AU2008201447A1 - Production of chlorine-free chlorine dioxide - Google Patents

Description

AUSTRALIA

Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

00 00 oO oO Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: Name of Applicant: Superior Plus Inc.

Actual Inventor(s): Peter David Dick, Edward J. Bechberger Address for Service and Correspondence: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: PRODUCTION OF CHLORINE-FREE CHLORINE DIOXIDE Our Ref: 825495 POF Code: 1649/463989 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): -1eooeq 00 TITLE OF INVENTION SPRODUCTION OF CHLORINE-FREE CHLORINE DIOXIDE 00 SThis application claims priority from US Application No.60/907,390 filed on 30 March 2007, the contents of which are to be taken as incorporated herein by this reference.

FIELD OF INVENTION [0001] This invention relates to the production of chlorine dioxide.

00 BACKGROUND OF THE INVENTION o[00021 Chlorine dioxide solution is utilized in the production of bleached 0 pulp from pulp mills. A variety of procedures have been described in the prior art for the production of chlorine dioxide.

[0003] In one such procedure, as described, for example in Canadian Patent Nos. 913,328 and 1,195,477, sodium chlorate is reduced with hydrochloric acid in an aqueous medium maintained at its boiling point under a subatmospheric pressure. Chlorine dioxide is removed from the reaction zone in admixture with steam and chlorine. The gaseous admixture is processed to from an aqueous solution of chlorine dioxide along with some chlorine.

[0004] In one embodiment of the latter process, the chlorine dioxide generator is integrated with a chlorate cell, in which sodium chloride precipitated in the chlorine dioxide generator is electrolytically converted to sodium chlorate for feed to the chlorine dioxide generator. Hydrogen produced as a by-product of the chlorate cell is reacted with chlorine recovered from the off-gas of the chlorine dioxide generator to form hydrogen chloride, which is dissolved in water to form hydrochloric acid for the chlorine dioxide generation.

[0005] One advantage that this use of hydrochoric acid as the reducing agent provides is that the by-product of the process is sodium chloride, which may be used to form fresh sodium chlorate for the chlorine dioxide generating process.

00 [0006] The presence of chlorine in the chlorine dioxide solution product from the chlorine dioxide generator, however, is considered to be undesirable for environmental reasons.

00 SUMMARY OF THE INVENTION [0007] The present invention provides a method of producing chlorine dioxide solution substantially free from dissolved chlorine with no hydrochloric acid present in a chlorine dioxide generation process in which added chloride ions, preferably in the form of hydrochloric acid, are used as the reducing 00 agent for chlorate ions.

[0008] In the present invention, the off-gas from the chlorine dioxide generator, comprising steam, chlorine dioxide and chlorine, is cooled to condense the steam. The gaseous mixture of chlorine dioxide and chlorine then is contacted with an aqueous solution of hydrochloric acid to dissolve the chlorine dioxide and some of the chlorine. The presence of the hydrochloric acid in the aqueous absorption medium has the effect of suppressing the solubility of the chlorine in the aqueous absorption medium.

[0009] The chlorine dioxide solution then is air stripped to remove the chlorine dioxide which then is reabsorbed in water to provide the final aqueous solution of chlorine dioxide containing very little chlorine and no hydrochloric acid. Prior to the air stripping step, the mixture of chlorine dioxide and chlorine may be contacted with hydrogen peroxide to reduce the remaining chlorine.

BRIEF DESCRIPTION OF DRAWINGS [0010] Figure 1 is a schematic flow sheet of an integrated chlorine dioxide generating process based on hydrochloric acid; [0011] Figure 2 is a schematic flow sheet of one embodiment of the process of the invention; and 00 S[0012] Figures 3A and 3B are detailed schematic flow sheets of the integrated chlorine dioxide generating process of Figure 1 and embodying the C procedure of Figure 2.

00 DESCRIPTION OF PREFERRED

EMBODIMENT

[0013] Referring first to Figure 1, an integrated chlorine dioxide generating plant 10 has a chlorine dioxide generator 12, a sodium chlorate plant 14 and a hydrochloric acid plant 16.

00 [0014] Chlorine dioxide is generated in the generator 12 by reduction of aqueous sodium chlorate solution fed by line 18 with hydrochloric acid fed by P line 20 at the boiling point of the reaction medium under a subatmospheric pressure. Sodium chloride is deposited in the generator 12 and is passed by line 22 to the sodium chlorate plant 14 wherein, in aqueous solution made with water fed by line 24, the sodium chloride is converted by power fed by line 26 to aqueous sodium chlorate solution, which is recycled by line 18 to the chlorine dioxide generator 12.

[0015] The chlorine dioxide generating medium produces a gaseous admixture of steam, chlorine dioxide and chlorine which is processed according to the procedure described below with respect to Figure 2 to b produce chlorine-free chlorine dioxide solution and chlorine gas. The chlorine-free chlorine dioxide solution is removed from the generator 12 by line 28 while the chlorine gas is passed by line 30 from the chlorine dioxide generator 12 to the hydrochloric acid plant 16.

[0016] The gaseous chlorine is reacted in the hydrochloric acid plant 16 with by-product hydrogen fed by line 32 from the chlorate plant 14 along with make-up chlorine fed by line 34 to form hydrogen chloride, which is absorbed in water fed by line 36 to form hydrochloric acid, which ,then is fed by line 20 to the generator 12.

[0017] As noted above, the chlorine dioxide generator is operated at the boiling point of the reaction solution under a subatmospheric pressure.

The temperature of the reaction solution should be below the temperature 00 Sabove which substantial decomposition of chlorine dioxide occurs. It is preferred to operate the generator 12 at temperatures of about 60 °C to about 0 80 0

C.

S[0018] The subatmospheric pressure applied to the generator is sufficient to maintain the reaction medium under boiling condition at the temperature of the reaction medium.

S[0019] The hydrochloric acid process may be operated at low acidities, 00 generally below about 1 N, preferably below about 0.5 N, more preferably Sabout 0.1 N to about 0.2 N. It is preferred to operate the process of the invention at concentrations where the CI.:CIO3 molar ratio is approximately below 0.3.

[0020] The basic reaction providing chlorine dioxide in the generator 12 is depicted by the equation: NaCIO3 2HCI CI02 1/2CI2 NaCI H 2 0 [0021] The reaction in the chlorate plant 14 is depicted by the equation: NaCI 3H 2 0 NaCIO3 3H 2 S and the reaction in the hydrochloric acid plant 16 is depicted by the equation:

H

2 CI2 2HCI [0022] Complete details of the integrated chlorine dioxide generating plant 10 are shown in the flow sheets of Figures 3A and 3B.

[00231 Turning now to Figure 2, which is a simplified flow sheet of one embodiment of the processing of the gaseous admixture of steam, chlorine dioxide and chlorine to produce chlorine-free aqueous chlorine dioxide solution containing no hydrochloric acid. Complete details are shown in Figures 3A and 3B.

[00241 The gaseous admixture of steam, chlorine dioxide and chlorine is fed under vacuum by line 100 to an indirect contact cooler 102 where the 00 gas stream is cooled by indirect contact with chilled water fed by line 104 to condense the steam. The condensate is removed by line 106 from the cooler C and may be added to the product chlorine dioxide solution, as described 00 0 below.

[0025] The resulting gaseous mixture of chlorine dioxide and chlorine is fed under vacuum by line 108 to an absorber 110 wherein the gases are countercurrently contacted with a dilute aqueous hydrochloric acid solution Cfed by line 112. The aqueous hydrochloric acid solution may have a 00 concentration and temperature which suppress absorption of chlorine to a desired degree while absorbing the chlorine dioxide to provide an aqueous P solution having a desired concentration of chlorine dioxide.

[0026] In general, when processing an off-gas stream from a chlorine dioxide generating process in which added chloride is used to reduce chlorate ions and a gaseous admixture of chlorine dioxide and chlorine is produced and the gaseous admixture, after condensation of the steam, is absorbed in water, an aqueous solution of chlorine dioxide and chlorine is produced which contains about 8 to 10 gpl of chlorine dioxide and about 1.5 to 1.8 gpl of chlorine.

[0027] In operating the absorber 110 utilizing aqueous hydrochloric acid as the absorbant, the conditions are generally chosen to provide an aqueous solution of chlorine dioxide and chlorine having a concentration of about 8 to 10 gpl of chlorine dioxide and about 0.5 gpl of chlorine. This result may be achieved by using a 0.1N hydrochloric acid solution which has been cooled at about 5°C. Operation at up to 0.5 N hydrochloric acid may be used to minimize the bleed stream referred to below.

[00281 The gaseous chlorine is vented from the absorber 110 by line 114 and is forwarded to the hydrochloric acid plant 16.

[0029] The absorber 110 is maintained under the same vacuum pressure as the chlorine dioxide generator. Details of the vacuum pump are shown in Figure 3B.

00 [0030] The aqueous hydrochloric acid solution of chlorine dioxide and chlorine is passed by lines 116 and 118 to a chlorine dioxide stripper 120.

00 The chlorine dioxide solution in line 116 may be preheated to a temperature of 00 about 200 c to 30°C by stripped dilute hydrochloric acid in a preheater (not shown but seen in Figure 3B) and further heated at a temperature of about 0 C to 40 0 C in a second preheater (not shown but seen in Figure 3B). The temperature of the chlorine dioxide solution entering the stripper 120 is about to 15 0 C higher than employed conventionally in order to provide the 00 driving force to strip the chlorine dioxide from the solution by air fed by line 122 and produce a gas stream of sufficient chlorine dioxide concentration to produce the desired concentration of chlorine dioxide upon reabsorption.

[0031] Hydrogen peroxide fed by line 122 is mixed with the hydrochloric acid solution in line 116 to reduce the residual chlorine to hydrochloric acid.

[0032] The flow of the air in the stripper 120 is controlled by vacuum such that the concentration of chlorine dioxide in the air stream at the exit from the stripper 120 is below a concentration above which substantial decomposition of chlorine dioxide occurs, preferably about 90 to about 100 mm Hg partial pressure. The mixture of air and chlorine dioxide is forwarded by line 124 to a reabsorber 126.

I

[0033] The reabsorber 126 and the stripper 120 are generally maintained at the same vacuum level as the chlorine dioxide generator and the absorber 110 by a separate vacuum pump, as seen in Figure 3B.

[0034] The aqueous dilute hydrochloric acid is recycled from the stripper by lines 128 and 112 to the absorber 110. For energy economy, the solution may first pass through the chlorine dioxide solution pre-heater referred to above and then chilled to about 5°C in an absorber cooler (see Figure 3B for details).

[0035] The concentration of HCI in the recycling'dilute hydrochloric acid increases with the addition of hydrogen peroxide to convert chlorine to HCI and that concentration may be allowed to build up to a desired concentration, such as about 0.5 N. Thereafter, a bleed of 0.5 N hydrochloric acid may be 00 made by line 130 to prevent a further increase in acid concentration. The tbleed may be made to dilute HCI or the chlorine dioxide reabsorber 126. A Scorresponding bleed in of chilled water is required to balance the water in the 00 bleed stream 130.

[00361 The efficiency of the stripper 120 with respect to chlorine dioxide is important in that such efficiency affects the recirculation rate of the hydrochloric acid solution. However, it is not critical that all chlorine dioxide C ~be stripped from the solution. Any dissolved chlorine dioxide remaining in the hydrochloric acid solution in line 128 is left to recirculateand reduces the amount of C10 2 per liter of solution that can be absorbed in the next pass through the stripper 120, thereby increasing the recirculation rate of the dilute hydrochloric acid. As the concentration of chlorine dioxide is low and the temperature of the dilute hydrochloric acid being recycled to the top of the stripper 120 is low, there is little loss of chlorine dioxide at the top of the stripper 120.

[0037] In the reabsorber 126, the chlorine dioxide in the mixture of air and chlorine dioxide is contacted by water fed by line 132 toproduce an aqueous chlorine dioxide solution of the desired concentration in line 134.

This aqueous chlorine dioxide solution is free from chlorine and contains no HCI. As mentioned above, the condensate removed from the chamber 102 by line 106 may be added to the product solution. While this condensate contains some chlorine, the overall amount of chlorine present in the product is less than about 0.1 gpl of C12.

[0038] The following Table provides typical concentrations of chlorine dioxide, chlorine and HCI in the aqueous phase at various stages of the procedure in comparison to a standard product solution.

Table Standard

HCI

treated H2021HCI treated 8 to 10 2 gpl 8 to 10 8 to 10 -Cl gpl 1.5to 1.8 0.5 0

H

2 0 2 Product of the treated process of the invention 8 to 10 8 to 0 0.1 1.5 to 0 1.8 HC gpl 0 3.6 3.6 SUMMARY OF DISCLOSURE [0039] In summary of this disclosure, a procedure is provided whereby an integrated chlorine dioxide generating process utilizing hydrochloric acid as the acid source may be used to produce a virtually chlorine-free aqueous chlorine dioxide solution with no hydrochloric acid present by processing the off-gas from the chlorine dioxide generator. Modifications are possible within the scope of the invention.

Claims (12)

1. A process for the production of an aqueous chlorine dioxide solution, which includes: 00 0 reducing chlorate ions with chloride ions in an aqueous acid reaction medium contained in a reaction zone, in which the acidity and chloride ions are provided by hydrochloric acid, said aqueous acid reaction medium being maintained at its boiling point under a subatmospheric pressure, to produce Oan off-gas stream comprising steam, chlorine dioxide and chlorine, 0O removing the off-gas stream from the reaction zone and cooling the off- Ogas stream to condense the steam, contacting the resulting gaseous mixture of chlorine dioxide and chlorine with an aqueous solution of hydrochloric acid to dissolve the chlorine dioxide and some the chlorine therein to form a chlorine dioxide solution, air stripping the chlorine dioxide solution to remove the chlorine dioxide therefrom, and reabsorbing the stripped chlorine dioxide in water to produce a chlorine dioxide solution substantially free from dissolved chlorine and containing no hydrochloric acid.

2. A process as claimed in claim 1, in which the mixture of chlorine dioxide and chlorine is contacted with hydrogen peroxide prior to the air stripping step to reduce the chlorine.

3. A process as claimed in claim 1 or 2, in which the aqueous acid reaction medium contains less than 1N hydrochloric acid.

4. A process as claimed in claim 3, in which the aqueous acid reaction medium contains less than 0.5 N hydrochloric acid. A process as claimed in claim 4, in which the aqueous acid reaction medium contains less than 0.1 to 0.2 N hydrochloric acid. 00

6. A process as claimed in any one of claims 1 to 5, in which the molar Sratio of chloride ions to chlorate ions in the aqueous acid reaction medium is tbelow 0.3. 00

7. A process as claimed in any one of claims 1 to 6, in which said contacting of the resulting gaseous mixture of chlorine dioxide and chlorine with hydrochloric acid is effected at a hydrochloric acid concentration and temperature to preferentially permit absorption of chlorine dioxide in Spreference to chlorine. 00

8. A process as claimed in claim 7, in which the aqueous solution of chlorine dioxide and chlorine formed in the contacting step contains 8 to gpl of chlorine dioxide and 0.5 gpl of chlorine.

9. A process as claimed in claim 8, in which said hydrochloric acid has a concentration of 0.1 to 0.5 N. A process as claimed in any one of claims 1 to 9, in which said air stripping is effected using vacuum of the same pressure as the reaction zone.

11. A process as claimed in claim 10, in which said vacuum is 90 to 100 mm Hg.

12. A process as claimed in any one of claims 1 to 11, which is integrated with a sodium chlorate generating plant wherein by-product sodium chloride from the chloride dioxide generating reaction is electrolytically converted to sodium chlorate for feed to the chlorine dioxide generating reaction zone, and with a hydrochloric acid generation plant wherein by-product hydrogen from the sodium chlorate generating plant is reacted with by-product chlorine and resulting HCI is absorbed in water to form hydrochloric acid which is fed to the chlorine dioxide generating reaction zone. o00

13. A process for the production of an aqueous chlorine dioxide solution as claimed in claim 1 substantially as hereinbefore with reference to the Examples. 00

14. A process for the production of an aqueous chlorine dioxide solution as claimed in claim 1 substantially as hereinbefore described and as illustrated in the accompanied drawings. An aqueous chlorine dioxide solution whenever prepared by a process 00 as claimed in any one of claims to 4. ras claimed in any one of claims Ito 14.