Note: Descriptions are shown in the official language in which they were submitted.
107606iS
Ihe present invention relates to a novel diaphragm, particularly
for use in electrolysis cells, and to a novel method of m~king said
diaphragm. ~he invention is an ~mprovement in the diaphragm disclosed in
our Canadian patent 1,037,671, issued September 5, 1978.
The prior patent describes a method of making porous diaphragms -
of asbestos which is deposited and consolidated by a fluorinated polymeric
i resin, characterized in that a stable, homogeneous suspension is formed
by adding a fluorinated polymeric resin latex and a pore-forming agent to
; a suspension of asbestos fibers in water, in the presence of a sulphonic
anionic surfactant, followed by placing said suspension in a filtration
mold, drying and fritting the resulting casting at a temperature above the
crystaIline melting point of said fluorinated polymeric resin, and with the
pore-forming agent being finally eleminated by decomposition or chemical
action.
One of the requirements of the diaphgragm of zn electrolytic cell
is that it behave as a porous medium: (1) on the one hand, it allows
current to flow with a slight ohmic drop, and on the other hand, (2) it
permits an even flow of electrolyte from one compartment of an electrolytic
cell to the other. There is, consequently, a co~bination of mechanical,
electrical and hydraulic conditions which are all the more critical now
that electrolytic cells are required to function with high current density,
sub~ect to tolerzting prohibitive ohmic drops. The properties required
are rather contradictory. Thus, from a mechanical point of view, the
diaphra m must have a well-defined,
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107606S
relatively permanent geometry or shape. The diaphragm must
not be subject to swelling and being permanently deformed
during use. In other words, such a diaphragm must h~v~ good
mechanical strength. However, it must also have good wettability
properties, enabling gases to be released and electxolyte to be
circulated, while preventing diffusion of hydroxide ions in the
opposite direction to the liquid flow. Such diffusion is respon-
- sible for the formation of chlorate which leads to a drop in
yield.
To express the siiuation differently, a diaphragm for
electrolysis must have low relative resistance. Relative resis-
tance is understood as being the quotient of the resistance of
a medium consisting of the diaphragm impregnated with electrolyte
and the resistance of the same medium consisting of electrolyte.
It has been observed that the relative resistance is bound up
with the porosity of the diaphragm, but also with the shape of
the flow passa~es.
Attempts have therefore been made to associate a
plurality of elements so as to meet all the above requirements
simultaneously. Thus, U. S. patent No. 3,694,281 proposes forming
a mechanical support consisting of plastic filaments and arranged
on at least one face of the diaphragm, or making a laminate with
threads, fibers or particles inserted in the interface between
the asbestos and the substrate.
French patent No. 2,170,247 proposes a simple arrange-
ment which makes use of a support consisting of a cloth, e.g.,
of polypropylene.
Belgian patent No. 814,510 proposes improving the
rigidity of a diaphra~m by applying the diaphragm to at least
one sheet of chrysotile asbestos. In a special embodiment of the
diaphragm, the sheet comprising the diaphragm is sandwiched between
and bonded to two sheets of chrysotile asbestos.
10760~;5
However, the main difficulty encountered in providing a preferred
diaphragm is to maintain the advantages of an open structure as provided by
the diaphragm produced in accordance with our prior Canadian Patent 1,037,671,
while improving its mechanical properties. When such diaphragms are used in-
dustrially, they are in fact found to have the disadvantage of showing struc-
tural deformation, particularly localized loss of cohesion, and to result in
performance which cannot be perfectly reproduced.
It has been found that this defect can be avoided if the diaphragm
obtained according to Patent 1,037,671 is associated with a reinforcing element,
by integrating it with the anode face of the porous membrane, the reinforcing
element having a structure which is at least as open, relative to the passage
of liquid during electrolysis, as the porous membrane.
In one general aspect, the present invention comprises an electro-
lytic cell diaphragm in accordance with Canadian Patent 1,037,671 which has
associated with it a reinforcing element integrated with the anode face of
the porous electrolytic cell diaphragm. The reinforcing element shall have a
structure which is at least as open or porous relative to the passage of
liquid during electrolysis as the porous membrane.
It has unexpectedly been observed that if the same reinfor^ing ele-
ment is placed on the cathode face rather than the anode face, the above-men-
tioned disadvantages of the prior art membranes will still be experienced.
Accordingly, the invention provides a method of manufacturing por-
ous electrolytic cell diaphragms which are consolidated by a fluorinated poly-
mer resin, comprising agitating a suspension of asbestos fibers in water, a sul-
fonic anionic surfactant, a fluorinated polymer resin latex and a solid mineral
pore-former, to form a stable, homogeneous suspension, depositing and filter-
ing said suspension on a grid or screen to form a preform, drying the result-
ing preform, sintering it by heat at a temperature above the crystalline melt-
ing point of the fluorinated polymer resin, removing said solid minerql pore-
former to form a uniform, porous membrane having uniform pore size, combiningsaid resulting uniform, porous, homogeneous membrane with a reinforming ele-
ment by integrating the said reinforcing element with the anode face of said
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~076065
uniform, porous, homogeneous membrane, wherein said reinforcing element has a
structure which is at least as open with respect to the passage of liquor as
said uniform, porous, homogeneous membrane.
The reinforcing 01ement advantageously comprises a cloth or sheet
of asbestos fibers.
The asbestos cloth from 0.5 to 1 mm. thick and less than 500 grams
per square meter in weight may advantageously be associated with a micro-
porous membrane produced in accordance with our Canadian Patent 1,037,671, in
which the proportion of charge of pore-forming agent relative to the weight
lQ of asbestos is at least about 100% and preferably between about 250 and 600%.
In a preferred embodiment the diaphragm according to the invention
comprises as asbestos cloth from 0.5 to 1 mm., weighing less than 500 grams per
square meter and incorporated in the anode face of a microporous membrane in
which the proportion of charge comprising calcium carbonate or other solid
mineral pore-forming agent, relative to the weight of asbestos, is from about
250 to 600~. The diaphragm has a relative resistance of less than 4, a liquid
flow rate through the diaphragm of from about 0.25 to 0.02 ml./min./cm.2 under
50 g./cm of liquid charge at 20C., and a thickness of about 2.5 to 5 mm. and
preferably from about 3 to 4 mm.
Such a diaphragm can be obtained by applying a method similar to
that described in Canadian Patent 1,037,671.
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~gJI
` 1076065
In practice, an asbestos suspension is prepared by
dispersing by agitation a mixture which contains (by weight):
1 part of asbestos
about 5 to 100 parts of water
about 0~015 to 0.1 part of anionic surfactant.
The asbestos used is preferably made up of 0.5 to 50
millimeter fibers. The surfactant, a sulfonic anionic surfac-
- tant and preferably a sodium sulphosuccinate, i6 used in pure
.
form or in solution in alcohol. Vigorous agitation gives a
stable, well-dispersed asbestos suspension.
Other sulfonic anioni¢ surfactants may be empLoyed
such as alkyl sulfonates, sulfosuccinates and sulfosuccinamates.
The latex of a fluorinated polymeric resin and the
pore-forming agent are added to ~he suspension so as to give
:.
the following proportions by weight:
100 parts asbestos
about 60 to 200 parts of fluorinated polymeric
~, resin, on dry basis
about 100 to 1400 parts of pore-forming agent.
Agitation is then continued for about 1 to 20 minutes,
- preferably 5 to 10 minutes, depending upon conditions employed,
particularly of speed. The final concentration of the suspension
may be adjusted by adding water, at the end of the agitation
process, in proportions best adapted to the depositing condi-
tions observed.
; The latex of fluorinated polymeric resin, preferably
of polytetrafluoroethylene, is preferably a suspension of approxi-
mately 60% polytetrafluoroethylene in water. It may be replaced
by other latices of fluorinated resins (a polymerized mixture OL
tetrafluoroethylene-hexafluoropropylene, or polymerized poly-
chlorotrifluoroethylene etc.)~
1076065
The pore-forming agent used is desirably a mineral
material and may he calcium carbonate, colloidal alumina, metal
oxides or any substances adapted to be eliminated by a solvent
or by destruction at the end of the operation. It must have a
well-defined particle size. It is preferable to use a calcium
carbonate consisting of particles with an average diameter OL
2 to 25 microns.
When a flat diaphragm is being made, the stable,
homogeneous mixture of the various constituents is poured onto
the reinforcing element in a quantity such that the desired
thickness is obtained. It is then filtered under vacuum then
dried. Drying takes place at a temperature of over 100C., at
approximately 150C. for 3 to 24 hours.
The sheet is then fritted or sintered by placing it in
a furnace at a temperature above the crystalline melting point
of the fluorinated polymer resin, preferably about 25 to 75C.
above it, for a period of 2 to 20 minutes and preferably about
6 to 10 minutes. The optimum temperature chosen depends some-
what on the fritting time but also on the thickness and compo-
sition of the diaphragm.
; When the diaphragm has cooled and employs calcium
carbona*e as the pore-forming agent, it is immersed in a 10 to
20% by weight aqueous solution of a weak acid, in the presence
of a wetting agen~, for a period ranging from 24 to 72 hours
depending on the thickness. Acetic acid is preferred, but other
weak acids may be used with the same degree of success. Where
ctherpoxe-forming agents are employed, another suitahle solvent
or reagent for decomposing and dissolving the pore-forming agent
may be employed. Thus, depending upon the pore-former employed,
any agent which will dissolve or decompose the pore-former may
be employed, provided it does not dissolve or decompose the
~076065
fluorinated polymer resin. If the mineral pore-former is
alumina, acid o~ alkali solutions may be employed. ~ith other
metal oxides, other dissolving agents well known to the chemist
may be employed.
The diaphragm obtained is then washed in water to
eliminate the acid, and kept under water.
Specific Description of the Invention
In order to disclose more clearly the nature of the
present invention, the following examples illustrating the
invention are given. It should be understood, however, that
this is done solely by way of example and is intended neither
to delineate the scope of the invention nor limit the ambit of
the appended claims. In the examples which follow and through-
out the specification, the quantities of ma~erial are expressed
in terms of parts by weight, unless otherwise specified.
:
EXAMPL~ 1
This example is a control experiment which differs
from the method of the invention, in that the reinforcing element
is arranged on the cathode face of the diaphragm.
In this experiment, a suspension of asbestos fibers
is prepared, containing:
100 g. of asbestos fibers of an average length
of 1 to 2 mm.
2500 g. of water
2.5 g. of a 75% solution of sodium dioctylsulpho-
succinate in alcohol.
Dispersion is effected by agitating for 60 minutes using
an agitator of the drum roller type.
The following are then added:
300 g. of polytetrafluoroethylene polymer in
the form of a latex with 60% of dry polymer
560 g. of calcium carbonate, marketed under
the registered trademark "BLE O~1YA".
1076065
The mixture is agitated for 5 minutes.
170 g. of the suspension is drained on an asbestos
cloth measuriny 1 square decimeter, using the following vacuum
program:
1 1 minute of decantation
2 minutes at a pressure of 200 mm. of Hg. (mercury)
2 minutes at 300 mm. of Hg.
10 minutes at 740 mm. of Hg.
The asbestos cloth used has a thickness of 0.76 mm.
for a weight of 255 grams per square meter. It comprises warp
and weft threads of 111 tex (weight in grams of 1,000 meters of
thread) and has 13 threads/cm. in the warp and 7 threads/cm. in
the weft.
The form obtained, thus combined with the asbestos
cloth, is dried in an oven at 150C. for 5 hours. The reinforced
- form is then fritted in a furnace which is brought to 360C. for
7 minutes. The carbonate is eliminated in a 20% by weight solu-
tion of acetic acid in water for 72 hours. The diaphragm thus
o,btained is washed with water.
This gives a diaphragm which has the following pro-
perties:
weight in grams per 170
square decimeter
thickness in mm. 3,55
liquid flow rate in ml./ 0.15
min./cm.2 (wlder a charge
of 50 g./cm.2)
relative resistance R/Ro 2.6
The diaphragm is used as a separator in electrolysis
0 of a solution o~ sodium chloride under the ~ollowing conditions:
asbestos cloth placed at cathode side
electrodes formed by a grating (platinum-
plated titanium at anode side and iron at
cathode side) 7 mm. apart.
1076065
current density--25 amperes per square decimeter
temperature--85C.
The following properties are observed during operation
of the cell:
loss of cohesion on the anode face
tension of cell in equilibrium 3.44 volts
composition of liquor:
soda g./1. = 125
chlorate g./l. - 1
liquid charge on diaphragm = 15 cm. water (H)
~ EXAMPLE 2
- This example is identical with the previous one, except
that the same asbestos cloth is arranged at the anode side.
The liquor is found to have the same composition of
soda and chlorate, but the equilibrium tension is found to have
decreased to 3.17 volts; relative resistance is 2.1 and the
liquid charge on the diaphragm is 12 cm. of water.
. EX~PLES 3 to 14
In all these examples the proportions of the various
-constituents are as follows:
asbestos fibers 100 g.
water 2500 g.
- polytetrafluoroethylene 300 g.
resln
sodium dioctylsulphosuccinate 2.5 g.
The other conditions and the results in electrolysis
under the same conditions as in Examples 1 and 2 are set forth
in the table which follows.
1076065
O_ -- C~ O N _ O _ l O __
Zo _~ o ~ _~ o t~ _I N
~ _ _ _ _ _ _ _ _ _ __ _ _
O _~Ul O O O U~ U~ 00 U~ O
bON N ':t N ~) N O N l O
_ _ _ _ _ _ _ _ ~d- _ ~d- c~- .
~I~ ~ d' X N ~t N Il~ b O ~ h
_ _ _ _ _ _ _ _l O _ O O
oo ~ L~ O I~
E-~O ' t~ ro t-~ t~ t~ t~ t~ t~ ~
O _ _ _ _ _ _ _ _ _ _ _ _
~: ~ a~ ~ ~D cr~ O ~ _1 O u~ 00
1~ _I N N t-~ N t ) t~ ~ ~ I~ O O~
_ _ _ _ _ _ _ _ _ _ _
N N .,
q~ ~ O Ln n I~ _l
1-~ _~ t~ ';t C~ n N O _l O 1~ O O
.~, h N N O O O O O O O O O O
h _ _ _ _ _ _ _ _ _ _ _ _
~: h ~
~0 ~. ~ o~ ~ _1 ~ Ln I~ el~ oo ~ n ~ `D
~ ~ t ~ u~ ~ t~ t~ t~ N t ~ N t~
_ _ _ _ _ _ _ _ _ _ _ _
~0
~ rl h ~
.,~ ~, a) O O O O O O O Q O O O O
ID ~ 1~ ~D ~ O ~ a~ N ~0 a- N ~ 01 N
--~ ~1 N --I _I N _~ _I N _~ ~1 N
_ _ _ _ _ _ _ _ _ _ _ _
0~0 O O O O O O O O O O O O
C~ 1: N `D ~7 N N N ~t ~ ~
_ _ _ _ _ _ _ _ _ _ _ _
X ¦ t~l ~ u~ ~0 1~ oO O- O _~ N ~ ~:t
~11 , _ _ _ _ _ _ _ _1 _1 _~ _1 ~
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1076065
The foregoing table shows the properties and advan-
tages of the diaphragm according to the invention.
Thus, it will be noted that the charge valu~s for
Examples 10 to 14 are very high or too great, making the dia-
phragms virtually useless in electrolysis.
Number 9 is an example of a diaphragm which tolerates
a high proportion of chlorate, thus making it unsuitable.
The examples show that a diaphragm structure which is
too closed gives poor results in electrolysis.
Conversely, too open a structùre leads to fairly good
results in electrolysis, but the diaphragm obtained is fragile,
which means that a thicker diaphragm has to be used; this is
detrimental to application in electrolysis, as can be seen from
Example 3.
It will be seen from Examples 4 to 8 that there is a
preferred range which gives an excellent overall result.
EX~MPLE 15
This example is carried out under the same conditions
as previously and in the preferred range as defined above. It
illustrates an experiment with a reinforcing element on the
cathode face, different from that described in Example 1.
In this example, the charge of pore-forming agent
(CaCO3) is 560 g. and the quantity of material filtered 120 g.
The thickness of the diaphragm is 3.10 mm.
Relative resistance is 2.25, tension in equilibrium is
3.30 volts, the liquid charge, H, is 2.5 cm. and the composition
of the liquor:
soda = 116 g./l.
chlorate = 1.7 g./l.
These conditions are unacceptable in electrolysis;
the liquid charge is too small and the chloratè content too high.
107606S
Moreover, it will be noted, e.g., that the tension of the cell
corresponds to that in Example 5, where the quantity of material
filtered is 200 g., the liquid charge 24, and the quantity of
chlorate 0.9.
It has been seen that the solution comprising increasing
the quantity of solid in the porous membrane to obtain a lower
chlorate content and an adequate liquid charge leads to a high
equilibrium tension, whereas according to the invention with a
larger quantity of filtered material, 140 g. instead of 120 g.
(see Example 4), the liquid charge on the diaphragm is greater,
the proportion of chlorate reduced, and the tension of the cell
weaker.
Even with a larger quantity of pore-forming filler,
better results are obtained in electrolysis (Examples 2 and 3).
The examples thus illustrate the new and advantageous
~; effects of the invention, such as the absence of any loss of
cohesion and the very marked improvement in performance in
electrolysis.
It should also be pointed out that the following
effects are observed with diaphragms according to the invention:
(1) the diaphragms of the invention have greater
resistance, at the anode side, to abrasion by gases;
(2) the diaphragms of the invention have greater resistance
to bending and punching, operations which are sometimes necessi-
tated by cell technology; they are also less vulnera~le to
handling, particularly during the shaping or forming of the
diaphragms;
(3) in situ clearing operations are facilitated.
Above all, it should be noted tha~, in contrast with
what usually happens, the mechanical, hydrodynamic and electro-
chemical re~uirements of electrolysis are all unexpectedly met
better by diaphragms according to the invention.
1076065
The terms and expressions which have been employed
are used as terms of description and not of limitation, and
there is no intention in the use of such terms and expressions
of excluding any equivalents of the eatures shown and described
or portions thereof, but it is recognized that various modifi-
cations are possible within the scope of the invention claimed.
