Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B13/00—Diaphragms; Spacing elements
  • C25B13/04—Diaphragms; Spacing elements characterised by the material

Definitions

• the present invention relates to the manufacture of permeable diaphragms based on inorganic fibers such as asbestos, 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.
• the 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, anode-cathode differences clearly greater than the optimum value required for a cell in operation.
• the invention therefore relates to a method of manufacturing a permeable diaphragm for an electrolysis cell of aqueous solutions of alkali metal halides, in which a sheet comprising inorganic fibers and a polymer in divided solid form is formed and heated. the sheet so as to melt the polymer; according to the invention, the polymer is chosen from copolymers (polyacids) of unsubstituted alpha-olefins and of carboxylic monomers which are insoluble in aqueous solutions of alkali metal halides.
• the inorganic fibers are chosen so as to resist the corrosive action of the electrolysis baths. They advantageously consist of asbestos fibers, such as chrysotile asbestos fibers or amphibole asbestos fibers, in particular crocidolite or anthophyllite.
• polyacids having from 0.1 to 50, and more particularly from 1 to 20, acid groups per 100 carbon atoms.
• these polyacids comprise monomer units comprising one or more carboxylic groups which can be derived from unsaturated carboxylic acids, such as acrylic, alpha-chloroacrylic, methacrylic, alpha-hydroxy-acrylic and fumaric acids, comprising up to ten carbon atoms per molecule.
• unsaturated carboxylic acids such as acrylic, alpha-chloroacrylic, methacrylic, alpha-hydroxy-acrylic and fumaric acids, comprising up to ten carbon atoms per molecule.
• the preferred polycarboxylic acids advantageously comprise olefinic monomeric units derived from unsubstituted alpha-olefins comprising from 2 to 10 carbon atoms, and preferably from 2 to 6, in their molecule.
• the polycarboxylic acids contain at least 50% by weight of such olefinic monomer units. The best their results are obtained with polycarboxylic acids comprising only such olefinic monomer units and carboxylic monomer units.
• alpha-olefins from which the olefinic monomer units can be derived mention may be made of ethylene, propylene, butene-1, 4-methylpentene-1, and octene-1. Among these, propylene and even more ethylene are very particularly advantageous because they lead to polycarboxylic acids with low melting point.
• the polycarboxylic acids used in the process according to the invention are copolymers of unsubstituted alpha-olefins and of unsaturated carboxylic monomers. These copolymers can be of the statistical, block or grafted type. The latter, whose main chain is of polyolefinic structure and the side chains carry carboxylic groups, are preferred.
• polycarboxylic acids can be used in any form - and in particular the form of salts - ionizable during the manufacture of diaphragm, it is preferred to use them in the form of acids or anhydrides.
• Polymers which are particularly suitable are polyolefins containing from 0.3 to 1% by weight of carboxylic monomer units, in particular polyethylene and polypropylene containing from 0.3 to 1%, preferably from 0.5 to 0.7%. approximately, maleic anhydride.
• polyacid in the process according to the invention, it is advisable to incorporate into the sheet a sufficient quantity of polyacid so that the latter can be interposed between the individual inorganic 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 polyacid required depends on the nature of the polyacid and can be readily determined by experience. In general, at least 1%, preferably at least 5%, by weight of polyacid is incorporated into the diaphragm sheet. It is preferable not to exceed 70% by weight of polyacid in the diaphragm. In general, good results are obtained by incorporating into the sheet preferably 5 to 20% by weight of polyacid.
• the polyacid 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 polyacid is incorporated in the form of fibrils.
• fibrils is understood to mean a specific structure consisting of an aggregate of a multitude of very fine filaments, of dandruff appearance, connected together so as to form a three-dimensional network. With fibrous appearance, the fibrils 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 polyacid fibrils used in accordance with this variant of the invention can be manufactured by any process known per se, in particular by grafting monomeric units comprising ionizable groups, onto thermoplastic polymers and more particularly onto polyolefins derived from ethylene or propylene.
• the process described in Belgian patent 847,491 of October 21, 1976, in the name of the Applicant, can advantageously be used, according to which the monomer units comprising ionizable groups are grafted onto a thermoplastic polymer in the molten state in a solvent, then subjects the mixture of solvent and grafted polymer to sudden expansion, capable of causing the instantaneous vaporization of the solvent and the solidification of the polymer in the state of fibrils.
• the duration and the temperature of the heating to which the sheet is subjected depend on the nature of the polyacid and on the state in which it is incorporated in the sheet of the diaphragm. They must be chosen so that a sufficient fusion of the polyacid 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 60 minutes.
• the process according to the invention is equally applicable to the production of preformed permeable diaphragms, obtained for example according to the papermaking techniques, as well as to the production of diaphragms in situ on an openwork support (which may be, for example, the cathode openwork of a diaphragm cell), by applying the technique described in United States patent 1865152 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.
• a flat coherent sheet for example according to a technique used in stationery, of inorganic fibers and of polyacid, for example in the form of fibrils.
• the sheet is then wrung, for example by calendering, dried, then heated to a sufficient temperature and for a time sufficient to melt the polyacid.
• a sheet of inorganic fibers and of polyacid can be produced on an openwork support while aspirating to through the support an aqueous suspension of inorganic fibers and of polyacid, so as to form a felt which follows the contours of the openwork support.
• an aqueous medium is used to disperse the inorganic fibers and the polyacid, 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 a beating while the fibers and the polyacid are introduced therein, using for this purpose the process 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 polyacid at least superficially, so as to thus weld the inorganic fibers together.
• the polyacid is advantageously used in the form of fibrils.
• the choice, in accordance with the invention, of a polyacid as polymer in the diaphragm greatly improves both the homogeneity of the diaphragm, its stability of shape and dimensions and its ability to be wetted by aqueous electrolytes during use in an electrolysis cell.
• the invention thus has the advantage of improving the permeability of the diaphragms. It also provides the appreciable advantage of no longer requiring a surfactant for the manufacture of the diaphragms and thereby avoiding the emission of foam in the electrolysis cells. Finally, it does not involve heating the diaphragm to high temperatures.
• the cathode consisted of a mild steel lattice, the face of which opposite the anode was covered with a diaphragm.
• a diaphragm manufactured in accordance with the process according to the invention was applied to the cathode of the cell.
• an aqueous suspension of chrysotile asbestos fibers and polyethylene fibrils grafted with approximately 0.6% maleic anhydride was first prepared.
• the fibrils used had a length of not more than 2 mm and their average specific surface was equal to 14 m 2 / g.
• aqueous suspension 200 g of fibrils were first 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 2440 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 by addition of a supplement of alkaline brine.
• the cathode of the cell was immersed therein and said aqueous suspension was sucked through the mesh of the cathode, creating a regularly increasing depression up to a value corresponding approximately to a 500 mm column of mercury 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 1.42 kg / m 2 .
• the anode-cathode distance was set at 6 mm.
• an aqueous solution containing approximately 250 g of sodium chloride per liter was electrolyzed at 80 ° C. under an anodic current density equal to 2 kA / M 2 .
• An electrolysis voltage equal to 3.2 V was noted at the terminals of the cell, and the chlorine yield of the electrolysis was 93.5%.
• the energy consumption, per tonne of chlorine produced, was equal to 2580 kWh.
• 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 operated at a much lower temperature.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Manufacture Of Macromolecular Shaped Articles (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Chemical Or Physical Treatment Of Fibers (AREA)
• Electrolytic Production Of Metals (AREA)

Applications Claiming Priority (2)

Publications (2)

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ID=19728737

Family Applications (1)

Country Status (13)

Families Citing this family (4)

Family Cites Families (10)

• 1977
  • 1977-10-19 LU LU78350A patent/LU78350A1/xx unknown
• 1978
  • 1978-10-04 ZA ZA00785610A patent/ZA785610B/xx unknown
  • 1978-10-05 US US05/948,777 patent/US4204941A/en not_active Expired - Lifetime
  • 1978-10-05 AU AU40436/78A patent/AU4043678A/en active Pending
  • 1978-10-12 PT PT68647A patent/PT68647A/pt unknown
  • 1978-10-16 EP EP78200242A patent/EP0001664B1/fr not_active Expired
  • 1978-10-16 DE DE7878200242T patent/DE2860625D1/de not_active Expired
  • 1978-10-18 BR BR7806881A patent/BR7806881A/pt unknown
  • 1978-10-18 ES ES474298A patent/ES474298A1/es not_active Expired
  • 1978-10-18 AT AT747678A patent/AT357175B/de not_active IP Right Cessation
  • 1978-10-18 CA CA313,696A patent/CA1114778A/fr not_active Expired
  • 1978-10-19 JP JP12901578A patent/JPS5474281A/ja active Pending
  • 1978-10-19 IT IT28917/78A patent/IT1101663B/it active

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