Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF INVENTION
NITRIC ACID BA8ED CHLORINE DIOXIDE (3ENER.ATION PROCESS
FIELD OF THE INVENTION
The present invention is concerned with the
production of chlorine dioxide from chlorate ions using
hydrogen peroxide or methanol as the preferred reducing
agents and, in particular, with the production of
chlorine dioxide where the reaction medium is
substantially free of sulfate ions and where the
primary source of acidity is nitric acid.
BACKGROUND TO THE INVENTION
It is known to produce chlorine dioxide by
reduction of an aqueous acid chlorate solution using
various reducing agents, such as methanol, hydrogen
peroxide and sulfur dioxide, wherein the acidity
required for the chlorine dioxide generation reaction
is supplied primarily by sulfuric acid. The process
can be carried out either in the crystallizing mode,
typically in a single vessel generator-evaporator-
crystallizer at subatmospheric pressure with the sodium
sulfate being precipitated from the reaction medium, or
in a non-crystallizing mode where the conversion of
chlorate takes place in at least one but typically two
reaction vessels with the secondary vessel yielding a
liquid acid effluent containing sulfuric acid and
alkali metal sulfate along with unreacted chlorate and
other by-products.
The single vessel sulfuric acid-based processes
employing methanol as a reducing agent are described,
for example, in US Patents 4,081,520 (Swindells et al),
4,473,540 (Fredette) and 4,770,868 (Norell), while the
use of hydrogen peroxide as a reducing agent in
chlorine dioxide generation is described, for example,
in US Patents 5,091,166 (Engstrom et al), 5,091,167
(Engstrom et al) and 5,366,714 (Bigauskas).
One of the deficiencies of sulfuric acid based
chlorine dioxide generators is a low production rate at
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low acidities. The lower limit of acidity below which
the reaction rate becomes commercially unacceptable is
somewhat dependent on the type of reducing agent used.
Typically an acidity of at least about 2 N sulfuric
acid is required with the preferred acidity being at
least about 4 N and at least about 5 N in the processes
involving the use of hydrogen peroxide and methanol,
respectively.
Another problem associated with the sulfuric acid
based generators is the cost of disposal of excess
saltcake by-product, especially in the case of non
crystallizing generators. There is a need, therefore,
to provide a chlorine dioxide generation process
operating at a very low acidity, preferably below about
2 N, without a co-production of saltcake by-product.
While sulfate-free chlorine dioxide generators have
been described in the prior art, none of them offers
high efficiency and high production rate at low
acidities (i.e., below about 2 N) without the necessity
of a significant increase in the chlorate ion
concentration.
US Patents 5,174,868 and 5,284,553 (Lipsztajn, et
al) describe an operation involving the addition of
chloric acid as a sole source of hydrogen ions whereby
the high efficiency and production rate are achieved
due to the presence of a dead load of sodium chlorate,
resulting in the chlorate ion concentration in the
reaction medium in the range of about 6 to about 9 M.
An analogous process disclosed in US Patent 5,486,344
(Winters et al) requires a preferred range of chlorate
ion concentration of from about 2 M up to about 5 M.
US Patent 5,523,072 (Falgen et al) teaches a
sulfate-free process in which the acidity is provided
by phosphoric acid. The latter process requires an
acidity of above about 2 N and a preferred range of
chlorate ion concentration above about 2 M. The
aforementioned US Patent 5,366,714 (Bigauskas) teaches
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both a sulfate-based and sulfate-free operation wherein
the latter one involves the addition of a strong
mineral acid, such as nitric acid, hydrochloric acid,
perchloric acid or chloric acid. For a low acidity
operation (below about 5 N), this patent suggests the
use of a high concentration of chlorate ion, from about
2 M up to saturation, preferably about 3 to 4 M.
SUMMARY OF THE INVENTION
Surprisingly it has been found that by employing
nitric acid as a source of acidity a sulfate-free
chlorine dioxide generation process can be operated
very efficiently and at a high production rate, even in
the low acidity range (i.e. below about 2 N) and at low
concentrations of chlorate (i.e. below about 3 M,
preferably in the range of about 2 to 3 M). Chlorate
concentrations higher than about 3 M may also be
employed, especially in the process involving the use
of methanol as a reducing agent. Such a combination of
acidity and chlorate concentration has not been
previously disclosed for a nitric acid based chlorine
dioxide generation processes.
Accordingly, the present invention provides an
improvement in a process for the production of chlorine
dioxide by the reduction of chlorate ions in an aqueous
acid reaction medium, the improvement being nitric acid
providing the acidity to the reaction medium.
GENF,RAL DESC1~IPT~ON OF INVENTION
As noted above, the present invention provides a
chlorine dioxide generating process which utilizes
nitric acid as a source of acidity for the process.
Such a process has been found to be much less dependent
on variations in acidity than other chlorine dioxide
generating processes known in the art, thus enabling a
very stable, continuous operation to be provided, which
is less sensitive to the process upset conditions. In
particular, the chlorine dioxide generating process may
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be operated at low acid normalities below about 2 N,
preferably about 0.5 to about 2 N.
The nitric acid based chlorine dioxide generating
process provided herein can be performed both in a
crystallizing and non-crystallizing mode at both
atmospheric and sub-atmospheric pressures. When
operating under subatmospheric conditions, which is the
preferred mode of operation, the aqueous acid reaction
medium generating the chlorine dioxide is maintained at
its boiling point at a temperature of about 50 to about
90°C, preferably about 70 to about 80°C, while a
subatmospheric pressure is applied to the reaction zone
of about 80 to about 400 mmHg (about 11 to about 53
kPa), preferably about 100 to about 250 mmHg (about 13
to about 33 kPa).
When operating in a crystallizing mode, a by-
product nitrate corresponding to the cation of the
chlorate reactant is crystallized from the aqueous acid
reaction medium in the reaction zone and is
continuously or periodically removed therefrom. In the
non-crystallizing mode of operation, an aqueous stream
is continuously or periodically removed from the
reaction zone for processing to remove by-product
nitrate salt.
A wide selection from the known reducing agents
for reduction of chlorate ions to generate chlorine
dioxide can be employed in the process of the
invention, with hydrogen peroxide and methanol being
the preferred ones. A combination of several reducing
agents also may be employed, if desired.
Chlorate ions reduced by the reducing agent to
chlorine dioxide in the process of the invention may be
provided by an alkali metal chlorate, usually sodium
chlorate, although chloric acid and mixtures of chloric
acid and alkali metal chlorate may be used. However,
in contrast to the prior art processes (see, for
example, US Patent 5,296,108 (Kaczur et al)), the
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presence of chloric acid is not required in order to
achieve high efficiencies at high production rates,
when nitric acid is employed as a source of acidity, in
accordance with the present invention. The
5 concentration of chlorate ions in the aqueous acid
reaction medium depends to some extent on the reducing
agent employed. For example, when hydrogen peroxide is
the reducing agent, the chlorate ion concentration is
generally below about 3 M, preferably about 2 to 3 M.
When methanol is employed as the reducing agents,
higher chlorate concentration of up to about 4,
preferably about 2 to about 3 M may be employed.
When operating the chlorine dioxide generating
process in a single vessel generator-evaporator
crystallizer at subatmospheric pressures, it is
possible to modify the process conditions used in the
process of the invention so that the concentrations in
the reaction medium remain below the saturation level
with respect to both alkali metal chlorate and alkali
metal nitrate.
In such a case it is preferred to continuously or
periodically withdraw the reaction medium from the
single vessel and to subject the withdrawn reaction
medium to electrochemical acidification in the anodic
compartment of an electrochemical cell, which enables
the regeneration of acidity values and at the same time
the co-production of alkali metal hydroxide, preferably
sodium hydroxide, in the cathodic compartment of the
electrochemical cell. The acidified product withdrawn
from the anodic compartment can be recycled back to the
chlorine dioxide producing reaction vessel.
Such an operation enables there to be achieved the
minimization of the quantity of the externally added
nitric acid. If desired, all the acidity requirements
can be sustained by the acid regenerated in the
electrochemical cell from effluent from the chlorine
dioxide generator.
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Any suitable electrochemical cell can be employed
to effect such acidification, including a standard two-
compartment cell equipped with a suitable separator,
for example, a cation exchange membrane, or a three-
s compartment cell equipped with two cation exchange
membranes. In the latter case, it is preferred to
direct the reaction mixture exiting the chlorine
dioxide generator into the central compartment of the
cell while circulating any suitable mineral acid, for
example, nitric acid, sulfuric acid or perchloric acid,
in the anode compartment as an anolyte. Hydrogen ions
generated in the anode compartment of the three-
compartment cell, are transferred through the cation
exchange membrane into the central compartment while
alkali metal ions are transferred through the second
cation exchange membrane from the central compartment
into the cathode compartment. By effecting such a
three-compartment operation, there is prevented an
oxidation of the various components of the reaction
medium removed from the chlorine dioxide generator, for
example, chlorate ions, chloride ions, hydrogen
peroxide, methanol and formic acid, at the anode of the
electrochemical cell.
Alternative cell configurations involve the use of
both anion and cation exchange membranes, as well as
bipolar membranes.
Due to the very high solubility of alkali metal
nitrates and the low acidity requirements in the
chlorine dioxide generation step, a very high current
efficiency for the electrochemical acidification
process can be achieved. It is believed that this
phenomenon can be attributed to a low concentration
ratio of hydrogen ions to alkali metal ions in the
acidified stream. The ratio of hydrogen ions to alkali
metal ions generally may vary from about 1:100 to about
5:1, preferably about 1:30 to about 2:1.
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When operating a subatmospheric type generator in
a crystallizing mode for the generation of chlorine
dioxide by the process of the invention, it is also
possible to combine the chlorine dioxide generation
process with the electrochemical acidification process.
In such a case, the alkali metal nitrate crystals which
precipitate as a by-product of the process and which
are continuously or periodically removed from the
generator, can be dissolved, optionally with alkali
metal chlorate, and the resulting solution can be
subjected to an electrochemical acidification step,
using one of the electrochemical processes described
above, and the resulting acidified solution can be
directed to the generator. Such electrochemical
acidification may be effected in a manner to coproduce
an alkali metal hydroxide, preferably sodium hydroxide,
in the electrochemical cell as described above.
The alkali metal nitrate is a high value by
product of the crystallizing mode of operation, which
can be readily used, for example, as a nutrient for
biological effluent treatment or as a fertilizer. When
the alkali metal nitrate comprises sodium nitrate, a
further conversion, for example, via metathesis, to
other nitrates, for example, potassium nitrate or
ammonium nitrate, is also possible.
When compared to the conventional sulfuric acid-
based chlorine dioxide generating process, the nitric
acid-based process of the present invention offers some
additional advantages in terms of the composition of
the crystalline by-product precipitated in the
generator in the crystallizing mode of operation. While
the conventional generators often co-produce an acidic
sulfate, such as sodium sesquisulfate, having regard to
the operating acid normality the process of the present
invention always yields a neutral salt irrespective of
the total acid normality of the reaction medium, thus
minimizing the acid and alkali consumption. A neutral
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by-product stream is also easier to handle due to the
lower corrosiveness when compared to conventional
procedures.
A non-crystallizing, atmospheric type nitric acid-
s based chlorine dioxide generation process can be
operated in a single reaction vessel or in a multi
vessel arrangement. In the latter case, the effluent
from a primary reactor is directed to the secondary
reactor enabling a further conversion of unreacted
chlorate with an additional feed of the reducing agent.
The acidity level in the secondary reactor may be
increased, if desired, to higher values, typically up
to about 7 N, preferably about 5 to about 7 N, in order
to ensure a substantially complete conversion of
chlorate ions, thus minimizing losses of chlorate with
the effluent. The effluent from the primary or
secondary reactor may also be directed (cascaded) to a
single vessel subatmospheric type process.
Alternatively, the effluent from the primary or
secondary generator can be subjected to electrochemical
acidification in the manner described in more detail
above and recycled to the generator. It is possible, if
desired, to effect hydrochloric acid addition to the
secondary reactor in order to lower the residual
chlorate ion concentration.
If desired, an effluent from the non-crystallizing
chlorine dioxide generating process described above
containing unreacted nitric acid and alkali metal
nitrate may be used as a nutrient for biological
effluent treatment or for pH control in the bleach
plant of a pulp mill. Since essentially all nitrates
are highly soluble, the use of nitric acid may be
beneficial as compared to sulfuric acid due to the
absence of scale forming deposits.
Both the crystallizing and non-crystallizing type
nitric acid-based chlorine dioxide generation
operations provided in accordance with the present
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invention, can operate in the substantial absence of
added chloride ions. However, small additions of
chloride can be made, if desired.
The chlorine dioxide generating process provided
herein can operate with the optional addition of a
catalyst selected from the group of catalysts known in
the art, such as those described in US Patent 4,421,730
(Isa et al) or US Patent 4,770,868 (Norell). Examples
of typical catalytic ions which may be employed include
Ag, Mn, V, Mo, Pd and Pt. While the premixing of the
reducing agent (for example, hydrogen peroxide or
methanol) with the other feedstocks to the generator
can be employed, it is generally not required.
The above-mentioned embodiments of the process of
the present invention involve the generation at either
atmospheric or subatmospheric pressures. However, it
is also possible to operate the process at super
atmospheric pressures, especially in water treatment
applications.
In some applications of the process of the present
invention, it is possible to replace at least part of
the nitric acid with another strong acid, for example,
perchloric acid. However, in general, it is preferred
to operate the process with nitric acid alone as the
source of acidity for the process.
In a case when the entire nitric acid feed is
replaced by a perchloric acid feed, it is possible to
effect an integration of the chlorine dioxide generator
with the sodium chlorate manufacturing plant.
The invention is illustrated by the following
Example.
EXAMPLE
A subatmospheric pressure pilot plant chlorine
dioxide generator had a maximum volume capacity of 20
L. The chlorine dioxide generator was operated to
generate chlorine dioxide under the following
conditions:
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Reducing agent . hydrogen peroxide
Liquor Total [H'] : 1.0 to 2.0 N
Liquor [C103-] . 1.0 to 2.0 M
Temperature . about 70°C
5 Pressure . about 165 mmHg (about 22 kPa)
Liquor (N03-] . 7.9 to 8.9 M (crystallization
of NaN03 observed) .
Chemical feeds were made to the aqueous acid reaction
medium (liquor) as the chlorine dioxide generator was
10 operated on a continuous basis at the boiling point of
the liquor under the subatmospheric pressure. The
chemical feeds were:
70~ w/w HN03 commercial, reagent grade solution
300 g/L HzOz solution prepared from industrial
grade 50~ w/w H202.
6 M NaC103 solution prepared from crystallized,
industrial grade NaC103
A gaseous mixture of chlorine dioxide, steam and
oxygen was removed from the chlorine dioxide generator
and an aqueous solution of chlorine dioxide formed from
the gaseous mixture. The chemical efficiency of
chlorine dioxide production based on gas analysis by
gas chromatography, consumption of hydrogen peroxide
and rates of chlorine dioxide generation were
determined to be as follows:
Chemical efficiency . greater than 95~
H202 consumption . 0.25 to 0.3 t H20z/t C102
Rate of CLOZ generation . 1.4 g.min-1L-1
As may be seen from these results, highly
efficient chlorine dioxide generation is achieved at
commercially acceptable production rates.
SUMMARY OF DISCLOSURE
In summary of this disclosure, the present
invention provides a process for the production of
chlorine dioxide in which chlorate ions are reduced in
the presence of nitric acid as at least the primary
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source of acidity for the process. Modifications are
possible within the scope of this invention.
