EP2188417A2

EP2188417A2 - Combination of a cathode element with a diaphragm

Info

Publication number: EP2188417A2
Application number: EP08804000A
Authority: EP
Inventors: Briac Morin; Jean-Claude Kiefer
Original assignee: Chloralp SAS
Current assignee: Vencorex France SAS
Priority date: 2007-09-14
Filing date: 2008-09-11
Publication date: 2010-05-26
Also published as: FR2921073B1; FR2921073A1; WO2009034116A3; WO2009034116A2

Abstract

The invention relates to a combination of a composite cathode element with a diaphragm, and to a method for producing the same. The invention also relates to the use of this combination in a cell for the electrolysis of aqueous solutions of alkaline metal halides. The association of a composite cathode element with a diaphragm according to the invention is characterised in that it comprises an elementary cathode covered with a pre-cathode consisting of a volumetric ply of fibres containing at least carbon or graphite fibres bound by a fluorinated polymer, said ply being made more conductive using an electrocatalytic agent, and in that the pre-cathode is covered by a diaphragm comprising a ply of fluorinated polymer fibres in which are imbedded particles of an inorganic finely divided refractory material not soluble in water.

Description

ASSOCIATION OF A CATHODIC ELEMENT AND A DIAPHRAGM.

The present invention relates to a combination of a composite cathode element and a diaphragm.

It also relates to its manufacturing process. The invention also relates to the use of this combination in an electrolysis cell of aqueous solutions of alkali metal halides.

Conventionally, the production of chlorine and sodium hydroxide is carried out by electrolysis of aqueous solutions of alkali metal halides in an electrolysis cell comprising a cathode element comprising an elementary cathode optionally covered with a microporous material and a diaphragm whose essential function is to separate the anode compartment from the cathode compartment.

Whether it is the cathode or the diaphragm, they must both be made of materials meeting many requirements in order to obtain the reliability and profitability required by an industrial operation. It is known that the materials used for producing the cathode element must meet several qualities. They must have a low electrical resistivity compatible with the operation of the electrolysis cell equipped with such a cathode element at an acceptable energy level. They must have a small thickness of about 0.1 to 10 mm while they must have a large surface area that may exceed several square meters.

In addition these materials must be obtainable by deposition of a rigid structure which has opening rates and large hole diameters. As for the qualities required for the diaphragm, they are also very numerous.

The diaphragm in an electrolysis cell behaves like a medium allowing on the one hand the passage of the current with a low ohmic drop and on the other hand the uniform flow of the electrolyte from one compartment to another.

This results in a set of mechanical and hydraulic conditions all the more critical that we are called in the electrolysis cells modern to operate with a high current density except to tolerate prohibitive ohmic drops.

The qualities required for a diaphragm are contradictory. Indeed, from a mechanical point of view, the diaphragm must have a defined and persistent geometry, it must be homogeneous in size and structure. The phenomenon of swelling of the diaphragm should be avoided.

From an electrical point of view, the diaphragm is characterized by its relative resistance. By relative resistance is meant the quotient of the resistance of a medium constituted by a diaphragm soaked with electrolyte to the resistance of the same medium consisting solely of electrolyte.

It has been observed that this relative resistance is related to the porosity of the diaphragm but also to the shape of the flow channels of the electrolyte.

Finally, it is advantageous to avoid diffusion phenomena from one medium to another through the diaphragm, in particular in the case of the electrolysis of a sodium chloride solution, a flow of OH ions must be avoided. ^" In the opposite direction of the liquid current which is responsible for the formation of chlorate, therefore the yield drop.

This disadvantage can be overcome by increasing the thickness of the diaphragm and decreasing its porosity, but then increasing the voltage drop in the diaphragm.

Finally, from a hydraulic point of view, the permeability of a diaphragm must be such that the pressure drop is low, but this permeability is a function of the pore size, but for the reasons stated above, an excessively large pore diameter does not occur. can be tolerated. A last requirement that we meet is that of reliability over time. Indeed, the technology is moving towards relatively long-lived cells. In this respect, diaphragms used are expected to have a rigorous stability in the time in their properties.

Therefore, the implementation of a precathode should not interfere with the intrinsic qualities of the diaphragm.

In addition, the industry is constantly confronted with the problems of reducing operating costs including the cost of electricity.

Thus, efforts are continually made to increase the efficiency of electrolysis. The object of the present invention is to provide a solution for reducing energy consumption while respecting the constraints imposed on the cathode / diaphragm electrolytic system. The object of the invention is a combination of a composite cathode element and a diaphragm characterized in that it comprises an elementary cathode covered with a precathode consisting of a volume sheet of fibers comprising at least fluorinated polymer or graphite, said web being rendered more conductive by an electrocatalytic agent and in that the precathode is covered with a diaphragm consisting of a web of fluoropolymer fibers in which nested particles of a finely divided inorganic refractory material insoluble in water. Depending on the embodiment of the various coatings on the cathode, there may be the presence of other additives such as, in particular, residual amounts of thickeners and blowing agents used in the preparation of the coatings.

Another object of the invention lies in the manufacturing process of the combination.

The originality of the present invention lies in the combination of two coatings which are deposited on the elementary cathode.

According to the invention, it is therefore proposed to cover the elementary cathode with two layers of fibers, the first layer with conductive fibers which are totally or partially carbon or graphite fibers thus constituting the precathode and the second layer with fibers. preferably insulating polytetrafluoroethylene fibers constituting the diaphragm. The diaphragm is deposited successively on the elementary cathode, the precathode and is deposited directly thereon, which is an insulating material thus making it possible to ensure the separation of the anode and cathode compartments.

Thus, in proposing the association as claimed, it could be feared that the carbon or graphite fibers render the diaphragm conductive. In this case, hydrogen could have been generated at the anode and reach explosive areas of damage to the cell.

It has been found that the association defined according to the invention can operate on an industrial scale and makes it possible to obtain gains in terms of energy consumption.

According to the invention, the elementary cathode is covered with a precathode defined above. The term "elementary cathode", a metal part consisting essentially of a mesh or a piece of perforated metal and acting as a cathode in an electrolysis cell.

It is a metal surface having a mesh gap or perforations whose size is generally between 20 microns and 5 mm.

The elementary cathode is most often in the form of a grid.

It is made of iron or one of its alloys (eg bronze) that can withstand its environment. The precathode which covers the elementary cathode is a microporous material which comprises a sheet of fibers bonded by a fluoropolymer.

By "fiber web" is defined a three-dimensional assembly or stack of fibers whose thickness is substantially smaller than the smallest of the other dimensions, said assembly may or may not have two parallel surfaces. This sheet may be in various forms, in particular determined by the shape of the elementary cathode with which it is associated.

Generally, the thickness of this layer may be between 0.1 and 5 mm, one of the large dimensions may substantially correspond to the height of the cathode element and reach 1 m or more.

One of the constituents of the web according to the invention consists of fibers, part of which is electrically conductive fibers.

The term "conductive fibers" denotes any material in the form of a filament whose diameter is generally less than 1 mm and preferably between 10 ^-5 and 0.1 mm and whose length is greater than 0.5 mm and preferably between 1 and 20 mm, said material having a resistivity of less than or equal to 0.4 Ω.cm.

According to a preferred embodiment of the invention, the conductive fibers are carbon or graphite fibers. However, the invention does not exclude the case where a portion of the carbon or graphite fibers, for example from 1% to 30% by weight, is substituted by non-conductive fibers such as, for example, mineral fibers such as glass, quartz, zirconia or organic fibers including polypropylene fibers, polyethylene fibers or fluoropolymers such as polytetrafluoroethylene.

Fibers, entirely of carbon or graphite, are preferred. They are in the form of filaments whose dimensions are those mentioned above in the definition of conductive fibers. Preferably, these carbon or graphite fibers have a monodisperse length distribution, that is to say a distribution of lengths such that the length of at least 80% and, advantageously at least 90% of the fibers corresponds to the average length to within ± 20%, and preferably within ± 10%.

As mentioned above, the fibers are bound in the form of a web with a fluoropolymer.

The term "fluoropolymer" currently refers to a homopolymer or copolymer derived at least in part from olefinic monomers substituted by fluorine atoms, or substituted by a combination of fluorine atoms and at least one of the chlorine atoms. , bromine or iodine per monomer.

Examples of fluorinated homopolymers or copolymers may consist of polymers and copolymers derived from tetrafluoroethylene, hexafluoropropylene, chlorotrifluoroethylene, bromotrifluoroethylene.

Such fluorinated polymers may also contain up to 75 mole percent of units derived from other unsaturated ethylenic monomers containing at least as many fluorine atoms as carbon atoms, such as, for example, vinylidene difluoride, vinyl and perfluoroalkyl, such as perfluoroalkoxyethylenes.

Naturally, it is possible to use in the invention several homopolymers or fluorinated copolymers as defined above. It goes without saying that it would not be outside the scope of the invention by associating with these fluoropolymers a small amount, for example up to 10 or 15% by weight of polymers whose molecule does not contain carbon atoms. fluorine, such as polypropylene.

The fluoropolymer is preferably polytetrafluoroethylene. The fluorinated polymer may represent up to 30% of the total weight of the sheet, this level being more generally between 5 and 20%. The fluoropolymer according to the invention is advantageously in the form of an aqueous dispersion (latex) containing, in general, from 30 to 70% of dry polymer, having a particle size of between 0.1 and 5 μm, and preferably , between 0.1 and 1 μm.

It is not beyond the scope of the present invention using a fluoropolymer in the form of dry powder or fiber.

According to the invention, the fiber web may contain one or more electrocatalytic agents which are preferably uniformly distributed in the web. These catalysts are most often in the form of a powder whose particle size varies for example between 1 and 100 microns.

As electrocatalytic agents, mention may be made especially of platinum and palladium, but said agent is preferably chosen from the group consisting of Raney metals and Raney alloys from which most of the easily removable metal (metals) has been removed. (s) and mixtures thereof.

By Raney metal (or Raney type) is meant essentially particular catalytic forms of large surface of the following metals: nickel, cobalt, iron and copper, optionally doped or stabilized by at least one metal selected in the group consisting of: chromium, cobalt, titanium, molybdenum, tungsten, vanadium and manganese.

However, it is not excluded that the Raney metal is likely to be transformed both chemically and physically as a result of treatment during the preparation of the material and / or during its use.

These particular large surface catalytic forms are generally obtained, in a manner known per se, from an alloy containing one or more of these catalytically active metals and one or more easily removable metals such as aluminum, silicon or magnesium. and zinc; said catalytically active metal being present in the "dissolved" state in the easily removable metal. The starting alloy (or precursor) may also contain minor amounts generally not exceeding 5% by weight of the active metal (s) (s), one or more stabilizing or doping metals selected from chromium , cobalt, titanium, molybdenum, tungsten, vanadium and manganese. During the alkaline leaching of such a precursor, most of the easily removable metal (s) is removed. It should be specified here that by the expression "Raney alloys, from which most of the easily removable metal (s) has been removed," in the context of the present invention is meant alloys "Precursors" which have just been discussed, but which have been treated in order to eliminate nevertheless, after treatment, up to 20% by weight of these easily removable metals.

Reference can be made in particular to the following works for obtaining various preparations: Chemical Technology Review No. 94 "Hydrogenation Catalysts" and RJ. Peterson / Noyes Data Corp. 1977, "Preparation of Nickel Hydrogenation Catalyst" pages 3-10.

It should be noted that the presence of nickel in the pre-cathodic layer covering the cathode makes it possible to obtain the additional advantage of reducing very remarkably the chlorate content in a ratio of 1 to 10, usually from 1 to 5, of sodium hydroxide produced by the electrolysis of the sodium chloride solution.

As an indication, the amount of electrocatalytic agent, in whatever form it may present may represent up to 30% of the weight of the bonded web and more generally from 1 to 20% of this weight.

As mentioned above, the fiber web can contain residual amounts of additives generally not exceeding 5% of its weight which can be added simultaneously or successively during the preparation of the precathode. Thus, they may contain traces of surfactants, porogenic agents whose role is to regulate the porosity of the material constituting the precathode, and / or thickeners although in principle said agents are decomposed or eliminated during the development of said material. Their nature will be specified later in the description of the invention. According to the invention, the elementary precathode is covered with a diaphragm which consists essentially of insulating fibers associated with an inorganic filler.

By "insulating fibers" is meant any material in the form of a filament which is not an electrically conductive fiber. As insulating fibers, mention may be made of the fluoropolymer fibers in the sense indicated above and especially the polytetrafluoroethylene fibers.

The polytetrafluoroethylene fibers, hereinafter referred to as PTFE fibers, used in the context of the present invention are insulating fibers which may have variable dimensions: their diameter (D) is generally between 10 and 500 μm and their length (L ) is such that the L / D ratio is between 5 and 500. Preferably, PTFE fibers are used, the average dimensions of which are between 1 and 10 mm for the length and between 50 and 200 μm for the diameter. . Their preparation is described in US Pat. No. 4,444,640 and this type of PTFE fiber is known to those skilled in the art.

Also involved in the diaphragm is a mineral filler which is an inorganic refractory material, insoluble in water and in the form of finely divided particles. Examples of suitable materials for the preparation of the diaphragm include oxides, borides, carbides, nitrides or silicates of the following metals: zirconium, vanadium, chromium, niobium, molybdenum, hafnium, tantalum, titanium and tungsten and mixtures thereof. According to a preferred embodiment of the invention, use is made of zirconium oxide, zirconium silicate or mixtures thereof.

The particles of said material are of essentially spherical shape. Their size, expressed by their diameter, advantageously varies between 0.5 and 10 μm and is preferably between 1 and 5 μm.

The amount of the refractory material can represent up to 75% of the weight of the fluoropolymer fibers and more generally from 20 to 50% of this weight

The essential components of the diaphragm have been specified above, but it goes without saying that it can contain as for the precathode residual amounts of additives used in its manufacture.

The diaphragm according to the present invention advantageously has a weight per unit area of between 3 and 7 kg / m ² , preferably between 4 and 6 kg / m ² . The thickness of the diaphragm generally varies between 1 and 5 mm and is preferably between 2 and 4 mm.

The association, which is the subject of the invention, is in a way, an assembly from one face to the other, of three layers, namely an elementary cathode, a precathode in the form of a first conductive fibrous sheet of the electricity and a diaphragm in the form of a sheet of insulating fibers.

The subject of the present invention is also the process for preparing said combination essentially comprising the following steps: a - the deposition of a precathodic layer by programmed vacuum filtration of an aqueous dispersion comprising at least carbon or graphite fibers, the fluoropolymer, the electrocatalytic agent, a thickening agent, a pore-forming agent and, where appropriate, other additives through an elementary cathode constituted by a wire mesh or perforated metal surface; b - the elimination of the liquid medium and, where appropriate; the drying of the sheet thus formed, c - the deposition of a sheet of consolidated fibers constituting the diaphragm by programmed vacuum filtration of an aqueous dispersion comprising at least the fluoropolymer fibers, the refractory material, a thickening agent , through the porous precathodic material, d - the elimination of the liquid medium and, where appropriate, the drying of the sheet thus formed, e - the sintering of the assembly. Thus, the method of the invention comprises the preparation of aqueous dispersions in order to deposit the various successive coatings.

The dispersion (or suspension) allowing the deposition of the precathodic layer is generally strongly diluted. Its dry matter content (fibers, fluoropolymer, electrocatalytic agent, additives) varies most often between 1 and 5% and is preferably between 2 and 4% by weight.

The relative amounts of the essential constituents of the precathodic material to be introduced into the dispersion are substantially the same as those found in the material. In general, the dispersion contains the amounts expressed as solids of the following main constituents:

from 15% to 40% by weight of carbon or graphite fibers,

from 5% to 20% by weight of the fluorinated polymeric binder,

from 1% to 30% by weight of electrocatalytic agent (s), from 40% to 55% by weight of a pore-forming agent,

from 3% to 6% by weight of a thickening agent.

By weight of "fluorinated polymeric binder" is meant the weight of the fluoropolymer type binder plus the weight of the fluoropolymer fibers in the case where they are introduced as a partial substitution of carbon or graphite fibers.

For a good implementation of the invention, the preferred contents are as follows:

from 20% to 30% by weight of carbon or graphite fibers,

from 5% to 10% by weight of the fluorinated polymeric binder, from 1% to 20% by weight of electrocatalytic agent (s),

from 45% to 50% by weight of a blowing agent,

from 4% to 5% by weight of a thickening agent.

The sheet contains a porogenic agent whose role is to regulate the porosity of the sheet, porosity that influences the flow of liquids and the evacuation of gases.

A compound is used which can then be removed by leaching or by chemical or thermal decomposition.

Those which are removed during the sintering operation are preferred.

As porogenic agents, there may be mentioned in particular alkali or alkaline earth salts, such as halides, sulphates, sulphites, bisulphites, phosphates, carbonates, bicarbonates; amphoteric alumina or a derivative based on silica: the latter being preferred. According to the invention, the term "silica-based derivatives" is intended to mean precipitated silicas and fumed or pyrogenic silicas.

It is also possible to use mixtures of said blowing agents. Advantageously, the silicas have a specific surface area

BET of between 100 m ² / g and 300 m ² / g and / or a granulometry determined by the COULTER ^® counter between 1 and 50 microns and preferably between 1 and 15 microns.

These derivatives behave as excellent porogens providing substantially no deconsolidation of the electroconductive microporous material when used in the amounts of the present invention.

In order to obtain a homogeneous deposit on the cathode, it is advantageous to introduce into the dispersion a thickening agent in order to increase the viscosity of the dispersion. By "thickener" is meant according to the present invention, a compound which increases the viscosity of the solution and which has water retentive properties. Natural or synthetic polysaccharides are generally used.

There may be mentioned biopolymers obtained by fermentation of a carbohydrate under the action of microorganisms and more particularly the Xanthan gum obtained by fermentation of a sugar using Xanthomonas Campestris.

Xanthan gum is commercially available in the form of a powder having a particle size generally between 50 and 250 microns and a bulk density greater than about 0.7.

As other types of thickening agents which may be used, mention may also be made of carrageenan gum Kappa (Aubytel X52) or carrageenan Iota, pectin (Unipectin HMI), guar gum, locust bean gum, Wellan gum alginates and mixtures thereof. It should be noted that various additives may be added to the dispersion, in particular surfactants, biocidal agents. Their content is low and generally represents less than 5% of the total weight of dry matter.

In the present invention, at least one surfactant is preferably added during the preparation of the suspension in step a).

The maximum amount of surfactant present is generally less than 3% and is preferably between 0.5 and 2%. Preferably, a nonionic surfactant is used. As nonionic surfactants, it is especially possible to use ethoxylated alcohols or fluorocarbon compounds with functional groups alone or as a mixture; these alcohols or these fluorocarbon compounds generally have carbon chains from Ce to C2. Preferably, ethoxylated alcohols which are ethoxylated alkylphenols, such as in particular octoxynols, are used. (Triton X-100 ^® ).

Examples of biocidal agents include bactericides (triclosan ...), 1, 2-benzisothiazolin-3-one (Procel ^® ), fungicides. The amount of biocide is generally at most 2% by weight and is preferably between 0.01 and 0.05%.

According to the process of the invention, the preparation of the aqueous dispersion comprising the constituents intended for preparing the precathode is carried out by introducing into the water, in the following preferred order: the surfactant and the thickening agent.

It will be specified that the viscosity measured at ambient temperature (20 ^° C.) at the BROOKFIELD viscometer at a speed of 50 rpm varies between 20 and 60 centipoise, preferably between 30 and 50 centipoise.

The fibers are then introduced, the fluoropolymer preferably a PTFE latex, the blowing agent and lastly the electrocatalytic agent. The constituents are mixed with stirring. The elementary cathode which consists of a metal surface having a mesh gap or perforations of between 20 μm and 5 mm is dipped in the aqueous dispersion described above. The web is then formed by vacuum filtration of the dispersion through the cathode.

The vacuum program can be continuous or stepwise, from atmospheric pressure to final pressure (0.01 to 0.5 bar absolute).

After removal of the liquid medium, the sheet thus formed is dried in air, at a temperature preferably between 70 ⁰ C and 120 ° C; for a period ranging from 1 to 12 hours.

In a next step, the diaphragm is deposited on the precathode by soaking the precathode in an aqueous dispersion of insulating fibers as defined above. The dispersion (or suspension) for the deposition of the diaphragm is an aqueous dispersion having a dry matter concentration which most often varies between 10 and 30% by weight and is preferably between 15 and 25%. The dispersion of the insulating fibers composed of fibers of a fluorinated polymer, preferably of PTFE, is introduced by introducing into water a thickening agent and various additives such as sodium hydroxide, biocidal agent and antifoam agent, for example an antifoam agent. of silicone type and then the fibers.

The use of the thickener makes it possible to increase the viscosity of the dispersion.

The amount of thickening agent in the dispersion is generally between 0.1 and 0.3% by weight. It will be specified that at this stage, the viscosity measured at room temperature

(20 ^° C.) at the BROOKFIELD viscometer at a speed of 1 rpm varies between 800 and 3000 centipoise, preferably between 1000 and 2800 centipoise.

It may be mentioned that the medium may also contain caustic soda used to preserve the cathode during the impregnation. 0.5 to 2% by weight of soda expressed relative to a commercial solution of sodium hydroxide at 30% by weight are added.

A mixture comprising from 35 to 60% by weight of fluoropolymer fibers, preferably PTFE, from 35 to 60% of sodium chloride and from 5 to 35% of zirconia or any other refractory material, is then introduced. The preferred amounts used in the mixture are from 40 to

55% by weight of fluoropolymer fibers, 40 to 55% of sodium chloride and 10% to 30% of zirconia or any other refractory material.

A quantity of mixture is used such that the fibers represent from 3.5 to 18% of the weight of the dispersion and preferably from 4 to 16.5% by weight.

It should be noted that the presence of sodium chloride is inherent in the manufacture of PTFE fibers. Other inert and granular solid materials can be used in their manufacture in place of sodium chloride and there may be mentioned in particular, alumina, calcium carbonate or sand.

The precathode is soaked in the aqueous dispersion described above. It should be noted that, given the thickness of the chosen diaphragm, most of the time, it is necessary to make several successive impregnations, for example up to 10. The web is then formed by vacuum filtration programmed of the dispersion to through the cathode and precathode.

The vacuum program can be continuous or stepwise, from atmospheric pressure to final pressure (0.01 to 0.5 bar absolute). After removal of the liquid medium, the sheet thus formed can be dried in air, at a temperature preferably between 70 ⁰ C and 120 ⁰ C, for a period of from 1 to 12 hours.

Generally, the drying is carried out simultaneously with a sintering operation which comprises heating in air at a temperature above the melting point or softening point of the fluoropolymer, for example from 5 to 50 ° C above this point.

The sintering temperature is advantageously between 120 and 370 ^° C.

The duration of this operation advantageously varies between 1 hour and 24 hours, more precisely between 1 hour and 12 hours.

According to the invention, there is obtained an elementary cathode covered with a precathode and a diaphragm that can be implemented in an electrolysis cell dedicated in particular to the electrolysis of the alkali metal halide solutions and more particularly sodium chloride.

The electrolysis is carried out on solutions with a high concentration of sodium chloride which is usually between 290 and 320 g / l.

It is made in an electrolysis cell most often made of steel and copper with as electrodes, the cathode defined according to the invention and covered with a diaphragm and an anode which is generally an anode made of activated titanium.

An electric current is supplied to the anode at a rate of 0.6 to 2.5 kA per square meter of anode.

Electrolysis is generally conducted continuously although a batch operation is not excluded.

Chlorine is recovered at the anode and sodium hydroxide and hydrogen at the cathode.

The concentration of the electrolytic sodium hydroxide obtained is between 70 and 150 g / l.

The combination of a cathode composite element and diaphragm as defined according to the invention, provides improved performance during the implementation for the electrolysis of alkali metal halide solutions and more particularly sodium chloride . In the latter case, there is a lower energy consumption. By comparing the combination of the invention and the case where the elementary cathode is not coated with the precathode, it is found by implementing the means of the invention, a voltage gain of the order of 50 to 300 mV expressed with respect to an operating voltage of 3 volts. Furthermore, it has been found, advantageously, a decrease in the chlorate content.

An exemplary embodiment of the invention given for illustrative and non-limiting purposes is given below.

Example

In this example, the preparation of the pre-cathode bath is carried out to effect the impregnation on the cathode and the preparation of the diaphragm bath for its coupling on the precathode.

Prebathodic bath impregnation on cathode.

A suspension comprising:

- 2.04 g of a surfactant aqueous solution octoxynol to 25% by weight (Triton X-100 ^®),

- 4.6 g of guar gum and 0.046 g of biocide agent 1, 2- benzisothiazolin-3-one (Proxel ^®)

22.9 g of carbon fiber of length 1 to 5 mm and diameter 5 to 30 μm, 13.3 g of PTFE latex at 60% by weight,

50.3 g of silica, with a particle size D ₅₀ = 5 μm,

24.0 g of a 60% aqueous suspension of Raney nickel containing 92 ± 2% nickel,

- water, so as to have a final volume of 3 liters of suspension. The total volume of water is calculated so that the weight percentage of solids is about 3%.

The various ingredients are then added with slow stirring.

Between each addition, there are 15 minutes.

After adding nickel, the solution is stirred strongly for 15 minutes, then gently for the next hour.

The solution is then left at rest for at least 24 hours before use and visual verification of its appearance.

To carry out sampling for the deposit, the gentle agitation is reset for 30 minutes. The appropriate volume of solution is taken so as to make a deposit of the precathode of the order of 0.4-0.5 kg / m ² . The filtration is conducted under a programmed vacuum increasing on a volumic cathode so as to obtain a spin vacuum of 600-700 mbar (final pressure: 0.3 -0.4 bar) on the precathode.

The fibrous web is then dried in air at 100 ^° C. for 1 hour.

Impregnation bath diaphragm on precathode.

To obtain the coupling of this asbestos-free precathode with the diaphragm, it is necessary to prepare the suspension of insulating fibers composed of PTFE fibers. This suspension is prepared by successive addition of the following quantities and materials, with stirring.

0.72 g of antifoam agent of the silicone type,

3 ml of an aqueous solution of sodium hydroxide at 30% by weight,

- 6.6 g of thickening agent, the Wellan gum, - 6.6 g of biocide 1, 2-benzisothiazolin-3-one (Proxel ^®)

- 528 g of a mixture of PTFE fibers, sodium chloride and zirconia in the respective weight proportions of 40%, 44% and 16%,

- water, so as to have a final volume of 3 liters of suspension. After gently stirring the antifoam and the soda, the thickener is then added with vigorous stirring shear so as to create the viscosity.

The aqueous suspension has a viscosity of 1800 centipoises measured at room temperature (20 ^° C.) on the BROOKFIELD viscometer at a speed of 1 rpm.

After shearing this medium and then adding the biocide, the material is added with gentle stirring.

The volume of water of the medium is calculated so as to have a concentration of PTFE + Zirconia fibers of 100 g / l. About Vi h stirring is necessary to have a homogeneous medium. The solution can be used directly.

In the opposite case, the solution is then kept under slight agitation to avoid any irremediable settling.

A sample is taken to deposit between 4 and 6 kg / m ² on said precathode prepared initially.

The impregnation is carried out under controlled vacuum increasing by successive dipping to have a clogging and reduce the thickness of the system. The final vacuum of the source is 900 mbar (final pressure: 0.1 bar). After obtaining the wet coupling, it is dried in air at 100 ⁰ C and then sintered at 355 ° C for a total cycle of 1 Vi h.

The performance of the various composite materials, the manufacture of which has just been described, has been evaluated in an electrolysis cell which has the following characteristics and whose operating conditions are indicated below:

- Titanium anode activated surface 49 cm ²

- Surface steel cathode 100 cm ² - Assembled cell according to filter press type with anode-cathode distance of 3 mm

- Current density of 2.5 kA / m ²

- Temperature: 80 ⁰ C

Concentration of the electrolytic soda: 110 g / l A voltage gain of 150 mV is obtained with respect to an initial voltage of 3.23V.

It is specified for comparison that the voltage gain is 5% compared to an elementary cathode / diaphragm electrolytic system.

Claims

1 - Association of a composite cathode element and a diaphragm characterized in that it comprises an elementary cathode covered with a precathode consisting of a fiber volume sheet comprising at least carbon fibers or graphite fibers bonded by a fluoropolymer, said web being rendered more conductive by an electrocatalytic agent and in that the precathode is covered by a diaphragm consisting of a web of fluoropolymer fibers in which particles of a material are interwoven inorganic refractory finely divided and insoluble in water.

2 - Association according to claim 1 characterized in that the elementary cathode is a metal part consisting essentially of a mesh or a perforated metal piece having a mesh gap or perforations whose size is generally between 20 microns and 5 mm .

3 - Association according to one of claims 1 and 2 characterized in that the carbon or graphite fibers are in the form of filament whose diameter is generally less than 1 mm and preferably between 10 ^{~ 5} and 0, 1 mm and whose length is greater than 0.5 mm and preferably between 1 and 20 mm.

4 - Association according to claim 3 characterized in that the carbon or graphite fibers have a length distribution such that the length of at least 80% of the fibers corresponds to the average length within ± 20%.

5 - Association according to one of claims 1 to 4 characterized in that the fluoropolymer is polytetrafluoroethylene.

6 - Association according to one of claims 1 to 5 characterized in that the fluoropolymer represents up to 30% of the total weight of the precathodic layer, this level being preferably between 5 and 20%.

7 - Association according to one of claims 1 to 6 characterized in that the electrocatalytic agent is selected from platinum, palladium and in the group consisting of Raney metals and Raney alloys of which there is eliminated most of the easily removable metal (s), and their mixtures.

8 - Association according to claim 7 characterized in that the Raney metal is Raney nickel.

9 - Association according to one of claims 7 and 8 characterized in that the amount of electrocatalytic agent represents up to 30% of the weight of the bonded web and preferably from 1 to 20% of this weight.

10 - Association according to claim 1 characterized in that the fluoropolymer fibers are polytetrafluoroethylene fibers.

11 - Association according to claim 10 characterized in that one uses PTFE fibers whose average dimensions are between 1 and 10 mm for the length and between 50 and 200 microns for the diameter.

12 - Association according to one of claims 10 and 11 characterized in that the refractory mineral material is selected from oxides, borides, carbides, nitrides or silicates of the following metals: zirconium, vanadium, chromium, niobium, molybdenum, hafnium, tantalum, titanium and tungsten and their mixtures.

13 - Association according to claim 12 characterized in that the refractory mineral material is zirconium oxide, zirconium silicate or mixtures thereof.

14 - Association according to one of claims 10 to 13 characterized in that the amount of the refractory material is up to 75% by weight of the fluoropolymer fibers and preferably from 20 to 50% of this weight.

15 - Association according to one of claims 10 to 14 characterized in that the thickness of the diaphragm varies between 1 and 5 mm and preferably between 2 and 4 mm.

16 - Process for preparing the combination described in one of claims 1 to 15 characterized in that it essentially comprises the following steps: a - the deposition of a precathodic layer by programmed vacuum filtration of an aqueous dispersion comprising at least the carbon or graphite fibers, the fluoropolymer, the electrocatalytic agent, a thickening agent, a pore-forming agent and, if appropriate other additives through an elementary cathode constituted by a wire mesh or perforated metal surface, b - the elimination of the liquid medium and, where appropriate, the drying of the sheet thus formed, - the deposition of a layer of fibers consolidated membrane constituting the diaphragm by vacuum filtration of an aqueous dispersion comprising at least the fluoropolymer fibers, the refractory material, a thickening agent, through the porous precathodic material, - the elimination of the liquid medium and, where appropriate where appropriate, drying the web thus formed, e-sintering the assembly.

17 - Process according to claim 16, characterized in that the aqueous dispersion intended for depositing the precathode has a solids content of between 1 and 5%, preferably between 2 and 4% by weight.

18 - Process according to claim 17 characterized in that the dispersion contains the amounts expressed as solids of the following main constituents: - from 15% to 40% by weight of carbon or graphite fibers,

from 5% to 20% by weight of the fluorinated polymeric binder,

from 1% to 30% by weight of electrocatalytic agent (s),

from 40% to 55% by weight of a blowing agent,

from 3% to 6% by weight of a thickening agent.

19 - Process according to claim 18 characterized in that said dispersion contains the amounts expressed as solids of the following main constituents:

from 20% to 30% by weight of carbon or graphite fibers, from 5% to 10% by weight of the fluorinated polymeric binder,

from 1% to 20% by weight of electrocatalytic agent (s),

from 45% to 50% by weight of a blowing agent,

from 4% to 5% by weight of a thickening agent. 20 - Process according to one of claims 16 to 19 characterized in that the blowing agent is selected from halides, sulfates, sulfites, bisulfites, phosphates, carbonates, alkali metal or alkaline earth bicarbonates; amphoteric alumina or preferably a silica-based derivative: the latter being preferred.

21 - Process according to claim 20 characterized in that the blowing agent is a precipitated silica or combustion or pyrogenic having a BET specific surface area of between 100 m ² / g and 300 m ² / g and / or a particle size evaluated at COULTER ^® meter between 1 and 50 μm and preferably between 1 and 15 μm.

22 - Method according to one of claims 16 to 21, characterized in that the thickening agent is a natural or synthetic polysaccharide.

23 - Method according to one of claims 16 to 21 characterized in that the thickening agent is Xanthan gum, carrageenan gum, Kappa or Iota, pectin, guar gum, locust bean gum, gum Wellan, alginates or mixtures thereof.

24 - Method according to one of claims 16 to 23 characterized in that a surfactant is added in an amount of at most 3% and preferably between 0.5 and 2%.

25 - Method according to one of claims 16 to 24 characterized in that a biocide is added in an amount of at most 2% by weight and preferably between 0.01 and 0.05%.

26 - Method according to one of claims 16 to 25 characterized in that the aqueous dispersion for the deposition of the diaphragm to a solids content between 10 and 30%, preferably between 15 and 25% by weight.

27 - Process according to claim 26 characterized in that a thickening agent is added to the dispersion in a proportion of 0.1 and 0.3% by weight.

28 - Method according to one of claims 26 and 27 characterized in that the aqueous dispersion has a viscosity measured at room temperature (20 ^° C.) at the BROOKFIELD viscometer at a speed of 1 rpm, between 800 and 3000 centipoise, preferably between 1000 and 2800 centipoise.

29 - Process according to one of claims 26 to 28, characterized in that sodium hydroxide is added in amounts of 0.5 to 2% by weight of sodium hydroxide expressed relative to a 30% commercial sodium hydroxide solution. weight.

30 - Process according to one of claims 26 to 29 characterized in that it uses a mixture comprising from 35 to 60% by weight of fluoropolymer fibers, preferably PTFE, 35 to 60% of sodium chloride and 5 to 35% zirconia or any other refractory material.

31 - Process according to claim 30 characterized in that one implements a mixture of 40 to 55% by weight of fluoropolymer fibers, 40 to 55% of sodium chloride and 10% to 30% of zirconia or any other refractory material.

32 - Process according to one of claims 30 and 31 characterized in that one implements a quantity of mixture such that the fibers represent from 3.5 to 18% of the weight of the dispersion and preferably from 4 to 16.5% by weight.

33 - Method according to one of claims 26 to 32 characterized in that one adds an anti-foam agent, a biocidal agent.

34 - Use of the combination according to one of claims 1 to 15 in an electrolysis cell of alkali metal halide solutions.

35 - Use according to claim 34 characterized in that the alkali metal halide is sodium chloride.