NO771758L - MEMBRANE DEVICE FOR ELECTROLYSIS CELLS AND PROCEDURES FOR MANUFACTURE THEREOF - Google Patents

Description

Membrane separator for electrolysis cells

and method of production thereof.

The invention generally relates to the conversion of a standard diaphragm electrolysis cell used for the production of chlorine and caustic materials (sodium hydroxide) into an electrolysis cell with a membrane for the same type of chemical production and with the inherent advantageous properties of a membrane electrolysis cell. The invention relates more particularly to a method of forming a membrane over a standard diaphragm electrolysis cell cathode by vacuum forming a mat material on the perforated electrode with subsequent application of a membrane material over the top of the mat material which is melted to form a thin and uniform, essentially hydraulically impermeable film. Such a method will enable manufacturers who have standard diaphragm electrolysis cell equipment in use to convert this equipment into membrane electrolysis cells with small capital outlays to achieve a saving in the operating costs associated with the use of membrane electrolysis cells.

Electrochemical preparation methods are becoming increasingly important for the chemical industry due to the methods' greater ecological acceptability, opportunities for energy conservation and the possible cost reductions obtained. A large part of research and development therefore concerns electrochemical processes and the production equipment for these. This effort has led to technical

advances, such as the dimensionally stable anode and different coating agents for this which enable increasingly narrow electrode distances so that the electrolysis cell has become more efficient for use in electrochemical processes. The hydraulically impermeable membrane has also contributed greatly to the possible use of electrolysis cells due to of the fact that it enables a selective migration of different ions over the membrane surface, thereby excluding contamination from the product obtained so that a number of costly purification and concentration treatment steps are avoided.

A significant commercial opportunity for these advances in terms of electrolysis cells will be in the production of chlorine and caustic materials. Chlorine and caustic materials are important technical products of which large volumes are required and which are basic chemicals that are in demand by all industrial enterprises. They are produced almost exclusively electrolytically from aqueous solutions of alkali metal chlorides, with a main proportion of this production being obtained from electrolysis cells of the diaphragm type.

In the diaphragm cell process, salt solution (sodium chloride solution) is continuously supplied to the anode compartment and flows through. the diaphragm is usually made of asbestos and supported by the cathode. To do one back migration of the hydroxyl ions then. small as possible, a current that is stronger than the conversion rate is almost always maintained so that the catholyte solution obtained will contain unused alkali metal chloride. The hydrogen ions are discharged from the solution and the cathode in the form of molecular hydrogen gas. The cathode solution containing caustic materials, unreacted sodium chloride and other impurities has usually been concentrated and purified at a later stage to obtain a salable sodium hydrogen product and a sodium chloride salt which can be reused in a chlorine and alkali electrolysis cell for further production of sodium hydroxide.

The dimensionally stable anode is used today by a large number of chlor-alkali producers, but the extensive commercial use of the hydraulically impermeable membrane has been at least partially counteracted by the significant investments required to convert diaphragm electrolysis cells into membrane electrolysis cells. This is due to the difficulty in placing a more or less planar membrane on the cathode assembly, which is generally a three-dimensional assembly on which the asbestos diaphragm is placed by vacuum forming from a slurry. The diaphragm is. has been improved by adding to the slurry from which the diaphragm is deposited on the cathode assembly, a polymeric material to act as a binder to improve the chemical resistance of the diaphragm material. However, the diaphragm which has been modified with dimensionally stable polymer is not a hydraulically impermeable membrane. Another attempt has been to form a membrane on the electrode surface itself. The problem with this experiment is that most currently used cathode assemblies are perforated, and such porosity makes it very difficult to deposit a membrane material so that a film is formed directly on the cathode assembly.

It is therefore taken in the invention, duty to provide

a method by forming a membrane over a standard diaphragm cell electrode assembly to thereby avoid the substantial investment currently required to convert a diaphragm electrolysis cell to a membrane electrolysis cell.

The invention also aims to provide a method for forming a membrane over a standard diaphragm cell electrode assembly so that the advantages of the membrane can be achieved using existing diaphragm electrolysis cell equipment.

The invention also aims to provide a membrane separator for a standard diaphragm electrolysis cell.

The invention thus relates to a membrane separator for a standard diaphragm electrolysis cell, and the membrane separator is characterized by the fact that it comprises a perforated standard diaphragm cell electrode, a layer of mat material on the surface of the perforated standard diaphragm cell electrode with such a thickness that the perforated standard, diaphragm electrode. porosity is substantially reduced, and on the surface of the mat material a thin and uniform, hydraulically impermeable cation exchange membrane consisting essentially of a film of a copolymer with the repeating structural units of the formula

in which R denotes the group -CF-CF2-0 (-CFY-CF20-) in which R denotes a fluorine atom or a perfluoroalkyl group with 1-10 carbon atoms, Y denotes a fluorine atom or the trifluoromethyl group, m denotes 1, 2 or 3, n denotes 0 or 1, X denotes a fluorine atom, a chlorine atom or the trifluoromethyl group, X<1> denotes X or CF-, (- CF~ ± 0-,

and a denotes 0 or an integer from 1 to 5, and the units of formula (1) are present in such an amount that a copolymer with an -SO^H equivalent weight of 800 - 1600 is obtained.

The invention relates to a membrane separator of the type described above, which is characterized by the thin and. uniform, hydraulically impermeable cation exchange membrane essentially consisting of a film of a copolymer having the repeating structural units of the formula

hyori R denotes the group

where R denotes

a fluorine atom or a perfluoroalkyl group with 1-10 carbon atoms,

Y denotes a fluorine atom or the trifluoromethyl group. m denotes

a

1, 2 or 3, n denotes O or 1, X denotes fluorine atom, a chlorine atom or the trifluoromethyl group, X"*" denotes X or<CF>3 -fCF^) aO-}-, and a denotes 0 or an integer from 1 to 5, in which R^ denotes an ion exchange group selected from the group of oxyacids and salts or esters of carbon, nitrogen, silicon,. phosphorus, sulphur, chlorine, arsenic, selenium or tellurium. The invention also relates to a method for manufacturing a membrane over a perforated standard diaphragm cell electrode, and the method is characterized by the fact that a mat material is suspended in a liquid medium, a perforated electrode is introduced into the suspension, a layer of the suspended mat material is vacuum formed over the surface of the perforated electrode to substantially reduce the porosity of the perforated electrode material, a layer of a thermoplastic material comprising "Nafion" particles in the sulfonyl fluoride form is applied to the surface of the perforated electrode with the applied mat material, the perforated electrode is fired with the layers in place until the thermoplastic material has been melted into a thin and uniform film on the surface of the mat material which is essentially impervious to hydraulic flow, and the applied "Nafion" is hydrolyzed to change the sulfonyl fluoride state to the sulfonic acid cation exchange state. The invention also relates to a method as described above and which is characterized by the fact that a layer of a thermoplastic material comprising a material containing carboxyl ion exchange groups is applied to the surface of the perforated electrode with the applied mat material, after which the perforated electrode with the layers in place are burned until the thermoplastic material has been melted into a thin and uniform film on the surface of the mat material which is essentially impenetrable to. hydraulic current. The membrane separator for a standard diaphragm electrolysis cell electrode assembly and the method for the formation of such a membrane will overcome a number of the disadvantages with which the above-mentioned known embodiments are burdened and provide the advantages of the use of a membrane in an electrolysis cell without them. significant capital costs which have hitherto been associated with the conversion of a diaphragm electrolysis cell into a membrane electrolysis cell. ;Most of these currently used diaphragm electrolysis cells are of two general types. Both consist of an outer steel jacket with a cylindrical or rectangular shape that supports a cathode of a perforated iron plate or, of a woven iron cloth inside the jacket and which is usually referred to as a perforated electrode element. This constitutes the cathode assembly. The relevant cathode surfaces are usually lined with an asbestos layer in the form of a paper wrapped around the cathode surfaces, or with fibers that have been deposited under vacuum. The type of cathode assembly for which the present invention is socially useful is the cathode assembly type known as "the Diamond Shamrock Cell", in which the cathode assembly consists of a surrounding rectangular steel jacket with an internal assembly of laterally arranged rows of vertically flattened deviator tubes on which the diaphragm has been deposited by suction from a suspension of asbestos fibers in the cell fluid. As these perforated electrode assemblies usually have high porosity, it is necessary to reduce the porosity by vacuum deposition of one or another type of mat material on the perforated electrode surface before the membrane material is applied. The mat material can be an asbestos carrier material made from chrysotile asbestos fibers mixed with 5% by weight of fluorinated ethylene/propylene copolymer particles, or any other material that will form ;er\. sufficient mat on the perforated electrode. Another example is a cellulose material. Sheets of a material, such as filter paper, can also be wrapped around the electrode tube. It is believed that the exact type of mat material is not of. great importance as it is usually of a transient nature to support the polymer materials for forming a film on the perforated electrode. It is believed that any fiber that can be deposited will be able to serve as a sufficient carrier material as it is not necessary that this be inert to the environment in a chlorine cell. As the thickness of the carrier material affects the cell potential, it is desired to obtain the thinnest mat material that is compatible with the purpose that the porosity of the perforated electrode material should be strong. is reduced. One way of building up a mat that is often used in industry is to suspend the mat material in a fluid, and when using asbestos fibers this is usually made up of the cell fluid. The perforated electrode material can then be suspended in the slurry of the mat material and a vacuum applied to the inside of the perforated electrode material so that the fibers of the mat material will be drawn towards the surface of the perforated electrode. This carrier material will then provide a smooth surface onto which the slurry of thermoplastic materials, including "Nafion" can be applied. As soon as the thermoplastic material has been applied and melted, the carrier material is no longer needed and the film acts as a membrane. The mat structure itself must have a sufficiently low porosity that it will hold particles with a size of less than 1 ym on the surface without these being drawn into the inner parts of the mat material. The "Nafion" material used in the execution of the present method is a fluorinated copolymer with suspended sulfonic acid groups. The fluorinated copolymer is derived from monomers with the formula; in which the attached -SO-jF groups are converted to -SO^H groups, and monomers with the formula in which R denotes the group - ; , in which R"^ denotes a fluorine atom or a perfluoroalkyl group with 1-10 carbon atoms, Y denotes a fluorine atom or the trifluoromethyl group, m denotes 1, 2 or 3, n denotes 0 or 1, X denotes a fluorine atom, a chlorine atom or the trifluoromethyl group, and X"*" denotes X or CF3-fCF24-a0-, wherein a denotes 0 or an integer from 1 to 5.

This leads to copolymers with the repeating structural units

In the copolymer, sufficient repeating units according to formula (3) above should occur so that a -SO^H equivalent weight of 800 - 1600 is obtained. Materials with a. water absorption rate which is approx. 25% or more is preferred as higher cell potentials are required for a given current density when using materials with a lower water absorption capacity.

Similarly, materials with a film thickness (unlaminated)

of approx. 0.2 mm or more require higher cell potentials, and this leads to a lower power yield.

Polymeric materials of this type are described in more detail

in the following patents: US Patent No. 3,041,317,

No. 3,282,875, No. 3,560,569, No. 3,624,053 and No. 3,718,627 and British Patent No. 1,184,321. Polymeric materials as described above are sold under the trademark "Nafion".

Polymeric materials according to formulas 1 and 2 can also be prepared where the ion exchange group can have. a number of other structures than a sulfonic acid substituent. A special type of structure is a carboxyl group that ends in an acid, an ester or a salt while forming one ion exchange group similar to the sulfonic acid ion exchange group. In such a group, instead of the group SO„F the group COOR 2 will be in its place, in which R 2 denotes a hydrogen atom, an alkali metal ion or an organic group. These polymer materials can currently be obtained from E.I. du Pont de Nemours & Co. Furthermore, it has been found that a substrate material, such as "Nafion", having any ion exchange group or functional group which can be converted into an ion exchange group or a functional group into which an ion exchange group can be readily introduced will include such groups as oxyacids, salts or esters of carbon, nitrogen, silicon, phosphorus, sulphur, chlorine, arsenic, selenium or tellurium.

"Nafion" together with any fillers used may be applied using any method which will result in the formation of a thin, uniform film, as described above, to form a sufficient membrane over the top of the . the. deposited mat material.. Among the methods that it. are believed to be suitable, mention may be made of deposits from a slurry, drawing the material onto the surface of the mat material using a vacuum, pouring the slurry over the mat material, brushing on a solution or spraying on in some way, as in plasma spraying:. Vacuum formation from a slurry can be the most economical method as the equipment used to carry out such a method will be the same as that equipped as. used to apply the asbestos diaphragms.

A typical slurry for deposition on a. perforated cathode when the mat has been applied thereto can be prepared by using particulate "Nafion" having an equivalent weight of 1208 or 1073 and mixed with a suitable solvent, such as 1,1,2-trichlorotrifluoroethane sold under the trade name "Freon 113". This solvent will give a good result because it softens the "Nafibn" particles so that it becomes easier to reduce the particle size by applying shear force to obtain a very uniform dispersion of "Nafion". It is also believed that an aqueous slurry of "Nafion" together with an alkaryl polyether alcohol sold under the trade name "Hyonic PE260" as a wetting agent will give a good result. The "Nafion" material is in the sulfonyl fluoride state. or in thermoplastic sand and is then, in contrast to when the sodium and acid conditions are present, completely fusible into the polymer film. A typical method for producing such a suspension is to use a stirrer which is equipped with a chamber surrounded by a jacket and a reflux condenser and to which the particulate "Nafion" material is added together with the solvent "Freon 113". The system is heated with hot water to boil the Freon 113 solvent, and cold water is passed through the reflux condenser to condense the Freon 113 solvent. When the mixture of 'Nafion' and 'Freon 113' is refluxed for about 10 minutes and then subjected to shear forces for 30 minutes while the solvent 'Freon 113' is still refluxed, a good slurry of 'Nafion' is obtained from which a film can be deposited on the surface of the mat material surface Various thermoplastic materials compatible with "Nafion" can be used as fillers in the slurry to lower costs while obtaining a good film.

Examples of such fillers are a fluorinated ethylene/propylene copolymer or a perfluoroalkoxy material.

The slurry-of "Nafion" can be applied to carrier material-

light in different ways, the purpose being to produce a continuous smooth film after melting. After application, the "Freon 113" solvent evaporates and the particulate material is then melted into a film. This is done by firing the entire perforated electrode assembly in an oven usually maintained at a temperature of 240 - 300° C. More than one application and subsequent melting of a slurry of a thermoplastic material may be required to obtain a hole-free continuous film.

Immediately a thin and uniform film has been formed on the surface

of the mat material which is essentially impervious to hydraulic flow, the film can be hydrolysed to the infusible sulfonic acid ion exchange state. It is relatively easy to hydrolyze or saponify "Nafion" to convert the sulfonyl fluoride form into the free acid form. This can be done by immersing the film in a sodium hydroxide solution, a solution of sodium hydroxide in dimethylsulfoxide, a potassium hydroxide solution or a potassium hydroxide solution in dimethylsulfoxide. All these treatments seem to work equally well even at different temperatures and

times are necessary for. to carry out the hydrolysis. As soon as this has been carried out, the electrode is ready for use in a standard diaphragm electrolysis cell. The cell conditions should be changed so that the cell is operated as a membrane electrolysis cell.

Various means of improving these substrate materials have been investigated, and one of the most effective of these means is to subject the surface of the substrate itself to a chemical treatment. These treatments generally consist of reacting the suspended sulfonyl fluoride groups with materials such as will give a min-

dre polar bond and thereby absorb fewer water molecules via

hydrogen bonding. This tends to narrow the pore openings through which the cations pass, so that less water of hydration is transferred with the cations through the membrane. An example of such. treatment is to react the ethylenediamine with the pendant groups in the sulfonyl fluoride form to bind two of the pendant groups together via two nitrogen atoms in the ethylenediamine. For a film thickness of 0.18 mm, this surface treatment will usually take place up to a depth of approx. 0.05 mm on. one side of the film by regulating the reaction time.. This will lead to good electrical conductivity and penetration of cations with fewer hydroxyl ions and associated reversed migration of water.

The obtained membrane electrolysis cell will give a high current yield, a lower concentration of sodium chloride in the obtained sodium hydroxide solution compared to the concentration obtained in a standard diaphragm cell, a higher sodium hydroxide concentration, a good utilization of the existing cell space, a longer life for the cell and a lower cell voltage.

The invention will be described in more detail in the examples below.

Example 1

For examination in a laboratory cell, a membrane was formed over an asbestos mat using a Buchner funnel as a framework over which the mat material was to be formed. The mat material consisted of asbestos plus 5% fluorinated ethylene/propylene copolymer as a binder in water to form a suspension. A vacuum was applied to the Buchner funnel to draw the mat material towards the perforated surface of the funnel until the porosity had been so reduced that particles of a size of approx. 1 ym was captured. A slurry of thermoplastic material was prepared from a particulate "Nafion" resin material of equivalent weight 1208 mixed with the solvent "Freon 113". The mixture was stirred at 47.8° C, and 10 g of slurry was obtained. This was then filtered through the mat material over the top of the Buchner funnel. The "Nafion" resin particles were melted by burning the mat material with the "Nafion" particles on its surface for 60 minutes at a temperature of 300° C. This resulted in the formation of a thin and uniform film which was substantially impervious to hydraulic flow. Then, it was. used "Nafion" hydrolyzed in a solution of dimethyl sulfoxide and water plus 10% sodium hydroxide for 70 minutes at a temperature of 70°C to convert the sulfonyl fluoride state to the sulfonic acid cation exchange state.

This membrane over the mat material was then placed in a laboratory test cell in which a flow hase-3 2 was used

thickness of 0.82 cm per my. and dm cathode surface, a pressure column of 35.6 cm saline, a potential of 4.29 V and a current of 6.28 A at 92° C, and a current yield of 64% was obtained over a period of 37 days .

Example 2

An asbestos mat was formed over a Buchner funnel as described in Example 1. A slurry of a thermoplastic material was prepared using particulate "Nafion" resin material with an equivalent weight of 1073 1 of the solvent "Freon 113" and the slurry was refluxed. for 10 minutes at 50° C. The resulting slurry was then poured over the asbestos mat. After the solvent "Freon 113" was evaporated, the "Nafion" resin particles were melted into a thin and uniform film for 30 minutes at a temperature of 275° C. The film was then hydrolyzed for 1 week in.. 3.5% sodium hydroxide .

The resulting membrane over the mat material was then introduced into a laboratory test cell in which the conditions according to example 1 were applied, and a maximum current yield of 50% was obtained.

Example 3

An asbestos slurry was formed over a Buchner funnel according to Example 1. A slurry of a thermoplastic material was prepared from particulate "Nafion" resin material having an equivalent weight of 1073 in the solvent "Freon 113". and the slurry was reflux distilled as. described in Example 2. The slurry was then drawn towards the mat material using a vacuum of 660 mm. The "Nafion" resin particles were melted for 30 minutes at a temperature of 250°C, another coating of the slurry was applied using a dropper pipette to close holes in the film, and a melt was again performed. The material was then hydrolysed in dimethylsulfoxide and sodium hydroxide for 70 minutes at a temperature of 80°C.

The obtained membrane on the mat material. was then placed in a laboratory test cell in which the conditions according to example 1 were applied, and a current yield of

53% over a period of 78 days.

. Example 4

A mat material was applied over a Buchner funnel and dried for 45 minutes at 150°C as described in Example 1. A slurry of a thermoplastic material was prepared as described in Example 1 and applied sequentially as four coats, the first two of which were brushed over the entire surface, and the third and fourth coats were partial coats to cover holes. The film was melted for 30 minutes at a temperature of 250°C and examined with air to determine if leaks occurred after each coating had been melted. The resulting film was then hydrolysed in a solution of dimethylsulfoxide and sodium hydroxide for 70 minutes at a temperature of 80°C.

The resulting membrane was then placed in a laboratory research cell in which the conditions, according to example 1, were used, and a current yield of approx... 60% was obtained over a period of 6 7 days.

Claims (15)

1. Procedure for forming a membrane over a perfo-:. standard diaphragm cell electrode, characterized in that a mat material is suspended in a fluid, a perforated electrode is introduced into the suspension, a layer of the suspended mat material is formed over the surface of the perforated electrode to thereby greatly reduce the porosity of the perforated electrode material, a layer of a thermoplastic material comprising one material containing carboxyl ion exchange groups is applied to the upper surface of the perforated electrode with the applied mat material, and the perforated electrode is fired/with the layers in place until the thermoplastic material has been melted into a thin and uniform film. on the surface of the mat material which is essentially impervious to hydraulic flow. 2. Method - according to claim 1, characterized in that the thermoplastic material is applied, by creating a vacuum from a slurry of the thermoplastic material in the solvent "Freon 113". 3. Method according to claim 1, characterized in that the thermoplastic material is applied by plasma spraying. 4. Method according to claims 1-3, characterized in that the firing is carried out at a temperature of 24 0 - 3 0 0° C. 5. Method according to claims 1-4, characterized in that asbestos or cellulose is used as mat material. 6. Membrane separator for a standard diaphragm electrolysis cell, characterized in that it comprises a perforated standard diaphragm cell electrode, a layer of mat material on the surface of the perforated standard diaphragm cell electrode and of such a thickness that the porosity of the perforated standard diaphragm electrode is substantially reduced, and on the surface of the mat material a thin and uniform hydraulically impermeable cation exchange membrane consisting substantially of a film of a copolymer having the repeating structural units with the formula: where R denotes the group , in which R"*" is a fluorine atom or a perfluoroalkyl group with 1-10 carbon atoms, Y denotes a fluorine atom or the trifluoromethyl group, m denotes 1, 2 or 3, n denotes 0 or 1, X denotes a fluorine atom, a chlorine atom or the trifluoromethyl group, X"<*>" denotes X or CFo4CF0-)- 0-, and a denotes 0 or an integer from 1 to 5 , and in which R denotes an ion exchange group from the group consisting of oxyacids, salts or esters of carbon, nitrogen, silicon, phosphorus, sulphur, chlorine, arsenic, selenium or tellurium. 7. Membrane separator according to claim 6, characterized in that the membrane material is surface treated to improve the selectivity of the membrane material. 8. Method for forming a membrane over a perforated standard diaphragm cell electrode, characterized in that a mat material is suspended in a fluid, a perforated electrode is introduced into the suspension, a layer of the suspended mat material is vacuum-formed over the top of the perforated electrode. surface to greatly reduce the porosity of the perforated electrode material, a layer of a thermoplastic material comprising "Nafion" particles in the sulfonyl fluoride form is applied to the surface of the perforated electrode with the applied mat material, the perforated electrode is fired with the layers in place until the thermoplastic material is melted into a thin and uniform film on the surface of the mat material which is essentially impervious to hydraulic current, and the "Nafion" used is hydrolyzed to convert the sulfonyl fluoride form to the sulfonic acid cation exchange form. 9. Method according to claim 8, characterized in that the thermoplastic material is applied by vacuum formation from a slurry of a thermoplastic material in the solvent "Freon 113". 10. Method according to claim 8, characterized in that the thermoplastic material is applied by plasma spraying. 11.. Method according to claims 8-10, characterized in that the firing is carried out at a temperature of 24 0 - 300° C. 12. Method according to claims 8 - 11, characterized in that the hydrolysis is carried out in a solution containing as active ingredients sodium hydroxide, potassium hydroxide, sodium hydroxide in dimethyl sulfoxide or potassium hydroxide in dimethyl sulfoxide. 13. Method according to claims 8-12, characterized in that asbestos or cellulose is used as mat material. 14. Membrane separator for a standard diaphragm electrolysis cell, characterized in that it comprises a perforated standard diaphragm cell electrode, a layer of a mat material on the surface of the perforated standard diaphragm cell. electrode and of such a thickness that the porosity of the perforated standard diaphragm cell electrode is greatly reduced, and on the surface of the mat material a thin and smooth, hydraulically impermeable cation exchange membrane essentially consisting of a film of a copolymer with the repeating structural units of the formula wherein R denotes the group in which R''" denotes # ILL a fluorine atom or ^perfluoroalkyl group with 1 - 10 carbon atoms, Y denotes a fluorine atom or the trifluoromethyl group, m denotes 1, 2 or 3, n denotes 0 or 1, ;X denotes a fluorine atom, a chlorine atom or the trifluoromethyl group, X"*" denotes X or CF34CF24-aO-, and a denotes 0 or an integer from 1 to 5, the units of formula (1) being present in such an amount that a copolymer with a -SO3 H equivalent weight of 800 - 1600 is obtained. 15. Membrane separator according to claim 14, characterized in that the membrane material is surface treated to improve the selectivity of the membrane material.