Unitary frame and membrane for electrolytic cells

A unitary frame and membrane assembly particularly useful in a filter press electrolytic cell arrangement is provided by the present invention. The assembly comprises a rigid, peripheral frame member having separable peripheral frame portions. A permselective membrane member is positioned between the separable peripheral frame portions, and the membrane member and frame portions are secured together to form a unitary frame and membrane assembly. The present assembly is adapted for use as the barrier component in electrolytic cells which utilize a separatory membrane barrier between the cell electrodes.

BACKGROUND OF THE INVENTION 
The present invention relates to the construction of improved electrolytic 
cells useful as units of a filter press cell arrangement. More 
particularly, the present invention relates to a unitary frame and 
membrane member which facilitates cell assembly and membrane replacement. 
Electrolytic cells are particularly useful in the electrolysis of alkali 
metal chloride, such as sodium chloride, to produce alkali metal 
hydroxides, such as sodium hydroxide, together with chlorine and hydrogen. 
A filter press arrangement typically consists of a plurality of separate 
cell units having planar electrode elements generally mounted in a 
vertical position separated along their active faces by a barrier, such as 
a diaphragm or membrane layer. The filter press cell units may be 
monopolar or bipolar and may be appropriately connected in series or 
parallel to form a cell circuit or bank. 
Chlorine and alkali metal hydroxides are essential and large volume 
commodities as basic industrial chemicals. Plants producing 500 to 1000 
tons of chlorine per day are not uncommon. Such plants typically utilize a 
large number of individual electrolytic cells having current capacities of 
several hundred thousand amperes. Thus, minor improvements in individual 
cell operation or performance have major economic benefits because of the 
volume of the products produced. 
Upon the application of direct, electrolyzing current to an electrolytic 
cell containing an aqueous solution of an alkali metal chloride as the 
electrolyte, hydrogen and alkali metal hydroxide are produced at the 
cathode, and chlorine is produced at the anode. 
Electrolytic cells that are commonly employed commercially for the 
conversion of alkali metal halides into alkali metal hydroxides and 
halides may be considered to fall into the following general types: (1) 
diaphragm, (2) mercury and (3) membrane cells. 
Diaphragm cells utilize one or more diaphragms permeable to the flow of 
electrolyte solution but impervious to the flow of gas bubbles. The 
diaphragm separates the cell into two or more compartments. Although 
diaphragm cells achieve relatively high product per unit floor space, at 
low energy requirements, and at generally high current efficiency, the 
alkali metal hydroxide product, or cell liquor, must be concentrated and 
purified. Such concentration and purification is usually accomplished by a 
subsequent evaporation step. 
Mercury cells typically utilize a moving or flowing bed of mercury as the 
cathode and produce an alkali metal amalgam in the mercury cathode. Halide 
gas is produced at the anode. The amalgam is withdrawn from the cell and 
treated with water to produce a high purity alkali metal hydroxide. 
Membrane cells utilize one or more membranes or barriers separating the 
catholyte and the anolyte compartments. The membranes are permselective, 
that is, they are selectively permeable to either anions and cations. 
Generally, the permselective membranes utilized are cationically 
permselective. Usually, the catholyte product of the membrane cell is 
relatively high purity alkali metal hydroxide ranging in concentration 
from about 250 to about 350 grams per liter. 
The introduction of dimensionally stable anodes has permitted ever 
narrowing of the space, or gap, between the electrodes of a cell, thereby 
facilitating progressively higher cell efficiency. The advent of 
dimensionally stable anodes and suitable membrane materials has made 
possible the construction of electrolytic cells having a thin separating 
partition positioned between planar electrodes, and the combination of a 
number of individual cell units, usually between about 10 and about 100, 
to form a cell circuit or bank arranged in the manner of a filter press. 
For example, in the case of a monopolar arrangement, the components 
typically would comprise a plurality of anodes mounted in anode frames and 
cathodes mounted in cathode frames. The anodes and cathodes are separated 
along their active faces by a permeable barrier, or membrane, and along 
the inner periphery of the frames by a pliable or elastic gasket member. 
The assembly is completed by coupling or pressing the components together, 
hydraulically or by means of threaded connectors, such as tie rods, to 
compress the gasket members to form gas and liquid-tight seals between the 
individual units. 
The term "membrane", as used herein, is meant to encompass separating 
partitions or barriers in the form of sheets or fabrics of chemically 
resistant materials which are ion conducting, for example, permselective 
resin materials, asbestos fibers, mixtures thereof, and includes 
microporous materials. 
From time to time during the operation of electrolytic cells equipped with 
a membrane barrier, the membrane component requires replacement. The 
replacement process typically entails removal of the cell circuit from 
service, disassembly of the circuit, disassembly of the individual cell 
unit, removal and replacement of the membrane and the subsequent 
reassembly of the components into an operative cell circuit. However, the 
membrane is typically attached to an electrode member, by clamping or 
other means, and the membrane replacement operation requires the 
time-consuming steps of removing the usually heavy electrode member, 
generally by a lifting means, removal of the membrane, positioning and 
clamping the new membrane on the electrode member and the replacement and 
repositioning of the electrode member in the cell circuit. 
Although the present invention provides a means of shortening the time 
required to initially assemble a cell circuit, its more important aspect 
is that the present invention substantially shortens the down time 
required for replacement of membranes in operating circuits, thereby 
substantially increasing the production time. 
GENERAL DESCRIPTION OF THE INVENTION 
A unitary frame and membrane assembly particularly useful in a filter press 
electrolytic cell arrangement is provided by the present invention. The 
assembly comprises a rigid, peripheral frame member having separable 
peripheral frame portions. A permselective membrane member is positioned 
between the separable peripheral frame portions, and the membrane member 
and frame portions are secured together to form a unitary frame and 
membrane assembly. The present assembly is adapted for use as the barrier 
component in electrolytic cells which utilize a separatory membrane 
barrier between the cell electrodes. 
The frame portions of the present frame member are rigid, that is, they are 
substantially dimensionally stable when hand-held. In assembling the 
present unit, the membrane member is initially sandwiched between the 
separated frame portions, the frame portions are aligned, and the membrane 
and frame portions secured together by suitable fastening means, such as 
threaded connectors or by a tongue and groove arrangement in the frame 
portions, to provide a rigid frame and membrane assembly. 
Assemblies of the present invention may be replaced in a cell circuit 
without completely disassembling the circuit. The circuit clamping or 
holding means, for example, tie rods, are loosened, the faulty membrane 
mounted in its frame and membrane assembly is removed and a new assembly 
substituted. The circuit is then reclamped and placed back in service. The 
present invention is particularly useful in that it facilitates storage of 
prepared, spare membrane components. In cases where the membrane material 
is to be chemically treated prior to placing the membrane in service, the 
present invention allows the mounted membrane to be pretreated and stored 
ready for use, or, if desired, the frame and membrane assembly may be 
stored in a suitable treatment liquid until the assembly is needed.

DETAILED DESCRIPTION OF THE INVENTION 
Looking now at FIG. 1, the unitary frame and membrane assembly consists of 
flat membrane 1 mounted around its periphery in rigid peripheral frame 
member 2. As shown in FIG. 1, frame member 2 consists of separable 
portions 3 and 4 adapted to receive and hold membrane 1 therebetween. 
Frame portions 3 and 4 are secured together by fastening means, suitably 
screws or bolts 5. The circuit components include a plurality of electrode 
members, such as 8, having active faces, such as 9. The present frame and 
membrane assemblies are adapted to be positioned between, and to separate, 
adjacent electrode members. A circuit of cells may be assembled by 
placement of suitable gaskets between each of the individual components, 
compressing or clamping the whole to form a plurality of separate cell 
units, each having a pair of electrodes separated by a frame and membrane 
assembly. 
FIG. 2 shows in detail a portion of a frame and membrane assembly, such as 
shown in FIG. 1, fastened with threaded connectors around the periphery of 
the frame member. Frame portions 3 and 4 with membrane member 1 mounted 
therebetween are joined together with bolt 10 and nut 11. In order that 
the sides of the frame member present a relatively flat surface to 
facilitate cell assembly, it is preferred that frame portions 3 and 4 be 
recessed in the areas adapted to receive bolt 10 and nut 11. 
FIG. 3 shows an alternative means of mounting membrane 1 in the frame 
member. In this mode, frame portions 12 and 13 clamp membrane 1 between 
peripheral tongue portion 14 and peripheral groove portion 15. Tongue and 
groove portions 14 and 15 are preferably fabricated to tightly fit 
together in an interlocking manner. Areas of frame portions 12 and 13 in 
locking contact with membrane 1 are preferably rounded in order that 
damage to the membrane is not incurred. 
FIG. 4 shows a further means of mounting membrane 1 in the frame. In this 
embodiment, membrane 1 is clamped between frame portions 16 and 17. Frame 
portion 16 is fabricated of a non-conducting material and has a peripheral 
projection 18. Frame portion 17 is fabricated of metal having spring 
properties and is sized to tightly fit periphery projection 18. Projection 
18 may be tapered slightly outward towards its outer surface to provide 
constricted portion 19 along its inner peripheries. Constricted portion 19 
provides a means of snap fitting frame portions 16 and 17 together. 
Preferably, all corners of frame portions 16 and 17 which come in contact 
with membrane 1 are rounded to avoid damaging the membrane during clamping 
of the membrane and frame portions. 
In order to provide electrical insulation between cell components, at least 
one portion of the present frame member is fabricated of a suitable 
non-conducting material. Non-conductive plastic materials which are 
resistant to corrosion by the electrolyte and can withstand the operating 
temperatures of the cell can be used. Examples of such suitable materials 
are various thermoplastic or thermosetting resins, such as polypropylene, 
polybutylene, polytetrafluoroethylene, after chlorinated or rigid FEP, 
chlorendic acid based polyesters, and the like. 
Membranes suited to use in the present invention include those fabricated 
of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a sulfonated 
perfluorovinyl ether. More specifically, such suitable membrane materials 
are fabricated of a hydrolyzed copolymer of tetrafluoroethylene and a 
fluorosulfonted perfluorovinyl ether of the general formula: FSO.sub.2 
CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF.dbd.CF.sub.2. Normally, the 
membrane wall thickness will range from about 0.02 to about 0.5 mm., and, 
preferably, from about 0.1 to about 0.3 mm. When mounted on 
polytetrafluoroethylene, asbestos, or other suitable network for support, 
the network filaments or fibers will usually have a thickness of from 
about 0.01 to about 0.5 mm., and, preferably, from about 0.05 to about 
0.15 mm. 
While there have been described various embodiments of the invention, the 
apparatus described is not intended to be understood as limiting the scope 
of the invention as it is realized that changes therewithin are possible, 
and it is intended that each element recited in any of the following 
claims is to be understood as referring to all equivalent elements for 
accomplishing the same results in substantially the same or equivalent 
manner, it being intended to cover the invention broadly in whatever form 
its principle may be utilized.