CA2018507C - Electrochemical processing of aqueous solutions - Google Patents
Electrochemical processing of aqueous solutionsInfo
- Publication number
- CA2018507C CA2018507C CA 2018507 CA2018507A CA2018507C CA 2018507 C CA2018507 C CA 2018507C CA 2018507 CA2018507 CA 2018507 CA 2018507 A CA2018507 A CA 2018507A CA 2018507 C CA2018507 C CA 2018507C
- Authority
- CA
- Canada
- Prior art keywords
- alkali metal
- sodium
- ions
- chlorate
- aqueous solution
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
- 239000007864 aqueous solution Substances 0.000 title claims description 54
- 238000012545 processing Methods 0.000 title description 4
- 238000000034 method Methods 0.000 claims abstract description 122
- OSVXSBDYLRYLIG-UHFFFAOYSA-N dioxidochlorine(.) Chemical compound O=Cl=O OSVXSBDYLRYLIG-UHFFFAOYSA-N 0.000 claims abstract description 116
- -1 hydrogen ions Chemical class 0.000 claims abstract description 103
- HEMHJVSKTPXQMS-UHFFFAOYSA-M Sodium hydroxide Chemical compound [OH-].[Na+] HEMHJVSKTPXQMS-UHFFFAOYSA-M 0.000 claims abstract description 84
- 229910052783 alkali metal Inorganic materials 0.000 claims abstract description 75
- 230000008569 process Effects 0.000 claims abstract description 72
- PMZURENOXWZQFD-UHFFFAOYSA-L Sodium Sulfate Chemical compound [Na+].[Na+].[O-]S([O-])(=O)=O PMZURENOXWZQFD-UHFFFAOYSA-L 0.000 claims abstract description 70
- 239000004155 Chlorine dioxide Substances 0.000 claims abstract description 58
- 235000019398 chlorine dioxide Nutrition 0.000 claims abstract description 58
- 229910052938 sodium sulfate Inorganic materials 0.000 claims abstract description 58
- 235000011152 sodium sulphate Nutrition 0.000 claims abstract description 58
- XTEGARKTQYYJKE-UHFFFAOYSA-M Chlorate Chemical compound [O-]Cl(=O)=O XTEGARKTQYYJKE-UHFFFAOYSA-M 0.000 claims abstract description 49
- 229910052739 hydrogen Inorganic materials 0.000 claims abstract description 42
- 239000001257 hydrogen Substances 0.000 claims abstract description 41
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 claims abstract description 40
- 239000000203 mixture Substances 0.000 claims abstract description 34
- 229910001415 sodium ion Inorganic materials 0.000 claims abstract description 32
- 229910052936 alkali metal sulfate Inorganic materials 0.000 claims abstract description 23
- 239000006227 byproduct Substances 0.000 claims abstract description 22
- 229940005989 chlorate ion Drugs 0.000 claims abstract description 16
- 239000000243 solution Substances 0.000 claims description 59
- BZSXEZOLBIJVQK-UHFFFAOYSA-N 2-methylsulfonylbenzoic acid Chemical compound CS(=O)(=O)C1=CC=CC=C1C(O)=O BZSXEZOLBIJVQK-UHFFFAOYSA-N 0.000 claims description 54
- 239000012528 membrane Substances 0.000 claims description 40
- 238000004519 manufacturing process Methods 0.000 claims description 31
- 239000002253 acid Substances 0.000 claims description 30
- 238000006243 chemical reaction Methods 0.000 claims description 27
- 150000001340 alkali metals Chemical class 0.000 claims description 23
- 238000005341 cation exchange Methods 0.000 claims description 23
- 239000012429 reaction media Substances 0.000 claims description 22
- OKKJLVBELUTLKV-UHFFFAOYSA-N Methanol Chemical compound OC OKKJLVBELUTLKV-UHFFFAOYSA-N 0.000 claims description 18
- 229910001413 alkali metal ion Inorganic materials 0.000 claims description 18
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 claims description 16
- 230000020477 pH reduction Effects 0.000 claims description 14
- 239000012266 salt solution Substances 0.000 claims description 14
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 14
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims description 13
- 239000001301 oxygen Substances 0.000 claims description 13
- 229910052760 oxygen Inorganic materials 0.000 claims description 13
- 239000002002 slurry Substances 0.000 claims description 13
- 239000011734 sodium Substances 0.000 claims description 13
- 239000000460 chlorine Substances 0.000 claims description 12
- 229910052801 chlorine Inorganic materials 0.000 claims description 12
- ZAMOUSCENKQFHK-UHFFFAOYSA-N Chlorine atom Chemical compound [Cl] ZAMOUSCENKQFHK-UHFFFAOYSA-N 0.000 claims description 11
- IYGFDEZBVCNBRU-UHFFFAOYSA-L disodium sulfuric acid sulfate Chemical compound [H+].[H+].[H+].[H+].[Na+].[Na+].[O-]S([O-])(=O)=O.[O-]S([O-])(=O)=O.[O-]S([O-])(=O)=O IYGFDEZBVCNBRU-UHFFFAOYSA-L 0.000 claims description 11
- QAOWNCQODCNURD-UHFFFAOYSA-L Sulfate Chemical compound [O-]S([O-])(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-L 0.000 claims description 10
- 239000011260 aqueous acid Substances 0.000 claims description 10
- 238000007254 oxidation reaction Methods 0.000 claims description 10
- DGAQECJNVWCQMB-PUAWFVPOSA-M Ilexoside XXIX Chemical compound C[C@@H]1CC[C@@]2(CC[C@@]3(C(=CC[C@H]4[C@]3(CC[C@@H]5[C@@]4(CC[C@@H](C5(C)C)OS(=O)(=O)[O-])C)C)[C@@H]2[C@]1(C)O)C)C(=O)O[C@H]6[C@@H]([C@H]([C@@H]([C@H](O6)CO)O)O)O.[Na+] DGAQECJNVWCQMB-PUAWFVPOSA-M 0.000 claims description 9
- 239000002609 medium Substances 0.000 claims description 9
- 230000003647 oxidation Effects 0.000 claims description 9
- 230000009467 reduction Effects 0.000 claims description 9
- 229910052708 sodium Inorganic materials 0.000 claims description 8
- 150000008044 alkali metal hydroxides Chemical class 0.000 claims description 7
- 239000002585 base Substances 0.000 claims description 7
- GPRLSGONYQIRFK-UHFFFAOYSA-N hydron Chemical compound [H+] GPRLSGONYQIRFK-UHFFFAOYSA-N 0.000 claims description 6
- 230000007935 neutral effect Effects 0.000 claims description 6
- 239000000376 reactant Substances 0.000 claims description 6
- 229920006395 saturated elastomer Polymers 0.000 claims description 6
- MYMOFIZGZYHOMD-UHFFFAOYSA-N Dioxygen Chemical compound O=O MYMOFIZGZYHOMD-UHFFFAOYSA-N 0.000 claims description 4
- 238000009835 boiling Methods 0.000 claims description 4
- 238000010349 cathodic reaction Methods 0.000 claims description 4
- 239000013078 crystal Substances 0.000 claims description 4
- 229910001882 dioxygen Inorganic materials 0.000 claims description 4
- 239000000446 fuel Substances 0.000 claims description 3
- 239000012736 aqueous medium Substances 0.000 claims 1
- 229910001948 sodium oxide Inorganic materials 0.000 claims 1
- FKNQFGJONOIPTF-UHFFFAOYSA-N Sodium cation Chemical compound [Na+] FKNQFGJONOIPTF-UHFFFAOYSA-N 0.000 abstract description 14
- 239000000463 material Substances 0.000 abstract description 6
- 230000003247 decreasing effect Effects 0.000 abstract description 5
- 230000015572 biosynthetic process Effects 0.000 abstract description 3
- 235000011121 sodium hydroxide Nutrition 0.000 description 24
- 239000000047 product Substances 0.000 description 10
- 238000005868 electrolysis reaction Methods 0.000 description 9
- XTEGARKTQYYJKE-UHFFFAOYSA-N chloric acid Chemical compound OCl(=O)=O XTEGARKTQYYJKE-UHFFFAOYSA-N 0.000 description 8
- 229940005991 chloric acid Drugs 0.000 description 8
- 230000000694 effects Effects 0.000 description 7
- 239000012527 feed solution Substances 0.000 description 7
- MHAJPDPJQMAIIY-UHFFFAOYSA-N Hydrogen peroxide Chemical compound OO MHAJPDPJQMAIIY-UHFFFAOYSA-N 0.000 description 4
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 4
- 239000007844 bleaching agent Substances 0.000 description 4
- QAOWNCQODCNURD-UHFFFAOYSA-M hydrogensulfate Chemical compound OS([O-])(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-M 0.000 description 4
- 239000003014 ion exchange membrane Substances 0.000 description 4
- 125000000129 anionic group Chemical group 0.000 description 3
- 125000002091 cationic group Chemical group 0.000 description 3
- 230000007423 decrease Effects 0.000 description 3
- 238000002474 experimental method Methods 0.000 description 3
- 239000007789 gas Substances 0.000 description 3
- 150000002500 ions Chemical class 0.000 description 3
- 230000005012 migration Effects 0.000 description 3
- 238000013508 migration Methods 0.000 description 3
- 238000001556 precipitation Methods 0.000 description 3
- 150000003839 salts Chemical class 0.000 description 3
- 229910000342 sodium bisulfate Inorganic materials 0.000 description 3
- 159000000000 sodium salts Chemical class 0.000 description 3
- 229920000557 Nafion® Polymers 0.000 description 2
- FAPWRFPIFSIZLT-UHFFFAOYSA-M Sodium chloride Chemical compound [Na+].[Cl-] FAPWRFPIFSIZLT-UHFFFAOYSA-M 0.000 description 2
- 238000005299 abrasion Methods 0.000 description 2
- 150000001450 anions Chemical class 0.000 description 2
- 230000008901 benefit Effects 0.000 description 2
- 230000003139 buffering effect Effects 0.000 description 2
- 239000003518 caustics Substances 0.000 description 2
- 239000003638 chemical reducing agent Substances 0.000 description 2
- 238000010960 commercial process Methods 0.000 description 2
- 230000001351 cycling effect Effects 0.000 description 2
- 230000001419 dependent effect Effects 0.000 description 2
- 238000003487 electrochemical reaction Methods 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-M hydroxide Chemical compound [OH-] XLYOFNOQVPJJNP-UHFFFAOYSA-M 0.000 description 2
- 230000006872 improvement Effects 0.000 description 2
- 230000001965 increasing effect Effects 0.000 description 2
- 239000007788 liquid Substances 0.000 description 2
- 238000002156 mixing Methods 0.000 description 2
- 238000012986 modification Methods 0.000 description 2
- 230000004048 modification Effects 0.000 description 2
- 229910052759 nickel Inorganic materials 0.000 description 2
- 239000002244 precipitate Substances 0.000 description 2
- 238000004064 recycling Methods 0.000 description 2
- 239000011833 salt mixture Substances 0.000 description 2
- 238000004448 titration Methods 0.000 description 2
- VEXZGXHMUGYJMC-UHFFFAOYSA-M Chloride anion Chemical compound [Cl-] VEXZGXHMUGYJMC-UHFFFAOYSA-M 0.000 description 1
- 229920003935 Flemion® Polymers 0.000 description 1
- ZWBFTAKXOQWXLQ-UHFFFAOYSA-L S(=O)(=O)(O)O.S(=O)(=O)(O)O.[Na+].S(=O)(=O)([O-])[O-].S(=O)(=O)(O)O.[Na+].S(=O)(=O)(O)O Chemical compound S(=O)(=O)(O)O.S(=O)(=O)(O)O.[Na+].S(=O)(=O)([O-])[O-].S(=O)(=O)(O)O.[Na+].S(=O)(=O)(O)O ZWBFTAKXOQWXLQ-UHFFFAOYSA-L 0.000 description 1
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 1
- 230000002378 acidificating effect Effects 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- 239000003513 alkali Substances 0.000 description 1
- 238000004061 bleaching Methods 0.000 description 1
- 150000001768 cations Chemical class 0.000 description 1
- 238000005660 chlorination reaction Methods 0.000 description 1
- 150000001804 chlorine Chemical class 0.000 description 1
- 239000012141 concentrate Substances 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 238000001816 cooling Methods 0.000 description 1
- 239000002178 crystalline material Substances 0.000 description 1
- 238000000354 decomposition reaction Methods 0.000 description 1
- 239000003085 diluting agent Substances 0.000 description 1
- 238000004090 dissolution Methods 0.000 description 1
- 230000005684 electric field Effects 0.000 description 1
- 238000000909 electrodialysis Methods 0.000 description 1
- 239000003792 electrolyte Substances 0.000 description 1
- 230000002708 enhancing effect Effects 0.000 description 1
- 238000001704 evaporation Methods 0.000 description 1
- 230000008020 evaporation Effects 0.000 description 1
- 125000000524 functional group Chemical group 0.000 description 1
- 150000002431 hydrogen Chemical class 0.000 description 1
- 125000002887 hydroxy group Chemical group [H]O* 0.000 description 1
- 238000005342 ion exchange Methods 0.000 description 1
- 239000012452 mother liquor Substances 0.000 description 1
- 229920000642 polymer Polymers 0.000 description 1
- 230000001376 precipitating effect Effects 0.000 description 1
- 238000002360 preparation method Methods 0.000 description 1
- 238000000746 purification Methods 0.000 description 1
- 238000009877 rendering Methods 0.000 description 1
- WBHQBSYUUJJSRZ-UHFFFAOYSA-M sodium bisulfate Chemical compound [Na+].OS([O-])(=O)=O WBHQBSYUUJJSRZ-UHFFFAOYSA-M 0.000 description 1
- 239000011780 sodium chloride Substances 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 238000006467 substitution reaction Methods 0.000 description 1
- 229910052717 sulfur Inorganic materials 0.000 description 1
- 239000011593 sulfur Substances 0.000 description 1
- 150000003467 sulfuric acid derivatives Chemical class 0.000 description 1
- TXEYQDLBPFQVAA-UHFFFAOYSA-N tetrafluoromethane Chemical compound FC(F)(F)F TXEYQDLBPFQVAA-UHFFFAOYSA-N 0.000 description 1
- 210000000689 upper leg Anatomy 0.000 description 1
Classifications
-
- Y02E60/366—
Landscapes
- Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
- Inorganic Compounds Of Heavy Metals (AREA)
Abstract
The formation of sodium sulfate by-product in sulfuric acid-based chlorine dioxide generating processes is decreased and preferably eliminated entirely, by effecting electrochemical treatment of sodium ion-containing feed materials for the generator to remove sodium ions and add hydrogen ions. Sodium hydroxide is produced as a by-product. The process is generally applicable to the electrochemical treatment of alkali metal chlorates, alkali metal sulfates and mixtures thereof to produce acidified solutions useful in providing chlorate ion-containing feeds to such chlorine dioxide generating processes.
Description
a~_ 2 0 18 5 0 7 ELECTROCHEMICAL PROCESSING OF AQUEOUS SOLUTIONS
The present invention relates to the electrochemical processing of aqueous solutions to produce acidic media for a plurality of applications, in particular in the production of chlorine dioxide.
Chlorine dioxide, useful as a pulp mill bleaching agent, is produced chemically by reduction of an acid aqueous chlorate solution in accordance with the a l0 equation:
C103' + 2H; + e' -~ C102 + HZO
where the electron e' is supplied by various reducing agents, for example, methanol, chloride ion and hydrogen peroxide. In many commercial processes for effecting this reaction, the acidity for the process is provided by sulfuric acid while the chlorate ions are provided by sodium chlorate. The presence of these species leads to the formation of some form of sodium sulfate as a by-product.
One particular embodiment of a commercial process is the so-called "R8" process of the assignee of this application, as described in U.S. Patent No.
4,081,520, assigned to the applicant herein.
Improvements in and modifications to that process also are described in the appl.icant's U.S. Patents Nos.
4,465,658, 4,473,540 and 4,E~27,969.
In that chlorine dioxide generating process, the reaction medium is at a Thigh total acid normality of sulfuric acid and is maintained at its boiling point under a subatmospheric pressure applied thereto.
Methanol is used as a reducing agent for chlorate ions, resulting in the formation of chlorine dioxide in a substantially pure form. The boiling nature of the reaction medium produces stsaam which acts as a diluent for the gaseous chlorine dioxide, so as to prevent decomposition of the chlorine dioxide.
The sodium sulfate by-product builds up in the reaction medium after start:-up until the solution is saturated with sodium su7lfate, whereupon the sodium sulfate precipitates from the reaction medium. A slurry of the sodium sulfate is removed from the reaction vessel, the crystalline sodium sulfate is filtered therefrom and the mother liquor is recycled to the reaction zone after the addition of make-up quantities of sodium chlorate, sulfuric acid and methanol.
This process is highly efficient and rapidly produces chlorine dioxide in commercial quantities. As may be concluded from the above equation, for each mole of chlorine dioxide produced a mole of chlorate ion and hence of sodium ion is introduced to the reaction medium.
The sodium ions combine with the sulfate ions introduced with the sulfuric acid, to produce a sodium sulfate, which may be sodium bisulfai:e or, more normally under the conditions of an R-8 process, the double salt sodium sesquisulfate, i.e., Na,H(S04)Z (or NaHS04.NaZS04), depending on the acidity of the solution.
Another sulfuric acid-based chlorine dioxide generating process, a low acidity "R3" process, as described in U.S. Patent No. 3,864,456, produces neutral sodium sulfate as the by-product.
Such by-product sodium sulfate and sodium sesquisulfate (sometimes termed "saltcake"), generally have been employed to make up sulfur losses in the pulp mill.
However, the adoption of high substitution of chlorine by chlorine dioxif~,e in the chlorination stage .. 201807 of the bleach plant has led to saltcake by-product production from the chlorine. dioxide generating process exceeding the mill make-up requirements.
There exists a need, therefore, for a chlorine dioxide generating process which possesses the attributes of, for example, the R8 process, while, at the same time, producing less sodium sulfate by-product for the same level of production of chlorine dioxide.
It is even more advantageous if, in addition to a lower saltcake production, caustic soda solution is co-produced together with C102, thus minimizing an NaOH/C12 imbalance presently existing in pulp mills.
It has previously been suggested in U.S.
Patent No. 4,129,484 to treat aqueous effluent from chlorine dioxide generating processes electrolytically to form an acid-enriched fraction from the original solution, which then may be recycled to the chlorine dioxide generator.
In order to utilize the by-product saltcake, it was proposed in the prior art to employ an electrochemical process to convert sodium sulfate into sulfuric acid and caustic soda solution in a three compartment electrolytic cell, equipped with a cation exchange membrane facing t:he cathode and an anionic membrane or a diaphragm facing the anode, wherein the saltcake solution is fed to the middle compartment. In an electric field, sodium and sulfate or hydrogen sulfate ions are transferred to the cathodic and anodic compartments respectively where they recombine with electrolytically-generated hydroxyl and hydrogen ions to form caustic soda and sulfuric acid, respectively.
Analogously, in a simplified process, a two-compartment electrolytic cell equipped with a cation exchange membrane was proposed to generate a mixture of sulfate and sulfuric acid in an anodic compartment along with caustic soda solution in the cathodic compartment.
The present invention relates to the electrochemical processing of aqueous solutions to produce acidic media for a plurality of applications, in particular in the production of chlorine dioxide.
Chlorine dioxide, useful as a pulp mill bleaching agent, is produced chemically by reduction of an acid aqueous chlorate solution in accordance with the a l0 equation:
C103' + 2H; + e' -~ C102 + HZO
where the electron e' is supplied by various reducing agents, for example, methanol, chloride ion and hydrogen peroxide. In many commercial processes for effecting this reaction, the acidity for the process is provided by sulfuric acid while the chlorate ions are provided by sodium chlorate. The presence of these species leads to the formation of some form of sodium sulfate as a by-product.
One particular embodiment of a commercial process is the so-called "R8" process of the assignee of this application, as described in U.S. Patent No.
4,081,520, assigned to the applicant herein.
Improvements in and modifications to that process also are described in the appl.icant's U.S. Patents Nos.
4,465,658, 4,473,540 and 4,E~27,969.
In that chlorine dioxide generating process, the reaction medium is at a Thigh total acid normality of sulfuric acid and is maintained at its boiling point under a subatmospheric pressure applied thereto.
Methanol is used as a reducing agent for chlorate ions, resulting in the formation of chlorine dioxide in a substantially pure form. The boiling nature of the reaction medium produces stsaam which acts as a diluent for the gaseous chlorine dioxide, so as to prevent decomposition of the chlorine dioxide.
The sodium sulfate by-product builds up in the reaction medium after start:-up until the solution is saturated with sodium su7lfate, whereupon the sodium sulfate precipitates from the reaction medium. A slurry of the sodium sulfate is removed from the reaction vessel, the crystalline sodium sulfate is filtered therefrom and the mother liquor is recycled to the reaction zone after the addition of make-up quantities of sodium chlorate, sulfuric acid and methanol.
This process is highly efficient and rapidly produces chlorine dioxide in commercial quantities. As may be concluded from the above equation, for each mole of chlorine dioxide produced a mole of chlorate ion and hence of sodium ion is introduced to the reaction medium.
The sodium ions combine with the sulfate ions introduced with the sulfuric acid, to produce a sodium sulfate, which may be sodium bisulfai:e or, more normally under the conditions of an R-8 process, the double salt sodium sesquisulfate, i.e., Na,H(S04)Z (or NaHS04.NaZS04), depending on the acidity of the solution.
Another sulfuric acid-based chlorine dioxide generating process, a low acidity "R3" process, as described in U.S. Patent No. 3,864,456, produces neutral sodium sulfate as the by-product.
Such by-product sodium sulfate and sodium sesquisulfate (sometimes termed "saltcake"), generally have been employed to make up sulfur losses in the pulp mill.
However, the adoption of high substitution of chlorine by chlorine dioxif~,e in the chlorination stage .. 201807 of the bleach plant has led to saltcake by-product production from the chlorine. dioxide generating process exceeding the mill make-up requirements.
There exists a need, therefore, for a chlorine dioxide generating process which possesses the attributes of, for example, the R8 process, while, at the same time, producing less sodium sulfate by-product for the same level of production of chlorine dioxide.
It is even more advantageous if, in addition to a lower saltcake production, caustic soda solution is co-produced together with C102, thus minimizing an NaOH/C12 imbalance presently existing in pulp mills.
It has previously been suggested in U.S.
Patent No. 4,129,484 to treat aqueous effluent from chlorine dioxide generating processes electrolytically to form an acid-enriched fraction from the original solution, which then may be recycled to the chlorine dioxide generator.
In order to utilize the by-product saltcake, it was proposed in the prior art to employ an electrochemical process to convert sodium sulfate into sulfuric acid and caustic soda solution in a three compartment electrolytic cell, equipped with a cation exchange membrane facing t:he cathode and an anionic membrane or a diaphragm facing the anode, wherein the saltcake solution is fed to the middle compartment. In an electric field, sodium and sulfate or hydrogen sulfate ions are transferred to the cathodic and anodic compartments respectively where they recombine with electrolytically-generated hydroxyl and hydrogen ions to form caustic soda and sulfuric acid, respectively.
Analogously, in a simplified process, a two-compartment electrolytic cell equipped with a cation exchange membrane was proposed to generate a mixture of sulfate and sulfuric acid in an anodic compartment along with caustic soda solution in the cathodic compartment.
The main drawback of these prior proposals was that the sulfuric acid solution produced had a low acid strength (less than 10 w~t% HZS04) , which imposes an excessive evaporative load on the chlorine dioxide generator, thereby rendering the process uneconomical and impractical.
Although higher aulfuric acid concentrations can be achieved in the electrochemical splitting of saltcake in the manner described in the prior art, the current efficiency for such a process is prohibitively low due to the leakage of H; ions through the ion-exchange membrane. Such migration of hydrogen ions towards the cathode is related to a very high mobility of this ion relative to Na' io;ns.
For example, in the aforementioned U.S. Patent No. 4,129,484, current efl:iciencies as low as 9% for production of about 1 normal caustic-soda solution and 39% for production of about 2M sulfuric acid were reported.
In order to count~.ract the undesired migration of hydrogen ions towards the cathode and hence maximize the current efficiency for an acidification process, the electrolytic process is eff~acted, in accordance with the invention, while maintaining a high concentration ratio of Na+/H~ in the anolyte.
In one embodiment of the present invention, there is provided an electrochemical process for the treatment of sodium sulfate,isodium chlorate mixtures, so as to acidify the same and provide acid for the chlorine dioxide generating process, while coproducing aqueous sodium hydroxide solution. Although the co-production of aqueous sodium hydroxide solution is most desired for the pulp mill applications, any other suitable cathodic reactions, for example, an electroreduction of oxygen to hydrogen peroxide and sodium hydroxide, can be carried z o~s~o7 out in combination with the anodic acidification of the salt mixtures.
The process of the invention is more widely applicable than to the treatment of such mixtures.
Although higher aulfuric acid concentrations can be achieved in the electrochemical splitting of saltcake in the manner described in the prior art, the current efficiency for such a process is prohibitively low due to the leakage of H; ions through the ion-exchange membrane. Such migration of hydrogen ions towards the cathode is related to a very high mobility of this ion relative to Na' io;ns.
For example, in the aforementioned U.S. Patent No. 4,129,484, current efl:iciencies as low as 9% for production of about 1 normal caustic-soda solution and 39% for production of about 2M sulfuric acid were reported.
In order to count~.ract the undesired migration of hydrogen ions towards the cathode and hence maximize the current efficiency for an acidification process, the electrolytic process is eff~acted, in accordance with the invention, while maintaining a high concentration ratio of Na+/H~ in the anolyte.
In one embodiment of the present invention, there is provided an electrochemical process for the treatment of sodium sulfate,isodium chlorate mixtures, so as to acidify the same and provide acid for the chlorine dioxide generating process, while coproducing aqueous sodium hydroxide solution. Although the co-production of aqueous sodium hydroxide solution is most desired for the pulp mill applications, any other suitable cathodic reactions, for example, an electroreduction of oxygen to hydrogen peroxide and sodium hydroxide, can be carried z o~s~o7 out in combination with the anodic acidification of the salt mixtures.
The process of the invention is more widely applicable than to the treatment of such mixtures.
5 Rather the invention is broadly directed to the treatment of aqueous solutions of certain alkali metal salts and mixtures thereof to form an aqueous acidified medium.
In one aspect, the present invention is broadly directed to a method for the production of an aqueous acidified chlorate ion-containing solution which is useful on a feed stream for a chlorine dioxide generating process wherein chlorate ions are reduced to chlorine dioxide in an aqueous acid medium.
An aqueous solution of at least one alkali metal salt selected from the group consisting of alkali metal chlorate, alkali metal sulfate and mixtures of alkali metal chlorate and alkali metal sulfate is electrochemically acidified.
Alkali metal is>ns are electrochemically removed from the aqueous solution to produce an acidified alkali metal salt solution which constitutes the aqueous acidified chlorate ion-containing solution when the alkali metal salt is selected from the group consisting of alkali metal chlorate and mixtures of alkali metal chlorate and alkali metal sulfate.
When the alkali metal salt is alkali metal sulfate, alkali metal chlorate is added to the acidified alkali metal salt solution to provide the aqueous acidified chlorate ion-containing solution.
One manner of effecting the procedure employs an electrolytic cell comprising a cation-exchange membrane dividing the cell into an anode compartment and a cathode compartment. The aqueous solution of at least one alkali metal salt is fed to the anode compartment and hydrogen ions are elect.rolytically produced in the anode compartment while alkali metal ions are transferred from the anode compartment through the cation-exchange membrane to the cathode compartment.
The acidified alkali metal. salt solution is removed from the anode compartment.
Alternatively, an.y other oxidation reaction producing hydrogen ions, for example, hydrogen gas oxidation to hydrogen ions, may be employed as an anodic reaction. Such anodic hydrogen gas oxidation may be combined with oxygen gas electroreduction as a cathodic reaction, to provide a fuel cell operation wherein, in addition to acidification of anolyte and production of alkali metal hydroxide solution in the catholyte, electrical energy is generated.
When an anodic o~,;idation of hydrogen gas to hydrogen ions is combined with a cathodic reduction of water to hydroxyl ions and hydrogen gas, the latter gas may be used as an anodic feed and, at the same time, a substantial energy savings nnay be achieved, as a result of the difference in electrochemical reaction potentials. Similarly, an analogous energy and material savings is achieved when an anodic oxidation of water to hydrogen ions and oxygen is combined with cathodic reduction of the oxygen gas stream.
In accordance with one embodiment of the present invention, there is provided an electrochemical process for the treatment of an aqueous solution of alkali metal salt mixtures, which comprises a plurality of steps. The aqueous so7.ution is fed to the anode compartment of an electrolytic cell having a cation-exchange membrane dividing the cell into an anode compartment and a cathode compartment.
Hydrogen ions are electrolytically produced in the anode compartment and hydroxyl ions are electrolytically produced in the cathode compartment.
Simultaneously alkali metal cations are transferred .__ Zp ~g5o7 across the cation-exchange membrane from the anode compartment to the cathode compartment.
This migration of alkali metal cations and the electrochemical reaction ;producing hydrogen ions and hydroxyl ions have the effect of producing an alkali metal hydroxide solution in the cathode compartment and an acid of the anion of the alkali metal salts in the anode compartment. The respective aqueous solutions are removed from the compartments of the cell.
In order to achieave high current efficiencies of at least about 70%, prefcarably at least about 80%, and thereby provide an economic: process, the molar ratio of [Na+]:[H+] in the anolyte generally varies from about 1000:1 to about 1:5, prefcarably about 1000:1 to about 1:2, throughout the electrolytic reaction.
In another aspect of the present invention, the method for the production of aqueous acidified chlorate ion-containing solution is, integrated into a chlorine dioxide generating process by providing an aqueous acid chlorine dioxide-generating reaction medium containing sulfuric acid and alkali mEatal ions in a reaction zone, feeding the aqueous acidified chlorate ion-containing solution to the reaction zone to provide chlorate ion and hydrogen ion reactants to the aqueous acid chlorine dioxide-generating reaction medium, and recovering an alkali metal sulfate from t:he reaction zone.
As noted above, the present invention is directed generally to the electrochemical treatment of aqueous solutions of alkali metal salts and mixtures thereof to generate an aqueous acidified medium. The procedure of the present invention is particularly useful in sulfuric acid-based chlorine dioxide generating processes, since the invention enables the quantity of by-product alkali metal sulfate co-produced with the chlorine dioxide to be decreased and even eliminated entirely, while maintaining the same level of chlorine dioxide production.
In one embodiment of the present invention, an aqueous solution of at least one alkali metal salt which is alkali metal chlorate, alkali metal sulfate or, preferably, mixtures thereof: is introduced to the anode compartment of a cation-exchange membrane divided electrochemical cell and alkali metal ions are transferred from the anode compartment to the cathode compartment. This procedure may be used in the present invention in different ways to achieve the decreased alkali metal sulfate output in a chlorine dioxide generating process. It is usual for chlorine dioxide generating processes to employ sodium salts, and hence in the following description of specific embodiments of the invention reference will be made to those sodium salts.
However, it is apparent that the principles of the invention are applicable to salts of other alkali metals.
In a preferred embodiment of the invention, a combination of sodium chlorate and sodium sulfate, particularly in the form of aodium sesquisulfate, is fed to the anode compartment of 'the cell, since the presence of both salts enables the [rfa+]/ [H'] molar ratio in the anode compartment to be maximized, resulting in a high current efficiency and thereby lower power consumption. In addition, a higher overall current efficiency decreases the capital cost, since a lesser number of cells is required for the same production rate.
It is preferred to employ a saturated feed solution or a slurry, in order to minimize the resistance of the electrolylte, while maintaining a high [Na+]/[H+] ratio in the anode compartment. When the feed is in the form of a slurry, it usually is preferred to provide an additional separator or separators, such as an ion-exchange membrane or a diaphragm, between the feed stream and the anode t:o prevent abrasion and wear on the anode from occurring. Such additional separator also may be employed, if deaired, if the feed does not comprise a slurry.
The cell produces an acidified solution from the mixed sodium chlorate/sodium sulfate feed in the anode compartment and a sodium hydroxide solution in the cathode compartment. The acidified solution then is fed to the chlorine dioxide generating process as an acid and chlorate source therefor.
When there is no~ need for the saltcake in particular pulp mill, it is preferred to operate the cell so as to remove from the mixed sodium chlorate/sodium sulfate feed a quantity of sodium ions corresponding to the quantii~y of sodium ions introduced with the sodium chlorate. The requirements of the chlorine dioxide generator for acid and chlorate thereby are completely satisfied, so that no additional sodium sulfate by-product is formed. Therefore, in effect, a dead load of sodium sulfate cycles between the chlorine dioxide generator and the cell.
When operating the process of the invention with sodium chlorate/sodium sulfate mixtures, it is preferred to dissolve sodium chlorate in the sodium sulfate solution. Alternatively, the sodium sulfate, in slurry or crystalline form, may be added to an aqueous sodium chlorate solution or, if desired, aqueous 5 solutions of sodium sulfate and sodium chlorate may be mixed in any required proportion.
The composition of the anolyte feed may be prepared by a one-time mixing of the individual components, namely sodium sulfate and sodium chlorate, 10 or by a gradual addition of one component to another in the course of electrolysis. For example, sodium sulfate can be pre-acidified in an electrolytic cell prior to the addition of sodium chlorate. In fact, the process can be carried out in such. a way that neutral sodium sulfate or sodium sesqu.isulfate is acidified to bisulfate, NaHS04, and then this solution is used to dissolve sodium chlorate, in order to take advantage of an increased solubility of bisulfate with respect to other sulfates. The mixture can be forwarded to the generator or further acidified in an electrolytic cell.
From this discussion, it will be apparent that any combination of sodium ulfate and sodium chlorate may be selected to meet the requirements of a particular pulp mill. Production of sodium sulfate by-product may be permitted at any level desired by the pulp mill.
The presence of sodium sulfate (sesquisulfate) in the anolyte improves the [Na+]/[H+] ratio not only due to an increase in sodium ion concentration but also as a result of a buffering action of the sulfate ions, which tend to recombine with hydrogen ions to form bisulfate, thereby decreasing the concentration of free hydrogen ions. In addition, a combined acidified chlorate/sulfate feed effectively decreases the amount of water introduced to the chlorine dioxide generator, as a result of a water "sharing" effect.
1:1 While operating the process using sodium chlorate/sodium sulfate mixitures provides a practically unlimited flexibility in selecting the extent of production of a by-product. saltcake corresponding to between 0 to 100% recycling of sulfate, it also is possible to achieve a partial decrease in saltcake production by electrolytically acidifying a pure sodium chlorate feed solution, i.e. without any addition of sulfate. The overall reaction for such a process may be depicted as:
xNaC103 + 3H20 1 (x-2) NaC103 + 2HC103 + 2NaOH
+ z02 + H2 Such a partial acidification. of sodium chlorate leads to a mixture of sodium chlorate and chloric acid, in which the content of chloric acid determines the extent of reduction in sulfuric acid addition to the chlorine dioxide generator and, consequently, the production of sodium sulfate by-product. Since the [Na+]/[H+] ratio constitutes a determining factor for the current efficiency of the acidification process, only partial conversion of sodium chlorate into chloric acid is feasible in this case. Usually, up to about a 20%
reduction in saltcake by-product production can be achieved by acidification of a pure sodium chlorate feed at practical current efficiencies.
The aqueous sodium hydroxide solution which is co-produced in the present invention is readily used by the pulp mill in its bleach plant operations for the purification of bleached pu:Lp. When pulp mills have an on-site electrolytic process for producing aqueous sodium hydroxide, usually from sodium chloride, Such process results in the co-production of chlorine. The ability to utilize such co--produced chlorine, however, is often limited, which leads to a caustic/chlorine imbalance in the pulp mill. By utilizing the process of the invention, not only is the problem of co-production x,018507 of excess sodium sulfate in the chlorine dioxide generation process overcome, but also the problem of co-production of chlorine in sa~dium hydroxide production is overcome. The concentration of sodium hydroxide solution produced in the cathode compartment may be adjusted to any reasonable level, by adjusting flow rates and recycling product solution.
As noted above, tile current efficiency of the process is largely dependent on the [Na+]/[H+] ratio in the anode compartment, which. usually requires a deadload of sodium ion cycling between the cell and the chlorine dioxide generator which, i.n the embodiment disclosed above, may be provided by sodium sulfate as the deadload. However, any other sodium salt which does not otherwise adversely affect the production of chlorine dioxide may be employed.
The anode employed in the electrolytic cell may take any desired form, but it is preferred to employ a low overpotential one with respect to the oxygen evolution reaction, for example, a DSA-02~ electrode.
Similarly, any convenient material of construction may be used for the cathode, for example, nickel.
The cation-exchange membrane may be formed of any convenient material which enables cations to selectively pass therethrough in preference to anions.
Preferably, the cation-exchange membrane is formed of perfluorocarbon polymer having pendant cation-exchange functional groups, such as those sold under the trademarks "NAFION" (DuPont) or "FLEMION" (Asahi Glass).
~ In the event a:n additional ion exchange membrane or membranes is used in the anode compartment, such membrane may be formed of any convenient ion-exchange material.
In one embodiment: of the invention in which electrodialysis utilizing bipolar membranes is employed, the aqueous solution of a mixture of alkali metal salts zols~o7 1:3 is processed in a plurality of unit cells, with each unit cell being separated from the adjacent ones by bipolar membranes. The bipolar membranes have an anionic face in the base compartment of one cell and a cationic face in the acid compartment of an adjacent cell. The individual cells are divided by a cation exchange membrane.
With the plurality of cells separated by bipolar membranes, gas evolution does not take place in the acid and base compartments, and the overall reaction may be represented by the equation:
(x+y) NaC103 +y H20 -~ x rfaC103 +y HC103 +y NaOH
The plurality of cells is terminated at both ends by cationic membranes. A separate electrode rinse solution, such as a sodium sulfate solution, is circulated between the cathodic and anodic compartments adjacent to the cathode and anode respectively. A
single electrical current :Feed, therefore, is used to effect acidification in parallel in a plurality of unit cells, with gaseous evolution occurring only in the end anode and cathode compartments.
Bipolar membranes and their operation are well known and are described, for example, in U.S. Patents Nos. 4,024,043, 4,180,87L5, 4,057,481, 4,355,116, 4,116,889, 4,253,900, 4,584,246 and 4,673,454 and reference may be had to such patents for details thereof.
The parameters of: operation of the cell are not critical to the proce:>s of the invention and may vary widely. For example, the electrolytic process may be effected under any desired electrolytic conditions, generally at a membrane current density of about 0.01 to about 10 kA/m2, preferably about 1 to about 5 kA/m2.
Similarly, the process may be carried out over a wide range of temperatures, generally from about 0°
to about 150°C, preferably i_rom about 15° to about 90°C.
2o~s~o7 Generally, higher temperatures are preferred, in view of the generally greater solubility of the alkali metal salts at higher temperatures, thereby enhancing the [Na+]/[H+] ratio. This greater solubility at high temperature is particularly true of sodium chlorate, so that, when such salt is included in the feed to the cell, higher temperature operation, above about 60°C, is preferred. The acidified) product stream from the electrolysis retains a high concentration of sodium chlorate, so that cooling of that product stream prior to passage to the chlorine dioxide generator may lead to precipitation of sodium chlorate, which can be removed and recycled to the cell feed.
As already described, one important parameter influencing current efficiency is the molar ratio of [Na+]/[H+] in the anode compartment. Generally, this molar ratio varies from about 1000:1 to about 1:5, preferably about 1000:1 to about 1:2. Such a ratio is dependent on the concentration of the feed solution to the anode compartment and the extent to which sodium ions are transferred from the anode compartment to the cathode compartment. Accordingly, it is preferred to employ a feed solution having a concentration of about 0.1 to about 15M in sodium ions and to remove from the anode compartment for feed to the chlorine dioxide generation process an acidified solution having a concentration of about 0.1 to about 12M in sodium ions.
The electrolytic process increases the total acid normality of the alkali metal salt solution.
Depending on the initial total acid normality and the degree of electrolysis effected on the feed material, it may be desirable to concentrate, such as by evaporation, the product stream to increase its total acid normality, prior to feed to the chlorine dioxide generation process. Generally, the total acid normality of the feed solution varies from about neutral to about 12 .~ 2018507 normal, preferably about 0.1 to about 10 normal, and the electrolysis is effected to such a degree as to provide a product stream from the: anode compartment having a total acid normality generally from about 0.2 to about 15 5 normal, preferably about 0,.5 to about 6 normal.
The invention is described further, by way of illustration, with reference to the accompanying drawings, in which:
Figure 1 is a flaw sheet of a chlorine dioxide 10 generating process provided in accordance with one embodiment of the invention;
Figure 2 is a schematic illustration of a bipolar membrane cell which may be employed with the chlorine dioxide generating process of Figure 1 ; and 15 Figure 3 contains a graphical representation of experimental data, illustrating the relationship of acid normality to current efficiency for different concentrations of alkali metal salt.
Referring to the drawings, Figure 1 depicts an embodiment of the application of the principles of the present invention to the production of chlorine dioxide.
There is shown therein a chlorine dioxide generating operation 10 comprising a chlorine dioxide generating process 12 wherein sodium chlorate, sulfuric acid and methanol are reacted at the boiling point of the reaction medium under a ~:ub-atmospheric pressure, to produce gaseous chlorine dioxide in line 14 (i.e. the R8 process).
The chlorine dioxide generating process in the generator 12 results in the precipitation of a by-product sodium sulfate, once the reaction medium has become saturated after start-up. The form of the by-product sodium sulfate, namely neutral sodium sulfate, sodium sesquisulfate, sodium bisulfate or mixtures thereof, depends on the total acid normality of the reaction medium, which may vary from about 2 to about 11 normal.
15a The crystalline sodium sulfate (sometimes termed "saltcake" ) is removed from the reaction medium in the form of a slurry with ;spent reaction medium by line 16 and is passed to a filter 18 wherein the crystalline material is separated from the spent reaction medium.
The separated spent reaction medium in line 20 is recycled to the generator 12 with make-up reactants, namely sodium chlorate, sulfuric acid and methanol, being fed to the recycle stream i.n line 20 by lines 22, 24 and 26 respectively.
20~.8~07 The crystalline aodium sulfate separated in the filter 18 may be removed by line 28 for use elsewhere in the pulp mill in whatever proportion (including zero) of the sodium sulfate in the slurry 16 is desired.
The portion of the sodium sulfate not removed by line 28 is forwarded by line 30 to a dissolving tank 32, wherein the sodium sulfate is dissolved in water fed by line 34 to form an aqueous solution which is preferably saturated. This solution is forwarded by lines 36 and 38 to a membrane-divided electrolysis cell 40. It is not necessary that the sodium sulfate be completely dissolved in water, but may be fed as a slurry thereof. Where such a slurry is employed, it is generally desirable to isolate the anode from the slurry in the cell 40, to avoid excessive wear resulting from abrasion.
The cell 40 comprises a cation-exchange membrane 42, which divides the interior of the cell 40 into an anode compartment 44 and a cathode compartment 46 in which are located cell anode 48 and cell cathode 50 respectively.
Sodium chlorate i:~ added to the sodium sulfate solution in line 36 by line 52 to form in line 38 a feed solution for the anode' compartment 44 of the cell 40. An electrolyte is fed by line 54 to the cathode compartment 46. A current is applied between the anode 48 and the cathode 50. The sodium chlorate may be added by line 52 in the i=orm of an aqueous solution thereof or as solid sodium chlorate.
In the cell 40, several reactions occur simultaneously. At the anode 48, water is electrolyzed to oxygen and hydrogen ion, as follows:
H20 -~ X02 + 2H+ + 2e-while at the cathode 50 water is electrolyzed to hydrogen and hydroxyl ion, as follows:
~o~s~o7 e- + H20 ~ iH2 + OH-At the same time, sodium ions in the aqueous solution or slurry of a mixture of sodium sulfate and sodium chlorate migrate under then influence of the applied current from the anode compartment 44 across the cation-exchange membrane 42 to the cathode compartment 46. In effect, therefore, the electrolytically-produced hydrogen ions replace the sodium ions in the anode compartment 44 and the transferred sodium ions are available to combine with the electrolytically-produced hydroxyl ions in the cathode: compartment 46.
The sodium sulfate contained in the solution feed in line 38 to the cell 40 can be considered to be a deadload circulating via the generator 12 in a closed loop, so that the overall reaction in cell 40 can be considered to be representedl, as follows:
xNaC103 + 3H20 -~ (x-2) NaC103 + 2HC103 + 2NaOH
+ X02 + H2 where x is the molar amount of sodium chlorate which is processed.
The resulting chlo~ric acid-containing solution then is recycled by line 54 to the recycle line 20, to provide acid and chlorate ion for the chlorine dioxide generator 12. The proportion of sodium sulfate removed from the system by line 28 corresponds to the proportion of the sulfuric: acid and sodium chlorate reactants fed to the chlorine dioxide generation from external sources, namely by line 24 for sulfuric acid and by unconverted sodium chlorate in line 24 and in line 22. Under steady state operating conditions for a process precipitating sodiunn sesquisulfate and where no sodium sulfate product is required, the processing of the aqueous solution of a mixture of sodium sesquisulfate and sodium chlorate in the cell 40 provides all the hydrogen ions and chlorate ions 20~18~07 necessary to sustain the reaction and additional sulfuric acid and sodium chlorate are not required.
In effect, therefore, sodium chlorate fed to the cell 40 is converted, at: least partially, to chloric acid, so that the sodium sulfate component of the feed solution 38 is a dead-load cycling between the cell 40 and the chlorine dioxide generator 12.
Oxygen is vented from the anode compartment 44 by line 56. Alternatively,, the product stream may be recycled by line 58, with oxygen being vented from a gas-liquid separator 60 by line 62, wherein recycle of anolyte is required to achieve the desired acid normality of the acidified ~;olution 54.
The sodium hydroxide produced in the cathode compartment 46 is recycled by line 64, through a gas liquid separator 66 from which hydrogen is vented by line 68, until the desired concentration of sodium hydroxide solution is achieved. The resulting aqueous sodium hydroxide solution is removed as an aqueous product stream in line 70. This solution has considerable utility in a pulp mill, particularly as a chemical employed in th~a purifying and bleaching operations effected in the: bleach plant of the pulp mill. The gaseous by-products, namely hydrogen and oxygen, also can be utilized. in the pulp mill.
High current efficiency for the electrolytic process effected in the cell 40 as described above can be attributed to the high [lKa+]/[H+] molar ratio in the anode compartment 44 and also to the buffering effect of S042- ions towards hydrogen ions, which tends to maintain the free hydrogen ion concentration in the anode compartment low, thereby tending to maintain the high [Na+]/[H+] molar ratio.
As may be seen from the above description, the process of Figure 1 producer chlorine dioxide by the R8 process and hence retains 'the benefits thereof. More importantly, the process doEa not produce any excess by-product sodium sulfate requiring disposal. The amount of sodium sulfate which is produced can be tailored to the mill requirement, or may be eliminated entirely. In effect, when there is no requirement for the saltcake in the pulp mill, the sodium sulfate is maintained in a closed loop within the process and the sodium ions introduced to the chlorine dioxide generating process with the sodium chlorate, exit the process in the form of aqueous sodium hydroxide solution. The acid for the chlorine dioxide generai~ing process is produced electrolytically from water, which co-produces the hydroxyl ions required to combine with the sodium ions to form the sodium hydroxide..
Referring to Figure 2, there is shown therein the utilization of a bank 100 of unit cells, with the individual cells 102 producing an acidified mixture in line 104 for feeding to a chlorine dioxide generator, as described with respect to Figure 1, from an aqueous feed mixture of sodium sesquisulfate and sodium chlorate in line 106. The number of unit cells 102 in the bank of cells may vary widely, depending on the required production capacity and typically may vary from about 20 to about 500.
Each unit cell 100 is separated from each adjacent unit cell by bipolar membranes 108, 110. The bipolar membrane 108 has its cationic face in an acid compartment 112, so as to form hydrogen ions under the influence of the electric current applied to the bank of cells 100, thereby acidifying the feed mixture, while sodium ions are transported from the acid compartment 112 across a cation-exchange membrane 114 to a base compartment 116.
The bipolar membrane 110 has its anionic face in the base compartment 116, so as to form hydroxyl ions from the aqueous feed i;.hereto in line 118 under the influence of the electrical current applied to the bank of cells 100. In this way, sodium hydroxide is formed in the base compartment 116 and is removed by line 120.
Only a single anode 122 and a single cathode 5 124 are required for the bank 100 of unit cells 102.
Oxygen and hydrogen respectively are formed at the electrode surfaces and vented from the terminal unit cells.
The invention is illustrated by the following 10 Examples:
Electrochemical e~;;periments were carried out in a two-compartment MP cell, supplied by Electrocell AB, Sweden equipped with an oxygen-evolving anode (DSA-OZO), nickel cathode and a catio:n exchange membrane (NAFION
15 427) dividing the cell into an anode compartment and a cathode compartment. The anode, cathode and membrane each had an area of 100 sq, cm.
In the experiments, a current density of 3 kA/m2 was mainly employed (occasionally 2 kA/m2), the 20 anolyte was NaC103 or NaC103/NaZS04/H2S04 mixtures, the catholyte was 1N aqueous sodium hydroxide solution and the temperature was 40° to 50°C.
Example 1 In a first set of experiments, aqueous solutions of sodium chlorate. of various concentrations were used as the anolyte. During the course of electrolysis, the anolyte became enriched with hydrogen ions as the sodium chlorate was partially converted to chloric acid. Current efficiencies were determined at various product chloric acid concentrations for the various initial sodium chlorate concentrations and were plotted graphically. These results appear as Figure 3.
It will be seen from the data presented in this Figure that the current efficiency declines with increasing H' to Na' moles ratio in the solution electrolyzed.
~,018~07 Example 2 Various mixtures of sodium chlorate and Na2S04/H2S04 were prepared and electrolyzed.
(a) 1 L of an approximately 2M Na3H(S04)2 aqueous solution was prepared by mixing Na2S04 and H2S04 at a molar ratio of 3:1. Acidii~y was measured by titration with NaOH and the solution was determined to have an initial concentration of 1.94 normal. The catholyte was 0.5L of 1N NaOH.
Current was imposed between the electrodes at a current density of 3 kA/m2 and sodium chlorate crystals, in a total amount corresponding to the preparation of a 2M aqueous solution, were added slowly to the anolyte. A very rapid dissolution of sodium chlorate was observed. The increase in acidity of the anolyte was monitored by titration and the electrolysis was terminated when the acidity of the anolyte reached 4.12 N. The basicity of they catholyte was determined to be 4.54 N NaOH. The total tame of electrolysis was 8583 seconds with the total charge passed being 257,490 A.sec.
In the calculation of the current efficiency, the volume changes of the anolyte and catholyte were taken into account, since 'water is transported to the catholyte together with Na+ ions and also is consumed in the electrolysis. The current efficiency based on the increase in acidity of the anolyte was determined to be 79~ while that based on the increase in basicity of the catholyte was 80~.
The overall conversion in the anolyte can be depicted as:
2 M Na3H(S04)2 + 2 M NaC103 ~ 2 M Na3H (S04)2 + 2 M HC103 with an equivalent amount of caustic being co-produced in the cathode compartment:. Hydrogen (cathode) and oxygen (anode) were the gaseous by-products. All the reactants and products depicted by this equation are largely dissociated in the solution into the ionic species Na', H+, HS04'- 504~~' and C103'~ However, since sesquisulfate contributes a prevailing form of the precipitate formed in the chlorine dioxide generator, chloric acid (HC103 = H~ + C:L03'~ is liberated from such a mixture upon precipitation of sesquisulfate.
(b) Other mixtures of sodium chlorate and sodium sesquisulfate were processed in a similar way to that described in experiment (a), with quantitative (1 to 4) or partial (5) conversion of the sodium chlorate to chloric acid. The results obtained are reproduced in the following Table I:
22a O oW
.,..y.
~7 W O ~ ~ tMD 0~0 U
N
y N~
H
N M M N M
A
U
~o o~ o .-.
z ~ M M M
N N
v H
O ~ O N
d' O 01 N e-~ rl e-i O
~ tf7 U~ rl N N N d' z N r-i H H r-1 .~.7 z rl N M d' tt7 :~2b It will be seen from this: Table and the data in (a) above, that an improvement in current efficiency is obtained by the presence of sulfate ion in conjunction with a high [Na+]/ [H+] ratio, as compared with the results obtained in Example 1. Fo:r example, l.5 M HC103 can be obtained at about 60% C,.E. in pure aqueous sodium chlorate solution and at about 80% C.E. in the mixtures of sodium sesquisulfate and sodium chlorate.
In summary of this disclosure, the present invention provides a novel electrochemical process which enable the quantity of by-product sodium sulfate produced by a chlorine dioxide generating process to be decreased.
Modifications are possible within the scope of this invention.
In one aspect, the present invention is broadly directed to a method for the production of an aqueous acidified chlorate ion-containing solution which is useful on a feed stream for a chlorine dioxide generating process wherein chlorate ions are reduced to chlorine dioxide in an aqueous acid medium.
An aqueous solution of at least one alkali metal salt selected from the group consisting of alkali metal chlorate, alkali metal sulfate and mixtures of alkali metal chlorate and alkali metal sulfate is electrochemically acidified.
Alkali metal is>ns are electrochemically removed from the aqueous solution to produce an acidified alkali metal salt solution which constitutes the aqueous acidified chlorate ion-containing solution when the alkali metal salt is selected from the group consisting of alkali metal chlorate and mixtures of alkali metal chlorate and alkali metal sulfate.
When the alkali metal salt is alkali metal sulfate, alkali metal chlorate is added to the acidified alkali metal salt solution to provide the aqueous acidified chlorate ion-containing solution.
One manner of effecting the procedure employs an electrolytic cell comprising a cation-exchange membrane dividing the cell into an anode compartment and a cathode compartment. The aqueous solution of at least one alkali metal salt is fed to the anode compartment and hydrogen ions are elect.rolytically produced in the anode compartment while alkali metal ions are transferred from the anode compartment through the cation-exchange membrane to the cathode compartment.
The acidified alkali metal. salt solution is removed from the anode compartment.
Alternatively, an.y other oxidation reaction producing hydrogen ions, for example, hydrogen gas oxidation to hydrogen ions, may be employed as an anodic reaction. Such anodic hydrogen gas oxidation may be combined with oxygen gas electroreduction as a cathodic reaction, to provide a fuel cell operation wherein, in addition to acidification of anolyte and production of alkali metal hydroxide solution in the catholyte, electrical energy is generated.
When an anodic o~,;idation of hydrogen gas to hydrogen ions is combined with a cathodic reduction of water to hydroxyl ions and hydrogen gas, the latter gas may be used as an anodic feed and, at the same time, a substantial energy savings nnay be achieved, as a result of the difference in electrochemical reaction potentials. Similarly, an analogous energy and material savings is achieved when an anodic oxidation of water to hydrogen ions and oxygen is combined with cathodic reduction of the oxygen gas stream.
In accordance with one embodiment of the present invention, there is provided an electrochemical process for the treatment of an aqueous solution of alkali metal salt mixtures, which comprises a plurality of steps. The aqueous so7.ution is fed to the anode compartment of an electrolytic cell having a cation-exchange membrane dividing the cell into an anode compartment and a cathode compartment.
Hydrogen ions are electrolytically produced in the anode compartment and hydroxyl ions are electrolytically produced in the cathode compartment.
Simultaneously alkali metal cations are transferred .__ Zp ~g5o7 across the cation-exchange membrane from the anode compartment to the cathode compartment.
This migration of alkali metal cations and the electrochemical reaction ;producing hydrogen ions and hydroxyl ions have the effect of producing an alkali metal hydroxide solution in the cathode compartment and an acid of the anion of the alkali metal salts in the anode compartment. The respective aqueous solutions are removed from the compartments of the cell.
In order to achieave high current efficiencies of at least about 70%, prefcarably at least about 80%, and thereby provide an economic: process, the molar ratio of [Na+]:[H+] in the anolyte generally varies from about 1000:1 to about 1:5, prefcarably about 1000:1 to about 1:2, throughout the electrolytic reaction.
In another aspect of the present invention, the method for the production of aqueous acidified chlorate ion-containing solution is, integrated into a chlorine dioxide generating process by providing an aqueous acid chlorine dioxide-generating reaction medium containing sulfuric acid and alkali mEatal ions in a reaction zone, feeding the aqueous acidified chlorate ion-containing solution to the reaction zone to provide chlorate ion and hydrogen ion reactants to the aqueous acid chlorine dioxide-generating reaction medium, and recovering an alkali metal sulfate from t:he reaction zone.
As noted above, the present invention is directed generally to the electrochemical treatment of aqueous solutions of alkali metal salts and mixtures thereof to generate an aqueous acidified medium. The procedure of the present invention is particularly useful in sulfuric acid-based chlorine dioxide generating processes, since the invention enables the quantity of by-product alkali metal sulfate co-produced with the chlorine dioxide to be decreased and even eliminated entirely, while maintaining the same level of chlorine dioxide production.
In one embodiment of the present invention, an aqueous solution of at least one alkali metal salt which is alkali metal chlorate, alkali metal sulfate or, preferably, mixtures thereof: is introduced to the anode compartment of a cation-exchange membrane divided electrochemical cell and alkali metal ions are transferred from the anode compartment to the cathode compartment. This procedure may be used in the present invention in different ways to achieve the decreased alkali metal sulfate output in a chlorine dioxide generating process. It is usual for chlorine dioxide generating processes to employ sodium salts, and hence in the following description of specific embodiments of the invention reference will be made to those sodium salts.
However, it is apparent that the principles of the invention are applicable to salts of other alkali metals.
In a preferred embodiment of the invention, a combination of sodium chlorate and sodium sulfate, particularly in the form of aodium sesquisulfate, is fed to the anode compartment of 'the cell, since the presence of both salts enables the [rfa+]/ [H'] molar ratio in the anode compartment to be maximized, resulting in a high current efficiency and thereby lower power consumption. In addition, a higher overall current efficiency decreases the capital cost, since a lesser number of cells is required for the same production rate.
It is preferred to employ a saturated feed solution or a slurry, in order to minimize the resistance of the electrolylte, while maintaining a high [Na+]/[H+] ratio in the anode compartment. When the feed is in the form of a slurry, it usually is preferred to provide an additional separator or separators, such as an ion-exchange membrane or a diaphragm, between the feed stream and the anode t:o prevent abrasion and wear on the anode from occurring. Such additional separator also may be employed, if deaired, if the feed does not comprise a slurry.
The cell produces an acidified solution from the mixed sodium chlorate/sodium sulfate feed in the anode compartment and a sodium hydroxide solution in the cathode compartment. The acidified solution then is fed to the chlorine dioxide generating process as an acid and chlorate source therefor.
When there is no~ need for the saltcake in particular pulp mill, it is preferred to operate the cell so as to remove from the mixed sodium chlorate/sodium sulfate feed a quantity of sodium ions corresponding to the quantii~y of sodium ions introduced with the sodium chlorate. The requirements of the chlorine dioxide generator for acid and chlorate thereby are completely satisfied, so that no additional sodium sulfate by-product is formed. Therefore, in effect, a dead load of sodium sulfate cycles between the chlorine dioxide generator and the cell.
When operating the process of the invention with sodium chlorate/sodium sulfate mixtures, it is preferred to dissolve sodium chlorate in the sodium sulfate solution. Alternatively, the sodium sulfate, in slurry or crystalline form, may be added to an aqueous sodium chlorate solution or, if desired, aqueous 5 solutions of sodium sulfate and sodium chlorate may be mixed in any required proportion.
The composition of the anolyte feed may be prepared by a one-time mixing of the individual components, namely sodium sulfate and sodium chlorate, 10 or by a gradual addition of one component to another in the course of electrolysis. For example, sodium sulfate can be pre-acidified in an electrolytic cell prior to the addition of sodium chlorate. In fact, the process can be carried out in such. a way that neutral sodium sulfate or sodium sesqu.isulfate is acidified to bisulfate, NaHS04, and then this solution is used to dissolve sodium chlorate, in order to take advantage of an increased solubility of bisulfate with respect to other sulfates. The mixture can be forwarded to the generator or further acidified in an electrolytic cell.
From this discussion, it will be apparent that any combination of sodium ulfate and sodium chlorate may be selected to meet the requirements of a particular pulp mill. Production of sodium sulfate by-product may be permitted at any level desired by the pulp mill.
The presence of sodium sulfate (sesquisulfate) in the anolyte improves the [Na+]/[H+] ratio not only due to an increase in sodium ion concentration but also as a result of a buffering action of the sulfate ions, which tend to recombine with hydrogen ions to form bisulfate, thereby decreasing the concentration of free hydrogen ions. In addition, a combined acidified chlorate/sulfate feed effectively decreases the amount of water introduced to the chlorine dioxide generator, as a result of a water "sharing" effect.
1:1 While operating the process using sodium chlorate/sodium sulfate mixitures provides a practically unlimited flexibility in selecting the extent of production of a by-product. saltcake corresponding to between 0 to 100% recycling of sulfate, it also is possible to achieve a partial decrease in saltcake production by electrolytically acidifying a pure sodium chlorate feed solution, i.e. without any addition of sulfate. The overall reaction for such a process may be depicted as:
xNaC103 + 3H20 1 (x-2) NaC103 + 2HC103 + 2NaOH
+ z02 + H2 Such a partial acidification. of sodium chlorate leads to a mixture of sodium chlorate and chloric acid, in which the content of chloric acid determines the extent of reduction in sulfuric acid addition to the chlorine dioxide generator and, consequently, the production of sodium sulfate by-product. Since the [Na+]/[H+] ratio constitutes a determining factor for the current efficiency of the acidification process, only partial conversion of sodium chlorate into chloric acid is feasible in this case. Usually, up to about a 20%
reduction in saltcake by-product production can be achieved by acidification of a pure sodium chlorate feed at practical current efficiencies.
The aqueous sodium hydroxide solution which is co-produced in the present invention is readily used by the pulp mill in its bleach plant operations for the purification of bleached pu:Lp. When pulp mills have an on-site electrolytic process for producing aqueous sodium hydroxide, usually from sodium chloride, Such process results in the co-production of chlorine. The ability to utilize such co--produced chlorine, however, is often limited, which leads to a caustic/chlorine imbalance in the pulp mill. By utilizing the process of the invention, not only is the problem of co-production x,018507 of excess sodium sulfate in the chlorine dioxide generation process overcome, but also the problem of co-production of chlorine in sa~dium hydroxide production is overcome. The concentration of sodium hydroxide solution produced in the cathode compartment may be adjusted to any reasonable level, by adjusting flow rates and recycling product solution.
As noted above, tile current efficiency of the process is largely dependent on the [Na+]/[H+] ratio in the anode compartment, which. usually requires a deadload of sodium ion cycling between the cell and the chlorine dioxide generator which, i.n the embodiment disclosed above, may be provided by sodium sulfate as the deadload. However, any other sodium salt which does not otherwise adversely affect the production of chlorine dioxide may be employed.
The anode employed in the electrolytic cell may take any desired form, but it is preferred to employ a low overpotential one with respect to the oxygen evolution reaction, for example, a DSA-02~ electrode.
Similarly, any convenient material of construction may be used for the cathode, for example, nickel.
The cation-exchange membrane may be formed of any convenient material which enables cations to selectively pass therethrough in preference to anions.
Preferably, the cation-exchange membrane is formed of perfluorocarbon polymer having pendant cation-exchange functional groups, such as those sold under the trademarks "NAFION" (DuPont) or "FLEMION" (Asahi Glass).
~ In the event a:n additional ion exchange membrane or membranes is used in the anode compartment, such membrane may be formed of any convenient ion-exchange material.
In one embodiment: of the invention in which electrodialysis utilizing bipolar membranes is employed, the aqueous solution of a mixture of alkali metal salts zols~o7 1:3 is processed in a plurality of unit cells, with each unit cell being separated from the adjacent ones by bipolar membranes. The bipolar membranes have an anionic face in the base compartment of one cell and a cationic face in the acid compartment of an adjacent cell. The individual cells are divided by a cation exchange membrane.
With the plurality of cells separated by bipolar membranes, gas evolution does not take place in the acid and base compartments, and the overall reaction may be represented by the equation:
(x+y) NaC103 +y H20 -~ x rfaC103 +y HC103 +y NaOH
The plurality of cells is terminated at both ends by cationic membranes. A separate electrode rinse solution, such as a sodium sulfate solution, is circulated between the cathodic and anodic compartments adjacent to the cathode and anode respectively. A
single electrical current :Feed, therefore, is used to effect acidification in parallel in a plurality of unit cells, with gaseous evolution occurring only in the end anode and cathode compartments.
Bipolar membranes and their operation are well known and are described, for example, in U.S. Patents Nos. 4,024,043, 4,180,87L5, 4,057,481, 4,355,116, 4,116,889, 4,253,900, 4,584,246 and 4,673,454 and reference may be had to such patents for details thereof.
The parameters of: operation of the cell are not critical to the proce:>s of the invention and may vary widely. For example, the electrolytic process may be effected under any desired electrolytic conditions, generally at a membrane current density of about 0.01 to about 10 kA/m2, preferably about 1 to about 5 kA/m2.
Similarly, the process may be carried out over a wide range of temperatures, generally from about 0°
to about 150°C, preferably i_rom about 15° to about 90°C.
2o~s~o7 Generally, higher temperatures are preferred, in view of the generally greater solubility of the alkali metal salts at higher temperatures, thereby enhancing the [Na+]/[H+] ratio. This greater solubility at high temperature is particularly true of sodium chlorate, so that, when such salt is included in the feed to the cell, higher temperature operation, above about 60°C, is preferred. The acidified) product stream from the electrolysis retains a high concentration of sodium chlorate, so that cooling of that product stream prior to passage to the chlorine dioxide generator may lead to precipitation of sodium chlorate, which can be removed and recycled to the cell feed.
As already described, one important parameter influencing current efficiency is the molar ratio of [Na+]/[H+] in the anode compartment. Generally, this molar ratio varies from about 1000:1 to about 1:5, preferably about 1000:1 to about 1:2. Such a ratio is dependent on the concentration of the feed solution to the anode compartment and the extent to which sodium ions are transferred from the anode compartment to the cathode compartment. Accordingly, it is preferred to employ a feed solution having a concentration of about 0.1 to about 15M in sodium ions and to remove from the anode compartment for feed to the chlorine dioxide generation process an acidified solution having a concentration of about 0.1 to about 12M in sodium ions.
The electrolytic process increases the total acid normality of the alkali metal salt solution.
Depending on the initial total acid normality and the degree of electrolysis effected on the feed material, it may be desirable to concentrate, such as by evaporation, the product stream to increase its total acid normality, prior to feed to the chlorine dioxide generation process. Generally, the total acid normality of the feed solution varies from about neutral to about 12 .~ 2018507 normal, preferably about 0.1 to about 10 normal, and the electrolysis is effected to such a degree as to provide a product stream from the: anode compartment having a total acid normality generally from about 0.2 to about 15 5 normal, preferably about 0,.5 to about 6 normal.
The invention is described further, by way of illustration, with reference to the accompanying drawings, in which:
Figure 1 is a flaw sheet of a chlorine dioxide 10 generating process provided in accordance with one embodiment of the invention;
Figure 2 is a schematic illustration of a bipolar membrane cell which may be employed with the chlorine dioxide generating process of Figure 1 ; and 15 Figure 3 contains a graphical representation of experimental data, illustrating the relationship of acid normality to current efficiency for different concentrations of alkali metal salt.
Referring to the drawings, Figure 1 depicts an embodiment of the application of the principles of the present invention to the production of chlorine dioxide.
There is shown therein a chlorine dioxide generating operation 10 comprising a chlorine dioxide generating process 12 wherein sodium chlorate, sulfuric acid and methanol are reacted at the boiling point of the reaction medium under a ~:ub-atmospheric pressure, to produce gaseous chlorine dioxide in line 14 (i.e. the R8 process).
The chlorine dioxide generating process in the generator 12 results in the precipitation of a by-product sodium sulfate, once the reaction medium has become saturated after start-up. The form of the by-product sodium sulfate, namely neutral sodium sulfate, sodium sesquisulfate, sodium bisulfate or mixtures thereof, depends on the total acid normality of the reaction medium, which may vary from about 2 to about 11 normal.
15a The crystalline sodium sulfate (sometimes termed "saltcake" ) is removed from the reaction medium in the form of a slurry with ;spent reaction medium by line 16 and is passed to a filter 18 wherein the crystalline material is separated from the spent reaction medium.
The separated spent reaction medium in line 20 is recycled to the generator 12 with make-up reactants, namely sodium chlorate, sulfuric acid and methanol, being fed to the recycle stream i.n line 20 by lines 22, 24 and 26 respectively.
20~.8~07 The crystalline aodium sulfate separated in the filter 18 may be removed by line 28 for use elsewhere in the pulp mill in whatever proportion (including zero) of the sodium sulfate in the slurry 16 is desired.
The portion of the sodium sulfate not removed by line 28 is forwarded by line 30 to a dissolving tank 32, wherein the sodium sulfate is dissolved in water fed by line 34 to form an aqueous solution which is preferably saturated. This solution is forwarded by lines 36 and 38 to a membrane-divided electrolysis cell 40. It is not necessary that the sodium sulfate be completely dissolved in water, but may be fed as a slurry thereof. Where such a slurry is employed, it is generally desirable to isolate the anode from the slurry in the cell 40, to avoid excessive wear resulting from abrasion.
The cell 40 comprises a cation-exchange membrane 42, which divides the interior of the cell 40 into an anode compartment 44 and a cathode compartment 46 in which are located cell anode 48 and cell cathode 50 respectively.
Sodium chlorate i:~ added to the sodium sulfate solution in line 36 by line 52 to form in line 38 a feed solution for the anode' compartment 44 of the cell 40. An electrolyte is fed by line 54 to the cathode compartment 46. A current is applied between the anode 48 and the cathode 50. The sodium chlorate may be added by line 52 in the i=orm of an aqueous solution thereof or as solid sodium chlorate.
In the cell 40, several reactions occur simultaneously. At the anode 48, water is electrolyzed to oxygen and hydrogen ion, as follows:
H20 -~ X02 + 2H+ + 2e-while at the cathode 50 water is electrolyzed to hydrogen and hydroxyl ion, as follows:
~o~s~o7 e- + H20 ~ iH2 + OH-At the same time, sodium ions in the aqueous solution or slurry of a mixture of sodium sulfate and sodium chlorate migrate under then influence of the applied current from the anode compartment 44 across the cation-exchange membrane 42 to the cathode compartment 46. In effect, therefore, the electrolytically-produced hydrogen ions replace the sodium ions in the anode compartment 44 and the transferred sodium ions are available to combine with the electrolytically-produced hydroxyl ions in the cathode: compartment 46.
The sodium sulfate contained in the solution feed in line 38 to the cell 40 can be considered to be a deadload circulating via the generator 12 in a closed loop, so that the overall reaction in cell 40 can be considered to be representedl, as follows:
xNaC103 + 3H20 -~ (x-2) NaC103 + 2HC103 + 2NaOH
+ X02 + H2 where x is the molar amount of sodium chlorate which is processed.
The resulting chlo~ric acid-containing solution then is recycled by line 54 to the recycle line 20, to provide acid and chlorate ion for the chlorine dioxide generator 12. The proportion of sodium sulfate removed from the system by line 28 corresponds to the proportion of the sulfuric: acid and sodium chlorate reactants fed to the chlorine dioxide generation from external sources, namely by line 24 for sulfuric acid and by unconverted sodium chlorate in line 24 and in line 22. Under steady state operating conditions for a process precipitating sodiunn sesquisulfate and where no sodium sulfate product is required, the processing of the aqueous solution of a mixture of sodium sesquisulfate and sodium chlorate in the cell 40 provides all the hydrogen ions and chlorate ions 20~18~07 necessary to sustain the reaction and additional sulfuric acid and sodium chlorate are not required.
In effect, therefore, sodium chlorate fed to the cell 40 is converted, at: least partially, to chloric acid, so that the sodium sulfate component of the feed solution 38 is a dead-load cycling between the cell 40 and the chlorine dioxide generator 12.
Oxygen is vented from the anode compartment 44 by line 56. Alternatively,, the product stream may be recycled by line 58, with oxygen being vented from a gas-liquid separator 60 by line 62, wherein recycle of anolyte is required to achieve the desired acid normality of the acidified ~;olution 54.
The sodium hydroxide produced in the cathode compartment 46 is recycled by line 64, through a gas liquid separator 66 from which hydrogen is vented by line 68, until the desired concentration of sodium hydroxide solution is achieved. The resulting aqueous sodium hydroxide solution is removed as an aqueous product stream in line 70. This solution has considerable utility in a pulp mill, particularly as a chemical employed in th~a purifying and bleaching operations effected in the: bleach plant of the pulp mill. The gaseous by-products, namely hydrogen and oxygen, also can be utilized. in the pulp mill.
High current efficiency for the electrolytic process effected in the cell 40 as described above can be attributed to the high [lKa+]/[H+] molar ratio in the anode compartment 44 and also to the buffering effect of S042- ions towards hydrogen ions, which tends to maintain the free hydrogen ion concentration in the anode compartment low, thereby tending to maintain the high [Na+]/[H+] molar ratio.
As may be seen from the above description, the process of Figure 1 producer chlorine dioxide by the R8 process and hence retains 'the benefits thereof. More importantly, the process doEa not produce any excess by-product sodium sulfate requiring disposal. The amount of sodium sulfate which is produced can be tailored to the mill requirement, or may be eliminated entirely. In effect, when there is no requirement for the saltcake in the pulp mill, the sodium sulfate is maintained in a closed loop within the process and the sodium ions introduced to the chlorine dioxide generating process with the sodium chlorate, exit the process in the form of aqueous sodium hydroxide solution. The acid for the chlorine dioxide generai~ing process is produced electrolytically from water, which co-produces the hydroxyl ions required to combine with the sodium ions to form the sodium hydroxide..
Referring to Figure 2, there is shown therein the utilization of a bank 100 of unit cells, with the individual cells 102 producing an acidified mixture in line 104 for feeding to a chlorine dioxide generator, as described with respect to Figure 1, from an aqueous feed mixture of sodium sesquisulfate and sodium chlorate in line 106. The number of unit cells 102 in the bank of cells may vary widely, depending on the required production capacity and typically may vary from about 20 to about 500.
Each unit cell 100 is separated from each adjacent unit cell by bipolar membranes 108, 110. The bipolar membrane 108 has its cationic face in an acid compartment 112, so as to form hydrogen ions under the influence of the electric current applied to the bank of cells 100, thereby acidifying the feed mixture, while sodium ions are transported from the acid compartment 112 across a cation-exchange membrane 114 to a base compartment 116.
The bipolar membrane 110 has its anionic face in the base compartment 116, so as to form hydroxyl ions from the aqueous feed i;.hereto in line 118 under the influence of the electrical current applied to the bank of cells 100. In this way, sodium hydroxide is formed in the base compartment 116 and is removed by line 120.
Only a single anode 122 and a single cathode 5 124 are required for the bank 100 of unit cells 102.
Oxygen and hydrogen respectively are formed at the electrode surfaces and vented from the terminal unit cells.
The invention is illustrated by the following 10 Examples:
Electrochemical e~;;periments were carried out in a two-compartment MP cell, supplied by Electrocell AB, Sweden equipped with an oxygen-evolving anode (DSA-OZO), nickel cathode and a catio:n exchange membrane (NAFION
15 427) dividing the cell into an anode compartment and a cathode compartment. The anode, cathode and membrane each had an area of 100 sq, cm.
In the experiments, a current density of 3 kA/m2 was mainly employed (occasionally 2 kA/m2), the 20 anolyte was NaC103 or NaC103/NaZS04/H2S04 mixtures, the catholyte was 1N aqueous sodium hydroxide solution and the temperature was 40° to 50°C.
Example 1 In a first set of experiments, aqueous solutions of sodium chlorate. of various concentrations were used as the anolyte. During the course of electrolysis, the anolyte became enriched with hydrogen ions as the sodium chlorate was partially converted to chloric acid. Current efficiencies were determined at various product chloric acid concentrations for the various initial sodium chlorate concentrations and were plotted graphically. These results appear as Figure 3.
It will be seen from the data presented in this Figure that the current efficiency declines with increasing H' to Na' moles ratio in the solution electrolyzed.
~,018~07 Example 2 Various mixtures of sodium chlorate and Na2S04/H2S04 were prepared and electrolyzed.
(a) 1 L of an approximately 2M Na3H(S04)2 aqueous solution was prepared by mixing Na2S04 and H2S04 at a molar ratio of 3:1. Acidii~y was measured by titration with NaOH and the solution was determined to have an initial concentration of 1.94 normal. The catholyte was 0.5L of 1N NaOH.
Current was imposed between the electrodes at a current density of 3 kA/m2 and sodium chlorate crystals, in a total amount corresponding to the preparation of a 2M aqueous solution, were added slowly to the anolyte. A very rapid dissolution of sodium chlorate was observed. The increase in acidity of the anolyte was monitored by titration and the electrolysis was terminated when the acidity of the anolyte reached 4.12 N. The basicity of they catholyte was determined to be 4.54 N NaOH. The total tame of electrolysis was 8583 seconds with the total charge passed being 257,490 A.sec.
In the calculation of the current efficiency, the volume changes of the anolyte and catholyte were taken into account, since 'water is transported to the catholyte together with Na+ ions and also is consumed in the electrolysis. The current efficiency based on the increase in acidity of the anolyte was determined to be 79~ while that based on the increase in basicity of the catholyte was 80~.
The overall conversion in the anolyte can be depicted as:
2 M Na3H(S04)2 + 2 M NaC103 ~ 2 M Na3H (S04)2 + 2 M HC103 with an equivalent amount of caustic being co-produced in the cathode compartment:. Hydrogen (cathode) and oxygen (anode) were the gaseous by-products. All the reactants and products depicted by this equation are largely dissociated in the solution into the ionic species Na', H+, HS04'- 504~~' and C103'~ However, since sesquisulfate contributes a prevailing form of the precipitate formed in the chlorine dioxide generator, chloric acid (HC103 = H~ + C:L03'~ is liberated from such a mixture upon precipitation of sesquisulfate.
(b) Other mixtures of sodium chlorate and sodium sesquisulfate were processed in a similar way to that described in experiment (a), with quantitative (1 to 4) or partial (5) conversion of the sodium chlorate to chloric acid. The results obtained are reproduced in the following Table I:
22a O oW
.,..y.
~7 W O ~ ~ tMD 0~0 U
N
y N~
H
N M M N M
A
U
~o o~ o .-.
z ~ M M M
N N
v H
O ~ O N
d' O 01 N e-~ rl e-i O
~ tf7 U~ rl N N N d' z N r-i H H r-1 .~.7 z rl N M d' tt7 :~2b It will be seen from this: Table and the data in (a) above, that an improvement in current efficiency is obtained by the presence of sulfate ion in conjunction with a high [Na+]/ [H+] ratio, as compared with the results obtained in Example 1. Fo:r example, l.5 M HC103 can be obtained at about 60% C,.E. in pure aqueous sodium chlorate solution and at about 80% C.E. in the mixtures of sodium sesquisulfate and sodium chlorate.
In summary of this disclosure, the present invention provides a novel electrochemical process which enable the quantity of by-product sodium sulfate produced by a chlorine dioxide generating process to be decreased.
Modifications are possible within the scope of this invention.
Claims (50)
1. A method for the production of an aqueous acidified chlorate ion-containing solution useful as a feed stream for a chlorine dioxide generating process wherein chlorate ions are reduced to chlorine dioxide in an aqueous acid medium, which comprises:
electrochemically acidifying an aqueous solution of at least one alkali metal salt selected from the group consisting of alkali metal chlorate, alkali metal sulfate and mixtures of alkali metal chlorate and alkali metal sulfate, electrochemically removing alkali metal ions from said aqueous solution to produce an acidified alkali metal salt solution as said aqueous acidified chlorate ion-containing solution when said at least one alkali metal salt is selected from the group consisting of alkali metal chlorate and mixtures of alkali metal chlorate and alkali metal sulfate, and, when said at least one alkali metal salt is alkali metal sulfate, adding alkali metal chlorate to said acidified alkali metal salt solution to provide said aqueous acidified chlorate ion-containing solution.
electrochemically acidifying an aqueous solution of at least one alkali metal salt selected from the group consisting of alkali metal chlorate, alkali metal sulfate and mixtures of alkali metal chlorate and alkali metal sulfate, electrochemically removing alkali metal ions from said aqueous solution to produce an acidified alkali metal salt solution as said aqueous acidified chlorate ion-containing solution when said at least one alkali metal salt is selected from the group consisting of alkali metal chlorate and mixtures of alkali metal chlorate and alkali metal sulfate, and, when said at least one alkali metal salt is alkali metal sulfate, adding alkali metal chlorate to said acidified alkali metal salt solution to provide said aqueous acidified chlorate ion-containing solution.
2. The method of claim 1 wherein said electrochemical acidification of said aqueous solution of at least one alkali metal salt and electrochemical removal of alkali metal ions are effected simultaneously by:
providing an electrolytic cell comprising a cation-exchange membrane dividing said cell into an anode compartment and a cathode compartment, feeding said aqueous solution of at least one alkali metal salt to said anode compartment, electrolytically producing hydrogen ions in said anode compartment while simultaneously effecting transfer of alkali metal cations from said anode compartment through said ration-exchange membrane to said cathode compartment, and removing said acidified alkali metal salt solution from said anode compartment.
providing an electrolytic cell comprising a cation-exchange membrane dividing said cell into an anode compartment and a cathode compartment, feeding said aqueous solution of at least one alkali metal salt to said anode compartment, electrolytically producing hydrogen ions in said anode compartment while simultaneously effecting transfer of alkali metal cations from said anode compartment through said ration-exchange membrane to said cathode compartment, and removing said acidified alkali metal salt solution from said anode compartment.
3. The method of claim 2 wherein said alkali metal sulfate is selected from neutral alkali metal sulfate, alkali metal sesquisulfate, alkali metal bisulfate and mixtures thereof.
4. The method of claim 3 wherein said alkali metal is sodium.
5. The method of claim 4 wherein said aqueous solution of at least one alkali metal salt comprises an aqueous solution of sodium sulfate and sodium chlorate.
6. The method of claim 5 wherein said sodium sulfate comprises sodium sulfate from a sulfuric acid-based chlorine dioxide generation process.
7. The method of claim 6 wherein said sodium sulfate is formed as a by-product of said chlorine dioxide generating process from sodium ions and sulfate ions fed to said process at least in part in the form of said feed stream.
8. The method of claim 7 wherein said sodium sulfate comprises sodium sesquisulfate.
9. The method of claim 5 wherein said aqueous solution of sodium sulfate and sodium chlorate contains sufficient sodium ions to provide in said anode compartment a molar ratio range of [Na+]: [H+] of 1000:1 to 1:5 during said electrolytic production of hydrogen ions.
10. The method of claim 9 wherein said molar ratio range of [Na+]:[H+] is 1000:1 to 1:2.
11. The process of claim 2 wherein said anode compartment contains sufficient alkali metal ions to provide a current efficiency of at least 70% during said electrolytic production of hydrogen ions.
12. The method of claim 11 wherein said current efficiency is at least 80%.
13. The method of claim 5 wherein said aqueous solution of sodium sulfate and sodium chlorate is saturated.
14. The method of claim 13 wherein said saturated aqueous solution is a slurry containing crystals of at least sodium sulfate.
15. The method of claim 2 wherein said anode compartment is provided with an additional separator between an anode in the anode compartment and the feed stream.
16. The method of claim 5 wherein said electrolytic cell comprises one unit cell of a multiple number of unit cells each separated by bipolar membranes and having an anode and a cathode in terminal compartments located one at each end of the multiple member of unit cells, and said aqueous solution of sodium sulfate and sodium chlorate is fed in parallel streams to an acid compartment of each unit cell and said acidified solution is removed in parallel streams from the acid compartment of each unit cell.
17. The method of claim 16 wherein hydroxyl ions are electrolytically produced in a base compartment of each unit cell simultaneously with the electrolytic production of hydrogen ions in the acid compartment of each unit cell, and an aqueous sodium hydroxide solution is removed from each base compartment of each unit cell.
18. The method of claim 2 wherein said electrolytic production of hydrogen ions is effected at a membrane current density of 0.01 to 10 kA/m2.
19. The method of claim 18 wherein said current density is 1 to 5 kA/m2.
20. The method of claim 2 wherein said acidified alkali metal salt solution is recirculated through the anode compartment until the desired acidity is achieved.
21. The method of claim 2 wherein hydroxyl ions are electrolytically produced in said cathode compartment simultaneously with said electrolytic production of hydrogen ions in said anode compartment, and an alkali metal hydroxide solution is removed from said cathode compartment.
22. The method of claim 21 wherein said alkali metal is sodium.
23. The method of claim 22 wherein said sodium hydroxide solution is recirculated through the cathode compartment until the desired concentration of aqueous sodium hydroxide solution is produced.
24. The method of claim 1 wherein said electrochemical acidification of said aqueous solution of at least one alkali metal salt is effected by anodic hydrogen gas oxidation to hydrogen ions and addition of said hydrogen ions to said aqueous solution, and said electrochemical removal of alkali metal ions is effected by transfer of alkali metal ions from said aqueous solution across a cation-exchange membrane to a recipient medium.
25. The method of claim 24 wherein cathodic electroreduction of oxygen gas is effected whereby said anodic and cathodic reactions constitute a fuel cell producing electrical energy.
26. The method of claim 24 wherein cathodic reduction of water is effected in said recipient medium to produce hydrogen and hydroxyl ions to combine with said transferred alkali metal ions to form an aqueous alkali metal hydroxide solution, and said hydrogen is fed to said anodic hydrogen gas oxidation.
27. The method of claim 1 wherein said electrochemical acidification of said aqueous solution of at least one alkali metal salt is effected by anodic oxidation of water in said aqueous solution to hydrogen ions and oxygen and said electrochemical removal of alkali metal ions is effected by transfer of alkali metal ions from said aqueous solution across a cation-exchange membrane to a recipient medium.
28. The method of claim 27 wherein cathodic electroreduction of said oxygen is effected.
29. An electrochemical process for the treatment of an aqueous solution of a mixture of alkali metal salts, at a current efficiency of at least 70%, which comprises:
feeding said aqueous solution to an anode compartment of an electrolytic cell having a cation exchange membrane dividing said cell into the anode compartment and a cathode compartment, electrolytically producing hydrogen ions in said anode compartment at a molar ratio range of alkali metal ion to hydrogen ion of 1000:1 to 1:5 to form an acidified aqueous solution therein and electrochemically producing hydroxyl ions in said cathode compartment while simultaneously effecting transfer of alkali metal cations from said anode compartment through said cation-exchange membrane to said cathode compartment to form an aqueous alkali metal hydroxide therein, removing said aqueous alkali metal hydroxide solution from said cathode compartment and removing said acidified aqueous solution from said anode compartment.
feeding said aqueous solution to an anode compartment of an electrolytic cell having a cation exchange membrane dividing said cell into the anode compartment and a cathode compartment, electrolytically producing hydrogen ions in said anode compartment at a molar ratio range of alkali metal ion to hydrogen ion of 1000:1 to 1:5 to form an acidified aqueous solution therein and electrochemically producing hydroxyl ions in said cathode compartment while simultaneously effecting transfer of alkali metal cations from said anode compartment through said cation-exchange membrane to said cathode compartment to form an aqueous alkali metal hydroxide therein, removing said aqueous alkali metal hydroxide solution from said cathode compartment and removing said acidified aqueous solution from said anode compartment.
30. The process of claim 30 wherein said molar ratio range is 1000:1 to 1:2.
31. The process of claim 30 wherein said mixture of alkali metal salts comprises a mixture of an alkali metal chlorate and an alkali metal sulfate which is selected from the group consisting of neutral alkali metal sulfate, alkali metal sesquisulfate, alkali metal bisulfate, and mixtures thereof.
32. The process of claim 31 wherein said alkali metal is sodium.
33. A method for the production of chlorine dioxide, which comprises:
providing an aqueous acid chlorine dioxide-generating reaction medium containing sulfuric acid and alkali metal ions in a reaction zone, electrochemically acidifying an aqueous solution of at least one alkali metal salt selected from the group consisting of alkali metal chlorate, alkali metal sulfate and mixtures of alkali metal chlorate and alkali metal sulfate, electrochemically removing alkali metal ion from said aqueous solution to produce an acidified alkali metal salt solution, when said at least one alkali metal salt is selected from the group consisting of alkali metal chlorate and mixtures of alkali metal chlorate and alkali metal sulfate, forwarding said acidified alkali metal salt solution as an acidified chlorate ion-containing solution to said reaction zone to provide chlorate ion and hydrogen ion reactants to said aqueous acid chlorine dioxide-generating reaction medium, when said at least one alkali metal salt is alkali metal sulfate, adding alkali metal chlorate to said acidified alkali metal salt solution and then forwarding the resulting aqueous acidified chlorate ion-containing solution to said reaction zone to provide chlorate ion and hydrogen ion reactants to said aqueous acid chlorine dioxide-generating reaction medium, and recovering from said reaction zone an alkali metal sulfate.
providing an aqueous acid chlorine dioxide-generating reaction medium containing sulfuric acid and alkali metal ions in a reaction zone, electrochemically acidifying an aqueous solution of at least one alkali metal salt selected from the group consisting of alkali metal chlorate, alkali metal sulfate and mixtures of alkali metal chlorate and alkali metal sulfate, electrochemically removing alkali metal ion from said aqueous solution to produce an acidified alkali metal salt solution, when said at least one alkali metal salt is selected from the group consisting of alkali metal chlorate and mixtures of alkali metal chlorate and alkali metal sulfate, forwarding said acidified alkali metal salt solution as an acidified chlorate ion-containing solution to said reaction zone to provide chlorate ion and hydrogen ion reactants to said aqueous acid chlorine dioxide-generating reaction medium, when said at least one alkali metal salt is alkali metal sulfate, adding alkali metal chlorate to said acidified alkali metal salt solution and then forwarding the resulting aqueous acidified chlorate ion-containing solution to said reaction zone to provide chlorate ion and hydrogen ion reactants to said aqueous acid chlorine dioxide-generating reaction medium, and recovering from said reaction zone an alkali metal sulfate.
34. The method of claim 33 wherein said electrochemical acidification of said aqueous solution of at least one alkali metal salt and electrochemical removal of alkali metal ions are effected simultaneously by:
providing an electrolytic cell comprising a cation-exchange membrane dividing said cell into an anode compartment and a cathode compartment, feeding said aqueous solution to said anode compartment, electrolytically producing hydrogen ions in said anode compartment while simultaneously effecting transfer of alkali metal cations from said anode compartment through said cation-exchange membrane to said cathode compartment, and removing said acidified alkali metal salt solution from said anode compartment.
providing an electrolytic cell comprising a cation-exchange membrane dividing said cell into an anode compartment and a cathode compartment, feeding said aqueous solution to said anode compartment, electrolytically producing hydrogen ions in said anode compartment while simultaneously effecting transfer of alkali metal cations from said anode compartment through said cation-exchange membrane to said cathode compartment, and removing said acidified alkali metal salt solution from said anode compartment.
35. The method of claim 34 wherein said aqueous acid reaction medium is maintained at its boiling point while a subatmospheric pressure is applied to the reaction zone.
36. The method of claim 35 wherein said alkali metal sulfate is crystallized from said aqueous acid reaction medium in said reaction zone and is removed in crystalline form from said reaction zone.
37. The method of claim 36 wherein said alkali metal is sodium and chlorine dioxide is formed from said aqueous acid reaction medium by reduction of chlorate ions therein.
38. The method of claim 37 wherein said reduction is effected by feeding methanol to said reaction zone.
39. The method of claim 38 wherein said reaction medium has a total acid normality of from 2 to 11.
40. The method of claim 39 wherein said reaction medium has a total acid normality such that the sodium sulfate crystals comprise sodium sesquisulfate.
41. The method of claim 40 wherein at least part of said recovered sodium sesquisulfate crystals are dissolved in an aqueous medium to form an aqueous solution thereof and said latter aqueous solution provides at least part of said aqueous alkali metal salt solution feed to said anode compartment.
42. The method of claim 41 wherein said latter aqueous solution also has sodium chlorate dissolved therein prior to the feeding thereof to said anode compartment.
43. The method of claim 42 wherein said latter aqueous solution comprises a slurry of said sodium sesquisulfate.
44. The method of claim 41 wherein said acidified alkali metal salt solution comprises an acidified sodium sesquisulfate solution and sodium chlorate is dissolved therein prior to the feeding thereof to said reaction zone.
45. The method of claim 44 wherein the aqueous solution resulting from the addition of sodium chlorate is subjected to further electrolytic acidification prior to the feeding thereof to said reaction zone.
46. The method of claim 33 wherein said electrochemical acidification of said aqueous solution of at least one alkali metal salt is effected by anodic hydrogen gas oxidation to hydrogen ions and addition of said hydrogen ions to said aqueous solution, and said electrochemical removal of alkali metal ion is effected by transfer from said aqueous solution across a cation-exchange membrane to a recipient medium.
47. The method of claim 41 wherein cathodic electroreduction of oxygen gas is effected whereby the anodic and cathodic reactions constitute a fuel cell producing electrical energy.
48. The method of claim 41 wherein cathodic reduction of water is effected in said cathode compartment to produce hydrogen and hydroxyl ions to combine with said transferred alkali metal ions to form an aqueous alkali metal hydroxide solution, and said hydrogen is, fed to the anodic hydrogen gas oxidation.
49. The method of claim 33 wherein said electrochemical acidification of said aqueous solution of at least one alkali metal salt is effected by anodic oxidation of water in said aqueous solution to hydrogen ions and oxygen, and said electrochemical removal of alkali metal ions is effected by transfer of alkali metal ions from said aqueous solution across a cation-exchange membrane to a recipient medium.
50. The method of claim 49 wherein cathodic electroreduction of said oxygen is effected.
Priority Applications (20)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2018507 CA2018507C (en) | 1990-06-07 | 1990-06-07 | Electrochemical processing of aqueous solutions |
| BR919106511A BR9106511A (en) | 1990-06-07 | 1991-05-23 | PROCESS FOR THE PRODUCTION OF WATER ACID SOLUTION CONTAINING ION CHLORATE AND THE PROCESS FOR THE PRODUCTION OF CHLORINE DIOXIDE |
| DE69117180T DE69117180D1 (en) | 1990-06-07 | 1991-05-23 | ELECTROCHEMICAL PRODUCTION OF ACID CHLORATE SOLUTIONS |
| EP91909631A EP0532535B2 (en) | 1990-06-07 | 1991-05-23 | Electrochemical production of acid chlorate solutions |
| JP91509058A JPH05506481A (en) | 1990-06-07 | 1991-05-23 | Electrochemical production method of acid chlorate solution |
| AU78772/91A AU644179B2 (en) | 1990-06-07 | 1991-05-23 | Electrochemical production of acid chlorate solutions |
| CA002084183A CA2084183C (en) | 1990-06-07 | 1991-05-23 | Electrochemical processing of aqueous solutions |
| RU92016631A RU2108413C1 (en) | 1990-06-07 | 1991-05-23 | Method for production of aqueous acidified solution containing chlorate ions, method for electrochemical treatment of aqueous solution of mixture of salts of alkali metals, method for production of chlorine dioxide |
| AT91909631T ATE134184T1 (en) | 1990-06-07 | 1991-05-23 | ELECTROCHEMICAL PRODUCTION OF ACIDIC CHLORATE SOLUTIONS |
| PCT/CA1991/000175 WO1991018830A1 (en) | 1990-06-07 | 1991-05-23 | Electrochemical production of acid chlorate solutions |
| NZ238362A NZ238362A (en) | 1990-06-07 | 1991-05-31 | Electrochemical production of an aqueous acidified chlorate ion-containing solution for use in the production of chlorine dioxide |
| ZA914242A ZA914242B (en) | 1990-06-07 | 1991-06-04 | Electrochemical processing of aqueous solutions |
| CS911711A CS171191A3 (en) | 1990-06-07 | 1991-06-05 | Process for preparing aqueous acidified chlorate solution |
| PT97915A PT97915B (en) | 1990-06-07 | 1991-06-07 | PRODUCTION PROCESS OF AN ACIDIFIED AQUEOUS SOLUTION CONTAINING IONS CHLORIDE AND CHLORINE DIOXIDE |
| AR91319888A AR247254A1 (en) | 1990-06-07 | 1991-06-07 | Electrochemical production of acid chlorate solutions |
| CN91104562A CN1054657C (en) | 1990-06-07 | 1991-06-07 | Electrochemical processing of aqueous solutions |
| MYPI91001012A MY107909A (en) | 1990-06-07 | 1991-06-07 | Electrochemical processing of aqueous solutions. |
| TW080104963A TW270151B (en) | 1990-06-07 | 1991-06-26 | |
| NO92924687A NO924687L (en) | 1990-06-07 | 1992-12-04 | ELECTROCHEMICAL PRODUCTION OF ACID CHLORATE SOLUTIONS |
| FI925554A FI118535B (en) | 1990-06-07 | 1992-12-07 | Electrochemical Production of Acid Chlorate Solutions |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2018507 CA2018507C (en) | 1990-06-07 | 1990-06-07 | Electrochemical processing of aqueous solutions |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2018507A1 CA2018507A1 (en) | 1991-12-07 |
| CA2018507C true CA2018507C (en) | 1999-12-21 |
Family
ID=4145159
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2018507 Expired - Lifetime CA2018507C (en) | 1990-06-07 | 1990-06-07 | Electrochemical processing of aqueous solutions |
Country Status (2)
| Country | Link |
|---|---|
| CA (1) | CA2018507C (en) |
| ZA (1) | ZA914242B (en) |
-
1990
- 1990-06-07 CA CA 2018507 patent/CA2018507C/en not_active Expired - Lifetime
-
1991
- 1991-06-04 ZA ZA914242A patent/ZA914242B/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| ZA914242B (en) | 1992-05-27 |
| CA2018507A1 (en) | 1991-12-07 |
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| Date | Code | Title | Description |
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| EEER | Examination request | ||
| MKEX | Expiry |