SE438165B - BIPOLE Electrode for Electrolytic Processes - Google Patents

Description

8ÛÜ5Û75-0 10 15 20 25 30 BJ is dehydrated 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, e.g. fluoride solutions, or in seawater the breakdown voltage of the protective oxide layer on the exposed anodic oxide film-forming metal base is too small and said 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, e.g. electrolysis of molten metal salts, typically for electrolysis of molten fluoride baths, e.g. those used to make aluminum from molten cryolite. In this special electrolytic process, which is of great economic importance, carbon anodes are consumed at a rate of about 450-500 kg of carbon per tonne of aluminum produced and expensive constant adjustment equipment is required to maintain a small and uniform gap between the corrosive anode surfaces. and the liquid aluminum cathode. It can be estimated that over 6 million tonnes of coal anodes are consumed in one year by aluminum manufacturers. The carbon anodes burn out due to the reaction: Aho, + “/ 2c_9 zAl + a / zco, but the actual consumption rate is much higher due to the brittleness and breakdown of carbon particles and intermittent sparking, which takes place through the anode gas films, which are often formed 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.

English Patent 1,295,117 discloses anodes for molten cryolite baths consisting of a sintered ceramic oxide material 5-12; 80050754) consisting essentially of SnO2 with minor amounts of other metal oxides, namely oxides of Fe, Sb, Cr, Nb, Zn, W, Zr, Ta in a concentration of up to 20%. Although electrically conductive sintered SnO 2 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 alternating glass melting furnaces (see U.S. Pat. 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 obtained from samples of the compositions described in the said patents when used in molten cryolite electrolyte at 3000 A / m 2. The high wear rate of sintered electrodes of SnO2 appears to be due to several factors: a) a chemical attack of halogens, and in fact Snïv produces complex compounds of high coordination numbers with halogen ions, b) reduction of SnO2 by aluminum dispersed in electrodes blemishes and c) mechanical erosion by anode gas evolution and salt deposition within the pores of the material.

Japanese Patent 112,589 (Laid-Open Publication No. 62114 of 1975) 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 metal oxides of the spinel and / or perovskite type. 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 electro-conductive and electrocatalytic anode coating materials for dimensionally stable anodic oxide film-forming metal anodes (see U.S. Patents 3,711,382 and 3,711,381,382 .297, “FÄÚåo1r, r-sï_fsw 8005075-0 Belgian Patent 780,303).

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 the spinels 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 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 off from the corroded substrate. Furthermore, spinels and perovskites are not chemically or electrochemically stable in molten halide salt electrolytes and exhibit considerable wear rates due to halide 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 solution which are deposited on the cathode together with the metal generated and the impurity content of the recovered metal is high enough that the metal can no longer be used for applications which requires 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 melting 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 as well as 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 e.g. about 200-1100 ° C, 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 fluorite salt electrolysis, the applicant has systematically examined the properties of a very large number of sintered substantially ceramic electrodes of various compositions.

A very efficient bipolar electrode for electrolytic processes according to the invention is essentially characterized in that the self-supporting body comprises sintered powders of yttrium oxide and an electroconductive material, a major part of the powder being yttrium oxide and a smaller part being the electroconductive material, the self-supporting the body over at least a portion of its cathode surface has a layer of cathodic electrocatalytic material selected from the group consisting of metal carbides, borides, nitrides, sulfides and carbonitrides and mixtures thereof.

The sintered yttrium oxide electrodes are particularly useful in electrowinning processes used in the production of various metals, such as aluminum, magnesium, sodium, calcium, potassium, lithium and other metals, from molten salts. Yttrium oxide and at least one electroconductive material, when used as an anode in DC electrolysis of molten metal electrolytes, have been found to be unusually stable as an inert, dimensionally stable anode of sufficient electrical conductivity and, when provided on a surface with an oxide electrocatalyst, such as pp -w-rs' POOR QUEÄLET * 10 15 20 25 30 8Û05075 ~ G CO3Oq, Niaür, MnO2, Rh2Oa, IrQ2, Ru02, Ag¿Q etc. it has a high electrocatalytic activity especially for chlorine development.

The term "sintered yttrium oxide" is intended to mean a self-supporting, substantially rigid body consisting essentially of yttrium oxide and produced by any of the known methods used in the ceramic industry, e.g. by applying temperature and pressure to a powder mixture of yttrium oxide and other materials to shape the mixture to the desired size and shape or by casting the material into molds, by extrusion or by using adhesives, etc. and then sintering the shaped body at high temperature to a self-supporting electrode. The electrical conductivity of the sintered ceramic electrodes is improved by adding to the composition 0.1-20% by weight of at least one electroconductive material selected from that group. consisting of (A) doping oxides, typically metals with a valence lower than or higher than the valence of the metals in the oxides constituting the matrix, e.g. the alkaline earth metals Ca, Mg, Sr and Ba, and such metals as Zn, Cd, Ing, Tlz, Asz, Sbz, Bi; and Sn, (B) oxides showing electroconductivity due to internal redox systems, e.g. spinel oxides, perovskite oxides, etc., (C) oxides having electroconductivity due to metal-to-metal bonds such as CrO2, MnO2, TiO, Ti2O3, etc., borides, silicides, carbides and sulfides of 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 Agl - or alloys thereof. mixtures of (A) and / or (B) and / or (C).

By adding to the powder in the matrix material a minor amount, typically 0.5-about 30%, of powder of a suitable soluble electrocatalytic material and by sintering the mixture into the self-supporting body shown when used as an electrode, it has satisfactory electroconductive and electrocatalytic properties which maintain its chemical stability, even though the added catalyst would not be resistant in itself to the electrolysis conditions.

The electrocatalyst may 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 MnO2, Co3Oq, Rh2O3, IrO2, RhO2, AqgÛ, Agzog, Åggøg, ÅS2Û3, 813203, 231203, C0MI12Û | H CORhgOi, and mixtures of said powder metals and oxy compounds.

It has proved particularly advantageous to add to the yttrium oxide a second material, e.g. stannous oxide, zirconia or the like and to also 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, both mechanical the properties and electrical conductivity of the sintered yttrium oxide electrodes are improved without appreciable reduction in their chemical and electrochemical corrosion resistance.

These additives are added in powder form and mixed with the yttria powder in percentages which can range from 40 to 1% based on the weight of the metal content. Optionally, additional other organic and / or inorganic compounds may be added to the powder mixture to improve the bonding of the particles during both casting and sintering.

Anodes containing a major constituent YZO3 have a high melting point well above the temperature of the molten salt electrolytes used and they do not undergo any phase change under working conditions in the electrolysis. Furthermore, the coefficient of thermal expansion is not too different from that of the halide salts used in the molten salt baths, which helps to maintain the proper electrode gap between the anode and the cathode and avoids expansions and contractions which would cause the salt crust on top of the molten metal electrolyte to rupture. in the normal aluminum electroplating process. The conductivity of the sintered yttrium oxide electrodes according to the invention is comparable to that of graphite. The matrix has acceptable functionality in shaping and sintering and in use forms a thin layer of oxyhalides on its surface under anodic conditions. The release energy of the outer triium oxide is more negative than the oxide release energy of the molten salt electrolyte of the corresponding halide phase, so that these sintered yttrium oxide anodes have a high degree of chemical stability.

The sintered yttrium oxide electrodes according to the invention can also be used as bipolar electrodes. In the latter embodiment, the sintered yttrium oxide electrodes may conveniently be made in the form of a wafer or 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 Oq, Ni 3 O 4, MnO 2, Rh 2 O 3 , IrO 2, RuO 2, Ag 2 O, etc. and the second major surface is provided with a layer containing suitable cathode materials, such as carbides, borides, nitrides, sulfides, carbonitrides, etc. of metals, especially 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 yttrium oxide, can be prepared by grinding the materials or separately grinding preferably to a grain size between 50 and 500 microns to obtain 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 holding pad 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 800-1800 ° C for a period of 1-30 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 2 (80%), as the powder mixture is composed essentially of yttrium oxide with a minor amount of other metal oxides or metals.

Since the powder mixture also contains metal powder, it is preferable to carry out the heat treatment in an oxidizing atmosphere at least during a part of the heat treatment cycle to promote the oxidation of 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.

Poor Quinn 8005075-0 10 The molding process may be accompanied by the high temperature sintering process as mentioned above or the molding process and the sintering process may 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 forming, compressing and sintering the yttria powder mixture can, of course, be used.

Due to the cost, the electrical catalyst, which is usually applied to the electrode surface, 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 RuO2 - TiO2, 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 to be preferred. Other oxides for use such as electrocatalysts may be oxides of platinum, palladium and lead.

Toxins to suppress unwanted anodic reactions can be used, e.g. to suppress oxygen evolution from chloride electrolytes. Toxins, which have a high acid 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 resp. weight of metals.

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

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

Alternatively, by suitable powder mixing techniques for shaped electrocatalytic oxides and optionally preformed poison oxides, the powder mixture can be ground and fed during the casting of the electrodes in such a way that the outer layers of the cast electrodes are enriched with electrocatalyst oxide powder and optionally with the poison oxides. the sintering surface of the electrodes will already be provided with the electrocatalyst.

The sintered yttrium oxide electrodes are used as bipolar electrodes. Thereby, the yttrium electrodes are provided on one 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, is provided with a layer of metal carbides, borides, nitrides, sulphides and / or carbonitrides of yttrium, tantalum, titanium, zirconium, etc.

A preferred way to apply a layer is through plasma p »va. .___ POOR QUALITY 10 15 20 25 30 8005375-0 12 radiation technology, whereby powders of selected materials are sprayed and adhered to the surface of the sintered yttrium body with a flame under a controlled atmosphere. Alternatively, the selected powder materials may be added to the yttria powder mixture during the molding process and then sintered together, whereby the cathode surface of the bipolar electrode is provided with a layer of the selected cathode material.

The sintered yttrium oxide activated with suitable electrocatalyst can be used as a non-consumption electrode in electrolysis of molten salts and for other processes in which an electric current is passed through an electrolyte to decompose the electrolyte to perform oxidations and reductions of organic and inorganic compounds or to apply a cathode potential to a metal structure to be protected from corrosion, as well as to primary and secondary batteries containing molten salts, such as aluminum halide alkali metal halides. In bipolar electrodes, one surface or end of the electrode acts as an anode and the opposite surface or end of the electrode acts as a cathode relative to the electrolyte which is in contact with each respective surface of the electrode, as is known in the electrolytic art.

The following examples describe various embodiments to illustrate the invention. The percentages of the electrode components are calculated as a percentage by weight of metal based on the total metal content of the composition.

Example [Approx. 250 g of a mixture of the matrix material and the additive material according to 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 raised to about 1500 kg / cm 2 for 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 1200 ° C under a nitrogen atmosphere for a period of 24 hours and kept at the maximum temperature for 2-5 hours and then cooled to 300 ° C for the following 24 hours. The sintered samples were then taken from the oven and after cooling to room temperature they were weighed and their apparent density and electrical conductivity at 25 ° C and at 100,000 were measured. The results are shown in Table I.

Table I Sample Composition Sintering time Rails Electricity conductive no.%% At max. Temp. density at 1000 C at 250 C (hr.) g / ana S1 1 cm-1 Sf * cm-1 A YZOQ 75 5 5.2 Ti? 03 25 0.001 - YZO3 65 1 Ti2O3 15 5 5.2 0.2 - Rh2O3 20 ZnO; Y2O3- 30 2 WO 10 5 5.5 0.4 - A920 10 RuO2 10 ZnO 10 YZO3 50 YFO 15 3 Y 15 2 5.9 11 2.5 IrO2 10 Agzo 10 »w MN- ..,; - ~ 1- _ '' jvw-Mfw ~ u; ä fl íllßlllm. i 10 15 20 25 '30 35 8005075-0 14 “_ Table I (continued.) Test Knmposition Sintering time Apparent Electrical conductivity no weight% at max.tenp. density at 1000 C at 250 C (hr) g / af: fl mi * n "af * Y2O3 60 NiCo2O @ 20 4 CdO 5 2 5.7 7 0.2 CaO 1 RhgO3 19 The data in Table I show that the electrical conductivity for the sintered ceramic electrodes at high temperatures of 1000 ° C is 5-10 times higher than the electrical conductivity at 2500. The addition of oxides with conductivity equivalent to metals to the substantially non-conductive ceramic oxides in the matrix increases the conductivity of the electrodes of the order of 102 as shown by electrodes A and No. 1. The addition of a metal which is stable to molten salts, such as yttrium or molybdenum, etc., to the ceramic electrodes according to the invention increases the electrical conductivity of the electrodes 2- 5 times.

Example 2 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 applied to the bottom and a melt consisting of cryolite (80-82%), alumina (5-10%) and AlF3 (from 1-5%) was poured on top. The test electrodes with a working surface of 3 cm 2 prepared according to the procedure of Example 1, in which a Pt wire was welded to provide a light means for electrical connection, were immersed in the salt melt and kept at a distance of about 1 cm from the liquid aluminum layer. The crucible was maintained at a temperature of 950-1050 ° C and the current density was 0.5 A / cm 2 and the cell was operated for 2000 hours. Experimental data obtained are shown in Table II. a 10 15 20 25 30 8005075-0 15 The sample number indicates that the tested electrode corresponds to the sample described in Table I with the same number.

Table II Sample Aluminum removes fi Weight loss of anode no (b / w) (Q / Gnz) V 0.48 0.1 2 0.50 0.02 The test electrodes were operated successfully as anodes in the cryolite melt and the observed degrees of wear appear to be completely acceptable for electrolyte production of aluminum from molten cryolite. Both tested electrodes showed a low degree of wear during 2000 hours of operation. 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 3 Electrode No. 1 according to Table I was used as an anode for electrolysis of a molten aluminum chloride electrolyte in the sample cell described in Example 2. Electrolysis conditions were as follows: Electrolyte: AlCl3 31-35% NaCl 31-35% BaCO3 31-35% Electrolyte temperature: 690-720 ° C Anode current density: 2000 amp / m2 Cathode: molten aluminum Electrode gap: 1 cm The tested electrodes worked successfully and the weight losses after 2000 hours of operation were negligible. Example 4 Sintered anodes consisting of 70% by weight of Y2O3 and 30% by weight of ZnO2 with the dimensions 10x10x10 mm were impregnated with an aqueous solution of chloride salt of the metals according to Table III and then heated in air at 300- 650 ° C. The process was repeated until the amount of metal catalyst was 10 g / mA calculated as metal. The electrodes were then used as anodes in electrolysis of a 5: 1 mixture of AlCl 3 -NaCl at a current density of 1000 A / m 2 at 75000. The initial anode potential as well as the potential and the degree of wear after 100 hours were determined. The results are shown in Table III.

Table III Electrocatalyst Anode Potential V (RCGE) Wear rate g / m2 initial after 100 hours - after 100 hours Rh2O3 0.2 0, not noticeable IrO2 0.0 0.0 "0.0 n CO3Oq Û, Û The electrodes showed excellent properties in electrolysis of molten aluminum chloride.

Example 5 Sintered anode plates 10x10x10 mm consisting of 70% by weight YZO 3 and 30% by weight ZrO 2 were impregnated with the metals of Table IV according to the process of Example 4. The anodes were used for electrolysis of molten metal carbonate fluoride salt and the wear and anode potential was determined as in Example 4. The results are shown in Table IV. 10 15 20 25 30 8005075 * 0 17 Table IV Electrocatalyst Anode Potential V (RCGE) Wear rate g / m2 initial after 100 hours - after 100 hours when AZOX x> 1 0.0 0.0 no IrO2 0.1 0.2 "MnO2 0.2 0.2" The anodes were successfully operated for electrolysis of molten carbonate fluoride salts, in which oxygen evolved at the anode.

Example Gel 6 Electrode sample No. 4 in Example I was alternatively used as the anode and cathode in the electrolysis of synthetic seawater in a sample cell, in which the electrolyte was pumped through the 3 mm electrolyte gap at a speed of 3 cm / s. The current density was maintained at 1500 A / m2 and the consumed electrolyte contained 0.8-2.4 sodium hypochlorite with a faraday efficiency of more than 88%. The weight loss for the electrode after 200 hours of operation was negligible.

Example 7 Electrode sample No. 3 of Example 1 was used as the anode in electrolysis of an acidic cupric sulfate solution in a cell with a titanium cathode substance. The electrolyte contained 150-200 g / l sulfuric acid and 40 g / l cupric sulphate such as metallic copper and the anode current density was 300 A / cm 2. The electrolyte temperature was 60-80 ° C and an average of 6 mm of copper was deposited on the flat cathode with a faraday efficiency of 92-98%.

The quality of the metal deposit was good and free of denitrite and the anode overpotential was low, from 1.81 to 1.95 V (NHE).

Example gel 8 A block of sintered Y2O3: SnO2: Y metal in weight ratio 7: 2: 1 with respect to free metals, a block of sintered Y203: ZrO2: Zr metal in the weight ratios 6: 3: 1 free metals and a block of sintered Y 2 O; Pd metal in the weight ratio 9: 1 free metals was activated by impregnating the sintered samples with an aqueous solution of Co 2 Cl 3 followed by drying and heating in air at 300-650 ° C to convert the chloride to Co 3 O; The cycle was repeated to obtain a final coating on the electrodes of 15 g / m 2 Co 3 O 4 on the anode surface. The activated anodes were used for electrolysis of molten AlCl3 + NaCl electrolyte and the anode potential and degree of wear are shown in Table V.

Table V Sample Anode Potential V (SCGE) Wear rate g / m2 initial after 100 inches after 100 inches Y20; -SnO2-Y 0.1 0.1 none YZO3-ZrO2-Zr 0.1 0.1 "Y2O3 ~ Pd 0.1 0.0 0.5 A block of sintered YZOQ, ZrO 2 and Pd metal in the weight ratio 7: 2.5: 0.5 for free metals was impregnated on one of its larger surfaces with 15 g / m2 of the surface with Co3Or with using the method of Example 8. The opposite larger surface was coated with a layer of 1 mm thick zirconium diboride applied by flame spraying under a nitrogen atmosphere.The block was placed between two counter electrodes of graphite in an electrically conductive ratio and at intervals therefrom. the root cavities were filled with molten AlCl 3 + NaCl and the graphite counter electrode facing the zirconium boride coated surface of the sintered bipolar electrode was connected to the positive pole of the DC source and the graphite counter electrode facing the Co 3 O 3 activated the surface of the sintered bipolar electrode was connected to the negative pole of direct current llan. The sintered electrode acted as a bipolar electrode and molten aluminum flowed down the zirconium diboride coated surface and was recovered at the bottom thereof, while chlorine developed on the Co3Ou activated surface of the electrode.

The electrolysis process was carried out satisfactorily for a period of 28 hours, when the sample cell, which was mainly made of graphite, broke down. After this operating period, the bipolar electrode showed no signs of destruction and no wear was detected.

Other electrocatalysts which can be used in the electrolysis of molten halide salts for halide ion release are RuO2, and oxides such as As2O3, Sn2O3 and Bi2O3 can be added in percentages up to 10% by weight of the free metal based on the total metal content without increasing the oxygen excess potential. affect the halide ion release potential.

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

.TiOg.

The components of the anodes according to the examples are calculated in weight percent free metal based on the total metal content of the anode composition.

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, primary 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 generate molten aluminum.

These electrodes can be used in place of graphite anodes in standard aluminum electroplating cells with either aluminum ore feed in the cryolite bath or with aluminum chloride feed in a predominantly aluminum chloride bath.

The use of these sintered yttrium oxide anodes to recover the desired metals from molten salts of the metals to be recovered results in a reduced power consumption per unit weight of metal produced and in purer recovered metals. The electrodes are dimensionally stable in operation and therefore do not require any frequent intervention to restore optimal distance from the cathode surface, as is necessary with the known consumption anodes.

The sintered yttrium oxide anodes of the invention can also be used in aqueous or non-aqueous solutions of electrolytes to recover one or more constituents of the electrolytes.

Claims (9)

1. 0 25 10 ä, auosefzs-o l? ... e_i = _._ s..ß__ë_, ls_š_ë_if Bipolar electrode for electrolytic processes, comprising a self-supporting body and an anodic and a cathodic electrocatalyst on its respective electrode surfaces, characterized in that the self-supporting body comprises sintered powder of yttrium oxide and an electroconductive material, a major part of the powder being yttrium oxide and a minor part being the electroconductive material, the self-supporting body over at least a part of its cathode surface having a layer of cathodic electrocatalytic material selected from the group consisting of metal carbides, borides, nitrides, sulphides and carbonitrides and mixtures thereof. An electrode according to claim 1, characterized in that the electroconductive material is a minor component of the sintered electrode body and is an oxide of one of the metals zirconium and tin. Electrode according to claim 1, characterized in that the electroconductive material is a minor component of the sintered electrode body and is at least one of the metals chromium, molybdenum, zirconium, tantalum, tungsten, cobalt, nickel, manganese, rhodium, iridium , ruthenium, silver or mixtures thereof. 4. An electrode according to claim 1, characterized in that the electrocatalyst is an oxide of cobalt, nickel, manganese, rhodium, iridium, ruthenium, silver or mixtures thereof. 5. An electrode according to claim 4, characterized in that the electrocatalyst is formed in situ on the sintered electrode body from a solution of respective metal salts, which are converted to oxides on the sintered electrode body. 3 _ f'§ "'*” lF lrr fl w: lï ”' ~ - g X-'ÄIY JJ-Ü. J, Tb-: w 10 8005075-0 6. An electrode according to claim 4, characterized in that the electrocatalyst 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 outer cathode surfaces of the electrode. 9. An electrode according to claim 1, characterized in that the cathode material is selected from the group consisting of carbides, borides, nitrides, sulphides and carbonitrides of at least one of the metals yttrium, titanium and zirconium.