Method of producing chlorine dioxide

Details Method of producing chlorine dioxide Method of producing chlorine dioxide

1995-06-30

2003-06-10

Nguyen, Ngoc-Yen (Department: 1754)

Chemistry of inorganic compounds

Halogen or compound thereof

Chlorine dioxide

C423S272000

Reexamination Certificate

active

06576213

ABSTRACT:

The present invention relates to a method of producing chlorine dioxide from chlorate ions and hydrogen peroxide in an aqueous acidic reaction medium.
Chlorine dioxide used in aqueous solution is of considerable commercial interest, mainly in pulp bleaching, but also in water purification, fat bleaching, removal of phenols from industrial wastes etc. It is therefore desirable to provide processes in which chlorine dioxide can be efficiently produced.
There are numerous different processes for chlorine dioxide production. Most processes in commercial use involve continuous reaction of alkali metal chlorate in an acidic medium with a reducing agent such as hydrogen peroxide, methanol, chloride ions or sulfur dioxide, chlorine dioxide gas being withdrawn from the reaction medium. Generally, the acidity is provided by addition of sulfuric acid and the sulfate is withdrawn as a by-product in the form of solid alkali metal sulfate or dissolved in depleted reaction medium.
Hydrogen peroxide is a particularly effective reducing agent offering high reaction rate at low acidity. Thus, the U.S. Pat. Nos. 5,091,166 and 5,091,167 disclose processes for production of chlorine dioxide from alkali metal chlorate and hydrogen peroxide operated at subatmospheric pressure, International patent specification WO 93/21105 suggests that phosphonic acids increase the production rate of chlorine dioxide when hydrogen peroxide is used as a reducing agent, International patent specification WO 93/25470 discloses chlorine dioxide production from chloric acid and hydrogen peroxide, and EP patent application 94200268.4 discloses a process for production of chlorine dioxide from chlorate ions and hydrogen peroxide operated at substantially atmospheric pressure. The disclosure of the above patents and patent applications are incorporated herein by reference.
It has been found that use of hydrogen peroxide as a reducing agent results in foaming in the chlorine dioxide reactor, particularly in subatmospheric processes involving evaporation of water from the reaction medium. If too much foam forms, it takes up a great deal of the total volume of the reactor, decreasing the production of chlorine dioxide. International patent specification WO 93/25469 discloses that the foaming can be eliminated if the hydrogen peroxide is premixed with the chlorate solution fed to the reactor which, however, has been found not to be effective.
The object of the present invention is to provide a process of producing chlorine dioxide from chlorate ions and hydrogen peroxide with an acceptable degree of foaming in the reactor. It is normally not necessary to totally eliminate the foaming, but it is desirable to keep the degree of foaming as low as possible.
According to the invention, it has surprisingly been found that additives and impurities normally present in hydrogen peroxide greatly affect the foaming in chlorine dioxide reactors. Technical grade hydrogen peroxide always contains small amounts of stabilizers, the most common system being a combination of alkali metal stannate and one or several phosphorus compounds such as phosphonic acids or salts thereof. Typically, technical hydrogen peroxide contains from about 100 to about 1000 mg Na
2
SnO
3
(from about 28 to about 558 mg Sn) per liter 50% H
2
O
2
-solution, and from about 200 to about 2000 mg of phosphonic acids or a salts thereof per liter 50% H
2
O
2
-solution.
The acidic environment in a chlorine dioxide reactor causes most tin to be oxidized to Sn
4+
which has been found to accumulate in the reaction medium of the reactor, building up rather high concentrations. It has also been found that the tin forms complexes with many organic phosphorus compounds, which complexes significantly increase the foaming. Even if some of the tin supplied with the hydrogen peroxide leaves the reactor with products and by-products, it has been found that the content in the reaction medium at steady state is sufficiently high to cause an unacceptable degree of foaming.
According to the invention, the problems of foaming are solved by a method of producing chlorine dioxide comprising the steps of: Providing a reactor with an aqueous acidic reaction medium containing chlorate ions; reacting said chlorate ions with hydrogen peroxide as a reducing agent in such proportions that chlorine dioxide is formed; withdrawing the chlorine dioxide from the reaction medium; wherein the content of tin in the reaction medium at steady state is maintained below 20 mg Sn per kg reaction medium. Preferably, the content of tin is maintained below about 15 mg Sn, most preferably below about 10 mg Sn, particularly preferably below about 5 mg Sn per kg reaction medium. The absolutely best results are achieved if the content of tin is maintained below about 1 mg Sn per kg reaction medium or if the process is performed in the substantial absence of tin in the reaction medium.
Suitably the process is continuous, thus involving continuous supply of chlorate ions and hydrogen peroxide. The chlorate ions can be added in the form of alkali metal chlorate, chloric acid or mixtures thereof. Any mineral acid can be used as an acid source but normally sulfuric acid is preferred. If both alkali metal chlorate and sulfuric acid are supplied to the reactor, alkali metal sulfate is preferably withdrawn, either as a solid salt cake or dissolved in depleted reaction medium leaving the reactor. Alkali metal sulfate withdrawn from the reactor may be used as a by-product, but can also be electrochemically acidified and recirculated to the reaction medium as an acid source, as described in U.S. Pat. 4,129,484 or EP patent application 94200267.6, the disclosure of which are incorporated herein by reference.
In order to keep the tin content in the reaction medium sufficiently low, the amount of tin compounds added with the hydrogen peroxide, such as sodium stannate, should be as low as possible. Suitably, the hydrogen peroxide solution supplied contains less than about 20 mg Sn per kg H
2
O
2
, preferably less than about 15 mg Sn per kg H
2
O
2
, most preferably less than about 10 mg Sn per kg H
2
O
2
, particularly less than about 5 mg Sn per kg H
2
O
2
. The absolutely best results are achieved if the hydrogen peroxide solution contains less than about 1 mg Sn per kg H
2
O
2
or if it is substantially free from tin or tin compounds. If it is assumed that most of the tin is in the form of sodium stannate, 1 g Sn corresponds to about 1.8 g Na
2
SnO
3
. Preferably, the hydrogen peroxide is stabilized with one or more phosphonic acids or salts thereof, for example alkali metal salts, suitably in an amount from about 0.1 to about 10 g/liter hydrogen peroxide solution, preferably from about 1 to about 5 g/liter hydrogen peroxide solution. As examples of useful phosphonic acids the following can be mentioned: 1-hydroxyethylidene-1,1-diphosphonic acid, 1-aminoethane-1, 1-diphosphonic acid, aminotri (methylenephosphonic acid), ethylene diamine tetra (methylenephosphonic acid), hexamethylene diamine tetra (methylenephosphonic acid), diethylenetriamine penta (methylenephosphonic acid) and diethylenetriamine hexa (methylenephosphonic acid). A particularly preferred stabilizer is diethylenetriamine penta (methylenephosphonic acid) or salts thereof. It is to be understood that the phosphonic acids or salts thereof can be used in combination with other stabilizers as long as the content of tin is less than the values mentioned above.
It is also preferred that the hydrogen peroxide contains as small amounts as possible of organic impurities, for example rests of the working solution used in the production according the anthraquinone process. Preferably, the amount of organic impurities is less than about 150 mg per kg H
2
O
2
, most preferably less than about 100 mg per kg H
2
O
2
, particularly less than about 50 mg per kg H
2
O
2
.
If suitable, it is also possible to add other reducing agents such as methanol, formaldehyde, formic acid, sugar alcohols, sulfur dioxide and chloride.
It is also possible to add small amounts of catalysts t

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