Note: Descriptions are shown in the official language in which they were submitted.
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~04744~ 1
Back~round of the Invention
Diaphragm cells useful for the elec~rolysis of brines and the
formation of chlorine and caustic soda have an anolyte chamber and a
catholyte chamber. The anolyte chamber contains an anolyte solution
containlng sodium chloride at a pH of from about 2 to about 4.5. Within
the anolyte chamber is an anode at which chlorine is evolved. The
catholyte chamber contains catholyte liquor at a p~ of from about lD to
about 12.5. The catholyte llquor contalns sodium hydroxide, and sodium
chloride. Sodlum hydroxide generated in the catholyte and hydrogen gas
i8 evolved at the cathode.
In the operatlon of the dlaphragm cell, brine containing approximately
300 to 315 grams per liter of sodium chloride is fed into the anolyte
chamber. At the anode, the reaction 2Cl ~ C12+2e takes place.
The anolyte liquor passes from the anolyte chamber through the
diaphragm into the catholyte chamber and a catholyte product containing
from approximately 110 to about 130 grams per liter of sodium hydroxide
and from approximately 110 to about 170 grams per liter of sodium chloride
i8 formed. In the catholyte chamber, the reactions Na+ + OH ~ NaGH,
and 2H + 2e ~~~~ H2 take~ place.
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1047441
Typically, diaphragms for chlorine cells have been constructed
of chrysotile asbestos. The chrysotile asbestos provides a diaphragm
having a thickness of from about l/8 of an inch to about lt4 of an inch.
Chrysotile asbestos diaphragms generally have a service life on the
order of from about 4 to about 8 months, and most frequently about 6 months.
It is therefore necessary to periodically remove the diaphragm
cell from service in order to remove the old diaphragms from the cathodes,
and install new diaphragms on the cathodes. This periodic removal of the
cell from service results in a consequent loss of production in order to
carry out a labor intensive cell renewal operation. In the past, when
diaphragm cells used graphite anodes, diaphragm renewal could be coordinated
with anode renewal. However, metallic anodes, e.g., coated titanium anodes,
have replaced such graphite anodes to a considerable degree. While graphite
anodes have a life of from about 4 to about 8 months, the metallic anodes
now being used have a life far in excess of 4 to 8 months, e.g., 3 or 4
or re years. Accordingly, the renewal of diaphragms has become a prin-
cipal factor in cell outage.
Summary of the Invention
It has now unexpectedly been found that a particularly long-lived
dlaphragm may be provided by a chrysotile asbestos diaphragm which has
been treated with a water-soluble silicate on the anolyte side thereof.
According to this invention, a conventional chrysotile asbestos diaphragm
i8 treated with a water-soluble siliLate which is rendered substantially
insoluble during or after deposition thereby providing a tough, adherent,
electrolyte permeable coating or sizing on the anolyte side thereof. Such a
diaphragm has a long life, e.g., in excess of 6 months, and frequently in excess
of 1 year of longer. Additionally, according to this invention, an asbestos
diaphragm may be coated or sized with a water-soluble silicate several
1047441
times over the course of its life thereby considerably extending the
servlce life of a diaphragm.
Detailed Description of the Invention
~ .
According to this invention, a diaphragm for chlor-alkali electrolytic
cells i9 provided having a long service life. Such an asbestos diaphragm
is one that has been treated with a water-soluble silicate on the anolyte
slde thereof.
According to one exemplification, the diaphragm is a deposited
asbestos diaphragm. For example, the deposited asbestos may be chrysotile
asbestos where the individual fibers have a fiber length of from about
0.25 inch to about 1.50 inch and a f~ber diameter in the range of from
about 0.015 microns to about O.OS0 microns.
Adtitionally, fine asbestos powders, such as Union Carbide "Calidria"
(T.M.) asbestos may be present with the asbestos fibers.
Typically, a deposited chrysotile asbestos diaphragm will have a
weight of from about 0.2 pounds of asbestos per square foot of surface
area to about 0.4 pounds of asbestos per square foot of exterior surface
area.
According to another exemplification of this invention, the asbestos
diaphragm may be an asbestos paper diaphragm having a thickness of from
about 30 mils to about 60 mils and a weight of from about 30 grams of
asbestos per square foot of exterior surface area to about 100 grams of
asbestos per square foot of exterior surface area.
According to this invention, the anolyte-facing surface of the
diaphragm is treated with a water-soluble silicate which is rendered sub-
stantially insoluble to acidic media during deposition or treatment to
provide a strong, adherent, electrolyte-permeable fiber coating. While the
1l)~7~
preforrul ~ nt~ ~re r~erred to here.Ln n~ belng ~olubLe prior to
traatm~nt -Y or d~po~LtLon on the n~b~cto~, they Qr~ Al~o chnrncterlzed
by th~lr ~blllty to torm ~ronK binder~ or ~1ze~, ndherent to a~be~toH.
Watur~ho:Lubl~ 4l.11c~tuu aru tho~c s:ll1cateH hav1ng th~ formuln
~Ma20) (~'2) wh~r~ Mu 1H an ~lk~ll m~tal and m nnd n nre a~ de0cribed hBre1n
aet~r. llo alk~J.L meLaL~ Incllldc litll.Lum, ~odlum, potu~:Lum, rubidium, and
ea~lum, Mo~t ~rquently, M~ 18 llthlum, ~odlum or pota~0ium. Llthium
prOVld~A a part LCIJ1arlY ~:lex.Lb.l.e tough, ndherent co~ting on the aabestou.
Nowuvur, th~ voltaK~ dr)p I~cro~ a Lith1um uilic~te cont~d n~besto~ din-
pllra~m lu lIKhtly ll.L~hor than the voltago drop ~cro~s ~n nsbestos dia-
phr~m h~v.ln~ an uqulvulunt amount Oe a ~odlum ~lllcnt~ or potn~oium
t0 eo~t.LIlK thorqon.
Mo~t ~r~quentl.y I:Lthlum, ~odium, or potass1um ~111cntc will be
tha pr~err~d w~t~r-~olub1~ ~.Lllc~to. ~cller~l1y, ~or re~00n~ of co~t,
a.~ he cout of tha Hll.Leato and el0ctricaL power co~t0, sodlum slllcntc
1~ pr~r~rrod over alth~r llthlum ~lllcato or pota~lum ~lLlc~to. However,
~ntirq1y ~atlae~ctory ra~ult~ m~y b~ obtalned with eltllor Llthium ~llicate
or pot~auium ~ neo.
Th~ ~I0~ moloty may oIther be a mono~ lic~t~, a poly-~illcate or
a poly-nI11~ cld. Tho formula for a wat~r-~oluble ~illcat~, (MH20)m(5iO2)n
do~ not raear to an aceu~l ctructurfll formula of the sllicate, but rathor
to tha atoichiomu~ry of tha uillc~t4. For ~xampl~, the ~illcato may be
~odium orthoPilicnto, ~N~20)2.(S102), ~odlum metA~llLcaC~, ~N~20) . ~S102),
or ~odlum poly~lIIauto tNa~0) . ~S102~ whHre n Is in tha ran~e of 2 to 4.
An d ~onaraL rula, eho ~roator tho ratlo of n to m tl.e., the smaller
th~ ratlo of m to n) th~ bottar th~ ~olubl~ ~llicato i0 for the u~e horein
lntan~l. A~ A ~OIl~rAl ru.L~, howavdr~ th~ uppor llmlt of tho ratio of n to
m ~I.o., th~ Iowor ;Ilmlt ~ th~ r~tio o~ m to n) i~ that level at uhich a
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solution ceases to form and the silicate no longer remains adhesive.
The preferred silicates are those having a ratio of n ts m below the
level at which a solution ceases to form and thè silicate loses its
adhesion.
The silicate itself is a complex composition of silicate ions
and radicals, poly-silicic acid ions and radicals, oxides and ions of the metal,
water, and colloidal silica. The silicate ions and radicals may be
present in the form of ions or colloidal silica. The colloidal silica
may further be present in the form of hydrated micelles and non-hydrated
micelles, As a general rule, micelle formation occurs when the ratio of
m to n is less than one-quarter (i.e., when the ratio of n to m is greater
than 4).
The preferred water-soluble silicates useful in providing the
diaphragms of this invention are those wherein the ratio of m to n is less
than 1 and preferably 1/3 or less. Particularly good results are obtained
when the ratio of m to n is in the range of from about 1/2 to about 1/4.
Satisfactory results are also obtained when the ratio of m to n is even
less then 1/5, for example, as small as 1/10 or less.
According to this invention, the silicate is coated on the anolyte
surface of the asbestos diaphragm. The amount of silicate present is
from about 0.01 to about 5.0 grams per square foot of external surface
area, generally in excess of 0.1 gram per square foot. and preferably from
about 0.5 to about 2.0 grams per square foot.
Moreover, for any given porosity, pore size distribution, and
thickness of diaphragm, best results are obtained if the silicate extends
at least as far into the diaphragm from the anolyte surface as the "gel
layer" in an untreated diaphragm of like porosity, pore size distribution,
and thlckness. This "gel layer" is described by Klrcher, "Electrolysis of
Brines in Diaphragm Cells," in Sconce, ed., Chlorine, A.C.S. Monograph
Serles, No. 154, Reinhold Publishing Co., New York (1962), at Page 105,
~()47441
as a layer "formed within the asbestos mat which is sensitive to pH
and which tends to dissolve, precipitate and reform depending upon flow
rate and salt content and pH of the flowing liquor."
Typically, the "gel layer" extends approximately 0.08 to about
0.12 inch into the diaphragm. ThereforeJ an optimal depth of penetration
of the silicate is at least 0.08 inch, and preferably about 0.15 inch,
or even to the full thickness of the diaphragm, especially when the
diaphragm is less than about 0.15 inch thick.
According to this invention, a sllicate treated asbestos diaphragm
is provided that is useful for chlor-alkali diaphragm cell service where
concentrated unsaturated brine, e.g., water containing from 250 to about
310 grams per liter of salt, or saturated brine, e.g., water containing
in excess of 310 to 315 grams per li~er of sodium chloride, is fed to an
anolyte chamber of the cell and electrolyzed to form chlorine, hydrogen,
nd a liquid composition containing sodium chloride and sodium hydroxide.
Within the anolyte chamber, sodium chloride is present as sodium ion and
chloride ion. At the anode within the anolyte chamber the reaction
2Cl ~ C12 + 2e~ takes place. The anolyte liquor, including sodium
chloride and sodium hydroxide, flows through the diaphragm under the
driving force of a hydrostatic head of anolyte into the catholyte chamber.
In the catholyte chamber, the reaction Na + OH NaOH takes
place, while at the cathode within the catholyte chamber, the reaction
2H + 2e ~ ~ Hz takes place. A cell liquor containing from about 100
grams per liter to about 135 grams per liter of sodium hydroxide and from
about 160 grams per liter to about 200 grams per liter sodium chloride is
obtained.
The diaphragms prepared according to this invention typically have
a service life in excess of 8 months and frequently in excess of 1 year or
even longer, compared with a service life of 4 to 6 months for conventional
asbestos diaphragms.
~047441
According to another exemplification of this invention, a con-
ventional asbestos diaphragm may be treated with a water-soluble silicate
prior to or when actually showing signs of incipient deterioration.
Typically, such treatment will consist of brushing, rolling, spraying,
or pulling a water-soluble silicate onto and through the anolyte surface
of the diaphragm to provide thereon a coating of water-soluble silicate
of from about 0.01 to about 5.0 grams per square foot.
According to this invention, a diaphragm may be prepared having
a silicate on the anolyte surface by first preparing an asbestos diaphragm
i~ the conventional manner, and thereafter applying a solution of a water-
soluble silicate to the diaphragm. Thus, a diaphragm may be prepared by
dlspersing from about 0.5 weight percent to from about 2.0 weight percent
of chrysotile asbestos fibers in a cell liquor solution containing about
100 to 150 grams per liter of sodium hydroxide and about 150 to 225 grams
per liter of sodium chloride, and aging the asbestos in the cell liquor
for from approximately 1 day to about 5 days. Thereafter, the asbestos may
be applied to a cathode structure in the conventional manner, e.g., by
drawing a vacuum across the cathode and causing the asbestos fibers to be
deposited on the cathode.
Thereafter, the water-soluble silicate may be applied to the
asbestos diaphragm. A liquid composition of the water-soluble silicate
in water may be prepared. Typically, the liquid composition should contain
as much s'licate as possible for effective coating or si~ing of the surface
layers of the asbestos diaphragm and binding of the fibers together, but
still allowing adequate porosity for flow of the electrolyte through the
diaphragm. For example, the liquid composition may contain from about 0.01
to about 5 weight percent of the water-soluble silicate in water, or even
higher, for example, up to about 10 weight percent of the water-soluble
silicate.
1047~41
The solution may then be applied to the asbestos diaphragm by
various methods. Thus, the solution containing the water-soluble
silicate may be applied to the diaphragm by painting, brushing, spraying,
or drawing the solution through the diaphragmO
According to one preferred exemplification of this invention,
a small amount of air is drawn through the diaphragm while applying the
water-soluble silicate. It is believed that this serves to maintain the
porosity and electrolyte-permeability properties of the diaphragm.
According to still another exemplification of this invention, a
diaphragm may be prepared by first preparing a dispersion of chrysotile
asbestos in a dilute aqueous solution of a water-soluble silicate in water.
For example, a solution may be prepared containing 1 weight percent sodium
silicate in water. Then sufficient chrysotile asbestos may be added to
provide a dispersion containing approximately 1 to 2 weight percent of
chrysotile asbestos. Thereafter, a cathode structure may be inserted
., .
in the liquid containing the asbestos in a dilute solution of water-sol~ble
silicate and the asbestos and silicate caused to be deposited on the cathode
structure .
After applying the silicate to the chrysotile asbestos diaphragm,
the diaphragm may be treated to further set the silicate. This may be
accomplished by drying and heating the diaphragm, e.g., to from about 80
degrees Centigrade to about 140 degrees Centigrade. Alternatively the
diaphragm may be exposed to acidic anolyte liquor. The exposure to acidic
~nolyte liquor appears to convert the water-soluble silicate into an
amorphous, anolyte resistant, hydrated silica coating and bonding material.
The amorphous hytrated silica appears to protect the asbestos fibers from
the leaching action of the anolyte, i.e., the removal of magnesium from
the asbestos, and from the erosive action of the turbulent chlorine containing
anolyte.
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104744~
The following examples are illustrative.
EXAMPLE I
A diaphragm having a silicate-treated anolyte surface was pre-
pared and utilized in the electrolysis of brine.
The asbestos was drawn from a slurry of about 120 grams per liter
of sodium hydroxide and about 150 grams per liter of sodium chloride, and
2 grams per liter of G.A.F. Grade Hl-H2 chrysotile asbestos in water.
The liquid composition of sodium hydroxide, sodium chloride, and asbestos
had been aged for about 5 days at the time the diaphragm was drawn.
The diaphragm was then heated to about 90 degrees Centigrade for 19.5 hours.
The resulting asbestos diaphragm weight 0.50 pounds per square foot.
The cathode-diaphragm assembly was then treated with a sodium meta-
sillcate solution. First the dried diaphragm was wetted with 30 grams of
water per square foot. The sodium metasilicate solution containing l percent
by weight sodium metasilicate in distilled water was applied to the diaphragm.
The solution was brushed onto the asbestos diaphragm at the rate of l gram
of silicate per square foot with air being pulled through the diaphragm while
brushing.
Thereafter, the cathode and cathode chamber was joined to an anode
chamber having a titanium mesh anode with a Bishop A-l platinum coating.
An electrolyte containing 315 grams per liter of sodium chloride
was fed to the cell and voltage was established across the cell. Chlorine
was rapidly generated at the anode, and the pH of the anolyte solution rapidly
dropped to pH 2-3. Thereafter, chlorine was recovered from the anolyte chamber,
and hydrogen gas and a cell liquor product containing on a time average 160 to
180 grams per liter of sodium chloride and 100 to 130 grams per liter of sodium
hydroxide was recovered from the catholyte chamber. The range of voltage
_ g _
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drops across the diaphragm was calculated to be 0.53 to 0.78 volt with
a range of voltage drops across the cell of 3.31 to 3.45 volts.
After 4 days of electrolysis, the hydrostatic head across the
diaphragm was 6.1 inches of brine and the voltage across the cell was 3.31
volts. After 76 days of electrolysis, the cell voltage was 3.41 volts,
and the hydrostatic head was 7.4 inches. At this point, the cell was
shut down. The diaphragm was visually determined to be in satisfactory
- condition.
EXAMPLE II
~` Three diaphragms were sub~ected to side by side life testing in
substantially identical cells under substantially identical conditions.
The cells each had a 35 cubic inch capacity catholyte chamber
fabrlcated of 1/4 inch by 1-1/8 inch by 1-1/8 inch angle iron with a
10 gauge steel back and a 35 square inch iron cathode. Each cell also
had a 35 cubic inch capacity anolyte chamber fabricated of 1/4 inch
PVDC with a 1 inch PVDC frame and a 35 square inch Bishop platinized
tltanium anode. The anodes were spaced 1/8 inch from the cathodes by
"Neoprene" gaskets.
Three slurries were prepared. Each slurry contained 33.1 grams
of Johns-~anville 4K-12 chrysotile asbestos, 5 grams of Solka-Floc alpha-
cellulose, and 1600 milliliters of cell liquor of twelve weight percent
sodium hydroxide and twelve weight percent sodium chloride. Each slurry
was agitated for 30 minutes with a nitrogen stream and 20 minutes with an
ultrasonic generator.
The indlvidual diaphragms were deposited by inserting a cathode
in the slurry and drawing the slurry through the cathode screen at a vacuum
of one inch of mercury until the liquid appeared clear. The vacuum was
then pulled to 10 inches of mercury, and subsequently to 17 inches of
-- 10 --
:
~0474~1
mercury and maintained at 17 inches of mercury for 20 minutes. The dia-
phragms were then dried in air for 7 days.
Run A
One diaphragm was subsequently utilized in an electrolytic cell
without further treatment. After fourteen days of electrolysis at 190
Amperes per square foot, and an anode cathode gap of 1/8 inch, the dia-
phragm falled. The average cell voltage was 3.14 volts.
Run B
The second diaphragm was coated with 28.3 milliliters of a one
weight percent solution of sodium metasilicate by brushing and dried at
85 degrees Centigrade for 40 hours. After seventy-three days of electrolysis
at 190 Amperes per square foot, and a'n anode-cathode gap of 1/8 inch, the
current efficiency had dropped four percent, but no holes were observed.
The average cell voltage was 3.29 volts.
Run C
The third diaphragm was coated with Lithium Corporation of America
"Lithsil-S". "Lithsil-S" is a solution containing 1.9 weight percent Li20,
1.2 weight percent Na20, 18.9 weight percent SiO2, and the balance water,
and having a density of 1.187 grams per liter. The coating solution was
obtained by diluting 18.9 milliliters of the "Lithsil-S" solution with 100
milliliters of water. Twenty-eight and three tenth milliliters (28.3 ml)
of this ~olution were brushed onto the ~hird diaphragm. After twenty-
seven days of electrolysis at 190 Amperes per square foot and an anode-
cathode gap of 118 inch, the diaphragm failed. The average cell voltage
was 3 95 volts. I
EXAMPLE III
Sodium metasilicate was used to rejuvenate an asbestos diaphragm
in an operational electrolytic cell.
The operational electrolytic cell had an asbestos diaphragm that
had been pulled from an aqueous slurry of sodium hydroxide, sodium chloride,
~047441
and 2 weight percent Johns-Manville 3T-4T asbestos. The diaphragm was
uset in an electrolytic cell having coated titanium anodes. After twenty-
three days of electrolysis, the concentration of hydrogen in the chlorine
gas increased to a high level indicating deterioration of the asbestos
diaphragm. At this point one weight percent sodium metasilicate was added
to the brine feed. The hydrogen content of the chlorine gas returned to
normal and the cell voltage remained normal.
Although the present invention has been described with reference to
specific details of particular embodiments thereof, it is not intended thereby
to limit the scope of the invention except insofar as the specific details
are recited in the appended claims.
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