IL45883A - Electrodes for electrochemical processes - Google Patents

Description

ELECTRODES FOR ELECTROCHEMICAL PROCESSES MD26540/MD26791 The present invention relates to electrodes for electrochemical processes.

More particularly it relates to electrodes comprising a support member made of a film-forming metal or a film-forming alloy carrying an electro-catalytically active coating.

UK Patent No.1402414 In the specification of our -e-o-pen-d-i-ng-Wr .App-l-iea-t-i-on-No--43 7i--(-nΘW--p-ub1-e-hed--a-s- (Belgian Patent Specification 788883) there is described and claimed an electrode for electrochemical processes having good resistance to damage by short-circuiting which comprises a support member made of a film-forming metal or a film-forming metal alloy and an electrocatalytically active coating thereon, which coating consists of a matrix of. electroconducting material having electrocatalytic properties and embedded in the said matrix a non-conducting particulate or fibrous refractory material.

Suitable refractory materials mentioned in the aforesaid specification for embedding in the matrix of the electroconducting material included glass, zirconia, alumina, Silica fibres (for example quartz wool), thorium dioxide, titanium dioxide and alumino-silicates .

We have now found further refractory materials which may be advantageously embedded in the matrix of the electroconducting material.

According to the present invention we provide an electrode for electrochemical processes, which comprises a support member of a film-forming metal or a film-forming metal alloy and an electrocatalytically active coating thereon, which coating consists of a matrix of electroconducting material having electrocatalytic properties and embedded in the said matrix a non-conducting particulate or fibrous refractory material, and wherein the said refractory material is selected from the group consisting of oxides -G-apfo-i-d-es-, sulphides,- nitrides and fluorides, the said oxides bein other than those claimed and/or described Specification 788883).

By a "film-forming metal" we. mean one of the metals titanium, zirconium, niobium, tantalum or tungsten. By "a film-forming metal alloy" we mean an alloy based on one of the said film-forming metals and having anodic polarisation properties similar to those of the commercially pure film-forming metal.

The support member of the electrode is made of one of the film-forming metals, titanium, zirconium, niobium, tantalum or tungsten or a film-forming metal alloy.

Preferably the support member is made of titanium or an alloy based on titanium and having anodic polarisation properties similar to those of titanium.

The matrix of the electrode coating. may be formed of any electroconducting material which has electro- catalytic properties, ie which is active in transferring electrons from an electrolyte to the underlying film- forming metal or alloy structure of the electrode, and which is resistant to anodic attack in an aqueous electrolyte containing chloride ions. It may for instance consist of one or more of the platinum group metals, ie platinum, rhodium, iridium, ruthenium, osmium and palladium and/or oxides of one or more of these metals, rhenium, rhenium trioxide, magnetite, titanium nitride and the borides, phosphides and silicides of the platinum group metals. It may consist of one or more of the said platinum group metals and/or oxides thereof in admixture with one or ' > more non-noble metal oxides. Alternatively, it may consist of one or more non-noble metal oxides alone or a mixture of one or more non-noble metal oxides and a non-noble metal chlorine discharge catalyst. Suitable non-noble metal oxides are, for example, oxides of the said film-forming metals, tin dioxide, germanium dioxide and oxides of antimony. Suitable chlorine-discharge catalysts include the difluorides of manganese, iron, cobalt, nickel and mixtures thereof, for example as described in the specification of our UK Patent No 1,277,033- Especially suitable electro-conducting materials according to the invention include platinum itself and those based on ruthenium dioxide/ titanium dioxide and ruthenium dioxide/tin dioxide/ titanium dioxide.

The non-conducting refractory materials which are suitable for use according to the present invention are any particulate or fibrous materials which are chemically stable and resistant to melting at the temperatures employed during the preparation of the coating (for example in the region of 400°C to 500°C), are resistant, to electrochemical attack, are non-conducting, and have electrical properties which are not in the form used in this invention signif cantly changed by chemical interaction with the electrocatalytically active material used in the matrix during the preparation process.

By the. term "non-conducting" is usually meant insulating^ materials with an electrical resistivity at room temperature in the range about 101" to about 1022 ohm-cm as distinct from good conductors having a resistivity of about 10 5 ohm-cm and semi-conductors having a resistivity of about 10 2 to about lO9 ohm-cm (cp. 'Introduction to Solid State Physics', by C Kittel Wiley and Sons, New York, 1953). In the present specification, we mean that the refractory material is non-conducting relative to the electroconducting material used in the matrix, and such refractory materials include those having a resistivity greater than about 10 ohm-cm, and preferably in the region 1010 to about 1022 ohm-cm.

By the term "embedded" we include any coatings in which the non-conducting refractory particles are bound together by the electroconducting material of the matrix. · By the term "oxides" we include single oxides, binary oxides, ternary' oxides and more complex oxides.

Suitable single oxides for use as refractory non-conducting materials according to the invention include eerie oxide, hafnium oxide and ditantalum pentoxide ( a20s). Other suitable single oxides Patent No.1402414 disclosed in UK •Ap-pli-eerfe- eft-- e- l-^ 0-7- include zirconia, alumina, silica, thorium oxide and titanium dioxide.

Suitable binary oxides include magnesium aluminate, for example spinel Mg0.Al203 , ullite (AI2O3 ) 3 (Si02 ) 2 , zirconium silicate ZrSiCu, calcium silicate, for example bellite (CaO)2Si02 j calcium aluminate, calcium titanate, for example perovskite CaTi03, and calcium zirconate CaO.Zr02. Aluminosilicates in. general are Patent No.1402414 disclosed in UK -Ap-pl -ee-fei©f - e- -3-1 ©- l.

Suitable ternary oxides and complex oxides include attapulgite, kaolinite, asbestos, mica, cordierite and bentonite.

■STJrtab _e--caTb"ide^"inclIde"rfiobi Suitable sulphides include dicerium trisulphide, and suitable nitrides include boron nitride and silicon nitride.

Suitable fluorides include calcium fluoride.

The aforesaid refractory materials may be present in either their naturally occurring form or as synthetic materials.

The preferred refractory material is zirconium gilicate, which is conveniently used in particulate form.

The zirconium silicate may be present as the naturally occurring zircon or as the synthetic compound obtained, for example, by heating a mixture of the component oxides, Zr02 and S1O2 , or by heating a mixture of compounds which give rise to the component oxides on heating.

The zirconium silicate may be admixed with zirconia, whence it is preferred to use a mixture of zirconium silicate particles and zirconia fibres (for example zirconia fibres prepared as described in our copending 1425934, 1445331, 1360197 UK Patents-^iitr- ioTre--No-s--]:2.G-&¾-A73 "566 5" 72V"i1'36 ri Non-fibrous particulate refractory materials of ^' a wide size range may be used, for example from 0.05 to 200 microns, although the particulate refractory materials are preferably in the size range 0.1 to 75 microns.

. Preferably the refractory fibres employed are such that no dimension of the individual fibre exceeds 1 mm.

The proportion of particulate or fibrous refractory material embedded in the matrix of the coating is preferably between 5% and 95% by volume calculated on the total volume of the components in the coating as defined . below. In general, increasing the proportion of particulate or fibrous refractory material leads to a continuous improvement in the shorting resistance of the coating thus obtained, although even quite low proportions of refractory material (for example 5% to ■ 20% by volume) still have a beneficial effect with respect to shorting resistance, especially if the - refractory material is added to the final surface layer or layers. The preferred proportions of refractory material are in the range 20% to 90% by volume calculated on the total volume of the components in the coating.

The volume percentage values of the particulate or " fibrous refractory material are based on the volumes of the. components of the coating, the said volumes being calculated from the known weights of the various components in the coating and the specific gravities of these components (for example,, as given in "The Handbook of Chemistry and Physics", 53rd edition, 1 72-3 published by the Chemical Rubber Company). No account is taken of porosity in making this . calculation .

The. electrodes of the invention may be prepared by the painting and firing technique, wherein a coating of metal and/or metal oxide is formed on a film-forming metal support member by applying a layer of a paint composition comprising thermally-decomposable compounds of each of the metals that are to feature in the finished coating in a liquid vehicle to a cleaned and/or etched surface of the support member, drying the paint layer by evaporating the liquid vehicle and then firing the paint layer by heating the coated support member, suitably at 250°C to 800°C, to decompose the metal compounds of the paint and form the desired coating. The refractory particles or fibres may be mixed into the aforesaid paint composition before it is applied to the support member. Alternatively, refractory particles or fibres may be applied on to a layer of the aforesaid paint composition while this is still in the fluid state on the surface of the support member, the paint layer then being dried by evaporation of the liquid vehicle and fired in the usual manner.

The coated electrodes are preferably built up by applying a plurality of paint layers on the support member, each layer being dried and fired before applying the next layer. Preferably this same techniqur^ of applying a plurality of paint layers and drying and firing each, layer is employed in preparing electrodes according to the present invention using either of the methods described above.

The refractory particles or fibres may be present in each of the layers of paint that are applied to build up the coating.

When the refractory material is in the form of fibres of median length greater than 50 microns and is deposited, on the surface of the paint film after this has been applied to the support member, and while it is still in the fluid state, it is preferred to add the fibres to only the first layer or the first two layers of paint that are applied to the support member, ie any subsequent layers of paint are then laid down without any further addition of the refractory material to the coating. When the refractory material is in non-fibrous particulate form or in the form of very short fibres (less than 50. microns median length), it is preferred to incorporate the material in the paint composition before the paint is applied to the support member and to include the . refractory material in all or in the final layers of paint that are applied to build up the coating.

In preferred electrodes according to the present invention, the matrix of the coating comprises at least one platinum group metal in the elementary and/or the oxidised state and an oxide of at least one film-forming metal. For the manufacture of these preferred electrodes, suitable thermally-decomposable compounds, of the platinum group metals for use in the aforesaid paint compositions are the halides and halo-acid complexes of the platinum group metals, eg RuCl3, RhCl3, H2 tCl6, H2l Cl6 and organo-compounds of the platinum group metals, eg resinates and alko-oxides of these metals. Suitable thermally-decomposable compounds of the film-forming metals are alkoxides, alkoxy-halides in which the halogen is chlorine, bromine or fluorine and resinates of these metals. Most preferred, especially when the electrode support member that is to be coated consists of titanium or a titanium alloy, are the alkyl ortho-titanates , partially-condensed (hydrolysed) derivatives of these, which are usually referred to as alkyl polytitanates , and alkyl halotitanates wherein the halogen is chlorine, bromine or fluorine, especially those compounds of these classes wherein the alkyl groups contain two to four carbon atoms each.

The paint composition is made by dissolving or dispersing a thermally-decomposable compound of at least one platinum group metal and a thermally-decomposable compound of at least one film-forming metal in a liquid vehicle, preferably a lower alkanol, eg an alkanol containing two to six carbon atoms per molecule. The refractory particles or fibres are suspended in this paint composition if they are to be applied to the electrode support member at the same time as the paint film.

When the platinum group metal is to be present in the matrix of the finished coating wholly or preponderantly in the elementary state, a reducing agent, eg linalool, is included in the paint composition and the temperature at which each paint layer is fired is restricted to approximately Ί50°0 maximum.

The coating of the finished electrode very suitably consists of a mixture of platinum group metal oxide and film-forming metal oxide containing 5% to 65% (preferably 25% to 50%) by weight of platinum group metal oxide forming the aforesaid matrix, together with particulate or fibrous refractory material embedded in the matrix in amount between 5% and 95% by volume calculated on the total volume of the components in the coating as defined hereinbefore.

The most preferred electrodes according to the invention for use as anodes in a mercury-cathode cell comprise a support member of titanium or an alloy based on titanium and a coating thereon which comprises 20% to 90% by volume as defined above of the nonconducting particulate or fibrous refractory material, especially zirconium silicate, in a matrix of ruthenium dioxide and titanium dioxide containing 50% to 75% by weight of titanium dioxide (most suitably 65% to 70% by weight of titanium dioxide). According to one modification of this embodiment of the invention, ^ however, up to 50% by weight of the ruthenium dioxide and titanium dioxide in the said matrix may be replaced by one or more of tin dioxide, germanium dioxide and oxides of antimony as described and claimed in the specification of our UK Application No 1,354,8 7.

Preferred coatings of this modified type consist of a matrix which is a three-component mixture of 27% to 45% by weight ruthenium dioxide, 26% to 50% by weight titanium dioxide and 5% to 48% by weight tin dioxide together with 20% to 90% by volume of particulate or fibrous refractory material, especially zirconium silicate.

As a further modification coatings may comprise tin dioxide, germanium dioxide and oxides of antimony which may further include a chlorine-discharge catalyst other than a noble-metal or noble-metal oxide as hereinbefore descibred. Preferred coatings of this type consist of a matrix which is a three component mixture of tin dioxide and oxides of antimony (calculated as Sb203) in the weight ration Sn02:Sb203 from 5:1 to 100:1 with 0.1% to 1.0% by weight manganese difluoride, together with 20% to 90% by volume of non-conducting particulate or fibrous refractory material, especially zirconium silicate.

These modified coatings are suitably obtained by including thermally-decomposable compounds of one or more of tin, germanium and antimony in the paint composition. Suitable thermally-decomposable compounds of tin, germanium and antimony include the alkoxides of these respective elements, their alkoxy-halides wherein the halogen is chlorine; bromine or fluorine and antimony halides.

It will be understood that the relative proportions of thermally-decomposable compounds of platinum group metal, of film-forming metal, and/or of tin and/or germanium and/or antimony in the paint composition employed to form the matrix of the electrode coating will be adjusted to correspond on a chemically equivalent basis with the relative proportions of these elements and/or their oxides desired in the matrix.

While the electrodes of the present invention are particularly useful as anodes in mercury-cathode . cells electrolysing alkali-metal chloride solutions, they can also be used in other electrochemical processes, including other electrolytic processes, electrocatalysis as for instance in fuel cells, electrosynthesis and cathodic protection.

The invention is further illustrated by the following following Examples: Example 1 3 gm o ruthenium trichloride supplied by Johnson Matthey Chemicals Limited and containing 0%. by weight of ruthenium was dissolved in 18.75 m of n-pentanol.

To this solution was added 12 gm of tetra n-butyl orthotitanate and ^.5 gm of "Zircosil 5" - a zirconium silicate of median particle size 1.25 microns, made by Associated Lead Manu acturers Limited ('Zircosil' is a registered. trade mark). This weight composition was selected to give a composition by volume in the final coating of approximately 53% of ZrSiO^ and ^ % of titanium and ruthenium dioxides. The paint was. mixed very thoroughly and applied by spraying to a previously etched experimental titanium anode section consisting of 6 parallel blades each 140 mm long x 6 mm high and 1 mm thick. The upper edges of the blades were fixed at one end to a current lead-in section of 3 mm thick titanium and at the other end to an angle piece of 2 mm thick titanium so that the blades were rigidly supported and remain aligned parallel.

When one coat of the paint had been applied to the titanium anode section the paint was dried at l80°C and then fired in air at 4 0°C to convert the paint to the ruthenium and titanium oxides. After cooling, a further coat of paint was applied, dried and fired. This was repeated until a sufficient number of coats of paint had been applied. The total loading of oxides plus zirconium silicate after firing was equivalent to 75 gm of coating per metre square of projected area of anode.

A similar . experimental titanium anode section was coated in the same way but this time omitting the "Zircosil 5" from the paint. When electrolysing normally, both anodes passed the same current under identical conditions of temperature, brine strength, cell voltage etc. However, when immersed in the mercury cathode to a depth of 4 mm, the sample with "Z rcosil 5" in the coating passed only 260 amps whereas the sample coated with the oxides of ruthenium and titanium alone took a shorting current in excess of 1000 amps.

The current taken by the sample coated with "Zircosil 5" in a matrix of mixed ruthenium and titanium oxides could be entirely accounted' for by the electrolysis of the thin film of brine surrounding the blades; hence virtually no shorting current arising from direct anode to mercury amalgam cathode electronic contact was obtained.

Example 2 26.7 gm of 'Hanovia 05X' liquid bright platinum paint manufactured. by Engelhard Industries Limited was diluted with 13.3 m of thinning essence. To this solution .5 gm "Zircosil 5" was added. The paint was thoroughly mixed and applied to an etched experimental titanium anode section similar to that described in Example 1.

In this case the sample was dried at l80°C and then fired at 50°C after each application of paint so as to produce a coating consisting of a matrix of electrocatalytically active platinum metal in which was dispersed the inorganic refractory additive. The total final loading was equivalent to 36 gm of coating (platinum plus ZrSiC ) per metre square projected anode area. This loading corresponded to a composition by volume of approximately 9% platinum and 91% zirconium silicate (ZrSiOi,). The titanium strips thus coated had a low overpotential for chlorine evolution (80 mV at 10 kA/m2) and passed a current of only 2 to 4 amps/cm of titanium strip when immersed to a depth of ^ mm in flowing mercury with an applied voltage of Jj .2 volts. A simil.ar coating prepared from 'Hanovia 05X ' paint but this time without the addition of "Zircosil 5", allowed a heavy current (greater than 100 amps/cm), to flow as soon as the anode sample touched the mercury surface.

Example 3 ■ A paint was mixed as in Example 1 except that 9 gm of "Zircosil F" was added in place of ¾ .5 gm of "Zircosil 5". "Zircosil F" is a zirconium silicate of median particle size. of 25 microns made b Associated Lead Manufacturers Limited. This weight composition corresponded to a composition by volume in the final coating of approximately J>1% of titanium and ruthenium dioxides plus 69% of ZrSiOi, . The paint was applied in the same manner as in Example 1, and an equally satisfactory coating in respect of the magnitude of the current drawn under short circuit conditions was obtained.

Example A paint of the same composition as in Example 3 was applied to a full size (0.1 m2) anode. The total loading consisting of the oxides of ruthenium and titanium plus zirconium silicate was 7.5 gm. This anode as installed in a mercury cell alongside an anode which was in every way similar except that no. "Zircosil F" was included. in the coating. During a short duration contact between these anodes and the mercury cathode, the anode with "Zircosil F" in its coating passed to kA whereas under the same conditions the anode without "Zircosil F" in its coating passed 17 kA. Example 5 ' A paint was made from 3 gm ruthenium trichloride (containing ^0% Ru by weight), 1.8.75 gm n-pentanol, 12 gm tetra n-butyl orthotitanate , 3 gm "Zircosil 5" and 2 gm "Saffil" (a zirconia-containing fibre, diameter 2 microns, median length 20 microns prepared Patents Nos.1425934, 1445331 and 1360197) as described in UK Appld^titm»-l«&8yjf5-,--5^37'-7-2--ant5l ' 29-9 9--7 H) to give a coating comprising 19% Zr02 , 35% ZrSi04 and 6% Ru02/Ti02. A number of coats of this paint were applied to an experimental anode as in Example 1. When electrolysing normally this anode sample passed the same current as the anode sample incorporating "Zircosil 5" described in Example 1.

When immersed in mercury this sample passed a low shorting current as did the sample incorporating a "Zircosil 5". Furthermore, to illustrate the resistance to short-circuit of these coatings the contact resistance between mercury and the anode surface was measured under standard conditions for the coatings consisting of (1) ruthenium and titanium oxides alone, (2) the coating containing ruthenium and titanium oxides and "Zircosil 5" as in Example 1, and (3) the coating described in this example.

The contact resistances were respectively 0.011 ohm cm2, 0.11 ohm cm2 and 1.96 ohm cm2. The higher the contact resistance between mercury and the anode surface the lower will be the shorting current.

Example 6 A paint was made from.3 gm ruthenium trichloride and 12 gm of tetra n-butyl orthotitanate in 25 gm of n-pentanol and to this was added 0.211 gm of "Zircosil 5". A number of coats of this paint were applied to a titanium anode section as in Example 1. This paint composition was formulated to yield a coating comprising 5% by volume of zirconium silicate particles in a matrix of 95% by volume of ruthenium and titanium dioxides. A strip of this coated section was immersed to a depth of 4 mm in a static pool of mercury under 21.5% w/w NaCl brine and a voltage of 3.5 volts was applied. The total current drawn was 1.12 amps per cm length of the titanium anode strip. Another anode section was also coated with a similar paint composition except, in this case, the "Zircosil 5" was omitted; this section passed 2.9 amps per cm length of strip under identical test conditions.

Example 7 An alternative method for preparing snorting-resistant coatings is to include the particulate refractory material in only the outer layers of the coating. Two anode blade sections were coated in the manner of Example .1, but omitting the refractory additive, ie with ruthenium and titanium Oxides alone. The total loading was 52 gm/m2 projected area. Two and three coats of paint comprising 3 gm ruthenium trichloride, 12 gm of tetra n-butyl orthotitanate , 4 gm of "Zircosil F" and 25 gm of n-pentanol were then applied, dried and fired in the manner of the previous examples. This procedure yielded coatings containing, in toto, approximately 1225 and 17% by volume of zirconium silicate. When subjected to the test described in Example 6, these anode samples passed currents of 1.41 and 1.06 amps per cm length of titanium strip respectively.

Example 8 A paint was prepared from 3 gm ruthenium trichloride, 12 gm tetra n-butyl orthotitanate in 50 gm n-pentanol and to this was added 77.4 gm "Zircosil F". A number of coats of this paint were applied to an anode blade section and fired as in Example 1. This paint formulation was calculated to give a coating .comprising 5% by volume "Zircosil F" and 5% by volume of ruthenium and titanium oxides. This sample was tested in a mercury pilot cell by immersing in the mercury stream flowing at 30 cm/sec surface speed. At 3 mm depth of immersion and with an applied voltage of .2 volts, a current of 133 amps was drawn. Under similar test conditions a sample coated as above with ruthenium and titanium oxides but with no zirconium silicate present passed currents of over 1000 amps.

Example 9 A paint was prepared from 3 gm ruthenium trichloride, 12 gm tetra n-butyl orthotitanate in 25 gm n-pentanol. To this was added 0. 27 gm of "Saffil" (a zirconia containing fibre prepared as described in Patents Nos.1425934, 1445331, 1360197 our copending UK -App- 1i-e-a-t-ierte--Ne-s--¾?Θ-8-8-Α7 —-r66^yh _-(5j 9- -7i--afl-d--2-9- 0 - -7-O) of diameter 2 microns and median length 20 microns. This paint composition was formulated to yield a coating comprising % by volume of zirconia in 95 $ by volume of titanium and ruthenium oxides. An anode section was coated with this paint as in Example 1 and the sample was subjected to the shorting test described in Example 8 . At just over 1 mm depth of immersion and with an applied voltage of 4 . 2 volts, a current of 600 amps was drawn. A similar test using an anode section coated with only ruthenium and titanium dioxides passed a current of over 700 amps.

Example 10 A paint was made from 3 gm ruthenium trichloride, 12. gm tetra n-butyl orthotitanate in 25 gm of n-pentanol To this was added 9 gm of 'Zircosil 200 ' . This is a zirconium silicate powder of a somewhat coarser grade than "Zircosil F" ': whereas 'Zircosil F' is milled to pass a British Standard screen of aperture size 53 microns, "Zircosil 200" is milled to pass a British Standard screen of aperture size 75 microns. This paint was applied in a number of coats to a titanium anode- section in the manner of Example 1.. The paint composition was formulated to yield a coating comprising 69 % by volume of zirconium silicate and 31% by volume of ruthenium and titanium dioxides. A strip of the anode section was immersed to a depth of 4 mm in a static pool of mercury in a similar experiment to that described in Example 6. A current of 0.88 amps/cm length of titanium strip was drawn whereas a coating containing no added zirconium silicate passed 2,9 amps/ cm length of strip under identical test conditions. Example 11 To a paint comprising 3 gm ruthenium trichloride, 12 gm of tetra n-butyl orthotitanate and 18.75 gm of n-pentanol was added 3 gm of 'Micro-Cote' which is a commercial grade of attapulgite - a complex hydrated magnesium aluminium silicate ('Micro-Cote' is a registered trademark of the Floridin Company, USA). The median particle size of the attapulgite powder is 3.- 3 microns. This paint was formulated to give a coating containing 58% by volume of attapulgite in a matrix of ruthenium and titanium dioxides which occupy ^ 2 % by volume of the total coating. The paint was applied to a titanium anode section as described in Example 1. A 3 cm strip of this coated section was immersed to a depth of 4 mm in a flowing mercury cathode under 21.5% w/w N.aCl brine with an applied voltage of 4.2 volts. A total current of between 11 and 13 amp was drawn. A similar strip but coated with only ruthenium and titanium dioxides, under identical test conditions passed a current of over 30 amps.

Example 12 A paint was made from 3 gm of ruthe.nium trichloride 12 gm of tetra n-butyl orthotitanate and 25 gm of n-pentanol. To this was added 9 gm of 'Tioxide CL/D 718 - a commercial titanium dioxide powder (of rutile form) of median particle size 0.3- micron (which is about 30 times the size of the RUO2/T1O2 crystallites of a typical RUO2/T1O2 matrix), as supplied by British Titan Products Limited ('Tioxide' is a registered trademark). The paint was thoroughly mixed and applied to. a titanium anode section in the manner described in Example 1. The paint formulation was designed to yield a coating containing 70% by volume of the Ti02 particles in a matrix of ruthenium and titanium dioxides of proportion 30% by volume. A strip of the coated anode section was tested for its resistance to shorting in the experiment described in Example 6. With an applied voltage of 3.5 volts the total current drawn was 0.71 amps/cm of titanium strip; whereas a strip coated in a similar manner but containing no added particulate titanium dioxide passed 2.9 amps/cm length of strip under identical test conditions.

Example 12 A coating consisting of the oxides of antimony. and tin and manganese fluoride was prepared and applied to an etched titanium anode section according to the following procedure. l8 gm of antimony trioxide were boiled in concentrated nitric acid until evolution of oxides -of nitrogen ceased. 84 gm of metallic tin were dissolved in concentrated nitric acid with heating, and the precipitated tin dioxide formed was thoroughly mixed with the precipitate of antimony oxide and heated for a further period in concentrated nitric acid. The precipitated mixture was washed free from acid and dried in air at 200°C. To the dried mixed oxides was added 3% by weight of manganese difluoride. The resultant mixture was pressed into pellets (100 lb/in2) and fired in air in a furnace at 800°C for 4 hours. After firing, the mixture was crushed and the particle size reduced to less than 60 microns. It was subsequently recompacted into pellets and. fired as before at 1000°C for 24 hours. The resultant material was crushed and the particle size reduced to less than 5 microns by ball milling.

A solution of an alkoxy-tin compound was prepared by boiling under reflux for 24 hours a mixture of 15 gm of stannic chloride and 55 gm of n-amyl alcohol. Into the resultant solution were dissolved 2.13 gm of an t i mon t r c 1ori de .

" A composition suitable for coating on to an electrode support was prepared by suspending 0.17 gm of the above mixed fluoride/oxide material and 0.67 gm of "Zircosil 5" in 3.6 gm of the antimony-trichloride-alkoxy-tin solution. This coating composition was painted on to a strip of titanium which had been immersed overnight in a hot acid solution to etch the surface, and then washed and dried. The coating of paint was dried in an oven at 80°C and heated in a furnace in air at 450°C for 15 minutes to convert the coating substantially into a matrix of the oxides of antimony and tin with manganese difluoride in which is embedded zirconium silicate particles. The whole coating operation and final heating in air at 450°C was then repeated three times to increase the thickness of the coating. The coating comprised approximately 59% by volume of zirconium silicate in l% by volume of Sn02 /Sb203 /MnF2 (in the proportions by weight of 85%, l % and 1% respectively).

A section of this coated strip was then tested for its resistance to shorting in mercury amalgam as described in Example 6. With an applied voltage of 3.5 volts under 21.5% w/w NaCl brine, the total current drawn was 0.20 amps/cm length of titanium strip.

Example l^ A paint was prepared from 3 gm ruthenium trichloride, 12 gm tetra n-butyl orthotitanate in 25 gm of n-pentanol. To this was added and thoroughly mixed 5 gm of hafnium oxide of a median particle size 10.6 microns (supplied by British Drug Houses Limited).

This paint composition was formulated to yield a coating comprising 37% by volume of hafnium oxide, in 63% by volume of titanium and ruthenium oxides. An anode section was coated with this paint in a similar manner to that described in Example 1 and the sample was subjected to the shorting test described in Example 6. At a depth of 4 mm in a- static pool of mercury under 21.5% w/w NaCl brine a current of 0.88 amp/cm length of the titanium strip was drawn for an applied voltage of 3·5 volts.

Example 15 A coating was prepared in a manner similar to that described in Example 1 using a paint- comprising 3 gm of ruthenium trichloride, 12 gm of tetra n-butyl orthotitanate, 25 gm of n-pentanol and 9 gm of eerie oxide. The eerie oxide, which was supplied by British Drug Houses Limited, was of a median particle size 10.5 microns. The paint composition should yield a coating comprising 59% by volume of eerie oxide in 1% by volume of titanium and ruthenium oxides. When a- sample of the coating. on titanium strip was subjected to the shorting test described in Example 6, a current of 1.06 amp/cm length of titanium strip was drawn for an applied voltage of 3.5 volts. This may be directly compared to the current of 2.9 amp/cm length of strip k obtained from strips coated with ruthenium and titanium ^ oxides alone and not containing a refractory nonconducting additive.

Example 16 A paint was prepared from 3 g ruthenium trichloride, 12 gm of tetra n-butyl titanate in 75 gm of n-pentanol and to this was added 9 gm of boron . nitride of median particle size 12.0 microns. The paint was applied to a titanium anode section and dried and fired in the manner described in previous examples. The . final coatings should contain 82% by volume of boron nitride in a matrix of 18% by volume of ruthenium and titanium dioxides . When the sample was. subjected to the shorting test described in Example 6 a current of 0.6 amp/cm length of titanium strip was drawn for an applied voltage of 3.5 volts. The improved shorting resistance of this coating was also' exemplified by the relatively high contact resistance (2.47 x 10 1 ohm cm2) measured across the coating/mercury interface (cf Example 5).

Example 17 A paint comprising 3 gm ruthenium trichloride, 12 gm of tetra n-butyl titanate, 25 gm of n-pentanol and 5.26 gm of silicon nitride (median particle size I6.5 microns) was made up, applied to a titanium anode section and dried and fired in the manner of Example 1. The paint composition yielded a coating containing approximately 67% by volume of silicon nitride embedded in a matrix of ruthenium and titanium dioxides 33% by volume. When the titanium anode section was tested for its resistance to direct mercury contact by lowering into a static pool of mercury under 21..5% w/w NaCl brine, a current of 0.69 amp/cm length of titanium strip was drawn for a 4 mm depth of immersion at 3.5 volts. Coatings containing no refractory particles and only consisting of RUO2/T1O2 under the same test conditions pass currents in excess of 2.5 amps/.cm length of titanium strip.

Example l8 3 in of ruthenium trichloride supplied by Johnson Matthey Chemicals Limited and containing 40% by weight of ruthenium was dissolved in 25 gm of n-pentanol. To this solution was added 12 gm of tetra n-butyl orthotitanate and 9 gm of mullite, an alumino-silicate having the formula (Al203 ) 3 (Si02)2, and having a median particle size 39 microns, made by Cawoods Refractories Limited. This weight composition was selected to give a composition by volume in the final coating of approximately 76% of mullite and 24% of titanium and ruthenium dioxides. The paint was mixed very thoroughly and applied by spraying to a previously etched experimental titanium anode section consisting of 6 parallel blades each 140 mm long x 6 mm high and 1 mm thick. The upper edges of the blades were fixed at one end of the current lead-in section of 3 mm thick titanium and at the other end to an angle piece of 2 mm thick titanium so that the blades were rigidly supported and remain aligned parallel.

When one coat of the paint had been applied to the titanium anode section the paint was dried at l80°C and then fired in air at ^50°C to convert the paint to the ruthenium and titanium oxides. After cooling, a. further coat of paint was applied, dried and fired.

This was repeated until a sufficient number of coats of paint had been applied. The total, loading of oxides plus mullite after firing was equivalent to 75 gm of coating per metre square of projected area of .anode.

A strip' of this coated anode section was subjected to a shorting test which consisted of immersing the strip to a depth of 4 mm in a static pool of mercury under 21.5% w/w NaCl brine and applying a voltage of 3.5 volts. The total current drawn was 1.2 amp/cm length of. the titanium . strip .

By way of comparison, another anode section was also coated with a similar paint composition except, in this case, the mullite was omitted. This section passed 2.9 amps per cm length of strip under identical test conditions.

Example 19 A paint was prepared from 3 gm ruthenium trichloride, 12 gm tetra n-butyl orthotitanate in 25 gm of n-pentanol. To this was added and thoroughly mixed 9 gm of calcium silicate of a median particle size 15.5 microns (supplied by Crosfield Chemicals). This paint composition was formulated to yield a coating comprising^' 80% by volume of calcium silicate in 20% of titanium and ruthenium oxides. An anode section was coated with this paint in a similar manner to that described in Example 18 and the sample was subjected to the shorting test described in Example l8. At a depth of M mm in a static pool of mercury under 21.5% w/w NaCl brine a current of 0.62 amp/cm length of the titanium strip was drawn for an applied voltage of 3.5 volts.

Example 20 A paint was prepared from 3 gm ruthenium trichloride 12 gm tetra n-butyl orthotitanate in 25 gm of ' n-pentanol . To this was added and thoroughly mixed 9 gm of kaolinite of a median particle size 17 microns (supplied by Hopkin and Williams Limited). This paint composition was formulated to yield a coating comprising 79% by volume of kaolinite in 21% of titanium and ruthenium oxides. An anode section was coated with this paint in a similar manner to that described in Example l8 and the sample was subjected to the shorting test described in Example l8. At a depth of ¾ ram in a static pool of mercury under 21.5 w/w NaCl brine a current of 0.41 amp/cm length of the titanium strip was drawn for an applied voltage of 3.5 volts.

Example 21 A paint was prepared from 3 gm ruthenium trichloride 12 gm tetra n-butyl orthotitanate in 25 gm of n-pentanol.

To this was added and thoroughly mixed 9 gm of bentonite of a median particle size 22 microns (supplied by Production Chemicals Limited) . This paint composition was formulated to yield a coating comprising 80% by volume of bentonite in 20% of titanium and ruthenium oxides. An anode section was coated with this paiUft. in a similar manner to that described in Example 18 and the sample was subjected to the shorting test described in Example 18. At a depth of 4 mm in a static pool of mercury under 21.5% w/w NaCl brine a current of 0.54 amp/cm length of the titanium strip was drawn for an applied Voltage of 3.5 volts'.

A paint was prepared from 3 gm ruthenium trichloride, 12 gm tetra n-butyl orthotitanate in 25 gm of n-pentanol. To this was added and thoroughly mixed 9 gm of calcium fluoride of a median particle size 15.5 microns (supplied by Hopkin and Williams Limited). This paint composition was formulated to yield a coating comprising 76% by volume of calcium fluoride in 2 % of titanium and ruthenium oxides. An anode section was coated with this paint in a similar manner to that described in Example l8 and the sample was subjected to the shorting test described in Example 18. At a depth of 4 mm in a static pool of mercury under 21 . 5 % w/w NaCl brine a current of 0.66 amp/cm length of the titanium strip was drawn for an applied voltage of 3.5 volts.

Claims (1)

1. we claim is:- · An electrode for electrochemical processes, which ^ comprise a support member of a film-forming metal or a film- orming metal alloy and. an electro-catalytically active coating thereon, which coating consists of a matrix of electroconducting material having electrocatal tic properties and embedded in the said matrix a non-conducting particulate or fibrous refractory material, and wherein the said refractory material is selected from the group consisting of oxides, -G-a-pb-i-d-e-s-5 sulphides, nitrides and fluorides, the said oxides being other than those claimed and/or described in Patent No.1402414 our copending UK Jp lic-a:ti-on--Ne- -3-19O7/- l (Belgian Specification 788883). An electrode according to Claim 1 wherein the oxide is a single oxide selected from the . group consisting of eerie oxide, hafnium oxide and ditantalum pentoxide.. An electrode according to Claim 1 wherein the oxide is a binary oxide selected from the group consisting of magnesium aluminate, mullite, zirconium silicate, calcium silicate, calcium aluminate, calcium titanate, and calcium zirconate. An electrode according to Claim 1 wherein the oxide is a ternary or complex oxide selected from the group consisting of attapulgite, kaolinite, asbestos, mica, cordierite and bentonite. -5 τ -AH--e-teet-po4e--a-c-e-θr-d-i-n-g- x -Gla-i-ffl-1--wh-er -i-ft-the— -ea^ de--i-&-Ri€>te-iHHi--e- pfe--i r 5. J&-. An electrode according to Claim 1 wherein the sulphide is dicerium trisulphide. 6. -?·. An electrode according to Claim 1 wherein the nitride is selected from the group consisting of • boron nitride and silicon nitride. 7. . An electrode according to Claim 1 wherein the fluoride is calcium fluoride. 8.· ··. An electrode according to Claim 1 wherein the refractory material is a mixture of zirconium silicate and zirconia. 8 9. to-. An electrode according to Claim 9- wherein the refractory material is a mixture of zirconium silicate particles and zirconia fibres. 10. -_ri. An electrode according to any one of the preceding claims wherein the refractory material consists of non-fibrous particles in the size range 0.05 to 200 microns. 10 11. K?-. An electrode according to Claim i÷ wherein the refractory material is in the size range 0.1 to 75 microns. 12. 1-5-.. An electrode according to any one of the preceding claims wherein the refractory material consists of fibres wherein no dimension of the individual fibres exceeds 1 mm. 13. 3r . An electrode according to any one of the preceding claims wherein the proportion of particulate or fibrous refractory material embedded in the L matrix of the coating is 5% to 95% by volume . ^ calculated on the total volume of the components in the coating. 14. . An electrode according to Claim 3-4 wherein the proportion of particulate or fibrous refractory material embedded in the matrix is 20% to 90% by volume calculated on the total volume of components in the coating. 15. 3-6. An electrode according to any one of the preceding claims wherein the said support member is made of titanium or an alloy based on titanium and having anodic polarisation properties similar to those of titanium. 15 16. tf. An electrode according to Claim i& wherein the said matrix comprises at least one platinum group metal and/or oxides of at least one platinum group metal. 17. i-σ. An electrode according to Claim ¥† wherein the said matrix consists of at least one platinum group metal and/or oxides thereof in admixture with at least one non-noble metal oxide. 17 18. 1-9. An electrode according to Claim 3r8- wherein the non-noble metal oxide portion of the said matrix consists of at least one oxide selected from oxides of titanium, zirconium, niobium, tantalum and tungsten, tin dioxide, germanium dioxide and oxides of antimony. 18 19 £6. An electrode according to Claim ϊ-9" wherein the said coating consists of a matrix of platinum group metal oxide and an oxide of titanium, zirconium, niobium, tantalum or tungsten containing. 5% to 65 % by weight of platinum group metal oxide, and embedded in the matrix non-conducting particulate or fibrous refractory material in amount between 5% and 95% by volume calculated on the total volume of the components of the coating. 19 20. An electrode according to Claim 2-Θ wherein the said matrix contains 25% to 50% by weight of the platinum group metal oxide. 20. 21 . An electrode' according to Claim B÷- wherein the coating on the support member consists of a matrix of ruthenium dioxide and titanium dioxide containing 50% to 15% by weight of titanium dioxide and embedded in the matrix 20% to 90% by volume of non-conducting particulate or fibrous refractory material calculated on the total volume of the components of the coating. 21 22 ; An electrode according to Claim 3* wherein the said matrix contains 65 % to 0% by weight of titanium dioxide. 18 23. -H* . An electrode according to Claim ·3τ9 wherein the coating on the support member consists of a matrix which is a three-component mixture of 27% to ^ 5 % by weight ruthenium dioxide, 26 % to 50% by weight- titanium dioxide and 5% to ^8% by weight tin dioxide and embedded in the matrix 20% to 90% by volume of non-conducting particulate or fibrous V refractory material calculated on the total volume of the components in the coating. 24. 2 > An electrode according to any one of Claims 1 to /S wherein the said matrix consists of at least one oxide selected from tin dioxide, germanium dioxide and the oxides of antimony. 24 25. 2-6. An electrode according to Claim wherein the said matrix consists of at least one oxide selected from tin dioxide, germanium dioxide, and the oxides of antimony in admixture with a .chlorine discharge catalyst . 25 26. 2-?-. An electrode according to Claim 2-6 wherein the chlorine discharge catalyst is manganese difluoride. 26 27. . An electrode according to Claim wherein the said coating consists of a matrix of tin dioxide, the oxides of antimony and manganese difluoride containing tin dioxide and antimony oxides • (calculated as Sb203) in the weight proportions of from 5:1 to 100:1 and with 0.1% to 1.0% by weight of manganese difluoride and embedded in the matrix 20% to 90% by volume of non-conducting particulate or fibrous refractory material calculated on the total volume of the components in the coating. 28. 5^9". Electrodes according to Claim 1 and substantially as hereinbefore described with reference to Examples 1 to 17 Electrodes according to Claim 1 and substantially as hereinbefore described with reference to Examples 18 to 2-5·. 22. A method for the manufacture of an electrode as claimed in any one of the preceding claims wherein' the electrocatalytically-active electro-conducting coating is formed on a support member made of a film-forming metal or a film- forming metal alloy by heating on the said support member at least one applied layer of a paint composition containing a metal compound or a . plurality of metal compounds that is or are thermally-decomposable to form the said coating, the said method comprising including in the said paint composition, a non-conducting particulate or fibrous refractory material, drying each paint layer by heating the coated support member at 250 to 800°C to convert the said metal compound or compounds into an electrocatalytically active electroconducting matrix in which the non-conducti particulate or fibrous refractory material is embedded. 30 A method according to Claim rt wherein the said refractory material is in the form of non-fibrous particles and is added to the paint composition forming the final surface layers of the coating. 30 A method according to Claim ¾i wherein the said refractory material is in the form of fibres of mean length less than 50 microns and is added to the paint composition forming the final surface layers of the coating. 33. 3^.· A method for the manufacture of an electrode as 29 claimed in any one of Claims 1 to 5θ wherein an electrocatalytically active electroconducting coating is formed on a support member made of a film-forming metal or a film-forming metal alloy by heating on the said support member an applied layer of a paint composition containing a metal compound or a plurality of metal compounds that is or are thermally-decomposable to . form the said coating, the said method comprising depositing non-conducting particulate or fibrous refractory material; on to the said layer of paint composition while this is in the fluid state on the said support member, drying the paint layer by evaporating its liquid vehicle and then firing the paint layer by heating the coated support member 250 to 800°C to convert the said metal compound or compounds into an electrocatalytically active electroconducting matrix in which the non-conducting particulate or fibrous refractory material is embedded. 34.: A method as claimed in Claim wherein a plurality of layers of the said paint composition is applied to the said support member, each layer being dried by evaporation of its liquid vehicle and then fired by heating the coated support member at 250 I to 800WC, the said r.e'f .ctory material is in the form of fibres of length greater than 50 microns and is al&¾d to only the first layer of the first two MSw&i&s of paint composition by depositing it on the said first layer or the said first two layers while these respective layers are in the fluid state. 35. ½&. A method for the manufacture of electrodes according 30 34 to any one of Claims to ¾· substantially as . hereinbefore described with reference to Examples 1 to 17. ■ 36. 5^·- A method for the manufacture of electrodes according to any one of Claims substantially as hereinbefore described with reference to Examples 18 to 23. ·