FR2465797A1 - POROUS DIAPHRAGM FOR ELECTROLYSIS CELL OF ALKALINE CHLORIDE SOLUTIONS - Google Patents

Abstract

THE INVENTION RELATES TO A DIAPHRAGM FOR AN ELECTROLYSIS CELL. IT RELATES TO A POROUS DIAPHRAGM INCLUDING A THERMOPLASTIC SUPPORT ETOFFE IMPREGNATED WITH PARTICLES OF A SILICE COMPOSITION OF DETERMINED FORMULA, ONE FACE OF THE SUPPORT ETOFFE HAVING A CONDUCTING ZONE OF ELECTRICITY, AND A VISIBILITY AY - WITH RESPECT TO AQUEOUS SOLUTIONS. FURTHERMORE, SILICEOUS COMPOSITIONS IMPREGNANT WITH ETOFFE MAY SUBJECT TO HYDRATION WHEN IN CONTACT WITH AT LEAST ONE IONIZABLE COMPOUND PRESENT IN THE ELECTROLYSIS CELL. APPLICATION TO ELECTROLYSIS CELLS OF ALKALINE CHLORIDE BRINE.

Description

 The present invention relates to diaphragm electrolysis cells, used for the electrolysis of aqueous solutions of ionizable compounds. More precisely, it relates to the diaphragms for diaphragm electrolysis cells.

 In a diaphragm electrolysis cell, the latter represents the constituent which allows the cell to function by the formation of products, such as chlorine, alkali hydroxide, hydrogen and oxygen, when the electrolyte is an aqueous solution of an ionizable compound, with current yields which are sufficiently high for the operation to be profitable.

The separation properties, indicated by the current yields, can be increased, for example by increasing the thickness or the density of the diaphragm. However, these changes usually cause an increase in the electrical resistance of the diaphragm as indicated for example by an increase in the tension coefficient. Favorable cell profitability is based on increasing or maintaining the current efficiency at a high level, with minimization or control of the increase in the voltage coefficient.

 For years, industrial type diaphragm cells have been used for the production of chlorine and alkali hydroxides, hydrogen and oxygen, having a porous diaphragm of asbestos fibers. It is considered that, in the use of asbestos diaphragms, the effective material is a layer of a gel formed in the asbestos felt. This gel layer is formed by decomposition of asbestos fibers in contact with electrolytes in the cell. During the electrolysis of aqueous saline solutions, asbestos fibers not only undergo chemical decomposition during operation but also exhibit dimensional instability because they deform by swelling due to dissolution. Porous asbestos diaphragms, good that they allow satisfactory production, for example of chlorine and solutions of alkali hydroxides, have a limited duration in the cells and, when they have been removed from a cell, they cannot be reused. In addition, asbestos is considered to pose a health risk.

 Diaphragms having an excellent operating life but formed from materials which are durable and inexpensive, are therefore very desirable.

 The invention relates to a diaphragm having effective separation properties and giving a low cell voltage during operation.

 It also relates to a diaphragm having excellent stability and a very long operating time when it is used for the electrolysis of aqueous solutions of ionizable compounds.

 The invention also uses non-polluting materials, acceptable from an ecological point of view, for the formation of diaphragm compositions.

 The invention also relates to a diaphragm having support materials which are physically and chemically stable during electrolysis.

 The invention also relates to a diaphragm which can be easily manipulated when mounting and removing an electrolysis cell.

 More specifically, the invention relates to a porous diaphragm for the cell for the electrolysis of aqueous solutions of ionizable compounds, the diaphragm comprising a support fabric impregnated with particles of a siliceous composition having the formula (X) m (Si) p (O ) g (H) r. nH2O I in which X is at least one metal chosen from the group which comprises Be, Mg, Ca, Sr, Ba, Ti, Zr, Al, Zn and their mixtures, p is a number between 1 and 16 approximately, m is comprised between 0 and p approximately, q is a number comprised between 2 and 5p + r approximately, r is comprised between 0 and 4p approximately and n is comprised between 0 and 30 approximately. Siliceous compositions can undergo hydration when they are in contact with at least one of the lonisable compounds present in the electrolysis cell, the support fabric having an electrically conductive zone along one face.

Other characteristics and advantages of the invention will emerge more clearly from the description which follows, given with reference to the annexed drawings in which
Figure 1 is a perspective of an embodiment of diaphragm according to the invention
FIG. 2 is a perspective view of an embodiment of a diaphragm according to the invention which can be used with several electrodes
FIG. 3 is a perspective of another embodiment of a diaphragm according to the invention, intended to be used with several electrodes
FIG. 4 is a photomicrograph of a section of an embodiment of support fabric used in the diaphragms according to the invention (with a magnification of 30 times)
FIG. 5 is a photomicrograph of a planar section of an embodiment of support fabric having bundles of fibers (with a magnification of 30)
Figure 6 is a section of Figure 5, taken along line 6-6; and
Figures 7 to 9 show sections of several embodiments of support fabric.

 FIG. 1 represents a diaphragm according to the invention suitable for covering a cathode. The diaphragm 1, formed of a fabric, has end portions 10 fixed for example by sewing to the body 12 of the diaphragm.

The latter is a hollow rectangle mounted on a cathode not shown so that it surrounds the cathode on all sides. The end portions 10 have holes 14 allowing the attachment of these portions to the walls of the cell (not shown).

 FIG. 2 represents a diaphragm suitable for several electrodes. A fabric panel 20 has fabric housings 22 fixed in a direction substantially perpendicular to the plane of the panel 20. The fabric housings 22 are suitably spaced from each other and are attached to the panel 20 for example by sewing . The panel 20 has holes, not shown, corresponding to the region in which the housings 22 are fixed so that the electrodes can be introduced into the housings 22.

FIG. 3 represents another embodiment of a diaphragm according to the invention. The fabric panel 30 in
U has portions 32 of ends intended for attachment to the walls of a cell (not shown). The fabric housing 34 is fixed to the U-shaped panel 30, for example by sewing. The orifice, not shown, formed at the lower part of the housing 34 allows the arrangement of the diaphragm on an electrode placed vertically.

 Figure 4 is a section through a support fabric 40 formed by a polytetrafluoroethylene felt, having fibers 42 randomly oriented.

 The embodiment of the support fabric 40 shown in Figure 5 has bundles 44 of fibers, at regular intervals, substantially perpendicular to the plane of the outer surface of the fabric 40. The bundles 44 penetrate the entire width of the support fabric 40.

This is a fabric in the form of a polytetrafluoroethylene felt, the magnification indicated on the photomicrograph corresponding to 30 times the original.

 FIG. 6 represents bundles 44 of fibers, in section along line 6-6 of FIG. 5, the magnification being equal to 120.

 FIG. 7 shows an embodiment of support fabric 40 in which groups 44 of fibers partially penetrate only into the support fabric and leave a portion 46 having fibers oriented generally in a vertical direction.

 The support fabric 50 formed of layers shown in Figure 8, has a first layer 52 formed of a very porous fabric. A second layer 54 is contiguous with the first layer 52 and has groups 44 of fibers, diagonal to the fibers 42 of generally vertical orientation.

 The embodiment of diaper support fabric 50 of Figure 9 comprises a first layer 52 of a very porous fabric which is not adjacent to a second layer 54. The second layer 54 has groups 44 of fibers which partially penetrate this second layer 54.

The part 46, having fibers of generally vertical orientation, is adjacent to the first layer 52.

 More specifically, the diaphragm according to the invention comprise a support fabric which is impregnated with the siliceous composition.

 The fabric used is prepared from materials which chemically resist the gases and electrolytes present in the electrolysis cell and which are dimensionally stable under these conditions. The support in the form of fabric practically does not swell, does not conduct and does not dissolve practically during the operation of the electrolysis cell. The support fabric is not rigid either and is flexible enough to be shaped into an electrode if necessary.

 Suitable support fabrics are those which can be handled easily without undergoing physical deterioration. Handling includes treatment before and after impregnation with the active ingredient. Suitable support fabrics can be removed from the cell after electrolysis and then treated or repaired if necessary and returned to the cell for further use without significant degradation or disturbance.

 The support fabrics having a uniform permeability in the fabric are very suitable for the diaphragms according to the invention. Figure 4 shows support fabrics of this type. Before impregnation with the siliceous composition of formula I, these support fabrics must have a permeability to gases such as air which is for example between approximately 1.5 and 150, preferably between approximately 6 and 60 and very advantageously between approximately 9 and 30 m3 / min.m2. Uniform permeability in the fabric is not, however, necessary and it may be advantageous if the permeability is greater in part of this fabric. When it has been impregnated, this part can be placed as close as possible to the anode of the electrolysis cell, for example. The layered structures can thus be used in the form of support fabrics having a first layer which, when the diaphragm is in place in the cell, is in contact with the lanolyte, and a second layer which is in contact with the catholyte. The first layer may, for example, have an air permeability of between approximately 30 and 150 m3 / min.m2.

The first layer may for example be formed from a gauze having orifices which are slightly larger than the particle size of the active constituent by which it is impregnated.

 The second layer which is placed in contact with the catholyte when the assembly is in the cell, may for example have an air permeability of between approximately 1.5 and 30 m 3 / min.m2. The multi-layer support fabric can be prepared by attaching, for example, a gauze to a felt fabric, for use with a selected size of silica containing an active ingredient. The gauze allows the particles to pass and they are retained on the felt.

 The permeability values of the support fabrics can be determined, for example, by the standard method for determining the air permeability of textile fabrics of the ASTM D737-75 standard of the American Society for Testing Materials.

 The carrier fabrics can be prepared in any convenient manner. pes advantageous forms are those which facilitate the absorption of the active ingredient, and in particular spongy type fabrics. Particular types of support fabrics are felt plies, that is, fabrics having a very large intermingling or interconnection of fibers, the plies not usually being woven. When using a felt as a support fabric, fluids passing through the fabric travel a sinuous path between the very tangled fibers which are randomly distributed. The permeability of these fabrics is of general type, that is to say that it is neither linear nor controlled.

 The permeability of the support fabrics can be increased by a device or means which modifies the structure of this fabric. As indicated in FIGS. 5 to 9, the support fabrics are modified by a device giving linear permeability, for example bundles of fibers distributed in the fabric. The bundles of fibers, arranged at regular or irregular intervals, increase the permeability by forming regions in which the streams of fluids, for example brines of alkali chlorides, are practically laminar. Laminar flow reduces turbulence or mixing of fluids in this region and causes the formation of a homogeneous fluid in the region.

 The fabric may for example be needled or perforated at certain intervals along its surface so that the fiber bundles are formed in the fabric.

The needling depth can be adjusted so that the beams pass through the fabric as shown in Figures 5, 6 and 8, or so that they only partially penetrate the fabric as shown in Figures 7 and 9.

 The fiber bundles can be placed with any suitable inclination from the plane of the outer surface of the support fabric. For example, the angle of the fiber bundles can be between approximately 90 and 450 relative to the vertical and preferably between approximately 90 and 600.

 Fiber bundles contain several fibers, for example up to several hundred fibers.

The beams are distributed in the support fabric which can comprise several tens of beams per square centimeter of fabric.

 The fiber bundles give a linear permeability which practically increases the permeability of the fabric. Any suitable permeability of the whole fabric can be obtained by incorporating fiber bundles. For example, fiber bundles can give about 15 to 70% of the fabric permeability. Preferably, these bundles form about 20 to 50% and advantageously about 30 to 40% of the permeability of the support fabric.

 The fiber bundles not only improve the permeability but also facilitate the impregnation of the fabric by the siliceous composition and further facilitate the more uniform distribution of the siliceous composition in the support fabric.

 Another advantage of the presence of the fiber bundles in the support fabric is that the electrical resistance is reduced.

 The materials which are suitable for the formation of support fabric are in particular thermoplastic materials such as polyolefins which are polymers of olefins having approximately 2 to 6 carbon atoms in the primary chain, as well as their chlorinated and fluorinated derivatives.

 Examples are in particular polyethylene, polypropylene, polybutylene, polypentylene, polyhexylene, polyvinyl chloride, polyvinylidene chloride, polytetrafluoroethylene, fluorinated copolymers of ethylene and propylene, polychlorotrifluoroethylene, polyvinyl fluoride, fluoride polyvinylidene and copolymers of ethylene and chlorotrifluoroethylene.

 Advantageous olefins are in particular chlorinated and fluorinated derivatives such as polytetrafluoroethylene, fluorinated copolymers of ethylene and propylene, polychlorotrifluoroethylene, polyvinyl fluoride and polyvinylidene fluoride.

 Other suitable carrier materials are fabrics of polyaromatic compounds such as polyarylene compounds. Such compounds include in particular derivatives of polyanthracene, poly naphthylene and polyphenylene. For example, polyarylene sulfides such as polyphenylene sulfide or polynaphthylene sulfide can be used. Polyarylene sulfides are well known compounds, the preparation and properties of which are described in the Encyclopedia of Polymer Sciences and Technology (Interscience Publishers) vol. 10, pages 653-659. In addition to the main compounds, derivatives having chloro, fluoro or alkyl substituents can be used, for example poly (perfluorophenylene) sulfide and poly (methylphenylene) sulfide.

 The fabrics formed by the mixtures of polyolefin fibers and polyarylene sulfide fibers can be advantageously used as well as support fabrics having layers, the first layer being a polyolefin, for example polytetrafluoroethylene, and the second layer being a polyarylene sulfide, for example polyphenylene sulfide.

 The support fabric is impregnated with a siliceous composition having the formula (X) m (Si) p (O) q (H) rr nH2O I in which X represents at least one metal chosen from the group which comprises Be, Mg , Ca, Sr, Ba, Ti, Zr, Al, Zn and their mixtures, p is a number between 1 and 16 approximately, m is between 0 and p approximately, q is a number between 2 and 5p + r approximately , r is between 0 and 4p approximately and n is between 0 and 30 approximately.

 Siliceous compositions of formula I are especially those in which m is a positive number and X is at least one metal chosen from group IIA of the Periodic Table of the Elements. Examples which are suitable are beryllium, magnesium, calcium, strontium or barium silicates, the ratio of metal to silicon not exceeding about 1/1. The compositions also include mineral materials containing magnesium such as sepiolite, sea foam, augites, talcs and vermiculites, mineral materials containing calcium such as wollastonite as well as mineral materials such as tremolite having the formula CaMg3 (SiO3) 4. Other suitable materials are synthetic silicates such as commercial magnesium silicates having the approximate composition 2MgO. 3SiO2. 2H2O, as well as hydrated calcium silicate having the approximate composition CaO. 3.5 SiO2. 1.8H20.

 Other suitable materials are the synthetic clay materials described, for example, in US Patents 3,586,478, 3,671,190, 4,040,974, 4,054,537, 3,666,407, 3,844,979 and 3,855 147.

 Representative examples of siliceous compositions of formula I, in which the metal is Ti or Zr, are zirconium silicates and benitoite BaTiSi3O9.

 When X represents aluminum, the suitable siliceous compositions of formula I are in particular aluminum silicates, mineral materials such as albites, feldspars and labradorites, micro turkeys, nephelines, orthoclases, pyrophyllites and sodalites, as well as natural and synthetic zeolites.

Synthetic mineral materials based on silicates, for example described in state patents
United States of America Nos. 3,252,757 and 3,252,889 are also suitable as compositions containing aluminum.

 Other suitable materials are mineral compositions in which X represents zinc, for example zinc silicates.

 Advantageous embodiments of siliceous compositions of formula I are those in which m is a positive number and X is at least one metal chosen from the group which comprises Mg, Ca and Al as well as their mixtures, the siliceous compositions of formula I in which X represents Mg, Al or their mixtures being the most advantageous. Examples of advantageous embodiments are sepiolite or sea foam.

 The siliceous compositions of formula I can also comprise additional elements such as vanadium, niobium and rare earth elements, from the lanthanide series, germanium, tin and tungsten. In addition, alkali metals such as sodium, potassium and lithium and their oxides are often present in siliceous materials suitable as compositions of formula I. The abovementioned additional elements, when they are present in these compositions, do not represent X and are therefore not incorporated for the determination of m.

 In formula I, when m and r are equal to zero, the siliceous compositions are materials containing silica which are suitably represented by sand, quartz, silica sand, silica col loidale as well as cristobalite, tripolite and chalcedony. The term "sand" denotes compositions having a silicon dioxide content of at least 95% by weight approximately.

 As indicated by formula I, the siliceous compositions can be in the form of a hydrate, the quantity of hydration water present can be diverse.

The siliceous compositions of formula I can be formed in situ by interaction of Be, Mg,
Ca, Sr, Ba, Ti, Zr, Al and Zn with for example silica or an alkaline silicate. When X represents magnesium, magnesium compounds such as acetate, aluminate, carbonate, chloride or peroxide can be used.

Thus, a mixture of suitable quantities of MgO magnesia with silica in the presence of an electrolyte of a cell, for example an alkali hydroxide, forms a siliceous composition of formula I which can be used in the porous diaphragm according to the invention.

 The presence of metals other than those included in formula I or described above as additional elements can be tolerated at low concentrations. For example, the concentration of metals such as Fe, Ni, Pb, Ag as well as other heavy metals which may be present in the alkali chloride brines suitable for electrolysis, is preferably less than 1 ppm. When these metals are present in mineral materials suitable as a siliceous composition of formula I, it is advantageous that their concentration is less than approximately 5% of the concentration of silicon present in the material.

 Likewise, non-metallic materials such as ammonia as well as organic compounds where appropriate should be limited to moderate and preferably low concentrations.

 The degree of hydration of the siliceous composition of formula I forms the basis for the selection of suitable particle sizes. In the case of compositions which hydrate easily in the electrolyte solutions used or formed in the cell, a particle size as large as about 100 microns is satisfactory. When the component hydrates less easily, the particle size can be significantly reduced. In these compositions, particles whose size is between about 75 microns and about 0.1 microns are the most advantageous.

 Aqueous solutions of ionizable compounds which are suitable as electrolytes - include for example alkali chlorides and alkali hydroxides.

 In one embodiment of the invention, the electrically conductive area can be formed by applying a coating of an electrically conductive metal to one side of the support fabric. The coatings of conductive metals are applied to the support fabric by implementing known methods such as electroplating, catalytic chemical deposition (chemical deposition of the compounds), as well as by painting or spreading the metal or a compound. metal if applicable.

 The metal coating is applied so that it forms a non-continuous covering of the fabric and so that the diaphragm retains the desired porosity or permeability.

The coatings are essentially surface coatings and the penetration of the electrically conductive metal into the fabric is minimal. The penetration of the metal into the support advantageously does not exceed approximately 10% and preferably 1 to 5% approximately of the thickness of the fabric.

 It may be desirable for the evaluation of the coating that its electrical conductivity be measured at different stages during the application of the metal.

The operation can be carried out for example with two electrodes coated with nickel for example, analogous to needles and connected to an ohmmeter. The electrodes which are separated by a distance of one centimeter and have a contact surface of one square millimeter, are pushed against the face of the diaphragm which is coated with metal, with a pressure of 10 N / cm, and the resistance is measured.

Suitable coated diaphragms have a resistance of less than about 30 ohms.

 The support fabric may be coated with the conductive metal before or after impregnation with the siliceous composition of formula I. However, it is often more convenient for the non-continuous coating of the metal to be applied to the fabric before it is impregnated.

 In another embodiment, the conductive zone is formed by fixing filaments of a conductive metal on at least one face of the support fabric.

 The term "filaments" used herein refers to fibers, threads and fibrils. The filaments can be formed from electrically conductive metals themselves or from materials which can be coated with a conductive metal.

 Filaments can be attached to the support fabric individually, in arrangements comprising adjacent filaments but spaced apart maintaining the desired porosity of the porous diaphragm. The filaments can be interconnected in the form of a network or a sheet.

The operation can be carried out by fixing individual filaments with the desired arrangement or by forming a substrate such as a fabric which comprises the filaments.

Any suitable substrate can be used and in particular a substrate of "Nylon" or polytetrafluorethylene. Coated with carbon or silver for example.

 In an advantageous embodiment, a substrate in the form of a sheet is chosen and, after deposition of the conductive metal, it has a structure and a porosity similar to those of the support fabric which is impregnated with the siliceous composition. In this way, the brine can circulate almost uniformly through the porous diaphragm without undesirable change in the flow of liquid flowing to or from the conductive area.

 When a substrate in the form of a sheet or a sheet, for example a cloth coated with silver, is used, the conductive metal can be deposited on the substrate before the fixing of the latter to the support cloth .

 The filaments can be fixed to the support fabric, for example by sewing or by needling. Thermoplastics can be attached using heat energy sources or ultrasonic waves.

You can also fix the filaments with an adhesive.

 When manufacturing the electrically conductive zone, any conductive metal which is stable under the conditions prevailing in the cell and which does not interact with other constituents of the cells can be used.

 Suitable electrically conductive metals include nickel and its alloys, molybdenum and its alloys, vanadium and its alloys, iron and its alloys, cobalt and its alloys, magnesium and its alloys, tungsten and its alloys, gold and its alloys, platinum group metals and their alloys. The term "platinum group metal" used herein refers to a member of the group which includes platinum, ruthenium, rhodium, palladium, osmium and iridium.

Other materials which are suitable for the formation of the electrically conductive zone are metallic compounds such as spinels. These are compounds of oxygen and one or more metals characterized by an original crystal structure, a certain stoichiometric relationship and a particular pattern of X-ray diffraction. These oxygen-containing compounds, having the structure of spinels, can be represented by the formula AB2O4 in which A represents a metal having a valence +2 and
B represents a metal having a valence + 3. Metals A and
B can be a single metal, for example in the case of Mn304 or can be different metals, for example in the case of NiA1204. Metals which can be used in spinel structures as constituents A include Mg, Mn, Fe, Co, Ni, Cu and Zn. Component B can represent, among other things, one of the metals of the group which includes Al, Cr, Fe, Mn, Co, Ti and V.

 Advantageous conductive metals are nickel and its alloys, molybdenum and its alloys, cobalt and its alloys and platinum group metals and their alloys. It is very advantageous that, when the side of the diaphragm carrying the conductive zone is in contact with an ionizable compound such as an alkali hydroxide, the metal of the conductive coating is formed of nickel or of one of its alloys, of molybdenum or one of its alloys, or cobalt or one of its alloys. When the side of the diaphragm carrying the conductive zone is in contact with an ionizable compound such as an alkali chloride, the coating of conductive metal is advantageously formed from a platinum group metal or an alloy of such a metal.

 Other materials, in addition to the conductive metal filaments themselves, can be used for the coating forming the conductive zone.

 The materials which can be coated with these conductive metals are, for example, metals such as silver or copper, plastics such as polyarylene sulfides, polyolefins obtained from alfines having 2 to 6 carbon atoms approximately. , and their chlorinated and fluorinated derivatives, "Nylon", melamine resins, resins of the acrylonitrile-butadiene-styrene ABS type and mixtures thereof.

 Other materials which are suitable for coating are cellulose, carbon and in particular graphite, and glass filaments.

 When the filaments or the support fabric to be coated do not conduct electricity, sensitization of the filaments by application of a metal such as silver, aluminum or palladium, by known methods, may be necessary. The conductive metals are then deposited on the sensitized parts of the filaments or of the fabric.

 The electrically conductive area has insufficient surface for the porous and impregnated diaphragm to conduct electricity. No contact or electric wire is fixed to the diaphragm during the operation of the cell and the porous diaphragm cannot play the role of an electrode, for example in an electrolysis cell of alkali chlorides intended to form chlorine and an alkali hydroxide.

 The porous diaphragms according to the invention can be placed at a distance from the electrodes in the cell.

In an advantageous embodiment, the face of the diaphragm carrying the conductive zone is placed very close to an electrode. For example, when filaments of a metal such as nickel are attached to one face of the support fabric for the formation of a conductive zone, this zone is placed in intimate contact with the cathode so that the overvoltage of hydrogen at the cathode is reduced and the cell voltage is reduced.

 Although the support fabric can be impregnated with the siliceous composition before or after the formation of the conductive zone, it is advantageous that the impregnation takes place after this formation.

 Aqueous solutions of ionizable compounds suitable as electrolytes are in particular solutions of chlorides and alkali hydroxides.

 The support fabrics can be impregnated with the siliceous composition of formula I in any way chosen from among several. For example, a suspension of the composition in a solution, for example of an alkali metal hydroxide or chloride, is prepared and the fabric is impregnated by soaking in the suspension.

Another method includes attaching the support fabric to the cathode and immersing the cathode in the suspension, the fabric being used as the filtration support. A suction device is used to pass the suspension through the fabric, the solid particles permeating the fabric and the filtrate being removed.

 In another embodiment, the support fabric can be impregnated with the siliceous composition using a device, for example rollers, ensuring contact of the support fabric with the suspension.

 It is not essential that a solution or suspension is used for the impregnation. For example, the mineral siliceous composition of formula I can be used for the formation of a fluidized bed. Vacuum is used to vacuum the particles into the backing fabric to the desired degree of impregnation.

After impregnation, the diaphragm according to the invention contains approximately 10 to 100 mg of siliceous composition per square centimeter of support fabric and preferably
2 approximately 25 to 75 mg / cm and very advantageously approximately
2 30 to 50 mg / cm2 of composition.

 After impregnation with the siliceous composition of formula I, the diaphragms have a permeability with respect to brines of alkaline chlorides, of between approximately 100 and 1000 cm3 / min.m2 and preferably between about 200 and 500 cm3 / min.m2 diaphragm, for hydrostatic pressure, between the anolyte and the catholyte, corresponding to a brine height of between 0.25 and 50 cm approximately.

 Deposited asbestos diaphragms, when required to provide similar permeability to brines, require greater density so that electrical resistance is increased as indicated by the increased voltage coefficients for comparable conditions Operating. The diaphragms according to the invention have a higher energy efficiency than that of asbestos diaphragms and reduce the costs of the energy required.

 The diaphragms according to the invention have handling properties which are much higher than those of asbestos for example. The supporting diaphragms can be removed from the cell, washed or treated so that the flow properties are restored, and then replaced in the cell without physical deterioration. During the operation of the cell, the diaphragms according to the invention keep dimensional stability, the support material does not swell and does not dissolve or deteriorate in the presence of the electrolyte, the siliceous composition or the products formed. in the cell.

 Electrolysis cells in which the diaphragms according to the invention can be used are in particular those which are used industrially for the manufacture of chlorine and alkali hydroxides by electrolysis of the brines of alkali chlorides. Such electrolysed brines are aqueous solutions having high concentrations of alkali chlorides. Thus, when the alkali chloride is sodium chloride, suitable concentrations are those of brines containing approximately 200 to 350 g / l and preferably approximately 250 to 320 g / l of NaCl. The cells have an anode assembly containing several perforated graphite or metal anodes, a cathode assembly having several perforated metal cathodes, the diaphragm according to the invention separating the anodes from the cathodes. Electrolysis cells which are suitable by using diaphragms according to the invention, are in particular those which are described in the patents of the United States of America Nos. 1,862,244, 2,370,087, 2,987,463, 3,247,090, 3,477,938, 3,493,487, 3,617,461 and 3 642,604.

 Diaphragms according to the invention can also advantageously be used in cells, for example which electrolyze alkaline hydroxides with formation of hydrogen and oxygen. When used in electrolysis cells, the diaphragms according to the invention are sufficiently flexible so that they can be mounted on an electrode, for example a cathode, or supported by such an electrode.

 During the electrolysis of alkali chloride solutions, the siliceous compositions of formula I give a gel-like formation which is permeable to alkaline axions. Although these gel-like formations can be formed throughout the diaphragm, they normally form in the support fabric, in the portion adjacent to the side of the anolyte. The extent of gel formation in the support fabric varies for example with the thickness of the support fabric and the concentration of alkali hydroxide in the catholyte liquor. Advantageous diaphragms are those which have a gel-free part in contact with the catholyte. The formation of the gel undoubtedly takes place during the hydration of the siliceous composition. The gel is probably soluble in the liquor forming the catholyte and it is desirable that the rate of dissolution be reduced so that there remains a suitable balance between the formation of the gel and its dissolution, allowing the efficient functioning of the cell. The presence of the metals of the compounds represented by X in formula I, in the gel, is considered to be a means of increasing the stability of the gel and thus reducing its speed of dissolution. Another means appears to be the selection of suitable particle sizes for the mineral composition. Diaphragm parts without gel are formed for example by adjusting the surfaces of the support fabric which are impregnated with the siliceous composition or by adjusting the concentration of electrolytes in the anode and cathode compartments. is effective when the equilibrium is set enough to form a caustic liquor containing silica in amounts between about 10 and 150 ppm. The operation can be obtained by periodically adding the mineral siliceous composition of formula I to the brine, in suitable quantities. Brines of alkali chlorides, used in electrolysis operations, normally contain silica concentrations of between about 10 and 30 ppm, and thus the brine can provide enough silica to maintain equilibrium, the additional addition of mineral composition which may then be superfluous.

 The following examples describe examples of porous diaphragms according to the invention, by way of illustration only and without limitation.

EXAMPLE 1
A silver metallization paint is sprayed onto a section of polytetrafluoroethylene felt with a thickness equal to 1.73 mm. This silver paint is applied in a manner which forms a non-continuous coating on the felt and which minimizes the penetration of the paint into the felt. The electrical conductivity of the painted fabric is determined by bringing the painted surface into contact with two needle-coated nickel-coated electrodes, each having a contact surface 2 of 1 mm 2. The electrodes, each connected to an ohmmeter, are pressed against the painted face with a pressure of 10 Pa. The distance between the two electrodes is 1 cm. The silver spraying is stopped when the resistance is less than about 0.1 ohm. After drying, the painted felt is immersed in an electroplating bath containing an aqueous solution of nickel deposit which comprises
nickel sulfate 300 g / l
nickel chloride 60 g / l
boric acid 6 g / l
sodium molybdate 0.3 g / l
vanadyl sulfate 0.4 g / l
2
A current of 0.02 kA / m2 circulates in the solution for approximately 4 h and the current is then brought to 0.1 kA / m2 for an additional 2 h. The electrodeposition is finished with a current of 0.4 to 0.6 kA / m2 for approximately 2 h. After removal from the deposition bath, the felt, coated on one side with a nickel-molybdenum-vanadium alloy, is rinsed with tap water and then washed with a 20% solution of caustic soda. The felt is placed on a cathode formed by a grid of expanded steel, the coated face being in contact with the surface of the cathode.

The cathode covered by the felt is immersed in a sodium chloride brine (295 to 305 g / l of NaCl) in which about 5% by volume of sepiolite are dispersed. The composition of the sepiolite corresponds to the formula H4Mg2Si3O10. nH2O, the indicated oxides of the following elements being present in weight percentages
Si 79.1, Mg 9.3, K 4.8, Ca 4.8, Al 1.4 and Fe 1.4.

 A vacuum is then used so that the felt is impregnated with the dispersion until the vacuum reached is 75 to 175 torr. The vacuum is then stopped and the operation is repeated three times.

The cathode covered with felt and impregnated is then placed in an electrolysis cell having an anode in the form of a titanium grid coated with ruthenium oxide and containing a sodium chloride brine having a pH equal to 12, with a concentration of 315 to 320 g of
NaCl per liter, at a temperature of 900C. Current 2 flows in the brine with a density of 2.0 kA / m of anodic surface. The hydrostatic pressure of the initial brine is higher, in the anode compartment, than that of the cathode compartment with a brine height of between 12.5 and 25 mm. The permeability of the impregnated diaphragm is between approximately 200 and 250 cm 3 / m2 of diaphragm, this value being obtained by measuring the flow rate of catholyte liquor produced. The cell operates for 6 weeks and gives a catholyte liquor having a concentration of 131 to 188 g / l NaOH for a range of cathode current yields of 87 to 94%. The cell voltage is between 3.1 and 3.2 V. The catholyte produced has a concentration of sodium chloride between 130 and 170 g / l.

COMPARATIVE EXAMPLE A
A polytetrafluoroethylene felt having a thickness of 1.73 mm is impregnated with sepiolite using the method of Example 1. However, the felt was not previously coated on one side with the alloy of nickel. The impregnated felt is then placed on a cathode formed by a grid of expanded steel and the electrolysis of sodium chloride is carried out in the same cell and under identical conditions, with the same concentration of brine. A catholyte liquor with a concentration equivalent to that of Example 1 is obtained with current yields of 87 to 94%, but the cell voltage is between 3.2 and 3.4 V.

 The use of the diaphragm of Example 1, having a non-continuous metallic coating, causes a significant reduction in cell tension, compared to that observed with an uncoated diaphragm.

EXAMPLE 2
A section of fabric is coated in the form of a polypropylene felt, having an air permeability of approximately 140 l / min, with a thin non-continuous coating of nickel, by implementing the deposition process. Example 1. The polypropylene felt coated with nickel is then impregnated with sepiolite, by using the suspension of sodium chloride brine and the evacuation operations of Example 1. After impregnation, the diaphragm is mounted on a cathode of expanded steel so that the face coated with nickel is adjacent to the anode. When used in the cell of example 1 and by carrying out the electrolysis of this example, the diaphragm functions satisfactorily for one month with a cell voltage between 3.4 and 3.7 V, and it gives a catholyte liquor having a concentration of 135 + 20 g / l NaOH with a cathodic current yield of 90 + 2
COMPARATIVE EXAMPLE B
A section of polypropylene felt identical to that used in example 2 is impregnated with sepiolite as described in this example 2. However, the felt is not coated with an electrically conductive metal . When used in the cell of Example 1, by implementing the electrolysis process described in this example, the polypropylene felt diaphragm is destroyed in less than a week of operation of the cell.

 Example 2 thus shows that a coating of electrically conductive metal, on the face of the diaphragm facing the anode, notably increases the duration of use of the diaphragms.

EXAMPLE 3
r
A sheet of "Nylon" coated with silver is sewn onto one side of a section of polytetrafluoroethylene felt having a thickness of 2.41 mm. The sheet has a thickness which is roughly equal to a tenth of that of the felt and a 2 specific surface of 4 m per gram of fibers. The felt sensitized by silver is immersed in an electroplating bath which contains 450 g / l of nickel sulfamate and 30 g / l of boric acid, for a pH between 3 and 5. An electric current, corresponding to a density of 0.5 kA / m2, circulates in the solution for approximately 2 h. During electrodeposition, an electrically conductive coating is deposited on the silver-coated fibers and forms a porous diaphragm having a nickel coating on parts of a surface. After removal of the coating bath, the felt is rinsed with tap water and then washed with a 20% solution of caustic soda. The felt section (150 cm2) is then mounted on a cathode formed by a steel grid, the coated surface being in contact with the surface of the cathode.

The felt is then impregnated with sepiolite by implementing the method of Example 1, and the assembly forms a porous diaphragm having 50 mg / cm of sepiolite.

 The cathode impregnated and covered with felt is then placed in an electrolysis cell, having an anode formed from a titanium grid coated with ruthenium oxide and with a sodium chloride brine at a pH of 2 + 0. , 5, for a concentration of 350 + 5 g of NaCl per liter, at a temperature of 85 + 50C. A polytetrafluoroethylene spacer is placed between the anode and the diaphragm of porous felt and leaves a space of 6.35 mm. The spacer also pushes the felt against the cathode so that it makes contact between the nickel-coated surface of the porous diaphragm and the surface of the cathode. The current flows through the brine with a density of 2.0 kA / m2 of anodic surface. The initial hydrostatic brine pressure is higher in the anode compartment than that of the cathode compartment, with a brine height of between 12.5 and 25 mm. The permeability of the impregnated diaphragm 32 is between approximately 250 and 350 cm / m of diaphragm, the value being obtained by measuring the flow rate of catholyte liquor produced. The cell operates for 15 days and gives a catholyte liquor having a concentration of 135 + 12 g / l of NaOH with a range of cathodic current yields of 90 to 9d t. The cell voltage is 2.85 + 0.05 V. The catholyte liquor produced has a sodium chloride concentration of between 180 + 15 g / l. The energy consumption is calculated to be between 2270 and 2440 kWh / t of chlorine produced.

EXAMPLE 4
A section of polytetrafluoroethylene felt is coated with a sheet of "Nylon" fibers coated with silver sewn on one side, with nickel on the sensitized side, by implementing the method of Example 3.

The porous diaphragm is mounted on a steel cathode, the metal coated surface being in contact with the steel cathode. The felt is impregnated with sepiolite by implementing a process identical to that of Example 1. The porous impregnated diaphragm is then mounted in the electrolysis cell used in Example 3. A polytetrafluoroethylene spacer is placed between the anode and the porous diaphragm so that the nickel-coated face of the diaphragm and the cathode are in intimate contact. The space between the anode and the cathode is 12.7 mm. A sodium chloride brine (305 + 5 g / l NaCl) at a pH of 2 + 0.5 and at a temperature of 85 + 50C enters the anode compartment. The electrolysis cell operates with four different densities of current and voltages obtained are shown in the table below.

COMPARATIVE EXAMPLE C
Example 4 is repeated exactly using a section of a polytetrafluoroethylene felt of the type used in this example but without the sheet of "Nylon" coated with silver, fixed on it. The felt section is not coated with the nickel alloy applied in Example 4.

After mounting the porous diaphragm on the cathode, the electrolysis of sodium chloride is carried out with the same current densities as in Example 4. The results are shown in the table below.

BOARD
Current density Cell voltage (Volts)
(kA / m) Example 4 Comparative example C
1.0 '2.3 2.65
2.0 2.76 3.05
3.0 3.15 3.55
4.0 3.55 4.10
As this table indicates, the cell voltage during electrolysis is significantly reduced when the diaphragm is coated with nickel alloy in contact with the cathode, compared to the uncoated diaphragm. For each current density used, the cell voltage is reduced by at least 0.29 Then from the use of the diaphragm having an electrically conductive area.

Claims (10)

 1. Porous diaphragm for cell for electrolysis of aqueous solutions of ionizable compounds, said diaphragm being characterized in that it comprises a thermoplastic support fabric impregnated with particles of a siliceous composition of formula (X) m (Si) p ( O) q (H) r.  pq nH2O in which X represents at least one metal chosen from the group which comprises Be, Mg, Ca, Sr, Ba, Ti, Zr, Al, Zn and their mixtures, p is a number between 1 and 16 approximately, m is between 0 and p approximately, q is a number between 2 and 5p + r approximately, r is between 0 and 4p approximately and n is between 0 and 30 approximately, the siliceous compositions being capable of undergoing hydration when they are placed in contact with at least one of the ionizable compounds present in the electrolysis cell, the support fabric having an electrically conductive zone along one side of this fabric, the latter having a device giving linear permeability to aqueous solutions. 2. Diaphragm according to claim 1, characterized in that X is at least one metal chosen from the group which comprises Be, Mg, Ca, Sr, Ba, aluminum and their mixtures, and m is a positive number. 3. Diaphragm according to claim 2, characterized in that the support fabric is a polyolefin chosen from the group which comprises polypropylene, polytetrafluoroethylene, fluorinated ethylene-propylene copolymers, polychlorotrifluoroethylene, polyvinyl fluoride and fluoride of polyvinylidene. 4. Diaphragm according to claim 3, characterized in that the electrically conductive zone is formed by a face of the support fabric which carries a non-continuous coating of an electrically conductive metal. 5. A diaphragm according to claim 2, characterized in that the electrically conductive zone comprises filaments having an electrically conductive metal, fixed to a support fabric. 6. Diaphragm according to claim 5, characterized in that the filaments are chosen from the group which comprises "Nylon", melamine resins, acrylonitrile-butadiene-styrene copolymers, cellulose, glass, carbon and their mixtures, the filaments having a coating of the electrically conductive metal. 7. Diaphragm according to claim 2, characterized in that the support fabric is a polyarylene sulfide chosen from the group which comprises polyphenylene sulfide, polynaphthalene sulfide, poly (perfluorophenylene) sulfide, and sulfide poly (methylphenylene). 8. A diaphragm according to claim 7, characterized in that the siliceous composition is chosen from the group which comprises magnesium silicates, sepiolite and sea foam. 9. Diaphragm cell intended for the electrolysis of aqueous solutions of ionizable compounds, the cell having an anode assembly containing several anodes, a cathode assembly having several cathodes, a diaphragm separating anode assembly from the cathode assembly, and a cell body housing the anode assembly and the cathode assembly, said cell being characterized in that it contains a porous diaphragm formed of a thermoplastic support fabric impregnated with particles of a siliceous composition of formula  (X) m (Si) p (O) q (H) r nH2O  pqq nH2O in which X represents at least one metal chosen from the group which comprises Be, Mg, Ca, Sr, Ba, Ti, Zr, Al, Zn and their mixtures, p is a number between 1 and 16 approximately m is included between 0 and p approximately, q is a number between 2 and 5p + r approximately, r is between 0 and 4p approximately, and n is between 0 and 30 approximately, the siliceous compositions being able to undergo hydration when they are at contact of at least one of the ionizable compounds in the electrolysis cell, the support fabric having an electrically conductive zone along one of its faces, this fabric having a device giving it a linear permeability to aqueous solutions. 10. Diaphragm according to one of claims 4 and 5, characterized in that the electrically conductive metal is chosen from the group which comprises nickel and its alloys, molybdenum and its alloys, cobalt and its alloys, vanadium and its alloys, tungsten and its alloys, gold and its alloys, and platinum group metals and their alloys.