Note: Descriptions are shown in the official language in which they were submitted.
~CI S85S~i
This invention relates to the electrolytic manufacture
of dithionites, More specifically, it is of a process for
making alkali metal dithionite from alkali metal chloride and
sulfur dioxide, utilizing a combination of electrolytic cells,
one having three compartments and the other having two compart-
ments, the compartments of each being separated by a cation-
active permselective membrane which, in the best embodiments of
the invention, is of a hydrolyzea polymer of a perfluorinated
hydrocarbon and a fluorosulfonated perfluorovinyl ether or is
a sulfostyrenated perfluorinated ethylene propylene polymer.
The cation-active membranes mentioned allow ~ .
proportio~ of hydroxyl ion generated at the cathodes of the cells
to migrate to the buffer compartmen~ of the three-compartment
cellsand to the anode compartmen~ of the two-compartment cells~
In the former case this portion of the hydroxyl generated i5
reacted with sulfur dioxide to produce sulfite and in the latter
case may be converted to oxygen, thereby interfering with the
efficiency of the two-compartment cell portion of the process.
However, the proportion of hydroxide entering the anode compart-
ment of the two-compartment cell is very low because it is
consumed in the catholyte of that cell by reaction with sulfur
dioxide therein to form larger anions, such as sulfite and
dithionite, which do not readily.penetrate the cation-active
permselective membrane. Thus, dithionite and sulfite ions are
~,
.. ' -. , '
, ~ . .....
1~5~3555
preven~ed from migrating from the catholyte or buffer solution
to the buffer solution or anolyte, chloride is prevented from
migrating from the anolyte to the buff~r or catholyte compart-
ments and hydroxyl ion is effectively prevented from passing
~5 into the anolytes.
^ Dithionites and in particular, alkali metal dithionites,
especially sodium dithionite, are useful bleaching agents and
have been found to brighten or bleach wood pulps appreciably.
Such a brightening or bleaching operation is an essential portion
of many papermaking processes. Usually, the dithionite employed
in the past has been zinc dithionite but to prevent water
pollution the discharging of zinc ions into streams has been
limited. Therefore, it has been found desirable to utilize other
dithionites which are less objectionable. It has been suggested
that dithionites could be made by the electrolysis of acidic
solutlons of sulfur dioxide, utili~ing separating permselective
membranes between anode and cathode compartments~ Such a process
ha~ been described in Pulp and Paper Magazine of Canada~ in the
issue of Dec. 19, 1969, at pages 73-78. Such methods are
feasible to some extent but the process of the present invention
is far superior. It eIectrolytically produces hydroxide employ-
ed to make sulfite reactant, manufactures useful chlorine
simultaneously, rather than useless oxygen,and makes a hydroxide
and the bleaching product, both of which are low in chloride
2S content. Such low chloride contents are advantageous since the
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~q~585S5
proportion of chloride which may be discharged into streams and
ground water is also limited. Although sulfite accompanies the
dithionite, it may be usefully employed with it and is useful in
making white liquor, utilized in papermaking processes. A
~5 special advantage of the present invention is in the utilization
of the various products of the process in industrial plants,
such as papermaking plants. The chloride-free hydroxide,
dithionite, sulfite and chlorine are all useful products for
papermaking and are produced in usable forms, without objection-
able contaminants. They are made from a limited number of
starting materlals, primarily sources of chloride, e.g., salt,
and sulfur dioxide, which may be obtained from the burning of
sulfur or sulfur-containing ores.
In accordance with the present invention a method for
electrolytically manufacturing a dithionite, chlorine, hydroxide
and a sulfite from sulfur dioxide and a chloride comprises
feeding chloride solution to the anode compartment of an electro-
lytic cell having anode, buffer and cathode compartments separated
b~ cation-active permselective membranes, an anode in the anode
compartment and a cathode in the cathode compartment and feeding
sulfur dioxide to the buffer compartment, withdrawing chlorine
from the anode compartment, hydroxide from the cathode compart-
; ment and ~ulfite from the buffer compartment, feeding such
sulfite and sulfur dioxide to the cathode compartment of a two-
compartment electrolytic cell having ~n anode in an anode
~ 3
~L~5~555
compartment, a cathode in a cathode compartment and a cation-
~-` active permselective membrane dividing the compartm~nts, feeding
chloride to the anode compartment thereof and withdrawing
chlorine from the anode compartment and dithionite and sulfi~e
from the cathode compartment.
Important advantages of this process include the maml-
facture of chloride-free, high concentration caustic in the three
compartment cell at a high current efficiency, together with use-
ful chlorine from both cells, and the production of sodium di-
thionite in the cathode compartment of the two compartment cellat a pH which is about neutral, preferably about 6 to 8, in
which range the dithionite is comparatively stable, so that it
may be used commercially as the aqueous solution produced, with
sulfite, for ~he bleaching of wood pulp and other analogous pro-
cesses.
According to another aspect of the invention there is
provided an electrolytic cell system for manufacturing a dithionite,
chlorine, a hydroxide and a sulfite from sulfur dioxide and a
chloride which comprises a first cell comprising a housing
having an anolyte compartment, containing an anode adapted to be
connected to a positive terminal of an electrical input source
a catholyte compartment containing a cathode, and a buffer com-
partment be~ween said anolyte compartment and said catholyte
compartment defined by a pair of spaced apart cation-active perm-
selective membranes, and at least a second electrolysis cel.l
comprising a housing having an anode compartment containing an
anode adapted for connection to a positive terminal of an
electrical input source; and a cathode compartment containing a
cathode, said anode compartment being separated from said
cathode compartment by a cation permselective membrane; said
buffer compartment having a first inlet for sulfur dioxide and
a first outlet for sulfite communicating with said cathode com-
~5~555
partment of said second cell, said anolyte and anode compartmentseach including an inlet for chloride solution and an outle~ for
gaseous chlorine, said catholyte compartment including an outlet
for hydroxide, said cathode compartment including an inlet for
sulfur dioxide and an outlet for sulfite and dithionite.
The invention will be readily understood by reference
to the following description of an embodiment thereof, taken in
conjunction with the drawing of apparatuses utilized in carrying
out the inventive process.
In the drawing:
The FIGURE is a schematic representation of a pair of
electrolytic cells and auxiliary equipment for producing
dithionite by the method of this inve~ion.
In electrolytic cell 11, outer wall 13 and bottom 15
enclose anode 17, -cathode 19 and conductive means 21 and 23,
respectively, for connecting the anode and cathode to sources
of positive and negative electrical potentials, respectively.
~.. ,~
~0513S~5
Cation-active permselective membranes 25 and 27 divide the cell
volume into anode or anolyte compartment 29, buffer compartment
31 and cathode or catholyte compartment 33. An acidic aqueous
solution of a halide or brine is indicated as passîng into the
anode compartment through line 35. Such brine is used for
initial charging of the anolyte and for make-up eed, although
make-up may also be added before recirculated anolyte is admit~
ted to the resaturator, to be descrihed. Also, it may be
desirable to dispense with brine line 35 and charge the cell
initially and ~eed make-up through the resaturator piping. The
chloride solution ~or the anolyte compartment, which may be
maintained at a desired acidity by add~ions of acid, e.g., HCl,
by conventional means, not shown, is circulated from the anode
compartment through resaturator 37 via line 39 and exits from
; 15 the resaturator through line 41, from whence it returns to the
anode compartment. In a normal opera~ion, utilizing sodium
chloride solution or other alkali metal chloride, ~he anolyte
compartment is charged with a suitable chloride r e.g., a 25%
salt solution, and that withdrawn for r~satur~ion is at ~
lower concentration, e.g., about 22~ NaCl. Chlorine, generated
in the anode compartment by electrolysis of the halide solu-
tion, is taken off through line 43.
Water may be added to the cathode compar~ment 33
through piping 45 to maintain the desired level thereof ~nd of
the buffer compartment. Hydrogen is removed from this
. .
- 6
~5~555
s
compartment through ventin~ means 47. The bufer compar~ment
has sulfur dioxide and water added to it through lines 4g and -
51, respectively, and alkaline sodium sulfite is taken off
through piping 53, through which it is transmitted to cathode
; 5 compartment 55 of two-compartment electrolytic cell 57.
~ To increase circulation in the buffer compartment,
effectively increase the volume of the compartment and to
allow greater reaction times between the caustic and sulfur
dioxide there may be provided a recirculation loop, for the
bufer compaxtment including lines 50, 52 and 54g pump 56 and
"holding tank" 58. The volume of such system may be 10 to
100,000 times that of t~.e bu-Efer compartment, preferably from 100
to 10,000 times such volume. High strength sodium hydroxide
is removed from the cell through take-off piping 40, at a con-
lS centration of about 20 to 30% hydroxide, as sodium hydroxide,
in water, and with a low chloride content, usually less than
one gram per liter of NaCl. Some of the hydroxide produced ~
in the cathode compartment 33 penetrates the cation-active perm-
selective membrane 27 and passes into buffer compartment 31,
wherein it reacts with the sulfur dioxide to produce sodium
sulfite. The passage of the hydroxide into the buffer compart-
ment is represented by arrow 42. Because of the reaction of
the hydroxide in th buffer compartment and because the sulfite
ion and S02 do not penetrate the membrane 25, very little
h,droxide passes into the anode compa~tment 29 and thereLore,
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~58SS5
the chlorine efficienc~ is maintained high. Also, of course,
chloride ion does not pass from the anolyte into the buffer
compartment, due to the repulsive effect of the permselective
membrane. Additionally, the membranes and buffer zone prevent
; 5 hydrogen or other cathode-produced gases from being mixed with
chlorine, preventing the production of combustible gas
mixtures.
Two-compartment cell 57 has sides 59 and bottom 61
enclosing anode 63 and cathode 65, which are connected to
sources of positive and negative electrical potentials,
respectively, through conductive means 67 and 69. Cation-
acti~7e permselective membrane 71 divides the two-compartment
cell volume into anode or anolyte compartment 73 and cathode
or catholyte compartment 55. Acidic aqueous halide, e.g.,
chloride solution or brine passes into the anode compartment
through line 77 for initial charging of the anolyte andt i~
desired, for make-up feed. ~he ha-ide or chloride solution
for the anolyte compartment, also maintained at desired
acidity in the same manner described for the three-compart-
ment cell, is taken off from the anode compartment through
line 79 and passes through resaturator 81, exiting through
line 83 and returning to the anode compartment. Concentrations
of chloride solution taken off and returned to that compaxt-
ment are about the same as with respect to the three-compart-
ment cell, already described. As with the three-compartment
~ ~ .
.
~5~5~5
cell operation the use of the separate brine line may be
discontinued in favor of utilization of the resaturator
elements instead, to feed brine and make-up for any losses
thereof. Also, i~stead of separate resaturatoxs and attendant
. 5 lines a single resaturator and appropriate piping may be used
to maint~ain halide concentrations in both cell anolytes.
Chlorine generated in the anode compartment of the two-compart-
ment cell is removed therefrom through piping 85.
Cathode compartment 55 is~charged with gaseous sulfur
dioxide through line 87 and water is added through line 89. A
mixture of dithionite and sulfite is removed via piping 91
and any hydrogen or other gases which may be produced in the
cathode compartment are vented off via venting means 93.
Analogously to the buffer solution recirculation in the three-
compartment cell, catholyte of the two-compartment cell may also
be recirculated, utilizing lines 60, 62.and 64, tank 66 and
pump 68. The ratio of the total circulating system volume to
that of the cathode compartment may be from 2:1 to 100,000:1
and is preferably 100:1 to 10,000:1.
During operations of the cells high concentration,
low chloride caustic is taken off from the three-compartment-cell
and is ready for use in wood pulping, bleaching or other opera-
tions and chlorine removed from the anode compartment of the
three-cvmpartment cell is useful in the bleaching of wood pulp or
for other pulp and paper mills' industrial purposes. The sulfite,
produced in alkaline form due to the content ol hydroxide thorein,
_ g _
' , . - .
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~3585~5
is converted in the two-compartment cell to dithionite and addi-
tional sulfite is made by reaction of sulfur dioxide with
hydroxide generated in the cathode compartment. As is clear from
the diagram, the two-compartment cell also makes chlorine, useful
in pulp bleaching. The sulfibe made by reaction of the sulfur
dioxide with hydroxide in the cathode compartmen~ is useful in
pulping operations and may be converted to white liquor after
completion of bleaching of pulp by the accompanying dithionite.
The sulfur dioxide performs the important function of regulati~g
the pH in the cathode compartment of the two-compartment cell so
- as to maintain it in the range of 6 to 8, thereby stabilizingthe dithionite produced. Although the mechanism of the reaction
. has been described, applicants should not be considered as being
bound by this description, since it may also be theorized that
the sulfur dioxide charged is reduced to dithionic acid, which is -
then neutralized by hydroxyl present to form dithionite. In
such case, the presencs of the sulfite can help to exert a
buffering effect to maintain the desired pH. . .
As is illustrated schematically ~y arrow 95 the
~20 dithionite (and sulfite) ions do not penetrate the permselectivemembrane 71 and therefore, are held in the.cathode compartment
55. Similarly, halide ions, the path of which is indicated by
an arrow identified by numeral 97, do not pass from the anolyte
to the catholyte of the two-compartment cell. However, cations
:25 such as alkali metal ions, e~g., Na+, indicated by M~ in the
illustration, the direction of which is represented by the ar-
row99 headed toward the right on the right side of the
.
iC~S15~S55
drawing, may pass from anolyte to catholyte. A small propoxtion
of hydroxyl ion may penetrate the membrane 71 but usually the
concentration of free hydroxyl is low in the catholyte, due to
reaction with sulfur dioxide and reduction of the pH to the 6 to
8 range, so that the hydroxyl entering the anolyte, if any, has
li~tle effect on chloride current efficiency.
By the described process, utilizing a co~bination of
three~compartment and two-compartment cells, the sulfur dioxide
feed to the buffer compartment of the three-compartment cell ties
up the sodium hydroxide penetrating the membrane between the
catholyte and buffer solution and prevents it from reaching the
anode; where it could be converted to useless oxygen, thereby
decreasing current efficiency. At the same time, high strength,
chloride-free caustic is made, which is important in various
chemical operations, e.g., pulp bleaching, where chloride dls-
charges from industrial plants are undesirable and may be
strictly limited.
The chlorine and chloride-free caustic made are both
useful chemicals for many industrial processesr including wood
pulpiny and pulp bleaching. Thus, the invention has a distinct
advantage over an electrolytic method for producing dithionite
by charging sulfite or sulfur dioxide to a two-compartment cell
and producing dithionite in the cathode compartment by reduction
of sulfite or reduction of sulfur dioxide, followed by neutrali-
zation to the dithionite. That is~ the sulfur dioxide which would
_, ... , ,., . . . .. .... . . . . . . , ... .... .. .......... .... .. . , . . , ~
1058555
be required to make sulfite for the two-compartment cell
electrolytic reaction, makes the sulfite in the buffer compart-
ment of the three-compartment cell while chloride-free caustic
is made in the cathode compartment, and increases chlorine
current efficiency of the cell. These additional advantages
improve the efficiency of the present process and make it
commercially advantageous over similar or related processes.
Instead of adding suIfur dioxide to the cathode compart-
ment, wherein it acts as a source of sulfite for reduction to
dithionite and at the same time serves to help regulate the pH
in the desired 6 to 8 range, sulfite may be fed to the catholyte,
with other means employed for pH regulation. By such a process,
although the results may not be as satisfactory as with that
previously described, utilizing sulfur dioxide, dithionite can
be made. However, unless the means of reducing the àlkaline pH
caused by the presence of the hydroxide generated at the ca~hode
is a chemical which produces a useful proauct ~and which is non-
; interfering with the dithionite process), there will ~e a waste
of hydroxide and po~sibly, even sreation of a disposal problem~
The halide solution fed to the anode compartment of both
cells is an aqueous solution of a water soluble metal chloride
in the usual case, preferably of sodium chloride. The concentra-
tion thereof is generally in the range of 200 to 320 grams/liter
for sodium chloride and 200 to 360 g./1. ~or potassium chloride.
Preferably such solutions contain 20 to 25% of the alkali metal
.
- 12
~S85SS
halide salt, as the solutions are charged to the cell or deliver-
ed to it from the resaturator. Generally the chloride content
will be reduced to 5 to 30% less than the original content,
preferably to 10 to 20% less and normally, as with sodium
`5 chl~ride, ~he concentration of the halide xemoved from the anode
compartment for resaturation and return to such compartment is
about 22~, as NaCl, or equivalent. Although the anolyte may be
nsutral, it is often acidified so as to be of a pH in the range
of about 1 to 6, preferably 2 to 4, with acidification normally
being effected with a suitable acid, such as hydrochloric acid.
Water utilized to make the initial brine charge or added as
make-up feed to the ano~e compartments and watex added to the
other compartments of the cells will preferably be deionized,
; containing less than 10 p.p.m. hardness, as CaCO3, although tap
water of comparatively low hardnesst è.g., under 150 p.p.m.,
preferably under 50 p.p.m., can be used.
The sulfur dioxide charged to the buffer compartment of
the three-compartment cell is usually s~bstantlally pure, e.g.,
over 90~ SO2, but lower concentrations thereof, e.g., as low as
20%, are usable because of the desirable attributes of the
membrane material in preventing gas in~erchanges between cell
portions. Thus, the unreacted gas, e.g., 2~ N2, may be removed
from line 53 at a suitable point, before the sulfite produced is
charged to the cathode compartment of the two-compartment cell.
In the three-compartment cell hig~ concentration
.
13
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.
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~0585S5
hydroxide solution, such as alkali metal hydroxide, preferably
sodium hydroxide, is produced, normally of 20 to 30% hydroxide,
although lesser concentrations may also be made, e.g., down to
as low as 5%. The chloride content thereof is low, usually
being less than 5 g./l. and often less than 1 g.fl. The
concentration of the hydroxide may be regulated by controls of
the rate of feed of water to the catholyte, flow of elec~ric
current and, in some cases, nature o the feed to the cathode
compartment (dilute caustic may somètimes be fed in at least
partial replacement of water).
The sulfite produced by reaction of the sodium
hydroxide and sulfur dioxide in the bu~fer compartment may be
of any of various concentrations. These are controllable by
regulating the feed of sulfur dioxide to the buffer compartment.
m e more sulfur dioxide charged, the greater the quantity of
sulfite in the buffer effluent, in comparison to that of the
hydroxide. Generally, the sulfite will be an aqueous solution
of 1 to 15% strength and the hydroxide removed from the buffer
compartment will also be a corresponding 15 to 1% soluticn r
with more sulfite than hydroxide in the buffer compartment.
Preferably the sulfite and hydroxide concentrations total
about 10 to 20%, e.g., about 15%, and in more preferred
embodiments of the invention the concentration of hydroxide is
maintained at less than 5~ while that of the sulfite is up to
about 10%.
In the two-compartment cell the feeds to the catholyte
_ 14
~L058SSS
of sulfite-hydroxide solution from the buffer compartment and SO2
are so regulated as to maintain the desired pH for the formation
of a stable dithionite. Such a pH should be in the range of
about 6 to 8, preferably 6 to 8 and most preferably about 7.
~5 It may be regulated by controlling the feed of sulfur dioxide,
which has the additional beneficial effect of diminishin~ the
hydroxide concentration to a very small proportion, preventing
all but a very minor proportion of the hydroxide generated at the
cathode ~rom migrating through the membrane to the anolyte, where
it could have been converted to oxygen, with a loss of electrical
efficiency. The effluent from the cathode compartment is a
mixture of dithionite and sulfite and the concentrations of these
components are usually in the ranges of 0.5 to 30% sulfite and
0.5 to 10% dithionite. Within such ranges the noxmal ranges
are from 10 to 20% of sulfite and 1 to 5~ of dithionite. The
conversion of sulfite or sulfur dioxide to dithionite will usually
.
be at a current efficiency of *rom about 40 to 80%, normally
within the 60 to 75% range. The dithionite removed from the
cathode compartment of the two-compartment cell will generally
have a concentration of 10 to 70 g./l., within which range 30
to 50 g./l. is usual. From 100 to 250 g./l. will be the concen-
tration of the sulfite drawn off wikh it.
To obtain the desired operation of these cells, as
described, the voltage drop across the three-compartment cell
is maintained at about 3 to 6 volts, preferably 4 to 5 volts
.. _ ... . .. . .. . . . . . . . .. . . . .. . . . . . . .... .
~51~SS5
and that across the two-compartment cell is about 3 to 5 vol~s,
pre~erably 3.S to 4.5 volts. The current density for the three-
compartment cell is about 1 to 3 amperes/sq. in., preferably 1.5
to 2.5 a.s.i., and that of the two-compartment cell is 0.1 to
2 a.s.i., preferably 0.2 to 1 a.s.i. The operating temperature
of the three-compartment cell is about 50 to 100C.~ pre~erably
80 to 100C., whereas that of the two-compartment cell is 3 to
40C., preferably 3 to 25C. A low temperature is desirable for
operation of the two-compartment cell because of the ~reater
stabilit~ of the dithionite at such low temperatures~
The anodes employed are preferably dimensionally stable
anodes o~ a material selected from the group consisting of noble
metals, noble metal alloys, noble metal oxides, mixtures of
noble metal oxides with valve metal oxides and mixtures thereof,
on a valve metal, whereas the cathodes are preferably o stainless
steel. Instead of the dimensionally stable anodes, anodes of
noble metals or oxides thereof may also be employed, e.g~,
platinum, iridium, ruthenium or rhodium. Alternatively, other
anodes resistant to the anolytes can be used, although t~ey are
not usually preferred. The anodes and cathodes.may be connected
to sources of electrical potential by conductive metals, such as
copper, silver, aluminum~ steei and iron but these materials
are normally shielded fro~. contact with the electrolytes.
Pre~erable dimensionally stable anode surfaces, all on titanium
or tantalum substrates, are ruthenium oxide-titanium oxide
, . . . . . , . .. ....... . . . . . . . , , _ _ _
5~555
mixtures, platinum, ruthenium, platinum oxide and mixtures of
ruthenium and platinum and mixtures of their oxides. A preferred
dimensionally stable anode is a ruthenium oxide-titanium dioxide
mixture on a titanium substrate, connected to a source of positive
electrical potential by a titanium-clad copper conductor.
The cathodes employed should be resistant to the
corrosive catholyte and therefore it has been found that no~le
metal, noble metal oxide and stainless steel cathodes are prefer-
red. Ordinary iron or steel cathodes soon become deteriorated
in use, although they may be employed for short term operations.
Graphite cathodes are not preferred because of their poorer
; conductivity and other physical properties. Of the noble metals,those previously described are satisfactory and of the stainless
steels those containing small proportions of molybdenum, in
addition to chromium, nickel and iron, are preferred. These
include Stainless Steel Types Nos. 316 and 317. However,
other stainless steels of high resistances to corrosion by
the catholyte environments may also be employed, many of which
may contain about 18% of chromium and 8~ of nickel. The various
stainless steels from which corrosion-resistant anodes may
be made are described in Section 24 of the Steel Products
Manual, issed by the American,Iron and Steel Institute in
February, 1949, under the heading 'IStainless and ~eat-Resisting
Steels". A summary of such steel formulations and correspond-
ing type numbers is found in the Handbook of Engineering
Fundamentals by Eshbach, Second Edition r published in 1952 by
John Wiley & Sons, Inc., New York, page 12-40 and discussions
.
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8S55
of such s-teels and their corrosion resistances is at page
12-39. In addition to the s-tainless steels, other corrosion
resistant steels such as silicon steels, nickel steels,
and other conduc-tive materials resistant to corrosion may
also be employed as cathode materials or surfaces.
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 th~se o;f-2 toi3lca~rbon
atoms, e.g., tetrafluoroethylene, hexafluoropropylene. The
sulfonated perfluorovinyl ether ~hich is most useful is that
of -the formula FS02CF2-CF20~F(CF3)CF20CF=CF2- Such a
j material, named as perfluoro e2-(2-fluorosulfonylethoxy)-
propyl vinyl ether~, referred to henceforth as PSEPVE, may
be modified to equivalent monomers, as by modifying the inter-
nal perfluorosulfonylethoxy component to the corresponding
propoxy component and by altering the propyl to ethyl or
butyl, plus rearranging positions of substi-tution of the sul-
fonyl 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
~s~s~
is described in Example XVII o U.S. patent 3,~82,875 and an
alternative method is mentioned in Canadian patent 849,670,
which also discloses the use of the finished men~rane in fuel
cells, characterized therein as electrochemical cells. In
short, the copolymer may be made by reacting PSEPVEor equiva-
lent 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 wei~ht is indeter-
minate but the equivalent weight is about 900 to 1,600 preferably
1,100 to 1,400 and the percentage of PSEPVEor corresponding
compound is about 10 to 3~/O preferably 15 to 2~/o 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
-SO2F groups to -S03H groups, as by treating with l~/o 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 3,0~1,317.
Because it has been found that some expansion
-- 19 -
~58~i5
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 applied to the backing polytetrafluoroethylene
filaments or other suitable filaments prior to hydrolysis, when it is
still thermoplastic, and the film of copolymer covers each filament,
penetrating into the spaces between them and even around behind them,
the films becoming slightly thinner in the process, where they cover
the filaments.
The membrane described is far superior in the present
processes to all other previously suggested membrane materials. It
is more stable at elevated temperatures, e.g., above 75CC. It lasts
for much longer time periods in the medium of the electrolyte and the
caustic product and does not become brittle when subjected to chlorine
at high cell temperatures. Considering the savings in time and
fabrication costs, the present membranes are more economical. The
voltage drop through the membranes is acceptable and does not become
inordinately high, as it does with many other membrane materials,
when the caustic concentration in the cathode compartment increases
to above about 200 g./l. of caustic. The~selectivity of the membrane
and its compatibility with the electrolyte do not decrease
- 20 -
~s~ss
detrimentally as the hydroxyl concentration in the catholyte
liquor increases, as has been noted with other membrane materi-
als. Furthermore, the caustic efficiency of the electrolysis
does not diminish as significantly as it does with o~her
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 ltlOo to 1,400 being most preferred, some useful resinous
membranes produced by the present me~hod may be o~ equivalent
weights from 500 to 4,003. 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 operations.
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,
~he 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 Q.Ol mm.
Caustic efficiencies of the inven~ed processes r usiny su~h
modified versions of the present improved membranes can
- increase about 3 to 20%, often about 5 to 15%. Exemplary of
- 21 - ~
.
..... ~ ' :, " ''
;5S
such treatments is th~t described in French patent publication
2,i52,194, in which one side of the membrane is treated with
NH3 to form SO2NH2 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 usefu~ for making satisfactory
cation-active permselective memhranes for use in the present
electrolytic processes it has been established that perfluori-
nated ethylene propylene polymer (FEP) which is styrenated and
sulfonated makes a useful membrane. Whereas useful lives of
as much as three years or more (that of the preferred
lS copolymers) may not be obtained,the sulfostyrenated FEP~s are
surprisingly resis~ant to hardening and otherwise failing in
- use under the present process conditions.
To manufacture the sulfostyrenated FEP membranes
a stanaard 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 thick-
ness of about 0.02 to 0.5 mm., preferably 0.05 to 0.15 mm.,
is dipped into the solution. Aiter removal it is sub~ected
,
'.
.'.
'
- 22
'
- :. - ~ ... .
.. .
__ __ __ .. ~ _ ._ _ _ _ _ .. __ ... _ __ _ _ .. _ .. _ ... _ _ .. . .. _ . ... _ . . .. .. . _ .. _ ._ .. .... _ _ _ _ _ _ . _ ..
_ . ; . ...... _ .... ..
ss~
to radiation treatment, using a cohalt60 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
megarad. 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,
uming sulfuric acid or S03. Preferably, chlorosulfonic acid
in chloroform is utilized and the sulfonation is completed
in about 1/2 hour.
~xamples of useful membranes made by the described
process are products of RAI Research Corporation, ~auppauge,
New York, identified as 18ST12S and 16ST13S, the form~r being
18~ styrenated and having 2/3 of the phenyl groups mono~
sulfonated and the latter being 16% s~yrenated 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 ~he
preferred copolymers previously described, givi~g 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 rom
the copolymer.
- ~ ~r~ ~R~k
~ . . ' ''
.'
,
_ 23
_,, , , . . .. . . . .. . .. . . ... . , .. .. . .: ... .... . ., .. . _, .
~L~ 8 ~S ~j
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 the membrane is mounted for support on a network oF fila-
ments or fibers of polytetrafluoroethylene perfluorinated ethylene
propylene polymer, polypropylene, asbestos, titanium, niobium and
noble metals or other suitable network filaments, the filaments or
fibers of the network will usually have a thickness of 0.01 to 0.5
mm., preferably 0.05 to 0.15 mm., corresponding to up to the thick-
ness 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 f;lm may also be successfully employed,
e.g., 1.1 to five times the film thickness. The networks, screens
or cloths have an area percentage of openings therein from about
~3 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 elipses, 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 comp-
ression 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
- 24 -
3 C~S8~SS
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 Pibers mixed
with synthetic organic polymeric fibers may be employed. ~owever,
when such diaphragms are used efforts should be made to remove hard-
ness ions and other impurities from the feed to the cell so as to
prevent these from prematurely depositing on and blocking the
diaphragm5
The material of construction of the cell body may be con-
ventional, including concrete or stressed concrete lined with mastics,
rubbers, e.g., neoprene, polyvinylidene chloride, FEP, chlorendic
acid based polyester, polypropylene, polyvinyl chloride, TFE polymers
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 processes of this invention obtain good current
efficiencies for the manufacture of chlorine and acceptable
- 25 -
~-1
~ ,
~5~35S~
current efficiencies for producing hydroxide, sulfite and
dithionite. In preferre~ embodiments of the invention, when
sodium chloride is utilized and sodium sul~ite and sodium
dithionite are made, the current efficienc~for the productions
. 5 of chlorine in both cells ~e from 90 to 99%, usually be1ng ~4
to 97%, e.g., 96%-.The production of caustic in the ~hree-compart-
ment cell, including caustic produced in the cathode compartment,
whether removed therefrom or from the buffer compartment and
whether removed from the buffer compartmPnt as caustic or sulfite,
is at a current efficiency or sodium ion efficiency of about 70
or 75 to 90%. Approximately 5 to 50~ of the hydroxide produced
in the cathode compartment migrates to the buffer compartment and
usua~y this will be from 5 to 25%. In the two-compartment cell
the current efficiency for the production of the dithionite will
normally be from 40 to 80%, u sually 60 to 75~, with the conver-
sion of sulfur dioxide or sulfite to dithionit~ being about 20
to 50%. Such efficiencies are acceptable and althou~h ~he
efficiency for the manufacture of dithionite might appear low,
considering that useful sulfite is also made, it is satisfactory~
The present cells may be incorporated in large ox small
electrochemical plants, those producing bleaching dithionite
and accompanying sulfite while also making from 20 to 1,000 tons
per day of chlorine or equivalent derivative. In all cases the
efficiencies obtainable are such as to make the processes
economically desirable. It is highly preferred, however, that
- 26
.
.
~58555
the installation should be located near to and should be used in
conjunction with a groundwood or woodpulp bleaching plant so
that the dithionite produced can be employed promptly as a bleach
and the other chemicals may also be us~d for pulping or bleaching
purposes without the need to ship ~hem long distances ~o ultimate
consumers. Of course, if desired, the chlorine and caustic may
be so shipped or may be chemically converted to other matexials.
In some instances the chlorine may be liquefied and the caustic
may be evaporated to a higher concentration so as to facilitate
shipment or transfer.
The following examples illustrate but do not limit the
invention~ Unless otherwise indicated, all parts are by weight
and all temperatures are in C.
EXAMPLE 1
lS Utilizing the apparatus illustrated in the FIGURE,
useful sodium dithionite in a~ueous solution, accompanied by
sodium sulfite, is produced and is successfully employed in the
bleaching of groundwood pulp.
The materials of construction of the ~hree-compartment
and two-compartment cells include as a preferred material,
asbestos filled polypropylene. The anodes are dimensionally stable
anodes of titanium having ruthenium-titanium oxide coatings. The
titanium mesh-based anodes are connec~ed to sources o~ electricity
by titanium-clad copper rods. The cathodes are of Typ- 316
' ' , . " ~
'
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- 27
.
~63 5~35S5
.
stainless steel. In other experiments, yielding essentiall~ the
same results, the internal cell walls are of such materials as
chlorinated polyethylene or chlorinated polypropylene, the anodes
are o~ platinum or platinum-ir~dium alloy and the cathodes are
; 5 of Type 317 stainless steel.
The cation-active permselective membranes employed have
a wall thic~ness of 7 mils (about 0.2 mm.) and the membrane portion
thereof is joined to a backing or supporting network of polytetra-
1uoroethylene tTe~lon~ filaments having a diameter of about 0~1
mm. and woven into cloth form such that the ar~ percentage of
openings therein is of 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 ~he screen or cloth
by high temperature, high compression pressing, with portions of
the membranes actually flowing around the filaments during the
fusion processes to lock onto the cloth. The described perm-
selective membranes are obtainable from E. I. Du Pon~ de Nemours
and Company, Inc., Plastics Department, Wilmington, Delaware
19898, as XR Perfluorosulfonic Acid Membranes. The material
thereof is a hydrolyzed copolymer of a perfluorinated hydrocarbon
and a fluorosulfonated perfluorovinyl ether. The hydrolyzed
copolymer is of tetrafluoroethylene and FSO2CF2CF2OCFtCF3)CF2-
OCF-CF2 and has an equivalent weight in the 1,100 to 1,40~ range,
about 1,250.
Although in the FIGURE, for clarity of presentation,
- 28
- ,, -;;'- ;'
.; , . .
1C~585S5
the electrodes are apart from the membranes, in the practice of
~he present process the electrodes are in contact with the
membranes in the three-compartment cell, with the "flatter" sides
of the membranes facing the contacting electrodes. In ~he three-
` 5 compartment cell the buffer compartment volume is about 10~ ofthe total of the anode and cathode compartment volumes, which are
of about the same volume. In the two-compartment cell, cell
volumes are about equal and the electrodes are about 1/4 inch or
6.3 mm. apart.
The feeds to the anode compartments of both cells are
25% sodium chloride solutions in water and the depleted anolytes
in both cases are at 22% sodium chloride contents, with circula-
tions of the depleted anolytes through the resaturators (ox a
single resaturator) being controlled by sensors, valves and pumps
to maintain this desired difference in concentration between feed
and take-off solutions to/from the anode compartment.
In the case of the three-compartment cell the feed
of sulfur dioxide to the buffer compartment is regulated so as to
produce an effluent from that compartment comprising about 10% of
sodium sulfite and 10% of sodium hydroxide i~ water. Water feed
to the buffer compartment and wzter feed and caustic producing
conditions in the cathode compartment may also be regulated to
adjust the proportion of sulfite to hydroxide leaving the buffer
compartment. The pH of such solution is that of the caustic, 14.
Under best operating conditions of the three-compartment cell ~he
~ 29
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- . . . ^. :- .
. _, .. . . .. . .. . . . . . . . ... . . . . ., . _ ... _ . _ ~ .. .. .. . . . . . _
~L~51!~5S5
proportion of hydroxide passing from the ca~hode compartment to
the buffer compartment is or averages about 25~ of that produce~
at the cathode and this ratio is in the range of 5 to 50%. The
high concentration, low chloride content hydroxide taken off from
;s 5 the cathode compartment is a 25% hydroxide and has a chloride
content of about 0.05~. The temperature of the electrolyte is
maintained at about 90C. during the process, with 4.5 volts
impressed across the electrodes and a current density of 2 a.s.i.,
the current flow being 90 kiloamperes.
In the two-compartment cell khe feed to the catholyte
is the effluent from the buffer compartment of a three-compartment
cell and preferably it is cooled en route by cooling means, not
illustrated in the drawing, so as ~o enter the cathode compartment
of the two-compartment cell at the desired cell temperature r about
20C. (within a range of 15 to 35C.), Sulfur dioxide is added
to the cathode compartment at such a rate as to maintain the pH
of the catholyte at 7, although it may vary between 6 and 8.
Under flow rates described,about 60% of the cathodic current is
utilized in the production of dithionite and about 40% to make
sulfite from hydroxide and sulfur dioxide. The effluent from the
cathode compartment is an aqueous solution containing 16% of
sodium sulfite and 3,7~ of sodium dithionite.
The installation described produ~es 0.36 ton per day
of sodium dithionite, in a 3.7~ concentration aqueous solution,
with 33% conversion of sulfur dioxidP to dithionite and with the
- 30
1~35~S~5
dithionite obtained at a 75c current efficiency, calculated on the
basis of useful products obtained. The chlorine produced from the
two-compartment cell is at the rate of 0.3 ton per day and the
current efficiency is 95~. With respect with the three-compartment
S cell, the chl~rine production is at ~he rate of 3 tons per day,
also with a 95% current efficiency. The sodium hydroxide taken
off the cathode ~ompartment of the three-compartment cell is
produced at the rate of 2.28 tons per day and is in 25% aqueous
solution. The sulfur dioxide feed to the buffer compartment cell
is 0.49 ton per day with production of sodium sulfite from ~hat
compartment being at 0.97 ton per day and with 0.39 ton per day
of sodium hydroxide accompanying it. Current efficiency for the
production of sulfite and hydroxide in the three-compartment cell,
or sodium ion efficiency, is about 90%.
The solution of dithionite and sodium sulfite from the
cathode compartment of the t~o-compartment cell is continuously
employed to bleach groundwood pulp, aft r ailution to a 1%
dithionite solution. The groundwood charge is an 85.15 mixture
of West Coast hemlock and balsam, the rate of applicatior. is
1.1% of sodium dithionite, on a dry pulp basis and the pulp is in
- a 3~ aqueous slurry buffered to a pH of about 6.5 with potassium
hydrogen phosphate before addition of the dithionite. A b~ight-
ness increase of about 10 units is obtained at a brightening
temperature of 60-70C. after about 30 minutes treatment.
Reversion in such cases is about 2 units.
'
.
- 31
1~5~55~
The bleach liquor is recovered and mixed with black
liquor which is subsequently converted to white liquor used in
pulping.
E_AMPLE 2
The procedure of Example 1 is followed except for the
addition of sulfur dioxide to the catholyte of the two-compart-
ment cell. Instead of the sulfur divxide, additional sulfi.te is
added and the desired pH of 7 is maintained in the cathode compart-
ment by continuous addition of sulfuric acid, sodium bisulfatP,
sodium bisulfite or any other suitable acidic or alkaline
neutralizing agent or buffer. A~though the current efficiency
is not as good as in the processes utilizing a sulfur dioxide
feed to the catholyte of the two-compartment cell, thè pxocess
is operative and production of dithionite and other product is
at essentially the same rate as previously described. The
dithionite solution obtained is effective for groundwood bleaching,
as described in Example 1, and is useful for other bleaching
purposes, too.
In variations of this process and that of Example 1
the sulfur dioxide is fed to the buffer compartment of the three-
compartment cell and to the cathode compartment of the two-
compartment cell as aqueous solutions containing about 8% of
sulfur dioxide. Utilizing the solutions fewer problems of gas
bubbling and interference with electrode reactions are experienced
- 32
.-
~L~58S~S
but weaker product is obtained. In other modifications of the
experiments, batch and continuous processes are employed. The
continuous processes, sometimes with recycles o~ each of the
compartment contents, are generally superior, yielding a more
consistent product a n d readily lending themselves to automatic
control.
The invention has been described with respect ko
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.
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- 33 - ~
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