U.S. patent number 3,925,174 [Application Number 05/411,620] was granted by the patent office on 1975-12-09 for electrolytic method for the manufacture of hypochlorites.
This patent grant is currently assigned to Hooker Chemicals & Plastics Corporation. Invention is credited to Jeffrey D. Eng, Cyril J. Harke.
United States Patent |
3,925,174 |
Eng , et al. |
December 9, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Electrolytic method for the manufacture of hypochlorites
Abstract
Hypochlorites, such as alkali metal hypochlorites, are made by
electrolyzing brine in a cell having three or more compartments or
zones therein, wherein anode and cathode compartments are separated
from at least one intervening buffer compartment by cation-active
permselective membranes of a hydrolyzed copolymer of
tetrafluoroethylene and a fluorosulfonated perfluorovinyl ether or
of a sulfostyrenated fluorinated ethylene propylene polymer or by
such a permselective membrane on the cathode side plus a porous
asbestos diaphragm, while feeding chlorine gas to the buffer zone
at such a rate and under such conditions as to produce hypochlorite
therein. The hypochlorite may be converted to chlorate externally
of the cell or, in a variation of the process, may be converted to
chlorate in the buffer compartment.
Inventors: |
Eng; Jeffrey D. (North
Vancouver, CA), Harke; Cyril J. (Burnaby,
CA) |
Assignee: |
Hooker Chemicals & Plastics
Corporation (Niagara Falls, NY)
|
Family
ID: |
23629666 |
Appl.
No.: |
05/411,620 |
Filed: |
November 1, 1973 |
Current U.S.
Class: |
205/500; 204/257;
204/265 |
Current CPC
Class: |
C25B
13/04 (20130101); C25B 1/26 (20130101) |
Current International
Class: |
C25B
1/00 (20060101); C25B 1/26 (20060101); C25B
13/04 (20060101); C25B 13/00 (20060101); C25B
001/26 (); C01B 011/06 (); C25B 013/08 (); B01K
001/00 () |
Field of
Search: |
;204/86,87,92,93,295,296 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Edmundson; F. C.
Attorney, Agent or Firm: Casella; Peter F.
Claims
What is claimed is:
1. A method for electrolytically manufacturing a hypochlorite which
comprises electrolyzing an aqueous solution containing chloride
ions in an electrolytic cell having at least three compartments
therein, being anode and cathode compartments and at least one
buffer compartment, 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 a sulfostyrenated
perfluorinated ethylene propylene polymer, defining a cathode-side
wall of a buffer compartment between the anode and cathode, an
anode-side wall of a buffer compartment being defined by such a
cation-active permselective membrane or a porous diaphragm, and
such walls, with walls thereabout, defining anode, cathode and
buffer compartments, while feeding gaseous chlorine into a buffer
compartment and regulating the rate of feed thereof and reaction
conditions to produce hypochlorite in the buffer compartment.
2. A method according to claim 1 wherein the permselective
membrane(s) is/are of a hydrolyzed copolymer of tetrafluoroethylene
and a sulfonated perfluorovinyl ether of the formula FSO.sub.2
CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF.times.CF.sub.2, which
copolymer has an equivalent weight of about 900 to 1,600.
3. A method according to claim 2 wherein the pH of the aqueous
buffer compartment solution is maintained in the range of about 6
to 11, the temperature thereof is less than about 105.degree.C. and
the cell contains a single buffer compartment.
4. A method according to claim 3 wherein the anode side and cathode
side walls of the buffer zone are of the permselective membrane,
which is of a hydrolyzed copolymer of tetrafluoroethylene and a
sulfonated perfluorovinyl ether of the formula of claim 4, the
membrane walls are from about 0.02 to 0.5 millimeter thick and the
buffer solution pH is from 8 to 11.
5. A method according to claim 4 wherein the membranes are mounted
on a netework of a material selected from the group consisting of
polytetrafluoroethylene, asbestos, perfluorinated ethylene
propylene polymer, polypropylene, titanium, tantalum, niobium and
noble metals, which has an are percentage of openings therein from
about 8 to 80%.
6. A method according to claim 5 wherein the temperature is from
60.degree. to 95.degree.C., the network is a screen or cloth of
polytetrafluoroethylene filaments having a thickness of 0.01 to 0.5
mm., being less than or equal to the thickness of the membrane
mounted thereon and the area percentage of openings in the screen
or cloth is from about 10 to 70%.
7. A method according to claim 6 wherein the membrane walls are
from 0.1 to 0.3 mm. in thickness and the temperature of the
electrolyte is regulated at least in part by the recirculation of
compartment contents.
8. A method according to claim 1 wherein the hypochlorite made is
sodium hypochlorite and chloride ions are from sodium chloride.
Description
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 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 hydrolyzed polymer of a perfluorinated hydrocarbon and a
fluorosulfonated perfluorovinyl ether or a sulfostyrenated
perfluorinated ethylene propylene polymer.
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 and to 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 societies, 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 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 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 perfluorovinyl 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 modifications of the sulfonic group, to make them
more conductive and efficient. Also, sulfostyrenated 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 improving
manufacturing methods therein is well recognized, before the
present invention the process thereof had not been practiced and
the advantages of it had not been obtained.
In accordance with the present invention a method of
electrolytically manufacturing a hypochlorite comprises
electrolyzing 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 sulfostyrenated perfluorinated ethylene propylene
polymer, defining a cathode-side wall of a buffer compartment
between the anode and cathode, an anode-side wall of said buffer
compartment being defined by such a cationactive perselective
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 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, 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 catholyte and buffer
compartments, and the membranes are mounted on networks of
supporting materials such as polytetrafluoroethylene or asbestos
filaments. In some preferred aspects of the invention the
hypochlorite is converted to chlorate, either externally or
internally of the cell. The described preferred copolymers may be
further modified to improve their activities, as by surface
treating, modifying the sulfonic group or by other such mechanism.
Such varieties of the polymers are included within the generic
description given.
The invention will be more readily understood by reference to the
following descriptions of embodiments thereof, 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 is an electrolytic cell by a
method of this invention and subsequently converting it to chlorate
outside the cell;
FIG. 2 is a schematic view of an electrolytic cell in which
chlorate is produced internally; 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
illustrated. 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
metal halide is fed to the anolyte compartment through line 33 and
chlorine gas is fed to the buffer compartment through line 35.
Recirculated 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, liquid 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 reactor 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 41 until the chlorate-chloride concentration
is increased to a useful level. Chloride may be removed by
precipitation and if desired, chlorate may be crystallized out by
installation of the appropriate 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 produce
the hypochlorite for direct use, it is removed through line 39,
together with alkali metal chloride. Some of it may subsequently be
converted to chlorate. Hydrogen is obtainable from line 40.
In the operation of the invented process chlorine is generated at
the anode and alkali metal hydroxide and hydrogen are produced at
the cathode. The normal tendency for alkali metal halide to move
into the catholyte and increase the halide content of the hydroxide
made is counteracted by the cationic permselective membrane 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 exact of the
terms employed but all are used. Thus, if sodium hydroxide 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
hypochlorite, 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
percentage 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.
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 lines 47 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 proportioning 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 materials 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
concentrations throughout their respective compartments.
Alternatively, 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. Low 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 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 all the time it efficiently
prevents undesirable migration of hypochlorite from the buffer
compartment and prevents the chloride ions of the 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 halogen 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 parallelepipedal construction, various other shapes,
including curves, e.g., ellipsoids, 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.
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 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 compartment. 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 production by the reaction of chlorine
with the caustic to form 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 Hypochlorite Generator for Treatment of Combined Sewer
Overflows (Water Pollution Control Research Series 11023 DAA 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 preferable 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 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 2 to 6, acidification 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 g./l. In recycle charges to the buffer
compartment, if utilized, the concentration will normally be less
than 50 or 100 g./l., 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
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 perfluoro-lower 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. 3,282,875 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 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 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 resistances, all of which are important to
the present electro-chemical 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 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. 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 Pat. No. 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 3 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 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
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 the 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 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 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
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. 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 employed. However, when
such 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 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,
polypropylene or phenol formaldehyde resins may be employed,
preferably reinforced with molded-in fibers, cloths or webs.
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 previously 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 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.
The feeding of gaseous chlorine into the buffer compartment 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 controlled in response to
variations in caustic transmission into the buffer compartment.
Additions may be in response to pH fluctuations in the buffer zone.
Normally, to produce hypochlorite, possibly with some chlorate
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 employed,
also. On the average, it is 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 105.degree.C.,
preferably being from 20.degree. to 95.degree.C., more preferably,
50.degree. to 95.degree.C. and most preferably, about 60 to
85.degree.C. or 95.degree.C. Similar temperatures apply to the
electrolyte in the anolyte and catholyte compartments. However, 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
recirculating liquid may also be utilized. For example, the feeds
of water, brine and recirculated electrolyte or mixtures of these
entering the anode compartment or any of the other compartments may
be cooled about 5.degree. to 40.degree.C. below their otherwise
obtained temperatures or to about 10.degree.C. 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 recovered from the liquid product the
aqueous medium may be 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
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 g./l. thereof. The hypochlorite concentration will
normally be from 50 g./l. to its solubility limit and the chlorate
concentration producible, either in the cell or external thereto,
is 150 to 450 g./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 the rate of caustic
take-off. Alternatively, more concentrated caustic solutions may be
made by evaporation of comparatively dilute solutions produced.
When more concentrated caustic is made in the catholyte the
hpochlorite 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 making from 20
to 1,000 tons per day of chlorine or equivalent and in all cases
efficiencies obtainable 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., chlorine dioxide.
The following examples illustrate but do not limit the invention.
Unless otherwise indicated, all parts are by weight and all
temperatures are in .degree.C.
EXAMPLE 1
To produce hypochlorite electrolytically and externally convert it
to chlorate the apparatus illustrated in FIG. 1 is employed, with
the electrolytic cell having steel walls. The anode compartment is
lined with polyester resin 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 .times. 6 to the inch and is communicated to a negative
electrical source or sink by a copper conductor. The walls
separating the anode and cathode compartments, and together with
walls of the cell, defining the buffer compartment, are of a
cation-active permselective 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 title 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) 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, with some of the membrane actually flowing
around the filaments during the fusion process to lock onto the
cloth.
The material of the premselective membrane is a hydrolyzed
copolymer of a perfluorinated hydrocarbon and a fluorosulfonated
perfluorovinyl ether. The hydrolyzed 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.
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 experiment the electrodes
are in contact with the buffer membranes, with the "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 addition to
being taken off for use or sale as chlorine, some thereof is fed to
the buffer compartment and an additional proportion is utilized to
help to convert sodium 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 1 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 and to
some extent, water. The pH in the anode compartment is 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
retention 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.
TABLE 1
__________________________________________________________________________
EXAMPLES 1-1 1-2 1-3 1-4 1-5
__________________________________________________________________________
Average Anolyte NaCl Conc. (g./l.) 270.0 270.0 270.0 270.0 270.0
Av. Anolyte NaClO.sub.3 Conc. (g./l.) < 1 g./l. < 1 g./l.
< 1 g./l. < 1 g./l. < 1 g./l. Av. Buffer Compartment NaOH
Conc. (g./l.) 36.20 21.80 48.32 58.12 19.32 Av. Catholyte NaOH
Conc. (g./l.) 339.80 238.40 325.68 395.36 236.88 Av. Catholyte
NaClO.sub.3 Conc. (g./l.) 1.3 2.95 0.80 0.45 2.65 Av. Catholyte
NaCl Conc. (g./l.) 0.78 1.33 1.27 1.40 2.76 Av. Anolyte Flow Rate
(l./min.) 0.81 0.76 0.94 0.80 0.95 Av. Buffer Inlet Flow Rate
(ml./min.) 20.0 19.5 19.8 -- -- Av. Buffer Exit Flow Rate
(ml./min.) 19.1 19.5 19.0 20.0 15.2 Av. Catholyte Exit Flow Rate
(l./min.) 0.151 0.115 0.082 0.054 0.438 Anolyte Volume in System
(l.) 121.910 118.339 123.695 121.650 24.000 Anode Compartment
Volume (l.) 3.840 3.840 3.840 3.840 3.840 Cathode Compartment
Volume (l.) 3.884 3.884 3.884 3.884 3.884 Buffer Compartment Volume
(l.) 0.520 0.520 0.520 0.520 0.520 Av. Anolyte Temperature
(.degree.C.) 72 72 64 81 95 Av. Anolyte pH 4.47 3.90 4.40 5.35 3.95
Av. Catholyte Temperature (.degree.C.) 68 63 54 79 81 Av. Current
Density (a.s.i.) 1.204 1.052 0.740 1.500 1.667 Av. Cell Voltage
5.163 4.943 4.798 5.407 4.894 Anode NaCl Efficiency (%) 97.70 92.26
91.81 92.34 87.2. Anode Current Efficiency (%, from gas analysis)
94.10 92.82 -- -- 90.69 NaOH Accounted for (or NaOH Efficiency, %)
98.36 92.46 99.56 97.27 91.38 NaClO.sub.3 Efficiency (%) 95.40
90.72 98.54 93.67 90.99 Operational Cell Time (hours) 23.00 19.00
22.50 16.08 3.00
__________________________________________________________________________
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 in bleaching of groundwood pulp and
greatly improve the color thereof.
In a modification 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 compartment. Otherwise, the experiments are essentially
the same and the product resulting is the same.
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 crystallize out in the cell or other portions of the
apparatus.
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, maintaining the buffer
compartment pH at about 10.5 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 70.degree.C., and the process is continuous. The chlorine
feed to the buffer compartment is at the rate of 0.89 ton perday of
the 3 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 sodium chloride in the effluent from that
compartment is 22%.
EXAMPLE 5
Using a commercial apparatus like that of FIG. 2 and maintaining
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 hypochlorite is
present in the product. The pH is maintained by addition of more
chlorine 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 place of the
described membrane there are substituted 18ST12S and 16ST13S RAI
membranes of about twice the thickness of the XR perfluorosulfonic
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 resistant 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 to utilize substitutes and equivalents without departing from
the spirit of the invention or the scope of the claims.
* * * * *