CA1076060A

CA1076060A - Electrolytic method for the manufacture of hypochlorites

Info

Publication number: CA1076060A
Application number: CA212,574A
Authority: CA
Inventors: Cyril J. Harke; Jeffrey D. Eng
Original assignee: Hooker Chemicals and Plastics Corp
Current assignee: Occidental Chemical Corp
Priority date: 1973-11-01
Filing date: 1974-10-29
Publication date: 1980-04-22
Also published as: JPS5099994A; US3925174A

Abstract

ABSTRACT
Hypochlorites, such as alkali metal hypochlorites, are made by electrolyzing brine in a cell having three or more compart-ments or zones therein, wherein anode and cathode compartments are separated from at least one intervening buffer compartment by ration-active permselective membranes of a hydrolyzed copolymer of tetra-fluoroethylene and a fluorinated perfluorovinyl ether or of a sulfostyrenated fluorinated ethylene propylene polymer or by such a permeselective membrane on the cathode side plus a porous asbestos diaphragm on the anode side when feeding chlorine gas to the buffer zone at such a rate and under such conditions as to produce hypo-chlorite therein. The hypochlorite may be converted to chlorate externally of the cell or, in a variation of the process, may be converted to chlorate in the buffer compartment.

Description

3 . ' ~

~076~60 - This invention relates to the electrolytic manufacture of hypochlorites. More specifically, it is of a process for ; making alkali metal hypochlorite from chlorine and aqueous alkali metal hydroxide solution,both of which reactants are produced in S an electrolytic cell containing anode, cathode and buffer compartments,with means provided for separating the cathode and buffer compartments being a cation-active permselective membrane which is a hydr~lyzed polymer of a perfluorinated hydrocarbon t and a fluorosulfonated perfluorovinyl ether or a sulfostyrenated ;10 perfluorinated ethylene propylene polymer.
i Such cation-permeable membranes permit flow of hydroxyl ion from the catholyte to the buffer zone but do not allow chloride ion to pass through andto mix with the hydroxyl in the cathode compartment. Thus, when chlorine is added to a buffer compartment,hypochlorite is produced therein, consuming the hydroxyl ion and preventing it from flowing to the anolyte and at the same time a chloride-free alkali metal hydroxide is made in the cathode compartment.
Chlorine and caustic, essential and very large volume chemicals required by all industrial societiesl are commercially produced by the electrolysis of aqueous salt solutions. Improved !~
electrolytic methods utilize dimensionally stable anodes, which - include noble metals, alloys or oxides or mixtures thereof on Yalve metals. The concept of employing permselective diaphragms to separate anolyte from catholyte during electrolysis is not a -' ' ' .. ...... . ..... _ . .. . . .. . . .. . , . . _ , l~ .
~`

~ " 1076~60 .
new one and plural compartme~t electrolytic cells have been suggested in which one or more of such membranes is employed.
Recently, improved membranes which are of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluoro-vinyl ether have been described and in some experiments these have been used as the membranes between the catholyte and buffer zones of chlorine-caustic cells. Such membranes have been further improved by surface treatments, preferably by modifica-., ~ . .
tions of the sulfonic group, to make them more conductive and efficient. Also,sulfos ty renated perfluorinated ethylene propylene polymers have been made into useful membranes.
Although the electrolysis of aqueous salt solutions is a technologically advanced field of great commercial interest in which~much research is performed, and the importance of im-proving manufacturing methods therein is well recognized, before the present invention the process thereof had not been practicéd and the advantages of it had not been obtained.
In accordance with the present invention a method of electrolytically manufacturing a hypochlorite comprises electro-lyzing an aqueous solution containing chloride ions in an electrolytic cell having at least three compartments therein, an anode, a cathode, at least one cation-active permselective membrane selected from the group consisting of a`hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether, and of a sulfost y renated perfluorinated ethylene propylene polymer, defining a cathode-side wall of a .

buffer compartment between the anode and cathode, an anode-side wall of said buffer compartment being defined by such a cation-active permselective membrane or a porous diaphragm, and such walls, with walls thereabout, defining anode, buffer and cathode compartments, while feeding gaseous chlorine into the buffer compartment and regulating the rate of feed thereof and reaction conditions to produce hypochlorite in the buffer compartment.
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 FS02CF2CF20CF(CF3)CF20CF=CF2, which has an equivalent weight of about 900 to 1,600, at least two such membranes are employed, at least one of which separates the anolyte and buffer compartments and the other of which separates the catho-lyte and buffer compartments, and the membranes are mounted on networks of 9upporting materials such as polytetrafluoroethylene or asbestos filaments.
In some preferred aspects of the invention the hypo-chlorite is converted to chlorate, either externally or internally of the cell. The described preferred copolymers may be further modified to improve thei~ 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.
According to another aspect of the invention there is provided an electrolytic cell for manufacturing a hypochlorite or chlorate which comprises a housing having at least three com-partments therein comprising an anolyte compartment, containing an anode adapted to be connected to a positive terminal of an electrical input source, a catholyte compartment containing a cathode, and at least one buffer compartment between said anolyte compartment and said catholyte compartment defined by at least a pair of spaced apart barriers including a cathode-side wall barrier and an anode-side wall barrier said cathode-~ide wall barrier comprising a cation-active permselective membrane selected from the group consisting of a copolymer of a perfluori-nated hydrocarbon and a fluorosulfonated perfluorovinyl ether, and a sulfostyrenated perfluorinated ethylene propylene polymer, said anode-side wall barrier of the buffer compartment being defined by a cation-active permselective membrane of said group or a porous diaphragm; said anolyte compartment including inlet means for an aqueous solution of chloride ions; said buffer com-partment including inlet means for a regulated flow of gaseous chlorine and outlet means for removing hypochlorite or chlorate from said buffer compartment.
The invention will be more readily understood by - reference to the following descriptions of embodiments thereof, ~ - 4 _ ~076060 taken in conjunction with the drawing of apparatuses and means for effecting the invented processes.
In the drawing:
FIG. 1 is a schematic representation of the arrangement of equipment for producing hypochlorite in an electrolytic cell by a method of this invention and subsequently converting it to chlorate outside the celli FIG. 2 is a schematic view of an electrolytic cell in which chlorate is produced internallyi and FIG. 3 is a schematic view of apparatus like that of FIG. 1, including means to remove chloride from the chlorate made and to recirculate chlorate through the cell buffer compartment to increase chlorate content in the product stream.
In the FIGURES, to facilitate understanding of the process, the flows of typical and preferred reactants and products are illust-rated. M stands for alkali metal, preferably sodium, but other halide-forming cations may also be employed and in some instances bromine may be at least partially substituted for chlorine.
In FIG. 1 electrolytic cell 11 includes outer wall 13, anode 15, cathode 17 and conductive means 19 and 21 for connecting the anode and the cathode to sources of positive and negative electrical potentials, respectively. Inside the walled cell cation-.
active permselective membranes 23 and 25 divide the volume into anode or anolyte compartment 27, cathode or catholyte compartment 29 and buffer compartment 31. An acidic aqueous solution 32 of alkali 10~6a60 metal halide is fed to the anolyte compartment through line 33 and chlorine gas is fed to the buffer compartment through l.ne 35. Re-circulated buffer solution may also be fed into the buffer compartment, through a separate line, 36' or a common line with the chlorine or water. Water may be admitted through line 36 to maintain the desired liquid level in the buffer compartment. Of course, liqu;d levels should be maintained in all compartments and this is often effected with known feed-overflow techniques, the apparatus for effecting which is known and therefore, is not illustrated.
Halogen, e.g., chlorine gas, is removable from the anolyte compartment through line 34 and aqueous sodium hydroxide is removable from the catholyte compartment through line 37. An aqueous solution of alkali metal hypochlorite, with some dissolved alkali metal chloride, is removable through line 39 and may be passed through that line to reaction vessel or mixer 41 in which it is mixed with halogen, e.g., chlorine gas, from line 34. The chlorine passes through line 43, and may be pulled through that line by low pressure created by pumping hypochlorite-chloride or hypochlorite-chlorate-chloride solution 45 through line 47, pump 49 and return line 51, through eductor reaction 53, in which intimate mixing is effected.
Chlorine gas in the upper portion of reaction and retention vessel 41 may be vented off or may be recycled, too. The chlorate made is removed as an aqueous solution, with alkali metal chloride, through discharge line 55. The chlorate-chloride solution may be circulated through lines 47 and 51, pump 49, reactor 53 and vessel ~1 0 7 6 0 6 0 41 until the chlorate-chloride concentration is increased to a useful level. Chloride may be removed by precipitation and if de- !
sired, chlorate may be crystallized out by installation of the appropr;ate apparatus in lines 47 and 51. Because sodium chloride is relatively insoluble, compared to sodium chlorate, it should be removed before chlorate crystals are manufactured; otherwise chloride solids can block orifices, etc., during manufacturing.
In some aspects of the invention, when it is preferred to ~rad~lce the hy~chlarite for direct use, it is removed through ro l~ne 39, together w~th a7ka7i meta7 ch70ride. Some of it may subsequent~ be converted to chlorate. Hydrogen is obt~inab7e ~rom -: 7ine ~
In the operation af the in~ented process chlorine is generated ~t the anode and alkali metal hydroxide and hydrogen are produced at the cathode. The norma1 tendency for a7ka7i metal halide to move into the catholyte and increase the halide content ; of the hydroxide m~de iS counteracted by the cationic permselectivemembrane 25 and this prevention of chloride flow is aided by the presence of the additional permselective membrane 23. Yet, alkali metal hydroxide may migrate from catholyte to the anolyte in ordinary ; cells and such migration can interfere with the caustic or sodium . ion current efficiency if the product made is useless or is not recovered. Caustic, sodium ion or cathode current efficiency is the percentage of useful product made, compared to 100% maximum, with the current flow employed. Sodium ion efficiency may be the most ~076~60 exact of the terms employed but all are used. Thus, if sodium hydr-oxide is chemically reacted to make recoverable sodium hypochlorite or sodium chlorate, coulombs are not wasted, as they are when hydroxyl ions are electrolytically converted to useless oxygen at the anode.
Anode or chlorine efficiencies are figured in the same general way.
In the present cell the addition of chlorine to the buffer zone causes the alkali metal hydroxide migrating through the membrane, as illustrated, to be converted to hypochlorite and chloride, which do not pass through the permselective membranes. Therefore, the process satisfactorily produces a chloride-free caustic at satisfactory high current efficiency and additionally makes a desired byproduct, the hypochlorite, which may be further converted to chlorate, when desired. -In FIG. 2 the manufacture of chlorate in the buffer zone is shown, using a cell like that of FIG. 1. The only difference in operation is in the employment of sufficient chlorine to diminish the pH further, favoring formation of chlorate rather than of hypo-chlorite, which is normally produced at a higher pH. Acids and bases may also be used to regulate the pH. A liquid medium such as recirculated buffer solution or other chlorate-chloride water ,, solution may be added to the buffer zone through line 36 so as to help control the temperature, and sometimes, to increase the per-centage of chlorate in the buffer zone and in the recirculating ,-~ liquid to such a level that after removal of chloride, chlorate may be crystallized out.

~ ;
~..' In FIG. 3 external manufacture of chlorate is illustrated, with buildup of chloride and chlorate concentrations by recirculation, followed by removal of the chloride, which may then be followed by crystallization of the chlorate. As is illustrated, chloride-chlorate solution may be recirculated through vessel 41 via lines47 and 51' with the solution passing through pump 49 and reactor 53.
During recirculation additional reaction with MOCl from the cell is effected in reactor 53 and the concentrations of the hypochlorite, and chloride resulting from such reaction are increased. Because the chloride is less soluble and is produced to a greater extent, it will soon crystallize out in the reactor or retention vessel, causing processing difficulties. Accordingly, it is removed in separator or crystallizer 61 and more pure, more concentrated chlorate is continually circulated and ultimately, is drawn off from the retention vessel 41, possibly for further concentration and/or crystallization out as the solid. At junction 63 a pro-portioning valve may be located and the concentration of chlorate in the circulating system may be further increased by returning a proportion of it through line 65 to cell 11. Desired pH's at various parts of the system may be controlled by regulating the proportions of chlorine utilized at such different locations.
Although some circulations and recirculations of mate-rials of the process are illustrated, others may also be effected.
Thus, anolyte, buffer compartment solution and catholyte recirculation may be utilized to maintain the various solutions at the same con-centration throughout their respective compartments. Alternatively, g ', 10'76060 once-through processes and "hybrid" processes are also useful.
Similarly, recirculation of chlorate-chloride solutions may be to the anolyte compartment, at least in part, to convert the chloride thereof to chlorine and thus reduce the concentration of it in the chlorate-chloride mixtures.
In the preferred embodiments of the invention the buffer zone or compartment has two opposing boundaries or walls thereof, dividing it from anode and cathode compartments, respectively, both of the described hydrolyzed copolymer membranes, usually supported on an open network, screen or cloth of electrolyte- and product-resistant material which is preferably filamentary in form.
The cationic membranes oppose or prevent the passage of anions such as halide, hypohalite and halate ions, while allowing the passage of cations, e.g., alkali metal and hydrogen ions. ~ow ~ 15 molecular weight anions, such as hydroxyl, may also pass through ; the cationic membranes.
The selective ion-passing effects of cationic membranes have been noted in the past but the membranes of this invention ~ have not been employed in the present processes before and their -~ 20 unexpectedly beneficial effects have not been previously obtained or suggested. Thus, with the use of a comparatively thin membrane, ; prefer~bly supported as described herein, several years of operation under commercial conditions are obtainable without the need for removal and replacement of the membrane, while all the time it efficiently prevents undesirable migration of hypochlorite from the buffer compartment and prevents the chloride ions of the .

~ 10-~ 076060 anolyte from entering the buffer compartments, while also stopping any chloride in the buffer zone from transferring to the catholyte.
Together with the use of the buffer zone between the anolyte and catholyte zones, it prevents hydrogen formed on the cathode side from escaping into the hydrogen formed on the anode side. In this respect the present membranes are superior to prior art membranes because they are more impervious to the passage of hydrogen, even in comparatively thin films, than are various other polymeric materials. Also important is their ability to prevent transfer of chlorine gas into the hydrogen produced at the cathode, especially when chlorine is fed to the buffer compartment, since when chlorine is present in hydrogen an explosive mixture may be formed. The superiority of the preferred membranes of the described copolymer (including modified or surface treated versions thereof) over the prior art membranes in the various described aspects is also evident, usually to a lesser degree, in the sulfostyrenated fluorinated ethylene propylene polymers.
Although the preferred embodiments of the invention utilize a pair of the described membranes to form the three compartments of the present cells it will be evident that a greater number of compartments, e.g., 4 to 6, including plural buffer zones may be employed. Similarly, also, while the compartments will - usually be separated by flat membranes and will usually be of -.
substantially rectilinear or parallelpipedal construction, various - 25 other shapes, including curves. e.g., ellipsoids, irregular surfaces, e.g., sawtoothed or plurally pointed walls, may also be utilized.

'''-`

` ~076060 In another variation of the invention the buffer zone(s), formedby the plurality of membranes, will be between bipolar electrodes, rather than the monopolar electrodes which are described herein.
Those of skill in the art will know the variations in structure that will be made to accommodate bipolarS rather than monopolar electrodes, and therefore. these will not be described in detail.
Of course, as is known ;n the art, pluralities of the individual cells will be employed in-multi-cell units, often having common feed and product manifolds and being housed in unitary structures.
Again, such constructions are known to those in the art and need not be discussed herein.
For most satisfactory and efficient operations the volume of the buffer compartment(s) will usually be from 1 to 100%, preferably from 10 to 70% that of the sum of the volumes of the anode and cathode compartments.
- Although the utilization of the present cationic or cation-active membranes to define the buffer compartment(s) and separate it/them from the anolyte and catholyte sections is highly preferred it is possible to operate with a conventional diaphragm separating the anode compartment from the buffer com-partment. However, the membrane will be employed to separate the catholyte from the buffer zones in order to produce the highly desirable salt-free caustic. Otherwise, even if such a membrane was employed to separate the anolyte from the buffer zone, halide present in the buffer section due to addition of brine or pro-duction by the reaction of chlorine with the caustic to form ,:~

. .

" 1076060 hypochlorite, could pass through the diaphragm to contaminate the caustic. In many applications salt-free caustic is highly desirable and therefore, 3-compartment cell structures having a cathode-side porous diaphragm, such as illustrated in the U.S. Environmental Protection Agency publication entitled Hypoch10rite Generator for Treatment of Combined Sewer Overflows (Water Pollution Contro1 Research Series 11023 DM 03/72) are unsatisfactory. Additionally, the conventional diaphragms, which are usually of deposited asbestos fibers, tend to become blocked with insoluble impurities from the brine and have to be cleaned periodically, usually necessitating shutdown of the cell and often, replacement of the diaphragm.
; The aqueous solution containing chloride ions is normally a water solution of sodium chloride, although potassium and other soluble chlorides, e.g., magnesium chloride and ammonium chloride, may be utilized, at least in part. However, it is preferably to employ the alkali metal chlorides and of these sodium chloride is the best. Sodium and potassium chlorides include cations which ` form soluble salts or precipitates and which produce stable hydr-; oxides. The concentration of sodium ehloride in a brine charged - 20 will usually be as high as feasible, normally being from 200 to 320 grams per liter for sodium chloride and from 200 to 360 9./1.
for potassium chloride, with intermediate figures for mixtures of sodium and potassium chlorides. The electrolyte may be neutral or acidified to a pH in the range of about 2 to 6, acidification normally being effected, with a suitable acid such as hydrochloric acid. Charging of the brine is to the anolyte compartment. The solid sodium chloride added to the liquid medium in the anolyte results in a sodium chloride concentration from 200 to 320 g./l.
and most preferably of 250 to 300 9./l. In recycle charges to the buffer compartment, if utilized, the concentration will normally be less than 50 or 100 9./1., although chlorate contents may be higher, and usually the chloride contents of the buffer liquids - will be less than such limits, too.
Water may be charged to the buffer compartment and in some cases it may be desirable to charge water with brine to the anolyte compartment. Dilute caustic may be recirculated to the catholyte compartment but this is not usually done. For the most part the liquid level in that zone is maintained by transfer to it of material(s) charged to the anolyte and/or buffer zone, ; plus water.
The presently preferred cation permselective membrane ~ is of a hydrolyzed copolymer of perfluorinated hydrocarbon and a - fluorosulfonated perfluorovinyl ether. The perfluorinated hydro-carbon is preferably tetrafluoroethylene, although other perfluori-nated and saturated and unsaturated hydrocarbons of 2 to 5 carbon atoms may also be utilized, of which the monoolefinic hydrocarbons i~ are preferred, especially those of 2 to 4 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 FS02CF2CF20CF(CF3)CF20CF=CF2.
Such a material, named as perfluoro[2-(2-fluorosulfonylethoxy)-propyl vinyl ether], referred to henceforth as PSEPVE, may bemodified 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 re-arranging positions of substitution of the sulfonyl thereon and utilizing isomers of the perfluoro-lower alkyl groups, respectively.
However, it is most preferred to employ PSEPYE.
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 849,670, which also discloses the use of the finished membrane in fuel cells, characterized therein as electrochemical cells. In short, the copolymer may be made by reacting PSEPVE or equivalent with tetra- :
fluoroethylene or equivalent in desired proportions in water at elevated temperature and pressure for over an hour, after which ; 20 time the mix is cooled. It separates into a lower perfluoroether 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 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 -S02F groups to -S03H groups, as by treating with 10~ sulfuric acid or by the methods of the patents previously mentioned. The presence of the -S03H groups may be verified by titration, as described in the Canadian patent. Addi-10 tional details of various processing steps are described inCanadian patent 752,427 and U.S. Patent 3,041,317.
Because it has been found that some expansion accompanies hydrolysis of the copolymer it is preferred to position the copolymer membrane after hydrolysis onto a frame or other support which will 15 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 a backing of filaments of polytetrafluoro-ethylene or other suitable filaments prior to hydrolysis, when it is still thermoplastic so that the film of copolymer covers each , 20 filament, penetrating into the spaces between them, the copolymer membrane films becoming slightly thinner in the process, where - they cover the filaments.
The membrane described is far superior to the present - processes to all other previously suggested membrane materials.
,:

', ~ 16 ~

a -1076060 ' It is more stable at elevated temperatures, e.g., above 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 temperatures. 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./1. 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 materials. Furthermore, the caustic efficiency of the electrolysis does not diminish as significantly 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 the more preferred copolymers are those having equivalent weights of 900 to 1,600, with 1,100 to 1,400 being most preferred, some useful resinous membranes produced by the present method may be of equivalent weights from 500 to 4,000.
The medium equivalent weight polymers are preferred because they are of satisfactory strength and stability, enable better selective ion exchange to take place and are of lower internal `:

~076060 ~

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 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. Caust;c 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%.
Exemplary of such treatment is that described in French patent publication 2,152,194 of March 26, 1973 in which one side of the membrane is treated with NH3 to form S02NH2 groups.
~,.
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 application in the present processes. Although it appears that tetrafluoroethylene (TFE) polymers which are sequentially styrenated and sulfonated are not useful for making sa~isfactory 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 sulfo-; 25 nated makes a useful membrane. Whereas useful lives of as much as three years or more (that of the preferred copolymers) may not 1076a60 be obtained the sulfostyrenated FEP's are surprisingly resistuntto 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 radiation treatment, using a cobalt60 radiation source. The rate of application may `, be in the range of about 8,000 rads/hr. and a total radiation -~ application is about 0.9 megarads. After rinsing with water 15 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. Prefer-., , ably, chlorosulfonic acid in chloroform is utilized and the ,, `1 sulfonation is complete 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 18ST12~ and 16ST13~, 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% styrenation a solution of *Trade Mark _ 19 _ 1~76060 17-1/2% of styrene in methylene chloride is utilized and to obtain the 16% styrenation a solution of 16% of styrene in methylene chloride is employed.
The products resulting compare favorably with the preferred 5 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.
The membrane walls will normally be from 0.02 to 0.5 mm. thick, preferably from 0.1 to 0.5 mm. and most preferably 0.1 10 to 0.3 mm. When the membrane is mounted on a polytetrafluoro-ethylene, asbestos, perflorinated ethylene propylene polymer, r titanium, tantalum, niobium and noble metals or other suitable i network, for support, the network filaments or fibers will usually have a thickness of 0.01 to 0.5 mm., preferably 0.05 to 0.15 mm., 15 corresponding to up to the thickness of the membrane. Often it will be preferably for the filaments to be less than half the film thickness but filament thicknesses greater than that of the film may also be successfully employed, e.g., 1.1 to 5 times the film thickness. The networks, screens or cloths have an area percentage of openings therein from about 8 to 80%, preferably 10 to 70% and most preferably 30 to 70%. Generally the cross sections of the filaments will be circular but other shapes, such as ellipses, squares and rectangles, are also useful. The supporting network is preferably a screen or cloth and although it may be cemented to - 25 the membrane it is preferred that it be fused to it by high tempe-rature, high pressure compression before hydrolysis of the copolymer.
Then, the membrane-network composite can be clamped ' ~.' 10~76060 or otherwise fastened in place in a holder or support. It is preferred to employ the described backed membranes as walls of the cell between the anolyte and catholyte compartments and the buffer compartment(s) but if desired, that separating the anolyte and buffer compartments may be of conventional diaphragm material, e.g., deposited asbestos fibers or synthetic polymeric fibrous material (polytetrafluoroethylene, polypropylene). Also, treated asbestos fibers may be utilized and such fibers mixed with synthetic ,organic polymeric fibers may be meployed. However, when such - 10 diaphragms are used efforts should be made to remove hardness ions `and other impurities from the feed to the cell so as to prevent these from prematurely depositing on and blocking the diaphragms.
The material of construction of the cell body may be conventional, including concrete or stressed concrete lined with -~l15 mastics, rubbers, e.g., neoprene, polyvinylidene chloride, FEP, chlorendic acid based polyester, polypropylene, polyvinyl chloride, TFE or other suitable plastic or may be similarly lined boxes of -other structural materials. Substantially self-supporting structures, such as rigid polyvinyl chloride, polyvinylidene chloride, poly-propylene or phenol formaldehyde resins may be employed, preferably reinforced with molded-in fibers, cloths or webs.
The electrodes of the cell ~an be made of any electri-cally conductive material which will resist the attack of the 1~76~;0 ~

various cell contents. In general, the cathodes are made of graphite, iron, lead dioxide on graphite or titanium, steel or noble metal, such as platinum, iridium, ruthenium or rhodium.
Of course, when using the noble metals, they may be deposited as surfaces on conductive substrates, e.g., copper, silver, aluminum, steel, iron. The anodes are also of materials or have surfaces of materials such as noble metals, noble metal alloys, noble metal oxides, noble metal oxides mixed with valve metal oxides, c~ e.g., ruthenium oxide plus titanium dioxide, or mixtures thereof, 0 on a substrate which is conductive. Preferably, such surfaces are ` on or in contact with a valve metal and connect to a conductive metal such as those previously described. Useful surfaces are platinum, platinum on the valve metal titanium, platinum oxide on titanium, mixtures of ruthenium and platinum and their oxides on titanium and similar surface coating materials on other valve metals, e.g., tantalum. The conductors for such materials may be c, aluminum, copper, silver, steel or iron, with copper being much ` preferred. A preferable dimensionally stable anode is ruthenium oxide-titanium dioxide mixture on a titanium substrate, connected to a copper conductor.
The voltage drop from anode to cathode is usually in the range of about 2.3 to 5 volts, although sometimes it is slightly more than 5 volts, e.g., up to 6 volts. Preferably, it is in the range of 3.5 to 4.5 volts. The current density, while it may be from 0.5 to 4 amperes per square inch of electrode surface, is preferably from 1 to 3 amperes/sq. in. and ideally about 2 amperes/sq. in. The voltage ranges given are for perfectly aligned electrodes and it is understood that where such alignment is not exact, as in laboratory units, the voltages can be up to about 0.5 volt higher.

'"~ o The feeding of gaseous chlorine into the buffer compart-ment is at such a rate as to enable it to react with the sodium hydroxide entering such compartment from the catholyte and to convert substantially all of it to hypochlorite (or further, to chlorate), thereby preventing it from migrating further into the anolyte. It will be evident that the rate of feed is control-led in response to v~r~rations in caustic transmission into the buffer compartment. Additions may be in response to p~ fluctua-tions in the buffer zone. Normally, to produce hypochlorite, possibly with some c~.lorate therein, in the buffer zone, a pH of 8 to 11 will be maintained whereas to produce chlorate therein this will be lowered to 6 to 7.5, preferably 6 to 7. Control of the pH may be and preferably is by chlorine addition but other acidifying agents may be e~ployed, alsO. On the average, it is ~15 considered that from 5 to 20% of the caustic produced in the catholyte compartment migrates to the buffer compartment and therefore, the stoichiometric amount of chlorine to convert this caustic to hypochlorite will be employed, plus an excess when desired, e.g., from 5 to 20% of chlorine, to adjust the pH. In addition to controlling the pH of the buffer zone electrolyte to obtain the desired product, temperature is also controlled.
Normally, it is maintained at less than 105C., preferably being from 20 to 95C., more preferably, 50 to 95C. and most preferably, about 60 to 85C. or 95C. Similar temperatures apply to the electrolyte in the anolyte and catholyte compartments. However, .
,~ .

_ 23 the pH of the buffer solution and catholyte are different from those of the anolyte, being about 14, compared to about 1 to 5, preferably 2 to 4 for the anolyte. The temperature of the electrolyte may be controlled by recirculation of various portions thereof, in the anolyte, catholyte and buffer zones. Also, it is affected by the proportion of feed to such zones and the temperatures thereof. Feeds will be regulated to obtain the desired temperatures, previously mentioned. Of course, when the temperature cannot be lowered sufficiently by recirculation, refrigeration of the re-circulating liquid may also be utilized. For example, the feedsof water, brine and recirculated electrolyte or mixtures of these entering the anode compartment or any of the other compartments -may be cooled about 5 to 40C. below their otherwise obtained ` temperatures or to about 10C. before admissions to such compartment(s).
When the hypochlorite is being produced in the buffer compartment or a mixture of hypochlorite and chlorate is being made therein the hypochlorite content may be converted to chlorate - externally of the cell by addition of chlorine or other acidifying agent to lower the pH from 8 to 11 to the range of 6 to 7.5, preferably 6 to 7. The chlorine employed is chlorine produced in the cell. It is a preferred acidifying agent for this reason and because byproduct chloride can be reused. Whether the chlorate is made externally or internally or whether the hypochlorite is removed for use, excess chlorine sent to the buffer zone is also recoverable and reusable. Similarly, if chlorate is r :
recovered from the liquid product the aqueous medium may be i returned to the buffer zone, preferably after removal of chloride, ; too.
The processes of this invention realize greatly improved current efficiencies due to their prevention of the wasteful production of oxygen in the anolyte compartment. Anolyte pH is kept low, to prevent oxygen release, by neutralization of hydroxyl !
ions and in the present process the chlorine in the buffer solution diminishes hydroxyl in the anolyte markedly. Thus, chlorine current - 10 efficiencies of from 90 to 97% are obtainable, together with caustic current efficiencies of from 75 to 85% or higher. Also, the caustic made is free of chloride, normally containing as little as 0.1 to 10 9./1. thereof. The hypochlorite concentration will normally be from 50 g./l. to its solubility limit and the chlorate concentration lS producible, either in the cell or external thereto, is 150 to 450 9./l. The sodium hydroxide concentration from the catholyte can be increased by feeding dilute sodium hydroxide, recirculating sodium hydroxide solution previously taken off, increasing the electrolysis time or diminishing caustic solutions may be made by evaporation of comparatively dilute solutions produced. When more concentrated caustic is made in the catholyte the hypochlorite or chlorate made in the buffer zone will also be more concentrated.
The present cells may be incorporated in large and small plants, those producing hypochlorite or chlorate while also ~ 0~6060 r making from 20 to 1,000 tons per day of chlorine or equivalent ' and in all cases efficiencies obtaina~le are such as to make the process economically desirable. It is highly preferred however, that the installation should be located near to and be used in conjunction with a pulp bleaching plant, so that the hypochlorite or chlorate can be employed as a bleach or in the production of bleaching agent, e.g., c~.lorine dioxide.
`, The following examples illustrate but do not limit the ,~ invention. Unless otherwise indicated, all parts are by weight and all temperatures are in C.

EXA~LE 1 To produce hypochlorite electrolytically and externally ccr.v~rt it to chlorate the apparatus .llust.a~ed in FIG. 1 s ~ employed, with the electrolytic cell having steel walls. The L5 anode compartment is lined with polyester resi,n and the buffer compartment is lined with polypropylene. The anode is of an expanded diamond-shaped titanium mesh (1 mm. in thickness and expanded to 50% open area with strand thickness and width being equal), coated with a mixture of ruthenium oxide and titanium oxide 0.1 mm. thick, in a ratio of 1:3. The titanium mesh is communicated with a positive direct current electrical source through a titanium-clad copper conductor. The cathode is of mild steel woven wire mesh 2.2 mm. in diameter and 6 x 6 to the inch and is communicated to a negative electrical source or -!5 sink by a copper conductor. The walls separating the anode and cathode compartments, and together with walls of the cell, :"

` 1076060 , . . .
defining the buffer compartment, are of a cation-ac~ive perm-selective membrane manufactured by E. I. DuPont de Nemours &
Company and sold under the trade name Nafion. Characteristics of such membranes are described in a DuPont New Product Information Bulletin of 10/1/69 under the titie XR Perfluorosulfonic Acid Membranes. The walls of the membrane are seven mils thick (about 0.2 mm.) and it is joined to a backing or supporting network of polytetrafluoroethylene (Teflon~3) filaments having a diameter of about 0.1 mm. and arranged in a screen or cloth form so that the area percentage of openings therein is about 25%. The cross-sectional shape of the filaments is substantially circular and the membranes mounted on them are originally flat and are fused onto the screen or cloth by high temperature, high compression pressing, Wi~l some of tha r.embrane actually flcw-ng around the filaments during the fusion process to lock onto the cloth.
The material of the permselective membrane is a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether. The hydrolyzed copolymer is of tetrafluoroethylene and FSO2CF2CF2OCF(CF3)CF2OCF=CF2 and has an equivalent weight in the 900 to 1,600 range, about 1,250.
In the electrolytic cell illustrated in FIG. 1, for clarity of presentation and in accord with conventional cell illustrations, spaces are shown between the buffer compartment membranes and the electrodes but in the practice of this _ 27 : - 1076060 .
` experiment the electrodes are in contact with the buffer membranes, with thè "flatter" sides of the membranes facing the contacting electrodes. The buffer compartment between them is 1/4 inch (6.4 mm.) wide, for minimum voltage drop at satisfactory production rates and the interelectrode distance is essentially the same, although gaps of 7/16 inch are also successfully used.
The anode compartment is filled with a saturated salt solution or brine and the cathode and buffer compartments are filled with water, initially containing a small quantity of salt or brine to improve conductivity. Then the current is turned on and chlorine is fed to the buffer compartment to convert any sodium hydroxide transmitted thereto to sodium hypochlorite and sodium chloride. Chlorine is removed from the anode compartment and, in addi'ion to being taken off for use or sale as chlor ne, some thereof is fed to the buffer compartment and an additional - proportion is utilized to help to convert s~dium hypochlorite to sodium chlorate externally of the cell. Hydrogen gas is removed from the cathode compartment and, after it reaches a satisfactory concentration, sodium hydroxide is also taken off from that compartment and is essentially free of chloride ions, containing .
about one g./l. of sodium chloride.
During operation of the cell the pH in the buffer compartment is maintained in the range of 8 to 11, at about 10, to promote formation of hypochlorite. Control of the pH in the buffer compartment is maintained by adjusting the feed of chlorine : 1076060 ', and to some extent, water. The pH in the anode compartment ;s held at about 4 and acidification control is maintained by addition ' of small proportions of hydrochloric acid. Of course, the pH in `! the cathode compartment is 14.
The solution of sodium hypochlorite and sodium chloride is conveyed from the electrolytic cell to a retention vessel from which it is pumped continuously in a cycle through a reactor wherein the hypochlorite is treated with chlorine to produce sodium chlorate and more sodium chloride. The mixture is drawn off from the retent;on vessel and the sodium chloride is subsequently separated from the sodium chlorate so that the chlorate may be utilized in pulp bleaching without stream pollution by the accompanying chloride.
In a modification of the described process means are provided for removing sodium chloride from the circulating stream from the retention vessel and chlorate liquor, essentially free of chloride is partly returned to the retention vessel through the reactor, where a small proportion of sodium hypochlorite present therein is reacted with chlorine to produce additional chlorate, and another portion of the chloride-free chlorate is removed from the system, to be crystallized to solid chlorate or to be employed as a chlorate liquor. When crystallized, the mother liquor is returned to the buffer compartment of the electrolytic cell. The following table describes the operation of the process (unmodified) of this example in a number of variations of the described process.

` 1076060 ...
~, , a~ o o O ~ ~ 00 ~ r o oo U~
.~ ~O ~1 ~ ~D ~ ~ OU~ O el~
,,,~
~ 11 ~ ~ o o :
~ ~ O O U~
o ~ ~ r . oo o o ~o ~o ,~ o _~ o o o t _Ir~

.~ , .
~ oo r~
~1 O ts t~ D CO ~ ~ CO O O
o ,~ co In O ~1 0C~
_II`~ I ~ ~ ~I~I
~ ;' : ~
n o O OU) ~ ~D ~ ~ ~r IO ~ N _I O~ ~ O CO ~'1 _II~~ ~ ~ ~ ~~1 ~ V ~
,.
O O
O O ~ ~
.~, o ~ ~ CO ~ ~ COO _I ~ G~ ;
k ,~
I o ~ o o o ~ o ,1 ~
_l,~, V ~ "~ . _l .
:~
~ .
~ .

,-~ ~ U ~ ~ ~ ~~3 ~

-- O `~
t) d U
` g o Z U o U --a ~ ; o o ~ o o a~ 3 ,~
o C~ ~~J o ~ ~ ~
d ~ ~ ~ W--I
Z O )I O O O ~
3 ~ X
Z Z Z 0 ~3~ W a~

0 0 ~ ~ ~ ~ ~ ~WX ~ ~
a) o o o ~ 0a~ o ~ ~-~ ~`~ ~ o 0o ~ ~ ~ J~ O ~ ~ ~ a~
~; m u u u ~: m mu ~ ~
0 ~ o o In o ~o ~ .

o I~
N If 1 ~ C~ 01 O
If) ~ ~D or~ N ~ O
~r O ~~ ~1 _I ~t` O ~ O

, ~r o ot`
t~ U) O O ~
IO ~1 Ir) ~~ N ~D O
~. ~P. . . . .I
. I~) O~ N r--er o oa:~ ' 00 N O ~ra~ r-l ~ ~ O
p~ ~
0 ~r ~ ~ O ~r_I ~ ~ N
,~ ~ ~ a~ o~ ~ N

~r o ~ ~
) N O 10 ~ DN~D N O
0;~In CS~ O ~ NC~~ r~ O
N
l~ O ~~) ~) ~1 ~rN N N O ~

_ ~ O ~r~
~J a~ N ~ O~O OO~O O O
_ 0~ ~ Nr~ O
~1 o ~
O _I I~ w a~ N
. ~ .
.. :11~ d.
~¢ . ~;
E~
~, ~a) , O r _ ~ ~
_~ O _ ~ m a) o .~1 ~ O ::~
o o ~ ~ a) o _ a) a t) --a ~ ~ h E~

C) O ~ :>~ O 0 ~ ~ O` ~ ~:

a) o o ~ s~ ~I z o o ~1 0 ~ a~ ~ ~ ~
O ~ 0 1~]
" ~ ; 3 3 ~ Cl. .

In O Irl ~1 ` '3 o .

In the procedure described the feed of sodium chloride to the anolyte compartment is at about 25~ sodium chloride concentration and in the effluent from the anolyte the chloride concentration is about 22~. The chloride-free caustic is taken off from the cathode compartment and the buffer compartment material is either employed as hypochlorite or, as illustrated in FIG. 1, is fed to a reactor and then to a holding tank equipped with means to lower the chloride concentration during recirculation. In the holding tank, wherein the pH is held at 6.5, the hypochlorite is converted to chlorate with a typical concentration and that of this example being 430 g./l. of sodium chlorate,with 140 g./l.
sodium chloride. In some runs as much as 500 g./l. of the chlorate and as little as 100 g./l. of the chloride are ~ produced. The hypochlorite and chlorate produced are used i in bleaching of groundwood pulp and greatly improve the , , color thereof.
In a modlfication of the preceding process the apparatus of FIG. 2 is employed and the rate of chlorine feed to the buffer compartment is regulated so that the pH of the buffer solution is maintained at about 6.5, at which pH the hypochlorite is converted to chlorate in the buffer compart-ment. Otherwise, the experiments are essentially the same and the product resulting is the same.

_ 32 o The procedure of Example 1 is followed with the exception that the apparatus of FIG. 3 is employed and sodium chloride is continuously removed from recirculating chlorate, which circulates through a chloride removal apparatus and also back to the buffer compartment. By this method chlorate is continuously removed from the holding vessel and chloride content is maintained low enough so that it does not crystal-lize out in the cell or other portions of the apparatus.
.

~10 EXAMPLE 4 Using a commercial size three-compartment cell like that of FIG. 1 chlorate is formed externally at the rate of 0.42 ton per day of sodium chlorate, at 95~ conversion, main-taining the buffer compartment pH at about 1~.5 and the reactor and holding vessel pH at about 6.5. The current is 90 kiloamperes and the current density is 2 amperes/sq. in., at a direct current potential of 4.5 volts and at 70C., and the process is continuous. The chlorine feed to the buffer compartment is at the rate of 0.89 ton per day of the three tons per day of chlorine produced at the anode at 95% current efficiency. Sodium hydroxide produced is at a 25% concentration - and is made at the rate of 2.28 tons per day. Sodium chloride solution charged to the anode compartment is a 25% solution and the concentration of sodi~m chloride in the effluent from t~at compartment is 22%.

~7 ~3 EXAMPLE S
Using a commercial apparatus like that o FIG. 2 and mainta1ning the buffer compartment pH at 6.5, 0.4 ton per day of sodium chlorate is made in situ in the buffer compartment at a 90% conversion rate. A small proportion (about 5%) of hypo-chlorite is present in the product. The pH is maintained by -addition of more chlorin~ -.tO the compartment. Other conditions are the same as described in Example 4. In a modification a batch --process is employed with essentially the same results. When in 10~ place of the described membrane there are substituted 18STl2S
and 16ST13S RAI membranes of about twice the thickness of the XR
perfluorsulfonic acid membranes employed in the other examples the same reactions are effected and the desired products also result. However, in such cases it is noted that the RAI
membranes are not as reslstant to the electrolyte and the products of electrolysis and do not last as long in use until replacement becomes desirable. This is especially true when thinner membranes, such as those of 7 mil thickness are employed.
The invention has been described with respect to working examples and illustrative embodiments but is not to be limited to these because it is evident that one of ordinary skill in the art will be able ~ utilize substitutes and equivalents without departing from the spirit of the invention or the scope of the claims.
k . .

.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for electrolytically manufacturing a hypo-chlorite which comprises electrolyzing an aqueous solution containing chloride ions in an electrolytic cell having at least three com-partments therein, being anode and cathode compartments and at least one buffer compartment, an anode, a cathode, at least one cation-active permselective membrane selected from the group consisting of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether, and a sulfostyrenated per-fluorinated ethylene propylene polymer, defining a cathode-side wall of a buffer compartment between the anode and cathode, an anode-side wall of a buffer compartment being defined by such a cation-active permselective membrane or a porous diaphragm, said walls defining anode, cathode and buffer compartments, while feeding gaseous chlorine into a buffer compartment and regulating the rate of feed thereof and reaction conditions to produce hypochlorite in the buffer compartment.

2. A method according to Claim 1 wherein the hypochlorite is removed from a buffer compartment and is converted to chlorate externally of the cell.

3. A method according to Claim 2 wherein chlorate produced is sent to the cell buffer compartment to increase the concentration thereof, and alkali metal hydroxide, and chlorine further react where-by the hydroxide is converted to hypochlorite in the cell, and hypochlorite is converted to chlorate externally of the cell and recovered.

4. A method according to claim 1 wherein the perm-selective membrane(s) is/are of a hydrolyzed copolymer of tetra-fluoroethylene and a sulfonated perfluorovinyl ether of the formula FSO2CF2CF2OCF(CF3)CF2OCF=CF2, which copolymer has an equivalent weight of about 900 to 1,600.

5. A method according to claim 4 wherein the pH of the aqueous buffer compartment solution is maintained in the range of about 6 to 11, the temperature thereof is less than about 105°C. and the cell contains a single buffer compartment.

6. A method according to claim 5 wherein the anode and cathode side walls of the buffer zone are of the permselective membrane, the membrane walls are from about 0.02 to 0.5 millimeter thick and the buffer solution pH from 8 to 11.

7. A method according to claim 6 wherein the membranes are mounted on a network of a material selected from the group consisting of polytetrafluoroethylene, asbestos, perfluorinated ethylene propylene polymer, polypropylene, titanium, tantalum, niobium and noble metals, which has an area percentage of openings therein from about 8 to 80%.

8. A method according to claim 7 wherein the temperature is from 60 to 95°C., the network is a screen or cloth of polytetra-fluoroethylene filaments having a thickness of 0.01 to 0.5 mm., being less than or equal to the thickness of the membrane mounted thereon and the area percentage of openings in the screen or cloth is from about 10 to 70%.

9. A method according to claim 8 wherein the membrane walls are from 0.1 to 0.3 mm. in thickness and the temperature of the electrolyte is regulated at least in part by the recircula-tion of compartment contents.

10. A method according to claim 2 wherein the hypo-chlorite is converted to chlorate externally of the cell by adjustment of the pH of electrolyte removed from the cell to the range of about 6 to 7.5 by addition of chlorine gas.

11. A method according to claim 9 wherein the hypo-chlorite is converted to chlorate externally of the cell by adjustment of the pH of electrolyte removed from the cell to the range of about 6 to 7.5 by addition of chlorine gas and maintain-ing it at such pH for from about 10 minutes to one hour.

12. A method according to claim 1 wherein the hypo-chlorite made is sodium hypochlorite an d chloride ions are from sodium chloride.

13. A method according to claim 11 wherein the hypochlorite made is sodium hypochlorite and the chloride ions are from sodium chloride.

14. A method according to claim 13 wherein the surface of the cathode is of a material selected from the group consisting of platinum, iridium, ruthenium, rhodium, graphite, iron and steel and the surface of the anode is of a material selected from the group consisting of noble metals, noble metal alloys, noble metal oxides, mixtures of noble metal oxides with valve metal oxides, and mixtures. thereof, on a valve metal, the aqueous sodium chloride solution being electrolyzed is at a concentration of from 200 to 320 grams NaCl per liter, the pH of the anolyte is about 1 to 5, the temperatures of the compartment contents are in the range of 60° to 95°C., the voltage is from about 2.3 to 5 volts and the current density is from about 0.5 to 4 amperes per square inch of electrode surface.

15. A method according to claim 14 wherein the copolymer equivalent weight is from about 1,100 to 1,400, the cathode is of steel and the anode is of ruthenium oxide on titanium, the aqueous sodium chloride solution being electrolyzed is at a concentration of 250 to 300 grams per liter and the pH
of the anolyte is from about 2 to 4.

16. A method for electrolytically manufacturing a chlorate which comprises electrolyzing an aqueous solution containing chloride ions in an electrolytic cell having at least three compartments therein, being anode and cathode compartments and at least one buffer compartment, an anode, a cathode, at least one cation-active permselective membrane selected from the group consisting of hydrolyzed copolymers of a per-fluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether, and of a sulfostyrenated perfluorinated ethylene propylene polymer defining a cathode-side wall of a buffer compartment between the anode and cathode, an anode-side wall of a buffer compartment being defined by such a permselective membrane or a porous diaphragm, and said walls, with walls thereabout, defining anode, cathode and buffer compartments, while feeding gaseous chlorine into the buffer compartment and regulating the rate of feed thereof and reaction conditions to produce chlorate in the buffer compartment, and removing the chlorate so produced.

17. A method according to claim 16 wherein chloride produced in the buffer compartment in the reaction by which chlorate is made therein is removed from the chlorate externally of the cell and the chlorate is recirculated to the cell buffer compartment to increase the concentration thereof resulting from the process.

18. A method according to claim 16 wherein the permselective membrane(s) is/are of a hydrolyzed copolymer of tetrafluoroethylene and a fluorosulfonated perfluorovinyl ether of the formula FSO2CF2CF2OCF(CF3)CF2OCF=CF2, which copolymer has an equivalent weight of about 900 to 1,600.

19. A method according to claim 18 wherein the pH of the aqueous buffer compartment solution is maintained in the range of about 6 to 7.5, the temperature thereof is less than about 105°C.
and the cell contains a single buffer compartment.

20. A method according to claim 19 wherein the anode-side and cathode-side walls of the buffer zone are permselective membranes, which are of a hydrolyzed copolymer of tetrafluoroethylene and a sulfonated perfluorovinyl ether of the formula of claim 18 and the membrane walls are from about 0.02 to 0.5 millimeter thick.

21. A method according to claim 19 wherein the membranes are mounted on a network of a material selected from the group con-sisting of polytetrafluoroethylene, perfluorinated ethylene propylene, polypropylene, titanium, tantalum, niobium and noble metals, which has an area percentage of openings therein from about 8 to 80%.

22. A method according to claim 21 wherein the temperature is from 60° to 95°C., the network is a screen or cloth of polytetra-fluoroethylene filaments having a thickness of 0.01 to 0.5 mm., being less than or equal to the thickness of the membrane mounted thereon, and the area percentage of openings in the screen or cloth is from about 10 to 70%.

23. A method according to claim 22 wherein the membrane walls are from 0.1 to 0.3 mm. in thickness and the temperature of the electrolyte is regulated at least in part by the recirculation of compartment contents.

24. A method according to claim 16 wherein the chlorate made is sodium chlorate and the chloride ions are from sodium chloride.

25. A method according to claim 23 wherein the chlorate made is sodium chlorate and the chloride ions are from sodium chloride.

26. A method according to claim 25 wherein the surface of the cathode is of a material selected from the group consisting of platinum, iridium, ruthenium, rhodium, graphite, iron and steel and the surface of the anode is of a material selected from the group consisting of noble metals, noble metal alloys, noble metal oxides, mixtures of noble metal oxides with valve metal oxides, and mixtures thereof, on a valve metal, the aqueous sodium chloride solution being electrolyzed is at a concentration of from 200 to 320 grams NaCl per liter, the pH of the anolyte is about 1 to 5, the temperatures of the compartment contents are in the range of 60° to 95°C., the voltage is from about 2.3 to 5 volts and the current density is from about 0.5 to 4 amperes per square inch of electrode surface.

27. A method according to claim 26 wherein the copolymer equivalent weight is from about 1,100 to 1,400, the cathode is of steel and the anode is of ruthenium oxide on titanium, the aqueous sodium chloride solution being electrolyzed is at a concentration of 250 to 300 grams per liter and the pH
of the anolyte is from about 2 to 4.

28. A method for electrolytically manufacturing a hypochlorite or chlorate which comprises electrolyzing an aqueous solution containing chloride ions in an electrolytic cell having at least three compartments therein, being anode and cathode compartments and at least one buffer compartment, an anode, a cathode, at least one cation-active permselective membrane selected from the group consisting of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluoro-vinyl ether, and a sulfostyrenated perfluorinated ethylene propylene polymer, defining a cathode-side wall of a buffer compartment between the anode and cathode, an anode-side wall of a buffer compartment being defined by such a cation-active permselective membrane or a porous diaphragm, and such walls, with walls thereabout, defining anode, cathode and buffer compartments, while feeding gaseous chlorine into a buffer compartment and regulating the rate of feed thereof and reaction conditions to produce hypochlorite or chlorate in the buffer compartment, and removing the hypochlorite or chlorate so produced.