThO2 20% 5,8 0,7 - r1¿, o3 20% Rh203 20% POOÉ QUALITY 800-50 76 ~ 0 10 20 25 20 6 Zr02 Y203 S102 Zr20CI Bi203 A920 Ru02 Cu0 2 7 Zrüz Y 03 Sn02 Ir0 Cu0 8 ZrO2 Yb203 T502 T1203 Sn02 Co304 9 Ti02 ^ 12 ° 3 T10203 Sn0 5Mn + 2 10 Zr02 Y 0 2 3 YOF Ru0 40% 5% 5% 15% 10% 15% 5%, 5% 50% 30% 10% 2% 30% 10% 10% 25% 15% 10% 20% 20% 20% 20% 20% 30% 5% 25% 20% 20% 5.3 5.9 5, 8 5.4 5.1 1.2 1.0 0.8 2.1 0.5 IU 15 20 75 8005076-8 21 11 T102 20% Ta2O5 30% V02 10% 2 5.7 5 0.9 Fe203 10 % C0304 10% Co 20% 12 T102 40% TiOC 25% SnO2 15% 5 6.3 12 1.6 RuO2 5% Mo 10% Ti 5% 13 T102 40% Ta205 10% T1203 10% 5 6.1 10 2 , 5 Co304 20% Mo 10% ASZO3 10% 14 Zr02 40% Y203 5% V02 25% 2 6,5 15 2 .5 A90 20% Pd 1% Mo 9% Datana 1 tabe11 V shows that the electric conductivity of the sintered ceramic electrodes at high temperatures around 100006 is 5 - 10 times higher than the electric conductivity at 2506.

PÖOR QUALITY 8005076- 8 10 20 25 22 The addition of oxides, with conductivity equivalent to metals, to the substantially non-conductive ceramic oxides in the matrix increases the conductivity.for the electrodes of the order of 102. The addition of a metal which is stable to melt salts, such as yttrium or molybdenum, etc., to the ceramic electrodes according to the invention, increase the electrical conductivity of the electrodes 2 - 5 times.

Example 5 The operating conditions of an electrolytic cell for producing aluminum metal from a molten cryolite bath were simulated in a laboratory sample cell. In a heated graphite crucible, a layer of liquid aluminum was formed on the bottom and a melt consisting of cryolite (80 - 85%), alumina (5 - 10%) and AlF3 (from 1 - 5%) was poured on top of the same. The sample electrodes with a working surface of 3 cm were prepared according to the procedure described in Example 4 and a platinum wire was soldered to provide a suitable means for electrical connection and the electrodes were immersed in the salt melt and kept at a distance of about 1 cm from the liquid aluminum. - the minium layer. The crucible was maintained at a temperature of 950 - 105000 and the current density was 0.5 A / cm 2 and the cell was operated for 2000 hours. The sample experimental data as obtained were shown in Table VI.

The sample number indicates that the tested electrode corresponded to the sample described in Table V with the same number.

Table VI Sample No. Aluminum Weight loss for manufactured (g / h) anodes (9 / cm 2) l 0.49 0.02 2 0.50 0.12 3 0.49 0.04 4 0.49 0.02 10 20 25 48005076-8 23 5 0.48 0.01 6 0.49 0.04 7 0.49 0.06 8 0.46 0.18 9 0.46 0.2 The test electrodes were successfully driven as anodes in the cryolite melt and the observed wear rates appears to be perfectly acceptable for the electrolytic production of aluminum from narrow cryolite.

All the tested electrodes showed a low degree of wear during 2000 operating hours. In general, the degree of wear for electrodes containing heat stabilizers, such as oxy compounds of the metals of Group III of the Periodic Table, is about 10 times less than for electrodes without heat stabilizers.

Example 6 Electrodes Nos. 4 and 5 in Table V were used as anodes for electrolysis of a molten aluminum chloride electrolyte in the sample cell described in Example 5. The electrolysis conditions were as follows: Electrolyte 1 AlCl 3 31 - 35% NaCl 31 - 35% BaCO 3 31 - 35% Temperature for the electrolyte 690 -720 ° C 2000 amp / m2 Anode current density Cathode: molten aluminum Electrode spacing: 1 cm The tested electrodes worked successfully and the weight losses after 2000 hours of operation were negligible. fPeoR rubber 8005076-8 l0 _ 20 25 - _. q-WFVPWW ”íåëâšääå íÉ = J¿n fi» 24 Other electrocatalysts, which can be used in the electrolysis of molten halide salts for halide ion release, are RuO2 and such oxides as ASZO3, Sn203 and Bi203 can be added in percentages of up to 10% by weight the free metal based on the total metal content to increase the oxygen overpotential without affecting the halide ion release potential.

For anodes to be used in molten fluoride electrolytes where oxygen is evolved, the catalyst may be that shown in Example 5 or Rh 2 O 3, PbO 2 and IrO 2. Ti02.

The components of the anodes according to the examples are calculated in% by weight of free metal based on the total metal content of the anode composition- 9 fi l0 fl8fl.

The electrolyte may contain salts other than those used in the examples such as alkali metal chloride or fluoride as well as the salt of the metal undergoing the electrolysis. The metal halides are effective in reducing the melting point of the salt undergoing electrolysis, while allowing the use of lower temperatures while maintaining the salt bath in the molten or melting state.

The above examples include molten metal salt electrolysis, primarily electrolysis of molten aluminum chloride or fluoride salts. In a similar manner, molten chlorides of other metals such as alkali metals or alkaline earth metals can be electrolyzed using the indicated anodes and in accordance with standard practice. In addition, other molten salts, such as molten nitrates, can be electrolyzed in the same manner. A molten alumina cryolite electrolyte or similar alkali metal aluminum fluoride can be electrolyzed to produce molten aluminum. x___ _____ ... fq qw fi 3 "'_., 8005076- 8 25 These electrodes can be used in the stand for graphite anodes in standard aluminum electrode recovery cells with either aluminum feed or the crystallite bath with aluminum chloride feed in a predominantly aluminum chloride bath.

The use of these sintered metal anode nodes for the recovery of the desired metals from forged salts of the metals to be recovered results in a reduced power consumption per unit weight of metal produced] and in cleaner recovered metals. The electrodes are dimensionally stable in function and therefore do not require any frequent intervention to restore optimal distance from the cathode surface as is necessary with the known consumption anodes.

Claims (4)

1. 3005076-8 g 1 n 50 çQUïiïš-lï §_a ~ t_§ "ntkra V 1. Bipolar electrode for melt salt electrolysis, comprising a self-supporting body of sintered powders, a layer of cathode material on its cathode surface and an anode catalyst on its anode surface , characterized in that the self-supporting body is a matrix of sintered powders comprising a main constituent of an oxy compound and a minor part of an electroconductive material, the oxy compound comprising at least one element selected from the group consisting of titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminum, silicon, tin, chromium, molybdenum, tungsten, lead, manganese, beryllium, iron, cobalt, nickel, platinum, palladium, osmium, iridium, rhenium, technetium, rhodium, ruthenium, gold, silver, cadmium, copper, zinc, grmanium, arsenic, antimony, bismuth, boron, scandium and metals from the lanthanoid and actinoid series and that the cathode surface is provided with a layer of cathode material selected from the group metal carbides, - borides, nitrides, sulphides and carbonitrides and mixtures thereof. 2. An electrode according to claim 1, characterized in that the electroconductive material is an oxide of zirconium and / or tin. 3. An electrode according to claim 1, characterized in that the electroconductive material is at least one of the metals yttrium, chromium, molybdenum, zirconium, tantalum, tungsten, cobalt, nickel, palladium and silver. 4. An electrode according to claim 1, characterized in that the electrocatalyst is an oxide of the metals cobalt, nickel, manganese, rhodium, iridium, ruthenium, silver and landings thereof. I>. Electrode according to claim 4, k ä n n e t e c k n a d av, ..__, .. i ..___ + _..___ _ *. “Tï LI! 8-7505076-8 that the electrocatalyst is formed in situ on the sintered electrode body from a solution of salts of said metals, which are converted to oxides on the sintered electrode body. 6. An electrode according to claim 1, characterized in that the electrode catalyst consists of oxide powder of said metals sintered in the outer layers of the electrode. 7. An electrode according to claim 1, characterized in that the layer of the cathode material is applied by flame spraying. 8. An electrode according to claim 1, characterized in that the layer of the cathode material consists of powder of the cathode material sintered in the cathodic outer surfaces of the electrode. Electrode according to claim 1, characterized in that the cathode material consists of carbides, borides, nitrides, sulphides and carbonitrides or at least one of the metals yttrium, titanium and zirconium, 'man Quazm' J-Ul »'. «