EP0018034B1 - Process for manufacturing a porous diaphragm for an electrolytic cell - Google Patents

Process for manufacturing a porous diaphragm for an electrolytic cell Download PDF

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Publication number
EP0018034B1
EP0018034B1 EP80200312A EP80200312A EP0018034B1 EP 0018034 B1 EP0018034 B1 EP 0018034B1 EP 80200312 A EP80200312 A EP 80200312A EP 80200312 A EP80200312 A EP 80200312A EP 0018034 B1 EP0018034 B1 EP 0018034B1
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Prior art keywords
polymer
process according
suspension
diaphragm
sheet
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EP80200312A
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German (de)
French (fr)
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EP0018034A1 (en
Inventor
Giuseppe Niccolini
Luigi Reginato
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Solvay SA
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Solvay SA
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Priority to AT80200312T priority Critical patent/ATE5489T1/en
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    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25BELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
    • C25B13/00Diaphragms; Spacing elements
    • C25B13/04Diaphragms; Spacing elements characterised by the material

Definitions

  • the present invention relates to the manufacture of permeable diaphragms based on asbestos fibers, intended for cells for the electrolysis of aqueous solutions of alkali metal halides such as sodium or potassium chloride.
  • It relates more particularly to a process for obtaining diaphragms of this type, having good dimensional stability, that is to say the thickness of which remains practically unchanged during their use in an electrolysis cell.
  • the diaphragm in particular cells for the electrolysis of sodium chloride brine, the diaphragm usually consists of a layer or sheet of asbestos, applied to an openwork iron structure forming the cell cathode.
  • Known asbestos diaphragms have the disadvantage of being unstable in size over time. At the start of electrolysis, these known diaphragms tend to swell, this swelling being followed by a gradual compaction under the effect of the hydrostatic pressure prevailing in the cell.
  • These variations, over time, in the volume and shape of the diaphragm are unfavorable for electrolysis. They also have a disadvantageous influence on the geometry to be adopted for the cell. In particular, the swelling of the diaphragm at the start of its operation makes it necessary to provide for the cell, gaps between the anodes and the cathodes clearly greater than the optimum value required for a cell in operation.
  • the diaphragms obtained by these two known methods generally have insufficient permeability to aqueous electrolytes and high electrical resistivity.
  • the object of the invention is to provide a new process for the manufacture of permeable diaphragms based on asbestos fibers, which avoids the aforementioned drawbacks of the known processes by allowing in particular the removal of surfactants and leading to the production of diaphragms. which exhibit both optimum permeability to aqueous electrolytes, low electrical resistance, as well as great homogeneity and excellent characteristics of cohesion and dimensional stability.
  • the invention therefore relates to a method for manufacturing a diaphragm permeable in situ on an openwork cathode for an electrolysis cell of aqueous solutions of alkali metal halides, in which a sheet is formed comprising asbestos fibers and a thermoplastic polymer in solid form divided from an aqueous suspension of these products which are aspirated through the perforated cathode and the sheet is heated so as to melt the polymer; according to the invention, before melting the polymer, it is subjected to an oxidation treatment.
  • This treatment and the polymers used are defined below.
  • the asbestos fibers are chosen so as to resist the corrosive action of the electrolysis baths. They advantageously consist of chrysotile asbestos fibers or amphibole asbestos fibers, in particular crocidolite or anthophyllite.
  • the polymers used in the process according to the invention are the polymers derived from alpha- unsubstituted olefins comprising from 2 to 10, and preferably from 2 to 6 carbon atoms in their molecule, such as ethylene, propylene, butene-1, 4-methylpentene-1 and octene-1.
  • alpha- unsubstituted olefins comprising from 2 to 10, and preferably from 2 to 6 carbon atoms in their molecule, such as ethylene, propylene, butene-1, 4-methylpentene-1 and octene-1.
  • the homopolymers of these olefins are well suited, it is preferred, according to a particularly advantageous embodiment of the process according to the invention, to use copolymers of at least two of these olefins. Particularly advantageous results have been obtained with the copolymers of ethylene and butene-1 or propylene.
  • a sufficient quantity of polymer is generally incorporated into the sheet so that the latter can be interposed between the individual asbestos fibers and bind them effectively once they have melted, so as to oppose thus to swelling of the diaphragm during its use in an electrolysis cell.
  • the minimum amount of polymer depends on the nature of it and can be easily determined by experience. Generally, at least 1%, preferably at least 3%, by weight of polymer is incorporated into the diaphragm sheet. It is preferable not to exceed 70% by weight of polymer in the diaphragm. In general, good results are obtained by incorporating into the sheet preferably 3 to 20% by weight of polymer.
  • the polymer can be incorporated into the diaphragm sheet in any divided solid form, for example in the form of particles such as grains, flakes or short fibers.
  • the polymer is incorporated in the form of fibrils.
  • fibrils is intended to denote a specific structure of the polymeric material.
  • the fibrils consist of an aggregate of a multitude of very fine filaments, with a film-like appearance, connected together so as to form a three-dimensional network.
  • the fibrillated aggregates In appearance flaky, the fibrillated aggregates have an oblong shape; their length varies from 0.5 to 50 mm approximately and their diameter from a few microns to approximately 5 mm. They are characterized by a high specific surface, greater than 1 m 2 / g and even, in many cases, 10 m 2 / g.
  • the fibrils used within the framework of the process according to the invention can, for example, be produced by subjecting a mixture of a polymer in the molten state and of a solvent, to a sudden expansion through an appropriate orifice, as described in particular in French patents 1,596,107 of December 13, 1968, 2,148,449 and 2,148,450 of August 1, 1972 and in Belgian patents 811,778 of March 1, 1974 and 824,844 of January 17, 1975, all in the name of the Applicant.
  • the fibrils used in the context of the invention can also be manufactured by other methods, for example by one or other of the methods described in French patents 1,214,157 of June 10, 1958 and 1,472 989 of September 24, 1965, in the name of E. 1. du Pont de Nemours and Co. In these manufacturing processes, continuous fibrillated wicks are obtained, which must then be shredded, for example by grinding.
  • oxidize the polymer before the sheet is formed use can be made of any known, suitable technique, such as treatment with ozone or irradiation in the presence of air.
  • oxidize the polymer by means of an oxidizing agent in solution, for example by means of an aqueous solution of hydrogen peroxide, calcium hypochlorite or of alkali metal, of perborate.
  • an aqueous solution of hydrogen peroxide, calcium hypochlorite or of alkali metal, of perborate for example by means of an aqueous solution of hydrogen peroxide, calcium hypochlorite or of alkali metal, of perborate.
  • alkali metal or alkali metal percarbonate in the case where the polymer is chosen from polyolefins, excellent results have been obtained with aqueous solutions of ammonium persulfate or of alkali metal.
  • the amount of oxidant to be used depends on various factors, and in particular on the nature of the polymer, the nature and dimensions of the inorganic fibers of the diaphragm, the nature of the oxidant and the technique used to treat the polymer with the oxidant. It can be determined in each individual by routine laboratory work.
  • the oxidation of the polymer is adjusted to obtain an active oxygen content at least equal to 0.5 g, and preferably greater than 1 g, per 100 g of polymer.
  • the invention is not limited by a maximum limit value of the oxidation rate, it is generally chosen, for economic considerations, not to exceed an oxidation rate corresponding to 10 g of active oxygen per 100 g of polymer. Particularly suitable values are generally those between 2 and 5 g.
  • the oxidation of the polymer is carried out before its melting.
  • the oxidation of the polymer can be carried out either before or after having given it the divided solid form desired to form the sheet.
  • the oxidation treatment can be carried out directly on the fibrils or, as a variant, before forming the fibrils, for example before subjecting a solution of the polymer to a sudden expansion through a suitable opening to form the fibrils.
  • the duration and the temperature of the heating to which the sheet is subjected depend on the nature of the polymer and the state in which it is incorporated in the sheet of the diaphragm. They must be chosen so that a sufficient fusion of the polymer results so that the latter partially coats the inorganic fibers and binds them together. Usually temperatures between 130 and 250 ° C are sufficient as well as times between 1 and 100 minutes.
  • the diaphragm is produced in situ on the perforated cathode by applying the technique described in United States patent 1,865,152 in the name of KE STUART, of June 28, 1932, or in United States patent 3 344,053 in the name of NEIPERT et al., Of May 4, 1964.
  • an aqueous suspension of the asbestos fibers and of the polymer is sucked through the support, so as to form a felt which follows the contours of the perforated cathode.
  • an aqueous medium is used to disperse the asbestos fibers and the polymer, which advantageously consists of an aqueous solution of sodium hydroxide; preferably a caustic brine obtained by electrolysis of a sodium chloride brine in a diaphragm cell is used.
  • the latter can advantageously be subjected to threshing while the fibers and the polymer are introduced therein, using for this purpose the method and the device described in French patent 2,308,702 filed April 25, 1975, in the name of the Applicant.
  • the sheet thus obtained is then dried, then heated, on its support, to a sufficient temperature and for a time sufficient to melt the polymer at least superficially, so as to thus weld the inorganic fibers together.
  • the polymer is advantageously used in the form of fibrils.
  • the cathode consisted of a mild steel lattice, the face of which opposite the anode was covered with a diaphragm.
  • Fibrils were prepared by an appropriate technique, not forming part of the invention, from the polymer known under the name ELTEX A3400 (Solvay & Cie), which is a copolymer of ethylene and butene.
  • the fibrils used had a length of not more than 2 mm and their average specific surface was equal to 14 m 2 / g.
  • the fibrils were subjected to an oxidation treatment, in accordance with the invention.
  • the fibrils were dispersed in demineralized water at 100 ° C., at a rate of 10 g of fibrils per liter of water, and the dispersion was subjected to intense turbulence by means of a stirrer. propeller.
  • potassium persulfate was gradually added to the suspension, up to about 50% of the weight of the fibrils.
  • 200 g of the fibrils thus obtained were dispersed in 70 l of an alkaline brine containing approximately 8% by weight of sodium hydroxide and 16% by weight of sodium chloride.
  • the resulting dispersion was then subjected to stirring for 5 minutes, then 2,440 g of asbestos was dispersed therein and stirring was continued for 9 seconds.
  • the homogeneous suspension thus obtained was then diluted to an overall volume of 525 l, by addition of a supplement of alkaline brine.
  • the cathode of the laboratory cell described above was immersed therein, and said aqueous suspension was sucked through the mesh of the cathode, creating a regularly increasing depression until 'to a value corresponding approximately to a column of mercury of 500 mm, for ten minutes.
  • the cathode was then extracted from the suspension and the diaphragm was successively dried at 90 ° C for one hour, then heated at 160 ° C for one hour, to melt the fibrils.
  • the diaphragm obtained at the end of the treatment had a grammage equal to 1.44 kg / m 2 .
  • the anode-cathode distance was set at 6 mm.
  • an aqueous solution containing about 250 g of sodium chloride per liter was electrolyzed at 85 ° C, with an anodic current density equal to 2kA / m 2.
  • the diaphragm After the 78-day trial period, the diaphragm maintained excellent behavior and was kept in service.
  • Example 1 By way of comparison, the test of Example 1 was repeated, but omitting to introduce polymeric fibrils into the aqueous suspension of asbestos, used to manufacture the diaphragm. Furthermore, the heat treatment at 160 ° C. has been eliminated.
  • anode-cathode distance equal to 6 mm was imposed, as in the test of Example 1. From the start of the electrolysis, the diaphragm swelled until it occupied almost entire space between the anode and the cathode, so that it was impossible to reach a stationary operating state of the cell.
  • Example 1 in accordance with the invention
  • Examples 2 and 3 shows the advantage of the diaphragms obtained by the process according to the invention as regards the size and the energy efficiency of the electrolysis cells.
  • example 1 further shows that in the process according to the invention the heating of the diaphragm can be carried out at a much lower temperature.

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  • Chemical & Material Sciences (AREA)
  • Organic Chemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Engineering & Computer Science (AREA)
  • Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
  • Manufacture Of Macromolecular Shaped Articles (AREA)
  • Chemical Or Physical Treatment Of Fibers (AREA)
  • Electrodes For Compound Or Non-Metal Manufacture (AREA)
  • Manufacture Of Porous Articles, And Recovery And Treatment Of Waste Products (AREA)
  • Separation Using Semi-Permeable Membranes (AREA)
  • Electrolytic Production Of Metals (AREA)

Abstract

1. Process for the manufacture of a permeable diaphragm, in situ, on a perforated cathode of a cell for the electrolysis of aqueous solutions of alkali metal halides, in which an aqueous suspension of asbestos fibres and of a thermoplastic polymer in the divided solid state is prepared, the suspension is sucked through the perforated cathode in order to form thereon a sheet containing the fibres and the polymer, and the sheet is heated on the support in order to melt the polymer, characterized in that the polymer used is selected from amongst polymers derived from unsubstituted alpha-olefines containing from 2 to 10 carbon atoms in their molecule, and in that, before the suspension is sucked through the perforated cathode, the polymer is subjected to an oxidation treatment controlled so as to produce a proportion of at least 0,5 % by weight of active oxygen in the polymer.

Description

La présente invention a pour objet la fabrication de diaphragmes perméables à base de fibres d'amiante, destinés à des cellules pour l'électrolyse de solutions aqueuses d'halogénures de métaux alcalins tels que le chlorure de sodium ou de potassium.The present invention relates to the manufacture of permeable diaphragms based on asbestos fibers, intended for cells for the electrolysis of aqueous solutions of alkali metal halides such as sodium or potassium chloride.

Elle concerne plus particulièrement un procédé pour l'obtention de diaphragmes de ce type, présentant une bonne stabilité de dimensions, c'est-à-dire dont l'épaisseur reste pratiquement inchangée pendant leur utilisation dans une cellule d'électrolyse.It relates more particularly to a process for obtaining diaphragms of this type, having good dimensional stability, that is to say the thickness of which remains practically unchanged during their use in an electrolysis cell.

Dans les cellules à diaphragme connues, en particulier les cellules pour l'électrolyse d'une saumure de chlorure de sodium, le diaphragme est habituellement constitué d'une couche ou d'une feuille en amiante, appliqué sur une structure ajourée en fer formant la cathode de la cellule. Les diaphragmes connus en amiante présente le désavantage d'être de dimensions instables au cours du temps. Au début de l'électrolyse, ces diaphragmes connus ont tendance à gonfler, ce gonflement étant suivi d'un tassement progressif sous l'effet de la pression hydrostatique régnant dans la cellule. Ces variations, au cours du temps, du volume et de la forme du diaphragme sont défavorables à l'électrolyse. Elles ont en outre une influence désavantageuse sur la géométrie à adopter pour la cellule. En particulier, le gonflement du diaphragme au début de son exploitation impose de prévoir pour la cellule, des écarts entre les anodes et les cathodes nettement supérieurs à la valeur optimum requise pour une cellule en régime.In known diaphragm cells, in particular cells for the electrolysis of sodium chloride brine, the diaphragm usually consists of a layer or sheet of asbestos, applied to an openwork iron structure forming the cell cathode. Known asbestos diaphragms have the disadvantage of being unstable in size over time. At the start of electrolysis, these known diaphragms tend to swell, this swelling being followed by a gradual compaction under the effect of the hydrostatic pressure prevailing in the cell. These variations, over time, in the volume and shape of the diaphragm are unfavorable for electrolysis. They also have a disadvantageous influence on the geometry to be adopted for the cell. In particular, the swelling of the diaphragm at the start of its operation makes it necessary to provide for the cell, gaps between the anodes and the cathodes clearly greater than the optimum value required for a cell in operation.

Pour renforcer la cohésion des diaphragmes et amiante et améliorer leur stabilité de forme et de dimensions, on propose, dans la demande de brevet allemand 2 140 714 de Tokuyama Soda K. K., déposée le 13 août 1971, d'ajouter aux fibres d'amiante du diaphragme un liant constitué d'un polymère fluoré mis en oeuvre sous forme d'une dispersion aqueuse. On doit chauffer ensuite le diaphragme ainsi obtenu à une température élevée pour fondre le polymère (au moins 295°C selon les exemples).To strengthen the cohesion of the diaphragms and asbestos and improve their stability of shape and dimensions, it is proposed, in the German patent application 2 140 714 by Tokuyama Soda KK, filed on August 13, 1971, to add to the asbestos fibers diaphragm a binder consisting of a fluoropolymer used in the form of an aqueous dispersion. The diaphragm thus obtained must then be heated to an elevated temperature to melt the polymer (at least 295 ° C. according to the examples).

Dans une variante de ce procédé connu, décrite dans le brevet belge 809 822 de Diamond Shamrock Corporation, déposé le 16 janvier 1974, on forme une suspension aqueuse de fibres d'amiante et de particules d'un polymère thermoplastique généralement fluoré, on aspire la suspension à travers la cathode ajourée pour y déposer un diaphragme formé d'un mélange sensiblement homogène des fibres d'amiante et du polymère, et on chauffe également le diaphragme à haute température, par exemple supérieure à 300°C, pour fondre le polymère et lui permettre de lier les fibres d'amiante entre elles.In a variant of this known process, described in Belgian patent 809 822 of Diamond Shamrock Corporation, filed on January 16, 1974, an aqueous suspension of asbestos fibers and particles of a generally fluorinated thermoplastic polymer is formed, the suspension through the perforated cathode to deposit there a diaphragm formed of a substantially homogeneous mixture of asbestos fibers and of the polymer, and the diaphragm is also heated to high temperature, for example above 300 ° C., to melt the polymer and allow it to bond the asbestos fibers together.

Les diaphragmes obtenus par ces deux procédés connus présentent généralement une perméabilité insuffisante aux électrolytes aqueux et une résistivité électrique élevée.The diaphragms obtained by these two known methods generally have insufficient permeability to aqueous electrolytes and high electrical resistivity.

Pour remédier à cet inconvénient, on suggère, dans le brevet Etats-Unis 4 065 534, publié le 27 décembre 1977 et cédé à PPG Industries Inc., d'exécuter le chauffage en atmosphère oxydante, de manière à oxyder le polymère pendant qu'on le fond.To remedy this drawback, it is suggested, in United States patent 4,065,534, published on December 27, 1977 and assigned to PPG Industries Inc., to carry out the heating in an oxidizing atmosphere, so as to oxidize the polymer while we melt it.

Ces procédés connus présentent divers inconvénients sérieux. Tout d'abord, ils mettent en oeuvre des polymères fluorés qui sont difficiles à fabriquer. Ensuite, ils impliquent de traiter le diaphragme à des températures élevées ce qui est particulièrement dommageable lorsque le diaphragme est formé sur la cathode car celle-ci subit des contraintes thermiques exagérées et se déforme fréquemment lors du traitement thermique. Ils imposent par ailleurs l'emploi d'agents tensioactifs pour assurer une dispersion satisfaisante du polymère parmi les fibres inorganiques, ce qui conduit généralement à une émission importante d'écume dans les cellules d'électrolyse utilisant de tels diaphragmes. Enfin, les diaphragmes obtenus par ces procédés connus manquent généralement d'uniformité, leurs propriétés intrinsèques sont difficilement reproductibles et ils n'ont pas encore une stabilité de dimensions suffisante.These known methods have various serious drawbacks. First of all, they use fluoropolymers which are difficult to manufacture. Then, they involve treating the diaphragm at high temperatures, which is particularly damaging when the diaphragm is formed on the cathode because the latter undergoes exaggerated thermal stresses and frequently deforms during heat treatment. They also require the use of surfactants to ensure satisfactory dispersion of the polymer among the inorganic fibers, which generally leads to a significant emission of foam in the electrolysis cells using such diaphragms. Finally, the diaphragms obtained by these known methods generally lack uniformity, their intrinsic properties are difficult to reproduce and they do not yet have sufficient dimensional stability.

L'invention a pour objectif de fournir un procédé nouveau pour la fabrication de diaphragmes perméables à base de fibres d'amiante, qui évite les inconvénients précités des procédés connus en permettant notamment la suppression des agents tensioactifs et en conduisant à l'obtention de diaphragmes qui présentent à la fois une perméabilité optimum aux électrolytes aqueux, une faible résistance électrique, ainsi qu'une grande homogénéité et d'excellentes caractéristiques de cohésion et de stabilité de dimensions.The object of the invention is to provide a new process for the manufacture of permeable diaphragms based on asbestos fibers, which avoids the aforementioned drawbacks of the known processes by allowing in particular the removal of surfactants and leading to the production of diaphragms. which exhibit both optimum permeability to aqueous electrolytes, low electrical resistance, as well as great homogeneity and excellent characteristics of cohesion and dimensional stability.

L'invention concerne dès lors un procédé de fabrication d'un diaphragme perméable in situ sur une cathode ajourée pour cellule d'électrolyse de solutions aqueuses d'halogénures de métaux alcalins, dans lequel on forme une feuille comprenant des fibres d'amiante et un polymère thermoplastique sous forme solide divisee à partir d'une suspension aqueuse de ces produits qu'on aspire à travers la cathode ajourée et on chauffe la feuille de manière à fondre le polymère; selon l'invention, avant de fondre le polymère, on le soumet à un traitement d'oxydation. Ce traitement et les polymères utilisés sont définis ci-après.The invention therefore relates to a method for manufacturing a diaphragm permeable in situ on an openwork cathode for an electrolysis cell of aqueous solutions of alkali metal halides, in which a sheet is formed comprising asbestos fibers and a thermoplastic polymer in solid form divided from an aqueous suspension of these products which are aspirated through the perforated cathode and the sheet is heated so as to melt the polymer; according to the invention, before melting the polymer, it is subjected to an oxidation treatment. This treatment and the polymers used are defined below.

Dans le procédé selon l'invention, les fibres d'amiante sont choisies de façon à résister à l'action corrosive des bains d'électrolyse. Elles consistent avantageusement en fibres d'amiante chrysotile ou des fibres d'amiante amphibole, notamment de la crocidolite ou de l'anthophyllite.In the process according to the invention, the asbestos fibers are chosen so as to resist the corrosive action of the electrolysis baths. They advantageously consist of chrysotile asbestos fibers or amphibole asbestos fibers, in particular crocidolite or anthophyllite.

Les polymères utilisés dans le procédé selon l'invention sont les polymères dérivés d'alpha- oléfines non substituées comprenant de 2 à 10, et de préférence de 2 à 6 atomes de carbones dans leur molécule, telles que l'éthylène, le propylène, le butène-1, le 4-méthylpentène-1 et l'octène-1. Bien que les homopolymères de ces oléfines conviennent bien, on préfère selon une forme d'exécution particulièrement avantageuse du procédé selon l'invention, faire usage de copolymères d'au moins deux de ces oléfines. Des résultats particulièrement avantageux ont été obtenus avec les copolymères d'éthylène et de butène-1 ou de propylène.The polymers used in the process according to the invention are the polymers derived from alpha- unsubstituted olefins comprising from 2 to 10, and preferably from 2 to 6 carbon atoms in their molecule, such as ethylene, propylene, butene-1, 4-methylpentene-1 and octene-1. Although the homopolymers of these olefins are well suited, it is preferred, according to a particularly advantageous embodiment of the process according to the invention, to use copolymers of at least two of these olefins. Particularly advantageous results have been obtained with the copolymers of ethylene and butene-1 or propylene.

Dans le procédé selon l'invention, on incorpore généralement à la feuille une quantité suffisante de polymère pour que ce dernier puisse s'interposer entre les fibres d'amiante individuelles et les lier efficacement entre elles une fois fondu, de manière à s'opposer ainsi à un gonflement du diaphragme pendant son utilisation dans une cellule d'électrolyse. La quantité minimum de polymère dépend de la nature de celui-ci et peut être aisément déterminée par l'expérience. D'une manière générale, on incorpore à la feuille du diaphragme au moins 1 %, de préférence au moins 3%, en poids de polymère. Il est préférable de ne pas dépasser 70% en poids de polymère dans le diaphragme. D'une manière générale, on obtient de bons résultats en incorporant dans la feuille de préférence de 3 à 20% en poids de polymère.In the process according to the invention, a sufficient quantity of polymer is generally incorporated into the sheet so that the latter can be interposed between the individual asbestos fibers and bind them effectively once they have melted, so as to oppose thus to swelling of the diaphragm during its use in an electrolysis cell. The minimum amount of polymer depends on the nature of it and can be easily determined by experience. Generally, at least 1%, preferably at least 3%, by weight of polymer is incorporated into the diaphragm sheet. It is preferable not to exceed 70% by weight of polymer in the diaphragm. In general, good results are obtained by incorporating into the sheet preferably 3 to 20% by weight of polymer.

En variante, on peut aussi incorporer, dans la feuille du diaphragme, d'autres constituants habituels des diaphragmes perméables, tels que des additifs destinés à conférer des propriétés particulières au diaphragme. On peut par exemple lui incorporer, à titre d'additif, des particules de fer, d'oxyde de fer, de cuivre ou d'oxyde de cuivre, comme décrit dans le brevet belge 773 918 du 14 octobre 1971, au nom de la Demanderesse.As a variant, it is also possible to incorporate, into the diaphragm sheet, other usual constituents of permeable diaphragms, such as additives intended to impart particular properties to the diaphragm. It is possible, for example, to incorporate therein, as an additive, particles of iron, iron oxide, copper or copper oxide, as described in Belgian patent 773,918 of October 14, 1971, in the name of Applicant.

Suivant l'invention, le polymère peut être incorporé dans la feuille du diaphragme sous n'importe quelle forme solide divisée, par exemple à l'état de particules telles que des grains, des paillettes ou des fibres courtes. De préférence, le polymère est incorporé sous forme de fibrilles.According to the invention, the polymer can be incorporated into the diaphragm sheet in any divided solid form, for example in the form of particles such as grains, flakes or short fibers. Preferably, the polymer is incorporated in the form of fibrils.

On entend désigner par fibrilles une structure spécifique de la matière polymère. Les fibrilles consistent en un agrégat d'une multitude de filaments très ténus, d'aspect pelliculaire, connectés entre eux de manière à former un réseau tridimensionnel. D'aspect floconneux, les agrégats fibrillés ont une forme oblongue; leur longueur varie de 0,5 à 50 mm environ et leur diamètre de quelques microns à 5 mm environ. Ils sont caractérisés par une surface spécifique élevée, supérieure à 1 m2/g et même, dans beaucoup de cas, à 10 m2/g.The term “fibrils” is intended to denote a specific structure of the polymeric material. The fibrils consist of an aggregate of a multitude of very fine filaments, with a film-like appearance, connected together so as to form a three-dimensional network. In appearance flaky, the fibrillated aggregates have an oblong shape; their length varies from 0.5 to 50 mm approximately and their diameter from a few microns to approximately 5 mm. They are characterized by a high specific surface, greater than 1 m 2 / g and even, in many cases, 10 m 2 / g.

Les fibrilles utilisées dans le cadre du procédé suivant l'invention peuvent, par exemple, être fabriquées en soumettant un mélange d'un polymère à l'état fondu et d'un solvant, à une détente brusque au travers d'un orifice approprié, comme décrit notamment dans les brevets français 1 596 107 du 13 décembre 1968, 2 148449 et 2 148 450 du 1 er août 1972 et dans les brevets belges 811 778 du 1 er mars 1974 et 824 844 du 17 janvier 1975, tous au nom de la Demanderesse.The fibrils used within the framework of the process according to the invention can, for example, be produced by subjecting a mixture of a polymer in the molten state and of a solvent, to a sudden expansion through an appropriate orifice, as described in particular in French patents 1,596,107 of December 13, 1968, 2,148,449 and 2,148,450 of August 1, 1972 and in Belgian patents 811,778 of March 1, 1974 and 824,844 of January 17, 1975, all in the name of the Applicant.

En variante, les fibrilles utilisées dans le cadre de l'invention peuvent aussi être fabriquées par d'autres procédés, par exemple par l'un ou l'autre des procédés décrits dans les brevets français 1 214 157 du 10 juin 1958 et 1 472 989 du 24 septembre 1965, au nom de E. 1. du Pont de Nemours and Co. Dans ces procédés de fabrication, on obtient toutefois des mèches fibrillées continues, qu'il convient ensuite de déchiqueter, par exemple par broyage.As a variant, the fibrils used in the context of the invention can also be manufactured by other methods, for example by one or other of the methods described in French patents 1,214,157 of June 10, 1958 and 1,472 989 of September 24, 1965, in the name of E. 1. du Pont de Nemours and Co. In these manufacturing processes, continuous fibrillated wicks are obtained, which must then be shredded, for example by grinding.

Pour oxyder le polymère avant formation de la feuille, on peut faire usage de n'importe quelle technique connue, approprié, telle qu'un traitement à l'ozone ou une irradiation en présence d'air.To oxidize the polymer before the sheet is formed, use can be made of any known, suitable technique, such as treatment with ozone or irradiation in the presence of air.

On préfère toutefois, selon l'invention, oxyder le polymère au moyen d'un oxydant se trouvant en solution, par exemple au moyen d'une solution aqueuse de peroxyde d'hydrogène, d'hypochlorite de calcium ou de métal alcalin, de perborate de métal alcalin ou de percarbonate de métal alcalin; dans le cas où le polymère est choisi parmi les polyoléfines, d'excellent résultats ont été obtenus avec des solutions aqueuses de persulfate d'ammonium ou de métal alcalin.However, according to the invention, it is preferred to oxidize the polymer by means of an oxidizing agent in solution, for example by means of an aqueous solution of hydrogen peroxide, calcium hypochlorite or of alkali metal, of perborate. alkali metal or alkali metal percarbonate; in the case where the polymer is chosen from polyolefins, excellent results have been obtained with aqueous solutions of ammonium persulfate or of alkali metal.

La quantité d'oxydant à mettre en oeuvre dépend de divers facteurs, et notamment de la nature du polymère, de la nature et des dimensions des fibres inorganiques du diaphragme, de la nature de l'oxydant et de la technique mise en oeuvre pour traiter le polymère avec l'oxydant. Elle peut être déterminée dans chaque ces particulier par un travail de routine au laboratoire.The amount of oxidant to be used depends on various factors, and in particular on the nature of the polymer, the nature and dimensions of the inorganic fibers of the diaphragm, the nature of the oxidant and the technique used to treat the polymer with the oxidant. It can be determined in each individual by routine laboratory work.

Selon le procédé de l'invention on règle l'oxydation du polymère pour obtenir une teneur en oxygène actif au moins égale à 0,5 g, et de préférence supérieure à 1 g, pour 100 g de polymère. Bien qu'en principe l'invention ne soit pas limitée par une valeur limite maximum du taux d'oxydation, on choisit généralement, pour des considérations d'ordre économique, de ne pas dépasser un taux d'oxydation correspondant à 10 g d'oxygène actif pour 100 g de polymère. Des valeurs particulièrement adéquates sont en général celles comprises entre 2 et 5 g.According to the process of the invention, the oxidation of the polymer is adjusted to obtain an active oxygen content at least equal to 0.5 g, and preferably greater than 1 g, per 100 g of polymer. Although in principle the invention is not limited by a maximum limit value of the oxidation rate, it is generally chosen, for economic considerations, not to exceed an oxidation rate corresponding to 10 g of active oxygen per 100 g of polymer. Particularly suitable values are generally those between 2 and 5 g.

Selon l'invention, l'oxydation du polymère est opérée avant sa fusion.According to the invention, the oxidation of the polymer is carried out before its melting.

L'oxydation du polymère peut être opérée indifféremment avant ou après lui avoir conféré la forme solide divisée, souhaitée pour former la feuille.The oxidation of the polymer can be carried out either before or after having given it the divided solid form desired to form the sheet.

Dans le cas particulier où le polymère est utilisé dans la feuille à l'état de fibrilles, on peut opérer le traitement d'oxydation directement sur les fibrilles ou, en variante, avant de former les fibrilles, par exemple avant de soumettre une solution du polymère à une détente brusque à travers un orifice approprié pour former les fibrilles. Dans cette variante de l'invention, on peut par exemple oxyder le polymère à l'état de poudre, de granules ou de paillettes, puis le dissoudre dans un solvant approprié et soumettre la solution résultante à une détente brusque à travers un orifice approprié pour produire les fibrilles, suivant la technique décrite dans les brevets français 1 596 107 du 13 décembre 1968, 2 148 449 et 2 148 450 du 1 er août 1972 et dans les brevets belges 811 778 du 1 er mars 1974 et 824 844 du 17 janvier 1975, tous au nom de la Demanderesse.In the particular case where the polymer is used in the sheet in the form of fibrils, the oxidation treatment can be carried out directly on the fibrils or, as a variant, before forming the fibrils, for example before subjecting a solution of the polymer to a sudden expansion through a suitable opening to form the fibrils. In this variant of the invention, it is possible, for example, to oxidize the polymer in the form of a powder, granules or flakes, then dissolve it in an appropriate solvent and subject the resulting solution to sudden expansion through an orifice suitable for produce the fibrils, according to the technique described in French patents 1,596,107 of December 13, 1968, 2,148,449 and 2,148,450 of August 1, 1972 and in Belgian patents 811,778 of March 1, 1974 and 824,844 of January 17, 1975, all in the name of the Applicant.

Dans le procédé suivant l'invention, la durée et la température du chauffage auquel on soumet la feuille dépendent de la nature du polymère et de l'état sous lequel il est incorporé dans la feuille du diaphragme. Elles doivent être choisies de manière qu'il en résulte une fusion suffisante du polymère pour que celui-ci enrobe partiellement les fibres inorganiques et les soude entre elles. D'habitude, des températures comprises entre 130 et 250°C suffisent de même que des durées comprises entre 1 et 100 minutes.In the process according to the invention, the duration and the temperature of the heating to which the sheet is subjected depend on the nature of the polymer and the state in which it is incorporated in the sheet of the diaphragm. They must be chosen so that a sufficient fusion of the polymer results so that the latter partially coats the inorganic fibers and binds them together. Usually temperatures between 130 and 250 ° C are sufficient as well as times between 1 and 100 minutes.

Dans le procédé suivant l'invention on fabrique le diaphragme in situ sur la cathode ajourée en appliquant la technique décrite dans le brevet Etats-Unis 1 865 152 au nom de K. E. STUART, du 28 juin 1932, ou dans le brevet Etats-Unis 3 344 053 au nom de NEIPERT et al., du 4 mai 1964.In the process according to the invention, the diaphragm is produced in situ on the perforated cathode by applying the technique described in United States patent 1,865,152 in the name of KE STUART, of June 28, 1932, or in United States patent 3 344,053 in the name of NEIPERT et al., Of May 4, 1964.

On aspire à travers le support une suspension aqueuse des fibres d'amiante et du polymère, de manière à former un feutre qui épouse les contours de la cathode ajourée. De préférence, on utilise un milieu aqueux pour disperser les fibres d'amiante et le polymère, qui consiste avantageusement en une solution aqueuse d'hydroxyde de sodium; on utilise de préférence une saumure caustique obtenue par électrolyse d'une saumure de chlorure de sodium dans une cellule à diaphragme. Pour assurer une dispersion efficace des fibres d'amiante et du polymère dans le milieu aqueux, on peut avantageusement soumettre celui-ci à un battage pendant qu'on y introduit les fibres et le polymère, en utilisant à cet effet le procédé et le dispositif décrits dans le brevet français 2 308 702 déposé le 25 avril 1975, au nom de la Demanderesse.An aqueous suspension of the asbestos fibers and of the polymer is sucked through the support, so as to form a felt which follows the contours of the perforated cathode. Preferably, an aqueous medium is used to disperse the asbestos fibers and the polymer, which advantageously consists of an aqueous solution of sodium hydroxide; preferably a caustic brine obtained by electrolysis of a sodium chloride brine in a diaphragm cell is used. To ensure effective dispersion of the asbestos fibers and of the polymer in the aqueous medium, the latter can advantageously be subjected to threshing while the fibers and the polymer are introduced therein, using for this purpose the method and the device described in French patent 2,308,702 filed April 25, 1975, in the name of the Applicant.

La feuille ainsi obtenue est ensuite séchée, puis chauffée, sur son support, à une température suffisante et pendant un temps suffisant pour fondre au moins superficiellement le polymère, de manière à souder de la sorte les fibres inorganiques entre elles.The sheet thus obtained is then dried, then heated, on its support, to a sufficient temperature and for a time sufficient to melt the polymer at least superficially, so as to thus weld the inorganic fibers together.

On utilise avantageusement le polymère sous forme de fibrilles.The polymer is advantageously used in the form of fibrils.

L'intérêt du procédé selon l'invention va apparaître à la lecture des exemples d'application qui suivent.The advantage of the method according to the invention will appear on reading the application examples which follow.

Dans les exemples d'application suivants, on a procédé à quelques essais d'électrolyse dans une cellule de laboratoire à électrodes verticales, séparées par un diaphragme. L'anode a consisté en une plaque en titane portant un revêtement actif de cristaux mixtes constitués de 50% en poids de bioxyde de ruthénium et de 50% en poids de bioxyde de titane.In the following application examples, some electrolysis tests were carried out in a laboratory cell with vertical electrodes, separated by a diaphragm. The anode consisted of a titanium plate carrying an active coating of mixed crystals consisting of 50% by weight of ruthenium dioxide and 50% by weight of titanium dioxide.

La cathode a consisté en un treillis d'acier doux dont la face disposée en regard de l'anode a été recouverte d'un diaphragme.The cathode consisted of a mild steel lattice, the face of which opposite the anode was covered with a diaphragm.

Exemple 1Example 1

On a préparé, par une technique approprié, ne faisant pas partie de l'invention, des fibrilles au départ du polymère connu sous le nom ELTEX A3400 (Solvay & Cie), qui est un copolymère d'éthylène et de butène. Les fibrilles utilisées avaient une longueur ne dépassant pas 2 mm et leur surface spécifique moyenne était égale à 14 m2/g.Fibrils were prepared by an appropriate technique, not forming part of the invention, from the polymer known under the name ELTEX A3400 (Solvay & Cie), which is a copolymer of ethylene and butene. The fibrils used had a length of not more than 2 mm and their average specific surface was equal to 14 m 2 / g.

Les fibrilles ont été soumises à un traitement d'oxydation, conformément à l'invention. A cet effet, on a dispersé les fibrilles dans de l'eau déminéralisée à 100°C, à raison de 10 g de fibrilles par litre d'eau, et on a soumis la dispersion à une turbulence intense au moyen d'un agitateur à hélice.The fibrils were subjected to an oxidation treatment, in accordance with the invention. For this purpose, the fibrils were dispersed in demineralized water at 100 ° C., at a rate of 10 g of fibrils per liter of water, and the dispersion was subjected to intense turbulence by means of a stirrer. propeller.

Tout en maintenant l'agitation on a ajouté progressivement du persulfate de potassium à la suspension, jusqu'à concurrence d'environ 50% du poids des fibrilles.While maintaining stirring, potassium persulfate was gradually added to the suspension, up to about 50% of the weight of the fibrils.

Après environ 40 minutes d'agitation intense, on a filtré la suspension et on a recueilli les fibrilles.After about 40 minutes of vigorous stirring, the suspension was filtered and the fibrils were collected.

On a consigné au tableau I la distribution granulométrique des fibrilles recueillies à l'issue du traitement d'oxydation, établie conformément aux normes de Tyler (Chemical Engineers Handbook, J. H. Perry, McGraw-Hill Book Company, 1963, p. 21-50 et 21-51).

Figure imgb0001
The particle size distribution of the fibrils collected after the oxidation treatment is recorded in Table I, established in accordance with the standards of Tyler (Chemical Engineers Handbook, JH Perry, McGraw-Hill Book Company, 1963, p. 21-50 and 21-51).
Figure imgb0001

On a dispersé 200 g des fibrilles ainsi obtenues, dans 70 1 d'une saumure alcaline contenant approximativement 8% en poids d'hydroxyde de sodium et 16% en poids de chlorure de sodium. On a ensuite soumis la dispersion résultante à une agitation pendant 5 minutes, puis on y a dispersé 2 440 g d'amiante et on a poursuivi l'agitation pendant 9 secondes. La suspension homogène ainsi obtenue a ensuite été diluée jusqu'à un volume globale de 525 I, par addition d'un complément de saumure alcaline.200 g of the fibrils thus obtained were dispersed in 70 l of an alkaline brine containing approximately 8% by weight of sodium hydroxide and 16% by weight of sodium chloride. The resulting dispersion was then subjected to stirring for 5 minutes, then 2,440 g of asbestos was dispersed therein and stirring was continued for 9 seconds. The homogeneous suspension thus obtained was then diluted to an overall volume of 525 l, by addition of a supplement of alkaline brine.

Après formation de la suspension aqueuse de la manière décrite ci-dessus, on y a immergé la cathode de la cellule de laboratoire décrite plus haute, et on a aspiré ladite suspension aqueuse à travers le treillis de la cathode en créant une dépression régulièrement croissante jusqu'à une valeur correspondant approximativement à une colonne de mercure de 500 mm, pendant une dizaine de minutes. La cathode a ensuite été extraite de la suspension et le diaphragme a été successivement séché à 90°C pendant une heure, puis chauffé à 160°C pendant une heure, pour fondre les fibrilles.After forming the aqueous suspension in the manner described above, the cathode of the laboratory cell described above was immersed therein, and said aqueous suspension was sucked through the mesh of the cathode, creating a regularly increasing depression until 'to a value corresponding approximately to a column of mercury of 500 mm, for ten minutes. The cathode was then extracted from the suspension and the diaphragm was successively dried at 90 ° C for one hour, then heated at 160 ° C for one hour, to melt the fibrils.

Le diaphragme obtenu à l'issue du traitement présentait un grammage égal à 1,44 kg/m2.The diaphragm obtained at the end of the treatment had a grammage equal to 1.44 kg / m 2 .

Pour l'essai d'électrolyse, la distance anode-cathode a été réglée à 6 mm.For the electrolysis test, the anode-cathode distance was set at 6 mm.

On a procédé, dans la cellule, à l'électrolyse d'une solution aqueuse contenant environ 250 g de chlorure de sodium par litre, à 85°C, sous une densité de courant anodique égale à 2kA/m2. In the cell, an aqueous solution containing about 250 g of sodium chloride per liter was electrolyzed at 85 ° C, with an anodic current density equal to 2kA / m 2.

Au cours de l'essai d'électrolyse, on a relevé périodiquement la tension aux bornes de la cellule, le rendement de courant correspondant au chlore produit et l'énergie consommée par tonne de chlore produite. Les résultats sont consignés au tableau II.

Figure imgb0002
During the electrolysis test, the voltage across the terminals of the cell, the current efficiency corresponding to the chlorine produced and the energy consumed per tonne of chlorine produced were periodically noted. The results are reported in Table II.
Figure imgb0002

A l'issue de la période d'essai de 78 jours, le diaphragme conservait un excellent comportement et il a été maintenu en service.After the 78-day trial period, the diaphragm maintained excellent behavior and was kept in service.

Exemple 2Example 2

A titre comparatif, on a répété l'essai de l'exemple 1, mais en omettant d'introduire des fibrilles polymériques dans la suspension aqueuse d'amiante, utilisée pour fabriquer le diaphragme. On a par ailleurs supprimé le traitement thermique à 160°C.By way of comparison, the test of Example 1 was repeated, but omitting to introduce polymeric fibrils into the aqueous suspension of asbestos, used to manufacture the diaphragm. Furthermore, the heat treatment at 160 ° C. has been eliminated.

Pour l'essai d'électrolyse, on a imposé une distance anode-cathode égale à 6 mm, comme dans l'essai de l'exemple 1. Dès le début de l'électrolyse, le diaphragme a gonflé jusqu'à occuper la quasi totalité de l'espace séparant l'anode de la cathode, de sorte qu'il a été impossible d'atteindre un état de fonctionnement stationnaire de la cellule.For the electrolysis test, an anode-cathode distance equal to 6 mm was imposed, as in the test of Example 1. From the start of the electrolysis, the diaphragm swelled until it occupied almost entire space between the anode and the cathode, so that it was impossible to reach a stationary operating state of the cell.

Exemple 3Example 3

On a répété l'essai de l'exemple 2, mais en imposant cette fois une distance anode-cathode égale à 12 mm, pour compenser le gonflement du diaphragme. On a relevé les résultats d'électrolyse suivants:

  • tension aux bornes: 3,45 V
  • rendement de courant: 95,7%
  • consommation d'énergie: 2 726 kWh/t chlore.
The test of Example 2 was repeated, but this time imposing an anode-cathode distance equal to 12 mm, to compensate for the swelling of the diaphragm. The following electrolysis results were noted:
  • terminal voltage: 3.45 V
  • current efficiency: 95.7%
  • energy consumption: 2,726 kWh / t chlorine.

Une comparaison des résultats de l'exemple 1 (conforme à l'invention) avec ceux des exemples 2 et 3 fait apparaître l'avantage des diaphragmes obtenus par le procédé suivant l'invention en ce qui concerne l'encombrement et le rendement énergétique des cellules d'électrolyse.A comparison of the results of Example 1 (in accordance with the invention) with those of Examples 2 and 3 shows the advantage of the diaphragms obtained by the process according to the invention as regards the size and the energy efficiency of the electrolysis cells.

Comparé au procédé décrit dans le brevet belge 809 822 précité, l'exemple 1 montre en outre que dans le procédé suivant l'invention le chauffage du diaphragme peut être opéré à une température beaucoup plus basse.Compared to the process described in the aforementioned Belgian patent 809,822, example 1 further shows that in the process according to the invention the heating of the diaphragm can be carried out at a much lower temperature.

Claims (7)

1. Process for the manufacture of a permeable diaphragm, in situ, on a perforated cathode of a cell for the electrolysis of aqueous solutions of alkali metal halides, in which an aqueous suspension of asbestos fibres and of a thermoplastic polymer in the divided solid state is prepared, the suspension is sucked through the perforated cathode in order to form thereon a sheet containing the fibres and the polymer, and the sheet is heated on the support in order to melt the polymer, characterised in that the polymer used is selected from amongst polymers derived from unsubstituted alpha-olefines containing from 2 to 10 carbon atoms in their molecule, and in that, before the suspension is sucked through the perforated cathode, the polymer is subjected to an oxidation treatment controlled so as to produce a proportion of at least 0.5% by weight of active oxygen in the polymer.
2. Process according to Claim 1, characterised in that the oxidation of the polymer is controlled so as to obtain a proportion of active oxygen of more than 1 g of active oxygen per 100 g of polymer.
3. Process according to Claim 1 or 2, characterised in that an aqueous solution of ammonium persulphate or an alkali metal persulphate is used for the oxidation treatment.
4. Process according to any one of Claims 1 to 3, characterised in that the polymer used is chosen from amongst polymers of unsubstituted olefines containing from 2 to 6 carbon atoms in their molecule.
5. Process according to Claim 4, characterised in that the polymer is a copolymer of ethylene and but-1-ene or propylene.
6. Process according to any one of Claims 1 to 5, characterised in that the sheet comprises from 1 to 70% by weight of polymer.
7. Process according to any one of Claims 1 to 6, characterised in that the polymer is used in the form of fibrils.
EP80200312A 1979-04-17 1980-04-08 Process for manufacturing a porous diaphragm for an electrolytic cell Expired EP0018034B1 (en)

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