Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR THE PRODUCTION OF CHLORINE DIOXIDE
The present invention relates to the production of
chlorine dioxide, a chemical used in the bleaching of
wood pulp, in particular to improving the efficiency of
production of chlorine dioxide using hydrogen chloride.
Chlorine dioxide may be produced by reaction of
aqueous sodium chlorate and hydrochloric acid at the
boiling point of the reaction medium under a
subatmospheric pressure applied to the chlorine dioxide
generator, as described, for example, in U.S. Patent No.
3,929,974, assigned to the applicant hereof. By-product
sodium chloride crystallizes from the reaction medium
once saturation of the reaction medium is reached.
The by-product sodium chloride may be removed from
the generator and formed into an aqueous solution
thereof, which is forwarded to a sodium chlorate cell
wherein the aqueous sodium chloride is electrolyzed to
aqueous sodium chlorate which is returned to the
generator, as described, for example, in the above-noted
U.S. Patent No. 3,929,974. Sodium dichromate
(Na2Cr2O7. 2H20) is generally used to improve the
efficiency of the sodium chlorate production in such
electrolysis procedure. When chlorate cell liquor is
forwarded to the chlorine dioxide generator, the sodium
dichromate present in the cell liquor enters the
chlorine dioxide generator.
In accordance with the present invention, such
integrated chlorine dioxide generating process is
operated in a manner so as to increase the concentration
of sodium dichromate in the reaction medium in the
chlorine dioxide generator and thereby increase the
efficiency of chlorine dioxide production from the
reaction medium as a result of catalysis of the chlorine
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dioxide generating reaction by the relatively
concentrated sodium dichromate.
The efficiency of chlorine dioxide production is
the proportion of sodium chlorate used to make chlorine
dioxide by reaction (1) rather than less desired
chlorine from reaction (2):
NaC103 + 2HC1 ~ C102 +'-~ C12 + NaCl + H20 (1)
NaC103 + 6HC1 ~ 3C12 + NaCl + 3H20 (2)
The greater the proportion of sodium chlorate which
reacts to form chlorine dioxide by reaction (1), the
greater is the efficiency of chlorine dioxide production
and the efficiency is expressed as a percentage.
Although the catalytic effect of sodium dichromate
on chlorine dioxide production is known and is described
in U.S. Patent No. 3,563,702, it has not heretofore been
possible to take full advantage of this effect in an
integrated chlorine dioxide generating system, as
described in the aforementioned U.S. Patent No.
3,929,974, with cell liquor being forwarded from the
chlorate cells to the chlorine dioxide generator. The
reasons for this are that the chlorate cells must
operate at near pH 6.8 for optimum performance and
cannot operate at lower pHs, otherwise too much chlorine
gas will be formed in the chlorate cells, leading to an
explosive C12-H2 mixture.
A high level of sodium dichromate in the chlorine
dioxide generator forms a buffer, as follows:
Alkali pH> 7 Neutral pH Acid pH< 7
2HC1 HC1
2 Na2Cr2O4 -~ Na2Cr2O7 ~ NaHCr2O7
For each 10 gpl Na2Cr2O7.2H20 added to the chlorine
dioxide generating reaction medium, the operating
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acidity of the generator rises approximately 0.1 N as a
result of this buffering effect.
In accordance with one aspect of the present
invention, the concentration of sodium dichromate (as
Na2Cr2O7.2H20) in the chlorine dioxide generating reaction
medium is at least about 20 grams per liter (gpl),
generally about 20 to about 200 gpl, preferably greater
than about 100 gpl. Accordingly, in one aspect of the
present invention, there is provided a process for the
production of chlorine dioxide, which comprises reacting
sodium chlorate and hydrochloric acid in an aqueous acid
reaction medium in the presence of at least about 20 gpl
of sodium dichromate.
At the concentration levels of sodium dichromate
discussed above, the quantity of acid removed from the
generator and sent to the chlorate cells with by-product
sodium chloride in an integrated operation is too high for
efficient chlorate cell operation, particularly if there is
no crystallization of sodium chloride in the generator.
In a process wherein sodium chloride is crystallized
from the generator, the sodium chloride by-product may be
separated from the generator liquor containing the sodium
dichromate by washing the solids on a filter, thereby
returning little or no acid to the chlorate cells. In
practice, some acid is desirable and this amount may be
controlled by returning a controlled amount of generator
liquor to the cells rather than the whole solution, as is
the case of a non-crystallizing system.
The controlled amount of generator liquor forwarded to
the chlorate cells also serves to return sodium dichromate
to the chlorate cells. The chlorate cells generally
operate at a concentration of sodium dichromate of about 2
to about 7 gpl, in contrast to the concentration of sodium
dichromate in the chlorine dioxide generator of at least
about 20 gpl, preferably
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at least about 100 gpl dichromate, employed in the
present invention.
Accordingly, in a further aspect of the invention,
there is provided a continuous integrated process of
forming chlorine dioxide by reacting sodium chlorate
with hydrochloric acid to form chlorine dioxide and
chlorine and electrolytically producing the sodium
chlorate from sodium chloride produced by the chlorine
dioxide-forming reaction, which comprises feeding
sodium chloride and hydrochloric acid to an aqueous
acid reaction medium in a reaction zone; effecting
reaction of sodium chlorate and hydrochloric acid in
the presence of at least about 20 gpl of sodium
dichromate to produce chlorine dioxide and chlorine in
a reaction zone, said aqueous acid reaction medium
being at the boiling point of the aqueous acid reaction
medium while a subatmospheric pressure is applied to
the reaction zone, and precipitating by-product
crystalline sodium chloride in said reaction zone;
removing a gaseous admixture of chlorine dioxide,
chlorine and steam from the reaction zone and forming
an aqueous solution of chlorine dioxide and chlorine
and residual gaseous chlorine therefrom;
electrolytically forming sodium chlorate by
electrolysis of an aqueous solution of sodium chloride
containing about 2 to about 7 gpl of sodium dichromate
at a pH of about 6 to about 7 to produce an aqueous
solution of sodium chlorate, sodium chloride and sodium
dichromate; forwarding said aqueous solution of sodium
chlorate, sodium chloride and sodium dichromate to said
aqueous acid reaction medium as the feed of sodium
chlorate thereto; removing said by-product crystalline
sodium chloride from said reaction zone in a slurry
with entrained aqueous acid reaction medium; removing
at least a substantial proportion of said entrained
aqueous acid reaction medium from said recovered by-
product crystalline sodium chloride; forming an aqueous
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solution of sodium chloride from the resulting purified
by-product crystalline sodium chloride and forwarding
said aqueous solution of sodium chloride to said
electrolysis step for electrolysis therein; and
5 forwarding removed entrained aqueous acid reaction
medium to said reaction zone.
In the process of the present invention, the
chlorine dioxide generation is preferably effected at
the boiling point of the aqueous acid reaction medium
lo while a subatmospheric pressure is applied to the
chlorine dioxide generation vessel. The process may be
carried out at a temperature of about 550 to about 80 C,
preferably about 60 to about 75 C, and at a
subatmospheric pressure of about 120 to about 250 mmHg,
preferably about 125 to about 200 mmHg.
The chlorine dioxide generating process may be
carried out at a total acid normality of about 0.05 to
about 2 N, preferably about 0.05 to about 1.5 N.
Electrochemical production of sodium chlorate may
be effected by electrolysis of an aqueous solution of
sodium chloride having a concentration of sodium
chloride from about 90 to about 320 gpl, preferably
about 100 to about 200 gpl, and containing about 2 to
about 7 gpl of sodium dichromate at a pH of about 6 to
about 7 and a temperature of about 60 to about 100 C.
Such electrolysis results in an aqueous solution of
sodium chlorate, sodium chloride and sodium dichromate
which is passed to the chlorine dioxide generation step
and generally contains about 400 to about 600 gpl,
preferably about 450 to about 550 gpl, of sodium
chlorate, about 90 to about 200 gpl, preferably about
100 to about 150 gpl, of sodium chloride, and about 2 to
about 7 gpl of sodium dichromate.
The invention is described further, by way of
illustration, with reference to the accompanying
drawing, wherein:
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Figure 1 is a schematic flow diagram of an
integrated chlorine dioxide generating operation
provided in accordance with one embodiment of the
present invention.
Referring to the drawing, chlorate cells (1)
operating near pH 6 to 7 in reactor (2) convert aqueous
sodium chloride to aqueous sodium chlorate and by-
product hydrogen (6) while the coproduction of C12
and/or 02 is maintained below the explosive limit of
reaction with hydrogen. Hydrogen (6) is fed to an HC1
burner (3) wherein the hydrogen is reacted with
chlorine (27) and water (28) to form hydrochloric acid
(29). The hydrochloric acid (29) is forwarded via pump
(4) to a combined chlorine dioxide generator-
evaporator-crystallizer (5) . Cell liquor (30) from the
reactor (2) containing sodium chlorate, sodium chloride
and sodium dichromate, is forwarded by pump (7) to the
chlorine dioxide generator (5). Typically, the
concentrations of materials in the cell liquor are 500
gpL NaC103, 100 gpL NaCl and 2 to 6 gpl Na2Cr207. H20.
In the combined chlorine dioxide generator-
evaporator-crystallizer (5), the HC1 and NaC103 react
according to the reactions shown in reactions (1) and
(2) above, to form chlorine dioxide and chlorine which
are removed from the chlorine dioxide generator (5) in
admixture with steam (31), which is formed by
maintaining the aqueous acid reaction medium in the
chlorine dioxide generator (5) at its boiling point
under a subatmospheric pressure.
The gases (31) are forwarded to an absorber (8)
after passing through a cooler (9) to condense out most
of the steam. In the absorber (8), chilled water
absorbs the C102 and some of the C12 to form a solution
of chlorine dioxide (11), typically containing about 10
gpl C102 and about 2 gpl C12. The balance of the
chlorine is forwarded by an ejector (12), which creates
the vacuum on the chlorine dioxide generator (5), and
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via a seal pot (13) to the HC1 burner (3). If desired,
the ejector (12) may be replaced by a vacuum pump. Make-
up chlorine (14) is added to the chlorine supplied to
the HC1 burner (3) to replace chlorine removed by
absorption in the chlorine dioxide solution.
Sodium chloride precipitates in the chlorine
dioxide generator (5) once the sodium chloride by-
product of the reactions between NaC103 and HC1
saturates the aqueous acid reaction medium after start
up and is a by-product of the chlorine dioxide
generation. The evaporation of water from the aqueous
acid reaction medium not only increases the
concentration of NaCl formed by reaction (1) and (2) and
NaCl fed from chlorate reactor (2) to form the
crystalline NaCl but also concentrates the sodium
dichromate fed from reactor (2).
The slurry of sodium chloride crystals and aqueous
acid reaction medium containing at least about 20 gpL,
preferably greater than about 100 gpL, of sodium
dichromate, is removed from the generator (5) and passed
via a pump (15) to a hydrocyclone (16) to concentrate
the slurry, generally from about 10 to about 30% v/v to
about 70 to about 90% v/v, typically from about 20 to
about 80% v/v. The concentrated slurry is fed from the
hydrocyclone (16) to a filter (17) where the remaining
liquid containing sodium chlorate, sodium chloride and
sodium dichromate is separated from the solid sodium
chloride crystals.
Gases and liquids flow from the filter (17) to a
separator (18) from where the gases pass to an ejector
(19), and from where liquid, in the form of aqueous
sodium chlorate, sodium chloride and sodium dichromate,
returns to the generator (5) through line 32. The
separated crystalline sodium chloride from the filter
(17) is fed to a dissolving tank (21) via a chute (20).
Water (22) or a condensate from cooler (9) is added to
the dissolving tank (21) to dissolve the crystalline
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sodium chloride to form an aqueous solution of
substantially pure sodium chloride (33), which then is
fed via a pump (23) to the chlorate reactor (2) for
generation of further sodium chlorate.
In order to control the amount of acid and sodium
dichromate which is recycled to the chlorate reactor (2)
from the chlorine dioxide generator and to permit sodium
chlorate to be produced under optimum conditions, a
controlled flow of slurry from pump (15) is forwarded to
the dissolving tank (21) by line (24). Alternatively, a
portion of the slurry from the hydrocyclone (16) may be
passed directly from the hydrocyclone to the dissolving
tank (21), by-passing the filter (17). In general, the
sodium dichromate concentration in the slurry is at
about 20 gpL or more, whereby the amount of acid
forwarded to the chlorate cells (1) provides an
acceptable pH in the reactor (2) for safe and efficient
operation. At sodium dichromate concentrations of above
about 100 gpL, it may be necessary to provide some
sodium hydroxide via line (25) to maintain the pH in the
chlorate reactor (2) at safe and efficient levels.
In the event of addition of such sodium hydroxide,
the neutralization reaction proceeds in accordance with
reaction (3):
NaOH + HC1 -+ NaCl + H20 (3)
In order to prevent accumulation in the overall process
of sodium chloride formed by reaction (3), sodium
chloride may be bled by line (26) and disposed of. In
order to prevent loss of sodium dichromate for economic
and environmental reasons, it is preferable for line
(24) to be closed during disposal of NaCl via line 26.
It will be seen, therefore, that sodium dichromate
fed by pump (7) from chlorate reactor (2) into the
chlorine dioxide generator (5) may be concentrated to
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any desired level in the aqueous acid reaction medium in
the chlorine dioxide generator (5) by removing only pure
sodium chloride at filter (17) and returning all the
separated liquor (32) to the chlorine dioxide generator
(5). The concentration of sodium dichromate in the
chlorine dioxide generator is prevented from reaching
excessively high levels and may be controlled at any
desired concentration in the chlorine dioxide generator
via bleed line (24). If desired, some of the sodium
dichromate may be bled from the chlorine dioxide
generator (5) by not washing or only partially washing
the sodium chloride on filter (17) and permitting such
sodium dichromate to pass to the chlorate cells (1).
A safe and effective level of acidity in the
chlorate reactor (2) is provided by the bleed (24) which
prevents excess chlorine formation in the chlorate
reactor in the presence of the hydrogen. As noted
earlier, excess chlorine and the potential for forming
explosive mixtures with the hydrogen, can occur at low
pH values. The reactions involved in the formation of
sodium chlorate by electrolysis of an aqueous solution
of sodium chloride are illustrated by the following
reactions:
electrolysis pH 6-7
3 Nacl -4 3 NaOH + 3/2C12 _+ 3 NaOC1+3/2H2
step 1 Step 2
3 NaOCl ~ NaC103 + 2 NaCl
step 3
The excess chlorine can result where step 2 does not
occur properly, as at low pH.
An alternative process to the integrated procedure
described in detail above with respect to Figure 1, is
to operate a free-standing chlorine dioxide generating
process in which chlorine dioxide and chlorine are
formed by reaction of sodium chlorate and hydrochloric
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acid in an aqueous acid reaction medium at the boiling
point of the reaction medium under a subatmospheric
pressure. An initial charge of sodium dichromate is
made to the reaction medium to provide a desired steady
5 state concentration of sodium dichromate in the aqueous
acid reaction medium. Aqueous sodium chlorate (which
may be formed from crystal sodium chlorate or supplied
by cell liquor) and hydrochloric acid are continuously
fed to the reaction medium to replace chemicals
10 consumed by the reactions. A de-chromated cell liquor
feed may be employed, if desired.
Crystalline by-product sodium chloride removed
from the chlorine dioxide generator forms a slurry with
aqueous acid reaction medium and is thoroughly washed
to free the crystalline by-product sodium chloride from
reaction medium, particularly sodium dichromate, before
disposal of the sodium chloride or reuse thereof. The
removed aqueous reaction medium is returned to the
generator. In this way, the loss of sodium dichromate
values from the generator is minimized. Any losses of
Cr (VI) values, however, which may result from
incomplete washing of the crystalline sodium chloride,
may be compensated for by employing a cell liquor feed
containing dichromates.
Although sodium dichromate is fed to the
generator, some of the feed may be converted to
trivalent chromium ions (Cr3+), especially at higher
acidities. However, both hexavalent and trivalent
chromium compounds have catalytic properties in the
chlorine dioxide generating reaction. Cr3+ is
reoxidized back to Cr (VI) in the chlorate cells due to
the action of hypochlorite.
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EXAMPLES
Example 1:
This Example illustrates the catalytic effect of
high concentrations of sodium dichromate on the
efficiency of chlorine dioxide generation.
A 10L chlorine dioxide pilot plant generator was
operated at the boiling point of the reaction medium
under subatmospheric pressure to form chlorine dioxide
by reaction of sodium chlorate and hydrochloric acid.
The generator conditions were are follows:
Reaction medium: Total [H+] - from 0.67N to 3.19N
[C103-] - from 0.23 M to 0.01 M
[Na2Cr207.2H20] - 70 g/L
Temperature - 68 C
During the run, 12N HC1 was fed to the generator
at an average rate of 25mL/min in order to increase
acidity while generating chlorine dioxide at a rate of
approximately 6 to 7 g/min. The sodium chlorate input,
in the form of either 6M NaC103 or 5.2M NaC103 + 1.9M
NaCl solution was made to match production rate demand
while decreasing chlorate liquor concentration.
Chemical efficiencies, based on analysis by gas
chromatography, as high as 92.24% were found while,
with the sodium dichromate concentration below 20 gpL,
chemical efficiencies were found to range from 85.9 to
86.90.
Example 2:
This Example also illustrates the catalytic effect
of high concentration of sodium dichromate on the
efficiency of chlorine dioxide production.
The same pilot plant generator from Example 1 was
operated under the following process conditions to form
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chlorine dioxide by reaction of sodium chlorate and
hydrochloric acid.
The generator conditions were as follows:
Reaction medium: Total [H+] - 0.05N to 3.6N
[C103] - 1.62 M to 0.1M
[Na2Cr2O7.2H20] - 150 g/L
Temperature - 68 C
The run was made in a similar manner to Example 1, with
liquor composition being driven from 0.05N [H+]/1.62M
[C103-] to 3. 6N [H+] /0. 1M [C103-] over the course of a
run of more than 1.5 hours.
Chemical efficiencies, based on gas analysis by
gas chromatography, as high as 96.24% were found while,
under similar conditions at concentrations of sodium
dichromate below 20 gpL, chemical efficiencies were
found in a range of 85.9 to 86.9%.
In summary of this disclosure, the present
invention provides a novel method of producing chlorine
dioxide at high efficiency from sodium chlorate and
hydrochloric acid by using high concentrations of
sodium dichromate to catalyze the process, particularly
in a procedure which is integrated with sodium chlorate
generation. Modifications are possible within the
scope of this invention.
