FI60577B - FOERFARANDE FOER FOERYNGRING AV EN TILLTAEPPT JONBYTAREMEMBRAN IN-SITU I EN ELEKTROLYSCELL FOER EN SALTLOESNING - Google Patents

Description

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Patent and Registration Office .... ...... ... , . ,, .., -. . f441 Date of registration and date of issue. - ratani · och registerstyrelsen 'Anaekan utltgd oeh uti.akriftan pubikarad 30.10.81 (32) (33) (31) Pyy * · »/ atuolkau * - Bagird prlorltat 29.10.71 * USA (US) 5I89O3 (71) Hooker Chemicals & Plastics Corp., PO Box 189 »Niagara Falls,

New York 1 ^ 302, USA (US) (72) Jeffrey D. Eng, North Vancouver, British Columbia, Canada (CA) (7 * 0 Berggren Oy Ab (5 * 0 Method for regenerating a clogged ion exchange membrane in an in-situ saline electrolysis cell - Förfarande The present invention relates to a method for regenerating a clogged ion exchange membrane in situ in an electrolysis cell of a saline solution, the method of which comprises regenerating the membranes of three-compartment electrolytic membrane cells to reduce the voltage drop of the cell required for electrolysis,

In diaphragm-type electrolytic cells used for the electrolysis of saline solutions, their diaphragms tend to clog or coat in an undesirable manner with precipitates during electrolysis. Such deposits, which are thought to be due to impurities in the feeds entering the cell, tend to block the passage of liquid and ion currents through the conventional asbestos diaphragms used. The result is a greater required voltage drop to achieve the desired electrolysis and a corresponding increase in the amount of energy consumed during electrolysis. Especially today, when there is a shortage of energy, it is important to save it. As a result, diaphragm cells are sometimes treated with an acidic medium other than an electrolyte to help remove such high resistance fines and to improve the electrical conductivity and flow of electrolyte ions through the diaphragm.

Membrane cells have recently been used experimentally and commercially to prepare chlorine and base from saline. In such cells, membranes, preferably cationic, selectively permeable membranes, separate the anolyte from the catholyte and in some cases delimit the buffer compartment between the anode compartment and the cathode compartment. It has now been found that although the materials of the membranes are chemically and physically very different compared to the membranes of normal asbestos diaphragms used in diaphragm cells, the voltage drop across the membrane cell also increases during continuous use, leading to higher energy consumption. This is apparently due to the formation of an insulating layer on the surface of the membrane.

It has now been found that such a layer can be removed and the membrane can be regenerated by the invented method.

The method of the present invention for regenerating a clogged polymeric ion exchange membrane adjacent to the anolyte compartment in an in-situ event in an electrolytic cell of a saline solution is characterized by what is set forth in the independent claim.

The invention will be readily understood by reference to the accompanying description of its preferred embodiments, taken in conjunction with the drawing, in which:

Figure 1 is a cross-sectional view of a three-compartment electrolytic cell in which the membrane between the anode and buffer compartments is regenerated to lower the cell voltage; Figure 2 is a similar view of a two-compartment cell in which a membrane is regenerated; and Figure 3 is a cross-sectional view of a container in which a plurality of membranes removed from electrolytic cells for regeneration are treated with acid.

The three-compartment electrolytic cell 11 comprises a housing or tank part 13, positive and negative voltage sources 15 and 17, conductors 19 and 21 for conducting electricity to the anode 23 and cathode 25, respectively, and separating membranes 27 and 29 defining an anolyte or anode compartment 33 and a catholyte or between the cathode compartment 35. The supply pipes 37, 39 and 41 are, respectively, 3 60577 for the supply of saline solution, for the supply of water and for the supply of water and / or a weak base. Exhaust pipes 43 and 45 are for removing liquid and base in the buffer compartment, respectively, and gas outlet pipes 47 and 49 are for removing chlorine and hydrogen, respectively.

In addition, although not shown to maintain the clarity of the drawing, various connections to facilitate the recovery or recycling of different electrolytes are often also included, as are means for maintaining the circulation of different electrolytes within the electrolyte compartments. External means for adding sodium chloride to the consumed saline solution, which is shown through the removable outlet pipe 51, are conventional and are not shown.

Figure 2 shows a two-compartment cell, which largely corresponds in element to the cell of Figure 1. The two-compartment cell 53 comprises a tank or cell body 55, positive and negative electrical voltage sources 57 and 59, conductor rods 6l and 63, anode 65 and cathode 67 and membrane 69, which divides the cell into anode or anolyte compartment 71 and cathode or catholyte compartment 73. The saline inlet pipe 75 and the water inlet pipe 77 to the anode and cathode compartments, respectively, are shown at the bottom of the cell. The alkaline solution discharge 79 and the spent anolyte discharge pipe 111 are shown at the top of the cell, as are the chlorine and hydrogen discharge pipes 83 and 85, respectively. In Figure 3, the reservoir 87 includes attachment means 89 for supporting the membranes 91 in the regenerating acid 93- In use when it is observed that the voltage drop across the cell or series of such cells has risen to 0.2-0.5 V / cell above the initial drop, which is found in a good start, it is a sign that the process of this invention should be implemented. However, even before such a reduction is noted, it may be desirable to take preventive measures. Thus, as part of preventive maintenance, it may be desirable to perform that process once a week, every two weeks, or every month. The more often preventive maintenance is performed, the shorter the times required for each procedure may be. Normally, unless a preventive maintenance procedure is followed, it is desirable to renew the membranes every 2-6 months. In order to save unnecessary openings of the cells for on-site regeneration, on-site treatment is recommended, although, especially when the cell is dismantled for other reasons, it is possible to remove the membranes and immerse them in the regenerating solution.

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Extranembrane treatment is a simple operation that requires only the removal of the membrane from the cell and its immersion in an acidic solution such as aqueous hydrochloric acid. Other acids can be used, such as citric, acetic, sulfuric, phosphoric, and gluconic acids, and mixtures thereof, but HCl is clearly the best in those applications. However, despite the simplicity of the procedure, the electrolysis process must be interrupted and the cell must be disassembled and then reassembled. This makes somewhat more complex on-site operations recommended.

Central to in-situ regeneration of the membrane to reduce cell voltage is that the anode side of the membrane must be treated with acid to remove hard deposits, slurry and other undesirable substances that may have partially inactivated it. Due to the nature of the precipitate on the surface of the membrane and its possible extension to its subsurface parts, the mere use of acid on the anode surface does not usually remedy the situation. The precipitate or slurry turns out to be a complex mixture of calcium, magnesium, and sometimes iron oxides, hydroxides, halides, and its complexes that adhere to the surface of the anode (often ruthenium oxide on the surface of titanium).

However, it has been found that if the base concentration on the opposite side of the membrane is reduced away from the anode and the current density is reduced (ranges of usable acids, concentrations and current densities are given later), the slurry or other coating is removed and the membrane regenerates significantly.

It is found that multi-compartment cells with three, four or more compartments (but usually no more than four are used) allow for easier "cleaning" or regeneration of membranes because the base concentration against the membrane side away from the anode is lower in such cells. Thus, the regenerating acid is not partially neutralized due to the undesired passage of some hydroxyl ions across the membrane into the anode compartment. This is because normally the buffer zones of such cells are intended to hold a fluid with a lower hydroxyl content close to the anode buffer membrane. The times of the renewal process can be shortened by using buffer compartments. An additional advantage of buffered membrane cells is their lower tendency to accumulate sludge deposits. The precipitates turn out to be alkaline and therefore the less alkali leakage into the anolyte compartment occurs, the less chance there is of precipitation of the alkaline slurry on the membrane surface. A significant portion is believed to be due to the calcium and magnesium salts in the feed salt solution feed, which may contain from 1 to 100,000 ppm of such insoluble salt-forming compounds, such as iron compounds as their oxides. Normally a good feed of saline solution contains less than 30 ppm of such salts, e.g. 3-3Ο ppm, but this amount can still cause membrane problems.

During the regeneration operation, the flow of the stream is reduced so that smaller amounts of base solution are produced and thus acidification is facilitated. Such a reduction in current density produces greatly reduced amounts of chlorine and base and therefore reduces the capacity of the cell. In a three- or four-compartment cell, the electrolyte in the buffer compartment can be diluted while still keeping the catholyte fairly concentrated, and the dilute buffer solution produced can be recycled to the catholyte to further improve or "concentrate" it. Therefore, it is recommended that this regeneration method be performed in a three-compartment cell for best results, although since two-compartment cells are more common, it may be necessary to perform most of the process in such cells.

Reducing the base concentration adjacent to the membrane to be treated is a simple matter that requires only a reduction in current flow through the cell and dilution of the catholyte or buffer electrolyte with water. Once experience has been gained with the cell structure and membrane in question, it is known how long the regeneration process must be performed to drop the cell voltage to the desired amount. However, the time normally required is from one hour to one day, usually 3-10 hours, preferably 6-8 hours and most preferably about 7 hours (one shift). The pH is lowered to 1-4, with a range of 1.5 ~ 3.5 being the normally used range and a pH of about 2-3 or 3.3 being preferred. The pH is normally lower than the pH of the electrolyte during normal use, and the pH of said electrolyte is usually 3-5 and preferably 3 ~ 4. The normal base concentration in the buffer zone of a three-compartment cell is about 50 g / l and in the catholyte compartment of a two-compartment cell it is about 150 g / l (it is also about 150 g / l in the catholyte compartment of a three-compartment cell). It is desirable that this concentration be preferably reduced to less than half for higher base concentrations and less than 3/4 for lower concentrations. Thus, if the concentration of the catholyte is 150 g / l of sodium hydroxide, it is desirable to reduce this to less than 75 g / l, whereas if the concentration of the buffer compartment is 6 60577 50 g / l, it only needs to be reduced to less than 38 g / l, e.g. To 20-35 g / l.

The recommended range of reduced base concentrations is 20-45 g / l. The current density 2, which is normally 0.2-0.5 A / cm, is usually calculated as n.

2 2 to 0.02-0.1 A / cm, preferably 0.03-0.07 A / cm and most preferably 2 mm to about 0.05 A / cm during the treatment period.

As a result of the regeneration, when the cell is restored to regular operation, its voltage is usually 3.5-4.5 V, which is usually 0.2-n. 0.5 V lower than required to maintain the previous operating current density. The electrolysis is usually restarted at a pH of the anolyte of about 3-4, e.g. 4, with a base concentration of the buffer compartment of about 50-100 g / l and a current density of about 0.1-0.5 A / cm, e.g.

2 0.3 A / cm. In a two-compartment cell, the base concentration can be 100-160 g / l, with the current density, voltage drop and anolyte pH being the same as in the three-compartment cell already described.

The preferred acid used for acidification, the aqueous hydrochloric acid solution, may be concentrated or slightly diluted so that its pH is usually about 2 or less so that it can satisfactorily acidify the anolyte. The amount used is such that it provides the desired pH in the anolyte. To avoid any possible damage to the membrane, it is recommended that the acid be completely mixed with the anolyte before it enters the cell. Therefore, it is usually recommended to mix the acid with the electrolyte outside the cell after adding salt to the consumed saline solution, which has been removed from the cell and returned to it as feed. The normal concentration of the saline solution is about 25% sodium chloride, with the range of 21-23% being approximately the concentration in the consumed saline solution.

Thus, 2-4% sodium chloride plus sufficient hydrochloric acid is usually added to change the pH of the anolyte to the desired acidity. Hydrochloric acid is recommended because it leaves no useless residue in the anolyte, with the conversion of chlorine ion to chlorine being recovered as a valuable product. In some cases it may be desirable to add other substances with hydrochloric acid, e.g. sequestering agents, but normally this is neither necessary nor desirable.

Operating temperatures during acidification and other recovery treatments can be maintained in the usual range of 65-95 ° C, preferably about 80-92 ° C. Although the anolyte may contain amounts of calcium and magnesium salts that produce an undesired slurry or surface layer on the membrane surface, e.g., 1-8% as calcium and magnesium compounds, it is recommended that the amounts of these materials be severely limited and kept below At $ 0.1 or $ 1. As this is not always possible, the method in question is important. Even when the amount of alkaline earth metal and magnesium salts that are sludge formers is kept low, such as below $ 1, such sludge or surface layer still accumulates on and / or within the membrane, requiring regeneration by this method, although longer time. In cases where magnesium and calcium salt concentrations are above 1%> there is a real need for periodic regeneration of the membrane (s) by the method in question in order to avoid costly energy loss.

When membranes are treated with acid outside the electrolytic cell instead of using in situ treatment, care must be taken to prevent the membranes from drying out while still in place in the cell body or frame, as this may cause excessive shrinkage and rupture of the membrane despite the use of polytetrafluoroethylene support nets or other structures. to improve membrane strength. If it is apparent that the membranes will dry out, it is suggested that they be first treated with a suitable solvent or solvent system, such as glycerol saline, to prevent excessive shrinkage prior to acidification. The acidification treatment can be carried out while still keeping the membranes in place in the parts of the cell body, but normally they are removed and a large number of them are treated in one vessel. The mixing can be used to help remove sludge or surface layer from the membranes. Ultrasonic mixing, ordinary agitation, and mixing by circulating a liquid medium are all useful. In both the on-site and in-tank regeneration process, any sludge formed on the bottom of the cell or tank can then be removed to separate the calcium and magnesium compounds and, when dissolved in the acidic anolyte, can be recovered and removed by neutralization processes. Otherwise, it is possible that if the magnesium and / or calcium compounds remain in the anolyte, they could re-deposit on the surface of the membrane and adversely affect its conductivity.

The currently preferred cation-selectively permeable membrane is a hydrolyzed copolymer of perfluorocarbon and fluorosulfonated perfluorovinyl ether 8,60577. The perfluorinated hydrocarbon is preferably tetrafluoroethylene, although other perfluorinated and saturated and unsaturated hydrocarbons having 2 to 5 carbon atoms may be used, of which monoolefinic hydrocarbons are preferred, especially those containing 2 to 4 carbon atoms and especially 2-3 carbon atoms, e.g. tetrafluoroethylene. and hexafluoropropylene. The most useful sulfonated perfluorovinyl ether is that of formula FSO 2 CF 2 CF 2 CF (CP 2) 0Ρ-OCF 2 Fg. Such a material, designated perfluoro-2- (2-fluorosulfonylethoxy) -propyl vinyl ether and hereinafter referred to as PSEPVE, can be modified to the corresponding monomers, for example, by modifying the internal perfluorosulfonylethoxy moiety to the corresponding propoxy component and replacing the propyl component with a propyl group. substituents of the -yl group and using isomers of perfluoro-lower alkyl groups. However, the use of PSEPVE is most recommended.

A process for preparing a hydrolyzed copolymer is described in Example XVII of U.S. Patent 3,282,865 and an alternative process is mentioned in Canadian Patent 849,670, which also discloses the use of a final membrane in fuel cells known therein as electrochemical cells. The disclosures of these patents are hereby incorporated by reference into this specification. Briefly, a copolymer can be prepared by reacting a PSEPVE compound or the like with tetrafluoroethylene or the like in the desired proportions in water at elevated temperature and pressure for more than one hour, after which the mixture is cooled. It separates into a lower perfluoroether layer and an upper layer of aqueous medium in which the desired polymer is dispersed. The molecular weight is not determinable, but the equivalent weight is about 900-1600 and preferably 1100-1400 and the percentage of PSEPVE or similar compound is about 10-30%, preferably 15-20% and most preferably about 17%. The non-hydrolyzed copolymer can be extruded at high temperature and pressure to form films or membranes that can range in thickness from 0.02 to 0.5 mm. These are then further treated to hydrolyze the dependent -SO 2 F groups to -SO 2 H groups, e.g., by treatment with 10 51 sulfuric acid or by the methods of the aforementioned patents. The presence of -30jH groups can be detected by titration as disclosed in the Canadian patent. Further details of the various process steps are set forth in Canadian Patent 752,427 and U.S. Patent 3,041,317, which are also incorporated herein by reference.

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Since it has been found that the hydrolysis of the copolymer involves some expansion, it is often advisable to place the copolymer membrane after hydrolysis in a frame or other support that holds it in place in the electrolysis cell. It can then be tightened or glued in place and is accurate without dents. Preferably, the membrane is attached to the rear tetrafluoroethylene or other suitable filaments prior to hydrolysis while still thermoplastic, and the copolymer film covers each filament by penetrating the spaces between and even around them, slightly thinning the films in the process where they cover the filaments.

The membrane described is clearly superior to all other previously proposed membrane materials in these processes. It is more stable at elevated temperatures, e.g. above 75 ° C. It lasts for much longer periods of time in the electrolyte and alkaline product medium and does not become brittle when exposed to chlorine at high cell temperature. In terms of time and manufacturing cost savings, the membranes in question are more economical. The voltage drop across the membrane is acceptable and does not become unreasonably high, as occurs with many other membrane materials when the base concentration in the cathode compartment rises above about 200 g / L of base. The selectivity of the membrane and its compatibility with the electrolyte do not adversely deteriorate as the hydroxyl concentration in the catholyte fluid increases, as has been observed with other membrane materials. Besides, the base efficiency of electrolysis does not decrease as significantly as that of other membranes as the hydroxyl ion concentration in the catholyte increases. Although more preferred copolymers are those having equivalent weights in the range of 900-1600, with 1100-1400 being most preferred, some of the useful resinous membranes prepared by the method in question may have equivalent weights of 500-4000. Polymers of medium weight equivalent are preferred because they have satisfactory strength and stability, allow better selective ion exchange to occur, and have lower internal resistances, all of which are important considerations for the electrochemical cells in question.

Improved modifications of the copolymers described above can be made by chemical treatment of their surfaces, e.g., by treatments that modify the -SO, H group therein. For example, the sulfone group may be modified on the membrane surface to provide a concentration gradient or may be partially replaced by a phosphorus or phosphone group. Such changes can be made in the manufacturing process or after the manufacture of the membrane. When implemented as a subsequent surface treatment of the membrane, the treatment depth is usually 0.001 to 0.01 mm. In some cases, it may be desirable to convert the sulfonyl or sulfonic acid group of the membrane on one side (usually the anode side) to a more hydrophilic sulfonamide, as may be accomplished as described in U.S. Patent 3,784,399, which is incorporated herein by reference. The membrane may also be in a laminated form, which is now most preferred with layers having a thickness of between 0.07-0.17 mm on the anode side and 0.01-0.07 mm on the cathode side, the equivalent weights of the layers being between 1000-1200 and 1350, respectively. -1600. The recommended thickness of the layers in the anode side is in the range 0.07 to 0.12 mm, and most preferably has a thickness of approx. 0.1 mm, the cathode layers, the preferred thickness of 0.02 to 0.07 mm, and most preferably approx. 0.05 mm. The recommended and most preferred equivalent weights are 1050-1150 and 1100 and 1450-1550 and 1500, respectively. The higher the equivalent weight of the private layer, the smaller the recommended thickness for use within the given limits.

In addition to the copolymers described above, including their modifications, it has been found that another type of membrane material is also superior to previously used films for application to the processes in question. Although tetrafluoroethylene (TFE) polymers sequentially styrened and sulfonated have been shown not to be useful in making satisfactory cationic, selectively permeable membranes for use in the electrolysis processes in question, perfluorinated ethylene-propylene, forms a useful membrane. Sulfostyrenated PEP materials are amazingly resistant to curing and other degradation in use under the process conditions involved.

To prepare sulfostyrenated FEP membranes, a standard FEP material such as E.I. Du Pont de Nemours & Co. Inc., the styrenated and styrenated polymer is then sulfonated. A solution of styrene in methylene chloride or benzene is prepared at a suitable concentration of about 10-20% and a film of FEP polymer having a thickness of about 0.02-0.5 mm and preferably 0.05-0.15 mm is dipped in the solution. Once removed therefrom, it is subjected to an irradiation treatment using a co-bolt radiation source. The amount of radiation can be in the order of about 8000 rad / h 11 60577 and the total radiation dose is about 0.9 megarads. After rinsing the film with water, the phenyl rings of the styrene portion of the polymer are monosulfonated, preferably by treatment with chlorosulfonic acid, fuming sulfuric acid or SO 2 in their para position. It is recommended to use chlorosulfonic acid in chloroform and the sulfonation is completed in about 1/2 hour.

Examples of useful membranes made by the process described are those of RAI Research Corporation, Hauppauge, New York, known as 18ST12S and 16ST13S, the former being 18 56 styrenated and having 2/3 of the phenyl groups monosulfonated and the latter being 16% styrenated and 13/16 of its phenyl groups are monosulfonated. A 17 1/2% methylene chloride solution of styrene is used to effect styrenation of 18 # and a $ 16 methylene chloride solution of styrene is used to effect styrenation of 16 56.

The products obtained are preferably comparable to the preferred copolymers described above, each giving a voltage of about 0.2 volts. . . 2 reductions in the cells in question at a current density of 0.3 A / cm, which is the same as that obtained with the copolymer.

Membrane walls are normally 0.02-0.5 mm, preferably 0.07-0.4 mm and most preferably 0.1-0.2 mm thick. Thickness ranges for portions of the laminated membranes described above have already been provided. When the membrane is mounted on the surface of polytetrafluoroethylene, asbestos, titanium or other suitable mesh for support purposes, the thickness of the filaments or fibers of the mesh is usually 0.01-0.5 mm and preferably 0.05 * 0.15 mm corresponding to a maximum thickness of the membrane. It is often recommended that the fibers be less than half the film thickness, but filament thicknesses greater than the film thickness, e.g., 1.1-5 times the film thickness, may also be used successfully. The nets, screens or strainers have a percentage area of openings in them of about 8-80 56, preferably 10-70% and most preferably 30-70%. Generally, the cross sections of the filaments are circular, but other shapes such as ellipses, squares, and rectangles are also useful. The support web is preferably a screen or strainer and although it may be glued to the membrane, it is recommended that it be melted onto it by high temperature and high pressure compression prior to hydrolysis of the copolymer. The membrane mesh assembly can then be tightened or otherwise secured in place on the holder or support device.

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The construction material of the cell body may be conventional including concrete or prestressed concrete coated with mastic, rubbers such as neoprene, polyvinylidene chloride, FEP material, polyester made of chlorendic acid construction, other propylene or polyvinyl chloride, similarly coated boxes. Substantially self-supporting structures such as unplasticized polyvinyl chloride, polyvinylidene chloride, polypropylene, or phenol-formaldehyde resin may be used, preferably reinforced with molded fibers, fabrics, or fabrics, such as glass filaments, steel, nylon, etc. - or bipolar, is made of an electrolyte-resistant polymeric material, such as molded polypropylene, preferably reinforced with asbestos, mica or calcium silicate fibers or scales.

The anodes used are a suitable material with openings through which any chlorine formed during electrolysis can escape. The active surface materials of the anodes may be noble metals, precious metal alloys, noble metal oxides, noble metal oxides mixed with valve metal oxides, e.g. ruthenium oxide plus titanium dioxide, or mixtures thereof, normally with a substrate sufficiently conductive for electrolysis. It is recommended that such surfaces be on an electrolyte resistant metal metal such as titanium and bonded therethrough to a metal conductor such as copper, silver, aluminum, steel or iron normally coated, galvanized or otherwise protected with a surface layer of a similar electrolyte resistant material. It is particularly desirable that the open parts of the electrodes, with the exception of the conductors, be titanium which is activated on one of its surfaces or completely (due to the evolution of chlorine from this surface) with a noble metal and a noble metal oxide such as ruthenium oxide, platinum oxide, ruthenium or platinum. Instead of titanium, another useful valve metal is tantalum.

The anode is usually a stretched titanium mesh with a surface coated with ruthenium oxide. In all cases, the conductive material of the conductor is preferably copper coated with titanium.

The cathodes used can be any electrically conductive material that resists the effect of different contents of the cell. The cathodes are preferably made of a steel mesh connected to a copper conductor, but other cathode materials and conductive materials may be used, of which iron, graphite, lead oxide on the graphite surface, lead dioxide on the titanium surface or precious metals such as platinum, iridium, rutium are suitable for the cathode. When precious metals are used, they may be layered on surfaces with conductive substrates such as copper, silver, aluminum, steel or iron. The cathodes are preferably a screen or a stretched metal mesh, and like the anodes they are planar or other compatible shape so that the distances between the electrodes are approximately the same at each location.

The following examples are provided to illustrate, but not limit, the invention. Unless otherwise indicated, all parts are by weight and all temperatures are in ° C.

Example 1

In a three-compartment membrane cell of the type shown in Figure 1, continuous operation of the cell results in the deposition of a cationic, selectively permeable membrane adjacent to the apparently anolyte compartment and apparently also within it calcium, magnesium, iron and other metal salts apparently composed of oxides, hydrides complexes and other inorganic salts. These salts may also react with cationic sites on the membranes. It is found that after several months of use of the cell to produce chlorine, base and hydrogen, the voltage drop across the cell increases by about 0.3 volts to about 4.4 volts instead of the 4.1 volts that it has on the new membrane.

The membrane used in the cell is a hydrolyzed copolymer of perfluorocarbon and fluorosulfonated perfluorovinyl ether in a laminated form with a layer about 0.1 mm thick on the side closer to the cathode and 0.05 mm on the side opposite the anode. This arrangement is the same for both membranes. Membrane copolymers have been made by reacting PSEPVE with tetrafluoroethylene to produce polymers of different molecular weights, each containing 17% PSEPVE. These polymers having equivalent weights of 1100 and 1500 for a thicker and thinner layer, respectively, are hydrolyzed to convert its dependent -SO 2 F groups to -SO 2 H groups according to the method described in the above specification.

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The anode is ruthenium oxide on the surface of a stretched titanium mesh and the cathode is a cast steel mesh. The conductors are titanium-coated copper and copper rods in the same order and carry electricity from its source or external connector to the electrodes. The cell body is molded polypropylene reinforced with asbestos and mica scales. The cell is part of a cell assembly, not shown, in which it is connected to an external bipolar connection.

Operating conditions are 85-90 ° C, 4.1 volts at start-up (4.4 V membrane

O

0.3 A / cm for the production of 150 g / l sodium hydroxide solution for the catholyte and 50 g / l sodium hydroxide solution for the buffer compartment using a feed containing 25% sodium chloride and 15 ppm mixed salts of calcium, magnesium and iron. as oxides (in proportions of approx.

3: 1, 3: 0.7). The salts are mostly present as chlorides and sulfates, although some oxides and hydroxides are also present, e.g., 30% chlorides, 60% sulfates, and 10% oxides and hydroxides.

During continuous operation of the cell for about a month, apparently due to a small backflow of hydroxyl ion through the membrane, its surface and part of its interior become contaminated with insoluble, precipitated or adhering material and it is observed that the voltage drop across the cell increases to 4.4 volts. To reduce the voltage drop, concentrated hydrochloric acid is added to the feed tube in front of the cell in an amount sufficient to change the pH of the anolyte to 2.5. At the same time, the current flow into the cell array is reduced from 60 kiloampers to a current density of 10 kiloampers or about 0.05 A / cm, and more water is fed to the buffer compartment to lower the base concentration therein to 32 g / l of sodium hydroxide. The cell is operated for seven hours under these conditions, and after this time a voltage drop (0.3 V) is obtained when the cell operates at 4.1 volts under normal conditions.

It is considered that reducing the current to a low value further aids the desired dilution effect of the base in the buffer compartment to reduce hydroxyl transfer to the anolyte side of the membrane and natural backwash due to osmosis. Also, the continuous production of chlorine at the anode helps to maintain the circulation of the anolyte, which can also be assisted by a mechanical or pumping device and thus helps to remove material from the membrane once it has been removed from it.

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Upon completion of the regeneration treatment, the anolyte is removed from the cell, replaced with new anolyte, conditions are returned to normal, and chlorine and base production is resumed. However, thereafter, as every week, the cell is operated for one hour under the described experimental conditions to prevent membrane fouling. At the end of the seven and one hour treatments, the cell can be continued to be acidified with anolyte and, if appropriate, this acidity can be reduced in the appropriate order and the pH raised to normal operating pH (3-4).

Example 2

Instead of treating the membranes in situ as in Example 1, several anode compartment membranes are removed from the cells when they become dirty, such as the anode membrane of Example 1, and immersed in an aqueous solution of hydrochloric acid at room temperature for up to 10 hours. During immersion, the solution is kept in slight motion by means of a circulating pump, which is not shown in the drawing. After 6-8 hours, the membranes are sufficiently regenerated to be returned to the cells from which they were removed. In this case, these cells operate at a lower voltage, which is usually 0.1-0.5 V lower than that obtained by interrupting the operation of the cell and removing the membranes.

In some variations of this invention, the membranes are held in frames or holders when subjected to an external acidification treatment, and in others they are first removed from the frames or holders. In both cases, satisfactory regeneration is obtained when the treatment pH is 1.5-3.5, e.g. 2.5-

Example 3

The experiment of Example 1 is repeated using the two-compartment cell of Figure 2.

The parts of the different cells are the same as the corresponding parts shown in Example 1, but the operation of the cell takes place at a base concentration of 130-200 g / l, which means on average about 160 g / l of sodium hydroxide. The on-site treatment performed is the same as in Example 1 and after seven hours the membrane is regenerated.

The pH of the anolyte obtained in the modifications of the above examples, the current densities used, and the concentration of sodium hydroxide in the second compartment (not the anolyte) adjacent to the anolyte vary within the ranges given in the above specification and useful regeneration is achieved within the time limits set forth. For example 605 77, the pH of the anolyte may be 1.5 »1.8, 2.2 and 2.5, the current densities may be 0.02, 0.04 and 0.1 A / cm, the base concentrations may be be 25, 45 or 75 g / l in a two-compartment cell and processing times can be one hour, three hours or ten hours. Similarly, the membrane material can be changed to a 0.5 mm thick sulfostyrenated FEP membrane of RAI Research Corporation types 18ST12S and 16ST13S, and good regeneration is achieved. If desired, external acidification of the "catholyte" membranes of the three-compartment cells can be accomplished, but this is usually not necessary for efficient cell operation. In the above examples, the slurry removed from the membranes is also filtered off to prevent it from reprecipitating later. In those cases where it is completely dissolved, it is neutralized outside the cell, e.g. by treatment with sodium hydroxide to obtain a sufficiently neutral or basic pH to precipitate. However, even when such a separation is not implemented, the voltage characteristics of the cell are improved by the described treatments.