CA2037522A1 - System for electrolytically generating strong solutions of halogen oxyacids - Google Patents
System for electrolytically generating strong solutions of halogen oxyacidsInfo
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- CA2037522A1 CA2037522A1 CA 2037522 CA2037522A CA2037522A1 CA 2037522 A1 CA2037522 A1 CA 2037522A1 CA 2037522 CA2037522 CA 2037522 CA 2037522 A CA2037522 A CA 2037522A CA 2037522 A1 CA2037522 A1 CA 2037522A1
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- cathode
- alkali metal
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Abstract
SYSTEM FOR ELECTROLYTICALLY GENERATING
STRONG SOLUTIONS OF HALOGEN OXYACIDS
Abstract The present invention resides in a process and apparatus for the electrolytic production of strong solutions of halogen oxyacids, more specifically for the production of such acids having a normality of about 0.1 to about 3.6 from the corresponding alkali metal salts of such acids. The present invention comprises establishing a solution of the corresponding alkali metal salt having a molar concentration less than that at which precipitation of said salt occurs. An electrolytic cell is provided comprising an anode compartment containing an anode, a cathode compartment containing a cathode, and a middle feed compartment intermediate the anode compartment and cathode compartment. The feed compartment is separated from the anode compartment by a diaphragm or an anion-selective membrane, and from the cathode compartment by a cation-selective membrane. Means are provided for introducing said alkali metal Walt solution into said middle feed compartment and for applying a voltage between the anode and cathode. Under the influence of the applied voltage, the alkali metal ions migrate through the cation-selective membrane to the cathode, reacting with hydroxyl ions to form alkali metal hydroxide, and the oxyhalogen ions migrate through the diaphragm or anion-selective membrane to the anode reacting with protons to form halogen oxyacid. Means are provided for maintaining the cell at a temperature in the range of about 10°C to about 40°C.
STRONG SOLUTIONS OF HALOGEN OXYACIDS
Abstract The present invention resides in a process and apparatus for the electrolytic production of strong solutions of halogen oxyacids, more specifically for the production of such acids having a normality of about 0.1 to about 3.6 from the corresponding alkali metal salts of such acids. The present invention comprises establishing a solution of the corresponding alkali metal salt having a molar concentration less than that at which precipitation of said salt occurs. An electrolytic cell is provided comprising an anode compartment containing an anode, a cathode compartment containing a cathode, and a middle feed compartment intermediate the anode compartment and cathode compartment. The feed compartment is separated from the anode compartment by a diaphragm or an anion-selective membrane, and from the cathode compartment by a cation-selective membrane. Means are provided for introducing said alkali metal Walt solution into said middle feed compartment and for applying a voltage between the anode and cathode. Under the influence of the applied voltage, the alkali metal ions migrate through the cation-selective membrane to the cathode, reacting with hydroxyl ions to form alkali metal hydroxide, and the oxyhalogen ions migrate through the diaphragm or anion-selective membrane to the anode reacting with protons to form halogen oxyacid. Means are provided for maintaining the cell at a temperature in the range of about 10°C to about 40°C.
Description
~7~22 SYS~ OR E~ECTRO~YTI~T-T-Y GEN~RATING
S~RONG SOLUTIONS OF HALO OE ~ O~YAC DS
sackqround o~ th~ In~ention Teahnical Fi ld The present invention relates to a proces~ and apparakus for electrolytically generating ~trong solutions of halogen oxyacids from the corresponding alkali metal 0alt. The present invention will be particularly described with reference to generating a chloric acid ~HClO3) solution o high normality from sodium chlorate (NaC103). However, it will be apparent to those skilled in the axt that the present invention i8 al80 applicable to the generation of other oxyacid~, for instance perchloric acid (HC104) from sodium perchlorate (NaCl04).
Description of the Prior Art One problem with halogen oxyacids such a~ chloric acid i~ that they are unstable and sub~ect to decomposition, particularly at elevated temperatures.
This prevents them from being easily stored and shipped re~uiring that they be made at a point of use ra~her than , - .
S~RONG SOLUTIONS OF HALO OE ~ O~YAC DS
sackqround o~ th~ In~ention Teahnical Fi ld The present invention relates to a proces~ and apparakus for electrolytically generating ~trong solutions of halogen oxyacids from the corresponding alkali metal 0alt. The present invention will be particularly described with reference to generating a chloric acid ~HClO3) solution o high normality from sodium chlorate (NaC103). However, it will be apparent to those skilled in the axt that the present invention i8 al80 applicable to the generation of other oxyacid~, for instance perchloric acid (HC104) from sodium perchlorate (NaCl04).
Description of the Prior Art One problem with halogen oxyacids such a~ chloric acid i~ that they are unstable and sub~ect to decomposition, particularly at elevated temperatures.
This prevents them from being easily stored and shipped re~uiring that they be made at a point of use ra~her than , - .
2 ~ 2 2 on a large industrial scale. Present commercial methods ~or generating chloric acid on site acidify sodium chlorate with ~ulfuric acid. This produces an Lmpure product stream containing ~odium sulfate which has to be removed, and which is of littla value as a by-product.
U.S. Pa~ent ~o. 4,798,715, assigned to the a~ignee of the pre~ent application discloses the production of chloric acid from sodium chlorate using an ion exchange resin. Chlorine dioxide i8 then manu~actured by reducing the chloric acid in an electrochemical cell. It is disclosed in the patent thPt the chloric acid feed to the electrochemical cell can ha~e a normality of about O.S up to about 4.5. However, it is de~irable to feed chloric acid to the electrochemical cell at a relatively high normality, ~or in~tance, above about 1.5, in order to obtain reduction of the chloric acid to chlorine dioxide at optimum efficiency.
U.S. Pa~ent No 3,810,969 also discloses the manufacture of chloric acid by reacting an alkali metal chlorate with a stoichiometric exces~ of a cation exchange re~in. One problem with the use of a cation exchange resi.n ~8 that such re~ins have a relatively short lifetime, incrQasing ths cost of manufacture of chloric acid.
~t i~ known to produce acids using an electroly~ic cell. U.S. Patent No. 4,115,217 disclose~ the u~e of a three-compartment electrolytic cell for the preparation of ~odium chlorita (NaC102) from sodium chlorate (NaC103), , . . : . . - . '-., : . .
.. . .
.
., ' ' ' '` , . ` ' ' . ' ` ' , .
U.S. Pa~ent ~o. 4,798,715, assigned to the a~ignee of the pre~ent application discloses the production of chloric acid from sodium chlorate using an ion exchange resin. Chlorine dioxide i8 then manu~actured by reducing the chloric acid in an electrochemical cell. It is disclosed in the patent thPt the chloric acid feed to the electrochemical cell can ha~e a normality of about O.S up to about 4.5. However, it is de~irable to feed chloric acid to the electrochemical cell at a relatively high normality, ~or in~tance, above about 1.5, in order to obtain reduction of the chloric acid to chlorine dioxide at optimum efficiency.
U.S. Pa~ent No 3,810,969 also discloses the manufacture of chloric acid by reacting an alkali metal chlorate with a stoichiometric exces~ of a cation exchange re~in. One problem with the use of a cation exchange resi.n ~8 that such re~ins have a relatively short lifetime, incrQasing ths cost of manufacture of chloric acid.
~t i~ known to produce acids using an electroly~ic cell. U.S. Patent No. 4,115,217 disclose~ the u~e of a three-compartment electrolytic cell for the preparation of ~odium chlorita (NaC102) from sodium chlorate (NaC103), , . . : . . - . '-., : . .
.. . .
.
., ' ' ' '` , . ` ' ' . ' ` ' , .
-3- 2~3~
sulfuric acid, and sulfur dioxide. A product of the proces~ of this patent is enriched sulfuric acid (H2S04) instead of chloric acid. ThP process compri~es reacting the sodium chlorate in a reactor with the æulfur dioxide to produce a residual solution of sodium ~ulfate and sulfuric acid. Chlorine dioxide tC102) i~ also formed in the reactor and is removed in an inert gas stream. The re~idual selution containing sodium sulfate and sulfuric acid is fed into the middle compar ment of the electrolytic cell. The middle compartment is defined on one side by an anion selective membrane and on the oppo~ite Ride by a ca~ion selective membrane. The end compartment~ of the cell are an anode compartment separated from the middle compar~ment by the anian selective membrane and a cathode compartment ~eparated ~rom the middle compartment by ~he cation æelective membrane. When a voltage is applied to the cell, ~ulfate ions migrate from the middle compartment through the anion selective membrane into the anode compartment. At the anode, water is decomposed with the evolution of oxygen and generation of hydrogen ion~ which react with the migrated sulfate ions to form sulfuric acid. The chlorine dioxide formed in the reactor is fed into the catholyte of the three-compartment cell and is reduced to chlorite ions (C102-) at the cathode. The cation~ in the middle cempartment, mainly sodium and hydrogen ions, migrate through the cation selective membrane. The sodium ion~
react with the chlorite ions formed at the cathode to form sodium chlorite which is precipita~ed in the cathode compartment when saturation is reached.
Problem~ arise, however, when an electrolytic cell such as that di~closed in U.~. Patent No. 4,115,217 is attempted ~o be used in the manufacture of a ~trong halogen nxyacid such as chloric acid. For one, such cells are known to generate substantial amounts of heat because of solution and separator resistance, which can lead to acid decomposition. In addition, the halogen oxyacids at high temperature are highly corro4ive preventing many m~terials conventionally employed in electrolytic cells ~rom being u~ed in association with the oxyacid~.
U.S. Patent No. 3,222,267 also describes a three-compartment cell. An electrolytic ~olution is electrolyzed in such a manner a~ to produce salt-free prodllct hydroxide and the corre~ponding acid salt of sodium bi~ul~ate. By way of example, a 10% sodium sulfate solution was introduced into a center feed compartment.
The ~low rate and pressure of the solution is sufficient for the solution to percolate through a Rorous diaphra~m into an anode compartment. The flow rate and pressure al~o prevents back migration or diffusion of protons toward the cell cathode. Water is introduced into the cathode compartment. Elec~rolysis in the cell produces a 2N sodium hydroxide catholyte effluent and a .075N acid anode effluent. AB in Patent No. 4,115,217, the compo~itions o~ the solutions, were not such that -5~
decomposition o~ th~ effluents, or corrosion of materials co~ventionally used in such a cell, were a problem.
A disclosure sLmilar to that of Patent NoO 3,222,267 is contained in U.S. Patent No. 3,523,7S5.
U.S. Patent No. 2,82g,095 discloses a process for the pxoduction of acidic solutions in an electrolytic cell using a plurality of anion exchange and cation exchange membranes. There is no disclosure concerning the produckion of halogen oxyacids.
U.S. Patent No. 4,504,373 discloses a three-compartment electrodialytic cell and feeding alkali metal sul~ate values to the cell.
a.s. Patent No. 4,740,281 discloses ~upplying a salt and acid to one compartment of an electrodialysis apparatus and a liquid containing water to a ~econd compartment of the apparatus. The process is for regenerating acids from stainle3s steel pickling baths.
Summary of the Invention The present invention re~ides in a process and apparakus for the electrolytic production of strong solutions of halogen oxyacids. The pre~ent invention comprises e~tablishing a solution of the corresponding alkali metal salt. An electrolytic cell is provided comprising an anode compartment containing an anode, a cathode compartment containing a cathode, and a middle feed cQmpartment in~ermediate the anode compartment and cathode compar~ment. The middle feed compartment is separated ~rom the anode compartment by a separator which .
.~ ' ' ' .
2Q3~,2 is either a porous diaphragm or a membr~ne selective to the migration of oxyhalogen ions, and from the cathode compartment by a cation selective membrane. Means are providPd for introducing said alkali metal salt solution into said feed compartment and for applying a voltage between the anode and the cathode. Under the influence of the applied voltage, the alkali metal ion~ formed by the di~a~sociation o~ the salt migrate through the cation Relective membrane to the cathode, reacting with electxochemically produced hydroxyl ions to form alkali metal hydroxide. The oxyhalogen ions foxmed by the di~as~ociation of the salt migrate through the porou~
diaphragm or membrane separator between the middle compartment and the anode compartment, to the anode, reacting with electrochemically formed proton~ to form halogen oxyacid. Mean~ are provided for cooling the electrolytic cell. It wa~ found that reducing the temperature~ in the electrolyklc cell allowed the use of membxanes, diaphragm~ and other materials of constxuction that would not otherwise be allowed, and also inhibit3 decompo~ition of the halogen oxyacid, that might otherwi~e occur. ~he electrolytic cell and acid product preferably are cooled to a tempexature in the range of about 10C to about 40C.
_7~ 2 ~.
Brief De~cription of the Drawin~
Further feature~ of the pres2nt invention will bacome apparent to tho~e skilled in the ar~ to which the present invention relates from readiny the following ~pecification with reference to the accompanyiny drawing in which the ~igure is a flow diagram illustrating the process Qf the presQnt invention.
De~cri~tion of Preferred ~mbodiment In tha ~ollowing description, the principals of the pre~ent invention will be disclosed by reference to a ~peci~ic halogen oxyacid product and ~ ~pecific alkali metal salt feed. The halogen oxyacid hereinafter disclosed iQ chloric acid and the alkali metal salt feed i~ ~odium chlorate. It will be apparent to those skilled in the art that the principal3 of the pre~ent invention, a~ herelnafter disclosed, are applicable to the generation of other halogen oxyacid~ u~ing as feed other alkali metal salt~.
Referring to the Figure, a chloric acid generator is disclosed. A ~odium chlorate solukion i~ contained in ~eed tank 12. This ~olution can be obtained by dissolving ~odium chlorata crystal3 in water, or by diluting a concentrated solution of soclium chlorate, by way of example. The ~odium chlorate solution is pumpad at a controlled feed rate, by a pump 14, through line 16 into ths middle feed compartment 18 of a thre2-compartment electrolytic cell 20. One type of pump 14 that can be used i~ a positive displacement pump.
-8- 2~7~
The electrolytic cell 20 comprises an anode compartment 22, containing an oxygen or chlorine evol~ing anode 24, and a ca~hode compartment 26 containing a cathode 28. The anode compar~ment 22 is separated from the middle compar~nent 18 by a separator 30. The separator 30 can be either a porous diaphra~m or a membrane which is selective to the migration of oxyhalogen ions. The cathode compartment 26 is separated from the middle compartment 18 ~y a cation-selective membrane 32, which is selective to the migration of cations.
In operation, the sodium chlorate in solution disassociates into positively charged sodium ions and negatively chaxged chlorate ions per the following equa~ion:
NaCl03 ~ Na~ + Cl03 ~l) Upon the influence of an impressed direct electric current in the cell 20, the cation constituents of the soditum chlorate ~olution, namely, positive sodium ions, pas~ through the cation-selective membrane 32 into the cathode compartment 26. Hydroxyl ions produced at the cathode 28 by the electrolysls of water react with the ~odium ions to produce sodium hydroxide, per the following reactions:
2~0 ~ 2e- ~ ~2 ~ 20H- (2) _9_ ~37~22 20~I- + 2Na~ ~ 2NaO~I (3) Overall: 2H20 + 2e~ + 2Na+ - H2 + 2NaOH (4) Under the influence of an Lmpressed direct electric current in the cell 20, some water is carried into the cathode compartment 26 with the sodium ions. This dilutes the sodium hydroxide, the diluted sodium hydroxide being withdrawn from the cathode compartment 26 through a catholyte ef~luent line 34. In the embodiment illustrated in the Figure, the catholyte effluent line 34 lead~ by way of example, to a sodium hydroxide storage tank 36.
Hydrogen i~ also produced in the cathode compartment 26 by the electroly~i~ of water, and is vented from the compaxtment by means of a hydrogen vent 38.
The negatively charged chlorate ions in the middle ~eed compartment 18 mlgrate through the separator 30 into the anode compartment 22. In the anode compartment, the negatively charged chlorate ions combine with protons produced at the anode 24 by the electroly~is of water to produce chloric acid. The following reac~ion~ take place:
H20 ~ 2H~ ~ 2 2 ~ 2e~ (5 2 ~
Cl03 ~ H+ ~ HCl03 ( Oxygen evolved in the anode compartment 22 can be vented to a~mo~phere in oxygen vent line 40. Chloric acid (~Cl03) is withdra~m from the anode compar~nent 22 in anolyte effluQnt line 42 to acid storage tank 44. Acid pxoduct is withdrawn from the acid ~torage tank in acid product line 46.
The overall reaction for the production of chloric acid is 5 3H20 ~ 2NaCl03 - 2HCl03 + H2 + 2NaO~I t 2 2 (7 C~loric acid i8 a ~trong oxidizing acid, and the separator 30, broadly, has to be resi~tant to this acid.
Further, the separator 30 should have such properties that it c~use~ a relatively low voltage drop in the acid generator, and allows anions to pas~ easily ~o that a high current density can be achieved. Broadly, the separator 30 can be either of a hydraulically porous nature, such as a diaphragm, or can ~e essentially hydraulically :~npervious to the bulk transport of electrolyte, such as an anion-~elective membrane.
In the generator 20 r proton~ are generated at the anode 24. Thera i8 a tendency, under the influence of an impre~ed direct electric current, for these proton~ to migrate to the cathode, which reduces the g~nerator efficiency by direct reaction with OH- ions generated at the cathode. Preferably, the diaphragm ~where the separator 30 is a diaphragm) has a pore ~ize and pore den~ity that creates a high fluid v~locity such as to re~ist migration of the protons through the diaphragm, while at the s~me time sufficient to allow the transport of chlorate ions from the middle compartment 18 into the anode compartment 22.
Where the ~eparator 30 is a porous diaphragm, a number of well known diaphragm materials which have resistance to oxidiz~ng acids and have good electrical properties can be employed. A preferred porous diaphragm is one made of polyvinylidens fluoride (PVDF).
Polyvinylidene fluoride ha~ good resistance to chemical attack by chloric aci~. The polyvinylidene fluoride diaphragms have recommended maximum service temperatures up to expected temperatures for the generator. The electrical and wetting properties of polyvinylidene fluoride are suitable for the process and apparatus of the pre~ent invention.
One ~uitable polyvinylidene ~luoride (PVDF) diaphragm i8 marketed by Porex Technologie~ Corp. under tha trademark POREX. The polyvinylidene fluoride (PVDF) is marketed by Pennwalt Corp. under the trademark KYNAR. The POREX diaphragm~ typically have an average pore ~ize of about 25 micron~, a void volume of about 40%, and a densi~y of about 1.05 grams per cubic centimeter. Ano~her -12~ 2 suitable polyvinylidene fluoride (PVDF) diaphragm is one marketed by Millipore Corporation under the trademark DURAPORE.
Examples of other material~ having re~i~tance to chloric acid are polytetrafluoroPthylene (PTFE), fibergla~s, polyvinyl chloride (PVC), ~tyrene-acrylonitrile, and ceramics. Mo~t hydrocarbons, such as rubber, are readily at~acXed by strong oxidizing agents.
One porous polyvinyl chloride (PVC) diaphragm commercially available is marketed by Microporous Products Division of Amerace Corporation under the trademark AMERSI~. Porous polytetrafluoroethylene (PTFE) diaphragm~
are commercially available from Millipore Corporation under the trademark "F~UOROGARD", and from Norton Company under the trademark "ZITEX". The wottability of polytetra~luoroethylene (PTFE) or other ~luorocarbon~ can be improved by treating the polytetra~luoroethylene or fluorocarbon with a ~urfactant such a~ ZONYL (trademark, E. I. DuPont de Nemours & Company). Alternatively, it i-~
possible to compound into the polytetrafluoroethylene(P~FE), in the manufacture of the diaphragm, a wettable acid resi~tant ~iller such as ground NAFION (trademark, E.
I. DuPont de Nemours ~ Company) or a ceramic such as boro~ilicate glass. NAFION is the trademark for a perfluorocarbon copolymer marXeted by E. I. DuPont de Nemours & Co. It 19 also possible to improve the wettability of polytetrafluoroethylene or other fluorocarbon diaphragms by treating the diaphragms with a -13~ 2 ~ ~ 7 ~ 2 2 N~FION solution. The diaphragms can also be porous NAFION.
Where ~he separator 30 is a membrane, the back migration of proton~ to the cathode can be minLmized by using a membrane which has ion-selective ~ualities necessary to ~uppress the flow of protons. An example of one such me~brane i~ a perfluorinated membrane marketed by Tosoh under the trademar~ TOSFLEX. Other mem~ranes selective to the migration of oxyhalogen ions include a fluoroethylene polymer/tetrafluoroethylene marketed by RAI
under khe trade designation "R4030", a tetrafluoroethylene membrane maxketed by RAI under trade de~ignation "R1030"
and a membrane marketed by Asahi glass under the trade designation "A~V".
The cation selective membrane 32 is commonly of the type consi~ting of a cation exchange resin prepared in the form of thin ~heets. A preferred membrane is a perfluorinated copolymer having pendant cation exchange functional groups. Broadly, these perfluorocarbons are a copolymer of at least two monomers with one monomer being ~elected from A group including vinyl fluoride, hexafluoropropylene, vinyliclene fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro (alkylvinyl ether), tetrafluoroethylene and mixtures thereof.. The second monomer often i~ selected ~rom a group of monomers usually containing an SO2F or sulfonyl fluoride pendant group. One suitable membrane i9 a -14~ 2 2 perfluorocarbon membrane marketed by E. I. DuPont de Nemours & Company under the trademark NAFION.
Since chloric acid is a strong oxidi~ing agent, many materials used for separator 30 will be more ~u~ceptible to corrosion by the chloric acid at higher temperatures than at lower temperatures. Substantial electrical resi~tance and heat build-up, particularly in the separator 30 and acro~ ~he middle compartmenk 18, at high current den~ities, can occur in the cell 20. An aspect of the pre~ent invention is cooling the cell 20 so as to maintain the separator 30 within a temperature range of abouk 10C to about 40C, preferably at a temperature near to room temperature of about 20C (68F).
The Figure discloses one method for cooling the cell 20. Referring to the Figure, a portion of the chloric acid in tank 44 is withdrawn in line 50 through circulation pump 52 to coil 54 o~ heat axchanger 56. The he~t exchanger 1~ maintained at a temperature effective to cool the acid in coil 54. The cooled acid i8 then recirculated to the anolyte compartment 22 by means of line 58. Line 58 contain~ a rotometer 60 which measures the c~mount of acid recirculated, and a flow control val~e 62 to control the amount recirculated.
In combina~ion with acid cooling, the chloric acid generator can compri~e a recirculation line 70 leading from the middle feed compartment 18 of the cell 20 to a coil 74 of hea~ exchanger 56, via recirculation pump 7~.
The coil 74 functions to cool at least a portion of the -15- 2~ 2~
sodium chlorate in the middle feed compar~ment 18. The cooled ~odium chlorate from coil 74 is retl~rn0d to the middle feed compartment 18 by mean~ of return line 77 which feed~ into feed line 16 of the generatox. In the e~bodLment illustrated in the Figure, the eombined flows of line 77 and line 16 lead to the middle compartment 18 of cell 20 by means of line 76.
It i~ po~sible that some decomposition of chloric acid can occur at elevated temperatures. If de3ired, additional pro~ision can be made for cooling the chloric aeid in aeid storage tank 44. Thi~ can be aecomplished by mean~ of reeirculation line 64 which takes part of the ~low from the acid cooling coil 54 and returns it directly ~o ~he aeid storage tanX 44. ln this way, ~he temperature of the chloric acid is maintained in a eooled state in both the cell 20 and in the acid ~torage tank 44.
The cooling medium in the heat exehanger 56 can be any conventional eooling medium. A chilled bath type heat exchanger can be employed. Preferably, the recirculation rate~ and rates of eooling the chloric acid and reciraulated chlorate solution are effeetive to maintain both ~he ehlorate ~olution and anolyte (chlorie acid) at a temperature in the range of about 10 to about 40C, more preerably about 20C (room temperature).
~he eathode 28 can be any ~uitable material eonventionally employed a~ a cathode. Preferred such materials are a niekel, steel or titanium expanded metal me~h ox sheet. ~lternatively, the eathode ean bg a gas -16- 2 ~J~
diffusion electrode such as disclosed in prior Patent No.
sulfuric acid, and sulfur dioxide. A product of the proces~ of this patent is enriched sulfuric acid (H2S04) instead of chloric acid. ThP process compri~es reacting the sodium chlorate in a reactor with the æulfur dioxide to produce a residual solution of sodium ~ulfate and sulfuric acid. Chlorine dioxide tC102) i~ also formed in the reactor and is removed in an inert gas stream. The re~idual selution containing sodium sulfate and sulfuric acid is fed into the middle compar ment of the electrolytic cell. The middle compartment is defined on one side by an anion selective membrane and on the oppo~ite Ride by a ca~ion selective membrane. The end compartment~ of the cell are an anode compartment separated from the middle compar~ment by the anian selective membrane and a cathode compartment ~eparated ~rom the middle compartment by ~he cation æelective membrane. When a voltage is applied to the cell, ~ulfate ions migrate from the middle compartment through the anion selective membrane into the anode compartment. At the anode, water is decomposed with the evolution of oxygen and generation of hydrogen ion~ which react with the migrated sulfate ions to form sulfuric acid. The chlorine dioxide formed in the reactor is fed into the catholyte of the three-compartment cell and is reduced to chlorite ions (C102-) at the cathode. The cation~ in the middle cempartment, mainly sodium and hydrogen ions, migrate through the cation selective membrane. The sodium ion~
react with the chlorite ions formed at the cathode to form sodium chlorite which is precipita~ed in the cathode compartment when saturation is reached.
Problem~ arise, however, when an electrolytic cell such as that di~closed in U.~. Patent No. 4,115,217 is attempted ~o be used in the manufacture of a ~trong halogen nxyacid such as chloric acid. For one, such cells are known to generate substantial amounts of heat because of solution and separator resistance, which can lead to acid decomposition. In addition, the halogen oxyacids at high temperature are highly corro4ive preventing many m~terials conventionally employed in electrolytic cells ~rom being u~ed in association with the oxyacid~.
U.S. Patent No. 3,222,267 also describes a three-compartment cell. An electrolytic ~olution is electrolyzed in such a manner a~ to produce salt-free prodllct hydroxide and the corre~ponding acid salt of sodium bi~ul~ate. By way of example, a 10% sodium sulfate solution was introduced into a center feed compartment.
The ~low rate and pressure of the solution is sufficient for the solution to percolate through a Rorous diaphra~m into an anode compartment. The flow rate and pressure al~o prevents back migration or diffusion of protons toward the cell cathode. Water is introduced into the cathode compartment. Elec~rolysis in the cell produces a 2N sodium hydroxide catholyte effluent and a .075N acid anode effluent. AB in Patent No. 4,115,217, the compo~itions o~ the solutions, were not such that -5~
decomposition o~ th~ effluents, or corrosion of materials co~ventionally used in such a cell, were a problem.
A disclosure sLmilar to that of Patent NoO 3,222,267 is contained in U.S. Patent No. 3,523,7S5.
U.S. Patent No. 2,82g,095 discloses a process for the pxoduction of acidic solutions in an electrolytic cell using a plurality of anion exchange and cation exchange membranes. There is no disclosure concerning the produckion of halogen oxyacids.
U.S. Patent No. 4,504,373 discloses a three-compartment electrodialytic cell and feeding alkali metal sul~ate values to the cell.
a.s. Patent No. 4,740,281 discloses ~upplying a salt and acid to one compartment of an electrodialysis apparatus and a liquid containing water to a ~econd compartment of the apparatus. The process is for regenerating acids from stainle3s steel pickling baths.
Summary of the Invention The present invention re~ides in a process and apparakus for the electrolytic production of strong solutions of halogen oxyacids. The pre~ent invention comprises e~tablishing a solution of the corresponding alkali metal salt. An electrolytic cell is provided comprising an anode compartment containing an anode, a cathode compartment containing a cathode, and a middle feed cQmpartment in~ermediate the anode compartment and cathode compar~ment. The middle feed compartment is separated ~rom the anode compartment by a separator which .
.~ ' ' ' .
2Q3~,2 is either a porous diaphragm or a membr~ne selective to the migration of oxyhalogen ions, and from the cathode compartment by a cation selective membrane. Means are providPd for introducing said alkali metal salt solution into said feed compartment and for applying a voltage between the anode and the cathode. Under the influence of the applied voltage, the alkali metal ion~ formed by the di~a~sociation o~ the salt migrate through the cation Relective membrane to the cathode, reacting with electxochemically produced hydroxyl ions to form alkali metal hydroxide. The oxyhalogen ions foxmed by the di~as~ociation of the salt migrate through the porou~
diaphragm or membrane separator between the middle compartment and the anode compartment, to the anode, reacting with electrochemically formed proton~ to form halogen oxyacid. Mean~ are provided for cooling the electrolytic cell. It wa~ found that reducing the temperature~ in the electrolyklc cell allowed the use of membxanes, diaphragm~ and other materials of constxuction that would not otherwise be allowed, and also inhibit3 decompo~ition of the halogen oxyacid, that might otherwi~e occur. ~he electrolytic cell and acid product preferably are cooled to a tempexature in the range of about 10C to about 40C.
_7~ 2 ~.
Brief De~cription of the Drawin~
Further feature~ of the pres2nt invention will bacome apparent to tho~e skilled in the ar~ to which the present invention relates from readiny the following ~pecification with reference to the accompanyiny drawing in which the ~igure is a flow diagram illustrating the process Qf the presQnt invention.
De~cri~tion of Preferred ~mbodiment In tha ~ollowing description, the principals of the pre~ent invention will be disclosed by reference to a ~peci~ic halogen oxyacid product and ~ ~pecific alkali metal salt feed. The halogen oxyacid hereinafter disclosed iQ chloric acid and the alkali metal salt feed i~ ~odium chlorate. It will be apparent to those skilled in the art that the principal3 of the pre~ent invention, a~ herelnafter disclosed, are applicable to the generation of other halogen oxyacid~ u~ing as feed other alkali metal salt~.
Referring to the Figure, a chloric acid generator is disclosed. A ~odium chlorate solukion i~ contained in ~eed tank 12. This ~olution can be obtained by dissolving ~odium chlorata crystal3 in water, or by diluting a concentrated solution of soclium chlorate, by way of example. The ~odium chlorate solution is pumpad at a controlled feed rate, by a pump 14, through line 16 into ths middle feed compartment 18 of a thre2-compartment electrolytic cell 20. One type of pump 14 that can be used i~ a positive displacement pump.
-8- 2~7~
The electrolytic cell 20 comprises an anode compartment 22, containing an oxygen or chlorine evol~ing anode 24, and a ca~hode compartment 26 containing a cathode 28. The anode compar~ment 22 is separated from the middle compar~nent 18 by a separator 30. The separator 30 can be either a porous diaphra~m or a membrane which is selective to the migration of oxyhalogen ions. The cathode compartment 26 is separated from the middle compartment 18 ~y a cation-selective membrane 32, which is selective to the migration of cations.
In operation, the sodium chlorate in solution disassociates into positively charged sodium ions and negatively chaxged chlorate ions per the following equa~ion:
NaCl03 ~ Na~ + Cl03 ~l) Upon the influence of an impressed direct electric current in the cell 20, the cation constituents of the soditum chlorate ~olution, namely, positive sodium ions, pas~ through the cation-selective membrane 32 into the cathode compartment 26. Hydroxyl ions produced at the cathode 28 by the electrolysls of water react with the ~odium ions to produce sodium hydroxide, per the following reactions:
2~0 ~ 2e- ~ ~2 ~ 20H- (2) _9_ ~37~22 20~I- + 2Na~ ~ 2NaO~I (3) Overall: 2H20 + 2e~ + 2Na+ - H2 + 2NaOH (4) Under the influence of an Lmpressed direct electric current in the cell 20, some water is carried into the cathode compartment 26 with the sodium ions. This dilutes the sodium hydroxide, the diluted sodium hydroxide being withdrawn from the cathode compartment 26 through a catholyte ef~luent line 34. In the embodiment illustrated in the Figure, the catholyte effluent line 34 lead~ by way of example, to a sodium hydroxide storage tank 36.
Hydrogen i~ also produced in the cathode compartment 26 by the electroly~i~ of water, and is vented from the compaxtment by means of a hydrogen vent 38.
The negatively charged chlorate ions in the middle ~eed compartment 18 mlgrate through the separator 30 into the anode compartment 22. In the anode compartment, the negatively charged chlorate ions combine with protons produced at the anode 24 by the electroly~is of water to produce chloric acid. The following reac~ion~ take place:
H20 ~ 2H~ ~ 2 2 ~ 2e~ (5 2 ~
Cl03 ~ H+ ~ HCl03 ( Oxygen evolved in the anode compartment 22 can be vented to a~mo~phere in oxygen vent line 40. Chloric acid (~Cl03) is withdra~m from the anode compar~nent 22 in anolyte effluQnt line 42 to acid storage tank 44. Acid pxoduct is withdrawn from the acid ~torage tank in acid product line 46.
The overall reaction for the production of chloric acid is 5 3H20 ~ 2NaCl03 - 2HCl03 + H2 + 2NaO~I t 2 2 (7 C~loric acid i8 a ~trong oxidizing acid, and the separator 30, broadly, has to be resi~tant to this acid.
Further, the separator 30 should have such properties that it c~use~ a relatively low voltage drop in the acid generator, and allows anions to pas~ easily ~o that a high current density can be achieved. Broadly, the separator 30 can be either of a hydraulically porous nature, such as a diaphragm, or can ~e essentially hydraulically :~npervious to the bulk transport of electrolyte, such as an anion-~elective membrane.
In the generator 20 r proton~ are generated at the anode 24. Thera i8 a tendency, under the influence of an impre~ed direct electric current, for these proton~ to migrate to the cathode, which reduces the g~nerator efficiency by direct reaction with OH- ions generated at the cathode. Preferably, the diaphragm ~where the separator 30 is a diaphragm) has a pore ~ize and pore den~ity that creates a high fluid v~locity such as to re~ist migration of the protons through the diaphragm, while at the s~me time sufficient to allow the transport of chlorate ions from the middle compartment 18 into the anode compartment 22.
Where the ~eparator 30 is a porous diaphragm, a number of well known diaphragm materials which have resistance to oxidiz~ng acids and have good electrical properties can be employed. A preferred porous diaphragm is one made of polyvinylidens fluoride (PVDF).
Polyvinylidene fluoride ha~ good resistance to chemical attack by chloric aci~. The polyvinylidene fluoride diaphragms have recommended maximum service temperatures up to expected temperatures for the generator. The electrical and wetting properties of polyvinylidene fluoride are suitable for the process and apparatus of the pre~ent invention.
One ~uitable polyvinylidene ~luoride (PVDF) diaphragm i8 marketed by Porex Technologie~ Corp. under tha trademark POREX. The polyvinylidene fluoride (PVDF) is marketed by Pennwalt Corp. under the trademark KYNAR. The POREX diaphragm~ typically have an average pore ~ize of about 25 micron~, a void volume of about 40%, and a densi~y of about 1.05 grams per cubic centimeter. Ano~her -12~ 2 suitable polyvinylidene fluoride (PVDF) diaphragm is one marketed by Millipore Corporation under the trademark DURAPORE.
Examples of other material~ having re~i~tance to chloric acid are polytetrafluoroPthylene (PTFE), fibergla~s, polyvinyl chloride (PVC), ~tyrene-acrylonitrile, and ceramics. Mo~t hydrocarbons, such as rubber, are readily at~acXed by strong oxidizing agents.
One porous polyvinyl chloride (PVC) diaphragm commercially available is marketed by Microporous Products Division of Amerace Corporation under the trademark AMERSI~. Porous polytetrafluoroethylene (PTFE) diaphragm~
are commercially available from Millipore Corporation under the trademark "F~UOROGARD", and from Norton Company under the trademark "ZITEX". The wottability of polytetra~luoroethylene (PTFE) or other ~luorocarbon~ can be improved by treating the polytetra~luoroethylene or fluorocarbon with a ~urfactant such a~ ZONYL (trademark, E. I. DuPont de Nemours & Company). Alternatively, it i-~
possible to compound into the polytetrafluoroethylene(P~FE), in the manufacture of the diaphragm, a wettable acid resi~tant ~iller such as ground NAFION (trademark, E.
I. DuPont de Nemours ~ Company) or a ceramic such as boro~ilicate glass. NAFION is the trademark for a perfluorocarbon copolymer marXeted by E. I. DuPont de Nemours & Co. It 19 also possible to improve the wettability of polytetrafluoroethylene or other fluorocarbon diaphragms by treating the diaphragms with a -13~ 2 ~ ~ 7 ~ 2 2 N~FION solution. The diaphragms can also be porous NAFION.
Where ~he separator 30 is a membrane, the back migration of proton~ to the cathode can be minLmized by using a membrane which has ion-selective ~ualities necessary to ~uppress the flow of protons. An example of one such me~brane i~ a perfluorinated membrane marketed by Tosoh under the trademar~ TOSFLEX. Other mem~ranes selective to the migration of oxyhalogen ions include a fluoroethylene polymer/tetrafluoroethylene marketed by RAI
under khe trade designation "R4030", a tetrafluoroethylene membrane maxketed by RAI under trade de~ignation "R1030"
and a membrane marketed by Asahi glass under the trade designation "A~V".
The cation selective membrane 32 is commonly of the type consi~ting of a cation exchange resin prepared in the form of thin ~heets. A preferred membrane is a perfluorinated copolymer having pendant cation exchange functional groups. Broadly, these perfluorocarbons are a copolymer of at least two monomers with one monomer being ~elected from A group including vinyl fluoride, hexafluoropropylene, vinyliclene fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro (alkylvinyl ether), tetrafluoroethylene and mixtures thereof.. The second monomer often i~ selected ~rom a group of monomers usually containing an SO2F or sulfonyl fluoride pendant group. One suitable membrane i9 a -14~ 2 2 perfluorocarbon membrane marketed by E. I. DuPont de Nemours & Company under the trademark NAFION.
Since chloric acid is a strong oxidi~ing agent, many materials used for separator 30 will be more ~u~ceptible to corrosion by the chloric acid at higher temperatures than at lower temperatures. Substantial electrical resi~tance and heat build-up, particularly in the separator 30 and acro~ ~he middle compartmenk 18, at high current den~ities, can occur in the cell 20. An aspect of the pre~ent invention is cooling the cell 20 so as to maintain the separator 30 within a temperature range of abouk 10C to about 40C, preferably at a temperature near to room temperature of about 20C (68F).
The Figure discloses one method for cooling the cell 20. Referring to the Figure, a portion of the chloric acid in tank 44 is withdrawn in line 50 through circulation pump 52 to coil 54 o~ heat axchanger 56. The he~t exchanger 1~ maintained at a temperature effective to cool the acid in coil 54. The cooled acid i8 then recirculated to the anolyte compartment 22 by means of line 58. Line 58 contain~ a rotometer 60 which measures the c~mount of acid recirculated, and a flow control val~e 62 to control the amount recirculated.
In combina~ion with acid cooling, the chloric acid generator can compri~e a recirculation line 70 leading from the middle feed compartment 18 of the cell 20 to a coil 74 of hea~ exchanger 56, via recirculation pump 7~.
The coil 74 functions to cool at least a portion of the -15- 2~ 2~
sodium chlorate in the middle feed compar~ment 18. The cooled ~odium chlorate from coil 74 is retl~rn0d to the middle feed compartment 18 by mean~ of return line 77 which feed~ into feed line 16 of the generatox. In the e~bodLment illustrated in the Figure, the eombined flows of line 77 and line 16 lead to the middle compartment 18 of cell 20 by means of line 76.
It i~ po~sible that some decomposition of chloric acid can occur at elevated temperatures. If de3ired, additional pro~ision can be made for cooling the chloric aeid in aeid storage tank 44. Thi~ can be aecomplished by mean~ of reeirculation line 64 which takes part of the ~low from the acid cooling coil 54 and returns it directly ~o ~he aeid storage tanX 44. ln this way, ~he temperature of the chloric acid is maintained in a eooled state in both the cell 20 and in the acid ~torage tank 44.
The cooling medium in the heat exehanger 56 can be any conventional eooling medium. A chilled bath type heat exchanger can be employed. Preferably, the recirculation rate~ and rates of eooling the chloric acid and reciraulated chlorate solution are effeetive to maintain both ~he ehlorate ~olution and anolyte (chlorie acid) at a temperature in the range of about 10 to about 40C, more preerably about 20C (room temperature).
~he eathode 28 can be any ~uitable material eonventionally employed a~ a cathode. Preferred such materials are a niekel, steel or titanium expanded metal me~h ox sheet. ~lternatively, the eathode ean bg a gas -16- 2 ~J~
diffusion electrode such as disclosed in prior Patent No.
4,377,496 entitled ~Ga~ Diffusion Electrode and Proces~
A gas diffusien electrode, as di~closed in these patents, changes the cathode reaction to eliminate production of hydrogen while continuing ~o produce hydroxyl gxoups.
Therefore, hydrogen is no longer evolved. The cell voltage is also ubstantially reduced resulting in a lower co~t of electrical energy, as is reported in Patent No.
4,377,496. The disclosure of this prior patent is incorporated herein by reference.
The anode can ~e either chlorine evolving or oxygen evolving dapending on the electrolyte com~osition. A
preferred anode is dimensionally stable. That is, th~
thickne~s of the anode does not decrease significantly during use. Such anodes usually comprise a film-forming valva me~al ~ubstrate, such as titanium, tantalum, zirconium, niobium tungsten, and alloys thereof, which has the capacity to conduct an electrolyte current in the cathodic direction and to re~ist the passage o~ current in the anodic direction. These metals are also resistant to corrosion in tha electrolytes at conditions u~ed within an electrolytic cell. A preferred valvQ metal, based on C08t, availability, and electrical and chemical propertiss, i~ titanlum. It i5 well known that in the anodic direction, the valve metals passivate, that is the resistancQ o~ the valve metals to the passage of current goes up rapidly due to the formation of an oxide layer thereon. It is therefore customary to apply -17~
electrs~chemically active coatings to the valve metal sllbstrate. The electrochemically active coatings have the capacity to continue to conduct current to the electrolyte, for example by the evolution of oxygen, over 5 long period~ of time without becoming pas~ivated. Such coatings are those provided from platinum or other platinum group metals or they can be repre~ented by active oxide coatings such as platinum group metal oxide~, magnetite, ferrite, ~pinels, e.g. cohalt oxide, or mixed metal oxide coatings. The coatings also preferably c4ntain at least one oxids o~ a valve metal with at least one oxide of a platinum group metal including platinum, palladium, xhodium, iridium, and ruthenium or mixtures thereof and with other metals.
An example of one such dimensionally stable oxygen evolving anode i8 a titanium sub5 trate which has been coated with a precious metal oxide and valve metal oxide coating. This anode is marketed by the assignee of $he pre~ent application under the trade designation EC-600.
The anode 24 may be in the form of a sheet or expanded metal mesh. Examples of suitable chlorine evolving anodes that can be used in the present invention are di~closed in U.S. Patents No~. 3,632,498; 3,751,296; 3,778,307;
3,840,443 and 3~,933,616.
For the production of perchloric acid, a lead oxide or platinum anode should be used.
-78~
In operation, the flow diagram for the chloric acid generator i5 essentially the same where the separator 30 is either a diaphxagm or a membrane.
Where the separator 30 is a diaphragm, the concentration of the sodium chlorate solution in feed line 16 can be a function primarily of the concentration of the acid de~ired in lines 42, 46. Chloric acid, at xoom temperature can decompose spontaneously at a concentration above about 3.6N. To produce chloric acid at a concentration less than 3.6N, it is necessary to feed to the middle compartment 18 a sodium chlorate ~olution having a normality le~s than about 4. The concentration of the chloric acid in lines 42, 46 is preferably above about 0.1. This requires a sodium chlorate feed having a normality more than about 0.1. Thus, the concentration of the ~odium chloràte feed preferably is in the range of about O.lN to about 4N.
Howevert an alternate method of operation is to ~eed a more concentrated chlorate solution to middle compartment 18, but to d11ute the acid in lines 42, 46 with water 80 that the acld does not decompose. The alternate method has ~he ad~antage that it reduces the coll voltage re~uired, ~ince ~he conductivity of chlorate ~olu~ion is higher at higher concentrations.
In thi~ alternate method of operation, the concentration of the sodium chlorate ~olution in feed linQ
16 ~hould be leas than that at which precipitation of sodi~m chlorate occurs. Sodium chlorate ha~ a maximum -19- 2~'3~
solubility of 7.4M at 0C and 10.7~ a~ 23~C. The middle compartment 18 lo es water with the migr~tion of sodium ion3 to the cathode 28, and due to inefficiencies. The concen~ration of the chlorate ~olution in feed line 16 should take into account this loss of water, and thus can be near but should be ~omewhat less than the solubility lLmits, e.g., 10.7M, assuming the temperature in the cell to be about room temperature. This alternate method is partlcularly applicable where the separator 30 is a membrane.
For the production of chlorine dioxide in an electrolytic cell as disclosed in prior Patent No.
4,798,715, the chloric acid concentration pre~erably is above about 1.5, more preferably above about 2. This requires tha~ the sodiurn chlorate solution feed in line 16 pre~erably ha~e a nonnality of at least about 2.
The feed rate o~ the sodiu~l chlorate solution in line 16 is a function of the amount of chlorate ion in the product lines 42, 46, the concentration of the sodiurn chlorate solution, and the water balance in the generator.
The f~ed rate and concentration of the ~odi~n chlorate ~olution can both be ad~usted depending upon chlorate ion and water balances. An aspect of the pre~ent in~ention is that the feed of the ~odlum chlorate solution in lins 16, when the separator 30 iB a diaphra~n, suppresses the back-migratlon of protons through the diaphra~n 30 to the cathode 28~ ~his is important where an acid product of high normality is desired. The higher the norrnality o~
-20- ~J7~22 the acid, the higher the proton content in the anode compartment 22~ and the greater the likelihood of back-migration of protons ~o the cathode 28.
Preferably~ ~he generator of the present invention is operated at a relatiYely high current density, to reduce capital costs. Satisfactory results can be obtained with current densities in the range of about 2-5 kiloamp~ per s~uare m~tex.
The following Examples illustrate the present invention.
Example 1 A chloric acid generator 20 a~ shown in the Figure was operated to convert ~odium chlorate into ~odium hydroxide and chloric acid. The generator 20 employed an expanded mesh titanium anode 24 having a precious metal oxi.de coating marketed by the assignee of the present application under the trade de~ignation EC-600. The cathode 28 wa~ titanium mesh. The diaphragm 30 was a ~heet o~ porous polytetra~luoroethylene ("Kynar") marketed by Porex Technologies Corp. under the trademark Porex.
The diaphragm typically has an average void volume of about 40~ and a density of about 1.05 grams per cubic centimeter. Average pore size is about 25 microns. The diaphragm ha~ a service temperature up to about 300F
(149C). The membrane 32 was made of NAFION 324 (trademark, B. I. DuPont de Nemours & Co.). The generator wa~ constructed of chlorinated polyvinylchloride. The men~rane 32 and diaphragm 30 had an active area o~ 20 -21- 2~3~2~
square centimetexs. The middle compartment 18 gap was 0.25 inches (0~64 cm).
The generator was operated under the following conditionss Cuxrenk Density 4 kiloamps per square meter Volkage 8.5 volts The following Table 1 shows measured temperatures, concenkrations~ and flow rates at various points in the generator. The chloric acid and sodium hydroxide product concentrations were measured to determine current efficiency at the anode and at the cathode.
~r r~ D
D ~ 0 o o o ~ ~ ~~ ~D 2 ~
o~D
U~ C~ o . . .
U ~D rAI M O O
NO
~rI U
u~ ~
'~I O ~ ~ o. .
~a ~ o ~ OOO-DO
cn ~ P. ~
~D
~1~ ~ ~ O
o ~! ~ a~o o ~ ~ ~1 h~ UIn O ~ ~ o o ~ o _lO
N
~o ~
~ ~ o~Co O ~ ~ U7~ooo N
C) U
I O Zæ~z ~ . .
h 1~ U 1!4 U~ o _~3_ 2~
Table 1 shows that both cooled acid and cooled salt were recircula~ed from the heat exchanger 56, in lines 64 and 77, re~pectively, at 16C. This maintained the anolyte in line 42 at about 26.5C ~the cell 20 nd diaphragm 30 being at about the same temperature), and the chloric acid product in line 46 at about 18.6C. The concentration of the chloric acid product obtained in line 46 was 2.06N. Cathode and anode current efficiencies were determined by dividing the actual production rate~ by the theoretical rates and multiplying by 100. Theoretical rates were ~a~ed on amperage.
The following cell efficiencies were obtained:
Cathode (NaOH) CE~ 29.1 Anode lHC103) CE~ 23.6 The generator was disassembled at the end of 200 hours on line and the Porex diaphragm wa~ in excellent condition. The generator has ~een succe~sfully operated for longer period~.
~ample 2 The generator described in Example 1 was u~ed. It was operated at the same current density of 4 Xilo~mps pex squaxe meter as ~n Example 1. The voltage drop in the call was 7.5 as compared to 8.5 in Ex~mple 1. The following ~abla 2 gives mea~ured temperature, concentrations and flow rate~ at various point3 in the generator. ~he ma~or difference of operation from Example 1 was ths absence of cooled recirculated chlorate ~olution in line 77 from heat exchanger 56. Thus, the cell was -24- 2~ 2~5 operated at a higher temperatnre of about 59C, as evidenced by th~ temperature in anolyte product line 42.
. .
~: o ~
" ~ a) ~ _, ,, O o r~l O
;o O~ o o ~.~ Ln o~
a~
r '~ U ~ ~D
~ x ~ u ~r~ o~ ~ o . .
` o ~r ~ ~o c~ ~ u ~ ~ ~ o oo ~ ~ a) ~ cO Oo ~ ~ o ~
~ ~ ~rl ~ _~_100 E~
r~
r~
~ o a o ~1 V In ~ ~ O
o a~ 0 u~ ~ooo ~:: h~ ~'1 o ~
2;V0 Ul ~1 O _l El C~ ~, In O
: ', . . .
..
. ... .
., .
.
.
. . . ~
2 ~ 2 -~6-The generator functioned for only 36 hsurs before exces~ive corro~ion of the diaphragm 30 occurred. Current efficiencies were determined~ as followss Cathode (NaOH) CE% 28.6 Anode (HC103) CE% 11.3 ~he co~centration of the chloric acid in line 46 wa~
only 1.8N. Thus, although the c811 efficiency increased slightly for sodium hydroxide production when compared with Example 1, the cQll efficiency wa~ significantly less ~or acid production. The comparative data of Example 1 illu~tr~te~ the importance of maintaining the generator 20, in the production of oxyhalogen acids, at a relati~ely low temperatura. Example 1 also demonstrate~ that by sufficiently cooling the cell 20, the life of the diaphra~m 30 can be 3ignificantly extended.
One principle use for chloric acid is as a precursor in the manufacture o chlorine dioxide (Cl02). ~hlorine dioxide is a strong oxidizing agent. Some of the market areas or chlorine dioxide are: water treatment, pulp and paper processing, flour processing, municipal waste treatment, petroleum well in~ection, crop and meat storage, and bleaching such materials as textiles, oils, shellacs, varnishes, waxes, and straw products.
For water treatment, the use of chlorine dioxide is particularly attractive a~ environmental regulations are being tight~n~d concerning the production of chlorinated organics and trihalomethanes ~TH~8). Trihalomethanes are signi~icantly reduced with the use of chlorine dio~ide in~tead o chlorina as a reactant.
. .
-27~
One chlorine dioxide generAting proces~, known as the MathiQson process, reacts sulfur dioxide with ~odium chlorate (NaCl03) and sulfuric acid to produce chlorine dioxide. This reaction also produce~ an undesirable 5 sodium bi~ulfate ~alt cake (NaHSO4). When this process is used in the pulp and paper industry, excess salt cake causes an im~alance for mills ~rying to reduce sulfur emissions.
An advantage of the chloric acid process of the present in~ention is that it can be integrated into the production of chlorine dioxide, for the pulp and paper indu~try, without ~he production of unwanted sodium bisulfate salt cake. When the separator 30 of the electrolytic cell of the pre~ent invention is an anionic membrane, a substantially pure chloric acid stre~m is produced in lins 46. The chloric acid can then chemically react with sulfur dioxide to produce a chlorine dioxide product ~tream, and a by-product stream which i9 sulfuric acid. Sulfuric acid, in contraqt with sodiwn bisulfate, is a useful by-product. If a diaphragm is used as the separator 30, in place of an anionic membrane, some sodium chlorate rom the middle feed compartment 18 of the chloric acid generator flows into the anode compar~ment 22 making a mixture, in product line 46, of chloric acid and ~odi~n chlorate. The sodium chlorate reacts with sulfur dloxide and sulfuric acid, a~ in the Mathie~on process, to ~orm some salt cake. However, the sulfur dioxide can be reacted preferentially with the chloric acid and sulfuric -28- 2~3~
acid will be produced with very little salt cake. ~he sodium bisulfate salt ~hat i5 produced can be sepàrated from the sulfuric acid and recycled to the feed compartment 18 of the chloric acid generator of the present invention. In the chloric acid yenerator, the sodium bisulfate is converted to additional sodium hydroxide and sulfuric acid.
Processes similar to the Mathieson process, which use sodium chlorate and sulfuric acid as feed components, are known as the Solvay, R-2 and SVP proce~ses. In the Solvay process, sodium chlorate i~ reacted wi-th sulfuric acid and a reducing agent such as methanol to produce chloxine dioxide. Thi~ proce~s produce~, as a by-product, sodium sulfate (Na2S04) which is of little value. By the present invention, chloric acid can be reacted directly with a reducing a~ent, such as methanol, to produce chlorine d~oxlde without producing the ~alt by-product. In the R-2 and SVP processes, sodium chlorate, sodium chloride, and ~ul~uric acid are reacted to produce chlorine dioxide.
The processes also produce chlorine gas (C12) and sodium sulfate as by-products. The chlorine gas is a particularly undesirable by-product.
The production of chlorine dioxide (C102) contaminated with chlorin~ gas (C12) is also di~closed in two patents, 2S No. 4,806,215 "Combined Process for Production of Chloxine Dioxide and Sodium Hydroxide", and No. 4,853,0g6 "Production of Chlorine Dioxide in an Electrolytic Cell".
~he first patent is a three compartment cell which ~37~2?.
produces NaOH, Cl2 and Cl02 from HC1 and ~aCl03. The second paten~, in Fig. 3, de~eribes a proeess for reducing, but not eliminating, the chlorine that contaminates the chlorine dioxide.
In the pre~ent invention, th2 ehloric acid in line 46 can be reduced directly to chlorine dioxide by feeding the chloric acid to an eleetrochemieal cell as disclosed in U.S. Patent No. 4~798,715, discuRsed abo~e, assigned to the a~signee of the present application. An advantage of this reduction i~ that the ehlorine dio~ide is produced completely free of chlorine by-product ga~. The di~closure of U.S. Patent No. 4,798,715 i9 ineorporated by referenee herein.
Other u~es ~or chloric acid are as a catalyst in the polymerization of acrylonitrile, and in the production of perchloric acid. In Kirk Othmer, Vol. 5, page 656, it i5 disclosed that chloric acid can be electrolytically oxidized to perehloric aeid in an electrochemical proce~s.
Perchlorie acid uses are in medicine, in analytical chemistry, a~ a eatalyst in the manufacture of various esters, as an ingredient of an electrolytic bath in the depo~itioll of lead, in electro-polishing, and in the manufacture of Qxplosive~. The conventional mathod for produeing ammonium perehlora~e, diselosed in Kirk Othmer, Vol. 5, pg~ 660, is to reaet sodium perchlorate, ammonia, and hydxoehloric acid to produce ammonium perchlorate and by~produet sodium chloride:
-30- ~3~
NaC10~ + NH3 + HCl ~ NH4~104 ~ NaCl (8) In accordance with the present invention, æodium chlorate can be converted to chloric acid and odium hydroxide with the three eompartmPnt cell as described above in reactions 1 through 7. The chloric acid is then eleetrolytieally oxidized to perchlorie aeid using the electrochemical proees~ described abovs and on pg. 656, Vol. S of Kirk Othmer. The perchloric acid ean then be reaeted with ammonia to produee ammonium perchlorate:
NH3 J~ HCl 04 ~ N~I4 ~10~ ( 9 ) A~ an altexnative, sodium perchlorate ean be eon~erted to perehlorie aeid and sodium hydroxide in the three compartment aeid generator. The perchloric acid can then ba reaeted with ammonia to produee ammonium perehlorate a~ in reaetion (9).
From the above deseription of a preferred embodiment o~ the invention, those skilled in the art will perceive improvements, ehanges and modification~. Such improvements, changes and modifieations within the skill of the art are intended to be eovered by the appended elalm~.
A gas diffusien electrode, as di~closed in these patents, changes the cathode reaction to eliminate production of hydrogen while continuing ~o produce hydroxyl gxoups.
Therefore, hydrogen is no longer evolved. The cell voltage is also ubstantially reduced resulting in a lower co~t of electrical energy, as is reported in Patent No.
4,377,496. The disclosure of this prior patent is incorporated herein by reference.
The anode can ~e either chlorine evolving or oxygen evolving dapending on the electrolyte com~osition. A
preferred anode is dimensionally stable. That is, th~
thickne~s of the anode does not decrease significantly during use. Such anodes usually comprise a film-forming valva me~al ~ubstrate, such as titanium, tantalum, zirconium, niobium tungsten, and alloys thereof, which has the capacity to conduct an electrolyte current in the cathodic direction and to re~ist the passage o~ current in the anodic direction. These metals are also resistant to corrosion in tha electrolytes at conditions u~ed within an electrolytic cell. A preferred valvQ metal, based on C08t, availability, and electrical and chemical propertiss, i~ titanlum. It i5 well known that in the anodic direction, the valve metals passivate, that is the resistancQ o~ the valve metals to the passage of current goes up rapidly due to the formation of an oxide layer thereon. It is therefore customary to apply -17~
electrs~chemically active coatings to the valve metal sllbstrate. The electrochemically active coatings have the capacity to continue to conduct current to the electrolyte, for example by the evolution of oxygen, over 5 long period~ of time without becoming pas~ivated. Such coatings are those provided from platinum or other platinum group metals or they can be repre~ented by active oxide coatings such as platinum group metal oxide~, magnetite, ferrite, ~pinels, e.g. cohalt oxide, or mixed metal oxide coatings. The coatings also preferably c4ntain at least one oxids o~ a valve metal with at least one oxide of a platinum group metal including platinum, palladium, xhodium, iridium, and ruthenium or mixtures thereof and with other metals.
An example of one such dimensionally stable oxygen evolving anode i8 a titanium sub5 trate which has been coated with a precious metal oxide and valve metal oxide coating. This anode is marketed by the assignee of $he pre~ent application under the trade designation EC-600.
The anode 24 may be in the form of a sheet or expanded metal mesh. Examples of suitable chlorine evolving anodes that can be used in the present invention are di~closed in U.S. Patents No~. 3,632,498; 3,751,296; 3,778,307;
3,840,443 and 3~,933,616.
For the production of perchloric acid, a lead oxide or platinum anode should be used.
-78~
In operation, the flow diagram for the chloric acid generator i5 essentially the same where the separator 30 is either a diaphxagm or a membrane.
Where the separator 30 is a diaphragm, the concentration of the sodium chlorate solution in feed line 16 can be a function primarily of the concentration of the acid de~ired in lines 42, 46. Chloric acid, at xoom temperature can decompose spontaneously at a concentration above about 3.6N. To produce chloric acid at a concentration less than 3.6N, it is necessary to feed to the middle compartment 18 a sodium chlorate ~olution having a normality le~s than about 4. The concentration of the chloric acid in lines 42, 46 is preferably above about 0.1. This requires a sodium chlorate feed having a normality more than about 0.1. Thus, the concentration of the ~odium chloràte feed preferably is in the range of about O.lN to about 4N.
Howevert an alternate method of operation is to ~eed a more concentrated chlorate solution to middle compartment 18, but to d11ute the acid in lines 42, 46 with water 80 that the acld does not decompose. The alternate method has ~he ad~antage that it reduces the coll voltage re~uired, ~ince ~he conductivity of chlorate ~olu~ion is higher at higher concentrations.
In thi~ alternate method of operation, the concentration of the sodium chlorate ~olution in feed linQ
16 ~hould be leas than that at which precipitation of sodi~m chlorate occurs. Sodium chlorate ha~ a maximum -19- 2~'3~
solubility of 7.4M at 0C and 10.7~ a~ 23~C. The middle compartment 18 lo es water with the migr~tion of sodium ion3 to the cathode 28, and due to inefficiencies. The concen~ration of the chlorate ~olution in feed line 16 should take into account this loss of water, and thus can be near but should be ~omewhat less than the solubility lLmits, e.g., 10.7M, assuming the temperature in the cell to be about room temperature. This alternate method is partlcularly applicable where the separator 30 is a membrane.
For the production of chlorine dioxide in an electrolytic cell as disclosed in prior Patent No.
4,798,715, the chloric acid concentration pre~erably is above about 1.5, more preferably above about 2. This requires tha~ the sodiurn chlorate solution feed in line 16 pre~erably ha~e a nonnality of at least about 2.
The feed rate o~ the sodiu~l chlorate solution in line 16 is a function of the amount of chlorate ion in the product lines 42, 46, the concentration of the sodiurn chlorate solution, and the water balance in the generator.
The f~ed rate and concentration of the ~odi~n chlorate ~olution can both be ad~usted depending upon chlorate ion and water balances. An aspect of the pre~ent in~ention is that the feed of the ~odlum chlorate solution in lins 16, when the separator 30 iB a diaphra~n, suppresses the back-migratlon of protons through the diaphra~n 30 to the cathode 28~ ~his is important where an acid product of high normality is desired. The higher the norrnality o~
-20- ~J7~22 the acid, the higher the proton content in the anode compartment 22~ and the greater the likelihood of back-migration of protons ~o the cathode 28.
Preferably~ ~he generator of the present invention is operated at a relatiYely high current density, to reduce capital costs. Satisfactory results can be obtained with current densities in the range of about 2-5 kiloamp~ per s~uare m~tex.
The following Examples illustrate the present invention.
Example 1 A chloric acid generator 20 a~ shown in the Figure was operated to convert ~odium chlorate into ~odium hydroxide and chloric acid. The generator 20 employed an expanded mesh titanium anode 24 having a precious metal oxi.de coating marketed by the assignee of the present application under the trade de~ignation EC-600. The cathode 28 wa~ titanium mesh. The diaphragm 30 was a ~heet o~ porous polytetra~luoroethylene ("Kynar") marketed by Porex Technologies Corp. under the trademark Porex.
The diaphragm typically has an average void volume of about 40~ and a density of about 1.05 grams per cubic centimeter. Average pore size is about 25 microns. The diaphragm ha~ a service temperature up to about 300F
(149C). The membrane 32 was made of NAFION 324 (trademark, B. I. DuPont de Nemours & Co.). The generator wa~ constructed of chlorinated polyvinylchloride. The men~rane 32 and diaphragm 30 had an active area o~ 20 -21- 2~3~2~
square centimetexs. The middle compartment 18 gap was 0.25 inches (0~64 cm).
The generator was operated under the following conditionss Cuxrenk Density 4 kiloamps per square meter Volkage 8.5 volts The following Table 1 shows measured temperatures, concenkrations~ and flow rates at various points in the generator. The chloric acid and sodium hydroxide product concentrations were measured to determine current efficiency at the anode and at the cathode.
~r r~ D
D ~ 0 o o o ~ ~ ~~ ~D 2 ~
o~D
U~ C~ o . . .
U ~D rAI M O O
NO
~rI U
u~ ~
'~I O ~ ~ o. .
~a ~ o ~ OOO-DO
cn ~ P. ~
~D
~1~ ~ ~ O
o ~! ~ a~o o ~ ~ ~1 h~ UIn O ~ ~ o o ~ o _lO
N
~o ~
~ ~ o~Co O ~ ~ U7~ooo N
C) U
I O Zæ~z ~ . .
h 1~ U 1!4 U~ o _~3_ 2~
Table 1 shows that both cooled acid and cooled salt were recircula~ed from the heat exchanger 56, in lines 64 and 77, re~pectively, at 16C. This maintained the anolyte in line 42 at about 26.5C ~the cell 20 nd diaphragm 30 being at about the same temperature), and the chloric acid product in line 46 at about 18.6C. The concentration of the chloric acid product obtained in line 46 was 2.06N. Cathode and anode current efficiencies were determined by dividing the actual production rate~ by the theoretical rates and multiplying by 100. Theoretical rates were ~a~ed on amperage.
The following cell efficiencies were obtained:
Cathode (NaOH) CE~ 29.1 Anode lHC103) CE~ 23.6 The generator was disassembled at the end of 200 hours on line and the Porex diaphragm wa~ in excellent condition. The generator has ~een succe~sfully operated for longer period~.
~ample 2 The generator described in Example 1 was u~ed. It was operated at the same current density of 4 Xilo~mps pex squaxe meter as ~n Example 1. The voltage drop in the call was 7.5 as compared to 8.5 in Ex~mple 1. The following ~abla 2 gives mea~ured temperature, concentrations and flow rate~ at various point3 in the generator. ~he ma~or difference of operation from Example 1 was ths absence of cooled recirculated chlorate ~olution in line 77 from heat exchanger 56. Thus, the cell was -24- 2~ 2~5 operated at a higher temperatnre of about 59C, as evidenced by th~ temperature in anolyte product line 42.
. .
~: o ~
" ~ a) ~ _, ,, O o r~l O
;o O~ o o ~.~ Ln o~
a~
r '~ U ~ ~D
~ x ~ u ~r~ o~ ~ o . .
` o ~r ~ ~o c~ ~ u ~ ~ ~ o oo ~ ~ a) ~ cO Oo ~ ~ o ~
~ ~ ~rl ~ _~_100 E~
r~
r~
~ o a o ~1 V In ~ ~ O
o a~ 0 u~ ~ooo ~:: h~ ~'1 o ~
2;V0 Ul ~1 O _l El C~ ~, In O
: ', . . .
..
. ... .
., .
.
.
. . . ~
2 ~ 2 -~6-The generator functioned for only 36 hsurs before exces~ive corro~ion of the diaphragm 30 occurred. Current efficiencies were determined~ as followss Cathode (NaOH) CE% 28.6 Anode (HC103) CE% 11.3 ~he co~centration of the chloric acid in line 46 wa~
only 1.8N. Thus, although the c811 efficiency increased slightly for sodium hydroxide production when compared with Example 1, the cQll efficiency wa~ significantly less ~or acid production. The comparative data of Example 1 illu~tr~te~ the importance of maintaining the generator 20, in the production of oxyhalogen acids, at a relati~ely low temperatura. Example 1 also demonstrate~ that by sufficiently cooling the cell 20, the life of the diaphra~m 30 can be 3ignificantly extended.
One principle use for chloric acid is as a precursor in the manufacture o chlorine dioxide (Cl02). ~hlorine dioxide is a strong oxidizing agent. Some of the market areas or chlorine dioxide are: water treatment, pulp and paper processing, flour processing, municipal waste treatment, petroleum well in~ection, crop and meat storage, and bleaching such materials as textiles, oils, shellacs, varnishes, waxes, and straw products.
For water treatment, the use of chlorine dioxide is particularly attractive a~ environmental regulations are being tight~n~d concerning the production of chlorinated organics and trihalomethanes ~TH~8). Trihalomethanes are signi~icantly reduced with the use of chlorine dio~ide in~tead o chlorina as a reactant.
. .
-27~
One chlorine dioxide generAting proces~, known as the MathiQson process, reacts sulfur dioxide with ~odium chlorate (NaCl03) and sulfuric acid to produce chlorine dioxide. This reaction also produce~ an undesirable 5 sodium bi~ulfate ~alt cake (NaHSO4). When this process is used in the pulp and paper industry, excess salt cake causes an im~alance for mills ~rying to reduce sulfur emissions.
An advantage of the chloric acid process of the present in~ention is that it can be integrated into the production of chlorine dioxide, for the pulp and paper indu~try, without ~he production of unwanted sodium bisulfate salt cake. When the separator 30 of the electrolytic cell of the pre~ent invention is an anionic membrane, a substantially pure chloric acid stre~m is produced in lins 46. The chloric acid can then chemically react with sulfur dioxide to produce a chlorine dioxide product ~tream, and a by-product stream which i9 sulfuric acid. Sulfuric acid, in contraqt with sodiwn bisulfate, is a useful by-product. If a diaphragm is used as the separator 30, in place of an anionic membrane, some sodium chlorate rom the middle feed compartment 18 of the chloric acid generator flows into the anode compar~ment 22 making a mixture, in product line 46, of chloric acid and ~odi~n chlorate. The sodium chlorate reacts with sulfur dloxide and sulfuric acid, a~ in the Mathie~on process, to ~orm some salt cake. However, the sulfur dioxide can be reacted preferentially with the chloric acid and sulfuric -28- 2~3~
acid will be produced with very little salt cake. ~he sodium bisulfate salt ~hat i5 produced can be sepàrated from the sulfuric acid and recycled to the feed compartment 18 of the chloric acid generator of the present invention. In the chloric acid yenerator, the sodium bisulfate is converted to additional sodium hydroxide and sulfuric acid.
Processes similar to the Mathieson process, which use sodium chlorate and sulfuric acid as feed components, are known as the Solvay, R-2 and SVP proce~ses. In the Solvay process, sodium chlorate i~ reacted wi-th sulfuric acid and a reducing agent such as methanol to produce chloxine dioxide. Thi~ proce~s produce~, as a by-product, sodium sulfate (Na2S04) which is of little value. By the present invention, chloric acid can be reacted directly with a reducing a~ent, such as methanol, to produce chlorine d~oxlde without producing the ~alt by-product. In the R-2 and SVP processes, sodium chlorate, sodium chloride, and ~ul~uric acid are reacted to produce chlorine dioxide.
The processes also produce chlorine gas (C12) and sodium sulfate as by-products. The chlorine gas is a particularly undesirable by-product.
The production of chlorine dioxide (C102) contaminated with chlorin~ gas (C12) is also di~closed in two patents, 2S No. 4,806,215 "Combined Process for Production of Chloxine Dioxide and Sodium Hydroxide", and No. 4,853,0g6 "Production of Chlorine Dioxide in an Electrolytic Cell".
~he first patent is a three compartment cell which ~37~2?.
produces NaOH, Cl2 and Cl02 from HC1 and ~aCl03. The second paten~, in Fig. 3, de~eribes a proeess for reducing, but not eliminating, the chlorine that contaminates the chlorine dioxide.
In the pre~ent invention, th2 ehloric acid in line 46 can be reduced directly to chlorine dioxide by feeding the chloric acid to an eleetrochemieal cell as disclosed in U.S. Patent No. 4~798,715, discuRsed abo~e, assigned to the a~signee of the present application. An advantage of this reduction i~ that the ehlorine dio~ide is produced completely free of chlorine by-product ga~. The di~closure of U.S. Patent No. 4,798,715 i9 ineorporated by referenee herein.
Other u~es ~or chloric acid are as a catalyst in the polymerization of acrylonitrile, and in the production of perchloric acid. In Kirk Othmer, Vol. 5, page 656, it i5 disclosed that chloric acid can be electrolytically oxidized to perehloric aeid in an electrochemical proce~s.
Perchlorie acid uses are in medicine, in analytical chemistry, a~ a eatalyst in the manufacture of various esters, as an ingredient of an electrolytic bath in the depo~itioll of lead, in electro-polishing, and in the manufacture of Qxplosive~. The conventional mathod for produeing ammonium perehlora~e, diselosed in Kirk Othmer, Vol. 5, pg~ 660, is to reaet sodium perchlorate, ammonia, and hydxoehloric acid to produce ammonium perchlorate and by~produet sodium chloride:
-30- ~3~
NaC10~ + NH3 + HCl ~ NH4~104 ~ NaCl (8) In accordance with the present invention, æodium chlorate can be converted to chloric acid and odium hydroxide with the three eompartmPnt cell as described above in reactions 1 through 7. The chloric acid is then eleetrolytieally oxidized to perchlorie aeid using the electrochemical proees~ described abovs and on pg. 656, Vol. S of Kirk Othmer. The perchloric acid ean then be reaeted with ammonia to produee ammonium perchlorate:
NH3 J~ HCl 04 ~ N~I4 ~10~ ( 9 ) A~ an altexnative, sodium perchlorate ean be eon~erted to perehlorie aeid and sodium hydroxide in the three compartment aeid generator. The perchloric acid can then ba reaeted with ammonia to produee ammonium perehlorate a~ in reaetion (9).
From the above deseription of a preferred embodiment o~ the invention, those skilled in the art will perceive improvements, ehanges and modification~. Such improvements, changes and modifieations within the skill of the art are intended to be eovered by the appended elalm~.
Claims (22)
1. A process for the production of strong solutions of halogen oxyacids from the corresponding alkali metal salt of the oxyacid comprising the steps of:
(a) establishing a solution of said alkali metal salt;
(b) providing an electrolytic cell comprising an anode compartment containing an anode, a cathode compartment containing a cathode, and a middle feed compartment separated from the anode compartment by a separator selected from the group consisting of a diaphragm or an anion-selective membrane and from the cathode compartment by a cation-selective membrane;
(c) introducing said alkali metal salt of the oxyacid into said middle feed compartment; and (d) applying a voltage across said cell to cause migration of alkali metal ions to the cathode and reaction of said alkali metal ions with hydroxyl ions to form an alkali metal hydroxide, and migration of the oxyhalogen ions to the anode and reaction of said oxyhalogen ions with protons to form a halogen oxyacid product.
(a) establishing a solution of said alkali metal salt;
(b) providing an electrolytic cell comprising an anode compartment containing an anode, a cathode compartment containing a cathode, and a middle feed compartment separated from the anode compartment by a separator selected from the group consisting of a diaphragm or an anion-selective membrane and from the cathode compartment by a cation-selective membrane;
(c) introducing said alkali metal salt of the oxyacid into said middle feed compartment; and (d) applying a voltage across said cell to cause migration of alkali metal ions to the cathode and reaction of said alkali metal ions with hydroxyl ions to form an alkali metal hydroxide, and migration of the oxyhalogen ions to the anode and reaction of said oxyhalogen ions with protons to form a halogen oxyacid product.
2. The process of claim 1 wherein said cell is maintained at a temperature in the range of about 10°C to about 40°C.
3. The process of claim 2 wherein said halogen oxyacid is selected from the group consisting of chloric acid and perchloric acid.
4. The process of claim 1 wherein said halogen oxyacid is chloric acid and said alkali metal salt is sodium chlorate including the step of preparing a solution of sodium chlorate having a molar concentration in the range of about 0.1N to about 4N, said chloric acid having a concentration in the range of about 0.1N to about 3.6N.
5. The process of claim 4 wherein said chloric acid has a normality above about 1.5 and said sodium chlorate has a normality above about 2.
6. The process of claim 5 wherein said anode is dimensionally stable and said separator is a diaphragm, said diaphragm comprising a material selected from the group consisting of polyvinylidene fluoride, a perfluoro carbon, and polytetrafluoroethylene, and said cell is operated at a current density of about 2-5 kiloamps per square meter.
7. The process of claim 1 wherein said halogen oxyacid is perchloric acid and said alkali metal salt is sodium perchlorate, said anode being a lead oxide or platinum electrode.
8. The process of claim 2 comprising the steps of circulating a portion of said halogen oxyacid product through a cooler and back to the anode compartment to cool said anode compartment; and circulating a portion of the alkali metal salt feed from said middle feed compartment through a cooler and back to said middle feed compartment to cool said middle feed compartment.
9. The process of claim 8 further comprising the step of circulating a portion of said halogen oxyacid product to a cooler and directly back to said halogen oxyacid product to cool said halogen oxyacid product.
10. The process of claim 1 wherein said separator is a diaphragm, the feed of alkali metal salt into said middle feed compartment being effective to suppress back-migration of protons from the anode compartment to the cell cathode.
11. The process of claim 10 wherein said diaphragm comprises a material selected from the group consisting of polyvinylidene fluoride, a perfluorocarbon and polytetrafluoroethylene.
12. A process for the production of chlorine dioxide comprising the steps of:
(a) establishing a solution of sodium chlorate having a molar concentration less than that at which precipitation of the sodium chlorate occurs;
(b) providing an electrolytic cell comprising an anode compartment containing an anode, a cathode compartment containing a cathode, and a middle feed compartment separated from the anode compartment by a diaphragm or an anion-selective membrane and from the cathode compartment by a cation-selective membrane;
(c) introducing said sodium chlorate solution into said feed compartment;
(d) applying a voltage across said cell to cause migration of sodium ions to the cathode and reaction of the sodium ions with hydroxyl ions to form sodium hydroxide, and migration of chlorate ions to the anode and reaction of the chlorate ions with protons to form chloric acid, said chloric acid having a normality in the range of about 1.5 to about 3.6;
(e) maintaining said cell at a temperature in the range of about 10°C to about 40°C;
(f) providing a second electrolytic cell; and (g) feeding said chloric acid to said second electrolytic cell and electrolyzing said chloric acid into hydrogen and chlorine dioxide in said second electrolytic cell.
(a) establishing a solution of sodium chlorate having a molar concentration less than that at which precipitation of the sodium chlorate occurs;
(b) providing an electrolytic cell comprising an anode compartment containing an anode, a cathode compartment containing a cathode, and a middle feed compartment separated from the anode compartment by a diaphragm or an anion-selective membrane and from the cathode compartment by a cation-selective membrane;
(c) introducing said sodium chlorate solution into said feed compartment;
(d) applying a voltage across said cell to cause migration of sodium ions to the cathode and reaction of the sodium ions with hydroxyl ions to form sodium hydroxide, and migration of chlorate ions to the anode and reaction of the chlorate ions with protons to form chloric acid, said chloric acid having a normality in the range of about 1.5 to about 3.6;
(e) maintaining said cell at a temperature in the range of about 10°C to about 40°C;
(f) providing a second electrolytic cell; and (g) feeding said chloric acid to said second electrolytic cell and electrolyzing said chloric acid into hydrogen and chlorine dioxide in said second electrolytic cell.
13. The process of claim 12 wherein said chlorine dioxide is chlorine-free.
14. An apparatus for the production of a halogen oxyacid from the alkali metal salt of said oxyacid comprising:
(a) an electrolytic cell comprising an anode compartment containing an anode, a cathode compartment containing a cathode, and a middle feed compartment separated from the anode compartment by a separator selected from the group consisting of a diaphragm or an anion-selective membrane and from the cathode compartment by a cation-selective membrane;
(b) means for introducing said alkali metal salt into said middle feed compartment;
(c) means for applying a voltage across said cell to cause migration of alkali metal ions to the cathode and reaction of said alkali metal ions with hydroxyl ions to form alkali metal hydroxide, and migration of oxyhalogen ions to the anode and reaction of said oxyhalogen ions with protons to form halogen oxyacid;
and (d) means for maintaining said cell at a temperature in the range of about 10°C to about 40°C.
(a) an electrolytic cell comprising an anode compartment containing an anode, a cathode compartment containing a cathode, and a middle feed compartment separated from the anode compartment by a separator selected from the group consisting of a diaphragm or an anion-selective membrane and from the cathode compartment by a cation-selective membrane;
(b) means for introducing said alkali metal salt into said middle feed compartment;
(c) means for applying a voltage across said cell to cause migration of alkali metal ions to the cathode and reaction of said alkali metal ions with hydroxyl ions to form alkali metal hydroxide, and migration of oxyhalogen ions to the anode and reaction of said oxyhalogen ions with protons to form halogen oxyacid;
and (d) means for maintaining said cell at a temperature in the range of about 10°C to about 40°C.
15. The apparatus of claim 14 wherein said separator is a diaphragm comprising a material selected from the group consisting of polyvinylidene fluoride, a perfluoro carbon, and polytetrafluoroethylene.
16. The apparatus of claim 14 comprising:
a cooler;
means for circulating a portion of said halogen oxyacid product through said cooler and back to the anode compartment to cool said anode compartment; and means for circulating a portion of the alkali metal salt from said middle feed compartment through said cooler and back to said feed compartment to cool said middle feed compartment.
a cooler;
means for circulating a portion of said halogen oxyacid product through said cooler and back to the anode compartment to cool said anode compartment; and means for circulating a portion of the alkali metal salt from said middle feed compartment through said cooler and back to said feed compartment to cool said middle feed compartment.
17. The apparatus of claim 16 further comprising an acid storage tank and means for circulating a portion of said halogen oxyacid product from said acid storage tank to said cooler and directly back to said storage tank to cool said halogen oxyacid product.
18. An apparatus for the production of chloric acid having a normality of about 0.1 to about 3.6 from sodium chlorate comprising:
(a) an electrolytic cell comprising an anode compartment containing an anode, a cathode compartment containing a cathode, and a middle feed compartment separated from the anode compartment by a separator selected from the group consisting of a diaphragm or an anion-selective membrane and from the cathode compartment by a cation-selective membrane;
(b) means for introducing said sodium chlorate into said middle feed compartment, said sodium chlorate having a concentration less than that at which precipitation of the sodium chlorate occurs in said middle feed compartment;
(c) means for applying a voltage across said cell to cause migration of sodium ions to the cathode and reaction of said sodium ions with hydroxyl ions to form sodium hydroxide, and migration of chlorate ions to the anode and reaction of said chlorate ions with protons to form chloric acid; and (d) means for maintaining said cell at a temperature in the range of about 10°C to about 40°C.
(a) an electrolytic cell comprising an anode compartment containing an anode, a cathode compartment containing a cathode, and a middle feed compartment separated from the anode compartment by a separator selected from the group consisting of a diaphragm or an anion-selective membrane and from the cathode compartment by a cation-selective membrane;
(b) means for introducing said sodium chlorate into said middle feed compartment, said sodium chlorate having a concentration less than that at which precipitation of the sodium chlorate occurs in said middle feed compartment;
(c) means for applying a voltage across said cell to cause migration of sodium ions to the cathode and reaction of said sodium ions with hydroxyl ions to form sodium hydroxide, and migration of chlorate ions to the anode and reaction of said chlorate ions with protons to form chloric acid; and (d) means for maintaining said cell at a temperature in the range of about 10°C to about 40°C.
19. A process for the production of strong solutions of halogen oxyacids from the corresponding alkali metal salt of the oxyacid, comprising the steps of:
(a) establishing a solution of said salt having a molar concentration less than that at which precipitation of said salt occurs;
(b) electrolytically causing the migration of oxyhalogen ions to an electrolytic cell anode for reaction with protons generated at said anode to form halogen oxyacid product; and (c) maintaining said solution of said salt and said halogen oxyacid product at a temperature in the range of about 10°C to about 40°C.
(a) establishing a solution of said salt having a molar concentration less than that at which precipitation of said salt occurs;
(b) electrolytically causing the migration of oxyhalogen ions to an electrolytic cell anode for reaction with protons generated at said anode to form halogen oxyacid product; and (c) maintaining said solution of said salt and said halogen oxyacid product at a temperature in the range of about 10°C to about 40°C.
20. An apparatus for the production of strong solutions of halogen oxyacids according to the process of claim 19, comprising:
(a) an electrolytic cell comprising an anode compartment containing an anode, a cathode compartment containing a cathode, and a middle feed compartment separated from the anode compartment by a diaphragm or an anion-selective membrane and from the cathode compartment by a cation-selective membrane;
(b) means for introducing said alkali metal salt into said middle feed compartment;
(c) means for applying a voltage across said cell to cause migration of alkali metal ions to the cathode and reaction of said alkali metal ions with hydroxyl ions to form alkali metal hydroxide, and migration of oxyhalogen ions to the anode and reaction of said oxyhalogen ions with protons to form halogen oxyacid;
and (d) means for maintaining said cell at a temperature in the range of about 10°C to about 40°C.
(a) an electrolytic cell comprising an anode compartment containing an anode, a cathode compartment containing a cathode, and a middle feed compartment separated from the anode compartment by a diaphragm or an anion-selective membrane and from the cathode compartment by a cation-selective membrane;
(b) means for introducing said alkali metal salt into said middle feed compartment;
(c) means for applying a voltage across said cell to cause migration of alkali metal ions to the cathode and reaction of said alkali metal ions with hydroxyl ions to form alkali metal hydroxide, and migration of oxyhalogen ions to the anode and reaction of said oxyhalogen ions with protons to form halogen oxyacid;
and (d) means for maintaining said cell at a temperature in the range of about 10°C to about 40°C.
21. The apparatus of claim 20 comprising:
a cooler;
means for circulating a portion of said halogen oxyacid product through said cooler and back to the anode compartment to cool said anode compartment, and means for circulating a portion of the alkali metal salt from said middle feed compartment through said cooler and back to said middle feed compartment to cool said middle feed compartment.
a cooler;
means for circulating a portion of said halogen oxyacid product through said cooler and back to the anode compartment to cool said anode compartment, and means for circulating a portion of the alkali metal salt from said middle feed compartment through said cooler and back to said middle feed compartment to cool said middle feed compartment.
22. The apparatus of claim 21 comprising an acid storage tank and means for circulating a portion of said halogen oxyacid product from said acid storage tank to said cooler and directly back to said storage tank to cool said halogen oxyacid product.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US49703890A | 1990-03-21 | 1990-03-21 | |
| US497,038 | 1990-03-21 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2037522A1 true CA2037522A1 (en) | 1991-09-22 |
Family
ID=23975215
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2037522 Abandoned CA2037522A1 (en) | 1990-03-21 | 1991-03-04 | System for electrolytically generating strong solutions of halogen oxyacids |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2037522A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5407547A (en) * | 1993-10-06 | 1995-04-18 | Eka Nobel Ab | Process for production of acidified process streams |
-
1991
- 1991-03-04 CA CA 2037522 patent/CA2037522A1/en not_active Abandoned
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5407547A (en) * | 1993-10-06 | 1995-04-18 | Eka Nobel Ab | Process for production of acidified process streams |
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