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

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

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.

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.

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).

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.

The diaphragms obtained by these two known methods generally have insufficient permeability to aqueous electrolytes and high electrical resistivity.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

According to the invention, 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.

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.

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.

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.

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.

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 advantage of the method according to the invention will appear on reading the application examples which follow.

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.

The cathode consisted of a mild steel lattice, the face of which opposite the anode was covered with a diaphragm.

Example 1

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. 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.

While maintaining stirring, potassium persulfate was gradually added to the suspension, up to about 50% of the weight of the fibrils.

After about 40 minutes of vigorous stirring, the suspension was filtered and the fibrils were collected.

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).

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.

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.

The diaphragm obtained at the end of the treatment had a grammage equal to 1.44 kg / m ² .

For the electrolysis test, the anode-cathode distance was set at 6 mm.

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.

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.

After the 78-day trial period, the diaphragm maintained excellent behavior and was kept in service.

Example 2

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.

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.

Example 3

• 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.

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.

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.