CA1049711A - Chlorine dioxide generation - Google Patents
Chlorine dioxide generationInfo
- Publication number
- CA1049711A CA1049711A CA208,075A CA208075A CA1049711A CA 1049711 A CA1049711 A CA 1049711A CA 208075 A CA208075 A CA 208075A CA 1049711 A CA1049711 A CA 1049711A
- Authority
- CA
- Canada
- Prior art keywords
- chlorine
- chlorine dioxide
- sodium
- continuously
- equation
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
- D21C9/10—Bleaching ; Apparatus therefor
- D21C9/12—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds
- D21C9/14—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds with ClO2 or chlorites
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C11/00—Regeneration of pulp liquors or effluent waste waters
- D21C11/06—Treatment of pulp gases; Recovery of the heat content of the gases; Treatment of gases arising from various sources in pulp and paper mills; Regeneration of gaseous SO2, e.g. arising from liquors containing sulfur compounds
- D21C11/063—Treatment of gas streams comprising solid matter, e.g. the ashes resulting from the combustion of black liquor
- D21C11/066—Separation of solid compounds from these gases; further treatment of recovered products
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P40/00—Technologies relating to the processing of minerals
- Y02P40/40—Production or processing of lime, e.g. limestone regeneration of lime in pulp and sugar mills
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- Life Sciences & Earth Sciences (AREA)
- Engineering & Computer Science (AREA)
- Wood Science & Technology (AREA)
- Paper (AREA)
Abstract
Abstract of the Disclosure Chlorine dioxide production is integrated with a pulp mill system. A gaseous mixture of chlorine dioxide and chlorine is formed in a chlorine dioxide generating zone, an aqueous solution of chlorine dioxide wherein no more than about 10% of the available chlorine of the aqueous solution is provided by chlorine and the remainder of the available chlorine is provided by the chlorine dioxide is formed from the gaseous mixture, the aqueous chlorine dioxide solution is used to bleach cellulosic fibrous material pulp, hydrogen chloride is formed by reaction between the chlorine separated from the mixture and hydrogen and the hydrogen chloride is forwarded to the chlorine dioxide generating zone for use in forming the gaseous mixture of chlorine dioxide and chlorine.
Description
The present invention relates to the generation of chlorine dioxide, more particularly, to the integratio~
of chlorine dioxide generatlon with bleach plant operations.
It is well known to form chlorine dioxide by reduction of a chlorate in an acid medium in accordance with the equation:
C103- + Cl + 2H+ ~ C102 ~ ~C12 ~ H2O
Typical processes are disclosed in Canadian Patents Nos.
825,084 and 826,577, wherein sodium chlorate is reduced by chloride ions, provided by sodium chloride,hydrogen chloride or mixtures thereof, in the presence of sulphuric acid.
Other procedures are disclosed in Canadian Patents Nos.
913,328 and 908,944 wherein sodium chlorate is reduced by -., .
hydrochloric acid which also provides the acid requirement.
A further procedure is disclosed in Canadian Patent ~o. 920,773 ~; -wherein sodium chlorate is reduced by chloride ions in the presence of sulphuric acid, sodium sulphate is deposited from the reaction mixture, and an a~ueous solution of the i sodium sulphate is treated with hydrogen chloride to form sodium chloride and sulphuric acid, the latter being recycled to the reaction mixture.
In the production of bleached cellulosic fibrous materlal pulp it is known to bleach cellulosic fibrous material ~ ~
~; pulp, formed in any convenient manner, typically by digestion ~ -of wood chips using the Kraft process with sodium hydroxide and sodium sulphide as the active pulping chemicals, with -chlorine-containing bleaching chemicals, especially chlorine - -dioxide and chlorine.
~, The present invention is directed to a novel
of chlorine dioxide generatlon with bleach plant operations.
It is well known to form chlorine dioxide by reduction of a chlorate in an acid medium in accordance with the equation:
C103- + Cl + 2H+ ~ C102 ~ ~C12 ~ H2O
Typical processes are disclosed in Canadian Patents Nos.
825,084 and 826,577, wherein sodium chlorate is reduced by chloride ions, provided by sodium chloride,hydrogen chloride or mixtures thereof, in the presence of sulphuric acid.
Other procedures are disclosed in Canadian Patents Nos.
913,328 and 908,944 wherein sodium chlorate is reduced by -., .
hydrochloric acid which also provides the acid requirement.
A further procedure is disclosed in Canadian Patent ~o. 920,773 ~; -wherein sodium chlorate is reduced by chloride ions in the presence of sulphuric acid, sodium sulphate is deposited from the reaction mixture, and an a~ueous solution of the i sodium sulphate is treated with hydrogen chloride to form sodium chloride and sulphuric acid, the latter being recycled to the reaction mixture.
In the production of bleached cellulosic fibrous materlal pulp it is known to bleach cellulosic fibrous material ~ ~
~; pulp, formed in any convenient manner, typically by digestion ~ -of wood chips using the Kraft process with sodium hydroxide and sodium sulphide as the active pulping chemicals, with -chlorine-containing bleaching chemicals, especially chlorine - -dioxide and chlorine.
~, The present invention is directed to a novel
- 2 - ~ -integration of a chlorine dioxide-producing process and a bleach plant operation to achieve efficient and economic : utilization of chemicals. In accordance with the present invention, there is provided a continuous pulp mill process to provide bleached pulp which comprises: continuously generating chlorine dioxide in a reaction vessel by reduc-tion of sodium chlorate using sodium chloride in an aqueous acid reaction medium containing sulphuric acid and having an acidity from about 2 to about 4.8N in accordance with the equation: .
NaClO3 + NaCl ~ H2S4 ~ C1O2 + 1/2cl2 + H2O + Na2SO4 -(1);
continuously maintaining the aqueous reaction medium at its boiling point at a temperature below that above which sub-stantial decomposition of chlorine dioxide occurs while maintaining the reaction vessel under a subatmospheric pressure to form the chlorine dioxide in gaseous admixture with water vapour and chlorine and to deposit sodium sulphate from the reaction medium; continuously removing the gaseous mixture of chlorine dioxide, chlorine and water vapour from the reaction vessel; continuously separating the removed gaseous mixture into an aqueous solution of chlorine dioxide containing no more than 10% of the available chlorine thereof as chlorine and the remainder of the available chlorine as chlorine dioxide, and gaseous chlorine; continuously sub-jecting washed and otherwise untreated wood pulp to bleaching utilizing the aqueous solution of chlorine dioxide to provide bleached pulp having a predetermined brightness; continuously removing the deposited sodium sulphate from the reaction vessel; continuously contacting the removed sodium sulphate with hydrogen chloride to form sodium chloride and sulphuric acid in accordance with the equation:
; Na2SO4 + 2HCl - 2NaCl ~ H2SO4 ~ (2) ~ .
.
" 1049711 continuousl~ ~oxwaxding the sulphuric acid and substantially one-half the molar amount of the sodium chloride to the reac-tion vessel along with unreacted sodium sulphate to provide the sulphuric acid and sodium chloride requirements of the chlorine dioxi:de generating reaction of equation l; the unreacted sodium sulphate depositing from the reaction medium and cycling as a dead load between the reaction medium and the contacting with hydrogen chloride; continuously electro-lyzing an aqueous solution of the remainder of the molar amount of sodium chloride to form an aqueous solution of sodium chlorate and hydrogen in accordance with the equation:
NaCl + 3H2O (3);
continuously forwarding the aqueous solution of sodium chlorate to the reaction vessel to provide the sodium chlorate : requirement of the chlorine dioxide generating reaction of - ~
equation l; continuously reacting substantially one-third :
the molar amount of hydrogen formed in the electrolysis reac-tion of equation 3 with chlorine in accordance with the :
equation:
; 20 H2 + C12 ~ 2HCl - (4) : the gaseous chlorine separated from the gaseous mixture of chlorine dioxide, chlorine and water vapour providing part -:
of the molar amount of chlorine required by equation 4; and continuously utilizing the hydrogen chloride formed by . equation 4 as the hydrogen chloride requirement of equation 2. :
The process of the invention is described further, : by way of illustration, with reference to the accompanying drawing, which is a schematic flow sheet of one embodiment of the present invention.
Referring first to the drawing, chlorine dioxide and chlorine are formed in a chlorine dioxide generation zone ~ .
, -:. . '; . , . ., : - . . , , ~- , .
.
10. The generation zone 10 contains a chlorine dioxide generator 12 wherein an aqueous reaction medium is maintained at its boiling point, preferably with the generator 12 being maintained under a reduced pressure. The aqueous B
, . . . . .. - -. . . . , . . - . , - :
. .
reaction medium is formed from sodium chlorate solution fed by line 14, sulphuric acid and sodium chloride fed by line 16.
Sodium sulphate is precipitated from the reaction medium and the water vapor formed by the evaporation forms a gaseous mixture with the chlorine dioxide and chlorine generated from the reaction medium. The gaseous mixture of chlorine dioxide, chlorine and steam is removed from the chlorine dioxide generation zone 10 by line 18 to a separator 20.
In the separator 20, the water vapor is condensed and the chlorine dioxide is separated substantially completely from the chlorine to provide an aqueous solution of chlorine dioxide in line 22. The separator may be of any convenient type, typically one in which a stream of water contacts the gaseous mixture. The separation of chlorine dioxide and J chlorine preferably is as complete as possible, and in any .. . .
event no more than about 10% of the total available chlorine of the chlorine dioxide solution in line 22 should be provided by chlorine, the remainder of the available chlorine being provided by the chlorine dioxide.
The chlorine dioxide solution in line 22 is passed to a bleach plant 24 wherein it is used to bleach wood pulp fed thereto by line 26 to provide bleached pulp of the required brightness in line 28.
In the bleaching of pulp, the pulp generally is :.
~, subjected to a series of alternating bleaching and purification , steps with the pulp usually being washed after each step.
The purification step generally involves caustic extraction ; using sodium hydroxide solution. The bleaching steps have been carried out in a number of ways, and usually the first `~ 30 ~ 4 _ .. . .: .. , : . . .
bleaching step has involved the use of chlorine or mixtures of chlorine and chlorine dioxide. A commonly-employed bleach plant sequence is known as the CEDED sequence, where C represents bleaching with chlorine, E represents caustic extraction and D represents bleaching with chlorine dioxide.
In the present invention, the first bleaching step is carried out using the aqueous chlorine dioxide solution in line 22.
Preferably, sufficient chlorine dioxide solution is forwarded -~
by line 22 to provide all the bleaching agen`t required by the bleach plant 24.
The sodium sulphate which is precipitated in the chlorine dioxide generator 12 is removed therefrom by line 30, made up into a saturation aqueous solution thereof and passed to a reaction tank 32. The form of the sodium sulphate re-moved by line 30 depends on the reaction conditions of the generator 12. At high acidities, typically around 10 to 12N
the sodium sulphate is in the form of sodium bisulphate whereas at low acidities, typically about 2 to 4.8N, the sodium sulphate is in the form of neutral sodium sulphate (Na2S04).
In the reaction tank 32, the sodium sulphate is reacted with hydrogen chloride fed by line 34, thereby forming sodium chloride, which precipitates from the solution, and sulphuric acid, in accordance with the equatlon:
2 4 ~ H2S04 + 2NaCl :
The hydrogen chloride fed by line 34 to the reaction tanks 32 may be in gaseous form or as hydrochloric acid. In the latter case, the total quantity of water present in the reaction tank should be insufficient to dissolve the sodium .
10497~1 chloride and hence precipitation of the sodium chloride occurs. Similarly, the aqueous solution of sodium sulphate fed to the reaction tank 32 may be less than saturated provided that the quantity of solution is not sufficient to dissolve all the sodium chloride formed.
Since it is not possible to convert all the sodium sulphate to sodium chloride in the reaction tank 34 due to some solubility of the sodium chloride in the aqueous phase, in the interests of maintaining stoichiometry, an excess ~ -of sodium sulphate preferably recycles in the loop between the generator 12 and the reaction tank 32. Therefore, in the liquid effluent from the reaction tank 32, which provides the '3,~ feed line 16 to the chlorine dioxide generator 12, there aredissolved quantities of sodium sulphate in addition to the sulphuric acid.
Sodium chloride precipitated in the reaction tank 32 and formed in accordance with the above equation is utilized in two different ways. Thus, one half the amount is used as the reducing agent for the sodium chlorate in the generator 12 and is made up into an aqueous solution for feed to the generator, typically in admixture with the sulphuric acid feed in line 16~ -~i The remainder of the sodium chloride is formed into . ~ . .
~ an aqueous solution and is passed by line 36 to a chlorate ;9 cell 38 wherein it is subjected to electrolysis to provide the sodium chlorate feed in line 14 to the chlorine dioxide generator 12.
Alternatively, all the precipitated sodium chloride from the reaction tank 32 may be formed into an aqueous solution - . . : . . :
104~711 and passed by line 36 to the chlorate cell 38 wherein half the sodium chloride is electrolysed to sodi~m chlorate ~nd hence the aqueous solution in line 14 contains both the sodium chlorate and sodium chloride requirements of the generator 12 and the feed line 16 contains only the sulphuric acid requirement.
Hydrogen is formed as a by-product gas in chlorate cell 38 and passes therefrom by line 40, approximately one-third of the mole amount passing by line 42 to a hydrogen chloride reactor 44 and the remaining approximately two-thirds of the mole amount being discharged by line 46. The dis-charged hydrogen in line 46 may be used to form water, if desired.
; The chlorine separated from the chlorine dioxide , in the separator 20 passes by line 48 to the hydrogen chloride reactor 44 together with additional chlorine fed by line 50.
In a continuous stoichiometric operation, the chlorine in line 48 provides up to half the chlorine requirement of the reactor 44, depending on the efficiency of the separation of the chlorine dioxide and chlorine in the separator 20. The ' remainder of the chlorine requirement is provided by the chlorine in line 50.
The stoichiometry of the operation described above ~, with reference to Figure 1 is demonstrated by the following :~ . equations:
chlorine dioxide .
i : generator 12 - NaC103 + NaCl + H2S04 ~ C102 + ~C12 + H2ot~so~ .
Chlorate cell : 38 - NaCl + 3H20 - ~ NaC103 + 3H2 ':
~ .
.. , ,, -. . . . .
Reaction Tank 32 - Na2SO~ + 2HCl - ~ 2NaCl + H2SO4 Line 22 - C102 Line 48 - ~C12 Line 50 - ~C12 HCl reactor 44 - H2 + C12 ~ 2~Cl : In the above-described embodiment of the invention, chlorine dioxide is formed in admixture with chlorine in the ~; chlorine dioxide generation zone 10, the chlorine dioxide and chlorine are separated in the separator 20 to provide an aqueous solution of chlorine dioxide in line 22 wherein no more than 10% of the available chlorine content of the aqueous solution is provided by chlorine, the a~ueous solution of chlorine dioxide is used in the bleach plant 24 to bleach . -.
the pulp fed by line 26, the chlorine separated from the chlorine dioxide is used to form hydrogen chloride in the reactor 44 by reaction with hydrogen fed by line 42 ànd the hydrogen chloride formed from the chlorine in line 48, together with the hydrogen chloride formed from the chlorine in line 50, :- is used in the chlorine dioxide-producing reaction, in the -embodiment illustrated by feeding to the chlorine dioxide ~ .
1, generation zone 10 to convert product sodium sulph~te from l the chlorine dioxide.generator to sodium chloride for recycle - .
to the reaction zone.
¦:: Other forms of chlorine dioxide generation zone 10 may be used. For example, the chlorine dioxide generator 12 ~:
may be one in which sodium chlorate solution fed by line 14 is reduced by hydrogen chloride and the acid reaction medium is provided by hydrochloric acid. This may be achieved by ~:
eliminating the reaction tank 32, in which event the hydrogen ~ - 8 - `
i ' , ~' ~ ' . ' ~ ' ' .
, chloride in line 34, preferab]y in the form of hydrochloric acid, is fed directly to the chlorine dioxide generator 12.
Thus, the hydrogen chloride in this embodiment formed from the chlorine separated from the chlorine dioxide is used directly in the chlorine dioxide-producing reaction, rather than indirectly as in the embodiment described above with reference to Figure 1.
In the latter embodiment, the chlorine dioxide generator 12 preferably is operated under the same conditions as discussed with reference to Figure 1, namely, at the boiling point of the reaction mixture under a reduced pressure These conditions result in precipitation of sodium chloride which, after separation from the generator 12 and formation into an aqueous solution, forms the sodium chloride solution feed to the chlorate cell 38 in line 36.
Another embodiment of the invention utilizes the reduction of sodium chlorate with chloride ions in an aqueous medium using sulphuric acid and resulting in precipitation of sodium sulphate. In this embodiment, the hydrogen chloride in line 34 is used to provide up to half the acid requirement, ' the remainder being provided by the sulphuric acid, and up to all the reducing agent. Where the hydrogen chloride provides only part of the reducing agent requirement additional ! :
quantities of chloride may be provided by sodium chloride.
Sodium sulphate again is recovered and, in this embodiment, may be used for make up chemicals in the pulp mill recovery and regeneration operations. In this embodiment, additional chlorine in line 50 is unnecessary, but a source of sodium chloride is required to form the sodium chlorate in line 14.
_ 9 _ .
.
Alternatively, the sodium chlorate solution in line 14 may : be provided from purchased solid sodium chlorate.
Modifications are possible within the scope of the invention.
A
, I .
~, - 10 - , ' ~3 '' "''~ ' ,. "', . ' ~' ., ~' ' ' . , . ' .
NaClO3 + NaCl ~ H2S4 ~ C1O2 + 1/2cl2 + H2O + Na2SO4 -(1);
continuously maintaining the aqueous reaction medium at its boiling point at a temperature below that above which sub-stantial decomposition of chlorine dioxide occurs while maintaining the reaction vessel under a subatmospheric pressure to form the chlorine dioxide in gaseous admixture with water vapour and chlorine and to deposit sodium sulphate from the reaction medium; continuously removing the gaseous mixture of chlorine dioxide, chlorine and water vapour from the reaction vessel; continuously separating the removed gaseous mixture into an aqueous solution of chlorine dioxide containing no more than 10% of the available chlorine thereof as chlorine and the remainder of the available chlorine as chlorine dioxide, and gaseous chlorine; continuously sub-jecting washed and otherwise untreated wood pulp to bleaching utilizing the aqueous solution of chlorine dioxide to provide bleached pulp having a predetermined brightness; continuously removing the deposited sodium sulphate from the reaction vessel; continuously contacting the removed sodium sulphate with hydrogen chloride to form sodium chloride and sulphuric acid in accordance with the equation:
; Na2SO4 + 2HCl - 2NaCl ~ H2SO4 ~ (2) ~ .
.
" 1049711 continuousl~ ~oxwaxding the sulphuric acid and substantially one-half the molar amount of the sodium chloride to the reac-tion vessel along with unreacted sodium sulphate to provide the sulphuric acid and sodium chloride requirements of the chlorine dioxi:de generating reaction of equation l; the unreacted sodium sulphate depositing from the reaction medium and cycling as a dead load between the reaction medium and the contacting with hydrogen chloride; continuously electro-lyzing an aqueous solution of the remainder of the molar amount of sodium chloride to form an aqueous solution of sodium chlorate and hydrogen in accordance with the equation:
NaCl + 3H2O (3);
continuously forwarding the aqueous solution of sodium chlorate to the reaction vessel to provide the sodium chlorate : requirement of the chlorine dioxide generating reaction of - ~
equation l; continuously reacting substantially one-third :
the molar amount of hydrogen formed in the electrolysis reac-tion of equation 3 with chlorine in accordance with the :
equation:
; 20 H2 + C12 ~ 2HCl - (4) : the gaseous chlorine separated from the gaseous mixture of chlorine dioxide, chlorine and water vapour providing part -:
of the molar amount of chlorine required by equation 4; and continuously utilizing the hydrogen chloride formed by . equation 4 as the hydrogen chloride requirement of equation 2. :
The process of the invention is described further, : by way of illustration, with reference to the accompanying drawing, which is a schematic flow sheet of one embodiment of the present invention.
Referring first to the drawing, chlorine dioxide and chlorine are formed in a chlorine dioxide generation zone ~ .
, -:. . '; . , . ., : - . . , , ~- , .
.
10. The generation zone 10 contains a chlorine dioxide generator 12 wherein an aqueous reaction medium is maintained at its boiling point, preferably with the generator 12 being maintained under a reduced pressure. The aqueous B
, . . . . .. - -. . . . , . . - . , - :
. .
reaction medium is formed from sodium chlorate solution fed by line 14, sulphuric acid and sodium chloride fed by line 16.
Sodium sulphate is precipitated from the reaction medium and the water vapor formed by the evaporation forms a gaseous mixture with the chlorine dioxide and chlorine generated from the reaction medium. The gaseous mixture of chlorine dioxide, chlorine and steam is removed from the chlorine dioxide generation zone 10 by line 18 to a separator 20.
In the separator 20, the water vapor is condensed and the chlorine dioxide is separated substantially completely from the chlorine to provide an aqueous solution of chlorine dioxide in line 22. The separator may be of any convenient type, typically one in which a stream of water contacts the gaseous mixture. The separation of chlorine dioxide and J chlorine preferably is as complete as possible, and in any .. . .
event no more than about 10% of the total available chlorine of the chlorine dioxide solution in line 22 should be provided by chlorine, the remainder of the available chlorine being provided by the chlorine dioxide.
The chlorine dioxide solution in line 22 is passed to a bleach plant 24 wherein it is used to bleach wood pulp fed thereto by line 26 to provide bleached pulp of the required brightness in line 28.
In the bleaching of pulp, the pulp generally is :.
~, subjected to a series of alternating bleaching and purification , steps with the pulp usually being washed after each step.
The purification step generally involves caustic extraction ; using sodium hydroxide solution. The bleaching steps have been carried out in a number of ways, and usually the first `~ 30 ~ 4 _ .. . .: .. , : . . .
bleaching step has involved the use of chlorine or mixtures of chlorine and chlorine dioxide. A commonly-employed bleach plant sequence is known as the CEDED sequence, where C represents bleaching with chlorine, E represents caustic extraction and D represents bleaching with chlorine dioxide.
In the present invention, the first bleaching step is carried out using the aqueous chlorine dioxide solution in line 22.
Preferably, sufficient chlorine dioxide solution is forwarded -~
by line 22 to provide all the bleaching agen`t required by the bleach plant 24.
The sodium sulphate which is precipitated in the chlorine dioxide generator 12 is removed therefrom by line 30, made up into a saturation aqueous solution thereof and passed to a reaction tank 32. The form of the sodium sulphate re-moved by line 30 depends on the reaction conditions of the generator 12. At high acidities, typically around 10 to 12N
the sodium sulphate is in the form of sodium bisulphate whereas at low acidities, typically about 2 to 4.8N, the sodium sulphate is in the form of neutral sodium sulphate (Na2S04).
In the reaction tank 32, the sodium sulphate is reacted with hydrogen chloride fed by line 34, thereby forming sodium chloride, which precipitates from the solution, and sulphuric acid, in accordance with the equatlon:
2 4 ~ H2S04 + 2NaCl :
The hydrogen chloride fed by line 34 to the reaction tanks 32 may be in gaseous form or as hydrochloric acid. In the latter case, the total quantity of water present in the reaction tank should be insufficient to dissolve the sodium .
10497~1 chloride and hence precipitation of the sodium chloride occurs. Similarly, the aqueous solution of sodium sulphate fed to the reaction tank 32 may be less than saturated provided that the quantity of solution is not sufficient to dissolve all the sodium chloride formed.
Since it is not possible to convert all the sodium sulphate to sodium chloride in the reaction tank 34 due to some solubility of the sodium chloride in the aqueous phase, in the interests of maintaining stoichiometry, an excess ~ -of sodium sulphate preferably recycles in the loop between the generator 12 and the reaction tank 32. Therefore, in the liquid effluent from the reaction tank 32, which provides the '3,~ feed line 16 to the chlorine dioxide generator 12, there aredissolved quantities of sodium sulphate in addition to the sulphuric acid.
Sodium chloride precipitated in the reaction tank 32 and formed in accordance with the above equation is utilized in two different ways. Thus, one half the amount is used as the reducing agent for the sodium chlorate in the generator 12 and is made up into an aqueous solution for feed to the generator, typically in admixture with the sulphuric acid feed in line 16~ -~i The remainder of the sodium chloride is formed into . ~ . .
~ an aqueous solution and is passed by line 36 to a chlorate ;9 cell 38 wherein it is subjected to electrolysis to provide the sodium chlorate feed in line 14 to the chlorine dioxide generator 12.
Alternatively, all the precipitated sodium chloride from the reaction tank 32 may be formed into an aqueous solution - . . : . . :
104~711 and passed by line 36 to the chlorate cell 38 wherein half the sodium chloride is electrolysed to sodi~m chlorate ~nd hence the aqueous solution in line 14 contains both the sodium chlorate and sodium chloride requirements of the generator 12 and the feed line 16 contains only the sulphuric acid requirement.
Hydrogen is formed as a by-product gas in chlorate cell 38 and passes therefrom by line 40, approximately one-third of the mole amount passing by line 42 to a hydrogen chloride reactor 44 and the remaining approximately two-thirds of the mole amount being discharged by line 46. The dis-charged hydrogen in line 46 may be used to form water, if desired.
; The chlorine separated from the chlorine dioxide , in the separator 20 passes by line 48 to the hydrogen chloride reactor 44 together with additional chlorine fed by line 50.
In a continuous stoichiometric operation, the chlorine in line 48 provides up to half the chlorine requirement of the reactor 44, depending on the efficiency of the separation of the chlorine dioxide and chlorine in the separator 20. The ' remainder of the chlorine requirement is provided by the chlorine in line 50.
The stoichiometry of the operation described above ~, with reference to Figure 1 is demonstrated by the following :~ . equations:
chlorine dioxide .
i : generator 12 - NaC103 + NaCl + H2S04 ~ C102 + ~C12 + H2ot~so~ .
Chlorate cell : 38 - NaCl + 3H20 - ~ NaC103 + 3H2 ':
~ .
.. , ,, -. . . . .
Reaction Tank 32 - Na2SO~ + 2HCl - ~ 2NaCl + H2SO4 Line 22 - C102 Line 48 - ~C12 Line 50 - ~C12 HCl reactor 44 - H2 + C12 ~ 2~Cl : In the above-described embodiment of the invention, chlorine dioxide is formed in admixture with chlorine in the ~; chlorine dioxide generation zone 10, the chlorine dioxide and chlorine are separated in the separator 20 to provide an aqueous solution of chlorine dioxide in line 22 wherein no more than 10% of the available chlorine content of the aqueous solution is provided by chlorine, the a~ueous solution of chlorine dioxide is used in the bleach plant 24 to bleach . -.
the pulp fed by line 26, the chlorine separated from the chlorine dioxide is used to form hydrogen chloride in the reactor 44 by reaction with hydrogen fed by line 42 ànd the hydrogen chloride formed from the chlorine in line 48, together with the hydrogen chloride formed from the chlorine in line 50, :- is used in the chlorine dioxide-producing reaction, in the -embodiment illustrated by feeding to the chlorine dioxide ~ .
1, generation zone 10 to convert product sodium sulph~te from l the chlorine dioxide.generator to sodium chloride for recycle - .
to the reaction zone.
¦:: Other forms of chlorine dioxide generation zone 10 may be used. For example, the chlorine dioxide generator 12 ~:
may be one in which sodium chlorate solution fed by line 14 is reduced by hydrogen chloride and the acid reaction medium is provided by hydrochloric acid. This may be achieved by ~:
eliminating the reaction tank 32, in which event the hydrogen ~ - 8 - `
i ' , ~' ~ ' . ' ~ ' ' .
, chloride in line 34, preferab]y in the form of hydrochloric acid, is fed directly to the chlorine dioxide generator 12.
Thus, the hydrogen chloride in this embodiment formed from the chlorine separated from the chlorine dioxide is used directly in the chlorine dioxide-producing reaction, rather than indirectly as in the embodiment described above with reference to Figure 1.
In the latter embodiment, the chlorine dioxide generator 12 preferably is operated under the same conditions as discussed with reference to Figure 1, namely, at the boiling point of the reaction mixture under a reduced pressure These conditions result in precipitation of sodium chloride which, after separation from the generator 12 and formation into an aqueous solution, forms the sodium chloride solution feed to the chlorate cell 38 in line 36.
Another embodiment of the invention utilizes the reduction of sodium chlorate with chloride ions in an aqueous medium using sulphuric acid and resulting in precipitation of sodium sulphate. In this embodiment, the hydrogen chloride in line 34 is used to provide up to half the acid requirement, ' the remainder being provided by the sulphuric acid, and up to all the reducing agent. Where the hydrogen chloride provides only part of the reducing agent requirement additional ! :
quantities of chloride may be provided by sodium chloride.
Sodium sulphate again is recovered and, in this embodiment, may be used for make up chemicals in the pulp mill recovery and regeneration operations. In this embodiment, additional chlorine in line 50 is unnecessary, but a source of sodium chloride is required to form the sodium chlorate in line 14.
_ 9 _ .
.
Alternatively, the sodium chlorate solution in line 14 may : be provided from purchased solid sodium chlorate.
Modifications are possible within the scope of the invention.
A
, I .
~, - 10 - , ' ~3 '' "''~ ' ,. "', . ' ~' ., ~' ' ' . , . ' .
Claims (4)
1. A continuous pulp mill process to provide bleached pulp which comprises continuously generating chlorine dioxide in a reaction vessel by reduction of sodium chlorate using sodium chloride in an aqueous acid reaction medium containing sulphuric acid and having an acidity from about 2 to about 4.8N in accordance with the equation:
NaClO3 + NaCl + H2SO4 C1O2 + 1/2Cl2 + H2O + Na2SO4 - (1) continuously maintaining said aqueous reaction medium at its boiling point at a temperature below that above which substantial decomposition of chlorine dioxide occurs while maintaining said reaction vessel under a subatmospheric pres-sure to form said chlorine dioxide in gaseous admixture with water vapour and chlorine and to deposit sodium sulphate from said reaction medium, continuously removing said gaseous mixture of chlorine dioxide, chlorine and water vapour from said reaction vessel, continuously separating said removed gaseous mixture into an aqueous solution of chlorine dioxide containing no more than 10% of the available chlorine thereof as chlorine and the remainder of the available chlorine as chlorine dioxide, and gaseous chlorine, continuously subjecting washed and otherwise un-treated wood pulp to bleaching utilizing said aqueous solution of chlorine dioxide to provide bleached pulp having a pre-determined brightness.
continuously removing said deposited sodium sulphate from said reaction vessel, continuously contacting said removed sodium sulphate with hydrogen chloride to form sodium chloride and sulphuric acid in accordance with the equation:
Na2SO4 + 2HCl2NaCl + H2SO4 -(2) continuously forwarding said sulphuric acid and substantially one-half the molar amount of said sodium chloride to said reaction vessel along with unreacted sodium sulphate to provide the sulphuric acid and sodium chloride requirements of said chlorine dioxide generating reaction of equation 1, said unreacted sodium sulphate depositing from the reaction medium and cycling as a dead load between said reaction medium and said contacting with hydrogen chloride, continuously electrolyzing an aqueous solution of the remainder of said molar amount of sodium chloride to form an aqueous solution of sodium chlorate and hydrogen in accordance with the equation:
NaCl + 3H2ONaClO3 + 3H2 -(3) continuously forwarding said aqueous solution of sodium chlorate to said reaction vessel to provide the sodium chlorate requirement of said chlorine dioxide generating reaction of equation 1, continuously reacting substantially one-third the molar amount of hydrogen formed in said electrolysis reaction of equation 3 with chlorine in accordance with the equation;
H2 + Cl22HCl -(4) said gaseous chlorine separated from said gaseous mixture of chlorine dioxide, chlorine and water vapour providing part of the molar amount of chlorine required by equation 4, and continuously utilizing the hydrogen chloride formed by equation 4 as the hydrogen chloride requirement of equation 2.
NaClO3 + NaCl + H2SO4 C1O2 + 1/2Cl2 + H2O + Na2SO4 - (1) continuously maintaining said aqueous reaction medium at its boiling point at a temperature below that above which substantial decomposition of chlorine dioxide occurs while maintaining said reaction vessel under a subatmospheric pres-sure to form said chlorine dioxide in gaseous admixture with water vapour and chlorine and to deposit sodium sulphate from said reaction medium, continuously removing said gaseous mixture of chlorine dioxide, chlorine and water vapour from said reaction vessel, continuously separating said removed gaseous mixture into an aqueous solution of chlorine dioxide containing no more than 10% of the available chlorine thereof as chlorine and the remainder of the available chlorine as chlorine dioxide, and gaseous chlorine, continuously subjecting washed and otherwise un-treated wood pulp to bleaching utilizing said aqueous solution of chlorine dioxide to provide bleached pulp having a pre-determined brightness.
continuously removing said deposited sodium sulphate from said reaction vessel, continuously contacting said removed sodium sulphate with hydrogen chloride to form sodium chloride and sulphuric acid in accordance with the equation:
Na2SO4 + 2HCl2NaCl + H2SO4 -(2) continuously forwarding said sulphuric acid and substantially one-half the molar amount of said sodium chloride to said reaction vessel along with unreacted sodium sulphate to provide the sulphuric acid and sodium chloride requirements of said chlorine dioxide generating reaction of equation 1, said unreacted sodium sulphate depositing from the reaction medium and cycling as a dead load between said reaction medium and said contacting with hydrogen chloride, continuously electrolyzing an aqueous solution of the remainder of said molar amount of sodium chloride to form an aqueous solution of sodium chlorate and hydrogen in accordance with the equation:
NaCl + 3H2ONaClO3 + 3H2 -(3) continuously forwarding said aqueous solution of sodium chlorate to said reaction vessel to provide the sodium chlorate requirement of said chlorine dioxide generating reaction of equation 1, continuously reacting substantially one-third the molar amount of hydrogen formed in said electrolysis reaction of equation 3 with chlorine in accordance with the equation;
H2 + Cl22HCl -(4) said gaseous chlorine separated from said gaseous mixture of chlorine dioxide, chlorine and water vapour providing part of the molar amount of chlorine required by equation 4, and continuously utilizing the hydrogen chloride formed by equation 4 as the hydrogen chloride requirement of equation 2.
2. The process of claim 1 wherein said washed and otherwise untreated wood pulp is subjected to a plurality of bleaching and purification operations to provide said bleached and purified pulp of a predetermined brightness, said plurality of operations including alternate bleaching by said aqueous chlorine dioxide solution and purification by aqueous sodium hydroxide solution, the pulp in addition being washed after each said bleaching and purification step.
3. The process of claim 1 including forming said removed sodium sulphate into a saturated aqueous solution thereof and contacting said saturated aqueous solution with hydrogen chloride to precipitate sodium chloride from said solution, separating said precipitated sodium chloride from the resulting sulphuric acid and unreacted sodium sulphate and forwarding said resulting sulphuric acid and sodium sulphate to said reaction vessel as said forwarded sulphuric acid and unreacted sodium sulphate.
4. The process of claim 3 including forming said separated sodium chloride into an aqueous solution, subjecting the latter aqueous solution to electrolysis to provide said aqueous sodium chlorate solution forwarded to said reaction vessel and containing said substantially one-half molar amount of sodium chloride forwarded to said reaction vessel.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA305,442A CA1053860A (en) | 1973-08-30 | 1978-06-14 | Chlorine dioxide generation and pulp bleaching |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB4086373 | 1973-08-30 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1049711A true CA1049711A (en) | 1979-03-06 |
Family
ID=10416999
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA208,075A Expired CA1049711A (en) | 1973-08-30 | 1974-08-29 | Chlorine dioxide generation |
Country Status (3)
| Country | Link |
|---|---|
| JP (1) | JPS5072893A (en) |
| CA (1) | CA1049711A (en) |
| FR (1) | FR2242514A1 (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS61289193A (en) * | 1985-06-14 | 1986-12-19 | 神崎製紙株式会社 | Bleaching of pulp |
-
1974
- 1974-08-29 CA CA208,075A patent/CA1049711A/en not_active Expired
- 1974-08-30 JP JP49099812A patent/JPS5072893A/ja active Pending
- 1974-08-30 FR FR7429740A patent/FR2242514A1/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| JPS5072893A (en) | 1975-06-16 |
| FR2242514A1 (en) | 1975-03-28 |
| FR2242514B1 (en) | 1982-02-19 |
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