CA1124210A - Sintered electrodes with electrocatalytic coating - Google Patents
Sintered electrodes with electrocatalytic coatingInfo
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- CA1124210A CA1124210A CA275,175A CA275175A CA1124210A CA 1124210 A CA1124210 A CA 1124210A CA 275175 A CA275175 A CA 275175A CA 1124210 A CA1124210 A CA 1124210A
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
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Abstract
ABSTRACT OF THE DISCLOSURE
Sintered electrodes for electrolytic processes com-prising a self sustaining body or matrix of sintered powders of an oxycompound of at least one metal selected from the group consisting of titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminum, silicon, tin, chromium, molybdenum, tungsten, lead, manganese, beryllium, iron, cobalt, nickel, platinum, palladium, osmium, iridium, rhenium, technetium, rhodium, ruthenium, gold, silver, cadmium, copper, zinc, germanium, arsenic, antimony, bismuth, boron, scandium and metals of the lanthanide and actinide series and at least one electroconductive agent, the said electrodes being provided over at least a portion of their surface with at least one electrocatalyst for electrolysis reaction and bipolar elect-rodes, electrolytic cells containing said electrodes and elect-rolytic processes using the said electrodes as anodes and/or electrodes. Oxycompounds include oxides, multiple oxides, mixed oxides, oxyhalides and oxycarbides and mixtures thereof.
Sintered electrodes for electrolytic processes com-prising a self sustaining body or matrix of sintered powders of an oxycompound of at least one metal selected from the group consisting of titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminum, silicon, tin, chromium, molybdenum, tungsten, lead, manganese, beryllium, iron, cobalt, nickel, platinum, palladium, osmium, iridium, rhenium, technetium, rhodium, ruthenium, gold, silver, cadmium, copper, zinc, germanium, arsenic, antimony, bismuth, boron, scandium and metals of the lanthanide and actinide series and at least one electroconductive agent, the said electrodes being provided over at least a portion of their surface with at least one electrocatalyst for electrolysis reaction and bipolar elect-rodes, electrolytic cells containing said electrodes and elect-rolytic processes using the said electrodes as anodes and/or electrodes. Oxycompounds include oxides, multiple oxides, mixed oxides, oxyhalides and oxycarbides and mixtures thereof.
Description
~..2~210 ST~TE OF THE AI~T
Dimensionally stable electrodes for anodic and cathodic xeactions in electrolysis cells have recently become of general use in the electrochemical industry replacing the consumable electrodes of carbon, graphite and lead alloys.
They are particularly useful in flowiny mercury cathode cells and in diaphragm cells for the production of chlorine and caustic, in mctal electrowinnin~ cells wherein pure metal is recovered from aqueous chloride or sulfate solution as well as for the cathodic protection of ships' hulls and other metal structures.
Dimensionally stable electrodes generally comprise a valve metal base such as mi, Ta, Zr, Hf, Nb and W, which - under anodic polarization develop a corrosion-resistant but non-electrically conductive oxide layer or "barrier layer", coated over at least a portion of their outer surface with an electrically conductive and electrocatalytic layer of platinum group metal oxides or platinum group metals (see U.S. Patents No. 3,711,385, No. 3,632,498 and No. 3, 846,273). Electro-conductive and electrocatalytic coatings made of or containing platinum group metals or platinum group metal oxides are, however, expensive and are eventually subjected to consump-tion or deactivation in certain electrolytic processes and, therefore, reactivation or recoating is necessary to reactivate exhausted electrodes.
Furthermore, electrodes of this type are not operable in a number of electrolytic processes. For example, in mab/
1~.2~Z~o molten salt electrolytes, the valve metal support is rapidly dissolved, sillce the thin protective oxide layer is either not formed at all or is rapidly destroyed by the molten electrolyte with the consequent dissolution of the valve metal base and loss of the catalytic noble metal coating. Moreover, in several aqueous electrolytes, such as fluoride solutions or in sea-water, the breakdown voltage of the protective oxide layer on the exposed valve metal base is too low and the valve metal base is often corroded under anodic polarization.
10Recently, other types of electrodes have been sug-gested to replace the rapidly consumed carbon anodes and car-bon cathodes used up to now in severely corrosive applications such as the electrolysis of molten metal salts, typically for - the electrolysis of molten fluoride baths such as those used to produce aluminum from molten cryolite. In this particular electrolytic process which is of great economic importance, carbon anodes are consumed at a rate of approximately 450 to 500 kg of carbon per ton of aluminum produced and expensive constant adjustment apparatus is needed to maintain a small and uniform gap between the corroding anode surfaces and the -liquid aluminum cathode. It is estimated that over 6 million tons of carbon anodes are consumed in one year by aluminum producers. The carbon anodes are burned away according to the reaction:
A1203 + 3/2 C ~ 2Al + 3/2 C02 but the actual consumption rate is much higher due to fragi-lization and brcaking away of carbon particles and to inter-.
mab/
~.2~210 mittent sparking which takes place across anodic gas films which often- form over areas of the anode surface since carbon - is poorly wetted by the molten salts electrolytes, or to short circuiting caused by "bridges" of conductive particles coming from the corroding carbon anodes and from dispersed particles of the depositing metal.
British Patent No. 1,295,117 discloses anodes for molten cryolite baths consisting of a sintered ceramic oxide material consisting substantially of SnO2 with minor amounts of other metal oxides, namely, oxides of Fe, Sb, Cr, Nb, Zn, W, Zr, Ta in concentration of up to 20%. While electrically conducting sintered SnO2 with minor additions of other metal oxides, such as oxides of Sb, Bi, Cu, U, Zn, Ta, As, etc., has been used for a long time as a durable electrode material in alternating current glass smelting furnaces (see U.S.
Patents No. 2,490,825, No. 2,490,826, No. 3,287,284 and No. 3,502,597), it shows considerable wear and corrosion when used as an anode material in the electrolysis of molten salts.
We have found wear rates of up to 0.5 grams per hour per cm2 from samples of the compositions described in the patents mentioned above when operated in fused cryolite electrolyte at 3000 A/m2. The high wear rate of sintered SnO2 electrodes is thought to be due to several factors: a) chemical attack by the halogens, in fact SnIV gives complexes of high co-ordination numbers with halogen ions; b) reduction of SnO2 by aluminum dispersed in the electrolyte; and c) mechanical erosion by anodic gas evolution and salt precipitation within the pores of the material.
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Japancse Patent Pub]ications No. 62,11~ of 1975 (U.S.
Patent No. ~,039,~01) discloses electrodcs having a conductive support of titanium, nickel or copper or an alloy thereof, carbon graphit~ or other conductive material coated with a layer consisting substantially of spinel and/or perovskite type metal oxides and alternatively electrodes obtained by sintering mixtures of said oxides. Spinel oxides and perovskite oxides belong to a family of metai oxides which typically show good electronic conductivity and have been proposed previously as 10 suitable electroconductive and electrocatalytic anodic coat-ing materials for dimensionally stable valve-metal anodes tsee U.S. Patents No. 3,711,382 and No. 3,711,297; Belgian Patent No. 780,303).
- Coatings of particulate spinels and/or perovskites have been found, however, to be mechanically weak as the bond-ing bPtween the particulate ceramic coating and the metal or carbon substrat:e is inherently weak, because the crystal structures of the spinels and of the perovskites are not iso-morphous with the oxides of the metal support and various 20 binding agents such as oxides, carbides, nitrides and borides have been tried with little or no improvement. In molten salt electrolytes, the substrate material is rapidly attacked due to the inevitable pores through the spinel oxide coating and the coating is quickly spalled off the corroding substrate.
Furthermore, spinels and perovskites are not chemically or electrochemically stable in molten halide salt electrolytes and show an appreciable wear rate due to halide ion attack and to the reducing action of dispersed metal.
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~.2~Z10 In the electrolytic production of metals from molten halide salts, ~he mentioned anodes of the prior art have been found to have another disadvantage. The appreciable dissolu-tion of the ce;ramic oxide material brings metal cations into the solution which deposit on the cathode together with the metal which is being produced and the impurity content in the recovered metal is so high that the metal can no longer be used for applications requiring electrolytic grade purity.
In such cases, the economic advantages of the electrolytic pxocess which are due, to a large extent, to the high purity attainable compared to the smelting processes are partially or entirely lost.
An electrode material to be used successfully in severely corrosive conditions such as in the electrolysis of molten halide salts and particularly of molten fluoride salts, should primarily be chemically and electrochemically stable at the operating conditions. It should also be catalytic with respect to the anodic evolution of oxygen and/or halides, so that the anode overpotential is lowest for high overall ef-ficiency of the electrolysis process. The electrode shouldalso have the thermal stability at operating temperatures of, i.e., about 200 to 1100C, good electrical conductivity and be sufficiently resistant to accidental contact with the molten metal cathode. Excluding coated metal electrodes, since hardly any metal substrate could resist the extremely cor-rosive conditions found in molten fluoride salt electrolysis, we have systematically tested the performances of a very large number of sintered substantially ceramic electrodes of dif-ferent compositions.
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~.24210 _BJECTS OF TIIE INV}~NTION
It is an object of the invention to provide novel s.intered electrodes comprisi.ng an oxymetal matrix containincJ an electroconducti.ve acJent and provided ovcr at least a part of its surface with an electrocatalyst.
It is another object of the invention to provide novel electrolytic cells wherein the anode is comprised of an oxy-metal matrix containing an electroconductive agent and is provided over at least a portlon of its surface with an electrocatalyst.
It is a further ob~ect of the invention to provide novel bipolar electrodes as well as novel electrolytic processes.
These and other objects and advantages of the invention will become obvious from the following detailed description.
THE INVENTION
The novel electrodes of the invention are comprised of a self-sustaining matrix of sintered powders of an oxycompound of at least one metal selected from the group consisting of titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminum, silicon,-tin,-chromium, molybdenum, tungsten, lead, manganese, beryllium, iron, cobalt, nickel, platinum, palladium, osmium, iridium, rhenium, technetium, rhodium, ruthenium, gold, silver, cadmium, copper, zinc, germanium, arsenic, antimony, bismuth, boron, scandium and metals of the lanthanide and actinide series and at least one electro-conductive agent, the said electrodes being provided over at least a portion of their surface with at least one electrocatalyst.
Preferred metals of lanthanide and actinide series are lanth~num, D
.
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~.242~0 terbium, erbium, thorium and ytterb.ium.
The "sintered" electrode is meant to describe a self-sustaining~ essentially rigid body consisting principally of an oxymetal compound and at least one electroconductive agent and produced by any of the known methods used in the ceramic industry such as. by the application of temperature and pressur:e to a powdered mixture of the materials to shape the mixture to the desired size and shape, or by casting the material in molds, by extrusion, or by the use of bonding agents and so forth? and then sintering the shaped body at high tempe.rature. into a self-sustaining ele.ctrode..
The preferred sintered ceramic electrodes are the compositions containing ~0.1 to 2Q% by weight of at least one electroconductive agent selected from the group consisting of (A) doping oxides, typically of metals having a valence which is lower or 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, In, Tl, As, Sb, Bi, Zr and Sn; (B.) oxides s:howing electroconductivity: dle to intrinsic Redox syste.m such as spinel oxides, perovskite oxides etc.; (C) oxides showing electroconducti.vity due to metal to metal bonds such as CrO2, MnO2, TiO~, Ti2Q3, etc.; (D) borides, silicides, carhides and sulfides of the valve metals such as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W; (E) the metals Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Pd and Ag or alloys th.ereof, and (F) mixtures of any of the. above.
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By admixing with the powder of the matrix material, a minor amount, typically from 0.5 to about 30%, of powders of a suitable electro-catalytic material and by sintering the mixture into a self-sustaining body, one obtains an elec-trode with satisfactory electroconductive and electrocatalytic properties which retains its chemical stability even though the admixed catalyst would not be resistant per se to the conditions of the electrolysis.
The electrocatalyst may be a metal or an inorganic oxycompound. The preferred admixed catalyst powders are the powdered metals Ru, Rh, Pd, Ir, Pt, Fe, Co, Ni, Cu and Ag, especially the platinum group metalsi powdered oxy-compounds of Mn, Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt, Ag, As, Sb, Pb and Bi and especially oxycompounds of the platinum group metals.
Specifically preferred are ~MnO2, Co304, Rh203, IrO2, RhO2, Ag20, Ag O Ag o As 03~ Sb203, Bi203~ CoMn204, NiMn2 4, 2 4 2 4 and mixtures of said powdered metals and oxycompounds.
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It has been found to be especially advantageous to add to the oxymetal compound a second material such as stannous oxide, zirconium oxide or the like and that also by adding a small amount of at least one metal belonging to the group comprising yttrium, chromium, molybdenum, zirconium, tantalum, tungsten, cobalt, nickel, palladium, and silver, both the mechanical properties and the electrical conductivity of the sintered electrodes are improved without appreciably decreas-ing thei.r 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 in terms of weight of the metal content. Optionally, yet other organic and/or inorganic com-pounds may be added to the powder mixture to improve on the bondin~ of the particles during both the moulding and sinter-ing processes.
The anodes have a high melting point well above the temperature of the molten salt electrolytes being used and they undergo no phase change under working conditions of the electrolysis. Moreover, the thermal elongation co-efficient is not far different from that of the halide salts used in the molten salt baths, which helps preserve the proper elec-trode spacing between the anode and the cathode and avoids expansions and contractions which might break the salt crust on the top of the molten salt electrolyte il~ the normal aluminum electrowinning process.
The conductivity of the sintered electrodes of the invention is comparable with that of graphite. The matrix mab/
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' has acccptable work-ability in the forming and sintering operation and in use orms a thin layer of oxyhalides on its surface under anodic conditions. The metal oxycompound free formation energy is more negative than the oxide free forma-tion energy of the corresponding halide-phase fused salt e]ect-rolyte, so that these sintered anodes have a high degree of chemically stability.
The sintered metal oxycompounAd electrodes of the invention may also be used as bipolar el~ctrodes. According to this latter embodiment, the sintered electrodes may be conveniently produced in the foxm of a slab or plate whereby one of the two major surfaces of the electrode is provided with a layer containing the anodic electrocatalyst such as the oxides C0304, Ni304~ Mn2' ~h203, Ir2' 2' 2 and the other major surface is provided with a layer contain-ing suitable cathodic materials such as carbides, borides, nitrides, sulfides, carbonitrides etc. of me~als, particularly of the valve meta]s and most preferably of y~trium, titanium and zirconium.
The self-sustaining sintered body consisting of a majox portion of oxymetal compound may be prepared by grind-ing the materials together, or separately, preferably to a grain size between 50 and 500 microns, to provide a powder mixture which contains a range of grain sizes to obtain a better de~ree of compaction. According to one of the pre-ferred methods, the mixture of powders is mixed with water or with an oraanic binding agent to obtain a plastic mass having suitable flowing properties for the particular forming mab/
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~.242~0 process used, The material may be molded in known manner eithcr by ramming or pressing the mixture in a mold or by slip-casting in a plaster of Paris mold or the material may be extruded through a die into various shapes.
The molded electrodes are then subjected to a drying process and heated at a temperature at which the desired bonding can take place, usually between 800 to 1800C for a period of between 1 to 30 hours, normally followed by slow cooling to room temperature. The heat treatment is prefer-ably carried out in an inert atmosphere or one that isslightly reducing, for example in H2 + N2 (80~), when the powdered mixture is composed essentially of oxymetal com-pound with a minor portion of other metal oxides or metals.
When the powdered mixture contains also metallic powders, it is preferable to carry out the heat treatment in an oxidizing atmosphere, at least for a portion of the heat treatment cycle to promote the oxidation of metallic particles in the outside layers of the electrodes. The metallic particles remaining inside the body of the sintered material improve the electrical conductivity properties of the elec-trode.
The forming process may be fol]owed by the sintering process at a high temperature as mentioned above or the form-ing process and the sintering process may be simultaneous, that is, pressure and temperature may be applied simultane-ously to the powder mixture, for example by m~ans of elect-rically-heated molds. Lead-in connectors may ~e fused into the ceramic electrodes during the molding and sintering pro-mab/
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cess or attached to the electrodes after slntering or molding. Other methods of shaping, compressing and slntering the powder mixture may of course be used.
The electrocatalyst, usually applied to the electrode surface rather than dispersed throughout the electrode due to costs, should have a high stability, a low anodic overpotential for the wanted anodic reaction, and a high anodic overpotential for non-wanted reactions. In the case of chlorine evolution, oxides of cobalt, nickel, iridium, rhodium, ruthenium or mixed oxides thereof such as Ru02 - Ti02 etc. can be used, and in the case of fluoride containing electrolytes wherein oxygen evolution is the wanted anodic reaction, oxides of silver and manganese are preferable. Other oxides for use as electrocatalysts may be oxides of platinum, palladium and lead.
Poisons for the suppression of unwanted anodic reactions may be used, such as, for example, to suppress oxygen evolution from chloride electrolytes.
Poisons which present a high oxygen overpotential should be used and suitable materials are the oxides of arsenic, antimony and bismuth. These oxides which are used in small percentages may be applied together with the electro-catalyst oxides in percentage of 1 to 10% of the electrocatalyst calculated in terms of the respective metals weight.
The application of the electrocatalyst, and optionally of the poisoning agent may be effected by any of known coating methods. Preferably the electro-catalyst, and optionally the poisoning agent, are applied to the sintered electrodes as a solution of decompasable salts of the metals.
~.Z42~Lai Thc sintered body is impregnated wlth the solution containing the appropriate metal salts and dried. I~ence the electrode is heated in air or in otherwise oxygen containing atmosphere to convert the salts into the wanted oxides.
Vsually the porosity of the sintered body and the method used to impregnate the surface layers of the sintered body with the metal salts should provide for the penetration of the solution down to a depth of at least 1 to 5 millimeters, preferably 3 mm, inward from the surface of the electrode so that after the heat treatment the electrocatalysts are pre-sent in the pores of the sintered body down to a certain depth inward from the surface of the electrodes.
Alternatively, by appropriate powder mixing techniques, preformed electrocatalyst oxides and optionally preformed poisoning oxides, may be ground into powder form and added to the powder mixture during the moulding of the electrodes in such a way that the external layers of the moulded electrodes are enriched with powders of the electrocatalyst oxides, and optionally of the poisoning oxides, during the forming process whereby after sintering the surface of the electrodes will already have been provided with the electrocatalyst.
The sintered electrodes of the invention may be used as bipolar electrodes. According to this embodiment of the invention, electrodes may be provided over one surface with the anodic electrocatalyst, and optionally with the poisoning agent for the unwanted anodic reaction by one of the methods disclosed above while the other surface may be provided with a coating of suitable cathodic material. For example, the mab/
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surface of t:he }:~:ipo:lar electrode which wl].J fuoclion as a ca~hoc1e during the process of electrolysis may be p:rovi.dc~d wil:ll a layer of metal carhi.des, l~orides, nitrides, sulfides and/or carboni.lxides o~ yttrium, tantalum, titanium, zi~conium, e1c.
One preferred method to apply a layer is by plasma~jet techni.que whereby powders of the selected materi.als are sprayed and adhere to the surface of the sintered body with a flame under controlled atmosphere. Alternatively, the selected powdered material may be added during the forming process to the powder mixture and the mixture then be sintered whereby the cathodic surface of the bipolar electrode is provided with a layer of the selected cathodic material.
Tlle electrodes may be used effectively for the electrolysis of many electrolytes. They are especially advant-ageous when used as anodes in electrolytic cells used for electro-lyzing molten salt electrolytes such as molten cryolite baths, molten halides of aluminum, magnesium, sodium, potassium, calcium, lithium and other metals. Thus, aluminum halides may he electro-lyzed according to the Hall process or processes disclosed in U.S. Patents No. 3,464,900, 3,518,712 or 3,755,099 using the electrodes herei.n described as anodes. The temperature of electrolysis is high enough to melt an(1 maintain the salts of the metal to be recovered in a molten state and the metal is deposited in the rnolten state and usually collected as a molten cathode Wit]
molten metal being withdrawn from the molten cathode.
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~Z42~0 -The electrodes may also bc used effectively as anodes and/or cathodes in direct current electrolysis of other molten salt electrolytes typically containing halides, oxides, carbonates or hydrates for the production of aluminum, beryllium, calcium, cerium, lithium, sodium, magnesium, potassium, barium, strontium, cesium and other metals.
When the electrodes of the invention are used as bipolar electrodes for molten salt electrolysis, the com-position of the cathode portion of the elec~rodes must be such that it will not be reduccd by the cathodic reaction or attacked by the metal being deposited at the cathodes, particularly when the electrode composition is an oxycompound.
For this reason, it is desirable to have the composition of the cathode side of the bipolar electrode irlert to the cath-odic reaction and the reducing action of the molten metal.
The electrodes may also be used as anodes and/or as cathodes in electrochemical processes such as: the electro-lysis of aqueous chloride solutions for the production of chlorine, caustic, hydrogen, hypochlorite, chlorates and perchlorates; the electrowinning of metals from aqueous sul-fate or chloride solutions for the production of copper, zinc, nickel, cobalt and other metals; the electrolysis of molten metal salt electrolytes typically containin~ halides, oxides, carbonates or hydrates for the production o~ aluminum, beryl-lium, calcium, cerium, lithium, sodium, mag~esium, potassium, ~arium, strontium, cesium and other metals and the electro-lysis of bromides, sulfides, sulfuric acid, ~ydrochloric acid and hydrofluoric acid. In general, the electrodes are useful for all electrolytic processes.
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~24Z10 The novcl electrolytic cell of the inv~ntion is com-prised of at least onc- anode and at least one cathode and means for imposing a direct electric current between the anode and cathode, the improvement residing in the anode being comprised of a self-sustaining matrix of sintered powders of an oxycompound of at least one metal selected from the group consisting of titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminum, silicon, tin, chromium, molybdenum, tungsten, lead, manganese, beryllium, iron, cobalt, nickel, platinum, palladium, osmium, iridium, rhenium, technetium, rhodium, ruthenium, gold, silver, cadmium, copper, zinc, germanium, arsenic, antimony, bismuth, boron, scandium and metals of the lanthanide and actinide series and at least one electroconductive agent, the said electrodes being provided over at least a portion of their surface with at least one electrocatalyst. The cell may also contain bipolar elect-rodes as described above.
The novel electrolysis method of the invention com-prises electrolyzing an electrolyte between an anode and a cathode, the improvement residing in the anode being com-prised of a self-sustaining matrix of sintered powders of an oxycompound of at least one metal selected from the group consisting of titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminum, silicon, tin, chromium, molybdenum, tungsten, lead, manganese, beryllium, iron, cobalt, nickel, platinum, palladium, osmium, iridium, rhenium, technetium, rhodium, ruthenium, gold, silver, cadmium, copper, zinc, mab/
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:~.Z4210 germanium, arsenic, antimony, bismuth, boron, scandium and -- metals of the lanthanide and actinide series and at least one electroconductive agent, the said electrodes being pro-vidcd over at least a portion of their surface with at least one electroca~alyst.
In t:he following examples there are described several preferred embodiments to illustrate the invention.
~owever, it should be understood that the invention is not in-tended to be limited to the specific embodiments. The per-centages of the components of the electrodes are calculated in percent by weight and calculated as free metal based on the total metal content of the composition.
Among the preferred anodes are those wherein the major portion of the self-sustaining body is tin dioxide alone or with up to 20% by weight of cobalt oxide provided with a coating of cobalt oxide which give electrodes of improved mechanical properties and electrocatalytic properties for chlorine evolution. Other preferred additives are Y203, Ti02
Dimensionally stable electrodes for anodic and cathodic xeactions in electrolysis cells have recently become of general use in the electrochemical industry replacing the consumable electrodes of carbon, graphite and lead alloys.
They are particularly useful in flowiny mercury cathode cells and in diaphragm cells for the production of chlorine and caustic, in mctal electrowinnin~ cells wherein pure metal is recovered from aqueous chloride or sulfate solution as well as for the cathodic protection of ships' hulls and other metal structures.
Dimensionally stable electrodes generally comprise a valve metal base such as mi, Ta, Zr, Hf, Nb and W, which - under anodic polarization develop a corrosion-resistant but non-electrically conductive oxide layer or "barrier layer", coated over at least a portion of their outer surface with an electrically conductive and electrocatalytic layer of platinum group metal oxides or platinum group metals (see U.S. Patents No. 3,711,385, No. 3,632,498 and No. 3, 846,273). Electro-conductive and electrocatalytic coatings made of or containing platinum group metals or platinum group metal oxides are, however, expensive and are eventually subjected to consump-tion or deactivation in certain electrolytic processes and, therefore, reactivation or recoating is necessary to reactivate exhausted electrodes.
Furthermore, electrodes of this type are not operable in a number of electrolytic processes. For example, in mab/
1~.2~Z~o molten salt electrolytes, the valve metal support is rapidly dissolved, sillce the thin protective oxide layer is either not formed at all or is rapidly destroyed by the molten electrolyte with the consequent dissolution of the valve metal base and loss of the catalytic noble metal coating. Moreover, in several aqueous electrolytes, such as fluoride solutions or in sea-water, the breakdown voltage of the protective oxide layer on the exposed valve metal base is too low and the valve metal base is often corroded under anodic polarization.
10Recently, other types of electrodes have been sug-gested to replace the rapidly consumed carbon anodes and car-bon cathodes used up to now in severely corrosive applications such as the electrolysis of molten metal salts, typically for - the electrolysis of molten fluoride baths such as those used to produce aluminum from molten cryolite. In this particular electrolytic process which is of great economic importance, carbon anodes are consumed at a rate of approximately 450 to 500 kg of carbon per ton of aluminum produced and expensive constant adjustment apparatus is needed to maintain a small and uniform gap between the corroding anode surfaces and the -liquid aluminum cathode. It is estimated that over 6 million tons of carbon anodes are consumed in one year by aluminum producers. The carbon anodes are burned away according to the reaction:
A1203 + 3/2 C ~ 2Al + 3/2 C02 but the actual consumption rate is much higher due to fragi-lization and brcaking away of carbon particles and to inter-.
mab/
~.2~210 mittent sparking which takes place across anodic gas films which often- form over areas of the anode surface since carbon - is poorly wetted by the molten salts electrolytes, or to short circuiting caused by "bridges" of conductive particles coming from the corroding carbon anodes and from dispersed particles of the depositing metal.
British Patent No. 1,295,117 discloses anodes for molten cryolite baths consisting of a sintered ceramic oxide material consisting substantially of SnO2 with minor amounts of other metal oxides, namely, oxides of Fe, Sb, Cr, Nb, Zn, W, Zr, Ta in concentration of up to 20%. While electrically conducting sintered SnO2 with minor additions of other metal oxides, such as oxides of Sb, Bi, Cu, U, Zn, Ta, As, etc., has been used for a long time as a durable electrode material in alternating current glass smelting furnaces (see U.S.
Patents No. 2,490,825, No. 2,490,826, No. 3,287,284 and No. 3,502,597), it shows considerable wear and corrosion when used as an anode material in the electrolysis of molten salts.
We have found wear rates of up to 0.5 grams per hour per cm2 from samples of the compositions described in the patents mentioned above when operated in fused cryolite electrolyte at 3000 A/m2. The high wear rate of sintered SnO2 electrodes is thought to be due to several factors: a) chemical attack by the halogens, in fact SnIV gives complexes of high co-ordination numbers with halogen ions; b) reduction of SnO2 by aluminum dispersed in the electrolyte; and c) mechanical erosion by anodic gas evolution and salt precipitation within the pores of the material.
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Japancse Patent Pub]ications No. 62,11~ of 1975 (U.S.
Patent No. ~,039,~01) discloses electrodcs having a conductive support of titanium, nickel or copper or an alloy thereof, carbon graphit~ or other conductive material coated with a layer consisting substantially of spinel and/or perovskite type metal oxides and alternatively electrodes obtained by sintering mixtures of said oxides. Spinel oxides and perovskite oxides belong to a family of metai oxides which typically show good electronic conductivity and have been proposed previously as 10 suitable electroconductive and electrocatalytic anodic coat-ing materials for dimensionally stable valve-metal anodes tsee U.S. Patents No. 3,711,382 and No. 3,711,297; Belgian Patent No. 780,303).
- Coatings of particulate spinels and/or perovskites have been found, however, to be mechanically weak as the bond-ing bPtween the particulate ceramic coating and the metal or carbon substrat:e is inherently weak, because the crystal structures of the spinels and of the perovskites are not iso-morphous with the oxides of the metal support and various 20 binding agents such as oxides, carbides, nitrides and borides have been tried with little or no improvement. In molten salt electrolytes, the substrate material is rapidly attacked due to the inevitable pores through the spinel oxide coating and the coating is quickly spalled off the corroding substrate.
Furthermore, spinels and perovskites are not chemically or electrochemically stable in molten halide salt electrolytes and show an appreciable wear rate due to halide ion attack and to the reducing action of dispersed metal.
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~.2~Z10 In the electrolytic production of metals from molten halide salts, ~he mentioned anodes of the prior art have been found to have another disadvantage. The appreciable dissolu-tion of the ce;ramic oxide material brings metal cations into the solution which deposit on the cathode together with the metal which is being produced and the impurity content in the recovered metal is so high that the metal can no longer be used for applications requiring electrolytic grade purity.
In such cases, the economic advantages of the electrolytic pxocess which are due, to a large extent, to the high purity attainable compared to the smelting processes are partially or entirely lost.
An electrode material to be used successfully in severely corrosive conditions such as in the electrolysis of molten halide salts and particularly of molten fluoride salts, should primarily be chemically and electrochemically stable at the operating conditions. It should also be catalytic with respect to the anodic evolution of oxygen and/or halides, so that the anode overpotential is lowest for high overall ef-ficiency of the electrolysis process. The electrode shouldalso have the thermal stability at operating temperatures of, i.e., about 200 to 1100C, good electrical conductivity and be sufficiently resistant to accidental contact with the molten metal cathode. Excluding coated metal electrodes, since hardly any metal substrate could resist the extremely cor-rosive conditions found in molten fluoride salt electrolysis, we have systematically tested the performances of a very large number of sintered substantially ceramic electrodes of dif-ferent compositions.
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~.24210 _BJECTS OF TIIE INV}~NTION
It is an object of the invention to provide novel s.intered electrodes comprisi.ng an oxymetal matrix containincJ an electroconducti.ve acJent and provided ovcr at least a part of its surface with an electrocatalyst.
It is another object of the invention to provide novel electrolytic cells wherein the anode is comprised of an oxy-metal matrix containing an electroconductive agent and is provided over at least a portlon of its surface with an electrocatalyst.
It is a further ob~ect of the invention to provide novel bipolar electrodes as well as novel electrolytic processes.
These and other objects and advantages of the invention will become obvious from the following detailed description.
THE INVENTION
The novel electrodes of the invention are comprised of a self-sustaining matrix of sintered powders of an oxycompound of at least one metal selected from the group consisting of titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminum, silicon,-tin,-chromium, molybdenum, tungsten, lead, manganese, beryllium, iron, cobalt, nickel, platinum, palladium, osmium, iridium, rhenium, technetium, rhodium, ruthenium, gold, silver, cadmium, copper, zinc, germanium, arsenic, antimony, bismuth, boron, scandium and metals of the lanthanide and actinide series and at least one electro-conductive agent, the said electrodes being provided over at least a portion of their surface with at least one electrocatalyst.
Preferred metals of lanthanide and actinide series are lanth~num, D
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~.242~0 terbium, erbium, thorium and ytterb.ium.
The "sintered" electrode is meant to describe a self-sustaining~ essentially rigid body consisting principally of an oxymetal compound and at least one electroconductive agent and produced by any of the known methods used in the ceramic industry such as. by the application of temperature and pressur:e to a powdered mixture of the materials to shape the mixture to the desired size and shape, or by casting the material in molds, by extrusion, or by the use of bonding agents and so forth? and then sintering the shaped body at high tempe.rature. into a self-sustaining ele.ctrode..
The preferred sintered ceramic electrodes are the compositions containing ~0.1 to 2Q% by weight of at least one electroconductive agent selected from the group consisting of (A) doping oxides, typically of metals having a valence which is lower or 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, In, Tl, As, Sb, Bi, Zr and Sn; (B.) oxides s:howing electroconductivity: dle to intrinsic Redox syste.m such as spinel oxides, perovskite oxides etc.; (C) oxides showing electroconducti.vity due to metal to metal bonds such as CrO2, MnO2, TiO~, Ti2Q3, etc.; (D) borides, silicides, carhides and sulfides of the valve metals such as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W; (E) the metals Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Pd and Ag or alloys th.ereof, and (F) mixtures of any of the. above.
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By admixing with the powder of the matrix material, a minor amount, typically from 0.5 to about 30%, of powders of a suitable electro-catalytic material and by sintering the mixture into a self-sustaining body, one obtains an elec-trode with satisfactory electroconductive and electrocatalytic properties which retains its chemical stability even though the admixed catalyst would not be resistant per se to the conditions of the electrolysis.
The electrocatalyst may be a metal or an inorganic oxycompound. The preferred admixed catalyst powders are the powdered metals Ru, Rh, Pd, Ir, Pt, Fe, Co, Ni, Cu and Ag, especially the platinum group metalsi powdered oxy-compounds of Mn, Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt, Ag, As, Sb, Pb and Bi and especially oxycompounds of the platinum group metals.
Specifically preferred are ~MnO2, Co304, Rh203, IrO2, RhO2, Ag20, Ag O Ag o As 03~ Sb203, Bi203~ CoMn204, NiMn2 4, 2 4 2 4 and mixtures of said powdered metals and oxycompounds.
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It has been found to be especially advantageous to add to the oxymetal compound a second material such as stannous oxide, zirconium oxide or the like and that also by adding a small amount of at least one metal belonging to the group comprising yttrium, chromium, molybdenum, zirconium, tantalum, tungsten, cobalt, nickel, palladium, and silver, both the mechanical properties and the electrical conductivity of the sintered electrodes are improved without appreciably decreas-ing thei.r 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 in terms of weight of the metal content. Optionally, yet other organic and/or inorganic com-pounds may be added to the powder mixture to improve on the bondin~ of the particles during both the moulding and sinter-ing processes.
The anodes have a high melting point well above the temperature of the molten salt electrolytes being used and they undergo no phase change under working conditions of the electrolysis. Moreover, the thermal elongation co-efficient is not far different from that of the halide salts used in the molten salt baths, which helps preserve the proper elec-trode spacing between the anode and the cathode and avoids expansions and contractions which might break the salt crust on the top of the molten salt electrolyte il~ the normal aluminum electrowinning process.
The conductivity of the sintered electrodes of the invention is comparable with that of graphite. The matrix mab/
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' has acccptable work-ability in the forming and sintering operation and in use orms a thin layer of oxyhalides on its surface under anodic conditions. The metal oxycompound free formation energy is more negative than the oxide free forma-tion energy of the corresponding halide-phase fused salt e]ect-rolyte, so that these sintered anodes have a high degree of chemically stability.
The sintered metal oxycompounAd electrodes of the invention may also be used as bipolar el~ctrodes. According to this latter embodiment, the sintered electrodes may be conveniently produced in the foxm of a slab or plate whereby one of the two major surfaces of the electrode is provided with a layer containing the anodic electrocatalyst such as the oxides C0304, Ni304~ Mn2' ~h203, Ir2' 2' 2 and the other major surface is provided with a layer contain-ing suitable cathodic materials such as carbides, borides, nitrides, sulfides, carbonitrides etc. of me~als, particularly of the valve meta]s and most preferably of y~trium, titanium and zirconium.
The self-sustaining sintered body consisting of a majox portion of oxymetal compound may be prepared by grind-ing the materials together, or separately, preferably to a grain size between 50 and 500 microns, to provide a powder mixture which contains a range of grain sizes to obtain a better de~ree of compaction. According to one of the pre-ferred methods, the mixture of powders is mixed with water or with an oraanic binding agent to obtain a plastic mass having suitable flowing properties for the particular forming mab/
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~.242~0 process used, The material may be molded in known manner eithcr by ramming or pressing the mixture in a mold or by slip-casting in a plaster of Paris mold or the material may be extruded through a die into various shapes.
The molded electrodes are then subjected to a drying process and heated at a temperature at which the desired bonding can take place, usually between 800 to 1800C for a period of between 1 to 30 hours, normally followed by slow cooling to room temperature. The heat treatment is prefer-ably carried out in an inert atmosphere or one that isslightly reducing, for example in H2 + N2 (80~), when the powdered mixture is composed essentially of oxymetal com-pound with a minor portion of other metal oxides or metals.
When the powdered mixture contains also metallic powders, it is preferable to carry out the heat treatment in an oxidizing atmosphere, at least for a portion of the heat treatment cycle to promote the oxidation of metallic particles in the outside layers of the electrodes. The metallic particles remaining inside the body of the sintered material improve the electrical conductivity properties of the elec-trode.
The forming process may be fol]owed by the sintering process at a high temperature as mentioned above or the form-ing process and the sintering process may be simultaneous, that is, pressure and temperature may be applied simultane-ously to the powder mixture, for example by m~ans of elect-rically-heated molds. Lead-in connectors may ~e fused into the ceramic electrodes during the molding and sintering pro-mab/
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cess or attached to the electrodes after slntering or molding. Other methods of shaping, compressing and slntering the powder mixture may of course be used.
The electrocatalyst, usually applied to the electrode surface rather than dispersed throughout the electrode due to costs, should have a high stability, a low anodic overpotential for the wanted anodic reaction, and a high anodic overpotential for non-wanted reactions. In the case of chlorine evolution, oxides of cobalt, nickel, iridium, rhodium, ruthenium or mixed oxides thereof such as Ru02 - Ti02 etc. can be used, and in the case of fluoride containing electrolytes wherein oxygen evolution is the wanted anodic reaction, oxides of silver and manganese are preferable. Other oxides for use as electrocatalysts may be oxides of platinum, palladium and lead.
Poisons for the suppression of unwanted anodic reactions may be used, such as, for example, to suppress oxygen evolution from chloride electrolytes.
Poisons which present a high oxygen overpotential should be used and suitable materials are the oxides of arsenic, antimony and bismuth. These oxides which are used in small percentages may be applied together with the electro-catalyst oxides in percentage of 1 to 10% of the electrocatalyst calculated in terms of the respective metals weight.
The application of the electrocatalyst, and optionally of the poisoning agent may be effected by any of known coating methods. Preferably the electro-catalyst, and optionally the poisoning agent, are applied to the sintered electrodes as a solution of decompasable salts of the metals.
~.Z42~Lai Thc sintered body is impregnated wlth the solution containing the appropriate metal salts and dried. I~ence the electrode is heated in air or in otherwise oxygen containing atmosphere to convert the salts into the wanted oxides.
Vsually the porosity of the sintered body and the method used to impregnate the surface layers of the sintered body with the metal salts should provide for the penetration of the solution down to a depth of at least 1 to 5 millimeters, preferably 3 mm, inward from the surface of the electrode so that after the heat treatment the electrocatalysts are pre-sent in the pores of the sintered body down to a certain depth inward from the surface of the electrodes.
Alternatively, by appropriate powder mixing techniques, preformed electrocatalyst oxides and optionally preformed poisoning oxides, may be ground into powder form and added to the powder mixture during the moulding of the electrodes in such a way that the external layers of the moulded electrodes are enriched with powders of the electrocatalyst oxides, and optionally of the poisoning oxides, during the forming process whereby after sintering the surface of the electrodes will already have been provided with the electrocatalyst.
The sintered electrodes of the invention may be used as bipolar electrodes. According to this embodiment of the invention, electrodes may be provided over one surface with the anodic electrocatalyst, and optionally with the poisoning agent for the unwanted anodic reaction by one of the methods disclosed above while the other surface may be provided with a coating of suitable cathodic material. For example, the mab/
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surface of t:he }:~:ipo:lar electrode which wl].J fuoclion as a ca~hoc1e during the process of electrolysis may be p:rovi.dc~d wil:ll a layer of metal carhi.des, l~orides, nitrides, sulfides and/or carboni.lxides o~ yttrium, tantalum, titanium, zi~conium, e1c.
One preferred method to apply a layer is by plasma~jet techni.que whereby powders of the selected materi.als are sprayed and adhere to the surface of the sintered body with a flame under controlled atmosphere. Alternatively, the selected powdered material may be added during the forming process to the powder mixture and the mixture then be sintered whereby the cathodic surface of the bipolar electrode is provided with a layer of the selected cathodic material.
Tlle electrodes may be used effectively for the electrolysis of many electrolytes. They are especially advant-ageous when used as anodes in electrolytic cells used for electro-lyzing molten salt electrolytes such as molten cryolite baths, molten halides of aluminum, magnesium, sodium, potassium, calcium, lithium and other metals. Thus, aluminum halides may he electro-lyzed according to the Hall process or processes disclosed in U.S. Patents No. 3,464,900, 3,518,712 or 3,755,099 using the electrodes herei.n described as anodes. The temperature of electrolysis is high enough to melt an(1 maintain the salts of the metal to be recovered in a molten state and the metal is deposited in the rnolten state and usually collected as a molten cathode Wit]
molten metal being withdrawn from the molten cathode.
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~Z42~0 -The electrodes may also bc used effectively as anodes and/or cathodes in direct current electrolysis of other molten salt electrolytes typically containing halides, oxides, carbonates or hydrates for the production of aluminum, beryllium, calcium, cerium, lithium, sodium, magnesium, potassium, barium, strontium, cesium and other metals.
When the electrodes of the invention are used as bipolar electrodes for molten salt electrolysis, the com-position of the cathode portion of the elec~rodes must be such that it will not be reduccd by the cathodic reaction or attacked by the metal being deposited at the cathodes, particularly when the electrode composition is an oxycompound.
For this reason, it is desirable to have the composition of the cathode side of the bipolar electrode irlert to the cath-odic reaction and the reducing action of the molten metal.
The electrodes may also be used as anodes and/or as cathodes in electrochemical processes such as: the electro-lysis of aqueous chloride solutions for the production of chlorine, caustic, hydrogen, hypochlorite, chlorates and perchlorates; the electrowinning of metals from aqueous sul-fate or chloride solutions for the production of copper, zinc, nickel, cobalt and other metals; the electrolysis of molten metal salt electrolytes typically containin~ halides, oxides, carbonates or hydrates for the production o~ aluminum, beryl-lium, calcium, cerium, lithium, sodium, mag~esium, potassium, ~arium, strontium, cesium and other metals and the electro-lysis of bromides, sulfides, sulfuric acid, ~ydrochloric acid and hydrofluoric acid. In general, the electrodes are useful for all electrolytic processes.
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~24Z10 The novcl electrolytic cell of the inv~ntion is com-prised of at least onc- anode and at least one cathode and means for imposing a direct electric current between the anode and cathode, the improvement residing in the anode being comprised of a self-sustaining matrix of sintered powders of an oxycompound of at least one metal selected from the group consisting of titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminum, silicon, tin, chromium, molybdenum, tungsten, lead, manganese, beryllium, iron, cobalt, nickel, platinum, palladium, osmium, iridium, rhenium, technetium, rhodium, ruthenium, gold, silver, cadmium, copper, zinc, germanium, arsenic, antimony, bismuth, boron, scandium and metals of the lanthanide and actinide series and at least one electroconductive agent, the said electrodes being provided over at least a portion of their surface with at least one electrocatalyst. The cell may also contain bipolar elect-rodes as described above.
The novel electrolysis method of the invention com-prises electrolyzing an electrolyte between an anode and a cathode, the improvement residing in the anode being com-prised of a self-sustaining matrix of sintered powders of an oxycompound of at least one metal selected from the group consisting of titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminum, silicon, tin, chromium, molybdenum, tungsten, lead, manganese, beryllium, iron, cobalt, nickel, platinum, palladium, osmium, iridium, rhenium, technetium, rhodium, ruthenium, gold, silver, cadmium, copper, zinc, mab/
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:~.Z4210 germanium, arsenic, antimony, bismuth, boron, scandium and -- metals of the lanthanide and actinide series and at least one electroconductive agent, the said electrodes being pro-vidcd over at least a portion of their surface with at least one electroca~alyst.
In t:he following examples there are described several preferred embodiments to illustrate the invention.
~owever, it should be understood that the invention is not in-tended to be limited to the specific embodiments. The per-centages of the components of the electrodes are calculated in percent by weight and calculated as free metal based on the total metal content of the composition.
Among the preferred anodes are those wherein the major portion of the self-sustaining body is tin dioxide alone or with up to 20% by weight of cobalt oxide provided with a coating of cobalt oxide which give electrodes of improved mechanical properties and electrocatalytic properties for chlorine evolution. Other preferred additives are Y203, Ti02
2 5 The electrocatalyst coating may be protected against wear by the simultaneous or subsequent application of a pro-tective agent such as a valve metal oxide like Ti02 and Ta205 or Si02 mixed oxides such as AgRe203, TiCo204 and AgxW03.
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Three sintered electrode samples, 80% SnO2 ~ 20~
cobalt (coupon A), ~0~ SnO2 + 10~ Co + 10% Mo (coupon B) and 100~ SnO2 (coupon C) were prepared and were then carefully washed with water and dried under vacuum. The resulting electrodes were then immersed under vacuum into the solution indicated in Table I and were then dried followed by heating at 370C for 20 minutes in a forced air circulation furnace.
The samples were then brushed with the same solution and heated for 15 minutes at 350C in the same furnace and this 10 procedure was repeated several times until the electrode had a weight gain of 5 g/m2.
The electrode samples were then used as anodes in a test cell for the electrolysis of aluminum chloride at 750C
and an anodic current density of 1000 A/m . The cell vol-tage was 5 volts and the electrolyte was a 5-1-1 mixture by weight of aluminum chloride, sodium chloride and potassium chloride. The anodic potential was determined initially and after 500 hours of operation and the weight loss of the electrode was determined after 500 hours. For comparative - 20 purposes, a standard graphite electrode was also used under the same conditions and the results are reported in Table I.
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_ 19 _ mab/
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.. : ' : ' ~.242~) TABLE I
. . ~ _ _ , _ ._ Solution : Anodic Potential Weight loss Coupon Solvent Salt fter 500 hrsl after 500 ... . ~ ~ _ ._ .
mide ~oC12 0.-5 ¦ 0.4 0.
. __ ' B hydro- r rC13 0.45 0.45 1 0.5 __ achiodri c .
-- -- -I --`- --'-1 .__ I -- '--- ----I -¦ C hydro- r rC13 0.5 0.6 0.2 ¦ chloric ~ ___ _ _ _ . A untreated 0.6 0.6 0.5 . I . _ ... .
B untreated 0.55 0.55 0.5 l _ _ .-_ l .. _ ._ _ C ¦untreated ¦ 1.0 ¦ 1.3 Nil i ... l .. I ~ __ ,, ._ *RCGE: Reference chlorine yraphite electrodr~.
The results of Table I show that t~e coated elect-rodes have an even lower overpotential for c~llorine evolu-tion without any substantial increase in weigllt loss. Sample C which had too high an overpotential withou~ the post-treat-ment was not suited for the electrolysis reac~tion while the treated sample C is. The average Faraday ef~iciency duriny the test was 96~. An ordinary graphite elec~rode used in the sa~e way and compared to the reference graphite elect-rode showed a voltage of about 0.8 volts.
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~.242:~0 EX~MPLE 2 Samples of 90~ by weight of tin dioxide and 10% by weight of cobalt were sintered and the electrodes were then provided with a coating of cobalt oxide as in Example 1 to obtain a layer of 10 g/m of cobalt oxide. The electrodes were then used to electrolyze the electrolytes of Table II
under the operating conditions recited therein. The anode potential after 300 hours of operation and the wear rate after 300 hours were determined and are reported in Table II.
TABLE II
. _._ _ . . . . .. .
31ectrolyte lectrolyte -Current Average Anodic ~eight -omposition emp. C Density Faraday Potential loss ~nd weight 2 ~Effic- after ~/m2 ratio _ A/m _ iency V(RCG3) (51C.1)3+KCl 750 lO00 92% 0.5 0.
. . __ . . .
'(5aC1)2+KCl 450 1000 94% 0.6 0.5 . I . .. . . _ 20oC12+KC1 ,450 1000 0 ~ 0.5 _ _ I _ _ _ _ Table II shows that the electrodes of the invention have a low wear rate and a low anode potential even after 300 hours of operation.
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~.242~
. ~XAMPLE 3 -l)isc-shaped electrodes with a diameter of 10 mm and a thickness of 5 mm were prepared from powders having a mesh number of 100 to 250. The powders were press-moulded at a pressure of 1000 Kg/cm2 and were then sintered in an induction furnace under the conditions reported in Table III
which also shows the compositions of the powders.
The sintering was conducted in a furnace through which the indicated gas was circulated or maintained at at-mospheric pressure. Thus at least the external surfaces, and perhaps some of the external pores, were exposed to an oxidizing atmosphere at the temperature indicated and the exposed metal in the surfaces were oxidized to form the electrocatalyst.
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~.24~0 TABLE III
. --------- .. , ............ . . .
Sample Components Sinterization Time No. and of Wt. Percentage Temp. C Atmosphere Heating . ----- . _ . .......... . __ _ . . . _ .~ _ . ._ 1 SnO2 80% Forced air Co 20% 1250circulation 2 hrs.
_ _ . .__ ' _ .
SnO2 80% Forced air 2 Co 10% 1500circulation 2 hrs.
Mo 10%
. _, . _ ..... _. . _ __ .
SnO2 80% Forced air
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Three sintered electrode samples, 80% SnO2 ~ 20~
cobalt (coupon A), ~0~ SnO2 + 10~ Co + 10% Mo (coupon B) and 100~ SnO2 (coupon C) were prepared and were then carefully washed with water and dried under vacuum. The resulting electrodes were then immersed under vacuum into the solution indicated in Table I and were then dried followed by heating at 370C for 20 minutes in a forced air circulation furnace.
The samples were then brushed with the same solution and heated for 15 minutes at 350C in the same furnace and this 10 procedure was repeated several times until the electrode had a weight gain of 5 g/m2.
The electrode samples were then used as anodes in a test cell for the electrolysis of aluminum chloride at 750C
and an anodic current density of 1000 A/m . The cell vol-tage was 5 volts and the electrolyte was a 5-1-1 mixture by weight of aluminum chloride, sodium chloride and potassium chloride. The anodic potential was determined initially and after 500 hours of operation and the weight loss of the electrode was determined after 500 hours. For comparative - 20 purposes, a standard graphite electrode was also used under the same conditions and the results are reported in Table I.
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_ 19 _ mab/
. . ~ . .
.. : ' : ' ~.242~) TABLE I
. . ~ _ _ , _ ._ Solution : Anodic Potential Weight loss Coupon Solvent Salt fter 500 hrsl after 500 ... . ~ ~ _ ._ .
mide ~oC12 0.-5 ¦ 0.4 0.
. __ ' B hydro- r rC13 0.45 0.45 1 0.5 __ achiodri c .
-- -- -I --`- --'-1 .__ I -- '--- ----I -¦ C hydro- r rC13 0.5 0.6 0.2 ¦ chloric ~ ___ _ _ _ . A untreated 0.6 0.6 0.5 . I . _ ... .
B untreated 0.55 0.55 0.5 l _ _ .-_ l .. _ ._ _ C ¦untreated ¦ 1.0 ¦ 1.3 Nil i ... l .. I ~ __ ,, ._ *RCGE: Reference chlorine yraphite electrodr~.
The results of Table I show that t~e coated elect-rodes have an even lower overpotential for c~llorine evolu-tion without any substantial increase in weigllt loss. Sample C which had too high an overpotential withou~ the post-treat-ment was not suited for the electrolysis reac~tion while the treated sample C is. The average Faraday ef~iciency duriny the test was 96~. An ordinary graphite elec~rode used in the sa~e way and compared to the reference graphite elect-rode showed a voltage of about 0.8 volts.
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~.242:~0 EX~MPLE 2 Samples of 90~ by weight of tin dioxide and 10% by weight of cobalt were sintered and the electrodes were then provided with a coating of cobalt oxide as in Example 1 to obtain a layer of 10 g/m of cobalt oxide. The electrodes were then used to electrolyze the electrolytes of Table II
under the operating conditions recited therein. The anode potential after 300 hours of operation and the wear rate after 300 hours were determined and are reported in Table II.
TABLE II
. _._ _ . . . . .. .
31ectrolyte lectrolyte -Current Average Anodic ~eight -omposition emp. C Density Faraday Potential loss ~nd weight 2 ~Effic- after ~/m2 ratio _ A/m _ iency V(RCG3) (51C.1)3+KCl 750 lO00 92% 0.5 0.
. . __ . . .
'(5aC1)2+KCl 450 1000 94% 0.6 0.5 . I . .. . . _ 20oC12+KC1 ,450 1000 0 ~ 0.5 _ _ I _ _ _ _ Table II shows that the electrodes of the invention have a low wear rate and a low anode potential even after 300 hours of operation.
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~.242~
. ~XAMPLE 3 -l)isc-shaped electrodes with a diameter of 10 mm and a thickness of 5 mm were prepared from powders having a mesh number of 100 to 250. The powders were press-moulded at a pressure of 1000 Kg/cm2 and were then sintered in an induction furnace under the conditions reported in Table III
which also shows the compositions of the powders.
The sintering was conducted in a furnace through which the indicated gas was circulated or maintained at at-mospheric pressure. Thus at least the external surfaces, and perhaps some of the external pores, were exposed to an oxidizing atmosphere at the temperature indicated and the exposed metal in the surfaces were oxidized to form the electrocatalyst.
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~.24~0 TABLE III
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Sample Components Sinterization Time No. and of Wt. Percentage Temp. C Atmosphere Heating . ----- . _ . .......... . __ _ . . . _ .~ _ . ._ 1 SnO2 80% Forced air Co 20% 1250circulation 2 hrs.
_ _ . .__ ' _ .
SnO2 80% Forced air 2 Co 10% 1500circulation 2 hrs.
Mo 10%
. _, . _ ..... _. . _ __ .
SnO2 80% Forced air
3 Ni 10% 1~50circulation 2 hrs SnO2 75%
4 Co2 3 150%%1500Ambient air 2 hFs.
_ ... . _ _ . _ _ _ _ SnO2 60%
Co2NiO4 30% 1500 Ambient air 2 hrs.
~_ ___ ___ . ._ ............. __ _ SnO2 60%
SiO~ 10%
6 Cu2Nlo4 10%1000 Ambient air 2 hrs.
! Mo 10%
1--- -- _ ~
l Sn2 95%
1 7 Co 2.5% 1500 Forced air Mo 2.5% ~ ~irculation 2 hrs.
. _ . 1. .
8` SnO2 100% 1500 ¦ Air 10 hrs.
._ --- !-----mab/
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The elccl-rocondu(t:i.vi-ty of the Samples l to 7, measured at 500C., ~as between 0.01 and l.00 5~ lcm l ~nd tlle density of the sinterized electrodes varied hetweerl 'i and 8,5 g/cm . The electrode samples were used as anodes in a test cell for the electrolysis of aluminum chloride at 750C. and an anodic current density of lO00 A/m . The cell voltage was 5 volts and the electrolyte was a 5-l-l mixture of aluminuM chloride, sodium chloride and potassium chloride. The anodic potential was deter-mined initially and after 500 hours of operation, and the weight loss of the electrode was determined after 500 hours. For COM-parative purposes, a reference yraphite electrode was also used under the same conditions and the results are reported in Table IV.
TABLE IV
Anodic Potential Weiyht Loss Af~er V. V. (RGE)* 500 Hrs. g/m Sample Initial After 500 Hrs.
_ . - --- ------I - .
___ ~ _.
l 0.6 0.6 0.5 2 0.55 0.55 0.5 3 0.60 0.6 0.8 4 0.55 0.6 0.5 0.6 0.6 Nil 6 0.65 0.65 0.5 7 0.55 0.6 Nil 8 l.0 1.3 Nil Graphite 0.85 0.85 105 *RGE Reference Graphite electrode - 2~ -`
~L~.24210 The results of Table IV show that clectrodes 1 to 7, containin~ a major portion of an oxide and a minor portion of r a metal, h-ave a low over-potential for chlorine evolution and a very low wear rate. Electrode 8, which did not contain any additive electroconductive metal, had a substantially higher over-potential for chlorine evolution and the reference grap-hite electrode had an over-potential above the values for electrodes 1 to 7 and a high wear rate. The reference graphite anode needed substantial adjustments durin~ the electrolysis and an early replacement. The average efficiency during the test was 97%. All of the samples 1 to 7, inclusive, were less brittle than Sample No. 8.
About 250 g of a mixture of the matrix material and additive materials indicated in Table V were ground in a mixer for 20 minutes and the powder mixtures were poured into cylin-drical plastic molds and pre-compressed manually with a steel cylinder press. Each mold was placed ;n an isostatic pressure chamber and the pressure was raised to about 1500 Kg/cm in 5 minutes and then reduced to zero in a few seconds. The samples were then taken out of the plastic molds and polished. The pressed samples were put into an electrically heated furnace and heated from room temperature to 1200C under a nitrogen atmosphere over a period of 24 hours,, held at the maximum temperature for 2 to 5 hours and then cooled to 300C over the following 24 hours. Tne sintered samples were then taken out of the furnace and after cooling to room temperature, they were weighed and their apparent density and electrical conductivity at 25Cc~d at 1000C were measured. The results are reported in Table V.
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The elccl-rocondu(t:i.vi-ty of the Samples l to 7, measured at 500C., ~as between 0.01 and l.00 5~ lcm l ~nd tlle density of the sinterized electrodes varied hetweerl 'i and 8,5 g/cm . The electrode samples were used as anodes in a test cell for the electrolysis of aluminum chloride at 750C. and an anodic current density of lO00 A/m . The cell voltage was 5 volts and the electrolyte was a 5-l-l mixture of aluminuM chloride, sodium chloride and potassium chloride. The anodic potential was deter-mined initially and after 500 hours of operation, and the weight loss of the electrode was determined after 500 hours. For COM-parative purposes, a reference yraphite electrode was also used under the same conditions and the results are reported in Table IV.
TABLE IV
Anodic Potential Weiyht Loss Af~er V. V. (RGE)* 500 Hrs. g/m Sample Initial After 500 Hrs.
_ . - --- ------I - .
___ ~ _.
l 0.6 0.6 0.5 2 0.55 0.55 0.5 3 0.60 0.6 0.8 4 0.55 0.6 0.5 0.6 0.6 Nil 6 0.65 0.65 0.5 7 0.55 0.6 Nil 8 l.0 1.3 Nil Graphite 0.85 0.85 105 *RGE Reference Graphite electrode - 2~ -`
~L~.24210 The results of Table IV show that clectrodes 1 to 7, containin~ a major portion of an oxide and a minor portion of r a metal, h-ave a low over-potential for chlorine evolution and a very low wear rate. Electrode 8, which did not contain any additive electroconductive metal, had a substantially higher over-potential for chlorine evolution and the reference grap-hite electrode had an over-potential above the values for electrodes 1 to 7 and a high wear rate. The reference graphite anode needed substantial adjustments durin~ the electrolysis and an early replacement. The average efficiency during the test was 97%. All of the samples 1 to 7, inclusive, were less brittle than Sample No. 8.
About 250 g of a mixture of the matrix material and additive materials indicated in Table V were ground in a mixer for 20 minutes and the powder mixtures were poured into cylin-drical plastic molds and pre-compressed manually with a steel cylinder press. Each mold was placed ;n an isostatic pressure chamber and the pressure was raised to about 1500 Kg/cm in 5 minutes and then reduced to zero in a few seconds. The samples were then taken out of the plastic molds and polished. The pressed samples were put into an electrically heated furnace and heated from room temperature to 1200C under a nitrogen atmosphere over a period of 24 hours,, held at the maximum temperature for 2 to 5 hours and then cooled to 300C over the following 24 hours. Tne sintered samples were then taken out of the furnace and after cooling to room temperature, they were weighed and their apparent density and electrical conductivity at 25Cc~d at 1000C were measured. The results are reported in Table V.
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l~.Z4Z10 The data in Table V shows that the electrical conduc-tivity of the sintered ceramic electrodes at high temperatures of 1000C is 5 to 10 times higher than the electrical con-ductivity at 25C. The addition of oxides having conductivity equivalent to metals to the substantially non-conductive cer-amic oxides of the matrix increases the conductivity of the electrodes by a magnitude of 102. The addition of a metal stable to molten salts such as yttrium or molybdenum, etc. to the ceramic electrodes of the invention increases the electrical conductivity of the electrodes by 2 to 5 times.
_ _ The conditions of operation of an electrolysis cell for the production of aluminum metal from a molten cryolite bath were simulated in a laboratory test cell. In a heated crucible of graphite, a layer of liquid aluminum was provided on the bottom and a melt consisting of cryolite (80 to 85%), alumina (5 to 10%) and AlF3 (from 1 to 5%) was poured on top thereof. The sample electrodes with a working surface area of 3 cm2 prepared according to the procedure described in Example 4 and to which a Pt wire was brazed to provide an easy means for electrical connection were dipped into the salt melt and held at a distance of about 1 cm from the liquid aluminum layer. The crucible was maintained at a temperature ranging from 950 to 1050C and the current density was 0.5 A/cm2 and the cell was operated for 2000 hours. The experi-mental data obtained is shown in Table VI. The sample mab/
'' ' ~.Z4Z10 number indicate~; tha~. ~he el.ectrode teste(l col:re.~pol~ds ~o the samp].e describe(l in Tab1e V wi.th the same nurnber, T~13LL VI
Sample ~luminum Weight loss of No. produced anode~
gLh? _ __ _ (g/cm ) _ _ 1 0.49 0.02 2 0~50 0.12 3 0.49 0.04 4 0-49 0.02 0.48 0.01
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l~.Z4Z10 The data in Table V shows that the electrical conduc-tivity of the sintered ceramic electrodes at high temperatures of 1000C is 5 to 10 times higher than the electrical con-ductivity at 25C. The addition of oxides having conductivity equivalent to metals to the substantially non-conductive cer-amic oxides of the matrix increases the conductivity of the electrodes by a magnitude of 102. The addition of a metal stable to molten salts such as yttrium or molybdenum, etc. to the ceramic electrodes of the invention increases the electrical conductivity of the electrodes by 2 to 5 times.
_ _ The conditions of operation of an electrolysis cell for the production of aluminum metal from a molten cryolite bath were simulated in a laboratory test cell. In a heated crucible of graphite, a layer of liquid aluminum was provided on the bottom and a melt consisting of cryolite (80 to 85%), alumina (5 to 10%) and AlF3 (from 1 to 5%) was poured on top thereof. The sample electrodes with a working surface area of 3 cm2 prepared according to the procedure described in Example 4 and to which a Pt wire was brazed to provide an easy means for electrical connection were dipped into the salt melt and held at a distance of about 1 cm from the liquid aluminum layer. The crucible was maintained at a temperature ranging from 950 to 1050C and the current density was 0.5 A/cm2 and the cell was operated for 2000 hours. The experi-mental data obtained is shown in Table VI. The sample mab/
'' ' ~.Z4Z10 number indicate~; tha~. ~he el.ectrode teste(l col:re.~pol~ds ~o the samp].e describe(l in Tab1e V wi.th the same nurnber, T~13LL VI
Sample ~luminum Weight loss of No. produced anode~
gLh? _ __ _ (g/cm ) _ _ 1 0.49 0.02 2 0~50 0.12 3 0.49 0.04 4 0-49 0.02 0.48 0.01
6 0.49 0.04
7 0 49 0.06
8 0.~6 0.18
9 0.46 0.2 The test sample electrodes operated successfully as anodes in the cryolite mel-t and the observed wear rates appear to be quite acceptable for the electrolytic production of aluminum from mo].ten cryolite. All the tested el.ectrodes showed a low wear rate during 2000 hours of operation.
1~24Z~O
EXAMPLE_6 Electrodes Nos. 4 and 5 described in Table V were used as anodes for the electrolysis of a molten aluminum chloride electrolyte in the test cell described in Example S. The electrolysis conditions were the following:
Electrolyte : AlC13 from 31 to 35~ b.w.t.
NaCl from 31 to 35% b.w.t.
BaC03 from 31 to 35~ b.w.t.
Temperature of Electrolyte : from 690 to 720C
Anodic current 2 density : 2000 Amp/m Cathode : Molten Aluminum Interelectrodic gap : 1 cm.
The tested electrodes operated successfully and the weight losses after 2000 hours of operation were neg-ligible.
mab/
2~0 EXA~PLE 7 Electrode samples Nos. 10 and 11 of Example 1 were used as anodes for the electrolysis of an aqueous bromide so~
lution to produce bromine using a test diaphragm type cell with an asbestos diaphragm to separate the cathode compartment with a steel cathode from the ancde compartment with the test elect-rode as the anode. The electrolysis was effected with an aqueous solution of 200-220 g/l of sodium bromide and the electrolyte temperature was 80 to 85C with a current den-sity of 2000 A/m . The current efficiency was 95~ and after 1000 hours of operation, the weight loss of the test elect-rode was negligible.
Electrode samples Nos. 10, 11 and 13 of Example 1 were used alternatively as anode and as cathode in the elec-trolysis of synthetic sea-water in a test cell in which the electrolyte was pumped through the electrodic gap of 3 mm at a speed of 3 cm/sec. The current density was maintained at 1500 A/m2 and the spent electrolyte contained 0.8 to 2.4 of sodium hypochlorate with a Faraday efficiency of more than 88~. The weight loss of the electrodes after 200 hours of operation was negligible.
mab/
. ~,~ .
~.Z42~0 Electrode samples Nos. 12 and 14 of Example 1 were used as anodes in the electrolysis of an aqueous acidic cupric sulfate solution in a cell with a titanium cathode blank.
The electrolyte contained 150 to 200 gpl of sulfuric acid and 40 gpl of cupric sulfate as metallic copper and the anode current density was 300 A/cm . The electrolyte temperature was 60 to 80C and an average of 6 mm of copper were deposited on the flat cathode at 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 V (NHE).
Other electrocatalysts which may be used in the electrolysis of molten halide salts for halide ion discharge are Ru02 and oxides such as As203, Sb203 and Bi203 may be added in percentages up to 10% by weight of free metal based upon the total metal content to raise the oxygen overpotential without affecting the halide ion discharge potential.
For anodes to be used in molten fluoride electrolytes where oxygen is evolved, the catalyst may be one of those listed in Example 5 or Rh203, Pb02 and Ir02.Ti02-The components of the anodes given in the Examples are calculated in percent by weight of free metal based upon the total metal content of the anode composition.
The electrolyte may contain other salts than those used in the Examples such as alkali metal chloride or fluoride as well as the salt of the metal undergoing electrolysis.
~ 37 -mab/
.
;: .
~z~z~
The metal halides are effective to reduce the melting point of the salt undergoing e]ectrolysis thus permittlng use of lower temperatures while one maintains the salt bath in molten or melted state.
The above examples include fused or molten metal salt electrolysis~
primarily the electrolysis of molten aluminum chloride or fluoride salts.
In a similar manner, the molten chlorides of other metals such as alkali metal or alkaline earth metals may be electrolyzed using the desigr~ated anodes, according to otherwise standard practice. In addition, otker molten salts, such as the molten nitrates, may be electrolyzed in the same way.
A molten alumina-cryolite electrolyte or the like-alkali metal aluminum fluoride may be electrolyzed to produce molten aluminum.
These electrodes may be used in place of graphite anodes in standard aluminum electrowinning cells with either aluminum ore feed into a cryolite bath or with a predominantly aluminum chloride bath.
The use of these sintered metal oxide anodes for the recovery of the desired metals from fused salts of the metals to be won results in reduced power consumption per unit weight of metal produced and in purer recovered metals. The electrodes are dimensionally stable in service and therefore 3~ -~.2~Z10 a do not require frequent interventions to restore the op-timum distance ~rom the cathode surface as is necessary with the consumablc anodes of the prior art.
Various modifications of the electrodes and pro-cesses of the invention may be made without departin~ from the spirit or scope of our invention and it is to be under-stood that the invention is intended to be limited only as defined in the appended claims.
mab/
1~24Z~O
EXAMPLE_6 Electrodes Nos. 4 and 5 described in Table V were used as anodes for the electrolysis of a molten aluminum chloride electrolyte in the test cell described in Example S. The electrolysis conditions were the following:
Electrolyte : AlC13 from 31 to 35~ b.w.t.
NaCl from 31 to 35% b.w.t.
BaC03 from 31 to 35~ b.w.t.
Temperature of Electrolyte : from 690 to 720C
Anodic current 2 density : 2000 Amp/m Cathode : Molten Aluminum Interelectrodic gap : 1 cm.
The tested electrodes operated successfully and the weight losses after 2000 hours of operation were neg-ligible.
mab/
2~0 EXA~PLE 7 Electrode samples Nos. 10 and 11 of Example 1 were used as anodes for the electrolysis of an aqueous bromide so~
lution to produce bromine using a test diaphragm type cell with an asbestos diaphragm to separate the cathode compartment with a steel cathode from the ancde compartment with the test elect-rode as the anode. The electrolysis was effected with an aqueous solution of 200-220 g/l of sodium bromide and the electrolyte temperature was 80 to 85C with a current den-sity of 2000 A/m . The current efficiency was 95~ and after 1000 hours of operation, the weight loss of the test elect-rode was negligible.
Electrode samples Nos. 10, 11 and 13 of Example 1 were used alternatively as anode and as cathode in the elec-trolysis of synthetic sea-water in a test cell in which the electrolyte was pumped through the electrodic gap of 3 mm at a speed of 3 cm/sec. The current density was maintained at 1500 A/m2 and the spent electrolyte contained 0.8 to 2.4 of sodium hypochlorate with a Faraday efficiency of more than 88~. The weight loss of the electrodes after 200 hours of operation was negligible.
mab/
. ~,~ .
~.Z42~0 Electrode samples Nos. 12 and 14 of Example 1 were used as anodes in the electrolysis of an aqueous acidic cupric sulfate solution in a cell with a titanium cathode blank.
The electrolyte contained 150 to 200 gpl of sulfuric acid and 40 gpl of cupric sulfate as metallic copper and the anode current density was 300 A/cm . The electrolyte temperature was 60 to 80C and an average of 6 mm of copper were deposited on the flat cathode at 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 V (NHE).
Other electrocatalysts which may be used in the electrolysis of molten halide salts for halide ion discharge are Ru02 and oxides such as As203, Sb203 and Bi203 may be added in percentages up to 10% by weight of free metal based upon the total metal content to raise the oxygen overpotential without affecting the halide ion discharge potential.
For anodes to be used in molten fluoride electrolytes where oxygen is evolved, the catalyst may be one of those listed in Example 5 or Rh203, Pb02 and Ir02.Ti02-The components of the anodes given in the Examples are calculated in percent by weight of free metal based upon the total metal content of the anode composition.
The electrolyte may contain other salts than those used in the Examples such as alkali metal chloride or fluoride as well as the salt of the metal undergoing electrolysis.
~ 37 -mab/
.
;: .
~z~z~
The metal halides are effective to reduce the melting point of the salt undergoing e]ectrolysis thus permittlng use of lower temperatures while one maintains the salt bath in molten or melted state.
The above examples include fused or molten metal salt electrolysis~
primarily the electrolysis of molten aluminum chloride or fluoride salts.
In a similar manner, the molten chlorides of other metals such as alkali metal or alkaline earth metals may be electrolyzed using the desigr~ated anodes, according to otherwise standard practice. In addition, otker molten salts, such as the molten nitrates, may be electrolyzed in the same way.
A molten alumina-cryolite electrolyte or the like-alkali metal aluminum fluoride may be electrolyzed to produce molten aluminum.
These electrodes may be used in place of graphite anodes in standard aluminum electrowinning cells with either aluminum ore feed into a cryolite bath or with a predominantly aluminum chloride bath.
The use of these sintered metal oxide anodes for the recovery of the desired metals from fused salts of the metals to be won results in reduced power consumption per unit weight of metal produced and in purer recovered metals. The electrodes are dimensionally stable in service and therefore 3~ -~.2~Z10 a do not require frequent interventions to restore the op-timum distance ~rom the cathode surface as is necessary with the consumablc anodes of the prior art.
Various modifications of the electrodes and pro-cesses of the invention may be made without departin~ from the spirit or scope of our invention and it is to be under-stood that the invention is intended to be limited only as defined in the appended claims.
mab/
Claims (18)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An electrode comprising a self-sustaining matrix of sintered powders of an oxycompound of at least one metal selected from the group consisting of titanium,tantalum, zirconium, vanadium, niobium, hafnium, aluminum, silicon, tin, chromium, molybdenum, tungsten, lead, manganese, beryllium, iron, cobalt, nickel, platinum, palladium, osmium, iridium, rhenium, technetium, rhodium, ruthenium, gold, silver, cadmium, copper, zinc, germanium, arsenic, antimony, bismuth, boron, scandium and metals of the lanthanide and actinide series and at least one electroconductive agent, different from said oxycompound, the said electrode being provided over at least a portion of to surface with from about 0.5 to about 30% by weight of at least one electrocatalyst.
2. The electrode of claim 1 wherein the electroconductive agent is a minor portion of the sintered electrode body and is an oxide of zirconium and/or tin.
3. The electrode of claim 1 wherein the electroconductive agent is a minor portion 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.
4. The electrode of claim 1 wherein the electrocatalyst is at least one metal selected from the group consisting of oxides of cobalt, nickel, manganese, rhodium, iridium, ruthenium and silver.
5. The electrode of claim 4 in which the electrocatalyst is comprised of powdered oxides sintered into the outer layers of said electrode.
6. A bipolar electrode comprising a self sustaining matrix of sintered powders of an oxycompound of at least one metal selected from the group consisting of titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminum, silicon, tin, chromium, molybdenum, tungsten, lead, manganese, beryllium, iron, cobalt, nickel, platinum, palladium, osmium, iridium, rhenium, technetium, rhodium, ruthenium, gold, silver, cadmium, copper, zinc, germanium, arsenic, antimony, bismuth, boron, scandium and metals of the lanthanide and actinide series and at least one electroconductive agent different from said oxycompound, the said electrode being provided over at least a portion of its surface with from about 0.5 to about 30% by weight of at least one electrocatalyst on its anodic surface and over at least a portion of its cathodic surface with a layer of cathodic material selected from the group consisting of metal carbides, borides, nitrides, sulfides and carbonitrides and mixtures thereof.
7. The electrode of claim 6 wherein the electroconductive agent is a minor portion of the sintered electrode body and is an oxide of zirconium and/or tin.
8. The electrode of claim 6 wherein the electroconductive agent is a minor portion 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.
9. The electrode of claim 6 wherein the electrocatalyst is selected from the group consisting of oxides of cobalt, nickel, manganese, rhodium, iridium, ruthenium, silver and mixtures thereof.
10. The electrode of claim 9 in which the electrocatalyst is comprised of powdered oxides of said metals sintered into the outer layers of said electrode.
11. The electrode of claim 6 wherein the layer of the said cathodic material comprises powders of said cathodic material sintered into the outer cathodic surfaces of said electrode.
12. The electrode of claim 6 wherein the cathodic material is selected from the group comprising carbides, borides, nitrides, sulfides and carbonitrides of at least one metal selected from the group consisting of yttrium, titanium and zirconium.
13. The process for effecting an electrolysis reaction with an anode and cathode, the improvement comprising using as the anode an electrode of claim 1.
14. The process of claim 13 wherein the electroconductive agent is a minor portion of the sintered electrode body and is an oxide of zirconium and/or tin.
15. The process of claim 13 wherein the electroconductive agent is a minor portion 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.
16. The process of claim 13 wherein the electrocatalyst is at least one metal selected from the group consisting of oxides of cobalt, nickel, manganese, rhodium, iridium, ruthenium and silver.
17. The process of claim 13 wherein the electrocatalyst was formed in situ on said sintered electrode body from a solution of salts of said metals which were converted to oxides on said sintered electrode body.
18. The process of claim 16 in which the electrocatalyst is comprised of powdered oxides sintered into the outer layers of said electrode.
Applications Claiming Priority (10)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US67227976A | 1976-03-31 | 1976-03-31 | |
| US672,279 | 1976-03-31 | ||
| US67346076A | 1976-04-05 | 1976-04-05 | |
| US673,460 | 1976-04-05 | ||
| US68127976A | 1976-04-28 | 1976-04-28 | |
| US68694376A | 1976-05-17 | 1976-05-17 | |
| US686,943 | 1976-05-17 | ||
| US774,101 | 1977-03-07 | ||
| US05/774,101 US4146438A (en) | 1976-03-31 | 1977-03-07 | Sintered electrodes with electrocatalytic coating |
| US681,279 | 1996-07-22 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1124210A true CA1124210A (en) | 1982-05-25 |
Family
ID=27542061
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA275,175A Expired CA1124210A (en) | 1976-03-31 | 1977-03-30 | Sintered electrodes with electrocatalytic coating |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1124210A (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2019005510A1 (en) * | 2017-06-29 | 2019-01-03 | Langley Justin | Zero emissions reforming operation |
| CN111807573A (en) * | 2020-07-16 | 2020-10-23 | 湖南中湘春天环保科技有限公司 | Treatment device and method for thallium-containing wastewater |
| CN114560536A (en) * | 2022-03-09 | 2022-05-31 | 四川塔菲尔环境科技有限公司 | Terbium rhenium modified Ti/RuO2Dimensionally stable anode, preparation method and application |
-
1977
- 1977-03-30 CA CA275,175A patent/CA1124210A/en not_active Expired
Cited By (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2019005510A1 (en) * | 2017-06-29 | 2019-01-03 | Langley Justin | Zero emissions reforming operation |
| US10988849B2 (en) | 2017-06-29 | 2021-04-27 | Blue Skies Research, Inc. | Electrochemical device for cascading reactive distillation |
| CN111807573A (en) * | 2020-07-16 | 2020-10-23 | 湖南中湘春天环保科技有限公司 | Treatment device and method for thallium-containing wastewater |
| CN111807573B (en) * | 2020-07-16 | 2022-06-14 | 湖南中湘春天环保科技有限公司 | Treatment device and method for thallium-containing wastewater |
| CN114560536A (en) * | 2022-03-09 | 2022-05-31 | 四川塔菲尔环境科技有限公司 | Terbium rhenium modified Ti/RuO2Dimensionally stable anode, preparation method and application |
| CN114560536B (en) * | 2022-03-09 | 2023-07-28 | 四川塔菲尔环境科技有限公司 | Terbium-rhenium modified Ti/RuO 2 Dimensionally stable anode, preparation method and application |
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