METHOD AND APPARATUS FOR GENERATING CHLORINE DIOXIDE
20210171347 · 2021-06-10
Inventors
Cpc classification
B01D2325/10
PERFORMING OPERATIONS; TRANSPORTING
B01D69/145
PERFORMING OPERATIONS; TRANSPORTING
International classification
Abstract
There is disclosed a method of forming chlorine dioxide comprising passing chlorous acid through a membrane including a catalyst suitable to catalyse the formation of chlorine dioxide from chlorous acid. There is also disclosed a membrane suitable for forming an aqueous solution of chlorine dioxide comprising a catalyst suitable to catalyse the formation of chlorine dioxide from chlorous acid or alkali metal chlorite.
Claims
1. A method of forming chlorine dioxide comprising: feeding an aqueous solution of chlorous acid to a first face of a hydrophobic microporous membrane wherein the membrane comprises a catalyst for catalyzing the formation of chlorine dioxide from chlorous acid; wherein the chlorous acid contacts the catalyst and oxidises to chlorine dioxide; wherein chlorine dioxide in gaseous form passes through the hydrophobic microporous membrane to a second face thereof wherein the second face is in fluid communication with an outlet for providing the chlorine dioxide formed.
2. The method of claim 1, further comprising converting an alkali metal chlorite to chlorous acid prior to feeding the chlorous acid to the first face of the hydrophobic microporous membrane, by passing the alkali metal chlorite through a cation exchange resin.
3. A method of forming chlorine dioxide comprising: feeding an aqueous solution of alkali metal chlorite to a first face of a hydrophobic microporous membrane wherein the membrane comprises a catalyst for catalyzing the oxidation of alkali metal chlorite to chlorine dioxide; wherein the alkali metal chlorite contacts the catalyst and oxidises to chlorine dioxide; wherein chlorine dioxide in gaseous form passes through the hydrophobic microporous membrane to a second face thereof wherein the second face is in fluid communication with an outlet for providing the chlorine dioxide formed.
4. The method of claim 1, further comprising the step of providing a pressure differential between the first face of the hydrophobic microporous membrane and the second face of the hydrophobic microporous membrane.
5. The method of claim 1, further comprising contacting a waste reactant liquor produced during the chloride dioxide formation with UV light in the wavelength range 350 to 365 nm to breakdown chlorine dioxide.
6. A hydrophobic microporous membrane for forming an aqueous solution of chlorine dioxide, the membrane comprising: a first face; a second face; and a catalyst for catalyzing the formation of chlorine dioxide from chlorous acid or alkali metal chlorite.
7. The hydrophobic microporous membrane of claim 6 comprising a material selected from the group consisting of polymerised high internal phase emulsion polymer (polyHIPE), ePTFE, PVDF, or ceramic.
8. The hydrophobic microporous membrane of claim 7 formed from ePTFE comprising the catalyst embedded therein.
9. The hydrophobic microporous membrane of claim 6 comprising a catalyst rich portion at or towards the first face.
10. The hydrophobic microporous membrane of claim 6, wherein the catalyst is based on platinum, palladium, manganese or molybdenum metal centers.
11. The hydrophobic microporous membrane of claim 7, wherein the catalyst is mixed with the material used to form the hydrophobic microporous membrane during production thereof and prior to extrusion of the material.
12. The hydrophobic microporous membrane of claim 7, wherein the catalyst is mixed with the material used to form the hydrophobic microporous membrane during extrusion of the material to form the membrane.
13. The hydrophobic microporous membrane of claim 6, wherein the catalyst is encapsulated, for instance on zeolite or ceramic.
14. An apparatus for increasing the purity of an aqueous recipient medium comprising: the hydrophobic microporous membrane of claim 6, a first housing in fluid communication with the first face of the hydrophobic microporous membrane, a second housing in fluid communication with the second face of the hydrophobic microporous membrane and in fluid communication with the aqueous recipient medium to be treated.
15. The apparatus of claim 14 comprising a pump or a pressure control apparatus for applying a pressure differential across the membrane between the first face and the second face.
16. The apparatus of claim 15 further comprising a third housing in fluid communication with the first housing and comprising a cation exchange resin, for converting alkali metal chlorite to chlorous acid.
17. The apparatus of claim 14, wherein the first housing further includes a mechanism to automatically block fluid communication between the first housing and the membrane.
18. The apparatus of claim 14 further comprising a UV chamber in fluid communication with the first housing, wherein the UV chamber generates UV light having a wavelength of 350 to 365 nm.
19. The method of claim 3, further comprising the step of providing a pressure differential between the first face of the hydrophobic microporous membrane and the second face of the hydrophobic microporous membrane.
20. The hydrophobic microporous membrane of claim 10, wherein the catalyst is selected from the group consisting of platinum oxide, palladium oxide, manganese dioxide, manganese porphyrins, manganese porphyrazines and molybdenum-based macrocycles.
Description
[0144] The present invention will now be described by way of example only with reference to the associated FIGURES in which:
[0145]
[0146]
EXAMPLES
[0147] Decomposition of Chlorous Acid to Chlorine Dioxide
[0148] Initial testing was carried out to determine the decomposition rate of chlorous acid to chlorine dioxide without the addition of a catalyst.
[0149] The reactions are shown below for the full process:
NaClO.sub.2+H.sup.+.fwdarw.HClO.sub.2+Na.sup.+ (1)
5HClO.sub.2.fwdarw.ClO.sub.2+HCl+2H.sub.2O (2)
[0150] Reaction (1) represents the exchange of sodium cation for a hydrogen ion when sodium chlorite is passed over the cation exchange resin.
[0151] Reaction (2) represents the breakdown of chlorous acid to chlorine dioxide.
Example 1A
[0152] Sodium chlorite precursor was diluted and passed over strong cation exchange resin to form chlorous acid. The chlorite concentration and pH of the diluted samples were tested before ion exchange. The pH was tested after ion exchange and any colour change (solution was initially clear) or other observations were noted.
TABLE-US-00001 Sodium Chlorite precursor Dilution Factor 1 10 100 Chlorite Conc (mg/L) 70167 7031 702.7 pH (after dilution) 9.2 8.9 8.7 pH (after ion 1.4 1.8 2.2 exchange) Colour Yellow/Green + Faint green + Clear change/observations gas evolution slight gas evolution
Example 1B
[0153] A 703 mg/L chlorite solution was passed over strong cation exchange resin and the chlorine dioxide concentration was measured over time. Chlorine dioxide concentrations were measured every hour for 5 hours following ion exchange.
TABLE-US-00002 Chlorine dioxide Time conc. (mins) (mg/L) 1 10 60 17 120 25 180 44 240 74 300 92
[0154] The results from examples 1A and 1b show that chlorous acid does breakdown over time to chlorine dioxide without the addition of a catalyst. This can be a very rapid or very slow reaction depending on the starting concentration of sodium chlorite.
[0155] A strong sodium chlorite solution will produce a strong chlorous acid solution after ion exchange, this solution will have a low pH (below 1.5 in our experimentation) and will rapidly form chlorine dioxide. This was seen in example 1A with the sample which was undiluted (dilution factor xi) by the yellow/green colour of solution and the gassing off of chlorine dioxide.
[0156] The opposite occurred when a weak. (dilution factor ×100) sodium chlorite solution was used to begin with. A sodium chlorite solution of approximately 700 mgL.sup.−1 was passed over ion exchange and resulted in a clear solution of chlorous acid. This solution was higher in pH, 2.2, and did not form chlorine dioxide rapidly. There was no observed yellow colour in the solution indicating chlorine dioxide and there was no gas being evolved from the solution.
[0157] To track the breakdown with time of chlorous acid, a diluted sodium chlorite precursor was used in Example 1B. A solution of approximately 700 mgL.sup.−1 sodium chlorite was passed over ion exchange to form a chlorous acid solution. Chlorine dioxide concentration in the chlorous acid solution was measured 1 minute after ion exchange, and then every hour for 5 hours.
[0158] The maximum concentration of chlorine dioxide expected is approximately 550 mgL.sup.−1 from the sample used in example 1B. The maximum chlorine dioxide measured was 92 mgL.sup.−1 after 5 hours, equivalent to approximately 17% yield.
[0159] Manganese Dioxide as Oxidation Catalyst
[0160] Static Testing without ePTFE Membrane
[0161] Static tests were carried out initially to determine an ideal quantity of manganese dioxide to use and an ideal chlorous acid precursor concentration needed.
Example 2A
[0162] Three 50 ml samples of 703.5 mgL.sup.−1 chlorous acid solutions were transferred to flasks. 0.1 g, 0.5 g and 1 g of manganese dioxide, MnO.sub.2, were added to one of the three samples. The flasks were stoppered and allowed to mix. The pH and chlorine dioxide concentrations were measured over 15 minutes.
TABLE-US-00003 Mass of MnO.sub.2 added = 0.1 g ClO2 Percentage time conc Yield (mins) pH (mg/L) (%) 0 2.1 0.0 1 2.6 276 49.8 5 2.61 306 55.2 10 2.56 301 54.3 15 290 52.3
TABLE-US-00004 Mass of MnO.sub.2 added = 0.5 g ClO.sub.2 Percentage time conc Yield (mins) pH (mg/L) (%) 0 2.13 0.0 1 2.6 347 62.6 5 2.55 352 63.5 10 2.5 354 63.8 15 292 52.7
TABLE-US-00005 Mass of MnO.sub.2 added = 1 g ClO.sub.2 Percentage time conc Yield (mins) pH (mg/L) (%) 0 2.11 0.0 1 2.52 273 49.2 5 2.48 331 59.7 10 2.47 290 52.3 15 313 56.4
Example 2B
[0163] Three 50 ml samples of chlorous acid were prepared at concentrations of approximately 700, 3500 and 7000 mgL.sup.−1. 0.1 g of MnO.sub.2 was added to each sample, the flasks stoppered, and each allow to mix. The pH and chlorine dioxide concentrations were measured over 15 minutes.
TABLE-US-00006 Chlorous Acid Concentration = 703.5 mg/L ClO.sub.2 Percentage time conc Yield (mins) pH (mg/L) (%) 0 2.25 9 1.6 1 2.66 284 51.2 5 2.77 316 57.0 10 2.7 275 49.6 15 2.61 340 61.3
TABLE-US-00007 Chlorous Acid Concentration = 3517.5 mg/L ClO.sub.2 Percentage time conc Yield (mins) pH (rng/L) (%) 0 1.75 99 3.6 1 2.01 1170 42.2 5 2.06 1350 48.7 10 2.04 1270 45.8 15 2.08 1210 43.6
TABLE-US-00008 Chlorous Acid Concentration = 7035 mg/L ClO.sub.2 Percentage time conc Yield (mins) pH (mg/L) (%) 0 1.56 147 2.7 1 1.8 1630 29.4 5 1.78 2192 39.5 10 1.8 2310 41.7 15 1.85 1890 34.1
[0164] 0.1 g of catalyst produced similar quantities of chlorine dioxide as 1 g of catalyst. Chlorine dioxide yields were approximately 50-60% within the first minute of the reaction between chlorous acid and MnO.sub.2, compared to chlorous acid decomposition without a catalyst which yielded approximately 2% ClO.sub.2 in the same period of time.
[0165] 0.1 g of MnO.sub.2 produced the highest yields when reacting with weaker chlorous acid solutions. ClO.sub.2 percentage yield of approximately 50% was achieved within one minute for a chlorous acid solution of approximately 700 mgL.sup.−1. Lower percentage yields of approximately 40% and 30% were achieved in the same time period for stronger chlorous acid solutions of approximately 3500 and 7000 mgL.sup.−1, respectively.
[0166] Recirculating Tests with ePTFE Membrane Treated to Incorporate a Catalyst
Example 2C
[0167] MnO.sub.2 catalyst has been immobilised in the ePTFE membrane for testing.
[0168] A 705 mgL.sup.−1 chlorous acid solution was prepared by passing sodium chlorite solution over strong cation exchange resin. The chlorous acid solution was then pumped through a ePTFE membrane containing MnO.sub.2, the chlorous acid was contained in a recirculating loop. Motive water was pumped counter-currently across the second face of the membranes.
[0169] Chlorine dioxide, chlorite ion and chlorate ion concentrations in the motive water toward the second face of the PTFE membrane were measured at defined time intervals after the beginning of the test. Purity of the solution was calculated.
TABLE-US-00009 ClO.sub.2 Solution Time conc Chlorite Chlorate Purity (mins) (mg/L) (mg/L) (mg/L) (%) 0 0 0 0.110 5 121 0.037 0.134 99.95 20 111 0.054 0.159 99.93 35 136 0.042 0.176 99.96 50 134 0.057 0.142 99.98 65 145 0.045 0.157 99.96
[0170] As can be seen from the results, chlorine dioxide solutions of greater than 99.93% purity were achieved throughout the test when recirculating a chlorous acid solution through an ePTFE membrane in which a suitable catalyst had been immobilised into the membrane.
Once-Through Tests with ePTFE Membrane Treated to Incorporate a Catalyst
Example 2D
[0171] MnO.sub.2 catalyst has been immobilised in the ePTFE membrane for testing.
[0172] A 693 mgL.sup.−1 Chlorous acid solution was prepared by passing sodium chlorite solution over strong cation exchange resin. The chlorous acid solution was continuously pumped through a ePTFE membrane containing MnO.sub.2. Motive water was pumped counter-currently across the second face of the membrane.
[0173] Chlorine dioxide, chlorite ion and chlorate ion concentrations in the motive water flowing across the second face of the PTFE membrane were measured at defined time intervals after the beginning of the test. Purity of the solution was calculated.
TABLE-US-00010 ClO.sub.2 Solution Time conc Chlorite Chlorate Purity (mins) (mg/L) (mg/L) mg/L) (%) 0 0 0 0.134 5 112 0.043 0.15 99.95 20 118 0.032 0.157 99.97 35 129 0.034 0.163 99.97 50 123 0.024 0.154 99.99 65 131 0.05 0.155 99.96
[0174] As can be seen from the results, chlorine dioxide solutions of greater than 99.95% purity were achieved throughout the test when pumping a chlorous acid solution through an ePTFE membrane in which a suitable catalyst had been immobilised into the membrane.
Platinum as Oxidation Catalyst
Recirculating Tests with ePTFE Membrane Treated to Incorporate a Catalyst
Example 3A
[0175] A platinum catalyst has been immobilised in the ePTFE membrane for testing.
[0176] A 705 mgL.sup.−1 Chlorous acid solution was prepared by passing sodium chlorite solution over strong cation exchange resin. The chlorous acid solution was then pumped through the ePTEE membrane containing the catalyst, the chlorous acid was contained in a recirculating loop. Motive water was pumped counter-currently over the second face of the membrane.
[0177] Chlorine dioxide, chlorite ion and chlorate ion concentrations in the motive water moving across the second face of the PTFE membrane were measured at defined time intervals after the beginning of the test. Purity of the solution was calculated.
TABLE-US-00011 ClO.sub.2 Solution Time conc Chlorite Chlorate Purity (mins) (mg/L) (mg/L) (mg/L) (%) 0 0 0 0.115 5 90 0.041 0.156 99.91 20 96 0.062 0.172 99.92 35 104 0.047 0.165 99.96 50 99 0.032 0.166 99.97 65 86 0.039 0.172 99.95
[0178] As can be seen from the results, chlorine dioxide solutions of greater than 99.91% purity were achieved throughout the test when recirculating a chlorous acid solution through an ePTFE membrane in which a suitable catalyst had been immobilised into the membrane.
Once-Through Tests with ePTFE Membrane Treated to Incorporate a Catalyst
Example 3B
[0179] A platinum catalyst has been immobilised in the ePTFE membrane for testing.
[0180] A 700 mgL.sup.−1 Chlorous acid solution was prepared by passing sodium chlorite solution over strong cation exchange resin. The chlorous acid solution was continuously pumped to the first face of the ePTFE membrane containing the catalyst. Motive water was pumped counter-currently over the second face of the membranes.
[0181] Chlorine dioxide, chlorite ion and chlorate ion concentrations in the motive water flowing across the second face of the PTFE membrane were measured at defined time intervals after the beginning of the test. Purity of the solution was calculated.
TABLE-US-00012 ClO.sub.2 Solution Time conc Chlorite Chlorate Purity (mins) (mg/L) (mg/L) (mg/L) (%) 0 0 0 0.099 5 97 0.032 0.129 99.94 20 94 0.051 0.136 99.94 35 90 0.053 0.139 99.94 50 87 0.06 0.125 99.95 65 89 0.046 0.138 99.93
[0182] As can be seen from the results, chlorine dioxide solutions of greater than 99.93% purity were achieved throughout the test when pumping a chlorous acid solution through an ePTFE membrane in which a suitable catalyst had been immobilised into the membrane.
[0183] Manganese Porphyrin as Oxidation Catalyst
[0184] Manganese porphyrins can be used as a catalyst for oxidising chlorite ion to chlorine dioxide. There is not requirement for ion exchange as catalysts in this group react directly with chlorite. The catalyst will also react with chlorine dioxide to further oxidise it to chlorate ion. The incorporation of manganese porphyrins into an ePTFE membrane reduces the potential for further oxidation of chlorine dioxide, as the ClO.sub.2 should be formed by the reaction of catalyst and chlorite ion then be transferred through the membrane to the motive water line.
[0185] Any chlorine dioxide that is further oxidised to chlorate will remain in the impure chemical line.
[0186] Static Testing without ePTFE Membrane
[0187] Static tests were carried out initially to determine an ideal quantity of manganese porphyrin/manganese porphyrazine to use and an ideal chlorite precursor concentration needed.
[0188] Example 4H exemplifies static testing of the catalyst that was then incorporated into the ePTFE membrane.
Example 4A-I
[0189] 2 litres of chlorite solution (concentration specified in each table) was prepared for each test by diluting commercially available sodium chlorite solution to the desired concentration. Either a manganese porphyrin or porphyrazine, was added to the diluted chlorite solution (mass added as specified in tables below) and allowed to react while stirring.
Example 4A
[0190] Manganese Porphyrin Used as Catalyst
TABLE-US-00013 Vol dilution water (L) 2 Vol of Sodium Chlorite (ml/L) 0.65 Initial Chlorite Conc (mg/L) 45.728 Mass of Catalyst (g) 0.012
TABLE-US-00014 Time % conversion % conversion % % conversion min to ClO.sub.2 to FAC CHLORITE to Chlorate 0 90.23 0.19 5 0.15 86.11 1.22 45 0.26 86.24 1.28 85 0.04 0.59 88.95 1.44 115 0.09 0.68 88.17 1.52 155 0.11 0.66 86.91 2.00 185 0.17 0.70 86.91 1.87 270 0.44 1.18 76.96 2.80 1440 0.59 1.60 52.81 6.45
Example 4B
[0191] Manganese Porphyrin Used as Catalyst
TABLE-US-00015 Vol dilution water (L) 2 Vol of Sodium Chlorite (ml/L) 0.65 Initial Chlorite Conc (mg/L) 45.728 Mass of Catalyst (g) 0.025
TABLE-US-00016 Time % conversion % conversion % conversion (min) to ClO.sub.2 to FAC % CHLORITE to Chlorate 0 88.67 0.78 25 0.42 88.67 1.80 75 0.44 90.56 1.63 110 0.42 93.19 1.51 150 0.09 0.55 88.26 1.56 220 0.20 0.72 23.66 0.98 330 0.33 0.17 80.63 2.50 390 0.37 1.18 78.71 3.01 4320 0.57 2.60 50.57 9.61
Example 4C
[0192] Manganese Porphyrin Used as Catalyst
TABLE-US-00017 Vol dilution water (L) 2 Vol of Sodium Chlorite (ml/L) 0.65 Initial Chlorite Conc (mg/L) 45.728 Mass of Catalyst (g) 0.05
TABLE-US-00018 Time % conversion % conversion to % conversion (min) to ClO.sub.2 FAC % CHLORITE to Chlorate 0 83.49 1.41 45 0.07 0.87 85.17 1.61 110 0.42 1.07 77.96 2.86 140 0.44 1.40 76.10 3.28 180 0.52 2.19 72.46 4.01 210 0.52 2.21 69.95 4.57 240 0.59 2.16 67.32 5.18 1440 0.66 3.00 39.55 11.75
Example 4D
[0193] Manganese Porphyrin Used as Catalyst
TABLE-US-00019 Vol dilution water (L) 2 Vol of Sodium Chlorite (ml/L) 0.65 Initial Chlorite Conc (mg/L) 45.728 Mass of Catalyst (g) 0.075
TABLE-US-00020 Time % conversion % conversion to % conversion (min) to ClO.sub.2 FAC % CHLORITE to Chlorate 0 86.05 1.39 30 0.09 1.16 86.77 1.74 60 0.09 1.53 83.77 2.03 120 0.44 2.01 73.98 3.13 170 0.57 2.76 70.95 4.34 230 0.68 2.84 64.99 5.87 260 0.70 2.76 61.63 6.28 350 0.68 3.74 57.21 7.54 1440 0.72 4.11 14.04 17.48
Example 4E
[0194] Manganese Porphyrin Used as Catalyst
TABLE-US-00021 Vol dilution water (L) 2 Vol of Sodium Chlorite (ml/L) 0.65 Initial Chlorite Conc (mg/L) 45.728 Mass of Catalyst (g) 0.105
TABLE-US-00022 Time % conversion % conversion to % conversion (min) to ClO.sub.2 FAC % CHLORITE to Chlorate 0 84.07 1.73 30 0.48 84.87 1.51 60 0.07 1.09 79.90 1.78 135 0.42 2.03 70.14 3.19 230 0.55 2.43 65.32 4.61 280 0.68 2.93 67.46 6.02 320 0.68 3.00 52.33 6.86 1440 0.79 3.74 20.90 15.27
Example 4F
[0195] Manganese Porphyrin Used as Catalyst
TABLE-US-00023 Vol dilution water (L) 2 Vol of Sodium Chlorite (ml/L) 6.5 Initial Chlorite Conc (mg/L) 457.275 Mass of Catalyst (g) 0.05
TABLE-US-00024 Time % conversion % conversion to % conversion (min) to ClO.sub.2 FAC % CHLORITE to Chlorate 0 86.86 8.75 30 87.74 9.39 60 0.11 0.70 126.07 10.47 120 0.20 1.29 126.07 10.47 280 0.24 1.29 86.21 4.55 360 0.26 1.20 85.77 3.51 4320 0.99 1.16 79.82 7.72
Example 4G
[0196] Manganese Porphyrin Used as Catalyst
TABLE-US-00025 Vol dilution water (L) 2 Vol of Sodium Chlorite (ml/L) 6.5 Initial Chlorite Conc (mg/l) 457.275 Mass of Catalyst (g) 0.05
TABLE-US-00026 Time % conversion % conversion to % conversion (min) to ClO.sub.2 FAC % CHLORITE to Chlorate 0 15 0.38 0.31 56.38 1.64 30 0.30 0.31 53.18 3.37 60 0.23 0.30 53.78 4.33 90 0.23 0.21 54.32 4.22 120 0.29 0.18 50.80 4.85 240 0.23 0.12 50.89 6.06 360 0.23 0.10 49.58 7.09 1020 0.24 0.07 44.85 7.87
Example 4H
[0197] Manganese Porphyrazine Used as Catalyst
TABLE-US-00027 Vol dilution water (L) 2 Vol of Sodium Chlorite (ml/L) 6.5 Initial Chlorite Conc (mg/L) 457.275 Mass of Catalyst (g) 0.05
TABLE-US-00028 Time % conversion % conversion to % conversion (min) to ClO.sub.2 FAC % CHLORITE to Chlorate 0 15 0.95 0.33 53.49 11.90 30 0.95 0.34 49.95 19.31 60 0.97 0.29 43.98 23.01 90 0.96 0.25 40.72 24.14 120 0.98 0.21 33.90 26.18 240 1.03 0.16 27.07 27.64 360 1.03 0.11 25.85 28.78 1020 1.03 0.08 22.57 30.77
[0198] The results from the above experiments show that suitable catalysts will produce chlorine dioxide directly from oxidation of chlorite ion. In all examples, there is further oxidation of chlorine dioxide to chlorate ion as the chlorine dioxide stays in contact with the catalyst.
Recirculating Tests with ePTFE Membrane Treated to Incorporate a Catalyst
Example 4I
[0199] A manganese porphyrazine catalyst has been immobilised in the ePTFE membrane for testing.
[0200] A 708 mgL.sup.−1 sodium chlorite solution was prepared by diluting commercially available sodium chlorite solution with de-ionised water. The sodium chlorite solution was then pumped to the first face of the ePTFE membranes containing the catalyst, the sodium chlorite solution was contained in a recirculating loop. Motive water was pumped counter-currently over the second face of the membranes.
[0201] Chlorine dioxide, chlorite ion and chlorate ion concentrations in the motive water flowing across the second face of the PTFE membrane were measured at defined time intervals after the beginning of the test. Purity of the solution was calculated.
TABLE-US-00029 ClO.sub.2 Time conc Chlorite Chlorate Solution (mins) (mg/L) (mg/L) (mg/L) Purity (%) 0 0 0 0.128 5 110 0.045 0.154 99.94 20 119 0.029 0.162 99.97 35 104 0.036 0.167 99.96 50 112 0.039 0.152 99.98 65 118 0.048 0.158 99.95
[0202] As can be seen from the results, chlorine dioxide solutions of greater than 99.94% purity were achieved throughout the test when recirculating a sodium chlorite solution through an ePTFE membrane in which a suitable catalyst had been immobilised into the membrane.
Once-Through Tests with ePTFE Membrane Treated to Incorporate a Catalyst
Example 4J
[0203] A manganese porphyrazine has been immobilised in the ePTFE membrane for testing.
[0204] A 707 mgL.sup.−1 sodium chlorite solution was by diluting commercially available sodium chlorite solution with de-ionised water. The sodium chlorite solution was continuously pumped to the first face of a ePTFE membrane containing the catalyst. Motive water was pumped counter-currently across the second face of the membranes.
[0205] Chlorine dioxide, chlorite ion and chlorate ion concentrations in the motive water flowing across the second face of the PTFE membrane were measured at defined time intervals after the beginning of the test. Purity of the solution was calculated.
TABLE-US-00030 ClO.sub.2 Time conc Chlorite Chlorate Solution (mins) (mg/L) (mg/L) (mg/L) Purity (%) 0 0 0 0.128 5 110 0.045 0.154 99.94 20 119 0.029 0.162 99.97 35 104 0.036 0.167 99.96 50 112 0.039 0.152 99.98 65 118 0.048 0.158 99.95
[0206] As can be seen from the results, chlorine dioxide solutions of greater than 99.94% purity were achieved throughout the test when pumping a sodium chlorite solution through an ePTFE membrane in which a suitable catalyst had been immobilised into the membrane.
Comparative Example 5
[0207] Comparative data was prepared using a process comprising the steps of: [0208] a. Diluting a sodium chlorite solution, [0209] b. Converting sodium chlorite to chlorous acids using an ion exchange column [0210] c. Oxidising chlorous acid to chlorine dioxide by passing shlorous acid over a platinum based catalyst.
TABLE-US-00031 Free Average ClO.sub.2 Chlorine Chlorite Chlorate Solution Solution Generation Sample Conc Conc Conc Conc Purity Purity Equipment No (mg/L) (mg/L) (mg/L) (mg/L) (%) (%) 1 1 1.81 0.2 0.974 2.461 33.24 40.35 2 1.85 0.23 0.287 1.531 47.46 2 1 146 2 4.2 73.2 64.77 66.78 2 220 5 72.9 62.3 61.08 3 207 3 16.1 51.8 74.49 3 1 718 6 7.7 251 73.06 61.03 2 655 90 16.5 575.6 48.99 4 1 800 4 816.7 172.3 44.62 52.23 2 1290 4 678.7 183.1 59.84 5 1 648 2 54.9 127.3 77.87 77.87 6 1 617 20 43.5 104 78.65 64.94 2 453 9 314.5 107.6 51.24 7 1 507 3 512.9 99.5 45.17 37.58 2 190 21 517.8 80.9 23.47 3 491 0 524.1 98.1 44.11 8 1 294 0 295 200.1 37.26 36.51 2 282 0 320.7 185.8 35.76
[0211] Various modifications and variations of the described aspects of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes of carrying out the invention which are obvious to those skilled in the relevant fields are intended to be within the scope of the following Claims.