Process for the production of chlorine dioxide

Abstract

The invention relates to a process for the production of chlorine dioxide comprising forming chlorine dioxide in a reaction medium in a reaction vessel and withdrawing chlorine dioxide as a gas from the reaction medium in the reaction vessel, the process further comprising adding chlorine dioxide to at least one raw material used in the process. The invention further relates to the use of chlorine dioxide for the reduction of the amount of chlorinated aromatic compounds occurring in a process for the production of chlorine dioxide.

Claims

1. A process for reducing the amount of chlorinated aromatic compounds in the reaction of chlorate ions and a reducing agent in an acidic aqueous reaction medium to form chlorine dioxide, the process consisting of adding chlorine dioxide to at least one raw material used in said reaction, wherein the at least one raw material includes water, an aqueous solution of an alkali metal chlorate, a reducing agent, or at least one mineral acid.

2. The process as claimed in claim 1, wherein said water includes water used for dissolving the alkali metal chlorate to form the aqueous solution thereof.

3. The process as claimed in claim 1, wherein said water includes water used for absorbing gaseous chlorine dioxide formed in the process.

4. The process as claimed in claim 1, wherein said at least one mineral acid is sulfuric acid.

5. The process as claimed in claim 1, wherein said at least one mineral acid is hydrochloric acid.

6. The process as claimed in claim 1, wherein the chlorine dioxide is added to said at least one raw material as an aqueous solution.

7. The process as claimed in claim 1, wherein the chlorine dioxide is added to said at least one raw material in an amount to obtain a concentration thereof in the raw material from about 1 mg/l to about 1.5 g/l.

8. The process as claimed in claim 1, wherein the chlorine dioxide is added to and mixed in-line with said at least one raw material.

9. The process as claimed in claim 1, wherein the at least one raw material to which chlorine dioxide has been added at some stage before being used in the reaction is brought to a temperature from about 5 to about 100 C.

10. The process according to claim 1 wherein the chlorinated aromatic compounds are chlorinated aromatic dibenzo-p-dioxin and dibenzo-furan.

11. The process as claimed in claim 2, wherein said at least one raw material includes the reducing agent.

12. The process as claimed in claim 1, wherein said at least one raw material includes the at least one mineral acid and the aqueous solution of the alkali metal chlorate.

13. The process as claimed in claim 12, wherein said at least one mineral acid is sulfuric acid.

14. The process as claimed in claim 3, wherein the chlorine dioxide is added to said at least one raw material in an amount to obtain a concentration thereof in the raw material from about 1 mg/l to about 1.5 g/l.

15. The process as claimed in claim 6, wherein the chlorine dioxide is added to said at least one raw material in an amount to obtain a concentration thereof in the raw material from about 1 mg/l to about 1.5 g/l.

16. The process as claimed in claim 6, wherein the chlorine dioxide is added to and mixed in-line with said at least one raw material.

Description

EXAMPLE

(1) Two trials were performed in a laboratory chlorine dioxide generator. In the first trial untreated river water was used for making a starting reaction medium and a sodium chlorate feed solution. The generator was operated continuously at a pressure of 25 kPa and a temperature of 67 C. with hydrochloric acid as acid and hydrogen peroxide as reducing agent, a system giving substantial formation of elemental chlorine. The process was run for 6 hours and after cooling the reaction medium (including formed solid NaCl particles) was collected. The reactor was then thoroughly cleaned and tubes were exchanged before the second trial.

(2) In the second trial, water taken at the same occasion from the same river as in the first trial, was pretreated by adding 5 ml of ClO.sub.2-water (5 g ClO.sub.2/1) to 2 liter of water, corresponding to an addition of 12 mg ClO.sub.2 per liter of water. The water was then left in room temperature for about 16 hours. A new starting reaction medium and a sodium chlorate feed solution were then made from the ClO.sub.2 treated water. The generator was then operated continuously for 6 hours under the same conditions as in the first trial and after cooling the reaction medium (including formed solid NaCl particles) was collected.

(3) The two samples of reaction medium were analysed in respect of chlorinated dibenzo-p-dioxins and dibenzofurans. The results, expressed as toxic equivalents of 2,3,7,8-tetrachlorodibenzo-p-dioxin, are shown in the table below:

(4) TABLE-US-00001 I-TEQ Lower I-TEQ 2005 Higher bound (pg/kg) bound (pg/kg) Reaction medium from trial 6.8 7.1 1 with untreated water Reaction medium from 2.9 3.7 trial 2 with ClO.sub.2 treated water
The results, expressed as the sum of all PCDD and PCDF congeners with 4 chlorine atoms or more, are shown in the table below:

(5) TABLE-US-00002 Reaction medium from Reaction medium from trial 1 with untreated trial 2 with ClO.sub.2 treated water (pg/kg) water (pg/kg) Sum PCDD 15 5.5 Sum PCDF 140 77
The results for the 17 most toxic congeners are shown in the following table:

(6) TABLE-US-00003 Reaction medium from Reaction medium from trial 1 with untreated trial 2 with ClO.sub.2 treated Congener water (pg/kg) water (pg/kg) 2378 TeCDD 0.69 ND(0.34) 12378 PeCDD ND(0.36) ND(0.42) 123478 HxCDD ND(0.61) ND(0.71) 123678 HxCDD ND(0.34) ND(0.4) 123789 HxCDD ND(0.49) ND(0.58) 1234678 HpCDD ND(0.53) ND(0.66) OCDD ND(0.72) ND(1) 2378 TeCDF 41 22 12378 PeCDF 4.6 1.8 23478 PeCDF 2.6 0.98 123478 HxCDF 2.1 0.91 123678 HxCDF 0.58 0.43 234678 HxCDF 0.67 0.52 123789 HxCDF 1.4 ND(0.49) 1234678 HpCDF 1.4 0.85 1234789 HpCDF ND(0.39) ND(0.51) OCDF ND(1.1) ND(1.6) ND(X.XX) is a notation that the concentration is below the detection limit, the X.XX in the parenthesis is the detection limit (LOD).

(7) As apparent from the above results the amount of PCDD and PCDF could be significantly reduced by operating the process according to the invention.