FI64815B - MED ELEKTROKATALYTISKT OEVERDRAG FOERSEDDA CINTRADE ELEKTRODER - Google Patents

Description

RSr ^ lr, ADVERTISEMENT £ ΑοΛΓ LBJ (· ') UTLAGGN1NGSSKR1 FT 64015 ^ (51) Kv.ik.V.ci.3 c 25 B 11 / 04- // C 25 c 3/12 FINLAND I — FINLAND pi) iwrt «Nw-h« «« eicnini 770937 (22) Hukumltpllv · —An * öknlng »dtg 24.03.77 ^^^ (23) Alkupllvi — Gtttlghuttdag 24.03-77 (41) Tullut | ulktMk> l —Bllvlt offuntlig 01.10. 77

Patent and Registration Office ... ________ . . . ,. p.

Ä 1. (44) NShtSvilcilpvton) «Date of issue. - 30.09.Ö3

Patents and Registration Cases and Orders of Public Administration (32) (33) (31) Privilege claimed — Begird prlorrtct 31 · 03.76 05.04.76, 28.04.76, 17.05.76 USA (US) 672279, 673460 , 681279, 686943 Proven-Styrkt (71) Diamond Shamrock Technologies SA, 3 Place Isaac Mercier, 1201 Geneva,

Switzerland-Switzerland (CH) (72) Vittorio De Nora, Nassau, Bahamas-Bahamas (BS),

Placido M. Spaziante, Milan, Antonio Nidola, Milan, Italy-Italy (lT) (74) Leitzinger Oy (54) Sintered electrodes with electrocatalytic coating - Med elektrokatalytiskt överdrag försedda cintrade elektroder

The invention relates to an electrode for a molten salt electrolysis unit, which electrode consists of a self-supporting body of sintered flour and having an oxide compound matrix of at least one of the following metals: titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminum, silicon, silicon, , 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 lanthanide and actinide series metals, and at least one electrically conductive substance.

In the electrochemical industry, the use of dimensionally durable electrodes in electrolytic cells in anodic and cathodic reactions has recently become widespread, replacing wearing carbon, graphite and lead alloy electrodes. They are particularly useful in cathode cells with mercury flow and in diaphragm cells for chlorine and lye production, in electronic metal recovery cells where pure metal is recovered from aqueous chloride or sulphate solution, and in the cathodic protection of ship hulls and other metal structures.

Dimensionally resistant electrodes generally comprise a valve-metallic substrate, such as Ti, Ta, Zr, Hf, Nb and W, which under anodic polarization develops a corrosion-resistant but non-conductive oxide layer or "barrier layer", the outer surface of which is coated with electrically conductive and electro -catalytic layer of platinum group metal oxides or platinum group metals (see U.S. Patents 3,711,385, 3,632,498 and 3,846,273). However, electrically conductive and electrocatalytic coatings made of or containing platinum group metals or oxides of platinum group metals are expensive and eventually wear or deactivate in certain electrolytic processes and therefore need to be reactivated or recoated to reactivate depleted electrodes. / In addition, electrodes of this type cannot be used in many electrolytic processes. For example, in molten salt electrolytes, the valve metal support dissolves rapidly because a thin protective oxide layer either does not form at all or is rapidly destroyed by the molten electrolyte, causing the valve metal substrate to dissolve and the catalytic noble metal coating to disappear. In addition, in many aqueous electrolytes, such as fluoride solutions or seawater, the decomposition voltage of the protective oxide layer on the bare valve metal substrate is too low, and the valve metal substrate often corrodes under anodic polarization.

It has recently been proposed that other types of electrodes replace fast-wearing carbon anodes and carbon cathodes hitherto used in highly corrosive applications, such as the electrolysis of molten metal salts, typically the electrolysis of molten fluoride baths, for example in the production of aluminum from molten cryolite. In this particular electrolytic process, which is very important economically, about 450-500 kg of carbon anodes are consumed per tonne of aluminum produced, and an expensive control device is required to maintain a small and uniform gap between the corrosive anode surfaces and the liquid aluminum cathode. Aluminum manufacturers are estimated to consume more than 6 million tons of carbon anodes in one year. The carbon anodes burn according to the following reaction:

Al2 ° 3 + 3/2 C - 2A1 + 3/2 C02 but the actual consumption rate is much higher due to the rupture and detachment of carbon particles and the intermittent sparking that occurs across the anodic gas films often formed on the anode surface portions because molten salt electrolytes wet the carbon poorly , or due to a short circuit caused by "bridges" of conductive particles from corrosive carbon anodes and dispersed particles of the deposited metal.

British Patent 1,295,117 discloses anodes for molten cryolite baths consisting of a sintered ceramic oxide material containing substantially SnO 2 and minor amounts of other metal oxides, namely oxides of the following metals: Fe, Sb, Cr, Nb, Zn, W, Zr, Ta % up. Although electrically conductive SnO 2 with minor amounts 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 AC glass melting furnaces (see U.S. Patents 2,490,825, 2,490,826, 3,297,294, and 3,502,597), it wears and corrodes considerably when used as an anode material in the electrolysis of molten salts. On the basis of samples taken from the mixtures described in the above patents, we have found that the wear rates have been up to 0.5 Gra per hour per cm 2 when used in molten cryolite electrolyte at a current density of 3000 A / m 2. The high wear rate of Sn2 electrodes in rint-ridden is believed to be due to several factors: a) halogens act chemically, in fact SnIV forms complexes with halogen ions with a high coordination number? b) aluminum dispersed in the electrolyte reduces SnO 2; and c) anodic gas evolution causes mechanical erosion and salt precipitates in the pores of the material.

French Patent Publication No. 2,246,657 discloses electrodes having a conductive support made of titanium, nickel or copper or an alloy thereof, graphite carbon or other conductive material coated with a layer consisting essentially of spinel and / or perovskite. t-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 are characterized by good electrical conductivity and have previously been proposed for use as electrically conductive and electrocatalytic anodic coating materials for dimensionally durable valve metal anodes (see U.S. Pat.

However, coatings made of particulate spinels and / or perovskites have been found to be mechanically weak due to the inherently weak bond between the particulate ceramic coating and the metal or carbon substrate. This is because the crystal structure of spinels and perovskites is not isomorphic to metal support oxides. Attempts have been made to use various binders such as oxides, carbides, nitrides and borides, but little or no improvement has occurred. In molten salt electrolytes, the effect on the substrate material is rapid due to the inevitable pores in the spinel oxide coatings, and the coating rapidly cracks off the corrodible substrate. In addition, spinels and perovskites are not chemically or electrochemically stable in molten halide-salt electrolytes and wear out considerably rapidly due to the reducing effect of the halide ion and the dispersed metal.

Said prior art anodes have been found to have another disadvantage in the preparation of metals from electrolytically molten halide salts. The considerable dissolution of the ceramic oxide material introduces into the solution metal cations which deposit on the cathode together with the metal to be manufactured, so that the amount of impurities in the recovered metal is so great that the metal can no longer be used in electrolyte purity applications. In such cases, the economic advantages of the electrolytic process are lost in part or in whole, largely due to the high purity that can be achieved compared to smelting processes.

The electrode material that can be successfully used under highly corrosive conditions, such as the electrolysis of molten halide salts and especially the electrolysis of molten fluoride salts, should primarily be chemically and electrochemically stable under the conditions of use. It should also be catalytic for the anodic evolution of oxygen and / or halides, with the anode having the lowest potential and the overall efficiency of the electrolysis process being high. The electrode should also be thermally stable at operating temperatures, i.e. about 200 to 1100 ° C, it should have good electrical conductivity and should withstand sufficient accidental contact with the molten metal roof. With the exception of coated electrodes, since hardly any metal substrate could withstand the extremely corrosive conditions that prevail in the electrolysis of a molten fluoride salt, we have systematically tested the performance of a large number of sintered, substantially ceramic, electrodes of varying composition.

It is an object of the invention to provide novel sintered electrodes comprising an oxide metal matrix containing an electrically conductive substance and at least a part of the surface of which is provided with an electrocatalyst.

It is a further object of the invention to provide novel electrolytic cells in which the anode consists of an oxide metal matrix containing an electrically conductive substance and at least a part of the surface of which is provided with an electrocatalyst.

It is a further object of the invention to provide new bipolar electrodes as well as new electrolytic processes.

These and other objects and advantages of the invention will become apparent from the following detailed description.

The novel electrodes of the invention consist of at least one metal 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, scandium and metals of the manure-nidium and actinide series, consisting of sintered powders of at least one electrically conductive substance of the oxy compound, 6481 a matrix, wherein at least a part of the surface of said electrodes is provided with at least one electrocatalyst. Recommended metals for the lanthanide and actinide series are lanthanum, terbium, erbium, thorium, and ytterbium.

By "sintered" electrode is meant a non-reinforced, substantially rigid body consisting essentially of oximetal and at least one electrically conductive material made by any known method used in the ceramic industry, such as applying a certain temperature and pressure to a powder mixture to mold or shape material into molds, extruding or using binders, etc., and then sintering the molded body at high temperature into a non-reinforced electrode. The oxyhalide compounds are preferably oxychlorides or oxyfluorides.

The electrical conductivity of sintered ceramic electrodes is improved by adding to the mixture 0.1 to 20% by weight of at least one electrically conductive material selected from the group consisting of (A) enhancement oxides, typically oxides of metals with lower or higher valences than matrix oxides. , the valence of metals, for example oxides of the alkaline earth metals Ca, Mg, Sr and Ba and metals such as Zn, Cd, I2, Tl2 / As2 '

Sb2 »BI2 and Sn oxides; (B) oxides that are electrically conductive due to internal Redox systems, such as spinel oxides, perovskite oxides, etc .; (C) oxides that are electrically conductive due to metal-to-metal bonds such as CrO 2 R MnO 2, TiO, Ti 2 ° 3, etc. · »Borides of valve metals such as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W , silicides, carbides and sulphides, or the metals Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Pd and Ag or their alloys or the compounds (A) and / or (B) and / or ( C) mixtures.

The preferred electrocatalysts are selected from the group consisting of ruthenium, rhodium, palladium, iridium, platinum, iron, cobalt, nickel, copper and silver and mixtures thereof, and the metal group comprising manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, iridium, platinum, silver, arsenic, antimony, lead and bismuth, oxides and their alloys.

7 6481 5

By mixing a small amount, typically 0.5 to about 30% of powders of a suitable electrocatalytic material with the powder of the matrix material and sintering the mixture into a non-reinforced body, a body is obtained which has satisfactory electrically conductive and electrocatalytic properties when used as an electrode and retains its chemical stability even when mixed would not in itself withstand electrolyte conditions.

The electrocatalyst may be a metal or an inorganic oxy compound. Preferred mixed catalyst powders include the powdered metals Ru, Rh, Pd, Ir, Pt, Fe, Co, Ni, Cu and Ag, especially platinum group metals; metals Mn, Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt, Ag,

As, Sb and Bi powdery oxy compounds and in particular oxy compounds of platinum group metals.

Particularly preferred are 6MnO 2, Co 3 O 4, Rh 2 O 3, IrO 2, RhO 2, Ag 2 O, Ag 2 ° 2 'Ag 2 ° 3' As 2 ° 3 / S ^ 2 ° 3 ' mixtures of powdered metals and oxy compounds.

It has been found particularly advantageous to add another material such as stannous oxide, zirconia or the like to the oxometallic compound. It has further been found that the addition of a small amount of at least one metal belonging to the group: yttrium, chromium, molybdenum, zirconium, tantalum, tungsten, cobalt, nickel, palladium and silver can improve both the mechanical properties and electrical conductivity of sintered electrodes without significantly reducing the chemical and electrochemical corrosion resistance.

These additives are added in powder form and mixed with powdered metal oxide in percentages which can vary from 40% to 1% by weight of the metal content. Alternatively, further organic and / or inorganic compounds may be added to the powder mixture to improve particle binding during both casting and sintering processes.

The melting point of the anodes is high, well above the temperature of the molten s-salt electrolytes used, and there is no phase change in the anodes under the operating conditions of the electrolysis. In addition, the coefficient of thermal elongation is not very different from the halide salts used in molten salt baths, which helps maintain the correct electrode between the anode and cathode and prevents expansions and contractions that could break the molten salt electrolyte salt in the normal aluminum electrode.

The conductivity of the sintered electrodes according to the invention is comparable to the conductivity of graphite. The matrix has sufficient processability in molding and sintering operations and in use forms a thin layer of oxyhalides on its surface under anodic conditions. The free formation energy of the oxy compounds of the metals is much more negative than the free formation energy of the oxide of the corresponding molten halide-phase salt electrolyte, so that these sintered anodes are chemically very stable.

The sintered metal oxy compound electrodes according to the invention can also be used as bipolar electrodes. According to the latter embodiment, the sintered electrodes can be easily manufactured as a plate or sheet, wherein one of the two main surfaces of the electrode is provided with a layer containing an anodic electrocatalyst such as oxides CO 2 Cl 2, Ni 2 Cl 2, MnO 2, R 1 -O 2 , IrO 2 'RuO 2' Α ^ 2 °, etc., and the second main surface is provided with a layer containing suitable cathodic substances such as carbides, borides, nitrides, sulphides, carbonitrides of metals, especially valve metals, and most preferably yttrium, titanium and zirconium and so on.

The unreinforced sintered body consisting of the major part of the oxometallic compound can be made by grinding the materials together or separately, preferably to a grain size between 50 and 500 microns, to obtain a powder mixture containing different sized granules for better compaction. According to one preferred method, the mixture of powders is mixed with water or an organic binder to obtain a plastic mass with flow properties suitable for the molding process used. The material can be cast in a known manner either by compressing or pressing the mixture into a mold or by casting into a gypsum mold, or the material can be extruded through a nozzle into various shapes.

The cast electrodes are then dried and heated to a temperature at which the desired bonding can occur, usually intermittently. Leave at 800 to 1800 ° C for 1 to 30 hours, after which it is usually cooled slowly to room temperature. The heat treatment is best performed in an inert atmosphere or a slightly reducing atmosphere, for example H 2 + N 2 (80%), when the powder mixture consists essentially of an oxometallic compound and contains a minor proportion of other metal oxides or metals.

When the powder mixture also contains metal powders, the heat treatment is best performed in an oxidizing atmosphere, at least for one portion of the heat treatment cycle, to better oxidize the metal particles in the outer layers of the electrodes. The metal particles remaining inside the sintered piece of material improve the electrical conductivity of the electrode.

The molding process may be followed by a high temperature sintering process, as mentioned above, or the molding process and the sintering process may be similar, i. a certain temperature and pressure can be applied to the powder mixture at the same time, for example by means of electrically heated molds. Lead connectors can be fused to ceramic electrodes during casting and sintering, or they can be attached to electrodes after sintering or casting. Of course, other methods can be used to form, compress and sinter the yttrium oxide powder mixture.

The electrocatalyst account, which is usually placed on the surface of the electrode for cost reasons, should have good stability, a low anodic overpotential for the desired anode reaction, and a high anodic overpotential for undesired reactions. In the case of chlorine evolution, cobalt, nickel, iridium, rhodium, ruthenium oxide or mixed oxides thereof such as RuO 2 -TiO 2, etc. can be used. In the case of fluoride-containing electrolytes in which oxygen evolution is the desired anode reaction, silver and manganese oxides are preferred. . Oxides of platinum, palladium and lead can also be used as electrocatalysts.

Toxins that suppress an undesired anode reaction can be used, for example, to suppress the evolution of oxygen from chloride electrolytes. In this case, toxins with high oxygen overpotential should be used, suitable materials are oxides of arsenic, anti-mon and bismuth. These oxides, which are used in small percentages, can be incorporated into the electrocatalyst oxides from 1 to 10% of the electrocatalyst, based on the weight of the corresponding metals.

10; 64815 f

The application of the electrocatalyst and any toxicant can be carried out by any known coating method. The electrocatalyst and any toxicant are preferably applied to the sintered electrodes as a solution of decomposing salts of metals. The sintered body is saturated with a solution containing the metal salts in question and dried. The electrode is then heated in air or other oxygen-containing atmosphere to convert the salts to the desired oxides.

The porosity of the sintered body and the method by which the surface layers of the sintered body are impregnated with metal salts should normally be such that the solution penetrates to a depth of at least 1-5 mm, preferably 3 mm inwards from the electrode surface. .

Alternatively, the preformed electrocatalyst oxides and any preformed toxic oxides can be ground to a powder form by a suitable powder mixing technique and added to the powder mixture during electrode casting so that the outer layers of the selected electrodes are enriched with electrocatalyst oxides and possible

The sintered electrodes according to the invention can be used as bipolar electrodes. According to this embodiment of the invention, the second surface of the electrodes may be provided with an anodic electrocatalyst and a possible undesired anode reaction poison by any of the above methods, while the second surface may be coated with a suitable cathode material. For example, the surface of the bipolar electrode, which acts as a cathode during the electrolysis process, may be provided with a layer of yttrium, tantalum, titanium, zirconium, etc. metal carbides, borides, nitrides, sulfides and / or carbon nitrides.

One preferred method of layer application is the plasma jet technique, in which powders of selected materials are sprayed and fixed to the surface of a sintered body in a flame-controlled atmosphere. Alternatively, the selected powdered material may be added to the powder mixture during the molding process and thus sintered together, whereby the cathode surface of the bipolar electrode obtains a layer of the selected cathode material.

Electrodes can be used effectively to electrolyse multiple electrolytes. They are particularly preferred when used as anodes in electrolytic cells used to electrolytic molten halide electrolytes such as molten cryolite baths, aluminum, magnesium, sodium, potassium, calcium, lithium and other metals. Aluminum halides can be electrolyzed by the Hall method or in the methods disclosed in U.S. Patents 3,464,900, 3,518,712, or 3,755,099 (incorporated herein by reference) using the electrodes described herein as anodes. The electrolysis temperature is so high that it melts and keeps the salts of the metal to be recovered in the molten state. The metal is deposited in the molten state by removing the molten metal from the molten cathode.

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

When the electrodes according to the invention are used as bipolar electrodes in the electrolysis of molten salt, the composition of the cathode part of the electrodes must be such that it is not reduced by the cathode reaction and is not affected by the metal deposited on the cathodes, especially when the electrode mixture is an oxy compound. For this reason, it is desirable that the cathode side of the bipolar electrode composition is an inert cathode and the molten metal of the reducing effect.

The electrodes can also be used as anodes and / or cathodes in, for example, the following electrochemical processes: Electrolysis of aqueous chloride solutions in the preparation of chlorine, alkali, hydrogen, hypochlorite, chlorates and perchlorates; electrochemical recovery of metals from aqueous sulphate or chloride solutions in the manufacture of copper, zinc, nickel, cobalt and other metals; electrolysis of molten metal salt electrolytes, typically containing halides, oxides, carbonates or hydrates, in the manufacture of aluminum, beryllium, calcium, cerium, 12 64 31 5 lithium, sodium, magnesium, potassium, barium, strontium, cesium and other metals, and bromide, and electrolysis of sulfuric acid, hydrochloric acid and hydrofluoric acid. In general, electrodes are useful in all electrolytic processes.

The new electrolyte cell according to the invention comprises at least one anode and at least one cathode and devices for supplying direct current between the anode and the cathode. The improvement consists in that the anode consists of at least one metal 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 latan and actinide series, an unreinforced matrix of sintered powders of an oxy compound and at least one electrically conductive substance, wherein at least a part of the surface of said electrodes is provided with at least one electrocatalyst. The cell may also contain bipolar electrodes as described above.

In the new electrolysis method according to the invention, the electrolyte is electrolyzed between the anode and the cathode. The improvement consists in that the anode consists of at least one metal selected from the group consisting of titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminum, silicon / tin, chromium, molvodene, 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 of the lanthanide and actinide series, oxy a non-reinforced matrix of sintered powders of a compound and at least one electrically conductive substance, wherein at least a part of the surface of said electrodes is provided with at least one electrocatalyst.

The following examples illustrate several preferred embodiments to illustrate the invention. However, it is to be understood that the invention is not intended to be limited to certain embodiments; the percentages of electrode components are calculated as weight percent free metal of the total metal content of the alloy.

13 6481 5

Preferred anodes include anodes in which the bulk of the unreinforced body is tin dioxide alone or contains up to 20% by weight of cobalt oxide in a cobalt oxide coating which improves the mechanical and electrocatalytic properties of the electrodes with respect to chlorine evolution. Other recommended additives are Y 2 O 2, TiO 2 Ta 2 ° 5 '

The electrocatalyst coating may be protected against wear by the simultaneous or subsequent use of a protective agent such as a valve metal oxide such as TiO 2 and Ta 2 O 3 or SiO 2 - mixed oxides such as AgR 6 -O 2, TiCl 2 O 2 and Ag 2 O 2 -.

Example 1

Three sintered electrode samples were prepared, 80% SnO 2 + 20% cobalt (piece A), 80% SnO 2 + 10% Co + 10% Mo (piece B) and 100%

SnO 2 (piece C), which was washed thoroughly with water and dried in vacuo. The resulting electrodes were then immersed in vacuo in the solution shown in Table I, after which they were dried and heated for 20 minutes at 370 ° C in an air-forced oven.

The samples were then brushed with the same solution and heated in the same oven for 15 minutes at 350 ° C. This treatment 2 was repeated several times until the weight gain of the electrode was 5 g / m 2.

Electrode samples were then used as anodes in a test cell for electrolyzing aluminum chloride at 750 ° C at an anode current density of 2,000 A / m. The cell voltage was 5 volts, and the electrolyte was a 5-1-1 weight ratio mixture of aluminum chloride, sodium chloride, and potassium chloride. The anode potential was determined at the beginning and after 500 operating hours and the weight loss of the electrode after 500 operating hours. For comparison, a standard graphite electrode was used under the same conditions. The results are reported in Table I.

14 6481 5

Table I

(---- 1 ------- 1-- _Solution__Anode potential _I Weight loss

Piece Solvent Salt At the beginning of V (RCGE) * 5 °° 2h 3alkene ____ after 500 h g / m _ A Water and CoCl »0.45 0.47 0.8 formamide B Hydrochloric acid IrCl-. 0.45 0.45 0.5 i _-3 _ C Hydrochloric acid IrCl ^ 0.5 0.6 0.2 A Untreated 0.6 0.6 0.5 B Untreated 0.55 0.55 0.5 C Untreated 1.0 1.3 No * RCGE: Chlorine-graphite reference electrode.

The results in Table I show that the excess potential of the coated electrodes for chlorine development is low without any significant increase in weight loss. Sample C, whose excess potential was too high without post-treatment, was not suitable for the electrolysis reaction, whereas treated Sample C was suitable. The mean faradi efficacy during the experiment was 96%. A normal graphite electrode was used in the same manner. Compared to the graphite reference electrode, its voltage was about 0.8 volts.

Example 2 Samples containing 90% by weight of tin dioxide and 10% by weight of cobalt were sintered and the electrodes were then coated with a cobalt oxide coating as in Example 1 to give a 10 g / m thick layer of cobalt oxide. The electrodes in Table II were then electrolysed with the electrodes under the operating conditions shown in Table II. Table II shows the anode volume 1 and the snow snopcus determined after 300 hours of use.

Table II

15 6481 5

Electrolyte Electro-Current Avg. Anode potency loss composition and lithium density Faradayal 300 h 2 weight ratio laptops. 2 power after g / m _ ° C 7 ___ V (RCGE) _ A1C13 + KC1 750 1000 92% 0.5 0.5 (5: 1) ____________

CaClp + KCl 450 1000 94% 0.6 0.5 j (5: 1) ________ | Pbl2 + KCl 450 1000 90% 0.6 0.5 1 (5: 1) ______________ L_

Table II shows that the electrodes of the invention have a low wear rate and a low anode potential after 300 hours of use.

Example 3

Powders with a mesh number of 100 to 250 were made into plate-shaped electrodes with a diameter of 10 mm and a thickness of 5 mm. Powders 2 were compression molded at 1000 kg / cm and then sintered in an induction furnace under the conditions shown in Table III. Table III also shows the compositions of the powders.

The sintering was carried out in a furnace through which said gas was recirculated or kept at atmospheric pressure. Thus, at least the outer surfaces and possibly some of the outer pores were exposed to an oxidizing atmosphere at said temperature, whereby the metal exposed to the surfaces was oxidized to an electrocatalyst.

Table III

16 6481 5 Sample Components and Sintering _______ Body: °% by weight | LampöiTT ^ C | Atmospheric sediment

SnO ^ 80% 1250 Without forced circulation 2 h 1 Co_ 20%

Sn02 80% 2 Co 10% 1500 Without forced circulation 2 h ___Mo___10% __________________

Sn02 80% 3 Mo 10% 1250 Without forced circulation 2 h

Ni_______________10% ___________ _ '

Sn02 75% 4 La20 ^ 10% 1500 Ambient air 2 h __________Co_________15% ______________________ __________

Sn02 6Ö% 5 Co2Ni0 ^ 30% 1500 Ambient air 2 h __ Co _10% ____ '____i ______________________________, Sn02 60%:; SiO2 10%

La203 6 Co2Ni0 ^ 10% 1000 Ambient air 2 h

Cu 10% ___Mo___ 10% _______ __________________ __........__

Sn0 „95%: 7 Co 2.5% 1500 Without forced circulation 2 h __Mo ___ 2.5%; _____; __________________________________ 8 Sn02 100% 1500 Air 10 h 6481 5 17 The electrical conductivity of samples 1-7 measured at 500 ° C ranged from 0.01 to 1.00 £ 2 ^ cm and the density of the sintered electrodes ranged from 3 to 5-8.5 g / cm. Electrode samples were used as anodes in a test cell in which aluminum chloride was electrolyzed at 750 ° C. The current density of the anode 2 was 1000 A / m. 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 at the beginning and after 500 hours of operation. The weight loss of the electrode was determined after 500 hours. A graphite reference electrode was also used for comparison purposes under similar conditions. The results are reported in Table IV.

Table IV

Sample V Anode potential V. (RGE) * Weight loss 1 · ° -——-—--— after 500 h at the beginning after 500 h g / i »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 No 6 0.65 0.65 0.5 7 0.55 0.6 No 8 1 .0 1.3 Ei

Graphite 0.85 0.85 105 -1 —-----------------— - 4 ——--------- RGE: Graphite reference electrode

The results in Table IV show that electrodes 1-7, which are mainly oxide and minor metal, have a low excess potential for chlorine evolution and a very low wear rate. Electrode 8, which did not contain any added electrically conductive metal, had a substantially higher overpotential with respect to chlorine evolution, and the graphitic reference electrode had a higher overpotential than electrodes 1 to 7 and a high wear rate. The graphite reference electrode had to be significantly adjusted during electrolysis and had to be replaced at an early stage. The mean efficacy during the experiment was 97%. Not all samples 1-7 were as brittle as sample # 8.

Example 4

About 250 g of the mixture containing the matrix material and additives shown in Table I was ground in a mixer for 20 minutes and the powder mixtures were poured into cylindrical plastic molds and pre-compressed by means of a steel cylinder press. Each mold was placed in an isostatic pressure chamber and the pressure was raised in 5 minutes to about 1500 kg / cm 2 and then lowered to zero in a few seconds. The samples were then removed from the plastic molds and polished. The formed samples were placed in an electrically heated oven where they were heated at room temperature to 1200 ° C under a nitrogen atmosphere for 24 hours. They were kept at maximum temperature for 2-5 hours and then cooled to 300 ° C over the next 24 hours. The screened samples were removed from the oven. After cooling to room temperature, they were weighed and their apparent density and electrical conductivity at 25 ° C and 1000 ° C were determined. The results are reported in Table V.

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23 1 64815

The values in Table V show that the electrical conductivity of sintered ceramic electrodes at temperatures as high as 100 ° C is 5 to 10 times higher than the electrical conductivity at 25 ° C. The addition of oxides whose conductivity corresponds to the conductivity of metals to the substantially non-electrically conductive ceramic oxides of the matrix increases the electrical conductivity of the electrodes 2 by a factor of 10. The addition of a metal resistant to molten salts such as yttrium or molybdenum, etc. to the ceramic electrodes of the invention increases the electrical conductivity of the electrodes by a factor of 2 to 5.

Example 5

In a laboratory-scale test cell, the operating conditions of an electrolysis cell used to make aluminum metal from a molten cryolite bath were mimicked. A layer of liquid aluminum was placed on the bottom of the heated graphite crucible and a melt containing cryolite (80-85%), alumina (5-10%) and AlF 2 2 ^ (1-5%) was poured on top of it. was 3 cm, prepared by the procedure described in Example 4 and to which a Pt-t wire was attached as a simple electrical connection, was immersed in a salt melt and kept at a distance of about 1 cm from the liquid aluminum layer. The crucible was maintained at a temperature between 950 and 1050 ° C. The current density was 0.5 A / cm 2, and the cell was used for 2000 hours. The experimental results obtained are shown in Table VI. The sample number means that the electrode tested corresponds to the sample marked with the same number in Table V.

6481 5 24

Table VI

Nf ^ te Aluminum formed Weight loss of anodes n * ° _ (q / h) ___ (q / cm ^) _ 1 0.49 0.02 2 0.50 0.12 3 0.49 0.04 4 0.49 0 .02 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 sample electrodes tested performed well as anodes in the cryolite melt, and the observed wear rates appear to be completely acceptable for the electrolytic production of aluminum from the molten cryolite. The wear rates of all electrodes tested were low during 2000 operating hours. The wear rate of electrodes containing thermal stabilizers, such as oxy compounds of metals of Group III of the Periodic Table, is generally about 10 times lower than that of electrodes without thermal stabilizers.

Example 6

Electrodes Nos. 4 and> 5 described in Table V were used as anodes when molten aluminum chloride electrolyte was electrolyzed in the test cell described in Example 5. The electrolysis conditions were as follows:

Electrolyte: AlCl3 31 to 35% by weight

NaCl 31-35

BaCO3 31 - 35

Electrolyte temperature: 690 - 720 ° C

2

Anode current density: 2,000 Amp / m

Cathode: Molten aluminum

Electrode spacing: 1 cm

The electrodes tested worked well and the weight losses after 2000 hours of operation were negligible.

6481 5 25

Example 7

Electrode Samples Nos. 10 and 11 of Example 4 were used as anodes in the preparation of bromine by electrolysis of an aqueous bromine solution using a diaphragm-type test cell in which an asbestos diaphragm separates a cathode chamber having a steel cathode from an anode chamber equipped with a test electrode. The electrolysis was performed with an aqueous solution containing 200 to 220 g / l of sodium bromide. The electrolyte temperature was 80-85 ° C and the current density was 2000 A / m 2. The current efficiency was 95% after 1000 hours of operation and the weight loss of the test electrode was negligible.

Example 8

Electrode samples No. 10, 11 and 13 of Example 4 were used alternately as an anode and cathode in the electrolysis of synthetic seawater in a test cell into which the electrolyte was pumped over a 3 mm electrode gap at a rate of 3 cm / sec. The current density was maintained at 1500 A / m and the waste electrolyte contained 0.8 to 2.4 sodium hypochlorite with a Faraday efficiency of more than 88%. The weight loss of the electrodes after 200 hours of operation was negligible.

Example 9

Electrode Samples Nos. 12 and 14 of Example 4 were used as anodes in the electrolysis of an aqueous acidic copper (11) sulfate solution in a cell having a titanium cathode. The electrolyte contained 150 to 200 g per liter of sulfuric acid and 40 g per liter of copper (II) sulfate as metallic copper. The current density of the anode was 300 A / cm. The electrolyte temperature was 60-80 ° C. An average of 6 mm of copper was deposited on the flat cathode with Faraday efficiencies ranging from 92 to 98%. The quality of the metal precipitate was good and did not contain dendrites. The voltage across the anode was very low, 1.81 - 1, 95 V (NHE).

Other electrocatalysts that can be used in the electrolysis of molten halide salts to decompose the halide ion include RuO 2 electrocatalysts. Oxides such as As 2 O 2, Sn 2 O 2 and ΒΪ2θ ^ η can be added up to 10% by weight as free metal based on the total metal content to increase the excess oxygen potential without affecting the halide ion discharge potential.

26 6 4 81 5

In the anodes used in molten fluoride electrolytes which evolve oxygen, the catalyst may be the same as that listed in Example 5 or Rh 2 O 3, PbCl 2 and IrCl 2 .TiCl 2 ·

The components of the anodes given in the examples are calculated as weight percent free metal of the total metal content of the anode mixture.

The electrocatalyst may contain salts other than those used in the examples, such as an alkali metal chloride or fluoride and an electrolyzable metal salt. Metal halides effectively reduce the melting point of the electrolyzable salt, making it possible to use lower temperatures while keeping the salt bath in the molten state.

The above examples have dealt with the electrolysis of molten or molten metal salts, mainly the electrolysis of molten aluminum chloride or fluoride salts. Similarly, molten chlorides of other metals, such as alkali metals or alkaline earth metals, can be electrolysed using anodes developed otherwise in accordance with normal practice. In the same way, other molten salts, such as molten nitrates, can be further electrolysed. To produce molten aluminum, molten alumina-cryolite electrolyte or a corresponding alkali metal aluminum fluoride can be electrolysed.

These electrodes can be used instead of graphite anodes in standard aluminum electron recovery cells, where either aluminum ore is fed to a cryolite bath or aluminum chloride is fed to a bath containing mainly aluminum chloride.

The use of these sintered metal oxide anodes in the recovery of the desired metals from the molten salts of the metals to be recovered reduces the power consumption per unit weight of the metal produced and produces cleaner recovered metals. The electrodes are dimensionally blocking in use and therefore often do not need to be repositioned to restore the optimum distance from the cathode surface, as is necessary when using prior art wear anodes.

Various variations can be made in the electrodes and methods of the invention without departing from the spirit or scope of the invention,

Claims (6)

27 6 4 8 1 5 and it is to be understood that the claims are limited only by the claims. An electrode for a molten salt electrolysis unit, which electrode consists of a self-supporting body of sintered flour and having an oxide compound matrix of at least one of the following metals: titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminum, silicon, tin, 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 lanthanide and actinide series metals, and at least one electrically conductive material, characterized in that at least part of the surface of the body is provided with an electrocatalyst of ruthenium, rhodium, palladium, iridium, platinum, iron, cobalt, nickel or silver metals and / or oxide compounds or oxide compounds of manganese, arsenic, lead, antimony and / or bismuth or mixtures thereof, the amount of electrocatalyst being from 0.5 to 30% by weight of the body p Aino. Electrode according to Claim 1, characterized in that the electrically conductive material forms a small part of the sintered electrode body and is an oxide of zirconium and / or tin. Electrode according to Claim 1, characterized in that the electrically conductive material forms a small part of the sintered electrode body and is at least one metal selected from yttrium, chromium, molybdenum, zirconium, tantalum, tungsten, cobalt, nickel, palladium and silver. Electrode according to Claim 1, characterized in that the electrocatalyst is at least one oxide of the following metals: cobalt, nickel, manganese, rhodium, iridium, ruthenium and silver. 28 6 4 81 5 An electrode according to claim 4, characterized in that the electrocatalyst consists of powdered oxides of said metals sintered in the outer layer of the electrode. Electrode according to any one of the preceding claims, characterized in that the electrocatalyst is formed in situ on said sintered electrode body from a brine of said metals, which metals are converted to oxides by a sintered electrode body.