DK155529B - ELECTRODE FOR MELTED SALT ELECTROLYSIS - Google Patents

Description

DK 155529B

Dimension-stable electrodes for anodic and cathodic reactions in electrolysis cells have recently come into general use in the electrochemical industry and replace the usable electrodes of carbon, graphite and lead alloys. They are particularly useful in cathode-5 flowing mercury cells and in diaphragm cells for chlorine and alkali production, in metal electro-recovery cells in which pure metal is extracted from aqueous chloride or sulfate solution, and for cathodic protection of ship hulls and other metal structures.

Dimension-stable electrodes generally comprise a base of meta such as Ti, Ta, Zr, Hf, Nb and W, where during anodic polarization a corrosion-resistant but not electrically conductive oxide layer or barrier relay is coated over at least a portion of their outer surface. with an electric

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or plate group metals (see U.S. Patents Nos. 3 · 711,385, 3,632,498, and 3,846,273). However, electroconductive and electrocatalytic coatings made from or containing platinum group metals or platinum group metal oxides are costly and are ultimately subjected to consumption or deactivation in certain electrolytic processes, and therefore reactivation or re-coating is required to reactivate spent electrodes.

Furthermore, electrodes of this type are not usable in some electrolytic processes. In molten salt electrolytes, the base metal, e.g. quickly dissolved, the thin protective oxide layer being neither formed nor rapidly destroyed by the molten electrolyte with the resulting dissolution of the base metal base, and loss of the catalytic coating of precious metal. Furthermore, in several aqueous electrolytes, such as fluoride solutions or in seawater, the decomposition voltage of the protective oxide layer on the exposed base metal is too low and the base metal is often corroded during anodic polarization.

Recently, other types of electrodes have been proposed to replace the rapidly used carbon anodes and carbon cathodes which have heretofore been used for severely corrosive purposes, such as electrolysis of molten metal salts, typically for electrolysis of molten fluoride baths, such as those used in manufacture of aluminum of 5 molten cryolite. In this particular electrolytic process, which is of great economic importance, carbon anodes are consumed at a rate of approx. 450 to 500 kg of carbon per tonnes of aluminum produced, and a costly constant adjusting apparatus are needed to maintain a small and uniform gap between the corrosive anode-0 surfaces and the liquid aluminum cathode. It is estimated that over 6 million tonnes of carbon anodes are consumed in one year by aluminum manufacturers. The carbon anodes are burned off after the reaction: A1203 + 3/2 C - ^ 2A1 + 3/2 C02 5, but the actual rate of consumption is much greater due to brittle formation and carbon particle breakage and due to intermittent sparking which occurs over anodic gas films. which are often formed over areas of the anode surface because carbon is wetted 0 poorly by the molten salt electrolytes, or as a result of short-circuiting 1 Λ 1 1 Ρ V r V r «r * Ί · Ρ» T N N VI Λ λ vi n <> \ / 3 Λ vi λ v \ «-> vi-f- A 1 **" "T Λ vi J /> vi 1 r Λ WI wi M u Λ JA Ir Λ u

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rotating carbon anodes and from dispersed particles of the deposited metal.

British Patent No. 1,295 * 117 discloses anodes for molten cryolite baths consisting of a sintered ceramic oxide material consisting essentially of SnO 2 with minor amounts of other metal oxides, namely oxides of Pe, Sb, Cr, Nb, Zn, W, Zr, Ta in concentration up to 20%. Electrically conductive sintered SnO2 with minor additions of other metal oxides, such as oxides of Sb, Bi, Cu, U, Zn, Ta, As, etc., has long been used as durable electrode material in AC glass melting furnaces (see US patents Nos. 2,490,825, 2,490,826, 3,287,284, and 3-502,597), but it exhibits considerable wear and corrosion when used as anode material for electrolysis of molten salts. Man., Have found wear rates up to 0.5 g / l. hour 2 pr. cm from samples of the compositions described in the above patents when used in a molten cryolite electrolyte at

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3000 A / m. The high wear rate of sintered SnO2 electrodes is thought to be due to two serious factors: a) chemical attack of the halogens, in which Sn "actually produces high coordination numbers with halo ions; b) reduction of SnO₂ of aluminum dispersed in the electrolyte; and c) mechanical erosion by anodic gas evolution and salt precipitation within the pores of the material.

Japanese Patent Application No. 112,589 (Publication No. 62,114 of 1975) discloses electrodes having a conductive support of 25 titanium, nickel or copper or an alloy thereof, carbon graphite or other conductive material coated with a layer consisting essentially of spinel and / or metal oxides. of the perovskite type and alternatively electrodes obtained by sintering mixtures of these oxides.

Spin oxides and 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. Patents Nos. 3-711-382 and 3-711-297 and Belgian Patent No. 780,303).

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However, coatings of particulate spinels and / or perovskites have been found to be mechanically weak as the bond between the particulate ceramic coating and the metal substrate or carbon mviilavil ¢ 31.0 + -i οίη o \ i on cnroir -P / norl -i Ti-oiro + cil o + riiiVfiivon α · Ρ eminoll avno

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and of the perovskites not isomorphic with the oxides of the metal support and various binders such as oxides, carbides, nitrides and borides have been tried 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 production of metals of molten halide salts, said anodes in the prior art have been found to have other disadvantages. The significant dissolution of the ceramic oxide material 15 dissolves metal cations deposited 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 an electrolytic purity degree. In that case, the economic benefits of the electrolytic process, which is largely due to the high purity attainable, compared to the melting processes, are lost in whole or in part.

An electrode material which can be used with good results under severely corrosive conditions, such as by electrolysis of molten halide salts and especially of molten fluoride salts, must first of all be chemically and electrochemically stable under the operating conditions.

It must also be catalytic with respect 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, i.e. ca. 200 to 1100 ° C, good electrical conductivity and sufficiently resistant to accidental contact with the molten metal cathode. Excluding coated metal electrodes, since hardly any metal substrate could withstand the extremely corrosive conditions encountered by electrolysis in molten fluoride salt, the properties of a very large number of sintered ceramic electrodes of various compositions have been systematically tested.

It has been found that very efficient insoluble electrodes are produced by sintering of yttria and at least one electroconductive agent.

DK 155529B

5 to a self-supporting body and providing at least one surface thereof with at least one electrocatalyst.

This is achieved with an electrode which according to the invention is characterized in that it comprises a self-supporting body of sintered powders of 5 99-60 wt% yttrium oxide and 1-40 wt% of at least one electroconductive agent selected from chromium, molybdenum, tantalum, tungsten , cobalt, nickel, palladium and silver, which electrode is provided on at least a portion of its surface with a layer of at least one electrocatalyst in the form of a metal or metal oxide.

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The sintered yttrium electrodes of the invention are particularly useful for electro-recovery processes used to prepare various metals, such as aluminum, magnesium, sodium, potassium, calcium, lithium, and other metals of molten salts. Yttrium oxide and at least one electroconductive agent, when used as anode in direct electrolysis by molten salt electrolytes, have been found to be unusually stable as an inert dimensionally stable anode with sufficient electrical conductivity and provided on the surface by oxide electrocatalysts such as CO ^, Ni- ^ O ^, MnC ^, Rh ^ O ^, 20 IrC ^ »RuC> 2> AgzO, etc., it has a high electrocatalytic effect, especially for chlorine development.

Thus, the "sintered" yttria electrode is a self-supporting, substantially rigid body consisting primarily of 25 yttria, and at least one electroconductive agent produced by one of the known methods used in the ceramic industry, such as by applying temperature and printing on a powdered mixture of yttrium oxide and other materials to mold the mixture to the desired size and shape or by molding the material into molds by pressing or using binders, etc., and then sintering the molded body by high temperature for a self-supporting electrode.

By mixing the powder of the matrix material with a smaller amount, 35

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typically from 0.5 to approx. 30%, powders of a suitable electrocatalytic material and by sintering the mixture to a self-supporting body, · when used as an electrode, it exhibits satisfactory electrically conductive and electrocatalytic properties which retain its chemical stability, although the mixed catalyst does not as such would be resistant to electrolysis conditions.

The catalyst may be a metal or metal oxide ·.

The preferred mixed catalyst powders are the powdered meta! 10 clays Ru, Rh, Pd, Ir, Pt, Fe, Co, Ni, Cu and Ag, especially plate group meta! the clay, powdered oxy compounds of Mn, Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt, Ag, As, Sb and Bi, and especially oxy compounds of platinum groups.

Particularly preferred are βΜη02, Co₂O₂, Rh₂O₂, IrO₂, RuO₂, Ag₂O, Ag₂O₂, Ag₂O ^, As₂O₂, Sb and mixtures of said powdered metals and oxy compounds.

It has been found particularly advantageous for the yttrium oxide to add a material such as stannous oxide, zirconium oxide or the like, and that upon addition also of a small amount of at least one metal belonging to the group comprising yttrium, chromium, molybdenum, zircon, tantalum, wool fram, cobalt, nickel, palladium and silver, both the mechanical properties and electrical conductivity of the sintered yttrium oxide 25 electrodes are improved without significantly reducing their chemical and electrochemical corrosion resistance.

These additives are added in powder form and mixed with the powdered yttrium oxide in percentages ranging from 40 to 1%, calculate 30 by weight% of the metal content. Optionally, other organic and / or inorganic compounds may also be added to the powder mixture to improve the bonding of the particles, both during forming and sintering.

Anodes containing a greater amount of Y₂O₂-k © high melting point, which are slightly above the temperature of the molten salt electrolytes used, and undergo no phase change under the operating conditions of the electrolysis. Furthermore, the heat elongation coefficient is not very different from the heat elongation coefficient of the halide salts used in the molten metal baths, which helps maintain the proper electrode gap between the anode and cathode and

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on top of the molten salt electrolyte in the normal aluminum electro extraction process.

The conductivity of the sintered yttrium oxide electrodes of the invention can be compared to the conductivity of graphite. The matrix has acceptable processability in molding and sintering and, upon use, forms a thin layer of oxyhalides on its surface under anodic conditions. The free formation energy for yttria is more negative than the free formation energy for the oxide of the car-like molten salt electrolyte in halide phase, so that these sintered yttria = 10 anodes have a high degree of chemical stability.

The sintered yttrium oxide electrodes of the invention can also be used as dipolar electrodes. In this latter embodiment, the sintered yttrium oxide electrodes can be conveniently prepared 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 CO₂, O₂, Ni₂O₂, MhO₂, Rh₂O₂, IrO₂, RuO₂, Ag₂ osv, etc., and the other larger surface are provided with a layer containing suitable cathodic materials such as carbides, borides, nitrides, sulfides, carbonitrides, etc. of 20 metals, especially of the base metals and most conveniently of yttrium, titanium and zirconium.

The self-supporting sintered body consisting of a greater amount of yttrium oxide can be prepared by grinding the materials together or separately, preferably to a grain size between 50 and 500 µm to form a powder mixture containing a grain size range to obtain 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 particular molding method used. The materials may be formed in known manner, either by stamping or pressing the mixture in a mold or by licking molds in a plaster mold, or the material may be extruded through a nozzle to various shapes.

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The molten electrodes are then subjected to drying and heated to a temperature at which the desired bonding can take place, usually between 800 and 1800 ° C for a period of between 1 and 30 hours, usually followed by slow cooling to room temperature. The heat treatment

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reducing, e.g. (80%) when the powdered mixture is composed essentially of yttrium oxide with a minor amount of other metals or oxides.

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

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

The electrocatalyst, which is usually applied to the electrode surface due to cost, must have a high stability, a 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, rho = dim, ruthenium or mixed oxides thereof, such as RuC2 - TiC2, etc., may be used, and in the case of fluoride-containing electrolytes in which oxygen = evolution is the desired anodic reaction is preferred to oxides of silver and manganese. Other oxides for use as electrocatalysts may be oxides of platinum, palladium and lead.

Poisons for suppressing undesirable anodic reaction may be used, e.g. to suppress oxygen evolution of chloride electrolytes. Poisons presenting a high oxygen surge must be used and suitable materials are the oxides of arsenic, antimony and bismuth. These oxides are used in small percentages and can be applied together with the electrocatalyst oxides in percentages of 1 to 10% of the electrocatalyst, calculated on the respective metal weights.

DK 155529 B

9 can be carried out by any known coating method. Preferably, the electrocatalyst and optionally the poisoning agent is applied to the sintered yttrium electrodes as a solution of decomposable salts of the metals. The sintered yttria body is impregnated with solution containing the desired metal salts and dried. Then the electrode is heated in air or in an oxygen-containing atmosphere to convert the salts to the desired oxides.

As a rule, the porosity of the sintered yttria body and the method used to impregnate the surface layers of the sintered body with the metal salts must penetrate the solution ne 10 to a depth of at least 1 to 5 mm, preferably 3 mm, inward from the surface of the electrode. so that, after the heat treatment, the electrocatalysts are present in the pores of the sintered yttria down to a certain depth inward from the surface of the electrodes.

Alternatively, by suitable powder blending technique, pre-formed electrocatalyst oxides and any pre-formed poisoning oxide may be ground into powder form and added to the powder mixture during molding of the electrodes in such a way that the outer layers of the formed electrodes are enriched with powders of the electrocatalyst oxides and even 2x the formation process whereby the surface of the electrodes after sintering is already provided with the electrocatalyst.

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 yttria body with a flame under a controlled atmosphere. Alternatively, the selected powder material may be added during the molding process to the yttria powder mixture and then sintered together, providing the cathodic surface of the bipolar electrode with a layer of the selected cathodic material.

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DK 155529 B

The sintered yttria activated by suitable electrocatalyst can be used as an unusable electrode in the electrolysis of molten salts and in other processes by which an electric current is passed through an electrolyte for the purpose of decomposing the electrolyte to carry out oxidation and reductions of organic and inorganic compounds. or to apply a cathode voltage to a metal structure to be protected against corrosion, as well as to primary and secondary batteries containing molten salts, such as aluminum halides-alkali metal halides. The electrodes of the invention can be polarized as anodes or cathodes and can be used as bipolar electrodes, one face or end of the electrode acting as anode, and the opposite surface or end of the electrode acting as a cathode to the electrolyte touching each side of the electrode. the electrode, as is known in the art of electrolysis.

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

Example 1.

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A block of sintered Y2 O3: Pd metal in a ratio of 9: 1 by weight calculated as free metals was activated by impregnating the sintered sample with an aqueous solution of Co Cl3 followed by drying and heating in air to 300 to 650 ° C to convert The circuit was repeated to form a final coating of the electrodes of 15 g / m COgO 2 anode surface. The activated anode was used for electrolysis of molten AlClg +

NaCl electrolyte and the anode potential and degree of wear are listed below.

2- - 2-

Anode potential Wear g / m

Sample Y (S.C.G.E.) __ after ____ Immediately After 100 hours _100 hours_ -y203-Pd. 0.0 0.0 - 0.5 • * 35

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Example 2.

A block of sintered YgO-, ° S PcL metal in weight ratio 7: 2.5: 0.5 calculated as free metals was impregnated on one of its large 5 surfaces with 15 g / m surface of Co in Example 8. The opposing large surface was coated with a 1 mm thick layer of zirconium diboride applied by flame spraying under nitrogen atmosphere. The block was placed between two graphite counter electrodes in electrically conductive relationship with them and at a distance therefrom. The electrode gaps were filled with molten AlCl 2 ^ O ^ activated surface of the sintered bipolar electrode, was connected to the negative pole of the DC source. The sintered electrode acted as a bipolar electrode, and molten aluminum metal sprayed down the zirconium diboride-coated surface and was recovered at the bottom thereof while chlorine was developed on the co-activated surface of the electrode. The electrolysis process was carried out satisfactorily for a period of 28 hours, at which time the search cell, made mainly with graphite, failed. After this operating time, the bipolar electrode showed no signs of deterioration and no wear was detected.

Other electrocatalysts which can be used for electrolysis of molten halide salts to expel halide ion are RuC 2, and oxide such as ° S Bi-2 3 can be added in percentages up to 10% by weight of the free metal, calculated on the total metal content to increase the oxygen overpotential. In addition to affecting the potential for the release of halide ion.

For anodes to be used in molten fluoride electrolytes where oxygen is generated, the catalyst may be Ag 2, 0x, (x> 1), IrO 2,

MnO2 or RHgOg.'PbOg and IrO2> TiO2.

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

λ o TPT dlr + n / iT τ; Η- + αη Iran -i n ri olo ηΊ Λ δ s aaT + .o enrl ri o rldri or1 nmmniH · i

DK 155529 B

12 examples, such as alkali metal chlorides or fluoride and the salt of the metal undergoing electrolysis. The metal halides are effective in reducing the melting point of the salt, which undergoes electrolysis and thus enable the use of lower temperatures while maintaining the salt bath in the molten state.

The above examples include electrolysis of molten metal salt, primarily electrolysis of molten aluminum chloride or fluoride salt. Similarly, the molten chlorides of other metals, such as alkali metals or alkaline earth metals, can be electrolysed using the aforementioned anodes in accordance with standard practice. Furthermore, other molten salts, such as molten nitrates, can be electrolysed in the same way. A molten electrolyte of alumina and cryolite or similar alkali metal aluminum fluoride can be electrolysed to produce molten aluminum.

These electrodes can be used instead of graphite anodes in standard aluminum electrowinning cells with either aluminum ore added to a cryolite bath or with aluminum chloride applied to a bath of predominantly aluminum chloride.

The use of these sintered yttria anodes to recover the desired metals of molten salts of the metals to be extracted results in reduced power consumption per weight unit made of metal and in recovered metals. The electrodes are dimensionally stable during operation and therefore do not require frequent interventions to restore the optimum distance from the cathode surface, as is necessary with usable anodes in the prior art. 1 10 35 ...

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

Claims (8)

A molten salt electrolysis electrode, characterized in that it comprises a self-supporting body of sintered powders of 99-60 wt% yttrium oxide and 1-40 wt% of at least one electroconductive agent selected from chromium, molybdenum, tantalum, tungsten, cobalt, nickel, palladium and silver, which electrode is provided with at least a portion of its surface with a layer of at least one metal or metal oxide catalyst. Electrode according to claim 1, characterized in that the electrocatalyst is selected from oxides of cobalt, nickel, manganese, rhodium, iridium, ruthenium and silver · Electrode according to claim 1 or 2, characterized in that the electrocatalyst is formed in situ on the sintered electrode body of a solution of salts of metals which 2o are converted to oxides on the sintered electrode body. Electrode according to claim 1 or 2, characterized in that the electrocatalyst comprises powdered oxides of the metals sintered into the outer layers of the electrode. 25 Bipolar electrode according to claims 1-4, characterized in that the electrode is provided with at least part of its anodic surface with at least one anodic electrocatalyst selected from oxides of cobalt, nickel, manganese, rhodium, 3 iridum, ruthenium , silver and mixtures thereof and at least part of its cathodic surface with a layer of cathodic material selected from metal carbides, borides, nitrides, sulfides and carbonitrides and mixtures thereof. DK 155529 B Bipolar electrode according to claim 5, characterized in that the layer of the cathodic material is applied by flame spraying. Bipolar electrode according to claim 5, characterized in that the layer of the cathodic material comprises powders of the cathodic material sintered into the outer cathodic surfaces of the electrode body. Bipolar electrode according to claim 5, characterized in that the cathodic material is selected from the group comprising carbides, borides, nitrides, sulfides and carbonitrides of at least one metal selected from the group comprising yttrium, titanium and zircon. 15 20 25 1 35