WO1983001940A1

WO1983001940A1 - Method for the manufacture of chlorine dioxide

Info

Publication number: WO1983001940A1
Application number: PCT/SE1981/000351
Authority: WO
Inventors: Ab Cellchem
Original assignee: Olson, Erik
Priority date: 1981-12-02
Filing date: 1981-12-02
Publication date: 1983-06-09

Abstract

A method for the manufacture of chlorine dioxide by reducing chlorate with an excess amount of sulphur dioxide in an aqueous solution of the chlorate in strong sulphuric acid to form a gas mixture containing chlorine dioxide and unreacted sulphur dioxide. Destruction of chlorine dioxide product is prevented by adding chlorine to the gas mixture and contacting the gas with water, whereby the unreacted sulphur dioxide is oxidized.

Description

METHOD FOR THE MANUFACTURE OF CHLORINE DIOXIDE

The present invention relates to a method for manu- facture of chlorine dioxide by reduction of chlorates, especially sodium chlorate, with sulphur dioxide in strong- 5. ly sulphuric acid solutions.

The chlorine dioxide is hereby manufactured by the so-called Mathieson method, i.e. a reaction sequence ini¬ tiated when a gaseous mixture of air and sulphur dioxide is brought into contact with a liquid solution of sodium 10. chlorate and sulphuric acid in a gas distribution vessel. This method for chlorine dioxide production is described for example in the Swedish applications 7807093-5 and 7704773-6.

The general formula for the production of chlorine 15. dioxide according to this method is normally written:

I. 2 NaClO^ + S0₂ + ^SO^ >2 C10₂ + 2 NaHSO^

The reaction is in reality much more complicated than suggested by this general formula. Several concurrent and mutually dependent reactions for reduction of the clo- rate takes place in the solution. A partial disproportiona- 20. tion of the chlorate according to the formula:

II. HC10 + HCl HC10₂ + HC10 is the primary reaction for the formation of chlorine di¬ oxide according to the reaction:

III. HCIO, + HC10 - 2 CIO2- + H20

Hydrochloric acid for the reaction according to for¬ mula II may be produced from HC10 according to the react- ■f. 25- ion:

, IV. HC10 + H₂S0 HC1 + r^SO_jj where the concentration of H_?S0, is maintained by a conti¬ nuous addition of sulphur dioxide to the reaction solution. The sulphur dioxide is added in the form of a gas, which will not react with anyone of the ions of the solution but 30. only with its water molecules according to the equilibrium:

V. SO, H₂0. H₂S0 The reactions II to IV above can theoretically give a 100-percent yield of chlorine dioxide from the chlorate according to general formula I. In practice the theoretical yield is not obtained which partly depends on a production 5. of .chlorine from HCIO according to the reaction:

VI. HCIO + HCl C1₂ + H₂0

To the degree this reaction takes place the reaction solu- .tion is depleted of HCIO for reaction IV as well as HCl for reaction II, which reaction is a condition for the chlo¬ rine dioxide production reaction III.

10. In order to suppress this chlorine production a sur¬ plus of sulphur dioxide is used, whereby a high H SO,-con¬ centration according to reaction V is secured in the react¬ ion solution and a corresponding high HCl-production accord¬ ing to reaction IV. A suitable excess of sulphur dioxide is

15. about 0.1 to 0.2 kg in addition to the 0.3 kg per kg sodium chlorate stoichiometrically- required according to formula I and preferably a total amount of 0.4 to 0.45 kg is used. Although the surplus of sulphur dioxide is an advan¬ tage in the reactor for suppressing unwanted reactions the

20. excess has the disadvantage of slowly reacting with the chlorine dioxide product in the presence of water according to the formula:

VII. 2 C10₂ + H₂S0₃ + HCl + 5 H₂S0 4

This reaction is of little problem in the reaction solution because it take place fairly slowly. Since the non-reacted

25. sulphur dioxide, however, will be present in the product • gas stream from the reactor it will accompany the chlorine dioxide to the absorption tower normally present after the reactor and in which the chlorine dioxide is absorbed in water for later use as a water solution. The remaining sul-

30. phur dioxide in the gas stream will also be absorbed in the water absorption tower and, although the reaction VII above is slow, the normal time delay before use of the chlorine di¬ oxide solution product is sufficient for all of the sulphur dioxide to react according to this formula whereby a substantial amount of

35. chlorine dioxide product is destroyed. For each kg of sulphur di¬ oxide absorbed about 0.42 kg of chlorine dioxide is destroyed. Because of this it has not been possible to fully make use of the advantages in the reactor of a sulphur dioxide surplus feed^¬ ing.

The object of the present invention is to provide a 5. method for making it possible to use an excess amount of sulphur dioxide in a chlorine dioxide reactor without nega¬ tive effects on the final chlorine dioxide product. A furth¬ er object is to accomplish this to a low cost and in a suit¬ ably integrated overall process.

10. This is obtained by the steps disclosed in the append¬ ed claims.

According to the invention the gases leaving the pri¬ mary reactor, which contains chlorine dioxide and residual sulphur dioxide, is supplied with- chlorine and contacted

15. with water in a step between the primary reactor and the chlorine dioxide absorption tower. Hereby the surplus of sulphur dioxide reacts with the chlorine according to the formula:

VIII. S0₂ + Cl₂ + 2 H₂0 ?^» 2 HCl + H^O^

This reaction is much faster, about 20 times faster, than

20. reaction VII, which means that the sulphur dioxide in this way can be effectively removed without much chlorine dioxi¬ de beeing destroyed in the same step. The reaction prefer¬ ably takes place in a contacting device for water in a re¬ action tower also called gas scrubber. In this way the be-

25- nefits of a surplus of sulphur dioxide in the primary re¬ actor can be made use of without negative effects on the chlorine dioxide product. In the gas scrubber a water solu¬ tion containing hydrochloric acid and sulphuric acid is produced. According to a preferred embodiment of the invent-

30. ion the reaction solution from the primary reactor, still containing some chlorate, is fed to a secondary reactor in which the remaining amounts of chlorate is reduced to chlo¬ rine dioxide, whereby the acid solution from the scrubber in this step is used as source of acid and reducing agent in

35. the form of chlorine ions. Hence the reaction in the second¬ ary reactor takes place without addition of sulphur dioxide according to the reaction: IX. 2 HCIO, + 2 HCl ^2 C10₂ + Cl₂ + H₂0

Since the gas leaving this reactor contains chlorine, the gas stream can preferably be used as chlorine source for the above mentioned scrubber reaction. By in this way clos- ing the system in respect of the gas scrubber and the se- 5. condary reactor not only the streams involved are better made use of but the efficient and economical recovery also allows a higher concentration of chlorate to be used in the primary reactor, which improves the overall efficiency of the process.

10. The chlorine dioxide gas is very explosive so that in processes of the present kind it is necessary to redu- ^• ce its partial .pressure in the gas phase in a known way to values below its explosive limits. By the addition of air a non-explosive, mixture is maintained in the gas phases of

15. the two reactors, the gas scrubber and the absorption tower for chlorine dioxide. The addition of thinning air suitably is made in the bottom of the reaction vessels so that it will have both an agitating and a stripping effect. The stripping effect is of special importance in the primary

20. reactor in order to suppress reaction VII.

The figure shows a preferred plant layout for a pro¬ cess according to the invention.

The plant of the figure contains a primary reactor A, a secondary reactor B, a gas scrubber C, an absorption towe

25. D, strippers E and F, exhaust gas scrubber G with circula¬ tion pump H and fan I.

To the primary reactor A is added a sodium chlorate solution through conduit 1. As said the invention allows a higher than usual concentration of chlorate to be maintain-

30. ed in the primary reactor without economical disadvantages or above about 40 g/1, preferably between 50 and 70 g/1. The reaction solution is kept 9 to 10-normal in respect of sulphuric acid, which is added in the form of a solution of about 95 percent strength through conduit 2. Sulphur di-

35- oxide gas is fed through 3, which is mixed with air from conduit 4. When necessary, e.g. when not enough chlorine is formed in the secondary reactor, chlorine gas may be added through 5. By use of the fan I gases are sucked from the primary reactor through conduit 6 to the gas scrubber C. The gases contains chlorine dioxide, residual sulphur di¬ oxide gas, chlorine and dilution air. The spent reaction solution from the primary reactor A, containing .sulphuric 5. acid, sodium sulphate and unreacted sodium chlorate, is via an overflow fed to the secondary reactor B through conduit 7. Dilution air is also added through conduit 8. To the secondary reactor B is also through 9 fe at least a part but preferably all the mixture of- sulphuric acid and hydro- 10. chloric acid formed in the scrubber C at the reaction bet¬ ween sulphur dioxide, chlorine and water. In the secondary reactor B residual chlorate is reduced by hydrochloric acid whereby chlorine dioxide and chlorine are formed. These gases, together with the dilution air, are sucked 15. through conduits 10 and 11 to the primary reactor where they mix with the previously mentioned gases and are drawn away through conduit 6. In the gas scrubber C the earlier mentioned reaction between sulphur dioxide, chlorine and water takes place. The water is added through conduit 12. 20. The necessary amount of water can be kept quite small in relation to the gas volume passed through the tower and it may be suitable to circulate the liquid through the to¬ wer. To some extent the hydrochloric acid formed in the reaction is stripped off by the gas passed through the to- 25. wer and is captured in the absorption tower. so that the pH of the product solution is lowered. By adding more water the hydrochloric acid can more effectively be kept in the acid solution but instead the amount of acid solution in¬ creases. A suitable acid concentration compromise is bet- Ά 30. ween 300 and 500, preferably between 350 and 450, g/1 of sulphuric acid in the solution leaving the contacting de¬ vice. At these sulphuric acid concentrations also the hydro¬ chloric acid is sufficiently well retained in the solution. Any gas/liquid contact device may be, used but preferably a 35^*. device for countercurrent flow of at least two theoretical plates is used, e.g. a packed column. In order to avoid damages in case of minor explosions the gas volume above the bed in the tower may be designed as a movable hood attached to the tower by a water seal. Chlorine dioxide, dilution air and possible residual chlorine are sucked through conduit 13 to the absorption tower D, to which is also fed water through conduit 14 in a suitable amount to solve the chlorine dioxide. The chlorine dioxide solution 5. product flows through conduit 15 to the user or to storage tanks. Through lβ the dilution air and residual small amounts of chlorine dioxide are sucked to the exhaust gas tower G, to which tower is also fed water via 17 and possibly also sodium hydroxide via 18 when necessary. Pump H circulates

10. the liquid through conduit 19. Fan I expels the purified dilution air from the system through conduit 20. Fan I shall have a sufficient capacity to maintaining a reduced pressur in all the devices of the system. The washing water surplus is ejected through conduit 21. From the secondary reactor

1 ♦ B the solution flows via an overflow and counduit 22 to the stripper E where it is degassed by air fed through 23. The air from the stripper contains chlorine dioxide and chlor¬ ine and is sucked through conduits 24 and 25 to the second¬ ary reactor B or through 11 directly to the primary reactor

20. A. The stripped residual solution can if desired be comple¬ tely freed from active chlorine by feeding it via 2β to a second stripper F to which is also fed air via 27 and a small amount of sulphur dioxide via 28. The sulphur dioxide containing gas stream leaving stripper F can suitably be

25- sucked via conduit 30 to exhaust gas tower G, where the sul phur dioxide is absorbed by the sodium hydroxide and helps to eliminating rests of chlorine dioxide and chlorine from the dilution air. Through overflow conduit 29 the residual solution is fed to a collecting tank.

Claims

1. Method for producing chlorine dioxide by reduct- ion of chlorate with an excess amount of sulphur dioxide

._.. in an aqueous reaction solution of the chlorate in strong

5. sulphuric acid in a primary reactor, whereby a gaseous mix ture containing chlorine dioxide and unreacted sulphur di¬ oxide is obtained, characterized in that the gaseous mix¬ ture is supplied with chlorine and contacted with water, whereby the -added amount of chlorine is sufficient for 10. oxidizing at least the major part of the sulphur dioxide content of the gaseous mixture.

2. The method of claim 1, characterized in that the

- gaseous mixture after the oxidation of the sulphur dioxide is separated from the water solution and fed to a step for 15. absorption of chlorine dioxide.

3. The method of claim 2, characterized in that the water solution separated from the gas is fed to a seconda¬ ry reactor and that the re'sidual reaction solution from th primary reactor is also fed into the secondary reactor whe

20. reby chlorate rests are reduced mainly with chloride ions to form a gas stream containing chlorine dioxide and chlo¬ rine.

4. The method of claim 3, characterized in that the gas stream from the secondary reactor is added to the gas-

25. eous mixture leaving the primary reactor.

5. The method of claim 3, characterized in that the spent reaction solution from the secondary reactor is fed to a stripper into which air is introduced and from which

^• a gas, containing chlorine dioxide and chlorine, is leav-

* 30. ing, and that the leaving gas is mixed with the gases from the primary and/or the secondary reactor.

6. The method of claim 5, characterized in that the stripped solution from the stripper is fed into a second .stripper into which air and sulphur* dioxide is introduced 35. and from which a gas, containing sulphur dioxide, is leav- ing, and that this gas is fed into an exhaust gas tower in¬ to which also water and the gases from the chlorine dioxide absorption tower are fed.

7. The method of any of the preceeding claims, chara terized in that an additional external addition of chlorin is made to the gases from the primary reactor.