Electrolysis Of Brine Using Permeable Membranes Comprising Fluorocarbon Copolymers

Abstract

Novel diaphragms for use in chlor-alkali cells and methods of making such diaphragms are disclosed. The diaphragms are electrolyte-permeable, asbestos diaphragms containing an organic, ion exchange resin and a second fibrous material. These diaphragms are especially permeable to the electrolyte.

Description

BACKGROUND

According to one process for the production of chlorine, alkali metal chlorides are electrolyzed to produce chlorine and alkali metal hydroxide in a diaphragm cell. Such electrolytic cells have a structure substantially as shown in U.S. Pat. No. 3,337,443 to C.W. Raetzsch et al., for ELECTROLYTIC CELL and in U.S. Pat. No. 3,022,244 to Carrol P. LeBlanc et al., for ELECTROLYTIC ALKALI-CHLORINE DIAPHRAGM CELL.

The diaphragm serves to separate the anolyte compartment from the catholyte compartment. An aqueous solution of an alkali metal chloride, i.e., brine, is fed into the anolyte compartment. In the anolyte compartment the chloride ion of the disassociated alkali metal chloride forms chlorine at the anode. The anolyte liquor, including alkali metal ion, hydroxyl ion, and chloride ion, travel through the diaphragm to the catholyte compartment. In the catholyte compartment alkali metal hydroxide and gaseous hydrogen are liberated at the cathode, and catholyte liquor containing alkali, metal chloride and alkali metal hydroxide is recovered from the catholyte compartment.

The diaphragm also serves to maintain a difference in pH between the two compartments. Typically, the electrolyte in the anolyte compartment will have a pH of from about 3.5 to about 5.5, while the electrolyte in the catholyte compartment will have a pH of 12.0 or greater. The diaphragm, in a typical electrolytic cell of the type hereinabove described, serves to maintain this difference in pH, while permitting the flow of electrolyte therethrough.

Typically, such diaphragms have been prepared from asbestos. In the electrolytic cels of the prior art, the asbestos has been of the type referred to in the literature as chrysotile asbestos. Chrysotile asbestos has a structure characterized as tubular fibers, and an empirical formula of 3MgO .sup.. 2SiO .sup.. 2H.sub.2 O.

Asbestos diaphragms generally have a weight of from about 0.30 to about 0.40 pounds per square foot of diaphragm surface area, a thickness of about one-eighth inch when installed and "swell" by about 100 percent to a thickness of about one-fourth inch when in service.

Such asbestos diaphragms of the prior art have a service life of about 4 months to about 7 months in electrolytic cell service where the anodes of the electrolytic cell are graphite. In the electrolytic cells wherein the anodes are dimensionally stable anodes (i.e., where the anodes are a metal substrate having an electrocatalytic metal or metal compound surface thereon), the service life of the diaphragm is typically from about 3 months to about 7 months. This contrasts with a service life typically in excess of 18 months for the dimensionally stable anodes themselves, thereby requiring several renewals of the asbestos diaphragm between each renewal of the anodes.

SUMMARY

It has now been found that diaphragms suitable for use in alkali metal chloride electrolytic cells of the diaphragm type particularly when such cells have dimensionally stable anodes, may be provided by an asbestos diaphragm that contains small amounts of a second fibrous material and has been treated with small amounts of an electrolyte-resistant, ion exchange resin.

Diaphragms of this invention are long lived in the electrolytic chlorine cell environment, being characterized by their increased life over conventional electrolytic cell diaphragms containing asbestos alone. Diaphragms of this invention are further characterized by being gas and electrolyte-permeable, functioning as a diaphragm rather than as a permionic membrane. Diaphragms of this invention are also characterized in that they may be prepared at lower cost than diaphragms of permionic membranes having as their principal constituent a synthetic, ion exchange resin.

Diaphragms of this invention are characterized in that they have a resistance voltage drop across the diaphragm of as much as 0.2 to 0.3 volt less than an untreated asbestos diaphragm of the same thickness, and as much as 10 to 20 volts less than a permionic membrane of the same thickness.

DESCRIPTION OF THE INVENTION

Diaphragms are used in alkali metal hydroxide chlorine cells to separate the anolyte, having a pH of about 3.5 to about 5.5, from the catholyte having a pH of 12 or greater. Such cells typically have an anolyte compartment or chamber containing an anode, a catholyte compartment or chamber containing a cathode, a diaphragm to separate the anolyte from the catholyte, and external power supply means for imposing an electromotive force between the anode and the cathode. The cathode is a catholyte resistant metal structure, e.g., iron, open to the flow of electrolyte. The cathode may be iron mesh, iron screen, or a perforate plate. The anode may be graphite, or it may be a valve metal, e.g., titanium, with an electrocatalytic surface, e.g., a platinum group metal.

In the operation of commercial chlorine-caustic soda diaphragm cells, brine containing from about 280 to about 315 grams per liter of sodium chloride is fed into the anolyte chamber of the cell. An electromotive force is established between the anode and the cathode. In the anolyte chamber chlorine is evolved at the anode. Additionally, anolyte liquor, containing sodium chloride, passes through the diaphragm to the catholyte chamber. In the catholyte chamber hydrogen is evolved at the cathode, and catholyte liquor containing from about 7 to about 15 weight percent of sodium chloride and from about 10 to about 15 weight percent of sodium hydroxide is recovered.

Diaphragms of this invention, having a thickness of from about 0.001 inch to about 0.250 inch, and a voltage drop of less than about 0.6 volt at a current density of 190 Amperes per square foot, can operate at an inter-electrode gap (i.e., a spacing between a cathode and the next adjacent anode in the cell) of less than about 0.50 inch and in some cases even less than 0.25 inch.

Moreover, inasmuch as the diaphragms of this invention are less subject to swelling than the diaphragms of the prior art, the anodes may be closer to the cathode than was previously the case.

Diaphragms of this invention are electrolyte-permeable asbestos members such as fibrous asbestos mats or sheets, that have been treated with small amounts of electrolyte-resistant, ion exchange resins and that contain a second fibrous member having a fiber diameter greater than the fiber diameter of the asbestos.

According to this invention, particularly satisfactory diaphragms may be prepared containing chrysotile asbestos and having a second fibrous material dispersed therethrough. Such diaphragms maintain a high porosity, on the order of from about 0.1 gallons per hour per square foot to about 3.0 gallons per hour per square foot, after treatment with ion exchange resin. Furthermore, when diaphragms are prepared having a second fibrous material they appear to have greater reproducibility and less variance in porosity, which is particularly advantageous in commercial operation.

The second fibrous material should have a fiber diameter at least two times greater than the fiber diameter of the chrysotile asbestos. Preferably, the second fibrous material has a fiber diameter of from about two to about 1,000 times greater than the fiber diameter of the chrysotile asbestos, with particularly good results being obtained when the second fibrous material has a fiber diameter of from about 200 to about 400 times the fiber diameter of the chrysotile asbestos.

Chrysotile has a fiber diameter of from about 0.015 to about 0.030 microns. Materials suitable for use as the second fibrous material include crocidolite or anthophyllite asbestos, having a fiber diameter of from about 0.060 to about 0.090 microns, fiberglass, having a fiber diameter of from about 8 to 12 microns, and alpha-cellulose, having a fiber diameter of from about 6 to about 15 microns. Alternatively, amosite, tremolite, or actinolite asbestos may be used.

Other materials, having some chemical resistance to the liquid used in preparing the asbestos slurry, and having fiber diameters of from about 0.060 micron to about 15 microns, may be used as the second fibrous material. Such materials include polyperfluoroethylene, polyvinylchloride, polyvinylidene chloride, polyvinylfluoride, polyvinylidene fluoride, and the like. By "chemical resistance" is meant that the material is not significantly attacked by the liquid during the time the slurry is being "aged."

The second fibrous material, when present, should be from about 1 to about 50 weight percent of the total weight of the diaphragm. Amounts less than about 1 weight percent do not impart any additional porosity over a diaphragm not containing the second fibrous material, while amounts greater than about 50 weight percent yield a diaphragm that is too porous.

Typically, the ion exchange resins are hydrophilic, cation-selective resins substantially inert to the electrolyte. By "hydrophilic" it is meant that the ion exchange resins are readily wetted by the electrolyte. By "cation selective" it is meant that the ion exchange resins useful in preparing diaphragms of this invention preferentially allow the migration of cations such as Na+ and H+ through the diaphragm to a greater extent than the migration of anions such as chloride ions (Cl.sup.-) through the diaphragm. By "inert" it is meant that the ion exchange resins useful in preparing the diaphragm of this invention are substantially immune to attack by the electrolyte, e.g., chloride ions, hypochlorite ions, hydroxyl ions, hydronium ions, sodium ions, and the like, and by the electrolysis products, e.g., chlorine.

The inert polymer chains of the ion exchange resins serve to impart physical strength and durability to the porous asbestos diaphragm structure. The cation-selective ion exchange groups on the ion exchange resins render the diaphragm hydrophilic, allowing the diaphragm to exhibit permeability to the electrolyte with a minimum hydrostatic head of anolyte.

Ion exchange resins having the properties of cation selectivity, wettability, and inertness are preferentially provided by fluorocarbons having acid groups, fluorocarbon interpolymers having fluorocarbon and fluorocarbon acid moieties, and interpolymers having aryl and arylacid moieties where the main chain is a fluorocarbon, e.g., fluoro styrenes.

One class of fluorocarbons useful in providing the ion exchange resins of this invention are those having the empirical formula: ##SPC1##

where m is from 2 to 10, the ratio of M to N is sufficient to provide an equivalent weight of from 600 to 2,000 as will be more fully elucidated hereinafter, and R is chosen from the group consisting of:

A,

--ocf.sub.2 --cf.sub.2 --.sub.p A where p is from 1 to 3, ##SPC2##

where p is from 1 to 3 and Y is --F, or a perfluoroalkyl group having from one to 10 carbon atoms, ##SPC3##

where p is from 1 to 3, Y is --F or a perfluoroalkyl group having from one 10 carbon atoms, and R.sub.f is --F or a perfluoroalkyl having from one to 10 carbon atoms,

--.phi.--A where .phi. is an aryl group, and

--CF.sub.2 --.sub.p A where p is from 1 to 3; and

where A is an acid group chosen from the group consisting of

--SO.sub.3 H,

--cf.sub.2 so.sub.3 h,

--ccl.sub.2 SO.sub.3 H,

--.phi..sup.1 so.sub.3 h,

--po.sub.3 h.sub.2,

--po.sub.2 h.sub.2,

--cooh, and

--.phi..sup.1 OH where .phi..sup.1 is an aryl group

When the fluorocarbon is one having short side chains, such as poly(perfluoroethylene-trifluorovinyl sulfonic acid) or --CF.sub.2 --CF.sub.2 --.sub.M --CF.sub.2 --CF(O--CF.sub.2 --CF.sub.2 --SO.sub.3 H)-- the ratio of N to M, that is, the ratio of the moles of fluorocarbon to the moles of the fluorocarbon acid, is typically about 8, thereby providing an equivalent weight of about 1,000 gram per mole of acid. In the case of such polymers having short side chains, the ratio of N to M is from about 5 to about 20, and preferably from about 6 to about 14. When the ratio of the moles of fluorocarbon to moles of fluorocarbon acid is below about 5, the ion exchange agent shows a decrease in physical strength and becomes subject to the abrasive effects of the evolved gas and the flowing electrolyte. When the ratio of the moles of halocarbon to the moles of halocarbon acid is in excess of about 20, the hydrophobic properties of the fluorocarbon member of the interpolymer begin to predominate, causing the diaphragm to behave as a permionic membrane.

The ion exchange resin is preferably fully fluorinated. By "fully fluorinated" is meant that both nuclear magnetic resonance examination and infrared spectroscopic examination of the interpolymer show less than 1 percent C--H bonds in the polymer. However, the polymer need not be completely fluorinated inasmuch as the asbestos is the structural member of the diaphragm and some minimal degradation of the ion-exchange resin can be tolerated.

In a preferred exemplification the ion exchange resin has the empirical formula: ##SPC4##

where M and N are as described above.

In another preferred exemplification, the ion exchange resin has the empirical formula: ##SPC5##

where M and N are as described above.

While the ion exchange resin is spoken as being a polymer, polyfunctional perfluoroalkyl acids may also be used in preparing diaphragms according to this invention. Such polyfunctional perfluoroalkyl acids include those having the empirical formula:

A --CF.sub.2 --.sub.Q A'

where A and A' are acid groups chosen from the group consisting of:

--SO.sub.3 H,

--cf.sub.2 so.sub.3 h,

--ccl.sub.2 SO.sub.3 H,

--.phi.'so.sub.3 h,

--po.sub.3 h.sub.2,

--po.sub.2 h.sub.2,

--cooh, and

--.phi.'OH,

where .phi.' is an aryl group, and q is greater than 8. A and A' may be the same acid groups or they may be different acid groups. Most frequently A is --SO.sub.3 H and A' is either a second --SO.sub.3 H group, a --COOH group, a .phi.'SO.sub.3 H group or a .phi.'OH group. When other combinations of acid groups are useful in providing diaphragms, they are not as readily available, and no significant additional benefit is gained by their use. The polyfunctional acid may also be an ether, having the formula A' -- (CF.sub.2).sub.q ' -- O -- (CF.sub.2).sub.q "--A where q'+q"=q.

The length of the perfluoroalkyl unit, q, is greater than 8, and generally between 8 and 20, most frequently between 10 and 16. While longer non-polymeric perfluoroalkyls may be used, they are not generally commercially available.

When, as is more frequently the case, the ion exchange resin is a polymer, the fluorocarbon moiety is a fluorinated olefin such as tetrafluoroethylene, hexafluoropropylene, octafluorobutylene, or further homologues thereof. Tetrafluoroethylene is preferred.

There may, also, be fluorocarbon moieties, present in the interpolymer, having as their precursors fluorinated acetylenes such as difluoroacetylene or fluorinated diolefins such as hexafluorobutadiene. Such fluoroacetylenes and fluorodiolefins may serve as cross-linking agents cross-linking the fluoroolefin polymers and in that way impart additional strength to the diaphragms of this invention.

The acid moiety ##SPC6##

may be a fluoroolefin acid such as the trifluoroethylene acids, the pentafluoropropylene acids, the heptafluorobutylene acids, and further homologues thereof. The pendant group may also be a poly(perfluoroether) or poly(perfluoroalkyl) side chain with a terminal acid group. The pendant acid group A is a cation-selective, ion exchange acid group such as a sulfonic --SO.sub.3 H--, a fluoromethylene sulfonic --CF.sub.2 SO.sub.3 H--, a chloromethylene sulfonic --CCl.sub.2 SO.sub.3 H--, a benzene sulfonic (--.phi.SO.sub.3 H), a carboxylic (--COOH), a phosphonic (PO.sub.3 H.sub.2), a phosphonous (PO.sub.2 H.sub.2), or a phenolic (--.phi.OH) acid group. When .phi. is used herein, it refers to the aryl group --C.sub.6 H.sub.4 --.

While the preferred acids are the trifluorovinyl acids, both with and without perfluorinated side chains, it is to be understood that other halocarbon acids may be used with entirely satisfactory results.

The preferred acid groups are the sulfonic acid groups including the benzene sulfonic acid group (--.phi.SO.sub.3 H), the fluoromethylene sulfonic acid group (--CF.sub.2 SO.sub.3 H), the chloromethylene sulfonic acid group (--CCl.sub.2 SO.sub.3 H), the sulfonic acid group (--SO.sub.3 H), and perfluoro side chains have terminal sulfonic acid groups, in that they have the greatest cation selectivity.

Particularly satisfactory ion exchange resins are the copolymers of fluoroolefins and trifluorovinyl sulfonic acid. A particularly satisfactory ion exchange resin useful in preparing diaphragms of this invention is a tetrafluoroethylene and trifluorovinyl sulfonic acid interpolymer, as disclosed, for example, in U.S. Pat. No. 3,624,053 to Gibbs and Griffin for TRIFLUOROVINYL SULFONIC ACID POLYMERS.

While the fluorocarbon ion exchange resin described above is illustrated as a polyolefin, it should be noted that other polymeric fluorocarbons may be used with equally satisfactory results. One particularly satisfactory group of ion exchange resins are the fluorocarbon-fluorocarbon acid vinyl ether polymers, such as those disclosed in U.S. Pat. No. 3,282,875 to Connolly and Gresham for FLUOROCARBON VINYL ETHER POLYMERS; British Pat. No. 1,034,197; and German Offenlegungsschrift No. 1,806,097 of D. P. Carlson, based on U.S. application Ser. No. 697,162 filed Oct. 30, 1967, now U.S. Pat. No. 3,432,103. Disclosed by Connolly and Gresham are fluorocarbon-fluorocarbon acid vinyl ether polymers prepared from monomers having the empirical formula: ##SPC7##

where R.sub.f is a radical selected from the group consisting of fluorine and perfluoroalkyl radicals having from one to 10 carbon atoms, Y is a radical selected from the group consisting of fluorine and perfluoroalkyl radicals having from one to 10 carbon atoms, n is an integer from 1 to 3, and m is a radical selected from the group consisting of fluorine, the hydroxyl radical, the amino radical, and radicals having the formula --OMe where Me is a radical selected from the group of alkali metals and the quaternary ammonium radicals.

According to this invention, a thin adherent coating is provided, for example, by contacting properly the asbestos diaphragm with the ion exchange resin. Quite small amounts of the resin provide effective treatment. The ion exchange resin is typically from about 0.01 weight percent to about 5 weight percent of the total diaphragm (basis a typical chlorine-caustic cell diaphragm), and preferably from about 0.3 to about 2.0 weight percent of the total diaphragm. Concentrations of ion exchange resin of less than about 0.01 weight percent, although beneficial, do not impart sufficient additional physical strength to the diaphragm to justify normally using such small amounts. Concentrations of ion exchange resin of greater than about 5.0 weight percent, while useful, are not usually recommended since they tend to decrease porosity and necessitate higher hydrostatic heads of anolyte for proper cell operation. High concentrations of ion exchange resin, e.g., amounts greater than about 22 weight percent, cause the diaphragm to behave as a permionic membrane, yielding a catholyte liquor containing less than 1.5 weight percent of sodium chloride.

The ion exchange resin may be dispersed through the asbestos mat or sheet, coating individual fibers within the asbestos mat or sheet. Alternatively, the ion exchange resin may only coat the fibers on the exterior surfaces of the asbestos mat or sheet, or the ion exchange resin may only coat bundles of asbestos fibers. In a preferred exemplification the ion exchange agent will be both dispersed through the asbestos member, coating the individual asbestos fibers and fiber bundles within the asbestos member as well as on the external surface of the asbestos member, between the asbestos body and the anolyte.

Moreover, it has been found that for any given porosity, pore size distribution and thickness of diaphragm, best results are obtained if the ion exchange resin extends at least as far into the diaphragm as the "gel layer" in an untreated diaphragm of like porosity, pore size distribution, and thickness. This "gel layer" is described by Kircher, "Electrolysis of Brines in Diaphragm Cells," in Sconce, ed., Chlorine, A.C.S. Monograph Series, No. 154, Reinhold Publishing Co., New York (1962), at Page 105, as a layer "formed within the asbestos mat which is sensitive to pH and which tends to dissolve, precipitate and reform depending upon flow rate and salt content and pH of the flowing liquor."

Typically, the "gel layer" extends approximately about 0.08 to about 0.12 inch into the diaphragm. Therefore, an optimal depth of penetration of the ion exchange resin is at least 0.08 inch, and preferably about 0.15 inch, or even to the full thickness of the diaphragm, especially when the diaphragm is less than about 0.15 inch thick.

While in terms of location and concentration the ion exchange resin is spoken of as being dispersed through the asbestos member, coating various individual asbestos fibers, and fiber bundles, the precise chemical relationship between the resin and the asbestos is believed to be more complicated.

Thus, one of the particularly effective types of diaphragms of this invention is obtained by forming a strongly adherent, protective film of coating of these polymers on the anolyte-facing surface of the asbestos diaphragm member. Particularly good adherence is accomplished by effecting what appears to be a chemical reaction between the acidic moieties of the polymer and the asbestos, notably magnesium of the asbestos. That is, when the polymer and asbestos are appropriately contacted it is possible to form a thin (possibly even a substantially monomolecular) tenacious, polymer coating on the surface of the asbestos fibers. In one embodiment, only a portion of these acidic moieties are involved in the reaction with the magnesium, leaving free acid moieties in the coating.

One way of assuring formation of such a coating is to apply a solution of the resin to the asbestos (whether in dispersed fiber form or as a diaphragm member) under conditions which do not interfere with the reaction or interaction between the polymer and the asbestos. For example, when the asbestos is drawn from cell liquor, the diaphragms are dried to precipitate sodium chloride crystals and then washed to remove the sodium chloride crystals before coating with the ion exchange resin.

Diaphragms of this invention, having an asbestos member with ion exchange resin therein, contain from about 0.01 grams of the ion exchange agent per square foot of exposed diaphragm area to about 5.0 grams of ion exchange agent per square foot of diaphragm area.

In providing diaphragms according to this invention, chrysotile asbestos may be used. Such asbestos has an empirical formula of 3MgO .sup.. 2SiO.sub.2 .sup.. H.sub.2 O. Alternatively, other forms of asbestos such as anthophyllite asbestos may be used with entirely satisfactory results.

The asbestos useful in providing the diaphragm of this invention typically has a fiber length of from about 1/32 inch to about 11/2 inches and a fiber diameter range of from about 0.01 micron to about 20 microns, and a mean fiber diameter of from about 0.015 to about 0.030 micron.

Thus, according to the exemplification a diaphragm is prepared having a weight of 0.40 pounds per square foot, containing 85 weight percent chrysotile asbestos, about 15 weight percent cellulose, and about 0.1 to about 1.0 grams per square foot of an ion exchange resin.

Diaphragms of this invention may be prepared according to a number of methods.

In one method, the asbestos and second fibrous material are drawn from a cell liquor slurry onto an electrolyte-permeable cathode by any of the conventional methods known in the art, the sodium ion present in the deposited asbestos as a result of the deposition process is removed, and the ion exchange resin in deposited on the asbestos.

According to this method, the asbestos is slurried in cell liquor. Typically, a slurry containing from about 0.3 to about 4 percent total solids by weight is prepared in a solution containing from about 10 to about 15 weight percent caustic soda and from about 10 to about 15 weight percent sodium chloride, thereby providing a solution of about 25 to about 30 weight percent total caustic soda and sodium chloride.

According to this method, diaphragms may be prepared having less of a tendency to "tighten" after treatment with the ion exchange resin. In this method, the diaphragm is drawn from a slurry containing both chrysotile asbestos and a second fibrous material. The second fibrous material present with the chrysotile asbestos is characterized in that the individual fibers have mean fiber diameters greater than the mean fiber diameter of chrysotile asbestos.

The mean fiber diameter of the second fibrous material should be at least two times as great as the mean fiber diameters of the chrysotile asbestos, and preferably at least two orders of magnitude (i.e., 100 times) greater than the mean fiber diameter of the chrysotile asbestos, and may even be as much as 1,000 or more times greater than the mean fiber diameter of the chrysotile asbestos. Particularly good results have been obtained when the mean fiber diameter of the second fibrous material is from about 200 to about 400 times greater than the mean fiber diameter of the chrysotile asbestos.

Chrysotile asbestos has a mean fiber diameter of from about 0.015 micron to about 0.030 micron. Therefore, according to this method, the second fibrous material may be crocidolite or anthophyllite asbestos, having a mean fiber diameter from about 0.060 micron to about 0.090 micron, fiberglass, having a mean fiber diameter of from about 8 to about 12 microns, or alpha-cellulose, having a mean fiber diameter of from about 6 to about 15 microns.

The slurry used in preparing diaphragms according to this method contains from about 0.3 weight percent to about 3.5 weight percent chrysotile asbestos, and from about 0.015 weight percent to about 3.5 weight percent of the second fibrous material. In this way, from about 5 to about 50 weight percent of the total solids in the slurry is the second fibrous material. The liquid used in preparing diaphragms according to this method may be cell liquor, aqueous sodium chloride, aqueous sodium hydroxide, water, water containing a surfactant, or an organic solvent.

The asbestos diaphragm is drawn onto the cathode in the conventional way, allowed to dry, and then washed or rinsed with water or an organic solvent to remove the solid sodium chloride precipitated in and on the diaphragm. The diaphragm is then treated with the ion exchange as described above. Thereafter the cathode, having an ion exchange resin treated asbestos diaphragm, is installed in an electrolytic cell, and electrolysis carried out.

Suitable slurries that provide a diaphragm containing from about 5 to about 20 weight percent alpha-cellulose may be prepared by adding cellulose and asbestos to cell liquor. Particularly satisfactory slurries contain from about 0.5 to about 2.0 weight percent chrysotile asbestos, and from about 0.10 to about 0.25 weight percent alpha-cellulose. By "cell liquor" is meant a solution containing from about 100 to about 135 grams per liter of NaOH and from about 150 to about 200 grams per liter of NaCl.

The slurry of asbestos and cellulose in cell liquor may be aged for from about 1 to about 10 days. Additionally air or nitrogen may be bubbled through the slurry.

The asbestos-cellulose diaphragm may be drawn onto the cathode structure by vacuum deposition, as is well known in the art.

EXAMPLE I

A cathode-diaphragm assembly was prepared having an ion exchange resin treated porous asbestos diaphragm.

Cell liquor was prepared containing 135 grams per liter of sodium hydroxide and 175 grams per liter of sodium chloride. To this cell liquor solution was added sufficient Johns-Manville 3T-4T asbestos to provide a slurry containing 1.5 weight percent asbestos. The slurry was aged for 3 days. Thereafter, sufficient Solka-Floc cellulose was added to the slurry to provide a cellulose concentration of 15 weight percent cellulose based on the total weight of the solids, i.e., asbestos and cellulose. The asbestos and cellulose were deposited onto an iron mesh cathode by drawing the slurry onto the cathode screen at a vacuum of 7 inches of mercury. The deposited diaphragm was then washed with water, and then maintained at a vacuum of 16 inches of mercury and dried at 110.degree.C. for 22 hours.

An ethanol solution was prepared containing 10 weight percent duPont "XR" (Trademark) resin, a polymeric, per-fluorinated, sulfonic acid ion exchange resin having an equivalent weight of 1,300 grams, and less than 1 percent C--H bonds as determined by infrared spectroscopy and nuclear magnetic resonance. The cathode-diaphragm structure was dipped into the resin-ethanol solution and the solution was allowed to permeate the diaphragm. Thereafter, the diaphragm-cathode assembly was heated at 100.degree.C. for 41/2 hours.

At a brine head of 9 inches, the diaphragm had a porosity of 1.10 gallons of brine per hour per square foot. The voltage across the diaphragm was then tested at a minimum anode to diaphragm gap, i.e., with the anode touching the diaphragm, at current densities of from 20 to 81 amperes per square foot. The resulting voltages were extrapolated to 190 amperes per square foot indicating a predicted cell voltage of 2.9 volts.

Thereafter, the cathode-diaphragm assembly was installed in a laboratory electrolytic cell.

The laboratory electrolytic diaphragm cell used in this example had a 1,000 cubic centimeter capacity catholyte compartment fabricated of 10 gauge steel sheet, and an anolyte compartment having a 1,000 cubic centimeter capacity fabricated of 1/2 inch thick Grade-1 titanium. The anode, measuring 5 inches by 7 inches, was 1/16 inch Grade-1 titanium mesh coated with platinum and iridium. The cathode was 6 by 6 mesh, 3/16 inch, number 13 steel screen. The gap between the anode and the diaphragm was one-fourth inch. After 7 days of electrolysis, the cell current efficiency was 96.5 percent, the cell liquor had between 0.01 and 0.02 weight percent NaClO.sub.3, the cell gas contained between 0.01 and 0.10 volume percent hydrogen, the cell voltage was 3.09 volts, and the cell liquor and diaphragm IR drop was 0.69 volts.

EXAMPLE II

A cathode-diaphragm assembly was prepared having an ion-exchange resin treated asbestos diaphragm.

An asbestos-cellulose slurry was prepared as described in Example I hereinabove. The asbestos and cellulose were drawn onto an iron mesh cathode as described in Exampe I hereinabove. The resulting diaphragm was not washed but was dried under a vacuum of 20 inches of mercury applied to the cathode side at 85.degree.C. for 72 hours.

A liquid composition containing 10 weight percent duPont "XR" (Trademark) resin described in Example I above, in ethanol, was poured onto the surface of the diaphragm and a vacuum of 17 inches of mercury was drawn within the cathode. The resulting resin-impregnated diaphragm, containing 2.0 grams of resin per square foot of diaphragm surface was heated at 110.degree.C. for 1 hour.

The porosity of the diaphragm was tested as described in Example III hereinabove and a porosity of 0.71 gallons per square foot per hour of brine was measured. Thereafter, the diaphragm IR drop at a minimum anode to diaphragm gap was measured as described in Example I and extrapolated to 190 amperes per square foot yielding a predicted cell voltage of 3.02 volts.

The cathode-diaphragm assembly was installed in a laboratory diaphragm cell. Electrolysis was commenced and chlorine was seen to be evolved. After 7 days of electrolysis the cell liquor contained 0.01 weight percent of NaClO.sub.3, the cell gas contained 0.01 volume percent hydrogen, the cell voltage was 3.40 volts, and the cell liquor and diaphragm IR drop was 1.01 volts.

EXAMPLE III

A cathode-diaphragm assembly was prepared having an ion exchange resin treated asbestos diaphragm.

A slurry of cellulose and asbestos in cell liquor was prepared and drawn onto an iron mesh cathode as described in Example I hereinabove. The diaphragm was not washed after pulling but was dried under a vacuum of 23 inches of mercury on the cathode side at a temperature of 85.degree.C. for 72 hours.

A 10 weight percent solution of duPont "XR" (Trademark) resin, described in Example I above, was poured onto the surface of the asbestos diaphragm and a vacuum of 17 inches of mercury was established in the cathode-diaphragm structure. The resulting diaphragm having approximately 3.0 grams of XR-resin per square foot was heated at a temperature of 110.degree.C. for 77 hours.

The resulting cathode-diaphragm assembly had a porosity of 0.61 gallons of brine per square foot per hour determined as described in Example III hereinabove. The minimum anode to diaphragm gap voltage drop, measured and calculated as described in Example I hereinabove, yield a predicted cell voltage of 3.0 volts at 190 amperes per square centimeter.

The cathode-diaphragm assembly was installed in a laboratory diaphragm cell. Electrolysis was commenced and chlorine was seen to be evolved. After 7 days of electrolysis the cell liquor contained 0.02 weight percent NaClO.sub.3, the cell was contained 2.4 volume percent hydrogen cell voltage was 3.24 volts, and the diaphragm-cell IR liquor voltage drop was 0.78 volts.

EXAMPLE IV

A cathode-diaphragm assembly was prepared having an ion exchange resin treated porous asbestos diaphragm.

A slurry of asbestos and cellulose in cell liquor was prepared as described in Example I hereinabove. The slurry was aged for 3 days. The asbestos and cellulose were drawn onto a mesh cathode as described in Example III hereinabove. The cathode-diaphragm assembly was not washed but was dried with a vacuum on the cathode side of 23 inches of mercury at 85.degree.C. for 72 hours.

A solution containing 10 weight percent duPont "XR" (Trademark) resin in ethanol was poured onto the surface of the asbestos diaphragm and a vacuum of 17 inches of mercury was established inside the cathode. The resulting diaphragm, which contained about 1.0 grams of the ion exchange resin per square foot of asbestos, was dried at 110.degree.C. for 72 hours.

The porosity of the diaphragm, determined as described in Example III hereinabove, was 0.74 gallons per square foot per hour. The minimum anode to diaphragm gap cell voltage was measured and calculated as described in Example I hereinabove and yielded a predicted cell voltage of 3.13 volts at 190 amperes per square foot.

The resulting cathode-diaphragm assembly was installed in a laboratory diaphragm cell. Electrolysis was commenced; chlorine was seen to be evolved. After 7 days of electrolysis, the cell liquor contained 0.01 weight percent NaClO.sub.3, the cell gas contained from about 0.3 to about 2.4 volume percent hydrogen, cell voltage was 3.39 volts, and the cell liquor and diaphragm IR drop was found to be 0.69 volt.

Although the invention has been described with reference to particular specific details and contains preferred exemplifications, it is not intended to thereby limit the scope of this invention except insofar as the details are recited in the appended claims.

Claims

We claim:

1. In a process for electrolyzing alkali metal chloride brines in an electrolytic cell having an anolyte compartment and a catholyte compartment separated therefrom by an alkali metal chloride brine permeable, fluorocarbon resin containing, chrysotile asbestos diaphragm, wherein alkali metal chloride brine is fed to said cell and catholyte liquor containing alkali metal chloride and alkali metal hydroxide is recovered from said cell, the imrovement wherein said diaphragm contains from about 1.0 to about 50.0 weight percent of a second fibrous material having a mean fiber diameter of from 2 to about 1,000 times the mean fiber diameter of chrysotile asbestos, and wherein said diaphragm further contains from about 0.01 to about 22 weight percent of fluorocarbon resin, the fluorocarbon resin being dispersed into the diaphragm to a depth of at least 0.08 inch from one surface thereof, said fluorocarbon resin providing a coating on individual asbestos fiber bundles, and said fluorocarbon resin having the empirical formula: ##SPC8##

where m is from 2 to 10, the ratio of M to N is sufficient to provide an equivalent weight of from 600 to 2,000, R is chosen from:

A,

--o--cf.sub.2 --cf.sub.2 --.sub.p A, ##SPC9## ##SPC10##

--.phi.a, and

--CF.sub.2 --.sub.p A;

where p is from 1 to 3, Y is chosen from the group consisting of --F and perfluoroalkyls having from one to 10 carbon atoms, R.sub.f is chosen from the group consisting of --F and perfluoroalkyls having from one to 10 carbon atoms, and .phi. is an aryl group;

and where A is an acid group chosen from the group consisting of:

--SO.sub.3 H,

--cf.sub.2 so.sub.3 h,

--ccl.sub.2 SO.sub.3 H,

--.phi.'so.sub.3 h,

--po.sub.3 h.sub.2,

--po.sub.2 h.sub.2,

--cooh, and

.phi.'OH where .phi.' is an aryl group.

2. The process of claim 1 wherein the chrysotile asbestos has a mean fiber diameter of from about 0.015 to about 0.030 microns and the second fibrous material has a mean fiber diameter of from about 0.060 to about 15 microns.

3. The process of claim 2 wherein the second fibrous material is chosen from the group consisting of crocidolite asbestos, anthophyllite asbestos, amosite asbestos, tremolite asbestos, actinolite asbestos, fiberglass, and cellulose.

4. The process of claim 1 comprising maintaining the concentration of alkali metal hydroxide in the catholyte liquor below about 15 weight percent.