Electrolytic cell with membrane and solid, horizontal cathode plate

An electrolytic cell is disclosed comprising an upper anode compartment and a lower cathode compartment partitioned by a cation exchange membrane, in which partitioning spacers are provided on a cathode plate in order to eliminate the troubles owing to non-uniform flow of catholyte liquor, non-uniformity of anode-cathode gap, coarse surface of the cathode plate and the like.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to an electrolytic cell for 
electrolysis of an aqueous alkali metal halide solution, especially an 
aqueous alkali metal chloride solution. More particularly, it relates to 
an apparatus for mainly obtaining a high quality caustic alkali more 
effectively with low cell voltage using a horizontal type electrolytic 
cell providing a cation exchange membrane as an electrolytic separator. 
2. Description of Prior Art 
The horizontal type electrolytic cell is partitioned by an asbestos 
diaphragm positioned substantially horizontal into an upper anode 
compartment and a lower cathode compartment and has been in considerably 
widespread use industrially, because of an advantage that the product, for 
example, caustic alkali is produced in the cathode compartment, therefore, 
does not move to the anode compartment through the diaphragm. 
The most typical horizontal electrolytic cell is a mercury electrolytic 
cell but destined to be shut down in the near future since mercury served 
as a cathode contaminates environment. When such a mercury cathode 
electrolytic cell is desired to be converted into a separator electrolytic 
process employing no mercury with a reduced cost, the separator 
electrolytic process should be of a horizontal type. In view of the 
situation, it is a significant matter the industry is now encountering to 
develop a process for producing a high quality product, not inferior to a 
product by the mercury process, with a high current efficiency using such 
horizontal type separator electrolytic cells. 
A process for retrofitting a mercury cell to a horizontal type separator 
cell is revealed in U.S. Pat. No. 3,923,614. In the patent, however, a 
porous membrane (asbestos diaphragm) is used to serve as a separator, 
having great water permeability and accordingly anolyte solution passes 
through the separator hydraulically to thus mingle in, for example, 
caustic alkali produced in the cathode compartment, thereby resulting in 
decreased quality. 
On the other hand, a cation exchange membrane called a nonporous membrane 
permits no passage of anolyte solution or catholyte liquor hydraulically, 
allowing only water molecules coordination-bonded to alkali metal ions 
transported electrically to pass, hence a high quality caustic alkali 
being obtained. To the contrary, a small quantity of water transported 
evaporates to cause electric conduction failure between a membrane and a 
cathode, in the long run to terminate electrolytic reaction. 
U.S. Pat. No. 3,901,774 proposes processes to solve the problems; one is a 
process for placing a liquid maintaining material between a cation 
exchange membrane and a cathode and another is a process for carrying out 
the electrolysis while supplying into a cathode compartment an aqueous 
caustic alkali solution in the form of mist or spray in order to maintain 
electric conductivity. 
Notwithstanding, the former process not only involves the problems 
including troubles for interposing the liquid maintaining material and the 
durability thereof, but increases cell voltage because the distance 
between electrodes is expanded by the liquid maintaining material located 
between the cation exchange membrane and the cathode, besides an increase 
in electric resistance of the liquid maintaining material per se. Hence it 
can not be an advantageous process. Moreover the latter process has some 
difficulties in practice on an industrial scale since the uniform supply 
of liquid is difficult when applied to a large-scale electrolytic cell 
such as employed commercially. 
In an attempt to eliminate the foregoing defects attendant on the 
conventional processes, a process and apparatus therefor has been 
developed by the present applicant and proposed by U.S. Ser. No. 434,737 
(EPC Appln. No. 82109528). This proposal involves an electrolytic process 
characterized in that hydrogen gas generated on a cathode is allowed to be 
enfolded in a stream of catholyte liquor and removed out of a cathode 
compartment, and electrolytic cell which is characterized by an upper 
anode compartment and a lower cathode compartment partitioned by a cation 
exchange membrane positioned substantially horizontal, said anode 
compartment having therein substantially horizontal anodes and being 
surrounded by a top cover, side walls positioned so as to enclose the 
anodes and the upper side of the membrane, and being provided with an 
inlet and an outlet of anolyte solution and an outlet of anode gas, said 
cathode compartment being surrounded by a cathode plate, side walls so as 
to enclose the cathode plate and the underside of the membrane, and being 
provided with an inlet of catholyte liquor and an outlet of a mixed stream 
of the cathode gas and the catholyte liquor. 
However, during the course of further study, it has been found out by the 
inventors; (1) Non-uniform flow of catholyte liquor (mixed stream) and 
dead space occur owing to adhesion of cathode gas to the membrane. This 
dead space causes variation of pressure difference (.DELTA.p) in the flow 
of catholyte liquor (mixed stream) between catholyte liquor inlet and 
mixed stream outlet, brings about vibration of the membrane and damages 
the membrane through collision of the membrane with electrodes. (2) 
Maintaining of uniform anode-cathode gap is difficult. (3) When the 
cathode plate is not substantially flat on its surface (for instance, 
cathode plate having a concave-convex surface or a coarse surface) the 
membrane contacts with and rubs against the cathode plate to thus result 
in damage of the membrane. As a result, stable operation for a long period 
is prevented. 
The present invention has been completed in order to eliminate the 
deficiencies attendant on the conventional processes as aforesaid and 
enables the retrofit of a mercury cell into a horizontal type cation 
exchange membrane cell with a relative ease, at the same time, achieving 
the production of a high quality caustic alkali with a high current 
efficiency. The present invention is, of course, useful in newly 
constructing a cell with new materials. 
SUMMARY OF THE INVENTION 
An object of the present invention is to obtain a high quality caustic 
alkali with high current efficiency using a horizontal type membrane 
electrolytic cell. 
Another object of the present invention is to provide an improved 
horizontal type membrane electrolytic cell with high performance providing 
partitioning spacers sandwiched between the cathode and the membrane. 
A further object of the present invention is to provide a horizontal type 
membrane electrolytic cell with high performance, a horizontal type cation 
exchange membrane electrolytic cell, in particular, made by retrofitting a 
mercury electrolytic cell. 
Other objects of the present invention will be made apparent from the 
following description.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention encompasses an electrolytic cell comprising an upper 
anode compartment and a lower cathode compartment partitioned by a cation 
exchange membrane positioned substantially horizontal, 
said anode compartment having therein substantially horizontal anode plates 
and being surrounded by a top cover, side walls positioned so as to 
enclose the anodes and the upper side of the membrane, and being provided 
with at least one inlet of anolyte solution and at least one outlet of 
anolyte solution and/or anode gas, and 
said cathode compartment being surrounded by a cathode plate on which 
partitioning spacers are arranged at a suitable interval, side walls so as 
to enclose the cathode plate and the underside of the membrane, and being 
provided with at least one inlet of catholyte liquor and at least one 
outlet of a mixed stream of cathode gas and catholyte liquor. 
Hereinafter, embodiments of the present invention will be explained in 
detail by referring to the drawings attached. The following explanation is 
referred, as a matter of convenience, to sodium chloride which is most 
popular in the industry and typical of alkali metal halides, and to 
caustic soda as an electrolytic product, but to which the present 
invention is not limited, the present invention being, needlessly, applied 
to the electrolysis of an aqueous solution of other inorganic salts, and 
the like. 
In FIG. 1 and FIG. 2, an electrolytic cell of the present invention is 
comprised of an anode compartment (1) and a cathode compartment (2) 
located thereunder, both compartments being of a rectangular shape having 
the greater length than the width, preferably several times the length. 
The anode compartment (1) and the cathode compartment (2) are separated 
from each other by a cation exchange membrane (3) positioned substantially 
horizontal by being sandwiched between side walls of the compartments. The 
word "substantially horizontal" also includes the cases where the membrane 
is positioned slightly slant (up to a slope of about 2/10). In FIG. 1, 
catholyte liquor inlet and outlet are omitted. 
The cation exchange membrane used suitably in the present invention 
includes, for example, membranes made of perfluorocarbon polymers having 
cation exchange groups. The membrane made of a perfluorocarbon polymer 
containing sulfonic acid groups as a cation exchange group is sold by E. 
I. Du Pont de Nemours & Company under the trade mark "NAFION" having the 
following chemical structure; 
##STR1## 
The equivalent weight of such cation exchange membranes is preferred in a 
range between 1,000 and 2,000, more preferably in a range between 1,100 
and 1,500. The equivalent weight herein means weight (g) of a dry membrane 
per equivalent of an exchange group. Moreover membranes whose sulfonic 
acid groups are substituted, partly or wholly, by carboxylic acid groups 
and other membranes widely used can also be applied to the present 
invention. These cation exchange membranes exhibit very small water 
permeability so that they permit the passage of only sodium ion containing 
three to four molecules of water, while hindering the passage of hydraulic 
flow. 
The anode compartment (1) is formed by being surrounded by a top cover (4), 
side walls (5) of the anode compartment located so as to enclose anodes 
comprising anode conducting rods (6), anode conducting rod covers (9) and 
anode plates (12) and the upper side of a cation exchange membrane (3). 
The anode conducting rods (6) are suspended by anode-suspending devices 
(7) located on the top cover (4) and connected electrically to one another 
by an anode busbar (8). The top cover (4) possesses holes (10) through 
which anode conducting rod covers (9) are inserted and the holes (10) are 
sealed airtight by sheets (11). To the lower ends of the anode conducting 
rods (6) are anode plates (12) secured. As such, the anode plates (12) are 
connected to the anode-suspending devices (7), so that those can be 
ascended and descended by the adjustment of the anode-suspending devices 
(7), thereby being positioned so as to come into contact with the cation 
exchange membrane (3). Of course, the anodes may also be suspended by 
other means, not being limited to the cases where those are suspended from 
the anode-suspending devices positioned to the top cover. Moreover the 
anode compartment is provided with at least one anolyte solution inlet 
(13), which may be positioned to the top cover (4) or side walls (5) of 
the anode compartment. On the other hand, at least one anolyte solution 
outlet (14) is provided and may be positioned to the side walls (5). 
Furthermore, to a suitable place of the top cover (4) or the side walls 
(5), anode gas (chlorine gas) outlet (15) is provided. Anode gas may also 
be removed through the anolyte solution outlet (14) together with anolyte 
solution. 
As the material for the top cover (4) and side walls (5) forming the anode 
compartment (1), a top cover and side walls of an anode compartment of a 
mercury electrolytic cell may also be served. and any chlorine-resistant 
material may be effectively used. Examples of such materials are 
chlorine-resistant metals such as titanium and an alloy thereof, 
fluorocarbon polymers, hard rubbers and the like. Moreover iron lined with 
the foregoing metals, fluorocarbon polymers, hard rubbers and the like may 
also be employed. 
As the anode plate (12) on which the anode reaction takes place, a graphite 
anode may also be used, but a dimensionally stable anode made of metals 
such as titanium and tantalum coated with platinum group metals, platinum 
group metal oxides or mixtures thereof is preferred to use. Of course, 
anode plates used in a mercury electrolytic cell may be directly diverted 
without altering dimensions and shapes. 
The cathode compartment (2), on the other hand, is formed by being 
surrounded by the underside of the cation exchange membrane (3), a cathode 
plate (16) on the surface of which partitioning spacers (24) are arranged 
in parallel and side walls (17) of the cathode compartment positioned so 
as to enclose the cathode plate along the periphery of the cathode plate. 
The side walls (17) of the cathode compartment may be made of those such 
as frames having some rigidity or may also be made of packing-like elastic 
materials such as rubbers, plastics and the like. Furthermore, as shown in 
FIG. 3 the portion of the bottom plate opposing the anodes through the 
cation exchange membrane is shaved off except the periphery and the 
remaining bank-like periphery of the cathode plate is served as the side 
walls of the cathode compartment. Moreover the cathode compartment may be 
formed as illustrated by FIG. 4; that is, a thin layer packing (23) is 
placed on the periphery of the cathode plate (16), the anode plates (12) 
are located upper than the lower flange level of side walls forming the 
anode compartment and the cation exchange membrane (3) is located along 
the iniside surfaces of the side walls of the anode compartment utilizing 
the flexibility of the membrane to thus form the cathode compartment. 
As the material for the side walls (17) of the cathode compartment, any 
material resistant to caustic alkali such as sodium hydroxide may be used 
including, for example, iron, stainless steel, nickel and an alloy thereof 
in addition to the materials listed above for the side walls of the anode 
compartment. Iron base material lined with alkaliresistant materials may 
also be suitably used. Materials such as rubbers and plastics may also be 
used. As those materials, there are exemplified rubbers such as natural 
rubber, butyl rubber and ethylene-propylene rubber (EPR), fluorocarbon 
polymers such as polytetrafluoroethylene, copolymers of 
tetralfuoroethylene and hexafluoropropylene and copolymers of 
etylene-tetrafluoroethylene, polyvinyl chloride and reinforced plastics 
(FRP). 
As the cathode plate (16) used in the present invention, a bottom plate 
used in a mercury electrolytic cell may be economically served. The 
surface of the bottom plate becomes coarse owing to corrosion, errosion 
caused by mercury, electrical short-circuit and the like, and therefore 
when the bottom plate is directly served, the cation exchange membrane 
occasionally rubs against the coarse surface to thereby be damaged. Hence, 
it is desired to smooth the surface before serving. The smoothing may be 
attained by plating with nickel, cobalt, chrome, molybdenum, tungsten, 
plutinum group metals, silver and the like, bonding of a thin metal plate 
made of nickel, austenitic stainless steel and the like, mechanical 
polishing or other suitable manners. It is a preferred embodiment to 
employ the cathode plate, the surface of which was subjected to plasma or 
flame spray with nickel cobalt, chrome, molybdenum, tungsten, platinum 
group metals, silver, alloys of foregoings or mixtures of foregoings to 
reduce hydrogen overvoltage. To the surface of the cathode plate, 
electro-plating or electro-dispersion plating may be also preferably 
applied to reduce hydrogen overvoltage with, for example, Raney nickel 
including or not including plutinum group metals such as plutinum, 
ruthenium, palladium and the like. 
On the foregoing cathode plate (16), are partitioning spacers (24) provided 
at a suitable interval. The size and the interval of the partitioning 
spacers (24) had better be optionally determined according to the 
construction of the cell and operating conditions. For example, strips 
having height of about 0.5 to about 5 mm and width of about 3 to about 15 
mm may be provided at an interval of about 10 cm to about 1 m. The 
partitioning spacers are preferably arranged in parallel, but are not 
necessarily limited thereto. The material of the partitioning spacers may 
include alkali-resistant rubbers and plastics, metals such as iron whose 
surface was, partly or wholly (at least, top), covered with the foregoings 
rubber, plastics and the like. Moreover, plastics having 
electro-conductivity may also be used. The partitioning spacers (24) may, 
for example, be sandwiched between the anode plates (12) and the cathode 
plate (16) or embedded in the cathode polate (16) by adhesives or 
mechanical means. Furthermore, when the spacers (24) united to the side 
walls of the cathode compartment (17), it is possible to provide the 
spacers concurrently with assembling of the side walls of the cathode 
compartment. One of preferred embodiments is to employ a packing-like 
elastic material served as the cathode compartment side walls (17) to 
which the partitioning spacers (24) are united, and to allow the side 
walls to function as packings (23) simultaneously, as exhibited by FIG. 5. 
In this example, a bottom plate used in a mercury electrolytic cell is 
served as the cathode plate (16) and bolt holes made originally are 
utilized as bolt holes (25) for assembly as well as catholyte liquor 
inlets (outlets) (25a), so that assembling of the cathode compartment side 
walls (17), packings (23), partitioning spacers (24), catholyte liquor 
inlet (19) and outlet (20) can be achieved at one stroke. 
The partitioning spacers (24) are arranged along the flow of catholyte 
liquor (mixed stream). The catholyte liquor inlet (19) and the mixed 
stream outlet (20) had better be provided so as to cause flowing of said 
mixed stream to take plaace. Accordingly, the mixed stream may be allowed 
to flow either along the longitudinal way or traverse way of a 
rectangular-shaped cell, but the latter is preferred since the pressure 
difference (.DELTA.p) resulting from non-uniform flow is reduced, the 
value of G/(L+G) (gas content contained in unit volume of mixed stream of 
catholyte liquor and cathode gas) is minimized, in consequence, 
reinforcement of the cathode plate and the top cover may be omitted or 
minimized. In an attempt to attain such purpose, a slit-like inlet is a 
preferred embodiment. Moreover, as shown by FIG. 3 to FIG. 5, the inlet 
(19) and the outlet (20) may be provided, respectively, to the end of the 
cathode plate (16). In the event that the inlet and the outlet are 
comprised of incontinuous holes such as depicted by FIG. 5, the interval 
of the partitioning spacers had better be agreeable to pitches of the 
holes or every two or three pitch. 
The partitioning spacers (24) may preferably be provided as well along the 
flowing of catholyte liquor (mixed stream) in the traverse direction 
rather than the longitudinal direction of the rectangular cell. The 
spacers (24) may not necessarily be continuous from the catholyte liquor 
inlet (19) to the mixed stream outlet (20), but intermittent. 
In FIG. 6, there is depicted a sectional view of a horizontal type cation 
exchange membrane electrolytic cell retrofitted from a mercury 
electrolytic cell according to the present invention, including a 
catholyte liquor circulating system. 
In FIG. 1 and FIG. 6, an anode compartment (1) is formed by being 
surrounded by a top cover (4), side walls (5) of the anode compartment 
provided so as to enclose a plurality of anode conducting rods (6) and 
anode plates (12) and the upper side of a cation exchange membrane (3) 
positioned by being sandwiched between the lower flange of anode 
compartment side walls (5) and cathode compartment side walls (not shown). 
The anode conducting rods (6) are suspended vertically by anode-suspending 
devices (7) located protruding at the top cover (4) and connected 
electrically to each other by a busbar (8). The anode compartment (1) is 
provided with an anolyte solution inlet (13), an anolyte solution outlet 
(14) and an anode gas outlet (15). 
On the other hand, a cathode compartment (2) is formed by being surrounded 
by a cathode plate (16), directly served from a bottom plate of a mercury 
electrolytic cell, smoothed, if required, on the surface of which 
partitioning spacers (24) are provided, cathode compartment side walls 
positioned at the periphery of the cathode plate (16) and the underside of 
the cation exchange membrane (3). The cathode plate (16) is connected to a 
cathode busbar (18). The cathode compartment (2) is provided with a 
catholyte liquor inlet (19) and an outlet (20) of a mixed stream of 
catholyte liquor and cathode gas. 
An approximately saturated brine is supplied through the anolyte solution 
inlet (13) into the anode compartment (1) and then electrolysed therein. 
Chlorine gas generated is removed through the anode gas outlet (15) and 
the depleted brine is discharged through the anolyte solution outlet (14). 
The catholyte liquor is supplied through the catholyte liquor inlet (19) 
into the cathode compartment (2) and mixed with hydrogen gas evolved in 
the cathode compartment to provide a mixed stream, discharged through the 
outlet (20) of the mixed stream, then the mixed stream being transported 
to a separating tank (21) in which hydrogen gas is separated from caustic 
liquor. The aqueous caustic alkali solution containing substantially no 
hydrogen gas is recirculated by use of a pump (22) through the catholyte 
liquor inlet (19) to the cathode compartment (2). 
The separating tank (21) and the pump (22) may be one, respectively, for a 
plurality of cells, otherwise, for each cell. 
The electric current is supplied to an anode busbar (8), passed through the 
bottom plate (16) of the cathode compartment (2) and then taken out from a 
cathode busbar (18). 
In the anode compartment (1), the following reaction takes place; 
##STR2## 
Sodium ions in the anode compartment (1) move through the cation exchange 
membrane (3) to the cathode compartment (2). In the cathode compartment 
(2), on the other hand, the following reaction occurs; 
##STR3## 
In the cathode compartment sodium hydroxide is produced by reaction of 
hydroxyl ions with sodium ions transparent through the cation exchange 
membrane (3) from the anode compartment (1), concurrently with evolution 
of hydrogen gas. 
In the electrolysis using a cation exchange membrane, a vertical type cell 
is commonly employed. In this case, cathode gas generated in the cathode 
compartment is rapidly removed behind the cathode (i.e., to an opposite 
direction to the cation exchange membrane), and hence a porous cathode 
fabricated of expanded metal sheets, perforated metal sheets, metal nets 
and the like with a view to reducing electric resistance of the catholyte 
liquor may be used. 
Nonetheless, in the case of a horizontal type cell it is impossible to 
remove cathode gas with a small specific gravity compared with catholyte 
liquor behind the cathode, i.e., under the cathode located extending to a 
horizontal way. 
Therefore, the greatest feature of the present invention lies in that into 
the cathode compartment comprised of the underside of the cation exchange 
membrane (3) and the cathode plate (16) with gas-liquid impermeability 
positioned adjacent thereto, catholyte liquor is supplied and the cathode 
compartment is filled therewith to thus form a mixed stream of catholyte 
liquor and cathode gas, with which the underside of the cation exchange 
membrane (3) is wetted to allow the electrolysis reaction to take place 
smoothly, at the same time, sodium hydroxide and cathode gas produced in a 
space between the cation exchange membrane (3) and the cathode plate (16) 
are enfolded in the stream, then discharged outside the cathode 
compartment (2). 
It is advantageous to recirculate back to the catholyte liquor inlet (19) 
at least a part of the catholyte liquor which is supplied into the cathode 
compartment, removed together with cathode gas and caustic soda produced 
and then separated from hydrogen gas by the separating tank (21), since 
the concentration of caustic soda can be increased optionally and adjusted 
by being diluted with water. 
As was stated earlier, the present invention is capable of retrofitting 
mercury electrolytic cells to cation exchange membrane electrolytic cells 
very feasibly, and therefore almost all existing equipments including 
busbars, rectifiers, disposal equipments of depleted brine and brine 
system equipments as well as electrolytic cells can be served without 
being scrapped. The present invention further prevents troubles due to 
non-uniform flow of catholyte liquor (mixed stream), non-uniformity of 
anode-cathode gap, coarse surface of the cathode plate and the like, to 
thus enable long-term stable operation.