CA2226651C - Process for the production of chlorine dioxide - Google Patents
Process for the production of chlorine dioxide Download PDFInfo
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- CA2226651C CA2226651C CA 2226651 CA2226651A CA2226651C CA 2226651 C CA2226651 C CA 2226651C CA 2226651 CA2226651 CA 2226651 CA 2226651 A CA2226651 A CA 2226651A CA 2226651 C CA2226651 C CA 2226651C
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- sodium chloride
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B11/00—Oxides or oxyacids of halogens; Salts thereof
- C01B11/02—Oxides of chlorine
- C01B11/022—Chlorine dioxide (ClO2)
- C01B11/023—Preparation from chlorites or chlorates
- C01B11/025—Preparation from chlorites or chlorates from chlorates without any other reaction reducing agent than chloride ions
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01D—COMPOUNDS OF ALKALI METALS, i.e. LITHIUM, SODIUM, POTASSIUM, RUBIDIUM, CAESIUM, OR FRANCIUM
- C01D3/00—Halides of sodium, potassium or alkali metals in general
- C01D3/04—Chlorides
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
- C25B1/00—Electrolytic production of inorganic compounds or non-metals
- C25B1/01—Products
- C25B1/24—Halogens or compounds thereof
- C25B1/26—Chlorine; Compounds thereof
- C25B1/265—Chlorates
Abstract
Chlorine dioxide is produced from sodium chlorate and hydrochloric acid in the presence of a significant amount (at least about 22 gpl) of sodium dichromate to improve the efficiency of chlorine dioxide production. The process is integrated with a chlorate cell producing sodium chlorate for the chlorine dioxide generation from sodium chloride by-product from a chlorine dioxide generating process. The integration is effected to maintain a pH of about 6 to 7 in the chlorate cell in the presence of about 2 to about 7 gpl of sodium dichromate.
Description
PROCESS FOR THE PRODUCTION OF CHLORINE DIOXIDE
The present invention relates to the production of chlorine dioxide, a chemical used in the bleaching of wood pulp, in particular to improving the efficiency of production of chlorine dioxide using hydrogen chloride.
Chlorine dioxide may be produced by reaction of aqueous sodium chlorate and hydrochloric acid at the boiling point of the reaction medium under a subatmospheric pressure applied to the chlorine dioxide generator, as described, for example, in U.S. Patent No.
3,929,974, assigned to the applicant hereof. By-product sodium chloride crystallizes from the reaction medium once saturation of the reaction medium is reached.
The by-product sodium chloride may be removed from the generator and formed into an aqueous solution thereof, which is forwarded to a sodium chlorate cell wherein the aqueous sodium chloride is electrolyzed to aqueous sodium chlorate which is returned to the generator, as described, for example, in the above-noted U.S. Patent No. 3,929,974. Sodium dichromate (Na2Cr2O7. 2H20) is generally used to improve the efficiency of the sodium chlorate production in such electrolysis procedure. When chlorate cell liquor is forwarded to the chlorine dioxide generator, the sodium dichromate present in the cell liquor enters the chlorine dioxide generator.
In accordance with the present invention, such integrated chlorine dioxide generating process is operated in a manner so as to increase the concentration of sodium dichromate in the reaction medium in the chlorine dioxide generator and thereby increase the efficiency of chlorine dioxide production from the reaction medium as a result of catalysis of the chlorine dioxide generating reaction by the relatively concentrated sodium dichromate.
The efficiency of chlorine dioxide production is the proportion of sodium chlorate used to make chlorine dioxide by reaction (1) rather than less desired chlorine from reaction (2):
NaC103 + 2HC1 ~ C102 +'-~ C12 + NaCl + H20 (1) NaC103 + 6HC1 ~ 3C12 + NaCl + 3H20 (2) The greater the proportion of sodium chlorate which reacts to form chlorine dioxide by reaction (1), the greater is the efficiency of chlorine dioxide production and the efficiency is expressed as a percentage.
Although the catalytic effect of sodium dichromate on chlorine dioxide production is known and is described in U.S. Patent No. 3,563,702, it has not heretofore been possible to take full advantage of this effect in an integrated chlorine dioxide generating system, as described in the aforementioned U.S. Patent No.
The present invention relates to the production of chlorine dioxide, a chemical used in the bleaching of wood pulp, in particular to improving the efficiency of production of chlorine dioxide using hydrogen chloride.
Chlorine dioxide may be produced by reaction of aqueous sodium chlorate and hydrochloric acid at the boiling point of the reaction medium under a subatmospheric pressure applied to the chlorine dioxide generator, as described, for example, in U.S. Patent No.
3,929,974, assigned to the applicant hereof. By-product sodium chloride crystallizes from the reaction medium once saturation of the reaction medium is reached.
The by-product sodium chloride may be removed from the generator and formed into an aqueous solution thereof, which is forwarded to a sodium chlorate cell wherein the aqueous sodium chloride is electrolyzed to aqueous sodium chlorate which is returned to the generator, as described, for example, in the above-noted U.S. Patent No. 3,929,974. Sodium dichromate (Na2Cr2O7. 2H20) is generally used to improve the efficiency of the sodium chlorate production in such electrolysis procedure. When chlorate cell liquor is forwarded to the chlorine dioxide generator, the sodium dichromate present in the cell liquor enters the chlorine dioxide generator.
In accordance with the present invention, such integrated chlorine dioxide generating process is operated in a manner so as to increase the concentration of sodium dichromate in the reaction medium in the chlorine dioxide generator and thereby increase the efficiency of chlorine dioxide production from the reaction medium as a result of catalysis of the chlorine dioxide generating reaction by the relatively concentrated sodium dichromate.
The efficiency of chlorine dioxide production is the proportion of sodium chlorate used to make chlorine dioxide by reaction (1) rather than less desired chlorine from reaction (2):
NaC103 + 2HC1 ~ C102 +'-~ C12 + NaCl + H20 (1) NaC103 + 6HC1 ~ 3C12 + NaCl + 3H20 (2) The greater the proportion of sodium chlorate which reacts to form chlorine dioxide by reaction (1), the greater is the efficiency of chlorine dioxide production and the efficiency is expressed as a percentage.
Although the catalytic effect of sodium dichromate on chlorine dioxide production is known and is described in U.S. Patent No. 3,563,702, it has not heretofore been possible to take full advantage of this effect in an integrated chlorine dioxide generating system, as described in the aforementioned U.S. Patent No.
3,929,974, with cell liquor being forwarded from the chlorate cells to the chlorine dioxide generator. The reasons for this are that the chlorate cells must operate at near pH 6.8 for optimum performance and cannot operate at lower pHs, otherwise too much chlorine gas will be formed in the chlorate cells, leading to an explosive C12-H2 mixture.
A high level of sodium dichromate in the chlorine dioxide generator forms a buffer, as follows:
Alkali pH> 7 Neutral pH Acid pH< 7 2 Na2Cr2O4 -~ Na2Cr2O7 ~ NaHCr2O7 For each 10 gpl Na2Cr2O7.2H20 added to the chlorine dioxide generating reaction medium, the operating acidity of the generator rises approximately 0.1 N as a result of this buffering effect.
In accordance with one aspect of the present invention, the concentration of sodium dichromate (as Na2Cr2O7.2H20) in the chlorine dioxide generating reaction medium is at least about 20 grams per liter (gpl), generally about 20 to about 200 gpl, preferably greater than about 100 gpl. Accordingly, in one aspect of the present invention, there is provided a process for the production of chlorine dioxide, which comprises reacting sodium chlorate and hydrochloric acid in an aqueous acid reaction medium in the presence of at least about 20 gpl of sodium dichromate.
At the concentration levels of sodium dichromate discussed above, the quantity of acid removed from the generator and sent to the chlorate cells with by-product sodium chloride in an integrated operation is too high for efficient chlorate cell operation, particularly if there is no crystallization of sodium chloride in the generator.
In a process wherein sodium chloride is crystallized from the generator, the sodium chloride by-product may be separated from the generator liquor containing the sodium dichromate by washing the solids on a filter, thereby returning little or no acid to the chlorate cells. In practice, some acid is desirable and this amount may be controlled by returning a controlled amount of generator liquor to the cells rather than the whole solution, as is the case of a non-crystallizing system.
The controlled amount of generator liquor forwarded to the chlorate cells also serves to return sodium dichromate to the chlorate cells. The chlorate cells generally operate at a concentration of sodium dichromate of about 2 to about 7 gpl, in contrast to the concentration of sodium dichromate in the chlorine dioxide generator of at least about 20 gpl, preferably at least about 100 gpl dichromate, employed in the present invention.
Accordingly, in a further aspect of the invention, there is provided a continuous integrated process of forming chlorine dioxide by reacting sodium chlorate with hydrochloric acid to form chlorine dioxide and chlorine and electrolytically producing the sodium chlorate from sodium chloride produced by the chlorine dioxide-forming reaction, which comprises feeding sodium chloride and hydrochloric acid to an aqueous acid reaction medium in a reaction zone; effecting reaction of sodium chlorate and hydrochloric acid in the presence of at least about 20 gpl of sodium dichromate to produce chlorine dioxide and chlorine in a reaction zone, said aqueous acid reaction medium being at the boiling point of the aqueous acid reaction medium while a subatmospheric pressure is applied to the reaction zone, and precipitating by-product crystalline sodium chloride in said reaction zone;
removing a gaseous admixture of chlorine dioxide, chlorine and steam from the reaction zone and forming an aqueous solution of chlorine dioxide and chlorine and residual gaseous chlorine therefrom;
electrolytically forming sodium chlorate by electrolysis of an aqueous solution of sodium chloride containing about 2 to about 7 gpl of sodium dichromate at a pH of about 6 to about 7 to produce an aqueous solution of sodium chlorate, sodium chloride and sodium dichromate; forwarding said aqueous solution of sodium chlorate, sodium chloride and sodium dichromate to said aqueous acid reaction medium as the feed of sodium chlorate thereto; removing said by-product crystalline sodium chloride from said reaction zone in a slurry with entrained aqueous acid reaction medium; removing at least a substantial proportion of said entrained aqueous acid reaction medium from said recovered by-product crystalline sodium chloride; forming an aqueous solution of sodium chloride from the resulting purified by-product crystalline sodium chloride and forwarding said aqueous solution of sodium chloride to said electrolysis step for electrolysis therein; and 5 forwarding removed entrained aqueous acid reaction medium to said reaction zone.
In the process of the present invention, the chlorine dioxide generation is preferably effected at the boiling point of the aqueous acid reaction medium lo while a subatmospheric pressure is applied to the chlorine dioxide generation vessel. The process may be carried out at a temperature of about 550 to about 80 C, preferably about 60 to about 75 C, and at a subatmospheric pressure of about 120 to about 250 mmHg, preferably about 125 to about 200 mmHg.
The chlorine dioxide generating process may be carried out at a total acid normality of about 0.05 to about 2 N, preferably about 0.05 to about 1.5 N.
Electrochemical production of sodium chlorate may be effected by electrolysis of an aqueous solution of sodium chloride having a concentration of sodium chloride from about 90 to about 320 gpl, preferably about 100 to about 200 gpl, and containing about 2 to about 7 gpl of sodium dichromate at a pH of about 6 to about 7 and a temperature of about 60 to about 100 C.
Such electrolysis results in an aqueous solution of sodium chlorate, sodium chloride and sodium dichromate which is passed to the chlorine dioxide generation step and generally contains about 400 to about 600 gpl, preferably about 450 to about 550 gpl, of sodium chlorate, about 90 to about 200 gpl, preferably about 100 to about 150 gpl, of sodium chloride, and about 2 to about 7 gpl of sodium dichromate.
The invention is described further, by way of illustration, with reference to the accompanying drawing, wherein:
A high level of sodium dichromate in the chlorine dioxide generator forms a buffer, as follows:
Alkali pH> 7 Neutral pH Acid pH< 7 2 Na2Cr2O4 -~ Na2Cr2O7 ~ NaHCr2O7 For each 10 gpl Na2Cr2O7.2H20 added to the chlorine dioxide generating reaction medium, the operating acidity of the generator rises approximately 0.1 N as a result of this buffering effect.
In accordance with one aspect of the present invention, the concentration of sodium dichromate (as Na2Cr2O7.2H20) in the chlorine dioxide generating reaction medium is at least about 20 grams per liter (gpl), generally about 20 to about 200 gpl, preferably greater than about 100 gpl. Accordingly, in one aspect of the present invention, there is provided a process for the production of chlorine dioxide, which comprises reacting sodium chlorate and hydrochloric acid in an aqueous acid reaction medium in the presence of at least about 20 gpl of sodium dichromate.
At the concentration levels of sodium dichromate discussed above, the quantity of acid removed from the generator and sent to the chlorate cells with by-product sodium chloride in an integrated operation is too high for efficient chlorate cell operation, particularly if there is no crystallization of sodium chloride in the generator.
In a process wherein sodium chloride is crystallized from the generator, the sodium chloride by-product may be separated from the generator liquor containing the sodium dichromate by washing the solids on a filter, thereby returning little or no acid to the chlorate cells. In practice, some acid is desirable and this amount may be controlled by returning a controlled amount of generator liquor to the cells rather than the whole solution, as is the case of a non-crystallizing system.
The controlled amount of generator liquor forwarded to the chlorate cells also serves to return sodium dichromate to the chlorate cells. The chlorate cells generally operate at a concentration of sodium dichromate of about 2 to about 7 gpl, in contrast to the concentration of sodium dichromate in the chlorine dioxide generator of at least about 20 gpl, preferably at least about 100 gpl dichromate, employed in the present invention.
Accordingly, in a further aspect of the invention, there is provided a continuous integrated process of forming chlorine dioxide by reacting sodium chlorate with hydrochloric acid to form chlorine dioxide and chlorine and electrolytically producing the sodium chlorate from sodium chloride produced by the chlorine dioxide-forming reaction, which comprises feeding sodium chloride and hydrochloric acid to an aqueous acid reaction medium in a reaction zone; effecting reaction of sodium chlorate and hydrochloric acid in the presence of at least about 20 gpl of sodium dichromate to produce chlorine dioxide and chlorine in a reaction zone, said aqueous acid reaction medium being at the boiling point of the aqueous acid reaction medium while a subatmospheric pressure is applied to the reaction zone, and precipitating by-product crystalline sodium chloride in said reaction zone;
removing a gaseous admixture of chlorine dioxide, chlorine and steam from the reaction zone and forming an aqueous solution of chlorine dioxide and chlorine and residual gaseous chlorine therefrom;
electrolytically forming sodium chlorate by electrolysis of an aqueous solution of sodium chloride containing about 2 to about 7 gpl of sodium dichromate at a pH of about 6 to about 7 to produce an aqueous solution of sodium chlorate, sodium chloride and sodium dichromate; forwarding said aqueous solution of sodium chlorate, sodium chloride and sodium dichromate to said aqueous acid reaction medium as the feed of sodium chlorate thereto; removing said by-product crystalline sodium chloride from said reaction zone in a slurry with entrained aqueous acid reaction medium; removing at least a substantial proportion of said entrained aqueous acid reaction medium from said recovered by-product crystalline sodium chloride; forming an aqueous solution of sodium chloride from the resulting purified by-product crystalline sodium chloride and forwarding said aqueous solution of sodium chloride to said electrolysis step for electrolysis therein; and 5 forwarding removed entrained aqueous acid reaction medium to said reaction zone.
In the process of the present invention, the chlorine dioxide generation is preferably effected at the boiling point of the aqueous acid reaction medium lo while a subatmospheric pressure is applied to the chlorine dioxide generation vessel. The process may be carried out at a temperature of about 550 to about 80 C, preferably about 60 to about 75 C, and at a subatmospheric pressure of about 120 to about 250 mmHg, preferably about 125 to about 200 mmHg.
The chlorine dioxide generating process may be carried out at a total acid normality of about 0.05 to about 2 N, preferably about 0.05 to about 1.5 N.
Electrochemical production of sodium chlorate may be effected by electrolysis of an aqueous solution of sodium chloride having a concentration of sodium chloride from about 90 to about 320 gpl, preferably about 100 to about 200 gpl, and containing about 2 to about 7 gpl of sodium dichromate at a pH of about 6 to about 7 and a temperature of about 60 to about 100 C.
Such electrolysis results in an aqueous solution of sodium chlorate, sodium chloride and sodium dichromate which is passed to the chlorine dioxide generation step and generally contains about 400 to about 600 gpl, preferably about 450 to about 550 gpl, of sodium chlorate, about 90 to about 200 gpl, preferably about 100 to about 150 gpl, of sodium chloride, and about 2 to about 7 gpl of sodium dichromate.
The invention is described further, by way of illustration, with reference to the accompanying drawing, wherein:
Figure 1 is a schematic flow diagram of an integrated chlorine dioxide generating operation provided in accordance with one embodiment of the present invention.
Referring to the drawing, chlorate cells (1) operating near pH 6 to 7 in reactor (2) convert aqueous sodium chloride to aqueous sodium chlorate and by-product hydrogen (6) while the coproduction of C12 and/or 02 is maintained below the explosive limit of reaction with hydrogen. Hydrogen (6) is fed to an HC1 burner (3) wherein the hydrogen is reacted with chlorine (27) and water (28) to form hydrochloric acid (29). The hydrochloric acid (29) is forwarded via pump (4) to a combined chlorine dioxide generator-evaporator-crystallizer (5) . Cell liquor (30) from the reactor (2) containing sodium chlorate, sodium chloride and sodium dichromate, is forwarded by pump (7) to the chlorine dioxide generator (5). Typically, the concentrations of materials in the cell liquor are 500 gpL NaC103, 100 gpL NaCl and 2 to 6 gpl Na2Cr207. H20.
In the combined chlorine dioxide generator-evaporator-crystallizer (5), the HC1 and NaC103 react according to the reactions shown in reactions (1) and (2) above, to form chlorine dioxide and chlorine which are removed from the chlorine dioxide generator (5) in admixture with steam (31), which is formed by maintaining the aqueous acid reaction medium in the chlorine dioxide generator (5) at its boiling point under a subatmospheric pressure.
The gases (31) are forwarded to an absorber (8) after passing through a cooler (9) to condense out most of the steam. In the absorber (8), chilled water absorbs the C102 and some of the C12 to form a solution of chlorine dioxide (11), typically containing about 10 gpl C102 and about 2 gpl C12. The balance of the chlorine is forwarded by an ejector (12), which creates the vacuum on the chlorine dioxide generator (5), and via a seal pot (13) to the HC1 burner (3). If desired, the ejector (12) may be replaced by a vacuum pump. Make-up chlorine (14) is added to the chlorine supplied to the HC1 burner (3) to replace chlorine removed by absorption in the chlorine dioxide solution.
Sodium chloride precipitates in the chlorine dioxide generator (5) once the sodium chloride by-product of the reactions between NaC103 and HC1 saturates the aqueous acid reaction medium after start up and is a by-product of the chlorine dioxide generation. The evaporation of water from the aqueous acid reaction medium not only increases the concentration of NaCl formed by reaction (1) and (2) and NaCl fed from chlorate reactor (2) to form the crystalline NaCl but also concentrates the sodium dichromate fed from reactor (2).
The slurry of sodium chloride crystals and aqueous acid reaction medium containing at least about 20 gpL, preferably greater than about 100 gpL, of sodium dichromate, is removed from the generator (5) and passed via a pump (15) to a hydrocyclone (16) to concentrate the slurry, generally from about 10 to about 30% v/v to about 70 to about 90% v/v, typically from about 20 to about 80% v/v. The concentrated slurry is fed from the hydrocyclone (16) to a filter (17) where the remaining liquid containing sodium chlorate, sodium chloride and sodium dichromate is separated from the solid sodium chloride crystals.
Gases and liquids flow from the filter (17) to a separator (18) from where the gases pass to an ejector (19), and from where liquid, in the form of aqueous sodium chlorate, sodium chloride and sodium dichromate, returns to the generator (5) through line 32. The separated crystalline sodium chloride from the filter (17) is fed to a dissolving tank (21) via a chute (20).
Water (22) or a condensate from cooler (9) is added to the dissolving tank (21) to dissolve the crystalline sodium chloride to form an aqueous solution of substantially pure sodium chloride (33), which then is fed via a pump (23) to the chlorate reactor (2) for generation of further sodium chlorate.
In order to control the amount of acid and sodium dichromate which is recycled to the chlorate reactor (2) from the chlorine dioxide generator and to permit sodium chlorate to be produced under optimum conditions, a controlled flow of slurry from pump (15) is forwarded to the dissolving tank (21) by line (24). Alternatively, a portion of the slurry from the hydrocyclone (16) may be passed directly from the hydrocyclone to the dissolving tank (21), by-passing the filter (17). In general, the sodium dichromate concentration in the slurry is at about 20 gpL or more, whereby the amount of acid forwarded to the chlorate cells (1) provides an acceptable pH in the reactor (2) for safe and efficient operation. At sodium dichromate concentrations of above about 100 gpL, it may be necessary to provide some sodium hydroxide via line (25) to maintain the pH in the chlorate reactor (2) at safe and efficient levels.
In the event of addition of such sodium hydroxide, the neutralization reaction proceeds in accordance with reaction (3):
NaOH + HC1 -+ NaCl + H20 (3) In order to prevent accumulation in the overall process of sodium chloride formed by reaction (3), sodium chloride may be bled by line (26) and disposed of. In order to prevent loss of sodium dichromate for economic and environmental reasons, it is preferable for line (24) to be closed during disposal of NaCl via line 26.
It will be seen, therefore, that sodium dichromate fed by pump (7) from chlorate reactor (2) into the chlorine dioxide generator (5) may be concentrated to any desired level in the aqueous acid reaction medium in the chlorine dioxide generator (5) by removing only pure sodium chloride at filter (17) and returning all the separated liquor (32) to the chlorine dioxide generator (5). The concentration of sodium dichromate in the chlorine dioxide generator is prevented from reaching excessively high levels and may be controlled at any desired concentration in the chlorine dioxide generator via bleed line (24). If desired, some of the sodium dichromate may be bled from the chlorine dioxide generator (5) by not washing or only partially washing the sodium chloride on filter (17) and permitting such sodium dichromate to pass to the chlorate cells (1).
A safe and effective level of acidity in the chlorate reactor (2) is provided by the bleed (24) which prevents excess chlorine formation in the chlorate reactor in the presence of the hydrogen. As noted earlier, excess chlorine and the potential for forming explosive mixtures with the hydrogen, can occur at low pH values. The reactions involved in the formation of sodium chlorate by electrolysis of an aqueous solution of sodium chloride are illustrated by the following reactions:
electrolysis pH 6-7 3 Nacl -4 3 NaOH + 3/2C12 _+ 3 NaOC1+3/2H2 step 1 Step 2 3 NaOCl ~ NaC103 + 2 NaCl step 3 The excess chlorine can result where step 2 does not occur properly, as at low pH.
An alternative process to the integrated procedure described in detail above with respect to Figure 1, is to operate a free-standing chlorine dioxide generating process in which chlorine dioxide and chlorine are formed by reaction of sodium chlorate and hydrochloric acid in an aqueous acid reaction medium at the boiling point of the reaction medium under a subatmospheric pressure. An initial charge of sodium dichromate is made to the reaction medium to provide a desired steady 5 state concentration of sodium dichromate in the aqueous acid reaction medium. Aqueous sodium chlorate (which may be formed from crystal sodium chlorate or supplied by cell liquor) and hydrochloric acid are continuously fed to the reaction medium to replace chemicals 10 consumed by the reactions. A de-chromated cell liquor feed may be employed, if desired.
Crystalline by-product sodium chloride removed from the chlorine dioxide generator forms a slurry with aqueous acid reaction medium and is thoroughly washed to free the crystalline by-product sodium chloride from reaction medium, particularly sodium dichromate, before disposal of the sodium chloride or reuse thereof. The removed aqueous reaction medium is returned to the generator. In this way, the loss of sodium dichromate values from the generator is minimized. Any losses of Cr (VI) values, however, which may result from incomplete washing of the crystalline sodium chloride, may be compensated for by employing a cell liquor feed containing dichromates.
Although sodium dichromate is fed to the generator, some of the feed may be converted to trivalent chromium ions (Cr3+), especially at higher acidities. However, both hexavalent and trivalent chromium compounds have catalytic properties in the chlorine dioxide generating reaction. Cr3+ is reoxidized back to Cr (VI) in the chlorate cells due to the action of hypochlorite.
Referring to the drawing, chlorate cells (1) operating near pH 6 to 7 in reactor (2) convert aqueous sodium chloride to aqueous sodium chlorate and by-product hydrogen (6) while the coproduction of C12 and/or 02 is maintained below the explosive limit of reaction with hydrogen. Hydrogen (6) is fed to an HC1 burner (3) wherein the hydrogen is reacted with chlorine (27) and water (28) to form hydrochloric acid (29). The hydrochloric acid (29) is forwarded via pump (4) to a combined chlorine dioxide generator-evaporator-crystallizer (5) . Cell liquor (30) from the reactor (2) containing sodium chlorate, sodium chloride and sodium dichromate, is forwarded by pump (7) to the chlorine dioxide generator (5). Typically, the concentrations of materials in the cell liquor are 500 gpL NaC103, 100 gpL NaCl and 2 to 6 gpl Na2Cr207. H20.
In the combined chlorine dioxide generator-evaporator-crystallizer (5), the HC1 and NaC103 react according to the reactions shown in reactions (1) and (2) above, to form chlorine dioxide and chlorine which are removed from the chlorine dioxide generator (5) in admixture with steam (31), which is formed by maintaining the aqueous acid reaction medium in the chlorine dioxide generator (5) at its boiling point under a subatmospheric pressure.
The gases (31) are forwarded to an absorber (8) after passing through a cooler (9) to condense out most of the steam. In the absorber (8), chilled water absorbs the C102 and some of the C12 to form a solution of chlorine dioxide (11), typically containing about 10 gpl C102 and about 2 gpl C12. The balance of the chlorine is forwarded by an ejector (12), which creates the vacuum on the chlorine dioxide generator (5), and via a seal pot (13) to the HC1 burner (3). If desired, the ejector (12) may be replaced by a vacuum pump. Make-up chlorine (14) is added to the chlorine supplied to the HC1 burner (3) to replace chlorine removed by absorption in the chlorine dioxide solution.
Sodium chloride precipitates in the chlorine dioxide generator (5) once the sodium chloride by-product of the reactions between NaC103 and HC1 saturates the aqueous acid reaction medium after start up and is a by-product of the chlorine dioxide generation. The evaporation of water from the aqueous acid reaction medium not only increases the concentration of NaCl formed by reaction (1) and (2) and NaCl fed from chlorate reactor (2) to form the crystalline NaCl but also concentrates the sodium dichromate fed from reactor (2).
The slurry of sodium chloride crystals and aqueous acid reaction medium containing at least about 20 gpL, preferably greater than about 100 gpL, of sodium dichromate, is removed from the generator (5) and passed via a pump (15) to a hydrocyclone (16) to concentrate the slurry, generally from about 10 to about 30% v/v to about 70 to about 90% v/v, typically from about 20 to about 80% v/v. The concentrated slurry is fed from the hydrocyclone (16) to a filter (17) where the remaining liquid containing sodium chlorate, sodium chloride and sodium dichromate is separated from the solid sodium chloride crystals.
Gases and liquids flow from the filter (17) to a separator (18) from where the gases pass to an ejector (19), and from where liquid, in the form of aqueous sodium chlorate, sodium chloride and sodium dichromate, returns to the generator (5) through line 32. The separated crystalline sodium chloride from the filter (17) is fed to a dissolving tank (21) via a chute (20).
Water (22) or a condensate from cooler (9) is added to the dissolving tank (21) to dissolve the crystalline sodium chloride to form an aqueous solution of substantially pure sodium chloride (33), which then is fed via a pump (23) to the chlorate reactor (2) for generation of further sodium chlorate.
In order to control the amount of acid and sodium dichromate which is recycled to the chlorate reactor (2) from the chlorine dioxide generator and to permit sodium chlorate to be produced under optimum conditions, a controlled flow of slurry from pump (15) is forwarded to the dissolving tank (21) by line (24). Alternatively, a portion of the slurry from the hydrocyclone (16) may be passed directly from the hydrocyclone to the dissolving tank (21), by-passing the filter (17). In general, the sodium dichromate concentration in the slurry is at about 20 gpL or more, whereby the amount of acid forwarded to the chlorate cells (1) provides an acceptable pH in the reactor (2) for safe and efficient operation. At sodium dichromate concentrations of above about 100 gpL, it may be necessary to provide some sodium hydroxide via line (25) to maintain the pH in the chlorate reactor (2) at safe and efficient levels.
In the event of addition of such sodium hydroxide, the neutralization reaction proceeds in accordance with reaction (3):
NaOH + HC1 -+ NaCl + H20 (3) In order to prevent accumulation in the overall process of sodium chloride formed by reaction (3), sodium chloride may be bled by line (26) and disposed of. In order to prevent loss of sodium dichromate for economic and environmental reasons, it is preferable for line (24) to be closed during disposal of NaCl via line 26.
It will be seen, therefore, that sodium dichromate fed by pump (7) from chlorate reactor (2) into the chlorine dioxide generator (5) may be concentrated to any desired level in the aqueous acid reaction medium in the chlorine dioxide generator (5) by removing only pure sodium chloride at filter (17) and returning all the separated liquor (32) to the chlorine dioxide generator (5). The concentration of sodium dichromate in the chlorine dioxide generator is prevented from reaching excessively high levels and may be controlled at any desired concentration in the chlorine dioxide generator via bleed line (24). If desired, some of the sodium dichromate may be bled from the chlorine dioxide generator (5) by not washing or only partially washing the sodium chloride on filter (17) and permitting such sodium dichromate to pass to the chlorate cells (1).
A safe and effective level of acidity in the chlorate reactor (2) is provided by the bleed (24) which prevents excess chlorine formation in the chlorate reactor in the presence of the hydrogen. As noted earlier, excess chlorine and the potential for forming explosive mixtures with the hydrogen, can occur at low pH values. The reactions involved in the formation of sodium chlorate by electrolysis of an aqueous solution of sodium chloride are illustrated by the following reactions:
electrolysis pH 6-7 3 Nacl -4 3 NaOH + 3/2C12 _+ 3 NaOC1+3/2H2 step 1 Step 2 3 NaOCl ~ NaC103 + 2 NaCl step 3 The excess chlorine can result where step 2 does not occur properly, as at low pH.
An alternative process to the integrated procedure described in detail above with respect to Figure 1, is to operate a free-standing chlorine dioxide generating process in which chlorine dioxide and chlorine are formed by reaction of sodium chlorate and hydrochloric acid in an aqueous acid reaction medium at the boiling point of the reaction medium under a subatmospheric pressure. An initial charge of sodium dichromate is made to the reaction medium to provide a desired steady 5 state concentration of sodium dichromate in the aqueous acid reaction medium. Aqueous sodium chlorate (which may be formed from crystal sodium chlorate or supplied by cell liquor) and hydrochloric acid are continuously fed to the reaction medium to replace chemicals 10 consumed by the reactions. A de-chromated cell liquor feed may be employed, if desired.
Crystalline by-product sodium chloride removed from the chlorine dioxide generator forms a slurry with aqueous acid reaction medium and is thoroughly washed to free the crystalline by-product sodium chloride from reaction medium, particularly sodium dichromate, before disposal of the sodium chloride or reuse thereof. The removed aqueous reaction medium is returned to the generator. In this way, the loss of sodium dichromate values from the generator is minimized. Any losses of Cr (VI) values, however, which may result from incomplete washing of the crystalline sodium chloride, may be compensated for by employing a cell liquor feed containing dichromates.
Although sodium dichromate is fed to the generator, some of the feed may be converted to trivalent chromium ions (Cr3+), especially at higher acidities. However, both hexavalent and trivalent chromium compounds have catalytic properties in the chlorine dioxide generating reaction. Cr3+ is reoxidized back to Cr (VI) in the chlorate cells due to the action of hypochlorite.
EXAMPLES
Example 1:
This Example illustrates the catalytic effect of high concentrations of sodium dichromate on the efficiency of chlorine dioxide generation.
A 10L chlorine dioxide pilot plant generator was operated at the boiling point of the reaction medium under subatmospheric pressure to form chlorine dioxide by reaction of sodium chlorate and hydrochloric acid.
The generator conditions were are follows:
Reaction medium: Total [H+] - from 0.67N to 3.19N
[C103-] - from 0.23 M to 0.01 M
[Na2Cr207.2H20] - 70 g/L
Temperature - 68 C
During the run, 12N HC1 was fed to the generator at an average rate of 25mL/min in order to increase acidity while generating chlorine dioxide at a rate of approximately 6 to 7 g/min. The sodium chlorate input, in the form of either 6M NaC103 or 5.2M NaC103 + 1.9M
NaCl solution was made to match production rate demand while decreasing chlorate liquor concentration.
Chemical efficiencies, based on analysis by gas chromatography, as high as 92.24% were found while, with the sodium dichromate concentration below 20 gpL, chemical efficiencies were found to range from 85.9 to 86.90.
Example 2:
This Example also illustrates the catalytic effect of high concentration of sodium dichromate on the efficiency of chlorine dioxide production.
The same pilot plant generator from Example 1 was operated under the following process conditions to form chlorine dioxide by reaction of sodium chlorate and hydrochloric acid.
The generator conditions were as follows:
Reaction medium: Total [H+] - 0.05N to 3.6N
[C103] - 1.62 M to 0.1M
[Na2Cr2O7.2H20] - 150 g/L
Temperature - 68 C
The run was made in a similar manner to Example 1, with liquor composition being driven from 0.05N [H+]/1.62M
[C103-] to 3. 6N [H+] /0. 1M [C103-] over the course of a run of more than 1.5 hours.
Chemical efficiencies, based on gas analysis by gas chromatography, as high as 96.24% were found while, under similar conditions at concentrations of sodium dichromate below 20 gpL, chemical efficiencies were found in a range of 85.9 to 86.9%.
In summary of this disclosure, the present invention provides a novel method of producing chlorine dioxide at high efficiency from sodium chlorate and hydrochloric acid by using high concentrations of sodium dichromate to catalyze the process, particularly in a procedure which is integrated with sodium chlorate generation. Modifications are possible within the scope of this invention.
Example 1:
This Example illustrates the catalytic effect of high concentrations of sodium dichromate on the efficiency of chlorine dioxide generation.
A 10L chlorine dioxide pilot plant generator was operated at the boiling point of the reaction medium under subatmospheric pressure to form chlorine dioxide by reaction of sodium chlorate and hydrochloric acid.
The generator conditions were are follows:
Reaction medium: Total [H+] - from 0.67N to 3.19N
[C103-] - from 0.23 M to 0.01 M
[Na2Cr207.2H20] - 70 g/L
Temperature - 68 C
During the run, 12N HC1 was fed to the generator at an average rate of 25mL/min in order to increase acidity while generating chlorine dioxide at a rate of approximately 6 to 7 g/min. The sodium chlorate input, in the form of either 6M NaC103 or 5.2M NaC103 + 1.9M
NaCl solution was made to match production rate demand while decreasing chlorate liquor concentration.
Chemical efficiencies, based on analysis by gas chromatography, as high as 92.24% were found while, with the sodium dichromate concentration below 20 gpL, chemical efficiencies were found to range from 85.9 to 86.90.
Example 2:
This Example also illustrates the catalytic effect of high concentration of sodium dichromate on the efficiency of chlorine dioxide production.
The same pilot plant generator from Example 1 was operated under the following process conditions to form chlorine dioxide by reaction of sodium chlorate and hydrochloric acid.
The generator conditions were as follows:
Reaction medium: Total [H+] - 0.05N to 3.6N
[C103] - 1.62 M to 0.1M
[Na2Cr2O7.2H20] - 150 g/L
Temperature - 68 C
The run was made in a similar manner to Example 1, with liquor composition being driven from 0.05N [H+]/1.62M
[C103-] to 3. 6N [H+] /0. 1M [C103-] over the course of a run of more than 1.5 hours.
Chemical efficiencies, based on gas analysis by gas chromatography, as high as 96.24% were found while, under similar conditions at concentrations of sodium dichromate below 20 gpL, chemical efficiencies were found in a range of 85.9 to 86.9%.
In summary of this disclosure, the present invention provides a novel method of producing chlorine dioxide at high efficiency from sodium chlorate and hydrochloric acid by using high concentrations of sodium dichromate to catalyze the process, particularly in a procedure which is integrated with sodium chlorate generation. Modifications are possible within the scope of this invention.
Claims (17)
1. A process for the production of chlorine dioxide, which comprises:
reacting sodium chlorate and hydrochloric acid in an aqueous acid reaction medium in the presence of at least 100 grams per liter (gpl) of sodium dichromate.
reacting sodium chlorate and hydrochloric acid in an aqueous acid reaction medium in the presence of at least 100 grams per liter (gpl) of sodium dichromate.
2. The process of claim 1 wherein said sodium dichromate is present in an amount of up to 200 gpl.
3. The process of claim 1 or 2 which is carried out at the boiling point of the aqueous acid reaction medium while a subatmospheric pressure is applied thereto.
4. The process of claim 3 wherein the boiling point is 55° to 80°C and said subatmospheric pressure is from 100 to 250 mmHg.
5. The process of claim 3 or 4 wherein said sodium chloride by-product precipitates from the aqueous acid reaction medium and is separated from the aqueous acid reaction medium.
6. The process of any one of claims 3 to 5 wherein chlorine dioxide and chlorine are continuously generated from the aqueous acid reaction medium in a reaction zone, said sodium chloride by-product is continually deposited in said reaction zone and recovered from the reaction zone in the form of slurry entrained with aqueous acid reaction medium, said recovered sodium chloride is freed from entrained aqueous acid reaction medium, and the removed entrained aqueous acid reaction medium is returned to the reaction zone.
7. The process of any one of claims 1 to 6 wherein said aqueous acid reaction medium has a total acid normality of 0.05 to 2 N.
8. The process of any one of claims 1 to 7 wherein said sodium dichromate concentration is provided by an initial charge of sodium dichromate to the reaction medium to provide a desired steady state concentration of sodium dichromate.
9. A continuous integrated process of forming chlorine dioxide by reacting sodium chlorate with hydrochloric acid to form chlorine dioxide and chlorine and electrolytically producing the sodium chlorate from sodium chloride produced by the chlorine dioxide-forming reaction, which comprises:
feeding sodium chloride and hydrochloric acid to an aqueous acid reaction medium in a reaction zone, effecting reaction of sodium chlorate and hydrochloric acid in the presence of at least 100 grams per liter (gpl) of sodium dichromate to produce chlorine dioxide and chlorine in a reaction zone, said aqueous acid reaction medium being at the boiling point of the aqueous acid reaction medium while a subatmospheric pressure is applied to the reaction zone, and precipitating by-product crystalline sodium chloride in said reaction zone, removing a gaseous admixture of chlorine dioxide, chlorine and steam from the reaction zone and forming an aqueous solution of chlorine dioxide and chlorine and residual gaseous chlorine therefrom, electrolytically forming sodium chlorate by electrolysis of an aqueous solution of sodium chloride containing 2 to 7 gpl of sodium dichromate at a pH of 6 to 7 at a temperature of 60° to 100°C to produce an aqueous solution of sodium chlorate, sodium chloride and sodium dichromate, forwarding said aqueous solution of sodium chlorate, sodium chloride and sodium dichromate to said aqueous acid reaction medium as the feed of sodium chlorate thereto, removing said by-product crystalline sodium chloride from said reaction zone in a slurry with entrained aqueous acid reaction medium, removing at least a substantial proportion of said entrained aqueous acid reaction medium from said recovered by-product crystalline sodium chloride, forming an aqueous solution of sodium chloride from the resulting purified by-product crystalline sodium chloride and forwarding said aqueous solution of sodium chloride to said electrolysis step for electrolysis therein, and forwarding removed entrained aqueous acid reaction medium to said reaction zone.
feeding sodium chloride and hydrochloric acid to an aqueous acid reaction medium in a reaction zone, effecting reaction of sodium chlorate and hydrochloric acid in the presence of at least 100 grams per liter (gpl) of sodium dichromate to produce chlorine dioxide and chlorine in a reaction zone, said aqueous acid reaction medium being at the boiling point of the aqueous acid reaction medium while a subatmospheric pressure is applied to the reaction zone, and precipitating by-product crystalline sodium chloride in said reaction zone, removing a gaseous admixture of chlorine dioxide, chlorine and steam from the reaction zone and forming an aqueous solution of chlorine dioxide and chlorine and residual gaseous chlorine therefrom, electrolytically forming sodium chlorate by electrolysis of an aqueous solution of sodium chloride containing 2 to 7 gpl of sodium dichromate at a pH of 6 to 7 at a temperature of 60° to 100°C to produce an aqueous solution of sodium chlorate, sodium chloride and sodium dichromate, forwarding said aqueous solution of sodium chlorate, sodium chloride and sodium dichromate to said aqueous acid reaction medium as the feed of sodium chlorate thereto, removing said by-product crystalline sodium chloride from said reaction zone in a slurry with entrained aqueous acid reaction medium, removing at least a substantial proportion of said entrained aqueous acid reaction medium from said recovered by-product crystalline sodium chloride, forming an aqueous solution of sodium chloride from the resulting purified by-product crystalline sodium chloride and forwarding said aqueous solution of sodium chloride to said electrolysis step for electrolysis therein, and forwarding removed entrained aqueous acid reaction medium to said reaction zone.
10. The process of claim 9 wherein said recovered by-product crystalline sodium chlorate is treated to be substantially free of entrained aqueous acid reaction medium.
11. The process of claim 9 wherein said recovered by-product crystalline sodium chloride is treated sufficiently to leave an amount of sodium dichromate entrained in the crystalline sodium chloride sufficient to provide the concentration of sodium dichromate required to be present during said electrolysis step.
12. The process of any one of claims 9 to 11 wherein said slurry of by-product crystalline sodium chloride and entrained aqueous acid reaction medium is forwarded to a hydrocyclone to concentrate the slurry from a concentration of 10 to 30 % v/v, to a concentration of 70 to 90 % v/v and then passed onto a filter to remove said entrained aqueous acid reaction medium from the crystalline sodium chloride.
13. The process of claim 12 wherein crystalline sodium chloride is dissolved in water in a dissolving tank to form said aqueous solution of sodium chloride forwarded to the electrolysis step and a controlled amount of said slurry of by-product crystalline sodium chloride and entrained aqueous acid reaction medium is forwarded to said dissolving tank rather than to said hydrocyclone, and, optionally at concentrations of sodium dichromate in said entrained aqueous acid reaction medium above 100 gpl, feeding sodium hydroxide to said dissolving tank in an amount to provide a pH of the aqueous solution of sodium chloride in said electrolysis step which is within acceptable limits for safe and efficient operation.
14. The process of claim 12 wherein crystalline sodium chloride is dissolved in water in a dissolving tank to form said aqueous solution of sodium chloride forwarded to the electrolysis step and a controlled amount of said slurry of by-product crystalline sodium chloride and entrained aqueous acid reaction medium is forwarded directly from the hydrocyclone to said dissolving tank by-passing the filter, and, optionally at concentrations of sodium dichromate in said entrained aqueous acid reaction medium above 100 gpl, feeding sodium hydroxide to said dissolving tank in an amount to provide a pH of the aqueous solution of sodium chloride in said electrolysis step which is within acceptable limits for safe and efficient operation.
15. The process of claim 13 or 14 wherein sodium chloride is removed from the integrated process cycle to prevent accumulation of sodium chloride resulting from neutralization of HCl in the aqueous solution of sodium chloride by said sodium hydroxide.
16. The process of claim 15 wherein said forwarding of said controlled amount of slurry to said dissolving tank is terminated during said sodium chloride removal step to avoid losses of sodium dichromate during such removal step.
17. The process of any one of claims 9 to 16 wherein hydrochloric acid feed for the aqueous acid reaction medium is provided by reacting hydrogen by-product from the electrolysis step with said residual gaseous chlorine and additional chlorine in the presence of water.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US3548897P | 1997-01-13 | 1997-01-13 | |
| US60/035,488 | 1997-01-13 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2226651A1 CA2226651A1 (en) | 1998-07-13 |
| CA2226651C true CA2226651C (en) | 2009-04-28 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2226651 Expired - Lifetime CA2226651C (en) | 1997-01-13 | 1998-01-12 | Process for the production of chlorine dioxide |
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| CA (1) | CA2226651C (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN106011916A (en) * | 2016-07-27 | 2016-10-12 | 广西博世科环保科技股份有限公司 | Sodium chlorate reactor applied to comprehensive-method chlorine dioxide preparation system |
-
1998
- 1998-01-12 CA CA 2226651 patent/CA2226651C/en not_active Expired - Lifetime
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN106011916A (en) * | 2016-07-27 | 2016-10-12 | 广西博世科环保科技股份有限公司 | Sodium chlorate reactor applied to comprehensive-method chlorine dioxide preparation system |
| CN106011916B (en) * | 2016-07-27 | 2018-09-25 | 广西博世科环保科技股份有限公司 | Sodium chlorate reactor for synthesis chlorine dioxide preparation system |
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| Publication number | Publication date |
|---|---|
| CA2226651A1 (en) | 1998-07-13 |
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