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United States Patent |
5,165,911
|
Cawlfield
,   et al.
|
November 24, 1992
|
Process for producing chlorine dioxide
Abstract
A process which comprises heating a reaction mixture comprised of an
aqueous solution containing perchlorate ions, chlorate ions and hydrogen
ions to produce chlorine dioxide and oxygen gas.
The novel process of the present invention provides a commercially viable
process for producing chlorine dioxide from mixtures of oxy-chlorine
species in the absence of a reducing agent. The process can be operated
without producing an acidic salt by-product while producing a chlorine
dioxide product which is substantially free of chlorine. In addition, the
process of the invention permits a reduction in the amount of acid fed to
the chlorine dioxide generator.
Inventors:
|
Cawlfield; David W. (Cleveland, TN);
Mendiratta; Sudhir K. (Cleveland, TN)
|
Assignee:
|
Olin Corporation (Cheshire, CT)
|
Appl. No.:
|
703432 |
Filed:
|
May 17, 1991 |
Intern'l Class: |
C01B 011/02 |
Field of Search: |
423/477,478
|
References Cited
U.S. Patent Documents
2157525 | May., 1939 | Cady | 423/462.
|
3810969 | May., 1974 | Schlumberger | 423/478.
|
4146578 | Mar., 1979 | Brennan et al. | 423/473.
|
4147761 | Apr., 1979 | Wojtowicz | 423/473.
|
4169134 | Sep., 1979 | Isa et al. | 423/478.
|
4798715 | Jan., 1989 | Hardee et al. | 423/478.
|
Foreign Patent Documents |
164634 | Sep., 1976 | CS.
| |
1261824 | Jul., 1960 | FR | 423/477.
|
WO90/05111 | May., 1990 | WO.
| |
Primary Examiner: Straub; Gary P.
Assistant Examiner: Lund; Valerie
Attorney, Agent or Firm: Meyer, Jr.; Allen A., Weinstein; Paul
Claims
What is claimed is:
1. A process for producing chlorine dioxide which comprises heating a
reaction mixture comprising an aqueous solution containing perchlorate
ions, chlorate ions and hydrogen ions in the presence of an
oxygen-evolving catalyst in solid form selected from group VIIIA of the
Periodic Table of Elements to produce chlorine dioxide and oxygen gas in
absence of a reducing agent.
2. The process of claim 1 in which the concentration of hydrogen ions is at
least 2 molar.
3. The process of claim 1 in which the chlorate ion concentration is at
least 0.02 molar.
4. The process of claim 1 in which the concentration of the chlorate ion is
from about 0.1 to about 3 molar.
5. The process of claim 1 in which the oxygen-evolving catalyst is a
platinum group metal, a platinum group metal oxide, or mixtures thereof.
6. The process of claim 1 in which the molar ratio of perchlorate ions to
chlorate ions is from about 0.5:1 to about 100:1.
7. The process of claim 1 in which the oxygen-evolving catalyst contains a
metal from Group VIIIa of the Periodic Table of Elements.
8. The process of claim 1 in which the source of chlorate ions is a
solution of chloric acid in a non-oxidizable inorganic acid.
9. The process of claim 8 in which the concentration of chloric acid in the
non-oxidizable inorganic acid is from about 5 to about 20 percent by
weight of HClO.sub.3.
10. The process of claim 8 in which the non-oxidizable inorganic acid is
selected from the group consisting of sulfuric acid, phosphoric acid,
perchloric acid, and mixtures thereof.
11. The process of claim 1 in which the source of perchlorate ions is an
aqueous solution of perchloric acid.
12. The process of claim 1 in which the reaction mixture is heated at a
temperature in the range of from about 40.degree. to about 90.degree. C.
at about atmospheric pressure.
13. The process of claim 1 in which the source of chlorate ions is a
solution of chloric acid.
14. A process which comprises heating a reaction mixture comprising an
aqueous solution containing chloric acid and perchlorate ions in the
presence of an oxygen evolving catalyst in solid form selected from group
VIIIA of the Periodic Table of Elements to produce chlorine dioxide and
oxygen gas, the aqueous solution being substantially free of ionic
impurities in the absence of a reducing agent.
15. The process of claim 14 in which the source of chlorate ions is a
solution of chloric acid.
16. The process of claim 14 in which the oxygen evolving catalyst contains
a metal from Group VIIIA of the Periodic Table of Elements.
17. The process of claim 16 in which the oxygen evolving catalyst is a
platinum group metal, a platinum group metal oxide and mixtures thereof.
18. The process of claim 17 in which the oxygen evolving catalyst is
selected from the group consisting of an oxide iridium, rhodium or
ruthenium or a mixture of the oxide with platinum group metals or alloys
of platinum group metals.
19. The process of claim 14 in which the source of perchlorate ions is
perchloric acid.
20. The process of claim 19 in which the concentration of chloric acid is
in the range of from about 5 to about 45 percent by weight of HClO.sub.3.
21. A process which comprises heating a reaction mixture consisting of
essentially pure chloric and perchloric acids in the presence of ruthenium
oxide to produce chlorine dioxide and oxygen gas in the absence of a
reducing agent.
22. The process of claim 21 in which the concentration of chlorate ion is
from about 0.1 to about 3 molar.
23. The process of claim 21 in which the molar ratio of perchlorate ions to
chlorate ions is form about 0.5:1 to about 100:1.
Description
BACKGROUND OF THE INVENTION
This invention relates to a process for producing chlorine dioxide. More
particularly, this invention relates to the production of chlorine dioxide
from a chloric acid solution.
Chlorine dioxide has found wide use as a disinfectant in water
treatment/purification, as a bleaching agent in pulp and paper production,
and a number of other uses because of its high oxidizing power. There are
a number of chlorine dioxide generator systems and processes available in
the marketplace. Most of the very large scale generators utilize a
chlorate salt, a chloride ion source or reducing agent, and a strong acid.
In the presence of chloride ion and acid, chlorate ion reacts to produce a
mixture of chlorine and chlorine dioxide. The chlorine present is an
undesired by-product.
Many processes have been developed to produce chlorine dioxide with lower
chlorine concentrations by adding a reducing agent. Reducing agents which
have been used include methanol or other organic compounds, sulfur, sulfur
dioxide or other sulfur-oxygen species having a sulfur valence of less
than +6, and carbon monoxide among others. When organic compounds are
used, unreacted volatile organics including formic acid are present in the
product gas.
Using sulfur containing reducing agents, the sulfate or sulfuric acid
produced accumulates as a waste product. When gaseous reducing agents such
as sulfur dioxide or carbon monoxide are employed, reactor designs and
process control systems must protect against unreacted reducing agent
leaving the system with the chlorine dioxide gas.
In addition, prior art processes for the production of chlorine dioxide
from chlorate salts require an excess of the acid used. This acid is
slowly neutralized by the accumulation of alkali metal ions that enter the
process with the chlorate salt. The accumulation of salts must be removed
as a waste stream, either liquid or solid, in every process currently
practised commercially.
To avoid the formation of an acidic alkali metal salt, it has been proposed
that chlorine dioxide be prepared from chloric acid. Chloric acid is,
however, not commercially available. Its preparation has been taught, for
example, in U.S. Pat. No. 3,810,969 issued May 14, 1974 to A. A.
Schlumberger. Schlumberger teaches a process for producing chloric acid by
passing an aqueous solution containing from 0.2 gram mole to 11 gram moles
per liter of an alkali metal chlorate such as sodium chlorate through a
selected cationic exchange resin at a temperature from 5.degree. to
40.degree. C. The process produces an aqueous solution containing from 0.2
gram mole to about 4.0 gram moles of HClO.sub.3.
K. L. Hardee et al, in U.S. Pat. No. 4,798,715 issued Jan. 17, 1989,
describe a process for chlorine dioxide which electrolyzes a chloric acid
solution produced by passing an aqueous solution of an alkali metal
chlorate through an ion exchange resin. The electrolysis is carried out
using an electrocatalytic cathode where the catalyst is, for example, one
or more valve metal oxides which may be combined with a platinum group
metal oxide, or a platinum group metal, or oxides of a platinum group
metal, magnetite, ferrite, or mixed metal oxides.
The electrolyzed solution contains a mixture of chlorine dioxide, and
chloric acid which is fed to an extractor in which the chlorine dioxide is
stripped off. The ion exchange resin is regenerated with hydrochloric acid
and an acidic solution of an alkali metal chloride formed. Hardee et al
teach that the electrocatalyst may also be used to convert the chloric
acid to chlorine dioxide in a catalytic reactor.
Processes which produce chloric acid in an ion exchange resin require the
regeneration of the ion exchange resin with acid to remove the alkali
metal ions and the use or treatment and disposal of the acidic salt
solution. In addition, the concentration of chloric acid which can be
produced by an ion exchange process is limited as more concentrated
chloric acid solutions attack the ion exchange resins used in the process.
SUMMARY OF THE INVENTION
Now a process has been discovered which produces chlorine dioxide from
mixtures of oxy-chlorine species in the absence of a reducing agent. The
process can be operated without producing an acidic salt by-product while
producing a chlorine dioxide product which is free of chlorine. In
addition, the process of the invention permits a reduction in the amount
of acid fed to the chlorine dioxide generator.
These and other advantages are accomplished in a process which comprises
heating a reaction mixture comprising an aqueous solution containing
perchlorate ions, chlorate ions and hydrogen ions to produce chlorine
dioxide and oxygen gas.
DETAILED DESCRIPTION OF THE INVENTION
Reaction mixtures suitable for use in the novel process of the present
invention are aqueous solutions containing chlorate ions, perchlorate ions
and hydrogen ions. The aqueous solutions are highly acidic and have a
hydrogen ion concentration of at least 2 molar and preferably at least 3
molar. The concentration of chlorate ions is at least 0.02 molar and
preferably from about 0.1 to about 3 molar. Concentrations of perchlorate
ions are those which provide a molar ratio of perchlorate ions to chlorate
ions of from about 0.5:1 to about 100:1, and preferably from about 3:1 to
about 20:1. These acidic solutions preferably are substantially free of
ionic impurities such as chloride ions, alkali metal and alkaline earth
metal ions.
Chlorate ions present in the reaction mixture may be provided by aqueous
solutions of chloric acid, mixtures of chloric acid and non-oxidizable
inorganic acids such as sulfuric acid, phosphoric acid or perchloric acid,
as well as mixtures of alkali metal chlorates and non-oxidizable
inorganics acids. Where it is desired to produce chlorine dioxide in the
absence of an acidic salt by-product, the chlorate ions are provided by
aqueous solutions of chloric acid or mixtures of chloric acid and
non-oxidizable inorganic acids. Suitable concentrations of chloric acid
used as the source of chlorate ions include those in the range of from
about 5 to about 45 percent, preferably from about 10 to about 40 percent
by weight of HClO.sub.3.
To suppress or minimize the auto-oxidation of chloric acid to perchloric
acid without the formation of oxygen gas, for example, where an
oxygen-evolving catalyst is employed, it is preferred to use as the source
of chlorate ions a mixture of chloric acid and a non-oxidizable inorganic
acid in which the concentration of chloric acid is low, for example, less
than about 20 percent by weight of the aqueous solution providing chlorate
ions.
High purity chloric acid solutions are produced by the oxidation of high
purity hypochlorous acid solutions. One process suitable for producing the
chloric acid solutions heats a hypochlorous acid solution, containing from
about 35 to about 60 percent by weight of HOCl, at a temperature in the
range of from about 25.degree. to about 120.degree. C.
This process is represented by the following reactions:
3HOCl.fwdarw.HClO.sub.3 +2HCl (1)
2HOCl+2HCl.fwdarw.2Cl.sub.2 +2H.sub.2 O (2)
5HOCl.fwdarw.HClO.sub.3 +2Cl.sub.2 +2H.sub.2 O (3)
Thermal oxidation of the hypochlorous acid takes place at ambient
temperatures and autogenous pressures. To increase the rate of production
of chloric acid the reactant may be decomposed at elevated temperatures.
The concentrated hypochlorous acid solution may be heated at temperatures,
for example, in the range of from about 50.degree. to about 120.degree.,
and preferably in the range of from about 70.degree. to about 110 .degree.
C. to increase the rate of decomposition of the hypochlorous acid and
hence the rate of production of chloric acid.
Another process for producing the high purity chloric acid solution
utilizes anodic oxidation of the high purity concentrated hypochlorous
acid solution in an electrolytic cell having an anode compartment, a
cathode compartment, and an cation exchange membrane separating the anode
compartment from the cathode compartment. In operation, the process
includes feeding an aqueous solution of hypochlorous acid to the anode
compartment, and electrolizing the aqueous solution of hypochlorous
solution at a temperature of from about 0.degree. to about 40.degree. C.
to produce the chloric acid solution.
The process is represented by the following equation:
HOCl+2H.sub.2 O.fwdarw.HClO.sub.3 +2H.sub.2 +4e (4)
Chloric acid solutions can be produced by these processes in any
concentrations desired up to about 45% by weight of HClO.sub.3. However,
preferred concentrations are those in the range of from about 15 to about
40% by weight of HClO.sub.3.
High purity HOCl solutions used in the production of chloric acid are
produced by a process in which gaseous mixtures, having high
concentrations of hypochlorous acid vapors and chlorine monoxide
(dichlorine monoxide, Cl.sub.2 O) gas and controlled amounts of water
vapor, are generated, for example, by the process described by J. p.
Brennan et al in U..S. Pat. No. 4,146,578, issued Mar. 27, 1979, or WO
90/05111 published May 17, 1990 by J. K. Melton, et. al. Each of these
disclosures are incorporated in their entirety by reference.
Hypochlorous acid solutions produced by these processes contain
concentrations of from about 35 to about 60, and more preferably from
about 40 to about 55 percent by weight of HOCl. The hypochlorous acid
solutions are substantially free of ionic impurities such as chloride ions
and alkali metal ions as well as metal ions such as nickel and copper or
mercury, among others.
Perchlorate ions present in the reaction mixture are supplied by mixing an
aqueous solution of perchloric acid, a mixture of perchloric acid and
chloric acid or an aqueous solution of an alkali metal perchlorate in a
non-oxidizable inorganic acid. Preferred as a source of perchloric acid is
an aqueous solution of perchloric acid or an aqueous solution containing a
mixture of perchloric acid and chloric acid.
A method of directly producing high purity perchloric acid initially begins
with high purity chloric acid solutions such as those described above. The
chloric acid is fed as the anolyte to the anode compartment of an
electrolytic cell which includes a cathode compartment, the anode
compartment, and a separator such as a cation exchange membrane positioned
between the anode compartment and the cathode compartment.
Perchlorate ions present in the reaction mixture, while not wishing to be
bound by theory, are believed to promote the formation of oxygen gas by
the following reaction:
2HClO.sub.3 .fwdarw.2ClO.sub.2 +1/20.sub.2 +H.sub.2 O
Chlorine dioxide production thus takes place in the absence of the reducing
agent which has been required in ClO.sub.2 processes commercially
practiced up to now.
The perchlorate ions are believed to serve as a "solvent" and provide an
acidic media in which ClO.sub.2 and O.sub.2 formation is favored.
To increase yields of chlorine dioxide and conversion efficiencies it is
preferred to carry out the process in the presence of a solid surface
which promotes oxygen evolution. Any solid surface may be used which
facilitates oxygen formation including oxygen-evolving catalysts. Suitable
as oxygen-evolving surfaces or catalysts are, for example, metals and
oxides of the elements of Group VIIIA of the Periodic Table of Elements
(Handbook of Chemistry and Physics. 68th Edition, CRC Press, Inc. Boca
Raton, Fla., 1987-88, inside cover). Thus metals such as the platinum
group metals including platinum, palladium, iridium, rhodium or ruthenium;
and mixtures or alloys of these platinum group metals may be employed.
Additionally oxides of platinum group metals such as iridium, rhodium or
ruthenium, as well as mixtures of these oxides with platinum group metals
or alloys of these precious metals could be suitably employed. Likewise,
iron alloys such as stainless steel, nickel or nickel based alloys, and
cobalt based alloys can be used as oxygen-evolving catalysts in the
process of the invention. Other oxygen-evolving catalysts include
semiconductive ceramics known as perovskites. The catalyst may be present
as particles suspended in the reaction mixture or supported on an inert
substrate. The oxygen-evolving catalysts may be used in the forms of a
packed bed, slurries, or any structure which will suitably promote mass
transfer. In a preferred embodiment of this invention, the catalyst is
supported on valve metal heat exchanger surfaces to facilitate evaporation
of water during the reaction. Suitable valve metals include titanium and
tantalum, among others.
During operation of the process of the invention the perchlorate ions are
not consumed. Where the process is operated using the oxygen-evolving
catalysts, the production of oxygen gas is increased and the
auto-oxidation of chloric acid or chlorate ions to perchloric acid or
perchlorate ions is minimized. The concentration of chloric acid present
in the reaction mixture can be increased and preferably is at least 30
percent, for example, from about 30 to about 40 percent by weight of
HClO.sub.3. Further, the oxygen-evolving catalysts are not removed, for
example, in by-product streams during operation of the process. Any
suitable amounts of the oxygen-evolving catalysts may be used which will
desirably increase the reaction rate.
The process is preferably carried out at temperatures in the range of from
about 40.degree. to about 90.degree., and preferably at temperatures of
from about 50.degree. to about 80.degree. C.
The product of the process of the invention is a mixture of gaseous oxygen,
chlorine dioxide and water vapor. Concentrations of chlorine dioxide
produced include those in the range of from about 0.5 to about 10, and,
preferably from about 1 to about 6 percent by volume. The gaseous mixture
contains varying concentrations of oxygen and water vapor. A typical ratio
of oxygen to ClO.sub.2 in the gaseous mixture is from about 1 mol of
O.sub.2 to about 4 mols of ClO.sub.2 by volume. The gaseous product
mixture contains amounts of chlorine which are considerably less than
those produced in presently operated commercial processes. For example the
concentrations of chlorine are less than 10%, and preferably less than 5%
by volume of the chlorine dioxide in the mixture.
The novel process of the invention may be operated batchwise or
continuously. When operated continuously, it is preferred to continuously
add chloric acid or an acidic solution of chlorate to the generator and
remove the gaseous mixture of ClO.sub.2, O.sub.2 and water vapor as
product from the generator in amounts or ratios which maintain a
concentrated perchloric acid solution in the generator. When operated
continuously, the process of the invention converts essentially all of the
chlorate ions to chlorine dioxide.
The novel process of the present invention is further illustrated by the
following examples with no intention of being limited thereby. All parts
and percentages are by weight unless otherwise indicated.
EXAMPLE 1
As the chlorine dioxide generating apparatus, a round bottom glass flask
was placed on a heating mantle containing a variable speed magnetic
stirring mechanism. A teflon encapsulated magnet provided aggitation
inside the flask. To the flask was connected a vacuum gauge, a
thermometer, and an eductor providing vacuum. The eductor was operated
using a solution of KI pumped from a tank to which the effluent from the
eductor was returned.
Into the eductor tank, 225 gms of KI and 15 liters of water were added.
Into the reactor, 50 gms of a solution containing 24.41% HClO3, and 28.89%
HClO4 in equimolar amounts. Also added to the reactor was 0.5 grams of
powdered ruthenium oxide (Aldrich Chemical Co.). After applying vacuum to
the reactor, the heater was energized and the power regulated until the
temperature was approximately 60 degrees C. and the pressure was
approximately 25 inches of mercury vacuum. Samples of the product tank
were removed and analyzed iodometrically for reacted chlorine and chlorine
dioxide. The reaction was essentially complete in 75 minutes. After five
hours, the remaining solution was analyzed for chloric and perchloric
acid.
The results, in which the product and generator solution are expressed in
milliequivalents, are as follows:
______________________________________
Generator
Time Product* Solutions*
(Min) C102 C12 HC103 HC104
______________________________________
0 0 0 138.4 148.9
20 63.22 0.15
75 101 -1.3
235 113.1 -4.3
300 118.2 -4.3 2.2 181.2
Difference -136.3 32.3
C102 Yield 86.8%
HC103 Conv. 98.4%
______________________________________
*milliequivalents.
EXAMPLE 2
To the same apparatus used in Example 1, was charged with 50 grams of a 1:2
molar mixture of chloric and perchloric acid to which 0.5 grams of
ruthenium dioxide was added. This mixture was heated under vacuum as in
Example 1 except that the temperature was allowed to rise to 68 degrees C.
near the end of the experiment. An overall yield of 78.9% was achieved
while an overall conversion of 98.7% was obtained after 2.5 hours.
The results are given below:
______________________________________
Generator
Time Product* Solutions*
(Min) C102 C12 HC103 HC104
______________________________________
0 0 0 75.9 276.2
30 41.1 0
90 59.3 2.4
150 59.1 3.25 1.0 272.8
Difference -74.9 -3.4
C102 Yield 78.9%
HC103 Conv. 98.7%
______________________________________
*milliequivalents.
EXAMPLE 3
Using the same apparatus and procedure of Example 1, the reaction was
carried out without the addition of ruthenium oxides as the
oxygen-evolving catalyst.
______________________________________
Generator
Time Product* Solutions*
(Min) C102 C12 HC103 HC104
______________________________________
0 0.0 0.0 144.5 143.8
60 8.6 1
180 18.8 5.1
360 50.5 4.3 59.6 164.9
Difference -84.9 21.1
C102 Yield 59.5%
HC103 Conv. 58.8%
______________________________________
*milliequivalents.
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