Liquid purification or separation – With preliminary chemical manufacture
Reexamination Certificate
1997-10-24
2001-01-09
Nguyen, Ngoc-Yen (Department: 1754)
Liquid purification or separation
With preliminary chemical manufacture
C210S758000, C423S477000
Reexamination Certificate
active
06171485
ABSTRACT:
BACKGROUND OF THE INVENTION
Aqueous solutions of chlorine dioxide, because of the high oxidation power of chlorine dioxide, are frequently used in water-treatment methods. The area of application ranges from the disinfection of drinking water and bathing water to treatment of service water and wastewater. In comparison with the classic oxidizing disinfectants chlorine and hypochlorite, chlorine dioxide is distinguished in use by an outstanding ecobalance. Thus, when chlorine dioxide is used, only small amounts of AOX (“adsorbable organic halogen compounds”; overall parameter for all chlorine, bromine and iodine compounds which can be adsorbed to activated carbon and have very different hazard potentials) and virtually no trihalomethane (TEM, haloforms) are formed.
It is known, for example from DE-C 843 999, for the preparation of chlorine dioxide, to proceed from a chlorite, for example sodium chlorite and to oxidize this to chlorine dioxide with an oxidizing agent, for example sodium peroxodisulphate, in aqueous solution according to the summation equation
2NaClO
2
+Na
2
S
2
O
8
→2ClO
2
+2Na
2
SO
4
.
To obtain a reaction rate as high as possible, the reaction solution in this known process is set to a pH of 5-9, if appropriate using a buffer, contains the oxidizing agent in a stoichiometric excess and can be heated to up to 65° C. for further acceleration of the reaction. The chlorine dioxide forming is continuously expelled from the reaction solution by passing in inert gas and is collected in an absorption tower.
Although this known process gives a chlorine dioxide of very high purity at good yields, based on the chlorite used, it is highly restricted in its applicability and is little suited to industrial application “on site”, not only because of the high cost of equipment for separating the chlorine dioxide, but also because of the high explosion hazard of the gaseous chlorine dioxide. Furthermore, for many applications, for example for drinking-water treatment, there is also no possibility of avoiding the separation of the chlorine dioxide and using the reacted reaction solution as such for the disinfection, since this solution still contains too much residual chlorite and, moreover, is contaminated by toxic chlorate formed as by-product.
SUMMARY OF THE INVENTION
The object of the invention is to develop the known process in the manner of a “one pot” reaction in such a manner that the reacted chlorine-dioxide-containing reaction solution can be used directly as such for the disinfection and is directly suitable for drinking-water treatment, for example. This object is achieved according to the invention by means of the fact that an aqueous reaction solution is prepared from chlorite and halogen-free oxidizing agent, which reaction solution contains the oxidizing agent at up to twice the stoichiometrically required amount and is set to a pH between 5.5 and 9.5 and is reacted at room temperature until the chlorite is at least virtually completely converted to chlorine dioxide.
The invention is based on the finding that it is possible, by means of systematic reaction procedure, to react the reaction solution to give a chlorine-dioxide-containing product solution which virtually no longer contains residual chlorite (in the ideal case no chlorite at all) and is also free of chlorate and other unwanted by-products. Surprisingly, it has been found that this is achieved by the interaction of a plurality of factors, namely
setting the pH of the solution to 5.5 to 9.5, preferably 6 to 9, as promptly as possible,
oxidizing agent excess in a molar ratio of chlorite to oxidizing agent between 1 and 2, preferably between 1.75 and 2,
sufficiently long reaction time at room temperature.
Commercially conventional chlorite solutions, for reasons of storage stability, are made highly alkaline, meaning that the reaction solution has a pH of about 12 in the absence of additional measures (at least in a relatively long initial phase). At such a high pH, the chlorine dioxide formed unavoidably disproportionates into chlorite and chlorate. It has now been found that, at pHs below 9.5, the tendency of the chlorine dioxide to disproportionate disappears, but the stability of the chlorite solution is ensured sufficiently, at least for the duration of the oxidation reaction. Only at pHs below 5.5 is the chlorite solution no longer sufficiently stable. Thus, the reaction solution is brought as quickly as possible to a pH range in which both the starting material (chlorite) and the product (chlorine dioxide) are stable and the oxidation reaction can proceed without interfering side reactions (neither as decomposition of the starting material chlorite nor as decomposition of the product chlorine dioxide). Higher temperatures, which promote chlorate formation, are avoided and sufficient oxidizing agent is made available. Thus virtually no chlorate is formed in the reaction mixture and the chlorite used is converted virtually quantitatively into chlorine dioxide.
To prepare the aqueous reaction mixture, a chlorite solution is expediently mixed with a solution of the oxidizing agent, the rapid setting of the pH of this mixture, which is important for the success of the process, being achieved using a proton donor, expediently present in dissolved form, which decreases the pH of the mixture. This proton donor can be added to the two reactants during the mixing process, but, in a preferred embodiment of the invention, is present in the solution of the oxidizing agent. A particular advantage of the invention in this case is that a reaction package consisting of two components can be made available to a user and the user only needs to mix these components in a preset ratio in order to prepare in-house a fresh chlorate-free chlorine dioxide solution at any time as required. If appropriate, the oxidizing-agent- and the proton-donor-containing component can also be supplied in solid form and can be brought into solution by the user, which is advisable, in particular, if the solution of the oxidizing agent cannot be kept sufficiently storage-stable per se, and, moreover, which decreases the shipping costs. In principle, the chlorite solution can also be prepared in-house by the user by dissolving solid sodium chlorite (stabilized with sodium chloride), but this is less expedient, since solid sodium chlorite is subject to the provisions on transport of hazardous materials.
The concentration of the chlorite and, correspondingly, of the oxidizing agent, in the reaction solution has no effect on the course of the process. However, excessively high concentrations should be avoided, in order that an impermissibly high concentration of chlorine dioxide does not form in the reacted solution.
The preferred oxidizing agents are peroxodisulphates, but other halogen-free oxidizing agents, such as permanganates, Fentons reagent or ozone, can be used. The oxidizing agent shall be of a type and in sufficient amount in the aqueous reaction mixture for complete oxidation of the chlorite and be present in the stoichiometric excess. It will then not be used completely in the chlorine dioxide formation reaction. This can be expedient in many applications, in particular for the excess oxidizing agent to reoxidize in situ the chlorine dioxide which was reduced during the use of the chlorine dioxide solution e.g. by organic contaminants present in the treated water, which improves the overall action of the disinfectant or treatment solution. This is a particular additional advantage of the invention, which does not occur in the known process.
Compounds which can be used as proton donor for the purposes of the invention are all substances which are compatible with the oxidizing agent and with the chlorite and can decrease the pH of the reaction mixture by releasing acid protons. Typical examples are sodium hydrogen sulphate, sodium dihydrogen phosphate and acid salts such as iron(III) chloride or aluminium chloride.
In further pursuance of the inventive concept, it is expedient to add to the aqueous chlorite solution
Nguyen Ngoc-Yen
Pendorf & Cutliff
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