Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
37~
E197 ELECTROLYTIC PRODUCTION OF CHLORINE DIOXIDE
The present invention relates to the production o~
chlorine dioxide by the electrolysis of highly acidic
sodium chlorate solutions.
Chlorine dioxide is used as a bleach in a variety
of environments, notably in the bleaching of wood pulp.
Various chemical processes for the generation of
chlorine dioxide by reduction of sodium chlorate in
aqueous acid media have been described in the past and
are in commercial operation. The chemical process may
be depicted by the equation (I):
2C103 + 2C1 ~ 4H -~ 2C102 + C12 + 2H20 (I)
U.S. Patent No. 4,426,263 (Hardee et al~ describes
an electrolytic process for producing chlorine dioxide
lS using an electrocatal`yst comprising a platinum group
metal oxide as a cathode coating in an electrolytic cell
containing sodium chlorate and sulphuric acid. This
patent` also describes the use of platinum group metal
oxides as a catalyst in the absence of applied
electrical current and indicates that this is the
; preferred embodiment.
Since the material used as the cathode coating is
itself a catalyst for the production of chlorine
dioxide, the effect of an applied current cannot readily
be determined but the data presented in the patent and
also in an article by Hardee describing the
electrochemical process (see "The Electrochemical
Generation of Chlorine Dioxide Utilizing Electrolytic
Oxide Coatings", Extended Abstracts, vol. 85-1, pp.617
to 618, The Electrochemical Society, 1985) suggest
little beneficial effect of the applied current on the
generation of chlorine dioxide.
According to the article, the better efficiencies
are observed at lower current values and hence at lower
contributions of electrolysis to the overall process of
generation of chlorine dioxide. In particular, current
efficiencies as low as 20% were observed at higher
current densities. The observed loss in efficiency was
~ 2
believed to arise from further reduction of chlorine
dioxide.
Poor results obtained in electrolytic e~periments
carried out at higher current densities are in a good
correspondence with the cyclic voltammograms reported in
the above-noted Hardee article, where the maximum
current density observed for the elec~roreduction of 0.5
M NaC103 is less than 10 mA/cm2, which is, by an order
of magnitude, lower than expected Eor such a high
concentration of reducible species.
The experimental data in the Hardee article
indicates to one skilled in the art that the process is
not limited by the electrochemical step involving
chlorate but rather by a chemical step in which an
electroactive species, different from chlorate, is
formed, which undergoes subsequently an
electroreduction. Accordingly, the rate of reduction of
chlorate ion to chlorine dioxide is limited by a
chemical reaction rather than an electrochemical one and
2~ this chemical reaction can be accelerated by the
presence of a catalyst, as described in the Hardee
patent and article. Although the platinum metal oxide
- catalyst appears to enhance the rate of chemical
conversion of chlorate ion to chlorine dioxide, its
electrocatalytic properties have a detrimental effect on
the electrochemical stability of the desired product,
namely chlorine dioxide, when practical current
densities are applied to electrodes having a surface of
such platinum metal oxides.
The Hardee article also claims that the platinum ~;
group metal oxides are the only materials which show
activity for the reduction of chlorate and data ls
presented showing the alleged ineffectiveness of
platinum.
In accordance with the present invention, there is
provided an electrochemical process for the production
of chlorine dioxide which is based on an autocatalytic
cycle utilizing part of the product, namely chlorine
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dioxide, for generation o~ the next portion of the same
prod~lct.
It has been surprisingly found that pure chlorine
dioxide, without any substantial contamination by
chlorine, can be produced from a chlorate-containing
aqueous acid solution having a total acidity greater
than that of about 7 normal sulphuric acid by passing a
cathodic current through the solution fro~ an elect.rode
constructed of an electrochemically-active makerial
whlch is also chemically inert with respect to the
production of chlorine dioxide from the solution and by
- maintaining a residual dissolved concentration of
.~ chlorine dioxide in the solution. -:
Accordingly, the present invention provides an
electrochemical process for the production of chlorine
; dioxide, which comprises passing a cathodic electrical
; current through an aqueous acid solution of chlorate
ions having a total acidity greater than that of about 7
normal sulphuric acid and containing dissolved chlorine
dioxide using a cathode constructed of an
electrochemically-active material which is also
chemically inert and non-catalytic with respect to the
production of chlorine dioxide from the aqueous acid
: solution; and removing generated chlorine dioxide from :
. 25 the aqueous acid solution.
The mechanism of generation of chlorine dioxide by
the electrochemical process of the invention is believed
to involve chemical reaction between chlorate ions and
electrolytically-produced short-lived chlorite ions to
form chlorine dioxide. Part of the chemically-produced
chlorine dioxide is electrochemically reduced to form
the chlorite ions, while the remainder is removed from
the solution as product.
The reactions which are thought to occur may be :
35 depicted, as follows: :;
: '~
~- -, ,
,~.~ . . ..
r'
Cl ~ - ,~ ClO2-
+
ClO2 ~ Cl03 + 2H ClO2 + ClO2 (~) + H20
overall: Cl03 + e + 2H -~ClO2 + H20
From these equations, it will be seen that the process
can be considered autocatalytic, in that generated
chlorine dioxide is used to produce the active species
for reduction of chlorate ions. A residual
concentration of chlorine dioxide must be maintained in
the aqueous acid solution to sustain the autocatalytic
cycle.
If the cell as a whole is considered, then the
anodic and cathodic reactions may be depicted as
follows:
Cathode : 2Cl03 + 2e ~ 4H --~2ClO2 + 2H20
Anode H20 - 2e ~ ~2 H
Cell 03 + 2H -~2C102 + ~2 + H20 (II)
As may be seen in equation (II) in comparison with
equation (I~, the electrochemical process of the
invention produces the same amount of chlorine dioxide
while half the amount of water is produced and half the
amount of acid is consumed, as compared to the chemical
process. The chlorine dioxide which is produced in the
process of the invention i5 substantially pure since the
reactions do not produce chlorine.
No chlorine dioxide is produced from the acidic
aqueous chlorate solution in the absence of an applied
cathodic current. The cathode which is used in ~he
process of the invention may be constructed of any
convenient electro-conductive material which is
chemically inert ti.e. has no catalytic properties) to
the chemical production of chlorine dioxide by reduction
of chlorate ions in the acid aqueous reaction medium, in
contrast to the materials described in U.S. Pa~ent No.
4,426,263 referred to above. Suitable cathode materials
include the platinum group metals and, preferably in
view of its cheapness and ease of use, carbon in any
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form, for example, graphite and vitreous carbon. The
use of a carbon cathode also is advantageous, since it
stabilizes the intermediate state, that is, the chlorite
ions, against further electroreduction to a lower
valency state, such as C10 or Cl .
As noted above, the chlorine dioxide which is
produced electrochemically in this invention is obtained
free from chlorine, since chlorine is not produced by
the reactions depicted by the equations given above.
Chlorine generation is possible only if chloride ions
are present in the reaction medium.
The possibility exists for the production of
- chloride ions by the acidic decomposition of chlorite
ions if the excess of chlorate ion in the acid medium is
lS insufficient, in accordance with the following equation:
5Cl2 + 4H - -~ 4C102 + Cl + 2H2o
The chloride ion produced in this way then can react
- chemically with the chlorate ions in accordance with the
- reaction depicted in equation (I) above to produce
2~ chlorine as well as chlorine dioxide. Although chlorine
dioxide still is formed, the coproduction of chlorine
represents an inefficiency with respect to the
production of chlorine dioxide from chlorate ions and
also is a source of current inefficiency.
The electrochemical process of the invention may be
carried out under a wide range of process conditions.
Essential to the present invention is the provision of
an aqueous acid electrolyte solution containing
dissolved chlorate ions and having a total acidity
greater than that of about 7 normal sulphuric acid. At
acidities corresponding to below about 7 normal
sulphuric acid, the production of pure chlorine dioxide
is not possible.
The acidity may be provided most conveniently by
sulphuric acid although any other strong mineral acid,
other than hydrochloric acid, or a mixture of acids, may
be employed, such as perchloric acid (HC104),
orthophosphoric acid (H3P04) or nitric acid (HN03).
Hydrochloric acid is avoided, since the introduction of
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37~
chloride ions would produce the undesired side chemical
reaction with chlorate ions to produce chlorine dioxide
and chlorine. An acid aqueous chlorate solution having
a total acidity corresponding to that of about 9 to
about 11 normal sulphuric acid is preferred.
The chlorate ions in the electrolyte are provided
preferably by sodium chlorate, since this chemical is
the most readily-available form of chlor~te. However,
other alkali metal chlorates, such as potassium
chlorate, lithium chlorate, rubidium chlorate and cesium
chlorate may be used, as well as alkaline earth metal
chlorates, such as beryllium chlorate, magnesium
chlorate, calcium chlorate, strontium chlorate, barium
chlorate and radium chlorate, and mixtures of two or
lS more of such chlorates. The concentration of chlorate
ions in the electrolyte may vary widely from about 0.001
to about 7 molar, preferably about 0.1 to about 2 molar.
In order to sustain the reactions which are thought
to be involved in the electrochemical process of the
invention, it is essential to maintain a dissolved
concentration of chlorine dioxide in the electrolyte.
Chlorine dioxide generation ceases if all the produced
chlorine dioxide is removed. In addition, some
dissolved chlorine dioxide is necessary at start up. A
concentration of dissolved chlorine dioxide in the range
of about 0.01 to about lS grams per litre (gpl) may be
employed, preferably about 0.1 to about 8 gpl, at the
initial startup and during the reaction.
In order to minimize side reactions and to maximize
the overall chemical efficiency of the productlon of one
mole of chlorine dioxide for each mole of chlorate ion
consumed, the concentration of chlorate ion in the
electrolyte should be in substantial excess to the
concentration of dissolved chlorine dioxide, generally a
molar excess of at least about 2:1, preferably at least
- about 10:1, usually up to about lO~0~
Generally, the concentration of dissolved chlorine
dioxide is maintained at a substantially uniform level
by stripping chlorine dioxide at the rate of its
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3.~37~3~5
formation. Chlorine dioxide spontaneously decomposes at
high partial pressures thereof and it is necessary to
dilute the chlorine dioxide well below the decomposition
partial pressure, usually below about 100 mmFlg. Any
convenient diluent gas, usually air, may be used ko
strip the generated chlorine dioxide from the
electrolytic cell and to provide the required dilution.
Chlorine dioxide may be recovered from the of~-gas
stream by dissolution in water.
~` 10 The electrical potential applied to the cathode
- during the electrochemical reaction depends on the
material of construction of the electrode and usually
varies from about +1.0 to about -0.5 Volts as compared
with a saturated calomel electrode (SCE). For a carbon
electrode, the preferred potential is approximately +0.4
Volts while for a platinum electrode, the preferred
potential is approximately +0.7 Volts. The process
usually is operated under constant voltage conditions
while the current also preferably is constant.
The temperature of operation of the cell affects
the purity of the chlorine dioxide ~as which is
obtained. Higher temperatures favour the formation of
; chloride ions by decomposition of chlorite ions, as
described above, in accordance with the equation:
5C102 -~ 4H --~ 4ClO2 + Cl + 2H2o
As discussed above, formation of chloride ions in this
way results in the formation of chlorine, with the
consequent loss of efficiency and chlorine dioxide
purity. Accordingly, it is preferred to operate at
temperatures below about 40C, more preferably at
ambient temperatures of about 20 to about 25C.
The process of the present invention may be carried
out in any convenient cell arrangement in which anode
and cathode electrodes are located and between which
current may be passed. The cell may be divided
physically into anolyte and catholyte chambers by any
convenient cation-exchange membraneO With a divided
cell arrangement, the aqueous acid chlorate solution is
fed to the cathode compartment while water is fed to the
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9 ~t7~3~5
anode compartment, the latter containing an electrolyte,
such as an acid solution.
However, operation in an undivided cell or a cell
with a simple non-membrane separator also is possible.
As noted above, the electrochemical reaction at the
cathode surface is believed to be the formation of
chlorite ion from chlorine dioxide. In an undivided
cell, such chlorite ions will attempt to migrate to the
anode but are consumed by the chlorate ions present in
large excess in the electrolyte to form chlorine
dioxide, so that the chlorite ions never reach the anode
and, in addition, their lifetime in the acidic medium is
very short.
The generation of chlorine dioxide by the process
of the invention is accompanied by the formation of
by-products. ~s noted earlier, the anodic reaction in
the cell produces gaseous oxygen, which may be vented in
any convenient manner. The other by-products are water
produced by the electrochemical reaction and a salt of
the cation of the chlorate and the anion of the acid
consumed in the process. These may be removed
respectively by any convenient procedure, such as by
evaporation and crystallization outside the cell.
The present invention, for the first time, provides
an electrochemical process for generating chlorine
dioxide from chlorate which does not rely on
chemically-catalytic electrode materials. Chlorine
dioxide is produced in pure form from an aqueous acid
chlorate solution by passing a cathodic current through
the solution from a cathode constructed of material
chemically inert with respect to the formation of
chlorine dioxide from the solution.
In the Examples which follow, reference is made to
the accompanying drawing, in which:
Figure 1 contains a series of three voltammetric
curves obtained in experiments described in these
Examples.
The invention is illustrated by the following
Examples:
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Example 1
Cyclic voltammetric studies were effected on an
aqueous solution containing about 0.1 g/L o~ chlorine
dioxide, 1 M NaC103 and 10 N H2S04, using, in one case,
a glassy carbon electrode and, in another case, a
platinum disc electrode, each having a surface area of
; 0.196 cm2. The current was plotted against the applied
potential and the results are reproduced as curves a
tglassy carbon) and b (platinum) in Figure 1. The
initial potential applied was +l.OV vs. SCE and a sweep
rate of 0.1 Vs 1 was used. A further run was made using
glassy carbon on a solution from which the sodium
chlorate was absent. These results are reproduced as
curve c in Figure 1.
- 15 It will be seen from the data presented in curves a
and b in Figure 1 that both the C102 reduction current
to C102 , which is proportional to the C102
concentration, and corresponding reoxidation current of
C102 back to C102, recorded during consecutive
~ potential scans between +l.OV and +0 6V vs. SCE,
increases substantially with the duration of the
multicyclic experiments. This rèsult indicates a
progressive accumulation or self-perpetuated
multiplication of chlorine dioxide in the proximity of
the electrode, both for the glassy carbon and platinum
electrode. In comparison, curve c of Figure 1 shows no
accumulation of chlorine dioxide in the absence of the
chlorate ions. ~-
Example 2
Electrolytic studies were carried out in a divided
H-cell using reticulated vitreous carbon foam as the
cathode material and platinum foil a~ the anode
material. A potentiostatic mode of operation was
adopted at a cathodic potential of +C.2 volt vs.
Hg/Hg2S04 as the reference electrode.
The catholyte of volume approximately 100 ml
contained about lON H2S04, about lM NaC103 and a~
variable initial dissolved C102 concentration. During
electrolysis, gaseous products, C102 and C12 were
~.~ .;
3 ~37~ o
stripped to a po~assium iodide (KI~ trap by bubbliny
nitrogen and by applying a low le~tel of vacuum. ~oth
the nitrogen flow and the vacuum were adjusted -to
maintain a substantially constant level of electrolyte
in the compartments and, at the same time, to maintain a
substantially constant concentration of dissolved
chlorine dioxide in the catholyte, so that the stripping
rate of chlorine dioxide was approximately equal to the
production rate of chlorine dioxide.
The electrolyte was analyzed for Cl02, C12, C103 ,
Cl , Cl02 and acidity both before and after
electrolysis while the KI trap was analyzed and changed
every 30 to 60 minutes.
The results obtained are set forth in the following
lS Table I:
:,
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c~2
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O S~, ~ ~ ~ æ ~ ~8
~ o~ s ¦ ~ S! ~ O O O
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~ ,s 4-o C~
., _, ~S ~ O O O ~ ~0 0
æ æ ~ ~ æ_æ ~o
S E C~ 8 _~ _. 8 ~ '
2~ o ~
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12
As may be seen from Table I, the chemical and
current efficiencies which were obtained were very high,
particularly at ambient temperature. The current
required was dependent on both dissolved chlorine
dioxide concentration and temperature, with -the
temperature effect being much more signi~icant. As may
be seen from run No. 4, a significant increase in sodium
chlorate concentration did not significantly affeck the
process.
The mole ratio of C102/C12, and hence the purity of
chlorine dioxide, although good in all the experiments,
was much better at ambient temperature and is in a good
correspondence with the temperature dependence of
current efficiency.
Chlorite ions postulated as short-lived
intermediates in the autocatalytic process were not
detected either in the catholyte and anolyte, before and
after electrolysis.
- Maintaining a residual chlorine dioxide
~0 concentration was critical for the operation of the
process. In an experiment where all the chlorine
dioxide was stripped from the solution, no further
electrogeneration of chlorine dioxide was observed and
the curxent measured under potentiostatic conditions
went to zero.
In summary of this disclosure, the present
invention provides a novel method of producing chlorine
dioxide by an autocatalytic cathodic electrochemical
- reduction of chlorate ions. Modifications are possible
within the scope o~ this invention.
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