U.S. patent number 3,899,403 [Application Number 05/411,619] was granted by the patent office on 1975-08-12 for electrolytic method of making concentrated hydroxide solutions by sequential use of 3-compartment and 2-compartment electrolytic cells having separating compartment walls of particular cation-active permselective membranes.
This patent grant is currently assigned to Hooker Chemicals & Plastics Corporation. Invention is credited to Edward H. Cook, Jr., Alvin T. Emery.
United States Patent |
3,899,403 |
Cook, Jr. , et al. |
August 12, 1975 |
Electrolytic method of making concentrated hydroxide solutions by
sequential use of 3-compartment and 2-compartment electrolytic
cells having separating compartment walls of particular
cation-active permselective membranes
Abstract
A concentrated hydroxide solution containing over 250 g./l. of
sodium hydroxide is made by a two-cell aqueous sodium chloride
electrolysis process wherein, in a first cell, containing at least
three (and preferably three) compartments, with a buffer
compartment separated from adjoining anode and cathode compartments
by walls of a cation-active permselective membrane, which is of a
hydrolyzed copolymer of a perfluorinated hydrocarbon and
fluorosulfonated perfluorovinyl ether or of a sulfostyrenated
perfluorinated ethylene propylene polymer, a concentrated sodium
hydroxide solution is made in the cathode compartment, a dilute
sodium hydroxide solution is produced in a buffer compartment, to
which compartment water is added during the electrolysis and
chlorine is generated in the anode compartment, after which the
dilute hydroxide is fed to the cathode compartment of a
two-compartment electrolytic cell in which sodium chloride solution
is electrolyzed in the anode compartment and concentrated sodium
hydroxide solution, at a concentration of more than 250 g./l., is
withdrawn from the cathode compartment. The caustic efficiency of
the combined process is above 70%, which is normally unattainable
using only a two-compartment membrane cell and yet, the sodium
hydroxide solution resulting is at a high concentration.
Inventors: |
Cook, Jr.; Edward H. (Lewiston,
NY), Emery; Alvin T. (Youngstown, NY) |
Assignee: |
Hooker Chemicals & Plastics
Corporation (Niagara Falls, NY)
|
Family
ID: |
23629662 |
Appl.
No.: |
05/411,619 |
Filed: |
November 1, 1973 |
Current U.S.
Class: |
205/345; 205/505;
205/524; 205/535 |
Current CPC
Class: |
C25B
1/26 (20130101); C25B 13/02 (20130101) |
Current International
Class: |
C25B
13/02 (20060101); C25B 1/00 (20060101); C25B
1/26 (20060101); C25B 13/00 (20060101); C01d
001/06 (); C01b 007/06 () |
Field of
Search: |
;204/98,128,296 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
"New Product Information from Research & Development Div. -
Plastics Dept.," E. I. Dupont De Nemours & Co., XR
Perfluorosulfonic Acid Membranes, 10-1-69, pp. 1-4..
|
Primary Examiner: Andrews; R. L.
Attorney, Agent or Firm: Casella; Peter F. Studley; Donald
C.
Claims
What is claimed is:
1. A method for electrolytically manufacturing a concentrated
hydroxide solution containing over 250 but less than 450 g./l. of
sodium hydroxide or equivalent hydroxide which comprises making
concentrated and dilute aqueous hydroxide solutions simultaneously
by electrolyzing an aqueous solution containing halide ions in an
electrolytic cell having at least three compartments therein, an
anode, a cathode, at least two cation-active permselective
membranes of a polymeric material selected from the group
consisting of a hydrolyzed copolymer of a perfluorinated
hydrocarbon and a fluorosulfonated perfluorovinyl ether and a
sulfostyrenated perfluorinated ethylene propylene polymer, defining
anode and cathode side walls of a buffer compartment or
compartments between anode and cathode compartments, and such
walls, with walls thereabout, defining anode and cathode
compartments, while adding water to the buffer compartment at such
a rate as to produce a dilute hydroxide solution therein at the
same time that a more concentrated hydroxide solution, containing
over 250 but less than 450 g./l. of sodium hydroxide or equivalent
is produced in the cathode compartment, while maintaining a high
caustic efficiency, removing the dilute hydroxide from the buffer
compartment and feeding it to the cathode compartment of a
two-compartment electrolytic cell, having anode and cathode
compartments separated by a cation-active permselective membrane of
a polymeric material selected from the group consisting of a
hydrolyzed copolymer of a perfluorinated hydrocarbon and a
fluorosulfonated perfluorovinyl ether, and a sulfostyrenated
perfluorinated ethylene propylene polymer, in which cell, in the
anode compartment thereof, an aqueous solution containing halide
ions is electrolyzed, and withdrawing from the catholyte
compartment of the two-compartment cell concentrated hydroxide
solution containing more than 250 but less than 450 g./l. of sodium
hydroxide or equivalent, so that the cathode compartment efficiency
of the combined processes is above 70%.
2. A method according to claim 1 wherein the electrolytic cells are
three-compartment and two-compartment cells, the walls between the
buffer compartment of the three-compartment cell and the anode and
cathode compartments thereof and the wall between the anode and
cathode compartments of the twocompartment cell are of a hydrolyzed
copolymer of tetrafluoroethylene and a fluorosulfonated
perfluorovinyl ether of the formula FSO.sub.2 CF.sub.2 CF.sub.2
OCF(CF.sub.3)CF.sub.2 OCF=CF.sub.2, which copolymer has an
equivalent weight of about 900 to 1,600, and the addition of water
to the buffer compartment of the three-compartment cell is at such
a rate that the dilute hydroxide removed from the buffer
compartment and fed to the cathode compartment of the
two-compartment electrolytic cell is at a concentration greater
than 50 g./l.
3. A method according to claim 2 wherein sodium hydroxide, chlorine
and hydrogen are made from an aqueous solution of sodium chloride,
the concentrated sodium hydroxide solution removed from the cathode
compartment of the three-compartment and two-compartment cells is
at a concentration over 350 g./l. and the concentration of the
sodium hydroxide solution withdrawn from the buffer compartment of
the three-compartment cell and fed to the cathode compartment of
the two-compartment cell is from 50 to 200 g./l., the permselective
membranes are about 0.02 to 0.5 mm. thick, the concentration of
sodium chloride in the anode compartments of the three-compartment
and two-compartment cells is from about 200 to 320 g./l., the pH of
the anolytes in both cells is about 1 to 5 and the temperatures of
the anolytes, catholytes and buffer compartment solution are less
than 105.degree.C.
4. A method according to claim 3 wherein the permselective
membranes are mounted on networks of a material selected from the
group consisting of polytetrafluoroethylene, asbestos,
perfluorinated ethylene propylene polymer, polypropylene, titanium,
tantalum, niobium and noble metals, which networks have area
percentages of openings therein from about 8 to 80%, the
temperatures of the anolytes, catholytes and buffer compartment
solution are in the range of 20.degree. to 95.degree.C, the
surfaces of the cathodes are of a material selected from the group
consisting of platinum, iridium, ruthenium, rhodium, graphite, iron
and steel, the surfaces of the anodes are of a material selected
from the group consisting of noble metals, noble metal alloys,
noble metal oxides, mixtures of noble metal oxides with valve metal
oxides, or mixture thereof, on a valve metal, the voltage is from
about 2.3 to 6 volts and the current density is from about 0.5 to 4
amperes per square inch of electrode surface.
5. A method according to claim 4 wherein the networks are screens
of cloths of polytetrafluoroethylene filaments having a thickness
of 0.1 to 0.3 mm., the membrane walls are from 0.1 to 0.3 mm.
thick, the polytetrafluoroethylene filament thickness is less than
or equal to that of the membrane walls, the copolymer equivalent
weight is from about 1,100 to 1,400, the cathodes are of steel, the
anodes are of ruthenium oxide on titanium, the aqueous sodium
chloride solution electrolytes are at a concentration of about 250
to 320 g./l., the pH of the anolyte is from 2 to 4 and the
temperatures of the anolytes, catholytes and buffer compartment
solution are from 65.degree. to 95.degree.C.
6. A method according to claim 5 wherein the aqueous solution of
sodium hydroxide withdrawn from the buffer compartment of the
three-compartment cell is at a concentration of about 100 to 150
g./l., the concentrations of the catholytes withdrawn from the
three-compartment and two-compartment cells are about 375 to 425
g./l. and the caustic efficiency of the combined process is over
75%.
7. A method according to claim 6 wherein a plurality of
three-compartment and two-compartment cells is employed, the
three-compartment cells operating at about 90% caustic efficiency
or more and producing buffer compartment solution containing about
125 g./l. sodium hydroxide and the two-compartment cells operating
at about 60% caustic efficiency or more, with the solution removed
from the cathode compartment of both cells being at about 400 g./l.
sodium hydroxide, the proportion of three-compartment cells to
two-compartment cells is about 1.5 and the buffer compartment
solution withdrawn from three three-compartment cells is fed to the
catholytes of two two-compartment cells.
8. A method according to claim 3 wherein the aqueous solution of
sodium hydroxide withdarwn from the buffer compartment of the
three-compartment cell is at a concentration of about 100 to 150
g./l., the concentrations of the catholytes withdrawn from the
three-compartment and two-compartment cells are about 375 to 425
g./l. and the caustic efficiency of the combined process is over
75%.
9. A method according to claim 3 wherein a plurality of
three-compartment and two-compartment cells is employed, the
three-compartment cells operating at about 90% caustic efficiency
or more and producing buffer compartment solution containing about
125 g./l. sodium hydroxide and the two-compartment cells operating
at about 60% caustic efficiency or more, with the solution removed
from the cathode compartments of both cells being at about 400
g./l. sodium hydroxide, the proportion of three-compartment cells
to two-compartment cells is about 1.5 and the buffer compartment
solution withdrawn from three three-compartment cells is fed to the
catholytes of two two-compartment cells.
Description
This invention relates to the electrolytic manufacture of hydroxide
solutions. More specifically, it is of a process for making alkali
metal hydroxides in concentrated liquid solution form by the
electrolysis of aqueous alkali metal halide solutions in two
different types of electrolytic cells, each of which utilizes one
or more cation-active permselective membranes of a particular
type.
Chlorine and caustic are essential and large volume commodities
which are required basic chemicals in all industrial societies.
They are commercially produced by electrolysis of aqueous salt
solutions and a major proportion of such production is by diaphragm
cells. Such cells have been improved by incorporation therein of
dimensionally stable anodes, which include noble metals, alloys or
oxides thereof or mixtures thereof, on valve metals.
The concept of employing permselective diaphragms to separate
anolyte from catholyte during electrolysis is not a new one and
plural compartment electrolytic cells have been suggested which
employ one or more of such membranes. Recently, improved membranes
have been described which are of a hydrolyzed copolymer of a
perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl
ether. In some experiments these membranes have been employed
between the anolyte and buffer zones of chlorinecaustic cells of
have been utilized to separate anolyte and catholyte zones of such
cells. Yet, although the electrolysis of aqueous salt solutions is
a technologically advanced field of very great commercial interest
in which much research is performed and although the importance of
improving manufacturing methods therein is well recognized, before
the present invention there had not been described such an improved
process by which high strength, low-chloride content caustic
solutions could be made at reasonably high caustic current
efficiencies.
In another patent application of the present inventors, filed
concurrently herewith and entitled Electrolytic Method for the
Simultaneous Manufacture of Concentrated and Dilute Aqueous
Hydroxide Solutions there is described the use, for the production
of sodium hydroxide, of electrolytic cells having at least three
compartments, including a buffer compartment, with the present
cation-active permselective membranes separating the buffer and
other compartments. However, by such a method there is produced in
the buffer compartment a dilute sodium hydroxide and although this
material is valuable, because of the presence of a large proportion
of water therein shipping costs are often prohibitive. The present
method allows the utilization of the dilute caustic in the
manufacture of a more concentrated sodium hydroxide solution so
that, even if there is no nearby plant or other installation which
can utilize the dilute caustic enough water has been "removed" from
it to allow it to be marketed as an article of commerce and to be
economically shipped over long distances.
In accordance with the present invention a method for
electrolytically manufacturing concentrated hydroxide solutions
containing over 250 g./l. of sodium hydroxide or equivalent
hydroxide comprises making concentrated and dilute aqueous
hydroxide solutions simultaneously by electrolyzing an aqueous
solution containing halide ions in an electrolytic cell having at
least three compartments therein, an anode, a cathode, at least two
cation-active permselective membranes of a polymeric material
selected from the group consisting of a hydrolyzed copolymer of a
perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl
ether and a sulfostyrenated perfluorinated ethylene proplyene
polymer, defining anode and cathode side walls of a buffer
compartment or compartments between anode and cathode compartments,
and such walls, with walls thereabout, defining anode and cathode
compartments, while adding water to the buffer compartment at such
a rate as to produce a dilute hydroxide solution therein at the
same time that a more concentrated hydroxide solution, containing
over 250 g./l. of sodium hydroxide or equivalent is produced in the
cathode compartment, while maintaining a high caustic efficiency,
removing the dilute hydroxide from the buffer compartment and
feeding it to the cathode compartment of a two-compartment
electrolytic cell, having anode and cathode compartments separated
by a cation-active permselective membrane of a polymeric material
selected from the group consisting of a hydrolyzed copolymer of a
perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl
ether, and a sulfostyrenated perfluorinated ethylene propylene
polymer, in which cell, in the anode compartment thereof, an
aqueous solution containing halide ions is electrolyzed, and
withdrawing from the catholyte compartment of the two-compartment
cell concentrated hydroxide solution containing more than 250 g./l.
of sodium hydroxide or equivalent, so that the cathode compartment
efficiency of the combined processes is above 70 %.
In preferred embodiments of the invention the permselective
membranes are of a hydrolyzed copolymer of tetrafluoroethylene and
a fluorosulfonated perfluorovinyl ether of the formula FSO.sub.2
CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF=CF.sub.2, hereafter
called PSEPVE, which polymer has an equivalent weight of about 900
to 1,600, only two such membranes are employed and the membranes
are mounted on networks of supporting material such as
polytetrafluoroethylene, perfluorinated ethylene propylene polymer,
polypropylene, asbestos, titanium, tantalum, niobium or noble
metals.
The invention will be more readily understood by reference to the
following descriptions of embodiments thereof, taken in conjunction
with the drawing of means for effecting the invented processes.
IN THE DRAWING
The FIGURE is a schematic diagram of a three-compartment
electrolytic cell for producing alkali metal hydroxide by the
electrolysis of brine, coupled with a two-compartment cell for
increasing the caustic concentration of the dilute caustic removed
from the buffer compartment of the first cell. The cells include
membranes of the described preferred hydrolyzed copolymer.
In the FIGURE, the points of addition and withdrawal of typical and
preferred reactants and products are illustrated. Although the
production of sodium hydroxide solutions is illustrated, other
halide-forming cations may also be employed and in some instances
bromine may be at least partially substituted for chlorine in the
halide.
In the FIGURE 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
permselective membranes 23 and 25 divide the volume into anode or
anolyte compartment 27, cathode or catholyte compartment 29 and
buffer compartment 31. An aqueous solution of alkali metal halide,
preferably acidic, is fed to the anolyte compartment through line
33, from saturator 35. During electrolysis chlorine gas is removed
from above the anode compartment through line 37 and hydrogen gas
is correspondingly removed from above the cathode compartment
through line 39. More concentrated hydroxide solution is withdrawn
from cathode compartment 29 through line 41 while the corresponding
solution of lower concentration is withdrawn from the buffer
compartment through line 43 and is delivered to the cathode
compartment 45 of twocompartment cell 47. Water may be added to
buffer compartment 31 of three-compartment cell 11 through line 49
to maintain the desired concentration of caustic in that
compartment so as to maintain a high current efficiency, cathode
efficiency, cathode compartment efficency, caustic efficiency or
sodium ion efficiency (all such terms being interchangeable) by
limiting the transmission of hydroxyl ions to the anolyte through
membrane 23. Solid sodium chloride or other source of chloride ions
may be fed to saturator 35 through line 51 to raise the chloride
concentration in the feed to the cells. The anolytes may be
recirculated back to the saturator for addition of salt to maintain
the desired concentration thereof in the anolyte. Proportioning
valve 53 controls the flow of rejuvenated electrolyte to anolyte
compartments 27 and 55 through lines 33' and 75.
In cell 47 anode 57 is connected to a source of positive electrical
potential via conductor 59 and cathode 61 is similarly connected
via a corresponding conductor 63. Cationactive permselective
membrane 65 separates compartments 45 and 55. Low concentration
sodium hydroxide solution from buffer compartment 31 passes through
line 67 into cathode compartment 45 to add its hydroxyl ion content
to that produced in the twocompartment cell by electrolysis
thereof. Thus, a high concentration sodium hydroxide solution is
produced in cathode compartment 45 and is withdrawn from it through
line 69. Chlorine and hydrogen are withdrawn from the anode and
cathode compartments, respectively, through lines 71 and 73.
The high concentration sodium hydroxide solution produced may be
sold, employed in chemical reactions, e.g., making chlorate, or may
be evaporated to still higher concentrations. The chlorine may be
reacted with the caustic to form hypochlorite or chlorate or may be
reacted with hydrogen made to produce hydrochloric acid. Otherwise,
it is salable, usually after liquefaction, in which process any
oxygen present is removed. The chlorine, hydrogen and high
concentration caustic solution streams from the three-compartment
and two-compartment cells may be kept separate or may be combined.
In preferred operations anolyte will be recirculated back to the
saturator and if the chlorate concentration therein becomes too
high, due to reaction of chlorine with caustic in the anode
compartment of the twocompartment cell, some of the anolyte may be
sent to a separator or crystallizer for removal of the chlorate,
preferably, in crystalline form, or it may be fed as a chlorate
cell electrolyte.
In the process described chloride-free, high strength caustic
solution can be made at a high caustic current efficiency, e.g.,
over 80 or 90%, in the three-compartment cell, due to the
interposition of the buffer compartment, which diminishes migration
of hydroxyl ions to the anolyte. Thus, the chlorine produced
contains less oxygen and the electric power is utilized more for
the production of caustic than in two-compartment cells. In the
two-compartment cell caustic current efficiencies of only 50 or 60%
(or more) are obtainable but there is no dilute caustic byproduct,
all of the caustic solution being produced as saltfree high
concentration caustic. The overall efficiency of the process is
generally over 70% and often over 75%, a startling increase over
the 50 or 60% efficiencies obtainable when utilizing the
two-compartment cell alone to make strong caustic.
In the drawing the feed from one three-compartment cell is shown
going to a single two-compartment cell. For simplicity of
presentation the drawing is shown thus but in practice, to balance
the various streams and obtain best properties in the product and
highest efficiencies, a plurality of three-compartment cells may be
headed together, both with respect to feeds and products, and the
products may be fed to a plurality of twocompartment cells. In
practice it is found that it is preferred to utilize from one to
two three-compartment cells per two-compartment cell, with the
ratio preferably being from 1.3 to 1.7 and most preferably about
1.5.
The high concentration catholyte hydroxide removed is at a
concentration over 250 g./l., preferably over 350 g./l. with a
maximum of about 450 g./l. Most preferably, it is of 375 to 425
g./l., e.g., 400 g./l. The dilute caustic taken off from the buffer
compartment is at a concentration over 20 g./l. and is usually in
the 50 to 200 g./l. range, preferably being over 75 g./l., more
preferably being from 100 to 150 g./l. and most preferably being
about 125 g./l.
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 prcesses before and their unexpectedly
beneficial effects have not been previously obtained or suggested.
Thus, with the use of a comparatively thin membrane, preferably
supported as described herein, several years of operation under
commercial conditions are obtainable without the need for removal
and replacement of the membrane, while it efficiently prevents
undesirable migration of chloride ions from the anolyte to the
catholyte. Simultaneously, it prevents hydrogen formed on the
cathode side from escaping into the halogen formed on the anode
side and prevents hydrogen from infiltrating the chlorine and
producing an explosive mixture. 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 known polymeric materials.
Although the preferred embodiments of the invention utilize a pair
of the described membranes to form the three compartments of the
present three-compartment 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 cell
compartments of both types of cells will usually be separated by
flat membranes and will usually be of substantially rectilinear or
parallelepipedal construction, various other shapes, including
curves, e.g., ellipsoids, and irregular surfaces, e.g., sawtoothed
or plurally pointed walls, may also be utilized. In another
variation of the invention the buffer zone(s), formed by the
plurality of membranes, will be between bipolar electrodes, rather
than the monopolar electrodes which are described herein. Bipolar
electrodes may also be employed for the two-compartment cells.
Those of skill in the art will know the variations in structure
that will be made to accommodate bipolar, rather than monopolar
electrodes, and therefore, these will not be described in detail.
Of course, as is known in 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 5 to 70% that of the sum of the volumes of the anode and
cathode compartments.
The aqueous solution containing chloride ions is normally a water
solution of sodium chloride, although potassium and other soluble
chlorides, e.g., magnesium chloride, sometimes also may be
utilized, at least in part. However, it is preferable to employ the
alkali metal chlorides and of these sodium chloride is the best.
Sodium and potassium chlorides include cations which do not form
insoluble salts or precipitates and which produce stable
hydroxides. The concentration of sodium chloride in a brine charged
will usually be as high as feasible, normally being from 200 to 320
grams per liter for sodium chloride and from 200 to 360 g./l. 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 1 to 6, acidification
normally being effected with a suitable acid such as hydrochloric
acid. Charging of the brine is to the anolyte compartment, usually
at a concentration of 200 to 320 g./l., most preferably of 250 to
300 g./l. Intracompartmental recirculations of the various
compartment contents are often desirable to maintain the
concentrations uniform throughout.
The presently preferred cation permselective membrane is of a
hydrolyzed copolymer of perfluorinated hydrocarbon and a
fluorosulfonated perfluorovinyl ether. The perfluorinated
hydrocarbon is preferably tetrafluoroethylene, although other
perfluorinated and saturated and unsaturated hydrocarbons of 2 to 5
carbon atoms may also be utilized, of which the monoolefinic
hydrocarbons are preferred, especially those of 2 to 4 carbon atoms
and most especially those of 2 to 3 carbon atoms, e.g.,
tetrafluoroethylene, hexafluoropropylene. The sulfonated
perfluorovinyl ether which is most useful is that of the formula
FSO.sub.2 CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF=CF.sub.2.
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 of the perfluorolower alkyl
groups, respectively. However, it is most preferred to employ
PSEPVE.
The method of manufacture of the hydrolyzed copolymer is described
in Example XVII of U.S. Pat. No. 3,282,865 and an alternative
method is mentioned in Canadian Pat. No. 849,670, which also
discloses the use of the finished membrane in fuel cells,
characterized therein as electrochemical cells. The disclosures of
such patents are hereby incorporated herein by reference. In short,
the copolymer may be made by reacting PSEPVE or equivalent with
tetrafluoroethylene or equivalent in desired proportions in water
at elevated temperature and pressure for over an hour, after which
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 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 --SO.sub.2 F groups to --SO.sub.3 H
groups, as by treating with 10% sulfuric acid or by the methods of
the patents previously mentioned. The presence of the --SO.sub.3 H
groups may be verified by titration, as described in the Canadian
patent. Additional details of various processing steps are
described in Canadian Pat. No. 752,427 and U.S. Pat. No. 3,041,317,
also hereby incorporated by reference.
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 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 the backing tetrafluoroethylene or
other suitable 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, thinning the films slightly 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., above 75.degree.C. 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 membranes 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 g./l. of caustic. The
selectivity of the membrane and its compatibility with the
electrolyte do 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 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 --SO.sub.3 H group thereon. For example, the sulfonic group may
be altered on the membrane to produce a concentration gradient or
may be replaced in part with a phosphoric or phosphonic moiety.
Such changes may be made in the manufacturing process or after
production of the membrane. When effected as 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%. Exemplary of such
treatments is that described in French patent publication 2,152,194
of Mar. 26, 1973 in which one side of the membrane is treated with
NH.sub.3 to form SO.sub.2 NH.sub.2 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
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 perfluorinated
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
hardening and otherwise failing in use under the present process
conditions.
To manufacture the sulfostyrenated FEP membranes a standard FEP,
such as manufacture 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
cobalt.sup.60 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 the phenyl rings of
the styrene portion of the polymer are monosulfonated, preferably
in he para position, by treatment with chlorosulfonic acid, fuming
sulfuric acid or SO.sub.3. Preferably, chlorosulfonic acid in
chloroform is utilized 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 two-thirds of the phenyl groups monosulfonated and the
latter being 16% styrenated and having thirteen-sixteenths of the
phenyl groups monosulfonated. To obtain 18% styrenation a solution
of 171/2% 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
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 to 0.3 mm.
When mounted on a polytetrafluoroethylene, asbestos, titanium or
other suitable network, for support, the network filaments or
fibers will usually have a thickness of 0.01 to 0.5 mm., preferably
0.05 to 0.15 mm., corresponding to up to the thickness of the
membrane. Often it will be preferable for the fibers to be less
than half the film thickness but filament thicknesses greater than
that of the film may also be successfully employed, e.g., 1.1 to 5
times the film thickness. The networks, screens or cloths have an
area percentage of openings therein from about 8 to 80%, preferably
10 to 70% and most preferably 30 to 70%. Generally the cross
sections of the filaments will be circular but other shapes, such
as ellipses, squares and rectangles, are also useful. The
supporting network is preferably a screen or cloth and although it
may be cemented to the membrane it is preferred that it be fused to
it by high temperature, high pressure compression before hydrolysis
of the copolymer. Then, the membrane-network composite can be
clamped or otherwise fastened in place in a holder or support.
The material of construction of the cell body may be conventional,
including concrete or stressed concrete lined with 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 selfsupporting structures, such
as rigid polyvinyl chloride, polyvinylidene chloride, polypropylene
or phenol formaldehyde resins may be employed, preferably
reinforced with molded-in fibers, cloths or webs of glass
filaments, steel, nylon, etc.
The electrodes of the cell can be made of any electrically
conductive material which will resist the attack of the 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, e.g., ruthenium oxide plus titanium
dioxide, or mixtures thereof, on a substrate which is conductive.
Preferably, such surfaces are on or with a valve metal and connect
to a conductive metal, such as those described. Especially useful
are platinum, platinum on titanium, platinum oxide on titanium,
mixtures of ruthenium and platinum and their oxides on titanium and
similar surfaces on other valve metals, e.g., tantalum. The
conductors for such materials may be 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 drops from anodes to cathodes are usually in the range
of about 2.3 to 5 volts, although sometimes they are slightly more
than 5 volts, e.g., up to 6 volts. Preferably, they are in the
range of 3.5 to 4.5 volts. The current densities, while they may be
from 0.5 to 4 amperes per square inch of electrode surface, are
preferably from 1 to 3 amperes/sq. in. and ideally about 2
amperes/sq. in. The voltage ranges 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.
Among the important advantages of the present invention is in the
production of concentrated caustic, low in chloride concentration,
without the need for dilute caustic being an end product of the
method. Yet, this is done at a comparatively high efficiency due to
the utilization of the three-compartment cell and the initial
manufacture of dilute caustic in the buffer compartment thereof.
The use of the buffer compartment and the presence of dilute
caustic in it diminishes the "pressure" on caustic ions to
penetrate into the anode compartment of that cell and thereby
improves efficiency because less oxygen or other relatively useless
product is manufactured in the anolyte than would be the case were
more hydroxide to penetrate into it. The pressre of the caustic can
also be diminished by feeding additional water to the compartment
and making a weaker caustic therein, e.g., one of as little as 25
to 50 g./l. concentration. However, the improved efficiency of this
operation must be balanced against the employment of so weak a
caustic as a feed to the catholyte of the two-compartment cell.
It is desirable that the anolyte of both cells should be acidic so
that it can react with any hydroxyl entering the anode compartment
from the buffer zone, preventing oxygen formation. A pH in the
ranges of 1 to 6 can be used, the range of 1 to 5 is preferred and
2 to 4 is best. Of course, the buffer solution and catholyte pH's
are 14. The temperature of the electrolyte will be maintained at
less than 105.degree.C., preferably being 20.degree. to
95.degree.C., more preferably being 50 to 95.degree.C. and most
preferably being about 65.degree. to 95.degree.C. Electrolyte
temperatures may be controlled by recirculation of portions thereof
and by regulations of proportions of feeds to the various zones and
the temperatures of such feeds. When temperatures cannot be lowered
sufficiently by recirculation or feed control, refrigeration or
other cooling means or liquids may also be employed. For example,
feeds of diluting water to the buffer compartment and dilute
caustic to the catholyte compartment of the two-compartment cell
and any recirculating anolyte employed may be cooled to about
10.degree.to 20.degree.C., preferably to about 10.degree.C., before
admission to the compartment and recirculating electrolyte moving
essentially "intracompartmentally" may also be cooled.
alternatively, cooling may be merely by exposure of the liquids to
ambient conditions before they enter or reenter the cells.
The greatly improved current efficiencies of the present process
are attributable in large part to the 90 to 97% chlorine current
efficiency and over 80%, often over 85% caustic current efficiency
obtainable in the three-compartment cell due to the use of the
buffer compartment therein. It has been found that caustic
efficiency (Faradaic) decreases as caustic concentration of the
buffer effluent increases, apparently being essentially a straight
line function of concentration from 90% at 73 g./l. to 82% at 150
g./l., then dropping off more sharply to 72% at 180 g./l.
The high concentration caustic solution made is free of chloride or
essentially free thereof, often containing as little as 0.1 to 10
g./l. and usually about one g./l. thereof, with the caustic
concentration in the 250 to 400 or 250 to 450 g./l. range. Such
caustic concentrations may be further increased by evaporation and
comparatively little thermal energy is needed to raise them to 50%.
A possible disadvantage of the present method, the production of
chlorate in the anolyte of the two-compartment cell, is even
convertible to an advantage, when that anolyte effluent is fed to
chlorate cells. Alternatively, the chlorate may be separated from
the recirculating anolyte and may be commercially utilized.
The present cells may be incorporated in large or small plants,
thus producing usable caustic while making from 20 to 1,000 tons
per day of chlorine or equivalent and in all cases efficiencies
obtained can be 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 chlorine and chlorate, if any of the latter salt
is produced, may be used together with the concentrated caustic in
wood pulp bleaching or in the production of bleaching agents, e.g.,
chlorine dioxide. Of course, the caustic and chlorine manufactured
may also be marketed.
The following examples illustrate but do not limit the innvention.
Unless otherwise indicated, all parts are by weight and all
temperatures are in .degree.C.
EXAMPLE 1
The three-compartment and two-compartment cells of the Figure are
employed, with the changes described herein, to produce chlorine,
hydrogen and concentrated sodium hydroxide solution from an aqueous
sodium chloride solution. As illustrated, for simplicity, a single
three-compartment and a single two-compartment cell are employed
but in actual practice the ratio of threecompartment cells to
two-compartment cells will be about 1.5, with the feeds to the
various cells and discharges from them often being through common
lines.
The cell walls are of asbestos filled polypropylene or
polypropylene or may be of steel lined with unplasticized polyvinyl
chloride and in some instances, polypropylene. Polyesters, such as
chlorendic acid based polyesters, e.g., Hetron, made by Hooker
Chemical Corp., may be used as a wall lining, too. Rubber or other
synthetic plastic gaskets may act as seals between cell walls,
covers and other parts. The electrodes are in contact with the
membranes separating the buffer compartment from the electrode
compartments of the three-compartment cell and such membranes are
cation-active permselective membranes manufactured by E. I. DuPont
de Nemours & Company, Inc. and sold as their XR-type membranes.
The membranes are 7 mils thick, (about 0.2 mm.) and are joined to a
backing or supporting network of polytetrafluoroethylene (Teflon)
filaments of a diameter of about 0.1 mm., woven into a cloth which
has an area percentage of openings therein of about 22%. They were
initially flat and were fused onto the screen or cloth of Teflon by
high temperature, high compression pressing, with some of the
membrane portions actually flowing around the filaments during the
fusion process to lock onto the cloth, without thickening the
membrane between the cloth filaments.
The material of the XR-type permselective membrane is a hydrolyzed
copolymer of a perfluorinated hydrocarbon and a fluorosulfonated
perfluorovinyl ether. The copolymer is of tetrafluoroethylene and
FSO.sub.2 CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF=CF.sub.2 and
has an equivalent weight in the 900 to 1,600 range, about 1,250.
The electrodes are in contact with the buffer membranes, with the
"flatter" sides of the membranes facing the contacting electrodes.
In some experiments spacings of 0.01 to 5 mm. between the
electrodes and the membranes are utilized and satisfactory results
are obtained but the present arrangement, and the absence of
spacings is preferred. The same membranes are employed in both the
three-compartment and two-compartment cells.
In the three-compartment cells the buffer compartment is about 6
mm. wide and the electrodes are positioned against the membranes,
making the interelectrode distance essentially the same. The volume
ratios of the anode compartment : buffer compartment : cathode
compartment are about 10:1:10 and the anode and cathode compartment
of the two-compartment cell are of about the same volume, with the
permselective membrane being centrally located and electrode gaps
(from anode to cathode) being 1 to 10 mm.
The anodes utilized are of a mixture of ruthenium and titanium
oxides on titanium communicated with current sources by
titanium-clad copper members. The titanium base for the anode is
titanium mesh, about 1 mm., in diameter and with about 50% open
area, coated with a 1:3 ruthenium oxide, titanium oxide mixture
about 1 mm. thick. The cathodes are of a mild steel wire mesh,
essentially 1 mm. in equivalent diameter, having about 35% open
area, and are communicated with negative electrical sources or
sinks by a copper conductor.
The anode compartments of the cells are filled with a nearly
saturated salt solution or brine of sodium chloride at about a 25%
concentration and the cathode and buffer compartment are filled
with water, initially containing a small quantity of salt or brine
to improve conductivity. The current is turned on and the chlorine
and hydrogen produced in the cells are taken off. Water is fed to
the buffer compartment of the three-compartment cell to maintain
the concentration of sodium hydroxide therein low and at the
desired concentrations dilute and more concentrated sodium
hydroxide solutions are removed from the buffer compartment and the
cathode compartment, respectively, of the three-compartment cell.
That taken off the buffer compartment is fed to the cathode
compartment of the two-compartment cell. The depleted anolyte of
the three-compartment cell is passed through a resaturator wherein
its sodium chloride content is increased and it is then returned to
the anode compartment. Generally, the sodium chloride content of
the withdrawn anolyte is about 22% by weight and that of the
returned anolyte from the resaturator is about 25%. A proportion of
the recirculating electrolyte may bypass the resaturator. As
illustrated in the drawing, a similar recirculation and
resaturation in the anolyte of the two-compartment cell is
effected, utilizing the same resaturator and by valve adjustment
the proportions of flows to each of the anolyte compartments of the
three-compartment and two-compartment cells are regulated. Of
course, if desired, separately circulating systems may be employed.
As illustrated, provision is made for taking off a proportion of
the anolyte, especially from the two-compartment cell, when
desired. The removal of anolyte may be effected to enable the
crystallization from it of any excess chlorate content.
In a plant rated at 100 tons of chlorine per day, including 20
150-kiloampere cells, 12 of which are three-compartment cells and 8
of which are two-compartment cells, 25% sodium chloride solution is
fed to the three-compartment cell anode compartment, 22% sodium
chloride content anolyte is removed from it and is recirculated
back to the anode compartment after rejuvenation thereof by
dissolving of solid salt in such solution in the resaturator.
Acidification is also effected at this time, to a pH of 3. A
current density of 2 amperes/sq. in. is applied to the electrodes
at a potential drop of 4.2 volts. Chlorine is produced at a current
efficiency of about 96% and it contains essentially no free oxygen.
The caustic efficiency of the three-compartment cell is about 90%
and it produces 36 tons of sodium hydroxide per day at a
concentration of 400 g./l. and 18 tons per day at a concentration
of 125 g./l. The more concentrated caustic contains only 0.5 g./l.
of NaCl. It is sent to an evaporator and is raised to 50%
concentration, although in some instances it is sold directly as
400 g./l. solution or is utilized in such form for woodpulp
treatment.
In the two-compartment cell the gap between the anode and cathode
is approximately 2 mm., the chloride solution feeds are the same as
for the three-compartment cell and in addition to the caustic
charge to the cathode compartment and the feed solution to the
anode compartment, sometimes a water feed is sent to the cathode
compartment to regulate the concentration of the product thereof.
Operating at 3.6 volts and 2 amperes/sq. in. the eight cells
produce a total of 42 tons of sodium hydroxide per day of a
concentration of 400 g./l. and a sodium chloride concentration of
0.8 g./l. It is noted that the total hydroxide production for the
20 cells is 78 tons of sodium hydroxide per day, which corresponds
to a total plant caustic efficiency of about 78%. The
three-compartment cells operate at a caustic efficiency of about
85-90% and the two-compartment cells operate at about 60% caustic
efficiency. When the anolyte is not acidified the chlorine produced
in the two-compartment cells may be kept separate from that of the
three-compartment cells because of its oxygen or impurity content
being more than would have been the case if it was acidified.
However, it contains less than 6% of oxyen. When hydrochloric acid
is added to the anolyte of the two-compartment cells in sufficient
quantity to neutralize the migrating caustic the chlorine from the
cell can be as pure as that from a threecompartment cell.
The products obtained are used in pulping and in pulp bleaching
operations, especially in the treatment of groundwood pulps. In
some instances the caustic and chlorine are reacted to form
bleaching agents, such as hypochlorite and chlorate, the latter of
which may be subsequently treated to produce chlorine dioxide for
pulp bleaching.
In a modification of the described process, after a buildup of
chlorate in the anolyte of the two-compartment cell (which is not
mixed in with anolytes from three-compartment cells) such anolyte
is fed to a chlorate cell or cells where the chloride therein is
converted to chlorate and the chlorate content is usefully
additive.
In other modifications of the process the thicknesses of the
membranes are increased to 10 to 14 mils, at which caustic
efficiencies increase but voltage drops are also greater. Thus,
although the membranes of greater thicknesses are operative it is
preferred to employ the 7 mil membranes in these reactions. When
the membrane's thickness is decreased to 4 mils the process is
satisfactory but caustic efficiency is diminished somewhat.
EXAMPLE 2
The process of Example 1 is repeated, employing 10 mil membranes of
membrane materials identified as 18ST12S and 16ST13S, respectively,
made by RAI Research Corporation, in replacement of the hydrolyzed
copolymer of tetrafluoroethylene and sulfonated perfluorovinyl
ether. The former of the RAI products is a sulfostyrenated FEP in
which the FEP is 18% styrenated and has two-thirds of the phenyl
groups thereof monosulfonated, and the latter is 16% styrenated and
has thirteen-sixteenths of the phenyl groups monosulfonated. Under
operating conditions the membranes stand up better than other
available cation-active permselective membranes but not as well as
the membranes of Example 1. After such use their characteristics,
e.g., physical appearances, uniformity, voltage drop, are better
than other cation-active permselective membrane materials available
(except the hydrolyzed copolymers of perfluorinated hydrocarbons
and fluorosulfonated perfluorovinyl ethers) but especially in the
anode compartment where chlorine attacks the membrane lengthening
of operating life is desirable. However, in the cathode compartment
the RAI membranes are decidedly better in operation than comparable
commercial products, except for those of the MX type.
In a variation of this procedure the operating temperature is
changed to 80.degree.C. and although efficiencies are somewhat
lower the reactions are satisfactorily operative. Similar good
results are also obtained when the surface of the cathode is
changed to platinum or graphite and the surface of the anode is
changed to platinum or ruthenium oxide (on titanium). The
concentrated caustic and chlorine of this example are used to make
sodium chlorate and the 400 g./l. caustic, without further
concentration, is employed for the pulping of groundwood.
EXAMPLE 3
The experiment of Example 1 is repeated, using a single
three-compartment cell and a single two-compartment cell and
feeding two-thirds of the dilute caustic production of the
three-compartment to the cathode compartment of the two-compartment
cell. Otherwise, the conditions obtaining are the same and the
results thereof are the same although production of concentrated
caustic falls off accordingly and some dilute caustic remains to be
disposed of, (it is used in the pulping of groundwood). When
further changes are made in the operating conditions, as by varying
the operating temperatures, chloride feed concentrations, ratios of
compartment volumes, the screen or cloth backing for the membranes,
the current densities, voltages and/or anolyte pH's, within the
ranges hereinbefore described, satisfactory products are made at
overall caustic current efficiencies greater than 70%. The
efficiencies in such methods are further increased by utilizing a
modified membrane of the hydrolyzed copolymer of a perfluorinated
hydrocarbon and a fluorosulfonated perfluorovinyl ether type, as
previously described. In other experiments plant sizes are
increased or diminished by utilization of more or fewer cells
and/or by the changings of cell sizes. In such experiments the high
concentration caustic solution produced contains from 375 to 425
g./l. of sodium hydroxide and the feed to the two-compartment from
the threecompartment cell is from 100 to 150 g./l. sodium
hydroxide. In still other variations of the examples the continuous
processes described are changed to batch operations and while
efficiencies drop, the processes are operative and produce the same
high concentration caustics of good purities.
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 to utilize substitutes and equivalents without departing from
the spirit of the invention or the scope of the claims.
* * * * *