Note: Descriptions are shown in the official language in which they were submitted.
2766
~2~'~7~
IMPROVED DIMENSIONALLY STABLE
ASBESTOS-POLYTETRAFLUOROETHYLENE DIAPHRAGMS FOR
CHLOR-ALKALI ELECTROLYTIC CELLS
Diaphragms for electrolytic cells used
to produce chlorine, and sodium hydroxide or
potassium hydroxid from brine (hereinafter gen-
erally referred to as "chlor-alkali" cells) are
conventionally asbestos fiber mat structures sup-
: ported directly by the cathode of the chlor-alkali
cell~ Such asbestos diaphragms suf~er the serious
disadvantage of swelling under load, sometimes, for
~ 10 example, swelling up to 800 percent. Such swelling
; can result in filling the anode diaphragm gap, there-
: : by increasing cell voltage and subjecting the dia-
; : phragm itself to at~ri~ion by gas released at the
anode surface proximate ~o the swollen diaphraqm.
15~ The result of severe swelling of the asbestos dia-
; : phragm and of attrition caused by gas relea~ed at
the anode proximate the swollen diaghragm is to
limit the lifetime of such diaphrayms, used commer-
cially, to approximately six mGnths. '
Many attempts and proposals for overcoming
the problems of the conventional asbes-tos diaphragms
involve polymer modification of the asbestos sheets.
However, the majority of existing diaphragm-type
cells are of complex geometric design; accordingly,
the composite sheets are necessarily formed exterior
to the cell and cannot be employed without signi-
ficantly reducing the available diaphragm surface
area. Moreover, such composite sheet diaphragms
must be used in the filter press or "sandwich" type
cell design to be useful.
$~
ane -qpecific ~ug~esti~n, ~r ~vercomin~
the problems of asbestos diaphra~s, in~olves
impregnating a preformed asbestos di~phragm with
monomer or polymer and subsequently polymerizing
~he monomer in situ, or curing the polymer. How-
ever, such impregnation of asbestos fiber dia-
phragms results in the formation of continuous
polymer coating on the surface of the asbestos
fibers; the continuous polymer coating eliminates
the water permeability Qroperties of the asbes~os
fibers. .~oreover, impregnation techni~ues which,
by design or by accident, form a con~inuous skin
on one surface of t~e asbestos, render the asbestos
impermeable to the elec~rolv~.
Another proposal for overcoming th~ dis-
advantages of asbestos dlaphra~ms invol~es a pro-
cess which includes depositlng a diaphragm from a
slurry of asbestos fibers and polyte~rafluorethylene
fibers on a foraminous cathode and heat treating
the deposit to physically blnd it and to strengthen
the diaphragm. However, the cost of these die-drawn
polytetrafluoroethvlene fibers of relati~relY largP
diameter i3 excessive and excee~s the cost of fibrids
: as described in the ins~an~ inven~ion. Likewise,
the amount of such fibers reouired in the diaphraym
for a given level of o~eration exceeds the amounts
reauired when fibrids are utilized.
SUMMARY OF T~E INVENTION
The invention i5 directed to new and im-
proved electrodes coated with fiber~ e polytetra-
fluoroe~hylene (PTF~) modified asbestos diaphragms,
to the production of these diaphragms, and to use
:~2~7~
-- 3 --
these diaphragms in chlor-alkali cells.
The new and improved diaphragms of the present
invention include a foraminous substrate which is
electrically conductive which is coated with a random
mixture of asbestos fibers and polytetra1uoroethylene
fibrids (described below) and which is subsequently
subjected to temperatures effective to cause the fibrous
P~FE component in the coating to shrink and form an
interlocking matrix.
The new and improved diaphragm of the present
invention is dimensionally stable and exhibits
substantially less swelling during usa than prior
diaphragms. Power efficiencies of cells incorporating the
new and improved diaphragm of the invention are
accordingly superior to power efficiencies o~ conventional
asbestos diaphragms when used in chlor-alkali cells.
Significantly, new and improved diaphragms of the present
invention exhibit substantially increased lifetimes
compared to conventional diaphragrns used in chlor-alkali
cells.
Therefore, the present invention provides for a
process for making a diaphragm coated cathode for an
electrolytic cell, said diaphragm containing
polytetra1uoroethylene fibrid and asbestos fiber,
comprising:
forming an aqueous slurry of asbestos fibers and
polytetrafluoroethylene fibrids, wherein said
polytetrafluoroethylene fibrids comprise from about 1 to
10 grams/liter of said slurry, and wherein said
polytetrafluoroethylene fibrids before shrinking are
between 0.1 microns to 100 microns in diameter, by
s
- 3A -
dissolving a solid, granular, inert material intimately
admixed with polytetrafluoroethylene fibrids and mixing
same with asbestos fiber, said disxolving occurring ~efore
or after said mixing;
S depositing said slurry of asbesto6 fibers and
polytetraEluoroethylene fibrids on a foraminous
electrically conductive substrate to form a mat for a
diaphragm while removing the majority of the dissolved
solid, granular, inert material, and
heating said deposited slurry to dry said mat, and
thereafter ~eating sai~ dry mat to tempera~ures < 400
C., to ~hrink and/or fuse said polytetrafluoroeth~lene
fibrids in said dried mat to form a matrix holding said
asbestos fibers in a polytetrafluoroethylene fibrid and
asbestos fiber containing diaphragm, said fused deposit
containing polytetrafluoroethylene fibrids comprising from
S to 25 percent by weight of said diaphragm.
The present invention also provides for a process
for making a diaphragm coa~ed electrode for an
electrolytic ~ell, said diaphragm containing a
polytetrafluoroethylene fibrid and asbestos fiber,
comprising:
forming said polytetrafluoroethylene fibrids by
com~ining particulate polytetrafluoroethylene with a fibrid
inducing su~strate, wherein said fibrid inducing substrate
comprises a solid, granular, inert material which is
easily separated from polytetrafluoroethylene fibrids~ and
subjecting said suspension to a compressive shearing
action at a temperature between 20 C. and 250 C.;
- 3B - ~
forming an aqueous slurry of asbestos fibers and
polytetrafluoroethylene fibrids in said fibrid inducing
substrate and dissolving said fibrid inducing substrate;
depositing said slurry of asbestos ibers and
polytetrafluoroethylene fibrids on a foraminous
electrically conductive substrate to form a mat for a
diaphragm while rernoving most of said dissolved fibrid
inducing substrate; and
heating said deposited slurry to dry said mat, and
thereafter heating said dry ma~ to temperatures ~ 400
C., to shrink and/or fuse said polytetrafluoroethylene
fibrids in said dried mat to form a matxix holding said
asbestos fibers in a polytetrafluoroethylene fibrid and
asbestos fiber containing diaphragm.
DETAILED SUBSCRIPTION OF THE INVENTION
The new and improved diaphragms of thè present
invention are produced by depositing a random mixture of
asbestos fibers and polytetrafluoroethylene fibrids (i.e~
fibrous material of various lengths and about 0.1 microns
to about 100 microns in d.iameter, which are distinct and
well separated and which are produced by a shearing action
on fibrillatable polytetrafluoroethylene as differentiated
from a fiber made by die-drawing) onto an electrically
foraminous substrate, and
~2~7'~
heating the deposit to temperatures sufficient to
fuse the deposit and to shrink the deposit. A
product of the present invention, resulting from
this process, can accordingly be described as a
foraminous electrode coated on its electrically
active surface with a porous~ fused, coherent, ad-
herent, dimensionally stable deposit of a random
mixture of asbestos fibers and polytetrafluoroethyl-
ene 1brids. The fused deposit contains polytetra-
fluoroethylene fibrids in an amount of at least 5
percent by weight and up to about 25 percent by
weight, based on the weight of the fused deposit.
The fused deposit may contain other fibers and
fibrids, in addition to those specified; for example,
the fused deposit may also contain conventional poly-
tetrafluoroethylene fibers, made by die-drawing.
The random mixture of asbestos fiber~ and
polytetrafluoroe~hylene fibrids can be prepared by
first forming the polytetrafluoroethylene fibrids
and then admixing the fibrids with the asbestos
fibers. Polytetrafluoroethylene fibrid formation
involves subjecting particulate polytetrafluoro-
ethylene to shear conditions. The particle dia-
meters of the particulate polytetrafluoroethylene
may range from about 0.01~, or less, to about 50~,
preferably between about 0.1~ to about 0.5~. The
particulate polytetrafluoroethylene can be either
in the form of a wet (water) dispersion or dry
powder. The concentration of polytetrafluoro-
ethylene in a water dis~ersion should be a concen-
tration sufficiently high to facilitate fibridformation as ~olytetrafluoroethylene fibrid ini-
tiation becomes difficult at extremely low, concen-
trations; but the concentration must be low enough
7~
to obviate large clump formation, By way of ex-
ample, it is noted that PTFE fibrid formation has
been induced in aqueous dispersions containing as
low as 1 percent by weight particulate polytetra-
fluoroethylene to concentrations of about 30 per-
cen~ by weight.
Certain commercially available products
contain particulate polytetrafluoroethylene parti-
cles having diamet~rs ranging up ~o about 0.5~ re-
quired for the production of fibrids. For example,both Fluon~CDl, sold b~ Imoerial Chemical Indus-
tries Ltd., and Teflon~30B, sold by E. I. duPont
deNemours ~ Co., have been found to be quite suit-
able, both contain about 60 percent solids in a
water dispersion, including about 6 percent wetting
agents, based on the weight of solids. These dis-
persions are described in U.S. Patent No. 4,047,537,
These
dispersions are hydrophilic, negatively charged
colloid dispersions, containing particles having dia-
meters preferably of about 0.05~ to about 0.5~, sus-
: pended in water Another commercially available
produc~, Teflon ~ype T-6 (also sold ~y DuPont) can
also be used to form the fibrids of the invention,
it is a powder agglomerate produced from Teflon 30B.
The primary diameters of particles of polytetrafluoro-
ethylene in the DuPont Teflon 30B and T-6 dispersions
average from about 0.2~ to about O.S~ , while powder
agglomerates sf the T-6 powder average about 50C
microns.
~IL2~ 5
Formation of the polytetrafluoroethylene
fibrids is effected by suspending a particulate
fibrid inducing substrate in a mass or dispersion
of particulate polytetrafluoroethy~ne and sub~
jecting the polytetrafluoroethylene particles to
shearing forces to form fibrids of polytetrafluoro-
ethylene. The fibrid inducing substrate comprises
coarse particles of suitable materials. The mate-
rials used as the fibrid inducing substrate are sub-
stantially physically and chemically inert to theparticulate polytetrafluoroethylene. By physically
and chemically inert, it is meant that the substrate
material will not absorb the polytetrafluoroethylene
dispexsion and will not chemically react with the
15~ polytetrafluoroethylene. The materials used as
the fibrid inducing substrate include any solid gran-
ular inert material which is easily separated from
the fibrids. Suitable fibrid inducing substrate
materials include alumina, limestone, salt, sugar,
sand, and graphite. Preferably salt that is pul-
verized sodium chloride, is used. Coarse particles
of suitable materials may be illustrated by noting
that the diameters of particles of particulate
alumina, when used as the fibrid inducing substrate,
usually range from abou~ 1~ to abou~ 800~, and pre-
ferably from about 100~ to about 200~.
AsbPstos fibers are admixed with the poly-
tetrafluoroethylene fibrids after fibrid formation.
These asbestos fibers may be any product used to
r~s
form conventional asbestos mat diaphragms,
Generally, in the e~periments repor~ed below,
asbestos fibers of standard length combinations
are employed, based on the Quebec Standard for
length. For instance, a standard length combina-
tion which can be used in accordance with theinvention comprises two parts short asbestos
fibers to one part long asbestos fibers. For
example, a mixture of VAG ~2 short fibers having
lengths xanging from 1/32 inch to 1 inch with an
average length of 1/4 inch and of VAG #l long
fibers having an average length of 1/2 inch may
be employed. Asbestos fibers are not generally
used as the fibrid inducing substrate.
Shearing conditions which affect fibrid
formation include the time, the temperature and
the shearing force applied to the mixture of par-
ticulate polytetrafluoroethylene and substrate.
The temperature of the shearing step is a tempera-
ture sufficient to render the polytetrafluoroethyl-
ene sufficiently plastic to form fibrids. The time
duration of the shearing action is temperature de-
pendent, and thus the polytetrafluoroethylene will
be maintained at the temperature of ~he shearing
step for time sufficient to allow subs~antial fibrid
formationO The temperature during the shearing
step may range from about 20C up to about 250C,
preferably from about 60C to about 200C. Most
preferably, the polytetrafluoroethylene is heated
to a temperature of from about 75C to about 100C
during the shearing step.
3~ 7~
The shearing action used to form the
polytetrafluoroethylene fibrids is generally a
compressive shearing action obtained by mulling
or stirring. Various means may be employed to
effect a compressive shearing action, including
a spatula and beaker, a mortar and pestle, ribbon
blade, a small kall mill, a double screw blender
and a Banbury~mixer or a Hobart~mixer. The re-
sult of the shearing action is the production
of fibrids which may be highly branched or sing-
ular fibers or a mixture of both. These fibridsare composed of poly~etrafluoroethylene particles
having diameters of from about 0.1~, or less, up
to about 100~. The leng~hs of the fibrids is
not critical; the fibrids of experiments reDorted
below are generally less than about one-half inch.
After polytetrafluoroethylene fibrid forma-
tion, a random mixture of the ~olytetrafluoroe~hy~
ene fibrids and the asbestos fibers is deposited on
the foraminous electrically conductive subs~rate.
This has been done by forming an aqueous slurry o
the polytetrafluoroethylene fibrids and asbestos
fibers in water, in cell liquor, in caustic, in a
salt solution or in admixtures thereof. The poly-
tetrafluoroethylene fibrid content of the slurry
can be quite variable ranging from about 1 to about
10 grams per liter of slurry volume. The slurry is
applied to the foraminous substrate by gravity feed
and/or by vacuum applied downstream from the site of
deposit. For example, the foraminous electrically
conductive substrate can be disposed in a vacuum fil-
tration funnel; vacuum facilitates removal o~ water
from the depoæit and matting of the deposit. There-
after the mat is dried.
~ ~ \
~2~37~
The electrically conductive foraminous
substrate is a metal mesh or a metal alloy mesh.
In other words, the substrate is a mesh electrode.
The mesh sizes of the substrate are not critical.
Below, a 6-mesh electrode or perforated screen,
specifically a mesh cathode, is described in the
examples. However, chemically stable metallic mesh
electrodes having in excess of 8 mesh to the linear
inch and width openings of less than 0.06 inch have
been used in chlor-alkali cells~ In the embodiment
of the invention which is directed to chlor-alkali
cells equipped with diaphragms of the invention, the
foraminous substrate may be any cathode currently
used in chlor-alkali cells. For example, mesh
cathodes, wire cathodes, or Ryerson cathodes (per-
forated steel plate) may be used.
After drying the PTFE fibrid-asbestos
fiber deposit at a temperature of about 100C,
it is heated to temperatures sufficient to fuse
the polytetrafluoroethylene of the deposit. Fusion
occurs at temperatures around the melting point of
polytetrafluoroethylene (327 ~ 10C). Preferably,
however, the temperature of fusion is at least
about 340C. Generally, fusion is undertaken at
temperatures ranging from about 340C ~o about
370C for about cne-quarter hour to two hours.
Although temperatures as high as 400C can be uti-
lized with appropriate shortening of the time, temper-
atures above 380C should be avoided as degradation
of the polytetrafluoroethylene starts at about that
temperature and interferes to a degree with the
effectiveness of the process.
7~S
As a result of fusion, the polytetrafluoro-
ethylene fibrids form a recticulate or matrix con-
figuration and shrinks. The network or matrix acts
to hold in or enclose asbestos fibers for improved
dimensional stability. As a result of shrinkage
S during fusion, the diaphragm is made more porous.
The increased porosity of the diaphragm so produced
reduces the electrical resistance in an operating
cell and results in consequent power savings.
The following examples are presented to
10 illustrate the invention and specific embodiments by
which the invention can be effected.
EX2~1PLE 1
To demonstrate the suitable need for fibrid
inducing substrate, CaCO3 powder (Fisher ~-20 microns)
15 was used in the following manner: To 98 parts by
weight of the CaCO3 was added 3.3 parts by weight of
a 60~ solids PTFE dispersion (Teflon 30B). Shearing
was app~ied by mulling in a mortar and pestle at 80C
for 10 min. The CaCO3 was then removed from the mix-
~ tuxe by leaching wi~h dilute HCl, and the resulting
fibrid residue was washed and then dried at 100C
for l hour to yield about 2.0 parts of Teflon fibrids.
The slurry used for deposition of the dia~
phragm consisted of the following components:
3.6 g fibrids made as above
~.6 g ~2 VAG Short Asbestos fibers
4.8 g #l VAG Long Asb~sto~ fibers
0.1 g of NOPCO PE 260, a no~-ionic dis-
persant, all in l liter of cell liquor
(11% NaOH and 16% NaCl solution).
A11 the above were then stirred vigorously with a
dispersator at moderate speeds for about 5 minutes.
The diaphragrn is constructed by taking an
aliquot portion, approximately 360 milliliters, and
35 passing it by gravity over a 6 mesh cathode (0.093l'
:~LZ~ 7~
.
steel wire calendared to a thickness of 0.155") cen-
tered in a 450 ml filtration funnel. A vacuum is
applied to the suction flask ranging from 0-2.5" of
mercury for about S minutes and gradually increased
over a five minute interval to 17" ~g vacuum and then
S holding for drying the deposited diaphragm for a
period of 10 minutes. ~he diaphragm was next heated
to 100C for one hour for additional drying, and
fused at 350C for one hour. The r~sulting ~at had
a den~ity of 1.25 g/sq inch (0.002755 lb/sq.in.) and contained
20% by weight of PTFE fibrids.
When this 3 sq inch assembly of diaphragm
a~d cathode is installed in a laboratory chlor-alkali
cell t it demonstrated yood dimensional stability with
a very good brine head and permeability (a~ judged
by the flow ratç through the diaphragm of .185
mls/min/sq inoh ~0.0065 ounce~/min./sq.in.3). Conditions o opera-
tion were 93C with the electrodes }:eing separated at ~" apart .
Voltage w~s measured and ~ound to be exoellent at 2.98 v~lts at one asi.
These results should be com~ared with an asbestos dia-
phragm made in a conventional way but containing nofibrids, in which the diaphragm showed instability,
lasting less than 2 days at 3.23 volts (see Table 1).
Compare also with a dia~hraom made with 2$~ of Teflon
fibers (6.6 de~ier) with the asbestos firbers, all other
conditions being the same; stability and voltage are
about the same, showing an equivalency of pexformance
using Teflon f.ibers and 20~ PTFE fibrids. See Table
1 for other examples.
EXAMPLE 2
To demonstrate the use of another fibrid
inducing substrate, 98 parts of granular NaCl (ap-
prox. 50 mesh) were added to 3.3 parts of a 60%
Teflon 30B dispersion. Fibrids were formed by mull~
7~5
12
ing in a mortar and ~estle for 30 minutes at 21C,
followed by continuous mulling at 130C for 3 minutes.
The salt substrate was removed by leaching with water,
washed and dried as above. A diaphragm was formed in
the same manner as above. It was dried and fused as
above. The resulting diaphragm had a mat density of
1.17 g/sq inch and contained 15~ by weight of fibrids.
It had good performance parameters as set forth in
Table 1.
EXAMoeLE 3
To demonstrate the use of other equipment
to form fibrids, a ~ower muller was ased. A 2 wheel
Cincinnati brand muller was used, with 1~1/2" wide
8" diameter wheels in a 12" pan. Fibrids were made
with a granular salt substrate at 21C for 40 minutes,
with a 1 kilogram mass on the wheels~ using about a
1 kilsgram charge. The fibrids were recovered in the
same manner as described above and a diaphragm made
in exactly the same manner as above, including the
drying and fusing. The resulting diaphragm had a
density of 1.22 g/sq inch and containedl 15~ fibrids
by weight. The diaphragm performance was comparable
to Examples 1 and 2, as set forth in Table 1.
EXAMPLE 4
Exactly the same procedure as in Example
3 was repeated, except that the mass was dried before
mulling. This yielded a diaphragm with very similar
parameters as above with a mat density of ,1.17 and
contained 15% by weight fibrids~
,
7~
13
Table 1
A Comparison of Asbestos 3iaphragms Modified
_y the Addition of Fi~rids in Chlor-Alkali Pexformance
Brine Days
Ex. % Head Volts NaO~ Current on
No. Fibrids Substrate ~inches) 1 asi q/l Eff.~ Stability Line
. . _ _
CaC03 3 2.98 124 92 Stable ~14
2 15 NaCl 3-1/4 3.04 128 92 Stable ~29
3 15 NaCl 5-1/2 3.03 128 93 Stable >61
4 15 NaCl 6-1/4 3.02 131 92 Stable >52
* 25 -- 2-1/2 2.98 130 93 Stable ~20
** none -- 3-1/2 3.23 130 93 Unstable < 2
Notes: * 25% Teflon fibers, 6.6 denier, all conditions
standard as per Example 1.
** No fibex or fibrid binder, made in the conventional
manner.
EXAMPLES 5 through 10
The diaphrc~ s of the following examples were
used using fibrids made from DuPont Teflon T-6 (the
solid agglomera~e particles made by evaporation of
Teflon 30B dispersion) and DuPont Teflon 30B (dis-
persion of PTFE particles). The performance results
of these 5 diaphragms are set foxth in Table 2.
EXAMPLE 5
-
Fibrids were made by using 2% Teflon solids
from Teflon K-20 (6.6g of a 30% Teflon solids disper-
sion) with 98% granular salt, heating to 130C for
one-half hour and then mulling the wet mix with a
spatula in the beaker for about 3 minu~es to induce
7~
fibrid formation. ~ibrids were recovered by leaching
out the salt with water, washing and drying. The
slurry mix was made by the formula of ExaMple 1,
sheared by disperator action for 3 minutes and a
diaphragm was deposited as before. The diaphragrn was
dried and fused as above. The resulting diaphragm had
a density of 1.00 g/sq inch and contained 25~
fibrids. Performance parameters were similar to those
described above.
EXAMPLE 6
In this experiment, Teflon type T-6
particulate PTFE powder was used. Twenty-five parts
of Teflon type T-6 and 7S parts of granular salt ~ere
added to a mortar. The mix was mulled with a pestle
for 60 minutes at 21C. The fibrids were recovered
by leaching out the salt; the fibrids were then w~shed
and dried.
Asbestos, the fibri~s made above, Teflon 6.6
denier fibers, and a small quantity of dispersant were
added to cell liquor. The resulting diaphragm, made
in the manner previously described, was dried and
fused at 350C Eor one hour. The resul-ting
diaphragm had 7% fibrids, and 15% Teflon fibers with a
mat density of 1.15 g/sq inch~ When the diaphragm was
installed in a chlor-alkali cell, the voltage,
concentration and current efficiency all showed
superior characteristics as set forth in Table 2.
EXAMPLE 7
In this experiment the use of a ball mill and
mulling was demonstrated. Eigh~ parts of Teflon 30B
(a ~0% solids dispersion) and 92 parts oE granular
salt, were ball-milled for 2 hours at 21C, and
7~
removed. The mixture was then preheated in an oven
to }-30c and mulled for 5 minutes to foL~ the com-
ple~ed fibrids. The fibrids wer~ recovered and dia-
phragms were made in the manner described above.
S The resulting diaphragm had a density of 1.21 and
contained 20% flbrids. The superior operating para-
meters obtained as set forth in Table. 2.
EXAMPLE 8
This expeximent illustrates the use of both
shearing equipment as wPll as the use of particulate
Teflon type ~6.In an automated power mortar and pes-
tle were added 5 parts of Teflon T-6 and 95 parts
granular salt, and mulled for 40 minutes at 21C.
The recovered fibrids were admixed with asbestos and
Teflon 6.6 denier fibPrs as described abo~e. The re-
sulting diaphragm made in the manner described above
had a density of 1.22 g/~q inch and contained 7%
fibrids and 15~ Teflon fibers. The superior opera-
ting parameters obtained are set forth in Table 2.
EXAMPLE 9
.
This experiment further demonstrates the
use of a power muller, such as the automated mor~ar
and pestle to make fibrids from Tefloh type T-6.
Two parts T-6 and 98 parts of granular salt were
placed in a power mortar and pestle. This mixture
was mulled at ~oo~ ~am~xatur~ for 4Q minutes and
then at 85C for } rinute. The resulting fi~brids
were made into a diaphragm wi~h asbestos fibers in
the manner described above ~hich had a density of
1.15 g/sq inch and contained 15% fibrids. The
superior operating parameters obtained are set forth
in Table 2.
~77r~
16
EXAMPLE 1 0
The procedure of Example 9 was repeated
except that the density was increased a little
at 1.22 g/sq inch. The superior operating para-
meters obtained are set forth in Table 2.
Table 2
Comparison of Asbestos Diaphragms Modified by
the Addition of Fibrids and Teflon Fibers
Made in Various Different Ways in
Chlor-Alkali Cell Performance
Brine Days
Percent Head Volts NaO~ ~xent on
E~le Fibrids Substrate (inches) 1 asi* ~ Eff.% S~ility Line
NaCl 1-1/2 3.01 129 91 Stable >29
6 7%+
15~ NaCl 2-1/4 3.02 129 93 Stable jl3
f~s
7 20 NaCl 4-1/2 3.05 130 94 Stable >51
8 7
15~
fikers NaCl 3-1/2 3.06 131 93 Stable >60
9 15 NaCl 4-V2 3.09 130 94 Stable >91
ld 15 NaCl 4 3.03 131 94 Stable >100
* amperes per square inch
The followinq exam~lss show th~ processin~ of
fibr'ds by pilot pla~t ~ca~e equipment.
;
s
17
EXAMPLE 11
3 . 59 pounds of salt were pulverized to
100 mesh by a hammermill,added to .~ pound of a
Teflon type 30B dispersion (60% Teflon solids dis-
S persion) and mixed dry. This is a 4% Teflon fi-
brid blend. The mixture was then compression
sheared in a 4 pound ~anbury brand mixer for 13
minutes with a partial ram pressure. The result-
ing fibrids were re~overed in the usual manner,
washed and dried, and diaphragms prepared in the
same manner as in Example 1, from an approximately
18 grams/liter slurry from cell liquor, again in
the usual manner. The density of the diaphragm was
1.30 g/sq inch with a fibrid content of 15% . The
superior operating parameters obtained in a chlor-
alkali cell are set forth in Table 3.
EXAMPLE 12
This is another example of pilot scale
fibrid production. A 5~ blend of Teflon 30B solids
(from a 60% Teflon solids dispersion) of 100 mesh
salt was processed in a Banbury brand mixer for 9
minutes as above with partial ra~. pressure. The
fibrids, recovered in the usual manner, were added
to the cell liquor with asbestos fiber and air/vacuum
2S agitated for 1 hour. The resulting diaphragm had a
density of 1.25 g/sq inch and a fibrid content of
15%. The superior operating parameters obtained in
a chlor-alkali cell are set forth in Table 3.
EXAMPLE 13
This experiment illustrates a va~iation
in the processing of fibrids. Teflon 30s was pre-
mixed with 100 mesh salt, as an 8% blend, in a
~z~q77rlls
18
ribbon blender at80-90C for 30 minutes. No
Banbury mixer was used. The diaphragm was made
in the manner described above and contained 20~
of the fibrids as recovered fxom the ribbon blender.
The superior operating parameters obtained in a
chlor-alkali cell are set forth in Table 3.
EXAMPLE 14
This experiment again is a variation in
processing equipment and conditions. Teflon 30B
was premixed with 100 mesh salt, as a 4% blend, in
a 1 cu ft. ribbon bl.ender at 80-90C for 45 minutes.
It was compression sheared in a Banbury ~ixer
for 13 minutes as described in Example 11. The fi-
brids salt mixture was dissolved in water, and the
major portion of the salt water was removed to yield
a saline slurry containing about-3.8 grams/liter of
fibrids. About 250 mls of this slurry (2.2 g of
fibrids) was added to 500 mls of!water, 250 mls of
standard cell liquor, 15.8 g of asbesto~ fibers and
mechanically sheared by a dispersator. A stable dia-
phragm was made from this slurry in the usual mannerand had a density of 1.20 g/l with 12~ fibrids. Its
performance in a chlor-alkalicell was au~ted byusing
approximately half the normal gap between electrodes
as well as the use of a porous nickel-coated steel
cathode to yield an unusually low voltage (2.64
volts vs. a normal 3.00 volts -- 1 asi).
.,
19
a~ a) a~ nD ~ ~ ~
rn , ~, ~ Y
~) ~
~:;rn r~ r~ r~ ,I rJ
~c
~ o o o o
S~ ~ ~ ~ ~ co
nJ ~ ~ ~ ~1 c~
~; ~i N N
S æ~
3 ~ ~
~ o ~ ~ o ~ ~
~-rl ~
O
n W .~
nJ O P~ ~ 1~ U In ~r ~ N
ro ~-~1 ~ cr~
o
rn r~,~
n ,S:I ~ h ~ ~ o o
o
r~
;~ rn-~
o ~ r~n ~ ~¦ ~ ~ ~D.
O ~ ~1
~:~,1 n~
0 ~
rl ~ O ~D
~ rn ~ O
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~0
rn
~2~77~
Thus it ls apparent that there has
been provided, in accordance with the invention,
a diaphragm coated foraminous electrode that fully
satisfies the objects, aims, and advantages set
forth above. While the invention has been
described in conjunction with specific embodiments
thereof, it is evident ~hat many alternatives,
modifications, and variations will be apparent to
those skilled in the art in light of the foregoing
description. Accordingly, it is intended to embrace
all such alternatives, modifications, and variations
as fall within the spirit and broad scope of the
appended claims.
,
