DK155531B - ELECTRODE FOR USE BY ELECTROLYSE, ISRAEL FOR ELECTROLYSE OF MELTED METAL - Google Patents

Description

in

DK 155531 B

Dimension-stable electrodes for anodic and cathodic reactions in electrolytic cells have recently found widespread use in the electrochemical industry to replace the usable electrodes of carbon, graphite and lead alloys. The 5 are particularly useful in flowing mercury cathode cells and in diaphragm cells for the production of chlorine and alkalis, in metal electro-recovery cells where pure metal is extracted from aqueous chloride or sulfate solution, and for cathodic protection of ship hulls and other metal structures.

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Dimensionally stable electrodes generally comprise a base of metal, such as Ti, Ta, Zr, Hf, Nb and W, where during anodic polarization a corrosion resistant but non-electrically conductive oxide layer or barrier layer is coated over at least a portion of their outer surface with an electrically conductive and electrocatalytic layer of platinum group oxides or pi that ingroup metal1 (see U.S. Patents Nos. 3,711,385, 3,632,498 and 3,846,273). However, electrodepositing and electrocatalytic coatings made of or containing platinum group metals or platinum group metal oxides are expensive and are eventually subjected to use or deactivation by certain electrolytic processes, and re-activation or re-coating is therefore required to reactivate spent electrodes.

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Furthermore, electrodes of this type are not usable for several electrolytic processes. In molten salt electrolytes, the base metal, e.g. quickly dissolved, as the thin protective oxide layer is either not formed at all or is rapidly destroyed by the molten electrolyte with resulting dissolution of the base metal and loss of the catalytic coating of precious metal. Furthermore, in several aqueous electrolytes such as fluoride solutions or in seawater, the degradation voltage of the protective oxide layer on the exposed base metal is too low and the base metal 35 is often corroded during anodic polarization.

Recently, other types of electrodes have been proposed to replace the rapidly used carbon anodes and carbon 2

DK 155531 B

fabric cathodes which have heretofore been used under severely corrosive conditions, such as electrolysis of molten metal salts, typically for electrolysis of molten fluoride baths, such as those used in the manufacture of aluminum of molten cryolite. In this particular electrolytic process, which is of great economic importance, carbon anodes are consumed at a rate of 450-500 kg of carbon per tonne of aluminum produced, and a costly and constant adjustment of the apparatus is necessary to maintain a small uniform gap between the corroding anode surfaces. -10 surfaces and the liquid aluminum cathode. It is estimated that more than 6,000,000 tons of cow 1 fabric anodes are consumed in 1 year by aluminum manufacturers. The carbon anodes are burned off after the reaction: A1203 + 3/2 C - »2A1 + 3/2 C02 15 but the actual rate of consumption is much greater due to the breaking and breaking of carbon particles and intermittent spark formation which takes place over anodic gas films which are often formed over areas of the anode surface because carbon 20 is poorly wetted by the molten salt electrolytes, or due to short circuits caused by bridges of conductive particles coming from the corrosive carbon anodes and from dispersed particles of that of the alloy metal. 1 2 3 4 5 6 7 8 9 10 11

British Patent No. 1,295,117 discloses anodes for molten 2 cryolite baths consisting of a sintered ceramic oxide material consisting mainly of SnO 2 with smaller amounts of other metals, namely oxides of Fe, Sb, Cr, Nb, Zn, W, Zr, Ta in a concentration up to 20%. Electrically conductive sintered SnO2 with 6 minor additions of other metal oxides, such as oxides of Sb, 7

Bi, Cu, U, Zn, Ta, As, etc. have been used for a long time as 8 durable electrode material in AC glass melting furnaces (see 9 U.S. Patent Nos. 2,490,825, 2,490,826, 3,287,284, and 10 3,502,597), but exhibit considerable wear and corrosion when used as anode material in electrolysis of molten salts.

Wear rates of up to 0.5 g / h / cm 2 have been found on samples of compositions described in the above patents when used in molten cryolite electrolyte at 3000 A / m 2. The 3th

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high Si velocity of sintered Sn02 electrodes is believed to be due to several factors: a) chemical attack of the halogens because Sn * V forms high coordination number complexes with halogen ions, b) reduction of Sn02 of aluminum dispersed in the electrolyte and c) mechanical erosion of due to anodic gas evolution and salt precipitation inside the material pores.

Japanese Patent Application No. 112,589 (Publication No. 62,114 of 1975) discloses electrodes having a conductive support of titanium, nickel or copper or an alloy thereof, carbon, graphite or other conductive material coated with a layer consisting essentially of of spinel and / or metal oxides of the perovskite type and alternatively electrodes obtained by sintering mixtures of these oxides. Spin oxides and 15 perovskite oxides belong to a family of metal oxides which typically exhibit good electronic conductivity and have previously been proposed as suitable electroconductive and electrocatalytic anodic coating materials for dimensionally stable metal anodes (see U.S. Patent Nos. 3,711,382 and 3,711,297 and 20 Belgian Patent No. 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 itself is weak because the crystal structure of the spinels and perovskites is not isomorphic with the oxides of the metal support. , and various binders such as oxides, carbides, nitrides, and borides have been tested with no or little improvement. In molten salt electrolytes, the substrate material is rapidly attacked due to the inevitable pores through the spinel oxide coating and the coating peels off rapidly from the corrosive substrate. Furthermore, spinels and perovskites are not chemically or electrochemically stable in molten halide salt electrolytes and exhibit a significant wear rate due to halide ion attack and due to the reducing effect of dispersed metal.

In the electrolytic preparation of metals of molten halide salts, the aforementioned known anodes have been found to have a 4

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other disadvantage. The significant dissolution of the ceramic oxide material dissolves metal cations which deposit on the cathode with the metal being produced, and the impurity content of the recovered metal is so high that the metal can no longer be used for purposes requiring purity of metal. electrolytic quality. In these cases, the economic benefits of the electrolytic process, which are largely due to the high purity that can be achieved compared to melting processes, are lost in whole or in part.

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An electrode material which can be used with good results under severely corrosive conditions, such as by electrolysis of molten halide salt tea and especially molten fluoride salts, must first and foremost be chemically and electrochemically stable under the operating conditions. It must also be catalytic in relation to the anodic evolution of oxygen and / or halides so that the anode overvoltage is lowest for the high overall efficiency of the electrolysis process. The electrode must also have heat stability at operating temperatures of approx. 200 to 1100 ° C, 20 good electrical conductivity and must be sufficiently resistant to accidental contact with the molten metal cathode. Excluding coated metal electrodes because there is hardly any metal substrate that could withstand the extremely corrosive conditions found by electrolysis of molten fluoride salt, the effect of a very large number of sintered ceramic electrodes of different compositions has been systematically tested.

According to the present invention, there is provided an electrode for use in electrolysis, especially for electrolysis of molten salt, and this electrode is characterized in that it comprises a self-supporting matrix of sintered powders of at least one metal oxide selected from titanium, tantalum , zirconium, vanadium, niobium, aluminum, silicon, tin, manganese, iron, cobalt, nickel, silver, arsenic, bismuth, lanthanum, outer bium, thorium and yttrium and at least one electroconductive metal or metal oxide provided in at least part of its surface with at least one electrocatalyst in the form of a metal, metal oxide or mixtures thereof.

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It is preferred that said electroconductive metal or metal oxide constitute less than 50% by weight of the sintered electrodes.

Thus, the "sintered" electrode is a self-supporting, substantially rigid body consisting essentially of an oxymetal compound and at least one electroconductive agent produced in any of the known ways used in the ceramic industry, such as by heating under pressure a powdered mixture of the materials to mold the mixture to the desired size and shape, or by molding the material into molds, by extrusion or by using binders, etc. and then sintering the shaped body at high temperature to a self-supporting electrode. The oxyhalide compounds are preferably oxychlorides or oxy fluorides.

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The electrical conductivity of the sintered ceramic electrodes is enhanced by the addition to the composition of 0.1-20 wt. Of at least one electroconductive agent selected by the group consisting of (A) doping oxides, typically of metals having a valence lower or lower. higher than the valence of the metals of the oxides constituting the matrix, such as the alkaline earth metals Ca, Mg, Sr and Ba and metals such as Zn, Cd, In2, Tl2, As2, Sb2, Bi2 and Sn, (B) oxides exhibiting electrolytic conductivity due to internal Redox system such as spinel oxides, perovskite oxides, etc., (C) oxides exhibiting electroconductivity due to metal-to-metal bonds such as CrO 2, M 2 O 2, TiO, Ti 2 O 2, etc., borides silicides, carbides and sulfides of metals such as Ti , Zr, Hf, V, Fb, Ta, Cr, Mo and ¥ or the metals Y, Ti, Zr, Hf, V, Fb, Ta, Cr, Mo, W, Pd and Ag or alloys thereof or mixtures of (A) and / or (B) and / or (C).

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The preferred electrocatalysts are selected from the group of metals consisting of ruthenium, rhodium, palladium, iridium, platinum, iron, cobalt, nickel, copper and silver and mixtures thereof and oxides of metals of the group consisting of manganese, iron, cobalt, nickel, ruthe -35 nium, rhodium, palladium, iridium, platinum, silver, arsenic, antimony, lead and bismuth and mixtures thereof.

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By mixing the powder of the matrix material with a smaller amount, typically from 0.5 to approx. $ 50, powders of a suitable electrocatalytic material and, by sintering the mixture to a self-supporting body, when used as an electrode, exhibit satisfactory electroconductive and electrocatalytic properties which retain its chemical stability, although the admixed catalyst itself could not resist the electrolysis conditions.

The electrocatalyst may be a metal or an inorganic oxy compound. The preferred mixed catalyst powders are pulverized metals Ru, Kb, Pd, Ir, Pt, Ee, Co, Ri, Cu and Ag, especially the plate group metals, pulverized oxy compounds of Min, le, Co, Ri, Ru, Rb, Pd, Ir , Pt, Ag, As, Sb and Bi and especially oxy compounds of the platinum group metals.

Particularly preferred are 3MnC> 2, Co 2, Rh 2, IrO 2, RhO 2, Ag 2 O, Ag 2, Ag 2 O 3, As 2 S 2 O 2, Bi 2 O 2, CoMn 2 O 2, NiC 2> 20 3 and mixtures of these powdered metals and oxy compounds.

It has been found particularly advantageous for the oxymetal compound 20 to add another material such as stannous oxide, zirconium oxide or the like, and by also adding a small amount of at least one metal belonging to the group comprising yttrium, chromium, molybdenum, zircon, tantalum, tungsten, cobalt, nickel, palladium and silver, both the mechanical properties and the electrical conductivity of the sintered electrodes are improved without appreciable reduction of their chemical and electrochemical corrosion resistance.

These additives are added in powder form and mixed with the powdered metal oxide in percentages which may range from 40 to 1 $, calculated by weight of the metal content. Optionally, other organic and / or inorganic compounds may be added to the powdered mixture to improve the bonding of the particles both during shaping and sintering. 1

The anodes have a high melting point slightly above the temperature of the molten sole electrolytes used, and they undergo no phase change under the working conditions of the electrolysis. Furthermore, the heat elongation coefficients are not very different from the one that applies to the heat exchanger.

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the logenide salts used in the molten salts, which help to preserve the proper electrode gap between the anode and cathode and avoid expansions and contractions that could break the salt crust on top of the molten salt electrolyte in the normal aluminum electrolysis process.

the conductivity of the sintered electrodes according to the invention is comparable to the conductivity of graphite. The matrix has acceptable workability in shaping and sintering and, upon use, forms a thin layer of oxyhalides on its surface under anodic conditions. The free formation energy of the metal oxy compounds is more negative than the free formation energy of the oxide in the corresponding molten salt electrolyte in the halide phase, so that these sintered anodes have a high degree of chemical stability.

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The sintered metal oxy compound electrodes of the invention may also be used as bipolar electrodes. According to this last embodiment, the sintered electrodes can be conveniently made in the form of a plate, one of the two large surfaces of the electrode being provided with a layer containing the anodic electrocatalyst such as the oxides Co 2 O 2, M 2 O 2, Kh 2 O 2, IrO 2, RuO 2 , Ag20, etc., and the other large surface is provided with a layer containing suitable cathodic materials such as carbides, borides, nitrides, sulfides, carbonitrides, etc. of metals, especially of the metals yttrium, titanium and zirconium.

The self-supporting sintered body consisting of a greater amount of oxide-metal compound can be prepared by grinding the materials together or separately, preferably to a grain size between 50 and 30,500 µm, to form a powder mixture containing a grain size range providing a better degree of compression.

According to one of the preferred methods, the mixture of powders is mixed with water or with an organic binder to form a plastic mass which has suitable flow properties for the specially used molding method. The material may be formed in known manner either by stamping or pressing the mixture into a mold or by licking molding in a plaster mold, or the material may be extruded through a nozzle to various shapes.

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The shaped electrodes are then subjected to drying and heated to a temperature at which the desired coating can take place, usually between 800 and 1800 ° 0 for a period between 1 and 30 hours, usually followed by slow cooling to room temperature. The heat treatment 5 is preferably carried out in an inert atmosphere or a slightly reducing atmosphere, e.g. in + Hg ($ 80) when the powdered mixture is composed essentially of oxymetal compound with a minor amount of other metal oxides or metals.

If the powdered mixture also contains metallic powders, it is preferred to carry out the heat treatment in an oxidizing atmosphere, at least in part of the heating circuit, to promote the oxidation of metallic particles in the outer layers of the electrodes. The metallic particles remaining inside the body of the sintered material improve the electrical conductivity of the electrode.

The molding process can be followed by the high temperature sintering process as mentioned above, or the molding process and sintering process can be simultaneous, i.e. pressure and temperature can be applied simultaneously to the powder mixture, e.g. using electrically heated molds. Insertion leads can be melted into the ceramic electrodes during molding and sintering, or connected to the electrodes after sintering or molding, internal methods of shaping, compressing and sintering the powdered mixture can of course also be used.

The electrocatalyst, which is usually applied to the electrode surface as a result of the cost, must have high stability, low anodic voltage for the desired anodic reaction, and a high anodic voltage for undesired reactions. In the case of chlorine development, oxides of cobalt, nickel, iridium, rhodium, ruthenium or mixed oxides thereof, such as RuOg-IiOg, etc., may be used, and in the case of fluoride-containing electrolytes in which oxygen evolution is the desired anodic reaction, oxides of

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silver and manganese · -Other oxides for use as electrocatalysts can be oxides of platinum, palladium and lead.

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G-ifte for undertaking undesirable anodic reactions can be used, e.g. to suppress oxygen evolution of chloride electrolytes. G-ifte exhibiting high oxygen overvoltage should be used and suitable materials are the oxides of arsenic, antimony and bismuth. These oxides, which are used in small amounts, can be applied together with the electrocatalyst oxides in percentages of 1-10 of the electrocatalyst, based on the weight of the respective metals.

The application of the electrocatalyst and optionally the poisoning agent 10 can be effected by any of the known coating methods. Preferably, the electrocatalyst and optionally the poisoning agent are applied to the sintered electrodes as a solution of decomposable salts of the metals. The sintered body is impregnated with the solution containing the desired metal salts and dried. Thereafter, electrode 15 is heated in air or another oxygen-containing atmosphere to convert the salts to the desired oxides.

The porosity of the sintered body and the method used to impregnate the surface layers of the sintered body with the metal salts shall provide penetration of the solution down to a depth of at least 1-5 mm, preferably 5 mm, inward from the surface of the electrode, thus after the heat treatment, the electrocatalysts are present in the pores of the sintered body down to a certain depth inward from the surface of the electrode.

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By suitable powder blending technique, alternatively pre-formed electrocatalyst oxides and optionally pre-formed poisoning oxides may be ground to powder and added to the powder mixture during shaping of the electrodes in such a way that the outer layers of the connected electrodes are enriched with powders of electrocatalyst oxides and, optionally, the poisoning oxides, of the electrodes after sintering is already provided with the electrocatalyst. The sintered electrodes of the invention can be used as bipolar electrodes. In this embodiment of the invention, electrodes may be provided on one surface with the anodic electrocatalyst and optionally with the poisoning agent for the undesirable anodic reaction in one of the ways described above, while the other surface may be provided with a coating of eg - 10

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net cathodic material. E.eks. For example, the surface of the Mpolar electrode acting as a cathode during the electrolysis process may be provided with a layer of metal carbides, borides, nitrides, sulfides and / or carbohydrates of yttrium, tantalum, titanium, zircon, and so on.

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A preferred method of applying a layer is by the plasma jet technique whereby powders of the selected materials are sprayed and adhered to the surface of the sintered body with a flame under controlled atmosphere. Alternatively, the selected powdered material may be added during the formation process to the powder mixture and then sintered together, providing the cathodic surface of the bipolar electrode with a layer of the cathodic material of choice.

The electrodes can be used effectively for the electrolysis of many electrolytes. They are particularly advantageous used as anodes in electrolytic cells used to electrolyze molten salt electrolytes such as molten cryolite baths, molten halides of aluminum, magnesium, sodium, potassium, calcium, lithium and other metals. Thus, aluminum halide can be electrolysed according to the Hall process or processes described in U.S. Patents Nos. 3 * 464,900, 3,518 * 712 or 3 * 755,099 using the electrodes described herein as anodes. The electrolysis temperature is high enough to melt and hold the salts of the metal to be recovered in the molten state, and the metal is deposited in the molten state and is generally collected as a molten cathode, the molten metal being extracted from the molten cathode.

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

If the electrodes of the invention are used as bipolar electrodes 35 for molten salt electrolysis, the composition of the cathode portion of the electrodes must be such that it is not reduced by the cathodic reaction or attacked by the metal deposited at the cathodes, especially when the electrode composition is an oxy compound. Of this 11

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For this reason, it is desirable that the composition of the cathode side of the bi-polar electrode be inert to the cathodic reaction and the reducing effect of the molten metal.

The electrodes may also be used as anodes and / or as cathodes in electrochemical processes such as: electrolysis of aqueous chloride solutions to produce chlorine, alkalis, hydrogen, hypochlorite, chlorates and perchlorates, electro-extraction of metals from aqueous sulfate or chloride solutions to produce copper, zinc, 10 nickel, cobalt and other metals, electrolysis of molten metal salt electrolytes, typically containing halides, oxides, carbonates or hydrates, for the production of aluminum, beryllium, calcium, cerium, lithium, sodium, magnium, potassium, barium , strontium, cesium and other metals, and electrolysis of bromides, sulfides, sulfuric acid, hydrochloric acid and hydrofluoric acid. In general, the electrodes are useful for all electrolytic processes.

In the following examples, several preferred embodiments are described to illustrate the invention. The percentages of the components of the 2q electrodes are calculated in% by weight and calculated as free metal on the basis of the total metal content of the composition.

Among the preferred anodes are those in which the majority of the self-supporting body is tin dioxide alone or with up to 20% by weight cobalt oxide provided with a coating of cobalt oxide which provides electrodes with improved mechanical properties and electrocatalytic properties for chlorine development. Other preferred additives are Y₂O₂, TiO₂ and I₂O₂.

Bone electrocatalytic coatings can be protected from abrasion by the simultaneous application or application of a protective agent such as a metal oxide such as LiO 2 and Ta 2 O 2 or SiO 2 mixed oxides such as AgRe 2 O, TiCo 2 O 4 and Ag 2 WO 2.

Example 1

Berries were prepared in three sintered electrode samples, 80% SnO2 + 20% cobalt (Sample A), 80% SnO2 + 10% Co + 10% Mo (Sample B) and 100% SnO2 (Sample C), and were carefully washed with water and dried in vacuo. The resulting electrodes were then immersed under vacuum in the solution.

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in Table I, and then dried followed by heating to 370 ° C for 20 minutes in a forced air oven. The samples were then brushed with the same solution and heated for 15 minutes to 350 ° C in the same oven, and this treatment was repeated several times until the electrode had a weight gain of 5 g / m.

The electrode samples were then used as anodes in a sample cell for electrolysis of aluminum chloride at 750 ° 0 and an anodic current density of 1000 A / m. The cell voltage was 5 volts and the electrolyte was a L0 5-1-1 mixture by weight of aluminum chloride, sodium chloride and potassium chloride. The anode voltage was determined immediately and after 500 hours of operation, and the weight loss of the electrode was determined after 500 hours. For comparison purposes, a standard graphite electrode was also used under the same conditions and the results are presented in Table I.

L5--

Table I

Anode voltage vf-Rprv ^ Weight loss after -Solution- after 500 500

Sample Solvent wrong Immediately hours g / nr_ A water and formamide CoClg 0.45 0.47 0.8 B hydrochloric acid IrCl ^ 0.45 0.45 0.5 C hydrochloric acid IrCl * 0.5 0.6 0.2> 5 0 A untreated 0.6 0.6 0.5 B untreated 0.55 0.55 0.5 C untreated 1.0 1.3 nothing ws RCG-E: Reference chloro graph the electrode.

The results in Table I show that the coated electrodes have an even lower voltage for chlorine evolution without any significant increase in weight loss. Sample C, which had an excessively high voltage without the post-treatment, was not suitable for the electrolysis reaction while the treated sample C is. Ben's average faraday efficiency during the trial was $ 96. A conventional graphite electrode used in the same way and compared to the reference graphite electrode showed a voltage |, q of approx. 0.8 volts.

Example 2

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Samples of 90 wt. Tin dioxide and 10 wt. Cobalt were sintered, and the electrodes were then provided with a coating of cobalt oxide as in Example 1, so that a layer of 10 g / m of cobalt oxide was obtained. The electrodes were then used to electrolyze the electrolytes in Table II under the operating conditions mentioned therein. The anode voltage after 300 hours of operation and the degree of wear after 300 hours were determined and are shown in Table II.

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label II

Electrolyte current composition. extra-high density Average Anode Voltage Voltage and Electrolyte Density paraday after 300 weight loss 15 weight ratio temp. ° C k / wr efficiency timer y (ROG-E) g / iir

AlCl ^ + KCl 750 1000 92 $ 0.5 0.5 (5: 1)

CaCl2 + KCl 450 1000 94 $ 0.6 0.5 (5: 1) 20

PbClp + KCl 450 1000 90 $ 0.6 0.5 (5: 1)

Table II shows that the electrodes according to the invention have a lower abrasion degree and a low anode potential even after 300 hours of operation.

Example 3

Disc shaped electrodes 10 mm in diameter and 30 mm thick were made from powders having a sieve size of 100 to 250. The powders were die-cast at a pressure of 1000 kg / cm and then sintered in an induction furnace under the conditions which are shown in Table III, which also shows the compositions of the powders.

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

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label III

Sample Components and - ^ - No. Weight Percent Rel. ° C Atmosphere Heating time 1 Sn02 80% 1250 forced air- 2 hours

Co 20% Circulation 5 2 SnOp 80% 1500 "2 hours

Co * 10%

Mo 10% 3 SnOp 80% 1250 »2 hours

Mo * 10%

Mi 10% 10 4 Sn02 75% 1500 ambient 2 hours

Iao0, $ 10 air

Oo2 5 15 $ 5 Sn02 60% 1500 "2 hours 15 OOpMO, 30%

Co 4 10% 6 SnOp 60% 1000 11 2 hours

SiO ^ 10%

IO0, 20 2 COPU10, 10%

Cu 4 10%

Mo 10% 7 Sn02 95% 1500 forced air- 2 hours

Co 2.5% circulation 25 Mo 2.5% 8 Sn02 100% 1500 air 10 hours

The electroconductivity of samples 1 to 7 measured at 500 ° C was between 0.01 and 1.00, and the density of the sintered electrodes varied between 5 and 8.5 g / cm. The electrode samples were used as anodes in a test cell for electrolysis of aluminum chloride at 750 ° C and an ano-current density of 1000 A / m. The cell voltage was 5 volts and electrolyte one was a 5-1-1 mixture of aluminum chloride, sodium chloride and 33 potassium chloride. The anode voltage was determined immediately and after 500 hours of operation, and the weight loss of the electrode was determined after 500 hours. For comparison purposes, a reference graphite electrode was also used under the same conditions and the results are shown in Table 17.

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label IT Anode Potential

Test VY (RSE) * Weight loss after 500 no. Immediately After 500 hours hours g / m ^ 1 0.6 0.6 0.5 2 0.55 0.55 0.5 5 5 0.60 0.6 0, 8 4 0.55 0.6 0.5 5 0.6 0.6 nothing 6 0.65 0.65 0.5 10 7 0.55 0.6 nothing 8 1.0 1.3 no graphite 0.85 0.85 105 1 R ae RGE: Reference graphite electrode

The results in Table IY show that electrodes 1 to 7 containing a larger amount of an oxide and a smaller amount of a metal have a low voltage for chlorine evolution and a very low degree of wear.

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The electrode 8, which contains no added electroconductive metal, has a significantly higher voltage for chlorine development, and the reference graphite electrode had a voltage over the values of the electrodes 1 to 7 and a high degree of wear. The reference graphite anode required substantial adjustment during electrolysis and early replacement. The average mean efficiency during the test was $ 97. All samples 1 to 7 inclusive were less brittle than sample # 8

Example 4 Approx. 250 g of a mixture of the matrix material and additive materials 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 manually with a steel cylinder press. Each mold was placed in an isostatic pressure chamber and the pressure was raised to approx. 1500 35 kg / cm in 5 minutes and then reduced to 0 in seconds. The samples were then removed from the plastic molds and polished. They pressed rw »/ 5iTT / a * K> <-« · + δ 4 — Λννττ * α * νι »ττΐΛ4 * λτr * n r \ rv rs-r ^ Faith 4- ο + ιιλ.

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temperature to 1200 ° C under a nitrogen atmosphere for a period of 24 hours, allowed to cool at the maximum temperature for 2-5 hours and then cooled to 500 ° C over the following 24 hours. The sintered samples were then taken out of the oven and after cooling to room temperature they were weighed and their apparent density and electrical conductivity at 25 ° C and at 1000 ° C were measured. The results are listed in Table Y.

Table Y

10 Supervision

Sintering- Electrical Conductivity

Sample Composition max. temp. filling at 1000 C at 25 C and weight $ (h) g / cnr Ω-1 cm ~ Ω-1 cm ~ 1 Zr02 60 $ 5 5.1 0.1 15 10 $ top 1056

Ir02 20 $ 2 Τα205 50 $ 5 5.3 0.4

LaJO ^ 5 $ 20 SiOo 5 $ Y0p 20 $ 0o ^ 04 20 $ 3 Zr02 50 $ 5 5.1 0.3

Ti205 20 $ 25 ZrOClp $ 20

Rb205 10 $ 4 Fb205 30 $ 5 5.6 0.5

TiOr, $ 20 YOE 20 $ 30 Ag20 20 $

Sb203 10 $ 5 T203 20 $ 5 5.8 0.7 la203 $ 20

Th.0o 20 $ 35 ^

Ti20 * $ 20

RligOj $ 20

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Table Y (continued)

Visible-

Sintering Charged Electrical Conductivity

time at fifth at 10 ° C at 25 ° C

Sample Composition max. temp. 3 µi -] No. and weight / (h) g / cnr fi-1 cm cm 6 ZrOp 40/5 5.3 1.2 Y20 ^ 5 /

SiOr, 5 /

ZrpOClp 15 /

BipO ^ 10/10 AgpO ^ 15 / °

RuOp 5 /

CuO 5/7 ZrOp 50/5 5.9 1.0 Y203 30/15 SnOp 10 /

IrOp 8 /

CuO 2/8 ZrO 2 30/5 5.8 0.8

Ybo0 * 10/20 2 3

Th.02 10 /

TipOj 25 /

Sn02 15 /

Co304 10/25 9 TiO2 20/5 5.4 2.1

AlpO ^ 20 /

TipO ^ 20 /

Sn02 20 / βΜη02 20/30 10 ZrOp 30/2 5.1 4 0.5 Y203 5 / YOE 25 / Y 20 /

RuO2 20/35 11 TiO2 20/2 5.7 5 0.9

TapO, - 30 / VOp 10 /

Eepo. 10 /

Co ^ o; 10/18

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Table 7 (continued)

Visible-

Sintering Electrical Conductivity j · n _ n vS3g U- · '.i i · - —w' 1

Sample Composition max. temp. fill at 1000 C at 25 C 5 no. and weight # (h) g / cnr Ω-1 about "1 Ω-1 cm-1 12 TiO'2 40 # 5 6.3 12 1.6

TiOC 25 #

SnOp 15 #

EuOp 5 #

Mo 1096 10 Ti 5 # 13 Ti02 40 $ 5 6.1 10 2.5

Take? 0c 10 #

Iip02 10 #

Co ~ 0? 20 # ις Mo9 4 10 # ASpO ^ 10 # 14 ZrOp 40 # 2 6.5 15 2.5 Y20 ^ 5 # vo2 25 # 20 AgO 20 #

Pd 1 #

Mo 9 #

The data in Table Y shows that the electrical conductivity of the sintered ceramic electrodes at high temperatures of 1000 ° C is 5-10 times higher than the electrical conductivity at 25 ° C. The addition of oxides having conductivity equivalent to metals to the substantially non-conductive ceramic oxides of the matrix increases the conductivity of the electrodes by a magnitude of 10. Addition of a metal which is stable to molten salts, such as yttrium or molybdenum, etc., to the ceramic electrodes of the invention increases the electrical conductivity of the electrodes 2-5 times.

Example 5 35

The operating conditions of an electrolytic cell for the production of metallic aluminum of molten cryolite were mimicked in a laboratory test cell. In a heated graphite crucible, a layer of liquid aluminum was placed on the bottom and a melt consisting of cryolite 4Π

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($ 80-85), alumina ($ 5-10) and Al i1- (from $ 1 to $ 5) were poured 2 · ζ on top. The test electrodes having a working surface of 3 cm made by the method described in Example 4, and to which a Pt wire was soldered to provide an electrical conduit, were immersed in the salt melt and kept at a distance of about 5 cm. 1 cm from the liquid aluminum layer. The crucible was kept at a temperature of 950 to 1050 ° C and the current density was 0.5 A / cm 2 and the cell operated for 2000 hours. The test data obtained are shown in Table 71. Sample number indicates that the tested electrode corresponds to the sample described in Table 7 with the same number.

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Table 71

Sample Produced Aluminum "Weight Loss of ^ Anodes 15 No. _ (g / hr) (g / cm) 1 0.49 0.02 2 0.50 0.12 3 0.49 0.04 20 4 0.49 0, 02 5 0.48 0.01 6 0.49 0.04 7 0.49 0.06 25 8 0.46 0.18 9 0.46 0.2

The sample electrodes worked well as anodes in the cryolite melt, and the observed wear appears to be perfectly acceptable for the electrolytic production of aluminum of molten cryolite. All the tested electrodes exhibited a low degree of wear during 2000 hours of operation. Generally, the degree of wear of electrodes containing heat stabilizers such as oxy compounds of Group III metals in the periodic system is about 10 times smaller than the electrodes without heat stabilizers.

20 DK 155531 B

Example 6

Electrodes Nos. 4 and 5 described in Table V were used as anodes for electrolysis of an electrolyte of molten aluminum chloride in the sample cell described in Example 5. The electrolysis conditions were as follows: 5 Electrolyte: AlCl to 35 'wt% ITaCl from 31 to 35 wt%

BaCO3 from 31 to 35:% by weight

1Q Temperature of electrolyte: from 690 to 720 ° C

ISLAND

Anodic current density: 2000 A / m

Cathode: Melted aluminum 15

Electrode gap: 1 cm.

The tested electrodes worked well and the weight loss after 2000 hours of operation was insignificant.

20

Example 7

The electrode samples Nos. 10 and 11 of Example 1 were used as anodes for electrolysis of an aqueous bromide solution to produce bromine, using a diaphragm sample cell with an asbestos diaphragm to separate the cathode compartment with a steel cathode from the anode compartment. the test electrode as anode. The electrolysis was performed with an aqueous solution of 200-220 g / l sodium bromide, and the electrolyte temperature was 80-85 ° 0 with a current density of 2000 A / m 2. The current-3Q efficiency was 95f ° and after 1000 hours of operation the weight loss of the test electrode was insignificant.

Example 8

Electrode Samples Nos. 10, 11 and 13 of Example 1 were alternatively used as anode and cathode for electrolysis of synthetic seawater in a sample cell in which the electrolyte was pumped through the 3 mm electrode gap at a rate of 3 cm / second. The current density became

DK 155531 B

held at 1500 A / m 2, and the spent electrolyte contained 0.8 to 2.4 sodium hypochlorate with a faraday efficiency of more than $ 88. The accuracy of the electrodes after 200 hours of operation was insignificant.

Example 9

The electrode samples Nos. 12 and 14 of Example 1 were used as anodes for electrolysis of an aqueous acidic cupric sulfate solution in a cell with a titanium cathode. The electrolyte contained 150 to 200 g / l sulfuric acid 10 and 40 g / l cupric sulfate as metallic copper, and the anode current density O r \ was 300 A / cm. The electrolyte temperature was 60-80 C and an average of 6 mm copper was deposited on the flat cathode with a Faraday efficiency ranging from 92 to $ 98. The quality of the metal deposit was good and free of dendrites, and the anode overvoltage was very low, ranging from 1.81 to 1.95 Υ (ΗΉΕ).

Other electrocatalysts which can be used in the electrolysis of molten halide salts for halide ion evaporation are RuO 2 and oxides such as As 2 20 free metal, calculated on the total metal content, to increase the oxy gene overvoltage without affecting the halogen ion ejection voltage.

For anodes to be used in molten fluoride electrolytes where oxygen is generated, the catalyst may be those used in Example 5> or Rh2 O2, PbO2 and IrO2, TiC> 2.

The components of the anodes listed in the Examples are calculated in weight free metal based on the total metal content of the anode composition.

30

The electrolyte may contain salts other than those used in the examples, such as alkali metal chloride or fluoride, and the salt of the metal undergoing electrolysis. The metal halides are effective in lowering the melting point of the salt which undergoes electrolysis, thus enabling the use of lower temperatures while keeping the salt bath in the molten state.

The above examples include electrolysis of molten metal salts, primarily electrolysis of molten aluminum chloride and 40 fluoride salts. Similarly, the molten chlorides of others may

Claims (6)

Electrode for use in electrolysis, in particular electrolysis of 30 molten salt, characterized in that it comprises a self-supporting matrix of sintered powders of at least one metal oxide selected from titanium, tantalum, zirconium, vanadium, niobium, aluminum, silicon, tin, manganese, iron, cobalt, nickel, silver, arsenic, bismuth, lanthanum, outer bium, thorium and 35 yttrium and at least one electroconductive metal or metal oxide, said electrode having at least part of its surface with at least one electrocatalyst in the form of a metal, metal oxide or mixtures thereof. DK 155531 B Electrode according to claim 1, characterized in that the electroconductive agent is a small amount of the sintered electrode body and is an oxide of at least one metal selected from zirconium and tin. 5 Electrode according to claim 1, characterized in that the electroconductive agent is a minor amount of the sintered electrode body and is at least one metal selected from yttrium, chromium, molybdenum, zirconium, tantalum, tungsten, cobalt, nickel, 10 pal 1 adium and silver. Electrode according to claim 1, characterized in that the electrocatalyst is selected from oxides of cobalt, nickel, manganese, rhodium, iridium, ruthenium and silver. 15 Electrode according to claim 1, characterized in that the electrocatalyst is formed in tu on the sintered electrode body by a solution of salts of said metals which are converted to oxides on the sintered electrode body. 20 An electrode according to claim 4, characterized in that the electrocatalyst comprises pulverized oxides of said metals sintered in the outer layers of the electrode. 25 1 35