EP0007674B1 - Method of elektrolysis of an aqueous alkali metal chloride solution in a diaphragm cell - Google Patents

Abstract

The invention relates to a permeable diaphragm for an electrochemical cell. The diaphragm consists of a sheet of fibrous, organic polymeric material, obtained from a suspension of the said polymeric material in an organic liquid. The invention applies to permeable diaphragms for cells for the electrolysis of aqueous sodium chloride solutions.

Description

The present invention relates to a process for the electrolysis of aqueous solutions of alkali metal chloride in a diaphragm cell.

In electrolysis processes of this type, it is known to use diaphragms based on organic polymers in a fibrous state.

Thus, United States patent 4,036,729 in the name of Patil et al., Filed April 10, 1975 and published July 19, 1977, relates to a permeable diaphragm, which consists of a felt formed on the perforated cathode of a cell of electrolysis from a dispersion of a fibrous organic polymeric material in an aqueous medium containing acetone and a surfactant.

This known diaphragm has the disadvantage of requiring the incorporation of a surfactant in the aqueous medium used to manufacture it, so as to allow sufficient dispersion of the polymeric material therein. It has the additional disadvantage of requiring the addition of an additive wettable by aqueous electrolytes to the fibrous polymeric material, to make the diaphragm sufficiently wettable by these electrolytes. Practice has shown that in the absence of a wettable additive (such as asbestos fibers, mica, talc, or particles of titanium dioxide), the diaphragm is generally too hydrophobic to allow its normal use in electrochemical cells for the treatment of aqueous electrolytes, in particular in cells for the electrolysis of aqueous solutions of alkali metal halides.

The known diaphragm described above also has the disadvantageous feature of being only achievable from a very specific and costly variety of polymer fibers, namely fibers obtained by extrusion of a polymer in the molten state. , subjected to an intense shearing force in an auxiliary liquid medium. This particular feature of the known diaphragm has the disadvantageous effect of adversely affecting its cost.

In Japanese patent application JP-A-3 919 673 filed on April 6, 1973 by Mitsubishi Rayon Co., Ltd, a method of manufacturing porous sheets is described, according to which a fibrous polymeric material, generally a fluoropolymer, is dispersed, in an organic liquid, for example, a halogenated hydrocarbon, and a felt is separated from the resulting organic suspension.

It has now been discovered that the porous sheets of fibrous organic polymeric material obtained by this known method have the advantageous characteristic of having excellent behavior when they are used as permeable diaphragms in cells for the electrolysis of aqueous solutions of metal chloride. alkaline.

The object of the invention is therefore to provide a process for the electrolysis of aqueous solutions of alkali metal chloride, in a diaphragm cell permeable in fibrous organic polymeric material which simultaneously exhibits excellent wettability by aqueous electrolytes and optimum permeability to these aqueous electrolytes during electrolysis.

Consequently, the invention relates to a process for the electrolysis of an aqueous solution of alkali metal chloride in a diaphragm cell, in which a diaphragm consisting of a porous sheet of fibrous fluoropolymer is used, obtained from 'a suspension of the fibrous polymer in a liquid halogenated hydrocarbon.

The fibrous organic polymer used in the context of the invention may be in the form of fibers or fibrils.

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. Flaky in appearance, 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 in the context of 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 particularly in french Patent 1,596,107 of 13 December 1968, 2,148,449 and 2,148,450 of August ^1, 1972 and in the Belgian patent 811 778 of ¹ March 1974 and 824 844 of 17 January 1975, all in the name of the Applicant.

As a variant, the fibrils used in the context of the invention can also be manufactured by other methods, for example by one or the 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 EI du Pont de Nemours and Co. In these manufacturing processes, continuous fibrillated wicks are obtained, which must then be shredded, for example by grinding.

In the case where the fibrous polymer is in the form of fibers, it is preferable, according to the invention, to use fibers whose diameter is substantially between 0.1 and 25 microns; fibers which are very suitable are those having a diameter of between 1 and 15 micrometers.

For the implementation of the process according to the invention, the choice of polymer is dictated by the need to obtain a diaphragm which resists the chemical and thermal conditions normally prevailing in the electrolysis cells. Polymers containing monomer units are advantageously chosen that fluorinated derivatives of olefins, for example ethylene or propylene, preferably polymers containing at least 50%, and more particularly at least 75%, of such monomeric units. Particularly suitable polymers are those containing only monomeric units derived from ethylene or propylene in which all the hydrogen atoms have been substituted by chlorine or fluorine atoms.

By way of example, polymers which are suitable in the case where the diaphragm according to the invention is intended for the electrolysis of sodium chloride brines, are those chosen from polytetrafluorethylene, polychlortrifluoroethylene, polyvinylidene fluoride , copolymers of ethylene and chlorotrifluoroethylene, copolymers of ethylene and tetrafluoroethylene, copolymers of tetrafluoroethylene and perfluoralkylvinylether, copolymers of chlorotrifluoroethylene and vinylidene fluoride, copolymers of hydropentafluorpropylene and vinylidene fluoride, copolymers hexafluoroisobutylene and vinylidene fluoride, copolymers of tetrafluoroethylene and perfluorvinylether sulfonyl.

The liquid used to form the suspension of the fibrous polymer can be any halogenated hydrocarbon which, under the working conditions used to manufacture the porous sheet, is liquid and does not substantially modify the fibrous structure of the polymer, in particular the structure of the fibrils in the case where the polymer is in the state of fibrils. It can therefore consist of a halogenated hydrocarbon which is normally liquid or gaseous under normal conditions of pressure and temperature. In the case where a halogenated hydrocarbon is used which is gaseous at the working temperature used to manufacture the porous sheet, it is obviously advisable to work under pressure, in an autoclave.

The halogenated hydrocarbon can for example be chosen from chlorinated or fluorinated hydrocarbons. It is advantageously chosen from saturated acyclic hydrocarbons and ethylene hydrocarbons, such as for example trichlorethylene, perchlorethylene, 1,1,1-trichloroethane, methylene chloride, carbon tetrachloride, trichlorotrifluoroethane.

The halogenated hydrocarbon can be used in pure form or, alternatively, it can be slightly diluted with water or another miscible medium. Generally, the amount of water or other miscible medium mixed with the halogenated hydrocarbon cannot exceed 10% of the weight of the pure halogenated hydrocarbon; it is preferably less than 5% of the weight of the pure halogenated hydrocarbon.

It is preferred, according to the invention, to choose a fibrous polymer and a halogenated hydrocarbon whose respective solubility parameters are different from each other by less than 10 229 (J / m ³ ) ¹¹² ₁ the solubility parameter of a substance being, by definition, the square root of its cohesion energy per unit of volume (Kirk-Othmer - Encyclopedia of Chemical Technology - 1971 - Supplement Volume - p. 889). The fibrous polymeric material and the organic liquid are advantageously chosen so that the difference between their respective solubility parameters is between 3,068.7 and 6,137.4 (J / m ³ ) ^½ , the solubility parameter of the halogenated hydrocarbon preferably being greater than that of the fibrous polymer.

All other things being equal, the ability of the diaphragm to be wetted by aqueous electrolytes, in particular by sodium chloride brines, is considerably improved, when a halogenated hydrocarbon is chosen whose surface tension does not exceed 0, 04 N / m and is preferably less than 0.03 N / m, such as, for example, carbon tetrachloride or 1,1,2-trichloroethane.

According to the invention, it is advantageous to adjust the dispersion of the fibrous polymer in the halogenated hydrochloride so that the weight content of polymer in the resulting suspension is between 0.2 and 25%, preferably between 0.5 and 15% . Weight contents which are very suitable are those between 1 and 10%.

According to the invention, it is also advantageous to subject the suspension to a beating which has the function of dispersing the fibrous polymer in the halogenated hydrocarbon and of conferring, using the halogenated hydrocarbon, an intrinsic mechanical cohesion to the porous sheet obtained after decantation or filtration of the halogenated hydrocarbon. In general, the intensity of the threshing is adjusted so that the sheet thus obtained has sufficient cohesion to allow its use as a diaphragm in an electrolysis cell. Values which are suitable for the intensity of threshing are those which give the suspension of fibrous polymer in the halogenated hydrocarbon, a state of dispersion corresponding to a Schopper-Riegler number of between 20 and 80, preferably 30 and 75, as defined in standards SCAN-M3: 65 and TAPPI T227m-58.

The threshing intensities required to obtain these values of the Schopper-Riegler number depend on various factors, including the nature of the fibrous polymer and the halogenated hydrocarbon, as well as the concentration and the temperature of the suspension subjected to threshing. They can be easily determined, in each particular case, by routine work.

During threshing, the organic suspension of the fibrous polymer can be kept at a temperature lower, equal to or higher than room temperature. In general, the suspension should be kept at all times at a temperature below the softening temperature of the fibrous polymer. The temperature and pressure should also be adjusted during threshing to avoid vaporization of the halogenated hydrocarbon. Generally speaking, temperatures which are very suitable are those between 15 and 100 ° C.

To execute the threshing of the suspension, one can for example use a sogger (or "Pulper" in Anglo-Saxon literature) of the type commonly used in the paper industry and a copy of which is described by way of example, in the treaty "Pulp and Paper Manufacture •, vol. III, Mc Graw- Hill Book Company, 1970, page 156.

To form the porous sheet, it suffices, in a manner known per se, to decant or filter the suspension which has been subjected to threshing. One can for example, for this purpose, form the sheet on an openwork fabric, from which the sheet is then removed to arrange it, after drying, as a diaphragm in the electrolysis cell.

However, it is preferred to form the sheet by filtering the above suspension directly through an openwork support of the diaphragm by applying a technique of the type commonly used for the manufacture of asbestos diaphragms and described in particular in United States patents 1,865,152 in the name of KE STUART, of June 28, 1932, and 3,344,053 in the name of NEIPERT et al., Of May 4, 1964. The perforated support of the diaphragm can advantageously be the perforated cathode of the electrolysis cell. This embodiment has the advantage of making it possible to manufacture the diaphragm in situ on cathodes of complicated shape, with non-developable surface, for example of the type equipping those of the electrolysis cells described in French patents 2,223,083 of 28.3. . 1973 and 2 248 335 of October 14, 1974 in the name of the Claimant.

According to a preferred variant of this embodiment of the diaphragm sheet, threshing of the suspension is carried out and then its filtration through the perforated support, in a unique device, known per se for the manufacture of asbestos diaphragms, of the type of that described in French patent 2308702 of 25.4.1975 (SOLVAY & Cie).

In the particular case where this embodiment of the invention is applied to the manufacture of the diaphragm in situ on a cathode mesh profiled in the form of parallel fingers, for example of the type equipping the electrolysis cells described in French patents 2 223 083 and 2 248 335 cited above, it is desirable to use the technique described in United States patent 3,970,041 filed on 2.4.1975, in the name of Morton S. Kircher, which consists in engaging separating elements between successive cathode fingers during the formation of the porous sheet of the diaphragm.

It is advantageous, after forming the sheet, to dry it. Any known drying technique can be used for this purpose, such as drying at room temperature, in a calm atmosphere or by vacuum. However, it is preferred, in the context of the invention, to dry the sheet by heating it to a temperature above 80 ° C. but below the softening temperature of the polymer. The duration of the drying must be sufficient for the residual content of organic liquid in the sheet to be reduced to 5%, preferably to 2% by weight.

According to an advantageous variant of the invention, after drying the sheet, it is subjected to annealing at a temperature above 100 ° C., but below the softening temperature of the fibrous polymer. The annealing is advantageously carried out at at least 200 ° C. for at least 1 hour, preferably at least 10 hours. Well suited annealing temperatures are those which are about 5 to 15 ° C lower than the softening temperature of the fibrous polymer.

The diaphragm used in the process according to the invention has the advantage of having good mechanical cohesion and of being of stable dimensions during the electrolysis. It has the advantageous property of an excellent wettability by aqueous electrolytes, in particular by sodium chloride brines.

In an advantageous embodiment of the diaphragm, the aforementioned porous sheet resulting from drying and annealing is subjected to a treatment with a liquid whose surface tension does not exceed 0.04 N / m and is preferably less than 0.03 N / m.

The diaphragm according to this particular embodiment is characterized by optimum wettability by aqueous solutions of chloride of alkali metals such as sodium chloride brines.

In this preferred embodiment of the diaphragm, the treatment of the porous sheet with the liquid at low surface tension can be carried out by any known impregnation technique, for example by washing or immersion. However, it is preferred to percolate the treatment liquid through the porous sheet.

In general, the treatment of the sheet with the liquid at low surface tension can be carried out at low temperature, at ambient temperature or at high temperature, under a pressure less than, equal to or greater than atmospheric pressure.

The liquid with low surface tension can be any organic or inorganic liquid which, under the conditions for processing the sheet, has a surface tension not exceeding 0.04 N / m and does not substantially modify the fibrous structure of the polymer of the sheet.

Liquids with low surface tension which are particularly suitable, in the context of the invention, are those whose surface tension is between 0.01 and 0.03 N / m, for example acetone, ethyl alcohol, methyl alcohol, isopropyl alcohol, chloroform, diethyl ether, carbon tetrachloride, ammonia.

Alternatively, it is also possible to use aqueous solutions having the required surface tension, for example water containing a surfactant in an amount sufficient to bring its surface tension to 0.04 N / m at most, and preferably at 0.03 N / m.

In addition to the advantageous properties mentioned above, the diaphragm used in the process according to the invention has the particularly advantageous characteristic of generally having a permeability to aqueous solutions of alkali metal chloride of the same order of magnitude as that of the asbestos diaphragms normally fitted to the electrolysis cells with sodium chloride brines, so that it is well suited to be substituted for the asbestos diaphragms of existing electrolysis cells, for example of the type described in French patents 2 164 623 of 12.12.1972, 2 223 083 of 28.3.1973, 2 230 441 from 27.3.1974 and 2 248 335 from 14.10.1974 (SOLVAY & Cie).

The diaphragm used in the process according to the invention also has the interesting and surprising characteristic of having, from the start of its use, the optimum required characteristics of wettability and permeability. This peculiarity of the diaphragm brings the appreciable advantage that the electrolysis cells of sodium chloride brines are henceforth able to function in normal regime, with an optimum energy yield, from the start of their commissioning with a new diaphragm.

The invention finds an interesting application for the electrolysis of sodium chloride brines.

In a particularly advantageous embodiment of the invention, suitable for the electrolysis of sodium chloride brines, the specific surface of the porous sheet of the diaphragm is between 0.5 and 40 m ² / g, preferably between 1 , 5 and 20 m ² / g; its permeability is also advantageously between 0.02 and 1 h ^-1 , preferably between 0.05 and 0.5 h ^-1 , the permeability of the porous sheet being defined by the flow rate (in cm ³ / h) of brine saturated with sodium chloride at 80 ° C, which crosses a surface of the sheet of 1 cm ² , under hydrostatic pressure corresponding to a brine column of 1 cm. Values of between 5 and 15 m ² / g for the specific surface and between 0.07 and 0.3 h ^-1 for the permeability are particularly suitable in the case of the electrolysis of concentrated brines of sodium chloride.

The adequate values of the specific surface and of the permeability of the sheet forming the diaphragm can easily be obtained by a judicious choice of the dimensions of the fibers or fibrils of the fibrous polymer, of the nature of the halogenated hydrocarbon, of the concentration of the suspension subjected to threshing and threshing energy used.

The diaphragm used in the process according to the invention may optionally contain, in addition to the fibrous polymer, other usual constituents of permeable diaphragms, such as inorganic fibers (for example asbestos fibers) or additives intended for reinforce certain properties of the diaphragm or give it additional properties, for example particles of titanium dioxide or barium titanate, or surface-active agents, preferably fluorinated, such as fluorinated or perfluorinated fatty acids, sulfonic acids fluorinated or perfluorinated or salts of these acids.

To incorporate these additional constituents into the diaphragm, it is possible, for example, to introduce them as is in the particulate state into the suspension of the fibrous polymer after or, preferably, before subjecting the latter to threshing. Alternatively, a fibrous polymer can be used which already contains such additives as a filler.

The advantage of the invention will emerge from the following examples of application.

Examples 1 to 6 which follow relate to electrolysis processes with diaphragms based on fibrils made from Halar brand polymers (sold by Allied Chemical Corp.), which are copolymers of ethylene and chlorotrifluoroethylene. The fibrils used were obtained by subjecting a biphasic mixture of the polymer in the molten state and of an appropriate solvent, to a sudden expansion through an orifice of small section, as described in the French patents 1,596,107 of December 13 1968 2148 449 and 2148 450 ¹ August 1972 and in the Belgian patents 811 778 ¹ March 1974 and 824 844 of 17 January 1975 (Solvay & Cie).

First series of tests (in accordance with the invention). Examples 1 to 3 which follow are in accordance with the invention Example 1

To manufacture the diaphragms, the fibrils, obtained as described above, were dispersed in perchlorethylene so as to form 9 I of a suspension containing 1% by weight of fibrils. The suspension was subjected, at ambient temperature and atmospheric pressure, to a threshing in a conventional paper industry sinterer, for 1 min 30. The organic polymer suspension exhibited, after threshing, a state of dispersion corresponding to a number of Schopper-Riegier between 55 and 60. The suspension was then filtered through a circular perforated fabric of 113 cm ² , causing for this purpose a depression of 50 mm under the trellis, for 3 minutes. The porous sheet collected on the canvas was dried thereon, by heating at 90 ° C for one hour.

• charge de rupture : 3,737 61 106 Pa/cm²
• taux d'allongement à la rupture : 12,2 %

The diaphragms collected after drying had a specific surface area of 3 m ² / g. Subjected to a tensile test, they presented, on average, the following mechanical characteristics:

• breaking load: 3,737 61,106 Pa / cm ²
• elongation at break rate: 12.2%

One of the diaphragms thus obtained was subjected to an electrolysis test in a laboratory cell. This included a 113 cm ² circular vertical cathode, formed of a mild steel lattice and an anode arranged vertically opposite the cathode and formed of a 113 cm ² circular titanium plate, carrying a active coating consisting of an equimolar mixture of ruthenium dioxide and titanium dioxide. The distance between the anode and the cathode was 5 mm and the diaphragm was placed as it was on the cathode, facing the anode.

où

• Q désigne le débit de saumure à travers le diaphragme (en cm³/h) ;
• S désigne la section du diaphragme (en cm²) ;
• H désigne la pression hydrostatique de saumure sur le diaphragme, exprimée en cm de colonne de saumure.

In the cell equipped with the diaphragm, electrolysis of a brine at 85 ° C. containing 255 g of sodium chloride per kg was carried out at a constant current density equal to 2 kA / m ² of anode. Before the start of electrolysis, the diaphragm had a permeability equal to 0.11 h ^-1 , as defined by the relation:

or

• Q denotes the flow of brine through the diaphragm (in cm ³ / h);
• S denotes the section of the diaphragm (in cm ² );
• H is the hydrostatic brine pressure on the diaphragm, expressed in cm of brine column.

The evolution of the permeability of the diaphragm, of the electrolysis voltage, recorded at the terminals of the cell and of the current efficiency, the caustic detergent leaving the cell containing 10% by weight, is recorded in table 1 below. sodium hydroxide.

At the end of the 100-day test period, the diaphragm was still in service.

Example 2

The test of Example 1 was repeated, with the only difference that after drying, the diaphragm sheet was annealed at 200 ° C for 24 h.

• charge de rupture : 9,162 54 · 106 Pa/cm²
• allongement à la rupture : 31,5 %

The diaphragms obtained had a specific surface equal to 1.9 m ² / g. They presented the following mechanical characteristics:

• breaking load: 9,162 54106 Pa / cm ²
• elongation at break: 31.5%

The electrolysis results are recorded in Table II below.
(see Table II on page 7)

At the end of the 46 days, the diaphragm was still in service.

Example 3

Was produced, as described in Example 1, a diaphragm containing 30% by weight of fibrils and 70% by weight of titanium dioxide particles. After threshing, which lasted 1 min 30, the organic suspension of fibrous polymer and titanium dioxide had a state of dispersion corresponding to a Schopper-Riegler number equal to 43.

• Charge de rupture : 1,21644 · 106 Pa/cm²
• Allongement à la rupture : 6 %

Subjected to a tensile test, the diaphragm collected after drying exhibited the following mechanical characteristics:

• Breaking load: 1.21644106 Pa / cm ²
• Elongation at break: 6%

The electrolysis results are shown in Table III.

At the end of the 7-day trial period, the diaphragm was still in service.

The results of the tests of Examples 1 to 3 show that the diaphragms used in the process according to the invention have good mechanical strength, allowing their normal use in electrolysis. The electrolysis results obtained with such diaphragms are at least comparable to those obtained with the best asbestos diaphragms. It is also observed that the permeability of the diaphragms and the electrolysis results remain practically unchanged throughout the duration of the electrolysis, which makes it possible to make the cells operate in normal regime from the moment of the start of electrolysis.
Second series of tests (comparative tests).

Example 4

A diaphragm was made by applying the method described in Example 1, but this time using, in contrast to the invention, water instead of perchlorethylene to form the suspension of fibrils. The water used contained 1% by weight of product FLUORAD FC-170 (3M Company), which is a fluorinated surfactant. The duration of the beating of the aqueous suspension of fibrils was 1 min 30, as in the tests of Examples 1 to 3.

Subject to a tensile test, the diaphragm collected after drying was characterized by a breaking load equal to 1.962 104 Pa / cm ² .

Example 5

The test of Example 4 was repeated, but by extending the duration of the threshing which, here, was one hour.

The diaohragm collected at the end of the drying did not exhibit a higher breaking load than that of the diaphragm of Example 4.

Example 6

The test of Example 4 was repeated, with the only difference that after drying, the diaphragm sheet was annealed at 200 ° C for 24 hours.

The diaphragm obtained exhibited a tensile breaking load equal to 1.1772 105 Pa / cm ² .

A comparison of the results of Examples 1 to 3 with those of Examples 4 to 6 immediately reveals the advantage of the invention.

Claims (7)

1. Process for the electrolysis of an aqueous solution of an alkali metal chloride in a diaphragm cell, characterised in that use is made of a diaphragm which consists of a porous sheet made of a fibrous fluoro polymer initially obtained from a suspension of the fibrous polymer in a liquid halogenated hydrocarbon.

2. Process according to Claim 1, characterised in that the fibrous polymer consists of fibrils.

3. Process according to Claim 1 or 2, characterised in that the fibrous polymer and the liquid halogenated hydrocarbon are chosen so that the difference between their respective solubility parameters is less than 10 229 (J/M3)112.

4. Process according to any one of Claims 1 to 3, characterised in that the liquid halogenated hydrocarbon is chosen from those with a surface tension not exceeding 0.04 N/m.

5. Process according to any one of Claims 1 to 4, characterised in that the porous sheet is obtained from a suspension of the fibrous polymer in the halogenated hydrocarbon containing from 1 to 10 % by weight of the said fibrous polymer.

6. Process according to any one of Claims 1 to 5, characterised in that the porous sheet is obtained by subjecting the suspension of the fibrous polymer to beating so that it has a Schopper-Riegler number of between 30 and 75.

7. Process according to any one of Claims 1 to 6, characterised in that use is made of a porous sheet which has successively undergone drying reheating at least 100 °C for at least one hour, and then treatment with a liquid with a surface tension not exceeding 0.04 N/m.