US3220941A

US3220941A - Method for electrolysis

Info

Publication number: US3220941A
Application number: US47331A
Authority: US
Inventors: Company Canada Trust; George T Miller
Original assignee: Hooker Chemical Corp
Current assignee: Occidental Chemical Corp
Priority date: 1960-08-03
Filing date: 1960-08-03
Publication date: 1965-11-30
Anticipated expiration: 1982-11-30

Description

Nov. 30, 1965 s. G. osBoRNE METHOD FOR ELECTROLYSIS 2 Sheets-Sheet 1 Filed Aug. 3, 1960 Naacog BRINE SOLUTION OUT Nov. 30, 1965 s. G. osBoRNE 3,220,941

METHOD FOR ELECTROLYS IS Filed Aug. .'3, 1960 2 Sheets-Sheet 2 METHD FR ELECTROLYSS Sidney G. sborne, deceased, late of St. Davids, Ontario, Canada, by Canada Trust Company, administrator, St. Catharines, ntario, Canada, and George T., li/iiier, Lewiston, NX., assignors to Hooker Chemical Corporation, Niagara Fails, NX., a corporation of New York Filed Aug. 3, 1960, Ser. No. $7,331

13 Ciaims. (Cl. 2MP-87) This application is a continuation-impart of our cpending application Serial No. 267,846, led January 23, 1952, now U.S. 2,967,807, and of our co-pending application Serial No. 327,182, tiled December 22, 1952, now abandoned.

This invention relates to improvements in the electrolytic decomposition of chemical compounds and the production of useful products therefrom. Further, this invention relates to novel methods for .advantageously employing the loss in current etiiciency inherent in the use of permselective diaphragms in electrolytic cells, caused by undesirable leakage, migration or adsorption of a useful product of electrolysis into or through the diaphragm, under the intluence `of a concentration gradient. More particularly, this invention involves a method which results in the production of an additional product of the cell in an added compartment of a multicompartment electrolytic cell, which comprises reacting a chemical reagent With the liquid or soluble products appearing in the additional compartment as a result of undesirable leakage, migration or adsorption into or through the permselective diaphragm. Still further, this invention relates to a novel structure of permselective diaphragms for use in the electrolytic decomposition of chemical compounds which avoids the loss of current eiciency .and provides for separate recovery of another product of the cell in which it is used. Still more particularly this invention relates to methods and apparatus for the production and separate recovery of hydrogen, chlorine, caustic soda and soda ash by the electrolytic decomposition of an aqueous solution of sodium chloride in an electrolytic cell separated into three compartments by permselective diaphragms.

Permselective diaphragms are characterized by their substantial impermeability to liquids and by their selective permeability to ions of one charge and substantial impermeability to ions of the opposite charge when wet with an electrolyte and under the infiuence of an electrical current. Permse'lective diaphragms which selectively permit the passage of `anions are designated as anionic; those which selectively permit the passage of cations are designated as cationic. The practical utilization of permselective diaphragms in electrolytic cells for the electrolytic decomposition yof chemical compounds, and the separate recovery of the products resulting therefrom, depend upon the presence of a polar medium, generally Water, being present in the pores of the diaphragm; that is to say, permselective diaphragms must be Wet with solvent in order to function in accordance with this invention. Such diaphragms and certain methods and apparatus for employing them in the electrolytic decomposition of chemical compounds are more fully set forth in our co-pending application for Letters Patent S.N. 267,- 846 tiled January 23, 1952.

In our co-pending application S.N. 267,846 there is dis- States Patent fiii Patented Nov. 3Q, 1965 ICC closed the fundamental concept of simultaneously controlling both molecular yand ionic migration during the electrolysis of chemical compounds between the electrodes of an electrolytic cell, by electrolyzing the chemical compound in a cell separated into electrode compartments, by one or more permselective diaphragms. By this means, and under ideal conditions, including substantially one hundred percent permselectivity of the permselective diaphragms employed and complete impermeability to solvents, substantially all of the electrical current introduced during electrolysis will be carried through the permselective diaphragm by ions of one charge, because permselective diaphragms do not permit the passage of the ions of opposite charge. However, neither one hundred percent permselectivity nor complete impermeability to solvents in such diaphragms has yet been achieved in practice.

It follows therefore, that in practice, even under the most perfect conditions, that there is a minor transfer of such polar medium or solvent through the diaphragm and a minor transfer of undesirable ions in the direction opposite to desired particularly high speed ions, because of the presence of such solvent; and that, this causes a decrease in ampere eiciency and a loss of other advantages. In addition, there is also a loss in the permselectivity of such .a diaphragm, with increasing concentration of the products of electrolysis contained in one or more of the electrode compartments. The decreasing permselectivity with increasing caustic concentration appears to follow the curve of typical adsorption isotherms. Thus, when permselective diaphragms .are used in electrolytic cells, the current efficiency decreases with increasing concentration of the pro-duct in its electrode compartment of the cell, corresponding to the proportional loss of permselectivity of the diaphragm. Although we do not Want to be held to this explanation, our invention may 'be more easily understood by illustrating the case of producing caustic soda in terms of it.

For example, when sodium hydroxide and elemental chlorine are produced by the electrolytic decomposition of an aqueous solution of sodium chloride brine in an electrolytic cell in which the brine at the anode is separated from the caustic soda produced at the cathode, by a cationic permselective diaphragm, the diaphragm operates in two distinct manners, the rst advantageously and the second adversely.

First, during electrolysis sodium ions migrate from the anolyte to the cathode, and upon reaching the cationic permselective diaphragm, which divides the cell into separate anode an-d cathode compartments, are permitted passage through the diaphragm from active point to active point in the pores of the cationic permselective diaphragm because such diaphragm permits passage of such sodium cations, thus permitting the isolation and separate recovery yof sodium hydroxide in the cathode compartment and also permitting an increase in its concentration with continued electrolysis. But the second mode oisets this advantage because, under the influence of the concentration gradient, sodium hydroxide diffuses into the diaphragm from the cathode compartment in the direction toward the anode by adsorption because of the presence of the polar solvent in the pores of the diaphragm, thus causing a serious loss in current eiciency and other disadvantages. The effects of this secon-d described mode of operation of permselective diaphragms are suicient to negate some of the important advantages resulting from the first describe-d method of operation for the permselective diaphragm.

This is vfurther illustrated, for example, by employing a presently available ca-tionic permselective diaphragm in a two compartment electrolytic cell for the electrolysis of an aqueous solution of sodium chloride to hydrogen, caustic soda and chlorine and observing the current etiiciencies at various concentrations of caustic in the catholyte. The current efficiency of such cell is found to decrease from 75 percent at a caustic concentration in the catholyte of ten grams sodium hydroxide per liter to 46 percent current efiiciency as the caustic concentration is increased in the catholyte to 500 grams sodium hydroxide per liter. The following tabulation gives these and other current efficiencies which We have experimental-ly determined at various -caustic concentrations:

Caustic concentration, g.p.l.: Current eiciency, percengt These data constitute a typical adsorption isotherm and at concentrations of caustic in the catholyte of 500 grams sodium hydroxide per liter and above, the current elliciency remains at 46 percent. This indicates that the percent permselectivity of the diaphragm has been reduced to zero percent and that the advantages to be realized by using the permselective diaphragms are offset by the deleterious effects of adsorption of caustic into the permselective diaphragm. Under these conditions of high caustic concentration in the catholyte, the diaphragm apparently operates simply as a porous diaphragm in the manner that an asbestos diaphragm will operate to show a current eiciency of about 46 percent irrespective of the concentration of caustic in the catholyte. Thus in the case of using a permselective diaphragm for making high concentration caustic or in the case of using an asbestos diaphragm for making caustic soda of any strength, ap-

proximately half the current input is being carried by ions of one charge while approximately the other half is carried by ions of the opposite charge.

In our co-pending application, S.N. 267,846, we have proposed to increase the percent permselectivity of the permselective diaphragmat a given caustic strength, that is to say, to offset the elfects of the undesirable adsorption of sodium hydroxide into the cationic diaphragm from the cathode compartment in the direction toward the anode compartment, by using two diaphragms in the electrolytic cell thereby forming a center compartment between them, into which water is introduced whereby the concentration of the caustic soda in contact with the diaphragm facing the anode, and subject to adsorption by such diaphragm, is very dilute in comparison to the caustic soda being produced in the cathode compartment, whereby the loss in permselectivity of the diaphragm facing the anode is correspondingly reduced and current efciency correspondingly increased, then transferring the weak caustic soda solution so produced in the center compartment to the cathode compartment, by way of a conduit in communication between the center and cathode compartments. Another technique for offsetting the ineiciency of permselective diaphragms, which is disclosed in our co-pending application for patent, involves employing a porous diaphragm facing the cathode and permitting a slow percolation of electrolyte through it in the' direction toward the cathode thereby protecting the cationic permselective diaphragm which faces the anode from the strong caustic in the cathode compartment. Still another technique for offsetting the ineliciency of the permselective diaphragms, disclosed in our co-pending application above referred to, based on our finding that the diffusion coefficient for sodium carbonate through a permselective diaphragm is of the order of about one tenth of the diffusion coefficient for sodium hydroxide through such diaphragms under the same conditions, involves passing carbon dioxide into the catholyte in the cathode compartment of the cell, or outside the cell, to cause the formation and separation of sodium carbonate rather than caustic soda.

We have now found that the losses in current efficiency may be minimized and an additional product of the cell may be realized by employing a permselective diaphragm structure of this invention in the electrolysis of chemical compounds in an electrolytic cell, said structure being composed of two diaphragms having the same kind of permselectivity separated from each other to form a center compartment containing an inlet for introducing a chemical reactive with the products of electrolysis migrating from the electrode compartments into said compartment and an outlet for removing the products so produced.

We have now also found that we can increase the percent permselectivity of permselective diaphragms in processes and apparatus for the electrolysis of alkali metal chloride brines by approximately the amount of permselectivity lost due to adsorption and with the realization of a fourth product of the cell, alkali metal carbonate, by effecting the electrolysis of an aqueous alkali metal chloride solution in an electrolytic cell divided into an anode compartment containing brine, a center compartment, and a cathode compartment, by two cationic permselective diaphragms, carbonating the solution in the center cornpartment with carbon dioxide and separately recovering the alkali metal carbonate so produced and alkali metal hydroxide produced in the cathode compartment, in addition to separately recovering the chlorine and hydrogen which are also produced.

We have also found that We can eifectively regulate the ratio of alkali metal carbonate to alkali metal hydroxide produced in accordance with our invention, by controlling the concentration of caustic soda producd in the catholyte. This finding is of vast commercial signiiicance because it allows for maintaining an economic balance in the production of these two basic commodities of commerce, according to the needs of commerce.

We have also found that the advantages of employing permselective diaphragms in electrolytic cells for the electrolytic decomposition of chemical compounds may be realized, without the concomitant undesirable effects resulting from the adsorption and diffusion of liquid or soluble products of electrolysis from one electrode compartment toward the opposite electrode, and with the production of a separately recoverable added product of the cell, by effecting the electrolysis of the chemical compound in an electrolytic cell divided into an anode compartment, an added compartment and a cathode compartment, by two permselective diaphragms, introducing a chemical reagent reactive with the liquid or soluble products of electrolysis produced in the electrode compartments, into the solution ofthe center compartment and separately recovering the added product of the cell so produced and the catholyte so produced, in addition to separately recovering any gaseous products which are also separately produced at the electrodes.

In order that this invention may be more readily understood it is described with specic reference to certain preferred embodiments thereof and with reference to FIG- URE I, which is a diagrammatic sketch illustrating the electrolysis of an aqueous solution of sodium chloride at the anode, the carbonation of the solution in the center compartment of our three compartment electrolytic cell with carbon dioxide, whereby hydrogen, chlorine, sodium carbonate and caustic soda are separately produced and recovered; and, with reference to FIGURE II which is a perspective view of a permselective diaphragm structure of this invention: however, our invention is not to be construed as limited thereto, as will be apparent from the modifications discussed hereinafter, except as dened in the appended claims.

Referring to FIGURE I, which is a diagrammatic sketch illustrating specific embodiments of this invention: the electrolytic cell comprises a vessel 1 separated into an anode compartment 2, a center compartment 3 and a cathode compartment 4 by two cationic

permselective diaphragms

5 and 6. The anode compartment contains an anode 7 in contact with sodium chloride brine 8. The anode compartment also contains a brine inlet 9, a brine outlet 10 and chlorine outlet 11. The cathode compartment contains a cathode 12 in contact with water or caustic 13. The cathode compartment also contains a water inlet 14, a caustic outlet 15 and a hydrogen outlet 16. The center compartment 3 is provided with inlets 17 for introducing water 13 and carbon dioxide 19 into the center compartment of the cell, which is positioned between the cationic permselective diaphragm facing the anode and cationic permselective diaphragm facing the cathode, designated herein as 5 and 6, respectively. The center compartment is also provided with an outlet 29 for removing the sodium carbonate solution which is produced under the conditions maintained during electrolysis. In addition the electrolytic cell contains any other necessary accessories for the given electrolysis.

Referring to FIGURE II which is a perspective view of a permselective diaphragm structure of this invention composed of two sheets or

membranes

5 and 6 having the same kind of permselectivity, i.e. both are cationic or both are anionic. The permselective diaphragms are separated from each other by a gasket type separator 21 containing inlet 17 and outlet 20. The separated diaphragms form compartment 3 which contains an electrolyte comprising a chemical reactive with the products of electrolysis migrating into said compartment from the electrode compartments during electrolysis, some of the unreacted migratory products of electrolysis and the reaction product of these substances with each other.

Referring back to FIGURE I, in accordance with this invention a saturated solution of sodium chloride brine 8 is fed into the anode compartment 2 of the cell 1 through brine inlet 9. Depleted brine is removed from the anode compartment through brine outlet 10. Chlorine produced is removed from the cell through outlet 11. A starting solution of sodium carbonate is introduced into compartment 3 in order to lower the voltage at the start of electrolysis, then water 1S and carbon dioxide 19 are fed into the center compartment of the cell through inlet 17. The sodium carbonate solution produced in the center compartment during the electrolysis is removed from the cell outlet 20. The caustic soda produced in the cathode compartment 4 is removed through outlet 15. Hydrogen which is also produced in the cathode compartment is removed through outlet 16. Water can be introduced into the cathode compartment through inlet 14 in predetermined amounts in order to produce caustic soda of the desired concentration. In an alternative mode of operation the cathode compartment may acquire water through the diaphragm 6 by electro-osmosis, especially if the particular diaphragms employed have minor leakage points. In such case the amount of water introduced through inlet 14 will be correspondingly minimized or entirely eliminated. Another alternative mode of operation involves introducing the carbon dioxide necessary to cause the formation of sodium carbonate, which is the fourth product of the cell in accordance with this invention, into the solution produced in the center compartment of the cell, at a point removed from the electrolytic cell, such as at a point subsequent to outlet 20 and circulating the sodium carbonate solution so produced back into the center compartment to favor maintaining a substantially uniform concentration of sodium carbonate throughout the solution and adding water as necessary to do this.

Our findings to the effect that the percent permselectivity of permselective diaphragms at a given caustic concentration can be increased by approximately the amount of permselectivity lost due to adsorption, and, with the realization of an additional product of the cell are best exemplied by data taken from a curve based on actual electrolysis of a saturated aqueous solution of sodium chloride, producing different concentrations of caustic in an electrolytic cell designed and operated after the principles depicted in the attached figure, in accordance with this invention and the foregoing description.

(B asis N 2120) Caustic Soda Ash Current Current Caustic Concentra- Eiciency Efficiency Total Current tion, gpl. N aOH and Percent and Percent Eiciency Production Production as as Caustic Soda Ash Percent Percent Percent These figures show that the deleterious effect of adsorption of a desired product into permselective diaphragms can be transformed into an advantage by producing another useful product and that the proportions of the two products produced, which can be separately recovered, can be varied as desired. More particularly the data also show that percent current efficiency can be consistently maintained during electrolysis in the electrolytic cell regardless of the concentration of sodium hydroxide in the catholyte being produced and that the lost current efciency based on caustic duc to adsorption is advantageously employed in making sodium carbonate. The proportion of caustic soda to soda ash which is produced under a given set of conditions of caustic concentration in the catholyte in an electrolytic cell having a total overall current efciency of 95 percent is also given by the data in the tabulation. For example, if it is desired to maintain approximately even distribution between the two products, under the conditions obtained during the electrolysis depicted, this can be realized by maintaining the concentration of sodium hydroxide in the catholyte at about 500 grams per liter.

It has long been desired to maintain an economic balance between the amount of chlorine and caustic soda produced electrolytically. However, because of the basic laws of electrochemistry, equivalent amounts of these commodities must necessarily be produced in the electrolysis of salt. The present invention provides economic and feasible means for electrolyzing brine so that chlorine is produced with the production of an equivalent amount of alkali in the form of caustic soda and soda ash in exible predetermined ratios, thereby allowing for an economic balance between these products as the demand varies.

The following examples are given to further illustrate this invention and should not be construed as limiting except as defined in appended claims.

Example l A saturated solution of sodium chloride brine was introduced into the anode compartment of a three compartment electrolytic cell designed after the principles depicted in the attached drawings. The anode compartment, which contained a graphite anode, was separated from the center compartment by a cationic permselective diaphragm and the cathode compartment, which contained a steel cathode, was separated from the center compartment by another cationic permselective membrane. The brine was continuously introduced into the cell and circulated within the anode compartment through a conduit in communication with the brine inlet and outlet. A solution of sodium carbonate containing about 138 grams per liter Na2CO3 was introduced into the center compartment of the electrolytic cell to lower the initial voltage of the cell. This solution was maintained in circulation in the center compartment through a conduit connecting the inlet and outlet. Carbon dioxide was introduced into the solution circulating in the center compartment at a point in the outside conduit which connected the center compartments inlet and outlet so that the solution circulated under the power of the gas lift thereby created and so that the concentration of sodium carbonate was substantially evenly distributed throughout the solution both inside and outside the cell. The cathode compartment was filled with water. The cell was operated to produce sodium carbonate, sodium hydroxide, chlorine and hydrogen. A cell temperature of about 60 degrees centigrade at a current density of 90 amperes per square foot on the diaphragms and a voltage of 4.65 volts were observed. The current efficiency on the caustic soda was measured by employing a copper coulometer and found to be 73.8 percent of theory at a NaOH concentration of 65.6 grams per liter of sodium hydroxide in the catholyte, and, the current efficiency on the sodium carbonate at a Na2CO3 concentration of 190 grams per liter was found to be 21 percent of theory. The over-all cell Current efficiency measured 94.7 percent of theory.

Example 2 In another experiment for the electrolysis of saturated solution of sodium chloride brine, in a manner after the foregoing example, the current eiciency on caustic soda was found to be 54.8 percent of theory at a NaOH concentration of 326.4 grams per liter of sodium hydroxide in the catholyte, and, the current efficiency on the sodium carbonate at a Na2CO3 concentration of 420 grams per liter was found to be 40.0 percent of theory. The overall cell current efciency measured 94.8 percent of theory.

The permselective diaphragms employed in the electrolytic decompositions described in the preceding examples can be constructed using ion exchange resins Which have been formed 4into continuous thin sheets. When a cation active ion exchange resin is employed a cationic permselective membrane is produced, i.e., one Which selectively permits passage of cations through its structure from one compartment of the cell to the next adjacent compartment in the direction toward the attraction of its electrode under the iniiuence of an impressed voltage and when wet with electrolyte. When an anion active ion exchange resin is employed an anionic permselective membrane is produced, i.e., one which selectively permits passage of lanions through its structure from one compartment of the cell to the next adjacent compartment in the direction toward the attraction of its electrode under the iniiuence of an impressed voltage and when wet with electrolyte. Successful diaphragms for use in electrolysis of alkali chloride brines have been constructed using cation exchange resins of an Amberlite type which have been formed into continuous sheets. These diaphragms .are described in United States Patents 2,681,319 and 2,681,320. Such sheets are continuous, nonporous, self-supporting, pliable, permselective membranes or pellicles which comprise a matrix-such as a synthetic hydrocarbon type plastic, or vulcanized, natural or synthetic rubber or polyethylene or polyisobutylene, or polyvinyl chloride, or copolymers of vinyl chloride and vinyl esters of lower aliphatic acids-having distributed intimately and uniformly therein particles of an insoluble, infusible ion-exchange resin, said particles being of such :a size as to pass through a United States Standard sieve No. 50 andbeing present in said diaphragm in an amount equal to twenty-live to seventy-five percent of the total weight of the diaphragm. The degree of perfection attainable in operation when using such diaphragms in electrolysis is among other things a function of its tightness, or the number of macropores or leakage points occurring between the resin particles in the sheet or fiber.

The permselective diaphragm structures of this invention may be made in a number of diiferent Ways, depending on various factors particularly in geometric design of the cell itself. One method is to use two sheets of membrane at least one of which is made in accordance with United States Patent 2,681,319 or United` States Patent 2,681,320, separated by a gasket-type spacer having the inlets and outlets to the resulting compartment positioned either in the spacer or through the membranes. Another method involves forming a continuous envelope from the said Amberlite type permselective diaphragm and allowing a space between the folded sheets to form the special compartment. When the cell design is such as to require large sheet-like permselective diaphragms to be spaced from each other by a gasket-type spacer We have found it desirable to reinforce at least one of the diaphragms by placing a grid against the face of the diaphragm in order to prevent buckling. The grid may consist of a rigid screen or 4similar type structure of suitable material of construction disposed across the entire surface of the diaphragm. If la diaphragm buckles inward toward the center compartment of the permselective diaphragm structure of this invention due to pressure from an adjacent compartment a grid is positioned in contact with the face of the diaphragm in said center compartment between the two diaphragms and is adapted to prevent buckling and contact lbetween the diaphragms. If a diaphragm Ibuckles outward toward an adjacent compartment due to pressure from the center compartment of the permselective diaphragm structure of this invention, a grid is positioned in the adjacent compartment, in contact with the face of the diaphragm and is adapted to prevent buckling and contact with the electrode if the next adjacent compartment is an electrode compartment.

The scope of this invention is not to be construed as limited to the specific electrolysis depicted in the above examples, but is to include the electrolysis of all those compounds which give products of fast moving ions which result in undesirable back-migration effects through irnperfect permselective membranes. Among the preferred chemical compounds which can be electrolyzed in accordance with the method of this invention are solutions of soluble inorganic salts such as the aqueous solutions of the alkali and alkaline earth metal bromides, chlorides, sulfates, acid sultes, etc. The ,alkali metal salts such as lithium and potassium which are very soluble and electrolyzable in water can be successfully electrolyzed in the same manner as the sodium salts depicted above. For example, in the electrolysis of sodium sulfate solutions las described in our copending application S.N. 267,846 led January 23, 1952, the salt is introduced into the center compartment of a three compartment electrolytic cell, separated from the cathode compartment by a cationic permselective membrane and separated from the anode compartment by an anionic permselective membrane, if the hydroxyl ions produced in the catholyte have a tendency to back-migrate due to an ineicient cationic permselective diaphragm, another cationic permselective membrane may be added between the center and cathode compartments forming, in effect, a fourth compartment whereby an acidic reagent such as carbon dioxide may be added to improve the over-all current efficiency and obtain other important advantages las discussed hereinbefore. (Similarly, if the hydrogen ions have a tendency to back-migrate due to .an inefficient anionic permselect1ve diaphragm an additional anionic permselective membrane may be added between the anode and center compartment forming in effect an additional compartment whereby a basic reagent may be added to improve the over-al1 current efficiency and obtain other important advantages as discussed hereinbefore.)

Thus, .a preferred embodiment of this invention comprises the use of a permselective diaphragm type structure in an electrolytic decomposition cell into which a reactive chemical selected from the group consisting of acidic and basic substances is introduced in the center compartment of said structure, which chemical is reactive with ions selected from the group consisting of hydroxyl and hydrogen or hydronium ions which leak, back-migrate, or are absorbed into said center compartment of the permselective diaphragm structure.

Chemical reagents other than carbon dioxide may be used in accordance with this invention for realizing the improvements described. The reagent selected will depend upon the salt to be electrolyzed and the product which is desired. Inorganic and organic acids or substances capable of reactint7 with bases such as SO2, HCl, H23, H5904, acetic acid, benzoic acid, etc. may all be used to react with the migrating hydroxyl ions coming from the cathode compartment through the imperfect cationic permselective membranes. Similarly where it is desired to add a base into an additional compartment to prevent fast moving hydrogen ion-s from reducing the current etiiciency by being lost because of leakage in the anionic permselective membrane, the choice of this base will depend upon the salt to be electrolyzed and the product which is desired, as above. Alkali metal hydroxides or carbonates are among the preferred compounds to be used. In either case the product need not .be water soluble, but this is desirable especially where the reagent is introduced directly into the cell compartment. The metallic salts corresponding to the acid or base introduced into the center compartment of the diaphragm structure of this invention will of course be produced lby the reaction between the acid or base and the hydroxyl or hydrogen ions. For example, when the reagent added is sulfur dioxide the additional product produced by the cell will be sodium suliite; by controlling the proportion of chemical reactant introduced, sodium bisulte may be produced.

The operating conditions such as current density, applied voltage, temperature, feed and product concentrations, various additives and other conditions familiar to those skilled in the art are considered to be unrelated in so far as limiting the scope of this invention.

The purity of the products in accordance with this invention are of exceptionally high standard and are in accordance with the disclosure of our co-pending application referred to above. By operating an electrolytic cell embraced within the scope of this invention, particularly as depicted in the foregoing examples and descr-iption with reference to the drawings, elemental chlorine having a purity of 99.0 percent can be consistently obtained and substantially pure hydrogen with no detectable impurities is also consistently obtained. The liquid products of the cell are likewise of exceptional purity. A typical analysis of these products is as follows:

Diaphragm Structure Center Com- Catholyte Solution partment Solution 43.0 percent NaOH 18.0 percent N :12003 4.0 p.p.m. Fe (soin. basis) 0.8 p.p.m. Fe (soln. basis) Thus, in accordance with this invention substantially pure products of the cell can be separately recovered in high concentrations.

Although we have described our invention with respect to certain specific embodiments thereof we do not intend to be limited thereto except as delined in the following claims.

We claim:

1. In the process for the electrolytic ldecomposition of an aqueous solution of an inorganic salt in an electrolytic cell having an anode compartment containing anode means separated from a cathode compartment containino cathode means by a diaphragm unit structure comprising two essentially continuous non-porous permselective diaphragm-s having t-he same kind of permselectivity and spaced from each other to form a central compartment of said unit structure, said central compartment having an inlet and an outlet which are in external communication with each other through a conduit, and at least one of said diaphragms comprising (a) a matrix which is polymeric material from the class consisting of polyethylene, polyisobutylene, vulcanized natural and synthetic rubber, polyvinyl chloride and copolymers of vinyl chloride and vinyl esters of lower aliphatic acids and (b) particles of an insoluble -infusible ion-exchange resin intimately and uniformly dispersed throughout said matrix, said particles 'being of such size as to pass through a United States Standard sieve size No. 50 and being present in said diaphragm in an amount equal to twenty-tive percent to seventy-tive percent of the total weigh-t of said diaphragm, the improvement which comprises: introducing the `said salt solution into an electrode compartment; effecting the electrolysis of said solution by impressing a decomposition voltage across the electrodes of said cell, whereby a product of electrolysis migrates into said central compartment; introducing into said external conduit a chemical compound reactive with said product of electrolysis to form a reaction product which under the conditions of electrolysis has a diusion coefficient through the diaphragm no greater than about the diffusion coefficient of carbonate ion through the diaphragm; and separately recovering the products of the cell so produced.

2. lhe process of claim 1 wherein the said reactive chemical compound introduced is one which when dissolved in water is a base, and the product of electrolysis is hydrogen ion.

3. The process of claim It wherein the said reactive chemical compound is one which when dissolved in water 1s an acid, and the product of electrolysis is hydroxyl ion.

4. The process of claim 3 wherein the compound is carbon dioxide.

5. The process of claim 3 sulfur dioxide.

6. In a process for the electrolytic decomposition of an aqueous solution of an alkali metal salt in an electrolytic cell having an anode compartment containing anode means separated from a cathode compartment containing cathode means by a diaphragm unit structure comprising two essentially continuous non-porous cationic permselective diaphragms and spaced from each other to form a central compartment of said unit structure, said central compartment having an inlet and an outlet which are an external communication with each other through a conduit, and at least the diaphragm facing the cathode compartment comprising (a) a matrix which is polymeric material from the class consisting of polyethylene, polyisobutylene, vulcanized natural and synthetic rubber, polyvinyl chloride and copolymers of vinyl chloride and vinyl esters of lower aliphatic acids and (b) particles of an insoluble infusible cation exchange resin in-timately and uniformly dispersed through said matrix, said particles being of such as size as to pass through a United States Standard sieve size No. 50 and being present in said diaphragm in an amount equal to twenty-five percent to seventy-tive percent of the total weight of said diaphragm, the improvement which comprises: introducing the said wherein the compound is salt Solution into the anode compartment; effecting the electrolysis of said solution by impressing a decomposition voltage across the electrodes of said cell, whereby hydroxyl ions migrate into said central compartment from said cathode compartment; introducing into said external c-onduit a chemical compound which when dissolved in water is an acid reactive with said hydroxyl ions -to form a reaction product which under the conditions of electrolysi-s has a diffusion coefficient through the diaphragm no greater than about the diffusion coeicient of carbonate ion through the diaphragm; and separately recovering the products of the cell so produced.

7. In a process for the electrolytic decomposition of an aqueous solution of an alkali metal chloride in an electrolytic cell to produce hydrogen, chlorine, alkali metal hydroxide and an additional product of the cell, having an anode compartment containing anode means separated from a cathode compartment containing cathode means by a diaphragm unit structure comprising two essentially continuous non-porous cationic permselective diaphragms and spaced from each other to form a central compartment of said unit structure, said central compartment having an inlet and an outlet which are an external communication with each other through a conduit, and at least the diaphragm facing the cathode cornpartment comprising (a) a matrix which is polymeric material from the class consisting of polyethylene, polyisobutylene, vulcanized natural and synthetic rubber, polyvinyl chloride and copolymers of vinyl chloride and vinyl esters of lower aliphatic acids and (b) particles of an insoluble infusible cation exchange resin intimately and uniformly dispersed throughout said matrix, said particles being of such size as to pass through a United States Standard sieve No. 50 and being present in said ilm in an amount equal to twenty-tive percent to seventy-five percent of the total weight of said diaphragm, the improvement which comprises: introducing the said alkali metal chloride solution into the anode compartment, effecting the electrolysis of said solution by impressing a decomposition voltage across the electrodes of said cell, whereby hydroxyl ions migrate into said central compartment from said cathode compartment; introducing into said external conduit carbon dioxide to react with said hydroxyl ions; and separately recovering the products of the cell so produced.

8. The method of effecting the electrolytic decomposition of an aqueous solution of an inorganic salt comprising introducing the said solution into the anode compartment of an electrolytic cell means having an anode compartment containing anode means separated from a cathode compartment containing cathode means by a diaphragm unit structure means comprising two essentially continuous non-porous permselective diaphragm means having the same kind of permselectivity and spaced from each other to form a central compartment of said unit structure means, said central compartment having an inlet means and an outlet means which are in external communication with each other through conduit means, and at least one of said diaphragms being formed by combining a supporting material with an ion-exchange resin dispersed therein and polymerized to an insoluble infusible condition and sealed to substantial impermeability to passage of liquids and gases and ions of a given sign while passing ions of the opposite sign and being composed of material having the characteristics of an ionized salt pair; effecting the electrolysis of said solution by impressing a decomposition voltage across the electrodes of said cell, whereby a product of electrolysis migrates into said central compartment; introducing into said external conduit means a chemical compound reactive with said product of electrolysis to form an additional product which under the conditions of electrolysis has a diffusion coeicient through the diaphragm no greater than about the diffusion coeflicient of carbonate ion through the diaphragm; and separately recovering the products so produced.

9. The method of effecting the electrolytic decomposition of an aqueous solution of an alkali metal chloride to hydrogen, chlorine, alkali metal hydroxide and an additional product comprising introducing the said solution into the anode compartment of an electrolytic cell means, having an anode compartment containing anode means separated from a cathode compartment containing cathode means by a diaphragm unit structure means comprising two essentially continuous non-porous cationic permselective diaphragm means and spaced from each other to form a central compartment of said unit structure means, said central compartment having an inlet means and an outlet means which are an external communication with each other through conduit means, and at least the diaphragm means facing the cathode compartment being formed by combining a supporting material with an ion-exchange resin dispersed therein and polymerized to an insoluble infusible condition and sealed to substantial impermeability of passage of liquids and gases and anions while passing cations and being composed of an ionized salt pair, effecting the electrolysis of said solution by impressing a decomposition voltage across the electrodes of said cell, whereby hydroxyl ions migrate into said central compartment from said cathode compartment; introducing into said external conduit carbon dioxide to react with said hydroxyl ions to form an additional product; and separately recovering the products so produced.

10. The method of effecting the electrolytic decomposition of an aqueous solution of an alkali metal salt to alkali metal hydroxide and an additional product comprising introducing the said salt and water into the anode compartment of an electrolytic cell means having an anode compartment containing anode means separated from a cathode compartment containing cathode means by a diaphragm unit structure means comprising two essentially continuous non-porous cationic permselective diaphragm means and spaced from each other to form a central compartment of said unit structure means, said central compartment having an inlet means and an outlet means which are an external communication with each other through conduit means, and at least the diaphragm means facing the cathode compartment being formed by combining a supporting material with an ion-exchange resin dispersed therein and polymerized to an insoluble infusion condition and sealed to substantial impermeability to passage of liquids and gases and anions while passing cations and being composed of material having the characteristics of an ionized salt pair, effecting the electrolysis of said solution by impressing a decomposition voltage across the electrodes of said cell, whereby hydroxyl ions migrate into said central compartment from said cathode compartment; introducing into said external conduit a chemical compound reactive with said hydroxyl ions to form an additional product which under the conditions of electrolysis has a diffusion coeflicient through the diaphragm no greater than about the diifusion coeflicient of carbonate ion through the diaphragm; and separately recovering the products so produced.

11. The process of claim 1 wherein the said reactive chemical compound is H28.

12. The process of claim 1 wherein the said reactive chemical compound is H3PO4.

13. The process of claim 1 wherein the said reactive chemical compound is C12.

References Cited by the Examiner UNITED STATES PATENTS 831,474 9/ 1906 Roberts 204-295 1,972,561 9/1934 Henbaum 204-180 2,057,232 lO/l936 Endell 204- 2,383,674 8/1941 Osborne 204-87 2,571,247 10/ 1951 Hoebotter 204-180 (Other references on following page) UNITED 13 14 STATES PATENTS FOREIGN PATENTS Bodamer 204-180 6,417 1887 Great Britain. Bodamer 204-180 Rosenberg 204-108 OTIER REFERENCES Van Hoek 204 1g0 5 Kalauch: Kollold Zeltschrlft, vol. 112 (1949), pp. De Haas Van Dorsser et al. 21-26- 04-180 Bodamer 2204 72 JOHN H. MACK, Primary Examiner. Osborne et al. 204-72 JOHN R. SPECK, JOSEPH REBOL'D, MURRAY A.

Osborne et al 204-98 10 TILLMAN, Examiners.

Claims

1. IN THE PROCESS FOR THE ELECTROLYTIC DECOMPOSITION OF AN AQUEOUS SOLUTION OF AN INORGANIC SALT IN AN ELECTROLYTIC CELL HAVING AN ANODE COMPARTMENT CONTAINING ANODE MEANS SEPARATED FROM A CATHODE COMPARTMENT CONTAINING CATHODE MEANS BY A DIAPHRAGM UNIT STRUCTURE COMPRISING TWO ESSENTIALLY CONTINUOUS NON-POROUS PERMSELECTIVE DIAPHRAGMS HAVING THE SAME KIND OF PERMSELECTIVITY AND SPACED FROM EACH OTHER TO FORM A CENTRAL COMPARTMENT OF SAID UNIT STRUCTURE, SAID CENTRAL COMPARTMENT HAVING AN INLET AND AN OUTLET WHICH ARE IN EXTERNAL COMMUNICATION WITH EACH OTHER THROUGH A CONDUIT, AND AT LEAST ONE OF SAID DIAPHRAGMS COMPRISING (A) A MATRIX WHICH IS POLYMERIC MATERIAL FROM THE CLASS CONSISTING OF POLYETHYLENE, POLYISOBUTYLENE, VULCANIZED NATURAL AND SYNTHETIC RUBBER, POLYVINYL CHLORIDE AND COPOLYMERS OF VINYL CHLORIDE AND VINYL ESTERS OF LOWER ALIPHATIC ACIDS AND (B) PARTICLES OF AN INSOLUBLE INFUSIBLE ION-EXCHANGE RESIN INTIMATELY AND UNIFORMLY DISPERSED THROUGHOUT SAID MATRIX, SAID PARTICLES BEING OF SUCH SIZE AS TO PASS THROUGH A UNITED STATES STANDARD SIEVE SIZE NO 50 AND BEING PRESENT IN SAID DIAPHRAGM IN AN AMOUNT EQUAL TO TWENTY-FIVE PERCENT TO SEVENTY-FIVE PERCENT OF THE TOTAL WEIGHT OF SAID DIAPHRAGM, THE IMPROVEMENT WHICH COMPRISES: INTRODUCING THE SAID SALT SOLUTION INTO AN ELECTRODE COMPARTMENT; EFFECTING THE ELECTROLYSIS OF SAID SOLUTION BY IMPRESSING A DECOMPOSITION VOLTAGE ACROSS THE ELECTRODES OF SAID CELL, WHEREBY A PRODUCT OF ELECTROLYSIS MIGRATES INTO SAID CENTRAL COMPARTMENT; INTRODUCING INTO SAID EXTERNAL CONDUIT A CHEMICAL COMPOUND REACTIVE WITH SAID PRODUCT OF ELECTROLYSIS TO FORM A REACTION PRODUCT WHICH UNDER THE CONDITIONS OF ELECTROLYSIS HAS A DIFFUSION COEFFICIENT THROUGH THE DIAPHRAGM NO GREATER THAN ABOUT THE DIFFUSION COEFFICIENT OF CARBONATE ION THROUGH THE DIAPHRAGM; AND SEPARATELY RECOVERING THE PRODUCTS OF THE CELL SO PRODUCED.