Note: Descriptions are shown in the official language in which they were submitted.
Background of the Invention
1. Field of the Invention
The present invention relates to a combined cathode
and diaphragm unit for electrolytic cells, and more parti-
cularly the present invention relates to such units for
electrolytic cells utilized for the electrolysis of sodium
chloride.
2. Prior Art
The making of diaphragms for brine-electrolysis
cells from asbestos has been widely practiced throughout the
world for many decades. Those skilled in the art are familiar
with the techniques involved, wherein asbestos is formed in
a slurry and drawn through the cathode screen in a vacuum
box to provide a diaphragm wherein the asbestos fibers are
entwined with the screen and with each other. In this way,
- a unit is obtained that will remain intact during operation
in spite of vigorous hydrogen generation at the cathode
screen. One of the disadvantages of this construction is
; that the physical intimacy and/or the interaction between
the asbestos and cathode screen interferes somewhat with the
generation of hydrogen at the cathode and tends to increase
electrical resistance due to bubble retention. Another
disadvantage is the use of asbestos, the use of which is
becoming increasingly regarded as a health hazard.
However, the use of substitutes for asbestos is a
relatively new technology, and improvements in the technology
of such substitutes is necessary to render the new technology
practical. The present invention relates to one of such
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improvements. One satisfactory substitute which has been
found and is known to the prior art is the use of relatively
inert synthetic plastic material which may be formed into
small fibers and deposited by known techniques to provide
fibrous diaphragms. An example of such a diaphragm is shown
in U.S. Patent No. 4,036,729. Other improvements have been
made in the use of these synthetic fibers to make satis-
factory diaphragms, involving the use of fluorinated hydro-
carbon resins, heat treatments, and the like in order to
render the diaphragms more satisfactory. In addition, these
synthetic fibers are hydrophobic and have presented diffi-
culties not present with hydrophilic asbestos fibers.
Accordingly, it is also known to utilize surfactants to
render the fiber diaphragms more wettable.
As noted above, U.S. Patent No. 4,049,841 dis-
closes a cathode having special surface characteristics
which may be combined with asbestos diaphragms and provide
an improvement in lowering the hydrogen overvoltage of the
cathode. Similarly, United States Patent Nos. 3,945,907 and
3,974,058 show special cathodes having a low hydrogen over-
voltage. However, it is believed that the advantages
achieved by the combined cathode and diaphragm unit of this
invention are not suggested by any of these prior art patents.
Summary of the Invention
In accordance with the invention, a cathode and
diaphragm unit is provided for electrolytic cells which
provides an improved efficiency of operation. First of all,
the use of the sprayed cathode provides a reduced hydrogen
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overvoltage that provides a considerable cost saving in
energy. Secondly, the use of the synthetic diaphragms
provides for improved contact between the diaphragm fibers
and the cathode. In addition, the combination of the elec-
trode, which has a roughened surface, and the synthetic
diaphragm allows for the diaphragm to be deposited on the
cathode with considerably less coverage thereof than in
asbestos diaphragms heretofore used with a resulting better
hydrogen generating surface, improvements in disengagement
of the hydrogen bubbles from the cathode, and an overall
improvement in cell energy efficiency as a result of the
combination.
In a copending Canadian patent-application, No. 291,077
filed November 17, 1977 and having common ownership with the
present invention, and as it is pointed out therein, the
fiber diaphragm in operation develops skin layers substan-
tially separating itself from the hydrogen generating cathode.
Hence, the gas bubbles formed between the cathode and the
- diaphragm do not have a tendency to be trapped. Thus, the
coated cathode and the diaphragm combination is synergisti-
cally efficacious.
~ Thus, the invention provides a chlor-alkali elec-
; trolysis cell having a cathode and the fibrous diaphragm
juxtaposed thereto, in which the cathode and diaphragm are
provided as a unit comprising a cathode screen and a fibrous
diaphragm composed of an organic thermoplastic polymer. The
cathode has a roughened surface obtained by spray coating a
powder metal on a ferrous metal substrate with the powder
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metal having a lower hydrogen overvoltage than said substrate,
and having a larger surface area than the area of said
substrate.
The invention also provides a process for manu-
facturing the chlor-alkali electrolysis cell containing the
cathode and diaphragm unit of this invention. In the method
form of the invention, the improvement comprises spray
coating a ferrous metal substrate with a powder metal having
a lower hydrogen overvoltage than said substrate to form a
cathode with the cathode surface having a larger true surface
area than the geometric surface area of the substrate, and
vacuum depositing thermoplastic polymer fibers from a slurry
onto the spray-coated surface of the cathode to form a
fibrous diaphragm adhered thereto.
Description of the Preferred Embodiments
As hereinbefore noted, the present invention
contemplates the spraying of a powder metal onto a conven-
tional cathode deployed in an electrolytic chlor-alkali
cell. The powder metal is either flame sprayed or plasma
` 20 sprayed onto the cathode.
i With more particularity, the present invention
contemplates the spraying of a powder metal onto a ferrous
metal cathode utilized in an electrolytic chlor-alkali cell.
The chlor-alkali cell can be either a monopolar or bipolar
cell. Furthermore, the cell employs a synthetic polymeric
deposited diaphragm such as those manufactured from per-
fluorinated polymers, chloro-substituted perfluorinated
polymers, sulfonated polymers and the like.
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As noted, the powder metal is either flame sprayed
onto the cathode or plasma sprayed onto the cathode. The
spraying of the metal onto the cathode surface provides a
hiyh degree of bonding while increasing the surface area of
the cathode. Furthermore, by spraying the coating onto the
surface, the resul~ing roughened surface provides the proper
conditions for efficient hydrogen bubble release. This is
to be contrasted with the prior art noted hereinbefore which
did not enhance the efficiency of the hydrogen bubble release.
Flame spraying and plasma spraying techniques, per
se, are known. Flame spraying generally comprises spraying
and fusing a powder metal onto a metallic surface with a
flame. Such flames are generated with a torch or similar
apparatus. Such apparatus and techniques are more compre-
hensively discussed in U.S. Patent Nos. 3,238,060; 2,786,779
and 3,220,068.
Plasma spraying generally comprises the utiliæation
of an electric arc discharge through which a plasma gas is
passed. As the gas passes the electric arc the gas is
ionized. Thus, there is achieved a plasma or ionized gas.
There is admixed with the plasma of ionized gas, a powder
metal suspended in a carrier gas therefor. Usually, a
plasma spray gun is utilized for the plasma spray coating.
Such guns are known. One such gun is depicted in U.S. Patent
No. 3,630,77Q.
In practicing the present invention, it is pre-
ferred to plasma spray coat the cathode. Plasma spraying
provides a higher temperature and a more non-oxidizing
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atmosphere than flame spraying and results in a greater
degree of bonding than flame spraying. In fact, plasma
spraying provides a substantially non-oxidizing atmosphere.
The gases employed in plasma spraying are nitrogen and
hydrogen, wherein hydrogen qas is ionized and the powder
metal is suspended in the nitrogen.
The powder metals which can effectively be employed
herein are those which have a lower hydrogen overvoltage
than the ferrous metal used in manufacturing the cathode.
Representative of the metals which can be used herein include,
for example, cobalt, nickel, platinum, molybdenum, tungsten,
manganese, iron, tantalum, niobium and mixtures thereof. In
addition, alloys of these metals can be used. Also, metallic
compounds such as carbides, nitrides and the like can be
used such as tungsten carbide, iron nitride and the like.
The pure metals can be used alone or can be admixed with the
alloys and the compounds. Also, the alloys and the metallic
compounds can be used alone. The only criteria attached to
the metal are that it be a powder capable of being sprayed
and have a lower hydrogen overvoltage than the cathode
material. In the practice of the present invention, the
preferred powder metal is nickel.
The metal is sprayed onto the cathode to a thick-
ness of about 0.001 to about 0.006 inches. Preferably, the
metal is deposited to a thickness of from about 0.002 to
about 0.005 inches. By spraying the metal powder onto the
cathode surface, the surface area is increased due to the un-
eveness of the sprayed particles and the inherent surface
porosity of the coating.
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The thermoplastic fibers contemplated for use
herein are the fluorinated hydrocarbon polymers, and in
particular fluorinated polyalkylenes. The fluorinated
polyalkylenes can bè additionally halogen substituted fluor-
inated polyalkylenes. Representative of the fluorinated
hydrocarbon polymers are polytetrafluoroethylene, fluorin-
ated ethylene-propylene copolymers, polychlorotrifluoro-
ethylene, polyvinylidine fluoride, polyethylenechlorotri-
fluoroethylene, polyethylenetetrafluoroethylene and tetra-
fluoroethylene perfluorovinyl ether sulfonylfluoride co-
polymers. Most preferredj are the homopolymer of chloro-
trifluoroethylene, and a copolymer containing chlorotri-
fluoroethylene and vinylidine fluoride with at least 80
percent of the copolymer being chlorotrifluoroethylene. It
is also possible to use these polymeric fibers along with
minor amounts of other fibers such as asbestos, potassium
titanate, glass, silica, zirconia fibers and silicate,
borate and phosphate fibers.
In general, the synthetic fibers may be prepared
by the procedures given in U.S. Patent No. 4,036,729 or for
the preferred fibers by the procedure given in the Canadian
patent application n 291,077 filed November 17, 1977.
Thus, the chemical content of one of the preferred
fibers to be utilized is a composition based upon a terpolymer
containing over 80 percent chlorotrifluoroethylene and minor
amounts of vinylidene fluoride and tetrafluoroethylene.
Such material is commercially available from Allied Chemical
Co. under the name "Aclon 2000"*. Another preferred fiber is
*(Trademark)
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made from the homopolymer of chlorotrifluoroethylene sold by
3M Company as "Kel-F 81"(Tradema~k).
Such material is put into the form of fibers
having a cross section on the order of one micron by four
microns and a length of approximately 0.25 to 0.5 milli-
meters in accordance with a modification of a process which
is adequately described in Belgium Patent No. 795,724. The
surface area of such fibers is five to 20 square meters per
gram as measured by nitrogen adsorption. There is thus
produced material which is, in effect, water soaked fiber
bundles, containing 80 to 90 percent by weight water, made
by draining the output of the process conducted according to
the above-mentioned Belgian patent on a perforated moving
bed.
As is known to those skilled in the art, fluorin-
ated hydrocarbon fibers, per se, are difficult to disperse
in an aqueous medium, thereby, rendering such fibers difficult
to deposit on a cathode screen or support. Thus, it is
customary to add a surfactant and disperse the fibers in an
aqueous-acetone medium. The surfactant is employed in
amount ranging from about 0.01 percent to about ten percent,
by weight, based on weight of the slurry all of which is
shown in the prior art.
The slurry is then vacuum deposited on a cathode
screen by any suitable method. A particularly preferred
method of depositing slurry involves the immersion of the
cathode screen, mounted in a vacuum box, into the slurry
which is maintained in the'state of agitation. Then, a
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series of increasing partial vacuums are applied across the
screen for a period of time followed by a full vacuum for a
predetermined pericd of time. Screen having the fibers
deposited thereon is, then, dried at a temperature higher
than room temperature, say, 100C for about one to three
hours to evaporate the water.
For a more complete understanding of the present
invention, reference is made to the following examples. In
the examples, which are intended to be illustrative only and
not limitative of the invention, all parts are by weight,
absent indications to the contrary.
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Example 1
A combined diaphragm and cathode unit was made
according to the pr~sent invention. The cathode used was a
mild steel screen plasma sprayed on both sides with nickel,
in accordance with our copending applications cross-referenced
above. The composition of the diaphragm was "Aclon 2000"*
polymer. The average cross-sPctional dimensions of the
fibers used to form the diaphragm were 1 micron by 4 microns,
further branched into finer fibers, with a length of 0.25 to
0.5 millimeters. Such fibers were suspended in water, to
the extent of 12.7 grams per liter (dry weight of fiber
employed), along with 4 grams per liter of dioctyl sodium
sulfosuccinate and 2 grams per liter of a fluorine-containing
surfactant, namely, that sold by 3M Company under the designa-
tion FLUORAD "FC-170"*
Fiber dispersion and slurry agitation were performed
- with the use of a propellor-type mechanical agitator driven
by a "Lightnin" mixer.
A two-layered web was formed by drawing two suc-
cessive volumes of slurry through the cathode screen at aratio of 8.3 milliliters of slurry per square centimeter of
screen area per layer according to the following schedule:
2 minutes at 25 millimeters of mercury difference from
atmospheric pressure, 3 minutes further at 50 millimeters of
mercury difference in pressure, and 2 minutes further at 100
; millimeters of mercury difference in pressure.
- The second layer was then applied: 3 minutes at
50 millimeters of mercury difference from atmospheric pressure,
8 minutes further at lO0 millimeters of mercury difference
* (Trademark)
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in pressure, and 2 minutes further at 150 millimeters of
mercury difference in pressure. The full vacuum of 615
millimeters of mercury was then applied for 20 minutes.
There was obtained a diaphragm having a gross thickness of
2.7 millimeters and having a permeability coefficient
1.7xlO 9 square centimeters. After being dried at 100C for
16 hours, the unit is ready for use in a chlor-alkali cell.
Example 2
A steel cathode screen was plasma spray coated on
one side with nickel powder to a thickness of 0.002 inches.
A fiber diaphragm was then vacuum deposited on the coated
side of the cathode in accordance with the procedure of
Example 1.
After 40 days of operation in a chlor-alkali cell,
the electrode potential was 1.23 to 1.27 volts versus standard
calomel electrode (SCE) at 80C and a current density of 160
ma per square centimeter. In previous measurements, an
uncoated steel cathode with an asbestos diaphragm had an
electrode potential of 1.35 to 1.40 volts versus SCE under
the same conditions. A cathode plasma coated with nickel on
one side with an asbestos diaphragm had an electrode potential
of 1.27 to 1.29 versus SCE.
Example 3
The procedure of Example 2 was repeated by coating
the cathode on both sides and the unit tested as in Example 2.
After 40 days of operation, the electrode potential of the
cathode was 1.22 to 1.27 volts versus SCE.
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Example 4
A steel cathode screen was electroplated with
bright nickel. An "Aclon" fiber diaphragm was then vacuum
deposited on the cathode screen using the procedure of
Example 1. After 15 days of operation in a chlor-alkali
cell, the cathode potential was 1.32 to 1.35 volts versus
SCE.
From the foregoing data, it is seen that the in-
vention provides improved electrical characteristics in a
chlor-alkali cell, and that both elements of the combination
contribute to the improvement.
* (Trademark)
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