CA1071570A - Prevention of metallic salt build-up in permselective membranes - Google Patents
Prevention of metallic salt build-up in permselective membranesInfo
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- CA1071570A CA1071570A CA215,183A CA215183A CA1071570A CA 1071570 A CA1071570 A CA 1071570A CA 215183 A CA215183 A CA 215183A CA 1071570 A CA1071570 A CA 1071570A
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Abstract
ABSTRACT OF THE DISCLOSURE
An electrolytic cell suitable for use in the electrolysis of ionizable chemical compounds, particularly alkali metal halide brines and hydrohalic acids, which comprises a cell having therein an anode compartment containing an anode, a cathode com-partment containing a cathode said compartments being separated from each other by at least one cation active perselective memberane which is substantially imprevious to liquids and gases, selected from a hydrolyzed copolymer of a perfluorinated hydro-carbon and a sulfonated perfluorovinle ether and a sulfostyrenated perfluorinated ethlene propylene polymer, said cathode and said inert protective barrier, thereby preventing physical contact of salts on the said membrane thereby extending the efficiency and useful life of the membrane. A polypropylene mesh screen is preferred as the protective barrier.
An electrolytic cell suitable for use in the electrolysis of ionizable chemical compounds, particularly alkali metal halide brines and hydrohalic acids, which comprises a cell having therein an anode compartment containing an anode, a cathode com-partment containing a cathode said compartments being separated from each other by at least one cation active perselective memberane which is substantially imprevious to liquids and gases, selected from a hydrolyzed copolymer of a perfluorinated hydro-carbon and a sulfonated perfluorovinle ether and a sulfostyrenated perfluorinated ethlene propylene polymer, said cathode and said inert protective barrier, thereby preventing physical contact of salts on the said membrane thereby extending the efficiency and useful life of the membrane. A polypropylene mesh screen is preferred as the protective barrier.
Description
FIELD OF THE INV~NTION
This invention relates to improvements in the electro-lytic decomposition of chemical compounds and the production of useFul products therefrom. Further, this invention relates to a novel method for extending the l;fe of permse1ective membranes used in electrolytic cel1s, especially in the electrolysis of alkali metal halide to produce halogen, hydrogen and alkali metal hydroxide. More particularly, this invention involves a method and apparatus for the protection of the permselective membrane adjacent to the cathode of the electrolytic cell, so that the life of the membrane is greatly extended and the overall cell current eFficiency is improved.
BACKGROUND OF THE INVENTION
Permselective membranes are characterized by their substantial impermeability to liquids and gases and by their selective permeability to ions of one charge and substantial imper-meability to ions of the opposite charge when wet with an electrolyte and under the influence of an electrical current. Permselective membranes which selectively permit the passage of anions are designated as anionic, those which selectively permit the passage of cations are designated as cationic. The practical utilization of permselective membranes in electrolytic cells for the electro-lytic decomposition of chemical compounds, and the separate re- ~ ;
covery of the products resulting therefrom, depend upon the presence of a polar medium, generally water, being present in the pores of the membrane, that is, to say, permselective membranes ; must be wet with electrolyte in t~
P7~
order to function in accordance with this invention. Such mem-branes and certain methods and apparatus for employing them in the electrolytic decomposition of chemical compounds are more fully set forth in Canadian Patent 988,051, issuea April 27, 1976, E. H. Cook Jr. et al, in which there i5 disclosed the fundamental concept of simultaneously controlling both mole-cular and ionic migration during the electrolytic decomposition of chemical compounds between the electrodes of an electrolytic cell, by electrolyzing the chemical compound in a cell separated into electrode compartments, by one or more permselective mem-branes. By this means, the electrical current introduced during electrolysis will be carried through the permselective membranes substantially by ions of one charge, because permselective mem~
branes inhibit the passage of the ions of opposite charge. For example, when sodium hydroxide and elemental chlorine are pro-dùced by the electrolytic decomposition o~ an aqueous solution of sodium chloride brine in an electrolytic cell in which the brine in the anolyte at the anode is iseparated from the caustic soda produced at the cathode, by a cationic active permselective membrane, the membrane operates in two distinct manners. During electrolysis, sodium ions migrate from the anolyte to the cathode and upon reaching the cation active permselective membrane, which divides the cell into separate anode and cathode compart-ments, are permitted passage through the membrane from active point to active point in the pores of the cation active perm~
selective membrane because such membrane selectively permits passage of such sodium cations while selectively prohibiting the passage of anions such as chloride ions and hydroxyl
This invention relates to improvements in the electro-lytic decomposition of chemical compounds and the production of useFul products therefrom. Further, this invention relates to a novel method for extending the l;fe of permse1ective membranes used in electrolytic cel1s, especially in the electrolysis of alkali metal halide to produce halogen, hydrogen and alkali metal hydroxide. More particularly, this invention involves a method and apparatus for the protection of the permselective membrane adjacent to the cathode of the electrolytic cell, so that the life of the membrane is greatly extended and the overall cell current eFficiency is improved.
BACKGROUND OF THE INVENTION
Permselective membranes are characterized by their substantial impermeability to liquids and gases and by their selective permeability to ions of one charge and substantial imper-meability to ions of the opposite charge when wet with an electrolyte and under the influence of an electrical current. Permselective membranes which selectively permit the passage of anions are designated as anionic, those which selectively permit the passage of cations are designated as cationic. The practical utilization of permselective membranes in electrolytic cells for the electro-lytic decomposition of chemical compounds, and the separate re- ~ ;
covery of the products resulting therefrom, depend upon the presence of a polar medium, generally water, being present in the pores of the membrane, that is, to say, permselective membranes ; must be wet with electrolyte in t~
P7~
order to function in accordance with this invention. Such mem-branes and certain methods and apparatus for employing them in the electrolytic decomposition of chemical compounds are more fully set forth in Canadian Patent 988,051, issuea April 27, 1976, E. H. Cook Jr. et al, in which there i5 disclosed the fundamental concept of simultaneously controlling both mole-cular and ionic migration during the electrolytic decomposition of chemical compounds between the electrodes of an electrolytic cell, by electrolyzing the chemical compound in a cell separated into electrode compartments, by one or more permselective mem-branes. By this means, the electrical current introduced during electrolysis will be carried through the permselective membranes substantially by ions of one charge, because permselective mem~
branes inhibit the passage of the ions of opposite charge. For example, when sodium hydroxide and elemental chlorine are pro-dùced by the electrolytic decomposition o~ an aqueous solution of sodium chloride brine in an electrolytic cell in which the brine in the anolyte at the anode is iseparated from the caustic soda produced at the cathode, by a cationic active permselective membrane, the membrane operates in two distinct manners. During electrolysis, sodium ions migrate from the anolyte to the cathode and upon reaching the cation active permselective membrane, which divides the cell into separate anode and cathode compart-ments, are permitted passage through the membrane from active point to active point in the pores of the cation active perm~
selective membrane because such membrane selectively permits passage of such sodium cations while selectively prohibiting the passage of anions such as chloride ions and hydroxyl
- 2 -ions, thereby enabling the isolation and separate recovery of sodium hydroxide in the cathode compartment, and permits an increase in NaOH concentration in the catholyte with continued electrolysis, as well as substantial freedom from sodium chloride contamination of said catholyte.
One of the major problems encountered using such cation-active permselective membranes in such cells results from the build-up of iron and/or other metallic salt contamination on the said membrane. It has been found that the cationic permselective membrane during the electrolysis has the tendency to expand and/or move to contact the cathode. At the points of contact of the membrane and cathode, the relatively fragile ~ membrane develops pin holes. At these points, at least, it has ; been found that iron and/or other metal salts deposit and build-up on and around the membrane surfaces. Such deposits and pinholing reduce the efficiency and active life of the membrane.
OBJECTS OF THE INVENTION
It is thus a principal object of the present invention to provide an improved apparatus suitable for the electrolysis of ionizable chemical compounds.
Another object is to devise an improved process for electrolyzing aqueous solutions of ionizable chemical compoundsy .uch as alkali metal halide brines and hydrohalic acids, which is not subject to many of the disadvantages wh;ch have heretofore been encountered in the prior art processes.
A more specific object is to provide an improved elec-trolytic apparatus which prevents or substantially reduces the .
., ' . .. . ~ '' : ~ : ' ~g~
build-up of iron or other metallic salt contamination on the membrane, These and other objects will become apparent to those skilled in this art from the ~ollowing description of the inventionO
According to the invention there is provided an electrolytic cell which comprises a cell body having an anode compartment containing an anode and a cathode compartment con-taining a cathode, said compartments being separated by at least one cation active permselective membrane which is impervious to liquids and gases, said cathode and said membrane being separated by a porous chemically inert protective barrier, the porous chemically inert protective barrier being mounted on the cathode.
According to another aspect of the invention there is provided a process for the electrochemical decomposition of an ioniæable chemical compound which comprises introducing an aqueous solution of an ionizable chemical compound into the anode compartment of the electrolytic cell of the invention, introducing a second aqueous solution into the cathode compart-ment o~ said cell and effecting the electrolytic decomposition of said ionizable chemical compound by passing an electric current between the anode and cathcde of said cell. In parti-cular the second aqueous solution is a dilute aqueous solution of an alkali metal hydroxide.
More particularly in a pre~erred embodiment the invention is concerned with an electrolytic cell, suitable ~or the electrol~sis of ionizable chemical compounds, particularly alkali metal halide brines and hydrohalic acids, which com-prises a cell body having an anode compartment containing ananode and a cathode compartment containing a cathode said compartments being separated from each other hy at least one ~; - 4 -cation-active permselective membrane which is substantially impervious to liquids and gases and is selected from a hydrolyzed copolymer of a perfluorinated hydrocarbon and a sulfonated perfluorovinyl ether and a sulfostyrenated per- -fluorinated ethylene propylene polymer, said cathode and said membrane being separated by a porous chemically resistant pro-tective barrier mounted on the cathode. The barrier is inter-posed between the cathode and the membrane; and prevents physical contact and thus damage of the membrane by the cathode thereby extending the efficiency and useful life of the membrane and preventing the build-up of iron and/or other metallic salts on the membrane.
By the use of electrolytic cells of this type, for example, for the electrolysis of alkali metal halide brine solu-tions, it has been found that highly concentrated al~ali metal hydroxide solutions which are significantly low in impurities can be produced with maximum electrical operating efficiency and that the membranes are useful over surprising long periods without undergoing iron and/or other metallic salt build-up on the surface thereof.
DETAILED DESCRIPTION OF THE INVENTION
In order that the invention may be readily understood it will be described w;th specific reference to certain preferred embodiments. Thus the following description will be made with reference to the electrolysis of alkali metal halide brine specifically sodium chloride brine. The invention is not limited to such, since other ionizable chemical compounds, can be elect-rolyzed in accordance with the process of the invention also.
Such other compounds include aqueous solutions of alkali and alkaline earth metal chloridesg bromides, sulfates, acid sulfites and the like. Sodium, lithium and potassium salts are included as well as hydrohalic acids such as hydrochloric acid and hydro-bromic acid.
Particular mention is made of alkali metal halide brines acidified with a hydrohalic acid.
In the drawings wh;ch are attached hereto and form a part thereof, Figure 1 is a schematic representation of a two compartment electrolytic cell of the present invention, and, Figure 2 is a schematic representation of a three comp~rtment cell containing one buffer co~partment.
The methods and apparatus of this invention will be further described with reference to the attached drawings. Re-ferring to Figure 1, there is shown an electrolytic cell including a cell body, 1, comprising an anode, 2, and a cathode, 3, separated by a cation-active permselective membrane, 4, to form an anolyte ~7~
compartment, 13, and a catholyte compartment 14. The cell body, 1, has an inlet, 5, in the anode compartment, 13, for the electrolyte and an outlet, 6, for the gaseous product formed at the anode.
There is also provîded an inlet, 7, for charg;ng liqu;d, such as dilute aqueous c~ust;c soda, to the cathode compartment, 14, an outlet, 8, for discharging concentrated catholyte liquor, such as concentr~ted aqueous caustic soda from the cathode compartment and an outlet, 9, for discharging gaseous products formed at the cathode, such as hydrogen. A porous chemically resistant protective barrier, 15, is mounted on the cathode, 3, separating the latter from the membrane, 4, and protecting said membrane during the electrolysis from physical erosion which may occur due to movement of the relatively fragile membrane against the surface of the relatively hard cathode surface.
Saturated brine is continuously circulated in the anolyte compartment, 13, by introducing brine through inlet, 5, and with-drawing it through over-flow, lO, to replenishing zone, ll, where the depleted brine solution is resaturated with sodium chloride and acidified with acid, if desired. The replenished electrolyte flows, via line 12, to reenter the cell body 1, at inlet, 5. At the same ~ime, diluke aqueous caustic soda is charged to the catholyte compart~ent, 14, through inlet, 7~ and concentrated aqueous caustic soda is withdrawn fro~ catholyte compartment, 14, through outlet, 8.
Although the porous protective barrier 15, is shown mounted on the face of cathode, 39 said barrier may be positioned, suspended or otherwise mechanically held spaced apart from the cathode between the membrane and the cathode.
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:
The porous chemically resistant barrier can be fabricated from polypropylene, polytetrafluoroethylene, copolymers of vinyli-dene chloride and vinyl chloride, copolymers of acrylonitrile and vinyl chloride and the like synthetic organic plastic material.
The porous barrier may be fabricated in the form of woven sheet cloth, screen, sintered plate or other suitable form. Pre-ferably polypropylene in the form of a mesh screen is used. One or more such barriers can be used. An especially preferred barrier can be prepared from two layers of 1/8 in. thick polypropylene mesh having a diamond shaped one inch open weave, the layers being overlaid so that the mesh pattern of one layer is offset in reference to that of the other layer.
In a preferred embodiment, sodium chloride brine solution containing from about 200 to about 320 gram per liter (gpl) sodium chloride is electrolyzed, in a cell having an anode compartment and a cathode compartment separated by a cation active permselective membrane, which is substantially impervious to liquids and gases, and which contains in the cathode compartment a porous chemically resistant barrier of the type described above mechanically held, suspended, or inserted between the cathode and the membrane so as to prevent any physical contact between the membrane and the cathode, by impressing a decomposition voltage across the electrodes disposed in each of said compartments, while maintaining the alkali metal hydroxide content in said cathode compartment above about 10 percent by weight, dnd preferably from about 24 to about 33 percent by weight, and recovering an alkali metal hydroxide product from said cathode compartment containing less than about one percent by weight of sodium chloride and chlorine from said anolyte compartment.
The cell can be opera~ed for extended periods under constant operating conditions without deterioration of the membrane and without any evidence of build-up of iron and/or other metalllc salts on the surface of the membrane.
Referr;ng now to Figure 2, this is a schematic re-presentation of a three compartment cell incorporating the improve-ment of the present invention. In this Figure, the cell body, 16, is formed into an anode compartment, 17, a cathode compartment, 18, and a buffer compartment, 19, which separates the anode and cathode compartments. An anode, 20, and a cathode, 21, are disposed within the anode and cathode compartments respectively. A porous chemically inert barrier~ 22, is disposed within the cathode compartment, also, between the cathode and the membrane, 24, forming one wall of the buffer compartment. Sa;d barrier thus prevents contact of said membrane with the metal surface of the cathode. Forming the buffer compartment, 19, and separating it from anode compartment, 17, and the cathode compartment, 18, are membranes; 23 and 24, respectively which membranes are formed of a cation active permselective membrane substantially impervious to liquids and gases as defined hereinabove.
The anode compartment~ 17, is provided with inlet, 25, through which the electrolyte, such as sodium chloride brine, is introduced into the cell, 16. An outlet, 26, is also provided in the anode compartment, through which the partially depleted electrolyte is removed from the anode compartment. Additionally, the anode com-partment is provided w;th a gas outlet, 27, through which the gaseous decomposition products formed at the anode, such as chlorine, are removed from the anode compartment. Although the electrolyte inlet, and gaseous product outlet are shown as being located in the upper portion of the anode compartment with the electrolyte outlet in the lower portion, other arrangements for these inlets and outlets may be utilized if desired.
Similarly, in the buffer compartment, 19, an inlet, 28, and an outlet, 29, are provided. When the cell is utilized for the electrolysis of a sodium chloride brine to produce chlorine and caustic soda, water will be introduced in the buffer compartment through the inlet, 28, and, if desired, a dilute solution of sodium hydroxide may be withdrawn through the outlet, 29. Additionally the cathode compartment, 183 contains an inlet, 30, and an outlet, 31, through which, respectively, in the preferred electrolysis of a sodium chloride brine, water or dilute aqueous caustic soda are introduced and a concentrated caustic soda liquor of high purity is recovered as a product of the process. The cathode compartment may also contain an outlet (not shown) for the gaseous by-products, such as hydrogen, which may be formed at the ca~hode. As in the case of the anode compartment, the inlets and outlets for the buffer and cathode compartments may, if desired, be positioned in other than the upper and lower portions, respectively, of the compartments as shown in Figure 2. Similarly, although the porous barrier, 22, is shown positioned between the membrane, 24, and the cathode, 21, said barrier may be p1aced on the face of the cathode facing said membrane, on the face of said membrane facing said cathode, or in any other position wherein contact between the membrane surface and the metal cathode is prevented.
In a preferred embodiment of the invention the permselective membranes are of a hydrolyzed copolymer of tetrafluoroethylene and a fluorosulfonated perfluorovinyl ether of the formula FS02CF2CF20CF(CF3)CF20CF=CF2, which has an equivalent weight of about 900 to 1,600 and the membranes may be mounted on networks of supporting materials such as polytetrafluoroethylene or asbestos filaments. The described preferred copolymers may be further modified to improve their activities, as by surface treating, modifying the sulfonic group or by other such mechanism. Such varities of the polymers are included within the generic descrip-tion given.
More specifically, the electrolytic cell of the present invention comprises a cell body or container formed of materials which, as such, or when provided with a suitable coating, will be electrically non-conductive and withstand the temperatures at which the cell may be operated and will also be resistant to the materials being processed in the cell, such as chlorine, sodium hydroxide, hydrochloric acid, and the like. Exemplary of materials which may be used are various polymeric materials, such as high temperature polyvinyl chloride, hard rubber, chlorendic acid based polyester reslns, and the like. Additionally, materials such as concrete, cement, and the like, may a7so be used. In the case of these latter materials, however, any interior exposed areas should have a coating which is resistant to hydrochloric acid, chlorine, caustic soda, or similar materials with which said surfaces will be in contact. Additionally, the cell body may be made of metal, such as steel, titanium, or the like, if the exposed surfaces are coated with a corrosion protective material and electrical insu-lation is provided where necessary.
The electrodes for the present electrolytic cell may be formed of any electr;cally conductive material which will res;st the corrosive attack of the various cell reactants and products with which they may come in contact, such as alkali metal hydroxides, hydrochloric acid, and chlorine. Typically, the cathodes may be constructed of graphite, iron, steel, or the like, with steel being generally preferred. Similarly, the anodes may be formed of graphite or may be metallic anodes. Typically, where metallic anodes are used, these may be formed of a so-called "valve" metal, such as titanium, tantalum or niobium as well as alloys of these in which the valve metal constitutes at least about 90% of the alloy. The surface of the valve metal may be made active by means of a coating of one or more noble metals, noble metal alloys, noble metal oxides, or mixtures of such oxides, either alone or with oxides of other metals. The noble metals which may be used include ruthen;um, rhodium, palladium, irridium and platinum. Particularly preferred metal anodes are those formed of titan;um and having a mixecl titanium oxide and ruthenium oxide coating on the surface, as is described in U.S.
Patent 3,632,498. Additionally, the valve metal substrate may be clad on a more electrically conductive metal core, such as aluminum, steel, copper, or the l;ke.
The cell body or container is formed into at least one set or unit of compartments made up of an anode compartment, con-taining the anode, and a cathode compartment, containing the cathode, and optionally at least one buffer compartment between the anode and cathode compartments. Typically, the electrolytic cell will contain a plurality of these sets, e.g., 20 to 30 or more, depending upon the size of the cell.
These compartments are separated from each other by a membrane which is substantially impervious to fluids and gases and composed essentially of a hydrolyzed copolymer of a perfluori-nated hydrocarbon and a fluorosulfonated perfluorovinyl ether.
The perfluorinated hydrocarbon is preferably tetrafluoroethylene, although other perfluorinated and saturated and unsaturated hydro-carbons of 2 to 5 carbon atoms may also be utilized, of ~hich the monoolefinic hydrocarbons are preferred, especially those of 2 to 4 carbon ato~s and most especially those of 2 to 3 carbon atoms, ~-e.g., tetrafluoroethylene, hexafluoropropylene. The sulfonated perfluorovinyl ether which is most useful is that of the formula FS02CF2CF20CF(CF3)CF20CF=CF2. Such materials, named as perfluoro L2(2-fluorosulfonylethoxy)-propyl vinyl ether], referred to hence-forth as PSEPVE, may be modified to equivalent monomers, as by modifiying the internal perfluorosulfonylethoxy component to the corresponding propoxy component and by altering the propyl to ethyl or butyl, plus rearranging positions of substitution of the sulfonyl thereon and utilizing isomers of the perfluoro-lower alkyl groups, respectively. However, it is most preferred to employ PSEPVE.
The method of manufacture of the hydrolyzed copolymer is descr;bed in Example XVII of U.S. Patent 3,282,875 and an alternative method is mentioned in Canadian Patent 849,670, which also discloses the use of the finished membrane in fuel cells, characterized therein as electrochemical cells. In short, the copolymer may be made by reacting PSEPVE or equivalent with tetrafluoroethylene or equivalent in desired proportions in water at elevated temperature and pressure ;~, ,~; ."-, for over an hour, aFter which time the mix is cooled. It separates into a lower per~luoroether layer and upper layer of aqueous medium with dispersed desired polymer. The molecular weight is indeter-mined, but the equivalent weight is about 900 to 1,600 preferably 1,100 to 1,400, and the percentage of PSEPVE or corresponding compound is about 10 to 30%, preferably 15 to 20% and most pre-ferably about 17%. The unhydrolyzed copolymer may be compression molded at high temperature and pressure to produce sheets or mem-branes, which may vary in thickness from 0.02 to 0.5 mm. These are then further treated to hydrolyze pendant -S02F groups to -S02H
groups, as by treating with 10% sulfuric acid or by the methods of the patents previously mentioned. The presence of the -S02H groups may be verified by titration, as described in the Canadian patent.
Additional details of various processing steps are described in Canadian patent 752,427 and U.S. patent 3,041,317.
Because it has been found that some expansion accompanies hydrolysis of the copolymer it is preferred to position the co-polymer membrane after hydrolysis onto a frame or other support which will hold it in place in the electrolytic cell. Then it may be clamped or cemented in place and will be true, without sags. The membrane is preferably joined to the backing tetrafluoroethylene or other suitable filaments prior to hydrolysis, ~hen it is still thermo-plastic, and the film of copolymer covers each filament, penetrating into the spaces between them and even around behind them, the films becoming slight7y thinner in the process, where they cover the filaments.
The membrane described is far superior in the present processes to all other previously suggested membrane materials.
It is more stable at elevated temperatures, e.g., about 75C.
Especially, when protected from physical damage and iron salt build-up in ~ccordance with the present invention it lasts for much longer time periods in the medium of the electrolyte and the caustic product and does not become brittle when subjected ts chlorine at high temperatures. Considering the savings in time and fabrication costs, the present membranes are more economical. The voltage drop through the membranes is acceptable and does not become inordinately high, as it does with many other membrane materials, when the caustic concentration in the cathode -compartment increases to above about 200 9./1. of caustic. The selectivity of the membrane and its compatibility with the electrolyte does n~t decrease detrimentally as the hydroxyl con-centrat;on in the catholyte liquor increases, as has been noted with other membrane mater;als. Furthermore, the caustic effi-ciency of ~he electrolysis does not diminish as significantly as it does with other membranes when the hydroxyl ion concent-ration in the catholyte increases. Thus, these differencesin the present process make it practicable, whereas previously described processes have not attained commercial acceptance.
While the more preferred copolymers are those having equivalent weights of 900 to 1,600 with 1,100 to 1,400 being most preferred, ~ -some useful resinous membranes produced by the present method may be of equivalent weights from 500 to 4,000. The medium equivalent weight polymers are preferred because they are of satisfactory strength and stability, enable better selective ion d~
exchange to take place and are of lower internal resistances, all of which are important to the present electrochemical cell.
Improved versions of ~he above-described copolymers may be made by chemical treatment of surfaces thereof, as by treatments to modify the -S03H group thereon. For example, the sulfonic group may be altered or may be replaced in part with other moieties. Such changes may be made in manufacturing process or after production o the membrane. When effected as a subsequent surface treatment of a membrane the depth of treatment will usually be from 0.001 to 0.01 mm.
Caustic effic;encies of the invented processes, using such modified versions of the present improved membranes, can increase about 3 to 20%, often about 5 to 15%. One such modification is described in French Patent 2,152,194. This patent discloses membranes consisting of a film of a fluorinated polymer having pendant side chains con-taining sulfony1 groups attached to carbon atoms bearing at leastone fluorine atom9 the majority of the sulfonyl groups one on surface of the film being in the form of -(S02NH2)mQ groups where Q is H, NH4, alkali or alkaline earth metal and m is the valence of Q and the sulfonyl groups on the other surface o~ ~he film belng in the form of -(S03)nMe groups where Me is a cation and n is the valence of Me, with the proviso the Me is H, Q is H.
In addition to the copolymers previously discussed, including mod;fications thereof, it has been found that another type of membrane material is also superior to prior art films for applications in the present processes. Although it appears that tetrafluoroethylene (TFE) polymers which are sequentially styrenated and sulfonated are not useful for making satisfactory cation-active 7~
permselective membranes for use in the present electrolytic process it has been established that perfluorinated ethylene propylene polymer (FEP) which is styrenated and sulfonated makes a useful membrane. Whereas useful lives of as much as three years or more (that of the preferred copolymers) may not be obtained the sulfostyrenated FEP's are surprisingly resistant to hardening and other wise failing in use under the present process conditions.
To manufacture the sulfostyrenated FEP membranes a standard FEP, such as manufactured by E. I. DuPont de Nemours & Co., Inc., is styrenated and the styrenated polymer is then sulfonated.
A solution of styrene in methylene chloride or benzene at a suitable concentration in the range of about 10 to 20% is prepared and a sheet of FEP polymer having a thickness of about 0.02 to 0.5 mm., pre-ferably 0.05 to 0.15 mm., is dipped into the solution. After removal it is subjected to radiation treatment, using a cobalt60 radiation source. The rate of application may be in the range of about 8,000 rads/hr. and a total radiation application is about 0.9 megarads.
After rinsing with water the phenyl rings of the styrene portion of the polymer are monosulfonated, preferably in the para position, by treatment with chlorosulfonic acid, fuming sulfuric acid or S03.
Preferably, chlorosulfonic acid in chloroform is utilized and the sulfonation is completed in about 1/2 hour.
Examples of useful membranes made by the described process are products of RAI Research Corporation, Hauppauge, New York, identified as 18ST12S and 16ST13S, the former being 18% styrenated and having 2/3 of the phenyl groups monosulfonated and the latter being 16% styrenated and having 13/16 of the phenyl groups mono-sulfonated. To obtain 18% styrenation a solution of 17-1/2% of styrene in methylene chloride is utilized and to obtain the 16%
styrenation a solution of 16% of styrene in methylene chloride is employed.
The products resulting compare favorably with the pre-ferred copolymers previously described, giving voltage drops ofabout 0.2 volt each in the present cells at a current density of 2 amperes/sq. in., the same as is obtained From the copolymer.
Desirably, these membranes are utilized in the form of a thin film, either as such, or deposited on an inert support or substrate, such as a cloth woven of polytetrafluoroethylene, glass fibers or the like. The thickness of such a supported membrane can be varied considerably, thicknesses of from about 3 to l5 mils being typical. These membr~nes may be fabricated into any desired shape, depending upon the configuration of the cell in which they are used. As has been noted, the membrane copolymer is initially obtained in a non-acid form, i.e., in the form of the sulfonyl fluoride. In this non-acid form, it is relatively soft and pliable and can be seam or butt welded to form welds which are as strong as the membrane material itself. Accordingly, it is preferred that the membrane material be shaped and formed in this non-acid state.
Once the membrane has been shaped or formed into the desired configuration, it is then conditioned for use by hydrolyzing the sulfonyl fluoride groups to free sulfonic acid or alkali metal sulfonate groups, for example by boiling in water or alkaline solution, such as caustic solution. This conditioning process may be carried out either before the membrane is placed in the cell or within the cell with the membrane in place. Typically, _ 1~ _ when the memb~ane is boiled in water for about 16 hours, the material undergoes swelling of about 28%, about 9% in each direction. Upon exposure to brine, during operation, the swelling is reduced to about 22%, resulting in a net tightening of the membrane during use.
In some instances, it has been found that it may be desirable to use a "sandwich" of two or more of these membranes, rather than only a single membrane. When such a sandwich is used in a chlor-alkali cell, ;t has been found that in some instances 0 there is an increase in the caustic efficiency of the cell, particularly when operating at catholyte liquor caustic con-centrations in excess of about 200 grams per liter. With the electrolytic cells of the present invention however, this increase in caustic efficiency may not be sufficiently great as to offset the increased material cost o~ using such a membrane sandwichO
Accordingly, although the use of such a sandwich is possible in the present cell, it may not always be preferred.
The anode compartment of each set or unit of compartments is fGrmed with an inlet for introducing a liquid electrolyte into the compartment, such as an aqueous alkali metal halide brine and an outlet for gaseous reaction products, such as chlorine. The cathode compartment of each set or unit is formed with an outlet for liquid reaction products9 such as aqueous solutions of alkali metal hydroxide, and also an outlet for gaseous by-products, such as hydrogen. If desired, the cathode compartment may also be formed with an inlet for a liquid electrolyte, such as water, dilute alkali metal hydroxide solutions, or the like. Additionally, each of the buffer compartments between the anode and cathode compartments, _ 19 _ .~
when present, i~ formed with an inlet for liquid electrolyte~ ~uch as water and, if desired may also have an outlet for liquid reaction products, such as dilute alkali meta1 hydroxide solutions. Prefer-ably, the inlets for liquid materials and the outlets for gaseous products in each of the compartments are located in the upper portion of the compartment while the outlets for liquid materials are po-sitioned in the lower portion of the compartments, although other locations may also be used.
These repeating sets or units of anode, buffer (when present), and cathode compartments may be formed into the total electrolytic cell of the present invention in any convenient manner. Thus, in a preferred embodiment, the cell is of the so-called "filter press"
type. In this embodiment, the anodes, cathodes, porous barriers9 and membranes are mounted in suitable mounting or frame members which are provided with suitable sealing gaskets and are formed so as to provide the desired spacing between the elements to form the anode, cathode and buffer compartments. These frame members are provided with the desired inlets and outlets, as have been described and are secured together by tie rods, bolts, or other suitable means as is known in the art. Typical of such a filter press configuration is that shown in U.S. Patent 2,282,058.
Alternatively, the cell body may be in the form of a box of a suitable material of construction in which anode, cathode and membrane are mounted to form the various compartments, such as that shown in U.S. Patent 3,32~,023. Additionally, the cell may be of the "conventional" chlor-alkali type having interleaved anodes and cathodes, wherein the deposited asbestos diaphragm is replaced -with the various membranes as have ~een described, to form the desired buffer compartments. Typical of such a cell structure is that shown in U.S. Patent 3,458,~11.
It is to be appreciated that the above are mere1y exemplary of the various cell con~igurations which may be used.
In all of these, of course, suitable materials of construction wil1 be used, as have been described hereinabove, and in each, there will be inserted between the cathode and the cation-active membrane, a porous chemically resistant barrier as described here-inabove. Additionally, it is further to be appreciated that theparticular configuration used in each instance will depend upon the various specific requirements for that particular cell.
In carrying out the process of the present invention, a solution of the ionizable compound to be electrolyzed is intro-duced into the anode compartment of the electrolytic cell. Ex-emplary of the various solutions of ionizable compounds which may be electrolyzed and the products produced are aqueous solutions of alkali metal halides to produce the alkali metal hydroxides and halogen; aqueous solutions of HCl to produce hydrogen and chlorine;
aqueous solutions of ammonium sulfate to produce persulfates;
aqueous solutions of borax to produce perborates, and the like.
Of these, the most preferred anoly~e solutions are the aqueous solutions of alkali metal halides, and particularly sodium chloride, and aqueous solutions of HCl.
In a typical process, utilizing a sodium chloride brine as the feed to -the anode compartment, the feed solution will contain from about 250 to 32~ grams per liter sodium chloride and, most preferably, about 320 grams per liter sodium chloride. The pH of this anolyte feed solution is typically within the range of about l.O to lO.O, with a pH of about 3.5 being preferred. These desired pH values in the anode compartment may be maintained by the addition of acid to the anolyte solution, preferably hydrochloric acid.
The anolyte overflow or depleted anolyte solution removed from the anode compartment will generally have a sodium chloride content of from about 200 to 295 grams per liter, with a sodium chloride content of about 250 grams per liter being typical.
l O In a three compartment cell such as shown in Figure 2, i.e., a cell having one or more repeating units of an anode com-partment and a cathode compartment separated by a single center or buffer compartment, water is introduced into the center or buffer compartment and a dilute solution of sodium hydroxide is removed from this compartment. Generally, this solution will have a sodium hydroxide content of from about 50 to 200 grams per liter with a sodium hydroxide content of about lOO grams per liter being typical. Preferablyg this dilute solution of sodium hydroxide is introduced into the cathode compartment, either with or without additional water, to form the catholyte liquor. From the cathode compartment there is obtained a more concentrated sodium hydroxide solution, having NaOH concentration bf from about 150 to 250 grams per liter, with the sodium hydroxide content of about l60 grams per liter being typical. Additionally, gaseous products of chlorine gas and hydrogen gas are obtained from the anode compartment and the cathode compartment~ respectively.
In an ~lternative method of operation, water is added to both the center or buffer compartment and to the cathode com-partment and there is recoverd from the buffer compartment a ~7~ 7a:~
product stream of dilute sodium hydroxide and, from the cathode compartment9 a product stream of more concentrated sodium hydroxide solution. When operating in this manner, the amount of dilute caustic soda solution recovered from the buffer compartment and the amount of concentrated caustic soda solution recovered from the cakhode compartment may be varied, depending upon the particular requirements for each type of solution. In a typical operation, approximately 50% of the sodium hydroxide will be recovered as a dilute solution from the buffer compartment with the other 50%
being recovered as ~he more concentrated solution from the cathode compartment. The concentration of the dilute caustic soda solution will generally be within the range of about 50 to 200 grams per liter with a concentration of about 100 grams per liter being typical. Similarly, the concentration of the more concentrated caustic solution from the cathode compartment will generally be within the range of about 200 to 420 grams per liter with a concentration of about 280 grams per liter being typical.
The electrochemical decomposition process of the present invention is typically carried out at ~ voltage within the range of about 3.4 to 4.9, with a voltage of about 4.2 being preferred.
Typically, the current densities are within the range of about 0.8 to 2.5 amps per square inch, with current densities of about 2 amps per square inch being particularly preferred. In general, the cell will be operated at temperatures within the range of about 90 to 105 degrees centigrade with temperatures of about 95 degrees centigrade being typical. When operating in this manner, it is found that chlorine or anode efficiencies of at least about 96%
~7~
and cathode or caustic soda efficiencies of at least 85% and frequently in excess of 90% are obtained. Additionally, the concentrated caustic soda solution obtained from the cathode compar~ment is found to be of high purity, at least approaching, if not equal, that of "Rayon grade" caustic soda. Typically, the purity of this sodium hydroxide is such that it is substan-tially free of sodium chlorate and contains less than one gram per liter of sodium chloride.
Where the process is carried out with cells having sections or repeating units which contain two or more buffer compartments9 the operation is similar to that which has been described hereinabove. Thus, water may be introduced into each of the buffer compartments and into the cathode compartment and a portion of the sodium hydroxide product values may be recovered from each of the buffer compartments, as a dilute solution of sodium hydride and from the cathode compartment as a more concen-trated sodium hydroxide solution. Preferably, however, the dilute sodium hydroxide solutions from each buffer compartment is introduced as at least a portion of the feed to the next succeeding buffer compartment9 and ultimately into the c~thode compartment so that there is obtained from the cathode compartment a concentrated sodium hydroxide product stream of high purity.
As has been indicated hereinabove, in addition to the electrolysis of sodium chloride brine solution9 to produce ch10rine and caustic soda, in another preferred operation, the electrolytic cells of the present invention may be used for the electrolysis of hydrochloric acid solutions, to form chlorine and hydrogen as the products of the process. In such an operation, the anolyte solution ` ~ \
introduced into the anode compartment is an aqueous solution of hydrochloric acid, desirably having an HCl content of from about 10% to 36% by weight and preferably having an HCl content of from about 15 to 25% by weight. Although the feed to the buffer com-partrnents, if present, and the cathode compartment may be wateralone, in the most preferred method of operation, the feed to both the buffer compartments and the cathode compartment is also an aqueous hydrochloric acid solution. Desirably, the ~Cl content o~ these feed solutions is from about l to 10% by weight with an HCl content of from about l to 5% by weight being preferred. A1-though it is preferred that the feed solution to the anode, buffer, when present, and cathode compartments be substantially free of contaminating ions, in many instances it has been found to be desirable to add alkali metal chlorides, such as sodium chloride to the anolyte, in order to minimize corrosion, particularly where a steel or similar corrodible cathode is used. In these instances, additions of sodium chloride in amounts within the range of about 12 to 25% by weight of the anolyte solution are typical.
In order that those skilled in the art may better under-stand the present invention and the manner in which it may be practiced, the following specific examples are given. In these examples, unless otherwise indicated, temperatures are in degrees centigrade and parts and percent are by weight. It is to be appreciated, however, that these examples are merely exemplary of the method and apparatus of the present invention and are not to be taken as a limitation thereof.
.
~7~ 7~
EXAMPLE I
A three comp~rtment pilot plant sized cell, of the type shown in Figure 2, not including the porous barrier was operated at 2400 amperes, an anode current density of 2 amps/sq.
inch and an average voltage of 4.9 volts. The cell was equipped with a metallic anode formed of titanium mesh with a ruthenium oxide coating, a steel mesh cathode, and two cation-active perm-selective membranes, one on each side separating the buffer compartment from the anode compartment and the cathode compartment.
The membranes were, each, a lO mil thick film of a hydrolyzed copolymer of tetrafluoroethylene and sulfonated perfluorovinyl ether, having an equivalent weight of about llO0 and prepared according to U.S. Patent 3,282,875.
Brine containing about 320 gpl NaC1 was circulated through the cathode compartment and water was added to both the buffer and cathode compartments. The pH of the anolyte was maintained at about 4.0 by the addition of hydrochloric acid to the circulating liquor.
The cell was operated under substantially constant conditions for about 72 hours (3 days) and thereafter the cell was disassembled. The membrane separating the buffer compartment from the cathode compartment was examined. This examination showed pin hole development at several points in the membrane and each of the points was stained with iron salts arising from contact of the membrane with the meta1 cathode.
EXAMPLE II.
The disasserrlbled cell of Example I was reassembled with new 10 mil thick membranes. In addition, two layers of l/8 inch .
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~7~
thick polypropylene mesh screen, having a diamond shaped one inch open weave were overlaid so that the mesh pattern of one layer was offset with reference to the pattern of the other layer, were placed between the bare steel mesh cathode face and the membrane surface 5 facing the cathode.
The cell was operated under substantially the same conditions set out in Example I, above, for twenty days. Thereafter, the cell was again disassembled and the membrane on the cathode side of the buffer compartment was examined. Inspection of the membrane showed no evidence of pin holing or other physical damage. Further there was no evidence of the build-up of iron or other metallic salts on the surface of the membrane.
While there has been described various embodiments of the invention, the methods and apparatus described are not intended to be understood as limiting the scope of the invention as changes therewithin are possible, and it is intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever ~orm its principle may be utilized.
One of the major problems encountered using such cation-active permselective membranes in such cells results from the build-up of iron and/or other metallic salt contamination on the said membrane. It has been found that the cationic permselective membrane during the electrolysis has the tendency to expand and/or move to contact the cathode. At the points of contact of the membrane and cathode, the relatively fragile ~ membrane develops pin holes. At these points, at least, it has ; been found that iron and/or other metal salts deposit and build-up on and around the membrane surfaces. Such deposits and pinholing reduce the efficiency and active life of the membrane.
OBJECTS OF THE INVENTION
It is thus a principal object of the present invention to provide an improved apparatus suitable for the electrolysis of ionizable chemical compounds.
Another object is to devise an improved process for electrolyzing aqueous solutions of ionizable chemical compoundsy .uch as alkali metal halide brines and hydrohalic acids, which is not subject to many of the disadvantages wh;ch have heretofore been encountered in the prior art processes.
A more specific object is to provide an improved elec-trolytic apparatus which prevents or substantially reduces the .
., ' . .. . ~ '' : ~ : ' ~g~
build-up of iron or other metallic salt contamination on the membrane, These and other objects will become apparent to those skilled in this art from the ~ollowing description of the inventionO
According to the invention there is provided an electrolytic cell which comprises a cell body having an anode compartment containing an anode and a cathode compartment con-taining a cathode, said compartments being separated by at least one cation active permselective membrane which is impervious to liquids and gases, said cathode and said membrane being separated by a porous chemically inert protective barrier, the porous chemically inert protective barrier being mounted on the cathode.
According to another aspect of the invention there is provided a process for the electrochemical decomposition of an ioniæable chemical compound which comprises introducing an aqueous solution of an ionizable chemical compound into the anode compartment of the electrolytic cell of the invention, introducing a second aqueous solution into the cathode compart-ment o~ said cell and effecting the electrolytic decomposition of said ionizable chemical compound by passing an electric current between the anode and cathcde of said cell. In parti-cular the second aqueous solution is a dilute aqueous solution of an alkali metal hydroxide.
More particularly in a pre~erred embodiment the invention is concerned with an electrolytic cell, suitable ~or the electrol~sis of ionizable chemical compounds, particularly alkali metal halide brines and hydrohalic acids, which com-prises a cell body having an anode compartment containing ananode and a cathode compartment containing a cathode said compartments being separated from each other hy at least one ~; - 4 -cation-active permselective membrane which is substantially impervious to liquids and gases and is selected from a hydrolyzed copolymer of a perfluorinated hydrocarbon and a sulfonated perfluorovinyl ether and a sulfostyrenated per- -fluorinated ethylene propylene polymer, said cathode and said membrane being separated by a porous chemically resistant pro-tective barrier mounted on the cathode. The barrier is inter-posed between the cathode and the membrane; and prevents physical contact and thus damage of the membrane by the cathode thereby extending the efficiency and useful life of the membrane and preventing the build-up of iron and/or other metallic salts on the membrane.
By the use of electrolytic cells of this type, for example, for the electrolysis of alkali metal halide brine solu-tions, it has been found that highly concentrated al~ali metal hydroxide solutions which are significantly low in impurities can be produced with maximum electrical operating efficiency and that the membranes are useful over surprising long periods without undergoing iron and/or other metallic salt build-up on the surface thereof.
DETAILED DESCRIPTION OF THE INVENTION
In order that the invention may be readily understood it will be described w;th specific reference to certain preferred embodiments. Thus the following description will be made with reference to the electrolysis of alkali metal halide brine specifically sodium chloride brine. The invention is not limited to such, since other ionizable chemical compounds, can be elect-rolyzed in accordance with the process of the invention also.
Such other compounds include aqueous solutions of alkali and alkaline earth metal chloridesg bromides, sulfates, acid sulfites and the like. Sodium, lithium and potassium salts are included as well as hydrohalic acids such as hydrochloric acid and hydro-bromic acid.
Particular mention is made of alkali metal halide brines acidified with a hydrohalic acid.
In the drawings wh;ch are attached hereto and form a part thereof, Figure 1 is a schematic representation of a two compartment electrolytic cell of the present invention, and, Figure 2 is a schematic representation of a three comp~rtment cell containing one buffer co~partment.
The methods and apparatus of this invention will be further described with reference to the attached drawings. Re-ferring to Figure 1, there is shown an electrolytic cell including a cell body, 1, comprising an anode, 2, and a cathode, 3, separated by a cation-active permselective membrane, 4, to form an anolyte ~7~
compartment, 13, and a catholyte compartment 14. The cell body, 1, has an inlet, 5, in the anode compartment, 13, for the electrolyte and an outlet, 6, for the gaseous product formed at the anode.
There is also provîded an inlet, 7, for charg;ng liqu;d, such as dilute aqueous c~ust;c soda, to the cathode compartment, 14, an outlet, 8, for discharging concentrated catholyte liquor, such as concentr~ted aqueous caustic soda from the cathode compartment and an outlet, 9, for discharging gaseous products formed at the cathode, such as hydrogen. A porous chemically resistant protective barrier, 15, is mounted on the cathode, 3, separating the latter from the membrane, 4, and protecting said membrane during the electrolysis from physical erosion which may occur due to movement of the relatively fragile membrane against the surface of the relatively hard cathode surface.
Saturated brine is continuously circulated in the anolyte compartment, 13, by introducing brine through inlet, 5, and with-drawing it through over-flow, lO, to replenishing zone, ll, where the depleted brine solution is resaturated with sodium chloride and acidified with acid, if desired. The replenished electrolyte flows, via line 12, to reenter the cell body 1, at inlet, 5. At the same ~ime, diluke aqueous caustic soda is charged to the catholyte compart~ent, 14, through inlet, 7~ and concentrated aqueous caustic soda is withdrawn fro~ catholyte compartment, 14, through outlet, 8.
Although the porous protective barrier 15, is shown mounted on the face of cathode, 39 said barrier may be positioned, suspended or otherwise mechanically held spaced apart from the cathode between the membrane and the cathode.
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:
The porous chemically resistant barrier can be fabricated from polypropylene, polytetrafluoroethylene, copolymers of vinyli-dene chloride and vinyl chloride, copolymers of acrylonitrile and vinyl chloride and the like synthetic organic plastic material.
The porous barrier may be fabricated in the form of woven sheet cloth, screen, sintered plate or other suitable form. Pre-ferably polypropylene in the form of a mesh screen is used. One or more such barriers can be used. An especially preferred barrier can be prepared from two layers of 1/8 in. thick polypropylene mesh having a diamond shaped one inch open weave, the layers being overlaid so that the mesh pattern of one layer is offset in reference to that of the other layer.
In a preferred embodiment, sodium chloride brine solution containing from about 200 to about 320 gram per liter (gpl) sodium chloride is electrolyzed, in a cell having an anode compartment and a cathode compartment separated by a cation active permselective membrane, which is substantially impervious to liquids and gases, and which contains in the cathode compartment a porous chemically resistant barrier of the type described above mechanically held, suspended, or inserted between the cathode and the membrane so as to prevent any physical contact between the membrane and the cathode, by impressing a decomposition voltage across the electrodes disposed in each of said compartments, while maintaining the alkali metal hydroxide content in said cathode compartment above about 10 percent by weight, dnd preferably from about 24 to about 33 percent by weight, and recovering an alkali metal hydroxide product from said cathode compartment containing less than about one percent by weight of sodium chloride and chlorine from said anolyte compartment.
The cell can be opera~ed for extended periods under constant operating conditions without deterioration of the membrane and without any evidence of build-up of iron and/or other metalllc salts on the surface of the membrane.
Referr;ng now to Figure 2, this is a schematic re-presentation of a three compartment cell incorporating the improve-ment of the present invention. In this Figure, the cell body, 16, is formed into an anode compartment, 17, a cathode compartment, 18, and a buffer compartment, 19, which separates the anode and cathode compartments. An anode, 20, and a cathode, 21, are disposed within the anode and cathode compartments respectively. A porous chemically inert barrier~ 22, is disposed within the cathode compartment, also, between the cathode and the membrane, 24, forming one wall of the buffer compartment. Sa;d barrier thus prevents contact of said membrane with the metal surface of the cathode. Forming the buffer compartment, 19, and separating it from anode compartment, 17, and the cathode compartment, 18, are membranes; 23 and 24, respectively which membranes are formed of a cation active permselective membrane substantially impervious to liquids and gases as defined hereinabove.
The anode compartment~ 17, is provided with inlet, 25, through which the electrolyte, such as sodium chloride brine, is introduced into the cell, 16. An outlet, 26, is also provided in the anode compartment, through which the partially depleted electrolyte is removed from the anode compartment. Additionally, the anode com-partment is provided w;th a gas outlet, 27, through which the gaseous decomposition products formed at the anode, such as chlorine, are removed from the anode compartment. Although the electrolyte inlet, and gaseous product outlet are shown as being located in the upper portion of the anode compartment with the electrolyte outlet in the lower portion, other arrangements for these inlets and outlets may be utilized if desired.
Similarly, in the buffer compartment, 19, an inlet, 28, and an outlet, 29, are provided. When the cell is utilized for the electrolysis of a sodium chloride brine to produce chlorine and caustic soda, water will be introduced in the buffer compartment through the inlet, 28, and, if desired, a dilute solution of sodium hydroxide may be withdrawn through the outlet, 29. Additionally the cathode compartment, 183 contains an inlet, 30, and an outlet, 31, through which, respectively, in the preferred electrolysis of a sodium chloride brine, water or dilute aqueous caustic soda are introduced and a concentrated caustic soda liquor of high purity is recovered as a product of the process. The cathode compartment may also contain an outlet (not shown) for the gaseous by-products, such as hydrogen, which may be formed at the ca~hode. As in the case of the anode compartment, the inlets and outlets for the buffer and cathode compartments may, if desired, be positioned in other than the upper and lower portions, respectively, of the compartments as shown in Figure 2. Similarly, although the porous barrier, 22, is shown positioned between the membrane, 24, and the cathode, 21, said barrier may be p1aced on the face of the cathode facing said membrane, on the face of said membrane facing said cathode, or in any other position wherein contact between the membrane surface and the metal cathode is prevented.
In a preferred embodiment of the invention the permselective membranes are of a hydrolyzed copolymer of tetrafluoroethylene and a fluorosulfonated perfluorovinyl ether of the formula FS02CF2CF20CF(CF3)CF20CF=CF2, which has an equivalent weight of about 900 to 1,600 and the membranes may be mounted on networks of supporting materials such as polytetrafluoroethylene or asbestos filaments. The described preferred copolymers may be further modified to improve their activities, as by surface treating, modifying the sulfonic group or by other such mechanism. Such varities of the polymers are included within the generic descrip-tion given.
More specifically, the electrolytic cell of the present invention comprises a cell body or container formed of materials which, as such, or when provided with a suitable coating, will be electrically non-conductive and withstand the temperatures at which the cell may be operated and will also be resistant to the materials being processed in the cell, such as chlorine, sodium hydroxide, hydrochloric acid, and the like. Exemplary of materials which may be used are various polymeric materials, such as high temperature polyvinyl chloride, hard rubber, chlorendic acid based polyester reslns, and the like. Additionally, materials such as concrete, cement, and the like, may a7so be used. In the case of these latter materials, however, any interior exposed areas should have a coating which is resistant to hydrochloric acid, chlorine, caustic soda, or similar materials with which said surfaces will be in contact. Additionally, the cell body may be made of metal, such as steel, titanium, or the like, if the exposed surfaces are coated with a corrosion protective material and electrical insu-lation is provided where necessary.
The electrodes for the present electrolytic cell may be formed of any electr;cally conductive material which will res;st the corrosive attack of the various cell reactants and products with which they may come in contact, such as alkali metal hydroxides, hydrochloric acid, and chlorine. Typically, the cathodes may be constructed of graphite, iron, steel, or the like, with steel being generally preferred. Similarly, the anodes may be formed of graphite or may be metallic anodes. Typically, where metallic anodes are used, these may be formed of a so-called "valve" metal, such as titanium, tantalum or niobium as well as alloys of these in which the valve metal constitutes at least about 90% of the alloy. The surface of the valve metal may be made active by means of a coating of one or more noble metals, noble metal alloys, noble metal oxides, or mixtures of such oxides, either alone or with oxides of other metals. The noble metals which may be used include ruthen;um, rhodium, palladium, irridium and platinum. Particularly preferred metal anodes are those formed of titan;um and having a mixecl titanium oxide and ruthenium oxide coating on the surface, as is described in U.S.
Patent 3,632,498. Additionally, the valve metal substrate may be clad on a more electrically conductive metal core, such as aluminum, steel, copper, or the l;ke.
The cell body or container is formed into at least one set or unit of compartments made up of an anode compartment, con-taining the anode, and a cathode compartment, containing the cathode, and optionally at least one buffer compartment between the anode and cathode compartments. Typically, the electrolytic cell will contain a plurality of these sets, e.g., 20 to 30 or more, depending upon the size of the cell.
These compartments are separated from each other by a membrane which is substantially impervious to fluids and gases and composed essentially of a hydrolyzed copolymer of a perfluori-nated hydrocarbon and a fluorosulfonated perfluorovinyl ether.
The perfluorinated hydrocarbon is preferably tetrafluoroethylene, although other perfluorinated and saturated and unsaturated hydro-carbons of 2 to 5 carbon atoms may also be utilized, of ~hich the monoolefinic hydrocarbons are preferred, especially those of 2 to 4 carbon ato~s and most especially those of 2 to 3 carbon atoms, ~-e.g., tetrafluoroethylene, hexafluoropropylene. The sulfonated perfluorovinyl ether which is most useful is that of the formula FS02CF2CF20CF(CF3)CF20CF=CF2. Such materials, named as perfluoro L2(2-fluorosulfonylethoxy)-propyl vinyl ether], referred to hence-forth as PSEPVE, may be modified to equivalent monomers, as by modifiying the internal perfluorosulfonylethoxy component to the corresponding propoxy component and by altering the propyl to ethyl or butyl, plus rearranging positions of substitution of the sulfonyl thereon and utilizing isomers of the perfluoro-lower alkyl groups, respectively. However, it is most preferred to employ PSEPVE.
The method of manufacture of the hydrolyzed copolymer is descr;bed in Example XVII of U.S. Patent 3,282,875 and an alternative method is mentioned in Canadian Patent 849,670, which also discloses the use of the finished membrane in fuel cells, characterized therein as electrochemical cells. In short, the copolymer may be made by reacting PSEPVE or equivalent with tetrafluoroethylene or equivalent in desired proportions in water at elevated temperature and pressure ;~, ,~; ."-, for over an hour, aFter which time the mix is cooled. It separates into a lower per~luoroether layer and upper layer of aqueous medium with dispersed desired polymer. The molecular weight is indeter-mined, but the equivalent weight is about 900 to 1,600 preferably 1,100 to 1,400, and the percentage of PSEPVE or corresponding compound is about 10 to 30%, preferably 15 to 20% and most pre-ferably about 17%. The unhydrolyzed copolymer may be compression molded at high temperature and pressure to produce sheets or mem-branes, which may vary in thickness from 0.02 to 0.5 mm. These are then further treated to hydrolyze pendant -S02F groups to -S02H
groups, as by treating with 10% sulfuric acid or by the methods of the patents previously mentioned. The presence of the -S02H groups may be verified by titration, as described in the Canadian patent.
Additional details of various processing steps are described in Canadian patent 752,427 and U.S. patent 3,041,317.
Because it has been found that some expansion accompanies hydrolysis of the copolymer it is preferred to position the co-polymer membrane after hydrolysis onto a frame or other support which will hold it in place in the electrolytic cell. Then it may be clamped or cemented in place and will be true, without sags. The membrane is preferably joined to the backing tetrafluoroethylene or other suitable filaments prior to hydrolysis, ~hen it is still thermo-plastic, and the film of copolymer covers each filament, penetrating into the spaces between them and even around behind them, the films becoming slight7y thinner in the process, where they cover the filaments.
The membrane described is far superior in the present processes to all other previously suggested membrane materials.
It is more stable at elevated temperatures, e.g., about 75C.
Especially, when protected from physical damage and iron salt build-up in ~ccordance with the present invention it lasts for much longer time periods in the medium of the electrolyte and the caustic product and does not become brittle when subjected ts chlorine at high temperatures. Considering the savings in time and fabrication costs, the present membranes are more economical. The voltage drop through the membranes is acceptable and does not become inordinately high, as it does with many other membrane materials, when the caustic concentration in the cathode -compartment increases to above about 200 9./1. of caustic. The selectivity of the membrane and its compatibility with the electrolyte does n~t decrease detrimentally as the hydroxyl con-centrat;on in the catholyte liquor increases, as has been noted with other membrane mater;als. Furthermore, the caustic effi-ciency of ~he electrolysis does not diminish as significantly as it does with other membranes when the hydroxyl ion concent-ration in the catholyte increases. Thus, these differencesin the present process make it practicable, whereas previously described processes have not attained commercial acceptance.
While the more preferred copolymers are those having equivalent weights of 900 to 1,600 with 1,100 to 1,400 being most preferred, ~ -some useful resinous membranes produced by the present method may be of equivalent weights from 500 to 4,000. The medium equivalent weight polymers are preferred because they are of satisfactory strength and stability, enable better selective ion d~
exchange to take place and are of lower internal resistances, all of which are important to the present electrochemical cell.
Improved versions of ~he above-described copolymers may be made by chemical treatment of surfaces thereof, as by treatments to modify the -S03H group thereon. For example, the sulfonic group may be altered or may be replaced in part with other moieties. Such changes may be made in manufacturing process or after production o the membrane. When effected as a subsequent surface treatment of a membrane the depth of treatment will usually be from 0.001 to 0.01 mm.
Caustic effic;encies of the invented processes, using such modified versions of the present improved membranes, can increase about 3 to 20%, often about 5 to 15%. One such modification is described in French Patent 2,152,194. This patent discloses membranes consisting of a film of a fluorinated polymer having pendant side chains con-taining sulfony1 groups attached to carbon atoms bearing at leastone fluorine atom9 the majority of the sulfonyl groups one on surface of the film being in the form of -(S02NH2)mQ groups where Q is H, NH4, alkali or alkaline earth metal and m is the valence of Q and the sulfonyl groups on the other surface o~ ~he film belng in the form of -(S03)nMe groups where Me is a cation and n is the valence of Me, with the proviso the Me is H, Q is H.
In addition to the copolymers previously discussed, including mod;fications thereof, it has been found that another type of membrane material is also superior to prior art films for applications in the present processes. Although it appears that tetrafluoroethylene (TFE) polymers which are sequentially styrenated and sulfonated are not useful for making satisfactory cation-active 7~
permselective membranes for use in the present electrolytic process it has been established that perfluorinated ethylene propylene polymer (FEP) which is styrenated and sulfonated makes a useful membrane. Whereas useful lives of as much as three years or more (that of the preferred copolymers) may not be obtained the sulfostyrenated FEP's are surprisingly resistant to hardening and other wise failing in use under the present process conditions.
To manufacture the sulfostyrenated FEP membranes a standard FEP, such as manufactured by E. I. DuPont de Nemours & Co., Inc., is styrenated and the styrenated polymer is then sulfonated.
A solution of styrene in methylene chloride or benzene at a suitable concentration in the range of about 10 to 20% is prepared and a sheet of FEP polymer having a thickness of about 0.02 to 0.5 mm., pre-ferably 0.05 to 0.15 mm., is dipped into the solution. After removal it is subjected to radiation treatment, using a cobalt60 radiation source. The rate of application may be in the range of about 8,000 rads/hr. and a total radiation application is about 0.9 megarads.
After rinsing with water the phenyl rings of the styrene portion of the polymer are monosulfonated, preferably in the para position, by treatment with chlorosulfonic acid, fuming sulfuric acid or S03.
Preferably, chlorosulfonic acid in chloroform is utilized and the sulfonation is completed in about 1/2 hour.
Examples of useful membranes made by the described process are products of RAI Research Corporation, Hauppauge, New York, identified as 18ST12S and 16ST13S, the former being 18% styrenated and having 2/3 of the phenyl groups monosulfonated and the latter being 16% styrenated and having 13/16 of the phenyl groups mono-sulfonated. To obtain 18% styrenation a solution of 17-1/2% of styrene in methylene chloride is utilized and to obtain the 16%
styrenation a solution of 16% of styrene in methylene chloride is employed.
The products resulting compare favorably with the pre-ferred copolymers previously described, giving voltage drops ofabout 0.2 volt each in the present cells at a current density of 2 amperes/sq. in., the same as is obtained From the copolymer.
Desirably, these membranes are utilized in the form of a thin film, either as such, or deposited on an inert support or substrate, such as a cloth woven of polytetrafluoroethylene, glass fibers or the like. The thickness of such a supported membrane can be varied considerably, thicknesses of from about 3 to l5 mils being typical. These membr~nes may be fabricated into any desired shape, depending upon the configuration of the cell in which they are used. As has been noted, the membrane copolymer is initially obtained in a non-acid form, i.e., in the form of the sulfonyl fluoride. In this non-acid form, it is relatively soft and pliable and can be seam or butt welded to form welds which are as strong as the membrane material itself. Accordingly, it is preferred that the membrane material be shaped and formed in this non-acid state.
Once the membrane has been shaped or formed into the desired configuration, it is then conditioned for use by hydrolyzing the sulfonyl fluoride groups to free sulfonic acid or alkali metal sulfonate groups, for example by boiling in water or alkaline solution, such as caustic solution. This conditioning process may be carried out either before the membrane is placed in the cell or within the cell with the membrane in place. Typically, _ 1~ _ when the memb~ane is boiled in water for about 16 hours, the material undergoes swelling of about 28%, about 9% in each direction. Upon exposure to brine, during operation, the swelling is reduced to about 22%, resulting in a net tightening of the membrane during use.
In some instances, it has been found that it may be desirable to use a "sandwich" of two or more of these membranes, rather than only a single membrane. When such a sandwich is used in a chlor-alkali cell, ;t has been found that in some instances 0 there is an increase in the caustic efficiency of the cell, particularly when operating at catholyte liquor caustic con-centrations in excess of about 200 grams per liter. With the electrolytic cells of the present invention however, this increase in caustic efficiency may not be sufficiently great as to offset the increased material cost o~ using such a membrane sandwichO
Accordingly, although the use of such a sandwich is possible in the present cell, it may not always be preferred.
The anode compartment of each set or unit of compartments is fGrmed with an inlet for introducing a liquid electrolyte into the compartment, such as an aqueous alkali metal halide brine and an outlet for gaseous reaction products, such as chlorine. The cathode compartment of each set or unit is formed with an outlet for liquid reaction products9 such as aqueous solutions of alkali metal hydroxide, and also an outlet for gaseous by-products, such as hydrogen. If desired, the cathode compartment may also be formed with an inlet for a liquid electrolyte, such as water, dilute alkali metal hydroxide solutions, or the like. Additionally, each of the buffer compartments between the anode and cathode compartments, _ 19 _ .~
when present, i~ formed with an inlet for liquid electrolyte~ ~uch as water and, if desired may also have an outlet for liquid reaction products, such as dilute alkali meta1 hydroxide solutions. Prefer-ably, the inlets for liquid materials and the outlets for gaseous products in each of the compartments are located in the upper portion of the compartment while the outlets for liquid materials are po-sitioned in the lower portion of the compartments, although other locations may also be used.
These repeating sets or units of anode, buffer (when present), and cathode compartments may be formed into the total electrolytic cell of the present invention in any convenient manner. Thus, in a preferred embodiment, the cell is of the so-called "filter press"
type. In this embodiment, the anodes, cathodes, porous barriers9 and membranes are mounted in suitable mounting or frame members which are provided with suitable sealing gaskets and are formed so as to provide the desired spacing between the elements to form the anode, cathode and buffer compartments. These frame members are provided with the desired inlets and outlets, as have been described and are secured together by tie rods, bolts, or other suitable means as is known in the art. Typical of such a filter press configuration is that shown in U.S. Patent 2,282,058.
Alternatively, the cell body may be in the form of a box of a suitable material of construction in which anode, cathode and membrane are mounted to form the various compartments, such as that shown in U.S. Patent 3,32~,023. Additionally, the cell may be of the "conventional" chlor-alkali type having interleaved anodes and cathodes, wherein the deposited asbestos diaphragm is replaced -with the various membranes as have ~een described, to form the desired buffer compartments. Typical of such a cell structure is that shown in U.S. Patent 3,458,~11.
It is to be appreciated that the above are mere1y exemplary of the various cell con~igurations which may be used.
In all of these, of course, suitable materials of construction wil1 be used, as have been described hereinabove, and in each, there will be inserted between the cathode and the cation-active membrane, a porous chemically resistant barrier as described here-inabove. Additionally, it is further to be appreciated that theparticular configuration used in each instance will depend upon the various specific requirements for that particular cell.
In carrying out the process of the present invention, a solution of the ionizable compound to be electrolyzed is intro-duced into the anode compartment of the electrolytic cell. Ex-emplary of the various solutions of ionizable compounds which may be electrolyzed and the products produced are aqueous solutions of alkali metal halides to produce the alkali metal hydroxides and halogen; aqueous solutions of HCl to produce hydrogen and chlorine;
aqueous solutions of ammonium sulfate to produce persulfates;
aqueous solutions of borax to produce perborates, and the like.
Of these, the most preferred anoly~e solutions are the aqueous solutions of alkali metal halides, and particularly sodium chloride, and aqueous solutions of HCl.
In a typical process, utilizing a sodium chloride brine as the feed to -the anode compartment, the feed solution will contain from about 250 to 32~ grams per liter sodium chloride and, most preferably, about 320 grams per liter sodium chloride. The pH of this anolyte feed solution is typically within the range of about l.O to lO.O, with a pH of about 3.5 being preferred. These desired pH values in the anode compartment may be maintained by the addition of acid to the anolyte solution, preferably hydrochloric acid.
The anolyte overflow or depleted anolyte solution removed from the anode compartment will generally have a sodium chloride content of from about 200 to 295 grams per liter, with a sodium chloride content of about 250 grams per liter being typical.
l O In a three compartment cell such as shown in Figure 2, i.e., a cell having one or more repeating units of an anode com-partment and a cathode compartment separated by a single center or buffer compartment, water is introduced into the center or buffer compartment and a dilute solution of sodium hydroxide is removed from this compartment. Generally, this solution will have a sodium hydroxide content of from about 50 to 200 grams per liter with a sodium hydroxide content of about lOO grams per liter being typical. Preferablyg this dilute solution of sodium hydroxide is introduced into the cathode compartment, either with or without additional water, to form the catholyte liquor. From the cathode compartment there is obtained a more concentrated sodium hydroxide solution, having NaOH concentration bf from about 150 to 250 grams per liter, with the sodium hydroxide content of about l60 grams per liter being typical. Additionally, gaseous products of chlorine gas and hydrogen gas are obtained from the anode compartment and the cathode compartment~ respectively.
In an ~lternative method of operation, water is added to both the center or buffer compartment and to the cathode com-partment and there is recoverd from the buffer compartment a ~7~ 7a:~
product stream of dilute sodium hydroxide and, from the cathode compartment9 a product stream of more concentrated sodium hydroxide solution. When operating in this manner, the amount of dilute caustic soda solution recovered from the buffer compartment and the amount of concentrated caustic soda solution recovered from the cakhode compartment may be varied, depending upon the particular requirements for each type of solution. In a typical operation, approximately 50% of the sodium hydroxide will be recovered as a dilute solution from the buffer compartment with the other 50%
being recovered as ~he more concentrated solution from the cathode compartment. The concentration of the dilute caustic soda solution will generally be within the range of about 50 to 200 grams per liter with a concentration of about 100 grams per liter being typical. Similarly, the concentration of the more concentrated caustic solution from the cathode compartment will generally be within the range of about 200 to 420 grams per liter with a concentration of about 280 grams per liter being typical.
The electrochemical decomposition process of the present invention is typically carried out at ~ voltage within the range of about 3.4 to 4.9, with a voltage of about 4.2 being preferred.
Typically, the current densities are within the range of about 0.8 to 2.5 amps per square inch, with current densities of about 2 amps per square inch being particularly preferred. In general, the cell will be operated at temperatures within the range of about 90 to 105 degrees centigrade with temperatures of about 95 degrees centigrade being typical. When operating in this manner, it is found that chlorine or anode efficiencies of at least about 96%
~7~
and cathode or caustic soda efficiencies of at least 85% and frequently in excess of 90% are obtained. Additionally, the concentrated caustic soda solution obtained from the cathode compar~ment is found to be of high purity, at least approaching, if not equal, that of "Rayon grade" caustic soda. Typically, the purity of this sodium hydroxide is such that it is substan-tially free of sodium chlorate and contains less than one gram per liter of sodium chloride.
Where the process is carried out with cells having sections or repeating units which contain two or more buffer compartments9 the operation is similar to that which has been described hereinabove. Thus, water may be introduced into each of the buffer compartments and into the cathode compartment and a portion of the sodium hydroxide product values may be recovered from each of the buffer compartments, as a dilute solution of sodium hydride and from the cathode compartment as a more concen-trated sodium hydroxide solution. Preferably, however, the dilute sodium hydroxide solutions from each buffer compartment is introduced as at least a portion of the feed to the next succeeding buffer compartment9 and ultimately into the c~thode compartment so that there is obtained from the cathode compartment a concentrated sodium hydroxide product stream of high purity.
As has been indicated hereinabove, in addition to the electrolysis of sodium chloride brine solution9 to produce ch10rine and caustic soda, in another preferred operation, the electrolytic cells of the present invention may be used for the electrolysis of hydrochloric acid solutions, to form chlorine and hydrogen as the products of the process. In such an operation, the anolyte solution ` ~ \
introduced into the anode compartment is an aqueous solution of hydrochloric acid, desirably having an HCl content of from about 10% to 36% by weight and preferably having an HCl content of from about 15 to 25% by weight. Although the feed to the buffer com-partrnents, if present, and the cathode compartment may be wateralone, in the most preferred method of operation, the feed to both the buffer compartments and the cathode compartment is also an aqueous hydrochloric acid solution. Desirably, the ~Cl content o~ these feed solutions is from about l to 10% by weight with an HCl content of from about l to 5% by weight being preferred. A1-though it is preferred that the feed solution to the anode, buffer, when present, and cathode compartments be substantially free of contaminating ions, in many instances it has been found to be desirable to add alkali metal chlorides, such as sodium chloride to the anolyte, in order to minimize corrosion, particularly where a steel or similar corrodible cathode is used. In these instances, additions of sodium chloride in amounts within the range of about 12 to 25% by weight of the anolyte solution are typical.
In order that those skilled in the art may better under-stand the present invention and the manner in which it may be practiced, the following specific examples are given. In these examples, unless otherwise indicated, temperatures are in degrees centigrade and parts and percent are by weight. It is to be appreciated, however, that these examples are merely exemplary of the method and apparatus of the present invention and are not to be taken as a limitation thereof.
.
~7~ 7~
EXAMPLE I
A three comp~rtment pilot plant sized cell, of the type shown in Figure 2, not including the porous barrier was operated at 2400 amperes, an anode current density of 2 amps/sq.
inch and an average voltage of 4.9 volts. The cell was equipped with a metallic anode formed of titanium mesh with a ruthenium oxide coating, a steel mesh cathode, and two cation-active perm-selective membranes, one on each side separating the buffer compartment from the anode compartment and the cathode compartment.
The membranes were, each, a lO mil thick film of a hydrolyzed copolymer of tetrafluoroethylene and sulfonated perfluorovinyl ether, having an equivalent weight of about llO0 and prepared according to U.S. Patent 3,282,875.
Brine containing about 320 gpl NaC1 was circulated through the cathode compartment and water was added to both the buffer and cathode compartments. The pH of the anolyte was maintained at about 4.0 by the addition of hydrochloric acid to the circulating liquor.
The cell was operated under substantially constant conditions for about 72 hours (3 days) and thereafter the cell was disassembled. The membrane separating the buffer compartment from the cathode compartment was examined. This examination showed pin hole development at several points in the membrane and each of the points was stained with iron salts arising from contact of the membrane with the meta1 cathode.
EXAMPLE II.
The disasserrlbled cell of Example I was reassembled with new 10 mil thick membranes. In addition, two layers of l/8 inch .
.
~7~
thick polypropylene mesh screen, having a diamond shaped one inch open weave were overlaid so that the mesh pattern of one layer was offset with reference to the pattern of the other layer, were placed between the bare steel mesh cathode face and the membrane surface 5 facing the cathode.
The cell was operated under substantially the same conditions set out in Example I, above, for twenty days. Thereafter, the cell was again disassembled and the membrane on the cathode side of the buffer compartment was examined. Inspection of the membrane showed no evidence of pin holing or other physical damage. Further there was no evidence of the build-up of iron or other metallic salts on the surface of the membrane.
While there has been described various embodiments of the invention, the methods and apparatus described are not intended to be understood as limiting the scope of the invention as changes therewithin are possible, and it is intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever ~orm its principle may be utilized.
Claims
THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
An electrolytic cell which comprises a cell body having an anode compartment containing an anode and a cathode compartment con-taining a cathode, said compartments being separated by at least one cation active permselective membrane selected from the group consisting of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a sulfo-nated perfluorovinyl ether and a sulfostyrenated perfluorinated ethyl-ene propylene polymer, which is impervious to liquids and gases, said cathode and said membrane being separated by a porous chemically inert protective barrier the improvement wherein the porous chemically inert protective barrier is mounted on the cathode.
A cell described in Claim 1 wherein said porous protective barrier is of synthetic organic polymeric plastic material.
A cell as described in Claim 2 wherein said material is polypropylene.
A cell as described in Claim 3 wherein said polypropylene material is a polypropylene mesh having a diamond shape open weave.
A cell as described in Claim 1 wherein said membrane is a hydrolyzed copolymer of tetrafluoroethylene and a sulfonated perfluoro-vinyl ether having the formula FSO2CF2CF2OCF(CF3)CF2OCF=CF2 which copolymer has an equivalent weight of from about 900 to about 1,600.
A cell as described in Claim 1, wherein said copolymer has an equivalent weight of from about 1100 to about 1400 and contains from about 10 to 30% of the ether compound.
A cell as described in Claim 1, wherein said membrane is a sulfostyrenated perfluorinated ethylene propylene polymer.
A cell as described in Claim 7, wherein said polymer is sty-renated to from about 16 to 18 percent by weight and from about 2/3 to 13/16 of the phenyl groups are monosulfonated.
A cell as described in Claim 8, wherein said porous pro-tective barrier is fabricated from polypropylene.
A cell as described in Claim 1, wherein said cell is a two compartment electrolytic cell and the anode and cathode compartments are separated by a single cation active permselective membrane.
A cell as described in Claim 1, wherein the cell is formed with at least one buffer compartment between the anode compartment and cathode compartment said compartments being separated from each other by cation active permselective membranes.
A process for the electrochemical decomposition of an ionizable chemical compound which comprises providing an electrolytic cell which comprises a cell body having an anode compartment containing an anode and a cathode compartment containing a cathode, said compartments being separated by at least one cation active permselective membrane selected from the group consisting of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a sulfonated perfluorovinyl ether and a sulfostyrenated perfluorinated ethylene propylene polymer, which is impervious to liquids and gases, said cathode and said membrane being separated by a porous chemically inert protective barrier, mounted on said cathode, introducing an aqueous solution of an ionizable chemical compound into the anode compartment of an electrolytic cell, introducing an aqueous solution of dilute alkali metal hydroxide into the cathode compartment of said cell, and effecting the electrolytic decomposition of said ionizable chemical compound by passing an electric current between the anode and cathode of said cell.
13. The process as claimed in claim 12, wherein the aqueous solution of an ionizable chemical compound is an aqueous solution of an alkali metal halide.
14. The process as claimed in claim 13, wherein the alkali metal halide is sodium chloride, chlorine is produced as the electrolytic decomposition product at the anode, the alkali metal hydroxide is sodium hydroxide, and a concentrated solution of sodium hydroxide, which is substantially free from sodium chloride, is produced as an electrolytic decomposition product at the cathode.
15. The process as claimed in claim 14, wherein the sodium chloride solution introduced into the anode compartment contains hydrochloric acid sufficient in amount to maintain a pH of the sodium chloride solution at between about 1.0 and about 10.
16. The process as claimed in claim 15, wherein the pH of the sodium chloride solution is maintained at about 3.5.
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
An electrolytic cell which comprises a cell body having an anode compartment containing an anode and a cathode compartment con-taining a cathode, said compartments being separated by at least one cation active permselective membrane selected from the group consisting of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a sulfo-nated perfluorovinyl ether and a sulfostyrenated perfluorinated ethyl-ene propylene polymer, which is impervious to liquids and gases, said cathode and said membrane being separated by a porous chemically inert protective barrier the improvement wherein the porous chemically inert protective barrier is mounted on the cathode.
A cell described in Claim 1 wherein said porous protective barrier is of synthetic organic polymeric plastic material.
A cell as described in Claim 2 wherein said material is polypropylene.
A cell as described in Claim 3 wherein said polypropylene material is a polypropylene mesh having a diamond shape open weave.
A cell as described in Claim 1 wherein said membrane is a hydrolyzed copolymer of tetrafluoroethylene and a sulfonated perfluoro-vinyl ether having the formula FSO2CF2CF2OCF(CF3)CF2OCF=CF2 which copolymer has an equivalent weight of from about 900 to about 1,600.
A cell as described in Claim 1, wherein said copolymer has an equivalent weight of from about 1100 to about 1400 and contains from about 10 to 30% of the ether compound.
A cell as described in Claim 1, wherein said membrane is a sulfostyrenated perfluorinated ethylene propylene polymer.
A cell as described in Claim 7, wherein said polymer is sty-renated to from about 16 to 18 percent by weight and from about 2/3 to 13/16 of the phenyl groups are monosulfonated.
A cell as described in Claim 8, wherein said porous pro-tective barrier is fabricated from polypropylene.
A cell as described in Claim 1, wherein said cell is a two compartment electrolytic cell and the anode and cathode compartments are separated by a single cation active permselective membrane.
A cell as described in Claim 1, wherein the cell is formed with at least one buffer compartment between the anode compartment and cathode compartment said compartments being separated from each other by cation active permselective membranes.
A process for the electrochemical decomposition of an ionizable chemical compound which comprises providing an electrolytic cell which comprises a cell body having an anode compartment containing an anode and a cathode compartment containing a cathode, said compartments being separated by at least one cation active permselective membrane selected from the group consisting of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a sulfonated perfluorovinyl ether and a sulfostyrenated perfluorinated ethylene propylene polymer, which is impervious to liquids and gases, said cathode and said membrane being separated by a porous chemically inert protective barrier, mounted on said cathode, introducing an aqueous solution of an ionizable chemical compound into the anode compartment of an electrolytic cell, introducing an aqueous solution of dilute alkali metal hydroxide into the cathode compartment of said cell, and effecting the electrolytic decomposition of said ionizable chemical compound by passing an electric current between the anode and cathode of said cell.
13. The process as claimed in claim 12, wherein the aqueous solution of an ionizable chemical compound is an aqueous solution of an alkali metal halide.
14. The process as claimed in claim 13, wherein the alkali metal halide is sodium chloride, chlorine is produced as the electrolytic decomposition product at the anode, the alkali metal hydroxide is sodium hydroxide, and a concentrated solution of sodium hydroxide, which is substantially free from sodium chloride, is produced as an electrolytic decomposition product at the cathode.
15. The process as claimed in claim 14, wherein the sodium chloride solution introduced into the anode compartment contains hydrochloric acid sufficient in amount to maintain a pH of the sodium chloride solution at between about 1.0 and about 10.
16. The process as claimed in claim 15, wherein the pH of the sodium chloride solution is maintained at about 3.5.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US42099373A | 1973-12-03 | 1973-12-03 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1071570A true CA1071570A (en) | 1980-02-12 |
Family
ID=23668733
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA215,183A Expired CA1071570A (en) | 1973-12-03 | 1974-12-02 | Prevention of metallic salt build-up in permselective membranes |
Country Status (2)
| Country | Link |
|---|---|
| JP (1) | JPS5086497A (en) |
| CA (1) | CA1071570A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN114008848A (en) * | 2019-06-14 | 2022-02-01 | 罗兰大学 | Polypropylene-or polyethylene-based separator for use in electrochemical cells for the production of alkali metal ferrate |
-
1974
- 1974-11-15 JP JP49132453A patent/JPS5086497A/ja active Pending
- 1974-12-02 CA CA215,183A patent/CA1071570A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN114008848A (en) * | 2019-06-14 | 2022-02-01 | 罗兰大学 | Polypropylene-or polyethylene-based separator for use in electrochemical cells for the production of alkali metal ferrate |
Also Published As
| Publication number | Publication date |
|---|---|
| JPS5086497A (en) | 1975-07-11 |
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