Note: Descriptions are shown in the official language in which they were submitted.
'7t~
This invention relates to a cell frame for use in making
electrolytic cells and assemblies thereof. More particularly, it
relates to a molded synthetic organic polymeric electrolytic
cell frame having molded-in parts and passageways for holding cell
elements, feeding and removing cell fluids and aligning and fastening
the cell frames together in assemblies.
For the manufacture of chlorine, sodium hydroxide and hydrogen
from aqueous sodium chloride solutions electrolytic cells employed
have in the past usually been of either the-diaphragm or mercury cell
types. In recent years, due to increases in the cost of mercury
; and pollution problems attending its use, diaphragm cells have been
of increasing importance. Most such cells have in the past been made
of concrete but the use of liners of various polymeric materials,
including polyvinyl chloride and polypropylene, has been suggested.
With the advent of dimensionally stable anodes and effective
permselective membranes it has now become possible to increase the
productivity of membrane cell assemblies. Because of the possibility
of using flat electrodes with a thin separating membrane between them the
manufacture of thin cell assemblies has become feasible. Accordingly,
massive cell wall constructions are no longer necessary or desirable
and efforts have been made to discover structural materials which can
be made into satisfactory electrolytic cells which are thin, readily
manufactured and capable of withstanding electrolysis condit10ns, reactants
and products. Efforts have been made to machine cells and cell frames
~5 from synthetic organic polymeric materials but this has often been expensiveand the product made has not been resistant to electrolytic conditions,
possibly because of strains developed in the plastic during machining or
because of incipient weaknesses created thereby. Machining of filled
polymers exposes surfaces of the filler and this may be undesirable for
long term electrolytic applications, especially in the presence of electrolyte
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; and cell products at elevated temperatures or when the temperature
varies over a range which is sufficiently wide to cause expansions
and contractions which can develop weaknesses in the filled polymer
surface where the filler is exposed.
Polypropylene has been utilized as an acid-resistant and caustic
resistant material of construction and has been filled with various
high temperature resistant materials, such as talc, mica and asbestos but
has not previously been employed for the manufacture of molded frames of
the present type for joinder together to form electrolytic cell assemblies.
Such prod~cts are comparatively simple to make, assemble and use and are
of long life, with the frames being highly resistant to checking, warping,
expanding, contracting, creeping and other distortions during the electrolysis
of aqueous sodium chloride solutions.
In accordance with the invention an electrolytic cell frame is a
synthetic organic polymeric frame for housing an anode and a cathode and
for supporting a membrane, which frame, in coniunction with a matching
contiguous frame containing an electrode of opposite sign to that nearer to
it in the present frame, forms an electrolytic cell having an anode, a
cathode and a membrane between them, said frame including inlets and
outlets for electrolytic process fluids to anolyte and catholyte
compartments so that the cell formed from two contiguous frames has at
least one inlet to the anolyte compartment, at least one outlet from the
anolyte compartment, at least one inlet to the catholyte compartment and
at least one outlet from the catholyte compartment, each of which inlets
and outlets is communicated with respective headers or manifolds integral
with the cell frame. In preferred embodiments of the invention the synthetic
organic polymer is polypropylene containing a filler such as asbestos,
mica, calcium silicate, talc or mixtures thereof, the frame is injection
molded, includes integral external alignment and fastening means for holding
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a plurality of frames together in a cell assembly and includes molded-in
mounting means for holding various cell parts in desired positions. Also,
within the invention are methods of electrolyzing, utilizing electrolytic
cells incorporatdng the present framesi methods for making the frames; and
intermediate mounting means or frames for electrodes to hold them in
desired positions in the present frames.
The invention will be readily understood from the description thereof
herein, taken in conjunction with the drawing in which:
FIG. 1 is a perspective view of a cell assembly incorporating a
plurality of frames of this invention;
FIG. 2 is a partial cutaway vertical sectional view of a pair of
frames of this invention and parts of other frames adjacent to them, with
two complete frames being combined to make an electrolytic cell;
FIG. 3 is a partially cutaway sectional elevational view along plane
3-3 of FIG. 2; and
FIG. 4 is a partial vertical sectional view of the T-shaped rectangular
"picture frame" intermediate mount for joining an electrode to the polymeric
plastic frame of this invention.
In FIG. 1 electrolytic cell assembly 11 comprises a base 13, individual
cell frames 15 mounted together with gaskets (not shown) between them for
sealing purposes, compression front and rear end plate members 17 and 19,
respectively, horizontal side supporting bars 21 and 23, held to base 13
and supported above it by vertical members 25, 27, 29 and 31, and rods
33, 35, 37, 39 and 41, each of which is threaded at both ends thereof
and each of which has tightening nuts thereon for pressing the end plates
together and thereby holding the frames together in fluid-tight relation-
ship, usually with the aid of the gaskets between them. Threaded ends
; 43 and 45 and tightening nuts 47 and 49 on rod 41 are illustrated and it
" 10~7699~
is evident that the other rods contain similar threaded ends and
tightening nuts. A roller which functions as a bearing to facilitate
sliding movement of plate 17 is identified by numeral 51. Plates
17 and 19 contain passages therein or connected to them for holding
the tightening rods in position and for transmitting tightening forces
to the plates. Cell frames 15, the structures of which will be further
illustrated in FIG'S. 2 and 3, contain external alignment means 53, which
comprises external members integral with the frames and molded (at 54)
to fit and rest on support bar 23 to facilitate assembly together of the
cell frames, which will be described later, in conjunction with FIG'S.
2 and 3.
In FIG. 2 wherein the combinations of cell frames to form electrolytic
cells are shown without the external parts thereof, except for electrical
connectors, and from which, for the sake of clarity, upper inlet and outlet
passages and manifolds are not illustrated (they are shown in FIG. 3),
cell frames 15 are held together, with gasket 61 or a plurality of gaskets
between them holding them in fluid-tight contact, thereby forming an
, electrolytic cell when electrodes and an interposed membrane are in place.
'~ Frames 15 have walls 63 which bound the cell created by joinder of the
frames and prevent leakage into the next adjoining cells, 65 and 67. Cell
; 69, formed by joinder of frames 15, includes anode 71, cathode 73 and
membrane 75, shown herein held to the inner, inactive face of anode 71.
Anode 71 is electrically and mechanically (by welding) connected to conductor
77 and cathode 73 is similarly connected to conductor 79. Both such
conductors are positioned substantially vertically and have upper ends
thereof passing through open~ngs 81 and 83, respectively, in the frames
and into contacts with connectors 85 and 87, through which electricity
is fed to cell 69 from the cathode conductor of cell 67 and is transmitted
from cell 69 to the anode conductor of cell 65. Sealing means 89 and 91
are provided to make a fluid-tight seal to prevent gas or liquid loss.
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As is shown for the anode conductor exit from cell 65, the seal
comprises a plurality of resilient neoprene washers or 0-rings 93 which
are compressed together against a ledge 95 by tightening of a cap 97
through threaded collar 99 against force transmitting rigid washer 101.
Thus~ the washers or 0-rings or fasteners of other suitable types,
such as wedges, are compressed against the anode conductor and hold it
tightly in place, while preventing fluid loss from the cell.
Conductors 77 and 79, and through them electrodes 71 and 73, are
supported in place by supporting brackets 103 and 105, which are fastened
by screws 107 and 109 to molded-in bosses 111 and 113. The conductors
may be welded or otherwise positively held to the brackets for additional
support.
As illustrated, cathode 73 fits into a prepared opening in the frame
at the cathode ends and is additionally supported by the pressure of the
gasket against it when the cell frames are tightened together. To improve
the rigidity of the anode, it is mounted on an angle frame or series of
angle brackets 115, which frame is held in place with respect to body
frame 15, by suitable means, including screws, cements, pressure holding by
gasket 61, etc. The use of the angle 115 or a T-shaped "picture frame" type
of inter~ediate mounting means, such as is illustrated in FIG. 4 and which is
interchangeable with the present angle, serves to provide additional strength
and prevents undesirable flexing of the anode material.
Membrane 75, in the present embodiment shown held against anode 71,
is maintained in position by being sealed in place between angle 115,
frame portions 117, 119 and 121 and also in suitable channels or grooves such
as those at 123 and 125 holding members 127 and 129, held in place on
frame 15 by screws. It will be noted that in the illustration given
the membrane is not perfectly vertical at all locations but is pressed
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107~9~
against by the anode to help tighten it in place. It is important that
the distance between the electrodes be maintained constant for best
electrolytic effects and it is also important for the membrane to be
held tightly against the anode to prevent chlorine gas generated at
the active side of the anode, away from the membrane, from collecting
between the anode and the membrane. Similarly, the membrane-cathode
distance should be maintained constant to prevent interference with
the removal of hydrogen gas generated and allow free flow of electrolyte.
It will be evident that in other embodiments of the invention, wherein
the membrane is positioned between the two electrodes with clearances
between the membrane and the electrode, it may be desirable to have the
active side of the anode facing the membrane or to have both sides of the
anode active or when the membrane is to be placed against the cathode
the structure employed may be modified accordingly.
An important feature of the present invention is in the molding-in
of the various means for adding and removing fluid from the electrolytic
cells produced, so that it is unnecessary to drill or bore such passageways
in the frame. In FIG. 2 the only such passageways illustrated are for
feeds to the anolyte and catholyte compartments and the anolyte feed
header or manifold. Other feed headers and discharge passageways and
headers are shown in FIG. 3. Anolyte header 131, molded into frame 15,
communicates through passageway 133 with anolyte compartment l35 of cell
69. At the end of passageway 133 nearer to the anolyte compartment an
orifice plug 137 is screwed into place in previously molded-in threads
in frame 15 or is otherwise fastened therein so as to restrict the
passageway to the orifice shown at 139. In a similar manner, a
catholyte feed manifold 134 (FIG. 3) communicates through passage 141
with catholyte compartment 143 and is stopped down by orifice 145.
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Use of the orifices, which are interchangeable with others of different
sizes, allows regulation of flow rates into the cell.
In FIG. 3 cell frame 15 and cell 69 include an additional optional
feed manifold 147, chlorine outlet manifold 151, hydrogen outlet manifold
149, caustic soda solution take-off header 153 and depleted anolyte take- ~
off header 155. It will be noted that overflow passageways 157 and 159 -
carry the respective catholyte and anolyte effluents to the headers from
anolyte 161 and catholyte 163 (behind the membrane). Frame lS also
contains optional anolyte feed header 167 and optional catholyte feed
header 165, both of which may be connected by channels or passages, not
shown, to the cell interior. Such passages may be drilled in the frame if
it is desired to use feeds from the top of the cell or may be molded in
and plugged when not in use. Similarlyj if it is desired not to use
bottom feeds, passages 133 and 141 may be plugged. Manifold 147 is useful
when one wants to employ a plurality of membrane, forming a buffer compartment,
to which feed is to be added. As was mentioned with respect to the other
optional headers, a passageway may be opened up or plugged, according to
the desired use.
Although not specifically illustrated, it is within the invention
to employ "tortuous passage" feed and outlet streams so as to increase
the resistance of any stream through which current leakage might occur.
Thus, with respect to feeds of liquids through orifices, because of the
smaller streams the resistances thereof will be greater and current
leakages will be diminished. This will be especially effective with
respect to the upper feed locations when orifices are employed which
are small enough to produce droplets of feed liquids which are discontinuous
,'
as they fall through the chlorine and hydrogen gas phases above the
electrolyte in the anode and cathode compartment, respectively. Such
j discontinuous streams prov1de very high resistance against current leakage.
.,
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7~99~
Similarly, such interrupted streams may be produced wherein the overflow
is withdrawn, with the cell liquor and depleted anolyte dropping from
the cell electrolyte levels to lower levels, in which dropping discontinuous
"streams" are created. Of course, the electrolytes may also be broken-
up when removed from the cell bank to prevent leakage through them. In
this respect, the polymeric plastic material of construction for the cell
frames is useful because it is non-conductive.
In FIG. 4 is shown a part of a T-shaped "picture frame" intermediate
mounting means for holding an electrode, such as expanded mesh anode 71
of FIG's. 2 and 3 to frame 15. In FIG. 2 the framework 115 is shown as
an angle and in FIG. 4 it is a T 169, having a horizontal leg portion
171, and upper vertical "top" part 173 and a lower vertical "top" part '
175. The upper part of the T top 173 is held to a body portion of frame
15 and the lower portion has the expanded mesh anode 177 held to it. As
shown, a clearance is created between anode 177 and membrane 179 which is
held between the upper part of the T and the gasket 181. Of course,
by changing proportions and locations, it can be arranged that this
clearance is greater, lesser or non-existent.
In modifications of the structure illustrated, a monopolar type of
cell is readily produced by merely changing the electrical connectors
so that ealch cell is independently charged with an electrolyzing voltage.
Similarly, electrical connections between bipolar cells may be made
internally, rather than externally of the cell. Instead of having the
electrical connections at the top of the cell they may be at the side
thereof. The major separators in the cell, shown as vertical central
structural members of the frames, may be replaced by pluralities of such
members, both being thinner, to conserve plastic. They may be moved
to either side of the cell frame but it is preferred to have them
substantially centrally located, as indicated, for dividing ~he cell into
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~769~4
approximately equal compa~tments and for greater strength and dimensional
stability of the molded plastic frame. Instead of using the neoprene
washers, which are squeezed tightly against the electrode conductors
by compression of a packing nut, other forms of seals may be employed,
including single 0-rings, cylinders, hollow cone wedges, etc. Such
materials may be of neoprene or other suitable polymers, e.g., poly-
tetrafluoroethylene and other fluorinated polymers. Instead of using
positively held means for fastening the membrane and/or electrodes in
- place, such means may be elastic, such as neoprene bands, pressing
the gasket or other parts against the frame during assembly, possibly
fitting into hollows in the frame. Such temporary means can be
employed until the various frames are tightly pressed together during
construction of the cell assembly. Alternatively, instead of employing
such means, a cement, such as neoprene cement, which holds the gasketsl
electrodes and membranes in place temporarily, can be used. Such cement
should be sufficiently strong to hold the various parts together in correct
relationship until "permanently" fastened by pressing together of the various
frames. Upon disassembly the cement would not so tightly bind the various
parts as to make them unremovable without permanent damage. In some cases,
it may be possible to dispense with the use of gaskets and employ mating
parts of the polypropylene frames which are so tightly held together by
the assembly compressing means as to maintain the various interposed parts
in desired "permanent" relationship without the need for softer gasketing
materials to prevent leakages. Where one dispenses with the neoprene
gaskets the resilience of the polypropylene frame may be increased by
including rubber or other elastomeric materials in the molding compositions,
e.g., 5 to 25% of ethylene propylene elastomer.
" , .
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107699~
The cells illustrated in FIG'S 2 and 3 are those of a centralportion of the cell assembly. It is evident that the terminal frames
are of a different design, being essentially half cell frames with the
exterior sides blanked off. It is not thought to be necessary to il-
lustrate such structures here because they are considered to be self-
evident.
: The only material of construction of the present cell frames which
has been found to be satisfactory in use and can be injection molded
readily is polypropylene, more particularly, polypropylene filled with
a suitable "filler" such as one selected from the group consisting of
asbestos, mica, calcium silicate, talc and mixtures thereof. The mix-
tures of fillers mentioned may be of two, three or four components but
it is considered especially beneficial to include both the calcium
silicate and mica in such mixtures. In some cases art-recognized
equivalent fillers may also be used. I~ only one of the fillers is to
be employed it is preferred that it is calcium silicate, of which the
, fibrous form is considered to be best. The polypropylene referred to
may be a normal resin intended for injection molding, such as the un-
modified copolymer, inert filled or impact (rubber modified) polypro-
pylenes that result in products of the properties given in 1973-1974
Modern Plastics Encyclopedia, at page 552. Thus, such resins, when
injection molded according to the methods described at pages 338-410
of that publication, produce useful electrolytic cell frames suitable
for employment in "filter press" assemblies of from about 10 to 60 of
such frames, for the electrolysis of aqueous sodium chloride solutions.
The frames described resist the electrolyte and products of electro-
'r,(
lysis and are satisfactory dimensionally stable during electrolysis, even
` over temperature variations from 40 to 95C. and pH variations from 3 to
14. Although it is possible to utilize polypropylene resin containing
_ 11
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' 1~'7~9~9~
no copolymer and no rubber, providing that the desired content of
"filler" sufficiently improves the dimensional stability, heat
resistance, etc., to make the material satisfactorily operative in
commercial chlorine cells, usually one will employ a mixture of homo-
polymer and copolymer. In such mixture the proportion thereof will
normally be from 10% to 40% of copolymer, such as that sold by Shell
Chemical Company as Shell 7525 and 10 to 40% of homopolymer, with
the total content of such components being 40 to 90% of the final
product. Instead of the homopolymer, copolymers of it with other
materials may be utilized, e.g., propylene-acrylic acid copolymers such
as Exxon D-561. The proportion of rubber impact modifier may be from
0 to 20% but it is preferably present and will normally be from 3 to 15
of the product. When the term "polypropylene" is used herein in a general
sense it is inclusive of copolymer and copolymers of homopolymer and
linking agents such as unsaturated lower organic acids, e.g., acrylic
acid, methacrylic acid and other monoalkenoic acids of 3 to 6 carbon
atoms and equivalents, as well as homopolymers.
The "filler" while it may talc, asbestos, calcium silicate or mica
or a mixture thereof, preferably includes calcium silicate fibers, such
as Wollastonite fibers or synthetic calcium silicate fibers and
may preferably include additionally, mica flakes or platelets. The
calcium silicate, either natural or synthetic, asbestos and talc
fibers may be of a variety of diameters commercially available, for the -
180 Angstroms for chrysotile asbestos to as much as one millimeter, although
usually the diameters will be less than 0.1 mm., e.g., 0.001 to 0.05 mm.
Fiber lengths will normally be in the range of 10 to one million times
the diameter, preferably 20 to 1,000 times, and will normally be in the range
of 1 mm. to 2 cm. Similarly, the mica employed will usually be finely
enough divided to pass through a 140 mesh screen, preferably through a
- 12 -
"' ~~ io ~99,~
.
200 mesh screen, (United States Standard Sieve Series). Although
f these sizes are mentioned as guides, it will be evident that in somecases it may be desirable to utilize different sizes of materials for
special effects.
The proportion of the described inorganic filler material in
the present injection molded frames is from 10 to 60%, preferably
15 to 50% and more preferably 20 to 40%. Preferably from 50 to 100%
thereof is calcium silicate fiber. However, in some embodiments of the
invention from 10 to 30% of asbestos will be employed with 90 to 70% of
polypropylene homopolymer or 5 to 20% of mica flake, 10 to 30% asbestos
fiber, 15 to 50% of polypropylene homopolymer and 20 to 60% of
polypropylene copolymer may be utilized and will produce satisfactory
products. Instead of some of the calcium silicate there may be substituted
~! an e~ual proportion, from 10 to 50% thereof, of talc powder. Also, the
calcium silicate and/or talc may be treated with silanes or silicones
to modify the silanol and siloxane groups thereon and to vary properties
of the molded polypropylene. Various rubber impact modifiers of known
types may be employed for their obvious purpose. The rubbers utilized may
be any of those normally acceptable for this purpose in the polymer art
! 20 and mixtures of these may also be employed but elastomers based on poly-
ethylene or polypropylene are preferred, as are those based on both
such polymers.
..,
Various other additives may be present in small proportions, usually
to the extent of no more than 10% and preferably no more than 5%, in the
present molded products, e.g., colorants, mold released agents and fire
retardant chemicals. Descriptions of such materials and of the polymers,
fillers and rubbers employed in making the injection moldable polypropylene
resin composition utilized in this invention are described in more detail
., .
- 13 -
9~
in the 1973-1974 Modern Plastics Encyclopedia and in the 1972-1973
eclition of this work and accordingly, are not described further herein.
The injection molding of the frames is in accordance with known
p~ocedures for molding large items (the frame size is about 1.1 meter
by 1.1 meter by 9 cm. and the distance between anode and cathode in the
cell formed by fitting together of two frames is normally about 1 cm.).
A description of such molding is found in the article entitled Giant,
Thick-Sectioned Plastic Parts Achieved by New Method, appearing at page
42 of the May 1972 issue of Product Engineering and in the article en-
10 titled Unusual Technique Makes 'Impossibl--e' Parts, at page 53 of the -
September 1971 issue of Modern Plastics. Such molding techniques are
those developed and practiced by Eimco Envirotech. Details of suitable
molding methods are given in the article.
The molded frame made, although produced from thermoplastic mate-
rials, does not soften at the operating temperatures of the presentelectrolytic cells, which can go as high as 95C. The cells operate
continuously for lengthy periods of time, e.g., six months, without
; warping, cracking, failing, sagging or otherwise showing objectionableevidence of lack of dimensional stability. Without the reinforcing in-
organic filler materials results are not as satisfactory but the molded
frames are still useful. It is considered that the molding operation,
which tends to cover all fibrous material near the surfaces of the molded
item, thereby prevents any exposure of such reinforcing material to the
contents of the electrolytic cell, thereby aiding in stabilizing the in-
25 jection molded frame. Thus, such products are considered to be superior `
to similar ones wherein a cell frame is machined from stock material of
reinforced polypropylene.
The materials of construction of the various components of thepresent cells are chosen for resistance to the particular environments
encountered. The anode, which preferably of expanded titanium mesh, al-
though other
_ 14
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.` ~ ' ' ~ . : ' `
~6~'7~
valve metals are also useful, e.g., tantalum, and which may be coated on
an active surface thereof with a noble metal or noble metal oxide, e.g.,
ruthenium oxide, is resistant to the chlorine and acidic brine of the
anolyte compartment. The conductor rods are utilized in such compartment
are preferably of copper for good conductivity and clad with titanium
for resistance to the electrolyte. The openwork portion of the anode
has openings of 2 to 700 sq./mm., preferably 100 to 600 sq./mm. and the
proportion of openings to the nominal single major face area of the anode
is in the range of about 25 to 70%, preferably 40 to 70%. The mesh is
normally from 0.2 to 2.5 mm. thick, preferably 1 to 2 mm. and the
strand or section width thereof is from 0.7 to 2.5 mm., preferably 1 to
2 mm. Preferably the anode is activated with ruthenium oxide on the
back surface thereof, away from the membrane.
The cathodes utilized may be of any electrically conductive material
that resists the attack of cell liquor, which is comparatively high in
sodium hydroxide content. Suitable cathodes are made of steel mesh and
they are joired to a copper conductor but other cathode materials and
conductor materials may also be employed, among which are iron, graphite,
lead dioxide on graphite, lead dioxide on titanium and noble metals, such
as platimum, iridium, ruthenium, and rhodium. The noble metals may be
deposited as surfaces on conductive substrates, such as those of copper,
silver, aluminum, steel and iron. The cathodes used preferably will be
of screen or expended metal mesh and, like the anodes, will be flat or
of other conforming shapes so that the inter-electrode distances will
be approximately the same throughout. The openings in the cathode screen
or mesh will normally be at least 25% of the surface area of a face
the cathode, preferably 30 to 80% thereof and most preferably about 45 to
65% thereof. The areas of openings are usually 0.5 to 1,000 sq. mm.,
- 15 -
10769~4
preferably 2 to 100 sq. mm. When a wire screen is employed the strands
thereof will preferably be about 0.5 to 3 mm. in diameter. Both the
electrodes will normally be maintained in perfectly vertical or sub-
stantially vertical position, usually not being more than 10 from the
vertical and preferably not more than 5 therefrom.
The presently preferred cation permselective membrane is of a
hydrolyzed copolymer of perfluorinated hydrocarbon and a fluorosulfo-
nated perfluorovinyl ether. The perfluorinated hydrocarbon is prefer-
ably tetrafluoroethylene, although other perfluorinated and saturated
and unsaturated hydrocarbons of 2 to 5 carbon atoms may also be utilized,
of which the monolefinic hydrocarbons are preferred, especially those
of 2 to 4 carbon atoms and most especially those of 2 to 3 carbon atoms,
e.g., tetrafluoroethylene, hexafluoropropylene. The sulfonated per-
fluorovinyl ether which is most useful is that of the formula
FS02CF2CF20CF(CF3)CF20CF=CF2. Such a material, named as perfluoro[2-
(2-fluorosulfonylethoxy)-propyl vinyl ether], referred to henceforth
as PSEPVE, may be modified to equivalent monomers, as by modifying theii
internal perfluorosulfonylethoxy component to the corresponding propoxy
component and by altering the propyl to ethyl or butyl, plus rearranging
positions of substitution of the sulfonyl thereon and utilizing isomers
of the perfluoro-lower alkyl groups, respectively. However, it is most
preferred to employ PSEPVE.
The method of manufacture of the hydrolyzed copolymer is described
. in Example XVII of U.S. patent 3,282,865 and an alternative method is
mentioned in Canadian patent 849,670, which also discloses the use of the
finished membrane in fuel cells, characterized therein as electroehemical
cells. In short, the copolymer may be made by reacting PSEPVE or equi-
valent with tetrafluoroethylene or equivalent in desired proportions in
- 16 -
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:
, ~;t~
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~o~
water at elevated temperature and pressure for over an hour, after
which time the mix is cooled. It separates into a lower perfluoro-
ether layer and an upper layer of aqueous medium with dispersed
desired polymer. The molecular weight is indeterminate but the
equivalent weight is about 900 to 1,600 preferably 1,100 to 1,400
and the percentage of PSEPVE or corresponding compound is about 10
to 30%, preferably 15 to 20% and most preferably about 17%. The
unhydrolyzed copolymer may be compression molded at high temperature
and pressure to produce sheets or membranes, which may vary in
thickness from 0.02 to 0.5 mm. These are then further treated to
hydrolyze pendant -S02F groups to -S03H groups, as by treating with
10% sulfuric acid or by the methods of the patents previously
mentioned. The presence of the -S03H groups may be verified by
titration, as described in the Canadian patent. Additional details
of various processing steps are described in Canadian Patent 752,427
and U.S. Patent 3,041,317.
Improved versions of the above-described copolymers may be
made by chemical treatment of surfaces thereof, as by treatments
to modify the -S03H group thereon. For example, the sulfonic
group may be altered on the membrane to produce a concentration
gradient or may be replaced in part with a phosphoric or phosphonic
moiety. Such changes may be made in the manufacturing process or
after production of the membrane. When effected as subsequent
surface treatment of a membrane the depth of treatment will usually
be from 0.001 to 0.01 mm. In some instances it may be desirable to
convert the sulfonyl or sulfonic acid group of the membrane on one
side (such as the anode side) to a sulfoamide, which is more hydro-
philic, which may be effected in the manner described in U.S. Patent
10'7699~
3,784,399. Also, the membrane may be in laminated form, which is
now most preferred, with the laminae being of a thickness in the
range of 0.07 to 0.17 mm. on the anode side and 0.01 to 0.07 mm.
on the cathode side, which laminae are respectively, of equivalent
weights in the ranges of 1,000 to 1,200 and 1,350 to 1,600. A
preferred thickness for the anode side lamina is in the range of
0.07 to 0.12 mm. thick and most preferably this is about 0.1 mm.,
.,. , - .
with the preferred thickness of the lamina on the cathode side
being O.Q2 to 0.07 mm., most preferably about 0.05 mm. The pre-
ferred and most preferred equivalent weights are 1,050 to 1,150
and 1,100 and 1,450 to 1,550 and 1,500, respectively. The higher
the equivalent weight of the individual lamina the lesser the
thickness preferred to be used, within the ranges given.
In addition to the copolymers previously discussed, including
modifications thereof, it has been found that another type of mem-
brane material is also superior to prior art films for applications
in the present processes. Although it appears that tetrafluoro-
ethylene (TFE) polymers which are sequentially styrenated and
sulfonated are not useful for making satisfactory cation-active
permselective membranes for use in the present electrolytic
processes it has been established that per~luorinated ethylene
propylene polymer (FEP) which is styrenated and sulfonated makes
a useful membrane. The sulfostyrenated FEP's are surprisingly
resistant to hardening and otherwise failing in use under the
present process conditions.
Examples of useful membranes made by the described process are
products of RAI Research Corporation, Hauppauge, New York, identified
as 18ST12S and 16ST13S, the former being 18% styrenated and having 2/3
of the phenyl groups monosulfonated and the latter being 16% styrenated
. .
- 18 -
.
. ~0~95~
and having 13/16 of the phenyl groups monosulfonated. To obtain 18%
styrenation a solution of 17-1/2% of styrene in methylene chloride is
utilized and to obtain the 16% styrenation a solution of 16% of styrene
in methylene chloride is employed.
The membrane walls will normally be from 0.02 to 0.5 mm. thick,
preferably from 0.07 to 0.4 mm. and most preferably 0.1 to 0.2 mm.
Ranges of thicknesses for the portions of the laminated membranes previously
described have already been given. When mounted on a polytetrafluoro-
r
ethylene asbestos, titanium or other suitable network, for support, the
network filaments or fibers will usually have a thickness of 0.01 to
0.5 mm., preferably 0.05 to 0.15 mm., corresponding to up to the thickness
of the membrane. Often it will be preferable for the fibers to be less
than half the film thickness but filament thicknesses greater than that of
~ the film may also be successfully employed, e.g., 1.1 to 5 times the film
; 15 thickness. The networks, screens or cloths have an area percentage of
openings therein from about 8 to 80%, preferably 10 to 70% and most
preferably 30 to 70%. Generally the cross-sections of the filaments will
be circular but other shapes, such as ellipses, squares and rectangles,
are also useful. The supporting network is preferably a screen or cloth
and although it may be cemented to the membrane it is preferred that it
be fused to it by high temperature, high pressure compression before
hydrolysis of the copolymer. Then, the membrane-network composite can be
clamped or otherwise fastened in place in a holder or support. To maintain
the desired spacings between membrane and one, the other or both electrodes,
gaskets and/or vertical wires or strips of suitable plastic material,
e.g., polytetrafluoroethylene, are utilized. Generally the cell width
will be from 0.3 to 1 cm. and the distance from the anode to the membrane
will be from 1.5 to 6 mm. but may be O mm., too. Similar measurements also
apply to cathode-membrane distances.
, 19 _
'
'
~,
- .
,6g9~
After the injection molding of the cell frame at elevated
temperature and pressure the frames are assembled into cells and into
a cell assembly, as shown in FIG'S. 1-4. When the T or right angle
picture frame for holding the electrode (and possibly also the membrane)
in place is utilized it will be of a material which withstands the electrolyte
with which it is brought into contact. Thus, when the frame is on the
anode side it will often be of titanium or titanium clad metal and when on
the cathode side will usually be of soft steel. Similar rules apply to
various molding devices, supporting brackets, etc.
After assembly of the frames into cells and tightening of these into
a cell assembly, as shown in FIG. 1, the cells are charged with electrolyte
and electrolysis is begun.
The anode compartment is filled with a nearly saturated salt solution
or brine of sodium chloride content of about 25% and the cathode compartment
15 is filled with water, initially containing a small quantity of salt or brine
to improve its conductivity. Current is turned on and chlorine and
hydrogen produced in the cells are taken off continuously, in some
embodiments of the invention with the chlorine and anolyte being separated
after removal from the cell rather than before. Usually sodium hydroxide
20 iS removed continuously during electrolysis but it may be removed at the
completion of an electrolytic cycle. Depleted anolyte is passed through
` a resaturator wherein sodium chloride content is increased and it is then
returned to the anode compartment. Generally, the sodium chloride content
of the withdrawn anolyte is about 22% by weight and that of the returned
25 anolyte from the resaturator is about 25%. The anolyte may be acidified
and preferably is of a pH in the range of 1 to 6, preferably 1 to 5
' and most preferably about 3 to 4.5, e.g., 3.9 to 4.3. Of course, the
catholyte pH is about 14, due to the high content of sodium hydroxide.
- 20 -
' , ~ . . ' -
~ 7~
The temperature of the electrolyte will be maintained at less than
105C., preferably being 20 to 95O and more preferably being about 80 to
95C. Electrolyte temperatures may be controlled by recirculation of
portions thereof, by regulation of proportions of feed for the various
zones and by changing the temperatures of the feeds. Refrigeration and
other cooling means may also be employed.
The voltage impressed per cell will usually be from 2~3 to 5 or 6 volts
and is preferably in the range of 3.3 to 4.3 volts. Sometimes in a
preferred method it may be as high as 4.5 volts. Most preferably the
voltage will be from 3.3 to 4.1 volts. The current density will generally
- be from 0.2 to 0.5 ampere/sq. centimeter, preferably about 0.3 ampere/sq.
cm. The take-off of caustic from the catholyte compartment is at such
a rate that the caustic produced is at a concentration of 5 to 45%, preferably
5 to 25% and most preferably about 8 to 12%.
The following examples illustrate but do not limit the invention.
Unless otherwise mentioned, all parts are by weight and all temperatures
are in C.
EXAMPLE 1
2~ A frame of the type illustrated in FIG'S. 2 and 3 is injection
molded by the Eimco Envirotech method for injection molding large plastic
pieces, at normal elevated injection molding temperatures and pressures.
The mold is so constructed that passageways, bosses, channels, ledges,
ribs, manifolds, alignment and tightening means are molded into the frame
produced. Where feasible, threaded or partly threaded openings are also
molded so as to be available for and receptive to threaded orifices or
other parts of the cell frame. The frames are molded of a mixture of
' 25% of calcium silicate fiber, 10% EP rubber impact modifier, 37.5%
- Exxon D-561 propylene-acrylic acid polymer and 27.5% Shell 75~5 poly-
'
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.,
.
-` 10'7~
propylene copolymer and the products made are examined and tested. It
is found that the surfaces of the frames are richer in resin than the
interiors and few or no fibers of inorganic material are at the surfaces,
uncovered by resin. Accordingly, when tested in representative aqueous
electrolyte suitable for use in an electrolytic cell for the production
of chlorine and caustic from sodium chloride solution, the frame shows
no significant weakening, possibly because there are no incipient cracks
or points at which the calcium silicate is exposed. Also, when tested
for dimensional stability at temperatures over the range of 40 to ~5C.,
which temperatures may be used in normal electrolysis with this type of
cell, the frames are stable and do not warp, crack, check, craze or
otherwise distort.
~The frames made are assembled into cells, using neoprene gaskets,
; titanium "picture frame" intermediate retainers for the anodes and similar
steel picture frame retainers for the cathode, with nylon screws and poly-
propylene rectangular "ring" members for holding the membrane in the
frame and tightly against the anode. The anode employed is an expanded
titanium mesh anode and the cathode is a steel screen. The conductor to
the anode is titanium clad copper and that to the cathode is copper. The
anode expanded mesh is of a thickness of about 2.0 mm. and the strand
width is about 2 mm. The mesh is of diamond configuration with a long
axis being horizontal and includes about 50% open area at a surface
thereof. Distances across the diamonds are 0.75 cm. and 1.25 cm. The
anode is coated on the side away from the membrane with an active coating
of ruthenium oxide 0.07 mm. thick, which is applied according to standard
methods known for making dimensionally stable anodes. The cathode is of
mild steel wire mesh, about 2.0 mm. in equivalent diameter, with about
f 50% open area.
:~ .
- 22 -
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.. .
.~ . . . . .
.
- ~0'~999~
The membrane is of the PSEPVE type (hydrolyzed) described in
the foregoing specification and is a laminate 0.17 mm. thick, of
which thickness 2/3 is of an equivalent weight of about 1,100 and 1/3
is of an equivalent weight of about 1,500. The high molecular weight side
of the membrane faces the cathode and the flatter, thicker, lower molecular
weight side, backed with a polytetrafluoroethylene supporting network of
filaments or threads of diameters of about 0.3 mm., woven into a screen
or cloth which has an area percentage of openings therein of about 22%,
is pressed tightly against the anode.
The frames and cell walls are about 1.1 meter x 1.1 meter and the cells
are about 11 cm. thick. The cell walls and other plastic parts thereof
in the cell, such as the bosses, walls defining the headers and passageways,
and ledges and channels are usually from 1 to 3 cm. thick.
Between the cathode and the membrane is a series of vertical
polytetrafluoroethylene flexible separating cylinders or lines, each
2.5 mm. in diameter, which are employed as vertical spacers every 15 cm.
across the membrane-cathode gap and together with the gasket, which is of
a thickness such as to produce about a 2.5 mm. gap between the cathode
and the membrane, these maintain the membrane at a regular distance from
the cathode and hold it tightly against the anode.
In the assembly of the cell the various frames are positioned on
the temporary aligning means or bar, the electrodes are applied and
the membranes are held in place by the temporary cement (Velcro or other
suitable temporary and permanent seals can also be used) and later by
the sealing gaskets. Before sealing the various frames together the
membrane is normally conditioned in position on the frame by application
of a suita61e solvent, e.g., glycerol, which prevents drying of the
membrane during cell assembly and is especially useful, as in the present
case, when a plurality of cells (35) is being joined in a cell assembly.
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. .
7~994
After assembly of the cells together in desired position, in
filter press type arrangement, they are drawn tight so as to pre~ent
flu-id leakages. See FIG. l for a view of the assembled cell stack or
bank. They are then connected to sources of feeds and electricity
and to discharge piping.
Next, the anode compartments are filled with a nearly saturated
acidified salt solution of about 25% concentration and of a p~ of about
4.1 and the cathode compartment is filled with water, initially containing
a small quantity of sodium hydroxide to improve its conductivity. The
current is turned on and a current density of 0.3 ampere/sq. cm. results.
The voltage drop across each cell is noted during equilibrium production
of aqueous sodium hydroxide solution, chlorine and hydrogen. Under such
conditions the cell voltage drop is about 4 volts, the load is about 3
kiloamperes and the total voltage is about 120 volts D.C. At an operating
15 temperature of about 90C and at other temperatures in the 85 to 95C range
the stack or cell assembly produces about 3 tons per day of chlorine at
a chlorine efficiency of about 96%, with acid addition, and a caustic
efficiency of about 90% at a caustic concentration of 80 to lO0 g./l.
Chlorine efficiency without acid addition is on the order of about 90%.
After about six months operation under the conditions described above the
cell stack is opened and the electrodes, membrane, frame, gaskets and
; fastening means are inspected. The electrodes and the membrane are still
; operative and the frame shows no signs of significant weakening or dis-
tortion, despite the fact that during the normal operation of the cells,
including some shutdowns, the temperature varies over the range of 40 to
95~C. The neoprene cement used is still readily removable from the membrane
and frame wall, so that the membranes can be employed again. In a
modification of the experiment, when Velcro fasteners are employed to hold
the membranes in position, they are readily separable and the membranes are
removable without damaging them.
. .
, - 24 -
- . ~
-:
~ , .
- 1~7~9~
In variations of this experiment, the filled polypropylene resin
employed is one made from a resin mix of 30% of the described calcium
silicate fiber, 10% EP rubber impact modifier, 45% Exxon D-561 propylene-
acrylic acid copolymer mix and 15% of Shell 7525 polypropylene copolymer.
The results obtained are almost as good as those of the formula previously
given except that it is noted that the frames are somewhat more susceptible
to cracking under extreme conditions. In a further variation, there is
employed 20% of asbestos and 80% of polypropylene homopolymer. Although
this frame is a useful one, it is not as good as that initially described.
However, an ;mprovement on it, made and tested the same way as described
above, is one in which the resin mix is of 20% of asbestos, 10~ mica,
40% of homopolymer and 30% of copolymer.
EXAMPLE 2
The procedure of Example 1 is repeated, utilizing the 30% calcium
silicate fiber - 10% EP rubber impact modifier - 45% Exxon D-561 propylene-
acrylic acid copolymer resin - 15% Shell 7525 polypropylene copolymer
formula but employing as the membrane the unlaminated PSEPVE hydrolyzed
copolymer having a thickness of about 0.2 mm. with the same type of rein-
forcing polytetrafluoroethylene screen. In a similar manner, there are
substituted for the membrane RAI Research Corporation membranes identified
as 18ST12s and 16ST13s, of equivalent thicknesses. Utilizing 0.3 ampere/sq.
cm. current densities in all such cases the cells operate satisfactorily
without damage to the frame material from either the acidic anolyte or
; alkaline catholyte. In modifications of the cells the ruthenium oxide on
titanium anode is changed so as to be active on all surfaces thereof and
the cathode-membrane distance is lowered to 2 mm. At such conditions
efficient electrolysis is effected without damage to the framing material.
- 25 -
. ~ . . . . . .
Also, when the electrodes are held by T-shaped intermediate holding
members they are less apt to sag than when merely gasket held, aithough -
angle frames are also satisfactorily rigid.
The cells of this example also work satisfactorily when the membranes
S are held against the cathodes or are positioned halfway between the anodesand cathodes, held in desired positions by Teflon line spacers l.S mm. in
diameter.
EXAMPLE 3
Frames are made by the method described in Example 1 from 35% of
polypropylene homopolymer, 35% of polypropylene copolymer and the remainder
(no impact modifiers present) of mica flakes (200 mesh), calcium silicate
fibers (Wollastonite), calcium silicate fibers (synthetic), talc or asbestos
(chroysotile). Unmodified homopolymer, copolymer and 50:50 homopolymer-
copolymer mixtures are also used. Frames are molded of each of these. When
tested by practical use test, in electrolytes under electrolyzing conditions,
it is found that the use of the calcium silicate fibers in a homopolymer-
copolymer mix is best and that the filled polymers are superior in physical
properties, especially dimensional stability and heat resistance, to the
unfilled materials. However, while the differences are important it is
possible to utillze all of the built polypropylene frames in commercial
application and the unbuilt frames, while inferior, are still operative.
Frames made of other polymers, such as polyvinyl chloride, polytetrafluoro-
ethylene and polyethylene, while not normally commercially acceptable
- for long term use, are operative for short term use in electrolytic cells
and are advantageous when they have molded into them by ordinary injection
molding techniques, when applicable, the various headers, passageways,
orifices, alignment, mounting and fastening means of the apparatuses of
this invention.
:',
- 26 -
.~
.. ' ~, .
7~99g~
Still, the combination of polypropylene frame and cation-active
permselective membrane of the DuPont Nafion XR type, especially when
the membrane is held directly to the frame, without intervening gaskets,
is best, with respect to dimensional stability, chemical resistance and
long life in use and is preferred.
EXAMPLE 4
The procedures of Example 1 are repeated but the temperatures,
voltages and pH's of the various processes described are varied over the
ranges given, from 20 to 95C, from 3.3 to 4.3 volts, from 3 to 4.5 anolyte
pH and from 0.2 to 0.4 ampere/sq. cm. Also, the electrodes are varied
over the open area size and proportion ranges and inter-electrode distances
given in the preceding specification. In such cases, adequate electrolysis
results and the frames are sufficiently stable to be commercially acceptable.
No significant problems are encountered in the operations of these cells
~, 15 and the cost of manufacture of the frames is diminished to an important
extent because of the ease of making the frames by injection molding.
Furthermore, with the pre-molded bosses, ledges, channels, etc. in the
frames, assembly of parts is facilitated and is accomplished in shorter
periods of time, decreasing the expense of celi assembly. Accordingly,
the present cells represent an important advance in the art of electrolysis
of aqueous hallde solutions.
The invention has been described with respect to illustrations and
specific working examples thereof but is not to be llmited to these because
it is evident that one of skill in the art with the present specification
before him will be able to utilize substitutes and equivalents without
departing from the invention.
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