CA1059942A - Process and apparatus for electrolysis - Google Patents

Abstract

Abstract of the Disclosure An electrolytic cell, suitable for use in electrolyzing alkali metal halide brines, which comprises a cell body having an anode compartment containing an anode, a cathode compartment con-taining a cathode and at least one buffer compartment between said anode and cathode compartments, said compartments being separated from each other by a carrier located between the anode and buffer compartments, cathode and buffer compartments and between individual buffer compartments, said barrier being substantially impervious to fluids and gases, selected from a hydrolyzed copolymer of tetra-fluoroethylene and a sulfonated perfluorovinyl ether having the formula:
FSO2CF2CF2OCF(CF3)CF2OCF=CF2 said copolymer having an equivalent weight of from about 900 to 1600, and a sulfostyrenated perfluorinated ethylene propylene polymer.

Description

.
'' ~oss942 This invention relates to an improved process and apparatus for the electrolysis of alkali metal halide brines and more particularly it relates to a process for the electrolysis of alkali halide brines in an electrolytic cell having at least three compartments, which cell utilizes a diaphragm or membrane which is substantially impervious to fluids and gases.
The production of numerous commercial chemicals by the electrolysis of various electrolyte solutions is well known. For example, chlorine and caustic soda are produced commercially by the electrolysis of sodium chloride brine solutions. Typically, this process is carried out in an electrolytic cell having an anode compartment and a cathode compartment, which compartments are separated by a fluid-permeable diaphragm, such as an asbestos dia-phragm The sodium hydroxide produced by this method is, however, relatively dilute and, because of the fluid permeable nature of the diaphragms used, it is further contaminated with various impurities, such as sodium chloride, sodium chlorate, iron and the like. It is, therefore, necessary to subject the sodium hydroxide product to various evaporation and purification steps in order to obtain a product which is suitable for many commercial uses. Moreover, with such electrolytic cells, there is an appreciable back migration of hydroxyl ions from the cathode compartment to the anode compartment which results in the production of hypochlorites wh;ch are oxidized to chlorates, with a consequent reduction in chlorine yield and further contamination of the sodium hydroxide. Additionally, depending upon the source of sodium chloride used in making up the brine electrolyte, brine purification systems must frequently be used to eliminate ions such as calcium, that may clog the fluid permeable diaphragms.

i Attempts have heretofore been made to overcome the afore-said difficulties in the operation of such d;aphragm cells by re-placing the fluid permeable asbestos diaphragms with permselective ion exchange membranes. In theory, the use of such membranes which, for example, would permit the passage of only sodium ions from the anode compartment to the cathode compartment, would eliminate the problems of contamination of the sodium hydroxide liquor in the cathode compartment and would prevent the back migration of the hydroxyl ions to the anode compartment. For this purpose, various resins, such as cation exchange resins of the "Amberlitei' (TM) type, sulfonated copolymers of styrene and divinylbenzene, and the like, have been proposed. In practice, however, the permselective ion ex-change membranes which have been used have generally been found not to be stable to the strong caustic and/or acidic solutions encountered in the cells at operating temperatures above 75 degrees C. so that they have had only a relatively short çffective life. Additionally, as the concentration of caustic soda in the catholyte liquor is increased, e.g., above about 200 grams per liter, it has frequently been found that the ion selectivity and chemical compatibility of the membrane decreases, the voltage drop through the membrane becomes unacceptably high and the caustic efficiency of the electrolysis process decreases.
Moreover, in many instances, the resins which have been used have been found to be relatively expensive so that the fabrication costs of the membrane has been unacceptably high. Attempts to overcome these drawbacks by utilizing one or more buffer compartments between the anode and cathode compartments of the cells have not solved the problem so that at the present time, there has been no appreciable utilization of membranes of this type for the commercial production of ~05994Z
various chem;cals, such as chlorine and caustic soda.
It is, therefore, an object of the present invention to provide an improved apparatus suitable for the electrolysis of alkali metal halide brines.
Another object of the present invention is to provide an improved process for electrolyzing aqueous solutions of ionizable chemical compounds, such as alkali metal halide brines, which is not subject to many of the disadvantages which have heretofore been encountered in the prior art processes.
A further object of the present invention is to provide an improved electro1ysis apparatus which utilizes ion selective membranes and to provide a process for electrolyzing alkali metal halide brine using such apparatus.
These and other objects will become apparent to those skilled in the art from the description of the invention which follows.
In the drawing which is attached hereto and forms a part hereof, Figure 1 is a schematic representation of a three compartment electrolytic cell of the present invention and;
Figure 2 is a schematic representation of a modification of the electrolytic cell shown in Figure 1.
Pursuant to the above objects, the present invention includes an electrolytic cell, suitable for use in electrolyzing alkali metal halide brines, which comprises a cell body having an anode compartment containing an anode connected to an electrical input source, a cathode compartment containing a cathode and at least one buffer compartment between said anode and cathode compartments, said compartments being separated from each other by at least one side wall barrier located between the anode and buffer com-partments, the cathode and buffer compartments and between individual buffer compartments, said barriers being substantially impervious to fluids and gases selected from a hydrolyzed copolymer of a perfluorinated hydrocarbon and a sulfonated perfluorovinyl ether and a sulfostyrenated perfluorinated ethylene propylene polymer. By the use of electrolytic cells of this type, it is found that highly concentrated alkali metal hydroxide solutions, which are significantly low in impurities, can be produced with maxi-mum electrical operating efficiency.
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 are 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 varieties of the polymers are included within the generic description 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 temperature 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 resins, and the like.
Additionally, materials such as concrete, cement, and the like, may also 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 insulation is provided where necessary.

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The electrodes for the present electrolytic cell ~ay be formed of any electrically conduct;ve material which will resist 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 wefl 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 oxides, or mixtures of such oxides, either alone or with oxides of other metals. The nsble metals which may be used include ruthenium, rhodium, pallad;um, ;ridium, and platinum. Particularly preferred metal anodes are those formed of titanium and having a mixed 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 like.
The cell body or container is formed into at least one set or unit of compartments made up of an anode compartment, containing the anode, a cathode compartment, containing the cathode, and 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 barrier or membrane which is substantially impervious to fluids and gases and composed essentially of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether. The perfluorinated hydrocarbon is preferably tetrafluoroethylene, although other perfluorinated and sat-urated and unsaturated hydrocarbons of 2 to 5 carbon atoms may also be utilized, of which the monoolefinic hydrocarbons are preferred, ,~,.,, ~;
.3~. J

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especially those of 2 to 4 carbon atoms and most especially those of 2 to 3 carbon atoms, e.g., tetrafluoroethylene, hexafluoropropylene. The sul-fonated perfluorovinyl ether which is most useful is that of the formula FS02CF2CF20CF(CF3)CF20CF=CF2. Such a material, named as perfluoro ~2(2-fluorosulfonylethoxy)-propyl vinyl ether], referred to henceforth as PSEPVE, may be modified to equivalent monomers, as by modifying 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 10 of the perfluoro-lower alkyl groups, respectively. However, it is most preferred to employ PSEPVE .
The method of manufacture of the hydrolyzed copolymer is des-cribed 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 15 the finished membrane in fuel cells, characterized therein as electro-chemical cells. In short, the copolymer may be made by reacting PSEPVE
or equivalent with tetrafluoroethylene or equivalent in desired propor-tions in water at elevated temperature and pressure for over an hour, after which time the mix is cooled. It separates into a lower perfluoroether 20 layer and an upper layer of aqueous medium with dispersed desired polymer.
The molecular weight is indeterminate 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 preferably about 17%. The unhydrolyzed copolymer may be compression mold-~5 ed at high temperature and pressure to product sheets or membranes, whichmay vary in thickness from 0.02 to 0.5 mm. These are then further treated to hydrolyze pendant -S02F groups to -S03H groups, as by treating with 10% sulfuric acid or by the methods of the paten~ previously mentioned.
The presence of the -S03H 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 ~05994Z
Because it has been found that some expansion accompanies hydrolysis of the copolymer it is preferred to position the copolymer membrane after hydrolysis onto a frame or other support which will hold it ~n 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 filaments of a tetrafluoroethylene backing or other suit-able filaments prior to hydrolysis, when it is still thermoplastic; and the film of copolymer covers each filament, penetrating into the spaces between them and even around behind them, the films becoming slightly 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., abo~e 75C. It lasts for much longer time periods in the medium of the electrolyte and the caustic product and does not become brittle when subjected to chlorine at high cell tempe-ratures. Considering the savings in time and fabrication costs, the present membranes are more economical. The voltage drop through the membrane 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./l. of caustic. The selectivity of the membrane and its compatibility with the electrolyte does not decrease detrimentally as the hydroxyl concentration in the catholyte liquor increases, as has been noted with other membrane mate-rials. Furthermore, the caustic efficiency of the electrolysis does not diminish as significantly as it does with other membranes when the hy-droxyl ion concentration in the catholyte increases. Thus, these differ-ences in the present process make it practicable, whereas previously described processes have not attained commercial acceptance. While the more preferred copolymer 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 1059~4Z

method may be of equivalent weights from 500 to 4,000. The medium equivalent weight polymers are preferred because they are of satis-factory strength and stability, enable better selective ion exchange to take place and are of lower internal resistances, all of which are important to the present electrochemical cell.
Improved versions of the 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 10 may be made in the manufacturing process or after productlon of 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 efficiencies of the invented processes, using such modified versions of the present improved membranes, can increase about 3 to 20%, often about 5 to 15%.
In addition to the copolymers previously discussed, including modifications 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 permselective membranes for use in the present electrolytic processes it has been established that per-fluorinated 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 hardenin3 and otherwise 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 ~ 1059942 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., preferably 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 appli-cation 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 10 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 monosulfonated. To obtain 18% sty-renation a solution of 17-1/2% of styrene in methylene chloride is uti-lized and to obtain the 16% styrenation a solution of 16% of styrene in methylene chloride is employed.
The products resulting compare favorably with the preferred co-polymers previously described, giving voltage drops of about 0.2 volt eachin 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 15 mills being typical. These membranes 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 co-polymer 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 pl~able 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, when the membrane 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, it has been found that in some instances there is an increase in the caustic efficiency of the cell, particularly when operating at catholyte liquor caustic concentrations in excess of about 200 grams per liter.
With the electrolytic cells of the present invention which have one or more buffer compartments between the anode and cathode compartment, how-ever, this increase in caustic efficiency may not be sufficiently great as to offset the increased material cost of using such a membrane sandwich. 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 formed with an inlet for introducing a liquid electrolyte into the com-partment, such as an aqueous alkali metal halide brine and an outlet for gaseous reaction products, such as chlorine. The cathode compartment J~059942 of each set or unit is formed with an outlet for liquid reaction products, such as aqueous solutions of alkali metal hydroxide, and also an outlet for gaseous by-products, such as hydrogen. If desired, the cathode com-partment may also be formed with an inlet for a liquid electrolyte, such as water, dilute alkali metal hydroxide solutions, or the like. Addi-tionally, each of the buffer compartments between the anode and cathode compartments is formed with an inlet for liquid electrolytes, such as water and, if desired may also have an outlet for liquid reaction pro-ducts, such as dilute alkali metal hydroxide solutions. Preferably, 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 positioned in the lower portion of the compartments, although other locations may also be used.
These repeating sets or units of anode, buffer, and cathode lS compartments may be formed into the total electrolytic cell of the present invention in any convenient manner. Thus, in a preferred embodi-ment, the cell is of the so-called "filter press" type. In this embodi-ment, the anodes, cathodes, 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 3,324,023. Additionally, the cell may be of the "conven-tional" chlor-alkali type having interleaved anode and cathodes, ~05994Z

wherein the deposited asbestos, diaphragm is replaced with the various membranes as have been described, to form the desired buffer cornpartments.
Typical of such a cell structure is that shown in U.S. patent 3,458,411.
It is to be appreciated that the above are merely exemplary of the various cell configurations which may be used. In all of these, of course, suitable materials of construotion will be used, as have been described hereinabove. Additionally, it is further to be appreciated that the particular configuration used in each instance will depend upon the various specific requirements for that particular cell.
Referring now to the drawings, in Figure 1, which is a sche-matic representation of a three compartment cell of the present invention, the cell body is shown at (1). The cell body (1) is formed into an anode compartment (3), a cathode compartment (7) and a buffer compartment (11) which separates the anode and cathode compartments. An anode (5) and a cathode (9) are disposed within the anode and cathode compartments, res-pectively. Forming the buffer compartment (11) and separating it from the anode compartment (3) and the cathode compartment (7) are barriers or membranes (13) and (15), respectively, which barriers are formed of a hydrated cationic exchange resin membrane which a film of a fluorinated copolymer having pendent sulfonic acid groups, as has been defined here-inabove.
The anode compartment (3) is provided with an inlet (17) through which the electrolyte, such as a sodium chloride brine, is introduced.
An outlet (19) is also provided in the anode compartment, through which outlet the depleted electrolyte is removed from the anode compartment.
Additionally, the anode compartment is provided with a gas outlet (21) through which the gaseous decomposition products of the electrolysis, such as chlorine. are removed from the anode compartment. Although the electrolyte inlet and gaseous product outlets 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 lnlets and outlets may be utilized if desired.
Similarly, in the buffer compartment (11), an inlet (23) and an outlet (27) are provided. Where the electrolytic cell is utilized for the electrolysis of a sodium chloride brine to produce chlorine and caustic soda, water will be introduced into the buffer compartment through the inlet (23) and, if desired, a dilute solution of sodium hydroxide may be withdrawn from the outlet (27). Additionally, 0 the cathode compartment (7) contains an inlet (29) and an outlet (25) through which, respectively, in the preferred electrolysis of a sodium chloride brine, water or dilute caustic soda solutions are introduced and a concentrated caustic soda solution, of high purity, is recovered as a product of the process. Additionally, the cathode compartment may also contain an outlet for gaseous by-products, such as hydrogen, (not shown). As in the case of the inlets and outlets for the anode compartment, the inlets and outlets for the buffer compartment and cathode compartment may, if desired, be positioned other than in the upper and lower portions, respectiYely, of the compartments, as is shown in Figure 1.
Referring now to Figure 2, this is a schematic representation of a modification of the electrolytic cell shown in Figure 1, in which the cell is provided with more than one buffer compartment between the anode and cathode compartments~ As is shown in this Figure, the cell body (2) is formed into an anode compartment (4~ and a cathode compart-ment (8), which compartments are separated by two intermediate or buffer compartments (12) and (14). An anode (6) and a cathode (10) are posi-tioned in the anode compartment (4) and cathode compartment (8), res-pectively. A series of barriers or membranes (16~, (18) and (20) form the buffer compartments (12) and (14) and separate them from the anode compartment and the cathode compartment. All three of these membranes ~05994Z
are formed of a film of a fluorinated copolymer having pendant sulfonic acid groups, as has been described hereinabove.
An inlet t22) and an outlet (24) are provided in the anode compartment for the introduction and removal of the electrolyte, such as a sodium chloride brine. Additionally, an outlet (26) is also provided in the anode compartment for the removal of gaseous decomposition products, such as chlorine. The buffer compartments (12) and (14) are each provided with inlets (30) and (32) respectively, and outlets (36) and (38), respectively. Where the cell is utilized for the electrolysis of a sodium chloride brine, typically water will be introduced into the inlets (30) and (32) and dilute solution of caustic soda will be removed from the outlets (36) and (38). Additionally, an inlet (40) and an outlet (34) are provided in the cathode compartment (8). Where a sodium chloride brine is being electrolyzed, a concentrated solution of caustic soda of high purity will be recovered from the outlet (34) and water or a dilute caustic soda solution may be introduced through the inlet (40).
As with the cell shown in Figure 1, an outlet for gaseous decomposition products, such as hydrogen, (not shown) may also be provided in the cathode compartment. Additionally, as with the cell configuration shown in Figure 1, the positions of the various inlets and outlets may be changed, depending upon the particular mode of operation which is desired.
In carrying out the process of the present invention, a solution of the ionizable compound to be electrolyzed is introduced into the anode compartment of the electrolytic cell. Exemplary of the various solutions ~5 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 ~059942 these, the most preferred anolyte solutions are the aqueous solutions of alkali metal halides, and particularly sodium chlorlde, 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 325 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 1.0 to 10.0, with a pH of about 3.5 being preferred. These desired pH values in the 10 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.
In a three compartment cell, i.e., a cell having one or more repeating units of an anode compartment 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 100 grams per liter being typlcal.
Preferably, 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 of from about 150 to 250 grams per liter, with the sodium hydroxide content of about 160 grams per liter being typical. Addi-tionally, gaseous products of chlorine gas and hydrogen gas are obtained from the anode compartment and the cathode compartment, respectively.

In an alternative method of operation, water is added to both the center or buffer compartment and to the Gathode compartment and there is recovered from the buffer compartment a product stream of dilute sodium hydroxide and, from the cathode compartment, 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 cathode compartment may be varied, depending upon the particular requirements for each type of solution. In a typi-cal operation, approximately 50% of the sodium hydroxide will be re-covered as a dilute solution from the buffer compartment with the other 50% being recovered as the 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 o~ 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 in-vention is typically carried out at a voltage within the range of about 3.4 to 4.8, 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% and cathode or caustic soda efficiencies of at least 85% and frequently in excess of 90% are obtained. Additionally, lOS994;~
the concentrated caustic soda solution obtained from the cathode com-partment is found to be of high purity, at least approaching, it not equal, that of "Rayon grade" caustic soda. Typically, the purity of this sodium hydroxide is such that it is substantially 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 compartments, 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 hydrnxide product values may be recovered from each of the buffer compartments, as a dilute solu-tion of sodium hydroxide and from the cathode compartment as a more con-centrated 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 compartment, and ultimately into the cathode 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 electro-lysis of sodium chloride brine solutions, to produce chlorine and causticsoda, 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 compartments and the cathode compartments may be water alone, 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 HCl content of these feed solutions is .,.
"~

~059942 from about 1 to 10% by weight, with an HCl content of from about 1 to 5% by weight being preferred. Although it is preferred that the feed solution to the anode, buffer 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 understand the present invention and the manner in which it may be practiced, the following specif~c examples are given. In these examples, unless other-wise 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.
Example 1 A three compartment laboratory size cell was operated at 120 amperes, an anode current density of 2 amps/square inch and a voltage of 4.1 volts. The cell was equipped with a metallic anode formed of ~-~ titanium with an Ru02 coating, a steel cathode and two cation exchange membrane barriers, one on each side, separating the intermediate or buffer compartment from the anode compartment and the cathode com-partment. The membrane was a 10 mil thick film of a hydrolyzed co-polymer of tetrafluoroethylene and sulfonated perfluorovinyl ether, having an equivalent weight of about 1100 and prepared according to U.S. Patent 3,282,875. Brine containing 320 gramslliter NaCl was circulated through the anode compartment and water was added to both the buffer compartment and the cathode compartment. HCl was added to the anolyte to maintain the anolyte pH at about ~Ø The effluent lOS994Z
from the-buffer compartment contained about 116 grams/liter NaOH and that from the cathode compartment contained about 3~4 grams/liter NaOH.
Upon blending the two effluent streams together, there was obtained a solution which contained about 197 grams/liter NaOH and which was substantially free of sodium chlorate and contained less than about 1 gram/liter NaCl. Over a period of 16.5 hours of operation, the caustic or cathode current efficiency was about 85.7% and the chlorine or anode current efficiency was about 97%.
Example 2 lû A commercial size three-compartment cell, of the type described in Example 1, was operated at 150 KA, an anode current density of 2 amps/square inch, a voltage of 4.1 volts and a temperature of about 96C. An aqueous brine containing about 320 grams/liter NaCl was introduced into the anode compartment and an anolyte overflow was 15 obtained from the anode compartment which had a pH of about 3.5 and contained about 250 grams/liter NaCl, HCl being added as required to maintain the anolyte pH at about 3.5. Water was fed to the buffer compartment and a buffer compartment effluent was obtained whi.ch con-tained about 110 grams/liter NaOH. This effluent was fed to the cathode 20 compartment and there was produced a catholyte effluent containing about 160 grams/liter NaOH, 0.5 grams/liter NaCl and no detectable (< 0.1 grams/liter) NaC103. During the time of operation, the cathode current.
efficiency was 85% and the anode current efficiency was 96%.
Example 3 The three-compartment cell of Example 2 was operated at 150 KA, an anode current density of 2.0 amps/square inch, a voltage of 4.2 volts and a temperature of about 94C. The aqueous brine feed to the anode compartment contained about 320 grams/liter NaCl and the anolyte overflow was at a pH of about 4.0 and contained about 250 grams/
30 liter, HCl being added as required to maintain the anolyte pH at about 4Ø ~later was fed to both the buffer compartment and the cathode ~05994Z
compartment. A weak caustic effluent containing about 100 grams/liter NaOH was obtained from the buffer compartment and a strong caustic effluent containing about 280 grams/liter NaOH was obtained from the cathode compartment. The cell produced about 2.3 tons/day NaOH, 5 as the weak caustic liquor, and about 2.4 tons/day NaOH, as the strong caustic liquor, at a cathode current efficiency of 86% and an anode current efficiency of 96%.
While there have 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 there-within 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 form its principle may be utilized.

Claims

THE EMBODIMENTS OF THE INVENTION IS 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 connected to an electrical input source, a cathode compartment containing a cathode and at least one buffer compartment between said anode and cathode compartments,said compartments being separated from each other by a side wall barrier located between each of the anode and buffer compartments, the cathode and buffer compartments and between individual buffer compartments, said barriers being substantially impervious to fluids and gases, selected from a hydrolyzed copolymer of a perfluorinated hydrocarbon and a sulfonated perfluorovinyl ether and a sulfostyrenated perfluori-nated ethylene propylene polymer.

The electrolytic cell as claimed in Claim 1 wherein the barrier is a hydrolyzed copolymer of tetrafluoroethylene and a sulfonated per-fluorovinyl ether having the formula FSO2CF2CF2OCF(CF3)CF2OCF=CF2 which copolymer has an equivalent weight of from about 900 to 1,600.

The electrolytic cell as claimed in Claim 2 wherein the co-polymer has an equivalent weight of from about 1100 to about 1400 and contains from about 10 to 30% of the ether compound.

The electrolytic cell as claimed in Claim 1 wherein the barrier is a sulfostyrenated perfluorinated ethylene propylene polymer.

The electrolytic cell as claimed in Claim 4 wherein the co-polymer is styrenated to from about 16 to 18 percent by weight and from about 2/3 to 13/16 of the phenol groups are monosulfonated.

The electrolytic cell as claimed in Claim 1 wherein the anode is a metallic anode.

The electrolytic cell as claimed in Claim 3 wherein the anode is a metallic anode.

The electrolytic cell as claimed in Claim 5 wherein the anode is a metallic anode.

The electrolytic cell as claimed in Claim 1 wherein the cell is formed with at least two buffer compartments between the anode com-partment and the cathode compartment.

The electrolytic cell as claimed in Claim 3 wherein the cell is formed with at least two buffer compartments between the anode com-partment and the cathode compartment.

The electrolytic cell as claimed in Claim 5 wherein the cell is formed with at least two buffer compartments between the anode com-partment and the cathode compartment.

The electrolytic cell as claimed in Claim 6 wherein the cell is formed with at least two buffer compartments between the anode com-partment and the cathode compartment.

A process for the electrochemical decomposition of an aqueous solution of an ionizable chemical compound which comprises introducing an aqueous solution of a alkali metal halide into the anode compartment of the electrolytic cell, introducing water into the buffer and cathode compartments of the cell and effecting the electrolytic decomposition of said ionizable solution by passing an electric current between the anode and cathode of the cell, which cell comprises a cell body having an anode compartment containing an anode, a cathode compartment con-taining a cathode and at least one buffer compartment between said anode and said cathode compartments, said compartments being separated from each other by a side wall barrier located between each of the anode and buffer compartments, the cathode and buffer compartments and between individual buffer compartments, said barriers being substantially im-pervious to fluids and gases, selected from a hydrolyzed copolymer of a perfluorinated hydrocarbon and a sulfonated perfluorovinyl ether and a sulfostyrenated perfluorinated ethylene propylene polymer.

The process as claimed in Claim 13 wherein the alkali metal halide is sodium chloride, chlorine is produced as the electrolytic decomposition produce at the anode, a dilute solution of sodium hydr-oxide is produced as an electrolytic decomposition produce at the cathode.

The process as claimed in Claim 13 wherein the aqueous solution of an ionizable chemical is an aqueous solution of HCl, chlorine is pro-duced as the electrolytic decomposition produce at the anode and hydrogen is produced as the electrolytic decomposition produce at the cathode.

The process as claimed in Claim 14 wherein the dilute solution of sodium hydroxide from the buffer compartment is introduced into the cathode compartment as at least a portion of the aqueous catholyte solution.

The process as claimed in Claim 14 wherein the aqueous sodium chloride solution introduced in the anode compartment contains from about 250 to 325 grams per liter NaCl and has a pH within the range of about 1.0 to 10.0, the cell is operated at a voltage within the range of about 3.4 to 4.8 volts and an anode current density within the range of about 0.8 to 2.5 amps per square inch, the concentration of the sodium hydroxide solution obtained from the buffer compartment is within the range of about 50 to 200 grams per liter of NaOh and the concentration of the sodium hydroxide solution produce obtained from the cathode compartment is within the range of about 200 to 420 grams per liter NaOH.

The process as claimed in Claim 16 wherein the aqueous sodium chloride solution introduced into the anode compartment has a pH of from about 1.0 to 10.0 and contains from about 250 to 325 grams per liter NaCl, the cell is operated at a voltage within the range of about 3.4 to 4.8 volts and an anode current density within the range of about 0.8 to 2.5 amps per square inch and the concentration of the sodium hydroxide solution produce obtained from the cathode compartment is within the range of about 150 to 250 grams per liter NaOH.

The process as claimed in Claim 15 wherein the aqueous HCl solution introduced into the anode compartment contains from about 10 to 36 percent by weight HCl and the cell is operated at a voltage within the range of about 3.4 to 4.8 volts and an anode current density within the range of about 0.8 to 2.5 amps per square inch.

The process as claimed in Claim 13 wherein the cell in which the electrolysis is carried out contains at least two buffer compartments between the anode compartments and the cathode compartment.

The process as claimed in Claim 14 wherein the electrolytic cell in which the electrolysis is carried out contains at least two buffer compartments between the anode compartment and the cathode compartment.

The process as claimed in Claim 15 wherein the electrolytic cell in which the electrolysis is effected contains at least two buffer compartments between the anode compartment and the cathode compartment.

The process compartment as claimed in Claim 16 wherein the electrolytic cell in which the electrolysis is effected contains at least two buffer compartments between the anode compartment and the cathode compartment.