CA1124015A - Diffuser oxygen bleaching - Google Patents
Diffuser oxygen bleachingInfo
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- CA1124015A CA1124015A CA325,828A CA325828A CA1124015A CA 1124015 A CA1124015 A CA 1124015A CA 325828 A CA325828 A CA 325828A CA 1124015 A CA1124015 A CA 1124015A
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Abstract
ABSTRACT OF THE DISCLOSURE
A method of treating cellulose material to produce oxygen bleached pulp without the need for all of the complicated equipment used in the production of oxygen bleached pulp by conventional methods. Cellulose material is continuously digested in a conventional di-gester to form pulp having a solids concentration of about 6 to 15 percent. The pulp is refined without significant dilution, and passed into the inlet of a conventional diffusion washer having one or more bleaching stages and a washing stage. The pulp is maintained at a pressure of about 50 to 150 psi in the diffusion washer, and the upwardly moving pulp being subjected to an oxygen agent in a bleaching stage by diffusion of the oxygen bleaching agent into the pulp. After passing through one or more such bleaching stages, the upwardly moving pulp is sub-jected to treatment with a washing liquid in the washing stage of the diffusion washer, and then the oxygen bleached washed pulp is expelled from the diffusion washer.
Liquid withdrawn from the different stages of the diffusion washer is either recycled to the diffusion washer, or returned to the digester to serve as a wash liquid in the wash zone of the digester. Oxygen gas is introduced in the recycled withdrawn liquid, the liquid being saturated with the oxygen gas, before return of the liquid to the diffusion washer, the oxygen saturated liquid comprising the oxygen bleaching agent.
A method of treating cellulose material to produce oxygen bleached pulp without the need for all of the complicated equipment used in the production of oxygen bleached pulp by conventional methods. Cellulose material is continuously digested in a conventional di-gester to form pulp having a solids concentration of about 6 to 15 percent. The pulp is refined without significant dilution, and passed into the inlet of a conventional diffusion washer having one or more bleaching stages and a washing stage. The pulp is maintained at a pressure of about 50 to 150 psi in the diffusion washer, and the upwardly moving pulp being subjected to an oxygen agent in a bleaching stage by diffusion of the oxygen bleaching agent into the pulp. After passing through one or more such bleaching stages, the upwardly moving pulp is sub-jected to treatment with a washing liquid in the washing stage of the diffusion washer, and then the oxygen bleached washed pulp is expelled from the diffusion washer.
Liquid withdrawn from the different stages of the diffusion washer is either recycled to the diffusion washer, or returned to the digester to serve as a wash liquid in the wash zone of the digester. Oxygen gas is introduced in the recycled withdrawn liquid, the liquid being saturated with the oxygen gas, before return of the liquid to the diffusion washer, the oxygen saturated liquid comprising the oxygen bleaching agent.
Description
~2~
BACKGROUND AND SIJ2~MARY OF THE INVENTION
The invention relates to a method of treating cellulose material to provide oxygen-bleached, washed pulp as the end product. Cellulose material is digested in a conventional continuous digester, and is fed to a diffusion washer in which oxygen bleaching is effected, the pulp being Xept under substantially digester pressure during bleaching in the diffusion washer.
At present, displacement bleaching of pulp in a diffusion washer is known, ~U.S.Patent No, 3,59g,449 and "Kamyr Diffusion Washers", Kamyr Bulletin No. 400DR, February 1975), however, the bleaching is accomplished -with C102 or the like at atmospheric pressure, and essen-tially all bleaching takes place in the diffusion washer itself so that the diffusion washer must thus be sized ~and the throughput con~rolled) accordingly to lnsure adequate bleaching. ~o chlorine is added to the pulp before re~ining thereof, nor is an initial stage of bleaching established in the digester associated with the diffusion washer. Since the diffuser pressure is less than the digester pressure (even considering normal transmission losses), some pressure reduction ~etween digester discharge and diffuser discharge must be effected.
~4~
Conventionally, oxygen bleaching of pulp is accomplished by utilizing an oxygen reactor, such as sho-~n in U.S. Patent No. 3,660,225, along with a large amount of accessory, expensive equipment~ such as shown in "Oxygen Bleaching" by John Kalisn, Pulp and Paper In~er-national~ March, 1975, pp. 53-59. The oxygen bleaching is done at relatively high consistency (i.e., 20-30~), and therefore it is necessary to concentrate the pulp between digester and bleaching. Also, separate washing and bleaching vessels are utilized, and normally the pulp is concentrated before it is refined. There have been recent proposals for the oxygen bleaching of pulp substantially at digester consistency (U.S. Patent No. 3,963,561), however, when bleaching is accomplished according to such proposals, washing and bleachin~ must still be done in separate containers, and a special type of ~essel must be used for the bleaching, such vessels not being readily - obtainable on the market.
According to the present invention, it is be-lieved that all of the above-mentioned drawbacks associated with conventional oxygen-bleaching techniques axe eliminated.
Ac ording to the present invention, refining is accom-plished substantially at digester consistency (and oxygen may be added before refining and an emulsion produced), and bleaching is accomplished at digester consistency.
_ 3 _ 4(~
Bleaching and washing take place in the same vessel, and the equipment that is used is conventional equipment readily available on the market today, only minor modi-fications th~reto being necessary in order to ef~ect oxygen bleaching. Also, caustic (NaO~) may be added at numerous positions in the various pulp and treating--agent lines, and therefore the concentration o the caustic added at each position may be minimized, thereby also minimizing the chances o~ random degradation of the pulp (which can occur when subjected to a high concentration o~ caustic). Also, the organic products from initial bleaching may readily be returned to the digester to provide ~supplement) the wash liquid in the wash zone of the digester~ and oxygen may be added to this return line to effect the pro~ision o an initial bleaching zo~e in the digester itself. Since reining and bleaching both take place at conventional digester pulp pressure (minus transmission line losses) and consist~ncy, there is no need to reduce the pressure or increase the concen-tration of the pulp before feeding it into the oxygentreatment diffuser, and thus much of the e~uipment associated with conventional oxygen-bleachin~ systems is eliminated, with attendant cost savings.
According to the teachings of the present in-vention, cellulose material is continuously digested in a ~ ,:
~L~L2~5 continuous digester to form pulp having a solids con-centration of about 6 to 15 percent, and the pulp is refined without significant dilution thereof. The pulp is passed into the inlet of a conventional diffusion washer having at least two stages, a bleaching stage and a washing stage, without dilution of the pulp, and the pulp is moved upwardly in the diffusion washer while subjecting the pulp to an oxygen bleaching agent in he bleaching stage of the pulp by difusing of the oxygen bleaching agent into the pulp. The pressure of the pulp in the diffusion washer is normally maintained at about 50 to 150 psi. Af-ter the pulp passes through one or more bleaching stages.in the diffusion washer, the pulp is subjected to a washing liquid in the washing stage of the diffusion washer, by diffusion of washing liquid into the pulp. The oxygen-bleached, washed pulp is then expelled from the diffusion washer, and any gaseous products of the bleaching process may be vented after expulsion from the diffusion washer.
According to the method of the present invention, oxygen may be aaded to the pulp both before ~he pulp is refined, and the li~uid withdra~l from each stage of the continuous washer is saturated with oxygen, and then recycled to the diffusion washer, whereby bleaching takes place over an extended period, and an emulsion of oxygen S
and pulp is produced. Also, liquid withdrawn from the first bleaching stage can be returned to the digester, and oxygen added to the return line, whereby initial bleaching can be institut~d in the wash zone of the 5 continuous digester. Additionally, caustic (NaOH) may be added to the pulp both before refining. thereof, and in each of the recycled lines. Thus, according to the method of the present invention, relatively inexpensive, readily a~ailable equipment is used for the effective oxygen bleaching and washing of pulp.
It is the primary object of the present invention to provide a simplified method for the oxygen bleaching and washing of digested pulp~ This and other objects of . the invention will become clear from an inspection of the detailed description of the invention, and from the . appended claims.
BRIEF DESCRIPTION OF THE DR~WINGS
Exemplary apparatus for practicing the method according to the present invention is shown diagramtically 2G in the drawing.
; - 6 -DET~ILED DESCRIPTION OF T~E DRAWINGS
The reference numerals in the drawing refer to the ~ollowing elements 10 General system 12 Conventional digester 13 Digester wash zone 13' Inlet for returned ~ash liquid 14 Pulp discharge from 15 Blow unit digester 16 Coarse refiner 16' Fine refiner 17 Digester discharge line 18 Inlet to diffusion washer 1~ Conventional di~fusion 20 1st bleaching stage washer 21 Second bleachi.ng stage 22 3rd bleaching stage 23 Diffusion washer shat 25 Recycle and return line pumps 24 Pressurized seal filtrate tanks 30 Wash stage 32 Source of oxygen gas 33-38 Oxygen addition lin~s 39 Source of wash liquid 40 Source of caustic 41-45 Caustic addition lines 50 Air pad 53 Washed, bleached pulp discharge 55 Blow unit 56 Pu~p discharge from blow unit 57 Ga5 discharge from blow 120 Bleaching agent inlet ~nit for 1st bleaching stage 121 Bleaching agent inlet 122 Bleaching agent inlet for 2nd bleaching stage for 3rd bleachiny stage of diffuser of diffuser ~241~S
130 Washing li~uid inlet 220 1st bleaching stage inlet for diffuser liquid withdrawal washing stage 221 2nd bleaching stage 222 3rd bleaching stage liquid withdrawal liquid withdrawal 223 Washing stage iiquid 320 Digester return line withdrawal from 1st bleaching stage liquid withdrawal 321 Recycle line from 2nd 322 Recycle line from 3rd bleaching stage ex- bleaching stage ex-traction to first traction to 2nd bleaching stage shower bleaching stage shower.
330 Recycle line from washing stage to 3rd bleaching stage shower The digester 12 and the refiners 16, 16' are conventional structures such as shown in Kamyr Bulletin ~o.
1100, November 1972, and the diffusion washer 19 is a conventional diffusion washer such as shown in U.S. Patent Nos. 3,599,449, and 3,815,386. The pulp is maintained at - a pressure of about 50 to 150 psi in the diffusion washer 19 by any suitable means, such as closing the top of the diffusion washer and establishing an air pad 50. Alter-natively, the diffusion washer 19 may be pressured hydraulicallY-Cellulose material is digested in digester 12, and is discharged through discharge means 14 and blow unit 15 into discharge line 17. Preferably, two refiners 16, 16' - a coarse refiner and a fine refiner - are dis-posed in the line 17, and oxygen is added from lines 37 and 38 to the pulp before refining, an emulsion of pulp ~z~r~ ~
and oxygen ~eing produced during refining. The Pulp emulsion is then fed to the inlet of the pressurized diffusion washer 19. Since the pressure of the pulp upon discharge from the digester 12 is nor~ally about S 150 to 300 psi, and since the pulp may be refined at such a pressure by the refiner 16, 16', after normal line losses without specifically relieving the pressure of the pump when it fed to the inlet 18 of the diffusion washer, it is at a pressure of about 50 to 150 psi. This is approximately the pressure desirable for oxygen bleaching (to proYide sufficient oxygen solubility), and thus normally ; no re-pressurization of the pulp itself is necessary before it is fed into inlet 18. Also, the concentration of the pulp is approximately 6 to 16 percent when discharged from the digester 12, and the pulp need not be diluted or conc ntrated when passed through the refiner 16 and 16' and into the diffusion washer 19, proper oxygen bleaching taking place in the diffusion washer 19 with the consistency of the pulp about 6 to 15 percent.
As the pulp moves upwardly in the diffusion washer 19, it passes through at least one bleaching stage 20, and a washing stage 30. Preferably, as shown in the drawing~ two or more accessory bleaching stages 21, 22, are provided in the diffusion washer. Bleaching agent for first bleaching stage 20 is added to the rotating vertical g s tubes associated with the first bleaching stage through an inlet 120 which passes down the middle shaft to the vertical tubes associated with the first bleachin~ stage 20 (as shown in U.S. Patent Nos. 3,815,386 and 3,599,449).
Similarly, bleaching agent for second bleaching stage 21 is added at 121, bleaching agent for third bleaching stage 22 is added at 122, and wash liquid (wa~er) for washing stage 30 is added at inlet 130. Displaced liquid from the pulp flowing through the diffusion washer l9 is with-drawn from ea~h of the difusion washer stages at liquidoutlets 220, 221, 222, and Z30, respectively, associated with stages 20, 21, 22 and 30. The withdrawn liquid from at least one bleaching stage is recylced to the inlet for a previous bleaching stage, and oxygen and caustic are added to the recycled liquid, a bleaching agent being formed~ The amount of oxygen that is added to the recycled liquid is enough to saturate the liquid at its existing pressure. In the preferred embodiment shown in the drawing, withdra~m liquid from outlet 221 passes through line 321 through a conventional pressurized filtrate tank 24, oxygen and caustic are added thereto lines 34 and 41~ respectively, and the liquid is trans-ported by pump 25 back to inlet 120 for the stage 20.
Liquid withdrawn through outlet 222 is passed into line 25 322, through 'conventional pressurized riltrate tank 24~
-- 10 -- .
~2~5 oxygen and caustic are added thereto by lines 35, 42, respectively, and the liquid is transported by pump 25 to inlet ~1 for stage 21. Liquid withdrawn from the wash stage 30 through outlet 223 passes through line 323 back to inlet 122 for the final bleaching stage, again caustic and oxygen ~eing added. In order to increase the effectiveness of the bleaching, and to save water, chemicals, and organic material, the liquid withdrawn from first bleaching stage 20 through outlet 220, passes through line 320 back to inlet 13' in the wash zone 13 or the digester 12, oxygen being added by line 33 to line 320. The liquid added at 13' in the digester 12 flows countercurrent to the cellul~se material flowing down-wardly in digester 12, to provide a was'n li~uid in wash zone 13, and also to effect initial bleaching of the digested pulp.
The washed, oxygen-bleached pulp is expelled from the top of the diffusion washer 19 at 53, and if there are any gaseous products that are produced as a result of the bleaching process (i.e., turpentine i the pump is produced mainly from pine, or carbon monoxide if the p~mp is produced mainly from certain hardwoods), those gaseous products are vented at 57 by feeding the pul~ into a blow unit 55.
.
. _ ~24~
It will thus be seen that according to the present invention effective o~ygen bleaching of pulp over an extended area, with a minLmum amount of equipment, is effected; cellulose material is continuously digested in a con~entional digester to form a pulp having a solids concentration of about 6 to 15 percent, the pulp is refined wlthout significant dilution or pressure reduction thereof (oxygen is added to the pulp before rerining so that an emulsion is formed), and the pulp is passed into the inlet of a diffusion washer having at least two stages, a bleaching stage and a washing stage, without dilu_ion of the pulp, or without pressure reduction thereof other than that which normally occurs in the transport of a suspension o~er a distance. The pulp is maintained at a pressure to about 50 to 150 psi in the diffusion washer, as it is passed upwardly in the diffusion washer through the bleaching stage and then the washing stage. In the bleaching st2ge, the pulp is subjected to an oxygen bleaching agent by diffusion of the oxygen bleaching agent into the pulp, and in thP wash stage the pulp is subjected to a washing liquid by diffusion of washing liquid into the pulp. Caustic is added at numerous different points~
and thereby the concentration of the caustic may be reduced, and the chances of random degradation minimized.
The withdrawn liquid from the first bleaching stage may ~2~5 be returned to the wash zone of the digester after addition of oxygen to it, whereby initial bleaching of the pulp may take place in the digester, thereby increasing the efficiency o~ the bleaching operation.
While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment thereof, it will be apparent to one of ordinary skill in the art that man~
modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent processes and methods.
BACKGROUND AND SIJ2~MARY OF THE INVENTION
The invention relates to a method of treating cellulose material to provide oxygen-bleached, washed pulp as the end product. Cellulose material is digested in a conventional continuous digester, and is fed to a diffusion washer in which oxygen bleaching is effected, the pulp being Xept under substantially digester pressure during bleaching in the diffusion washer.
At present, displacement bleaching of pulp in a diffusion washer is known, ~U.S.Patent No, 3,59g,449 and "Kamyr Diffusion Washers", Kamyr Bulletin No. 400DR, February 1975), however, the bleaching is accomplished -with C102 or the like at atmospheric pressure, and essen-tially all bleaching takes place in the diffusion washer itself so that the diffusion washer must thus be sized ~and the throughput con~rolled) accordingly to lnsure adequate bleaching. ~o chlorine is added to the pulp before re~ining thereof, nor is an initial stage of bleaching established in the digester associated with the diffusion washer. Since the diffuser pressure is less than the digester pressure (even considering normal transmission losses), some pressure reduction ~etween digester discharge and diffuser discharge must be effected.
~4~
Conventionally, oxygen bleaching of pulp is accomplished by utilizing an oxygen reactor, such as sho-~n in U.S. Patent No. 3,660,225, along with a large amount of accessory, expensive equipment~ such as shown in "Oxygen Bleaching" by John Kalisn, Pulp and Paper In~er-national~ March, 1975, pp. 53-59. The oxygen bleaching is done at relatively high consistency (i.e., 20-30~), and therefore it is necessary to concentrate the pulp between digester and bleaching. Also, separate washing and bleaching vessels are utilized, and normally the pulp is concentrated before it is refined. There have been recent proposals for the oxygen bleaching of pulp substantially at digester consistency (U.S. Patent No. 3,963,561), however, when bleaching is accomplished according to such proposals, washing and bleachin~ must still be done in separate containers, and a special type of ~essel must be used for the bleaching, such vessels not being readily - obtainable on the market.
According to the present invention, it is be-lieved that all of the above-mentioned drawbacks associated with conventional oxygen-bleaching techniques axe eliminated.
Ac ording to the present invention, refining is accom-plished substantially at digester consistency (and oxygen may be added before refining and an emulsion produced), and bleaching is accomplished at digester consistency.
_ 3 _ 4(~
Bleaching and washing take place in the same vessel, and the equipment that is used is conventional equipment readily available on the market today, only minor modi-fications th~reto being necessary in order to ef~ect oxygen bleaching. Also, caustic (NaO~) may be added at numerous positions in the various pulp and treating--agent lines, and therefore the concentration o the caustic added at each position may be minimized, thereby also minimizing the chances o~ random degradation of the pulp (which can occur when subjected to a high concentration o~ caustic). Also, the organic products from initial bleaching may readily be returned to the digester to provide ~supplement) the wash liquid in the wash zone of the digester~ and oxygen may be added to this return line to effect the pro~ision o an initial bleaching zo~e in the digester itself. Since reining and bleaching both take place at conventional digester pulp pressure (minus transmission line losses) and consist~ncy, there is no need to reduce the pressure or increase the concen-tration of the pulp before feeding it into the oxygentreatment diffuser, and thus much of the e~uipment associated with conventional oxygen-bleachin~ systems is eliminated, with attendant cost savings.
According to the teachings of the present in-vention, cellulose material is continuously digested in a ~ ,:
~L~L2~5 continuous digester to form pulp having a solids con-centration of about 6 to 15 percent, and the pulp is refined without significant dilution thereof. The pulp is passed into the inlet of a conventional diffusion washer having at least two stages, a bleaching stage and a washing stage, without dilution of the pulp, and the pulp is moved upwardly in the diffusion washer while subjecting the pulp to an oxygen bleaching agent in he bleaching stage of the pulp by difusing of the oxygen bleaching agent into the pulp. The pressure of the pulp in the diffusion washer is normally maintained at about 50 to 150 psi. Af-ter the pulp passes through one or more bleaching stages.in the diffusion washer, the pulp is subjected to a washing liquid in the washing stage of the diffusion washer, by diffusion of washing liquid into the pulp. The oxygen-bleached, washed pulp is then expelled from the diffusion washer, and any gaseous products of the bleaching process may be vented after expulsion from the diffusion washer.
According to the method of the present invention, oxygen may be aaded to the pulp both before ~he pulp is refined, and the li~uid withdra~l from each stage of the continuous washer is saturated with oxygen, and then recycled to the diffusion washer, whereby bleaching takes place over an extended period, and an emulsion of oxygen S
and pulp is produced. Also, liquid withdrawn from the first bleaching stage can be returned to the digester, and oxygen added to the return line, whereby initial bleaching can be institut~d in the wash zone of the 5 continuous digester. Additionally, caustic (NaOH) may be added to the pulp both before refining. thereof, and in each of the recycled lines. Thus, according to the method of the present invention, relatively inexpensive, readily a~ailable equipment is used for the effective oxygen bleaching and washing of pulp.
It is the primary object of the present invention to provide a simplified method for the oxygen bleaching and washing of digested pulp~ This and other objects of . the invention will become clear from an inspection of the detailed description of the invention, and from the . appended claims.
BRIEF DESCRIPTION OF THE DR~WINGS
Exemplary apparatus for practicing the method according to the present invention is shown diagramtically 2G in the drawing.
; - 6 -DET~ILED DESCRIPTION OF T~E DRAWINGS
The reference numerals in the drawing refer to the ~ollowing elements 10 General system 12 Conventional digester 13 Digester wash zone 13' Inlet for returned ~ash liquid 14 Pulp discharge from 15 Blow unit digester 16 Coarse refiner 16' Fine refiner 17 Digester discharge line 18 Inlet to diffusion washer 1~ Conventional di~fusion 20 1st bleaching stage washer 21 Second bleachi.ng stage 22 3rd bleaching stage 23 Diffusion washer shat 25 Recycle and return line pumps 24 Pressurized seal filtrate tanks 30 Wash stage 32 Source of oxygen gas 33-38 Oxygen addition lin~s 39 Source of wash liquid 40 Source of caustic 41-45 Caustic addition lines 50 Air pad 53 Washed, bleached pulp discharge 55 Blow unit 56 Pu~p discharge from blow unit 57 Ga5 discharge from blow 120 Bleaching agent inlet ~nit for 1st bleaching stage 121 Bleaching agent inlet 122 Bleaching agent inlet for 2nd bleaching stage for 3rd bleachiny stage of diffuser of diffuser ~241~S
130 Washing li~uid inlet 220 1st bleaching stage inlet for diffuser liquid withdrawal washing stage 221 2nd bleaching stage 222 3rd bleaching stage liquid withdrawal liquid withdrawal 223 Washing stage iiquid 320 Digester return line withdrawal from 1st bleaching stage liquid withdrawal 321 Recycle line from 2nd 322 Recycle line from 3rd bleaching stage ex- bleaching stage ex-traction to first traction to 2nd bleaching stage shower bleaching stage shower.
330 Recycle line from washing stage to 3rd bleaching stage shower The digester 12 and the refiners 16, 16' are conventional structures such as shown in Kamyr Bulletin ~o.
1100, November 1972, and the diffusion washer 19 is a conventional diffusion washer such as shown in U.S. Patent Nos. 3,599,449, and 3,815,386. The pulp is maintained at - a pressure of about 50 to 150 psi in the diffusion washer 19 by any suitable means, such as closing the top of the diffusion washer and establishing an air pad 50. Alter-natively, the diffusion washer 19 may be pressured hydraulicallY-Cellulose material is digested in digester 12, and is discharged through discharge means 14 and blow unit 15 into discharge line 17. Preferably, two refiners 16, 16' - a coarse refiner and a fine refiner - are dis-posed in the line 17, and oxygen is added from lines 37 and 38 to the pulp before refining, an emulsion of pulp ~z~r~ ~
and oxygen ~eing produced during refining. The Pulp emulsion is then fed to the inlet of the pressurized diffusion washer 19. Since the pressure of the pulp upon discharge from the digester 12 is nor~ally about S 150 to 300 psi, and since the pulp may be refined at such a pressure by the refiner 16, 16', after normal line losses without specifically relieving the pressure of the pump when it fed to the inlet 18 of the diffusion washer, it is at a pressure of about 50 to 150 psi. This is approximately the pressure desirable for oxygen bleaching (to proYide sufficient oxygen solubility), and thus normally ; no re-pressurization of the pulp itself is necessary before it is fed into inlet 18. Also, the concentration of the pulp is approximately 6 to 16 percent when discharged from the digester 12, and the pulp need not be diluted or conc ntrated when passed through the refiner 16 and 16' and into the diffusion washer 19, proper oxygen bleaching taking place in the diffusion washer 19 with the consistency of the pulp about 6 to 15 percent.
As the pulp moves upwardly in the diffusion washer 19, it passes through at least one bleaching stage 20, and a washing stage 30. Preferably, as shown in the drawing~ two or more accessory bleaching stages 21, 22, are provided in the diffusion washer. Bleaching agent for first bleaching stage 20 is added to the rotating vertical g s tubes associated with the first bleaching stage through an inlet 120 which passes down the middle shaft to the vertical tubes associated with the first bleachin~ stage 20 (as shown in U.S. Patent Nos. 3,815,386 and 3,599,449).
Similarly, bleaching agent for second bleaching stage 21 is added at 121, bleaching agent for third bleaching stage 22 is added at 122, and wash liquid (wa~er) for washing stage 30 is added at inlet 130. Displaced liquid from the pulp flowing through the diffusion washer l9 is with-drawn from ea~h of the difusion washer stages at liquidoutlets 220, 221, 222, and Z30, respectively, associated with stages 20, 21, 22 and 30. The withdrawn liquid from at least one bleaching stage is recylced to the inlet for a previous bleaching stage, and oxygen and caustic are added to the recycled liquid, a bleaching agent being formed~ The amount of oxygen that is added to the recycled liquid is enough to saturate the liquid at its existing pressure. In the preferred embodiment shown in the drawing, withdra~m liquid from outlet 221 passes through line 321 through a conventional pressurized filtrate tank 24, oxygen and caustic are added thereto lines 34 and 41~ respectively, and the liquid is trans-ported by pump 25 back to inlet 120 for the stage 20.
Liquid withdrawn through outlet 222 is passed into line 25 322, through 'conventional pressurized riltrate tank 24~
-- 10 -- .
~2~5 oxygen and caustic are added thereto by lines 35, 42, respectively, and the liquid is transported by pump 25 to inlet ~1 for stage 21. Liquid withdrawn from the wash stage 30 through outlet 223 passes through line 323 back to inlet 122 for the final bleaching stage, again caustic and oxygen ~eing added. In order to increase the effectiveness of the bleaching, and to save water, chemicals, and organic material, the liquid withdrawn from first bleaching stage 20 through outlet 220, passes through line 320 back to inlet 13' in the wash zone 13 or the digester 12, oxygen being added by line 33 to line 320. The liquid added at 13' in the digester 12 flows countercurrent to the cellul~se material flowing down-wardly in digester 12, to provide a was'n li~uid in wash zone 13, and also to effect initial bleaching of the digested pulp.
The washed, oxygen-bleached pulp is expelled from the top of the diffusion washer 19 at 53, and if there are any gaseous products that are produced as a result of the bleaching process (i.e., turpentine i the pump is produced mainly from pine, or carbon monoxide if the p~mp is produced mainly from certain hardwoods), those gaseous products are vented at 57 by feeding the pul~ into a blow unit 55.
.
. _ ~24~
It will thus be seen that according to the present invention effective o~ygen bleaching of pulp over an extended area, with a minLmum amount of equipment, is effected; cellulose material is continuously digested in a con~entional digester to form a pulp having a solids concentration of about 6 to 15 percent, the pulp is refined wlthout significant dilution or pressure reduction thereof (oxygen is added to the pulp before rerining so that an emulsion is formed), and the pulp is passed into the inlet of a diffusion washer having at least two stages, a bleaching stage and a washing stage, without dilu_ion of the pulp, or without pressure reduction thereof other than that which normally occurs in the transport of a suspension o~er a distance. The pulp is maintained at a pressure to about 50 to 150 psi in the diffusion washer, as it is passed upwardly in the diffusion washer through the bleaching stage and then the washing stage. In the bleaching st2ge, the pulp is subjected to an oxygen bleaching agent by diffusion of the oxygen bleaching agent into the pulp, and in thP wash stage the pulp is subjected to a washing liquid by diffusion of washing liquid into the pulp. Caustic is added at numerous different points~
and thereby the concentration of the caustic may be reduced, and the chances of random degradation minimized.
The withdrawn liquid from the first bleaching stage may ~2~5 be returned to the wash zone of the digester after addition of oxygen to it, whereby initial bleaching of the pulp may take place in the digester, thereby increasing the efficiency o~ the bleaching operation.
While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment thereof, it will be apparent to one of ordinary skill in the art that man~
modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent processes and methods.
Claims (15)
1. A method of treating cellulose material comprising the steps of sequentially:
(a) continuously digesting cellulose material in a continuous digester to form pulp having a solids concentration of about 6-15%, (b) refining the pulp without signifi-cantly changing the concentration thereof, (c) passing the pulp upwardly into the inlet of a diffusion washer having at least two stages, a bleaching stage and a washing stage, with-out significantly changing the concentration of the pulp, (d) subjecting the upwardly moving pulp in the diffusion washer to an oxygen bleaching agent in said bleaching stage by diffusion of said oxygen bleaching agent into the pulp, (e) subjecting the upwardly moving pulp in the diffusion washer to a washing liquid in said washing stage by diffusion of said washing liquid into the pulp, and (f) withdrawing bleached, washed pulp from the diffusion washer.
(a) continuously digesting cellulose material in a continuous digester to form pulp having a solids concentration of about 6-15%, (b) refining the pulp without signifi-cantly changing the concentration thereof, (c) passing the pulp upwardly into the inlet of a diffusion washer having at least two stages, a bleaching stage and a washing stage, with-out significantly changing the concentration of the pulp, (d) subjecting the upwardly moving pulp in the diffusion washer to an oxygen bleaching agent in said bleaching stage by diffusion of said oxygen bleaching agent into the pulp, (e) subjecting the upwardly moving pulp in the diffusion washer to a washing liquid in said washing stage by diffusion of said washing liquid into the pulp, and (f) withdrawing bleached, washed pulp from the diffusion washer.
2. A method as recited in claim 1 comprising the further step of adding oxygen to the pulp before refining thereof.
3. A method as recited in claim 1 wherein three bleaching stages and one washing stage are provided in the diffusion washer, and comprising the further steps of subjecting the upwardly moving pulp in the diffusion washer to an oxygen bleaching agent in all three bleaching stages by diffusion of said oxygen bleaching agent into the pulp, before passing the pulp to said washing stage.
4. A method as recited in claim 3 comprising the further steps of withdrawing liquid from each of said bleaching stages and said washing stage, recycling said withdrawn liquid from the second and third bleaching stages to inlets to bleaching stages via recycle lines, and adding oxygen gas under pressure to the recycled withdrawn liquid in the recycle lines.
5. A method as recited in claim 4 comprising the further step of adding caustic to each of the recycle lines of the diffusion washer, and adding caustic to the pulp before introduction thereof into the diffusion washer.
6. A method as recited in claim 4 wherein the digester for effecting digestion has a high heat washing zone and comprising the further step of with-drawing liquid from said first bleaching stage, and returning said liquid to the bottom of the digester via a return line, the withdrawn and returned liquid providing a countercurrent wash of the cellu-lose material in the digester washing zone, and adding oxygen to the liquid in the return line whereby a bleaching zone is established in the washing zone of the digester.
7. A method as recited in claim 4 comprising the further step of recycling liquid withdrawn from the washing stage to the final bleaching stage.
8. A method as recited in claim 4 wherein enough oxygen is added to the liquid in the recycle lines to saturate the liquid at its existing pressure.
9. A method as recited in claim 1 wherein said step of refining consists of the step of coarse refining and then fine refining.
10. A method as recited in claim 9 comprising the further step of adding oxygen to the pulp before coarse refining thereof.
11. A method as recited in claim 10 comprising the further step of adding oxygen to the pulp before fine refining thereof.
12. A method as recited in claim 9 comprising the further step of adding oxygen to the pulp before fine refining thereof.
13. A method as recited in claim 1 wherein the pressure of pulp introduced into the diffusion washer is about 50-150 psig, and wherein this pres-sure is maintained in the diffusion washer.
14. A method as recited in claim 13 wherein the pressure is controlled in said diffusion washer by providing an air pad over the pulp at the top of the washer.
15. A method as recited in claim 1 comprising the further step of blow-venting the bleached-washed pulp from the diffusion washer as it is withdrawn from the diffusion washer to vent any gaseous products produced during bleaching.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US89845978A | 1978-04-20 | 1978-04-20 | |
| US898,459 | 1978-04-20 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1124015A true CA1124015A (en) | 1982-05-25 |
Family
ID=25409491
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA325,828A Expired CA1124015A (en) | 1978-04-20 | 1979-04-19 | Diffuser oxygen bleaching |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1124015A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4543155A (en) * | 1983-01-31 | 1985-09-24 | The Boc Group, Inc. | Method for bleaching wood pulp including dissolving oxygen into the dilution water of an extraction stage |
-
1979
- 1979-04-19 CA CA325,828A patent/CA1124015A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4543155A (en) * | 1983-01-31 | 1985-09-24 | The Boc Group, Inc. | Method for bleaching wood pulp including dissolving oxygen into the dilution water of an extraction stage |
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