Process for producing chlorine dioxide

Details Process for producing chlorine dioxide Process for producing chlorine dioxide

2001-06-25

2004-09-14

Nguyen, Ngoc-Yen (Department: 1754)

Chemistry of inorganic compounds

Halogen or compound thereof

Chlorine dioxide

C422S129000

Reexamination Certificate

active

06790427

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a process for producing chlorine dioxide from chlorate ions, acid and hydrogen peroxide
BACKGROUND OF THE INVENTION
Chlorine dioxide is used in various applications such as pulp bleaching, fat bleaching, water purification and removal of organic materials from industrial wastes. Since chlorine dioxide is not storage stable, it must be produced on-site.
Chlorine dioxide is usually produced by reacting alkali metal chlorate or chloric acid with a reducing agent in an aqueous reaction medium. Chlorine dioxide may be withdrawn from the reaction medium as a gas, as in the processes described by U.S. Pat. Nos. 5,091,165, 5,091,167 and EP patent 612686. Normally, the chlorine dioxide gas is then absorbed into water to form an aqueous solution thereof.
For production of chlorine dioxide in small-scale units, such as for water purification applications or small bleaching plants, it is favourable not to separate chlorine dioxide gas from the reaction medium but to recover a chlorine dioxide containing solution directly from the reactor, optionally after dilution with water. Such processes are described in U.S. Pat. Nos. 2,833,024, 4,534,852, 5,895,838 and in WO 00/76916, and have in recent years become commercial. However, there is still a need for further improvements. Particularly, it has been found difficult to obtain solutions with sufficiently high concentration of chlorine dioxide as required for some applications, like recycle paper bleaching, bagasse bleaching, or small-scale pulp bleaching. A high concentration of chlorine dioxide also may be useful to any application where minimising the water flow is important.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a process enabling direct production of chlorine dioxide in an aqueous solution of high concentrations.
It is another object of the invention to provide a process for direct production of chlorine dioxide in an aqueous solution with high production capacity.
It is still another object of the invention to provide an apparatus for performing the process.
BRIEF DESCRIPTION OF THE INVENTION
It has surprisingly been found possible to meet these objects by providing a process for continuously producing chlorine dioxide comprising the steps of.
feeding chlorate ions, acid and hydrogen peroxide as aqueous solutions to a reactor:
reducing chlorate ions in the reactor to chlorine dioxide, thereby forming a product stream in the reactor containing chlorine dioxide;
feeding motive water to an eductor comprising a nozzle;
bringing the motive water to flow through the nozzle and causing it to flow further through the eductor in an at least partially, preferably substantially, spiral or helical manner,
transferring the product stream from the reactor to the eductor and mixing it with the motive water and thereby forming a diluted aqueous solution containing chlorine dioxide, and;
withdrawing the diluted aqueous solution containing chlorine dioxide from the eductor.
The chlorate ions can be fed to the reactor as an aqueous solution comprising chloric acid and/or a metal chlorate, preferably alkali metal chlorate. The alkali metal may, for example, be sodium, potassium or mixtures thereof, of which sodium is most preferred. Unless chloric acid is used, another acid must be fed to the reactor, preferably a mineral acid such as sulfuric acid, hydrochloric acid or nitric acid, of which sulfuric acid is most preferred. The molar ratio H
2
O
2
to ClO
3
−
fed to the reactor is suitably from about 0.2:1 to about 2:1, preferably from about 0.5:1 to about 1.5:1, most preferably from about 0.5:1 to about 1:1. Metal chlorate and chloric acid always contain some chloride as an impurity, but it is fully possible also to feed more chloride to the reactor, such as metal chloride or hydrochloric acid. However, in order to minimize the formation of chlorine it is preferred to keep the amount of chloride ions fed to the reactor low, suitably below about 1 mole %, preferably below about 0.1 mole %, more preferably less than about 0.05 mole %, most preferably less than about 0.02 mole % Cl
−
of the ClO
3
−
.
In a particularly preferred embodiment alkali metal chlorate and hydrogen peroxide are fed to the reactor in the form of a premixed aqueous solution, for example a composition as described in WO 00/76916, which hereby is incorporated by reference. Such a composition may be an aqueous solution comprising from about 1 to about 6.5 moles/liter, preferably from about 3 to about 6 moles/liter of alkali metal chlorate, from about 1 to about 7 moles/liter, preferably from about 3 to about 5 moles/liter of hydrogen peroxide and at toast one of a protective colloid, a radical scavenger or a phosphoric acid based complexing agent, wherein the pH of the aqueous solution suitably is from about 0.5 to about 4, preferably from about 1 to about 3.5, most preferably from about 1.5 to about 3. Preferably, at least one phosphonic acid based complexing agent is present, preferably in an amount from about 0.1 to about 5 moles/liter, most preferably from about 0.5 to about 3 moles/liter, if a protective colloid is present, its concentration is preferably from about 0.001 to about 0.6 moles/liter, most preferably from about 0.02 to about 0.05 moles/liter. If a radical scavenger is present, its concentration is preferably from about 0.01 to about 1 moles/liter, most preferably from about 0.02 to about 0.2 moles/liter. Particularly preferred compositions comprise at least one phosphonic acid based complexing agent selected from the group consisting of 1-hydroxyethylidene-1,1-diphosphonic acid, 1-aminoethane1,1-diphosphonic acid, amino (methylenephosphonic acid), ethylene diamine tetra (methylenephosphonic acid), hexamethylene diamine tetra (methylenephosphonic acid), diethylenetriamine penta (methylenephosphonic acid), diethylenetriamine hexa (methylenephosphonic acid), and 1-aminoalkane-1,1-diphosphonic acids such as morphollnomethane diphosphonic acid, N,N-dimethyl aminodimethyl diphosphonic acid, aminomethyl diphosphonic acid, or salts thereof, preferably sodium salts. Useful protective colloids include tin compounds, such as alkali metal stannate, particularly sodium stannate (Na
2
(Sn(OH)
6
). Useful radical scavengers include pyridine carboxylic acids, such as 2,6-pyridine dicarboxylic acid. Preferably the amount of chloride ions is below about 0.5 moles/liter, most preferably below about 0.1 mmoles/liter, particularly below about 0.03 mmoles/liter.
In the case that sulfuric acid is used as a feed, it preferably has a concentration from about 70 to about 98 wt %, most preferably from about 75 to about 85 wt % and preferably a temperature from about 0 to about 80° C., most preferably from about 20 to about 60° C., as it then may be possible to operate the process substantially adiabatically. Preferably from about 2 to about B kg H
2
SO
4
, most preferably from about 3 to about 5 kg H
2
SO
4
is fed per kg ClO
2
produced. Alternatively, the equivalent amount of another mineral acid may be used.
The net reaction resulting in chlorine dioxide generation can be described by the formula;
2ClO
3
−
+2H
+
+H
2
O
2
→ClO
2
+2H
2
O+O
2
The exact mechanism is complex and is believed to involve a first reaction between chlorate and chloride (even if not added separately always present in sufficient amount as an impurity in chlorate) to give chlorine dioxide and chlorine, followed by reaction of the chlorine with hydrogen peroxide back to chloride. However, considering the net reaction hydrogen peroxide is normally regarded as a reducing agent reacting with the chlorate ions.
The reduction of chlorate ions to chlorine dioxide results in formation of a product stream in the reactor, normally comprising both liquid and foam, and containing chlorine dioxide, oxygen and, in most cases, some remaining unreacted feed chemicals. Chlorine dioxide and oxygen may be present both as dissolved in the liquid and as gas bubbles. When a metal chlorate and a mineral a

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