CA1053860A - Chlorine dioxide generation and pulp bleaching - Google Patents
Chlorine dioxide generation and pulp bleachingInfo
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- CA1053860A CA1053860A CA305,442A CA305442A CA1053860A CA 1053860 A CA1053860 A CA 1053860A CA 305442 A CA305442 A CA 305442A CA 1053860 A CA1053860 A CA 1053860A
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- 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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Abstract
ABSTRACT OF THE DISCLOSURE
A pulp mill procedure is described in which chlorine dioxide is formed in admixture with chlorine in a chlorine dioxide generation zone, the chlorine dioxide and chlorine are separated to provide an aqueous solution of chlorine dioxide wherein no more than 10%
of the total available chlorine content is provided by chlorine, the aqueous solution of chlorine dioxide is used in a bleach plant to bleach pulp, the chlorine separated from the chlorine dioxide is used to form hydrogen chloride by reaction with hydrogen and hydrogen chloride is used in the chlorine dioxide generation.
A pulp mill procedure is described in which chlorine dioxide is formed in admixture with chlorine in a chlorine dioxide generation zone, the chlorine dioxide and chlorine are separated to provide an aqueous solution of chlorine dioxide wherein no more than 10%
of the total available chlorine content is provided by chlorine, the aqueous solution of chlorine dioxide is used in a bleach plant to bleach pulp, the chlorine separated from the chlorine dioxide is used to form hydrogen chloride by reaction with hydrogen and hydrogen chloride is used in the chlorine dioxide generation.
Description
iO5~86V
The p~esent invention relates to the generation of chlorine dioxide, more particularly, to the integration of chlorine dioxide generation with bleach plant operations.
This application is a division of application Serial No. 208,075 filed August 29, 197~.
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 > ClO2 + 1/2 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 disolosed in Canadian Patent No. 920,773 wherein sodium chlorate is reducea by chloride ions in the presence of sulphuric acid, sodium sulphate is deposited from the reaction mixture, and an aqueous solution of the sodium sulphate - is treated with hydrogen chloride to form sodium chloride ana sulphuric acid, the latter being recycled to the reaction mixture.
In the production of bleached cellulosic fibrous material pulp it is known to bleach cellulosic fibrous material pulp, formed in any convenient manner, typically by digestion of wood ships 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 p~esent invention relates to the generation of chlorine dioxide, more particularly, to the integration of chlorine dioxide generation with bleach plant operations.
This application is a division of application Serial No. 208,075 filed August 29, 197~.
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 > ClO2 + 1/2 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 disolosed in Canadian Patent No. 920,773 wherein sodium chlorate is reducea by chloride ions in the presence of sulphuric acid, sodium sulphate is deposited from the reaction mixture, and an aqueous solution of the sodium sulphate - is treated with hydrogen chloride to form sodium chloride ana sulphuric acid, the latter being recycled to the reaction mixture.
In the production of bleached cellulosic fibrous material pulp it is known to bleach cellulosic fibrous material pulp, formed in any convenient manner, typically by digestion of wood ships 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.
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105386~ :
The present invention is directed to a novel integration of a chlorine dioxide-producing process and a bleach plant operation to achieve efficient and economic utilization of chemicals.
In accordance with this invention, there is provided a continuous pulp mill process to provide bleached pulp which comprises: continuously digesting wood chips with a pulping liquor contalning sodium sulphide and sodium hydroxide as active pulping chemicals to form a wood pulp, continuously separating the wood pulp from spent ~ .
pulping liquor, continuously washing the separated pulp free from spent pulping liquor to provide a washed but otherwise untreated wood pulp, continuously subjecting the spent pulping liquor to recovery and regeneration opera-tions to form fresh pulping liquor, continuously rec~cling the fresh pulping liquor as at least part of the pulping liquor, continuously incorporating sodium sulphate into .. ..
the spent pulping liquor to make up chemical losses in -the recovery and regeneration steps, the recovery and ~ .
regeneration steps including furnacing spent pulping .
liquor to provide a solid mass and a flue gas stream containing entrained sublimed solids, continuously intro-ducing sodium chloride into the spent pulping liquor whereby the entrained solids consist mainly of sodium sulphate and sodium chloride, continuously removing the entrained solids ~rom the flue gas stream, continuously leaching -the removed solids with water to dissolve substantially all the sodium chloride values thereof along with part of the sodium sulphate values thereof and leave a solid mass consisting essentially of sodium sulphate, continuously recycling the solid mass to the spent pulping liquor,
105386~ :
The present invention is directed to a novel integration of a chlorine dioxide-producing process and a bleach plant operation to achieve efficient and economic utilization of chemicals.
In accordance with this invention, there is provided a continuous pulp mill process to provide bleached pulp which comprises: continuously digesting wood chips with a pulping liquor contalning sodium sulphide and sodium hydroxide as active pulping chemicals to form a wood pulp, continuously separating the wood pulp from spent ~ .
pulping liquor, continuously washing the separated pulp free from spent pulping liquor to provide a washed but otherwise untreated wood pulp, continuously subjecting the spent pulping liquor to recovery and regeneration opera-tions to form fresh pulping liquor, continuously rec~cling the fresh pulping liquor as at least part of the pulping liquor, continuously incorporating sodium sulphate into .. ..
the spent pulping liquor to make up chemical losses in -the recovery and regeneration steps, the recovery and ~ .
regeneration steps including furnacing spent pulping .
liquor to provide a solid mass and a flue gas stream containing entrained sublimed solids, continuously intro-ducing sodium chloride into the spent pulping liquor whereby the entrained solids consist mainly of sodium sulphate and sodium chloride, continuously removing the entrained solids ~rom the flue gas stream, continuously leaching -the removed solids with water to dissolve substantially all the sodium chloride values thereof along with part of the sodium sulphate values thereof and leave a solid mass consisting essentially of sodium sulphate, continuously recycling the solid mass to the spent pulping liquor,
- 3 -1~531~60 continuously generating chlorine dioxide in a reaction - vessel by r~duction of sodium chlorate using sodium chloride in an aqueous acid reaction medium containing sulphuric acid and having an acidity of about 2 to about 4.8N in accordance with the equation:
NaC103 + NaCl + H2SO~ ~ C12 + 1/2 C12-~ H20 + Na2so4 continuously maintaining the aqueous reaction medium at its boiling point at a temperature below that above which substantial decomposition of chlorine dioxide occurs while maintaining the reaction vessel under a reduced pressure to form the chlorine dioxide in gaseous admixture with water vapour and chlorine and to deposit sodium sulphate produced by the chlorine dioxide generating reaction o~
equation 1 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 subjecting the washed but 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 forwarding part of the removed sodium sulphate to the spent pulping liquor as at least part of the make up sodium sulphate, continuously forming a substantially saturated aqueous solution of the remainder of the sodium sulphate, mixing the latter saturated aqueous solution of sodium sulphate with the aqueous solution resulting from : . , ~05386~
the le~ching, continuously contacting the latter mixture - -with hydrogen chloride to deposit therefrom sodium chloride and to form sulphuric acid in accordance with the equation: .
Na2 4 ~ 2NaCl + H2SO4 -(2) continuously separating the sodium chloride from the sulphuric acid, continuously forwarding the sulphuric acid containing dissolved sodium chloride to the reaction vessel ~o provide the sulphuric acid requirement of the chlorine dioxide generating reaction of equation 1, contin-uously recovering from the separated sodium chloride an amount of solid substantially pure sodium chloride substantially equal to the amount present in the solution resulting from the leaching, continuously forming an aqueous solution of the remainder of the separated sodium chloride, continuously electrolyzing the latter aqueous solution to form an aqueous solution of sodium chlorate ~ `
containing the stoichiometric requirement of sodium chlorate of the chlorine dioxide producing reaction of equation 1, and hydrogen in accordance with the equation:
NaCl + 3H2O ~ NaClO3 + 3H2 ~(3) continuously forwarding the aqueous solution of sodium ~ -chlorate containing the stoichiometric requirement of ::
sodium chloricle of the chlorine dioxide producing reaction of equation 1 not provided by the sodium chloride contained in the forwarded sulphuric acid, continuously reacting substantially one-third of the molar amount of hydrogen ~:
~ .
formed in the electrolysis reaction of equation 3 with chlorine .in accordance with the equatlon:
H2 + C12 ~ 2HCl -~4) the gaseous chlorine separated from the gaseous mixture of . ' . .
lOS3861D
chlorine dioxide, chlorine and water vapour providin ;
part of the molar amount of chlorine required by equatio~
NaC103 + NaCl + H2SO~ ~ C12 + 1/2 C12-~ H20 + Na2so4 continuously maintaining the aqueous reaction medium at its boiling point at a temperature below that above which substantial decomposition of chlorine dioxide occurs while maintaining the reaction vessel under a reduced pressure to form the chlorine dioxide in gaseous admixture with water vapour and chlorine and to deposit sodium sulphate produced by the chlorine dioxide generating reaction o~
equation 1 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 subjecting the washed but 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 forwarding part of the removed sodium sulphate to the spent pulping liquor as at least part of the make up sodium sulphate, continuously forming a substantially saturated aqueous solution of the remainder of the sodium sulphate, mixing the latter saturated aqueous solution of sodium sulphate with the aqueous solution resulting from : . , ~05386~
the le~ching, continuously contacting the latter mixture - -with hydrogen chloride to deposit therefrom sodium chloride and to form sulphuric acid in accordance with the equation: .
Na2 4 ~ 2NaCl + H2SO4 -(2) continuously separating the sodium chloride from the sulphuric acid, continuously forwarding the sulphuric acid containing dissolved sodium chloride to the reaction vessel ~o provide the sulphuric acid requirement of the chlorine dioxide generating reaction of equation 1, contin-uously recovering from the separated sodium chloride an amount of solid substantially pure sodium chloride substantially equal to the amount present in the solution resulting from the leaching, continuously forming an aqueous solution of the remainder of the separated sodium chloride, continuously electrolyzing the latter aqueous solution to form an aqueous solution of sodium chlorate ~ `
containing the stoichiometric requirement of sodium chlorate of the chlorine dioxide producing reaction of equation 1, and hydrogen in accordance with the equation:
NaCl + 3H2O ~ NaClO3 + 3H2 ~(3) continuously forwarding the aqueous solution of sodium ~ -chlorate containing the stoichiometric requirement of ::
sodium chloricle of the chlorine dioxide producing reaction of equation 1 not provided by the sodium chloride contained in the forwarded sulphuric acid, continuously reacting substantially one-third of the molar amount of hydrogen ~:
~ .
formed in the electrolysis reaction of equation 3 with chlorine .in accordance with the equatlon:
H2 + C12 ~ 2HCl -~4) the gaseous chlorine separated from the gaseous mixture of . ' . .
lOS3861D
chlorine dioxide, chlorine and water vapour providin ;
part of the molar amount of chlorine required by equatio~
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 accompan~ing drawing, which is a schematic flow sheet of one embodiment of the invention.
Referring to the drawing, wood chips, or other cellulosic fibrous material, are fed by line 110 to a digester 112 wherein they are cooked to form a pulp by the action of a pulping liquor fed by line 114 containing sodium hydroxide and sodium sulphide as the active pulpin~ chemicals.
The pulp is washed and separated from the spent pulping liquor (black liquor) in a brown stock washer 116 and is passed by line 118 to a bleach plant 120. In the bleach plant 120, the pulp is bleached to the required brightness and removed therefrom by line 122.
The bleaching oper~tions occurring in the bleach plant 120 are carried out by an aqueous solution of chlorine `
dioxide fed by line 124. The precise sequence of steps utilized in the bleach plant may vary widely, although, it is preferred to employ~a series of alternate bleaching and caustic extraction operations in which bleaching is carried out with the chlorine dioxide solution fed by line 124 and the caustic extraction is carried out using aqueous sodium hydroxide solution. Washing commonly is ;
carried out after each bleaching and caustic extraction step.
The black liquor leaving the brown stock washer ~0~i3860 by line 126 is subjected to recovery and regeneration operations to form white liquor in line 128 which is recycled to provide at least part of the pulping liquor in line 114. The black liquor first is evaporated in an evaporator 130 and the concentrated black liquor is passed by lines 132 and 134 to a furnace 136 wherein the black liquor is burned.to provide a solid smelt which contains sodium sulphide and sodium carbonate. The smelt is dissolved in water to form green liquor which is passed by line 138 to a causticizer 140 wherein the sodium carbonate values of the green liquor are converted to sodium hydroxide, generally using lime, and the resulting white liquor is recycled by line 128.
Sodium sulphate, or other source of sodium-.and ~.
sulphur- values, is introduced to the black liquor ~o make up losses of sodium and sulphur values during the recovery and regeneration operations. Typically, sodium sulphate is added to the concentrated black liquor in line 132 by line 142. .
The furnace 136 produces a flue gas stream which . .-contains entrained quantities of sublimed solids and the flue gas in line 144 generally i9 passed through a ..
precipitator 146 wherein the entrained solids are removad from the flue gas prior to the discharge of the latter to `: .
atmosphere by iine 148, after further treatment to remove .
any noxious or odorous gases, if desired.
The black liquor in line 126 may contain dissolved quantities of sodium chloride arising from various sources. For example, in coastal regions where logs are floated in sea water, sodium chloride is introduced to the mill with the wood chips in line 110.
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-1~3860 The presence of sodium chloride in the black liquor may arise from other sources, for example, from the bleach plant 120, if effluents therefrom, such as spent wash water is introduced to the blac]c liquor.
The entrained solids :in the flue gas in line 144 and hence the fume deposit obta:ined in the precipitator 146 contains mainly sodium sulphate and sodium chloride.
Small amounts of sodium carbonate also may be present.
The fume deposit is passed by line 150 to a leacher 152 wherein the solid is leached with water fed by line 154, preferably in an amount sufficient to dissolve all the sodium chloride values of the fume deposit. The resulting solid mass consisting mainly of sodium sulphate is recycled to the black liquor by line 156.
The preferably sàturated solution of sodium chloride and sodium sulphate resulting from the leacher 152 is passed by line 158 to a chlorine dioxide generation ~one 160. In the chlorine dioxide generation zone 160, there is provided a chlorine dioxide generator 162 contain-ing an aqueo~s reaction medium which contains sodium chlorate, sodium chloride and sul~huric acidy The reaction medium generally is malntained at its boiling -point while the generator 162 is maintained under a reduce~ pressure in any convenient manner. The gaseous mixture of chlorine dioxide, chlorine and steam formed in the generator 162 is passed by line 164 to a separator 166 of any convenient construction, wherein the chlorine -dioxide is formed into an aqueous solution which passes by line 124 to the bleach plant 120. The separation - 30 achieved in the separator 166 should be such that no more than about 10% of the available chlorine in the chlorine lOS3860 dioxide solution in line 124 is provided by chlorine, with the remainder of the available chlorine being provided by chlorine dioxide.
The sodium sulphate which is precipitated in the generator 162 is removed therefrom by line 168, partly is passed by lines 169 and 142 to the concentrated black liquor as the make up chemical and partly by line 170 to a reaction tank 172. The sodium sulphate may be forwarded by line 169 in solid or aqueous solution form and is passed to the reaction tank 172 by-line 170 prQferably as a substantially saturated aqueous solution thereof.
In the reaction tank 172, the aqueous medium provided by the feed solutions in line 158 and 170 is contacted with hydrogen chloride fed by line 174. This causes reàction of the sodium sulphate to sodium chloride, which precipitates, and sulphuric acid, in accordance with the equation:
Na2S4 + 2HC1 ~ 2Nacl + H2SO4 - ~ ~-The hydrogen chloride fed by line 174 to the reaction tank 172 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 all the sodium chloride. Similarly, the aqueous sol~tions in lines 158 and 170 may be less th~n saturated provided the total quantity of water is insufficient - to dissolve all the ~odium chloride.
The liquid effluènt from the reaction tank 172 contains sulphuric acid and sodium chloride as is passed by line 176 to the chlorine dioxide generator 162. The solid sodium chloride deposited in the reaction tank 172 is removed therefrom by line 178, partially is recovered _ g _ ;.
1053~6~
in substantially pure form by llne 180 and partially is made up into an aqueous solution which is passed by line 182 to a chlorate cell 184.
In the chlorate cell :L~4, the aqueous sodium chloride solution is electrolysed to form a sodium chlorate solution which passes by line 186 to the chlorine dioxide generator 162. Gaseous hydrogen also is formed in the electrolysis and is removed from the cell 184 by :~
line 188. Approximately two-thirds of the mole amount of hydrogen is vented to atmosphere by line 190 or burned to form water, if desired. The remaining approximately one-third mole amount is forwarded by line 192 to a hydrogen -:
chloride reactor 194.
The chlorine separated from the chlorine dioxide in the separator 166 is passed by line 196 to the hydrogen chloride reactor 194, wherein, together with chlorine fed by line 198, the chlorine is reacted with the hydrogen fed by line 192 to form hydrogen chloride which passes by line 174 to the reaction tank 172.
To preserve the stoichiometry of the system described with reference to Figure 2, and where the chlorine dioxide generation zone 160 constitutes the sole source of make up sodium sulphate in line 142, additional sulphuric acid may be required in the chlorine dioxide generator 162 in addition to that provided from the reaction tank 172 and such additional sulphuric acid may be provided by line 200.
- Where, however, an additional external source of sodium sulphate is available as make up chemical by 30 . line 202 the sodium sulphate in lines 170 and 15~ may be sufficient to provide the total sulphuric acid r~quirement.
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1~53860 Since it may not be possible to convert all the sodium sulphate to sodium chloride in the reaction tank 172, an excess over stoichiometry may be continuously cycled between the reaction tank 172 and the generator 162.
The amount of sodium chloride present in the solution forwarded by line 176 to the chlorine dioxide generator 162 generally is insufficient to provide all the stoichiometric requirement of sodium chloride for the pro-duction of chlorine dioxide. The amount of sodium chloride ~-fed by line 176 depends on the quantity present in the flue gas 144. To provide the required amount of sodi;um chloride in the generator 162, part of the sodium chloride in line 176 may be fed to the gènerator 162, usually as ,-an aqueous solution thereof. Alternatively, the required amount is present in the aqueous solution fed to the cell 1~4 by line 182 and hence is present in the sodium chlorate solution fed by line 186. The latter mode of operation is the preferred.
The quantity of pure sodium chloride recove~ed by line 180 corresponds to the quantity of sodium chloride - present in the solution in line 158 in a stoichiometric continuous operation. Hence, this embodiment of the invention provides separation of the sodium chloride present in the fume deposit in a pure form without discard of the~ ;
other useful chemicals contained in the fume deposit.
The sodium chloride recovered in line 180 may ~e sold as such or used in a variety of ways. For example, the sodium chloride may be used to provide the sodium hydroxide requirement of the bleach plant 120 and the chlorine requirement of line 198.
Therefore, the process of the invention provides -- 11 ~ ':
lOS3~i~0 a process in which chlorine dioxide is formed in admixture with chlorine in the chlorine di.oxide genera,ion zone 160, the chlorine dioxide and chlorine are separated in a separator 166 to provide an aqueous solution of chlorine dioxide in line 124 wherein no more than 10~ of the available chlorine content of the aqueous solution is provided by chlorine, the aqueous solution of chlorine dioxide is used in the bleach plant 120 to bleach pulp in all bleaching stages, the chlorine separated from the chlorine dioxide is used to form hydrogen chloride in the reactor 194 by reaction with hydrogen fed by line 192, and the hydrogen chloride formed thereby is used in the chlorine dioxide generation zone 160.
- Modifications are possible within the scope of the invention.
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The process of the invention is described further, by way of illustration, with reference to the accompan~ing drawing, which is a schematic flow sheet of one embodiment of the invention.
Referring to the drawing, wood chips, or other cellulosic fibrous material, are fed by line 110 to a digester 112 wherein they are cooked to form a pulp by the action of a pulping liquor fed by line 114 containing sodium hydroxide and sodium sulphide as the active pulpin~ chemicals.
The pulp is washed and separated from the spent pulping liquor (black liquor) in a brown stock washer 116 and is passed by line 118 to a bleach plant 120. In the bleach plant 120, the pulp is bleached to the required brightness and removed therefrom by line 122.
The bleaching oper~tions occurring in the bleach plant 120 are carried out by an aqueous solution of chlorine `
dioxide fed by line 124. The precise sequence of steps utilized in the bleach plant may vary widely, although, it is preferred to employ~a series of alternate bleaching and caustic extraction operations in which bleaching is carried out with the chlorine dioxide solution fed by line 124 and the caustic extraction is carried out using aqueous sodium hydroxide solution. Washing commonly is ;
carried out after each bleaching and caustic extraction step.
The black liquor leaving the brown stock washer ~0~i3860 by line 126 is subjected to recovery and regeneration operations to form white liquor in line 128 which is recycled to provide at least part of the pulping liquor in line 114. The black liquor first is evaporated in an evaporator 130 and the concentrated black liquor is passed by lines 132 and 134 to a furnace 136 wherein the black liquor is burned.to provide a solid smelt which contains sodium sulphide and sodium carbonate. The smelt is dissolved in water to form green liquor which is passed by line 138 to a causticizer 140 wherein the sodium carbonate values of the green liquor are converted to sodium hydroxide, generally using lime, and the resulting white liquor is recycled by line 128.
Sodium sulphate, or other source of sodium-.and ~.
sulphur- values, is introduced to the black liquor ~o make up losses of sodium and sulphur values during the recovery and regeneration operations. Typically, sodium sulphate is added to the concentrated black liquor in line 132 by line 142. .
The furnace 136 produces a flue gas stream which . .-contains entrained quantities of sublimed solids and the flue gas in line 144 generally i9 passed through a ..
precipitator 146 wherein the entrained solids are removad from the flue gas prior to the discharge of the latter to `: .
atmosphere by iine 148, after further treatment to remove .
any noxious or odorous gases, if desired.
The black liquor in line 126 may contain dissolved quantities of sodium chloride arising from various sources. For example, in coastal regions where logs are floated in sea water, sodium chloride is introduced to the mill with the wood chips in line 110.
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-1~3860 The presence of sodium chloride in the black liquor may arise from other sources, for example, from the bleach plant 120, if effluents therefrom, such as spent wash water is introduced to the blac]c liquor.
The entrained solids :in the flue gas in line 144 and hence the fume deposit obta:ined in the precipitator 146 contains mainly sodium sulphate and sodium chloride.
Small amounts of sodium carbonate also may be present.
The fume deposit is passed by line 150 to a leacher 152 wherein the solid is leached with water fed by line 154, preferably in an amount sufficient to dissolve all the sodium chloride values of the fume deposit. The resulting solid mass consisting mainly of sodium sulphate is recycled to the black liquor by line 156.
The preferably sàturated solution of sodium chloride and sodium sulphate resulting from the leacher 152 is passed by line 158 to a chlorine dioxide generation ~one 160. In the chlorine dioxide generation zone 160, there is provided a chlorine dioxide generator 162 contain-ing an aqueo~s reaction medium which contains sodium chlorate, sodium chloride and sul~huric acidy The reaction medium generally is malntained at its boiling -point while the generator 162 is maintained under a reduce~ pressure in any convenient manner. The gaseous mixture of chlorine dioxide, chlorine and steam formed in the generator 162 is passed by line 164 to a separator 166 of any convenient construction, wherein the chlorine -dioxide is formed into an aqueous solution which passes by line 124 to the bleach plant 120. The separation - 30 achieved in the separator 166 should be such that no more than about 10% of the available chlorine in the chlorine lOS3860 dioxide solution in line 124 is provided by chlorine, with the remainder of the available chlorine being provided by chlorine dioxide.
The sodium sulphate which is precipitated in the generator 162 is removed therefrom by line 168, partly is passed by lines 169 and 142 to the concentrated black liquor as the make up chemical and partly by line 170 to a reaction tank 172. The sodium sulphate may be forwarded by line 169 in solid or aqueous solution form and is passed to the reaction tank 172 by-line 170 prQferably as a substantially saturated aqueous solution thereof.
In the reaction tank 172, the aqueous medium provided by the feed solutions in line 158 and 170 is contacted with hydrogen chloride fed by line 174. This causes reàction of the sodium sulphate to sodium chloride, which precipitates, and sulphuric acid, in accordance with the equation:
Na2S4 + 2HC1 ~ 2Nacl + H2SO4 - ~ ~-The hydrogen chloride fed by line 174 to the reaction tank 172 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 all the sodium chloride. Similarly, the aqueous sol~tions in lines 158 and 170 may be less th~n saturated provided the total quantity of water is insufficient - to dissolve all the ~odium chloride.
The liquid effluènt from the reaction tank 172 contains sulphuric acid and sodium chloride as is passed by line 176 to the chlorine dioxide generator 162. The solid sodium chloride deposited in the reaction tank 172 is removed therefrom by line 178, partially is recovered _ g _ ;.
1053~6~
in substantially pure form by llne 180 and partially is made up into an aqueous solution which is passed by line 182 to a chlorate cell 184.
In the chlorate cell :L~4, the aqueous sodium chloride solution is electrolysed to form a sodium chlorate solution which passes by line 186 to the chlorine dioxide generator 162. Gaseous hydrogen also is formed in the electrolysis and is removed from the cell 184 by :~
line 188. Approximately two-thirds of the mole amount of hydrogen is vented to atmosphere by line 190 or burned to form water, if desired. The remaining approximately one-third mole amount is forwarded by line 192 to a hydrogen -:
chloride reactor 194.
The chlorine separated from the chlorine dioxide in the separator 166 is passed by line 196 to the hydrogen chloride reactor 194, wherein, together with chlorine fed by line 198, the chlorine is reacted with the hydrogen fed by line 192 to form hydrogen chloride which passes by line 174 to the reaction tank 172.
To preserve the stoichiometry of the system described with reference to Figure 2, and where the chlorine dioxide generation zone 160 constitutes the sole source of make up sodium sulphate in line 142, additional sulphuric acid may be required in the chlorine dioxide generator 162 in addition to that provided from the reaction tank 172 and such additional sulphuric acid may be provided by line 200.
- Where, however, an additional external source of sodium sulphate is available as make up chemical by 30 . line 202 the sodium sulphate in lines 170 and 15~ may be sufficient to provide the total sulphuric acid r~quirement.
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1~53860 Since it may not be possible to convert all the sodium sulphate to sodium chloride in the reaction tank 172, an excess over stoichiometry may be continuously cycled between the reaction tank 172 and the generator 162.
The amount of sodium chloride present in the solution forwarded by line 176 to the chlorine dioxide generator 162 generally is insufficient to provide all the stoichiometric requirement of sodium chloride for the pro-duction of chlorine dioxide. The amount of sodium chloride ~-fed by line 176 depends on the quantity present in the flue gas 144. To provide the required amount of sodi;um chloride in the generator 162, part of the sodium chloride in line 176 may be fed to the gènerator 162, usually as ,-an aqueous solution thereof. Alternatively, the required amount is present in the aqueous solution fed to the cell 1~4 by line 182 and hence is present in the sodium chlorate solution fed by line 186. The latter mode of operation is the preferred.
The quantity of pure sodium chloride recove~ed by line 180 corresponds to the quantity of sodium chloride - present in the solution in line 158 in a stoichiometric continuous operation. Hence, this embodiment of the invention provides separation of the sodium chloride present in the fume deposit in a pure form without discard of the~ ;
other useful chemicals contained in the fume deposit.
The sodium chloride recovered in line 180 may ~e sold as such or used in a variety of ways. For example, the sodium chloride may be used to provide the sodium hydroxide requirement of the bleach plant 120 and the chlorine requirement of line 198.
Therefore, the process of the invention provides -- 11 ~ ':
lOS3~i~0 a process in which chlorine dioxide is formed in admixture with chlorine in the chlorine di.oxide genera,ion zone 160, the chlorine dioxide and chlorine are separated in a separator 166 to provide an aqueous solution of chlorine dioxide in line 124 wherein no more than 10~ of the available chlorine content of the aqueous solution is provided by chlorine, the aqueous solution of chlorine dioxide is used in the bleach plant 120 to bleach pulp in all bleaching stages, the chlorine separated from the chlorine dioxide is used to form hydrogen chloride in the reactor 194 by reaction with hydrogen fed by line 192, and the hydrogen chloride formed thereby is used in the chlorine dioxide generation zone 160.
- Modifications are possible within the scope of the invention.
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Claims (2)
1. A continuous pulp mill process to provide bleached pulp which comprises:
continuously digesting wood ships with a pulping liquor containing sodium sulphide and sodium hydroxide as active pulping chemicals to form a wood pulp, continuously separating said wood pulp from spent pulping liquor, con-tinuously washing said separated pulp free from spent pulping liquor to provide a washed but otherwise untreated wood pulp, continuously subjecting said spent pulping liquor to recovery and regeneration operations to form fresh pulping liquor, continuously recycling said fresh pulping liquor as at least part of said pulping liquor, continuously incorporating sodium sulphate into said spent pulping liquor to make up chemical losses in said recovery and regeneration steps, said recovery and regen-eration steps including furnacing spent pulping liquor to provide a solid mass and a flue gas stream containing entrained sublimed solids, continuously introducing sodium chloride into said spent pulping liquor whereby said entrained solids consist mainly of sodium sulphate and sodium chloride, continuously removing said entrained solids from said flue gas stream, continuously leaching said removed solids with water to dissolve substantially all the sodium chloride values thereof along with part of the sodium sulphate values thereof and leave a solid mass consisting essen-tially of sodium sulphate, continuously recycling said solid mass to said spent pulping liquor, continuously generating chlorine dioxide in a reaction vessel by reduction of sodium chlorate using sodium chloride in an aqueous acid reaction medium contain-ing sulphuric acid and having an acidity of about 2 to about 4.8N in accordance with the equation:
NaC103 + NaCl + H2SO4 ?C102 + 1/2 C12 + 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 reduced pressure to form said chlorine dioxide in gaseous admixture with water vapour and chlorine and to deposit sodium sulphate produced by said chlorine dioxide genera-ting reaction of equation 1 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 said washed but otherwise untreated wood pulp to bleaching utilizing said aqueous solution of chlorine dioxide to provide bleached pulp having a predetermined brightness, continuously removing said deposited sodium sulphate from said reaction vessel, continuously forwarding part of said removed sodium sulphate to said spent pulping liquor as at least part of said make up sodium sulphate, continuously forming a substantially saturated aqueous solution of the remainder of said sodium sulphate, mixing the latter saturated aqueous solution of sodium sulphate with the aqueous solution resulting from said leaching, continuously contacting said latter mixture with hydrogen chloride to deposit therefrom sodium chloride and to form sulphuric acid in accordance with the equation:
Na2SO4 + 2HCl ? 2NaCl + H2SO4 -(2) continuously separating said sodium chloride from said sulphuric acid, continuously forwarding said sulphuric acid containing dissolved sodium chloride to said reaction vessel to provide the sulphuric acid requirement of said chlorine dioxide generating reaction of equation 1, continuously recovering from said separated sodium chloride an amount of solid substantially pure sodium chloride substantially equal to the amount present in the solution resulting from said leaching, continuously forming an agueous solution of the remainder of said separated sodium chloride, continuously electrolyzing said latter aqueous solution to form an aqueous solution of sodium chlorate containing the stoichiometric requirement of sodium chlorate of said chlorine dioxide producing reaction of equation 1, and hydrogen in accordance with the equation;
NaCl + 3H2O ? NaC103 + 3H2 -(3) continuously forwarding said aqueous solution of sodium chlorate containing the stoichiometric requirement of sodium chloride of said chlorine dioxide producing reaction of equation 1 not provided by the sodium chloride contained in said forwarded sulphuric acid, continuously reacting substantially one-third of the molar amount of hydrogen formed in said electrolysis reaction of equation 3 with chlorine in accordance with the equation:
H2 + C12 ? 2HCl -(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.
continuously digesting wood ships with a pulping liquor containing sodium sulphide and sodium hydroxide as active pulping chemicals to form a wood pulp, continuously separating said wood pulp from spent pulping liquor, con-tinuously washing said separated pulp free from spent pulping liquor to provide a washed but otherwise untreated wood pulp, continuously subjecting said spent pulping liquor to recovery and regeneration operations to form fresh pulping liquor, continuously recycling said fresh pulping liquor as at least part of said pulping liquor, continuously incorporating sodium sulphate into said spent pulping liquor to make up chemical losses in said recovery and regeneration steps, said recovery and regen-eration steps including furnacing spent pulping liquor to provide a solid mass and a flue gas stream containing entrained sublimed solids, continuously introducing sodium chloride into said spent pulping liquor whereby said entrained solids consist mainly of sodium sulphate and sodium chloride, continuously removing said entrained solids from said flue gas stream, continuously leaching said removed solids with water to dissolve substantially all the sodium chloride values thereof along with part of the sodium sulphate values thereof and leave a solid mass consisting essen-tially of sodium sulphate, continuously recycling said solid mass to said spent pulping liquor, continuously generating chlorine dioxide in a reaction vessel by reduction of sodium chlorate using sodium chloride in an aqueous acid reaction medium contain-ing sulphuric acid and having an acidity of about 2 to about 4.8N in accordance with the equation:
NaC103 + NaCl + H2SO4 ?C102 + 1/2 C12 + 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 reduced pressure to form said chlorine dioxide in gaseous admixture with water vapour and chlorine and to deposit sodium sulphate produced by said chlorine dioxide genera-ting reaction of equation 1 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 said washed but otherwise untreated wood pulp to bleaching utilizing said aqueous solution of chlorine dioxide to provide bleached pulp having a predetermined brightness, continuously removing said deposited sodium sulphate from said reaction vessel, continuously forwarding part of said removed sodium sulphate to said spent pulping liquor as at least part of said make up sodium sulphate, continuously forming a substantially saturated aqueous solution of the remainder of said sodium sulphate, mixing the latter saturated aqueous solution of sodium sulphate with the aqueous solution resulting from said leaching, continuously contacting said latter mixture with hydrogen chloride to deposit therefrom sodium chloride and to form sulphuric acid in accordance with the equation:
Na2SO4 + 2HCl ? 2NaCl + H2SO4 -(2) continuously separating said sodium chloride from said sulphuric acid, continuously forwarding said sulphuric acid containing dissolved sodium chloride to said reaction vessel to provide the sulphuric acid requirement of said chlorine dioxide generating reaction of equation 1, continuously recovering from said separated sodium chloride an amount of solid substantially pure sodium chloride substantially equal to the amount present in the solution resulting from said leaching, continuously forming an agueous solution of the remainder of said separated sodium chloride, continuously electrolyzing said latter aqueous solution to form an aqueous solution of sodium chlorate containing the stoichiometric requirement of sodium chlorate of said chlorine dioxide producing reaction of equation 1, and hydrogen in accordance with the equation;
NaCl + 3H2O ? NaC103 + 3H2 -(3) continuously forwarding said aqueous solution of sodium chlorate containing the stoichiometric requirement of sodium chloride of said chlorine dioxide producing reaction of equation 1 not provided by the sodium chloride contained in said forwarded sulphuric acid, continuously reacting substantially one-third of the molar amount of hydrogen formed in said electrolysis reaction of equation 3 with chlorine in accordance with the equation:
H2 + C12 ? 2HCl -(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 but otherwise untreated wood pulp is subjected to a series 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 chlorine dioxide solution and purification by aqueous sodium hydroxide solution, the pulp in addition being washed after each said bleaching and purification step.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB4086373 | 1973-08-30 | ||
| CA208,075A CA1049711A (en) | 1973-08-30 | 1974-08-29 | Chlorine dioxide generation |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1053860A true CA1053860A (en) | 1979-05-08 |
Family
ID=25667674
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA305,442A Expired CA1053860A (en) | 1973-08-30 | 1978-06-14 | Chlorine dioxide generation and pulp bleaching |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1053860A (en) |
-
1978
- 1978-06-14 CA CA305,442A patent/CA1053860A/en not_active Expired
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