Note: Descriptions are shown in the official language in which they were submitted.
a~_ 2 0 18 5 0 7
E437 53-632 MIS 305 1990 06 06 D6
ELECTROCHEMICAL PROCESSING OF AQUEOUS SOLUTIONS
The present invention relates to the
electrochemical processing of aqueous solutions to
produce acidic media for a plurality of applications, in
particular in the production of chlorine dioxide.
Chlorine dioxide, useful as a pulp mill
bleaching agent, is produced chemically by reduction of
an acid aqueous chlorate solution in accordance with the
a
l0 equation:
C103' + 2H; + e' -~ C102 + HZO
where the electron e' is supplied by various reducing
agents, for example, methanol, chloride ion and hydrogen
peroxide. In many commercial processes for effecting
this reaction, the acidity for the process is provided by
sulfuric acid while the chlorate ions are provided by
sodium chlorate. The presence of these species leads to
the formation of some form of sodium sulfate as a by-
product.
One particular embodiment of a commercial
process is the so-called "R8" process of the assignee of
this application, as described in U.S. Patent No.
4,081,520, assigned to the applicant herein.
Improvements in and modifications to that process also
are described in the appl.icant's U.S. Patents Nos.
4,465,658, 4,473,540 and 4,E~27,969.
In that chlorine dioxide generating process,
the reaction medium is at a Thigh total acid normality of
sulfuric acid and is maintained at its boiling point
under a subatmospheric pressure applied thereto.
Methanol is used as a reducing agent for chlorate ions,
resulting in the formation of chlorine dioxide in a
substantially pure form. The boiling nature of the
reaction medium produces stsaam which acts as a diluent
for the gaseous chlorine dioxide, so as to prevent
decomposition of the chlorine dioxide.
The sodium sulfate by-product builds up in the
reaction medium after start:-up until the solution is
20 1850 7
2
saturated with sodium su7lfate, whereupon the sodium
sulfate precipitates from the reaction medium. A slurry
of the sodium sulfate is removed from the reaction
vessel, the crystalline sodium sulfate is filtered
therefrom and the mother liquor is recycled to the
reaction zone after the addition of make-up quantities of
sodium chlorate, sulfuric acid and methanol.
This process is highly efficient and rapidly
produces chlorine dioxide in commercial quantities. As
may be concluded from the above equation, for each mole
of chlorine dioxide produced a mole of chlorate ion and
hence of sodium ion is introduced to the reaction medium.
The sodium ions combine with the sulfate ions introduced
with the sulfuric acid, to produce a sodium sulfate,
which may be sodium bisulfai:e or, more normally under the
conditions of an R-8 process, the double salt sodium
sesquisulfate, i.e., Na,H(S04)Z (or NaHS04.NaZS04),
depending on the acidity of the solution.
Another sulfuric acid-based chlorine dioxide
generating process, a low acidity "R3" process, as
described in U.S. Patent No. 3,864,456, produces neutral
sodium sulfate as the by-product.
Such by-product sodium sulfate and sodium
sesquisulfate (sometimes termed "saltcake"), generally
have been employed to make up sulfur losses in the pulp
mill.
However, the adoption of high substitution of
chlorine by chlorine dioxif~,e in the chlorination stage
.. 201807
3
of the bleach plant has led to saltcake by-product
production from the chlorine. dioxide generating process
exceeding the mill make-up requirements.
There exists a need, therefore, for a chlorine
dioxide generating process which possesses the
attributes of, for example, the R8 process, while, at
the same time, producing less sodium sulfate by-product
for the same level of production of chlorine dioxide.
It is even more advantageous if, in addition to a lower
saltcake production, caustic soda solution is co-
produced together with C102, thus minimizing an NaOH/C12
imbalance presently existing in pulp mills.
It has previously been suggested in U.S.
Patent No. 4,129,484 to treat aqueous effluent from
chlorine dioxide generating processes electrolytically
to form an acid-enriched fraction from the original
solution, which then may be recycled to the chlorine
dioxide generator.
In order to utilize the by-product saltcake,
it was proposed in the prior art to employ an
electrochemical process to convert sodium sulfate into
sulfuric acid and caustic soda solution in a three
compartment electrolytic cell, equipped with a cation
exchange membrane facing t:he cathode and an anionic
membrane or a diaphragm facing the anode, wherein the
saltcake solution is fed to the middle compartment. In
an electric field, sodium and sulfate or hydrogen
sulfate ions are transferred to the cathodic and anodic
compartments respectively where they recombine with
electrolytically-generated hydroxyl and hydrogen ions to
form caustic soda and sulfuric acid, respectively.
Analogously, in a simplified process, a two-
compartment electrolytic cell equipped with a cation
exchange membrane was proposed to generate a mixture of
sulfate and sulfuric acid in an anodic compartment along
with caustic soda solution in the cathodic compartment.
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4
The main drawback of these prior proposals was
that the sulfuric acid solution produced had a low acid
strength (less than 10 w~t% HZS04) , which imposes an
excessive evaporative load on the chlorine dioxide
generator, thereby rendering the process uneconomical and
impractical.
Although higher aulfuric acid concentrations
can be achieved in the electrochemical splitting of
saltcake in the manner described in the prior art, the
current efficiency for such a process is prohibitively
low due to the leakage of H; ions through the ion-
exchange membrane. Such migration of hydrogen ions
towards the cathode is related to a very high mobility of
this ion relative to Na' io;ns.
For example, in the aforementioned U.S. Patent
No. 4,129,484, current efl:iciencies as low as 9% for
production of about 1 normal caustic-soda solution and
39% for production of about 2M sulfuric acid were
reported.
In order to count~.ract the undesired migration
of hydrogen ions towards the cathode and hence maximize
the current efficiency for an acidification process, the
electrolytic process is eff~acted, in accordance with the
invention, while maintaining a high concentration ratio
of Na+/H~ in the anolyte.
In one embodiment of the present invention,
there is provided an electrochemical process for the
treatment of sodium sulfate,isodium chlorate mixtures, so
as to acidify the same and provide acid for the chlorine
dioxide generating process, while coproducing aqueous
sodium hydroxide solution. Although the co-production of
aqueous sodium hydroxide solution is most desired for the
pulp mill applications, any other suitable cathodic
reactions, for example, an electroreduction of oxygen to
hydrogen peroxide and sodium hydroxide, can be carried
z o~s~o7
out in combination with the anodic acidification of the
salt mixtures.
The process of the invention is more widely
applicable than to the treatment of such mixtures.
5 Rather the invention is broadly directed to the
treatment of aqueous solutions of certain alkali metal
salts and mixtures thereof to form an aqueous acidified
medium.
In one aspect, the present invention is
broadly directed to a method for the production of an
aqueous acidified chlorate ion-containing solution which
is useful on a feed stream for a chlorine dioxide
generating process wherein chlorate ions are reduced to
chlorine dioxide in an aqueous acid medium.
An aqueous solution of at least one alkali
metal salt selected from the group consisting of alkali
metal chlorate, alkali metal sulfate and mixtures of
alkali metal chlorate and alkali metal sulfate is
electrochemically acidified.
Alkali metal is>ns are electrochemically
removed from the aqueous solution to produce an
acidified alkali metal salt solution which constitutes
the aqueous acidified chlorate ion-containing solution
when the alkali metal salt is selected from the group
consisting of alkali metal chlorate and mixtures of
alkali metal chlorate and alkali metal sulfate.
When the alkali metal salt is alkali metal
sulfate, alkali metal chlorate is added to the acidified
alkali metal salt solution to provide the aqueous
acidified chlorate ion-containing solution.
One manner of effecting the procedure employs
an electrolytic cell comprising a cation-exchange
membrane dividing the cell into an anode compartment and
a cathode compartment. The aqueous solution of at least
one alkali metal salt is fed to the anode compartment
and hydrogen ions are elect.rolytically produced in the
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6
anode compartment while alkali metal ions are
transferred from the anode compartment through the
cation-exchange membrane to the cathode compartment.
The acidified alkali metal. salt solution is removed
from the anode compartment.
Alternatively, an.y other oxidation reaction
producing hydrogen ions, for example, hydrogen gas
oxidation to hydrogen ions, may be employed as an anodic
reaction. Such anodic hydrogen gas oxidation may be
combined with oxygen gas electroreduction as a cathodic
reaction, to provide a fuel cell operation wherein, in
addition to acidification of anolyte and production of
alkali metal hydroxide solution in the catholyte,
electrical energy is generated.
When an anodic o~,;idation of hydrogen gas to
hydrogen ions is combined with a cathodic reduction of
water to hydroxyl ions and hydrogen gas, the latter gas
may be used as an anodic feed and, at the same time, a
substantial energy savings nnay be achieved, as a result
of the difference in electrochemical reaction
potentials. Similarly, an analogous energy and material
savings is achieved when an anodic oxidation of water to
hydrogen ions and oxygen is combined with cathodic
reduction of the oxygen gas stream.
In accordance with one embodiment of the
present invention, there is provided an electrochemical
process for the treatment of an aqueous solution of
alkali metal salt mixtures, which comprises a plurality
of steps. The aqueous so7.ution is fed to the anode
compartment of an electrolytic cell having a cation-
exchange membrane dividing the cell into an anode
compartment and a cathode compartment.
Hydrogen ions are electrolytically produced in
the anode compartment and hydroxyl ions are
electrolytically produced in the cathode compartment.
Simultaneously alkali metal cations are transferred
.__ Zp ~g5o7
7
across the cation-exchange membrane from the anode
compartment to the cathode compartment.
This migration of alkali metal cations and the
electrochemical reaction ;producing hydrogen ions and
hydroxyl ions have the effect of producing an alkali
metal hydroxide solution in the cathode compartment and
an acid of the anion of the alkali metal salts in the
anode compartment. The respective aqueous solutions are
removed from the compartments of the cell.
In order to achieave high current efficiencies
of at least about 70%, prefcarably at least about 80%, and
thereby provide an economic: process, the molar ratio of
[Na+]:[H+] in the anolyte generally varies from about
1000:1 to about 1:5, prefcarably about 1000:1 to about
1:2, throughout the electrolytic reaction.
In another aspect of the present invention, the
method for the production of aqueous acidified chlorate
ion-containing solution is, integrated into a chlorine
dioxide generating process by providing an aqueous acid
chlorine dioxide-generating reaction medium containing
sulfuric acid and alkali mEatal ions in a reaction zone,
feeding the aqueous acidified chlorate ion-containing
solution to the reaction zone to provide chlorate ion and
hydrogen ion reactants to the aqueous acid chlorine
dioxide-generating reaction medium, and recovering an
alkali metal sulfate from t:he reaction zone.
As noted above, the present invention is
directed generally to the electrochemical treatment of
aqueous solutions of alkali metal salts and mixtures
thereof to generate an aqueous acidified medium. The
procedure of the present invention is particularly useful
in sulfuric acid-based chlorine dioxide generating
processes, since the invention enables the quantity of
by-product alkali metal sulfate co-produced with the
chlorine dioxide to be decreased and even eliminated
entirely, while maintaining the same level of chlorine
dioxide production.
20 18507
F3
In one embodiment of the present invention, an
aqueous solution of at least one alkali metal salt which
is alkali metal chlorate, alkali metal sulfate or,
preferably, mixtures thereof: is introduced to the anode
compartment of a cation-exchange membrane divided
electrochemical cell and alkali metal ions are
transferred from the anode compartment to the cathode
compartment. This procedure may be used in the present
invention in different ways to achieve the decreased
alkali metal sulfate output in a chlorine dioxide
generating process. It is usual for chlorine dioxide
generating processes to employ sodium salts, and hence in
the following description of specific embodiments of the
invention reference will be made to those sodium salts.
However, it is apparent that the principles of the
invention are applicable to salts of other alkali metals.
In a preferred embodiment of the invention, a
combination of sodium chlorate and sodium sulfate,
particularly in the form of aodium sesquisulfate, is fed
to the anode compartment of 'the cell, since the presence
of both salts enables the [rfa+]/ [H'] molar
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9
ratio in the anode compartment to be maximized,
resulting in a high current efficiency and thereby lower
power consumption. In addition, a higher overall
current efficiency decreases the capital cost, since a
lesser number of cells is required for the same
production rate.
It is preferred to employ a saturated feed
solution or a slurry, in order to minimize the
resistance of the electrolylte, while maintaining a high
[Na+]/[H+] ratio in the anode compartment. When the
feed is in the form of a slurry, it usually is preferred
to provide an additional separator or separators, such
as an ion-exchange membrane or a diaphragm, between the
feed stream and the anode t:o prevent abrasion and wear
on the anode from occurring. Such additional separator
also may be employed, if deaired, if the feed does not
comprise a slurry.
The cell produces an acidified solution from
the mixed sodium chlorate/sodium sulfate feed in the
anode compartment and a sodium hydroxide solution in the
cathode compartment. The acidified solution then is fed
to the chlorine dioxide generating process as an acid
and chlorate source therefor.
When there is no~ need for the saltcake in
particular pulp mill, it is preferred to operate the
cell so as to remove from the mixed sodium
chlorate/sodium sulfate feed a quantity of sodium ions
corresponding to the quantii~y of sodium ions introduced
with the sodium chlorate. The requirements of the
chlorine dioxide generator for acid and chlorate thereby
are completely satisfied, so that no additional sodium
sulfate by-product is formed. Therefore, in effect, a
dead load of sodium sulfate cycles between the chlorine
dioxide generator and the cell.
When operating the process of the invention
with sodium chlorate/sodium sulfate mixtures, it is
2418507
preferred to dissolve sodium chlorate in the sodium
sulfate solution. Alternatively, the sodium sulfate, in
slurry or crystalline form, may be added to an aqueous
sodium chlorate solution or, if desired, aqueous
5 solutions of sodium sulfate and sodium chlorate may be
mixed in any required proportion.
The composition of the anolyte feed may be
prepared by a one-time mixing of the individual
components, namely sodium sulfate and sodium chlorate,
10 or by a gradual addition of one component to another in
the course of electrolysis. For example, sodium sulfate
can be pre-acidified in an electrolytic cell prior to
the addition of sodium chlorate. In fact, the process
can be carried out in such. a way that neutral sodium
sulfate or sodium sesqu.isulfate is acidified to
bisulfate, NaHS04, and then this solution is used to
dissolve sodium chlorate, in order to take advantage of
an increased solubility of bisulfate with respect to
other sulfates. The mixture can be forwarded to the
generator or further acidified in an electrolytic cell.
From this discussion, it will be apparent that
any combination of sodium ulfate and sodium chlorate
may be selected to meet the requirements of a particular
pulp mill. Production of sodium sulfate by-product may
be permitted at any level desired by the pulp mill.
The presence of sodium sulfate (sesquisulfate)
in the anolyte improves the [Na+]/[H+] ratio not only
due to an increase in sodium ion concentration but also
as a result of a buffering action of the sulfate ions,
which tend to recombine with hydrogen ions to form
bisulfate, thereby decreasing the concentration of free
hydrogen ions. In addition, a combined acidified
chlorate/sulfate feed effectively decreases the amount
of water introduced to the chlorine dioxide generator,
as a result of a water "sharing" effect.
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1:1
While operating the process using sodium
chlorate/sodium sulfate mixitures provides a practically
unlimited flexibility in selecting the extent of
production of a by-product. saltcake corresponding to
between 0 to 100% recycling of sulfate, it also is
possible to achieve a partial decrease in saltcake
production by electrolytically acidifying a pure sodium
chlorate feed solution, i.e. without any addition of
sulfate. The overall reaction for such a process may
be depicted as:
xNaC103 + 3H20 1 (x-2) NaC103 + 2HC103 + 2NaOH
+ z02 + H2
Such a partial acidification. of sodium chlorate leads to
a mixture of sodium chlorate and chloric acid, in which
the content of chloric acid determines the extent of
reduction in sulfuric acid addition to the chlorine
dioxide generator and, consequently, the production of
sodium sulfate by-product. Since the [Na+]/[H+] ratio
constitutes a determining factor for the current
efficiency of the acidification process, only partial
conversion of sodium chlorate into chloric acid is
feasible in this case. Usually, up to about a 20%
reduction in saltcake by-product production can be
achieved by acidification of a pure sodium chlorate feed
at practical current efficiencies.
The aqueous sodium hydroxide solution which is
co-produced in the present invention is readily used by
the pulp mill in its bleach plant operations for the
purification of bleached pu:Lp. When pulp mills have an
on-site electrolytic process for producing aqueous
sodium hydroxide, usually from sodium chloride, Such
process results in the co-production of chlorine. The
ability to utilize such co--produced chlorine, however,
is often limited, which leads to a caustic/chlorine
imbalance in the pulp mill. By utilizing the process of
the invention, not only is the problem of co-production
x,018507
12
of excess sodium sulfate in the chlorine dioxide
generation process overcome, but also the problem of co-
production of chlorine in sa~dium hydroxide production is
overcome. The concentration of sodium hydroxide
solution produced in the cathode compartment may be
adjusted to any reasonable level, by adjusting flow
rates and recycling product solution.
As noted above, tile current efficiency of the
process is largely dependent on the [Na+]/[H+] ratio in
the anode compartment, which. usually requires a deadload
of sodium ion cycling between the cell and the chlorine
dioxide generator which, i.n the embodiment disclosed
above, may be provided by sodium sulfate as the
deadload. However, any other sodium salt which does not
otherwise adversely affect the production of chlorine
dioxide may be employed.
The anode employed in the electrolytic cell
may take any desired form, but it is preferred to employ
a low overpotential one with respect to the oxygen
evolution reaction, for example, a DSA-02~ electrode.
Similarly, any convenient material of construction may
be used for the cathode, for example, nickel.
The cation-exchange membrane may be formed of
any convenient material which enables cations to
selectively pass therethrough in preference to anions.
Preferably, the cation-exchange membrane is formed of
perfluorocarbon polymer having pendant cation-exchange
functional groups, such as those sold under the
trademarks "NAFION" (DuPont) or "FLEMION" (Asahi Glass).
~ In the event a:n additional ion exchange
membrane or membranes is used in the anode compartment,
such membrane may be formed of any convenient ion-
exchange material.
In one embodiment: of the invention in which
electrodialysis utilizing bipolar membranes is employed,
the aqueous solution of a mixture of alkali metal salts
zols~o7
1:3
is processed in a plurality of unit cells, with each
unit cell being separated from the adjacent ones by
bipolar membranes. The bipolar membranes have an
anionic face in the base compartment of one cell and a
cationic face in the acid compartment of an adjacent
cell. The individual cells are divided by a cation
exchange membrane.
With the plurality of cells separated by
bipolar membranes, gas evolution does not take place in
the acid and base compartments, and the overall
reaction may be represented by the equation:
(x+y) NaC103 +y H20 -~ x rfaC103 +y HC103 +y NaOH
The plurality of cells is terminated at both ends by
cationic membranes. A separate electrode rinse
solution, such as a sodium sulfate solution, is
circulated between the cathodic and anodic compartments
adjacent to the cathode and anode respectively. A
single electrical current :Feed, therefore, is used to
effect acidification in parallel in a plurality of unit
cells, with gaseous evolution occurring only in the end
anode and cathode compartments.
Bipolar membranes and their operation are well
known and are described, for example, in U.S. Patents
Nos. 4,024,043, 4,180,87L5, 4,057,481, 4,355,116,
4,116,889, 4,253,900, 4,584,246 and 4,673,454 and
reference may be had to such patents for details
thereof.
The parameters of: operation of the cell are
not critical to the proce:>s of the invention and may
vary widely. For example, the electrolytic process may
be effected under any desired electrolytic conditions,
generally at a membrane current density of about 0.01 to
about 10 kA/m2, preferably about 1 to about 5 kA/m2.
Similarly, the process may be carried out over
a wide range of temperatures, generally from about 0°
to about 150°C, preferably i_rom about 15° to about 90°C.
2o~s~o7
14
Generally, higher temperatures are preferred, in view of
the generally greater solubility of the alkali metal
salts at higher temperatures, thereby enhancing the
[Na+]/[H+] ratio. This greater solubility at high
temperature is particularly true of sodium chlorate, so
that, when such salt is included in the feed to the
cell, higher temperature operation, above about 60°C, is
preferred. The acidified) product stream from the
electrolysis retains a high concentration of sodium
chlorate, so that cooling of that product stream prior
to passage to the chlorine dioxide generator may lead to
precipitation of sodium chlorate, which can be removed
and recycled to the cell feed.
As already described, one important parameter
influencing current efficiency is the molar ratio of
[Na+]/[H+] in the anode compartment. Generally, this
molar ratio varies from about 1000:1 to about 1:5,
preferably about 1000:1 to about 1:2. Such a ratio is
dependent on the concentration of the feed solution to
the anode compartment and the extent to which sodium
ions are transferred from the anode compartment to the
cathode compartment. Accordingly, it is preferred to
employ a feed solution having a concentration of about
0.1 to about 15M in sodium ions and to remove from the
anode compartment for feed to the chlorine dioxide
generation process an acidified solution having a
concentration of about 0.1 to about 12M in sodium ions.
The electrolytic process increases the total
acid normality of the alkali metal salt solution.
Depending on the initial total acid normality and the
degree of electrolysis effected on the feed material, it
may be desirable to concentrate, such as by evaporation,
the product stream to increase its total acid normality,
prior to feed to the chlorine dioxide generation
process. Generally, the total acid normality of the
feed solution varies from about neutral to about 12
.~ 2018507
normal, preferably about 0.1 to about 10 normal, and the
electrolysis is effected to such a degree as to provide
a product stream from the: anode compartment having a
total acid normality generally from about 0.2 to about 15
5 normal, preferably about 0,.5 to about 6 normal.
The invention is described further, by way of
illustration, with reference to the accompanying
drawings, in which:
Figure 1 is a flaw sheet of a chlorine dioxide
10 generating process provided in accordance with one
embodiment of the invention;
Figure 2 is a schematic illustration of a
bipolar membrane cell which may be employed with the
chlorine dioxide generating process of Figure 1 ; and
15 Figure 3 contains a graphical representation of
experimental data, illustrating the relationship of acid
normality to current efficiency for different
concentrations of alkali metal salt.
Referring to the drawings, Figure 1 depicts an
embodiment of the application of the principles of the
present invention to the production of chlorine dioxide.
There is shown therein a chlorine dioxide
generating operation 10 comprising a chlorine dioxide
generating process 12 wherein sodium chlorate, sulfuric
acid and methanol are reacted at the boiling point of the
reaction medium under a ~:ub-atmospheric pressure, to
produce gaseous chlorine dioxide in line 14 (i.e. the R8
process).
The chlorine dioxide generating process in the
generator 12 results in the precipitation of a by-product
sodium sulfate, once the reaction medium has become
saturated after start-up. The form of the by-product
sodium sulfate, namely neutral sodium sulfate, sodium
sesquisulfate, sodium bisulfate or mixtures thereof,
depends on the total acid normality of the reaction
medium, which may vary from about 2 to about 11 normal.
20 18507
15a
The crystalline sodium sulfate (sometimes
termed "saltcake" ) is removed from the reaction medium in
the form of a slurry with ;spent reaction medium by line
16 and is passed to a filter 18 wherein the crystalline
material is separated from the spent reaction medium.
The separated spent reaction medium in line 20
is recycled to the generator 12 with make-up reactants,
namely sodium chlorate, sulfuric acid and methanol, being
fed to the recycle stream i.n line 20 by lines 22, 24 and
26 respectively.
20~.8~07
16
The crystalline aodium sulfate separated in
the filter 18 may be removed by line 28 for use
elsewhere in the pulp mill in whatever proportion
(including zero) of the sodium sulfate in the slurry 16
is desired.
The portion of the sodium sulfate not removed
by line 28 is forwarded by line 30 to a dissolving tank
32, wherein the sodium sulfate is dissolved in water fed
by line 34 to form an aqueous solution which is
preferably saturated. This solution is forwarded by
lines 36 and 38 to a membrane-divided electrolysis cell
40. It is not necessary that the sodium sulfate be
completely dissolved in water, but may be fed as a
slurry thereof. Where such a slurry is employed, it is
generally desirable to isolate the anode from the slurry
in the cell 40, to avoid excessive wear resulting from
abrasion.
The cell 40 comprises a cation-exchange
membrane 42, which divides the interior of the cell 40
into an anode compartment 44 and a cathode compartment
46 in which are located cell anode 48 and cell cathode
50 respectively.
Sodium chlorate i:~ added to the sodium sulfate
solution in line 36 by line 52 to form in line 38 a
feed solution for the anode' compartment 44 of the cell
40. An electrolyte is fed by line 54 to the cathode
compartment 46. A current is applied between the anode
48 and the cathode 50. The sodium chlorate may be
added by line 52 in the i=orm of an aqueous solution
thereof or as solid sodium chlorate.
In the cell 40, several reactions occur
simultaneously. At the anode 48, water is electrolyzed
to oxygen and hydrogen ion, as follows:
H20 -~ X02 + 2H+ + 2e-
while at the cathode 50 water is electrolyzed to
hydrogen and hydroxyl ion, as follows:
~o~s~o7
17
e- + H20 ~ iH2 + OH-
At the same time, sodium ions in the aqueous solution or
slurry of a mixture of sodium sulfate and sodium
chlorate migrate under then influence of the applied
current from the anode compartment 44 across the cation-
exchange membrane 42 to the cathode compartment 46. In
effect, therefore, the electrolytically-produced
hydrogen ions replace the sodium ions in the anode
compartment 44 and the transferred sodium ions are
available to combine with the electrolytically-produced
hydroxyl ions in the cathode: compartment 46.
The sodium sulfate contained in the solution
feed in line 38 to the cell 40 can be considered to be a
deadload circulating via the generator 12 in a closed
loop, so that the overall reaction in cell 40 can be
considered to be representedl, as follows:
xNaC103 + 3H20 -~ (x-2) NaC103 + 2HC103 + 2NaOH
+ X02 + H2
where x is the molar amount of sodium chlorate which is
processed.
The resulting chlo~ric acid-containing solution
then is recycled by line 54 to the recycle line 20, to
provide acid and chlorate ion for the chlorine dioxide
generator 12. The proportion of sodium sulfate removed
from the system by line 28 corresponds to the
proportion of the sulfuric: acid and sodium chlorate
reactants fed to the chlorine dioxide generation from
external sources, namely by line 24 for sulfuric acid
and by unconverted sodium chlorate in line 24 and in
line 22. Under steady state operating conditions for a
process precipitating sodiunn sesquisulfate and where no
sodium sulfate product is required, the processing of
the aqueous solution of a mixture of sodium
sesquisulfate and sodium chlorate in the cell 40
provides all the hydrogen ions and chlorate ions
20~18~07
18
necessary to sustain the reaction and additional
sulfuric acid and sodium chlorate are not required.
In effect, therefore, sodium chlorate fed to
the cell 40 is converted, at: least partially, to chloric
acid, so that the sodium sulfate component of the feed
solution 38 is a dead-load cycling between the cell 40
and the chlorine dioxide generator 12.
Oxygen is vented from the anode compartment 44
by line 56. Alternatively,, the product stream may be
recycled by line 58, with oxygen being vented from a
gas-liquid separator 60 by line 62, wherein recycle of
anolyte is required to achieve the desired acid
normality of the acidified ~;olution 54.
The sodium hydroxide produced in the cathode
compartment 46 is recycled by line 64, through a gas
liquid separator 66 from which hydrogen is vented by
line 68, until the desired concentration of sodium
hydroxide solution is achieved. The resulting aqueous
sodium hydroxide solution is removed as an aqueous
product stream in line 70. This solution has
considerable utility in a pulp mill, particularly as a
chemical employed in th~a purifying and bleaching
operations effected in the: bleach plant of the pulp
mill. The gaseous by-products, namely hydrogen and
oxygen, also can be utilized. in the pulp mill.
High current efficiency for the electrolytic
process effected in the cell 40 as described above can
be attributed to the high [lKa+]/[H+] molar ratio in the
anode compartment 44 and also to the buffering effect of
S042- ions towards hydrogen ions, which tends to
maintain the free hydrogen ion concentration in the
anode compartment low, thereby tending to maintain the
high [Na+]/[H+] molar ratio.
As may be seen from the above description, the
process of Figure 1 producer chlorine dioxide by the R8
process and hence retains 'the benefits thereof. More
2018507
19
importantly, the process doEa not produce any excess by-
product sodium sulfate requiring disposal. The amount
of sodium sulfate which is produced can be tailored to
the mill requirement, or may be eliminated entirely. In
effect, when there is no requirement for the saltcake in
the pulp mill, the sodium sulfate is maintained in a
closed loop within the process and the sodium ions
introduced to the chlorine dioxide generating process
with the sodium chlorate, exit the process in the form
of aqueous sodium hydroxide solution. The acid for the
chlorine dioxide generai~ing process is produced
electrolytically from water, which co-produces the
hydroxyl ions required to combine with the sodium ions
to form the sodium hydroxide..
Referring to Figure 2, there is shown therein
the utilization of a bank 100 of unit cells, with the
individual cells 102 producing an acidified mixture in
line 104 for feeding to a chlorine dioxide generator, as
described with respect to Figure 1, from an aqueous feed
mixture of sodium sesquisulfate and sodium chlorate in
line 106. The number of unit cells 102 in the bank of
cells may vary widely, depending on the required
production capacity and typically may vary from about 20
to about 500.
Each unit cell 100 is separated from each
adjacent unit cell by bipolar membranes 108, 110. The
bipolar membrane 108 has its cationic face in an acid
compartment 112, so as to form hydrogen ions under the
influence of the electric current applied to the bank of
cells 100, thereby acidifying the feed mixture, while
sodium ions are transported from the acid compartment
112 across a cation-exchange membrane 114 to a base
compartment 116.
The bipolar membrane 110 has its anionic face
in the base compartment 116, so as to form hydroxyl
ions from the aqueous feed i;.hereto in line 118 under the
20 1850 7
influence of the electrical current applied to the bank
of cells 100. In this way, sodium hydroxide is formed in
the base compartment 116 and is removed by line 120.
Only a single anode 122 and a single cathode
5 124 are required for the bank 100 of unit cells 102.
Oxygen and hydrogen respectively are formed at the
electrode surfaces and vented from the terminal unit
cells.
The invention is illustrated by the following
10 Examples:
Electrochemical e~;;periments were carried out in
a two-compartment MP cell, supplied by Electrocell AB,
Sweden equipped with an oxygen-evolving anode (DSA-OZO),
nickel cathode and a catio:n exchange membrane (NAFION
15 427) dividing the cell into an anode compartment and a
cathode compartment. The anode, cathode and membrane
each had an area of 100 sq, cm.
In the experiments, a current density of 3
kA/m2 was mainly employed (occasionally 2 kA/m2), the
20 anolyte was NaC103 or NaC103/NaZS04/H2S04 mixtures, the
catholyte was 1N aqueous sodium hydroxide solution and
the temperature was 40° to 50°C.
Example 1
In a first set of experiments, aqueous
solutions of sodium chlorate. of various concentrations
were used as the anolyte. During the course of
electrolysis, the anolyte became enriched with hydrogen
ions as the sodium chlorate was partially converted to
chloric acid. Current efficiencies were determined at
various product chloric acid concentrations for the
various initial sodium chlorate concentrations and were
plotted graphically. These results appear as Figure 3.
It will be seen from the data presented in this
Figure that the current efficiency declines with
increasing H' to Na' moles ratio in the solution
electrolyzed.
~,018~07
21
Example 2
Various mixtures of sodium chlorate and
Na2S04/H2S04 were prepared and electrolyzed.
(a) 1 L of an approximately 2M Na3H(S04)2 aqueous
solution was prepared by mixing Na2S04 and H2S04 at a
molar ratio of 3:1. Acidii~y was measured by titration
with NaOH and the solution was determined to have an
initial concentration of 1.94 normal. The catholyte was
0.5L of 1N NaOH.
Current was imposed between the electrodes at
a current density of 3 kA/m2 and sodium chlorate
crystals, in a total amount corresponding to the
preparation of a 2M aqueous solution, were added slowly
to the anolyte. A very rapid dissolution of sodium
chlorate was observed. The increase in acidity of the
anolyte was monitored by titration and the electrolysis
was terminated when the acidity of the anolyte reached
4.12 N. The basicity of they catholyte was determined to
be 4.54 N NaOH. The total tame of electrolysis was 8583
seconds with the total charge passed being 257,490
A.sec.
In the calculation of the current efficiency,
the volume changes of the anolyte and catholyte were
taken into account, since 'water is transported to the
catholyte together with Na+ ions and also is consumed in
the electrolysis. The current efficiency based on the
increase in acidity of the anolyte was determined to be
79~ while that based on the increase in basicity of the
catholyte was 80~.
The overall conversion in the anolyte can be
depicted as:
2 M Na3H(S04)2 + 2 M NaC103 ~ 2 M Na3H (S04)2
+ 2 M HC103
with an equivalent amount of caustic being co-produced
in the cathode compartment:. Hydrogen (cathode) and
oxygen (anode) were the gaseous by-products. All the
20 1850 7
22
reactants and products depicted by this equation are
largely dissociated in the solution into the ionic
species Na', H+, HS04'- 504~~' and C103'~ However, since
sesquisulfate contributes a prevailing form of the
precipitate formed in the chlorine dioxide generator,
chloric acid (HC103 = H~ + C:L03'~ is liberated from such a
mixture upon precipitation of sesquisulfate.
(b) Other mixtures of sodium chlorate and sodium
sesquisulfate were processed in a similar way to that
described in experiment (a), with quantitative (1 to 4)
or partial (5) conversion of the sodium chlorate to
chloric acid. The results obtained are reproduced in the
following Table I:
20 1850 7
22a
O oW
.,..y.
~7
W O ~ ~ tMD 0~0
U
N
y N~
H
N M M N M
A
U
~o o~ o
.-.
z ~ M M M
N N
v
H
O ~ O N
d' O 01
N e-~ rl e-i O
~ tf7
U~ rl N N N d'
z
N r-i H H r-1
.~.7
z
rl N M d'
tt7
20 1850 7
:~2b
It will be seen from this: Table and the data in (a)
above, that an improvement in current efficiency is
obtained by the presence of sulfate ion in conjunction
with a high [Na+]/ [H+] ratio, as compared with the results
obtained in Example 1. Fo:r example, l.5 M HC103 can be
obtained at about 60% C,.E. in pure aqueous sodium
chlorate solution and at about 80% C.E. in the mixtures
of sodium sesquisulfate and sodium chlorate.
In summary of this disclosure, the present
invention provides a novel electrochemical process which
enable the quantity of by-product sodium sulfate produced
by a chlorine dioxide generating process to be decreased.
Modifications are possible within the scope of this
invention.
