CA1043736A - Electrolytic process for manufacturing chlorine dioxide, hydrogen peroxide, chlorine, alkali metal hydroxide and hydrogen - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Chlorine dioxide, hydrogen peroxide, chlorine, alkali metal hydroxide and hydrogen are produced from alkali metal chloride, alkali. metal chlorate, sulfuric acid and water, utilizing an electrolytic cell having anode and cathode compartments separated by two intermediate buffer compartments, the boundaries between the anode and cathode compartments and the buffer compartments being of cation-active permselective membranes which are resistant to attack by the medium And the buffer compartments being separated by a suitable anion-active permselective membrane. On electrolysis, with sulfuric acid fed to the anode compartment, chloride and chlorate fed to the buffer compartment adjacent to the cathode compartment and water fed to the cathode compartment there are produced hydrogen and alkali metal hydroxide in the cathode compartment,chorine dioxide and chlorine in the buffer compartment adjacent to the anode compartment and persulfuric acid in the anode compartment. The persulfuric acid is hydrolyzed to produce hydrogen peroxide. Hydrogen peroxide, alkali metal hydroxide, chlorine and chlorine dioxide are useful pulp mill chemicals especially suited for pulping wood and bleaching wood pulp.

Description

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The present invention is directed to the preparation of chlorine dioxide, hydrogen peroxide, chlorine, aqueous alkali metal hydroxide solution which is substantially free of alkali metal halide, and hydrogen and an electrolytic cell therefor.
More particularly, the invention is of such a process and cell which utilizes a relatively simple four compartment electrolytic cell having anion-active and cation-active membranes separating compartments thereof.
Chlorine dioxide, hydrogen peroxide, chlorine, and salt-free aqueous alkali metal hydroxide are chemicals that are fre-quently employed in pulp mill operations, especially for the pulping of wood chips and bieaching of wood pulps. It has long been desired, ` for reasons of economy and convenience, to prepare these chemicals ;
together at a single site, preferably adjacent to the pulp mills.
However, the known methods of producing each of these chemicals require comparatively costly and complex apparatuses and multi-`~ ~ plicities of reaction stages, so that single-site productions of -;
¦ these reagents has heretofore proved impractical. For example, in the well known Day-Kesting process for making chlorine and chlorine dioxide, aqueous alkali metal chloride is electrolyzed . . - ~
to the chlorate, which is treated with hydrogen chloride to form ; ~, chlorine and chlorine dioxide, which are separated by treatment with water in an absorption tower. This process, however, employs `~
a very slow countercurrent contact of chlorate solution and hydrogen ',1 25 chloride so that, in addition to an electrochemical cell, the pro-, cedure requires a costly array of cascading reactors i 5 3~

with a large storage tank for holding the chlorate solution prior to its reaction ~ith hydrogen chloride [see the article by W. H. Rap~on, Canadian Journal of Chemical Engineering Vol. 36, p. 6 (1958)]. Furthermore, this process does not produce hydrogen S peroxide or a substantially salt-free alkali metal hydroxide, i.e., aqueous sodium hydroxide containing less than about one percent of alkali metal chloride.
The foregoing disadvantages of typical prior art proces-ses are overcome by the present invention, which provides a novel method, utilizing a relatively simple reaction apparatus, for co-producing chlorine dioxide, hydrogen peroxide, chlorine, substan-tially chloride-free alkali metal hydroxide solution and hydrogen, from aqueous alkali metal chlorate, aqueous alkali metal chloride, sulfuric acid, water and electric power. This method comprises `15 electrolyzing in a cell having an anode compartment with anode therein, a cathode component with c~thode therein and intermediate buffer compartments, Bl and B2, the anode compartment being sepa-~rated from Bl by a cation~active permselective membrane, M , the ~ cathode compartment ~eing separated from B2 by a cation-active permselective membrane, Mc 2, and Bl and B2 being separated from each other by an anion-active permselective membrane, Ma, solutions resultlng from feeding sulfuric acid to the anode compartment and alkali metal chloride and alkali metal chlorate to B2, so that with the passage of electric current through the cell hydrogen , ion selectively diffuses or passes from the anode compartment to through Mc 1, chloride and chlorate anions selectively difuse or pass from B2 to Bl through Ma and alkali metaI cations selectively ~............... - ~ ~

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diffuse or pass from B to the cathode compartment through Mc sulfuric acid is oxidized at the anode to produce a sulfuric acid solution of persulfuric acid in the anode compartment, hydrogen chloride and aqueous chlorate anions are reacted to produce chlorine and chlorine dioxide in Bl, and water and aqueous alkali metal cation are reacted at the cathode to pro-duce aqueous, substantially alkali metal chloride-free alkali metal hydro~ide and hydrogen in the cathode compartment, after which the persulfuric acid solution, chlorine dioxide, chlorine, hydrogen and aqueous alkali metal hydroxide are removed from the cell compartments. Subsequently, the aqueous persulfuric acid solution is converted to sulfuric acid and hydrogen peroxide.
According to another aspect of the invention there is provided an electrolytic cell which can be used in the above described process of the invention, which comprises anode and cathode compartments and adjacent buffer compartments disposed intermediate said anode and cathode compartments, with ~ ~;
boundaries between the anode and cathode compartments and adjacent , -buffer compartments being of cation-active permselective membranes which are resistant to attack by the medium and with at least two buffer compartments being separated by an anion-active perm~
selectiYe membrane.
The invention will be readily understood by reference i to descriptions of the embodiments thereof herein, taken in conjunction with the drawing of means for carrying out a pre- ;
ferred embodiment of the procese. ~ ~

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In the Drawing:
The FIGURE is a schematic diagram of a four-compart-ment electrochemical cell for converting water, alkali metal chloride, alkali metal chlorate and sulfuric acid to chlorine dioxide, chlorine, aqueous alkali metal hydroxide, hydrogen a-nd persulfuric acid. The FIGURE also includes hydrolysis means for converting the persulfuric acid to hydrogen peroxide by reaction with water in the form of steam.
In the FIGURE the points of addition and withdrawal 3~

of typical and preferred reactants and products are illustrated.
Although the production of sodium hydroxide solutions, using sodium chloride and sodium chlorate reactants is illustrated, other alkali metal cations, such as potassium, may also be employed. Furthermore, although the hydrolysis means illustrated is a steam distillation apparatus, it will be appreciated that other suitable vessels or apparatuses for reacting the persulfuric acid solution with water can also be used.
In the FIGURE 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 a cation-active permselective membrane Mc 1 23, anion-active permselective membrane Ma 25, and cation-active permselective membrane Mc 2 27, divide the volume into an anode or anolyte compartment 29, a buffer compartment Bl 31, a buffer compartment B 33, and a cathode or catholyte compartment 35. Aqueous sulfuric acid is fed to the anode compartment through line 37. Aqueous sodium chlorate and aqueous sodium chloride are fed to B2 through line 39 and water is fed to the cathode compartment through line 41. During electrolysis sulfuric acid in the anode compartment is oxidi~ed at the anode to form persulfuric acid which is with-drawn as an aqueous sulfuric acid solution through line 43. Also during electrolysis, hydrogen ions selectively diffuse or pass from the cathode compartment through cation-active membrane Mc 1 into buffer compartment Bl while chlorate and chloride anions selectively pass from buffer compartment B2 through anion-active membrane Ma into buffer compartment Bl. In Bl the aqueous hydrogen chloride introduced by the aforementioned diffusion processes reacts with the chlorate anions to produce chlorine dioxide and chlorine, which are withdrawn through line 45.
Under the electric potential of the electrolysis process sodium cat;ons selectively diffuse from buffer compartment B2 through -~ :
cation-active membrane Mc 2 into the cathode compartment where lo they are reacted with water to form hydrogen, which is withdrawn ; through line 47, and aqueous sodium hydroxide, which is withdrawn through line 49. The aqueous sulfuric acid solution of per-sulfuric acid which is recovered from the anode compartment is fed to a steam distillation apparatus 51 and is hydrolytically distilled with steam fed to ~he apparatus through line 53. The resulting steam distillate, an aqueous hydrogen peroxide solution, is withdrawn from the steam distillation apparatus through line 55 and the steam distilland, an aqueous sulfuric acid, is withdrawn from the apparatus through line 57.
In the present process the overall electrolytic cell reaction is represented by Equation (1) 4H2S04 + 2MC103 + 2 MCl - 2H20 2H2S28 + 2C12 + C12 + 2H2 + 4MOH
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wherein M represents an alkali metal cation such as sodium or potassium. The hydrolytic conversion of persulfuric acid to hydrogen peroxide proceeds by the known reaction represented by Equation (2).

(2) 2H2S20~ + 4H20 ~ ~ 2H22 + 4H2S4 In initiating the electrolytic process of the invention the anode compartments of the cell are charged with sufficient sulfuric acid, ;n aqueous solution, as to initiate the electro-lytic oxidation of the H2S04 to H2S208, while the buffer com-partments are charged with sufficient alkali metal chlorate and/or alkali metal chloride, also in aqueous solution, to avoid depletion and concentration polarization. Additionally, an aqueous solution containing about 0.1 to 1% of alkali metal hydroxide is charged into the cathode compartments. Advanta-geously, the cell is filled so as to provide a small free space,e.g.,~about 1 to 10%, preferably 1 to 5% of the cell volume, above the compartments so as to facilitate collection and withdràwal of the gaseous products, chlorinè dioxide, chlorine and hydrogen. On connection of the conductive means to sources 20~ f positive and negative électrical potentials to initiate a direct current electrolysis, sulfuric acid, alkali metal chlorate and alkali metal chloride are fed to the cell at rates sufficient to establish-concentrations which will effect the electrolysis .,~,.

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according to Equation (1). Typically, these will be in molar proportioned rates, of about 2:1:1, with the usual variance from these of about - 20%, preferably - 10% and most preferably about - 2%. During electrolysis water is charged at a sufficient rate to maintain the desired caustic concentration.
The cell is operated at a temperature above the freezing point of the liquid contents of the cell, i.e., above about 2 to 5C. and below about 60C. or the temperature at which the rate of electrolytic formation of persulfuric acid from sulfuric acid is about eqùal to the rate of hydrolytic decomposition of the preacid.
Preferably, the cell is operated at a temperature of about 5 to 40C., more preferably at about 20 to 35C. and most preferably at about 30 to 35C.
The sulfuric acid charged to the anode compartment is generally aqueous sulfuric acid containing at least about 80% by ~ ;
weight sulfuric acid and is preferably "concentrated" sulfuric acid, "aqueous" sulfuric acid containing about 90 to 100%, usually 93 to 97~ sulfuric acid. If desired and useful, stronger, even non-aqueous sulfuric acids and sometimes, even oleums can be successfully employed.
The alkali metal chloride and alkali metal chlorate are generally charged in aqueous solution or solutions at concen~
trations of from about 1 normal up to about the saturation solubility of the salts. Preferably the concentrations of the ~ ;
aqueous alkal; metal chlorate charged are about 3 N. The chlorate ' '7~
and chloride salts may be charged in individual feed streams to compartment B but preferably the salts are charged in the same feed solution.
The sulfuric acid solution of persulfuric acid produced in the anode compartment is reacted with water at about 60C. to 100C., preferably at about 100C., to produce hydrogen peroxide, in accord with known processes for the hydro-lytic conversion of persulfuric acid to hydrogen peroxide. At least about two molar portions of water per mol of persulfuric acid are employed in the hydrolysis in accord with the stoichio~
metry of Equation (2) above. Advantageously, the water is charged in excess, e.g., 10 to 300% or 20 to 100%. Preferably, the water which ;s charged to the hydrolysis operation is in the form of steam. In an especially preferred embodiment of the invention the persulfuric acid solution is subjected to steam distillation to prepare hydrogen peroxide, the distillation being effected in a steam distillation apparatus comprising a still and a condenser of the types conventionally used for the manufacture of hydrogen peroxide from persulfuric acid. In ~ :
accord with this preferred embodiment of the invention the hydrogen peroxlde is recovered from the steam distillation apparatus as an aqueous steam distillate, with the concentration of the hydrogen peroxide in the distillate being determined by the amount o~ water used in the steam distillation. The proportion of water may be regulated to produce the peroxide in best form for _ g_ .

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use, e.g., in b1eaching, especia11y of woodpulp. The distilland remaining is aqueous sulfuric acid which can be concentrated, if desired, by addition of stronger sulfuric acid, oleum or sulfur trioxide, and may then be recycled to the sulfuric acid feed stream to the anode compartment of the present electrolytic cell.
Alternatively, it may be sent to that compartment directly.
The chlorine and chlorine dioxide produced in buffer compartment B are recovered as a gaseous mixture. If desired, these products can be separated by contacting the mixture with water to preferentially dissolve the chlorine dioxide. Advanta-geously this separation can be effected by contacting the chlorine dioxide-chlorine mixture with a countercurrent stream of water in a conventional absorption tower of the type utilized for separation of chlorine dioxide and chlorine in the previously discussed Day-Kest;ng process. If desired, the chlorine dioxide and chlorine may remain together and be employed in such mixture. Of course, the separate or mixed products are useful as bleaching agents, especially for woodpulps.
The aqueous alkali metal hydroxide solution recovered from the cathode compartment generally contains about 60 to 250 9./1., usually about 80 to 120 g./l. of alkali metal hydroxide ;
and is free or substantially free of alkali metal chloride, i.e., the product solution generally contains less than about 1% alkali metal chloride and under most preferred operating conditions, less than about 0.1%. Thus, the aqueous caustic product is often suitable, without further purification, for many applications 3~

wherein substantially salt-free aqueous alkali metal hydroxides or caustic is desirable or necessary, ~or example, in pulping wood chips, neutralizing acids, peroxide bleaching, making caustic sulfites and regenerating ion-exchange resins.
The present electrolytic cells operate at a voltage of about 2.3 to 5 volts, preferably about 2.5 to 4 volts, and most preferably, about 3 volts. The current density in the cells is about 0.5 to 4, preferably about 1 to 3, more preferably about 3 amperes per square inch of electrode surface. The current efficiency of the present cell is generally at least about 70%, ` and, under preferred operating conditions, is about 75 to 80%
or greater. The caustic efficiency of the electrolytic cell is generally greater than about 75% and, under preferred operating conditions may be 85 to 90% or greater.
The membranes utilized in the invention to divide the electrolytic cell into compartments and to provide selective ion diffusion are preferably mounted in the cell on networks or screens of supporting material such as polytetrafluoroethylene, perfluorinated ethylene-propylene copolymer, polypropylene, asbestos, titanium, tantalum, niobium or noble metals. Preferably, ~;~
polytetrafluoroethylene screening ;s used.
The cation-active and anion-active perselective mem- ~ -branes used are of known classes of proprietary organic polymers, initially often being thermoplastics, which are substituted with a multiplicity of ionogenic substituents and which, in thin film form, are permeable to a certain type of ion. Certain ions, ,:, -. .

apparently by means of ion exchange with the ionogenic substi-tuents on the polymer film, are able to pass through the polymer membrane, while other ions, of opposite sign, are not able to do so .
Cation-active permselective membrane materials which selectively permit passage or diffusion of cations generally contain a multiplicity of sulfonate or sulfonic acid substituents or, in some instances, carboxylate or phosphonate substituents.
Cation-active membranes can be prepared by introducing the cation-exchanging substituents, e.g., sulfonate, into a thin film of polymer, e.g., phenol formaldehyde polymer, by chemical reaction, e.g., sulfonation. Other polymers which can be sulfonated in this manner to obtain cation-active membrane materials include ~`~
polystyrene, styrene-divinyl benzene copolymer, polyvinyl chloride, vinyl chloride-styrene copolymers, polyethylene, and styrene-butadiene rubbers. Alternatively a homo- or copolymer containing the cation-exchanging group(s) can be prepared by polymerizing a monomer substituted with the group(s3. For example, phenol sulfonic acid can be substituted for some or all of the phenol normally `
used as a reactant 1n preparing a phenol formaldehyde polymer to obtain polysulfonated phenol formaldehyde polymer. In another example of this type of procedure, acrylic, methacrylic or maleic acid or its anhydride can be polymerized or copolymerized, e.g., with divinyl benzene, to obtain a cation-active membrane material in which the cation exchanging substituents on the pslymer base are carboxylate groups.

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Anion-active permselective membranes permit selective passage or diffusion of anions and are impermeable or substantially impermeable to cations. In such membranes, the anion exchanging substituents on the polymer base are generally quaternary ammonium substituents wherein the substituent groups on the nitrogen atoms can be lower alkyl groups, i.e., alkyl groups of 1 to 6 carbon atoms, such as methyl, ethyl, t-butyl and isopropyl; aralkyl groups, such as benzyl; aryl grnups such as phenyl or tolyl; or hetero-cyclics, such as hydrocarbyl-nitrogen ring-containing compounds, e.g., those containing pyridine groups. Anion-active membrane materials can be made by conventional aminations of thin films of polymer base, e.g., phenol-formaldehyde polymer, polyethylene, polyvinyl chloride and the like, followed by quaternizing of the amino substituents by conventional reaction with an alkylating agent, e.g., a lower alkyl halide, such as methyl iodide or di-lower alkyl sulfate such as dimethyl sulfate. Alternatively, thin films of polymer bases such as polystyrene, polyethylene and styrene-divinyl benzene copolymers can be haloalkylated, for example, by conventional chloromethylation, to introduce the -group -CH2Cl, and thereafter may be reacted with a tertiary amine, such as trimethyl amine, to produce the quaternary ammonium sub-stituted anion active membrane. Additionally, polymer bases which contain replaceable halogen substituents such as polyvinyl chloride, - -chlorinated polyethylene, and chlorinated rubber, can be condensed with polyalkylene polyamines, such as tetraethylene pentamine, to produce anion-active polymeric membranes. The cation-active and ::
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anion-active polymeric membranes used for selective diffusion of ions are further classified as homogeneous, i.e., polymers visually appearing to be of only one phase, or as heterogeneous, i.e., polymers visually appearing to include more than one phase because of the presence of a matrix material in which the ion exchange polymer is embedded in powdered form.
The preparation and structure of cation and anion-active permselective membranes are discussed in greater detail in the chapter entitled "Membranes" in the "Encylopedia of Polymer Science and Technology" published by J. Wiley and Sons, New York, 1968, at Vol. 8, pages 620 to 638, and in the chapter entitled "Synthetic Resin Membranes" in Diffusion and Membrane Technolo~y, by S. B. Tuwiner, published by Rheinhold Publishing Corporation, New York, 1962, at pages 200 to 206.
In addition to the examples of anion-active permslective membranes listed above, the following proprietary compositions are anion-active permselective membranes, and may also be considered as representative of preferred membranes of such type: AMFion~ 310 series - anion type quaternary ammonium substituted fluorocarbon polymer, manufactured by American Machine and Foundry Co.; and Ionac~ types MA 3148, MA 3236 and MA 3475R-quaternary ammonium substituted polymers derived from heterogeneous polyvinyl chloride, manufactured by the Ritter-Pfaudler Corp., Permutit Division.
In addition to the examples of cation-active permselective membranes previously discussed, ~he following proprietary composi-tions are representative examples of cation-active permselective ~L~ 3 ~

membranes which may be used in practicing the present invention:
Ionac~ MC 3142, MC 3235, and MC 3470 XL types - polysulfonate- ~
-; substituted heterogeneous polyvinyl chloride, manufactured by ` ~ -the Ritter-Pfaudler Corp., Permutit Division; NafionEDXR type -5 hydrolyzed copolymer of perfluorinated olefin and a fluoro-sulfonated perfluorovinyl ether, manufactured by E. I. DuPont de Nemours and Company, Inc.; Nafion XR, modifled - Nafion XR treated on one side with ammonia to convert S02F groups to S02NH2, which are then hydrolyzed to S02NHNa; RAI Research Corporation membranes 10 such as types 18ST125 and 16ST13S - sulfostyrenated perfluorinated -ethylene propylene copolymers.
Preferred cation-active permselective membranes of the invention are the hydrolyzed copolymer of perfluoroolefins and fluorosulfonated perfluorovinyl ether, the -S02NHNa modifications 15 thereof and the sulfostyrenated perfluoroethylene-propylene co-polymers. ;
The sulfostyrenated perfluoroethylene-propylene polymers useful as cation-active membranes in a preferred embodiment of the invention are generally those which have 2/3 to 11/16 of the phenyl 20 groups therein monosulfonated and which are about 16 to 18% sty-; renated. To manufacture the sulfostyrenated perfluoroethylene propylene copolymer membrane materials, a standard perfluoroethylene-propylene copolymer (hereinafter referred to as FEP), such as is ~` manufactured by E. I. DuPont de Nemours & Company, Inc., is styrenated 25 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 megarad. 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 sulfur;c ~ ;~
acid or S03. Preferably, chlorosulfonic acid in chloroform is utilized and the sulfonation is completed in about 1j2 hour.
Examples of useful membranes made by the described process are the RAI Research Corporation products previously mentioned, 18ST12S and 16ST13S, the former being 18% styrenated and having 2/3 of the phenyl groups monosulfonated and the latter being 16% styrenated and having 13/16 of the phenyl groups mono-sulfonated. To obtain 18% styrenation a solution of 17-1/2% of -styrene in methylene chloride is utilized and to obtain 16% sty-renation a solution of 16% of styrene in methylene chloride is employed. -The especially preferred cation-active permselective membranes of the invention are of a hydrolyzed copolymer of per-fluorinated hydrocarbon, e.g., an olefin, and a fluorosulfonated perfluorovinyl ether. The perfluorinated olefin is preferably tetrafluoroethylene, although other perfluorinated hydrocarbons .

of 2 to 5 carbon atoms may also be uti1ized, of which the mono-olefinic hydrocarbons 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-L2-(2-fluorosulfonYlethoxy)-propyl vinyl ether], referred to hence-forth as PSEPVE, may be modified to equivalent monomers which are represented by the formula FS02CFRlCF20(CFYCF20)nCF=CF2, wherein Rl is a radical selected from the group consisting of fluorine and perfluoroalkyl radicals having from 1 to 10 carbon atoms, Y is a radical selected from the group consist;ng of fluorine and the trifluoromethyl radical, and n is an integer from 1 to 3, inclusive.
However, it is most preferred to employ PSEPVE.
The method of manufacture of the hydrolyzed copolymer is described in Example XVII of the U.S. Patent 3,282,875 and an al-ternative 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 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 equiva-lent weight is about 900 to 1,600 preferably 1,100 to 1,400, e.g., 1,250, and the percentage of PSEPVE or corresponding compound is about 10 to 30%, preferably 15 to 20% and most preferably about 17%. The unhydroly7ed 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 khen further treated to hydrolyze pendant -S02F groups to -S03H groups, as by treating with 10% sulfuric acid or by the methods of the patents previosuly mentioned. The presence of the -S03H groups may be verified by titration, as describecl in the Canadian patent.
Additional deta;ls of various processing steps are described in Canadian Patent 752,427 and U.S. Patent 3,041,317.
Because it has been found that some expansion accom- -panies 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, w;thout 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, penetrating into the spaces between them and even around behind them, thinning the films slightly in the process, where they cover the filaments.

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The aminated and hydrolyzed improvements or modifications of the polytetrafluoroethylene PSEPVE copolymers are made, as pre-viously mentioned, by treatment with ammonia of one side of the copolymer, before hydrolysis thereof, and then hydrolyzing with caustic or other suitable alkali. Acid forms may also be utilized.
The final hydrolysis may be conducted after the membrane is mounted on its supporting network or screen. The membranes so made are fluorinated polymers having pendant side chains containing sulfonyl groups which are attached to carbon atoms bearing at least one fluorine atom, with sulfonyl groups on one surface being in -(S02NH)nM form, where M is H, NH4i alkali metal or alkyline earth - ; -metal and n is the valence of M, and the sulfonyls of the polymer on the other membrane surface being in -(S03)pY form or -S02F, wherein Y is a cation and p is the valence of the cation, with the requirement that when Y is H, M is also H. In use the sulfonamide side faces the cathode.
A complete description of methods for making the above ;mproved membrane is found in French Patent No. 2,152,194 of E. I.
DuPont de Nemours and Company, Inc., corresponding to U.S. Patent application S.N. 178,782, filed September 8, 1971 in the name of Walther Gustav Grot.
The membranes of hydroly~ed copolymer of perfluorlnated olefin and fluorosulfonated perfluorovinyl ether and the one-side hydrolyzed aminated modifications thereof described are far superior in the present processes to various other cation-active membrane '' ~ .

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;'3,~t.3 materials. The RAI type membranes are also generally superior to those previously known. The preferred membranes last for much longer time periods in the medium of the cell electrolytes and do not become brittle when subjected to long term contact with chlorine, chlorine dioxide and persulfuric acid. Consid-ering the savings in time and fabrication costs, the present membranes are more economical. The voltage drops through the membranes are acceptable and do not become inordinately high, as they do with many other cation-active 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 do not decrease detrimentally as the hydroxyl concentration in the `
catholyte liquor increases, as has been noted with other cation-active 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 or the alkal;nity in the catholyte increases. Thus, these differences . .
in the present process make it practicable, whereas previously 0 described processes have not attained commercial acceptability. ;~
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 employable in present methods may `~
be of equivalent weights from 500 to 4,000. The medium equivalent weight polymers are preferred because they are of satisfactory ', ~
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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's improved operation.
The improved versions of the TFE - PSEPVE copalymers, made by chemical treatment of surfaces thereof to modify the -S03H group thereon, may have the modification only on the surface or extending up to as much as halfway through the membrane. The -depth of treatment will usually be from 0.001 to 0.2 mm., e.g., 0.01 mm. Caustic and other efficiencies of the invented processes, using such modified versions of the present improved membranes, can increase about 3 to 20%, often about 10 to 20%, over the un-modified membranes.
The me~branes Mc 1 and Mc 2 may, if desired, be composed of different cation-active permselective membrane materials but preferably both are of the same polymer.
The walls of membranes used in the present process will normally be from 0.02 to 0.5 mm. thick, preferably 0.1 to 0.4 mm.
thick. When mounted on a polytetrafluoroethylene, asbestos, ~
titanium or other suitable network~ for support, the network ~ -filaments or fib~rs will usually have a thickness of 0.01 to 0.5 mm., preferably 0.05 to 0.15 mm., corresponding to up to the thickness of the membrane. Often it will be preferable for the fibers to be less than half the film thickness but filament thick-nesses 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 about 10 to 70% and most preferably about 20 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 the membrane -it is preferred that it be fused to it by high temperature, high pressure compression before hydrolysis of the copolymer. Then, the membrane-network composite can be clamped or otherwise fasten-ed in place ;n a holder or support. ~ u ` The electrodes of the cell and the conductive means `~
connected thereto can be made of any electrically conductive mat-erial which will resist the attack of the various cell contents.
In general, the cathodes are made of graphite, iron, lead dioxide, iron in graphite, lead dioxide on graphite, 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. Preferably, the cel1 cathode is of mild steel, although ;
graphite, especially high density graphite, i.e., graphite having a density of about 1.68 to 1.78 grams per milliliter may also be used, particularly in a bipolar configuration. The conductive means attached to the cathode may be aluminum, copper, silver, - -steel or iron, with copper being much preferred. The anode should ,,: . . . , . ~ :

be resistant to attack by persulfuric acid and accordingly should often be of persulfuric acid-inert noble metal. The anode pre-` ferably is platinum or platinum-clad tantalum, with platinum being much preferred. The conductive means attached to the anode.
is also desirably protected against the persulfuric acid in the cathode compartment and preferably is tantalum encased in platinum.
The material of construction of the cell body is con-ventional, including steel, concrete, stressed concrete or other suitably strong material, lined with mastics, rubbers, e.g., neoprene, polyvinylidene chloride, FEP, chlorendic acid based polyester, polypropylene, polyvinyl chloride, polytetrafluoro-ethylene, or other suitable plastics, usually being in tank or -box form. Substantially self-supporting structures, such as rig;d polyvinyl chlor;de, polyv;nylidene chloride, polypropylene or phenol formaldehyde resins may be employed, preferably reinforced with molded-in fibers, cloths or webs, such as asbestos fibers.
Wh;le the compartments of the present cell will usually be sèparated from each other by flat membranes and w;ll usually be o~ substant;ally rectilinear or parallelepipedal construction, various other shapes, ;ncluding curves, e.g.~ cyl;nders, spheres, ellipso;ds; and irregular surfaces, e.g., sawtoothed or plurally . ~
- po;nted walls, may also be utilized. In accord with conventional electrochemical practice, pluralities of individual cells of the invention can be employed in mult;-cell un;ts, often having common feed and product manifolds and being housed in unitary structures ~ -;:
or in a f;lter press assembly, or the like.

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: ~ , , For satisfactory and efficient operation of the present cell the volumes of the buffer compartments Bl an B2 will be about the same and the combined volume of both buffer compartments will normally be from 1 to 100% that of the sum of the volumes of the anode and cathode compartments, preferably from 5 to 70%, and the ~ ~
anode and cathode compartment volumes will be approximately the ~ : -same.
The present process provides efficiently, without excess-ive costly reaction equipment being needed, important woodpulp bleaching reagents, hydrogen peroxide, chlorine dioxide and chlorine together with aqueous caustic which is useful in pulping wood chips.
Even the hydrogen produced can be used as a fuel to heat materials for bleaching or pulping. Since the present process requires at most only two or three reaction vessels, it can be readily set up at a single location, which advantageously should be near pulp-manufacturing and pulp-bleaching facilities, so as to take advan-tage of its efficient production of the described pulping chemicals.
However, it is also useful for off-site production, too.
The following examples illustrate but do not limit the invention. All parts are by we;ght and all temperatures are in C~, unless otherwise indicated.

A four-compartment electrolytic cell, as illustrated in the FIGURE, is utilized to produce chlorine, chlorine dioxide, aqueous, substantially salt-free sodium hydroxide, hydrogen and ,' ' , , , . . ' ' . ' . ' ~ . , 1 ~ ~f L~

persulfuric acid, wh;ch is of platinum mesh which is communicated with a positive direct current electrical source through a platinum-clad tantalum conductor rod. The cathode is of mild steel, and is communicated with a negative direct current sink through a copper conductor rod. The anode and cathode are each about two inches wide and about thirty inches high. The cell walls are of asbestos-filled polypPopylene.
The two cation-active permselective membranes Mc 1 and Mc 2, are Nafion~ membranes manufactured by E. I. DuPont de Nemours 10 and Company, Inc., and sold as their XR-type membranes. The membranes are 7 mils thick (about 0.2 mm.) and are joined to a backing or supporting network of polytetrafluoroethylene (Teflon~ filaments of a diameter of about 0.1 mm., woven into cloth which has an area percentage of openings therein of about 22%. The membranes are ~ `
15 initially flat and are fused onto the Teflon cloth by high tem-perature, high compression processing, with some of the membrane portions actually flowing around the filaments during the fusion process to lock onto the cloth without thickening the membrane between the cloth filaments.
The material of Nafion-XR permselective membranes contain a multiplicity of sulfonate substituents and is a hydrolyzed co-polymer of tetrafluoroethylene and FS02CF2CF20CF(CF3)CF20CF-CF2 which has an equivalent weight in the 900 to 1,600 range, about 1,250.
The anion-active permselective membrane Ma is derived from a heterogeneous polyvinyl chloride polymer containing a , . - , . ~

. . , ~ `'3 multiplicity of quaternary ammonium substituents. The anion-active membrane ;s Ionac~ type MA-3475R membrane ~manufactured by Ritter- ~;
Pfaudler Corporation, Permutit Division), having a thickness of about 14 mils (0.4 mm.), which is mounted on a T~flon cloth similar to that employed as a supporting network for the cation-active permselective membranes.
The cell electrodes are in contact with the cation-active permselective membranes, with the "flatter" side of the membranes facing and contacting the electrodes. In some experiments spacings of about 0.01 to 5 mm. between the electrodes and the membranes are utilized and satisfactory results are obtained but the present arrangement, with no spacings, is preferred. The interelectrode distance and the total width of the two buffer compartments, Bl and B , are about 6 mm. and the volume ratio of anode compartment: `
buffer compartment Bl: buffer compartment B2: cathode compartment is about 10:~.5:0.5:10.
The cell is filled with water to about 99% of usual capac;ty, a small open volume, about 5%, being left at the top of the cell to fac;litate collection of gaseous products from buffer ~ ~
compartment Bl and the cathode compartment. In small amounts ~ ~ ;
sulfuric acid is introduced into the anode compartment, sodium ~ ~ ;
chlor;de is charged to buffer compartments Bl and B2 and sod;um hydrox;de is introduced into the cathode compartment, to provide about a 1% concentration of~these electrolytes in the indicated compartments and thereby to provide conduction of electric current through the cell. The cell is externally cooled by circulating water to mainta;n the cell contents at a temperature of about 30 to 35C. during electrolysis.
Electrolysis is initiated by passage of direct current through the cell, concentrated aqueous sulfuric acid (containing about 93% sulfuric acid) is continuously fed to the anode compart-ment, an aqueous solution containing about 3 equivalents per liter, i.e., 3 N, of sodium chlorate and about 3 equivalents per liter of sodium chloride is fed continuously to buffer compartment B2 10 and water is continuously added to the cathode compartment. The rates of addition of sulfuric acid, chlorate and chloride are adjusted so that the mol ratio of acid, chlorate, and chloride feed rates is about 2:1:1. Water is charged continuously to the cathode compartment at a rate sufficient to maintain the liquid 15 level in the cell substantially constant. During electrolysis the voltage drop in the cell is about 3 volts and the current density is about 2 amperes per square inch of electrode surface.
A sul~uric acid solution of persulfuric acid is contin-uously withdrawn as product from the anode compartment. This 20 sol~ution is subjected to distillation with steam at 100C. in -stoichiometric excess, in a conventional glass steam distillation apparatus, including a still pot equipped with an inlet tube for introducing steam below the surface of liquid in the pot, agitation means, a water-cooled condenser and a distillate receiver.
25 The steam distillate recovered from the steam distillation is aqueous hydrogen peroxide containing about ~% of the peroxide.

- 27 - ;~

... ..

; The distilland recovered from the steam distillation still pot is about 50% aqueous sulfuric acid which is adjusted to the con-centration of the sulfuric acid feed stream for the anode compartment by addition of oleum and then is combined with the sulfuric acid feed stream for recycling to the electrolytic cell.
A gaseous mixture of chlorine dioxide and chlorine containing about 0.63 parts of chlorine dioxide per part of chlorine is continuously withdrawn as product from buffer com-partment B . The m;xture is introduced into the base of a conventional chlorine dioxide absorption tower or column of the type illustrated in FIG. 4 of the Canadian Journal of Chemical Engineering, Vol. 36 (1958), page 3, and is contacted with a downwardly flowing countercurrent stream of water at ambient temperature to remove chlorine dioxide as about a 3% aqueous solution which is recovered from the base of the tower, the purified chlorine gas being recovered from the top of the tower.
When des;red, the aqueous chlorine dioxide product solution is cooled enough so as to precipitate chlorine dioxide as a solid hydrate containing about 16% chlorine dioxide, which can be re-covered by filtration or decantation.
Gaseous hydrogen and aqueous sodium hydroxide arecontinuously withdrawn as products from the cathode compartment during electrolysis. The aqueous caustic product contains about 80 grams per liter of sodium hydroxide and less than about 0.1% sodium chloride. The cell operates at a caustic efficiency of about 90% and a current efficiency of about 75%.
~.

.. . .

, - : :~

3?
In mod;fications of the above laboratory cell for large scale operation the thicknesses of the cation-active permselective membranes can be increased to 10 to 14 mils, at which thicknesses the caustic efficiency increases but the voltage drop also increases. Accordingly, although cation-active membranes of greater thicknesses are operative in the ~ -present process, it is preferred to employ the 7 mil membranes.
Cation-active membranes which are 4 mils thick are also used and are satisfactory although caustic efficiency is decreased slightly.
The cation-active membranes of the present experiment do not show any deterioration in appearance or operating effi-ciency or adverse selectivity toward ion diffusion, even after operation in electrolytic processes in contact with oxidizing chemicals such as chlorine and chlorates, ~or as long as three years. They withstand the present cell's harsh environment very well and require fewer replacements than other non-preferred membranes. More frequent replacements of the anion-active membranes may be needed but the process efficiency is satisfactory because only 1/3 of the membranes used by this method are anion-active.

The procedure of Example 1 is repeated substantially as described except that the anion-active permselective membrane employed is an AMFion 310 series anion type membrane (manufactured by American Machine and Foundry Co.). This membrane, which has a , . . . . .
,~ , .

thickness of about 6 mils ~about 0.17 mm.), is a proprietary fluorocarbon polymer containing a multiplicity of quaternary ammonium substituents as anion-exchanging groups. The cell using this anion-active membrane is operated continuously with substantially no or little membrane deterioration and with ex-cellent operating results, substantially similar to those ob-tained in Example 1.

The procedure of Example 1 is followed and essentially the same results are obtained, utilizing as cation-active mem-branes RAI Research Corporation membranes identified as 18ST12S
and 16ST13S, respectively, and DuPont "improved" membranes made by the method prev;ously described, instead of the hydrolyzed copolymer of tetrafluoroethylene and sulfonated perfluorovinyl ether. The former of the RAI products is a sulfostyrenated FEP
in which the FEP is 18% styrenated and has 2/3 of the phenyl groups thereof monosulfonated, and the latter is 16% styrenated and has 13/16 of the phenyl groups monosulfonated. The membranes stand up well under the described operating conditions and after ~ -operation for several weeks are significantly better in appearance and operating characterisitics, e.g., physical appearance, uni-formity, voltage drop, than other cation-active permselective membranes available (except the hydrolyzed copolymers of per-fluoro-olefins and fluorosulfonated perfluorovinyl ethers of the `
type utilized in Example 1).
,,, ,, '`'" ~ ~

. . ~ . . .. . , . - , .
", ;, , ;, , ' ' . , ;~ ,' . . " ,.' ' , ' ' . ~ `

~3 ~ hen utilizing the RAI Research Corporation membranes descr;bed above, the anion-active membranes are also changed, to Amberlite* resins of the same thickness, also supported on polytetrafluoroethylene and polypropylene screening. The Amber-lites* utilized are made by Dow Chemicals Corp., and are ammoniumand quaternary ammonium functionalized styrenes grafted onto polymeric bases, such as those of FEP, TFE, PVE, PE, nylon and polypropylene. In other experiments, such anion-active perm-selective membranes are employed with cation-active membranes other than the RAI products, including the Ionacs* and Nafions*
and the electrodes are both platinum, in one series, or platinum-clad tantalum, in another. In some such instances, two or four additional buffer compartments are employed, inserted between Bl and B2 and maintained in the same order as B1 B2. The reactions described produce the desired products, with the sodium hydroxide being even lower in chloride content when additional buffering compartments are utilized. However, the Amberlite* resins do not appear to resist deterioration by the e1ectrolyte as well as the Ionac* and Nafion* (and modified Nafion*~
resins previously discussed.
The invention has been described with respect to working examples and illustrative embodiments but it is not to be limited to these because it is evident that one of skill in the art will be able to utilize substitutes and equivalents without departing from the spirit of the invention or going beyond its scope.
* trademark products

Claims

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

A method of manufacturing chlorine dioxide, hydrogen peroxide, chlorine, hydrogen and substantially alkali metal chloride-free aqueous alkali metal hydroxide from aqueous alkali metal chloride, aqueous alkali metal chlorate, sulfuric acid and water which comprises electrolyzing in a cell having an anode compartment with an anode therein, a cathode compartment with a cathode therein and intermediate buffer compartments, B1 and B2, the anode compartment being separated from B1 by a cation-active permselective membrane, Mc-1, the cathode compartment being separated from B2 by a cation-active permselective membrane, Mc-2, and B1 and B2 being separated from each other by an anion-active permselective membrane, Ma, solutions resulting from feeding sulfuric acid to the anode compartment, alkali metal chloride and alkali metal chlorate to B2, and water to the cathode compartment so that with the passage of electric current through the cell hydrogen ions selectively pass from the anode compartment to B1 through Mc-1, chloride and chlorate anions selectively pass from B2 to B1 through Ma, and alkali metal cations selectively pass from B2 to the cathode compartment through Mc-2, sulfuric acid is oxidized at the anode to produce persulfuric acid in the anode compartment, chloride and chlorate ions react to produce chlorine and chlorine dioxide in B1, and water and alkali metal cation react at the cathode to produce an aqueous substantially alkali metal halide-free alkali metal hydroxide and hydrogen in the cathode compartment, recovering the persulfuric acid solutions, chlorine dioxide, chlorine, hydrogen and aqueous hydroxide produced and reacting the persulfuric acid solution with water to produce sulfuric acid and hydrogen peroxide.

A method according to Claim 1 wherein the alkali metal chloride, the alkali metal chlorate and the alkali metal hydroxide are sodium chloride, sodium chlorate and sodium hydroxide respect-ively, the Mc-1 and Mc-2 cation-active membranes are of the same cation-exchange material, the cell is operated at a temperature below about 60°C. and the persulfuric acid solution recovered from the anode compartment is reacted with at least about two moles of water per mol of persulfuric acid in the solution.

A method according to Claim 2 wherein the material of the anion-active membrane is selected from the group consisting of quaternary ammonium group-substituted fluorocarbon polymers and quaternary ammonium-substituted polymers derived from hetero-geneous polyvinyl chloride, the cation-active membranes are selected from the group consisting of hydrolyzed copolymers of perfluorinated olefin and a fluorosulfonated perfluorinated vinyl ether, fluorinated polymers having pendant side chains containing sulfonyl groups which are attached to carbon atoms bearing at least one fluorine atom, with sulfonyl groups on one surface being in -(SO2NH)nM form where M is H, NH4, alkali metal or alkaline earth metal and n is the valence of M, and the sulfonyls of the polymer on the other membrane surface in -(SO3)pY
form wherein Y is a cation and p is the valence of the cation and when Y is H, M is also H, or being -SO2F, and sulfostyrenated perfluorinated ethylene propylene copolymers, the sulfuric acid charged to the anode compartment is aqueous sulfuric acid con-taining above about 80% thereof of sulfuric acid by weight and the sodium chloride and sodium chlorate are charged as aqueous solutions.

A method according to Claim 3 wherein the anode is of a persulfuric acid-inert noble metal, the cathode is a material selected from the group consisting of platinum, iridium, ruthenium, rhodium, graphite, iron and steel, the hydrolyzed copolymer is derived from tetrafluoroethylene and fluorosulfonated perfluoro-vinyl ether of the formula FSO2CF2CF2OCF(CF3)CF2OCF=CF2 and an equivalent weight of about 900 to 1,600, the fluorinated polymer with different side materials is a perfluorinated co-polymer of tetrafluoroethylene and FSO2CF2CF2OCF(CF3)CF2OCF=CF2 in a molar ratio of about 7:1, M and Y are both sodium and n and p are both 1, and the sulfostyrenated perfluorinated ethylene propylene copolymer is about 16 to 18% styrenated and has from about 2/3 to 13/16 of the phenyl groups therein monosulfonated, the thicknesses of the cation-active membranes and the anion-active membrane are between about 0.02 to 0.5 mm., the concen-tration of sulfuric acid in the sulfuric acid feed solution to the anode compartment is about 93 to 97% by weight, the concen-trations of sodium chlorate and sodium chloride in the feed to B2 are from about 1 N to the saturation solubility for each salt in water, and the sulfuric acid, sodium chloride and sodium chlorate are fed to the cell in molar proportioned rates of about 2:1:1.

A method according to Claim 4 wherein the cell operates at a temperature of from about 20 to 35°C., the hydrolyzed copolymer is utilized and has an equivalent weight of from about 1,100 to 1,400, the cation-active and anion-active membranes are mounted on networks of material(s) selected from the group consisting of polytetrafluoroethylene, asbestos, perfluorinated ethylene-propy-lene copolymer, polypropylene, titanium, tantalum, niobium and noble metals, which have area percentage(s) of openings therein from about 8 to 80% and the persulfuric acid solution recovered from the anode compartment is reacted with water at a temperature of about 60 to 100°C. to produce hydrogen peroxide.

A method according to Claim 5 wherein the cell operates at a voltage of about 2.3 to 5 volts and a current density of about 0.5 to 4 amperes per square inch of electrode surface, the anode is of platinum or platinum on titanium, the cathode is of a mild steel and the substantially sodium chloride-free hydroxide solution contains about 60 to 250 grams per liter of sodium hydroxide.

A method according to Claim 6 wherein the cell operates at a voltage of about 2.5 to 4 volts, a current density of about 1 to 3 amperes per square inch of electrode surface and a tem-perature of about 30 to 35°C., the membranes are from about 0.1 to 0.4 mm. thick, and are mounted on a network of polytetrafluoro-ethylene filaments with the area percentage of openings in the network being from 10 to 70%, and the concentration of sodium hydroxide in the aqueous hydroxide solution recovered from the cathode compartment is about 80 to 120 grams per liter.

A method according to Claim 7 wherein the cation-active membranes are of the hydrolyzed copolymer having an equivalent weight of about 1,250, the cell operates at about 3 volts and a current density of about 2 amperes per square inch of electrode surface, the anode is of platinum, the concentrations of sodium chlorate and sodium chloride in the feed solution to B2 are each 3 N, the hydroxide solution recovered from the cathode compartment contains about 100 grams per liter of sodium hydroxide, and the aqueous sulfuric acid distill and is recycled to the sulfuric acid feed to the anode compartment.

A method according to Claim 8 wherein the anion-active membrane is a quaternary ammonium substituted fluorocarbon polymer.

A method according to Claim 8 wherein the anion-active membrane is a quaternary ammonium substituted polymer derived from a heterogeneous polyvinyl chloride.

A method of manufacturing chlorine dioxide, hydrogen peroxide, chlorine, substantially alkali metal chloride-free aqueous alkali metal hydroxide and hydrogen from alkali metal chloride, alkali metal chlorate, sulfuric acid and water which comprises electrolyzing with a direct current, in a cell having an anode in an anode compartment, a cathode in a cathode compartment and a plurality of intermediate buffer com-partments, with the anode compartment being separated from a buffer com-partment by a cation-active permselective membrane, the cathode compart-ment active permselective membrane and at least one buffer compartment being separated from another by an anion-active permselective membrane, electrolytes resulting from feeds of sulfuric acid to the anode com-partment, alkali metal chloride and alkali metal chlorate to a buffer compartment nearer to the cathode compartment than another buffer com-partment, and water to the cathode compartment, so that with the passage of direct electric current through the cell hydrogen ions selectively pass from the anode compartment to an adjacent buffer compartment through the cation-active permselective membrane, chloride and chlorate ions selectively pass from a buffer compartment nearer to the cathode compartment to another buffer compartment through an anion-active permselective membrane, and alkali metal cations selectively pass from a buffer compartment to the cathode compartment through a cation-active permselective membrane, sulfuric acid is converted to persulfuric acid in the anode compartment, chloride and chlorate ions react to produce chlorine and chlorine dioxide in a buffer compartment nearer to the anode compartment than the buffer compartment into which chlorine and chlorate are fed, and water and alkali metal cations are converted to aqueous, substantially alkali metal halide-free alkali metal hydroxide and hydrogen in the cathode compartment, and removing from the electrolytic cell the persulfuric acid solution, chlorine dioxide, chlorine, aqueous alkali metal hydroxide and hydrogen produced.

12. An electrolytic cell for the manufacture of chlorine dioxide, chlorine, alkali metal hydroxide, persulfuric acid and hydrogen from alkali metal chloride, alkali metal chlorate, sulfuric acid and water, which comprises anode and cathode compartments and adjacent buffer compartments disposed intermediate said anode and cathode compartments, with boundaries between the anode and cathode compartments and adjacent buffer compartments being of cation-active permselective membranes which are resistant to attack by the medium and with at least two buffer compartments being separated by an anion-active perm-selective membrane.

13. An electrolytic cell according to claim 12 wherein there are present two buffer compartments, the anion-active membrane is selected from the group consisting of quater-nary ammonium-substituted fluorocarbon polymers and quaternary ammonium-substituted polymers from heterogeneous polyvinyl chloride, and the cation-active membranes are selected from the group consisting of hydrolyzed copolymers of perfluorinated olefin and a fluorosulfonated perfluorinated vinyl ether, fluorin-ated polymers having pendant side chains containing sulfonyl groups which are attached to carbon atoms bearing at least one fluorine atom, with sulfonyl groups on one surface being in -(SO2NH)nM
form where M is H, NH4, alkali metal or alkaline earth metal and n is the valence of M, and the sulfonyls of the polymer on the other membrane surface being in -(SO3)pY form wherein Y is a cation and p is the valence of the cation and when Y is H, M is also H, or being -SO2F, and sulfostyrenated perfluorinated ethylene propylene copolymers.