Electrolytic process for manufacturing chlorine dioxide, hydrogen peroxide, chlorine, alkali metal hydroxide and hydrogen

Abstract

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, chlorine 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

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. More particularly, the invention is of such a process 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 frequently employed in pulp mill operations, especially for the pulping of wood chips and bleaching 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 multiplicities 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 chloride so that, in addition to an electrochemical cell, the procedure requires a costly array of cascading reactors with a large storage tank for holding the chlorate solution prior to its reaction with hydrogen chloride [see the article by W. H. Rapson, Canadian Journal of Chemical Engineering Vol. 36, p. 6 (1958)]. Furthermore, this process does not produce hydrogen 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 processes 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, substantially 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 electrolyzing in a cell having an anode compartment with anode therein, a cathode component with cathode therein and intermediate buffer compartments, B.sup.1 and B.sup.2, the anode compartment being separated from B.sup.1 by a cation-active permselective membrane, M.sup.c.sup.-1, the cathode compartment being separated from B.sup.2 by a cation-active permselective membrane, M.sup.c.sup.-2, and B.sup.1 and B.sup.2 being separated from each other by an anion-active permselective membrane, M.sup.a, solutions resulting from feeding sulfuric acid to the anode compartment and alkali metal chloride and alkali metal chlorate to B.sup.2, so that with the passage of electric current through the cell hydrogen ion selectively diffuses or passes from the anode compartment to B.sup.1 through M.sup.c.sup.-1, chloride and chlorate anions selectively diffuse or pass from B.sup.2 to B.sup.1 through M.sup.a and alkali metal cations selectively diffuse or pass from B.sup.2 to the cathode compartment through M.sup.c.sup.-2, 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 B.sup.1, and water and aqueous alkali metal cation are reacted at the cathode to produce aqueous, substantially alkali metal chloride-free alkali metal hydroxide 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.

The invention will be readily understood by reference to descriptions of the embodiments thereof herein, taken in conjunction with the drawing of means for carrying out a preferred embodiment of the process.

In the Drawing:

The FIGURE is a schematic diagram of a four-compartment electrochemical cell for converting water, alkali metal chloride, alkali metal chlorate and sulfuric acid to chlorine dioxide, chlorine, aqueous alkali metal hydroxide, hydrogen and 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 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 M.sup.c.sup.-1 23, anion-active permselective membrane M.sup.a 25, and cation-active permselective membrane M.sup.c.sup.-2 27, divide the volume into an anode or anolyte compartment 29, a buffer compartment B.sup.1 31, a buffer compartment B.sup.2 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 B.sup.2 through line 39 and water is fed to the cathode compartment through line 41. During electrolysis sulfuric acid in the anode compartment is oxidized at the anode to form persulfuric acid which is withdrawn 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 M.sup.c.sup.-1 into buffer compartment B.sup.1 while chlorate and chloride anions selectively pass from buffer compartment B.sup.2 through anion-active membrane M.sup.a into buffer compartment B.sup.1. In B.sup.1 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 cations selectively diffuse from buffer compartment B.sup.2 through cation-active membrane M.sup.c.sup.-2 into the cathode compartment where 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 persulfuric acid which is recovered from the anode compartment is fed to a steam distillation apparatus 51 and is hydrolytically distilled with steam fed to the 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),

(1) 4H.sub.2 SO.sub.4 + 2MClO.sub.3 + 2MCl + 2H.sub.2 O .fwdarw. 2H.sub.2 S.sub.2 O.sub.8 + 2ClO.sub.2 + Cl.sub.2 + 2H.sub.2 + 4MOH

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) 2H.sub.2 S.sub.2 O.sub.8 + 4H.sub.2 O .fwdarw. 2H.sub.2 O.sub.2 + 4H.sub.2 SO.sub.4

In initiating the electrolytic process of the invention the anode compartments of the cell are charged with sufficient sulfuric acid, in aqueous solution, as to initiate the electrolytic oxidation of the H.sub.2 SO.sub.4 to H.sub.2 S.sub.2 O.sub.8, while the buffer compartments 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 percent of alkali metal hydroxide is charged into the cathode compartments. Advantageously, the cell is filled so as to provide a small free space, e.g., about 1 to 10 percent, preferably 1 to 5 percent of the cell volume, above the compartments so as to facilitate collection and withdrawal of the gaseous products, chlorine dioxide, chlorine and hydrogen. On connection of the conductive means to sources of positive and negative electrical 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 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 percent, preferably .+-.10 percent and most preferably about .+-.2 percent. 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.degree. to 5.degree.C. and below about 60.degree.C. or the temperature at which the rate of electrolytic formation of persulfuric acid from sulfuric acid is about equal to the rate of hydrolytic decomposition of the peracid. Preferably, the cell is operated at a temperature of about 5.degree. to 40.degree.C., more preferably at about 20.degree. to 35.degree.C. and most preferably at about 30.degree. to 35.degree.C.

The sulfuric acid charged to the anode compartment is generally aqueous sulfuric acid containing at least about 80 percent by weight sulfuric acid and is preferably "concentrated" sulfuric acid, "aqueous" sulfuric acid containing about 90 to 100 percent, usually 93 to 97 percent 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 concentrations of from about 1 Normal up to about the saturation solubility of the salts. Preferably the concentrations of the aqueous alkali metal chlorate charged are about 3 N. The chlorate and chloride salts may be charged in individual feed streams to compartment B.sup.2 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 60.degree. to 100.degree.C., preferably at about 100.degree.C., to produce hydrogen peroxide, in accord with known processes for the hydrolytic 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 stoichiometry of Equation (2) above. Advantageously, the water is charged in excess, e.g., 10 to 300 percent or 20 to 100 percent. Preferably, the water which is 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 peroxide 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 of water used in the steam distillation. The proportion of water may be regulated to produce the peroxide in best form for use, e.g., in bleaching, especially of woodpulps. 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.sup.1 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. Advantageously 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-Kesting 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 g./l., 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 percent alkali metal chloride and under most preferred operating conditions, less than about 0.1 percent. Thus, the aqueous caustic product is often suitable, without further purification, for many applications wherein substantially salt-free aqueous alkali metal hydroxides or caustic is desirable or necessary, for 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 cell 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 percent, and, under preferred operating conditions, is about 75 to 80 percent or greater. The caustic efficiency of the electrolytic cell is generally greater than about 75 percent and, under preferred operating conditions may be 85 to 90 percent 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 is used.

The cation-active and anion-active permselective membranes 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 substituents 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 substituent, 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(s). For example, phenol sulfonic acid can be substituted for some or all of the phenol normally used as a reactant in 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 polymer base are carboxylate groups.

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 groups such as phenyl or tolyl; or heterocyclics, 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 dilower 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 -CH.sub.2 Cl, and thereafter may be reacted with a tertiary amine, such as trimethyl amine, to produce the quaternary ammonium substituted 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 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 "Encyclopedia 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 Technology, by S. B. Tuwiner, published by Rheinhold Publishing Corporation, New York, 1962, at pages 200 to 206, the pertinent subjects matter of which references are hereby incorporated by reference.

In addition to the examples of anion-active permselective 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 flurocarbon polymer, manufactured by American Machine and Foundry Co.; and Ionac types MA 3148, MA 3236 and MA 3475-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, the following proprietary compositions are representative examples of cation-active permselective 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; Nafion XR type hydrolyzed copolymer of perfluorinated olefin and a fluorosulfonated perfluorovinyl ether, manufactured by E. I. DuPont de Nemours and Company, Inc.; Nafion XR, modified - Nafion XR treated on one side with ammonia to convert SO.sub.2 R groups to SO.sub.2 NH.sub.2, which are then hydrolyzed to SO.sub.2 NHNa; RAI Research Corporation membranes such as types 18ST12S 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 -SO.sub.2 NHNa modifications thereof and the sulfostyrenated perfluoroethylene-propylene copolymers.

The sulfostyrenated perfluoroethylene-propylene polymers useful as cation-active membranes in a preferred embodiment of the invention are generally those which have two-thirds to eleven-sixteenths of the phenyl groups therein monosulfonated and which are about 16 to 18 percent styrenated. 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 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 percent 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 cobalt.sup.60 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 sulfuric acid or SO.sub.3. Preferably, chlorosulfonic acid in chloroform is utilized and the sulfonation is completed in about one-half hour.

Examples of useful membranes made by the described process are the RAI Research Corporation products previously mentioned, 18ST12S and 16 ST13S, the former being 18 percent styrenated and having two-thirds of the phenyl groups monosulfonated and the latter being 16 percent styrenated and having thirteen-sixteenths of the phenyl groups monosulfonated. To obtain 18 percent styrenation a solution of 171/2 percent of styrene in methylene chloride is utilized and to obtain 16 percent styrenation a solution of 16 percent of styrene in methylene chloride is employed.

The especially preferred cation-active permselective membranes of the invention are of a hydrolyzed copolymer of perfluorinated 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 utilized, of which the monoolefinic 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 FSO.sub.2 CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF=CF.sub.2. Such a material, named as perfluoro-[2-(2-fluorosulfonylethoxy)-propyl vinyl ether], referred to henceforth as PSEPVE, may be modified to equivalent monomers which are represented by the formula FSO.sub.2 CFR.sup.1 CF.sub.2 O(CFYCF.sub.2 O).sub.n CF=CF.sub.2, wherein R.sup.1 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 consisting 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 U.S. Pat. No. 3,282,875 and an alternative method is mentioned in Canadian Pat. No. 849,670, which also discloses the use of the finished membrane in fuel cells, characterized therein as electrochemical cells. The disclosures of such patents are hereby incorporated herein by reference. In short, the copolymer may be made by reacting PSEPVE or equivalent with tetrafluoroethylene or equivalent in desired proportions in water at elevated temperature and pressure for over an hour, after which time the mix is cooled. It separates into a lower perfluoroether layer and an upper 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, e.g., 1,250, and the percentage of PSEPVE or corresponding compound is about 10 to 30 percent, preferably 15 to 20 percent and most preferably about 17 percent. 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 -SO.sub.2 F groups to -SO.sub.3 H groups, as by treating with 10 percent sulfuric acid or by the methods of the patents previously mentioned. The presence of the -SO.sub.3 H groups may be verified by titration, as described in the Canadian patent. Additional details of various processing steps are described in Canadian Pat. No. 752,427 and U.S. Pat. No. 3,041,317, also hereby incorporated by reference.

Because it has been found that some expansion accompanies hydrolysis of the copolymer it is preferred to position the copolymer membrane after hydrolysis onto a frame or other support which will hold it in place in the electrolytic cell. Then it may be clamped or cemented in place and will be true, without sags. The membrane is preferably joined to the backing tetrafluoroethylene or other suitable filaments prior to hydrolysis, when it is still thermoplastic, and the film of copolymer covers each filament, penetrating into the spaces between them and even around behind them, thinning the films slightly in the process, where they cover the filaments.

The aminated and hydrolyzed improvements or modifications of the polytetrafluoroethylene PSEPVE copolymers are made, as previously 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 -(SO.sub.2 NH).sub.n M form, where M is H, NH.sub.4, 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 -(SO.sub.3).sub.p Y form or -SO.sub.2 F, 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 improved membrane is found in French Pat. No. 2,152,194 of E.I. DuPont de Nemours and Company, Inc., corresponding to U.S. Pat. application Ser. No. 178,782, filed Sept. 8, 1971 in the name of Walther Gustav Grot, which disclosures are hereby incorporated herein by reference.

The membranes of hydrolyzed copolymer of perfluorinated 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 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. Considering 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 g./l. 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 alkalinity in the catholyte increases. Thus, these differences in the present process make it practicable, whereas previously 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 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 copolymers, made by chemical treatment of surfaces thereof to modify the -SO.sub.3 H 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 percent, often about 10 to 20 percent, over the unmodified membranes.

The membranes M.sup.c.sup.-2 and M.sup.c.sup.-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 fibers 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 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 percent, preferably about 10 to 70 percent and most preferably about 20 to 70 percent. 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 fastened in place in a holder or support.

The electrodes of the cell and the conductive means connected thereto can be made of any electrically conductive material 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 cell 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 preferably 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 conventional, 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, polytetrafluoroethylene, or other suitable plastics, usually being in tank or box form. Substantially self-supporting structures, such as rigid polyvinyl chloride, polyvinylidene chloride, polypropylene or phenol formaldehyde resins may be employed, preferably reinforced with molded-in fibers, cloths or webs, such as asbestos fibers.

While the compartments of the present cell will usually be separated from each other by flat membranes and will usually be of substantially rectilinear or parallelepipedal construction, various other shapes, including curves, e.g., cylinders, spheres, ellipsoids; and irregular surfaces, e.g., sawtoothed or plurally pointed walls, may also be utilized. In accord with conventional electrochemical practice, pluralities of individual cells of the invention can be employed in multi-cell units, often having common feed and product manifolds and being housed in unitary structures or in a filter press assembly, or the like.

For satisfactory and efficient operation of the present cell the volumes of the buffer compartments B.sup.1 and B.sup.2 will be about the same and the combined volume of both buffer compartments will normally be from 1 to 100 percent that of the sum of the volumes of the anode and cathode compartments, preferably from 5 to 70 percent, and the anode and cathode compartment volumes will be approximately the same.

The present process provides efficiently, without excessive 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 advantage 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 weight and all temperatures are in .degree.C., unless otherwise indicated.

EXAMPLE 1

A four-compartment electrolytic cell, as illustrated in the FIG., is utilized to produce chlorine, chlorine dioxide, aqueous, substantially salt-free sodium hydroxide, hydrogen and persulfuric acid, which is subsequently hydrolyzed to hydrogen peroxide. The anode 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 polypropylene.

The two cation-active permselective membranes M.sup.c.sup.-1 and M.sup.c.sup.-2, are Nafion membranes manufactured by E. I. duPont de Nemours 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 percent. The membranes are initially flat and are fused onto the Teflon cloth by high temperature, 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 copolymer of tetrafluoroethylene and FSO.sub.2 CF.sub.2 CF.sub.2 OCF(CE.sub.3)CF.sub. 2 OCF=CF.sub.2 which has an equivalent weight in the 900 to 1,600 range, about 1,250.

The anion-active permselective membrane M.sup.a is derived from a heterogeneous polyvinyl chloride polymer containing a multiplicity of quaternary ammonium substituents. The anion-active membrane is 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 Teflon 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, B.sup.1 and B.sup.2, are about 6 mm. and the volume ratio of anode compartment:buffer compartment B.sup.1 :buffer compartment B.sup.2 : cathode compartment is about 10:0.5:0.5:10.

The cell is filled with water to about 99 percent of usual capacity, a small open volume, about 5 percent, being left at the top of the cell to facilitate collection of gaseous products from buffer compartment B.sup.1 and the cathode compartment. In small amounts sulfuric acid is introduced into the anode compartment, sodium chloride is charged to buffer compartments B.sup.1 and B.sup.2 and sodium hydroxide is introduced into the cathode compartment, to provide about a 1 percent 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 maintain the cell contents at a temperature of about 30 to 35.degree.C. during electrolysis

Electrolysis is initiated by passage of direct current through the cell, concentrated aqueous sulfuric acid (containing about 93 percent sulfuric acid) is continuously fed to the anode compartment, 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 B.sup.2 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 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 sulfuric acid solution of persulfuric acid is continuously withdrawn as product from the anode compartment. This solution is subjected to distillation with steam at 100.degree.C. 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. The steam distillate recovered from the steam distillation is aqueous hydrogen peroxide containing about 4 percent of the peroxide. The distilland recovered from the steam distillation still pot is about 50 percent aqueous sulfuric acid which is adjusted to the concentration 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 compartment B.sup.1. The mixture 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 counter-current stream of water at ambient temperature to remove chlorine dioxide as about a 3 percent aqueous solution which is recovered from the base of the tower, the purified chlorine gas being recovered from the top of the tower. When desired, the aqueous chlorine dioxide product solution is cooled enough so as to precipitate chlorine dioxide as a solid hydrate containing about 16 percent chlorine dioxide, which can be recovered by filtration or decantation.

Gaseous hydrogen and aqueous sodium hydroxide are continuously 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 percent sodium chloride. The cell operates at a caustic efficiency of about 90 percent and a current efficiency of about 75 percent.

In modifications 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 efficiency or adverse selectivity toward ion diffusion, even after operation in electrolytic processes in contact with oxidizing chemicals such as chlorinde and chlorates, for 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 one-third of the membranes used by this method are anion-active.

EXAMPLE 2

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 excellent operating results, substantially similar to those obtained in Example 1.

EXAMPLE 3

The procedure of Example 1 is followed and essentially the same results are obtained, utilizing as cation-active membranes RAI Research Corporation membranes identified as 18ST12S and 16St13S, respectively, and DuPont "improved" membranes made by the method previously 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 percent styrenated and has two-thirds of the phenyl groups thereof monosulfonated, and the latter is 16 percent styrenated and has thirteen-sixteenths 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 characteristics, e.g., physical appearance, uniformity, voltage drop, than other cation-active permselective membranes available (except the hydrolyzed copolymers of perfluoro-olefins and fluorosulfonated perfluorovinyl ethers of the type utilized in Example 1).

When utilizing the RAI Research Corporation membranes described above, the anion-active membranes are also changed, to Amberlite resins of the same thickness, also supported on polytetrafluoroethylene and polypropylene screening. The Amberlites utilized are made by Dow Chemical Corp., and are ammonium and 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 permselective 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 B.sup.1 and B.sup.2 and maintained in the same order as B.sup.1 and B.sup.2. 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 electrolyte 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.

Claims

What is claimed is:

1. 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, B.sup.1 and B.sup.2, the anode compartment being separated from B.sup.1 by a cation-active permselective membrane, M.sup.c.sup.-1, the cathode compartment being separated from B.sup.2 by a cation-active permselective membrane, M.sup.c.sup.-2, and B.sup.1 and B.sup.2 being separated from each other by an anion-active permselective membrane, M.sup.a, solutions resulting from feeding sulfuric acid to the anode compartment, alkali metal chloride and alkali metal chlorate to B.sup.2, 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 B.sup.1 through M.sup.c.sup.-1, chloride and chlorate anions selectively pass from B.sup.2 to B.sup.1 through M.sup.a, and alkali metal cations selectively pass from B.sup.2 to the cathode compartment through M.sup.c.sup.-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 B.sup.1, 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.

2. 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 respectively, the M.sup.c.sup.-1 and M.sup.c.sup.-2 cation-active membranes are of the same cation-exchange material, the cell is operated at a temperature below about 60.degree.C. and the persulfuric acid solution recovered from the anode compartment is reacted with at least about 2 moles of water per mol of persulfuric acid in the solution.

3. 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 heterogeneous 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 -(SO.sub.2 NH).sub.n M form where M is H,NH.sub.4, 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 -(SO.sub.3).sub.p Y 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 -SO.sub.2 F, and sulfostyrenated perfluorinated ethylene propylene copolymers, the sulfuric acid charged to the anode compartment is aqueous sulfuric acid containing above about 80 percent thereof of sulfuric acid by weight and the sodium chloride and sodium chlorate are charged as aqueous solutions.

4. 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 perfluorovinyl ether of the formula

FSO.sub.2 CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF=CF.sub.2

and an equivalent weight of about 900 to 1,600, the fluorinated polymer with different side materials is a perfluorinated copolymer of tetrafluoroethylene and FSO.sub.2 CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF=CF.sub.2 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 percent styrenated and has from about two-thirds to thirteen-sixteenths 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 concentration of sulfuric acid in the sulfuric acid feed solution to the anode compartment is about 93 to 97% by weight, the concentrations of sodium chlorate and sodium chloride in the feed to B.sup.2 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.

5. A method according to claim 4 wherein the cell operates at a temperature of from about 20.degree. to 35.degree.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-propylene copolymer, polypropylene, titanium, tantalum, niobium and noble metals, which have area percentage(s) of openings therein from about 8 to 80 percent and the persulfuric acid solution recovered from the anode compartment is reacted with water at a temperature of about 60.degree. to 100.degree.C. to produce hydrogen peroxide.

6. 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 mild steel and the substantially sodium chloride-free hydroxide solution contains about 60 to 250 grams per liter of sodium hydroxide.

7. 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 temperature of about 30.degree. to 35.degree.C., the membranes are from about 0.1 to 0.4 mm. thick, and are mounted on a network of polytetrafluoroethylene filaments with the area percentage of openings in the network being from 10 to 70 percent, and the concentration of sodium hydroxide in the aqueous hydroxide solution recovered from the cathode compartment is about 80 to 120 grams per liter.

8. 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 B.sup.2 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 distilland is recycled to the sulfuric acid feed to the anode compartment.

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

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

11. 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 compartments, with the anode compartment being separated from a buffer compartment by a cation-active permselective membrane, the cathode compartment being separated from a buffer compartment by a cation-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 compartment, alkali metal chloride and alkali metal chlorate to a buffer compartment nearer to the cathode compartment than another buffer compartment, 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 chloride 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.