CA1072056A - Diaphragms for electrolytic cells - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An improved diaphragm coated cathode comprises a foraminous cathode bearing on the cathodically active surfaces thereof, a uniform, adherent, and coherent dimensionally stable diaphragm consisting essentially of asbestos fibers having a discontinuous polymer coating on the fiber surface, the polymer binding the fibers together, the improved cathode permits a reduction in the space between the anode and the cathode which reduces the electrical potential required for the electrolysis.

Description

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This i~vention relates to a diaphragm coated cathode, an electrolytic cell containing such a cathode, and a method for the electrolysis of brine employing such a cathode.
This is a division of Canadian Patent Application S. N. 225,379, filed April 24, 1975, John T. Rucker.
The earliest commercial electrolytic cells for the :: .
production of chlorine utilized a diaphragm. An example is the Griesheim cell, about 1866, that contained a diaphragm made by ;

mixing portland cement with brine acidified with muriatic acid.

Once the diaphragm set, it was leached with water to remove `
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soluble salts, leaving a thick porous diaphragm.
~ argraves, U. S. 596,157 teaches a method of prepar-in~ a cell diaphragm by filtering a mixture of asbestos and milk of lime on a deckle frame to produce a thick board which was then coated with sodium silicate. Asbestos paper and coated asbestos paper were the major forms of diaphragms until Stuart about 192~ developed the vacuum deposited diaphragm, : .
U, S. patents 1,855,497; 1,862,244 and 1,865,152 The next pertinent development in deposited diaphragm ~`
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technology was impregnating the asbest~s with resinous material. ~-.
This development gave the diaphragm stability and improved the ` separatory abilities. Examples of such diaphragms are~ Carlin, U, S. 3,057,794; LeDuc, U. S. 3,694,281 and 3,723j264, Pall, 3,238,056 and 3,245,767; Hacker, 3,583,891 and Korach & Foster, 3,853,720 and Darlington & Foster, 3,853,721.
The present invention is an improvement in the method of producing a resin impregnated asbestos diaphragm useful in `

electrolytic cells.
There is disclosed a method of forming an impregnated ~-asbestos diaphragm directly on a foraminous cathode member of an electrolytic cell, which method comprises the following steps:
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~1) forming a slurry,suitably an aqueous or cell liquor slurry, of fibrous asbestos,

(2) placing a ~oraminous cathode member to be coated in the slurry and depositing a relatively uniform layer of asbestos on the cathode member by means of a vacuum,

(3) removing the asbestos coated cathode from the slurry, ~ -drying under vacuum, and placing it in a second slurry, suit-ably an aqueous or cell liquor slurry, of thermoplastic polymer --particles and impregnating the asbestos coating on the cathode member with thermoplastic polymer particles by means of a vacuum, ~4) removing the treated cathode from the second slurry and subjecting the cathode to a temperature sufficient to cause the thermoplastic polymer particles to fuse and thus join adjacent fibers of asbestos together, (5) cooling the treated cathode member, suitably to ambient temperature to obtain a cathode coated with asbestos which in turn is impregnated with a polymer material in a ~ `
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discontinuous phase.
According to the invention there is provided a diaphragm coated cathode comprising a foraminous cathode bearing on the cathodically active surfaces thereof, a uni-form, adherent, and coherent dimensionally stable diaphragm consisting essentially of asbestos fibers having a discontinuous thermoplastic polymer coating on the fiber surface, said polymer .. ~ .
binding said fibers together.
According to another aspect of the invention there is provided a method of electrolyzing brine in an electrolytic cell which comprises electrolyzing the brine in a cell having a diaphragm coated cathode of the invention spaced apart from an anode.

According to a further aspect of the invention there is provided an electrolytic cell containing a diaphragm coated cathode of the invention.
The present invention allows the use of less asbestos ~ `
than previous other methods of producing resin~impregnated diaphragms. ~ normal loading on the cathode is 0.3 pound per square foot. Utilizing the method of the present invention it has been found that the loading can be lessened to as much as 60%, i.e., 0.18 pounds per square foot. However, to insure a L0 uniform layer of asbestos without thin spots, it has been found that dependable results are consistantly obtained when the amount of asbestos is reduced to the 75 to 85% range, i,e. -~
0.22 to 0.26 pounds per foot of cathode area.
The use of less asbestos allows a reduction in the spacing of the anode and cathode within the cell. This spacing is termed the brine gap. Lessening the brine gap reduces the amount of electrical potential requixed to cause decomposition of the salt, alkali metal halide, in the cell. Normal brine gap in the usual asbestos diaphragm cell is about 3.5 mm. With ' `,, 6 :~
the diaphragm of the present invention a reduction as low as 3.2 mm.
is possible. However, at such reduction, difficulties may be encountered in cell assembly and operation and a reduction to about 5.6 mm. has been found to be eminently useful.
The present method utilizes an intermediate drying step wherein a vacuum is maintained on the asbestos coated cathode. This step appears to set the asbestos and provide the porosity required -for the resin addition step. The resin addition enters the preformed pores formed by the drying step and also in the interstices of the -~
layer of asbestos~ The resin entering the interstices will enter as far as possible. The diaphragm of the present invention has resin impregnated throughout, however, it has been found that there ;~
is less resin at the interface of the metal cathode and the ad~jacent asbestos. This distribution gradient occurs because the asbestos 5 alone in initially added to the cathode and in the resin addition step resin that is pulled into depth is significantly less than on the outer layers. The advantage being that with the coated cathodes of the present invention the coating may be removed with normal . .
washing procedures and no special equipment or heating is required.
DETAILED DESCRIPTION OF THE INVENTION
The present diaphragms are particularly adapted to use on cathode members utilized in conventional, commêrcial electrolytic diaphragm cells. Such cathodes are constructed of foraminous sheet ~etal, expanded metal screens or woYen metal screens. The cathode member 25 of an electrolytic cell generally extends substantially the width of the cell with spacing between each cathode member adapted to receive an anode member. The diaphragm member separates the active surface of the cathode member and the anode member. Frequently the ~ ~`
cathode member is in the form of a can or box and encloses a 30 cathode chamber within the cell. ~;

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Various nlethods of forming impregnated asbestos diaphragms -~
have been proposed. Those which form the diaphragm separately and subsequently place it on the cathode member are not satisfactory because of the problems in fitting and attaching the diaphragm to the cathode member or cell wall and by failure w;thin the cell if the diaphragm becomes loosened from the cathode during cell operation.
Diaphragms formed with mixtures of asbestos and polymeric materials .,~
in single-step applications suffer from the fact that a uniform slurry of varying size particles of different material is difficult to obtain and maintain as the cathode member is coated and subsequent fusing of the `~
polymeric material glves a diaphragm that is spotty, that is it has ~ -impervious areas and substantially untreated areas. In use, such diaphragms are "tight" in spo~s and tend to swell and flake in untreated areas. The result is tha-t such diaphragms are not wholly dependable over lon9 periods Or use and require repiacement nlore often than the diaphragms of the present invention. In accord with . .
the present i~nvention these deficiencies are overcome by utilizing two separate treatment steps. In the first step the cathode member is `-, substantially uniformly coated with asbestos fibers and in the second ..
step the asbestos coating is substantially unifornnly impregnated with particulate thermoplastic polymer particles which are subsequently fused to give a coated cathode member having a substantially uniform, but discontinuous polymer phase dispersed throughout the asbestos phase.
The first step in the process of the present invention 1S the preparatîon of a slurry of asbestos fibers and the applicat-ion thereof to a cathode member. Suitably the asbestos is deposited in the conventional manner, that is, the asbestos fibers are placed in a tank containing water or solutions of brine or cell liquor. The mixture ~`
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is agitated suitably by means of a pump to ~orm a uniform slurry.
The cathode member is then immersed in the slurry and a vacuum applied in the inside of the cathode box or chamber. The vacuum initially is pulled in the range of from about 1-10 inches of ~ercury with a gradual increase to about 25 inches. Agitation may be discontinued as the vacuum becornes ~reater. This method is substantially that described in the Sturart patents previously cited above.
The asbestos diaphragm coating is allowed to dry under vacuum for a period of from about 15 minutes to one hour to substantially remove the liquid phase of the slurry. The diaphragm still appears slightly damp to the touch. A period of about thirty minutes has been found to be aptly ~_~ suited to this drying step. An asbestos layer of from 25 to 125 mils is obtained.
While still under vacuum the asbestos coated cathode member is immersed in a tank containing a dilute slurry of thermoplastic resin powder. ~gitation of the resin powder slurry is preferred to yield a unifornl penatration of the asbestos layer. The slurry is then pulled down to a pre-calculated level drop to deposit the desired amount of resin ln the asbestos. Generally the time ranges from 5 minutes to _ O one hour for this operation, depending upon the slurry concentration and the permeability of the asbestos layer, but a period of about fifteen to thirty minutes is usually sufficient to disperse the resin in the asbestos layer.
The thus-treated cathode member is then reMoved from the tank and subjected to a drying step similar to that described above after the ;~
asbestos layer was deposited. The thus-treated coated cathode member is then subjected to a heat treatment to complete the drying and to fuse the thermoplastic polymer particles, in situ, that is dispersed throughout the asbestos layer in a substantially discontinuous phase.
The temperature of fusion is dependent upon the thermoplastic material employed. The temperature ;s sufficient to cause the thermoplastic particle to flow but is not above the decomposition point of the thermoplastic material.

107.2056 The therlnoplastlc po1ymer;c materials that are particularly useful in the present invention are those ~hich will withstand the physical and chemical environment of an electrolytic cell. Preferably the polymer;c material is particulate with a size range of from 0.2 to 100 microns, suitably an average size of about 70 microns is well adapted for use in the present invention. The materials should have a softening point of greater than about 105 C, typical cell operating temperature and should have a softening point less than about ~00 C, .
as at such temperatures warpage of the foraminous cathode member may occur.
When applied in accord with the present invention a uniform, adherent and coherent dimensionally stable diaphragm is obtained which consists J essentially of asbestos fibers having a dispersed polymeric material phase therein binding the fibers together.
Although any th~rmoplastic resin that will withstand the cell conditions are suited to use in the present invention, fluorine-containing polymers and copolynlers appear best sui~ed. ExaMples Or sui~able polylneri~
materials are polytétrafluoroethylene, polyhexafluoropropylene, poly-- . .~ .
chlorotrifluoroethylene, polyvinylidene fluoride, such polylllers alone or as copolymers w;th each other, or as copolyrners with ethylene vinyl chloride or other hydrocarbon monomers. Particularly suited are copolym rs 7~ 7~ o~ æ
of ethylene and chlorotrifluoroethylene in a l~ e~ or polyvinyliden ~
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The heat tréating step is suitably carhied out in a batch type oven.
Preferably the oven is large enough to occommodate two or three cathodes at once and the cathode members are positioned in the oven to give about one foot spacing around the members for good air circulation. In a preferred embodiment using a copolymer of ethylene and chlorotrifluoro-ethylene in a 1:1 ratio as the thermoplastic polymer material, the oven is initially heated to 105-125C and held in this range for about three hours. This minimizes the risk of swelling the asbestos by too rapid an evolution of steam and trapped gases and also minimizes the possible , .

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~7,Z056 warpage of the cathode member due to rapid temperature change. The temperature is then raised over a two-three hour period to 270C and held for one hour at 265-275C. The oven is then allowed to cool to ambient temperature for convenient handling of the cathode members. -The temperature ranges described are dependent upon the thermoplast;c resin material that is utilized, however, they are typical as a material with a fusion point below the cell operating temperatures is not suitable and although a thermoplastic material having a higher flow or fusion point could be used, it would require additional heating ~-and additional costs which are not Justified.
The following examples further illustrate the invention. However, ~J it is to be understood that the examples are presented for the purpose of better i}lustrating the invention and are not to be construed as unnessarily limiting to the scope of the invention as set forth in the foregoing disclosure and the appended c-laims. -EXAMPLE I
~: , A w1re mesh cathode was lowered into a depositing tank containing an agitated slurry of 1.5% by weight of asbestos fibers in cell liquor.
An initial vacuum of about 2-3 inches of Mercury was pulled on the inside -~e~ of the cathode member. After a period of five minutes, the vacuum was increased to about 28 inches for about ten minutes. The cathode member as then removed from the slurry and a full vacuum maintained for a thirty minute hanging time. While still maintained under vacuurn the asbestos - ;~
coated cathode was lowered into a second depositing tank containing an agitated slurry of about 0.15,~' by weight of Halar powder (a trademark /~-of Allied Chemical Corp. for a 1:1 weight ratio of chlorotrifluoroethylene J
and ethylene)which had been previously wetted utilizing a 0.~ by weight of Triton X-lOO (trademark of Rohm & Haas for a nonionic octyl phenoxy polyethoxy ethanol surfactant) in aqueous liquor. As the cathode member ~;
is immersed, the agitation was discontinued. After five minutes the cathode member was removed and dried under vacuum for a two hour hanging time. From the makeup required in the resin depositing tank, 8.8 pounds , ~

~7;~56 of resin, and the weight of the d~posited diaphragm, 175 pounds, it was calculated that the diaphragm contained about 5% by weight of resin. The coated cathode member was then placed in an oven and positioned to m~intain ~ood air flow around the member. The oven was heated to 105-125C for three hours. The temperature was then raised over a three hour period to 270C and held at between 265-275C for one hour.
The oven was then allowed to cool to ambient temperature. The cured ;~
coated cathode member was then removed and subsequently installed in an -electrolytic chlor-alkali cell.
By sirnilar steps with variances in the resin concentration in the ;
slurry, resin composition in the diaphragn~ was varied from 1 to 20%.
The ~ollowing table shows the improvement in electrical potential when diaphragms of the present invention are utilized in a chlor-alkali electrolytic cell utilizing varying brine gaps.
~5 EXAMPLE II
~ Electrical Potential RunBrine Gap. Wt. Asb~stos % by Wt. 1 ampe~e 1.5 ampere No. mrn. lbs _ t. Resin~__ft. _per ft.
1) 8.5 0.30 0.0 3.~0 3.79 ~;
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2) 8.5 0.30 5.0 3.31 3.66 _ 3) 8.5 0.21 5.0 3.25 3.57 ` -.,.
4) 8.5 0.17 5.0 3.21 3.50 5) 5.6 0.21 5.0 3.13 3.39

6) 5.6 0.17 5.0 3.08 3.30 ;

7) 3.2 0.21 5.0 3.09 3-33 `

8) 3.2 0.17 5.0 3.04 3.24 Thus in run 1 the anode and cathode of an electrolytic cell were set at a distance of 8.5 mm apart. A layer of asbestos of 0.30 pounds per square foot was deposited in the conventional manner on the cathode in accord wi~h the normal depositing procedure described in foregoing. No resin was added. ~;
The potential at 1 ampere per square inch was found to be 3.40 and at ', g ., .

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1~5 ampere per square inch was found to be 3.79. In run 3, the brine gap remained at 8.5 mm, the weight of the asbestos deposited on the cathode was 0.21 pounds per square foot, a 5% by weight of Halar powder was deposited in the asbestos layer in accord with the procedure described above. In operation the potential was found to be 3.25 at 1 ampere per square inch and 3.57 at 1.5 ampere per square inch.
EXAMPLE III
',, An electrolytic cell comprising a cell top, a cell bottorn and sides ~orming an enclosure, a series of cathode members with asbestos impregnated resin diaphragms made in accord with Example I on the cathodically active surfaces thereof, and inter-positioned with the cathode members a series . .
of anode members. The brine gap was fixed at 5.6 mm. A sodium chloride `~' brine was fed into the cell having a concentration of about 3.25 grams per liter of sodium chloride. The brine had a pH of about 8.0 and was .
preheated to 70C. A current of 31,000 amperes was applied to the cell giving a current density of about 1 ampere per square foot. The cell `
operated at a current efficiency of 94.5~. Chlorine gas was produced at the anode and exited the cell top and was collected in a header line.
Hydrogen and cell liquor were produced in the cathode compartn1ent.
The cell l;quor contained about 150 grams per liter sodium hydroxide together ~ `
~Ef wi~th the unconverted sodium chloride.
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It is understood that the present disclosure is for the purpose oF
illustration only and that this invention includes modifications and equivalents which fall within the scope of the appended claims.

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Claims (15)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A diaphragm coated cathode comprising a foraminous cathode bearing on the cathodically active surfaces thereof, a uniform, adherent, and coherent dimensionally stable dia-phragm consisting essentially of asbestos fibers having a discontinuous thermoplastic polymer coating on the fiber surface, said thermoplastic polymer binding the fibers together, said diaphragm having asbestos in a range of between 0.18 and 0.3 pounds per square foot of active cathode area.

2. A cathode according to Claim 1 wherein the coated diaphragms impregnated with thermoplastic polymer particles in a range between about 0.2 microns and about 100 microns in size.

3. A cathode according to Claim 2, wherein the thermo-plastic polymer has a softening point between about 105°C.
and about 400°C.

4. A cathode according to Claim 1, 2 or 3 wherein the thermoplastic polymer is fluorine containing.

5. A cathode according to Claim 1, 2 or 3 wherein the thermoplastic polymer is a copolymer of ethylene and chlorotri-fluoroethylene.

6. A cathode according to Claim 1, 2 or 3 wherein the thermoplastic polymer is polyvinylidene fluoride.

7. A cathode according to Claim 1, 2 or 3 wherein the thermoplastic polymer is polytetrafluoroethylene.

8. A cathode according to Claim 1, 2 or 3 wherein the thermoplastic polymer content is from about 1 to about 20 percent by weight of the asbestos.

9. A cathode according to Claim 1, 2 or 3 wherein said asbestos is in an amount of 0.22 to 0.26 pounds per square foot of active cathode area.

10. An electrolytic cell containing a diaphragm coated cathode as defined in Claim 1.

11. An electrolytic cell according to Claim 10 wherein the spacing between anode and the diaphragm coated cathode in the cell is from 3.2 mm to 5.6 mm.

12. A diaphragm coated cathode comprising a foraminous cathode bearing on the cathodically active surfaces thereof, a uniform, adherent and coherent dimensionally stable diaphragm consisting essentially of asbestos fibers having a discontinuous thermoplastic polymer coating on the fiber surface, said polymer having a distribution gradient whereby the concentration at the interface of the cathode and adjacent diaphragm is less than the outer surface of the diaphragm, said diaphragm having asbestos in a range between 0.18 and 0.3 pounds per square foot of active cathode area.

13. A cathode according to Claim 12 wherein the asbestos content is between 0.22 and 0.26 pounds per square foot of active cathode area.

14. A cathode according to Claim 12 or 13 wherein the polymer is a copolymer of ethylene and chlorotrifluoroethylene.

15. A cathode according to Claim 10 or 11 wherein the polymer is polytetrafluoroethylene.