Note: Descriptions are shown in the official language in which they were submitted.
3 . ' ~
~076~60
- This invention relates to the electrolytic manufacture
of hypochlorites. More specifically, it is of a process for
; making alkali metal hypochlorite from chlorine and aqueous alkali
metal hydroxide solution,both of which reactants are produced in
S an electrolytic cell containing anode, cathode and buffer
compartments,with means provided for separating the cathode and
buffer compartments being a cation-active permselective membrane
which is a hydr~lyzed polymer of a perfluorinated hydrocarbon t
and a fluorosulfonated perfluorovinyl ether or a sulfostyrenated
;10 perfluorinated ethylene propylene polymer.
i Such cation-permeable membranes permit flow of
hydroxyl ion from the catholyte to the buffer zone but do not
allow chloride ion to pass through andto mix with the hydroxyl
in the cathode compartment. Thus, when chlorine is added to a
buffer compartment,hypochlorite is produced therein, consuming
the hydroxyl ion and preventing it from flowing to the anolyte
and at the same time a chloride-free alkali metal hydroxide is
made in the cathode compartment.
Chlorine and caustic, essential and very large volume
chemicals required by all industrial societiesl are commercially
produced by the electrolysis of aqueous salt solutions. Improved !~
electrolytic methods utilize dimensionally stable anodes, which
- include noble metals, alloys or oxides or mixtures thereof on
Yalve metals. The concept of employing permselective diaphragms
to separate anolyte from catholyte during electrolysis is not a
-' ' '
.. ...... . ..... _ . .. . . .. . . .. . , . . _ ,
l~ .
~`
~ " 1076~60
.
new one and plural compartme~t electrolytic cells have been
suggested in which one or more of such membranes is employed.
Recently, improved membranes which are of a hydrolyzed copolymer
of a perfluorinated hydrocarbon and a fluorosulfonated perfluoro-
vinyl ether have been described and in some experiments these
have been used as the membranes between the catholyte and buffer
zones of chlorine-caustic cells. Such membranes have been
further improved by surface treatments, preferably by modifica-
., ~ . .
tions of the sulfonic group, to make them more conductive and
efficient. Also,sulfos ty renated perfluorinated ethylene
propylene polymers have been made into useful membranes.
Although the electrolysis of aqueous salt solutions is
a technologically advanced field of great commercial interest
in which~much research is performed, and the importance of im-
proving manufacturing methods therein is well recognized, before
the present invention the process thereof had not been practicéd
and the advantages of it had not been obtained.
In accordance with the present invention a method of
electrolytically manufacturing a hypochlorite comprises electro-
lyzing an aqueous solution containing chloride ions in an
electrolytic cell having at least three compartments therein, an
anode, a cathode, at least one cation-active permselective
membrane selected from the group consisting of a`hydrolyzed
copolymer of a perfluorinated hydrocarbon and a fluorosulfonated
perfluorovinyl ether, and of a sulfost y renated perfluorinated
ethylene propylene polymer, defining a cathode-side wall of a
.
1076060
buffer compartment between the anode and cathode, an anode-side
wall of said buffer compartment being defined by such a cation-
active permselective membrane or a porous diaphragm, and such
walls, with walls thereabout, defining anode, buffer and cathode
compartments, while feeding gaseous chlorine into the buffer
compartment and regulating the rate of feed thereof and reaction
conditions to produce hypochlorite in the buffer compartment.
In a preferred embodiment of the invention the perm-
selective membranes are of a hydrolyzed copolymer of tetra-
fluoroethylene and a fluorosulfonated perfluorovinyl ether of
the formula FS02CF2CF20CF(CF3)CF20CF=CF2, which has an equivalent
weight of about 900 to 1,600, at least two such membranes are
employed, at least one of which separates the anolyte and
buffer compartments and the other of which separates the catho-
lyte and buffer compartments, and the membranes are mounted on
networks of 9upporting materials such as polytetrafluoroethylene
or asbestos filaments.
In some preferred aspects of the invention the hypo-
chlorite is converted to chlorate, either externally or
internally of the cell. The described preferred copolymers may
be further modified to improve thei~ 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.
According to another aspect of the invention there is
provided an electrolytic cell for manufacturing a hypochlorite
or chlorate which comprises a housing having at least three com-
partments therein comprising an anolyte compartment, containing
an anode adapted to be connected to a positive terminal of an
electrical input source, a catholyte compartment containing a
cathode, and at least one buffer compartment between said anolyte
compartment and said catholyte compartment defined by at least
- 3 -
1076060
a pair of spaced apart barriers including a cathode-side wall
barrier and an anode-side wall barrier said cathode-~ide wall
barrier comprising a cation-active permselective membrane
selected from the group consisting of a copolymer of a perfluori-
nated hydrocarbon and a fluorosulfonated perfluorovinyl ether,
and a sulfostyrenated perfluorinated ethylene propylene polymer,
said anode-side wall barrier of the buffer compartment being
defined by a cation-active permselective membrane of said group
or a porous diaphragm; said anolyte compartment including inlet
means for an aqueous solution of chloride ions; said buffer com-
partment including inlet means for a regulated flow of gaseous
chlorine and outlet means for removing hypochlorite or chlorate
from said buffer compartment.
The invention will be more readily understood by
- reference to the following descriptions of embodiments thereof,
~ - 4 _
~076060
taken in conjunction with the drawing of apparatuses and means
for effecting the invented processes.
In the drawing:
FIG. 1 is a schematic representation of the arrangement
of equipment for producing hypochlorite in an electrolytic cell
by a method of this invention and subsequently converting it to
chlorate outside the celli
FIG. 2 is a schematic view of an electrolytic cell in
which chlorate is produced internallyi and
FIG. 3 is a schematic view of apparatus like that of
FIG. 1, including means to remove chloride from the chlorate made
and to recirculate chlorate through the cell buffer compartment to
increase chlorate content in the product stream.
In the FIGURES, to facilitate understanding of the process,
the flows of typical and preferred reactants and products are illust-
rated. M stands for alkali metal, preferably sodium, but other
halide-forming cations may also be employed and in some instances
bromine may be at least partially substituted for chlorine.
In FIG. 1 electrolytic cell 11 includes outer wall 13,
anode 15, cathode 17 and conductive means 19 and 21 for connecting
the anode and the cathode to sources of positive and negative
electrical potentials, respectively. Inside the walled cell cation-
.
active permselective membranes 23 and 25 divide the volume into
anode or anolyte compartment 27, cathode or catholyte compartment 29
and buffer compartment 31. An acidic aqueous solution 32 of alkali
10~6a60
metal halide is fed to the anolyte compartment through line 33 and
chlorine gas is fed to the buffer compartment through l.ne 35. Re-
circulated buffer solution may also be fed into the buffer compartment,
through a separate line, 36' or a common line with the chlorine or
water. Water may be admitted through line 36 to maintain the desired
liquid level in the buffer compartment. Of course, liqu;d levels
should be maintained in all compartments and this is often effected
with known feed-overflow techniques, the apparatus for effecting
which is known and therefore, is not illustrated.
Halogen, e.g., chlorine gas, is removable from the anolyte
compartment through line 34 and aqueous sodium hydroxide is removable
from the catholyte compartment through line 37. An aqueous solution
of alkali metal hypochlorite, with some dissolved alkali metal
chloride, is removable through line 39 and may be passed through
that line to reaction vessel or mixer 41 in which it is mixed with
halogen, e.g., chlorine gas, from line 34. The chlorine passes
through line 43, and may be pulled through that line by low pressure
created by pumping hypochlorite-chloride or hypochlorite-chlorate-
chloride solution 45 through line 47, pump 49 and return line 51,
through eductor reaction 53, in which intimate mixing is effected.
Chlorine gas in the upper portion of reaction and retention vessel
41 may be vented off or may be recycled, too. The chlorate made
is removed as an aqueous solution, with alkali metal chloride,
through discharge line 55. The chlorate-chloride solution may be
circulated through lines 47 and 51, pump 49, reactor 53 and vessel
~1 0 7 6 0 6 0
41 until the chlorate-chloride concentration is increased to a
useful level. Chloride may be removed by precipitation and if de- !
sired, chlorate may be crystallized out by installation of the
appropr;ate apparatus in lines 47 and 51. Because sodium chloride
is relatively insoluble, compared to sodium chlorate, it should be
removed before chlorate crystals are manufactured; otherwise chloride
solids can block orifices, etc., during manufacturing.
In some aspects of the invention, when it is preferred
to ~rad~lce the hy~chlarite for direct use, it is removed through
ro l~ne 39, together w~th a7ka7i meta7 ch70ride. Some of it may
subsequent~ be converted to chlorate. Hydrogen is obt~inab7e ~rom -
: 7ine ~
In the operation af the in~ented process chlorine is
generated ~t the anode and alkali metal hydroxide and hydrogen are
produced at the cathode. The norma1 tendency for a7ka7i metal
halide to move into the catholyte and increase the halide content
; of the hydroxide m~de iS counteracted by the cationic permselectivemembrane 25 and this prevention of chloride flow is aided by the
presence of the additional permselective membrane 23. Yet, alkali
metal hydroxide may migrate from catholyte to the anolyte in ordinary
; cells and such migration can interfere with the caustic or sodium
. ion current efficiency if the product made is useless or is not
recovered. Caustic, sodium ion or cathode current efficiency is
the percentage of useful product made, compared to 100% maximum, with
the current flow employed. Sodium ion efficiency may be the most
~076~60
exact of the terms employed but all are used. Thus, if sodium hydr-
oxide is chemically reacted to make recoverable sodium hypochlorite
or sodium chlorate, coulombs are not wasted, as they are when hydroxyl
ions are electrolytically converted to useless oxygen at the anode.
Anode or chlorine efficiencies are figured in the same general way.
In the present cell the addition of chlorine to the buffer
zone causes the alkali metal hydroxide migrating through the membrane,
as illustrated, to be converted to hypochlorite and chloride, which
do not pass through the permselective membranes. Therefore, the
process satisfactorily produces a chloride-free caustic at satisfactory
high current efficiency and additionally makes a desired byproduct,
the hypochlorite, which may be further converted to chlorate, when
desired. -
In FIG. 2 the manufacture of chlorate in the buffer zone
is shown, using a cell like that of FIG. 1. The only difference
in operation is in the employment of sufficient chlorine to diminish
the pH further, favoring formation of chlorate rather than of hypo-
chlorite, which is normally produced at a higher pH. Acids and
bases may also be used to regulate the pH. A liquid medium such
as recirculated buffer solution or other chlorate-chloride water
,,
solution may be added to the buffer zone through line 36 so as to
help control the temperature, and sometimes, to increase the per-
centage of chlorate in the buffer zone and in the recirculating
,-~ liquid to such a level that after removal of chloride, chlorate
may be crystallized out.
- 8 -
~ ;
~..'
1076060
In FIG. 3 external manufacture of chlorate is illustrated,
with buildup of chloride and chlorate concentrations by recirculation,
followed by removal of the chloride, which may then be followed
by crystallization of the chlorate. As is illustrated, chloride-
chlorate solution may be recirculated through vessel 41 via lines47 and 51' with the solution passing through pump 49 and reactor 53.
During recirculation additional reaction with MOCl from the cell is
effected in reactor 53 and the concentrations of the hypochlorite,
and chloride resulting from such reaction are increased. Because
the chloride is less soluble and is produced to a greater extent,
it will soon crystallize out in the reactor or retention vessel,
causing processing difficulties. Accordingly, it is removed in
separator or crystallizer 61 and more pure, more concentrated
chlorate is continually circulated and ultimately, is drawn off
from the retention vessel 41, possibly for further concentration
and/or crystallization out as the solid. At junction 63 a pro-
portioning valve may be located and the concentration of chlorate
in the circulating system may be further increased by returning
a proportion of it through line 65 to cell 11. Desired pH's at
various parts of the system may be controlled by regulating the
proportions of chlorine utilized at such different locations.
Although some circulations and recirculations of mate-
rials of the process are illustrated, others may also be effected.
Thus, anolyte, buffer compartment solution and catholyte recirculation
may be utilized to maintain the various solutions at the same con-
centration throughout their respective compartments. Alternatively,
g
',
10'76060
once-through processes and "hybrid" processes are also useful.
Similarly, recirculation of chlorate-chloride solutions may be to
the anolyte compartment, at least in part, to convert the chloride
thereof to chlorine and thus reduce the concentration of it in the
chlorate-chloride mixtures.
In the preferred embodiments of the invention the buffer
zone or compartment has two opposing boundaries or walls thereof,
dividing it from anode and cathode compartments, respectively,
both of the described hydrolyzed copolymer membranes, usually
supported on an open network, screen or cloth of electrolyte- and
product-resistant material which is preferably filamentary in form.
The cationic membranes oppose or prevent the passage of anions
such as halide, hypohalite and halate ions, while allowing the
passage of cations, e.g., alkali metal and hydrogen ions. ~ow
~ 15 molecular weight anions, such as hydroxyl, may also pass through
; the cationic membranes.
The selective ion-passing effects of cationic membranes
have been noted in the past but the membranes of this invention
~ have not been employed in the present processes before and their
-~ 20 unexpectedly beneficial effects have not been previously obtained
or suggested. Thus, with the use of a comparatively thin membrane,
; prefer~bly supported as described herein, several years of operation
under commercial conditions are obtainable without the need for
removal and replacement of the membrane, while all the time it
efficiently prevents undesirable migration of hypochlorite from
the buffer compartment and prevents the chloride ions of the
.
~ 10-
~ 076060
anolyte from entering the buffer compartments, while also stopping
any chloride in the buffer zone from transferring to the catholyte.
Together with the use of the buffer zone between the anolyte and
catholyte zones, it prevents hydrogen formed on the cathode side
from escaping into the hydrogen formed on the anode side. In this
respect the present membranes are superior to prior art membranes
because they are more impervious to the passage of hydrogen, even
in comparatively thin films, than are various other polymeric
materials. Also important is their ability to prevent transfer of
chlorine gas into the hydrogen produced at the cathode, especially
when chlorine is fed to the buffer compartment, since when chlorine
is present in hydrogen an explosive mixture may be formed. The
superiority of the preferred membranes of the described copolymer
(including modified or surface treated versions thereof) over the
prior art membranes in the various described aspects is also evident,
usually to a lesser degree, in the sulfostyrenated fluorinated
ethylene propylene polymers.
Although the preferred embodiments of the invention
utilize a pair of the described membranes to form the three
compartments of the present cells it will be evident that a greater
number of compartments, e.g., 4 to 6, including plural buffer zones
may be employed. Similarly, also, while the compartments will
- usually be separated by flat membranes and will usually be of -.
substantially rectilinear or parallelpipedal construction, various
- 25 other shapes, including curves. e.g., ellipsoids, irregular surfaces,
e.g., sawtoothed or plurally pointed walls, may also be utilized.
'''-
`
` ~076060
In another variation of the invention the buffer zone(s), formedby the plurality of membranes, will be between bipolar electrodes,
rather than the monopolar electrodes which are described herein.
Those of skill in the art will know the variations in structure
that will be made to accommodate bipolarS rather than monopolar
electrodes, and therefore. these will not be described in detail.
Of course, as is known ;n the art, pluralities of the individual
cells will be employed in-multi-cell units, often having common
feed and product manifolds and being housed in unitary structures.
Again, such constructions are known to those in the art and need
not be discussed herein.
For most satisfactory and efficient operations the
volume of the buffer compartment(s) will usually be from 1 to
100%, preferably from 10 to 70% that of the sum of the volumes
of the anode and cathode compartments.
- Although the utilization of the present cationic or
cation-active membranes to define the buffer compartment(s) and
separate it/them from the anolyte and catholyte sections is
highly preferred it is possible to operate with a conventional
diaphragm separating the anode compartment from the buffer com-
partment. However, the membrane will be employed to separate the
catholyte from the buffer zones in order to produce the highly
desirable salt-free caustic. Otherwise, even if such a membrane
was employed to separate the anolyte from the buffer zone, halide
present in the buffer section due to addition of brine or pro-
duction by the reaction of chlorine with the caustic to form
,:~
- 12 -
. .
" 1076060
hypochlorite, could pass through the diaphragm to contaminate the
caustic. In many applications salt-free caustic is highly desirable
and therefore, 3-compartment cell structures having a cathode-
side porous diaphragm, such as illustrated in the U.S. Environmental
Protection Agency publication entitled Hypoch10rite Generator for
Treatment of Combined Sewer Overflows (Water Pollution Contro1
Research Series 11023 DM 03/72) are unsatisfactory. Additionally,
the conventional diaphragms, which are usually of deposited asbestos
fibers, tend to become blocked with insoluble impurities from the
brine and have to be cleaned periodically, usually necessitating
shutdown of the cell and often, replacement of the diaphragm.
; The aqueous solution containing chloride ions is normally
a water solution of sodium chloride, although potassium and other
soluble chlorides, e.g., magnesium chloride and ammonium chloride,
may be utilized, at least in part. However, it is preferably to
employ the alkali metal chlorides and of these sodium chloride is
the best. Sodium and potassium chlorides include cations which
` form soluble salts or precipitates and which produce stable hydr-
; oxides. The concentration of sodium ehloride in a brine charged
- 20 will usually be as high as feasible, normally being from 200 to
320 grams per liter for sodium chloride and from 200 to 360 9./1.
for potassium chloride, with intermediate figures for mixtures of
sodium and potassium chlorides. The electrolyte may be neutral
or acidified to a pH in the range of about 2 to 6, acidification
- 13 -
1076060
normally being effected, with a suitable acid such as hydrochloric
acid. Charging of the brine is to the anolyte compartment. The
solid sodium chloride added to the liquid medium in the anolyte
results in a sodium chloride concentration from 200 to 320 g./l.
and most preferably of 250 to 300 9./l. In recycle charges to the
buffer compartment, if utilized, the concentration will normally
be less than 50 or 100 9./1., although chlorate contents may be
higher, and usually the chloride contents of the buffer liquids
- will be less than such limits, too.
Water may be charged to the buffer compartment and in
some cases it may be desirable to charge water with brine to the
anolyte compartment. Dilute caustic may be recirculated to the
catholyte compartment but this is not usually done. For the
most part the liquid level in that zone is maintained by transfer
to it of material(s) charged to the anolyte and/or buffer zone,
; plus water.
The presently preferred cation permselective membrane
~ is of a hydrolyzed copolymer of perfluorinated hydrocarbon and a
- fluorosulfonated perfluorovinyl ether. The perfluorinated hydro-
carbon is preferably tetrafluoroethylene, although other perfluori-
nated and saturated and unsaturated hydrocarbons of 2 to 5 carbon
atoms may also be utilized, of which the monoolefinic hydrocarbons
i~ are preferred, especially those of 2 to 4 carbon atoms and most
.~
'''
:
- 14 -
1076060
especially those of 2 to 3 carbon atoms, e.g., tetrafluoroethylene,
hexafluoropropylene. The sulfonated 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 bemodified 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 re-
arranging positions of substitution of the sulfonyl thereon and
utilizing isomers of the perfluoro-lower alkyl groups, respectively.
However, it is most preferred to employ PSEPYE.
The method of manufacture of the hydrolyzed copolymer
is described 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 the finished membrane in fuel cells,
characterized therein as electrochemical cells. In short, the
copolymer may be made by reacting PSEPVE or equivalent with tetra- :
fluoroethylene or equivalent in desired proportions in water at
elevated temperature and pressure for over an hour, after which
; 20 time the mix is cooled. It separates into a lower perfluoroether
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
., .
,
- 15 -
1076060
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. Addi-
10 tional details of various processing steps are described inCanadian patent 752,427 and U.S. Patent 3,041,317.
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
15 hold it in 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 a backing of filaments of polytetrafluoro-
ethylene or other suitable filaments prior to hydrolysis, when it
is still thermoplastic so that the film of copolymer covers each
, 20 filament, penetrating into the spaces between them, the copolymer
membrane films becoming slightly thinner in the process, where
- they cover the filaments.
The membrane described is far superior to the present
- processes to all other previously suggested membrane materials.
,:
',
~ 16 ~
a -
1076060 '
It is more stable at elevated temperatures, e.g., above 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 temperatures. 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./1. 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 materials. Furthermore, the
caustic efficiency of the electrolysis does not diminish as
significantly as it does with other membranes when the hydroxyl
ion concentration in the catholyte increases. Thus, these
differences in the present process make it practicable, whereas
previously described processes have not attained commercial
- acceptance. While the more preferred copolymers 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 method may be of equivalent weights from 500 to 4,000.
The medium equivalent weight polymers are preferred because they
are of satisfactory strength and stability, enable better
selective ion exchange to take place and are of lower internal
`:
- 17 -
~076060 ~
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 may be made in the manufacturing process or after
production 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. Caust;c 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%.
Exemplary of such treatment is that described in French patent
publication 2,152,194 of March 26, 1973 in which one side of the
membrane is treated with NH3 to form S02NH2 groups.
~,.
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 application in the present processes. Although it appears
that tetrafluoroethylene (TFE) polymers which are sequentially
styrenated and sulfonated are not useful for making sa~isfactory
cation-active permselective membranes for use in the present
electrolytic processes it has been established that perfluorinated
ethylene propylene polymer (FEP) which is styrenated and sulfo-
; 25 nated makes a useful membrane. Whereas useful lives of as much
as three years or more (that of the preferred copolymers) may not
1076a60
be obtained the sulfostyrenated FEP's are surprisingly resistuntto hardening 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 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
-~ application is about 0.9 megarads. After rinsing with water
15 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. Prefer-
.,
, ably, chlorosulfonic acid in chloroform is utilized and the
,,
`1 sulfonation is complete 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 18ST12~ and 16ST13~, 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% styrenation a solution of
*Trade Mark
_ 19 _
1~76060
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 products resulting compare favorably with the preferred
5 copolymers previously described, giving voltage drops of about 0.2
volt each in the present cells at a current density of 2 amperes/sq.
in., the same as is obtained from the copolymer.
The membrane walls will normally be from 0.02 to 0.5
mm. thick, preferably from 0.1 to 0.5 mm. and most preferably 0.1
10 to 0.3 mm. When the membrane is mounted on a polytetrafluoro-
ethylene, asbestos, perflorinated ethylene propylene polymer, r
titanium, tantalum, niobium and noble metals or other suitable
i 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.,
15 corresponding to up to the thickness of the membrane. Often it
will be preferably for the filaments 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
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
- 25 the membrane it is preferred that it be fused to it by high tempe-
rature, high pressure compression before hydrolysis of the copolymer.
Then, the membrane-network composite can be clamped
- 20 -
' ~.'
10~76060
or otherwise fastened in place in a holder or support. It is
preferred to employ the described backed membranes as walls of
the cell between the anolyte and catholyte compartments and the
buffer compartment(s) but if desired, that separating the anolyte
and buffer compartments may be of conventional diaphragm material,
e.g., deposited asbestos fibers or synthetic polymeric fibrous
material (polytetrafluoroethylene, polypropylene). Also, treated
asbestos fibers may be utilized and such fibers mixed with synthetic
,organic polymeric fibers may be meployed. However, when such
- 10 diaphragms are used efforts should be made to remove hardness ions
`and other impurities from the feed to the cell so as to prevent
these from prematurely depositing on and blocking the diaphragms.
The material of construction of the cell body may be
conventional, including concrete or stressed concrete lined with
-~l15 mastics, rubbers, e.g., neoprene, polyvinylidene chloride, FEP,
chlorendic acid based polyester, polypropylene, polyvinyl chloride,
TFE or other suitable plastic or may be similarly lined boxes of
-other structural materials. Substantially self-supporting structures,
such as rigid polyvinyl chloride, polyvinylidene chloride, poly-
propylene or phenol formaldehyde resins may be employed, preferably
reinforced with molded-in fibers, cloths or webs.
The electrodes of the cell ~an be made of any electri-
cally conductive material which will resist the attack of the
- 21 -
1~76~;0 ~
various cell contents. In general, the cathodes are made of
graphite, iron, lead dioxide on graphite or titanium, steel or
noble metal, such as platinum, iridium, ruthenium or rhodium.
Of course, when using the noble metals, they may be deposited as
surfaces on conductive substrates, e.g., copper, silver, aluminum,
steel, iron. The anodes are also of materials or have surfaces
of materials such as noble metals, noble metal alloys, noble
metal oxides, noble metal oxides mixed with valve metal oxides,
c~ e.g., ruthenium oxide plus titanium dioxide, or mixtures thereof,
0 on a substrate which is conductive. Preferably, such surfaces are
` on or in contact with a valve metal and connect to a conductive
metal such as those previously described. Useful surfaces are
platinum, platinum on the valve metal titanium, platinum oxide
on titanium, mixtures of ruthenium and platinum and their oxides
on titanium and similar surface coating materials on other valve
metals, e.g., tantalum. The conductors for such materials may bec,
aluminum, copper, silver, steel or iron, with copper being much
` preferred. A preferable dimensionally stable anode is ruthenium
oxide-titanium dioxide mixture on a titanium substrate, connected
to a copper conductor.
The voltage drop from anode to cathode is usually in
the range of about 2.3 to 5 volts, although sometimes it is
slightly more than 5 volts, e.g., up to 6 volts. Preferably, it
is in the range of 3.5 to 4.5 volts. The current density, while
it may be from 0.5 to 4 amperes per square inch of electrode
surface, is preferably from 1 to 3 amperes/sq. in. and ideally
about 2 amperes/sq. in. The voltage ranges given are for perfectly
aligned electrodes and it is understood that where such alignment
is not exact, as in laboratory units, the voltages can be up to
about 0.5 volt higher.
- 22 -
'"~ o
1076060
The feeding of gaseous chlorine into the buffer compart-
ment is at such a rate as to enable it to react with the sodium
hydroxide entering such compartment from the catholyte and to
convert substantially all of it to hypochlorite (or further,
to chlorate), thereby preventing it from migrating further into
the anolyte. It will be evident that the rate of feed is control-
led in response to v~r~rations in caustic transmission into the
buffer compartment. Additions may be in response to p~ fluctua-
tions in the buffer zone. Normally, to produce hypochlorite,
possibly with some c~.lorate therein, in the buffer zone, a pH of
8 to 11 will be maintained whereas to produce chlorate therein
this will be lowered to 6 to 7.5, preferably 6 to 7. Control of
the pH may be and preferably is by chlorine addition but other
acidifying agents may be e~ployed, alsO. On the average, it is
~15 considered that from 5 to 20% of the caustic produced in the
catholyte compartment migrates to the buffer compartment and
therefore, the stoichiometric amount of chlorine to convert this
caustic to hypochlorite will be employed, plus an excess when
desired, e.g., from 5 to 20% of chlorine, to adjust the pH. In
addition to controlling the pH of the buffer zone electrolyte to
obtain the desired product, temperature is also controlled.
Normally, it is maintained at less than 105C., preferably being
from 20 to 95C., more preferably, 50 to 95C. and most preferably,
about 60 to 85C. or 95C. Similar temperatures apply to the
electrolyte in the anolyte and catholyte compartments. However,
.
,~ .
_ 23
1076060
the pH of the buffer solution and catholyte are different from
those of the anolyte, being about 14, compared to about 1 to 5,
preferably 2 to 4 for the anolyte. The temperature of the
electrolyte may be controlled by recirculation of various portions
thereof, in the anolyte, catholyte and buffer zones. Also, it is
affected by the proportion of feed to such zones and the temperatures
thereof. Feeds will be regulated to obtain the desired temperatures,
previously mentioned. Of course, when the temperature cannot be
lowered sufficiently by recirculation, refrigeration of the re-
circulating liquid may also be utilized. For example, the feedsof water, brine and recirculated electrolyte or mixtures of these
entering the anode compartment or any of the other compartments -
may be cooled about 5 to 40C. below their otherwise obtained
` temperatures or to about 10C. before admissions to such compartment(s).
When the hypochlorite is being produced in the buffer
compartment or a mixture of hypochlorite and chlorate is being
made therein the hypochlorite content may be converted to chlorate
- externally of the cell by addition of chlorine or other acidifying
agent to lower the pH from 8 to 11 to the range of 6 to 7.5,
preferably 6 to 7. The chlorine employed is chlorine produced in
the cell. It is a preferred acidifying agent for this reason and
because byproduct chloride can be reused. Whether the chlorate
is made externally or internally or whether the hypochlorite is
removed for use, excess chlorine sent to the buffer zone is also
recoverable and reusable. Similarly, if chlorate is
- 24 -
1076060
r
:
recovered from the liquid product the aqueous medium may be
i returned to the buffer zone, preferably after removal of chloride,
; too.
The processes of this invention realize greatly improved
current efficiencies due to their prevention of the wasteful
production of oxygen in the anolyte compartment. Anolyte pH is
kept low, to prevent oxygen release, by neutralization of hydroxyl
!
ions and in the present process the chlorine in the buffer solution
diminishes hydroxyl in the anolyte markedly. Thus, chlorine current
- 10 efficiencies of from 90 to 97% are obtainable, together with caustic
current efficiencies of from 75 to 85% or higher. Also, the caustic
made is free of chloride, normally containing as little as 0.1 to
10 9./1. thereof. The hypochlorite concentration will normally be
from 50 g./l. to its solubility limit and the chlorate concentration
lS producible, either in the cell or external thereto, is 150 to 450
9./l. The sodium hydroxide concentration from the catholyte can
be increased by feeding dilute sodium hydroxide, recirculating
sodium hydroxide solution previously taken off, increasing the
electrolysis time or diminishing caustic solutions may be made by
evaporation of comparatively dilute solutions produced. When more
concentrated caustic is made in the catholyte the hypochlorite or
chlorate made in the buffer zone will also be more concentrated.
The present cells may be incorporated in large and
small plants, those producing hypochlorite or chlorate while also
~ 0~6060 r
making from 20 to 1,000 tons per day of chlorine or equivalent
' and in all cases efficiencies obtaina~le are such as to make the
process economically desirable. It is highly preferred however,
that the installation should be located near to and be used in
conjunction with a pulp bleaching plant, so that the hypochlorite
or chlorate can be employed as a bleach or in the production of
bleaching agent, e.g., c~.lorine dioxide.
`, The following examples illustrate but do not limit the
,~ invention. Unless otherwise indicated, all parts are by weight
and all temperatures are in C.
EXA~LE 1
To produce hypochlorite electrolytically and externally
ccr.v~rt it to chlorate the apparatus .llust.a~ed in FIG. 1 s
~ employed, with the electrolytic cell having steel walls. The
L5 anode compartment is lined with polyester resi,n and the buffer
compartment is lined with polypropylene. The anode is of an
expanded diamond-shaped titanium mesh (1 mm. in thickness and
expanded to 50% open area with strand thickness and width being
equal), coated with a mixture of ruthenium oxide and titanium
oxide 0.1 mm. thick, in a ratio of 1:3. The titanium mesh is
communicated with a positive direct current electrical source
through a titanium-clad copper conductor. The cathode is of mild
steel woven wire mesh 2.2 mm. in diameter and 6 x 6 to the
inch and is communicated to a negative electrical source or
-!5 sink by a copper conductor. The walls separating the anode
and cathode compartments, and together with walls of the cell,
- 26
:"
` 1076060
, . . .
defining the buffer compartment, are of a cation-ac~ive perm-
selective membrane manufactured by E. I. DuPont de Nemours &
Company and sold under the trade name Nafion. Characteristics of
such membranes are described in a DuPont New Product Information
Bulletin of 10/1/69 under the titie XR Perfluorosulfonic Acid
Membranes. The walls of the membrane are seven mils thick
(about 0.2 mm.) and it is joined to a backing or supporting
network of polytetrafluoroethylene (Teflon~3) filaments having a
diameter of about 0.1 mm. and arranged in a screen or cloth
form so that the area percentage of openings therein is about
25%. The cross-sectional shape of the filaments is substantially
circular and the membranes mounted on them are originally flat and
are fused onto the screen or cloth by high temperature, high
compression pressing, Wi~l some of tha r.embrane actually flcw-ng
around the filaments during the fusion process to lock onto the
cloth.
The material of the permselective membrane is a
hydrolyzed copolymer of a perfluorinated hydrocarbon and a
fluorosulfonated perfluorovinyl ether. The hydrolyzed copolymer
is of tetrafluoroethylene and FSO2CF2CF2OCF(CF3)CF2OCF=CF2 and
has an equivalent weight in the 900 to 1,600 range, about 1,250.
In the electrolytic cell illustrated in FIG. 1, for
clarity of presentation and in accord with conventional cell
illustrations, spaces are shown between the buffer compartment
membranes and the electrodes but in the practice of this
_ 27
: - 1076060
.
` experiment the electrodes are in contact with the buffer membranes,
with thè "flatter" sides of the membranes facing the contacting
electrodes. The buffer compartment between them is 1/4 inch
(6.4 mm.) wide, for minimum voltage drop at satisfactory
production rates and the interelectrode distance is essentially
the same, although gaps of 7/16 inch are also successfully used.
The anode compartment is filled with a saturated salt
solution or brine and the cathode and buffer compartments are
filled with water, initially containing a small quantity of salt
or brine to improve conductivity. Then the current is turned on
and chlorine is fed to the buffer compartment to convert any
sodium hydroxide transmitted thereto to sodium hypochlorite and
sodium chloride. Chlorine is removed from the anode compartment
and, in addi'ion to being taken off for use or sale as chlor ne,
some thereof is fed to the buffer compartment and an additional
- proportion is utilized to help to convert s~dium hypochlorite to
sodium chlorate externally of the cell. Hydrogen gas is removed
from the cathode compartment and, after it reaches a satisfactory
concentration, sodium hydroxide is also taken off from that
compartment and is essentially free of chloride ions, containing
.
about one g./l. of sodium chloride.
During operation of the cell the pH in the buffer
compartment is maintained in the range of 8 to 11, at about 10,
to promote formation of hypochlorite. Control of the pH in the
buffer compartment is maintained by adjusting the feed of chlorine
- 28
: 1076060
',
and to some extent, water. The pH in the anode compartment ;s
held at about 4 and acidification control is maintained by addition
' of small proportions of hydrochloric acid. Of course, the pH in
`! the cathode compartment is 14.
The solution of sodium hypochlorite and sodium chloride
is conveyed from the electrolytic cell to a retention vessel from
which it is pumped continuously in a cycle through a reactor wherein
the hypochlorite is treated with chlorine to produce sodium chlorate
and more sodium chloride. The mixture is drawn off from the retent;on
vessel and the sodium chloride is subsequently separated from the
sodium chlorate so that the chlorate may be utilized in pulp bleaching
without stream pollution by the accompanying chloride.
In a modification of the described process means are
provided for removing sodium chloride from the circulating stream
from the retention vessel and chlorate liquor, essentially free of
chloride is partly returned to the retention vessel through the
reactor, where a small proportion of sodium hypochlorite present
therein is reacted with chlorine to produce additional chlorate,
and another portion of the chloride-free chlorate is removed from
the system, to be crystallized to solid chlorate or to be employed
as a chlorate liquor. When crystallized, the mother liquor is
returned to the buffer compartment of the electrolytic cell. The
following table describes the operation of the process (unmodified)
of this example in a number of variations of the described process.
` 1076060
...
~, , a~ o o
O ~ ~ 00 ~ r o oo
U~
.~ ~O ~1 ~ ~D ~ ~ OU~ O el~
,,,~
~ 11
~ ~ o o :
~ ~ O O U~
o ~ ~ r . oo o o ~o
~o ,~ o _~ o o o
t _Ir~
.~ , .
~ oo r~
~1 O ts t~ D CO ~ ~ CO O O
o ,~ co In O ~1 0C~
_II`~ I ~ ~ ~I~I
~ ;' : ~
n o
O OU) ~ ~D ~ ~ ~r
IO ~ N _I O~ ~ O CO ~'1
_II~~ ~ ~ ~ ~~1
~ V ~
,.
O O
O O ~ ~
.~, o ~ ~ CO ~ ~ COO _I ~ G~ ;
k ,~
I o ~ o o o ~ o ,1 ~
_l,~, V ~ "~ . _l .
:~
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~ .
,-~ ~ U ~ ~ ~ ~~3 ~
-- O `~
t) d U
` g o Z U o U --
a ~ ; o
o ~ o o a~ 3 ,~
o C~ ~~J o ~ ~ ~
d ~ ~ ~ W--I
Z O )I O O O ~
3 ~ X
Z Z Z 0 ~3~ W a~
0 0 ~ ~ ~ ~ ~ ~WX ~ ~
a) o o o ~ 0a~ o ~ ~-
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0o ~ ~ ~ J~ O ~ ~ ~ a~
~; m u u u ~: m mu ~ ~
0 ~ o o
In o
~o ~ .
1076060
o I~
N If 1 ~ C~ 01 O
If) ~ ~D or~ N ~ O
~r
O ~~ ~1 _I ~t` O ~ O
,
~r o ot`
t~ U) O O ~
IO ~1 Ir) ~~ N ~D O
~. ~P. . . . .I
. I~) O~ N r--
er o oa:~ '
00 N O ~ra~ r-l ~ ~ O
p~ ~
0 ~r ~ ~ O ~r_I ~ ~ N
,~ ~ ~ a~ o~ ~ N
~r o ~ ~
) N O 10 ~ DN~D N O
0;~In CS~ O ~ NC~~ r~ O
N
l~ O ~~) ~) ~1 ~rN N N O ~
_ ~ O ~r~
~J a~ N ~ O~O OO~O O O
_ 0~ ~ Nr~ O
~1
o ~
O _I I~ w a~ N
. ~ .
.. :11~ d.
~¢ . ~;
E~
~,
~a) ,
O r
_ ~ ~
_~ O _ ~ m
a) o .~1 ~ O ::~
o
o ~ ~ a) o _ a)
a t) --
a ~ ~ h E~
C) O ~ :>~ O 0 ~ ~ O` ~ ~:
a) o o ~ s~ ~I z o o ~1
0 ~ a~ ~ ~ ~
O ~ 0 1~]
" ~ ; 3 3 ~ Cl. .
In O Irl
~1
` '3 o
1076060
.
- EXAMPLE 2
In the procedure described the feed of sodium
chloride to the anolyte compartment is at about 25~ sodium
chloride concentration and in the effluent from the anolyte
the chloride concentration is about 22~. The chloride-free
caustic is taken off from the cathode compartment and the
buffer compartment material is either employed as hypochlorite
or, as illustrated in FIG. 1, is fed to a reactor and then to
a holding tank equipped with means to lower the chloride
concentration during recirculation. In the holding tank,
wherein the pH is held at 6.5, the hypochlorite is converted
to chlorate with a typical concentration and that of this
example being 430 g./l. of sodium chlorate,with 140 g./l.
sodium chloride. In some runs as much as 500 g./l. of the
chlorate and as little as 100 g./l. of the chloride are
~ produced. The hypochlorite and chlorate produced are used
i in bleaching of groundwood pulp and greatly improve the
, ,
color thereof.
In a modlfication of the preceding process the
apparatus of FIG. 2 is employed and the rate of chlorine feed
to the buffer compartment is regulated so that the pH of the
buffer solution is maintained at about 6.5, at which pH the
hypochlorite is converted to chlorate in the buffer compart-
ment. Otherwise, the experiments are essentially the same
and the product resulting is the same.
_ 32
o
1076060
EXAMPLE 3
The procedure of Example 1 is followed with the
exception that the apparatus of FIG. 3 is employed and sodium
chloride is continuously removed from recirculating chlorate,
which circulates through a chloride removal apparatus and also
back to the buffer compartment. By this method chlorate is
continuously removed from the holding vessel and chloride
content is maintained low enough so that it does not crystal-
lize out in the cell or other portions of the apparatus.
.
~10 EXAMPLE 4
Using a commercial size three-compartment cell like
that of FIG. 1 chlorate is formed externally at the rate of
0.42 ton per day of sodium chlorate, at 95~ conversion, main-
taining the buffer compartment pH at about 1~.5 and the
reactor and holding vessel pH at about 6.5. The current is 90
kiloamperes and the current density is 2 amperes/sq. in., at
a direct current potential of 4.5 volts and at 70C., and
the process is continuous. The chlorine feed to the buffer
compartment is at the rate of 0.89 ton per day of the three
tons per day of chlorine produced at the anode at 95% current
efficiency. Sodium hydroxide produced is at a 25% concentration
- and is made at the rate of 2.28 tons per day. Sodium chloride
solution charged to the anode compartment is a 25% solution and
the concentration of sodi~m chloride in the effluent from t~at
compartment is 22%.
- 33
~7 ~3
1076060
EXAMPLE S
Using a commercial apparatus like that o FIG. 2 and
mainta1ning the buffer compartment pH at 6.5, 0.4 ton per day of
sodium chlorate is made in situ in the buffer compartment at a
90% conversion rate. A small proportion (about 5%) of hypo-
chlorite is present in the product. The pH is maintained by -
addition of more chlorin~ -.tO the compartment. Other conditions
are the same as described in Example 4. In a modification a batch --
process is employed with essentially the same results. When in
10~ place of the described membrane there are substituted 18STl2S
and 16ST13S RAI membranes of about twice the thickness of the XR
perfluorsulfonic acid membranes employed in the other examples
the same reactions are effected and the desired products also
result. However, in such cases it is noted that the RAI
membranes are not as reslstant to the electrolyte and the
products of electrolysis and do not last as long in use until
replacement becomes desirable. This is especially true when
thinner membranes, such as those of 7 mil thickness are
employed.
The invention has been described with respect to
working examples and illustrative embodiments but is not to be
limited to these because it is evident that one of ordinary
skill in the art will be able ~ utilize substitutes and
equivalents without departing from the spirit of the invention
or the scope of the claims.
k
. .
- 34
.
