WO1993016217A2

WO1993016217A2 - Separators for electrolytic cells and processes for making

Info

Publication number: WO1993016217A2
Application number: PCT/US1993/000878
Authority: WO
Inventors: Neal A. Grob; John P Mcgraw, Jr.; John W. Gross, Sr.
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1992-02-13
Filing date: 1993-02-02
Publication date: 1993-08-19
Anticipated expiration: 1994-08-13
Also published as: WO1993016217A3; AU3603393A

Abstract

A process for forming an electrolyte-permeable diaphragm on a foraminous structure for use in an electrolytic cell, comprising the steps of i) making a slurry from an aqueous medium and containing as undissolved, particulate materials polyfluoroethylene fibers, a polymeric solids dispersion, and talc, wherein the ratio by weight in the slurry of talc to the polymeric solids of said polymeric solids dispersion is less than 4.5, ii) depositing a diphragm on the foraminous structure by drawing the slurry therethrough, iii) drying the diaphragm, and iv) sintering at least the polymeric solids from said dispersion in the diaphragm.

Description

SEPARATORS FOR ELECTROLYTIC CELLS AND PROCESSES FOR MAKING

The present invention is directed to electrolyte-permeable separators for electrolytic cells and, in one aspect, to diaphragms for chlor-alkali cells.

Chlorine and alkali metal hydroxide ("caustic") are produced electrolytically from an aqueous solution of sodium chloride, e.g. brine, which is fed into an anolyte chamber of a chlor-alkali cell. An electrolyte-permeable diaphragm divides the cell into the anolyte chamber and a catholyte chamber. An effective diaphragm must satisfy a variety of well-known criteria, some of which are in conflict. The diaphragm must maintain a pH difference between the two cell chambers and must also permit liquid flow, but prevent gas exchange, between the two chambers. The diaphragm must have a desired porosity, yet exhibit adequate mechanical strength, resistance to chemicals, and resistance to varying cell conditions over long periods of time, including fluctuations in temperature and electrical differentials.

A variety of diaphragms are presently known or used, and these can be generally classified as the predominant asbestos or asbestos-containing diaphragms, polymer-modified asbestos diaphragms, and non-asbestos diaphragms. These known diaphragms are conventionally formed on some type of structure, such as a foraminous cathode, by vacuum drawing from a slurry of diaphragm-forming materials in a draw vat.

Commonly-owned United States Patent No. 4,606,805 to Bon (Bon) discloses an effective electrolyte-permeable diaphragm of the non-asbestos type and a process for making the diaphragm. The process comprises: (a) producing an aqueous slurry containing as particulate materials a major portion of a water-wettable, inert, inorganic, micron-size material (e.g. talc, various metal silicates, the alkali metal zirconates and titanates, and magnesium aluminates such as spinel), polyfluoroethyiene fibers, and a polyfluoroethyiene solids dispersion to obtain a slurry concentration of about 170 to about 200 grams of particulate material per liter of aqueous medium; (b) drawing the slurry through a foraminous structure to deposit the particulate materials on such structure in the form of a diaphragm; (c) drying the diaphragm, and (d) heating the diaphragm to sinterthe polyfluoroethyiene dispersion particles in the diaphragm

"Polyfluoroethyiene" as used in Bon means any polymer of a halogenated ethylene wherein the halogen atoms consist of at least one fluorine atom and the balance, if any, of chlorine "Fiber" means a product in elongated form which may or may not be branched or feathered, with a diameter of from 1 to 10 microns and varying in length from 1/32 inch (0 08 cm) to 1/2 inch (1 27 cm)

The polyfluoroethyiene dispersion in Bon comprises small particles of polyfluoroethyiene dispersed in an aqueous medium which may include various wetting or dispersing agents The preferred aqueous medium is cell effluent The slurry is drawn on commercial size cathodes, and the drawing process is begun at a very low vacuum which is gradually increased as the diaphragm begins to form After the diaphragm is dried, it is heated to sinter the polyfluoroethyiene particles or to soften them to the point that they adhere to one another and to the inert inorganic particles and the polyfluoroethyiene fibers One of the problems encountered with Bon's diaphragms is the tendency of at least some intermediate- and production-scale diaphragms to develop cracks therein This cracking phenomenon is not observed in laboratory-scale diaphragms, however

The present invention can, in one significant aspect, be viewed as an improvement on the non-asbestos diaphragms of Bon wherein visible cracks are reduced or substantially eliminated in intermediate- and production-scale diaphragms The present invention in this aspect teaches the formation of a separator, e g but not limited to a diaphragm for a chlor-alkali cell, by a process comprising making a slurry from an aqueous medium and containing as undissolved particulate materials polyfluoroethyiene fibers, a polymeric solids dispersion, and talc, wherein the talc to polymeric solids ratio by weight in the slurry is less than 4 5, depositing a diaphragm on the foraminous structure by drawing the slurry therethrough, drying the diaphragm, and sintering at least the polymeric solids from said dispersion in the dried diaphragm

"Polyfluoroethyiene" as used above and elsewhere herein possesses the same meaning as in Bon Accordingly, "polyfluoroethyiene" means any polymer of a halogenated ethylene wherein the halogen atoms consist of at least one fluorine atom and the balance ι f any, of chlorine "Fiber" means a product in elongated form which may or may not be branched or feathered, with a diameter generally ranging from 1 to 25 microns and ranging in length from 1/64 inch (0 04 cm) to 1/2 inch (1 27 cm)

In other aspects, the present invention provides for agglomerating the fine particulate materials in the slurry so that a significant degree of uniformity and homogeneity in the diaphragm can be achieved even at low slurry solids concentrations, and for pretreatmg diaphragms prepared according to the present invention to make them more effective initially.

In a preferred embodiment, the process provided by the present invention for making an electrolyte-permeable diaphragm for use in a chlor-alkali cell comprises making a slurry using an aqueous medium and certain undissolved particulate materials, depositing the diaphragm on a foraminous cathode for example by vacuum drawing, drying the diaphragm, and sintering at least the polymeric solids from the dispersion in the diaphragm so that the particulate materials from the slurry are all knit together through the polymeric solids contained therein. Preferably the aqueous medium of the slurry will be a synthetic cell effluent or caustic/salt solution in water, in which the sodium hydroxide concentration is from 80 to 190 grams per liter of solution, preferably is at least 100 grams per liter of solution and most preferably is at least 160 grams up to the 190 grams per liter figure. A typical salt concentration will be approximately 160 grams per liter of solution. A preferred slurry will include as undissolved particulate materials polyfluoroethyiene fibers, polymeric solids in the form of a polymeric solids dispersion or latex, and talc. The polyfluoroethyiene fibers will preferably comprise from 5 to 20 percent by weight of the particulate materials in the slurry, and will preferably be of a relatively small diameter, for example from 3.2 denier per filament to 6.7 denier per filament. The polymeric solids dispersion is preferably comprised of micron-sized polyfluoroethyiene solids, for example poiytetrafluoroethylene solids, polyvinylidene fluoride solids, polychlorotrifluoroethylene solids or mixtures of one or more of these.

The talc employed can be any of the asbestos-free commercially-available grades, but will preferably have an average particle size in the range of 1 1/2 to 2 microns. The drying of the wet diaphragm can be accomplished by any conventionally- employed method, for example through the use of a vacuum, or preferably through oven- drying.

Sintering of the diaphragms involves heating the diaphragms to permit at least the polymeric materials associated with the polymeric solids dispersion, and possibly also the polyfluoroethyiene fibers, to begin to flow together and to form a cohesive structure with the talc and polyfluoroethyiene fibers without at the same time being degraded. Where the polyfluoroethyiene solids in a preferred dispersion consist of poiytetrafluoroethylene solids, a temperature between 330 degrees Celsius and 350 degrees Celsius will preferably be employed, and most preferably the sintering temperature will be 335 degrees Celsius. For polychlorotrifluoroethylene solids (e.g., those exemplified below and commercially-available under the mark ACLON'"), the preferred sintering temperature will be 230 degrees Celsius, and for dispersions including polyvinylidene fluoride solids (e.g., the KYNAR^™ PVDF/PTFE blends exemplified below) the preferred sintering temperature will be 180 degrees Celsius For substantially preventing the cracking seen in the intermediate- and production-scale diaphragms of Bon, it is presently thought that the ratio by weight of talc to the polymeric solids in the polymeric solids dispersion should preferably be no higher than 4 5 Preferably, the ratio will be between 2 5 and 4 0, more preferably will be from 2 5 to 3 8, and most preferably will be 3 8 At these ratios, and within the preferred slurry formulations described above, it is considered that an appropriate balance of chemical resistance, hydrophil city, strength and other important properties may generally be obtained without the undue cracking seen in Bon's intermediate- and production-scale diaphragms

Preferred diaphragms will be 120 to 160 mils (3 mm to 4 mm) thick, and will be conditioned before being placed in operation in a chlor-alkali cell according to one of the conditioning methods described in detail and exemplified below

A particularly preferred diaphragm-making process of the present invention comprises i) making a slurry from an aqueous medium and polyfluoroethyiene fibers, a polymeric solids dispersion and talc at temperatures sufficient to effect an agglomeration of these particulate materials, then n) depositing a diaphragm on a foraminous cathode by drawing the slurry through the cathode, preferably while still at these elevated temperatures in) drying the thus-deposited diaphragm, and iv) sintering at least the polymeric material in the diaphragm

As in the former, preferred process, the apparatus for performing this particularly preferred process and the manner and means by which these steps are performed may be those conventionally employed in the art for making slurries, drawing, drying and sintering diaphragms of this type

The particularly preferred process described above enables uniform and homogeneous diaphragms to be drawn even at low slurry solids concentrations, e g , from 40 to 120 grams of the undissolved particulate solids per kilogram of slurry, although preferably the slurry solids will be from 60 to 100 grams per kilogram of the slurry, and most preferably will be from 70 to 85 grams per kilogram of the slurry to prevent settling out of appreciable amounts of solids in the slurry

Agglomeration of the slurry solids in the particularly preferred embodiment (i e , the polyfluoroethyiene fibers, polymeric solids in dispersion, and talc) can be accomplished typically at temperatures in the range of 85 degrees Fahrenheit (29 4 deg C) to 120 degrees Fahrenheit (48 9 deg C), although the degree and strength of agglomeration may vary to an extent based on the formulation (e g , on the amount of the polymeric solids added via the dispersion), on how much caustic is present (higher concentrations generally are associated with less agglomeration), and the age of the slurry (older slurries tend toward greater agglomeration), for example

In contrast to the process of the Bon patent wherein initially the vacuum is very low and is gradually increased, the vacuum drawing of the diaphragms in this particularly preferred process can be done at initially higher vacuums and without the need for gradual increases in vacuum. Preferably, the drawing of the diaphragms in this process involves controlling the flow of residual slurry materials through a vacuum flow line at no more than 0.25 gallons per minute per square foot (0.088 liters per minute per square meter) of cathode area. Higher flow rates may be possible in some circumstances, but preferably the flow rate is not so high as to cause undesirable pinholing in the diaphragm.

In practical terms, this controlling of the flow rate of the slurry through the cathode is accomplished by opening a valve in the vacuum flow line. After the initial 0.25 gpm/ft² flow is established and as the diaphragm materials are continually deposited on the cathode, it will be necessary to progressively open the valve to maintain the target 0.25 gpm/ft² flow rate. At a particular point in the drawing process, the valve is fully open, and the flow rate across the cathode and through the flow line is limited not by the valve but by the diaphragm materials already deposited on the cathode.

One slurry formulation demonstrating a tendency to agglomerate involves natural Teflon*" fibers having rayon therein, a dispersion of poiytetrafluoroethylene solids in water with a surfactant, and talc in an aqueous medium containing the 80-190 gram per liter concentrations of sodium hydroxide earlier specified for the synthetic cell effluent. A presently-preferred dispersion is commercially available as Teflon^*" 30 PTFE dispersion, and consists of poiytetrafluoroethylene solids in water with a nonionic, octylphenoxypolyethanoxyethanol surfactant (Triton X-100™ surfactant, Rohm & Haas).

The precise nature and causes for the agglomeration observed in this particular, preferred system are not known. It is suspected, however, that the shared hydroxyl functionality in rayon, the surfactant and talc may in a caustic/salt solution have some role ι n the agglomeration. In this regard it is noted that the same materials also agglomerate in heated water, but the strength cohesion of the agglomeration appears to be at least in part a function of the amount of Teflon'" 30 dispersion incorporated in the slurry. Low concentrations of the dispersion generally involve heightened degrees of agglomeration, while higher concentrations may generally involve lesser degrees of agglomeration. Notwithstanding the lack of a definite and complete understanding of the reasons for agglomeration, however, it is nevertheless believed that those skilled in the art (when equipped with the instant disclosure) will be able to achieve an agglomeration of other combinations of polyfluoroethyiene fibers and polymeric solids dispersions with talc or with some other water-wettable, inorganic particulate material without undue experimentation Diaphragms made in accordance with the teachings of the present invention are preferably conditioned to remove loosely adhering particles, salts and alkaline solids therefrom (e.g., sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium chloride and magnesium hydroxide) before the chlor-alkali cell in which they are placed is energized Unless some sort of conditioning is undertaken of the diaphragms, high caustic concentrations (grams per liter or gpl NaOH) are produced in the on-line production or cell effluent soon after startup that can spike up to greater than 200 gpl (grams per liter) NaOH and that can typically stay above 150 gpl NaOH for several months During this time, the caustic current efficiency is low One preferred process for conditioning a duphragm of the present invention involves flowing an aqueous buffer solution (prepared by bubbling carbon dioxide through brine or water) through the diaphragm at a controlled flowrate, until the pH of t he et fluent from the buffer's flowing through the diaphragm is from 7 to 9 The diaphragm is then allowed to soak for a period of time, for example 12-16 hours, in the solution during whu-h any alkaline solids are dissolved or leached from the diaphragm (although in another embodiment the soaking step can precede the flowing step) The controlled flow can be restarted until the effluent pH again reaches from 7 to 9 (with a pH of 8 5 being most preferred) and to complete the conditioning step, whereby on start-up the on-line production effluent contains from 70 to 140 grams per liter of sodium hydroxide or more preferably contains from 100 to 120 grams per liter of sodium hydroxide In various other embodiments one or more additional soaking steps accomplish the desired conditioning with one or more flowing steps, with the soaking and flowing steps alternating in each instance with one another

Other suitable processes for conditioning the diaphragms might include water washes, flushing and/or soaking with solutions of surfactants in water, and combinations of one or more of these flushing and soaking solutions, for example a buffer solution having a suitable surfactant incorporated therein

The present invention is more particularly illustrated in the examples which follow

EXAMPLE 1 About 49 7 grams of particulate talc (hydrated magnesium silicate, commercially available U S grade Mistron Vapor talc from Cyprus Industrial Minerals) and about 5 6 grams of 1/4" (0 64 cm) Teflon'" flock fibers (E I DuPont de Nemours & Co , Inc ), 3 2 DPF were added to 438 millihters of a 160 gpl (grams per liter) sodium hydroxide and 160 gpl NaCI aqueous solution in a bottle and mixed to make a slurry About 24 5 grams of Teflon '" 30 dispersion (also from E I DuPont de Nemours & Co , Inc ) were added and the slurry mixed again Teflon '" 30 dispersion is a commercially available aqueous dispersion containing surfactant and abou t 60% by weight of poiytetrafluoroethylene particles ranging in size from 0 05 microns to 0 5 microns, so that the slurry in this Example possessed a talc to Teflon'" 30 solids ratio of 3 4

A test cathode (5" ( 12 7 cm) by 6" (15 2 cm) perforated metal about 1/10" (0 25 cm) thick) weighing 205 7 grams was positioned in a draw vat The slurry was poured over the cathode After an initial period of gravity draining, a variable vacuum starting at zero inches (0 bars) of mercury was applied beneath the cathode Flow was limited during the beginning of the draw with a valve on a flow line to the vacuum flask The vacuum under the cathode eventually reached about 24 inches (0.81 bars) after the liquid was drained from the draw vat. At the end of the draw, a diaphragm was deposited on the cathode having about 40 grams of water remaining therein. The cathode and the new diaphragm (wet) weighed about 319.6 grams. The diaphragm was then dried at about 120°C overnight and bonded. The diaphragm was bonded so that it would hold together better during normal electrolytic cell operation by placing it in an oven initially at a temperature of 120°C, ramping the temperature up at a rate of 2°C per minute to 335°C, and then holding at this temperature for 15 to 20 minutes. Then the diaphragm was slowly cooled without heat to ambient temperature. This diaphragm was installed in a lab chlor-alkali cell and operated at 12 amps,

75°C, with a dimensionally stable anode and acidified brine feed. Performance is summarized in the table below.

Day 14 Day 34 Avg. over 60 days

Voltage is in volts. "CCE" is caustic current efficiency - the ratio of the actual amount of caustic produced to the expected theoretical amount under the same conditions. A duplicate diaphragm was made for destructive testing using the same amount of diaphragm materials and same draw process. Wet Mullenburst strength of the diaphragm was 17 psi (1.2 kg/cm²) and the final formulation in the diaphragm (neglecting the residual caustic and salt from drying and bonding the wet diaphragm) was 73% talc solids, 17% Teflon™ 30 solids and 10% Teflon'" fiber solids by weight. A diaphragm prepared according to a preferred formulation of United States Patent No. 4,606,805 to Bon, with about 80% talc solids and about 20% total Teflon'" material had a burst strength of about 8 psi (0.56 kg/cm²).

EXAMPLE 2 About 40.2 grams of particulate talc (hydrated magnesium silicate, commercially available U.S. grade Mistron Vapor talc from Cyprus Industrial Minerals) and about 7.2 grams of 1/4 inch (0.64 cm) Teflon'" flock fibers (6.7 DPF in diameter, natural brown fibers having a rayon content of from about 4 to about 6%) were added to about 550 milliliters of water and mixed together in a bottle by shaking. About 21 grams of Teflon'" 30 dispersion were added to the bottle and mixed with its contents by shaking, producing a 98 grams solids/kilogram slurry mixture. Some of the Teflon'" fibers floated to the top of the bottle.

About 76 grams of 100% standard sodium hydroxide beads were added to the mixture in the bottle and the entire contents shaken to mix them together, producing heat. The mixture's temperature rose to near about 120°F (48.9 degrees Celsius)(from an initial temperature of about 70°F (21 degrees Celsius)) The liquid was clarified, indicating that substantially all of the talc combined with the other solids Several large masses with fibers, were formed The masses settled in the bottle and large clumps of solid materials were present The resulting mixture was placed in a Waring blender for about 10 seconds at a "low" setting to break up the masses and clumps of fibers

A test cathode (as described below in Example 3) weighing about 210 1 grams was positioned in a draw vat The slurry was poured over the cathode, and a variable vacuum starting at zero inches of mercury (0 bars) and eventually reaching about 25 inches of mercury (0 85 bars) was applied beneath the cathode from the start of a draw process like that described in Example 1 At the end of the draw process, a diaphragm was deposited on the cathode having about 50 grams of water remaining therein The cathode and the new diaphragm (wet) weighed about 303 98 grams The diaphragm was then dried (overnight in oven at 120°C) and bonded with a bonding cycle as described in Example 1, but with a final temperature of about 355°C The diaphragm materials exhibited an excessively high degree of agglomeration in the slurry and in the draw process, so that the pore structure of the resulting diaphragms was large The diaphragms had current efficiencies of less than 90% when the cell liquor was at a caustic concentration of 100 grams per liter caustic

EXAMPLE 3 A diaphragm was produced as described in Example 2 above [but a fiber mixture of 1/4" (0 64 cm) long fibers (5 2 grams) and 1/64" (0 04 cm) long fibers (2 grams) was employed and the blender was not used] on a cathode that weighed 208 9 grams initially With a diaphragm deposited thereon (wet), the cathode and diaphragm together weighed 306 9 grams The test cathode was made from 13 gaugethick carbon steel and had 1/10" (0 25 cm) perforations on 5/32" (0 4 cm) staggered centers with a total open area of about 30% The final level of the vacuum used in deposition and drying was at about 21 to 22 inches of mercury (0 71 to 0 74 bars) Flow was controlled as described above in Example 1 After oven drying the diaphragm weighed 268 6 grams and, after bonding, 266 6 grams The diaphragm was about 90 mils (2 3 mm) thick The bonding cycle was like that of Example 1 , but with a final temperature of about 355°C

The diaphragm was soaked in a bucket of water for about 3 hours to leach out caustic solids and salts (salts from the slurry that remain in the diaphragm after drawing) and to thereby test the diaphragm's strength due to deposited solids other than the caustic solids The soaked diaphragm was accordingly removed from the cathode and severed and its Mullenburst strength was 18 psi (1 27 kg/cm²)

EXAMPLE 4 An aqueous slurry for forming a diaphragm was produced by mixing about 5 grams of 1/4" (0 64 cm) long Teflon'" flock fiber, 6 7 DPF in diameter, with about 40 grams of talc. To this about 550 milli ters of warm (near 35°C) tap water were added and the resulting mixture shaken. About 10 grams of Teflon'" 30 dispersion were added and again the resulting mixture was shaken. About 30 grams of KYNAR'" latex 32 (Pennwalt Chemical Corp , about 20% polyvinylidene fluoride solids of about 0 3 microns in size) were finally added with shaking The slurry thus formed was drawn through a cathode weighing 210 grams dry, with a final vacuum at 20 inches of mercury (0.68 bars) The resulting cathode with a wet diaphragm deposited thereon weighed 308 8 grams The cathode was placed in an oven at about 1 10°C for about two hours and fifteen minutes, and then the temperature was ramped for about ten minutes to about 180°C The dried cathode with the diaphragm was then subjected to the

10 180°C sintering temperature for about thirty minutes The dried cathode and diaphragm weighed about 261 8 grams, and the diaphragm had an average burst strength of about 9 7 psi (0.68 kg/cm²)

EXAMPLE 5 A slurry for forming a diaphragm was produced by adding about 6 grams of 1/4"

15 (0.64 cm) natural brown Teflon'" flock fibers, 3.2 DPF in diameter, to about 40 grams of talc in about 550 millihters of water at about 45°C About 10 grams of Teflon'" 30 material were added while lightly shaking the mixture, with the temperature of the slurry cooling to 40°C About 25 grams of ACLON'" polychlorotrifluoroethylene solids (about 0.15 microns in size) in dispersion were added, and the resulting mixture was shaken at 40°C. Vigorous shaking

20 produced foam on top of the slurry About 2 grams of commercially available Dow Corning Antifoam 1430 antifoam agent was added, and the slurry was again shaken vigorously and then mixed in a Lightnin"" mixer for about 1 minute on "medium" speed. The diaphragm was drawn as in Example 1, but with a final draw vacuum at about 15 inches of mercury (0 51 bars) A diaphragm of this slurry was successfully drawn on a cathode, but was not bonded

25 EXAMPLE 6

A diaphragm was made using a slurry made up in a mix tank Caustic-salt solution (160 gpl NaOH and 150 gpl NaCI) was pumped into the tank and solids were added to a concentration of about 70 grams of solids per kilogram of slurry. The solids consisted of (by weight): talc (about 74%), natural brown Teflon'" flock fibers, 1/4" long, about 3 2 DPF (about

30 7%), and Teflon'" 30 solids (about 19%, for talc to Teflon'" 30 solids ratio of 3 9)

The slurry was initially mixed with a Lightnin' '" mixer to facilitate the addition of the solids and was then recirculated through a draw vat and the mix tank using a centrifugal pump

A production cell cathode was hooked up to a draw pan and placed in the draw

35 vat. The vat was filled through its corners to cover the cathode with the slurry at about 95°F (35 deg. C). During drawing, automatic raker tubes traveled between pockets on the cathode to enhance flow and circulation between pockets with re-circulated slurry flowing therefrom, to prevent unwanted buildup on the cathode screen, and to increase deposition uniformity Once the cathode was completely submerged, a valve on a flow line to a vacuum tank was opened, and a flow of about 0.25 gallons per minute per square foot (0.088 liters per minute per square meter) of cathode was commenced therethrough. Within about 1 to 2 minutes a vacuum reading on the cathode began to increase from 0 to about 6 to 8 inches of mercury (0 to about 0.2 to about 0.28 bars) and the flow through the diaphragm decreased to near about 0.05 gallons of slurry per minute per square foot (0.018 liters per minute per square meter), indicating that the cathode was coated with a cake of solids from the slurry. This flow condition was maintained for about 3 to 5 minutes. The vat was then emptied after the vacuum on the vacuum tank had increased from about 18 inches of mercury (0 61 bars) to about 21 inches of mercury (0.71 bars). In other embodiments during the emptying of the vat, the vacuum on the cathode was controlled at about 3 inches of mercury (0.1 bars) to prevent pinholing of the wet diaphragms. After liquid cleared from the flow line, the vacuum on the cathode rose to about 21 inches of mercury (0.71 bars), a condition that was held for about 10 minutes to help dry the diaphragm. The diaphragm thus produced was placed in a drying oven at an inlet air temperature of about 330 to 350°F ( 166 to 177 degrees Celsius) for about 8 to 12 hours. The oven door was kept slightly open to keep the temperature lower and for uniform drying The diaphragm was turned at about 2 hour intervals. When the diaphragm was about 80% or more dry (determined by weighing the diaphragm at intervals), it was placed in a bonding oven to reach a temperature of about 335°C between pockets in the diaphragm, and to thus sinter the Teflon™ material. The diaphragm was slowly ramped up to the sintering temperature and sintered over the space of about 2 hours and 10 minutes, with the diaphragm being above about 330°C for about 10 to 20 minutes of this time. To prevent degradation of the diaphragm and to avoid a caustic-Teflon'" material exotherm, the air temperature was kept at about 355°C as a maximum. The dried diaphragm was cooled slowly at a rate of about 2 degrees Celsius per minute, and did not show the sort of cracking seen in the earlier Bon diaphragms.

Significant cracking was found to occur or is thought to be likely where diaphragms are cooled down at rates significantly above 2 degrees Celsius per minute (e.g., above 3 degrees Celsius per minute), or where a significant air-diaphragm temperature gradient exists over portions of the diaphragm, or where the talc to polymeric solids ratio approaches or exceeds the preferred values specified for this ratio herein. The kind of cracking seen in the Bon diaphragms is thus considered as related both to slurry formulation (and more particularly to the ratio of talc to the polymeric solids in the dispersion) and to the manner and environment in which the dried diaphragms are cooled after sintering. EXAMPLE 7

Mistron Vapor RP6 is a grade of talc available in Europe which comes from the same ore as does U.S. grade Mistron Vapor talc, but which is ground on different equipment and which has slightly different properties for this reason. The surface area of RP6 is slightly lower and the particle size is 2.1 microns versus 1.7 microns for the U.S. grade Mistron Vapor talc. Another difference is loose density which is 8 Ib/ft3 (0.1 kg/nrι3) for the U.S. grade talc vs 15 Ib/ft3 (0.19 kg/m3) for the RPδ talc.

A diaphragm was made using a slurry prepared in a mix tank. Caustic-salt solution (160 gpl NaOH and 160 gpl NaCI) was added into the tank and solids were added to a concentration of 80 grams solids per kilogram of slurry. The solids consisted of (by weight): Mistron Vapor RP6 talc (about 70%), natural brown Teflon™ flock fibers, 1/4" long, 3.2 DPF (about 8%), and Teflon™ 30 solids (about 21 %). The Teflon™ 30 material was again added as a dispersion of about 60% solids, and the talc to Teflon™ 30 solids ratio was about 3.3. The slurry was initially mixed and heated to a temperature of about 104°F (40 degrees Celsius), with addition of the Teflon™ 30 dispersion just before drawing.

A full-size production cell foraminous cathode was hooked up to a draw pan and placed in the draw vat. The slurry was transferred to a holding tank by vacuum and then gravity fed into the draw vat. During drawing, automatic raker tubes traveled between the pockets on the cathode to enhance draw vat mixing and slurry suspension. Unlike Example 6, however, re-circulated slurry did not flow from the raker tubes. The vat was filled to cover the cathode with the slurry at about 104°F (40 deg. C). Once the cathode was submerged, a valve on a line to a vacuum tank was opened and a flow therethrough of about 0.25 gpm/ft2 (0.088 liters per minute per square meter) commenced. Within 1 to 3 minutes a vacuum reading on the cathode began to increase from 0 to 10 inches of mercury (0 bars to 0.34 bars) and the flow drawn across the diaphragm and through the flowline decreased to about 0.05 gallons per minute per square foot (0.0176 liters per minute per square meter) of cathode, indicating that the cathode was coated with a cake of solids. This flow condition was maintained for about 3 to 5 minutes. The vat was then emptied and the vacuum under the cathode controlled at about 6 to 8 inches of mercury (0.20 bars to 0.27 bars) for 10 minutes while in the draw vat to remove excess water from the diaphragm. The diaphragm was removed from the draw vat and placed onto a vacuum pan which held a vacuum of about 5 inches mercury (0.17 bars) for 10 minutes, and then ramped slowly to a full vacuum of 18 to 22 inches of mercury (0.61 bars to 0.74 bars) and held up to 60 minutes. The diaphragm was dried and bonded generally as in Example 6, with no significant cracking.

EXAMPLE 8 Four samples of a diaphragm drawn using the process of Example 6 were evaluated in a laboratory cell. After the diaphragm was bonded, sections of the diaphragm were cut using a band saw (the diaphragm remained on the cathode). The samples were sealed and then mounted in a laboratory cell. Before energizing the cells, a buffer solution was used to flow through the diaphragm samples. The buffer solution was made by bubbling carbon dioxide through water, keeping the solution pH in the range of 5 to 6. This solution was fed to one of the cells and flowed through the diaphragm for 20 hours at a flowrate equivalent to about 12 ml/hr/in (1.86 ml/hr/cm²). The flowing was stopped at intervals and the diaphragm soaked in the solution overnight, with the flow being re-started the next morning. Flow of the buffer solution was maintained until a final end point pH of 7.2 was reached when measuring the test effluent from the catholyte chamber of the cell. A similar flush/soak technique was used on the other three samples, and the flows therethrough were stopped when the test effluents from the cell reached respective pH's of about 7.6, 9.6, and 9.8. The cells were then drained and filled with alkaline brine.

Electrolysis was started by energizing the various cells and operating at about 14 inches (35.6 cm) of brine head (level difference between anolyte and catholyte). The concentration of sodium hydroxide (grams per liter NaOH) in the cell on-line production effluent and CCE (caustic current efficiency) measured 24 hours after reaching operating amperage indicated significant differences as seen in the table below:

The sodium hydroxide concentration in the cell on-line production effluent and CCE (caustic current efficiency) measured after reaching operating amperage ("days elapsed") are indicated below:

EXAMPLE 9 Samples of a diaphragm drawn using the process of Example 7 were evaluated in a laboratory cell. After the diaphragm was bonded, sample sections were cut (the diaphragm remained on the cathode) and sealed and mounted in a laboratory cell. Several different solutions were used to pre-condition the diaphragms before energizing. Two samples were soaked in alkaline brine. Two samples were soaked in water for two days and flushed with water for four hours. Two samples were soaked in surfactant solution (0.5% by weight of Triton DF-12'" available from Rohm and Haas) for two days, and flushed with the same solution for four hours. Two samples were flushed with buffer solution (made from bubbling carbon dioxide through water), soaked, and then flushed again. The six samples that were thus pre¬ conditioned were drained and filled with alkaline brine after the pre-conditioning was complete. Samples of the feed solution and on-line production effluent from the catholyte were taken from each of the cells at timed intervals to be analyzed. Electrolysis was started on the cells by operating at 10.8 amps, 75°C, and 10 inch (25.4 cm) brine head. Average flow and concentration data taken over the first five days of operation on alkaline brine are shown in the table below:

Method Cell Flowrate

(ml/hr)

Alkaline Brine A 124 Soak B 36 Water Soak/Flush C 117 D 118

While certain preferred embodiments have been described and/or exemplified herein, those skilled in the art will readily appreciate that numerous changes can be made in these embodiments without departing from the spirit and the scope of this invention, as more particularly defined by the claims below.

Claims

CLAIMS:

1. A process for forming an electrolyte-permeable diaphragm on a foraminous structure for use in an electrolytic cell, comprising the steps of: making a slurry from an aqueous medium and containing as undissol ed, particulate materials polyfluoroethyiene fibers, a polymeric solids dispersion, and talc, wherein the ratio by weight in the slurry of talc to the polymeric solids of said polymeric solids dispersion is less than 4.5; depositing a diaphragm on the foraminous structure by drawing the slurry therethrough; drying the diaphragm; and sintering at least the polymeric solids from said dispersion in the diaphragm.

2. The process of claim 1 , wherein the ratio by weight of talc to the polymeric solids in the polymeric solids dispersion is in the range of from 2.5 to 4.0.

3. The process of claim 2, wherein the ratio by weight of talc to the polymeric solids in the polymeric solids dispersion is in the range of from 2.5 to 3.8.

4. The process of any of claims 1-3, wherein the polymeric solids in said polymeric solids dispersion consist of polyfluoroethyiene solids and the aqueous medium of the slurry is an aqueous solution of sodium hydroxide and sodium chloride.

5. The process of any of claims 1-3, wherein the polymeric solids in said polymeric solids dispersion are a mixture of polyvinylidene fluoride and poiytetrafluoroethylene solids.

6. The process of any of claims 1-5, wherein the polyfluoroethyiene fibers have a diameter ranging from 3.2 denier per filament to 6.7 denier per filament.

7. The process of any of claims 1-6, wherein the talc has a particle size ranging from 1 1/2 to 2 microns.

8. The process of claim 1, wherein the polyfluoroethyiene fibers and the polymeric solids in said dispersion are both sintered to knit the diaphragm together

9. A process for forming an electrolyte-permeable diaphragm on a foraminous structure for use in an electrolytic cell, comprising the steps of: making a slurry from an aqueous medium and polyfluoroethyiene fibers, a polymeric solids dispersion, and talc, with such slurry being prepared at temperatures sufficient to effect an agglomeration of said materials; depositing a diaphragm on the foraminous structure by drawing the slurry therethrough while still at such elevated temperatures; drying the diaphragm; and sintering at least the polymeric solids from said dispersion in the diaphragm.

10. The process of claim 9, wherein the polymeric solids dispersion comprises micron-sized polyfluoroethyiene solids, a surfactant, and the remainder of water.

11. The process of claim 9 or of claim 10, further comprising the step of controlling the flow of slurry through a flowline from the foraminous structure at no more than 0.088 liters per minute per square meter of surface area of such structure.

12. The process of any of claims 9-11 , wherein the slurry contains from 40 to 120 grams of undissolved particulate materials per kilogram of slurry.

13. A process for conditioning a diaphragm for use in an electrolytic cell, comprising the steps of: flowing an aqueous buffer solution through the diaphragm until effluent from the buffer's flowing through the diaphragm has a pH of from 7 to 9; and soaking the diaphragm in the buffer solution.

14. The process of claim 13, further comprising restarting the flow after soaking the diaphragm in the buffer solution, until the pH of the effluent from the buffer solution's flowing through the diaphragm is in the range of from 7 to 9.

15. The process of claim 13 or of claim 14, wherein the aqueous buffer solution contains a surfactant.