Note: Descriptions are shown in the official language in which they were submitted.
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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
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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,
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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. ~-
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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
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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.
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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
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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
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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
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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
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the present i~nvention these deficiencies are overcome by utilizing two
separate treatment steps. In the first step the cathode member is `-
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substantially uniformly coated with asbestos fibers and in the second
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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,
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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-
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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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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
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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
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~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
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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
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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 ` -
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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
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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
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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
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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
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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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