Note: Descriptions are shown in the official language in which they were submitted.
~ o o
lOq340'~
. This invention relates to the electrolytic manufacture
. of chlorates. More.specifically, it is of a process for making
alkali metal chlorate, alkali metal hydroxide, chlorine and
hydrogen from aqueous alkali me~al chloride solution by electro-
, 5 lysis of the solution in a two compartment cell equipped with an
effèctive cation-active permselective membrane divider, with the
production of some chlorate in the anolyte, and subsequent
electrolysis of the-anolyte in a chlorate cell. Polymeric
material found to be effective as the membrane is a hydrolyzed
'10 . polymer of a perfluorinated hydrocarbon and a fluorosulfonated
perfluorovinyl ether or is a sulfostyrenated perfluorinated
ethylene propylene polymer.
An advantage of this invention is that ~kali metal ~
i:~ chlorate produced by transmission of hydroxide through-the ~. -
~15 membrane into the anolyte is recovered and the "contaminated" ;- ~
.: tchlorate-containing) anolyte is subsequently utilized as a feed ~ ~.
, ~ ~ to a chlorate cell. Satisfactory efficiencies are obtained under
the.process conditions and little or no hydrochloric acid is
- needed for the treatment of.the anolyte. ,
,20 . ~n accordance with the present invention a method for
~: . -
. electrolytically manufacturing hydroxide, chlorine and chlorate
comprises electrolyzing an aqueous solu~Dn containing chloride
.,~ .
: ions in an electrolytic chlorine cell having anode and cathode
compartments, an anode, a cathode, a cation-active permselective
membrane of a hydrolyzed copolymer of a perfluorinated hydrocarbon
~, . .
~ .
,f
`` 10~340~
and a fluorosulfonated perfluorovinyl ether, or of a sulfostyre-
nated perfluorinated ethylene propylene polymer, defining a
boundary between the cathode and anode compartments and between
the anode and the cathode, to produce an aqueous hydroxide solu-
tion and hydrogen in the cathode compartment and chlorine and
chlorate in the anode compartment, removing the aqueous hydroxide
solution and anolyte liquor containing chlorate and further
electrolyzing the liquor in a chlorate cell to convert chloride
therein to chlorate.
According to another aspect of the invention there is
provided an electrolytic cell system for electrolytically manu-
facturing hydroxide, chlorine and chlorate which comprises an
electrolytic chlorine cell comprising a housing having an anolyte
compartment, containing an anode adapted to be connected to an
electrical input source; a catholyte compartment containing a
cathode, a cation-active permselective membrane of a hydrolyzed
copolymer of a perfluorinated hydrocarbon and a fluorosulfonated
perfluorovinyl ether, or of a sulfostyrenated perfluorinated
ethylene propylene polymer, defining a boundary between the
catholyte and the anolyte compartments and between the anode and
the cathode, outlet means from said anolyte compartment for
removing anolyte liquor containing chlorate; and a chlorate cell
for converting chloride to chlorate; said chlorate cell having
inlet means connected to said outlet means for passage of said
anolyte liquor from said anolyte compartment to said chlorate
~; cell.
The invention will be readily understood by reference
to the following descriptions of embodiments thereof, taken in
conjunction with the drawing in which:
The FIGURE is a schematic representation of the
arrangement of cells and equipment for producing chlorate with
hydroxide, chlorine and hydrogen.
- 2 -
`` 107340Z
,
In the FIGURE electrolytic chlorine 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 membrane 23
divides the volume into anode or anolyte compartment 27 and
cathode or catholyte compartment 29. Alkali metal halide solu-
. tion is fed to the anolyte compartment through line 33 and
chloride, separated out from chlorate-chloride`solution produced
in chlorate cell 35, is employed to make up halide feed to the
.
.
~'`.; .
` :. - - ~: :
- - .
; ~ ~
.`a ~ :
~'~
:~ :
'
~ ' ' ' ' - `' ','
~I '
.
~ - 3 -
.. . . . .
~ `, o
lOq340~ ,
anode compartment, being added with other chloride to resaturator
36. Water is fed to the cathode compartment 29 through line 37.
Of course, additions of electrolyte and water should be such as
to maintain the desired liquid levels in the anode and cathode
` 5 compartments and this may often be effected with feed-overflow
devices and similar techniques, which are known in the art and
therefore, are not illustrated.
In the pre sent cell halogen, e.g., chlorine gas, is
removable from the anolyte compartment through line 39 and
hydrogen is removable from the catholyte compartment through
line 42. A comparatively high concentration of aqueous hydroxide
solution may be taken off from the catholyte through exit 43.
Because the cation-active permselective membrane 23 allows some
hydroxyl ions to migrate through it from the catholyte to the
anolyte these can react to produce chlorate in the anolyte and the
cell liquor resulting, a mixture of chlorate and chloride in
aqueous solution, is removed at 45 and transferred through line
47 to the interior 49 of chlorate cell 35. In such cell, due
to the absence of any diaphragms or separators between anode 51
and cathode 53 the products of electrolysis, chlorine and
- hydroxide, interact and form chlorate, possibly also with the
formation of some hypochlorite. By controlling the pH in tne
chlorate cell to be within the 6 to 7.5 range the production o~
chlorate can be favored. If desired, hypochlorite may be
produced in the cell and converted externally to chlorate, which
. . .
- 4
-. :
1 0 ~ 3~
may then be processed further as described herein.
The chlorate and chloride in solution are removed from
chlorate cell 35 through line 55 after the concentration of
chlorate has increased sufficiently and that of chloride is lo~J
enough so as not to be salted out in the chlorate cell. Under
the conditions employed the chloride is removed from the chlorate
solution as a solid in separator 57 and then passes through line
59 or other suitable transfer mechanism to resaturator 36 and
back to chlorine cell ll. The chlorate is crystallized out of
the solution in crystallizer 61, being removed at 63. The mother
liquor remaining is sent back to the chlorine cell through line
64, preferably via resaturator 36 or is recycled via line 65.
; The chlorine taken off from chlorine cell 11 may be
burned in the hydrogen produced to form hydrochl~oric acid, which
may be used to adjust pH in the resaturator, cell or separating
; and crystallizing apparatuses. Also, the chlorine may be reacted
externally of the cell with hydroxide removed from the cell, to
produce chlorate.
Various recirculations of compartment contents, pre-
.,
"b 20 ferably intracompartmentally, may also be employed. Although
~' continuous processes such as illustrated are highly preferred,
~` once-through processes, batch method and "hybrid" processes are
also useful. -
-~ The ~ain aspect of the present invention is in its
utilization of a single permselective membrane chlorine cell to
.
~ ) o
` 10~340Z
.
make high purity, high strength caustic solution and at the same
time produce some chlorate in the anolyte, which is subsequently
made useful by processing the anolyte liquor through a chlorate
cell and recycling back to the chlorine cell non-chiorate products.
s It has been found that this process can be effected at anode
i efficiencies of over 85% and even over 90% and at caustic
' efficiencies over 75% and often over 80~.
- The selective ion-passing effects of cationic membranes ~ f
! : .' .
h a v e been noted in the past but the membranes of this invention
have not been employed in the present processes before and the
.. . .
unexpectedly beneficial effects resulting have not been previous-
ly obtained or suggested. Thus, with the use of a comparatively
`; thln membrane, preferably supported as described herein, several
,~ years o. opera~iûn under co~me,-cial conditions ar~ obtair,able
.. ;
' lS without the need for removal or replacement of the membrane,
:~,
while all the time it efficiently prevents undesirable migration
of chloride from the anode compartment to the cathode compartment,
thus allowing production of high purity caustic. It prevents
; hydrogen formed on the cathode _ide from escaping into the
halogen formed on the anode side and vice versa. In this respect
the present membranes are superior to prior art membranes, being
- more impervious to the passage of gases, even when the membranes
are very thin, than various other polymeric materials. The
prevention of hydrogen mixing with chlorine is important since
these materials form explosive mixtures, especially in the Rresence
, .
: - ,
.,
: .
.
. .. .
o
07340Z
of oxygen, such as may be produced in the present processes. The
superiority of the preferred membranes, including modified or
surface treated versions thereof, over prior art membranes in
the various described aspects is also evident, usually to a lesser
degree, in the sulfostyrenated fluorinated ethylene propylene
; polymers.
The membranes employed are normally thin flat sheets,
normally rectangular, but various other shapes and configurations
may be employed. Plural membranes may be used together but
usually this is of no special advanta.ge. Buffer compartments
may be formed but are detrimental to the carrying out of the
present processes. Instead of monopolar electrodes, bipolar
electrodes may be utilized.
-, The aqueous solution containing chloride ions is
~15 normally a water solution of sodium chloride, although potassium `
and other soluble chlorides, e.g., magnesium chloride, may be
utilized, at least in part. However, it is preferred to employ
the alkali metal chlorides and of these, sodium chloride is
-; the best. Similarly, the chlorates made will preferably be
alkali metal chloratesand the hydroxides will be alkali metal
hydroxides, most preferably all being sodium compounds.
The concentration of sodium chloride in a charge to the
anolyte and in the anolyte 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, wi-th
. - - -....... . .
10~
intermediates for mixtures thereof. The electrolyte may be
acidified to a pH in the range of 2 to 6 but in many applications,
as when hydrochloric acid is not readily available for acidification,
instead of the addition of acid to provide a pH range of 2 to 6,
the pH may be from 4 to 7, preferably about 6, and may be that
resulting from generation of chlorine in the anolyte and its
neutralization, at least in part, by sodium hydroxide. A most
preferred concentration of sodium chloride in water is 250 to 300
9./1. Because the chlorine cell anolyte, containing chlorate
made therein, is the feed to the chlorate cell, the chloride content
of the chlorate cell liquor is less than that of the chlorine cell
anolyte fed to it, due to conversion of some of the chloride to
chlorine and thence to chlorate in the chlorate cell. After removal
.~ of chloride and chlorate from the withdrawn chlorate cell liquor
the chloride concentration will be still lower and the mother liquor
' which may be returned to the resaturator will normally be less than
50 or 100 g./l. concentration. It may contain a small-er quantity
of chlorate, too.
The presently preferred cation permselective membrane is of a
:''d 20 hydrolyzed copolymer of perfluorinated hydrocarbon and a fluoro-
sulfonated 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.
-- 8 --
'~
..
lOq3402
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 FS02CF2CF20CF~CF3)CF20CF=CF2. Such a material,
named as perfluoro[2-(2-fluorosulfonylethoxy)-propyl vinyl
ether], referred to henceforth as PSEPVE, may be modified
to equivalent monomers~ as by modifying the internal per-
; fluorosulfonylethoxy component to the corresponding propoxy
component and by altering the propyl to ethyl or butyl, plus
rearranging positions of substitution of the sulfonyl thereonand 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. Patent 3,282~875 and
an alternative method is mentioned in Canadian Patent
849,670, which also discloses the use of the finished
membrane in fuel cells, characterized therein as electro-
chemical cells. 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 by
the equivalent weight is about 900 to 1,600 preferably 1,100
. . . ~ . .
1073~02
to 1,400 and the percentage of PSEPVE or corresponding
compound is about 10 to 30%, preferably 15 to 20% and most
preferably about 17%. The unhydrolyzed copolymer may be
compression molded at high temperature and pressure to
produce sheets or membranes, which may vary in thickness
from 0.02 to 0.5 mm. These are then further treated to
hydrolyze pendant -S02F groups to -S03H groups, as by
treating with 10% sulfuric acid or by the methods of the
patents previously mentioned. The presence of the -S03H
groups may be verified by titration, as described in the
Canadian patent. Additional details of various processing - -
steps are described in Canadian Patent 752,427 and U. S.
~` Patent 3,041,317.
Because it has been found that some expansion accomp-
anies hydrolysis of the copolymer it is preferred to posi-
tion the copolymer membrane after hydrolysis onto a frame
or other support which will hold it in place in the electro-
lytic 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
filiments prior to hydrolysis, when it is still thermoplas-
tic so that the film of copolymer covers each filament,
penetrating into the spaces between the filaments and even
around behind said filaments the films becoming slightly
thinner in the process, where they cover the filaments.
The membrane described is far superior in the present
processes to all other previously suggested membrane mater-
- ials.
'
''
. ~ .
,
lOq340Z
It is more stable at elevated temperatures, e.g., about 75C.
It lasts for much longer time periods in the medium of the
electrolytè and the caustic product and does not become brittle
when subjected to chlorine at high cell temperatures. Considering
the savings in time and fabrication costs, the present membranes
are more economical. The voltage drop through the membrane is
acceptable and does not become inordinately high, as it does with
; many other membrane materials, when the caustic concentration in
the cathode compartment increases to above about 200 9./1. of
caustic. The selectivity of the membrane and its compatibility
with the electrolyte 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 descrlbed 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 resino~s
membranes employable in present methods 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.
- . , .
111~340Z
Improved versions of the above-described copolymers may
be made by chemical treatment of surfaces thereof, as by treatments
to modify the -S03H group thereon. For example, the sulfonic group
may be altered on the membrane to produce a concentration gradient.
Such a change 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 March 26, 1973 in which one side of the
membrane is treated with NH3 to form S02NH2 groups.
In addition to the copolymers previously discussed,
including modifications thereof, it has been found that another
type of membrane material is also superior to prior art films
for 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 sulfo-
nated makes a useful membrane. Whereas useful lives of as much
~ as three years or more (that of the preferred copolymers) may not
'~'
- lZ -
,`
o
lOq3402
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 manufach~edby E. I. DuPont de Nemours &
Co. Inc., is styrenated and the styrenated polymer is then
sulfonated. A solution of styrene in methylene chloride or
benzene at a suitable concentration in the range of about 10 to
20% is prepared and a sheet of FEP polymer having a thickness of
about 0.02 to 0.5 mm., preferably 0.05 to 0.15 mm., is dipped
into the solution. After removal it is subjected to radiation
treatment, using a cobalt60 radiation source. The rate of appli-
` cation may be in the range of about 8,000 rads/hr. and a total
radiation application is about 0.9 megarad. After rinsing with
water the phenyl rings of the styrene portion of the polymer are
monosulfonated, preferably in ~ ~aposition, by treatment
with chlorosulfonic acid, fuming sulfuric acid or SO3. Prefer-
ably, chlorosulfonic acid in chloroform is utilized and the
sulfonation is completed in about 1/2 hour.
- 20 Examples of useful membranes made by the described
process are products of RAI Research Corporation, Hauppauge,
New Yor~, identified as 18ST12S and 16ST13S, the former being
18% styrenated and having 2/3 of the phenyl groups monosulfonated
~nd the latter being 16~ styrenated and having 13/16 of the phenyl
groups monosulfonated. To obtain 18% styrenation a solution of
clemark
D
!, - 13
.~ . .
.
10'~3~0Z
17 1/2X of styrene in methylene chloride is utilized and
to obtain the 16X styrenation a solution of 16X of styrene
in methylene chloride is employed.
The products resulting compare favorably with the
` 5 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 ob- -
tained from the copolymer.
The membrane wall will nor~ally be from 0.02 to 0.5
- 10 mm. thick, preferably from 0.1 tc 0.5 mm. and most prefer-
~ ably 0.1 to 0.3 mm. When the membrane is mounted on a net-
`; work of material selected from the group of polytetrafluoro-
ethylene, asbestos, perfluorinated ethylene, propylene
polymer, polypropylene, tantalum, titanium, niobium and
nob~e metals, for support, the network filaments or fibers
will usually have a thickness of 0.01 to 0.5 mm., prefer-
` ably 0.05 to 0.15 mm., corresponding to up to the thickness
of the membrane. Often it will be preferable for the fila-
menbs to be less than half the film thickness but filament
thicknesses greater than that of the film may also be suc-
cessfully employed, e.g., 1.1 to 5 times the film thick-
ness. The networks, screens or cloths have an area per-
centage of openings therein from about 8 to 80%, preferably
10 to 70X and most preferably 30 to 70X. 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 a.lthough 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 copoly-
mer. Then,the membrane-network composite can be clamped
, ,
-14-
s~ -
\~ o
:10~34~;~
, ' .
or otherwise fastened in place in a holder or support.
The material of construction of the cell body may be
conventional, including steel, 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. Substantial-
ly 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 migration of caustic into the anolyte compartment
of the chlorine cell may be controlled by regulating the con-
centration of caustic in the catholyte or by using a membrane of
different thickness, more caustic being tranferred when the thick-
ness is diminished. The rate of feed will determine to some ex-
tent the pH in the anolyte and it is desirable to keep this in the
range of 3 to 7.5, preferably 3 or 4 to 7, so as to make chlorate.
Recirculation rates may also be adjusted to help maintain the pH in
the desired range and,as was mentioned previously, acids (or
-~ bases) may also be utilized. On the average it is considered
that a controlled proportion, from 5 to 50~ of the caustic
~! produced in the catholyte compartment, as desired, may migrate
'
~,1
-- 15
.
`'' . .
~-) o
`
10'~3~02
. , .
to the anolyte compartment and the cell design and components may
` be varied to obtain the particular proportion most preferred.
The pH of the chlorate cell may be regulated in similar manner.
In addition to controlling the pH's of the chlorine
cell anolyte and the electrolyte of the chlorate cell the tempe-
~ ratures of these are also usually regulated. Normally they are
- maintained at less than 105C., preferably being from 20 to
95C., more preferably from 50 to 95~C. and most preferably about
60 or 65~C. to 85 or 95C. Usually `the chlorate cell is operated
at about 70C. Electrolyte temperatures may be controlled by
~ recirculation of various portions thereof or by changing the
-i proportions of feed. When the temperature cannot be lowered
sufficiently by recirculation, refrigeration of the recirculating
liquid(s) may also be utili~ed.
The processes of this invention, utilizing a single
-~.!
cation-active permselective diaphragm in a two compartment
chlorine cell, operate at comparatively high efficiencies. By
holding the anolyte p~ in the range given, preferably at about
4.5, efficiencies of 80~ (caustic efficiency) and more an~ of 90%
(anode efficiency) and more are obtainable. Under such conditions, *
80% caustic efficiency and 90~ anode efficiency, two-compartment
.. . . .
cellsof the type described, rated at a capacity of 1 ton per
day of chlorine, will produce about 0.9 T/d of sodium
t hydroxide, 0.8 T/d of chlorine and 0.05 T/d of sodium chlorate.
--,?t 25 The amount of chlorate produced will be increased, in accordance
.:'.
- with the invention, by feeding the chlorine cell chlorate plus
, .. . .
chIoride into a chlorate cell. Of course, additional chloride
- 16
. ~ . '
~ ! o
.
.
107340Z
.
may also be added to such cell.
The caustic made in the chlorine cell is free of
chloride, normally containing as little as 0.1 to 10 g./l.
thereof. Its concentration, which is usually from 250 to 450
g./l., may be increased by feeding dilute sodium hydroxide to the
cathode compartment, recirculating sodium hydroxide solution
previously taken off, increasing the electrolysis time or
diminishing the rate of caustic takeoff. Alternatively, more
concentrated solutions may be made by evaporation of the caustic
10 produced. Of course, when more concentrated caustic is made in
the catholyte the production of chlorate in the anolyte will be
increased,since more caustic escapes to the anolyte and reacts.
The present-cells lend themselves to use in both large
and small plants, e.g., from 5 to 1,000 tons per day of chlorine
15 or equivalent, based on the chlorine cells'production. In such
cases the efficiencies described are obtainable so as to make
the process economically desirable. It is highly preferred
however, that the installation should be located near to and the
product should be used in conjunction with a pulp bleaching
20 plant, so that chlorate made can be employed as a bleach or in
the production of a bleaching agent, e.g., chlorine dioxide,
and the caustic may be employed in wood pulping operations.
The following examples illustrate but do not limit
the invention. Unless otherwise indicated, all parts are by
25 weight and all temperatures are in 'C.
~ 17
, ' - ' ' . . .
~ - .. ,,, .. -
1073~o2
EXAMPLE i
Employing the apparatus illustrated in the Figure,
sodium chlorate, essentially chloride-free sodium hydroxide,
chlorine and hydrogen are manufactured from an aqueous sodium
chloride solution electrochemically, sequentially utilizing ~-
a chlorine cell and a chlorate cell. The chlorine cell is of
asbestos-filled polypropylene, equipped with a dimensionally
stable anode, and a steel cathode and the anode and cathode
compartments of this two-compartment cell are separated by
a cation-active permselective membrane. The single anode
employed is of ruthenium oxide on titanium, with the titanium
base being titanium mesh, 1 mm. in diameter, of about 50%
open area~ and the coating of ruthenium oxide being about
1 mm. thick. The anode is connected to a current source
through titanium-clad copper rods. The steel cathode is of
mild steel wire mesh, essentially 1 mm. in equivalent -
~' diameter, having about 35% open area, and it is communi-
- cated with a negative electrical sink by a copper conductor.
The membrane is manufactured by E. I. DuPont de Nemours
t
Company, Inc. and is sold by them as their XR-type membrane.
i` It is 7 mils thick (about 0.2 mm.) and is 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%. The membrane is initially flat and is fused
onto the screen or cloth of Teflon by high temperature, high
.
~ - 18 -
',
~3
.. ~ . .. . .
. . . .
' ;3 ` O
107:~0Z
compression pressing, with some of the membrane portions
actually flowing around the filaments during a fusion process
to lock onto the cloth, without thickening the membrane between
the cloth filaments, although it is thinned somewhat where it is
`5 pressed against the filaments.
The material of the XR-type perms~ective membrane is
a hydrolyæed copolymer of a perfluorinated hydrocarbon and a
fluorosulfonated perfluorovinyl ether. The copolymer is of
tetrafluoroethylene and FSO2CF2CF2OCF(CF3)CF2OCF=CF2 and has an
equivalent weight in the 900 to 1,600 range, about 1,250. The
electrodes are each separated about 1/8 inch from the membrane
although in some processes this is increased to as much as 1/4
inch (about 6 mm.), with little change in voltage drop.
In a chlorine cell OI a type described, rated for the
production of 10 tons of c h l o r i n e per day and operating
.~ .
~ at 90% anode efficiency, a potential drop of 4 volts, a current
3 density of two amperes per square inch, an anolyte NaCl
concentration of about 22% (25~ NaCl solution is fed to the
anode compartment), an anolyte pH of about 4.5 and an electro- r
lyte temperature of about 90~C., a caustic efficiency of 80%
is obtained and the chlorine cell, in a continuous operation,
produces d a i 1 y 9 to~ of sodium hydroxide, 8 tons of chlorine,
-~5` (containtng about 5.3~ oxygen)and 0 .5 ton of sodium chIorate.
The hydroxide i9 an aqueous solution containing 300 g.~l.
NaOH and about 0.3g./1. NaCl. The anolyte contains 250 g./l.
of NaCl and 100 g./l. of NaClO3.
The anolyte is agitated within the anode compartment,
which agitation may be effected by reclrculating it to and from
such compartment directly, not illustrated in the drawing, so as
~'
_ 19
107340Z
.
to help to combat polarization within the compartment. A proportion
of the anolyte is removed and fed to a chlorate cell for conversion
of chloride therein to chlorate, and for recovery of chlorate
produced in the chlorine cell anolyte.
`~ 5 The chlorate cell is a monopolar single-compartment cell
having steel walls which serve as the cathode. The anode is platinum-
iridium or titanium mesh, like that described for the chloride cell,
with the exception that the titanium is coated with a platinum-
iridium mixture, containing about three times as much platinum as
iridium. The cathode, of mild steel, is like that of the chloride
cell. The cell cover is of a plastic or fiberglass reinforced
plastic, such as an after-chlorinated polyvinyl chloride, e.g.,
Trovidur HT, made by Dynamit Nobel, preferably externally reinforced
with a polyester resin, more preferably of the chlorendic ester type,
e.g., Hetrono3 polyester, made by Hooker Chemical Corp. The chlorate
;............... cell, rated at 100 kiloamperes, operates at 4.2 volts and 4 amperes/
~ Sq. in. current density and at a temperature of 90C. 94~ Current
. ~,
`-~ efficiency is obtained. It produces 1.7 tons per day of sodium~i chlorate in an aqueous solution containing 430 9./l. of sodium
chlorate and 120 9./1. sodium chloride. Bipolar chlorate cells are
` also used and graphite anodes can be employed. For the metal anodes
preferred operating conditions used are pH: ~-6.5; temperatures:
60-80C.i preferably about 70C.; current density: 1-6 a.s.i.; and
- voltage: 3-4.8 v. For graphite anodes the ranges are 6.5-7; 30-50C.,
preferably about 40C.; 0.5-1.5 a.s.i.; and 3.5-4~5 v.
The aqueous solution of chloride and chlorate removed from
the chlorate cell is passed to a separator wherein chloride is
removed, and thence to a crystallizer, from which chlorate is
removed from the mother liquor. The solid chloride is returned to
~- 30 the anode compartment of the chlorine cell after
- 20 -
f? 0
lOq340Z
being passed througll the resaturator and is used to increase the
chloride concentration of the feed to the anode compartment to
about 25% sodium chloride. The mother liquor remaring after
p~oduction of solid sodium chlorate is recycled back to the
chlorate cell and includes about l/3 of the feed of chloride to
;~ the cell. Some of the chlorine produced is burned in hydrogen
to make hydrochloric acid, which is utilized to adjust the pH of
the anolyte to that desired.
` -By the described process chloride-free, high strength
caustic solution (containing less than l~ NaCl, on a solids basis)
is made electrolytically in the chlorine cell (because the cation-
active permselective membrane prevents chloride ion from entering
the catholyte) and the chlorate made in the anode compartment due
~; to reaction of migrating hydroxide is not wasted, being additive
,15 to the product of the el~strol~sis of the chlorir.e c~all-anoly~e
fed to the chlorate cell. The products made, chlorine, caustic
il and chlorate, are subsequently utilized in the bleaching of
groundwood pulps and in processing and pulping operations for-
the manufacture of papers and paperboard products.
20 In variations of this process, when the sodium ~hloride
1 concentration in the chlorine cell anode compartment is varied
i,, .
over the range of from 200 to 320 grams per liter, e.g., 220 g./l. -
and 310 g./l., the anolyte pH of the chlorine cell is varied -
over the range of 3 to 7.5, e.g., 4, 5.5 and 7, the temperatures
~25 of both cells are varied over the range of 50 to 95C., e.g.,
60C., 70C. and 85C., the voltage drops of both cells are
varied over the 2.3 to 6 volt range, e.g., 3 and 5 volts for each
cell, and the current density is varied in the 0.5 to 6 amperes
:
- 21
~:~ o
10'~340~
.
per square inch range, e.g., 1 and 3 a.s.i. for the chlorine cell
` and 2 and 6 a.s.i. for the chlorate cell, caustic, chlorine,
hydrogen and chlorate are produced at satisfactory rates, with
the chlorine containing less than 7.5% of oxygen, the chlorine
cell operating at an anode efficiency over 85~ and a caustic
efficiency over 75%, and the chlorate cell operating at a current
efficiency over 90~. Such operating conditions also result when
\
the anode is replaced with a noble metal, a noble metal alloy,
a noble metal oxide or a mixture of noble metal oxide and valve
metal oxide, e.g., platinum, platinum-ruthenium oxide, platinum-
titanium oxides,any of which is a coating on a valve metal such as
titanium or tantalum. The cathode is changed to be entirely
~ graphite, iron or steel or to have surfaces of platinum, iridium,
;,- ruthenium, rhodium or other noble metal on a base metal, such as
copper or steel. Such c~anges in the electrodes do not adversely
~ affect the operations of the chlorine and chlorate cells or of
;1 the overall process of Example 1. Similarly, when the materials
! of construction of the cell walls are changed to polyvinylidene
chloride, synthetic rubber, polypropylene-or similar useful
substance or other such lining which is resistant to the
~ . , .
electrolyte and the electrochemical reaction, the processes are
also successful.
, . . .
EXAM2LE 2
,. . .
When the cation-active permselective membrane of the
chlorine cell of Example 1 is replaced with any of various
modifications thereof, having equivalent weights in the 900 to
- 22
.
.
.
O
`
10~3402
1,600 ran~e, e.g., 1,100, 1,400, or when the surfaces are modified
to a depth of 0.002Or 0.005 mm., as by chemical reactions with
pèndant groups or additional copolymerizations, utilizing products
available from the manufacturer,satisfactory chloride-free caustic
`~ 5 having a content of less than l~ sodium chloride on a sodium
-hydroxide solids basis is made and with the modified NF membrane
the current efficiency is improved by about 5~. Successful
processes are also carried out, following the method of Example
1, when the backing network for the membrane is titanium mesh
~10 or polypropylene, FEP or nylon cloth of free area in the range
of from 15 to 60%, e.g., 15%, 30% and 55~, with filament sizes
being about 0.1 mm. Similarly, when the thickness of the membrane
is varied to 4 mils or 14 mils, the processes also are operative
in the same manner as in Example 1. Using the thinnest of the
~mentioned membranes, the backing network may be coated on both
sides with the membrane, in a variation of this invention.
'
EXAMPLE 3 ~-
The process of Example 1 is repeated except for the
~- .
replacement of the membrane with 10 mil membranes identified
l~ 20as 18ST12S and 16ST13S, respectively, made by RAI Research
. - .
Corporation. The same efficiencies are obtained and satisfactory
caustic, chlorate and chlorine production result, as reported
in Example i. The former of the RAI products is a sulfostyrenated
FEP in which the FEP is 18% styrenated and has 2/3 of the phenyl ~-
groups thereof monosulfonated, and the latter is 16% styrenated
'
.
- 23
~ o
1073402
and has 13/1`6 of the phenyl groups monosulfonated. The membranes
do not stand up as well under the described operating conditions
as do those of Examples 1 and 2 although they are significantly
better for a longer time in appearance and operating character-
` 5 istics, e.g., physical appearanCe, uniformity, voltage drop, than
the various other cation-active permselective membrane materials
available. The membranes do not split in use but do give increased
voltage drops as use continues.
..~
' The process of this example is carried out as a continuous
process , like those of Example 1 and 2 but it is also operative -
, as a batch or once-through process. In such latter methods, as
.~` with continuous operations, circulation in the various compart-
, .~
-~ ments may be obtained by recirculating the electrolyte (merely
~J removing it from the particular compartment and pumping it back
into the compartment).
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
i~ skill in the art will be able to utilize substitutes and
`~ 20 equivalents without departing from the spirit of the invention
or the scope of the claims.
'' ~.
.
~', ,
- 24
' `
