EP0642602A1

EP0642602A1 - Method for the preparation of a microporous diaphragm

Info

Publication number: EP0642602A1
Application number: EP94911227A
Authority: EP
Inventors: Frédéric Kuntzburger; Jean-Claude Magne
Original assignee: Rhodia Chimie SAS; Rhone Poulenc Chimie SA
Current assignee: Rhodia Chimie SAS
Priority date: 1993-03-26
Filing date: 1994-03-28
Publication date: 1995-03-15
Also published as: BG99292A; RU94046344A; FR2703075B1; BG61878B1; CN1106615A; CA2136763A1; FR2703075A1; JPH08505189A; US5547550A; PL306376A1; KR950701692A; KR970007100B1; WO1994023093A1

Abstract

Diaphragm for use in cells for the electrolysis of alkaline halide solutions comprising: 100 parts by weight of asbestos fibres, 30 to 70 parts by weight of silica-based derivatives and 20 to 60 parts by weight of fluorinated polymers, deposited on a porous material. The weight ratio of the fluorinated polymers and the silica-based derivatives is between 0.6 to 1.2 and preferably between 0.6 to 0.9 with the exception of a diaphragm obtained by depositing a suspension comprising 100 parts by dry weight of asbestos fibres, 30 parts by dry weight of silica-based derivatives, 25 parts by dry weight of fluorinated polymers and 1.5 parts by dry weight of a thickening agent. The invention also concerns a method for the preparation of an optionally microporous diaphragm. The diaphragms of the invention are especially useful in aqueous alkaline halide solutions electrolysis cells.

Description

PROCESS FOR THE PREPARATION OF MICROPOROUS DIAPHRAGM

The present invention relates to a diaphragm capable of being used in cells for the electrolysis of solutions of alkali halides.

It also relates to a method for preparing a diaphragm, possibly microporous. The diaphragm cited above is capable of being obtained by this process. Finally, it relates to the use of this diaphragm in an electrolysis cell for aqueous solutions of alkali halides.

The aqueous solutions of alkaline halides generally electrolyzed are those of sodium chloride to obtain chlorine and soda.

In general, it is known to prepare this type of material by depositing asbestos fibers on a support, consolidating them with a polymer inert with respect to the electrolyte and optionally adding a porophore which is broken down into end of operation to give them the necessary porosity.

Diaphragms based on asbestos fibers known and obtained by this process do not have all the qualities of mechanical and chemical resistance required for optimal electrolysis conditions. Indeed, these diaphragms can either have unsatisfactory hydraulic and / or electrical performance due in particular to a hydrophobic nature of the diaphragms, and this from the start of their use in electrolysis, or deteriorate over time, in particular by deconsolidation of the structure , when using these diaphragms in electrolysis, inducing a decrease in hydraulic and / or electrical performance.

It is described in French patent n ° 73 18805 filed on May 18, 1973 by the company Rhône-Progil, a process for the preparation of porous diaphragms made from an aqueous suspension of asbestos fibers, a resin latex fluorinated, a porophore and sulfonic anionic surfactants. It is more particularly preferred specific quantities of fluorinated resin, porophore and asbestos, which lead to microporous diaphragms whose performance in electrolysis appear to be unsatisfactory; these unsatisfactory performances correspond to a poor flow of the electrolyte from one compartment to another of the system and / or to an increase in the voltage without however noting an increase in the yield of soda. In addition, the anionic surfactants present in the diaphragms react with the cations present during the manufacture or during the use of these diaphragms in electrolysis, which decreases their electrical and hydraulic performance. An object of the present invention is therefore to provide microporous diaphragms having, when used in the electrolysis of aqueous solutions of alkali halides, satisfactory transport of the soluble species present in the electrolyte, while having a reduced flux of soda to through the separator of given geometry.

Another object of the present invention is to provide microporous diaphragms having, during their use in electrolysis, a uniform flow of the electrolyte from one compartment to another.

Another object of the invention is to provide a method for manufacturing microporous diaphragms whose hydraulic and electrical performance in electrolysis are satisfactory, while respecting the energy consumption of the system in kilowatt hours.

These and other objects are achieved by the present invention which relates to a diaphragm, comprising: - 100 parts by weight of asbestos fibers,

- 30 to 70 parts by weight of silica-based derivatives,

- 20 to 60 parts by weight of fluoropolymer; deposited on a porous material, and in that it has a ratio by weight of the fluoropolymer to the silica-based derivatives of between 0.6 and 1.2, and preferably between 0.6 and 0.9, with the exception of a diaphragm obtained by depositing a suspension comprising 100 parts by dry weight of asbestos fibers, 30 parts by dry weight of silica-based derivatives, 25 parts by dry weight of fluoropolymer, 1, 5 parts by dry weight of a thickening agent.

Another subject of the invention consists of a diaphragm comprising: - 100 parts by weight of asbestos fibers,

- 30 to 70 parts by weight of silica-based derivatives,

- 20 to 60 parts by weight of fluoropolymer; deposited on a porous material, having a weight ratio of the fluoropolymer to the silica-based derivatives of between 0.6 and 1, 2, and preferably between 0.6 and 0.9, and being obtained by depositing 'a suspension whose nature and constituents will be indicated below, said suspension possibly comprising a thickening agent in an amount less than 1.5 parts by dry weight, per 100 parts by dry weight of asbestos fibers.

The present invention likewise relates to a process for the preparation of a diaphragm essentially comprising the following steps: a) preparation of an aqueous suspension comprising, in addition if necessary, a thickening agent:

- 100 parts by dry weight of asbestos fibers, - 30 to 60 parts by dry weight of silica-based derivatives,

- 20 to 60 parts by dry weight of fluoropolymer; b) deposition of a sheet by programmed vacuum filtration of said suspension through a porous material; c) elimination of the liquid medium and drying of the sheet thus formed; d) sintering of the sheet; the suspension prepared having a specific weight ratio of the fluoropolymer to the silica derivatives, such that the diaphragm prepared has a weight ratio of the fluoropolymer to the silica derivatives after step c), inclusive between 0.6 and 1.2, and preferably between 0.6 and 0.9, with the exception of a diaphragm obtained by depositing a suspension comprising 100 parts by dry weight of asbestos fibers, 30 parts by dry weight of silica-based derivatives, 25 parts by dry weight of fluoropolymer, 1.5 parts by dry weight of a thickening agent.

The invention also relates to a process for the preparation of a diaphragm essentially comprising the following steps: a) preparation of an aqueous suspension comprising:

- 100 parts by dry weight of asbestos fibers,

- 30 to 60 parts by dry weight of silica-based derivatives,

- 20 to 60 parts by dry weight of fluoropolymer, - 0 to less than 1.5 parts by dry weight of a thickening agent; b) deposition of a sheet by programmed vacuum filtration of said suspension through a porous material; c) elimination of the liquid medium and drying of the sheet thus formed; d) sintering of the sheet; the suspension prepared having a specific weight ratio of the fluoropolymer to the silica derivatives, such that the diaphragm prepared has a weight ratio of the fluoropolymer to the silica derivatives after step c), inclusive between 0.6 and 1, 2, and preferably between 0.6 and 0.9.

However, other advantages and characteristics will appear more clearly on reading the description and the examples which follow.

According to a first embodiment, the subject of the present invention is a diaphragm comprising:

- 100 parts by weight of asbestos fibers,

- 30 to 70 parts by weight of silica derivatives, - 20 to 60 parts by weight of fluoropolymer; with the exception of a diaphragm obtained by depositing a suspension comprising in particular 100 parts by dry weight of asbestos fibers, 30 parts by dry weight of silica derivatives, 25 parts by dry weight of fluoropolymer, 1.5 parts by dry weight of a thickening agent

According to a second embodiment of the invention, the diaphragm comprises - 100 parts by weight of asbestos fibers, - 30 to 70 parts by weight of silica-based derivatives,

- 20 to 60 parts by weight of fluoropolymer, deposited on a porous material, having a weight ratio of the fluoropolymer on the silica-based derivatives of between 0.6 and 1, 2, and preferably between 0.6 and 0.9, and being obtained by depositing a suspension optionally comprising a thickening agent in an amount of less than 1.5 parts by dry weight, per 100 parts by dry weight of asbestos fibers

Preferably, the diaphragms according to these two embodiments of the invention comprise

- 100 parts by weight of asbestos fibers, - 30 to 60 parts by weight of silica-based derivatives,

- 25 to 50 parts by weight of fluoropolymer

According to a variant of the invention, the diaphragms are obtained by depositing a suspension comprising, in addition to the other constituent elements described below, 0 to less than 1.5 parts by dry weight and more particularly from 0 to 1 part in dry weight of a thickening agent per 100 parts by dry weight of asbestos fibers

The diaphragms according to the present invention preferably comprise at least one surfactant. This surfactant is present in amounts of between 0.5 and 10, preferably between 0.6 and 5 parts by weight per 100 parts by dry weight of asbestos fibers. Preferably, a nonionic surfactant is used. As nonionic surfactant, it is possible in particular to use ethoxylated alcohols or fluorocarbon compounds with functionalized groups alone or as a mixture, these alcohols or these fluorocarbon compounds generally have carbon chains from C6 to

C20- Preferably, ethoxylated alcohols are used which are ethoxylated alkylphenols, such as in particular octoxynols

The diaphragms according to the present invention advantageously have a weight per unit area of between 0.4 and 3 kg / m ² and, preferably, 0.7 and

2 kg / m ² . The present invention also relates to a method for preparing a diaphragm.

According to a first embodiment of the invention, the process is suitable for the preparation of diaphragm, with the exception of that obtained from a suspension comprising 100 parts by dry weight of asbestos fibers, 30 parts by dry weight of silica-based derivatives, 25 parts by dry weight of fluoropolymer, 1.5 parts by dry weight of a thickening agent.

The method according to this first mode essentially comprises the following steps: a) preparation of an aqueous suspension comprising, in addition if necessary, a thickening agent:

- 100 parts by dry weight of asbestos fibers,

- 30 to 60 parts by dry weight of silica-based derivatives, - 20 to 60 parts by dry weight of fluoropolymer; b) deposition of a sheet by programmed vacuum filtration of said suspension through a porous material; c) elimination of the liquid medium and drying of the sheet thus formed; d) sintering the web; the suspension of step a) has a specific weight ratio of the fluoropolymer to the silica derivatives, so that the diaphragm prepared has a weight ratio of the fluoropolymer to the silica derivatives, after step c), between 0.6 and 1.2, and preferably between 0.6 and 0.9.

According to a second embodiment, the method according to the invention essentially comprises the following steps: a) preparation of an aqueous suspension comprising:

- 100 parts by dry weight of asbestos fibers,

- 30 to 60 parts by dry weight of silica-based derivatives,

- 20 to 60 parts by dry weight of fluoropolymer, - 0 to less than 1.5 parts by dry weight of a thickening agent; b) deposition of a sheet by programmed vacuum filtration of said suspension through a porous material; c) elimination of the liquid medium and drying of the sheet thus formed; d) sintering of the sheet; the suspension prepared having a specific ratio by weight of the fluoropolymer to the silica-based derivatives, such that the diaphragm prepared has a weight ratio of the fluoropolymer to the silica-based derivatives, after step c) , between 0.6 and 1.2, and preferably between 0.6 and 0.9.

According to an important characteristic of the invention, the suspension prepared in each of the two embodiments mentioned has a weight ratio of the fluoropolymer to the silica-based derivatives which is to be adjusted so that the diaphragm obtained after 1 step c) has such a ratio of between 0.6 and 1.2, and preferably between 0.6 and 0.9. Indeed, this ratio can vary depending on the respective stopping rate of these two compounds on the material with high porosity. A person skilled in the art will be able to determine, by means of simple tests, the quantity of dry matter to be dispersed in the suspension as a function of the rate of stopping observable on the porous material through which the dispersion is filtered under the conditions vacuum programmed.

Preferably, and in a manner suitable for both embodiments, the preparation of an aqueous suspension according to step a) of the process of the present invention essentially comprises, in addition to the thickening agent if one is used : - 100 parts by dry weight of asbestos fibers, - 35 to 50 parts by dry weight of silica-based derivatives,

- 30 to 40 parts by dry weight of fluoropolymer. The method according to the present invention preferably uses a suspension comprising at least one surfactant.

This surfactant is generally present in amounts of between 0.5 and 10, preferably between 0.6 and 5, parts by weight per 100 parts by weight of asbestos fibers.

Preferably, this surfactant is nonionic.

The surfactants advantageously usable are those mentioned above.

It has been found that, according to this process, it is possible to prepare microporous diaphragms whose electrical and hydraulic performance are satisfactory and stable over time; and this is advantageously observed when these diaphragms obtained are used, in brine electrolysis cells under high current densities of up to 40 A / dm ² and more. The diaphragms thus obtained also make it possible to work with high sodium hydroxide concentrations (of the order of 140 to 200 g / l, or even more) in the catholyte, which limits the energy consumption useful for the subsequent concentration of sodium hydroxide.

As mentioned before, the suspension prepared during step a) may include a thickening agent.

More particularly, the amount of thickening agent can vary between 0 and less than 1.5 parts by dry weight per 100 parts by dry weight of asbestos fibers.

Preferably, the amount of thickening agent varies between 0 and 1 part by dry weight relative to the above reference.

In general, the thickening agents are chosen from polysaccharides of natural or synthetic origin. Preferably, the thickening agents are chosen from natural polysaccharides, such as biogums, obtained by fermentation of a carbohydrate under the action of a microorganism. By way of example of such compounds, mention may in particular be made of xanthan gum, gellan, rhamsan and welan. It has been found that such amounts of thickening agent, more especially in the preferred range, are particularly suitable for the preparation of diaphragms on an industrial scale. Indeed, these contents of thickening agent, if however the suspension comprises one, make it possible to stabilize the suspension to be deposited, and therefore to obtain homogeneous diaphragms, while keeping a time necessary for the deposition, compatible with industrial objectives.

The asbestos fibers used in the composition of the suspension to be deposited are advantageously those commercially available. Mention may in particular be made of chrysotile asbestos fibers of length between 1 to 5 mm and chrysotile asbestos fibers of length less than 1 mm.

The material binder according to the invention consists of fluorinated polymers.

By "fluorinated polymers" is meant homopolymers or copolymers derived at least in part from olefinic monomers substituted pa: fluorine atoms, or substituted by a combination of fluorine atoms and at least one of chlorine atoms , bromine or iodine per monomer.

Examples of homopolymers or fluorinated copolymers can be constituted by polymers and copolymers derived from tetrafluoroethylene, hexafluoropropylene, chlorotrifluoroethylene, bromotrifluoroethylene. Such polymers can also comprise up to 75 mol% of units derived from other ethylenically unsaturated monomers containing at least as many fluorine atoms as carbon atoms, such as for example vinylidene (di) fluoride, esters vinyl and perfluoroalkyl, such as perfluoroalkoxyethylene.

The fluoropolymer is advantageously according to the invention in the form of an aqueous dispersion containing, in general, from 30 to 70% of dry polymer, with a particle size between 0.1 and 5 micrometers and preferably between 0.1 and 1 micrometer.

Preferably, the fluoropolymer used is a polytetrafluoroethylene.

By "silica derivatives" is meant according to the invention precipitated silicas and combustion or pyrogenic silicas.

Advantageously, the silicas have a BET specific surface area between 100 m ² / g and 300 m ² / g and / or a particle size evaluated with the COULTER® counter between 1 and 50 microns and, preferably, between 1 and 15 microns.

These derivatives behave like excellent blowing agents which do not bring practically any deconsolidation of the microscopic material when they are used in the amounts of the present invention. These derivatives are also agents forming latex networks constituted by the binder present. In practice, the aqueous suspension prepared in step a) of the process according to the present invention comprises 500 to 10,000 parts of water per 100 parts by weight of asbestos fibers.

When the diaphragm is used in electrolysis, the diaphragm should preferably be in the form of a microporous diaphragm, that is to say a diaphragm substantially devoid of silica-based derivatives.

The process of the present invention then comprises a step e) corresponding to the elimination of the silica-based derivatives.

The elimination of the silica-based derivatives can be carried out by attack in an alkaline medium. This elimination of the silica-based derivatives can be done before the use of the diaphragm in electrolysis, but, in a practical and advantageous manner, the silica-based derivatives can be eliminated "in situ" in the electrolyser, by dissolution in the medium. alkaline, especially during the first hours of electrolysis.

Thus, the treatment is advantageously carried out in contact with an aqueous solution of sodium hydroxide whose concentration is between 40 and 200 g / l and at a temperature between 20 and 95 ° C.

According to the method of the present invention, the web is therefore formed by filtration under programmed vacuum of said suspension through a porous material. These porous materials can in particular be fabrics and / or grids whose void, the perforations or the porosity can be between 1 μm and 5 mm, and preferably between 20 μm and 2 mm.

In the case where the diaphragm according to the invention is used in the cells for the electrolysis of alkali halides or more specifically of sodium chloride, the porous material may correspond to a porous metallic surface which then constitutes the elementary cathode of the cell d 'electrolysis. This elementary cathode may have one or more planar or cylindrical surfaces, commonly called "thimble", having an open surface.

According to a preferred variant of the invention, prior to the deposition of the diaphragm, the cathode is covered with a precathode sheet. This preliminary step is carried out by programmed vacuum filtration through the elementary cathode, consisting of a metal surface having a mesh vacuum or perforations of between 1 μm and 5 mm, and preferably between 20 μm and 2 mm, d '' an aqueous suspension of fibers at least part of which is electrically conductive, of a binder based on a fluoropolymer in the form of particles and optionally additives, followed by the elimination of the liquid medium, the possible drying of the sheet thus formed and any sintering of this sheet. The sintering of the precathode layer is preferably carried out at this stage of the process only insofar as the binder of this layer is different from the binder of the suspension prepared in step a) of the process according to the present invention.

The precathode layer thus obtained then constitutes the porous material through which the suspension prepared in step a) of the process according to the present invention can be filtered.

The additional technical details and the variants of the precathode layer mentioned above are described in particular in European patent applications Nos. 132,425 and 412,916; these European patent applications are incorporated by reference to avoid further development on said cathode elements. Thus, the additives can in particular correspond to derivatives based on silica, as described above for the diaphragm, or else to electrocatalytic agents chosen from the group consisting of Raney metals and Raney alloys, the major of which will be eliminated. easily removable part (s) of metal (s), and mixtures thereof.

The different vacuum programs described above, whether for depositing the precathode layer or the diaphragm according to the invention, can be carried out continuously or in stages, from atmospheric pressure to final pressure (0.01 to 0.5 bar). absolute). The above-mentioned sintering (or consolidation) steps are generally carried out at a temperature above the melting or softening point of the fluoropolymers, binders of said layers. .

The following examples illustrate the invention, without however limiting its scope. In the following, the percentages will be expressed by weight, unless otherwise stated.

EXAMPLES

For all of the examples, the procedure for preparing a diaphragm is as follows:

A suspension is prepared, with stirring, of:

A - deionized water, the amount of which is calculated to obtain approximately 4 liters of suspension and an extract of approximately 4.5%; B - Z g of surfactant;

C - 100 g of chrysotile asbestos fibers of lengths less than 1 mm;

D - X g of polytetrafluoroethylene (thereafter: PTFE) in the form of latex at approximately

60% by weight of dry extract; E - Y g of Tixosil 33 J ® (silica manufactured and marketed by Rhône-Poulenc).

The suspension is allowed to stand for at least 24 hours. The suspension is stirred for 30 minutes before use.

The volume of solution required is withdrawn so that it contains the quantity of dry extract which it is intended to deposit to form the diaphragm (of the order of 1 to 2 kg / m ² ).

Filtration is carried out under programmed vacuum on a cathode on which a precathode sheet has been previously deposited which will be described below.

The depression is established and increases by 50 mbar per minute to reach approximately 800 mbar.

The vacuum is maintained for 15 minutes at 800 mbar.

The assembly is then sintered, after optional drying at approximately 100 ° C., bringing the cathode and diaphragm assembly to 350 ° C., with a bearing at a temperature of approximately 315 ° C., the whole for approximately 1 hour and a half. .

The silica is then eliminated by an alkaline attack with electrolytic soda during the first moments of the electrolysis (elimination "in situ").

The precathode layer is prepared in the following manner: In 7 liters of deionized water, 30 g of asbestos fibers less than 1 mm in length are introduced with stirring, 82 ml of triton X 100® at 40 g / l of Rohm and Haas company.

Then, after stirring, 70 g of graphite fibers (of monodisperse length of about 1.5 mm), 35 g of PTFE in the form of latex, 100 g of Tixosil 33 J ® silica, 2.1 g of xanthan gum are added and 60.5 g of Raney nickel. The suspension is left to stand for approximately 48 hours. This suspension is then deposited on a metal grid having a mesh void of 2 mm.

The depression is established and increases by 10 mbar per minute to reach around 200-300 mbar.

The vacuum is maintained for 15 minutes at 200-300 mbar. It is dried for 1 hour at 120 ° C.

The electrolysis cell for measuring performance has the following characteristics and operating conditions: - expanded metal anode coated with Ru0 ₂ - Ti0;

- cathode, see description above;

- inter electrode distance of 7 mm;

- active surface of 0.5 dm ² ; - Intensity of 12.5 A;

- brine feed in the anode compartment at 305 g / l, the chloride concentration of the anolyte is kept constant and equal to 4.8 moles. H;

- the cell temperature is 85 ° C.

In the tables given in the examples:

- the permeability corresponds to the flow of the electrolyte from one compartment to another, this flow is evaluated by the simple difference in heights observed between the anode and cathode compartments;

- ΔU, expressed in volts, corresponds to the voltage across the electrolyser at 12.5 A.

Comparative examples 1 to 3

The suspensions prepared are as follows: X = 20 g of PTFE; Y varies: 20, 23 and 27 g of silica; Z = 1.2 g of triton X 100 ® from the company Rohm and Haas (30 ml of triton® X 100 at 40 g / l).

These examples therefore correspond to an unsuitable silica content, taking into account the relatively low content of PTFE.

The stop rates on the precathode layer which constitutes the porous material are 100% (stop rate observed by simple material balance: determination of the elements F, Mg and Si by fluorescence with X-rays and / or by weighing).

The results are collated in Table 1 below.

Examples 4 and 5

The suspensions prepared are the following X = 20 g of PTFE; Y varies: 30 and 50 g of silica; Z = 1.2 g of triton ® X 100 from Rohm and Haas (30 ml of triton ^® X 100 at 40 g / l).

The results are collated in Table 1 below.

The results of Comparative Examples 1, 2 and 3 show that the hydraulic and / or electrical performance is unsatisfactory.

It can also be seen that when the PTFE / Silica ratio is equal to 0.4 (example 5) the ΔU and the permeability obtained are less satisfactory than when this ratio is 0.67 (example 4), although the yields are comparable. The results obtained by this last example 4 turn out to be good, but at the border of a critical zone by insufficient quantity of PTFE and narrowness of the zone of use.

TABLE 1

EXAMPLES 1 2 3 4 5

Composition of the 100/20/20 100/20/23 100/20/27 100/20/30 100/20/50 asbestos suspension / PTFE / Sii-lcε

Deposited weight 0.88 1.23 1.57 1.96 1.53 1.5 0.96 1.35 1.57 1.92 0.92 1.25 1.57 1.95

Permeability very low unstable unstable good high

ΔU final (volt) 3.32 3.33 3.3 3.51 3.20-3.40 unstable 3.20-3.40 unstable 3.05 3.07 3.17 3.27 3.3 3, 34 3.58 3.73

Yield (%) of NaOH at 3.5 N 87 ND ND ND 93 94 96 95 38 93.5 95 99 98.5 95 in NaOH at 5 N 77 ND 82.5 75 85 87 84.5 85 88.5 88 85 92 91, 5 88

G

"

s

Examples 6 to 11

The suspensions prepared are as follows: X varies: 15, 30 and 40 g of PTFE; Y = 30 g of silica; Z = 1.2 g of triton X ® 100 from Rohm and Haas (30 ml of triton ® X 100 at 40 g / i).

Consequently, Examples 6 and 7 corresponding to 15 g of PTFE per 100 g of asbestos fibers in the suspension and Examples 10 and 11 corresponding to a PTFE / Silica ratio equal to 1.33 are comparative examples.

The results are collated in Table 2 below.

The low PTFE content in Examples 6 and 7 induces a high and unstable permeability, but above all there is the risk of a deconsolidation of the diaphragm after a few hours of operation in electrolysis (result found, but not apparent from the table), this which is incompatible with industrial development.

Conversely, when the PTFE content is high and the PTFE / Silica ratio is equal to 1.33 (examples 10 and 11), the diaphragm has a hydrophobic character (high tension and low permeability), the soda yield is low and energy consumption is high.

TABLE 2

SUBSTITUTE SHEET RULE 26 Example 12

Long-term electrolysis test ^•

The suspension prepared is as follows: X = 25 g of PTFE; Y = 35 g of silica;

Z = 1.2 g of triton ® X 100 from Rohm and Haas (30 ml of triton ® X 100 at 40 g / l).

The stop rates on the precathode layer which constitutes the porous material are 100% (stop rate observed by simple material balance: determination of the elements F, Mg and Si by fluorescence with X-rays and / or by weighing). The deposited weight is 1.59 kg / m2.

The hydraulic and electrical performance of the diaphragm during a long-term test is satisfactory.

The concentration of sodium hydroxide produced is gradually varied from 2 to 5 moles. M during the first 300 hours of operation.

The test is extended for 2500 hours and a production of sodium hydroxide concentration of 5N ± 0.2N is maintained.

After a transition phase of approximately 800 hours during which the permeability increases slightly and the tension passes through a minimum, the performances stabilize until the end of the test and are as follows: NaOH = 5N ± 0.2N, Voltage = 3.25 V, Permeability: correct, Soda yield 5N = 89-90%. Energy consumption of 2700-2750 kilowatt hours per tonne of chlorine produced.

Examples 13 to 15

Influence of the surfactant used

The suspension prepared is as follows: X = 20 g of PTFE; Y = 30 g of silica;

These examples consist in modifying the nature of the surfactant and its content in the suspension.

Thus, in these examples, the newt is replaced entirely or partially by sodium dioctyl sulfosuccinate (sulfimel) which is an anionic surfactant. The weights deposited are identical: 1.34 kg / m2

The stop rates on the precathode layer which constitutes the porous material are 100% (stop rates observed by simple material balance: determination of the elements F, Mg and Si by fluorescence with X-rays).

The results are collated in Table 3 below.

In this table, the ratio of sulfimel to newt is expressed by weight.

Table 3

Examples 13 14 15 sulfimel / triton 0/1, 2 1/1, 2 1 / 0.4 permeability correct low low

ΔU 3, 1 3.7 3.5 yield 88% 72% 79% at NaOH = 5N at NaOH = 5N at NaOH = 4.3N

Sulfimel therefore contributes to the joint increase in flow resistance and electrical resistance.

Examples 17 to 30

The procedure is as in the previous examples, the metallic mesh vacuum grid of 2 mm is however replaced by an electrolysis cell of the Hooker S3B® type with a surface area of 20 m2 which is in the form of a "thermowell".

This industrial embodiment results in decreased rates of discontinuation PTFE - ^"and silica which are 80% and 90%, respectively, discontinuation rates are recorded by single material balance in the bath of suspension ^• determination of the elements F, Mg and Si by X-ray fluorescence. The intensity is 34 kA.

The dry matter content in the suspension is approximately 4.1%. The compositions, the conditions and the results are collated in the following table 4.

The deposited weight corresponds to the dry weight of the precathode (approximately 5 kg) and that of the deposited diaphragm. The examples "bis" correspond to the examples having the same number, the only difference lies in the weight deposited.

TABLE 4

Ex. COMPOSITION OF THE PTFE / SiLC-yf REPORT WEIGHT PERFORMANCE

SUSPENSION DEPOSITED (KG)

Asbestos Si CB PTFE SUSPEN¬ DIAPHRA¬ NaOH perméa¬ yield

(g) (g) (g) SION GME (V) (g / D (%) bility REMOVAL

17 100 42 37 0.88 0.78 25 3.25 132 94.3 correct

17 29.5 3.25 135 91.9 bis 100 41 36 0.88 0.78 36 3.22 153 84.7

18 33 3.24 141 92.3

18 100 35 29 0.83 0.75 33 3.19 119 96.1 bis 28 3.15 111 96.8

19 100 30 27 0.90 0.81 33 3.36 141 94.1 tee

19 31 3.47 135 95.1 bis 100 31 30 0.97 0.87 36.5 3.53 153 91, 1

20 30 3.11 163 75.9

20 bis

21

21 bis

TABLE 4 (CONTINUED)

Examples 31 to 33

The suspensions prepared are the following: X = 50 g of PTFE;

Y varies: 30, 50 and 70 g of silica;

Z = 1.2 g of triton ® X 100 from Rohm and Haas (30 ml of triton ® X 100 at

40 g / l). .

The example corresponding to the diaphragm with 50 g of PTFE and 30 g of silica is therefore a comparative example, the PTFE / Silica ratio being 1.7.

The results are collated in the following table 5.

The low silica content compared to PTFE induces a very low permeability and a very high tension. The soda yield is very low and the production of a soda with a titre of between 3.3 and 4.5 N is impossible (for a deposited weight of

1.3 kg / m2) in an acceptable range of hydraulic head between the anode and cathode compartments.

TABLE 5

SUBSTITUTE SHEET RULE 26) Examples 34 and 35

The purpose of these tests is to measure the time necessary for the deposition of the suspension to obtain the diaphragm (flow time).

A suspension is prepared, in accordance with Example 1, with stirring, of:

B = 1.2 g of triton ® X 100 from Rohm and Haas (30 ml of triton ® X 100 at

40 g / l). C = 100 g of asbestos fibers; D = 25 g of PTFE; E = 30 g of silica;

The suspension of Example 34 also comprises 1.5 g of xanthan gum, that of Example 35 does not contain any. The filtration of the suspension is carried out under programmed vacuum on a volume cathode prepared according to example 7 of European patent application no. 296,076, as follows:

- 1 minute at a vacuum of -5 to -10 mbar of relative pressure compared to atmospheric pressure, - rise of the vacuum at the rate of 50 mbar / min.

The flow time measured is 40 minutes for Example 34 and 5 minutes for Example 35.

These results show that amounts of xanthan gum less than 1.5 parts by weight relative to 100 parts by weight of asbestos fibers are preferable for obtaining a diaphragm preparation process according to the invention compatible with industrial exploitation.

Claims

1- Diaphragm, characterized in that it includes

- 100 parts by weight of asbestos fibers, - 30 to 70 parts by weight of silica-based derivatives,

- 20 to 60 parts by weight of fluoropolymer; deposited on a porous material, and in that it has a weight ratio of the fluoropolymer to the silica-based derivatives of between 0.6 and 1.2, and preferably between 0.6 and 0.9, with the exception of a diaphragm obtained by depositing a suspension comprising 100 parts by dry weight of asbestos fibers, 30 parts by dry weight of silica-based derivatives, 25 parts by dry weight of fluoropolymer, 1, 5 parts by dry weight of a thickening agent

2- Diaphragm, characterized in that it comprises: - 100 parts by weight of asbestos fibers,

- 30 to 70 parts by weight of silica-based derivatives,

- 20 to 60 parts by weight of fluoropolymer, deposited on a porous material, in that it has a weight ratio of the fluoropolymer on the silica-based derivatives of between 0.6 and 1, 2, and preferably between 0.6 and 0.9, and in that it is obtained by depositing a suspension optionally comprising a thickening agent in an amount less than 1.5 parts by dry weight of a thickening agent, per 100 parts dry weight of asbestos fibers.

3- diaphragm according to any one of the preceding claims, characterized in that it comprises:

- 100 parts by weight of asbestos fibers,

- from 30 to 60 parts by weight of silica-based derivatives,

- from 25 to 50 parts by weight of fluoropolymer

4- diaphragm according to any one of the preceding claims, characterized in that it is obtained by depositing a suspension comprising 0 to 1 part by dry weight of a thickening agent, per 100 parts by dry weight of fibers asbestos.

5- diaphragm according to one of the preceding claims, characterized in that it comprises at least one surfactant in an amount between 0.5 and 10, preferably between 0.6 and 5 parts by weight per 100 parts by weight of asbestos fibers. 6- Diaphragm according to the preceding claim, characterized in that the surfactant is nonionic

7- Diaphragm according to any one of the preceding claims, characterized in that the microporous material is a porous metallic surface constituting an elementary cathode

8- Diaphragm according to the preceding claim, characterized in that the microporous material is an elementary cathode coated with a precathode layer

9- Method for preparing a diaphragm according to any one of claims

1 and 2 to 8, characterized in that it essentially comprises the following stages a) preparation of an aqueous suspension comprising, in addition if necessary, a thickening agent - 100 parts by dry weight of asbestos fibers,

- 30 to 60 parts by dry weight of silica-based derivatives,

- 20 to 60 parts by dry weight of fluoropolymer, b) deposition of a sheet by filtration under vacuum program of said suspension through a porous material, c) elimination of the liquid medium and drying of the sheet thus formed, d) sintering of the web, the suspension prepared having a specific weight ratio of the fluoropolymer to the silica derivatives such that the diaphragm prepares has a weight ratio of the fluoropolymer to the silica derivatives after step c ), between 0.6 and 1.2, and preferably between 0.6 and 0.9, with the exception of a diaphragm obtained by depositing a suspension comprising 100 parts by dry weight of fibers d asbestos, 30 parts by dry weight of silica derivatives, 25 parts by dry weight of fluoropolymer, 1, 5 parts by dry weight of a thickening agent

10- A method of preparing a diaphragm according to any one of claims

2 to 8, characterized in that it essentially comprises the following steps a) preparation of an aqueous suspension comprising

- 100 parts by dry weight of asbestos fibers

- 30 to 60 parts by dry weight of silica-based derivatives - 20 to 60 parts by dry weight of fluoropolymer

- 0 to less than 1.5 parts by weight of a thickening agent, b) deposition of a sheet by program vacuum filtration of said suspension through a porous material, c) elimination of the liquid medium and drying of the sheet thus formed; d) sintering of the sheet; the suspension prepared having a specific weight ratio of the fluoropolymer to the silica derivatives, such that the diaphragm prepared has a weight ratio of the fluoropolymer to the silica derivatives after step c), inclusive between 0.6 and 1, 2, and preferably between 0.6 and 0.9.

11- Preparation process according to claim 9, characterized in that the aqueous suspension of step a) comprises 0 to less than 1.5 parts by dry weight of a thickening agent.

12- Method according to any one of claims 9 to 11, characterized in that the aqueous suspension of step a) comprises:

- 100 parts by dry weight of asbestos fibers, - from 35 to 50 parts by dry weight of silica-based derivatives,

- from 30 to 40 parts by dry weight of fluoropolymer.

13- Method according to any one of claims 9 to 12, characterized in that the aqueous suspension of step a) comprises 0 to 1 part by dry weight of a thickening agent.

14- Method according to any one of claims 9 to 13, characterized in that the aqueous suspension comprises at least one surfactant.

15- Method according to any one of claims 9 to 14, characterized in that the fluoropolymer used is a polytetrafluoroethylene.

16- Method according to one of claims 9 to 15, characterized in that the microporous material is a microporous metallic surface having a void of mesh or perforations of between 1 μm and 5 mm.

17- A method according to claim 16, characterized in that a precathode layer is deposited, prior to deposition according to step b), this deposition is carried out by filtration under vacuum programmed through the metal surface of an aqueous suspension of fibers of which at least part is electrically conductive, of binder based on a fluoropolymer in the form of particles and optionally additives, followed by the elimination of the liquid medium, the possible drying of the sheet thus formed and possible sintering of this layer. 18- A method of preparing a microporous diaphragm, characterized in that the steps of the process according to one of claims 9 to 17 are implemented, then a step e) allowing the elimination of the silica-based drifts

19- Method according to the preceding claim, characterized in that the elimination of the silica-based derivatives is carried out by attack in an alkaline medium.

20- Use of a diaphragm according to one of claims 1 to 8 in an electrolysis cell of aqueous solutions of alkali halides.