IL45141A - Process and apparatus for electrolysis - Google Patents

Abstract

1468942 Electrolytic diaphragm cells HOOKER CHEMICALS & PLASTICS CORP 15 July 1974 [19 July 1973 15 Aug 1973] 31179/74 Heading C7B An electrolytic cell, for electrolyzing aqueous solutions of alkali metal halides e.g. NaCl to produce alkali metal hydroxides and halogen e.g. NaOH and Cl 2 , of HCl to produce H 2 and Cl 2 , of ammonium sulphate to produce persulphate and of borax to produce perborates, comprises a housing 1, an anode compartment 3 and a cathode compartment 7 containing anode 5 and cathode 9 and at least one buffer compartment 11 therebetween, at least the anode compartment and the adjacent buffer compartment being separated from each other by a permselective barrier 13, impervious to liquids and gases and comprising a hydrolyzed copolymer of a perfluorinated hydrocarbon, e.g. tetrafluoroethylene or hexafluoropropylene, and a fluorosulphonated perfluorovinyl ether, e.g. FSO 2 CF 2 CF 2 OCF(CF 3 )CF 2 OCF = CF 2 (as disclosed in Specification 1431733), in which some of the sulphonic acid groups may be replaced or modified, or sulphostyrenated perfluorinated ethylene propylene polymer. All compartments may be separated by the barrier. When at least two buffer compartments are used Figs. 2, 3 (not shown) at least the anode compartment and the adjacent buffer compartment are separated from each other by the permselective barrier and at least the cathode compartment and its adjacent buffer compartment may be separated by a porous diaphragm e.g. of asbestos or polypropylene. Dilute NaOH solution removed for example from the buffer compartment(s) may be introduced into the cathode compartment. The anode may be of RuO 2 coated Ti and the cathode may be of steel.

Description

45141/2 Process and apparatus for electrolysis EDWARD H. COOK, Jr. and ALVIN T. EMBRT C. 43236 Case 304 / 058 3218/AM/mcs " PROCESS AND APPARATUS FOR ELECTROLYSIS 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 diaphragm. 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 which are oxidized to chlorates, with a consequent reduction in chlorine yield and further contamination of the sodium hydroxide. Additionally, depending upon the source φτ 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.

Attempts have heretofore been made to overcome the aforesaid difficulties in the operation of such diaphragm cells by replacing the fluid permeable asbestos diaphgrams with permselective ion exchange membranes. Tn 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 hydrox 1 ions to the anode compartment. For this purpose, various resins, such as the cation exchange resins of the "Amberlite" type, sulfonated copolymers of styrene and divinylbenzene, and the like, have been proposed. In practice, however, the permselective ion exchange 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 i 75 degrees C so that they have had only a relatively short effective 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 use 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 various chemicals, 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 electrolysis apparatus which utilizes ion selective membranes and to provide a process for electrolyzing alkali metal halide brines using such apparatus.

These and other objects will become apparent to those skilled in the art from the description of theinvention which follows .

In the drawings which are attached hereto and form a part hereof, Figure 1 is a schematic representation of a three compartment electrolytic cell and Figure 2 is a schematic representation of a four compartment electrolytic cell of the present invention and; Figure 3 is a schematic representation of a modification of the electrolytic cell shown in Figure 1 and Figure 2 is a schematic representation of a modification of the electrolytic cell shown in Figure A? 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, a cathode compartment containing a cathode and at least one to two or more buffer compartments between said anode and cathode compartments, said compartments being separated from each other by a barrier which is substantially impervious to fluids and gases selected from a hydrolyzed copolymer of a perfluorinated hydrocarbon and a sulfonated perfluorovinyl ether and a sulfostyrenated per- fluorinated ethylene propylene polymer and at least said cathode compartment and the next adjacent buffer compartment being separated from each other by a porous diaphragm. 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 maximum electrical operating efficiency.

In a preferred embodiment of the invention the perm- selective membranes are of a hydrolyzed copolymer of tetra- fluoroethylene and a fluorosulfonated perfluorovinyl ether of the formula which has an equivalent weight of about 900 to 1,600 and the membranes aie counted on networks of supporting materials such as polytetra-fluoroethylene 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 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 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.

The electrodes for the present electrolytic cell ' may be formed of any electrically conductive 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 hydroxide, 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 oxides, or mixtures of such oxides, either alone or with oxides of other metals. The noble metals which may be used include ruthenium, rhodium, palladium, irridium, 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 elad 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 saturated and unsaturated hydrocarbons of 2 to 5 carbon atoms may also be utilized, of which the monolefinic hydrocarbons are preferred, especially those of 2 to carbon atoms 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 CF20CF=CF2. Such a material, named as perfluoro /~2(2Τ>ΠΛΙΟΓΟ-sul onylethoxy)-propyl vinyl etherJ7* 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 of the perfluoro-lower alkyl groups, respectively. However, it is most preferred to employ PSEPVE.

The method of manufacture of the hydrolyzed copolymer is described in Example XVII of U. S. Patent 3,282,875 and an alternative method is mentioned in Canadian Patent 8^9,670, which also discloses the use of the finished membrane in fuel cells, characterized therein as electrochemical cells. The disclosures of such patents are hereby incorporated herein by reference. 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 perfluoroether layer and an upper laye 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 Υΐ%. The unhydrolyzed copolymer may be compression molded at high temperature and pressure to produce sheets or membranes, which may vary in thickness from 0.02 to 0.5 mm. These are then further treated to hydrolyze pendant -SO2F groups to -SO3H groups, as by treating with 10%" sulfuric acid or by the methods of the patents previously mentioned. The presence of the -S0.-,H S**oups may verified by tetration, 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, also hereby incorporated by reference.

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 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, when it is still thermoplastic; and the film of copolymer covers each filament, penetratin into the spaces between them and even around behind them, thinning the films slightly 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., above 75°C 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 temperatues. 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 g./l. of caustic. The selectivity of the membrane and its comparti-bility with the electrolyte does not decrease detrimentally as the hydroxyl concentration in the catholyte liquor increases, as has been noted with other membrane materials. Furthermore, the caustic efficiency of the electrolysis does not diminish as signi icantly as it does with other membranes when the hydroxyl ion concentration in the catholyte increases. Thus, these differences in the present process make it practicable, whereas previously described processes have not attained commercial acceptance. While themore 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 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 -SO3H 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 the manufacturing process or after production 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 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 sulfo-styrenated FEP's are surprisingly resistant to hardening 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 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 60 radiation treatment useing a cobalt radiation source. The rate of application may be in the range of about 8000 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 SO3. 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 mono-sulfonated and the latter being l6# styrenated and having 13/16 of the phenyl groupsmonosulfonated. To obtain l8# styrenation a solution of 17-1/2$ of styrene in methylene chloride is utilized and to obtain the l6# styrenation a solution of 16$ of styrene in methylene chloride is employed.

The products resulting compare favorably with the preferred copolymers previously described, giving voltage drops of about 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 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 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 plyable 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 y^~^ 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 2896, 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, however, 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 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 products, 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 is formed with an inlet for liquid electrolytes, such as water and, if desired may also have an outlet for liquid reaction products, 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 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, 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 been described, to form the desired buffer compartments.

Typical of such a cell structure is that shown in U. S.

Patent 3, 4 8 ,^11.

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 construction will be used, as have been described hereinabove. Additionally, it is further to be appreciated that the particular con iguration 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 schematic 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 compartment respectively. 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 / pendant sulfonic acid groups, as has been defined hereinabove.

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) s 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 inlets 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, 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, respectively, 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 c&ll 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 compartment (8 ) , which compartments are separated by two intermediate or buffer compartments (12) and (14). An anode (6) and a cathode ( 10 ) are positioned in the anode compartment (4 ) and cathode compartment (8 ) , respectively. 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 are formed of a film of a fluorinated copolymer having pendant sulfonic acid groups, as has been described hereinabove.

An inlet (22 ) 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 theinlets (30) and (32) and a dilute solution of caustic soda will be removed from the outlets (36) and (3§).

Additionally, an inlet ( 0) and an outlet (3½) 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 (3* 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 con iguration 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.

Referring now to Figure 3, this is a schematic representation of a modification of the cell shown in Figure 2, in which the cell is provided with our buffer compartments between the anode and cathode compartments . As is shown in this Figure, the cell body (1) is formed into an anode compartment (3) and a cathode compartment (13). These compartments are separated by four buffer compartments (5), (7), (9) and (11). An anode (15) and a cathode (17) are positioned in the anode compartment (3) and the cathode compartment (13) , respectively. A series of membrane or diaphragm barriers (19)* (21), (23) y (25) and (27) form the buffer compartments and separate them from the anode compartment and the cathode compartment. The barriers (19) , (21) and (23) are membranes formed of a film of a fluorinated copolymer having pendant sulfonic acid groups, as has been described hereinabove, while the barriers (25) and (27) are porous asbestos diaphragms.

An inlet (29) and an outlet (31) are provided in the anode compartment for the introduction and removal of the electrolyte, such as a sodium chloride brine. Additionally, an outlet (33) is also provided in the anode compartment for the removal of gaseous decomposition products, such as chlorine. The buffer compartments (5), (7),'- (9) and (11) are each provided with inlets (35), (37), (39) and (4l), respectively, and outlets (45), (47), (49) and (51), respectively. Wte re the cell is utilized for the electrolysis of a sddium chloride brine, typically water will be introduced into the inlets and a dilute solution of caustic soda will be removed from the outlets.

Additionally, an inlet ( 3) and an outlet (53) are provided in the cathode compartment (13)· Where a sodium chloride brine is being electrolyzed, a concentrated solution of caustic soda of high purity will be recovered from the outlet (53) and water or a dilute caustic soda solution may be introduced through the inlet (43). 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 2, 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. Examplary 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 anolyte solutions are the aqueous solutions of alkali metal halides, and particularly sodium chloride, and aqueous solutions of HCl.

In another mode of operation, water is introduced into each of the center or buffer compartments and a dilute solution of sodium hydroxide is removed from each of these compartments. Generally, these solutions will vary in concentration, with the most dilute solutions coming from the compartments closest to the anode compartment. Sodium hydroxide contents of from about 50 to 200 grams per liter for these solutions are typical. Preferably, one or more of these dilute solutions of sodium hydroxide are introduced into the cathode compartment, either with or without additional water, to form the catholyte liquor. These solutions may be combined or introduced separately into the cathode compartment. 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. From the cathode compartment thereis obtained a more concentrated sodium hydroxide 45141/2 solution.having an NaOH concentration of from about 150 to 250 grams per liter, with the sodium hydroxide content of about 160 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. 45141/2 f 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 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 typical. 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 concen-trated sodium hydroxide solution, having NaOH concentration of 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. further In fin- alternative method of operation, water is added to both the center or buffer compartment and to the cathode 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 typical operation, approximately 0$ of the sodium hydroxide will be recovered 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 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 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 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# &nd 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 compartment 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 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 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 concentrated 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 electrolysis of sodium chloride brine solutions, to produce chlorine 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 ¾y 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 compartment 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 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 t 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 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.

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 compartment. The membrane was a 10 mil thick film of a hydrolyzed copolymer of tetra-fluoroeth lene and sulfonated perfluorovinyl ether, having an equivalent weight of about 1100 and prepared according to U. S. Patent 3,282,875. Brine containing 320 grams/liter NaCl was circulated through the anode compartment and water was added to both the buffer compartment and the cathode compartment. HC1 was added to the anolyte to maintain the anolyte pH at about 4.0. The effluent from the buffer compartment contained about 116 grams/liter NaOH and that from the cathode compartment contained about 384 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 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 96°C. An aqueous brine containing about 320 grams/liter NaCl was introduced into the anode compartment and an anolyte overflow was obtained from the anode compartment which had a pH of about 3.5 and contained about 250 grams/liter NaCl, HC1 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 which contained about 110 grams/ liter NaOH. This effluent was fed to the cathode 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 effir ciency 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 94°C. 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/liter, HCl being added as required to maintain the anolyte pH at about 4.0.

Water was fed to both the buffer compartment and the cathode compartment. A weak caustic effluent containing about 100 grams/liter NaOH was obtained from the buffer compartment and a strong caustic effluent containing about 28Ο grams/liter NaOH was obtained from the cathode compartment. The cell produced about 2.3 tons/day NaOH, 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 the anode current efficiency of 96 .

EXAMPLE 4 A four compartment laboratory size electrolytic cell was constructed having an anode compartment and a cathode compartment, separated by two buffer compartments. The anode compartment was formed of a chlorendic acid based polyester, sold under the trademark Hetron^j the two buffer compartments were formed of polypropylene; and the cathode compartment was formed of mild steel. The anode compartment and the first buffer compartment and the first and second buffer compartments were separated from each other by a cation exchange membrane , barrier. This membrane was a 10 mil thick film of a hydrolyzed copolymer of tetrafluoroethylene and sulfonated perfluorovinyl ether, having an equivalent weight of about 1100 and prepared according to U. S. Patent 3,282,875. The second buffer compartment and the cathode compartment were separated from each other by conventional asbestos diaphragm. Brine containing about 320 grams/liter NaCT was circulated through the anode compartment, which was equipped with a metallic anode formed of titanium with an RuO^ coating, and water was added to the first buffer compartment. The effluent from the first buffer compartment was pumped into the second buffer compartment and the solution in this compartment flowed through the porous asbestos diaphragm into the cathode compartment, which was equipped with a steel cathode. The cell was operated at 120 amperes, an anode current density of 2.0 amps/square inch and a voltage 4.4 volts. The NaOH concentration of the solution in the first buffer compartment was 120 grams/liter; in the second, l87 grams/liter; and 209 grams/ liter in the cathode compartment which latter solution had an NaCl content of about 0.5 grams/liter. When operating under these conditions, the anode chlorine efficiency was 96$ and the cathode caustic efficiency was 93$, with hydrochloric acid being added to the anolyte in stoichiometric amounts to compensate for difference between the anode and cathode efficiency.

EXAMPLE 5 Using the procedure as has been described in Example 4, the cell of Example 1 was operated at 120 amperes, an anode current density of 2.0 amps/square inch and a voltage of 4.3 volts. The solution in the first buffer compartment contained l4o grams/liter NaOH; that in the second buffer compartment contained 226 grams/liter; and the effluent from the cathode compartment contained 240 grams/liter NaOH and about 0.6 grams/liter NaCl. The anode chlorine efficiency was 96$ and the cathode caustic efficiency was 90$.

EXAMPLE 6 The cell of Example was modified by replacing the asbestos diaphragm between the second buffer compartment and the cathode with a porous perfluorosulfonic acid membrane supplied by duPont and identified as ESL323. Using the procedure of Example 1, this cell was operated with a caustic concentration in the first buffer compartment of 100 grams/liter NaOH; in the second buffer compartment of l40 grams/liter NaOH; and in the cathode compartment effluent of 213 grams/liter NaOH. The anode chlorine efficiency was 96$ and the cathode caustic efficiency was 9$.

EXAMPLE 7 The cell of Example 4 was modified by replacing the porous asbestos diaphragm with a porous polypropylene film, identified as Cel ard^ . Using the procedure of Example 1, this cell was operated under the following conditions and the indicated results were obtained: NaOH CONCENTRATION First Buffer Second Buffer Cathode Cathode Compartment Compartment compartment Efficiency 110 grams/liter NaOH 135 grams/liter NaOH 191 grams/liter NaOH 93$ 125 grams/liter NaOH 159 grams/liter NaOH ^2 grams/liter NaOH 92$ 135 grams/liter NaOH 173 grams/liter NaOH 2h grams/liter NaOH 91 1 0 grams/liter NaOH 21 grams/liter NaOH 303 grams/liter NaOH 89$

Claims (1)

1. WHAT IS CLAIMED IS: -1- An electrolytic cell which comprises a cell body having an anode compartment containing an anode, a cathode compartment containing a cathode and at least one buffer compartment between said anode and said cathode compartments, said compartments being separated from each other by a barrier, which is substantially impervious to fluids and gases, selected from a hydrolyzed copolymer of a perfluorinated hydrocarbon and a sulfonated per-fluorovinyl ether and a sulfostyrenated perfluorinated ethylene propylene polymer. -2- The cell of Claim 1 wherein there are at least 2 buffer compartments and at least the anode compartment and the next adjacent buffer compartment are separated from each other by said impervious barrier and at least said cathode compartment and the next adjacent buffer compartment are separated from each other by a porous diaphragm. -3- The electrolytic cell as claimed in Claim 1 and 2 wherein the barrier is a hydrolyzed copolymer of tetrafluoroethylene and a sulfonated perfluorovinyl ether having the formula: which copolymer has an equivalent weight of from about 900 to l600. -k- The electrolytic cell as claimed in Claim 3 wherein the copolymer has an equivalent weight of from about 1100 to about 1*1-00 and contains from about 10 to 30$ of the ether compound. The electrolytic cell as claimed in Claim 1 and 2 wherein the barrier is a sulfostyrenated perfluorinated ethylene propylene polymer. -6- The electrolytic cell as claimed in Claim 5 wherein the copolymer is styrenated to from about 16 to 18 percent by weight and from about 2/3 to 13/16 of the phenol groups are mono-sulfonated. -7- The electrolytic cell as claimed in Claim 1 and 2 wherein the anode is a metallic anode. -8- The electrolytic cell as claimed in Claim 5 wherein the anode is a metallic anode. -9- The electrolytic cell as claimed in Claim 1 wherein the cell is formed with at least two buffer compartments between the anode compartment and the cathode compartment. -10- The electrolytic cell as claimed in Claim 2 wherein the porous diaphragm is asbestos. -11- A process for the electrochemical decomposition of an aqueous solution of an ionizable chemical compound which comprises introducing an aqueous solution of a ionizable chemical compound into the anode compartment of the electrolytic cell as claimed in Claim 1, introducing a second aqueous solution into the buffer and cathode compartments of said cell and effecting the electrolyti decomposition of said ionizable solution by passing an electric current between the anode and cathode of said cell. The process as claimed in Claim 11 wherein the aqueous solution of an ionizable chemical compound is an aqueous solution of an alkali metal halide and the second aqueous solution introduced into the buffer and cathode compartments is water. -13- The process as claimed in Claim 12 wherein the alkali metal halide is sodium chloride, chlorine is produced as the electrolytic decomposition product at the anode, a dilute solution of sodium hydroxide is produced in the buffer compartment and a concentrated solution of sodium hydroxide is produced as an electrolytic decomposition product at the cathode. -14- The process as claimed in Claim 11 wherein the aqueous solution of an ionizable chemical is an aqueous solution of HC1, chlorine is produced as the electrolytic decomposition product at the anode and hydrogen is produced as the electrolytic decomposition product at the cathode. -15- The process as claimed in Claim 13 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. -16- The process as claimed in Claim 13 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 i 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 product obtained from the cathode compartment is within the range of about 200 to 420 grams per liter NaOH. -17- The process as claimed in Claim 15 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 · ^ 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 product obtained from the cathode compartment is within the range of about 150 to 2 0 grams per liter NaOH. -I8- The process as claimed in Claim 14 wherein the aqueous HC1 solution introduced into the anode compartment contains from about 10 to 36 percent by weight HC1 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. -19- The process as claimed in Claim 11 wherein the cell in which the electrolysis is carried out contains at least two buffer compartments between the anode compartments and the cathode compartment. -20- The process as claimed in Claim 13 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. -21- The process as claimed in Claim I wherein the electrolytic cell in which the electrolysis is effected contains at least two buffer compartments between the anode compartment and the cathode compartment. -22- The process compartment 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. -23- A process for the electrochemical decomposition of an aqueous solution of an ionizable chemical compound which comprises introducing an aqueous solution of a ionizable chemical compound into the anode compartment of the electrolytic cell as claimed in Claim 2, introducing a second aqueous solution into the buffer and cathode compartments of said cell and effecting the electrolytic decomposition of said ionizable solution by passing an electric current between the anode and cathode of said cell. -24- The process as claimed in Claim 23 wherein the aqueous solution of an ionizable chemical compound is an aqueous solution of an alkali metal halide and the second aqueous solution introduced into the buffer and cathode compartments is water. The process as claimed in Claim 24 wherein the alkali metal halide is sodium chloride and wherein chlorine is produced as the electrolytic decomposition product at the anode, a dilute solution of sodium hydroxide is produced in the buffer compartments and a concentrated solution of sodium hydroxide is produced as an electrolytic decomposition product at the cathode. -26- The process as claimed in Claim 23 wherein the aqueous solution of an ionizable chemical is an aqueous solution of HC1, chlorine is produced as the electrolytic decomposition product at the anode and hydrogen is produced as the electrolytic decomposition product at the cathode. -27- The process as claimed in Claim 25 wherein at least a portion of the dilute solution of sodium hydroxide from each buffer compartment is introduced into the next succeeding buffer compartment and ultimately into the cathode compartment as at least a portion of the aqueous catholyte solution. -28- The process as claimed in Claim 25 wherein the aqueous sodium chloride solution introduced in the anode compartment contains from about 250 to 32 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 solutions obtained from the buffer compartments is within the range of about 0 to 200 grams per liter of NaOH and the concentration of the sodium hydroxide solution product obtained from the cathode compartment is within the range of about 200 to 420 grams per liter NaOH. -29- The process as claimed in Claim 27 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 product obtained from the cathode compartment is within the range of about 150 to 0 grams per liter NaOH. -30- The process as claimed in Claim 26 wherein the aqueous HC1 solution introduced into the anode compartment contains from about 10 to 36 percent by weight HC1 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. -31- The process as claimed in Claim 23 wherein the porous diaphragm in the electrolytic cell in which the electrolysis is carried out is asbestos. -32- r The process as claimed in Claim 25 wherein the porous diaphragm in the electrolytic cell in which the electrolysis is carried out is asbestos. -33- The process as claimed in Claim 26 wherein the porous diaphragm in the electrolytic cell in which the electrolysis is effected is asbestos. - 34 - The process compartment as claimed in Claim 27 wherein the porous diaphragm in the electrolytic cell in which the electrolysis is effected is asbestos.