FI61725B - NYA YTTRIUMOXIDELEKTRODER OCH DERAS ANVAENDNINGSSAETT - Google Patents

Description

I- —____ r „, .... ANNOUNCEMENT ^ 179 ^ 3¾¾ W <, 1> UTLAG6NINGSSKWFT ° '' Z 5 Π Patent granted September 10, 1932 * i5v ^ ^ Patent meddelat ^ '(51) K» .lk.3 /lnt.CI.3 C 25 B 11/0 * ENGLISH I - FI H LAN D (21) P * t * n «lh» kGinu «—ft * enttn» etu »inj 770938 (22) Hakamiipilvi - Ai» 6knlnp <Ug 2U.03.77 (23) AlkupWvt — Glklghatadai 2U.03-77 (41) Tullut | ulkl * «ksl - Bllvft off« Mllg 01.10.77

Patent, if rekl.terih.Hitu. NlhtfvIJuIpwon | when the date of «L | ullul» n. - 31.05.82

U.S. Pat. No. 3,822,76, U.S. Pat. No. 3,822,76, U.S. Pat. No. 6,272,76, U.S. Pat. 673U60, 681279, 6869U3 (71) Diamond Shamrock Technologies SA, 3 Place Isaac Mercier, 1201 Geneva, S vei tsi -Sc hwei z (CH) (72) Vittorio de Nora, Nassau, Bahamas-Bahamas (BS), Placido M.

Spaziante, Milan, 'Antonio Nidola, Milan, Italy-Italy (IT) (7U) Leitzinger Oy (5U) New yttrium oxide electrodes and their uses -Nya yttrium oxide electrodes and derivation methods

The invention relates to an electrode in the production of which sintered powders of at least one electrically conductive substance have been used and in the surface of which at least a part is provided with at least one electrocatalyst.

In the electrochemical industry, dimensionally resistant electrodes have recently been widely used in electrolytic cells for anodic and cathodic reactions, replacing consumable carbon, graphite and lead alloy electrodes. They are particularly useful in cathode cells with mercury flow and in diaphragm cells for chlorine and alkali 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 2,61725 electrically non-conductive oxide layer or "barrier layer" and at least a portion of the outer surface of which is electrically coated and an 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 is either not formed at all or is rapidly destroyed by the molten electrolyte, causing the metal base of the valve to dissolve and the catalytic precious 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-metallic 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 in 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. Carbon anodes burn according to the following reaction: A1203 + 3/2 C «♦ 2A1 + 3/2 C02 0 * but the actual consumption rate is much higher due to rupture and detachment of carbon particles and intermittent sparking across the anodic gas films often formed on the anode surface on the parts because the molten salt electrolytes dampen the carbon 61725 poorly, or due to a short circuit caused by the "bridges" of conductive particles from the corrovated carbon anodes and the 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, 3,287,284, and 3,502,597), it wears and corrodes considerably when used as an anode material in the electrolysis of molten salts. Based on samples taken from the mixtures described in the above patents, we have found that wear rates have been up to 0.5 g per hour per cm when used in molten cryolite electrolyte at a current density of 3000 A / m. The high wear rate of sintered SnO 2 electrodes is believed to be due to several factors: a) the halogens act chemically, in fact Sn 2 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 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 metal oxides of the spinel and / or perovskite type, and alternatively is obtained by sintering mixtures of said oxides. Spinel oxides and perovskite thioxides belong to a group of metal oxides which are characterized by good electrical conductivity and which have been previously proposed for use as electrically conductive and electrocatalytic anodic coating materials.

617 2 5

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 with 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 sponge 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 remarkably 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, part or all of the economic advantages of the electrolytic process are lost, 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 in particular 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 lowest potential potential of the anode and high overall efficiency of the electrolysis process. 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 very large number of sintered, 61725, substantially ceramic electrodes of various compositions.

It has now been found that highly efficient insoluble electrodes can be made by sintering yttrium oxide and at least one electrically conductive substance into a non-reinforced body and providing at least one surface thereof with at least one electrocatalyst.

The sintered yttrium oxide electrodes of the invention are particularly useful in electrochemical recovery processes used in the preparation of various metals such as aluminum, magnesium, sodium, potassium, calcium, lithium and other metals from molten salts. Yttrium oxide and at least one electrically conductive substance, when used as an anode in direct current electrolysis of molten salt electrolytes, have been found to be unusually stable as an inert, dimensionally stable anode with sufficient electrical conductivity. When provided with oxide electrocoaters such as CO 2 Cl 2, Ni 2 O 2, MnO 2, Rh 2 O 3, IrO 2, N 2 O 2, N 2 O 3, 3ne · ', they have a high electrocatalytic activity, especially with respect to chlorine evolution.

The term "sintered yttrium oxide" refers to a non-reinforced, substantially rigid body consisting essentially of yttrium oxide and at least one electrically conductive material, made by any known method used in the ceramic industry, such as subjecting a powder mixture of yttrium oxide and other materials to a certain temperature. that the mixture obtains the desired size and shape, or by casting the material into molds, extruding or using binders, etc., and then sintering the molded body at high temperature into a non-reinforced electrode.

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 having a low valence or higher valence than the oxides forming the matrix. the valence of metals, for example oxides of the alkaline earth metals Ca, Mg, Sr and Ba and metals such as Zn, Cd, In2, Tl2, As2,

Sb2 »Bi2 and Sn oxides; (B) oxides that are electrically conductive due to 6,61725 internal Redox systems, such as spinel oxides, perovskite oxides, etc .; (C) oxides that are electrically conductive due to metal-to-metal bonds, such as CrCl 2, MnCl 2, TiO, etc .; borides, silicides, carbides and sulphides of valve metals such as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, or metals Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Pd and Ag or their alloys or mixtures of compounds (A) and / or (B) and / or (C).

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 catalyst 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; powdered oxy compounds of the metals Mn, Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt, Ag, As, Sb and Bi, and in particular oxy compounds of platinum group metals.

Particularly preferred are 0MnO2, Co ^ O ^, R1 ^ Oj, IrO2, RuO2 'A <? 20' A <^ 2 ° 2 'a ^ 2 ° 3' As2 ° 3 'S ^ 2 ° 37 Bi2 ° 3' CoMn2 ° 4 'NiMn 2 O 2, CoRh 2 O 2 and NiCl 2 O 4 and mixtures of said powdered metals and oxy compounds.

It has been found to be particularly advantageous to add a substance such as stannous oxide, zirconia and the like to yttrium oxide. It has further been found that the addition of a small amount of at least one metal from the group consisting of yttrium, chromium, molybdenum, zirconium, tantalum, tungsten, cobalt, nickel, palladium and silver improves both mechanical and electrical properties of sintered yttrium oxide electrodes without significantly reducing their chemical and electrochemical corrosion resistance.

y These additives are added in powder form and mixed with powdered yttrium oxide in a percentage which may vary from 40 to 1% by weight of the metal content. Alternatively, some other organic and / or inorganic compounds may be added to the powder mixture 61725 to improve particle binding during both casting and sintering processes.

Anodes containing Y 2 O 3 as the main component have a high melting point, well above the temperature of the molten salt electrolytes used, and do not undergo phase changes under the operating conditions of the electrolysis. In addition, the coefficient of thermal elongation does not differ much from the coefficients of halide salts used in molten salt baths, which helps maintain the correct electrode gap between anode and cathode and prevents expansions and contractions that could break the molten salt electrolytic salt shell in normal aluminum electrochemical chemistry.

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

The sintered yttrium oxide electrodes according to the invention can also be used as bipolar electrodes. According to the latter embodiment, the sintered yttrium oxide electrodes can be easily manufactured as a plate or sheet, one of the main surfaces of the electrode being provided with a layer containing an anodic electrocatalyst such as oxides CO-JO 2, Νΐ- ^ Ο ^, MnO 2, Rh 2 ° 3 'IrO 2, RuO 2. 'A920, etc., and the second main surface is provided with a layer containing suitable cathodic substances, such as carbides, borides, nitrides, sulfides, carbonitrides, etc. of metals, especially valve metals and most preferably yttrium, titanium and zirconium.

An unreinforced sintered body consisting of a major portion of yttrium oxide can be prepared by grinding the materials together or separately, preferably to a grain size of between 50 and 500 microns to give a powder mixture. containing granules of different sizes to provide secure compaction. According to a preferred method, the mixture of powders is mixed with water or an organic binder so as to obtain a plastic mass 61725 8 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 binding can occur, usually between 800 and 1800 ° C for 1 to 30 hours, followed by slow cooling 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 yttrium oxide 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 in one part of the heat treatment cycle, in order to better oxidize the metal particles in the outer layers of the electrodes. The metal particles inside the sintered piece of material improve the electrical conductivity of the electrode.

The molding process may be followed by a sintering process at a high temperature, as mentioned above, or the molding process and the sintering process may be simultaneous, 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 RuCl 2 -TiO 2, etc. can be used. oxides of silver and manganese. 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 excess potential should be used, suitable materials are oxides of arsenic, antimony and bismuth. These oxides, which are used in small percentages, can be introduced into the electrocatalyst oxides from 1 to 10% of the electrocatalyst, based on the weight of the corresponding metals.

The delivery of the electrocatalyst and possibly the toxicant can be performed by any known coating method. The electrocatalyst and any toxicant are best applied to the sintered yttrium oxide electrodes as a solution of decomposing salts of metals. The sintered yttrium oxide block 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.

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

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 cast electrodes are enriched with electrocatalyst oxides

The sintered yttrium oxide electrodes of the invention can be used as bipolar electrodes. According to this embodiment of the invention, one surface of the yttrium oxide electrodes may be provided with an anodic electrocatalyst and possibly an undesired anode reaction poison by any of the methods described above, while the other surface may be provided with a coating of a suitable cathode material. For example, the surface of the bipolar electrode acting as a cathode during the electrolysis process may be provided with metal carbides, borides, nitrides, sulfides and / or carbonitrides of yttrium, tantalum, titanium, zirconium, etc.

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

Sintered yttrium oxide activated with a suitable electrocatalyst can be used as a non-wearing electrode in molten salt electrolysis and other processes in which an electric current is passed through the electrolyte to decompose the electrolyte, oxidize or reduce organic and inorganic compounds. molten salts such as aluminum halides - alkali metal halides. The electrodes of the invention may be polarized as anodes or cathodes, or may be used as bipolar electrodes, with one surface or end of the electrode acting as an anode and the opposite surface or end of the electrode acting as a cathode with respect to an electrolyte in contact with each end of the electrode.

The electrolysis cell according to the invention is a cell provided with at least one separate anode and cathode array and devices for supplying the electrolysis current to the cell, the anode being a dimensionally durable, three-component electrode according to the previous discussion. The cell is best used for the electrolysis of molten metal salts such as aluminum chloride.

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

Example 1

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 hand with 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 3a and then lowered to zero in a few seconds. The samples were then removed from the plastic molds and polished. The compressed samples were placed in an electrically heated oven and heated for 24 hours from room temperature to 1200 ° C under a nitrogen atmosphere. The maximum temperature was maintained for 2-5 hours, after which cooling to 300 ° C took place over the next 24 hours. The sintered samples were then removed from the furnace and, after cooling to room temperature, weighed and measured for apparent density and electrical conductivity at 25 ° C and 1000 ° C. The results are reported in Table I.

«I.

12 61725

Table I

Sample Composition Sintering Appearance Electrical conductivity __ No.% by weight and max. Thermal density, at 1000 ° C at 250 ° C at a temperature, h g / cm3 Ω “1 cm” 1 Ω-1 cm-1 A Y203 75 5 5.2

Ti_0, 25 0.001 j ——-_-----—--— -'-------- Y203 65 1 Ti2 ° 3 15 5 5.2 0.2

Rh203 20 1 - -. , I. - - I -. - | «- -" -----------

Zr02 30! Y2 ° 3 30 2 WO 10 5 5.5 0.4

Ag20 10

Ru02 10

ZnO 10,! Y2 ° 3 50 YOF 15; 3 Y 15 2 5.9 11 2.5

Ir02 10

Ag 2 O 10 Y 2 ° 3 60

Ni00204 20 4 CdO 5 2 5.7 7; 0.2

CaO 1 j j

Rh203 19 ’61725 13

The values in Table I show that the electrical conductivity of sintered ceramic electrodes at 100 ° C temperatures is 5 to 10 times higher than the electrical conductivity at 25 ° C. The addition of oxides having a conductivity similar to that of the electrically non-conductive ceramic 2 oxides of the matrix increases the conductivity of the electrodes by a factor of 10, as is observed in the case of electrodes A and No. 1. When molten salt-resistant metal such as yttrium or molybdenum, etc. is added to the ceramic electrodes of the invention, the electrical conductivity of the electrodes increases 2-5 times.

Example 2

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_ was poured on top of it.

2 J

(1 - 5%). Sample electrodes with an operating clamp area of 3 cm, prepared by the procedure described in Example 1 and to which a Pt wire was attached as a simple electrical connection, were 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 irradiation density was 0.5 A / cm, and the cell was used for 2000 hours. The experimental results obtained are shown in Table II. The sample number means that the electrode tested corresponds to the sample marked with the same number in Table I.

Table II

Sample Formed Anode weight no. Aluminum loss ___ (g / h) __ (g / cm2) __ 1 0.48 0.1 2 0.50 0.02

The sample electrodes tested performed well as anodes in the cryolite melt, and the observed wear rates appear to be perfectly acceptable for the electrolytic production of aluminum from the molten cryolite. The wear rate of both electrodes tested was low during 2000 operating hours. For electrodes containing thermal stabilizers, such as oxy compounds of metals of Group III of the Periodic Table, the wear rate is generally about 10 times lower than for electrodes without thermal stabilizers.

Example 3

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

Electrolyte: AlCl 2 31-35% by weight

NaCl 31 to 35% by weight

BaCO ^ 31 to 35% by weight

Electrolyte temperature: 690 - 720 ° C

2

Anode current density: 2000 Amp / m

Cathode: Molten aluminum

Electrode spacing: 1 can

The electrode tested worked well and the weight loss after 2000 hours of operation was negligible.

Example 4

The sintered anodes, which contained 70% by weight of Y-> 0 and 30% by weight of ZrO 2 and had a size of 10 x 10 x 10 mm, were saturated with an aqueous solution of the chloride salt of the metals of Table III and then heated in air at 300-65 ° C. This was repeated until the amount of catalyst 2 was 10 g / m calculated as metal. The electrodes were then used as anodes in the electrolysis of a 5: 1 mixture of AlCl 2 -NaCl at a current density of 100 K / vcT at 750 ° C. The anode potential at the beginning and the potential as well as the wear rate after 100 hours of wedging were determined. The results are reported in Table III.

Table III

15 61 725

Electrocatalyst Anode potential Wear rate after 100 h __V (RCGE), g / m ^ after initial 100 h

Rh2 ° 3 0.2 0.2 not detectable

IrO2 0.0 0.0 co3o4 0.0 0.0

The properties of the electrodes were excellent when electrolysing molten aluminum chloride.

Example 5

Sintered anode plates having a size dii of 10 x 10 x 10 mm and containing 70% by weight of Y 2 O 7 7a ^ 0% by weight of ZrO 2 were impregnated by the method of Example IV with the metals shown in Table IV. The anodes were electrolysed with molten metal carbonate fluoride salt and the wear rates and anode potentials were determined as in Example 4. The results are shown in Table IV.

Table IV

Electrocatalyst Anode potential Wear rate _V (RCGE) _ after 100 h ____ at the beginning after 100 h g / cm ^ __

Ag ~ 0 A 1 0.0 0.0 none

IrO 2 0.1 0.2

MnO2 0.2 0.2 "

The anodes performed well in the electrolysis of molten carbonate-fluoride salts, resulting in the evolution of oxygen at the anode.

Example 6

Electrode Sample No. 4 of Example 1 was 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 2 16 61725 3 cm / second. The current density was maintained at 1500 A / ra, and the waste electrolyte contained 0.8 to 2.4% sodium hypochlorate with a faradi efficiency of more than 88%. The weight loss of the electrode after 200 hours of operation was negligible.

Example 7

Electrode Sample No. 3 of Example 1 was used as an anode in the electrolysis of an aqueous acidic copper (II) sulfate solution in a cell having a titanium cathode. The electrolyte contained 150-200 g / l sulfuric acid and 40 g / l copper (II) sulfate as metallic copper. Anode current

A

the density 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 a faradi efficiency of 92-98%. The quality of the metal layer was good and it did not contain dendrites. The anode overvoltage was very low, 1.81 to 1.95 V (NHE).

Example 8

A piece of sintered ^ 203: SnO 2: Y metal with a weight ratio of free metals of 7: 2: 1; a piece of sintered Υ20 ^: Ζγ02: Ζγ metal with a weight ratio of free metals of 6: 3: 1; and a piece of sintered Y 2 O 3: Pd metal with a free metal weight ratio of 9: 1 was activated by impregnating the sintered samples with aqueous CoCl 2, then dried and heated in air at 300-650 ° C to convert the chloride to Co 2 O 2. The cycle was repeated so that the electrodes contained a final coating of 15 g / m 2 Co 2 O 2 on the anode surface. Activated anodes were used in the electrolysis of molten AlCl 2 + NaCl electrolyte. Anode potentials and wear rates are given in Table V.

Table V

17 61 72 5 Sample Anode potential Wear rates _V (SCGE) _ after 100 h ^ __ at the beginning _100 h after _______ ^ m _____ Y202 ~ Sn02 ~ Y 0.1 0.1 none Y203-Zr02-Zr 0.1 0.1 Y203-Pd 0.0 0.0 0.5

Example 9

A piece of sintered Y 2 O 3: ZrO 2: Pd metal containing free metals in a weight ratio of 7: 2.5: 0.5 was impregnated on another larger surface area by the method of Example 8 with 15 g of Co 3 O 3 per square meter of surface. The opposite large surface was coated with a 1 mm thick layer of zirconium diboride by a flame spray method in a nitrogen atmosphere.

Electrically conductive was placed between two opposite graphite electrodes separately from them. The electrode gaps were filled with AlCl 3 + NaCl melt and a graphite counter electrode facing the zirconium diboride coated surface of the sintered bipolar electrode was connected to the positive terminal of the DC source. A graphite counter electrode facing the Co 3 O 2 -activated surface of the sintered bipolar electrode was connected to the negative terminal of the DC source. The sintered electrode acted as a bipolar electrode, and molten aluminum metal flowed down the zirconium dichloride coated surface, and was recovered at the bottom. At the same time, chlorine evolved on the Co 3 O 2 -activated surface of the electrode. The electrolysis process worked well for 28 hours after which the test cell, which was mainly coated with graphite, failed. After this period of operation, the bipolar electrode showed no signs of deterioration or wear.

Other electrocatalysts that can be used in the electrolysis of molten halide salts to decompose the halide ion are RuCl 2. Oxides such as As 2 O 3, Sn 2 O 3 and Bi 2 O 3 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.

61 725 18

In the anodes used in molten fluoride electrolytes which evolve oxygen, the catalyst may be the same as that listed in Example 5, or R 1 O 2, PbO 2 or IrCl 2 .TiCl 2 ·

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

The electrolyte may contain salts other than those used in the examples, such as alkali metal chloride or fluoride, and a salt of the metal to be electrolysed. Metal halides effectively reduce the melting point of the electrolyzable salt, making it possible to use lower boiling points 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. In this way, molten chlorides of other metals, such as alkali metals or alkaline earth metals, can be electrolyzed using otherwise developed anodes in accordance with otherwise normal practice. In the same way, other molten salts, such as molten nitrates, can be further electrolysed. The molten alumina-cryolite electrolyte or similar alkali metal aluminum fluoride can be electrolyzed to produce molten aluminum.

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 yttrium 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 durable in use and therefore do not need to be repositioned to restore the optimum distance from the cathode surface, as is necessary when using prior art wear anodes.

I Γ. '

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

Claims (6)

19 617 25 in the recovery of one or more components of electrolytes. The claims An electrode made of sintered powders of at least one electrically conductive substance and having at least part of its surface provided with at least one electrocatalyst, characterized in that the electrode consists of a non-reinforced body, the main part of which is made of sintered yttrium oxide powders sintered electrode body. Electrode according to Claim 1, characterized in that the electrically conductive substance is an oxide of zirconium and / or tin. An electrode according to claim 1, characterized in that the electrically conductive material forms a smaller part of the sintered electrode body and is at least one metal selected from the group consisting of yttrium, chromium, molybdenum, zirconium, tantalum, tungsten, cobalt, nickel, palladium and silver. An electrode according to claim 1, characterized in that the electrocatalyst is at least one member of the group consisting of oxides of cobalt, nickel, manganese, rhodium, iridium, ruthenium and silver and mixtures thereof. An electrode according to claim 4, characterized in that the electrocatalyst is formed in situ on a sintered electrode body from a solution of said metal salts which are converted to oxides on the sintered electrode body. An electrode according to claim 4, characterized in that the electrocatalyst consists of powdered oxides of said metals sintered in the outer layers of the electrode.