Note: Descriptions are shown in the official language in which they were submitted.
.
'' ~oss942
This invention relates to an improved process and apparatus
for the electrolysis of alkali metal halide brines and more particularly
it relates to a process for the electrolysis of alkali halide brines
in an electrolytic cell having at least three compartments, which cell
utilizes a diaphragm or membrane which is substantially impervious
to fluids and gases.
The production of numerous commercial chemicals by the
electrolysis of various electrolyte solutions is well known. For
example, chlorine and caustic soda are produced commercially by the
electrolysis of sodium chloride brine solutions. Typically, this
process is carried out in an electrolytic cell having an anode
compartment and a cathode compartment, which compartments are
separated by a fluid-permeable diaphragm, such as an asbestos dia-
phragm The sodium hydroxide produced by this method is, however,
relatively dilute and, because of the fluid permeable nature of the
diaphragms used, it is further contaminated with various impurities,
such as sodium chloride, sodium chlorate, iron and the like. It is,
therefore, necessary to subject the sodium hydroxide product to
various evaporation and purification steps in order to obtain a
product which is suitable for many commercial uses. Moreover, with
such electrolytic cells, there is an appreciable back migration of
hydroxyl ions from the cathode compartment to the anode compartment
which results in the production of hypochlorites wh;ch are oxidized
to chlorates, with a consequent reduction in chlorine yield and
further contamination of the sodium hydroxide. Additionally,
depending upon the source of sodium chloride used in making up the
brine electrolyte, brine purification systems must frequently be
used to eliminate ions such as calcium, that may clog the fluid
permeable diaphragms.
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105994Z
i
Attempts have heretofore been made to overcome the afore-
said difficulties in the operation of such d;aphragm cells by re-
placing the fluid permeable asbestos diaphragms with permselective
ion exchange membranes. In theory, the use of such membranes which,
for example, would permit the passage of only sodium ions from the
anode compartment to the cathode compartment, would eliminate the
problems of contamination of the sodium hydroxide liquor in the
cathode compartment and would prevent the back migration of the
hydroxyl ions to the anode compartment. For this purpose, various
resins, such as cation exchange resins of the "Amberlitei' (TM) type,
sulfonated copolymers of styrene and divinylbenzene, and the like,
have been proposed. In practice, however, the permselective ion ex-
change membranes which have been used have generally been found not
to be stable to the strong caustic and/or acidic solutions encountered
in the cells at operating temperatures above 75 degrees C. so that they
have had only a relatively short çffective life. Additionally, as the
concentration of caustic soda in the catholyte liquor is increased,
e.g., above about 200 grams per liter, it has frequently been found
that the ion selectivity and chemical compatibility of the membrane
decreases, the voltage drop through the membrane becomes unacceptably
high and the caustic efficiency of the electrolysis process decreases.
Moreover, in many instances, the resins which have been used have
been found to be relatively expensive so that the fabrication costs
of the membrane has been unacceptably high. Attempts to overcome
these drawbacks by utilizing one or more buffer compartments between
the anode and cathode compartments of the cells have not solved the
problem so that at the present time, there has been no appreciable
utilization of membranes of this type for the commercial production of
~05994Z
various chem;cals, such as chlorine and caustic soda.
It is, therefore, an object of the present invention to provide
an improved apparatus suitable for the electrolysis of alkali metal halide
brines.
Another object of the present invention is to provide an improved
process for electrolyzing aqueous solutions of ionizable chemical compounds,
such as alkali metal halide brines, which is not subject to many of the
disadvantages which have heretofore been encountered in the prior art
processes.
A further object of the present invention is to provide an improved
electro1ysis apparatus which utilizes ion selective membranes and to provide
a process for electrolyzing alkali metal halide brine using such apparatus.
These and other objects will become apparent to those skilled in
the art from the description of the invention which follows.
In the drawing which is attached hereto and forms a part hereof,
Figure 1 is a schematic representation of a three compartment electrolytic
cell of the present invention and;
Figure 2 is a schematic representation of a modification of the
electrolytic cell shown in Figure 1.
Pursuant to the above objects, the present invention includes an
electrolytic cell, suitable for use in electrolyzing alkali metal halide
brines, which comprises a cell body having an anode compartment containing
an anode connected to an electrical input source, a cathode compartment
containing a cathode and at least one buffer compartment between said anode
and cathode compartments, said compartments being separated from each other
by at least one side wall barrier located between the anode and buffer com-
partments, the cathode and buffer compartments and between individual buffer
compartments, said barriers being substantially impervious to fluids and
gases selected from a hydrolyzed copolymer of a perfluorinated hydrocarbon
and a sulfonated perfluorovinyl ether and a sulfostyrenated perfluorinated
ethylene propylene polymer. By the use of electrolytic cells of this type,
105994Z
it is found that highly concentrated alkali metal hydroxide solutions,
which are significantly low in impurities, can be produced with maxi-
mum electrical operating efficiency.
In a preferred embodiment of the invention the permselective
membranes are of a hydrolyzed copolymer of tetrafluoroethylene and a
fluorosulfonated perfluorovinyl ether of the formula FS02CF2CF20CF-
(CF3)CF20CF=CF2, which has an equivalent weight of about 900 to
1,600 and the membranes are mounted on networks of supporting
materials such as polytetrafluoroethylene or asbestos filaments. The
described preferred copolymers may be further modified to improve
their activities, as by surface treating, modifying the sulfonic
group or by other such mechanism. Such varieties of the polymers
are included within the generic description given.
More specifically, the electrolytic cell of the present
invention comprises a cell body or container formed of materials which,
as such, or when provided with a suitable coating, will be electrically
non-conductive and withstand the temperature at which the cell may be
operated and will also be resistant to the materials being processed
in the cell, such as chlorine, sodium hydroxide, hydrochloric acid,
and the like. Exemplary of materials which may be used are various
polymeric materials, such as high temperature polyvinyl chloride,
hard rubber, chlorendic acid based polyester resins, and the like.
Additionally, materials such as concrete, cement, and the like, may
also be used. In the case of these latter materials, however, any
interior exposed areas should have a coating which is resistant to
hydrochloric acid, chlorine, caustic soda, or similar materials with
which said surfaces will be in contact. Additionally, the cell body
may be made of metal, such as steel, titanium, or the like, if the
exposed surfaces are coated with a corrosion protective material and
electrical insulation is provided where necessary.
lOS99~Z
The electrodes for the present electrolytic cell ~ay be formed of
any electrically conduct;ve material which will resist the corrosive attack
of the various cell reactants and products with which they may come in
contact, such as alkali metal hydroxides, hydrochloric acid, and chlorine.
Typically, the cathodes may be constructed of graphite, iron, steel, or the
like, with steel being generally preferred. Similarly, the anodes may be
formed of graphite or may be metallic anodes, Typically, where metallic
anodes are used, these may be formed of a so-called "valve" metal, such as
titanium, tantalum or niobium as wefl as alloys of these in which the valve
metal constitutes at least about 90% of the alloy. The surface of the valve
metal may be made active by means of a coating of one or more noble metals,
noble metal oxides, or mixtures of such oxides, either alone or with oxides
of other metals. The nsble metals which may be used include ruthenium,
rhodium, pallad;um, ;ridium, and platinum. Particularly preferred metal
anodes are those formed of titanium and having a mixed titanium oxide and
ruthenium oxide coating on the surface, as is described in U.S. patent
3,632,498. Additionally, the valve metal substrate may be clad on a more
electrically conductive metal core, such as aluminum, steel, copper, or
the like.
The cell body or container is formed into at least one set or
unit of compartments made up of an anode compartment, containing the anode,
a cathode compartment, containing the cathode, and at least one buffer
compartment between the anode and cathode compartments. Typically, the
electrolytic cell will contain a plurality of these sets, e.g., 20 to 30
or more, depending upon the size of the cell.
These compartments are separated from each other by a barrier or
membrane which is substantially impervious to fluids and gases and composed
essentially of a hydrolyzed copolymer of a perfluorinated hydrocarbon and
a fluorosulfonated perfluorovinyl ether. The perfluorinated hydrocarbon
is preferably tetrafluoroethylene, although other perfluorinated and sat-
urated and unsaturated hydrocarbons of 2 to 5 carbon atoms may also be
utilized, of which the monoolefinic hydrocarbons are preferred,
,~,.,, ~;
.3~. J
~OS994Z
especially those of 2 to 4 carbon atoms and most especially those of 2 to
3 carbon atoms, e.g., tetrafluoroethylene, hexafluoropropylene. The sul-
fonated perfluorovinyl 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 the
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
10 of the perfluoro-lower alkyl groups, respectively. However, it is most
preferred to employ PSEPVE .
The method of manufacture of the hydrolyzed copolymer is des-
cribed in Example XVII of U.S. patent 3,282,875 and an alternative method
is mentioned in Canadian patent 849,670, which also discloses the use of
15 the finished membrane in fuel cells, characterized therein as electro-
chemical cells. In short, the copolymer may be made by reacting PSEPVE
or equivalent with tetrafluoroethylene or equivalent in desired propor-
tions in water at elevated temperature and pressure for over an hour, after
which time the mix is cooled. It separates into a lower perfluoroether
20 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 mold-
~5 ed at high temperature and pressure to product sheets or membranes, whichmay 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 paten~ 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
~05994Z
Because it has been found that some expansion accompanies
hydrolysis of the copolymer it is preferred to position the copolymer
membrane after hydrolysis onto a frame or other support which will hold
it ~n place in the electrolytic cell. Then it may be clamped or cemented
in place and will be true, without sags. The membrane is preferably
joined to the filaments of a tetrafluoroethylene backing or other suit-
able filaments prior to hydrolysis, when it is still thermoplastic; and
the film of copolymer covers each filament, penetrating into the spaces
between them and even around behind them, the films becoming slightly
thinner in the process, where they cover the filaments.
The membrane described is far superior in the present processes
to all other previously suggested membrane materials. It is more stable
at elevated temperatures, e.g., abo~e 75C. It lasts for much longer
time periods in the medium of the electrolyte and the caustic product and
does not become brittle when subjected to chlorine at high cell tempe-
ratures. Considering the savings in time and fabrication costs, the
present membranes are more economical. The voltage drop through the
membrane is acceptable and does not become inordinately high, as it does
with many other membrane materials, when the caustic concentration in the
cathode compartment increases to above about 200 9./l. of caustic. The
selectivity of the membrane and its compatibility with the electrolyte
does not decrease detrimentally as the hydroxyl concentration in the
catholyte liquor increases, as has been noted with other membrane mate-
rials. Furthermore, the caustic efficiency of the electrolysis does not
diminish as significantly as it does with other membranes when the hy-
droxyl ion concentration in the catholyte increases. Thus, these differ-
ences in the present process make it practicable, whereas previously
described processes have not attained commercial acceptance. While the
more preferred copolymer are those having equivalent weights of 900 to
1,600 with 1,100 to 1,400 being most preferred, some useful resinous
membranes produced by the present
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1059~4Z
method may be of equivalent weights from 500 to 4,000. The medium
equivalent weight polymers are preferred because they are of satis-
factory strength and stability, enable better selective ion exchange
to take place and are of lower internal resistances, all of which
are important to the present electrochemical cell.
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 or may be replaced in part with other moieties. Such changes
10 may be made in the manufacturing process or after productlon of the
membrane. When effected as a subsequent surface treatment of a membrane
the depth of treatment will usually be from 0.001 to 0.01 mm. Caustic
efficiencies of the invented processes, using such modified versions of
the present improved membranes, can increase about 3 to 20%, often about
5 to 15%.
In addition to the copolymers previously discussed, including
modifications thereof, it has been found that another type of membrane
material is also superior to prior art films for applications in the
present processes. Although it appears that tetrafluoroethylene (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-
fluorinated ethylene propylene polymer (FEP) which is styrenated and
sulfonated makes a useful membrane. Whereas useful lives of as much as
three years or more (that of the preferred copolymers) may not be obtained
the sulfostyrenated FEP'~s are surprisingly resistant to hardenin3 and
otherwise failing in use under the present process conditions.
To manufacture the sulfostyrenated FEP membranes a standard
FEP, such as manufactured by E. I. DuPont de Nemours & Co. Inc., is
styrenated and the styrenated polymer is then sulfonated. A solution of
styrene in methylene chloride or benzene at a suitable concentration in
~ 1059942
the range of about 10 to 20% is prepared and a sheet of FEP polymer
having a thickness of about 0.02 to 0.5 mm., preferably 0.05 to 0.15 mm.,
is dipped into the solution. After removal it is subjected to radiation
treatment, using a cobalt60 radiation source. The rate of application
may be in the range of about 8,000 rads/hr. and a total radiation appli-
cation is about 0.9 megarads. After rinsing with water the phenyl rings
of the styrene portion of the polymer are monosulfonated, preferably in
the para position, by treatment with chlorosulfonic acid, fuming sulfuric
acid or S03. Preferably, chlorosulfonic acid in chloroform is utilized
10 and the sulfonation is completed in about 1/2 hour.
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 and
having 13/16 of the phenyl groups monosulfonated. To obtain 18% sty-
renation a solution of 17-1/2% of styrene in methylene chloride is uti-
lized and to obtain the 16% styrenation a solution of 16% of styrene in
methylene chloride is employed.
The products resulting compare favorably with the preferred co-
polymers previously described, giving voltage drops of about 0.2 volt eachin the present cells at a current density of 2 amperes/sq.in., the same
; as is obtained from the copolymer.
Desirably, these membranes are utilized in the form of a thin
film, either as such, or deposited on an inert support or substrate, such
as a cloth woven of polytetrafluoroethylene, glass fibers or the like.
The thickness of such a supported membrane can be varied considerably,
thicknesses of from about 3 to 15 mills being typical. These membranes
may be fabricated into any desired shape, depending upon the configuration
of the cell in which they are used. As has been noted, the membrane co-
polymer is initially obtained in a non-acid form, i.e., in the form of
the sulfonyl fluoride. In this non-acid form, it is relatively soft and
- 10-
105994Z
pl~able and can be seam or butt welded to form welds which are as strong
as the membrane material itself. Accordingly, it is preferred that the
membrane material be shaped and formed in this non-acid state.
Once the membrane has been shaped or formed into the desired
configuration, it is then conditioned for use by hydrolyzing the sulfonyl
fluoride groups to free sulfonic acid or alkali metal sulfonate groups,
for example by boiling in water or alkaline solution, such as caustic
solution. This conditioning process may be carried out either before
the membrane is placed in the cell or within the cell with the membrane
in place. Typically, when the membrane is boiled in water for about 16
hours, the material undergoes swelling of about 28%, about 9% in each
direction. Upon exposure to brine, during operation, the swelling is
reduced to about 22%, resulting in a net tightening of the membrane
during use.
In some instances, it has been found that it may be desirable
to use a "sandwich" of two or more of these membranes, rather than only
a single membrane. When such a sandwich is used in a chlor-alkali cell,
it has been found that in some instances there is an increase in the
caustic efficiency of the cell, particularly when operating at catholyte
liquor caustic concentrations in excess of about 200 grams per liter.
With the electrolytic cells of the present invention which have one or
more buffer compartments between the anode and cathode compartment, how-
ever, this increase in caustic efficiency may not be sufficiently great
as to offset the increased material cost of using such a membrane
sandwich. Accordingly, although the use of such a sandwich is possible
in the present cell, it may not always be preferred.
The anode compartment of each set or unit of compartments is
formed with an inlet for introducing a liquid electrolyte into the com-
partment, such as an aqueous alkali metal halide brine and an outlet for
gaseous reaction products, such as chlorine. The cathode compartment
J~059942
of each set or unit is formed with an outlet for liquid reaction products,
such as aqueous solutions of alkali metal hydroxide, and also an outlet
for gaseous by-products, such as hydrogen. If desired, the cathode com-
partment may also be formed with an inlet for a liquid electrolyte, such
as water, dilute alkali metal hydroxide solutions, or the like. Addi-
tionally, each of the buffer compartments between the anode and cathode
compartments is formed with an inlet for liquid electrolytes, such as
water and, if desired may also have an outlet for liquid reaction pro-
ducts, such as dilute alkali metal hydroxide solutions. Preferably, the
inlets for liquid materials and the outlets for gaseous products in each
of the compartments are located in the upper portion of the compartment
while the outlets for liquid materials are positioned in the lower portion
of the compartments, although other locations may also be used.
These repeating sets or units of anode, buffer, and cathode
lS compartments may be formed into the total electrolytic cell of the
present invention in any convenient manner. Thus, in a preferred embodi-
ment, the cell is of the so-called "filter press" type. In this embodi-
ment, the anodes, cathodes, and membranes are mounted in suitable mounting
or frame members which are provided with suitable sealing gaskets and
are formed so as to provide the desired spacing between the elements to
form the anode, cathode and buffer compartments. These frame members are
provided with the desired inlets and outlets, as have been described
and are secured together by tie rods, bolts, or other suitable means as
is known in the art. Typical of such a filter press configuration is
that shown in U.S. patent 2,282,058.
Alternatively, the cell body may be in the form of a box of a
suitable material of construction in which anode, cathode and membrane
are mounted to form the various compartments, such as that shown in
U.S. patent 3,324,023. Additionally, the cell may be of the "conven-
tional" chlor-alkali type having interleaved anode and cathodes,
~05994Z
wherein the deposited asbestos, diaphragm is replaced with the various
membranes as have been described, to form the desired buffer cornpartments.
Typical of such a cell structure is that shown in U.S. patent 3,458,411.
It is to be appreciated that the above are merely exemplary of
the various cell configurations which may be used. In all of these, of
course, suitable materials of construotion will be used, as have been
described hereinabove. Additionally, it is further to be appreciated
that the particular configuration used in each instance will depend upon
the various specific requirements for that particular cell.
Referring now to the drawings, in Figure 1, which is a sche-
matic representation of a three compartment cell of the present invention,
the cell body is shown at (1). The cell body (1) is formed into an anode
compartment (3), a cathode compartment (7) and a buffer compartment (11)
which separates the anode and cathode compartments. An anode (5) and a
cathode (9) are disposed within the anode and cathode compartments, res-
pectively. Forming the buffer compartment (11) and separating it from
the anode compartment (3) and the cathode compartment (7) are barriers
or membranes (13) and (15), respectively, which barriers are formed of a
hydrated cationic exchange resin membrane which a film of a fluorinated
copolymer having pendent sulfonic acid groups, as has been defined here-
inabove.
The anode compartment (3) is provided with an inlet (17) through
which the electrolyte, such as a sodium chloride brine, is introduced.
An outlet (19) is also provided in the anode compartment, through which
outlet the depleted electrolyte is removed from the anode compartment.
Additionally, the anode compartment is provided with a gas outlet (21)
through which the gaseous decomposition products of the electrolysis,
such as chlorine. are removed from the anode compartment. Although
the electrolyte inlet and gaseous product outlets are shown as
- 13 -
1059942
being located in the upper portion of the anode compartment with the
electrolyte outlet in the lower portion, other arrangements for these
lnlets and outlets may be utilized if desired.
Similarly, in the buffer compartment (11), an inlet (23)
and an outlet (27) are provided. Where the electrolytic cell is
utilized for the electrolysis of a sodium chloride brine to produce
chlorine and caustic soda, water will be introduced into the buffer
compartment through the inlet (23) and, if desired, a dilute solution
of sodium hydroxide may be withdrawn from the outlet (27). Additionally,
0 the cathode compartment (7) contains an inlet (29) and an outlet (25)
through which, respectively, in the preferred electrolysis of a sodium
chloride brine, water or dilute caustic soda solutions are introduced
and a concentrated caustic soda solution, of high purity, is recovered
as a product of the process. Additionally, the cathode compartment
may also contain an outlet for gaseous by-products, such as hydrogen,
(not shown). As in the case of the inlets and outlets for the anode
compartment, the inlets and outlets for the buffer compartment and
cathode compartment may, if desired, be positioned other than in the
upper and lower portions, respectiYely, of the compartments, as is
shown in Figure 1.
Referring now to Figure 2, this is a schematic representation
of a modification of the electrolytic cell shown in Figure 1, in which
the cell is provided with more than one buffer compartment between the
anode and cathode compartments~ As is shown in this Figure, the cell
body (2) is formed into an anode compartment (4~ and a cathode compart-
ment (8), which compartments are separated by two intermediate or buffer
compartments (12) and (14). An anode (6) and a cathode (10) are posi-
tioned in the anode compartment (4) and cathode compartment (8), res-
pectively. A series of barriers or membranes (16~, (18) and (20) form
the buffer compartments (12) and (14) and separate them from the anode
compartment and the cathode compartment. All three of these membranes
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~05994Z
are formed of a film of a fluorinated copolymer having pendant sulfonic
acid groups, as has been described hereinabove.
An inlet t22) and an outlet (24) are provided in the anode
compartment for the introduction and removal of the electrolyte, such as
a sodium chloride brine. Additionally, an outlet (26) is also provided
in the anode compartment for the removal of gaseous decomposition products,
such as chlorine. The buffer compartments (12) and (14) are each provided
with inlets (30) and (32) respectively, and outlets (36) and (38),
respectively. Where the cell is utilized for the electrolysis of a sodium
chloride brine, typically water will be introduced into the inlets (30)
and (32) and dilute solution of caustic soda will be removed from the
outlets (36) and (38). Additionally, an inlet (40) and an outlet (34)
are provided in the cathode compartment (8). Where a sodium chloride
brine is being electrolyzed, a concentrated solution of caustic soda
of high purity will be recovered from the outlet (34) and water or a
dilute caustic soda solution may be introduced through the inlet (40).
As with the cell shown in Figure 1, an outlet for gaseous decomposition
products, such as hydrogen, (not shown) may also be provided in the
cathode compartment. Additionally, as with the cell configuration shown
in Figure 1, the positions of the various inlets and outlets may be
changed, depending upon the particular mode of operation which is desired.
In carrying out the process of the present invention, a solution
of the ionizable compound to be electrolyzed is introduced into the anode
compartment of the electrolytic cell. Exemplary of the various solutions
~5 of ionizable compounds which may be electrolyzed and the products produced
are aqueous solutions of alkali metal halides to produce the alkali metal
hydroxides and halogen; aqueous solutions of HCl to produce hydrogen and
chlorine; aqueous solutions of ammonium sulfate to produce persulfates;
aqueous solutions of borax to produce perborates, and the like. Of
- 15 -
~059942
these, the most preferred anolyte solutions are the aqueous solutions
of alkali metal halides, and particularly sodium chlorlde, and aqueous
solutions of HCl.
In a typical process, utilizing a sodium chloride brine as
the feed to the anode compartment, the feed solution will contain from
about 250 to 325 grams per liter sodium chloride and, most preferably,
about 320 grams per liter sodium chloride. The pH of this anolyte
feed solution is typically within the range of about 1.0 to 10.0,
with a pH of about 3.5 being preferred. These desired pH values in the
10 anode compartment may be maintained by the addition of acid to the
anolyte solution, preferably hydrochloric acid. The anolyte overflow
or depleted anolyte solution removed from the anode compartment will
generally have a sodium chloride content of from about 200 to 295
grams per liter, with a sodium chloride content of about 250 grams per
liter being typical.
In a three compartment cell, i.e., a cell having one or more
repeating units of an anode compartment and a cathode compartment separated
by a single center or buffer compartment, water is introduced into the
center or buffer compartment and a dilute solution of sodium hydroxide
is removed from this compartment. Generally, this solution will have a
sodium hydroxide content of from about 50 to 200 grams per liter with
a sodium hydroxide content of about 100 grams per liter being typlcal.
Preferably, this dilute solution of sodium hydroxide is introduced
into the cathode compartment, either with or without additional water,
to form the catholyte liquor. From the cathode compartment there is
obtained a more concentrated sodium hydroxide solution, having NaOH
concentration of from about 150 to 250 grams per liter, with the sodium
hydroxide content of about 160 grams per liter being typical. Addi-
tionally, gaseous products of chlorine gas and hydrogen gas are obtained
from the anode compartment and the cathode compartment, respectively.
- 16 -
105994Z
In an alternative method of operation, water is added to both
the center or buffer compartment and to the Gathode compartment and
there is recovered from the buffer compartment a product stream of
dilute sodium hydroxide and, from the cathode compartment, a product
stream of more concentrated sodium hydroxide solution. When operating
in this manner, the amount of dilute caustic soda solution recovered
from the buffer compartment and the amount of concentrated caustic soda
solution recovered from the cathode compartment may be varied, depending
upon the particular requirements for each type of solution. In a typi-
cal operation, approximately 50% of the sodium hydroxide will be re-
covered as a dilute solution from the buffer compartment with the other
50% being recovered as the more concentrated solution from the cathode
compartment. The concentration of the dilute caustic soda solution will
generally be within the range of about 50 to 200 grams per liter with a
concentration of about 100 grams per liter being typical. Similarly,
the concentration of the more concentrated caustic solution from the
cathode compartment will generally be within the range o~ about 200 to
420 grams per liter with a concentration of about 280 grams per liter
being typical.
~ The electrochemical decomposition process of the present in-
vention is typically carried out at a voltage within the range of about
3.4 to 4.8, with a voltage of about 4.2 being preferred. Typically,
the current densities are within the range of about 0.8 to 2.5 amps
per square inch, with current densities of about 2 amps per square inch
being particularly preferred. In general, the cell will be operated at
temperatures within the range of about 90 to 105 degrees centigrade
with temperatures of about 95 degrees centigrade being typical. When
operating in this manner, it is found that chlorine or anode efficiencies
of at least about 96% and cathode or caustic soda efficiencies of at
least 85% and frequently in excess of 90% are obtained. Additionally,
- 17 -
lOS994;~
the concentrated caustic soda solution obtained from the cathode com-
partment is found to be of high purity, at least approaching, it not
equal, that of "Rayon grade" caustic soda. Typically, the purity of this
sodium hydroxide is such that it is substantially free of sodium chlorate
and contains less than one gram per liter of sodium chloride.
Where the process is carried out with cells having sections
or repeating units which contain two or more buffer compartments, the
operation is similar to that which has been described hereinabove. Thus,
water may be introduced into each of the buffer compartments and into the
cathode compartment and a portion of the sodium hydrnxide product values
may be recovered from each of the buffer compartments, as a dilute solu-
tion of sodium hydroxide and from the cathode compartment as a more con-
centrated sodium hydroxide solution. Preferably, however, the dilute
sodium hydroxide solutions from each buffer compartment is introduced as
at least a portion of the feed to the next succeeding buffer compartment,
and ultimately into the cathode compartment so that there is obtained
from the cathode compartment a concentrated sodium hydroxide product
stream of high purity.
As has been indicated hereinabove, in addition to the electro-
lysis of sodium chloride brine solutions, to produce chlorine and causticsoda, in another preferred operation, the electrolytic cells of the present
invention may be used for the electrolysis of hydrochloric acid solutions,
to form chlorine and hydrogen as the products of the process. In such
an operation, the anolyte solution introduced into the anode compartment
is an aqueous solution of hydrochloric acid, desirably having an HCl
content of from about 10% to 36% by weight and preferably having an HCl
content of from about 15 to 25% by weight. Although the feed to the
buffer compartments and the cathode compartments may be water alone,
in the most preferred method of operation, the feed to both the buffer
compartments and the cathode compartment is also an aqueous hydrochloric
acid solution. Desirably, the HCl content of these feed solutions is
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.,.
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~059942
from about 1 to 10% by weight, with an HCl content of from about 1 to
5% by weight being preferred. Although it is preferred that the feed
solution to the anode, buffer and cathode compartments be substantially
free of contaminating ions, in many instances it has been found to be
desirable to add alkali metal chlorides, such as sodium chloride to
the anolyte, in order to minimize corrosion, particularly where a steel
or similar corrodible cathode is used. In these instances, additions
of sodium chloride in amounts within the range of about 12 to 25% by
weight of the anolyte solution are typical.
In order that those skilled in the art may better understand
the present invention and the manner in which it may be practiced, the
following specif~c examples are given. In these examples, unless other-
wise indicated, temperatures are in degrees centigrade and parts and
percent are by weight. It is to be appreciated, however, that these
examples are merely exemplary of the method and apparatus of the present
invention and are not to be taken as a limitation thereof.
Example 1
A three compartment laboratory size cell was operated at 120
amperes, an anode current density of 2 amps/square inch and a voltage
of 4.1 volts. The cell was equipped with a metallic anode formed of
~-~ titanium with an Ru02 coating, a steel cathode and two cation exchange
membrane barriers, one on each side, separating the intermediate or
buffer compartment from the anode compartment and the cathode com-
partment. The membrane was a 10 mil thick film of a hydrolyzed co-
polymer of tetrafluoroethylene and sulfonated perfluorovinyl ether,
having an equivalent weight of about 1100 and prepared according to
U.S. Patent 3,282,875. Brine containing 320 gramslliter NaCl was
circulated through the anode compartment and water was added to both
the buffer compartment and the cathode compartment. HCl was added to
the anolyte to maintain the anolyte pH at about ~Ø The effluent
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lOS994Z
from the-buffer compartment contained about 116 grams/liter NaOH and
that from the cathode compartment contained about 3~4 grams/liter NaOH.
Upon blending the two effluent streams together, there was obtained a
solution which contained about 197 grams/liter NaOH and which was
substantially free of sodium chlorate and contained less than about 1
gram/liter NaCl. Over a period of 16.5 hours of operation, the caustic
or cathode current efficiency was about 85.7% and the chlorine or anode
current efficiency was about 97%.
Example 2
lû A commercial size three-compartment cell, of the type described
in Example 1, was operated at 150 KA, an anode current density of 2
amps/square inch, a voltage of 4.1 volts and a temperature of about
96C. An aqueous brine containing about 320 grams/liter NaCl was
introduced into the anode compartment and an anolyte overflow was
15 obtained from the anode compartment which had a pH of about 3.5 and
contained about 250 grams/liter NaCl, HCl being added as required to
maintain the anolyte pH at about 3.5. Water was fed to the buffer
compartment and a buffer compartment effluent was obtained whi.ch con-
tained about 110 grams/liter NaOH. This effluent was fed to the cathode
20 compartment and there was produced a catholyte effluent containing about
160 grams/liter NaOH, 0.5 grams/liter NaCl and no detectable (< 0.1
grams/liter) NaC103. During the time of operation, the cathode current.
efficiency was 85% and the anode current efficiency was 96%.
Example 3
The three-compartment cell of Example 2 was operated at
150 KA, an anode current density of 2.0 amps/square inch, a voltage
of 4.2 volts and a temperature of about 94C. The aqueous brine feed
to the anode compartment contained about 320 grams/liter NaCl and the
anolyte overflow was at a pH of about 4.0 and contained about 250 grams/
30 liter, HCl being added as required to maintain the anolyte pH at about
4Ø ~later was fed to both the buffer compartment and the cathode
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~05994Z
compartment. A weak caustic effluent containing about 100 grams/liter
NaOH was obtained from the buffer compartment and a strong caustic
effluent containing about 280 grams/liter NaOH was obtained from
the cathode compartment. The cell produced about 2.3 tons/day NaOH,
5 as the weak caustic liquor, and about 2.4 tons/day NaOH, as the strong
caustic liquor, at a cathode current efficiency of 86% and an anode
current efficiency of 96%.
While there have been described various embodiments of the
invention, the methods and apparatus described are not intended to be
understood as limiting the scope of the invention as changes there-
within 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 substantially 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.
